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.     

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 properties of soy protein crosslinked with organic calcium silicate hydrate hybrids

The objective of this work was to investigate if inorganic calcium silicate hydrate (CSH) hybrids would improve soy protein wet adhesion properties. 3‐aminopropyltriethoxysilane (APTES) was used as a crosslinking agent to make covalent linkage between organic soy protein and inorganic CSH phases. Soy protein–calcium silicate hydrate (MSP‐CSH) composites with different mole ratio of APTES were prepared and the effect of crosslinking reaction on physicochemical properties such as thermal, rheological, FTIR spectroscopic, and morphological and adhesion properties were studied with physical aging effect. Covalent linkage was observed between CSH and soy protein using the FTIR technique. With aging effect, the denaturation temperature (T_d) and enthalpies (ΔH_d) of each fraction of soy protein increased in DSC thermograms, representing higher thermal stability, and the viscoelasticity of the composites also increased. The roughly coated surface of the MSP‐CSH composite was observed in SEM images. All these changes further confirmed the interaction between CSH and soy protein molecules. Dry and wet adhesion strength of the MSP‐CSH composites was higher than the control MSP alone. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40693.     

PSA performances and viscoelastic properties of SIS‐based PSA blends with H‐DCPD tackifiers

Hotmelt pressure sensitive adhesives (PSAs) usually contain styrenic block copolymers like styrene–isoprene–styrene (SIS), SBS, SEBS, tackifier, oil, and additives. These block copolymers individually reveal no tack. Therefore, a tackifier is a low molecular weight material with high glass transition temperature (T_g), and imparts the tacky property to PSA. The SIS block copolymer with different diblocks was blended with hydrogenated dicyclopentadiene (H‐DCPD tackifier), which has three kinds of T_g. PSA performance was evaluated by probe tack, peel strength, and shear adhesion failure temperature. PSA is a viscoelastic material, so that its performance is significantly related to the viscoelastic properties of PSAs. We tested the viscoelastic properties by dynamic mechanical analysis and the thermal properties by differential scanning calorimeter to investigate the relation between viscoelastic properties and PSA performance. © 2006 Wiley Periodicals, Inc. J Appl PolymSci 102: 2839–2846, 2006     

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.     

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.     

The effect of hydrolyzed soy protein isolate on the structure and biodegradability of urea–formaldehyde adhesives

The hydrolyzed soy protein isolate (HSPI) was used to partially substitute urea to synthesis modified urea–formaldehyde (UF) adhesives via copolymerization process, in order to reduce the dependency on petroleum-based chemicals and mitigate possible environmental pollution. The soy protein isolate (SPI), HSPI, and modified UF adhesives were characterized by attenuated total reflection Fourier transform infrared spectroscopy, ¹H nuclear magnetic resonance (¹H NMR), and thermo-gravimetric analysis (TGA). The bonding strength, adhesive properties, biodegradability, and micrographs of the UF and HSPI-modified UF after degradation were also measured. The results show that the SPI native structure is unfolded during the treatment with sodium hydroxide. The thermal stability of HSPI is better than SPI. HSPI can incorporate into the structure of cured UF adhesives with three different feeding methods. And the best bonding strength of modified UF adhesives is 1.31?MPa when HSPI is added at the first step. The formaldehyde emission of modified UF adhesives is lower compared with UF. The earlier the HSPI is added, the better the properties for modified UF adhesives can be obtained. The degradation rate of modified UF adhesives improved nearly two times compared to the UF after six months of degradation in biologically active soil. There are microorganisms adhering to the surface of modified UF from the SEM micrographs.     

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.     

