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Influence of different exposition of larch wood facade models on their surface degradation processes

Influence of different exposition of larch wood facade models on their surface degradation processes Wood, as a building material, is nowadays more often used outdoors. From the point of view of environment care, wood constructions and use of renewable materials belongs between modern increasing trends in industry. Wooden facades, more often used without surface treatment, are the important part of this trend. In Central Europe, European larch (Larix decidua) and Siberian larch (Larix sibirica) are especially popular materials for wooden facade elements. The aim of this study is to characterize the surface degradation of untreated facade models from both European and Siberian larch wood. The wood species, orientation to the sides of the world and construction type of the facade were the evaluation factors, which were regularly examined during 24 months of outdoor exposure via measuring the changes of surface colour, gloss, wettability and visual appearance in the form of cracks and resin leaking. The influence of all evaluated factors on the measured properties was determined. The results of this work can help to proper use of untreated larch wood on facade elements in practice. Key words: colour changes; facades; larch wood; surface degradation; weathering Editor: Bohdan Konôpka processes of untreated wood during weathering. Gener- 1. Introduction ally, the prediction of service life of wood components is Wood, as a building material, is well known for many the main factor of research, which requires careful exami- advantages. But there is a need to use it in a way to protect nation of material´s properties (Brischke et al. 2008; it against some specic fi factors, which negatively affect its Gupta et al. 2011). Current trends even favor facade ele- service life, functionality and appearance. This process ments without surface treatment, even though the visual is called bio-degradation (Hrapková 2012). According appearance of untreated wood changes over time due to to study of Gielen (1997), the production of all building weather conditions (Feist & Hon 1984; Hirche 2014; materials is responsible for 8 – 12% of all CO emissions Lesar et al. 2016). in Western Europe. The use of wood as a building mate- Wooden facades are usually made from softwoods rial can reduce this number (Goverse et al. 2001; Bribiàn and used more and more often without surface treatment et al. 2011). Already in the last century the trend of eco- (Ingo 2011). According to Connell (2004), during last logical construction became familiar and the modern decade there was a trend of increasing application of wood constructions are the proof that wood still belongs domestic wood species. The reasons for that are the lower between popular and abundantly used building materials costs of transport, lower emission of CO and negative (Ingo 2011; Kržišnik 2018). perception of tropical wood use, which is often connected Untreated wood, as a material for facade, can be with deforestation (Sohngen et al. 1999). In the area of a cost-effective alternative to materials with an applied Czech Republic, there are several domestic wood species, coating system, as it requires limited maintenance. On which are suitable for facade production – spruce, fir, the other hand, more information about n fi al costs, envi - larch and Douglas fir among softwoods and oak, Black ronmental impact of building components and service life locust or chestnut among hardwoods. But practically, are needed (Feist & Hon 1984). In the study of Gupta et al. (2011), the FTIP analysis was used to estimate the pre- hardwoods are not used at all for this purpose. The rea- son are more disadvantages of more durable hardwoods diction of service life of wooden facade elements. Based on their results, the key is to understand the degradation (for example Quercus sp. or Robinia pseudoacacia) – their *Corresponding author. Miloš Pánek, e-mail: panekmilos@fld.czu.cz, phon e: +420 224 383 867 I. Štěrbová et al. / Cent. Eur. For. J. 67 (2021) 45–53 higher density (Požgaj et al. 1993) and thus increased lignin is decomposed. As a result, in indoor applications facade weight, higher price and also higher susceptibil- the wood turns dark, but in outdoors the decomposed ity of lumber to shape deformations and formation of parts are washed out by rainwater from the surface (Tolvaj larger cracks. Furthermore, various types of thermally 1995; Pandey 2005). Due to that process, the light shade modie fi d wood are used, preferably made of Scots pine or of wood caused by light colour of non-degraded cellulose Norway spruce (Reinprecht 2016). The most important is formed, but it is immediately disrupted by deposition factors for the selection of suitable wood species are the of dust particles and pollution into the porous structure number and width of annual rings – the denser annual of wood surface (Evans 2008). The well known greying rings stands for higher durability of wood (Ingo 2011). of wood surface in exterior takes place. After leaching the Frequently used larch wood belongs to the durability products of photodegradation the layers of wood cells class 3 according to EN 350 (2016). European larch are further exposed and eroded (Feist 1982; Williams & (Larix decidua Mill.) is characterized with distinguished Feist 1999; Reinprecht 2008). The other factors affect- sapwood and heartwood, sapwood is narrow and yellow- ing the intensity of surface degradation are temperature, ish, heartwood is from red-brown to red-purple colour acid rains and wind (Feist 1990; Williams 2005; Evans and turns dark on the air (Wagenführ 2003; Gierlinger 2008; Teacà et al. 2013). The effect of wind is evident in et al. 2003). European larch wood does not have a lot of formation of typical plastic structure of wood in exterior, resin canals but they can be recognized in all directions which is manifested by increased roughness of surface. well known for wood structure – radial, tangential and The uneven surface is caused mainly by the different den- longitudinal. This wood species is dimensionally stable sity of early wood and late wood, which is more obvious a relatively resistant to acids, climatic changes and attack at softwoods (Williams 1999). of insects and fungi (Musil 2007). Sapwood of Siberian