The Surface Coating of Polyurethane Substrates, Part 3: Some Properties of Polyurethane Surfaces and Interfaces

Abstract

The adhesion properties developed between a PU substrate coated with a PU-based paint are normally excellent. However, the industrial tendency to readily accept such adhesion without relating it to the theories of surface and interfacial science, means that commercial systems often will underperform. In a long research programme, attempts have been made to identify and quantify the order and magnitude of interfacial properties of coated PU substrates used commercially: some of this work is reported here. The approach was to produce conditions that would ‘age’ a substrate in some appropriate way. It was hoped this would allow the isolation of particular interfacial phenomena: this has had some success. Electron microscopy and spectro-analysis of PU/PU surfaces and interfaces were used, although x-ray photospectrometry (XPS) and secondary ion mass spectrometry (SIMS) techniques need to be refined for the molecular subtlies of joined PU systems. Research provided some predictions as to the types of bond and their strengths that may be expected for particular PU/PU systems. Such predictions made with some free interpretation of test findings, married together current interfacial theories. With a ‘well-made’ post coated product there will be expectations of most interfacial bonding types contributing to the total adhesive properties, although electrical double layer effects are a matter of debate. With in-mould coated systems there are indications that chemisorption may occur along with every other interfacial effect, especially with two-pack and possibly with one-pack paints.