A-Methylstyrene-acrylonitrile copolymers (AMSAN) are polymers which are highly compatible with PVC and have a high glass transition temperature. Such a copolymer (Luran KR 2556) has been employed by BASF since as early as the 1970s for the production of heat-resistant ABS plastics (Terluran HH 106 and Terluran HH 112; the number after the abbreviation HH specifies the Vicat B50 softening temperature of the High Heat type in question).Depending on the level of heat resistance desired, AMSAN here replaces a proportion of the usual SAN component (Vicat temperature of SAN is approx. 107°C, that of AMSAN is approx. 120°C).
The idea of using AMSAN - or even SAN or ASA - in order to increase the heat resistance of PVC has already been discussed in the industry for a relatively long time. New studies conducted by the Süddeutsches Kunststoff-Zentrum (SKZ = South German Plastics Centre) and BASF are now helping to adapt the heat resistance of PVC to particularly demanding applications in cost-effective manner without the need to take the CPVC route.
Producing Mixtures Under MildmConditionsIn order to be in a position to evaluate the effect that the AMSAN component of the mixture has on the mechanical properties of the polyvinyl chloride, care was taken to produce the blends under controlled mixing temperatures. In this way any possible decomposition of the polymer due to local overheating during the mixing process,which could diminish the mechanical properties of the blend, is minimised. This is ensured in optimum manner by the use of a planetary gear extruder.
The extruded strand is briefly cooled in a water bath, dried (Rieter SE/100) and pelletised (Rieter Primo 100). The PVC processed was an impact-modified, calcium-zinc stabilised PVC from Solvay whose mean melt temperature at the tip of the spindle was approx. 190°C. From the blends obtained in this way test specimens were produced which were subjected, inter alia, to the following tests: measurement of the Vicat softening temperature (ISO 306, method B50), measurement of thermal stability in accordance with DIN 53381-1 (test B) as well as measurement of impact resistance at room temperature (ISO 179- 1) and elongation at break (ISO 527-2).
Initial trials with proportions of AMSAN in the blend confirmed that the Vicat temperature of the blends can be raised by adding this copolymer. For every 10 % by weight of AMSAN added the Vicat temperature increased by approximately 4°C.
As expected this technically significant value is above those achievable by incorporating equivalent quantities of "classic" SAN copolymers as well as ASA varieties having correspondingly lower glass transition temperatures.
It is common to all the blends that they exhibit lower thermal stability than the starting polymer.While blends composed of PVC and Terluran HH 106 (ABS) or 112 and Luran S 778 T (ASA) are notable for slightly, with higher additions sometimes even dramatically, improved levels of impact resistance, the gain in Vicat temperature and hence in heat resistance as a result of AMSAN modification is simultaneously associated with certain losses in impact resistance.
The impact resistance of (PVC+AMSAN) blends, however, can be raised to the usual level by resorting to tried and tested methods of PVC formulation development.