Other important factors influencing weathering of larch (Larix sibirica) is yellow, heartwood is red-brown. wood are biotic factors and combination of wood product Siberian larch wood is also known for high mechanical application and construction or design solution (Sandak resistance and strength in compression and relatively low et al. 2017). When applying untreated wood elements on water absorption. Due to the high content of terpenoids it facade, the principles of construction protection should is durability to biotic and abiotic factors (Gierlinger et al. be respected in order to signic fi antly reduce the weather - 2003). Siberian larch is highly resistant to atmospheric ing process (EN 335; Ganne-Chédeville et al. 2012). The factors, in practice, it is considered as durable wood, basic point of construction protection is keeping the opti- which can be used in construction without any protec- mal shape and connection of wooden facade element in tion. Both European and Siberian larch are character- order to ensure the rainwater drain. Other principles are ized with high strength, which gives a high percentage of suitable roof overhangs, which can significantly reduce use in furniture and construction (Gierlinger et al. 2003; the exposition of wood facade to water, or avoiding the Musil 2007). contact of wood with ground (Ingo 2011). Geographic Wood exposed to outdoor conditions is subjected and climatic factors plays an important role as well in to process of natural weathering, when wood degrades weathering. In the study of Mohebby & Saei (2015) due to the effect of atmospheric factors and changes its the influence of different cardinal points on weather- surface properties both on macroscopic and microscopic ing was confirmed – facades exposed to south side are level (Williams 2007). The degree of degradation is gen- more affected by intensive solar radiation and changes of erally influenced by wood density, amount of sapwood relative humidity and temperature, therefore the higher and heartwood, annual ring´s orientation and content protection of facade is recommended for this side. The of lignin and extractives (Williams 2005). Abiotic fac- intensity of solar radiation is lower at north side, on the tors cause atmospheric degradation of wood, which other hand it is more exposed to higher humidity and negatively affects only its surface layers (Temiz et al. slower drying out of wood, which makes suitable condi- 2005; Evans 2008; Oberhofnerová et al. 2017) but it can tions for growth of wood-destroying fungi and moulds be beginning of more dangerous degradation caused by causing colour changes of wood (Reinprecht 2016). In biotic factors – wood-destroying fungi and insects (Feist the climatic conditions of Central Europe, the western and Hon 1984; Reinprecht 2012). The changes take place side is more influenced by the predominant direction only to the depth of few millimetres and do not affect the of winds and incident rainfall compared to the eastern important mechanical properties, service life or even the exposure. The western side is more influenced by the function of wooden element (Gobakken et al. 2011). The predominant direction of winds and incident rainfall most significant factors influencing the rate of degrada- compared to the eastern exposure in the climatic condi- tion are solar radiation and water, acting synergistically tions of Central Europe. (Tolvaj& Faix 1995; Hon & Shiraishi 2001; Müller et al. The aim of this study is to determine the influence 2003). Ultraviolet (UV) radiation provokes photochemi- of wood species, different exposure to cardinal points cal reactions, which cause the decomposition of lignin, (south and north) and different type of construction extractives and partly hemicellulose (Feist & Hon 1984; on the surface changes of untreated wood facade ele- Pandey 2005; Reinprecht 2008). In the first phase, the ments. Facade models were made from European (Larix 46 I. Štěrbová et al. / Cent. Eur. For. J. 67 (2021) 45–53 decidua) and, still more frequently used, Siberian larch The natural weathering test took place in Czech (Larix sibirica) and they were exposed to weathering con- Republic at Suchdol (50° 07´49,68 “N; 14° 22´13,87” ditions in Prague (Czech Republic) in Central Europe for E, 285 m elevation above sea level) for 24 months based 24 months. The results of this study could help to under- on the EN 927–3 (2006). Testing stand with facade mod- stand behaviour of larch wood in exterior and its applica- els was placed at the roof of Pavilion of Wood Sciences of tion in the form of facade cladding and to understand aes- Faculty of Forestry and Wood Sciences in Prague. thetical changes of untreated wood during weathering. 2. Materials and methods 2.1. Preparation of samples Based on the analysis of currently used wooden cladding, the two types of construction profiles (type A and B) and their placement were designed in software AutoCad. The accent was put on the elimination of rainwater and its faster drain from the wood surface (Fig. 1). The facade models were made from European larch (EL) and Sibe- rian larch (SL), which were prepared in dimensions of cladding of 400 × 145 × 20 mm (length x width x thick- Fig. 2. The placement of facade models in the testing stands ness). The overall size of the facade models was 400 in exterior in 90°. × 600 mm (width × length). The facade models were marked by the wood species, type of construction and The selected surface properties were regularly exam- exposure (f. e. SL–B–S, see Table 1). ined before and after 6, 12 and 24 months of exposure: changes of gloss and colour, wettability (via measur- Table 1. Marking of the facade models. ing contact angle) and visual evaluation focused on the Wood species Construction type Exposure cracks forming and resin leaking. Siberian larch Type A × Type B (South) S × (North) N (SL) × European larch (EL) Profiled cladding was sanded by sandpaper with 120 2.2. Colour measurements grain and then attached by fasteners to spruce wood sup- Colour parameters L*a*b* were determined using porting construction from the south (S) and north (N) the spectrophotometer CM-600d (Konica Minolta, side. These facade models were subsequently exposed in Japan) at the same marked places on the specimens. The exterior stands in the inclination of 90° (Fig. 2). Before device was set to an observation angle of 10°, d/8 geome- exposition, the