Anisotropy in injection‐molded ethylene‐propylene‐diene rubbers. Part II: Disks delaminated by a water‐jet cutting technique

Abstract

Anisotropy and molecular orientation are well known phenomena in the field of thermoplastics, but only a few studies have described anisotropy in rubber materials. It has been shown that injection molding gives rise to a higher degree of anisotropy than compression molding. The anisotropy in the rubber material was strengthened by carbon black and is presumably due to molecular orientation. This paper describes the anisotropy of injection‐molded ethylene‐propylene‐diene rubbers. The two polymers had different molecular weight distributions and thus different rheological properties. The compounds were injected into center‐gated 4mm thick disks. The disks were subsequently split into three layers using a water‐jet cutting technique. Measurement of mechanical and swelling properties in the different layers and directions showed that the anisotropy varied through the thickness of the disk. By X‐ray scattering it was shown that rubber molecules had a preferred direction and thus, that the anisotropy was probably predominantly due to molecular orientation created during the mold filling.

Anisotropic swelling of rubber: extension of the Flory theory

“…Therefore, for a generic rubber with anisotropic swelling with principal swelling ratios x , y and z , the average molecular weight between cross-links is given for the first time as (12) x…”

“…This leads to rubber products where there is a residual alignment of the chains in one or more preferential directions, depending on the manufacturing process. During sheet rolling processes and injection moulding, the rubber is subjected to a complex set of deformation in the three directions simultaneously, imparting generic anisotropy to the network structure [11,12]. Compression-moulded rubber sheets are somewhat simpler, manifesting in-plane isotropy [13,14] Thus, anisotropy appears to be an intrinsic part of rubber processing despite the lack of a suitable theory to determine cross-link density for anisotropic systems.…”