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How to efficiently select oligomers (resins) in UV coating formulations

Release time:

2014-05-30 16:55

How to efficiently select oligomers (resins) in UV coating formulations

The Role of Oligomers in UV Coatings

Oligomer: An oligomer used in UV-curable coatings, also known as a prepolymer. Earlier, it was sometimes translated as "oligomer." Key characteristics include: low molecular weight, distinctive polymerizable functional groups, and relatively high viscosity.

It serves as the main body and skeleton of light-curable coatings (affecting many physicochemical properties of the coating film).

Characteristics of UV-curing reactions

UV curing is an addition polymerization reaction involving 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 extensively is free-radical polymerization (and this lecture is entirely based on free-radical polymerization). The resulting C-C crosslinked structure is a rigid crosslink.

Polymerization mechanism

Free-radical polymerization is characterized by: rapid reaction; significant shrinkage; small variation in degree of polymerization; and substantial inhibition effects (as little as 0.01–0.1% inhibitor can effectively halt the reaction).

—The most detrimental effect on coatings is shrinkage. According to research by W.J. Bailey and others from abroad, the longer the time interval between the unreacted double bonds, the more pronounced the shrinkage once polymerization occurs. During polymerization, covalent bonds are formed, reducing the intermolecular spacing and causing volume contraction. The shrinkage of unsaturated polymerizable double bonds can reach as high as 11%.

The complexity of UV coating formulations

1. Many types of monomers

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

An Introduction to Commonly Used Resins in UV Coatings, Classified by Function

Hard resin—high Tg

High hardness, good chemical resistance, and most exhibit fast curing speeds.

1. Standard Bisphenol A-type EA;

2. High-functional PUA and low-molecular-weight 2fPUA;

3. High-functional-group amino acrylate;

4. Methacrylate oligomer.

Soft resin—low Tg

Good flexibility, low curing speed, and low crosslink density.

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

2. Long-chain polyester acrylates;

3. PUA with a straight-chain structure and a molecular weight exceeding 1200;

4. Partially pure acrylic oligomers

Polar resin

Oligomers containing active hydrogen or those readily forming hydrogen bonds, which alter polarity or surface tension.

1. Phosphate ester acrylate

2. Organosilicon Oligomers—Special

3. Carboxylate Acrylate Oligomers

Water-based UV oligomer

Emulsion type, water-dispersible type, water-soluble type

1. Polyurethane-based—primarily;

2. Epoxy acrylates;

3. Polyester acrylates.

Application of Non-Crosslinked Resins in UV Light

Auxiliary effects in areas such as filling, improving cross-linking density, enhancing adhesion, modifying flexibility, and boosting wettability.

1. Long-oil alkyd resin;

2. Thermoplastic acrylic resin;

3. Aldehyde-ketone resin;

4. Petroleum resins, etc.

How to Select Resins When Formulating UV Coatings

Before designing a coating formulation, you should clearly define:

1. Types of coatings in the paint application process—clearly distinguish between primer, topcoat, and tinted paint.

2. Understand the basic properties of the substrate material—such as polarity (surface tension), whether it is crystalline, and whether it is thermoplastic or thermoset.

Selection of Primer Resin

1. Adhesion requirements: This is a general characteristic of primer resins. Relatively speaking, the adhesion that currently poses greater challenges includes:

A. Glass—Select a combination of low-molecular-weight methacrylate oligomers, non-film-forming resins, and certain special polar resins—a thiol-siloxane system (though water resistance currently poses an obstacle to the formulation).

B, Metals: To distinguish among different types of metals, the coating industry generally employs destructive cross-linking methods to enhance adhesion to metals. The internationally accepted method is phosphating treatment. Currently, for UV coatings, the most common approach involves combining phosphate esters with certain pure acrylic formulations.

