Geometric changes of TMP fibres due to thermo-hydrolytic disintegration of waste MDF evaluated by three fibre analysers

Thermo-hydrolytic disintegration of medium-density fibreboard (MDF) is a promising method for recovering fibres. This process, however, may affect the geometry and size distribution of the applied fibres initially obtained by thermomechanical pulping (TMP), and thus, the properties of MDF produced thereof. Optical methods based on image acquisition are increasingly common for geometrical characterisation to assess fibre quality. In this study, we analysed the size distribution of recovered fibres (RF) obtained by thermo-hydrolytic disintegration of urea-formaldehyde-bonded waste MDF and subsequent hammer-milling by using three optical fibre analysers, FibreShape Pro, QICPIC and Valmet FS5, while comparing them to virgin fibres (VF). The analysers use different evaluation methods and therefore gave different absolute values. Thus, size distributions could not be directly compared. Nevertheless, the results provided an indication of the changes that occurred during the manufacturing and recycling process. The RF displayed a similar fibre geometry to the VF, in spite of process-related shortening. Mean fibre length and length-based distribution of the VF were always greater than those of the RF, while fibre width of the hammer-milled RF was slightly greater than that of the VF. The analysers, however, provided substantially different results, especially between FibreShape Pro and QICPIC, although the general tendencies observed for the process-related changes were consistent. It can be concluded that the thermo-hydrolytic disintegration applied to the UF-bonded MDF did not cause any substantial changes in fibre geometry (i.e. fibre shortening). All fibre analysers are capable of detecting these changes, but the absolute values provided can vary widely between the instruments.