Status & Prospect of Multi-layer Co-extruded High Barrier Film
1. Manufacturing technique of multi-layer co-extruded film
Generally speaking, multi-layer co-extruded technology is to melt and extrude different resin raw materials with different functions by 5 or more extruders, integrating advantages of various materials. It helps to control cost effectively and adjust the film structure based on demand with wide application scope.
The processing method of multi-layer co-extruded film is divided into: upper blown & air cooling, down blown & water cooling, and casting.
1) Upper Blown & Air Cooling
The film is cooled by air with slow cooling speed and higher crystallinity, realizing high strength but low softness and transparency, hard to control the film extrusion stability and leading to greater thickness difference.
2) Down Blown & Water Cooling
The film is cooled by water with fast cooling speed and lower crystallinity, realizing high softness and transparency, but hard to control the film extrusion stability, leading to greater thickness difference.
3) Casting
The film is cooled by cooling roller with fast cooling speed, realizing high softness and transparency, stable extrusion forming, and less thickness difference.
2. Difference between casting and extrusion blow moulding
1) Comparison of production process
A. Production difficulty
Casting is more suitable for the production of multi-layer co-extruded films than blowing. Particularly when co-extruding with various kinds of materials, it is easier to control the processing parameters of casting. The design of the die head can effectively ensure the uniform distribution of various materials in the product. However, the co-extruded die head technology of blowing is more difficult, the structure is more complex and the technology is not mature.
B. Cooling and forming
The casting hot-melt film is short which can be quickly cooled and formed under the action of vacuum exhaust hood, air knife and edge locator, generating products with stable and good quality. However, the blown hot-melt film will go through a process of blowing and inflating, and get cooled and formed by air or water, leading to a long forming time and great changes after forming. It is susceptible to external influences in the melting phase, resulting in hard product quality control and poor stability.
C. Raw material selection
Casting and blowing have great difference in raw material selection. When multi-layer co-extruding, it is requested to select combination of materials with similar fluidity. When the fluidity of materials are great different, there will be lack of adjustment. That means, once one material is chosen, a few options of other co-extruded materials are available, which largely restricts the advantages of multi-layer co-extrusion. But during casting process, it can solve most problems by adjusting splitter roller and swing blade.
2.Comparison of product features
The main advantages of cast film include: good surface gloss, high flatness, good mechanical extension, good flexibility, better transparency than blown film.
A. Heat resistance
The blown film will go through a process of blowing and inflating, leading to different stretching of molecular at both verticality or horizontality. The heat resistance and heat sealing performance of the product will be greatly affected. When the product is required to retort and sterilized, the molecular chain stretched in forming will have a significant retraction after heating, resulting in serious deformation and greatly reducing the strength of the heat sealing connection.
B. Transparency & uniformity
When producing the cast film, the molecular arrangement is orderly, and the cooling speed of the product is fast, which is conducive to improving the transparency, gloss and thickness uniformity of the product. The impact resistance, heat resistance and low temperature adaptability of cast film are better than blown film.
2) Production efficiency
Casting has high production efficiency and stable product quality. The waste in the production process is less, and it is easier to realize online recycling of cutting edge, waste film, etc., resulting in higher materials utilization.
3) Thickness tolerance
The casting process adopts a flat die head equipped with a special hold-up tank, which can ensure even material flow and automatically control the uniformity of the die lip gap by automatic adjustment device. The control accuracy is very high and the thickness tolerance of thick film can be controlled below ±3%.
The blowing process adopts ring die head and multi-layer die core. The requirements of machining accuracy and assembly accuracy are very high and particularly difficult to control. Therefore, the thickness tolerance of blown film can be controlled around +8%~10%.
4) Quality of corona
When in down blowing and water cooling, Incomplete drying and Improper selection of corona material will lead to negative corona or severe corona attenuation, resulting in poor adhesion fastness of ink and decolorization. In casting process, the steel casting roller is used to cool the products, and the moisture will not contact product directly, so it is easier to ensure the quality of the corona treatment of the products.
