BIOBASED PHENOLIC ADHESIVE USING UNMODIFIED LIGNIN AND GLYOXAL By Sasha Emmanuel A THESIS Submitted to Michigan State University in partial fulfillment of the requirement s for the degree of Forestry Œ Master of Science 2020 ABSTRACT BIOBASED PHENOLIC ADHESIVE USING UNMODIFIED LIGNIN AND GLYOXAL By Sasha Emmanuel Phenolic adhesives , primarily made of petroleum -based phenol and formaldehyde , have been used for many decades to manufacture wood composite s such as plywood and oriented strand board (OSB) due to their superior performance . Phenol and formaldehyde are both made of fossil -fuel chemicals , and formaldehyde ™s toxicity and carcinogenic activity caused many researchers to focus on finding ways to replace it with biobased , less toxic raw materials. This study was focused on using lignin , a natural plant -based polymer , to replace phenol and used glyoxal , a biobased non -toxic chemical , to replace formaldehyde . The phenol was entirely replaced with an unmodified enzymatic hydrolysis corn stover lignin , while at the same time formaldehyde was substituted from 0 to 100% in increments of 10 with glyoxal. The r esin s were formulated under alkaline conditions using lignin to formaldehyde and/or glyoxal with a 1:2 molar ratio . The propert y and performance of resins and adhesives were measured and compared to a phenol -formaldehyde (PF) adhesive formulated in the lab. The pH, alkalinity, solid -content, free formaldehyde content of the formulated adhesive s was similar to a commercially available PF adhesive. The dry lap shear strength of the developed lignin -glyoxal (LG) adhesive was 3. 3 ± 0.4 MPa , which was comparable to the dry adhesion strength of the laboratory formulated P F adhesive (3.4 ± 0. 2 MPa ). iii ACKNOWLEDGMENTS First and foremost, I want to thank God for His ever -present favor in my life. Without Him, this would not have been possible . I want to thank my a dvisor , Dr. Mojgan Nejad, for giving m e this opportunity . It all s tarted back in 2017 when I came in as a Summer Research Opportunity Program (SROP ) intern from Brooklyn with no research experience and no plans for graduate school. You saw my potential and gave me a chance to tak e my academic career beyond just my bachelor ™s degree . Thank you for always being available and thank you for pushing me to b e a great research er and even a better student. I wan t to thank my committee members Dr. Laurent Matuana, and Dr. Pascal Kamdem for sharing your knowledge , advice and just willingly being part of this journey. This journey would not have been possible without my parents , Helena Emmanuel and Hilary Moise , my sounding boards . For showing me every day, the hard work, ambition , and determination needed to succeed in this life. Thank you mom , for your constant pray ers , continuously encouraging m e when I was at my lowest. You prayed for me even when I could not pray for myself , when I doubted my place in this world , you showed me the mark I already made and its continued growth. Thank you both for being fantastic parents. I also want to thank my undergraduate mentors at Medgar Evers College , Dr. Dere ck Skeete , and Dr. Christopher Boxe , for believ ing i n me . They made it their duty to ensure I was exposed to as many opportunities as possible to advance my academic career. I owe load s of gratitude to the excellent Forestry Graduate Secretary Katie James ; from the very beginning , you have been an advocate and a iv friend. To Dr. Rothstein , from the beginning , you have been there, championing for the graduate students in all aspects , and for that , I will forever be grateful. To my fantastic lab members Christian , Mona, Saeid, Mohsen , Akash , and of course , Isal , thank you for all the help with research from collecting to analyzing data , classes , and just being great friends. You have all made this jou rne y here at MSU an am azing one. To my closest friends, thank you for daily encouragements and mental push. To you my dearest, thank you for the prayers, the love and understanding, the kind words of encouragement. I will always be grateful for your help during this journey. v TABLE OF CONTENTS LIST OF TABLES .......................................................................................................................... vi LIST OF FIGURES ....................................................................................................................... vii KEY TO ABBREVIATION S .......................................................................................................... viii Chapter 1: BACKGROUND ........................................................................................................... 1 1.1 Phenolic Adhesive ........................................................................................................1 1.2 Research Motivation ....................................................................................................7 Chapter 2: LITERATURE REVIEW ................................................................................................. 9 2.1 Tannin ..........................................................................................................................9 2.2 Lignin ..........................................................................................................................10 2.2.1 Lignin Modificat ion .............................................................................................14 2.3 Partial or Total Replacement of Phenol ......................................................................15 2.4 Partial or Total Replacement of Formaldehyde ..........................................................17 2.5 Possible Challenges and Drawbacks ...........................................................................20 2.6 Objectives ...................................................................................................................20 Chapter 3: MATERIALS AND METHODS .................................................................................... 22 3.1 Materials ....................................................................................................................22 3.2 Methodology ..............................................................................................................23 3.2.1 Lignin Characterization ........................................................................................23 3.2.2 Resin and Adhesive Formulation .........................................................................26 3.2.3 Resin and Adhesive Properties ............................................................................28 3.2.4 Resin and Adhesive Performance ........................................................................30 Chapter 4: RESULTS AND DISCUSSION ...................................................................................... 36 4.1 Lignin Characterization ...............................................................................................36 4.2 Resin and Adhesive Properties and Performance ......................................................39 Chapter 5: CONCLUSIONS AND FUTURE RECOMMENDATION .................................................. 48 5.1 Conclusions ................................................................................................................48 5.2 Future Recommendation ............................................................................................49 REFERENCE S ............................................................................................................................. 51 vi LIST OF TABLES Table 1. Properties of Novolac Resins Vs. Resole Resins ..............................................................2 Table 2. Lignin Source And Its Monolignols Content (%) 64 .........................................................11 Table 3. Lignin sources and its lignin content (%) 63 ....................................................................12 Table 4. 31 P-NMR chemical shifts of lignin's functional groups ..................................................26 Table 5. Ash Content (%), Sulfur Content (%), Moisture Content (%), Molecular weight and PDI of Lignin Sample ........................................................................................................................36 Table 6. 31P-NMR analysis results for corn stover lignin. ............................................................38 Table 7. Physical and Chemical Properties of Formulated Resins ...............................................39 Table 8. Physical and chemical properties of formulated adhesives ..........................................43 Table 9. Wet lap shear strength and wood failure result for LF, PF, and LG adhesives. ..............47 vii LIST OF FIGURES Figure 1. Reaction Mechanism for the Synthesis of Novolac Phenolic Resins. (Formaldehyde = F, Phenol = P) ..................................................................................................................................2 Figure 2. Reaction Mechanism for the Synthesis of Resole Phenolic Resin. (Formaldehyde = F, Phenol = P) ..................................................................................................................................3 Figure 3. Lignin Str ucture ..........................................................................................................10 Figure 4. Three basic phenylpropane units ( Monolignols) ........................................................11 Figure 5. Resin formulation equipment setting, equipped with a thermometer, condenser, dry bath stacker, and digital heater and stirrer. ...............................................................................27 Figure 6. Image of veneer specimens for lap shear strength testing. .........................................31 Figure 7. Universal Instron testing of pressed veneer specimen ................................................32 Figure 8. Wood specimen adjusted in Photoshop. 7.5 × 7.5 inches of veneer specimen was cropped for analysis. .................................................................................................................33 Figure 9. Wood Specimen adjusted in Photoshop. Image (left) were adjusted for auto contrast and auto color and then adjusted for brightness and contrast (100%) (middle and right) ..........34 Figure 10. ImageJ software analysis of veneer for wood failure. The tiff file of the image was opened in Image J software (left), and using the software (middle) image was made binary for analysis. (right). .........................................................................................................................34 Figure 11. ImageJ software analysi s of veneer for wood failure. Set measurements (left) are chosen for specific results needed to determine the wood failure of wood samples. (right) .....35 Figure 12. ImageJ Software Analysis of Wood Failure. Image Is Analyzed (Left) And Value Displayed Under % Area Represents Resin Failure(Right). .........................................................35 Figure 13. 