Composition and Characteristics of Light-Curing Coatings
1. Composition of Light-Curing Coatings
Composition of light-curing coatings Despite the wide variety of light-curing coatings, performance varies, its main components generally include: photoinitiators, active diluents, oligomers and various types of additives. The raw materials of light-curing coatings will be described in detail in Chapter 3, here only on the composition and performance characteristics of light-curing coatings are briefly described.
2. Role and Classification of Photoinitiators in Light-Curing Coatings
Light-curing coatings in the photoinitiator is equivalent to ordinary coatings in the catalyst, light-curing coatings through the photoinitiator absorption of ultraviolet light and produce free radicals or cations, triggering oligomer and active diluent polymerization and cross-linking reaction, the formation of network structure of the coating film. Photoinitiators can be divided into two categories: free radical photoinitiators and cationic photoinitiators because of the different active intermediates produced. Free radical photoinitiators can be categorized into cleavage-type photoinitiators and hydrogen-capturing photoinitiators because of the different mechanisms of generating free radicals. Cleavage type photoinitiators are mostly aryl alkyl ketone compounds, mainly benzene and its derivatives, benzene and its derivatives, acetophenone and its derivatives, α-hydroxy alkyl acetophenone, α-amino alkyl acetophenone, acyl phosphine oxides, etc. The free radical photoinitiators are mainly aryl alkyl ketone compounds. Hydrogen-capturing photoinitiators include benzophenone or heterocyclic aromatic ketone compounds, mainly benzophenone and its derivatives, thioxanthones, anthraquinones, etc. The co-initiators with hydrogen-capturing photoinitiators are tertiary amine compounds, such as aliphatic tertiary amines, ethanolamine tertiary amines, tertiary amine benzoate, active amines, etc. Cationic photoinitiators are mainly aryl diazonium salts, diaryl iodocumene salts, triaryl sulfur paving salts, aryl iron paving salts and so on. In addition, hybrid photoinitiators, water-based photoinitiators, visible light photoinitiators and maleimide – vinyl ether photoinitiators are also new hot spots for photoinitiators. Although the proportion of photoinitiators in the coating formula is small (only 3%~5%), but the role is very critical.
3. Active Diluents in Light-Curing Coatings
Light-curing coatings in the active diluent is equivalent to ordinary coatings in the solvent, but it is in addition to the dilution effect, regulating the viscosity of the system, but also to participate in the light-curing reaction, affecting the rate of light-curing coatings and coating film mechanical properties, in the structure of the organic compounds it is a light-curing groups. Usually light-curing coatings in the oligomer determines the main properties of the cured film, but oligomer viscosity is often higher, some room temperature up to 10Pa-s, difficult to paint, the need for active diluents to adjust its viscosity. Selection of active diluent should take into account the following factors: low viscosity, low toxicity, low irritation, low volatility, low color, low volume shrinkage, high reactivity, intermiscibility with the resin and photoinitiator, high purity, high glass transition temperature of cured products, good thermal stability, inexpensive. The above indicators to reactivity and the performance of the cured product is the most prominent two factors, but if it is used in special emphasis on health and safety of paint formulations, diluent physiological irritation and toxicity should be given priority consideration, the screening criteria are also more stringent. Acrylate monomers are commonly used as active diluents because of their high reactivity, including monofunctional active diluents, difunctional active diluents and polyfunctional active diluents. By ethoxylated or propoxylated modified acrylic ester is the second generation of acrylic ester active diluent, it is to improve the first generation of acrylic ester active diluent exists skin irritation, toxicity and curing shrinkage of the disadvantages of the large, while still maintaining a faster rate of light curing. Vinyl ether is a new class of active diluents, it contains vinyl ether or propylene-based ether structure, high reactivity, can be used for free radical curing system, cationic curing system and free radical and cation hybrid system. Containing methoxy terminal group (methyl) acrylate is the third generation of active diluent, it has a monofunctional active diluent of low shrinkage and high conversion rate, but also has a high reactivity. In addition, the active diluent with special function not only participates in the light-curing reaction, but also has the function of improving the adhesion to the substrate (metal, plastic, etc.) and the rate of light curing, and improving the dispersion of pigment.
