Abstract
The main project was focused on a long-term ambition of New Zealand Steel to convert their coil coating line from a solventborne system to a waterborne system for environmental and economic purposes. A related goal was for the new waterborne coating to cure at lower temperatures so that oven temperatures could be lowered and line speeds increased resulting in lower gas costs and improve productivity. To achieve both aims, the binder system was proposed to utilise commercial waterborne polycarbodiimides and their low temperature reactivity with carboxyl groups in waterborne functionalised resins.
The polycarbodiimide crosslinkers were found to be small polymers with molecular weights of less than 5,000 through gel permeation chromatography (GPC) and static light scattering (SLS). Extensive NMR and 2D-NMR revealed the exact aliphatic diisocyanates used for carbodiimidisation and the final capping agents; additionally, the oligomers' hydrophilic tails, vital for dispersibility, were identified. The main functionalised base resin used in the project was a water reducible dispersion of a methyl methacrylate-ethyl acrylate copolymer with a carboxyl equivalent weight of 1006 g solid mol-1. A second base resin studied was an aliphatic polycarbonate urethane anionic dispersion with 3468 g solid mol−1 carboxyl functionality available for crosslinking.
The subsequent phase of the project was to perform coating experiments on pretreated steel to determine the optimal formulations and curing conditions for the polycarbodiimide-carboxylated binder systems. Variables considered were: the carboxyl-carbodiimide functional ratio, the peak metal temperature (PMT) during curing, the pH of the formulation using ammonia, and the effect of various tertiary amines. The performance of the coated samples were evaluated by coil coating specific MEK double rubs, pencil hardness and T-bends to determine solvent resistance/cure, hardness and elongation/formability respectively. Trends from these tests were further explored by ATR-IR spectroscopy to derive some chemical justifications for the observed coating attributes.
The formulations of the polycarbodiimide and acrylic resin displayed a correlation between improved solvent resistance with higher crosslinker concentrations and increasing PMTs. For a constant pH, addition of a small quantity of triethylamine (TEA) generally improved solvent resistance while addition of the hydrochloride salt or dimethylethanolamine (DMEA) significantly retarded cure. The optimal pH range for solvent resistance was determined to be 8.7-8.9. There were no significant trends seen in coatings with hardness and elongation, and most coatings achieved the required hardness and formability values. The polycarbodiimide-polycarbonate urethane formulations overall yielded coatings with exceptional solvent resistance, with some coatings achieving over 200 double rubs at PMTs of 190°C and 208°C.
The positive correlation between PMT and solvent resistance manifested in the IR spectra as increased carbodiimide reactivity and lower anhydride formation, which implicitly suggested greater N-acylurea formation as an intermediate. However, degradation of some N-acylurea functionality materialised as the appearance of isocyanate and amide functional groups at higher temperatures. The isocyanate was expected to react rapidly with nearly any functional group and it was not possible to identify the final reaction products. These high PMT isocyanate-derived products occurred at higher levels in coatings at pH 8.9 compared to pH 9.5, which correlated with the coatings having the highest solvent resistance. In conclusion, limited degradation of the N-acylurea crosslinks yielded coatings with the best properties that met New Zealand Steel's performance criteria while still curing at lower PMTs than typically used on coil coating lines.