Water absorption is due to the hydrogen bonding of water molecules to the hydroxyl groups on the cells walls of the wood or lignocellulosic fibers.4870 The long-term water absorption as a function of time for the various LPCs at room temperature is shown in Fig. 13.5. All composites tested absorbed water during the incubation peri­od and no distinct saturation levels were achieved after 872 h of soaking (Fig. 13.5). The HDPE and HDPE-MAPE samples exhibited inconsequential weight gains (i. e., less than a 1% increase) after the immersion incubation time (872 h) compared to the biocomposites (Table 13.4; Fig. 13.5). Absorption of water by composites is a crucial factor in evaluating the ability of biocomposite to be commercially used.87172 To improve the resistance to water absorption inclusion of MAPE into the compos­ite formulation is routinely conducted.7172 However, we did not confirm this situa­tion for the composites used in this study. In fact, in some cases inclusion of MAPE in the formulation actually resulted in greater water absorption than that from com­posites without MAPE. For example, HDPE-25DDGS and HDPE-25DDGS-MAPE exhibited weight gains of 1.1 and 1.8%, respectively. This trend was observed for several STDDGS, PW and STPW formulations (Table 13.4; Fig. 13.5). These re­sults are somewhat surprising, since several other investigators have reported that inclusion of maleate olefins with the composite blend considerably reduces wa­ter absorption when using bio-fillers such as with Paulownia wood, loblolly wood, pine wood, sisal fiber, and wheat.437173 Practically no difference in water absorp­tion rates occurred between the HDPE-PW and HDPE-PW-MAPE formulations or the HDPE-STPW and HDPE-STPW-MAPE formulations (Table 13.4; Fig. 13.5). This observation seems counterintuitive and contrary to prior observations yet it is clearly demonstrated in this study with the PW formulations employed.437173 One explanation for the discrepancy between this study and others could be the thick­ness of the tensile bar formulations, method of processing and/or injection molding procedures employed in various studies.

The various composites exhibited distinctly different tensile bar thickness. Gen­erally, the gate section of the tensile bar is slightly thicker than the neck, which in turn is thicker than the end section (Table 13.5). This is due to the method the plastic resin and composite blend is injected into the mold; the gate portion is subjected to more injection time than the end portion and therefore contains more plastic resin than the other portions of the tensile bar. For example, the HDPE-25DDGS com­posite tensile bar exhibits an initial gate, neck and end section thickness of 3.18, 3.13 and 3.11 mm, respectively (Table 13.5). No significant increase in thickness for the tensile bar sections (gate, neck or end) of the neat HDPE and HDPE-MAPE were observed. However, increases in thickness could occur in the various com­posite formulations tested when given a soaking treatment (Table 13.5). Signifi­cant increases in thickness for the tensile bar gate, neck and end sections occurred in the HDPE-DDGS composites (e. g., HDPE-25DDGS and HDPE-25STDDGS, HDPE-25DDGS-MAPE and HDPE-25STDDGS-MAPE (Table 13.5). In contrast, the HDPE-PW composites (HDPE-25PW, HDPE-25PW-MAPE, HDPE-25STPW, and HDPE-25STPW-MAPE) only exhibited significant increases in the end section of the tensile bar. We attribute this occurrence to the presence of more PWF and less HDPE in the end portion of the tensile bar compared to that occurring in the gate and neck sections. Bio-fillers are hydrophilic in nature due to the presence of the abundant hydroxyl groups on the cellulose, lignin and hemicellulose which readily interacts with water molecules by hydrogen bonding.43 DDGS composites were ini­tially thicker than PW composites and when soaked for 872 h exhibited higher per­centages in thickness increases than PW composites (Table 13.5). We can conclude that DDGS composites are less dimensionally stable than PW composites. Inclusion of MAPE into the DDGS or PW composites did not notably alter the thickness measurements of the initial tensile bars compared to formulation without MAPE. Soaked composites containing MAPE exhibited slightly less thickness increases in terms of percent thickness increases than composites without MAPE (Table 13.5). Similarly this trend was also observed for “combination” composite mixtures (HDPE-12.5STDDGS/12.5PINEW and HDPE-12.5STDDGS/12.5PINEW-MAPE) where the inclusion of MAPE in the formulation reduced the thickness increase compared to the formulation without MAPE. This observation conforms to previous observations where inclusion of a maleate polyolefin in the formulation increases dimensional stability of the resulting composites through the binding of the cou­pling agent with the hydroxyl groups of the filler thereby preventing fillers binding to water molecules.43

