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Plastic packaging film materials

Plastic packaging and plastic packaging products account for an increasing share in the market, especially composite plastic flexible packaging, which has been widely used in food, medicine, chemical industry and other fields, among which food packaging accounts for the largest proportion, such as beverage packaging, quick-frozen food packaging, cooking food packaging, fast food packaging, etc., these products have brought great convenience to people's lives

several common films

1. Biaxially oriented polypropylene film (BOPP)

biaxially oriented polypropylene film is made by CO extrusion of polypropylene particles to form a sheet, and then stretching in both vertical and horizontal directions. Due to stretching molecular orientation, this kind of film has good physical stability, mechanical strength, air tightness, high transparency and gloss, toughness and wear resistance. It is the most widely used printing film at present, with a thickness of 20 ~ 40 μ m. When the batch number is no more than 50, the most widely used is 20 μ m 。 The main disadvantage of biaxially stretched polypropylene film is poor heat sealing, so it is generally used as the outer film of composite film. For example, after compounding with polyethylene film, moisture resistance, transparency, strength, stiffness and printability are ideal, which is suitable for holding dry food. Because the surface of biaxially oriented polypropylene film is non-polar, with high crystallinity and low surface free energy, its printing performance is poor and its adhesion to ink and adhesive is poor. Surface treatment is required before printing and compounding

2. Low density polyethylene film (LDPE)

LDPE film is generally made by blow molding and tape casting. The thickness of cast polyethylene film is uniform, but it is rarely used at present because of its high price. Blow molded polyethylene film is made of blow molded PE particles by blow molding machine, with low cost, so it is the most widely used. Low density polyethylene film is a translucent, glossy and soft film, which has excellent chemical stability, heat sealing, water resistance and moisture resistance, freezing resistance and boiling ability. Its main disadvantage is that it has poor barrier to oxygen. It is often used as the inner film of composite flexible packaging materials. It is also the most widely used and most used plastic packaging film at present, accounting for more than 40% of the consumption of plastic packaging film

due to the absence of polar groups in polyethylene molecules, high crystallinity and low surface free energy, the film has poor printing performance and poor adhesion to ink and adhesive, so surface treatment is required before printing and compounding

3. Polyester film (PET)

polyester film is a film material made of polyethylene terephthalate by extrusion and biaxial stretching. It is a colorless, transparent and glossy film with excellent mechanical properties, high rigidity, hardness and toughness, puncture resistance, friction resistance, high and low temperature resistance, chemical resistance, oil resistance, air tightness and fragrance retention. It is one of the commonly used barrier composite film substrates. However, the price of polyester film is relatively high, with a general thickness of 12mm. It is often used as the outer material of cooking packaging, with good printability

4. Nylon film (PA)

nylon film is a very tough film, with good transparency, good gloss, high tensile strength and tensile strength, good heat resistance, cold resistance, oil resistance and organic solvent resistance, excellent wear resistance, puncture resistance, relatively soft, excellent oxygen resistance, but poor barrier to water vapor, high moisture absorption and moisture permeability, With a small amount of watch oil, the heat sealing property is poor, which is suitable for packaging hard goods, such as greasy food, meat products, fried food, vacuum packaged food, cooked food, etc

5. Cast polypropylene film (CPP)

cast polypropylene film is a polypropylene film produced by casting process, which can be divided into ordinary CPP and cooking CPP. It has excellent transparency, uniform thickness, and uniform longitudinal and transverse performance. It is generally used as the inner layer material of composite film. The thickness of ordinary CPP film is generally 25 ~ 50 μ M, after compounding with OPP, the transparency is better, the surface is bright, and the hand feel is firm. This material is generally used in gift bags. The film also has good heat sealing property. The thickness of cooking grade CPP film is generally 60 ~ 80 μ M, it can withstand high temperature cooking at 121 ℃ for 30 minutes, has good oil resistance and air tightness, and has high heat sealing strength. Generally, the inner layer of meat packaging adopts cooking grade CPP film

6. Aluminized film

at present, the most widely used aluminized films are polyester aluminized film (VMPET) and CPP aluminized film (VMCPP). Aluminized film has the characteristics of both plastic film and metal. The function of aluminum plating on the surface of the film is to block light and prevent ultraviolet radiation, which not only prolongs the shelf life of the contents, but also improves the brightness of the film, replacing aluminum foil to a certain extent, but also has low price, beautiful appearance and good barrier performance. Therefore, aluminum plating film is widely used in composite packaging. At present, it is mainly used in dry and expanded food packaging such as biscuits, as well as the outer packaging of some medicines and cosmetics

basic performance requirements of plastic film materials

generally speaking, the following factors should be considered in the selection of plastic film materials

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 aluminum coating of the aluminum coating should be uniform, and no obvious bright bars, 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 shall meet the requirements, and the thickness shall be uniform, the horizontal and vertical thickness deviation shall be small, and the deviation distribution shall be relatively uniform. 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

4. Physical and mechanical properties

as plastic film is subjected to mechanical force in the process of printing and compounding, it is required that the film material 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 quality of water vapor per square meter of M plastic film 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

indicates 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 oxygen permeability of various film materials is also different

7. Geometric dimension stability

plastic film must have a certain geometric dimension 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 show wrinkles, curls and other problems, seriously affecting 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 all chemicals. Therefore, plastic films must have certain 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, thus affecting the quality of printing 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, plastic films must be surface treated before printing or compounding to improve their surface tension, and can be printed and compounded smoothly

the influence of film surface characteristics on printing and compounding

the surface characteristics of film have a great influence on film printing and compounding, and there are many influencing factors, which are briefly described as follows

1. Nonpolarity of molecular structure

generally, the adsorption and adhesion of ink and adhesive on the surface of plastic film are mainly realized by the force between the two molecules with a high degree of automation of this equipment. 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 show that the surface of the prepared sample is smooth, 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 its surface, so the adhesion effect of ink, adhesive, etc. on the surface of the film is not ideal

4, weak surface layer

in the process of processing polyolefin and other films, in order to make the films have better properties such as opening, antistatic, aging resistance, UV irradiation, etc., a certain amount of additives, such as opening agents, antistatic agents, plasticizers, stabilizers, etc., are often added. These additives are very easy to precipitate and migrate to the surface to 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 conditions 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,

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