Soy-based,tannin-modified plywood adhesives

In this research, two different types of commercial tannins, namely a hydrolysable tannin (chestnut) and a condensed flavonoid tannin (mimosa), were used to prepare two types of soy-based (soy flour (SF) and soy protein isolate) adhesives for making plywood. Thermogravimetric properties (TGA) and its derivative as function of temperature (DTG) of different soy-based adhesive were measured in the range 40°C–300°C. Thermomechanical analysis (TMA) from 25°C to 250°C was done for the different resin formulations. Duplicate three-ply laboratory plywood panels were prepared by adding 300 g/m² of the adhesives’ total resin solid content composed of SF or isolated soy protein (ISP), urea, chestnut, and mimosa tannin extracts with hexamine as hardener. Based on the results obtained, tannins can improve SF adhesion properties. The TMA showed that chestnut tannin extract appeared to react well with SF, while mimosa tannin extract appeared to react well with ISP. Matrix-assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry also showed that among other reactions, the soy protein amino acids reacted with the tannins. Furthermore, delamination and shear strength test results showed the good water resistance of plywood bonded with soy-based tannin modified adhesive.     

Soy-based adhesives for wood-bonding – a review

Over recent years, the interest in bio-adhesives, including soy-based adhesives, has increased rapidly. Among natural renewable resources suitable for industrial use, soy is a reasonable choice due to its high production volume and the small use of soy meal-based products for human food consumption. Soy flour can be an ideal raw material for the manufacturing of wood adhesives due to its low cost, high protein content and easy processing. There are also more concentrated forms of soy proteins, i.e. concentrates and isolates, which are also suitable raw materials for adhesive production except that their prices are higher. Extensive research has been carried out on improving the cohesive properties, especially water resistance, of soy-based adhesives. However, there is insufficient experimental data available for understanding the influences of modification methods on the structure of soy proteins and therefore for understanding the influences of structural changes on the adhesion. In this paper, some experimental techniques are proposed to be used for analysing soy-based adhesives to enable better understanding of those factors and improve future development. This review of soy-based adhesives is made with the focus on soy proteins’ chemical composition, soy protein product types (raw materials for adhesive production), modification methods for improving the adhesive properties of soy-based adhesives, and commercial soy-based adhesives.     

Preparation of a New Type of Polyamidoamine and Its Application for Soy Flour-Based Adhesives

The most commonly used curing agents for soy-based adhesives are polyamines, which have the problem of low solid content and/or high viscosity. To overcome this problem, a new type of polyamidoamine (PADA) resin was synthesized and applied to soy flour-based adhesives to improve their water resistance. The PADA solution obtained had a high solid content of 50 wt% and low viscosity of 270 cP. The optimum weight ratio of soy flour/PADA/maleic anhydride to prepare adhesive was 40/7/1.68. The wet strength of plywood prepared at the optimum weight ratio was 0.82 MPa, which meant the plywood could be used as type II plywood according to the Chinese National Standard GB/T 9846.7-2004. The results of water-insoluble solid content measurement and SEM observation demonstrated that cured soy flour–PADA–maleic anhydride adhesive had a 16 % greater water-insoluble solid content than soy flour–NaOH adhesive. The cross-linking network formed by the reactions of PADA and MA would increase the water-insoluble solid contents and improve water resistance of cured soy flour-based adhesives.     

Soy-based adhesive cross-linked by melamine–glyoxal and epoxy resin

To develop a soy-based adhesive with good water resistance, non-toxic melamine–glyoxal resin (MG) prepared in the laboratory was used as a cross-linker of soy-based adhesive. The FT-IR and ESI-MS results showed that there was a reaction between melamine and glyoxal. The resulted –CH–OH– groups could be the possible reactive groups for the cross-linking of soy-based adhesive. The wet shear strength of soy-based plywood indicated that the water resistance of soy adhesive cross-linked by MG improved with respect to that with no cross-linker, although it was not good enough to satisfy the relative standard. With the optimized preparation procedures for plywood, specifically, press temperature 180?°C, press time 3 min and resin loading 280 g m^?2, type I soy-based plywood could be prepared with a hybrid cross-linker, namely 12%MG + 2% epoxy resin (EPR). The DSC results showed that the reaction between soy-based adhesive and the hybrid cross-linker MG + EPR was very complex.     

