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THESlS

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[999

«uniIiiﬂillﬂijiiiii‘iﬁimull ”

12 7047

This is to certify that the

dissertation entitled

Interactions of Copper-Amine
Preservatives with Southern Pine

presented by

Jun Zhang

has been accepted towards fulﬁllment
of the requirements for

Ph.D. degree in Forestry

ﬁA/pﬂg (14% ‘34.de

 

 

Major professor

Date @JU‘iqqq

MS U is an Affirmative Action/Equal Opportunity Institution 042771

 

 

 

 

LIBRARY

Michigan State

University

 

 

PLACE IN RETURN Box to remove this checkout from your record.
TO AVOID FINES return on or before date due.
MAY BE RECALLED with earlier due date if requested.

 

DATE DUE

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ma mmmj4

INTERACTIONS OF COPPER-AMINE PRESERVATIVES WITH SOUTHERN PINE

By

Jun Zhang

A DISSERTATION

Submitted to
Michigan State University
in partial fulﬁllment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Forestry

1 999

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ABSTRACT
INTERACTIONS 0F COPPER-AMINE PRESERVATIVES WITH SOUTHERN PINE
By

Jun Zhang

The study of the interactions between copper amine preservatives (Cu-EA) and
wood substrate is extremely important, since it impacts greatly both on the performance
and the environmental consideration of treated wood. The objectives of this research are
to investigate copper amine-wood interactions, examine the copper bonding sites and
elucidate the copper ethanolamine ﬁxation mechanism. To achieve the objectives, the
eﬂ‘ects of copper source, amine ligand and amine to copper molar ratio on copper
retention and leaching were studied. The copper amine treated wood samples were
characterized by FTIR, electron paramagnetic resonance (EPR), X-ray diffraction (XRD)
and X-ray photoelectron spectroscopy (XPS).

The results of copper-amine treatment reveal that the retention and leachability of
copper in copper amine treated southern pine (SP) are inﬂuenced by the formulation and
the composition of copper amine treating solutions. The sources of copper used,
Cu(OH)2, CuCO3, CuSO4 and Cu(N03)2, in the copper amine formulations affect the
leachability of copper. Copper amine formulations made from CuSO4 and Cu(N03)2 show
less copper loss during laboratory water leaching than those ﬁ'om Cu(0H)2 and CuCOa.
Increasing amine to copper molar ratio increases the copper retention in wood and the

leaching of copper. The nature of amine ligands has some effects on copper retention and

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copper leaching. As the molecular weight of amine ligand increases, the copper loss during
leaching decreases.

FTIR analyses show that treatment of SP with Cu-EA causes a signiﬁcant
reduction in the band at 1739 cm'1 attributed to carbonyl vibration from carboxylic acid
groups and an increase in band at 1596 cm'1 ﬁom carbonyl in carboxylate. The same result
is observed in Cu-EA treated holocellulose. Cu-EA treatment of lignin results in a
reduction in the aromatic ester band at 1712 cm'l and an increase in carbonyl from
carboxylate at 1595 cm]. Bands at 1370 cm'1 and 1221 cm", assigned to phenolic
hydroxyl groups, exhibit a decrease in intensity aﬁer Cu-EA treatment.

EPR axial spectra are obtained for all Cu-amine treated samples irrespective of the
formulations. The values of A" and gu of the axial EPR spectra indicates that the stereo-
structure of copper complexes in copper amine treated wood is either tetragonal-based
octahedral or square-based pyramidal. Comparison of electronic parameters of A" and g" in
Cu-amine treated wood with those of the Cu-amine treating solution and the values
reported in the literature suggests that the copper complexes in both treating solution and
treated wood are in the form of CuN202, where copper is ligated with 2 nitrogen and 2
oxygen in the equatorial plane. XRD does not ﬁnd any crystalline copper compound in
Cu-EA treated wood, and XPS indicates that the valency state of copper in treated wood

was cupric.

To my parents and my lovely wife Hua

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ACKNOWLEDGMENTS

This work was made possible only through the help and support of many people. I
would like to express my sincere appreciation to Dr. D. Pascal Kamdem, my major
professor, for his guidance, patience and friendship. I have learned a great deal from him.
His unselﬁsh support and encouragement through my lengthy research program has been
greatly appreciated. I would also like to thank the members of my guidance committee Dr.
Craig R. McIntyre, Dr. Douglas A. Gage and Dr. Raymond C. Francis for their invaluable
suggestions, advice, guidance and constructive critiques of my work. Additionally, I want
to thank Dr. Rui H. Huang for helping me with the X-ray dif’r‘ractometer and Dr. John
McCracken for his valuable suggestions in EPR spectrum interpretation.

Successﬁrl completion of my research is owed in large part to the other members
of the wood science lab: Justin Zyskowski, Maldas Debesh, Weining Cui, Ismail Jusoh,
and Wanli Ma. They provided not only moral support, but also a sounding board for ideas
and insights for data interpretation.

Funding for this research was provided by the USDA-CSREES Eastern Hardwood
Utilization Program in the Department of Forestry at Michigan State University. The
continued ﬁnancial support throughout my research program was grateﬁrlly
acknowledged.

Finally, but not the least important, my deepest gratitude goes to my wife Hua for

her love, encouragement, patience and support during this journey.

LIST 1
LIST (

CRAP

CHAPT

Re

TABLE OF CONTENTS

LIST OF TABLES ............................................................................................................ ix
LIST OF FIGURES ........................................................................................................... X
CHAPTER 1 INTRODUCTION ...................................................................................... 1
1.1 Overview .................................................................................................... 1
1.2 Wood Anatomy and Its Chemistry .............................................................. 3
1.2.1 Anatomical Asoects ......................................................................... 3
122 Chemical Composition and Its Distribution ...................................... 4
1.3 Wood Deterioration .................................................................................... 11
1-4 Role of Copper as a Fungicide .................................................................... 12
1.5 Interactions of Wood with Copper-Based Wood Preservatives ,,,,,,,,,,,,,,,,,,,, 14
1-5-1 Ion-Exchange Theory ...................................................................... 14
1-5-2 Cooper Forms in Treated Wood ...................................................... 16
1.5.3 Physiochemical Analysis of Copper-Based Preservative Treated
Wood .............................................................................................. 18
1.6 Chemistry of Copper-Amine Wood Preservative System ,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 20
1-7 Objectives ................................................................................................... 23
References 24

ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo

CHAPTER 2 EFFECT OF COPPER-AMINE COMPOSITION ON COPPER

ABSORPTION AND LEACHING ............................................................. 31
2.1 Abstract ...................................................................................................... 31
2.2 Introduction ................................................................................................ 31
2.3 Materials and Methods ............................................................................... 33
2.3.1 Formulation of Copper Amine (Cu-EA) Treating Solutions ,,,,,,,,,,,,, 33
2.3.2 Treatment ....................................................................................... 33
2.3.3 Leaching ......................................................................................... 34
2.4 Results and Discussion ................................................................................ 34
2-4-1 Effect of Cooper Sources ................................................................ 34
24-2 Effect of Amine Ligands .................................................................. 38
2-4-3 Effect of Amino to Cooper Molar Ratios ......................................... 45
2-4-4 Cooper Fixation Mechanism ............................................................ 50
2.5 Conclusions ................................................................................................ 56
References ............................................................................................................. 57

C HA.

CHA

CHAP

CHAPI

CHAPTER 3 EFFECT OF WOOD COMPOSITION ON COPPER ABSORPTION ,,,,,,,, 59

3.1 Abstract ...................................................................................................... 59
3 .2 Introduction ................................................................................................ 59
3.3 Materials and Methods ............................................................................... 60
331 Materials ......................................................................................... 60
332 Treatment ....................................................................................... 61
3-3-3 Analysis of Phenolic Hydroxyl Groups ............................................ 61
3.4 Results and Discussion ................................................................................ 62
References ............................................................................................................. 71

CHAPTER 4 INVESTIGATION OF COPPER BONDING SITES BY FOURIER
TRANSFORM INFRARED (FTIR) SPECTROSCOPIC ANALYSIS 73

4.1 Abstract ...................................................................................................... 73
4.2 Introduction ................................................................................................ 74
4.3 Materials and Methods ............................................................................... 75
431 Materials ......................................................................................... 75
4.3.2 Oxidation of Cellulose ..................................................................... 75
4.3.3 Treatment ....................................................................................... 77
4.3.4 Fourier Transform Inﬁ'ared Spectroscopy (FTIR) ,,,,,,,,,,,,,,,,,,,,,,,,,,,, 77
4.4 Results and Discussion ................................................................................ 77
4.5 Conclusions ................................................................................................ 90
References 91

OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO

CHAPTER 5 ELECTRON PARAMAGNETIC RESONANCE SPECTROSCOPIC

(EPR) ANALYSIS OF COPPER-AMINE TREATED WOOD ,,,,,,,,,,,,,,,,,, 93
5.1 Abstract ...................................................................................................... 93
5.2 Introduction ................................................................................................ 93
5.3 Theoretical Principles of the EPR of Copper Complexes ,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 95
5.4 Materials and Methods ............................................................................... 97
541 Materials ......................................................................................... 97
542 Treatment ....................................................................................... 99
5.4.3 Electron Paramagnetic Resonance Spectroscopy (EPR) ,,,,,,,,,,,,,,,,,,, 99
5.5 Results ....................................................................................................... 99
5.6 Discussion .................................................................................................. 1 14
5.7 Conclusions ................................................................................................ 1 19
References ............................................................................................................. 120

CHAPTER 6 X-RAY DIFFRACTION (XRD) AND X-RAY PHOTOELECTRON
SPECTROSCOPIC (XPS) CHARACTERIZATION OF COPPER-

vii

CPU

AMINE TREATED WOOD SUBSTRATES

oooooooooooooooooooooooooooooooooooooooooooooo

122

6.1 Abstract ...................................................................................................... 122
6.2 Introduction ................................................................................................ 122
6.3 Materials and Methods ............................................................................... 123
6.3.1 Treatment ....................................................................................... 123

63-2 X-tay Diffractomctry ....................................................................... 124

63-3 X-tay Photoelectron Spectroscorry .................................................. 124

6.4 Results and Discussion ................................................................................ 124
64.1 X-tay Diffraction (XRD) Analysis ................................................... 124

6.4.2 X-ray Photoelectron Spectroscopic (XPS) Analysis ,,,,,,,,,,,,,,,,,,,,,,,,, 128

6.5 Conclusions ................................................................................................ 136
References ............................................................................................................. 13 7
CHAPTER 7 CONCLUSIONS ........................................................................................ 138

viii

Table

Table

Table

Table

Table

Table .
Table :
Table :

Table l

Table 1.1

Table 2.1

Table 2.2

Table 3.1

Table 3.2

Table 4.1

Table 5.1

Table 5.2

Table 6.1

LIST OF TABLES

Percentage of wood components in cell wall ............................................... 5

pH of copper monoethanolamine treating solutions from diﬂ‘erent copper
compounds with amino to c0pper molar ratio of 4 ....................................... 36

pH of copper amine treating solutions containing 0.5% copper ﬁ'om copper
hydroxide and different amine ligands ......................................................... 54

Copper absorption in wood substrates treated with 0.5 wt% copper amine
solution ...................................................................................................... 67

Copper absorption in wood substrates treated with 1.0 wt% copper amine

solution ...................................................................................................... 68
Assignments of infrared absorption bands in wood ...................................... 79
Copper amine formulations used for wood substrate treatment ,,,,,,,,,,,,,,,,,,,, 98
EPR parameters of Southern pine treated with copper amine solutions ,,,,,,,, 112

Atomic composition in the surface of wood and treated wood by ESCA
analysis ....................................................................................................... 135

ix

Figure 1.

Figure 1.

Figure 2

Figure 2

Figure 2

Figure ‘.

Figurg

 

Figure 1.1

Figure 1.2

Figure 2.1

Figure 2.2

Figure 2.3

Figure 2.4

Figure 2.5

Figure 2.6

Figure 2.7

Figure 2.8

Figure 3.1

Figure 3.2

Figure 4.1

Figure 4.2

LIST OF FIGURES

Stereo-chemical structure of cellulose ......................................................... 7

The building units of lignin: (a). p-coumaryl alcohol, (b). coniferyl alcohol
and (c)- sinapyl alcohol ............................................................................... 9

Eﬁ‘ect of copper sources on copper retention. Copper amine solutions were
made by mixing monoethanolamine with different copper sources ,,,,,,,,,,,,,,, 3 5

Effect of copper sources on copper leaching from copper amine treated
samPles ....................................................................................................... 39

Effect of amine ligands on copper retention. Copper amine solutions were
made by mixng c0pper hydroxide with different amines .............................. 41

Eﬂ‘ect of amine ligands on copper leaching from copper amine treated
sarnples ....................................................................................................... 43

Effect of amine to copper molar ratios on copper retention. Copper amine
solutions were made by mixing copper hydroxide with amines at different
ratios 46

oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo

Effect of amine to copper molar ratios on copper leaching ﬁom copper
amine treated samples ................................................................................. 48

Top: Amine ligands. Bottom: Copper amine complexes ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 50

Copper amine-wood interactions.
(A). Ligand exchange and (B)- Complexation ............................................. 52

Eﬁ‘ect of wood components and exposure time on copper absorption ﬁ'om
05% copper amine treating solution ........................................................... 63

Effect of wood components and exposure time on copper absorption from

10% copper amine treating solution ........................................................... 65
Oxidation of cellulose into oxidized cellulose .............................................. 76
FTIR spectra of (A). Wood and (B). Cu-MEA treated wood ,,,,,,,,,,,,,,,,,,,,,, 78

Figure s‘

Figure I

Figure l

Figure 4.3

Figure 4.4

Figure 4.5

Figure 4.6

Figure 4.7
Figure 5.1
Figure 5.2
Figure 5.3
Figure 5.4
Figure 5.5
Figure 5.6
Figure 5.7
Figure 5.8

Figure 5.9

Figure 5.10

Figure 5.11

Figure 5.12

Figure 5.13

Figure 6.1

Figure 6.2

Subtraction of spectrum of untreated wood from that of Cu-MEA treated
wood ..........................................................................................................

F TIR spectra of (A). Cellulose and (B). Cu-MEA treated cellulose ,,,,,,,,,,,,,

FTIR spectra of (A). Cellulose; (B). Oxidized cellulose and (C). Cu-MEA
treated oxidized cellulose ............................................................................

F TIR spectra of (A). Holocellulose and (B). Cu-MEA treated
holocellulose ...............................................................................................

F TIR spectra of (A). Lignin and (B). Cu-MEA treated lignin ,,,,,,,,,,,,,,,,,,,,,,
EPR spectrum of Cu-MEA frozen solution ................................................
EPR spectrum of wood treated with Cu-MEA at pH of 10.8 ,,,,,,,,,,,,,,,,,,,,,,,
EPR spectrum of wood treated with Cu-MEA at pH of 10.1 ,,,,,,,,,,,,,,,,,,,,,,,
EPR spectrum of wood treated with Cu-MEA at pH of 9.3 ,,,,,,,,,,,,,,,,,,,,,,,,,
EPR spectrum of wood treated with Cu-MEA at pH of 9.1 ,,,,,,,,,,,,,,,,,,,,,,,,,
EPR spectrum of wood treated with Cu-MeEA ..........................................
EPR spectrum of wood treated with Cu-DMeEA ........................................
EPR spectrum of wood treated with CuSO4 solution ...................................
EPR Spectrum of wood treated with CU(N03)2 solution ..............................
EPR spectrum of lignin treated with Ctr-MBA ............................................
EPR spectrum of oxidized cellulose treated with Cu-MEA ,,,,,,,,,,,,,,,,,,,,,,,,,,
Orbital energy diagram for Cu(H) complexes ..............................................

Correlation of All and gll .............................................................................

XRD patterns of (A). Untreated wood; (B). Cu-MEA treated wood; (C).

Cu-MeEA treated wood; (D). Cu-DMeEA treated wood and (E). Cu-MEA

treated lignin ...............................................................................................

XRD patterns of post-steamed (A). Untreated wood; (B). Cu-MEA treated
wood; (C). Cu-MeEA treated wood; (D). Cu-DMeEA treated wood and
(P3)- Cit-MBA treated lignin ........................................................................

81

82

83

84

85

100

101

102

103

104

105

106

107

108

109

110

115

118

125

127

Figure

Figure

Figure 6.3 XPS survey spectra of (A). Untreated wood and (B). Cu-MEA treated
wood .......................................................................................................... 129

Figure 6.4 XPS Cu2p spectra of (A) CuSO4 treated wood and (B). Cu-MEA treated
wood 132

oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo

xii

 

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Chapter 1

Introduction

1.1 Overview

Wood, a remarkable material of great importance in the world economy, is widely
used as a structural material, fuel, and industrial raw material. However, it is subject to
biodegradation through ﬁrngal and/or insect attack under a warm and highly humid
environment. So, as a renewable natural resource, wood is also biodegradable. The longer
it lasts, the less we harvest the forest. Preservative chemicals are commonly used to
enhance wood durability. It is estimated that the US industry treated about 727.8 million
cubic feet of wood products with chemicals in 1997 (Micklewright, 1998). The
preservative treatment increases the service life of wood products by a factor of 10 to 20
times. According to an estimate by the Southern Forest Products Association, pressure
treatment of wood material with preservative chemicals saves 226,000,000 trees each
year in the US alone. Without the use of preservative-treated wood, the wood ﬁber
demands might have exhausted our forest resources years ago.

Preservative treatment of wood will not only extend its service life under severe
outdoor conditions, such as high moisture, oxygen, UV irradiation and insect and fungus
attacks, but also improve its market potential in the areas where wood products are facing
competition from other polymeric materials. However, some concerns about the use of

preservative treated wood are the bioefﬁcacy against fungus and insect attack, and the

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environmental acceptability. As a result, more and more research in the area of wood
preservation has been directed to develop new preservatives with acceptable efﬁcacy and
low toxicity to humans and environment.

Copper amine is one of the copper-based wood preservative systems. It is used as
a source of copper for several new preservative systems including ammoniacal copper
quat-type D (ACQ-D), copper dimethyldithiocarbamate (CDDC), and copper azole
(AWPA, 1998). Copper amine has gained more and more attention in wood preservation
due to its low mammalian toxicity, its ﬁxation ability and the absence of odor. However,
relatively little research has been done on the ﬁxation, retention and leaching of copper in
copper amine treated wood. The retention and the bonding of copper are considered
extremely important because they may inﬂuence the performance and the environmental
impact of the treated wood products.

Chemically speaking, the copper amine (Cu-EA) system can be water-home
copper complexes in which different amines are used as ligands or chelating agents. Cu-
EA is formulated by mixing copper source from copper hydroxide, copper carbonate,
copper sulfate or copper nitrate, with aqueous amine ligands. The ligands include mono-
ethanolamine (primary amine), 2-methyl-amino-ethanol (secondary amine) and N, N-
dimethyl-ethanolamine (tertiary amine). The copper source, amine ligand and ligand to
metal ratio may inﬂuence the copper retention and leachability of copper in Cu-EA
treated wood. In this study, the effects of copper source, amine ligand, and molar ratio of
amine to copper on the retention and leaching of copper have been investigated. The
ﬁxation mechanism, the bonding sites for copper and the interactions of wood with

copper have also been examined.

