How much do you know about the use of polyurethane adhesives?
A polyurethane adhesive refers to an adhesive containing a urethane group or an isocyanate group in a molecular chain. According to the reaction composition classification, the reaction composition can be divided into a polyisocyanate adhesive, an isocyanate-based polyurethane adhesive, a hydroxyl-containing polyurethane adhesive and a polyurethane resin adhesive. According to the purpose and characteristics, according to the purpose and characteristics, can be divided into general-purpose adhesives, food packaging adhesives, shoe adhesives, paper-plastic composite adhesives, construction adhesives, structural adhesives, ultra-low temperature adhesives, foaming adhesives, and boring Oxygen adhesives, conductive adhesives, hot melt adhesives, pressure sensitive adhesives, sealable adhesives, water based adhesives, and sealing adhesives. However, no matter which kind of polyurethane adhesive, the isocyanate groups in the system react with substances in the system or containing active hydrogen to form polyurethane groups or polyureas, so that the strength of the system is greatly improved and the purpose of bonding is achieved. .
So far, in addition to the original rubber types, non-solvent polyurethane structural adhesives, reactive polyurethane hot melt adhesives and other foreign rubber grades are now available in China. Although China's polyurethane industry has a considerable scale, there is still a big gap compared with developed countries, mainly because the output is not large and the technical level is still low. What are the characteristics of polyurethane adhesives? What areas does it apply to? I will give you some brief introductions today.
First, the characteristics of polyurethane adhesive
1. Strong bonding force, large initial adhesion, wide application range
Since -NCO in the molecular chain of the polyurethane adhesive can react with a variety of active hydrogen-containing functional groups to form an interfacial chemical bond, it has excellent adhesion to a variety of materials. It can not only bond porous materials, such as foam, ceramics, wood, fabrics, etc., but also can bond a variety of metals, inorganic materials, plastics, rubber and leather. It is a wide range of adhesives.
2, outstanding low temperature resistance
At very low temperatures, the general polymer material is converted to a glassy state and becomes brittle, while the polyurethane adhesive maintains a high peel strength even below -250 ° C, and its shear strength decreases with temperature. Significantly increased.
Second, the shortcomings and improvement methods of polyurethane adhesives
1. The free isocyanate monomer in the polyurethane adhesive has greater toxicity. An epoxy-based polyurethane can be obtained by using a hydroxyl-terminated polyvalent epoxy compound and a terminal isocyanate-based polyurethane prepolymer, and then a polyurethane compound can be formulated into a polyurethane adhesive. The adhesive combines the excellent properties of epoxy resin and polyurethane, and contains almost no free isocyanate monomer and has a high solid content. Polyurethane adhesives are moving towards water-based and solvent-free green adhesives.
2, polyurethane adhesives usually cure slowly at room temperature, sometimes fully cured for several days, such as multi-component compounding can significantly shorten the room temperature curing time, more convenient and applicable.
3. The high temperature resistance of polyurethane adhesives is far less than its low temperature resistance. If used in food packaging composite film, it needs to have retort resistance. Introducing nanoparticles can significantly improve its heat resistance, tensile strength, shear strength and peel strength, etc., but the addition of nanomaterials should be appropriate. If added too much, it may destroy the orderly arrangement of hard segments in some macromolecules. And the mechanical properties are degraded.
Third, the application of polyurethane adhesive
1. Polyurethane adhesive for automobiles
The introduction of a large amount of lightweight metals, composites and plastics in new automotive structures has led to a growing demand for automotive adhesives and sealants. The most widely used polyurethane adhesives in automobiles are mainly one-component moisture-curing polyurethane sealant for windshield, structural adhesive for bonding glass fiber reinforced plastic and sheet molding compound, and two-component polyurethane adhesive for interior parts. And waterborne polyurethane adhesives. Automotive interior parts are also an area where adhesive usage is growing rapidly. In the past, for a long period of time, China's automobile manufacturing industry has been importing polyurethane windshield glue from abroad. Until the "Eighth Five-Year Plan" period, the development of this rubber was included in the national key scientific and technological projects.
