Manufacture of Rigid PVC/Wood-Flour Composite Foams Using Moisture Contained in Wood as Foaming Agent LAURENT M. MATUANA* and FATIH MENGELOGLU School of Forestry and Wood Products Michigan Technological University Houghton, Michigan 49931-1295 Relationships between the density of foamed rigid FVC/wood-flour composites and the moisture content of the wood flour, the chemical foaming agent (CFA) content, the content of all-acrylicfoam modifier, and the extruder die temperature were determined by using a response surface model based on a four-factor central composite design. The experimental results indicated that there is no synergistic effect between the CFA content and the moisture content of the wood flour. Wood flour moisture could be used effectively as foaming agent in the production of rigid WC/wood-flour composite foams. Foam density as low as 0.4 g/cm3 was produced without the use of chemical foaming agents. However, successful foaming of rigid WC/wood-flour composite with moisture contained in wood flour strongly depends upon the presence of all-acrylic foam modifier in the formulation and the extrusion die temperature. The lowest densities were achieved when the all-acrylic foam modifier concentration was between 7 phr and 10 phr and extruder die temperature was as low as 170°C. INTRODUCTION E€F xtruded PVC/wood-fiber composite profile is in owing demand as a substitute for solid wood components in the building industry because of the favorable performance and cost attributes of both wood fibers and WC matrix (1-5). The higher density, however, at approximately 1.4 g/cm3 for WC/wood-fiber composites, combined with their brittleness and lower impact resistance compared to unfilled W C and/or ~ t u r a solid l wood (Table 11, may prevent this emerging class of materials from capturing their full market potential in applications such as decking, siding, and window and door frames. Fortunately, the foaming of WC/wood-fiber composites has been proven to overcome some of these shortcomings (6-9). In the past few years, several investigators have examined the foamability of WC/wood-fiber composites through batch (6-10) or continuous extrusion processes (11,12).Recently, Einley (12)patented a foamed composite member comprised of vinyl polymer and wood fiber. According to his approach, WC/wood-&r composites containing up to 50% of wood fiber can be foamed by using physical or chemical foaming agents (12). The potential of using water as a blowing agent has also been reported (13). Moisture occurs in wood *Corresponding author. Current address: Department of Forestry. Michigan State University. East Lansing, M-i 48824-1222. 264 in three Merent forms: (i)as a free water contained in the cell lumens, (ii) as a bound water located within the cell walls, and (iii) as water of constitution, which is part of the molecular structure of the wood, in the forms of hydrogen and hydroxyl groups (14-16). According to Rizvi et aL (13),the moisture available in the wood fibers can be transformed into the gaseous state at temperatures exceeding 100°C. Bound water is quickly converted into gas and lost from the extruder’s hopper; the remaining water of constitution (almost 3% based on oven-dry weight of wood) is utilized as a foaming agent. PS/wood-fiber and HIPS/woodfiber composites were successfully foamed using this approach (13).However, higher processing temperature was needed in the extruder barrel (205°C) because of the high-energy requirement to dislodge water of constitution, which is linked to the cell walls within the fiber (15.16).In addition, the incorporation of a chemical foaming agent was needed to increase the volume expansion of foamed samples. The feasibility of foaming all-acrylic modified rigid WC/wood-flour composites using wood flour moisture as foaming agent has also been reported (11).Since an all-acrylic foam modifier traps the evolving gas and prevents bubbles from coalescjng during the foaming process (17-19). it is hypothesized that the