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, and the substrates include paper, wood, metal, plastic, glass, ceramics, textiles, etc. Therefore, correctly grasping the performance of the ink is an indispensable prerequisite for smooth screen printing. The ink for screen printing is formed by pigments, connecting materials, fillers and auxiliary materials according to the suitability requirements of the screen printing process and printing substrate materials, so that the pigments are evenly dispersed in the connecting material composed of polymer compounds. A stable suspended colloidal substance. Whether the screen printing ink is in a stable state depends to some extent on the dispersion of the pigment and the viscosity of the binder, and more importantly depends on whether the pigment particles are surrounded by the surface active material in the binder to form a protective layer . The role of the pigment in the ink is to give the ink color and determine the ink concentration according to the amount used; give the ink a certain thickness and other physical properties; maximize the ink to have strong durability; in a certain To a certain extent, it affects the drying of ink, and the most obvious is oxidative conjunctiva drying. The connecting material of the screen printing ink is a fluid medium with a certain viscosity that plays the role of dispersing the pigment in the ink. Its role in the ink is to give the ink a certain viscosity, viscosity, flow performance, thixotropy; Ink drying type and drying speed; determine ink gloss, abrasion resistance, firmness, etc. One of the most prominent problems of screen printing is the fastness of the ink to the substrate after printing. If there is not a good adhesion fastness, it is equivalent to no printing, and it will cause a lot of waste. The problem of fastness involves many variable factors such as the adhesion mechanism of ink and substrate, the formulation of ink composition, and the quality of chemical raw materials. The factors affecting the fastness of screen printing ink adhesion are as follows. 1. The influence of resin in screen printing ink Various resins are an important component of the screen printing ink connecting material. The adhesion of the ink is good or bad. How the effect is closely related to the performance of the resin. Resins are divided into natural resins and synthetic resins, which are polymers or condensates of some organic compounds. Natural resins include rosin, peach gum, gum arabic, etc., which are amorphous or crystalline structures. The characteristic of the resin is that it gradually softens when heated and generates a viscous and stable melt. Another distinctive feature is its ability to dissolve or melt in organic solvents. The resins commonly used in ink binders are phenolic resins, alkyd resins, maleic anhydride resins, polyamide resins, etc. Their functions are: â‘ Improve the gloss of the imprint, it is not easy to yellow after the conjunctiva, and the light resistance is good; â‘¡Enhance the adhesion fastness, fluidity and thixotropy of imprinting. For example, the alkyd resin ink is fastened on the paper, and the wear resistance after drying is also good; â‘¢Vegetable oil modified alkyd resin is a good wetting agent. Add more pigments to the ink containing alkyd resin, and the viscosity does not change much, and the fluidity is good; â‘£After adding resin to the ink connecting material, the ink layer has strong adhesion, soft quality, good drying performance, bright conjunctiva, and has good printing performance. The composition of the resin: the functional groups and branched structure in the resin molecule directly affect the adhesion effect of the ink on the substrate, so the special ink for screen printing is more and more specific for different substrates, and a single resin often cannot meet the special screen printing The need for ink. In fact, the use of binary or ternary copolymer resins, mixed resins, or modification of resins can be used to formulate excellent-quality inks, and the selection of appropriate resin components is very beneficial to the firmness of the ink. The molecular weight of the resin: If the molecular weight of the resin is too close to the molecular weight of the substrate, the ink solvent is not easy to choose. The molecular weight of the resin is too large. In order to make the ink reach the viscosity required for screen printing, the content of the solid resin in the ink is too small, resulting in a waste of solvent. The molecular weight is large, the melting point is high, and it is difficult to dissolve in the solvent, which is detrimental to the ink adhesion effect; on the contrary, if the molecular weight of the resin is too small, then the cohesion and compatibility are too poor, and the adhesion effect will be poor. Therefore, it is very important to choose the appropriate molecular weight resin, which must be adapted to it. Glass transition temperature: the polymer segment motion has a significant diffusion speed only above the glass transition temperature. The entire chain movement can only be carried out at a higher temperature, and the glass transition temperature of the resin cured at room temperature should be very low, which is obviously beneficial to the adhesion effect. For thermosetting inks, the heating temperature should be higher than the glass transition temperature and lower than the thermal deformation temperature of the substrate. Therefore, the ink adhesion effect is related to the glass transition temperature. In general, in practical applications, epoxy resins can produce chemical bonds to many substances such as wood, metals, and certain plastics, so many special screen printing inks are often formulated with epoxy resins to achieve the desired adhesion effect. Second, the influence of the solvent in the ink binder In the