Waterborne polyurethane adhesives, which are widely used in automobiles, are adhesives formed by dissolving polyurethane in water or dispersed in water. Most of the waterborne polyurethanes are linear thermoplastic polyurethanes. Because their coatings are not crosslinked and their molecular weight is low, their water resistance, solvent resistance, film strength and other properties are still poor. They must be modified to improve their properties. performance. The automotive refinish varnish prepared by mixing the polyester and acrylic miscellaneous and dispersion with uretdione and isocyanurate does not require high-speed stirring equipment, is easy to mix together, and has good adhesion properties.
2, polyurethane adhesive for wood
With the rapid reduction of the world's forest resources and the implementation of China's natural forest resources protection project, small wood slabs require adhesive bonding strength and durability and weather resistance to be superior to the wood itself. The amount of adhesive used has become a measure of the level of technological development in the wood industry. In the past, most of the wood adhesives used were urea-formaldehyde resins, phenolic resins and melamine-formaldehyde resins with formaldehyde as the main raw material. However, due to the presence of free formaldehyde, formaldehyde gas was gradually emitted to the surroundings during use, causing environmental pollution. The wood processing industry has begun to look to the new environmentally friendly adhesive polyurethane glue to reduce environmental pollution.
The one-component moisture-curing polyurethane adhesive used in the woodworking industry is liquid and is used at room temperature. It is generally high in bond strength, flexibility and water resistance, and can be bonded to many non-wood substrates (such as textile fibers, metals, plastics, rubber, etc.). The one-component polyurethane adhesive exhibited better dry and wet strength in the test than the phenolic adhesive. Prior to bonding, the application of a methylol resorcinol (HMR) coupler to the surface of the bonded substrate improves the bond strength, and HMR enhances the bond strength of all thermoset wood adhesives. When the wood surface is pre-coated with HMR coupling agents, the strength and durability of the one-component polyurethane adhesive can meet most of the stringent test requirements.
3, polyurethane adhesive for shoes
China is a big country in shoemaking. After three generations of development of shoe adhesives, with the increase of global environmental awareness and the intensification of the oil crisis, the emergence of the fourth generation of environmentally friendly solvent-free and water-based shoe adhesives . In recent years, the preparation process of waterborne polyurethane has become increasingly mature. For the bonding of some low-polarity shoes such as SBS, the peel strength of polyurethane adhesives is not up to standard. By adding a tackifier resin or the like, it is possible to develop a polyurethane shoe adhesive having high crystallinity, high crystallization speed, high cohesive strength, and excellent peel strength.
4. Polyurethane adhesive for packaging
Flexible packaging, also known as soft can, has its unique comprehensive properties such as light weight, long shelf life, hygiene, easy storage and transportation, easy disassembly, low waste and good shelf effect. It has now surpassed hard packaging such as plastic and glass bottles. Cans, etc. Due to its excellent performance, polyurethane adhesives can bond film materials of different properties together to obtain composite films for flexible packaging which are resistant to cold, oil, chemicals, transparency and abrasion. At present, in the domestic and foreign markets, polyurethane adhesives have become the main adhesive for the processing of composite films for flexible packaging. In the domestic adhesive market, in the composite film manufacturing industry for packaging, the amount of polyurethane adhesive is second only to the footwear industry.
There are a wide variety of polyurethane adhesives for packaging, such as water-based polyurethane adhesives, hot melt polyurethane adhesives, solvent-based polyurethane adhesives, and solvent-free polyurethane adhesives. Among them, commonly used polyurethane hot melt adhesives can be divided into two categories, one is thermoplastic polyurethane elastomer hot melt adhesive, and the other is reactive hot melt adhesive. The main disadvantage of thermoplastic hot melt adhesives is their high viscosity, which has a great influence on the apparent quality of the coating. Reactive polyurethane hot melt adhesive is a new type of adhesive developed on the basis of traditional hot melt adhesive. It not only has the characteristics of good initial tack and post-cure performance of traditional hot melt adhesive, but also has the composition and structure of polyurethane. The advantage of large adjustment range is excellent adhesion to a variety of substrates.
