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.     

Soy proteins as plywood adhesives: formulation and characterization

Soybean proteins have great potential as bio-based adhesives. The objectives of our study were to develop and characterize formaldehyde-free soybean wood adhesives with improved water resistance. Second-order response surface regression models were used to determine the effects of soy protein isolate concentration, sodium chloride, and pH on adhesive performance. All three variables affected both dry and wet strengths of bonded wood specimens. The optimum operation zone for preparing adhesives with improved water resistance is at a protein concentration of 28% and pH 5.5. Sodium chloride had negative effects on adhesive performance. Soy adhesives modified with 0.5% sodium chloride had dry strength, wet strength, and boiling strength of bonded specimens comparable to nonmodified soy adhesives. Rheological study indicated that soy adhesives exhibited shear thinning behavior. Adhesives modified with sodium chloride showed significantly lower viscosity and yield stress. Sodium chloride-modified soy adhesives formed small aggregates and had low storage moduli, suggesting reduced protein–protein interactions. These formaldehyde-free soy adhesives showed strong potential as alternatives to commercial formaldehyde-based wood adhesives.     

Physicochemical Properties of Soy Protein Adhesives Obtained by In Situ Sodium Bisulfite Modification During Acid Precipitation

Successful industrial applications of soy protein adhesives require high adhesion strength and low viscosity at high solid protein concentration. This study examined the effects of β-conglycinin (7S) and glycinin (11S) ratios on the physicochemical properties of soy protein adhesives. Soy protein adhesives with various 7S/11S ratios were extracted from soy flour slurry modified with sodium bisulfite using the acid precipitation method, which is based on the different solubilities of 7S and 11S globulins. Seven glycinin-rich soy protein fractions and six β-conglycinin-rich soy protein fractions were obtained. The external morphology of the samples changed from the viscous cohesive phase to the clay-like phase without cohesiveness. The viscous cohesive samples had good flowability and good water resistance with a wet adhesion strength of 2.0–2.8 MPa. They were stable for up to several months without phase separation at room temperature. Based on the results, we suggest that proper protein–protein interaction, hydration capacity (glycinin-rich soy protein fractions), and certain ratios of 7S and 11S (β-conglycinin rich soy protein fractions) in the soy protein sample are crucial to continuous protein phase formation. Hydrogen bonding, electrostatic forces, and hydrophobic interactions are involved in maintaining the protein viscous cohesive network, whereas disulfide bonds do not exert significant effects. This study describes a new way to investigate viscous cohesive soy protein systems with high solid protein content, thus alleviating the disadvantages of traditional methods for studying the adhesive properties of soy protein isolates, which tend to have poor water resistance, low solid contents, and short storage life.     

Soy Protein Adhesive Blends with Synthetic Latex on Wood Veneer

Environmental pollution has prompted an interest in and a need for bio-based wood adhesives. Modified soy protein has shown adhesion properties similar to those of formaldehyde based adhesives. The objective of this research was to investigate the compatibility of a modified soy protein (MSP) with six commercial synthetic latex adhesives (SLAs). Four different blending ratios of MSP and SLAs were studied. Adhesion; structural change; and rheological, thermal, and morphological properties of the MSP/SLAs blends were characterized. Dry adhesion strength of MSP, SLAs and their blends were all similar with 100% wood cohesive failure. Water resistance of all six SLAs was improved by blending with MSP in terms of the wet adhesion strength. The wet adhesion strength of MSP/PBG (40/60) blends was 6.416 MPa, as compared to 4.66 MPa of pure PBG (press bond glue, urea formaldehyde based resin). Viscosity of MSP/SLAs blends was reduced significantly and reached the lowest value at 40–60% MSP. Infrared spectra, thermal properties, and morphological images indicated that chemical reactions occurred between soy protein and PBG molecules. The MSP provided some functional groups, such as carboxylic (–COOH), hydroxyl (–OH) and amino groups (–NH₂), that cross-linked with hydroxymethyl groups (–CH₂–OH) of PBG, and also acted as an acidic catalyst for the self-polymerization of urea formaldehyde based resin.     

