Publication II Lehto , J . H . 2004 . Characterisation

Basic fiber properties can be classified into four categories: Size distribution, shape, structure of the cell wall and the fiber surface. This approach was used when long fiber fractions separated from 10 different mechanical pulps and unrefined and refined chemical pulp samples were compared in order to shed light on the different reinforcing abilities of chemical and mechanical pulp fibers. The long fiber fractions were analyzed using a variety of fiber analyzers and testing methods. The commercial analyzers for determining the size distribution and shape of the fibers were the FS-200, MorFi, FiberMaster and CyberSize analyzers. In addition, light microscopy was used for several purposes. The structure of the cell walls was studied using the CyberFlex analyzer (flexibility was analyzed using the Steadman method) and the FSP (fiber saturation point), FBW (free bound water), NFW (nonfreezing water) and WRV (water retention value) were analyzed. The internal fibrillation was evaluated using Simons staining (light microscopy). The zero-span method was used for fiber strength analysis and the RBA (relative bonded area) was determined using the a CyberBond analyzer. The chemical composition (lignin and extractives coverage) of the fiber surfaces was analyzed using ESCA. The results indicate that the most essential differences between chemical and mechanical pulp fibers can be found in the cell wall structure rather than in the fiber dimensions. The cell walls of chemical pulp fibers are more porous and internally fibrillated and, as a result, more flexible, conformable and swollen than those of mechanical pulp fibers. They are also stronger when analyzed using the zero-span method. In addition, surface lignin or extractives do not limit bonding as in the case of mechanical pulps. Thus, chemical pulp fibers can be more active in sheet forming and consolidation than mechanical pulp fibers. The possibilities for modifying the properties of mechanical pulp fibers in order for them to perform more like chemical pulps will be studied in the further research. INTRODUCTION Wood-containing papers are usually reinforced with chemical pulp fibers in order to ensure that the paper web is durable enough during the manufacturing process and downstream operations. Long-fiber mechanical pulp can be used to partially substitute chemical pulp in order to obtain the same toughness. However, there is evidence that mechanical pulp fibers have relatively worse reinforcement properties than chemical pulp fibers. One indication of this is that coated papers that contain TMP as a mechanical pulp component contain almost the same amount of chemical pulp as those composed of GW or PGW, even though the average fiber length in a TMPbased furnish is much higher than in a groundwoodbased one. Figure 1. The fracture energy as a function of the average fiber length of a pulp furnish that contains TMP and different long fiber fractions. Chemical pulp (SW kraft) and its long fiber fractions increase the fracture energy much more than do mechanical pulp fibers. In the previous study (1), it was shown that chemical pulp fibers give the pulp furnish a much higher tear index and fracture energy than do mechanical pulp fibers, even at equal fiber length. In the study in question, the fiber properties were characterized only by rather conventional methods such as fiber length and coarseness analysis and standard hand sheet testing. This approach enabled speculation as to the possible reasons for the differences in the reinforcement ability but no definite answer. Heikkurinen et al. (2) have suggested that the basic fiber properties can be classified into four classes. Size distribution Shape Structure of the cell wall Fiber surface This classification has been used in this study. The objective of this study was to investigate in which respects chemical pulp and mechanical pulp fibers differ the most from each other and, in that way, to get ideas as to the reasons for their different abilities for reinforcing mechanical pulp. 6 7 8 9 10 11 12 13 14 15 16 1,5 1,6 1,7 1,8 1,9 2 2,1 2,2 Fiber length, mm F ra ct u re e n er g y, J m /k g TMP 16 TMP 30 Kraft Kraft 16 Kraft 30 MATERIALS AND METHODS The long fiber fractions were separated from different mechanical pulps as well as from refined and unrefined chemical pulps. All the pulps were mill-scale pulps. The mechanical pulps were thermomechanical pulps (TMP; five different pulps, of which three were from Finland and two from France), groundwood (GW) pulp, pressure groundwood (PGW) pulp, pulp for medium density fiberboard (MDF) and TMP reject (abbreviations in the graphs: unrefined and refined; TREJu and TREJr). One of the TMP pulps (no. 5) had been manufactured in the RTS process. The TMP pulps were sampled from the second refining stage outlet. The chemical (kraft) pulps were sampled prior to and after a mill refiner (abbreviations in the graphs: BKPu and BKPr; bleached kraft pulp unrefined and refined). Fractionation was performed using a Bauer-McNett classifier with 10-mesh, 16-mesh and 30-mesh sieves. The amount of pulp was 40 g and the fractionation time 30 minutes. The shives were removed from the 10-mesh fraction using a Somerville apparatus (a screen plate with 0.15-mm slots) before being admixed to a mixture of the 16and 30-mesh fractions. Thus, the long fiber fraction of this study could be labelled as the R30 fraction. The fractions pulps were analyzed at various institutes for their properties as shown in Table 1. The sample preparation and FS-200 and WRV analyses were performed at UPM-Kymmene’s Kaukas Research Center in Lappeenranta, Finland. The FiberMaster analyses were performed by PFI, Norway, and the MorFi, CyberSize, CyberFlex and CyberBond analyses by CTP, France. KCL, Finland, carried out the light microscopy analyses for internal and external fibrillation and performed the zero-span measurements and the flexibility analysis using the Tam Doo & Kerekes method. The FSP, FBW, NRW and ESCA analyses were performed at Helsinki University of Technology. All the test methods and analyzers used in this study, are discussed in detail in the thesis by Huurre (3). In addition, some of the commercial analyzers used in this study were recently described and compared in the paper by Turunen et al. (4). Table 1 The measured fiber properties and analyzers or methods classified according to the basic fiber property category. Basic fiber property category Measured property Analyzer or method Size distribution Fiber length FS-200, FiberMaster, MorFi Fiber width CyberSize, FiberMaster, MorFi Cell wall thickness Light microscopy Coarseness MorFi Shape External fibrillation Light microscopy, CyberSize Curl CyberSize, FiberMaster (Shape factor), MorFi Kinks CyberSize, FiberMaster, MorFi Structure of cell wall Flexibility, stiffness CyberFlex (Steadman), Tam Doo & Kerekes Fiber wall porosity FSP, FBW, NFW, WRV Internal fibrillation Simons staining, light microscopy Fiber strength Zero-span RBA CyberBond Conformability CyberFlex (Steadman) Fiber surface Chemical composition ESCA (Extractives and lignin coverage)