What Is Urethane Rubber?
Urethane rubber is a polymer with urethane bonds and is a type of synthetic rubber with excellent elasticity and rigidity (Figure 1).
It has the advantage of long-term use, especially in mechanical strength and wear resistance. The properties of urethane rubbers vary greatly depending on the composition of the material and the bonding method.
There are two main types of urethane rubbers: polyester urethane rubbers (AU), which have ester bonds in the main chain, and polyether urethane rubbers (EU), which have ether bonds in the main chain.
AU has ester bonds and is easily hydrolyzed, but it has excellent oil resistance and mechanical strength.
EU, on the other hand, is inferior to AU in terms of oil resistance and mechanical strength, but is resistant to hydrolysis and has excellent water resistance.
Applications of Urethane Rubber
Urethane rubber is used for rolls, tires, conveyor belts, and soles of sports shoes. A typical example that takes advantage of its abrasion resistance is belt compares used in factories.
However, care must be taken when using it in acidic or high humidity environments, as it will deteriorate. The high elasticity of the material is also used for the soles of sports shoes.
The reason for this wide variety of applications is that the elasticity and other properties can be freely changed depending on the composition of the material and the manufacturing method.
Causes and Countermeasures for Urethane Rubber Degradation
Causes of urethane rubber degradation in the natural environment include light, heat, oxygen, water, and microorganisms. Among these, degradation due to light and heat is decomposition by oxidation reaction, which causes cleavage of urethane bonds and degradation of physical properties.
To counteract this degradation, the addition of UV absorbers or antioxidants is effective. The next example of water-caused degradation is hydrolysis of urethane bonds, and polyester urethane rubbers are more susceptible to degradation than polyether urethane rubbers.
In the case of polyester systems, ester bonds are also subject to degradation due to hydrolysis. This type of degradation can be prevented by adding carbodiimide compounds.
Microbial degradation caused by fungi is a typical example. It is known that urethane rubber is more susceptible to microbiological degradation than other high-molecular compounds such as plastics.
Also, in this type of degradation, this is true for polyester urethane rubber, which is more susceptible to degradation than polyether urethane rubber.
The reported mechanisms include hydrolysis of ester bonds by esterases produced by microorganisms.
Principle of Urethane Rubber
Types and Characteristics of Urethane Rubber
Urethane rubbers are used in a wide variety of applications, and their properties can also be controlled by manufacturing methods.
The synthesis method uses a polyaddition reaction between polyol, represented by glycol, and diisocyanate. The following three manufacturing methods are mainly employed.
Mirable type: This is one of the most common manufacturing methods used to process synthetic rubber. After generating a dough from raw materials, it is poured into a mold and pressed to form.
This is a very simple method that allows for high production efficiency. On the other hand, it has disadvantages such as susceptibility to hydrolysis and poor material properties.
Thermoplastic type: This is a manufacturing method that applies a resin processing method and is also called the injection type. After heating and melting urethane rubber pellets, they are injected into a mold and molded. This method has the advantage of low-cost production, but the manufacturing equipment is expensive, so mass production is a prerequisite for this method.
Castable urethane type: Liquid urethane rubber is mixed with a curing agent and vulcanized in the mold. Vulcanization cross-links urethane rubber molecules to obtain high elasticity.
After one day of this process, the product is molded and manufactured. Many companies have introduced this method because the manufacturing facilities are inexpensive and complex shapes can be processed at the same time. However, manufacturing efficiency is low because of the time required for production.