The market for new durable products of modified wood has increased substantially during the last few years, especially in Europe. This increased interest depends partly on the restricted use of toxic preservatives due to increased environmental concern, as well as the need for reduced maintenance for wood products that are mainly for exterior use. Furthermore, as sustainability becomes a greater concern, the environmental impact of construction and interior materials should be included in planning by considering the entire life cycle and embodied energy of the materials used. As a result, wood modification has been implemented to improve the intrinsic properties of wood, widen the range of sawn timber applications, and acquire the form and functionality desired by engineers, without bringing environmental friendliness into question. The different wood modification processes are at various stages of development, and the challenges that must be overcome to expand to industrial applications differ amongst them. In this paper, three groups of wood modification processes are discussed and exemplified with modified wood products that have been newly introduced to the market: (i) chemical processing (acetylation, furfurylation, resin impregnation etc.); (ii) thermo-hydro processing (thermal treatment); and (iii) thermo-hydro-mechanical processing (surface densification). Building on these examples, the paper will discuss the environmental impact assessment of modification processes and further development needs.
As a natural renewable resource, wood is in general a non-toxic, easily accessible and inexpensive biomass-derived material. Since ancient times, wood has been used by mankind based on its inherent properties, meaning that a particular species or part of the tree was utilised to achieve the best performance. Aside from drying, modification of sawn timber has been rare from a historical perspective. Nevertheless, as wood is a natural product that originates from different individual trees, limits are imposed on its use, and the material needs to be transformed to acquire the desired functionality. Modification is applied to overcome weak points of the wood material that are mainly related to moisture sensitiveness, low dimensional stability, hardness and wear resistance, low resistance to bio-deterioration against fungi, termites, marine borers, and low resistance to UV irradiation.
Nowadays, wood modification is referred to as a process used to improve the physical, mechanical, or aesthetic properties of sawn timber, veneer or wood particles used in the production of wood composites. This process produces a material that can be disposed at the end of a product’s life cycle without presenting any environmental hazards greater than those that are associated with the disposal of unmodified wood.
The wood modification industry is currently undergoing major developments, driven in part by environmental concerns regarding the use of wood treated with certain classes of preservatives. Several “new” technologies, such as thermal modification, acetylation, furfurylation, and different impregnation processes, have been successfully introduced in the market and demonstrate the potential of these modern technologies.
The main reasons for the increased interest during the last decades in wood modification with regard to research, the industry, and society in general can be summarised as:
a change in wood properties as a result of changes in silvicultural practices and the way of using wood (
awareness of the use of rare species with outstanding properties, such as durability and appearance;
awareness and restrictions by law of using environmental non-friendly chemicals for increased durability and reduced maintenance of wood products;
increased interest from the industry to add value to sawn timber and by-products, from the sawmill and refining processes;
EU policies supporting the development of a sustainable society; and
the international dimension on climate change and related activities mainly organised within the frame of the United Nations (UN), such as the Paris Agreement under the United Nations Framework Convention on Climate Change (
The purpose of this review is to present state-of-the-art wood modification processes recently introduced in European market with a specific focus on chemical processing (acetylation, furfurylation, impregnation modifications) and thermo-hydro and thermo-hydro-mechanical processing (thermal treatment, surface densification). Continuous surface densification is given as an example of a modification process that has been under development for a long time, but is now close to industrial application. The importance of integrating environmental impact assessments into the optimisation of industrial processes and the development of new modification processes is discussed. The dynamics of the research interest these modification treatments have received is presented with an overview of the number of publications published in the field during the period 1990-2016.
Scientific articles related to wood modification have been increasing over the past decade.
This review of the Scopus® database shows that wood modification area is receiving an increased interest in the scientific community, much of which has been driven by environmental concerns and increased wood use in novel applications to replace fossil-based materials. However, it is important that environmental impact assessments, such as life cycle assessments (LCAs) of wood products, especially of modified wood, are included in the development of new treatments, wood-based materials, and products.