31PNMR Spectra of Lignin Sample ............................................................................38 Figure 14. Alkalinity and pH of formulated adhesives. ...............................................................40 Figure 15. Image of precipitated lignin in LG resin during formula tion ......................................44 Figure 16. Dry lap shear strength results for formulated adhesives ...........................................45 Figure 17. Percent wood failure, Image analysis results ............................................................46 viii KEY TO ABBREVIATION S 31P-NMR 31 Nuclear Magnetic Resonance CPF Commercial Phenol -Formaldehyde DMF Anhydrous Dimethylformamide DSC Differential Scanning Calorimetry FT-IR Fourier -Transform Infrared G Guaiacyl GPC Gel Permeation Chromatography H Hydroxyphenyl H2SO4 Sulfuric Acid HCL Hydrochloric Acid HMTA Hexamethylenetetramine HPLC High Performance Liquid Chromatography HSO 3 Hydrogen Sulfite IB Internal Bond iCAP Inductively Coupled Argon Plasma LD50 Lethal Dose, 50% LFG Lignin Formaldehyde Glyoxal LG Lignin Glyoxal LVL Laminated Veneer Lumber MDF Medium Density Fiberboard ix MF Melamine Formaldehyde Mn Average molecular number Mw Average molecular weight NaOH Sodium hydroxide OSB Oriented Strand Board PF Phenol Formaldehyde PG Phenol Glyoxal PLF Phenol Lignin Formaldehyde pMDI - Polymeric diphenylmethane diisocyanate PRF Phenol Resorcinol Formaldehyde S Syringyl SO2 Sulfur Dioxide Tg Glass Transition Temperature TGA Thermogravimetric Analysi s THF Tetrahydrofuran UF Urea Formaldehyde 1 CHAPTER 1: BACKGROUND 1.1 Phenolic Adhesive Phenolic resins , also known as phenol -formaldehyde (PF) resins 1 were the fir st synthetic polymer ever developed .1,2 With their first commercial use in 1908 ,3 phenolic resins are used in numer ous applications , such as fiber -reinforced composites , electric laminates , molding compounds , and adhesives. 4 Phenolic adhesives are primarily used as binders due to its high moisture and chemical resistance, mechanical and thermal stability, and electrical insulation properties. 4Œ8 In the adhesive industry , phenolic adhesives are widely used in wood composites , including laminated veneer lumber (LVL) , plywood , and o riented strand board (OSB) .1 They are primarily made from the reaction of formaldehyde with phenol , and based on the molar ratio of formaldehyde to phenol and the type of catalyst used, the resin can be classified as either resole or novolac .1,2,9 An increase in the molar ratio of formaldehyde to phenol incr eases the reactivity of the phenol to formaldehyde, hardening rate, degree of branching, and cross -linking of the resin. 1 Table 1 tabulates the differences between novolac and resole resin properties. Novolac phenolic resins are formed under acidic conditions, pH ranging between 4 to 6 2 with phenol to formaldehyde ratio higher than one , as seen in Figure 1.2,10 Bisphenol F is the purest form of novolac and occurs when formaldehyde is reacted with large amounts of phenol. 11 Novolacs are two-stage curing resins, cured at high temperatures (>150 °C),12 along with the addition of a hardener like hexamethylenetetramine (HMTA) or paraformaldehyde to aid in forming additional methylene bridg es. 13,14 2 Figure 1. Reaction Mechanism for the Synthesis of Novolac Phenolic Resins. (Formaldehyde = F, Phenol = P) Without the introduction of the curing agent or hard ener , novolac resins are thermoplastics ,9 and can be used as thermoplastic polymer s for the pr oduction of belts, tube s, and tires. 13 However, when cured using HMTA , novolac resi ns go from a thermoplastics resin to a thermoset. 15 Table 1. Properties of Novolac Resins Vs. Resole Resins Novolac Resin Resole Resin Acidic catalysts (H 2S04) with a pH range 1 Œ 4 Basic catalyst (NaOH) with a pH range 7 to 13 Phenol in excess Formaldehyde in excess Requires hardener to cure Requires only heat to cure Two -stage resin Single -stage resin Produces a thermoplastic polymer before adding HMTA 15 Produces a 3 -D cross -linked insoluble polymer 16 Resole phenolic resins are produced by a condensation polymerization reaction between formaldehyde and phenol at temperatures ranging from 40°C to 100 °C,13 with the pH ranging from 10 to 13 1 using an alkaline catalys t, usually sodium hydroxide , with a molar ratio of phenol 3 to formaldehyde of less than o ne.2 Resole resin production occurs in two phases , show n in Fig ure 2; addition and condensation. 2 Below 60 °C, the addition phase occurs where the formaldehyde will react with the phenol to prod uce hydroxymethyl phenols on either the two para positions or on the o rtho position , which will then react with methylene glycol to form methylol phenols compounds. 9 At temperatures above 60 °C, methylolphenol reacts with both phenol and othe r methylolphenol compounds in the condensation phase producing a prepolymer. 2,11 Unlike novolac resins, resole resins only require heat to start curing and crosslinking. 14 Figure 2. Reaction Mechanism for the Synthesis of Resole Phenolic Resin. (Formaldehyde = F, Phenol = P) 4 It must be noted that there are some difference s between the resin and the adhesive. The resin is th e prepolymer produced by reacting the phenol with formaldehyde and is uncured. 17 The resin is then used to formulate the phenolic adhesive , also known as glue mix , which will then be used to manufacture bio -composite like OSB and plywood . Plywood and OSB are the primary wood composites that are made using phenol ic adhesives .2 Typically , it is made with southern yellow pine and Douglas fir , but sometimes they are also ma de from western hemlock, western pines, red pines (in Michigan ), maples , and yellow poplar. 18 Plywood is produced using up t o at least three layers of veneers , which are adhered together with an adhesive under high pressure about 220psi (1500kPa) , for dense species and 110 psi (750 kPa) for low -density species .18 They are pressed under temperature ranging from 132°C to 165 °C and 107° to 135 °C for softwood and hardwood , respectively. 18,19 It can be used for both interior and exterior application s.20 Oriented strand board (OSB) are made of thin strands of wood with low to medium density like southern pine, yellow poplar, sassafras , white birch , and aspen. 18 Core and surface boards are typically 4™ × 8™ with s trands arranged in the cross -machine direction or parallel to the length of the board in the core layer and are placed in the machine direction or parallel to the width of the board in the surface boards. 21 This arrangement allows for better mechanical properties. These boards will then be compressed under pressure ranging from 4.8 to 5.5 MPa (700 to 800 psi ) at temperatures ranging from 177 to 204 °C (350 to 400 °F) for 3 to 6 minutes .18 At this time, pressure and temperature, the adhesive curing , or hardening process will occ ur. OSB is widely used for roof ing , siding, subfloor s, wall sheathing , and web -stock for wood I-beams .2 Majority ( 85%) of the world ™s OSB production is produced in the United State s and Canada with a growing interest in Europe an countries like France, Germany, 5 Spain, and Poland. 22 Typically , in the Unite d States, there are two types of resin s used in OSB: the core resin for the middle layers made with polymeric diphenylmethane diisocyanate ( pMDI) and the surface resin for the outer layers are usually phenolic resin s.20 However, European manufacturers primarily use pMDI in their OSB production due to their stricter regulations for formaldehyde usage. 23 The composition and chemistry of wood var y based on the species , properties of the wood (adherend ) like extractives content , porosity, moisture content, grain orientation, and density , which can all affect the performance of the adhesive .24 Extractives, which are non -structural substances 25 in the wood, can disperse to the surface when exposed to high temperatures, reducing the adhesives™ ability to penetrate the wood and form interfacial bonds. 26 For instance, wood species with high amount s of extract ive s like starches, alcohols, tannins , resinous materials , and proteins can affect the adhesive's wettability .20,26 Another effect resinous extractive has on the performance of the adhesive is the ability to repel water (hydrophobic properties) , which can pose a problem for water -based adhesives. 26 Extractives can also impact the adhesive's bond -ability and the adherend through its acidi c level , 27 which will decreas e the curing rate of alkaline phenolic resins while increasing the cure rate of acid ic-phenolic resins like urea -formaldehyde (UF) resins. 25 The wettability of the adhesive is another critical factor that will impact the adhesion performance . Wettability is measure d as the speed by which the adhesive will wet the surface or spread over the surface of adherend (wood) .24 This happens when the contact angle between the surface of the adherend (wood) and a drop of adhesive approaches zero .24,26 The wettability of a wood surface with an adhesive is the initial step in creating the bon d.25 Many factors can affect the wetting of the wood ; consequently , the a dhesion 6 performance of the adhesive (glue mix) . Penetration is also a critical factor in the application of the adhesive on the wood specimen. Porous wood specie s can show high amounts of overpenetration of the adhesive . There are two types of adhesive penetration: 1) gross /lumen penet ration and 2) cell wall penetration. 28 Gross or lumen penetration occurs when the adhesi ve flows into and through the pores of the wood filling the lumens. 24,28 This type of penetration depends on the adhesive ™s contact angle on the wood specimen and its viscosity .29 Selecting hardwood specimens may lead to higher gross penetration due to its more porous nature and larger vessel presence. 18 Cell wall penetration depend s more on the adhesive™s molecular weight .24 The c ell wall penetration can affect the mechanical strength between the adhesive and wood but cell wall penetration of adhesives in the cells cou ld lead to nanomechanical interlock ing by enhancing the contact area of the adhesive and the wood .24,26 A wood specimen ™s moisture content, density , and grain orientation can affect its wettability .24 The m oisture content of wood ranging from 6% to 14% allows for satisfactor y bonding with the adhesive , and below 3% will reduce adhesive wetting. 