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Thermally modified birch wood interaction with liquids
Dace Cirule
1*, Anrijs Verovkins
1, Ingeborga Andersone
1, Edgars Kuka
1,2, and Bruno Andersons
11Laboratory of Wood Biodegradation and Protection
Latvian State Institute of Wood Chemistry Riga, LV-1006, Latvia
2Faculty of Material Science and Applied Chemistry
Riga Technical University Riga, LV-1048, Latvia
ABSTRACT
Large research work is currently being performed concerning different elaborated new wood protection methods. However, combining industrially well-approbated processes is also considered potentially quite promising and such approach is being actively studied. This study is part of a project that is aimed at improving wood service properties by combining thermal modification (TM) and impregnation with Cu-organic preservatives. The objective of the present study was to investigate peculiarities of interaction between liquids and TM birch wood (Betula spp.). This knowledge is essential for proper TM wood post-treatments involving its impregnation as well as for evaluation of potential wood moisture dynamics in outdoor applications.
Changes caused by TM (150-170°C) in a closed system under elevated pressure in wood wettability, permeability, liquid absorption capacity, and drying characteristics were evaluated. The results concerning absorption capacity, which is mainly related to wood anatomical features and is density dependent, indicated to reduced absorption capacity of TM wood compared with unmodified birch of similar density. Permeability, which characterises the ease with which liquid is transported through wood porous system, was evaluated by capillary absorption tests through the specimens’ tangential and radial surfaces. TM made birch wood less permeable through both surfaces as well as less anisotropic regarding transverse absorption rates. Moreover, TM caused also decrease in drying rates for wood impregnated with both water as well as biocide solution. Reduction in permeability influences impregnation process of boards and not full saturation was detected for TM boards when impregnation schedule providing complete saturation for unmodified boards was applied. On the other hand, less water was absorbed by TM boards exposed to rain on outdoor weathering racks.
1.INTRODUCTION
Thermal modification (TM) is a commercial wood treatment method aimed at enhancement of wood dimensional stability and biological durability without using any chemicals. Several industrial-scale TM processes differing in the treatment parameters such as environment, temperature, time are developed and industrially implemented (Hill 2011). Despite process differences, all TM causes complex reactions including certain destruction of low-molecular substances and hemicelluloses, reorganisation of lignin and cellulose and evaporation of volatile compounds resulting in wood weight losses and alteration of its properties (Militz and Altgen 2014).
It is well recognized that regardless of the applied TM process and treatment parameters, reduction of certain degree in wood hygroscopicity, which characterizes the interaction between wood and water vapour, takes place. Changes in wood sorptive behaviour in various humidity ranges have been intensively studied and numerous results concerning different aspects of wood equilibrium moisture content (EMC) alteration due to TM are available. It is found that the reduction of EMC considerably varies depending on the used TM method, temperature, time as well as on the treated wood species and characteristics of the individual specimen (Chirkova et al. 2007, Militz and Altgen 2014, Willems et al. 2015).
Much less focus has been on studying peculiarities of interaction between TM wood with liquids. Moreover, the reported results are quite fragmentary and, contrary to the observed similar trend of reduced EMC due to the TM, no common trend has been found and both increase and decrease in wood liquid water uptake are reported for TM wood (Metsa -Kortilainen et al. 2006, Johansson et al. 2006, Pfriem 2011). In addition, majority of this research is done on softwoods. Another wood characteristic influencing its interaction with liquids is surface wettability which is evaluated by measuring contact angle. However, results regarding TM caused alteration of wood surface wettability also are inconsistent. Alongside with usually observed increase in contact angle due to the TM also decrease in contact angle for TM wood treated at lower temperatures are reported (Hakkou et al. 2005, Kocaefe et al. 2008, Metsä-Kortelainen and Viitanen 2012). Nevertheless, when used in exterior applications wood quite often is exposed to
* Corresponding author: E-mail: dace.cirule@edi.lv
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occasional contact with liquid water in a form of rain or dew. Consequently water absorption of certain extent can take place which will inevitably influence wood moisture content. Alteration of wood moisture content causes dimensional changes that can result in surface check and crack formation. Moreover, elevated wood moisture content negatively affects its mechanical strength. In addition, moisture content is crucial for the germination and growth of decay fungi. Therefore, the time of wood wetness above certain critical level is of great importance. This time depends on both water absorption and desorption velocities implying the importance of drying rate when long wood service life expectations in outdoor use are considered. Impregnation with biocide or fire-retardant water solutions is another area for which knowledge about wood interaction with liquid water is of greater significance (Rapp et al. 200, Van Acker et al. 2015).
