EXTRUDED WOOD-FLOUR POLYPROPYLENE COMPOSITES: EFFECT OF A MALEATED POLYPROPYLENE COUPLING AGENT ON FILLER-MATRIX BONDING AND PROPERTIES

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1 In: Caulfield, D.F.; Passaretti, J.D.; Sobczynski, S.F., eds. Materials interactions relevant to the pulp, paper, and wood industries: Proceedings, Materials Research Society symposium; 1990 April 18-20; San Francisco, CA. Pittsburgh, PA: Materials 67 Research Society; 1990: Vol EXTRUDED WOOD-FLOUR POLYPROPYLENE COMPOSITES: EFFECT OF A MALEATED POLYPROPYLENE COUPLING AGENT ON FILLER-MATRIX BONDING AND PROPERTIES GEORGE E. MYERS,* PAUL C. KOLOSICK,** ICHWAN S. CHAHYADI,** CAMDEN A. COBERLY,** JAMES A. KOUTSKY,** AND DONALD S. ERMER** *USDA Forest Products Laboratory, 1 Madison, WI **University of Wisconsin-Madison, Madison, WI ABSTRACT Full factorial studies were conducted to determine the effects of a coupling agent (a low molecular weight maleated polypropylene (MAPP)) and other composition and processing variables on the mechanical properties of a wood-flour-filled polypropylene (PP) composite. Effects of MAPP on the bonding between PP and wood veneer were also examined. At less than 1 percent by weight, MAPP produced useful increases in strength and modulus properties of the composite, and this effect was somewhat enhanced by small-particle-size wood flour and multiple extrusions. However, MAPP caused small losses in notched impact energy. High extrusion temperature (190 C to 250 C) had little influence on strength, but it decreased notched impact energy. Peel force between PP and wood veneer was increased by pretreatment with MAPP for aspen, but not for birch, aspen being more porous than birch. The effectiveness of MAPP may therefore be related to its ability to penetrate the wood and form a strongly held hydrophobic layer that is attractive to the PP, thereby increasing both the effective bonding area and mechanical interlocking. INTRODUCTION Wood-polyolefin composites suffer from inherently poor bonding between the hydrophilic wood filler and the hydrophobic polymer matrix. We initiated a research program aimed at achieving better understanding, control, and improvement of the wood-polyolefin bond and interphase. The program has two complementary approaches. The first approach involved an assessment of bonding strength at the micro or molecular level by determining the force required to peel a wood veneer-polypropylene laminate [l]. We combined this with a macroscopic approach that measured mechanical properties of extruded composites. In both approaches, the effects of surface modification or coupling agent were examined. This paper compares results of both approaches to determine the effects of a particular maleated polypropylene (MAPP) additive in the wood-polypropylene system. It is postulated that the MAPP additive provides coupling between the wood and polypropylene (PP) as a result of reaction of its anhydride groups with the wood and of the compatibility of its polypropylene segments with the polypropylene matrix [2,3,4]. A paper on the details of some of these results (denoted here as Experiment A) has been submitted for publication [2]. However, the peel force data are preliminary. 1 The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. Mat. Res. Soc. Symp. Proc. VoI Materials Research Society