26 The high amount of moisture above the fiber saturation point in wood , which is averaged about 30%, can also lead to loss of adhesive due to the reduced amount of adh esive and water that can be absorbed by the wood. 26,30 This loss occurs when pressure is applied to the adhered wood veneers together causing the adhesive to be more fluid leading to adhesive spill age on the sides of the panel (squeez e-out) .26 Changes in the moisture content of the wood can also lead to shrinking and swelling , which can result in interfacial stress es between the adhered wood. 31 The density of the wood species can also affect the adhesive ™s performance and penetration. The low -density wood species have large lumen volumes and thinner walls ,18 which allows for better adhesive penetration to create strong interfacial bonding 26 while also 7 resultin g in loss of adhesive on the bond line due to excessive penetration . Although adhesive penetration is vital in forming strong bonds, over -penetration can occur, resulting in weaker bonding between the wood and adhesive and resulting in a waste of adhesive. 24,26 Adhesive shear strength of joint wood is the strongest when the grain runs parallel to the applied force. 32 The lap shear strength testing , which is one way of determining the pressed panels' mechanical propertie s, measures the amount of stress it can withstand when force is applied in opposite directions . The great er the lap shear strength , the stronger the bond between the wood and the adhesive , leading to greater wood failure . Percentage of wood failure can help determine the bonding strength between the wood and the adhesive , where a higher percent .33 This can be determined by visual analyzing the panels after lap shear testing or using image analysis software. The more visible damage seen on the panels, the stronger the adhesive , which can be seen easily with the PF adhesives due to its darker color. The alkali content of the resin is also an essential parameter . The higher the a dhesives' alkalinity , the greate r its reactivity and hardening rate, resulting in the need for shorter press time. 1 1.2 Research Motivation According to market research, global formaldehyde consumption has increased exponentially .34 By the end of 2026, it is anticipated to increase to 36.6 million tons , with the majority of it being used for formulating resin s such as phenol -formaldehyde (PF), melamine -formaldehyde (MF) , and urea -formaldehyde (UF) .34 This increase in consumption of formaldehyde will result in higher use and dependency on petr ochemicals . Formaldehyde is produced from the cat alytic oxidation of methanol and is used as a precursor of PF, UF, and MF resins .2 Moreover, f ormaldehyde is used in many sectors ranging from automotive to healthcare industries .2 It is a highly flammable 8 colorless gas . Although th ere are serious concern s about formaldehyde 's impact on human health, it is still being used in many different applications .4,7,35 Œ38 Formaldehyde is considered a highly toxic chemical with a lethal dosage (LD50) higher than 100mg/kg in rats and 42mg/kg in mice .2,36,37,39,40 Formaldehyde is known to cause certain health risk s like eye, nose , throat , and skin irritation at a concentration higher than one ppm (1.25 mg/m 3).2,12 In 2010, formaldehyde was classified as carcinogenic to humans through inhalation exposure by the Environmental Protection Agency (EPA ).41 Formaldehyde has been concluded to have a ca usal association with nasopharyngeal, leukemia, nasal , and paranasal cancer s.41 This very concern is why many countries are introducing stricter restrictions and regulations for the use of formaldehyde. Countries like Japan , Germany , Austria , Sweden , and the United States all began forging more stringent regulations on reducing formal dehyde emissions .42 For decades, Eu ropean countries have been gradually decreas ing the acceptable formaldehyde emissions level in OSB, medium -density fiberboard (MDF), and particleboard with the highest emission level permitted at 8mg/100g board. 23 While countries like the U S continue to use formaldehyd e in the production of OSB panels, albeit with stricter regulations, in Europe , almost all OSB producers use pMDI , especially in the core layer .23 The United States Congress passed a bill in July 2010 , setting restrictions on the number of formaldehyde emis sions allowed in plywood, MDF , and particleboard manufacturing .43 Formaldehyde emissions were set to 0.05ppm for plywood , 0.11 ppm for MDF , and 0.09 for particleboard. 43 Formaldehyde emissions have decreased significantly due to all these regulations , which will be subjected to even stricter regulations since formaldehyde was classified as a carcinogenic chemical .23 9 CHAPTER 2: LITERATURE REVIE W In recent years, many researchers have found comparable replacements to help alleviate our dependency on nonren ewable petroleum -based chemicals. 6,10,44 Œ50 Biomass sources like a cashew -nut shell , tannin , and lignin have been used as substitutes for phenol to formulate phenolic adhesive. 2,20,49 These biomass feedstocks are used as replacements due to their renewability, availability typically as waste biomass , and their comparatively lower price s.51 2.1 Tannin Tannin has been a favorable raw material used to partial ly or entir ely substitute phenol in the phenolic adhesive formulations due to its phenolic structure .48,52 Œ54 Tannins are biomaterials extractable from leaves, bark, and wood from various wood species .51,55 Due to the phenolic nature of tannins and its ability to form cross -linkages with formaldehyde , it has been a suitable substitute for phenol . Its reactivity is determined by the reactive position sites . There are two different types of tannins, condensed , also called polyflavonoid tannins and hydroly zable tannins. 56 Condensed (polyflavonoid) tannins comprise 90% of the world™s commercially produced tannins , which is up to 200,000 tons per year. 55,57 They are mad e of flavonoid units with two types of phenolic n uclei A- ring , which includes phloroglucinol and resorcinol , and B -ring , which includes cate chol and pyrogallol .55,57 Depending on the tannin type , they can contain up to 8 reactive locations to react with formaldehyde and form methylene brid ges .55 The type A rings have higher reactivity toward aldehydes like formaldehyde than the B -type tannin. The most commonly used tannin type is the condensed tannins and is mainly extracted from wattle or mimosa bark ( Acacia ), hemlock bark (Tsuga ), quebracho ( Schinopsis ), sumach ( Rhus ), and 10 different Pinus bark species. 54,57 Hydro lyzable tannins are nonpolymeric structure s that comprise simple phenols such as gallic, digallic , and ell agi c acid and sugar esters , usually in the form of glucose .57 These tannins are less common in adhesive production due to their lower reactivity, limited availability, and higher price than condensed tannins .58 2.2 Lignin Lignin is another major naturally available polymer that has been used to replac e phenol in the formulation of phenolic adhesives due to its phenolic structure . Lignin is one of the most commonly found biobased amorphous polymeric material second to cellulose in the world .47,59 Œ61 Figure 3. Lignin Structure ns: Karol007e -mail: kamikaze007 (at) tlen.pl - own work from: Glazer, A. W., and Nikaido, H. (1995). Microbial Biotechnology: fundamentals of applied microbiology. San Francisco: W. H. Freeman, p. 340. ISBN 0-71672608 -4This W3C -unspecified vector image was created with Inkscape., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1993633 11 Lignin ™s complex structure seen in Figure 3 consists of thre e basic phenyl propane units called monolignols , which includes sinapyl alcohol, coniferyl alcohol , and p-coumaryl alcohol . The se three monolignols shown in Figure 459,63 produc e aromatic residues , including syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) units , respectively .59 The presence of these m onolignols and aromatic residues differ s depending on the lignin source in various plants ,64 shown in Table 2. Figure 4. Three basic phenylpropane units ( Monolignols ) The lignin's performance and suitability for different applications are contingent on the source of the lignin and the isolation method. 65 The lignin content varies based on the source hardwood (angiosperms), softwood (gymnosperms), and annual crops (Table 3) .63 Table 2. Lignin Source And Its Monolignols Content (%) 64 Lignin Source Con iferyl Alcohol (%) Sinapyl alcohol (%) p -coumaryl alcohol (%) Hardwoods 25 Œ 50 45 Œ 75 0 Œ 8 Softwoods > 95 0 < 5 Herbaceous crops 35 Œ 80 20 Œ 55 5 Œ 35 12 Table 3. Lignin sources and its lignin content (%) 63 Lignin Source Lignin Content (%) Hardwood 18 - 25 Softwood 25 - 40 Herbaceous crops (grasses) 10-20 In pulp and paper manufacturing , the wood's cellulose portion is used to prepare pulp and paper products. In consequence , more than 95% of lignin and other lignocellulosic waste from the wood are burnt to generate energy for the pulping proces s.64,66 To be able to utilize the lignin component of the waste ; first , lignin needs to be isolated from pulping liquor. The pulping process technique s can be divided into two categories : sulf ur-based or sulf ur-free isolation techniques .67 Sulfur -based process es include kraft pulping and sul fite pulping ..67,68 Sulfur -free method s include soda pulping and organosolv p ulping processes .68 The lignins derived from these processes are considered technical lignins 68, including kraf t, sulfite , soda , and organosolv lignin s.60 Kraft lignin is developed by the kraft p rocess , which in volves the delignification of wood and other biomass > 95% , under high temperature around 170°C with a solution of sodium sul fide and sodium hydroxide .64,69 The resulting kraft lignin makes up 85% of the world™s total lignin produc tion .70 This process results in lignin with high amounts of phenolic hydroxylic groups and condensed structures and a low number -average molecular weight (Mn) ranging from 1000 -3000 g.mol -1.67 Kraft lignin has a low sulfur content (2 -3%) despite the high sulfur extraction method used and also has high levels of phenolic hydroxyl groups .68 Sulfite lignin or lignosulfonate lignin is derived from the sulfite pulping process of wood using a high temperature of 140-170 °C with 13 sulfur dioxide (SO 2- ) and hydrogen sulfite (HSO 3-).68 Lignosulfonate consequently contains high amounts of sulfur , resulting in higher ash content leading to a darker color . It is water -soluble, with a broad polydispersity index ranging from 6 to 8 , and a higher number -average molecular weight ranging from 15,000 to 50,000 g. mol -1 than the kraft lignin. 