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Expansion mechanisms of plastic/wood-flour composite foams with moisture, dissolved gaseous volatiles, and undissolved gas bubbles.

INTRODUCTION

Wood-flour is gaining increasing acceptance as a filler material, as it significantly increases the stiffness and lowers the cost of the resulting composites (1-12). However, the applicability of plastic/ wood-flour composites is limited because of their lower ductility, reduced impact strength (3, 13-15), and a higher density (16-19) in comparison with those of unfilled thermoplastics and natural wood. If plastic/ wood-flour composites could be foamed properly with a fine-celled structure, their ductility, impact strength, and weight per unit volume would all improve (13). thereby significantly enhancing their overall utility. Thus, it is desirable to produce plastic/wood-flour composite foams having a fine-celled structure. Since extrusion is one of the most cost-effective processes for production of plastics, the commercial utility of foamed plastic/wood-flour composites would be further elevated, if these could be manufactured using a continuous extrusion-based technology.

The moisture present in wood-flour becomes a major concern because it is generally known to cause deterioration of the cell structure (20). Some obvious effects of moisture on foam processing include corrosion of equipment, poor cell structure, blistered surface, and. contraction due to water condensation. The moisture is released during the healing (plasticating) stage of extrusion, and is retained in the melt in a gaseous or liquid state as it has a very low solubility (21). Once the extrudate exits the die, moisture will be vaporized immediately because of the pressure drop while the temperature of the extrudate is high. This can cause both a poor cell structure and blistered surface because of the low nuclei density developed by moisture. Consequently, a non-uniform cell distribution and a large average cell size usually characterize the obtained foams. In addition, when the foam cools down. the water vapor condenses at around 100[degrees]C, forming vacuums within the cells of the foam, resulting in shrinkage of the extrudate.

No literature could be found that systematically describes the effects of moisture on the foaming and expansion mechanisms of plastic/wood-flour composite materials in extrusion processing. Although the effect of moisture has been pointed out in some earlier studies (20, 22), the science behind the effect of moisture in plastics/wood-flour composites is still uncertain. Issues involved include whether a better foam structure can always be obtained with lesser amount of moisture, what is the critical moisture level for obtaining fine-celled structure, and how the moisture can be adequately removed to the critical level. This research area requires systematic study that would characterize the effects of moisture and other volatiles.

Dambauld (23) foamed polyolefin elastomer blends by using water as a blowing agent and reported density reductions of 10% to 70% while using 0.1 to 10 weight percent of water. However, attempts to foam pure PE or PP with water were unsuccessful. Other authors (24-28) also describe the importance of removing moisture for producing non-foamed plastic/ wood-flour composite materials. Nevertheless, it is obvious that the critical moisture content directly depends upon the plastics-wood-flour combination: i.e., if the percentage of wood-flour in the plastics/wood-flour composites varies, the respective critical moisture content would also vary.

Wood contains four main constituents: cellulose (44%-50%), hemicelluloses (20%-25%), lignin (16%-33%). and extractives (5%-10%) (29). The main fibrous components cellulose and hemicelluloses are bound together by the lignin. Extractives are other extraneous materials that are present but do not contribute to the structural formation of the wood.

Upon being heated, wood/wood-flour releases volatiles, in addition to the adsorbed moisture. The adsorbed moisture varies according to the ambient humidity to which wood is subjected to during storage and is the first constituent to be removed upon heating. When the temperature of wood/wood-flour is raised beyond 100[degrees]C, additional [H.sub.2]O and other volatiles are released (29). This [H.sub.2]O is part of the chemical composition of these components and is released upon their degradation because of heating. The cellulose, hemicelluloses, and lignin, the chemically well-defined constituents of wood, are known to start to give out moisture while being heated to temperatures higher than 100[degrees]C in addition to other possible emissions such as C[O.sub.2] (29). However, the extractives, which are the fourth constituent of wood, comprise of a number of different kinds of materials such as resins, waxes, and tannins (29). These materials are also volatilized while being subjected to elevated temperatures. Therefore, prior to undertaking a study of foaming of plastic/wood-flour composite materials, the effect of moisture/volatiles, which are released from the wood-flour during heating, should be clearly understood.

