57 Les/Wood, Vol. 69, No. 1, June 2020 1 INT R ODUC TION 1 UVOD Wood is a versatile raw material that is wide- ly used for indoor and outdoor applications. Con- sumption of wood and wood products has in- UDK 629.1:544.527.25 Original scientific article / Izvirni znanstveni članek Received / Prispelo: 12. 5. 2020 Accepted / Sprejeto: 27. 5. 2020 Vol. 69, No. 1, 57-70 DOI: https://doi.org/10.26614/les-wood.2020.v69n01a06 Abstract / Izvleček PHO T OS T ABILIZ A TION OF RUBBER W OOD USING CERIUM O XIDE NANOP AR TICLE S P AR T 1: CHARA C TERIZ A TION AND C OL OUR CHANGE S F O T OS T ABILIZ A CIJ A LE S A KA V ČUK O V CA Z NANODEL CI CERIJEVE GA DIOK SID A 1. DEL: KARAKTERIZ A CIJ A IN SPREMEMBE B AR VE Kavyashree Srinivasa 1,2* , Krishna Kumar Pandey 1 , Marko Petrič 2 1 Wood Processing Division, Institute of Wood Science and Technology, Bengaluru, India 2 Department of Wood Science and Technology, Biotechnical Faculty, University of Ljubljana, Slovenia * e-mail: kavyashree.srinivasa@bf.uni-lj.si (ORCiD 0000-0002- 0197-2413) Abstract: Light induced darkening and deterioration of wood used outdoors is undesirable. Photoprotection of wood could be achieved by using additives that reflect or absorb harmful radiation responsible for degradation. Nano met- al oxides have strong absorption in the UV range of solar radiation and good transparency in the visible region. They offer unique benefits in protecting coatings and coated substrates from being degraded by UV radiation. However, to exploit the properties of nanoparticles, homogenous dispersion without agglomeration is necessary. In the present work, the photostabilization of rubberwood surfaces coated with cerium oxide (CeO 2 ) was studied. The nanoparticles were surface functionalized with an organic alkoxy silane (3-glycidyloxypropyltrimethoxy silane) to improve the ho- mogenous distribution in coatings, and the modified nanoparticles were dispersed in isopropanol and polyurethane (PU) coating. Rubberwood surfaces coated with dispersed nanoparticles (concentration 0.5 % to 6 % w/v) were ex- posed to a fluorescent UVA light source (λ=340 nm) at 60 °C in an accelerated weathering tester for 500 h and 1000 h. Colour changes due to UV light exposure were monitored using a spectrocolourimeter. Dispersion of CeO 2 nanopar- ticles in PU coatings (concentration >2 %) restricted the photoyellowing of wood polymers. K e y w or ds: Rubberwood, PU coating, Nanoparticles, Cerium oxide, Photostability, Colour stability Izvleček: Degradacija in potemnitev lesa, ki je v uporabi na prostem, je nezaželena. Les je pred učinki svetlobe možno zaščititi z dodatki, ki odbijajo ali absorbirajo škodljivo sevanje, ki povzroča njegovo razgradnjo. Nano kovinski oksidi izkazujejo močno absorpcijo svetlobe v UV območju sončnega sevanja in dobro transparentnost v vidnem območju. Zato ponujajo edinstvene prednosti pri zaščiti premazov in površinsko obdelanih substratov, ki so občutljivi na UV sevanje. Vendar pa je za izkoriščanje zaščitnih lastnosti nanodelcev potrebna njihova homogena disperzija, tako da se ne tvorijo aglomerati. V tem prispevku poročamo o raziskavah fotostabilizacije površin lesa kavčukovca z nanodelci cerijevega dioksida (CeO 2 ). Nanodelci so bili površinsko funkcionalizirani z organskim alkoksi silanom (3-glicidiloksi propiltrimetoksi silan) za izboljšanje homogene porazdelitve v premazih. Modificirane nanodelce smo dispergirali v izopropanolu in v poliuretanskem (PU) premazu. Površine lesa kavčukovca, na katere smo nanesli dispergirane nano- delce (koncentracija od 0,5 % do 6 % m/v), smo v napravi za umetno pospešeno staranje za 500 ur in 1000 ur in pri 60 °C izpostavili UVA svetlobi (λ = 340 nm). Barvne spremembe zaradi izpostavitve UV svetlobi smo spremljali s kolorime- trom. Disperzija nanodelcev CeO 2 v PU premazih (koncentracija >2 %) je zmanjšala foto-rumenenje lesnih polimerov. Ključne besede: kavčukovec, PU premaz, nanodelci, cerijev dioksid, fotostabilnost, stabilnost barve creased due to concern about the environment (Rowell, 2005; Hill, 2006). Wood has gained lot of attention because of its low embodied energy, which also acts as carbon sink and contributes to climate change mitigation. Being a biological ma- terial, unprotected wood is susceptible to deg- radation due to a combination of environmental factors (sunlight, moisture, heat, atmospheric pollution, chemicals and biological agents) (Feist & Hon, 1984; Williams, 2005; Evans, 2013). Some of the limitations associated when wood is used 58 Les/Wood, Vol. 69, No. 1, June 2020 Srinivasa, K., Pandey, K. K., & Petrič, M.: Fotostabilizacija lesa kavčukovca z nanodelci cerijevega dioksida. 