The Effects of Slicing Parameters on Surface Quality of European Beech Wood

• The main objective of this work was to evaluate the effects of ﬂ at slicing processes on wood surface characteristics of the European beech (Fagus sylvatica L.). The relation between wettability, roughness and machining methods were studied. Two different wood thickness (3.4 and 4.0 mm) and three levels of compression during slicing (67.5 %, 57.5 % and 47.5 % of desired veneer thickness) were used to prepare surfaces prior to testing. The smaller variation of the thickness of thinner veneers was observed. No signi ﬁ cant impact of compression on variation of the thickness was found. The contact angle was lower when roughness measured parallel to the grain was higher. The in ﬂ uence of selected compression on roughness of European beech veneers measured perpendicular to the grain was con ﬁ rmed. This indicated that the in ﬂ uence of the set of machining processes, such as pressure bar setting during slicing, is signi ﬁ cant for wooden veneers surface properties.


INTRODUCTION 1. UVOD
According to the European Federation of the Parquet Industry, Poland is the leader in the production of fl ooring materials in Europe, reaching almost 17 % of the European production, which gives approx. 29.5 million m 2 of fl ooring produced each year. Most of manufactured fl oor materials are layered fl oorboards (EPLF/GFA, 2020). The signifi cance of proper management of raw materials leads to savings and reducing of production costs and forced modifi cation of current-forest in Europe is estimated to cover ca 14-15 Mha (excluding the Caucasian mountains) with the largest areas in France, central and southern Germany, and the southeast European mountains (Carpathians, Dinaric and Balkan mountains) (Hahn and Fanta, 2001;Standovár and Kenderes, 2003). In Poland only, the stock of thick beech wood is about 90 million m 3 . It is estimated that the market share of beech wood will systematically increase due to the observed expansion of this species (Tarasiuk, 1999;Dobrowolska, 2011). European beech is an expansive, fast-growing tree species gradually expanding its area of occurrence, whose wood can be an alternative to the dominant European oak in fl ooring.
The main objective of this work was to evaluate the effects of slicing processes on wood surface characteristics of the European beech (Fagus sylvatica L.). The relations between wettability, roughness and slicing parameters were studied. Two different wood thicknesses and three levels of compression were used during slicing to prepare surfaces prior to testing of selected characteristics.

