Waterborne polyurethane adhesives (PU glues) are an important class of waterborne adhesives. With their excellent adhesion, outstanding oil resistance, impact resistance, wear resistance, and low temperature resistance, they have developed rapidly in recent years.
1 Definition and Classification of Waterborne PU Adhesives
Water-based PU glue refers to an adhesive formed by dissolving polyurethane in water or dispersing in water. There are many methods of classification, and they can be divided into three categories according to the appearance and particle size, namely polyurethane emulsion, polyurethane dispersion and polyurethane aqueous solution. The most practical applications are polyurethane emulsions and dispersions [1].
1.1 polyurethane emulsion
Polyurethane emulsion refers to a polyurethane dispersion containing an emulsifier in an aqueous dispersion. It can be obtained by external emulsification. Its particle size is more than 0.1 μm, and its appearance is cloudy. Since this polyurethane is not easily soluble in water, it is necessary to force the polyurethane to be emulsified and dispersed in water by means of strong stirring, relying on shear force and a large amount of emulsifiers. Most of the product size of the external emulsified polyurethane emulsion is coarse, and the residue of the hydrophilic small molecule emulsifier will affect the performance of the cured polyurethane film. Now, it has gradually developed to the direction of self-emulsifying polyurethane dispersion.
1.2 Polyurethane dispersion
Polyurethane dispersions that do not contain emulsifiers are generally referred to as aqueous polyurethane dispersions or polyurethane dispersions, having a particle size of 0.001-0.1 μm and a translucent appearance, which can be obtained by internal emulsification or self-emulsification. Hydrophilic polyurethane salts are formed by reacting substances with salt-forming hydrophilic groups with the -NCO groups of the prepolymer. This polyurethane salt can be directly dispersed in water to obtain a translucent dispersion without adding an emulsifier. . According to the different hydrophilic groups introduced in polyurethane molecules, they can be divided into anionic, cationic and non-ionic. In the process of film formation, moisture is gradually eliminated, and the molecular chain asks for regular arrangement among ion groups. It not only has electrostatic effect and hydrogen bonding force, but also cross-linking reaction between molecules to form a network structure. Due to the absence of emulsifiers, these particles are not sensitive to mechanical stirring, heating or dilution, and are resistant to electrolytes; the adhesive film is strong and elastic and has strong adhesion.
1.3 Polyurethane aqueous solution
Aqueous polyurethane solution refers to a solution of the produced polyurethane molecules dissolved in water. Its particle size is less than 0.001μm, its appearance is transparent and its stability is good. However, since the hydrophilicity of the polymer in the aqueous polyurethane solution is good, the water resistance and other properties of the film are inferior to those of the former two, and the process is complicated and the cost is high, and its application is limited.
2 Status and Application of Waterborne Polyurethane Adhesives
In the 1950s, foreign countries began research on water-based PU adhesives. It was only in the 1980s that breakthroughs in technology were achieved, and they were introduced to the market in the 1990s [2]. Research in China began in the early 1970s. In recent years, research work has been very active, but it is still in the development stage compared with foreign standards [3].
With the rapid economic development, environmental protection requirements are getting higher and higher. Many domestic units are actively investing in the research and development of waterborne polyurethanes. According to incomplete statistics, there are currently hundreds of research and production units for waterborne PU adhesives in China. Water-based PU adhesives have the advantages of low temperature resistance, flexibility, good adhesive properties, and wide adjustable range of adhesive film properties. They have been used in flocking and bonding of various laminated products (including films, fabrics, cotton or chemical fiber, and non-woven fabrics). Paper, paper, food packaging composite plastic film, etc.), automotive interior materials, bonding, woodworking, pressure sensitive adhesives, footwear and other fields are widely used.
2.1 flocking adhesive
Waterborne polyurethane has excellent flexibility, performance exceeds acrylic emulsion adhesive, and can be used as flocking adhesive. The epoxy-modified polyurethane cationic emulsion system developed by Xu Gewen et al. [5] and others has excellent performance in terms of adhesion, water resistance, solvent resistance, and hand feel.
