There are many kinds of plastic flexible packaging materials. I have also discussed this issue with you in the previous article. So what impact does plastic flexible packaging have on the printing and compounding of packaging bags? In this article, packaging bag manufacturers continue to share with you. 1. The non-polar molecular structure of plastic flexible packaging materials In general, the adsorption and adhesion of inks and adhesives on the surface of plastic films are mainly achieved by the force between the two molecules. Most plastic film packaging materials have basically no polar groups or only weak polar groups in their molecular structures, with small critical surface tension and strong inertness. They are typical non-polar polymer materials, inks and adhesives. The wettability and affinity of its surface are poor, and the intermolecular force between the two is very weak. 2. Crystallinity of plastic flexible packaging materials Many plastic films not only have smooth surfaces, but also have high crystallinity. The molecules are in a thermodynamically stable state and have stable chemical properties. It is difficult for ink or adhesive molecules to diffuse on the surface, so that the The adhesion effect of ink, adhesive, etc. on the surface of the film is very unsatisfactory. 3. Surface energy of plastic flexible packaging materials The surface energy of plastic films is usually very low. For example, the critical surface tension of polyethylene is only 3.1 * 10-2N/m, and the critical surface tension of polypropylene is only 3.4 * 10-2N/m. The surface energy is low, and it is not easy to be wetted by ink or adhesive. Therefore, the critical surface tension of the plastic film must be greater than or equal to the surface tension of the ink or adhesive to ensure that the ink or adhesive can be obtained on its surface. Adequate wetting. Fourth, the cleanliness of plastic flexible packaging materials If the environmental sanitation conditions are relatively poor and the air cleanliness is low, the surface of the plastic film is very easy to absorb dust, grease and other substances in the air, and these adsorbents will hinder the ink, adhesive and film. The mutual contact of the surfaces affects their wetting on the surface of the plastic film. Therefore, attention must be paid to the cleanliness of the film surface. 5. In the process of processing polyolefin and other films of the weak surface layer of plastic flexible packaging, in order to make the film have good opening, antistatic, aging resistance, anti-ultraviolet radiation and other properties, a certain amount of additives are often added, such as opening These additives are easily precipitated and migrate to the surface to form oil stains, so that a weak surface layer with very low strength is formed on the surface of the film. Since the cohesive strength of the weak surface layer is much lower than that of the bulk, printing and lamination processes can be adversely affected. The commonly used surface treatment methods for plastic films mainly include chemical treatment, solvent treatment, coating treatment, flame treatment, corona treatment and ultraviolet irradiation, among which corona treatment is currently the most commonly used. Corona treatment is to use the principle of tip discharge to treat the surface of the plastic film with high frequency high voltage or medium frequency high voltage, so that the surface is activated and porous, and the surface area is increased to improve the adhesion to ink and adhesives. The surface of the plastic film is oxidized, and the surface energy of the plastic film is increased, thereby improving the printability and composite performance of the film. The above factors have a great influence on the adhesion fastness of printing ink and the bonding strength of composite film. Therefore, before printing or composite, surface treatment of plastic film is generally required. By surface treatment of the plastic film, polar groups are introduced on the surface of the non-polar plastic film, the chemical structure of the surface of the plastic film is changed, the polarity of the surface of the plastic film is improved, the surface energy is increased, the crystallinity of the surface is reduced, and the The cohesive strength of the surface layer is not only conducive to improving the wetting properties of the ink and the adhesive on the film surface, but also increases the force between the two molecules, thereby improving the adhesion of the ink and the adhesive on the film surface. To ensure the smooth progress of production.
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