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The Impact of Paraffin-Thermal Modification of Beech Wood on Its Article The Impact of Paraffin-Thermal Modification of Beech Wood on Its Biological, Physical and Mechanical Properties Ladislav Reinprecht * and Miroslav Repák Department of Wood Technology, Faculty of Wood Sciences and Technology, Technical University in Zvolen, T. G. Masaryka 24, SK-96001 Zvolen, Slovakia; [email protected] * Correspondence: [email protected]; Tel.: +421-45-520-6383 Received: 5 November 2019; Accepted: 26 November 2019; Published: 2 December 2019 Abstract: The European beech (Fagus sylvatica L.) wood was thermally modified in the presence of paraffin at the temperatures of 190 or 210 ◦C for 1, 2, 3 or 4 h. A significant increase in its resistance to the brown-rot fungus Poria placenta (by 71.4%–98.4%) and the white-rot fungus Trametes versicolor (by 50.1%–99.5%) was observed as a result of all modification modes. However, an increase in the resistance of beech wood surfaces to the mold Aspergillus niger was achieved only under more severe modification regimes taking 4 h at 190 or 210 ◦C. Water resistance of paraffin-thermally modified beech wood improved—soaking reduced by 30.2%–35.8% and volume swelling by 26.8%–62.9% after 336 h of exposure in water. On the contrary, its mechanical properties worsened—impact bending strength decreased by 17.8%–48.3% and Brinell hardness by 2.4%–63.9%. Keywords: beech; paraffin; thermal modification; fungi; swelling; mechanical properties 1. Introduction European beech (Fagus sylvatica L.) is one of the most popular commercial broad leaved tree species in Central Europe [1,2]. Beech wood is preferred due to its properties; especially because it is easily workable and impregnable. On the contrary, beech wood suffers from low resistance to fungi and insects, and also from high volume shrinking, i.e., during drying it warps and splits, and under changing weather conditions its dimensions change significantly [3]. Low decay and mold resistance is considered one of the basic disadvantages of beech wood. According to the Standard EN 350 [4], it is non-durable and in exterior cannot be used without convenient treatments, mainly for structural elements. In comparison to traditional chemical treatment of wood with toxic biocides, the processes of its thermal, biological and chemical modification do not usually affect the environment; therefore, investigation of new wood modification modes is very prospective [5,6]. Thermal modification of wood at high temperatures is connected with changes in its molecular structure. These changes are associated primarily with the hemicelluloses degradation, creation of hemicelluloses–lignin linkages and extinction of some hydroxyl groups [7–9]. An increase in the resistance of wood to water and biotic agents is the main aim of its thermal modification [5,10]. Types and extent of changes in molecular structure and subsequently in properties of thermally modified wood depend not only on the temperature and its duration, but also on the wood species, its initial moisture content, as well as on parameters of air, nitrogen, plant oil or other heating medium [5,6,10,11]. Biological resistance of wood is not affected positively by the thermal modification carried out in the atmosphere at lower temperatures ranging from 130 to 160 ◦C[10,12,13]. In practice, an increase in wood resistance to wood-decaying fungi and insects results from the application of higher temperatures Forests 2019, 10, 1102; doi:10.3390/f10121102 www.mdpi.com/journal/forests Forests 2019, 10, 1102 2 of 14 of air ranging from 160 to 220 ◦C, or also from temperatures up to 260 ◦C at the limited quantity of oxygen present [5,12,14,15]. In comparison to standard thermal modification of wood in hot air, higher resistance to decaying-fungi and water is ensured when hot plant oils and other hydrophobic agents are applied [12–14,16–18]. Forests 2019, xx, x FOR PEER REVIEW 2 of 14 Waxes are due to hydrophobic properties used to protect wood against water, as the water sorption kineticsquantity of wood of oxygen is reduced present and [5,12,14,15]. dimensional stability is improved [19]. Waxes also improve resistance of wood toIn termites comparison [20,21 to]. standard Mechanical thermal properties, modification as compressive of wood in strengthhot air, higher [22], bending resistance strength to [21] and hardnessdecaying-fungi [21,22], and of woodwater is treated ensured with when waxes hot plan dot oils not and change other or hydrophobic increase slightly. agents are Moreover, applied waxes [12–14,16–18]. are almost non-toxic and environmentally acceptable, so they can also be used in other sectors, e.g., Waxes are due to hydrophobic properties used to protect wood against water, as the water pharmaceutical,sorption kinetics beauty of wood and food-processingis reduced