How to improve the water resistance of TPU?

TPU has outstanding wear resistance, bending fatigue resistance, and high elasticity. It can be used in a variety of automotive parts, such as for shift lever handle, a variety of coupling bushings and washers, a variety of wire harness connectors, spiral expansion wire, cable sheathing, toothed belt, steering tie rod sheathing and gaskets, suspension joint hinges, hydraulic pneumatic, suspension diaphragm, shock absorbing diaphragm, shock absorber, spring limit block, etc. It can also be used with other plastic or rubber compound for car bumpers, airbags and dust cover, etc.

TPU contains many polar groups, which can easily absorb the moisture in the air. On the one hand, the absorbed water acts as a plasticizer and causes the physical properties of TPU to decrease, 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 the water is absorbed.

On the other hand, under certain conditions, it 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 main way to improve the water resistance of TPU is to improve the water resistance of polyester-type TPU, generally speaking, there are several methods as follows.

01 Adding anti-hydrolysis stabilizer

The addition of carbodiimide (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 reacts with PCD to produce acylurea derivatives, thus suspending the auto-catalytic effect of the carboxyl group. What’s more, because each PCD molecule contains multiple carbodiimides, it can reconnect the end carboxyl polymer formed by the broken chain, thus playing the role of chain repair. The commonly used anti-hydrolysis agent is HyMax 1010 or HyMax 220 (liquid). The dosage is 0.5-2.5wt% of the polyol amount.

02 Change the structure of polyester

Increase the length of the carbon chain of polyester, which can make its hydrolysis stability improved. Introduce branched chains in the polyester structure, or introduce rigid ring structure. Can also improve its hydrolytic stability. For adipic acid series polyesters, polyesters synthesized with hexanediol or neopentyl glycol have better water resistance than polyesters synthesized with ethylene glycol or butanediol.

Since the soft segment of polyolefin diol does not provide the strong electronegativity element required to form hydrogen bonds, the synthesized TPU has excellent hydrolysis resistance and poor oxidation resistance due to the presence of double bonds in the molecule; TPU synthesized using polycaprolactone diol and polycarbonate diol has better water resistance than the polyadipate series, but the cost is much higher.

-The polyester synthesized by using a mixture of adipic acid and terephthalic acid in a certain ratio has better water resistance.

-The introduction of a certain proportion of polyether in the polyester structure can be random copolymerization or block copolymerization into polyether esters can make a great improvement in water resistance.

-The process of synthesizing polyester, control its acid value below 0.1, can also maintain its performance is basically stable.

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