UV printing ink formula structure UV printing ink is mainly composed of prepolymer, active diluent, photosensitizer, pigment filler and various additives. The characteristics and functions of each component are as follows:
1.
Prepolymer Prepolymer is the binder and main film-forming material of UV printing ink, and is closely related to the operation suitability and quality suitability such as the transferability of ink on the printing machine, the curing speed of the printing film, the reproducibility of the printing dots, the gloss, adhesion, wear resistance and resistance of the printing film.
Currently, there are two types of photocurable prepolymers that are widely used in inks: one is unsaturated polyesters, which are usually linear polymer compounds with unsaturated double bonds in the molecular chain formed by the condensation of a saturated or unsaturated diol and an unsaturated or saturated dicarboxylic acid (anhydride); the other is acrylic resins, such as polyester acrylates, polyether acrylates, epoxy acrylates, polyurethane acrylates, etc.
Performance advantages and disadvantages of common prepolymers:
● Unsaturated polyester Low price, fast curing, good wettability to pigments and fillers, high toughness Low gloss, poor resistance, easy to emulsify
● Polyester acrylate Low price, easy to adjust ink properties, good wettability to pigments and fillers, good comprehensive properties Poor chemical resistance
● Polyether acrylate High adhesion, good gloss, good flexibility Poor resistance, easy to emulsify
● Epoxy acrylate Low price, fast curing, good adhesion, high gloss Poor flexibility, easy to emulsify
● Polyurethane acrylate Toughness and wear resistance, easy to adjust ink properties, excellent comprehensive performance High price, high viscosity
2.
Reactive diluents Reactive diluents have the dual functions of curing, crosslinking and solvent dilution in the composition system of UV printing inks. Their molecules contain active functional groups (such as double bonds) that can be initiated by photosensitive free radicals and can further prepolymers to reflect crosslinking, ensuring the reasonable curing and drying of UV printing inks while taking into account other printing quality suitability; at the same time, this type of viscous liquid compound has good solubility and dilution properties for prepolymers, giving the ink good fluidity and thus meeting the suitability of printing operations.
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 their molecular structure, such as styrene used in saturated polyesters; the other type is difunctional or multifunctional compounds, which contain two or more vinyl groups in their molecular structure, mainly polyol acrylate compounds.
3. Photosensitizer
According to Einz's law of photochemical equivalence, the energy of a photon is inversely proportional to its wavelength. Ultraviolet light is usually a continuous spectrum with a wavelength in the range of 200nm-400nm, and the corresponding photon energy is 598-299KJ/mol. In order to make the ink system undergo a photocuring reaction quickly and efficiently within this range, in addition to prepolymers and reactive diluents, compounds that are easily decomposed or excited by light, called photosensitizers (or sensitizers), need to be added to the UV printing ink system.
This triggers the polymerization induction period and increases the drying speed of the ink. Photosensitizers usually absorb light with longer wavelengths than prepolymers and reactive diluents, decompose to produce free radicals; or light excitation triggers secondary reactions to produce free radicals, both of which trigger photopolymerization.
Common photosensitizers include:
● Carbonyl compounds (main absorption wavelength 360~420nm) Benzoin, benzophenone, w-bromoacetophenone, benzoquinone, anthraquinone
● Azo compounds (main absorption wavelength 340~400nm) Azobisisobutylcyanide, diazo compounds
● Organic sulfur compounds (main absorption wavelength 280~400nm) Thiol, diphenyl sulfide, tetraethylthiuram
● Redox system -------- Fe2+/ROOH
● Halogen compounds (main absorption wavelength 300~400nm) Silver halide, CBr4, HgBr2
● Sensitizing dyes (main absorption wavelength 400~700nm) Eosin/stupid amine, riboflavin, methylene blue, rose bengal, cyanine dyes
● Organic metal compounds (main absorption wavelength 300~450nm) Alkyl metals, such as tetraethyl lead
● Carbonyl metal compounds (Main absorption wavelength 360~440nm) Manganese (Mn(CO)10)
Among them, carbonyl compounds are the main photosensitizers used in UV inks, including ketones, benzoin and its ethers. Their common feature is that they all contain carbonyl groups in their molecular structures, and they are aromatic ketone groups; the difference is that benzoin itself can initiate polymerization of olefin components through photochemical action, so it is both a photosensitizer and an initiator. After photochemical action, benzophenones often need to react with other molecules before initiating polymerization of olefin components. So strictly speaking, it is only a photosensitizer, not an ideal initiator. To further initiate polymerization of olefin components, it must be used in combination with other compounds. Nevertheless, the common feature of these two types of photosensitizers is that the carbonyl group in the aromatic ketone group is photosensitized first.
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