Prepress for plastic film

First, the printability of printing materials on the amount of ink transfer

The printing effect is achieved by the reasonable adhesion of the ink on the substrate, so the physical and chemical state of the surface of the substrate directly affects the amount of ink transferred thereon.

Second, improve the printability of plastic film

1. The necessity of improving the printability of plastic film surface

The physicochemical theory shows that good printability should be obtained on the substrate, and 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

Plasma is ionized gas. It consists of three components: electron, ion, and neutral particle. The total number of charges of the v-substance and the 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 processing apparatus is shown in the following diagram (omitted). 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

Before printing, the surface of PP and PE plastic film is treated with an oxidizing agent to form polar groups such as hydroxyl groups and carbonyl groups on the surface, and a certain degree of roughening is obtained at the same time to improve the surface fastness of the ink and the plastic film.

The potassium dichromate-sulphuric acid solution is often used to treat the surface of the polyolefin. The formulation of the treatment solution, 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) Photochemical treatment

Generally, ultraviolet radiation is used to irradiate the surface of the polymer to cause chemical changes, thereby improving the surface tension and improving the wettability and adhesion. 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.

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. Simple measurement method of surface tension

The known surface tension measurement solution is applied on the corona-treated film with a cotton wool ball. The coating area is about 30 mm2, and it shrinks into a water sphere within 2 seconds. The corona treatment strength of the film is insufficient and it is necessary to increase the power again. Halo intensity strikes again. If the test solution does not shrink in water mark within 2 seconds, it indicates that the film has reached the treatment effect. Film printing requires a surface tension of 38 x 10-3 N/m (38 dyn/cm).

Source: Internet