Effects of thermal modification on the water resistance and colour stability of wood species from Bosnia and Herzegovina

The effect of UV irradiation on different wood species represents an open subject of research. Six species, half softwoods and half hardwoods, all important for both historic and modern furniture, were subjected to the action of artificial UV radiation at a constant temperature of 40 °C. The species were: fir (Abies alba), pine (Pinus sylvestris), larch (Larix decidua), ash (Fraxinus excelsior), cherry (Prunus/Cerasus avium) and walnut (Juglans regia). Previous research showed that important and rapid changes occur within the first 24 h of exposure, and therefore, a short exposure time was adopted in this research, where the specimens were subjected to the action of UV-rich artificial light (UVB–UVA–Vis, 295–600 nm, UVA-Spot 400T source), at a constant temperature of 40 °C, in a climatic chamber. The effect of exposure was measured after 6, 12, 18 and 24 h. The test samples were covered with black cardboard on half of their surface to block the access of UV radiation and allow the investigation of temperature effect with none or limited UV penetration. The experiments proved that UV radiation caused rapidly colour changes (after 6 h), as darkening, monitored by direct observations and colour measurements in the CIE Lab system. The overall colour difference ΔE evolved in time, reaching, after 24 h UV exposure at 40 °C, a maximum for pine (15.93) and minimum for ash (7.05). FTIR–ATR analyses were performed to determine surface chemical structure changes associated with the measured colour changes. UV radiation caused photodegradation phenomena as lignin degradation and photooxidation processes leading to the formation of carbonyl containing chromophores, even after short-time exposure periods (6 h), which evolved in time following patterns influenced by the wood species. The moderate temperature had only a limited effect on the covered specimens, on both colour and surface chemistry changes, compared to the effect combined with UV irradiation, but particularities were observed in the behaviour of the wood species studied. The combined analysis of FTIR–ATR results with CIE Lab measurements showed only a slight trend in case of yellowness, which was associated with lignin degradation, but none for redness, associated mostly with wood extractives. The different behaviour of the wood species studied may be related to the differences in their chemical composition, especially lignin and extractives content.

Simple SummaryOver the past thirty years, the thermal modification of wood has become a universally recognised and commercialised wood modification process. Thermal modifications may affect wood properties, either positively (dimensional stability and decay resistance) or negatively (mechanical properties). The combination of the impregnation of specific reagents with thermal modification may help to overcome the negative effects on wood properties. In this study, we evaluate the effect of a combination of two zwitterionic buffers, bicine and tricine, and thermal modification of two wood species (beech and spruce) against subterranean termites and their symbiotic fauna. Bicine and tricine treatments alone had a clear influence on wood mass loss and termite survival. The flagellate protist symbiotic community was affected by the treatments and responded differently to them, as a highly adaptable community. However, the combination of bicine with the thermal modification showed a negative effect on termites and their symbionts on both wood species. The combination of these different factors should be further investigated, as these results seem to be promising with regard to the enhancement of the termite resistance of wood.The majority of thermal modification processes are at temperatures greater than 180 °C, resulting in a product with some properties enhanced and some diminished (e.g., mechanical properties). However, the durability of thermally modified wood to termite attack is recognised as low. Recent attempts at combining thermal modification with chemical modification, either prior to or directly after the thermal process, are promising. Buffers, although not influencing the reaction systems, may interact on exposure to certain conditions, potentially acting as promoters of biological changes. In this study, two zwitterionic buffers, bicine and tricine, chosen for their potential to form Maillard-type products with fragmented hemicelluloses/volatiles, were assessed with and without thermal modification for two wood species (spruce and beech), with subsequent evaluation of their effect against subterranean termites (Reticulitermes grassei Clément) and their symbiotic protists. The effect of the wood treatments on termites and their symbionts was visible after four weeks, especially for spruce treated with tricine and bicine and heat treatment (bicine HT), and for beech treated with bicine and bicine and heat treatment (bicine HT). The chemical behaviour of these substances should be further investigated when in contact with wood and also after heat treatment. This is the first study evaluating the effect of potential Maillard reactions with zwitterionic buffers on subterranean termite symbiotic fauna.

Measurement of three wood materials against weathering during long natural sunlight exposure

The effect of thermal modification and extracts of Scots pine sapwood and heartwood, and Norway spruce on the colonisation by the bacterium, Escherichia coli was studied. All wood samples caused more rapid decrease of bacterial numbers compared to glass, which was used as reference material. Pine sapwood caused somewhat faster decrease of bacterial count than the other wood types. On the other hand, both thermal modification and extraction increased the bacterial count on all the samples compared to untreated wood samples. Neither the amount of extractives nor the faster drying of the surface, to which the bacterial inoculum was added, could alone explain this result; rather it is likely that this is due to a combination of both factors.

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.

