The laminating film composite method using adhesives has become a major production process of China's flexible packaging composite materials due to its simple process equipment, wide application range, and rapid development. Polyurethane adhesives contain a large number of isocyanate groups and urethane groups, which are highly polar and highly chemically reactive due to their main resin molecules. They have excellent chemical bonding properties with active hydrogen-containing materials, as well as synthetic raw materials and additives. With a wide variety of materials, the resin composition structure and adhesive formulations have a wide range of variations, which can meet the requirements of different substrates for bonding and use conditions, and are suitable for laminated composite films of various materials. The domestic film compounding polyurethane glue is about 8,000 tons/year, accounting for 8% of the total amount of polyurethane glue, but the production and application technology of the glue lags behind. The low-end products are the main products, and the product mix is not standardized. A considerable part of the product still needs import.
1 Classification of Polyurethane Adhesives
Laminated composite technology The production of flexible packaging film composite adhesives is a key factor affecting product quality. However, flexible packaging composite manufacturers are currently confused about the classification of polyurethane adhesives and the type of commercial adhesives. Polyurethane adhesives can be divided into three major types from the viewpoint of chemical mechanism: polyisocyanate adhesives, which are dissolved in an appropriate proportion of a solvent by a polyisocyanate monomer and can be modified by mixing with rubber; reaction of isocyanates with polyhydroxy compounds The polyurethane prepolymer adhesives with isocyanate groups at the end are made into one-component and two-component adhesives, and the solutions, emulsions, films, powders and the like produced by the full reaction of the isocyanate and the polyhydroxy compounds are prepared. . From the polyhydroxy compound raw materials used Polyester polyester and Polyether type, polyester type bonding is better, polyether type hydrolysis stability is good, there is a strong flexibility, low cost, but the initial viscosity is poorer than the polyester type . The polyisocyanates used differ in aromatic types such as TDI, MDI, PAPI and aliphatic types such as HDI and IPDI2. General factories are classified according to the variety of products sold: a. Polyisocyanate solution; b. Mixing of polyisocyanate and rubber solution; c. Reactive type of isocyanate prepolymer and hydroxy compound (including water) (single and two components) ); d. closed type; e. elastomer type; f. emulsion type; g. pressure sensitive adhesive; h. hot melt adhesive.
2 Application of Polyurethane adhesive in Process of Flexible Packaging Composite Film
The selection and use conditions of the adhesive in the flexible packaging composite process depend on the composite substrate, the use of the composite product, and the processing conditions of the laminating machine. The predominant use of a. to c. The type, composition, concentration, coating amount, and drying conditions of the adhesive are interrelated, and the key is to select the appropriate type of Polyurethane Adhesive to adjust the solvent concentration to the appropriate speed of the laminating machine. The drying conditions must ensure that the solvent can be fully evaporated and dried. The general control of the cloth amount CW is 1.5 to 5.0 g•m-2, which can be calculated by CW=KμρW. In the formula, K is the transfer rate, and the empirical value is 1/4 to 1/6, with the micro-hole of the coating roll and the pressure of the rubber roll. Squeegee angle, film surface state, adhesive viscosity varies; μ is the depth of the coating roll micropore; ρ is the adhesive density; W is the adhesive concentration used (including solids%).
2.1 Universal solvent-based polyurethane adhesive
Solvent-based adhesives account for a large proportion of adhesives for flexible packaging in China due to their ease of adjustment and ease of application. Their production and use technologies are relatively mature. Generally contains 30% to 50% solid resin (a few more than 50%), with low toxicity of esters, ketones, alcohols as a solvent, the glue can be simply divided into single-component and two-component.
The single component is generally a polyether-type NCO group that forms a substituted urea chain or crosslinks into a macromolecule under the action of moisture. At the same time, the solvent is volatilized and the film is solidified, and the chain extender can be appropriately added to improve the bonding strength. Paper-plastic composite products, solvents are non-toxic esters, the rubber and rubber-solvent type, high bonding strength, transparent and bright, no bad smell of benzene.
