Solvent-free compound technology and adhesive for flexible packaging
At present, the three most important environmental protection technologies for international flexible packaging are: solvent-free compound technology, flexographic printing and co-extrusion compound, among which solvent-free compound technology is the most environmentally friendly.
At present, solvent-free compound technology has occupied a very important position in the European and American markets, accounting for 80% to 90% of the newly added compound machine equipment. In China, the solvent-free compound technology began in the mid-to-late 1990s. It has developed slightly in recent years. The number of existing installations does not exceed 40 (accounting for no more than 2% of the total number of compound machines), which is far lower than that of developed countries. Under this situation, how to make domestic flexible packaging enterprises and technicians engaged in flexible packaging work have a correct understanding of solvent-free composite technology and its products, and then expand the share of solvent-free composite technology in the domestic market has become the current domestic soft package The industry focuses on issues.
Solvent-free lamination (solventfreelamination) is relatively dry solvent-based lamination, is another composite process used in the flexible packaging industry. Solvent-free soft lamination is the use of solvent-free adhesives and special composite equipment to bond film-like substrates (plastic film or paper, Al foil, etc.) to each other, and then after the chemical curing reaction of the adhesive, the layers of the substrate are made Composite method of bonding together. Both solvent-free compounding and solvent-based dry compounding use adhesives to make compound films. The difference between the two types of process is that the solvent-free compound adhesive does not contain any solvent, so the two-layer substrates do not need to exclude the solvent in the drying tunnel like the solvent dry compound process before bonding. Solvent-free compounding and dry compounding are competing and complementary industrial production methods in plastic film compounding.
The solvent-free composite process was first developed by Herberts in Germany in 1974. Due to the economics, safety and environmental protection advantages of its process, it has gradually become an important processing method for plastic composite films. In developed European and American industrial countries with strong awareness of environmental protection and energy saving, the number of composite films produced by solvent-free composite processes has clearly exceeded that of dry-process composite processes; since the 21st century, the number of new solvent-free composite equipment in Europe has accounted for the new More than 90% of the total composite equipment; since the introduction of solvent-free composite technology in Japan in 1977, more than 60 production lines have been built to the beginning of the 21st century, occupying a dominant position in the production of composite films; the amount of solvent-free adhesives in the United States accounted for 10% in 1977. In 1980, it reached 50%, and it rose to more than 90% in the 1990s. Due to the failure of popularization of solvent-free composite equipment in China, its applications are few.
1. Domestic solvent-free composite equipment. Currently, there are mainly manufacturers of solvent-free composite equipment in the world: W & H in Germany, Nordmecanica in Italy, Schiavi, Bielloni, DCM in France and Comexi in Spain. Many compound equipment manufacturers in China have also invested in the development and research of this equipment, and have made some progress. In 1985, Beijing Chemical Industry Research Institute introduced the first solvent-free coating compound testing machine manufactured by DCM company, engaged in scientific research and production; then in 1996 Tianjin Dingzheng Printing and Packaging Company introduced solvent-free equipment from Italian Schiavi company, and in the same year Commissioning was successfully put into production in August. Subsequently, Guangzhou Jinweilong Company, Dalian Shengdao Packaging, Shanghai Tiger Brand, Sanying, Hubei Salt Industry and other manufacturers successively introduced a total of dozens of solvent-free composite equipment from Italy, France, Germany and other countries. With the understanding and mastery of the new technology of solvent-free compounding by domestic compound packaging companies, this new type of compounding process has a clear trend of accelerated development.
2. The characteristics of solvent-free equipment. The typical structure of solvent-free laminating machine includes: two unwinding devices, one winding device and solvent-free adhesive coating compound device. Its biggest feature is that it directly enters the compound after passing through a special coating device, and does not require a drying tunnel. Figure 1 and Figure 2 show schematic diagrams of the structure of the solvent-free composite equipment and the coating unit. The solvent-free compound equipment should be suitable for the application of solvent-free adhesives, so it has some obvious differences from the solvent dry compound equipment. The main performance is that the coating device has a heating and temperature control system, and the viscosity of the adhesive is adjusted by adjusting the temperature to facilitate the coating construction. The take-up device is equipped with a closed-loop tension controller to strictly control the winding tension to prevent the "tunnel" effect and other disadvantages in the production process of the composite film.
The feeding device has a high-precision mixing nozzle (to meet the needs of two-component adhesive feeding) and a humidifier (to ensure the single component adhesive is fully cured).
