Application of Waterborne Polyurethane Adhesive in the Manufacturing of Composite Films (in)

March 29, 2019

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.

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