This paper presents the results of one such study, in which the objective is to determine the effects of moisture/volatiles on the plastic/wood-flour composite foams produced in an extrusion process. First, the extractive contents in the wood-flour were determined using acetone as the extracting solvent. Then a series of TGA experiments were carried out on the wood-flours, extractives, and acetone-extracted wood-flours to study their weight loss behaviors during heating. Finally, wood-flours and HDPE were mixed and processed in a tandem extrusion system to study the foaming behaviors in extrusion processing. The expanding mechanisms of wood-flour composite foams due to moisture and gaseous volatiles have been also analyzed.

EXPERIMENTAL

Drying of Wood-Flour

The wood-flour was dried in an oven at various designated temperatures for 12 hours tinder vacuum. Vacuum was maintained while moisture and other volatiles were released from the wood-flour. At the end of the drying time, the vacuum was released under a 99.99% pure nitrogen environment, to prevent reabsorption of moisture from the atmosphere. This wood-flour was used in all extraction, TGA, and extrusion foaming experiments described below. The dried wood-flour was kept under pure nitrogen environment at all times.

Acetone Extraction

The acetone extraction was carried out in a Soxhlet apparatus (30), The weighed wood-flour was enclosed in sample bags made from fine porous 'lint free' wipe paper and subjected to extraction for 6 hours (30). The heating rate was adjusted to allow at least tour boiling/condensation cycles per hour. The acetone containing the extractives was then evaporated in a bottle of known weight using a Buchi Rotary Evaporator, so that only the extractives remained in the bottle, which was then dried and weighed again to determine the extractive content.

Thermogravimetric Analysis of Wood-Flour

Thermogravimetric analysis (TGA) was conducted to study thermal devolatilization of wood-flour using TGA model SDT 2060 from TA Instruments. For the first set of experiments, the temperature was abruptly increased to the desired level and isothermal conditions were maintained for the duration of the experiment. These sample fibers were not oven-dried. For the second set of experiments, the wood-flour was initially heated isothermally for 4 minutes at 75[degrees]C, to achieve the same drying states for all samples, and then the temperature was suddenly raised to the desired isothermal conditions for the duration of the experiment. The oven was continuously purged with nitrogen during the experiment to remove the released volatiles and to maintain dry conditions. The purging also prevented the degradation of wood-flour as a result of oxidation. The same procedure was followed for the extractives and the acetone-extracted wood-flour.

Extrusion Foaming

Materials

All the experiments were carried out using 50 wt% plastic pellets and 50 wt% wood-flour. The plastic used was HDPE Grade 58G from Nova Chemicals. having a melting temperature [T.sub.m] = 129.5[degrees]C, a crystallization temperature [T.sub.c] = 115.4[degrees]C. and a density of 0.956 g/[cm.sup.3]. The MFI was 0.9 g/10 min. The wood-flour was standard softwood (pine) grade 12020, supplied by American Wood Fibers. Fifty weight-percent (50 wt%) of these fibers passed through the sieve of 120 mesh size (125 microns) and were retained on 140 mesh (106 microns). The moisture content and initial specific gravity of the fibers were 8% and 0.4, respectively. The coupling agent used for improving the adhesion between the hydrophobic HDPE and the hydrophylic wood-flour was maleated polyethylene Fusabondi[R]E Modified PE (E-MB-100D) supplied by DuPont, Canada. The chemical blowing agent (CBA) used was Celogen OT from Uniroyal Chemicals, which is known to decompose at 158.9[degrees]C, and to liberate 125 [cm.sup.3]/g of 91% [N.sub.2] and 9% [H.sub.2]O. Different amounts of CBA (from 0% to 2% of total weight) were used. All the materials were used as supplied.