1. del: Karakterizacija in spremembe barve outdoors are the low durability of many species, dimensional instability with change in moisture content, low resistance against fungi and insect attack and photodegradation of wood (Rowell, 2005). The colour stability of natural wood against light exposure is an important issue from aes- thetic point of view. Reducing or eliminating the damaging effects of solar and artificial UV radi- ation is a major challenge for material scien- tists. One of the most widely used methods of UV protection is the dispersion of UV-absorbing molecules into a material (George et al., 2005). Photoprotection of wood can be achieved by ad- ditives that reflect or harmlessly absorb the light responsible for photodegradation or terminate the free radicals that degrade wood constituents. Inorganic particles can block light from reaching wood substrates and protect wood from photo- degradation. Small particles below a certain size are thus able to scatter UV light while having little effect on the visible component of the spectrum. These properties of nanoparticles and their abil- ity to absorb UV light underpins the use of metal oxides (titanium dioxide, iron and zinc oxides) as transparent photoprotective agents for coatings applied onto wood. Recently, many studies have focused on im- proving the UV absorption characteristics of wood coatings by incorporation of nanoparticles (Aloui et al., 2007; Clausen et al., 2010; Auclair et al., 2011; Nikolic et al., 2015). Moreover, some stud- ies use nanoparticles along with or in contrast to organic UV absorbers to protect wood from pho- todegradation (Forsthuber et al., 2013). But the majority of such studies report the use of zinc oxide or titanium dioxide as the nano additives in coatings for UV protection (Allen et al., 2002; Cristea et al., 2010; Fufa et al., 2012; Wang et al., 2014; Miklečić et al., 2015), and very few use cerium oxide as a UV stabilizer (Liu et al., 2010; Blanchard & Blanchet, 2011; Schaller et al., 2011; Saha et al., 2013). The photostability of yellow cedar veneers pre-treated with micronized iron oxide and ce- rium oxide nanoparticles was examined by Liu et al. (2010). The results revealed that, in com- parison to iron oxide, the cerium oxide nano- particles were not as effective at restricting the weight loss, tensile strength losses, and prevent- ing the photodiscolouration of exposed veneers. Blanchard and Blanchet (2011) studied the colour stability of ZnO and CeO 2 nanoparticles in com- parison with UV absorbers dispersed in a water- borne UV curable polyurethane / polyacrylate resin. The inorganic absorbers performed better in comparison with the organic UV absorbers, while zinc and cerium oxides efficiently reduced yellowing. A similar comparison study was carried out by Schaller et al. (2011) for a longer exposure time. The poor performance of CeO 2 nanoparti- cles that was found was attributed to the pres- ence of aggregates and lower concentration. The acrylic polyurethane coatings modified with CeO 2 nanoparticles alone or in combination with lignin stabilizer and/or bark extracts showed better pro- tection of thermally treated jack pine compared to coatings containing organic UV absorbers (Saha et al., 2013). In the present study, the photostability of rub- berwood coated with different concentrations of CeO 2 nanoparticles exposed to UV-A light under ac- celerated weathering conditions is discussed. Col- our changes occurring due to UV light irradiation were regularly monitored and analysed. Rubber- wood (Hevea brasiliensis) is a low durable, light yel- lowish-brown plantation grown, easy to work, hard wood species. It finds applications in furniture, toys, kitchen accessories, pulp and paper products, and fibreboards. 2 MA TERIA LS AND METH ODS 2 MA TERIA LI IN MET OD E 2.1 MA TERIAL S 2.1 MA TERIAL I Specimens of rubberwood (Hevea brasiliensis) of size (150 mm × 75 mm × 5 mm) (length × width × thickness) were prepared from defect-free wood for the evaluation of photostability. Wood speci- mens were air dried followed by drying in a hot air oven at 65 °C and stored at room temperature. Ce- rium oxide nanoparticles (~25 nm) were purchased from Sigma Aldrich, 3-glycidyloxypropyltrimethoxy silane (GPTMS) from Gelest Inc., and polyurethane (PU) coating material (without any additives) was procured from Asian Paints, Mumbai. Other chemi- cals used in the study were of AR grade. 59 Les/Wood, Vol. 69, No. 1, June 2020 Srinivasa, K., Pandey, K. K., & Petrič, M.: Photostabilization of rubberwood using cerium oxide nanoparticles. Part 1: Characterization and colour changes 2.2 SU RF A CE MODIFICA TION AND DISPER SION OF NANOP AR TICLE S 2.2 PO VR ŠINS KA MODIFIKA CIJ A IN DISPER GIRANJE NANOD EL CEV In order to obtain a homogenous distribution of nanoparticles in solution, alkoxy silane 3-glycidy- loxypropyltrimethoxy silane (GPTMS) was used as a surface modifier. The process used for dispersion of CeO 2 nanoparticles was carried out as per the procedure discussed elsewhere in detail (Srinivas & Pandey, 2017). 