What chemical changes have taken place in the use of thermosetting powder coatings

"The difference between thermosetting powder coating and thermoplastic powder coating is that thermosetting powder coating will not be heated and solidified again after heating and curing, while thermoplastic powder coating will be softened again if it is heated and plasticized again after heating and curing, then why won't thermosetting powder coating be heated and softened again after heating and curing? The key problem is that the chemical reaction of thermosetting powder coating takes place when it is heated, which makes the linear small molecules become a network of three-dimensional macromolecular structures So what are the chemical changes of thermosetting powder coatings when they are heated? The following is a further description of different resin systems An epoxy polyester system The main film-forming materials of the system are epoxy resin and polyester resin with carboxyl group The curing reaction is shown as follows [upload = GIF] / powder / UploadFile / 20049718151750595 GIF [/ upload] is essentially an esterification reaction At the same time, it can be seen that the alkali resistance of the system is limited to some extent With the understanding of the reaction mechanism, the curing rate of the resin can be changed by adding some accelerants The high functionality of the system determines the high crosslinking density, that is to say, a TGIC molecule can react with multiple COOH to form a three-dimensional network structure 2 Primid XL 552 (hydroxyalkylamides) [upload = GIF] / powder / UploadFile / 20049718161633970 GIF [/ upload] due to the release of small molecular water during curing, the film is not easy to be too thick In addition, the content of anti pinhole agent (benzoin, etc.) should be increased Because there are many curing agents for epoxy powder coating, dicyandiamide is only taken as an example to show that the reaction activity of dicyandiamide is very low, and it generally needs 200 ℃ / 30min to cure completely, so curing accelerator is generally used to make it complete curing at 180 ℃ / 15 ~ 20min The film-forming materials of tetraurethane system polyurethane system are hydroxypolyester and blocked isocyanate Their reactions are expressed as follows [upload = GIF] / powder / UploadFile / 20049718165012409 GIF [/ upload] due to the free low molecular sealer in the reaction, the amount of defoamer (benzoin) must be increased appropriately, and it is not easy to make too thick film Because acrylic powder coatings are seldom used in China, their introduction is not introduced one by one.