Bonding mechanisms between polypropylene and wood: coupling agent and crystallinity effects

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1 Bonding mechanisms between polypropylene and wood: coupling agent and crystallinity effects In: Wood-fiber/polymer composites: Fundamental concepts, processes, and material options: Proceedings of 1st wood fiber-plastic composite conference; Madison, WI and 45th annual meeting of the Forest Products Society; New Orleans, LA. Madison, WI: Forest Products Society; 1993: Paul C. Kolosick, Graduate Student, Chemical Engineering Dept., University of Wisconsin, Madison, Wisconsin George E. Myers, Research Chemist, USDA Forest Products Laboratory, Madison, Wisconsin James A. Koutsky, Professor, Chemical Engineering Dept., University of Wisconsin, Madison, Wisconsin Abstract Peel testing (90 ) of birch and aspen veneers laminated with polypropylene was performed. The samples included untreated veneers and veneers treated with emulsified Epolene E-43 (a maleated polypropylene wax). All the samples had finite peel forces. However, the E-43 treated samples had lower peel forces than the untreated samples. Optical microscopy of the peeled surfaces showed no gross differences in the failure mechanism between untreated and treated surfaces. A failure mechanism based on the evidence to date is proposed. The crystallization of polypropylene at the interface with the treated and untreated wood was investigated using differential scanning calorimetry. Results indicate that polypropylene nucleation occurs at the untreated wood surface but not at the E-43 treated surface. This difference may explain the poorer adhesion in E-43 treated samples. Introduction This paper details part of our continuing investigation into the adhesion between lignocellulosic materials and polyolefins and the influence of that adhesion on the behavior of composites made from lignocellulosics and polyolefins. The specific objective of this phase of our studies was to determine the adhesion mechanism between polypropylene and lignocellulosic materials and the effect thereon of emulsified Epolene E-43, a maleated polypropylene wax (Eastman Chemical Co.). Other work has demonstrated that small amounts of either emulsified or nonemulsified E-43 provide significant improvements in the mechanical properties of polypropylene/lignocellulosic composites (2-5). To investigate the adhesion, we have been looking at the interface between polypropylene and wood veneer. This allows us to consider surface interactions without the mixing difficulties that arise in particulate-filled systems, and it provides a more readily observable model of such systems. We have employed three techniques. First, the force required to peel a laminated film of polypropylene from a veneer surface was measured using a 90 peel test. Second, the failure surfaces of the peel samples were examined using optical microscopy. Third, the influence of the wood on nucleation of polypropylene crystallization was investigated using differential scanning calorimetry (DSC). Experimental procedures Two wood species (aspen and birch) were treated at 5 levels of emulsified E-43 (0.0, 0.1, 0.4, 1.2, and 1.5 g per panel, each panel being 180 by 180 mm). The E-43 was spray-coated onto the surface of the veneer at the desired level. The E-U-treated veneer sheets were then vacuum dried (7O C, 30-mm Hg vacuum) for 24 hours. A polypropylene film (melt flow index 2.5 g/l0 min.) was then laminated onto the veneer sheets. The lamination step was performed in a Carver press at 175 C and 40 psi at pressing times of 3 and 6 minutes. The resulting panels were cut into 25 by 180 mm strips parallel to the grain. The average 90 peel force was then measured for each strip using the testing head shown in Figure 1 at a peel rate of 100 mm/mm. The fracture surfaces of the samples were then examined using optical microscopy. Polypropylene crystal nucleation at the aspen and birch surfaces was evaluated using DSC. The wood was first ground to -40 mesh in a Wiley mill. Portions of the resultant flours were treated with emulsified E-43 (2.5% by weight) by evaporating a slurry. All of the flours were conditioned to approximately 6 percent moisture content before placing samples of approximately 8 mg in DSC capsules. Approximately 5 mg of polypropylene powder (the same used to form the film used in the peel tests) was then placed on top of the wood flour in the capsule and the sample was sub- 15