2.2 Laminate and composite packaging
Polyurethane adhesives contain isocyanate (-Nc0) and urethane (-NHCOO-) groups, which have excellent adhesion to active hydrogen-containing materials, and hydrogen bond interactions with adhered materials, making them more adhesive. firm. The formula of polyurethane adhesive is adjustable, and the adhesive layer can be arbitrarily adjusted from flexible to rigid, and it is suitable for the requirements of composite packaging and lamination of multiple materials. In addition, because of its small odor and non-toxicity, the film is particularly suitable for composite film for food packaging and composite adhesive for decoration.
The PU-5080 waterborne polyurethane adhesive developed by the Key Laboratory of Green Polymer Materials of Anhui University can be used for magnetic lamination, fabric processing, material bonding, etc. Solid content (30±1)%, PH value 6-9, elongation at break Rate> 400%, tensile strength> 13 MPa.
2.3 Wood adhesive
Wood processing is the largest application area for adhesives. Plywood, fiberboard, particleboard commonly used adhesives are urea formaldehyde, melamine-formaldehyde, phenolic resin and so on. The use of 'trialdehyde resin' for the manufacture of composite sheet, generally requires wood moisture content within 2%, and untreated wood moisture content of about 10% or more, need to be dried before the composite processing, or suppression It may burst. In addition, trialdehyde resins generate irritating odor in the process of bonding and the use of products, causing environmental pollution. Urea-formaldehyde-bonded products have poor water resistance, polyvinyl acetate emulsions have poor water resistance and heat resistance, and are easily penetrated by rubber when hot-pressed. The use of vinyl water-based polyurethane adhesive can overcome the above shortcomings, can glue wood at room temperature, has a high initial viscosity, and can adapt to wet wood. The adhesive layer has good heat resistance and aging resistance.
Japan first developed a water-based vinyl PU wood adhesive, abbreviated as API, and later changed its name to water-based polymer-isocyanate wood adhesive (WPI). This kind of adhesive has high initial viscosity, can be glued at room temperature, and finally has high bonding strength. The glue layer has good water resistance and durability. It has short pressing time when bonding wood, and it is easy to operate. The adhesive is neutral and has no pollution to the wood [6] .
2.4 Shoe adhesives
A few years ago, some of China's three funded shoe factories have begun using water-based PU glue. At present, the water-based PU adhesives used by the four famous brands (Nike et al.) that are exported from China alone to foreign countries each year reach 20,000 tons [2]. As the domestic water-based PU glue is inferior to solvent-based glue in terms of curing speed, initial viscosity, heat resistance, etc., the water-based PU for shoes on the domestic market is basically an imported product. According to reports, the PUWD26210 water-based adhesive developed by the Shanxi Provincial Institute of Applied Chemistry and produced by Yunyang Huayang Industry and Trade Co., Ltd. of Shanxi Province has good adhesiveness to shoe materials [2].
2.5 pressure sensitive adhesive
The purpose of the emulsion pressure sensitive adhesive is to manufacture adhesive tapes, self-adhesive labels, etc. It can also be used directly and widely used in offices, buildings, furniture, vehicles and other fields. At present, acrylic emulsions are used more often. Considering factors such as aging resistance, cold resistance, adhesive strength, transparency, and non-toxicity, polyurethane can be a new type of water-based pressure-sensitive adhesive. PU emulsion molecular mass should not be too large. Japanese Patent Laid-Open No. Sho 62-297375 reports an anionic PU emulsion developed by Nippon Kasei Co., Ltd. The pressure-sensitive adhesive obtained by cross-linking with epoxy has good water resistance, high strength, and good stability [8]. The epoxy modified water-based PU pressure-sensitive adhesive developed in this experiment has a stable emulsion and is used for bonding aluminum plates with films and sponges and sponges, and has very good initial and sticky properties.