A New Approach to Measurement of Effective Residual Ink Concentration of Deinked Papers, 2007 TAPPI 8th Research Forum on Recycling

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The measurement of effective residual ink concentration (ERIC) in recycled papers is prone to errors. TAPPI Provisional Method T 567 pm-97 is based on application of the Kubelka Munk (KM) theory to diffuse reflection from papers measured once with the black backing and again with a thick backing of the same papers. At high opacities the two results become indistinguishable, leading to an undetermined value for ERIC when the full KM theory is applied. This is handled by use of an approximate value for the KM scattering coefficient to remove the ambiguity. However, the approximate value may be in error by 10%. An alternate approach avoids the uncertainty inherent in the approximation by applying the KM theory to the measurement of diffuse reflection and transmission in single sheets. The measurement is valid at any opacity for which the percentage transmission through the sheet is accurately determined in the near-infrared spectral region. Coefficients of variation (COV) are as low as or lower than those from the standard ERIC measurement throughout the range of interest. They decrease with increasing opacity to a low of 8% for a sheet having 1000 ppm ERIC, compared to a COV of 16% for the standard measurement based on an average value of scattering coefficient. The proposed method promises to be a superior way to monitor deinking in recycled papers.