Composites of Rice Husk/Wheat Straw with pmdi Resin and Polypropylene

Transcription

1 Composites of Rice Husk/Wheat Straw with pmdi Resin and Polypropylene Masoud Frounchi 1, Susan Dadbin 2, Javad Jahanbakhsh 1 and Mojtaba Janat-Alipour 1 1 Department of Chemical and Petroleum Eng., Sharif University of Technology, Tehran, Iran 2 Radiation Applications Research School, Nuclear Science and Technology Research Institute, Tehran, Iran Received: 16 December 2006 Accepted: 25 June 2007 SUMMARY The application of agricultural fibers for making particleboards has been studied in two types of composites: (i) type I particleboards were composites of wheat straw and rice husk fibers with polymeric methylene di-phenyl diisocyanate resin (pmdi) as a binder made by compression moulding; (ii) type II particleboards were composites of rice husk and polypropylene made by extrusion and injection moulding. In type I particleboards, the effects of varying the resin content and various combinations of wheat straw/rice husk fibers were investigated and characterised in terms of physical and mechanical properties of particle boards such as modulus of rupture, modulus of elasticity, compression strength, density, dimensional stability and water absorption. Results indicated that with increasing binder content, the physical and mechanical properties were improved until an optimum level of binder content was reached. Particleboards made of wheat straw blended with 6% pmdi showed a maximum MOE. Dimensional stability and water absorption of particle-boards made by combining wheat straw and rice husks were superior to those made only from wheat straw. In type II particleboards, the non-polar polypropylene had poor affinity with rice husk, which resulted in weak interfacial adhesion and low mechanical properties. Tri-methoxy vinyl silane as coupling agent and maleic anhydride-grafted polypropylene (MAPP) as compatibiliser were used to obtain improved interfacial adhesion. The interface-treated composites were characterised by measuring tensile strength, tensile modulus, impact strength and optical microscopy. The combination of silane and MAPP proved effective in improving interfacial adhesion between polypropylene and rice husk. The improved interface allowed effective transfer of mechanical stress through boundary regions as verified by better mechanical properties. The latter was confirmed by the improved interface morphology of composites. The best impact strength result was obtained at a combination of 1.5 wt.% silane and 5 wt.% MAPP in all rice husk composites. INTRODUCTION Agricultural fibers (agrofibers) such as wheat straw and rice husks may be considered as renewable alternatives to wood fibers for making particleboards. These alternatives could ease the demand for wood and thus reduce the rate of deforestation and mitigate its negative impact on the environment. Studies on particleboards made from wheat straw and other agricultural fibers using different adhesive resins such as urea formaldehyde, methylene diisocyanate and soybean protein have been reported 1-5. In these 1 Corresponding author. frounchi@sharif.edu Smithers Rapra, 2007 composites the solid adhesive content is 4-15 wt.%. The straw particleboards are now competing with wood fiber particleboards for floor underlays, furniture, and cabinet construction 3. There is also a great interest in utilising natural fibers in thermoplastic polymers such as polypropylene (PP) and high density polyethylene 6-9. Agrofibers such as flax, sisal, kenaf and hemp have recently been investigated as substitutes for wood fibers in composites intended for automobile parts. As these materials are similar to wood fibers in composition, they would be expected to impart similar properties to the composites. However, these properties are not optimised because the wood fibers and agrofibers do not bind to non-polar polypropylene to any significant extent The objective of the present work was to characterise mechanical and physical properties of particleboards made from rice husk, wheat straw and a thermosetting resin, namely polymeric methylene di-phenyl diisocyanate (pmdi) and also particleboards made from rice husk and polypropylene. The use of a coupling agent and polymer compatibiliser for achieving improved adhesion between polypropylene and rice husk have been of prime 619