4. The Role and Selection of Oligomers in Light-Curing Coatings
Light-curing coatings in the oligomer equivalent to ordinary coatings in the resin, are film-forming materials, their performance on the performance of the coating plays a major role in the structure of the oligomer must have light-curing groups, such as unsaturated double bond or epoxy group, belonging to the light-sensitive resin. Among the components of light-curing coatings, oligomer is the main body of light-curing coatings, and its performance basically determines the main performance of the cured material, therefore, the synthesis and selection of oligomer is undoubtedly an important link in the design of light-curing coatings formulations. Free radical light-curing oligomer is mainly various types of acrylic resins, such as epoxy acrylic resin, polyurethane acrylic resin, polyester acrylic resin, polyether acrylic resin, acrylate resin and so on. Among them, epoxy acrylic resin and polyurethane acrylic resin are the most used in practice. Cationic light-curing coatings with oligomers with epoxy groups or vinyl ether groups, such as epoxy resin, vinyl ether resin. Light-curing coatings in the choice of oligomer to consider the following factors: low viscosity, fast light curing rate, good physical and mechanical properties, glass transition temperature, curing shrinkage, low toxicity and low irritation. Unsaturated polyester is generally used for light-curing wood coatings, low cost, almost comparable with traditional solvent-based coatings, with other varieties of oligomer prices continue to decrease, performance is much higher than the unsaturated polyester system, its market share has been gradually reduced. Epoxy acrylic resin synthesis is relatively easy, the price has been gradually approaching the level of unsaturated polyester, in the curing rate, curing film hardness, solvent resistance, corrosion resistance, tensile strength and adhesion properties of most substrates are excellent, with a high cost-effective, has become the preferred raw materials for light-curing coatings formulations. The defect of epoxy acrylate is that its curing product is hard and brittle, usually used in conjunction with isooctyl acrylate, the second generation of acrylate active diluent or flexible resin. Polyurethane acrylate according to its structural characteristics, both high hardness curing film can be obtained, but also to obtain a good flexibility of the coating, coatings are generally used on the soft section of the longer urethane acrylate structure, urethane acrylate is mainly characterized by the cured film has excellent flexibility, adhesion to most substrates as well as excellent corrosion resistance, but its overall cost of the synthesis of higher, in addition, the rate of light curing In addition, the light curing rate is slightly lower than that of epoxy acrylate. Polyurethane acrylates are usually mixed with epoxy acrylates and polyfunctional acrylate reactive diluents. Polyester acrylate viscosity is lower, the cost is also lower, light curing rate is general, the use of high-functionality polyester acrylate light curing rate can be improved, containing long-chain alkanes or chain segments of the polyester acrylate on the pigment wettability is good, can be used in the light-curing pigment paint. Polyester acrylate in light-curing coatings are less often used alone, often with epoxy acrylate, polyurethane acrylate and other commonly used main resin used in conjunction. Acrylic resin oligomer in the light-curing coatings industry is also less often used alone, because of its molecular weight, viscosity is higher, but contains acryloyloxy acrylic resin oligomer although the light-curing rate is not high, but the curing shrinkage rate is low, to improve the adhesion of the cured film is helpful, carboxy-modified acrylic resin oligomer can play a stabilizing effect of dispersion of pigments in the pigment coloring system, so the oligomer can be used as a functional auxiliary resin in the light-curing coating, and can be used with epoxy acrylate, polyurethane acrylate and other common body resins. Therefore, these oligomers can be used as functional auxiliary resins in light-curable coating formulations. In recent years, in order to meet the needs of special coatings, light-curing coatings, a number of new structures of oligomers have been gradually developed and applied, such as silicone acrylic resins, waterborne UV oligomers, hyperbranched oligomers, double curing oligomers, self-initiated oligomers, aliphatic and alicyclic epoxy acrylate, low viscosity oligomers, UV-curable powder coatings with oligomers, hybrid oligomers and so on.
5. Role and Considerations of Additives in Light-Curing Coatings
Additives in the coatings in the use of largely the same, all need to use pigments, fillers, a variety of additives, only light-curing coatings additives used to minimize the absorption of ultraviolet light, so as not to affect the light-curing reaction. Light-curing coatings often add inorganic fillers to reduce oligomer, active diluent photopolymerization caused by volume contraction, to improve adhesion is beneficial, and enhance the hardness of the cured film, abrasion resistance, heat resistance. In such light-curing floor coatings and other wear-resistant requirements of the occasion, often add talc, silica micropowder and other fillers, the addition of inorganic fillers may lead to the curing coating flexibility decreased, the coating flexibility requirements of higher occasions, should be used with caution. The addition of inorganic fillers often lead to a significant increase in coating viscosity, mixing and dispersion process produces a large number of bubbles is difficult to quickly eliminate themselves, adding defoamer is very necessary. At the same time, a certain amount of light-curing coatings must be added to the polymerization inhibitor to ensure the stability of light-curing coatings during production, storage, transportation and construction.