Key points of functional adhesives for soft composite materials

The application of soft composite packaging materials in food, medicine and other fields has a history of four to fifty years. The era is advancing, science and technology are developing rapidly, and various types of substrates with different properties are bonded together to make a unified overall composite material, and adhesives with good properties are required. The objective and practical requirements are also increasingly diversified and rigorous. As a result, the adhesives used in composite materials are no longer limited to ordinary properties such as normal temperature use and moisture-proof and moisture-proof, and are increasingly used in composite packaging. The so-called functional adhesives, in addition to the general properties of common adhesives such as normal temperature use and moisture and temperature protection, can also meet the requirements under special conditions and special performance. Specifically, it includes corrosion resistance to chemical media, high versatility, high transparency, good temperature resistance, and fast curing. Here are some brief descriptions of their characteristics and applications.

First, anti-chemical media erosion adhesive

Chemical media are diverse. In food there are salty, sour, spicy, spices, alcohol, water, sugar, sulfur, carbon dioxide gas, oil, protein, etc.; in addition to water in cosmetics, there are flavors, spices, polyols, pigments, emulsifiers , surfactants, various additives, etc.; in the chemical solvents, pesticides, acids, alkalis, salts and so on. These chemical substances have different characteristics, but they have one thing in common that they have a corrosive effect on the adhesive layer of the composite material and cause the laminate bags to separate, break, and fail. This requires the adhesive to have a good chemical resistance to aggressiveness, that is, to resist the above chemical media, the glue can maintain good adhesion and ensure sufficient peel strength. This kind of adhesive can resist acid, anti-hot and salty because the molecular chain contains a lot of active elements that can interact with the surface of aluminum foil, in addition to the same urethane and carboxylate structure as common polyurethane. A special chemical that generates a chemical bond by chemical reaction causes the adhesive and aluminum foil to chemically react to form a strong bond. Ordinary adhesives work with van der Waals force, double-elastic force, dipole moment force, or dispersive force. These adhesives need chemical bond strength, which is much stronger than simple adhesion. , better withstand the erosion of various media. Therefore, the mustard bags made with it have better quality.

Second, the versatility of the adhesive

The versatility is strong, that is, the adhesive is required to have good adaptability, and can be used for the composite of aluminized film and aluminum foil in addition to the common ordinary optical film such as CPP, OPP, PE, PET, and nylon. In addition, it is also required that the adhesive be used on a low speed machine; it can also be used for high speed recombination. Compared with ordinary adhesives, these adhesives introduce functional groups in the molecule; they change the structure of the molecular chain, and in addition to having the same urethane and carboxylate structure as ordinary polyurethane, they also contain energy. Chemical reactions with aluminum metal surface active elements to form chemical bonds, so that the adhesive layer and the aluminum layer are transformed into a relatively strong bond. This bond is like a tight bond between the adhesive layer and the aluminum layer. The bonding force of the bridges and ordinary adhesives mainly comes from intermolecular cohesion; including van der Waals forces, dipole moments, dispersive forces, etc. These forces are relatively weak compared to the chemical bonds. Therefore, this new functional adhesive can provide much stronger composite fastness than ordinary adhesives, and is fully qualified for the composite of aluminum foil, aluminized film and other various substrates.

Third, high transparency adhesive

With the continuous improvement of product quality and to meet the needs of export, transparency has gradually become an important indicator of customer requirements. Especially in food and clothing, there are many applications for high-transparency packaging, and there are many factors that affect the transparency of composite packaging products. In addition to plastic substrates, the main causes related to the adhesive itself are insufficient adhesive fluidity. Poor flatness, resulting in the glue on the surface of the substrate can not be well wet, the formation of water marks traces affect the transparency of the product.

