Core Tip: Polyurethane adhesive is a molecular chain containing urethane groups and / or isocyanate type adhesive. Since polyurethane has excellent flexibility, low temperature resistance, abrasion resistance, chemical resistance and good adhesion to various substrates and other characteristics, It is widely used in coatings, adhesives, inks and other fields. In the past, adhesives and coatings, Solvent-based products account for a large proportion. However, since the solvent-free polyurethane containing volatile organic compounds and pollution of the environment, So that its application is limited. Although waterborne polyurethane has many excellent properties, But there are still many shortcomings. First of all, The aqueous polyurethane emulsion solids due to the low dry film. 

Polyurethane adhesive is a molecular chain containing urethane groups and / or isocyanate type adhesive. Since polyurethane has excellent flexibility, low temperature resistance, abrasion resistance, chemical resistance and good adhesion to various substrates and other characteristics, It is widely used in coatings, adhesives, inks and other fields. In the past, adhesives and coatings.

Solvent-based products account for a large proportion. However, since the solvent-free polyurethane containing volatile organic compounds and pollution of the environment, So that its application is limited.

Although waterborne polyurethane has many excellent properties, But there are still many shortcomings. First of all, The aqueous polyurethane emulsion solids due to the low dry film is slow, self-thickening poor early adhesive power and low defects; In addition, the presence of aqueous polyurethane water resistance is poor, poor solvent resistance, low hardness, surface gloss poor poor film feel shortcomings after the emulsion film.

Because of these disadvantages aqueous polyurethane, Requiring their modification. Modification methods at home and abroad are mainly acrylic modified, epoxy-modified silicone organic fluorine modification, nanomaterials modification, composite modification. In recent years, major modification of waterborne polyurethane toward hyperbranched prepolymer-modified nano modified cellulose and other direction.

1、Waterborne polyurethane adhesives and synthetic methods of classification

1.1、The aqueous polyurethane adhesive classification

The aqueous polyurethane emulsion can be divided according to the appearance of polyurethane, aqueous polyurethane dispersions and water-soluble polyurethane. Press isocyanate polyurethane material points, Aromatic isocyanates can be divided into type, aliphatic type isocyanate, alicyclic isocyanate type. The aqueous polyurethane according to its main or side chains contain ionic groups are divided into anionic polyurethane dispersion of cationic polyurethane dispersions and nonionic polyurethane emulsion. 

1.2、Synthesis of waterborne polyurethane

Synthesis of waterborne polyurethane emulsion method can be divided into external and internal emulsion method. The introduction of the molecular chain of the outer emulsion method contains a small amount is not sufficient self-emulsifying hydrophilic segment or group, Or completely without introducing a hydrophilic component, to add an emulsifier and vigorous stirring into the polyurethane emulsion or dispersion. Emulsifying law is the introduction of hydrophilic groups in the polyurethane molecule having a hydrophilic group or chain extender, And then salified directly dispersed in an aqueous medium, without an emulsifier to form a stable emulsion. Emulsifying method can be divided into the prepolymer method, acetone, melt dispersion method, the end groups and ketimine Protection Act - ketazine law. Wherein the prepolymer method and acetone is more mature.

2、The aqueous polyurethane adhesive modified method

2.1、modified epoxy resin

Traditional aqueous polyurethane molecular chain containing ionic groups, and the molecular chain structure is linear, branched, rarely crosslinked, resulting in poor film chemical resistance and water resistance.Thus, it was modified with epoxy. There are two methods for the preparation of waterborne polyurethane modified by epoxy resin: Mechanical total hybrid copolymerization. There was no chemical bond between the epoxy resin and the polyurethane in the blending method. Waterborne polyurethane modified epoxy resin will be many excellent properties of epoxy resin, such as easy to cure, high mechanical strength, strong adhesion, low mold shrinkage, good chemical stability, good electrical insulation, low cost, high strength and thermal stability, etc., a good combination with the aqueous polyurethane. Therefore, it can improve the mechanical properties, heat resistance, water resistance and solvent resistance of the film.

2.2、Acrylate modification 

Preparation of waterborne polyurethane modified acrylate has the following five kinds:

①PA and General direct physical blending;

②Plus cross-linking agent to form a polyurethane - acrylate hybrid emulsion blend;

③Polyurethane emulsion as seed emulsion, acrylic emulsion be formed with a core - PUA composite emulsion shell structure;

④Two emulsions molecular lines of mutual penetration, and then react to form an interpenetrating polymer network of PUA composite emulsion;

⑤Graft copolymerization. 

Acrylate having excellent light resistance, resistance to ultraviolet radiation, has good acid and alkali salt corrosion, excellent flexibility.Waterborne polyurethane modified polyurethane may be a higher tensile strength and impact strength, excellent abrasion resistance and light resistance acrylate resin, weather resistance combined organic and aqueous polyurethane modified cost decrease. Accordingly modified waterborne polyurethane acrylate is widely used in coatings, leather finishing agents, shoes with plastic and other industries. 

