Polyurethane adhesives contain urethane groups (-NHCOO-) or isocyanate groups (-NCO) in the molecular chain, and are divided into two categories: polyisocyanates and polyurethanes. Polyurethane adhesives, all through the system of isocyanate groups and the system or the system outside the reaction of substances containing active hydrogen, to generate polyurethane groups or polyurea, which makes the system much stronger and achieve the purpose of bonding.
Adhesives are formulated with various curing agents, plasticizers, fillers, solvents, preservatives, stabilizers, coupling agents and other additives as the main agent of adhesives. In recent years, with the rapid improvement in the level of material development, a variety of adhesives with greater applicability have been introduced one after another, greatly enriching the adhesive market.
1. Development status
Polyurethane adhesive is a high-grade adhesive, with excellent flexibility, impact resistance, chemical resistance, abrasion resistance, the important thing is its low temperature resistance. By adjusting the raw materials and formulations, it is possible to design polyurethane adhesives of various types suitable for bonding between various materials and for different purposes. Polyurethane adhesives were used in the military field as early as 1947, when Bayer successfully applied triphenylmethane triisocyanate to the bonding of metal and rubber and used it on the tracks of tanks, laying the foundation for the polyurethane adhesive industry. Japan introduced German and American technology in 1954 and started the production of polyurethane adhesives in 1966, and successfully developed vinyl-based polyurethane water-based adhesives and put them into industrial production in 1981. At present, Japan is very active in the research and production of polyurethane adhesives, and has become a major producer and exporter of polyurethane together with the U.S. and Western Europe. after the 1980s, polyurethane adhesives have developed rapidly and have become a wide variety and widely used industry.
In 1956, China developed and produced triphenylmethane triisocyanate (Lekna glue), and soon produced toluene diisocyanate (TDI), two-component solvent-based polyurethane adhesives, which are still the most productive varieties of polyurethane adhesives in China. After that, China successively introduced many advanced production lines and products from abroad, which required a large number of imported polyurethane adhesives to match with them, thus promoting domestic research units to accelerate the development of polyurethane adhesives. Especially after 1986, China’s polyurethane industry entered a period of rapid development. In recent years, the price of polyurethane adhesive has been decreasing, and the current price of polyurethane adhesive is only about 20% higher than that of neoprene, which provides conditions for polyurethane adhesive to occupy the neoprene market.
1.1 Production method
The production methods of polyurethane adhesive include solution method and solid resin dissolution method. At present, there are more than one hundred enterprises producing polyurethane adhesive for shoes in mainland China, among which there are more than twenty large scale enterprises, with the production capacity of Nam Kwong, Baili and other companies above 10,000 tons/year. Basically, they use the resin imported from Spain, Germany, the United States and Taiwan of China to dissolve and produce polyurethane adhesive. Given that there are many producers and the competition is getting fierce, in order to have technical and price advantages, some large adhesive factories have started to build solution method production polyurethane adhesive devices, so the proportion of dissolution method polyurethane adhesive production will be reduced.
At present, the common polyurethane adhesive is solvent-based. Like neoprene, it has the problems of toxicity and environmental pollution. In recent years, polyurethane adhesives without “triphenylene” have appeared one after another, i.e., using a mixture of ketones and esters, etc. The toxicity has been reduced, but the problem has not been solved fundamentally.
1,2 Classification of composition
Polyurethane adhesives can be classified into polyisocyanate adhesives, polyurethane adhesives with isocyanate group, polyurethane adhesives with hydroxyl group and polyurethane resin adhesives according to the composition of the reaction.
According to the classification of use and characteristics, it can be divided into general-purpose adhesive, food packaging adhesive, shoe adhesive, paper-plastic composite adhesive, construction adhesive, structural adhesive, ultra-low temperature adhesive, foam adhesive, anaerobic adhesive, conductive adhesive, hot melt adhesive, pressure-sensitive adhesive, closed adhesive, water-based adhesive and sealing adhesive.
