Weathering Resistance of Poplar Wood Coated by Organosilane Water Soluble Nanomaterials

The potential use of organosilane nanomaterials (nano-zycosil and nano-zycofi l) for improving the weathering resistance of poplar wood was evaluated in comparison to common clear coatings (nitrocellulose and polyester lacquer). A 250 μm coating layer was applied by an automatic fi lm applicator at the speed of 150 mm/s. The coated specimens were exposed to a 1000 W xenon arc light source at 65 % relative humidity and temperature of 20 oC inside a weather-o-meter for 1440 hrs. Among coating materials, nano-zycosil showed the best performance to improve the weathering resistance. Compared to the lacquer-coated samples, the roughness of nanoparticle-coated ones was less affected by weathering. Contact angle measurements indicated that nanozycosil coating had a pronounced decreasing effect on the surface wettability. The combined analyses of SEM and EDX demonstrated that the nanoscale silane layer covered the whole wood surface homogeneously, whereas nitrocellulose and polyester coatings were deposited preferentially in the surface depressions.


INTRODUCTION 1. UVOD
Wood surfaces exposed to outdoor conditions are rapidly degraded due to the combined action of weather factors, such as oxygen, ultraviolet light and relative humidity (RH).Several paints and varnishes are commonly used as coating materials to prevent the degradation.However, wood coated with common clear coatings is still susceptible to photo discoloration (Lung Chou et al., 2008;Bulian et al., 2017).The photo discoloration is unavoidable even if wood is coated with non-yellowing or durable clear coatings, such as aliphatic polyurethane ones (Chang and Chou, 1999).The coating performance depends on several factors, in particular the substrate, the coating system and the interactions between them (Bulian and Graystone, 2009).Nowadays, coating is an area of signifi cant research in nanotechnology.Nanoparticle based coating systems can provide better weathering resistance (Clausen et al., 2010;Nguyen-Tri et al., 2018) and conservation than the conventional techniques because of very small pigment particles with very high specifi c surface.Some appropriate nanoparticles, such as silica (SiO 2 ), alumina (Al 2 O 3 ) (Busseya et al., 2018; Mori et al., 1998;Powell et al., 1997) and TiO 2 (Nanetti, 2006;Li et al., 2005) have been previously used to improve the wood weathering resistance.For example, Veronovski et al. (2013) found that the surface treatment of wood with nano-TiO 2 (rutile) incorporated in water-based acrylic coatings improves the weathering resistance.Organosilane nanomaterials have been mainly developed for waterproofi ng of wood because they can block the pores by agglomeration and prevent the penetration of water into wood (Tshabalala and Gangstad, 2003;Godnjavec et al., 2012).For concrete, Nanozycosil with size of up to 6 nm can enhance the waterproofi ng property of the surfaces by fi lling the microcracks and nanopores (Taghiyari, 2013).Gholamiyan et al., (2012,2016) also reported that the organosilane nanomaterials (zycosil and zycofi l) can be used as water vapor diffusion retarders for wood.The present study was, therefore, conducted to evaluate if organosilane nanoparticles can signifi cantly contribute to improvement of weathering resistance of wood.

MATERIJALI I METODE
2.1 Wood sampling and coating materials 2.1.Uzorkovanje drva i premazni materijali Flat-sawn boards of poplar wood (Populous nigra) with dimensions of 50 by 100 by 200 mm (T, R and L directions, respectively) at moisture content of 12 % were selected for the study.Sealer and nitrocellulose lacquer and polyester lacquer were used as clear coatings.They were purchased from Dorsa Chemistry (Brilliant) Co.The technical properties of the lacquers are summarized in Table 1.Organosilane water soluble nanomaterials (nano-zycosil and nano-zycofi l) were purchased from Zydex Company.The most important properties of the nanomaterials are presented in Table 2.

