15 Les/Wood, Vol. 71, No. 1, June 2022 SORPTION PROPERTIES OF WOOD IMPREGNATED WITH THE FIRE RETARDANT BURNBLOCK SORPCIJSKE LASTNOSTI LESA, IMPREGNIRANEGA Z OGNJEZADRŽEVALNIM SREDSTVOM BURNBLOCK Miha Humar 1* , Boštjan Lesar 1 , Davor Kržišnik 1 UDK 630*843:630*812.7 Received / Prispelo: 21. 12. 2021 Original scientific article / Izvirni znanstveni članek Accepted / Sprejeto: 14. 2. 2022 . Abstract / Izvleč ek Abstract: The sorption properties of wood have a characteristic influence on some of its properties, such as the mechanical properties and susceptibility to fungal decay. Moist wood is more susceptible to fungal decay, and wood is often impregnated in order to protect it from fungal decomposition, photodegradation or fire. In particular, inorganic salts affect the sorption properties of wood. For this purpose, the sorption properties of Norway spruce wood impregnated with Burnblock refractory (uptake 38 kg/m³) were investigated. The microscopic analysis confirmed the presence of crystals of this in the cell lumina of wood tissue. Sorption properties were determined using an instrument capable of dynamic vapour sorption (DVS) assessment. DVS analysis confirmed that the sorption properties of impregnated spruce wood are comparable to those of non-impregnated spruce wood. However, the higher hysteresis at higher relative humidity is probably due to the presence of crystals in the cell lumina. Keywords: fire retardants, Burnblock, wood, sorption properties, laser confocal microscopy Izvleček: Sorpcijske lastnosti lesa imajo značilen vpliv na nekatere njegove lastnosti. V največji meri vplivajo na mehanske lastnosti in dovzetnost lesa na glivni razkroj. Vlažnejši les je bolj dovzeten za glivni razkroj, zato ga pred glivnim razkrojem, fotodegradacijo ali gorenjem pogosto impregniramo. Predvsem anorganske soli vplivajo na sorpcijske lastnosti lesa. S tem namenom smo preiskali sorpcijske lastnosti smrekovine, impregnirane z ognjezadrževalnim sredstvom Burnblock (navzem 38 kg/m³). Mikroskopska analiza je potrdila prisotnost kristalov sredstva v celičnih lumnih lesnega tkiva. Sorpcijske lastnosti smo določili z opremo, ki vrednoti dinamično sorpcijo vodne pare (DVS). DVS analiza je potrdila, da so sorpcijske lastnosti impregniranega smrekovega lesa primerljive s sorpcijskimi lastnostmi neimpregnirane smrekovine. Večja histereza pri višjih relativnih zračnih vlažnostih je verjetno posledica prisotnosti kristalov anarganskih soli, ki so se izločili v celičnih lumnih po impregnaciji. Ključne besede: ognjezadrževalna sredstva, Burnblock, les, sorpcijske lastnosti, laserska konfokalna mikroskopija 1 INTRODUCTION 1 UVOD Wood is hygroscopic due to its specific chem- ical composition and large internal surface area. Therefore, the moisture content of wood oscillates depending on the varying climatic conditions. Un- der stable conditions, wood reaches hygroscopic equilibrium or equilibrium moisture content (EMC). The interactions between wood and water have been studied scientifically for more than a centu- ry (Engelund et al., 2013). The moisture content of wood has a significant effect on some relevant properties, especially the mechanical properties (Gerhards, 1982) and service life of wood used out- doors (Meyer et al., 2016). Fungi can decompose wood if the moisture content is above a certain limit. The moisture content of wood must be high enough to promote the flow path for the reaction products of the enzymes, but low enough to pre- vent waterlogging. A wide variety of data on this is available in the literature. In the first set of data, it is indicated that the MC limits for fungal decay de- Vol. 71, No. 1, 15-22 DOI: https://doi.org/10.26614/les-wood.2022.v71n01a02 1 Univerza v Ljubljani, Biotehniška fakulteta, Jamnikarjeva 101, 1000 Ljubljana, SLO * e-mail: miha.humar@bf.uni-lj.si 