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1.2 Wood Anatomy and Its Chemistry

Wood is one of the most easily used products. It can be cut and shaped with ease
and fastened readily. At the same time, wood is one of the most complex materials. It is
made up of specialized tiny cells with deﬁned structure. The anatomical structure and the
chemical composition of wood probably inﬂuence the interactions between wood and
copper-based preservatives.

1.2.1 Anatomical Aspects

Wood is the secondary xylem formed by cell division in the vascular cambium of
both soﬁwood and hardwood. Softwood species are known as coniferous trees and
hardwood as deciduous trees. Softwood contains relatively simple structure with 90-95%
tracheids that are long and slender cells with ﬂattened or tapered closed edges. The
tracheids are arranged in radial ﬁles, and their longitudinal extension is oriented in the
direction of the stem axes. Hardwood has a basic tissue for strength containing ﬁber, ﬁber
tracheids, vessel and parenchyma. Within the strengthening tissue are distributed
conducting vessels, often with large lumina (F engel and Wegener, 1984). These vessels
are long pipes ranging ﬁom a few centimeters up to some meters in length and consisting
of single element with open or perforated ends.

In all species of wood, the cells composing the axial system and the cells of the
ray system are tightly integrated to produce a rigid material. The strength properties of
wood are determined by the thickness of the cell walls and the microﬁbrillar angle of the
cellulose component. Additionally, the elongated nature of the axial cells that make up

the greatest proportion of the cell types present makes a signiﬁcant contribution to

 

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strength. In contrast, the ray cells have little strengthening ﬁrnction either in living trees
or in commercial timber because of their radial elongate nature.

Wood cell walls are complex in structure. Most are composed of primary (P) and
secondary (S) wall layers. A detailed structure of the cell wall shows that the secondary
cell wall is composed of several layers, 81, 82 and 83 with different microﬁbril
orientation. The primary wall, developing ﬁrst as cells, undergoes differentiation, while
the thicker secondary wall is laid down inside the primary wall. Between cells there is a
thin layer, the middle lamella (M) which glues the cells together to form tissues. The
transition from the middle lamella to the adjacent cell wall layers is not very clear, so a
term, compound middle lamella, is used to describe parts encompassing the middle
lamella and the two neighbor cell walls (Eaton and Hale, 1993).

1.2.2 Chemical Composition

Major wood components include cellulose, hemicellulose and lignin. Cellulose is
the most abundant component of wood and constitutes slightly less than one-half of the
wood in both hardwood and softwood. The proportion of lignin and hemicellulose varies
widely among species and between hardwood and softwood. Table 1.1 shows the
approximate percentage of dry weight of each in hardwood and softwood. In addition,
wood also contains small amount of extractives that may vary from O to 10%.
1.2.2.1 Cellulose

The structure of cellulose was thoroughly surveyed by Purves (1954), and
Marchessault and Sundararajan (1983). Cellulose is present as the main structural

component in wood cell walls in the form of microﬁbrils. The cellulose microﬁbrils

Table 1.1 Percentage of wood components in cell wall (Haygreen and Bowyer,
1996)

 

Type Cellulose Hemicellulose Lignin

 

Hardwood 40-44 15-35 18-25

Softwood 40-44 20-32 25-35

 

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impart strength to the cell wall.

Cellulose is a polydisperse linear natural polymer. The basic monomeric unit of
cellulose is D-glucose, which is linked through glycosidic bonds in the beta conﬁguration
between carbon 1 and carbon 4 of adjacent units to form a long chain of l, 4-B-linkages
(Figure1.1). Each B-D-glucopyranose unit within a cellulose chain has three types of
hydroxyl groups, one primary (Cs-0H) and two secondary (Cz-OH and C3-OH). The
hydroxyl groups frequently form intra- and inter-molecular hydrogen bonds within and
among molecules. These bonds, together with other secondary force, such as Van der
Waals force, aggregate portions of the molecular chains into various degrees of lateral
order ranging from perfect geometrical packing of the crystal lattice (the so-called
crystalline region) to the random fraction (amorphous region).

Although there are OH-groups at both ends of the cellulose chains, these OH-
groups show different behaviors. The Cl-OH is an aldehyde hydrate group deriving from
the ring formation by an intra-molecular hemiacetal linkage. This is why the OH-groups
at the Cl-end have a reducing property, while the OH-groups at the C4-end of the
cellulose chains are alcoholic hydroxyl and therefore non-reducing. The topochemistry of
cellulose actually controls the chemical reactivity of cellulose. Essentially, the hydroxyl
groups located in the amorphous regions react readily in many chemical reactions. In the
crystalline regions, where there are close packing and strong inter-chain bonding, the
hydroxyl groups are not readily accessible to reactant.
1.2.2.2 Lignin

Next to cellulose, lignin is the most abundant and important polymeric organic

substance in wood. It exists as one of the essential wood components ranging in amount

HO

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ﬁom 20 to 30% (F engel and Wegener, 1984). The distribution of lignin within the cell
wall and the lignin content in different parts of a tree are not uniform. It is thought that
lignin is a polymer formed by the enzymatic dehydrogenation of phenylpropanes
followed by radical coupling (Eaton and Hale, 1993). Softwood lignin is mainly
composed of guaiacyl units originating from the predominant precursor, trans-coniferyl
alcohol (Figure 1.2b), while hardwood lignin is composed of both guaiacyl and syringyl
units derived from trans-coniferyl and trans-sinapyl alcohol (Figure 1.2c). Analysis of
structural elements of lignin demonstrates that carbonyl groups in lignin exists in
different types including aldehydes, unconjugated ketones, conjugated ketones and
carboxyl groups. The phenolic groups and aliphatic hydroxyl groups of lignin have been
quantitatively determined by various methods (Sarkanen and Schuerch, 1955; Adler and
Hemestam, 1955; Robert and Brunow, 1984).

Ultraviolet (UV) spectroscopy, particularly when used along with chemical
modiﬁcation, has contributed to estimating the frequencies of functional groups and
linkage types. Lignin shows a strong absorption spectrum in the UV region because of its
aromatic nature. In general, softwood lignin shows a maximum absorption at 280.285nm,
and hardwood lignin at 274-276nm (Terashima, 1978). Aulin-Erdtman (1963) and Aulin-
Erdtman and Sanden (1968) applied UV spectroscopy to estimate effectively the amounts
of the phenolic hydroxyls of lignin.

Inﬁ'ared spectroscopy has also been used for the characterization of lignin, and the
major absorption band frequencies and the most probable assignment of each band in
guaiacyl and guaiacyl-syringyl lignins have been summarized by Hergert (1971). The

absorption bands at 1605-1595 cm", 1515-1505 cm", and 1450-1420 cm’1 are assigned to

CHon (ISHon (IZHZOH

I

CH CH CH

u u u
HC H0 H0

OCH3 H300 OCH3

OH 0H 0H

a b c

Figure 1.2 The building units of lignin: (a). p-coumaryl alcohol, (b). coniferyl alcohol
and (c). sinapyl alcohol (F engel and Wegener, 1984)

 

aron'
hgnl
unhs
byth
andg
1984
guan
gave

band

resea
Ludv
Vaﬁo
dehy
(heqt
bunt
“ﬁbc
Carbc

1.2.2

ASpiI

molec

10

aromatic skeletal vibration. IR spectroscopy has also been used to estimate the content of
lignin (Kolboe and Ellefsen, 1962) and the ratio of syringylpropane to guaiacylpropane
units in hardwood lignin (Sarkanen et al., 1967a and b). The lignin content was estimated
by the difference of spectra between the original wood and holocellulose at 1515 cm",
and gave 28% to 29% that is in agreement with other estimations (Fengel and Wegener,
1984). Sarkanen et al. (1967a and b) compared the spectra of speciﬁcally deuterated
guaiacyl and syringyl model compounds with those of undeuterated lignin models, and
gave several new band assignments. The 1340-1380 cm'1 and 1250-1150 cm'1 absorption
bands were assigned to phenolic hydroxyl groups.

The magnetic resonance spectroscopic tool is very useful in lignin chemistry
research. The ﬁrst 1I-INMR spectra of lignin and its model compounds were obtained by
Ludwig et al. (1964a and b). The chemical shifts of the NMR signals from protons in
various model compounds have been determined (Ludwig et al., 1964 b). The NMR of
dehydrodiconifyl alcohol indicates a cis-conﬁguration in its ﬁrran ring and the
diequatorial configuration of pinoresinol (Ludwig et al., 1964a and b). Lﬁdemann and
Nimz (1974) studied the ”CNMR spectra of lignins. The chemical shifts of various
carbons in lignin model compounds, and the effect on the chemical shifts of the aromatic
carbon atoms from methoxyl ortho to the 4-phenolic hydroxyl group were examined.
1.2.2.3 Hemicellulose

The chemistry of hemicelluloses (polyoses) has been reviewed extensively by
Aspinall (1959) and Timell (1964 and 1965). Hemicelluloses differ from cellulose in the
composition of various sugar units, molecular chains and branching of the chain

molecules. The basic sugar units making up the hemicelluloses are grouped into

careg

arabi
hemi

poly<

than

gr 0”]

1.3

insec
majo
resul‘
cons:
destr.

SOﬁr

leaVe

andC

11

categories such as pentoses, hexoses, hexuronic acids and deoxy-hexoses (Fengel and
Wegener, 1984). The sugar units contain glucose, mannose, galactose, xylose, and
arabinose. Softwood and hardwood differ in the different percentages of total
hemicelluloses but also in the percentage of individual polyoses and composition of these
polyoses. Softwood tends to have a higher proportion of mannose and galactose units
than hardwood, while hardwood has a higher proportion of xylose units and more acetyl

groups than softwood.

1.3 Wood Deterioration

Wood products, when used out of doors, are subjected to decay, fungal stains, and
insect infestation, all of which cause the deterioration of wood. Biological agents are the
major causes of wood deterioration. The greatest ﬁnancial losses from biodeterioration
result ﬁ'om wood decay fungi. These fungi feed on the compounds of the cell wall and
consequently weaken the structure of the wood to such an extent that wood breaks. Wood
destroying ﬁJngi can be classiﬁed into three categories, namely brown rots, white rots and
soft rots.

The brown rots selectively attack the cellulose and hemicellulose of the cell and
leave the darker lignin more or less intact. Wood, after attack, seriously degraded and
remained an abnormally brownish color. The surface can become badly broken by deep
transverse and longitudinal cracks. The most common brown rots are often found
attacking softwood timbers and the lighter hardwoods.

In the white rot type of ﬁJngi, the fungi have the ability to degrade both the lignin

and cellulosic components of the cell although the lignin is usually decayed at a faster

rate. T
its stre

becomi

physic:
fungi d
commc
axis of
$011 an

suscep:

Proper
Of all r

Essenr

1.4

Either ‘
TESpira
aCllV’it:
interac

mediﬁ

12

rate. The affected wood eventually becomes much lighter in color and weight, and loses
its strength properties. White rotted wood usually retains its shape but may eventually
become a ﬁbrous spongy mass.

Soft rot fungi most often attack wood that is wet or contact with ground. The
physical and chemical character of the form of wood cell attack caused by this type of
fungi differs markedly from that of the above two types of ﬁmgi. Decomposition
commonly results from the organism making longitudinal cavities in and parallel to the
axis of the cell wall. In wet wood, the presence of soft rot is evident if surface layers are
soft and may be readily scraped away. Hardwoods are thought to be naturally more
susceptible to soft rots than soltwoods though no wood is completely resistant.

In addition to wood destroying fungi, some other agents that affect wood
properties are staining fungi, moulds, wood borers, termites and so forth. The existence
of all these factors could dramatically reduce the service life of wood products.

Essentially, Fungicide is widely used to protect wood products from deterioration.

1.4 Role of Copper as a Fungicide

Many fungicidal chemicals used as wood preservatives interfere with respiration
either by inhibiting the formation of acetyl coenzyme A (CoA) or by interrupting
respiratory chain phosphorylation (Eaton and Hale, 1993). The mode of fungicidal
activity of copper-based wood preservatives is assumed to involve metal-enzyme
interactions, generation of highly reactive free radical by copper ions, and the

modiﬁcation of DNA (Hertzberg and Dervan, 1984).

 

oxic

enr.

enz;

The
8117—?
cata
to b
met
1 96'
ﬁrm

cup:

radic;

Koba

13

Divalent cations of the ﬁrst transition series, such as Cu2+, can inhibit the
oxidative activity of alcohol dehyogenase 1 (ADHl) from Saccharomyces cerevisiae
(Gastaldi et al., 1993). Copper ions interact with both enzyme-cofactor (ECI) and
enzyme-cofactor-substrate (ECIS), which produce the enzyme-inhibitor complex and the
enzyme-substrateinhibitor complex (Gastaldi et al., 1993).

Superoxide radicals (02 ") and H202 are produced in many biological systems.
These are relatively harmless as they react with biomolecules at low rates, and speciﬁc
enzymes exist to remove them (Simpson et al., 1988). Free Cu2+ ions are expected to
catalyze the conversion of 02 " and H202 to the hydroxyl radical (OH ') which is known
to be highly reactive (373 kJ mol"). The formation of hydroxyl radical leads to
uncontrolled oxidation processes (Hartmann and Weser, 197 7; Peisach and Blumberg,
1969; Simpson et al., 1988). For this reason, biological cells usually keep metal ions very
ﬁrmly bound in less reactive forms (Halliwell and Gutteridge, 1986). The reactions of

cupric salts with H202 and 02 " are as follows:

cu2+ + H202 ___s Cu+ + 1102- + H+
Cu+ + H202 .__, OH’ + OH' + cu2+
Cu” + 02 -' ___, ctr“ + 02

CU+ + 02 '. ___) CU2+ + H202

The presence of ﬁee cupric ions induce the generation of highly reactive hydroxyl
radical (OH'). The hydroxyl radical can damage the protein molecules (Hunt et al., 1988;

Kobayashi et a1. , 1990). Hydroxyl radical attacks both the deoxyribose sugars arrayed

along 1?
constru
copper
highly l

Hertzbt

perforn
chemic
Wood c
Hartfor
1989)

ﬁxatiol
199] a]
and 19:

1.5.1

Carbox
Lignin
Contain
15) (Sj

wOOd l

14

along the surface of DNA (Hertzberg and Dervan, 1984) and the bases of which are
constructed the DNA molecules (Inoue and Kawanishi, 1987). As a fungicide, cupric
copper ion (valence state is 2+) is effective, because it can induce the generation of
highly reactive free radical which damages the DNA and enzyme (Hunt et al., 1988;

Hertzberg and Dervan, 1984).

1.5 Interactions of Wood with Copper-Based Wood Preservatives

The interactions of wood and copper-based preservatives impact both the
performance and the environment aspects of treated wood. In the last few decades, the
chemical and/or the physical interactions between copper-based wood preservatives and
wood components have been extensively studied (Dahlgren, 1972; Dahlgren and
Hartford, 1972; Pizzi, 1981 and 1982; Cooper, 1991 and 1998; Ostmeyer et al., 1988 and
1989). Many researchers in wood preservation area have attempted to determine the
ﬁxation mechanisms of copper and predict its retention and migration in wood (Cooper,
1991 and 1998; Kamdem and McIntyre, 1998; Craciun and Kamdem, 1997; Pizzi, 1981
and 1982).
1.5.1 Ion-Exchange Theory

Wood contains various ionizable functional groups. Hemicelluloses carry
carboxylic acid groups, which can be ionized in neutral or weakly acidic conditions.
Lignin contains phenolic groups, which can be ionized at pH above 10. Cellulose
contains alcoholic hydroxyl groups, which are ionized only in very strong base (pH above
15) (Sjostrom, 1989). All these functional groups are potential ion exchange sites when

wood is treated with preservatives containing metal ions.

 

and Ha
mﬂspl
that in '
phenon
chromi:
througl

1962; C

exchang
the dlSSl
PKa val
and the

CC A tre
lOWQYpI
carbOXy]
reSponsil

MltCl'lle,

15

Ion exchange is usually deﬁned as a fast reaction. Dahlgren (1972) and Dahlgren
and Hartford (1972) performed extensive studies on CCA ﬁxation in sapwood from pine
and spruce. They found that the H+ activity of the CCA was lowered to less than 20% of
that in the original solution within the ﬁrst 3 minutes, and they attributed this
phenomenon to ion exchange occurring between wood components and copper/
chromium cations. Other studies also suggested that the copper absorption by wood
through cation exchange appeared to be instantaneous after treatment (Eadie and Wallace
1962; Gray and Dickinson, 1988; Wilson, 1971).

The pH value of the treating solution plays a critical part in ion exchange. The ion
exchange capacity of wood depends on the pH of the treating solution. The pH controls
the dissociation of weak acid groups in wood. The acid groups in wood have different
pKa values. The carboxylic groups have a pKa of around 4, the phenolic groups 10-12,
and the alcoholic groups 13-15 (Sjostrom, 1989). The amount of adsorbed copper during
CCA treatment increases as the pH increases (Gray and Dickinson, 1988; Pizzi, 1983). At
lower pH or neutral conditions, carboxylic acid groups in wood are dissociated. The
carboxylic acid groups in hemicelluloses of the wood cell wall (uronic acids) are mainly
responsible for cation exchange (Rennie et al., 1987; Cooper, 1991; Knight et al., 1961;
Mitchie, 1961). As the pH increases to above 10, the phenolic groups in lignin may
become dominant sites responsible for ion exchange (Rennie et al., 1987; Pizzi, 1982).
Lebow and Morrell (1993) reported that phenolic groups from wood extractives also
provided ion exchange sites for copper, which caused high copper adsorption in the
heartwood of Douglas ﬁr. At high pH above 15, even alcoholic groups can provide

additional ion exchange sites (Rennie et al., 1987).

charact
adsorpti
Langmu
chromiu
exchang
Langmu
data of c
1.5.2 1
S
excellen
investig
With W0
Wood a:

Cellulos

16

Chen et al. (1996) suggested that ion exchange adsorption of metal ions can be
characterized using Langmuir’s adsorption equation (Adamson, 1990) or Freundlich
adsorption isotherm (Freundlich, 1926). Mitchie (1961) and Pizzi (1981) used
Langmuir’s monolayer adsorption isotherm to describe the adsorption of copper and
chromium on wood. Cooper (1991 and 1998) conducted extensive research on ion
exchange adsorption of various copper-based preservatives on wood and concluded that
Langmuir’s monolayer adsorption isotherm equation correlates well the experimental
data of copper adsorption.

1.5.2 Copper Forms in Treated Wood

Several commercial inorganic wood preservatives contain copper because of its
excellent ﬁlngicidal characteristics. Although tremendous efforts have been directed to
investigate the chemical interactions and the performance of copper based preservatives
with wood, the forms of copper and the structure of copper complexes or precipitates in
wood are not well documented. Copper might be present in treated wood as copper-
cellulose complex, copper-lignin complex, and crystalline or amorphous inorganic/
organic copper compounds. Knowledge of these forms of copper is of great importance
because it may lead to improve the ﬁxation process, the leaching resistance and the
biological efﬁcacy.

