Application of Waterborne Polyurethane Adhesives in Fabrication of Composite Films (2)

In view of the above, combined with the characteristics of waterborne polyurethane adhesives, the shortcomings of the application of waterborne polyurethane adhesives in the preparation of composite films are analyzed and the following improvements are provided:

1 Because of the hydrophilic groups in the molecular structure, its water resistance and solvent resistance are not good. To improve its water resistance and solvent resistance, there are many research measures, of which the most effective one is through cross-linking. Cross-linking methods include internal cross-linking and diplomatic alliances. Internal cross-linking means that bridging and cross-linking have been completed during the preparation process. Intramolecular cross-linking is often caused by excessive cross-linking, which prevents the prepolymer from dispersing in water. If dispersed and cross-linked at the same time, precipitates tend to form and stratify. Even if none of these phenomena occur, water-based polyurethanes with a network structure tend to have poor wettability and film-forming properties at room temperature. The Diplomatic Union refers to the addition of a cross-linking agent to water-borne polyurethanes, which is equivalent to a two-component system, that is, adding a cross-linking agent component to an aqueous polyurethane base agent before use to produce a chemical reaction under suitable dry film forming conditions. Crosslinked film. Compared with the internal cross-linking method, the obtained emulsion has good performance, and the properties of the film can be adjusted according to different types and amounts of the cross-linking agent. Diplomatic law is currently the most widely used method in industry. Cross-linking can improve the solvent resistance, heat-resistant creep, and the mechanical properties of adhesive bonding of waterborne polyurethane films. According to reactive functional group, suitable cross-linking systems include ionic cross-linking agents containing carboxyl-(COOH) group melamine/formaldehyde, polyfunctional aziridines, carbodiimides, epoxy resins, zinc or zirconium salts. Hydroxy-(OH) group melamine/formaldehyde, epoxy resin, screened isocyanate cross-linking agent; amine-(NH) group melamine/formaldehyde, epoxy resin screened isocyanate cross-linking agent, water-dispersible polyisocyanate, Aziridine crosslinker. Table 1 below compares commonly used cross-linking agents. The cross-linking temperatures and speeds listed in the table may vary with the amount of cross-linking agent added and the conditions of use.

2 The low initial viscosity of aqueous polyurethane adhesive is also one of the factors that hinder its wide application. In addition to the addition of tackifiers, Dainippon Ink and Chemicals introduced water-based polyurethanes made of epoxy resin, exhibiting good initial tack, and its water resistance, solvent resistance, heat-resistant creep resistance, and adhesion performance were also improved. improve.

3 Due to the specific heat of water and latent heat of evaporation, waterborne polyurethanes have poor drying properties. Increasing the waterborne polyurethane solids improves its dryness. Trying to raise the solids content to more than 50% is one of the research topics abroad. Because the increase of solid content often leads to instability of the product, researchers mostly solve the problem from the aspects of reaction technology and unit operation. An aqueous polyurethane with a solids content of 55% made by Goldschmidt, Germany, is stable to electrolytes and freezes. The company uses polyoxyalkylene and polyester diols containing both terminal hydroxyl groups and sulfonic acid side groups in the molecule as polyurethane raw materials, so that the hydrophilic groups in the polymer can self-emulsify and have stability to the emulsion. Significant improvement. It must be noted that after emulsifying and dispersing, it is necessary to add a chain extender that is more reactive with water than isocyanate groups in order to increase the molecular weight of the polymer. When reactive groups are contained, the self-reaction must be suppressed as much as possible. Otherwise, it will result in Degradation of product performance.

