Pilot-scale production and material properties of extruded straw-plastic composites based on...

Abstract

The predominant filler used in the commercial extrusion of natural fiber reinforced thermoplastic composites in North America is wood flour. Fibers such as wheat straw (Triticum aestivum L.) represent a promising filler alternative. In this investigation, untreated and fungal-treated

wheat straw was employed as filler for extruded high-density polyethylene (HDPE) based composites. Fungal treatment of straw was accomplished with the white-rot fungus Pleurotus oetreatus (Jacq. ex Fr. Kummer) to improve adhesion between straw and HDPE and thus mechanical properties of straw-plastic composites (SPC). The straw used in this research was not sterilized prior to fungal treatment for 6 and 12 weeks to achieve maximum cost-efficiency of large-scale SPC production. Our results indicate that the mechanical properties of SPC produced with untreated straw are comparable to those of a wood-plastic composite based on pine flour. In the temperature range with the most relevance to the extrusion process (100[degrees] to 300[degrees]C), fungal-degraded straw appeared thermally less stable than untreated straw, but this did not negatively affect composite manufacture. Complete dominance of P. ostreatus on the straw was not achieved under the non-sterile conditions applied in this study. Furthermore, treatment did not have a statistically significant ([alpha]-value of 0.05) influence on either modulus of rupture or modulus of elasticity of SPC. Hence, under the conditions applied in this study, degraded straw offered no advantages compared to untreated straw. At the same time, it was demonstrated that untreated wheat straw offers potential as a substitute for wood fillers in the extrusion of thermoplastic composites.

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Extruded wood-plastic composites (WPC) have experienced tremendous market growth in North America, primarily for application as deckboard (Wolcott and Englund 1999, Clemons 2002). Wheat straw (Triticum aestivum L.) could potentially be used instead of wood as filler in formulations resulting in the manufacture of straw-plastic composites (SPC). The use of straw in the manufacturing of various fiber-based composite materials has been previously suggested and evaluated (White and Ansell 1983, Sauter 1996, Simonsen 1996, Zhang et al. 2003, Boquillon et al. 2004). Simonsen (1996) suggested the use of rye grass straw as filler in polyethylene and polystyrene; however, composites were produced by compression-moulding on a small scale whereas in the present investigation, wheat straw- high-density polyethylene (HDPE) composites were manufactured using a commercial-scale extrusion process.

Wheat straw is an annually renewable agricultural byproduct. In the United States alone, 60 million metric tons of wheat straw are currently produced every year, 99 percent of which are either returned to the field by burning, tilling into the soil, or used as an on-farm fuel source (Cheng et al. 2004). The use of wheat straw in the production of natural fiber-reinforced thermoplastic composites potentially generates a new revenue stream for wheat producers, provides an incentive to reduce air pollution caused by field-burning, and accomplishes soil conservation goals in arid climates.

The present investigation succeeds a study by Houghton et al. (2004) in which non-sterile wheat straw stems were treated for 12 weeks with the white-rot fungus Pleurotus ostreatus (Jacq. ex Fr. Kummer) to estimate the variation in chemical composition of the straw with variations in initial moisture content (MC) and inoculum. The fungal straw treatment was intended for application in thermoplastic extrusion. Treatment of wheat straw with P. ostreatus was expected to cause a limited degradation of the waxy cuticle, lignin, and hemicelluloses in the straw without much cellulose removal (Valmaseda et al. 1991, Moyson and Verachtert 1991, Gamble et al. 1996). Fungal degradation of these specific straw components was expected to 1) improve adhesion between straw and HDPE and thus mechanical properties of SPC; 2) increase the internal pore volume and surface area within the straw particles through selective removal of hemicelluloses and lignin; and 3) reduce the amount of HDPE required in the formulations for SPC and therefore, the cost of the SPC. However, fungal treatment may potentially affect the thermal stability of the straw due to the selective degradation of lignin and hemicelluloses, and hence restrict the polymer class of potential thermoplastics to polyolefins (Wolcott and Englund 1999). The primary criterion used in the selection of a thermoplastic for production of a wood- or straw-plastic composite is that the melting or softening temperature of the thermoplastic is less than the thermal degradation temperature of the wood or straw filler (ca. 210[degrees]C for wood and undegraded wheat straw).