2 Masoud Frounchi, Susan Dadbin, Javad Jahanbakhsh and Mojtaba Janat-Alipour interest. To our knowledge, the use of rice husks with pmdi resin and with polypropylene has not been reported and the utilisation of rice husk has mainly been concentrated on being a source of silica in plastics such as polyethylene and polypropylene In this work, rice husk fiber was directly used in polypropylene matrix and also in combination with wheat straw in pmdi resin. EXPERIMENTAL Type I: Rice Husk/Wheat Straw/pMDI Composites Materials Rice husk and wheat straw are domestic agricultural wastes. The average composition of straw and rice husks is shown in Table 1. Rice husks flakes are 3 to 5 mm in average diameter and wheat straws are 3 to 12 mm in length. Polymeric MDI was obtained from Bayer of Germany, having wt.% NCO content with a viscosity of 300 mpa.s at 25 C. The resin is a translucent brown liquid mixture containing monomeric MDI isomers and oligo-isocyanates. Oligoisocyanates are products having a few phenyl rings, as indicated below: Particleboard Preparation and Evaluation Wheat straw fibers were soaked in a 5% bleach solution at 50 C for 30 min to remove the wax and ash from the straw surface. The treated straw was then washed with 50 C water and airdried at room temperature. Polymeric MDI was added to the straw and/or rice husks at 4, 6 and 8 wt.% levels. Composites of mixed wheat straw and rice husks were 60% wheat straw and 40 wt.% rice husks and vice versa. The resin-treated straws and husks were pressed into particleboards using a 160 mm 160 mm stainless steel mould and a hot press. The press conditions were 6 MPa at 175 C for 5 min. The initial moisture content of the straw and husks was 8 wt.%. The mass of straw and husks was adjusted so that the finished particleboards had a thickness of about 8 mm and bulk density of about 0.7 g/cm 3. Particleboards were cut into 76 m 160 mm rectangular strips for threepoint bend measurement, into 25 mm 30 mm for compression strength, and into 50 mm 50 mm squares for swelling tests. Modulus of rupture (MOR) and modulus of elasticity (MOE) were obtained from three-point bend experiment, and compression strength (CS) from compression tests in accordance with ASTM D1037 using a Hounsfield testing machine (Model H10KS). The crosshead speeds were 1.63 mm/min for three-point bending and 0.13 mm/min for compression test. The span between the supports in the three-point bending experiments was mm. Dimensional stability and water swelling of particleboards were determined at 20 C for 2 h and 24 h by soaking the samples in distilled water. Wet measurements of length, thickness, and weight were recorded immediately after removing the samples from the water. Throughthickness swelling and water content of the samples were calculated. Type II: Rice Husk-Filled Polypropylene Composites Materials The polypropylene was an injection grade (R40) product of Iran Petrochemical. Maleic anhydridegrafted polypropylene (MAPP), supplied by a local company, having 1 wt.% grafted maleic anhydride was used as compatibiliser. Trimethoxy vinyl silane CH 2 =CHSi (OCH 3 ) 3 from Merck was used as coupling agent. Particleboard Preparation and Evaluation Composites of polypropylene/rice husks were prepared containing 10, 20 and 40 wt.% rice husks by melt mixing in a single extruder and then injection moulded. The temperature was set from 180 C up to 210 C in different zones of the injection moulding machine. This temperature range didn t cause any degradation of rice husks in the extruder and the injection moulding machine as evidenced by the optical photographs of the composites in Figures The rice husks were vacuum dried at 70 C for 2 h before mixing. A series of samples with rice husks surface-treated with 1.5 and 3% silane and or adding 5% MAPP were prepared and designated according to the Table 2. Tensile tests were carried out according to ASTM D638 at a crosshead speed of 0.5 mm/s. Impact tests were conducted according to ASTM D256 in Charpy mode. The morphology of the fracture surfaces was studied by means of an Olympus SZH10 optical microscope. RESULTS AND DISCUSSION Table 1. The composition of straw and rice husks Straw Rice husks 29-35% cellulose 35% cellulose 26-32% hemicelluloses 25% hemicelluloses 16-21% lignin 20% lignin 2-5% extraction substances 3% crude protein 3-7% mineral compounds 17% ash of 94% silica Type I: Rice Husk/Wheat Straw/pMDI Composites The modulus of rupture of the particleboards is shown in Figure 1. Particleboards with more straw had higher MOR. Polymeric MDI was an effective binder and the MOR was 620