67,68 Soda lignin is produced through a soda pulping process that is usually used for non -wood fibrous feedstocks such as straw, flax, and sugarcane .68,71 Sodium hydroxide solution is used to digest the fibrous feedstocks at a temperature of 160°C or lower .68 Soda lignin s are insoluble in water , sulfur -fre e, and have low molecular weight and broad molecular weight distribution ranging from 800 -300 0 g/mol e.67,68,72 Soda lignin extracted from herbaceous sources also contains high silicate and nitrogen contents. 71,73 Organosolv lignin is produced through the organosolv pulping process using an acidic or nonacidic organic solvent like ethanol or methanol with water and acetic acid, with the addition of small amounts of acids like hydrochloric acid (HCL ) or sulfuric acid (H2SO4) used as a catalyst. 67 The resulting lignin is highly soluble in organic solvents , but because of its hydrophobic nature , they are insoluble in water and is considered the purest commercial lignin .67,68 Diluted Œacid enzymatic hydrolysis lignin , which is the lignin utilized in this research , is a primary by-product of the bioethanol pro cess resulting in lignin yield ranging from 10 -30% of the lignocellulosic biomass depending on the source .74,75 In this method , t he biomas s is pretreated using dilute acid (H2SO4) and then processed enzymatically using cellulo lytic enzymes like exo -glucanases , endo -glucanases , and -glucosidases .75 Pretreatment of lignocellulosic material is typic ally performed before conducting the enzymatic hydrolysis to increase the efficiency of the process in extracting the sugars by weakening the structure of the biomass. 76Œ79 This allows for the tightly packed cellulose and hemicellulose content between layers of lignin to 14 be more exposed and available for the enzymes and chemicals .76 This step is essential because lignin and hemicellulose reduce the eff ectiveness of enzymes from separating the cellulose component during the enzymatic hydrolysis process .80 Diluted -acid pretreatment , the most beneficial pretreatment technique which is performed at high temperatures for a short time (<30 min) or low temperature for longer processing time (30-90 min ).80 This pretreatment procedure may be used as either the main method in hydrolyzing lignocellulosic material to sugars or used as a precursor to the enzymatic hydrolysis of the material. 75 Corn stover lignin extract ed using dilute acid enzymatic hydrolysis tend to have fewer ether linkages resulting in added reactive sites (H and G) .81 Also note , the phenolic content of enzymatic hydrolysis corn stover pretreated using dilute acid can also be higher by increasing the pretreatment conditions like reaction temperature and residence time. 78 2.2.1 Lignin Modification Lignin™s complex structure, however, poses limitations for its use as a phenol replacement in these adhe sives. 10,82 Due to steric hindrance in lignin structure, formaldehyde ™s reactivity is significantly lower. Compared to monomeric phenol , which has three available reactive sites (one para and two ortho), t here a re fewer available reactive aromatic sites in the lignin structure for react ion with formaldehyde .83,84 This will then reduce the degree of cross -linking during the polymerization stage ,83 which may result in unfavorable adhesive properties like lower mechanical strength and higher free formaldehyde content. To combat this issue, lignin modification has been introduced to increase lignin's reactivity with formaldehyde through hydroxy methylation, demethylation , and phenolation. 47,85 Methylation , also known as hydroxymethyl ation , is the most straightforward modification technique .86 In this technique , 15 lignin i s dissolved in an a lkaline medium , then react s with formaldehyde , which result s in hydroxy methyl groups attached that are attached to the aromatic rings of lignin via the Lederer -Man asse reaction .65,67,86 Phenolation , also known as phenolysis , entails thermally treating lignin with phen ol in an acidic solution like sulfuric acid (H 2SO4).86 The p henolation process does not only increas e the phenoli c content ; it also decreases the lignin ™s molecular weight by cleaving ether bonds during the process .86 Demeth oxylation , which is the most expensive process ,50 increases lignin ™s reactivity by removing one or two methoxy groups from the ortho positions , creating available sites for reaction with formaldehyde .84 Subsequently , increasing lignin ™s reactivity with formaldehyde or other aldehydes by adding additional hydroxyl groups .84,86 Despite the ability to increase l ignin ™s reactivity with formaldehyde and other aldehyde s, these modification methods require added cost , time , and toxic organic chemicals , making it undesirable to industry and also impeding the purpose of developing sustainable means of reducing our dependency on petroleum -based chemicals. 47,82 2.3 Partial or Total Replacement of Phenol In many research studies, partial replacement of phenol was conducted to develop resol e phenolic adhesives .6,10,44,45,47,87 Khan et al .44 developed a lignin phenol -formaldehyde adhesive replacing phenol with Eucalyptus bark lignin (10, 25, 35, 50 , and 60% (w/w%) ). They concluded that replacing 50% of phenol with eucalyptus bark based on a 1:2 ratio , lignin -phenol to formaldehyde using a 1 0 wt.% catalyst (NaOH) , at 80°C for 4 hours would produce an adhesive with enhanc ed mechanical properties comparable to PF adhesives. This study noted that as the lignin concentration increased , the non -volatile solid content decreased and remained constant after 2 5 wt.% substitution s; gel t ime also decreased when lignin content was increased from 0 to 16 50 wt.%. Also , as the lignin concentration increased , there was an increase in the shear and adhesive strength . This study showed that an increase in cross -linking occurred which was also confirmed b y the decrease in gel ation time .44 Yang et al .45 conducted a study using four different biorefinery technical lignin s, wheat straw , corn cob, and two poplar lignin s to replace 50% of phenol in PF adhesive formulation . They reported that a dhesives made with high molecular weight lignins like the wheat straw and corn co b showed significantly higher viscosity than commercial PF adhesives . The solid content of the formulat ed adhesives was higher than the commercial PF too . The cure temperature of resins analyzed via differential s canning calorimetry (DSC ) showed slightly higher (124 Œ 130°C) than the cure temperature of commercial PF ( 118°C). The bonding strength of LPF adhesive formulated using corn cob lignin was the highest at 1.18 MPa compared to CPF adhesive 1.53 MPa , which is extremely lower than the adhesion strength of phenol -formaldehyde adhesives (3.5MPa) . Jin et al .5 also used modified enzymatic hydrolysis cornstalk lignin to replace 5 to 2 0% of phenol with formaldehyde to prepare an adhesive . This study showed that by replacing 20% of phenol with EHL , the dry shear strength was 1.5 MPa and 1.80 MPa for the wet strength , which met the Chinese standards of 1 and 0.7 , respectively . They were able to determine that by increasing the NaOH c ontent from 2.5 to 5% , the dry strength increases for the 20 % phenol replacement adhesive , but with the same formulation , the wet strength decreases .5 Many resea rchers have made significant improvements by successfully replac ing phenol with modified or unmodified lignin up to 50%. However, Kalami et al .47 ( our group) were the first , which successfully replace d 100% of phenol with an unmodified enzymatic hydrolysis corn stover lignin producing an adhesive with mechanical properties similar to that of a commercially 17 formulated phenol resorcinol formaldehyde (PRF) adhesive . The dry lap shear strength of 3.4 MPa an d the wet shear strength of 2.6 MPa showed no significant difference when compared to the formulated PRF adhesives , which was 3.6 MPa and 3.0 MPa , respectively. Furthermore, by substituting phenol with the unmodified corn stover lignin, formaldehyde consumption was reduced by 50%. The use of corn stover lignin as a suitable phenol replacement was further proven to be best by another study performed by Kalami et al .10 This study analyzed different lignin sources and various extraction methods to determine the most suitable lignin for replac ing phenol. 2.4 Partial or Total R eplacement of Formaldehyde Glyoxal , C2H2O2, (40 wt.%) is the simplest dialdehyde , which has been used as a substitution for formaldehyde .7,36,88,89 It is produced from oxidat ion (gas -phase) of ethylene gly col with a copper or silver catalyst , or oxidation (liquid phase) of acetaldehyde with nitric acid or as a secondary product of biological processes .90,91 It is considered non -volatile based on the Henry Law constant ( 3.38 × 10 -4 Pa.m 3/mol ) in its aqueous state ,91Œ93 biodegradable ,7,88,91 and is less toxic than formaldehyde. 92,94 Glyoxal is considered to be a non -toxic chemical due to its low acute toxicity levels . According to studies conducted on rats, the lethal or al dosage (LD 50) in rats ranged from 3000 to 9000 mg/kg body weight , and the dermal LD 50 was greater than 2000mg/kg body weight. 91 As mentioned earlier, formaldehyde™s toxicity (100mg/kg in rats and 42mg/kg in mice ) is much higher than glyoxal (LD50) .2 Also, with its two adjacent carbonyl groups , g lyoxal ™s high reactivity with phenol , which is similar to that of formaldehyde and phenol, makes it a suitable formaldehyde replacement. 51,93,95 Glyoxal is used in many applications including, the textile industry, paper coating , adhesives, pharmaceuticals , cosmetics and electronics .96 According to 18 global market trends , glyoxal production was USD 265 million in 2019 and is pre dicted to increase by 2024 to USD 326 million at a comp ound annual growth rate (CAGR ) of 4.3% . Researchers have attempted and succe eded in partially replacing formaldehyde with glyoxal in phenolic resin formulations .7,35,36,88,89 Hussin et al .35 successfully developed a lignin -based phenolic adhesive replacing 100% of formaldehyde with glyoxal and up to 30% of phenol using two hibiscus cannabinus (kenaf core) extracted through kraft and soda processes . They reported that their 30% soda kenaf lignin -phenol -glyoxal (SLPG) had higher glass transition temperature ( Tg) and higher phenolic -OH content than kenaf kraft lignin , which resulted in greater crosslinking and producing a favorable adhesive. Hussin and his colleagues also reported that their soda lignin -based adhesive performed better tha n their kraft lignin -based adhesive with higher viscosity , solid content , and tensile strength .35 Ballerini et al . 