The main way to improve the ink fastness

From the analysis of the formation mechanism and test methods of the printing adhesion fastness, we can better understand the ways to improve the fastness of printing adhesion (mainly discussed on poly-hydrocarbon films).
1. Raw material selection Raw materials containing no or less plastic additives should be selected when producing plastic films. We can judge the condition of containing additives based on the technical data of the resin's grade, performance, and application.
When raw materials containing more auxiliaries have to be used, they should also be blended with raw materials without auxiliaries.
The reason why this choice is made is that the auxiliary agents that are folded out during the molding process and the placement process will form an oil film, thereby weakening the adhesion of the ink on the film.
2. Modification of raw materials 1) Blend modification: The blending of polyolefins with polymers with good printing properties to improve printing performance is a recently developed attractive method. For example, the modification of high-density polyethylene with polyacrylic resin greatly improves the adhesion of the ink to polyethylene. When 5-20% of poly(methyl methacrylate) (PMMA) is incorporated therein, the above-mentioned adhesion is increased by a factor of seven. Another example is the use of chlorinated polyethylene with more chlorine content to blend, only adding a small amount can significantly increase the adhesion of polyethylene and oil black. When blended with 5% CPE-55, the blend has a three times higher adhesion to ink than pure HDPE. According to relevant reports, some plastic polymer processing aids (rheological modifiers, etc.), in addition to improving production and eliminating melt fracture, also have the effect of improving printing adhesion.
Adding polyacrylic resin or chlorinated polypropylene can improve the printability of HDPE based on their poor compatibility with HDPE. When they form a disperse phase in the blend system of HDPE as a matrix and leave the film surface, obviously Advantageous bonding with ink.
2) Graft modification: Graft modification is the use of radiation and other methods to make the grafting reaction of the polyolefin component so that the polyene macromolecule chain with polarity and improve the wettability of the material to the ink. The use of maleic anhydride or acrylic grafted linear density polyethylene improves its printability.
3) Modification by other methods: If a small amount of maleic acid or its anhydride is incorporated in the polyolefin, the ink fastness of the polyolefin film can be improved to a great extent.
3. Printing inks also play a decisive role in the problem of ink fastness. Of course, the corresponding ink should be used for different substrates. For example, a nitrocellulose resin type ink may be used for cellophane; a chlorinated polypropylene resin type ink may be used for the polypropylene film; a polyamide resin type ink may be used for the polyethylene film, and so on.
In production, we often add a certain amount of solvent in order to adjust the ink viscosity and volatilization rate. There are two issues that should be noted here:
1) When adding the solvent, the solvent with lower surface tension should be used as far as possible in order 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, which will affect the ink fastness. On the other hand, if too much solvent is added, the effective components that the ink adheres to the film will be relatively reduced, which also causes a decrease in the fastness.
In order to solve the contradiction between the fastness of the ink layer and the volatilization rate of the ink, a resin (rubber) or other suitable compatible adhesive resin or rubber corresponding to the ink may be added to the diluent or the ink. This can also improve the ink fastness to some extent. At the same time, if the ink is left for a long time or mixed with impurities, it will cause deterioration of the connecting material. Therefore, fresh ink should be used as much as possible or the changed ink should be replaced in time.
4. Surface treatment From the analysis of the mechanism of the adhesion of the ink, it can be known that further modification of the ink and the film can improve the adhesion of the ink, but for cost considerations, the commonly used method is Printed film surface treatment.
For films such as polyvinyl chloride, cellophane, polyester, nylon, etc., due to their large surface tension, they can be directly printed when the printing product is not very demanding (actually due to the increase in customer requirements in recent years, these materials are also To be treated), the surface tension of the untreated polyolefin film is very low, such as 31 dyne for PE and 29 dyne for PP, which should be increased to 38 dyne or more for printing. This requires the surface of the film to be processed. deal with.
The plastic surface treatment methods include flame treatment, chemical treatment, solvent treatment, and corona treatment, but corona treatment is widely used for polyolefin films. The following uses polyethylene film as an example to analyze the reason why corona treatment improves ink fastness.
1) As a result of ionization, oxygen in the air generates ozone, thereby oxidizing the surface. The molecular chains of the surface polyethylene may generate terminal P active poles such as carbonyl groups, carboxyl groups, and hydroxyl groups. The nitrogen in the air is also ionized into a plasma and interacts with polyethylene molecules to form amine-type genes on the polyethylene chains and can continue to oxidize to produce a series of nitrogen compounds. As a result, the polarity of the surface molecules of the polyethylene film is enhanced and the wettability is improved, thereby improving the adhesion of the ink.
2) Eliminate some oily substances (impurities, additives, oligomers, etc.) that are invisible to the naked eye on the surface of the film, so that the ink and the film can be in close contact.
3) Due to the impact, the surface is roughened and the mechanical wedge effect is achieved.
4) As the surface of the film is heated, the activity of the molecular chains is increased, that is, the degree of surface activity is increased.
5) In the non-conductive plastic film, under the action of high-voltage electric field, the charge in the molecule is displaced, and an induced charge is formed on the surface. The presence of this induced charge can be induced with the polar ink molecules, which is for the printing ink. The improvement of the layer fastness also has a certain effect.
The following issues should be noted when performing corona treatment:
1) After corona treatment, use it in a short period of time. Otherwise, if the polar group binds to the water molecules in the air, the wettability will be reduced and the adhesion of the printing ink layer will be impaired. Therefore, the qualified film should be stored in a dry, low-temperature warehouse;
2) For cold film, it is necessary to use a higher power surface electronic treatment. Cold film should be preheated with tungsten iodide prior to treatment.
3) For single-side printing, only one-side corona treatment is performed. Otherwise, sticking problems easily occur after printing.
4) If the treatment time is too long under high temperature conditions, the adhesion tends to decrease. This may be due to decomposition of the surface and the formation of a "weak boundary layer."
Wetting tension of common plastic film (untreated) Unit mN/m
Name Wetting Tensile Polyethylene 31
Polyester 43
Polypropylene 29
Nylon 46
PVC 39
Polyvinylidene chloride 40
Sai 42

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