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Frictional studies and high strain rate testing of wood under refining conditions
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
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. , 88 p.
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 31
Keyword [en]
Friction, High strain rate testing, Wood, Mechanical pulping, Tribology, Refining, Energy consumption
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:miun:diva-8895ISBN: 978-91-85317-64-6 (print)OAI: oai:DiVA.org:miun-8895DiVA: diva2:216096
Public defence
(English)
Supervisors
Available from: 2013-04-16 Created: 2009-05-06 Last updated: 2013-04-16Bibliographically approved
List of papers
1. Frictional testing of wood: Initial studies with a new device
Open this publication in new window or tab >>Frictional testing of wood: Initial studies with a new device
2009 (English)In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 42, no 1, 190-196 p.Article in journal (Refereed) Published
Abstract [en]

In the chip refining process used for mechanical pulp production, wood fibers are treated in a flat and narrow gap between rotating plates. The process is very energy consuming and much of the electrical energy supplied to the refiner is transferred to the fiber material through friction forces. Even though friction has been discussed frequently over the years among pulp and paper researchers worldwide and held to be of great importance, little has been proven due to the complexity of the refining equipment and the extreme conditions prevailing during operation. This paper presents a new apparatus for studying the frictional properties of wood, in lab-scale, under chip refining conditions. Friction tests can be carried out in a steam atmosphere under high temperature/pressure with maximum sliding velocity as high as 150 m/s. Initial studies at room temperature showed that the coefficient of friction between spruce wood and smooth steel increased linearly with the moisture content of the wood specimens. Impregnation by wood extractives lowered the friction coefficient for dry wood surfaces sliding at high speed. When tests were performed in a saturated steam environment, pressurized to various degrees, the frictional properties were strongly affected and varied by the temperature of the surroundings � which always has been considered as constants in analytical and numerical models of process operation.

Keyword
friction, wood, mechanical pulping, wood extractives
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-6455 (URN)10.1016/j.triboint.2008.03.009 (DOI)000265142800024 ()2-s2.0-56249138948 (Scopus ID)4536 (Local ID)4536 (Archive number)4536 (OAI)
Available from: 2009-01-28 Created: 2009-01-28 Last updated: 2016-09-23Bibliographically approved
2. Sliding Friction Between Wood and Steel in a Saturated Steam Environment
Open this publication in new window or tab >>Sliding Friction Between Wood and Steel in a Saturated Steam Environment
2006 (English)In: Journal of Pulp and Paper Science (JPPS), ISSN 0826-6220, Vol. 32, no 1, 38-43 p.Article in journal (Refereed) Published
Abstract [en]

To understand friction under refining conditions, measurements of sliding friction between wood and steel were made in a steam environment. To achieve various testing temperatures (100-170°C), the steam pressure was set to various levels. Two wood species, Norway spruce and Radiata pine, were examined in this study. The friction coefficients of native wood increased with temperature up to a critical point where fibres started to wear from the surface. This point was observed at a lower temperature for Radiata pine than for Norway spruce. After extracting the samples with acetone and dichloromethane, friction-augmenting effects were observed and local friction maxima appeared for both spruce and pine at ∼125°C. It is suggested that these local maxima are due to energy-dissipative processes and related to the softening temperature of lignin. Experiments with sulphonated samples of Norway spruce supported this suggestion.

Keyword
FRICTION FACTOR, PICEA ABIES, PINUS RADIATA, SATURATED STEAM, STEEL, TEMPERATURE, WOOD
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-4044 (URN)000237632600008 ()2-s2.0-33646820341 (Scopus ID)4531 (Local ID)4531 (Archive number)4531 (OAI)
Available from: 2008-09-30 Created: 2009-06-08 Last updated: 2016-09-28Bibliographically approved
3. Frictional properties of native and chemically modified birch wood
Open this publication in new window or tab >>Frictional properties of native and chemically modified birch wood
2007 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 22, no 3, 325-330 p.Article 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.

