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Frictional properties of native and chemically modified birch wood
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences. (FSCN – Fibre Science and Communication Network)ORCID iD: 0000-0003-3381-5516
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences. (FSCN – Fibre Science and Communication Network)
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
Responsible organisation
2007 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 22, no 3, p. 325-330Article in journal (Refereed) Published
Abstract [en]

The chemithermomechanical pulping of hardwoods has attracted increasing interest in the past decade. This study investigates the frictional behavior of both native and chemically treated birch under simulated chip-refining conditions (hot saturated steam, high sliding speed). Chip pretreatment, i.e. chemical impregnation and pre-heating done when birch chemithermomechanical pulp is produced, affects energy consumption during refining; the study found that frictional forces may be an important parameter in this respect. The coefficient of friction peaks at approximately 115°C for both native and extracted birch. It was shown that sulfonating native birch decreased its coefficient of friction over the temperature range investigated (100�170°C). The coefficient of friction had a broad and flat peak in the 130�150°C temperature interval for the specimens treated with 3% sodium sulfite at pH 13.5. Furthermore, it was clear that pH had a profound influence on the coefficient of friction. A high pH lowered the coefficient of friction, likely due to well lubricated contacts. The friction arising between birch (Betula verrucosa) and steel was shown to be higher than that between spruce (Picea abies) and steel. The higher stiffness and density of the birch wood, together with a more porous bulk structure leading to rough test surfaces, are some factors possibly accounting for these high friction coefficients. Also, the differences in the amount and composition of the extractive substances in birch and spruce may contribute to the differing frictional behaviors of these species.

Place, publisher, year, edition, pages
2007. Vol. 22, no 3, p. 325-330
Keywords [en]
Birch, Friction, Mechanical pulping, CTMP, Sodium sulfite, Sodium hydroxide, Wood extractives, Energy consumption
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:miun:diva-4045DOI: 10.3183/NPPRJ-2007-22-03-p325-330ISI: 000250025000006Scopus ID: 2-s2.0-34948882653Local ID: 4533OAI: oai:DiVA.org:miun-4045DiVA, id: diva2:29077
Available from: 2008-09-30 Created: 2009-06-08 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Frictional studies and high strain rate testing of wood under refining conditions
Open this publication in new window or tab >>Frictional studies and high strain rate testing of wood under refining conditions
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

When producing thermomechanical pulps (TMP), wood chips and fiber material are loaded mechanically in a disc-refiner to separate the fibers and to make them flexible. In the process, much of the energy supplied is transferred to the fiber material through cyclic compression, shear and friction processes. Therefore, compression and friction characteristics are needed in order to gain a better grasp of the forces acting during refining. To this end, in this thesis, the compressive and frictional behaviors of wood were investigated under simulated chip refining conditions (i.e., hot saturated steam, high strain rate compression, and high sliding speed). Two new, custom-designed, experimental setups were developed and used. The equipment used for compression testing was based on the split Hopkinson pressure bar (SHPB) technique and the friction tester was a pin-on-disc type of tribotester (wear rig). Both pieces of equipment allow a testing environment of hot saturated steam.

 

In the wood–steel friction investigation, the influence of the steam temperature (100-170°C) was of primary interest. The wood species chosen for the friction tests were spruce (Picea abies), pine (Pinus sylvestris, Pinus radiata), and birch (Betula verrucosa). When performing measurements in the lower-temperature region (100-130°C), the friction coefficients registered for the softwoods were generally low and surface properties such as lubrica­tion were suggested to have a great influence on the results; however, in the higher-tempera­ture region (~130 -170°C), the friction coefficients of all investigated wood species were probably determined by bulk properties to a much greater extent. When most of the wood extractives had been removed from the specimens, testing results revealed distinct peaks in friction at similar temperatures, as the internal friction of the different wood species are known to have their maxima at ~110–130°C. One suggested explanation of these friction peaks is that reduced lubrication enabled energy to dissipate into the bulk material, causing particularly high friction at the temperature at which internal damping of the material was greatest. During the friction measurements in the higher-temperature region, the specimens of the different wood species also started to lose fibers (i.e., produce wear debris) at different characteristic temperatures, as indicated by peaks in the coefficient of friction. In refining, the generally lower shives content of pine TMP than of spruce TMP could partly be explained by a lower wear initiation temperature in the pine species.