A new environmentally friendly bioadhesive mainly modified by propanetriol

In this paper, a series of new environmentally friendly bioadhesives with improved bonding strength were quickly synthesized via urea, sodium dodecyl sulfate (SDS) and propanetriol are mixed with soy isolate protein. The results showed that the bonding strength of the modified adhesives was changed with the increasing content of propanetriol. The maximum dry shear strength of the plywood bonded with the resultant adhesive was increased to 2.45 MPa when the propanetriol content was 20 ml. While the maximum wet shear strength of the plywood bonded with the resultant adhesive arrived 1.32 MPa, which is acceptable for industrial application in plywood fabrication according to the national standards of the People’s Republic of China (≥0.7 MPa). In addition, the orthogonal experiment suggested that the obtained material with pH of 9 for 5 h mixing at the hot pressing temperature of 120 °C exhibited the best comprehensive performance. Also, the FTIR, SEM and DSC measurements showed that the adhesives had a compact structure with stable thermal property.     

High temperature performance of soy-based adhesives

We studied the high temperature performance of soy meal processed to different protein concentrations (flour, concentrate, and isolate), as well as formulated soy-based adhesives, and commercial nonsoy adhesives for comparison. No thermal transitions were seen in phenol-resorcinol-formaldehyde (PRF) or soy-phenol-formaldehyde (SoyPF) or in as-received soy flour adhesive during differential scanning calorimetry scans heating at 10?°C/min between 35 and 235?°C. Heat flow rates decreased in the order soy flour (as received)?>?SoyPF?>?PRF?>?emulsion polymer isocyanate (EPI). In thermogravimetric analysis (TGA) scans from 110 to 300?°C at 2?°C/min, total weight loss decreased in the order soy flour (as-received)>SoyPF?>?PRF?>?casein?>?maple?>?EPI. For bio-based materials, the total weight loss (TGA) decreased in the order soy flour (as-received) > concentrate, casein?>?isolate. Dynamic mechanical analysis from 35 to 235?°C at 5?°C/min of two veneers bonded by cured adhesive showed 30–40% decline in storage modulus for maple compared to 45–55% for the adhesive made from soy flour in water (Soy Flour) and 70–80% for a commercial poly(vinyl acetate) modified for heat resistance. DMA on glass fiber mats showed thermal softening temperatures increasing in the order Soy Flour?<?casein?<?isolate?<?concentrate. We suggest that the low molecular weight carbohydrates plasticize the flour product. When soy-based adhesives were tested in real bondlines in DMA and creep tests in shear, they showed less decrease in storage modulus than the glass fiber-supported specimens. This suggests that interaction with the wood substrate improved the heat resistance property of the adhesive. Average hot shear strengths (ASTM D7247) were 4.6 and 3.1?MPa for SoyPF and Soy Flour compared to 4.7 and 0.8?MPa for PRF and EPI and 4.7 for solid maple. As a whole, these data suggest that despite indications of heat sensitivity when tested neat, soy-based adhesives are likely to pass the heat resistance criterion required for structural adhesives.     

Physicochemical Properties and Adhesion Performance of Canola Protein Modified with Sodium Bisulfite

The objective of this research was to study the adhesion properties of sodium bisulfite (NaHSO₃)-modified canola protein. Protein was extracted from canola meal through alkali solubilization and acid precipitation methods, then modified with different concentrations of NaHSO₃ (0–15 g/L) during the isolation process. As NaHSO₃ concentration increased, canola protein purities decreased. Amino acid profiles showed that the hydrophobic amino acids in canola protein constituted only 27% of total protein, indicating that canola protein is mostly hydrophilic. The reducing effects of NaHSO₃ were exerted on canola protein through the breaking of disulfide bonds in both its cruciferin and napin components, as reflected by the protein electrophoresis profile, DSC data, and morphological images. The wet protein isolates were used as adhesives. The greatest wet shear strength of canola protein adhesive without modification was 3.97 MPa with 100% wood cohesive failure (WCF), observed at a curing temperature of 190 °C. NaHSO₃ had slight weakening effects on the adhesion performance of canola protein. Canola protein modified with 3 g/L NaHSO₃ exhibited wet shear strength similar to the control at 190 °C and higher strength at 150 and 170 °C. The NaHSO₃ modification significantly improved handling and flowability of canola protein adhesives.     