2.3 CHARA C TERIZ A TION OF SILANE MODIFIED NANOP AR TICLE S 2.3 KARAKTERIZ A CIJ A NANODEL CEV , MODIFICIR ANIH S SILAN OM Surface modified nanoparticles were charac- terized using UV-visible absorption spectroscopy, X-ray diffraction (XRD) and the dispersion of nan- oparticles by Dynamic Light Scattering (DLS) and Scanning Electron Microscopy (SEM). The UV-Vis spectra of surface modified nan- oparticles in powder form were measured using an Ocean Optics HR 4000 UV-Vis spectrophotom- eter (UV-Vis-NIR light source, DT-MINI-2-GS, Jaz detector) at Kuvempu University, Shimoga, Karna- taka. The baseline of UV spectra was set by using standard BaSO 4 . The dried nanoparticle samples were packed tightly in a circular opening (diame- ter 0.4 cm) with a thickness of 0.5 mm on a glass plate. The UV spectra of samples were recorded using the optical fibre held exactly at 90° to the sample. XRD analysis was carried out to know the phase and size of surface modified nanoparticles. XRD patterns were recorded from 10° to 90° with a PANalytical X’pert pro diffractometer using Cu Kα (λ=1.5418 Å) with a nickel filter. Data were col- lected from modified nanopowder with a counting rate of 5° per min. Dynamic light scattering was used to deter- mine the size distribution profile of particles in PU suspension using BIC Zeta PALS. DLS analysis was done at concentration levels of 0.01 % of nanopar- ticles in liquid suspension (PU base material). PU alone was also analysed to assess any interference in DLS analysis when nanoparticles are used, and the findings showed that it did not have any signif- icant monomer/oligomeric structure which could interfere in the results. The distribution of nanoparticles in polyure- thane was also examined using a high resolution scanning electron microscope (Gemini Ultra 55, with ESB detector at 5.0 kV). Thin films of nanoparticle dispersed in PU were prepared by pouring dispersed solution on a clean plastic sheet. After drying at room temperature, the thin films were pulled from the plastic sheets and kept in a vacuum desiccator for 72 hours before analysis. A thin layer of gold (9 nm) was sputtered onto the thin films mounted on a metal grid using carbon tape to make the sample conductive. XRD, DLS and SEM analysis were carried out at CeNSE, Indian Institute of Science, Bengaluru. 2.4 C O A TING OF W OOD WIT H NANOP AR TICLE S DISPER SED IN ISOPR OP ANOL/PU 2.4 PO VR ŠINS KA OBDELA V A LE S A Z NANODEL CI, DISPER GIRANIMI V IZ OPR OP ANOL U/PU In order to know the effects of nanoparticles alone, one set of wood samples were coated with modified nanoparticles dispersed in isopropanol and another set with nanoparticles dispersed in PU coat- ing. Different concentrations (0.5 %, 1.0 %, 2.0 %, 4.0 % and 6.0 %) of silane modified CeO 2 nanoparticles were added to isopropanol or PU, subjected to ho- mogenisation in a homogeniser (IKA T25 digital UL- TRA-TURRAX) for 20 minutes at 10 krpm. Wood sur- faces were coated with two coats of homogenized solution of nanoparticles using sprayer with an inter- mittent drying time of one hour and dried overnight at room temperature. A coating thickness of ~50 µm was achieved. All the measurements made on wood samples coated with nanoparticles dispersed in iso- propanol were done carefully to avoid loss of the na- noparticle layer from the wood surface. 2.5 PHO T OS T ABILITY OF W OOD SURF A CE S C O A TED W ITH CE O 2 NAN OP AR TICLE DISPER SED IN ISOPR OP ANOL/PU 2.5 F O T OS T ABILNOS T PO VR ŠIN LE S A , OBDELANIH Z NANODELCI, DISPER GIRANIMI V IZ OPR OP ANOL U/PU The photostability of wood was assessed using a weatherometer (Qlab QUV accelerated weather- ing tester, UVA-340 lamp) at an irradiance of 0.68 W/m 2 , chamber temperature of 60 °C. Initially samples coated with different concentrations of nanoparticle were exposed to UV light. Four repli- cas of wood samples per treatment were used in 60 Les/Wood, Vol. 69, No. 1, June 2020 Srinivasa, K., Pandey, K. K., & Petrič, M.: Fotostabilizacija lesa kavčukovca z nanodelci cerijevega dioksida. 1. del: Karakterizacija in spremembe barve the study. Forty-eight samples were kept in a sin- gle run of 500 h. The samples were removed from the weathering tester after exposure of 50 h, 100 h, 150 h, 200 h, 250 h, and 500 h and were analysed for colour changes. Based on the results, only wood samples coated with higher nanoparticle concen- trations (2 %, 4 % and 6 %) were exposed to UV light for another 500 h along with control wood samples. 2.6 C OL OUR CHANGE S 2.6 SPREMEMBE B AR VE Changes in the colour of wood surfaces due to irradiation were measured using a Hunter lab - Lab scan XE model spectrocolourimeter (10° standard observer, D65 standard illuminant, xenon flash lamp source and CIELAB system). The CIELAB sys- tem is characterized by three parameters L*, a*, b*. L* axis represents the lightness, a* and b* are the chromaticity coordinates, a* varies from red (+) to green (-) and b* varies from yellow (+) to blue (-). Coordinates L*, a* and b* were measured on each sample before and after accelerated weathering exposure. Measurements were taken at six differ- ent locations for each sample; the mean value and standard deviation were calculated. Changes in col- our coordinates after UV exposure were measured and changes in colour due to exposure were calcu- lated as the ∆L*, ∆a* and ∆b* values. These values were used to calculate the total colour change ∆E* as a function of the weathering time, according to the following equation 1 (CIE 1986), ∆E* = (∆L* 2 + ∆a* 2 + ∆b* 2 ) 1/2 (1) The ∆L*, ∆a* and ∆b* values given in eqn. 1, are the changes in L*, a* and b* parameters due to irradiation with respect to unirradiated and irradi- ated wood specimens. 