Soy proteins as plywood adhesives: formulation and characterization 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.
This article was downloaded by: [Colorado College] On: 09 February 2015, At: 17:56 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Journal of Adhesion Science and Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tast20 Soy proteins as plywood adhesives: formulation and characterization Xiaoqun Mo & Xiuzhi Susan Sun a a Department of Grain Science and Industry , Bio-Materials and Technology Lab, BIVAP, Kansas State University , Manhattan , KS , 66506 , USA Published online: 10 Aug 2012 To cite this article: Xiaoqun Mo & Xiuzhi Susan Sun (2013) Soy proteins as plywood adhesives: formulation and characterization, Journal of Adhesion Science and Technology, 27:18-19, 2014-2026, DOI: 10.1080/01694243.2012.696916 To link to this article: http://dx.doi.org/10.1080/01694243.2012.696916 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content This article may be used for research, teaching, and private study purposes Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/termsand-conditions Journal of Adhesion Science and Technology, 2013 Vol 27, Nos 18–19, 2014–2026, http://dx.doi.org/10.1080/01694243.2012.696916 Soy proteins as plywood adhesives: formulation and characterization Xiaoqun Mo and Xiuzhi Susan Sun* Department of Grain Science and Industry, Bio-Materials and Technology Lab, BIVAP, Kansas State University, Manhattan, KS 66506, USA Downloaded by [Colorado College] at 17:56 09 February 2015 (Received 20 January 2011; final version received 30 August 2011; accepted June 2012) 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 Keywords: soybean protein adhesives; bond strength; water resistance; rheology Introduction The wood industry used more than six billion lb adhesive resins, and about 90% of which contained formaldehyde in various forms, such as phenol–formaldehyde (PF), urea–formaldehyde (UF), and melamine–formaldehyde (MF) [1] Formaldehyde emission during adhesive production and application causes environmental pollution and is detrimental to human health Moreover, formaldehyde adhesives are produced from petroleum, a limited resource In contrast, soy-based adhesives are made from soybean seed, which is abundant and renewable Soy-based adhesives have shown great potential as alternatives to petroleum-based adhesives Soy-based adhesives have been used to bind veneer panels and to fabricate fiberboard and straw particleboard [2–4] Columbia Forest Products, NC, the largest producer of decorative interior panels in the USA is currently converting fully to soy-based adhesives in their production of veneer-core panels [5] However, overall performance of soy-based adhesives remains inferior to commercial formaldehyde resin-based adhesives, especially in bond strength and water resistance *Corresponding author Email: xss@ksu.edu Ó 2012 US Government Downloaded by [Colorado College] at 17:56 09 February 2015 Journal of Adhesion Science and Technology 2015 Recent efforts have been devoted to improving the bond strength of soy-based adhesives Modifying soy protein using denaturation reagents such as urea, guanidine hydrochloride, and sodium dodecyl sulfate has led to improved bond strength [3,6,7] Soy protein modified with enzymes including trypsin [8], papain and urease [9], cross-linking reagent glutaraldehyde [10], or cationic detergents [11] showed improved bond strength Polyamide epichlorohydrin (PAE), a wet-strength resin widely used in the paper and pulp industry, also has been used to modify soy protein Wood panels bonded with soy protein–PAE adhesives had dry bond strength of 6.4 MPa, wet bond strength of 3.9 MPa, and boiling bond strength of about MPa [12,13], and performed better than that of commercial UF adhesives Liu and Li developed modified soy protein adhesives, which involved two modification steps [14] First, soy protein isolate (SPI) was modified with maleic anhydride (MA) to form MA-grafted SPI, which was then modified by polyethylenimime (PEI) Optimum MSPI–PEI adhesive could be made from 20% PEI and 80% MSPI, gave a dry bond strength of about 6.8 MPa and boiling bond strength of 1.5 MPa, and demonstrated potential for indoor applications Wescott et al [15] converted soy flour into adhesives using a three-step process: denaturation of soy flour, modification with formaldehyde, and conversion via copolymerization with a cross-linking agent The adhesive containing 40% soy flour showed comparable performance to commercial formaldehyde resin for binding random strandboard panels In our group, the new viscous cohesive soy protein adhesive system modified with sodium bisulfide was successfully developed, with high