2. Main application areas
2.1 Woodworking industry
With the sharp decline in global forest resources and the implementation of China’s natural forest resources protection project, small wood to put together a large board requires adhesives bonding strength and durability, weather resistance and other properties better than the wood itself. The amount of adhesives has become a measure of the level of technical development of the wood industry. In the past, people use wood adhesives are mostly formaldehyde as the main raw material of urea-formaldehyde resin, phenolic resin and melamine formaldehyde resin, but due to the presence of free formaldehyde, the product will gradually emit formaldehyde gas to the surrounding area during use, resulting in environmental pollution. The woodworking industry has begun to focus on new environmentally friendly adhesives – polyurethane adhesives, with a view to reducing environmental pollution. The one-component moisture-curing polyurethane adhesives used in the woodworking industry are liquid and used at room temperature. They usually have high adhesive strength, flexibility and water resistance, and can bond with many non-wood substrates (e.g. metals, plastics, rubber, etc.). One-component polyurethane adhesives have shown better dry and wet strengths than phenolic adhesives in tests. Before bonding, the surface of the bonding substrate coated with hydroxymethylresorcinol (HMR) coupling agent can improve the bonding strength.
2.2 Shoe industry
China is a major shoe manufacturing country, the development of shoe adhesives after three generations, with the global awareness of environmental protection, as well as the intensification of the oil crisis, prompting the emergence of the fourth generation of environmentally friendly solvent-free and water-based shoe adhesives. In recent years, the preparation process of water-based polyurethane has become more and more mature. For the bonding of some low-polar shoe materials, such as SBS and other materials, the peel strength of polyurethane adhesives cannot meet the requirements. By adding tackifying resins and other modifications, polyurethane shoe adhesives with high crystallinity, fast crystallization, high cohesive strength and more ideal peel strength can be developed.
2.3 Packaging industry
Flexible packaging with its light and convenient, long shelf life, health, easy storage and transportation, easy to open, less waste and good shelf effect and other unique comprehensive performance, now more than rigid packaging, such as plastic, glass bottles and cans. Due to its excellent performance, polyurethane adhesive can bond film materials of different properties together to get a variety of properties such as cold resistance, oil resistance, drug resistance, transparency, wear resistance and other composite films for flexible packaging. At present, polyurethane adhesives have become the main adhesives for flexible packaging laminated film processing in domestic and international markets.
In the domestic packaging composite film manufacturing industry, the amount of polyurethane adhesive ranks second only to the shoe industry. A wide variety of polyurethane adhesives for packaging, such as water-based polyurethane adhesives, hot-melt polyurethane adhesives, solvent-based polyurethane adhesives, solvent-free polyurethane adhesives. Among them, the commonly used polyurethane hot melt adhesives can be divided into two categories: thermoplastic polyurethane elastomer hot melt adhesives and reactive hot melt adhesives. In addition, in terms of water-based polyurethane adhesives for packaging, their water resistance is reduced due to the use of emulsifiers or the introduction of hydrophilic ionic groups in the molecule. In recent years, research on improving its water resistance has become a hot spot. At the same time, due to the large heat capacity of water, so how to increase its solid content and thus improve its drying speed is also one of the urgent problems to be solved.
2.4 Building pavement
In addition to non-toxic, non-polluting, easy to use and other advantages, polyurethane adhesives also have the advantages of other adhesives can not be compared, namely, excellent low temperature resistance, solvent resistance, aging resistance, ozone resistance and bacterial resistance, in the application of architectural paving materials play an important role. It is widely used in elastic rubber floor mats, hard rubber floor tiles and paving plastic runway sports fields. The new two-component polyurethane adhesive breaks through the contradiction between shear strength and peel strength of traditional adhesive, and can make both of them reach high strength at the same time, which shows excellent performance in the bonding of steel plates for construction, with firm bonding and not easy to produce deformation, and adjustable curing speed at room temperature, which makes the polyurethane adhesive easy to use and widely used. Polyurethane adhesives are widely used in the bonding of PVC materials for construction, sandwich panel production and building waterproof coatings.
