Scanner-based dirt specks and stickies analysis - Results of a ZELLCHEMING round robin test

Based on the experience of a pre-trial done in 2006, the TECH (Recycling Technology) technical sub-committee of the RECO (Recovered Paper Utilisation) technical committee Of ZELLCHEMING started two round robin tests (RRT) to measure dirt specks and stickies during recovered paper treatment. The RRT started in April 2007 and involved 13 companies/institutes. The tests aimed to establish the present situation of dirt speck and sticky measurements especially regarding the absolute comparability of results, and to derive measures for improvement. The RRT were done using the measuring methods available at the participating companies/institutes (different measuring systems, different flat-bed scanners). Sample materials were provided by the technical sub-committee, and had been chosen or prepared on lob scale to cover all main process stages of recovered paper recycling for graphic paper production. Dirt speck analyses were performed on RK handsheets prepared from the recycled fibres obtained, and on recycled-fibre based paper samples taken from full-scale production lines. Sticky samples were prepared according to INGEDE Method 4. The RRT on dirt speck evaluation revealed strong correlations between the measurements of individual participants, but the absolute dirt speck levels determined by the 13 partners differed by +/- 100 % on average. The causes of these deviations were analysed in detail, with commercial scanner types being identified as the predominant influence here. The authors show that it is possible to adapt the dirt speck data of scanners via their software programs- provided there are no overdrive effects during scanning. However this makes it necessary to adjust each single scanner - a solution which is not really practicable in view of the relatively short service lives (between one and two years) of commercial office and administration scanners. A viable expedient is seen in the use of measuring scanners which may be adapted for each specific measuring job and ensure a high comparability of results within their device class. A scanner system meeting these demands - but which has not yet been optimised - was included in the studies. The results of comparative sticky measurements illustrate that the significantly different image data produced by UMAX/MICROTEK and EPSON scanners give rise to significant deviations in the sticky areas detected by the DOMAS system when using identical threshold files. Adapting these files has been found to minimise these differences, leading to a variation range of sticky areas of less than +/- 10 % (with few exceptions). Because contrasting the sticky particles with white corundum powder is known to have significant effects on the sticky area detected, we tried to minimise the corundum loss caused by the handling of samples in the RRT by renewed application thereof. In some cases, however this was even found to decrease the sticky area detected. This can be attributed to subjective factors, i.e. the person performing the test had removed too much of what he or she had thought to be excess corundum powder.