solid content of 38%, good flowability, long shelf life, and excellent water resistance comparable to formaldehyde-based adhesives [16] Those modifications have significant effects on soy protein, which consists of two major components, β-conglycinin and glycinin Together, they constitute about 50–90% of total soybean seed protein β-conglycinin is a trimer with a molecular mass of 150–200 kDa and glycinin is a hexamer with a molecular mass of about 340–375 kDa [17] The quaternary structures of soy proteins are affected by pH and ionic strength Glycinin forms a hexamer at pH 7.6 and ionic strength of 0.5, but it exists mainly as a trimer at pH 3.8 and ionic strength of 0.03 [18] In the pH range from to 10 and ionic strength higher than 0.1, β-conglycinin is a trimeric glycoprotein consisting of three subunits in at least six different combinations At an ionic strength less than 0.1, β-conglycinin exists as a hexamer at pH and higher but dissociates into smaller molecular weight fractions at pH 2–5 [19] Soy protein functional properties including waterbinding capacity, solubility, viscosity, and gelation also are affected by pH and ionic strength [20] The objectives of this study were to: (i) investigate the effects of SPI concentration, sodium chloride concentration, and pH on soy adhesives performance; (ii) develop soy adhesives with improved bond strength; and (iii) characterize the adhesives’ rheological properties Materials and methods 2.1 Materials Defatted soy flour with a protein dispersion index of 90 was obtained from Cargill (Cedar Rapids, IA, USA) Cherry wood samples with dimensions 50 mm (width) Â 127 mm (length) Â mm (thickness) were obtained from Veneer One (Oceanside, NY, USA) Orientation of the wood grain was perpendicular to the length of the wood samples 2.2 Preparation of soy protein isolate (SPI) Soy flour was extracted with 15-fold water at pH 8.0 and centrifuged to remove insoluble material The pH of the extract was adjusted to 4.2 with N HCl Precipitates were collected, washed twice with distilled water, freeze-dried, then milled (Cyclone Sample Mill, UDY 2016 X Mo and X.S Sun Downloaded by [Colorado College] at 17:56 09 February 2015 Corp., Fort Collins, CO, USA) into powder, and collected as SPI, which had a protein content of 93% (dry basis, db) and moisture content of 5.8% 2.3 Experimental design The central composite design approach was used to study the effects and interactions of SPI concentration, sodium chloride, and pH on bond performance The central composite design is a type of response surface methodology that focuses on characteristics of the fit response function where the optimum estimated response values occur [21] Three independent variables were selected for this study: SPI content range from 9.8 to 38.8% (total mixture weight basis), sodium chloride concentration range from to 4%, and pH range from 3.5 to 8.3 The ranges of variables were selected based on preliminary experiments The levels of variables were defined by central composite design The design had 18 treatments with four replications at the center point Three responses were measured: dry bond strength, wet bond strength, and boiling bond strength Data were analyzed by the response surface regression procedure of SAS software (version 9.0, SAS Institute, Cary, NC, USA), and the final regression equations for the three responses were derived by using backward stepwise selection to drop terms that were insignificant at P P 0.1% 2.4 Adhesive and specimen preparation SPI adhesives were prepared with different sodium chloride concentration levels The pH of adhesives was adjusted to various values by adding N NaOH or N HCl Cherry wood samples were preconditioned in a controlled environment chamber (Electro-Tech Systems, Inc., Glenside, PA, USA) at 23 °C and 50% relative humidity (RH) for at least seven days before use The soy protein adhesive (600 mg) was then brushed onto a marked area (50 Â 127 mm) of the wood sample Two wood pieces were prepared, allowed to rest at room temperature for min, then assembled, and pressed using a hot press (Model 3890 Auto ‘M’; Carver Inc., Wabash, IN, USA) that had been preheated to 190 °C After pressing for 10 at 4.9 MPa, the specimen was removed promptly from the hot press and cooled to room temperature Wood specimens were preconditioned at 23 °C and 50% RH for three days, cut into 20 Â 50 mm pieces, and further conditioned for four days before testing for dry bond strength 2.5 Shear bond strength measurements Wood specimens for shear bond strength testing were prepared and tested using an Instron (Model 4465, Canton, MA, USA) according to the standard test method for strength properties of adhesives in two-ply wood construction in