ANISOTROPY IN INJECTION-MOLDED ETHYLENE-PROPYLENE-DIENE RUBBERS .2. DISKS DELAMINATED BY A WATER-JET CUTTING TECHNIQUE

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