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STATE UNIVERSITY
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This is to certify that the
thesis entitled

PHOTODEGRADATION OF CROP TRELLISING PLASTIC TWINE

presented by
MARIA-PAZ GONZALEZ MULET

has been accepted towards fulfillment
of the requirements for the

Master in degree in Packaging
Science

 

 

 

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Date

 

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PHOTODEGRADATION or= CROP TRELLISING PLASTIC TWINE
By

Maria-Paz Gonzalez Mulet

A THESIS

Submitted to
Michigan State University
In partial fulfillment of the requirements
For the degree of

MASTER OF SCIENCE
School of Packaging
2004

ABSTRACT

PHOTODEGRADATION OF CROP TRELLISING PLASTIC TWINE

By

Maria Paz Gonzalez Mulet

In Guatemala, snow peas and broccoli share the same field and are
planted in alternating cycles. Snow peas are a creeping plant and need to have
support on a number of lines that actually are currently made from black
polypropylene twine. After some time, when the twine photodegrades, hair-
shaped twine fragments get entangled in the growing heads of the broccoli.
When the growers want to pack the broccoli they have to remove all these
filaments because they are considered a contaminant and not allowed in exports.
This study aims to find replacement for the existing twine to avoid a packing
problem after harvesting.

Experiments were conducted in a QUV apparatus where UV light was
produced to simulate photodegradation in the field. Black and white
polypropylene twine and four different nylon monofilaments were tested.

Two of the nylon samples performed best and can be considered as a
solution for the problem. A field study is recommended to validate the results
and correlate the behavior of the twines and monofilaments exposed in the UV

apparatus with those exposed outdoors.

Dedication

To Luis, Jorge, Mireille, Marichu and Natalia.

iii

Acknowledgements

Thanks to
Dr. Diana Twede for all the knowledge and support.
Dr. Susan Selke and Dr. Larry Busch for the knowledge and support.
The School of Packaging staff and faculty.

Partnerships for Food Industry Development - Fruits & Vegetables PF lD-F&V,
Michigan State University, USAID and Asociacién Gremial de Exportadores no-
Tradicionales (AGEXPRONT)

My husband
My family

My friends

iv

Table of Contents

Chapter 1. Introduction

 

 

Chapter 2. Literature Review

 

Plastics

 

- Polypropylene

 

- Nylon
Ultra Violet light

 

 

Light absorption and reflectance

 

Polymer degradation

 

- Photo oxidation

Chapter3. Methodology

 

 

Materials

 

Sample preparation

 

Experiment performed

 

- Pilot experiment

 

- Black twine experiment

 

- Black and white twine experiment

 

- Nylon experiment

Chapter 4. Results

 

 

Chapter 5. Conclusions and recommendations

 

Appendix 1

 

Bibliography

10
14
16
19
22
23

26
26

35
36
36
36
37

38
49
51

61

List of Tables

Table 1. Snow peas and broccoli harvest times

 

 

Table 2. UV light ranges and characteristics

Table 3. Comparison of the cost of PP twine and nylon

 

Table 4. Load (in pounds) of black polypropylene twine as function of time -

Table 5. Elongation, in inches, for black twine during the pilot experiment
and black twine experiment

 

 

Table 6. Initial strength of black and white twine

 

Table 7. Elongation of black and white twine

 

Table 8. Load in relation to time for N1

 

Table 9. Load in relation to time for N2

Table 10. Load in relation to time for N3

 

Table 11. Load in relation to time for N4

 

Table 12. Percentage load loss of black twine

 

 

Table 13. Percentage extension of black twine

 

Table 14. Percentage load loss of nylons

Table 15. Percentage extension loss of nylons

 

Table 16. Elongation in inches of black twine during pilot experiment -------

Table 17. Elongation in inches of black twine during confirmatory
experiment

 

 

Table 18. Elongation in inches of N1

Table 19. Elongation in inches of N2

 

 

Table 20. Elongation in inches of N3

Table 21. Elongation in inches of N4

 

vi

17
49

52

52
53
53
54
54
55
55
56
56
58
57

57

58
59
59
6O
60

List of Figures

 

Figure 1. Guatemala Map

 

Figure 2. Picture of snow peas

 

Figure 3. A broccoli head

Figure 4. Representation of PP twine and stages of degradation -----

 

Figure 5. Polypropylene twine

 

Figure 6. Different types of bonds possible in polymers

 

Figure 7. Isotactic Polypropylene

 

Figure 8. The most important properties of PP

 

Figure 9. Nylon 6,6 reaction

 

Figure 10. Division of light waves

 

Figure 11. Model of color in a sphere
Figure 12. Specular, diffuse and spread reflection from a surface ----

Figure 13. Refraction

 

 

Figure 14. Mechanism of antioxidant action

Figure 15. Samples exposed for two years provided by the growers
in Guatemala and samples exposed after 1007 hours in the QUV
machine

 

Figure 16. QUV machine mounting fixtures

 

 

Figure 17. View of the QUV Machine

 

Figure 18. Lamp Rotation

Figure 19. Solar Eye panel

 

 

Figure 20. lnstron Machine and how was use it.

Figure 21. Side-to—side Rotation

 

vii

GOD-b

11
12
14
16
19
20
20

24

26
28
29
31
32
33
35

Figure 22. Average breaking load of black twine during the pilot
experiment and confirmatory experiment.

 

Figure 23. Average extension of black twine during the pilot
experiment and confirmatory experiment.

 

Figure 24. Comparison of load and extension percentage lost during

 

exposure time

 

Figure 25. Average load of four nylons

 

Figure 26. Nylon percentages of tensile strength loss

Figure 27. Average extension of four nylon twines

 

 

Figure 28. Nylon percentages of extension loss

Figure 29. N3 Stress-Strain curve

 

 

Figure 30. N4. Stress Strain curve
Figure 31. Average load of four nylons and black twine

 

Figure 32. Average extension of four nylons and black twine ----------

viii

38

39

4O
41
42
43
44
45

46
47

48

Chapter 1
Introduction

Between May and August of 2002, a subsector analysis of the frozen
vegetable sector in Guatemala was developed by Michigan State University and
PFID-F&V (Partnerships for Food Industry Development- Fruits & Vegetables)
aiming at the establishment of a sound business strategy for the next several
years in the US, European and Asian markets. Part of the study included also
the identification of key specific problems in the production and distribution chain
affecting exports. This research was done to address a specific problem
identified in the snow pea and broccoli industry in Guatemala concerning the use
of a plastic twine as a trellising material.

Located south of Mexico, as Figure 1 represents, Guatemala's mountains,
valleys and lowlands give the country its own range of climates and great
biodiversity. The North Central region along the eastern slopes of the
Cuchumatanes Plateau, with its abundant rainfall, cooler climate and ample
supply of labor, is the nation’s primary snow pea and broccoli production area.

Figure 1. Guatemala Map
Source: Lonely Planet, 2003

 

 

 

 

 

Snow peas and broccoli share the same conditions. Therefore, they have
the following growing areas (regions) in commons: Quiche, Baja Verapaz,
Sécatepequez, Guatemala, Santa Rosa, Solola, Chimaltenango and Totonicapan
(MAGA 2001 ).

Crop cycling is a recommended practice in agriculture to break down the
pest life cycles of a single product and, in the case of Guatemala, to take
advantage of different market windows in the export industry. Snow peas and
broccoli plantations are cycled on the same land because they need the same
cold weather, and requiring temperatures below 80° F. (Ohio State University
Extension Fact Sheet 2004), in alternating seasons as is shown in Table 1.
Snow peas are harvested from December to January and again in October.
Broccoli is harvested in March to April and July through August.

Table 1. Snow peas and broccoli harvest times.
Source: Ministerio de Agricultura de Guatemala, MAGA, 2001

 

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

\__a(\ " \\ / A 7% //

. / "’ /

\\$‘________—’/ “
—- -— Snow Peas —— Broccoli

Because snow peas are a vine type plant, they need the help of a trellis of
several lines where they can climb and thrive making the product cleaner by
avoiding it touching the ground, and also making the harvest easier for farmers.
In Figure 2 an image of a snow pea field is shown the use of the twine to hold the

plant in a vertical position can be observed.

 

 

. . ._ _ .. Figure 2. Snow pea plants
Source: www.floridata.com 2004

Currently, farmers are using black polypropylene twine, and even when
they remove it from the trellising poles between crops, the degraded old twine is
left in the fields at the ends of the rows. This means that when snow peas have
been harvested, the twine remains in the fields before, during and after the
broccoli season. The main reason for not removing the twine between crops is
that such activity would be labor intensive, thus increasing production costs
forthe grower.

In Contrast to snow peas, broccoli is not a creeping plant and the heads
start developing at a very low height, close to the ground where the twine

remnants have not been removed. In Figure 3 a broccoli head is shown.