Bond quality of soy-based phenolic adhesives in southern pine plywood

Increased demand for wood adhesives, environmental concerns, and the uncertainty of continuing availability of petrochemicals have led to recent attention on protein-based adhesives. This study was conducted to investigate the glue bond qualities of soy-based phenolic adhesive resins for southern pine plywood. Two types of soy-based resins were formulated and tested. The first was made by cross-linking soy flour with phenol-formaldehyde (pf) resins at neutral pH. The second type was obtained by cross-linking soy flour hydrolyzates with pf resin under alkaline conditions. Plywood bonded with the neutral phenolic soy resins containing 70% soy flour and 30% 1.6 g/cm² pf without the use of extenders and fillers compared favorably with the traditional southern pine pf glue mixes. Plywood bonded with alkaline phenolic soy resins, containing 40 or 50% 0.5 g/cm² PF with the addition of extender (19% corn-cob powder), provided better adhesive glue bond properties than traditional southern pine pf glue mixes. These results suggest that soy-based phenolic adhesive resins have potential for the production of exterior southern pine plywood.     

An All‐Natural Adhesive for Bonding Wood

A novel adhesive that is solely based on natural materials of defatted soy flour (SF) and magnesium oxide (MgO) has been investigated for preparation of five‐ply plywood panels. The resulting plywood panels met the industrial water‐resistant requirement for interior plywood. In this study, mechanisms by which an aqueous mixture of SF and MgO served as a strong and water‐resistant adhesive for bonding wood were investigated. SF was first fractionated into soy protein isolates (SPI), a water‐soluble fraction, and insoluble carbohydrates (ICs) that were mixed with MgO, respectively, for preparation of maple laminates. The water resistance of the resulting maple laminates was evaluated by a three‐cycle water‐soaking‐and‐drying (WSAD) test and a two‐cycle boiling‐water test (BWT). The mixture of MgO and the soluble fraction was not able to bond maple veneers together. The shear strengths of the resulting maple laminates before and after WSAD and BWT all had the following order: MgO–SPI > MgO–SF > SF only > MgO–IC. The water solubility of SF in the heat‐cured SF–MgO mixture was much lower than that of the heat‐cured SF. We believe that the low water solubility of SF–MgO and close interactions between MgO and soy proteins instead of soy carbohydrates were responsible for the superior strengths and high water resistance of the soy‐MgO adhesive.     

Rheological behaviors exhibited by soy protein systems under dynamic aqueous environments

In this study, rheological behaviors of soy protein and soy flour as powders, pastes, and dispersions are characterized over a range of water contents and temperature to understand their processing in adhesives or paints. At ambient temperature, soy protein samples were characterized by low critical strain values (<0.1%), whereas soy flour samples exhibit linear viscoelastic regions >1% strain with 30–90% water content. On heating, the aqueous soy protein and soy flour compositions have complex rheological behaviors due to plasticization by water and the thermal denaturing and crosslinking of protein and carbohydrate with increasing temperature. Below 100 °C, soy protein rheological behaviors were attributable to the glass transition of the 7S and 11S soy globulin fractions, polymer flow, and plasticization by residual moisture. Above 100 °C, the onset of protein crosslinking was observed with this shifting to higher temperatures for samples still dehydrating. With soy flour samples, the residual moisture present above 100 °C similarly increase protein crosslinking to higher temperatures (125–148 °C) for samples with initial water contents of 30–90%. These results provide a basis for understanding why soy systems undergoing heat processing and rapid dehydration will require higher temperature and longer processing time to attain a cured, crosslinked state. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45513.     

