What is polyurethane adhesives: pros&cons, applications

1st March 2024

Adhesives are one of the six major types of polymer materials (adhesives, coatings, plastics, rubber, fibers, and polymer-based composites). According to their chemical composition, adhesives are mainly divided into organosilicone adhesives, epoxy resin adhesives and polyurethane adhesives.

What is Polyurethane Adhesives?

Two-component polyurethane adhesive is one of the most important categories within polyurethane adhesives. Typically, it consists of two components, referred to as part A and part B, which are separately packaged and need to be mixed in certain proportions before use. Part A (the main component) contains hydroxyl groups, while Part B (the curing agent) contains components with free isocyanate groups.

Advantages of Polyurethane Adhesives

(1) Possessing higher reactivity, polyurethane adhesives can form covalent bonds with many surfaces containing active hydrogen atoms, such as metals, rubber, fibers, wood, leather, plastics, etc. After curing, they contain urethane, urea, and strongly polar bonds and groups, facilitating secondary bonding with substrates. The combined action of chemical and physical adhesion results in high bonding strength between adherents.

(2) Typical polyisocyanate compounds have low molecular weights, making them soluble in most organic solvents. This property allows them to easily spread onto substrate surfaces and penetrate into porous substrates, thereby enhancing adhesive performance.

(3) This type of adhesive can cure at room temperature or under heating conditions, readily forming cross-linked structures, exhibiting good heat resistance and solvent resistance.


Disadvantages of Polyurethane Adhesives

(1) The free monomers of isocyanates in polyurethane adhesives are highly toxic. Epoxy-based polyurethanes can be synthesized by reacting hydroxyl-terminated polyols with isocyanate-terminated polyurethane prepolymers, which are then formulated with polyamine compounds to produce polyurethane adhesives.

(2) Polyurethane adhesives typically cure slowly at room temperature, sometimes taking several days for complete curing. However, using multi-component formulations can significantly shorten the room temperature curing time, making them more convenient to use.

(3) Due to their low molecular weight and high NCO group content, the cured adhesive layer tends to be hard and brittle. Therefore, they are often modified using rubber solutions, polyethers, polyesters, etc., or used as cross-linking agents for various adhesives.

What is polyurethane adhesive used for?

Polyurethane adhesives are used to bond various materials and maintain their physical and chemical properties even at low or ultra-low temperatures after bonding. Polyurethane adhesives typically form a strong bond with a variety of materials, including metals, plastics, wood, and composites. They are known for their excellent resistance to heat, chemicals, and environmental factors.

Doxu provides polyisocyanate crosslinkers for adhesive in footwear. 

SA-1003 Isocyanate for adhesives

NCO Content% 7.2±0.3
Non-volatile content approx.40
Density at 20’C approx.1.04
Viscosity at 23C approx.11
Solvent Ethyl acetate
Flash point -5
Monomeric content <0.10