16 Les/Wood, Vol. 71, No. 1, June 2022 Humar, M., Lesar, B., & Kržišnik, D.: Sorpcijske lastnosti lesa, impregniranega z ognjezadrževalnim sredstvom Burnblock pend predominantly on the fungal species. For ex- ample, Schmidt (2006) reported that the minimum MC of wood was 25% for Coniophora puteana and Serpula lacrymans and 30% for Fibroporia vaillantii and Gloeophyllum trabeum. However, recent data suggest that the limiting moisture content for fun- gal growth depends on the wood species and fun- gal species studied. For example, the limiting mois- ture content for C. puteana growth on thermally modified Scots pine (Pinus sylvestris L.) sapwood is 12.1%, while the limiting moisture content for fungal decay on the same species of wood is 24.4% (Meyer et al., 2016). Similar to wood-decaying fungi, MC has a sig- nificant effect on the growth and development of sap stain fungi. Sap stain fungi are mainly associat- ed with Ascomycetes and Fungi imperfecti, and are characterised by the pigmentation of the hyphae walls, which leads to discolouration of the wood. Suitable conditions for the growth and reproduc- tion of the various mould and sap stain fungi vary. Some thrive at relatively low air relative humidity (RH = 75%), while most fungi require higher RH lev- els and consequently higher wood MC for optimal growth. Different building materials have different susceptibilities to mould growth (Isaksson et al., 2010). The relationship between the EMC and RH is expressed by sorption isotherms obtained by progressive equilibration in the adsorption or de- sorption process. Differences in hygroscopic and sorption isotherms result from the wood species, chemical composition of the wood, the amount of microcracks in the cell walls, density, possible hydrothermal and chemical treatment, and stress conditions (Hartley et al., 1992; Willems, 2018). Sorption isotherms can be divided into three regions. The first represents the EMC from an abso- lutely dry state to the equilibrium state reached at RH between 20% and 30% (Mitchell, 2018). In this interval, the adsorption of water molecules con- tinues gradually until the outer surface of the cell wall is completely covered by a water monolayer. The wood MC changes more rapidly in the upper part of the region, but slows as it approaches a dry state (Lesar et al., 2009). The second region begins when the first layer is saturated. The adsorption of water molecules on the first layer and the result- ing formation of additional layers is a feature of this region, and the isotherms here are quasi-lin- ear (Mangel, 2000). In the third region, capillary condensation of water occurs in microcapillaries. Water molecules form large groups, while the bonds between hydroxyl groups and the first layer of water molecules become weaker, and thus the water molecules can move in clusters (Khali & Ra- wat, 2000). The water concentration in this region is sufficient for liquid water to form in the pores by capillary condensation, so the microcapillary water forms a continuous phase. In the third hygroscopic region, sorption properties are also influenced by low-molecular secondary heartwood compounds such as polyphenols (flavonoids, lignans, tannins), biocides (boric acids) and fire retardants (Blahovec & Yanniotis, 2008). As mentioned earlier, the hygroscopic proper- ties of wood can be affected by various treatment processes, like the use of wood biocides and fire re- tardants. Wood impregnated with various inorgan- ic salts is usually more hygroscopic than untreated wood, especially at high RH. The increase in EMC of such wood depends on the chemicals used, re- tention, and wood species (White & Dietenberger, 2010). The EMC of impregnated wood and the ef- fects of preservative retention on the equilibrium point are still unknown. High EMC is problematic because it promotes leaching of active ingredients, corrosion of metals, and the creation of favourable conditions for the growth of fungi and especially moulds, and presents difficulties in surface treat- ment and gluing of moist wood (Lesar et al., 2009). In this study, the sorption properties of wood im- pregnated with the fire retardant Burnblock were investigated throughout the hygroscopic range dur- ing the adsorption and desorption process. 