Influence of Finishing Materials on Viscous Elastic Properties of Wooden Structures

In this study, the effect of fi nishing process on the mechanical properties of oak and pine wood elements were investigated. A special test stand was used for this purpose. Specimens were divided into subgroups, the resonance frequency of the specimens was determined (when they vibrate in mode of a theoretic isotope beam), and the modulus of elasticity (MOE) and damping coeffi cient were estimated. It was determined that MOE of oak and pine specimens was 8415-10570 MPa and 8220-14104 MPa, respectively, while damping coeffi cients were 0.014-0.019 r. u. and 0.013-0.026. Afterwards, some specimens were varnished (pentaftal varnish was used), while the specimens from others subgroups were oiled (water-based outdoor wood oil was used) and after each processing step, the viscous elastic properties of the specimen were recorded. The specimens were fi nished four times – fi rst, one side was fi nished in two layers, and then the other side. For the determination of elastic properties of the fi nishing materials, fi lms were prepared separately. The fi lm formation was carried out by casting liquid on a smooth, siliconised surface of a test panel. The tensile test was carried out using the universal testing machine. It was established that MOE of fi lms varied in the range of 4-7 MPa. After the varnishing process, the MOE of oak and pine elements decreased by 5 % and 7 %, respectively, while the damping coeffi cient decreased (in varnish case) by 60 %. The impact of oil on viscous elastic properties of the specimens was quite marginal.


INTRODUCTION 1. UVOD
The elements of wood and wood-based materials are widely used in building structures and interior products.The durability of wood for outdoor use is not only affected by climatic factors but also by mechanical properties of wood, which to a large extent depend on the fi nish.The complex effect of the mechanical and physical properties of wood and environmental factors determines the durability of wooden structures and other wood-based products.Depending on the purpose, wooden structures must exhibit appropriate mechanical properties, which are often subject to legislation, building regulations and standards.In some cases, the structures must be elastic and resistant to various mechanical loads, in other cases, on the contrary, they must be able to dampen vibrations and sounds, as well as have inhibitory properties (Forssén et. al., 2008;Botterman et. al., 2018).
The mechanical properties of these kinds of structures are determined by the mechanical properties of their individual elements and the composition method of their interconnection (Taghiyari et Albrektas and Vobolis, 2004).It is known that the modulus of elasticity (MOE) and damping coefficient of a glued wood panel depends on the properties of the glued wood scantlings (Albrektas and Vobolis, 2003; Albrektas and Vobolis, 2004).Once a scantling, which is characterised by a higher MOE, is glued to the panel, the MOE of the whole panel becomes higher, and vice versa.A lower MOE and a higher damping coeffi cient of the glued product than the average value of the total number of glued scantlings show that the product probably contains defects, e.g. that the seams were poorly glued together, etc.
The viscous elastic properties of the elements of wooden structures can be altered, e.g. by changing the surface area of the element (i.e. by creating grooves, notches, etc.) (Ono, 1993;Molin et. al., 1984;Molin et. al., 1988), by soaking them (Endo et. al., 2010) and using fi nishing materials of different properties.
It was determined that different components of wooden composites and manufacturing technology can also have an effect on their mechanical properties (Taghiyari et al., 2017).
Also, overlays have a large effect on the mechanical properties of particleboard (Vobolis and Albrektas, 2012).Depending on the orientation/direction of the overlay material (especially on sliced veneer of natural wood), the MOE of a fi nished particleboard can be increased or decreased in the direction concerned.This can be explained by the fact that the viscous elastic properties of natural wood can vary up to 20 times in different fi bre directions.
Nowadays, the market offers various fi nishing materials of wood.Therefore, there is a lack of comprehensive research that evaluates the infl uence of fi nishing materials on the properties of wood-based products.
The aim of this study is to evaluate the infl uence of fi nishing materials on viscous elastic properties of wooden structures and their elements.

