Enhancement of mechanical and electrical performances of insulating presspaper by introduction of nanocellulose

To enhance the insulating performance of presspaper used in power transformers, unmodified nanofibrillated cellulose (UNFC), cationic nanofibrillated cellulose (CNFC), anionic nanofibrillated cellulose (ANFC) and cellulose nanocrystals (CNC) were investigated as the potential nano-additives. Concentration of the nano-additives was 10 wt%. Functional groups and crystallinities of the obtained samples were characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). Tensile strength, DC volume resistivity, dielectric response and breakdown behavior of the nanomodified and conventional presspaper were analyzed. Results show that nanofibrillated cellulose (NFC) can remarkably enhance tensile strength of insulating presspaper. Compared to ANFC, CNFC has a greater improvement which is mainly ascribed to the electrostatic attraction between the positively charged NFC and negatively charged softwood fiber. In addition to mechanical strength, introduction of nanocellulose can also obviously improve both AC and DC breakdown performances. The reason can be attributed to the reduction of partial discharges inside the composites due to the presence of nanocellulose in the original air voids. Moreover, and importantly, the negative effect of nanocellulose on DC volume resistivity and dielectric response can be neglected. On the basis of the overall performance, we conclude that NFCs are promising nano-additives for insulating presspaper, especially for CNFC.