Overview of the composition of UV curable coatings

The main components of UV curable coatings generally include oligomers, photoinitiators, active diluents, and additives. The following briefly describes the composition and performance characteristics of UV curable coatings.

The oligomers in UV curable coatings are equivalent to resins in ordinary coatings, and they are film-forming substances that play a major role in the performance of coatings.

Structurally, oligomers must have photocurable groups, such as unsaturated double bonds or epoxy groups, and belong to photosensitive resins. The selection of oligomers is an important step in the formulation design of UV curable coatings.

The oligomers used in free radical photocurable coatings are mainly various acrylic resins, such as epoxy acrylic resin, polyurethane acrylic resin, polyester acrylic resin, polyether acrylic resin, acrylic ester resin, etc. The most commonly used ones are epoxy acrylic resin and polyurethane acrylic resin.

The oligomers used in cationic photocurable coatings have epoxy or vinyl ether groups, such as epoxy resin and vinyl ether resin.

The selection of oligomers in Cationic UV coating should comprehensively consider the following factors: low viscosity, fast UV curing rate, good physical and mechanical properties, glass transition temperature, curing shrinkage rate, low toxicity and low irritation.

Unsaturated polyester is generally used for UV cured wood coatings, with lower costs and almost equivalent to traditional solvent based coatings. With the continuous decrease in prices of other types of oligomers, their performance is much higher than that of unsaturated polyester systems, and their market share has gradually decreased.

The synthesis of epoxy acrylate is relatively easy, and its price has gradually approached the level of unsaturated polyester. It has excellent performance in curing rate, curing film hardness, solvent resistance, corrosion resistance, tensile strength, and adhesion performance to most substrates. It has a high cost performance ratio and has become the preferred raw material for current UV curable coating formulations.

The defect of epoxy acrylate is that its curing product is hard and brittle, usually used in combination with isooctyl acrylate, new active diluents of acrylate, or resins with good flexibility.

Polyurethane acrylate can obtain both high hardness curing films and coatings with good flexibility based on its structural characteristics. Generally, polyurethane acrylate structures with longer soft segments are used on coatings. The main characteristic of polyurethane acrylate is that the curing film has excellent flexibility, adhesion to most substrates, and corrosion resistance. However, its overall synthesis cost is high. In addition, the UV curing rate is slightly lower than that of epoxy acrylate.

Polyurethane acrylate is usually mixed with epoxy acrylate and multifunctional acrylate active diluents.

Polyester acrylate has low viscosity, low cost, and average UV curing rate. The use of high functional polyester acrylate can improve the UV curing rate. Polyester acrylate containing long-chain alkanes or segments has good wetting properties for pigments and can be used in UV cured paints. Polyester acrylate is less commonly used alone in UV curable coatings and is often used in combination with commonly used main resins such as epoxy acrylate and polyurethane acrylate.

Alicyclic epoxy resin oligomers are also less commonly used alone in the UV curing coating industry due to their large molecular weight and high viscosity. However, although acrylic resin oligomers containing acryloxy groups have a low UV curing rate, their curing shrinkage rate is low, which helps improve the adhesion of curing films. Carboxyl modified acrylic resin oligomers can play a stabilizing and dispersing role in pigment coloring systems. Therefore, these oligomers can be used as functional auxiliary resins in UV curing coating formulations as appropriate.

In recent years, in order to meet the needs of special coatings, some new structured oligomers have gradually been developed and applied, such as organosilicon acrylic resin, waterborne UV oligomers, hyperbranched oligomers, double cured oligomers, self initiated oligomers, aliphatic and alicyclic epoxy acrylates, low viscosity oligomers, oligomers for UV cured powder coatings, hybrid oligomers, etc.

The photoinitiator in photocurable coatings is equivalent to the catalyst in ordinary coatings. Photocurable coatings absorb ultraviolet light through the photoinitiator and produce free radicals or cations, which trigger polymerization and cross-linking reactions between oligomers and active diluents, forming a coating film with a network structure. Although the proportion of photoinitiators in the formulation of UV curable coatings is small (only 3% -5%), their role is crucial.

Photoinitiators can be divided into two types: free radical photoinitiators and cationic photoinitiators due to the different active intermediates produced.

