Soy proteins as plywood adhesives: formulation and characterization

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. 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Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-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 (Received 20 January 2011; final version received 30 August 2011; accepted 7 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 neg-ative 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 formal-dehyde-free soy adhesives showed strong potential as alternatives to commercial formalde-hyde-based wood adhesives. Keywords: soybean protein adhesives; bond strength; water resistance; rheology 1. Introduction The wood industry used more than sixbillion lb adhesive resins, and about 90% of which contained formaldehyde in various forms, such as phenol–formaldehyde (PF), urea–formalde-hyde (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 con-trast, 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 pro-duction 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 2012 US Government 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 6.4MPa, wet bond strength of 3.9MPa, and boiling bond strength of about 2MPa [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 6.8MPa and boiling bond strength of 1.5MPa, and demonstrated potential for indoor applications. Wescott et al. [15] converted soy flour into adhesives using a three-step process: denaturation of soy flour, modi-fication with formaldehyde, and conversion via copolymerization with a cross-linking agent. The adhesive containing 40% soy flour showed comparable performance to commercial form-aldehyde resin for binding random strandboard panels. In our group, the new viscous cohe-sive 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 resis-tance 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 soy-bean seed protein. β-conglycinin is a trimer with a molecular mass of 150–200kDa and glycinin is a hexamer with a molecular mass of about 340–375kDa [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 2 to 10 and ionic strength higher than 0.1, β-conglycinin is a trimeric glyco-protein 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 5 and higher but dissociates into smaller molecular weight fractions at pH 2–5 [19]. Soy protein functional properties including water-binding 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 adhe-sives with improved bond strength; and (iii) characterize the adhesives’ rheological properties. 2. 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 50mm (width)127mm (length) 5mm (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 2N HCl. Precipitates were collected, washed twice with distilled water, freeze-dried, then milled (Cyclone Sample Mill, UDY 2016 X. Mo and X.S. Sun 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 vari-ables were selected for this study: SPI content range from 9.8 to 38.8% (total mixture weight basis), sodium chloride concentration range from 0 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 equa-tions for the three responses were derived by using backward stepwise selection to drop terms that were insignificant at PP0.1%. 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 1N NaOH or 1N 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 sevendays before use. The soy protein adhesive (600mg) was then brushed onto a marked area (50127mm) of the wood sample. Two wood pieces were prepared, allowed to rest at room temperature for 5min, 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 press-ing for 10min at 4.9MPa, 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 threedays, cut into 2050mm pieces, and further conditioned for fourdays before test-ing 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 proper-ties of adhesives in two-ply wood construction in shear by tension loading [22]. Crosshead speed for testing was 1.6mm/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 48h. 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 4h, dried at 63°C for 20h, subjected to boiling water again for another 4h, and then, cooled in running water at room temperature for 1h. Specimens were tested for shear strength immediately after cooling and the shear strength was reported as boiling strength. 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 exper-iment, apparent viscosity was measured as shear rate ranged from 0.5 to 50s1. Small defor-mation oscillatory measurements were performed using strain sweep tests from 0.1 to 100% at 25°C, and a frequency of 1Hz was used to determine the linear viscoelastic region of sam-ples. 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 0.10–10Hz. 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 tempera-ture 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). 3. 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 interac-tion, 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 sub-units: α′, α, and β subunits. During thermal curing, acidic polypeptides are released from gly-cinin [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-con-glycinin 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 per-formance 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): ... - tailieumienphi.vn