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Surface Densification of Solid Wood: Paving the Way Towards Industrial Implementation
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.ORCID iD: 0000-0002-1489-0839
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Surface densification of a piece of solid wood results in an increase in density and in hardness in the whole or a part of the densified material, and is one of the ways of improving the properties and value of low-density wood species. Despite efforts for many years, mass commercialisation of either bulk- or surface-densified wood products has not yet been achieved. Most of the previously tested densification methods have limitations in terms of processing speed and integration into the largely continuous wood processing chain, which leads to high production costs. Established methods to eliminate the set-recovery rely either on technologically complex close-system methods or on open-system methods that require relatively long periods of high energy input. For this reason, impregnation with adhesives is used in almost all commercially available densified wood products, and none of them have risen above their status of being niche products.

Based on this background, three objectives for this project were formulated: (1) the development of a method for selecting the most suitable wood species for surface densification, (2) showing that surface densification can be carried out in a continuous manner at high process speeds, (3) and researching a fast open-system method to reduce the set- recovery.

The method developed for selecting the most suitable wood species for surface densification was based on Lean principles, and it confirmed the suitability of previously studied wood species, such as Scots pine, spruce and poplar. In addition, several suitable alternatives from different parts of the world and from different types of forest were identified. This suggests a high potential for establishing such wood products on a global market level.

Two studies using a continuous roller press showed that solid wood can be successfully surface-densified at process speeds of up to 80 m min-1, and that some defects, such as knots, are acceptable in the raw material, but the problem of set-recovery could not however be solved.

A screening experiment testing different open-system approaches to reduce the set-recovery highlighted the potential of a novel method using ionic liquids as a plasticiser prior to the surface densification of solid Scots pine. The set-recovery could be reduced to 10%, with the time of high energy input being less than 10 minutes. The Brinell hardness was increased by a factor of 2.7 over that of undensified wood. A study with thermo-gravimetric analysis and digital image correlation showed that the set-recovery almost exclusively happens in the transition zone between the densified and undensified wood cells, where there is less penetration of the ionic liquids.

The work accomplished in this project has successfully addressed several gaps in the field of wood densification, firstly, by employing a continuous surface densification process using a roller press, and secondly, by developing and studying a fast open-system pre-treatment with ionic liquids, which greatly reduces the set-recovery. Research will continue on a new band press, facilitating a swift transfer of knowledge between small- scale studies and the continuous surface densification of production-size wooden boards.

Place, publisher, year, edition, pages
Luleå University of Technology, 2019. , p. 123
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Wood Science Other Mechanical Engineering
Research subject
Wood Science and Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-75794ISBN: 978-91-7790-435-9 (print)ISBN: 978-91-7790-436-6 (electronic)OAI: oai:DiVA.org:ltu-75794DiVA, id: diva2:1347640
Public defence
2019-10-08, Auditorium A, Skellefteå, 09:00 (English)
Opponent
Supervisors
Available from: 2019-09-02 Created: 2019-09-02 Last updated: 2019-09-12Bibliographically approved
List of papers
1. A new methodology to select hardwood species for wooden products
Open this publication in new window or tab >>A new methodology to select hardwood species for wooden products
2015 (English)In: Wood Material Science & Engineering, ISSN 1748-0272, E-ISSN 1748-0280, Vol. 10, no 4, p. 344-352Article in journal (Refereed) Published
Abstract [en]

In general, only a few wooden species are used or even considered for any given wooden product, even though hundreds of alternative wood species are available. In many cases, the reason for this is tradition rather than availability, technical or aesthetic considerations and it can result in endangered species being used to an unnecessarily high degree. The purpose of this study was to develop a structured, quantifiable, and easy-to-use methodology to identify suitable hardwood species for a specific product. The methodology combines processes based on quality function deployment and multivariate data analysis in a three-step workflow, and takes different criteria into consideration. To verify the methodology, it was applied to an example product: an electric guitar. It was shown that the methodology was easy to use and provided useful and quantifiable results. Expansion of the underlying wood species data-set will be necessary to improve the performance in the future.

