How to Improve the Performance of Waterborne Polyurethane

The properties of any polymer material are determined by its structure. The structure of polyurethane includes two aspects: chemical structure and aggregate structure. The chemical structure, i.e., the molecular chain structure, is a key factor to be considered in formula design at the initial stage of synthesis. The aggregate structure refers to the packing state of macromolecular segments, which is affected by molecular chain structure, synthesis process, service conditions, etc. Studying the influence of structural factors on properties provides a way to improve the performance of waterborne polyurethane. The specific impacts are as follows:

The soft segments of polyurethane elastomers mainly affect the elasticity of the material and contribute significantly to its low-temperature performance and tensile properties. Generally, polyester-based polyurethane elastomers have better physical and mechanical properties than polyether-based ones, while polyether-based polyurethane exhibits better hydrolysis resistance and low-temperature flexibility. Polyether soft segments have a lower glass transition temperature, thus enabling a wider range of low-temperature applications. The regularity of polyether or polyester soft chains can increase their crystallinity, thereby improving the tear resistance and tensile strength of the material, while also increasing the hysteresis of the polymer.

The hard segment structure basically consists of low-molecular-weight urethane or urea groups. The nature of these groups largely determines the intermolecular interactions of the elastomer and the physical crosslinking structure resulting from microphase separation and hydrogen bonding.

The structure of isocyanate raw materials plays a key role in the properties of polyurethane elastomers, mainly because their bulky volume causes significant intermolecular steric hindrance, endowing the material with high tear strength and modulus. Prolingheuer et al. conducted a comparative study on the properties of NDI/polyester/BDO polyurethane elastomers, confirming the existence of this effect. In addition, Schollenberger's research showed that the high symmetry of MDI gives the polymer a higher modulus.

Polyurethane elastomers are basically thermoplastic resins with linear molecular characteristics, but a certain degree of crosslinking can be introduced through polyfunctional chain extenders or urea groups. Appropriate crosslinking can improve the physical and mechanical properties of the material and enhance the water resistance and weatherability of polyurethane. However, some studies have shown that a high degree of crosslinking leads to a decrease in the modulus of polyurethane elastomers in the rubbery state. The reason is that crosslinking in the hard segment domains hinders the optimal packing of segments and reduces the content of glassy or paracrystalline domains.

The special properties of polyurethane stem from its distinct microphase separation structure. Hard segments of different macromolecular chains aggregate into crystalline domains, acting as physical crosslinks and improving the toughness, temperature resistance, and wear resistance of the system. The hard segment domains are combined with the soft segment matrix through hydrogen bonds and other forms, thus acting as active fillers, which is the source of the material's toughening. Many factors affect the microphase separation of polyurethane, including the polarity, molecular weight, chemical structure, composition ratio of soft and hard blocks, the interaction tendency between soft and hard segments, thermal history, and sample synthesis methods. There is also segment mixing in the separated microphases, leading to an increase in the glass transition temperature of soft segments and a decrease in that of hard segments, narrowing the service temperature range of the material and reducing its heat resistance.

Hydrogen bonds can form between hard segments and between hard and soft segments in polyurethane elastomers. At room temperature, approximately 75% to 95% of the NH groups in polyurethane molecules form hydrogen bonds. The role of hydrogen bonds is to enable polyurethane to withstand higher service temperatures and maintain the modulus in the rubbery state at elevated temperatures.

Andreas et al. believe that the destruction of hydrogen bonds may lead to a loss of mechanical strength, because hydrogen bonds promote the aggregation of hard chains and better combine soft and hard segments, acting as an endogenous source of physical crosslinking, thereby improving the elasticity and modulus of rubbery polyurethane.

Samules et al., on the other hand, argue that the role of hydrogen bonds lies in influencing the degree of microphase separation, thereby affecting the mechanical properties of polyurethane.

After decades of development abroad, the supporting raw materials for waterborne polyurethane are relatively complete in variety and stable in quality, with excellent product performance and obvious differentiation. At present, the performance of domestic waterborne polyurethane is still relatively single, with most products being combinations of MDI or TDI with PPG and DMPA, while other varieties are rare. The high product price and low demand have inhibited the research and application of waterborne polyurethane in China.

However, Julong Chemical Co., Ltd. upholds the philosophy of 'Advocating Technology, Professional Service' and has always been at the forefront of waterborne polyurethane research and development. Currently, Julong Chemical is one of the few enterprises in China capable of independently researching and developing waterborne polyurethane products. Facing the future, Julong Chemical firmly believes that only technology is the competitive weapon for chemical enterprises, only technology can optimize the structure of China's polyurethane products, and only technology is the only way to reverse the trade deficit of China's polyurethane products.