Pyrolysis–GC–MS used to study the thermal degradation of polymers containing chlorine III. Kinetics and mechanisms of polychloroprene pyrolysis. Selected ion current plots used to evaluate rate constants for the evolution of HCl and other degradation products

• Published in: Polymer degradation and stability 2000-01, Vol.70 (3), p.395-407
• Main Authors: Lehrle, R.S, Dadvand, N, Parsons, I.W, Rollinson, M, Horn, I.M, Skinner, A.R
• Format: Article
• Language: English
• Subjects: Applied sciences; Chemical reactions and properties; Degradation; Exact sciences and technology; Kinetics; Mechanisms; Organic polymers; Physicochemistry of polymers; Polychloroprene (Neoprene); Pyrolysis–GC–MS; SIC plots; Thermal degradation
• ISSN: 0141-3910; 1873-2321
• DOI: 10.1016/S0141-3910(00)00134-8

The thermal degradation of polychloroprene [poly(2-chlorobutadiene)/Neoprene] has been studied at 387°C by pyrolysis–GC–MS techniques. First, the overall rate of production of volatile products was measured utilising total ion current (TIC) curves obtained from sequence pyrolysis experiments in which the products were not chromatographically resolved. This overall rate decreases abruptly during the course of the pyrolysis. The early stage (Stage 1) was calculated to have a rate constant k 1=0.065 s −1, and in Stage 2 this falls to k 2=0.013 s −1. Kinetic analyses of these results show that these stages are the consequence of fast and slow independent parallel degradation processes, in which the fast process is limited in some way so that after about 40 s it is virtually complete, and then only the slow process remains. The kinetics also indicate that the limiting yield (number fraction) of molecules produced by the fast process is approx. 60% of the total molecules produced by the end of the pyrolysis. Selected Ion Current (SIC) curves corresponding to the mass numbers in the MS cracking pattern of hydrogen chloride (i.e. m/ z=35, 36, 37, and 38) were then extracted from the above data, and these confirmed that HCl is a pyrolysis product. Despite the fact that the HCl is unresolved in the sequence pyrograms, its yields can be calculated from the areas of its SIC peaks, and on this basis the first order rate constant for HCl evolution ( k HCl) was calculated as 0.095 s −1 at 387°C. This high value for HCl formation shows that this process must be contributing significantly to the fast overall rate. SIC curves for a wide range of other possible degradation products were examined, and these revealed that monomeric and linear dimeric-type products from polychloroprene are also produced with fast rates ( k mon=0.090 s −1 and k dim=0.113 s −1 respectively). The formation of such products with specific rates which are identical (within experimental uncertainty) with that for HCl evolution, suggests that the loss of HCl molecules is associated with the loss of monomer and dimer molecules. The cumulative yield curves for HCl, monomeric, and linear dimeric products indicated that their total limiting yield was approx. 50% of the total products, which is close to the estimate of 60% for fast products based on the kinetic analysis of the overall yield. The overall rate constant for the formation of products by slow processes was then estimated from data obtained by su