shear by tension loading [22] Crosshead speed for testing was 1.6 mm/min Stress at maximum load was recorded Reported results were the average of five samples 2.6 Water resistance measurements Wet bond strength was evaluated according to standard test methods for determining effects of moisture and temperature on adhesive bonds [23] Preconditioned specimens were soaked in water at 23 °C for 48 h Wet bond strength was measured immediately after soaking The boiling test was carried out according to ASTM D5572 method [24] Preconditioned Journal of Adhesion Science and Technology 2017 specimens were soaked in boiling water for h, dried at 63 °C for 20 h, subjected to boiling water again for another h, and then, cooled in running water at room temperature for h Specimens were tested for shear strength immediately after cooling and the shear strength was reported as boiling strength Downloaded by [Colorado College] at 17:56 09 February 2015 2.7 Rheological measurements Both large deformation and small deformation shear rheological experiments were conducted on a Bohlin Rheometer System CVOR 150 (Bohlin Rheology Inc., Cranbury, NJ, USA) with cone-plate geometry All samples were maintained at 25 °C during testing Silicone oil was applied around the plate edges to prevent sample dehydration In the large deformation experiment, apparent viscosity was measured as shear rate ranged from 0.5 to 50 sÀ1 Small deformation oscillatory measurements were performed using strain sweep tests from 0.1 to 100% at 25 °C, and a frequency of Hz was used to determine the linear viscoelastic region of samples Frequency sweeps were measured at 25 °C and a strain of 1%, which was found within the linear viscoelastic region, in a frequency range of 0.10–10 Hz The corresponding storage modulus (G′) and loss modulus (G′′) were measured 2.8 Transmission electron microscopy (TEM) Protein adhesives were diluted to 1% and adsorbed for approximately 30 s at room temperature onto Formvar/carbon-coated 200-mesh copper grids (Electron Microscopy Science, Fort Washington, PA, USA) The samples were stained with 2% (w/v) uranyl acetate (Ladd Research Industries, Inc., Burlington, VT, USA) for 60 s at room temperature before being viewed by TEM (model CM 100, FEI Company, Hillsboro, OR, USA) Results and discussion 3.1 Effects of SPI content, sodium chloride, and pH on the shear bond strength: a model study Bonding between adhesive and wood is attributed to a combination of three mechanisms: mechanical interlocking, physical interaction, and covalent chemical bonding [25] When applied to wood, protein adhesives spread, wet, and penetrate the wood surface; achieve close contact with different molecules in wood; and form mechanical interlocking, physical interaction, and chemical bonding during the thermal curing process Soy protein is composed of an array of polypeptides with different molecular sizes One of the major components, glycinin, consists of six subunits in which six acidic and six basic polypeptides are joined by disulfide bonds The secondary major component, β-conglycinin, is composed of three types of subunits: α′, α, and β subunits During thermal curing, acidic polypeptides are released from glycinin [26] and trimeric β-conglycinin completely dissociates into its subunits [27] Basic polypeptides in glycinin are polymerized through disulfide bonds and the β subunits in β-conglycinin associate among themselves and also with a basic polypeptide through secondary forces, while acidic polypeptides and α′ and α subunits form oligomers [28] After curing, protein polymers are entangled with each other and bond with wood through mechanical interlocking, hydrogen bonding, van der Waals forces, or covalent bonds [25] Adhesive performance depends on both adhesiveness and cohesiveness of protein polymers Shear strength testing was carried out to evaluate adhesive performance The regression equation showed that protein, pH, and sodium chloride all had significant effects on dry bond strength as expressed in Equation (1): 2018 X Mo and X.S Sun (MPa) Dry Bond Strength 4 40 (%) ion t a r 10 ent onc IC P S 20 Figure Dry bond strength as affected by pH and SPI concentration at 0.5% sodium chloride (dry bond strength = 3.226 + 0.1059x + 0.1867yÀ0.00312y2, R2 = 0.68, where x = pH, y = SPI concentration%) Dry bond strength ẳ 3:363 ỵ 0:1059x ỵ 0:1867y 0:00312y2 0:2726z; R2 ẳ 0:68; 1ị where x = pH, y = SPI concentration%, and z = sodium chloride concentration% At 0.5% sodium chloride, dry bond strength increased as pH increased (Figure 1) However, SPI content affects dry bond strength differently: dry bond strength increased as SPI concentration increased, but only up to 25–30% concentration (maximum level), beyond