engleski

Dynamic thermogravimetric degradation of PVC/CPE blends stabilized with different Ca/Zn stabilisers

Poly(vinyl chloride)/chlorinated polyethylene blends (PVC/CPE) are used for the production of geosynthetics, marine and hydroisolation foils and the like. Both polymers, PVC and CPE, can be subjected to termooxidative degradation either during processing or application. PVC and CPE are chemically similar, but CPE has superior thermal stability to PVC. Moreover, the investigated polymers have different degradation mechanism, as a consequence of differences in the microregularity of the corresponding polymer chains. Adding of heat stabilisers is necessary in all PVC formulations to prevent the decomposition of PVC by heat and share during processing. Ca/Zn stabilisers are one group of commercially available PVC heat stabilisers. They are more expensive and less effective than many other stabiliser systems and can cause rapid degradation reactions. In spite of this, Ca/Zn stabilisers are interesting because of their physiological harmlessness. The heat stabilisers recommended for CPE are similar to those used for stabilising PVC, although their effectiveness is not directly comparable. The effect of three different Ca/Zn stabilisers on thermooxidative degradation of PVC, CPE and PVC/CPE blend 50/50 was investigated: Ca stearate/Zn stearate (CaSt2/ZnSt2) (white powder, Ca:Zn = 2,1:1), CaSt2/ZnSt2 in homogenous suspension with soya-bean oil (Ca:Zn = 1,6:1) and CaSt2/ZnSt2 in organic solvent (Ca:Zn = 1,2:1). From the thermogravimetric curves (TG), obtained by means of dynamic thermogravimetry it is evident that the thermooxidative degradation of all investigated samples occurs through two basic degradation steps. The first degradation step, which spans the temperature range to 400oC, corresponds to the mass loss by the dehydrochlorination of PVC and/or CPE. To estimate the thermal stability of the investigated samples, the following characteristics of TG curves for the first basic degradation step were used: the onset temperature (T1o), the temperature at the maximal rate of degradation (T1m), the degree of conversion at the corresponding maximal rate (ƒŃ1m), and the mass loss at the end of the first (ƒ´m1) degradation step. These characteristics as well as the shape of the derivative thermogravimetric (DTG) curves also indicate that the investigated Ca/Zn stabilisers have different influence on thermal degradation of PVC and CPE. The activation energy of the thermooxidative degradation of the nonstabilised and stabilised PVC, CPE and PVC/CPE blend was determined by applying Flynn-Wall-Ozawa isoconversional method. The shape of dependence of activation energy on the conversion indicates the complexity of thermooxidative degradation mechanism of the investigated polymers and different stabilising effect of Ca/Zn stabilisers on PVC and CPE.

Ca/Zn stabilizers; CPE; PVC; therooxidative degradation

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