Adhesion of a rigid polyurethane foam to zinc phosphated steel

Abstract

Rigid polyurethane foams were formed on rough zinc phosphate treated steel substrates. The interface between the two materials was investigated using knife peeling, immersion in a solvent which removes amorphous material predominantly, stud pull tests to observe bond failure, and X-ray scattering studies. There were three regions at the interface: an unbonded bare zinc phosphated surface, solid polyurethane areas where foaming did not occur, and foamed regions. These various interfacial regions result from temperature gradients during polymerization and foaming, and poor wetting of the substrate by the polymer. After immersion in a solvent, the solid polyurethane layer bonded to the substrate was completely removed, implying that this area only weakly adheres to the steel substrate. By contrast, the removal of the foamed area exhibited a well-ordered crystalline phase underneath polymer residues. In grazing angle X-ray scattering from the foamed region, a sharp peak from polyurethane crystallites was found on the shoulder of the amorphous peak; this re ection did not appear in the scans for unfoamed solid polymer areas. It is suggested that a greater number of these crystallitesresults in higher bond strength. A bond failure model was proposed in which fracture takes place along the non-connected regions, with cohesive failure in the foamed areas.

Interfacial Structures of Polyurethane Thin Films on Various Substrate Materials

“…The steel surface, treated using a zinc phosphate solution, consists of a lot of pits, cracks, and other defects. 18 This rough steel surface caused the structural variation of all of the polyurethane films (see Figure 6). The rough surface could provide a lot of nucleation sites where the polyurethane hard segments sit to arrange into the periodic registry.…”

“…This variation of crystallinity along with the film depth correlates with the variation of d (100) shown in Figure 8 such that the samples with a constant crystallinity through entire film exhibits a simultaneous constant d (100) . Factor et al 18 suggested that 'the d that is halfway between the bulk and surface values provides a rough estimate for the depth to which the surface effects persist'. In fact, polymer molecules near the substrate surface are subject to be strained, since the substrate surface restricts the motion of the polymer chains.…”

Structure analysis in polyurethane foams at interfaces

“…The adhesion of a PU foam on a thermoplastic (TP) material depends significantly on the interfacial structures of both materials. Kim et al 10 showed by X-ray scattering on an interface between a rigid PU foam and zinc phosphated steel some crystallite structures, and they found that these crystallites contribute to the interface strength. The same authors also claimed that the number of these crystalline structures is much more important than their size.…”