Existing water-based adhesives are not 100% solvent-free and may contain limited volatile organic compounds as additives to their water-based media in order to control viscosity or fluidity.The advantages are mainly non-toxic, no pollution, no combustion, safe use, easy to achieve clean production technology, etc., disadvantages include slow drying speed, poor water resistance, poor frost resistance.

PART 01

Classify

1.Classification by material

Polyvinyl alcohol water-based adhesives, vinyl acetate water-based adhesives, acrylic acid water-based adhesives, polyurethane water-based adhesives, epoxy water-based adhesives, phenolic water-based adhesives, silicone water-based adhesives, rubber water-based adhesives.

2.Classification by application

Water-based adhesive for construction, packaging, automobile, shoemaking, daily use, water-based adhesive.

3.according to the solvent classification

Water soluble and emulsion adhesive two kinds.

PART 02

Properties and modification of water-based adhesives

Waterborne polyurethane (APU) adhesive

APU is a new type of polyurethane system in which water replaces organic solvent as dispersion medium, also known as water-dispersed polyurethane, water-based polyurethane or water-based polyurethane. Waterborne polyurethane with water as solvent, no pollution, safe and reliable, excellent mechanical properties, good compatibility, easy to modify.

APU adhesive is non-toxic and pollution-free, and its high molecular weight enables it to form an excellent adhesive film. It has the advantages of low temperature resistance, flexibility, good bonding performance and wide adjustable range of the physical property of the adhesive film. Its molecular structure has flexible molecular chain, showing excellent impact resistance, fatigue resistance, low temperature resistance, but also has the performance of designability, the adhesive layer from flexible to rigid arbitrary adjustable, can meet the bonding needs between different materials; At present, the synthesis method of waterborne polyurethane adhesive has been relatively mature.

There are mainly the following methods:

(1)Introduce some hydrophilic groups into the structure of polyurethane resin to make it self-emulsifying. 

(2)Acetone method: the high viscosity prepolymer is prepared first, and then acetone is added to reduce the viscosity, and then the hydrophilic monomer is added to the water under high speed stirring, and it is dispersed in the water through strong shear. After emulsification, the decompressive distillation to recover the solvent is prepared PU water dispersion system.

(3)End-sealing isocyanate method: select an appropriate sealer, protect the end-NCO group of the water-sensitive isocyanate, make it lose its activity, and then add chain extender and crosslinking agent co-emulsion to make emulsion use, through a certain temperature and special catalyst, regenerated into -NCO end-group, chain extension to prepare waterborne polyurethane.

(4)End-sealing isocyanate method: select an appropriate sealer, protect the end-NCO group of the water-sensitive isocyanate, make it lose its activity, and then add chain extender and crosslinking agent co-emulsion to make emulsion use, through a certain temperature and special catalyst, regenerated into -NCO end-group, chain extension to prepare waterborne polyurethane.

PART 03

Disadvantages

Waterborne polyurethane adhesives have some problems, such as slow drying speed, poor wettability to non-polar substrates, low initial viscosity and poor water resistance. At the same time, the hardness and mechanical properties of the latex film are far from sufficient.

(1) Adjust the structure on the premise of ensuring the stability of the emulsion, the content of hydrophilic groups as low as possible, can improve the water resistance of the film

(2) Improved process using emulsifying equipment with excellent process performance for heat treatment, can improve the strength of the film and water resistance.

(3) In order to improve some properties of water-based polyurethane, it can be mixed with other water-based resins such as epoxy resin emulsion and urea-formaldehyde resin to make a new water-based adhesive, and its properties have been improved. 

(4) Grafting modification by adding appropriate amount of styrene monomer to polyurethane emulsion, polyurethane styrene grafting emulsion was prepared by in-situ polymerization, which improved the adhesion and water resistance. 

(5) Vinyl emulsion dispersion modification can disperse polyisocyanate (or its prepolymer) organic solution in the aqueous solution of polyvinyl alcohol, carboxymethyl cellulose. 

(6) Crosslinking modification is an effective method to improve the performance of waterborne polyurethane, which can improve the water resistance, heat resistance and bonding strength of the film.