Evaluation of cotton gin waste as a lignocellulosic substitute in woodfiber plastic composites.
Rising raw material costs and shortage of woody materials necessitate alternative sources for lignocellulosic material in woodfiber plastic composites (WPC). This study was conducted to evaluate cotton gin waste (CGW), an agricultural residue as a cellulosic substitute in WPC. Samples were fabricated with approximately 4 percent of zinc stearate, 48 percent of high-density polyethylene obtained from recycled plastics and 48 percent of lignocellulosic material by mass. The composition of the cellulosic material was changed from 0 to 100 percent CGW at 20 percent increments while the remainder was wood flour. The extruded sampled were tested for properties including SG, water absorption, linear coefficients of thermal expansion (LCTE), and strengths under compression, shear and bending. Except water absorption, the CGW composite samples exhibited similar or better properties than samples with no CGW. The CGW in the composite samples tended to increase yield strengths, ultimate strengths and water absorption, while decreasing LCTE and SG. Overall, mechanical properties of extruded CGW composites were within the range of reported values for various commercial WPC products. Therefore, cotton gin waste will be a viable alternative for lignocellulosic material in WPC intended for applications in decking, fencing, roofing, OEM, and other non-structural applications.
Wood plastic composites (WPC) are durable and low maintenance compared to wood, and hence ideal for non-structural applications. Wood flour is the major source of lignocellulosic filler in the polymer matrix in WPC. The price of good quality wood flour has been steadily increasing--as much as 300 to 500 percent in five years. The current reported price of wood flour is approximately 22 to 27 cents per kg. Demand for WPC has been steadily increasing in the last decade, with a market demand of 1.95 billion kg in 2006 (Freedonia Group, Inc. 2004). Considering a typical composition of 50 percent wood in WPC, the current demand of 0.5 billion kg of wood flour is expected to grow to approximately 0.85 billion kg in 2010 based on the estimated demand for WPC at 1.7 billion kg in North America based on 8-year average growth rate of 14 percent (Morton et al. 2003). However, the growth of WPC product reached 25 percent since 1998 (Morton et al. 2003). The price of the polymer has been soaring in the past couple of years because of escalating oil prices. With the growth in WPC demand coupled with mounting raw material price is expected to lower the profitability margin significantly if alternate sources of raw materials are not identified. Natural fibers have been used as a good source of lignocellulose, with 13 percent of the lignocellulose-polymer composites produced in 2002 being natural-fiber composites (Morton et al. 2003). Agriculture waste is an untapped resource that can provide an inexpensive alternative for lignocellulose in WPC. Several commercial WPC products use recycled lignocellulosic materials such as pallets and furniture waste, rice hull flour, sawmill waste, pine scrap, recycled fiber, recycled paper fiber, and natural fibers (Winandy et al. 2004).
Cotton gin waste (CGW) is a renewable agricultural waste that contains natural fibers. Approximately 2 to 3 million metric tons of CGW are generated each year across the cotton belt of the United States (Thomasson et al. 1998, Holt et al. 2000). More CGW is available globally since cotton is produced in many other countries, with China and India being the major producers. CGW has limited applications for processing into value-added products. The major reported uses of CGW in the united States include composting, bedding for dairy cattle, livestock feed, and land application as a soil amendment (Holt et al. 2000, Roberts and Pittaway 2000, Huitink et al. 2002). CGW has been researched as a source of solid fuel (Holt et al. 2003, Holt et al. 2004) and biogas (Macias-Corral et al. 2005). A 1990 regulatory change made the incineration of CGW illegal. Similarly, there are restrictions against the use of gin waste (except the seeds) as animal feed. The cost associated with the production of other fuels (methanol, ethanol, biodiesel) is prohibitive. In spite of the vast amount of research done on possible end uses of CGT (Thomasson 1990), the only successful applications currently adopted on a limited basis are composting and land application as a soil amendment. Approximately 63 percent of domestic CGW is disposed at a cost of $2 per ton (Buser 2001). The current practice to pile up CGW near the gin has caused environmental concerns due to reported incidents of fire and smoldering and resulting environmental pollution. Given the environmental concerns and current cost associated with proper disposal, it is desirable to find a more economical end use for cotton gin waste.
