The Surface Coating of Polyurethane Substrates, Part 2: A Theoretical Approach to the Interfacial Bonding of Polyurethane Coatings on Polyurethane Substrates

Abstract

In the first paper of this series a number of manufacturing techniques were reviewed from first hand observations of such practices. Comments were made of the techniques which had evolved on a trial and error basis to optimise the properties of the paint and PU foam substrate composite. In this paper, the wetting and interfacial bonding phenomena developed in these composites is examined on the basis of currently accepted surface science theories. Wetting and spreading over a surface, followed by interfacial bonding will depend on the initial affinity between the liquid and the solid, although there might be variation from that predicted because of composition changes as the paint system dries. It is suggested for a PU foam/PU surface coating composite, that good adhesive bonding results from a combination of dispersion, polar and covalent bonding interactions. However, because of moulding practice, boundary layers (e.g. release agent residues) and other phenomenon will interfere with the wetting and bonding in some of these systems. In-mould surface coating has certain advantages over other methods of painting a moulding. In particular the chemical activity of freshly produced surfaces minimises the possibility of boundary layer effects. The spreading pressure, π, which influences surface tension in a system, will be affected by the moulding pressure. Later papers will report on the examination of various PU/PU combinations, describing the influences and optimisation of the adhesive bond between coating and foam substrate.