Belford et a1. (1957, 1958) reported that metallic salts reacted with cellulose to
form metallo-cellulose complexes when wood was impregnated with aqueous solutions
of copper sulfate and copper sulfate-potassium dichromate mixtures. Hulme (1979)
proposed the formation of copper-cellulose complex as a result of hydrogen bonding.

Kubel and Pizzi (1982) studied the reactions of CCA with cellulose and its model

compOl

that the

compor
copper.
role in i
reportel
methyl

ammon.
lignin Ct
investig
hh'dYOX}
(1987)(

reacted

Bailey 1
reaCtion
CraciUn
form Co;
and Pase
“'Ood We

unhoxyl

17

compounds. They observed an initial absorption of copper on cellulose and postulated
that the absorption was physical in nature.

Bland (1963) reported that copper was concentrated in the region of the
compound middle lamella and suggested that lignin was a signiﬁcant bonding site for
copper. Studies by other researchers also provided evidence that lignin plays a signiﬁcant
role in absorbing copper (Gray and Dickinson, 1988; Rennie er al., 1987). Pizzi (1982)
reported that copper might complex with ortho-dihydroxy phenols and ortho-hydroxy-
methyl phenols in wood after CCA treatment. Using lignin model compounds and
ammoniacal copper preservatives, Xie et al. (1995) proposed that the formation of copper
lignin complex was a key reaction during ﬁxation. Lebow and Morrell (1995)
investigated the interactions of ACZA with Douglas ﬁr and suggested that phenolic
hydroxyl groups provided primary reaction bonding sites for copper. Daniel and Nilsson
(1987) observed that syringyl lignin had low copper retention and suggested that CCA
reacted preferentially with guaiacyl lignin.

The reaction between copper ion and carboxylic acid has also been studied.
Bayley (1960) suggested the formation of metal organic complexes that resulted from the
reaction of metal ions with the acid groups of non-cellulosic constituents of wood.
Craciun et al. (1997) proposed that copper ion reacted with carboxylic acid groups to
form copper carboxylate when wood was treated with copper preservatives. Thomason
and Pasek (1997) reported that the adsorbed copper decreased when the acid groups in
wood were reduced by heating, and concluded that adsorbed copper was bound to the

carboxylic acid groups within hemicellulose. Other studies (Pizzi, 1993a and 1993b) on

the inte

were 1.15

copper

exchang
most of
with lig
Dahlgre
ﬁxation
EV'apora
(1995)]
copperl
in the cl
meiatil
dimerh)
Chelate

Kamdel

1.5.3

18

the interaction mechanism showed the same ﬁndings when copper based preservatives
were used for wood impregnation.

The formation of inorganic copper precipitates also plays an important role in
copper ﬁxation. In CCA ﬁxation, precipitation reactions occur after the initial ion
exchange and adsorption (Pizzi, 1981 and 1982). Pizzi (1981 and 1982) postulated that
most of CCA becomes copper chromates or chromium arsenate that either complexed
with lignin or physically precipitated into the cellulose as inorganic salts. In addition,
Dahlgren and Hartford (1972) reported the precipitation of copper arsenates. In ACZA
ﬁxation, copper forms inorganic compounds through precipitation as the ammonia
evaporates from the wood (Hulme, 1979; J in and Archer, 1991). Lebow and Morrell
(1995) reported that the majority of copper precipitated in the wood was in the forms of
copper carbonates, copper oxides, or copper arsenate compounds. The ﬁxation of copper
in the copper dimethyldithiocarbamate (CDDC) system is mainly ascribed to the
formation of CDDC crystals. Reaction of copper ethanolamine and sodium
dimethyldithiocarbamate (SDDC) leads to an insoluble copper dimethyldithiocarbamate
chelate with a 1:2 molar ratio of copper to dithiocarbamate (Cooper and Stokes, 1993;
Kamdem and McIntyre, 1998).

1.5.3 Physiochemical Analysis of Copper-Based Preservative Treated Wood
Many researchers have employed physiochemical analysis to study the ﬁxation
chemistry of copper on wood (Ostmeyer et al., 1988 and 1989; Hughes et al., 1992
andl994; Craciun and Kamdem, 1997; Kamdem et al., 1991; Kamdem and McIntyre,
1998; Ruddick et al., 1992). In a study of ﬁxation of copper ammoniacal system, Xie et

al.(1995) used Electron paramagnetic resonance (EPR), X-ray diffraction (XRD), and

Fourier

formed

 

et al. ( 1‘1
reﬂectar
Souther.
were ox

Yamam

groups '

19

Fourier transform infrared spectroscopy (FTIR) to examine the structure of the complex
formed by reaction of copper solution with a lignin model compound, vanillin. Ostmeyer
et al. (1988 and1989) used X-ray photoelectron spectroscopy (XPS) and diﬂilse
reﬂectance Fourier transform infrared spectroscopy (DRIFT) to evaluate CCA treated
Southern pine. They concluded that the carbon-hydrogen bonds of the aromatic rings
were oxidized with possible formation of hexavalent chromate esters. Other studies by
Yamamoto and Ruddick (1990) did not provide evidence of the oxidation of hydroxyl
groups to carbonyl groups.

The reaction of preservative with wood components should result in the formation
of new chemical bonds. FTIR can be used to determine the formation of new bonds by
comparing sample spectra before and after treatment. Michell (1993) used F TIR to study
the reaction of wood and of lignin model compounds with inorganic chromium trioxide
and concluded that inorganic preservatives reacted with wood via the aromatic ring of
lignin. In an FT IR study of CDDC treated wood, Craciun et a1. (1997) postulated that the
wood/Cu-complex interaction occurred partially through a ligand exchange reaction
between wood carboxylate groups and 2-ethanolamine of the copper complex.

Since Cu2+ is paramagnetic, electron paramagnetic resonance spectrosc0py (EPR)
can be used to investigate the bonding environment or the modiﬁcation of copper
complexes in treated wood samples. Placket et al. (1987) used EPR to study Radiata pine
treated with CCA and copper sulfate. They observed no evidence of a copper-lignin
complex, but rather that copper ions appeared to be hydrated and stored within ﬁxed sites
of the wood. The forms of copper present in Pinus sylvestris treated with copper based

preservatives were analyzed using EPR by Hughes et al. (1994). The authors found that

immobi'
irrespec
4 nitrog

a diston

treated

waterbc
was for
carbon

treated

can be
al. (1 9
interac
(1975 )
treated
“Gate:

the for

1.6

20

immobile Cu2+ with anisotropic conﬁguration existed in all treated wood samples
irrespective of formulation. They further suggested that copper was complexed with 3 or
4 nitrogens in copper amine treated wood and copper was complexed with four oxygen in
a distorted octahedral conﬁguration in nitrogen free systems.

Like EPR, XPS is another useﬁll tool, which can be performed directly on the
treated sample. Craciun and Kamdem (1997) applied XPS, along with FTIR, to study the
waterborne copper naphthenate wood preservatives. They found that a copper complex
was formed through the interaction of copper ion from the treating solution and
carboxylic and/or carbonyl groups of wood. The oxidation state of copper present in
treated wood was as Cu2+ ion.

XRD is a non-destructive technique and can be used to examine treated wood. It
can be used to identify and quantify crystalline compounds present in a matrix. Creely et
al. (1978) used XRD to study the complexes of cellulose with secondary diamines. The
interaction of wood with inorganic solvents was studied with XRD by Shiraishi et a1.
(1975). Craciun et a1. (1997) used XRD to identify CDDC crystal formation in CDDC
treated wood. Sutter er al. (1983) conﬁrmed the formation of copper oxalate in wood
treated with a copper-based preservative by using XRD. Kamdem et a1. (1997) observed

the formation of a crystal and identiﬁed it as cuprous oxide using XRD.

1.6 Chemistry of Copper-Amine Wood Preservative System
Although the copper amine system as a wood preservative is fairly new in the
wood preservation area, researchers in the area of inorganic chemistry have studied

copper amine chemistry for a long time. The copper amine system is copper complexes

with dil
been ce
bidenta
1981; I
Davis a
investig
salts re.
and die
COpper
Djurdjl
comple
depron
study <
Tauler
3 hi ghi
1981).
mOno:

diSits

21

with different types of amines as ligands or chelating agents. So far, most research has
been centered on copper ethanolamine complexes in which ethanolamines act as
bidentate ligands through amino and hydroxyl groups (Davis and Patel, 1963; Hancock,
1981; Djurdjevic and Bjerrum, 1983). Copper monoethanolamine was ﬁrst studied by
Davis and Patel (1963). They performed conductimetric and potentiometric titration to
investigate the structure of the resulting chelating complex. They observed that cupric
salts reacted with monoethanolamine to form a non-conducting complex under high pH,
and diethanolamine and triethanolamine behaved similarly. The formation constants of
copper ethanolamine complexes in water were determined (Davis and Patel, 1968;
Djurdjevic and Bjerrum, 1983). The formation constants of copper ethanolamine
complexes vary with the pH of the solution. Hancock (1981) pointed out that
deprotonation of alcoholic group occurred with high pH by conducting a glass electrode
study of the complexes of ethanolamine with Cu(II). His ﬁndings were later conﬁrmed by
Tauler and Casassas (1986). Electronic spectra of cupric ethanolamine solution show that
a higher number of ligands leads to the formation of more stable complexes (Hancock,
1981). Using ESR, Tauler and Casassas (1986) observed four species of copper
monoethanolamine with the stoichiometriesl-l-O, 1-2-0, 1-2-1, 1-2-2, where the ﬁrst two
digits represent the numbers of metal and ligand, respectively, and the last digit the
numbers of proton released. Jensen (1971) used UV spectroscopy to identify the
coordinated alcoholate in copper monoethanolamine complexes dissolved in organic
solvents, and estimated the acidic constant of the alcohol group of the ligand. Research

by Casassas et al. (1989) demonstrated that at pH below 12 mono- and di-ethanolamine

can forr

 

followe
l
Sone (1‘
the mol:
added it
amine s
monoet
with EF
aminoe'
aminoe‘
(1991)
copper
PreCipil
the initi
(above
Hancoc
alky] an

Solutio1

"1018: l-

ConSlde

22

can form four species by stepwise addition of two molecules of the ligand to the metal
followed by deprotonation of two hydroxyl groups.

In the visible spectra of the aqueous copper-ethanolamine system, Ojima and
Sone (1961) observed a band at 520 nm. This band increases in intensity as a function of
the molar ratio of the aminoalcohol to copper, or when strong base (pH above 12) was
added to the system. This band was assigned to the formation of the deprotonated copper
amine species. The molecular and electronic structure of the CuClz-complexes with
monoethanolamine and triethanolamine in solid and in DMF solutions was examined
with EPR (Hedewy, 1986). The results indicated that a dimeric structure of CuClz-
aminoethanolate complexes with very weak Cu-Cu coupling, and the CUClz-
aminoethanolate complexes were inﬂuenced by the molecules of solvent. Kadoshnikova
(1991) conducted research on precipitating residual cupric ion from aqueous solutions of
copper nitrate by adding ethanolamine into the solution. The amount of copper in solid
precipitate varies with the molar ratio of amine to copper. Copper precipitate increases as
the initial molar ratio increases. Further increase in the molar ratio of amine to copper
(above 1.6) leads to the dissolution of the solid phase as a result of complex formation.
Hancock (1981) and Handcock and Nakani (1984) reported that an unsubstituted
alkylamine, such as ethylamine, precipitated the hydroxide when added to cupric
solutions because of the steric hindrance to coordination caused by the alkyl group.

The stability and structure of complexes vary with the pH values, copper to amine
molar ratio and solvent molecules. The formulation of copper-amine complexes should

consider all these parameters for the stability and the efﬁcacy.

1.7 ‘

betweel

mechan

solid te

amine t

23

1.7 Objectives

The objectives of this study are to examine the physical and chemical interactions
between copper-amine wood preservatives and Southern pine, and provide a feasible
mechanism for copper ﬁxation in copper-amine treated wood. To achieve the objectives,
solid techniques, such as FTIR, EPR, XRD and XPS, will be applied to study the copper

amine treated wood samples.

Referer
Adamsc

Adler, I
1
America
Aspinal
l

Aulin-E

Aulin-E

Bayley
Belforc‘
Belfon
Bland,
Casass
Chen,
Coolie

Coops

24

References
Adamson, A. W. 1990, Physical chemistry of surface. Fifth Ed. John Wiley & Son, Inc.

Adler, E. and S.Hemestam. 1955. Estimation of phenolic hydroxyl groups in 1igninI.,
periodate oxidation of guaiacol compounds. Acta Chem. Scand. 19: 3 19-3 34

American Wood Preservers’ Association (AWPA). 1997 . Book of standards. Granbury,
Texas

Aspinall, G. O. 1959. Structural chemistry of the hemicelluloses. Advan. Carbohydr.
Chem. 141429

Aulin-Erdtman, G. 1953. Spectrographic contributions to lignin chemistry HI,
investigation on model compounds. Svernsk Papperstidn. 56: 91-101

Aulin-Erdtman, G and R. Sanden. 1968. Spectrographic contributions to lignin chemistry
IX, absorption properties of some 4-hydroxyphenyl, guaiacyl, and 4-hydroxy-3,
5-dimethoxyphenyl type model compounds for hardwood lignins. Acta. Chem.
Scand. 22:1187—1209

Bayley, CH. 1960. Metal organic complexes formed during the treatment of wood with
metal salts. Nature. 4709: 3 13-314

Belford, BS. and RD. Preston. 1957. Timber preservation by copper compounds.
Nature. 180: 1081-1083

Belford, D.S., A. Myers and R. D. Preston. 1958. Electron diffraction study of adsorbed
metal ions on the surface of cellulose microﬁbrils. Nature. 181: 1516-1518

Bland, DE. 1963. Sorption of copper by wood constituents. Nature. 200:267

Casassas, E., L.L. Guetems and R. Tauler. 1989. Spectrophotometric study of complex
formation in copper(II) mono-, di-, and tri-ethanolamine systems. J. Chem. Soc.
Dalton Trans. 4: 569-573

Chen, I. S. Yiacoumi and T.G. Blaydes. 1996. Equilibrium and kinetic studies of copper
adsorption by activated carbon. Separation Technol. 6: 133-146

Cooper, P. A 1991. Cation exchange adsorption of copper on wood. Wood Protection.
1(1): 9-14

Cooper, P. A. and D.Stokes. 1993. Leaching characteristics and ﬁxation of copper
dimethyldithiocamamate treated wood. Proc. Am. Wood Preservers’ Assoc. 89:
192-203

Cooper
Craciur
Craciur

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Davis,
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DJUrdj

25

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11

 

 

 

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B

Rl

29

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3O

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preservative ﬁxation using electron spin resonance. IRG documents:
IRG/WP3 701

 

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Chapter 2

Effect of Copper-Amine Composition on Copper Absorption and Leaching

2.1 Abstract

The absorption and leachability of copper in copper amine (Cu-EA) treated wood
were inﬂuenced by the formulation of the copper amine treating solutions. The sources of
copper used, Cu(OH)2, CuCO3, CuSOa and Cu(N03)2, in the copper amine complex
formulation affected the leachability of copper. Data showed that copper amine ﬁ'om
CuS04 and Cu(N03)2 treated wood had less copper loss during laboratory water leaching
than ﬁom Cu(OH): and CuCO; treated wood. Increasing the amine to copper molar ratio
increased the copper retention by wood, but reduced the leaching resistance of copper.
The types of amine ligand, such as monoethanolamine (primary amine), 2-methylamino-
ethanol (secondary amine) and N, N-dimethyl-ethanolamine (tertiary amine), had some
effect on copper retention and copper leaching. As the molecular weight of amine ligands

increased, copper loss during leaching decreased.

2.2 Introduction

. More and more research in the area of wood preservation has been directed in the
development of environmentally benign preservative systems with low mammalian
toxicity and acceptable efficacy. The copper amine system is one of the new emerging

copper-based preservatives that are receiving a lot of attention. It is one of the main

31

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32

ingredients of ammoniacal copper quat-type D (ACQ—D), copper dimethyl-dithio-
carbamate (CDDC), and copper azole (AWPA, 1998).

The ﬁxation mechanism of copper in chromated copper arsenate (CCA) and
ammoniacal system has been extensively studied. The effect of formulation on the
efﬁcacy and performance, the ﬁxation chemistry and the reaction kinetics of copper ion
in CCA treated wood have been greatly examined (Fahlstrom er al., 1967; Hagar, 1969;
Dahlgren and Hartford, 1972; Pizzi, 1982). The general theory of ﬁxation of copper in an
ammoniacal system is that the cupriammonium ion is ﬁxed through cation adsorption
reactions with the wood substrate and through precipitation of copper inorganic
compounds (Hulme, 1979; Jin and Archer, 1991). Hartford (1972) suggested that
insoluble copper precipitates were formed after the evaporation of ammonia in solvent.
Lebow and Morrell (1993) proposed that the phenolic wood extractives provided reactive
sites for copper ﬁxation in ACZA treated wood.

While the ﬁxation mechanisms of other copper-based wood preservatives have
been widely studied, relatively little research has been done on the ﬁxation, retention and
leaching of copper in copper amine treated wood. It is reasonable to presume that the
copper source, the amine ligand and the ligand-metal molar ratio inﬂuence the copper
ﬁxation in copper amine treated wood. In this study, the effects of copper source, such as
copper sulfate, copper nitrate, copper carbonate and copper hydroxide, the amine ligand,
such as monoethanolamine (primary amine), 2-methylamino-ethanol (secondary amine)
and N, N-dimethyl-ethanolamine, and the amine to copper molar ratio on the retention

and the leaching of copper in treated wood were investigated.

2.3

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33

2.3 Materials and Methods
2.3.1 Formulation of Copper Amine (Cu-EA) Treating Solutions

To evaluate the effect of different copper sources on wood-copper interaction,
four different copper compounds, namely, copper hydroxide, copper carbonate, copper
nitrate and copper sulfate, were used in copper amine formulations. Copper amine
treating solutions were made up by mixing these copper compounds with an aqueous
solution of monoethanolamine (MEA). The molar ratio of amine to copper was set at 4.

To evaluate the effect of amine ligands on copper ﬁxation, copper amine treating
solutions were prepared by mixing copper hydroxide with three different ethanolamines
including a primary amine monoethanolamine (MBA), a secondary amine 2-
methylamino-arnine (MeEA), and a tertiary amine N, N-dimethyl-ethanolamine
(DMeEA). The amine to 00pper molar ratio was set at 4.

To evaluate the effect of amine to copper molar ratio on copper ﬁxation, copper
amine treating solutions were formulated by mixing copper hydroxide and MBA, MeEA
and DMeEA at a molar ratio of amine to copper varying from 3 to 8.

