Multi-layer co-extrusion barrier film refers to a functional composite film made by using multiple extruders to extrude the melts of resins with higher barrier properties and other resins through a common die.
First, the multilayer coextruded barrier film is a plastic film. In the field of plastic products, we usually refer to flat plastic products with a thickness of less than 0.2 mm as plastic films, those with a thickness of 0.2 to 0.7 mm as plastic sheets, and those with a thickness greater than 0.7 mm as plastic sheets.
The multi-layer co-extruded barrier film should have certain gas barrier properties. The barrier property here refers to the shielding ability of plastic products (containers, films) to small molecular gases and odors. We usually use gas permeability to measure the size of the barrier properties of plastic products.
General-purpose plastics such as PE and PP have large gas permeability values, that is, the gas barrier properties are poor. For resin materials such as PA, PVDC, EVOH, etc., the gas permeability value is much lower than that of general-purpose plastics, and the barrier performance is better. Therefore, we usually refer to the multi-layer co-extruded film containing at least one resin material of PA, PVDC and EVOH as the multi-layer co-extruded barrier film.
Multilayer co-extrusion of PE, PA, TIE, EVOH and other resins can be used for vacuum packaging of dairy products, jams, meat products, etc.
The multi-layer co-extruded film mostly adopts the “ABCBA” 5-layer symmetrical structure, with PA or EVOH as the barrier layer, and polyethylene as the heat-sealing layer. Adhesive resins are used to space the disconnected barrier and heat seal layers. PA or EVOH is a resin that is extremely sensitive to humidity, protected by a polyethylene layer, and its excellent oxygen barrier properties are fully exerted.
In general, the structure of a multilayer coextruded film depends on the functional requirements of the film. On the premise of meeting the process requirements, through the combination of different polymers, it can meet the functional requirements of packaging materials such as barrier, heat sealing, body strength, thermal puncture, environmental resistance, secondary processing characteristics, extended storage and shelf life. From the analysis of functional requirements, the combination formed by five polymers is sufficient.
However, co-extruded composite films with seven layers, nine layers, eleven layers and even more layers have been applied in the market, making it a trend and developing rapidly. The structural design of the co-extruded film is gradually required to systematically achieve the ideal state of integrating function, technology, cost, environmental protection, safety and secondary processing.
So what are the structural features of the multi-layer co-extruded film formed by dividing the barrier layer and the heat-sealing layer through the functional division of more layers? The following takes 7-layer co-extruded film as an example to compare with 5-layer co-extruded film:
1. Cost comparison
Replacing expensive polymers with cheaper polymers on the surface layer can reduce the cost of the product, and has the heat-sealing properties and high moisture barrier properties of ionic polymers. With the same barrier properties and heat-sealability, 7-layer co-extruded blown film is more economical than 5-layer.
The two surface layers are divided into two groups, and one group is made of cheap materials, which not only maintains the strength of the film, but also saves costs. For example, compare two 1KG films. The material cost for the 5-layer film is 19% higher than the cost for the 7-layer film. Replacing the ionomer with inexpensive EVA in a 5-layer coextruded film could equal the cost of a 7-layer film product. But the problem is that the heat seal strength is insufficient.
Using two different polymers to replace a single type of polymer on the barrier layer can greatly improve its barrier properties. For example, combining an EVOH layer with a typical nylon material not only maintains the penetration resistance of the PA, but also increases the strength of the EVOH and improves the crack resistance of the EVOH because the EVOH layer is sandwiched between two layers of PA amine. , making it a film with high barrier properties, which cannot be achieved by 5-layer co-extruded films.
The added cost of adding EVOH can be added to the total consumption of the structure. The oxygen transmission rate of the 5-layer co-extruded film with 20% PA structure was 3.5 units, but under the same conditions, EVOH was added to the 7-layer film, and the transmission rate was 0.13 units.
3. Crack resistance
The performance of PA co-extruded films with less than five layers can be improved by using co-extruded films with more layers. For example, the use of an additional adhesive layer can improve the barrier properties of the film by increasing the water vapor barrier of the film. Another advantage obtained at the same time is that the film can be made softer, has a good hand and has good crack resistance.
With the continuous progress of polymer synthesis technology, new polymers with unique physical and mechanical properties can be widely used to meet the needs of packaging. The function and structure of the multi-layer co-extruded composite substrate film will have greater flexibility and economy. Through the application and improvement of molding equipment and processes, combined with the unique and effective design of the composite structure, film manufacturers will revolutionize their pursuit of concepts such as diversification of packaging functions, rationalization of packaging structures, and maximization of packaging benefits. effect.
However, the scientific use of raw materials, the design of reasonable product structure and the close coordination with the processing technology are the never-ending challenges facing each of our composite film manufacturers. Because only when the technical resources of processing equipment, processing raw materials, and structural design (processing technology) are fully utilized and reach the best state, can the packaging substrate be used in the most economical, reasonable and sufficient way in its final product. , the cheapest form, and meet the needs of the market.