Chemical etching exposes the surface of the film to a reactive chemical solvent, causing it to undergo a chemical change, thereby forming a fine, rough surface that changes the surface wettability and adhesion properties of the film.
The principle of the chemical etching method is that the film undergoes strong oxidation or corrosion by the treatment liquid and the polymer material, and the molecules on the surface thereof are oxidized or some molecules are torn off, thereby introducing polarities such as carbonyl groups, carboxyl groups, and sulfonic acid groups on the surface of the material. The group increases the adhesion of the surface to the ink. At the same time, as some molecules are torn off, the surface roughness increases, the non-polarity and wettability of the film are improved, and the adhesion of the ink is increased.
Different sheets should use different corrosive agents, ie PET: carbon chloride or aqueous solvents;
·PVC: alcohol solvents;
· PC: Alcohols or aqueous solvents;
PE: carbon chloride or chromic acid mixture.
Although chemical etching is effective in thin film processing, it also has some drawbacks, such as the surface of the treated film tends to become dark or dark.
2) Radiation method
The plastic film is placed in some polymerizable monomers (such as styrene, fumaric acid, methacrylate, etc.) and irradiated with Co-60 to chemically graft the monomers on the surface of the plastic film. So that the surface of the polymer material to form a layer of easy-to-bond graft polymer. After grafting, the surface is roughened, the bonding surface area is increased, and the adhesion strength is improved. This method has the advantages of simple operation, short processing time and high speed, but the durability of the modified surface is poor, and the Co-60 radiation source is more harmful to humans.
3) Plasma treatment
Plasma is formed by means of a controlled alternating or direct current to ionize the air. It is a collection of charged atoms that move randomly. Within this set of atoms, it is electrically neutral because it achieves a certain particle density. Plasma can be divided into hot plasma, mixed plasma and cold plasma.
For high polymers, the cold plasma method is a significant and effective method. This technology consists mainly of a reaction center, a radio frequency (RF) generator, a gas flow unit, and a process controller that supplies the required energy for excitation. RF energy separates electrons from gaseous materials, creating free electrons, ions, and excited molecules. These active particles are continuously recombined to form ground state gas molecules and photons, and react with the plasma to produce deposition and grafting on the surface of the film, thereby modifying the surface of the film.
The advantage of plasma treatment compared to other high-energy radiation is that it only acts on the surface. Most plasma-induced reactions do not exceed the depth of 1 μm in the surface layer and introduce only the desired interfacial properties on the surface. Affecting the nature and appearance of the inner layer, it eliminates the wet processing, drying, waste water treatment and other processes, low equipment investment and operating costs, easy protection, easy operation, clean, efficient, and safe. Mainly applied to PET, PVC and PE films.
4) Corona treatment
Corona treatment is to discharge the electrode by applying high-frequency and high-voltage power to the electrode. For plastic film surface treatment, the voltage is generally between 10 kV and 13 kV, and the frequency is between 10 kHz and 30 kHz. Various energy particles generated after gas ionization are generated. (such as positive and negative ions, electrons, photons, etc.) under the action of a strong electric field, accelerating the impact of the surface of the polymer between the electrodes, so that the surface molecules of the chemical bond rupture and degradation, increase the surface roughness. Corona discharge produces large amounts of ozone. Ozone is a strong oxidant, which can partially oxidize the molecular chains of the polymer surface to form carbon-based compounds, hydrogen-based compounds, and hydrogen peroxide compounds, thereby allowing the surface of the film to be roughened. In addition, corona treatment also removes oil and grime.
This treatment method is mainly applied to PET, PVC and PE films.
5) Ultraviolet Irradiation
After the plastic film surface is irradiated with ultraviolet rays, the wetting property and the adhesive property of the plastic film can be improved. The effect of ultraviolet irradiation treatment is similar to the effect of plasma treatment to a certain degree, which can cause the linkage of surface molecules on the polymer surface to be broken, crosslinked, and surface oxidized.
The use of photosensitizers can increase the effect of UV treatment. For example, a 5% solution of benzophenone in methylene chloride is sprayed onto the surface of a plastic film, and the surface is then treated with ultraviolet light. Benzophenone decomposes to generate free radicals, making the surface of the film polar, this treatment effect is more durable than other common surface modification methods, and is mainly applicable to PE film.
6) Degreasing and dust removal
The presence of oil or dust on the surface of the plastic film will affect the adhesion of the ink to the film. Therefore, the sheet film must be degreased and dusted. Through the use of alkaline solution, surfactants, solvents to clean the surface of the film to achieve the purpose of surface cleaning, degreasing.
Degreasing and dusting treatment should be a reasonable choice of solvents, can not use a good solvent for the solubility of plastic film. Commonly used lower alcohols (such as ethanol, isopropyl alcohol) or n-hexane wipe, this can soak up the plastic film surface, so that the ink is easy to penetrate, is conducive to the adhesion of ink. In addition, ultrasonic cleaning can also be used, and the vortex energy generated by the ultrasonic wave is transmitted to the contaminant particles to accelerate the exfoliation of the contaminant particles from the surface. The frequency range of the ultrasonic wave is 20000-50000Hz, and the ester can be sprayed twice more.
The presence of dust not only affects the adhesion of the ink, but also affects the printing effect. In general, surfactants may also be used for cleaning, and static electricity may be used for dust removal.
After the sheet film is processed, the surface tension can be tested to determine whether the surface tension of the plastic film before printing reaches a specified standard. In general, a reasonable range of surface tension is 38 to 41 mN/m. When the tension is less than 35mN/m, the use requirement cannot be generally satisfied.
Source: "Printing World" Author: Chi Chuan Zhao Xiaojing
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