The overall goal of the study reported herein was to evaluate the feasibility of generating cost-effective large-quantities of P. ostreatus-degraded wheat straw for use in the commercial production of SPC. The specific objectives were to 1) evaluate the impact of fungal degradation on selected mechanical, physical, and thermal properties of extruded SPC; and 2) compare fungal-treated and untreated SPC to wood-flour-based thermoplastic composites.

Material and methods

Wheat straw

A hard red spring variety of wheat straw (Triticum aestivum L., var. Westbred 936), obtained from Grant 4-D Farms (Rupert, ID) during the year 2000 cropping season was used in all experiments. The internodal straw stems were mechanically separated from the leaves, sheaths, awns, and nodes (Hess et al. 2003), and stored indoors at 21 [+ or -] 2[degrees]C and 13 percent MC prior to the beginning of this study (Houghton et al. 2004). Straw composition is reported in Houghton et al. (2004). The internodal straw stems, or material derived thereof, were used in all experiments reported herein.

Preparation of fungal inoculum

Pleurotus ostreatus ATCC 32,783 was chosen based on its ability to selectively degrade the hemicellulose and lignin fractions in lignocellulosic substrates (Hadar et al. 1993). Fungal stock cultures were maintained at Utah State University on agar slants containing 41 g yeast-malt (YM) agar (Becton, Dickinson and Company, Sparks, MD) per L of water. Fresh slants were prepared and inoculated every 2 weeks. Slants were incubated at room temperature with caps loosely attached. Fungal inoculum from 2- to 3-week-old slants were used to inoculate 100 mL of culture medium in 500-mL Erlenmeyer flasks equipped with cotton plugs. The culture medium was prepared by adding 10 mL of stock mineral solution per L of sterilized YM broth containing 21 g of YM (Becton, Dickinson and Company, Sparks, MD) per L of water. The stock mineral solution (100x) consisted of 3.0 g of MgS[O.sub.4] *7[H.sub.2]O, 0.5 g of MnS[O.sub.4] *[H.sub.2]O, 1.0 g of NaCl, 0.1 g of FeS[O.sub.4]*7[H.sub.2]O, 0.1 g of CoS[O.sub.4], 0.1 g of Ca[Cl.sub.2]*2[H.sub.2]O, 0.1 g of ZnS[O.sub.4]*7[H.sub.2]O, 0.01 g of NaMo[O.sub.4]*2[H.sub.2]O, 0.01 g of CuS[O.sub.4]*5[H.sub.2]O, 0.01 g of A1K[(S[O.sub.4]).sub.2]*12[H.sub.2]O, 0.01 g of [H.sub.3]B[O.sub.3] per one L of water (all chemicals were standard laboratory grade). To this solution, 1.5 g/L nitrilotriacetic acid (NTA) was added and the pH adjusted to 6.5 with KOH. The stock solution was filter-sterilized and stored at 4[degrees]C.

Starter cultures were incubated for 2 to 3 days at room temperature in a metabolic shaker (New Brunswick Scientific Series 25, Edison, NJ) at ~200 rpm. After sufficient growth, the starter cultures were aseptically transferred to 2.8 L Fernbach flasks containing 1.5 L of liquid culture media (same as for starter cultures) and incubated as previously described for 3 to 4 days. Fungal pellets were separated from medium by centrifugation (Sorvall RC-5B) at ~10,000 xg for 10 to 15 minutes and transferred to sterile 500-mL bottles with sufficient spent medium to submerge the pellets. These bottles were shipped under refrigeration to the Idaho National Laboratory (INL, Idaho Falls, ID) and stored at 4[degrees]C until use (no longer than 2 wk from date of arrival).