Metode nanošenja premaznog materijala
The wood surfaces were sanded with 150-grade sandpaper before coating.Six different coating systems were applied as shown in Table 3.A 250 μm wet precure coating layer was applied by an automatic fi lm applicator at the speed of 150 mm/s.The nanomaterialcoated samples were oven dried at temperature of 103 ± 2 °C for 24 h, and the others were dried in a conditioned room (T = 25 °C and RH=65 %) for about 20 minutes.The lacquers were diluted by a lacquer thinner (1:2) before application.

Weathering test 2.3. Izlaganje vremenskim utjecajima
The coated specimens were exposed to a 1000 W xenon arc light source at 65 % RH and chamber temperature of 30 ºC in a weather-o-meter for 1440 hrs.Spectrophotometry was applied to measure brightness (L*), redness (b*) and yellowness (a*) variables of specimens in the CIE-LAB system before and after exposure to the accelerated weathering.Ten boards were tested for each type of coating system.The spectrophotometry was performed using a miniscan EZ spectrophotometer with the aperture diameter of 25 mm and with a standard illuminant D65 and a 10° standard observer.The parameters of ∆E* (color difference), C* (color saturation) and h* were calculated using the following equations: ( (2)

Measuring the surface roughness 2.5. Mjerenje hrapavosti površine
A Mitutoyo SJ-201P instrument was employed for measuring the surface roughness.Three roughness parameters characterized by ISO 4287 standard (1997), namely average roughness (Ra), mean peak-to-valley height (Rz), and maximum peak-to-valley height (Ry) were considered to evaluate the surface characteristics of coatings before and after weathering.

Determination of we ttability 2.6. Ispitivanje kvašenja
The wetting behavior of coated samples conditioned at 65 % RH and 20 °C was characterized by static contact angle (CA) measurement and deionized water as test liquid.The CA values were determined by sessile drop method using a KSV Cam-101 Scientifi c Instrument (Helsinki, Gottingen University, Germany).The measurements were carried out for 1 and 10 seconds after deposition of the water drop on the surface.The average CA value was obtained with fi ve drops for each sample.

SEM, EDX and ATR-FTIR studies SEM, EDX i ATR-FTIR istraživanje
The surface morphology of coated samples was characterized by a scanning electron microscopy (SEM).The surface chemical composition of speci-mens was examined by energy dispersive X-ray analysis (EDX).Fourier transform infrared spectrophotometer (ATR-FTIR; Bruker model Confocheck) was also applied to identify the formed chemical bonds.According to the measured color data, it can be noted that all coatings are more red, yellow and saturated (c*) than the uncoated wood both before and after weathering (Table 5).The color saturation after weathering ranged from 48.7 for sealer and nitrocellulose lacquer-coating to 35.6 for uncoated wood.Similar results were also found for hue angle (h*).

Surface roughness 3.2. Hrapavost površine
The surface roughness values (Ra, Rz and Rq) for each coating are depicted in Figure 1.After coating, the CSC+CPS and polyester (CPS) coatings were smoother than the other ones, probably due to rigid fi lm formation on the wood surface (Gholamiyan et al., 2012).After weathering, the surface roughness increased for all coatings expect CZ.Increase in the surface roughness of wood after weathering is believed to be due to the erosion of primary cell wall caused by lignin degradation (Bulcke et al., 2007;Meincken and Evans, 2009).It is claimed that the surface roughness of nano- coatings is not signifi cantly affected by weathering because of small size and high surface to volume ratio of nanoparticles (Li et al., 2001).Thus, the smaller size of nano-zycosil particles (10-30 nm) compared to nanozycofi l (90-300 nm) ones may play an important role in the surface roughness.

Kvašenje
The results showed that the wettability of wood decreased due to coating by all used coating systems (Figure 2), and the contact angle was increased significantly with the applied coatings.However, weathering had a decreasing effect on the contact angle for all coatings.The contact angle on the control sample dropped from 43º to 38º after 1440 h weathering and that of CZ from 87º to 67º.Among coatings, CZ and CSC+CPS exhibited the highest contact angle after the weathering exposure.In agreement with our results, waterproofi ng of organosilane nanomaterials has been reported, previously (Tshabalala and Gangstad, 2003;Godnjavec et al., 2012).