2 68 EXPERIMENTAL METHODS Experiment Design and Data Analysis Experiment A involved the following four-variable, two-level matrix: (1) Ratios of wood flour (WF) to total polymer were 45/55 and 55/45 by weight (total polymer = PP + MAPP), (2) WF particle sizes were nominal 20 and 40 mesh, (3) MAPP levels were 0 and 2.5 weight percent of total system, and (4) numbers of extrusions were 1 and 3. The matrix was replicated and experiments were performed in random order. The data were analyzed by standard statistical procedures for the main effects of the variables and the interactions between variables [5]. Experiment B involved the following two-variable, four-level matrix: (1) MAPP levels were 0, 0.5, 2.0, and 5.0 weight percent of total system and (2) extrusion temperatures were 190 C, 210 C, 230 C, and 250 C. This experiment was also replicated and trials performed in random order. These data were analyzed by analysis of variance to determine the main effects of the variables and the interactions between variables. Data were also analyzed by Duncan s multiple range test to determine which levels of MAPP concentration or extrusion temperature produced significant differences in properties [6]. In Experiment C, the peel strength between wood veneer and polypropylene laminates was measured on specimens from a four-variable, two-level matrix: (1) Wood veneer species were aspen and birch, (2) laminating pressures were 0.14 and 0.28 MPa, (3) laminating times were 3 and 6 min, and (4) surface treatments were none and MAPP treated. Data from three replications of the untreated design were available, but the tests of treated panels were replicated only at the higher pressure. Data were analyzed by regression for main effects and interactions [5]. Materials The wood flours were American Wood Fibers, Inc., 2 no. 402, nominal 40-mesh pinepredominantly ponderosa (Pinus ponderosa), jeffrey (Pinus jeffreyi), and lodgepole (Pinus contorta)-and no. 202, nominal 20-mesh loblolly pine (Pinus taeda). The veneers were 3-mm-thick yellow birch (Betula alfeghaniensis) and quaking aspen (Populus tremuloides). The polypropylene was Soltex Fortilene 9101 homopolymer spheres with a density of g/ml and a melt flow index of 2.5 g/10 min (230 C/2160 g). Stabilizers and processing aids were added to the polymer prior to processing: 0.10 percent Irganox-1010 (a tertiary butyl hydroxyhydrocinnamate) from McKesson Chemical, 0.20 percent Ionol (a butylated hydroxytoluene) from Ciba-Geigy Corporation, 0.10 percent GMS (monoand diglycerides of fatty acids) from ICI United States, Inc., and 0.20 percent distearyl thiodipropionate (DSTP) from Witco Chemical. The MAPP was Epolene E-43 powder from Eastman Chemical Products, Inc.; this material has a density of g/ml, acid no. 47, and an approximate molecular weight of 4,500. Composite Processing and Testing (Experiments A and B) Wood flour was vacuum dried at 50 C to 60 C for 24 h to a moisture content of 1 to 2 percent and then stored over desiccant in sealed containers. All other ingredients were 2 The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service.

3 69 added to the dried flour and dry blended. Experiment A trials were extruded in random order with a Brabender 2503 Plasti-Corder 19-mm, single-screw extruder (3-mm-diameter by 58-mm-long die) at 200 C barrel and die temperatures, with a residence time of about 2 min. The extruded rod was pelletized and either stored dry immediately or reextruded and repelletized two additional times. Experiment B trials were extruded in random order with a Killion 25-mm, single-screw extruder with a residence time of about 2 min. The extruded rod was pelletized and stored dry. For both Experiments A and B, test specimens were prepared in the same random order as the extrusions, using a Frohring Mini-Jector model SP50 (Newbury Industries, Inc.) plunger injection molding machine at 215 C with an average residence time of 1 to 2 min, ram pressure of 8.9 MPa, and mold temperature of 25 C to 30 C. After molding, the specimens were stored over desiccant at 23 C for at least 3 days before testing at that temperature. We report here the results for flexural (three-point bending) maximum strength and modulus and for notched impact energy, although other properties were measured. Flexural specimens were 127 by 12.7 by 3.2 mm and were tested in conformance with ASTM D790 84a [7], using a support span of 102 mm and a crosshead rate of 5 mm/min. Impact specimens were 64 by 12.7 by 6.4 mm and were tested in conformance with ASTM D [7], using an Izod impact tester. Peel Specimen Preparation and Testing (Experiment C) A polypropylene film was prepared by extruding through a slit die at 180 C and calendering to a 2-mm thickness. Veneer sheets (175 by 175 by 3 mm) were dried for 12 h at 70 C under vacuum and then cooled and stored over desiccant. Immediately before laminating, the MAPP-modified veneer was coated on the side to be laminated with 1 g of powdered MAPP (approximately 1 percent by weight of the PP film). The film was laminated against the veneer by pressing at 175 C in an electrically heated press. Each laminated sheet was cut into six strips (25 by 175 mm) parallel to the wood grain for peel force measurement. The 90 peel force was measured continuously along the length of each strip, using an Ametek universal testing machine at a peel rate of 100 mm/min [1]. A mean peel force for each panel was obtained by averaging the peel force over the middle 100 mm of all six strips. RESULTS Figures 1 to 5 provide a qualitative or semiquantitative presentation of results. Statistically quantitative presentations are given in Tables I to IV. Properties of Extruded Composites For Experiment A, Figures 1 and 2 illustrate the observed effects for strength and impact. The largest effect was the increase in flexural strength (Table I and Fig. 1) brought about by adding the MAPP (20-percent increase, averaged over the other three variables). Smaller wood-flour particles had a further postive effect on the strength, particularly in the presence of MAPP where the increase approached 30 percent (Fig. 1). The largest effect on modulus was from increasing the wood-flour content (Table I), but both MAPP and small particle size also had positive effects. The MAPP level and a larger number of extrusions showed a small negative interaction for modulus (Table I). All observed effects on notched impact were small, negative ones (Table I and Fig. 2).

