The main components of the UV ink are a polymerizable prepolymer, a photosensitive monomer, a photoinitiator, and auxiliary components such as coloring pigments, fillers, additives (leveling agents, polymerization inhibitors), and the like.
1. Polymeric Prepolymer
Polymeric prepolymer is an important component to determine the properties of UV light oil coating, is the most basic component in UV ink, and is a film-forming substance. The performance plays an important role in the curing process and the properties of the ink film after curing. Generally classified according to the skeleton structure. Skeleton structure affects coating hardness, abrasion resistance, adhesion, light resistance, chemical resistance, and water resistance.
Prepolymers Structurally, the oligomers are all low-molecular resins containing "C=C" unsaturated double bonds, such as acrylic acyl, methyl acryl, vinyl, allyl, and the like. There are several types of epoxy resin, such as epoxy acrylate resin, urethane acrylate resin, polyester acrylate resin, polyether acryl resin, polyacrylic propyl ester, and unsaturated polyester resin. Under the same conditions, the speed of photo-curing acryloyl group is the fastest, so the oligomer is mostly acrylic resin.
2, photosensitive monomer (active diluent)
UV inks and UV varnish oils need to be adapted to the viscosity of the coating machine during coating. Generally, the viscosity of the prepolymer is lowered by adding 20% â€‹â€‹to 80% of the monomer, and the monomer itself undergoes polymerization to become a cured film. Part.
Reactive diluents, also called cross-linking monomers, are functional monomers whose role in the ink is to adjust the viscosity of the ink, the cure speed, and the properties of the cured film. Reactive diluents also contain structurally "C=C" unsaturated double bonds, which may be acryloyl, methacryloyl, vinyl and allyl. In view of the fastest photo-setting rate of acryloyl groups, the currently used reactive diluents are mostly acrylic monomers. Due to the different amounts of acryloyl groups, they can be divided into monofunctional groups and difunctional groups. The release efficiency and curing speed of reactive diluents of different functional groups are all different. In general, the more the functionality, the faster the curing rate, but the worse the dilution effect.
Traditional reactive diluents, such as styrene, first-generation acrylate monomers, etc., are very toxic and some acrylate monomers have a strong irritating effect on the skin. In order to reduce the irritation of the active diluent on the skin, there are usually two methods: one is to increase the molecular weight of the monomers by ring-opening polymerization of ethylene oxide, propylene oxide and hexyl ester; second, to change the structure of the monomer ester group; The species is changing the previous method of using alcohol esterification. When the addition of alcohol to acryloyl group is used, the skin irritation of the multifunctional monomer is greatly reduced. For example, when neopentyl glycol diacrylate is synthesized by esterification, the PH value (skin irritation index) is 4.96, and When synthesizing, the pH drops to 0.3.
Recently, some well-performing monomers have been developed, such as: alkoxy acrylates, carbonate monoacrylates, imidazolyl monoacrylates, cyclocarbonate monoacrylates, epoxy silicone monomers, silicone acrylics, etc. Ester and vinyl ether monomers. When selecting monomers, follow these guidelines:
a, low viscosity, good dilution effect;
b, fast curing;
c. Good adhesion on the material;
d. It is less irritating to the skin and less toxic;
e. No odor is left in the coating.
Photoinitiators are materials that absorb radiant energy and produce chemically reactive intermediates with the ability to polymerize by initiators, and are also a major component of any UV curing system. Photoinitiators can be divided into hydrogen abstraction type and cracking type; hydrogen abstraction type is required to cooperate with an active hydrogen-containing compound (generally called co-initiator) to form free radicals by hydrogen abstraction reaction, which is a bimolecular photoinitiator. Cleavage type is a photoinitiator which is a single molecule photoinitiator after photoinitiator is decomposed into free radicals by the laser.
(1) Hydrogen abstraction: Take dimethyl xanthone (BP) as an example. When benzophenone alone is used, the vinyl monomer cannot be photopolymerized, and the photoinitiator needs to be different. The reaction mechanism is different alkyl and aryl groups. When hydrogen atoms are extracted from alcohols and ethers, oxygen can easily quench the excited benzophenone. When a hydrogen atom is extracted from an amine, since the ketone forms an excited complex with the amine immediately after the formation of the excited state, energy transfer to the oxygen molecule is avoided, so that the amine system is not easily quenched by oxygen, compared with the alcohol ether system. It also reduces the possible type of energy transfer to the monomer. Therefore, in practical applications, amine systems are generally used. In addition to benzophenone, photoinitiators of this type include oximinothiones, such as 2-isopropyl thioxanthone, commonly used in UV inks.
(2) Cleavage type: Taking benzoin ether as an example, benzoin ether has been the most widely used photoinitiator. It is characterized in that the excited state g is directly decomposed into total free radicals of both parties. The resulting radicals can all initiate polymerization of the monomers. The excited state of benzoin ether is short-lived, not easily quenched by oxygen, nor quenched by styrene, so it can be used for the polymerization of styrene. However, benzoin ethers have different degrees of thermal decomposition even if they are not exposed to light. Storage stability is not good, and stabilizers and polymerization inhibitors are generally added. Currently, dibenzothiophene is commonly used.
The principle that should be followed in the selection of photoinitiators:
a. High UV absorption efficiency;
b, relative stability;
c. Low cost.
4, other additives
Auxiliaries are mainly used to improve the performance of inks. Commonly used additives in UV inks include stabilizers, leveling agents, defoamers, dispersants, and waxes.
(1) Stabilizers: Stabilizers are used to reduce thermal polymerization during storage and improve ink storage stability. Hydroquinone, p-methoxyphenol, p-benzoquinone, 2,6-di-tert-butylcresol and the like are commonly used.
(2) Leveling agent: Leveling agent is used to improve the leveling of the ink layer, prevent the formation of shrinkage holes, make the ink film surface smooth, and also increase the gloss of the ink printing.
(3) Defoamer: Defoamer is used to improve the leveling of the ink layer, prevent shrinkage, smooth the surface of the ink film, and increase the gloss of the ink printing.
(4) Dispersing agent: The dispersant can make the pigment in the ink very well wetted in the linking material, so that the pigment has good dispersion in the ink, shortens the grinding time when the ink is manufactured, and reduces the oil absorption of the pigment. Produces a high concentration of ink; prevents the coagulation and precipitation of pigment particles in the ink. Dispersants are generally surfactants.
(5) Wax: The main role of wax is to change the rheology of the ink, improve the water resistance and printing properties (such as adjusting the viscosity), reduce defects such as smearing, pulling the paper, and can form a smooth surface on the dried ink film. The wax film 2 improves the abrasion resistance of the printed material. In UV inks, the wax also acts as a barrier to air, reducing oxygen inhibition and favoring surface cure. However, adding an excessive amount of wax to the ink and selecting the wrong wax type will reduce the gloss of the ink, destroy the ink transfer performance, and extend the drying time.
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