ANISOTROPY IN INJECTION-MOLDED STYRENE-BUTADIENE RUBBERS .2. DISKS DELAMIN ATED BY WATER-JET CUTTING TECHNIQUE

Anisotropy in the physical properties of moulded rubber materials was studied by some research groups during the 1970s and 1980s. It has bee n found that injection moulding gives a higher anisotropy than does co mpression moulding, and it has been suggested that the anisotropy is a n effect of molecular orientation rather than of differences in the st ate-of-cure. Further, carbon black has been shown to strengthen the an isotropy. This study is devoted to the anisotropy in mechanical and sw elling properties of injection-moulded centre-gated 4 mm thick discs. The materials used were carbon black-filled and unfilled compounds of two styrene-butadiene rubbers with different molecular weight distribu tions, and hence different rheological properties. Dumb-bell samples w ere taken radially and tangentially with respect to the centre of the disc, and then delaminated by an abrasive water-jet technique into thr ee layers. Methods used to characterise these samples were: stress-str ain measurements, volume and dimensional swelling, and thermogravimetr ic analysis measurements. The anisotropy measured in the discs was pro bably due to molecular orientation, a relic from the mould-filling pro cess, reflecting the flow pattern in the mould. The anisotropy in Youn g's modulus was most prominent in the core, i.e. the central layer of the disc. The modulus was higher in the outermost layers than in the c ore.