2.3、Organic fluorine modification

Due to the small fluorine atom radius, electronegativity strong carbon-fluorine bond can be high, the introduction of a fluorine-containing polymer emulsion prepared fluorinated alkyl side chains on the polymer main chain, Its latex film to maintain the original characteristics of the body on the basis, with the outstanding surface properties. Since the fluorine-containing side chains oriented and outward orientation of the main chain molecules can form and internal shielding, Effectively improve the polymer coating of low surface energy, lubricity, chemical resistance oleophobic and hydrophobic, stain resistance and good biocompatibility. 

After the aqueous polyurethane fluorine-modified organic fluorine atoms migrate to the surface during film formation of the film, So that the surface of the film can be remarkably reduced, the film water resistance and chemical resistance is improved. Therefore, the aqueous polyurethane modified organic fluorine has good surface properties, it is widely used as leather and waterproof and breathable fabric coating agent. 

2.4、Silicone modified

The most notable feature is silicone polymer oxidation resistance and low surface energy, that good weather resistance and can produce excellent hydrophobicity. Silicone-modified waterborne polyurethane hydrolysis can compensate somewhat less defects, the modified waterborne polyurethane exhibit good hydrophobicity, surface enrichment, low temperature flexibility, biological stability. Silicone-modified waterborne polyurethane mainly Blending and copolymerization methods. Blending by waterborne polyurethane and silicone emulsions physically mixed to achieve; and copolymerization polysiloxane oligomers with a reactive functional group through both ends, Such as silicone oil, hydroxy silicone oil, amino or alkoxy end closure of a silane coupling agent with the polyisocyanate by gradual addition, polymerization of the block copolymer.

After the performance of silicone-modified waterborne polyurethane three aspects can be improved:

①Water resistance and chemical resistance;

②Thermal stability;

③Mechanical properties.

2.5、Nano-materials modified

Nano-materials with special properties of surface effect, small size effect, an optical effect, quantum size effect, macroscopic quantum size effect. Preparing nanocomposites are: Sol - gel method, in situ polymerization, blending, intercalation, radiation synthesis, self-assembly technology preparation method. After the addition of nano materials in an aqueous polyurethane modified, greatly improving the impact resistance of materials, adhesion, anti-aging, corrosion resistance and anti-UV properties. Thus, nano-materials modified aqueous polyurethane applied to aerospace, marine and automotive and other high-end industries.

2.6、Composite Modified。

The so-called composite material is to use a common modification modified acrylic, epoxy and silicone, etc., waterborne polyurethane compound combines the advantages of the modified acrylate-modified, epoxy-modified, silicone-modified or the like, so that all aspects of its performance have been improved, modified hot research in recent years.

2.7、Hyperbranched prepolymer-modified

Hyperbranched prepolymer is a new class of polymers, such polymers having a great end functionality, and since the end of a great group activity so that the reaction of such polymers are highly active, and therefore the substrate bonding performance than it is good. Because of the spherical or dendritic structure, molecular chains are not entangled, so hyperbranched polymer has a low melting point, low viscosity, easy to dissolve, excellent performance. After hyperbranched prepolymer aqueous polyurethane-modified after compared with traditional linear polyurethane, water resistance, mechanical properties, thermal stability are improved.

3、Development Trend of Waterborne Polyurethane Adhesive

Waterborne polyurethane modified after decades of development, has become an important material for the preparation of adhesives. Especially in the manufacturing sector is highly developed in the 21st century, water-based polyurethane adhesive performance requirements are more demanding. For less than the current modified waterborne polyurethane adhesive existence of future work mainly in the following aspects.

High solid content of the modified waterborne polyurethane. Commercially available modified waterborne polyurethane adhesive solid content is generally 15% to 35%, thus increasing the cost of transport and dry, while the impact of the bond. So try to improve solid content become a hot research topic at home and abroad. Currently, synthetic high solid content of the aqueous polyurethane mainly used as a hydrophilic chain extender sulfonate, sulfonate salt is also mixed with a carboxylic acid as a hydrophilic chain extender.

Design will be introduced by the molecular ion group into the molecular chain end groups prepared by a low content of ion-containing aqueous polyurethane high solid stability, and development of high solids content of the aqueous polyurethane provides a new way of thinking .

UV-curable waterborne polyurethane modified. UV curable modified waterborne polyurethane adhesive with water as a diluent, non-toxic, non-flammable, environmentally friendly; and free of diluent monomer, will help improve the coating adhesion to the substrate. Therefore, vigorous development in recent years.

Natural polymer modified waterborne polyurethane. Such as nano-modified cellulose. Cellulose is one of the natural polymer material nature's most abundant, and nano-cellulose has a higher strength. Therefore, the aqueous polyurethane modified so that it can not only reduce costs, but you can get good mechanical properties.