The coating system is different, there is no mixing pump; there is no drying channel, and there is no need for drying; Zhang Li control system is more accurate than the solvent-based system. Compared with the dry method compound, the solvent-free compound has the following main differences in equipment : (1) The longer drying oven used in the solvent of the volatile adhesive is removed, the equipment structure is compact, and the equipment parts are easy to clean and maintain; (2) The distance of the material through the film is shortened, so that the material tension control is more accurate. Due to the compact structure of the equipment, all the control keys are concentrated on the main control panel, which is convenient for the operator to monitor; 3) The glue unit is the central part of the solvent-free compound machine, where the adhesive is mixed, heated, metered and coated (4) glue The head is generally designed as a four-roller or a five-roller, which are rubber storage roller, metering roller, (floating roller), rubber transfer roller and rubber coating roller. There is an accurately calculated angle between the rollers to ensure accurate and constant glue application. 3 Solvent-free compounding characteristics, compared with traditional dry method compounding, solvent-free compounding has many advantages in reducing costs, improving product quality, environmental protection, production safety and stability, etc. It is a production method worthy of advocacy and highly practical value .
3.1 Reduce production cost Solvent-free composite equipment does not require a drying tunnel, so it has a compact structure, takes up little space, and has low equipment cost, which can significantly reduce the cost of composite film production.
3.2 Less sizing material, low cost of sizing material The coating amount of solvent-free composite unit area adhesive is about 40% of dry compounding. Although the relative cost of solventless composite adhesive is relatively high, the overall cost is reduced by 30%. The production cost of area composite products is low.
3.3 The energy consumption of the equipment is low. The solvent-free equipment does not have a drying tunnel and does not need to pass through the drying tunnel to remove the solvent in the adhesive, which reduces energy consumption. At the same time, the longer drying oven used for the solvent in the volatile adhesive is eliminated, thereby eliminating the material curling problem. The material travels short on the equipment, reducing waste generation.
3.4 The production line has high speed, and the maximum linear speed of solvent-free compounding is up to about 480re / rain, generally about 300m / rain. The compounding speed is fast and the equipment production efficiency is high.
3.5 The product quality is guaranteed and the efficiency is improved. The adhesive used for solvent-free compounding does not contain solvent, so there is no pollution to the packaging, and it is more suitable for food packaging; at the same time, because the substrate does not deform after compounding, the dimensional stability of the composite film structure Good; In addition, the solvent-free composite equipment has an online corona treatment unit, which has good composite performance for films with reduced surface tension and high lubricant content.
3.6 Reduce labor and safety protection costs, equipment structure is compact, fewer operators, reducing labor costs. In the solvent-free compounding process, there is no solvent volatilization, and almost no three wastes are generated, which avoids environmental pollution and is conducive to cleaner production.
3.7 Good safety, good solvent-free compound safety, can minimize the fire and explosion hazards, no special fire prevention and explosion-proof measures are needed in the factory area and production workshop. In summary, the cost of solvent-free compound processing is significantly lower than that of dry method compound, and the production efficiency is improved. It has no pollution to food packaging, which is very beneficial to improve the market competitiveness of enterprises.
4 The development of solventless composite adhesives, many countries in the world have conducted in-depth research on solventless polyurethane adhesives, and developed a variety of practical products that can meet the various performance requirements of food packaging composite film materials.
The development of this series of adhesives can be roughly divided into the following stages: (1) The first generation is a single-component solvent-free polyurethane adhesive, which can be divided into polyether polyurethane polyisocyanate and polyester polyurethane polyisocyanate, which relies on water in the air The water attached to the film and the film to be coated reacts with it to solidify. The water supply is insufficient to cause poor curing. At the same time, the CO2 generated during curing is likely to generate bubbles. (2) The second generation is a solvent-free two-component polyurethane adhesive, which relies on the two components to mix and react with each other to form macromolecules to achieve cross-linking and curing. In terms of use, films and aluminum foils other than EVA and Nylon can be used.
(3) The third-generation solvent-free adhesive is an improved two-component system based on the second-generation (the use of aromatic polyols improves the high-temperature resistance), which can withstand high-temperature cooking in a composite film containing an aluminum foil structure , It has the advantages of high initial adhesion, low viscosity, low operating temperature, and no limit to the composite substrate, which can solve the problem of poor heat sealing of E and Nylon by the second generation adhesive, and the film with strong barrier is easy to wrinkle Or the problem of easy peeling has been solved. 4) The fourth generation of UV-curable solventless adhesives, due to the continuous speed-up of the laminating machine, makes it difficult to adapt to the curing speed of general-purpose adhesives, so UV-curable adhesives came into being. The currently used cationic curing products are mainly acid-catalyzed epoxy systems. Under ultraviolet irradiation, the photoinitiator of the adhesive decomposes to initiate epoxy block polymerization to obtain a three-dimensional cross-linked structure of epoxy resin. Under the premise of selecting an appropriate UV generator, this adhesive can quickly compound polyester film, and can reach a fairly high bonding strength within a few minutes to an hour, making the post-processing process smooth.