Free radical photoinitiators can be divided into two types: cracking photoinitiators and hydrogen withdrawing photoinitiators due to their different mechanisms of generating free radicals.

Cracking free radical photoinitiators are mostly aromatic alkyl ketone compounds, mainly including benzoin and its derivatives, benzoyl and its derivatives, acetophenone and its derivatives, alpha hydroxyalkylacetophenone, alpha aminoalkylacetophenone, acyl phosphine oxides, etc.

Hydrogen withdrawing photoinitiators include benzophenone or heterocyclic aromatic ketone compounds, mainly including benzophenone and its derivatives, thioanthraquinone, anthraquinone, etc.

The co initiators used in combination with hydrogen withdrawing photoinitiators are tertiary amine compounds, such as aliphatic tertiary amines, ethanolamine tertiary amines, tertiary amine benzoates, active amines, etc.

Cationic photoinitiators mainly include aryl diazonium salts, diaryl iodonium salts, triaryl thioonium salts, aryl iron onium salts, etc. In addition, hybrid photoinitiators, water-based photoinitiators, visible light photoinitiators, and maleimide vinyl ether photoinitiators are also new research hotspots in photoinitiators.

The active diluent in UV cured coatings is equivalent to a solvent in ordinary coatings, but it not only has a diluting effect and adjusts the viscosity of the system, but also participates in UV curing reactions, affecting the UV curing rate of the coating and the mechanical properties of the coating film. Structurally, it is an organic compound with UV curing groups.

Usually, oligomers in UV curable coatings determine the main properties of the curing film, but the viscosity of oligomers is often high, with some reaching up to 10Pa · s at room temperature, making it difficult to apply and requiring active diluents to adjust their viscosity. The selection of active diluents should comprehensively consider the following factors: low viscosity, low toxicity, low irritation, low volatility, low volume shrinkage, high reactivity, compatibility with resins and photoinitiators, high purity, high glass transition temperature of cured products, good thermal stability, and affordable price.

The two most prominent factors among the above indicators are reaction activity and the performance of cured products. However, if used in coating formulations that emphasize hygiene and safety, the physiological irritancy and toxicity of diluents should be given special consideration, and the screening criteria should be stricter.

Acrylate monomers are widely used as active diluents due to their high reactivity, including monofunctional active diluents, bifunctional active diluents, and multifunctional active diluents.

Acrylates modified by ethoxylation or propionation are a new type of acrylic active diluent. They are designed to improve the skin irritation, high toxicity, and high curing shrinkage of the first generation of acrylic active diluents, while still maintaining a fast curing rate.

Vinyl ethers are a new type of active diluents that contain ethylene or propylene ether structures, have high reactivity, and can be used in free radical curing systems, cationic curing systems, and free radical and cationic hybrid systems.

(Methacrylate) containing methoxy end groups is a third-generation active diluent, which not only has low shrinkage and high conversion rate as a monofunctional active diluent, but also has high reactivity.

In addition, active diluents with special functions not only participate in UV curing reactions, but also have functions such as improving adhesion and UV curing rate to substrates (metals, plastics, etc.), and improving pigment dispersion.

The additives used in UV cured coatings are generally the same as those used in ordinary coatings, requiring pigments, fillers, and various additives. However, the additives used in UV cured coatings should minimize their absorption of ultraviolet light to avoid affecting the progress of the UV curing reaction.

Inorganic fillers are often added to UV curable coatings to reduce volume shrinkage caused by oligomers and active diluents photopolymerization, which is beneficial for improving adhesion and enhancing the hardness, wear resistance, heat resistance, etc. of the cured film.

In situations where wear resistance is required, such as UV curable floor coatings, fillers such as talcum powder and silica fume are often added. The addition of inorganic fillers may lead to a decrease in the flexibility of the cured coating. In situations where high requirements for coating flexibility are required, caution should be exercised when using them. The addition of inorganic fillers often leads to a significant increase in the viscosity of coatings, and the large number of bubbles generated during the stirring and dispersion process are difficult to quickly eliminate on their own. It is necessary to add defoamers.

At the same time, a certain amount of polymerization inhibitor must be added to the UV cured coating to ensure the stability of the coating during production, storage, transportation, and construction.