PTFE initiators : their influence on the physical properties of the polymer

Abstract

The Fluorochemical Expansion Initiative (FEI) program was implemented to encourage the local beneficiation of South Africa’s abundant fluorspar resources for the benefit of the economy. Fluoropolymer production was identified as an area in which technical expertise was decidedly lacking and which promised many opportunities for research and skills development. Polytetrafluoroethylene (PTFE), also commonly referred to by its DuPont trade name, Teflon™, is the most well-known of the fluoropolymers. It is decidedly beneficial to develop expertise in the area of synthesis and characterisation of fluoropolymers, particularly PTFE. This polymer is synthesised via free radical polymerisation of TFE and can be initiated by a number of free radical initiators. A commonly used initiator is ammonium persulfate (APS). PTFE produced in this way experiences significant discolouration when subjected to temperatures in the region of 380 °C during the moulding process. The cause of the discolouration is unreported in the scientific literature, despite this discolouration being general knowledge within the fluoropolymer industry. The research detailed herein was aimed at determining the cause of colour changes in thermally processed PTFE homopolymers. The research started with an in-depth review of the literature regarding the homopolymerisation of tetrafluoroethylene and included the types of end groups that could be formed from initiators as well as the infrared- and Raman spectroscopic identification of these end groups. PTFE was synthesised in an autoclave via aqueous conventional radical polymerisation using ammonium- and metal persulfates, H2O2, KMnO4 and di-tert-butyl peroxide, benzoyl peroxide as well as azo-based initiators. The synthesised PTFE was then subjected to differential scanning calorimetry, thermogravimetric analysis and Fourier-transform infrared- and Raman spectroscopy both before and after sintering. Care was taken to ensure the polymers were not contaminated with any initiator- or buffer residues. It was found that the discolouration is caused by the decomposition of carboxylate end groups at elevated temperatures which deposit carbon into the polymer matrix. It was found to be generally true that higher initiator concentrations lead to more discolouration, because of more carbon being deposited into the matrix by eliminated end groups. It was also found that the use of buffering agents and the type of buffering agent used influence the type of end groups formed on the polymer chains.