TOFSIMS Studies of Cleaning Procedures and Silane Surface Treatments of Steels

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Abstract
Surface contaminants can affect properties such as wet-tability and reactivity in subsequent surface preparation processes. Cleaning of metal substrates has usually been studied by techniques such as AES and XPS. These techniques do not give any information on the molecular structure of the contaminants; however, Time-of-Flight Secondary Ion Mass Spectrometry (TOFSIMS) with its monolayer sensitivity, has been effective in providing structural, molecular information of organic contaminants which may be useful in identifying the source of the contamination. Industrial alkaline cleaning and degreasing processes leave mainly a siloxane-type of contaminant that deteriorates the adhesion of metals to epoxies. Phosphating leaves the surface clean and microrough, which improves the adhesion of adhesives to cold-rolled steel (CRS). Epoxies when cured at high temperatures give better adhesion values as compared to room temperature epoxies. Organofunctional silanes such as styrylamino aminopropyltrimeth-oxysilane (SAAPS) were found to have a specific orientation on the metal substrate as a function of cleanliness and mode of application. The orientation of the vinylbenzyl group towards the epoxy adhesive is shown to improve adhesion. A combined silicate and silane post-rinse of phosphate coatings is shown to be effective in giving the silanes the required orientation for bonding to epoxies, thus improving the adhesion.

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