Nevertheless TM imparts improved bio-resistance to wood, adequate wood protection against biodegradation is ensured only by wood treatment at temperatures that cause significant reduction of wood mechanical strength (Kamdem et al. 2002, Metsä-Kortelainen and Viitanen 2010, Candelier et al. 2017). Consequently, the application area of TM wood with high bio-durability is greatly restricted due to its declined strength properties. On the other hand, the most efficient biocide compositions containing chromium and arsenic, that were traditionally most widely used for wood impregnation to upgrade its bio-resistance, have been banned because of concerns about their environmental effects. As a result impregnation with copper-based formulations has become the main preservation method in use (Freeman and McIntyre 2008). However, the service properties of wood materials impregnated with these formulations are often unsatisfactory unless high biocide dose is introduced into wood. Furthermore, despite numerous investigations, no other simple wood protection method has been found yet for fully satisfactory result regarding wood potential performance in its end-use.
Therefore different complex treatment processes, including pre- or post-treatment of TM wood, aimed at prolonging the service life of wood materials are now intensively investigated and the results reported (Ahmed et al. 2013, Wang et al. 2013, Baysal et al. 2014, Salman et al. 2016, Turkoglu et al. 2016).
The objective of the present study was to evaluate TM caused changes in birch (Betula spp.) wood interaction with liquid water and copper-based preservative solutions. This study is a part of the research project evaluating the possibility to improve wood bio-durability combining wood thermal treatment at mild conditions and impregnation with a commercial biocide. In the project two the most commercially important wood species of Latvia are used: the dominant softwood pine (Pinus sylvestris L.) and the dominant hardwood birch.
2. MATERIALS AND METHODS 2.1. MATERIALS
Kiln-dried boards of birch (Betula spp.) wood measuring 700 × 100 × 25 mm were used for the thermal modification (TM). The modification was carried out in a multifunctional wood modification device of the WTT (Denmark) production. The boards were thermally treated in a closed system in a water-vaper medium under elevated pressure (0.6 – 0.8 MPa depending on the TM temperature) for 1 h at the peak temperature. Three peak temperatures were used: 150, 160 and 170 °C. The designations used in the present paper for wood treated at these temperatures are as follows: TM150, TM160, TM170. The TM boards were conditioned (RH 65 ± 5 %; 20 ± 2 °C) for at least two weeks before preparing the specimens for tests as well as subjecting them to impregnation.
2.2. IMPREGNATION
Impregnation was performed with boards (700 × 100 × 25 mm ) as well as specimens of two sizes, namely, 20 × 20 × 5 mm and 20 × 20 × 100 mm (Rad × Tg × L) which were conditioned before impregnation. Impregnation was performed with the vacuum-pressure method in a laboratory autoclave (vacuum1 kPa for 30 min, pressure 0.8 MPa for 1 h). For impregnation water as well as copper-azole type biocide solutions of three concentration (0.5%, 0.85%, 1.2%) were used.
2.3. CONTACT ANGLE
Contact angle was measured with a goniometer Dataphysics OCA20 (Germany) by applying sessile-drop method and using water droplet with volume of 10 µl. For each specimen contact angles were recorded for 10 droplets on the wood tangential surface during 30 s at a fixed interval of 1 s and the average contact angle values were calculated. Contact angles were measured for ten pre-conditioned (RH 65 ± 5 %; 20 ± 2 °C) specimens of each wood type.
