Model to predict the mechanical behaviour of oriented rigid PVC

The mechanical properties of PVC sheets can be modified substantially by both uniaxial and biaxial stretching of the material above its glass transition temperature. Previous experimental studies have established a clear pattern in the relationship between tensile properties of oriented PVC products and imposed strains. Several mathematical models have been scrutinised to assess whether the established pattern of behaviour can be modelled and predicted. Of these, "the filament theory", proposed by Turner, emerged as the best candidate. The filament theory has been refined and developed further into "the composite model". In its present form it gives a good correlation between predicted and measured yield stress values of oriented rigid PVC sheets and is also capable of predicting the "established pattern" of property dependence upon imposed strain

Biaxial orientation of poly(vinyl chloride) compounds Part 2 –Structure–property relationships and their time dependency

X-ray diffraction and thermomechanical analysis have been used, respectively, to examine structural order and shrinkage behaviour for oriented samples of rigid and flexible poly(vinyl chloride) (PVC). Results were compared with previously measured tensile properties and structure–property relationships explored. X-ray diffraction showed that drawing produces planar crystallite orientation in PVC sheets. If drawing and subsequent annealing conditions are held constant, but draw ratio is varied, there is good correlation between structural order measured by X-ray diffraction and tensile strength. Increased annealing time and temperature improve crystallite order and dimensional stability, while tensile strength is unchanged. The greatest enhancement in tensile strength is achieved by stretching PVC towards its maximum draw ratio at 90°C, but optimum thermal stability of the oriented structure is achieved when higher annealing temperatures are used. Room temperature recovery is observed for flexible PVC when the material has a glass transition temperature below ambient. This can be delayed by increased annealing time and temperature, and by increased draw ratio

Melt compounding of rigid PVC formulations with hydrotalcites

Hydrotalcites, compounds of magnesium–aluminium–hydroxycarbonate, are promoted as environmentally safe materials for costabilisation of PVC products. Commercial grades of hydrotalcites have been added to rigid PVC formulations, containing a range of different stabiliser types, to evaluate their contribution to heat stability and their effect on mechanical properties. Hydrotalcites are confirmed to be effective costabilisers for rigid PVC: static thermal stability (through oven testing) and dynamic thermal stability (via torque rheometry) of PVC compounds are modified by their presence. The extent of change is determined by the primary stabiliser type and the grade of the hydrotalcite. Detailed analysis of mechanical properties has been carried out on compression moulded samples and on extruded compounds. With the former, no significant change in tensile or impact performance was observed, for hydrotalcite levels up to 5 phr. Charpy impact data on extruded PVC have shown significant increase in performance of compounds containing an acrylate modifier when hydrotalcites are used

Inorganic impact modifier and processing aid for PVC

An amorphous silica, SIDISTAR has been investigated as an impact modifier for rigid PVC. Optimal dispersion of the silica as primary particles is essential and this can be achieved by incorporating it at an early stage of the mixing process before other fillers are added. This ensures that the PVC grains are coated with SIDISTAR. This material is able to function as an impact modifier via the cavitation mechanism and it will also improve the dispersion of the other ingredients in the compound. It was found that SIDISTAR delayed gelation of the PVC compound, but this could be avoided by minor changes in formulation and processing conditions. Reduction in gelation time resulted in improved impact performance. Furthermore, compounds containing SIDISTAR possessed a wide processing window