注塑成型中聚乙烯诱导结晶机理的研究

发布时间：2018-04-18 00:40

本文选题：聚乙烯 + 共混　；参考：《太原理工大学》2017年硕士论文

【摘要】：注塑成型是聚乙烯最主要的加工方式,在成型过程中控制聚乙烯的微观结构演变能够直接影响制品的性能。注塑成型的聚乙烯制品通常具有力学性能较差、结晶度较低等缺陷。增加成核密度能够提高结晶度并使晶形发生改变,从而改善制品性能。本文依据诱导结晶的试验现象,通过不同分子量诱导结晶和剪切应力诱导结晶两种方式,对注塑成型过程中聚乙烯的结晶机理进行探究,主要研究内容包括:(一)采用直接共混法将不同种类(分子量不同)的聚乙烯用双螺杆挤出机挤出,制备共混物。考察了不同比例、不同种类聚乙烯混合料对制品的影响。表征了共混物的熔体流动性、分子量分布情况、制品力学性能和结晶度等性能。通过对共混物流动性和分子量变化的分析,探讨了分子量的改变以及长链和短链在诱导结晶中的作用。结果表明,在中等分子量聚乙烯(5000S)中添加4%的低分子量聚乙烯(2911)或者4%的高分子量聚乙烯(1158)能大幅提高制品的结晶度。同时向5000S中添加4%2911和4%1158不仅能大幅提高制品的结晶度,而且制品的拉伸性能有了巨大的提升,伸长率达到846%。(二)采用控制剪切应力的方式对中等分子量聚乙烯进行试验,通过改变注塑机的注射压力和模具温度来调整聚乙烯熔融体受到的剪切应力。利用电子万能试验机、差示量热扫描仪和扫描电镜对试样进行表征,对剪切应力与晶形的关系进行了分析。通过对聚乙烯结晶过程的分析探讨,阐述了受剪切作用的剪切层和芯层取向及结晶状况。结果表明,当熔融体受到持续的剪切时,制品的剪切层会持续变厚,力学性能也会随之提升。剪切速率能够直接对晶形的结构、形状和大小产生影响。当剪切速率大于一定值时,分子链开始发生明显的取向。(三)运用分子模拟软件,利用分子链模型和粗粒化模型对聚合物分别受到剪切和拉伸作用的情况进行模拟,研究了聚合物分子链在剪切应力场中的取向行为以及在拉伸应力作用下微观结构的变形情况。在力学测试中,采用两种不同的系综来考察拉伸速率与应力应变曲线之间的关联。根据非键势能和总能量的变化情况来分析外部应力对晶形的影响。结果表明,拉伸时采用NVT系综能够有效避免空穴现象的产生。在剪切应力或拉伸应力的作用下,分子链结构均会发生明显的取向行为。
[Abstract]:Injection molding is the main processing method of polyethylene. Controlling the microstructure evolution of polyethylene during the molding process can directly affect the properties of products.Injection molding polyethylene products usually have poor mechanical properties, low crystallinity and other defects.Increasing nucleation density can improve the crystallinity and change the crystal shape, thus improving the properties of the products.Based on the experimental phenomena of induced crystallization, the crystallization mechanism of polyethylene in injection molding process was studied by means of different molecular weight induced crystallization and shear stress induced crystallization.The main research contents are as follows: (1) different kinds of polyethylene (with different molecular weight) were extruded by twin-screw extruder by direct blending method to prepare the blends.The effects of different proportion and different kinds of polyethylene mixture on the products were investigated.The melt fluidity, molecular weight distribution, mechanical properties and crystallinity of the blends were characterized.Based on the analysis of the fluidity and molecular weight of the blends, the changes of molecular weight and the roles of long and short chains in the induction of crystallization were discussed.The results show that the crystallinity of the product can be greatly improved by adding 4% low molecular weight polyethylene (LMWPE) 2911) or high molecular weight polyethylene (HMWPE) 4% to 5000S).At the same time, the crystallinity and tensile properties of the products were greatly improved by adding 4C11 and 4C58 to 5000S, and the elongation reached 8466a.(2) the medium molecular weight polyethylene (MMWPE) was tested by controlling the shear stress, and the shear stress of the melt was adjusted by changing the injection pressure and mold temperature of the injection molding machine.The relationship between shear stress and crystal shape was analyzed by means of electronic universal testing machine, differential scanning calorimeter and scanning electron microscope.Based on the analysis of the crystallization process of polyethylene, the orientation and crystallization of shear layer and core layer subjected to shear action are described.The results show that when the melt is shearing continuously, the shear layer will continue to become thicker and the mechanical properties will be improved.The shear rate can directly affect the structure, shape and size of the crystal shape.When the shear rate is greater than a certain value, the molecular chain begins to have obvious orientation.(3) using molecular simulation software, using molecular chain model and coarse graining model to simulate the effect of shear and tensile on polymer, respectively.The orientation behavior of polymer chains in shear stress field and the deformation of microstructure under tensile stress were studied.In mechanical testing, two different ensembles are used to investigate the relationship between tensile rate and stress-strain curve.The influence of external stress on crystal shape is analyzed according to the change of non-bond potential energy and total energy.The results show that NVT ensemble can effectively avoid the hole phenomenon.Under the action of shear stress or tensile stress, the orientation behavior of the molecular chain structure is obvious.
【学位授予单位】：太原理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TQ325.12

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