MATERIJALI I METODE
The research study used conditioned specimens of oak (Quercus robur) and pine (Pinus sylvestris) wood, which were stored in a climatic chamber for 336 hours at a temperature of 20 ± 1 °C, and a relative humidity of 60 ± 2 % (standard EN 408,chapter 8).The dimensions of the researched specimens were 700 mm ×100 mm ×16 mm, with moisture content ranging between 10.4 and 11.2 % (standard EN 13183-2).According to the standard EN 323, the density of the specimens was measured and the results showed that oak wood had a density ranging between 650 and 760 kg/m 3 , whereas the density of pine wood ranged between 460 and 570 kg/m 3 .
For fi nishing the specimens, two types of material were used: pentaftal varnish and water-based outdoor wood oil, based on water and natural oils.The amount of non-volatile substances of the varnish is 46 ± 3 %, whereas the amount of non-volatile substances of the oil is 21 %.The pentaftal varnish, which is based on alkyd resins, covers the wood surface with an elastic fi lm that suppresses liquids; however, it allows steam to penetrate and it is resistant to atmospheric agents.The water-based wood oil soaks into the wood surface and serves as protection from humidity and dirt.
A special test stand (Figure 1) was used to determine the MOE and damping coeffi cient on the basis of non-destructive testing (transverse resonant vibrations) method, which also allowed assessing the mechanical properties of the specimens (Albrektas and Vobolis, 2003;Albrektas and Vobolis, 2004).Loudspeaker 4, controlled by the generator of electric oscillations 5, excites resonance oscillations of the specimen 1.For this purpose, the frequency of the generator's oscillations is changed.These oscillations are recorded by the sensor 6, fi xed on the specimen.Their amplitude is measured using device 7. To ascertain the direction of specimen bending, the phase of oscillations is measured by a phase meter 9.The phase meter receives the signal from the measuring device and generator.For a more accurate ascertainment of the form of bending, several zones of specimen were chosen.In these zones the measuring element was fi xed and oscillations were recorded, i.e. their amplitude and phase were measured.By determining the corresponding (fi rst mode) frequency, a MOE is calculated.By determining two other frequencies, when the vibrational amplitude decreases by 0.7 times, the damping coeffi cient is calculated.The studies were performed at a frequency of 20-2000 Hz.
The MOE was calculated by the following Eq. 1 (Timoshenkо et al., 1985): (1) Where: E -modulus of elasticity, f rez -frequency of transverse vibrations, ρ -density of wood, s -crosssectional area, l -beam length, I -cross-sectional moment of inertia, A -method of fastening represented by a coeffi cient.
The viscous properties of studied specimens were evaluated by damping coeffi cient, calculated by the following Eq.2: (2) Where: f rez -frequency of transverse vibrations, Δffrequency bandwidth, when the amplitude of vibrations decreases by 0.7 times.
In order to evaluate the infl uence of fi nishing materials on viscous-elastic properties of wooden structures, the specimens were covered with two types of products for wooden surfaces.The fi nishing materials were applied to the specimens following the recommendations of the manufacturers.Prior to application, the specimens were stored at 23 ± 2 °C temperature and 50 ± 5 % relative humidity for 24 h.The surface of the specimen intended for covering was clean and free from distortion and other defects.The fi nishing material was applied with a brush in 2 layers (stepwise) on both sides of the specimen.After each application of the layer, the specimen was dried for 24 hours under the above temperature/humidity conditions in accordance with manufacturer's instructions without prejudice to EN 23270.
For the determination of elastic properties of fi nishing materials, fi lms were prepared separately, consisting of wood varnish and oil, which were used for the study.The fi lm was formed by casting liquid on a smooth, siliconised surface of a test panel.The casting was dried for 7 days at 23 ± 2 °C temperature and 50 ± 5 % relative air humidity.The fi lm formation process was completed by removing it from the test surface.Afterwards, the fi lm-coated specimens were cut to determine the modulus of elasticity.Ten specimens of varnish and the same number of oil specimens were cut for tensile test.The uniformity of fi lm specimens was ensured by measuring the thickness, which was on average 0.30 mm in the case of varnish and 0.19 mm in the case of oil.The tensile test was carried out using the universal testing machine BTI FB-050 TN (Zwick).The gripping distance was 100 mm and the constant speed for the grips was 100 mm/min.

