UV coating formulation: How to efficiently select oligomers (resins)

The role of oligomers in UV coatings

Oligomer: Oligomers, also known as prepolymers, are used in light-curing coatings. In earlier years, they were translated as oligomers. Their salient features are: small molecular weight, characteristic polymerization group, and high viscosity. Oligomers are the main body and skeleton of light-curing coatings (many of the physical and chemical properties of the coating film).

Characteristics of UV curing reaction

UV curing is an addition polymerization reaction between unsaturated molecules. According to the initiation mechanism of the initiator, there are free radical polymerization and cationic polymerization. However, the polymerization we have studied more is free radical polymerization (this lecture is based on free radical polymerization). This structure with C-C crosslinks at the end is a rigid crosslink.

Polymerization mechanism

Radical polymerization has the following characteristics: rapid reaction; large shrinkage; small change in degree of polymerization; and a high degree of influence from polymerization inhibitors (0.01-0.1% of the inhibitor can stop the reaction).

The most detrimental effect on coatings is shrinkage. According to research by W.J. Bailey and others, if the time between the unpolymerization of double bonds is long, and once polymerization occurs to form covalent bonds, the spacing is shortened, causing a decrease in volume. The shrinkage of all unsaturated polymerized double bonds is as high as 11%.

The complexity of UV coating formulations

1. There are many types of monomers

2. There are many types of basic oligomers (resins). Currently, they are classified according to the functional groups during synthesis: unsaturated polyester-based PE, epoxy-based EA, polyurethane-based PUA, polyester-based PEA, amino-based, polyether-based, silicone-based, phosphate-based, and hybrid types.

A brief introduction to the commonly used resins in UV coatings, classified according to their functions

Hard resins—high Tg

high hardness, good chemical resistance, and fast curing speed for most

1. Standard bisphenol A type EA;

2. High-function PUA and small molecular weight 2fPUA;

3. High-function amino acrylate;

4. Methacrylate oligomer.

Soft resin—small Tg

good flexibility, low curing speed, and low cross-linking density.

1. Modified epoxy – epoxy soybean oil acrylate, etc.;

2. Long-chain polyester acrylate;

3. PUA with a straight-chain structure and a mass fraction of more than 1200;

4. Some pure acrylate oligomers

Polar resins

Oligomers containing active hydrogen or easily forming hydrogen bonds, changing polarity or surface tension

1. Phosphate ester acrylate

2. Silicone oligomers – especially

3. carboxyl acrylate oligomers

Waterborne UV oligomers

Emulsion type, water dispersible type, water soluble type

1. polyurethane type – the main type;

2. epoxy acrylate type;

3. polyester acrylate type.

Application of non-crosslinking resins in UV

Filling effect, improving crosslink density, increasing adhesion, changing flexibility, enhancing wettability and other auxiliary effects

1. long-oil alkyd resins;

2. thermoplastic acrylate resins;

3. aldehyde-ketone resins;

4. petroleum resins, etc.

Designing a UV coating formula How to choose a resin

Before designing a coating formula, you should be clear about

1. the types of coatings in the coating process—identify primers, topcoats, and paints;

2. understand the basic properties of the material to be coated—polarity (surface tension), crystallinity, thermoplastic or thermosetting;

Selection of primer resin

1. Adhesion requirements: This is the general property of primer resins. Relatively speaking, the more difficult adhesion currently includes

A. glass – choose methacrylate oligomers and non-film-forming resins and some special polar resins – mercapto siloxane systems (but water resistance is currently a barrier to the formulation);

B. Metal: Distinguish between the types of metal. In the paint industry, the basic method for improving adhesion to metal is to destroy the cross-links. The international standard is phosphating. The most common UV method is to combine phosphates with some acrylics.

C. Plastics (including plasticized paper and other types of paint finishes), which is currently a relatively large and particularly complex category, mainly because of the complex structure of plastics, various crystalline forms, and different surface tensions, which makes it relatively difficult to work with plastics such as BMC, PET, and PP. There is no unified formula that can be used for all. Generally speaking, a combination of soft PUA, pure acrylic and some non-film-forming resins and polar resins can have a certain effect. However, attention should be paid to the chemical and water resistance when matching the relevant resins.

D. Oily wood products: Currently, it is mainly some hard sandalwood such as kronox, red sandalwood, green-stemmed mulberry, and okapi wood, etc. The adhesion of wood oil is relatively difficult. There are few cases of pure UV on the market that can seal the oil. You can first seal it with PU and then apply a UV adhesion primer. The adhesion can be achieved mainly by using some polar resins or monomers and filler resins.

2. Wetting: Wetting of the pigment and filler and wetting of the substrate are two different functions, as it cannot be guaranteed that the surface tension of the substrate is exactly the same as that of the pigment and filler.

A. Wetting of the pigment and filler can ensure the storage stability of the paint and the transparency of the film compatibility, for example, some PUAs, PEAs and epoxy soybean oil acrylates have this effect;

B. Wetting of the substrate, such as amino resins and PEA, has a better effect.

3. Flexibility: related to sandability and interlayer adhesion.

Generally, standard EA and some PEA and some monomers are used to coordinate flexibility, thereby adjusting sandability and interlayer adhesion.

Currently, the market also has hardening primers that emphasize hardness—pay attention to the curing of the hard resin and the amount of coating applied, otherwise the paint film is likely to burst;

The market also requires so-called flexible primers – use more flexible resins, preferably polyester-based PUAs. Polyether-based resins are not very flexible and have insufficient mechanical modulus.

Selection of topcoat resins

1. fullness and leveling

To achieve this, you must choose a resin and monomer with good compatibility, improve wetting and leveling with the primer, appropriately increase the degree of cross-linking, and use a resin with a higher refractive index.

Generally, high-functional PUAs and amino resins are used, with standard EA as the main resin.

2. Toughness (hardness and wear resistance):

These two film properties are closely related, but not necessarily identical.

Hardness: Apart from the traditional woodwork with a thick film of 80-120Unm and some thick spray coatings, a large part of the hardness in this case comes not from the film itself, but from other sources. Some of this apparent hardness should be given sufficient attention, such as the substrate, primer, surface feel, etc. A typical example is roller finish and thin spray, which can be improved by using some silicone resins or silicone additives in addition to the aforementioned high-ranking resins.

Wear resistance: Generally, PUA is better than other resins. Hydrogen bonds provide some toughness to increase wear resistance. However, the wear resistance of thin coatings cannot be solved by resins.

3. Interlayer adhesion

Solve the problems of wetting, leveling and resin polarity matching, and interlayer adhesion can be solved. In special cases, some methacrylate resins can be selected.

4. Chemical resistance

EA and PUA (polyester) have good chemical resistance, while PE and polyether are less so

5, yellowing resistance

Generally, it is currently believed that aliphatic PUA, pure polyether acrylate, pure acrylic, and amino groups all have very good yellowing resistance. The first type is the most commonly used, but the yellowing resistance is not the best. The latter two types are less commonly used because they lack certain properties, but the amino group has the best overall yellowing resistance.

6. Matte type

Currently, some resins with a slightly lower molecular weight or even huge ones are effective, and some polyurethanes are also very effective (currently, there is a two-functional polyurethane with good hardness on the market that is competitive).