CGW is a good source of lignocellulose. Physical and chemical compositions of CGW vary slightly from region to region, and with method of harvesting. CGW consists of approximately 8.4 percent sticks, 48.6 percent burs, 11.1 percent lint, and the rest composed of leaves, soil particles, mote and other plant materials (Schacht and LePori 1978). The amount of lignin in CGW from Virginia gins varied from 25 to 42 percent in various samples (Jeoh 1998). CGW from stripper-harvested cotton contained 26 percent of cellulose, 18 percent ofhemicellulose, 21 percent lignin, 14 percent extractives and 13 percent ash on a dry basis (Thomasson 1990). Cotton fibers in the CGW impart excellent mechanical properties, which is an added advantage toward its application in WPC. Cotton linters have a high aspect ratio of 165:1, which is higher than that of hardwood (50:1), softwoods (100:1), jute (100:1) and kenafbast fiber (135:1) (Hunter 1991). The significantly high aspect ratios of the cotton linters could impart useful strength properties to the plastic composites. Therefore, this study was undertaken with the goal of testing CGW as an alternative source of filler in lignocellulosic composite materials. The specific objective was to understand the feasibility of using cotton gin waste in wood plastic composites by evaluating the mechanical and physical properties that are important for WPC products.
Materials and methods
WPC samples were extruded from a mixture of high-density polyethylene (HDPE), fresh CGW, wood flour and Zinc stearate (ZnSt). Recycled plastic pellets made from milk jugs, with a melt flow index of approximately 0.6, were used as a source of HDPE. Wood flour used in the study was 40 to 60 mesh southern yellow pine, obtained from a commercial source. This is a common size used by most commercial WPC manufacturers and was assessed to provide the best WPC performance (Stark and Berger 1997). Zinc stearate was used as a lubricant. Fresh CGW was obtained from a cotton gin at Lonoke, Arkansas. The experimental design consisted of 6 combinations oflignocellulose, 4 percent ZnSt and 48 percent HDPE. The six ratios of wood flour and CGW were approximately 0:100, 20:80, 40:60, 60:40, 80:20 and 100:0 by weight (Table 1). Three sets of samples were extruded in separate batches under each treatment. The actual compositions of samples under each treatment are shown in Table 1.
A protocol was developed to fabricate ligno-cellulosic plastic composite samples from CGW, wood flour and plastic (Fig. 1). Fresh CGW and wood flour were dried separately in a convection dryer at 60[degrees]C to a MC of less than 2 percent. The CGW was then ground to 10 mesh size, and returned to the dryer for storage until use. A 10-mesh (2 mm) size was used for cotton gin waste since the cotton linter fibers are soft and small, and tend to form bolls when smaller mesh sizes were used. Mesh size larger than 10-mesh will break down the fibers and reduce its aspect ratio, compromising it mechanical properties. For sample preparation, all four ingredients were mixed in the correct proportion and preheated to approximately 100[degrees]C. The preheated mix was then manually mixed in a heated chamber until the material temperature reached 160 [+ or -] 5[degrees]C. The samples were then extruded into blanks with a square cross section of 19 mm using a ram type extruder. The laboratory scale ram extruder consisted of a cylinder with 62.4 mm diameter and 103.5 mm length, with 19 mm square die. The sample coupons were extruded at a speed of 0.5 mm/sec under a constant pressure on the ram, and then air-cooled. To ensure equivalent surface qualities between batches, air-cooled samples were planed on all sides to a final cross section of 13 mm square. Samples were then cut to the desired lengths for testing according to the recommendation of the ASTM standard followed.
[FIGURE 1 OMITTED]
The extruded samples were tested for SG, water absorption, linear coefficient of thermal expansion (LCTE), bending properties, shear strength, and parallel compressive properties. SG of the samples was tested according to the ASTM D 6111-97 standard test method for bulk density and SG of plastic lumber and shapes by displacement. SG tests were conducted as specified in the standard with the exception that measurements were made at 19[degrees]C. Water absorption by the samples was calculated according to the standard test method for water absorption of plastics (ASTM D 570-98). Water absorption was measured using a 24-hour immersion test conducted at 19[degrees]C. Sample dimensions for both tests were 13 mm by 13 mm by 305 mm.