3 highest (29.7 MPa) at 8% pmdi content and lowest (18.6 MPa) at 4% binder content. MDI can effectively wet the straw surface, and can penetrate and bond to the straw hydroxyl groups by forming polyurethane covalent bonds, so that a high amount of binder will produce a strong and rigid composite. The possible reaction is shown below: straw -OH + OCN-R- NCO -OCONH-R-NCO -OCONH-R-NHCOO- Table 2. Samples of PP/rice husks composites designation and description Sample designation Description PP/husks Untreated husk surface PP/husks-1.5% silane Husk surface treated with 1.5% silane PP/husks-3% silane Husk surface treated with 3% silane PP/MAPP/husk 5% MAPP added to PP PP/MAPP/husks-1.5% silane 5% MAPP added to PP and husk surface treated with 1.5% silane PP/MAPP/husks-3% silane 5% MAPP added to PP and husk surface treated with 3% silane Figure 1. Modulus of rupture of straw/husk/pmdi composites In addition, the isocyanate groups of MDI can react with moisture in the straw to produce cross-linked polyureas for better mechanical bonding. OCN-R-NCO + H 2 O H 2 N-R- NH 2 + CO 2 n OCN-R-NCO + n H 2 N-R-NH 2 OCN-[-R-NHCONH-] n -R-NCO The MOR of 60% straw/40% husk at 6% pmdi was as high as 27.5 MPa. The MOE of particleboard samples followed a similar trend with addition of pmdi and was highest with straw fibers, particularly at 6% pmdi (2.97 GPa) as shown in Figure 2. The compression strength of the straw-husk particleboards increased as the binder content increased, Figure 3. The maximum CS of 17.9 MPa was reached at 8% pmdi content with 100% straw particleboards. The pmdi content has been shown to be a significant factor influencing the dimensional stability and water resistance (Figures 4-5). At 8% pmdi content, the strawboards had the lowest through-thickness swelling, and at 4% pmdi content, the strawboards had the highest. This was expected because the high resin binder content in the strawboard would coat more surfaces, leading to lower water absorption. Water absorption showed a similar trend to thickness swelling (Figures 6 and 7). Thus, with higher binder content, more bonding sites are made available to provide the internal strength to resist the swelling forces, giving lower thickness swelling. The use of husks at the outer Figure 2. Modulus of elasticity of straw/husk/pmdi composites surfaces of the particleboards gave a smoother surface and the throughthickness swelling of 40% straw/60% husk boards at 8% pmdi was the lowest (11.9%) even lower than the 100% straw samples (13.7%). Type II: Rice Husk-Filled Polypropylene Composites The tensile strength of the composites (Figure 8) decreased with rising rice husks content. Addition of silane made 621

4 Masoud Frounchi, Susan Dadbin, Javad Jahanbakhsh and Mojtaba Janat-Alipour Figure 3. Compression strength of straw/husk/pmdi composites Figure 4. Thickness swelling in water of straw/husk/pmdi composites after 2 h at 25 C Figure 5. Thickness swelling of straw/husk/pmdi composites after 24 h at 25 C the decrease in strength more severe, but the combination of MAPP and silane showed some improvement, although the resulting strength was still less than that of the untreated samples. The decrease in tensile strength is because of poor adhesion between polypropylene and rice husks so the stress doesn t transfer effectively between the two phases. Agglomeration of cellulose fibers due to formation of hydrogen bonds between them may play a role in poor adhesion of these fillers to the matrix. The agglomeration creates weak sites inside fibre bundles in the composite (the optical photographs confirm the agglomeration of rice husks). The use of silane and MAPP as coupling agent and compatibiliser did not enhance the tensile strength of the composites, it reduced it. This was more pronounced in