Discussion on the problems of printing adhesion and film degumming
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Packaging printing products produced by packaging and printing companies often have the following problems:
1 Packaging printed products, there is no problem during printing or use in the low temperature season, but when printing or using in the high temperature season, the phenomenon that the graphic ink films adhere to each other may occur.
2 Packaging printed products have no problem when filming in the high temperature season, and there is a phenomenon of film degumming of the product packaging when used in the low temperature season.
The author believes that the former is caused by the softening point of the thermoplastic coconut in the ink system or the excessive dry solvent in the ink remaining in the printed ink film. The latter is caused by improper or excessive use of dispersing aids in the ink or compound system. This article will be discussed in this article, only for reference by industry colleagues.
1 Use pressure to solve the problem of adhesion caused by the ink layer and the adhesive layer being too thick.
By pressurizing the printing machine and the compound coating machine, the adhesion between the printed ink film layer and the coated composite rubber layer can be closely matched and the ink and glue can flow under pressure to facilitate the ink. The molecules of the system and the molecular diffusion and penetration of the glue system enhance the adhesion strength of the ink to the substrate, the glue and the printed ink film.
Of course, the pressure should not be deformed by the ink film and the glue film. That is to say, the purpose of pressurization is also to discharge excess ink or rubber to reduce the thickness of the ink layer and the adhesive layer to ensure that the entire ink layer and the adhesive layer are uniform and dense, and to prevent voids caused by partial ink or under glue. Prevent some inks and composite glue from generating bubbles due to the discharge of volatiles during heat curing or cooling and curing.
2 Solve the solvent residue problem with a blower.
Usually, we use infrared drying, UV drying, hair cooling or even drying in the packaging and printing process. The purpose of drying is to increase the solvent evaporation in the ink and glue system to facilitate the operation of the ink film forming and glue compounding process. It also allows the ink and glue to wet the substrate surface to facilitate its diffusion, penetration and adsorption. Therefore, inks and glues containing solvents require sufficient blowing, infrared baking, etc., to promote solvent evaporation and evaporation. Solvent-free inks and glues can be film bonded immediately after printing and application.
The cooling time of infrared, ultraviolet, blowing, baking, etc. depends on the evaporation rate and evaporation rate of the solvent, while the UV type depends on the light intensity. Time should not be too long to avoid hardening and curing of the ink film and film. The adhesion is not good, the bonding is not strong and should not be too short, so that the solvent volatilization, evaporation or cross-linking is not enough, resulting in poor adhesion of the ink and glue. The film strength is reduced.
We know that printing and laminating can be carried out at room temperature or under infrared, ultraviolet, heat-bake curing or cold-air curing, or between infrared and cold air, or between infrared and ultraviolet. process alternately. This is selected depending on the curing (aging) temperature of the ink and the glue. In particular, it is scientific to remove the film after the film formation of the ink and the curing of the composite glue (curing).
3 Solve the problem of leveling adhesion of ink and glue with a wetting dispersant.
Practice tells us that no matter what the ink and composite glue adhesion interface is, the complete infiltration of the surface of the adhesive material (substrate or substrate) of the ink and the composite glue is a prerequisite for obtaining the adhesion strength, but the main factors affecting the wetting are mainly It is the inherent characteristics of the ink and the composite adhesive, followed by the roughness of the adherend, the porosity, the degree of contamination of the adsorbed layer, and the length of the wetting time.
In order to improve the wetting efficiency of inks and composite glues, high-solids, low-drilling inks and compound adhesives should be used when selecting packaging printing inks and composite adhesives. The corresponding wetting agent and coupling agent are also selected according to the characteristics of the ink and the compound.
Because inks and composites are a multiphase dispersion between organic and inorganic pigments, between organic and inorganic fillers, between pigments and fillers, between fillers and binders (resins), resins and solvents. There will be competitive adsorption between them, so it is necessary to pay attention to the order of addition of pigments, fillers, adsorbents on the surface of the resin, and the adsorption of ink, composite glue and substrate surface.
It can be known from the adsorption theory that the thinner the coating layer of the ink layer and the composite glue of the packaged printing product, the higher the adhesion strength and the higher the bonding strength. Therefore, the ink layer and the adhesive layer should be as thin as possible while ensuring that the printed image ink film dots are not lost or the composite coating is not under-coated. It is generally considered that the thickness of the ink layer and the adhesive layer should be controlled to be 8-100 μm.
If the drilling degree of the binder or the composite glue in the ink system is too large, it is not easy to make the sizing coating uniform, and it is also unfavorable for the penetration and diffusion of the printing ink on the printing ink film by the printing substrate or the glue.
In addition, the adhesion strength is also related to the degree of surface roughness. A truly smooth solid surface for inks or compound glues. It is not an ideal adhesive surface. Only with a certain degree of roughness can the adhesion area be increased and ink or composite
Mechanical "engagement" occurs between the glues.
