Aluminum coating not only has the characteristics of plastic film, but also to some extent replaces aluminum foil, playing a role in improving product grade, and relatively low cost. Therefore, it is widely used in the packaging of biscuits and snack foods. However, in the production process, there is often a problem of aluminum layer transfer, which leads to a decrease in the peeling strength of the composite film, resulting in a decrease in product performance, and even seriously affecting the quality of the packaging content. What are the reasons for the transfer of aluminum coating? What should be paid attention to in the operation of composite technology?
Why is the aluminum coating prone to delamination?
At present, the most commonly used aluminum plating films are CPP aluminum plating film and PET aluminum plating film, and the corresponding composite film structures include OPP/CPP aluminum plating, PET/CPP aluminum plating, PET/PET aluminum, and so on. In practical applications, the most problematic aspect is PET composite PET aluminum plating.
The main reason for this is that as a substrate for aluminum plating, CPP and PET have significant differences in tensile properties. PET has a higher rigidity, and once compounded with materials that also have great rigidity, during the curing process of the adhesive film, the presence of cohesion can easily cause damage to the adhesion of the aluminum coating, leading to the migration of the aluminum coating. In addition, the permeation effect of the adhesive itself also has a certain impact on it.
Precautions during composite process operation
In the operation of composite processes, attention should be paid to the following aspects:
(1) Select appropriate adhesives. When composite aluminum coating, be careful not to use adhesives with low viscosity, as low viscosity adhesives have a small molecular weight and weak intermolecular forces, resulting in strong molecular activity and are prone to damaging their adhesion to the substrate through the aluminum coating of the film.
(2) Enhance the softness of the adhesive film. The specific method is to reduce the amount of curing agent when preparing the working adhesive, so as to reduce the degree of crosslinking reaction between the main agent and the curing agent, thereby reducing the brittleness of the adhesive film and maintaining good flexibility and extensibility, which is conducive to controlling the transfer of the aluminum coating.
(3) The amount of glue applied should be appropriate. If the amount of adhesive applied is too small, it will undoubtedly result in lower composite fastness and easy peeling; But if the amount of adhesive applied is too large, it is not good. Firstly, it is not economical. Secondly, the large amount of adhesive applied and the long curing time have a strong penetration effect on the aluminum plating layer. So a reasonable amount of glue should be chosen.
(4) Control the tension properly. When unwinding aluminum plating, the tension must be well controlled and not too high. The reason is that the aluminum coating will stretch under tension, resulting in elastic deformation. The aluminum coating is correspondingly easy to loosen and the adhesion is relatively reduced.
(5) Maturation speed. In principle, the curing temperature should be increased to accelerate the curing speed, so as to enable the adhesive molecules to quickly solidify and reduce the penetration damage effect.
The main reasons for aluminum plating transfer
(1) Causes of internal stress in glue
During the high-temperature curing process of the two-component adhesive, the internal stress generated by the rapid crosslinking between the main agent and the curing agent causes aluminum plating transfer. This reason can be demonstrated through a simple experiment: if the composite aluminum coating is not put into the curing room and is cured at room temperature (it takes several days to fully cure, without practical production significance, just an experiment), or is cured at room temperature for several hours before entering the curing room, the phenomenon of aluminum transfer will be greatly alleviated or eliminated.
We found that using a 50% solid content adhesive to composite aluminum plating films, even with a low solid content adhesive, would result in much better transfer behavior. This is precisely because the network structure formed by low solid content adhesives during the crosslinking process is not as dense as the network structure formed by high solid content adhesives, and the internal stress generated is not so uniform, which is not enough to densely and uniformly act on the aluminum coating, thereby alleviating or eliminating the phenomenon of aluminum transfer.
Except for the slight difference between the main agent and ordinary adhesive, the curing agent for general aluminum plating adhesive is generally less than ordinary adhesive. There is also a purpose to reduce or alleviate the internal stress generated by adhesive crosslinking during the curing process, in order to reduce the transfer of the aluminum plating layer. So personally, I believe that the method of "using high-temperature rapid solidification to solve the transfer of aluminum coating" is not feasible, but rather counterproductive. Many manufacturers now use water-based adhesives when composite aluminum plating films, which can also be evidenced by the structural characteristics of water-based adhesives.
(2) Reasons for stretching deformation of thin films
Another obvious phenomenon of aluminum plating transfer is generally found in three-layer composites, especially in PET/VMPET/PE structures. Usually, we first composite PET/VMPET. When composite in this layer, the aluminum coating is generally not transferred. The aluminum coating only undergoes transfer after the third layer of PE is composite. Through experiments, we found that when peeling a three-layer composite sample, if a certain amount of tension is applied to the sample (i.e. artificially tightening the sample), the aluminum coating will not transfer. Once the tension is removed, the aluminum coating will immediately transfer. This indicates that the shrinkage deformation of the PE film produces an effect similar to the internal stress generated during the adhesive curing process. Therefore, when composite products with such a three-layer structure, the tensile deformation of the PE film should be minimized as much as possible to reduce or eliminate the phenomenon of aluminum transfer.
The main reason for aluminum plating transfer is still film deformation, and the secondary reason is the adhesive. At the same time, aluminum plated structures are most afraid of water, even if a drop of water penetrates into the composite layer of the aluminum plated film, it will cause serious delamination.
Post time: Oct-28-2023
