Improvement of rheological properties and printability of UV offset inks
(1) Pigment Wettability
The linker of UV Offset Ink is mainly composed of high polarity oligomer and active diluent, and most of the pigments used in UV Offset Ink are organic pigments with low polarity. The affinity of these high polarity oligomer systems to the surface of organic pigment particles with low polarity is poor, and it is difficult to provide good pigment wettability. At the same time, the main body acrylate oligomer used in UV offset ink linker has a low molecular weight, usually 1000 to 2000 (while the molecular weight of rosin-modified phenolic resin used in traditional offset ink is 20,000 to 50,000). In short, compared with traditional offset inks, the resin and linker system of UV offset inks are characterized by low molecular weight and high polarity, which cannot provide satisfactory pigment wettability and dispersion stability.
In order to improve the pigment wettability and dispersion of UV offset inks and increase the production efficiency, the first step is to select a suitable main body oligomer resin. Fatty acid modified acrylate oligomer has good pigment wettability due to its high molecular weight and low polarity, which is especially suitable for offset ink systems with high pigment content. However, because part of the acrylic functional group is replaced by fatty acid, fatty acid modified acrylate has poor reactivity and low Tg, which will affect the curing rate of the ink and the physical properties of the ink film, and usually must be used in conjunction with low viscosity, high functionality oligomers. Selection of suitable pigments is also one of the key factors to improve the dispersion performance of the ink, but in addition to UV ink carbon black, there are very few organic pigments specifically used in conjunction with UV offset inks, the final determination of organic pigments can only rely on a large number of experiments to carry out. However, there is a principle for the selection of pigment for UV system, that is, the pigment applicable to UV Offset Ink should have certain polarity.
(2) Water-ink balance performance
In the actual printing process, UV Offset Ink has little tolerance for the amount of water in the fountain solution, and the water-ink balance is difficult to control, so it is difficult to achieve the printability of traditional offset ink. The reasons for the poor water-ink balance of UV Offset Ink are mainly in the following two aspects, the first is that the acrylate oligomer and active diluent used in UV Offset Ink usually have high polarity groups such as hydroxyl, carboxyl and amino groups, which have strong affinity for the fountain solution, and the emulsification possibility is greater compared with that of the oil-based connecting material with small polarity. Secondly, the printing substrate of UV offset ink is mainly based on non-absorbent materials such as synthetic paper, gold and silver cardboard and plastics, which are poorly absorbed by the fountain solution, which adds more variable factors to the ink-water balance in printing. Experience shows that the use of high residual hydroxyl value and polarity of reactive oligomer (including the adjustment of resins, active diluents, additives), must be very careful.
The selection of the host oligomer is the key to improving the ink-water balance of UV offset inks. Due to epoxy acrylate and polyurethane acrylate prepolymer molecular chain contains hydroxyl, amino and other functional groups, has a strong hydrophilicity, and the loss of ink resistance, so from the point of view of improving the ink ink balance performance, polyester acrylate has a better resistance to water, become the best choice. At present, from the point of view of ink performance, most of the UV offset ink system to polyester acrylate as the main resin, to improve the ink-ink balance performance is also one of the main selection basis. But polyester acrylate oligomer in the hardness, friction resistance and solvent resistance is still to be further improved. In addition, under the condition of keeping the viscosity of the ink system similar, the use of higher molecular weight acrylate oligomer can improve the anti-emulsification performance of the ink, but this is the case when the high molecular weight resin in the ink formula reaches a certain proportion, may be due to the high molecular weight resin through the entanglement of each other can form a larger physical mesh structure, which will effectively impede the penetration of water molecules into the ink system, thus improving the ink emulsion performance. It will effectively hinder the penetration of water molecules into the interior of the ink system, thus improving the emulsification performance of the ink. However, the polymer quantization of acrylate oligomers is at the expense of reduced functionality and loss of curing speed, which requires comprehensive consideration when designing ink formulations to compensate for the loss of other properties that deteriorate in order to improve the ink’s water-ink balance properties.
Epoxy acrylate oligomers are the main factor affecting the emulsification rate of inks. Therefore, under the premise of ensuring the curing speed and ink viscosity, should control the amount of epoxy acrylate oligomers, so that UV offset ink can maintain a strong water repellent to ensure that in the actual printing production has a good printing suitability.
(3) Reduce the flying ink phenomenon
Compared with traditional offset ink, there are significant differences in the rheological properties of UV offset ink. First of all, acrylate oligomer pigment wettability is poor, so that the UV offset ink in low shear speed shows a stronger structure, and the ink in the ink tank of the printing machine in the flow of adverse effects. Secondly, acrylate oligomers have special rheological properties and low molecular weight, resulting in poor cohesion of UV offset inks, which are sensitive to temperature changes and exhibit high viscosity, low viscosity and long filament head. Under high-speed printing conditions, due to the lack of viscoelasticity of the ink system, the phenomenon of flying ink is prone to occur. If the pigments with easy dispersion or high mobility are chosen for the sake of fluidity and easy dispersion of the ink, such problems will be more prominent.
At present, most of the methods used to add non-reactive additives to improve the system’s anti-rheological properties, to improve the UV offset ink anti-flying performance. Usually in the ink formula to add gas-phase SiO2, talc, organic bentonite and silicone resins have a certain thickening effect, can improve the thixotropy of the filler, by increasing the cohesion and viscoelasticity of the system to prevent flying ink. However, the effect of these additives is very limited, can not solve the problem fundamentally, and may have an adverse effect on the ink gloss, adhesion and other properties.
Improvement of the rheological properties of the oligomer resin itself is the most effective way to solve the rheological problems of UV offset inks. From a theoretical point of view, the introduction of 100% reactive oligomer resins with rheology control in ink formulations will not only improve the printability of the ink, but also improve the final curing film properties of the ink.
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