The wood of five European species: black poplar (Populus nigra L.), European beech (Fagus sylvatica L.), European ash (Fraxinus excelsior L.), European oak (Quercus robur L.), and Scots pine (Pinus sylvestris L.) was subjected to thermal modification in nitrogen atmosphere at 190 °C during 6 h. Native and modified wood was varnished and oiled in industrial conditions. Thermally modified (TM) wood was characterized by a greater absorption of varnish and oil when applying the first layer to the surface, which finally resulted in higher application values compared to native wood. In particular, after varnishing, there was a significant increase in gloss and radical change of colour. Regardless of the wood species, finishing process (varnishing, oiling), the ΔE values were close to or higher than 6, which proves high colour changes. Modified poplar, ash, and oak after varnishing had a different colour (ΔE higher than 12). The surface colour changes as a result of UV photoaging was individual, depending on the wood species and the method of finishing. In the case of the thickness of varnish coatings, the wood structure was important, i.e., on ring-porous hardwood and softwood they were thicker. In the case of wood species with a lower density, i.e., black poplar and pine, the thermal modification in nitrogen atmosphere process did not reduce the resistance of the varnish coat, and in the case of species with a higher density (oak, ash, beech) it decreased by one level. Thermal modification reduced the Brinell hardness of wood with wide rays (oak and beech) by 11%. The applied process of surface finishing by double varnishing or oiling did not significantly change the hardness of tested wood.

The ageing of materials is an irreversible, in-time occurring complex phenomenon, which affects both wood surfaces and the coating materials. This paper focuses on the light-induced natural ageing of wood-coated surfaces in indoor conditions. Two wood species: European maple (Acer pseudoplatanus) and European walnut (Juglans regia) and two types of waxes: bees wax and Chinese wax were employed in the experiments presented in this paper. Uncoated and coated wood samples were exposed to the natural sunlight filtered by window glass in a simulated indoors natural ageing test for a total period of 7 years. Colour measurements in the CIE-Lab system and FTIR-ATR investigation were employed to evaluate the ageing phenomena. The uncoated wooden samples underwent progressive colour changes perceived as darkening for European maple and lightening for European walnut, corresponding to total colour differences values (ΔE) after 7 years of exposure of 12.54 and 11.66, respectively. Coating of wood samples with the two types of waxes differently influenced the total colour changes for the two wood species: reduced colour changes corresponding to ΔE values of 4.79–6.44 were determined for European maple, whilst increased colour changes corresponding to ΔE values of 13.80–20.83 were determined for European walnut. FTIR analysis highlighted different surface chemistry changes for the uncoated and wax-coated wood samples.

Although the effect of thermal modification (TM) on teak wood color is well documented, few studies have been carried out on closed-system processes, and it remains unclear what the effect is of different processes on the same material. This work aimed to verify the effect of closed- and open-system processes of TM on the color of short-rotation teak wood. Thermally modified wood (TMW) was evaluated in a closed system at 160 °C (CS160) and in an open system at 185 °C and 210 °C (OS185 and OS210). We measured the moisture content (initial and final) of the wood and the corrected mass loss (CML). The chemical analyses encompassed the contents of alpha-cellulose, hemicelluloses, lignin, and extractives (total, in acetone and dichloromethane). Wood color was measured before and after TM according to the CIEL*a*b* color space. It was possible to achieve the same color using different processes of thermal modification (CS160 and OS210). TM reduced wood lightness (L*), red–green chromaticity coordinate (a*), and yellow–blue chromaticity coordinate (b*). L* and a* had the biggest and smallest variations, respectively. TMW color was significantly changed, even at the mildest condition tested (OS185, 0.33% CML).

In this research, the effect of thermal modifications at 170°C, 190°C, 210°C and 230°C on the wettability of sapwood and heartwood of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) was studied by measuring the static contact angles of distilled water on the surfaces as a function of time. The results were compared to industrially kiln-dried reference samples. The thermal modification at the lower temperatures of 170°C and 190°C increased the wettability of all wood materials with the exception of the heartwood of pine that had been thermally modified at 170°C, which was the most water-repellent material in the whole study. Thermal modification at the very high temperature of 230°C was needed to decrease the wettability of wood. The differences in water repellency between sapwood and heartwood were greater for pine than for spruce.

Densification and thermal modification change wood properties in different ways depending on the treatment conditions and the wood species. In the presented investigations, densification and thermal modification were applied consecutively. The primary objective of this treatment combination was the compensation of reduced mechanical properties due to the thermal modification by densification. The combined processes were applied to five European wood species: poplar (Populus nigra L.), beech (Fagus sylvatica L.), Norway spruce (Picea abies Karst.), English oak (Quercus robur L.) and European ash (Fraxinus excelsior L.). Depending on the mean density of the species, a thermo-mechanical densification of 43 or 50% was imposed to improve mechanical strength parallel to the grain. Subsequently, the densified material was thermally modified in the so-called Vacu3-process at 230 °C and 20 or 80% vacuum and at 240 °C and 20% vacuum. The thermal modification resulted in changing wood colour, mechanical strength, hardness, dimensional stability and durability. All the wood modification processes were carried out at industrial scale after pre-tests at laboratory scale. The modified material was characterized regarding flexural properties, static and dynamic hardness, structural integrity, abrasion resistance, moisture dynamics, dimensional stability, and durability against white, brown and soft rot fungi. In summary, the test results showed that the consecutive application of thermo-mechanical densification and thermal modification leads to significantly improved durability whilst mechanical properties at least for beech, ash and poplar remained and the material is dimensionally stable.

