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1.4 CHEMICAL SCIENCES
The intermolecular and intramolecular changes induced by thermal stress in an industrial rubber to metal coupling agent (the ‘green molecule’ or GM) are the subject of this thesis. The GM was analysed in-situ in a model application environment using vibrational spectroscopy. NMR spectroscopy was used in order to analyse the solution chemistry of the compound and how this changed as a result of thermal stress. The interaction of the GM and the substrate was analysed using a range of surface analysis techniques including XPS, AFMIR and EDX. An example of a complex substrate, the zinc phosphate conversion coating, was analysed using vibrational spectroscopy and EDX to determine the manner in which it behaves in the application environment. The effect of the industrial application environment was examined by preparing adhesion test pieces and analysing the manner in which the application temperature affected their performance. In tandem with the adhesion testing, the interaction of the substrate, the GM and the rubber in the application environment was examined. This was done by preparing test pieces that used the GM in isolation as the intermediate in rubber to metal bonding. It is typically used in a formulation. Stability testing of the GM in DMSO was carried out. Vibrational analysis of the GM revealed that urethane hydrogen bonding was playing an active role in directing the intermolecular state of the GM as a function of temperature. This intermolecular association was seen to have an effect on the manner in which the GM hydrolysed and condensed in the model application environment. Solution NMR of the GM before and after being subjected to thermal stress revealed that the GM was resistant to thermally induced hydrolysis at high concentration. This effect was a result of the urethane hydrogen bonding identified using the vibrational analysis. Surface analysis of mild steel substrates that were exposed to the GM at high temperature
showed that the GM binds to the substrate very sparingly. The mirror polished mild steel surfaces were visibly unchanged after thermal treatment in the presence of the pure GM. XPS analysis gave the only indication that any of the GM had bonded to the surface. IR analysis showed that the Henkel ZPCC dehydrated when subjected to thermal stress. EDX of thermally treated ZPCC showed that oxidation was occurring at the ZPCC coating however it is unclear whether this oxidation was occurring to metallic species contained in the coating or to the steel substrate underneath the coating. Adhesion testing showed that the rubber to metal adhesive formulation containing the GM formed stronger adhesive joints between the rubber and the metal at lower processing and curing temperatures than those typically used in industry. Analysis of test pieces prepared using the GM as the sole intermediate between the rubber and the metal showed that the GM nitroso moiety reacted upon mixing with the rubber. This interaction between the GM and the rubber was accompanied by sulphur release from the rubber which deposited as a sulphate on the iron substrate. The morphology of the sulphate deposit was temperature dependent, changing from crystalline to amorphous as the sample preparation temperature increased. Stability testing, where the hydrolysis of the GM induced by minute concentrations of water in DMSO-d6 was compare with common alkoxysilane precursors GPTMS and MAPTMS, showed that the GM was relatively stable in comparison to the alkoxysilane precursors that did not possess a urethane moiety in their molecular structure. The GM structure incorporates a urethane moiety that acts to stabilise the GM thermally and chemically. The hydrolysis and condensation behaviour of the GM is novel in comparison to common alkoxysilane precursors. The incorporation of the urethane moiety in the design of novel alkoxysilane precursors and also the preparation of urethane functionalised analogues of common organofunctional alkoxysilane compounds may open a doorway to improved alkoxysilane based surface treatments. These treatments will also be more easily ‘tuned’ to meet the requirements of a given application.
Barton, K. (2016). Analysis of an adhesion promoter for rubber to metal bonding. Doctoral thesis. Dublin Institute of Technology.