Ink is the most important material in screen printing and occupies an extremely important position in the entire printing process. Because the objects of screen printing are quite wide, such as paper, metal, plastic, glass, wood, ceramics and various textiles and so on. Therefore, the correct understanding of the performance of the ink is an indispensable condition for smooth screen printing.
One of the most prominent problems of screen printing ink is the clumping fastness of the ink on the substrate after printing. If there is no fix fastness, it means there is no printing, and it will cause a lot of waste. The problem of fastness to fixation involves the bonding mechanism of ink and substrate. At present, it is generally believed that the reasons that affect the fixing fastness of the ink are as follows.
1. The influence of resin. Resin is the main component of the ink binder, and the fixing effect of the ink is closely related to the performance of the resin.
A. Molecular weight. If the molecular weight of the resin is too close to that of the substrate, the ink solvent is not easy to choose. If the molecular weight of the resin is too large, in order to make the ink reach the viscosity required by the screen printing, the solid (resin) content in the ink is too small, resulting in a waste of solvent. The molecular weight is high, the melting point is also high, it is difficult to dissolve in the solvent, and it is unfavorable to the adhesion effect. If the molecular weight of the resin is too small, the cohesion and compatibility will be too poor, and the adhesion effect will be poor. The molecular star selection of the ink-mixing resin is important and must be appropriate.
B. Glass transition temperature. The movement of the polymer chain segment only has a significant diffusion speed above the glass transition temperature. The entire chain movement can only be carried out at higher temperatures. For the room temperature curing ink, the glass transition temperature of the resin should be very low, which is obviously beneficial to the adhesion effect. For thermal curing inks, the heating temperature should be higher than the glass transition temperature and lower than the thermal deformation temperature of the substrate. The adhesion effect is related to the operating (drying) temperature.
C. The components of the resin. The functional groups and branch structure in the resin molecule directly affect the fixing effect of the ink on the substrate. Therefore, the special ink for screen printing is more and more specific to different substrates. A single resin often cannot meet the needs of special screen printing inks. In fact, the use of binary or ternary copolymer resins, mixed resins, and resin modification methods can prepare inks with excellent texture. Choosing an appropriate resin component is beneficial to the fixing effect.
D. The element content of the resin monomer. The content of a certain element of the synthetic resin monomer changes, so that the performance of the resin also changes. When designing the ink formula, synthesizing some special resins will greatly improve the performance of the ink. A change in the content of a certain element of the resin monomer will result in a prominent change in a certain characteristic of the resin.
E. The polarity and crystallinity of the resin. The existence of polarity is conducive to the fixing effect, but linear polymers with high polarity have high crystallinity. Copolymerization and grafting techniques can be used to modify the resin to destroy crystallinity and maintain polarity.
Practice has shown that epoxy resin can produce chemical bonds to many substances such as wood, metal, and certain plastics. Therefore, many special screen printing inks are often formulated with epoxy resin in order to achieve the ideal fixing effect.
2. The influence of solvent. The solvent used to formulate the ink also affects the fixation effect. We know that it is appropriate that the solubility parameters of the resin and the solvent used are similar, and the solubility parameters of the substrate should also be considered similar. But for plastic screen printing inks, other screen printing effects must be considered (other special inks should also be the same)
A. The solubility parameters of resin and solvent are similar, which is good for dissolution.
B. The solubility parameters of plastics and plasticizers are similar to facilitate plasticization.
C. The solubility parameters of the ink solvent and the plastic are similar to the fixing effect, but the plastic surface should be prevented from being too corroded by the ink solvent (even causing plastic deformation).
D. The solubility parameters of ink solvents and plastic plasticizers are similar, which can easily cause the plasticizer to be extracted or drawn out.
E. Too much high-boiling point solvent reduces the fixing effect.
F. Improper selection of solvent has poor fixation effect. The use of solvents should be considered from many factors, so ink solvents often use more than three mixed solvents.
g. The solvent in the ink and the binder (high molecular polymer) are in contact with the substrate, swelling and dissolving the surface of the substrate, so that the substrate and the binder in the ink diffuse each other, forming a layer of the middle of the unbounded line after mixing Layer, leading to the disappearance of the interface, forming a strong bond between the ink and the substrate.
The diffusion process is the process in which the ink solvent dissolves the surface of the substrate, and the dissolution of the substrate depends on the polarity of the binder and the solvent in the ink. Only when the polarity of the ink and the substrate are similar can it dissolve and have the adhesion fastness.
The binder in the ink is generally a macromolecular polymer with a linear structure. The two ends of the molecule can easily diffuse into the substrate. Short side chains are not conducive to diffusion, and large branches are conducive to diffusion.
The binder has a large molecular weight and a long chain length, and its penetration and diffusion are slow. The molecular weight is small, the penetration is fast, the cohesive force is large, but the cohesive strength is small. Therefore, the high molecular polymer used as the linking material needs to have an optimal molecular weight, which is not only good for diffusion, but also has a certain cohesive strength.
The binder is more sensitive to temperature in the process of diffusion to the substrate. Increasing the temperature can promote the Brownian motion of macromolecules, which is conducive to the speed and ability of diffusion, and it is also conducive to the adhesion of ink.
3. The influence of additives. A wide variety of additives will greatly improve the performance of the ink. The additives that have an effect on the fixation effect are:
A. Crosslinking agent. The crosslinking agent can not only improve the fixing effect of the ink, but also greatly improve the hardness, solvent resistance, water resistance and drying speed of the ink.
B. Coupling agent. The coupling agent promotes the coupling effect between the resin and the polymer printing material in the ink, which can improve the fixing effect of the ink.
C. Plasticizer. The plasticizer can improve the flexibility of the ink film and is also beneficial to the fixing effect.
4. The influence of ink performance. The ink has good wettability, which is beneficial to the fixing effect. If the substrate has not been wetted and the ink has been dried or cured, the fixing effect is poor, which involves the problem of ink flowability. The flow performance is also closely related to other factors of silk screen printing, and only the fixation effect is discussed here.
A. When preparing inks, the volatilization speed of the solvent must be controlled. The presence of high boiling point solvents is beneficial to leveling.
B. Use appropriate molecular weight resin to prepare ink, the higher the viscosity of the resin, the more unfavorable for ink leveling.
C. Add appropriate leveling agent to the ink.
5. The influence of the substrate. The material of the substrate directly affects the fixing effect of the screen printing ink. The preparation of special inks for wooden substrates is a way to overcome this influence. By modifying the substrate, surface treatment and other methods, the fastness of the ink on the substrate can also be improved.
A. Improve the fixing effect of the ink by modifying the performance of the substrate. If some fillers are added to the olefins that are difficult to silk-screen, such as adding some BaSO4 to the polypropylene resin and mixing some random polypropylene at the same time, the fixing effect of the products on the ink will be greatly improved.
B. Cleaning the surface of the substrate can also improve the fastness of the ink on it. The presence of dust, grease and dirt on the surface of the substrate makes it difficult for the ink to be fixed and must be removed.
C. Oxidizing the surface of the substrate, changing the surface state of the substrate, etc., will greatly improve the fixing effect of the ink. Olefins, which are usually difficult to silk-screen, can be silk-screened smoothly after chemical treatment, flame treatment, and electronic shock.
D. Pretreatment of the substrate with a coupling agent to promote the activation of surface groups and also improve the fixing effect of the ink on the substrate.