88 studied the substitution of phenol and formaldehyde with pine tannin and glyoxal , respectively . They discovered that at pH around 8-9.5, tannin glyoxal had similar performance (gel time and rate of cure ) to that of tannin -formaldehyde adhesi ve at a pH range of 6 -7. Also , with the aid of pMDI, adhesive formulated using pine tannin and glyoxal (70% Tannin, 9% glyoxal , and 21% pMDI) internal bond ( IB) strength was increased from 0.44 MPa in the tannin and glyoxal (88%Tannin with 12%glyoxal) , to 0.60 MPa . Additionally , formaldehyde emission decreased dramatically from 4.7mg/100g 95% tannin /5% paraformaldehyde adhesiv e compared to 0.6mg/100g in the formulations without paraformaldehyde . Lei et al .48 formulated a formaldehyde -free adhesive using glyoxal and lignin (acetic acid wheat straw lignin) alo ng with pMDI 20 to 40%. In this study, mimosa tannin extract was also used as a replacement for PF resin in the formulation of the mixed adhesive. Lei and his colleagues focused on the properties of formulated resins when 19 formaldehyde is replaced with glyoxal and when low molecular weight lignins were used as phenol replacement. For these adhesives, 55 % and 60% glyoxalated lignin (GL) , which is pre -reacted lignin with glyo xal , were mixed with different ratios of pMDI (20,25, 30 , and 40%) and mimosa tannin (0, 15, 20 , and 25 ) was used to replace PF resin in the adhesive formulation. This study showed the adhesives formulated with 60% of glyoxalated lignin and with 40% pMDI (60/40 GL/pMDI) using neither PF resin and mimosa tannin had a higher internal bond strength of 0.53 MPa . This was compared to their control adhesives using glyoxalated lignin, pMDI , and PF resin 55/25/20 and 55/20/ 25, which had an internal bond strength of 0.35 MPa and 0.31 MPa , respectively. This study also used triacetin and resorcinol , which are used as an accelerator to help increase the performance of the adhesive. The addition of triacetin did not improve the perfo rmance and cure temperature of the GL/ MDI adhesive , but resorcinol did. With the growing interest in re ducing our dependency on petroleum -based chemica ls and reducing our exposure to toxic chemicals, few researchers have explored replacing both phenol and formaldehyde with lignin and glyoxal. Mansouri et al .36 were able to substitute 100% of formaldehyde with glyoxal , replacing phenol with modified lignin and mixed it with PMDI . This study showed that the formulated adhesive c ould meet the adhesion strength/IB strength required by the international standard for exterior grade panels. However, as a result of the low reactivity of both lignin and glyoxal , they used modified lignin . They used a combination of lignosulfonate with petro chemical adhesive diphenylmethane -diisocyanate (pMDI) with a ratio of glyoxal to pMDI of 60:40 . Diphenylmethane -diisocyanate was used as a n accelerator to aid in increasing the mechanical strength of the adhesive. 20 2.5 Possible Challenges and Drawbacks As stated earlier, replacing 100% of phenol with unmodified lignin is difficult due to lignin™s complex 3D-structure and its lower reactivity with formaldehyde than phenol . Kalami et al .10 compared nine different lignin samples as a phenol substitute and reported that the enzymatic hydrolys is corn stover lignin was the most suitable lignin for replacing 100% of phenol in the phenol -formaldehyde adhesive formulation . However , it is very challeng ing to replace 100% of both phenol and formaldehyde simultaneously with unmodified lignin and glyoxal that have lower reactivity than both phenol and formaldehyde . Due to t he two carbonyl groups in glyoxal and its tendency to undergo side reactions like the Cannizzaro reaction leading to a more acidic medium , more in-depth research is needed to determin e optimal parameters to address these concerns. 2.6 Objectives This study's objective was to replace the formaldehyde portion of lignin -based phenolic adhesive with a biobased aldehyde such as glyoxal in a way that would not negatively affect the performance of the formulated adhesive. This was accomplished by gradually substituting formaldehyd e with glyoxal in 10% increments starting from a 0% substitution, which was the control (LF and PF) to 100% substitution of formaldehyde. However, since both the phenol and formaldehyde portion s of the adhesive were replaced with biobased , less reactive co mpounds, the challeng e was to maintain the adhesion performance comparable with petroleum -based adhesive. The goal was to reduce the formaldehyde consumption in this biobased resin as low as possible by optimizing the resin formulation to achieve similar o r superior performance to the 21 commercially available phenolic resin. Any attempt in this line would improve the environmental aspect of the products by minimizing the adverse health effect for both workers at the manufacturing site and consumers using pro duced panels by significantly reducing the formaldehyde emission . This project's ultimate goal was to formulate a 100% biobased adhesive with similar or superior performance as of commercially formulated phenol -formaldehyde adhesive currently used in exterior grade plywood and OSB panels . 22 CHAPTER 3: MATERIALS AND METHOD S 3.1 Materials Phenol 99.5 wt.% was purchased from Acros Organics and used as received. The lignin sample used for this research was isolated from corn stover lignin provided by POET LLC , produced as a by-product of the bioethanol synthesis through dilute acid pretreatment 97 and enzymatic hydrolysis of corn stover. Corn stover entails the stalks, leaves , and husks left behind after harvest. 78 Corn stover (Zea Mays ) contains 15 -21% of lignin 97 and is the most produced popular biomass produced in the Un ited States .98 It contains high amounts of all three monoligno ls, including guaiacyl (G) , hydroxyphenyl ( H), and syringyl ( S). 97 Corn stover lignin was selected for this resea rch because , according to prev iously published work in our group 10, it was proven to be the best candidate for the substitution of phenol in the lignin -based phenolic adhesives since this lign in contains high p-coumaric and ferulic acid in addition to a higher amount of p-hydro xyphenyl (H-lignin) .10 Corn stover lignin processed through enzymatic hydrolysis process of corn stover contains high H -lignin , which has two vacant ortho positions in its phenolic structure that can react with aldehyde result ing in less free formaldehyde emission and higher bond strength .10 Formaldehyde 37 wt.% and glyoxal 40 wt.% were purchased from Acros Organics and Fisher Scientific Inc , respectively . Both chemicals were used as received . Sodium hydroxide (NaOH) is the most com mon basic catalyst used in the preparation of resole resin. Sodium hydroxide (1N) solution was purchased from Fisher Scientific Inc . and was used as a solvent to dissolv e the lignin and also as a catalyst . Wheat flour and plywood bark extender are used in adhesive formulations as a filler or extender to improve adhesive properties , control viscosity , 23 99and avoid excessive penetration of adhesive into the wood. 99 Southern yellow pine (SYP) veneer samples measuring 25.4 mm × 10 1.6 mm × 3.17 mm (1in × 4in × 0. 12in) were used for the lap shear strength testing of the formulated adhesives. 3.2 Methodology 3.2.1 Lignin Characterization Moisture Content The lignin sample's moisture content was measured by drying lignin samples in an oven at 105°C for 1 hour and at 80°C for 3 hours. Samples were dried at 80 °C to remove moisture (water) without partially degrading specific lignin components .10 A mass of 0.5 g of the lignin was weighed in a dry, pre -weighed , aluminum pan and heated in an oven for 3 hours . Samples were cooled in a desiccator to room temperature and then weighed . The analysis was done using five replicates , and m oisture content was calculated using equation 1. % = ×100% [Equation 1] Ash Content The ash content of the lignin was analyzed according to TAPPI -T 211 om-93 method . Crucibles were dried at 250 °C using a Thermolyne benchtop muffle furnace and which was then weighed to the nearest 0.1 mg after cooling to room temperature in a desiccator . Lignin samples were first dried for 3 hours at 105 °C, then 1 g of the oven -dried lignin was added to each weighed crucible . The c rucibles were then transferred to the furnace and heated at 525 °C for 4 hours. Using a d esic cator, s amples were then cooled to room temperature and weighed. This test was performed in five replicates. Ash content was calculated using equation 2. 24 % = () ()×100% [Equation 2] Elemental Analysis Elemental analysis of the lignin sample was determined by A&L, Great Lakes Laboratories (Fort Wayne, Indiana, USA ). Following U.S EPA method 3051A (SW -846), for microwave extraction , 100 the lignin sample was first mineral -digested via open vessel microwave . /Mineral content was then determined based on a method from the Association of Official Analytical Chemists (A OAC 985.01) using a Thermo Scientific ™s Inductively Coupled Argon Plasma (iCAP) Duo 6000 series instrument. Molecular Weigh t Distribution To determin e the molecular mass distribution of the lignin sample, it was first acetylated to increase its solubility in the tetrahydrofuran (THF) solution used as the mobile phase .65 One gram of oven -dried lignin was acetylated by adding it to 40 mL of a 50/50 v/v% solution of acetic anhydride and pyridine and was mixed at 600 rpm for 24 hours at room temperature. A total of 150 mL hydrochloric acid (pH=1) was used to precipitate the acetylated lignin. The precipitates were then vacuum filtered, and the residual solids were washed with hydrochloric acid (0.05M) solution three times and with deionized water several times. The washed acetylated lignin sample was then dried using a vacuum oven at 40 °C overnigh t. Gel permeation chromatography (GPC) was used to determine the molecular weight, molecular number, and polydispersity index (PDI) of the acetylated lignin. The dried acetylated lignin was dissolved in THF (HPLC grade, 5 mg/ml concentration) and was filte was used for GPC analysis. The GPC system from Waters, Milford, MA, USA included a separations module (Waters e2695 ), had a mobile phase (THF) at a flow rate of 1 mL/min at 35°C and three 25 col umns (300 mm × 7.8 mm Ultyragel THF 500 Å from Waters). Polystyrene standards were used for calibration standards with molecular weights from 162 , 370, 580, 945, 1440, 1920, 3090, 4730, 6320, 9590, 10400 to 16200 Da. Using a 2414 Refractive Index (RI) Detector, 25 lignin solution was injected into the GPC system, which was constantly maintained at 35 °C same as the columns. Chromatograms were analyzed using Empower GPC Software. Hydroxyl Content Using 31P-NMR Hydroxyl content of the lignin sampl e was quantitatively determined using phosphorous -31 nuclear m agnetic resonance (31P-NMR ). A mass of 40 mg of dried lignin was first dissolved in a anhydrous dimethylformamide (DMF). Cyclohexanol and chromium (III) acetylacetonate were purchased and used as internal standard and relaxation reagent .10 They were both dissolved in anhydrous pyridine and deuterated chloroform at a ratio of 1.6:1.0 (v/v) separately . A volume of (22mg/mL concentration) solution (5.8 mg/mL concentration) solution was added to the dissolved lignin mixture . Finally, (2-chloro -4,4,5,5 -tetramethyl -1,3,2 -dioxaphospholane, (TMDP) a phosphitylation reagent was used to assist with tagging the hydroxyl groups during the analysis . A total of of the mixture was then transferred to a 5mm NMR tube . 