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贝尔系统使用的电话电缆是以聚乙烯作护套的。对于不同工艺条件对这种护套环境应力开裂的影响作了调查研究。通过一台2吋半,20∶1长径比的实验室挤出机对三种聚乙烯均聚物进行加工。在各种不同的挤出温度和二种出料率(13磅/小时和120磅/小时)下,将每种材料制成0.125吋厚的扁平带。由这些带切成的样品,用弯曲条试验进行耐环境应力开裂的试验。发现低温、高出料率和浅计量段在降低每一种材料的耐环境应力开裂品质方面是很起作用的。材料耐应力开裂的排列次序是同用低长径比挤出机商业性生产聚乙烯电缆护套时得到的结果相同的。每种材料的应力开裂降解与由于加工而引起的平均分子量变化有关。这由熔融指数来计测,同时大概是由于机械作用而引起的。在涉及的试验条件范围内,计量段剪切应力水平代表这种作用的一个程度。
The Bell telephone cable is sheathed with polyethylene. The influence of different process conditions on the stress cracking of the sheath environment was investigated. Three polyethylene homopolymers were processed through a 2 inch, 20: 1 aspect ratio laboratory extruder. Each material was made into a 0.125 inch thick flat strip at a variety of extrusion temperatures and two discharge rates (13 lb / hr and 120 lb / hr). Samples cut from these tapes were tested for resistance to environmental stress cracking using a flexural strip test. It has been found that low temperatures, high reject rates and shallow metering sections play a role in reducing the environmental stress cracking resistance of each material. The material resistance to stress cracking is in the same order of results obtained with the commercially available polyethylene cable jacket using a low aspect ratio extruder. Stress Crack Degradation in each material is related to the change in average molecular weight due to processing. This is measured by the melt index, probably due to mechanical action. Within the scope of the experimental conditions involved, the level of shear stress in the metrological section represents a degree of this effect.