Performance requirements and influence of the hott

2022-08-14
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Performance requirements and influence of plastic film materials (Part 2)

requirements for the basic performance of plastic film materials

when purchasing plastic film materials, we must be clear about the requirements for the performance of plastic film materials in production, which will be briefly introduced below

1. Appearance

the surface of the plastic film should be flat and smooth, without wrinkles or only a small amount of live folds, without obvious bumps, black spots, impurities, crystal dots and rigid blocks, without stripes, spots, sharp ribs and other ills, without bubbles, pinholes and cracks. The aluminized layer of the aluminized film should be uniform, and no obvious bright strips, yin and Yang surfaces and other phenomena are allowed. In addition, the film surface is required to be clean and free of dust, oil, etc

2. Specification and deviation

the width, thickness and deviation of the plastic film should meet the requirements, and should be uniform in thickness, with small horizontal and vertical thickness deviation and relatively uniform deviation distribution. In addition, the thickness of aluminum coating of aluminum coating shall also meet the requirements

3. Transparency and gloss

for transparent plastic films, the light transmittance is required to be higher, which should generally reach more than 92%; For opaque plastic film, such as white film, it is required to have high whiteness and good opacity. In addition, the plastic film should also have good gloss to ensure that the printing and composite products are bright and beautiful

4. Physical and mechanical properties

because plastic films are subjected to mechanical forces in the process of printing and compounding, film materials must have certain mechanical strength and flexibility. The physical and mechanical properties of the plastic film mainly include tensile strength, elongation at break, tear strength, impact strength, etc. in addition, the firmness of the aluminum coating on the aluminum coating should also meet the requirements. 5. Moisture permeability

refers to the permeability of plastic film materials to water vapor under certain conditions. For example, when the relative humidity is 90% and the temperature is 30 ℃, the thickness within 24 hours is 25 μ The mass of water vapor per square meter of plastic film. It represents the moisture resistance of film materials to a certain extent. The moisture permeability of various film materials is different, which also determines its scope of application

6. Oxygen permeability

refers to the oxygen permeability of plastic film materials under certain conditions. For example, when the relative humidity is 90% and the temperature is 23 ℃, the thickness within 24 hours is 25 μ M is the volume of oxygen per square meter of plastic film. The moisture permeability of various film materials is also different

7. Geometric stability

plastic film must have a certain geometric stability, otherwise, its expansion rate is too large, and it is easy to produce expansion deformation under the action of mechanical force or heat in the process of printing and compounding, which will not only affect the overprint accuracy, but also wrinkle, curl and other problems, which will seriously affect the product quality and production efficiency

8. Chemical stability

plastic films need to contact inks, adhesives and some organic solvents in the process of printing and compounding. These are chemical substances. Therefore, plastic films must have certain additional resistance to these chemicals in contact, so as not to be affected by them

9. Surface tension

in order to make the printing ink and composite adhesive have good wettability and adhesion on the surface of plastic film, the surface tension of the film is required to reach a certain standard, otherwise it will affect the adhesion and adhesion of the ink and adhesive on its surface, and thus affect the quality of prints and composite products. For example, the surface tension of biaxially stretched polypropylene film and low-density polyethylene film is required to reach 3.8 × Above n/m, the surface tension of nylon film, polyester film and polyester aluminized film is generally required to reach 4.5 × Above n/m. Generally speaking, the plastic film must undergo surface treatment before printing or compounding, so that users can contact or contact the computer supplier or local joint insurance unit through us to improve its surface tension and be able to print and compounding smoothly

influence of film surface characteristics on printing and compounding

1 Nonpolarity of molecular structure

generally, the adsorption and adhesion of ink and adhesive on the surface of plastic film 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 inertia. They are typical non-polar polymer materials. Ink and adhesive have poor wettability and affinity to their surfaces, and the intermolecular force between them is very weak

2. Surface energy of plastic film

the surface energy of plastic film is usually very low. For example, the critical surface tension of polyethylene is only 3.1 × N/m, the critical surface tension of polypropylene is only 3.4 × N/m, low surface energy, not easy to be wetted by ink or adhesive. Therefore, the critical surface tension of plastic film must be greater than or equal to the surface tension of ink or adhesive to ensure that the ink or adhesive is fully wetted on its surface

3. Crystallinity of plastic film

many plastic films not only have smooth surfaces, but also have high crystallinity. The molecules are in a thermodynamic stable state and have stable chemical properties. It is difficult for ink or adhesive molecules to diffuse on their surfaces, so the adhesion effect of ink, adhesive, etc. on the film surface is not ideal

4. Weak surface layer

no protection, long service life and other characteristics. In the process of processing polyolefin and other films, in order to make the films have better properties such as opening, antistatic, aging resistance and UV radiation resistance, a certain amount of additives, such as opening agents, antistatic agents, plasticizers, stabilizers, etc., are often added. These additives are very easy to precipitate, migrate to the surface and form oil stains, so that the film surface forms a weak surface layer with very low strength, Since the cohesion strength of the weak surface layer is much lower than that of the main body, it will have an adverse impact on the printing and composite process

5. Cleanliness of plastic film surface

if the environmental sanitation statistics are relatively poor and the air cleanliness is low, the surface of plastic film is very easy to absorb dust, grease and other substances in the air, and these adsorbents will prevent the ink and adhesive from contacting with the film surface and affect their wetting on the plastic film surface. Therefore, we must pay attention to the cleanliness of the film surface

these factors have a great impact on the adhesion fastness of printing ink and the bonding strength of composite film. Therefore, the surface treatment of plastic film is generally required before printing or compounding. Through surface treatment of plastic film, polar groups are introduced on the surface of non-polar plastic film, which changes the chemical structure of plastic film surface, improves the polarity of plastic film surface, improves its surface energy, reduces its surface crystallinity, and improves the cohesion strength of surface layer, which is not only conducive to improving the wettability of ink and adhesive on its surface, but also increases the force between their molecules, So as to improve the adhesion of ink and adhesive on its surface, so as to ensure the smooth progress of production

the commonly used surface treatment methods of plastic film mainly include chemical treatment, solvent treatment, coating treatment, flame treatment, corona treatment and ultraviolet irradiation, among which the most commonly used is corona treatment. Corona treatment is to use the principle of tip discharge to treat the surface of plastic film with high frequency and high voltage or medium frequency and high voltage, so as to activate the surface and make it porous, increase the surface area and improve the adhesion to ink and adhesive, at the same time, oxidize the surface of plastic film and improve the surface energy of plastic film, so as to improve the printability and composite performance of film. Due to the limited space, the content of corona treatment will be described later

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