Fungal inoculation and incubation of wheat straw

The straw was inoculated at INL by spraying liquid inoculum of homogenized mycelia having known optical density (OD) and biomass concentration onto fresh stems using a pressurized garden sprayer (Houghton et al. 2004). Straw stems were inoculated rather than hammermilled fiber to simulate full-scale operations wherein hammermilling was viewed as not a feasible on-farm practice. Straw MC was adjusted for optimal fungal growth (Houghton et al. 2004), and the straw was packed into columns fabricated from glass process pipe. Following 6 weeks of incubation, the treated stems were removed from the glass columns and mixed by hand at 1:10 (w/w) with fresh, air-dried, uninoculated straw stems. Straw MC was brought to 1.6 g of water per g of straw as fresh stems were mixed with the inoculated straw from the columns. The straw was then packed into 208.5-L drums at approximately 7.5 kg dry weight of inoculated straw per drum and incubated for 6 and 12 weeks. Humidified air was supplied to each drum during incubation. Following incubation, the straw was shipped to the Wood Materials and Engineering Laboratory at Washington State University for extrusion and subsequent evaluation of material properties. Straw samples were referred to as neat (untreated), Degrade I (treated for 6 wk), and Degrade 2 (treated for 12 wk).

Thermogravimetric analysis of wheat straw

Ground straw samples were dried in a vacuum oven at room temperature overnight to obtain a straw MC between 5 and 7 percent. Thermogravimetric analysis was performed using a simultaneous thermal analyzer (Rheometric Scientific STA 625, Piscataway, NJ). Straw samples of approximately 4 mg weight were heated in an aluminum pan (Rheometric Scientific L7168, 2 mm, Piscataway, NJ) to 580[degrees]C at a heating rate of 60[degrees]C per minute. The maximum temperature (580[degrees]C) was held for 20 minutes, followed by cooling to 30[degrees]C at a rate of 60[degrees]C per minute. Each sample was run in duplicate. Weight losses were calculated based on the original sample weights with correction for the buoyancy effect of the air.

Extrusion of straw-plastic composites

A fractional factorial design was created to evaluate how the formulation components affect product performance (Table 1). In addition, runs 2, 12, and 17 were performed with southern yellow pine-flour (60 mesh) as filler instead of wheat straw (Table 2). Composite formulations were prepared by initially hammermilling the straw through a 0.69-mm-screen and ovendrying to 1 percent MC. Particle-size distribution of straw was determined using a Ro-Tap[R] sieve shaker (W.S. Tyler, Mentor, OH), with screen sizes ranging from 0 [micro]m to 830 [micro]m. The dried straw particles were blended with various amounts of HDPE (Equistar LB010000, Houston, TX), polyester-based wax (Honeywell OP-100, Morristown, NJ), and coupling agent (maleic-anhydride-grafted polyethylene, Honeywell 575 A, Morristown, NJ). The formulations were extruded with a 35-mm counter-rotating conical twin screw extruder (Cincinnati Milacron CMT 35, Batavia, OH), which produced a 9.53 by 38.1 [mm.sup.2] solid cross section.

Performance evaluation of straw-plastic composites

SPCs made from each formulation in the fractional factorial design (Table 1) were evaluated for flexural strength and stiffness (ASTM D 790-97) (ASTM 1997), and water sorption and thickness swell (ASTM D 1037, modified) (ASTM 1999). Statistical significance of fungal treatment on modulus of rupture (MOR) and modulus of elasticity (MOE) was determined using a General Linear Models Analysis of Variance (GLM ANOVA) approach with Number Cruncher Statistical Software (Kaysville, UT). Statistical differences in MOE and MOR among three formulations using either pine or wheat as a filler (Table 2) were also analyzed using a GLM ANOVA approach with density as a covariate at an alpha-value of 0.05.

[FIGURE 1 OMITTED]

Results and discussion Visual observations on fungal colonization of straw

Visual inspection of the incubated straw showed that P. ostreatus was not the dominating microorganism present in the straw as evident by the lack of white cottony growth characteristic of the fungus. Therefore, any straw modification observed was likely due to the degradation activity of a multitude of fungi comprising those that were naturally present on the straw prior to treatment, the inoculated fungus (P. ostreatus), as well as any microorganisms introduced during incubation.