samples were showed 18% of wood mois- try and D65 light source in L*a*b* colour space (accord- ture content. ing to the Commission International de l´Eclairage – CIE 1986). Forty measurements were carried out for each Fig. 1. Design of facade profiles – type A (left – Siberian larch) and B (right – European larch). Type A is characterized with clad- ding top cut in 45°, while type B was designed with inclination 30° and rounded top with radius r = 15 mm. 47 I. Štěrbová et al. / Cent. Eur. For. J. 67 (2021) 45–53 tested model – twenty for south exposure and twenty 2.5. Visual evaluation for north exposure. For the mathematical expression of The formation of cracks and degree of surface degrada- difference of two colours is used the Euclidian distance tion were evaluated before and after weathering using – called as total colour difference ΔE*. The total colour visual evaluation on the base of EN ISO 4628 (2003). difference was calculated using the following equation [Equation 1]: כ כ ଶ כ ଶ כ ଶ ܧ ൌ ඥ ሺ߂ ܮ ሻ ൅ሺ ߂ܽ ሻ ൅ሺ ߂ܾ ሻ  [1] 2.6. Statistical analysis where: The statistical evaluation was performed using Statistica L* is the lightness from 100 (white) to 0 (black), a* is the chro- 12 software (Statsoft, Palo Alto, USA) and MS Excel maticity coordinate from −60 (green) to + 60 (red), b* is the 2013 (Microsoft, Redmond, USA) using mean values, other chromaticity coordinate from −60 (blue) to +60 (yellow) standard deviations, whiskers plots and analysis of vari- (Sehlstedt-Persson 2003). ance (ANOVA). 2.3. Gloss measurements 3. results and discussion Gloss measurements were performed based on EN The total colour difference ΔE*, calculated from the ISO 2813 (2014) using glossmeter MG268-F2 (KSJ, measured values (L*, a*, b*), was the most significant Quanzhou, China). Forty measurements at a 60° angle factor indicating weathering of facade models. The spe- per sample (twenty for south side, twenty for north side) cic fi colour parameters ( L*, a*, b*) represents the change were performed during weathering. of colour more accurately. During exposure of face mod- els in exterior there was a significant change of colour, at the end of the experiment almost all the tested mod- 2.4. Contact angle measurements els were characterized with the total colour difference ΔE* > 12. This value is considered as different colour in The sessile drop method with static contact angle meas- comparison with the initial colour at the beginning of the urement (without external interference) was performed experiment. An exception was recorded for facade mod- using the methodology of Bastani et al. (2015). The wet- els SL–B–S and EL–B–S, which were characterized with tability measurements were taken using a goniometer the total colour change ΔE* = 9.9 and 11.2 respectively. Krüss DSA 30E device (Krüss, Hamburg, Germany) on These values are considered as high colour change only radial surfaces of wood samples before weathering and (Sehlstedt-Persson 2003). The most significant colour after 24 months. From each model, the samples from the changes were noted after 24 months of exposure for the south and north side after 24 months of exposure were facade models exposed to north side, specifically SL–A– prepared – in dimensions 70 × 30 × 20 mm (l × w × t). N, EL–A–N. Regarding south side, the most significant Ten measurements were taken for each sample, with colour difference was recorded for the model SL–A–S distilled water drops with a dosing volume of 5 μl. The with the values ΔE* = 19.3 (Fig. 3). contact angle values were determined after 5 s of drop When evaluating the colour parameter L *, a nega- deposition to reach the equilibrium point and prevent the tive difference of its values was initially observed, which risk of contamination with extractives. The phenomena means that the colour of the facade models gradually of spreading and absorption of drops on the wood sur- changed to a darker shade. This change is mostly caused face was investigated via comparison between initial and by the deposition of dust particles in exterior and by the weathered state of wood samples. Fig. 3. Total colour changes ΔE* of facade models during natural weathering. 48 I. Štěrbová et al. / Cent. Eur. For. J. 67 (2021) 45–53 Fig. 4. 2SD whiskers plots and Mean values of L* changes during natural weathering of facade models from European (left) and Siberian (right) larch. Fig. 5. 2SD whiskers plots and Mean values of a* changes during natural weathering of facade models from European (left) and Siberian (right) larch. Fig. 6. 2SD whiskers plots and Mean values of b* changes during natural weathering of facade models from European (left) and Siberian (right) larch. 49 I. Štěrbová et al. / Cent. Eur. For. J. 67 (2021) 45–53 photodegradation of extractives and lignin on wood sur- leaching of lignin (Pastore et al. 2004). All facade mod- face (Oberhofnerová et al. 2017). The most significant els exposed to natural weathering for 6 months showed difference in the values of the parameter ΔL* was clearly a trend of increasing values of the parameter a*. After observed at the facade models of type A, specifically in 12 months of exposure, all models showed a decrease in the Siberian larch (SL) on the north side (N) with ΔL* these values. The most signic fi ant decrease was recorded = −17.9. The second highest value was recorded for the for the EL–B–N and EL–A–N models. On the contrary, same model on the south side with ΔL * = −17.0. From the least significant decrease in a* values was measured the point of view of this indicator of lightness, the con- for the SL–B–S and SL–A–S models. Overall, the north struction type A model exposed to north side was less side and the European larch generally showed lower suitable for both wood species. For construction type values of a*. The highest value of the parameter b* was B, due to its specific inclination of the façade profiles recorded after 6 months of exposure for model SL–B–S greater than 90° (see Fig. 1), there was less deposition (b* = 27.1) and model SL–A–S (b* = 26.9). At the end of impurities in the porous structure and micro-cracks of the weathering, the most significant decrease of the of the wood. The European larch on the south side best parameter b* was measured at model EL–A–N (b* = 7.1) suited the construction type A of facade model. Facade and SL–B–N (b* = 7.9). According to the dimensional models SL–B–S and EL–A–S type showed the lowest colour model, the facade models approached the yellow th th differences in lightness values. hue in the 6 month and the blue hue from the 12 month The colour parameters a* and b* increased after 6 until the end of the test. The colour parameter b* showed months of exposure, which is caused by the photodeg- lower values of European larch compared to European radation of lignin (Müller 2003), but after 12 months larch, and lower values were also measured for the facade this trend was completely opposite and during the expo- models on the north side than on the south side. sure the values kept decreasing, which is due to gradual Fig. 7. Gloss changes of facade models during natural weathering. Fig. 8. Surface wetting of facade models before and after natural weathering. 