From the perspective of further improving efficiency, low-temperature curing powder coatings can significantly shorten curing time and improve production efficiency while maintaining a constant temperature. Therefore, achieving low-temperature curing of powder coatings has become one of the development directions in the powder coating industry.
For a long time, thanks to the efforts of colleagues engaged in powder coatings and related supporting fields such as raw materials and equipment, considerable progress has been made in low-temperature curing of powder coatings. Currently, pure epoxy system textured products can achieve curing at 130°C/15 minutes. Flat high-gloss epoxy systems can achieve curing at 140°C/15 minutes. However, epoxy has poor yellowing resistance and weather resistance and cannot be used outdoors. Polyester/epoxy hybrid systems, which have better yellowing resistance than epoxy systems, can achieve curing at 135°C/15 minutes for textured products, and 150°C/15 minutes for flat high-gloss products (with slightly poorer leveling). Pure polyester system textured products can achieve curing at 140°C/15 minutes, and flat high-gloss products can achieve curing at 160°C/15 minutes. However, achieving low-gloss flat products with pure polyester systems is still very difficult.
Achieving ultra-low-temperature curing of powder coatings presents significant technical challenges. First, the curing system of powder coatings is a low-temperature latent curing system. If this system has high reactivity at low temperatures, it will inevitably affect the powder manufacturing extrusion and storage stability. On the other hand, the resins and curing agents used in powder coatings are solids with high softening points. At low temperatures, the melt viscosity is high, making it difficult for the coating film to level during low-temperature curing, affecting the surface finish. While using resins and curing agents with lower softening points can reduce melt viscosity, it worsens the storage stability of the powder, requiring low-temperature refrigeration, which causes many inconveniences in use. Therefore, finding a resin and curing agent that is suitable for powder storage, has a high softening point, and has appropriate reactivity is key to preparing low-temperature curing powder coatings.
Development trends of low-temperature curing powder coatings
To lower the curing temperature of powder coatings, the following aspects will be discussed: main resin, curing agent, catalyst, infrared curing, ultraviolet curing, powder coating manufacturing methods using spray drying, and free radical reaction curing.
Powder coatings, whether thermoplastic or thermosetting, have a film-forming temperature of around 180-200°C and a long curing time (10-20 minutes). This limits their use to heat-resistant substrates such as metals, and the process is time-consuming and relatively energy-intensive. Lowering the curing temperature by 10°C saves approximately 10% in energy. To save energy, reduce costs, and expand the application range of powder coatings, and to better integrate with solvent-based coating lines, powder coatings must develop towards low-temperature curing. Low-temperature curing of powder coatings can be achieved through various means, including lowering the melting temperature, viscosity, and softening point of the resin itself, increasing the degree of crosslinking by increasing the functional groups of the resin, adding appropriate additives, and using appropriate catalysts. Lowering the curing temperature of powder coatings not only accelerates the production speed of automated production lines and improves production efficiency, saving energy, but also greatly expands the application range of powder coatings.
The key to determining the performance of powder coatings is the base resin. To achieve low-temperature curing powder coatings, unsaturated polyester, unsaturated acrylate resin, polyurethane acrylate resin, and vinyl ether resin types have been developed. Allnex and DSM both offer corresponding products. Unsaturated resins are the main film-forming substances in UV-curing or free-radical thermal-curing powder coatings and are the main components that determine the properties of the coating and the performance of the film. To achieve low-temperature curing, the resin must provide good storage stability to the powder, allowing it to be stored at 40°C for 3-6 months without caking; on the other hand, the raw materials used must have a low melt viscosity at lower temperatures (such as 100°C or lower) to ensure good flowability of the coating during the curing process. This requires that the glass transition temperature (Tg) of the selected resin should be between 50 and 70°C (at least above 40°C), with an average molecular weight of 1000-4000, and a narrow molecular weight distribution. Obtaining such a resin is not easy. Controlling the melting of resins with a Tg higher than 50°C is difficult because the C=C double bond can start to polymerize at 80°C, while below 80°C, the viscosity is too high to handle. Common methods to lower the resin melting temperature include synthesizing semi-crystalline resins, adding crystalline compounds, or amorphous oligomers. Synthesizing dendritic and hyperbranched semi-crystalline polymers through polymer structure design to prepare low-temperature curing unsaturated resins is also a feasible method.
