Additives for Improved Processing of Microcellular Shoe Soles and Integral Skin Systems *

Abstract

The manufacturers of polyurethane microcellular articles require better processing aids to meet the escalating pressures of environmental compliance, cost efficiency and complicated part design which have been imposed on them. Additives such as catalyst and surfactants play a key role in the production of microcellular systems. Intrinsic characteristics such as flowability and demouldability are affected by the type and amount of additives used during the production cycle. Surfactants and catalysts play a significant role in controlling foam cellular structure, reactivity, and physical properties and can also influence production scrap rates.

Environmental pressures have compelled producers of microcellular articles to eliminate or reduce the use of CFC's as blowing agents. In the case of microcellular/integral skin moulded articles, the replacement of CFC's has been difficult. The transition to soft halocarbons has created processing problems that the industry is striving to overcome. The obvious approach of replacing CFC's with water is not without its challenges. Water brings a host of processing and performance problems to the manufacturers of microcellular parts. The major difficulty to overcome is the inability to form a thick, tough, aesthetically attractive skin when water is used as the sole blowing agent. Cell structure, which impacts key physical properties such as tensile, tear and elongation, can also suffer.

Densities in polyurethane microcellular systems are also trending downward to provide lower cost and increased comfort. One of the technical challenges that the industry currently faces with the reduction of density is maintaining dimensional stability and part integrity. Additionally, articles produced with all water systems and at lower densities have a tendency to pinhole and void while possibly exhibiting non-uniformity in colour and grain definition.

This paper discusses and demonstrates the utilization of existing catalyst and surfactant technologies and their impact on the microcellular polyurethane producer to meet the current processing demands of today. Furthermore, Air Products and Chemicals, Inc. is developing new additive technologies that will provide reduced demould cycles, enhanced dimensional stability and improved skin definition to produce tough, aesthetically pleasing microcellular/integral skin parts for all water blown systems while maintaining or improving key physical property characteristics.