2 MATERIAL AND METHODS 2 MATERIALI IN METODE The analysis was carried out on wood treat- ed with fire-retardant, and specifically on Norway spruce (Picea abies (L.) Karst.) planks treated with Burnblock (Burnblock, København, Denmark) in a commercial impregnation plant using the full cell impregnation method. Five planks were delivered. The cross-section of the planks was approximately 23 mm × 100 mm and length 200 mm. The reten- tion of Burnblock was 38 kg/m³. Burnblock is made 17 Les/Wood, Vol. 71, No. 1, June 2022 Humar, M., Lesar, B., & Kržišnik, D.: Sorption properties of wood impregnated with the fire retardant Burnblock of ingredients that can be found in nature and are considered environmentally friendly. Treated wood is biodegradable and has no adverse environmental effects (Medved et al., 2019). Five parallel samples were conditioned at laboratory conditions (21 °C; RH 65%), then measured and weighted. The nomi- nal density of the wood was then calculated. Microscopic analysis was performed on cross-sections of the treated wood. The outer 6 mm of the wood that was fully impregnated with the fire retardant was analysed. Microscopic anal- ysis was performed using a confocal laser scanning microscope (Olympus OLS50-BSW, Tokyo, Japan) and a digital microscope (Olympus DSX1000, To- kyo, Japan). The surface was planed with a stainless steel blade. The MC of the wood was approximate- ly 12%. Dynamic water vapour sorption of treated and native (i.e. reference, non-treated) samples was performed using a gravimetric dynamic sorption analyser (DVS Intrinsic, Surface Measurement Sys- tems Ltd., London, UK). Samples were ground and homogenised into fractions smaller than 1 mm pri- or to analysis using a SM 2000 mill (Retsch GmbH, Haan, Germany) and a perforated sieve with a per- foration of 1 mm (Conidur˝). The ground samples were conditioned at 20 °C and 1 ± 1% RH. A small amount of the ground sample (≈400 mg) was used. The measurement was performed at a constant temperature of 25 ± 0.2 °C. A total of two sorption and desorption cycles were measured from 0% RH to 95% RH, and vice versa. 3 RESULTS AND DISCUSSION 3 REZULTATI IN RAZPRAVA The cross-sections of the wood planks indicate the typical structure of Norway spruce wood. The annual rings are about 2 mm to 3 mm wide (Fig- ure 1). The anatomical structure (Figure 2) shows the gradual transition between earlywood and latewood cells. The respective resin canals are bor- dered by 8 to 12 or more thick-walled epithelial cells (Wagenführ, 2014). The density of the air-dry planks examined was 420 kg/m³ (st. dev. 17 kg/m³). This is in line with the data in the literature (Gryc et al., 2011; Humar, 2013). As seen from microscopic analysis (Figure 3, Figure 4), Burnblock crystals are seen in the cell lu- mina. The presence of the crystals was confirmed using two independent microscopy techniques, confocal laser scanning microscopy and digital mi- croscopy. The presence of the crystals in the cell lumina is not surprising, as the retention of Burn- block and other fire-retardants is higher than the retention of wood preservatives. For example, the retention of typical copper-ethanol wood preserva- tives is about 20 kg/m³ (Nordic Wood Preservation Council 2021) (for in-ground use), while the reten- tion of classical CCA barely reaches 12 kg/m³ (Wil- leitner, 2001). The crystals in the cell lumina are rather significant. It can be assumed that they were at least partially damaged during cutting. In the graphs (Figure 5), the sorption curves of the untreated and treated spruce wood are plotted. As can be seen, both the untreated and Figure 1. Cross-section of spruce wood plank, used for microscopic and sorption analysis. Slika 1. Prečni prerez deske, uporabljene za mikroskopsko in sorpcijsko analizo. 