Experimental Setup

The experimental setup used in this study is schematically shown in Fig. 1. A twin-screw feeder (AccuRate 3006 from Arbo Engineering) with a controller (Danfoss Varispeed A2000) was used for feeding the dry, blended foamable mixture of plastic pellets and wood-flour into the first extruder--a counter-rotating twin-screw extruder (model D6/2 C from C. W. Brabender) driven by a 3.7 kW (5 HP) DC motor. The output of the first extruder was led into a second extruder--a 3/4" single-screw laboratory, extruder (model 3023-GR-8 from C. W. Brabender) equipped with a 5 hp motor (Allen Bradley: 1329 Inverter Duty Motor) and a variable speed drive unit (Allen Bradley 1336 Impact). The interconnection of the two extruders was open to the atmosphere so that the evaporated water and other emissions could be devolatilized (31). At the outlet of the second extruder, a diffusion-enhancing device containing a static mixer was placed to assist in dissolution of the gas generated from the CBA into the plastic matrix. It was further connected to a heat exchanger, which uniformly cooled the polymer/wood-flour composites by using an oil-bath based temperature controller. Finally, the extrudate passed through a nozzle die having a diameter of 1.27 mm (0.050") and length of 12.7 mm (0.500'). The process parameters were monitored and controlled by placing J type thermocouples (Omega). with temperature controllers (Omega CN9000A), and Dynisco PT462E-10M-6/18 pressure transducers, with pressure indicators (Omega DP25-S), at various important points along the tandem extrusion system.

[FIGURE 1 OMITTED]

Experimental Procedure

The foamable composite mixtures were prepared in the following consecutive steps. First, a predetermined amount of plastic pellets and 3 wt% of coupling agent. based on the total weight of HDPE and wood-flour, were mixed in a dry blender to ensure uniform distribution of the coupling agent pellets throughout the blend. Then, the oven-dried wood-flour (equal to the weight of HDPE) was added to the mixture and dry-blended, to achieve homogeneity. For experiments with CBA, 0.5, 1.0, 1.5 and 2.0 wt% Celogen OT (based on the weight of the plastic resin) was added along with the HDPE and coupling agent and dry. blended. Then, undried wood-flour (equal to the weight of HDPE) was included and dry blended again. The foaming experiments were conducted using the tandem extrusion system depicted in Fig. 1. The dry-blended composite mixture was fed into the hopper of the twin-screw extruder using a twin-screw feeder. The tandem extruders were starve-fed and a flow rate of 10 g/min was maintained. The main function of the twin-screw compounder was to provide effective mixing of the plastic with wood-flour and additives. The temperature profile at the first extruder was set to be 145[degrees]C, 150[degrees]C, and 155[degrees]C. The extrudate passed through the devolatilizing vent at the interconnection of the two extruders of the tandem system, and entered the second, single-screw extruder. The vent temperature was maintained at 160[degrees]C.

The maximum processing temperature in the second extruder was kept at 185[degrees]C in the third zone and in the diffusion-enhancing device to decompose all CBA. For purposes of comparison, the same temperature profile was used even when CBA was not used. At this higher temperature, the wood-flour emissions would be enhanced and the released gases, if soluble. would be dissolved in the polymer matrix under the action of high shear at an elevated temperature and pressure in these two sections of the extrusion system. The temperature of the melt is then lowered homogeneously in a heat exchanger to 150[degrees]C. The nozzle temperature was decreased from 15[degrees]C downwards (1-2[degrees]C at a time). At various stable conditions. samples were collected.

RESULTS AND DISCUSSION

Acetone Extraction

Table 1 shows the acetone-extractive contents obtained from the undried and oven-dried wood-flour as a percentage of unextracted wood-flour. The undried wood-flour had about 10% of these extractives. It can be seen that as the drying temperature was increased, the amount of extractives lost in drying increased. However, after 175[degrees]C, no further loss of extractives was observed, which indicates that the remaining extractives did not contain any volatile material in the temperature range under study. The extractives in the wood-flour under study had a volatile content of about 4%.

