Solvent free compounding technology and adhesives

Solvent free lamination technology and adhesives for flexible packaging at present, the three most important environmental protection technologies for flexible packaging in the world are solvent-free lamination technology, flexographic printing and coextrusion lamination, among which solvent-free lamination technology is the most environmentally friendly. At present, solvent-free compounding technology has occupied a very important position in the European and American markets, accounting for 80% - 90% of the new compounding machine equipment. In China, the solvent-free compounding technology began in the middle and late 1990s, and has developed slightly in recent years. The number of existing installations is no more than 40 (accounting for no more than 2% of the total number of compounding machines), which is far lower than that of developed countries []. In this situation, how to make domestic flexible packaging enterprises and technicians engaged in flexible packaging 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 a key exploration problem in the domestic flexible packaging industry. Solventfreelamination is another composite process applied in flexible packaging industry, which is relative to drylamination. Solvent free soft composite is a composite method that uses solvent-free adhesives and special composite equipment to make thin-film substrates (plastic film or paper, Al foil, etc.) adhere to each other, and then after the chemical curing reaction curing treatment of the adhesive, all layers of substrates are bonded together. Both solvent-free composite and solvent-based dry composite use adhesives to prepare composite films. The difference between the two kinds of processes is that the solvent-free composite adhesive does not contain any solvent, so the two-layer substrate does not need to remove the solvent in the oven like the solvent dry composite process before bonding. Solvent free compounding and dry compounding are practical industrial production methods that compete and complement each other in the production of plastic film compounding

the solvent-free composite process was first successfully developed by Herberts company of Germany in 1974. Because of its economy, safety and advantages in environmental protection, it has gradually become an important processing method of plastic composite film. In European and American industrial developed countries with strong awareness of environmental protection and energy saving, the number of composite films produced by solvent-free composite process has significantly exceeded that of dry composite process; Since the beginning of the 21st century, the number of new solvent-free compounding equipment in Europe has accounted for more than 90% of the total number of new compounding equipment; Since the introduction of solvent-free composite technology in 1977, Japan has built more than 60 production lines by the beginning of the 21st century, which plays a leading role in the production of composite membranes; In the United States, the consumption of solvent-free adhesives accounted for 10% in 1977, which had reached 50% in 1980 and increased to more than 90% in the 1990s. Solvent free compounding equipment is not widely used in China, so its application is less. 1. Domestic solventless compounding equipment. At present, the main manufacturers of solventless compounding equipment in the world are: wh in Germany, nordmecanica, SCHIAVI, bielloni in Italy, DCM in France and comexi in Spain. Many composite equipment manufacturers in China have also invested in the development and research of this kind of equipment, and have made some progress. In 1985, Beijing Research Institute of chemical industry introduced the first solvent-free coating compound testing machine manufactured by DCM company to engage in scientific research and production; Then in 1996, Tianjin dingzheng printing and packaging company introduced solvent-free equipment from SCHIAVI company in Italy, and successfully put into production in August of the same year. Subsequently, Guangzhou jinweilong company, Dalian Shengdao packaging, Shanghai tiger brand, Sanying, Hubei salt industry and other manufacturers successively introduced a total of more than a dozen solvent-free composite equipment from Italy, France, Germany and other countries. With the domestic composite packaging enterprises' understanding and mastery of the new solvent-free composite technology, this new composite technology has a significantly accelerated development trend. 2. 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 laminating device. Its biggest feature is that it enters the composite directly after passing through a special coating device, and there is no need to dry the oven. Figure 1 and Figure 2 show the structure of solvent-free compounding equipment and the schematic diagram of coating unit. Solvent free compounding equipment should meet the needs of solvent-free adhesive coating, so it has some obvious differences with solvent dry compounding equipment. The main performance is that the coating device has a heating and temperature control system, which depends on adjusting the temperature to adjust the viscosity of the adhesive, so as to facilitate the coating construction. The coiling device is equipped with a closed-loop tension controller to strictly control the coiling tension and prevent the disadvantages such as "tunnel" effect in the production process of composite film

the feeding device has high-precision mixing nozzle (to meet the needs of two-component adhesive feeding) and humidifier (to ensure the full curing of one-component adhesive), etc

the coating system is different, and there is no glue mixing pump; There is no drying channel, so there is no need to dry; Zhang Li control system is much more accurate than solvent-based system. Compared with dry compounding, solvent-free compounding has the following main differences in equipment: (1) the longer drying oven used to remove the solvent in the volatile adhesive has a compact structure, and the equipment components are easy to clean and maintain; (2) The film penetration distance of the material on the equipment is shortened, so that the material tension control is more accurate. Due to the compact structure of the equipment, all control keys are centralized on the main control panel, which is convenient for the operator to monitor; 3) The gluing unit is the central part of the solvent-free compounding machine, where the adhesive is mixed, heated, measured and coated. (4) the gluing head is generally designed with four or five rollers, including storage roller, metering roller, (floating roller), transfer roller and coating roller. There is an accurately calculated angle between rolls to ensure the accuracy and constant glue amount. 3. The characteristics of solvent-free compounding. Compared with the traditional dry compounding, solvent-free compounding has many advantages in reducing costs, improving product quality, environmental protection, production safety and stability, and is a production mode worthy of advocacy and of great practical value