Wood modification is an all-encompassing term to describe the application of chemical, mechanical, physical, or biological methods to alter the properties of the material. Such a definition of wood modification includes almost everything that happens within the wood material after it has left the forest.
To modify wood, four main types of processes can be implemented: (1) chemical treatments; (2) thermo-hydro (TH) and thermo-hydro-mechanical (THM) treatments; (3) treatments based on biological processes; and (4) physical treatment with the use of electromagnetic irradiation or plasma. In this review paper, only the two first processes will be discussed.
While chemical treatments are the most numerous type of processes, and the range of chemical agents is extremely broad, only heat and water are used during TH and THM treatments, albeit supplemented with mechanical forces in the latter case.
Modification of wood can involve active modifications, which result in a change of the chemical nature of the material, or a passive modification, in which a change in properties results without altering the chemistry of the material. Most active modification methods investigated to date have involved the chemical reaction of a reagent with cell-wall polymer hydroxyl groups. These hydroxyl groups play a key role in the wood-water interaction while simultaneously being the most reactive sites (
Several wood-treatment interaction mechanisms tend to occur at the same time. In thermal modification, as one example, parts of the cell-wall polymers are altered, which may lead to cross-linking, reduction of OH-groups, and undesired cleavage of the polymer chains.
It should be noted that most of the wood modification processes that are developed or under experimentation have full or partial origins in the pioneering research and seminal work of Alfred J. Stamm and his colleagues at the Forest Products Laboratory in Madison, Wisconsin, during the 1940s and 1950s.
Chemical modification of wood takes place when a chemical reaction of an agent occurs with the polymeric constituents of wood (lignin, hemicelluloses, or cellulose), thus resulting in the formation of a stable covalent bond between the reagent and the cell-wall polymers (
Consequently, chemical modification of wood is considered as an active modification because it results in a chemical change in the cell-wall polymers. Much is known about the modes of action of modified wood, which incorporates a combination of the following: (i) the equilibrium moisture content is lowered in modified wood, and it is harder for fungi to obtain the moisture required for decay; (ii) a physical blocking of the entrance of decay fungi from the micro pores of the cell walls; and/or (iii) inhibition of the action of specific enzymes (
Impregnation modification of wood is another type of modification. It implies that there is an impregnation of the cell wall of wood with a chemical, or a combination of chemicals, that reacts to form a material that is “locked” into the cell wall (
In Germany,
Wood acetylation using primarily acetic anhydride was first generated as a liquid phase reaction (
The reaction of acetic anhydride with wood polymers results in the esterification of accessible hydroxyl groups in the cell wall with the formation of a by-product, acetic acid (
Many scientists today believe that wood acetylation reduces the number of hydroxyl groups (-OH) that can absorb moisture by hydrogen bonding to largely reduce the equilibrium moisture content and fibre saturation point. Hence, the dimensional stabilisation of wood improves with increasing weight gain, due to acetylation reaction (
As far as biological resistance is concerned, several theories have been proposed to explain the high resistance of acetylated wood to fungal attacks. One theory that has gained large acceptance, is that enzyme penetration is prevented by physically blocking the cell-wall micropores (
On the contrary, Rowell has postulated that the mechanism of decay resistance in acetylated wood is based on “moisture exclusion”, the equilibrium moisture content of a highly modified wood is too low to support fungal attacks,
In his doctorate work,
However, recent research works (
Nowadays, the company Accsys Technologies in Arnhem (The Netherlands) industrially produces acetylated wood. This wood material is marketed under the commercial name Accoya®; the radiata pine (
The acetylation process currently applied by Accsys Technologies yields chemically modified timber, which has largely improved physical, mechanical, and biological material properties (
The biological durability of wood has been improved to the highest durability class (Class 1), similar to the extremely durable tropical species teak (
Acetylated wood has a fibre saturation point below 15%, and the cell wall attains a high moisture exclusion efficiency (
Acetylated wood has been proven to be exceptionally resistant to subterranean and Formosan termites (
At treatment levels >20% (acetyl content), Accoya wood has been found to possess excellent resistance to marine borer attacks even after 16 years of field exposure in the same or better order than CCA treated pine wood (
Acetylated wood can become 15-30% harder than untreated wood (
Acetylation technology has negligible impacts on the mechanical (strength) properties of wood material (
Acetylated wood is marketed today as a “green” product with several environmental benefits (
Due to the listed advantages of the material, Accoya wood has the potential to be widely used in certain applications. In Germany, Accoya wood, as a new wood material, has recently gained acceptance for use in exterior windows, by the German association of Windows and Facades (VFF). However, a limitation is that only radiata pine (
Research relating to chemical modification of wood with furfuryl alcohol (C5H6O2) was initiated by the renowned researchers A. Stamm and I. Goldstein (
Furfuryl alcohol is a liquid produced from agricultural wastes, such as sugar cane, and corn cobs. Furfurylation is executed by impregnating wood with a mixture of furfuryl alcohol and catalysts, and then heating it to cause polymerisation. The purpose of furfurylation is to improve resistance to biological degradation and dimensional stability by applying a non-toxic, proprietary, furfuryl alcohol polymer.
