Oligomers in Light-Curable Coatings: Properties, Selection Principles, and Performance Characteristics
The oligomer for light-curable coatings is a light-sensitive resin with a relatively low relative molecular mass, and has groups that can undergo light-curing reactions, such as various types of unsaturated double bonds or epoxy groups, etc. Among the components in light-curable coatings, oligomer is the main body of light-curable coatings, and its performance basically determines the main properties of the cured material. Among the components in light-curable coatings, oligomer is the main body of light-curable 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 formulation design of light-curable coatings. Free radical light-curing coatings with oligomers are “c-C” unsaturated double bond resins, such as acryloyloxy (CH2=CH-COO-), methacryloyloxy (CH2=C(CH3)-COO-), vinyl (c-d), allyl (CH2=CH-CH2-) and so on. According to the order of free radical polymerization reaction rate: acryloyloxy>methacryloyloxy>vinyl>allyl. Therefore, free radical light curing oligomer is mainly various types of acrylic resins, such as epoxy acrylic resins, polyurethane acrylic resins, polyester acrylic resins, polyether acrylic resins, acrylic ester acrylic resins or vinyl resins. Among them, the most used are epoxy acrylic resin and polyurethane acrylic resin.
| Oligomers | Curing rate | Tensile strength | Flexibility | Hardness | Chemical Resistance | Yellowing resistance |
| Epoxy acrylic resin (EA) | High | High | No good | high | Excellent | Medium |
| Polyurethane acrylic resin (PUA) | Adjustable | Adjustable | Good | Adjustable | Good | Adjustable |
| Polyester acrylic resin (PEA) | Adjustable | Medium | Adjustable | Medium | Good | Not good |
| Polyether acrylic resin | Adjustable | Adjustable | good | low | Not good | good |
| Pure acrylic resin | Slow | low | good | low | good | Very good |
| Vinyl Resin (UPE) | Slow | high | No good | high | Not good | Not good |
Properties of commonly used oligomers
Principle of selecting oligomer
The following factors should be considered for the selection of oligomers in light-curing coatings.
- Viscosity
Selection of low viscosity resin, can reduce the amount of active diluent, but low viscosity resin tends to be low relative molecular mass, will affect the physical and mechanical properties of the film.
- Light curing rate
Selection of light curing rate of resin is a very important condition, not only can reduce the amount of photoinitiator, but also to meet the requirements of light-curing coating production line fast curing. Generally speaking, the higher the functionality, the faster the light curing rate, epoxy acrylate light curing rate is fast, amine modified oligomer light curing rate is also fast.
- Physical and mechanical properties
The physical and mechanical properties of light-curing paint film is mainly determined by the performance of the oligomer curing film, and different varieties of light-curing coatings and its physical and mechanical properties of different requirements, the choice of oligomer is also different. The physical and mechanical properties of the paint film are mainly the following.
① hardness: epoxy acrylate and unsaturated polyester general hardness; oligomer containing benzene ring structure is also conducive to improving the hardness; high functionality, high crosslinking density, high Tg, high hardness.
② flexibility: polyurethane acrylic resin, polyester acrylic resin, polyether acrylic resin and pure acrylic ester are generally better flexibility; oligomer contains aliphatic long carbon chain structure, good flexibility; the larger the relative molecular mass, the better the flexibility; crosslinking density is low, the flexibility becomes good; low Tg, good flexibility.
③ Wear resistance: polyurethane acrylic resin has good wear resistance; oligomer intermolecular hydrogen bonding is easy to form, good wear resistance; crosslinking density is high, good wear resistance.
④ Tensile strength: epoxy acrylate and unsaturated polyester have high tensile strength, generally larger relative molecular mass, greater polarity, less flexibility and crosslinking degree of oligomers have higher tensile strength.
⑤ Impact resistance: polyurethane acrylic resin, polyester acrylic resin, polyether acrylic resin and pure acrylic ester have better impact resistance; low Tg,, good flexibility of the oligomer generally good impact resistance.
⑥ adhesion: low shrinkage oligomer, good adhesion to the substrate; oligomer containing -OH, -COOH and other groups of good adhesion to the metal. Low surface tension of the oligomer, the substrate wetting and spreading good, conducive to improving adhesion.
⑦chemical resistance: epoxy acrylate, polyurethane acrylic resin and polyester acrylic resin have good chemical resistance, but polyester acrylic resin alkali resistance is poor; improve the crosslinking density, chemical resistance enhanced.
⑧ yellowing resistance: aliphatic polyurethane acrylic resin, polyether acrylic resin and pure acrylic ester has good resistance to yellowing.
⑨ gloss: epoxy acrylate and unsaturated polyester have high gloss, crosslinking density increases, gloss increases; Tg high, refractive index of oligomer gloss is good.
⑩ pigment wettability: general fatty acid-modified and amine-modified oligomers have better pigment wettability; oligomers containing -OH and -COOH also have better pigment wettability.
- Glass transition temperature of oligomer Tg
Oligomer Tg is high, generally high hardness, good gloss; Oligomer TB is low, good flexibility, impact resistance is also good. Table 3-52 for the refractive index and glass transition temperature of commonly used oligomer.
- Curing shrinkage of oligomers
Low curing shrinkage is conducive to improving the adhesion of the cured film on the substrate, oligomer functionality increases, crosslinking density increases, curing shrinkage also increases.
- Toxicity and irritation
Oligomer due to the relative molecular mass are larger, most of the viscous resin, non-volatile, not flammable and explosive substances, its toxicity is also low, skin irritation is also low.