Gelation of rigid PVC-pipes

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Abstract
Balancing the properties of rigid PVC products requires an optimization of the gelation level. In this work different methods have been used to characterize the gelation level of a set of PVC pipes extruded at different temperatures. From capillary rheometry measurements at 145°C using a flat die, it is concluded that the gelation level should be related not only to one specific compound but to one specific reference process. The die swell was increased with the gelation level in about the same way as the flow pressure. The extrusion temperatures of the pipes ranged from 180 to 204°C and corresponded to gelation levels between 10 and 75% based on the roll mill reference curve. Methylene chloride swelling was small at 50% gelation level and negligible above 70%. Different impact tests showed a broad maximum at about 50%. The falling weight method “B 50,” however, showed a remarkably distinct maximum. Physical aging by slow cooling from above Tg of the pipe compound did not influence the process history of the pipes in terms of impact strength and methylene chloride swelling. Internal water pressure data at 60°C show a general trend toward increasing resistance at higher gelation levels. The considerable scattering due to the statistical nature of the process makes detailed interpretation difficult. Finally a high-temperature tensile test using rapidly prepared ring-sector-shaped test pieces with a central hole proved to be useful as gelation probe.

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