3 RE SUL T S AND DISCUSS ION 3 RE ZUL T A TI IN RAZPRA V A 3.1 SU RF A CE FUNC TIONALIZ A TION OF CEO 2 NAN OP AR TICLE 3.1 PO VR ŠINS KA FUNK CIONALIZ A CIJ A NANODELCEV CEO 2 Most of the widely used organosilanes (R-(CH 2 ) n -Si-X 3 ) have one organic substituent (R) and three hydrolyzable substituents (X). In most surface treatment applications, the alkoxy groups of the trialkoxy silanes are hydrolyzed to form si- lanol-containing species. The reaction of silane with nanoparticles involves hydrolysis of the three labile groups followed by their condensation to oligomers, the formation of hydrogen bond by oli- gomers with -OH groups of the substrate and finally a covalent linkage is formed with the substrate with loss of water during drying or curing. The R group (glycidyloxypropyl) remains available for covalent reaction or physical interaction with other phases (PU coating). Among alkoxy silanes, only methoxy silanes are effective without catalysis. In order to minimize agglomeration, nanoparticles were chem- ically modified with 3-glycidyloxypropyltrimethoxy silane (GPTMS) using ultrasonication. The chemical structure of GPTMS is shown in Fig. 1, as given by the provider. The basic principle of using ultrasoni- cation for dispersion is the cavitation in liquids cre- ated by ultrasonication, which accelerates chemical reactions by facilitating the mixing of reactants. It also enables the uniform dispersion of micron-size or nano-size materials (Suslick & Price, 1999). Figure 1. Chemical structure of 3-glycidyloxypropyl- trimethoxy silane (GPTMS). Slika 1. Kemijska struktura spojine 3-glicidiloksipro- piltrimetoksi silan (GPTMS). 3.1.1 Ultr a viole t - Visible Ab sorp tion spectr a 3.1.1 UV - vidni ab sorpcijski spek tri Nanoparticles modified with GPTMS were characterized using UV visible absorption spec- troscopy. The spectra of unmodified and modified nanoparticles are as shown in Fig. 2A. Absorption spectra showed a broad absorption in the region between (200-350) nm, which exhibit the strong tendency of nanoparticles to absorb UV radiation (Arul et al., 2011). The spectra also suggest that surface modification with silane has not altered or posed any interference in the UV absorption region of the nanoparticles, but was also observed to in- crease absorption in the visible region which affect- ed the transparency of the coating. 61 Les/Wood, Vol. 69, No. 1, June 2020 Srinivasa, K., Pandey, K. K., & Petrič, M.: Photostabilization of rubberwood using cerium oxide nanoparticles. Part 1: Characterization and colour changes 3.1.2 X -r a y diffr action (XRD) analy sis of nanoparticles 3.1.2 R en tg ensk a pr ašk o vna difr ak cija (XRD) nanodelcev X-ray powder diffraction is a rapid analyti- cal technique primarily used for phase identifica- tion of a crystalline material, and it can provide information on unit cell dimensions. X-ray dif- fraction is based on constructive interference of monochromatic X-rays and a crystalline sample. Various factors affect the broadening of diffrac- tion peaks, such as crystalline size, domain size distribution, crystalline facets (external defects) and micro-strain (deformation of the lattice). As the size of the nanocrystals decreases, the line width is broadened. The XRD pattern for CeO 2 nanoparticles is shown in Fig. 2B. The ‘hkl’ val- ues were compared with the standard JCPDS file (PCPDF 34-0394)20 (Arul et al., 2011). The exhib- ited XRD peaks correspond to the (111), (200), (220), (311), (222), (400), (331) and (420) of the cubic fluorite structure of CeO 2 . There is no spuri- ous diffraction peak found in the above samples. This confirmed that all the compounds were sin- gle phase. Further, the intensity of the XRD peaks of the sample reflects that the nanoparticles are crystalline and the broad diffraction peaks indi- cate very small size crystallites. The average crys- tallite size was estimated from the full width at half maximum (FWHM) of the diffraction peak of the powder samples, using Scherrer’s formula the average crystallite size of CeO 2 , and was found to be 84.08 nm. 3.1.3 Dynamic ligh t sc a tt ering (DLS) of nanoparticle disper sions 3.1.3 Dinamično sipanje s v e tlobe (DLS) disperzij nanodelcev Silane modified nanoparticles were dispersed in PU using ultrasonication and a homogenizer. The correlation functions of the intensity fluctuations were converted into intensity size distributions and are plotted in Fig. 3. A high proportion of the parti- cle sizes were found between (105-120) nm having a poly dispersity index of 0.144. 3.1.4 Sc anning Electr on Micr osc op y of nanoparticles disper sed in PU 3.1.4 V r s tična elek tr onsk a mikr osk opija nanodelcev, dispergiranih v PU A scanning electron microscope (SEM) uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. The signals derived from electron-sample interac- tions reveal information about the sample including the external morphology (texture), chemical com- position, and crystalline structure and orientation of materials making up the sample. In order to know the homogenous distribution of surface modified CeO 2 nanoparticles incorporated in PU coating, scanning Figure 2. A) UV visible absorption spectra (m-CeO 2 means functionalized CeO 2 ) and B) XRD pattern of GPTMS modified CeO 2 nanoparticles. Slika 2. A) UV-vis absorpcijska spektra (m-CeO 2 pomeni funkcionaliziran CeO 2 ) in B) rentgenski praškovni difraktogram z GPTMS modificiranih nanodelcev CeO 2 . 