14. The fixation fastness of screen printing ink.
Ink is the most important material in screen printing and occupies an extremely important position in the entire printing process. Because of the wide range of screen printing objects, such as paper, metal, plastic, glass, wood, ceramics and various textiles and so on. Therefore, proper control of ink performance is an essential condition for smooth screen printing.
The most prominent problem of screen printing ink is the fastness of the ink on the substrate after printing. Without solid fastness, it means that there is no printing, and it will also cause great waste. The fixation fastness problem involves the adhesion mechanism of the ink and the substrate. At present, it is generally believed that the following factors affect the fixation fastness of ink.
1 The effect of the resin. The resin is the main component of the ink binder, and the anchoring effect of the ink is closely related to the resin properties.
a. Molecular weight. If the molecular weight of the resin is too close to the molecular weight of the substrate, the ink solvent is not easily selectable. If the molecular weight of the resin is too large, in order for the ink to reach the required viscosity for screen printing, the solid (resin) content in the ink is too small, resulting in waste of solvent. It has a high molecular weight, a high melting point, and is difficult to dissolve in solvents, which is detrimental to the adhesion effect. If the molecular weight of the resin is too small, the cohesive force and compatibility are too poor, and the adhesion effect is poor. The molecular star selection of the ink distribution resin is important and must be suitable.
b. Glass transition temperature. The polymer segment movement only has a significant diffusion rate above the glass transition temperature. Whole chain movement can only be performed at higher temperatures. The glass transition temperature of the resin for room-temperature curing inks should be very low, which is obviously advantageous for the adhesion effect. For heat-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 (dry) temperature.
c. Resin components. The functional groups and branching structures in the resin molecule directly affect the fixation effect of the ink on the substrate. Therefore, the screen-printing ink is more and more specific to different substrates. A single resin can not always meet the needs of special screen printing inks. In fact, two or three-way copolymer resins, mixed resins, and resin modification methods can be used to formulate inks with excellent texture. The selection of a suitable resin component is advantageous for the fixation effect.
d. The monomer content of the resin monomer. The change in the content of a certain element in the synthetic resin monomer changes the performance of the resin. When designing an ink formulation, synthesizing some special resins will greatly improve ink performance. Changes in the content of a certain element of the resin monomer will lead to a prominent change in a certain characteristic of the resin.
e. The polarity and crystallinity of the resin. The presence of polarity facilitates the fixation effect, but the highly polar linear polymer is also high in crystallinity, and copolymerization and grafting techniques can be used to modify the resin to destroy the crystallinity and maintain the polarity.
Practice has shown that epoxy resins can produce chemical bonds on wood, metals, certain plastics, and many other substances. Therefore, many special screen printing inks are often formulated with epoxy resins in order to achieve an ideal fixation effect.
2 solvent effect. The ink formulation solvent also affects the fixation effect. We know that the solubility parameters of the resin and the solvent used are similar, and the solubility parameters of the substrate must be considered simultaneously. However, for plastic screen printing inks, we must consider them from other screen printing effects (other special inks should be used as well).
a. The solubility parameters of the resin and the solvent are similar, which is conducive to dissolution.
b. The solubility parameters of plastics and plasticizers are similar to plasticizing.
c. The solvent of the ink is similar to the solubility parameter of the plastic, which is beneficial to the fixation effect, but it is necessary to prevent the plastic surface from being excessively eroded by the ink solvent (even causing deformation of the plastic).
d. The solubility parameters of the ink solvent and the plastic plasticizer are likely to cause the plasticizer to be extracted or extracted.
e. High-boiling solvents use too much fixation effect to decrease.
f. Improper choice of solvent fixing effect. The use of solvents must be considered from a variety of factors, so ink solvents often use more than three mixed solvents.
g. The solvent and the binder (high molecular polymer) in the ink are in contact with the substrate to swell and dissolve the surface of the substrate, so that the binder in the substrate and the ink interdiffuses to form a mixed layer with an unbounded intermediate layer. This leads to the disappearance of the interface and the formation of a strong bond between the ink and the substrate.
Diffusion process is the process of ink solvent dissolved on the surface of the substrate, and the solubility of the substrate is determined by the polarity of the binder and solvent in the ink. Only when the ink and the substrate have similar polarities can they be dissolved and have adhesive fastness.
The binder in the ink is generally a macromolecular polymer having a linear structure. The molecular ends of the ink are easily diffused into the substrate. The short side chain is not conducive to diffusion, and the large branch chain is favorable for diffusion.
The linking material has a large molecular weight and a long chain length, and has slow penetration and slow diffusion. When the molecular weight is small, the permeation is quick and the binding force is large, but the cohesive strength is small in itself. Therefore, the high-molecular polymer as a linking material needs to have an optimum molecular weight, which facilitates diffusion and has a certain cohesive strength.
The binder is sensitive to the temperature during the diffusion process to the substrate. Increasing the temperature can promote the Brownian motion of the macromolecule, which is beneficial to the speed and capability of the diffusion, and also to the adhesion fastness of the ink.
3 additive effects. A wide variety of additives will greatly improve the performance of the ink. Additives that have an effect on fixation are:
a. Crosslinker. The cross-linking 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 of the resin and the macromolecule printing material in the ink, and can improve the fixing effect of the ink.
c. Plasticizers. The plasticizer can improve the softness of the ink film and at the same time, it is also advantageous for the fixing effect.
4 ink performance effects. The wettability of the ink is good, and it is beneficial to the fixing effect. If the substrate has not been wetted, the ink has dried or solidified, and the fixation effect is poor, which involves the problem of the flowability of the ink. The flow performance is also closely related to the other factors of screen printing, and only the fixation effect is discussed here.
a. To formulate the ink, the volatilization rate of the solvent must be controlled, and the presence of the high-boiling solvent facilitates the leveling.
b. The use of an appropriate molecular weight of the resin to formulate the ink, the higher the viscosity of the resin, the more unfavorable the ink leveling.
c. Add the appropriate leveling agent to the ink.
5 The influence of the substrate. Substrate material directly affects the fixation effect of screen printing ink. The formulation of special inks for wood and substrates is one way to overcome this effect. Through the modification of the substrate, surface treatment and other methods, it can also improve the ink's fixation fastness on the substrate.
a. Through the modification of the substrate, the fixing effect of the ink can be improved. If certain fillers are added to olefins that are difficult to screen-print, such as BaSO4 added to polypropylene resin, and some random polypropylene added, the fixation effect of the products on the ink will be greatly improved.
b. Cleaning the surface of the substrate can also improve the fixation fastness of the ink. The presence of dust, grease, and dirt on the surface of the substrate makes the ink difficult to fix and must be removed.
c. Oxidation of the surface of the substrate, changing the surface state of the substrate, etc. will greatly improve the effect of the ink. Screen printing olefins, which are difficult to screen, can be screen printed more smoothly by chemical treatment, flame treatment, and electronic impact.
d. Pretreatment of the substrate with a coupling agent promotes the activation of surface groups and also improves the anchoring effect of the ink on the substrate.
Screen printing ink performance solution - Solid fastness
June 18 07:10:20, 2019
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