Preparation and application of cationic blocked waterborne polyurethane as paper strength agent

Cationic blocked waterborne polyurethane (CBWPU) as paper strength agent was prepared by the reaction of N-[(1,1-dimethyl-2-acetyl)ethyl]-beta-dihydroxyethylamino propanamide(DDP), isophorone diisocyanate(IPDI), polycaprolactone glycol(PCL) and bisphenol A, with imidazole as blocking agent and N-methyldiethanolamine as hydrophilic chain-extending agent. Effects of the blocked ratio and the dosage of CBWPU on the physical properties of the treated paper were studied. The chemical structure of CBWPU and the surface morphology of the paper surface are characterized by Fourier transform infared spectrometer(FT-IR) spectra and scanning electron microscopy(SEM). The results show that the paper strength, especially the wet tensile index and the wet strength of paper can be obviously improved with the increasing of the blocked ratio and the dosage of CBWPU. FT-IR result shows that structures of tertiary amine group, imidazole ring and urethane group are found in CBWPU molecular structure. SEM obsevation of the treated paper reveals that the exiestence of a lot of filamentous materials and films, which can improve the crosslinking between fibers.