Chemical modification treatments (A and AM) performed on the HDPE-STD — DGS formulations (HDPE-25STDDGS/A, HDPE-25 STDDGS/A-MAPE, HDPE — 25STDDGS/AM, and HDPE-STDDGS/AM-MAPE) did not improve absorption rates (% weight gain) compared to untreated controls (HDPE-25STDDGS) (Table 13.4). Thickness of DDGS formulations that were chemically modified (A and AM) were less initially than the untreated formulation. However, both formulations in­creased significantly following soaking (Table 13.5). In contrast, chemical modifi­cation treatments (A and A/M) of HDPE-STPW formulations (HDPE-25STPW/A, HDPE-25STPW/A-MAPE, HDPE-25STPW/AM, and HDPE-STPW/AM-MAPE) exhibited less weight gain compared to un-modified controls (HDPE-25STPW and HDPE-25STPW-MAPE) (Table 13.4). Thickness of STPW formulations that were chemically modified (A and AM) were initially comparable to untreated controls (HDPW-25STPW). However, all soaked STPW composites whether chemically modified or not exhibited significant increases in the end section of the tensile bar only but not in the gate or neck sections, which were much less affected (Table 13.5). One explanation for the difference in responses between the two filler for­mulations was the abundance of protein content in the DDGS formulations which contains more hydroxyl groups that can interact with water molecules during the soaking process than present in the PW formulations.

An important attribute of WPC is their ability to retain their original color and this characteristic greatly contributes toward its commercial value.7197475 Weather­ing causes color changes in WPC which is both undesirable and irreversible.71974 Water soaking is an important weathering test and is useful in determining the du­rable nature of a thermoplastic composite.764’7176 Weathering (e. g., water soaking) causes HDPE-composites to undergo chemical reactions such as breakdown of lig­nins into water soluble products which form chromophoric functional groups such as carboxylic acids, quinones, and hydroperoxy radicals.74

Figure 13.6 compares color values (L*, a* and b*) of the original composites to the soaked composites. Almost all of the composites exhibited lightness (L*) following the soaking treatment. This trend has been observed in other immersion studies employing WPC.7 Coupling agents are included in the biocomposites to im­prove bio-filler binding to the thermoplastic resin and they may combat lightness (L* value) changes.197778 Similarly, in this study, composites containing MAPE are darker than the corresponding composites without MAPE and retained their L* val­ues to a greater extent following the soaking process (Table 13.6; Fig. 13.6). For example, following soaking the HDPE-25DDGS formulation exhibited a 16% light­ening response; while HDPE-DDGS-MAPE exhibited a 6% lightening response. Overall, DDGS formulations exhibited much higher color change values (L*, a* and b*) following soaking than PW formulations. This may be attributed to water induced chemical reactions in the DDGS; suggesting that DDGS are less chemically stable than PWF. Chemical modification of DDGS resulted in less change in color values compared to that occurring in the nonchemically modified DDGS formula­tions. Changes in redness (a*) and yellowness (b*) values generally followed the L* trends, but not always (Table 13.6). Changes in the C*ab (chromaticity, color qual­ity), and H*ab (hue) values also occurred when comparing the original and soaked composites. Notable changes in color values even occurred in the neat HDPE and HDPE-MAPE polymers (Table 13.6; Fig. 13.6).

presence of the asterisk “*” indicates significant difference between soaking treatments (p £ 0.05).

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FIGURE 13.6 Influence of soaking on color analysis of various PW/DDGS/PINEW composites.

Environmental stresses such as water soaking may cause changes in the me­chanical properties to occur which needs to be determined in order to assess the po­tential commercial value of a composite.7,64,70,73,76,79 For example, flexural properties have been reported to decrease when LPC are weathered.7,71,72 The response of the mechanical properties of composites as well as neat HDPE and HDPE-MAPE by water soaking are presented in Table 13.4. The neat HDPE and HDPE-MAPE blends exhibited significant changes in their oU and E values when given a soaking treat­ment (Table 13.4). Soaking caused oU values increased 3 and 3% for neat HDPE and HDPE-MAPE, respectively while %El values decreased 5 and 2% for neat HDPE and HDPE-MAPP, respectively. Soaking caused neat HDPE and HDPE-MAPE E values to change +4 and -5%, respectively. Changes in the mechanical properties for the composites varied considerably depending on the composition of the filler and MAPP concentration employed (Table 13.4). It is difficult to discern trends for the mechanical properties in the soaked and un-soaked formulations. Chemical modification of the PW and DDGS resulted in composites that when soaked main­tained their oU values (Table 13.4). However, several of these chemically modified formulations exhibited significant changes occurred for the E and %El values fol­lowing soaking. Absorption was found to be somewhat related to the variation of the mechanical properties of the composites. Formulations that maintained their me­chanical properties after prolonged soaking generally exhibited low absorption rates (Table 13.4). Even when significant differences occurred, less than a 5% change in values occurred (Table 13.4). Further work needs to be conducted to address how water absorbance affects the long-term mechanical properties of composites.