Abstract
This thesis presents a study of the chemistry and the kinetics of the gas phase fluorination of high density polyethylene (HDPE) sheets. Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) have been used. XPS and SIMS give complementary information on the surface chemistry whereas RBS allows elemental compositional/depth profiles to be determined, and with the aid of these techniques, the kinetics of fluorination have been studied. Reaction vessels have been designed especially for the fluonriation process with the possibility of fluorinating several samples at controlled reaction temperatures. In order to understand the Cl s XPS and SIMS spectra of the surface of the fluorinated HDPE samples, a database of the XPS spectra of commercial fluoropolymers has been established. The intensity of characteristic fragment ions in the SIMS spectra of the polymers has allowed a procedure to be established which enable the apparent fluorine concentration with the fluorinated HDPE surface to be established. RBS results show that the fluorine diffusion in HDPE follows a Case II diffusion behaviour, characterised by the existence of an induction period preceding the diffusion front of the fluorine. The induction period corresponds to the time during which, the material structure changes in order to allow the fluorine to diffuse into HDPE and involves substantial molecular re-arrangement to accommodate the fluorine. It has been observed that the induction period is temperature dependent. XPS and SIMS results reveal that the gas phase fluorination of HDPE follows a two step chemical reaction. The first step corresponds to the spontaneous substitution of one of the two hydrogen atoms on the carbon skeleton by a fluorine atom. The second step involves the substitution of the second hydrogen. These two steps take place during the induction period in a zone described as the "latent zone" which is the boundary between the unfluorinated and fluorinated parts of the polyethylene. The latent zone must reach a critical concentration before the fluorine can diffuse deeper into the polymer. This critical concentration is reaction temperature dependent and corresponds to the degree of substitution of fluorine atoms. At 25°C and 50°C, the chemical structure is C2F2H2. At 70°C and 90°C, it is respectively C4F5H3 and C2F3H2. Extrapolation of the experimental plot of the number of fluorine atom per repeat unit versus reaction temperature enable the prediction to be made that at 130°C a fully fluoro-substituted structure (PTFE) will be obtained. Finally, the results obtained from RBS allow a prediction of the experimental conditions necessary to form a fluorinated layer of HDPE with a given concentration and fluorine depth. This will enable the normal fluorination process to be reverse engineered to establish the optimum conditions for the desired physical and chemical properties.