31P NMR spectra were obtained using an Agilent DDR2 500 MHz NMR spectrometer combin ed with 7600AS, running VnmrJ 3.2A, and a pulse delay of 5 s and 128 scans was used. The different hydroxyl groups were determined using the chemical shifts reported in Table 4.101 26 Table 4. 31 P-NMR chemical shifts of lignin's functional groups Hydroxyl Group Chemical Shift Span (ppm) ( 31P- NMR Spectra) Aliphatic OH 149.1 -145.4 Condensed Phenolic OH 144.6 - 143.3 and 142.0 - 141.2 OH from Syringyl OH 143.3 - 142.0 OH from Guaiacyl OH 140.5 - 138.6 OH from Hydroxyphenyl 138.5 - 137.3 OH from Carboxylic acid 135.9 - 134.0 3.2.2 Resin and Adhesive Formulation For this research , phenolic resin was formulated by substituting 100% of phenol with an unmodified corn st over lignin and using different ratios of formaldehyde and/or glyoxal ( 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100% molar ratio ). The molar ratio of phenolic hydroxyl content of lignin to formaldehyd e/glyoxal was kept 1:2 for all the samples . Lignin formaldehyde (LF) resin was formulated using 100% unmodified lignin and formaldehyde . A mass of 15 g of lignin was dissolved gradually in 55mL of 0.1 N sodium hydroxide stirred at 150 rpm. The a mount s of lignin and formaldeh yde were measured based on the total phenolic content of our lignin sample , which was 3.89 mmol /g using a 1:2 molar r atio of lignin to formaldehyde . A round bottom flask equipped with a thermometer, condenser , stir bar , and dry bath stacker was used , as seen in Figure 5. A dry bath stack er was used to have a uniform temperature around the flask along with the thermometer. In a round bottom flask , 9.5 g of formaldehyde was added , followed by the addition of the dissolved lignin . Gradually t he temperature of the system was increased to 65 ° C within 30 minutes at constant stirring (600 27 rpm ). At 65° C, the system was kept constant for 10 minutes. The remaining NaOH solution , 20 mL (approximately 1/3 of the determined amount ), was added to the flask . The temperature was steadily increased to 85° C , kept at that temperature for 1 hr. The resin was then cooled to room temperature ; some of the resin was stored in a freezer to prevent further polymerization that can be used later to prepare adhesive. The remaining resin was used to measure its properties. Figure 5. Resin formulation equipment setting, equipped with a thermometer, condenser, dry bath stacker , and digital heater and stirrer. Lignin Formaldehyde and/or Glyoxal Resins The same procedure explained above was used for the formulation of lignin formaldehyde /glyoxal (LFG) . Formaldehyde was substituted in 10% increments from 0% to 100%. These formulations were denoted LFG -10, LFG -20, LFG -30, LFG -40, LFG -50, LFG -60, LFG -70, LFG -80, LFG -90, and LG. 28 Adhesive Preparation The formulated resin was used to prepare the adhesive using the same procedure recommended for commercial phenol -formaldehyde adhesive , also known as glue mix prep aration by industry , which was applied by Kalami et al. 10 First , 6.5 % (wt%) wheat flour was dissolved in 18 % (wt%) distilled water , followed by gradually adding a 6.5 % (wt%) plywood extender (modal ) to the mixture . Next, a mix of 3 % (wt%) sodium h ydroxide and 66 % (wt%) thawed resin was added to the solution gradually . The solution was stirred until the mixture became homogenous , using a high -speed mixer at 800 rpm for 5 minutes . 3.2.3 Resin and Adhesive Properties pH and Alkalinity The pH of the formulated resin and adhesives were analyzed using a Mettler Toledo S220 digital pH meter at room temperature. Samples were first mixed using a stir plate and stir bar for 10 seconds , and then pH was measured. The alkalinity of resins and adhesives determines the amount of acid needed to reduce the acidity to a pH of 3. 102 Alkalinity of samples were tested based on ASTM D1067 . About 6 g of sample was diluted in 1 00mL distilled wa ter . The solution was titrated t o a pH of 3.5 , using 0.1 N hydrochloric acid (HCl) . Alkalinity was then determined based on the consumed amount of HCl used , which was calculated using equation 3. % = ( )×. () ×100% [Equation 3] Solid content The solid content (SC %) or non -volatile content of the formulated resin and adhesive was analyzed following ASTM D4426 -01 procedure (5 replicates) . Aluminum pans were placed in a 29 digital oven a t 270 °C to burn off any excess oils on the pans from manufacturing. After cooling the pans to room temperature , the y were weighed and labeled . Then 1 g of the sample (resin or adhesive) was added to each pan a nd placed in an oven for 105 min utes at 125 °C. Samples were then cooled down to room temperature in a desiccator and weighed. The solid content was determined using equation 4. % = () ()×100% [Equation 4] Viscosity The v iscosity of the formulated resin w as measured at room temperature using a Brookfield DV2T Viscometer. Using spindle number 63, the viscosity of the adhesive was measured in triplicates and reported in centipoise (cp). Gelation Time The gelation time was measured by adding about 1g of the re sin in a glass test tube and immersed it in boiling wate r. T he t ime was measured instantly using a stopwatch. The gelation time w as recorded from the time the test tube was submerged in the boiling water to the time that the resin h eld to the rod. The resin was stirred in the tube and by raising and lowering the glass rod until a gel forms around the rod. This w as done in triplicate t o determine the average gel time for each sample. Free Formaldehyde Content To determine the free formaldehyde content (%CH 2O), a hydroxylamine hydrochloride method was used according to the European Standard DIN EN ISO 9397 . Five grams of sample was dissolved in 100m l distilled water and using 0.1N HCl, the p H of the solution was titrated to 30 pH 4.0 while stirred on a stir plate at 250 rpm. When the solution was at a stable pH of around 4.0, 20 m l of 10% (w/ v) hydroxylamine hydrochloride was added to the solution. The reaction between hydroxylamine hydrochloride with formald ehyde produce d formal doxime and hydrochloric acid :10 HCHO + NH 2OH · HCl H2O + CH2NOH + HCl . After 5 minutes of mixing, to ensure the reaction was complete, the solution was then titrated again to a pH of 4.0 using 0.1N sodium hydroxide . The amount of HCl formed was used to determine the amount of formaldehyde reacted with hydroxylamine hydrochloride. Free formaldehyde content was calculated using equation 5. % 2 = ( () × () × . ))( () ×100% [Equation 5] 3.2.4 Resin and Adhesive Performance Water Resistance of the Resin The formulated resin's water -resistance was measured using an industry -recommended technique to determine the quality of resin bond strength quickly. Samples passed the test if the cured mixture of sawdust and resin sample s remain ed intact and did not dissolve while submerged in the water for a specific period of time . Approximately 5 ml of resin was placed in aluminum dishes . In two dishes, 0.5 g of sawdust was mixed with the resin. The third dish , with only a resin sample , was used as a control. The samples were cured in a conventional oven for 1 hour at 130°C. Afterward, the cured samples were cooled, and 0.5g of each sample was submerged in a beak er containing 100 ml of distilled water , which was kept for one week at room 31 temperature . Formulated resin s amples were evaluated periodically after every hour for 4 hours , then every 24 hours for up to a week. Adhesion Strength of Adhesives The dry lap shear strengths of the produced adhesives were evaluated according to ASTM D5868 -01 standard test method to determine the adhesion strength of plywood samples when pressed under similar parameters as recommended by industry curing of commercial phenol resorcinol formaldehyde adhesive . Using southern yellow pine , veneer samples measuring at 25.4 mm × 101.6 mm × 5.6 mm (1 in × 4 in × 0. 22 in) , as shown in Figure 6. About 0.10 - 0.12 g of the prepared adhesive was applied on one -fourth of the veneer's surface (1 in 2). Ten replicates of each formulated adhesive were tested. Using a PHI heated press , two ve neers were pressed at 175°C under 1250 kPa (180 psi) for 4 min according to industry recommendation . Veneer samples were cooled down to room temperature in a desiccator for 24 hrs before testing. Figure 6. Image of veneer specimens for lap shear strength testing. The adhesion strength was determined using a n Instron 5565 universal testing machine . To measure the adhesion strength (maximum shear strength in PSI was recorded) of developed biobased adhesives . Specimens were loaded with a minimum of 1 inch of the veneer held in the test grip at each end . The s ample s were analyzed at a loading rate of 0.5 in/min , as seen in Figure 32 7. Results were recorded as maximum shear stress, calculated by dividing the maximum force by shear area in MPa. Figure 7. Universal Instron testing of pressed veneer specimen Wet Lap S hear Strength Wet lap shear strength of samples was evaluated according to ASTM D -3434 test method . Veneer samples were submerged in boiling water for 4 hours and then placed in an oven at 65 °C for 20 hours. Samples were then immers ed in boiling water again for another 4 hours . These samples were dried using a paper towel to remove excess water and tested right away using the same procedure performed for the dry lap shear strength. Image Analysis (Percent Failure) Using the procedure reported by Kalami et al .10, the percentage of wood failure for tested lap shear samples was determined quantitatively using ImageJ software for image analysis. This 33 analysis was conducted to determine what percentage of the veneer samples failed due to adhesive failure or wood failure. These samples were photographed, and the image was adjusted in Photoshop by first cropping the area to be analyzed using the cropping tool . Images were further cropped to specific dimensions of 7.5 ×7.5 inches using the ficanvas size fl in the fiimage fl tab seen in Figure 8. Figure 8. Wood specimen adjusted in Photoshop. 