Plastic film prepress processing

First, the printability of printing materials on the impact of the amount of ink transfer printing effect is achieved through the reasonable adhesion of the ink on the substrate, so the physical and chemical state of the surface of the substrate directly affect the amount of ink transferred on it.
Second, to improve the printability of plastic film 1. The need to improve the printability of plastic film surface The physics theory shows that in the substrate to obtain good printability, the surface tension of the substrate should be higher than the surface tension of the ink. This is the theoretical basis for the pretreatment and processing of various types of substrates before printing.
PP, PE film non-polar polymer materials, chemical properties, stability, surface tension, combined with synthetic resin added opening agent, antistatic agent, anti-aging agent, etc., difficult to bond with the ink.
2. Processing plastic film printability method (1) Plasma treatment The plasma is an ionized gas. It consists of three components: electron, ion and neutral particle. The total charge of electron and ion is basically the same, so the whole is electrically neutral. The electrons or ions in the ionized plasma are driven onto the surface of the substrate by the discharge device. On the one hand, long molecular chains of the material can be opened and high energy groups appear; on the other hand, fine pinholes appear on the surface of the film after striking. At the same time, it can also dissociate and re-dissolve surface impurities. The ozone released when ionized has a strong oxidizing property, and the attached impurities are oxidized to be removed, so that the free surface energy of the substrate is increased, and the purpose of improving the printing performance is achieved.
The structure of the plasma treatment device is shown in the figure below. One end of the electrode is 20,000 volt high voltage resistant wire with limited pressure. At the other end, a rubber-coated steel roller was used, and the length of the discharge electrode was 347 mm.
During the plasma treatment, the processing energy per unit area is:
W=P/v·L=uI/V·L
Where u - processing voltage;
I - processing current;
L - handle electrode width;
V - feed speed.
If the material is treated with different processing speeds, the relationship between the processing energy per unit area and the contact angle θ can be obtained, and the relation curve of θ and W(J/m2) can be drawn.
(2) Corona treatment The corona treatment is actually one kind of plasma treatment and is a low-density plasma treatment.
Using a high-frequency (intermediate frequency) high-voltage power supply, a corona release phenomenon occurs between the discharge blade holder and the blade gap. In this way, the plastic film is subjected to surface treatment before printing, called corona treatment, also called electronic shock or electricity. Spark treatment. Its processing role is:
1 Through the discharge, the oxygen between the two poles is ionized to generate ozone. Ozone is a strong oxidizer, which can immediately oxidize the surface molecules of the plastic film to convert from non-polar to polar, and the surface tension is improved.
2 After the electron impact, the surface of the film is generated with micro-cavities and dense holes to make the plastic surface rough and increase the surface activity.
(3) Chemical treatment The surfaces of PP and PE plastic films are treated with an oxidizing agent prior to printing to form polar groups such as hydroxyl groups and carbonyl groups on the surface, and a certain degree of roughening is obtained to improve the surface of the ink and the plastic film. Fastness.
The potassium dichromate-sulphuric acid solution is often used to treat the surface of the polyolefin. The formulation of the treatment solution and the time and temperature of the treatment process all have a certain influence on the treatment effect. In general, the processing time is long, the processing temperature is high, and the effect is good, but it should be noted that although the time is too long or the temperature is too high, although the polarity of the surface is increased, the oxidation and aging of the surface are also caused, and the bonding strength is reduced. .
The chemical treatment method is an earlier application of a surface treatment method. For the printing, the surface treatment effect of the pre-composite film is good, the use is simple and economical, but the longer processing time affects the production efficiency. In addition, the treatment liquid is generally chemically corrosive, causing environmental pollution and harm to the human body. At present, this process is rarely used, and this surface treatment process is generally used only when it is inconvenient to use other treatment methods.
(4) The photochemical treatment method generally uses ultraviolet rays to irradiate the surface of the polymer, causing it to cause chemical changes, thereby improving the surface tension and improving the wettability and adhesiveness. Like corona treatment, UV irradiation can also crack, cross-link, and oxidize the polymer surface.
In order to obtain better photochemical treatment effects, UV light of an appropriate wavelength must be selected. For example, irradiation of a polyethylene surface with ultraviolet rays having a wavelength of 184 mm can cross-link the surface of the polyethylene. However, if the wavelength is changed to 2537A, the same effect cannot be obtained.
