Low Temperature Crosslinking for Powder Coatings
Abstract: This paper will focus on low temperature cure of hydroxyl polyesters with polymeric blocked isocyanate crosslinkers, carboxyl polyesters cured with TGIC and a discussion about the future of IR/UV cured powder coatings.
Specific polymeric blocked isocyanates have been formulated for low temperature cure of various hydroxyl polyesters. Cure capabilities in the range of 130 C have been achieved. Polymeric isocyanate crosslinkers blocked with ε-caprolactam and ε-caprolactam free crosslinkers will be described. A novel approach to low temperature cure carboxyl terminated polyesters is described.The use of specific raw materials combined with selected catalysts has resulted in polyesters for triglycidyl isocyanurate,TGIC, cure which have cure capabilities at 121 C. Polymers and powder coating formulations which exhibit low temperature cure properties are described. This technology pushes conventional polyester powder resin raw material capabilities to the apparent limit of performance for low temperature cure.
For lower temperature cure than 120 C, ultraviolet, UV, cured powder coatings seem to be most feasible. Powder coating formulations melted with IR and cured with UV will be discussed.
Introduction: The powder coatings industry has continuously sought the reduction of cure temperature. Twenty-five years ago, low temperature cure for decorative powder coatings was considered to be temperatures below 200 C (392 F). Today, low temperature cure products can be generally classified as products which cure at relatively short times at temperatures at or below 160 C (320 F).
Hydroxyl polyesters and specific polymeric blocked isocyanate curing agents have been developed for low temperature cure. It has been observed that as the functionality of the hydroxyl polyester increases, the temperature required for cure decreases. Crosslinkers based on more reactive isocyanates like HDI, hexamethylene diisocyanate, and Desmodur?W, methylene-bis-4-cyclohexyl diisocyanate, H12 MDI, have shown better low temperature cure capability than other isocyanates like IPDI, isophorone diisocyanate. Crosslinkers formulated with more reactive isocyanates and low temperature blocking agents have shown cure activity at 130 C (266 F).
Typical approaches to low temperature cure carboxyl polyesters have focused on the use of catalysts which accelerate the reaction between the acid functionality on the polyester with the epoxy functionality on the TGIC,triglycidyl isocyanurate curative. This standard approach results in powder coatings with flow decreasing as the cure temperature is reduced. The utilization of a more reactive polymer structure and a latent catalyst 1 in combination, is the essence of the invention described in this paper.2 The optimized product described here has resulted in powder coatings which cure at 121 C (250 F). Formulation of a polyester with low melt viscosity to provide good flow at low temperature cure conditions and a glass transition temperature, Tg high enough to prevent sintering, is a goal of the polymer chemist. These properties are often in conflict and present challenges to the polymer chemist. For hydroxyl polyesters cured with polymeric blocked isocyanates, flow is maintained even at very low temperatures because the crosslinking does not begin to any extent until the blocking agent has left the polymeric diisocyanate. This is usually enough time to permit good flow to occur before crosslinking increases the viscosity of the powder coating. Low temperature curing systems can give faster cure at elevated temperatures. This can result in optimum coatings performance properties and permit faster line speeds.
Normal cure versus low temperature cure The maximum temperature that an oven can attain, or the heat stability limitations of the substrate may require specification of a low temperature cure powder coating. Typically, low temperature cure powder coatings have a baking temperature 20-40 degrees C below the equivalent normal curing powder coating. Powder coating suppliers do not recommend that the normal powder coating be cured at temperatures 20-40 C below what is typically recommended because an under cure situation would most likely be created or very long cure times would be needed. Low temperature curing powder coatings are often cured at higher temperatures, near typical baking temperatures, to induce faster cure. This is commonly done when an applicator has a short oven and wants to increase the production rate. If a standard cure powder is used in this manner, under cured parts would result from the short bake times. Low temperature curing powder coatings may be used to: ? Permit powder coating of temperature sensitive substrates or components. This may include assembled parts with temperature sensitive seals etc., wood components, certain plastic parts or components. ? Reduce energy costs by lowering baking temperatures. ? Provide a means of increasing the rate of production by baking at higher temperatures or normal temperatures for shorter periods of time. Limitations of low temperature cure powder coatings: ? Extremely high levels of catalyst in some low temperature curing powder coatings may cause powder package stability problems due to premature curing of the powder coating even at room temperature. (This instability could be measured by a change in gel time, a decrease in gel plate flow, GPF, and increased orange peel of the cured powder coating.) ? Lower softening point or glass transition temperature,Tg, associated with low temperature cure powder coatings often limit their use to regions that do not have high ambient temperatures. This is to prevent caking/sintering of the powder during transport, storage and application.
Refrigerated storage or transport may allow low temperature curing powders to be used in regions with high ambient temperatures or to extend the low temperature powder shelf life.