In the previous article, we discussed the characteristics of “TPU,” its merits and demerits, and applications of TPU in a wide range of industries. In this article, we will explain the types of TPU and the factors that cause “weak to moisture (hydrolysis)” and “yellowing (yellowing),” which are cited as disadvantages.
What is the difference between ester-based TPU and ether-based TPU?
First, let’s take a look at the different types of TPU, which are mainly classified into ester TPU and ether TPU according to the type of chemical bond (molecular structure). These are based on the type of polyol used as a raw material. Both types have similar characteristics, but their properties are slightly different. The following is a summary of the differences between ester-based TPU and ether-based TPU.
What is Ester-based TPU?
Ester-based TPU uses polyester polyol as raw material. It has the advantage of superior mechanical strength (mechanical properties), abrasion resistance, heat resistance, oil resistance, and lower manufacturing costs. On the other hand, it has lower resistance to hydrolysis.
What is Ether-based TPU?
Ether-based TPU is made from ether-based polyol. It has excellent hydrolysis resistance, low temperature resistance, and mold resistance, but is inferior to ester TPU in terms of mechanical strength.
Taking advantage of its extremely excellent abrasion resistance and shock absorption properties, ester-based TPU is used in a wide range of fields, from automobile parts to shoe sole parts, smartphone cases, and LCD protective films. Ether-based TPU also has excellent mold resistance and is relatively resistant to acidity and alkalinity, making it useful in the medical field. Each TPU is used in places that take advantage of its unique characteristics.
Disadvantages of TPU, hydrolysis and yellowing (yellowing)
Now, let’s take a closer look at the disadvantages of TPU, namely, hydrolysis and yellowing, which can occur not only with TPU but with any product made of polyurethane. The reason for this is the molecular structure of polyurethane. The molecular chains of polyurethane have a weak adhesive force, and the molecules tend to separate when exposed to external influences such as light (ultraviolet rays) and oxygen. The thermal dissociation temperature of polyurethane is also low, making it susceptible to degradation when exposed to heat. These degradation reactions appear as hydrolysis and yellowing.
Relationship between TPU, hydrolysis, and moisture
In the explanation of ester-based TPU and ether-based TPU, we wrote that “ether-based TPU has superior hydrolysis resistance.” Why is this difference? Let’s look at it using the bonding molecules and chemical formulas of both.
Ester TPU contains a bonding molecule (-COO-) in its structure. When the bonding molecule (-COO-) in ester TPU reacts with water (H₂O), it breaks down into an acid (-COOH) and an alcohol (-OH). Even if TPU and water do not come into direct contact, hydrolysis can occur when TPU reacts with moisture in the air.
Chemical formula
-COO- (ester bond) + H₂O (water) → -COOH (acid) + –OH (alcohol)
Ether-based TPU, on the other hand, contains a bonding molecule (-O-) in its structure, but this molecular structure is less affected by water, making hydrolysis less likely to occur.
TPU and UV
Let’s look at another weak point of TPU, yellowing, which is caused by light (UV), air, and (chemical) agents. Of these, light (UV) is probably the most susceptible. Generally, TPU is composed of two main raw materials: MDI (diphenylmethane isocyanate, hereafter MDI) and polyester polyol, as explained earlier. When this MDI is exposed to ultraviolet light, it undergoes an oxidation reaction, which causes yellowing. Light has “bond dissociation energy,” and this bond dissociation energy causes the bonds in TPU to break or separate, which is the cause of the yellowing.
In this article, we have explained ester-based and ether-based TPUs and the reasons why TPUs cause “hydrolysis” and “yellowing. However, there is no doubt that the resistance of TPU to hydrolysis and yellowing has been improved through the composition and blending of raw materials. For example, it has been proven that hydrolysis resistance can be improved by adding stabilizers to raw materials, and resistance to yellowing can be increased by changing polyol raw materials.
TPU is a material that can be further utilized in various combinations. We will continue to post stories related to TPU in the future.