4.2 The characteristics of solvent-free polyurethane adhesives, single-component solvent-free adhesives are generally NCO-terminated polyisocyanate prepolymers, which rely on moisture to complete the curing process shown in Figure 3, the single-component adhesives after crosslinking and curing are elastic, It is resistant to aging, and can make the compound have better bonding strength and mechanical properties. However, due to its low molecular weight and poor heat resistance (<100 ℃); the curing speed of this type of adhesive is relatively slow, and it is greatly affected by moisture, and it is easy to generate CO: bubbles, when the coating amount is large or the moisture supply is insufficient Poor curing. Single-component solvent-free adhesives are mainly used in paper / plastic composites. The two-component solvent-free polyurethane adhesive is composed of a main agent and a curing agent. When compounding, the main agent and curing agent are mixed in proportion, the viscosity is controlled in a suitable range by heating, and then applied to the substrate through a multi-roll coating system. The main components are polyether polyol and polyether polyol prepolymer modified with isocyanate or polyester polyol. The reaction mechanism similar to solvent-based adhesives is shown in Figure 4. Low molecular weight polyether polyols are generally polypropylene glycol, polyoxypropylene triol, tetrahydrofuran propylene oxide copolyether triol, polytetrahydrofuran, etc. and mixtures thereof. Isocyanate is generally selected from liquid linear hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI), and tin catalysts can also be used. The low molecular weight polyester polyol may be aliphatic polyol, cyclic hydrocarbon polyol, and aromatic polyol. Currently, solvent-free adhesives are generally mixed with polyester polyether. The viscosity of the pure polyester type is generally difficult to control, especially the solvent-free adhesive that is resistant to high temperature cooking, while the polyether type is limited in terms of adhesion performance and high temperature resistance. The third-generation two-component polyurethane solvent-free adhesive currently developed is aimed at approaching the performance of solvent-based adhesives. The third generation of solvent-free polyurethane adhesive products represented by Henkel Liofol already has the following performance characteristics: (1) high initial adhesion at room temperature / relative to the second generation product; (2) basically no anti-sealing effect ( anti-sealeffect); (3) no restrictions on composite substrates (high slip agent compatibility, EVA, white PE, etc.); (4) low isocyanate free monomer content; (5) boiling resistance, high temperature cooking . Solvent-free polyurethane adhesives, because there are no solvent small molecules, the intermolecular force is large, and hydrogen bonds are formed, and the viscosity is high at room temperature. The temperature can effectively reduce the viscosity. However, an excessively high temperature may easily cause gelation and generate a high molecular weight resin, which may cause difficulty in coating or uneven coating. Therefore, it is very important to control the temperature of adhesive application during construction. Generally, the temperature of single-component solventless adhesive is controlled at 70 ℃ ～ 100 ℃; the coating temperature of two-component adhesive is controlled at 40 ℃ ～ 80 ℃. 4.3 Solvent-free polyurethane adhesive supplier, the main manufacturer of solventless polyurethane adhesive in the world is Germany Henkel (Henkel) Group, Herberts Company of Germany, Morton Company of the United States, Toyo Ink Manufacturing Co., Ltd. of Japan and AtoFindley Company of France, etc. Domestically, Beijing Chemical Research Institute has also developed some solvent-free adhesives through product introduction. 5 Solvent-free compound development trends and prospects. After decades of development and application, solvent-free compound technology has become very mature. The use of solvent-free adhesive composite films can reduce energy consumption, increase production efficiency, improve product quality, and reduce overall production costs. Because it does not contain solvents, it is conducive to protecting the environment and clean production, and ensures that the compound does not pollute packaged foods and medicines. In recent years, as China's product quality and food hygiene and safety awareness have increased, solvent-free compounding has been increasingly valued by people in the composite film packaging industry. There have been many solvent-free compounding equipment in China, and most flexible packaging materials can be used at present. Produced with solvent-free adhesives. With the increasing domestic requirements for food and other packaging materials and the continuous improvement of production equipment, solvent-free compounding will show a vigorous development situation.
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