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SEM examination of the various composites showed little differences in their surface topography (Fig. 7). However, the cut edges showed the outlines of the particles sizes employed in the composition of the panels (Fig. 7). DDGS coating of the particles often obscured the wood particles (Fig. 7). Deeper fissures occurred in the micrographs of the DDGS-PW composites containing the larger particles sizes compared to that found in the other composites (Fig. 7 b and n). These fissures were often associated with poor interfacial adhesion between ingredients and resulted in lower flexural properties (Fig 6). The interaction between the DDGS and wood particle size was responsible for flexural properties of the composite. The DDGS matrix somewhat resembled a thermoset matrix, which mimics SBM74 matrix/adhesives. The mode of bio-based adhesion is unclear and complex (Frihart HW DO . 2010; Frihart 2011; Frihart and Birkeland 2014; Frihart HWDO . 2014). The adhesive nature of SBM is believed to be attributable to the protein composition (Frihart HWDO . 2010; Frihart 2011; Frihart and Birkeland 2014; Frihart HWDO . 2014). Soya seed proteins represent 30 to 50% of the seed mass with storage proteins accounting for 65 to 80% of the total proteins. The main storage proteins in SBM are quaternary globulins, glycinin, and conglycinin (Frihart and Birkeland 2014; Wolf 1970). DDGS is chemically dissimilar from SBM. In addition, yeast contributes 5.3% of the protein content of DDGS (Lim and Yildirim-Aksoy 2008). Corn meal proteins consist of 14% albumin/globulin, 35 to 40% zein (a prolamine protein), and 30% glutelin. It is conjectured that soy protein adhesive properties occur through the denaturation of the quaternary globulins into tertiary structures and crystalline secondary st ructures, α -helices, and β -sheets (Frihart 2010; Frihart and Birkeland 2014; Frihart HWDO 2014). These crystalline secondary protein structures are suspected to provide the optimum adhesive properties that bind protein to wood (Frihart 2010; Frihart and Birkeland 2014). No crystalline structures in the XRD studies with DDGS were observed which suggested a different method of protein adhesion may occur. Denaturation of proteins can be achieved through a variety of methods such as heat, alkali, or chemical modification (Frihart and Birkeland 2014; Frihart HWDO . 2014). DDGS powder contained 30% protein and was originally a solid and through the application of heat and pressure undergoes a “ phase change or transition ” and becomes a “liquid - gel” that binds with wood. These events are recognized as typical in the adhesive process (Adhesives.org 2017). Apparently, the DDGS proteins are denatured under pressure and heat to a state that can then bind to wood. Upon cooling, DDGSs transitions back into a solid, which cannot be melted again. It should be noted that employing a high moisture DDGS-PW formulation results in explosive stream generation and ultimately an unacceptable composite exhibiting excessive blistering. Therefore, a relatively dry DDGS-PW formulation was employed which was prepared by mixing the ingredients dry and not pre-stirring DDGS in any liquids as commonly reported in the fabrication of SBM biocomposites (Zhong HWDO . 2001; Amaral-Labat HW DO . 2008; Jeon HWDO . 2011; Reddy and Yang, 2011; Gu HWDO . 2013). In addition, in the preparation method steam was allowed to escape during the molding process by short releases of the molding pressure. Further, it was found that rapid cooling resulted in blistering and internal cracking of the DDGS-PW composites, while slow cooling the composite produced a non-blistered composite (Fig. 8). Blistering is a common problem in thermoset materials (Plenco 2015). Thermoset materials cure as a result of a chemical reaction and are affected by temperature and pressure (Plenco 2015). Blistering is often due to areas of gas trapped beneath the surface. One of the methods commonly employed to address this problem in injection molding is to decrease
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Tisserat et al . (2018). “DDGS - PW fiberboards” B io R esources 13(2), 2678-2701.
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