Properties and paper sizing application of waterborne polyurethane emulsions synthesized with TDI and IPDI

• WPU emulsions as surface sizing agents are synthesized with TDI and IPDI.
• Particle size increases as the NCO/OH and TDI/IPDI molar ratio increase.
• Viscosity increases as the NCO/OH molar ratio and DMPA content increase.
• Water resistance increases as the NCO/OH and TDI/IPDI molar ratio increase.
• The sized papers have much better mechanical properties than the unsized ones.

Waterborne polyurethane (WPU), used as surface-sizing agents, has received increasing concerns in the papermaking industry. In this work, using two-step synthesis techniques of pre-polymerization and chain-extending reaction, a series of new self-emulsifying WPUs have been obtained with toluene diisocyanates (TDI) and isophorone diisocyanate (IPDI) as the monomers, dimethylol-propionic acid (DMPA) as hydrophilic chain extender. The influences of the NCO/OH molar ratio, DMPA contents, and TDI/IPDI molar ratio on the physical properties of the resultant emulsions have been investigated in detail using various characterization methods. Furthermore, the emulsions synthesized have been used for paper surface sizing applications. After sized, the paper water resistance along with other properties has been improved greatly as compared with the unsized counterparts. In view of the paper sizing effect, well-performed WPUs may be synthesized with NCO/OH molar ratio of 1.6–1.8, DMPA of 5.0–7.0% and TDI/IPDI molar ratio of 5:1.

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