- Email: [email protected]
Utilizing straw as a filler in thermoplastic building materials
Construction
and Building Materials, Vol. 10, No. 6, pp. 435-440, 1996 Copyright 0 1996 Else-&r Science Ltd Printed in Great Britain. All rights reserved 09560618/96
$15.00+0.00
P11:S0950-0618(96)00016-5
ELSEVIER
Utilizing straw as a filler in thermoplastic building materials John Simonsen Department
of Forest Products, Oregon State University,
Received 5 January
Corvallis, Oregon, USA
1996; revised 3 June 7996; accepted 3 June 7996
A recent addition to the list of composite building materials is plastic lumber. While utilizing recycled plastics as building materials promotes recycling, plastic lumber itself is a poor replacement for solid wood. Research is underway to improve the mechanical properties of wood/polymer composites. This report investigates the use of Willamette Valley rye grass straw as a filler in the commodity plastics polyethylene (PE) and polystyrene (PSI. Since recycled plastics are often mixtures, blends of PE and PS were studied. A compatibilizer was used to improve the properties of the plastic blends. Composites of blends of plastics filled with straw showed a linear relationship in strength and stiffness. Straw performs similarly to wood as a filler in these systems. In composites containing only one plastic, the performance of straw as a filler seemed slightly superior to wood in polyethylene and slightly inferior in polystyrene. The properties observed here compare favourably to those of commercial products and suggest that improvements in these products are possible. Copyright 0 1996 Elsevier Science Ltd. Keywords: straw; plastic; polymer
The combination of three forces has created an opportunity for advanced composite materials. The first of these is the continuing population explosion, which has created a growing worldwide demand for building materials. The remaining forces are specific to the US, but have their counterparts in other areas of the world. The second force is the municipal solid waste (MSW) crisis. Probably the most popular response to the MSW problem has been recycling. The recycling of paper, glass, and metals is generally considered to be growing and on track with recycling rates of 30-60%. The picture for plastics is not so encouraging’. The poor rate of recycling for plastics is not because of public or industry indifference, but rather the technological, economic, and social problems involved:
6
Probably the greatest single block to plastics recycling is the lack of a suitable end market for mixed recycled plastics*. A large proportion of the recycled plastics stream is composed of ‘mixed’ plastics. Mixed plastics are the waste plastics that remain after all the directly recyclable, and therefore more valuable, plastics have been removed. Mixed plastics currently have few profitable uses. This problem can become an opportunity, however, when mixed plastics are viewed as a raw material resource.
The third force creating an opportunity for composites is the increased price and decreased availability of wood. Several factors, including environmental regulations and overharvesting have contributed to this situation-‘. The market for building products is enormous. If we cannot use as many trees to satisfy this market, what can we use? One raw material that exists in the enormous quantities comparable to previous amounts of wood-based materials is agricultural residues. Much of these materials must be disposed of, often by burning. Oregon rye grass straw is a typical example. If we could develop a technology for the manufacture of building products that incorporate rye grass straw as a reinforcing filler in recycled plastics, we could help to alleviate several problems at once: disposal of the straw, the lack of building materials caused by the reduced timber supply, and the lack of suitable and consistent end markets for recycled plastics, especially mixed plastics. Composites manufactured from alternative materials offer the promise of relieving the pressures on both the
MSW plastics are typically a mixture of resin types. The waste stream is also composed of many consumer products that are blends of resins or resin laminates. Separating different plastics once they are mixed is technically difficult. The low density of most plastics increases transportation costs for recyclers. Lack of infrastructure. Plastics recycling does not have the systems of collection, brokerage, and markets that exist for paper, glass and metals. Supply and demand for recycled plastics changes rapidly. This leads to volatile prices and unstable availability.
435
436
Straw/polymer
composites: J. Simonsen
landfills and the forests. However, in order to do this, the composites must possess material properties that allow them to substitute for traditional building materials. An alternative material that has received publicity recently is recycled mixed plastics extruded in the shape of dimension lumber, generically called ‘plastic lumber’4.j. This use of recycled plastics, both filled with wood or fibreglass and unfilled, as building materials is currently a fledgling industry. However, the material properties of plastic lumber are poor in comparison to solid wood. The tensile strength and stiffness are typically one fourth or less than those of solid woods. In addition, the poor creep properties of these materials have caused some in-field replacements in Florida’. This does not mean that the use of recycled plastics as a raw material for manufacturing building products is a bad idea; the product as it exists in the marketplace today, however, is not a satisfactory replacement for solid wood except in specialized niche markets, such as offset blocks in highway guard rails, signage, marine decking, etc. To expand the available markets for building products manufactured from recycled plastics, the problems of low strength and stiffness must be addressed. As with all materials, the engineering properties must be matched to the end use. We examined the feasibility of utilizing an agricultural residue as a filler in thermoplastics. We have also incorporated a compatibilizer in order to improve the properties of blends of plastics. The use of agricultural products as tillers in plastics is not new. Wood has been used since the dawn of the plastics age. Cotton, jute, sisal, and ground walnut and peanut shells are also used as fillers*. Both straw and wood are lignocellulosic materials. Straw has been used as a replacement for wood in many products. It is used for paper and corrugated medium production’ and actually pre-dates wood pulp as a raw material for papermakinglo. Oregon rye grass straw has also been used as the furnish for composition board. The resulting boards possessed properties similar to those of wood-based particleboard”. Because the composition of rye straw is similar to other fillers used in thermoplastics, its formulation chemistry in this use should also be similar. The techniques developed for dispersing wood in thermoplastic matrices should be directly applicable to straw. The ultimate goal of this research is to use recycled plastics as a component in composites designed for use as building materials. However, recycled plastics were not used in this study. This is because the properties of recycled plastics are usually more variable than those of virgin plastics. Thus, in order to reduce the variability of the experimental results, virgin plastics were used in this study. The use of recycled materials was left for future research.