Keywords
Forestry, agricultural sciences and landscape planning - Wood fibre and forest products, Skogs- och jordbruksvetenskap samt landskapsplanering - Träfiber- och virkeslära
National Category
Other Mechanical Engineering
Research subject
Wood Science and Engineering
Identifiers
urn:nbn:se:ltu:diva-15634 (URN)10.1080/17480272.2015.1046919 (DOI)000368718400004 ()2-s2.0-84945454628 (Scopus ID)f2b8de43-9f83-42c7-acfd-472616b0e91e (Local ID)f2b8de43-9f83-42c7-acfd-472616b0e91e (Archive number)f2b8de43-9f83-42c7-acfd-472616b0e91e (OAI)
Note

Validerad; 2015; Nivå 1; 20150614 (dicsan)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2019-09-12Bibliographically approved
2. Application of a new method to select the most suitable wood species for surface densification
Open this publication in new window or tab >>Application of a new method to select the most suitable wood species for surface densification
2016 (English)In: New Horizons for the Forest Products Industry: 70th Forest Products Society International Convention, June 26-29, Portland, Oregon, USA, Madison: Forest Products Society, 2016Conference paper, Published paper (Refereed)
Abstract [en]

Surface densification improves many properties of solid wood, and opens up new applications for low-density species e.g. flooring or wooden structures, and adds value to these species. Research into the surface densification of wood has been focused mainly on Scots pine, Norway spruce, aspen and poplar. In order to propagate the use of surface densification techniques, it is necessary to find more wood species that have a particularly high potential for an increase in value through such a treatment.The purpose of this study was to select wood species which are suitable for surface densification and for use in a specific product, by applying a systematic and quantifiable selection method. The method that was presented in detail in an earlier work, is designed around a workflow consisting of multiple stages, takes into account weighted selection criteria and provides a quantified ranking of the most suitable species.The results suggest that several species such as alder, basswood, cedar, and obeche that have not previously been considered for surface densification may be suitable candidates. Aspen, poplar and pine were confirmed as suitable, being among the top species in the ranking.

Place, publisher, year, edition, pages
Madison: Forest Products Society, 2016
Keywords
Forestry, agricultural sciences and landscape planning - Wood fibre and forest products, Skogs- och jordbruksvetenskap samt landskapsplanering - Träfiber- och virkeslära
National Category
Other Mechanical Engineering
Research subject
Wood Science and Engineering
Identifiers
urn:nbn:se:ltu:diva-40694 (URN)feba3b52-2111-4497-a812-5b7ae64e89a1 (Local ID)feba3b52-2111-4497-a812-5b7ae64e89a1 (Archive number)feba3b52-2111-4497-a812-5b7ae64e89a1 (OAI)
Conference
Forest Products Society International Convention : 26/06/2016 - 29/06/2016
Note

Godkänd; 2016; 20160712 (dicsan)

Available from: 2016-10-03 Created: 2016-10-03 Last updated: 2019-09-12Bibliographically approved
3. Development of a continuous wood surface densification process: the roller pressing technique
Open this publication in new window or tab >>Development of a continuous wood surface densification process: the roller pressing technique
2016 (English)In: Proceedings of the 59th International Convention of Society of Wood Science and Technology.: Forest Resource and Products: Moving Toward a Sustainable Future. / [ed] Susan LeVan-Green, Monona, WI: Society of Wood Science and Technology , 2016, p. 17-24Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
Monona, WI: Society of Wood Science and Technology, 2016
Keywords
Forestry, agricultural sciences and landscape planning - Wood fibre and forest products, Skogs- och jordbruksvetenskap samt landskapsplanering - Träfiber- och virkeslära
National Category
Other Mechanical Engineering
Research subject
Wood Science and Engineering
Identifiers
urn:nbn:se:ltu:diva-30743 (URN)4a861bab-bce6-4ee8-8387-7f46dd34cced (Local ID)978-0-9817876-6-4 (ISBN)4a861bab-bce6-4ee8-8387-7f46dd34cced (Archive number)4a861bab-bce6-4ee8-8387-7f46dd34cced (OAI)
Conference
International Convention of Society of Wood Science and Technology : Forest Resource and Products: Moving Toward a Sustainable Future 06/03/2016 - 10/03/2016
Note

