Flame retardant and mechanical properties of wood-plastic composites with multi-layer sandwich structures
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摘要: 针对传统膨胀石墨阻燃木塑复合材料(WPCs)阻燃剂添加量高、力学性能变差的问题,本文以杨木木粉(WF)、高密度聚乙烯(HDPE)、膨胀石墨(EG)与纳米二氧化硅(n-SiO2)为主要原料,通过层积热压工艺和结构优化设计制备了具有多层夹芯结构的阻燃增强木塑复合材料。采用锥形量热仪、垂直燃烧测试仪、极限氧指数(LOI)仪和万能力学试验机分别探究了单层、双层和三层夹芯结构对木塑复合材料阻燃和力学性能的影响。结果表明:相比于对照组(WPC-0),阻燃层中EG和增强层中n-SiO2的含量分别为10%和5%时,双层和三层夹芯结构木塑复合材料的热释放速率和总热释放、烟释放速率和总烟释放均有显著降低,残余物质量明显提升。其中三层夹芯结构木塑复合材料(WPC-E3B)的LOI值由20.8%提高至30.6%,UL-94达到V-0级。此外,相较于WPC-0,WPC-E3B的冲击强度提升了61.9%,拉伸与弯曲强度分别提高了16.2%和13.4%。Abstract: In order to solve the problems of high flame-retardant addition and deterioration of mechanical properties in the modification of wood-plastic composites (WPCs) by traditional expandable graphite, flame-retardant reinforced wood-plastic composites with a multilayered sandwich structure were prepared by laminated hot pressing process and structure optimization design using poplar wood flour (WF), high-density polyethylene (HDPE), expandable graphite (EG) and nano-silicon dioxide (n-SiO2) as the main raw materials. And the appropriate characterization and equipment such as cone calorimeter, vertical burner, limiting oxygen index (LOI) tester and mechanical testing machine were used to investigate the effects of single layer, double layer and triple layer sandwich structures on the flame retardant and mechanical properties of wood-plastic composites, respectively. The experimental results show that compared with the control wood-plastic composite (WPC-0), the multilayer structure wood-plastic composites exhibite significant reduction of heat release rate, total heat release, smoke production rate and total smoke production, and remarkable improvement of residue yield in the cone test when the contents of EG in the flame-retardant layer and n-SiO2 in the reinforcement layer are 10% and 5%, respectively. Among all the multilayer wood-plastic composites, WPC-E3B with a triple layer sandwich structure improves its LOI from 20.8% to 30.6% and passes the UL-94 test with a V-0 rating. Moreover, it also shows better mechanical properties compared with WPC-0, such as a 61.9% increase in impact strength and 16.2% and 13.4% increases in tensile and flexural strength, respectively.
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表 1 单层木塑复合材料(WPCs)各组分配方
Table 1. Formulation for the preparation of single-layer wood-plastic composites (WPCs)
Sample HDPE/wt% WF/wt% EG/wt% n-SiO2/wt% MAPE/wt% 530D/wt% Role in multilayer WPCs #0 45 50 0 0 3 2 — #1 45 35 10 5 3 2 — #2 45 40 10 0 3 2 — #3 45 45 0 5 3 2 Core layer #4 45 30 20 0 3 2 Surface layer #5 45 10 40 0 3 2 Surface layer Notes: HDPE—High-density polyethylene; WF—Wood flour; EG—Expandable graphite; n-SiO2—Nano-silicon dioxide; MAPE—Maleic anhydride grafted polyethylene; 530D—Lubricant type. 表 2 多层夹芯木塑复合材料的组成
Table 2. Composition of multilayer sandwich WPCs
Sample Core-layered Surface-layered WPC-E2 #3 (2 mm)*1 #4 (2 mm)*1 WPC-E3A #3 (2 mm)*1 #4 (1 mm)*2 WPC-E3B #3 (3 mm)*1 #5 (0.5 mm)*2 Note: The type, thickness and quantity of boards required for multilayer wood-plastic composites are shown in the table. For example, #5 (0.5 mm)*2 represents two samples of #5 with a thickness of 0.5 mm. 表 3 不同结构的木塑复合材料在锥形量热仪(CONE)试验所得燃烧数据
Table 3. Combustion data of wood-plastic composites with different structures in the cone calorimeter (CONE)
Sample TTI/s PHRR1/(kW·m−2) PHRR2/(kW·m−2) THR/(MJ·m−2) TSP/m2 MR/% WPC-0 15 510.3 553.8 129.8 11.3 15.2 WPC-Si 16 419.6 415.3 119.1 11.9 23.5 WPC-E1 25 361.2 488.1 107.1 11.0 35.6 WPC-E2 26 161.1 213.0 77.7 6.5 53.0 WPC-E3A 26 154.1 485.3 92.0 7.4 44.7 WPC-E3B 27 74.8 338.7 78.1 7.1 52.5 Notes: TTI—Time to ignition; PHRR1—Peak first heat release rate; PHRR2—Peak second heat release rate; THR—Total heat release; TSP—Total smoke production; MR—Mass of residue. 表 4 不同结构的WPCs的极限氧指数(LOI)与垂直燃烧等级
Table 4. Limiting oxygen index (LOI) and UL-94 rating of WPCs with different structures
Sample Vertical flame test LOI/% WPC-0 N-R 20.8 WPC-E1 N-R 25.7 WPC-E2 V-1 27.3 WPC-E3A V-0 29.1 WPC-E3B V-0 30.6 -
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