Thermomechanical Properties Of Rice Straw And Hygroscopicity Of Rice Straw-Based Particleboard

Rice straw as a newly-evaluated raw material for manufacturing composites was investigated in mechanical, chemical, thermal, and machining properties in this dissertation. With modified polymeric methylene diphenyl diisocyanate (MMDI) and wax emulsion as the binder and dimensional stabilizer, respectively, rice straw-based particleboards (RSPB) were experimentally fabricated. Mechanical properties, i.e., bending modulus (MOE) and strength (MOR), internal bond (IB) strength, and in-plane tensile modulus (ITM) and strength (ITS), as well as hygroscopic performances, i.e., water absorption (WA), linear expansion (LE), and thickness swelling (TS) of RSPB were tested according to ASTM D 1037. The tested results were assessed with ANSI A 208.1 for M3 wood-based particleboard. The influences of density, resin level, and wax content on above properties of RSPB were specifically discussed. Simultaneously, parallel wood- and agro-based particleboards were manufactured to clarify the advantageous and disadvantageous aspects of RSPB in mechanic-hygroscopic properties.This dissertation was composed of five sections as: 1) General Introduction; 2) The Influences of Heating on the Properties of Rice Straw; 3) Manufacture and Physic-mechanic-hygroscopic Properties of RSPB; and 4) Conclusions. Through above investigations, it was shown that:Rice straw had high tensile strength in fiber direction, which decreased slightly with heating. The abundant mineral and waxy substances in the surface layers of rice straw, namely silicon dioxide and long-chain hydrocarbons, prevented from effective bonding between synthetic resins and rice straw. By heating, the hydrocarbons were partially dissolved and hydroxyls inside were let out for resins to set up bonds. This was further proved by DSC tests in detecting the thermal reactivity of straw with MDI and UF resins. Simultaneously, particle geometry was optimized through heating.RSPB showed comparative mechanical properties as expected. Density and resin level were demonstrated to be the two most influential factors deciding the mechanical properties of RSPB. With 3 to 5% MDI, RSPB denser than 0.75g/cm~3 had MOEs and MORs higher than the ANSI-specified minimum values. And with 5%MDI, the 0.90g/cm~3 RSPB achieved IB specification in ANSI A 208.1. RSPB showed comparable tensile performances with medium-density wood-based particleboard. Wax-sizing showed no significant impacts on the mechanical properties of RSPB.Under moist conditions, RSPB showed evident dimensional instability. The three factors, i.e.,density, resin level, and wax content, exerted evident influences on LE and TS of RSPB. Sorption saturation point (SSP) was defined as the critical moisture content (MC) for RSPB to deform with moisture adsorption or desorption. The correlation between equilibrium moisture conten; (EMC) of RSPB and environmental relative humidity (RH) followed the sigmoid sorption isotherm found for wood and wood-based composites, which could be well modeled by Nelson's mathematical equation. Comparably, RSPB had evidently larger hysteresis ratios close to 1.0.(43 Figures; 18 Tables )...