REZULTATI I RASPRAVA
After completing the conditioning process, the specimens were divided into subgroups (oak specimens were divided into O.1 and O.2, whereas pine specimens were divided into P.1 and P.2), the resonance frequency of the specimens was determined (when they vibrate in mode of a theoretic isotope beam) (Timoshenkо et al., 1985), and the MOE and damping coeffi cient were estimated.The results are displayed in Table 1.
The estimated values of the MOE and damping coeffi cient of wood correspond to the values that are well-known in literature (Wagenführ, 2000;Wood Handbook, 2010).
The analysis of the viscous properties of the fi nishing fi lms showed that the MOE of wood varnish and oil varies in the range of 4-7 MPa with a variation of  Afterwards, the specimens from subgroups O.1 and P.1 were varnished, while the specimens from subgroups O.2 and P.2 were oiled using the test fi nishing materials, and after each processing step, the viscous elastic properties of the specimen were recorded.Each subgroup is appropriately represented by the variation of the MOE and damping coeffi cient of the specimens in Figures 2 and 3.
The values of the mechanical properties of all the specimens before and after the fi nishing treatment are provided in Tables 2 and 3.
According to these results (Table 2), it was estimated that between the minimum and maximum value in the same group of specimens, the MOE of oak wood specimens before and after varnish fi nish can vary up to 23 %, whereas the results before and after oil fi nish show that the difference between the maximum and minimum value is no larger than 15 %.Before using the varnish fi nish on oak wood specimens, the values of the damping coeffi cient in the same group can vary more than 50 %.After using the varnish fi nish, the difference between the minimum and maximum damping coeffi cient was reduced to 37 %.The use of this fi nishing treatment reveals that the damping coeffi cient of the specimens was reduced in all cases.The outcome was a little different in the group that was treated with oil fi nish.After applying the oil fi nish, the damping ratio was practically left unchanged, and in some cases, the results showed a slight increase or decrease.The equivalent results were retrieved by analysing the tests results of pine wood specimens.The use of oil fi nish on pine wood specimens proved to have no effect on the damping coeffi cient, since in nine out of ten cases, after applying the oil fi nish, the damping coeffi cient of the specimens remained unchanged.
It was determined that, after the varnishing process, the mass of oak wood specimens increased by 9-14  g (on average approximately 1.5 % of the specimen mass).In all cases, the MOE decreased by 1-5 %, whereas the damping coeffi cient decreased by 15-50 %.
The results show that the mass of the varnished pine wood specimens increased by 13-21 g (on average approximately 3.3 % of the specimen mass).A higher mass change also implied a change in the mechanical properties of the specimens -the MOE decreased by 2-7 %, while the damping coeffi cient decreased by 30-60 %.Pine wood could absorb more varnish, since it has a lower density and its capillary system can encompass a higher relative volume of the whole specimen.
It was established that the mass of the oiled oak wood specimens increased by 7 g, while in the case of pine, it increased by only 4 g, i.e. it altered less than 1 % of the specimen mass.It can be assumed that the impact of oil on the viscous elastic properties of the specimens was quite marginal.The MOE of oak specimens decreased by 1-4 %, the damping coeffi cient increased by 5 %, whereas the MOE and damping coeffi cient of pine specimens were ultimately left unchanged -the values varied within a 1 % range.
A clear reliance of the impact of the fi nishing material on a varied density of the same type of specimens was not determined.Also, there is no evidence of a linear relationship between the amount of fi nishing material for the specimen fi nishing procedure and the change of the viscous elastic properties.Moreover, there was also the infl uence of other factors, such as the uniformity of coating thickness of the fi nish, the depth of penetration, etc.
The decrease of the MOE of the fi nished specimens can be explained by taking into account that the MOE of the fi nishing materials is signifi cantly lower (4-7 MPa) than that of natural wood (8220-14104 MPa) (Table 1).Furthermore, the fi nishing materials, especially varnish, can improve the acoustic properties of wood elements (Ono, 1993).

CONCLUSIONS 4. ZAKLJUČAK
It was established that the fi nishing materials can alter the viscous elastic properties of wooden structure elements.Moreover, this effect is proportional to the amount of used material but not directly depend on the mass of the fi nishing materials.When the varnish formed 2 % (oak) -5 % (pine) of sample mass, the MOE decreased by 5-7 percent, respectively.The damping coeffi cient in these cases decreased to 30-60 percent.The oil formed a smaller part of the specimen and its infl uence on mechanical properties was lower.
It was estimated that the fi nishing material (oil and varnish) amounted to a considerable fraction of the total mass of the fi nished specimen (at least 1.5 %), and in all cases, its modulus of elasticity and damping coeffi cient decreased.This can be explained by the fact that the modulus of elasticity of fi nishing material is lower in relation to wood lengthwise fi bre.
It was determined that a substantial amount of the fi nishing material (varnish) can improve the acoustic properties of a wooden element.An unvarnished wooden structure can dampen the sound more effectively than a varnished one.