The formulation of PVC extrusion

During the extrusion of PVC, special attention must be paid to the formulation, the extruder design and the extrusion conditions, 

which all play an important part in determining whether or not a satisfactory product can be produced. The main points to be 

addressed concern the thermal stability and high melt-viscosity of unplasticised grades. At processing temperatures, PVC is damaged 

by dehydrochlorination, auto oxidation, and mechanochemical chain scission. The degradation is caused by simultaneous sequences 

of these reactions. The double bonds formed are conjugated in polyene sequences, which are active in secondary reactions that lead 

to cross-linking and to the formation of aromatic pyrolysates. Thermal dehydrochlorination of PVC in the presence of oxygen is faster 

than in an inert atmosphere. During processing, carbon radicals are formed from chain scission as a result of strong shear forces. 

These radicals then react with oxygen to form peroxy radicals, and finally, hydroperoxides.

The production of PVC articles generates substantial quantities of scrap material. It is, of course, important to be able to re-extrude scrap, 

but there are three main problems that can affect the ability of doing this successfully, namely: (1) degradation of the polymer and the 

additives during initial compounding and during the first and subsequent extrusions; (2) the effects of processing on the rheological 

behaviour of the material; and (3) the different particle shapes and sizes in the scrap compared to the original material. Retained properties 

and durability are among the most important factors when evaluating the possibility of mechanical recycling of plastic waste. In this investigation, 

profiles made of rigid, unplasticised PVC were studied over several repeated extrusions in order to gain knowledge of the changes in their properties, 

and on the effect on their durability after conventional processing.