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Tuesday, November 6, 2012

Why Measure ERIC at 950 nm and NOT 700 nm?

ERIC (Effective Residual Ink Concentration) at 950 nm was introduced through "Measuring the Concentration of Residual Ink in Recycled Newsprint" in the June 1994 edition of the Journal of Pulp and Paper Science.  The authors Jordan (Paprican) and Popson (Technidyne) point out that at 950 nm "the lignin and dye have a negligible contribution and the ink dominates the spectrum."  

It would have been easier, even for Jordan and Popson to use 700 nm or some other reflectance point in the visible spectrum (400-700 nm). However, they point out that in the 1000 nm range, "The absorption is almost entirely from the ink.  The absporption coefficient of the paper is only about 0.2 m2/kg and the absorption coefficient of the ink is ten to one hundred times that great" You can see this in Figure 10, below.  It is in this range that the measurement gives a clear picture and reliable information.

Also, they point out the added complication of different dyes in wastepaper. "We have measured many dyed papers and have not found interferences in the 950 to 1000 nm range with the exception of black art paper. For example, Fig. 14 shows the reflectance spectra of two coloured fine papers with strong absorption up to 700 nm, but with negligible absorption at 950 nm." (below)

There are many instruments and research papers attempting to use 700 nm as the measurement point for residual ink, however, there is no research that shows this as an acceptable replacement for the accurate measurement at 950 nm.  Keep in mind at 700 nm other constituents (bleach, dye, lignin) still contribute toward the absorption.  Inaccurate estimation of the absorption in the measurement of residual ink will lead to poor conclusions. This will not adequately characterized the bleaching, deinking and other processes and how to use them most efficiently. The best ROI will not be achieved if the measurement is not done correctly.

There are existing standards: ISO 22754, TAPPI T567 and PAPTAC E.8 which have been in existence for years. They all use the 950 nm measurements.