In addition, in the use of water-based polyurethane adhesives for packaging, the use of emulsifiers or the introduction of hydrophilic ionic groups in the molecule reduces the water resistance. In recent years, research on improving the water resistance has become a hot spot. At the same time, due to the large heat capacity of water, how to increase its solid content and increase its drying speed is also one of the urgent problems to be solved.
5. Polyurethane adhesive for construction
In addition to being non-toxic, non-polluting, and easy to use, polyurethane adhesives also have the advantages that other adhesives cannot match, namely excellent low temperature resistance, solvent resistance, aging resistance, ozone resistance and bacteria resistance, and are applied in building paving materials. Play an important role. Widely used in elastic rubber mats, hard rubber floor tiles and plastic track sports fields.
The new two-component polyurethane adhesive breaks the contradiction between the shear strength and the peel strength of the traditional adhesive, so that both can achieve high strength at the same time, and exhibit excellent performance in the bonding of steel sheets for construction, and the bonding is firm and not easy to deform. And the room temperature adjustable curing speed makes the polyurethane glue easy to use and widely used. Polyurethane adhesives are widely used in construction PVC bonding, sandwich panel production and architectural waterproof coatings.
6, polyurethane adhesive for ink
Water-based inks are listed as the first choice for environmentally friendly printing. The room temperature cross-linking waterborne polyurethane adhesive can produce one-component polyurethane water-based ink suitable for gravure printing. The water-based ink has high light resistance, strong adhesion, dry adjustable, bright color, clear layer, non-toxic, non-combustible, weather resistant, sticky and easy to control. Compared with other water-based inks, it has wide adaptability to various substrate materials.
Polyurethane-polyacrylate is used as a binder for aqueous inks. In the drying process of ink film prints, the properties of polyurethane adhesives and polyacrylates are combined, and the modified emulsions have good adhesion to the films.
The base film used in the color printing industry mainly includes polyester film, nylon film, polypropylene film, polyethylene film, etc. Experts from Tianjiang Chemical reminded that the key to selecting ink is that it should have good adhesion to these base films. Focus on. Luoyang Tianjiang Chemical New Material Co., Ltd. uses a vinyl chloride-hydroxyethyl acrylate copolymer containing hydroxyl groups in the molecule to react with macromolecular diol, small molecular chain extender and toluene diisocyanate to produce a new polyurethane adhesive. Effectively improve the quality of color printing inks.
7. Polyurethane adhesive for book binding
In the past 20 years, the application of hot melt adhesives has been rapidly developed in bookbinding. The main reason is that the production of low-priced paperback books has grown tremendously, and at the same time, the wireless binding process has been produced. Due to the good adhesion and elasticity of EVA hot melt adhesives, the bookbinding hot melt adhesives currently used in the domestic market. Most of them are EVA type hot melt adhesives. However, EVA type hot melt adhesives are expensive, and EVA type hot melt adhesives have the disadvantages of memory. Polyurethane hot melt adhesive can make up for the above defects. The adhesive has simple production process and low cost. The adhesive has good elasticity and toughness, high bonding strength, and the adhesive can be repeatedly melted and used. The product has high utilization rate and has wider application prospects.
Due to its excellent bonding properties, polyurethane adhesives play an increasingly important role in the bonding of aerospace equipment, cultural relics protection and restoration, military industry, stationery, and medical care.
Fourth, the conclusion
Polyurethane adhesives are widely used in various fields due to their excellent properties, and are increasingly becoming important synthetic materials in human life. With the gradual expansion of polyurethane use and the increasing awareness of human environmental protection, the world's polyurethane adhesive industry is now adapting to environmental protection, safety and health, and resource recovery. China's polyurethane adhesive industry has also shown rapid growth, and technology development has made great progress.