本文選題：聚乙烯 + 共混�。� 參考：《太原理工大學》2017年碩士論文

【摘要】：注塑成型是聚乙烯最主要的加工方式,在成型過程中控制聚乙烯的微觀結構演變能夠直接影響制品的性能。注塑成型的聚乙烯制品通常具有力學性能較差、結晶度較低等缺陷。增加成核密度能夠提高結晶度并使晶形發(fā)生改變,從而改善制品性能。本文依據(jù)誘導結晶的試驗現(xiàn)象,通過不同分子量誘導結晶和剪切應力誘導結晶兩種方式,對注塑成型過程中聚乙烯的結晶機理進行探究,主要研究內容包括:(一)采用直接共混法將不同種類(分子量不同)的聚乙烯用雙螺桿擠出機擠出,制備共混物。考察了不同比例、不同種類聚乙烯混合料對制品的影響。表征了共混物的熔體流動性、分子量分布情況、制品力學性能和結晶度等性能。通過對共混物流動性和分子量變化的分析,探討了分子量的改變以及長鏈和短鏈在誘導結晶中的作用。結果表明,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)能大幅提高制品的結晶度。同時向5000S中添加4%2911和4%1158不僅能大幅提高制品的結晶度,而且制品的拉伸性能有了巨大的提升,伸長率達到846%。(二)采用控制剪切應力的方式對中等分子量聚乙烯進行試驗,通過改變注塑機的注射壓力和模具溫度來調整聚乙烯熔融體受到的剪切應力。利用電子萬能試驗機、差示量熱掃描儀和掃描電鏡對試樣進行表征,對剪切應力與晶形的關系進行了分析。通過對聚乙烯結晶過程的分析探討,闡述了受剪切作用的剪切層和芯層取向及結晶狀況。結果表明,當熔融體受到持續(xù)的剪切時,制品的剪切層會持續(xù)變厚,力學性能也會隨之提升。剪切速率能夠直接對晶形的結構、形狀和大小產(chǎn)生影響。當剪切速率大于一定值時,分子鏈開始發(fā)生明顯的取向。(三)運用分子模擬軟件,利用分子鏈模型和粗粒化模型對聚合物分別受到剪切和拉伸作用的情況進行模擬,研究了聚合物分子鏈在剪切應力場中的取向行為以及在拉伸應力作用下微觀結構的變形情況。在力學測試中,采用兩種不同的系綜來考察拉伸速率與應力應變曲線之間的關聯(lián)。根據(jù)非鍵勢能和總能量的變化情況來分析外部應力對晶形的影響。結果表明,拉伸時采用NVT系綜能夠有效避免空穴現(xiàn)象的產(chǎn)生。在剪切應力或拉伸應力的作用下,分子鏈結構均會發(fā)生明顯的取向行為。
[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.
【學位授予單位】：太原理工大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：TQ325.12

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本文編號：1766063