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Analysis of the Types and Selection of Oligomers in Light-Curing Coatings

Release time:

2024-05-03 17:14

Analysis of the Types and Selection of Oligomers in Light-Curing Coatings
Analysis of the Types and Selection of Oligomers in Light-Curing Coatings

Light-curing coatings, a special type of coating, have increasingly gained favor in the market due to their environmental friendliness, efficiency, and superior performance. In recent years, they have demonstrated unique advantages and broad application prospects across multiple fields. With their high curing speed, exceptional hardness and abrasion resistance, as well as low volatile organic compound (VOC) emissions, these coatings are gradually becoming the new favorite in the coatings industry.

Light-curing coatings are primarily composed of the following key ingredients:

(1) Photopolymer resin (oligomer); (2) Photoinitiator; (3) Diluent; (4) Additive.

Among these components, oligomers constitute the main body of UV-curable coatings. Selecting the right oligomer is crucial, as it determines the key properties of the coating after curing, such as hardness, flexibility, adhesion, optical performance, and resistance to aging.

I. Selection of Oligomers

The oligomers in UV-curable coatings perform a function similar to that of resins in conventional coatings—they are both crucial film-forming components, and their properties have a decisive impact on the overall performance of the coating. Structurally, these oligomers must possess groups capable of undergoing UV-induced curing, such as unsaturated double bonds or epoxy groups; both of these belong to the category of photosensitive resins. Among all the components of UV-curable coatings, oligomers occupy a central position, and their characteristics largely determine the key performance attributes of the coating after curing. Therefore, the synthesis and selection of oligomers are critical to the formulation design of UV-curable coatings.

The oligomers commonly used in free-radical photopolymerizable coatings mainly include various acrylic resins, such as: (1) epoxy acrylate resin; (2) polyurethane acrylate resin; (3) polyester acrylate resin; (4) polyether acrylate resin; and (5) acrylate resin, among others.

In practical applications, epoxy acrylate resins and polyurethane acrylate resins are the most widely used. As for cationic UV-curable coatings, the commonly used oligomers include epoxy resins and vinyl ether resins.

When selecting an appropriate oligomer for UV-curable coatings, it is essential to comprehensively consider several factors: (1) viscosity; (2) UV curing speed; (3) excellent physical and mechanical properties; (4) glass transition temperature; (5) curing shrinkage rate; and (6) low toxicity and low irritation.

II. Types and Characteristics of Oligomers

1. Unsaturated polyester

A commonly used oligomer in UV-curable wood coatings has relatively low costs and is similar in price to traditional solvent-based coatings. However, as the prices of other types of oligomers continue to decline—and their performance far surpasses that of unsaturated polyester systems—the market share of this particular oligomer has been gradually shrinking.

2. Epoxy acrylate

Propylene glycol acrylate is produced by ring-opening esterification of epoxy resins with acrylic acid or methacrylic acid, and it is relatively easy to synthesize. It exhibits excellent adhesion and chemical resistance, making it the first-choice raw material for UV-curable coating formulations today. It is widely used in various coatings and inks. However, its cured products tend to be hard and brittle. To address this issue, it is commonly combined with 2-ethylhexyl acrylate, second-generation acrylate reactive diluents, or resins that offer better flexibility.

3. Polyester acrylate

Polyester acrylates are produced by the acrylation of low-molecular-weight polyester polyols. These oligomers provide excellent flexibility and durability in coatings, and they also feature low viscosity and cost, making them highly cost-effective in coating formulations. Although their photocuring rate is generally moderate, this rate can be significantly improved by using high-functional polyester acrylates. Meanwhile, polyester acrylates containing long-chain alkyl or long-chain segments exhibit excellent wetting properties toward pigments, making them particularly suitable for use in UV-curable tinted coatings. However, polyester acrylates are seldom used alone in UV-curable coatings; instead, they are typically combined with commonly used base resins such as epoxy acrylates and polyurethane acrylates to achieve optimal coating performance.

4. Polyurethane acrylate

Polyurethane acrylates combine the high abrasion resistance and toughness of polyurethanes with the excellent optical properties and weather resistance of polyacrylates. Depending on their structural characteristics, they can produce either highly rigid cured films or coatings with good flexibility; in coatings, polyurethane acrylates with longer soft segments are typically employed.

A notable feature of polyurethane acrylates is that their cured films exhibit exceptional flexibility while also demonstrating excellent adhesion to a wide variety of substrates and outstanding corrosion resistance. However, they tend to be relatively more expensive and have a slightly slower photopolymerization rate compared to epoxy acrylates. To enhance their performance, polyurethane acrylates are often blended with epoxy acrylates and multifunctional acrylate reactive diluents. They are commonly used in high-performance coating applications, such as surface coatings for automobiles, furniture, and marine vessels.

5. Acrylic resin

The carbon-carbon unsaturated double bonds contained in these resins are capable of undergoing UV-curing reactions. However, in the photopolymerizable coatings industry, they are seldom used alone—mainly because their relatively high molecular weight results in higher viscosity. Nevertheless, acrylic oligomers containing acryloyloxy groups, although not curing as rapidly under UV irradiation, exhibit lower shrinkage during curing, which helps enhance the adhesion of the cured film to the substrate. Moreover, these resins also possess excellent weather resistance and gloss, making them frequently employed in outdoor coatings and decorative coatings.

6. Other special oligomers

In recent years, to meet the diverse demands for specialized coatings, the field of UV-curable coatings has continuously developed and introduced a variety of novel oligomers with innovative structures. These new oligomers include, but are not limited to, organosilicone acrylate resins, waterborne UV oligomers, hyperbranched oligomers, dual-cure oligomers, oligomers with self-initiating functionality, aliphatic and cycloaliphatic epoxy acrylates, low-viscosity oligomers, oligomers specifically designed for UV-curable powder coatings, and hybrid oligomers. The introduction of these innovative materials has greatly expanded the application scope and enhanced the performance characteristics of UV-curable coatings.

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