The polymerisation of furfuryl alcohol in wood is a complex chemical reaction. Even today, the question of whether furfurylation is a distinct chemical modification process remains unanswered. Some scientists believe that it comprises a chemical modification process, since the furfuryl alcohol polymer reacts with itself and possibly reacts with the lignin in the cell walls (
Nowadays, the industrial production of furfurylated wood is carried out by Kebony AS (formerly Wood Polymer Technologies) in Norway. According to
Storage and mixing of chemicals: the treating solutions are mixed in a separate mixing tank where different chemicals (furfuryl alcohol, initiators/catalysts, buffering agents, surfactants, water) are added. The mixed solution is pumped to one of the buffer tanks.
Impregnation: the wooden material,
Reaction and curing:
Drying: final drying of the modified wood material in a kiln dryer is essential to minimise emissions and obtain a desirable final moisture content.
Cleaning: the emissions during the process are managed by cleaning the ventilated gases.
According to the literature (
The biological durability of wood is upgraded to “Class 1” (
The mechanical properties of wood, except for impact resistance, are enhanced when wood is treated with a furfuryl-alcohol polymer. Furfurylated wood is characterised by greater hardness, elasticity, and rupture moduli as compared to untreated wood; however, it is also more brittle (
Kebony wood, depending upon the loading, exhibits strong dimensional stability and resistance to weathering (
Furfurylated wood is extremely resistant to marine borers at high levels (>50%) of weight percentage gain (
Recent studies regarding ecotoxicology of furfurylated wood and leachates from furfurylated wood showed no significant ecotoxicity, while its combustion did not release any volatile organic compounds or polyaromatic hydrocarbons above the normal levels of wood combustion (
Furfurylated wood is a “green” wood product that holds an ecological label in the Scandinavian market, named “Swan”. Furfurylation of wood is, therefore, believed to be a safe process for the environment (
The company Kebony AS (Norway) currently produces two different furfurylated wood products:
Kebony Clear®: furfurylated wood, highly-loaded, dark, hard; currently used for flooring. The wood species used are radiata pine, southern yellow pine and maple.
Kebony Character®: furfurylated wood, more lightly-loaded; presently used as decking, siding, roofing and outdoor furniture. This is produced from Scots pine wood.
Nowadays, the company Kebony AS has an annual production of approximately 22.000 m3 (2016), and it is increasing its production capacity by building additional facilities in Belgium (
In addition, Kebony wood has been recently used in the production of exterior windows, like Accoya wood. Following a series of extensive quality tests in Germany, furfurylated wood is presently recommended by the German Association of Windows and Facades (VFF).
