May 29, 2024 Longchang Chemical

Analysis of the development status of polyurethane adhesives

In recent years, with the rapid improvement of the level of material development, a variety of adhesives with stronger applicability have come out one after another, greatly enriching the adhesive market. Polyurethane adhesive is a medium-high grade adhesive with excellent flexibility, impact resistance, chemical resistance, abrasion resistance, and most importantly, low temperature resistance. By adjusting the raw materials and formulations, it is possible to design many types of polyurethane adhesives that are suitable for bonding between various materials and for different purposes.

Polyurethane adhesives were first applied in the military field in 1947, when triphenylmethane triisocyanate was successfully applied to the bonding of metal and rubber by Bayer, which was used on the tracks of tanks, laying the foundation for the polyurethane adhesive industry. Japan in 1954, the introduction of German and U.S. technology, the production of polyurethane materials in 1960, the production of polyurethane adhesives began in 1966, the development of successful vinyl polyurethane water-based adhesives, and in 1981 into industrial production.

In 1956, China developed and produced triphenylmethane triisocyanate (Lechner adhesive), and soon produced toluene diisocyanate (TDI), two-component solvent-based polyurethane adhesive, which is still the largest output 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 and their matching, thus promoting the domestic research units to accelerate the development of polyurethane adhesives. Especially after 1986, China’s polyurethane industry entered a period of rapid development. For a long time, the price of polyurethane adhesive in China is too high, and for quite a long time, the price of polyurethane adhesive is 1 to 2 times of neoprene adhesive. In recent years, the price of polyurethane adhesive is decreasing, and at present, the price of polyurethane adhesive is only about 20% higher than that of neoprene adhesive, which provides conditions for polyurethane adhesive to occupy the market of neoprene adhesive.
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 footwear in mainland China, among which there are more than twenty large-scale enterprises, such as Nanguang and Baili, which have a production capacity of more than 10,000 tonnes/year, and basically all of them adopt the resins imported from Spain, Germany, the United States and Taiwan of China for dissolution and production of polyurethane adhesive. In view of the large number of manufacturers, the competition is getting more and more intense, in order to have technical and price advantages, some large adhesive factories have begun to build the solution method of production of polyurethane adhesive devices, so the proportion of the output of polyurethane adhesive by the dissolution method will be reduced.
At present, the common polyurethane adhesive is solvent-based. Like neoprene adhesive, there are problems of toxicity and environmental pollution. In recent years, there are no ‘three benzene’ polyurethane rubber, that is, the use of ketones, esters and other mixed solvents, toxicity is reduced, but the problem has not been fundamentally resolved.
Polyurethane adhesive according to the reaction composition can be divided into polyisocyanate adhesive, polyurethane adhesive containing isocyanate base, polyurethane adhesive containing hydroxyl polyurethane adhesive and polyurethane resin adhesive. According to the use and characteristics of classification, can be divided into general-purpose adhesives, adhesives for food packaging, adhesives for shoes, paper-plastic composite adhesives, construction adhesives, structural adhesives, ultra-low-temperature adhesives, foam-type adhesives, anaerobic adhesives, conductive adhesives, fusible adhesives, pressure-sensitive adhesives, closed-type adhesives, water-based adhesives, and sealing adhesives, and so on.