While it is recognized that sterilization of straw prior to fungal treatment increases overall costs, it may be necessary to achieve dominance of P. ostreatus. In a study conducted on the bench-scale in parallel to the one reported here, we determined that dominance of P. ostreatus over microorganisms present on the straw was achieved when the straw was sterilized prior to fungal inoculation (Schirp et al. 200_). Conversely, Houghton et al. (2004) report that dominance of P. ostreatus over existing microbes in non-sterilized straw was achieved when homogeneously inoculated laboratory-scale columns were used. Hence, if homogeneous inoculation of straw could be performed on a large scale, sterilization of the substrate may not be necessary.

Large-scale sterilization is currently applied in the biopulping industry where wood chips are commonly steam-sterilized prior to fungal inoculation (Messner and Srebotnik 1994, Scott and Akhtar 2003). Implementation of a sterilization process offers the advantage that the total amount of fungal inoculum required for straw treatment may be reduced, which in turn would increase cost efficiency.

In conclusion, it may be possible to achieve successful growth of P. ostreatus on non-sterile straw on a large scale but further research will be needed to determine under which conditions. In addition, alternative fungal species that have been successfully applied in biopulping (Messner and Srebotnik 1994, Scott and Akhtar 2003) and bioremediation (Aust 1990) could be tested as candidate strains for selective removal of hemicellulose and lignin in straw.

Thermogravimetric analysis of wheat straw

While treatment of wheat straw with a white-rot fungus may selectively degrade lignin, it could potentially reduce the thermal stability of the straw since lignin is the thermally most stable component in lignocellulosic materials (Fengel and Wegener 1989). Thermogravimetric analysis, a technique for determining weight loss of a material as a function of temperature, was used to evaluate if the thermal stability of straw was affected by fungal treatment.

Initial substrate weight losses below approximately 100[degrees]C are considered to be due to dehydration. Between 100[degrees]C and 250[degrees]C, the temperature range with the most relevance to the extrusion process, fungal-degraded straw appeared thermally less stable than non-inoculated straw (Fig. 1). This result could be expected since fungal degradation and depolymerization of a lignocellulosic substrate suggests that degraded straw has reduced thermal stability (Beall et al. 1976). However, the slight reduction in thermal stability of fungal-treated straw had no apparent influence on the extrusion process.

The highest weight loss rates occurred in the temperature range between 250[degrees] and 380[degrees]C (Fig. 1). Here, no differences were observed between Degrade 1, Degrade 2, and untreated straw. A less active pyrolysis stage, ranging from approximately 380[degrees] to 600[degrees]C, followed in which untreated straw appeared thermally less stable than inoculated straw. For example, 40 percent relative weight was reached at 378[degrees]C for the untreated and at 405[degrees]C for Degrade 1 (Fig. 1). This may indicate that in the treated straw, some thermally resistant polymers (e.g., condensed lignins and humic-like colloids (Blanco and Almendros 1994)) were degraded earlier during the thermal decomposition process, whereas these polymers were degraded later in the case of untreated straw. In addition, it is likely that fungal biomass in the treated straw had an influence on the thermal stability of straw. Overall, similar thermogravimetric analysis results as in the present study were reported for straw degraded by P. ostreatus after 20 and 40 days of incubation (Sharma 1990) and wheat straw compost (Blanco and Almendros 1994,1997; Sharma 1996).

Performance evaluation of straw-plastic composites

In general, the most significant factor governing flexural strength (MOR) of the SPC was the amount of wheat straw incorporated in the formulation (Table 1). Flexural strength decreased with increasing amounts of wheat straw used when the formulation did not contain any coupling agent. Optimum flexural strength was obtained when 60 to 65 percent straw filler was used in the formulation. Flexural strength was improved with the incorporation of HDPE and maleic-anhydride-grafted polyethylene (MAPE) in the formulations. The incorporation of a lubricant such as OP 100 compromised MOR and MOE of the composite, as was previously demonstrated by Harper (2003). Fungal treatment did not have a statistically significant impact on either MOR or MOE at an alpha-value of 0.05. Density was found not to be a statistically significant covariant in the analyses.

