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

PVC/copoly(ester-urethane) (Baymod PU(R)) and PVC/EVA copolymer blends have been analysed from the point of view of the effect of miscibility on the m echanical and rheological properties. The FTIR and NMR analysis of Baymod P U modifier leads to the conclusion that its miscibility with PVC (determine d by dynamic mechanical analysis) is due to the presence of 31.9% poly(epsi lon-caprolactone) and 38.4% adipates in the main chain of the copoly(ester- urethane) polymer. However the PVC/EVA system considered in this work resul ts 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 plastic iser (PVC/ DOP system), reveals that compatibility is not a determinant poi nt in what some ultimate properties are concerned. The molecular weight (re lated to the viscosity of pure polymers, which is similar for both polymeri c modifiers) seems to play a more important role than the miscibility. The stress-strain curves of both PVC/copoly(ester-urethane) (PU) miscible blend s 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 consid erably higher elongation at break and a lower Young's modulus than PVC mixe d with polymeric modifiers. Miscible systems, PVC/PU and PVC/DOP, show simi lar 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 adhesi on 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 a t shear rates such as those involved in calendering and milling, but is les s 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 th e viscosity with composition has been adjusted using an adaptation of the K olarik model, being this a novel approach to relate the viscous and mechani cal behaviour of immiscible but in some way compatible blends [1]. As expec ted miscible PVC/PU blends viscosity data follow a free volume additivity m odel. (C) 2000 Elsevier Science Ltd. All rights reserved.