2 jected to a series of four melting and recrystallizing steps (25 C ) 200 C 25 C at 10 C/min.). Results and discussion For both aspen and birch veneers, the E-43 produced no significant improvement in peel strength (Fig. 2). For the highest level of E-43 (1.5 g/panel), the peel force was actually lower than that of the untreated surface, indicating that the thick layer of E-43 on the wood surface had a low cohesive strength. In addition, optical micrographs of the fracture surfaces showed no obvious difference between the treated and untreated surfaces (Figs. 3 to 8). However, the micrographs of the fracture surfaces did reveal some significant features of the failure process, for example: 1. Open lumens were nearly parallel to the wood surface (Figs. 3-a, 4-a, 5-a, and 8-a). 2. Polypropylene tendrils extended from the open lumens (Figs. 3-b, 4-b, 5-b, and 8-b). 3. Necking occurred in the polypropylene tendrils (Fig. 6-c). 4. Fracture ends appeared in the polypropylene tendrils (Figs. 5-d and 6-d). 5. Regions of clean wood and clean polypropylene (Figs. 3-e to 8-e). 6. Regions with wood fibrils embedded in the polypropylene film (not shown here). DSC scans of polypropylene in the presence of aspen and birch (40 mesh flour) showed shifts in the crystallization temperature, indicating nucleation of polypropylene by the wood surfaces (Figs. 9 and 10). These shifts were not large enough to indicate a transition from the beta crystal form to the alpha crystal form. The beta to alpha shift is approximately 15 (6). The shift in peak temperature occurs gradually over the four melting and crystallizing cycles. This gradual change is attributed to the gradual increase in surface contact between polypropylene and the wood surfaces as the polypropylene melts. It should be noted that the wood samples that were treated with Epolene E-43 do not exhibit this shift, presumably because the E-43 forms a layer between the nucleation sites on the wood and the polypropylene. Table 1 summarizes the crystallization peak temperatures for the polypropylene-birch and polypropylene-aspen samples. Proposed failure mechanism during peel Based on the results to date, we propose a three-step failure process. The significance of each step for the magnitude of failure strength and/or energy has yet to be determined and may vary depending on surface type, coupling agent, and formation conditions. Before outlining the failure mechanism, we note that it is based on evidence of polypropylene flow into open lumens in the wood surface during formation of the laminate (Figs. 3-a, 4-a, 5-a, and 8-a). This evidence for mechanical adhe- Figure peel apparatus. Figure peel test results. 16

3 Figure 3. - Birch veneer surface after peel testing, 1.5 g E-43, magnified 4X. (a) open failure at Figure 4. - Birch veneer surface after peel testing, no E43, magnified 4X. (a) open lumens parallel to the wood surface; (b) polypropylene tendrils originating from open lumens: failure at Figure 5. - Birch veneer surface after peel testing, no E43, magnified 20X. (a) open (d) fracture end of polypropylene tendril; failure at 17

4 Figure 6. - Aspen veneer surface after peel testing, 1.5 g E-43, magnified 20X. (c) polypropylene tendril exhibiting necking; (d) fracture end of polypropylene tendril; (e) exposed wood surface, indicating failure at Figure 7. - Aspen veneer surface after peel testing, no E43, magnified 4X. (e) exposed wood surface, indicating failure at wood-polypropylene interface. Figure 8 - Aspen veneer surface after peel testing, no E-43, magnified 20X, (a) open failure at 18

5 Figure 9. - Polypropylene crystallization in the presence of aspen flour. Polypropylene crystallization peak after four remelting cycles. Scanning rate 10 C per minute. (Note the shift of crystallization peak in the presence of untreated aspen and the absence of a shift in the presence of Epolene E-43 treated aspen.) Figure Polypropylene crystallization in the presence of birch flour. Polypropylene crystallization peak after four remelting cycles. Scanning rate 10 C per minute. (Note the shift of crystallization peak in the presence of untreated aspen and the absence of a shift in the presence of Epolene E-43 treated aspen.) sion supports the findings of Goto (1). The three steps of the proposed failure process are: 1. Weak interface failure at: a) Wood surface-polypropylene interface. In Figures 3-e to 8-e, ample evidence of exposed wood surface can be seen. The matching polypropylene surfaces show no evidence of wood material. This is indicative of failure at the b) Polypropylene film-polypropylene tendril interface. Evidence of a failure zone between the polypropylene film and the polypropylene tendrils (Figs. 3-b, 4-b, 5-b, and 8-b) is indicated by the distinct formation of tendrils. These tendrils were originally part of the polypropylene film. c) Wood-wood interface. Optical micrographs have shown the presence of wood fibrils in the polypropylene film, indicating wood failure. 2. Polypropylene tendril stretching and necking. Figure 6-c shows polypropylene tendrils that have exhibited necking, a highly energy absorbing process. 3. Polypropylene tendril fracture. Figures 5-d and 6-d show the fractured end of a polypropylene tendril. 1. Aspen and birch surfaces nucleate polypropylene crystallization. 2. E-43 coating of wood surfaces interferes with the nucleation of polypropylene crystallization on wood surfaces. 3. E-43 influences the failure process only by altering the relative importance of the steps in the failure mechanism. Literature cited Conclusions The following preliminary conclusions can be made at this time: 19