Thermally Modified Timber Research Articles

Impact bending strength of thermally-modified timber

Abstract The article offers a perspective on how thermal-modification affects the impact bending strength of five different wood species, an aspect that has not received as much attention as the well-studied static load behavior of thermally-modified timber (TMT). Since the TMTs are mainly employed as outdoor materials, where they may encounter impact forces, a comparative investigation into the flexibility and strength of these materials under impact is useful. This article evaluates different aspects of the TMT, such as deflection, strain in the impact region, the maximum force needed to initiate cracks, and the energy required for rupture. Wood planks from ash, beech, larch, oak, and spruce were thermally modified at 180 and 220 °C. They were cut into test specimens, while a separate set of unmodified specimens from each wood species served as the reference group. The specimens were subjected to an impact 3-point bending test, and an ultra-high-speed camera meticulously recorded the results. The images were processed by the digital image correlation (DIC) method to determine the deflection and strain distribution of the beams during the impact test. The deflection, maximum force, maximum longitudinal strain, and required work for rupture of each group were determined. The results showed that thermal-modification decreases the wood deflection and maximum longitudinal strain by approximately 50 %. In addition, the impact bending strength decreased by nearly 60 %. However, the impact bending strength did not exhibit a statistically significant decrease at 180 °C; in some cases, it even increased.

Colour as a quality indicator for industrially manufactured ThermoWood®

ABSTRACT Since its market entry in 2001, ThermoWood® has become the most established form of thermal modification globally. Whilst its properties have been widely reported, there is still a need for a readily-applied quality control method to ensure treatments meet the criteria outlined by the International ThermoWood Association. One such method for quality control has focussed on the use of colour measurements. In the largest study of its kind, colour measurement data have been evaluated for Norway spruce and Scots pine subjected to the Thermo-D ThermoWood® process at twelve industrial plants between 2007 and 2018. This showed that the colour measurement according to the CIELAB* colour space on newly planed surfaces of thermally modified timber (TMT) may be used for quality control of the ThermoWood® Thermo-D process with regard to process intensity, i.e. the combined effect of temperature (212 ± 3°C) and time. In order to obtain more robust control, only the L* parameter (lightness) should be used as a quality indicator, as both a* and b* parameters for the TMT showed little variation from those of the unmodified wood and too high a scatter in values.

Effects of two-year weather exposure on thermally modifiedPicea abies,Pinus sylvestris, andFraxinus excelsiorwood

The use of thermally modified timber (TMT) in outdoor applications is well established, but its performance against the stresses of outdoor conditions is not comprehensively understood. This study investigates the changes in density, surface chemical composition, color, equilibrium moisture content (EMC), checking, hardness, and cupping of thermally modified boards of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) H. Karst.), and European ash (Fraxinus excelsior L.) during a two-year weather exposure test in Eastern Finland. Unmodified pine, spruce, and ash and copper salt impregnated pine boards were used as controls. The results show that a two-year weather exposure period causes degradation of hydrophobic lignin, and the leaching of the degraded compounds changed the color, increased the EMC, and decreased the hardness of specimens. Although the EMC of TMT specimens was lower compared with unmodified and impregnated ones during weathering, the changes were more obvious in TMT than in the controls. More checks occurred in the TMT specimens than in unmodified ones after the exposure. On the other hand, the TMT specimens had a lower degree of cupping. These findings also indicate that an increase in modification temperature induces more checks but improves the durability of all studied species by reducing the chemical degradation, color change, cupping, and hygroscopicity during weather exposure.

Quality Control of Thermally Modified Timber Using Dynamic Vapor Sorption (DVS) Analysis

The importance of thermal modification is increasing worldwide. Increased use of thermally modified timber (TMT) has resulted in a need for reliable quality control, comprising control of variation of the production within defined limits, allowing third-party control in the case of certification and the regulation of customer complaints and claims. Techniques are thus needed to characterise the modification of quality in terms of improved target properties of TMT during industrial production, and of TMT products that have been in service for an arbitrary time. In this study, we aimed to utilise dynamic vapor sorption (DVS) for this purpose. Norway spruce (Picea abies) and European beech (Fagus sylvatica) samples were thermally modified at different temperatures according to different heat treatment techniques: (1) the Silvapro process based on an initial vacuum; (2) an air heat treatment, whereby samples were wrapped in aluminium foil; (3) thermal modification of wood samples in the ambient atmosphere in a laboratory oven. Wood samples from closed processes were analysed for validation. TMT was characterised with respect to mass loss, colour and density. Mass loss of wood due to modification (MLTM) was correlated with factors derived from DVS analysis. The present DVS measurements suggest that the equilibrium wood moisture content (EMC95% RH), the time to reach 10% wood moisture content (t10% MC), and the elongation factor, c, derived from a logarithmic function, can serve as alternative parameters to characterise the quality of several thermal modification processes. Further studies are recommended using other wood species, different modification processes and further parameters gained from DVS measurements to understand the robustness and the predictive power of the applied technique.