Of course, it is simply unclear that it is possible to achieve good adhesion of the ink or the composite glue as long as the surface of the substrate is rough. Because the surface is too rough or even uneven, the film adsorption of the ink and the adhesion of the composite adhesive are also disadvantageous. Defects may be formed in the place where the interface of the substrate or the printing ink film is too rough, and the defects may cause the ink or the composite adhesive to enter the function of not adhering, and instead cause stress concentration to greatly reduce the ink and the substrate, The adhesive adhesion strength of the composite adhesive to the printing ink film. If the amount of ink used for the above surface is slightly less, the ink dot of the graphic ink will be lost, and the amount of the glue will be too small to form the undergum, which will also lower the bonding strength of the film.
4 Infiltrate the surface porosity of the substrate with ink and composite glue.
Some substrates (including the printed ink film surface), there are many open pores that are invisible to the naked eye. If the ink or compound glue can infiltrate the pore walls, the air should be allowed to penetrate into the pores and be "inlaid" in the holes. Conducive to improve the adhesion strength.
The depth of the ink or composite glue infiltrated into the hole is related to the shape and size of the pore, and also the adhesion of the ink to the substrate, the adhesion of the composite adhesive to the surface of the printed ink film, the fineness of the ink, the drilling degree and curing time of the composite adhesive, and the like. Related factors.
5 control the drilling degree of the ink and composite glue and the amount of plasticizer.
The degree of adhesion between the substrate and the printed image has a great relationship with the degree of penetration of the ink and the composite adhesive. Therefore, this indicator must be controlled.
The substrate and the printed graphic ink film contain some oligomers. The flowability of such low molecular substances is very easy to seep out of the surface of the substrate, or it is easy to seep the surface of the printed ink film, so that the surface tension is reduced. In order to prevent and eliminate these effects, the surface of the substrate and the surface of the printed ink film must be treated before or before the printing.
The purpose of using a plasticizer in inks and composite adhesives is mainly to obtain toughness of the ink ink film and the composite bonding layer. Secondly, the ink and composite rubber with high degree of privacy can also reduce the cohesive force of the base material of the system, and increase the diffusion ability of the base material of the system to improve the adhesion strength. However, when choosing to use plasticizers, the compatibility of the plasticizer with the binder and the substrate should be considered, and the amount of the plasticizer should be noted. The amount of plasticizer used in inks and composites should generally be controlled to an excess of less than 10%, which would reduce the adhesion of the product.
6 Use an appropriate amount of filler to make up for the lack of ink and glue.
The use of fillers in printing inks and composite adhesives not only saves the base material, reduces the material cost, but also reduces the shrinkage of the ink and composite adhesive joints, which greatly improves the bond strength of the printing ink film and the composite adhesive.
Commonly used fillers are: reduced silver powder or electrolytic silver powder, zinc oxide powder, activated silica (white carbon black), mica powder, calcium carbonate and the like.
In addition, in order to prevent the adverse effects of the ink and the composite glue, their storage time should also be controlled.
The adhesion of inks and composite adhesives tends to decrease with the increase of storage period. This is because the ink and composite adhesives have surface aging during storage, and this aging phenomenon is more serious if exposed to air. Therefore, the ink and compound glue should be stored in a container that is isolated from the air, and the time should not be too long.
7 Master the curing temperature and improve adhesion.
The increase of curing temperature increases the ability of the binder molecules in the ink and composite rubber system to accelerate the diffusion speed, which is beneficial to infiltration and adsorption, enhance adhesion and improve bonding strength.
Of course, the curing temperature is not as high as possible. Be sure not to exceed the softening point (temperature) of the substrate to avoid deformation of the composite. If the temperature is too high, the ink and the composite glue flow to the microvoids, and bubbles are easily generated to lower the adhesion strength.
On the other hand, the bond strength between the packaging printing ink and the printing material and the composite adhesive and the printed printing ink film tends to increase with the curing time, but the adhesion strength will not be reached when a certain value is reached. Increased again. The curing temperature and curing time are mutually restrictive. In short, increasing the temperature and increasing the light energy and light intensity can shorten the curing time. To reduce the temperature and reduce the light intensity, it is necessary to extend the curing time.
8 Use antistatic additives to solve the problem of not easy to ink and difficult to apply glue.
In the modern packaging printing process technology, especially in the process of continuous printing and continuous sizing film coating, due to the friction between the substrate and the printing machine and the coating machine, static electricity is generated, and some substrates are The static charge of the electrical insulator will accumulate more and more, and eventually the ink will be lost due to static electricity, and the dot of the printed image will be lost and the glue will be difficult to be evenly coated, even deinked, degummed, and stripped. To this end, it is recommended to add an antistatic agent to the printing ink and glue system in an appropriate amount to improve the ink transfer rate and the quality of the glue coating.