18 Les/Wood, Vol. 71, No. 1, June 2022 Humar, M., Lesar, B., & Kržišnik, D.: Sorpcijske lastnosti lesa, impregniranega z ognjezadrževalnim sredstvom Burnblock Burnblock-treated wood show typical sorption isotherms of type II. The differences between the EMC at 95% RH of untreated and treated wood are negligible. For example, in the first sorption cycle, the EMC of untreated spruce wood (23.09%) is slightly higher than the EMC of Burnblock-treated Figure 2. Annual ring of spruce wood plank Slika 2. Branika v lesu smrekove deske Figure 3. Cell lumina of all cells are filled with crys- tals of the fire retardant Burnblock. Microscopy was performed with a digital microscope. Colours are not always representative. Slika 3. Mikroskopska slika, ki prikazuje zapolnje- nost vseh celičnih lumnov z ognjezadrževalnim sredstvom Burnblock. Slika je posneta z digitalnim mikroskopom. Barve niso vedno reprezentativne. Figure 4. Cell lumina filled with crys- tals of the fire retardant Burnblock. The image was obtained with confocal scanning laser microscope (field of view 128 µm × 128 µm). Colours are not al- ways representative. Slika 4. Celični lumni, zapolnjeni s kristali ognjezadrževalnega sredstva Burnblock. Slika je bila posneta s konfokalnim la- serskim vrstičnim mikroskopom (vidno polje = 128 µm × 128 µm). Barve niso vedno reprezentativne. 19 Les/Wood, Vol. 71, No. 1, June 2022 Humar, M., Lesar, B., & Kržišnik, D.: Sorption properties of wood impregnated with the fire retardant Burnblock wood (22.79%). However, in the second sorption cycle, the EMC of Burnblock-treated wood was slightly higher (23.70%) than that of untreated spruce wood (22.47%). Normally, the second EMC at 95% RH for lignocellulosic materials is lower than the first (Glass et al., 2018), but in this case it was slightly higher for the Burnblock-treated sample. As DVS analysis was performed in controlled condi- tions it enables a reliable comparison, but statisti- cal analysis was not performed due to low number of measurements (Glass et al., 2018). The interpretation of the sorption curves is that the surfaces of the analysed wood samples are more polar than water molecules, and therefore Figure 5. Results of the sorption analysis of the (A) reference Norway spruce wood (PiAb), (B) Burnblock-im- pregnated Norway spruce wood (BB) in two sorption and desorption cycles. In graph (C), hysteresis is plotted. In contrast, in graph (D) differences in equilibrium wood moisture content between treated and untreated spruce in the first and second sorption cycles are presented. Negative values indicate that the MC of treated wood was lower than that of untreated wood. Slika 5. Rezultati sorpcijske analize (A) referenčne smrekovine (PiAb), (B) smrekovine, impregnirane z og- njezadrževalnim sredstvom Burnblock v dveh sorpcijskih in desorpcijskih ciklih. Slika C prikazuje histerezo, slika D pa razliko v vlažnosti med impregnirano in neimpregnirano smrekovino v prvem in drugem sorp- cijskem ciklu. Negativne vrednosti nakazujejo, da je bila vlažnost impregniranega lesa nižja od vlažnosti neimpregniranega lesa. 20 Les/Wood, Vol. 71, No. 1, June 2022 Humar, M., Lesar, B., & Kržišnik, D.: Sorpcijske lastnosti lesa, impregniranega z ognjezadrževalnim sredstvom Burnblock show increased water uptake at low RH (0 to 10%). Once a single (mono-)layer of water has formed, additional adsorption increasingly resembles