Pretreatments of fluoropolymers to enhance adhesion

2015-11-16T12:53:19Z (GMT) by Isla Mathieson
The aim of the project was to gain a better understanding of the factors that affect adhesion of fluoropolymers. This was achieved by employing various analytical techniques to the treated and untreated polymers. The effects of novel pretreatments, and established treatments, on Polytetrafluoroethylene, PTFE, Poly (vinyl fluoride), PVF, and poly (vinylidene fluoride) PVdF, were characterised using: adhesion tests, X-ray photoelectron spectroscopy (XPS), including derivatisation reactions, Fourier Transform Infrared (FTIR), contact angles and scanning electron microscopy (SEM) For untreated PVF and PTFE it was found that a certain degree of adhesion improvement was achievable without any chemical modification of the surfaces. This was observed when the substrates were repeatedly bonded. It is proposed that weakly cohesive material was present in the polymers and these acted as weak boundary layers when bonded. Removal of weak boundary layers alone was found to be insufficient to obtain high adhesion with PTFE. Surface functionality, increased wettabiIity and favourable topography all contributed to the high bond strengths observed with 'Tetra-Etch' treated PTFE. 'Tetra-Etch' treatment is used commercially on PTFE but prior to this programme was unreported on PVF and PVdF. The treatment was effective at promoting adhesion for PVF though at a· much slower rate than for PTFE. Additional mechanisms to that for PTFE (Le. electron transfer) are proposed for the action of 'Tetra-Etch' on PVF. These are dehydrohalogenation through electron transfer and an elimination reaction. The same mechanisms are proposed for PV dF. Flame and Iow pressure plasma treatments·,w7re carried out on PVF and PTFE. Flame was found to be ineffective for PTFE but with PVF chemical modification (oxidation) occurred at the carbon/hydrogen sites. No defluorination was observed; this was in contrast to the mechanism of oxidation via plasmas on PVF, where defluorination, oxidation, ablation, and crosslinking may have all contributed to the high bond strength obtained. Certain plasma treatments were effective at improving the adhesion of PTFE but were slower and caused less modification. Removal of weak boundary layers was proposed as the major factor since oxidation was often slight. Reaction with solutions of potassium hydroxide (KOH), sodium hydroxide (NaOH) and lithium hydroxide (LiOH) were effective as adhesion pretreatments for PVF and PVdF but not for PTFE. For PVF and PVdF rates of reaction and chemical modification varied with time, temperature, molarity of solution and the nature of the solution i.e. aqueous or alcoholic. The greatest improvement in rate and effectiveness of the treatment for adhesion improvement was on the addition of a phase transfer catalyst to the aqueous solution. It was found for PVF that substantial surface oxidation could be achieved without improving the adhesion. It was suggested that oxidation occurred at sites present in a weakly cohesive layer. Mechanisms of the reactions were considered in terms of neucloephilic substitution and elimination; for PVF and PV dF both are likely. The mechanism of the phase transfer catalyst was investigated and found to be complex. It was found not to be simply a wetting agent but had inherent reactivity on its own. A combination of mechanisms was proposed.