 

Figure 3. A broccoli head
Source: MAGA 2003

With exposure to the sun, the polypropylene twine will start to degrade,
changing color to a faded gray and then cracking. This twine is a multifilament
twine in a net type structure with unions at some points. Figure 4 is a
representation of the twine and how it degrades.

Figure 4. Representation of the polypropylene twine and the stages of
degradation.

 

Polypropylene “xx J4
Twine _/_ .\
Exposed to the Sun" \\

 
   

Polypropylene I ». U56

Twine
Exposed to the Sun \/\

When the polypropylene twine used in the field starts to photodegrade
under UV light, it looks like loose black human hair. These hair-like fibers find
their way to the broccoli heads through wind or rain, and are very hard to remove
mechanically at the time of harvesting or when controlling quality prior to export.
Finding such material on an edible product is considered by quality and safety
inspectors as physical contamination and a threat to human health.

Once in the processing plant, broccoli for export that has these hairs like ~-
filaments has to be removed, and the process is labor intensive and costly. It is
also not 100% effective. This contamination is highly penalized in the Japanese
market, where Guatemala has lost export opportunities to other countries such
as Mexico or Ecuador, since these countries do not use the twine in their fields or
are capable of removing the traces of the twine in a total and efficient manner. It
has been declared by some industry people, that totally removing such fibers
would cost each processing plant an increase in payroll due to the need to hire
more people for quality control. Such a situation cannot be afforded in a sector
that is challenged every day by more efficient competitors, demanding the
exporters use cost-reduction methodologies all along the production and supply
chain.

Three major objectives for this study are to find a replacement for the
polypropylene twine that: (1) must stay in place for two years (2) can degrade in
a way other than a hair-shape (3) is cost effective. The price of a pound of snow
peas sold by the farmer in bulk is not higher than US $ 0.10. The actual cost of a

10-pound roll of twine in Guatemala is equivalent to $7.48 at the current

exchange rate of Q 7.751 per US. dollar (Banco de Guatemala, January 27,

2003). Rolls are shown in Figure 5.

  

-._ ,- .IFigure 5. Polypropylene twine
Source: Olefinas.com 2003

The experiment exposes the polypropylene twine and possible
alternatives in the Q-UV light apparatus for a predetermined period. The
samples are then tested periodically for tensile strength following the procedures
of ASTM Standard D638-02 (“Standard Test Method for Tensile Properties of
Plastics")

The materials tested are the black polypropylene twine with carbon black
as a UV stabilizer, currently used in Guatemala; a white polypropylene twine with
titanium as a UV stabilizer; and 30 pounds test nylon monofilaments.

White twine was considered I this study as a possible alternative to black
twine based on the following hypothesis: (1) Black color absorbs more UV-Light
than white color that refracts more light. (2) Black twine will behave differently
than white twine because the presence of the carbon black UV stabilizer (Carlson
et al 1984). Nylon was selected for proving the last hypothesis, (3) Polypropylene
multifilament will degrade differently than nylon monofilament.

This literature review discusses the characteristics of polypropylene,

nylon, and the effects of ultraviolet light on polymers.

 

' Q. Stands for Quetzales, the Guatemala currency.

Chapter 2
Literature Review

Plastics

Plastics are a specific category of high molecular weight materials that
belong to the family of polymers. The first man-made plastic was the cellulose
nitrate, which was used in films and was created in 1838. After the Second
World War, polyethylene began to be produced in large quantities. As years
went by, many other plastics were developed with different properties and were

substitute for different uses.

Polymers can be homopolymers, meaning that they made from only one
type of monomer, or copolymers, those made from different types of monomers.
These can be block, alternating, random or graft. Linear and branched polymers
are called thennoplastics; they can be melted and shaped into different forms
repeated, using a combination of pressure, heat and time (almost all the
packaging plastics are thermoplastics). Thermoset plastics are those plastics
with a cross-linked structure.

These are two different types of bonds in the polymer chains: the primary
bonds that hold the monomers together shown in Figure 6, which can be
covalent bonds or ionic bonds, and the secondary bonds that hold with each

other.

Figure 6. Different types of bonds possible in polymers.
Source: Brigdson 1999

 

/ I“, ’ \. ./ R r". -\.
I. . r, . . .‘l I. . II
Kg /' \\_ _/ \\ _// \K ,x/
/” I" \" ./ \g/ \-.,
r' I'. l' ' II I
V, O ‘_ I. O + O O X.
XX '3 _---’/ \,.H___../K\\__ ._,./'
Covalent Bond lonchond
- Strongest of chemical bonds - Complete transfer Of electrons
- Formed by the sharing of a pair of - Results in one positive Ion and
electrons one negative ion
- Sharing can be equal or unequal
- Can be single, double or triple.

 

 

 

Secondary bonds are weaker than primary bonds, and solids experience
the strongest secondary bonds. Dispersion secondary bonds are the weakest,
induction secondary bonds are of medium strength, and the strongest are dipole
bonds. UV light will start by disrupting the weaker bonds. (Rodriguez 1996)

There are six different things that can affect the physical properties of a
polymer. (1) The type of monomer making up the polymer chains of the plastic. ,
(2) the average chain length (3) the distribution of the molecular weight, (4) the
type of chains, (5) the arrangement of the chains and (6) additives that will affect
the other five and enhance different properties (Brigdson 1999).

The morphology of a polymer refers to the shape, the arrangement and
the physical state of the molecules. This will define how crystalline or amorphous
is a polymer because the polymer molecules tend to move towards an

arrangement that is the lowest energy state possible, and that will define a crystal

form. Crystallinity is obtained when a polymer is capable of packing in a close,
regular, predictable and parallel pattern and this will be related to the
conformation of the polymer. Many things can interfere with this configuration
but the most common will be the presence of chiral carbons that disrupts the

symmetry of the molecule.

- Polypropylene

There are two main ways to produce a polymer. The first one is called
addition polymerization and is usually based on a free radical reaction. Ones the
reaction has been started the molecules start to add one at the time to the
growing chain. Polypropylene belongs to this category. Polypropylene is a
linear, addition polymer developed by Ziegler, (Germany), Natta (Italy) and
Peace and Roedel in the USA in 1950. Cracking petroleum products and fraction
distillation of the 03 produces PP.

The second broad category is the condensation polymerization where the
units are added but they loose one part that is a sub product of the reaction.
Generally is water or alcohol. Nylons are part of this last family of polymers.
Figure 8 shows the three steps for addition polymerization. The initiation step
begins with one molecule called the initiator that breaks into two free radicals,
and from there it starts to join to new monomers. The initiators tend to be
peroxides (ROO), hydroperoxides (ROOH), azo compounds (-N=N-), ultraviolet
light, or heat. The second step is the propagation, when the monomers start to
be added to the chain, in the third step, termination something happens and
disrupts the chain growth.

Tacticity is the way pendant groups are arranged along the backbone
chain of a polymer; for polypropylene the CH3 groups are those pendant groups.
They are three broad categories of tacticity: (1) Isotactic when the pendant
groups are always in one side of the molecule chain; (2) Syndiotactic is when the

pendant group alternates side of the chain in a regular pattern, and (3) Atactic

10

when there is not a specific pattern in the position of the pendant group in
relation to the chain.

Commercial polypropylene tends to be isotactic with a “head to tail"
configuration since it has the best properties for general applications. This is
used in fibers, films and injection molding. The structure is shown in Figure 7. It
is made with a Ziegler-Natta catalyst under controlled temperature and pressure.
Syndiotactic polypropylene which can be made with a single site catalyst is less
crystalline than isotactic, with less tensile strength and heat resistance and
greater susceptibility to solvents. Atactic polypropylene, without a established
pattern, is less crystalline than the two mentioned above. Atactic polypropylene
is amorphous, soft and rubbery, and is not used much in packaging. (Rodriguez

1996)

Figure 7. Isotactic Polypropylene
Source: Rodriguez 1996

 

ll-l oil—i H Cll-ls l-ll Cll-l 1'4 0|le H Cll-la 1'4 cTa
—c|:— clz—clz—c—c—c—c—c— c—c—c—c—
l | l l l
H H H H H H II III III H H II

 

 

 

Polypropylene is a linear hydrocarbon polymer, thermoplastic and the
major packaging plastic with the lowest density, ranging from 0.90 to 0.96 g/cm3.
It is typically 60% crystalline with a glass transition temperature (T9) of -10°
Celsius, which makes it tend to be brittle at lower temperatures. It is stiffer than

polyethylene, a melting point (Tm) of 176° Celsius, high tensile strength and good

11

transparency and gloss. Polypropylene is a low cost polymer and has good
chemical resistance. These properties are essential in the twine because the
creeping of the plants will be less difficult than on polyethylene, and it will have

resistance to fertilizers and pesticides.