4 Since the volatility of water is worse than that of organic solvents, its film forming ability is low and it needs to be improved. Solvents (generally called co-solvents or latent solvents), such as acetone, methyl ethyl ketone, or toluene, can be used to reduce the viscosity of the reactants, and can also act as short-acting plasticizers to promote the formation of films during the drying process. , It can also improve the fluidity of the emulsion and the wettability to the adherend and improve the bonding strength. Water-based polyurethanes produced in Europe and the United States often contain about 10% of high-boiling N-methyl-2-pyrrolidone that can be blended with water to overcome the disadvantages of low boiling point and have a good composite effect in the production of dry composite membranes. It is also possible to improve the film forming property by adding a high boiling point phosphoric ester. If the water-based polyurethane is viewed as microspheres dispersed in a continuous aqueous phase, the harder the microspheres, the more difficult it becomes to agglomerate into a continuous film, and the more necessary it is to use a co-solvent. If the microspheres are made soft, that is, they are synthesized into a soft polyurethane, the situation can be reversed.

ICIResinsUS has developed two water-based polyurethane adhesives that do not require the use of latent solvents, with a solids content of approximately 40% and a viscosity of less than 0.25 Pa·s. The softening point of polyurethane is 65°C and 107°C. Aziridine, carbodiimide, melamine, or tertiary amines can be used for curing. Designed for low surface energy plastic film composites or composites of vinyl and rigid plastics.

5 When using dry water-based laminates for general water-based polyurethane adhesives, it is necessary to heat to remove moisture and activate the adhesive layer. In the case of heat-sensitive materials, deformation is caused by heating, and the heat activation temperature needs to be reduced. To reduce the activity temperature, plasticizers or other resins were added but the bond strength was compromised. Bayer has developed a water-based polyurethane (polyester type) adhesive, which can reduce the activation temperature to 40 °C, heat resistance up to 80 °C or more; and the adhesive layer is not easy to change color, suitable for bonding light-sensitive materials.

6 The surface tension of water is large, so its wettability to the adherend is poorer than that of solvent polyurethane. The surface tension of water is about 7.3 x 10-4 N/cm, which is 3 times that of a typical organic solvent. The hydrophilic group-containing polymer emulsion holds certain surface activities and its surface tension is generally (3~4)*10<-4 >N/cm. A liquid with a small surface tension can wet a surface of the adherend having a large surface tension. If it is desired to reduce the surface tension of the aqueous polyurethane, it is possible to add a surfactant (preferably a fluorinated active agent) for this purpose.

7 The higher cost is one of the keys to hinder the popularization of water-based polyurethane adhesives. The blending technology is an effective method that can reduce investment costs and improve product performance. It is widely used. ICIResinsUS blends general-purpose aqueous polyurethane adhesives with polyacrylate emulsions to reduce costs for composite films.

4 Development and Prospects of Waterborne Polyurethanes for Composite Films

In the field of studying the adhesives for manufacturing food composite packaging materials, two-component waterborne polyurethanes have been used as composite film dry composite adhesives, so that the products still have good composite strength and flexibility. Table 2 shows a comparative example of the adhesive strength of a composite plastic film-bonded aqueous adhesive and a solvent-based adhesive.

