Development of Plastic Gravure Ink (2)

   3 Results and Discussion

3.1 Preparation of comb prepolymer UV curing adhesive

The binder plays a decisive role in the performance of the ink. The manufacturing processability of the ink, the suitability of the printing operation and the suitability of the printing quality all depend on the properties of the binder. The binder is the fluid component in the ink composition. The wetting property of the pigment filler affects the degree of pigment dispersion and the stability in the ink system after dispersion; the characteristics of the binder material determine the viscosity, fluidity, and print transfer of the ink. Sex, dryness, gloss, adhesion, and resistance of the film. The linking material of UV ink is mainly composed of prepolymer and reactive diluent. The curing and drying is initiated by ultraviolet irradiation to initiate the photosensitizer in the ink system, further initiating the unsaturated double bond in the reactive diluent and interacting with the prepolymer. The completion of free radical chain polymerization cross-linking occurs. Therefore, the development of UV-cured plastic gravure ink is mainly combined with printing requirements, the synthesis and selection of prepolymers and reactive diluents, and to achieve the optimal combination of the two, to meet the requirements of ink manufacturing, printing and use of the link material .

3.1.1 Synthesis of comb-type prepolymers Since plastics gravure printing inks are mainly non-polar polymer films, the lack of fastness of the printing film on the plastic surface is a technical difficulty for such products. Ink prepolymers have the problem of insufficient adhesion fastness, while polyamides have better adhesion to plastics, especially to polyolefin films, and at the same time the imine groups in the polyamide backbone structure (-HNCO-) Hydrogen has a high reactivity and lays the foundation for the reaction of acrylic acid monoisocyanate, and also creates the conditions for the synthesis of comb-type prepolymers.

The molecular weight of the polyamide resin should not be too high. Otherwise, the solubility and reactivity of the resin are too poor to be used in the synthesis of the prepolymer. The molecular weight of the polyamide is mainly regulated by monobasic acid. Since Xn=2/(2-pf), Xn is the number average polymerization degree of the polymer, p is the degree of polycondensation reaction, f is the average functionality of the reaction system, and the monobasic acid is added. Reducing the functionality of the reaction system, the greater the added amount, the lower the average functionality, and when the degree of reaction is certain (polyamide condensation p→1), the number average polymerization degree of the final polymer formation is smaller.

The direct reaction between TDI and low-molecular-weight polyamide makes it easy to cross-link, making the reaction system uncontrollable. When the comb-type prepolymer is synthesized, it is only equimolarly reacted with hydroxyethyl acrylate by TDI to obtain an isocyanate intermediate, and then with a low molecular weight. The polyamide can be further reacted before the comb-type prepolymer having the polyamide main chain and the urethane acrylate as the side branch can be successfully obtained.

3.1.2 Reactive diluent selection Reactive diluents have the dual function of curing cross-linking and solvent dilution in the UV ink composition system. At present, reactive diluents for UV-curing printing inks are mainly classified into two types of monofunctional compounds and bis(or) functional compounds. When selecting, monofunctional compounds are used to emphasize the solvent dilution function for adjusting the viscosity of the linking material; the use of double (or more) functional compounds to focus on the curing cross-linking function to increase the UV ink drying speed. It was decided that the principle of selection of reactive diluents is a compound route of multiple components. The available monofunctional compounds are mainly styrene, 2-ethylhexyl acrylate (EHA), hydroxyethyl acrylate (HEA). Compounds of bis(or more) functional groups are hexanediol diacrylate (HDDA), neopentyl glycol diacrylate (NPGDA), triethylene glycol diacrylate (TEGDA), trimethylolpropane triacrylate (TMPTA). In a comprehensive comparison, reactive diluents suitable for the preparation of comb-type prepolymer UV-curable plastic gravure ink are compounded by hydroxyethyl acrylate (HEA) and trimethylolpropane triacrylate (TMPTA).

3.2 The choice of pigments and pigments Pigments are the essential components of UV printing inks. The addition of pigments will change the rheological properties of the ink, and it will also have a greater impact on the gloss of the film. For UV printing inks, the addition of pigments will cause absorption, reflection, or scattering of UV light, and reflection and scattering will reduce the absorption of UV light by the ink system, reduce the photoinitiation efficiency of the photosensitizer, and lead to slower curing and drying of the ink. The commonly used color pigments for UV inks include benzidine yellow, Hansha yellow, indigo, indigo green, permanent red, big red powder, achromatic pigment titanium dioxide, high-pigment carbon black, and the like. The overprinting ink requires that the color ink layer of the rear color sequence has enough ultraviolet light transmittance to ensure that the following ink layer has sufficient ultraviolet light irradiation, so as to avoid the occurrence of printing failures where the ink is dry and the bottom ink is not dry. Analysis of a typical four-color version of the UV inks (magenta, yellow, cyan, black) showed that the magenta ink had the greatest transmission of light at 313.1 nm, the value was 50%; the yellow ink had the light at 320 nm The maximum absorption rate is 32%; the cyan ink has the maximum transmissivity to 285 nm light and its value is 25%; the black ink has almost zero transmittance to light. Therefore, when four-color printing is used for overprinting, the printing color sequence of the four-color printing ink is: black ink-cyan ink-yellow ink-magenta ink, if a UV light source that matches the maximum transmittance of each ink can be set after each color You will get better printing results.