From a physicochemical perspective, the rate of chemical reactions can be expressed using the Arrhenius equation: k = Aexp(-Ea/RT) (exponential form). k is the rate constant, R is the molar gas constant, T is the thermodynamic temperature, Ea is the apparent activation energy, and A is the pre-exponential factor (also called the frequency factor). Another common form is: lnk = lnA - Ea/RT (logarithmic form). To increase the reaction rate at low-temperature curing, the chemical reaction mechanism can be analyzed. It can be seen that Ea, the apparent activation energy, is a very important factor. Activation energy refers to the minimum energy required for reactant molecules to reach the activated state in a chemical reaction. The rate of a chemical reaction is closely related to the magnitude of its activation energy; the lower the activation energy, the faster the reaction rate. Therefore, lowering the activation energy effectively promotes the reaction. Promoters accelerate some inherently slow chemical reactions by lowering the activation energy (actually by changing the reaction pathway).
To improve the effectiveness of accelerators in promoting chemical reactions, solid compounds with good compatibility with the resin and low melting points (80–120°C) can be added during powder preparation via melt co-extrusion, as this method achieves good dispersion. For solid or liquid compounds with poorer compatibility and higher melting points, the so-called masterbatch method is commonly used, where they are pre-mixed into a molten carrier (such as epoxy resin or polyester resin) for molecular dispersion. This clearly helps in achieving uniform mixing of the additives. The choice of accelerator depends on the nature of the crosslinking and curing system. Dicyandiamide-cured epoxy systems are catalyzed by imidazole, imidazoline, amidine, and BF3 complexes; epoxy/polyester mixed systems and polyester/TGIC systems use imidazole, imidazoline, quaternary ammonium, quaternary phosphonium, and amidine compounds; and polyurethane (PU) systems use organotin compounds such as dibutyltin dilaurate, tin octoate, and dibutyltin oxide.
Imidazole, 2-methylimidazole, 2-phenylimidazoline, 2-isopropylimidazole, 2-propylimidazole, and a few containing long-chain substituents such as undecyl or heptadecyl groups are mainly used as reaction accelerators or catalysts. Imidazole-based curing agents are a class of highly active curing agents that can cure epoxy resins in a short time at moderate temperatures. Therefore, single-component systems composed of these curing agents and epoxy resins have a short shelf life. Chemical modification is necessary, introducing larger substituents into the molecule to form sterically hindered imidazole derivatives, or reacting with inorganic salts of transition metals such as Cu, Ni, Co, and Zn to form corresponding imidazole salt complexes, in order to become latent curing agents with a certain shelf life at room temperature. Research on imidazole-based latent curing agents is relatively limited in China, while the international market has more products available. Japan's Daiichi Kogyo Seiyaku Co., Ltd. produces blocked products by reacting various imidazoles with toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and hexamethylene diisocyanate (HDI). This weakens the activity of the amino group on the imidazole ring, resulting in a longer shelf life. When the temperature rises above 100°C, the blocking effect is released, and the imidazole regains its activity to cure the epoxy resin.
Lewis acid-amine complexes are a class of effective latent curing agents for epoxy resins, formed by the complexation of Lewis acids such as BF3, AlCl3, ZnCl2, and PF3 with primary or secondary amines. As curing agents for epoxy resins, these complexes are quite stable at room temperature, but they rapidly cure epoxy resins at 120°C. Boron trifluoride-amine complexes are the most extensively studied among them.
Microencapsulated epoxy resin latent curing agents utilize a physical method to encapsulate room-temperature two-component curing agents with a fine oil droplet film, forming microcapsules that temporarily block the curing activity of the curing agent. Heating or pressurizing the capsules causes them to rupture, releasing the curing agent and thus curing the epoxy resin. The film-forming agents for microencapsulated epoxy resin latent curing agents include cellulose, gelatin, polyvinyl alcohol, and polyesters. Due to the strict requirements of the preparation process, the thickness of the capsule membrane can have varying degrees of impact on storage, transportation, and use.
Saves time and space.