bound water, which requires less energy to remove from fiber cell wall than water of constitution (15, 16). could effectively be used as a foaming agent (11). In addition, JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, DECEMBER 2002, Vol. 8, No. 4 M a n u f a e of Rigid WC/Wood-FlourFoam Table 1. Denlsities of Various Materials. ~~ Table 2. Experimental Formulations Used in Rigid PVCMlood-Flour Composites. ~~~ Materials Density (9/cm3) HDPE/wood-fiber composite lumber' PVC/wood-fiber composite lumber' HDPE' PVC' Eastem white pine2 Red maple2 0.70-1 2 0 0.70-1.36 0.95 1.37 0.35 0.54 'Commercial products. 2Values obtained from reference (14) Ingredients Concentrations (phr) PVC (PolyOne C o p ) Tin stabilizer (PlastiStab) Calcium stearate (Synpro) Paraffin wax (Gulf Wax) Processingaid (Paraloid K-120)' Processingaid (Paraloid K-175)' Wood flour (American Wood Fibers) All-acrylic foam modifier (Paraloid K-400)' AZRV CFA (Uniroyal Chemicals) 100 2 1.2 1 1.2 1 50 0-10 0-1 'Paraloid K-120, K-175 and K-400 were supplied by Rohm B HaasCo. owing to the lower energV needed to remove bound water from the cell wall, the processing temperature could be substantiallly lower during the foaming process. The present study was conducted to verify these hypotheses. The main objectives of this investigation are to examine the effects of wood flour moisture content, allacrylic foam modifier content, and extruder die temperature on the faunability of rigid WC/wood-flour composites. The eOlect of incorporating a chemical foaming agent (CFA) during foaming process is also examined to evalu.ate possible synergistic effect between the CFA and the moisture contained in the wood flour. Since the number of variables and ranges of interest are large, the rehti~lonshipsamong the factors and levels cannot be fully understood with simple linear regression models. Fkesponse surface methodology, capable of opthizing a response of interest influenced by several variables, is required to effectively model these complex interactions. The central composite design (CCD) is used in this study because it provides higher level models such as quadratic response models without the reihndancy of a full factorial design with three levels (20-22). IEXPERIHENTAL lmterhb WC resin with a K value of 57 was used as the polymer matrix. Commercial wood flour from hardwood maple species (100-mesh size) was utilized as filler. The moisture from wood flour and modified amdicarbonamide WRV) were used as foaming agents. Allacrylic foam modifier was consumed as a processing aid. Other ingredients incorporated into the experimental formulations and the suppliers of the materials used in this study are listed in Table 2. Wood flour with Werent moisture contents was obtained by conditioning wood flour in a walk-in temperature/humidity-controlled environmental chamber for several days until the desired moisture content was attained for processing. The temperature and relative humidity in the room were set at 22°C (72°F)and 65%, respectively, to yield the equilibrium moisture content (EMC)of 12%. ~ e n t aDeaign l A four-factor central composite design (CCD) was used to develop a response surface model (RSM) to study the foamability of rigid WC/wood-flour composites. Wood flour moisture content, CFA content, allacrylic foam modifier content, and extruder die temperature were the four treatments studied: the density of the foamed composites was the response variable observed in this investigation. The CCD design matrix was generated by Design-Expert@software-v.6 (StatEase Corp., Minnesota). The Design-Expert@software provides two different methods of displaying the levels of the factors in a design of experiments: 1) the actual levels of the factors (i.e., the actual values in the experiment) and 2) the coded factor levels (i.e., as -1 for low levels, + 1 for high levels, and 0 for centerpoint). The coded factor levels are defined as (20-23): coded factor levels = Acfual value - Factor mean (Range of