Chromatographic Analysis of the Reaction of Soy Flour with Formaldehyde and Phenol for Wood Adhesives

The desire to make more biobased and lower-cost bonded wood products has led to an interest in replacing some phenol and formaldehyde in wood adhesives with soybean flour. Improved knowledge of the soy protein properties is needed to relate resin chemistry to resin performance before and after wood bonding. To expose the soy protein’s functional groups, it needs to be disrupted, with minimal hydrolysis, to maximize its incorporation into the final polymerized adhesive lattice. The best conditions for alkali soy protein disruption were to maintain the temperature below 100 °C and react the soy flour with sodium hydroxide at pH 9–12 for about 1 hour. A gel permeation chromatography procedure was optimized to determine conditions for selectively breaking down the high molecular weight soy protein fragments that contribute to high adhesive viscosity. This method and extraction data were used to evaluate the reaction of the disrupted soy flour protein with formaldehyde and phenol to provide a stable adhesive. The results were used to develop more economical adhesives that are ideally suited for the face section of oriented strandboard.     

Soy flour dispersibility and performance as wood adhesive

Soy flour adhesives using polyamidoamine-epichlorohydrin (PAE) resin as the curing agent are being used commercially to make bonded wood products. The original studies on the soy-PAE adhesives used purified soy protein isolate, but the much lower cost soy flour is now used commercially. We examined the performance of commercially available soy flours that have their proteins either mainly in their native (90 protein dispersibility index (PDI)) or denatured (70 and 20 PDI) states. We expected that the more native state soy proteins with their better dispersibility would provide better adhesion to wood surfaces and enhanced reaction with PAE resin. Small-scale wood bonding tests showed that neither of these effects was observed without and with a low level of PAE. In these tests, the solids content of the soy formulations had a large influence on adhesive viscosity but little influence on bond strength. Additionally, little difference was observed in any of the adhesive or viscosity properties between the soy flours having either a 0.152 or 0.075?mm (100 or 200 mesh) particle size.     

Influence of phosphoric acid etching on the dentin bond durability of universal adhesives

This study aimed to compare the microshear bond strength (MSBS) of three universal adhesives and a three-step conventional adhesive to dentin after 24-hour and one-year storage in water. A new fluoride-releasing universal adhesive (Clearfil Universal Bond Quick: CUQ) and two commercially available adhesives (ScotchBond Universal: SBU and All-Bond Universal: ABU) were evaluated with phosphoric acid etching (PA-etch mode) or without it (self-etch mode). All-Bond 3 (AB3) served as control group. After bonding composite cylinders to dentin discs obtained from caries-free human teeth, the specimens were stored in deionized water at 37?°C for either 24 hours or one year (n?=?14) before MSBS measurement. Two-way ANOVA analysis of the results showed that the adhesives, storage time and their interactions had a significant effect on MSBS (p?<?0.01). In self-etch mode, there was no significant difference among universal adhesives at the baseline. In PA-etch, the CUQ and SBU showed significantly higher MSBS compared with AB3 (p?<?0.05). At baseline, no difference was found between the two modes for each universal adhesive (p?>?0.05). After one year, CUQ in self-etch mode showed a slight increase in nominal MSBS (p?>?0.05) and Weibull characteristic strength, which was significantly higher than SBU and ABU in the corresponding mode. There was no difference among the three universal adhesives in PA-etch mode after one year (p?>?0.05). In conclusion, the durability and reliability of dentin bonding with universal adhesives in different application modes depended on the material; and the self-etch approach showed promising results for the tested fluoride-releasing universal adhesive.     

Preparation and evaluation of particleboard bonded with a soy flour-based adhesive with a new curing agent

Urea-formaldehyde (UF) resin is one of the most commonly used wood adhesives for making particleboards. However, UF emits carcinogenic formaldehyde and is derived from nonrenewable petrochemicals. In this study, a new formaldehyde-free wood adhesive that is based on soy flour and a renewable material-based curing agent (CA) were prepared and evaluated for the preparation of M-2 grade particleboards. The new CA was derived from ammonia and epichlorohydrin that can be derived from renewable glycerol. The composition of the adhesive was soy flour/sodium hydroxide/CA at a dry weight ratio of 9/0.3/1.0. The modulus of rupture, modulus of elasticity, and internal bond strength met the minimum industrial requirements of M-2 particleboards using the following variables: hot-press temperature of 190?°C, hot-press time of 240?s, the adhesive usage of the face particles of 12?wt.%, the adhesive usage of the core particles of 10?wt.%, and the target particleboard density of 0.80?g/cm³.     

