摘要
以聚乳酸和玉米秸秆纤维为原料制备食品包装材料既能解决不可降解材料给环境带来的污染问题又能缓解石化资源短缺问题,同时还能提高农业副产品的利用率,遵循可持续发展的原则。聚乳酸/玉米秸秆纤维食品包装材料的开发具有重大的应用价值和现实意义。
通过熔融混炼,模压发泡的方法制备聚乳酸发泡材料。首先引入马来酸酐、1,2,4-苯三酸酐、均苯四甲酸酐、甲基丙烯酸缩水甘油酯,分析熔体黏度对聚乳酸发泡材料表观密度和膨胀率的影响,从而优选出最佳增塑剂——GMA。其次,探索了GMA的加入对聚乳酸分子结构和界面形态的影响,发现GMA与PLA发生了反应。再者,考察了不同的熔体黏度对聚乳酸发泡材料表观密度、膨胀率、孔隙率、力学性能和热性能的影响,结果显示聚乳酸发泡需要合适的熔体黏度,熔体黏度过高,气泡膨胀生长所需克服的阻力较大,气泡膨胀生长较为困难；熔体黏度过低,聚合物熔体难以包裹住支撑膨胀的气泡而导致气泡破裂、崩塌、.合并,当GMA添加量为4%,熔体黏度为233Pa.S时材料的密度及其性能最好。
研究了纤维的加入对聚乳酸/玉米秸秆纤维复合发泡材料分子结构和界面形态的影响；优化了热压工艺参数；考察了不同玉米秸秆纤维含量对PLA/玉米秸秆纤维发泡材料的表观密度、膨胀率、力学性能、热性能和吸水性能的影响。结果表明纤维与聚乳酸之间有一定的界面相容性；热压发泡的最佳工艺参数为：热压压力15MPa,热压温度200℃,保压时间5min；当纤维添加量为15%时,所得发泡材料表观密度最小为0.568g/cm3,膨胀率最大为61%,拉伸强度最大为8.031MPa,纤维的加入降低了PLA/玉米秸秆纤维复合发泡材料的热降解温度。
根据经典发泡理论,选用细胞模型为物理模型,利用质量守恒定律、动量守恒定律、能量守恒定律、Dewitt本构方程和理想气体状态方程等,探讨了气泡膨胀生长中的数学模型,并对数学模型进行实验验证。结果显示,该模型可以应用在聚乳酸发泡体系中,但是当体系较为复杂时,仍需进一步改进实验方法和模型方程。
The developing of food packaging material based on polylactic acid (PLA)/corn straw fiber not only solve the environment pollution caused by undegradable material but also remission energy shortage, otherwise it also improve the use ratio of agriculture by-product, follow the principle of sustainable development.
PLA foams were prepared through melt mixing and hot pressing. Firstly MAH, TMA, PMDA and MAH were introduced as plasticizer and the effect of melt viscosity with different plasticizer on apparent density and expansion ratio of foams were measured as well, among these four plasticizers GMA was the optimal one. And then, the effect of GMA on the molecular and interfacial structures were discussed, it was revealed that there was a react between PLA and GMA. Besides, the apparent density, expansion ratio, void fraction, mechanical properties and thermal properties with different GMA content were investigated, it was showed that the foam of PLA need a suitable melt viscosity, High melt viscosity prevents the gas diffusion and the cell expansion while low viscosity accelerates gas loss and cell collapse phenomenon, the density and other properties reached maximum values while GMA mass was 4% and the melt viscosity was 233Pa.S.
The effect of fiber introduction on molecular and interface structures of PLA/corn fiber composite foams were studied firstly. Then, the parameters of hot-pressing process were optimized. Moreover, the effect of corn fiber concentration on apparent density, expansion ratio, mechanical thermal and moisture properties of foams were investigated. It turned out that, to some extent compatibility between PLA and corn fiber was observed; the most optimal parameters were hot-pressing pressure 15MPa, temperature 200℃, keeping-time 5min and the most ideal materials were obtained when fiber content was 15%.
A mathematical model for the bubble growth of PLA foams was established on the basis of classic foaming theory, cell-model, law of conservation of mass and conservation of momentum, ideal gas equation and so on. After compared with experiment data we got the conclusion that this model was suitable for PLA foam system, but when the foams were complicated it still need further calculation and simplification.
引文
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