Viscoelastic and adhesive properties of single‐component thermo‐resistant acrylic pressure sensitive adhesives

Poly(butyl acrylate‐vinyl acetate‐acrylic acid) based acrylic pressure sensitive adhesives (PSAs) were synthesized by solution polymerization for the fabrication of high performance pressure sensitive adhesive tapes. The synthesized PSAs have high shear strength and can be peeled off substrate without residues on the substrate at temperature up to 150°C. The PSAs synthesized in the present work are single‐component crosslinked and they can be used directly once synthesized, which is convenient for real applications compared to commercial multi‐component adhesives. The results demonstrated that the viscosity of the PSAs remained stable during prolonged storage. The effects of the preparation conditions such as initiator concentration, cross‐linker amount, organosiloxane monomer amount and tackifier resin on the polymer properties, such as glass transition temperature (T_g), molecular weight (M_w), surface energy and shear modulus, were studied, and the dependence of the adhesive properties on the polymer properties were also investigated. Crosslinking reactions showed a great improvement in the shear strength at high temperature. The addition of tackifier resin made peel strength increase compared to original PSAs because of the improvement of the adhesion strength. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40086.     

Production and characterization of high strength,thin-layered,pulp fiberboard using soy protein adhesives

Making thin-layered fiberboard and recycling the fiberboard materials are two major approaches to save quantities of wood fiber in fiberboard manufacture, which offer both environmental and economic benefits to the society and industry. The objective of this research was to develop high-strength, thin-layered pulp fiberboards (TLPBs) using sodium dodecyl sulfate (SDS)-modified soy protein adhesives for packaging applications. SDS-modified soy protein adhesives demonstrated significantly higher bonding strength than did unmodified soy protein adhesive. Results showed that the TLPB with SDS-modified soy flour adhesive (0.05?g/cm² area density and 0.6?mm of thickness) had stronger tensile strength, similar burst index, and similar or better water soaking properties in comparison to commercial solid fiberboard (1.24?g/cm² area density and 1.7?mm thickness).     

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.     

Novel conductive adhesives for surface mount applications

Electrically conductive adhesives (ECAs) have been explored as a tin/lead (Sn/Pb) solder alternative for attaching encapsulated surface mount components on rigid and flexible printed circuits. However, limited practical use of conductive adhesives in surface mount applications is found because of the limitations and concerns of current commercial ECAs. One critical limitation is the significant increase of joint resistance with Sn/Pb finished components under 85°C/85% relative humidity (RH) aging. Conductive adhesives with stable joint resistance are especially desirable. In this study, a novel conductive adhesive system that is based on epoxy resins has been developed. Conductive adhesives from this system show very stable joint resistance with Sn/Pb‐finished components during 85°C/85% RH aging. One ECA selected from this system has been tested here and compared with two popular commercial surface mount conductive adhesives. ECA properties studied included cure profile, glass transition temperature (T_g), bulk resistivity, moisture absorption, die shear adhesion strength, and shift of joint resistance with Sn/Pb metallization under 85°C/85% RH aging. It was found that, compared to the commercial conductive adhesives, our in‐house conductive adhesive had higher T_g, comparable bulk resistivity, lower moisture absorption, comparable adhesion strength, and most importantly, much more stable joint resistance. Therefore, this conductive adhesive system should have better performance for surface mount applications than current commercial surface mount conductive adhesives. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 399–406, 1999     

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.     

Influence of acrylamide copolymerization of urea–formaldehyde resin adhesives to their chemical structure and performance

To lower the formaldehyde emission of wood‐based composite panels bonded with urea–formaldehyde (UF) resin adhesive, this study investigated the influence of acrylamide copolymerization of UF resin adhesives to their chemical structure and performance such as formaldehyde emission, adhesion strength, and mechanical properties of plywood. The acrylamide‐copolymerized UF resin adhesives dramatically reduced the formaldehyde emission of plywood. The ¹³C‐NMR spectra indicated that the acrylamide has been copolymerized by reacting with either methylene glycol remained or methylol group of UF resin, which subsequently contributed in lowering the formaldehyde emission. In addition, an optimum level for the acrylamide for the copolymerization of UF resin adhesives was determined as 1%, when the formaldehyde emission and adhesion strength of plywood were taken into consideration. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Novel block ionomers. III. Mechanical and rheological properties