2.3.2 Treatment

Defect-ﬂee 5 by 10 by 180cm (2 by 4 inch by 6 feet) kiln-dried sapwood boards
of Southern pine (SP) were used in this study. Cubes measuring 19mm (0.75 inch) were
prepared ﬁ'om these boards and stored in a conditioning room maintained at 65% relative
humidity (RH) and 20 °C (68 °F) until they reached an equilibrium moisture content
(EMC) of 12 :t 3%. The conditioned cubes were then pressure-treated with copper amine
solutions. The copper contents in the solutions were 0, 0.25, 0.5, .075 and 1.0% by

weight. The treating procedure included an initial vacuum at 84.6 kPa (25 inch Hg) for 5

minute
vacuun
weeks I
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34

minutes, followed by a pressure level of 690kPa (100 psi) for lhour, and then a ﬁnal
vacuum for 10 minutes. Treated samples were conditioned at room temperature for 2
weeks before ﬁlrther test. The copper contents in treated wood samples were analyzed by
using a Perkin-Elmer atomic absorption spectrometer (AAS) model 3110, as described in
the AWPA standard (AWPA, 1998).
2.3.3 Leaching

After the conditioning of treated cubes, the leaching test was carried out to
determine the amount of copper leached from treated wood. Three treated wood cubes
weighing 10 grams were placed in a 200ml ﬂask and immersed in 100ml deionized
water. The ﬂasks were positioned on a horizontal-shaking tray with continuous mild
shaking, and an equal amount of fresh deionized water was renewed every day for 8 days.

The copper content in water leachate and in leached cubes was analyzed by AAS.

2.4 Results and Discussion
2.4.1 Effect of Copper Sources

AAS analyses indicated that the source of copper inﬂuenced the level of copper
absorbed during the treatment of Southern pine with Cu-EA (Figure 2.1). At lower
concentration of copper in solution (about 0.25%), no signiﬁcant difference was observed
for the level of copper absorbed by wood for all copper sources. As the copper
concentration increased, more copper was absorbed. Solutions made from copper
hydroxide and copper carbonate yielded higher copper retention levels than those made
from copper sulfate and copper nitrate. The pH of the treating solutions is tabulated in

Table 2.1. It is noticed ﬁ'om Figure 2.1 and Table 2.1 that the retention of copper in wood

Figure 2.1

35

Effect of copper sources on copper retention. Copper amine solutions were
made by mixing monoethanolamine with difference copper sources

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Table 2.1 pH of copper monoethanolamine treating solutions ﬁ'om different copper
compounds with amine to copper molar ratio of 4

 

 

 

 

 

 

Copper Copper concentration in solution (wt %)
compounds 0.25 0.5 0.75 1.0
Cu(OH)2 10.6 10.8 10.8 10.8
CuCO3 10.0 10.1 10.2 10.3
CuSO4 9.2 9.3 9.3 9.3
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38

increases as the pH of treating solutions increases. High pH solution systems resulted in
higher copper absorption. These ﬁndings generally agree well with the previous study
reported by Cooper (1998). A high pH solution might increase the dissociation of
carboxylic and phenolic groups, which in turn promote the retaining ability of copper.

Copper source also plays an important role in copper leaching resistance (Figure
2.2). After an 8-day water leaching of wood cubes treated with 0.5% copper solution,
about 12% copper from copper hydroxide system was lost, 10% from copper carbonate,
7% from copper sulfate and 6% ﬁom copper nitrate (Figure 2.2). The copper leaching
resistance shows an opposite trend to copper absorption. Copper from copper sulfate and
copper nitrate is more leaching resistant than copper ﬁom copper hydroxide and copper
carbonate. For copper amine complexes, high pH solution systems lead to higher copper
retention, and higher copper loss during leaching (Figure 2.2 and Table 2.1).
2.4.2 Effect of Amine Ligands

The role of amine ligands in copper ﬁxation is important because ligands can
affect the stability, polarity, and solubility of copper amine complexes. Figure 2.3
illustrates the inﬂuence of ligands on copper absorption. As the molecular weight of the
amine ligands increases, less copper is absorbed. Cu-EA made with primary amine
(MBA) solution system resulted in higher level of copper absorption than those made
from secondary amine (MeEA) and tertiary amine (DMeEA). As the copper
concentration of treating solution increases, this tendency becomes more noticeable.

Figure 2.4 shows the result of laboratory water leaching test on different copper
amine treated wood samples. It is noted that amine ligands inﬂuence copper leachability

signiﬁcantly. As the molecular weight of amine increases, the leaching resistance of

39

Figure 2.2 Effect of copper sources on copper leaching from copper amine treated
samples. (—) 1.0% copper treated; (“’“‘ ) 0.5% copper treated

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copper increases. Tertiary amine DMeEA system exhibits a minimum percentage of
copper loss, while primary amine MEA formulation treated samples lose more copper.
After an 8-day water leaching of the cubes treated with 0.5% copper solution, about 5%
of the absorbed copper leached into solution for the tertiary amine DMeEA system, 8%
for secondary amine MeEA and 12% for primary amine MEA.
2.4.3 Effect of Amine to Copper Molar Ratios

Increasing amine to copper molar ratio improves the stability of copper amine
complex (Hancock 1981), and hence increases the penetration and retention of the copper
amine complex in wood. Change in the molar ratio of amine to copper can alter the
copper absorption and leaching resistance. Figure 2.5 represents the copper absorption in
wood pressure-treated with copper amine solutions with amine to copper molar ratios
varying ﬁom 3 to 8. The molar ratio of amine to copper affects copper absorption greatly
in Cu-MeEA and Cu-DMeEA formulation systems. At amine to copper molar ratios
below 4, the retention of copper in wood increases rapidly as the molar ratio increases,
and then copper absorption tends to saturate as the amine to copper molar ratio further
increases. The inﬂuence of amine to copper molar ratio is not so pronounced in the Cu-
MEA system as in the Cu-DMeEA and Cu-MeEA formulations. Increase in amine to
copper molar ratio ﬁ'om 3 to 8 in Cu-MEA formulation only causes a slight increase of
copper absorbed in wood. Increased copper absorption may be due to easier diffusion of
copper in wood with the increase in amine content.

Figure 2.6 displays the percentage of copper loss after 8-day water leaching at
different amine to copper molar ratios. Higher molar ratio accelerates copper loss during

leaching. The percentage of copper loss is approximately proportional to the molar ratio

46

Figure 2.5 Effect of amine to copper molar ratios on copper retention. Copper amine
solutions were made by mixing copper hydroxide with amines at different
ratios.—(— ) 1.0% copper treated; {—m ) 0. 5% copper treated

47

ea 2:9”.

0:9 5.2: .258 2 05.5.

d n m m w

 

 

 

$220-30 LT
$22.30 III
(ms—.30 IOI

 

 

 

 

 

 

 

 

dd

.1.

%1M ‘poom parcel; u! luaruoo Jeddoo

48

Figure 2.6 Effect of amine to copper molar ratios on copper leaching from copper
amine treated samples. (——) 1.0% copper treated; (........-) 0.5% copper
treated

49

ea 2:9”.

2:: 8.9: .638 9 m:_E<
o m e m

 

 

 

 

5020-30 I11
$024.0 Ill
(MEIDO 1.1

 

 

 

 

 

 

om

poom won
peqoeel Jeddoo paqorsqe Allelilul lo %

 

 

 

50

of ligand to copper for all the formulations. This suggests that an increase in the amine to
copper molar ratio reduces the ability of copper to ﬁx in wood, and therefore increases
the leaching of copper in treated samples.
2.4.4 Copper Fixation Mechanism

Cupric ions form a 5-member ring complex with ethanolamine through amino and
hydroxyl groups in aqueous solution as proposed in Figure 2.7. Previous study showed
that the number of ligand molecules to one cupric metal ion was two at both low and high
ligand-to-metal ratios (Casassas et al., 1989). As solution pH increases, deprotonation of
hydroxyl groups occurs. At neutral or low basic conditions, only one proton ﬁom
hydroxyl groups of the amine ligand is released (Figure 2.7). As the pH further increases,
another proton ﬁom hydroxyl groups of the amine ligand is released and the complex
becomes a non-conducting species with no charge (Figure 2.7C). Deprotonation enhances
the stability of the copper amine complex (Jensen, 1971; Hancock, 1981; Hancock and
Nakani, 1984 and Casaccas et al., 1989). The three species of copper amine complex
maintain a dynamic equilibrium in aqueous solution and the proportion of these species
varies with the pH of the solution. According to Tauler and Casassas (1986), complex B
is the main species when the pH of the solution is around 8.5, and complex C becomes
dominant when the pH is above 10.

When wood is pressure treated with copper amine solutions, the copper amine
complexes penetrate into the wood substrate. The penetration of copper into wood is
related to the stability of the copper amine complexes. A stable complex should result in

a deep penetration of the complex and higher absorption of copper. A less stable copper

51

IR1
HOHZC—CH2N\
R2
Monoethanolamine: R1 = R2 = H
2-Methylamino-ethanol: R1=H, R2 = CH;
N, N-dimethyl-ethanolamine: R1 = R2 = CH3

H R1. ,R2 2+ H R1. ’R2 1+ R1. ’R2

Q IN\ Q IN\ Q IN\
Hzc/ “c" ('81: pH=8. H26/ \c.’ rlsrh pH=10.o H2'C/ x“: <|:l-l2

I , U‘ V—- l ’ [1‘ ﬁ— ” ‘

HzC\ -’ x 61-12 -H+ Hzc\ -’ \ ,cH, -H HzG\ . . ,Cl-lz

/N\ if /N o /N\ o

1 R2 1 R2 R1 R2

A B C

Figure 2.7 Top: Amine ligands. Bottom: Copper amine complexes

52

amine complex tends to react with wood readily. The reaction is a neutralization of
anionic groups. This may lower the swelling at the wood surface and retard the
penetration of additional treating solution.

If the copper amine complex exists in wood by physical interaction, it will be
leached out easily by water. To be ﬁxed in the wood cell wall, the copper amine
complexes interact with wood through chemical reactions. Wood is a weak acidic
substrate, in which functional groups, such as carboxylic groups and phenolic hydroxyl
groups, are active sites for interactions with copper. Two types of reaction mechanisms,
namely, ligand exchange and complexation, are proposed here.

In the ligand exchange reaction mechanism, copper amine complexes exchange
ligands with wood and release one amine molecule (Figure 2.8A). This ligand exchange
mechanism is proposed because of the presence of the non-conducting species of copper
amine complexes (Figure 2.7C). This assumption was previously proposed by Kamdem
et al. (1996) and Thomason and Pasek (1997). In the second possible reaction
mechanism, non-charged species of copper amine complexes are transformed into
charged species during the treatment of wood. Functional groups, such as wood-COOH
and wood-phenolic, can complex with the charged species to form a stable wood-copper
amine complex (Figure 2.8B).

These reactions are inﬂuenced by the stability and the pH values of copper amine
complexes. For the ligand exchange reaction, the less stable the copper amine complex,
the easier the complex disassociation will be, and therefore the reaction with wood will

be driven forward. For the complexation mechanism, the lower the pH of the treating

53

 

 

 

 

(A).
n1 [Ra R1 R2
\ — \N
_ Hzo/o.\ /,N\CH2 Wood-C 0..“ \CH2
Wood—O + l 'ZCu', | —> ,CU.‘ I + Amine
H20\N" “‘0’“? Wood-C-o" ‘~ ’0”?
/
R1 R2
(3)-
1+ 2+
0 R.N,R2 H R1‘nl'R2 S R..N,R2
a/ a ‘22— tar/“at" ‘22 are ‘2:
”W; V... "ac-a" eca ”7“; ac»:
/ / \ H
R1 R2 R1 R2 R4 R2
Wood-COO
Woodcoo 0'
0'
v “2,,
I I
0‘ 9 N 0.\? I,”
~.‘. .’ 'gcu;
.-"C‘L~. N" i ‘o
N O 0
Wood

Figure 2.8 Copper amine —wood interactions
A). Ligand exchange and B). Complexation

54

solutions, the more charged species of complex (Figure 2.7A and 2.7B) will be present,
and hence the interaction between wood and complex will be more likely.

The pH of treating solution varies with the copper source (Table 2.1) and amine
concentration (Table 2.2). As mentioned above, the higher the pH of the treating solution,
the more stable the copper amine complex is. The complexes from copper hydroxide and
copper carbonate system are more stable than those from copper sulfate and copper
nitrate. As a result, high retentions of copper were obtained from copper hydroxide and
copper carbonate. However, a stable complex resulting from high pH will not promote a
good interaction between copper and wood according to the above reaction mechanisms.
This explains the low copper loss from copper sulfate and copper nitrate formulations.

The stability of the complex is also inﬂuenced by the amine ligand and the amine
to copper molar ratio. The copper complex of primary amine is more stable than those of
secondary amine and tertiary amine due to the steric hindrance of methyl groups.
Hancock and Nakani (1984) pointed out that the steric effect outweighed the inductive
effect when copper ion (Cur') formed complex with amine ligands. The high stability of
copper complex with primary amine or with high amine to copper molar ratio will lead to
good penetration of the complex in wood and hence increase the retention of copper. Due
to its low stability, capper DMeEA complex is easier to react with wood than copper
MEA. This explains lower copper loss from DMeEA system, and high copper loss ﬁom
copper MEA during laboratory leaching test. Increasing the molar ratio of amine to
copper enhances the stability of complex and increases the pH of treating solution (Table
2.2), and therefore improves the penetration of complexes. At higher molar ratio, copper

amine complexes tend to exist as non-charged species and may block the ligand exchange

55

Table 2.2 pH of copper amine treating solutions containing 0.5% copper from
copper hydroxide and different amine ligands

 

Amine to copper molar ratio

 

 

 

Amine Ligands 3 4 6 8
MBA 10.6 10.8 11.1 11.2
MeEA 10.7 10.8 11.2 11.3

 

DMeEA 10.5 10.6 11.0 11.1

 

 

 

 

 

 

 

56

reaction and complexation reaction, and therefore block the interactions of wood
functional groups with the copper amine complexes. This accounts for lower leaching

resistance when the molar ratio of amine to copper increases.

2.5 Conclusions

The copper retention and leaching of Cu-EA treated wood were inﬂuenced by the
copper amine formulations. High pH formulation system resulted in higher copper
retention in wood, but lower copper leaching resistance. Increase in the molar ratio of
amine to copper improved copper penetration into the wood, and therefore increased the
copper retention. However, high amine to copper ratio caused high copper loss during
water leaching. As a general rule, in formulating a copper amine treating solution, a
balanced pH, amine ligand, and amine to metal ratio should be taken into consideration to
improve copper retention while minimizing the copper loss during leaching. The

biological impact of these parameters needs to be investigated.

57

References

American Wood Preservers’ Association (AWPA). 1998. Book of standards. Granbury,
Texas.

Casassas, E., L.L. Guetems and R. Tauler. 1989. Spectrophotometric study of complex
formation in copper(II) mono-, di-, and tri-ethanolamine systems. J. Chem. Soc.
Dalton Trans. 4: 569-573

Copper, PA. 1998. Diffusion of copper in wood cell walls following vacuum treatment.
Wood Fiber Sci. 30(4): 382—395

Dahlgren, SE. and W. H. Hartford. 1972. Kinetics and mechanism of ﬁxation of Cu-Cr-
As wood preservatives. Part 1H. Fixation of Tanalith C and comparison of
different preservatives. Holzforschung. 26: 142-149

F ahlstrom, GB. RB. Gunning and J. A. Carlson. 1967. Copper-chrome-arsenate wood
preservatives: a study of the inﬂuence of composition on leachability. Forest
Prod. J. 17(7): 17-22

Hagar, B. 1969. Leaching tests of copper-chrome-arsenic preservatives. Forest Prod.
J. 19(10): 21-26

Hancock, RD. 1981. The chelate effect in complexes with ethanolamine. Inorganica
Chimica Acta. 49(2): 145-148

Hancock, RD. and BS. Nakani. 1984. Some factors inﬂuencing the stability of
complexes with ligands containing neutral oxygen donor ligands, including crown
ethers. J. Coord. Chem. 13: 309-314

Hartford, W.H. 1972. Chemical and physical properties of wood preservatives and wood
preservative systems. In wood deterioration and its prevention by preservative
treatments. Vol. 2. Preservatives and Preservatives Systems. Syracuse University
Press

Hulme, MA. 1979. Ammoniacal wood preservatives. Rec. Ann. Conv. Brit. Wood
Preservers’ Assoc. pp38-50

Jensen, HP. 1971. Copper monoethanolamine complexes. Acta Chemica. Scand. 25:
1753-1757

J in, L. and K. Archer. 1991. Copper based wood preservatives: observation on ﬁxation,
distribution and performance. Proc. Am. Wood Preservers’ Assoc. 87 : 169-184

Kamdem, D.P., R.Craciun, C. Weitasacker and M. Freeman. 1996. Investigation of
copper-bis-dimethyldithiocarbamate (CDDC) treated wood with environmental

58

electron microscopy and other spectroscopic techniques. Proc. Am. Wood
Preservers’ Assoc. 92:112

Lebow, ST. and J .J . Morrell. 1993. ACZA ﬁxation: the roles of copper and zinc in
arsenic precipitation. Proc. Am. Wood Preservers’ Assoc. 89: 133-146

Pizzi, A. 1982. The chemistry and kinetic behavior of Cu-Ar-As/B wood preservatives.
II. Fixation of the Cu/Cr system on wood. IV. Fixation of CCA to wood. J.
Polym. Sci. Chem. Ed. 20: 707-724. 20:739-764

Rennie, P.M.S., S.M. Gray and DJ. Dickinson. 1987. Copper based water-borne
preservatives: Copper adsorption in relation to performance against soft-rot.
IRG documents: IRG/WP 3452

Tauler, R. and E. Casassas. 1986. The complex formation of Cu(II) with mono- and di-
ethanolamine in aqueous solution. Inorganica Chimica Acta. 114: 203-209

Thomason, SM. and EA. Pasek. 1997. Amine copper reaction with wood components:
Acidity versus copper adsorption. IRG documents: IRG/WP 97-30161

Chapter 3

Effect of Wood Composition on Copper Absorption

3.1 Abstract

The effect of individual wood components on copper absorption when treated
with copper amine solution was investigated. Data showed that the copper absorption by
wood substrates experiences a rapid increase at the ﬁrst 30 minutes and then reaches a
plateau. Lignin and xylan exhibit high copper absorption, suggesting that hemicellulose
and lignin in wood play a signiﬁcant role in bonding copper, while the role of cellulose in
retaining copper is negligible. Removal of extractives from wood also decreases the

amount of copper absorbed.