50 I. Štěrbová et al. / Cent. Eur. For. J. 67 (2021) 45–53 The wood of both types of tested larch had a matt weathering compared to exposure in the inclination of surface (EN ISO 2813 2014) even before the start of the 45° according to EN 927–3 (2006). weathering (Fig. 7). During the test, there was a slight Visual evaluation of facade models confirmed more increase after 12 months of weathering and a decrease signic fi ant formation of cracks at SL–A–S in comparison again to almost original values after 24 months of expo- with other models. These visual changes were observed sure. Despite some slight differences in the initial values after 12 months of weathering (Fig. 9). In this Figure, it and the course of changes, the gloss values are so low that is also possible to see typical greying of wood in exterior. they do not play an important role in the visual perception In some cases, resin leaking was observed, especially of the facade cladding for the external observer. in the first 12 months of exposure (Fig. 10). This char - The contact angle of surface wettability with water acteristic was the most pronounced at SL–B–S model. showed considerable variability due to the inhomoge- The colour changes were observed even by naked neous structure of larch wood even before weathering eye, which confirms the values of total colour difference (Fig. 8). This inhomogeneity was slightly lower at SL measured by spectrophotometer (Fig. 3). with higher density of annual rings, where the depos- ited drop with a volume of 5 μl always partially affected the zone of early wood and late wood. For EL with wider 4. Conclusion annual rings, the variability was higher. The places with Two construction types of facade models were made a higher resin content were other reason for the varia- from European Larch (Larix decidua Mill.) and Sibe- bility of results (see photos in Fig. 1). After weathering, rian Larch (Larix sibirica). These models were subjected significant decrease in contact angle values (CA°) and to natural weathering for 24 months on the south and a decrease in variability of EL were observed. This action north exposure. The colour changes at all models were was due to the gradual photodegradation and leaching significant, the lowest changes were observed for SL– of hydrophobic lignin from wood surfaces (Pastore et al. B–S model. But in general, the most significant overall 2004). In general, lower CA° values were measured for colour changes were paradoxically shown by the facade facade model B (lowest for EL–B–N), but the differences models made of Siberian larch. In terms of the indicator were not statistically significant (Fig. 8). The decrease is of total colour difference, the European larch reached in not as significant as in study of Oberhofnerová & Pánek more cases better results than Siberian. (2016), which is, however, due to the 90° inclination of In terms of type of exposure, the north side showed the wood in this experiment, where there is no such rapid higher colour changes compared to the south side. Fig. 9. The formation of cracks – visual appearance of Siberian larch after 12 months of exposure to south side, construction type A. Fig. 10. The resin leak after 12 months of weathering at Siberian larch exposed to south side, construction type B. 51 I. Štěrbová et al. / Cent. Eur. For. J. 67 (2021) 45–53 Another evaluation parameter was the gloss, where, Feist, W. C., 1990: Outdoor wood weathering and pro- however, due to its low initial values and small changes, tection. In: Archaeological wood: properties, chem- the visual characteristics of the facade models were not istry, and preservation. Advances in Chemistry Series significantly affected. A more significant decrease in the 225. Proceedings of 196th meeting of the American Chemical Society, p. 25–28. contact angle of wettability was observed for European Feist, W. C., Hon, D. N.-S., 1984: Chemistry of weath- larch and for facade model B, but the differences after weathering were not statistically significantly differ- ering and protection. In: R. M. Rowell (ed.): The Chemistry of Solid Wood (Washington, DC: Ameri- ent due to the large variability of the measured values. can Chemical Society), p. 401–451. There was a higher formation of cracks on Siberian larch Ganne-Chédeville, Ch., Jääskeläinen, A. S., Froidevaux, wood in comparison with European larch. Cracks were J., Hughes, M., Navi, P., 2012: Natural and artificial observed on both types of exposure (N×S) and construc- ageing of spruce wood as observed by FTIR-ATR tion models (A×B) but the most at the facade model A on and UVRR spectroscopy. Holzforschung, Berlin, the south side. The occurrence of resin leaking during 66:163–170. outdoor exposure was also observed, especially in the Gielen, D., 1997: Building Materials and CO2 – Western first 12 months of exposure. European Emissions Reduction Strategies. MAT- Finally, the main result of this work is that changes TER Project Report (Petten: Netherlands Energy of larch façade appearance during exterior exposure Research Foundation (ECN)). depend not only on kind of larch wood, north or south Gierlinger, N., Jacques, D., Schwanninger, M., Wim- exposure, but important factor is also design of profiles mer, R., Pâques, L. E., 2003: Heartwood extractives in construction. and lignin content of different larch species (Larix sp.) and relationships to brown-rot decay-resistance. Trees, 18:230–236. Acknowledgements Gobakken, L. R., Høibø, O. 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American Journal of Agricultural Economics, 81:1–13. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Forestry Journal de Gruyter