LCTE was determined using the standard test method for determination of the LCTE of plastic lumber and plastic lumber shapes between -30[degrees]F and 140[degrees]F (-34.4[degrees]C and 60[degrees]C) (ASTM D6341-98). Samples were 13 mm by 13 mm by 305 mm and the LCTE was measured in the axial or extruded direction. The actual low and high temperatures used for this test were 26[degrees]F and 168[degrees]F, respectively.
Flexural properties of the composite samples were determined following the standard test methods for flexural properties of unreinforced and reinforced plastic lumber (ASTM D 6109-97). With the sample depth-to-width ratio less than two, a typical four-point bending test configuration was used. A test jig was constructed in accordance with the above ASTM standard. The load span was 68 mm with a support span of 204 mm. Samples had a 13 mm square cross section with a length of 254 mm. Both modulus of rupture (MOR) and modulus of elasticity (MOE) were calculated as specified by the ASTM standard.
Shear properties were tested using the standard test method for shear properties of plastic lumber and plastic lumber shapes (ASTM D 6435-99). Samples were tested in double shear in the plane perpendicular to the axis of extrusion. A strain rate of 0.75 mm/min was used, as this was the closest realizable strain rate. The maximum shear strength (MSS) was calculated as the ultimate strength of the samples under shear test.
Parallel compressive properties of extruded samples were determined using the standard test method for compressive properties of plastic lumber and shapes (ASTM D 6108-97). Blanks tested had 13 mm square cross sections and were cut to 25 mm in the axial/extruded direction. Compressive stress was applied along the axis of extrusion with a loading rate of 0.03 (mm/mm/min). Compressive strength of samples was calculated as the ultimate strength of samples under compression.
The effect of substituting wood flour with CGW on the mechanical properties of the composite was tested with a general linear model (GLM) procedure for completely randomized block design in SAS (SAS 1999). The means of various mechanical properties under different levels of CGW substitution were compared with Fishers least significant difference (LSD) analysis. All tests were considered significant at an alpha value of 10 percent.
Mechanical properties of the new composite samples were compared to two commercially available WPCs. The two commercial WPCs used were Trex (Trex Company, Inc.) and Rhinodeck (Master Mark Plastics). The mechanical properties and standards used for testing these two products were obtained from data published on the company website (Master Mark Plastics 2004a, Trex Company, Inc. 2005). Trex produces a WPC lumber composed of approximately 40 percent to 50 percent HDPE and 50 percent to 60 percent wood fiber (Trex Company, Inc. 2003). Master Mark's Rhino Deck product line contains 30 percent to 50 percent HDPE, 50 percent to 65 percent wood flour (Master Mark Plastics 2004b).
Results and discussions
The extruded sample coupons of the new ligno-cellulosic composite material were visually inspected before and after planning the surface. There were no surface defects apparent on the new composite material. Its appearance was comparable to most commercially available WPCs. The only exception was that smaller grains were visible on the planed surface of WPC samples made with CGW compared to the commercially manufactured and in house samples that contained no CGW.
The new lignocellulosic composite had SG in the range of 0.94 to 0.96. A SG of less than unity is preferred for composite materials. Cotton gin waste decreased the SG, as the composite material showed a decreasing tendency with increases in the quantity of CGW (Table 2). Composite with 100 percent of wood flour substituted by CGW showed the lowest SG of 0.94, and the samples with no CGW showed the highest SG of 0.96. Lower SG for samples with higher quantity of CGW is expected as cotton linters are lighter in weight compared to wood flour. Lower SG is preferred by manufacturers as it will increase the potential applications of the WPC. In general, SG showed a positive linear relationship with the quantity of CGW in the composite at 10 percent significance level (Table 3). The CGW-plastic composite exhibited SGs comparable to Trex (0.91 to 0.95), but lower than many other commercial available products such as Rhinodeck (1.08) and Fiberon (1.4). Variations in measured values of SG could occur due to the different methods (ASTM standards) used to measure this property.
