At present, the three most important environmental protection technologies of international flexible packaging are: solvent-free composite technology, flexographic printing and co-extrusion compounding, among which solvent-free composite technology is the most environmentally friendly. At present, the solvent-free composite technology has occupied a very important position in the European and American markets, accounting for 80% to 90% of the new composite machine equipment. In China, the solvent-free composite technology began in the mid-to-late 1990s, with about 70 existing installations. Fortunately, driven by many flexible packaging raw material manufacturers who regard environmental protection and sustainable development as their responsibility, they have started to grow rapidly in recent years.
Relying on more than 80 years of intensive research in the field of adhesives, Henkel's experts maintain close contact with mechanical equipment, film and printing ink suppliers in R & D centers around the world, not only in flexible packaging composite adhesive technology In terms of keeping ahead, it is also very familiar with the development of solventless bonding for the entire flexible packaging company.
1. Solvent-free adhesives Henkel has set up specialized Liofol laboratories in major global markets to ensure and improve product performance, and continuously improve compounding and coating technologies. A major technological breakthrough in Liofol's laboratory on reactive polyurethanes has resulted in solvent-free adhesives.
Solvent-free adhesives are truly environmentally friendly adhesives, and their outstanding properties are:
(1) Does not contain or use solvents. Because solvent-free adhesives are 100% active ingredient content, they are coated with different coating systems and do not use solvents, so there is no solvent volatilization throughout the compounding process. There is no solvent residue in the composite section, which is more in line with food safety requirements, and has no emissions, and meets emission reduction and environmental protection requirements.
(2) Non-flammable and explosive dangerous goods have good safety, there is no danger of fire and explosion, and no solvent explosion-proof measures are required; solvent-free adhesives are not classified as flammable, explosive, high corrosion and other dangerous goods. Explosion-proof considerations are not required for storage. At the same time, the solvent-free adhesive is 100% solids and occupies less storage space. And solvent-based adhesives, because they contain solvents, must be considered flammable and explosive.
(3) Less coating amount Due to the better cold flow characteristics of solventless, the coating amount of solventless adhesive is generally between 0.8 ~ 2.5 g / m2. For special materials such as paper, the coating amount can reach 4 g / m2. The coating amount of the solvent-based adhesive is generally 2.0 to 4.5 g / m2. This shows the cost advantage of solventless adhesives.
(4) The molecular weight of solventless adhesives is relatively small compared to solvent-based adhesives of the same grade, which makes the adhesive wet the surface of the substrate at a suitable temperature without the need for solvent participation. Diffusion and penetration are possible.
two. Types and development of solventless polyurethane adhesives The solventless composite process, equipment and adhesives were developed in the 1970s, and the first generation of products has developed to the third generation.
The first generation of solvent-free adhesives were single-component moisture-curing adhesives. The chemical structure of the one-component moisture-curable polyurethane solventless adhesive is a polyester or polyether containing a relatively short isocyanate end group, isocyanate (-NCO), and moisture in the substrate or environment. A chemical reaction occurs and CO2 is released for curing.
In order to overcome the shortcomings of one-component adhesives, two-component polyurethane solvent-free adhesives have been developed. Two-component polyurethane solvent-free adhesives can be divided into the second generation and the first according to the initial adhesion and cooking resistance. Three generations of two-component polyurethane solvent-free adhesives.
The second generation of solventless adhesives was first applied to composite packaging materials in 1980.
The third-generation solventless adhesive overcomes the shortcomings of the second-generation solventless adhesive. It has the advantages of high initial tack, low viscosity, low operating temperature, and no restrictions on the composite substrate. It can solve the second-generation adhesive. The problem that the mixture has poor heat sealing of EVA and Nylon, and the problem that the barrier film is easy to wrinkle or easy to peel off are also solved. It can be applied to the compounding of various plastic films and foils, which reduces the Content, excellent content resistance, adhesive strength and heat resistance of the adhesive have also been improved, aluminum-plastic composite film products may have high temperature cooking performance.
The main features of the current third-generation products are:
(1) Adopt anti-catalysis system.
(2) The viscosity of the adhesive is reduced, the wettability of the adhesive to the film is improved, and the coating dispersion performance of the adhesive on the film surface is improved. Therefore, not only the quality of the solvent-free adhesive composite product can be improved, but also the machine speed can be increased and the production efficiency can be increased.
The development of solvent-free polyurethane adhesives has a history of nearly 30 years, and the performance of its products has gradually matured. Under the environment of green environmental protection and low-carbon economy, solvent-free polyurethane adhesives have excellent economical, Safety and environmental protection advantages, as well as the continuous development of composite processes and equipment, have been favored in Germany, the United States, Japan and the world. In developed countries such as Europe and the United States, the use of solventless polyurethane adhesives has increased rapidly from 10% 20 years ago to more than half today. Since the introduction of the first solvent-free dry laminating machine in Japan in 1977, after less than 20 years, it has now grown to nearly 100 solvent-free laminating equipment.
Two-component refers to prepolymers with isocyanate-terminated prepolymers as the first component and hydroxyl-terminated ethyl-component prepolymers terminated with the hydroxyl group, resulting in a urethane reaction to form a urethane, which can be regarded as a reactive The first step; excess isocyanate on the higher molecular weight chain reacts with moisture to form urea, which can be regarded as the second step to the end of the reaction. The addition polymerization reaction of the entire adhesive is very complicated, and can generally be regarded as the above two-step reaction, which can help and guide the compound production. The reaction mechanism is as follows:
Two-component solvent-free reaction mechanism As a market pioneer of polyurethane solvent-free adhesives, Henkel not only excels in today's third-generation general-purpose solvent-free adhesive products, but also achieves and exceeds the high-speed, cost-saving, and low-energy requirements of the market. Consumption, no emissions, etc .; continue to innovate and develop intelligent fast curing solventless adhesives that comply with FDA and other international food safety laws, hybrid solventless adhesives that can run at speeds up to 500 meters per minute, low viscosity A series of innovative solutions, such as simple solventless adhesives for room temperature applications, meet the rapidly changing solventless adhesives market needs.
