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  • 1. Kymalainen, M.
    et al.
    Hautamaki, S.
    Lillqvist, Kristiina
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Segerholm, Kristoffer
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Rautkari, L.
    Surface modification of solid wood by charring2017In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 52, no 10, p. 6111-6119Article in journal (Refereed)
    Abstract [en]

    Most wooden structures for outdoor applications require repetitive maintenance operations to protect the surfaces from adverse effects of weathering. One-sided surface modification of boards with a relatively fast charring process has the potential to increase the durability and service life of wooden claddings. To assess some weathering-related effects on surface charred wood, spruce and pine sapwood were subjected to a series of long charring processes (30-120 min) at a moderate temperature of 250 A degrees C and to a short one (30 s) at a high temperature of 400 A degrees C. The wettability and contact angles of treated samples were investigated, and the heat transfer was measured along with the micromorphological changes taking place in the material. The result revealed an increased moisture resistance of charred spruce sapwood and an increased water uptake of pine sapwood. The contact angles of both wood species improved compared to references. Heat conduction measurement revealed that only a thin section of the wood was thermally modified. Some micromorphological changes were recorded, especially on the inside walls of the lumina. The results show that spruce sapwood has an improved resistance towards moisture-induced weathering, but more studies are needed to unlock the potential of surface charred wood.

  • 2.
    Källbom, Susanna
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Laine, Kristiina
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Sedighi Moghaddam, Maziar
    SP Technical Research Institute of Sweden.
    Rohumaa, Anti
    Aalto University, Department of Forest Products Technology.
    Segerholm, Kristoffer
    SP Technical Research Institute of Sweden.
    Wålinder, Magnus
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    The influence of log soaking temperature and thermal modification on the properties of birch veneers2016In: IRG Annual Meeting, IRG Documents , 2016Conference paper (Other academic)
    Abstract [en]

    In veneer manufacture the logs are routinely soaked in heated water baths in order to soften the wood prior to peeling. The temperature of the water may vary greatly between batches; however, the influence of log soaking temperature on veneer properties has had little research attention. Uncontrolled moisture is known to cause problems in wood-based materials, while thermal modification offers a method to control the interaction between wood and water. Therefore it might be beneficial to utilise thermally modified veneers in plywood manufacture. Yet, thermal modification is expected to also change other wood properties which might influence the possibility to utilise thermally modified veneers for wood-based-panels. The purpose of this study was to investigate the influence of log soaking temperature (70 °C and 20 °C) and thermal modification (8h in steam conditions) on selected properties of birch veneers, which are relevant in plywood manufacture. The surface area and surface free energy was studied with inverse gas chromatography (IGC). The surface free energy was found to be slightly higher for the unmodified veneers, however, no major difference was found in the dispersive part of the surface free energy between the log soaking temperatures or between unmodified or thermally modified veneers. The wetting of the veneers was investigated with the Wilhelmy plate method utilising the multicycling technique. It was found that lower log soaking temperature produced veneers with more hydrophobic nature. Also, thermal modification increased the hydrophobicity of the veneers. The bond strength was measured with an automatic bond evaluation system (ABES) using phenol formaldehyde (PF) resin. In general, the lower log soaking temperature resulted in slightly higher bond strength (however, the result was statistically insignificant), while thermal modification slightly lowered the bond strength. Based on these initial results thermally modifying the veneers prior to plywood manufacture might be useful.In veneer manufacture the logs are routinely soaked in heated water baths in order to soften the wood prior to peeling. The temperature of the water may vary greatly between batches; however, the influence of log soaking temperature on veneer properties has had little research attention. Uncontrolled moisture is known to cause problems in wood-based materials, while thermal modification offers a method to control the interaction between wood and water. Therefore it might be beneficial to utilise thermally modified veneers in plywood manufacture. Yet, thermal modification is expected to also change other wood properties which might influence the possibility to utilise thermally modified veneers for wood-based-panels. The purpose of this study was to investigate the influence of log soaking temperature (70 °C and 20 °C) and thermal modification (8h in steam conditions) on selected properties of birch veneers, which are relevant in plywood manufacture. The surface area and surface free energy was studied with inverse gas chromatography (IGC). The surface free energy was found to be slightly higher for the unmodified veneers, however, no major difference was found in the dispersive part of the surface free energy between the log soaking temperatures or between unmodified or thermally modified veneers. The wetting of the veneers was investigated with the Wilhelmy plate method utilising the multicycling technique. It was found that lower log soaking temperature produced veneers with more hydrophobic nature. Also, thermal modification increased the hydrophobicity of the veneers. The bond strength was measured with an automatic bond evaluation system (ABES) using phenol formaldehyde (PF) resin. In general, the lower log soaking temperature resulted in slightly higher bond strength (however, the result was statistically insignificant), while thermal modification slightly lowered the bond strength. Based on these initial results thermally modifying the veneers prior to plywood manufacture might be useful.