thefactorial udue/2) ( 1) The design levels in terms of actual and coded factor levels and associated response values are given in Table 3.The average of twenty replicates was used for each datum used in the RSM. Compounding and E&nsaion Forming According to the experimental design, wood flour with different moisture contents, chemical foaming agent and all-acrylic foam modifier, at various loading levels, were dry-blended with WC matrix in a 20-L high-intensity mixer (papenmeier, "ype TGAHK20) for 15 minutes. The addition level of wood flour in the mixtures was fixed at 50-phr based on the oven-dry weight of wood flour. The amount of wood flour incorporated into the formulations was corrected by taking into account the moisture contained in the wood flour. This correction was performed by using the following equation ( 16): w,= w,. (l+E) where W, is the moist weight, W, is the oven-dry weight, and MC is the percent moisture of wood. JOURNAL OF VINWL 8 ADDITIVE TECHNOLOGY, DECEMBER 2002, Vol. 8, No. 4 265 Lawent M. Matuana and Fatih Mengeloglu Table 3. Central Composite Design (CCD) Matrix in Terms of Actual and Coded Factor Levels Generated by the Design-Expert@Software. Response Factors Wood flour MC Experiment number Types 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Axial Fact Fact Fact Fact Fact Fact Fact Fact Axial Axial Axial Center Center Center Center Center Center Axial Axial Axial Fact Fact Fact Fact Fact Fact Fact Fact Axial (%I1 CFA content (PW’ Acrylic content (PW 0.5 (0) 0.25(-1) 0.25(- 1) 0.75(+1) 0.75(+1) 0.25(-1) 0.25(-1) 0.75(+1) 0.75(+1) 0.5 (0) 0.0 (-2) 0.5 (0) 0.5 (0) 0.5 (0) 0.5 (0) 0.5 (0) 0.5 (0) 0.5 (0) 0.5 (0) 1.0 (+2) 0.5 (0) 0.25(-1) 0.25(-1) 0.75(+1) 0.75(+1) 0.25(-1) 0.25 (-1) 0.75(+1) 0.75(+ 1) 0.5 (0) 5.0(0) 2.5(-1) 2.5(-1) 2.5(-1) 2.5(-1) 7.5(+1) 7.5(+1) 7.5(+1) 7.5(+1) 0.0(-2) 5.0(0) 5.0(0) 5.0(0) 5.0(0) 5.0(0) 5.0(0) 5.0(0) 5.0(0) 5.0(0) 5.0(0) 10(+2) 2.5(-1) 2.5(-1) 2.5(-1) 2.5 (-1) 7.5(+1) 7.5(+1) 7.5(+1) 7.5(+1) 5.0(0) Die temperature (“C)’ Density gkm3 170 (-2) 180 (-1) 180 (-1) 180 (-1) 180 (-1) 180 (-1) 180 (-1) 180 (-1) 180 (-1) 190 (0) 190 (0) 190 (0) 190 (0) 190 (0) 190 (0) 190 (0) 190 (0) 190 (0) 190 (0) 190 (0) 190 (0) 200 (+1) 200 (+1) 200 (+1) 200 (+1) 200 (+1) 200 (+1) 200 (+1) 200 (+1) 210 (+2) 0.48 0.80 0.70 0.81 0.80 0.60 0.44 0.62 0.41 0.98 0.65 0.82 0.66 0.65 0.64 0.64 0.65 0.66 0.61 0.74 0.55 0.89 0.88 0.96 0.84 0.71 0.57 0.76 0.66 0.88 ‘The values in parenthesesrepresentthe coded factors. After blending, the compounded mixtures were extruded through a 19.1 mm single screw extruder with a L/D ratio of 30:1 (C.W. Brabender Instruments, Inc.) driven by 5 hp Prep-Center@(C.W. Brabender Instruments, Inc.) to produce foams. The screw speed was set at 40 rpm throughout the experiments. The extruder barrel temperature profile for the first three heating zones was set at 165/170/180°C. Since the die temperature was one of the independent variables, it was varied according to the experimental design. Foam CharactdzOtion The densities of rigid PVC/wood-flour composite foams were measured by the water displacement technique (ASTM standard D792). SEM micrographs were taken from a JEOL JSM-35C scanning electron microscope to determine average cell sizes of foamed samples. Samples were gold coated to provide the necessary conductivity to the surface for the electron microscope. the WpOllSe sllrfarrc lyIodel A central composite design was used to develop a response surface model (RSM) relating the density of foamed rigid WC/wood-flour composites to four 266 i 8 Of + + Density = + 0.67 - 0.053A 0.01923 - 0.12C 0.0790 + 0.014A2 0.027C2 - 0.023AC + RESULTS AND DISCUSSION W d C dm factors: wood flour moisture content, all-acrylic foam modifier content, CFA content, and extruder die temperature. A regression analysis was performed on the density measurements from the extrusion foaming experiment to obtain the best-fit model equation for the experimental data (Table4). A standard analysis of variance (ANOVA) showed that the density data was best fit with a quadratic model (values of ’Rob > F” less than 0.0001). Because the model contained significant and non-significant terms (Table4). it was simplified by dropping insignificant terms. This modification did not affect the adequacy of the model because the R2 and adjusted R2 for this reduced model were satisfactory and the predicted R2 value improved (Table5).Therefore, the derived regression equation describing the relationship between the density and the four variables (in terms of coded factors)was reduced to: (3) where A is the wood flour moisture content, B is the chemical foaming agent content, C is the all-acrylic foam modifier content, and D is the extruder die temperature. JOURNAL OF VINYL & ADDITlVE TECHNOLOGY, DECEMBER 2002, Vol. 8, No. 4 M a n u f m e of Rigid WC/Wood-FlourFoam Table 4. Analysis of Variance (ANOVA) for Responae Surface Quadratic Model. Source Sum of squares Degrees of freedom Mean square F-Value Prob > F1 Quadratic model 0.580 14 0.042 35.78 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0.067 8.43E-003 0.320 0.150 7.52E-003 3.66E-003 0.023 1.67E-0.003 5.63E-005 8.56E-0.003 7.56E-0.004 6.25E-006 3.06E-0.004 1.801E-003 57.63 7.24 274.16 127.64 6.45 3.14 19.87 1.44 0.048 7.34 0.65 0.005 0.26 1.55 < o.Ooo1 < o.oO01 A B C D A2 82 C2 D2 AB AC AD BC BD CD 0.067 8.43E-003 0.320 0.150 7.52E-003 3.66E-003 0.023 1.67E-0.003 5.63E-005 8.56E-0.003 7.56E-0.004 6.25E-006 3.06E-0.004 1.801E-003 0.0168 < 0.0001 < o.oO01 0.0226 0.0965 0.0005 0.2494 0.8291 0.0162 0.4332 0.9426 0.6258 0.2324 'Values of "Prob > F' less than I).O500 indicate model terms am significant. In t h s case A, 6,C, D. A2, C2,and AC are the mgnikmi model terms. Table 5. Values of Coefficient of Regression (Rz) for the Full arid Reduced Quadratic M o d e l s Selected From ANOVA Analysis. Types of R R* Adjusted R2 Predicted R2 Full 0.9707 0.9438 0.8354 Models Reduced 0.9582 0.9449 0.9249 Notice that all the first-order main effects (A, B, C, and D), most significant second-order main effects (A2 and C2).and the only significant two-factor interaction term (AC) (Table 4) are included in this equation. The relative effect of each factor in this equation is expressed by its coeficient and its algebraic sign (23,24). It is seen from Eq 3 that the density of foamed composites decreases with all-acrylic foam modifier content (factor C) and wood flour moisture content (factor A) increase (negative algebraic sign in Eq 3).In contrast, the density of foamed composites increases with the CFA content (factor B) and die temperature (factor D) increase (positive algebraic sign in Eq 3). The perturbation plot of density against all four investigated variables (FQ. I) provides supporting evidence of the relative importance of all-acrylic foam modifier (factor C) and wood flour moisture content (factor A) on the density of foamed rigid WC/woodflour composites. Figure I shows changes in the density as each factor moves from the chosen reference, with all other factors held constant at the middle of the design space (the coded zero level of each factor) Perturbation 'i 0.9 Hg. 1 . Perturbation,dotof demity againstfow~~trtedvariables. A represents the peirent moisture o~ntainedin w o o d j b ~ :B is the chemical foaming cgent content; c is the all-acrylic.foam modLfier content: and D is the ex&uder die tempemhrre. I -1.o I I I -0.5 0.0 0.5 I 1.o Dewation from Reference Point JOURNAL OF ViNKL & ADDitrVE TECHNOLOGY, DECEMBER 2002, Voi. 8, No. 4 267 Laurent M . Matuana and Fatih Mengeloglu (22).Once again, the perturbation graph clearly isolates all-auylic foam modifier (factor C) and wood flour moisture (factorA) as the two most important variables reducing the density of rigid WC/wood-flour composites. Chemical foaming agent (factorB) produces a relatively small effect whereas the density increases with the increased die temperature (factorD). Since the two main factors reducing the density of composites (A and C) are part of the only significant interaction (AC) (Eq 3).it would be inappropriate to investigate these factors individually because the effect of one factor will depend upon the level of the other (25). - '"14 Figure 2 shows the interaction graphs of the variation in density as a function of the interaction between all-acrylic foam modifier content and wood flour moisture content (interaction AC) for samples produced without CFA (Figs.2a-b) and by using 1-phr CFA (Figs. 2c- d ) .The extruder die temperature effect is also illustrated in these figures (170°C for Figs. 2a and 2c and 210'C for Figs. 2b and 24. 