Comparison of test medium preparation methods for residual ink analysis

Abstract

ERIC (950 nm) and residual ink (700 nm) are commonly employed in paper recycling for estimating the amount of ink in pulp. Determination of ERIC and residual ink are assessed from the measured near-infrared (NIR) reflectance and scattering coefficient. Pads and sheets can be used for the determination, but when the opacity of measurement media exceeds 97%, the scattering coefficient cannot be measured and a constant value has to be used instead. This paper studies the feasibility of various test medium preparation methods for residual ink analysis. Studied methods consisted of an opaque pad filtered on paper, low grammage sheet filtered on wire screen, and low gram¬mage sheet filtered on high-retention filter paper (the most novel method). The comparison is based on measured retention, NIR scattering coefficient, reflectance, and residual ink values. Results showed that higher retention, scat¬tering, and residual ink values can be obtained when filter paper is used during sheet preparation. However, the opaque pad from which the measurement of scattering coefficient is prevented gives the lowest reflectance. A method involving sheet preparation on paper is seen as a good compromise for high retention of ink and fine materi¬al, while also enabling the measurement of scattering coefficient.