the condensation of water. At high RH, i.e. above 70%, adsorption is enhanced due to the presence of tiny surface pores (mesopores, with pore diameters of 2 to 50 nm). These attract water molecules on more than one side, i.e. by capillary condensation. This leads to hysteresis in this humidity region caused by the reluctant release of the adsorbed water (Mangel, 2000). As the hysteresis between the sorption and desorption curves for Burnblock-treated wood in- creases at the higher sorption range (Figure 5), this indicates that there are more condensation sites present in the Burnblock-treated wood than in the reference spruce wood. This can be ascribed to the presence of crystals in cell lumina, as clearly seen from microscopic analysis (Figure 4). 4 CONCLUSIONS 4 ZAKLJUČKI The sorption properties of Burnblock-treated wood are comparable to those of untreated wood, while the moisture content of Burnblock-treated wood is comparable to that of untreated Norway spruce. The only difference can be found in the hysteresis between the sorption and desorption curves at higher relative humidities. This can be as- cribed to the presence of the crystals of Burnblock in the wood cell lumina. It can be thus presumed that the Burnblock-treated wood with retentions up to 38 kg/m³, exhibits the same susceptibility to- wards staining fungi. 5 SUMMARY 5 POVZETEK Sorpcijske lastnosti lesa imajo značilen vpliv na nekatere lastnosti lesa. V največji meri vplivajo na mehanske lastnosti in dovzetnost lesa na gliv- ni razkroj. Vlažnejši les je bolj dovzeten za glivni razkroj. Mejna vrednost za glivni razkroj je tri do pet odstotnih točk pod točko nasičenja celičnih sten. Po drugi strani dovzetnost lesa na pojav gliv plesni in gliv modrivk pogosto opišemo s kritično relativno zračno vlažnostjo, pri kateri se pojavijo plesni. Za večino lesnih vrst ta meja znaša okoli 75 %. V prime- ru bolj higroskopnih lesnih vrst je ta meja lahko tudi nižja. Za zaščito pred glivnim razkrojem, fotode- gradacijo ali gorenjem les pogosto impregniramo. Predvsem anorganske soli vplivajo na sorpcijske lastnosti lesa. To je še posebej značilno za les, im- pregniran z borovo kislino, boraksom ali natrijevim kloridom. Zelo higroskopna so tudi nekatera ognje- zadrževalna sredstva. S tem namenom smo prei- skali sorpcijske lastnosti smrekovine, impregnirane z ognjezadrževalnim sredstvom Burnblock (navzem 38 kg/m³). Burnblock sodi med okolju prijazne re- šitve, brez znanih negativnih vplivov na okolje. Impregniran les smo preiskali z dvema mikroskop- skima tehnikama (konfokalna laserska vrstična mi- kroskopija in digitalna mikroskopija). Mikroskopska analiza je potrdila prisotnost kri- stalov v celičnih lumnih. Sorpcijske lastnosti smo določili z opremo, ki omogoča dinamično sorpcijo vodne pare (DVS). Les smo zmleli in homogenizirali ter ga izpostavili dvema cikloma navlaževanja in su- šenja v območju med 0 % in 95 % relativne zračne vlažnosti pri 25 °C. Rezultati so pokazali, da so sorp- cijske lastnosti lesa, obdelanega z Burnblockom, primerljive s sorpcijskimi lastnostmi neobdelanega lesa smreke. Vsebnost vlage v lesu, impregniranim z Burnblockom, je primerljiva z vsebnostjo vlage neobdelane smrekovine. Edina razlika je v histerezi med sorpcijskimi in desorpcijskimi krivuljami pri viš- ji relativni vlažnosti. To lahko pripišemo prisotnosti kristalov v lumnih lesnih celic. Tako lahko domne- vamo, da je les, obdelan z Burnblock, z navzemom do 38 kg/m³, primerljivo dovzeten za delovanje gliv modrivk in plesni kot neimpregniran les. ACKNOWLEDGEMENT ZAHVALA The authors acknowledge the financial sup- port of the Slovenian Research Agency (ARRS) within the research program P4-0015 (Wood and lignocellulosic composites) and the infrastructural centre (IC LES PST 0481-09). 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