A collaborative study of the structure and rheological properties of EVOH/SMA blends produced by reactive extrusion (technical report)

Blends of ethylene-vinyl alcohol copolymer (EVOH) and styrene-maleic anhydride copolymer (SMA) have been produced by reactive extrusion and characterised in terms of their rheology, phase morphology and the degree of covalent bonding interaction tester linkages) between phases, as determined by FTIR. Strongly synergistic behaviour in terms of viscosity and melt elasticity was seen in all blends, for low frequency dynamic and low shear rate capillary tests, the largest increases occurring in blends containing 50-60% EVOH. Finely dispersed droplet phase morphologies were seen in most of the blends, the minor phase forming the dispersed phase. However the blends containing 50-60% EVOH were co-continuous or partly co-continuous; it was also notable that the blends containing 60% EVOH exhibited a surprisingly low value of Melt Index. Evidence of covalent bonding was apparent in all blends, the highest interaction levels again being seen for the 50-60% EVOH blends, despite a very low anhydride/alcohol molar ratio (only 0.05). Increased mixer speed and vacuum venting during processing also improved reaction levels. These findings suggest that the availability of reactive groups is restricted by the phase morphology of the blend during production. As a result, more reaction occurs at or near co-continuous phase compositions, where the surface area is maximised. The synergistic behaviour in viscosity and elasticity can be seen as consistent with the observed increased bonding levels, which causes more efficient stress transfer into the dispersed phase.