4 70 Figure 1. Maximum flexural strength as a function of wood flour particle size, number of extrusions, and MAPP content. Experiment A. (All data points are the average of results for 45/55 and 55/45 weight ratios of wood flour to total polymer. Percentage figures on axes represent the increase above the control.) Figure 2. Notched impact energy as a function of wood flour particle size, number of extrusions, and MAPP content. Experiment A. (All data points are the average of results for 45/55 and 55/45 weight ratios. MAPP levels are weight percentages of the total system. Percentage figures on axes represent the increase above the control.)

5 71 Figure 3. Maximum flexural strength as a function of extrusion temperature and MAPP level. Experiment B. (MAPP levels are weight percentages of the total system.) Figure 4. Notched impact energy as a function of extrusion temperature and MAPP level. Experiment B. (MAPP levels are weight percentages of the total system.)

6 72 Figure 5. Peel force as a function of veneer species, pressing time, and treatment with MAPP. Experiment C. (Percentage figures on axes represent the increase above the control.) Experiment B was conducted to determine whether the 2.5-percent MAPP and the 200 C extrusion temperature used in Experiment A yielded an optimum balance of properties. Increased MAPP levels or extrusion temperatures, or both, for example, might degrade the wood or might enhance the MAPP-wood reaction. Low extrusion temperature might lead to incomplete MAPP-wood reaction and to poor filler dispersion. The results are summarized in Tables II and III and Figures 3 and 4. We note also that the composite was darker in color at increased MAPP levels and at high extrusion temperatures. Extrusion temperature had a small positive effect on flexural strength (Table II and Fig. 3) but no clear effect on flexural modulus (Table II). In contrast, high extrusion temperature significantly decreased (about 15 percent, pooled) the notched impact energy (Fig. 4). It appeared that the 200 C temperature used in Experiment A represented a reasonable compromise. No significant advantage was gained in flexural strength beyond a MAPP level of 2.0 percent or in flexural modulus beyond a MAPP level of 0.5 percent (Table 111). However, notched impact energy steadily decreased with increasing MAPP (Fig. 4). Peel Force of Laminates Overall, the largest effects on peel force were due to species (birch less than aspen) and pressing time; the MAPP level had a small positive effect on peel force (Fig. 5 and Table IV). Significant interactions occurred between species and laminating pressure and between species and MAPP. Figure 5 plots the data averaged over the two pressures (0.14 and 0.25 MPa). The MAPP clearly improves the peel strength with the more porous aspen but has no effect with birch. For both species, longer pressing time improved the peel force, with and without MAPP.