Thermogravimetric Analysis of Wood-Flour

Thermal Analysis of Wood-Flour at Constant Heating Rates

Figure 2 shows the thermograms of wood-flour being subjected to heating rates of 5[degrees]C, 10[degrees]C, and 20[degrees]C per minute. At the higher heating rates, the amount of weight loss al any temperature is less. This suggests that a high heating rate and a low residence time would help in decreasing the volatile emissions. Figure 3 also suggests that different volatile emissions are released in each temperature range because the slopes of the weight loss curves are clearly distinct in each range. For example, it is believed that the rapid weight loss below 75[degrees]C was due to the demoisturization of the adsorbed water. But from 75[degrees]C to 150[degrees]C, the weight loss was almost linearly decreasing at a very low slope. This implies that devolatilization was not active, and one or very few volatile emissions were released in this temperature range. But above 150[degrees]C, the weight loss curve became sleeper, indicating that devolatilization became very active and many kinds of volatile emissions were released.

[FIGURES 2-3 OMITTED]

It is not clear when the release of the adsorbed water was finished and when the thermal degradation began to occur from the thermograms. It seems that slight degradation started to occur at around 80[degrees]C, and that serious degradation began around 150[degrees]C. Since the extractives and the other constituents of wood will show different degradation temperatures, their weight loss behaviors were separately investigated, as described in the following sections.

Isothermal Analysis of Wood-Flour at Low Temperatures

The devolatilizing behavior of volatiles from wood-flour during drying was simulated using a TGA. The amount of moisture released from the wood-flour was measured at low isothermal temperatures of 50[degrees]C, 75[degrees]C, and 100[degrees]C, and the results are shown in Fig. 3. It can be seen that at 50[degrees]C the weight loss was rather gradual, but at the higher temperatures of 75[degrees]C and 100[degrees]C, the weight loss rates became steeper. In fact. the weight loss behaviors became similar for both these temperatures, and after remaining at isothermal conditions for a few minutes, no significant differences in weight drop and weight drop rate were observed at these temperatures.

The results of Fig. 3 indicate that the adsorbed moisture can be substantially removed by isothermal heating at 75[degrees]C in 2 to 3 minutes. Consequently. isothermal heating for 4 minutes was adopted as a standard procedural step for achieving oven-dried conditions of the wood-flour samples in the TGA experiments other than those shown in Figs. 3 and 4. It should be noted that the weight loss due to the devolatilization of adsorbed moisture depends on the humidity of atmosphere unless dried wood-flour is used for TGA.

[FIGURE 4 OMITTED]

Isothermal Analysis of Wood-Flour at High Temperatures

Since the wood-flour is subjected to high temperatures during the foaming extrusion process, in order to understand the devolatilization behavior of wood-flour in extrusion, the weight change of wood-flour at various isothermal conditions was investigated using TGA at an interval of 25[degrees]C starting from 100[degrees]C. The TGA thermograms of wood-flour are summarized in Fig. 4. It is interesting to note that the weight loss did not increase uniformly with increasing the temperature. For example, after one hour, the weight loss at 125[degrees]C was only about 0.25% more than that at 100[degrees]C, whereas the weight loss at 150[degrees]C was about 1.50% more than that at 125[degrees]C. For the next 25[degrees]C increase in temperature, i.e., at 175[degrees]C, the weight loss increased by about 1%. From 175[degrees]C onwards, the weight loss increased at an ever-increasing rate. This suggests that the weight loss in wood-flour occurred as a result of the devolatilization of different components, which decomposed or evaporated at different temperatures, and with different rates.

