Applications of hyperbranched polymers in UV-curable powder coatings

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There are mainly two ways in which hyperbranched polymers are used in UV-curable powder coatings: one is that the hyperbranched polymers themselves are in powder form and can serve as the main film-forming component of the coating； Another approach is to add hyperbranched polymers to conventional powder coatings to participate in the film-forming reaction of the coatings. When using traditional powder coatings, in order to keep them stable at ambient temperature, the Tg of the resin used must be above 60°C; however, to achieve excellent performance, the application temperature must be at least 50°C higher than the Tg. This means that the minimum application temperature for powder coatings is around 110°C, which limits their use on heat-sensitive substrates such as wood, plastic, and medium-density fiberboard. Crystalline chain segments are introduced via reaction at the end groups of hyperbranched polymers, thereby endowing the new hyperbranched polymers with semicrystallinity; or, through ring-opening polymerization with certain compounds, crystallizable end groups are imparted to hyperbranched polyesters, allowing the hyperbranched polymers to be powdered, with a lower film-forming temperature compared to conventional powder coatings. Claesson et al. modified the Boltorn-H30 type hyperbranched polyester using caprolactone to introduce long-chain end groups with semicrystalline properties; these end groups were then modified with methacryloyl anhydride to produce UV-curable powder coatings containing unsaturated double bonds. These coatings could form a film at 80°C, and the melting point of the resin in these coatings ranged from 34–50°C. Kou Huiguang and others modified hyperbranched polyesters with terminal hydroxyl groups using acryloyl chloride, hexadecanoyl chloride, or octadecanediisocyanate, respectively. Studies have found that these hyperbranched polymers, with a molecular structure featuring an amorphous \"core\" and a crystalline \"shell,\" possess high melting points. The acrylated hyperbranched polyester modified with octadecyl isocyanate has a higher melting point than the one modified with hexadecanoyl chloride. Moreover, due to the presence of numerous UV-curable acrylate end groups, they can cure rapidly upon melt irradiation. Hyperbranched polyesters modified with hexadecanoyl chloride or a small amount of octadecanediisocyanate yield waxy products, while powdered products are obtained only when a larger number of terminal hydroxyl groups are modified with octadecanediisocyanate. Hydroxyl-terminated hyperbranched polymers can be added directly to conventional powder coatings without any modification of their terminal groups; after reacting with other functional groups in the coating resin, these hydroxyl groups confer new properties on the coating. Benthem synthesized hyperbranched polymers with a polyamide-ester structure using cyclocarboxylic anhydrides, namely hexahydronaphthalic anhydride or phthalic anhydride, and diisopropanolamine as raw materials. It is used as a curing agent for carboxyl-containing polyester powder coatings, participating in the cross-linking and curing reaction of the polyester powder coating film. When conventional powder coating cures, if volatiles such as water or ethanol are present during the curing and cross-linking process, bubbles will appear in thick coatings (when the coating thickness is greater than 100μm) once the gel point is reached. This indicates that such coatings have poor fluidity and surface optical properties, as well as a lower bubble formation threshold (less than 100μm). This is a common problem in solid powder coatings that affects the surface quality of the coatings. Using hyperbranched polyamide ester as a crosslinking agent resolved this issue; the paint film showed no bubbles even in coatings with thicknesses of 8–150 μm, the bubble formation threshold was significantly increased, and a smoother surface was obtained. Hong et al. modified the terminal groups of hyperbranched polyesters using caprolactone to obtain hydroxyl-containing, flexible long-chain terminal groups. When used as a modifier in cationic UV-curing systems with epoxy resin as the main film-forming component, it significantly improves the flexibility and impact resistance of the cured products. (Weihai Chenyuan Tree Branch Molecules)