the case of silane and can be explained by the fact that silane acts in the interphase regions. The silane reacts with the husks surfaces by hydrolysis of the methyl ether, followed by hydrogen bonding to the hydroxyl groups on the husk surface, followed by polymerisation of the silane residues. The silane has a carbon-carbon double bond, which tends to react with polypropylene in presence of radicalproducing peroxides. Because no peroxide compound was used in this work, no covalent bond was developed between polypropylene and the rice husks. This was reflected in poor the mechanical properties of silane treated composites without MAPP. Therefore, it can be concluded that the use of silane, instead of strengthening, weakens the interphase adhesion. It has been reported that addition of MAPP to cellulose-fiber-filled composites helped to improve the tensile strength. Indeed, the maleic anhydride group on the molecular chains of polypropylene reacts with the hydroxyl groups on the surface of the fibers while the polypropylene part of the MAPP is mixed into the bulk of the polypropylene matrix of the composite. Thus, MAPP bridges the cellulose fibers to the matrix and 622

5 facilitates the transfer of tensile stress between fibers and matrix. This results in high tensile strength of composites. However, addition of MAPP to the husk-filled composites in this work was not so effective and the tensile strength of the composites remained low (Figure 8). However, the combination of silane coupling agent and MAPP resulted in better properties than that of PP/ MAPP/husk. This could arise from an interaction between maleic group and the hydroxyl groups on the surface of the husks and also on the silane itself. Thus, we might expect to obtain higher tensile strengths by optimising the level of MAPP and silane in the composite. The tensile modulus of the composites increased with increasing rice husk content (Figure 9). The PP/huskssilane samples showed a lower modulus than PP/husks samples. However, the combination of MAPP and silane improved it significantly. Addition of husks caused a reduction in the impact strength of the composites. This can be attributed to weak adhesion between the surface of the fibers and the matrix. The impact strength of composites with and without coupling agents is shown in Figure 10. While silane proved ineffective as a coupling agent to improve the impact strength of the composites, MAPP showed better performance. The combination of MAPP and silane improved the impact strength significantly. The PP/MAPP/ husk-1.5% silane sample showed maximum impact strength at 10% filler. In addition, the impact strength at 20% rice husks was only slightly lower than that of the 10% case. Figure 6. Water absorption of straw/husk/pmdi composites after 2 h at 25 C Figure 7. Water absorption of straw/husk/pmdi composites after 24 h at 25 C Figure 8. Tensile strength of rice husk/pp composites with different surface treatments The surface of a rice husk, Figure 11, reveals a regular indented surface. It was expected that the rough surfaces of rice husks would provide better adhesion to the polymeric matrix. The fracture surface of all the tensile and impact specimens showed a rough surface. The fracture surface of tensile specimens of PP/husk-1.5% 623

6 Masoud Frounchi, Susan Dadbin, Javad Jahanbakhsh and Mojtaba Janat-Alipour Figure 9. Modulus of rice husk/pp composites with different surface treatments silane and PP/MAPP/husk-1.5% silane composites are shown in Figures 12 and 13 respectively. It can easily be seen that PP wetted the surface of the rice husks in PP/MAPP/husk-1.5% silane sample to some extent. In some places the matrix had not detached from rice husk and instead the matrix was ruptured. In contrast in silane treated samples (without MAPP) the matrix was detached from the rice husk surface. The latter is consistent with the by poor mechanical properties of the silane treated composites. Figure 10. Impact strength of rice husk/pp composites with different surface treatments CONCLUSIONS Rice Husk/Wheat Straw/pMDI Composites A particleboard was developed from a combination of wheat straw, rice husks, and a small amount of pmdi. The thickness swelling of the straw/ husks samples