As can be seen from the above table, waterborne polyurethanes are suitable for the preparation of composite films and their performance is even better than that of solvent-based ones. In foreign countries, continuous research results have appeared for waterborne polyurethane as a composite film adhesive, and the relatively simple preparation method is mostly acetone method. For example, in 1999, Fricke et al. developed a method for preparing an aqueous polyurethane adhesive for composite packaging film adhesives: 0.15 mol DMPA was used as an anionic hydrophilic chain extender, and 0.25 mol TDI and 0.1 mol PPG (hydroxyl value: 56) were mixed and 0.1 was added. g Dibutyl tin dilaurate is used as a catalyst. It is reacted at 95° C. for 5 hours. Then it is cooled to 30° C., 200 g of acetone and triethylamine are added for salt formation, and then 410 g of water is used for stirring and mixing to emulsify. Finally, the acetone solvent is distilled off. For flexible packaging composite film glue [7]. This method is relatively simple and has good reproducibility, and a linear aqueous polyurethane having a particle size of 10 nm to 5000 nm is obtained. The product has high molecular weight and excellent quality. Due to the non-cross-linking type, the solvent resistance is poor; using a large amount of low-boiling acetone (40% to 70%) results in poor safety and economy. In addition to the acetone process, another major method for preparing aqueous polyurethane adhesives is the prepolymer method. Prepolymer mixing method does not require a lot of solvents, it is urgently required by environmental protection. At present, the prepolymer method has become a more mature method for producing adhesives in industry. The ionic group is introduced simultaneously with the preparation of the isocyanate-terminated polyurethane prepolymer, and the ionomer is formed after neutralization, and then water is added with stirring. At this point, two phenomena occur. First, polyurethane prepolymers are dispersed into waterborne polyurethanes in water, and second, isocyanate groups react with water to extend the polyurethane molecules. If the reaction is not properly controlled, gels can easily form. For this reason, aliphatic isocyanates are generally used to synthesize prepolymers, which have low reaction activity and moderate reaction in water, and the obtained products have outstanding light resistance performance, and the adhesion performance is not bad. 280 g of poly(ester-ether) glycol (molecular weight 2000) and 15.4 g of DMPA were reacted with 11.3 g of triethylamine, 29.4 g of MDI, 11.1 g of IPDI, and 36.6 g of TMXDI at 70° C. for 3 hours, and the resulting neutralized isocyanate was tailed. The prepolymer was dispersed in 557 g of water with rapid stirring and then a solution of 3.05 g of ethanolamine and 4.8 g of ethylenediamine in 30 g of water was added slowly. A stable translucent aqueous polyurethane dispersion is obtained. The aqueous polyurethane dispersion is coated on a surface-treated polypropylene, polyethylene, polyester, and aluminum foil with a roller, dried, heat-composited, and has good adhesion. nature. However, currently there are few kinds of isocyanate available in China, which limits the development of polyurethane adhesives. Lightfast aliphatic isocyanates such as HDI, IP DI, H12MDI, etc. are widely used in overseas polyurethane adhesive products. There are only a few kinds of domestic small-lot trial production, and these small amounts of high-priced raw material import channels are not smooth. Therefore, most domestic use only TDI diisocyanate raw materials. Water-based polyurethane adhesives for composite film adhesives are still in its infancy in China. However, as China's environmental protection regulations and packaging requirements for foods and pharmaceuticals become more stringent, the tendency for waterborne polyurethane adhesives to replace solvent-based polyurethane adhesives for composite film preparation will become more pronounced. The more obvious it is. In 2001, Rohm and Haas Company of the United States introduced its water-based composite adhesive, which has achieved great success in the European and American markets, into the Chinese market, bringing an environmental revolution to China's food packaging industry. Rohm and Haas company's water-based adhesives are mainly single-component and two-component two series of products. The single-component product does not need to be proportioned when gluing, and can be directly used. The process is simple, the cost is low, and the efficiency is high. The composite packaging material produced has excellent transparency; the finished product can be cut, bagged, etc. without curing. Especially suitable for food manufacturers to shorten the supply cycle needs. In addition, water-based single-component adhesives have good versatility and can be used in areas such as bulk food, biscuits, confectionery, condiments and pharmaceuticals and other light packaging areas. Two-component waterborne adhesives have made breakthroughs in technical strengths such as high peel strength, heat resistance, and chemical resistance. At present, more than 90% of Rohm and Haas's adhesives are water-based products, which are growing at a rate of 15% to 20% in the Chinese market.

As people's requirements for quality of life continue to increase, waterborne polyurethane adhesives that combine technological and environmental advantages will become an inevitable choice for food packaging materials. Therefore, tracking international advanced technologies and developing waterborne polyurethane adhesives used in this field will be actively carried out in the future. In particular, in the future, the development of non-yellowing polyurethane adhesives based on polyester polyurethane adhesives and aliphatic isocyanates can be emphasized.