1.0- (a) 170°C and no CFA T 3% 0.8 - 0.8- 0 0 Y Y 0, 0, v v .b Effect of Au-amylic foam modifier Contsntl wood Flour 1yIoimtara content Intarpcuan (Interadon AC) on the Density of Rigid PVCIWood-FIour compomites 2 0.6- In C In n 0 0.6- C m 0, 0.5- 0.5- 2 10°C and no CFA -/ - 0.3 0.3 I 2.5 I I I I ' 3.8 5.0 6.3 7.5 2.5 All-acrylic processing aid content (phr) 3.8 5.0 6.3 7.5 All-acrylic processing aid content (phr) 1.0- (c) 170°C and 1phr CFA - 0.8- 0.8 0 Y 0, v 0.7- 32 w3 tn C m a I 21OoC and 1 phr CFA -L 0.51 0.3 4 4 0.3 1 2.5 I I I I 3.8 5.0 6.3 7.5 All-acrylic processing aid content (phr) ' 2.5 3.8 5.O 6.3 7.5 All-acrylic processing aid content (phr) Fig. 2. Interaction graphs of the uariation in density as afunctionof the interaction between all-acrylicfoam nwd@er content and luwdflour moisture content (MC)for samples produced without CFA (Figs.2a and 2b) and by using 1 phr of CFA (Figs. 2c and 24. "he exbuder die temperature eflfect is also illustrated in thesejQures (1 70°Cfor Figs. 2a and 2c and 21 0°C for Figs. 2b and Zd). 268 JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, DECEMBER 2002, V d . 8, No. 4 Manufacture of Rigid WC/Wood-FlourFoam Interaction between all-acrylic foam modifier content and wood flour moisture content has important effects on the density of rigid WC/wood-flour composite foams (Figs.:?a-d).A strong density dependence on both all-acrylic foam modifier content and wood flour moisture content is observed. The density of foamed compositt: decreases with all-acrylic foam modifier content and wood flour moisture content increase. However, the effect of wood flour moisture content is strongly d.ependent on the concentration of all-acrylic foam modifier. Rgures 2a-d illustrate that the wood moisture content has little effect when the concentration of all-acrylic foam modifier is a t the lowest level. The effect of wood flour moisture content becomes large at higher acrylic processing aid contents. This behavior is clearly seen by comparing the magnitude of the difference between the lowest (3%) and highest (9%) wood flour moisture content lines (Rgs.2a-d).The difference is small at low all-acrylic foam modaer content whereas opposite trend is observed at higher concentration of all-acrylic foam modifier. These results imply that less amount of all-acrylic foam modifier is required for a given level of foam density when wood flour with high moisture content is used in the formulation. The above-descrilxd interaction between all-acrylic foam modifier content and wood flour moisture content was expected because the all-acrylic foam modifier improved melt elasticity of the matrix (17-19), so that evolving gas is trapped and the bubbles are prevented from coalescing during the foaming process. With the increase of wood flour moisture content, the amount of gas dissolving in the molten polymer matrix for nucleation ;md cell growth also increases. As a result, nucleated bubbles can grow more easily and the foam density is reduced. Moreover, the changes in the average cell size of foamed composite samples due to the effect of wood flour moisture content illustrate this phenomenon. The results listed in Table 6 shows that foamed samplles with dried wood (sample 1) produced smaller cell :sizeswhen compared to the samples with 12% moisture content (sample 2).These results imply that the moiisture present in wood flour acts as an effective