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Effect of Metal Oxides/Chlorides on the Thermal Degradation of Poly(vinyl chloride), Poly(bisphenol A carbonate), and Their Blends G. Sivalingam and Giridhar Madras* Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India
The miscibility and thermal degradation of blends of poly(bisphenol A carbonate) (PBC) with poly(vinyl chloride) (PVC) have been studied in inert atmosphere by a differential thermal/ thermogravimetry analyzer. The blend showed a single glass transition temperature consistent with the Fox equation prediction suggesting the complete miscibility of the polymers. In the degradation of binary blend, PVC is stabilized slightly while PBC is destabilized significantly. Fourier transform infrared studies of the blend showed the presence of chlorine even after the dehydrochlorination stage. Thus, chlorine radicals or hydrogen chloride formed during the dehydrochlorination of PVC migrates and abstracts hydrogen from the PBC leading to the destabilization. The degradation of polymers with various metal oxides such as ZnO, Fe2O3, Co3O4, and TiO2 and metal chlorides such as AlCl3, ZnCl2, FeCl3, and CoCl2 was studied. Experimental data indicate that the increased degradation of the polymer blend in the presence of metal oxides is due to the formation of metal chlorides during the dehydrochlorination of PVC. These results indicate the high reactivity of chlorine radical or hydrogen chloride formed leading to the destabilization of polymers. Introduction Polymer blending is an important technique that can significantly alter the physical and mechanical properties of polymers, which are influenced by their morphology and miscibility.1 The low thermal stability of poly(vinyl chloride) (PVC) requires modification by blending with other more stable polymers.2,3 Poly(bisphenol A carbonate) (PBC) is an amorphous polymer with superior dimensional stability, good electrical properties, good thermal stability and excellent impact strength.4 It also offers excellent moldability and extrudability, low-temperature toughness, and use as flame-retardant. Hence the physical blend by solution blending2 of PBC and PVC has been chosen for the present investigation. PVC is known to be miscible with various thermoplastic polymers having ester linkages and ether linkages.2,5-7 Due to the similarity in structure of carbonate linkage to ester and ether linkages, PVC is expected to be miscible with PBC. Differential scanning calorimetry (DSC) studies on PVC/PBC showed the presence of dipolar interactions between the polymers, indicating complete miscibility.8 The liquid crystalline polycarbonate is reported be completely miscible with PVC over all compositions.1 However, a study on the dynamic mechanical analysis of this blend reports immiscibility of polycarbonate with PVC.9 Such contradictory observations make the study of this system interesting. The kinetics and thermal degradation of PVC has been widely investigated over a range of temperatures. The thermal degradation has been shown to occur in two distinct stages by most investigators,10 though degradation in three stages has also been reported.11 At lower temperatures ( TiO2 > AlCl3. Metal Chloride Catalytic Activity Over Metal Oxides. To check the hypothesis of metal chloride formation and increased catalytic activity of the metal chlorides formed, experiments were conducted with pure metal oxides and the metal chlorides under identical conditions. Figure 7a shows the effect of 16% loading of ZnO and ZnCl2 on the degradation of PBC, PVC, and 50/50 PBC/PVC under identical conditions. The maximum degradation temperatures of PVC in pure form and in the blend in the presence of ZnO are 208 and 204 °C, respectively, while in the presence of ZnCl2 they are 192 and 179 °C, respectively. The maximum decomposition temperatures of PBC in pure form and blend in the presence of ZnO are 395 and 354 °C, respectively, while in the presence of ZnCl2 they are 351 and 349 °C, respectively. The decomposition temperatures of PVC are different for ZnO and ZnCl2 aided degradations. This implies that ZnCl2 is being formed during dehydrochlorination during the degradation of PVC with ZnO, as the decomposition temperatures of PVC with ZnO and
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Figure 8. Plot showing the effect of cobalt and ferric oxides and chlorides on PBC, PVC, and their blends. (ax) PBC-oxide; (bx) PVC-oxide; (cx) 50/50 PBC/PVC-oxide; (ax′) PBC-chloride; (bx′) PVC-chloride; (cx′) 50/50 PBC/PVC-chloride. x ) 1 for Fe3+ and x ) 2 for Co2+.
Figure 7. (A) Effect of metal oxides and metal chlorides on the degradation of polymers and their blends. (a) PBC-ZnO; (b) PVCZnO; (c) 50/50 PBC/PVC-ZnO; (a′) PBC-ZnCl2; (b′) PVC-ZnCl2; (c′) 50/50 PBC/PVC-ZnCl2. (B) Effect of metal oxides and metal chlorides on the degradation of polymers and their blends. (a) PVAC-ZnO; (b) PVC-ZnO; (c) 50/50 PVAC/PVC-ZnO; (a′) PVACZnCl2; (b′) PVC-ZnCl2; (c′) 50/50 PVAC/PVC-ZnCl2.
PVC with ZnCl2 are closer. This can be clearly seen in the case of PBC degradation. The decomposition temperature of PBC in the presence of PVC alone is 441 °C, while the addition of ZnO to the blend reduces the degradation temperature to 354 °C. The decomposition temperature of PBC in the presence of ZnCl2 alone is 351 °C, and the decomposition temperature of PBC with PVC/ZnCl2 blends is 349 °C. The decomposition temperature of PBC in the presence of ZnO alone is 395 °C. These observations demonstrate the formation of ZnCl2 during the dehydrochlorination, and this also explains indirectly the migration of chlorine radical or hydrogen chloride to PBC in the PBC/PVC blend leading to the destabilization of PBC. It can also been from the plot that the degradation temperatures of PBC in the presence of metal chlorides are lower than the degradation temperatures in the presence of blends/metal chloride or blends/metal oxides. The present observation can also explain the degradation of PVAC/PVC in the presence of metal oxides,2 wherein the degradation temperature of PVAC decreased in the presence of PVC alone and PVC/metal oxides. Here, we have carried out the degradation
studies of PVC, PVAC, and 50/50 PVC/PVAC with ZnO and ZnCl2 (Figure 7B). It can be concluded that the metal oxides are able to destabilize the polymer to a lesser extent compared to metal chlorides, and this supports the postulate of metal chloride formation and migration of Cl• and HCl to the more stable polymer chain. Figure 8 shows a similar observation for the PBC/ PVC blends in the presence of various metal oxides such as Co3O4 and Fe2O3 and their corresponding metal chlorides CoCl2 and FeCl3. This also again confirms the formation of metal chlorides during dehydrochlorination of PVC. Thus the dehydrochlorination step also forms the metal chlorides if the polymers contain the metal oxides, leading to more degradation of polymer/metal composites. Conclusions The thermal stability and miscibility of PBC/PVC have been examined. The blend was completely miscible. In the blend, PVC is stabilized in the presence of PBC while the PBC undergoes significant destabilization due to the presence of PVC. The destabilization of PVC is attributed to the migration of chlorine radical or hydrogen chloride to the PBC matrix. The addition of metal oxides increases the degradation rate of polymers and can be attributed to the Lewis acidity of the oxides. The catalytic activity of the metal chlorides is higher than the metal oxides for the degradation of PBC. A similar effect was observed in the presence of metal oxides and metal chlorides during the degradation of PVC and PVC-PBC blends, indicating the formation of metal chlorides from metal oxides and liberated chlorine from PVC degradation. Acknowledgment G.S. thanks the General Electric Co. for a fellowship during the study, and the authors thank the Department of Science and Technology, India, for financial support. Supporting Information Available: SEM of a 50% PVC/PBC blend (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review May 15, 2004 Revised manuscript received September 1, 2004 Accepted September 11, 2004 IE049590Q
