Year

2000

Degree Name

Doctor of Philosophy

Department

Department of Materials Engineering

Abstract

Electrochemical polymerisation (ECP) of non conducting polymers to form thin, passive films on electrodes has received attention in recent years as a potential adhesion promotion technique. Work on carbon electrodes with applications in the carbon fibre composite industry have shown that it is possible to enhance adhesion of carbon fibres to the composite matrix through ECP pretreatments. ECP to produce passive coatings on metals has also been studied, however much of this work is concerned with production of corrosion inhibiting films, rather than adhesion promoting layers. ECP methods are a promising alternative to the chromate pretreatments and primers presently used to improve the adhesion and corrosion resistance of organic coatings on metals. This is because they can be non toxic, offer the possibility of pinhole free coatings and can be produced insitu, preventing the need for separate processing.

Thin, uniform, reproducible polymer coatings based upon poly (methyl methacrylate) (PMMA) and poly (glycidyl acrylate) (PGA) were produced on stainless steel cathodes from dilute aqueous sulphuric acid containing potassium persulphate. Electrochemical activities of the solutions were identified by cyclic voltammetry. The polymers were confirmed to be PMMA and PGA by Fourier transform infrared spectroscopy (FTIR). The thickness of the coatings (between 30 and 400 nm) were measured by ellipsometry, and found to increase with increasing treatment time and with less negative electrode potential. The porosity of the coatings was assessed electrochemically and after an annealing treatment the porosity and coating thickness were both found to decrease for the PMMA coatings and remain relatively unchanged for the PGA coatings. The morphology of the ECP coatings were studied by various microscopy techniques.

Further characterisation of the PMMA coatings was undertaken in order to clarify the polymerisation mechanism. This involved differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR), gel permeation chromatography (GPC) and solubility studies. A free radical polymerisation mechanism has been proposed in which the radicals are long lived resulting in post electrolysis polymerisation and very high molecular weights.

The PMMA and PGA ECP coatings were used as tie layers in adhesion tests by adhering aluminium studs onto the ECP surface with the desired adhesive and subsequently pulling the stud off at a reproducible rate with an Instron tensile testing machine.

It was conclusively shown that the adhesion strength between the stainless steel and a cyanoacrylate adhesive was improved by the presence of the PMMA ECP tie layer. In contrast the adhesion of the PMMA ECP tie layer to a standard epoxy adhesive was very poor. The adhesion of the PGA ECP tie layers to the stainless steel substrate was too great to be measured by the standard epoxy adhesive, therefore a toughened adhesive was employed to measure the strength of the interaction between the PGA layers and the stainless steel.

This study demonstrates that the ECP technique can be used to improve adhesion of metals to adhesives if the tie layers are chemically compatible with both the metallic oxide and the adhesive. The adhesion is proposed to be improved by increased primary and/or secondary bonding at these interfaces and by excellent interfacial contact between the metallic electrode and the electropolymerised tie layer.

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