foaming agent, but the addition of allacrylic modifier is needed in the formulation for effective bubble growth. The effect of incorporating a chemical foaming agent (CFA) during foaling process was also examined to Table 6. The Effect of Different Processing Conditions on the Average Cell Size OF the Rigid PVCMlood-Flour Composites Foamed Without Using Chemical Foaming Agents. Average cell size Sample identification' (WY 0% MC-5 phr K40(l-170°C (Sample 1) 12% MC-5 phr K400-170°C (Sample 2) 12% MC-5 phr K400-210°C (Sample 3) 60 254 87 'MC IS the moisture content of wood flour; K-400 IS the allacrylic foam modher; and the temperature descnbes the wtruder die temperature used dunng the expenments *Average cell sizes were measured directlyfmm SEM mlcmgraphs evaluate possible synergistic effect between the CFA and the moisture contained in wood flour. Comparison of Fig. 2a with Fig. 2c shows that rigid WC/woodflour composite can dectively be foamed without using chemical foaming agent in the formulation, regardless of the extruder die temperature (Fig. 2b vs. Rg. 2d). The result suggests that there is no synergistic d e c t between the CFA and the moisture contained in wood flour. The amount of gas produced from wood flour moisture is sufficient to produce rigid WC/wood-flour composite foams. However, the density increases with increased die temperature (Fig.2a vs. 2b and Fig. 2c vs. 2d).independent of the CFA content. It is believed that processing at higher extruder die temperature combined with the addition of all-acrylic foam modifier in the formulation lowered the viscosity of the matrix too much; resulting in an improper cell growth due to the excessive gas loss during foaming process (7, 9, 11, 23).The results listed in Table 6 (sample 2 vs. sample 3) show that samples foamed at higher temperature produced smaller average cell size.This result implies that rigid WC/wood-four composites could effectively be foamed at lower processing temperature, using bound water, which requires less energy to remove from wood cell wall, as foaming agent. CONCLUSIONS The dependence of the density of rigid WC/woodflour composite foams on the all-acrylic foam modifier content, wood flour moisture content, chemical foaming agent content, and extruder die temperature was investigated using a central composite design. The complex relationship among the variables was explained with quadratic response model and following conclusions can be drawn: i. There is no synergistic effect between the CFA and the moisture content of the wood flour. Rgid WC/wood-flour composite can effectively be foamed without using any chemical foaming agents in the formulation since the amount of gas produced from the moisture contained in the wood flour is sufficient for bubble nucleation and growth during the foaming process. However, successful foaming of the composite with moisture content strongly depends on the presence of all-acrylic foam modifier in the formulation. ii. Less amount of all-acrylic foam modifier is required for a given level of foam density when wood flour with high moisture content is used in the formulation. This is due to the trapping of evolving gas and prevention of bubble coalescence during the foaming process by all-acrylic foam modifier. iii. R@d WC/wood-four composites could effectively be foamed at a lower processing temperature, suggesting that bound water, which requires less energy to remove from wood cell wall, can be used as the foaming agent. JOURNAL OF WNKL & ADDl77WE TECHhlOLOGY, DECEMBER 2002, V d . 8, No. 4 269 Lament M. Matuana and Fatih Mengeloglu ACKNOWLEDGMENTS Partial funding for this project was provided by the CSREES-USDA-NRI (Agreement No. 99-35103-8601). REFERENCES 1. J.R Patterson, J.Vinyl Addit. TechnoL,7 (2).138 (2001). 2. R. Donnell, Zhber Roc., (November issue),28 (2000). (2000). 3. V.Wigotsky, Plastics Eng., 86 (12),26 4.J. H.Schut, Plastics TechnoL.46.46(1999). 5 . D. Crosby, J. 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