Combining mineralisation and thermal modification to improve the fungal durability of selected wood species

Impact of thermal modification on the chemical changes and impact bending strength of European oak and Norway spruce wood

The study investigated and compared the behaviour of four wood species, originating from Europe and China, in terms of temperature-induced artificial ageing. It was conducted at 100 °C for a total period of 288 h. Ageing effects were evaluated by colour measurements in the CIE Lab system and by FTIR analysis. Colour changes were then related to chemical changes in the wood. The investigated wood species were European ash (Fraxinus excelsior), European walnut (Juglans regia), Chinese ash (Fraxinus mandshurica) and Chinese walnut (Juglans mandshurica). Colour changes were maximum for European ash and minimum for Chinese ash, while European walnut and Chinese walnut evolved quite similarly. Main chemical changes due to temperature ageing were reduction of hydroxyl groups, increase of the unconjugated carbonyl groups and an apparent slight increase of lignin, more evident for European ash and delayed for European walnut. Formation of aromatic carbonyl conjugated groups as quinoid structures as a result of oxidative reactions was revealed especially for European ash. The different behaviour of the studied wood species may be explained by their different chemical composition, especially hemicelluloses, lignin and extractives content.

The mass loss kinetics of thermally modified wood species was analyzed as a time–temperature function. European beech (Fagus sylvatica L.), English oak (Quercus robur L.), Norwegian spruce (Picea abies L. Karst.) and Scots pine (Pinus sylvestris L.) wood specimens of dimensions 20 × 20 × 10 mm3 were thermally modified at 140 °C, 160 °C, 180 °C, 200 °C and 220 °C for 1–6 h using atmospheric pressure and superheated steam environment. The process intensity was determined by mass loss (ML), based on oven-dry mass before and after the thermal modification. Furthermore, the equilibrium moisture content (EMC) was determined before and after thermal modification to analyze the effect of mass loss on the sorption properties. Measured mass loss data were compared with the three-dimensional analytical function and its applicability to mass loss prediction was verified. For the studied wood species, the ML was found to be less than 1–1.5% when temperature of 140 °C and 160 °C was applied. Differences between studied species were more significant at temperatures higher than 160 °C. With the highest tested temperature (220 °C), mass loss reached 13.5% (beech), 18.8% (oak), 6.7% (spruce) and 13.5% (pine). According to the results, hardwoods have been shown to be more sensitive to the thermal degradation than softwoods as demonstrated by the higher mass loss recorded for the same modification time and temperature. The three-dimensional analytical function was confirmed as valid for all the species studied (R2 = 0.89–0.99) and relevant for the mass loss prediction using fitted parameters. The EMC was reduced after thermal modification within the range of 4–48%, 0.4–47%, 1–32% and 0.7–40% for beech, oak, spruce and pine, respectively. Further, the EMC correlates exponentially (R2 = 0.91–0.95) with the decrease in the specimens’ mass depending on the wood species used and modification temperature applied. However, the EMC seems to be almost stabilized beyond a limit value of approximately 10–12% of mass loss. The results provide a better insight into the mass loss and EMC kinetics of thermally modified wood species and can be used as a tool for prediction of mass loss values and required material properties (EMC) for designed wooden products.

Stabilising the natural colour of wood species exposed to light in indoor conditions is a challenge that could be better addressed based on a deeper understanding of the occurring phenomena and influencing factors. This paper investigates comparatively the light-induced colour changes for three hardwood species, namely, European maple (Acer pseudoplatanus L.), European ash (Fraxinus excelsior L.) and European walnut (Juglans regia L.), as well as the influence of finishing with three types of clear, colourless waterborne lacquers: acrylic-polyurethane (F1), acrylic (F2) and polyurethane (F3) on their colour stability. Colour measurements in the CIELab system in conjunction with an artificial accelerated ageing test under the action of UV-VIS radiation, simulating natural light passing through window glass, and two types of test samples were employed to highlight the influence and contributions of the wood substrate and of the coating films to the global colour modifications. Coating films applied on 1 mm clear glass slides were employed as a sort of “detachable” finish for this purpose. Direct exposure to UV-VIS light caused visible colour changes for both uncoated and coated wood surfaces, the values of the calculated colour differences (ΔE) after a 72 h exposure being dependant on both the wood species and the coating material. Excepting two situations for walnut, statistically significantly higher colour differences were obtained for the uncoated samples: maple (9.36 units), ash (8.39 units), walnut (6.20 units), compared to the coated ones: maple (4.92–5.71 units), ash (2.25–3.94 units), walnut (4.74–7.70 units). The wood substrates underlying the coating films were found to bring the maximum contribution to the overall colour changes in the clear coated surfaces, while the coating films employed in this research demonstrated a fairly good colour stability to UV-VIS light exposure, with maximum colour changes (∆E) up to only 1.30 units. Overall, the wood species and the type of coating were found as influencing factors in interaction with the light-induced colour changes in wood surfaces in indoor conditions.