Film made of polyvinyl chloride, polyethylene, polypropylene, polystyrene, and other resins, used for packaging, as well as used as a laminating layer. Plastic packaging and plastic packaging products accounted for an increasing share of the market, especially composite plastic flexible packaging, has been widely used in food, medicine, chemicals and other fields, including food packaging accounted for the largest proportion, such as beverage packaging, frozen food packaging, cooking food packaging, fast food packaging, etc., these products have brought great convenience to people's lives.
Types of film
PVA coated high barrier film
PVA coated high barrier film is added to the nanometer inorganic PVA coated in polyethylene film after printing, composite and become, without substantially increasing the cost of the premise, its barrier properties are not only significantly better than the five-layer *** extrusion of EVOH film, but also a substantial reduction in packaging costs, which Not only can ensure that the packaged goods on the aseptic packaging of all the quality requirements, but also significantly reduce the cost of aseptic packaging of food processing enterprises, can be used for packaging beverages, fruit juices, milk, soy sauce vinegar and so on.
Biaxially oriented polypropylene film (BOPP)
Biaxially oriented polypropylene film is made from polypropylene particles by **** extrusion to form a sheet, and then by the longitudinal and transverse direction of the stretching of the two produced. Because of the stretching molecular orientation, so this film's physical stability, mechanical strength, airtightness is better, higher transparency and gloss, tough and wear-resistant, is the most widely used printing film, the general use of the thickness of 20 ~ 40 μ m, the most widely used for the 20 μ m. The film is made from polypropylene particles that are *** extruded to form a sheet, and then stretched in both directions. Biaxially oriented polypropylene film is the main disadvantage of poor heat sealing, so it is generally used as a composite film of the outer layer of the film, such as composite with the polyethylene film moisture resistance, transparency, strength, stiffness and printability are more ideal, suitable for holding dry food. Because the surface of the biaxially oriented polypropylene film is non-polar, high crystallinity, low surface free energy, therefore, its printing performance is poor, poor adhesion to ink and adhesive, in printing and composite before the need for surface treatment.
Low-density polyethylene film (LDPE)
Low-density polyethylene film is generally made by blow molding and casting two processes. Cast polyethylene film has a uniform thickness, but is most widely used because it is more expensive and less costly. Low-density polyethylene film is a semi-transparent, glossy, softer texture film with excellent chemical stability, heat sealing, water resistance and moisture resistance, freezing resistance and can be boiled. Its main drawback is the poor barrier to oxygen, commonly used in composite flexible packaging materials for the inner layer of the film, but also the most widely used, the largest amount of a plastic packaging film, accounting for more than 40% of the consumption of plastic packaging film.
Because the polyethylene molecule does not contain polar groups, and high crystallinity, low surface free energy, therefore, the film's printing performance is poor, poor adhesion to ink and adhesive, so in the printing and composite before the need for surface treatment.
Polyester film (PET)
Polyester film is a film material made of polyethylene terephthalate as raw material, which is extruded into a thick sheet by extrusion method, and then made by bi-directional stretching. It is a colorless transparent, glossy film with excellent mechanical properties, high rigidity, hardness and toughness, puncture resistance, friction resistance, high and low temperature resistance, chemical resistance, oil resistance, airtightness and good fragrance retention, is one of the commonly used permeability barrier composite film substrate. However, polyester film is more expensive, the general thickness of 12 μ m, commonly used as the outer layer of the steaming packaging materials, better printability.
Nylon film (PA)
Nylon film is a very tough film, good transparency, and has a good gloss, tensile strength, tensile strength is high, but also has good heat resistance, cold resistance, oil resistance and resistance to organic solvents, abrasion resistance, puncture resistance is excellent, and more soft, oxygen barrier is excellent, but the barrier of water vapor is poor, moisture absorption, moisture permeability, heat sealing is poor. Larger, poorer heat sealability, suitable for packaging hard goods, such as greasy food, meat products, fried food, vacuum-packed food, boiled food.
