Study On Structure And Properties Of PVC/Rice Husks Composites

The physical mechanical preperties of PVC/rice husks composites can be equivalent to hardwood, which was prepared by modified rice husks and plastics to blend with suitable methods. It not only has the natural ligneous feeling of wood, but also has more excellent natural behavior than wood. It also can be drilled, cut, planed, milled and napped by the tools and techniques for wood. Also, it can be burnished, variegated and decorated to get needed handsome effect as to process wood. And it can be broadly used to product imitative wood floor board, architecture molding board, cloacae cover, railway crosstie, guardrail, hypaethral furniture and so on.In this paper, many surface treated agents were used to modify rice husks, and they were blended with PVC to prepare wood-plastics composites (WPC). The primary research works are as follows:Nano-CaCO₃, titanate coupling agent (TM-38), silane coupling agent (WD-20), methyl methacrylate (MMA) and phenol formaldehyde (PF) were used as modification agents to modify rice husks. The heat resistance of rice husks can be enhanced after the rice husks was modified by nano-CaCO₃ or in-situ reaction CaCO₃. The coupling agent can react with hydroxyl of rice husks, and moisture absorption of modified rice husks can be reduced. When MMA is added into rice husks or wood flour, a semi-interpenetrating networks structure modified materials can be formed by microwave irradiation. The moisture absorption ratio of the MMA or PF modified rice husks is weak.The different modification agent has different effect on the mechanical properties, moisture absorption, liner expansion ratio after absorbing water of PVC/rice husks composites. MMA is the most excellent modification agent to enhance the tensile strength of the composites. WD-20 is the most effectual to enhance impact strength of the composites, and PF has hardly increasing to mechanical properties of the composites, but much more effectual by using WD-20 and PF together. The water absorption ratio of composites filled with surface modification rice husks is obviously lower than that of non-surface modification. And the water absorption ratio of the composites is lower which modified by MMA and PF than that of other modification. The mechanical properties can be obviously enhanced by adding some elastomer into composites, such as CPE and POE-g-MAH. When elastomer is added into PVC, the toughness of the composites is enhanced. Elastomer is more available to the increasing of impact strength of the composites in the system of PP/POE-g-MAH/rice husks than coulping agent. When POE-g-MAH content is increased from 0 phr to 12 phr, the impact strength of the composites is increased from 5.2KJ/m² to 9.1KJ/m², e.i. enhanced 75%. The impact strength of the composites is increased with the same change of CPE, from 5.2KJ/m² to 9.1KJ/m². When the same POE-g-MAH and CPE content is added into PVC matrix, respectively, the former tensile strength of is higher. At the same time, pessure has great effect on the mechanical properties of the composites. When the process pressure reach to 30MPa, the mechanical properties of the composites will be no matter to surface modification agent. The moisture absorption ratio will be obviously decreased under high pressure. So, high pressure process would be a potential and viable method in WPC.The moduli and rigidity of the composites increase with the increasing of rice husks content, and decreased with the temperature increasing. When the rice husks content is 40 phr and 80 phr, the inner loss peak temperature is 95.2℃and 96.8℃, respectively. The inner loss peak of pure PVC is 92.8℃, which mean the temperature of glass transition of the composites get some increasing.There are two methods to prepare PVC/rice husks foamed composites, which are extrusion and compression moulding process. The maximal content of rice husks in PVC matrix is 60 phr by extrusion foam process, but less than 40 phr by compression moulding process. The gradient foam composites of PVC/rice husks were realized by using a temperature gradient in moulding.The outer layer of the composites is compact, which has much higher surface hardness and mechanical behavior (cortical bone). The inner has a structure with micro-vesicle which is similar to wood-cell (cancellous bone), and the midle of board is hollow (medullary cavity), which is a bionic structure (femur). The cell structure of foamed composites is ellipsoid by extrusion process, and the compression moulding foam is global. Mechanical properties of PVC/rice husks foamed composites prepared by extrusion is much better than that of compression moulding foam, but the liner thermal expansion coefficient is bigger.Through the optimization of lubricant dosage can control surface wood-like grain size of WPC. When the lubricant dosage is 2 phr in wood-like grain additive and 1.5 phr in basic materials, the extrusion effect of wood-like grain is well by single or twin screw extruder.The rheological test results of the PVC/rice husks show that the melt of PVC/rice husks is non-Newtonian fluid, and the shear viscosity decrease with the increasing of shear rate. The extrusion swelling ratio of the composites is decreased obviously through increasing of rice husks content and extruder temperature. When the shear rate is up to 230s^-1, the surface of the extrusion composites is smooth. But the shear rate is up to 324s^-1, the surface of the extrusion materials is of some sharkskines with slight moire aberration as honeycomb-shaped. When the shear rate reachs to 523s^-1, it is easily to find the sharkskin aberration, and the average wave and depth of the extrusion materials are up to 90μm and 20μm, respectively. Through adding lubricant and decreasing shear rate, the appearance of those defections can be avoided.