The main role of the filler in the ink is to improve the rheological properties of the ink and reduce the price of the ink. The addition of filler can also cause the ink to hinder the absorption of UV light, reduce the drying speed of the ink, and should be fully considered in the selection. The fillers commonly used in UV printing inks include calcium carbonate, aluminum hydroxide, silica, talcum powder, kaolin, and the like.

3.3 Selection of Photosensitizers According to Einstein's law of photochemical equivalents, the energy of a photon is inversely proportional to the wavelength. Ultraviolet light is usually a continuous wave spectrum with a wavelength in the range of 200 to 400 nm. The corresponding photon energy is 598 to 299 KJ/mol. In this range, the ink system undergoes a photocuring reaction quickly and efficiently. In addition to the prepolymer and the reactive diluent, a compound that is easily decomposed or excited by light is added to the UV printing ink system, and is called a photosensitizer (or Sensors). Photosensitizers typically absorb longer wavelength light than prepolymers and reactive diluents and decompose to generate free radicals; or photoexcitation initiates secondary reactions to generate free radicals, both of which initiate photopolymerization. Among them, carbonyl compounds are mainly used as photosensitizers in UV inks, including ketones and benzoin and ethers thereof, such as benzophenone, benzoin, benzoin methyl ether and the like.

3.4 Selection of adjuvants

In order to improve the printability and quality suitability of UV printing inks, various auxiliary agents need to be added in the UV ink formulation. UV inks have the advantages of less dosage, more varieties, and higher efficiency. For the requirements of plastic gravure to UV ink properties, the necessary adjuvants are:
(1) Inhibitors. In order to prevent the UV printing ink from cross-linking during the storage period, the auxiliary agent added to improve the storage stability of the ink. The commonly used polymerization inhibitors include hydroquinone, p-methoxyphenol, p-benzoquinone, 2.6-di-tert-butylphenol, etc. These polymerization inhibitors do not inhibit the photopolymerization reaction and are only added to prevent the thermal polymerization reaction. of.
(2) Defoamer. Gravure ink is a kind of thin ink with low viscosity. In the process of using it, the pumping method is very easy to cause bubbles in the ink, which will cause many printing failures. Therefore, the defoamer is an indispensable auxiliary agent for UV-curing plastic ink. Commonly used defoamers include silicone oils (F810, FN) and silicone acrylates (R2500).
(3) Auxiliary wax. In order to improve the rheology of the ink, improve the water resistance of the ink, enhance the abrasion resistance of the ink, etc., the wax is usually added for modification.

3.5 Design of Target Formula Using Orthogonal Experiments Gravure inks are thin, low-viscosity, quasi-Newtonian fluids, the printing dots are not pressed, the printing film is the thickest in all printing methods, and the plastic concave ink is adsorbed on the plastic surface and printed. The drying speed is the technical focus to be solved. Combining the characteristics of plastic gravure and plastic concave ink, the low-molecular-weight polyamide, comb prepolymer and active prepolymer were selected for the targeted formulation of UV-curable plastic gravure inks. Comb prepolymer UV-cured plastic gravure ink adhesion, drying speed and gloss, to determine the best link material formula; then, with the link material, photosensitizer and pigment filler and then carry out three factors and three levels of orthogonal test Focused on the adhesion, drying speed, and gloss of the comb-type prepolymer UV-cured plastic gravure ink, and finally determined the best UV-curable plastic gravure ink formulation.

3.6 Ink test results In accordance with the corresponding testing standards, the developed comb-type prepolymer UV-curing plastic printing inks were tested. Each of the testing indicators met the requirements of plastic gravure printing for ink performance, especially the ink Under the irradiation of ultraviolet light, it can be instantaneously dried (2~3S), which is very favorable for high-speed printing. There is no residual solvent after the printing film is cured, and the printing product meets the hygienic standard. At the same time, the other performance indexes of the ink are equal to or better than other plastic concave inks. The test indicators of this product are listed in Table 1.

4 Conclusion

A low molecular weight polyamide resin is obtained by polycondensation of dimer diacids, diamines, and monobasic acids, and is equimolarly reacted with hydroxyethyl acrylate and diisocyanate (TDI) to obtain ethyl acrylate urethane monoisocyanate. Addition of low-molecular-weight polyamide resin and ethyl acrylate urethane monoisocyanate to give a comb-type prepolymer having a polyamide main chain and ethyl acrylate dicarbamate as a side chain This prepolymer is properly compounded with suitable reactive diluents, photoinitiators, pigments and ink additives to meet the needs of gravure printing for plastics (PE, PP, PS, PET, PA ) UV-curing printing inks for films.

Author / Cui Jinfeng Zhou Yingping Source: Print World