62 Les/Wood, Vol. 69, No. 1, June 2020 Srinivasa, K., Pandey, K. K., & Petrič, M.: Fotostabilizacija lesa kavčukovca z nanodelci cerijevega dioksida. 1. del: Karakterizacija in spremembe barve electron micrographs (SEM) were recorded. SEM im- ages of unmodified CeO 2 nanoparticles are shown in Fig. 4A. Severe agglomeration was observed in the case of unmodified CeO 2 particles with an average particle size >500 nm. The surface modified CeO 2 nanoparticles showed a size distribution of around 100-150 nm and a uniform dispersion (Fig. 4B). SEM analysis showed that the surface modification with GPTMS silane has a significant effect in minimizing the formation of agglomerates. All the characteri- zation techniques show that the average size of the nanoparticles in the dispersing medium varies from (90-130) nm. But the performance of nanoparticles mainly depends on the particle size, morphology and uniform size distribution. Though the surface modi- fication with silane helped in avoiding the formation of agglomerates and encouraged an even distribu- tion of nanoparticles in the PU coating, it was not ef- fective to maintaining the particle size below 50 nm. Freeman and McIntyre (2008) reported that nano- particles having a size smaller than wood pores (100 µm) and intercellular pores ((400-600) nm) could penetrate the porous structure of wood and thereby influence wood protection against damaging agents. Hence the size specific properties of nanoparticles can be efficiently utilized in the coating formulations reported in this study. 3.2 PHO T OS T ABILITY OF W OOD C O A TED WITH NAN OP AR TICLE S 3.2 F O T OS T ABILNOS T LE S A , PO VR ŠINSK O OBDELANE GA Z NANODEL CI Wood specimens coated with modified nano- particles dispersed in isopropanol and another set with dispersed nanoparticles in PU coating were ex- posed to accelerated weathering. Analysis of wood coated with nanoparticles dispersed in isopropanol revealed the effects of nanoparticles alone on the wood surfaces, since isopropanol evaporated at room temperature. In contrast, the analysis of nan- oparticles dispersed in PU coating revealed the ef- fects of light on PU coating containing the inorganic absorbers as well as the wood surface. Figure 4. SEM images of A) unmodified and B) GPTMS modified CeO 2 nanoparticles dispersed in PU coating. Slika 4. SEM mikrografi A) nemodificiranih in B) nanodelcev CeO 2 , dispergiranih v PU premazu. Figure 3. DLS analysis of the particle size distribu- tion of modified CeO 2 nanoparticles in PU coating Slika 3. Analiza DLS za določitev porazdelitve ve- likosti modificiranih nanodelcev CeO 2 v PU premazu A B 63 Les/Wood, Vol. 69, No. 1, June 2020 Srinivasa, K., Pandey, K. K., & Petrič, M.: Photostabilization of rubberwood using cerium oxide nanoparticles. Part 1: Characterization and colour changes 3.2.1 E ff ects of UV irr adia tion on c olour parameters (L*, a* and b*) of nano CeO 2 c oa t ed w ood 3.2.1 V pliv UV ob se v anja na par ame tr e bar v e (L*, a* and b*) lesa, površinsko obdelanega z nanodelci CeO 2 Wood when exposed to light initially changes colour, showing the degradation of wood compo- nents by light absorption. By measuring the colour change of the clear coated wood with inorganic ab- sorbers during artificial weathering, it is possible to obtain information on the performance of photo- stabilization. Changes in colour parameters of wood surfaces coated with nanoparticles of CeO 2 and ex- posed to UV radiation are shown in Fig. 5. Uncoated wood changes its colour within a few hours of irradi- ation due to photodegradation of chemical compo- nents mainly lignin present in wood (Tolvaj & Faix, 2009; Rosu et al., 2010; Müller et al., 2013). The control wood becomes darker and yellower with an Figure 5. Colour changes after 1000 h of UV exposure of (a) uncoated, and coated with (b) 2 %, (c) 4 % and (d) 6 % CeO 2 (Top: without PU; Bottom: in PU coating). Slika 5. Spremembe barve po 1000 urah izpostavitve UV svetlobi (a) površinsko neobdelanega lesa in lesa, obdelanega z (b) 2 %, (c) 4 % in (d) 6 % CeO 2 (zgoraj: brez PU; spodaj v PU premazu). a b c d 64 Les/Wood, Vol. 69, No. 1, June 2020 Srinivasa, K., Pandey, K. K., & Petrič, M.: Fotostabilizacija lesa kavčukovca z nanodelci cerijevega dioksida. 