Polypropylene is a polymer with a disruption in the crystallinity due to the
presence of Ca in the chain. This disruption increases the crystalline melting
point, decreases the crystalline lattice, and increases in the transparency.
Polypropylene molecules cannot crystallize in a zigzag pattern like polyethylene
but rather in a helix with 3 molecules to complete one loop. Polypropylene can
be in a right handed helix or left handed or both. Polypropylene has a density of
0.90 g/cm3, has a higher softening point, apparently is free from environmental
stress cracking, and is more brittle than PE and more susceptible to oxidation.
Figure 8 summarizes polypropylene’s properties (Bridgson 1999).

Figure 8. The most important properties of PP.
Source: Down Chemical 2003

 

Good
Optical Excellent
Lower Properties Chemical

Resistance

 
    

Density

   

EXCé'lent PP Excellent
Fatigue Properties ESCR
Resrstance
Limited
Excellent Weathering
Impact Requires Exceflent Resistance

Balance NO Drying t0 Moisture
Process Barrier

 

 

 

12

The presence of the tertiary carbon makes the polymer more susceptible
to oxidation and photodegradation. Heat leads to chain scission rather than
cross-linking with different consequences that will be discussed later.

The most important additives in polypropylene are: fillers, rubbers to decrease
brittleness, pigments, carbon black as an ultraviolet absorber, antioxidants and

nucleating agents. (Brigdson 1999)

13

- Nylon

The other important category of polymer is the condensation polymers.
They have a “step-wise-reaction” normally slower than the addition
polymerization reaction. They are normally formed of at least two different
monomers but they are not considering being a copolymer. These monomers
have to have at least two reactive groups like an alcohol and an amine to
produce nylon, or an acid and an alcohol to produce polyester. As part of the
condensation reaction, a byproduct is produced, often is water. Polyethylene
terephthalate, polyethylene naphthalate, and nylons belong to the same
condensation family.

Nylons are also called polyamides; they are thermoplastics with a linear
structure. The nomenclature at the end of the name will indicate the size of the
amides and acids they are formed from. Figure 9 will show the chemical reaction
for producing nylon 6, 6.

Figure 9. Nylon 6, 6 reaction
Source: Rodriguez 1996

 

 

 

 

 

 

 

 

 

O O H H O (|)|

 

l l l
.C’ (CH2) " Cg—H_] + N " (CH2) " N -> N ”(CH2) - N - c - (CH2)4" C+2H20

H H H H

 

 

 

The structure of polyamides is crystalline, with a density of 1.14glcm3.
Some nylons have an elongation rate of 150% and a tensile strength of 7600 psi.

The melting point is 490° F. It is translucent with a good resistance to ultraviolet

14

light and solvents but poor resistance to alkalis and solvents. Also it is a very low
weight material but very strong. (Rodriguez 1996)

During the course of the experiments and looking for a possible solution
for the problem, nylon was chosen because of its properties, the strength and
ultraviolet light absorption capacity, and also because it is available in all the
Guatemalan markets at an affordable price. The 1000 meter rolls range in price
approximately, from 0.10.00 to 0.15.00, equivalent to $1 to 2 US. (Hiper Paiz
2003).

15

Ultra-violet light and fluorescent lamps

Ultraviolet light (UV light) is part of the light energy spectrum emitted
naturally by the sun. The spectrum includes gamma and x-rays, UV light, visible
light, infrared rays, microwaves, and radio waves, listed in order of decreasing
frequency and increasing wavelength. Figure 10 shows the division of light
waves included in UV light. These waves are shorter than visible light; they
cannot be detected by the human eye. UV light is invisible, but can cause other

substances to glow or fluoresce in a variety of colors visible to the human eye.

Figure 10. Division of the light waves

Source: International Lights 2003

X-Rays
_

     
  

Microwave
—

, n
Ultravrolet , -. Infrared
1 00-400 nm » 770-1.000,000 nm

   

2 3 4 5 6 7 8 9
1 00 1 .000 (nanometers) 10.000
Energy 7 . ‘0 Wavelength

.... ...i

UV light is divided, into four broad categories: short, medium, long, and
vacuum wave radiation. Long wave UV has a wavelength of 315 to 400
nanometers and is the less damaging and closer to the visible range. Medium
wave has a wavelength of from 280 to 315 nanometers and it is responsible for

polymers photodegradation. A nanometer is the unit of length used to define

wavelengths in the electromagnetic spectrum, and is equal to one millionth of a

millimeter. Table 2 lists the division of UV light.

Table 2: UV light ranges and characteristics.

Source: International Lights 2003

 

 

 

 

 

 

Range IC [Wavelength Eauses for humanI

Name haracteristics Range (nm) xposure

UVA [Long waves 315-400 Sun tanning

UVB IMedium waves 280-315 Sun burn and cancer

UVC Short waves 200-280 Absorbed by DNA producing
mutations

acuum Vacuum UV (VUV) 100-200 Strongly absorbed by water

UV (VUV) nd air and transmitted in

Eacuum

 

 

 

 

 

UV-A is the least harmful and most commonly found type of UV light,
because it has the least energy. UV-A light is often called black light, and is used
for its relative harmlessness and its ability to cause fluorescent materials to emit
visible light - thus appearing to glow in the dark. Most phototherapy and tanning

booths use UV-A lamps.

UV-B typically is the most destructive form of UV light, because it has
enough energy to damage biological tissues, yet not quite enough to be
completely absorbed by the atmosphere. UV-B is known to cause skin burns
and cancer. For this experiment UV-B lamps were chose because they are the

ones emitting the wavelength that destroys the polymer structure.

Short wavelength UV-C is almost completely absorbed in air within a few
hundred meters. When UV-C photons collide with oxygen atoms, the energy

exchange causes the formation of ozone. UV-C is almost never observed in

17

nature, since it is absorbed so quickly. Gerrnicidal UV-C lamps are often used to

purify air and water, because of their ability to kill bacteria.

Although the sun is the principal source of UV light, it can be reproduced
by different kinds of devices like the UV tanning devices that emit the UVA region
and are suitable for human exposure. Another important device emitting UV light
is the arc welding equipment, which emits very strong levels of UV light due to
the excitation of atoms and molecules in the air. Without the correct protection, it
can cause burns in the cornea and retina. Arc welding emits UV-B waves in

combination with visible light and infrared (Ryer 2003).

18

Light absorption, reflection and refraction

Incident light can be reflected, scattered or absorbed by polymers. The
first law of photochemistry of Grotthus-Draper said that only the absorbed part of
light would lead to photochemical transformation.

Most models of perceived color contain three components: hue,
saturation, and lightness. Generally, color is modeled as a sphere, with lightness
comprising the linear transform from white to black, and hues modeled as
opposing pairs, with saturation being the distance from the lightness axis, as
shown in Figure 11 where black and white are opposites in lightness.

Figure 11. Model of colors in a sphere
Source: Ryer 2000.

  

Light can be absorbed, reflected or refracted by a material. It can be
reflected by a mirror or by color with a high content of metal particles. Reflection
can be spread, diffuse or specular as is shown in Figure 12, where the material is

not absorbing light, just redirecting it.

Figure 12. Specular, diffuse and spread reflection from a surface

Source: Ryer 2000

Specular Diffuse Spread

   

 

 

Refraction is when a material absorbs a quantity of light and lets it pass
through the material. The rest of the energy is redirected at a different angle.
Because a white color is full saturation of light, it refracts more light than black,
while black is zero saturation and absorbs all light. Figure 13 shows how
refraction works.

Figure 13. Refraction.
Source: Ryer 2000

 

 

 

 

 

Absorption of light will depend on the degree of saturation of light of a
particular color. Black materials will absorb more light and will get hotter than

white materials (South Florida Test Services 1993).

20

The other important issue about color is that this color can come from a
specific additive added to the polymer during processing. In the case of black,
the additive is carbon black which can be used as an antioxidant, as an antistatic,
as an organic pigment and as a UV light absorber. White is commonly created
by adding titanium dioxide which is a pigment but does not have the UV light
stabilizer properties. Other colorless stabilizers can be added, but the cost of
production will increase. Carbon black is the least expensive of the additives

(Gatcher & Miiller 1985).

21

Polymer Degradation

Processes involving chain polymerization, addition, or condensation
reactions are used to manufacture most commercial plastics, such as
polypropylene. These processes are generally controlled to produce individual
polymer molecules with a defined average molecular weight (or molecular weight
distribution), degree of branching, and composition that will determine the

specific characteristics of each polymer.

During this process or by further exposure to heat, light, air, water,
radiation or mechanical loading, chemical reactions will tend to produce
modifications in the polymer chain resulting in a change in molecular composition
and molecular weight due to chain scission and cross linking, and the specific
characteristics of each polymer will tend to change. The stability of a polymer
can be affected by its own chemical makeup, its processing history and
parameters, and the additives and contaminants present in the polymer. This is
why the presence of the CH3 group in the polypropylene molecule will make it

more susceptible for oxidation and photo degradation.

