Surface analysis of printed plastics

For printing on non-absorbent plastic films, many printing suitability problems need to be solved in relation to printing on paper. The biggest issue is the adhesion and affinity of plastic films to printing inks:

At present, the basic materials for plastic film are mainly polypropylene (PP), polyhexene (PE), polyvinyl chloride (PVC), polyester (PET), etc., whose surface characteristics are due to the polar groups of the molecular structure substrate, The degree of crystallinity and the chemical stability of the plastics are different and very different. These factors have a great influence on the adhesion fastness of the printing ink layer. For example, the polyethylene material has a molecular structure of ∈CH2-CH2∈n. Contains polar groups, high chemical stability, resistance to tribasic acid dibasic, even hydrofluoric acid corrosion, dissolved in common organic and inorganic solvents at room temperature, but the stability of hydrocarbons, oils is poor, may Causes swelling or discoloration. It can dissolve in solvents such as chlorinated hydrocarbons and tetrahydronaphthalene at temperatures above 70°C. It also polymerizes from ethylene to polyethylene. In the process of manufacturing polyethylene pellets, it incorporates a certain amount of aids according to different requirements. Agents, additives, under the opening agent, when the suction film is set, these additives float on the film surface, forming a visible oil layer, can make the film anti-aging, transparency, gloss increase, softness, but these pairs Printing is not at all Of the adhesive film surface is not easy. Before printing, other substances should be determined depending on the characteristics of the plastic surface to be surface treated to improve the fastness of their printing ink.

For PS (polystyrene) and PVC, which are polar structures, no surface pretreatment is required before printing. However, PP, PE, PET, etc., whose surface structure is non-polar, have extremely high chemical stability and are not easy to be large. Most ink solvents penetrate and dissolve, and the bonding fastness to ink printing is very low. Therefore, the surface treatment must be performed before printing. The purpose of the surface treatment is to make the plastic surface layer activated to generate new chemical bonds to roughen the surface, thereby improving the ink and plastic. Surface adhesion fastness.

In the general case, the common methods of plastic surface treatment are the following:

1. Chemical method: It is a method of strong oxidation because the membrane surface is eroded to form carbonyl (>C=0) bond. The chemical oxidation treatment is to first degreas the film with carbon tetraoxide, then immerse it in H2SO4 88.5%, K2Cr207 4.4% or H2CrO4 7.1% solution in several parts, after being washed with water and dried, send it as soon as possible.

2. Solvent treatment: Use some organic solvents or chlorinated hydrocarbons to treat the surface, so that the non-crystalline part of the polyolefin will have different degrees of melting, forming a rough surface, and then drying with hot air, usually with the following The surface treatment of the solvent: xylene 50% mixed with 50% ethanol, containing a certain concentration of dibenzoyl peroxide and so on.

3, corona treatment:

1 corona treatment principle and method: corona treatment is often used in the film blowing temperature line, because the plastic film often add antioxidants and smoothing agents and other additives, these additives are sometimes effective, corona treatment, Instant printing works best. The corona treatment device includes a high-voltage AC motor, an output transformer and two electrodes; one of which is connected to a high-voltage metal pole (pressure resistance 10-40KV) and a high-frequency generator (1-4KHZ); the other electrode is Grounding steel wheel or knife, it and the high voltage electrode to maintain a distance of 1.5 --- 3mm, the width of the metal plate is slightly narrower than the width of the film to be treated 5 --- 10mm, to avoid direct discharge short circuit, the film will be processed continuous Between the two electrodes, due to high-voltage electricity, oxygen in the air is highly ionized to generate ozone, so that the surface of the film is subjected to oxidation treatment. Ozone is a strong oxidant that can oxidize the film surface and generate radicals at active sites. The lumps, which readily accept the ink, undergo polarization on the treated surface, generate electrostatic adsorption, and produce a finer graininess on the film surface, thereby changing the chemical and mechanical bonding of the film surface to the printing ink. The corona treatment effect mainly depends on the output power of the oscillator, the distance between the electrode and the film surface, the material of the film and the speed of the film through the electrode, and the intensity of the corona discharge is adjusted to form a cloud-like purple dark spark. Best state.
2 The effect and effect of corona treatment on the surface of the film.

