Water-based adhesives are environmentally friendly adhesives prepared by using natural or synthetic polymers as the adhesive material and water as the solvent or dispersant, replacing toxic organic solvents that pollute the environment. Existing water-based adhesives are not 100% solvent-free and may contain a limited amount of volatile organic compounds as additives to their water-based media to control viscosity or fluidity. The main advantages are that it is non-toxic, pollution-free, non-flammable, safe to use, and easy to achieve clean production processes, etc. The disadvantages include slow drying speed, poor water resistance, and poor frost resistance.
Classification of water-based adhesives
1. Classified by material:
Polyvinyl alcohol-based water-based adhesives, vinyl acetate water-based adhesives, acrylic water-based adhesives, polyurethane water-based adhesives, epoxy water-based adhesives, phenolic water-based adhesives, silicone water-based adhesives, rubber water-based adhesives.
2. Classified by application:
Water-based adhesives for construction, packaging, automobiles, shoe-making, daily-use
3. Characteristics and modifications of water-soluble and emulsion adhesives classified by solvents
1. Waterborne polyurethane (APU) adhesive: APU is a new type of polyurethane system that uses water instead of organic solvents as the dispersion medium. It is also known as water-dispersed polyurethane, water-based polyurethane or water-based polyurethane. Waterborne polyurethane uses water as the solvent, is pollution-free, safe and reliable, has excellent mechanical properties, good compatibility and is easy to modify.
APU adhesive is non-toxic and pollution-free. Its high molecular weight enables it to form a high-performance adhesive film. It has the advantages of low-temperature resistance, flexibility, good bonding performance, and a wide range of adjustable physical properties of the adhesive film. Its molecular structure features flexible molecular chains, demonstrating excellent impact resistance, fatigue resistance, and low-temperature resistance. It also has the designability of performance, with the adhesive layer adjustable from flexible to rigid at will, which can meet the bonding requirements between different materials. The synthesis methods of waterborne polyurethane adhesives have become relatively mature, mainly including the following methods:
(1) Introduce some hydrophilic groups into the structure of polyurethane resin to enable self-emulsification. [1]
(2) Acetone method: First, a high-viscosity prepolymer is prepared. Then, acetone is added to reduce the viscosity. Next, a hydrophilic monomer is used to extend the chain. Under high-speed stirring, it is added to water. Through strong shearing, it is dispersed in water. After emulsification, the solvent is recovered by vacuum distillation to obtain the PU water dispersion system.
(3) End-capping isocyanate method: Select an appropriate blocking agent to protect the -NCO groups at the ends of the water-sensitive isocyanate, rendering it inactive. Then, add chain extenders and crosslinking agents to co-emulsion it into an emulsion for use. Through a certain temperature and special catalyst for unblocking, regenerate the -NCO end groups and extend the chain to prepare waterborne polyurethane
Precautions for water-based adhesives
Water-based glue must be stored in a well-ventilated indoor environment at 5-40℃ away from light. 2. Water-based adhesives are odorless, non-flammable, and have a yellowing resistance of over 4.5 grades. They are suitable for light-colored materials and those with high requirements for yellowing resistance. The rubber brush can be cleaned with warm water. 3. Water-based adhesives are suitable for bonding all materials used in footwear except PE and PP materials. 4. Water-based glue can be mixed with no more than 5% water, but it must be thoroughly stirred evenly; otherwise, the glue will stratify and affect the bonding effect. The glue mixed with water must be used up on the same day. 5. Water-based glue should be stored in plastic containers and not in iron ones, otherwise it will affect the yellowing resistance coefficient of the glue. 6. Due to the slow evaporation of water-based glue, 5% water-based curing agent must be added and stirred for 5 to 8 minutes to ensure thorough and uniform mixing. Glue with curing agent added must be used up within 4 hours and cannot be used the next day. 7. The drying temperature for water-based glue should be between 50 and 60 degrees Celsius, and the drying time is about 3 to 5 minutes. 8. Oil-based curing agents must not be added to water-based adhesives; otherwise, it may cause the adhesives to become dead or form lumps, making them unusable. 9. When applying water-based glue, it should be thin and even, not too thick. After drying, the base layer of the material should be clearly visible. If a white layer is seen, it indicates that the glue was applied too thickly. 10. If the glue used for bonding shows stringing after drying, it indicates that the glue has not dried thoroughly. This could be due to insufficient temperature, too short drying time, too little curing agent added, or insufficiently uniform stirring. 11. Generally, only one coat of water-based glue is needed. If the material is loose and porous, two thin coats of glue can be applied. Each coat of glue must be completely dry before applying the second coat; otherwise, if the base glue is not completely dry, it will affect the bonding performance and may even cause delamination. 12. For shoes that have come apart, it is best to use a special glue for filling. For small areas, the glue will dry at room temperature for 5 minutes and can be pressed together by hand. For large areas, it needs to be placed in an oven and then pressed together by a press. 13. To use water-based adhesives, the following tools should be provided: a. Pneumatic or electric stirrer b. Plastic glue mixing container c. Plastic glue holding container.