Influence of different exposition of larch wood facade models on their surface degradation processes

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Abstract

Wood, as a building material, is nowadays more often used outdoors. From the point of view of environment care, wood constructions and use of renewable materials belongs between modern increasing trends in industry. Wooden facades, more often used without surface treatment, are the important part of this trend. In Central Europe, European larch (Larix decidua) and Siberian larch (Larix sibirica) are especially popular materials for wooden facade elements. The aim of this study is to characterize the surface degradation of untreated facade models from both European and Siberian larch wood. The wood species, orientation to the sides of the world and construction type of the facade were the evaluation factors, which were regularly examined during 24 months of outdoor exposure via measuring the changes of surface colour, gloss, wettability and visual appearance in the form of cracks and resin leaking. The influence of all evaluated factors on the measured properties was determined. The results of this work can help to proper use of untreated larch wood on facade elements in practice. Key words: colour changes; facades; larch wood; surface degradation; weathering Editor: Bohdan Konôpka processes of untreated wood during weathering. Gener- 1. Introduction ally, the prediction of service life of wood components is Wood, as a building material, is well known for many the main factor of research, which requires careful exami- advantages. But there is a need to use it in a way to protect nation of material´s properties (Brischke et al. 2008; it against some specic fi factors, which negatively affect its Gupta et al. 2011). Current trends even favor facade ele- service life, functionality and appearance. This process ments without surface treatment, even though the visual is called bio-degradation (Hrapková 2012). According appearance of untreated wood changes over time due to to study of Gielen (1997), the production of all building weather conditions (Feist & Hon 1984; Hirche 2014; materials is responsible for 8 – 12% of all CO emissions Lesar et al. 2016). in Western Europe. The use of wood as a building mate- Wooden facades are usually made from softwoods rial can reduce this number (Goverse et al. 2001; Bribiàn and used more and more often without surface treatment et al. 2011). Already in the last century the trend of eco- (Ingo 2011). According to Connell (2004), during last logical construction became familiar and the modern decade there was a trend of increasing application of wood constructions are the proof that wood still belongs domestic wood species. The reasons for that are the lower between popular and abundantly used building materials costs of transport, lower emission of CO and negative (Ingo 2011; Kržišnik 2018). perception of tropical wood use, which is often connected Untreated wood, as a material for facade, can be with deforestation (Sohngen et al. 1999). In the area of a cost-effective alternative to materials with an applied Czech Republic, there are several domestic wood species, coating system, as it requires limited maintenance. On which are suitable for facade production – spruce, fir, the other hand, more information about n fi al costs, envi - larch and Douglas fir among softwoods and oak, Black ronmental impact of building components and service life locust or chestnut among hardwoods. But practically, are needed (Feist & Hon 1984). In the study of Gupta et al. (2011), the FTIP analysis was used to estimate the pre- hardwoods are not used at all for this purpose. The rea- son are more disadvantages of more durable hardwoods diction of service life of wooden facade elements. Based on their results, the key is to understand the degradation (for example Quercus sp. or Robinia pseudoacacia) – their *Corresponding author. Miloš Pánek, e-mail: panekmilos@fld.czu.cz, phon e: +420 224 383 867 I. Štěrbová et al. / Cent. Eur. For. J. 67 (2021) 45–53 higher density (Požgaj et al. 1993) and thus increased lignin is decomposed. As a result, in indoor applications facade weight, higher price and also higher susceptibil- the wood turns dark, but in outdoors the decomposed ity of lumber to shape deformations and formation of parts are washed out by rainwater from the surface (Tolvaj larger cracks. Furthermore, various types of thermally 1995; Pandey 2005). Due to that process, the light shade modie fi d wood are used, preferably made of Scots pine or of wood caused by light colour of non-degraded cellulose Norway spruce (Reinprecht 2016). The most important is formed, but it is immediately disrupted by deposition factors for the selection of suitable wood species are the of dust particles and pollution into the porous structure number and width of annual rings – the denser annual of wood surface (Evans 2008). The well known greying rings stands for higher durability of wood (Ingo 2011). of wood surface in exterior takes place. After leaching the Frequently used larch wood belongs to the durability products of photodegradation the layers of wood cells class 3 according to EN 350 (2016). European larch are further exposed and eroded (Feist 1982; Williams & (Larix decidua Mill.) is characterized with distinguished Feist 1999; Reinprecht 2008). The other factors affect- sapwood and heartwood, sapwood is narrow and yellow- ing the intensity of surface degradation are temperature, ish, heartwood is from red-brown to red-purple colour acid rains and wind (Feist 1990; Williams 2005; Evans and turns dark on the air (Wagenführ 2003; Gierlinger 2008; Teacà et al. 2013). The effect of wind is evident in et al. 2003). European larch wood does not have a lot of formation of typical plastic structure of wood in exterior, resin canals but they can be recognized in all directions which is manifested by increased roughness of surface. well known for wood structure – radial, tangential and The uneven surface is caused mainly by the different den- longitudinal. This wood species is dimensionally stable sity of early wood and late wood, which is more obvious a relatively resistant to acids, climatic changes and attack at softwoods (Williams 1999). of insects and fungi (Musil 2007). Sapwood of Siberian Other important factors influencing weathering of larch (Larix sibirica) is