1. del: Karakterizacija in spremembe barve increase in the irradiation time. Wood coated with a lower concentration of CeO 2 nanoparticles also showed colour changes with time. Wood coated with concentrations of (2 %, 4 % and 6 %) of nan- oparticle loadings showed very less colour change when compared to that seen with lower loadings. Similarly, wood samples coated with different concentrations of nanoparticles dispersed in PU were subjected to UV light irradiation in a QUV test- er. The performance of the nanoparticles dispersed in PU coating on wood surfaces was compared with that of PU coating without nanoparticle. Wood coat- ed with PU alone (without nanoparticles) exhibited severe colour changes, which increased with irradi- ation time. This indicates that PU coating without any UV stabilizer gets degraded rapidly upon UV light irradiation. Incorporation of nanoparticles in PU limited the colour changes. Figure 5 illustrates the colour changes in wood coated with different concentrations of CeO 2 nanoparticles embedded in PU coatings after 1000 h of UV irradiation. Colour changes in wood coated with CeO 2 dispersed PU were significantly reduced. The presence of CeO 2 gave a lighter colour to the coating, which was re- tained even after 1000 h of UV exposure. These visual observations were supported with spectro- scopic analysis. Coating of wood surfaces with CeO 2 nanoparticles makes wood surfaces lighter in col- our, which is indicated by the successive increase in L* values and decrease in the values of a* and b*. Higher concentrations of nanoparticles dispersions showed stability against photoyellowing. The variations in colour coordinates of uncoat- ed and CeO 2 dispersed coatings before UV exposure are given in Table 1. It is seen that the L* value in- creases while the a* and b* values decrease with an increase in the nanoparticle concentration, which is due to the increase in opacity of the coating. It was also observed that coating of wood with PU has ef- fects on the colour parameters. Wood samples coat- ed with PU had a darker appearance. L* values were observed to be decreased and a* and b* increased in the PU coated wood compared to the corresponding coatings with isopropanol. This may be attributed to the coating material, as it was obtained without any additives. Upon light exposure, colour changes in the uncoated wood surface is indicated by a decrease in the value of lightness (L*) and increase in the yellow- ness b* (Fig. 6). The decrease in the L* parameter indicates severe darkening of the control wood sam- ple. The b* values of uncoated wood increased. The increase in value can be attributed to the formation of a quinone-like structure from lignin degradation (Feist & Hon 1984). The lightness index (L*) of un- coated wood decreased with an increase in irradia- tion time from 74.23 ± 1.68 (0h) to 69.18 ± 1.13 (500 h) and in wood coated with 0.5 % and 1% CeO 2 the L* values were 72.96 ± 1.98 and 73.73 ± 0.50 after 500 h of exposure. In the case of wood treated with 2 % CeO 2 nanoparticles the L* value varied from 78.33 ± 1.87 to 77.06 ± 1.60, similarly for wood treated with 4 % and 6 % of CeO 2 the values varied from 78.12 ± 1.86 to 76.37 ± 0.58 and 79.70 ± 0.59 to 79.63 ± 0.43, respectively, after 500 h of UV exposure. This shows there was no appreciable decrease in L* val- ues in comparison with uncoated wood and wood coated with lower nanoparticle concentrations. This indicates that the wood coated with a concentration of 2 % and more nano CeO 2 reduces the darkening of the wood surface due to light irradiation. The chromaticity coordinates a* and b* in the case of uncoated wood increased with an increase in exposure time. This can be attributed to the pho- toyellowing of the wood surfaces upon light irradi- ation. The a* value of uncoated wood increased from 6.63 ± 0.66 to 10.65 ± 0.30 after 500 h of UV exposure, in the case of wood coated with lower concentrations of CeO 2 ., 0.5 % and 1.0 %, the a* values increased as in the control wood. However, in wood samples coated with 2 %, 4 %, and 6 % nanoparticles, the a* value varied from 5.39 ± 1.21 to 5.25 ± 0.70, 4.64 ± 0.85 to 6.63 ± 0.55 and, 4.15 ± 0.66 to 5.25 ± 0.69 for the respective nanoparti- cle concentrations after 500 h of UV exposure. Yel- lowness induced in wood due to UV light exposure can be evaluated from b* values. The chromaticity coordinate b* values in the case of uncoated wood increased along with the exposure time. Uncoated wood became darker and yellower as the exposure time increased. In uncoated wood, the b* values increased from 21.31 ± 0.99 to 29.38 ± 1.52 after 500 h of UV exposure. Even in wood coated with 0.5 % and 1.0 % CeO 2 , b* values increased with the time of exposure, but in the case of wood coated with 2 % and 4 % CeO 2 , the increase in b* values was much lower in comparison to those seen with the control wood. In wood coated with 6.0 % CeO 2 , b* values were found to decrease initially from 65 Les/Wood, Vol. 69, No. 1, June 2020 Srinivasa, K., Pandey, K. K., & Petrič, M.: Photostabilization of rubberwood using cerium oxide nanoparticles. Part 1: Characterization and colour changes 14.15 ± 0.44 to 11.93 ± 1.39 (50 h) but increased to 13.51 ± 1.34 after 500 h of exposure. Similarly, for wood coated with PU coatings, the lightness index L* of wood coated with PU alone was found to darken with time, and the L* values decreased from 63.26 ± 0.60 to 55.23 ± 1.60 after 500 h of UV exposure. The yellowness index b* was observed to increase from 33.25 ± 1.37 to 49.13 ± 1.62 after 500 h of exposure. In the case of wood coated with nano CeO 2 in PU, the L* values varied from 67.08 ± 0.41, 69.15 ± 1.87 and 70.76 ± 1.73 to 62.79 ± 0.28, 66.10 ± 1.15 and 67.77 ± 0.31 for 2 %, 4 % and 6 % nanoparticle loadings respectively after 500 h of exposure. The