Performance of Thermally Modified Spruce Timber in Outdoor Above-Ground Conditions: Checking, Dynamic Stiffness and Static Bending Properties

Previous studies have shown that thermally modified wood (TMW) performs well in outdoor, above-ground conditions in terms of resistance to wood-decaying fungi. Yet, little is known about the development of defects such as checks and the corresponding mechanical properties of TMW in this condition. This experiment focused on the effect of 30 months outdoor above-ground exposure (weathering) on the degree of checking, dynamic stiffness and static bending properties of thermally modified timber (TMT) of Norway spruce. Two board pairs per log were cut from 190 logs; one board of each pair was thermally modified and the other used as control. Then, 90 board pairs were exposed to the weather in south Sweden. Surface checking and axial stiffness were monitored at six-month intervals by using digital photography and non-destructive tests (time-of-flight and resonance method) to monitor changes in the material upon weathering. Finally, all boards were tested destructively in a 4-point static bending test following EN 408 standard. Results showed that weathering had no significance influence on static bending properties of TMT even though the degree of checking was considerably higher in TMT than unmodified timber after weathering. In particular, checks along growth rings were deeper, longer and more common in TMT after weathering, especially on the pith side of boards. The maximum depth of these checks did not depend on board orientation (i.e., which side was exposed) and exceeded limits given in strength grading standards for 7% of the modified boards included. Axial dynamic stiffness determined at 6-month intervals was less influenced by fluctuations in moisture content for TMT compared to unmodified timber, but did not confirm the increase in the degree of checking of TMT. The presence of checks from weathering did influence failure modes in TMT; horizontal shear failure became more frequent and some boards failed in compression.

Crack formation, strain distribution and fracture surfaces around knots in thermally modified timber loaded in static bending

The effect of thermal modification (TM) on the chemistry, anatomy and mechanical properties of wood is often investigated using small clear samples. Little is known on the effect of growth-related and processing defects, such as knots and checks, on the bending strength and stiffness of thermally modified timber (TMT). Nine boards of Norway spruce with different combinations of knot types were used to study the combined effects of checks and knots on the bending behaviour of TMT. Digital image correlation (DIC) measurements on board surfaces at sites of knots subjected to bending allowed to study strain distribution and localise cracks prior to and after TM, and to monitor development of fracture (around knots) in TMT to failure. DIC confirmed that checking in knots was increased after TM compared to kiln-dried timber, specifically for intergrown knots and intergrown parts of encased knots. Effects appear local and do not affect board bending stiffness at these sites. Bending failure in TMT initiated mainly at knot interfaces or besides knots and fractures often propagated from checks. Scanning electron microscopy analyses of fracture surfaces confirmed this, and fractures were typically initiated around knots and at knot interfaces due to crack propagation along the grain in the longitudinal–radial plane (TL fracture) under mixed mode I and II loading, such that boards failed in simple tension like unmodified timber. Images of fracture surfaces at the ultrastructural level revealed details of the brittle behaviour of TM wood. This was especially apparent from the smooth appearance of transwall failure under mode I loading across the grain.

Prediction of bending strength of thermally modified timber using high-resolution scanning of fibre direction

The market share of thermally modified wood (TMW) has increased in Europe during the past few years as an environmentally friendly and durable building product. However, TMW products of today are not permitted for use in structural applications, because the reduction in strength that is caused by thermal treatment cannot be accounted for. The purpose of this paper was to investigate the bending properties of thermally modified timber (TMT) of Norway spruce, and to explore possibilities to predict the bending properties of TMT. A sample of 100 boards from a 2X-log sawing pattern of 100 logs was thermally modified according to the ThermoWood® process, while the mirror 100 boards served as an unmodified control sample. Two non-destructive methods were employed: (1) a novel method based on scanning of fibre directions to obtain the lowest edgewise bending modulus of elasticity (MOE) along a board, and (2) a conventional excitation method to determine the first axial resonance frequency used to calculate the axial dynamic MOE. Finally, the boards were bent to failure according to European standard EN 408. Despite the fact that bending strength was reduced by 42% due to thermal treatment, the type and location of failure in TMT remained related to the presence of knots. Prediction of bending strength based on local fibre direction and axial dynamic MOE gave coefficients of determination of 0.51 for the thermally modified boards and 0.69 for the control boards, whereas axial dynamic MOE alone gave 0.46 and 0.57, respectively. These results indicate that although Norway spruce TMT has lower bending strength compared to unmodified timber, predictions of the bending strength can be made with good accuracy.