yellow, heartwood is red-brown. wood are biotic factors and combination of wood product Siberian larch wood is also known for high mechanical application and construction or design solution (Sandak resistance and strength in compression and relatively low et al. 2017). When applying untreated wood elements on water absorption. Due to the high content of terpenoids it facade, the principles of construction protection should is durability to biotic and abiotic factors (Gierlinger et al. be respected in order to signic fi antly reduce the weather - 2003). Siberian larch is highly resistant to atmospheric ing process (EN 335; Ganne-Chédeville et al. 2012). The factors, in practice, it is considered as durable wood, basic point of construction protection is keeping the opti- which can be used in construction without any protec- mal shape and connection of wooden facade element in tion. Both European and Siberian larch are character- order to ensure the rainwater drain. Other principles are ized with high strength, which gives a high percentage of suitable roof overhangs, which can significantly reduce use in furniture and construction (Gierlinger et al. 2003; the exposition of wood facade to water, or avoiding the Musil 2007). contact of wood with ground (Ingo 2011). Geographic Wood exposed to outdoor conditions is subjected and climatic factors plays an important role as well in to process of natural weathering, when wood degrades weathering. In the study of Mohebby & Saei (2015) due to the effect of atmospheric factors and changes its the influence of different cardinal points on weather- surface properties both on macroscopic and microscopic ing was confirmed – facades exposed to south side are level (Williams 2007). The degree of degradation is gen- more affected by intensive solar radiation and changes of erally influenced by wood density, amount of sapwood relative humidity and temperature, therefore the higher and heartwood, annual ring´s orientation and content protection of facade is recommended for this side. The of lignin and extractives (Williams 2005). Abiotic fac- intensity of solar radiation is lower at north side, on the tors cause atmospheric degradation of wood, which other hand it is more exposed to higher humidity and negatively affects only its surface layers (Temiz et al. slower drying out of wood, which makes suitable condi- 2005; Evans 2008; Oberhofnerová et al. 2017) but it can tions for growth of wood-destroying fungi and moulds be beginning of more dangerous degradation caused by causing colour changes of wood (Reinprecht 2016). In biotic factors – wood-destroying fungi and insects (Feist the climatic conditions of Central Europe, the western and Hon 1984; Reinprecht 2012). The changes take place side is more influenced by the predominant direction only to the depth of few millimetres and do not affect the of winds and incident rainfall compared to the eastern important mechanical properties, service life or even the exposure. The western side is more influenced by the function of wooden element (Gobakken et al. 2011). The predominant direction of winds and incident rainfall most significant factors influencing the rate of degrada- compared to the eastern exposure in the climatic condi- tion are solar radiation and water, acting synergistically tions of Central Europe. (Tolvaj& Faix 1995; Hon & Shiraishi 2001; Müller et al. The aim of this study is to determine the influence 2003). Ultraviolet (UV) radiation provokes photochemi- of wood species, different exposure to cardinal points cal reactions, which cause the decomposition of lignin, (south and north) and different type of construction extractives and partly hemicellulose (Feist & Hon 1984; on the surface changes of untreated wood facade ele- Pandey 2005; Reinprecht 2008). In the first phase, the ments. Facade models were made from European (Larix 46 I. Štěrbová et al. / Cent. Eur. For. J. 67 (2021) 45–53 decidua) and, still more frequently used, Siberian larch The natural weathering test took place in Czech (Larix sibirica) and they were exposed to weathering con- Republic at Suchdol (50° 07´49,68 “N; 14° 22´13,87” ditions in Prague (Czech Republic) in Central Europe for E, 285 m elevation above sea level) for 24 months based 24 months. The results of this study could help to under- on the EN 927–3 (2006). Testing stand with facade mod- stand behaviour of larch wood in exterior and its applica- els was placed at the roof of Pavilion of Wood Sciences of tion in the form of facade cladding and to understand aes- Faculty of Forestry and Wood Sciences in Prague. thetical changes of untreated wood during weathering. 2. Materials and methods 2.1. Preparation of samples Based on the analysis of currently used wooden cladding, the two types of construction profiles (type A and B) and their placement were designed in software AutoCad. The accent was put on the elimination of rainwater and its faster drain from the wood surface (Fig. 1). The facade models were made from European larch (EL) and Sibe- rian larch (SL), which were prepared in dimensions of cladding of 400 × 145 × 20 mm (length x width x thick- Fig. 2. The placement of facade models in the testing stands ness). The overall size of the facade models was 400 in exterior in 90°. × 600 mm (width × length). The facade models were marked by the wood species, type of construction and The selected surface properties were regularly exam- exposure (f. e. SL–B–S, see Table 1). ined before and after 6, 12 and 24 months of exposure: changes of gloss and colour, wettability (via measur- Table 1. Marking of the facade models. ing contact angle) and visual evaluation focused on the Wood species Construction type Exposure cracks forming and resin leaking. Siberian larch Type A × Type B (South) S × (North) N (SL) × European larch (EL) Profiled cladding was sanded by sandpaper with 120 2.2. Colour measurements grain and then attached by fasteners to spruce wood sup- Colour parameters L*a*b* were determined using porting construction from the south (S) and north (N) the spectrophotometer CM-600d (Konica Minolta, side. These facade models were subsequently exposed in Japan) at the same marked places on the specimens. The exterior stands in the inclination of 90° (Fig. 2). Before device was set to an observation angle of 10°, d/8 geome- exposition, the samples were showed 18% of wood mois- try and D65 light source in L*a*b* colour