Keyword
Birch, Friction, Mechanical pulping, CTMP, Sodium sulfite, Sodium hydroxide, Wood extractives, Energy consumption
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-4045 (URN)10.3183/NPPRJ-2007-22-03-p325-330 (DOI)000250025000006 ()2-s2.0-34948882653 (Scopus ID)4533 (Local ID)4533 (Archive number)4533 (OAI)
Available from: 2008-09-30 Created: 2009-06-08 Last updated: 2016-09-26Bibliographically approved
4. An Encapsulated Split Hopkinson Pressure Bar for Testing of Wood at Elevated Strain Rate, Temperature and Pressure
Open this publication in new window or tab >>An Encapsulated Split Hopkinson Pressure Bar for Testing of Wood at Elevated Strain Rate, Temperature and Pressure
2008 (English)In: Experimental techniques (Westport, Conn.), ISSN 0732-8818, E-ISSN 1747-1567, Vol. 32, no 5, 44-50 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents a device, based on a split Hopkinson pressure bar (SHPB) setup, by which it is possible to obtain stress vs. strain for a wood specimen at high deformation rate, high temperature and high steam pressure. The need for determining the mechanical properties of wood not only at high deformation rate but also at high temperature and pressure is motivated by the need to model the wood chip refining process in mechanical pulp-ing. At mechanical chip refining, e.g. in thermomechanical pulping processes, preheated wood chips together with added water are fed into the centre of a refiner which in essence consists of two circular discs. Most often one disc is stationary and the other is rotating. The wood chips are transported radially between the discs due to inertia. On their way, due to impacts from radial bars on the discs, they are eventually broken down to individual fibres and fibre fragments. The device presented here is an SHPB set-up, modified so that the bars and the specimen are encapsulated in a pressure vessel within which the temperature is constant. In this way effects of temperature gradients in the bars are avoided. Pilot tests have been carried out which verify the intended per-formance of the device.

Keyword
dynamic testing
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:miun:diva-1692 (URN)10.1111/j.1747-1567.2008.00318.x (DOI)000259524500006 ()2-s2.0-54749107120 (Scopus ID)5058 (Local ID)5058 (Archive number)5058 (OAI)
Note
VR-Available from: 2008-11-30 Created: 2008-11-17 Last updated: 2011-04-06Bibliographically approved
5. High Strain Rate Compression and Sliding Friction of Wood under Refining Conditions
Open this publication in new window or tab >>High Strain Rate Compression and Sliding Friction of Wood under Refining Conditions
2007 (English)In: International Mechanical Pulping Conference 2007, TAPPI, 2007, Vol. 2, 983-994 p.Conference paper, Published paper (Other academic)
Abstract [en]

Compression and friction characteristics are needed in order to gain a better grasp of the forces acting during refining. To this end, both stress-strain relations and frictional behavior of wood were investigated under simulated chip refining conditions (hot saturated steam, high strain rate compression, and high sliding speed). Two new, custom-designed, experimental set-ups were used. Both pieces of equipment allow a testing environment of hot saturated steam. The wood-steel friction investigations indicate that when making measurements in the lower temperature region (100C-130C), surface properties such as lubrication have a great influence on the coefficients of friction. Traces of lubricating layers, comprising fatty acids, were found on friction-tested pine surfaces using a staining technique and light microscopy; in the higher-temperature region no traces of lubrication could be detected in this way.

Keyword
Friction, Compression
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-4279 (URN)2-s2.0-55849096900 (Scopus ID)5060 (Local ID)978-160560293-6 (ISBN)5060 (Archive number)5060 (OAI)
Conference
International Mechanical Pulping Conference 2007, TAPPI; Minneapolis, MN; United States; 6 May 2007 through 9 May 2007; Code 73991
Available from: 2008-09-30 Created: 2009-06-08 Last updated: 2016-09-23Bibliographically approved
6. High Strain Rate Compression of Spruce Wood in a Saturated Steam Environment
Open this publication in new window or tab >>High Strain Rate Compression of Spruce Wood in a Saturated Steam Environment
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-9399 (URN)
Available from: 2009-07-16 Created: 2009-07-16 Last updated: 2011-05-29Bibliographically approved

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