| Wood plastic composites is composed mainly of wood or wood cellulose as base material with one or more plastics. The abbreviation for wood plastic composites is WPCs. WPCs is a kind of the high performance, and high value-added environmental protection material, and WPCs plays an important role in protecting the environment and saving energy. However, poor creep properties seriously affected and restricted the development of the application of WPCs. This paper takes composites reinforced high density polyethylene (HDPE) with poplar wood fiber as a research object, analyses the influence of fiber size and distribution on mechanical properties and creep resistance of WPCs.Seven kinds of different mesh size fiber reinforced HDPE composite was prepared. The seven kinds of reinforced fibers of different size includes four kinds of single mesh size fibers (80-120mesh size,40-80mesh size,20-40mesh size and10-20mesh size) and three mixed fibers of single mesh size fibers. The flexural properties, impact resistance properties, rheological properties, dynamic mechanical properties,24hours creep-24hours recovery and1000hours creep properties of the resulted WPCs was studied. The results were described as bellow.(1) Excessively large or small fiber proved unconducive to the enhancement of the strength and modulus of composites. The strengthening effect of fibers of20-40mesh size was best. The flexural strength and the flexural modulus of WPCs with fiber of80-120mesh size were minimum values. The flexural strength and the flexural modulus of resulted WPCs with fibers of four mesh size were maximum. Reasonable collocation of fiber with different lengths, uneven thickness reinforced HDPE composites can fill the gap or space between fibers, and can extend the area of effective contact between fibers and matrix. It is helpful to improve the bonding strength of the material interface, enhanced mechanical properties.(2) The modified ROM model by introducing an parameter as contribution factor of component WPCs to flexural properties can be used to predict the flexural mechanical properties of mixed fiber of different mesh size reinforced HDPE composites. The results of variance analysis and paired samples T test proved that the modified ROM model is better than the ROM model, IROM model and Hirsch model. The model is verified by the fracture strength of the resulted WPCs.(3) The results of24hours creep-24hours recovery tests and1000hours creep tests show that for the single mesh fiber reinforced HDPE composites, the creep performance of the WPCs with80-120mesh size is the worst. Residual flexural properties of WPCs with fiber of80-120mesh size decreased maximum after the creep experiment. Therefore it was not suitable for long-term work in the load conditions. The increase of the fiber length had a positive impact on the creep properties of WPCs. At a smaller load, creep resistance properties of WPCs reinforced by fiber of40-80mesh size was best. While at a higher load (higher than30%of maximum flexural loads of WPCs), creep resistance properties of WPCs reinforced by fiber of20-40mesh size was best, and effect of loading on its creep resistance properties was smaller.(4) Creep resistance properties of WPCs reinforced by fiber of mixed mesh size were higher than that of WPCs reinforced by fiber of single mesh size. For the WPCs with fiber of mixed mesh size, the creep resistance properties of WPCs reinforced by mixed fiber of20-40mesh size and40-80mesh size were best, while the creep resistance properties of WPCs reinforced by mixed fiber of80-120mesh size and10-20mesh size were worst. After creep experiments at different load levels, recovery rates of WPCs were no change (81.81%-85.58%).(5) Findley’s power law model, a simpler two-parameter power law model and Burgers model were used to describe the24hour creep curve of seven WPCs. Through model testing, parameter testing, the SSE value of four elements Burgers model was successfully simulated with the creep resistance properties of WPCs prepared by hot pressing. The four element model was established to stimulate the recovery process of WPCs, and it can be applied to prediction of the recovery performance of WPCs.(6) The flexural properties of WPCs whose upper layer was reinforced fiber of40-80mesh size and the lower layer was20-40mesh size, were greatest. While the flexural properties of WPCs whose upper layer was reinforced fiber of80-120mesh size and the lower layer was10-20mesh size, were worst. For composites, the flexural properties of fiber of long-span in length, whether different fibers are hierarchically distributed or uniformly distributed, reinforced HDPE composites were lower than that of fiber of continuous length reinforced HDPE composites. The elastic strain, creep speed, and creep strain24hours of the WPCs reinforced by medium-length fiber distributed hierarchically were least, and after creep experiment, its residual flexural properties were greatest.(7) The WPCs whose upper layer was reinforced fiber of40-80mesh size and the lower layer was20-40mesh size, were most suitable for long-term load to work. While the WPCs with shorter fiber of80-120mesh size were not suitable for long-term load to work. The more content of short fiber in WPCs is, the worse creep performance of WPCs is. The creep resistance of the WPCs reinforced by mixed uniformly fiber of different mesh size better than that of the WPCs reinforced by different mesh size fiber of hierarchical distribution. The24hour recovery rate of the former was higher than that of the latter. The effect of flexural load on the elastic strain and24hours strain of WPCs with mixed uniformly fiber of different mesh size was more significant than that of WPCs with different mesh size fiber of hierarchically distribution. The effect of the increase of flexural load on the instantaneous recovery rate of WPCs with different mesh size fiber of hierarchically distribution was more significant than that of WPCs with mixed uniformly fiber of different mesh size.(8) Findley’s power law model, a simpler two-parameter power law model and Burgers model were used to describe the24hour creep curve of six resulted double layer WPCs. Through model testing and parameter testing, the SSE value of four elements Burgers model was successfully simulated with the creep resistance properties of resulted double layer WPCs prepared by slab paving molding method. The four element model was established to stimulate the recovery process of double layer WPCs, and it can be applied to prediction of the recovery performance of WPCs.(9) The safety coefficient of WPCs were investigatedm. Mixed fibers of40-80mesh size and20-40mesh size reinforced HDPE composite showed the most serviceability safety. |
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