Select rheological (dynamic viscoelastic) and mechanical properties of novel block cationomers and anionomers and their blends have been investigated. The block ionomers were linear di‐ and triblocks, and symmetric three‐arm stars comprising hydrophobic polyisobutylene (PIB) blocks attached to ionized poly(methacrylic acid) (PMAA^?X⁺, where X⁺ = Na⁺, Zn²⁺) and poly[2‐(dimethylamino)ethyl methacrylate] (PDMAEMA⁺I^?) blocks. The specific structures investigated were the well‐defined diblocks PIB‐b‐PMAA^? and PIB‐b‐PDMAEMA⁺ and their blends, the triblocks PMAA^?‐b‐PIB‐b‐PMAA^? and PDMAEMA⁺‐b‐PIB‐b‐PDMAEMA⁺ and their blends, and the three‐arm star anionomer Φ(PIB‐b‐PMAA^?)₃. For comparison, the properties of the precursor PIBs and unionized blocks have also been studied. Hydrogen bonding between the carboxyl groups of the PMAA blocks in PIB‐b‐PMAA diblocks leads to inverse micelles. Neutralization of the PMAA by Zn(AcO)₂ and quaternization of the PDMAEMA segments by CH₃I in the triblock copolymers and star copolymers yielded ionic domains, which self‐assemble and produce physical networks held together by coulumbic interaction. The physical/chemical characteristics of the domains control the viscoelastic behavior and mechanical properties of these block ionomers. The mechanical properties of the various block ionomers were significantly enhanced relative to the precursors, and they were thermally stable below the transition temperature. Further, the thermomechanical properties of these novel materials were satisfactory even above 200°C. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1516–1525, 2003     

Effect of natural and synthetic tackifiers on viscosities,adhesion properties and thermal stability of SNR and DPNR solution adhesives

Styrene-grafted natural rubber (SNR) and deproteinized natural rubber (DPNR) latexes were formulated with coumarone-indene (CI), gum rosin and petro resin (PR) tackifiers into solution adhesives with toluene as a solvent. The solution viscosities were evaluated by a Brookfield viscometer DV-II Plus with spindle No. 3. Pressure sensitive adhesives (PSAs) films were made and the adhesion properties were evaluated with loop tack, peel strength and shear strength tests. Thermal stability of the film was evaluated via Perkin-Elmer Pyris 6^TM thermogravimetric analysis at temperatures ranging from 30 to 600?°C at a heating rate of 10?°C per minute in nitrogen environment. Results indicate that as the tackifiers content increased, the solution viscosities increased with SNR/PR and DPNR/PR formulations showing the highest viscosities. Adhesion test also indicates that loop tack and peel strength of the adhesive solution increased but their shear strength decreased; increase of CI tackifier loadings conferred the highest peel strength for both SNR- and DPNR-based PSAs. Thermal analyses show that the addition of 40 phr CI tackifiers improved thermal stability of SNR adhesives based on their higher T_max and integral procedural decomposition temperature properties.     

Role of isophorone diisocyanate in the optimization of adhesion tendency of polyurethane pressure sensitive adhesives

The present work describes the role of accurate selection of diisocyanate on the adhesion strength of polyurethanes (PUs). The concentration of diisocyanate induces the hard segment (HS) in the main architecture of PUs which decides the viscoelastic properties of the polymers. A balanced ratio of viscoelastic properties ultimately determines the adhesion strength. The composition of the polymers consists of a blend of macrodiol of hydroxyl-terminated polybutadiene and polypropylene glycol with different molecular weights. Isophorone diisocyanate (IPDI) is used to develop the urethane linkages by maintaining its contribution from 28 to 67% as HS contents. It determines the adhesion strength of the final product. The adhesion strength is evaluated by texture analyzer and 180° peel test. The probe tack analysis shows maximum adhesion energy of 156.2 J cm⁻² and 180° peel test shows 18.80 N/25 mm peel force. The glass-transition (T _g) values obtained through differential scanning calorimetry are in good agreement with theoretically calculated Flory–Fox temperature. The proportion of the loss tangent to the storage modulus (tan δ/E′) shows the optimum value of 2.80 MPa⁻¹. The ideal concentration of IPDI results to achieve better adhesion properties of PU pressure sensitive adhesives. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47124.     