a* values in PU control and wood coated with <1 % nanoparticle loadings showed an increase with time, whereas wood coat- ed with >2 % loadings, a* values slightly decreased or remained constant. The yellowness index b* also increased with irradiation time in samples coated with (0.5 - 1.0) % CeO 2 and PU alone (33.25 ± 1.38 to 49.13 ± 1.62 after 500 h). The samples coated with 2 % nano CeO 2 showed an increase in b* value (40.44 ± 1.23), and in wood coated with 4 % (30.94 ± 1.53) and 6 % CeO 2 (25.89 ± 2.03) samples the b* values showed minor increase after 500 h of light exposure. Similar colour change results were observed for wood protected by depositing CeO 2 coating on surface (Lu et al., 2013; Nair et al. 2018). The changes in lightness (ΔL*) and yellowness (Δb*) for uncoated and nano coated wood are shown in Fig. 6. Δa* values are not discussed, and only the lightness and yellowness indexes are discussed to explain the UV stabilization of wood. The maximum changes were observed in the case of uncoated control wood, which increased with the length of exposure. The negative ΔL* values indicated the darkening of wood due to degradation from UV light. The ΔL* values were negligible in wood coat- ed with 6 % of CeO 2 even after 500h of UV expo- sure. The control wood showed an increase in Δb* values with time. In contrast, negative Δb* values were observed in wood coated with 4 % and 6 % CeO 2 . This shows the effectiveness of CeO 2 nano- particles at concentrations >2 % to stabilize wood surfaces against UV light induced photo-yellowing. The ΔL* and Δb* values of PU coating with / without CeO 2 nanoparticles after 500 h of UV ex- posure are presented in Fig. 6. It was observed that changes in ΔL* values were negative for wood coated with PU alone. ΔL* values in wood coated with 2 %, 4 % and 6 % CeO 2 in PU remained con- stant. The Δb* values in control wood increased with exposure time, and wood coated with 4 % and 6 % CeO 2 showed significantly less changes in yel- lowness (Blanchard & Blanchet, 2011; Saha et al., 2013). These results indicate that the UV resistance of wood coatings increases with the increase in concentration of nanoparticles. Table 1. Colour parameters of wood coated with nanoparticles dispersed in isopropanol and PU coating before UV exposure. Preglednica 1. Barvni parametri lesa, obdelanega z disperzijo nanodelcev v izopropanolu in s PU prema- zom z nanodelci pred izpostavljenostjo UV. Coa ting CeO 2 in Isopropanol CeO 2 in PU L* a* b* L* a* b* Control 74.23 ± 1.68 7.63 ± 0.66 21.31 ± 0.99 63.26 ± 0.60 14.10 ± 0.08 33.25 ± 1.37 0.5 % CeO 2 75.92 ± 2.36 6.35 ± 1.54 20.45 ± 0.87 66.47 ± 0.68 10.53 ± 0.56 31.38 ± 1.30 1 % CeO 2 77.00 ± 1.06 6.02 ± 0.56 20.01 ± 0.69 67.01 ± 0.18 10.26 ± 0.90 30.96 ± 1.31 2 % CeO 2 78.33 ± 1.87 5.39 ± 1.21 17.30 ± 0.89 67.08 ± 0.41 10.03 ± 0.20 28.30 ± 1.74 4 % CeO 2 78.12 ± 1.86 4.64 ± 0.84 16.26 ± 0.67 69.15 ± 1.87 9.58 ± 0.59 23.46 ± 0.79 6 % CeO 2 79.70 ± 0.59 4.15 ± 0.65 14.15 ± 0.44 70.76 ± 1.73 9.17 ± 0.22 21.41 ± 0.15 66 Les/Wood, Vol. 69, No. 1, June 2020 Srinivasa, K., Pandey, K. K., & Petrič, M.: Fotostabilizacija lesa kavčukovca z nanodelci cerijevega dioksida. 1. del: Karakterizacija in spremembe barve In order to verify the stability of these nano coatings, wood samples which showed good UV resistance were exposed to another 500 h along with control samples. The changes in colour pa- rameters after 1000 h of UV exposure are shown in Fig. 7. With an increase in concentration, na- noparticles may form aggregates and thereby decrease the photostabilization efficacy of coat- ings (Blanchard & Blanchet, 2011). However, the results revealed that the photostability of wood coated with more nanoparticles was not altered even after longer exposure durations. It was ob- served that the changes in colour parameters af- ter 1000 h exposure remained constant or only a slight variation was seen compared to the corre- sponding values after 500 h of exposure. It can be concluded that colour changes are more drastic in the initial hours and after a certain time they be- come less pronounced. Figure 6. Changes in the ΔL* and Δb* values of control and wood surfaces coated with CeO 2 plotted against time of UV exposure. A) Without PU coating, B) with PU coating. Slika 6. Vrednosti ΔL* in Δb* kontrolnih vzorcev in površin lesa z nanodelci CeO 2 , ki so bili izpostavljeni UV svetlobi. 