Historically, the first experiments on impregnation modification of wood using formaldehyde-based resins were carried out by A. Stamm and his colleagues at the Forest Products Laboratory in Madison, Wisconsin, during the 1940s. Their initial research work included impregnation of wood with phenol-formaldehyde resins with up to 100% resin addition, which resulted in an improved dimensional stability (anti-shrink efficiency, ASE, up to 58%) and improved resistance to biodeterioration against fungi, termites, and marine borers. Initial experiments using wood veneers by applying impregnation with phenol-formaldehyde (PF) resins, heat, and compression were implemented by
Compreg manufacturing is currently realised at a number of industrial sites in the USA, Pakistan, and India under different brand names. Compreg and related products (Fibron
Research on the impregnation modification of wood with melamine-formaldehyde (MF) resins has increased in recent decades, especially in Europe, with positive results with respect to dimensional stability and biological resistance to brown-rot fungi (
This technology was transferred from the treatment of non-wood systems. Furthermore, it involves the impregnation of pine wood, a known highly porous species, with the reagent 1.3-dimethylol-4.5-dihydroxyethyleneurea (DMDHEU). This reagent (
Since then, the process has undertaken considerable improvements by Militz and co-workers (
This technology is considered to be an innovative modification process under which pine wood, typically Scots pine, is impregnated under high pressure (12-14 atm) and polymerised by curing. The entire process is based on simple production stages. The first stage consists of penetrating the wood with a proprietary DMDHEU solution, an aqueous solution of a chemical agent (
The modified product has highly reduced hygroscopic properties (
Belmadur wood production is still sparse in Germany. The producing German company, Münchinger, belongs to the BASF group; and to date, it is focusing on the German market. Key applications thus far have been decking and garden furniture. However, a laminated Belmadur product has gained acceptance by the German association of Windows and Facades (VFF) for use in exterior windows. The resistance of Belmadur wood to marine borers is substantial (
The “Indurite process” has been developed from a comprehensive survey of possible reactions of wood cell walls with polymer systems. The technology was scaled-up by the company Engineered Wood Solutions in New Zealand (
According to
Vinyl monomer impregnation of wood, followed by
According to the nature of the monomer used, polymerisation can take place either in the cell lumens, the cell wall, or both (
One group of emerging wood treatments involves the combined use of temperature and moisture through which force can be applied: thermo-hydro (TH) and thermo-hydro-mechanical (THM) processes. In an orthodox definition, no additives are used in the processes beyond water in combination with wood, heat, and external forces to shape the wood. Procedures including impregnation or gluing to lock a shape are, however, usually included in these modification processes.
Human beings have demonstrably been using heat, moisture, and force for the modification of wood since ancient times; it can be assumed that they had long recognised the effects of fire and water on timber, utilising them for their own purposes (
Thermally modified timber is wood at which the composition of the cell wall material and its physical properties are modified by exposure to temperatures greater than 160 °C and conditions of decreased oxygen availability. There are various procedures to accomplish this process, most of which differ according to the way they exclude air/oxygen from the system (
The effect of thermal modification on wood properties has been reported in the literature from the beginning of the 20th century, when it was found that drying wood at a high temperature increased its dimensional stability and reduced its hygroscopicity and strength (
Thermally modified timber was introduced by
Wood degrades faster when heated by steam or water (
The complexity of the process increases as temperature is altered throughout the process. Degrade products form act as catalysts for further reactions, and moisture available for both hydrolysis and catalysts form continuously, moves from the interior to the surfaces of the timber, and evaporates from the material during treatment. Both the physical and chemical environment inside the wood will change throughout the process.
Thermal modification significantly influences the properties of wood,
Most thermal modification processes, even at mild temperatures, decrease the hygroscopicity of wood,
Thermal modification of sawn timber, which has been investigated for many years, is now commercialised, mainly in Europe. The first commercial thermal modification unit in Europe was based on the research of
The Plato process (Proving Lasting Advanced Timber Option) was developed in the 1980s by Royal Dutch Shell in the Netherlands, and is now used by the Plato Company in the Netherlands. The retification process for thermal modification was developed in France in the late 1980s. A second French process is named
As the boiling points of many natural oils and resins are greater than the temperature required for the thermal modification of timber, the thermal modification in a hot oil bath is a feasible option. The oil heat treatment (OHT) process was developed in Germany, and the process is performed in a closed process vessel.
