“In the production process of shoe materials, polyurethane adhesives are widely used. So what are the characteristics of polyurethane adhesives for shoes? And what factors will affect the performance of the adhesives?”

“In the production process of shoe materials, polyurethane adhesives are widely used. What are the characteristics of polyurethane adhesives for shoes? And what factors will affect the performance of the adhesives?”
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 often used in situations requiring normal temperature, rapid curing and flexibility, and is especially suitable for bonding dissimilar materials with different expansion coefficients.
The performance requirements of adhesives for shoemaking vary depending on shoe materials, shoe types and bonding processes, but overall, shoe adhesives should have the following properties:
It has sufficient bonding strength for dissimilar materials and materials with different crystallinity, and the peeling strength should be high;
The initial bonding tack is high, adapting to the requirements of the shoemaking production line; especially the peeling strength must be high;
The sizing process is simple, easy to operate, and the service life is adjustable, adapting to the needs of the shoemaking production line;
​Has moderate heat resistance, sufficient water resistance and bonding durability.
Polyurethane adhesive for shoes can meet the above requirements, but its initial tack performance is worse than that of chloroprene adhesive. However, researchers at home and abroad have proposed methods to improve the initial tack, which has been basically solved. Soft and modified polyvinyl chloride is one of the main materials for chemical shoes in recent years, and polyurethane adhesive has the function of bonding such materials. It not only has the wide applicability and excellent elasticity, softness and flex resistance of chloroprene adhesive, but also has better water resistance, grease resistance and heat resistance than the latter.
In addition, polyurethane adhesives can bond shoemaking materials that are difficult for general adhesives, such as microporous polyurethane elastomers, foam materials and synthetic leather with excellent wear resistance, cyano rubber with excellent oil and gasoline resistance, and thermoplastic rubber (SBR). , SBS, SIS, etc.). These materials have been widely used in the shoemaking industry as sole materials. They are combined with polyvinyl chloride with high plasticity or polyurethane synthetic leather upper materials with good breathability to make beautiful, lightweight and comfortable shoes and boots.
​ In shoemaking, the main factors affecting the performance of adhesives are:
The influence of polyester varieties on glue properties. In the polyadipic acid polyurethane adhesive system, the crystallinity and corresponding initial viscosity of the adhesive increase in the following order: ethylene glycol < L, 4-butanediol < 1, 6-hexanediol synthesized from diol with even number of carbon atoms. In polyurethane, the hydrogen-donating groups (such as NH—) and electron-donating groups (such as -C-O-) in the molecule are relatively easy to access, and can easily form more hydrogen bonds, making the polymer molecular chain easier to crystallize. Ethylene glycol with less than 4 carbon atoms is not suitable for use as an adhesive for shoemaking due to poor initial bonding strength. In view of the price factor of raw materials, polyadipic acid-L, 4-butanediol is generally selected as the main raw material of polyurethane adhesive for shoes.
The influence of polyester molecular weight on glue properties. Adhesives made of polyesters with different molecular weights have a great influence on the bonding performance. The higher the molecular weight of the polyester, the greater the viscosity of the glue. However, if the molecular weight of the polyester is too high, it will be detrimental to the penetration and adhesive properties of the glue. Selection of appropriate polyester molecular weight is necessary to improve bonding strength. Generally, polyester polyols with a molecular weight of 2000-3000 (for polybutylene adipate-1,4-diol) are used.
The influence of adhesive molecular weight on adhesive properties. When the molecular weight of the adhesive reaches about 100,000, the adhesive strength is the highest. When the molecular weight is less than 90,000, the adhesive strength decreases significantly. When the molecular weight is 110,000-130,000, the adhesive strength does not decrease much, but the stability of the glue is poor.
The effect of isocyanate index on the molecular weight and storage stability of the adhesive. The isocyanate index R (ie, the molar ratio of NCO/OH) has a direct impact on the molecular weight and storage stability of the adhesive. A ruler value of less than 0.8 has little impact on the molecular weight of the adhesive; a ruler value of 0.85-0.95 has a certain impact on the molecular weight of the adhesive; and a value of 0.95-1.0 has a greater impact on the molecular weight of the adhesive. Regarding the storage stability of adhesives, the adhesives synthesized when the scale value is less than 0.95 have good storage stability regardless of whether they are glue particles or liquid. The adhesive synthesized when the ruler value is between 0.95 and 1.0 has better storage stability than glue particles.
The effect of white carbon black on glue properties. Adding 1.6%-2.0% fumed silica to the glue after the hydroxyl polyurethane particles are dissolved can increase the viscosity of the glue by 300-400mPa·s. It also improves the bonding strength and initial tack of the polyurethane glue.
The influence of chain extenders on glue properties. Adding a chain extender can increase the content of the urethane group, increase its cohesive strength, polarity, and activity, so that the polyurethane adhesive can form physical adsorption and chemical combination with the adhered material, and increase the initial bonding strength and heat resistance of the polyurethane adhesive.
The effect of curing agent on glue properties. Curing agents usually use JQ-1, JQ-4 and 7900, and the general addition amount is 3%-10%. When using JQ-l, the glue layer is dark in color and will appear red, contaminating the upper. It is slightly better to use 79m, but it is not ideal. It is best to use JQ-4.