3.1 reduce the production cost. The solvent-free composite equipment does not need a drying oven, so it has a compact structure, takes up less space, and the equipment cost is low, which can significantly reduce the cost of composite film production

3.2 less glue is applied, and the cost of glue consumption is low. The coating amount of adhesive per unit area of solvent-free composite is about 40% of that of dry composite. Although the relative cost of solvent-free composite adhesive is higher, its comprehensive cost is reduced by 30%, and the production cost of composite products per unit area is low

3.3 the equipment consumes low energy. The solvent-free equipment has no drying oven, so it does not need to pass through the oven to remove the solvent in the adhesive, reducing energy consumption. At the same time, the longer drying oven for the solvent in the volatile adhesive is eliminated, thereby eliminating the problem of material curling. The distance of materials on the equipment is short, which reduces the generation of waste materials

3.4 the production line speed is high, and the maximum line speed of solvent-free compounding is about 480re/rain, which is generally about 300m/rain. The compounding speed is fast, and the production efficiency of the equipment is high

3.5 the product quality is guaranteed, the efficiency is improved, and the adhesive used for solvent-free compounding does not contain solvent, so it does not pollute the packaging, and is more suitable for food packaging; At the same time, because the composite substrate will not deform, the dimensional stability of the composite membrane structure is good; In addition, the solvent-free compounding equipment has corona treatment unit, which has good compounding performance for films with reduced surface tension and high lubricant content

3.6 reduce labor and safety protection costs, compact equipment structure, fewer operators, and reduce labor costs. In the solvent-free compounding process, there is no solvent volatilization, and almost no three wastes are produced, which avoids environmental pollution and is conducive to cleaner production

3.7 it has good safety and solvent-free compound safety, which can minimize the hidden dangers of fire and explosion. Special fire and explosion-proof measures are not required in the factory and production workshops. To sum up, the cost of solvent-free compounding is significantly lower than that of dry compounding. At the same time, the production efficiency is improved, and there is no pollution to food packaging, which is very beneficial to improve the market competitiveness of enterprises

4 the development of solventless polyurethane adhesive for composite, many countries in the world have carried out in-depth research on solventless polyurethane adhesive, and developed a variety of practical products, which can meet 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. It depends on the water in the air and the water attached to the coated film to react with it and cure. The water supply is easy to be insufficient, resulting in poor curing. At the same time, CO2 produced during curing is easy to generate bubbles. (2) The second generation is solvent-free two-component polyurethane adhesive, which is crosslinked and cured by the interaction of two components after uniform mixing to form macromolecules. In terms of application, films and aluminum foil except EVA and nylon can be used

(3) the third generation solvent-free adhesive is a two-component system improved on the basis of the second generation (aromatic polyols are used to improve the high temperature resistance). In the composite film containing aluminum foil structure, it can withstand high temperature cooking. It has the advantages of high initial viscosity, low viscosity, low operating temperature, unlimited to the composite substrate, and can solve the problem of poor heat sealing of the second generation adhesive to e and nylon, At the same time, the problem that the film with strong barrier is easy to wrinkle or peel off has also been solved. 4) The fourth generation ultraviolet improves the performance of waterproof materials by changing the formula. Due to the continuous acceleration of the laminating machine, the curing speed of the general adhesive is difficult to adapt, so the ultraviolet curing adhesive came into being. At present, the cationic curing products used are mainly acid catalyzed epoxy system. Under the irradiation of ultraviolet light, the photoinitiator of the adhesive decomposes and initiates epoxy block polymerization to obtain the three-dimensional cross-linking structure of epoxy resin. On the premise of selecting an appropriate UV generator, this adhesive can quickly compound polyester film, and can reach a fairly high bonding strength in a few minutes to an hour, so that the post-processing process can be carried out smoothly

4.2 characteristics of solvent-free polyurethane adhesive. One component solvent-free adhesive is generally NCO terminated polyisocyanate prepolymer, and its curing process is completed by relying on water. See Figure 3. The one component adhesive after crosslinking and curing is elastic, aging resistant, and can make the composite have better bonding strength and mechanical properties. However, due to its low molecular weight, its heat resistance is poor (100 ℃); The curing speed of this kind of adhesive is relatively slow, and it is greatly affected by water. It is easy to generate Co: bubbles. When the coating amount is large or the water supply is insufficient, the curing is poor. One component solvent-free adhesives are mainly used in paper/plastic composites. Two component solventless polyurethane adhesive is composed of main agent and curing agent. When compounding, the main agent and curing agent are mixed in proportion, the viscosity is controlled in the appropriate range by heating, and then coated on the substrate through the multi roll coating system. The main components are polyether polyol and polyether polyol prepolymer modified by isocyanate or polyester polyol. The reaction mechanism similar to that of 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 and their mixtures. Liquid straight chain hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI) is generally selected as isocyanate, and tin catalysts can also be selected. Low molecular weight polyester polyols can be aliphatic polyols, so that we can choose appropriate ones according to your actual situation