  • 3.
    Källbom, Susanna
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Lillqvist, Kristiina
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Spoljaric, Steven
    Seppälä, Jukka
    Segerholm, Kristoffer
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Rautkari, Lauri
    Hughes, Mark
    Wålinder, Magnus
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Wood-plastic composites made from thermally modified spruce wood components and effects of exposure to water-soaking-drying cyclesManuscript (preprint) (Other academic)
    Abstract [en]

    The over-all aim of this work is to gain more insight on the potential to use thermally modified wood (TMW) components in wood-thermoplastic composites (WPCs), ie a new type of biobased building material, here defined as TMWPCs, assumed to have significantly increased moisture resistance and durability related to conventional WPCs. The specific objective was to prepare lab-scale TMWPCs and WPC controls with unmodified wood (UW), and to expose these samples to a series of severe water-soaking-drying cycles to study the effects on the water sorption behavior and resulting dimensional and micromorphology changes. TMW was prepared by thermal modification of a spruce board in an atmosphere of superheated steam at atmospheric pressure with a peak temperature of 210°C (also matched with an UW board as control). TMW and UW components were then prepared by a Wiley mill and thereafter sifted into a smaller (0.20-0.40 mm) and a larger (0.40-0.63 mm) size fraction. A portion of the wood components were also hot-water extracted (HE) with liquid hot-water. Composite samples with these different wood components, polypropylene (PP) matrix, and maleated PP (MAPP) as coupling agent (50/48/2 wood/PP/MAPP ratio) were then prepared by using a Brabender mixer followed by hot-pressing. The matching micromorphology of the composites before and after the soaking-drying cycles was analyzed using a surface preparation technique based on UV-laser ablation combined with scanning electron microscopy (SEM). An effort was also made to study the wood-thermoplastic interfacial behavior in the composites by dynamic mechanical analysis (DMA). The results of the water absorption tests showed, as hypothesized, a significantly reduced water absorption and resulting thickness swelling for the TMWPCs compared with the controls. Similarly, the WPCs with HE-UW components showed a significant reduction in water absorption and thickness swelling compared with the controls. In contrast, the samples with HE-TMW components resulted in only minor moisture property changes. These observations were also in agreement with the micromorphology analysis of the composites before and after the moisture cycling which showed a more pronounced wood-plastic interfacial cracking (de-bonding) as well as other microstructure changes in the controls compared with those prepared with TMW and HE-UW components. The DMA indicated better dispersion and increased interfacial interaction for the WPCs with UW components with the smaller size fraction compared with the larger size fraction. The loss modulus and storage modulus were overall reduced for samples with HE and TMW components compared with those with UW components. Based on these observations it is suggested that a potential biobased building material with increased durability for applications in harsh outdoor environments may be tailored as a TMWPC with a well-defined and comparably small size fractions of TMW components.

  • 4.
    Källbom, Susanna
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Rautkari, Lauri
    Aalto University, Department of Forest Products Technology.
    Johansson, Leena-Sisko
    Aalto University, Department of Forest Products Technology.
    Wålinder, Magnus
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Segerholm, Kristoffer
    SP Technical Research Institute of Sweden.
    Jones, Dennis
    SP Technical Research Institute of Sweden.
    Laine, Kristiina
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials. Aalto University, Department of Forest Products Technology.
    Surface chemical analysis and water vapour sorpion of thermally modified wood exposed to increased relative humidity2015In: The Eighth European Conference on Wood Modification (ECWM8) 2015 / [ed] Mark Hughes, Lauri Rautkari, Tuuli Uimonen, Holger Militz and Brigitte Junge, 2015Conference paper (Other academic)
    Abstract [en]

