Five Keys of Screen Printing Ink Fastness

Ink is the most important material in screen printing. It occupies an extremely important position in the entire printing process because the objects of screen printing are quite extensive. The substrates include paper, wood, metal, plastic, glass, ceramics, and textiles. Therefore, the correct understanding of the ink's performance is an indispensable prerequisite for smooth screen printing.

The screen printing ink is made up of pigments, binders, fillers and auxiliary materials according to the suitability requirements of the screen printing process and the printing substrates, so that the pigments and the like are uniformly dispersed in the binder composed of the macromolecular compound and formed. A stable, suspended colloidal material. Whether the screen printing ink is in a stable state depends to a certain extent on the dispersion of the pigment and the viscosity of the binder, and more importantly, it depends on whether the pigment particles are surrounded by the surface active material in the binder. .

The role of pigments in inks is to impart color to the ink and determine the concentration of the ink according to the amount; to give the ink a certain thickness and other physical properties; to maximize the durability of the ink; in a certain degree To a certain extent, the ink dryness is affected, and the oxidative conjunctiva drying is the most significant.

The printing material for screen printing ink is a kind of fluid medium with a certain viscosity which acts as a dispersing pigment in the ink. Its role in the ink is to give the ink a certain viscosity, viscosity, flow properties, thixotropy; Drying type and drying speed of ink; determining ink glossiness, abrasion resistance, firmness, etc.

One of the most prominent problems in screen printing is the fastness of the ink on the substrate after printing. If there is not a good adhesion fastness, it means no printing, and it will also cause great waste. The problem of fastness involves many factors such as the adhesion mechanism of inks and substrates, the formulation of ink components, and the quality of chemical raw materials. There are several factors that affect the adhesion fastness of screen printing inks.

First, the effect of resin in screen printing ink

Various resins constitute an important component of the screen printing ink connection material, and the adhesion fastness of the ink is good and bad, and 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, and gum arabic, and are amorphous or crystalline structures. The characteristics of the resin are gradual softening upon heating and the formation of a viscous, stable melt. Another distinctive feature is the ability to dissolve or melt in organic solvents. The commonly used resins in the ink binders are phenolic resins, alkyd resins, maleic anhydride resins, polyamide resins, and the like, and their effects are:

1 to improve the glossiness of the blot, the yellowing is not easy after the conjunctiva, and the lightfastness is better;

2 enhance the imprinted fastness, fluidity, thixotropy, such as alkyd resin ink fixed faster on the paper, after drying is better;

3 The vegetable oil modified alkyd resin is a good wetting agent, adding more pigments to the ink containing alkyd resin, and the viscosity does not change much, and the fluidity is good;

4 After the resin is added into the ink connecting material, the ink layer has strong adhesion, soft quality, good drying performance, bright conjunctiva, and good printing performance.

Resin composition: The functional groups and the branched structure of the resin molecules directly affect the adhesion of the ink on the substrate. Therefore, screen printing inks are more and more specific to different substrates, and a single resin can not satisfy the special screen printing. The need for ink. In fact, the use of two-way or three-way copolymer resins, mixed resins, or modification of resins can produce inks with excellent textures, and the selection of suitable resin components is very favorable to the ink's firmness.

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 easily selectable. The molecular weight of the resin is too large. In order for the ink to reach the viscosity required for screen printing, the content of the solid resin in the ink is too small, resulting in waste of the solvent. On the other hand, if the molecular weight of the resin is too small, the cohesive force and compatibility are too poor, and the adhesion effect will be poor. Therefore, it is very important to choose the appropriate molecular weight resin and it must be adapted accordingly.

Glass transition temperature: The polymer segment movement only has a significant diffusion rate 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 attachment 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.

Elemental content of the resin monomer: The change in the content of a certain element of the synthetic resin monomer also changes the performance of the resin. When designing an ink process recipe, the synthesis of some special resins will greatly improve the performance of the ink. 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.

Under normal circumstances, epoxy resin can produce chemical bonds to many substances such as wood, metal, and certain plastics in practical applications. Therefore, many special screen printing inks are often formulated with epoxy resin to achieve the desired adhesion effect.

Second, the influence of the solvent in the ink binder

In the ink production and formulation process, the solvent also affects the adhesion 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, it must be considered from other screen printing effects.