Photosensitizers are used to make the photochemical treatment more effective. Benzophenone is a good photosensitizer. When treated with UV, it is sublimated and removed. Here, benzophenone catalyzes the photoinitiation reaction by generating resonance-stabilized benzophenone radicals. Alkyl radicals can continue free-radical propagation and radical termination reactions. The following table shows the adhesion of polyolefin films to UV radiation. UV radiation time/min Adhesion UV radiation time/min Adhesion LDPE HDPE PP LDPE HDPE PP 10 0 0 0 60 88 0 19 30 32 0 0 210 100 24 76
(5) The flame treatment method is applicable to the surface treatment of small plastic containers. Its purpose is to decontaminate the surface with high temperature, and to dissolve the surface of the film layer to improve the performance of the ink adhering to the surface.
The container to be treated is put into the gas flame, the length of the flame inner core is controlled at 6 to 19 mm, the top of the flame core to the surface is 6 to 25 mm, the contact time is 0.1 to 1 s, and the rotation speed is 100 to 150 revolutions/min. The flame contains radicals such as O, NO, OH, and NH in the excited state. They can extract hydrogen from the surface of the polymer, then undergo surface oxidation by a free radical mechanism, and introduce some polar oxygen-containing groups. , A broken chain reaction occurs.
After the polyolefin is treated with flame, polar groups are formed, the wettability is improved, and the improvement of the adhesiveness is relatively improved due to the improved wettability of the polar groups and the occurrence of chain scission.
The flame treatment effect is good, no pollution, and low cost, but the operating requirements are strict. If it is not careful, it will cause the product to be deformed and the finished product will be scrapped. At present, it is mainly applied to the surface treatment of thicker plastic products.
(6) Coating treatment method The coating treatment method is to apply a specific coating on the film to change its surface adsorption performance. Coating formulations are as follows:

The coating made of the above formula is coated on the surface of the plastic film, air-dried for 10s, and irradiated with a high-pressure mercury lamp for 15s. After curing, the surface tension of the film can be increased.
3. The simple method of measuring the surface tension is determined by measuring the known surface tension of the absorbent cotton ball (see the following table) and coating it on the corona-treated film. The coating area is about 30mm2, and it shrinks into a water sphere in 2s. Then, the corona treatment intensity of the film is insufficient, and the corona strength needs to be increased again for impact. If the test solution does not shrink within 2 seconds, the film has reached the treatment effect. Film printing requires a surface tension of 38 x 10-3 N/m (38 dyn/cm). Formamide/% Ethylene Glycol Ether/% Surface Tension/(dyn/cm)1 Formamide/% Ethylene Glycol Ether Surface Tension/(dyn/cm) 1 1 100 30 42.5 57.5 36 2.5 97.5 31 48.5 51.5 37 10.5 89.5 32 54 46 38 19 19 33 59 41 39 26.5 73.5 34 63.5 36.5 40 35 65 35
11dyn/cm=10-3N/m.
Third, anti-static treatment of static electricity in the printing of plastic film will bring a series of problems to the operation, a direct impact on the output and quality of printed products. For example, when printing small packaging plastic films, due to electrostatic adhesion, the films are in anoxic state, which will hinder the curing process of the plastic ink layer. In case of high temperature and high humidity environment, the adhesion of the ink layer is more likely to occur, and the color of the printing ink is lighter. Dyeing, increasing the difficulty of printing, slitting, finishing and other processes, when the film adheres to each other, tearing does not open, causing the printing products to be scrapped. In addition, the bag will be discharged during storage, storage and storage, which will not only affect the heat seal but also affect the transparency of the physical and space layers in the bag. When a large-format film is printed, a large number of static electricity is generated, and if the machine speed is high and the antistatic agent is not incorporated in the resin, it is likely to cause a fire or an explosion.
The static electricity of the plastic film is due to the excellent dielectric properties of PE and PP, high resistance, and poor conductivity. The film generates static electricity due to friction during the winding process of extrusion, which causes static electricity to be further generated and accumulated in the printing process. Release, causing the film surface to accumulate a large amount of static charge. After the printed film is wound, the film and the film are tightly wound together, so that the electric charge is not conducive to repulsion and is conducive to attraction, resulting in adhesion.
Paper printing can eliminate static electricity through static eliminators, but this method has little effect on plastic packaging printing.
Plastic packaging generally uses anti-static agent in the ink to eliminate static electricity. Commonly used antistatic agents include general surfactants, functional group polymers, and organic or inorganic salt compounds. These substances have weak ionization properties and can produce antistatic effects on plastic films. According to relevant data, Japan uses an ion generator to produce cations and anions on the surface of plastic films through nozzles, which can also achieve the effect of handling static electricity.