    The increased interest in environmentally friendly building materials is accompanied with an increased need for research on thermally modified wood. Products made from recycling or reusing of thermally modified residuals will have advantages in terms of environmental aspects. Surface characteristics of thermally modified wood play an important role for the development of applications involving bonding processes, for example when using thermally modified wood residuals in biocomposites. Surface chemistry characteristics are important in developing such materials. A technique used for surface chemical analysis of the outermost surface is X‑ray photoelectron spectroscopy (XPS). Some surface chemical analyses of wood and modified wood can be found in Nzokou and Kamdem (2005), Inari et al. (2006), Bryne et al. (2010), Johansson et al. (2012), Rautkari et al. (2012). Furthermore, the influence of water and moisture has crucial effect on the properties of wood and wood products. Water vapour sorption properties of hygroscopic materials can be studied using a dynamic vapour sorption (DVS) instrument. Previous studies on thermally modified wood exposed to several sorption cycles using DVS have shown an increase in hysteresis during the first cycle, compared with unmodified wood (Hill et al., 2012). However, during the second and the third sorption cycle a reduction in sorption hysteresis was observed.

     

    The objective of this work was to study the surface chemical composition and water vapour sorption properties of thermally modified wood. In particular, an effort was made to study any influence on such properties due to a previous exposure to a high relative humidity (RH). Interpretations of the results indicate a decrease of extractable or volatile organic components and a relative increase of non-extractable components, for the high humidity-exposed samples. This could be due to remaining extractives migrating towards or redistribution at the wood surface layer as a result of moisture diffusion. The DVS results show that the thermally modified wood samples that had been exposed to the high relative humidity condition revealed a slight decrease of the hysteresis of the sorption isotherms. The opposite trend was furthermore seen for the unmodified wood.

  • 5.
    Källbom, Susanna
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Rautkari, Lauri
    Department of Forest Products Technology, Aalto University.
    Wålinder, Magnus
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Johansson, Leena-Sisko
    Aalto University, Finland.
    Campbell, JM
    Department of Forest Products Technology, Aalto University.
    Segerholm, Kristoffer
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials. SP Technical Research Institute of Sweden, Sweden.
    Jones, Dennis
    SP Technical Research Institute of Sweden.
    Laine, Kristiina
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Water vapour sorption characteristics and surface chemical composition of thermally modified spruce (Picea abies karst)2016In: International Wood Products Journal, ISSN 2042-6445, E-ISSN 2042-6453, Vol. 7, no 3, p. 116-123Article in journal (Refereed)
    Abstract [en]

    The objective of this work was to study the hygroscopicity and surface chemical composition of thermally modified (TM) spruce. An effort was also made to study if those features were influenced by a previous exposure to a significant increase in relative humidity (RH). TM and unmodified Norway spruce (Picea abies Karst) samples, both in solid and ground form, were prepared. Water vapour sorption characteristics of the ground samples were obtained by measuring sorption isotherms using a dynamic vapour sorption (DVS). The surface chemical composition of the solid samples, both acetone extracted and non-extracted, were analysed using X-ray photoelectron spectroscopy (XPS). The DVS analysis indicated that the TM wood exposed to the 75% RH revealed a decrease in isotherm hysteresis. The XPS analysis indicated a decrease of acetone extractable or volatile organic components and a relative increase of non-extractable components for the samples exposed to the increased RH condition.

  • 6.
    Laine, Kristiina
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Segerholm, Kristoffer
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials. SP Tech Res Inst Sweden, Sweden.
    Wålinder, Magnus
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Rautkari, L.
    Hughes, M.
    Lankveld, C.
    Surface densification of acetylated wood2016In: European Journal of Wood and Wood Products, ISSN 0018-3768, E-ISSN 1436-736X, Vol. 74, no 6, p. 829-835Article in journal (Refereed)
    Abstract [en]

    The mechanical properties of wood can be improved by compressing its porous structure between heated metal plates. By adjusting the process parameters it is possible to target the densification only in the surface region of wood where the property improvements are mostly needed in applications, such as flooring. The compressed form is, however, sensitive to moisture and will recover to some extent in high humidity. In this study, therefore, acetylated radiata pine was utilised in the surface densification process in order to both reduce the set-recovery of densified wood and to improve the hardness of the acetylated wood. Pre-acetylation was found to significantly reduce the set-recovery of surface densified wood. However, after the second cycle the increase in set-recovery of acetylated wood was relatively higher than the un-acetylated wood. The acetylated samples were compressed by only 1 mm (instead of the target 2 mm), yet, the hardness and hardness recovery of the acetylated samples significantly increased as a result of densification. It was also discovered that rough (un-planed) surfaces may be surface densified, however, even if the surface became smooth to the touch, the appearance remained uneven.