Cast polypropylene film (CPP)
Cast polypropylene film is the use of cast process production of polypropylene film, can be divided into ordinary CPP and cooking grade CPP two kinds of transparency is very good, the thickness of the uniform, and longitudinal and transverse performance uniformity, is generally used as a composite film of the inner layer of material. Ordinary CPP film thickness is generally between 25 ~ 50μm, and OPP composite transparency is good, bright surface, feel firm, general gift bags are used in this material. This film also has good heat sealability. Cooking grade CPP film thickness is generally between 60 ~ 80 μ m, can withstand 121 ℃, 30 min of high temperature cooking, oil resistance, airtightness is better, and higher heat sealing strength, general meat packaging inner layer are used to cook grade CPP film.
Aluminized film
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 role of the film surface aluminizing is shading, anti-ultraviolet radiation, both to extend the shelf life of the contents, but also to improve the brightness of the film, to a certain extent, instead of aluminum foil, but also inexpensive, beautiful and better barrier properties, therefore, aluminized film in the composite packaging applications are very wide.
Surface properties
Plastic films are most widely used in the field of packaging. Plastic film can be used for food packaging, electrical product packaging, daily necessities packaging, clothing packaging and so on. They have a **** the same point, that is, the plastic film are to color printing, and as food packaging but also multi-layer composite or vacuum aluminizing and other process operations. Therefore, the requirements of the plastic film surface free energy to be high, wet tension to be large, in order to facilitate the printing ink, adhesive or aluminum layer and plastic film adhesion; in the production of plastic film winding and high-speed packaging process, it is required that the film surface has a certain degree of friction properties to prevent the film from adhering to the film or slippery; in the electrical appliances, electronic products, such as packaging, then the film is required to have a certain degree of anti-static properties, and so on.
Surface tension of plastic film
The surface tension of plastic film depends on the size of the surface free energy of the plastic film, and the surface energy of the film depends on the molecular structure of the film material itself. Most plastic films such as polyolefin films (LDPE, HDPE, LLDPE, PP) are non-polar polymers, their surface free energy is small, the surface wet tension is low, generally about 30 dynes/cm. Theoretically, if the surface tension of the object is lower than 33 dynes/cm, ordinary ink or adhesive will not be able to adhere to the firm, so it is necessary to treat its surface. Polyesters (PET, PBT, PEN, PETG) are polar polymers with high surface free energy and surface wet tension above 40 dynes/cm. But for high-speed color printing or to increase the bonding force between the vacuum aluminizing layer and the surface of the BOPET film, it is also necessary to surface treatment of BOPET film to further improve its surface wet tension.
Plastic film surface treatment methods are: corona treatment method, chemical treatment method, mechanical linting method, coating method, etc., which is most often used is the corona treatment method.
The basic principle of corona treatment method is: through the metal electrode and corona treatment roller (generally high temperature, ozone resistance, high insulation of the silicone rubber roller) between the application of high-frequency, high-voltage power supply to produce a discharge, so that the air ionization and the formation of a large amount of ozone. At the same time, high-energy electric spark impact film surface. In their *** with the role of the plastic film surface activation, surface energy increases. Through the corona treatment can make the polyolefin film wet tension increased to 38 dyne/cm; can make the polyester film surface wet tension to 52-56 dyne/cm or more. Corona treatment of plastic film surface wet tension of the size of the voltage applied to the electrode, the electrode and the distance between the corona treatment rollers and other factors. Of course, corona treatment should be moderate, not the higher the intensity of corona treatment is better. Here it is worth noting that the plastic film and corona treatment rollers should be avoided between the air entrapment, otherwise it is possible to make the reverse side of the film is also corona treatment. Reverse corona caused by the consequences of: 1 may produce ink printing anti-sticking phenomenon; 2 in the aluminum plating will occur in the transfer of aluminum plating layer, in the adhesive coating will occur in the transfer of adhesive coating layer. The main measure to prevent the reverse side of the film corona is to adjust the pressure of the rubber pressure roller before the corona treatment rollers, pressure on both ends of the pressure roller should be consistent and the size of the pressure should be appropriate. In addition, the corona roller and the pressure roller must be strictly static and dynamic balance test, radial runout requirements of less than 0.05 mm, the purpose is to ensure that the plastic film into the corona rollers, preventing air entrapment to avoid the phenomenon of reverse corona.