‘Polyurethane adhesives are widely used in the production process of shoe materials, so what are the characteristics of polyurethane adhesives for shoes? And what factors will affect the performance of the adhesive?’
Polyurethane adhesive for shoes, like other polyurethane adhesives, has excellent low-temperature performance, low curing temperature, excellent flexibility, impact resistance, wettability and adhesion to many materials. Therefore, it is commonly used in applications requiring normal temperature, fast curing and flexibility, and is particularly suitable for bonding dissimilar materials with different coefficients of expansion.
The performance requirements of adhesive for shoe-making vary according to the different materials, shoe types and bonding processes, but as a whole, adhesive for shoes should have the following properties:
Sufficient adhesive strength for heterogeneous materials and different crystalline materials, and high peel strength;
High initial adhesion to adapt to the requirements of the shoe production line; especially high peel strength;
The sizing process is simple, easy to operate, the use period can be adjusted to adapt to the needs of the shoe production line;
It has moderate heat resistance, sufficient water resistance and adhesive durability.
Polyurethane adhesive for shoes can meet the above requirements, and its initial tackiness is worse than that of neoprene adhesive, but at present, researchers at home and abroad have put forward methods to improve the initial tackiness, which has been basically solved. Soft and modified PVC is one of the main materials of chemical shoes in recent years, polyurethane adhesive has the function of bonding such materials. It not only has the wide applicability of neoprene adhesive as well as excellent elasticity, softness and flex resistance, and has more than the latter better water resistance, grease resistance and heat resistance.
In addition, the polyurethane adhesive is capable of bonding shoe-making materials that are difficult for general adhesives, such as microcellular polyurethane elastomers, foams and synthetic leathers with excellent abrasion resistance, and cyano rubbers and thermoplastic rubbers (SBR, SBS, SIS, etc.) with excellent oil and gasoline resistance. These materials have been widely used as sole materials in the shoe-making industry, with highly malleable polyvinyl chloride or breathable polyurethane synthetic leather upper materials, to make beautiful, lightweight and comfortable shoes and boots.
In shoe making, the main factors affecting the performance of the adhesive are:
The effect of polyester varieties on adhesive performance. Polyadipic acid class polyurethane adhesive system, the crystallinity of the adhesive, the corresponding initial adhesion in the following order of increasing: ethylene glycol < L, 4-butanediol < 1, 6-hexanediol even number of carbon atoms diol synthesised polyurethane, the molecule’s hydrogen-supplying groups (such as NH-) and electron-supplying groups (such as -C-O-) is easier to approach. More hydrogen bonds can be formed more easily, making the polymer molecular chain easier to crystallise. Glycols with less than 4 carbon atoms are not suitable for use as shoe adhesives because of their poor initial tack strength. In view of the price factor of raw materials, generally choose polyadipic acid-L,4-butanediol as the main raw material of polyurethane adhesive for shoes.
Polyester molecular weight on the performance of adhesive. Molecular weight of different polyester adhesive made of adhesive properties have a great impact, the higher the molecular weight of the polyester, the greater the viscosity of the glue, but the polyester molecular weight is too high for the penetration of the glue performance and bonding performance is unfavourable, the selection of the appropriate molecular weight of the polyester is necessary to improve the bonding strength. Generally, 2000-3000 molecular weight polyester polyol (for polyadipic acid-1,4-butanediol).
Adhesive molecular weight on the adhesive properties. When the molecular weight of adhesive reaches about 100,000, the adhesive strength of adhesive is the highest, and the adhesive strength decreases obviously when it is lower than 90,000. When the molecular weight of 11-130,000 when the adhesive strength is not reduced much, but the stability of the glue is poor.
Isocyanate index on the molecular weight of the adhesive and the effect of storage isocyanate index R (i.e., NCO / OH molar ratio) on the molecular weight of the adhesive, storage stability have a direct impact. Ruler value is less than 0.8, the molecular weight of the adhesive has a small impact; ruler value in 0.85-0.95, the molecular weight of the adhesive has a certain impact; and value in 0.95-1.0, the molecular weight of the adhesive has a greater impact. In terms of storage stability of adhesive, the ruler value is less than 0.95 when the synthetic adhesive, whether it is the glue particles of the glue, they have good storage stability. The ruler value of 0.95-1.0 when the synthesis of the adhesive, the glue than the storage stability of the glue particles.
The effect of silica on adhesive properties. In the hydroxyl polyurethane glue after the dissolution of the particles of the glue, add 1.6%-2.0% of the gas-phase silica, the viscosity of the glue can be increased by 300-400mPa-s, and also improve the adhesive strength of the polyurethane adhesive and the initial adhesion.
The effect of chain extender on the performance of the glue. Adding chain extender can improve the content of urethane base, increase its polymerisation strength, polarity and activity, so that the polyurethane adhesive can form physical adsorption and chemical bonding with the adhered material, and increase the initial adhesive strength and heat-resistant property of polyurethane adhesive.
The influence of curing agent on adhesive performance. Curing agent usually use JQ-1 and JQ-4 and 7900, the general additive amount of 3%-10%. When JQ-l is used, the adhesive layer is dark in colour and will appear red, contaminating the upper. The use of 79m is slightly better, but also not ideal JQ-4 is preferred.

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