The way to improve the fastness of printing ink

From the analysis of the formation mechanism and test method of the printing adhesion, we can better understand the way to improve the printing adhesion fastness (mainly on the discussion of the polyolefin film) 1 Raw material selection: the raw material that does not contain or contains less plastic additives shall be selected in the production of plastic film We can judge the situation of containing additives according to the relevant technical data of resin brand, performance and use When we have to use raw materials with more additives, we should also mix them with raw materials without additives The reason for this choice is that the auxiliary folded out in the process of molding and placement will form an oil film and weaken the adhesion of ink on the film 2 Raw material modification 1) blending modification: blending polyolefin with polymer with good printing performance to improve printing performance is a newly developed and attractive method For example, high-density polyethylene modified by polyacrylic resin greatly improves the adhesion of ink to polyethylene When 5-20% PMMA is added, the above adhesion is increased by seven times If the chlorinated polyethylene with more chlorine content is used for industrial mixing, the adhesion between the polyethylene and the oil black can be significantly improved by adding only a small amount of chlorine When 5% cpe-55 was added, the adhesion between the blends and ink was three times higher than that of HDPE In addition, it is reported that some processing aids (rheological modifiers, etc.) of plastic polymers can not only improve the output and eliminate melt fracture, but also improve the printing adhesion The reason why adding polyacrylic resin or chlorinated polypropylene can improve the printability of HDPE is that they have poor solubility with HDPE When they form a dispersed phase in the HDPE based blend system and occupy the surface of the film, it is obviously conducive to the adhesion with ink 2) Grafting modification: the grafting modification is to make the polyolefin component undergo grafting reaction by means of radiation, so as to make the polarity of the polyolefin macromolecule chain and improve the wettability of the material to the ink For example, the printability of LDPE was improved by grafting maleic anhydride or acrylic acid 3) Other methods of modification: for example, adding a small amount of maleic acid or its anhydride into polyolefin can also improve the ink fastness of polyolefin film to a large extent 3 Ink also plays a decisive role in the problem of printing ink layer fastness Of course, for different substrates should use their corresponding ink For example, nitrocellulose cellophane resin ink can be used for cellophane; chlorinated polypropylene resin ink can be used for polypropylene film; polyamide resin ink can be used for polyethylene film, etc In order to adjust the viscosity and volatilization speed of ink, we often add a certain amount of solvent Two problems should be paid attention to here: 1) when adding solvent, the solvent with lower surface tension should be selected as far as possible to reduce the surface tension of the whole ink system, so as to improve the wettability of printing film 2) when the working solid content is too high, the wetting effect will be relatively low, thus affecting the ink fastness On the contrary, if too much solvent is added, the effective components of ink and film adhesion will be relatively reduced, and the fastness will also be reduced In order to solve the contradiction between the fastness of ink layer and the volatilization speed of ink, the resin (rubber) corresponding to the ink or other suitable compatible adhesive resin or rubber can be added to the diluent or ink This can also improve the ink layer fastness to a certain extent At the same time, if the ink is placed for too long or mixed with impurities, it will lead to the deterioration of the connecting material, so it is necessary to use fresh ink or replace the deteriorated ink in time 4 Surface treatment from the analysis of the mechanism of ink adhesion, we can know that further modification of ink and film can improve the ink adhesion, but for the sake of cost, the commonly used method is to surface treatment of printing film For PVC, cellophane, polyester, nylon and other films, due to their high surface tension, they can be printed directly when the printing product requirements are not high (actually, due to the improvement of customer requirements in recent years, these materials need to be treated before printing), while the surface tension of untreated polyolefin films is very low, such as 31 dyne for PE and 29 dyne for pp in order to print, they should be improved Up to 38 dyne 2, this requires surface treatment of the film The methods of plastic surface treatment include flame treatment, chemical treatment, solvent treatment and corona treatment, but corona treatment is widely used for polyolefin film Next, take 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, which oxidizes the surface The molecular chain of surface polyethylene may form p-terminal active poles such as carbonyl, carboxyl and hydroxyl groups Nitrogen in the air is also ionized into plasma, and interacts with polyethylene molecules to form amine type genes on the polyethylene molecular chain, and can continue to oxidize into a series of nitrogen-containing compounds As a result, the polarity and wettability of the molecules on the surface of the polyethylene film are enhanced, thus improving the adhesion of the ink 2) Remove some invisible oil (impurities, additives, oligomers, etc.) on the surface of the film, so that the ink and the film can be in close contact 3) Because of the impact, the surface is coarsened, thus realizing the mechanical wedge head effect 4) Due to the heating of the film surface, the activity of the molecular chain is intensified, that is to say, the surface activity is increased 5) Under the action of high-voltage electric field, the charge in the molecule of non-conductive plastic film displaces, and the induced charge is formed on the surface The existence of the induced charge can generate a reservoir attraction with the polar ink molecule, which also plays a certain role in improving the fastness of the printing ink layer The following problems should be paid attention to in corona treatment: 1) after corona treatment, it should be used in a short time Otherwise, if the polar group is combined with water molecules in the air, it will reduce the wettability and destroy the adhesion fastness of the printing ink layer Therefore, the qualified film should be stored in a dry and low temperature warehouse, and the inspection and test should be carried out again when it is used 2) For the cold film, the surface electronic treatment with higher power is needed The cold film should be preheated with iodine tungsten infrared before treatment 3) Only one-sided printing is needed, and only one-sided corona treatment is carried out Otherwise, it is easy to produce adhesion after printing 4) At high temperature, if the treatment time is too long, the adhesion tends to decrease This may be due to the "weak boundary layer" formed by decomposition on the surface