Lap shear strength and thermal stability of diglycidyl ether of bisphenol a/epoxy novolac adhesives with nanoreinforcing fillers

Nanoreinforcing fillers have shown outstanding mechanical properties and widely used as reinforcing materials associated to polymeric matrices for high performance applications. In this study, a series of multiwalled carbon nanotubes (MWCNTs)‐, nano‐Al₂O₃‐, nano‐SiO₂‐, and talc‐reinforced epoxy resin adhesives composites were developed. The influence of different types and contents of nanofillers on adhesion, elongation at break, and thermal stability (under air and nitrogen atmospheres) of diglycidyl ether of bisphenol A (DGEBA)/epoxy novolac adhesives was investigated. A simple and effective approach to prepare adhesives with uniform and suitable dispersion of nanofillers into epoxy matrix was found to be mechanical stirring combined with ultrasonication. Transmission electron microscopic and scanning electron microscopic investigations revealed that nanofillers were homogeneously dispersed in epoxy matrix at optimized nanofiller loadings. Adhesion strength was measured by lap shear strength test as a function of nano‐Al₂O₃ and MWCNTs loadings. The results indicated that the lap shear strength was significantly increased by about 50% and 70% with addition of MWCNTs and nano‐Al₂O₃ up to a certain level, respectively. The highest lap shear strength was reached at 1.5 wt % of nano‐Al₂O₃ loading. MWCNTs at all loadings (except 3 wt %) and nano‐Al₂O₃ have enhanced onset of degradation temperature and char yield of the adhesives. By combined incorporation of 0.75 wt % nano‐Al₂O₃ and 0.75 wt % MWCNTs into the epoxy novolac/DGEBA blend adhesives a synergistic effect was observed in the thermal stability of the adhesives at high temperatures (800°C). © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 40017.     

Empirical Modeling of Butyl Acrylate/Vinyl Acetate/Acrylic Acid Emulsion‐Based Pressure‐Sensitive Adhesives

Summary: Butyl acrylate/vinyl acetate/acrylic acid (BA/VAc/AA) emulsion latexes were produced in a semi‐batch mode. The objective was to generate polymers with properties favoring their application as pressure‐sensitive adhesives. The influence of the individual monomer concentrations on final properties such as glass transition temperature (T_g), peel strength, shear strength and tack was investigated. To obtain the maximum amount of information in a reasonable number of runs, a constrained three‐component mixture design was used to define the experimental conditions. Latexes were coated onto a polyethylene terephthalate carrier and dried. Different empirical models (e.g. linear, quadratic and cubic mixture models) governing the individual properties (i.e. T_g, peel adhesion, shear resistance and tack) were developed and evaluated. In the given experimental region, no single model was found to fit all of the responses (i.e. the final properties). However, in all models the most significant factor affecting the final properties was the AA concentration, followed by the VAc concentration.

Shear strength contour lines over the investigated region.     

Urea-Modified Soy Globulin Proteins (7S and 11S): Effect of Wettability and Secondary Structure on Adhesion

This investigation characterized wettability and adhesive properties of the major soy protein components conglycinin (7S) and glycinin (11S) after urea modification. Modified 7S and 11S soy proteins were evaluated for gluing strength with pine, walnut, and cherry plywood and for wettability using a bubble shape analyzer. The results showed that different adhesives had varying degrees of wettability on the wood specimens. The 7S soy protein modified with urea had better wettability on cherry and walnut. The 11S soy protein modified with 1M urea had better wettability on pine. The 1M urea modification gave 11S soy protein the greatest bonding strength in all the wood specimens. The 3M urea modification gave 7S soy protein stronger adhesion on cherry and walnut than did 11S protein; but with pine, 11S soy protein had greater adhesion strength than 7S soy protein. Measurement of protein secondary structures indicated that the β-sheet played an important role in the adhesion strength of 3M urea-modified soy protein in cherry and walnut, while random coil was the major factor reducing adhesion strength of 7S soy protein modified with 1M urea.