UV-curing powder coatings (hereinafter referred to as UV powder coatings) are a new technology that combines traditional powder coatings with UV curing technology. The photocuring mechanism of UV powder coatings involves two types: free radical-initiated polymerization and cationic-initiated polymerization, each with its own advantages and disadvantages. The advantages of free radical-initiated polymerization are that water does not inhibit the polymerization and the curing speed is fast; the disadvantages are significant shrinkage and oxygen inhibition of the reaction. The advantages of cationic-initiated polymerization are minimal shrinkage and no oxygen inhibition; the disadvantages are that water inhibits the reaction, the curing time is long, and the molecular weight increases slowly. Photopolymerization of solid bisphenol A epoxy resins and vinyl ether resins can be achieved through cationic polymerization, but currently, most UV powder coatings still use free radical polymerization for photocuring, such as methacrylate polyester systems, unsaturated polyesters, and polyurethane acrylate systems. The biggest characteristic of UV-curing powder coatings is that the process is divided into two distinct stages: the coating does not undergo early resin curing during the melting and leveling stage, thus providing sufficient time for the coating to fully level and degas. UV curing significantly reduces the temperature of the heating and curing process (120–140°C), avoiding excessive heating of the substrate and opening up broader application areas for powder coatings, such as wood, plastics, paper, heat-sensitive alloys, and metal components containing heat-sensitive parts. However, the variety of UV-curing powder coatings is limited because:
Some organic pigments are not resistant to direct UV light irradiation, or opaque coloring pigments absorb UV light, resulting in poor film curing;
The deeper layers of the coating are difficult to cure, especially if the shape and structure of the coated object are complex, preventing direct and uniform UV light irradiation.
Unsaturated resin thermosetting powder coatings are generally composed of unsaturated resins, thermal initiators, leveling agents, fillers, and pigments. The curing mechanism of this type of unsaturated resin involves the thermal initiator decomposing to produce free radicals in the heated molten state. During the free radical polymerization process, growing chain free radicals abstract an atom from other molecules, terminating to form stable macromolecules, and the molecule that loses the atom becomes a new free radical, which then initiates new chain growth of the unsaturated double bond, allowing the polymerization reaction to continue. The resin undergoes a self-crosslinking curing reaction under the action of free radicals. The density of active double bonds in the resin, the decomposition temperature and amount of the thermal initiator all have a significant impact on the preparation and performance of powder coatings, and are fundamental and critical factors in powder coating formulation design.
Spray drying is a method for producing powder coatings by atomizing the powder coating slurry and contacting it with hot air to rapidly vaporize the water, resulting in a uniformly atomized powder coating with a uniform droplet size distribution. The supercritical fluid method VAMP (Vedoc Advanced Manufacturing Process) developed by Ferro Corporation involves adding the various components of the powder coating to a high-pressure reactor with mixing blades. Carbon dioxide is then introduced into the reactor to a supercritical state. The supercritical carbon dioxide fluidizes and mixes the various components of the coating to a uniform state, and then the mixture is sprayed through a nozzle to obtain a product with the desired particle size. The advantage of this process is that it eliminates the melting and extrusion mixing step, preventing gelation, expanding the range of applications, and allowing the use of raw materials that were previously difficult to use.
Additives are used in small quantities in powder coating formulations, but their role is crucial. Commonly used additives include leveling agents, degassing agents, matting agents, waxes, and edge coverage modifiers. These additives must remain stable in the powder coating to perform their intended functions. Therefore, the additives used must have good compatibility with resins such as epoxy, polyester, and acrylic resins. In the preparation of low-temperature curing powder coatings, the main function of leveling agents is to reduce the surface tension of the molten powder coating, allowing the coating to level quickly before curing and film formation, thus preventing surface defects such as orange peel and pinholes. Therefore, to ensure that a small amount of leveling agent is fully effective, the leveling agent in the formulation must be thoroughly and uniformly dispersed. Pre-dispersing the leveling agent into the resin carrier results in better dispersion and is more conducive to its function during the low-temperature melting and curing process.
In the preparation of low-temperature curing powder coatings, the main function of leveling agents is to reduce the surface tension of the molten powder coating, allowing the coating to level quickly before curing and film formation, thus preventing surface defects such as orange peel and pinholes. Therefore, to ensure that a small amount of leveling agent is fully effective, the leveling agent in the formulation must be thoroughly and uniformly dispersed. Pre-dispersing the leveling agent into the resin carrier results in better dispersion and is more conducive to its function during the low-temperature melting and curing process.
The purpose of using degassing agents such as benzoin and wax powder is to reduce or eliminate bubbles. During the low-temperature curing process, these agents quickly release bubbles from the coating, preventing surface defects such as pinholes. Proper selection of degassing agents is crucial; low-melting point or low-viscosity degassing aids are more effective in releasing bubbles from the coating.
Conventional matting agents are ineffective or show poor matting effects in low-temperature curing powder coating formulations, and the matting effect is unstable. Further research and development are needed to find suitable matting agents or additives that can provide matting effects at low temperatures.
When designing formulations for low-temperature curing powder coatings, due to the demanding production process conditions such as low-temperature extrusion and high-speed shearing, the selection of additives must be compatible with the production process. For example, low-temperature extrusion gelation can lead to formulation failure.