3.2 Introduction

Wood contains three major constituents: cellulose, hemicellulose, and lignin. In
addition, a small percent of extractives is also present. These wood components play an
important, but poorly understood, role in absorbing copper when wood is exposed to
copper based preservatives. Early study by Belford and Preston (1957) indicated that
cellulose in wood reacted with copper preservative to form metallo-cellulose complexes
when wood was impregnated with aqueous solutions of copper sulfate and copper sulfate-
potassium dichromate mixtures. By conducting a study of copper amine reaction with
wood, Thomason and Pasek concluded that hemicellulose was the major wood

constituent for selective absorbing copper. Pizzi (1982) and Xie et al. (1995), who

59

60

examined the interaction between copper and lignin on the basis of lignin model
compounds, proposed that lignin was responsible for ﬁxing copper by forming copper-
lignin complex.

In this experiment, copper amine preservatives were used to treat different wood
components and model compounds. The effect of the various functional groups on copper

absorption was studied.

3.3 Materials and Methods
3.3.1 Materials

In this study, unextracted and extracted Southern pine (SP) sapwood, cellulose,
lignin, holocellulose and a hemicellulose model compound, xylan, were used. Extractive
ﬁee wood samples were obtained by extracting SP with ethanol/toluene. Cellulose,
organosolv lignin and xylan were purchased form Aldrich Chemical Co.

Holocellulose was prepared by deligniﬁcation of wood with acidiﬁed sodium
chlorite using modiﬁed methods reported by Ona et al. (1995). 20 grams of extractive-
free wood powder and 300ml 6% sodium chlorite were added to a 500ml ﬂask, followed
by addition of 6ml glacial acetic acid (pH4.0). The mixture was heated to 70°C on a water
bath. The ﬂask was continuously stirred for 30 minutes, and then another 2 grams of
sodium chlorite and lml glacial acetic acid were added to the ﬂask. After 60 minutes, the
mixture was ﬁltrated with a glass-ﬁltering crucible and washed with cold distilled water
and acetone. The residue was dried under a high vacuum and holocellulose resulted.

Copper monoethanolamine (Cu-EA) treating solutions were made by dissolving

copper hydroxide in aqueous monoethanolamine with the molar ratio of amine to copper

61

of 4. The copper concentrations in the treating solutions were 0.5 and 1.0% by weight,
respectively.
3.3.2 Treatment

About 2.5 grams wood substrates were treated with 50 grams of 0.5 and 1.0 wt %
Cu-EA treating solutions in a 100ml ﬂask, sealed and agitated on a wrist-shaker for 0.2,
0.5, 2, 4, 8, 24 hours. After the agitation, samples were ﬁltered through glass ﬁber ﬁlters
and washed continuously with deionized water until the conductivity of the eluent water
was constant. The objective of continuous washing with deionized water was to minimize
physical absorption of copper in treated sample. The treated and water-washed samples
were air-dried for two weeks and analyzed by atomic absorption spectrometry (AAS) to
determine copper content.
3.3.3 Analysis of Phenolic Hydroxyl Groups

The procedure used in this research was a slightly modiﬁed method previously
described by Francis et a1. (1991). Approximate 100mg lignin or 200mg wood sawdust in
a 20 ml glass centriﬁlge tube was treated with 6m] sodium periodate solution (75 mg/ml),
to which lmL of distilled water containing 3 mg acetonitrile was added as an internal
standard. Both solutions were cooled to 4°C prior to addition to the sample. The
suspension was homogenized and kept in the dark at 4°C in a reﬁigerator with occasional
stirring.

The mixture was centrifuged to obtain a clear solution for GC analysis. Methanol
and the internal standard (acetonitrile) were determined with a 1.8-m x 0.32-cm stainless

steel column packed with Tenax GC. The GC was operated at 80°C with an injection-port

62

temperature of 150°C and a detector temperature of 250°C. a nitrogen ﬂow of 60mUmin

was maintained.

3.4 Results and Discussion

When wood substrates were exposed to copper-amine treating solution, copper
absorption by wood substrates except cellulose exhibited a rapid increase within the ﬁrst
30 minutes (Figure 3.1 and 3.2). After about 4 hours of exposure, copper absorption
tended to level off. This fast copper absorption during initial stages has also been
reported in other copper-based wood preservative systems, such as CCA (Cooper, 1991;
Dahlgren, 1972). The copper absorbed by cellulose did not noticeably change as exposure
time and copper concentration in treating solution increased.

Table 3. land 3.2 illustrate the copper absorption on different wood substrates.
Signiﬁcant copper absorption difference is exhibited among the different wood
substrates. Lignin was the most active material, retaining signiﬁcantly higher level of
copper than the other wood substrates. This organosolv lignin is expected to contain
about 4 times as much phenolic hydroxyl groups as the native lignin in softwood or
hardwood. In contrast, cellulose was the least reactive site for copper among the wood
substrates. Holocellulose, which consists of cellulose and hemicellulose, showed a
relatively higher copper absorption, suggesting that hemicellulose possesses higher
copper afﬁnity. As a model compound of hemicellulose, xylan displays higher copper
reactivity than holocellulose. The role of wood extractives in copper absorption cannot be
neglected. When the extractives were removed from wood, the extractive-free wood

showed less copper absorption than unextracted wood (Figure 3.1 and 3.2).

63

Figure 3.1 Effect of wood components and exposure time on copper absorption from
0.5% copper amine treating solution

 

 

68.2.00 lol
322.8201 1T
cm_>x IOI

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DOO>> III

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ON

rm 2:9”.

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l" r:

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ll

IT

+

l l\\

 

 

ddd

de

dod

dwd

2:.

%lM “lueluoo Jeddoo

65

Figure 3.2 Effect of wood components and exposure time on copper absorption from
1.0 % copper amine treating solution

66

 

 

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68.2.8201 1T
53x ll

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or or

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a i o
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67

Table 3.1 Copper absorption in wood substrates treated with 0.5 wt % copper amine

solution

 

Time Lignin Wood Ex. Wood Xylan Holocellulose Cellulose

 

Hours wt % copper

0.2 0.708 0.416 0.389 0.329 0.296 0.0589
0.5 0.731 0.430 0.399 0.331 0.305 0.0603
1.0 0.763 0.449 0.407 0.342 0.328 0.0604
2.0 0.792 0.466 0.432 0.352 0.332 0.0597
4.0 0.823 0.484 0.439 0.370 0.341 0.0609
8.0 0.836 0.492 0.446 0.379 0.348 0.0600
24.0 0.855 0.503 0.459 0.383 0.360 0.0610

 

68

 

 

Table 3.2 Copper absorption in wood substrates treated with 1.0 wt % copper amine
solution

Time Lignin Wood Ex. Wood Xylan Holocellulose Cellulose
Hours wt % copper
0.2 1.174 0.650 0.634 0.549 0.458 0.0618
0. 5 1.183 0.671 0.640 0.563 0.472 0.0633
1.0 1.201 0.702 0.671 0.582 0.493 0.0634
2.0 1.247 0.723 0.705 0.599 0.515 0.0627
4.0 1.312 0.760 0.711 0.631 0.530 0.0639
8.0 1.336 0.762 0.729 0.651 0.541 0.0630

24.0 1.390 0.779 0.747 0.659 0.560 0.0640

 

69

The lower copper absorption on cellulose may be due to the limited accessibility
of the cellulose molecules and lower reactivity of aliphatic hydroxyl groups in cellulose
molecules. Cellulose chains have many primary and secondary hydroxyl groups and these
hydroxyl groups form intra- and inter-molecular hydrogen bondings within and among
cellulose chains. These hydrogen bondings are arranged regularly, which results in a
Crystalline system of cellulose. The crystalline properties of cellulose limit the
accessibility of reactants into cellulose. Moreover, the aliphatic hydroxyl group is very
difficult to be ionized in aqueous solution, which makes it less likely to ﬁx copper
chemically. The small amount of copper absorbed by cellulose may be due to the
secondary forces, such as dipole-dipole and Van der Waals attractions, in amorphous
portion of cellulose.

Although hemicellulose in wood contains a great deal of aliphatic hydroxyl
groups, it contributes most of the carboxylic groups in wood, which tend to be very
reactive with copper. For instance, uronic acid groups in hemicellulose have been proven
to be active for copper absorption (Rennie e101,, 1987). This explains why holocellulose
and xylan absorbed more copper than cellulose. Since xylan possesses higher ratio of
carboxylic acids than holocellulose, more copper was absorbed in xylan than in
holocellulose (Figure 3.1 and 3.2).

Previous study by Bland (1963) indicated that, when wood was treated with
copper solution, copper was concentrated in the compound middle lamella, showing that
lignin is also responsible for the copper absorption. Lignin has been reported as a primary
reaction site for copper in alkaline solutions (Pizzi, 1982, Lebow and Morrell, 1995). The

high reactivity of lignin has been attributed to the phenolic hydroxyl (PhOH) groups in

7O

lignin. A study by Sjostrom (1989) demonstrated that phenolic hydroxyl groups were
ionized in alkaline conditions (pH 10-12) and these hydroxyls can react with copper ion.
The very high copper absorption by lignin in this study is due to the highly phenolic
nature of lignin. Aldrich Chemical Co. conﬁrmed that the lignin was extracted ﬁ'om a
mixture of hardwoods (birch, maple and poplar with 35: 50:15 by weight). This mixture
is the standard used in the research and development for the ALCELL Process (Goyal et
al., 1992). A mildly acidic ethanol / water mixture is used to hydrolyze some inter-
unitary ether bonds in the lignin (Goyal er al., 1992; Gallagher er al., 1989). This
depolymerizes the lignin and increases its phenolic hydroxyl content and its solubility in
ethanol/water. The GC analysis showed that the organosolv lignin and the Southern pine
used in this study had 64 PhOH/100 C9 units and 12.9 PhOH/lOO C9 units, respectively.
A value of 47 PhOH/100 C9 units have been reported for the dissolved or extracted lignin
from the ALCELL Process (Gallagher et al., 1989). Once the ethanol is evaporated, this
lignin precipitates out of the acidic aqueous solution. Native softwood lignin contains
approximately 12 PhOH/ 100 C9 units and the value is normally smaller for native
hardwood lignin (Francis et al., 1991).

Although only small percentage of extractives exists in wood, the role of
extractives in bonding copper is important (Figure 3.1 and 3.2). Wood extractives have a
certain amount of phenolic hydroxyl groups and carboxylic acid groups (F engel and
Wegener, 1984). These groups cause copper absorption between wood and extractive-

free wood treatment to differ.

71

References

Belford, D.S. and RD. Preston. 1957. Timber preservation by copper compounds.
Nature. 180: 1081-1083

Bland, DE. 1963. Sorption of copper by wood constituents. Nature. 200:267

Cooper, P. A. 1991. Cation exchange adsorption of copper on wood. Wood Protection.
1(1): 9-14

Dahlgren, SE. 1972. The course of ﬁxation of Cu-Cr-As wood preservatives. Rec. Ann.
Conv. Brit. Wood Preservers’ Assoc. 109-128

F engel, D. and G. Wegener. 1984. Wood: Chemistry, ultrastructure, reactions. New York

Francis, R.C., Y-Z. Lai, C.W. Dence and TC. Alexander. 1991. Estimating the
concentration of phenolic hydroxyl groups in wood pulps. Tappi J. 74(9): 219-224

Gallagher, D.K., H.L. Hergert, M. Cronlund and LL. Landucci. 1989. Mechanism of
deligniﬁcation in an auto-catalyzed solvoysis of Aspen wood. Proc. Intl. Symp.
Wood Pulping Chem. Raleigh, NC. pp709

Goya], G.C., J.H. Lora and BK. Pye. 1992. Auto-catalyzed organosolv pulping of
hardwoods: Effect of pulping conditions on pulp properties and characteristics of
soluble and residual lignin. Tappi J. 75(2): 110-116

Lebow, ST. and J .J .Morrell. 1995. Interactions of ammoniacal copper zinc arsenate
(ACZA) with Douglas-ﬁr. Wood Fib. Sci. 27(2): 105-118

Ona, T., T. Sonoda, M. Shibata and K. Fukazawa. 1995. Small-scale method to determine
the content of wood components from multiple eucalypt samples. Tappi J. 78(3):
121-126

Pizzi, A. 1982. The chemistry and kinetics behavior of Cu-Cr-As/B wood preservative.
I]. Fixation of the Cu/Cr system on wood. IV. Fixation of CCA to wood. J.
Polym. Sci. Polym. Chem. Ed. 20: 704-724, 739-764

Rennie, P.M.S., S.M. Gray and DJ. Dickinson. 1987. Copper based water-borne
preservatives: Copper adsorption in relation to performance against soft-rot.
IRG documents: IRG/WP/3452

Sjostrom, E. 1989. The origin of charge on cellulosic ﬁbers. Nordic Pulp and Paper
Research. 2: 90-93

72

Xie, G, J.N.R Ruddick, S.J.Rettig and F.G. Hering. 1995. Fixation of ammoniacal
copper preservatives: Reaction of vanillin, a lignin model compound with
ammoniacal copper sulfate solution. Holzforschung. 49: 483-490

Chapter 4

Investigation of Copper Bonding Sites by Fourier Transform Infrared
Spectroscopic Analysis

4.1 Abstract

The interactions of copper monoethanolamine (Cu-MBA) and wood components
were studied by using Fourier transform inﬁared spectroscopy (F TIR). In Cu-MEA
treated wood, signiﬁcant reduction was noticed on the band attributed to carbonyl
vibration ﬁom carboxylic groups at 173515 cm" and an increase in band intensity was
obtained from carbonyl in carboxylate at 1595¢5 cm". The same observation was made
in Cu-MEA treated holocellulose. The FTIR spectrum of cellulose did not change after
Cu-MEA treatment. When the carboxylic acid groups were introduced into cellulose by
oxidation, Cu-MEA treatment of oxidized cellulose resulted in the disappearance of the
carboxylic acid band, which further conﬁrmed that carboxylic groups were active
bonding sites for copper. Cu-MEA treated lignin resulted in a reduction in the aromatic
ester band at 1710¢S cm’1 and an increase in carbonyl from carboxylate at 159525 cm".
Bands at 1370 cm'l and 1221 cm", assigned to phenolic hydroxyl groups, exhibited a
decrease in intensity after the treatment. From these data, it is concluded that Cu-MEA
interacts with carboxylic groups, phenolic hydroxyl groups and ester groups ﬁom lignin

to form copper carboxylate and phenolate complexes.

73

74

4.2 Introduction

Copper-based preservatives are widely used in wood protection due to the
excellent fungicidal activity of copper. The interactions of copper-based wood
preservatives with wood and the bonding sites for copper have long been an active wood
preservation research area. As a result, a lot of hypotheses have been proposed. Pizzi
(1982) concluded that copper was physically absorbed by studying the reactions of
chromate copper arsenate (CCA) with cellulose, glucose, guaiacol and ﬁnely group pine
sapwood. Ion exchange reactions with wood have been postulated since wood contains
weak acid groups that are able to form complexes with copper cation (Dahlgren, 1972;
Pizzi, 1982). Cooper (1991) conducted extensive research on cation exchange adsorption
of copper on wood, conﬁrming that wood is a weak acid cation exchange material. At
low pH value, uronic acid groups in hemicellulose were reported to be the primary sites
for ion exchange (Rennie et al., 1987; Copper, 1991), while lignin became increasingly
important exchange sites at higher pH (Pizzi, 1982). Functional groups in wood, such as
aliphatic hydroxyl group, phenolic hydroxyl group and carboxylic group, have been
proposed as the potential bonding sites for copper. Research by Lebow and Morrell
(1993) demonstrated that the phenolic group in wood provided reactive sites for copper.
Carboxylic groups in wood have been reported as the bonding sites for copper (Craciun
and Kamdem, 1997; Thomason and Pasek, 1997).

Apart from all these investigations, spectroscopic analysis has also been
employed to obtain information about the chemical interactions between wood
components and preservatives. Fourier transform infrared spectroscopy is commonly

used to examine functional groups and components in wood sample (Ostmeyer et al.,

75

1989). The technique has also been used to identify and qualify major wood components
such as lignin, cellulose, glucose and their modiﬁed derivatives (Michell, 1995;
Rodrigues er al., 1998).

In the previous experiment, the role of individual wood components in copper
absorption was investigated. In this experiment, diffuse reﬂectance Fourier transform
infrared spectroscopy (DRIFT) was used to monitor the change in the spectra of wood
components after treatment with copper amine preservative, examine the interactions of
this preservative with various functional groups in wood and investigate the bonding sites

of copper in wood.

4.3 Materials and Methods
4.3.1 Materials

Kiln-dried southern pine (SP) sapwood was used for the current study. SP was
extracted with ethanol/toluene and then ground into sawdust to pass 60 mesh.
Holocellulose was prepared by deligniﬁcation of wood with acidiﬁed sodium chlorite
using modiﬁed methods reported by One et a1. (1995). Cellulose and organosolv lignin
were purchased form Aldrich Chemical Co. Copper monoethanolamine (Cu-MBA)
treating solution was made by dissolving copper hydroxide in aqueous MBA with the
molar ratio of amine to copper of 4.
4.3.2 Oxidation of Cellulose

Cellulose was ﬁrst oxidized to oxycellulose by sodium metaperiodate, and the

aldehyde group in oxycellulose was then oxidized to carboxylic group by chlorous acid

(Figure 4.1).

76

01-1on CH20H CH20H r
o 04‘ o of o o
Narogl HClOZ
0 OH 0 H H _’ 0 0H
J‘
r 0H 0 o .r 0 HO 0

Figure 4.1 Oxidation of cellulose into oxidized cellulose

77

4.3.3 Treatment

About 2.5 grams wood substrates were treated with 50 grams of Cu-MBA treating
solution containing 1.0% copper by weight in 100ml ﬂasks. The ﬂasks were sealed and
agitated on a wrist-shaker for 24 hours. After the agitation, samples were ﬁltered through
glass ﬁber ﬁlters and washed continuously with deionized water until the conductivity of
the eluent water was constant. The purpose of continuous washing with water was to
minimize physical absorption of copper in treated samples. The treated and water-washed
samples were air-dried for two weeks and analyzed by F TIR analyses.

4.3.4 Fourier Transform Infrared Spectroscopy (FTIR)

FTIR analyses were performed on a Nicolet Protégé 460 spectrometer equipped
with Spectra-Tech diffuse reﬂectance accessory. Potassium bromide (KBr) was used to
collect background. Air-dried samples were mixed with KBr before spectrum collection.
All spectra were collected using diffuse reﬂectance Fourier transform infrared
spectroscopic technique (DRIFT). Spectra were acquired for a total of 64 scans on a 400
to 4000 cm'1 wavenumber range with a resolution of 4 cm'l. All spectra were displayed in

absorbance and limited to 600-1800 cm'l region.

4.4 Results and Discussion

The interest region of all spectra is limited to 600-1800 cm". Figure 4.2 shows the
FTIR spectra of wood and Cu-MBA treated wood. The dotted line denotes the spectrum
of Cu-MBA treated wood. Major band assignments in the infrared spectra, listed in Table
4.1, were made according to those of Sarkanen et aI.(1967a, b), Michell et al. (1965),

Liang and Marchessault (1959), and Tolvaj and Faix (1995). The band at 1739 cm“1 in

78

\

 

     

13D 121)

.I..