These reactions will cause a structural change in the physical and optical
properties of the polymer. Any change in the polymer properties relative to the
initial, desirable properties is called degradation. In this sense, "degradation" is a
generic term for any number of reactions, that are possible in a polymer. A
polymer can suffer from mechanical degradation when it is exposed to different
kind of external forces, leading to fatigue and rupture. When it is exposed to the

action of different solvents or hydrolysis, the polymer will be exposed to chemical

22

degradation. Biodegradation refers to the exposure to microorganisms that

decompose the polymer and degrade it.

When a polymer is heated, thermal degradation occurs and chemical
bonds start to break or disrupt, forming new and different molecules. Thermal
degradation always occurs when transforming a polymer because the polymer is
exposed to heat. Polypropylene is normally processed at a range of
temperatures between 220 and 280° Celsius and needs a stabilizer to prevent

degradation.

The polypropylene twine used in this study is subject to some mechanical
forces that keep the twine in place under the plants’ weight, a maximum of 15
pounds per twine. Heat would not be a concern since this twine is used in the
highlands where the temperatures never exceed 23° to 25° C (INSIVUMEH

2003).
- Photoxidation degradation

Exposing polymers to sunlight and some artificial lights that generate UV
radiation can have destructive effects on plastic products. UV radiation can break
down the chemical bonds in a polymer. Photodegradation occurs when the
polymer have been exposed to UV light. In the air there is oxygen present and
simultaneously to Photodegradation, oxidation can occur. This phenomenon will
be called photoxidation. The UV light emitted by the sun is absorbed by the
polymer molecules, creating free radicals that decompose the polymer.
Polypropylene has C-C bonds and C-H bonds that both can be dissociated by
UV light energy (300 to 400 nm). (Selke 2003).

23

Titanium Dioxide is one of the most important white pigments for
polymers. Its most important characteristic is the ability to promote in presence of
UV light a oxidation process. The surface will pulverize and will exhibits a
phenomenon call chalking (Gatcher and Miller, 1983).

Photodegradation causes cracking, chalking, color changes and the loss
of physical properties observed in the samples from Guatemala. The black
polypropylene twine is no longer black but tends to have a grayish color, and
small filaments breaking and fall off the original net structure of the twine.
Figure 14 shows how free radicals are formed in a polymer that is exposed to

UV light as a consequence

Figure 14. Mechanism of antioxidant action
Source: Special Chem 2004
L H-Donor ]

 

 

Polymer

  
      

 

 

 

\ I
v
Radical
Hydroperoxide ’,ROOH [ Scavenger ]
decomposer

 

 

 

 

 

Photodegradation in the polyolefin family is generally due to the presence
of catalyst residues, hydroperoxide groups, carbonyl groups and insaturation of

the bonds introduced during the processing of the polymer. In polypropylene,

24

isolated hydroperoxides can be formed, and then they start grouping in different
way. Then oxidation occurs and may end in a chain scission. The B—scission
reaction has been considered mainly responsible for polypropylene degradation.
It is sometimes difficult to separate photodegradation from oxidation.

The aliphatic polyamides are polymers that only absorb a small amount in
the short wavelength region of the sun’s light spectrum. The absorption is due to
the impurities present in the polymer. Direct chain scission will occur and
photoxidation will occur in the outside layer of the polymer. Nylon monofilament
is made from an aliphatic polyamide, and was chosen as a possible solution
because it theoretically will behave differently than polypropylene, it can be found
in the Guatemalan market, and it is available at a reasonable price.

Black polypropylene twine has carbon black as an additive and a pigment
the degradation will be different than for white polypropylene that has titanium
dioxide as a pigment but does not have any UV light absorber (Stepek and Doust

1983)

25

Chapter 3
Methodology.

Materials
Growers in Guatemala provided samples of used and degraded
twine. Figure 15 (A) shows the material as it appears after having been exposed

to two years in the fields.

Figure 15. Samples exposed for two years provided by the growers in
Guatemala (A) and sample exposed after 1152 hours in the QUV machine
(3)-

   

(A) (B)

For this series of experiments the black polypropylene twine was
purchased in Guatemala City following the brand and the specifications of the
growers. To demonstrate hypothesis number two, that black polypropylene twine
absorbs light differently than white polypropylene, white polypropylene was also
purchased.

Black polypropylene had carbon black as additive to prevent
photodegradation, white polypropylene had white titanium dioxide as a pigment
and did not have any UV stabilizer (Salguero 2003). The quantity of each

additive could not be determined due to proprietary formulating.

26

Nylon was purchased in the United States from a local retailer. It was sold
as fishing line and comes in rolls of 20 yards. Knowing the weight that will be
exerted on the twine is equivalent to 15 pounds, line rated for 30 pounds
resistance was chosen, with the expectation that when it was degraded by half,
the plants would still be able to be supported by the nylon twine. N1 and N2
were fishing lines were labeled extra strength and with a greenish tint. N3 and
N4 were normal transparent fishing line, the same brand, and 75% less
expensive than the first two samples. N3 had twenty-five pound strength stated
in the label and N4 had thirty-pound strength stated. The other important aspect
to consider with the nylon fishing line is that it is a monofilament, contrary to the
“net-multi- polypropylene twine”. Since these fishing lines had the higher pound
test, also had a larger gauge than regular fishing line and would not be sharp and
cut the plants. In California and the Southern states galvanized iron is used for
this purpose, and the idea was to simulate the same gauge of the iron with the
fishing line. Previous experiments in Guatemala using very fine fishing line
produced good results for degradation, but cut the plants (Santizo 2003).

Weathering tests are very useful in packaging because some packages
will be exposed for certain amounts of time to different weather conditions, and it
is necessary to observe how they will behave under these different conditions.
For example, in the School of Packaging, polypropylene woven bags for food aid
have been analyzed for the USDA, to see the expected behavior in case they will
have to be stored outdoors. Other examples are plastic films and fabric used in

agriculture and sand bags and how they behave outdoors. These concerns

27

focus on how much time they can be in the fields, how they will degrade and
what kind of residues they will leave behind (SpecialChem 2003). For
packages, ultraviolet test devices made by Q-Panel Co. and Atlas Electric
Devices Co have been used. (ASTM D4329)

A QUV/SE Accelerated Weathering Tester, manufactured by Q-Panel
Company, Cleveland, Ohio, was used for this series of experiments. This
apparatus complies with 653 (ANSI 1992) and with ASTM D4329.

The QUV machine has 24 positions available for testing and has to be full
in order to operate. The samples were mounted on aluminum plates, tied in the
back into a knot and pressed into the holders by snap rings. Figure 16 shows
the different parts of the fixtures, and Figure 17 shows a view of the QUV

apparatus.

Figure 16. QUV machine mounting fixtures.

 

 

 

. \" Polypropylene

 

 

 

 

 

 
    

 

 

 

F‘ , ‘3‘ twrne
f, ’ Knotand tie liar ‘
. g‘ , .
a N
‘ El. Sna rin

 

 

Mounting fixtuia

 

_,Aluminum plate, _

 

 

 

 

 

Ith /\

 

Back view Front view

 

 

 

28

Figure 17. View of the QUV Machine

 

UV Lamps
Support for

fixtures

Base

 

Front VIeW Side view

 

 

 

There are four kinds of fluorescent lamps in a QU-V machine: two types of
UV-A lamps with the higher wavelength and two types UV-B lamps that produce
the UV-B wavelengths. They look like ordinary fluorescent lamps but produce
mainly UV light.

The UV—A lamps are recommended for tests comparing generic plastics,
and usually do not degrade materials as fast as UV-B lamps. UV-A 340 lamps
better simulates sunlight in the range between 365 nm and 295 nm and are the
default lamps proposed by the ASTM D4329. UV-A 351 lamps are used to
simulate sunlight filtered by a window and is generally used to test the behavior
of objects like automotive interiors.

The UV-B lamps reproduce the wavelengths that degrade polymers and

cause damage outdoors and this is why they are usually used for simulating

29

outdoor degradation. These lamps have demonstrated a good and accurate
correlation to outdoor exposures for evaluating material integrity of polymers.
These lamps provide the fastest and most cost efficient exposure. Two different
types of UV-B lamps are available; they produce the same UV wavelengths but
different amounts of energy. Both of them emit 313 nm of UV-B light spectrum,
UV-A light and some visible light. QSF-40, also known as FS-4O or F40UVB is
generally used to test automotive interiors and coatings.

UV-B 313 was designed for QUV testing: it produces higher UV output
and faster results than OPS-40. For this experiment, UV-B 313 lamps were
selected because they produce the UV-B wavelength and will produce faster and
more cost effective tests.

The main difference between weathering testers and outdoor exposure is
the reproducibility of the results. Light, temperature and moisture are controlled
by the QUV machine. For this experiment the condensation period was omitted
in favor of having UV light exposure. The reason for this decision was that
polypropylene was not expected to be degrading because of moisture. Nylons
can be susceptible to water, but since the samples were fishing lines, and to
replicate the experiments, the same considerations were taken for the nylon
testing.

