PROCESSING VARIABLES AND THEIR EFFECT ON THE FINAL PROPERTIES OF POLYETHYLENE CABLE JACKETS,
Abstract
Processing conditions can exert considerable influence on the final properties of a polyethylene after fabrication into a cable jacket. The loss of stress crack resistance occurs primarily as a result of the combination of reduction in molecular weight through shear, along with the retention of high levels of stress as a result of quick quenching. Factors which lead to degradation of molecular weight include: (1) low extrusion temperatures, (2) shallow screws (high total shear) and (3) high barrel back pressures. It is impossible to completely eliminate degradation of molecular weight in the extrusion process, but by utilizing long barreled extruders, screws of recommended metering depths and extrusion temperatures in the range of 450 F, degradation can be minimized. The effect of a decrease in molecular weight on stress crack resistance depends highly upon the particular polyethylene in question. The other factor which leads to stress crack failures in cable jackets is the level of retained orientation from the extrusion process. Molecular orientation occurs in the process as a result of the application of shear and drawdown. In order for the polyethylene melt to come to a relaxed state, the jacket must be allowed to cool rather slowly. It is well-known that re-entanglement of the polymer molecules occurs at a time/temperature dependent rate. If the temperature of the melt is suddenly reduced, re-entanglement will stop, thereby freezing in orientation. In order to prevent this situation from occurring, slower cooling of the jacket becomes necessary. (Author)
Document Details
- Document Type
- Technical Report
- Publication Date
- Dec 01, 1965
- Accession Number
- AD0479562
Entities
People
- David C. Sanders
- Robert L. Seeks