Thermoplastic polyurethane (TPU) is first polymer material discovered that has both elasticity of rubber and thermoplasticity of plastic. TPU has excellent physical and mechanical properties, high tensile strength and elongation, good wear resistance, so it is often used in manufacture of shoe soles and cable sheaths, it also has excellent tear resistance, and is elastic in temperature range.
Abrasion resistance, tear resistance, tensile strength and tear strength of polyester TPU are higher than polyester TPU, while polyester TPU is suitable for resistance to hydrolysis, microbial degradation and low temperature. The method has characteristics of two at same time, and performance is more excellent. It can be used to produce fire hoses, cable sheaths, films, etc.
1. Hydrolysis resistance
In process of obtaining TPU based on carboxylate polyester from polybutylene adipate diol, minor chains end with unreacted carboxyl groups (-CO2H) and are introduced into middle of diol component. When carboxylate polyester TPU is hydrolyzed, chain scission occurs in carboxylate (-COO-) chain to form some new chains bearing carboxyl ends. This carboxyl group, formed from two sources, autocatalyses further hydrolysis of TPU and accelerates this degradation process.
The polycarbodiimide structure has been proven to be a very effective hydrolysis stabilizer for polyether urethane. Its function is to react with resulting carboxyl groups to neutralize and repair broken hydrolysis chains in polymer.
Effect of adding polycarbodiimide (PCD) on hydrolytic stability of TPU
Because of their ability to react and neutralize carboxyl groups, polyepoxides can also be used as polyurethane hydrolysis stabilizers and can also be used to improve open chain TPU.
Lactone polyester TPUs made from poly(ε-caprolactone)diols are very similar in stability and stabilization to carboxylate polyester TPUs. In fact, macromolecular diol bond in polycaprolactam (PCL) is a carboxylate bond, but macromolecular diol is obtained by polymerization of lactone instead of condensation of dicarboxylic acid with diol.
For example, polyester-based poly(hexamethylenecarbonate)glycolcarbonate TPU has excellent resistance to hydrolysis, and ester bonds [such as carbonate bonds (-0-C0-0-)] are formed on broken chains with hydroxyl terminals and carbon gaseous dioxide, but non-terminal carboxyl for further hydrolysis autocatalyst.
Polytetramethylene glycol and TPU polyester have obvious hydrolytic stability, since ether bond of macromolecular glycol is very difficult to hydrolyze. In fact, urethane bond becomes most sensitive bond as far as hydrolysis of polyether urethane is concerned.
Another important factor that determines hydrolytic stability of TPU is degree of hydrophobicity and water permeability of TPU chain. Therefore, more hydrophobic TPU (for example, high hardness TPU), less water it absorbs, which is more resistant to hydrolysis.
2. UV resistance
The most important factor affecting aging of TPU in atmosphere is ultraviolet radiation with a wavelength of 330~410 nm. This energy, accompanied by solar radiation, initiates autoxidative degradation process of TPU, which causes intense chemical crosslinking of chains, making TPU brittle and insoluble, especially TPU aromatic urethane. Also, during this process, aromatic urethane TPU produces a pronounced yellow-brown color, while aliphatic urethane TPU has a more stable color.
It is believed that UV degradation process leads to formation of a quinoneimine structure in an aromatic carbamate containing a proquinone structure, and then a portion of active hydrogen can be added to the quinoneimine to form a cross-linked TPU chain.
There are still different views on UV degradation process of this aromatic carbamate, including photo-Fies rearrangements (photo-Fies rearrangements) as shown below to explain TPU discoloration and chain crosslinking.
The above two formulas of chemical reactions cannot be obtained in aliphatic urethane TPU, since latter does not have a quinone structure. In fact, a comparison of two structures shows that aliphatic urethane TPU exhibits better UV resistance than aromatic urethane TPU.
Undoubtedly, regardless of structure of carbamate groups in TPU, some degree of hydroperoxidation of TPU chains can also occur during UV-induced autoxidation. Based on nature of UV degradation process of TPU, UV absorber [2-(2'-hydroxy-3',5'-di-tert-pentylphenyl)benzotriazole] and antioxidant [tetra(sub-methyl-3(3' , 5'-di-tert-butyl-4'-hydroxyphenyl) propanoic acid) methane] provides a particularly effective UV stabilization system for TPU. In addition, a small amount of carbon black alone can shield UV light, and it is a very effective UV stabilizer for TPU.