In the extrusion processing, the wood-flour is heated as the plastic gels molten. This implies that the moisture and other volatiles are released from the wood-flour in the extrusion barrel. Since the formed solid bed is connected to the hopper al the atmospheric pressure while melting occurs in the barrel (32), the released emissions can easily escape through the hopper. As a result, a substantial amount of water vapor and other volatiles are automatically removed through the hopper before being trapped in the molten mixture and pushed further downstream into the die. In the second extruder, additional material is devolatilized as the temperature of the barrel is increased beyond the temperature of the vent. Additional gases and any residual moisture are also released as the extrudate undergoes extended residence time al elevated temperatures in the second extruder.

Thermograms of Acetone-Extracted Wood-Flour

Figure 5 shows the thermograms of acetone-extracted wood-flour. It can be seen that at temperatures below 200[degrees]C, the weight loss from these fibers was very low, less than 0.5%. Therefore, it can be inferred that after the loss of the adsorbed water, the major contribution to the volatile emissions from wood-flour during extrusion processing is from the extractives. In our extrusion setup, the moisture and volatiles are substantially removed because of the devolatilizing vent after the first extruder (31). This also indicates that if the temperature of plastic/wood-flour composites is kept under 200[degrees]C, the thermal degradation of the main constituent components of the wood-flour, i.e., cellulose, hemicelluloses and lignin, would tie negligible.

[FIGURE 5 OMITTED]

Thermograms of Extractives

Figure 6 shows the thermograms of the extractives. As expected from the results of Fig. 5, the extractives undergo a substantial amount of weight loss in the TGA (also, in the processing). It can be estimated that in addition to moisture, about 15% extactives or 1.5% of the weight of the original wood-flour, evolved into volatile emissions. As mentioned earlier, these extractives consist of a number of resins, waxes, tannins, etc. Some of these constituents are volatile and evaporate while being heated, and thereby participate in the foaming process. However, the constituent components of the volatiles from the extractives have not been known so far.

[FIGURE 6 OMITTED]

Extrusion of Plastic/Wood-Flour Composites

The plastic/wood-flour composites were processed on a tandem foaming extrusion system with a devolatilization vent (31). The same temperature profile was maintained in the extrusion system during all the experiments conducted.

Expansion Behavior of Oven-Dried Wood-Flour Composite Foams

Figure 7a shows the foam density of composites containing the undried wood-flour as well as the wood-flour oven-dried at various temperatures. The density reductions were obtained only from the volatile emissions given out by the wood-flour without using any other blowing agents. It can be seen that the foam density of undried wood-flour composites was lower than that of oven-dried wood-flour composites at high die temperatures, whereas the opposite was true at low temperatures.

[FIGURE 7 OMITTED]

The composites containing oven-dried wood-flour exhibited a small temperature window in which the foam density was reduced to a desired wood-density range of 0.6 g/[cm.sup.3] to 0.8 g/[cm.sup.3]. But, the temperature window was too narrow to derive any positive conclusions. Since the adsorbed moisture was removed from the wood-flour in the oven, most volatiles released from the wood-flour during extrusion would be soluble gases such as C[O.sub.2], CO, and organic volatiles (29) (although a small amount of moisture must also have been generated from the wood-flour). Therefore, one possible explanation could be: the volume expansion behaviors of the wood-flour composite foams could have been governed by the fundamental expansion mechanisms of foams blown by a normal blowing agent, i.e., gas loss through diffusion or crystallization (i.e., solidification) of plastic matrix due to the presence of the dissolved gas (33, 34).

The blowing-agent efficiency defined to be the ratio of the actual expansion ratio to the ideal expansion ratio can be estimated for the given blowing agent used. Figure 6 shows that the volatiles released from the wood-flour dried in the range of 100[degrees]C to 150[degrees]C would be 5 wt% to 15 wt% of extractives for a typical residence time of 10 minutes. This corresponds to 0.5 wt% to 1.5 wt% of wood-flour, or 0.25 wt% to 0.75 wt% of wood-flour composites. If all the volatiles are assumed to be C[O.sub.2]. then the ideal expansion ratio that can be calculated from Eq 1 (35) would be in the range of 5.2 to 13.7.

