Five key points for the adhesion of screen printing inks
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Ink is the most important material in screen printing, and it plays an extremely important role in the whole printing process, because screen printing has a wide range of objects, such as paper, wood, metal, plastic, glass, ceramics, textiles and so on. Therefore, correctly understanding the performance of the ink is an indispensable prerequisite for smooth screen printing.
The screen printing ink is formed by pigments, binders, fillers and auxiliary materials according to the suitability requirements of the screen printing process and the printing substrate, so that the pigments are uniformly dispersed in the binder composed of the polymer compound. A stable suspension of colloidal material. Whether the screen printing ink is stable depends to some extent on the dispersion of the pigment and the viscosity of the binder, and more importantly on whether the pigment particles are surrounded by the surface active material in the binder. .
The role of the pigment in the ink is: to give the ink a color, and to determine the concentration of the ink according to the amount of the ink; to give the ink a certain thickness and other physical properties; to maximize the ink has a strong durability; To a certain extent, it affects the drying property of the ink, and the drying of the oxidized conjunctiva type is most remarkable.
The connecting material of the screen printing ink is a fluid medium having a certain viscosity in the ink to act as a dispersing pigment, and its role in the ink is: giving the ink a certain viscosity, viscosity, flow property, thixotropy; The drying type and drying speed of the ink; determine the gloss, abrasion resistance, and firmness of the ink.
One of the most prominent problems in screen printing is the adhesion of the ink on the substrate after printing. If there is no good adhesion fastness, it means that there is no printing, and it will cause a lot of waste. The fastness problem involves a variety of factors such as the bonding mechanism of the ink and the substrate, the formulation of the ink composition, and the quality of the chemical raw material. The factors affecting the adhesion fastness of screen printing inks are as follows.
First, the influence of resin in screen printing ink
Various resins are important components of the screen printing ink connecting material, and the adhesion fastness of the ink is good and bad, and the effect is closely related to the performance of the resin.
The resin is divided into a natural resin and a synthetic resin, which are polymers or condensates of some organic compounds. Natural resins are rosin, peach gum, gum arabic, etc., which are amorphous or crystalline structures. The characteristic of the resin is that it gradually softens upon heating and forms a viscous, stable melt. Another striking property is that it can be dissolved or melted in an organic solvent. The commonly used resins in the ink binder are phenolic resin, alkyd resin, maleic anhydride resin, polyamide resin, etc., and their functions are as follows:
1 to improve the gloss of the imprint, after the conjunctiva is not easy to yellow, light resistance is better;
2 enhanced imprint adhesion fastness, fluidity, thixotropic, such as alkyd resin ink fixed fast on paper, wear resistance after drying is also better;
3 vegetable oil modified alkyd resin is a good wetting agent, adding more pigment in the ink containing alkyd resin, and the viscosity does not change much, and the fluidity is good;
4 After adding the resin in the ink connecting material, the ink layer has strong adhesion, soft texture, good drying performance, bright conjunct film, and good printing performance.
The composition of the resin: the functional group and the branched structure in the resin molecule directly affect the adhesion effect of the ink on the substrate. Therefore, the special ink for screen printing is more and more specific for different substrates, and the single resin often cannot meet the special screen printing. The need for ink. In fact, it is advantageous to use a two- or three-dimensional copolymer resin, a mixed resin or a resin modification to prepare an ink having excellent texture, and to select a suitable resin component, which is advantageous for the firmness of the ink.
Molecular Weight of 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 select. The molecular weight of the resin is too large. In order to achieve the viscosity required for screen printing, the content of the solid resin in the ink is too small, resulting in waste of solvent. The molecular weight is large, the melting point is also high, and it is difficult to dissolve in a solvent, which is unfavorable for the adhesion of the ink; on the contrary, if the molecular weight of the resin is too small, the cohesive force and compatibility are too poor, and the adhesion effect is poor. Therefore, it is very important to choose a resin with an appropriate molecular weight and it must be adapted to it.
Glass transition temperature: The polymer segment movement has a significant diffusion rate only above the glass transition temperature. The whole chain movement can only be carried out at a higher temperature, and the glass transition temperature of the resin for the room temperature curing ink 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. Therefore, the ink adhesion effect is related to the glass transition temperature.
Element content of the resin monomer: The change in the content of a certain element of the synthetic resin monomer also causes the performance of the resin to vary. When designing the ink process recipe, the synthesis of 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.