Development and characterization of a defatted soy flour-based bio-adhesive crosslinked by 1,2,3,4-butanetetracarboxylic acid

A new, environmentally friendly defatted soy flour-based bio-adhesive was developed by using the cross-linker 1,2,3,4-butanetetracarboxylic acid (BTCA). The reaction between BTCA and defatted soy flour was confirmed by assaying the free amino groups, by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, and ¹³C nuclear magnetic resonance (¹³C NMR). Three-ply plywood was fabricated to measure wet shear strength, and the cross-section and thermal behavior of resultant adhesives were characterized in detail. A decrease in the free amino group content, a new ester peak in the FTIR spectra, a weaker resonance at 65.4–76.2 ppm in ¹³C NMR, and a decrease in crystallinity confirmed that BTCA was successfully crosslinked with defatted soy flour. The resulting crosslinked bio-adhesive showed improved wet shear strength (1.36 MPa) and a reduced sol fraction (24.8%). The crosslinked bio-adhesive displayed enhanced thermal stability, and had a more uniform surface based on thermogravimetric analysis and scanning electron microscopy. The results suggest BTCA can be used to prepare high-performance environmentally friendly defatted soy flour-based bio-adhesives.     

Soy-oil-based waterborne polyurethane improved wet strength of soy protein adhesives on wood

Soy-oil-based waterborne polyurethane (WPU) is used to improve wet strength in shear test of wood bonded with an adhesive of soy protein isolate (SPI) by dispersing WPU into SPI slurry. WPU׳s effects on the physiochemical properties of WPU-SPI adhesives are characterized through Fourier transform infrared spectrum, transmission electron microscopy, thermal analysis, contact angle, and mechanical strength. Wet strength of the WPU-SPI adhesives increases by 65% compared to SPI control. Moreover, the microstructure of WPU has effects on the interactions between WPU and SPI. In this study, smaller and more uniform distributed WPU0002 is easier to interact and form stronger crosslinking network with protein than WPU0500. The stronger interaction between WPU0002 and protein results in increased viscosity and bond strength. The WPU-SPI blended adhesives show significantly improved wet strength, demonstrating their potential as wood adhesives.     

Development of High-Strength Soy Protein Adhesives Modified with Sodium Montmorillonite Clay

This study investigated the high strength of a soy protein adhesive system with good flowability at high protein concentration. Sodium montmorillonite (Na MMT), the most widely used silicate clay, was incorporated into viscous, cohesive soy protein adhesives at concentrations ranging from 1 to 11 % (dry basis, w/w). Hydroxyethyl cellulose was used as a suspension agent to stabilize the soy protein and nano clay to be the dispersion system. The interaction between soy protein and Na MMT was characterized by XRD, FTIR, Zeta potential and DSC. Results indicated that soy protein molecules were adsorbed on the surface of the interlayer of Na MMT through hydrogen bonding and electrostatic interaction. The soy protein/Na MMT adhesives had the intercalation structure with Na MMT contents ranging from 1 to 11 %. Adhesion strength, specifically wet adhesion strength, of soy protein adhesives at isoelectric point (pI) was significantly improved by the addition of Na MMT. It is believed that the physical cross‐linking reactions between soy protein and Na MMT mainly contribute to the improved adhesion performance of soy protein adhesives. Wet adhesion strength increased from 2.9 MPa of control soy protein adhesive to 4.3 MPa at 8 % Na MMT. An increase of pH beyond pI value resulted in decreased adhesion strength due to increased surface charges of soy protein and slightly reduced affinity of soy protein on the nano clay surface.     