MATERIJALI I METODE
Typical European beech (Fagus sylvatica L.) was selected as a test species, because of its wide application in the European wood industry for the production of fl oors, both from solid wood and glue laminated wood. Sawn wood of diameters of 20.5 cm × 12.0 cm × 100.0 cm (tangential × radial × longitude) was used. Material derived from Polish forest managed by the State Forests National Forest Holding. Beams were graded into two groups for fl at slicing thickness of about 3.4 and 4.0 mm.
The beech wood beams were hydrothermally treated before slicing. The treatment process included: 5 h of heating up to the temperature of 60 °C, 42 h of thermo-hydro treatment at a temperature of 90 °C and 1 h of cooling down to a temperature of 60 °C. The fl at slicing process was carried out on a FEZER Lumber Slicer FM 30 adapted for this purpose. The wood was sliced fl at with variable compression of 67.5 %, 57.5 % and 47.5 % of desired veneer thickness. Slicing speed was set at 100 m/min. Drying was carried out in a Vanicek 25 dryer. After drying, moisture content of wood was approx. 8.0 %.
The thickness variation of the obtained European beech veneers was determined using an optical microscope (SMZ 1500, Nikon) with image analysing software. The same microscope was used to prepare images of analysed veneer surfaces.
The contact angles of expanding droplets, i.e. advancing angles, were determined using a contact angle measuring device and using the Owens-Wendt methods (Owens and Wendt, 1969) with Petrič and Oven recommendations (Petrič and Oven, 2015) on a Goniometer Haas Phoenix 300 contact angle analyser connected to imaging software. The image analysis system calculated the contour of the drop from an image cap-ly used machining methods and development of new solutions, such as replacing fl at-sawing by more efficient (chipless) technology like slicing. A well-planned production process can provide more profi cient production by reducing the amount of waste. It is already known that slicing (or peeling) conditions clearly affect the process energy and forces as well as the quality of wood surface (Aguilera and Zamora, 2009; Aguilera and Muñoz, 2011; Thoma et al., 2015). Thus, the effects of different surfacing methods are of great interest in order to improve the preparation of materials. In every industrial production process, the occurrence of faults in processing is unavoidable. However, in the context of technological requirements, if the quality deviations are within certain limit values, then the detail is considered to have been properly processed. However, it occurs that the dimensions of the detail are within the limit values, and the problem lies in the quality of the machined surface. This is partly due to the fact that mechanical treatments change the chemical and morphological characteristics of wood surfaces (Liptákova and Kúdela, 1994), with the types of machining along with the characteristics of the raw material, or a combination of both these parameters, determining the surface quality and infl uencing the cost (Kilic et al., 2006;Mitchell and Lemaster, 2002).
In addition to the physical, mechanical, as well as the anatomical properties of wood, the surface quality of fi nished products is infl uenced by numerous factors such as: the direction of slicing, geometry of the blade and its sharpness, thickness of the cut part, any sharpening faults, as well as the technological parameters (speed of slicing, speed of movement, pressure bar settings, etc.) (Richter et al., 1995;Hernández and Cool, 2008;Haouzali et al., 2019). Surface properties are usually monitored in order to assess the quality of machining processes (Hernández and Cool, 2008). The surface roughness affects the wetting characteristics of materials, and the increase of surface roughness is associated with an increase of surface wettability. Surface roughness of veneer plays an important role in plywood manufacturing. Because of its effect on gluing and bonding characteristics, veneer surface roughness is an important factor in the production of glued timber products such as LVL (Dündar et al., 2008). Moreover, desired surface roughness is strongly and directly linked to or infl uenced by the future use condition, making it crucial in fl ooring production, especially in case of glue laminated fl oorboards. An effective control of surface roughness is important in production processes related to the adhesive bonding of wood elements and the fi nal processing of fi nished products (Lemaster et al., 1982;Keturakis, 2007;Arnold, 2010;Thoma et al., 2015). Relatively little research has been made concerning the effects of machining on the wetting characteristics of wood surfaces. Wood of deciduous porous structure has been tested more often (Gindl et al., 2001;Qin et al., 2014;Ugulino and Hernandez, 2015).
Beech, next to birch and oak, is the most abundant deciduous species in Europe (Boratyńska and Boratyński 1990). The total area of beech dominated .....Jankowska, Kozakiewi cz, Zbieć: The Effects of Slicing Parameters on Surface Quality...
The coeffi cient of variation (defi ned as the ratio of the standard deviation to the mean) was 1.8 % (mean value) for 3.4 mm thick veneers and 2.3 % (mean value) for 4.0 mm thick veneers. These values were in 5 % industrial range recommendation (Feihl, 1986) and so the slicing quality was good. A slight variation of the thickness of thinner veneers was most probably caused by lower tensions in material as slicing thin veneer generates lower cutting forces (Haouzali et al., 2019). Previous experiments (Rahayu, 2016) indicated that it is more diffi cult to maintain thickness regularity for thinner veneers. On the other hand, a study conducted on Fagus sylvatica L. wood (Daoui, 2011) confi rmed our conclusion. Probably, an observation of the inverse relation resulted from the use of relatively high material compression during cutting. It allowed to reduce tensions and made dimensions more stable. The regression analysis made for each veneer thicknesses did not confi rm the signifi cance of compression values.

Wettability and roughness of European beach veneers 3.2. Kvašenje i hrapavost bukova furnira
The results of measurements of contact angles as well as the results of measurements of roughness parallel and perpendicular to the grain are shown in Table 2. For the wood surfaces in case of each veneer thickness obtained with different compression, contact angle was in the range of 79.7° to 99.8° for tight side and 85.8° to 93.7° for loose side. There was no signifi cant difference in contact angle measurements on those sides.
The different machining process did not affect surface wettability, as determined by contact measurements. The ANOVA did not confi rm signifi cant dependence between veneer thickness, veneer side or use of different compression values (Table 3). I t was ob-tured using a video camera. The contact angle was calculated as the average of both sides of the droplets, to compensate for any horizontality variations. The initial contact angles recorded immediately after droplet deposition were used to estimate the wood surface energies using Berthelot's combining rule (Kwok and Neumann, 2000). The measurements were repeated 30 times. Distilled water was used for testing.
Roughness of the veneers was evaluated in accordance with the requirements of ISO 4287 (1997). As a part of the conducted research, the arithmetic mean deviation of the assessed profi le (R a ) was measured. The surface roughness was tested using the Surftest SJ-210 Series 178-Portable Surface Roughness Tester (Mitutoyo Corporation). The parameter R a was measured 30 times each in parallel and perpendicular to the grain direction for each tested variant. The analysis of wood roughness as well as wood surface wettability were done on the tight ("right") and loose ("left") side of the European beech veneers.
A statistical analysis of the test results was carried out using Statistica v. 10 software (StatSoft Inc.). The data were analysed and provided as the mean ± standard deviation. To determine the relations between the tested wood properties, simple regressions were used. Analysis of variance (ANOVA) was also made at the 0.05 level of signifi cance. The test factors and their variability levels are given in Table 1.

