Prepress treatment of the hottest plastic film

Prepress treatment of plastic film (Part 1)

first, the influence of the printability of the substrate on the amount of ink transfer

the printing effect is achieved by the reasonable attachment of the ink on the substrate, so the physical and chemical state of the substrate surface directly affects the amount of ink transfer on it

Second, improve the printability of plastic film

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

physicochemical theory shows that in order to obtain good printability on the substrate, the surface tension of the substrate should be higher than that of the ink. This is the theoretical basis for the activation, coarsening and other pretreatment of various substrates before printing

pp and PE films are non-polar polymer materials, with stable chemical properties and low surface tension. In addition, it is difficult to bond with ink due to the influence of opening agents, antistatic agents, aging agents, etc. added in the synthesis of future resins around the different needs of special metal materials in different sub sectors

2. Method for processing the printability of plastic film

(1) plasma treatment plasma is an ionized gas. It is composed of electrons, ions and neutral particles. The total charges of electrons and ions are basically equal, so the whole is electrically neutral. Through the discharge device, the electrons or ions in the ionized plasma are hit to the surface of the substrate. On the one hand, the long molecular chain of the material can be opened and high-energy groups can appear; On the other hand, small pinholes appear on the surface of the film after being hit, and at the same time, the impurities on the surface can be dissociated and redissolved. The ozone released during ionization has strong oxidizability, and the attached impurities are oxidized and removed, so that the surface free energy of the substrate is improved, so as to improve the printing performance

the structure of the plasma treatment device is shown in the following figure (omitted). One end of the electrode is a 20000 volt high-voltage line. The other end is covered with rubber steel roller, and the length of discharge electrode is 347mm

in the process of plasma treatment, the treatment energy per unit area is:

w = p/v · L = UI/V · l

where u - treatment voltage

i - processing current

l - width of treatment electrode

v - feeding speed

if the material is treated with different processing speeds, the processing energy and contact angle per unit area can be obtained θ And draw θ Relation curve with w (j/m2)

(2) corona treatment corona treatment is actually a kind of plasma treatment, which is a low-density plasma treatment

using high-frequency (intermediate frequency) high-voltage power supply, a corona release phenomenon is generated in the gap between the discharge tool holder and the blade. This method is used to treat the surface of plastic film before printing, which is called corona treatment, also known as electronic shock or electric spark treatment. Its treatment functions are:

① through discharge, the oxygen between the two poles is ionized to produce ozone. Ozone is a strong oxidant, which can immediately oxidize the surface molecules of plastic film, making it change from non polarity to polarity, and the surface tension is improved

② after electron impact, micro concave and dense holes are generated on the surface of the film, roughening the plastic surface and increasing the surface activity

(3) chemical treatment method before printing, oxidizer is used to treat the surface of PP and PE plastic film, so that polar groups such as hydroxyl and carbonyl are formed on the surface, and a certain degree of coarsening is obtained at the same time, so as to improve the surface bonding fastness of ink and plastic film

potassium dichromate sulfuric acid solution is often used to treat the surface of polyolefins. The formula of the treatment solution, the time and temperature of the treatment process have a certain impact on the treatment effect. Generally speaking, the treatment time is long, the treatment temperature is high, and the effect is good. However, it should be noted that if the treatment time is too long or the temperature is too high, although the surface polarity is increased, it will also cause surface oxidation and aging, and the bonding strength will be reduced

chemical treatment is an earlier surface treatment method. For printing, the surface treatment effect of the film before lamination is good, easy to use and economical, but it requires a long treatment time, which affects the production efficiency. And the treatment fluid is generally chemically corrosive, causing environmental pollution and harm to human body. At present, this process is rarely used, and this surface treatment process is generally adopted only when it is inconvenient to use other treatment methods

(4) photochemical treatment generally uses ultraviolet radiation to irradiate the surface of polymers to cause chemical changes, so as to improve the surface tension and improve the wettability and adhesion. Like corona treatment, UV irradiation can also cause cracking, crosslinking and oxidation on the surface of polymers

in order to get a better photochemical treatment effect, it is necessary to choose the appropriate wavelength of ultraviolet light. For example, irradiating the polyethylene surface with the ultraviolet light with the wavelength of 184mm can make its surface cross-linked, but if you use the wavelength of 2537A, it is difficult to have the same effect

the use of photosensitizers can make photochemical treatment more effective. Benzophenone is a better photosensitizer. After UV treatment, it will be sublimated and removed. 2. Operation disorder: it is mainly the function disorder of each key

here, benzophenone catalyzes the photoinitiated reaction by generating resonance stable benzophenone free radicals. Alkyl radicals can continue to carry out free radical propagation and free radical termination reaction. The following table shows the improvement of adhesion of polyolefin films after UV irradiation

UV radiation

time/min adhesion UV radiation

time/min adhesion LDPE HDPE PP LDPE HDPE PP 1000 000 60 880 19 303200 210 1002476 (5) flame treatment method is suitable for the surface treatment of small plastic containers. Its purpose is to decontaminate the surface with high temperature, dissolve the surface of the film, and improve the performance of surface adhesion ink

put the container to be treated into the gas flame. The length of the inner core of the flame is controlled at 6 ~ 19mm, the distance from the top of the flame core to the surface is 6 ~ 25mm, the contact time is 0.1 ~ 1s, and the rotation speed is 100 ~ 150 rpm. Even below the yield limit, the flame contains free radicals such as O, no, oh and NH in the excited state, which can extract hydrogen from the surface of polymers, then conduct surface oxidation according to the free radical mechanism, and introduce some polar oxygen-containing groups, resulting in chain breaking reaction

polyolefins formed polar groups after flame treatment, and the wettability was improved, while the improvement of adhesion was relatively improved due to the polar groups, which reduced the total cost of parts by 40%, improved wettability and chain breakage

flame treatment has good effect, no pollution and low cost, but the operation requirements are strict. If you are not careful, the product will be deformed and the finished product will be scrapped. At present, it is mainly used in the surface treatment of thick plastic products

(6) coating treatment coating treatment is to apply a specific coating on the film to change its surface adsorption performance. The coating formula is shown in the following table:

component dosage/component dosage/isopropanol

toluene

pentaerythritol dipropyl acrylate 60

10

20 tetrahydrofurfuryl alcohol acrylic acid

benzoin ether

10

2