C, plastics (including plasticized paper and other types of painted surfaces)—this is currently one of the largest and most complex categories, primarily due to the intricate structure of plastics, their varied crystalline forms, and differing surface tensions. This makes materials like BMC, PET, and PP particularly challenging. There’s no single universal formulation that works for all of them. Generally speaking, using soft PUA, pure acrylics, certain non-film-forming resins, and polar resins can achieve good results. However, when considering chemical resistance and water resistance, it’s crucial to pay close attention to the proper selection and combination of resins.

D. Oil-containing wood products: Currently, hard woods such as klon wood, rosewood, greenheart, and mahogany—whose wood contains natural oils—tend to have relatively poor adhesion for oil-based coatings. As of now, there are very few commercially available cases where UV coatings alone have achieved satisfactory adhesion. Therefore, it’s advisable to first apply a PU sealer and then use a UV-adhesion primer. To enhance adhesion, it’s effective to employ polar resins, monomers, and filler resins.

2. Wettability: Wetting of pigments and wetting of the substrate are two distinct functions, as it cannot be guaranteed that the surface tension of the substrate will exactly match that of the pigments.

A. Wetting of pigment fillers can ensure the storage stability of coatings and the transparency exhibited by the compatibility of the paint film—for example, certain PUA, PEA, and epoxy soybean oil acrylates possess this effect.

B. It exhibits good wetting performance on substrates such as amino resins and PEA.

3. Flexibility: Relates to grindability and interlayer adhesion.

Typically, standard EAs, some PEA, and certain monomers are used in combination to optimize flexibility, thereby controlling sanding performance and interlayer adhesion.

The market currently also offers hardening primers that emphasize hardness—pay attention to the curing of the hard resin and the amount of coating applied; otherwise, the paint film may easily crack.

The market also demands so-called flexible primers—using more resins with good flexibility, preferably PUA based on polyesters. Polyether-based resins don't offer very good flexibility and lack sufficient mechanical modulus.

Selection of topcoat resin

1. Fullness, leveling

To meet this requirement, it is essential to select resin and monomer combinations with good compatibility, enhance wetting and leveling with the primer, appropriately increase the degree of crosslinking, and choose resins with higher refractive indices.

Generally, high-functional PUA and amino resins are selected, with standard EA serving as the main resin.

2. Toughness (hardness and wear resistance): These two coating film properties are closely related, but they are not necessarily identical and should be treated differently.

Hardness: In addition to traditional wood coatings that feature thick paint films ranging from 80 to 120 μm and certain thick spray applications, the hardness in these cases stems not only from the paint film itself but also—more significantly—from apparent hardness effects that deserve adequate attention. These include factors such as the substrate, primer, surface texture, and other aspects. A typical example is roller-applied or thin-spray coatings; in addition to using the high-gloss resins mentioned earlier, one can also incorporate silicone resins or silicone additives to further enhance hardness.

Wear resistance: Generally, PUA performs better than other materials, primarily because hydrogen bonds provide some toughness that enhances wear resistance. However, even with thin coatings, wear resistance cannot be effectively improved solely by the resin itself.

3. Interlayer adhesion

By properly addressing wetting and leveling as well as the compatibility of resin polarities, adhesion issues can be resolved. In special cases, you can opt for certain methacrylate resins.

4. Chemical resistance

EA and PUA (polyester-based) both exhibit good chemical resistance, whereas PE and polyether-based materials are somewhat less resistant.

5. Yellowing resistance

Currently, it is generally believed that aliphatic PUAs—pure polyether acrylates, pure acrylic esters, and amino-based polymers—all exhibit excellent resistance to yellowing. The first category is the most widely used, though its resistance to yellowing isn't the best. The latter two categories are less commonly employed due to certain shortcomings, but among them, the amino-based polymers offer the best overall performance in terms of resistance to yellowing.

6. Matte type

Currently, both resins with slightly lower molecular weights and those with much larger molecular weights are effective. Notably, certain polyurethanes are also highly effective (currently, a two-functional, high-hardness polyurethane available on the market is quite competitive).

Source: Yu Zongping

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