Improving Cold Temperature Impact Performance with PVC Jackets

摘要

Cables designed for indoor/outdoor applications often use poly vinyl chloride (PVC) compounds as the cable jacket material for the multiple benefits these compounds provide. Specifically, PVCs typically provide resistance to flame spread, robust mechanical properties required for installation and long-term reliability, and ease of use in jacket processing. Along with these multiple benefits, there are also trade-offs associated with the use of PVC compounds. One notable disadvantage is that many PVCs have brittle fracture points at temperatures higher than those of other materials commonly used for jacketing applications. Improper material selection and processing can place these cables at risk of jacket damage when installed at temperatures near the lower end of the installation temperature range of industry standards. In conjunction with the release of a new industry standard for indoor/outdoor optical fiber cables, Insulated Cable Engineers Association (ICEA) S-104-696-2001, the issue of PVC compound performance and testing with respect to cold temperature impact was revisited. This paper will focus on the evaluation of indoor-outdoor cable designs using different PVC jacketing compounds to the cold temperature impact test requirements contained in ICEA 696. An internal rating method was created to help screen jacket compound candidates, especially to meet the ICEA 696 requirement of no visible cracks on either the inner or outer surface of the jacket [1]. ICEA 696 indicates that the presence of an interior or exterior crack after impact at cold temperature is a failure. This requirement provides a benchmark for cold temperature impact performance once a cable design is finalized. During the material selection and cable development process, more detailed information is required concerning crack severity to quantify the effect of a specific variable on cold impact performance. To better understand trends of cold impact performance for cables jacketed with PVCs, a crack rating scale was developed to track crack severity. This scale is based on average crack ratings which are obtained by evaluating nine double impact locations. The cold impact performance of a cable is related in part to jacket material properties. Using average crack ratings, a relationship was developed for PVCs between storage modulus and cold impact performance using Dynamic Mechanical Analysis (DMA). This relationship cannot be used to predict the temperature at which cold impact failures will occur because of the many variables, including wall thickness and inherent jacket stresses, which can affect performance. However, storage modulus curves can be used as a tool for comparing PVC jacket materials for their relative resistance to cold impact failures. Figure 1 shows the storage modulus curves for several PVC compounds.