and dimensio industries.nal stability is improved [19]. Waxes also improve Pararesistanceffin belongs of wood to to thetermites group [20,21]. of waxes. Mechanical It is properties, a mixture as ofcompressive solid linear strength aliphatic [22], bending hydrocarbons, especiallystrength straight [21] and chain hardness alkanes [21,22], synthesized of wood treate fromd with crude waxes oil do using not change Fischer–Tropsch or increase slightly. synthesis or from coalMoreover, tar. Para waxesffin are is almost cheap, non-toxic healthy and clean environmen and hydrophobictally acceptable, substance so they can used also in be beauty used in industry, candle-makingother sectors, business, e.g., pharmaceutical, civil engineering, beauty and as food-processing well as in preservation industries. of wood materials in order to Paraffin belongs to the group of waxes. It is a mixture of solid linear aliphatic hydrocarbons, reduce the hygroscopicity and to improve the dimensional stability. Paraffin and paraffin emulsions especially straight chain alkanes synthesized from crude oil using Fischer–Tropsch synthesis or from have beencoal usedtar. Paraffin to reduce is cheap, water healthy soaking clean and and tohydr improveophobic dimensionalsubstance used stability in beauty of industry, particleboards candle- [23,24] and inmaking hydrophobic business, treatment civil engineering, of solid as wood well as [19 in,21 preservation,25] over aof long wood period. materials Combining in order to thereduce technology of paratheffin hygroscopicity impregnation and of woodto improve with the subsequent dimensional thermal stability. modification Paraffin and inparaffin paraffi emulsionsn can have have a synergic effect resultingbeen used into reduce an effective water soaking improvement and to improve of selected dimensional wood stability properties of particleboards [21,26,27]. [23,24] and Thein hydrophobic aim of the experiment treatment of wassolid towood determine [19,21,25] the over eff aect long of period. thermal Combining modification the technology of European of beech paraffin impregnation of wood with subsequent thermal modification in paraffin can have a synergic wood ineffect the resulting presence in ofan effective paraffin improvemen in order tot of increase selectedits wood resistance properties to [21,26,27]. rot, mold and water and at the same time toThe evaluate aim of the the experiment effect of modificationwas to determine on selectedthe effect mechanicalof thermal modification properties of of European wood. beech wood in the presence of paraffin in order to increase its resistance to rot, mold and water and 2. Materialsat the same and time Methods to evaluate the effect of modification on selected mechanical properties of wood. 2.1. Wood2. Materials and Methods European2.1. Wood beech (Fagus sylvatica L.) heart-wood specimens of high quality, i.e., without rot, insect gallery, growth defects, tension wood or red-false wood were prepared from the sawn timber European beech (Fagus sylvatica L.) heart-wood specimens of high quality, i.e., without rot, insect naturally seasoned to a moisture content of 13.5% 2%. Three types of specimens were used in gallery, growth defects, tension wood and red-false wood± were or reaction—red-false wood were the experiment—typeprepared from the sawn (a): 25timber mm naturally25 mm seasoned5 mm to a inmoisture mycological content of tests 13.5% and ± 2%. in Three testing types the Brinell × × hardness;of specimens type (b): were 5 mmused in 50the mmexperiment—typ25 mme in (a): testing 25 mm the× 25 soakingmm × 5 mm and in swelling,mycological and tests type (c): × × 120 mmand in10 testing mm the10 Brinell mm in hardness; testing type the impact(b): 5 mm bending × 50 mm strength × 25 mm (Figurein testing1). the The soaking top and and bottom × × surfacesswelling, of specimens and type of (c): the 120 type mm (a)× 10 and mm type× 10 mm (b) werein testing milled. the impact bending strength (Figure 1). The top and bottom surfaces of specimens of the type (a) and type (b) were milled. FigureFigure 1. Types 1. Types of beech of beech wood wood specimens specimens used used in in the the paraparaffin-thermalffin-thermal modification modification and andfor testing for testing the selectedthe properties. selected properties. Note: Type Note: (a): Type 64 modified(a): 64 modifi anded 32and reference 32 reference specimens specimens for for attack attack by by wood-decaying wood- fungi; 32decaying modified fungi; and 32 modified 16 reference and 16 specimens reference specimens for attack for byattack the by mold the moldAspergillus Aspergillus niger niger; 54; 54 modified and sixmodified reference and specimens six reference for specimens testing for the testing Brinell the hardness. Brinell hardness. Type (b):Type 54 (b): modified 54 modified
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