SEM and EDX analysis 3.4. SEM i EDX analiza
Scanning electron microscopy (SEM) images of control and nano-zycosil coated fi lms are shown in Figure 3.As can be seen in Figure 4, most particles are in nanometric dimensions (<100 nm).SEM images confi rm that silane nanoparticles can effectively cover the wood surface after coating, which is similar to the results reported by a number of previous researchers (Li et al., 2010;Da Silva et al., 2012;Mahltig et al., 2010).A signifi cant silane peak in the EDX spectra of organosilane coated specimens confi rms the presence of nano-zycosil or nano-zycofi l particles (Figure 5).

Kemijska svojstva površine
The ATR-FTIR spectra of nano-zycosil coated and uncoated specimens are compared in Figure 6.The absorption bands at 1110-1090 cm −1 , 810-800 cm −1 and 480-470 cm −1 are attributed to the Si-O-Si asymmetric stretching, symmetric stretching and bending vibration, respectively.The spectrum of coated specimen 15.3 (0.9) 12.5 (0.8) Mean changes / prosječna promjena 2.4  shows bands resulting from Si-CH 3 bonds (1269 cm -1 ) (Tshabalala and Gangstad, 2003).The bands at broad absorption band of 3445-3425 cm −1 and 1640-1630 cm −1 are due to -OH groups of silane coatings (Rao et al., 2010).In addition, the weak bands centered around 2880 cm -1 due to the C-H stretch and 1460 cm -1 due to the C-H bending vibrations of the wood components were replaced by sharper and much stronger bands at 2921 cm -1 due to the C-H bending vibrations of the long hydrocarbon chain bound to the wood by the nano-zycosil coating (Tshabalala andGangstad, 2003 andWang et al., 2011).

CONCLUSIONS 4. ZAKLJUČAK
SEM images and EDX spectra showed that the wood surfaces can be effectively coated by the organosilane water soluble nanomaterials.SEM images revealed a signifi cant surface coverage by the nanoparticles, and the EDX analysis also confi rmed the presence of silane in the organosilane coated specimens.Among coating materials, nano-zycosil (CZ) exhibited the best performance for improving the weathering resistance of poplar wood.Different color parameters were observed between nano-zycosil and nano-zycofi l coat- ings.After weathering, all coatings except CZ and CZZF were darker than the uncoated wood.Compared to the clear coatings, the roughness of nano-zycosil and nano-zycofi l was less affected by weathering, and the former showed a better performance.This can be due to the very small size and high surface to volume ratio of nanoparticles.Further researches on the nanofi lm formation using sol-gel method are recommended for improving the weathering resistance of wood surfaces.

3. 1
Color changes 3.1.Promjene boje Ta ble 4 shows L*, a* and b* values and their changes on various coated wood samples due to weathering.Before weathering, L* for uncoated specimens was greater than that for all coated ones.The color parameters (L*, a* and b*) of nano-zycofi l (CZF)-and nano-zycosil (CZ)-coated specimens were found to be different.The color change of weathering for CZ was less than that for CZF.One of the main reasons for color change of clear coatings is the discoloration of wood substrate, affected by its individual chemical components, such as extractives (Fengel and Wegener, 1984).After coating, b* and a* decreased for all coatings, except CZ.Sigh et al. (2008) and Lung Chou et al. (2008) also reported a similar result for ye llowing of clear coatings.After weathering, the least change in lightness index was observed for CZ and CZZF coatings.The lightness of CZ and CSC+CPS coatings dropped from 75.6 to 74.3 and from 78.1 to 67.5, respectively.After weathering, all coatings except CZ and CZZF were darker than the control sample as shown by the decreased L* values (see Table 4 ).This L* reduction is due to destruction of chemical bonds and photochemical degradation of cross-linking reactions at the surface coatings (Vlad Cristea et al., 2011).After weathering, all coatings are more reddish and yellowish (indicated by increased b* and a* values, respectively, in Table4) than the control sample(Tsuchikawa et al., 2003 andPastore et al., 2004).The least change in a* and b* parameters was observed for CZZF.After coating, CZ showed the le ast color change (ΔE).