production process of ink production, the solvent will also affect the adhesion effect. We know that it is appropriate that the solubility parameters of the resin and solvent used are similar, and the solubility parameters of the substrate should also be considered to be similar. However, the plastic screen printing ink must be considered from other screen printing effects. â— The solubility parameters of resin and solvent are similar, which is good for dissolution. â— The solubility parameters of plastic and plasticizer are similar, which is beneficial to increase plasticity. â— The solubility parameters of the ink solvent and plastic are similar to the effect of adhesion, but it is necessary to prevent the plastic surface from being eroded by the ink solvent (even causing plastic deformation). â— Too much solvent with high boiling point will reduce the adhesion effect. â— Inappropriate selection of solvent has poor adhesion effect. The use of solvents should be considered from various factors, so the printing ink solvent should use more than three kinds of mixed solvents. â— The solvent and the connecting material (polymer) in the ink are in contact with the substrate to dissolve and dissolve the surface of the substrate, so that the substrate and the connecting material in the ink diffuse to each other, forming a layer of mixed unbounded middle Layer, leading to the disappearance of the interface, forming a strong combination of ink and substrate. 3. The influence of additives in ink There are many types of additives in the ink, mainly to improve the performance of the ink. The additives that have an effect on the adhesion effect are as follows: Cross-linking agent: The cross-linking agent can not only improve the adhesion effect of the ink, but also greatly improve the hardness, solvent resistance, water resistance and drying speed of the ink. Coupling agent: The coupling agent promotes the coupling effect of the resin and polymer printing material in the ink, which can improve the adhesion effect of the ink. Plasticizer: Plasticizer can improve the flexibility of the ink film, and it is also beneficial to the adhesion effect. Fourth, the impact of ink performance Screen printing ink has good wettability and is beneficial to the adhesion effect. If the substrate has not been wetted and the ink has been dried or cured, the adhesion effect is poor, which involves ink fluidity. The flow performance is also closely related to other factors of screen printing, and only the ink adhesion problem is discussed here. â— The speed of volatilization of the solvent must be controlled when preparing the ink. The presence of high-boiling solvents is conducive to leveling. â— Use proper molecular weight resin to formulate ink, the higher the viscosity of the resin, the more disadvantageous is the ink leveling. â— Add proper leveling agent to screen printing ink. Fifth, the impact of the substrate on the adhesion fastness during the printing process The material of the screen printing substrate directly affects the adhesion effect of the ink. Formulating special inks for different substrates is a good way to overcome this effect. Usually there are plastic ink, paper ink, fabric ink, wood ink, metal ink, leather ink, printed circuit board ink, ceramic, glass ink, etc. For different printing materials, choose the appropriate ink, and by changing the performance of the screen printing substrate, surface treatment and other means, it can also improve the adhesion of the ink on the substrate. â— When printing on the printing material, it is very important that the printing material should be strictly degreased and checked before printing. When oils, adhesives, dust, etc. are attached to the surface of the substrate, it will cause the ink to adhere to the substrate poorly. Insufficient surface treatment of plastic products before printing will also cause the problem of poor ink adhesion. Therefore, cleaning the surface of the screen printing substrate can also improve the adhesion of the ink on it, and the presence of dust, grease, and dirt on the substrate makes the ink difficult to adhere and must be cleaned. â— The screen printing material is a polyethylene film, in order to improve the adhesion performance with the ink during printing, surface flame treatment must be carried out; if it is a metal material, it must be degreased and dedusted before printing, and the metal cannot absorb ink after printing The film can only be oxidized and condensed, evaporated and dried, and dried according to the temperature required by the ink. If the drying process is improper, the ink film will fall off. In addition, in order to make textiles waterproof in textile printing, silicon processing is generally required to ensure good ink fastness during printing. â— Articles such as glass and ceramics should be grilled at high temperature after screen printing, so as long as the temperature treatment is appropriate, the adhesion of the ink layer will be good. A simple method to check the good and bad adhesion of ink film: glass and ceramic prints are exposed to rain and dew to see if the ink is easy to peel off and fade. When the printed substrate is paper, if you want to check the fastness of the ink, you can bend the printing surface repeatedly to see if the ink layer at the crease peels off. If the ink peels off, the ink fastness is poor, otherwise it is strong.
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Element content of resin monomers: Changes in the content of certain elements of synthetic resin monomers will also change the performance of the resin. When designing the ink process 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 cause a prominent change in a certain characteristic of the resin.
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Five key to the fastness of screen printing ink adhesion