7.5 × 7.5 inches of veneer specimen was cropped for analysis. The contrast and color were adjusted by first selecting fiauto contrast fl and fiauto color fl in the fiimage fl tab seen in the image on the left in Figure 9. Using the fiadjustment fl tab under the fiimage fl function shown in the middle image in Figure 9., the brightness and contrast were set to 100. The images were then saved as a tiff file for ImageJ analysis. 34 Figure 9. Wood Specimen adjusted in Photoshop. Image (left) were adjusted for auto contrast and auto color and then adjusted for brightness and contrast (100%) ( middle and right) In the ImageJ software, adjusted images were made binary by selecting fimake binary fl in the fiprocess fl tab , as seen in Figure 10. The n ext step was to sele ct the desired information in the results, which were area fraction, standard deviation , and area in the fiset measurements fl option in the fianalyze fl tab shown in Figure 11. Figure 10. ImageJ software analysis of veneer for wood failure. The tiff file of the image was opened in ImageJ software (left) , and using the software (middle) image was made binary for analysis. (right). 35 Figure 11. ImageJ software analysis of veneer for wood failure. Set measurements (left) are chosen for specific results needed to determine the wood failur e of wood samples. (right) After setting the desired measurements, the image was then analyzed using the fimeasurefl option in the fianalyzefl tab displayed in Figure 12. The result ing area fraction value was the total area shaded black, which corresponded to resin failure . The wood failure, which was shaded white, was determined by subtracting that value from 100 %, as seen in the right image in Figure 12. This wood failure analysis was performed on each wood specimen for each adhesive formulation and averaged . Figure 12. Image J Software Analysis of Wood Failure . Image Is A nalyzed (Left) And Value Displayed Under % Area Represents Resin Failure (Right). 36 CHAPTER 4: RESULTS AND DISCUSSION 4.1 Lignin Characterization Ash and Moisture Content The ash content of the corn stover lignin sample was at 0.7% ( Table 5). Although sulfuric acid was used during pretreatment of biomass and used to isolate pure lignin from lignin cake, the proper washing with distilled water helped ensure the removal of most residual ash. Moisture content (MC%) of the lignin was measured using two different parameters (times and temperatures). The moisture content of lignin was 3±0.04 % when measured at 100°C after 1 hour, whil e it was 2.3±0.2 % when measured at 80°C after 3 hours. As expected, drying lignin at 80°C for 3 hours had a lower moisture content than higher temperature s. This could be sue to potential lignin degradation at higher temperature and loss of VOC .47,103 Table 5. Ash Content (%), Sulfur Content (%), Moisture Content (%), Molecular weight and PDI of Lignin Sample Sampl e %N %S % MC: 100°C, 1h % MC: 80°C, 3h % Ash Content Mn (g/mol) Mw (g/mol) PDI Lignin 2.19 0.4 3.0 (0.04) 2.3 (0. 2) 0.7 (0.07) 1550 6400 4.12 Elemental Analysis Based on the elemental analysis performed on the lignin sample, 0.4% of sulfur was detected , which can also correspond with the low ash content (0.7%) of th is lignin. Sulfur content in lignin is dependent on the extraction method use d to isolate lignin from biomass . The nitrogen content of the lignin in this study was 2.19% , which was similar to an enzymatic hydrolysis corn 37 stover lignin reported by da Costa Sousa et al .104 Lignin isolated from annual crops contain higher nitrogen content probably due to the use of nitrogen -based fertilizers , or enzymes used in the bioethanol process. 104,105 Molar Mass Distribution Analysis of Lign in Sample The average molecular weight M w (6400 g/mol), the average molecular number M n (1550 g/mol), and the polydispersity index (PDI=4.2) of corn stover lignin were measured using GPC and are reported in Table 5. The high PDI of lignin might negatively impact the homogeneity of the formulated adhesives. Hydroxyl Functional Groups of Lignin ( 31PNMR Data) Using 31 phosphorous Nuclear magnetic reso nance spectroscopy ( 31P NMR), the lignin sample's hydroxyl content was determined, which can include aliphatic, phenolic, and carboxylic acid groups. For phenol -formaldehyde formulation, lignin's total phenolic hydroxyl content is needed to calculate the formulation's required formaldehyde amount . The total phenolic hydr oxyl content of the corn stover lignin used for this research, as shown in the 31P NMR spectra (Figure 13), was 2.37 mmol/g . 38 Figure 13. 31PNMR Spectra of Lignin Sample This value was used to measure the amount lignin needed to react with glyoxal and formaldehyde using a molar ratio of 1:2 lignin to formaldehyde or glyoxal. As aforementioned, based on the sourc e of the lignin and the extraction process, the reactivity of lignin with formaldehyde may differ. Corn stover lignin extracted via enzymatic hydrolysis had been shown to have a higher p-hydroxyphenyl content, H (0.66 mmol/g), and guaiacyl content, G (0. 93 mmol/g) compared to nine other lignins. 10 Table 6. 31P-NMR analysis results for corn stover lignin . Lignin ID Aliphatic OH (mmol/g) Syringyl (mmol/g) Guai acyl (mmol/g) p-hydroxy - phenyl (mmol/g) Carboxylic Acid (mmol/g) EH-CS 1.8 2 0.78 0.93 0.6 6 1.5 2 39 4.2 Resin and Adhesive Properties and Performance pH and Alkalinity For this research, the goal was to produce a resole phenolic adhesive, formulated under basic conditions typically in the range of 9 to 13. 20 The desired pH for lignin -based resins should be between the range of 9 to 11 to avoid lignin precipitation. 106 As shown i n Table 7, the pH of the PF and LF resins were 10.2 and 10.66 , respectively . Table 7. Physical and Chemical Properties of Formulated Resins Sample ID pH Alkalinity (%) Solid Content (%) Gelation Time (min) Free Formaldehyde Content (%) Water Resistance LF 10.66 ± 0.03 2.42 ± 0.04 19.98 ± 0.08 2.6 ± 0.02 0.79 ± 0.02 + 24 hours LFG -10 10.16 ± 0.01 2.44 ± 0.03 21.00 ± 0.14 3.04 ± 0.5 0.55 ± 0.01 Dissolved in <24 hrs LFG -20 9.71 ± 0.005 2.32 ± 0.03 21.20 ± 0.50 3.14 ± 0.7 0.91 ± 0.02 Dissolved in <24 hrs LFG -30 9.14 ± 0.02 2.05 ± 0.01 20.85 ± 0.28 3.93 ± 0.4 1.07 ± 0.001 Dissolved in <24 hrs LFG -40 8.16 ± 0.01 1.80 ± 0.03 20.79 ± 0.15 2.13 ± 0.9 1.55 ± 0.06 Dissolved in <24 hrs LFG -50 6.88 ± 0.02 1.71 ± 0.03 21.40 ± 0.03 Gelled 1.65 ± 0.02 Dissolved in <24 hrs LFG -60 6.35 ± 0.02 1.68 ± 0.03 21.03 ± 0.07 Gelled 1.76 ± 0.05 + 24 hrs LFG -70 6.12 ± 0.01 1.79 ± 0.05 21.44 ± 0.04 Gelled 2.06 ± 0.01 1 week LFG -80 5.82 ± 0.01 1.72 ± 0.05 22.12 ± 0.08 Gelled 1.79 ±0.01 1 week LFG -90 5.62 ± 0.01 1.72 ± 0.05 22.64 ± 0.27 Gelled 2.02 ± 0.07 1 week LG 5.52 ± 0.005 1.59 ± 0.0 1 22.09 ± 0.06 Gelled 1.61 ± 0.04 1 week PF10 10.2 2.6 ± 0.1 30 ± 0.5 18.7 ± 0.3 0.3 ± 0.03 1 week 40 Still, as the formaldehyde substitution increased, the pH gradually decreases to a recorded pH of 5.52 for the LG resin. This trend in decreasing pH may have occurred due to the Cannizzaro reaction. 107 Under alkaline conditions , typically with sodium hydroxide, two aldehydes molecules without alpha hydrogens will react with itself to produce a carboxylic acid and an alcohol, oxidation, and reduction products, respectively , as seen below. 107 However, the pH of the formulated adhesives , as seen in Figure 14, were all within the standard range. 106 Although our formulated resins resulted in lower pH, and the adhesive ( known by the industry as glue mix ) had pH higher than 9. 20 The formulated LF adhesive had a pH of 13.24 , which was similar to the pH of the formulated PF resin in the lab of 13.2 as reported by Kalami et al. 10 However, it should be noted that as the formaldehyde substitution increased, the pH of the formulated adhesive similar to resin decreased. Nonetheless, the LG adhesive had a pH of 12.54 , performing well above the expected range (10 to 13) for resole phenolic adhesives. The alkalinity of the adhesives ranged between 3.85% to 4.58% , with a slight decrease as the formaldehyde substitution increased. Figure 14. Alkalinity and pH of formulated adhesives. 13.2 13.2 13.1 12.9 13.0 12.7 12.7 12.4 12.3 12.0 12.5 13.2 4.6 4.5 4.5 4.3 4.2 4.1 4.0 3.9 3.9 3.9 4.0 0246810 1214LFLFG-10 LFG-20 LFG-30 LFG-40 LFG-50 LFG-60 LFG-70 LFG-80 LFG-90 LG-100 PFAlkalinity (%) and pH Sample ID Alkalinity and pH of Formulated Adhesives pH Alkalinity (%) 41 Solid Content The solid content (SC) of the resin had comparable results to a phenol -formaldehyde resin , as seen in Table 7. The solid content of the formulated adhesive s can impact mechanical interlocking and can also affect the adhesive spread rate . As shown in Table 7, the formulated resins showed a slight increase in solid content as the glyoxal incorporation increased from 19.98 % in the LF to 22.1% in the LG. However, these values are significantly lower than the solid content PF resin (30%). Although the percent solid content of LG adhesive increased to 30.2%, after the addi tion of extender and filler, it is still significantly lower than the solid content of PF adhesive (42%) and below the expected range of phenolic adhesives of 40 to 50%. 20 Nonetheless, if needed , the solid content can easily be adjusted by adding more fillers in the adhesive formulation . Viscosity and Gelation T ime The v iscosity of formulated adhesives was lower than the PF adhesive. The v iscosity for the LF adhesive was 672 cP (Table 8), while the viscosity of PF adhesive was 2179 cP as reported by Kalami et al .10 However , it is recommended that resole phenolic adhesive have a viscosity ranging between 400 and 600 cP for easy application. 