The QU-V apparatus can recreate outdoor exposure, but it is necessary to
remember that other elements as geographic position and topography can affect
the results (ASTM D 4329 1992). The areas where these crops are planted

have a range of temperature of 15 to 25° Celsius. The QU-V apparatus was set

30

to a temperature of 60° Celsius, with a irradiance level of 1.20 :I: .05 Wlm2lnm @
313 nm, the highest irradiance possible. These parameters were selected to
cause a faster rate of degradation, and to be able, in a short time, to correlate to
the two year degraded samples provided by the growers.

Control of iridescence, the rate at which light energy falls on a unit area of
surface, is extremely relevant because the smallest discrepancy will affect the
results (Q-Panel 2003). The QU-V machines are equipped with an automatic
irradiance control that uses a “solar eye” that allows the control of the irradiance
continuously and is maintained automatically. Four sensors placed in the
sample planes control and compensate for lamp aging and any other variability
by adjusting the power to the lamps. Rotating and replacing the two oldest lamps
every 400 hours also helps to compensate. Each lamp has a life of 1600 hours,
and replacement only takes five minutes.

Lamps were rotated during the different experiments every 400 hours and
then the solar eye was also recalibrated. Figure 18 shows the rotation of the
lamps and in Figure 19 a sample of the “solar eye” panel is displayed.

Figure 18. Lamp Rotation
Source: Q-Panel Technical Bulletin LU-8160. A choice of lamps.

(<9 <91
r G) @3
<9 <9
G) <9

31

 

 

 

 

Figure 19. Solar Eye Panel
Source: QU-V Bulletin Lu-8160 1994

The operator selects a desires level of irradiance by pressing

the arrow keys. The number selected Is the 'Set Point"

The controller displays irradiance
for each of the sensors

UV Light measurement

 

       
    
     

The oontroler campus
the irradiance to the

set point and adjusts
current to the lamps
accordingly

  

W Sensors

  
  
 

A

 

V
Bullt-In UV sensors
rrleasulas the Ight
from each pair of
lamps and transni
these readings to
the Solar eye controller

 

 

ASTM D 638-99 Standard Test Method for Tensile Properties of Plastics

procedure was followed for tensile testing. First 10 non-exposed samples were

32

tested on an lnstron machine model 4201, which has a constant-rate-of-
crosshead-movement mechanism and grips that hold the specimens. The grips
were separated by two inches, as shown in Figure 20. This machine is in

compliance with the requirements of ASTM D 638-99.

Figure 20. lnstron Machine and how was use it.

i

if
, II

 

3" 3.” 3." J

 

 

 

 

 

 

 

 

 

 

 

 

 

Step 1 Step 2 Step 3 sum
Open the pneumatic 9n” place one by one the “males. Move the floating grip up 3‘09 WM” sample "”69
orteedgefirstandclosethe WW'WDOIMOMNQMU
gripthesemndedgeand thedatazloadandextenslon
close the gnp

 

 

 

For each sample, the peak load and the extension were registered by the
machine. Every 144, hours 10 samples from one plate in the QU-V machine

were cooled to room temperature and tested for tensile strength in the lnstron.

33

Sample preparation

First the outside layer of the roll was discarded and the samples were cut
from an inside layer of the roll. Each sample measured 15 inches (38.1 cm). It
was wrapped around the aluminum plate of 0.635mm thickness as required by
ASTM 4329-92, and to tied to a nut. Then the samples were trimmed to 3 inches
after the nut and tie. The plates were mounted on the fixture and tightened with
the snap rings. On each plate five samples were mounted, since ASTM 4329-92
requires at least 3 replications of the samples. Then they were placed in the
QUV machine for running the experiments. The QUV machine is located in a
laboratory at Michigan State University at the School of Packaging with controlled
conditions between 18° and 27° degrees Celsius and is in compliance with the
same standard test procedures. Four different tests were run as described

below.

34

Experiments Performed

For this experiment a total of four experiments were done: a pilot test, black
polypropylene twine, one using black and white polypropylene twine and a last
one with four different nylon fishing lines. In all the tests the parameters and
procedures were the same. Every plate had 5 samples tied in the back and fixed
to the fixture by the snap rings. Every 144, hours fixtures were rotated as

suggested by ASTM D 4329 and as shown in Figure 21.

Figure 21 Side-to-Side Rotation

Source: ASTM 4329-92

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

iii
If

id

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ll
llllllll

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

During the course of the four experiments, temperature was constant at
60° C with a setting point of 110 on an irradiance level of 1.20 t .05 W/m2/nm @
313 nm. During the total test time, the QUV apparatus was in exposure mode
only, and used UV-B 313 lamps.

Except for the black and white polypropylene experiment that was run for
only 144 hours, the experiments were run for 1152 hours. Every 144, hours a
fixture was removed from the machine and the samples were tested for tensile

strength.

35

- Pilot experiment

For the pilot experiment all of the 48-aluminum plates were filled with the black
polypropylene twine, making a total of 240 samples, and mounted in the QUV

apparatus.

After 1152 hours of continuous exposure the samples began to look like
the samples brought from the fields and were already disintegrating in a hair-like
shape. Once the baseline exposure time was determined (1152 hours), a

second experiment was run.

- Black twine experiment

This experiment was a replication of the pilot test and was to correlate the
results after the different periods of time. The 48 aluminum plates were filled
with the black polypropylene twine, making a total of 240 samples, and mounted
in the QUV Machine.

Every 144 hours a sample was removed from the machine and tensile
tested. The experiment continued up to 1152 hours, time indicated by the

experiment.

- Black and white twines experiment
For this experiment, the QU—V Machine was loaded with half of the
mounting fixtures (12), 24 plates with black polypropylene twine and the other

half with white polypropylene twine, giving a total of 120 samples per twine.

36

After144 hours the experiment was terminated due to disintegration of the

white twine

- Nylon experiment

After the results of the white polypropylene twine experiments, a solution
still needed to be found and so an experiment was done with four (4) different
nylon fishing line. The procedure was identical but this time 6 fixtures were

loaded with 10 twines of each sample (N1, N2, N3 and N4)

37

Chapter 4
Results

Load (pounds) and extension (inches) as a function of time were the
parameters for the study. Load refers to how many pounds are necessary to
apply to the twine before rupture. Extension refers to the elongation of the twine
before rupture. These two elements were tested every 144 hours on ten
samples. The average of these ten results is reflected in the following figures.

In Figure 22, the results of the load in the pilot test and the confirmatory

black twine test are compared, and in Figure 23 extension results are shown.

Figure 22. Average breaking load of black polypropylene twine during the
pilot experiment and confirmatory black twine experiment.

90

\_ _—_ _ ‘—

70 A—TI'TT __ ___-_-

 

 

 

 

 

a)
o

I

I

l

|

I

I

I

 

 

g

l

l
.9 :
If I
l

I

l

m... Pilot
—+-Black

 

 

4o 43% g“ ‘ -8>_\"_,.,.~.-e

 

Load (pounds)
03
O
l

N
o
l
l
I
I
I
I
I
I
I
I
I

10 I» ~ ”ML,”

 

 

 

 

0 500 1000 1500
Time (hours)

 

 

 

38

Figure 23. Average extension at break of black polypropylene twine during
the pilot experiment and confirmatory black twine experiment.

 

080
070
060
050
040
(130
020
010
000

 

 

 

 

 

-——e——-Pmn
- +—Blacktwine

 

 

 

 

 

Extntion (inches)

 

0 200 400 600 800 1000 1200 1400

Time (hours)

 

 

 

Clearly, during both tests the black polypropylene had the same behavior,
for both load and extension.

The unexposed black twine has a tensile strength of 80 pounds. After 144
hours of exposure in the QUV apparatus, the black twine lost 12.5% of its original
tensile strength. It was not until 864 hours that black twine lost 50% of its original
tensile strength. After 1008 hours of exposure the black twine gained 3.75%
from the last tensile test. After 1152 hours it gained 2.5% more tensile strength.
This data is shown in Table 11 in Appendix 1.

The unexposed black twine has an extension at break of 0.75 inches.
After the first 144 hours of exposure, the black twine had 37.33% less extension
than the unexposed twine. Compared with the loss of tensile strength, the
flexibility suffered more during this period of time. After 576, hours the black
twine lost 50% of the original extension, 288 hours before the tensile strength
reached that same 50% loss. After 864, hours the black twine had the lowest

extension during the test with 62.66% loss. During the following 288 hours, the

39

extension increased by 2.66% of the most recent value for each 144-hours
period. These results are shown in Table 12 in Appendix 1, and a comparison
between these extension percentages and load percentages are shown in Figure
24.

Figure 24. Comparison of load and extension percentages lost during
exposure time.

 

 

 

 

_ . — + — Percentage of lead
us from the original

 

—-—— Percentage of
extension from the
1 original

 

 

 

 

 

 

 

 

 

 

0 500 1000 1500

Time (hours)

 

 

 

The percentage of load lost was higher than the loss in extension, but both
followed a similar pattern.