Preparation and Evaluation of Particleboard with a Soy Flour-Polyethylenimine-Maleic Anhydride Adhesive

A soy-based formaldehyde-free adhesive consisting of soy flour (SF), polyethylenimine (PEI), maleic anhydride (MA) and NaOH was investigated for making three-layer particleboard. The weight ratio of SF/PEI/MA/NaOH was 7/1.0/0.32/0.1. Hot-press temperature, hot-press time, particleboard density and adhesive usage were optimized in terms of enhancing the modulus of rupture (MOR), modulus of elasticity (MOE) and internal bond strength (IB) of the resulting particleboard. The MOR, MOE and IB met the minimum industrial requirements of M-2 particleboard under the following variables: hot-press temperature of 170 °C, hot-press time of 270 s, the adhesive usage of surface particles at 10 wt%, the adhesive usage of the core particles at 8 wt%, and the targeted particleboard density of 0.80 g/cm³.     

Comparison of Canola and Soy Flour with Added Isocyanate as Wood Adhesives

Canola is widely grown in the northern latitudes for its vegetable oil, generating large quantities of residual, low value canola flour used as animal feed. The common wood adhesive poly(diphenylmethylene diisocyanate) (pMDI) should react with the wide variety of functional groups in proteins. Therefore, it would seem that canola flour with added pMDI could be an effective adhesive. Two main questions are addressed in this study: How do the wood adhesive properties of canola flour compare to the better-studied soy flour? How well do proteins, which contain an abundance of functional groups, cure with the very reactive pMDI? These questions were addressed using the small-scale adhesive strength test ASTM D-7998, with various adhesive formulations and bonding conditions for canola flour plus pMDI compared to soy adhesives. The more challenging wet cohesive bond strength was emphasized because the dry strengths were usually very good. Generally, soy adhesives were better than canola ones, as was the polyamidoamine-epichlorohydrin cross-linker compared to pMDI, but these generalizations can be altered by the conditions selected. Three-ply plywood tests supported the small-scale test results.     

A new formaldehyde-free wood adhesive from renewable materials

A formaldehyde-free adhesive that consists of soy flour (SF) and a new curing agent (CA) was developed and evaluated for making interior plywood. Three types of plywood panels (seven-ply maple/white fir/pine/white fir/pine/white fir/maple, five-ply yellow poplar, and five-ply aspen) were prepared with the SF–CA adhesives and evaluated for their water resistance. The CA was derived from the reaction of epichlorohydrin (ECH) and ammonium hydroxide in water. Effects of the reaction time, reaction temperature, NaOH usage, heat treatment of CA, addition order of reactants in the preparation of the CA, and storage time of the CA on the water resistance of plywood panels bonded with SF–CA adhesives were investigated. The reaction time required for the completion of the reaction significantly decreased as reaction temperatures increased. The addition of NaOH to the SF–CA adhesive improved the water resistance and dry shear strength of the five-ply aspen panels. All plywood panels met the requirements for interior plywood when the CAs were prepared at 45–60 °C no matter whether the CA was heat-treated or not. Mixing ECH and ammonium hydroxide all at once resulted in better water resistance of the resulting plywood panels than adding either of ECH or ammonium hydroxide to the other dropwise. The viscosity of heat-treated CA was comparable to that of untreated CA when the CA was prepared at 50 °C. Both heat-treated and untreated CAs could be stored at room temperature for at least two months without compromising the water resistance of the resulting plywood panels.     

