Functional adhesive properties and applications

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 room temperature use and moisture and temperature resistance, but also satisfy special conditions and special conditions of use. Performance requirements. 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, the chemical medium that resists the erosion of the chemical medium is varied. In foods there are salty, sour, spicy, spices, alcohol, water, grains, carbon dioxide, oils, proteins, etc.; in addition to water, there are flavors, spices, polyols, pigments, emulsifiers, surface Active agents, various additives, etc. Chemicals include solvents, pesticides, acids, alkalis, and salts. These chemical substances have different characteristics but one thing in common is the corrosion and destruction of the adhesive layer of the composite material, which can cause separation, rupture and failure of the composite packaging bag. This requires the adhesive to have a good chemical resistance to aggressiveness, that is, the glue can maintain a good adhesive force in the case of resisting the above chemical medium, and ensure sufficient peeling degree. The adhesive is resistant to acids, salt and salt because it contains a small amount of urethane and carboxylic acid esters, as well as a small amount of activity on the aluminum foil surface. Elements react chemically to produce specific chemical bonds that allow 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 has strong versatility, that is, the adhesive is required to have good adaptability, in addition to composite ordinary optical film, such as CPP, OPP, PE, PET, nylon, can also be used for aluminum film, aluminum foil complex. 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, this kind of adhesives introduce functional genes into the molecules; they change the structure of the molecular chain, and they have the same urethane and carboxylate structure as ordinary polyurethanes. The aluminum metal surface active element chemically reacts to form a chemical bond chemical group, so that the adhesive layer and the aluminum layer are transformed into a relatively strong bond. This bond is like a tight connection between the adhesive layer and the aluminum layer. The adhesive force of bridges and ordinary adhesives mainly comes from intermolecular cohesion; it includes van der Waals forces, dipole distance forces, dispersive forces, etc. These forces are relatively weak relative to the chemical bonding forces. 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, the highly transparent adhesive With the continuous improvement of product quality and in order to meet the export needs, 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, the 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 so that the adhesive layer can be fully cured; 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 cycle to 24 hours or even shorter compression, thus speeding up the material flow, improve labor productivity and create good economic benefits for customers.
Fifth, high temperature resistant adhesive The so-called durable high temperature, including the long-term high temperature and instantaneous high temperature. 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 by 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. for 30 minutes. In 45 minutes, no deformation or separation occurred.
Sixth, the application of precautions Here to talk about how to determine the complex process conditions in order to allow the adhesive to obtain the best results.
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 made of medium and high temperature resistant packaging, the surface tension of the plastic film is preferably raised to 39mN/M or 40mN/M; and 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, stay with different amounts of glue. Simple plastic/plastic composite 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, to 3.5-4.0 grams per square meter. If you want to heat cooking, in addition to The very important surface tension is high, 4.5-5.0 grams per square meter.
The third is the control of the residual solvent. The amount of residual solvent not only affects the odor and the hygienic properties 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 best to be able 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.