The third test compared black polypropylene twine to white polypropylene
twine. They were manufactured by the same company, and the difference
between them is basically the additive and pigment they have. Black twine has
carbon black as a pigment, which at the same time is a UV light stabilizer
(Stepeck and Daoust, 1983). White twine has titanium dioxide as a pigment but
it is not a UV stabilizer. Because white absorbs less light than black it was
hypothesized that the UV light activity would not start oxidation in the white twine

as quickly, therefore resulting in a different pattern .of degradation. However,

40

after the first 144 hours of exposure, the white twine was pulverized, and the test
was discontinued.

The fourth test was analyzing four different nylon twines. In Figure 25 the
average load for each of the nylons is shown.

Figure 25. Average load of four nylon twines.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

50
540 ‘—~ —e—N1
§ 30 H— —+-N2
£20 - A--N3
N

------ N4

310- x

O I I

 

0 500 1 000 1 500

Time (hours)

 

 

N1 and N2 were labeled “Extra Strength” and N3 and N4 were regular
fishing lines. N1 had the highest initial tensile strength, 40 pounds, followed by
N2 with 33 pounds, N4 with 26 pounds and N3 with 23 pounds. N1 and N2 had
higher tensile strength through 144 hours of UV exposure; after 288 hours they
had lost almost half of their tensile strength. In contrast, N3 and N4 had a more
stable pattern of tensile strength. All four recovered some tensile strength after
720 hours of exposure. Because these are monofilaments, the outside layer of
the twine got degraded and felt “sticky” to the touch. After this first layer was

degraded, the inner layer was exposed and had similar properties to the original

41

 

non-exposed twine but with a smaller gauge, requiring less loads to rupture.
Figure 26 shows the percentages of tensile strength loss for the nylon samples.

Figure 26. Nylon percentages of tensile strength loss

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

__ M
A \ —e—N1
é \-__. —-I--N2
g ',x---'>F---x-T-:_:¥--,‘ —-a—- N3
' . "T ---><-~N4
1000 1500

 

 

Time (hours)

 

 

N4 will show the most stable pattern, compared to N1 and N2 that had a
very disrupt pattern. N3 has also an increasing pattern with a slight decrease
after 864 hours. After this decrease will increase and match N4 pattern.

At zero hours, N1, N2, N3, and N4 had a long elongation before rupture,

but after UV light exposure this elasticity decreased considerably, as shown in

Figure 27.

42

Figure 27. Average extension of four nylon twines.

 

 

 

 

 

 

 

 

 

 

 

 

 

'3
o
f, —o—N1
'5 —+-N2
C
,2 - .-- N3
3
3 ---x~-N4

 

 

 

 

 

0 200 400 600 800 1000 1200 1400
Tlme (hours)

 

 

 

N1 had the highest initial extension of 3.5 inches. N2 and N4 were very
close together with initial extensions of 2.72 and 2.53 inches, respectively. N3
had the lowest extension of the four samples with an initial extension of 1.65
inches. The four nylon samples had a significant drop in extension after the first
144 hours of exposure. After 576 hours, extension of the four samples was very
similar, ranging from 0.7 inches to 0.5 inches. Figure 28 shows a comparison of

the percentages of the extension lost over time for the four nylons.

43

Figure 28. Nylon percentages of extension loss

 

100

 

 

 

 

 

 

 

 

g —o—N1
8 —+-N2
E — a: —N3

-~---xN4

 

 

 

 

 

 

 

 

 

 

 

 

0 500 1000 1 500
Time (hours)

 

 

 

Again, N4 is the one that has a more stable behavior, but all four after 144
hours had lost at least 40% of their extension capacity. N2 has lost up to 60%
after the first 144 hours, but after 576 hours it will gain 40% of the extension. All
four after 576 hours will be close in their behavior ranging from 90 to 70% loss.
The total weight of the plants will not exceed fifteen pounds and
there are 5 lines. Dividing the total weight by number of lines, the maximum
force exerted per line is expected to be 3 pounds.
Stress is defined by the following formula:
Stress = Load (pounds) [Area (sq inches)
Stress = 3 /0.05
Stress for 3 pounds = 60 psi
As mentioned before, there is concern about the high elongation rate of
the nylon, but looking at Figure 29 and Figure 30, it can be observed that the

percentage of elongation will be less than 1% when the maximum expected

44

weight is applied. This is true for N3 and N4 both after 0 hours of exposure and
after 1152 hours of exposure.

Figure 29. N3 Stress-Strain curve

 

N3 Tensile Test

 

172368

137895

1 03421

Tensile Stress (psi)

 

 

 

 

0 10 20 3o
Tensile Strain (%)

 

— N3 exposed 0 Hours
— - ~ N3 exposed 1152 Hours

 

 

 

 

 

 

45

Figure 30. N4 Stress-Strain curve

 

 

    

 

 

 

 

 

N4TensileTest
172368 4 . I -- _ ...._- .. 9
137895
3; 103421 . , ,
‘Jl s ‘
9.’ I I l
I71 | . 1
g 68947 l-_- __- s i __
8 l 5 i
F“ ‘ i
34473 __ , l ,
, l i
0 10 20 30
Tensile Strain (96)
— N4exposed0Hours
-—-~ N4exposed1152l-lours

 

 

 

 

 

 

Since white was rejected as a possibility the black twine and the four
nylons were compared and analyzed. Figure 31 shows the behavior of these five

different twines regarding the load and Figure 31 the extension in function of

time.

46

Figure 31. Average load of four nylon twines and black twine.

 

 

1A

 

 

 

 

 

 

 

 

 

 

1A” 1‘ A
l

 

 

 

Load(pounds)
OSBSSBSB‘SB

 

 

0 200 400 600 800 1000 1200 1400
Time (hours)

 

 

 

Black twine supports a higher load than nylon. But, black twine has a
larger gauge than nylon. Black twine has almost doubled of the tensile strength
the nylon, but also has 3 times the gauge. It is important to notice that the five
materials have a similar behavior when exposed to UV light. They decrease
considerably their tensile strength at the beginning and they later recuperate part
of what was lost. The main difference with this material will be that the black
twine is a multifilament and the filaments are thin enough to rupture after the first
hours of exposure, and the nylons are a monofilament and it will take longer for
degradation to reach the inner layers.

After, UV light exposition the twine extension has a different behavior.

Figure 32 shows the average extension of nylon and black twine.

47

Figure 32. Average extension of nylon and black twine

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4
‘5?

a:

«5 —o—N1

E —-— N2

: _ .

.2 Ar N3

2 - -N4

g --0-——- Black Twine
|.l.l

 

 

 

 

 

0 500 1000 1500
Time (hours)

 

 

 

At the beginning, black twine has a lower capacity of extension, only 0.75
inches compared to nylons that range from 1.5 to 3.5 inches when before to UV
light exposition. But after 576 hours of exposure, all five samples have on
average 0.5 inches of extension. Thus, UV light not only affects the tensile
strength properties but also the elasticity properties. This is explained by the
fact that polypropylene is more brittle than nylon, and nylon is more flexible at
lower temperatures (Rodriguez 1996). The other explanation is that black twine is
a multifilament with a net structure, as some of the outside layer filaments start to
degrade and break will have less tensile strength and less capacity of elongation.

Compared to the black twine, nylon, especially N3 and N4, is more stable

on exposure to UV light.

48

Chapter 5
Conclusions and Recommendations.

After the experiments and analyzing the results, a solution to replace the
polypropylene twine by 25—30 pound test nylon fishing line, was identified. This
nylon monofilament should last more than 2 years in the fields, retaining more or
less 50% of it initial strength. Since the total weight of the plant will never exceed
15 pounds, they should not break. In Table 3, an economic comparison
between the nylon and the polypropylene twine is shown.

Table 3. Comparison of the costs of PP twine and nylon (1000 meters)

 

 

 

 

 

 

 

 

 

 

Year 1 Year 2 Year 3 Removing Total
filaments cost
PP Twine $ 7.50 $ 3.75 $7.50 Yes $ 18.75 + extra
packing costs
Nylon $ 2.00 0 0 No $. 2.00

 

If a monofilament did break, the farmers would have to replace the
damaged section with a newer one. This will lead to an operational and
administrative need to inspect and replace the line, when necessary. This
activity could be done during the regular inspections for pests and diseases,
during the harvest time or during the planting season of both crops: snow peas
and broccoli. It will be recommended that every time a worker is out in the fields
he carries with him a roll of the nylon monofilament for replacing lines as needed.
After 2 years of exposure, it is recommended to replace all the lines to avoid
collapsing plants and return to lines with full strength.

It is necessary to conduct field research to correlate these results with

natural exposure to UV light and other possible concerns. For this purpose, it is

49

recommended that the twine in one plot of a snow pea field be replaced by N3
and another with N4. Since the only correlation between the natural exposure
and the QU-V machine is that probably 1152 hours correlates to about two years,
the field should have to be monitored every week. Every week, observations
should be written in a notebook, especially when the black twine begins to break.
The time when this is happening can be clearly established, providing correlation
to the QUV machine.