  • 7.
    Laine, Kristiina
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Segerholm, Kristoffer
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials. SP Tech Res Inst Sweden.
    Wålinder, Magnus
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Rautkari, Lauri
    Hughes, Mark
    Wood densification and thermal modification: hardness, set-recovery and micromorphology2016In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 50, no 5, p. 883-894Article in journal (Refereed)
    Abstract [en]

    The density of wood can be increased by compressing the porous structure under suitable moisture and temperature conditions. One aim of such densification is to improve surface hardness, and therefore, densified wood might be particularly suitable for flooring products. After compression, however, the deformed wood material is sensitive to moisture, and in this case, recovered up to 60 % of the deformation in water-soaking. This phenomenon, termed set-recovery, was reduced by thermally modifying the wood after densification. This study presents the influence of compression ratio (CR = 40, 50, 60 %) and thermal modification time (TM = 2, 4, 6 h) on the hardness and set-recovery of densified wood. Previously, set-recovery has mainly been studied separately from other properties of densified wood, while in this work, set-recovery was also studied in relation to hardness. The results show that set-recovery was almost eliminated with TM 6 h in the case of CR 40 and 50 %. Hardness significantly increased due to densification and even doubled compared to non-densified samples with a CR of 50 %. Set-recovery reduced the hardness of densified (non-TM) wood back to the original level. TM maintained the hardness of densified wood at an increased level after set-recovery. However, some reduction in hardness was recorded even if set-recovery was almost eliminated.

  • 8.
    Laine, Kristiina
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Segerholm, Kristoffer
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials. SP Tech Res Inst Sweden.
    Wålinder, Magnus
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Rautkari, Lauri
    Department of Forest Products Technology, Aalto University.
    Hughes, Mark
    Department of Forest Products Technology, Aalto University.
    Källbom, Susanna
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Jones, Dennis
    SP Tech Res Inst Sweden.
    Hardness, set-recovery and micromorphology studies of densified and thermally modified wood2015Conference paper (Other academic)
    Abstract [en]

    The purpose of the work reported in this paper was to increase the density of Scots pine wood in order to improve its hardness. Density was increased by compressing the porous structure of wood between heated metal plates in the radial direction by 40, 50 or 60% of the thickness. The compressed state was stabilised by thermally modifying (TM) the samples at 200 °C under steam conditions for 2, 4 or 6h. Set-recovery was almost eliminated (<1%) with TM of 6h for samples compressed 40 and 50%. It was discovered that hardness of densified wood was in some cases even three times higher compared to untreated wood. However, the hardness of the densified, non-TM wood was reduced after soaking and drying back to the original untreated level, while TM of 4 and 6h maintained an increased level of hardness.

  • 9.
    Laine, Kristiina
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Segerholm, Kristoffer
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials. SP Tech Res Inst Sweden.
    Wålinder, Magnus
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Rautkari, Lauri
    Department of Forest Products Technology, Aalto University.
    Hughes, Mark
    Department of Forest Products Technology, Aalto University.
    Rowell, Roger
    Department of Biological Systems Engineering, University of Wisconsin, Madison, WI USA.
    Acetylation and densification of wood2015Conference paper (Other academic)
    Abstract [en]

    The purpose of this study was to explore the possibility to surface densify acetylated solid wood. The aim of surface densification is to improve mechanical properties, such as hardness, at the very surface of wood where the property improvements are mostly needed (e.g. in flooring and decking). However, when subjected to moisture, surface densified wood may swell back almost to the original dimensions. Therefore, acetylated and non-acetylated wood was surface densified in order to investigate whether the dimensional stability of densified wood may be improved by pre-acetylation. Surface densification was performed by compressing the acetylated radiate pine samples between metal plates with only one side heated (150°C) in order to target the deformation to one surface only. The original thickness of the samples was 20 mm and the target thickness 18 mm which was controlled by metal stops. The recovery of the deformation (set-recovery) was measured by soaking the samples in water and measuring the oven-dry thickness before and after soaking in repeated cycles. It was found that acetylated wood may be surface densified and indeed the set-recovery of the pre-acetylated wood was significantly lower (17.4 %) compared to non-acetylated wood (72.8 %). Further studies in adjusting the process parameters might lead to even higher reduction in set-recovery.