Variation in thickness 3.1. Varijacije debljine
The obtained veneers were characterised by a relatively small variation in the thickness (Figure 1).     served that the higher compression during slicing, the higher is the contact angle with water. Simple regression analysis confi rmed the infl uence of selected compression values on roughness of European beech veneers in both parallel and perpendicular to the grain directions (Table 4). Wood roughness was higher when the compression was lower. This indicates that the infl uence of the set of machining parameters, such as slicing, is visible in the surface quality. The same conclusion was made by de Moura and Hernández (2007), who tested sugar maple wood surfaces machined with the fi xed-oblique knife pressure-bar cutting system. The wood surface roughness is an important characteristic in terms of surface quality and properties, particularly in the case of fi nishing treatments (Buyuksari et al., 2011). Veneer with a rough surface can also cause excessive resin use and may result in resin bleeding through the face veneer (Dündar et al., 2008). The roughness of tested veneers varied significantly depending on the measurement direction (parallel and perpendicular to the grain) and veneer side. The roughness values measured perpendicular to the grain were twice as high as those measured parallel to the grain. ANOVA confi rmed signifi cance of veneer side on roughness values ( Table 3). The loose side was characterised by a greater roughness because more irregularities and cracks appeared on that side due to tensile stresses. Such stresses are an effect of veneer bending during the peeling process. According to previous studies (Candan et al., 2010;Bekhta et al., 2012), the improvement of surface quality of veneers is very important for bonding quality in bonded wood panels. Reducing the roughness, mainly on the loose side, reduces stress points between the wood surface and the adhesive, increasing the shear strength of the glue line (Rahayu, 2016).
Wood is not a homogeneous material, and its properties vary in different directions as a result of its anatomical structure (Li et al., 2018). The measurement perpendicular to the grain contains irregularities that are mainly caused by the size of the structural wood elements. In the case of deciduous wood species, these ele-a b Figure 2 Macrographs of tight side (a) and loose side (b) of tested veneers -magnifi cation 4x Slika 2. Izgled zatvorene (a) i otvorene (b) strane ispitivanih furnira -povećanje 4 puta a b Figure 3 Micrographs of tight side (a) and loose side (b) of tested veneers -magnifi cation 20x Slika 3. Mikrografi je zatvorene (a) i otvorene (b) strane ispitivanih furnira -povećanje 20 puta ments are primarily fi bres and vessels. The observed differences were due to the difference in texture in perpendicular and longitudinal direction of European beech wood. Moreover, wood quality depends on surface morphology (Jankowska et al., 2019). In case of tight side of veneers, only a few cell wall fi brillations roughed the surface. Microscopic images revealed that on loose side the wood fi bres and other cellular elements were torn out, mechanically destroyed and even crushed. In addition, loos side of veneers exhibited several cracks and some delamination (Figure 2 and 3).
Simple regression analysis was carried out to verify the existence of a relation between surface roughness measured perpendicular and parallel to the grain and wood wettability. According to results, the correlation between these properties was poor in case of roughness measured parallel to the grain (coeffi cient of correlation R^=0.719 was not signifi cant) and in case of roughness measured parallel to the grain R çç =0.851 was signifi cant. On the other hand, based on our results, it can be concluded that the contact angle is lower when roughness (value of R a parameter) is higher and that, consequently, wood wettability is also higher. The same inference was made by Arnold (2010), who tested differently machined solid wood surfaces regarding surface properties and coating performance.

ZAKLJUČAK
Based on the results of the research conducted, it can be concluded that compression is signifi cant for the surfaces of wood such as European beech slicing. Using pressure bar set (relatively high material compression) during wood slicing allowed to reduce internal stresses providing dimensions to be more stable. The smaller variation of the thickness was observed in thinner veneers. The contact angle was lower when roughness was higher, thus wood wettability was higher. The infl uence of selected compression factors on roughness of European beech veneers perpendicular to the grain was confi rmed. This indicates that the infl uence of the machining parameters of processes such as slicing is signifi cant for the surface properties of wooden veneers. This type of innovation has the potential to be used, among others, in the furniture industry and the production of wooden layered fl oors.

Acknowledgements -Zahvala
The presented research was made in the project: "Increasing the effi ciency of using wood raw material in production processes in industry". This project was co-fi nanced by the National Centre for Research and Development (NCBR) under Strategic Research and Development Programme "Environment, Agriculture and Forestry" -BIOSTRATEG, Project No. BI-OSTRATEG2/298950/1/NCBR/2016.