(1) [[PHI].sub.ideal] = 1 + [m.sub.gas] [v.sub.gas at Tc of HDPE]/ [m.sub.HDPE/wood-flour] [v.sub.HDPE/wood-flour at 25[degrees]C]

Since the actual expansion ratio, which is the unfoamed density (1.2 g/[cm.sup.3] divided by the foam density (0.7 g/[cm.sup.3]) at the optimum temperature, was approximately 1.7, the blowing-agent efficiency ranged from 12% to 33%. This means that only a small portion of the volatiles was used for actual blowing. Most of the gas must have been lost in the extrusion barrel during plastication of plastic or at the die exit during the expansion process. Especially, the weak interaction between the wood-flour and plastic must have expedited gas loss because of the fast diffusion through the interlace of the wood-flour and plastic (36).

The narrow temperature window shown in Fig. 7a indicates that a close control of temperature is required to achieve the desired foam density from the composites with oven-dried wood-flour only. Since accurate temperature control of industrial extrusion systems is difficult to achieve, it may not be possible to expect a respective density reduction due to the gaseous volatiles from the extractives of oven-dried wood-flour when using industrial extruders. Therefore, adding a blowing agent, either physical or chemical, is recommended to control the density of wood-flour composites (31, 37).

Expansion Behavior of Undried Wood-Flour Composite Foam

The cell growth mechanisms, occurring in the presence of water vapor, would be quite different from those occurring in the presence of other blowing agents. When moisture is involved, although the water vapor assumes the role of a blowing agent, it fails to dissolve in the plastic matrix because of its low solubility (21). Therefore, when a foam structure is developed at the die exit, the volume expansion attributable to moisture occurs promptly because the nondissolved moisture evaporates immediately at a lowered pressure. On the other hand, the expansion due to the diffusion of other dissolved gases into the nucleated cells will also occur. Consequently, cell growth in the presence of water vapor and dissolved gas(es) takes place in two stages. The first is rapidly promoted by the vaporization of moisture, whereas the second is more slowly promoted by the diffusion of dissolved gases, depending upon the diffusivities of the gases.

The undried wood-flour composite foams did not exhibit the sharp density reduction that was observed in the oven-dried wood-flour case. This was because the expansion mechanisms of wood-flour composite foams with a high moisture content would be entirely different from those of foams with large dissolved gases. When there was a large amount of moisture, many non-dissolved moisture pockets due to the low solubility of water remained as a separate liquid phase in the plastic melt during extrusion. These pockets will rapidly expand, through vaporization of water, upon exiting the die because of depressurization. Since this expansion occurs immediately, the hot melt will not have enough time to cool in the air (as in the case of foam processing with dissolved gas where the expansion occurs gradually because of the slow diffusion of dissolved gas from the melt). As a consequence, the temperature of the melt in its fully expanded state will be still high. Since the cell wall thickness gets thinner as the expansion ratio increases (35, 38), the cell-to-cell diffusion is expedited and the cell coalescence phenomenon will be very active in this early-expanded foam (35, 38). Especially, the cell walls of the wood-flour composite foams are easily ruptured by the wood-flour (13, 36), and therefore, the loss of the moisture will be more expedited through the openings of cell walls. Furthermore, the weak interaction between the wood-flour and plastic melt will also provide a channel for quick loss of moisture despite the use of a coupling agent (13, 36). As a consequence, the rapidly expanded foam will shrink immediately because of the loss of moisture and the final foam density will increase significantly.

Since a low die temperature will increase the stiffness of the plastic melt, and will decrease the vapor pressure, the initial expansion rate of foam will decrease as the die temperature decreases. As a result. the cell-to-cell diffusion will be retarded because of the decreased diffusivity at a lower temperature was because of the increased cell wall thickness. Also, the cells would be less coalesced. Therefore, in conclusion. the expansion ratio will increase, or conversely the foam density will decrease, al a lower temperature. However. Fig. 7a shows that the sensitivity of foam density with respect to the die temperature was very low. This indicates that even at a low temperature. close to the crystallization point, the initial expansion of the foams due to the vaporization of water at the die exit occurred vigorously, and most of the vapor and gaseous volatiles were lost at that moment.

