The effect of a miscible and an immiscible polymeric modifier on the mechanical and rheological properties of PVC

PVC/copoly(ester-urethane) (Baymod PU®) and PVC/EVA copolymer blends have been analysed from the point of view of the effect of miscibility on the mechanical and rheological properties. The FTIR and NMR analysis of Baymod PU modifier leads to the conclusion that its miscibility with PVC (determined by dynamic mechanical analysis) is due to the presence of 31.9% poly(ε-caprolactone) and 38.4% adipates in the main chain of the copoly(ester-urethane) polymer. However the PVC/EVA system considered in this work results in an immiscible blend, due to the low content of vinyl acetate, 33 wt%. The comparison of the mechanical and rheological properties of both blends with those of a PVC plasticised with a low molecular weight common plasticiser (PVC/DOP system), reveals that compatibility is not a determinant point in what some ultimate properties are concerned. The molecular weight (related to the viscosity of pure polymers, which is similar for both polymeric modifiers) seems to play a more important role than the miscibility. The stress–strain curves of both PVC/copoly(ester-urethane) (PU) miscible blends and PVC/EVA immiscible blends resemble each other and are very different with respect to PVC/DOP curves. The PVCs plasticised with DOP show a considerably higher elongation at break and a lower Young’s modulus than PVC mixed with polymeric modifiers. Miscible systems, PVC/PU and PVC/DOP, show similar values of the stress at break, slightly higher than those of immiscible PVC/EVA blends. The results of the latter two-phase system are well fitted to a two-parameter equivalent box model developed by Kolarik, considering a value very close to zero for the parameter, which accounts for the adhesion in between the phases. The reduction of the viscosity, induced by PU and EVA polymeric modifiers, with respect to that of pure PVC, is remarkable at shear rates such as those involved in calendering and milling, but is less significant at shear rates above 1000 s−1 which correspond to injection moulding processes. A better performance is obtained with DOP plasticiser at such high shear rates. For immiscible PVC/EVA blends the variation of the viscosity with composition has been adjusted using an adaptation of the Kolarik model, being this a novel approach to relate the viscous and mechanical behaviour of immiscible but in some way compatible blends . As expected miscible PVC/PU blends viscosity data follow a free volume additivity model.