2.4. LIQUIDS ABSORPTION
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Capillary water absorption through radial (Rad) and tangential (Tg) surfaces was evaluated by using ten cubic specimens (20 x 20 x 20 mm) per surface with annual ring and grain orientations strictly parallel to the edges. Four contiguous walls of that one intended for water absorption were sealed with waterproof coating. After conditioning (RH 65 ± 5 %; 20 ± 2 °C) the specimens were installed into a frame that restricted water evaporation from the container and fixed in a position in which the contact surface was 2 ± 0.2 mm under the water. The weight of the specimens was regularly recorded during six days period. The test was carried out in controlled environment (RH 65 ± 5 %; 20 ± 2 °C).
Liquid absorption capacity of wood was evaluated by impregnation of small specimens (20 × 20 × 5 mm), which ensured liquid penetration into the entire specimen volume. The amount of absorbed liquid was determined gravimetrically by recording mass of specimen before and after impregnation.
For evaluation of liquid penetration into the middle part of boards, 10 cm long section from board middle was cut out immediately after board impregnation. The section was divided in 12 similar pieces and moisture content was gravimetrically measured for each piece. Average moisture content for edge pieces and central pieces was calculated. For outdoor exposure boards measuring 700 × 100 × 25 with sealed end grains were installed on a weathering rack inclined at an angle of 45° to the horizontal and facing south. Collection of run-off rain water from each board was accomplished by fastening plastic edgings to prevent water miss and accumulating all water in a sealed bottle through a funnel fixed at the board end.
2.5. DRYING
Drying tests were performed on impregnated specimens of three size, namely, 20 × 20 × 5 mm, 20 × 20 × 100 mm, and 25 × 100 × 700 mm (boards). The specimens were impregnated according to the above described impregnation process and then exposed to drying at RH 65 ± 5 % and 20 ± 2 °C with regular mass control. The moisture content was calculated as the ratio to dry mass of the specimen.
3. RESULTS
3.1. CONTACT ANGLE
Wood surface wettability is influenced by many factors including its anatomical structure, density, and chemical composition. Certain alteration in these wood characteristics is typical for the wood subjected to the TM. Results of contact angle measurements for unmodified and TM birch wood are presented in Figure 1.
Figure 1: Contact angle of unmodified (UM) and thermally (TM) modified birch wood
As can be seen, the contact angles of the TM wood regardless of the treatment temperature are greater comparing with unmodified wood. This indicates to reduction in wettability of birch wood due to TM. Moreover, the higher the TM temperature the greater contact angle increase with more pronounced differences for TM wood treated at 170℃ (TM170). This somewhat contradicts with the results reported by Kocaefe et al. (2008) who observed insignificant influence of the TM treatment temperature on the wood contact angle.
40 45 50 55 60 65 70 75 80 5 10 15 20 25 30 Co ntact an gle, Time, s UM TM150 TM160 TM170
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3.2. LIQUID WATER ABSORPTION
When wood is exposed to liquid water, water absorption into wood occurs through water vapour diffusion and liquid water capillary flow with the latter being the governing process in wood impregnation. Capillary absorption without applying any additional force characterizes the ease of the liquid penetration into wood. Capillary absorption depends on such wood characteristics as species, density, location of its origin in the trunk (sapwood, heartwood, juvenile wood), pretreatment history, as well as the penetrating liquid (Thomas 1976, Siau 1984, Johansson 2006). However, wood anisotropic anatomical structure causes significant differences of liquid flow in wood principal directions regardless of its characteristics. Common feature of all woods is that the flow rate in the longitudinal direction is much greater than in the lateral directions (Siau 1984). However, transverse movement of fluid is pivotal for wood impregnation treatment, as normally the ratio of the board transverse sections area is rather small and the liquid penetration through it is of secondary importance. Moreover, in wood outdoor applications predominately its lateral surfaces are exposed to potential contact with liquid water. Therefore, in the present study capillary absorption tests through wood radial and tangential surfaces were performed for evaluation of TM effect on wood liquid uptake characteristics.