Godkänd; 2016; 20160309 (dicsan)

Available from: 2016-09-30 Created: 2016-09-30 Last updated: 2019-09-12Bibliographically approved
4. Continuous Wood Surface Densification: Chemical Treatments to Reduce the Set-Recovery
Open this publication in new window or tab >>Continuous Wood Surface Densification: Chemical Treatments to Reduce the Set-Recovery
Show others...
2016 (English)In: BIOCOMP 2016: The 13th Pacific Rim Bio-Based Composites Symposium - Bio-based composites for a sustainable future, Concepción: University of Concepción , 2016, p. 66-77Conference paper, Published paper (Refereed)
Abstract [en]

The hardness of the outer surface of solid wood can be improved by densification, and this opens up new fields of application for low-density species. So far, surface densification is carried out in time- and energy-consuming batch processes, and this means that potential advantages over more expensive hardwood species or non- renewable materials are lost. One of the crucial problems in all densification processes is to reduce the moisture-induced set-recovery of the densified wood cells. In a previous study, a new high-speed continuous surface densification process was introduced, where the surface of solid Scots pine boards could be densified at speeds of up to 80 m/min by a roller pressing technique. The aim of the present study was to integrate the roller pressing technique with different pre- and post-treatment methods to reduce the set- recovery. An aqueous solution of sodium hydroxide was used as a pre-treatment agent to activate the wood surface prior to densification, and a methacrylate ester monomer solution was used as an impregnation agent, both before and after densification. After densification and impregnation, the methacrylate monomers are polymerized by curing in an oven at 100°C in order to stabilize the compressed wood cells. The results show that the different treatments had no significant effect on the set-recovery, probably due to insufficient penetration into the wood material. Future work will focus on improving the treatment process and integrating all process steps into a fully continuous and automatic process. 

Place, publisher, year, edition, pages
Concepción: University of Concepción, 2016
Keywords
compression, sodium hydroxide, methacrylate, impregnation, roller pressing
National Category
Other Mechanical Engineering
Research subject
Wood Science and Engineering
Identifiers
urn:nbn:se:ltu:diva-60281 (URN)
Conference
13th Pacific Rim Bio-Based Composites Symposium, Concepción, Chile, 13-15 November 2016
Available from: 2016-11-10 Created: 2016-11-10 Last updated: 2019-09-12Bibliographically approved
5. Pre-treatment with sodium silicate, sodium hydroxide, ionic liquids or methacrylate resin to reduce the set-recovery and increase the hardness of surface-densified Scots pine
Open this publication in new window or tab >>Pre-treatment with sodium silicate, sodium hydroxide, ionic liquids or methacrylate resin to reduce the set-recovery and increase the hardness of surface-densified Scots pine
2017 (English)In: iForest : Biogeosciences and Forestry, ISSN 1971-7458, E-ISSN 1971-7458, Vol. 10, no 5, p. 857-864Article in journal (Refereed) Published
Abstract [en]

The hardness of the outer regions of solid wood can be improved by surface densification, and this opens up new fields of application for low-density species. So far, surface densification relies on time- and energy-consuming batch processes, and this means that the potential advantages over more expensive hardwood species or non-renewable materials are reduced. Using fossil-based plastics or applying wood densification processes with a high energy consumption has adverse effects on the environment. In a previous study, it was shown that the surface of wood can be densified by a continuous high-speed process, adopting a roller pressing approach. The desired density profiles could be obtained at process speeds of up to 80 m min-1, but an equally simple and fast method to eliminate the moisture-induced set-recovery of the densified wood cells is still required. For this reason, the goal of the present study was to evaluate the effect on the set-recovery and hardness of surface-densified Scots pine after a fast pre-treatment with solutions of sodium silicate, sodium hydroxide, methacrylate resin, and ionic liquids. The Scots pine specimens were pre-treated by applying the chemical treatment and impregnation agents to the wood surface with a paper towel, before the specimens were densified. For each type of treatment, 15 specimens were densified in a hot press. The set-recovery was measured after two wet-dry cycles, and 30 Brinell hardness measurements were carried out on each group of specimens. In general, the effect of the treatments on the set-recovery was rather low. Ionic liquid solutions appear to work as a strong plasticiser and the treatment led to a reduction in set-recovery by 25%. The treatments with sodium silicate, ionic liquids and methacrylate resin led to a greater hardness than in untreated and densified specimens. Further experiments are needed to improve the depth of penetration of the treatment solutions into the wood surface, as this was identified as one of the main causes of the rather weak effects.

