The role of oligomers in UV coatings
Oligomer: oligomers used in photocurable coatings, also known as prepolymers. In the early years, it was translated as oligomers, with significant characteristics: small molecular weight, characteristic polymer groups, and high viscosity. It is the main body and skeleton of photocurable coatings(many physical and chemical properties of paint films)
Characteristics of UV curing reactions
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. But the polymerization we have studied more is free radical polymerization (this lecture is based on free radical polymerization). This final C-C cross-linked structure is a rigid cross-linking.
Polymerization mechanism
Free radical polymerization has the following characteristics: rapid reaction; large shrinkage; small change in polymerization degree; large inhibition effect (0.01-0.1% inhibitor prevents the reaction).
–The most disadvantageous thing for coatings is shrinkage. According to the research of W.J.Bailey and others abroad, the distance between double bonds before polymerization is long. Once polymerized, covalent bonds are generated, the distance is shortened, causing the volume to shrink. All unsaturated polymerized double bonds shrink up to 11%.
Complexity of UV coating formula
1. Many types of monomers
2. Many types of basic oligomers (resins). At present, according to the functional groups during synthesis, there are unsaturated polyester PE, epoxy EA, polyurethane PUA, polyester PEA, amino, polyether, silicone, phosphate, mixed, etc.
Introduction to commonly used resins in UV coatings by function
Hard resin—high Tg
High hardness, good chemical resistance, most of them have fast curing speed
1. Standard bisphenol A EA;
2. High functional group PUA and low molecular weight 2fPUA;
3. High functional group amino acrylate;
4. Methacrylate oligomer.
Soft resin—small Tg
Good flexibility, low curing speed, low crosslinking density. 1. Modified epoxy — epoxy soybean oil acrylate, etc.;
2. Long-chain polyester acrylate;
3. PUA with a linear structure mass exceeding 1200;
4. Some pure acrylate oligomers
Polar resins
Oligomers containing active hydrogen or easy to form hydrogen bonds, changing polarity or surface tension
1. Phosphate acrylate
2. Silicone oligomers-especially
3. Carboxyl acrylate oligomers
Water-based UV oligomers
Emulsion type, water-dispersible type, water-soluble type
1. Polyurethane type—-mainly;
2. Epoxy acrylate;
3. Polyester acrylate.
Application of non-cross-linked resins in UV
Auxiliary effects in filling, improving cross-linking density, increasing adhesion, changing flexibility, enhancing wettability, etc.
1. Long oil alkyd resin;
2. Thermoplastic acrylate resin;
3. Aldehyde ketone resin;
4. Petroleum resin, etc.
How to choose resin when designing UV coating formula
Before designing coating formula, it should be clear:
1. Types of coatings in the coating construction process—-clearly define primer, topcoat, and color paint;
2. Understand the basic properties of the coated material—-polarity (surface tension), crystallization, thermoplasticity and thermosetting;
Selection of primer resin
1. Adhesion requirements: This is the common property of primer resin. Relatively speaking, the adhesion that is more difficult at present mainly includes:
A. Glass——select methacrylate oligomers and non-film-forming resins and some special polar resins-thiol siloxane system (but water resistance is an obstacle to the current formulation);
B. Metal, distinguish the types of metals. For the adhesion of metals, the basic application in the coating industry is the destruction of cross-linking methods. The internationally common method is phosphating treatment. The most common method for UV is phosphate ester combined with some pure acrylic.
C. Plastics (including plasticized paper and other types of paint finishing). This is currently a relatively large and particularly complex category, mainly because the plastic structure is complex, the crystal form is different, the surface tension is different, and the relatively difficult BMC, PET, PP, etc. There is no unified formula that can be used for everything. Generally speaking, soft PUA, pure acrylic, some non-film-forming resins and polar resins have certain effects, but attention should be paid to chemical resistance and water resistance. Pay strict attention to the matching of related resins;
D. Oil-containing wood: At present, it is mainly some hard sandalwood such as cloning wood, rosewood, green handle mulberry, hippopotamus wood, etc. The adhesion of wood oil is relatively difficult. There are few pure UV cases in the market for oil sealing. You can seal it with PU first and then make UV adhesion primer. Mainly use some polar resins or monomers and filling resins to achieve good adhesion.
2. Wettability: For the wetting of pigments and fillers and the wetting of substrates, these are two different functions, because it cannot be guaranteed that the surface tension of the substrate is exactly the same as that of the pigments and fillers.
A. The wetting of pigments and fillers can ensure the storage stability of the coating and the transparency of the compatibility of the paint film. For example, some PUA, PEA and epoxy soybean oil acrylate have this effect;
B. For the wetting of the substrate, such as amino resin and PEA, the effect is better.
3. Flexibility: It is related to polishing and interlayer adhesion.
Generally, standard EA, some PEA and some monomers are used to coordinate the flexibility, so as to adjust the polishing and interlayer adhesion.
At present, the market also has hardened primers that emphasize hardness-pay attention to the curing of hard resins and the amount of coating, otherwise it is easy to cause the paint film to burst;
The market also requires so-called elastic primers-use more resins with good flexibility, preferably polyester PUA, polyethers are not very tough, and the mechanical modulus is not enough.
Choice of topcoat resin
1. Fullness and leveling
To achieve this requirement, it is necessary to select resins and monomers with good compatibility, improve the wetting and leveling with the primer, appropriately increase the crosslinking degree, and select resins with higher refractive index.
Generally, high-functionality PUA and amino resins are selected, and standard EA is used as the main resin.
2. Toughness (hardness and wear resistance):
There are many inevitable connections between these two paint film properties, but they are not necessarily completely consistent, and they are treated differently.
Hardness: In addition to the 80-120Unm thick paint film and some thick spray coatings of traditional wood products, a large part of the hardness in this case comes from the paint film itself, and another part is the false hardness that should be paid enough attention to, such as the substrate, primer, surface feel, etc. Rolling and thin spraying are typical examples. In addition to the aforementioned high-function resin, some silicone resin or silicone additives can be used to improve it.
Wear resistance: Generally, PUA is better than others, mainly because hydrogen bonds provide some toughness to increase wear resistance. But the wear resistance of thin coatings cannot be solved by resin.
3. Interlayer adhesion
Solve the wetting and leveling and the polarity matching of resins, 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 worse.
5. Yellowing resistance
It is generally believed that aliphatic PUA, pure polyether acrylate, pure propylene, and amino all have good yellowing resistance. The first type is the most popular, but the yellowing resistance is not the best. The latter two types are rarely used because they have deficiencies, but they have the best overall yellowing resistance performance, amino-based.
6. Matt type
Currently, some resins with slightly smaller molecular weight or huge molecular weight 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).
Post time: Aug-28-2024
