How exactly can TPU’s water resistance be improved?
Release time:
13 Dec,2019
The TPU boasts outstanding wear resistance, flexural fatigue resistance, and high elasticity, making it suitable for a wide range of automotive components, such as shift lever handles, bushings and washers for various couplings, connectors for different wire harnesses, spiral retractable cables, cable jackets, toothed belts, protective sleeves and gaskets for steering tie rods, suspension joint hinges, hydraulic and pneumatic components, suspension diaphragms, shock-absorbing diaphragms, shock absorbers, spring stop blocks, and more. It can also be compounded with other plastics or rubbers for use in automotive parts like bumpers, airbags, and dust covers.
TPU contains numerous polar groups that readily absorb moisture from the air. The absorbed moisture acts as a plasticizer, which can lead to a deterioration in TPU’s physical properties. However, this effect is reversible: if the TPU is thoroughly dried, the absorbed water can be removed, and its physical properties can return to their original levels before moisture absorption occurred.
On the other hand, under certain conditions, TPU can also undergo degradation—a process that is irreversible and depends on the structure of the soft segment. Among soft segments, polyether structures exhibit stronger resistance to hydrolysis than polyester structures. Therefore, improving the water resistance of TPU primarily involves enhancing the water resistance of polyester-based TPU. Generally speaking, there are several approaches to achieve this:
I. Improve the polyester structure
Increasing the carbon chain length of polyesters can enhance their hydrolytic stability. Introducing branched chains or rigid ring structures into the polyester backbone can also improve their hydrolytic stability. For adipate-series polyesters, those synthesized using hexanediol or neopentyl glycol exhibit better water resistance than those synthesized using ethylene glycol or butanediol.
Since the soft segments of polyolefin glycols do not provide strongly electronegative elements necessary for hydrogen bonding, the TPU synthesized using these segments exhibits excellent hydrolytic resistance. However, due to the presence of double bonds in the molecular structure, its antioxidant resistance is relatively poor. TPUs synthesized using polycaprolactone diols and polycarbonate diols demonstrate better water resistance than those based on polycaprolactone diols, but their cost is significantly higher.
·The polyester synthesized by mixing adipic acid and terephthalic acid in a certain proportion exhibits good water resistance.
·Introducing a certain proportion of polyether into the polyester structure—whether through random copolymerization or block copolymerization—can significantly improve the water resistance of the resulting polyether ester.
·During the synthesis of polyester, maintaining its acid value below 0.1 can also help keep its performance essentially stable.
II. Add a hydrolysis-resistant stabilizer
Adding polycarbodiimide (PCD) to polyester-based TPU can improve its water resistance. The underlying mechanism is that the carboxyl groups present in the polyester, as well as those generated by hydrolysis, react with PCD to form acylurea derivatives, thereby halting the autocatalytic action of the carboxyl groups. Moreover, since each PCD molecule contains multiple carbodiimide units, it can re-link polymer chains terminated by carboxyl groups formed during chain scission, thus effectively repairing the chains. A commonly used hydrolysis-resistant agent is Stabaxol P200 (Bayer), at a dosage of 0.5–2.5 wt% based on the polyol content.
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