Definition and classification of UV inks and their applications
1. Overview of Ultraviolet Curing Drying (UV) Ink
UV curing drying flexo ink, referred to as UV ink, UV ink is essentially a liquid ink that can change from liquid to solid under the irradiation of ultraviolet rays of a certain wavelength.
Comparing the composition of UV ink with that of traditional ink, it will be found that the two are very different. The difference is that the film formation of UV ink is a chemical reaction from monomer to polymer; while the film formation of traditional ink is a physical action, the resin is already a polymer, and the solvent is a polymer that dissolves the solid polymer into a liquid. It helps to coat the ink on the substrate, and then the solvent is volatilized or absorbed, so that the liquid polymer can be restored to the original solid state. UV ink is mainly composed of pigments, prepolymers, reactive diluents, photoinitiators and additives.
The rapid development of the printing industry requires not only high quality and high efficiency, lower costs, but also no pollution to the environment. UV ink can be printed on any substrate, and the quality of the printed product is better than that of solvent-based and water-based flexo printing inks. good and low cost. UV ink has been widely used in foreign countries. In offset printing, screen printing, flexographic printing and letterpress printing, the consumption of UV ink is increasing year by year. The annual growth rate of UV inks in Europe, America and Japan is 9% to 12%.
2. the characteristics of UV light curing ink
Ultraviolet light is an electromagnetic wave with a shorter wavelength than visible light, and different wavelengths have different energies. Generally, the wavelength of ultraviolet light used for curing UV light-curable ink is 250~400nm. The commonly used ultraviolet light sources in the industry are high-pressure mercury lamps and metal halide lamps, which have the highest efficiency in the range of 200~300nm and 300~400nm respectively. Therefore, high-pressure mercury lamps are commonly used for colorless UV-curable inks, while metal halide lamps are commonly used for colored UV-curable inks.
The fundamental feature of UV light-curable ink that is different from other types of ink is that the ink layer is cured and formed into a film by strong ultraviolet light irradiation, and the film-forming substance is initiated by a photoinitiator and undergoes a rapid polymerization reaction, which is completed within a few seconds to tens of seconds. . If it is not exposed to strong ultraviolet light, even if it is heated, the ink will be in a sticky state for a long time and cannot be cured. The main advantages of UV light curing ink are: a. It can cure the thicker ink layer at one time, and the maximum thickness of the ink layer can reach 100~150μm; b. Short curing time, less energy consumption, saving printing space and space, and high production efficiency; c. No organic solvent, almost 100% cured film, no organic solvent volatilization, reducing environmental pollution; d. Applicable to the printing of substrates with large heat capacity; e. The prints are exquisite, of good quality and high grade, and the cured ink film has good heat resistance, solvent resistance and scratch resistance.
3. the composition characteristics of ultraviolet light curing ink
Ultraviolet light curing ink, also known as ultraviolet drying ink or UV ink. This is a kind of ink that absorbs light energy of a certain wavelength and produces a photochemical reaction under ultraviolet irradiation to quickly complete the conversion from liquid to solid.
There is no solvent in this ink, and only under the condition of ultraviolet irradiation can the photochemical reaction take place rapidly, so as to achieve the purpose of fast drying. It is precisely because of this feature that it overcomes the problem that the ink is prone to performance changes on the printing press, and also solves the problem of rapid drying after the ink is transferred to the surface of the substrate. Such inks not only meet the requirements of high-speed, multi-color printing at one time, but also provide convenience for the printing of non-absorbent materials.
Like other inks, UV light curing ink is composed of pigments, binders and auxiliary materials, but because it needs to rely on photochemical reactions to dry the ink, there are specific requirements for the use of ink binders, that is, the use of light Solid link material. The photo-curable binder is mainly composed of photosensitive resin or prepolymer, crosslinking agent, photoinitiator and other substances.
1) Pigment
The requirement of UV light-curing ink for pigments is that the mixing property is good, and the UV light-curing ink cannot be gelled within the effective storage period after mixing; when pigments are used together, the dosage of each pigment in the light-curing ink must be accurate; pigments The concentration of the ink should be high, the color should be bright, it should have excellent dispersibility and sufficient tinting strength; the pigment with low absorption rate of ultraviolet spectrum should be selected, so as not to hinder the curing speed of the light-setting ink; when the pigment is exposed to ultraviolet light or It is required not to change color during curing reaction.
