Birefringence of softwood mechanical pulp fines

Mechanical pulp fines which originate from the cell wall are birefringent while fines originating from the middle lamella are mostly composed of lignin and are not birefringent. A simple microscopic procedure has been developed to measure the fraction by number of birefringent material in a fines population. This proportion can vary by almost a factor of two between white water samples from various newsprint mills and can vary by the same amount with energy in RMP fines. Since fibre length analyzers like those of the Kajaani FS series detect particles by their birefringence, a large and variable proportion of mechanical pulp fines will not be detected by these instruments.