For further studies it will be interesting to conduct an experiment with
polypropylene twine with different quantities of carbon black to see the
differences in degradation correlated to the different percentages of carbon black
present in the twine. If the results are positive, this could lead the polymer
industry in the region to find markets, even if the price is higher.

Due to economic and time constraints, it was not possible to evaluate
performance in the snow pea and broccoli plots, but the laboratory results
obtained represents a guide to more research in the fields. Finally, for further
laboratory research, it is recommended to try a polypropylene monofilament and

compare the results with the nylon monofilament.

SO

Appendix 1
Results from tests (tables)

51

Table 4. Load (in pounds) of black polypropylene twine as function of time
during pilot experiment.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Pilot
Project
144 288 432 576 720 864 1152
Sample 0 Hours Hours Hours Hours Hours Hours Hours hours
1 82.15 65.25 60.25 46.89 48.67 36.99 40.37 38.99
2 72.5 63.03 54.3 47.55 40.11 38.57 35.67 34.76
3 77.34 62.04 58.99 58.99 52.43 48.56 39.99 35.66
4 85.06 66.01 54.03 59.87 52.46 43.76 46.84 50.87
5 82 75.76 56.98 55.76 50.01 42.56 40.14 40.23
6 83.9 70.35 51.99 48.56 49.99 40.18 39.23 45.78
7 85.77 67.15 55.54 52.47 50.65 43.45 45.66 34.42
8 86.89 73.99 44.98 59.78 52.65 50.11 42.79 43.76
9 79.47 67.43 49.89 51.23 50.65 48.37 33.65 48.9
10 86.78 72.89 64.57 54.87 48.99 39.99 32.45 49.3
Average 82.1 9 68.39 55.1 5 53.60 49.66 43.25 39.68 42.27
DVST 4.62 4.68 5.54 5.02 3.64 4.51 4.74 6.31

 

 

 

Table 5. Load (in pounds) of black polypropylene twine as function of time
during black twine experiment.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Black PP
Twine
144 288 432 576 720 864 1 152
Sample 0Hours Hours Hours Hours Hours Hours Hours hours
1 83.14 64.75 60.51 47.68 50.68 37.85 44.67 40.54
2 71.7 62.76 53.21 48.05 38.58 52.94 30.6 35.49
3 77.26 61.1 59.46 58.68 51.25 51.06 40.27 55.03
4 87.01 65.29 54.17 62.47 48.91 41.82 43.11 43.01
5 83.49 72.59 55.41 53.1 54.66 42.09 35.01 42.85
6 82.6 69.91 51.11 46.55 51.11 43.27 38.66 42.71
7 85.66 66.42 56.38 53.72 55.73 47.13 41.23 34.42
8 89.77 74.01 48.16 56.27 55.73 53.69 42.9 45.1
9 81.34 66.2 55.35 54.52 44.99 51.76 39.25 45.15
10 84.19 73.07 62.68 55.01 51.81 38.07 37.23 43.49
Average 82.62 67.61 55.64 53.61 50.35 45.97 39.29 42276
DVST 5.09 4.52 4.39 5.06 5.26 6.12 4.21 5.67

 

52

 

 

Table 6. Initial strength of black and white twine (144 hours)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Sample White Twine Black Twine

1 82.34 83.14
2 80.09 71.7
3 72.34 77.26
4 73.46 87.01
5 76.89 83.49
6 78.48 82.6
7 80.01 85.66
8 85.33 89.77
9 70.45 81.34
10 75.69 84.19

Average 77.51 82.62

DVST 4.65 5.09

 

 

 

 

Table 7. Elongation of black and white twine (144 hours)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Samples Black Twine White Twine

1 0.555 0.465

2 0.497 0.567

3 0.985 0.774

4 0.717 0.776

5 0.991 0.998

6 0.901 0.756

7 0.868 0.769

8 0.761 0.865

9 0.591 0.645

10 0.598 0.678
Average 0.75 0.73
STD 0.18 0.15
% 100 100

 

 

53

Table 8. Load (in pounds) in relation to time for N1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0 144 288 432 576 720 864 1 152
N1 Hours Hours Hours Hours Hours Hours Hours hours
1 42.3 17.81 35.67 9.981 11.46 12.25 18.9 13.61
2 41.21 33.44 29.85 8.284 10.89 11.002 18.78 14.14
3 42.28 31.48 17.211 10.65 11.85 12.5 20.51 15
4 39.49 14.24 14.2 21.03 15.3 16.78 14.23
5 43.87 33.9 30.56 8.757 6.776 10.85 21.7 12.71
6 39.44 32.78 7.25 8.44 24.03 16.85 20.36 14.54
7 36.64 30.2 17.84 11.11 7.29 10.54 18.58 14.88
8 36.4 31.57 15.72 12.56 7.52 11.36 17.36 14.96
9 42.47 35.73 8.961 9.879 6.389 10.25 20.67 14.09
10 38.93 31.78 8.601 14.59 6.448 15.28 15.219 15.63
Average 40.30 30.96 18.59 10.84 11.36 12.61 18.88 14.37
DVST 2.54 5.19 10.06 2.27 6.28 2.34 2.00 0.82
% 100 76.81 46.12 26.89 28.18 31.28 46.84 35.67
Table 9. Load (in pounds) in relation to time for N2.
0 144 288 432 576 720 864 1 152
N2 Hours Hours Hours Hours Hours Hours Hours hours
1 33.89 34.11 23.33 18.94 16.71 15.98 15.43 19.64
2 29.41 30.35 26.61 18.12 20.89 18.15 10.83 18.84
3 31.14 28.22 25.62 18.67 14.14 12.13 7.329 20.35
4 35.64 32.99 22.07 22.88 17.72 15.89 11.13 19.7
5 34.94 31.36 25.68 22.21 19.18 16.25 10.1 16.07
6 32.23 33.72 20.76 16.65 22.95 12.88 17.28 17.16
7 35.66 34.59 23.1 24.15 23.6 14.89 10.13 21.05
8 32.28 32.82 24.01 24.92 17.13 15.78 11.11 17.96
9 32.32 40.8 18.01 23.36 11.58 14.25 12.59 19.48
10 31.86 24.23 21.7 16.74 16.36 15.45 14.31 16.58
Average 32.94 32.32 23.09 20.66 18.03 15.17 12.02 18.68
DVST 2.05 4.35 2.60 3.16 3.76 1.73 2.93 1.67
% 100.00 98.14 70.11 62.75 54.74 46.05 36.50 56.73

 

54

 

Table 10. Load (in pounds) in relation to time for N3.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

144 288 432 576 720 864 1 152

N3 0 Hours Hours Hours Hours Hours Hours Hours hours

1 22.84 20.94 19.6 18.56 13.19 14.25 16.33 14.31

2 23.16 21.18 19.44 18.46 15.44 16.89 18.8 14.27

3 25.05 21.09 19.97 18.83 11.7 13.85 14.94 15.32

4 23.71 21.74 18.54 18.69 18.45 16.58 18.91 14.82

5 24.07 21.13 20.14 19.65 17.65 14.22 15.73 12.11

6 22.93 21.78 19.68 19.53 18.2 16.44 16.82 15.72

7 22.31 20.77 20.93 18.63 18.71 15.77 18.2 14.96

8 22.99 23.14 22.7 18.06 17.15 14.23 18.53 14.46

9 22.74 22.59 20.06 19.24 16.85 15.97 17.84 14.45

10 21.94 23.15 18.53 14.82 12.58 17.52 14.95
Averag

e 23.17 21.60 20.42 18.82 16.22 15.08 17.36 14.54

DVST 0.90 0.81 1.45 0.50 2.37 1.44 1.36 0.97

% 100 93.16 88.12 81.2 69.97 65.06 74.92 62.72

Table 11. Load (in pounds) in relation to time for N4.