The most common commercial thermal modification process, named ThermoWood®, started in Finland (1993). It has been licenced via the International ThermoWood Association, with many operations throughout Europe and a growing number outside Europe. For example, from 2003 to 2016, ThermoWood® global production grew nearly seven fold, from 25 797 m3 in 2003 to 179 507 m3 in 2016 (
The TERMOVUOTO process is a vacuum-based thermal modification technology that has been developed during recent years in an EU-Eco-Innovation initiative project (TV4NEWOOD ECO/12/333079). This thermo-vacuum modification process is an alternative technology for the thermal modification of timber, during which reduction of oxygen inside an air-tight cylinder (the reactor) is obtained by applying a vacuum, while volatiles and water vapour are continuously removed using a vacuum pump. This process exhibits high energy efficiency, lower rate of mass loss, and less corrosion of the process equipment compared to alternative thermal modification technologies (
Thermal modification processes have also been established in North America. The Perdure process (
Although thermally modified timber can now be used in many common applications, the market is still limited. Thermally modified timber is suitable for various uses, mainly in which it is exposed to weather and humidity variations above ground,
Densification,
In comparison to bulk densification, surface densification offers several advantages. From a structural perspective, surface-densified timber has a higher material usage efficiency. For some products, the improved dampening characteristics result from the unmodified part of the timber, which is deemed as an asset. In addition, treatments to avoid the moisture-induced recovery of the densified wood cells back to their original shape only need to affect the densified cells close beneath the surface, and not the entire piece of timber. This may allow a faster and, thereby, less costly treatment processes (
To create a high-density wood surface, an adequate volume of wood beneath the surface must be softened (
The first attempts to densify the surface of sawn timber are found in the work of
Past studies into surface densification of wood were aimed mainly at exploring different process approaches. Before performing the actual densification in a hot press,
The reported studies clearly show that it is possible to achieve a significant improvement in several wood properties with surface densification and stabilise the densification effectively, even upon repeated exposure to moisture. However, these approaches rely on time- and energy-consuming batch processes, which means that potential advantages over more expensive wood species or non-renewable materials are lost. For this reason, it is necessary to develop a high-speed surface densification process that is both cost- and energy-efficient.
In an on-going industrial-related project at the Department of Wood Science and Engineering at Luleå University of Technology, a continuous roller pressing approach was adopted to successfully densify the surface of Scots pine boards at a process speed of up to 80 m min-1 (
Though many aspects of wood modification treatments are known, the fundamental influence of the process on product performance, the environment, and end-of-life scenarios remain unknown. To contribute to the low-carbon economy and sustainable development, it is essential to integrate interactive assessments of process parameters, developed product properties, and environmental impacts. To perform objective environmental impact assessments of commercial modification processes and incorporate environmental impact assessments into wood modification processing and product development, including recycling and upgrading at the end of service life, the Life Cycle Assessments (LCA) should be applied.
The common LCA methodology is defined in ISO 14040 (
The number of LCA studies in the wood sector is relatively limited, geographically distributed, use a variety of databases, and impact assessment protocols (
Manufacturers of modified wood products, to some extent, have considered the environmental impacts related to their products; some companies have also obtained Environmental Products Declarations (EPDs). However, a more detailed consideration reveals that the global environmental impact of timber modification processing and further uses of the modified wood products are not yet included in the development of processes and products (
In this review paper, the development of modified wood according to chemical (acetylation, furfurylation, and impregnation modifications) and thermal-hydro-mechanical processes (thermal modification and surface densification) recently introduced on a European market were discussed. Though many aspects of these modifications are known, the fundamental influence of the process on product performance, the environment, and end-of-life scenarios are still to be included in the research and development of the wood modification technologies. This requires an analysis of the entire value chain, from forest through processing, installation, in-service, end-of-life, second/third life (cascading), and, ultimately, incineration with energy recovery.
