Development Of Thermal Insulation Material From Rice Straw Using High-frequency Heating

China is a agricultural country, abundant wheat and rice straw with low priceare produced every year. To expand new avenues for effective utilization of theagricultural straws, rice straw insulation material was developed usinghigh-frequency hot pressing. The rule of temperature distribution with thickness byhigh-frequency hot pressing was investigated. The effect of manufacturing process、material structure and ambient temperature on properties of rice straw insulationmaterial was analyzed. The effective thermal conductivity of insulation material wascalculated based on fractal theory. The results can provide theoretical basis andpractical experience for development of rice straw insulation material.The results are shown as following:1. The temperature-time curve can be divided into three stages: a rapidlytemperature rising stage, a water evaporating stage and a slowly temperatureincreasing stage. The internal temperature changes crossing the thickness section.Normally, the temperature of the central layer is higher than that of the surface layer.Comparing with the conventional hot pressing, the pressing time can be greatlyshorten by high-frequency hot pressing. Meanwhile, the internal temperature is moreuniform in the mat thickness direction. In the rapidly temperature rising stage, theinternal temperature is increased dramatically with raising the moisture content. Inthe moisture vaporizing stage, the time can be shorten by decreasing the moisturecontent. The moisture content has no influence on the slowly temperature increasingstage. The lower density of boards promotes the increase of the temperature and thedecreased of the time of water evaporation. The influence of the board thickness onthe internal temperature is similar to the board density.2.With the pressing time of240s, the moisture content of14%, the resin contentof8%, the mechanical properties of rice straw insulation materials can meet thestandard of EN and the thermal conductivity values range from0.051~0.053W/(m·K).The rice straw insulation materials can effectively replace the conventionalinsulation materials in the market, with low price and density、non-toxic、harmless、no pollution and other unique advantages, which is extremely interesting for energysaving and environment-friendly as the building insulation materials.3.Comparing with the conventional hot pressing, the IB and TS of boards inhigh-frequency hot pressing is higher, the MOR、MOE and WA of boards are lower.In addition, different manufacturing ways have no significant effect on the thermalconductivity of insulation boards.Select three types of adhesives(MDI、UF、PF）tomanufacture rice straw insulation material. The boards glued by MDI has higher physical and mechanical properties than the boards glued by PF and UF. However,the boards glued by different adhesives have no significant effect on the thermalconductivity of insulation boards.4.The effective thermal conductivity model of insulation material was calculatedbased on fractal theory：The result shows that the calculated value from this method is inconformity with theactual data, which proves the fractal model of effective thermal conductivity fairlyreasonable. Meanwhile, with increasing fractal dimension, the thermal conductivityof insulation materials decreased,the conclusions of validation experiments wasconsistent with the analysis of the model, which proves the credibility of the model.5.As the board density increasing, the physical and mechanical properties ofboards were reduced. Decreasing particle size in a certain range can improve theproperties of boards. As the board thickness increasing, the physical and mechanicalproperties were improved.6.The thermal conductivity of rice straw insulation material was increased withincreasing temperature. An increase of ambient temperature from10℃to50℃improved the thermal conductivity by about20%. The higher thetemperature,the trend of the thermal conductivity increased is more obviously.