Energy and environmental profile comparison of TMT production from two different companies - a Spanish/Portuguese case study

Life Cycle Assessment (LCA) is a technique for assessing the environmental aspects and potential impacts associated with a product and has been increasingly used to identify processes or stages in the wood chain with a high environmental impact or to highlight areas where environmental information is unknown. The main aim of this study was to quantify and compare the environmental impacts and the energy used for the production of one cubic meter of Thermally Modified Timber (TMT) by two different companies, one in Spain and the other in Portugal, using the LCA methodology. The LCA study was developed based on ISO 14040/44 standards. The inventory analysis and, subsequently, the impact analysis were performed using the LCA software SimaPro8.1.0.60. The method chosen for the environmental impact assessment was ReCiPe, and for energy use the Cumulative Energy Demand method was chosen. The results show that to produce 1 m3 of thermally modified pine timber the Portuguese company used 14.38 GJ of cumulative energy demand, of which 1.92 GJ was nonrenewable and 12.46 GJ renewable, and the Spanish company used a total of 17.55 GJ, of which 2.52 GJ was nonrenewable and 15.03 GJ renewable. The thermally modified pine timber produced by the Spanish company presented the best environmental results for 13 impact categories in comparison to the 5 best environmental results presented by the Portuguese company. From the weighting triangle, we can conclude that the Portuguese pine boards have a lower environmental impact than Spanish pine boards if a high weight (> 40%) is given to resources, while a weight of <80% is given to human health; otherwise the opposite is true. Regardless of the company, the energy used in the thermal treatment process was identified as the main factor responsible for climate change, acidification, eutrophication, photochemical oxidant formation, metal depletion and fossil depletion. This has to be expected as the treatment is based on heat production and no chemicals are added during the heat treatment process. The round wood production was identified as the leading process responsible for ozone depletion and also presented remarkable contributions to eutrophication and photochemical oxidant formation.

Performance of thermally modified wood during 14 years of outdoor exposure

The long-term outdoor performance of thermally modified timber (TMT) from four different industrial heat treatment processes was investigated. Graveyard tests (in-ground exposure, use class (UC) 4) and horizontal double layer tests (above ground, UC 3.2) with and without artificial shading were performed. Results from field tests after 14 years of exposure are compared with results from short-term laboratory tests with pure cultures of wood-destroying basidiomycetes and terrestrial microcosms (TMC). The thermally modified material used in the study was: Plato wood (Norway spruce, Picea abies, NL), Thermowood (Scots pine, Pinus sylvestris, FI), New Option Wood (NOW, Maritime pine, Pinus maritima, F), and oil-heat treated (OHT) wood (Scots pine, DE). In addition, Scots pine sapwood and heartwood, Douglas fir (Pseudotsuga menziesii), Sessile oak (Quercus petraea), and CCB treated Scots pine sapwood (0.7 and 2.8% CCB) were examined as control and reference materials. Based on results from laboratory standard tests with brown and white rot causing basidiomycetes all thermally modified materials were classified as durable to slightly durable (durability class (DC) 2–4). In TMC tests with unsterile soil against soft rot and other soil-inhabiting micro-organisms all TMT were classified as durable (DC 2). Consequently, all TMT showed remarkably increased durability compared to the untreated controls (DC 5) and reference species (DC 3–5). In contrast, according to the results of the graveyard tests all four TMTs were classified less durable (DC 4) as well as Douglas fir and Sessile oak. Also exposure above ground in horizontal double layers led to a significantly worse durability compared to the one calculated on the basis of lab test results: Plato, Thermowood, and NOW were classified as ‘less durable’ (DC 4), OHT performed significantly better. Since not all specimens had failed its durability was preliminarily classified as ‘very durable’ (DC 1). On the basis of 14 years outdoor testing under different exposure conditions durability of four industrially produced TMTs turned out to be clearly less than lab test results indicated. Neither in-ground nor above ground performance met the expectations based on testing with basidiomycete cultures and TMCs. Therefore, short-term durability test methods need improvements regarding their predictive capability for modified wood. In summary, the use of TMT in ground as well as under severe above ground conditions cannot be recommended based on the findings from this field test study.

