Impact strength and morphology in poly(vinyl chloride) window profiles - relationship with gelation level

The impact strength resistance of extruded PVC window profiles is a result of the combined effects of the interaction of their intrinsic material properties and processing/fabrication variables. Intrinsic variables include all the components of the formulation, such as the type and level of impact modifier and filler. As such, an appropriate level of toughness can be achieved by selecting the type and amount of rubber particles present in the matrix. However, the impact properties of poly(vinyl chloride) (PVC) profiles are also drastically affected by the thermal and shear history of the PVC matrix. The effect of processing on mechanical properties is explored by altering the temperature profile set on the extruder, and by varying the shear heating phenomena using different lubrication balances. The gelation level of any PVC formulation tends to increase with the level of work on the material, i.e. with increased melt temperature and shear history. The study reported in the present paper is intended to quantify the degree of fusion of the primary crystallites as a function of the melt temperature, and show the dependence of the toughness of the extruded profiles on the resulting free volume. Free volume in PVC extruded profiles depends on the degree of the gelation of the matrix and also on the cooling rate of the melt. As extrusion output increases, cooling of the melt is so fast that polymer chains have much less time to recover and reach a slate of minimum entropy. Upon physical aging, the free volume tends to decrease. The reduction in free volume changes the nonequilibrium state of the glass phase, thus reducing the toughness of the material, and causing ernbrittlement under certain test conditions. Finally, the effect of filler level and type of impact modifier (two intrinsic variables) on the impact strength of extruded profiles with various levels of free volume are presented.