which it declined (Figure 1) Similar trends were observed for dry bond strength at the other sodium chloride concentrations (data not shown) At a fixed SPI concentration of 28%, dry bond strength increased with pH but decreased as sodium chloride concentration increased (Figure 2) 7.0 Dry Strength (MPa) Downloaded by [Colorado College] at 17:56 09 February 2015 pH 30 6.5 6.0 5.5 pH ) l (% C Na Figure Dry bond strength as affected by pH and sodium chloride at 28% SPI concentration (Dry bond strength = 6.143 + 0.1059xÀ0.2726y, R2 = 0.68, where x = pH, y = sodium chloride concentration%) 2019 Wood is a polar cellulosic material with hydroxyl groups; therefore, polar adhesives yield good adhesion to wood Soy proteins are comprised of 20 amino acids with ionizable residues and have an isoelectric point around pH 4.2 At higher pH levels (e.g 8.0), proteins carry more negative charges and become more polar than at lower pH levels (e.g 6.0) At a high pH, more hydrogen bonds are formed between polar hydroxyl groups on the wood fibers and protein adhesives, resulting in increased dry bond strength Previous studies on soy protein adhesives for plywood usually applied the adhesive at about 11% SPI concentration [6], which is much lower than the optimum SPI concentration (i.e 25–30%) found in this study With higher SPI concentration, more protein molecules are available to interact with wood fibers This interaction forms increased amounts of mechanical interlocks and physical interactions and results in enhanced dry bond strength As SPI concentration increased beyond the optimum range, protein–protein interactions dominated over protein–wood interactions As a result, adhesive wettability, which is essential for adhesive penetration and diffusion, was reduced, resulting in reduced dry bond strength Water resistance is an important property that determines adhesive bond durability for exterior applications Wet test and boil test were used to evaluate the adhesive’s water resistance SPI concentration, pH, and sodium chloride all significantly affected wet bond strength as expressed in Equation (2): Wet bond strength ẳ 2:671 0:205x ỵ 0:1867y 0:0037y2 0:248z; R2 ẳ 0:76; 2ị (MPa) where x = pH, y = SPI concentration%, and z = sodium chloride concentration% Similar to dry bond strength, an optimum protein concentration range of 20–30% was identified for maximum wet bond strength (Figure 3) The regression equation also showed that a negative linear relationship existed between wet strength and sodium chloride Unlike dry strength, wet bond strength decreased as pH increased Regression Equation (3) showed that both protein and pH affected boiling bond strength, calculated as: Wet Bond Strength Downloaded by [Colorado College] at 17:56 09 February 2015 Journal of Adhesion Science and Technology 40 30 pH 20 nt ce 10 I SP n Co ) n(% io rat Figure Wet bond adhesion strength as affected by pH and SPI concentration at 0.5% sodium chloride (wet bond strength = 2.547À0.205x + 0.1867yÀ0.0037y2, R2 = 0.76, where x = pH, y = SPI concentration%) X Mo and X.S Sun ngh Boiling Bond Stre (MPa)t 2020 pH Downloaded by [Colorado College] at 17:56 09 February 2015 40 30 20 10 SPI tion ntra ce Con (%) Figure Boiling bond strength as affected by pH and SPI concentration (boiling bond strength = À3.159 + 0.8818xÀ0.07168x2 + 0.2231yÀ0.00394y2, R2 = 0.65, where x = pH, y = SPI concentration%) Boiling bond strength ẳ 3:159 ỵ 0:8818x 0:07168x2 ỵ 0:2231y 0:00394y2 ; R2 ẳ 0:65; 3ị where x = pH and y = SPI concentration% Sodium chloride was not included in the equation, suggesting that sodium chloride did not significantly affect boiling bond strength A pH range of 5–6 yielded maximum boiling bond strength Similar to dry and wet bond strengths, optimum SPI concentration was around 25–30% (Figure 4) Wet bond strength was measured immediately after glued wood veneer was soaked in water for 24 h During soaking, water molecules penetrate glued areas and interact with different molecules in wood and protein adhesives through hydrogen bonding, dipoles, and induced dipoles as well as dispersion forces [25] Water reduces the amount of hydrogen bonding between the adhesive and wood fiber, resulting in weakened adhesion bonds and reduced adhesion strength For example, maximum dry bond strength predicted by the model was about 6.75 MPa, whereas maximum predicted wet bond strength was about 4.0 MPa Bond strength decreased about 40% after water soaking, suggesting the important contribution of physical interactions, including hydrogen bonding and van der Waals forces between wood and adhesive for dry bond strength Addition of sodium chloride reduced soy-based adhesive wet bond strength but had no effect on boiling bond strength (Equations (2) and (3)) Sodium chloride acts on protein through interaction with charged and polar groups of amino acid residues by shielding effects [29], and may interfere with the ability of protein adhesives to cure and bond with wood substrate, thus reducing adhesion strength The boiling test is a harsh water resistance test that subjects samples to boil-dry-boil cyclic conditions Only adhesion bonds that survived the boiling cycle contributed to boiling bond strength Maximum predicted boiling bond strength was 2.8 MPa, about 70% of wet bond strength Some adhesion bonds that contribute to wet bond strength, including those affected by sodium chloride, were broken after the boiling test cycle As a result, sodium chloride showed no effect on boiling bond strength Downloaded by [Colorado College] at 17:56 09 February 2015 Journal of Adhesion Science and Technology 2021 Figure Contour plots of boiling bond strength overlaid on the contour plots of wet bond strength with no sodium chloride (A) and 0.5% sodium chloride (B); wet bond strength (—); boiling bond strength (- - -) 3.2 Optimum formulation of adhesive for water resistance One objective of this study was to develop soy protein-based adhesives with improved water resistance Both wet and boiling bond strengths were considered when formulating soy protein-based adhesives for optimum water resistance When contour plots for wet and boiling bond strengths were overlaid for adhesives with no sodium chloride addition (SPI) or with 0.5% sodium chloride (SPI–NaCl), optimum SPI concentration and pH zone (shaded area) for the adhesive formulations were located (Figure 5(A) and (B)) The optimum zone for SPI adhesives was located at about pH 5.0–6.7 and SPI content of 23–32%, while for SPI–NaCl adhesives, the zone was at about pH 4.9–5.8 and SPI content of 24–31% Within the optimum zone of interest in Figure 5, variables of pH 5.5 and 28% SPI content were selected to validate the experimental test on soy protein adhesives Data on the bond strength of SPI and SPI–NaCl (Table 1) showed that the dry, wet, and boiling bond strengths for both were close to values predicted by the regression equations Also, the bond strengths of the two soy adhesives were not significant (Table 1) Dry bond strengths of the soy protein adhesives were similar to that of commercial PF adhesive and higher than that of UF adhesive Water resistance of soy adhesives remained inferior to PF but was better than UF Wood veneer bonded by UF was delaminated after the boil test, whereas soy adhesive had boiling bond strength of about 2.7 MPa Therefore, soy protein adhesives with performance similar to or better than commercial UF can be produced for plywood applications Table Bonding properties of soy adhesives and commercial plywood adhesives (UF and PF) Bond strength (MPa) Adhesives SPI SPI with 0.5% NaCl UF PF Dry* a 6.52 (6.73) 6.57a (6.59) 5.00b 6.20a Wet b 3.67 (3.87) 3.55b (3.75) 3.46b 4.54a Values followed by the same letters in the same column are not significantly different (p < 0.05) SPI = soy protein isolate, UF = urea–formaldehyde; PF = phenol–formaldehyde *Predicted values from regression equations are reported in parentheses Boiling 2.84b (2.68) 2.66b (2.68) Delaminated 3.59a 2022 X Mo and X.S Sun Apparent Viscosity (Pa s) 160 140 120 Control 0.5% NaCl 100 80 60 40 20 0 10 20 30 40 50 -1 Downloaded by [Colorado College] at 17:56 09 February 2015 Shear Rate (S ) Figure 3.3 Apparent viscosity of soy adhesives as affected by shear rate Rheological properties Knowledge of rheological properties of protein adhesives is important for practical applications of adhesives Apparent viscosities of soy adhesives were evaluated across a range of shear rates Viscosity for soy adhesives decreased as shear rate increased (Figure 6) At low shear rates, not all protein molecules are oriented in the direction of flow, causing higher apparent viscosity At higher shear rates, molecules become oriented in the direction of flow, resulting in decreased apparent viscosity Rheology of soy adhesives can be expressed by the power law model η = K c_ nÀ1 , where c_ is the shear rate, K is the consistency index, and n is the flow index To have comparative results, η of samples at 10.77 sÀ1 were reported (Table 2) SPI–NaCl adhesive had apparent viscosity that was four times less than that of the SPI adhesive The lower flow behavior index and consistency index of SPI–NaCl adhesive also indicated that it was more shear thinning and easier to process and apply Through shielding effects, sodium chloride prevents water molecules from interacting with protein molecules, which decreases protein hydration potential and thus decreases hydrodynamic volume, resulting in reduced