Abstract:This paper analyzes the white point problem of composite films of PET/VMCPP and PET/VMPET/PE when they are composited,and introduces the corresponding solutions.
Aluminum coated composite film is a soft packaging material with an “aluminum luster” formed by compounding aluminum coated films (generally VMPET/VMBOPP, VMCPP/VMPE, etc., among which VMPET and VMCPP are the most commonly used) with transparent plastic films. It is applied to packaging of food, health products, cosmetics, and other products.Due to its excellent metallic luster, convenience, affordability, and relatively good barrier performance, it has been widely used (better barrier properties than plastic composite films, cheaper and lighter than aluminum-plastic composite films). However, white spots often occur during the production of aluminum plated composite films. This is particularly evident in composite film products with PET/VMCPP and PET/VMPET/PE structures.
1、 The causes and solutions of “white spots”
Description of the “white spot” phenomenon: There are obvious white spots on the appearance of the composite film, which can be randomly distributed and of uniform size. Especially for non printed composite films and full plate white ink or light color ink composite films, it is more obvious.
1.1 Insufficient surface tension on the aluminum plating side of the aluminum coating.
In general, surface tension testing should be carried out on the corona surface of the film used before composite, but sometimes the testing of the aluminum coating is ignored. Especially for VMCPP films, due to the possibility of precipitation of small molecular additives in the CPP base film, the aluminum plated surface of VMCPP films stored for a period of time is prone to insufficient tension.
1.2 Poor leveling of adhesive
Solvent based adhesives should choose the optimal working solution concentration according to the product manual to ensure optimal glue leveling. And viscosity testing control should be implemented during the continuous production composite process. When the viscosity increases significantly, solvents should be added immediately. Enterprises with conditions can choose enclosed automatic pump glue equipment. The optimal heating temperature for solvent-free adhesives should be selected according to the product manual. In addition, considering the issue of solvent-free activation period, after a long time, the glue in the measuring roller should be discharged in a timely manner.
1.3Poor composite process
For PET/VMCPP structures, due to the small thickness and easy extensibility of the VMCPP film, the lamination roller pressure should not be too high during lamination, and the winding tension should not be too high. However, when PET/VMCPP structure is composite, due to the fact that PET film is a rigid film, it is advisable to increase the laminating roller pressure and winding tension appropriately during composite.
Corresponding composite process parameters should be formulated based on the situation of the composite equipment when different aluminum coating structures are composite.
1.4Foreign objects entering the composite film causing “white spots”
Foreign objects mainly include dust, rubber particles, or debris. Dust and debris mainly come from the workshop, and are more likely to occur when the workshop hygiene is poor. Rubber particles mainly come from rubber discs, coating rollers, or bonding rollers. If the composite plant is not a dust-free workshop, it should also try to ensure the cleanliness and tidiness of the composite workshop, install dust removal or filtration equipment (coating device, guide roller, bonding device and other components) for cleaning. Especially the coating roller, scraper, flattening roller, etc. should be cleaned regularly.
1.5High humidity in the production workshop leads to “white spots”
Especially during the rainy season, when the workshop humidity is ≥ 80%, the composite film is more prone to “white spots” phenomenon. Install a temperature and humidity meter in the workshop to record changes in temperature and humidity, and calculate the probability of white spots appearing. Enterprises with conditions can consider installing dehumidification equipment. For multi-layer composite structures with good barrier properties, it is necessary to consider suspending production or producing single-layer multiple or intermittent composite structures. In addition, while ensuring the normal performance of the adhesive, it is recommended to reduce the amount of curing agent used appropriately, usually by 5%.
1.6Gluing surface
When no obvious abnormalities are found and the problem of “white spots” cannot be solved, the coating process on the aluminum coating side can be considered. But this process has significant limitations.Especially when VMCPP or VMPET aluminum coating is subjected to heat and tension in the oven, it is prone to tensile deformation, and the composite process needs to be adjusted. Additionally, the peel strength of the aluminum plating layer may decrease.
1.7Special explanation for the situation where no abnormalities were found after shutdown, but “white spots” appeared after maturation:
This type of problem is prone to occur in composite membrane structures with good barrier properties. For PET/VMCPP and PET/VMPET/PE structures, if the membrane structure is thick, or when using KBOPP or KPET films, it is easy to produce “white spots” after aging.
High barrier composite films of other structures are also prone to the same problem. Examples include using thick aluminum foil or thin films such as KNY.
The main reason for this “white spot” phenomenon is that there is gas leakage inside the composite membrane. This gas may be the overflow of residual solvents or the overflow of carbon dioxide gas generated by the reaction between the curing agent and water vapor. After the gas overflows, due to the good barrier properties of the composite film, it cannot be discharged, resulting in the appearance of “white spots” (bubbles) in the composite layer.
Solution: When compounding solvent based adhesive, the process parameters such as oven temperature, air volume, and negative pressure should be set well to ensure that there is no residual solvent in the adhesive layer. Control the humidity in the workshop and select a closed adhesive coating system. Consider using a curing agent that does not produce bubbles. In addition, when using solvent based adhesives, it is necessary to test the moisture content in the solvent, with a requirement of moisture content ≤ 0.03%.
The above is an introduction to the phenomenon of “white spots” in composite films, but there are various reasons that can cause such problems in actual production, and it is necessary to make judgments and improvements based on the actual production situation.
Post time: Dec-11-2023