The UV ink of UV printer has a unique curing method, that is, fast curing without relying on penetration and evaporation, and has been widely used in UV inkjet printing. UV ink has three major characteristics: energy-saving drying, broad printability to substrates (printing on almost all materials), and rapid curing, which can shorten the follow-up process time.
However, compared with the traditional ink and screen printing process, the price of UV ink is high, and the cost is too high, which directly affects the printing price of UV inkjet printing.
What are the main components of UV printing ink?
UV printing inks are mainly composed of prepolymers, reactive diluents, photosensitizers, pigments and fillers, and various auxiliaries, depending on the characteristics and functions of each component.
Workability and quality suitability of prepolymer or photocurable resin and UV printing ink
Such as the transfer of ink on the printing press, the curing speed of the printing film, the reproducibility of printing dots, and the gloss, adhesion, and abrasion resistance of the printing film are closely related. There are two types of photocurable prepolymers that are currently used in inks:
One is unsaturated polyesters, usually a linear polymer compound with unsaturated double bonds in the molecular chain formed by the polycondensation of a saturated or unsaturated dihydric alcohol and an unsaturated or saturated difar acid (anhydride)…
The second is acrylic resins, such as polyester acrylate, polyether acrylate, epoxy acrylate, polyurethane acrylate, etc.
2 Reactive diluents
Reactive diluents have dual functions of curing, cross-linking and solvent dilution in the composition system of UV printing inks, and their molecules contain reactive functional groups (such as double bonds) that can be initiated by photosensitive free radicals and can further reflect cross-linking by prepolymers. , which ensures the reasonable curing and drying of UV printing ink and takes into account other printing quality suitability; at the same time, this kind of viscosity liquid compound has good solubility and dilution for prepolymer, which endows the ink with good fluidity to meet the printing operation suitability. sex.
There are two main types of reactive diluents commonly used in UV printing inks
One type is monofunctional compounds, which contain a vinyl group in the molecular structure, such as styrene for saturated polyesters.
The other type is bifunctional or multifunctional compounds, which contain two or more vinyl groups in the molecular structure, and are mainly acrylate compounds of polyhydric alcohols.
According to Einstein’s law of actinic equivalence, the energy of photons is inversely proportional to the wavelength. Ultraviolet light is usually a continuous wave spectrum with a wavelength in the range of 200nm to 400nm, and the corresponding photon energy is 598 to 299KJ/mol. In order to make the ink system within this range, The photocuring reaction occurs quickly and efficiently. In addition to the prepolymer and reactive diluent, the UV printing ink system also needs to add compounds that are easily decomposed or excited by light, called photosensitizers (or sensitizers). This initiates the polymerization induction period and increases the drying speed of the ink. The photosensitizer usually absorbs longer wavelengths of light than prepolymers and reactive diluents, and decomposes to generate free radicals; or photoexcitation triggers secondary reactions to generate free radicals, both of which initiate photopolymerization. Among them, carbonyl compounds are the main photosensitizers used in UV inks, including ketones and benzoin and its ethers. Their common feature is that the molecular structure contains carbonyl groups and aromatic ketone groups; the difference is that the benzoin itself passes through photochemical action. It can initiate the polymerization of olefinic components, so it is both a photosensitizer and an initiator. After benzophenones pass through photochemical action, they often need to interact with other molecules to initiate the polymerization of olefinic components. So strictly speaking, it is only a photosensitizer, not an ideal initiator. To further initiate the polymerization of olefinic components, it must be used in conjunction with other compounds. Nevertheless, the common feature of these two types of photosensitizers is that the carbonyl group in the aromatic ketone group is the first to perform photosensitization.
4 Color fillers
The pigment is an indispensable component of UV printing ink. Pigment particles are dispersed in the binder for the expression of printing ink color. The addition of pigment will change the rheological properties of the ink, and will also have a greater effect on the gloss of the printing film. Influence. For UV printing inks, the addition of pigments will cause the absorption, reflection, or scattering of UV light, and the reflection and scattering will reduce the absorption of UV light by the ink system, reduce the photoinitiation efficiency of the photosensitizer, and cause the ink to cure and dry faster. The color pigments commonly used in UV inks are benzidine yellow, Hansa yellow, phthalocyanine blue, phthalocyanine green, permanent red, red powder, etc., and the achromatic pigments are titanium dioxide, high-pigment carbon black, etc. For overprinting ink, it is required that the color sequence ink layer has sufficient ultraviolet light transmittance to ensure that the lower ink layer has sufficient ultraviolet light irradiation, so as to avoid printing failures in which the surface ink is dry and the bottom ink is not dry.
In order to improve the printability and quality suitability of UV printing ink, in addition to the four components mentioned above in the composition of UV ink formulation, it is also necessary to add auxiliary agents. The auxiliary agents used in UV inks have the characteristics of less dosage, more varieties and specificity, and high efficiency. The main varieties are selected here for an overview.
In order to prevent the UV printing ink from cross-linking during the storage period and improve the storage stability of the ink, the auxiliary agent is added. Commonly used polymerization inhibitors are hydroquinone, p-methoxyphenol, p-benzoquinone, etc. These inhibitors will not inhibit the photopolymerization reaction and are only added to prevent the thermal polymerization reaction.
This adjuvant is used to eliminate the bubbles generated during the manufacturing and use of the ink, mainly for thin inks with low viscosity, such as UV inks for gravure printing and flexographic printing. Commonly used defoamers are silicone oils (F810, FN), silicone acrylates (R2500), and the like. ③Leveling agent:
It is used to improve the leveling of the ink layer, reduce the shrinkage of the printing film, and help to improve the gloss of the printing film. Commonly used leveling agents are silicone acrylate (R2100, R2200), silicone (Deqian 457), and so on.
UV printing ink is used for metal coil printing, after the printing is completed, the printed product needs to be cut, bent punched and other mechanical processing, and the printing film often breaks down during the mechanical processing. The performance of pressure processing is usually done by adding an impact agent. Commonly used impact agents such as (meth) acrylic acid phosphate (PM-1, PM-2, EB168, SR9008). In addition to those listed above, the auxiliary agents used in UV printing inks also have many varieties. For example, in order to improve the rheology of the ink, improve the water-resistance of the ink, and enhance the abrasion resistance of the ink, wax is usually added for modification. With the advancement of science and technology, auxiliary agents for UV printing inks will continue to be introduced.