'Tj'

'TTFfff'

'I'

17m m

181)

ﬁ

 

 

6
w 00."!"‘ B
tltiv tittil t
l «Iris 5
0 a. 11
5 IIIIIIIIIIII V I! 7
6 1
1|.
w
7 V .
1 \
d 4 u c 1 .- dl l d dlli d ilq l l d tiff 1d q

15D 14!)

1Q!)

Whit”
FTIR spectra of (A). Wood and (B). Cu-MBA treated wood

Figure 4.2

79

Table 4.1 Assignments of inﬁared absorption bands in wood.

 

Position in cm'1

Band Assignments

 

1739
1651
1595
1510
1470
1425
1370
1265

1230

1150, 1063

C=O stretching in COOH

C=O stretching of a-keto groups
Aromatic skeletal vibrations

Aromatic skeletal vibrations

C-H deformations (asymmetric)

Aromatic skeletal vibrations

C-H deformations (symmetric)

C-O-C asymmetric stretching vibration of
aryl ether linkages

C-O-C asymmetric stretching vibration of
aryl-alkyl ether linkages

C-O deformation

 

80

Figure 4.2A was assigned to carboxyl stretching vibration in carboxylic acid (Tolvaj and
Faix, 1995; Michell et al., 1965). After treatment, this band diminished and a weak band
at 1715 cm'1 was left as a shoulder. The intensity of the band at 1596 cm'l, relative to the
band at 1650 cm", was enhanced in Cu-MBA treated wood. The subtraction of spectrum
of untreated wood from that of Cu-MBA treated wood is illustrated in Figure 4.3. The
subtraction result conﬁrmed that the intensity decreased at 1739 cm'1 and the intensity
increased at 1596 cm'l. The disappearance of the band at 1739 cm'1 in treated wood can
be attributed to the dissociation of carboxylic acid into carboxylate anion and further
interaction with copper amine complex (Nakamoto, 1978). The increase of the band
intensity at 1596 cm'1 is due to carbonyl stretching vibration in carboxylate salt. It has
been widely reported that the carboxylic band at around 1735 cm“1 shifted to 1590 cm1
upon conversion to a carboxylate (Hergert, 1971). The weak band at 1715 cm'1 in Cu-
MEA treated wood may be due to carbonyl stretching from B-ketone groups in lignin
(Michell, 1965). All the information suggests that the carboxylic acid groups are very
active sites for complexing copper. Due to the intrinsic complexity of the structure of
wood, it is difficult to interpret the spectra in other regions.

To elucidate the interactions of Cu-MEA with wood, individual wood
components, such as cellulose, holocellulose and lignin, were treated with Cu-MBA and
their spectra were shown in Figure 4.4- 4.7. Figure 4.4 shows the spectra of cellulose and
Cu-MEA treated cellulose. There is no noticeable change in FTIR spectrum of cellulose
after being treated with Cu-MEA. Absorption test showed that only a minimal amount of

copper (Table 3.2) was absorbed by cellulose when cellulose was treated with 1.0% Cu-

81

 

 

1596
i
. 1739
T m‘oT' 'e'. «a. as nub an FT
Mbauﬁnmpwn

Figure 4.3 Subtraction of spectrum of untreated wood ﬁ'om that of Cu-MBA treated
twood

82

'-
...—o .0. .— 0‘-..

 

1&1)

Figure 4.4

FTIR spectra of (A). Cellulose and (B). Cu-MBA treated cellulose

83

 

 

l
1600
‘\
\\
C ‘a
1741
B .
\\
\ ‘a
A
4000 3500 3000 2500 2000 1500 1000 500
Wavenumbers

Figure 4.5 FTIR spectra of (A). Cellulose; (B). Oxidized cellulose and (C). Cu-MEA
treated oxidized cellulose

84

    

1595
1736 E 1510
it
five.

(---------

 

 

 

We (an-1)

Figure 4.6 FTIR spectra of (A).Holocellulose and (B). Cu-MEA treated holocellulose

85

1221

 

        

\.
P
O
6
O
l
--‘W \
l“.‘“
4 t
5 calf-5'
l """" I'll. ct ’.
l I- 'v flirted
‘3
ttttttttttttt
"i'v ”If!
.v
{If
an
as‘.
m ...... a. ..,
n
-"""-"v ”‘v I
o‘cllv .
"-' A“. II.
'3‘ ‘- "’
tntbflil'l
v 'I‘Mlv
llllllll V at... it...
3 it.V..r
0
6
l

I v v v j v v v 1' v ffT v v

1100

v " v v v

1200

W(m1)

1 v v

131)

r T v fv

14D

 

' T

.2-.-,2.
1500

v I v v

18D

 

 

  

1000 900 800

1700 151)

191)

FTIR spectra of (A). Lignin and (B). Cu-MBA treated lignin

Figure 4.7

86

MBA treating solution. This small amount of copper may be due to the result of physical
interactions, such as dipole-dipole, ion-dipole and Van der Waals interactions, of copper
with cellulose. These results imply that cellulose plays only a minor role in copper
absorption and aliphatic hydroxyl groups are inert in bonding copper in the treating
conditions since cellulose contains a great amount of aliphatic hydroxyl groups. Figure
4.5 demonstrates the FTIR spectra of cellulose, oxidized cellulose and Cu-MBA treated
oxidized cellulose. After oxidation of cellulose, a new band appeared at 1741 cm'1 due to
the introduction of carboxylic acid. When the oxidized cellulose was treated with Cu-
MEA solution, the band at 1741 cm'1 disappeared and the intensity of the band at 1600
cm'1 increased. The result conﬁrmed that the carboxylic acid groups are the active sites
for copper.

Figure 4.6 shows the spectra of holocellulose and Cu-MBA treated holocellulose.
As was the case in Cu-MBA treated wood, the intensity of the band at 1736 cm",
assigned to carboxylic groups, minimized and was shown as a shoulder after Cu-MEA
treatment. The intensity of the band at approximately 1595 cm'1 was also increased in
treated holocellulose due to the shift of the carboxylic band. It has been reported that
most carboxylic groups in wood are contributed by hemicellulose, for instance the uronic
acid (Sjostrom, 1989). These carboxylic groups can be attributed to copper absorption
(0.6%) in a 1.0% Cu-MBA treated holocellulose because holocellulose consists of
hemicellulose and cellulose and the copper absorbed by cellulose can be neglected. The
weak band at 1510 cm'1 in holocellulose (Figure 4.6A) indicates the presence of small
amount of lignin. After treatment, the band intensity was decreased. More detail on this

band is discussed in the following paragraph.

87

The FTIR spectra of lignin and Cu-MEA treated lignin are demonstrated in Figure
4.7. The assignment of the band at 1712 cm’1 in lignin (Figure 4.7A) has always been
difficult and controversial since unconjugated ketones, conjugated carboxylic acids and
their esters absorb in the 1712 cm’1 region. It has been reported that saturated open chain
ketones have a characteristic carbonyl stretching frequency of 1700-1715 cm'1 in the
solid state. The lignin model compounds, guaiacyl acetone, B-hydroxylconiferyl alcohol
and 1-ethoxy-l-guaiacyl-Z-propanone, showed absorption bands at 1705, 1709 and 1710
cm'l, respectively (Hergert, 1971). This supports that the 1712 cm'1 band is due to
carbonyl vibration in unconjugated ketones groups. However, Smith (1955) conducted
comprehensive study on this band and pointed out that this band was caused by the esters
of acids, such as p-hydroxybenzoic, vanillic, syringic, p-hydroxycinnamic and ferulic
acids. This result was supported by Sarkanen et al. (1967b). Sarkanen et al. (1967b) also
observed that treatment of ponderosa pine lignin with a buffer solution of pH 7 caused no
change in the IR spectrum, suggesting that this band is not the absorption of carboxylic
acid groups.

After treatment of lignin with Cu-MBA solution, the 1712 cm’1 absorption band
diminished and the absorption at 1603 cm'1 shifted to 1595 cm'1 with a small intensity
increase (Figure 4.7B). This case is very similar to that of Cu-MBA treated wood and
holocellulose. It is easy to take it for granted that the 1712 cm'1 band is due to carboxylic
acid. As mentioned above, this band is not caused by carboxylic acid. In addition, it has
been reported that carboxylic acid content in lignin was very low (SjoStrom, 1989). So,
the disappearance of band at 1712 cm'1 is not due to the change in carboxylic acid. It

should be noted that the pH of the Cu-MBA treating solution used in this study was about

88

10. With such an alkaline solution, the ester groups with absorption at 1712 cm'1 can be
hydrolyzed into aromatic carboxylic acid groups (Sarkanen et al., 1967b). When the
carboxylic acid groups form, they can ﬁrrther interact with copper ion, leading to the
formation of copper complex. This explains the decrease in band intensity at 1712 cm'1
and the increase in band intensity at 1595 cm".

Normally the absorption band at around 1735 cm'1 (carboxylic acid) for
hardwood, such as beech, is three times higher than the 1712 cm’1 absorption band (F aix,
1992). However, the 1712 cm'1 absorption band dominated over the 1735 cm’1 band in
this case. Theorganosolv lignin used in this study was obtained ﬁ'om ALCELL process.
Carboxylic acids would be expected to be esteriﬁed during ALCELL pulping in a mildly

acidic ethanol/water mixture at approximate 200 °C (Goyal et al., 1992). The expected

reaction is shown below:
H+
WOOd - COOH + CH3CH20H ____, WOOd - COOC2H5 + H20

It has also been reported that alkaline treatment of lignin could cause oxidative
cleavage (Hergert, 1960). Treatment of the model compounds, such as guaiacyl acetone
and the keto form of B-hydroxylconiferyl alcohol, with mild alkali in the presence of air
resulted in the loss of the 1712 cm'1 ketone carbonyl and its subsequent replacement upon
acidiﬁcation with a carboxylic acid group (Hergert, 1960). So if the absorption band at
1712 cm'1 in lignin (Figure 4.7A), is due to ketone carbonyl groups, it can also form
carboxylic groups through oxidation under the treating conditions, and the carboxylic

groups can further form carboxylate, causing an increase of band intensity at 1595 cm'l.

89

The absorption bands at 1603, 1514, and 1420 cm'1 in lignin (Figure 4.7A) were
assigned to aromatic skeletal vibration. The intensity and position of these bands are
sensitive to the nature of ring substituents (Hergert, 1971). After treatment, the band at
1514 cm'1 shifted to 1502 cm'1 and the relative intensity of the 1514 cm'1 ring-stretching
band and the band at 1462 cm'1 of C-H bonds was reversed. These changes can be
explained that treatment could alter the aromatic ring substituents, for instance, the
hydrolysis of aromatic esters and/or oxidative cleavage of ketone carbonyl groups.

The absorption bands at 1370 cm'1 and 1221 cm“1 in lignin (Figure 4.7A) have
been assigned to phenolic O-H deformation by a considerable number of investigators
(Kolboe and Ellefsen, 1962, Sarkanen et al., 1967a and Hergert, 1971), although C-H
deformation also absorbs in 1370 cm'1 and in 1220 cm'1 regions (Sarkanen et al., 1967a).
The intensities of these two bands decreased after treatment with Cu-MBA (Figure 4.7B),
which is due to the ionization of phenolic hydroxyl groups into phenolic anion. The
phenolic anion can further bond with copper to form copper phenolic complex. A new
weak band, exhibited in treated lignin at 1060 cm'1 (Figure 4.7B), could be assigned to C-
O stretching in copper phenolate complex. Another change in FTIR spectra after
treatment is the 1154 cm'1 absorption band. This band is normally assigned to C-0
stretching from ester groups (Colthup er al., 1964). The lowered intensity of this band in
Cu-MEA treated lignin is an indication of alkaline hydrolysis of ester groups.

Three main ionizable groups, namely carboxylic acid groups, phenolic hydroxyl
groups and alcoholic hydroxyl groups, are present in wood. Of these, the majority of
carboxylic groups are carried by hemicellulose, and lignin contains only a comparatively

few carboxylic groups (SjOStrOm, 1989). Lignin mainly contributes phenolic hydroxyl

90

groups and ester groups, and the ester groups can be hydrolyzed to carboxylic groups
under both acidic and alkaline conditions. Cellulose can only contribute alcoholic
hydroxyl groups. When wood is treated with copper amine preservative, these groups are
the possible bonding sites for copper ions. According to Serjeant and Dempsey (1979),
carboxylic acid groups are ionized in neutral or even weakly acidic conditions, a rather
high pH above 10 is needed for ionization of phenolic hydroxyl groups, and a very strong
alkali is needed for ionization of alcoholic hydroxyl groups. When wood is treated with
copper amine solution, carboxylic groups and phenolic groups can be easily ionized
during treatment. They are responsible for the bonding of copper. In addition, aromatic
esters in lignin can be hydrolyzed into aromatic carboxylic groups, which act as extra
bonding sites for copper. These hypotheses have been conﬁrmed by the FTIR spectra.
The dissociation of alcoholic hydroxyl groups was minimal in the treating solution used
in this study. This accounts for the negligible absorption of copper in cellulose and no

change in FTIR spectrum of cellulose after being treated with copper amine.

4.5 Conclusions

Diffuse reﬂectance FTIR (DRIFT) spectra of cellulose, hemicellulose and lignin
were analyzed. The carboxylic acid groups in hemicellulose and the phenolic groups in
lignin are the major bonding sites for copper. In addition, aromatic esters groups and/or
ketone groups, which can be changed to carboxylic acid groups through alkaline

hydrolysis or oxidative cleavage, are other potential bonding sites for copper.

91

References

Cooper, PA. 1991. Cation exchange adsorption of copper on wood. Wood Protect.
1(1):9-14

Dahlgren, SE. The course of ﬁxation of Cu-Cr-As wood preservatives. Rec. Ann. Brit.
Wood Preservers’ Assoc. 109-128

Colthup, N. B., L.H. Daly and E. Wiblerley. 1964. Introduction to Inﬁ'ared and Raman
spectroscopy. New York, Academic Press

Craciun, R. and PD. Kamdem. 1997. XPS and FTIR applied to the study of waterborne
copper naphthenate wood preservatives. Holzforschung. 51(3):207-213

Faix, O. 1992. Characterization in solid state. In: Methods in Lignin Chemistry. Bds. S.Y.
Lin and C.W.Dence. Springer-Verlag.

Goyal, G.C., J.H. Lora and BK. Pye. 1992. Auto-catalyzed organosolv pulping of
hardwoods: Effect of pulping conditions on pulp properties and characteristics of
soluble and residual lignin. Tappi J. 75(2): 110-116

Hergert, H.L. 1960. Inﬁared spectra of lignin and related compounds. 11. Conifer lignin
and model compounds. J.Org. Chem. 252405-413

Hergert, H.L. 1971. Infrared spectra. In: Lignins: Occurrence, Formation, Structure and
Reaction. Eds. K.V.Sarkanen and C.H.Ludwig. Wiley-Interscience.

Lebow, ST. and J.J.Morell. 1993. ACZA ﬁxation: the roles of copper and zinc in arsenic
precipitation. Proc. of the AWPA Preservers’ Assoc. 89: 133-146.Granbury, TX

Liang, C. Y. and RH. Marchessault. 1959. Infrared spectra of crystalline
polysaccharides. 11. Native celluloses in the region from 640 to 1700 cm-1.
392269-278

Lora, J .H., C.F. Wu, E.K. Pye and J .J . Balatinecz. 1988. Characteristics and potential
applications of lignin produced by the organosolve pulping process. In: Lignin,
Properties and Materials. Sarkanen, S., and G. Glasser, editiors

Jones, M. Jr. 1997. Organic chemistry. pp. 1023. W.W.Norton & Company, NewYork.
London

Michell, A.J. 1995. FTIR studies of sludges from copper-chrome-arsenic wood
Preservative formulation. Holzforschung. 49:217—221

Michell, A.J., A.J. Watson and HG. Higgins. 1965. An infrared spectroscopic study of
deligniﬁcation of eucalyptus regnans. Tappi. 48(9):520-532

92

Nakamoto, K. 1978. Inﬁared and Raman spectra of inorganic and coordination
compounds. 3“I edition. Wiley, New York

Ona, T., T. Sonoda, M. Shibata and K. Fukazawa. 1995. Small-scale method to determine
the content of wood components from multiple eucalypt samples. Tappi J.
78(3):121-126

Ostmeyer, J. G., Elder, T. J., Winandy, J. E., 1989. Spectroscopic analysis of southern
pine treated with chromated copper arsenate. II. Diffuse reﬂectanCe Fourier
transform infrared spectroscopy(DRIFT). J. Wood Chem. Tech. 9(1): 105-122

Pizzi, A. 1982. The chemistry and kinetic behavior of Cu-Cr-As/B wood preservatives.
H. Fixation of the Cu/Cr system on wood. IV. Fixation of CCA to wood. J.
Polym. Sci., Chem Ed. 20: 707-724, 739-764

Rennie, P.M.S., S.M. Gary and DJ. Dickinson. 1987. Copper based water-borne
preservatives: copper adsorption in relation to performance against soft-rot.
International Research Group on Wood Preservation. IRG document:
IRG/WP/3452

Rodrigues, J., O. F aix and H. Pereira. 1998. Determination of lignin content of eucabptus
globulus wood using FTIR spectroscopy. Holzforschung. 52:46-50

Sarkane, K.V., Hou-Min Chang and B. Ericsson. 1967a. Species variation in lignins. I.
Inﬁared spectra of guaiacyl and syringyl models. Tappi. 50(11):572-575

Sarkane, K.V., Hou-Min Chang and G.G. Allan. 1967b. Species variation in lignins. H.
Conifer lignins. Tappi. 50(12):583-587

Serjeant, E. P. and B. Dempsey. 1979. Ionization constants or organic acids in aqueous
solutions. IUPAC Chemical Data Series No. 23. Pergamon Press, Oxford

Sjostrom, E. 1989. The origin of charge on cellulosic ﬁbers. Nordic Pulp and Paper
Research Journal. 2:90-93

Smith, D.C.C. 1955. Ester groups in lignin. Nature. 1761267, 927

Thomason, S. M. and BA. Pasek. 1997. Amine copper Reaction with wood components:
Acidity versus copper adsorption. International Research Group on Wood
Preservation. IRG documents: IRG/WP/97-30161

Tolvaj, L and O.Faix. 1995. Artiﬁcial ageing of wood monitored by DRIFT spectroscopy
and CIE*a*b* color measurements. 1. Effect of UV light. Holzforschung. 49:397-
404

Chapter 5

Electron Paramagnetic Resonance Spectroscopic (EPR) Analysis of Copper Amine
Treated Wood

5.1 Abstract

The structures of copper complexes in copper amine treated wood samples were
elucidated by the application of electron paramagnetic resonance spectroscopy. Axial
spectra were observed for all treated samples irrespective of the formulations. The A" and
g” of all the axial spectra indicated that the stereo-structure of copper complexes in
copper amine treated wood is tetragonal-based octahedral or square-based pyramidal.
Comparison of electronic parameters of A“ and g” in treated wood with those in treating
solution and those reported in literature suggests that the interactions of wood with
copper amine is through complexation in which functional groups from wood complex
with copper amine perpendicularly. The copper complexes in both treating solution and

treated wood are in the form of CuN202.