Materials and methods Plustics The polystyrene (PS)was obtained from Dow Chemical Company as product Styron 685D. The polyethylene
(PE) is high density material and was obtained from Phillips Chemical Company as Marlex EHM 6007. A compatibilizer, Kraton 1652 (Kraton), was obtained from Shell Chemical Company. It was incorporated into the mix at a concentration of 0.1% of the blendedplastics phase. It was not used in samples containing only one plastic. All plastics were used as received. Str.ulL~ Straw was obtained locally in the Willamette valley, dried and stored outdoors under cover. It was ground to pass a 16-mesh screen in a Wiley mill and dried overnight at 105°C before use. The dried straw flour was stored over a silica gel desiccant. The straw flour was analysed with an optical measuring system manufactured by Micro Motion Systems, Inc. The average fibre length was found to be 0.7 mm, with a standard deviation of 0.6 mm. The ground straw was dusty. The dust was excluded from the particle measurements. The average aspect ratio of the fibres was 4.5, with a standard deviation of 4.7. Wood fihre Thermomechanical pulp (TMP) was obtained from the Smurtit Newsprint Corp. mill at Newburg, Oregon. The wood species was primarily hemlock (Tsugu heteroplzyllu) and a mixture of true firs, with a small amount of spruce. The fibre was obtained from the slurry stream prior to bleaching, air-dried, dried in vucuo at 60°C overnight, ground in a Wiley mill to pass a 16-mesh screen, and stored over a silica gel desiccant. The TMP was analysed optically. The average length was found to be 1.0 mm, with a standard deviation of 0.6 mm. The average aspect ratio was 22, with a standard deviation of 14. The TMP also contained dust, although not as much as the straw, which was not included in the measurements. A second sample was obtained by passing this sample through a 60-mesh screen. Wood flour made from Douglas-fir (Pseudotsugu menziesii (Mirb.) France) ground to 80-100 mesh was obtained from Menasha Wood Corporation, Olympia, Washington, as product T-14. The T-14 was dried in SCICLK~ at 60°C overnight and stored over a silica gel desiccant. This filler was analysed optically and found to have an average length of 0.9 mm with a standard deviation of 0.6 mm. The average aspect ratio was 9 with a standard deviation of 5. Sample preparation The plastics, fillers, and additives were blended in a Brabender Plasticorder set at 30 rpm and 177°C. The plastic(s) was added first and blended for three to five minutes to ensure complete melting. The Kraton was then added for those formulations requiring it. The tiller, either straw or wood, was then added and blended. The plasticorder contains a torque rheometer that indirectly measures the melt viscosity. Typically the melt viscosity rose when filler was added to the plastic melt. The viscosity then fell as the filler blended into the plastic. When the filler was completely blended, the melt
Straw/polymer
composites:
J. Simonsen
437
viscosity plateaued at a value higher than that of the plastic alone. When the melt viscosity plateaued, as determined by the torque rheometer, the blending was continued for an additional 2 min. We assumed that the dispersion was maximized at that point. The melt was then removed from the Plasticorder, cooled, and stored for grinding. The prepared blends were ground in a Wiley mill to a particle size of approximately 3 mm (0.1 in.). These powders were then compression-moulded in a thermostatted Carver laboratory press. The press conditions were 180°C (360°F) and 6.9 MPa (1000 psi) for 10 min. The sample size was approximately 2 X 13 X 55 mm (0.08 x 0.5 x 2.0 in.). The compression mould held five samples. At the higher filler contents, there were occasions where not all five samples were completely moulded. When that occurred, the number of samples included in the material properties test was reduced from five to three. Muterid properties
The modulus of elasticity (MOE) was determined on samples with dimensions of approximately 2 x 3 x 55 mm. They were tested in flexure with a three-point bending apparatus in accordance with ASTM standard D790-8612. Five samples were tested for each determination of MOE wherever possible. In some cases only three samples were used due to material limitations. The same samples and test procedure were used to determine the ultimate strength. Ultimate strength is defined in three different ways: Samples of pure polystyrene broke cleanly The proper reporting value in this case is the modulus of rupture (MOR). However, those samples containing more than 20% polyethylene typically did not break, but rather yielded under the stress. Yield strength is defined as the first point at which the stress-strain curve shows a slope of zero (Figure Z(B)). This point is reported for those samples that either yielded before breaking or did not break at all, i.e. they simply continued to bend to the limits of travel of the testing machine. The samples consisting of 100% polyethylene in the plastic phase and either 0 or 20% straw as filler showed neither a break nor a yield point (Figure l(C)). The stress-strain curve in these cases was a smooth curve to the limits of travel of the testing machine. In these cases the ultimate strength is reported as the 0.5% strain offset yield strength. This is calculated with the method specified in ASTM standard D790-86 and described in ASTM standard D638-8912. MOR.