Adhesion and Physicochemical Properties of Soy Protein Modified by Sodium Bisulfite

Soy protein adhesives with a high solid content (28–39 %) were extracted from soy flour slurry modified with sodium bisulfite (NaHSO₃) at different concentrations. 11S‐dominated soy protein fractions (SP 5.4) and 7S‐dominated soy protein fractions (SP 4.5) were precipitated at pH 5.4 and pH 4.5, respectively. The objective of this work was to study the effects of NaHSO₃ on adhesion and physicochemical properties of soy protein. The adhesion performance of NaHSO₃‐modified SP 4.5 was better than SP 5.4; the wet strength of these two fractions was from 2.5 to 3.2 MPa compared with 1.6 MPa of control soy protein isolate. SDS‐PAGE results revealed the reducing effects of NaHSO₃ on soy protein. The isoelectric pH of soy protein decreased as NaHSO₃ increased due to the induced extra negative charges (RS‐SO₃^?) on the protein surface. The rheological properties of soy protein adhesives were improved significantly. Unmodified samples SP 5.4 and SP 4.5 had clay‐like properties and extremely high viscosity, respectively; with 2–8 g/L NaHSO₃ modification, both SP 5.4 and SP 4.5 had a viscous cohesive phase with good flowability. Overall, NaHSO₃‐modified soy protein adhesives in our study have many advantages over the traditional soy protein isolate adhesive such as better adhesion performance, higher solid content but with good flowability and longer shelf life.     

Protein solubility characteristics of commercial soy protein products

Solubility characteristics of commerical soy protein products (flours, concentrates, and isolates) were determined under various conditions. From the solubility profiles at various pH values and NaCl concentrations, soy protein isolates can be divided into three groups. One group had high solubility near the pl. Another group had low solubility near the pl, but high solubility at pH 11. The third group had low solubility even at pH 11. Except for the hydrolyzed products, the protein solubilities of the soy protein products at various salt concentrations were very low. Temperature did not significantly affect the protein solubility, although a few products showed more than a 20% increase at temperatures >50°C. Soy protein concentrates and soy protein isolates showed similar solubility profiles. The proteins in all commercial products (except flours) tested were denatured, as evident from the solubility profiles in the presence of salt and the enthalpy values from DSC.     

Effects of Pressure from High-Pressure Homogenization on the Performance of Soybean Flour Based-Adhesive

A green and sustainable soybean flour (SF) adhesive is considered as a potential alternative to toxic formaldehyde-based resins. Nevertheless, poor bond stability and low bonding strength is caused by the uneven size distribution and low reactivity of SF. Herein, SF adhesives with excellent and stable performance are synthesized via the synergistic action of high-pressure homogenization (HPH) treatment by incorporating a green crosslinker. Specifically, an even distribution of the SF particles is obtained after the HPH treatment, from which large soy protein molecules are broken to several small and even single protein molecules. In this way, the adhesion stability is improved. Additionally, more active groups buried in proteins are exposed after the HPH treatment due to the unfolding of the protein molecules. Therefore, a more reactive SF is obtained and thus forms a denser crosslinking structure of resultant the adhesive, providing an increase in bonding strength. Particularly, the effects of homogenizing pressure on the adhesive performance are investigated. The results show that a 215.6% increase of wet bonding strength (1.01 MPa) is obtained after the HPH treatment with a homogenizing pressure of 20 MPa, meeting the standards (GB/T 9846-2015) for interior applications.     

PVAc乳胶/改性大豆分离蛋白共混胶黏剂的制备及性能

针对大豆蛋白胶黏剂耐水性差的缺点,用尿素初步改性大豆分离蛋白（SPI）,然后与白乳胶（PVAc）共混合成了共混改性大豆分离蛋白胶黏剂。采用正交实验方法考察了大豆蛋白胶与白乳胶质量比、共混时间、交联剂质量分数、交联时间对大豆蛋白胶黏剂剪切粘接强度的影响,确定了优化配比及制备工艺条件,并在此基础上采用正交试验优化了热压参数。结果表明：大豆蛋白胶与白乳胶质量比10∶1,共混时间1h,交联剂质量分数1.0%,交联时间1.5h,热压温度120℃,热压压强1.2MPa,热压时间2min/mm,涂胶量250g/m2时,测得胶黏剂的干态剪切粘接强度为2.01MPa,按照Ⅰ类胶合板标准测得湿态剪切粘接强度为1.04MPa,并对优化配方进行了结构与性能分析。     