Water absorption is an important property for composite materials that are exposed to environmental extremes. A lower water absorption rate is preferred as water absorption can lead to decay and shortened life of cellulosic materials in the composite. Water absorption ranged from 2.2 to 5.0 percent for the planed CGW-plastic composites. The upper limit of water absorption by CGW-plastic composite samples is relatively higher as compared to commercially available products. One reason for these high values is that sanded samples were used in the test. The sanded CGW samples are comparable to sanded Trex samples in water absorption property (Table 2). Water absorption property increased with the amount of CGW in the composite material with a highly significant [r.sup.2] value of 0.79 (Table 3). The sanded samples with high levels for CGW have more exposed CGW particles that are finer than wood particles, resulting in larger contact area and absorption surface for water to penetrate. Unsanded samples are usually sealed on the outer side by a coating of plastic which repels water. Water-phobic additives such as lubricants (Zinc Stearate), talc and coupling agents commonly used in commercially available WPC can substantially reduce water absorption and improve its mechanical properties (Stark and Rowlands 2003, Botros 2003). Commercial manufacturers typically use approximately 6 to 10 percent (by weight) of these additives to their products. Since we have used only 4 percent of lubricant, there is room for further improvement in water absorption and mechanical properties with the addition of these agents.
Linear coefficient of thermal expansion is an important property if the material is intended for use in outdoor environments, as the expansion and compression under extreme weather events would lead to failure of joints. LCTE for composite with only CGW and no wood flour was 26.2 [micro]m/m/[degrees]C, which was significantly lower than the LCTE for composite samples with no CGW (54.6 [micro]m/m/[degrees]C) (Table 2). The average LCTE for composites with CGW was comparable to the commercial products. The average LCTE of CGW composites showed a linear decrease with increase in CGW content, which was significant at 10 percent [alpha] level (Table 3).
The strength properties (ultimate strengths in parallel compression, transverse shear, and pure bending) of the composite samples increased with increasing proportions of gin waste (Table 2). This is an expected outcome as the greater aspect ratio of the cotton linters in CGW relative to that of the wood particles is expected to provide higher strength values. MOE determined using the 4-point bending tests indicates the toughness of the composite material. MOE for the new composite varied from 832 to 1199 MPa. This range of MOE is toward the lower side when compared to commercially available products such as Trex. However, it is possible to significantly increase the MOE of CGW composite by using extrusion pressures higher than we were able to create with the hand press. This is also supported by the fact that the composite samples produced with the same process with no CGW also showed a low MOE of 931 MPa. One caveat here is that the gain in MOE obtained through higher extrusion pressures will be at the cost of increased SG. However, we expect that the WPC made with CGW will have lower SG than those made with wood alone (as observed here) at comparable melt pressures, since cotton linters are lighter than wood fibers.
The average MOR per treatment for the CGW-plastic composite samples varied from 9.05 to 15.25 MPa, which was significantly higher than MOR for Trex, which was 9.8 MPa (Table 2). MOR, parallel compressive strength and MSS increased with increase with CGW content in the composite at a significant rate (Table 3). Ultimate parallel compressive strengths varied from 9.6 to 16.4 MPa. Compressive strength of Trex was in the middle of the range for CGW-plastic composite. Shear strength showed significant increases with the amount of CGW (Fig. 2). Mean shear strength values for each CGW composition was higher than the reported shear strength value for Trex and lower than that reported for Rhino Deck. It, however, must be noted that shear strength presented by both manufacturers are in the longitudinal plane whereas shear strength tests of gin waste samples were conducted in the transverse plane.
[FIGURE 2 OMITTED]
A study was conducted to test whether cotton gin waste could be a feasible lignocellulosic substitute in wood-plastic composites. A typical WPC composition of 48 percent lignocellulosic material, 48 percent plastic and 4 percent Zinc stearate was used for the study. Wood flour was substituted by CGW in increments of 20 percent by weight, varying from 0 percent (no CGW) to 100 percent CGW. The new composites exhibited low SGs, which decreased with increases in CGW content. Properties such as water absorption, LCTE, MOR, compressive strength and shear strength were comparable to commercially available WPC products. Strength properties (with the exception of MOE) and water absorption increased linearly with increases in the quantity of CGW in the composite. The tested properties (except water absorption) of CGW composite materials were similar or significantly better than the composite sample with no CGW. With better temperature and pressure control in the extrusion process, the properties of the new composite could be improved significantly. In conclusion, CGW is a renewable and viable alternative source of lignocellulosic material in lignocellulosic-plastic composites. CGW could be viable source of lignocellulosic material in non-structural plastic composites. While handling and processing of the material will present new challenges, its use could prove desirable to WPC manufacturers in cotton producing regions. As an inexpensive alternative to wood flour, there is economic incentive for WPC manufacturers. Use of CGW in composites could provide economical benefit to farmers and ginners, and environmental benefits to the communities surrounding cotton gins.