Place, publisher, year, edition, pages
Italian Society of Silviculture and Forest Ecology, 2017
National Category
Wood Science Other Mechanical Engineering
Research subject
Wood Science and Engineering
Identifiers
urn:nbn:se:ltu:diva-66300 (URN)10.3832/ifor2385-010 (DOI)000413842100004 ()2-s2.0-85039747852 (Scopus ID)
Note

Validerad;2017;Nivå 2;2017-10-31 (rokbeg)

Available from: 2017-10-29 Created: 2017-10-29 Last updated: 2019-09-12Bibliographically approved
6. Pre-treatment with ionic liquids or organic superbases to reduce the spring-back and set-recovery of surface-densified Scots pine.
Open this publication in new window or tab >>Pre-treatment with ionic liquids or organic superbases to reduce the spring-back and set-recovery of surface-densified Scots pine.
2018 (English)In: Proceedings of ECWM9 - The 9th European Conference on Wood Modification / [ed] Jos Creemers, Thomas Houben, Bôke Tjeerdsma, Holger Militz, Brigitte Junge and Jos Gootjes, Wageningen: SHR Wageningen, The Netherlands , 2018, p. 259-266Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
Wageningen: SHR Wageningen, The Netherlands, 2018
National Category
Wood Science
Research subject
Wood Science and Engineering
Identifiers
urn:nbn:se:ltu:diva-72586 (URN)978-90-829466-1-1 (ISBN)
Conference
ECWM9 - The 9th European Conference on Wood Modification, The Netherlands, Arnhem, September 17-18
Available from: 2019-01-16 Created: 2019-01-16 Last updated: 2019-09-12
7. The effect of ionic liquid and superbase pre-treatment on the spring-back, set-recovery and Brinell hardness of surface-densified Scots pine
Open this publication in new window or tab >>The effect of ionic liquid and superbase pre-treatment on the spring-back, set-recovery and Brinell hardness of surface-densified Scots pine
2019 (English)In: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434XArticle in journal (Refereed) Epub ahead of print
Abstract [en]

Compressing the surface of sawn timber results in a substantial increase in hardness, and this opens up new market opportunities of using low-density timber species as the raw material for high-value wood products. Unfortunately, widespread commercialisation is hindered by the lack of an industrially viable surface densification process, the major obstacle being the set- recovery of the densified wood cells upon exposure to moisture. Our hypothesis is that partial dissolution of the crystalline cellulose during densification will largely prevent the set- recovery of densified wood. We therefore evaluated the effect of ionic liquid or organic superbase pre-treatment on the elastic spring-back, set-recovery and Brinell hardness of surface-densified wood. Specimens of Scots pine were treated with solutions of ionic liquids or superbases, and then densified in a hot press at temperatures between 200°C and 270°C. The set-recovery was reduced from 90% for the control group to only about 10% for the treated materials. The treated and densified specimens exhibited a higher Brinell hardness than their untreated and densified counterparts. The method presented in this study is a precursor to the development of a continuous densification process adapted for an open system. Further studies are needed to understand the underlying mechanisms of the pre- treatment.

Place, publisher, year, edition, pages
Walter de Gruyter, 2019
Keywords
chemical treatment, surface modification, wood compression, wood modification
National Category
Wood Science Other Mechanical Engineering
Research subject
Wood Science and Engineering
Identifiers
urn:nbn:se:ltu:diva-75738 (URN)10.1515/hf-2019-0158 (DOI)2-s2.0-85073157805 (Scopus ID)
Available from: 2019-08-29 Created: 2019-08-29 Last updated: 2019-10-21

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