A Soy Flour-Based Adhesive Reinforced by Low Addition of MUF Resin

The desire to prepare a lower-cost soy-based adhesive has led to an interest in using the abundant and inexpensive soy flour (SF) as a substitute for expensive soy protein isolates (SPI) in wood adhesives. However, the weakness of this adhesive is poor water-resistance and bonding strength due to a low protein content, which limits its application in the wood industry. The objective of this research was to provide a simple and useful approach for improving the adhesion performance of SF-based adhesive by introducing a small addition of melamine-urea-formaldehyde (MUF) resin into the cured system. The optimum addition level of MUF resin, as well as the adhesion performance and conformation change of SF-based adhesive, were investigated. The analytical results indicated that the co-condensed methylene bridges were formed through the reaction of methylol groups of MUF resin with soy units during the hot-press process. The addition of MUF resin, not only significantly decrease the viscosity of SF-based adhesive but also increase its water-resistance and wet shear strength value. The SF-based adhesive containing 20% MUF resin, is a relatively low-cost adhesive, has a reasonable viscosity, and moreover can pass the Chinese Industrial Standard requirement (0.7 MPa) for interior plywood panels.     

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.     

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.     

Laccase/TEMPO-modified lignin improved soy-protein-based adhesives: Adhesion performance and properties

The aim of this study was to enhance the water resistance of soy protein (SP) adhesives using laccase/TEMPO-modified lignin. Kraft lignin was depolymerized by laccase enzyme in the presence of TEMPO to expand the oxidation reaction of both phenolic and non-phenolic compounds. This simplified process has the distinct advantage of enhancing lignin-protein interaction. Compared with SP adhesives, lignin-protein adhesives showed a stronger elastic modulus, higher thermal stability, and increased wet adhesion performance. Wet shear strength increased by 106% from 0.693 to 1.429 MPa, and partial wood failure was observed after the test. Better performance was also observed in the three-cycle soaking test. At the same time, the stronger interactions between -COO^- and -NH₂ groups of protein and lignin led to a decrease in flowability and spreadability.     

Comparative functionality of soy proteins used in commercial meat food products

The use of soy isolates, concentrates, and texturized flours in meat food products is discussed. Functional characteristics of soy products in relation to their market application are reviewed. Soy isolates find more limited usage in meat food systems (2%) than the concentrates and textured soy flours (8–12%). In weak meat systems containing large amounts of fat (30–45%), the concentrate emulsifier and isolate are more important than the texturized soy flour. In chopped meat systems with 18–25% fat, the textural properties of soy flour (extruded) are more important than the use of an isolate. However, combinations of concentrate emulsifier and texturized flour are used. The method of cooking, i.e. fresh, deep fat-fried, or char-broiled, will affect the usage of soy combinations. In comminuted cooked cured meat food mixes, soy concentrates, and textured flours currently are being used. Nutritional properties are improved by inclusion of available ingredients high in lysine and methionine. Functional measurements of textural properties have been completed using the Instron with a Lee Kramer cell. Both model emulsion systems and finished product results substantiate the accuracy of textural properties in soy-meat mixes using the Instron.     

Wood adhesive properties of cottonseed protein with denaturant additives

Most commercial wood adhesive use either formaldehyde-based resins or polyurethanes, both of which include potentially toxic chemicals in their formulations. As a result, proteins are being considered as greener and more sustainable wood adhesives. While most of the protein adhesive studies focus on soy proteins, there is also interest in exploring alternatives. In this work, testing of the adhesive performance of cottonseed protein isolate was undertaken in the presence of protein denaturants, i.e. guanidine hydrochloride (GuHCl), sodium dodecyl sulfonate (SDS), urea, and alkali. For comparison, soy protein isolate was also included in the study. At optimal dosage levels, the dry adhesive strength of cottonseed protein isolate could be enhanced by 38, 25, or 47% with SDS, GuHCl, or urea, respectively. The dry adhesive strength and hot water resistance of cottonseed protein isolate was generally superior to that of soy protein isolate, with or without the denaturants. Thus, the combination of cottonseed protein with an optimal concentration of a denaturant may be a potentially promising polymeric system for use as wood adhesives.