5.2 Introduction

Copper amine is one of the main ingredients of ammoniacal copper quat-type D
(ACQ-D), copper dimethyldithiocarbamate (CDDC), and copper azole (AWPA, 1998).
Chemically speaking, this system is a series of cupric complexes in which different type
of ethanolamines act as bidentate chelating agents through amino and hydroxyl groups.

At high pH solution system, deprotonation of the hydroxyl groups and formation of stable

93

94

chelate rings are proposed (Tauler and Casassas, 1986). The chemical interactions
between this copper amine chelating ring complex and wood are not well understood.

While a considerable number of methods have been employed to investigate the
interaction between copper-based wood preservatives and wood, only a few are suitable
to study the chemical and electronic structure of copper in treated wood. To date, FTIR
(Ostmeyer er al., 1989) and XPS (Kamdem et al., 1998) have been used to obtain
valuable information on bonding and valency state of the copper in treated wood.
However, these techniques cannot elucidate the stereochemistry of copper complexes
formed in copper-based preservative treated wood, such as the ligand ﬁeld information
and the conﬁguration of mono or divalent copper complexes. Previous work has shown
that electron paramagnetic resonance (EPR) can be successfully used to study the copper
bonding environment and the stereochemical structure of copper complexes in copper-
based preservative treated wood substrate (Plaket, 1987; Pohleven et al., 1994; Hughes et
al., 1994).

The EPR spectra of transition metal ion complexes contain a wealth of
information about their electronic structures. Plaket et al. (1987) have performed EPR
spectroscopic analysis of radiata pine treated with CCA and copper sulfate and they
found no evidence of copper complexation with lignin and concluded that copper (II) in
copper sulfate treated wood was in the form of hydrated copper (11) ion (Cu(H2O)62+)
bound to wood through hydrogen bonding. Hughes et al. (1994) observed the presence of
a number of different copper complexes in preservative treated Pinus sylvestris as a
function of the formulation of copper preservatives. Their study also showed the

existence of immobile copper ions in all treated samples. The EPR spectra of the

95

ammoniacal copper treated wood indicated that the A" and the g” tensors of copper were
different ﬁom those in the original ammoniacal copper solution, suggesting that the
ammonia ligands of copper in the original treating solution have been replaced by oxygen
(Ruddick, 1992).

EPR was used in the current study to investigate the interactions of copper amine
with wood and to describe the possible stereochemistry copper complex in treated wood.
Such a study is of great importance in understanding the retention, penetration and
ﬁxation of copper amine treated wood and in predicting the biological performance and

the stability of such a system.

5.3 Theoretical Principles of the EPR of Copper Complexes (Drago, 1992)
On application of a magnetic ﬁeld, different energy states arise from the
interaction of an unpaired electron spin moment (given by m. = 39/: for a free electron)

with the magnetic ﬁeld, the so-called electron Zeeman effect. The “Zeeman Hamiltonian’

that describes the interaction of an electron with a magnetic ﬁeld is:
I? = 3.685.

where g is Landé splitting factor (2.0023193 for a ﬁee electron)
B is the electron Bohr magneton, eh/2m.c, which has the value of 7.274x 10‘24
Joule/1‘ esla
B is the applied ﬁeld strength

S2 is the spin operator

96

when this Hamiltonian operates on a spin and [3 spin corresponding to m. = + ‘/2 and m. =
- ‘/2, the splitting occurs (Zeeman splitting). The energy difference AB for a single

electron (m, = 2%), found from the Hamiltonian, is given by:

AE=gﬂB

The direction of the ﬁeld has been taken as the axis of quantisation (z).

Divalent copper ion Cu2+ with a 3d9 electronic conﬁguration has one unpaired
electron in d-orbital. It has an effective spin of s = V2 and associated spin angular
momentum of m. = 21:1/2 resulting in a doubly degenerate spin energy state in the absence
of a magnetic ﬁeld. The spin energy degeneracy is removed after an application of a
magnetic ﬁeld. Besides the interaction of electron spin moment with the magnetic ﬁeld,
there is also an interaction between magnetic ﬁeld B and the magnetic moment due to the
orbital angular momentum of the electrons. The total interaction, assuming Russell-

Saunders coupling, is given by:

I? = ﬂB(L + gs,)

where L is the orbital angular momentum of the electrons.

The Zeeman splitting of the ground state can be observed in an electron spin
resonance experiment. The orbital angular momentum is “quenched” for the ground
states of most copper (II) complexes, but spin-orbit coupling mixes-in some contributions

from excited states, the extent being expressed by the multiplet splitting factor g in the

energy equation.

hv= gﬁB

97

Where h is Planck’s constant and v is frequency.

In practice, three types of EPR spectra of divalent copper are observed, namely,
isotropic spectra, axial spectra and rhombic spectra. Of these, axial is the most common
situation for copper complexes, where copper is in a tetragonally distorted environment.
Previous study has also shown that copper complexes in wood are in axial conﬁguration
(Packett et al., 1987). The axial spectra of copper complexes exhibit a strong absorption
to higher ﬁeld at gr and weak absorption to lower ﬁeld at g". The hyperﬁne splitting, A.l.,
due to the nuclear magnetic moment of the Cu” at g1 is too small to differentiate. The
hyperﬁne splitting All, arising from the nuclear magnetic moment of the Cu2+ at g”, is
usually much greater, having typical values from 150 to 250x 10" cm'l, and four features

at g" are often resolved.

5.4 Materials and Methods
5.4.1 Materials

Kiln-dried southern pine (SP) sapwood was used for the current study. SP was
ground into sawdust to pass 60 mesh. Organosolv lignin was purchased form Aldrich
Chemical Co. Oxidized cellulose was prepared by oxidation of cellulose as described in
Chapter 4. Copper amine treating solutions were made by mixing copper compounds
with ethanolamine, such as monoethanolamine (MBA), 2-methylamino-ethanol (DMBA)
and N, N-dimethyl-ethanolamine (DMeEA) (Table 5.1). The molar ratio of amine to

copper was kept constant at 4.

98

Table 5.1 Copper amine formulations used for wood substrate treatment *

 

 

Wood substrates Copper amine formulations pH
Wood Cu-MBA 10.8
Cu-MEA 10.1
Cu-MBA 9.3
Cu-MBA 9.1
Cu-MeEA 10.8
Cu-DMeBA 10.6
CuSOa solution 4.1
Cu(NOa)2 solution 4.0
Lignin Cu-MEA 10.8
Oxidized cellulose Cu-MEA 10.8

 

* The copper concentration in all the formulations is 1.0% by weight

99

5.4.2 Treatment

About 2.5 grams of wood substrates were mixed with 50 grams of copper amine
(Cu-BA) solution containing] .0 % elemental c0pper in 100ml ﬂasks. The ﬂasks were
sealed and agitated on a wrist-shaker for 24 hours. After the agitation, samples were
ﬁltered through glass ﬁber ﬁlters and washed continuously with deionized water until the
conductivity of the eluent water was constant. The purpose of continuous washing with
water was to minimize physical absorption of copper in treated samples. The treated and
water-washed samples were air-dried for two weeks and analyzed by EPR.
5.4.3 Electron Paramagnetic Resonance Spectroscopy (EPR)

EPR spectra were recorded on a Bruker ESP 300E X-band spectrometer at 9.5
GHz. The temperature was controlled either with ﬂowing liquid helium or with a
temperature controller. The operating conditions were: 100 kHz ﬁeld modulation, 4.014
G modulation amplitude and 10.24 ms time constant. For liquid copper amine solution,
the sample was put into the holder and cooled by liquid helium, and EPR spectrum was

acquired at 6 Kelvin. For solid samples, EPR spectra were obtained at room temperature.

5.5 Results

EPR axial spectra of all wood samples treated with the formulation used in this
study are illustrated in Figure 5.1 to 5.11. They all present a higher gJ. absorption and a
relative weaker g” absorption. The hyperﬁne splitting of A“ are well resolved and the A 2
is too small to be resolved. The G values, which is equal to (g..-2)/( gl-Z), are greater than
4. It has been reported that a G greater than 4 is speciﬁc to an axial type of EPR spectra

and less than 4 speciﬁc to a rhombic type of EPR spectra (Hathaway and Billing, 1970).

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The Hamiltonian parameters are listed in Table 5.2. The g" parameters for copper amine
solution and copper amine treated samples are in the range of 2.2 to 2.3.

EPR spectrum of copper monoethanolamine frozen solution is illustrated in
Figure 5.1. The Aand gvalues are given in Table 5.2. The value of A“ (17.1mK) is
smaller and the value of g" (2.263) is greater than those parameters in copper ammoniacal
solution where copper metal center is ligated with four nitrogen (CuN4) (Xie et al.,
1995). In copper amine solution, copper is ligated with 2 nitrogen and 2 oxygen
(CuN202) (Casassas et al., 1989). The replacement of N by 0 will reduce the value of All
and increase the value of g“ because nitrogen (N) is more electron dense than oxygen (0).

Figure 2 shows the EPR spectrum of copper amine treated wood with g“ of 2.271
and All of 16.9 mK. The g" and A" parameters are close to those of copper amine complex
in solution. Increase in the pH of the treating solution ﬁ'om 9.1 to 10.8 had a slight effect
on the g” and A” of copper in treated wood (Table 5.2). Figure 5.3-5.5 show the EPR
spectra of wood treated with copper amine at different pH conditions.

When the ligand was changed from MBA to DMBA and DeMEA, EPR spectra
exhibited a shoulder on gJ. absorption (Figure 5.6 and 5.7). Hughes et al. (1994) observed
the same phenomenon in their EPR study of copper complexes. They also noted a ﬁlrther
A“ splitting in the g" region, and they attributed this to the presence of two forms of
copper in the treated timber. In the current study, no splitting on A“ in the g" region were
observed. The shoulder absorption in the 8.1. region is likely due to the fourth feature of
hyperﬁne splitting from A" since only 3 A” features are observed in the g" region.
Compared to Cu-MEA treated wood, Cu-DMEA and Cu-DeMBA treated wood have a

slight decrease in g" and a slight increase All. This may be attributed to the addition of

112

 

 

Table 5.2 EPR parameters of Southern pine treated with copper amine solutions
Samples g" g; G' A“, me
Cu-MEA ﬁ'ozen solution 2.263 2.062 4.24 17.1
Wood treated with
Cu-MBA, pH = 10.8 2.271 2.059 4.59 16.9
Cu-MEA, pH = 10.1 2.267 2.061 4.38 17.3
Cu-MEA, pH = 9.3 2.265 2.063 4.21 17.6
Cu-MEA, pH = 9.1 2.264 2.065 4.06 17.5
Cu-MeEA, pH = 10.8 2.263 2.06 4.38 17.8
Cu-DMeEA, pH = 10.6 2.258 2.059 4.37 17.9
CuSO4 solution, pH = 4.1 2.413 2.074 5.58 14.1
Cu(NO3)2 solution, pH = 4.0 2.382 2.068 5.62 14.5
Lignin treated with
Cu-MBA, pH = 10.8 2.287 2.055 5.22 15.9
Oxidized cellulose treated with
Cu-MEA, pH = 10.8 2.259 2.053 4.89 17.4

 

a: G = (81'2) / (31- 2)

b: mK is milikaisers (thousandths of a wavenumber cm")

113

electron-donating methyl groups. The presence of methyl groups in DMBA and DeMBA
introduces steric effect that inﬂuences the stability of copper amine complex (Hancock
and Nakani, 1984). It was suggested that the EPR parameters of copper were determined
by the chemical nature and charge state of the close-lying ligand atoms to copper atom,
and were not directly correlated with thermodynamic parameters which govern stability
of metal-ligand complexes (Peisach and Blumberg, 1974).

To elucidate the effect of amine ligands on EPR parameters, copper sulfate and
copper nitrate treated wood was studied. Although copper sulfate and copper nitrate
treated wood exhibited axial spectra as illustrated in Figure 5.8 and 5.9, the value of A“ is
much smaller and g” is greater than those obtained with copper amine treated wood
(Table 5.2 and Figure 5.13). This may be explained by the presence of only oxygen-
copper bondings in copper sulfate and copper nitrate treated wood. With the small
hyperﬁne splitting, four features of A" are observed in the g“ region (Figure 5.8 and 5.9).

Figure 5.10 shows the EPR spectrum of copper amine treated lignin, which has a
relatively smaller A" and larger g". In addition, a strong free radical signal was also
observed in the spectrum. The formation of free radicals was tentatively attributed to the
abstraction of electrons from phenolate anions by Cu(II) to form phenoxy radicals and
Cu(I) (Landucci, 1978). The Cu(I) was subsequently oxidized to Cu(II) by oxygen in the
air.

No EPR signal was obtained on copper amine treated cellulose. This is due to the
lower absorption of copper by cellulose. Only 0.06% copper was absorbed in cellulose
when cellulose was treated with 1.0% Cu-MBA solution. However, when cellulose was

oxidized and carboxylic acid groups were introduced into cellulose, copper absorption

114

increased and the EPR spectrum in Figure 5.11 conﬁrmed the presence of copper. EPR
spectrum shows a similar A" and g“ to those obtained in treated wood, suggesting the

interactions between copper amine complex and carboxylic groups.

5.6 Discussion

Important information that the EPR spectra yield is the nature of the electronic
ground state of Cu”. A distinction between a ground state dxz-y2 and dz2 lies in that dz2
is indicated by a low g-value (<2.04), while dxz-y2 by an axial spectrum with G > 4.0
(Hathaway and Billing, 1970). The EPR parameters (Table 5.2) and the axial spectra
(Figure 5.1 to 5.11) are typical for the copper complexes with a dxz-y2 ground state. The
axial symmetry of copper complexes with a lowest g-value > 2.04 is usually consistent
with elongated tetragonal-octahedral (C4v), square-coplanar (D4h) or square-based
pyramidal stereochemistry (C4v) (Hathaway and Tomlinson, 1970). The energy diagrams
for these complexes are illustrated in Figure 5.12 (Addison et al., 1978). With such a
diagram, the EPR g-factors in axial symmetry can be calculated by the following

equations according to “magic pentagon”(Drago, 1997):

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Where 2. has a value of —829 cm'1 for Cu(II), g" involves a promotion of an electron from

ground state dxz-y2 orbital to dry orbital (Figure 5.12). Since A is negative, it gives g“ a

115

Figure 5.12 Orbital energy diagram for Cu (II) complexes

Energy

’ W -1—

 

116

larger value than the g value for a free electron, 2.0023. g4. involves a transition of an
electron from orbital dxz-y2 to orbital dxz (Figure 5.12). The energy needed for this
transition is very large so that the second term in g; is going to be very small. Therefore,
unlike g”, gJ. does not deviate signiﬁcantly from the g value of the free electron. Our data
agrees with these theoretical calculations well.

Peisach and Blumberg (1974) pointed out that the ligands of copper complexes
with axial symmetry are arranged in a environment about the metal ion with four ligands
lying in an approximate plane including the copper ion. The ligands are close to the metal
center and thus strongly bonded. The other ligands are arranged on a straight line
including the Cu2+ and perpendicular to the plane. Those ligands perpendicular to the
plane usually play only a minor role in both magnetic and optical properties of the copper
complex. So, the magnetic properties of copper complexes are governed by the four
equatorial atoms, which are bonded to metal center closely.

It has been reported that Cu” forms chelating complexes with ethanolamine in
aqueous solution where two amine ligands chelate one cupric ion center (CuN202)
(Tauler and Casassas, 1986; Davis and Patel, 1968 and Casassas et al., 1989). The value
of g" and A" of copper complexes in the treating solution are 2.263 and 17 .1 mK,
respectively. These values are in agreement with other CuN202 complexes reported in
literature (Freyberg et al., 1977 and Xie et al., 1995). The g" and A” values of copper
amine treated wood given in Table 5.2 are similar to those of copper complex in aqueous
solution and those reported in literature (F reyberg et al., 1977 and Xie e101,, 1995). From
these g and A values, the chemical interaction between copper amine and wood can be

suggested. In the previous chapter, several alternatives have been proposed for copper

117

amine-wood interaction as illustrated in Figure 2.8. Ifcopper amine ligand-exchanges
with wood as proposed in Figure 2.8a, the copper amine complex will change form from
CuN202 in solution to CuNlO3 in treated wood. Nitrogen is more electron-rich than
oxygen. Replacement of N by 0 should reduce the values of A" and increase the values of
g". In an extensive review on EPR analysis of copper complexes, Peisach and Bluberg
(1974) demonstrated that CuN202 has a g” value of 2.2-2.3 and an A-value of 16-20 mK,
while g" for CuNlO3 and CuO4 range in 2.3-2.4 and A" less than 16mK. In the current
study, copper complexes in solution and Cu-EA treated wood show a g-value of 2.25-
2.27 and an A-value of 17.0-18.0 mK except for copper amine treated lignin, which
suggests that copper complexes in treated wood are in the form of CuN202. The
correlation of A” and g“ is shown in Figure 5.13. Other CuN202 complex systems give
the similar relationship between A" and g“ (Peisach and Bluberg, 1974). This eliminates
the alternative of ligand exchange with the formation of CuNlO3 and supports the
hypothesis that the wood acts as the ligands which complex with copper metal center
perpendicularly without displacing the amine ligands (Figure 2.8b).

When wood is treated with copper amine, carboxylic groups and phenolic
hydroxyl groups can complex with copper amine from perpendicular direction as
suggested by Figure 2.8b. Depending on the pH of the system, one or two ligands can
attach to the copper center. If two ligands attach to copper amine, a tetragonal-octahedral
complex will be formed. If one ligand is added to copper amine, a tetragonal-pyramidal
configuration will be formed. The introduction of only one ligand does not change the
EPR parameters significantly because A and g are mainly determined by the four

equatorial atoms (Peisach and Blumberg, 1974).

 

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5.7 Conclusions

The stereo-structure of copper complexes formed in copper amine treated wood
was characterized by EPR. The axial spectra and the electronic parameters determined by
EPR suggest that copper amine interacts with wood to form a tetragonal-based octahedral

or square-based pyramidal complex when wood is treated with copper amine solution.