108,109 The g elation time s of the resins are reported in Table 7. After substituti ng 40% of formaldehyde with glyoxal , resin gel led right away . In the future, a rheometer can be used to determine the viscosity and gelation time of the formulated resins and a dhesives more accurately . Gelation time and viscosity are two critical parameters in determining the properties and performance of these adhesives . As gel time is the point where the resin is no longer a viscous liquid, a shorter gelation time can affect the application time , leaving less time for application and negatively affecting the 42 adhesives' mobility and ability to penetrate the wood . Additionally , a longer gelation time will require longer curing time , needing more energy and time during processing . Viscosity is also critical during processing affecting the application and penetration of the adhesive in the wood , which , along with the gelation time , can affect the mechanical properties of the adhesive. Both are essential for the application process ; therefore, they need more in-depth examination . Free Formaldehyde Content The free formaldehyde content of the resole phenolic adhesives relates to the unreacted formaldehyde that did not participate in the reaction with phenol or lignin in this case . As seen in Table 7, the fr ee formaldehyde is much higher than that of th e PF resin , which was 0.3% . This may be the result of the lower reactivity of both glyoxal and lignin. However , there is an unusual trend in the data which requires repeating this test . Nonetheless , it is safe to say that the free formaldehyde content increases as the formaldehyde substitution increases. Regarding the developed adhesives, the free formaldehyde content decreased significantly lower than that of the resin shown in Table 8. The free formaldehyde content reduced from 1.61 % in the LG resins to 0.17 % in the adhesive , which is slightly above the requirement (<0.1%) for phenolic resins .54 This decrease may be a result of further polymerization when formulating the adhesive . It must be noted that this test requires further investigation to determine th e cau se. Water Resistance of the Resin The water -resistance testing on the formulated resin was a simple qualitative industry performance test to determine its resistance in room temperature water . Based on the results shown in Table 7 above , the adhesive performanc e increased as the amount of glyoxal was 43 increased. PF, LF, and LFG-60 to LG did not dissolve in the water and remain ed intact for up to a week. LFG -10 to LFG -50 dissolved in less than 24hrs after being immersed in water . Table 8. Physical and chemical properties of formulated adhesives Lignin Glyoxal (L G) Optimized From 50 % formaldehyde substitution , there was a drop in pH resulting in an acidic resin. Due to this increased acidity of the resin, lignin began to precipitate , as show n in Figure 15. Gelation of the resin occurs at a point during the chemical reaction when the viscous liquid resi n converts irreversibly to a n elastic gel 110 and the molecular weight reaches a maximum , which is when the monomers are all now chemically bonded , forming one chain 111 causing the viscosity of the resin to reach infinity. 110,111 It is vit al to prevent the gelation of the resin during processing. Sample ID pH Alkalinity (%) Solid Content (%) Free Formaldehyde Content (%) Viscosity (c Ps) LF 13.2 ± 0.01 4.58 ± 0.03 29.9 ± 0.4 0.17 ± 0.01 672 ± 10 LFG -10 13.2 ± 0.02 4.52 ± 0.05 32 ± 0.4 0.17 ± 0.0 1 673 ± 6 LFG -20 13.1 ± 0.005 4.46 ± 0.05 30.8 ± 0.2 0.17 ± 0.0 1 688 ± 10 LFG -30 12.9 ± 0.005 4.30 ± 0.06 30.2 ± 0.4 0.18 ± 0.0 1 618 ± 7 LFG -40 13.0 ± 0.05 4.22 ± 0.03 29.6 ± 0.1 --- 665 ± 8 LFG -50 12.7 ± 0.0 1 4.12 ± 0.11 30.8 ± 0. 2 0.47 ± 0.02 746 ± 1 LFG -60 12.7 ± 0.0 1 3.97 ± 0.03 31.6 ± 0.2 0.30 ± 0.01 695 ± 4 LFG -70 12.4 ± 0.01 3.92 ± 0.04 31.1 ± 0.1 0.25 ± 0.01 437 ± 2 LFG -80 12.3 ± 0.01 3.86 ± 0.05 30.7 ± 0.1 0.26 ± 0.04 --- LFG -90 12 ± 0.02 3.85 ± 0.10 32.3 ± 0. 7 0.16 ± 0.01 157 ± 4 LG 12.5 ± 0.25 3.95 ± 0.02 30.2 ± 0. 5 0.17 ± 0.0 1 131 ± 3 PF10 13.1 --- 42 ± 0.6 --- 2180 44 This process should take place during heating after being applied to the veneer specimen. The onset of gelation reduced the viability of the adhesive during processing, which had adverse effects on the resin's mechanical properties, including adhesion and the processability of the resin .111 To solve this, the amount of NaOH solution was increased to help to reduce the viscosity, thereby preventing gelation of the resin. The NaOH addition was also added gradually at a slower rate. This change, along with the addi tion of lignin to the NaOH slowly at a slower pace , increased the time to dissolve the lignin in the NaOH. Also, changing the mixing speed of the stir plate from 150 to 250 rpm helped ensure that the lignin was dissolved entirely, which reduced the initial viscosity of the lign in solution. The time and temperature of the system were kept the same as LF formulation. Figure 15. Image of precipitated lignin in LG resin during formulation 45 Dry Lap Shear and Wood Failure Results The a dhesion strength of developed adhesives w as analyzed using a lap shear test. The analysis was performed on the plywood samples glued using the formulated lignin -based adhesives from 0 to 100% substitution of formaldehyde with glyoxal shown in Figure 16. Wood failure analysis results on the wood specimen after the lap shear test are presented in Figure 17. Dry lap shear strength of the lignin formaldehyde adhesive was 3.49 MPa , and wood failure was around 71%. In the LFG -10 adhesive, there was a sharp increase in shear strength of 4.51 MPa but a decline in the wood failure 62.7%. As the amount of formaldehyde substitution continued to increase, the adhesive shear strength decreased but was still within the industry standards for plywood specimens. Before optimization, the lignin glyoxal (LG) a dhesive had a lap shear strength of 2.83 MPa, the lowest shear strength, and a wood failure of 53%. Figure 16. Dry lap shear strength results for formulated adhesives 3.4 3.5 4.5 4.2 3.6 3.8 3.8 3.7 3.3 3.3 3.2 2.8 3.3 01 234 56PFLFLFG-10 LFG-20 LFG-30 LFG-40 LFG-50 LFG-60 LFG-70 LFG-80 LFG-90 LGOpt. LG Dry Lap Shear (MPa) Sample ID Dry Lap Shear Strength Results 46 After optimization, the LG adhesive had a lap shear strength of 3.26 MPa and a wood failure of 81%; these values are relatively close to the adhesion strength of PF adhesive formulated by Kalami et al .10 3.4 MPa and 88% wood failure . This improvement was due to the adhesive's increased viscosity, allowing lower penetration and possibly reducing glue line starvation, which resulted in improved adhesion strength. Figure 17. Percent wood failure , Image analysis results Wet Lap Shear Test for P F, L F, and L G. Wet lap shear testing was conducted on the P F, L F, and L G samples. PF and L F adhesives performed as expected , which is shown in Table 9. However, t he plywood veneer samples prepared with the LG adhesive performed poorly. After placing samples in boiling water , the veneer samples glued with LG adhesives separated in less than 30 minutes . Despite the high dry lap shear (3.26 MPa) reported for the optimized LG adhesive , the developed adhesive failed the wet lap shear strengt h. This could be due to the lower reactivity of glyoxal and lignin , which requires further investigation in the future. It must be noted that the glued veneers remained 88.0 71.2 62.7 65.3 73.4 75.7 77.0 64.9 51.4 68.2 51.5 53.3 81.4 020406080100 120PFLFLFG-10 LFG-20 LFG-30 LFG-40 LFG-50 LFG-60 LFG-70 LFG-80 LFG-90 LGOpt. LG % Wood Failure Sample ID Wood Failure Analysis Results 47 intact when the samples were immersed in the cold (room temperature) water. We believe the increased temperature (100°C) might have played a significant role in causing the LG adhesive to fail. Table 9. Wet lap shear strength and wood failure result for LF, PF, and LG adhesives . Sample ID Adhesive amount (g) Wet Lap Shear Stress (MPa) Wood Failure (%) Lignin Formaldehyde 0.12 2.03 ± 0.3 63 ± 7 Phenol Formaldehyde 0.12 3.0 ± 0.7 87 ± 5 Lignin Glyoxal 0.12 Failed ---- 48 CHAPTER 5: CONCLUSION S AND FUTURE RECOMMENDATION 5.1 Conclusions 1. For this study, an unmodified corn stover lignin extracted via dilute acid pretreatment -enzymatic hydrolysis was used. 100% of phenol was successfully substituted with an unmodified corn stover biorefinery lignin. Replacing 100% of phenol with unmodified l ignin reduced formaldehyde consumption by 58%. The formulated phenol -formaldehyde adhesive contained about 17g of formaldehyde , while the same amount of formulated lignin formaldehyde (LF) resin only contained 7g of formaldehyde. 2. Additionally, 100% of formaldehyde was replaced with non -toxic, biobased glyoxal to develop lignin glyoxal (LG) adhesive . This adhesive showed similar properties compared to that of commercial resole phenolic adhesive , while some properties like viscosity, gel time , and free formaldehyde content call for further optimization and analysis. The dry lap shear strength of the LG adhesive was 3.26 MPa , which performed similarly to the PF adhesive at 3.4 MPa. 3. Although the dry adhesion strength of the formulated LG adhesive was comparable to the PF adhesive, the developed LG adhesive failed the w et shear strength tests when submerged in hot water (in less than 30 minutes) during the first boiling stage of the wet shear strength test. Further investigation is required to better understand the reaction mechanism between lignin and glyoxal , which wil l help to improve the LG wet adhesive performance. 49 5.2 Future Recommendation This research resulted in a biobased adhesive that has comparable performance to the commercially available phenol -formaldehyde adhesive with a dry lap shear strength of 3.26 MPa. This is a n excellen t step in the right direction in eliminating the use of petroleum -based phenol and formaldehyde using biobased lignin and glyoxal. More analysis and optimization are needed to better understand the potential chemical rea ction betwee n lignin and glyoxal and to increase the performance of the developed LG adhesive. 1. An in -depth analysis of the cure kinetics is also important to better understand the characteristics of the adhesive. Differential Scanning Calorimetry (DSC) can be used to better understand the curing behavior and reaction kinetic of developed LG adhesives. Also, nuclear magnetic resonance (NMR) spectroscopy and Fourier -transform infrared (FT -IR) spectroscopy can be used to determine the extent of cure .112 2. A more accurate rheology analysis using a Rheometer will help to accurately measure the rheology and gel ation time of the resin s and adhesive s. This rheology analysis will aid in optimizing the amount of catalyst and fillers requir ed in the formulation of both the resin and the adhesive s. Using a Rheometer will reduce any margin of error , which is expected when using the gelation time procedure applied for this study. 3. The formulated resin and adhesive molecular weights can be analyzed by gel permeation chromatography (GPC) to understand the adhesive™s ability to penetrate into the wood ™s cell wall . Since lignin has a much larger mo lecular weight than phenol , it can impact the formulated biobased adhesive penetration .5 More op timization is needed to develop an 50 LG adhesive that can perform according to the industrial standards for the wet lap shear strength. This will require more elaborate analytical testing like DSC and TGA . 4. With further investigation, this 100% biobased resole adhesive can be a suitable replacement for phenol -formaldehyde adhesive for both interior and exterior applications. 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