 

Wood stiffness is known to decrease with temperature, when measured at low strain rates. The results presented in this thesis can confirm a similar behavior for high strain rate compression. The compressive strain registered during impulsive loading (using a modified split Hopkinson equipment) increased with temperature; because strain rate also increased with temperature. Accordingly, the strain rates should determine the strain magnitudes also in a refiner, since the impulsive loads in a refiner are of similar type. Larger strains would thus be achieved when refining at high temperatures. The results achieved in the compression tests were also considered in relation to refining parameters such as plate clearance and refining intensity, parameters that could be discussed in light of the stress–strain relations derived from the high strain rate measurements. Trials recorded using high-speed photography demonstrated that the wood relaxation was very small in the investigated time frame ~6 ms. As well, in TMP refining the wood material has little time to relax, i.e., ~0.04–0.5 ms in a large single disc refiner. The results presented here are therefore more suitable for comparison with the impulsive loads arising in a refiner than are the results of any earlier study. It can therefore be concluded that the modified SHPB testing technique combined with high-speed photography is well suited for studying the dynamic behavior of wood under conditions like those prevalent in a TMP system.

Place, publisher, year, edition, pages
Sundsvall: Mittuniversitetet, 2007. p. 88
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 31
Keywords
Friction, High strain rate testing, Wood, Mechanical pulping, Tribology, Refining, Energy consumption
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-8895 (URN)978-91-85317-64-6 (ISBN)
Public defence
(English)
Supervisors
Available from: 2013-04-16 Created: 2009-05-06 Last updated: 2013-04-16Bibliographically approved
2. High temperature CTMP from birch
Open this publication in new window or tab >>High temperature CTMP from birch
2006 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is intended to contribute to understanding of the chemithermomechanical pulping of birch intended for high freeness grades. It focuses on the effects that conditions during pre‐treatment, i.e. the chemical addition of sodium sulphite and sodium hydroxide and the temperature in pre‐heater, have on the energy consumption and process runnability in terms of disc clearance. Pulp properties are evaluated in regard to brightness and the relation between bulk and internal bond strength. Pilot trials showed that pre‐heating birch chips to high temperature prior to refining (HT CTMP > 140°C), facilitated defibration and considerably lowered energy consumption. This made it possible to produce pulp with very high freeness. Despite the low energy input at high pre‐heating temperature, shive content remained low or was even reduced in the high freeness range. Mill trials confirmed the positive effect of a high pre‐heating temperature on energy consumption and on pulp properties. Furthermore it was shown that the internal bond strength in sheets from birch CTMP, in terms of Scott‐Bond at a given bulk, compared well with that of Spruce CTMP. Moreover, the shive content of birch CTMP produced using the high temperature technique was lower than that of spruce CTMP at a given bulk. A new laboratory technique ʹthe shavings methodology was used in combination with multivariate data analysis to investigate the effect of various pre‐treatments on native wood brightness. This method looks directly on the changes in brightness of the green wood as such. It revealed that the brightness of green birch wood is sensitive to increases in relative humidity and temperature. It also indicated that using a relatively high pre‐heating temperature (~140–155°C) when manufacturing birch CTMP is not necessarily detrimental to pulp brightness, provided the chemical charge is properly adjusted. However, at very high temperature (>160°C), the time in the pre‐heater should be kept short. Measurement of frictional behaviour, at simulated CTMP conditions, showed that the coefficient of friction of birch was greatly affected by chemical modification. Thus extraction raised the coefficient of friction. This rise can probably be attributed to reduced lubrication by the extractive substances and to the higher moisture content in the extracted samples. Sulphonation of the birch samples with 3 % Na2SO3 and 2 % NaOH (pH 13.5) gave a local maximum around 140–155°C. The local peak may be correlated with the reduction in energy consumption when the pre‐heating temperature is increased in the production of birch CTMP. Birch wood and spruce wood are also shown to have distinct differences in frictional performance. The coefficient of friction between birch and steel is higher than that between spruce and steel. The high stiffness and density of the birch wood and differences in the amount and composition of birch and spruce extractive substances probably account for the observed variations.

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden University, 2006
Series
Mid Sweden University licentiate thesis, ISSN 1652-8948 ; 11
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-9109 (URN)91-85317-20-9 (ISBN)
Presentation
2006-05-10, Sundsvall, 00:00 (Swedish)
Opponent
Available from: 2009-06-08 Created: 2009-06-08 Last updated: 2009-07-13Bibliographically approved

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Svensson, BirgittaVesterlind, Eva-LottaHöglund, Hans

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