Different pigments have different absorption and reflection of ultraviolet spectrum, therefore, the curing speed of light-setting ink is also different due to different pigments. Pigments with good transparency are faster to fix due to their high UV transmittance; carbon black has a higher UV absorption capacity and cures the slowest; white pigments are highly reflective, which also hinders curing.
According to the above principles, the pigments used in UV light curing inks generally include titanium dioxide, benzidine yellow, phthalocyanine blue, permanent red, peach red, sapphire red, lightfast deep red, carbon black, etc.
2) Light-curing bonding material
In the ultraviolet light-curing ink, the photo-curable binder is particularly important. The requirements for it are: the photo-curable binder should be light in color and good in transparency; the photo-curing activity should be high, and it should be instantly dried under ultraviolet light; The gloss after the film should be good, the adhesion should be strong, and the toughness and impact resistance should be excellent; the acid value of the photo-curable binder should not be too high (generally below 20), otherwise it is easy to gel when the ink is stored; the wettability with the pigment Be good. For UV-curable binders, not only one photocurable resin can be selected, but two or more photocurable resins or prepolymers and crosslinking agents are mostly used. For example, one or more acrylic resins can be used as the base. The most commonly used materials are trifunctional configurations based on urethane, epoxy or polyester resins.
In practical applications, a large part is not actually a resin, but an ester of unsaturated compounds, also known as active monomers, such as TMPTA CAS 15625-89-5, NPGDA CAS 2223-82-7, HDDA CAS 13048-33-4, IBOMA CAS 7534-94-3, etc. However, they can be used in combination with photocurable resins to facilitate photopolymerization curing. Trimethylolpropane triacrylate is a highly reactive, low viscosity (generally 0.05～0.15Pa·s, 25℃) diluent, which can provide hardness and brightness, and can increase the fixing speed, it is especially suitable for For formulations requiring lower viscosity and higher reactivity.
Unsaturated polyester is a high polymer with a linear structure of unsaturated double bonds. Any monomer that can be cross-linked and copolymerized with this high polymer is called a cross-linking agent, also known as “bridging agent”. The polyester is cross-linked into a network structure. There are two main functions of the cross-linking agent: cross-linking and curing in photopolymerization; as a diluent, it reduces the viscosity of the system and improves the fluidity of the resin. The ideal crosslinking agent is also an ideal reactive diluent, which can be selected according to the following principles. It has better miscibility with resin, can dissolve and dilute unsaturated polyester, and can participate in photocuring reaction; it has good photosensitivity and excellent photocuring. Activity; improve the physical properties of the film after curing; low volatility, odorless, non-toxic; rich sources, cheap. To choose a suitable cross-linking agent, it is also necessary to consider the characteristics of the resin itself, integrate various factors, and learn from each other to obtain better results.
In the polymerization system, a compound that is easily decomposed or excited by light is added in addition to the monomer, called a photosensitizer (also called a sensitizer). The resulting photopolymerization is called photo-sensitized photopolymerization. The photosensitizer used to enhance photopolymerization usually absorbs light with a longer wavelength than the monomer and decomposes to generate free radicals; or photoexcitation causes secondary reactions to generate free radicals, both of which cause photopolymerization.
There are two main categories of photosensitizers commonly used in UV light curing inks: ketones, such as benzophenone, bibenzoyl, zhuchi ketone, p-phenylbenzophenone, halogenated acetophenone; benzoin and its ethers class, such as benzoin, benzoin methyl ether, benzoin ether, benzoin butyl ether, alpha-methyl benzoin, alpha-phenyl benzoin. Their common feature is that they all contain , base and aromatic ketone groups. The difference between them is that benzoins themselves can trigger olefin monomers through photochemical action, so they are both photosensitizers and photoinitiators; while benzoins often need to interact with other molecules after photochemical action. In order to initiate the polymerization of ethylenic monomers, it is strictly a photosensitizer, not a good photoinitiator, and often needs to be used in conjunction with other compounds. Nonetheless, the common feature of these two types of substances is that there are aromatic ketone groups in the group, which first perform photosensitization. The dosage of photosensitizer in UV light curing ink is generally 1% to 10% (mass fraction), preferably 3% to 6%. When the amount of photosensitizer is too small, the speed of light “bridging” is slowed down, which is not conducive to application. On the other hand, if the amount of photosensitizer is too much, the photo-curing speed will not necessarily increase, which is not only economically unreasonable, but also will reduce the strength of the cured conjunctiva and affect the performance of the ink film.
