STYRENICS MATERIALS AND CYCLOPENTANE - PROBLEMS AND PERSPECTIVES

摘要

The use of chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFC) has been greatly limited in recent years because of their high ozone depletion potential (ODP) (Montreal protocol). The manufacturers of refrigerators have tried various new blowing agents for polyurethane (PU) foams used as insulating panels, and currently the chosen organic compound in Europe seems to be cyclopentane (CP), due to its acceptable insulating power and null ODP and toxicity. Unfortunately the interaction of CP with high impact polystyrene (HIPS) panels of the refrigerators produces blisters and can possibly induce environmental stress cracking (ESC). The aim of this work is, then, to explain the growth of blisters with a theoretical calculation and also to investigate the mechanical behaviour of HIPS in contact with gaseous and liquid CP, in comparison with usually used Freon(R) 11 (F11). By means of the group contribution one can calculate with the Flory-Huggins equation an isothermal adsorption curve for the polystyrene (PS) matrix/organic system and with the Chow equation calculate the lowering of T-g. From the calculation it is evident that the lower CP vapour pressure causes a considerable absorption, and this is simultaneous to the lesser CP amount needed to reach a 23 degrees C T-g for the PS matrix: it is then possible to state that the blisters form from a sort of micro blow-moulding, induced in the plasticized PS by the thermal treatments that are introduced in the refrigerator production cycle. The use of small amounts of acrylonitrile (AN) in the PS matrix may be sufficient to avoid this inconvenience, as it has been confirmed by lab and industrial experiences. From a mechanical point of view two experimental set-ups were designed in order to evaluate the ESC resistance in gaseous and liquid environment: for the first case slow crack propagation (SCP) experiments were performed in controlled atmospheres on compact tension (CT) specimens, while in the second case the essential work of fracture (EWF) technique was applied to double edge notch (DEN) specimens fully immersed in the considered liquids at different temperatures. In the first case it can be immediately concluded that the ESC produced in the presence of the same pressure of the two different agents (CP and F11) is very different, the CP one being much more aggressive than the F11. However, if the data are plotted versus the thermodynamic activity, there is no meaningful difference between CP and F11, suggesting that this parameter controls the ESC in gaseous environment for the considered blowing agents. In the case of ESC in the presence of a liquid, that could have a practical relevance in refrigerators if condensation phenomena take place, one observes that ordinary HIPS becomes rapidly brittle, while an AN modified HIPS maintains an appreciable ductility also in the presence of the liquid solvents. [References: 36]