【作者】 曹岩；

【导师】 王清文； 王伟宏；

【作者基本信息】 东北林业大学 ， 生物材料工程， 2013， 博士

【摘要】 木塑复合材料,简称木塑(wood-plastic composites,缩写为WPCs),为生物质-聚合物复合材料的俗称,是一种由木质纤维材料与聚合物材料复合而制成的复合材料。它是新型的高性能、高附加值环保材料,在环境保护和节约能源等方面发挥了重要的作用。然而,抗蠕变性能差严重影响和制约了WPCs的拓展应用。论文以杨木纤维增强高密度聚乙烯(HDPE)复合材料为研究对象,重点分析了纤维尺寸和分布对WPCs的力学性质和抗蠕变性能的影响。采用10～20目、20-40目、40-80目和80-120目四种木纤维及它们的混合纤维制备了七种木纤维增强HDPE复合材料,对其弯曲性能、冲击强度、流变性能、动态热机械性能、24h蠕变-24h回复性能和1000h蠕变性能等进行分析,并引入数学模型拟合WPCs的蠕变-回复过程,取得如下结果：(1)纤维的尺寸过大或者过小都不利于WPCs的弯曲强度和模量的提高,增强效果以20-40目纤维为佳,而80-120目木纤维增强HDPE复合材料的弯曲强度和弹性模量均是最小值。四种目数木纤维混合增强HDPE复合材料的抗弯性能较好,长短不同,粗细不均的纤维搭配起来增强HDPE,既能填补纤维之间的间隙,又能扩大与基质的接触面积,有助于提高材料界面的结合强度,增强力学性能。(2)引入成分对弯曲力学性能的贡献因子λ作为参数修正ROM模型,通过方差分析和配对样本T检验证明该模型预测木纤维增强HDPE复合材料的弯曲性能优于三个传统ROM模型,通过材料的断裂强度验证了该模型。(3)24h蠕变-24h回复性能和1000h蠕变测试结果表明,单一目数木纤维增强HDPE复合材料中抗蠕变性能最差的是80-120目木纤维增强HDPE复合材料,该材料蠕变实验后弯曲性能值下降最大,不适合长期在负载的条件下工作。增加纤维长度有利于蠕变后弯曲性能的保留。在较小应力水平下,40-80目木纤维增强HDPE复合材料的抗蠕变性能最好；当载荷超过材料弯曲极限载荷的30%时,20-40目木纤维增强HDPE复合材料的抗蠕变性能最好且受载荷的影响较小(4)混合目数木纤维增强HDPE复合材料的抗蠕变性能优于单一目数木纤维增强HDPE复合材料,抗蠕变性能最好的是20-80目木纤维增强HDPE复合材料,最差的是80-120目和10-20目混合木纤维增强HDPE复合材料。材料在不同载荷水平蠕变后,回复率的变化不大(回复率范围81.81%～85.58%)。(5)分别利用Findley指数模型、两参数指数模型和四元件Burgers模型来拟合WPCs的24h蠕变曲线,经过模型检验和参数检验,四元件Burgers模型拟合效果最好,可以应用于热压成型的WPCs的蠕变性能的预测中。建立四元模型来模拟WPCs的回复过程,该回复模型模拟效果较好,可以应用于热压成型WPCs的蠕变-回复性能的预测中。(6)上层40～80目、下层20～40目木纤维增强HDPE复合材料的弯曲性能值最大,其次是四种目数纤维混合均匀分布增强HDPE复合材料,而上层80～120目、下层10～20目木纤维增强HDPE复合材料的弯曲性能最差。长度相差大的纤维无论是分层分布还是均匀分布,用其增强HDPE复合材料的弯曲性能值都小于长度相近的木纤维增强HDPE复合材料,后者弹性应变最小,蠕变速度较慢,24h应变最小,蠕变后剩余弯曲性能值均最大。(7)上层40～80目、下层20～40目木纤维增强HDPE复合材料最适合长期在载荷作用下工作。而含有短纤维(80～120目)的WPCs不适合长期在载荷作用下工作,短纤维含量越多,其抗蠕变性能越差。不同目数纤维均匀混合分布的WPCs抗蠕变性能均优于纤维分层分布的WPCs,且24h回复率也大于纤维分层分布的WPCS;不同目数纤维均匀混合分布的WPCs弹性应变和24h应变受加载力增加的影响较纤维分层分布的WPCs更敏感。(8)分别利用Findley指数模型、两参数指数模型和四元件Burgers模型来拟合叠层材料的24h蠕变曲线,并求出参数,经过模型检验和参数检验,四元件Burgers模型拟合效果最好,可以应用于叠层WPCs的蠕变性能的预测中。建立四元模型来模拟叠层WPCs的回复过程,该回复模型模拟效果较好,可以应用于叠层WPCs的蠕变回复性能的预测中。(9)对WPCs的安全系数进行考察,20～80目木纤维混合增强HDPE复合材料在弯曲性能和抗蠕变性能方面表现出最安全的使用性。更多还原

【Abstract】 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.更多还原