abstract |
Polyolefin fibers containing at least 70% by weight of polypropylene fibers having a denier of at least 0.5 and a birefringence of at least 2.5 × 10 - 2 are formed into a sheet. The sheet is then frictionally calendered in the dry state at a frictional ratio of at least 15% and at a temperature in the range from 90°C to 160°C whereby an insulating paper having excellent air-impermeability and oil-resistance is obtained. |