(Figure I (A)).
Results and discussion Ordinarily, PS and PE are immiscible. Blends of these plastics form a matrix of the larger volume component containing dispersed drops of the lesser component*s.
0
1
2
3
4
5
Deflection, mm
Figure 1 Examples of stress-strain (brittle fracture); (B) yield strength yield strength (ductile behaviour)
curves for plastics: (yielding behaviour);
(A)
MOR
(C) offset
The resulting mechanical properties of the blend are typically poor, but can be improved by incorporating a compatibilizer into the blend. A compatibilizer is usually a block copolymer consisting of blocks of the two immiscible phases. Increasing amounts of compatibilizer yield improved dispersionsi4. This study utilized Kraton 1652, a commercial product from Shell Chemical Company, as a compatibilizer. The resulting blends of PS and PE typically followed the rule of mixtures, i.e. the resulting blends showed properties that were the weighted averages of the components. This is shown in the lower line in Figure 2. Blends of PE and PS containing wood and straw fillers also showed a linear relationship between composition and material properties. The MOE of the blended-plastic composites generally followed the rule of mixtures in that the modulus varied linearly between PS and PE (Figure 2). The modulus increased with increasing filler content of up to 40%. The 60% filler did not show a significant increase over the 40% level. As the filler level increased, it reached a point where there was insufficient plastic to thoroughly wet the filler surface. At that point, load transfer from the matrix to the filler became less efficient, resulting in a drop in mechanical properties. The same behaviour was exhibited in the ultimate strength properties (Figure 3). Here, the situation was somewhat complicated by the yield behaviour of the composites. As the PE content increased, the sample yielding changed. Samples containing 0% straw yielded
Straw/polymer
43%
Straw Filler Content (%) -0 0 --n 20 ..__.-. + 40
3.5 S % ;
3.0
B 2.5
I
I
I
I
1
I
0
20
40
60
60
100
PE
in Plastic Phase (%)
A Modulus of rupture n Olfeet yield strength
~~j~4ctyS~ . , .I 0
20
40
60
80 100 PE in
1 0
6Wstraw 20
40
60
J. Simonsen
composites:
i 80 100
least squares regression of the appropriate data points. The regression is not assumed to have any analytical significance, but is used to show the relative trend of the data. The 40% straw composites showed the least change in ultimate strength with increasing PE content. This filler content seemed to be the most compatible with a variety of mixtures of PS and PE. The change in the ultimate strength with increasing straw content is caused by a greater decrease in the strengths of the high Ps-content composites relative to the high PE composites. This suggests that the behaviour of straw as a tiller is superior in FE compared to Ps. The MOE of straw-filled composites was about the same as for TMP composites. The behaviour of straw and wood fibre are compared in Figures 4 and F for tilled PE composites. Figure 4 indicates that the MOE of the TMP composites depended upon particle size. Yam et u/.15 found that shorter tibre lengths gave improved dispersion. This probably accounts for the higher MOE. Since the straw and the i&mesh TMP have approximately the same tibre length. we conclude that the straw performs about the same as wood fibre as a remforcing filler for PE in terms of flexural moduhLs. The ultimate strength properties are shown in Fi,qure .i. The straw-filled composites appeared to be slightly superior to the wood-fibre-filled composites. The h&mesh -rMP-filled samples are not shown, as the results were almost identical to those of the 16-mesh samples. The yield point of all the samples decreased at 40@60’%, loading, when it became difficult for the plastic matrix to completely wet the tiller surface. This can result in lower efficiencies in load transfer fromthe matrix to the tibre and consequently lower strengths.
2.5 -
Plastic Phase (%)
Figure 3 Ultimate strengths for VXIOUS stray loadings
nnd
plastic
blends
by fracture at 0 and 20% PE content. Samples L 40% PE did not break or yield. The ultimate strength is reported as the strength at 0.5% strain offset. Samples containing 20% straw all showed yield points, except for the lOO’/;~ PE sample. The 0.5’%1 strain offset yield strength is reported for that sample. The samples containing 40 and 60% straw all gave yield points. These data show approximately the same pattern of linear transition between PS and PE. While the stiff straw fibres increased the stiffness of the composition, as shown by an increase in MOE (Figure 2), the brittleness also increased. as indicated by a general decrease in ultimate strength (Figure 3). The lines shown in the figures are a linear
1.0’
I 0
I 10
( 20
I 30
I 40
I
I
50
60
Filler (%) Figure 4 Moduli of elasticity ethylene
(MOE)
of
straw
and wood
fibre in poly-
Straw/polymer
composites:
J. Simonsen
439
SO
I
-@-TMP.
t B-mesh
-B-TMP.