[FIGURE 2 OMITTED]

The particle size distribution of degraded wheat straw (Fig. 2) was influenced by the duration of incubation, which in turn, likely had an influence on the extrusion process and mechanical and physical properties of the composite. The 12-week degraded straw (Degrade 2) was primarily composed of smaller particles than the straw degraded for 6 weeks (Degrade 1). Maiti and Singh (1986) reported an improvement in yield strength of extruded wood-HDPE specimens with decreasing filler particle-size, which was attributed to improved adhesion between filler and thermoplastic since no coupling agents were used.

Of all variables, the flexural stiffness of the SPC was influenced most by MAPE. The addition of a coupling agent in combination with low amounts of straw filler caused an increase in stiffness of the composite. Improvements in material strength with the addition of coupling agents may be the result of improved fiber-plastic interaction, better dispersion of the filler, or changes in the thermoplastic morphology (Harper and Wolcott 2004). The extruding performance of some of the runs with MAPE and high levels of HDPE (runs 12, 17, 18, and 34) was marginal at best. The extrudate for these runs exhibited a "snake skin" or rough surface appearance. This was not due to incorporation of wheat straw since similar results were found when wood flour was used in place of the wheat straw in runs 12 and 17.

With the exception of the formulation used in run 12. MOE was not significantly different between straw- and pine-based composites at an alpha-value of 0.05 (Table 2). In addition, the modulus of rupture was not significantly different between straw- and pine-based composites at an alpha-value of 0.05, except for the formulation used in run #2 (Table 2). In conclusion, straw- and pine-filled thermoplastic composites are comparable with regard to their mechanical performance which is consistent with the results of Simonsen (1996).

[FIGURE 3 OMITTED]

Water sorption and thickness swell of SPC and WPC strongly depend on the amount of filler used in the formulation (Figs. 3 and 4). Generally, water sorption, thickness swell, and dimensional instability of the composites increased with the incorporation of higher amounts of filler. SPC specimens comprised of degraded straw demonstrated less resistance to the sorption of water than specimens in which neat straw or pine flour was used (Fig. 3). This may primarily reflect the fungal degradation of hydrophobic cell wall components (lignin and hemicelluloses) in treated straw, resulting in a relatively more hydrophilic substrate compared to neat straw. In addition, fungal treatment may have caused disruption of the waxy cuticle surrounding the straw, thus further reducing straw hydrophobicity and improving water sorption in the SPC.

When incorporating 55 percent fiber filler content, water sorption of the neat straw composite was higher than that of a WPC (Fig. 3a). However, at higher filler levels, water sorption of composites produced with neat straw displayed the lowest water sorption (Fig. 3b), with thickness swell values relatively comparable (Fig. 4b). The reasons for this behavior are not completely evident. Straw generally contains more cellulose and less hydrophobic lignin than wood (Sundstol and Owen 1984, Fengel and Wegener 1989). But the fact that the feedstock properties are not universally reflected in the composite behavior may suggest that the material structure of the composite is playing a role in controlling the moisture uptake. Note that the highest composite density was also found with the 75 percent neat straw composites (Table 2). The influence of material structure on water absorption has also been demonstrated by Clemons (2002) who showed that WPCs produced with different processing methods had substantially different water absorption behavior.

[FIGURE 4 OMITTED]

Conclusions

Inoculation of unsterilized straw with the white-rot fungus P. ostreatus did not have a statistically significant (alpha-value of 0.05) influence on either modulus of rupture (MOR) or modulus of elasticity (MOE) of an SPC under the conditions applied in the present study. In addition, SPCs incorporating degraded straw demonstrated less resistance to water sorption and thickness swell than SPCs in which neat straw was used. This may primarily reflect the fungal degradation of hydrophobic cell wall components (lignin and hemicelluloses) in treated straw, resulting in a relatively more hydrophilic substrate compared to neat straw. In the temperature range between 100[degrees] and 300[degrees]C, fungal degraded straw appeared thermally less stable than non-inoculated straw but this did not have any apparent effect on the extrusion process.

