Search

The densification, i.e., transverse compression of solid wood can lead to improvements in the mechanical properties, and this can expand the areas of application for low-density wood species. For the past one hundred years, many efforts have been made to mass-produce densified wood products, but despite being available on the market, they remain niche products with low annual production volumes. One of the main reasons for this is that all available densified wood products are produced in a batch-type process, which limits the achievable process speed and integration into the continuous wood processing chain. For this reason, we propose a continuous surface densification process using a bespoke belt press – similar in principle to those used to produce MDF panels. The belt-press is capable of densifying full-sized wood boards at processing speeds of up to 60 m min-1. The primary belt for densification can be heated to temperatures above 160°C, while a subsequent belt functions as a cooling stage. During the densification process, the belt press can log the pressing forces, moments, and temperature. Preliminary tests with Scots pine specimens of 120 mm in width and 38 mm in thickness resulted in a twofold increase in peak density, after a pressing time of two minutes at 120°C. The resulting density profiles were similar to those obtained in studies using a static hot press. As the belt press can be fed with a continuous stream of boards, it has a higher net throughput than a static hot press. Further studies continue with the aim to evaluate different aspects relevant to the large-scale industrial production of densified wood products. Funder: Luleå University of Technology, CT Wood

ISBN för värdpublikation: 978-609-02-1762-7

ISBN för värdpublikation: 978-952-94-4383-3

ISBN för värdpublikationen: 978-609-02-1762-7

Conference abstract for 62nd International Convention of Society of Wood Science and Technology, Pages 165-166

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.

Wood is a renewable, bio-based material with a mixture of different properties and qualities, used in numerous applications. Beside many species with high wood qualities several species suffer due to a number of disadvantages, where low hardness and abrasive resistance are characteristic for low-density species. This paper presents examples of on-going European research projects and industrial processes mostly related to wood densification methods. Wood densification is a classical thermohydro-mechanical (THM) wood treatment process, through which density is increased by mechanical compression of wood perpendicular to the grain, by impregnation of cell lumens or cell walls with solutions or melted substances (resins, waxes), or by a combination of both. The purpose is to produce newly designed and engineered materials and products with new property profiles, which would potentially find new markets. In general, the THM processes consist of three stages: plasticization of the wood cells, followed by the actualcompression, and finally solidification of the compressed wood in order to prevent elastic spring-back and the moisture-induced set-recovery. The wood densification process refers but is not limited to solid wood and might apply to whole wood pieces, or to local areas within given pieces only. Another THM method is the mechanical compression of wood parallel to the grain, which leads to a product with high flexibility. A European wood research network, represented by the authors of this contribution, has extended experience in many wood modification processes, as demonstrated through ongoing researches and case studies in this paper

Wood is a renewable, biological material used in numerous applications and it is growing in importance due to sustainable development efforts. Wood also suffers from a number of disadvantages, were low hardness and abrasive resistance are characteristic for low-density species. This paper presents state of the art on different wood densification processes as one emerging process technology for increased use of low-density species. The presentation is based on work by different European research groups in wood science, collaborating in the field mainly through different COST Actions. The main principles for processes are discussed, such as bulk and surface densification, as well as methods for reducing the shape memory effect of densified wood. The main challenges for the future are in the field of finding fast and environmental friendly method for elimination of the set-recovery and scaling up to profitable industrial applications. To provide a better understanding with this regard, some relevant applications of densified wood are presented. 2018-01-23 (rokbeg);Konferensartikel i tidskrift

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. Godkänd; 2016; 20160712 (dicsan)

For many decades, humans have been consuming more resources than the world has to offer in the long run. Increasing the use of renewable materials, such as wood, is essential if we are to achieve a sustainable use of the resources available to us. Densification of wood in general, and surface densification in particular, has the potential to improve the properties of widely available low-density wood species, opening up new fields of application, and fostering the use of wood products in general. The objective of this thesis is to put past research within the field of surface densification of solid wood and advancements within the present project into a context, and to suggest a path towards a performance-driven and cost-effective type of product. A prevalent research gap has been identified and this thesis summarizes how this gap can be filled by already conducted and future studies. Important aspects that need be handled in order to fulfill the overall vision of the large-scale commercialization of surface-densified wood products are considered. Past studies were successful in gaining a good understanding of different approaches to the densification of solid wood, such as using a hot press or a friction welding machine. Problems, such as the moisture-induced recovery of the densified wood cells could be solved from a technological perspective. However, those studies did not take into account industrial implementation. Existing methods are time- and/or energy-consuming. The use of roller-pressing equipment showed promising results, but also highlighted certain limitations. Solid wooden boards have been successfully surface-densified at speeds of up to 80 m/min. However, problems arose regarding the plasticization and stabilization of the densified wood cells, indicating that a continuous belt-press approach might be more viable with regard to the objective of industrial implementation. In parallel, a structured method to identify the most suitable wood species for surface densification has been presented, with the aim of propagating the use of surface-densified wood products and widening their fields of application. This method has confirmed the suitability of previously used wood species, and has also suggested alternatives not considered thus far. Previous studies into the surface densification of wood have provided a rather sound understanding of the densification process and have provided a useful foundation for the current project. The work done, thus far, has shown the viability of a continuous densification process and has indicated a sensible path towards the widespread establishment of surface-densified wood products on the market.

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.