1B-mesh
70
z
60
z. 2 tj m ii E .s 2
50
40
30
.”
d
50 2b
lb
3b
4b
5;
60
$0 Filler Content (% by weight)
Filler I%\ Figure 5 Ultimate strengths of straw and wood fibre in
Figure 7 Ultimate strengths for filled
PE
6
5
0 3
4
P
3
2
1
0
I
I
1
1
I
I
IO
20
30
40
50
60
PS
composites
increase the molecular weight for the pure PS samples, increasing the ultimate properties of the plastic. Increased molecular weight does not strongly influence the flexural modulus, and little difference was noted in these values between the blended and moulded-only PS samples. This effect is unlikely to be observed in the presence of a filler, however. In this case, it is likely that the filler would react with the free radical. One of these free-radical quenching reactions might be grafting of the PS molecule containing the free radical to the filler. If significant grafting to the filler were to occur, we would expect an improvement in mechanical properties. Whether or not the results presented here were influenced by grafting is not known. One might speculate that the high variability observed for the ultimate strength in the 20% straw-filled samples is due to varying degrees of grafting. Experimental techniques and equipment were not available to investigate the presence of grafting; thus these questions could not be addressed.
Filler Content (% by weight) Figure 6 Moduli of elasticity
(MOE)
for filled
PS
composites
Figures 6 and 7 present the data for filled PS systems. The straw appeared to be inferior to wood fillers in this case. Highly variable results at the 20% straw content also give cause for concern over the performance of straw-filled PS composites. This concern is intensified by the data shown in Figure 7. The ultimate strength of straw-filled PS composites was significantly lower than their TMP counterparts. The wide variation shown in the values for pure PS was probably caused by free radical polymerization reactions occurring during the heat and shear of the blending operation. The resulting reaction would
Summary and conclusions The feasibility of using straw as a filler in thermoplastics in lieu of wood is substantiated for certain systems. Straw seems to be slightly superior to wood in PE systems, but in PS systems, slightly inferior. Blends of PS and PE, either filled or unfilled, generally follow a linear relationship from one to the other, provided that a plastics compatibilizer is incorporated into the mixture. In general, increasing levels of filler increase stiffness while decreasing strength. The exception to this is low filler (up to 40%) and high PE contents (SO-lOO%), where the strength and stiffness increase with increasing filler content. Current plastic lumber products exhibit a modulus and strength (MOE and MOR from ASTM D79012) of
Straw/polymer
440
composites:
about 1.2 GPa and 10 MPa, respectivelyl6. The composites prepared here compare favourably with those values and indicate that improvements in filled thermoplastic composites for the building products industry are possible. These data also indicate that straw shows a potential as a filler for thermoplastics in these applications.
4
Acknowledgements The author thanks the State of Oregon Department of Agriculture for their financial support of this research via grant number G079. The author also thanks Mr Robert Gourley for his dedication and hard work in the sample preparation and testing involved in this project. This is Paper 3051 of the Forest Research Laboratory. Oregon State University, Corvallis.
IO
Recycling: is it worth the eifcxt” (brt.\lr~~.ftcf 1994. 59. (2). 92-98 Recycling supply sldc ib up. end markcth need boost /‘lc/\t fi:ji,c 1993. 49 (6). 6 Warren. D. D Production. Prlceh.. Employment. and Trade 111 Northwest F.orest Industrtes. Second Quarter 1992. USDA. Pacific Northwest Research StatIon. Portland. OR. Resource Bulletin PNW-RR-194. 1992
Hegberg, B. A.. Brenniman, G. R. and Hallenbeck, W. H. Mi.& Plustics Recycling Technology, Noyes Data Corporation, Park Ridge. NJ. 1992 Mackzo. J. An alternative to landfills for mixed plastic waste. P~uJ~. Eng. 1990, 46 (4). 51-53 Eskelsen. V. Oregon State University, 1993, personal communicatIon Beatty. C. Univcrslty of Florida, 1993, personal communicntion Seymour. R. B. and Carraher, C. E. Pulymrr Chemistry. Marcel Dekker, New York, 1992 Stephenson, J. N.. ed. Pulp und Paper Monufoclure. Volume 2, Pwpurotion ofStock/ur Pupermuking, McGraw-Hill, New York. 1951 Biermann, C. J. Use of Willamette Valley rye grass straw m the papermakmg process. prescntetl ut i/le Rrc.vcling Biohused :MureriuO
II
Conferrnce.
I?
I-l
IS
It!
Oregon Cit.v. OR, 12-13 Auglrst
O’Brien. M. and Tylman. strawboard. pre.ren/ecl (II
V the OR,
Meadowood,
1992
a medium
Recycling Biohu.red 12-13 Atrgusf IY92.
density Muterid\
Oregon Cit?;. (Contact Michael O’Brien, O’Brien and Associates, P 0. Box 963. Lake Oswego. OR 97034) American Society for Testmg dnd Materials. Annd Book o/ .4STM Siundurd~. Vol. 8.01, Philadelphia, 1994 Locke. C. E. and Paul. D. R. Graft copolymer modification of polyethylene-polystyrene blends. I. Graft preparation and characterization. J Appl Polym. Sci. 1973, 17. X97-2617 and Newman. S InterfacIal agents Paul, D. R (‘Compatihilizers’) for polymer blends. Chapter I2 m Po/Fmrr B/end\. Vol. 2. Academic Press. New York. 1978. pp. 35 67 Yam. K.. Gogoi. B., Lai, C. and Selke, S Composites from compounding wood fibers with recycled high density polyethylene P&n?. Enng. Sci., 1990. 30. (I I ). 693-699 Corlf~wnw.
13
References
J. Simonsen
Tre Y Wood-Polvmcr Compoxitr Ph_v.ticul und Propc’r/rr.\. Modil Chemical Company. Composite
Division.
