The components that make up the UV ink and their effects
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The main components of the UV ink are a polymerizable prepolymer, a photosensitive monomer, a photoinitiator, an auxiliary component, a coloring pigment, a filler, an additive (leveling agent, a polymerization inhibitor), and the like.
1. Polymeric prepolymer
The polymerizable prepolymer is an important component determining the performance of the UV varnish coating. It is the most basic component in the 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. The skeleton structure affects coating hardness, abrasion resistance, adhesion, light resistance, chemical resistance, and water resistance.
The prepolymer is structurally a low molecular weight resin containing a "C=C" unsaturated double bond, such as an acryloyl group, a methyl acrylate group, a vinyl group, an allyl group or the like. There are mainly resin types such as epoxy acrylate resin, urethane acrylate resin, polyester acrylate resin, polyether acrylic resin, polypropyl acrylate, and unsaturated polyester resin. Under the same conditions, the photocuring speed is the fastest, so the oligomers are mostly acrylic resins.
2. Photosensitive monomer (reactive diluent)
UV ink and UV varnish need to have the viscosity of the coating machine when coating, generally by adding 20% to 80% of the monomer to reduce the viscosity of the prepolymer, and the monomer itself polymerizes to become a cured film. portion.
Reactive diluent, also known as cross-linking monomer, is a functional monomer whose role in the ink is to adjust the viscosity, cure speed and cured film properties of the ink. The reactive diluent also structurally also contains a "C=C" unsaturated double bond which may be an acryloyl group, a methyl propyl acyl group, a vinyl group and an allyl group. In view of the fact that the acryloyl photocuring rate is the fastest, most of the reactive diluents currently used are acrylate monomers. Due to the different amounts of acryloyl groups, they can be divided into three types: monofunctional groups and difunctional groups. The release effect and curing speed of various functional group reactive diluents are different. In general, the more the functionality, the faster the cure, but the worse the dilution.
Traditional reactive diluents, such as styrene, first-generation acrylate monomers, etc., are highly toxic, and some acrylate monomers have a strong irritating effect on the skin. In order to reduce the skin irritation of reactive diluents, there are usually two methods: one is to use ethylene oxide, propylene oxide and hexyl ester ring-opening polymerization to increase the molecular weight of the monomer; the second is to change the monomer ester structure; The species is a change in the previously used alcohol esterification process. When alcohol is added to the acryloyl group, the skin irritation of the polyfunctional monomer is greatly reduced. For example, when the neopentyl glycol diacrylate is esterified, the pH value (skin irritation index) is 4.96, and When synthesized, the pH drops to 0.3.
Recently, some well-performing monomers have been developed, such as: alkoxy acrylates, carbonic acid monoacrylates, imidazolyl monoacrylates, cyclic carbonate monoacrylates, epoxy silicone monomers, silicone acrylics. Ester and vinyl ether monomers. When selecting a monomer, follow these guidelines:
a, low viscosity, good dilution effect;
b, curing fast;
c, good adhesion on the material;
d, it is less irritating to the skin and less toxic;
e, do odor is left in the coating.
3, photoinitiator
A photoinitiator is a substance that absorbs radiant energy and undergoes chemical changes to produce an active intermediate having an initiator polymerization ability, and is also a major component required for any UV curing system. Photoinitiators can be divided into hydrogen abstraction type and pyrolysis type; hydrogen abstraction type needs to cooperate with a compound containing active hydrogen (generally called co-initiator) to form a free radical by hydrogen abstraction reaction, which is a bimolecular photoinitiator. The cleavage type is a single-molecule photoinitiator which is decomposed into a radical in a molecule after being subjected to laser irradiation.
(1) Hydrogen abstraction type: Taking xylene ketone (BP) as an example, when benzophenone is used alone, the olefinic monomer cannot be photopolymerized, and the requirement for it to be a photoinitiator is different. The reaction mechanism is different alkyl and aryl groups. When extracting hydrogen atoms from alcohols and ethers, oxygen easily quenches the excited state of benzophenone. When a hydrogen atom is extracted from an amine, since the ketone forms an excited state complex with the amine immediately after the excited state is formed, the energy transfer to the oxygen molecule is avoided, so 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, an amine system is generally employed. In addition to benzophenone, such photoinitiators are also terpenoid thioxanthone, such as 2-isopropyl thioxanthone, which is commonly used in UV inks.
(2) Pyrolysis type: Taking benzoin ether as an example, benzoin ether is the most widely used photoinitiator, which is characterized by the direct decomposition of the excited state gram into two total free radicals. The resulting radicals can all initiate polymerization of the monomers. The benzoin ether has a short excited life, is not easily quenched by oxygen, and cannot be quenched by styrene, so it can be used for the polymerization of styrene. However, benzoin ether has different degrees of thermal decomposition even if it does not see light, and the storage stability is not good. Generally, stabilizers and polymerization inhibitors are added. At present, it is commonly used as benzoin.
The principles to follow when choosing a photoinitiator:
a, high efficiency of light absorption in the UV range;
b, relatively stable;
c, low cost.
4, other additives
Additives are mainly used to improve the performance of inks. The additives commonly used in UV inks are stabilizers, leveling agents, defoamers, dispersants, waxes, etc.
(1) Stabilizer: Stabilizer is used to reduce thermal polymerization during storage and improve ink storage stability. Commonly used hydroquinone, p-methoxyphenol, p-benzoquinone, 2,6-di-tert-butyl cresol and the like.
(2) Leveling agent: The leveling agent is used to improve the leveling of the ink layer, prevent the occurrence of shrinkage holes, smooth the surface of the ink film, and also increase the gloss of ink printing.
(3) Antifoaming agent: Defoaming agent is used to improve the leveling of the ink layer, prevent the occurrence of shrinkage cavities, smooth the surface of the ink film, and also increase the gloss of ink printing.
(4) Dispersing agent: The dispersing agent can make the pigment in the ink wet and wet very well in the binder, so that the pigment has good dispersibility in the ink, shorten the grinding time in the ink manufacturing, and reduce the oil absorption of the pigment. Producing a high concentration of ink; preventing coagulation and precipitation of pigment particles in the ink. The dispersing agent is generally a surfactant.
(5) Wax: The main function of wax is to change the rheology of the ink, improve the water resistance and printing performance (such as adjusting the viscosity), reduce the defects such as smudging and pulling paper, and form a smooth surface on the surface of the dried ink film. The wax film 2 improves the abrasion resistance of the printed matter and the like. In the UV ink, the wax also blocks air, reduces oxygen inhibition, and is beneficial for surface curing. However, adding an excessive amount of wax and a wrong wax to the ink will reduce the gloss of the ink, destroy the ink transfer performance, and prolong the drying time.