5) Removal of residue
In the plastic forming process, due to thermal decomposition or low molecular precipitation, it is easy to produce residue accumulation in the mold, which is easily removed in casting process. But the residue in the blowing process can only be removed by stopping the machine. If not removed in time, it is easy to entrap in the product, resulting in excessive impurity content or obvious scratches on the film surface. Because the blowing method itself has low output and large waste of startup, if the machine is frequently opened and stopped, the unit production cost will be greatly increased.
1. The cast film containing EVOH and PA as barrier layer has excellent barrier, mechanical properties, tensile thermoforming and heat sealing properties; no additional lamination is required and no ink, adhesive and auxiliary residue are involved. Therefore, it is a green and eco-friendly product with cost advantage, which can be used for packaging food, beverages and items that are easy to be oxidized. It can reduce the use of additives and preservative, and greatly extend the shelf life of food. Meanwhile, the material is easy to recycle with no harmful gases after incineration, which is known as green and eco-friendly packaging materials, and also becomes the market trend after "plastic limit directive". It also has the advantages of low production cost, simple process, low energy consumption, high production efficiency, and so on, according with the green and low carbon environmental protection development policy.
Here are several typical multi-layer co-extruded barrier film structures:
① Non-symmetrical structure: PA/TIE/PA/TIE/PE/PE/PE
Main performance: Good flexibility, transparency and low temperature resistance.
② Non-symmetrical structure: PA/TIE/PA/TIE/PP/PPIPP
Application: Frozen vacuum packaging; good puncture resistance.
Main performance: Good transparency, retortable above 121℃
③ Symmetrical structure: PP/TIE/PA/EVOHIPA/TIEIPP
Application: retort packaging; certain low temperature resistance
Main performance: High barrier, good transparency and aroma protection.
④ Non-symmetrical structure:PA/TIE/PA/EVOH/PA/TIE/PE
Application: Stretch thermoforming packaging
Main performance: High barrier, good transparency and easy to stretch thermoforming
⑤ Non-symmetrical structure: PA/PA/TIE/PA/EVOH/PA/TIE/PE/PE
Application: Stretch thermoforming packaging
Main performance: High barrier, good transparency and easy to stretch thermoforming
⑥ Non-symmetrical structure: PBT/TIE/PE/TIE/PA/EVOH/PA/TIE/PE/PE/PE
The mechanical property of structure ⑥ is better than structure⑤, avoid warping. The main performance of structure⑥ is similar with structure⑤.
Allocation principle of above structures:
A. PA, EVOH and PBT are polar materials, while PE and PP are non-polar materials. According to the principle of similarity and compatibility, TIE is needed between them to glue them together, and TIE is modified by PE and PP with good water resistance, so the PA in the inner layer will not reduce the barrier due to water absorption. Therefore, replacing the single-layer material with multi-layer of the same materials in the barrier layer can improve the stability of film barrier.
B. In the barrier layer, it can significantly improve the barrier property of the film by replacing a single material with two different materials. According to the principle of synergistic effect, PA/EVOH/PA has better oxygen barrier than EVOH.
C. Dividing PE or PP into two layers, and using cheap materials for one layer, and the functional materials for the other layer can reduce production costs.
D. Co-extruded film with more layers can improve the stress warping resistance of film below seven layers. Because PA is easy to deform after water absorption, resulting in film curling, and the above structure⑥ with PBT instead of PA will not generate curling.
1. Conclusion
According to authoritative statistics, there are currently more than 20 manufacturers of multi-layer co-extrusion high barrier film in China, among which there are only a dozen of production lines for multi-layer co-extruded high barrier cast film (most of which are introduced from Germany), with a total production capacity of less than 80,000 tons per year, and most of them are used in the packaging of low-end products. However, the use amount of plastic packaging film exceeds 10.7 million tons in 2020, while the amount of high barrier cast film accounts for less than 1%, which has huge market growth space.
Statement: This article is written by LD PACK and all rights reserved by LD PACK. The content is for reference only. Please correct any errors. Shall not be reproduced without permission, otherwise LD PACK will reserve the right to pursue its legal liability. If you have any technical questions, please email sales@ldpack.com for a discussion.
