tailieunhanh - Rubber Compounding - Chemistry and Applications Part 12

Tham khảo tài liệu 'rubber compounding - chemistry and applications part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 11 Vulcanization Frederick Ignatz-Hoover and Brendan H. To Flexsys America LP Akron Ohio . I. INTRODUCTION-TERMINOLOGY The following is a short list of terminology commonly used within rubber industry discussions of vulcanization of general-purpose elastomers. Where indicated reference is made to specific test methodologies. Vulcanization is the process of treating an elastomer with a chemical to decrease its plasticity tackiness and sensitivity to heat and cold and to give it useful properties such as elasticity strength and stability. Ultimately this process chemically converts thermoplastic elastomers into three-dimensional elastic networks. This process converts a viscous entanglement of long-chain molecules into a threedimensional elastic network by chemically joining cross-linking these molecules at various points along the chain. The process of vulcanization is depicted graphically in Figure 1. In this diagram the polymer chains are represented by the lines and the cross-links by the black circles. Scorch refers to the initial formation of an extensive three-dimensional network rendering the compound elastic. The compound is thus no longer plastic or deformable and cannot be shaped or further processed. Scorch safety is the length of time for which the compound Although based on ASTM D-1566-80b these definitions have been modified to fit this discussion. Copyright 2004 by Taylor Francis Vulcanized Rubber Figure 1 In vulcanization the randomly oriented chains of raw rubber become cross-linked as indicated diagrammatically at the right. can be maintained at an elevated temperature and still remain plastic. This time marks the point at which the plastic material begins the chemical conversion to the elastic network. Thus if the compound scorches before it is formed into the desirable shape or composite structure it can no longer be used. Time to scorch is thus important because it indicates the amount of time heat history the compound may be exposed to .