2.5 Polyurethane adhesives for automobiles
The polyurethane adhesives widely used in automobiles are mainly one-component wet-curing polyurethane sealants for windshield assembly; structural adhesives for bonding glass fiber reinforced plastics and sheet molding composites, two-component polyurethane adhesives for interior parts and water-based polyurethane adhesives. Automotive interior parts are also an area of growth in the amount of adhesives. Water-based polyurethane adhesives, which are widely used in automobiles, are adhesives formed by dissolving or dispersing polyurethane in water. Most waterborne polyurethanes are linear thermoplastic polyurethanes, and because their coating films are not cross-linked and have low molecular mass, their properties such as water resistance, solvent resistance, and adhesive film strength are still poor, and they must be modified to improve their performance. The heterogeneous and dispersions of polyester and acrylic acid with urea diketone and isocyanurate are prepared for automotive refinishing varnish, which does not require high-speed mixing equipment and is easily mixed together with good adhesion properties.
2.6 Others
In the past 20 years, the application of hot melt adhesive in bookbinding has gained rapid development. The reason for this is mainly the amazing growth in the production of low-cost paperback books, along with the production of wireless binding process, due to EVA hot melt adhesive has very good adhesive and elasticity, the vast majority of book binding hot melt adhesive used in the domestic market is EVA type hot melt adhesive. But EVA hot melt adhesive is expensive, and EVA hot melt adhesive has memory and other shortcomings. Polyurethane hot melt adhesive can make up for these defects. The adhesive production process is simple, low cost, good elasticity and toughness of the adhesive, high adhesive strength, can be repeatedly melted and used, high product utilization rate, has a broader application prospects.
Due to its excellent adhesive properties, polyurethane adhesives play an increasingly important role in the bonding of aerospace equipment, the protection and restoration of cultural relics, the military industry, stationery products, medical and health care. Polyurethane adhesives are widely used in various fields with their excellent performance, and gradually become an important synthetic material in human life. At present, the world polyurethane adhesive industry is developing in the direction of adapting to environmental protection, safety and health, resource recycling and so on. China’s polyurethane adhesive industry has also shown a rapid growth momentum, and great progress has been made in technology development.
3. Water-based polyurethane green adhesives and their development
At present, the entire polyurethane adhesive industry is still mainly solvent-based, with the strengthening of people’s awareness of safety and environmental protection, water-based polyurethane adhesives research has been developed rapidly.
3.1 Performance characteristics
Water-based polyurethane adhesives, in addition to maintaining the advantages of polyurethane, compared with solvent-based polyurethane adhesives, have the following unique features: water as the medium, non-toxic, non-flammable, non-polluting, non-hazardous, small odor, no pollution of the environment, energy saving, suitable for organic solvent-prone substrates; lower viscosity, and can be adjusted with water-soluble thickeners and water, easy to operate, easy to clean up the residual glue; can be mixed with a variety of It can be mixed with a variety of water-based resins to improve performance and reduce costs, but attention should be paid to the ionic nature and acidity and alkalinity to prevent cohesion; contains carboxyl, hydroxyl and other groups, which can participate in the reaction under suitable conditions to produce cross-linking and improve performance; poor wettability of non-polar substrates; slow drying speed, low initial viscosity, and poor water resistance.
High-performance water-based polyurethane adhesives have good water and media resistance; high adhesive strength, high initial adhesion; good storage stability; freeze-thaw resistance, higher temperature resistance; faster drying speed, good film formation under low ambient temperature; good construction process.
3.2 Classification
Water-based polyurethane adhesives can be classified according to a variety of methods.
(1) Classified by the structure of polyurethane. According to the raw material and structure, it can be divided into polyurethane emulsion, vinyl polyurethane emulsion, polyisocyanate emulsion and closed polyurethane emulsion; according to the molecular structure, it can be divided into linear polyurethane emulsion and cross-linked polyurethane emulsion. The cross-linked type is further divided into internal cross-linked type and external cross-linked type.
