RESINS FOR THERMOSETTING POWDER COATINGS.

Learn about the properties of the ideal resin and whether such a product exists.

Although both thermoplastic and thermosetting resins are used in powder coatings, thermosetting powder coatings comprise more than 90% of the market and account for almost all of the growth. It is no wonder then that the development of new resins or modification of existing types are reported in patents and trade journals on a regular basis. 'While the binder system (the resin and curing agent) determines the basic characteristics of the powder coating and the properties of the final film, the resin is typically the largest component of the binder and exerts the most influence on the properties of the coating.

Powder manufacturers wish there was such a thing as an ideal thermosetting powder coating resin. Here are the properties an ideal resin would have.

Solid, nonsintering and friable

These characteristics are interrelated and are fundamental requirements for a powder coating resin. To retain its particulate form during normal conditions of transportation and storage, a powder coating must have good blocking resistance or resistance to sintering. For amorphous resins normally used in powder coatings, the glass transition temperature ([T.sub.g]) should be above 40[degrees]C, preferably 50[degrees]C or more, to prevent sintering or clumping of powder particles (Table 1). Generally, aromatic monomers produce polymers with a higher [T.sub.g] than more linear aliphatic monomers. This is why most resins for powder coatings have a significant aromatic content.

The resins used in powder coatings have a relatively low molecular weight, in the range of about 1,000 to 10,000, so they are relatively friable and break up easily into fine particles during grinding. If the resin is not easily broken down during grinding, excessive heat may be generated, and the heat may be sufficient to melt the resin in some cases. The higher the resin's molecular weight, the more difficult it is to grind. Further, with most powder coating resins, the [T.sub.g ]increases with molecular weight, and resin manufacturers sometimes polymerize the resin to a higher molecular weight to increase the [T.sub.g]. In some cases, a higher-molecular-weight, …