The mechanical properties of SPCs produced with untreated straw were comparable to a WPC based on pine flour. Hence, large-scale modification of unsterilized straw with P. ostreatus is neither feasible nor necessary if untreated straw is used as an alternative to wood fillers. Further research would be necessary to demonstrate the feasibility and economic viability of using P. ostreatus for large-scale treatment of unsterilized straw if interest in this microorganism continues. Overall, untreated wheat straw is a promising alternative to wood fillers in the production of extruded thermoplastic composites.

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Anke Schirp*

Frank J. Loge

Karl R. Englund*

Michael P. Wolcott*

J. Richard Hess

Tracy P. Houghton

Jeffrey A. Lacey

David N. Thompson

The authors are, respectively, Senior Scientist, Univ. of Gottingen, Inst. of Wood Biology and Wood Technology, Gottingen, Germany (aschirp@gwdg.de); Associate Professor, Dept. of Civil and Environmental Engineering, Univ. of California Davis, Davis. CA (fjloge@ucdavis.edu); Associate in Research and Professor, Wood Materials and Engineering Lab., Washington State Univ., Pullman, WA (englund@wsu.edu; wolcott@wsu.edu); Bioenergy Program Lead, Renewable Energy and Power, Idaho National Lab. (INL), Idaho Falls, ID (JRichard.Hess@inl.gov); Research Scientist, Chemical Sciences, INL (Tracy.Houghton@inl.gov); Research Scientist, Biological Sciences, INL (Jeffrey.Lacey@inl.gov); and Research Engineer, Biological Sciences, INL (David.Thompson@inl.gov). The authors thank Professor Steven Aust and Paul Swaner of Utah State Univ. for maintaining and providing the fungal cultures of P. ostreatus, and David Dostal of Strandex Corp[c], formerly of the Wood Materials and Engineering Lab., Washington State Univ., for extruding straw-plastic formulations. This project was administered by the Idaho Dept. of Natural Resources Energy Division. This work was supported in part by the U.S. Dept. of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy under DOE Idaho Operations Office Contract DE-AC07-99ID13727. Additional support was provided by the Idaho Wheat Commission. Grant 4-D Farms, and Energy Products of Idaho Inc. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the supporting agencies. This paper was received for publication in April 2005. Article No. 10035.

*Forest Products Society Member.

Table 1. -- Formulations used in wheat straw extrusion. Values represent
weight-percentages required to make a 2-kg batch for extrusion. Five
replicates were run for each formulation. (a)

Run                     Formulation             Density
no.  Treatment  Wheat  HDPE   MAPE  Lubricant  Mean  %COV
                       (weight %)              (kg/[m.sup.3])

25   Degrade 1  55     38     4     3          1143   1.0
24   Degrade 1  55     41     4     0          1090  12.2
 5   Degrade 1  55     42     0     3          1110   0.5
10   Degrade 1  55     42     0     3          1111   0.4
29   Degrade 1  55     45     0     0          1126   0.6
 3   Degrade 1  60     36.25  3     0.75       1172   1.6
27   Degrade 1  60     36.25  3     0.75       1162   1.4
14   Degrade 1  60     38.25  1     0.75       1161   1.8
30   Degrade 1  60     38.25  1     0.75       1156   0.6
 9   Degrade 1  70     26.75  1     2.25       1181   0.7
26   Degrade 1  75     18     4     3          1200   0.6
32   Degrade 1  75     18     4     3          1187   1.8
33   Degrade 1  75     21     4     0          1228   1.4
 8   Degrade 1  75     22     0     3          1181   0.6
 1   Degrade 2  55     38     4     3          1140   2.0
20   Degrade 2  55     38     4     3          1143   1.4
11   Degrade 2  55     41     4     0          1126   0.9
 7   Degrade 2  55     42     0     3          1089   1.1
15   Degrade 2  55     45     0     0          1136   1.4
 6   Degrade 2  65     35     0     0          1182   0.9
16   Degrade 2  65     35     0     0          1194   1.2
31   Degrade 2  75     18     4     3          1229   2.0
 4   Degrade 2  75     21     4     0          1258   1.1
13   Degrade 2  75     22     0     3          1201   1.8
12   Neat       55     38     4     3          1127   0.4
17   Neat       55     41     4     0          1119   0.5
21   Neat       55     42     0     3          1063   0.7
18   Neat       55     45     0     0          1113   0.7
34   Neat       65     31     4     0          1183   1.0
22   Neat       65     31.5   2     1.5        1158   1.6
23   Neat       65     31.5   2     1.5        1156   0.6
19   Neat       65     32     0     3          1117   1.1
 2   Neat       75     18     4     3          1192   0.5
28   Neat       75     25     0     0          1188   1.0