Due to the continuous speed increase of the laminating machine, it is difficult to adapt the curing speed of general adhesives. Thanks to Henkel's research results and rich experience, the fourth generation of UV-curable solventless adhesives came into being. The currently used cationic curing products are mainly acid-catalyzed epoxy systems. Under ultraviolet radiation, the photoinitiator of the adhesive decomposes, and the epoxy block polymerization is initiated to obtain a three-dimensional crosslinked structure of the epoxy resin. Under the premise of selecting an appropriate UV generator, this adhesive can quickly compound polyester films, and can reach a relatively high bonding strength within minutes to an hour, making the post-processing process smoothly.
three. The characteristics of solvent-free composite equipment are different between solvent-based and solvent-free composite equipment. One aspect is that the coating system and the glue supply system of the solvent-free composite equipment are different. The other aspect is that the solvent-free composite equipment has no drying tunnel and structure. Compact, low energy consumption, low noise, small footprint.
Structural schematic diagram of solvent-free composite equipment:
Compared with solvent-based composite equipment, solvent-free composite equipment has the following advantages:
(1) No drying tunnel, low energy consumption. The drying capacity of the solvent-based compound machine is generally 150 ~ 250KW, and the average annual power consumption can reach RMB 500,000-700,000. Even if steam drying is used, the energy consumption is not. Decimal. The solvent-free machine has a low power and no drying tunnel. According to information provided by Nordmec, solvent-free energy consumption is almost one-twentieth of that of solvent-based.
(2) Fast compounding speed. Solvent-free compounding does not require drying, and there is no restriction on residual solvents, resulting in high production efficiency. The average compounding speed is 250m / min. Up to 600m / min, one compound machine can meet the production of two printing machines.
(3) High degree of automation The solvent-free laminating machine uses proprietary equipment to automatically feed glue, automatically control the proportion of the two components, and automatically control the temperature. The coating amount can be adjusted online. The equipment is compact and saves manpower.
The Liofol laboratory has an industrial-scale solvent-free laminating machine that can test materials and solvent-free adhesive systems under real-world conditions to develop optimal mix combinations and customized solutions for each application area.
Fourth, the main points of the process of solvent-free lamination At present, the common substrates for solvent-free lamination are mainly BOPP, PET, OPA, VMCPP, VMPET, CPP, PE, aluminum foil, etc. When compounding, generally put materials with high rigidity and good coating properties (PET, BOPP, OPA, VMPET, etc.) on the main unwinding station; put easily stretchable materials (PE, CPP, VMCPP, etc.) on the auxiliary unwinding station Station. But it is not static, and can be flexibly selected according to the actual production situation. For example, when the printed film is laminated with aluminized material, in order to ensure the quality of the composite and production efficiency, the aluminized material can be placed in the main unwinding station.
Solventless adhesives are mainly one-component moisture-curing adhesives, two-component cold-coating solventless adhesives, two-component reverse hot-coating adhesives, UV-curing adhesives, etc. . There are many factors to consider when selecting an adhesive. The first is the type of packaging content and the type of film material used. Second, if the printing ink is in contact with the adhesive, the compatibility of the two should also be considered. Third, peeling Strength requirements and heat-sealing conditions also greatly influence the choice of adhesive.
In addition, two issues need to be paid attention to: First, the MDI-type isocyanate will gradually migrate to the inner surface through the inner film, and react with water vapor to form a polyurea heat-resistant sealing layer, which affects the heat-seal of the packaging bag . Quality, this problem is prone to occur when the polyethylene film is of poor quality or the adhesive is not selected properly, especially when using two-component adhesives. Second, when the film with a higher slip agent content is used, the slip agent may migrate into the adhesive layer, which may cause problems such as poor peel strength, poor heat seal, and increased friction coefficient, affecting its operation on the packaging production line. Sex. Therefore, care must be taken in selecting the appropriate adhesive when laminating such films.
V. Prospects and Prospects of Solvent-free Adhesives After decades of development and application, solvent-free and solvent-free composite technologies have become very mature. The use of solvent-free adhesive composite films can reduce energy consumption, improve production efficiency, improve product quality, and reduce overall production costs. Because it does not contain solvents, it is beneficial to protect the environment and clean production, and to ensure that the compound has no pollution to packaged food and medicine.
It is believed that with the enhancement of China's composite product quality and food hygiene and safety awareness, the enhancement of energy conservation and environmental protection awareness, solvent-free compounding will surely receive more and more attention from people in the compound packaging industry . The good operation of the existing solvent-free composite equipment in China will play a demonstrative role in speeding up the conversion of the flexible packaging industry from solvent-based compounding to solvent-free compounding. Under the circumstances that companies in the composite film industry are willing to understand solventless compounding, research solventless compounding, and use solventless compounding, with the promotion of food and drug companies and national energy saving and emission reduction policies, solventless compounding is bound to usher in a vigorous development The new situation has become the main composite mode of China's composite film industry.
Henkel is a supplier of adhesive systems that regards innovation and sustainability as life. As a standard bearer of solvent-free polyurethane adhesives, Henkel is committed to providing society with high-quality adhesive products that meet economical, ecological, and social coexistence, helping packagers comply with regulations Bringing safe and convenient soft packaging to the wider family.
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