The main way to improve ink fastness
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From the aspect of the formation mechanism and test method of printing adhesion fastness, we can better understand the way to improve the adhesion fastness of printing (mainly discussed on the thin hydrocarbon film).
1. Raw material selection
In the production of plastic film, raw materials with no or little plastic additives should be selected. We can judge the presence of additives based on the technical data of the grade, performance and use of the resin. When it is necessary to use a raw material containing a large amount of auxiliary agents, it should also be blended with an auxiliary-free raw material. The reason for this choice is that the auxiliaries that are folded during the forming process and the placement process form an oil film that reduces the adhesion of the ink to the film.
2. Raw material modification
1) Blending modification: Blending of polyolefins with polymers with good printing properties to improve printing performance is a newly developed and compelling method. For example, the modification of high-density polyethylene with polyacrylic resin greatly improves the adhesion of the ink to polyethylene. When 5-20% poly(methacrylic acid) (PMMA) is blended therein, the above adhesion is increased by seven times. For example, when blending with a chlorinated polyethylene having a large amount of chlorine, the adhesion of the polyethylene to the oil black can be significantly improved by adding only a small amount. When blended with 5% CPE-55, the blend had an ink adhesion three times higher than that of pure HDPE. According to related reports, some processing aids (rheological modifiers, etc.) of plastic high polymers, in addition to improving yield and eliminating melt cracking, also have the effect of improving printing adhesion.
The addition of polyacrylic resin or chlorinated polypropylene, etc., is able to improve the printability of HDPE based on their poor compatibility with HDPE, when it forms a dispersed phase in the blend system of HDPE matrix and it is apparent that the film surface is present. Conducive to bonding with ink.
2) Graft modification: Graft modification is to use a method such as radiation to make a polyolefin grafting reaction to make the polyene macromolecular chain have a polarity and thereby improve the wettability of the material to the ink. If the linear density polyethylene is modified by maleic anhydride or acrylic acid grafting, the printability is improved.
3) Other methods of modification: If a small amount of maleic acid or its anhydride is incorporated into the polyolefin, the ink fastness of the polyolefin film can be improved to a large extent.
3. Printing ink layer fastness
Ink also plays a decisive role in the problem of printing ink layer fastness. Of course, the corresponding ink should be used for different substrates. For example, a nitrocellulose resin type ink can be used for the cellophane; a chlorinated polypropylene resin type ink can be used for the polypropylene film; a polyamide resin type ink can be used for the polyethylene film, and the like.
In production, we often add a certain amount of solvent in order to adjust the viscosity and volatilization speed of the ink. There are two issues to be aware of here:
1) When adding a solvent, a solvent with a low surface tension should be used as much as possible to reduce the surface tension of the entire ink system, thereby improving the wettability of the printed film.
2) When the working solid content is too high, the wetting effect will be correspondingly low, thus affecting the ink fastness. On the other hand, if too much solvent is added, the effective component of the ink adhered to the film is relatively reduced, which also causes a decrease in the fastness. In order to resolve the contradiction between ink layer fastness and ink volatilization speed, a resin (rubber) corresponding to the ink or other suitable compatible bonding resin or rubber may be added to the diluent or ink. This can also improve the ink layer fastness to some extent. At the same time, the ink is left for too long or mixed with impurities, which will cause the binder to deteriorate. Therefore, try to use fresh ink or replace the deteriorated ink in time.
4. Surface treatment
From the analysis of the adhesion fastness mechanism of the ink, it can be known that further modification of the ink and the film can improve the adhesion fastness of the ink, but for cost reasons, the commonly used method is to surface the printed film. deal with.
For polyvinyl chloride, cellophane, polyester, nylon and other films, because of their large surface tension, they can be directly printed when the printing product requirements are not high (actually due to the increase in customer requirements in recent years, these materials are also printed before To be treated), the untreated polyolefin film has a low surface tension, such as 31 dyne for PE and 29 dyne for PP, which should be increased to 38 dyne for printing, which requires filming. Surface treatment.
The methods of plastic surface treatment include flame treatment, chemical treatment, solvent treatment and corona treatment, but a widely used polyolefin film is a corona treatment method. The polyethylene film is taken as an example to analyze the reasons for the improvement of ink fastness by corona treatment.
1) Oxygen in the air generates ozone due to ionization, thereby oxidizing the surface. The molecular chain of the surface layer polyethylene may form a terminal P active electrode such as a carbonyl group, a carboxyl group or a hydroxyl group. Nitrogen in the air is also ionized into a plasma and acts with polyethylene molecules to form an amine-type gene on the polyethylene molecular chain and can continue to oxidize to form a series of nitrogen-containing compounds. As a result, the polarity of the surface layer of the polyethylene film is enhanced, and the wettability is improved, thereby improving the adhesion of the ink.
2) Eliminate some oils (impurities, additives, oligomers, etc.) that are invisible to the naked eye on the surface of the film, so that the ink and the film are in close contact.
3) The surface is roughened due to the impact, thereby realizing the mechanical wedge action.
4) As the surface of the film is heated, the activity of the molecular chain is intensified, that is, the degree of surface activity is increased.
5) Under the action of a high-voltage electric field, the non-conductive plastic film is displaced by the charge in the molecule, and an induced charge is formed on the surface. The presence of the induced charge can be correlated with the polarity of the ink molecules. The improvement of layer fastness also has a certain effect.
Pay attention to the following issues when performing corona treatment:
1) After corona treatment, it should be used in a short period of time. Otherwise, if the polar group is combined with water molecules in the air, the wettability will be lowered, and the adhesion of the printing ink layer will be destroyed. Therefore, qualified films should be stored in a dry, low-temperature warehouse; re-inspection tests should be performed during use.
2) For cold film, it is necessary to use surface power for processing with higher power. The cold film should be preheated with iodine-tungsten infrared before treatment.
3) Only one-sided printing is required, and only one-sided corona treatment is performed. Otherwise, adhesion problems are likely to occur after printing.
4) Under high temperature conditions, if the treatment time is too long, the adhesion tends to decrease. This may be due to the decomposition of the surface, forming a "weak boundary layer."