viscosity [29] Viscoelasticity of soy adhesives was studied by analyzing the storage (G′) and loss (G′′) moduli as a function of strain (Figure 7) At low strains, both G′ and G′′ were flat, indicating that the adhesives were in the linear viscoelastic region However, with increasing strain, nonlinearity set in for G′ Subsequently, G′ and G′′ crossed and at larger strains, G′′ became larger than G′ SPI adhesive had significantly higher storage and loss moduli than SPI–NaCl adhesive in the strain range studied The yield point was identified as the intersection point by drawing an asymptote through the low and large strain values of G′ (Figure 7) Yield Table Rheological properties of soy adhesives Adhesives SPI SPI with 0.5% NaCl Viscositya (Pa s) Flow behavior index n Consistency index K 19.0 5.0 75.8 24.5 0.483 0.425 SPI = soy protein isolates a Viscosity was measured at a shear rate of 10.77 sÀ1 Journal of Adhesion Science and Technology 2023 10 G', G" (Pa) 10 10 Yield strain 10 -3 -2 10 10 -1 10 Strain (%) Figure Effects of increasing strain amplitude on the storage modulus G′ and loss modulus G′′ of SPI adhesives (j, G′ of SPI; d, G′′ of SPI; Ã, G′ of SPI–NaCl; s, G′′ of SPI-NaCl) stress of SPI–NaCl adhesive was 68.7 Pa, about one-fourth that of SPI adhesive (287.8 Pa), whereas yield strain of SPI adhesives (0.141) was higher than that of SPI–NaCl adhesive (0.108) Both G′ and G′′ of soy adhesives exhibited frequency dependence (Figure 8) Lower G′ values were obtained at lower frequencies than those at higher frequencies because more protein–protein bonds have the opportunity to become stress free during the periodic deformation at larger experimental time scales Also, G′ was larger than G′′ over the entire frequency range observed The frequency spectrum of storage and loss modulus provides a signature of material state The slope of Log G′ vs Log frequency should be for pure elastomers [30] Adhesive with 0.5% NaCl showed a value of 0.26, while SPI adhesive had a value of 0.33, indicating that the former had a more gel-like structure than the latter SPI–NaCl showed a lower elastic modulus than SPI adhesive, indicating reduced protein interactions As shown by TEM, SPI adhesive formed large, irregular protein aggregates, while SPI–NaCl adhesive 10 G', G" (Pa) Downloaded by [Colorado College] at 17:56 09 February 2015 10 10 10 -1 10 10 10 Frequency (Hz) Figure Dynamic frequency spectra of storage modulus G′ and loss modulus G′′ of soy adhesives (j, G′ of SPI; d, G′′ of SPI; Ã, G′ of SPI–NaCl; s, G′′ of SPI–NaCl) 2024 X Mo and X.S Sun Downloaded by [Colorado College] at 17:56 09 February 2015 Figure TEM images of soy protein adhesives: (A) nonmodified SPI at pH 5.5 and (B) NaClmodified SPI at pH 5.5 had much smaller, dispersed aggregates (Figure 9) This finding suggests that NaCl reduces protein–protein associations, which is in agreement with the reduced storage modulus observed in the dynamic rheology study Conclusion The protein concentration and chemical environment (pH and NaCl) significantly affected adhesion and rheological properties of the soy adhesives The optimum adhesive protein concentration range and pH range for maximum adhesion strength were identified as SPI content of 23–32%, pH 5.0–6.7 Sodium chloride effectively shielded protein molecules’ multiple ionic charges, which had negative effects on dry and wet bond strengths, and reduced viscosity of soy adhesives by reducing the protein hydrodynamic volume Small deformation rheology and TEM study indicated that sodium chloride reduced adhesive storage modulus because of formation of small, dispersed protein aggregates Soy adhesives with 0.5% NaCl had bond strength comparable to that of soy adhesive with no NaCl added and an apparent viscosity of about Pa s, which meets specifications required for industrial adhesives [31] On cherry wood plywood panels, soy-based formaldehyde-free adhesives had higher dry strength than PF adhesives and better water resistance than UF Therefore, soy protein-based adhesive is an ideal candidate for plywood applications Acknowledgment Authors greatly appreciate the financial support from USDA Critical Agricultural Materials (This is a contribution no 08-379-J from the Kansas Agricultural Experiment Station, Manhattan, Kansas 66506.) 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Science and Technology, 2013 Vol 27, Nos 18–19, 2014–2026, http://dx.doi.org/10.1080/01694243.2012.696916 Soy proteins as plywood adhesives: formulation and characterization Xiaoqun Mo and Xiuzhi... decreases protein hydration potential and thus decreases hydrodynamic volume, resulting in reduced viscosity [29] Viscoelasticity of soy adhesives was studied by analyzing the storage (G′) and