TPU has outstanding abrasion resistance, bending fatigue resistance, and high elasticity, which can be used for various automotive parts. TPU contains many polar groups, which can easily absorb moisture in the air.
On the one hand, the absorbed water acts as a plasticizer, which will lead to a decrease in the physical properties of TPU, but this effect is reversible. If TPU is dried sufficiently, the absorbed water can be removed from TPU, and its physical properties can still be restored to the level before water absorption.
On the other hand, under certain conditions, the absorbed water will also play a degradation role, and this process is irreversible, which depends on the structure of the soft segment. The hydrolysis resistance of the polyether structure in the soft segment is stronger than that of the polyester structure. So the improvement of the water-resistance of TPU is mainly for the polyester type. Generally speaking, there are several methods as follows.
01 Change the structure of polyester
Increasing the length of the carbon chain of polyester can improve its hydrolysis stability. Introducing branched chains in the polyester structure, or introducing a rigid ring structure can also improve its hydrolytic stability.
-The introduction of a certain percentage of polyether in the polyester structure makes its water-resistance greatly improved.
-Controlling its acid value below 0.1 during the process of synthesizing polyester can also maintain its basic stability of performance.
02 Adding anti-hydrolysis stabilizer
The addition of carbodiimide or polycarbodiimide (PCD) to polyester TPU can improve its water resistance. The mechanism is that the carboxyl group contained in the polyester and the carboxyl group generated by hydrolysis react with carbodiimide or polycarbodiimide (PCD) to produce acylurea derivatives, so that the auto-catalytic effect of the carboxyl group is suspended. Because carbodiimide can reconnect the end carboxyl polymer formed by the broken chain, thus playing a role in repairing the chain.