Although 2K polyurethane coating systems containing a tin and/or bismuth catalyst,complexed with a molar excess of a complexing agent,have been known for many years and give coatings with acceptable properties,further improvement in the balance of curing speed,potlife and appearance is sought,especially in low VOC coatings.Using lower levels of curing catalyst will reduce risks of curing defects caused by entrapped solvent or gases,but extends drying times and reduces the coating hardness.On the other hand,higher levels of curing catalyst will shorten curing time,but can support foam stabilisation,leading to pinholes in the final coating.
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