Due to its ease of processing and toughness and not least on account
of its competitive price the material polyvinyl chloride (PVC) has captured a secure position in the building sector. Over half of the Western European PVC production, running to some 6 million metric tons, is used on building sites in the form of extruded window and door profiles, cable sheathing, floor coverings, pipes and conduits. Accordingly, PVC is one of the most important materials in this sector.
The potential of this material in this field has by far not yet been exhausted. One of the challenges constraining the wide use of PVC is the comparatively low heat resistance of this material. Most PVC products are recommended for operating temperatures of 60 to 65°C (peak temperatures of up to 80°C). At higher temperatures the material begins to soften.
This has negative effects on the dimensional stability of the material, in particular when the part in question is under mechanical load in use. In some circumstances this is already a problem in pipework carrying hot water or in dark coloured building panels (title picture) directly exposed to sunlight.
Increasing Heat ResistanceIt is possible to increase the softening temperature of polyvinyl chloride by postproduction chlorination of the main polymer chain. Chlorinated PVC (CPVC) having a chlorine content of 66 %, for example (by comparison with 57 % in the normal material), is suitable for continuous working temperatures of 90°C (peaks of 110°C). However, the higher density (of about 1.6 compared with 1.4 g/cm3 in normal PVC), poor processability and the relatively high price are disadvantages. A solution to this problem is found in blends composed of PVC and polymers characterised in comparison with polyvinyl chloride by a relatively high glass transition temperature.
Corresponding mixtures should exhibit higher heat resistance than pure PVC. A useful guide value for the heat resistance of a polymeric material is obtained by measuring its Vicat B50 softening temperature.
It is determined by a practical test method. In plastics having a low degree of crystallinity the Vicat temperature is correlated with the glass transition temperature which can be predicted on the basis of a simple relationship among the proportions of the components of a blend (Gordon-Taylor relationship). In order to increase the heat resistance of PVC it is useful to mix this plastic with proportionate amounts of a polymer which is inherently characterised by a high glass transition temperature.