Norwalk.
CT. 1993
Mrchunit
Products
ul
and Building Materials, Vol. 10, No. 6, pp. 435-440, 1996 Copyright 0 1996 Else-&r Science Ltd Printed in Great Britain. All rights reserved 09560618/96
$15.00+0.00
P11:S0950-0618(96)00016-5
ELSEVIER
Utilizing straw as a filler in thermoplastic building materials John Simonsen Department
of Forest Products, Oregon State University,
Received 5 January
Corvallis, Oregon, USA
1996; revised 3 June 7996; accepted 3 June 7996
A recent addition to the list of composite building materials is plastic lumber. While utilizing recycled plastics as building materials promotes recycling, plastic lumber itself is a poor replacement for solid wood. Research is underway to improve the mechanical properties of wood/polymer composites. This report investigates the use of Willamette Valley rye grass straw as a filler in the commodity plastics polyethylene (PE) and polystyrene (PSI. Since recycled plastics are often mixtures, blends of PE and PS were studied. A compatibilizer was used to improve the properties of the plastic blends. Composites of blends of plastics filled with straw showed a linear relationship in strength and stiffness. Straw performs similarly to wood as a filler in these systems. In composites containing only one plastic, the performance of straw as a filler seemed slightly superior to wood in polyethylene and slightly inferior in polystyrene. The properties observed here compare favourably to those of commercial products and suggest that improvements in these products are possible. Copyright 0 1996 Elsevier Science Ltd. Keywords: straw; plastic; polymer
The combination of three forces has created an opportunity for advanced composite materials. The first of these is the continuing population explosion, which has created a growing worldwide demand for building materials. The remaining forces are specific to the US, but have their counterparts in other areas of the world. The second force is the municipal solid waste (MSW) crisis. Probably the most popular response to the MSW problem has been recycling. The recycling of paper, glass, and metals is generally considered to be growing and on track with recycling rates of 30-60%. The picture for plastics is not so encouraging’. The poor rate of recycling for plastics is not because of public or industry indifference, but rather the technological, economic, and social problems involved:
6
Probably the greatest single block to plastics recycling is the lack of a suitable end market for mixed recycled plastics*. A large proportion of the recycled plastics stream is composed of ‘mixed’ plastics. Mixed plastics are the waste plastics that remain after all the directly recyclable, and therefore more valuable, plastics have been removed. Mixed plastics currently have few profitable uses. This problem can become an opportunity, however, when mixed plastics are viewed as a raw material resource.
The third force creating an opportunity for composites is the increased price and decreased availability of wood. Several factors, including environmental regulations and overharvesting have contributed to this situation-‘. The market for building products is enormous. If we cannot use as many trees to satisfy this market, what can we use? One raw material that exists in the enormous quantities comparable to previous amounts of wood-based materials is agricultural residues. Much of these materials must be disposed of, often by burning. Oregon rye grass straw is a typical example. If we could develop a technology for the manufacture of building products that incorporate rye grass straw as a reinforcing filler in recycled plastics, we could help to alleviate several problems at once: disposal of the straw, the lack of building materials caused by the reduced timber supply, and the lack of suitable and consistent end markets for recycled plastics, especially mixed plastics. Composites manufactured from alternative materials offer the promise of relieving the pressures on both the
MSW plastics are typically a mixture of resin types. The waste stream is also composed of many consumer products that are blends of resins or resin laminates. Separating different plastics once they are mixed is technically difficult. The low density of most plastics increases transportation costs for recyclers. Lack of infrastructure. Plastics recycling does not have the systems of collection, brokerage, and markets that exist for paper, glass and metals. Supply and demand for recycled plastics changes rapidly. This leads to volatile prices and unstable availability.
435
436
Straw/polymer
composites: J. Simonsen
landfills and the forests. However, in order to do this, the composites must possess material properties that allow them to substitute for traditional building materials. An alternative material that has received publicity recently is recycled mixed plastics extruded in the shape of dimension lumber, generically called ‘plastic lumber’4.j. This use of recycled plastics, both filled with wood or fibreglass and unfilled, as building materials is currently a fledgling industry. However, the material properties of plastic lumber are poor in comparison to solid wood. The tensile strength and stiffness are typically one fourth or less than those of solid woods. In addition, the poor creep properties of these materials have caused some in-field replacements in Florida’. This does not mean that the use of recycled plastics as a raw material for manufacturing building products is a bad idea; the product as it exists in the marketplace today, however, is not a satisfactory replacement for solid wood except in specialized niche markets, such as offset blocks in highway guard rails, signage, marine decking, etc. To expand the available markets for building products manufactured from recycled plastics, the problems of low strength and stiffness must be addressed. As with all materials, the engineering properties must be matched to the end use. We examined the feasibility of utilizing an agricultural residue as a filler in thermoplastics. We have also incorporated a compatibilizer in order to improve the properties of blends of plastics. The use of agricultural products as tillers in plastics is not new. Wood has been used since the dawn of the plastics age. Cotton, jute, sisal, and ground walnut and peanut shells are also used as fillers*. Both straw and wood are lignocellulosic materials. Straw has been used as a replacement for wood in many products. It is used for paper and corrugated medium production’ and actually pre-dates wood pulp as a raw material for papermakinglo. Oregon rye grass straw has also been used as the furnish for composition board. The resulting boards possessed properties similar to those of wood-based particleboard”. Because the composition of rye straw is similar to other fillers used in thermoplastics, its formulation chemistry in this use should also be similar. The techniques developed for dispersing wood in thermoplastic matrices should be directly applicable to straw. The ultimate goal of this research is to use recycled plastics as a component in composites designed for use as building materials. However, recycled plastics were not used in this study. This is because the properties of recycled plastics are usually more variable than those of virgin plastics. Thus, in order to reduce the variability of the experimental results, virgin plastics were used in this study. The use of recycled materials was left for future research.