Run                MOE        MOR
no.  Treatment  Mean  %COV  Mean    %COV
                   (MPa)        (Pa)

25   Degrade 1  4117   4.3  31,943   3.6
24   Degrade 1  4159   8.0  37,656   4.0
 5   Degrade 1  2678   8.8  23,708   9.9
10   Degrade 1  2411  21.9  21,208  14.3
29   Degrade 1  4072   5.6  32,959   2.2
 3   Degrade 1  4713   7.4  35,264   4.4
27   Degrade 1  4562   6.3  34,773   5.1
14   Degrade 1  4610   6.0  34,117   3.3
30   Degrade 1  4261   6.4  31,837   3.6
 9   Degrade 1  2782   5.2  19,808   7.0
26   Degrade 1  3205   7.5  17,987   8.2
32   Degrade 1  2694  15.4  16,428  14.7
33   Degrade 1  4500  13.5  28,496   9.4
 8   Degrade 1  1524  27.9  11,723  18.6
 1   Degrade 2  3616  14.3  30,532   4.8
20   Degrade 2  3621   7.8  31,208   4.1
11   Degrade 2  3507   5.7  34,848   2.4
 7   Degrade 2  2425   9.7  22,610   8.4
15   Degrade 2  3608   8.1  32,904   4.6
 6   Degrade 2  3772  11.9  27,868   6.6
16   Degrade 2  3671  10.8  26,854   7.0
31   Degrade 2  3067   6.1  18,217  11.0
 4   Degrade 2  4343  13.8  26,511  17.0
13   Degrade 2  2263  10.5  14,272  10.2
12   Neat       4355   6.4  35,025   3.8
17   Neat       3260  14.7  35,899   8.5
21   Neat       2101   5.9  18,991   8.7
18   Neat       3733   6.7  29,641   2.8
34   Neat       5486   8.2  39,602   4.4
22   Neat       4717  10.0  30,541   9.8
23   Neat       4726   8.6  31,827   3.8
19   Neat       1562  16.4  15,752   7.9
 2   Neat       3396  10.2  19,943   8.3
28   Neat       2650  14.1  17,188  11.8

(a) MOE = modulus of elasticity; MOR = modulus of rupture: COV =
coefficient of variation; HDPE = high-density polyethylene; MAPE =
maleic-anhydride-grafted polyethylene.

Table 2 -- Density, flexural strength (MOR), and stiffness (MOE) of
three extruded formulations with untreated pine and wheat flour as fiber
raw materials.

           Formulation
Fiber  HDPE  MAPE  Lubricant
           (weight %)

75     18    4     3
55     38    4     3
55     41    4     0

Fiber              Properties
type   Density          MOE           MOR
       (kg/[m.sup.3])   (MPa)         (Pa)

Straw  1192 (0.52) (a)  3396 (10.24)  19,943 (8.31)
Pine   1175 (0.94)      2995 (11.74)  19,926 (10.45)
Straw  1127 (0.42)      4355 (6.35)   35,025 (3.80)
Pine   1136 (1.94)      3816 (11.45)  34,864 (4.23)
Straw  1119 (0.52)      3260 (14.68)  35,899 (8.45)
Pine   1122 (0.69)      3235 (7.22)   35,828 (3.57)

(a) Values in parentheses are coefficients of variation in percent.