  • 10.
    Lillqvist, Kristiina
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Rohumaa, Anti
    LaBoMab - Ecole Nationale Supérieure d'Arts et Métiers.
    Källbom, Susanna
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Rautkari, Lauri
    Department of Bioproducts and Biosystems, Aalto University.
    Wålinder, Magnus
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    The influence of thermal modification on veneer bond strength2017Conference paper (Other academic)
  • 11.
    Linkosalmi, Lauri
    et al.
    Department of Forest Products Technology, Aalto University.
    Laine, Kristiina
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Rautkari, Lauri
    Department of Forest Products Technology, Aalto University.
    Life cycle impacts of modified wood products2015Conference paper (Other academic)
  • 12. Vahtikari, Katja
    et al.
    Rautkari, Lauri
    Noponen, Tuula
    Lillqvist (nee Laine), Kristiina
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Hughes, Mark
    The influence of extractives on the sorption characteristics of Scots pine (Pinus sylvestris L.)2017In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 52, no 18, p. 10840-10852Article in journal (Refereed)
    Abstract [en]

    The sorption behaviour of extracted and un-extracted Scots pine (Pinus sylvestris L.) heartwood was analysed using dynamic vapour sorption apparatus. In addition to the sorption isotherm and hysteresis, the moisture increments and decrements were determined as well as the rate of sorption. Parallel exponential kinetics model was used for further analysis. The effect of cyclic humidity loading on the sorption characteristics was studied by exposing samples to ten repeated sorption cycles and by determining the amount of accessible hydroxyl (OH) groups before and after the cyclic humidity loading. Removal of extractives led to an increase in EMC both in adsorption and in desorption. Hysteresis decreased due to the removal of extractives. Cyclic humidity loading reduced the sorptive capacity of wood material for both extracted and un-extracted wood, but was more pronounced in un-extracted wood. However, despite the decrease in the sorptive capacity, the amount of accessible OH groups increased after ten repeated dry-humid cycles.

  • 13. Čermák, Petr
    et al.
    Vahtikari, Katja
    Rautkari, Lauri
    Laine, Kristiina
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Horáček, Petr
    Baar, Jan
    The effect of wetting cycles on moisture behaviour of thermally modified Scots pine (Pinus sylvestris L.) wood2016In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 51, no 3, p. 1504-1511Article in journal (Refereed)
    Abstract [en]

    The moisture behaviour of thermally modified Scots pine (Pinus sylvestris L.) exposed to cyclic conditions was analysed. Specimens of dimensions 15 × 15 × 5 mm3 were thermally modified at 180 °C (TM1) and 220 °C (TM2) using atmospheric pressure and superheated steam. Radial, tangential, volumetric swelling and anti-swelling efficiency (ASE) were calculated during six consecutive drying–soaking cycles. Afterwards, additional specimens were exposed to ten relative humidity cycles (0 and 95 %) at temperature 25 and 40 °C in order to analyse its influence on sorption behaviour. Application of thermal modification led to significant reduction of swelling from original 18.4–13.3 % for TM1 and to 10.5 % for TM2. However, after exposure to six consecutive soaking–drying cycles, the swelling of control specimens slightly decreased, whereas the swelling of thermally modified specimens increased. Due to the increased swelling after repeated cycles, the original ASE (28.6 and 42.7 %) decreased to 22.5 % for TM1 and to 36.88 % for TM2. The presence of leachable compounds and release of internal stresses are mainly attributed to that phenomenon. The EMC of the reference specimens decreases over the repeated humidity cycles for approximately 1 %–units. Same trend was found for the mild thermal modification TM1, but decreasing only in the range of 0.5 %–units. However, the EMC of the TM2 specimens during humidity cycles behaved differently. The results provide a better insight into details of thermal modification of wood and its behaviour under cyclic conditions.

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