Two-component ordinary type is the most important type of compound glue, which is composed of two components of A and B. Generally, the main component of A component is polyester (or polyether) polyol and diisocyanate (such as TDI, MDI) in specific conditions. Under the following reaction for several hours to obtain urethane polymer (end group is -OH), supplemented with solvent to adjust its viscosity (0.5 ~ 6.5Pa.s) and solid content (35% ~ 75%), B component curing agent Diisocyanate and trimethylolpropane adducts with -NCO as the end group. When used, the A and B components are mixed in proper proportions. For uniform coating and favorable infiltration, most of them are diluted with ethyl acetate to a working concentration of about 20%. The adhesive has high initial tackiness, and has good adhesive properties to PE, OPP, CPP, PET, OPA film, aluminized film, and aluminum foil, and is suitable for dry-composite between them, and has both heat resistance and Anti-media erosion, composite film products can be packaged generally dry foods, drugs, etc., resistant to boiling water at 100 °C, but should not be used in high-temperature cooking. There are many series of products in China that can basically meet the needs. For example, the main raw materials are: PU170 of Huangyan Ink Chemical Factory uses MDI as the main raw material; Linhai Chemical 3rd Plant EST-B and Linhai Fine Chemicals E-460 use TDI as the basic raw material, and MDI is more reactive than TDI. High, low volatility in air, but MDI reactants are solid products with high viscosity, which is not conducive to coating. Zhongshan Xinhui XH-35, XH-50D are also used more, and imported products are mostly used in Japan AD-502 and Germany PU1860.
2.2 High temperature, low temperature polyurethane adhesive
Polyisocyanate cross-linked and cured polyurethane adhesives should not be used under high-temperature cooking conditions above 120°C. To prevent the aromatic polyurethane glue from separating the carcinogens aromatic amines from penetrating into the inner layer and endangering health, it is best to use fats to prevent the compound products from being sterilized during high temperature sterilization. Family HMDI or IPDI type glue. In addition, the use of copolymerization measures can also improve cooking performance. For example, it is prepared by copolymerization of ethylene carbonate, polyol, and TDI, and after mixing with an appropriate amount of curing agent HL (trimethylolpropane: 1:3 adduct of HMDI), the peel strength is measured at 120°C. The peel strength after boiling in water for 5 h decreased by less than 3%. 2Using a small amount of (MeO)2SiMe(CH)3NCO, polyester TDI copolymerization to obtain a polyurethane compound containing a silicon compound, mixed with an appropriate amount of curing agent HL, it is coated on a PET film and nylon composite, boiled in boiling water After 3 hours, the peel strength decreased by only 2% to 10%, while the glue without the above silicon compound decreased by 35%.
Domestic solvent-based polyurethane adhesives generally suffer from poor low-temperature resistance. When stored in cold areas (below 0°C) for a long period of time, they will crystallize and lose their fluidity. The higher the viscosity of the glue made from the polyester with higher crystallinity, the worse the flowability, and the easier the whitening gelation at low temperature. When the polyester is synthesized, the adhesive that is introduced into the ether bond and the aromatic ring and then reacted with the isocyanate will not freeze when stored at a low temperature. For example, 80 parts of polybutylene adipate and 20 parts of β-methyl valerolactone containing 20% to 10% of crystalline polyglycol (melting point 30-70°C) are reacted with diisocyanate. The resulting solution of butanone in the polymer (30%) maintained good fluidity after being stored for 3 months below 0°C, and its flowability deteriorated when it was not added. The domestic supply of rubber is basically aromatic, although individual plants can be used for cooking bags on product instructions, but they do not comply with FDA regulations for food packaging materials. Most of the cooking polyurethane glues were imported, mainly AD-1010 and AD-1080 from Japan Toyo Ink Chemical Co., Ltd., and UK3640 from HENKEL Germany, EPS-747 Japan Ink Chemical Company.