space (accord- ture content. ing to the Commission International de l´Eclairage – CIE 1986). Forty measurements were carried out for each Fig. 1. Design of facade profiles – type A (left – Siberian larch) and B (right – European larch). Type A is characterized with clad- ding top cut in 45°, while type B was designed with inclination 30° and rounded top with radius r = 15 mm. 47 I. Štěrbová et al. / Cent. Eur. For. J. 67 (2021) 45–53 tested model – twenty for south exposure and twenty 2.5. Visual evaluation for north exposure. For the mathematical expression of The formation of cracks and degree of surface degrada- difference of two colours is used the Euclidian distance tion were evaluated before and after weathering using – called as total colour difference ΔE*. The total colour visual evaluation on the base of EN ISO 4628 (2003). difference was calculated using the following equation [Equation 1]: כ כ ଶ כ ଶ כ ଶ ܧ ൌ ඥ ሺ߂ ܮ ሻ ൅ሺ ߂ܽ ሻ ൅ሺ ߂ܾ ሻ  [1] 2.6. Statistical analysis where: The statistical evaluation was performed using Statistica L* is the lightness from 100 (white) to 0 (black), a* is the chro- 12 software (Statsoft, Palo Alto, USA) and MS Excel maticity coordinate from −60 (green) to + 60 (red), b* is the 2013 (Microsoft, Redmond, USA) using mean values, other chromaticity coordinate from −60 (blue) to +60 (yellow) standard deviations, whiskers plots and analysis of vari- (Sehlstedt-Persson 2003). ance (ANOVA). 2.3. Gloss measurements 3. results and discussion Gloss measurements were performed based on EN The total colour difference ΔE*, calculated from the ISO 2813 (2014) using glossmeter MG268-F2 (KSJ, measured values (L*, a*, b*), was the most significant Quanzhou, China). Forty measurements at a 60° angle factor indicating weathering of facade models. The spe- per sample (twenty for south side, twenty for north side) cic fi colour parameters ( L*, a*, b*) represents the change were performed during weathering. of colour more accurately. During exposure of face mod- els in exterior there was a significant change of colour, at the end of the experiment almost all the tested mod- 2.4. Contact angle measurements els were characterized with the total colour difference ΔE* > 12. This value is considered as different colour in The sessile drop method with static contact angle meas- comparison with the initial colour at the beginning of the urement (without external interference) was performed experiment. An exception was recorded for facade mod- using the methodology of Bastani et al. (2015). The wet- els SL–B–S and EL–B–S, which were characterized with tability measurements were taken using a goniometer the total colour change ΔE* = 9.9 and 11.2 respectively. Krüss DSA 30E device (Krüss, Hamburg, Germany) on These values are considered as high colour change only radial surfaces of wood samples before weathering and (Sehlstedt-Persson 2003). The most significant colour after 24 months. From each model, the samples from the changes were noted after 24 months of exposure for the south and north side after 24 months of exposure were facade models exposed to north side, specifically SL–A– prepared – in dimensions 70 × 30 × 20 mm (l × w × t). N, EL–A–N. Regarding south side, the most significant Ten measurements were taken for each sample, with colour difference was recorded for the model SL–A–S distilled water drops with a dosing volume of 5 μl. The with the values ΔE* = 19.3 (Fig. 3). contact angle values were determined after 5 s of drop When evaluating the colour parameter L *, a nega- deposition to reach the equilibrium point and prevent the tive difference of its values was initially observed, which risk of contamination with extractives. The phenomena means that the colour of the facade models gradually of spreading and absorption of drops on the wood sur- changed to a darker shade. This change is mostly caused face was investigated via comparison between initial and by the deposition of dust particles in exterior and by the weathered state of wood samples. Fig. 3. Total colour changes ΔE* of facade models during natural weathering. 48 I. Štěrbová et al. / Cent. Eur. For. J. 67 (2021) 45–53 Fig. 4. 2SD whiskers plots and Mean values of L* changes during natural weathering of facade models from European (left) and Siberian (right) larch. Fig. 5. 2SD whiskers plots and Mean values of a* changes during natural weathering of facade models from European (left) and Siberian (right) larch. Fig. 6. 2SD whiskers plots and Mean values of b* changes during natural weathering of facade models from European (left) and Siberian (right) larch. 49 I. Štěrbová et al. / Cent. Eur. For. J. 67 (2021) 45–53 photodegradation of extractives and lignin on wood sur- leaching of lignin (Pastore et al. 2004). All facade mod- face (Oberhofnerová et al. 2017). The most significant els exposed to natural weathering for 6 months showed difference in the values of the parameter ΔL* was clearly a trend of increasing values of the parameter a*. After observed at the facade models of type A, specifically in 12 months of exposure, all models showed a decrease in the Siberian larch (SL) on the north side (N) with ΔL* these values. The most signic fi ant decrease was recorded = −17.9. The second highest value was recorded for the for the EL–B–N and EL–A–N models. On the contrary, same model on the south side with ΔL * = −17.0. From the least significant decrease in a* values was measured the point of view of this indicator of lightness, the con- for the SL–B–S and SL–A–S models. Overall, the north struction type A model exposed to north side was less side and the European larch generally showed lower suitable for both wood species. For construction type values of a*. The highest value of the parameter b* was B, due to its specific inclination of the façade profiles recorded after 6 months of exposure for model SL–B–S greater than 90° (see Fig. 1), there was less deposition (b* = 27.1) and model SL–A–S (b* = 26.9). At the end of impurities in the porous structure and micro-cracks of the weathering, the most significant decrease of the of the wood. The European larch on the south side best parameter b* was measured at model EL–A–N (b* = 7.1) suited the construction type A of facade model. Facade and SL–B–N (b* = 7.9). According to the dimensional models SL–B–S and EL–A–S type showed the lowest colour model, the facade models approached the yellow th th differences in lightness values. hue in the 6 month and the blue hue from the 12 month The colour parameters a* and b* increased after 6 until the end of the test. The colour parameter b* showed months of exposure, which is caused by the photodeg- lower values of European larch compared to European radation of lignin (Müller 2003), but after 12 months larch, and lower values were also measured for the facade this trend was completely opposite and during the expo- models on the north side than on the south side. sure the values kept decreasing, which is due to gradual Fig. 7. Gloss changes of facade models during natural weathering. Fig. 8. Surface wetting of facade models before and after natural weathering. 