A Formaldehyde-Free Water-Resistant Soy Flour-Based Adhesive for Plywood

The phasing out of the use of urea–formaldehyde adhesive in the fabrication of interior‐used hardwood plywood requires development of environmentally friendly bio‐based wood adhesives. We recently reported that phosphorylation of soy flour (SF) using phosphoryl chloride (POCl₃) greatly improved the moisture resistance of soy flour adhesive. In the present study, we investigated the effects of inorganic oxidizing agents, such as NaClO₂ and Ca(NO₂)₂, to further improve the wet bonding strength of phosphorylated SF (PSF) wood adhesive. We report that addition of 1.8 % (wet weight basis) Ca(NO₂)₂ to phosphorylated SF (PSF) adhesive formulation containing 25 % soy flour solids increased the wet bonding strength to greater than 3 MPa at 140 °C hot‐press temperature. The water resistance testing of the glued three‐ply hardwood plywood panels passed the three‐cycle soak/dry test recommended by the American National Standard for Hardwood and Decorative Plywood/Hardwood Plywood and Veneer Association protocol (ANSI/HPVA HP‐1‐2004). Since the process involves only inorganic chemistry and no petroleum‐based chemicals such as formaldehyde or polyamidoamine–epichlorohydrin are used, the PSF + Ca(NO₂)₂ adhesive is non‐toxic and environmentally safe.     

Thermoplastic polymers as modifiers for urea–formaldehyde wood adhesives. III. In situ thermoplastic‐modified wood composites

Acrylic monomers and free‐radical initiators were dispersed in an aqueous urea–formaldehyde (UF) suspension and polymerized in situ to afford a suspension containing 5 wt % thermoplastic (5 g of thermoplastic/100 mL of suspension). The viscosity of the thermoplastic‐modified UF suspension (65 wt % solids at 25°C) ranged from 240 to 437 cP versus 121 cP for the unmodified UF control. Wood‐flour composites (sugar maple and 50 wt % adhesive) were prepared with thermoplastic‐modified UF suspensions and cured with the same cycle used for the composites prepared with the unmodified UF adhesive (control). The effect of the thermoplastic‐modified UF adhesive was evaluated on the notched Izod impact strength and equilibrium moisture uptake of the wood‐flour composites. The notched Izod impact strength of the composites prepared with modified UF adhesives increased by as much as 94% above that of the control. The increase depended on the initiator and the monomer composition. The modification affected the equilibrium moisture uptake and rate of moisture uptake in the wood‐flour composites. Preliminary results for particleboard prepared with 10 wt % modified UF adhesive (5% thermoplastic in the UF resin) and unoptimized cure conditions confirmed a significant effect of the thermoplastic modification on both the internal‐bond strength and thickness swelling of the particleboard. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Jet cooking dramatically improves the wet strength of soy adhesives

The impact of jet cooking on shear strength of soy-and-water adhesives was investigated to understand the higher shear strength of commercial soy protein isolates compared to soy flours. Soy flour-based wood adhesives are appealing because of their bio-based content, low formaldehyde emission, and low cost, but their commercial application is limited by low wet cohesive strength. Previous researchers proposed that the process of jet cooking (steam injection with high turbulence followed by rapid cooling) was responsible for the high (~3 MPa) wet shear strength of adhesives made with commercially produced soy protein isolate, using the ASTM D 7998 test. In this work, we show that jet cooking did dramatically increase the wet strength of laboratory-produced, native-state soy protein isolate from 0.6 to 3 MPa, a strength similar to many commercial isolates. Jet cooking was far less effective at developing wet strength of soy flours, but greatly increased the viscosity of virtually all our soy materials. We hypothesize that the benefits of jet cooking are primarily a result of nonequilibrium protein aggregation states because subsequent wet autoclaving of jet cooked soy proteins dramatically decreased wet strength. The dramatic differences in adhesive properties between commercial soy protein isolates and soy flours suggests that the common practice of using results obtained with commercial isolates to predict the performance of soy flour adhesives is inappropriate.     

Effects of soy proteins containing trypsin inhibitors in long term feeding studies in rats

