TPU boasts exceptional wear resistance, flex fatigue resistance, and high elasticity, making it suitable for various automotive components, including shift lever handles, shaft sleeves and washers for various couplings, connectors for various wire harnesses, spiral stretch wires, cable sheaths, timing belts, protective sleeves and shims for steering rods, suspension universal joints, hydraulic and pneumatic systems, suspension diaphragms, shock absorber diaphragms, shock absorbers, and spring limit blocks. It can also be used in combination with other plastics or rubbers for parts like car bumpers, airbags, and dust covers.
TPU contains many polar groups, making it highly absorbent of moisture from the air. The absorbed moisture acts as a plasticizer, which can degrade the physical properties of TPU. However, this effect is reversible. By thoroughly drying the TPU, the absorbed moisture can be removed, restoring its properties to the pre-moisture level.
On the other hand, under certain conditions, it also exhibits degradation properties, a process that is irreversible and depends on the structure of the soft segments. The polyether structure within the soft segments has stronger hydrolysis resistance than the polyester structure, so improving the water resistance of TPU primarily involves enhancing the water resistance of polyester TPU. Generally, there are several methods for this:
I. Enhance Polyester Structure
Increasing the carbon chain length of polyester can enhance its hydrolytic stability. Introducing branching chains or rigid ring structures into the polyester structure can also improve its hydrolytic stability. For adipic acid series polyesters, those synthesized with adipic acid or neopentyl glycol have better water resistance than those synthesized with ethylene glycol or butylene glycol.
Due to the absence of strong electronegative elements required for hydrogen bonding in the polyether soft segments, the synthesized TPU boasts excellent hydrolysis resistance. However, due to the presence of double bonds in the molecule, its antioxidant properties are relatively poor. TPU made from polycaprolactone diols and polycarbonate diols has better water resistance than that from polyester series, but at a significantly higher cost.
The polyester synthesized from a mixture of adipic acid and terephthalic acid in a certain ratio exhibits good water resistance.
2. Introducing a certain proportion of polyether into polyester structures can significantly enhance water resistance, whether through random copolymerization or block copolymerization to form polyether esters.
During the process of producing polyester, maintaining its acid value below 0.1 can also ensure its basic stability.
Added anti-hydrolysis stabilizer
Adding polyimide (PCD) to polyester TPU improves its water resistance. The mechanism involves the reaction of carboxyl groups contained in the polyester and those produced by hydrolysis with PCD, forming acylurea derivatives, which stops the autocatalytic action of the carboxyl groups. Additionally, due to each PCD molecule containing multiple dicyandiamide groups, it can reconnect the end carboxyl polymer chains formed by chain breakage, thus repairing the chains. The commonly used antihydrolysis agent is Stabaxol P200 (Bayer), with a dosage of 0.5-2.5 wt% of the polyol content.
