Since the development of composite flexible packaging, reduction and removal of organic solvents in composites has become a common direction for the entire industry. At present, composite methods that can completely eliminate solvents include aqueous compounding and solvent-free compounding. Solvent-free compounding is currently in its infancy due to factors such as cost technology. The water-based adhesive can be used directly on the existing dry laminating machine, so it is welcomed by the domestic soft packaging manufacturers and has achieved rapid development abroad.
Water-based composites are classified into dry composites and wet composites. The wet composites are mainly used in the compounding of paper, plastic, and paper aluminum. White latex is popular in this field. In plastic-plastic composites and plastic aluminum composites, mainly water-based polyurethanes and waterborne acrylic polymers are used. Waterborne adhesives have the following advantages
(1) Large composite strength. Waterborne adhesives have a large molecular weight, which is several tens of times that of polyurethane adhesives. Its adhesive force is mainly based on van der Waals forces and belongs to physisorption. Therefore, a very small amount of glue can achieve a very high composite strength. For example, compared with two-component polyurethane adhesives, in the compounding process of the aluminized film, the amount of dry glue of 1.8 g/m2 can be used to achieve the composite strength when a two-component polyurethane adhesive is 2.6 g/m2 of dry glue.
(2) soft, more suitable for composite aluminum film. One-component waterborne adhesives are softer than two-component polyurethane adhesives. When they are completely set, the polyurethane adhesive is very stiff and the waterborne adhesive is very soft. Therefore, the softness and elasticity of the water-based adhesive are more suitable for the composite of the aluminized film, and it is not easy to cause the transfer of the aluminized film.
(3) It does not need to be ripened, and it can be cut after the machine is down. The single-component waterborne adhesive compound does not need to be matured, and subsequent processes such as slitting and bag making can be performed after the machine is released. This is because the initial adhesive strength of the waterborne adhesive, especially the high shear strength, ensures that the product will not have problems such as "tunneling" and wrinkles during compounding and slitting. In addition, the strength of the composite film using the water-based adhesive can be increased by 50% after being left for 4 hours. This is not the concept of ripening. Colloids themselves are not cross-linked. The main reason is that with the leveling of the glue, the composite strength also increases.
(4) Thin adhesive layer with good transparency. Due to the small amount of water-based adhesive sizing, and the higher concentration of the glue than the solvent-based adhesive, the amount of moisture that needs to be dried and discharged is much less than that of the solvent-based adhesive. After the water is completely dried, the film becomes very transparent. Because the adhesive layer is thinner, the composite transparency is also better than the solvent-based adhesive.
(5) Environmental protection, harmless to people. After the water-based adhesive is dried, there is no residual solvent. Many manufacturers use water-based adhesives to avoid the residual solvents that are compounded. Therefore, the use of water-based adhesives is safe and does not harm the health of the operators.