Materials and methods Plustics The polystyrene (PS)was obtained from Dow Chemical Company as product Styron 685D. The polyethylene
(PE) is high density material and was obtained from Phillips Chemical Company as Marlex EHM 6007. A compatibilizer, Kraton 1652 (Kraton), was obtained from Shell Chemical Company. It was incorporated into the mix at a concentration of 0.1% of the blendedplastics phase. It was not used in samples containing only one plastic. All plastics were used as received. Str.ulL~ Straw was obtained locally in the Willamette valley, dried and stored outdoors under cover. It was ground to pass a 16-mesh screen in a Wiley mill and dried overnight at 105°C before use. The dried straw flour was stored over a silica gel desiccant. The straw flour was analysed with an optical measuring system manufactured by Micro Motion Systems, Inc. The average fibre length was found to be 0.7 mm, with a standard deviation of 0.6 mm. The ground straw was dusty. The dust was excluded from the particle measurements. The average aspect ratio of the fibres was 4.5, with a standard deviation of 4.7. Wood fihre Thermomechanical pulp (TMP) was obtained from the Smurtit Newsprint Corp. mill at Newburg, Oregon. The wood species was primarily hemlock (Tsugu heteroplzyllu) and a mixture of true firs, with a small amount of spruce. The fibre was obtained from the slurry stream prior to bleaching, air-dried, dried in vucuo at 60°C overnight, ground in a Wiley mill to pass a 16-mesh screen, and stored over a silica gel desiccant. The TMP was analysed optically. The average length was found to be 1.0 mm, with a standard deviation of 0.6 mm. The average aspect ratio was 22, with a standard deviation of 14. The TMP also contained dust, although not as much as the straw, which was not included in the measurements. A second sample was obtained by passing this sample through a 60-mesh screen. Wood flour made from Douglas-fir (Pseudotsugu menziesii (Mirb.) France) ground to 80-100 mesh was obtained from Menasha Wood Corporation, Olympia, Washington, as product T-14. The T-14 was dried in SCICLK~ at 60°C overnight and stored over a silica gel desiccant. This filler was analysed optically and found to have an average length of 0.9 mm with a standard deviation of 0.6 mm. The average aspect ratio was 9 with a standard deviation of 5. Sample preparation The plastics, fillers, and additives were blended in a Brabender Plasticorder set at 30 rpm and 177°C. The plastic(s) was added first and blended for three to five minutes to ensure complete melting. The Kraton was then added for those formulations requiring it. The tiller, either straw or wood, was then added and blended. The plasticorder contains a torque rheometer that indirectly measures the melt viscosity. Typically the melt viscosity rose when filler was added to the plastic melt. The viscosity then fell as the filler blended into the plastic. When the filler was completely blended, the melt
Straw/polymer
composites:
J. Simonsen
437
viscosity plateaued at a value higher than that of the plastic alone. When the melt viscosity plateaued, as determined by the torque rheometer, the blending was continued for an additional 2 min. We assumed that the dispersion was maximized at that point. The melt was then removed from the Plasticorder, cooled, and stored for grinding. The prepared blends were ground in a Wiley mill to a particle size of approximately 3 mm (0.1 in.). These powders were then compression-moulded in a thermostatted Carver laboratory press. The press conditions were 180°C (360°F) and 6.9 MPa (1000 psi) for 10 min. The sample size was approximately 2 X 13 X 55 mm (0.08 x 0.5 x 2.0 in.). The compression mould held five samples. At the higher filler contents, there were occasions where not all five samples were completely moulded. When that occurred, the number of samples included in the material properties test was reduced from five to three. Muterid properties
The modulus of elasticity (MOE) was determined on samples with dimensions of approximately 2 x 3 x 55 mm. They were tested in flexure with a three-point bending apparatus in accordance with ASTM standard D790-8612. Five samples were tested for each determination of MOE wherever possible. In some cases only three samples were used due to material limitations. The same samples and test procedure were used to determine the ultimate strength. Ultimate strength is defined in three different ways: Samples of pure polystyrene broke cleanly The proper reporting value in this case is the modulus of rupture (MOR). However, those samples containing more than 20% polyethylene typically did not break, but rather yielded under the stress. Yield strength is defined as the first point at which the stress-strain curve shows a slope of zero (Figure Z(B)). This point is reported for those samples that either yielded before breaking or did not break at all, i.e. they simply continued to bend to the limits of travel of the testing machine. The samples consisting of 100% polyethylene in the plastic phase and either 0 or 20% straw as filler showed neither a break nor a yield point (Figure l(C)). The stress-strain curve in these cases was a smooth curve to the limits of travel of the testing machine. In these cases the ultimate strength is reported as the 0.5% strain offset yield strength. This is calculated with the method specified in ASTM standard D790-86 and described in ASTM standard D638-8912. MOR.