Table 1: Capillary water absorption (g/m2) of unmodified (UM) and thermally modified (TM) birch wood through radial (Rad)
and tangential (Tg) surfaces
Wood type Surface
Time, h 6 24 48 72 144 UM Rad 356 (46) 649 (73) 869 (88) 1011 (96) 1431 (114) Tg 480 (66) 949 (130) 1320 (183) 1585 (226) 2293 (342) TM150 Rad 172 (13) 327 (19) 462 (22) 562 (28) 859 (37) Tg 161 (11) 339 (21) 498 (30) 619 (37) 971 (56) TM160 Rad 174 (19) 342 (35) 487 (47) 596 (56) 914 (83) Tg 177 (20) 351 (31) 506 (40) 622 (48) 935 (71) TM170 Rad 91 (6) 201 (18) 301 (28) 370 (35) 556 (50) Tg 93 (8) 199 (17) 296 (25) 367 (29) 558 (44)
Standard deviations in parentheses
As it was expected, more water in equal time period was absorbed through the tangential surface (into radial direction) into unmodified birch wood (Table 1). This result agrees well with the findings that wood rays, which themselves enable liquid flow from the cambium towards the pith in living trees, are the dominant pathways for liquid water lateral ingress into wood. The predominant route for liquid transport through the radial surface (in tangential direction) goes through pits located in the radial walls of wood cells. However, hardwood fibres possess scanty pit structure which is the crucial obstacle influencing the tangential flow (Murmanis 1979, Siau 1984).
The results show that TM significantly retards liquid water uptake into birch wood through the both studied lateral surfaces. Johansson et al. (2006) detected reduction in water absorption rate also in longitudinal direction of TM birch wood. However, dissimilar extent of TM caused reduction in absorption rate was observed for water movement through the radial and tangential surfaces. Regardless of the applied TM temperature, greater decrease was recorded for capillary absorption through the tangential surface resulting in almost equal water uptake in both directions for the TM wood. These results demonstrate that TM makes birch wood more isotropic regarding lateral water absorption rate. Moreover, quite similar capillary absorption rate was detected for TM birch specimens treated at the two lower temperatures (TM150 and TM160). The highest used TM temperature (TM170) caused somewhat greater reduction through both surfaces but with the same trend of more reduction through the tangential surfaces resulting in similar water uptake regardless of the surface.
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The laboratory results stating reduced liquid water absorption tendency of TM birch wood were confirmed also by a test in which unmodified and TM boards were exposed outdoors on weathering racks. Water absorption behaviour of the boards was measured indirectly by collecting the run-off water. The results are presented in Figure 2. Because of big variation in the water volumes collected after individual rain episodes, average amounts were calculated for each treatment and the results expressed as relative volumes to the run-off water collected from the unmodified boards.
Figure 2: Relative amount of run-off water collected from unmodified (UM) and thermally modified (TM) boards exposed outdoors
In general more water was collected from the TM boards indicating that less water was absorbed by them. However, for certain rain episodes no difference was observed among TM boards treated at different temperatures as it could be expected from both the contact angle measurement and the capillary absorption test results. Moreover, just TM boards treated at the lowest temperature (TM150) absorbed less water which could be explained with wood structural and density peculiarities of individual boards. Anotherfactor influencing absorption is the level of board re-drying before the next rain episode.
Liquid absorption capacity is another important wood characteristic when its impregnation is considered. It depends mainly on both the total wood porosity as well as interconnectivity of pores. To ensure complete liquid penetration, small specimens with high surface are to volume ratio (specific surface) and high ratio of the transverse surfaces were used for evaluation of wood liquid absorption capacity. No substantial difference of the absorbed amount was detected for the studied liquids, namely water and biocide solutions. Therefore in Figure 3 are presented joint results
for absorbed amount of liquid depending on wood density without separating results of individual liquids.