The hardness of the outer areas of solid wood can be improved by surface densification,and this opens up new fields of application for low-density species. So far, surfacedensification relies on time- and energy-consuming batch processes, and this means thatpotential advantages over more expensive hardwood species or non-renewable materialsare lost. Using fossil-based plastics or applying wood densification processes with a highenergy consumption has adverse effects on the environment. The purpose of this project has been to study the viability of a high-speed continuouswood surface densification process on Scots pine boards using roller-pressing equipment.Based on the process parameters used in existing research into the surface densificationof wood, an experiment was conducted to study the potentials and limitations of the rollerpressing approach. Densification with a roller pressing technique resulted in a densified surface at feedspeeds up to 80 m/min, but there is a complex relation between the roller pressing processparameters and the properties of the densified wood surface. Even though immediatespringback was observed, the peak density increase reached up to 40%. The data indicatethat a short heating time at a high temperature to soften the wood yields the best results. Future work will continue on optimization of the densification process, and also onimproving the process from an economic and an environmental perspective. Godkänd; 2016; 20160309 (dicsan)

Until recently, furniture of high quality at a low price was a contradiction. The Modos modular furniture system, which was launched at the end of 2014, has the potential to break this contradiction. By using standardized aluminium connectors and rectangular wood based boards, various types of furniture can be assembled and disassembled many times – without the any tools. Despite its great potential the Modos system has a few shortcomings, caused by the aluminium connectors. Modos is not particularly cheap, and from an environmental perspective, there is room for improvement. For this reason, the objective of this project was the development of a similar modular furniture system, which would be entirely wood based. Wood is more eco-friendly, as well as arguably cheaper and more beautiful. A secondary objective was to evaluate the benefits of lean product development principles.The main challenge from a technological viewpoint was to achieve a sufficient structural strength of the wood based connectors, without significantly compromising the design. From a project planning perspective, the critical task was to manage the free flowing process of developing an innovative and design driven product.By applying lean principles, the product development process went smoothly and could be completed within the best-case time frame. The resulting product – called ‘Notofusy’ – met all the requirements as far they could be assessed. Notofusy is stable, flexible, eco-friendly, and most likely somewhat cheaper than Modos. However, before Notofusy can be launched, some details, such as precise manufacturing costs, have to be addressed and worked out. Validerat; 20150225 (global_studentproject_submitter)

Godkänd; 2015; 20150930 (dicsan)

Nowadays, low price furniture is often of low quality, while high quality furniture is too expensive for many people. Modularity can be an opportunity to manage the challenge to produce affordable furniture, i.e. furniture with the required quality, at an acceptable price. In 2014, the Modos modular furniture system was launched. It consists of standardized aluminium connectors and wood-based panels with which various types of furniture can be assembled many times, without the need for any tools. From an environmental, economic, and aesthetical perspective, the use of aluminium is questionable. By replacing it with wood-based materials, the situation can be improved. The objective of this study was to adapt the geometry of the aluminium connectors in a way that enables the use of wood-based materials, without considerably compromising the strength or the aesthetical appearance. The design of the connectors was based on customer needs, finite element analysis, and the results of bending tests on prototypes. Different design concepts were created, and in an iterative process the concepts were reduced to a single final design. A comprehensive prototype of the final design is presented in this paper; a stable and robust furniture system of sleek design, which has the potential to appeal to both furniture designers and the final customer. By applying different surface finishes or using different wood species, the design can be customized easily in order to attract a wide range of customers. Godkänd; 2015; 20150929 (dicsan)

Concepts for modular furniture systems are of growing interest from the perspective of bothproducers and customers, and they are now entering the area of mass customization. Modularity andstandard interfaces eliminate the interdependency of the components of a product, and allow themanufacturer and customer to change components without affecting the product architecture. In thiscontext, the connectors between components of the modular furniture systems are essential for thesystems. The objective of this study was to evaluate the suitability of different wood-based materialsfor the connectors of the modular furniture system, the focus being on the strength properties.13different types of wood-based material were tested in three-point bending tests, and the modulus ofelasticity (MOE) and the modulus of rupture (MOR) were determined. The tested materialsincorporated wood modifications such as compression, heat treatment and impregnation, as well asdifferent types of adhesives. It could be concluded that birch plywood bonded with soybean glue, andsolid cumaru wood are the most suitable types of wood-based material for the modular furnituresystem. Compressed plywood generally exhibited better strength properties than ordinary plywood,and the type of adhesive had no significant influence on either MOE or MOR. Self-bonding plywoodhad a high MOE, but a comparably low MOR. Solid cumaru wood exhibited both a high MOE and ahigh MOR. Materials which had reasonable strength properties and a comparably low environmentalimpact were considered suitable for the connectors of the modular furniture system, which meant thatnon-formaldehyde-emitting adhesives were favoured. Validerad; 2016; Nivå 1; 20151225 (dicsan)

Auto-adhesion is a phenomenon that is in general related to the manufacture of wood-basedfibreboards, e.g. to the Masonite process or similar processes based on lignocellulosic raw materials.Auto-adhesion as a mechanism for the bonding of solid wood or veneer has not met with the sameindustrial success, but interest is increasing for environmental reasons and as a result of theincreasing cost of adhesives in wood products. The temperature in the laminate is crucial for the autoadhesionprocess that will result in bonding between veneers during hot-pressing. This paper presentsa model for the temperature evolution during the hot-pressing of a porous material, which wasdeveloped and verified for a five-veneer beech laminate pressed at a temperature of 250°C and apressure of 6MPa in an open system for 280 seconds. The result shows good agreement between themodel and the experimental temperature data during the hot-pressing. It can be concluded that a goodcontrol of the temperature evolution during the manufacture of adhesive-free veneer boards is of majorimportance to reach the target properties of the product. Validerad; 2016; Nivå 1; 20151225 (dicsan)