Pancreatic hypertrophy that occurs in rats fed raw soy flour containing about 1200 mg tripsin inhibitor (TI)/100 g diet was reversed by switching the rats to control diets or to diets containing 30% toasted defatted soy flour. No pancreatic hypertrophy occurs in rats fed commercial, edible grade soy flours, concentrate or isolate from time of weaning to adulthood (ca. 300 days). TI content of the soy diets ranged from 178–420 mg/100 g. Except for pancreas enlargement in rats fed raw soy flour, gross and microscopic examination of pancreata revealed no abnormalities. The gross appearance of heart, kidney, spleen, and liver was normal. Soy flour, protein concentrate, and protein isolate in a formulated corn-soy diet provided optimum growth and maintained body weight only if supplemented with vitamin B-12 in long term feeding studies with rats. In the absence of such supplementation, rats fed soy diets initially grew at a rate equal to or greater than those fed a comparable corn-casein control diet; but, with continued feeding for ca. 300 days, body weight of rats fed the casein control was significantly greater than that of the soy flour-fed rats. Those fed soy isolate ceased to grow; and rats fed soy concentrate lost weight. No significant differences were found in organ weights between groups fed soy products and casein, except for increased kidney, liver, and testes weights relative to body weight with the group fed soy concentrate. Supplementation of the soy diets with vitamin B₁₂ stimulated growth to the greatest extent, calcium pantothenate or riboflavin had an intermediate effect, other vitamins had little or no effect; whereas a complete mineral mix was detrimental. Supplementation of the soy diets with vitamin B₁₂ stimulated growth to the greatest extent, calcium pantothenate or roboflavin had an intermediate effect, other vitamins had little or no effect; whereas a complete mineral mix was detrimental. Supplementation of the control diet was without effect. The dietary protein level in these diets was 20%, with casein or soy protein representing 75% of total protein. When fed continuously to rats from weaning to adulthood, properly processed soy protein products, when balanced with essential nutrients, can provide growth comparable to corn-casein diets.     

Adhesion properties of soy protein adhesives enhanced by biomass lignin

Soy protein adhesives have great potential as sustainable eco-friendly adhesives. However, low adhesion under wet conditions hinders its applications. The objective of this research was to enhance the water resistance of soy protein adhesives. The focus of this research was to understand the effect of protein to lignin ratio and lignin particle size i.e. large (35.66 μm), medium (19.13 μm), and small (10.26 μm) on the adhesion performance of soy protein adhesives as well as to characterize its rheological and thermal properties. Results showed that the lignin particle size and the protein to lignin ratio greatly affected the adhesion performance of soy protein adhesives. The addition of lignin slightly increased the viscosity, spreadability, and thermostability of soy protein adhesives. The wet strength of soy protein adhesives increased as lignin particle size decreased. Soy protein mixed with small size lignin at a protein to lignin ratio of 10:2 (w/w) at 12% concentration presented the lowest contact angle and the highest wet adhesion strength of 4.66 MPa., which is 53.3% higher than that of 10% pure soy protein adhesive. The improvements in adhesion performance and physicochemical properties of soy protein adhesives by lignin were ascribed to the interactions between protein and lignin. Lignin with smaller particle size increased the wet shear strength of soy protein adhesives because a larger surface area of lignin was available to interact with the protein.     

Sequential Fractionation of Cottonseed Meal to Improve Its Wood Adhesive Properties

For better understanding of the adhesive properties of different fractions of cottonseed protein, cottonseed meals from both glanded and glandless cotton varieties were separated into several fractions. Each meal was sequentially extracted with water and 1 M NaCl solution, or with phosphate buffer and NaCl solution. Adhesives were prepared from the recovered fractions and hot-pressed onto maple veneer strips and tested for their properties. The adhesive strength of the water- and buffer-washed solid fractions (i.e., the un-extractable residues of the meals) from the glanded seed ranged from 1.32 to 1.62 MPa and were unchanged or increased compared with the adhesive strength of the original meal that varied from 0.98 and 1.49 MPa. Soaking the wood specimens bonded at 80 °C revealed that the water resistance of these water- and buffer-washed adhesives was significantly improved in that they exhibited no delamination during soaking compared with the meal adhesive that showed some delamination (20–30 % of the samples). Furthermore, the water resistance of these fractions with wet shear strength around 1.5 MPa was comparable to that of cottonseed protein isolate (>90 % protein) when the joints were bonded at 100 °C. The preparations from glandless cottonseed meals showed similar adhesive performances. Additional extraction of the meals with NaCl solution reduced adhesive performance. The results suggest that water- or buffer-washed cottonseed meal fractions can be used as wood adhesives and would be less costly to prepare than cottonseed protein isolates.