The Synthesis and Characterisation of Fluorinated Ethylene Propylene (FEP) Copolymers

Abstract

This work details research on the semi-continuous copolymerisation of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) in aqueous medium using different monomer ratios. A main objective of this dissertation was to understand the influence of changing parameters, such as monomer feed ratio and initiator concentration, on the properties of the polymer. The copolymerisation of TFE and HFP was carried out in ultra-pure water using a 300 mL Parr 316 stainless steel high-pressure reactor. The experiments were done at 85 and 100 °C with pressures in the range of 11 - 20 bar using ammonium persulfate as the free radical initiator. Different monomer feed compositions were studied: 10, 19, 20, 22, 26, 38, 39, 40, 44 and 46 mol % HFP. The pressure of the reaction was found to be dependent on the amount of HFP in the feed stream. The following techniques were used to provide a comprehensive understanding of the as-polymerised FEP: FTIR, TGA-FTIR, SEM, DSC, XRD and 19F NMR. Solid state 19F NMR spectroscopy was used to elucidate the structure and determine the composition of FEP copolymers. The results revealed a successful incorporation of the HFP monomer in the copolymers by peaks at -67.44, -70.46 and -112.10 ppm due to –CF3 and –CF2 units of the HFP monomer and also by a peak at -183.42 ppm due to –CF unit of the HFP monomer. Results from DSC indicated that the melting temperature (determined by the highest peak of the first heating observed in the thermograms) and heat of fusion decreased from 324.28 to 278.71 °C and 73.05 to 36.60 J/g respectively as the mol % HFP increases in the copolymer. The XRD analysis showed that the FEP polymers are semi-crystalline with increase in the amorphous region as the concentration of HFP increases from 0.96 to 3.80 mol % HFP. SEM analysis showed that the FEP samples consists of spheres and rods at low HFP concentration in the monomer feed and spheres at high HFP. The TG analysis revealed that the copolymers are thermally stable up to 460 °C; after that they start to decompose. The decomposition can be resolved into a two-step process by taking the first derivative of the TGA curve. The evolved gas analysis using FTIR indicated that the first step corresponded to the elimination of HFP species (weak peak intensity at 1391 and 1790 cm-1 corresponding to C-F of CF3 in C3F6 and C=C of C2F6) in addition to strong bands at 1032, 1184 and 1328 cm-1 attributed to the CF2 moiety. The second corresponded to the degradation of the PTFE backbone chain to monomer component or unit.