By controlling the molecular weight, we keep the length of the molecular chain in a suitable range. Due to the high content and low viscosity; good fluidity, this greatly improves the transparency of the product. In addition, through the adjustment of production raw materials and processes, the amount of side impurities introduced into the colored impurities is controlled, and the transparency of the product is further improved. Fourth, fast curing adhesive

After the general two-component adhesive is well compounded, it should be placed in an environment of about 50° C. for 24-48 hours or more in order to fully cure the adhesive layer; only then can the next process be performed. This brings with it a problem: aging takes too much time and production efficiency is low, so customers have come up with the possibility of supplying a fast curing adhesive.

However, some people will worry about curing too quickly, so that gel phenomenon occurs in the plastic disk on the dry complex machine. In fact, this worry is not necessary. Because the selected functional component is inert at room temperature, only a certain temperature can stimulate its activity, that is, the special functional component will only work when passing through the drying tunnel and in the environment of the curing chamber. To shorten the curing time. After such improvements, the curing cycle can be compressed to 24 hours or even shorter, thereby speeding up material flow, increasing labor productivity and creating good economic benefits for customers.

Fifth, high temperature adhesive

The so-called high temperature, including long-term high temperature and high temperature two. The former refers to high-temperature cooking for a certain period of time during heating and pressure sterilization in a food processing process, and the latter is instantaneous high-temperature pressing at the time of bag-making sealing. To meet the requirements of high temperature resistance, that is, in both cases, the adhesive must be able to maintain a good bonding performance and high peel strength, no deformation, no bag breakage. It has been verified through experiments that under the condition that the glue amount reaches 4.5-5 g/m2, the PET and CPP composite film is used for bagging, and the hot and sour food is sealed, and then cooked at a high temperature of 121-125°C. In 30 minutes and 45 minutes, no deformation occurred. Stratification phenomenon.

Six, application notes

Here to talk about how to determine the complex process conditions in order to get the best adhesive effect.

The first is the substrate. Whether it is plastic film or aluminum foil, there is a common requirement that surface conditions. Here are two meanings. First, the surface should be dry and clean, free of dust, free from moisture, and free of oil. Second, the surface tension should be controlled at 38mN/M or more. If it is used for medium and high-temperature packaging, the surface tension of the plastic film should be improved to 39mN/M or 40mN/M. The surface cleanliness of the aluminum foil should be Class A, that is, When checked with distilled water, it should all be wetted; the surface tension is above 72mN/M.

Followed by the amount of glue. Different substrates and different final packaging requirements can have different amounts of glue. Simple plastic/plastic compound can be less, 2.5-3.0 grams per square meter can be, if the composite material containing aluminum foil, but also to resist erosion of media, 3.5-4.0 grams per square meter. If high-temperature cooking is required, except for the extremely important surface tension, 4.5-5.0 grams per square meter are required.

The third is the control of the residual solvent. The amount of residual solvent not only affects the odor and the hygienic performance of the food, but also has a significant effect on the transparency and peeling force. China's GB9683-88 standard stipulates that the residual solvent must not exceed 10 milligrams per square meter; but it is better to control below 5 milligrams per square meter. Therefore, in the choice of composite process, it must be determined from the factors of solvent type, drying tunnel temperature, start-up speed, blast volume and exhaust air volume.

The fourth is aging conditions. Because the adhesive is a two-component reaction type, proper conditions must be met to allow the main agent in the adhesive to fully react with the curing agent to achieve the best adhesion results. The functional adhesive, which can react chemically with active elements on the surface of the substrate, must have a certain temperature and time. According to our experience, this reaction is perfect with the increase of the ambient temperature and the extension of time. Such as high temperature cooking glue, at about 50 °C about five days and five nights (120 hours) to complete, but at about 80 °C; then three or four hours on the line. Of course, 80 °C drying room is impossible. In addition, due to the reaction between the curing agent and the main agent; it can also be carried out at room temperature, so some units do not mature into the drying room after being compounded, and it is not necessary to place after a week. The result of this low-temperature maturation is obviously not as good as the result of heat curing, especially for cooking bags. So, hope to pay attention to this.

(China Material Science and Technology)