Effect of Drying Temperature on Cell Morphology

The cell morphologies of composites made from wood-flour dried at 95[degrees]C, 105[degrees]C and 150[degrees]C are shown in Fig. 8. There were no significant differences between the cell structures of the foams produced with wood-flour dried al 95[degrees]C and 105[degrees]C. This indicates that the same amount of volatiles (i.e., the moisture) has been removed from the wood-flour at these temperatures and that the boiling point of water, 100[degrees]C, did not seem to have a significance on the emissions from the wood-flour. It should be noted that the previous TGA results measured at 75[degrees]C and 100d[degrees]C shown in Fig. 3 indicated the same. On the other hand, the SEM micrographs of the foam produced with wood-flour dried al 150[degrees]C showed the presence of bigger cells with a low cell density. This indicates that drying of wood-flour al 150[degrees]C actually decreased the soluble gaseous volatiles from the wood-flour (i.e., from the extractives) compared to the cases of drying at 95[degrees]C and 105[degrees]C. It is well known that as the amount of dissolved gas decreases, the bubbles get bigger because of the decrease in the cell density (39, 40).

[FIGURE 8 OMITTED]

Expansion Behavior of Undried Wood-Flour Composite Foams With a CBA

In order to practically achieve a desired foam density range of 0.6 to 0.8 g/[cm.sup.3] even from the non-dried wood-flour composite foams, additional experiments were conducted with a CBA (Celogen OT).

Figure 9a shows the density of plastic/wood-flour composite foams as a function of the amount of CBA. The CBA was decomposed in the second extruder because of the elevated temperature in the third zone and in the diffusion-enhancing device. The liberated blowing gases had a significant effect on lowering the foam density. This effect was greater at lower die temperatures, indicating that these gases too escaped easily at higher processing temperatures.

[FIGURE 9 OMITTED]

It is notable that the foam density of the undried wood-flour composite foams with the CBA decreased monotonously as the temperature decreased without showing a minimum as shown in Fig. 7a. Since the Celogen OT is known to generate [N.sub.2], which has a low solubility compared to that of C[O.sub.2], this generated gas would remain in a separate phase in the plastic melt. It is believed that the moisture generated from the wood-flour remains near or around its source, i.e., the wood-flour, whereas the evolved [N.sub.2] gas is finely dispersed in the plastic matrix because of the dispersion of the CBA particles. Although the presence of moisture would cause the same instantaneous expansion phenomena at the die exit, as described earlier, the finely dispersed [N.sub.2] bubbles would affect the cell morphology significantly. First of all, it is expected that the cells developed by the vaporization of moisture will be actively coalesced and collapsed as described before. In the meanwhile, the dispersed [N.sub.2] bubbles will also assist the initial expansion because most of the [N.sub.2] gas will be undissolved as in the case of moisture. However, coalescence of the expanded [N.sub.2] bubbles may not be as severe as that of the bubbles formed by the vaporized moisture, which are easily opened by the wood-flour (35, 38). Therefore, the [N.sub.2] bubbles may remain entrapped in the polymer matrix. At high temperatures, the diffusivity of the [N.sub.2] in these bubbles would be high enough to escape quickly to the environment through cell-to-cell diffusion, but at low temperatures, some of the [N.sub.2] will be saved in the bubbles, and thereby it will contribute to the reduction of the density of wood-flour composite foams. As evident from Fig. 9a, the greater the amount of blowing agent, the greater will be the entrapped [N.sub.2] and the greater will be the density reduction.

Figure 10 shows the cell morphology of composites foamed with different amounts of CBA at the same nozzle temperature of 140[degrees]C. Although the foam density decreased (or equivalently, the expansion ratio increased) with increasing amount of CBA, the cell density in general decreased with increasing blowing agent content. This implies that the degree of cell coalescence increased as the CBA content increased because of the increased expansion ratio. The best cell morphologies were observed when the blowing agent amount was 0.5%.