A. Surface Oxidation: Oxygen ionizes under the action of high-voltage electricity in the corona machine to form ozone (03). Ozone is a strong oxidizing agent that can immediately oxidize polyethylene molecules and can produce carbonyls, unsaturated bonds, etc. on polyethylene. The nitriding in the air can also be ionized into ions and polyethylene molecules to produce amine-type groups in the polyethylene chain, and can continue to be oxidized to generate a series of oxides. As a result of the surface oxidation, the polyethylene film is converted into polarity, and the low surface energy is converted into a high surface energy to increase the wettability and adhesion of the printing ink, thereby improving the print fastness.

B、Surface Polarization: Non-conductive plastic film under the action of high voltage and frequency electric field, the charge in the molecule is displaced, and the induced charge is formed on the surface. The existence of this sensory charge can be absorbed in the polar ink molecules .

C. Surface observation roughening: After the corona treatment, the surface of the film is roughened. Scanning micrographs show that the friction coefficient of the film surface of the plastic film increases, which can change the smooth shape of the original film surface, and is conducive to the drying and adhesion of the film printing. Attach fastness.

3, corona surface effect identification method:

A. Moisture test method: Due to the roughened surface after oxidation, if water or ink is dropped on the treated surface, it will quickly spread out from the surface.

B. Heat-seal Viscosity: The treated film feels particularly sticky, and the heat-insulating paper (such as glass) is directly proportional to its viscosity and fastness.

C. Scatch Adhesive Bag Test Method: press the pressure sensitive tape firmly to the printing surface of the film, then quickly pull and record the transfer amount of the ink. Generally, the migration amount of the ink does not exceed 1%. Considered qualified, you can also use adhesive tape or sticky tape on the printing ink surface for a sticky pull test to check the treatment effect.

D. Wrinkle test: The printing surface is rubbed or creased. The ink should not appear brittle or be peeled off into small pieces. Otherwise, it shows that the combination of ink and film surface is not satisfactory.

E. Wetting force test method for film corona treatment effect. The printing process stipulates that the product quality must be fast dry and the ink is thick and the layers are clear. However, sometimes there are cases where the printing film is decolored and the ink layer is stuck in the printing. The wetting force test method can be used to test the corona effect of the film. The method uses a cotton bat to draw a mixture of formamide and ethylene glycol ethyl ether on the surface of a plastic film of polyethylene or polypropylene, and coats the film to be tested with a coarse channel. When the liquid film is maintained for more than two seconds, Then use the following test mixture with high surface tension, when the liquid film breaks into small droplets within two seconds, test with the following mixture with low surface tension to select the appropriate surface tension of the mixture as the wetting tension. Samples. The wetting tension of the film is large, indicating that the adhesive property with the ink is good. When printing a small area pattern, the wetting tension of the plastic film is better at 360un/cm. The large color of the printed image and the wet tension of the solid film is at 380un/ Before cm is better.

Polyolefin Wetting Tension Determination Liquid Proportioning Table:
Formamide (%) Ethylene glycol ether (%) Wetting tension (un/cm) Remarks
19 81 330 Relative Humidity 50%-5%, Temperature 23°C+2°C Condition
26.5 73.5 340
35 65 350
42.5 57.5 360
48.5 51.5 370

In order to make the ink wet on the surface of polyethylene and other films, and to improve its imprinting fastness, it must be treated by corona treatment to increase the surface tension from the original 310un/cm to 380un/cm, so that the ink remains unbleached and fastened. That is, if the surface tension of the polyethylene film does not reach the value of the surface tension of the ink, the ink will discolor, and the plastic film having a low transfer property of the ink is corona-treated so that the active radical oxidized by the surface is oxidized. The oleophilic layer is produced to further improve the transferability of the ink, and the surface polarity and roughening caused by the discharge can be improved, so that the transfer performance of the ink can be improved.

Reproduced from: liutiefu.net