2.3 Low Solvent Polyurethane Adhesive
Polyurethane adhesives for flexible packaging usually have a solid content of 30% to 40% and a few of more than 50%. Increasing the solids content of the adhesive, thereby reducing the amount of solvent emitted to reduce pollution, is the current direction. Because of the high concentration, it is easy to produce gels and has a large viscosity. When high-speed composites are used, it is easy to produce flying silk during gluing, which causes difficulty in storage and coating. Therefore, the viscosity of the glue must be reduced. The basic approach is to use low molecular weights. Low viscosity polyester polyols are formulated with polyisocyanates. The molecular weight of the polyester is preferably 1500 to 2000, the molecular weight is too small, the cross-linking reaction rate is slow, the initial viscosity is poor, and the bonding strength is also low. The degree of alkyd condensation reaction can be controlled by determining the hydroxyl value in polyester synthesis. The final product is diluted with a specific diluent to obtain a suitable low molecular weight, low viscosity polyester to meet the coating requirements. Such as Lin Zhongxiang (5) control acid: alcohol ratio of 1:1.15, the synthesis of a hydroxyl value of 60 ~ 80mgKOH / g, acid value of less than 2mgKOH / g of low molecular weight, low viscosity polyester polyol, formulated with polyester 75% of the composite glue; Xiaodong Chen synthesized a molecular weight of 2018 polyester, formulated with a solid content of 50% to 70% of the composite glue, both the initial viscosity and adhesive strength to meet the requirements.
There are a few high solids products in China, such as Huangyan JPU-560 with a solid content of 50%, low viscosity, good initial adhesion strength, easy coating, acid resistance, heat resistance, salt resistance, high peel strength, and can be used in harsh conditions Mustard bags can also be used on imported machines. Beijing Chemical Research Institute uses UK Henkel's technology and equipment to produce UK2850/UK5015, UK2850/UK5000 with a solids content of 75%, viscosity of 6500mpas and 3500mpas respectively, and can be applied at high solids content of 38% or even 40% to 50%. Cloth, high bond strength, and plastic-plastic composite products can be resistant to sterilization at 121 °C 30min.
2.4 Water-based polyurethane adhesive
At present, polyurethane adhesives are mainly solvent-based, and the organic solvents used are flammable, explosive, volatile, odorous, toxic, and cause pollution when used. With many countries and departments promulgating laws and regulations to limit the use of volatile organic compounds (VOCs), water-based solvents are used as dispersion media. They are environmentally friendly, non-toxic, non-flammable, and can be used with existing equipment and equipment. The transition from solvent-based to water-based has become inevitable.
The properties of waterborne polyurethane adhesives depend primarily on the properties of the prepolymer. Prepolymers prepared from polyether polyols have good hydrolytic stability, good flexibility and elongation, and good low temperature resistance, whereas polyester polyol prepolymers have large cohesion and high bonding strength. Aromatic prepolymers have good strength but are not resistant to yellowing, while aliphatic prepolymers are excellent in yellowing resistance. In China, polyether-based TDI is the mainstream, with less other HDI and MDI, and polyester-based aliphatic (or cycloaliphatic)-based foreign countries. At present, waterborne polyurethanes are mainly anionic self-emulsification, 2,2-dimethylolpropionic acid (DM PA) is more used as hydrophilic chain extender, organic bases (such as triethylamine TEA), and TEA are used as salt forming agents. / DMPA can be obtained in the range of 0.73 ~ 0.78 viscosity, stability, good polyurethane emulsion. The glue prepared in this way can meet the use requirements directly on the existing solvent type dry coating machine. At present, the industrial application of waterborne polyurethane adhesives in dry compounding is still limited.
2.5 100% reactive polyurethane adhesive
100% Reactive Polyurethane Adhesive is a solvent-free adhesive with complete reaction and 100% solids content. It is mainly used in single-component and two-component types. It is used in dry solvent-free new dry process laminators without drying tunnels.
The first-generation single-component moisture curing products have a rapid viscosity change, slow curing speed, and easy generation of CO2 bubbles. Such as the domestic use of UR7505, UR7506 can be suitable for the combination of racetrack, pretreated polyethylene, nylon, polyester, paper and other films.
The second and third generation two-component adhesives are not only good in initial viscosity, but also resistant to high temperature cooking and can be applied to Al foils and other adhesives. The rapid development of the application of the adhesive in the manufacture of dry composite films has become one of the important characteristics of the development of polyurethane adhesives in recent years. Since the beginning of Germany in 1974, Europe and the United States currently account for more than 50% of the use of adhesive in dry composite films. By 1997, there were more than 60 production lines in Japan. China has successively introduced dozens of adhesives, and the demand for such adhesives has increased rapidly. . Such as domestic UR8150/UR7330 two-component solvent-free polyurethane adhesive, two groups were 100% of the -OH base and -NCO base film, a variety of materials such as plastic film, metal foil, etc. are extremely high Adhesive, good transparency and elasticity