(Figure I (A)).
Results and discussion Ordinarily, PS and PE are immiscible. Blends of these plastics form a matrix of the larger volume component containing dispersed drops of the lesser component*s.
0
1
2
3
4
5
Deflection, mm
Figure 1 Examples of stress-strain (brittle fracture); (B) yield strength yield strength (ductile behaviour)
curves for plastics: (yielding behaviour);
(A)
MOR
(C) offset
The resulting mechanical properties of the blend are typically poor, but can be improved by incorporating a compatibilizer into the blend. A compatibilizer is usually a block copolymer consisting of blocks of the two immiscible phases. Increasing amounts of compatibilizer yield improved dispersionsi4. This study utilized Kraton 1652, a commercial product from Shell Chemical Company, as a compatibilizer. The resulting blends of PS and PE typically followed the rule of mixtures, i.e. the resulting blends showed properties that were the weighted averages of the components. This is shown in the lower line in Figure 2. Blends of PE and PS containing wood and straw fillers also showed a linear relationship between composition and material properties. The MOE of the blended-plastic composites generally followed the rule of mixtures in that the modulus varied linearly between PS and PE (Figure 2). The modulus increased with increasing filler content of up to 40%. The 60% filler did not show a significant increase over the 40% level. As the filler level increased, it reached a point where there was insufficient plastic to thoroughly wet the filler surface. At that point, load transfer from the matrix to the filler became less efficient, resulting in a drop in mechanical properties. The same behaviour was exhibited in the ultimate strength properties (Figure 3). Here, the situation was somewhat complicated by the yield behaviour of the composites. As the PE content increased, the sample yielding changed. Samples containing 0% straw yielded
Straw/polymer
43%
Straw Filler Content (%) -0 0 --n 20 ..__.-. + 40
3.5 S % ;
3.0
B 2.5
I
I
I
I
1
I
0
20
40
60
60
100
PE
in Plastic Phase (%)
A Modulus of rupture n Olfeet yield strength
~~j~4ctyS~ . , .I 0
20
40
60
80 100 PE in
1 0
6Wstraw 20
40
60
J. Simonsen
composites:
i 80 100
least squares regression of the appropriate data points. The regression is not assumed to have any analytical significance, but is used to show the relative trend of the data. The 40% straw composites showed the least change in ultimate strength with increasing PE content. This filler content seemed to be the most compatible with a variety of mixtures of PS and PE. The change in the ultimate strength with increasing straw content is caused by a greater decrease in the strengths of the high Ps-content composites relative to the high PE composites. This suggests that the behaviour of straw as a tiller is superior in FE compared to Ps. The MOE of straw-filled composites was about the same as for TMP composites. The behaviour of straw and wood fibre are compared in Figures 4 and F for tilled PE composites. Figure 4 indicates that the MOE of the TMP composites depended upon particle size. Yam et u/.15 found that shorter tibre lengths gave improved dispersion. This probably accounts for the higher MOE. Since the straw and the i&mesh TMP have approximately the same tibre length. we conclude that the straw performs about the same as wood fibre as a remforcing filler for PE in terms of flexural moduhLs. The ultimate strength properties are shown in Fi,qure .i. The straw-filled composites appeared to be slightly superior to the wood-fibre-filled composites. The h&mesh -rMP-filled samples are not shown, as the results were almost identical to those of the 16-mesh samples. The yield point of all the samples decreased at 40@60’%, loading, when it became difficult for the plastic matrix to completely wet the tiller surface. This can result in lower efficiencies in load transfer fromthe matrix to the tibre and consequently lower strengths.
2.5 -
Plastic Phase (%)
Figure 3 Ultimate strengths for VXIOUS stray loadings
nnd
plastic
blends
by fracture at 0 and 20% PE content. Samples L 40% PE did not break or yield. The ultimate strength is reported as the strength at 0.5% strain offset. Samples containing 20% straw all showed yield points, except for the lOO’/;~ PE sample. The 0.5’%1 strain offset yield strength is reported for that sample. The samples containing 40 and 60% straw all gave yield points. These data show approximately the same pattern of linear transition between PS and PE. While the stiff straw fibres increased the stiffness of the composition, as shown by an increase in MOE (Figure 2), the brittleness also increased. as indicated by a general decrease in ultimate strength (Figure 3). The lines shown in the figures are a linear
1.0’
I 0
I 10
( 20
I 30
I 40
I
I
50
60
Filler (%) Figure 4 Moduli of elasticity ethylene
(MOE)
of
straw
and wood
fibre in poly-
Straw/polymer
composites:
J. Simonsen
439
SO
I
-@-TMP.
t B-mesh
-B-TMP.