● Solubility parameters of resin and solvent are similar, which is conducive to dissolution.

● The solubility parameters of plastics and plasticizers are similar to those for increasing plasticity.

● The solvent of the ink is similar to the solubility parameter of the plastic, which is favorable for the attachment effect, but it is necessary to prevent the plastic surface from being excessively eroded by the ink solvent (even causing the plastic to deform).

● The use of excessively high-boiling solvents will reduce the adhesion effect.

● Improper use of solvents has poor adhesion. The use of solvents must be considered from a variety of factors, so the solvent used for printing inks should use more than three kinds of mixed solvents.

● The solvent and binder in the ink (high molecular polymer) 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 mixed unbounded line. The layer causes the disappearance of the interface, forming a strong bond between the ink and the substrate.

Third, the impact of additives in the ink

The variety of additives in the ink is mainly used to improve the performance of the ink. Additives that have an effect on the adhesion are as follows:

Cross-linking agent: The cross-linking agent can not only improve the adhesion of the ink, but also can greatly improve the hardness, solvent resistance, water resistance and drying speed of the ink.

Coupling agent: Coupling agent to promote the coupling of resin and macromolecule printing materials in the ink, can improve the adhesion of the ink.

Plasticizers: Plasticizers can improve the softness of the ink film and at the same time have an advantageous effect on the adhesion.

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, the ink has dried or has been cured, the adhesion effect is poor, which involves the problem of ink fluidity. Flow performance is also closely related to other factors of screen printing. Here only ink adhesion problems are discussed.

● The ink must be formulated to control the volatilization rate of the solvent. The presence of a high-boiling solvent is beneficial to leveling.

● Use an appropriate molecular weight of the resin to formulate the ink, the higher the viscosity of the resin, the more unfavorable the ink leveling.

● Add appropriate leveling agent to the screen printing ink.

Fifth, the influence of substrates on the fastness of adhesion in the printing process

The material of the screen printing substrate directly affects the adhesion of the ink. Designing special inks for different substrates is a good way to overcome this effect. Plastic inks, paper inks, textile inks, wood inks, metal inks, leather inks, printed circuit board inks, ceramics, and glass inks are commonly used. For different printing materials, the ink of the corresponding adaptability is selected, and the fastness of the ink on the printing material can also be improved by means of changing properties of the screen printing material, surface treatment and the like.

● When printing on printed materials, it is very important that the printed materials are subjected to strict degreasing and pre-press inspection before printing. When the surface of the substrate adheres to oils, adhesives, dust, etc., the adhesion between the ink and the substrate is not firm. Inadequate surface treatment of plastic products prior to printing can also result in ink adhesion problems. Therefore, cleaning the surface of the screen printing substrate can also improve the adhesion of the ink on the surface, and the presence of dust, grease, and dirt on the substrate makes it difficult for the ink to adhere and must be removed.

●The screen printing material is a polyethylene film. In order to improve the adhesive performance with the ink during printing, surface flame treatment must be performed; if it is a metal material, it must be degreasing and dust removal before it can be printed. Since the metal cannot absorb ink after printing Membrane, only the use of oxidized conjunctiva, volatile drying methods, and in accordance with the required temperature of the ink drying process Caixing. Improper drying can also cause the ink film to fall off. In addition, in order to make the textile waterproof in the textile printing, silicon processing is generally required in order to ensure good ink adhesion during printing.

● The surface of the substrate will be oxidized and the surface state of the substrate will be changed. This will greatly increase the adhesion of the ink. Generally, for substrates that are difficult to screen, such as olefins, screen printing can be performed smoothly after chemical treatment, flame treatment, and electronic impact.

● Items such as glass and ceramics must be cooked at high temperatures after screen printing. Therefore, as long as the temperature is properly treated, the adhesion of the ink layer will be good. A simple method to check the adhesion of the ink film is good and bad: glass and ceramic prints are exposed to the rain, to see if the ink is easy to peel off. When the printed material is paper, in order to test the fastness of the ink, the printed surface can be repeatedly bent to see if the ink layer at the fold is peeled off. If the ink is peeled off, then the fastness of the ink is poor, and vice versa.

Reprinted from: China Ink Technology Network