0 144 288 432 576 720 864 1 152

N4 Hours Hours Hours Hours Hours Hours Hours hours

1 27.22 21.93 21.27 16.61 16.31 16.25 15.65 15.44

2 19.37 21.44 18.02 18.03 16.05 17.01 13.47 16.11

3 22.85 22.08 18.01 15.51 17.19 15.89 16.67 17.13

4 27.74 23.17 20.82 17.2 15.74 15.23 16.9 16.52

5 24.16 22.86 21.08 19.26 17.36 14.75 17.21 16.44

6 29.05 24.18 18.79 18.42 15.3 15.54 16.79 15.82

7 24.98 21.58 18.58 19.73 17.66 16.85 16.9 16.82

8 27.79 23.11 21.13 20.66 15.75 17.05 17.07 16.18

9 28.28 21.32 16.69 21.24 16.21 16.32 17.08 15.55

10 29.54 20.27 21.26 18.1 16.48 16.98 16.41 14.67

Avera e 26.10 22.19 19.57 18.48 16.41 16.19 16.42 16.07

DVST 3.21 1.14 1.72 1.79 0.77 0.82 1.13 0.72

% 100 96.08 74.96 70.79 62.85 62.02 62.89 61.56

 

55

 

 

 

 

 

 

 

 

 

 

 

 

Table 12. Percentage load loss of black twine
Difference in %
Percentage of
load loss from ”633356232: of
the original time
0 100 O
144 87.5 12.5
288 31.25 18.75
432 37.5 1.2
576 40 2.5
720 43.75 3.75
864 51 .25 8
1008 47.5 3.75
1152 45 2.5

 

 

 

 

Table 13. Percentage extension loss of black twine

 

Percentage of

Difference in %

 

 

 

 

 

 

 

 

 

 

 

 

. between the
extensron loss . .
. . reVlous rlod of
from the orlglnal p ti mp:

0 100 0
144 37.33 62.67
288 44 6.67
432 46 2.66
576 49.33 3.3
720 53.33 4
864 62.66 9.33

1008 60 2.66
1152 57.33 2.66

 

56

 

 

Table 14. Percentage load loss of nylons

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

N1 N2 N3 N4

0 100 100 100 100

144 23.17 1.88 6.77 14.98

288 53.87 29.9 11.73 25.01

432 73.1 37.23 18.77 29.18

576 71.31 45.26 29.99 37.12

720 68.7 53.94 34.91 37.96

864 53.15 63.5 25.07 37.08

1008 64.34 43.29 37.24 38.42

1152 62.13 42.28 34.18 33.79

Table 15. Percentage Extension loss of nylons
N1 N2 N3 N4

0 100 100 100 100
144 57.18 41.91 59.39 63.24
288 85.05 48.52 61.81 75.88
432 91.66 33.45 64.24 77.47
576 86.49 83.45 82.42 80.63
720 86.2 87.5 75.15 79.44
864 85.06 89.33 66.66 76.67
1008 86.78 83.45 72.72 77.86
1152 87.06 82.35 71.51 78.26

 

 

 

 

 

 

 

57

 

Table 16. Elongation in inches of black twine during pilot experiment

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Pilot test
Samples 0 144 288 32 I576 19 863 1007
1 0.459 0.555 0.555 0.478 0.398 0.302 0.233 0.35
0.556 0.465 0.401 0.459 0.356 0.333 0.256 0.349
3 0.876 0.453 0.499 0.432 0.378 0.358 0.299 0.3
4 0.777 0.478 0.465 0.435 0.356 0.376 0.298 0.299
5 0.999 0.501 0.423 0.403 0.432 0.405 0.279 0.274
6 0.987 0.456 0.412 0.416 0.345 0.332 0.299 0.265
0.801 0.5 0.399 0.399 0.475 0.342 0.282 0.234
'8 0.798 0.432 0.402 0.367 0.301 0.387 0.302 0.237
9 0.601 0.399 0.334 0.355 0.322 0.324 0.345 0.295
10 0.645 0.447 0.321 0.3 0.367 0.299 0.201 0.261
Average 0.75 0.47 0.42 0.40 0.37 0.35 0.28 0.29
sro 0.18 0.04 0.07 0.05 0.05 0.04 0.04 0.04

 

Table 17. Elongation in inches of black twine during confirmatory

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

experiment.

Samples 0 144 288 432 576 19 I663 1007
1 0.555 0.434 0.571 0.398 0.404 0.275 0.265 0.296
2 0.497 0.372 0.385 0.467 0.238 0.428 0.228 0.284
3 0.985 0.457 0.571 0.355 0.455 0.424 0.341 0.375
4 0.717 0.415 0.478 0.498 0.418 0.358 0.315 0.318
5 0.991 0.461 0.395 0.477 0.364 0.325 0.198 0.315
6 0.901 0.435 0.331 .316 0.383 0.264 0.271 0.297
7 0.868 0.425 0.445 0.408 0.375 0.351 0.284 0.251
8 0.761 0.518 0.324 0.373 0.428 0.454 0.324 0.3

19 0.591 0.375 0.475 0.337 0.345 0.441 0.321 0.316
10 0.598 0.465 0.539 0.325 0.449 0.298 0.248 0.331
Average 0.75 .44 0.45 0.40 0.39 0.36 0.28 0.31
STD 0.18 0.04 0.09 0.07 0.06 0.07 0.05 0.03

 

 

58

 

 

Table 18. Elongation in inches of N1.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Samples 0 144 288 432 '576 719 '863 1007
1 3.568 0.609 1.975 0.274 0.335 0.399 0.505 0.411
2 3.467 1.497 0.721 0.212 0.428 0.394 0.499 0.467
3 3.564 1.408 0.134 0.297 0.309 0.401 0.549 0.461
4 3.509 1.555 0.321 0.355 0.571 0.476 0.451 0.454
I5 4.117 1.439 0.717 0.234 0.504 0.436 0.575 0.445
6 3.873 1.625 0.404 0.298 0.556 0.455 0.545 0.478
7 3.262 1.63 0.179 0.307 0.598 0.501 0.208 0.465
8 3.1 1.66 0.367 0.279 0.511 0.567 0.425 0.494
9 2.568 1.825 0.191 0.271 0.488 0.569 0.625 0.481
10 3.767 1.634 0.224 .392 0.422 0.583 0.499 0.491
Average 3.48 1.49 0.52 0.29 0.47 0.48 0.49 0.46
STD 0.43 0.33 0.55 0.05 0.10 0.07 0.11 0.02
Table 19. Elongation in inches of N2.
Samples 0 144 288 432 I576 719 863 1007
1 2.892 1.377 0.498 0.934 0.431 0.459 0.339 0.447
2 1.725 1.207 2.061 1.362 0.455 0.362 0.277 0.457
3 2.159 1.405 2.025 0.784 0.477 0.256 0.155 0.474
4 2.915 1.491 3.572 1.835 0.409 0.301 0.291 0.458
|§ 2.471 1.641 1.415 0.581 0.485 0.347 0.245 0.414
l6 3.309 1.129 0.511 0.417 0.391 0.369 0.401 0.415
7 2.891 2.78 0.457 0.465 0.517 0.299 0.259 0.504
[8 3.108 1.225 1.049 1.748 0.486 0.348 0.281 0.45
2.999 1.839 0.731 0.601 0.432 0.325 0.322 0.494
10 2.691 1.749 0.859 0.395 0.455 0.334 0.371 0.415
Average 2.72 1.58 1.32 0.91 0.45 0.34 0.29 0.45
STD 0.48 0.48 0.99 0.55 0.04 0.05 0.07 0.03

 

 

 

 

 

 

 

 

 

 

59

 

 

Table 20. Elongation in inches of N3.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Samples 144 288 432 '576 719 863 1007
1 1.318 0.741 0.517 0.566 0.283 0.501 0.498 0.424
2 1.165 0.635 0.531 .556 0.285 0.446 0.584 .445
3 3.127 0.581 0.544 0.577 0.325 0.432 0.435 0.501
4 1.909 0.615 0.493 0.605 0.594 0.428 0.631 0.479
5 2.334 0.605 0.558 0.642 0.165 0.401 0.427 0.362
1.285 0.625 0.531 0.651 0.575 0.425 0.535 0.501
7 1.291 0.554 0.821 0.584 0.168 0.388 0.655 0.487
8 1.368 0.798 0.835 0.548 0.191 0.367 0.641 .445
9 1.461 0.764 0.585 0.607 0.139 0.384 0.565 0.431
10 1.221 0.769 0.879 0.579 0.165 0.345 0.549 0.439
Average 1.65 0.67 0.63 0.59 0.29 0.41 0.55 0.45
STD 0.63 0.09 0.15 0.03 0.17 0.04 0.08 0.04
Table 21. Elongation in inches of N4.
Samples 0 144 288 432 I576 19 863 1007
1 2.205 0.765 0.799 0.445 0.389 0.478 0.478 0.458
2 0.929 0.675 0.428 0.488 0.471 0.568 0.357 0.599
3 1.884 0.768 0.447 0.397 0.311 0.499 0.531 0.654
4 3.058 1.274 0.631 0.427 0.409 0.532 0.595 0.558
l5 1.975 1.195 0.951 .542 0.441 0.497 0.667 0.539
6 3.332 1.451 0.451 0.518 0.515 0.589 0.681 0.621
7 2.118 0.784 0.474 0.615 0.518 0.402 0.571 0.579
'8 2.791 1.215 0.615 0.805 0.414 0.566 0.655 0.568
9 3.529 0.637 0.395 0.933 1.064 0.542 0.681 0.599
10 3.449 0.585 0.865 0.488 0.375 0.529 0.655 0.434
Average 2.53 0.93 0.61 0.57 0.49 0.52 0.59 0.56
STD 0.84 0.31 0.20 0.17 .21 0.05 0.11 0.07

 

 

 

 

 

 

 

 

 

 

60

 

 

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