In the process of free radical polymerization, due to the existence of some small amounts of substances, the polymerization of monomers and unsaturated resins cannot occur, which is called polymerization inhibition, and the substances that cause this phenomenon are called polymerization inhibitors. 01%〜0. 20%。 The polymerization inhibitor should not inhibit the photocuring reaction, only to prevent the thermal polymerization reaction was added, therefore, the amount of unsaturated polyester 0. 01% ~ 0. 20%.
3) Auxiliary
Waxes are often used as auxiliary agents in inks to improve the friction resistance of inks and reduce viscosity. The basic requirements for the use of waxes in inks are high melting point and microcrystalline. The waxes that can be used in inks are microcrystalline waxes and polyethylene waxes. In addition, a small amount of tertiary amines, derivatives of phosphine, etc. are added as auxiliary agents.
4. Technical indicators and printability of UV-curable inks
1) Specifications of UV-curable ink
The color is similar to the standard sample; the fineness is 15~25μm; the fluidity is 20~35mm; the tinting strength is 90%~110%; the viscosity is 15~25 (32℃); the impact strength is 29.4N m; 1～2s.
2) Printability of UV-curable inks
Mixing of inks. Light-curable inks can be mixed with each other, but not with general inks. When changing from the original ink to light-setting ink, it should be washed before printing.
Ink cleaning. Dedicated product for light-curable inks applied to cleaning solvents. During printing, if the cleaning solvent remains on the ink roller, blanket and water roller, it will become the cause of poor ink drying and poor inking, so it must be completely removed.
Auxiliary material. There are viscosity reducers, driers and hardeners in the auxiliary materials. The viscosity reducer is used to reduce the viscosity of the ink, and the addition amount should be within 5%. If it exceeds this amount, the hardening of the ink will be reduced; the drier is used to increase the assimilation speed of the light-solid ink, and the addition amount should be within 2%; The maximum hardener is 10%, beyond which there is no effect. This adjuvant is a special product for light-curing ink, and general adjuvants cannot be used.
5. The main points of using UV light curing ink
Due to the drying characteristics of UV ink, it has a wide range of applications and is especially suitable for printing on non-absorbent materials. If the price factor is not considered, UV light curing ink can be used in printing machines of various printing methods, and the printing materials are paper, plastic and plastic film, aluminum foil, metal, etc.
When printing with UV light curing ink, the paper output part of each color group of the printing machine should be equipped with a UV light irradiation device, and the length and power of the lamp tube should be determined according to the printing format. Generally, the distance between the lamp tube and the printed matter is about 20cm or less. Generally equipped with special printing ink roller and blanket.
In the process of using UV ink, it should not be mixed with general ink. In printing, when converting from the original ink to UV light-curing ink, the original ink should be cleaned, and then replaced with UV light-curing ink, and special auxiliary agents must be used to adjust the adaptability of the ink. Such as viscosity reducer, hardener, drier, etc. The function of the special viscosity reducer is to reduce the viscosity of the UV light curing ink, and its addition amount should be controlled within 5%, otherwise the hardness of the ink will be reduced; the film hardener is to improve the strength of the ink layer, and the maximum limit is 10%; The role of the drying agent is to speed up the curing speed of the ink, and the addition amount should be controlled within 2%.
In the middle of printing or at the end of printing, it is necessary to clean the ink roller, blanket, printing plate, water roller and ink fountain. The special washing solvent for ultraviolet light curing ink should be used, and the cleaning must be thorough to avoid poor inking or poor drying of the ink. wait for failure.
The use and storage of UV light curing ink should avoid direct sunlight, and should be stored in a dark place at 20 ° C. The storage period should not be too long, and the general validity period is 6 months.
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