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Patrick J. Bourne
Sreekala G. Bajwa
Dilpreet S. Bajwa *
The authors are, respectively, Student, Dept. of Mechanical Engineering, and Assistant Professor, Dept. of Biological and Agri. Engineering, Univ. of Arkansas, Fayetteville, AR (firstname.lastname@example.org, email@example.com) and Research and Development Manager, Greenland Composites, Greenland, AR (firstname.lastname@example.org). This paper was received for publication in January 2006. Article No. 10158.
* Forest Products Society Member.
Table 1.--Recipe (compositions) of the lignocellulosic plastic composite under the six treatments. The treatments were to substitute wood flour in a WPC with cotton gin waste at levels of 0, 20, 40, 60, 80 and 100 percent. Amount of constituents in CGW-plastic composite Treatments HDPE ZnSt Wood flour CGW (g) 1 160 13.3 160 0 2 160 13.3 128 32 3 160 13.3 96 64 4 160 13.3 64 96 5 160 13.3 32 128 6 160 13.3 0 160 Table 2.--Mechanical properties of the new lignocellulosic composite material developed from CGW and two commercially available WPC materials. Numbers 1 to 6 for CGW indicate wood flour substitution by CGW at 0 to 100%. (a) Composite Sp. gr. at 19/ material 19[degrees]C Water absorp. LCTE (%) ([micro]m/ m/[degrees]C) 1 (CGW) 0.960a 2.2a 54.61a 2 (CGW) 0.957a 2.9a 45.40ab 3 (CGW) 0.956a 3.0a 44.60abc 4 (CGW) 0.956a 3.3a 44.53abc 5 (CGW) 0.946ab 4.9b 33.28bc 6 (CGW) 0.936b 5.0b 26.22c Trex 0.91 to 0.95 4.3 29.98 to 34.56 Rhino deck 1.08 na 35.82 ASTM standard D6111 (b) D570 (b) D6341 (b),(c),(d) D2395 (c),(d) D1037 (c) D4442 (d) Composite material MOE MOR MCS MSS --(MPa)-- 1 (CGW) 931a 9.29a 12.2abc 5.22a 2 (CGW) 1199a 10.79ab 11.2ab 4.81a 3 (CGW) 935a 9.05a 10.8ab 5.14 4 (CGW) 834a 10.49ab 9.6ba 5.66ab 5 (CGW) 862a 13.18bc 15.lbc 7.30ab 6 (CGW) 884a 15.25c 16.4c 8.28b Trex 1207 9.81 12.5 3.87 Rhino deck na 33.46-edge na 9.58 21.79-flat ASTM standard D6109 (b) D6109 (b) D6108 (b) D6435 (c) D4761 (c) D4761 (c),(d) D198 (c) D143 (c),(d) (a) Different letters along a column indicate a significant difference in the mean value at 5 percent significance level. (b) Standard used for CGW samples. (c) Standard used for Trex. (d) Standard used for Rhino Deck. (r) na = not applicable. Table 3.--Effect of cotton gin waste substitution on various physical and mechanical properties of a lignocellulosic composite with 48 percent plastic for n = 18. Model Model Treatment Model Property DF p-value p-value [r.sup.2] SG 7 0.0770 (a) 0.0743 (a) 0.65 (a) Water absorption 7 0.0094 (a) 0.0039 (a) 0.79 (a) LCTE 7 0.0992 (a) 0.0698 (a) 0.63 (a) MOE 7 0.6672 0.6408 (a) 0.33 (a) MOR 7 0.0173 (a) 0.0091 (a) 0.76 (a) Max. shear strength 7 0.1808 0.1236 (a) 0.56 (a) Max. compressive 7 0.0971 (a) 0.0639 (a) 0.63 (a) strength (a) Indicates that substitution with CGW significantly influenced the specific property at 10 percent significance level.
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|Author:||Bourne, Patrick J.; Bajwa, Sreekala G.; Bajwa, Dilpreet S.|
|Publication:||Forest Products Journal|
|Date:||Jan 1, 2007|
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