[FIGURE 10 OMITTED]

Behavior of Processing Pressures With Varying Die Temperature

Figure 7b shows the variations in processing pressures as the die temperature and the oven-drying temperature changed. As the die temperature decreased, the processing pressure increased because of the increased viscosity of melt. On the other hand, the lower drying temperature resulted in a lower pressure. For example, the extrudate containing the wood-flour dried at 150[degrees]C exhibited a sharp increase in pressure as the die temperature decreased. But at a low drying temperature, fewer volatiles were lost, and consequently, more volatiles were generated from wood-flours during extrusion (41). This in turn caused more plasticizing effects, so the resultant viscosity was lower, giving rise to the lower processing pressures. Similar behaviors were observed when the CBA content (and thereby the gas generated) was increased in extrusion loam processing using undried wood-flour. as shown in Fig. 9b.

SUMMARY AND CONCLUSIONS

For loosely packed wood-flour, drying at 75[degrees]C seemed to remove most of the adsorbed moisture content so that this temperature could be used to achieve uniform drying condition for all other experiments. The extraction experiments indicated that the wood-flour contains about 10% extractives, and 4% of these are volatile in nature. The TGA results suggest that different constituents of wood devolatilize at different temperatures. These results also indicate that in the temperature range from 100[degrees]C to 200[degrees]C, extractives undergo more weight loss than other constituents of wood-flour.

During extrusion processing of foamed plastic/ wood-flour composites, a better cell morphology was obtained by removing the adsorbed moisture. In this case, some gaseous emissions from the wood-flour seemed to have dissolved in the plastic melt and favorably contributed to the development of cell morphology. When a large amount of moisture was included, volume expansion of wood-flour composite foams occurred immediately at the die exit, owing to the vaporization of water, but through vigorous coalescence of cells, the foams collapsed, leading to higher-density foams regardless of the die temperature. When an exothermic CBA (Celogen OT) was used for foaming the undried wood-flour composites, the finely dispersed yet undissolved [N.sub.2] bubbles contributed to the density reduction. The use of CBA helped in reducing the foam density to the desirable range of 0.6 g/[cm.sub.3] to 0.8 g/[cm.sub.3] but did not improve the cell morphology.

NOMENCLATURE

[PHI] Expansion ratio

[[PHI].sub.ideal] Theoretical expansion ratio

[m.sub.gas] Mass flow rate of the blowing gas, g/s

[m.sub.HDPE/wood-flour] Mass flow rate of HDPE and wood-flour mixture, g/s

[[rho].sub.f] Foam density, g/[cm.sup.3]

[[rho].sub.P] Polymer density, g/[cm.sup.3]

[v.sub.gas] Specific volume of the blowing gas, [cm.sup.3]/g

[v.sub.HDPE/wood-flour] Specific volume of HDPE/wood-flour composite, [cm.sup.3]/g
Table 1. Extractive Contents From the Undried and
Oven-Dried Wood-Flour.

Drying Temperature ([degrees]C) Extractive Content (wt%)

 Undried 9.96
 100 9.56
 125 8.32
 150 7.82
 175 5.80
 200 5.72
 225 5.88


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G. M. RIZVI, C. B. PARK, W. S. LIN, G. GUO, and R. POP-ILIEV

Microcellular Plastics Manufacturing Laboratory,

Department of Mechanical and Industrial Engineering

University of Toronto

5 King's College Road

Toronto, Ontario, Canada M5S 3G8

This paper was presented at "Emerging Technologies for the New Millennium," held in Montreal. Dec. 10-11, 2001.
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Author:Rizvi, G.M.; Park, C.B.; Lin, W.S.; Guo, G.; Pop-Iliev, R.
Publication:Polymer Engineering and Science
Date:Jul 1, 2003
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