Figure 3. Absorbed liquid of unmodified (UM) and thermally modified (TM) birch wood depending on wood density
As it was expected, the general trend for both unmodified and TM wood was that the absorbed liquid amount per mass of wood correlates with wood density and less liquid is absorbed by wood of higher density. Comparing unmodified and TM wood of similar densities, less liquid was absorbed by the TM wood. Moreover, the higher the TM temperature the more reduction was recorded in the absorbed liquid by wood of similar density. Zauer et al. (2013) have reported about decrease in wood cell wall density due to TM. On the other hand, reduction of bulk wood density
0.90 0.95 1.00 1.05 1.10 1.15 1.20 1 2 3 4 5 6 Relativ e am ou nt of ru n-off w ater Rain episode UM (0.67 ± 0.03) TM150 (0.66 ± 0.02) TM160 (0.61 ± 0.04) TM170 (0.52 ± 0.04) 60 70 80 90 100 110 120 130 140 150 0.4 0.5 0.6 0.7 A bs or be d liq uid , % Density, g/cm3 UM TM150 TM160 TM170
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after TM also is reported (Widmann et al. 2012). Both these changes can have antipodal influence on the wood void volume which limits the potential amount of a liquid that can be introduced into wood. Another reason for reduction in absorption capacity could be formation of some closed cavities due to wood structural transformations caused by TM (Boonstra et al. 2006).
Penetration of the impregnation liquid into the whole volume is substantial prerequisite for high impregnation liquid retention and even distribution which are important indicators of treatment efficiency. Impregnation result depends mainly on wood permeability characteristics, impregnation solution chemical composition and the used treatment process (c-36). As it was demonstrated by the capillary absorption test, TM imparts reduced permeability to birch wood. However, during impregnation, the applied pressure not only aid liquid penetration into the wood through its capillary system but can also facilitate the liquid movement by creating extra pathways through entailing collapses in weaker wood anatomical structures. To evaluate the possible changes caused by the TM, boards were impregnated by applying treatment parameters found in preliminary experiments to ensure even liquid distribution throughout the whole board volume of unmodified birch wood. Similarly with the absorption capacity test, difference in introduced liquid amount depending on the used impregnation solution was not observed. In the Figure 4, average moisture content in centre and edges of board cross-section, calculated for all used impregnation solutions, are presented.
Figure 4: Moisture content in centre and edges of unmodified (UM) and thermally modified (TM) birch wood boards after impregnation
The lower moisture content of TM boards agrees with the finding of reduced absorption capacity comparing with unmodified wood. As it was expected, similar water content was detected in unmodified wood board central part and next to edges implying even distribution of the liquid through the board cross-section. The lowest treatment temperature (TM150) used in the present study did not cause substantial penetration unevenness in the impregnated boards. However, TM at higher temperatures impart wood characteristics that hinder penetration of liquid into wood not only in a case of capillary absorption but also when pressure is applied. Substantially lower moisture content detected in the board central part for TM wood treated at 160 ℃ and especially at 170 ℃ suggest necessity of longer impregnation time for satisfactory TM wood impregnation results. However, prolonged process time decreases production capacity and increases its costs.
3.2. WOOD DRYING 0 20 40 60 80 100 120 140 UM TM150 TM160 TM170 Mo istu re co nten t, % Edge Centre
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Wood drying is a complex process including free water transport caused by capillary forces and diffusion controlled evaporation (Siau, 1984). Proper drying rate is a favourable characteristic for wood materials that can come in contact with liquid water during their service life because extended desorption phase has a decisive influence upon the time with elevated moisture content (Rapp et al. 2000, Van Acker et al. 2015). Proper drying rate is a favourable characteristic for wood materials that can come in contact with liquid water during their service life. On the other hand, it was found that prolonged time of wood wetness after impregnation is advisable for copper fixation (Humar and Lesar 2009). In the present study, the focus was on the moisture exclusion from impregnated specimens up to wood moisture content (MC) of 25% which was suggested to be the limit of wood MC beyond which wood is subjected
to high degradation hazard (Rapp et al. 2000). The results of the drying tests are shown in Figure 5.
