Synthesis of novel low-molecularweight polytetrafluoroethylene and tetrafluoroethylene copolymers for pyrotechnic applications

Abstract

At present, the explosives industry in South Africa makes extensive use of heavy metal oxide pyrolants in the time-delay elements of detonators used in mining activities. There is a drive towards the use of safer and more environmentally benign pyrolants in the form of metal/fluorocarbon pyrotechnic formulations. However, high-molecular-weight polytetrafluoroethylene (PTFE) is the current industry standard and pyrolants made with this polymer cannot be easily processed as it cannot be melt-extruded owing to the exceptionally high melt viscosity of PTFE. The research detailed herein was aimed at developing a low-molecular-weight PTFE capable of being used for extrudable pyrotechnic formulations, without suffering the drawbacks of poor thermal stability associated with low-molecular-weight fluorocarbons. The end result of this endeavour was the development of a low-molecular-weight PTFE, marginally bridged with butanediol divinyl ether, having sufficiently low molecular weight to be classified as a wax, while retaining sufficient thermal stability to be useful in pyrolant formulations. Additionally, this polymer also showed increased reactivity towards silicon metal due to the liberation of small amounts of HF from the non-fluorinated end-groups and the bridging agent, which helped remove the passivation layer of SiO2 on the surface prior to the ignition of the metal fluorine exchange reaction. This research starts with an in-depth review of the English language literature regarding the homopolymerisation of tetrafluoroethylene and also details the design and construction of the equipment for the safe and facile generation of up to 100 g of tetrafluoroethylene as well as the equipment for the polymerisation of tetrafluoroethylene, both in a Carius tube, and in an autoclave. The work then relates the batch-type synthesis of low-molecular-weight PTFE by conventional free-radical polymerisation. The conventional process was unable to produce a polymer with a sufficiently low molecular weight, such that it could be easily melt-extruded. It was noticed that, although the molecular weight of the polymer decreased with initiator concentration, as evidenced by the TGA curves, the Mn calculated by DSC increased with initiator concentration. This discrepancy is due to the mass transfer effects present within the polymerisation reactor. An attempt was made at deriving a kinetic expression for the polymerisation of TFE under a masstransfer- limiting regime, but this endeavour was abandoned in favour of a more experimental solution to the low-molecular-weight problem. The use of a persistent-radical perfluorinated initiator capable of generating ∙CF3 radicals was investigated for the purpose of providing a tracer end-group that will permit the measurement of the molecular weight of the polymers directly by NMR spectroscopy. The usefulness of CF3 endgroups as labels for molecular weight determination in poly(CTFE-alt-iBVE) copolymers by 19F NMR spectroscopy was demonstrated and compared to results obtained by SEC. The persistentradical perfluorinated initiator was not applied to TFE homopolymers due to technical issues regarding NMR spectroscopic analysis. The penultimate part of this thesis relates the use of a RAFT/MADIX agent (O-ethyl-S-(1- methyloxycarbonyl)ethyl xanthate) for the control of the molecular weight of PTFE, as studied via GPC using the copolymer of tetrafluoroethylene and isobutyl vinyl ether for a model polymer system. The effectiveness of RAFT/MADIX techniques in the control of the molecular weight of TFE-based polymers was demonstrated. Finally, low-molecular-weight PTFE marginally bridged with butanediol divinyl ether was synthesised by RAFT/MADIX techniques. The tailored PTFE was tested as the fuel in a fluoropolymer/silicon metal mixture. The tailored PTFE showed enhanced reactivity towards the silicon mental as compared to commercial- and low-molecular-weight PTFE synthesised by conventional free-radical polymerisation. The PTFE developed here is of significant commercial importance to the South African fluorochemical industry and will enable the South African explosives industry to greatly improve the safety and environmental friendliness of their detonators. The work reported here is limited to the synthesis and characterisation of the polymers, and only briefly touches the pyrochemical behaviour. In depth investigation of this aspect, as well as the rheological characterisation of the product polymer is left for subsequent researchers.