Dry lamination is a process in which two or more kinds of films are bonded together by using dry laminating equipment and adhesives. Dry lamination is one of the most commonly used production methods in plastic composite flexible packaging. It plays an important role in the packaging industry due to its wide selection of substrates, high composite strength, convenient printing and fast production speed. However, bubbles often appear in dry composite products, which not only affect the beauty of the products, but also affect the oxygen resistance, air permeability and peel strength of the composite film. This problem has been troubling the flexible packaging manufacturers for a long time. There are many reasons leading to bubbles, including production environment, membrane materials, adhesives, coating, drying process, compounding, curing and so on.
Today, we will introduce the influence of production environment on bubbles.
When the ambient temperature is too high, the two-component adhesive is easy to be muddy and white, viscous and poor coating performance, resulting in bubbles. However, low temperature leads to poor dispersion of adhesive, poor wetting and adhesion to the film, and also leads to the generation of bubbles. Generally, the workshop temperature should be controlled at 18 ~ 25 ℃.
If the ambient humidity is too high (more than 70%), condensation will occur between the anilox roller and the cot, and the reaction between moisture and curing agent will produce carbon dioxide, which will eventually lead to bubbles. In addition, the plastic film is easy to absorb the moisture in the air, especially for the plastic film with high moisture absorption, such as nylon and cellophane, resulting in the formation of bubbles after compounding. The relative humidity should be kept at 60% - 70%.
There are a lot of dust, impurities and other particles in the air. These particles are adsorbed on the plastic film, or fall into the coating roller, composite roller and glue liquid, which may lead to the generation of bubbles. Therefore, it is necessary to keep the workshop clean and sanitary, reduce the floating dust in the air, and should often clean the drying channel, and try to keep the workshop in a good sealing state.
2、 The water content or other active hydrogen substances in the diluent are high.
1. The reaction process of two-component polyurethane adhesive is the cross-linking reaction between the active hydrogen in the main agent and isocyanate in curing agent to form a three-dimensional network structure. Isocyanate can easily react with substances (water, alcohol, amine) containing hydroxyl and amino groups. In the use of polyurethane adhesives, combined with the properties of solvents (boiling point, solubility), ethyl acetate is generally used as the diluent of adhesives. However, because ethyl acetate contains carboxyl group and has good affinity to water, it is inevitable that water will be mixed in the production process of ethyl acetate. According to the national standard gb3728-91 for industrial grade ethyl acetate, ethyl acetate is divided into several grades. The moisture content of superior product is less than 1000ppm (0.1%), and that of first grade is less than 2000ppm (0.2%).
The reaction formula of water with - NCO group is as follows:
—NCO+H20→—NHCOOH→—NH2+CO2
—NCO+—NH2→—NHCONH
It can be seen from the above formula that one water molecule consumes two isocyanate groups, that is, 18G water can react with 84g isocyanate group. Therefore, the quality of diluent should be strictly controlled when choosing and using diluent, and ethyl acetate with alcohol, ammonia or moisture greater than 0.2% should not be used. At the same time, the purity and moisture of ethyl acetate should be strengthened in the incoming inspection.
2. In the actual compound production process, a considerable part of the water and alcohols in the diluent will be volatilized in the drying channel, but if the water or alcohol in the diluent is slightly higher, the consumption of curing agent will be accelerated. Before entering the drying channel, part of the curing agent in the glue liquid will be consumed by water or alcohol, resulting in insufficient curing dose and incomplete curing.
Dongguan Beite packaging materials Co., Ltd. - located in Dongguan City, the world's manufacturing city, is a subsidiary of Beite packaging (China) Co., Ltd., with a plant area of 6000 square meters. It is a flexible packaging enterprise integrating R & D, production and sales. It has a complete set of domestic advanced production equipment, a scientific and humanized management system and a group of experienced technical personnel For backing, to provide customers with quality products and first-class service.
The company produces composite bags, roll film, fruit bags, aluminum foil bags, suction mouth bags, self-supporting bags, zipper bags, anti-static bags, special-shaped bags, etc., which are suitable for the internal and external packaging of food, daily chemicals, electronics, toys, medicine, cosmetics, industrial products and other industries.
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