Figure 5: Moisture content (MC) changes during drying at RH 65% and 20°C of impregnated unmodified (UM) and thermally modified (TM) birch wood specimens of different size: a) 20×20×5 mm, b) 20×20×100 mm, c) 25×100×700 (boards)
Quite similar release of water was recorded for both unmodified and TM wood small specimens which were characterised by large specific surface (6.0 cm-1) as well as high transverse surfaces proportion (67%) implying larger contribution of longitudinal water transport during drying (Figure 5a). Nevertheless slightly faster drying was observed for unmodified wood, TM wood treated at 170 ℃ reached MC of 25% most quickly because of the substantially less absorbed water during impregnation. The larger specimens were characterised by almost three times
smaller surface area to volume ratio (2.2 cm-1) and the transverse surfaces proportion of only 9%. For them not only
reduction of drying rate in general but also substantial difference in drying rate between unmodified and TM specimens was observed (Figure 5b). In this experiment, it took 24 hours for the unmodified wood specimens to reach the 25% MC limit comparing with six hours in the experiment with the smaller specimens even though the initial MC of the smaller specimens was for 10% higher. However, the most pronounced effect of the different size and ratio of specimens was observed for the TM wood for which drying was considerably impeded. For the TM wood regardless of treatment temperature almost three times longer drying time was needed to reach the 25% MC limit comparing with unmodified wood. Extremely slow drying was detected for the TM wood treated at 170 ℃.
Similar trend regarding significant differences of water release for unmodified and TM wood was observed also drying boards (Figure 5c). The unmodified boards reached the state of MC below 25% in less than one week while for TM boards it took four and more weeks depending on TM temperature. At 170 ℃ treated boards, likewise the specimens (20 × 20 ×100 mm) of the same treatment, released water most slowly. However, because of substantially lower initial MC, these boards reached the limit of 25% MC in shorter drying time compared with TM boards treated
at lower temperatures. The results of drying experiments carried out on specimens of different size suggest thatTM
causes retardation of water exclusion from deeper layers. We hypothesize that the slowdown of water release from impregnated TM specimens of larger size could be caused by surface clogging with TM degradation products dissolved in water and transported towards the water evaporation zone. However this hypothesis must be proved in further experiments.
4. CONCLUSIONS
The results show that in the present study applied thermal modification in a closed system under elevated pressure caused substantial alteration in birch wood interaction with liquids. Similar results were observed when both water
0 25 50 75 100 125 150 0 1 2 3 4 5 6 7 8 MC, % Drying time, h UM TM150 TM160 TM170 0 25 50 75 100 125 150 0 2 4 6 8 MC, % Drying time, h 0 25 50 75 100 125 150 0 20 40 60 80 100 MC, % Drying time, h 0 25 50 75 100 125 150 0 15 30 45 60 MC, %
Drying time, days
a)
b)
c)
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and copper-azole type biocide solutions of different concentration were used for evaluations. The results demonstrate that wood wettability which was assessed by contact angle measurements and permeability which was evaluated by capillary water absorption in transverse directions (through tangential and radial surfaces) were reduced due to thermal modification. These changes make satisfactory impregnation of thermally modified birch wood more difficult comparing with unmodified wood. On the other hand decreased absorption of rain water was observed for thermally modified wood during its outdoor exposure. The results of the drying tests show that thermally modified birch wood dry much slower comparing with unmodified birch wood except specimens with very high surface-to-volume ratio. The hindered drying could be quite a challenge when impregnation is considered.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support by the European Regional Development Fund project No. 1.1.1.1/16/A/133.
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