7 73 Table I. Experiment A: Statistically significant main effects and interactions of variables on mechanical properties of extruded-molded composites a Overall mean c Main effects of variables d Interactions of variables e WF/Pol MAPP Mesh Passes Flexural Flexural Notched impact strength modulus energy Variable b (MPa) (GPa) (J/m) MAPP Mesh MAPP Passes MAPP WF/Pol Passes (+1.6) a Main effects and interactions without parentheses are significant at the 95-percent confidence limit. Parenthetical values are at the 90-percent confidence limit. Absence of value indicates significance is below the 90-percent confidence limit. b WF is wood flour; Pol is total polymer (PP + MAPP); WF/Pol is wood flour/polymer weight ratio (45/55 to 55/45); mesh (20 to 40) is wood flour particle size; MAPP is Epolene E-43 content (0 to 2.5 percent); and passes is number of extrusions (1 to 3). c Mean of 32 trials. d Change in property resulting from the particular variable, averaged over all other variables [5]. e X Y interaction = (average effect of X at first level of Y minus average effect of X at second level of Y). X Y Z interaction = the difference between the X Y interactions at the two levels of Z [5]. (-0.008) DISCUSSION Approximately 1 percent MAPP in the extruded composite is adequate to achieve most of the benefits provided by that material in terms of flexural strength and modulus, with only a small sacrifice in impact behavior. The resultant improvements in flexural strength and modulus are sufficient to be commercially useful. At 1 percent MAPP, the total cost increase resulting from the approximately threefold greater cost of MAPP over PP would be small. Moreover, that cost increase can be partially compensated by increasing the WF/PP ratio, with little degradation in strength benefits or in impact and with small additional increases in stiffness (Table I). The increased flexural strength and modulus resulting from using MAPP indicates that it provides some coupling between the filler and matrix, thus allowing increased reinforcement by the wood filler. This is also consistent with the increased effectiveness of MAPP when small wood particles are present (Fig. 1). Increased coupling and rigidity of the system would also be expected to lower notched impact resistance, as observed (Figs. 2 and 4). Darkening of the composite during extrusion at increased MAPP levels or high extrusion temperature, or both, clearly indicates degradation of the wood particles under those

8 74 Table II. Experiment B: Analysis of variance for extruded-molded composites Observed F ratio for main effects a Flexural Flexural Impact Variable strength modulus energy MAPP Extrusion temperature a F is the variance of responses attributable to the particular variable divided by the variance as a result of experimental error. For the degrees of freedom available in this experiment, F > 3.3 indicates significance at the 95-percent confidence level [5]. For example, at that confidence level all effects are significant except for extrusion temperature on modulus. No significant interaction effects were observed. Table III. Experiment B: Duncan s multiple range test for extruded-molded composites MAPP b Property a Flexural strength (MPa) Flexural modulus (GPa) a Averaged over the four extrusion temperatures. Underlined property values are not significantly different at the 95-percent confidence level [6]. Analysis not performed on impact energy because both MAPP and extrusion temperature had statistically significant effects on that property. b Percent by weight of total system. conditions. This action by MAPP presumably arises from acidic catalysis of wood degradation by the carboxyl groups in MAPP. The expected embrittlement in the wood may account for the losses in impact energy seen at increased MAPP levels and high extrusion temperature. At present, we offer only a preliminary rationale for the observed peel force behavior. First, we assume that greater polymer penetration into aspen than birch is to be expected because aspen is more porous than birch (birch density about 0.6 g/ml compared to about 0.4 g/ml for aspen). Second, we assume the property changes seen in the extruded composite do in fact demonstrate that MAPP facilitates bonding between PP and wood. Thus, a greater increase in birch-pp peel force with longer press times (Fig. 5) and higher pressure (see interactions in Table IV) may arise because they are necessary to achieve sufficient

9 75 Table IV. Experiment C: Statistically significant main effects and interactions on peel force a Variable b Peel force (N) Overall mean c 2.21 Main effects Species of variables d Time 0.27 MAPP (0.16) Interactions Species Pressure 0.19 of variables Species MAPP a Main effects and interactions without parentheses are significant at the 95-percent confidence limit. Parenthetical values are at the 90-percent confidence limit. bspecies mean changes from aspen to birch; time mean from 3 to 6 min; MAPP mean from untreated to treated with MAPP; pressure mean from 0.14 to 0.28 MPa. c Mean of 32 trials. d Change in property resulting from the particular variable, averaged over all other variables [5]. polymer penetration into birch to provide effective mechanical interlocking. Furthermore, the greater efficacy of MAPP with aspen may simply indicate that MAPP enhances the wetting and penetration of PP into aspen by forming a hydrophobic layer on the wood surface, thereby increasing both the effective bonding surface area and mechanical interlocking. In contrast, the poor penetration of MAPP and PP into birch reduces that advantage, although MAPP does aid bonding to the more accessible areas of the wood surface. However, this interpretation needs confirmation. ACKNOWLEDGMENT We thank Jerome Saeman for his advice and aid. We also thank the American Woodstock Co. for providing financial support and the American Wood Fibers Co., Soltex Polymer Corp., and Eastman Chemical Co. for providing the materials. Polymer processing equipment was made available by Professor J. Rietveld, University of Wisconsin-Madison, Mechanical Engineering Department. REFERENCES

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