120

References

Addison, AW. and M. Carpenter, L.K-m. Lau and M. Wicholas. 1978. Coordination
sphere ﬂexibility at copper: Chemistry of a unipositive copper (II) macrocycle,
[Cu(cyclops)]+. Inorg. Chem. 17(6): 1545-1552

Casassas, E. , L.L. Gustems and R. Tauler. 1989. Spectrophotometric study of complex
formation in copper (H) mono-, di-, and tri-ethanolamine systems. J. Chem. Soc.
Dalton Trans. 4: 569-573

Davies, CW. and EN. Patel. 1968. Complexes of the cupric ion with mono, di-, and tri-
ethanolamine. J. Chem. Soc. (A). 8: 1824-1828

Drago, Russell. 1997. in Physical Methods for Chemists. Sec. Ed. Saunders College
Publishing

Freyberg, D.P., G.M.Mockler and E.Sinn. 1977. Crystal and molecular structures of [N,
N-bis((5-chloro-2-hydroxyphenyl)phenylmethlene)-4-thiaheptane-1 ,7-diamino]
copper(II). Inorg. Chem. 16(7): 1660-1665

Hancock, RD. and BS. Nakani. 1984. Some factors inﬂuencing the stability of
complexes with ligands containing neutral oxygen donor ligands, including crown
ethers. J. Coord. Chem. 13: 309-314

Hathaway, BI. and A.A.G. Tomlinson. 1970. Copper (H) ammonia complexes. Coordin.
Chem. Rev. 5: 1-43

Hathaway, BJ. and DE. Billing. 1970. The electronic properties and stereochemistry of
mono-nuclear complexes of the copper(II) ion. Coordin. Chem. Rev. 5: 143-207

Hughes, A.S., RJ. Murphy, J.F. Gibson and IA Cornﬁeld. 1994. Electron paramagnetic
resonance (EPR) spectroscopic analysis of copper based preservatives in Pinus
sylvestris. Holzforschung. 48: 91-98

Kamdem, DP, .1. Zhang and M.H. Freeman. 1998. The effect of post-steaming on copper
naphthenate treated southern pine. Wood Fiber Sci. 30(2): 210-217

Landucci, LL. 1978. Metal-catalyzed phenoxy radical generation during lignin oxidation.
Transactions. 25-29

Plackett, D.V., E.W. Ainscough and AM. Brodie. 1987. The examination of preservative
treated radiata pine using electron spin resonance spectroscopy. IRG documents.
IRG/WP-3423

121

Ostmeyer, J. G., T.J.Elder and J .E.Winandy. 1989. Spectroscopic analysis of southern
pine treated with chromated copper arsenate. II. Difﬁrse reﬂectance Fourier
transform infrared spectroscopy (DRIFT). J. Wood Chem. Technol. 9: 105-122

Peisach, J. and WE. Blumberg. 1974. Structural implications derived from the analysis
of electron paramagnetic resonance spectra of natural and artiﬁcial copper
proteins. Arch. Biochem. Biophys. 165: 691-708

Pohleven, F., M. Entjurc, M. Petric and F. Dagarin. 1994. Investigations of ammoniacal
copper (II) octanoate in aqueous solutions and its determination in impregnated
wood. Holzforschung. 48: 371-374

Ruddick, J NR. 1992. The ﬁxation chemistry of copper based wood preservatives. Proc.
Can. Wood Preserv. Assoc. 13: 116-137

Tauler, R. and E. Casassas. 1986. The complex formation of Cu(II) with mono- and di-
ethanolamine in aqueous solution. Inorganica Chimica Acta. 114 (2): 203-209

Xie, Changshi, J .R.Ruddick, S.J.Rettig and F .G. Herring. 1995. Fixation of ammoniacal
copper preservatives: reaction of vanillin, a lignin model compound with
ammoniacal copper sulphate solution. Holzforschung. 49: 483-490

Chapter 6

X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopic (XPS)
Characterization of Copper-Amine Treated Wood Substrates

6.1 Abstract

Copper amine treated wood samples were characterized by X-ray diffraction
(XRD) and X-ray photoelectron spectroscopy (XPS). XRD analysis demonstrated that
copper amine treatment did not alter the crystalline lattice structure of cellulose. Post
treatment steaming of copper amine treated wood causes the formation of a new crystal
that was identiﬁed as cuprous oxide by XRD. XPS survey spectrum indicated the
presence of C 1 s, 01s, Cu2p and N15 in copper amine treated wood. XPS analysis showed

that the oxidation state of copper in treated wood is in cupric form.

6.2 Introduction

X-ray diffraction and X-ray photoelectron spectroscopy techniques have been
widely employed to elucidate the nature of copper in treated wood in the area of wood
chemistry. XRD is a useful tool for investigating the compounds with ordered structure.
It can be used to identify and semi-quantify crystalline compounds present in a matrix.
Creely et al. (1978) used XRD to study the complexes of cellulose with secondary
diamines. In wood protection, Gallacher et al. (1995) used XRD to identify copper

dimethyldithiocarbamate (CDDC) crystal formation in CDDC treated wood. Sutter et al.

122

123

(1983) used XRD to conﬁrm the formation of copper oxalate in wood treated with a
copper-based preservative.

X-ray photoelectron spectroscopy (XPS), also referred as electron spectroscopy
for chemical analysis (ESCA), has been used widely for surface analysis in physical
chemistry. Ostmeyer and Elder (1988) applied XPS to examine the reaction of CCA with
southern pine. By using ESCA analysis, Williams and F eist (1984) investigated the
reactions occurring in wood and cellulose surface after treatment with aqueous chromium
trioxide, and proposed that chromic acid was ﬁxed to both wood and pure cellulose.

In the current study, XRD and XPS were used to monitor the change of copper
and wood in terms of crystal formation, valency state of copper or redox reaction aﬂer

copper amine treatment.

6.3 Materials and Methods
6.3.1 Treatment

Defect-free 5 by 10 by 180cm (2 by 4 inch by 6 feet) kiln-dried sapwood boards
of southern pine (SP) were used in this study. Cubes measuring 19mm (0.75 inch) were
prepared ﬁom these boards and stored in a conditioning room maintained at 65% relative
humidity (RH) and 20 °C (68 °F) until they reached an equilibrium moisture content
(EMC) of 12 :h 3%. The conditioned cubes were then pressure-treated with copper amine
(Cu-EA) solutions. The elemental copper content in the solutions was 1.0% by weight.
The treating procedure included an initial vacuum at 84.6 kPa for 5 minutes, followed by
a pressure level of 690kPa (100 psi) for lhour, and then a ﬁnal vacuum for 10 minutes.

Treated samples were conditioned at room temperature for 2 weeks before further test.

124

6.3.2 X-ray Diffractometry

XRD diffraction patterns were obtained on a Rigaku Rotaﬂex model CN-4148B2
X-ray diffractometer using Cu-Ka radiation (A= 1.5418A). The X-ray diffraction was
operated at 45kV and 100 mA. Diﬂ‘raction patterns were collected using DD and DS slot
widths of 05°. The difﬁ'action angle (20) was measured from 5° to 65° at speed of
2°/minute.

6.3.3 X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy was operated on a Perkin-Elmer Electronics
ESCA 5400 spectrometer. The spectrometer was run at 300 watts, using Mg-K standard
PHI 04-548 and an Al K toroidal monochromatic source PHI 10-410. The experiments
were performed at a pass energy of 89.45 eV for survey scans and 35.7 eV for high
resolution scans.

The wood samples were microtomed into thin slices (about 120 um) and ﬁ'eeze
dried prior to introduction into the spectrometer. The samples were mounted to the
sample stubs using double-sided tape and placed into the analysis chamber. Analysis
included the collection of a low resolution, wide window survey from 0 to 1100 eV to
identify the elements detectable on the surface, followed by a high resolution analysis of

15 to 20 eV in width.

6.4 Results and Discussion
6.4.1 X-ray Diffraction (XRD) Analysis

The XRD patterns of wood and the air-dried treated samples are illustrated in

Figure 6.1. The characteristic peaks of cellulose I resulted from the 101, 101, 002 and

125

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101 101

 

 

 

 

 

 

 

 

 

 

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f
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4, 11’
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Figure 6.1 XRD patterns of (A). Untreated wood; (B). Cu-MEA treated wood; (C).
Cu-MeEA treated wood; (D). Cu-DMeEA treated wood and (E). Cu-MEA treated lignin

126

040 reﬂections are present in all samples except lignin. The XRD patterns of copper
amine treated wood in Figures 6. IB, 6.1C and 6.1D are similar to that of untreated wood
in Figure 6.1A with respect to the crystal lattice type. The pressure treatment with copper
amine preservatives did not change or affect cellulose crystal lattice structure.

In wood preservation area, post treatment steaming is usually used to accelerate
the ﬁxation of copper-based preservatives in treated wood (Barnes, 1985; Anderson,
1990). Figure 6.2 shows the XRD patterns of post treatment steamed samples. No
significant modiﬁcation in XRD patterns was noticeable after post treatment steaming of
wood and lignin (Figure 6.2). However, new peaks appeared in the XRD patterns of Cu-
EA treated samples after post treatment steaming (Figure 6.2), suggesting the formation
of new crystal(s). The positions of the new peaks are located at 26 value of 365° and
421°. The crystal detected in the post treatment steamed Cu-EA treated samples were
identiﬁed as cuprous oxide (Cu20) (Kamdem et al., 1998).

The initial oxidation state of copper in Cu-EA treating solution is Cu”. The
oxidation state of copper in Cu20 formed during post treatment steaming of Cu-EA
treated wood is Cu”. This suggests that post treatment steaming may promote the
reduction of cupric into cuprous. It is well known that wood contains reducing agents,
such as aldehyde groups and reducing hemiacetal groups at the end of cellulose chains,
which are capable of reacting with Cu2+ to form Cu20 as suggested in Equation 6.1 under
a certain condition, such as the steaming conditioning. Since no Cu20 signal was
observed in XRD pattern of Cu-EA treated lignin after post treatment steaming (Figure
62E), the redox reaction may occur mainly between carbohydrate and copper.

R-CHO + 203* + 2H20 ____, R-COOH + CU2O + 4H“ Equation (6.1)

127

    

101 101

 

 

 

 

 

 

 

 

 

 

r l i r I

5 25 36.5 42.1 45 65
Diffraction angle 2-theta, degree

Figure 6.2 XRD patterns of post-steamed (A). Untreated wood; (B).Cu-MEA treated wood;
(C). Cu-MeEA treated wood; (D). Cu-DMeEA treated wood and (E). Cu-MEA treated lignin

128

6.4.2 X-ray Photoelectron Spectroscopic (XPS) Analysis

The surface composition of the specimens can be readily obtained from XPS
survey spectra (Figure 6.3). The XPS spectrum of untreated wood shows the presence of
Cls and 01s (Figure 6.3A), while the spectrum of Cu-MEA treated wood reveals the
presence of Cls, Ols, Cu2p and N13 (Figure 6.3B). The XPS Cu2p spectra are illustrated
in Figure 6.4. Cu2p XPS spectra contain two peaks with an energy difference of 20.0eV,
one at binding energy (BE) of 952.5eV corresponding to Cu2pm and the other at BB of
932.5eV corresponding to Cu2p3/2. These data agree with those previously reported
(Kamdem et al., 1996). The copper oxidation state is reﬂected by the BE of Cu2p3/2 and
the energy difference between Cu2p“; and Cu2p”. The oxidation state of copper in Cu-
MEA treated wood and in CuSO4 treated wood is similar due to the same Cu2p BE and
energy difference between Cu2pm and Cu2pm (Figure 6.4A and 6.4B). The difference
between these two peaks varies from 19.8eV for Cu° metal to 20.0eV for Cu2+ species
(Moulder et al., 1992). XPS data suggests that the oxidation state of copper in both Cu-
EA and CuSO4 treated wood is 2+.

The XPS analysis of individual elements in wood and treated wood is given in
Table 6.1. Table 6.1 shows that only the moiety of nitrogen and no copper are present in
untreated wood. After treatment with Cu-EA, the nitrogen content in wood increased
significantly. The leaching of treated sample caused very small percentage loss of
nitrogen and copper, suggesting that most of nitrogen and copper are bonded to wood.
With an amine free system, such as CuSO4 treated wood, leaching leads to a higher
percent loss of copper compared to Cu-EA treated wood. These ﬁndings conﬁrm the

importance of amine in the formulation of copper amine based preservatives.

129

Figure 6.3 XPS survey spectra of (A). Untreated wood and (B). Cu-MEA treated
wood

130

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Table 6.1 Atomic composition in the surface of wood and treated wood by ESCA

analysis

 

 

Wood samples C, % O, % Cu, % N, % O/C

Untreated 72.74 26.85 0.41 0.37

Cu-MEA treated 68.35 29.45 0.85 1.34 0.43 Unleached
pH = 10.8 69.37 28.74 0.78 1.11 0.41 Leached

Cu-DMEA treated 68.38 29.52 1.00 1.1 l 0.43 Unleached
pH = 10.9 70.61 27.49 0.94 0.96 0.42 Leached

Cu-DMeEA treated 67.37 30.29 1.18 1.16 0.45 Unleached
pH = 10.6 70.43 27.56 1.07 0.95 0.41 Leached

CuSO4 treated 73.77 25.15 0.70 0.38 0.36 Unleached
pH = 4.1 74.40 24.92 0.29 0.39 0.35 Leached

 

136

6.5 Conclusions

1). Cu-EA treatment did not alter the crystal lattice structure of cellulose in wood.

2). Post treatment steaming caused the redox reaction between Cu2+ and reducing
agents in wood, which resulted in the formation of Cu20. The reducing agents are mainly
from cellulose and/or hemicellulose.

3). XPS survey spectrum indicated the presence of Cls, Ols, Cu2p and N13 in
copper amine treated wood. XPS Cu2p spectra revealed that the oxidation state of copper

in treated wood was Cu(II).

137

References

Anderson, D.G. 1990. The accelerated ﬁxation of chromated copper preservative treated
wood. Amer. Wood Preservers’ Assoc. p129-151

Barnes, HM. 1985. Effect of steaming temperature and CCA retention on mechanical
properties of southern pine. Forest Product J. 35(6): 31-32

Creely, J .J ., RH. Wade and AD. French. 1978. X-ray diffration, thermal and physical
studies of complexes of cellulose with secondary diamines. Text. Res. J. 48(1):
37-43

Gallacher, A.C., CR McIntyre, M.H. Freeman, D.K Stokes and W.B. Smith. 1995.
Standard and new analytical techniques for CDDC preserved wood analysis. Proc.
Amer. Wood-Preservers’ Assoc. 91: 194-199

Kamdem, D.P., R.Craciun, C. Weitasacker and M. Freeman. 1996. Investigation of
copper-bis-dimethyldithiocarbamate (CDDC) treated wood with environmental
electron microscopy and other spectroscopic techniques. Proc. AWPA 92""
Annual Meeting

Kamdem, D.P., J. Zhang and M.H. Freeman. 1998. The effect of post-steaming on copper
naphthenate-treated southern pine. Wood Fiber Sci. 30(2): 210-217

Moulder, J. F., W.F. Stickle, P.E. Sobol and K.D. Bomben. 1992. Handbook of X-ray
photoelectron spectroscopy. Perkin-Elmer Co., Eden Prairee, MN. USA

Sutter, HR, BB. Gareth Jones and O. Walchli. 1983. The mechanism of copper
tolerance in Poria placenta (F r.) Cke. and Poria vaillantii (Pers.). Fr. Mat. und
Organismen. 18(4): 241-262

Willimas, RS. and WC. Feist. 1984. Application of ESCA to evaluate wood and
cellulose surfaces modiﬁed by aqueous chromium trioxide treatment. Colloid.
Surf. 9: 253-271

Chapter 7

Conclusions

In this research, the effects of copper source, amine ligand and amine to copper
molar ratio on copper retention and leaching of copper amine (Cu-EA) treated wood were
investigated. The roles of individual wood components in copper absorption during
copper amine treatment and the copper bonding sites in wood were also examined. The
Cu-EA treated wood and wood components were characterized by spectroscopic
methods. As a result, proposals were given for interactions between wood and copper
amine preservatives and the copper ﬁxation mechanism.

The copper retention and leaching of Cu-EA treated wood were inﬂuenced by the
formulation of copper amine complexes. Stable copper amine complexes can improve
copper penetration into wood and therefore increase the retention of copper in treated
wood. However, stable copper amine complexes would retard the chemical interaction
between the complexes and wood and consequently reduce the copper leaching
resistance. Less stable copper amine complexes tend to interact with wood readily and
increase the leaching resistance of copper. However, the fast interaction will block the
further penetration of copper amine treating solution and accordingly lower the copper
retention.

In general, three major factors, namely, pH of treating solution, amine ligand and

amine to copper molar ratio, can affect the stability of copper amine complexes. High pH

138

139

results in deprotonation of copper amine complexes and increases the stability of the
complexes. The copper complex of a primary amine (MBA) is more stable than those of
either a secondary amine (DMBA) or a tertiary amine (DMeEA) due to the steric
hindrance of the methyl groups in DMEA and DMeEA. Increasing the molar ratio of
amine to copper can increase the pH of the treating solution and the stability of copper
amine complexes. As a general rule, in formulating a copper amine treating solution, the
pH, amine ligand, and amine to metal ratio should be taken into consideration and
balanced to improve copper retention while minimizing the copper loss during leaching.

Studies of copper absorption on wood components showed that hemicellulose and
lignin in wood played signiﬁcant roles in absorbing copper. Tests with the model
compounds of hemicellulose and lignin conﬁrmed that hemicellulose and lignin were the
main copper absorption sites in wood during Cu-EA treatment. The role of cellulose in
retaining copper was negligible, which is reﬂected by minimal copper absorbed on
cellulose. Removal of extractives from wood also decreased the amount of copper
absorbed.

F TIR spectra indicated that carboxylic acid groups in hemicellulose were the
major reaction sites for copper. Ester groups in wood, which can be hydrolyzed into
carboxylic acid groups through alkaline hydrolysis, behaved similarly to the carboxylic
acid groups. These ester groups also provided reaction sites for copper. FTIR analysis of
Cu-EA treated holocellulose and Cu-EA treated oxidized cellulose veriﬁed the chemical
interactions between copper and carboxylic acid groups in wood. In addition, phenolic

hydroxyl groups in lignin provided another potential bonding sites for copper during

140

current treating conditions. Aliphatic hydroxyl groups in wood appeared to be inert to
react with copper during copper amine treatment.

After Cu-EA treatment, the oxidation state of copper in treated wood is cupric
(012+), which was conﬁrmed by XPS. XPS analysis also indicated the presence of Cl 3,
01s, Cu2p and le in treated wood. After leaching, the majority of copper and nitrogen
were still retained in wood. XRD investigation demonstrated that Cu-EA treatment did
not change the crystalline lattice structure of cellulose in wood. No redox reactions
between copper and wood were observed by XRD except when the treated wood samples
were post-steamed. Post-steaming of treated samples resulted in the formation of cuprous
oxide (Cu20) due to the occurrence of redox reaction between cupric copper (Cu2+) and
reducing groups in wood, such as aldehyde groups.

The structure of copper complexes in copper amine treated wood was elucidated
by the application of electron paramagnetic resonance spectroscopy. Anisotropic axial
spectra were observed for all treated samples irrespective of the copper amine
formulations. The axial spectra and EPR parameters of all samples indicated that the
stereo-structure of copper complexes in copper amine treated wood was tetragonal-based
octahedral with a symmetry of C4v or square-based pyramidal with a symmetry of D4h.
The interactions of wood with copper amine were through complexation reaction in
which functional groups ﬁ'om wood, such as carboxylic acid groups and phenolic
hydroxyl groups, complexed with copper amine ﬁ'om perpendicular direction. The copper
complexes in both treating solution and treated wood were in the form of CuN202 in the

equatorial plane of the complexes.

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