Under normal circumstances, in practical applications, epoxy resin can produce chemical bonds to many materials such as wood, metal, and certain plastics. Therefore, many special screen printing inks are often formulated with epoxy resin to achieve the desired adhesion effect.
Second, the influence of solvent in the ink binder
Solvents also affect the adhesion during ink production and formulation. We know that the solubility parameters of the resin and solvent used are similar, and the solubility parameters of the substrate should be considered at the same time. However, plastic screen printing inks should be considered from other screen printing effects.
● The solubility parameters of the resin and solvent are similar, which is good for dissolution.
● The solubility parameters of plastics and plasticizers are similar to increase plasticity.
● The solvent of the ink is similar to the solubility parameter of the plastic, which is advantageous for the adhesion effect, but the plastic surface is prevented from being excessively eroded by the ink solvent (even causing deformation of the plastic).
● High boiling point solvents use too much adhesion effect.
● Poor solvent adhesion is poor. The use of the solvent is considered from various factors, so that the solvent for printing the ink uses three or more mixed solvents.
● The solvent and the binder (polymer) in the ink are in contact with the substrate, and the surface of the substrate is dissolved and dissolved, so that the substrate and the binder in the ink are mutually diffused to form a layer of unbounded lines after mixing. The layer, which leads to the disappearance of the interface, forms a strong bond between the ink and the substrate.
Third, the influence of additives in the ink
There are many kinds of additives in the ink, mainly to improve the performance of the ink, and the additives that have an effect on the adhesion effect are as follows:
Crosslinking agent: The crosslinking agent can not only improve the adhesion 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 the polymer printing material in the ink, and can improve the adhesion effect of the ink.
Plasticizer: The plasticizer can improve the softness of the ink film and is also advantageous for the adhesion effect.
Fourth, the impact of ink performance
The screen printing ink has good wettability and is advantageous for the adhesion effect. If the substrate has not been wetted, the ink has dried or has been solidified, the adhesion effect is poor, which involves the problem of ink flowability. Flow performance is also closely related to other factors of screen printing. Only ink adhesion problems are discussed here.
● The formulation of the ink must control the evaporation rate of the solvent, and the presence of the high-boiling solvent is conducive to leveling.
● The ink is formulated with a resin of appropriate molecular weight. The higher the viscosity of the resin, the more unfavorable the leveling of the ink.
● Add an appropriate amount of leveling agent to the screen printing ink.
Fifth, effect of substrate on adhesion fastness during printing
The material of the screen printing substrate directly affects the adhesion of the ink. It is a good way to overcome this effect by formulating special inks for different substrates. There are usually plastic inks, paper inks, textile inks, wood inks, metal inks, leather inks, printed wiring board inks, ceramics, glass inks, and the like. For different printing materials, the ink corresponding to the suitability is selected, and the adhesion fastness of the ink on the substrate can also be improved by means of changing the performance of the screen printing substrate and surface treatment.
● When printing on printing materials, it is important to perform strict degreasing and pre-press inspection of the printing materials before printing. When the surface of the substrate is attached with grease, adhesive, dust, etc., the ink and the substrate are not firmly bonded. Insufficient surface treatment of plastic products before printing can also cause problems of poor ink adhesion. Therefore, cleaning the surface of the screen printing substrate can also improve the adhesion fastness of the ink thereon, and the presence of dust, grease, and dirt on the substrate makes the ink difficult to adhere and must be cleaned.
●The screen printing material is a polyethylene film. In order to improve the adhesion 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 it can be printed. After printing, the metal cannot absorb ink. The film is only dried by oxidized conjunctiva, volatile drying method, and dried according to the temperature required by the ink. If the drying process is improper, the ink film may fall off. In addition, in textile printing, in order to make the textile waterproof, it is generally necessary to carry out silicon processing, so as to ensure good adhesion of ink during printing.
● Items such as glass and ceramics must be baked at a high temperature after screen printing, so the adhesion of the ink layer will be good as long as the temperature is properly treated. A simple method to check the adhesion and fastness of the ink film: glass and ceramic prints are exposed to the rain and dew to see if the ink is easily peeled off and faded. When the printed substrate is paper, in order to check the fastness of the ink, the printed surface can be repeatedly bent to see if the ink layer at the crease is peeled off. If the ink is peeled off, the ink adhesion fastness is poor, and vice versa.