Novel color stabilization concepts for decorative surfaces of native dark wood and thermally modified timber

Abstract UV and visible light exposure of dark colored wood and thermally modified timber (TMT) is strongly connected with discoloration and photobleaching of the respective wood surfaces. Conventional and nanoscale UV absorbers as well as radical scavengers (HALS), mainly effective for the protection of softwood and bright hardwood, were found to be improper for the light stabilization of dark wood surfaces. In the current work discoloration mechanisms are discussed considering the role of wood extractives and the sensitivity of dark colored wood types to visible light and results are presented which were obtained by applying novel protecting systems for the light stabilization of transparently coated surfaces from dark colored wood types and TMT.

Production and markets of modified wood in Russia

Understanding the state of market for modified wood as well as the forces that affect its future development will be essential for those seeking to enter the Russian market. Information on modified wood from websites of companies, research and development organisations, woodworking-oriented magazines and forums, and official Russian statistics were explored, and finally verified by personal contacts with experts. A number of modification methods have been either researched, developed or pre-commercialised during the past 15 years, but only few of them have been adopted by users. Mostly, the modification methods have been copied, further developed and commercialised from variations of thermal modification originally introduced in Western Europe. Modified wood products are still considered premium goods among Russian consumers. Political initiatives aim at increasing the share of wood in construction and renovation but also at import minimisation, which reduces the penetration of foreign producers into the markets. There are approximately 20 producers of thermally modified timber (TMT) with an overall annual capacity of 60,000–65,000 m3. The current annual market for TMT is estimated to be approximately €35–40 million. In contrast to the European mainstream end uses of TMT, there is also a market for massive and glued thermally modified logs in Russia.

Dauerhaftigkeit und mechanische Stabilität von Terrassendielen bei erhöhter Temperatur – Ein Werkstoffvergleich: WPC, thermisch modifiziertes Holz und Massivholz

This article deals with the differences in the long-term durability and thermal stability under load of terrace deckings from various materials. The tested materials were deckings made of wood, thermally modified timber (TMT), and wood–polymer–composites (WPC). For the determination of the test temperatures for component testing according to EN 310, the surface temperatures of the decks during a normal hot summer day were measured. A cyclic test according to EN 321 was applied to all decking materials. Afterwards the component testing was repeated. All wooden samples reveal considerable cracks, some were twisted, and few were even broken. In particular regarding the optical appearance, wood decks show advantages against the TMT decks. Some WPC decks show very fine cracks on the face, which were additionally analyzed by means of X-ray computed tomography (CT). Except for the WPC deck with higher wood content, no WPC deck revealed significant changes after the cyclic test. The CT analysis was also suitable to find cracks inside the materials and illustrate them. Thus, the whole damage inside a sample could be characterized by calculating a kind of error pattern. No considerable cracks or failures could be observed on the WPC decks.

Non-destructive evaluation of checking in thermally modified timber

Non-destructive measurement techniques were used to study the characteristics of thermally modified timber (TMT), especially checking. Two non-destructive measurement techniques were evaluated: air-coupled ultrasound (ACU) and electrical impedance spectroscopy (EIS). In the studied TMT (N = 38), density and pith location in the cross-section were positively correlated with the number of checks in the cross-section. Several ultrasound signal features correlated with TMT characteristics, most strongly with number of checks. Severely checked samples and other samples were considered as two different classes of checking, and the potential of ultrasound for differentiating between the two classes was tested by using Bayesian classification method. The correct classification rate was 97 %. EIS parameters correlated with density and latewood content of TMT more strongly than the ACU parameters. Thus, for determination of checking and density or earlywood/latewood content, both EIS and ACU measurements are recommended.

Modeling the influence of thermal modification on the electrical conductivity of wood

Abstract A model has been developed aiming at the description of the effect of thermal modification on the electrical conductivity of wood. The intention was to calculate the moisture content (MC) of thermally modified timber (TMT) through the parameters electrical resistance R, wood temperature T, and CIE L*a*b* color data, which are known to correlate well with the intensity of a heat treatment. Samples of Norway spruce (Picea abies Karst.) and beech (Fagus sylvatica L.) samples were thermally modified in laboratory scale at 11 different heat treatment intensities and the resistance characteristics of the samples were determined. Within the hygroscopic range, a linear relationship between the resistance characteristics and the mass loss (ML) through the heat treatment was established. Based on this, a model was developed to calculate MC from R, T, and ML. To validate this model, color values of 15 different TMTs from industrial production were determined for estimation of their ML and fed into the model. MC of the 15 arbitrarily heat-treated TMTs was calculated with an accuracy of ±3.5% within the hygroscopic range. The material-specific resistance characteristics based on experimental data led to an accuracy of ±2.5%.

