Polyester-based and polyether-based waterborne polyurethane resins: a comparison of heat resistance performance

From a chemical perspective, waterborne polyurethane resin is essentially a-viscosity liquid composed of uniformly dispersed gel particles in water. During the of chain growth, the viscosity of the emulsion generally maintains a state of equilibrium, and its variation mainly comes from the increase in molecular weight of the particles themselves. In practical applications, when the temperature for film formation exceeds the melting point of the polymer particles, a uniformly distributed continuous film layer is formed between the particles. In the case of lower temperatures, the dried coating exhibits a discontinuous state of particle-to-particle adhesion.

When discussing the heat resistance of polymers, two important temperature indicators must be mentioned: the softening temperature and the thermal decomposition temperature. The softening temperature, as the name suggests, refers to the critical temperature at which the polymer transitions from the elastic state to the viscous flow state, i.e., the lowest point at which the polymer chains start to slide. Deformation that occurs at this temperature is irreversible. This temperature not only determines the range of processing through molding for the polymer but also sets the temperature limit for the use of polymer products. The thermal decomposition temperature is the lowest temperature at which chemical bonds in the polymer break during heating, and the long-term operating temperature of polymer products should be lower than this temperature. It is worth noting that the relationship between the thermal decomposition temperature and the softening temperature is not fixed and can be higher or lower than the softening temperature. For waterborne polyurethane, the thermal decomposition temperature is usually lower than the softening temperature, and the thermal decomposition process often intertwines with other degradation processes such as oxidation and hydrolysis, mutually promoting each other.

The thermal decomposition temperature of waterborne polyurethane emulsion is greatly influenced by the heat resistance of various functional groups in its macromolecular structure. For example, the thermal decomposition temperature of dimer urea and urethane methacrylate is significantly lower than that of urethane and urea. According to literature records, the thermal decomposition temperature of dimer urea is 120°C, while the decomposition temperature of urethane methacrylate is only 106°C. The thermal decomposition temperature of urethane is closely related to the structure of its parent compound, with aliphatic diisocyanates generally having better heat resistance than aromatic diisocyanates, and aliphatic alcohols having better heat resistance than aromatic alcohols (such as phenol). In aromatic diisocyanates, the order of heat resistance is usually PPDI > NDI > MDI > TDI.

In addition, there are significant differences in the thermal decomposition temperature of urethane-acrylate obtained from the reaction of different structures of fatty alcohols with the same isocyanate. Among them, the primary alcohol has the highest thermal decomposition temperature, while the tertiary alcohol has the lowest, and some may even start to decompose at 50°C. This is mainly because the bonds near the tertiary carbon atom and quaternary carbon atom are more fragile and prone to breakage. The structure of the soft segment also affects the thermal decomposition temperature. Due to the good thermal stability of carbonyl groups and the susceptibility of hydrogen on the α-carbon atom of ether groups to oxidation, polyester-based materials usually have better resistance to thermal air aging than polyether-based materials. In addition, the presence of double bonds in the soft segment will lower the heat resistance of the elastomer, while the introduction of isocyanurate rings and inorganic elements can effectively improve the heat resistance of the elastomer. Polyester polyols generally have better heat degradation performance than polyether polyols due to the larger intermolecular forces between the molecules. The thermal stability of polymers is significantly enhanced by the presence of silicon-oxygen bonds due to their high bond energy characteristics. Inorganic materials are often used to enhance the heat resistance of polymers due to their excellent thermal stability and mechanical strength.