50 I. Štěrbová et al. / Cent. Eur. For. J. 67 (2021) 45–53 The wood of both types of tested larch had a matt weathering compared to exposure in the inclination of surface (EN ISO 2813 2014) even before the start of the 45° according to EN 927–3 (2006). weathering (Fig. 7). During the test, there was a slight Visual evaluation of facade models confirmed more increase after 12 months of weathering and a decrease signic fi ant formation of cracks at SL–A–S in comparison again to almost original values after 24 months of expo- with other models. These visual changes were observed sure. Despite some slight differences in the initial values after 12 months of weathering (Fig. 9). In this Figure, it and the course of changes, the gloss values are so low that is also possible to see typical greying of wood in exterior. they do not play an important role in the visual perception In some cases, resin leaking was observed, especially of the facade cladding for the external observer. in the first 12 months of exposure (Fig. 10). This char - The contact angle of surface wettability with water acteristic was the most pronounced at SL–B–S model. showed considerable variability due to the inhomoge- The colour changes were observed even by naked neous structure of larch wood even before weathering eye, which confirms the values of total colour difference (Fig. 8). This inhomogeneity was slightly lower at SL measured by spectrophotometer (Fig. 3). with higher density of annual rings, where the depos- ited drop with a volume of 5 μl always partially affected the zone of early wood and late wood. For EL with wider 4. Conclusion annual rings, the variability was higher. The places with Two construction types of facade models were made a higher resin content were other reason for the varia- from European Larch (Larix decidua Mill.) and Sibe- bility of results (see photos in Fig. 1). After weathering, rian Larch (Larix sibirica). These models were subjected significant decrease in contact angle values (CA°) and to natural weathering for 24 months on the south and a decrease in variability of EL were observed. This action north exposure. The colour changes at all models were was due to the gradual photodegradation and leaching significant, the lowest changes were observed for SL– of hydrophobic lignin from wood surfaces (Pastore et al. B–S model. But in general, the most significant overall 2004). In general, lower CA° values were measured for colour changes were paradoxically shown by the facade facade model B (lowest for EL–B–N), but the differences models made of Siberian larch. In terms of the indicator were not statistically significant (Fig. 8). The decrease is of total colour difference, the European larch reached in not as significant as in study of Oberhofnerová & Pánek more cases better results than Siberian. (2016), which is, however, due to the 90° inclination of In terms of type of exposure, the north side showed the wood in this experiment, where there is no such rapid higher colour changes compared to the south side. Fig. 9. The formation of cracks – visual appearance of Siberian larch after 12 months of exposure to south side, construction type A. Fig. 10. The resin leak after 12 months of weathering at Siberian larch exposed to south side, construction type B. 51 I. Štěrbová et al. / Cent. Eur. For. J. 67 (2021) 45–53 Another evaluation parameter was the gloss, where, Feist, W. C., 1990: Outdoor wood weathering and pro- however, due to its low initial values and small changes, tection. In: Archaeological wood: properties, chem- the visual characteristics of the facade models were not istry, and preservation. Advances in Chemistry Series significantly affected. A more significant decrease in the 225. Proceedings of 196th meeting of the American Chemical Society, p. 25–28. contact angle of wettability was observed for European Feist, W. C., Hon, D. N.-S., 1984: Chemistry of weath- larch and for facade model B, but the differences after weathering were not statistically significantly differ- ering and protection. In: R. M. Rowell (ed.): The Chemistry of Solid Wood (Washington, DC: Ameri- ent due to the large variability of the measured values. can Chemical Society), p. 401–451. There was a higher formation of cracks on Siberian larch Ganne-Chédeville, Ch., Jääskeläinen, A. S., Froidevaux, wood in comparison with European larch. Cracks were J., Hughes, M., Navi, P., 2012: Natural and artificial observed on both types of exposure (N×S) and construc- ageing of spruce wood as observed by FTIR-ATR tion models (A×B) but the most at the facade model A on and UVRR spectroscopy. Holzforschung, Berlin, the south side. The occurrence of resin leaking during 66:163–170. outdoor exposure was also observed, especially in the Gielen, D., 1997: Building Materials and CO2 – Western first 12 months of exposure. European Emissions Reduction Strategies. MAT- Finally, the main result of this work is that changes TER Project Report (Petten: Netherlands Energy of larch façade appearance during exterior exposure Research Foundation (ECN)). depend not only on kind of larch wood, north or south Gierlinger, N., Jacques, D., Schwanninger, M., Wim- exposure, but important factor is also design of profiles mer, R., Pâques, L. E., 2003: Heartwood extractives in construction. and lignin content of different larch species (Larix sp.) and relationships to brown-rot decay-resistance. Trees, 18:230–236. Acknowledgements Gobakken, L. R., Høibø, O. 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Journal

Forestry Journal – de Gruyter

Published: Mar 1, 2021

Keywords: colour changes; facades; larch wood; surface degradation; weathering

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