(2) Classified by the nature of hydrophilic groups. Anionic: Most waterborne polyurethane adhesives are introduced with carboxyl or sulfonate expanded chains containing carboxyl or sulfonic acid ions; cationic: Generally, waterborne polyurethane containing ammonium or sulfur ions on the main chain or side chain; nonionic: The molecule contains nonionic hydrophilic chain segments or hydrophilic groups; hybrid: Both ionic and nonionic hydrophilic chain segments or groups exist in the molecular structure.
(3) Classification by preparation method: self-emulsification and external emulsification method; pre-polymer method, acetone method, melt dispersion method; ketime-methanone with nitrogen method.
(4) Classification according to the use of form: according to the use of form can be divided into one-component and two-component water-based polyurethane adhesives.
3.3 Research on performance improvement
Water-based polyurethane adhesives have the advantages of solvent-free, non-pollution, good film formation, strong adhesion, and other polymers, especially emulsion-type polymers are easy to mix and facilitate modification, etc. After the 1990s, they have been gradually used in automotive upholstery bonding, kitchenware manufacturing, composite film manufacturing, shoe sole and shoe upper bonding, garment processing, etc. However, there are still many shortcomings, which need to be improved by Various modifications are needed to improve its functions. In recent years, researchers have conducted a lot of research on water-based polyurethane adhesives with slow drying speed, low initial adhesion, poor wettability to non-polar substrates, poor water resistance, and poor heat resistance, and put forward a series of improvement measures, such as: water-based polyurethane itself contains hydrophilic groups, and to improve its water resistance, the content of hydrophilic groups should be reduced as much as possible under the premise of ensuring the stability of emulsions. Research on improving the performance of waterborne polyurethane mainly focuses on modification, such as cross-linking modification, blending modification, copolymerization modification and nano-modification.
3.3.1 Cross-linking modification
Cross-linking is an effective method to improve the performance of waterborne polyurethane, which can improve the water resistance, heat resistance and adhesive strength of the adhesive film. Crosslinking is divided into internal crosslinking and external crosslinking. Internal cross-linking refers to the introduction of cross-linking agents in the synthesis. Such as the use of partial tri-functional polyol or isocyanate, the introduction of amine or epoxy groups, closed isocyanate emulsions, multi-functional cross-linking agents, etc. can be made into internal cross-linked waterborne polyurethane. The disadvantage of the internal cross-linking method is that the prepolymer is very viscous and difficult to emulsify. The external cross-linking method is to add a cross-linking agent before the glue is used, and to form a cross-linked glue film by heating during or after the film-forming process. Compared with internal cross-linking, the resulting emulsion has good performance, but the disadvantage is that the two-component system is not as convenient as the one-component use. External crosslinking can be formaldehyde, melamine-formaldehyde resin, epoxy compounds for crosslinking agent, generally at 120 ~ 180 ℃ high temperature for crosslinking reaction. Can also aziridine, carbodiimide, polyisocyanate, metal salt compounds as cross-linking agent, at room temperature can be reacted, the carboxyl group can make aziridine open ring cross-linking reaction, but aziridine is more volatile, irritating, the respiratory system has a certain degree of toxicity, is not conducive to the occupational health of workers.
3.3.2 Blending modification
Blending modification can also improve and enhance the performance of waterborne polyurethane by using the complementary effects between different resins. Such as epoxy resin emulsion, acrylate emulsion, neoprene latex, polyvinyl acetate emulsion, urea-formaldehyde resin and other blends to produce new water-based adhesives with excellent overall performance. Among them, polyurethane and epoxy resin, polyurethane and acrylic resin blending research is important. At present, PU resin and epoxy resin, silicone, fluorine compounds, acrylates, carboxymethyl cellulose, polyvinyl alcohol, vinyl acetate and styrene-butadiene rubber copolymer modification have been studied. Among them, epoxy resins, silicones, fluorinated compounds and acrylates modified polyurethane resin research is active.
3.3.3 Nanomaterials
Nanomaterials have special properties such as surface effect, small size effect, optical effect, quantum size effect, etc., which can make materials obtain new functions.
For example, after the particle size enters the nanometer scale, the increase of active centers on the surface of the material can improve its chemical catalytic