Modified wood and the resultant products must place more emphasis on the interactive assessment of process parameters, developed product properties, and environmental impacts. Energy consumption considerably contributes to the environmental impact of modified wood. However, the improved properties during the use phase might reduce the environmental impact of the timber processing. It is important to note that the effective use of wood throughout its whole value chain from forest management, through multiple use cycles and end-of-life disposal, can lead to a truly sustainable development.
The authors acknowledge COST Action FP1407. Furthermore, the support of Wood Wisdom-Net+ and the Slovenian Ministry of Education, Science, and Sport of the Republic of Slovenia for their support of the Cascading Recovered Wood projects; European Commission for funding the project InnoRenew CoE (#Grant Agreement 7395 74) under the Horizon2020 Widespread-Teaming program, and infrastructure program IP-0035; the Swedish Research Council for Environment, the Agricultural Sciences and Spatial Planning (FORMAS), project 942-2016-64 and 2014-172 are also acknowledged.
Number of publications found in the Scopus® database of peer-reviewed articles 1990-2016 using keywords related to: (a) thermo-based modification methods; (b) chemical modification methods; and (c) “modified wood” and “LCA”, and “LCA” and “wood”.
Schematic diagram illustrating the effect of chemical modification (courtesy: Emil Engelund Thybring, University of Copenhagen, Denmark).
The main reaction of wood acetylation with acetic anhydride.
Cross section of radiata pine wood with cell walls containing furan polymer (in reddish areas), image through fluorescence microscopy (
The chemical formula of the reagent 1.3-dimethylol-4.5-dihydroxyethyleneurea (DMDHEU).
Classification of thermo-hydro (TH) and thermo-hydro-mechanical (THM) processes (
Cross-section views of poplar (
The principal setup of the integrated continuous surface densification manufacturing concept (side view). In addition to the plasticisation and densification stages, the pre- and post-treatment stages can be added for the stabilisation of the densified wood, impregnation, colouring, etc.
Main changes of properties for thermally modified timber compared with untreated timber.
Desirable property changes | Undesirable property changes |
---|---|
Lower equilibrium moisture content | Decreased MOR (and to some extent) MOE |
Greater dimensional stability | Decreased impact strength |
Greater durability against decay | Increased brittleness (e.g. complicates machining) |
Lower thermal conductivity | Decreased hardness (Brinell hardness) |
Lower density | |
Dark brown colour | |
Characteristic smell | |
Longer pressing time for gluing |
Examples of thermal modification processes and their processing conditions (modified after
Process | App.Year | Trademarks | InitialMC (%) | Temperature(‡)(°C) | Process duration (h) | Pressure (MPa) | Atmosphere | Comments |
---|---|---|---|---|---|---|---|---|
FWD | 1970 | - | 10-30 | 120-180 | ≈15 | 0.5-0.6 | Steam | Closed system |
Plato | 1980 | PlatoWood® | 14-18 | 150-180/170-190 | 4-5/70-120 up to 2 weeks | Super atmospheric pressure (partly) | Saturated steam/heated air | A four- stage process |
ThermoWood | 1990 | ThermoWood® | 10 togreen | 130/185-215/80-90 | 30-70 | Atmospheric | Steam | Continuous steam-flow through the wood under processing that removes volatile degradation products |
Le Bois Perdure | 1990 | Perdure® | green | 200-230 | 12-36 | Atmospheric | Steam | The process involves drying and heating the wood in steam. |
Retification | 1997 | Retiwood®, Bois Rétifié®, Réti®, Retibois®, Retitech®, Retifier® | ≈12 | 160-240 | 8-24 | - | Nitrogen or other gas | The nitrogen atmosphere guarantees a maximum oxygen content of 2% |
OHT | 2000 | OHT® | 10 togreen | 180-220 | 24-36 | - | Vegetable oils | Closed system |
TERMOVUOTO (thermo-vacuum treated [TVT] wood) | 2010 | VacWood® | 0% (at the TVT phase) | 160-220 | up to 25 | Vacuum 150-350 (1000) mbar | Vacuum | Closed system, initial low-temperature drying from initial MC in the TV cylinder |