was lower than that of 100% straw samples and they had a much smoother outer surface. The mechanical strength and modulus of the straw/husks samples were at the same level as, or only slightly lower than those of the 100% straw samples. Figure 11. Surface details of a rice husk, 10 Figure 12. Fracture surface of PP/husk composite (husk content 40%), 10 Figure 13. Fracture surface of PP/ MAPP/husk-1.5% silane (husk content 40%), 10 Rice Husk-Filled Polypropylene Composites Polypropylene/rice husk composites were investigated. Coupling agents were used to promote better adhesion between the two components of the composites. Husk surfaces treated with 1.5% and 3% silane-type coupling showed poor mechanical properties. However, good compatibility was obtained with the addition of 5% MAPP and 1.5% silane, resulting in high impact strength. This was attributed to interaction between the maleic groups of MAPP and hydroxyl groups on the surfaces of the husks and, as well as on the silane itself. REFERENCES 1. Mo X., Cheng E., Wang D. and Sun X.S, Physical properties of mediumdensity wheat straw particleboard 624

7 using different adhesives, Industrial Crops and Products, 18 (2003) G. S. Karr, E. Cheng and X.S. Sun, Physical properties of strawboard as affected by processing parameters, Industrial Crops and Products, 12 (2000), Girgoriou A.H., Straw-wood composites bonded with various adhesive systems, Wood Science and Technology, 34 (2000) Han G., Umemura K., Kawai S. and Kajita H., Improvement mechanism of bondability in UF-bonded reed and wheat straw boards by silane coupling agent and extraction treatments, Journal of Wood Science, 45 (1999) Pan Z., Cathcart A. and Wang D., Properties of particleboard bond with rice bran and polymeric methylene diphenyl diisocyanate adhesives, Industrial Crops and Products, 23 (2006) Jayaraman K., Manufacturing sisal/polypropylene composites with minimum fiber degradation, Composites Science and Technology, 63 (2003) Doan T., Gao S. and Mäder E., Jute/ polypropylene composites I. Effect of matrix modification, Composites Science and Technology, 66 (2006) Espert A., Vilaplana F. and Karlsson S., Comparison of water absorption in natural cellulosic fibers from wood and one-year crops in polypropylene composites and its influence on their mechanical properties, Composites Part A: Applied Science and Manufacturing, 35 (2004) Xie X.L., Li R.K.Y., Tjong S.C. and Mai Y-W., Structural properties and mechanical behavior of injection molded composites of polypropylene and sisal fiber, Polymer Composites, 23 (2002) Daan Feng, Caulfield D.F. and Sanadi A.R., Effect of compatibiliser on the structureproperty relationships of kenaf-fiber/ polypropylene composites, Polymer Composites, 22 (2001) Keener T.J., Stuart R.K. and Brown T.K., Maleated coupling agents for natural fiber composites, Composites Part A: Applied Science and Manufacturing, 35 (2004) Zafeiropoulos N.E., Baillie C.A. and Hodgkinson J.M., Engineering and characterization of the interface in flax fiber/polypropylene composite materials: Part II, The effect of surface treatments on the interface, Composites Part A: Applied Science and Manufacturing, 33 (2002) Qiu W., Zhang F., Endo T. and Hirotsu T., Preparation and characteristics of composites of high-crystalline cellulose with polypropylene: Effects of maleated polypropylene and cellulose content, Journal of Applied Polymer Science, 87 (2003) Panthapulakkal S., Sain M. and Law S., Effect of coupling agents on ricehusk-filled HDPE extruded profiles, Polymer International, 54 (2005) Panthapulakkal S., Law S. and Sain M., Enhancement of processability of rice husk filled high-density polyethylene composite profiles, Journal of Thermoplastic Composite Materials, 18 (2005) Ismail H., Hong H.B., Ping C.Y. and Abdul Khalil H.P.S, Polypropylene/silica/rice husk ash hybrid composites: A study on the mechanical, water absorption and morphological properties, Journal of Thermoplastic Composite Materials, 16 (2003) Siriwardena S., Ismail H., Ishiaku U.S. and Perera M.C.S., Mechanical and morphological properties of white rice husk ash filled polypropylene/ethylene-propylenediene terpolymer thermoplastic elastomer composites, Journal of Applied Polymer Science, 85 (2002)