1B-mesh
70
z
60
z. 2 tj m ii E .s 2
50
40
30
.”
d
50 2b
lb
3b
4b
5;
60
$0 Filler Content (% by weight)
Filler I%\ Figure 5 Ultimate strengths of straw and wood fibre in
Figure 7 Ultimate strengths for filled
PE
6
5
0 3
4
P
3
2
1
0
I
I
1
1
I
I
IO
20
30
40
50
60
PS
composites
increase the molecular weight for the pure PS samples, increasing the ultimate properties of the plastic. Increased molecular weight does not strongly influence the flexural modulus, and little difference was noted in these values between the blended and moulded-only PS samples. This effect is unlikely to be observed in the presence of a filler, however. In this case, it is likely that the filler would react with the free radical. One of these free-radical quenching reactions might be grafting of the PS molecule containing the free radical to the filler. If significant grafting to the filler were to occur, we would expect an improvement in mechanical properties. Whether or not the results presented here were influenced by grafting is not known. One might speculate that the high variability observed for the ultimate strength in the 20% straw-filled samples is due to varying degrees of grafting. Experimental techniques and equipment were not available to investigate the presence of grafting; thus these questions could not be addressed.
Filler Content (% by weight) Figure 6 Moduli of elasticity
(MOE)
for filled
PS
composites
Figures 6 and 7 present the data for filled PS systems. The straw appeared to be inferior to wood fillers in this case. Highly variable results at the 20% straw content also give cause for concern over the performance of straw-filled PS composites. This concern is intensified by the data shown in Figure 7. The ultimate strength of straw-filled PS composites was significantly lower than their TMP counterparts. The wide variation shown in the values for pure PS was probably caused by free radical polymerization reactions occurring during the heat and shear of the blending operation. The resulting reaction would
Summary and conclusions The feasibility of using straw as a filler in thermoplastics in lieu of wood is substantiated for certain systems. Straw seems to be slightly superior to wood in PE systems, but in PS systems, slightly inferior. Blends of PS and PE, either filled or unfilled, generally follow a linear relationship from one to the other, provided that a plastics compatibilizer is incorporated into the mixture. In general, increasing levels of filler increase stiffness while decreasing strength. The exception to this is low filler (up to 40%) and high PE contents (SO-lOO%), where the strength and stiffness increase with increasing filler content. Current plastic lumber products exhibit a modulus and strength (MOE and MOR from ASTM D79012) of
Straw/polymer
440
composites:
about 1.2 GPa and 10 MPa, respectivelyl6. The composites prepared here compare favourably with those values and indicate that improvements in filled thermoplastic composites for the building products industry are possible. These data also indicate that straw shows a potential as a filler for thermoplastics in these applications.
4
Acknowledgements The author thanks the State of Oregon Department of Agriculture for their financial support of this research via grant number G079. The author also thanks Mr Robert Gourley for his dedication and hard work in the sample preparation and testing involved in this project. This is Paper 3051 of the Forest Research Laboratory. Oregon State University, Corvallis.
IO
Recycling: is it worth the eifcxt” (brt.\lr~~.ftcf 1994. 59. (2). 92-98 Recycling supply sldc ib up. end markcth need boost /‘lc/\t fi:ji,c 1993. 49 (6). 6 Warren. D. D Production. Prlceh.. Employment. and Trade 111 Northwest F.orest Industrtes. Second Quarter 1992. USDA. Pacific Northwest Research StatIon. Portland. OR. Resource Bulletin PNW-RR-194. 1992
Hegberg, B. A.. Brenniman, G. R. and Hallenbeck, W. H. Mi.& Plustics Recycling Technology, Noyes Data Corporation, Park Ridge. NJ. 1992 Mackzo. J. An alternative to landfills for mixed plastic waste. P~uJ~. Eng. 1990, 46 (4). 51-53 Eskelsen. V. Oregon State University, 1993, personal communicatIon Beatty. C. Univcrslty of Florida, 1993, personal communicntion Seymour. R. B. and Carraher, C. E. Pulymrr Chemistry. Marcel Dekker, New York, 1992 Stephenson, J. N.. ed. Pulp und Paper Monufoclure. Volume 2, Pwpurotion ofStock/ur Pupermuking, McGraw-Hill, New York. 1951 Biermann, C. J. Use of Willamette Valley rye grass straw m the papermakmg process. prescntetl ut i/le Rrc.vcling Biohused :MureriuO
II
Conferrnce.
I?
I-l
IS
It!
Oregon Cit.v. OR, 12-13 Auglrst
O’Brien. M. and Tylman. strawboard. pre.ren/ecl (II
V the OR,
Meadowood,
1992
a medium
Recycling Biohu.red 12-13 Atrgusf IY92.
density Muterid\
Oregon Cit?;. (Contact Michael O’Brien, O’Brien and Associates, P 0. Box 963. Lake Oswego. OR 97034) American Society for Testmg dnd Materials. Annd Book o/ .4STM Siundurd~. Vol. 8.01, Philadelphia, 1994 Locke. C. E. and Paul. D. R. Graft copolymer modification of polyethylene-polystyrene blends. I. Graft preparation and characterization. J Appl Polym. Sci. 1973, 17. X97-2617 and Newman. S InterfacIal agents Paul, D. R (‘Compatihilizers’) for polymer blends. Chapter I2 m Po/Fmrr B/end\. Vol. 2. Academic Press. New York. 1978. pp. 35 67 Yam. K.. Gogoi. B., Lai, C. and Selke, S Composites from compounding wood fibers with recycled high density polyethylene P&n?. Enng. Sci., 1990. 30. (I I ). 693-699 Corlf~wnw.
13
References
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Tre Y Wood-Polvmcr Compoxitr Ph_v.ticul und Propc’r/rr.\. Modil Chemical Company. Composite
Division.
Norwalk.
CT. 1993
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ul