Moisture content determination of thermally modified timber by electrical and ultrasound methods

Thermal modification is an increasingly popular process that increases biological durability and dimensional stability of timber. The equilibrium moisture content of thermally modified timber (TMT) is lower than that of untreated timber, as is the fibre saturation point. For these reasons, traditional methods of moisture content determination have to be reviewed and calibrated specifically for TMT and novel methods may prove more suitable for online measurement systems. In this study, the moisture content (MC) of TMT was measured using conventional dielectric and resistance meters as well as by the air-coupled ultrasound method and electrical impedance spectroscopy (EIS). The timber was kept outdoors for a period of 10 weeks while the MC was determined every other week so that a wide range of MCs could be measured. It was seen from the results that the conventional meters were not readily calibrated for TMT and the measurement range of these methods is small due to lower fibre saturation point. The study showed the potential of EIS and air-coupled ultrasound for MC analysis of TMT. Best correlation with MC at higher MCs was achieved with the maximum amplitude of through transmission signals by the ultrasound method. Higher precision and a wider range of MCs could be measured by combining both EIS and ultrasound parameters.

Performance of thermally modified timber (TMT) in outdoor application - durability, abrasion and optical appearance.

Thermally modified timber (TMT) is increasingly offered in Europe as an alternative to preservative treated timber. TMT durability in field tests as well as its moisture sorption behaviour in facade application was determined so as to consider its suitability for outdoor use. Additionally, abrasion and crack-formation of TMT deckings were examined and the optical appearance of a TMT facade was evaluated after 5 years of service. After 7.5 years exposure in ground contact, the various TMT materials tested were classed as “slightly durable” to “not durable” whereas the classification in above ground exposure was “very durable” to “moderately durable”, which was in line with the reduced moisture sorption of TMT in weathered application. Moreover, the TMT-decking showed less abrasion and crack-formation compared to references, though the TMT facade revealed considerable discoloration by weathering. Hence, the suitability of TMT for above ground use is suggested, but a surface treatment is obligatory if discoloration is objectionable.

Influence of treatment temperature and duration on selected biological, mechanical, physical and optical properties of thermally modified timber

The impact of heat-treatment temperature (180, 200, 210, 220 and 240°C) and various heat-treatment durations on selected biological, mechanical, optical and physical properties of thermally modified timber (TMT) was determined. The suitability of different measures for prediction of the treatment intensity was also investigated. Resistance to impact milling (RIM), lightness L*, equilibrium moisture content (EMC), and antiswelling efficiency (ASE) were correlated with corresponding fungal resistance achieved by heat treatments. The results show that the decrease in mass by heat treatments is a suitable measurand to describe the treatment intensity, which is a product of treatment temperature and duration, where the impact of temperature is dominant over the impact of time. The properties examined showed a strong reciprocally proportional relationship with the decrease in mass. Different correlations were found for the various treatment temperatures: the higher the temperature applied, the lower the decrease in mass required for an equivalent improvement in certain wood properties, e.g. biological durability, EMC and dimensional stability. However, mass loss by Poria placenta correlated well with RIM, lightness L*, EMC and ASE of the different heat-treated specimens, depending on the heat-treatment temperature. Consequently, a reliable estimation of improved fungal resistance of TMT, as well as quality control of TMT in general, requires certain process information.

Durability of thermally modified timber from industrial-scale processes in different use classes: Results from laboratory and field tests

Material from four common European processes of industrial heat treatment of timber was examined comparatively with reference to biological laboratory standard tests and field tests in soil contact and above ground. The thermally modified timber (TMT) used in the study was: Plato wood from the Netherlands, ThermoWood from Finland, New Option Wood (NOW) from France, and oil–heat-treated wood (OHT) produced in Germany. Tests of resistance to basidiomycetes (EN 113, 1996) and tests of resistance to soft rotting microfungi and other soil-inhabiting microorganisms (EN 807, 1997) showed substantially lower mass losses of TMT compared with controls. Only slight differences in mass loss were found between the four thermally modified materials. Based on results from laboratory standard tests all tested heat-treated materials were classified as durable to moderately durable [durability class (DC) 2–4], analogous to the classification of natural durability (EN 350-1, 1994). In contrast, the classification of TMT samples after 5.5 years’ exposure in soil contact, in accordance with EN 252 (1990), was slightly durable to not durable (DC 4–5), whereas the classification obtained after 5.5 years’ exposure in double layers in European hazard class (EHC) 3 (EN 335-1, 1992) was very durable to moderately durable (DC 1–3). On the basis of 5.5 years’ field testing, thermally modified material (independent of the treatment process and supplier) appears not to be suitable for in-ground contact application. However, the suitability of TMT for use out of ground in EHC 3 was ascertained and is recommended.