67 Les/Wood, Vol. 69, No. 1, June 2020 Srinivasa, K., Pandey, K. K., & Petrič, M.: Photostabilization of rubberwood using cerium oxide nanoparticles. Part 1: Characterization and colour changes The total colour change (ΔE*) of control and nano coated wood at different time intervals is shown in Fig. 8. The ΔE* in the uncoated control wood and wood with 0.5 % CeO 2 increased rapidly with irradiation time. The rate of change was high- er during the initial exposure but later was found to decline. The ΔE* values were maximal for uncoat- ed wood (ΔE* = 10), which however were found to reduce with the addition of different nanoparticle concentrations. Wood coated with >2 % nanopar- ticles showed a much lower increase in ΔE* values (less than 6), which shows the better colour stabi- lization than at their respective lower concentra- tions. Wood samples coated with PU and CeO 2 dis- persed in PU also showed an increase in ΔE* values with an increase in time. However, the extent of this increase was lower in the case of wood coated with 4 % and 6 % CeO 2 loadings (ΔE*<6) in contrast to lower loadings (ΔE* = 18 in PU control wood). The total colour change in uncoated and wood coated with higher nanoparticle concentrations exposed for 1000 h of UV light did not vary much A B Figure 7. Changes in the ΔL*and Δb* values of control and wood surfaces coated with CeO 2 after 1000 h of UV exposure. A) Without PU coating, B) with PU coating. Slika 7. Vrednosti ΔL*in Δb* kontrolnih vzorcev in površin lesa z nanodelci CeO 2 , po 1000 urah izpostavitve UV svetlobi. Figure 8. Effects of UV irradiation on the ΔE* values of control and wood surfaces coated with different concentration of CeO 2 . Slika 8. Vpliv UV obsevanja na ΔE *vrednosti kontrolnih in površin lesa, prevlečenih z nanodelci z različnimi koncentracijami CeO 2 . 68 Les/Wood, Vol. 69, No. 1, June 2020 Srinivasa, K., Pandey, K. K., & Petrič, M.: Fotostabilizacija lesa kavčukovca z nanodelci cerijevega dioksida. 1. del: Karakterizacija in spremembe barve in comparison with 500 h of exposure. On visual observation, it can be noted that the yellowing of wood surfaces due to light exposure could not be controlled completely in wood coated with nano- particles dispersed in PU. This may be attributed to the particle size of CeO 2 that was >100 nm, and also due to the degradation of the PU coating on light exposure. In general, it was observed that in compar- ison to uncoated wood, wood with nanoparticles exhibits improved resistance to photodegradation, which increases along with the nanoparticle con- centration. However, higher concentrations of na- noparticles greatly affect the transparency of the coating material. 4 CONCLUSIONS 4 SKLEPI The efficacy of cerium oxide (CeO 2 ) nanopar- ticle-based coatings for photostabilization of rub- berwood (Hevea brasiliensis) surfaces was studied. Nanoparticles were surface functionalized with an organic alkoxy silane (3-glycidyloxypropyltrimeth- oxy silane) to achieve uniform dispersion of nano metal oxide in isopropanol and polyurethane coat- ings. Isopropanol or polyurethane coating with a dispersed surface of functionalized nanoparticles of different concentrations (concentration (0.5–6) %) were applied to rubberwood. The coated sam- ples were exposed to a UVA-340 nm light source in an accelerated weathering tester. Colour changes occurring due to UV light exposure were analysed at regular time intervals. Uncoated wood showed severe darkening and yellowing with the increase in exposure time, while the wood coated with na- no-dispersions showed less darkening and yellow- ing. The results revealed that formulations with ≥2 % of nanoparticles can stabilize wood surfaces against UV degradation. It was thus shown that dis- persion of nanoparticles in PU coatings can signifi- cantly restrict the colour changes and photodegra- dation of wood polymers. 5 SU MMAR Y Surface functionalization of nanoparticles us- ing 3-glycidyloxypropyltrimethoxy silane (GPTMS) was carried out. Modified nanoparticles were dis- persed in isopropanol and/or polyurethane (PU) coating. The modified nanoparticles and their dis- persion were characterized using UV-Visible ab- sorption spectroscopy, X-ray diffraction, dynamic light scattering (DLS) and scanning electron micros- copy (SEM). UV-Visible absorption spectra showed a broad and wide absorbance range for nanoparti- cles in the UV region. The results from SEM showed that modification with GPTMS was effective in reducing agglomeration and obtaining a homoge- nous distribution of nano metal oxides in the pol- ymer matrix. The efficacy of CeO 2 nanoparticles for photostabilization of rubberwood (Hevea brasilien- sis) surface was studied. Different concentrations of surface functionalized nanoparticles (concentra- tion (0.5–6) %) were dispersed in isopropanol and polyurethane clear finish, and the obtained for- mulations were applied on wood. The coated and uncoated samples were exposed to a UVA-340 nm light source in an accelerated weathering tester for up to 500 h and 1000 hours. Colour changes occur- ring due to UV light exposure were analysed at reg- ular time intervals. The dispersion of nanoparticles in coatings effectively restricted the colour changes and photodegradation of wood polymers, particu- larly at ≥2 % nanoparticle concentration. 5 PO VZETEK Izvedli smo površinsko funkcionalizacijo na- nodelcev s 3-glicidiloksipropiltrimetoksi silanom (GPTMS). Modificirane nanodelce smo dispergirali v izopropanolu in poliuretanskem (PU) premazu. Obdelane nanodelce in njihovi disperziji smo oka- rakterizirali z UV vidno spektroskopijo, rentgensko difrakcijo, metodo dinamičnega sipanja svetlobe (DLS) in z vrstično elektronsko mikroskopijo (SEM). UV-vidni absorpcijski spektri so za nanodelce poka- zali široko območje absorpcije UV svetlobe. Rezul- tati raziskav s SEM so pokazali, da je bila modifikaci- ja nanodelcev z GPTMS učinkovita pri zmanjševanju aglomeracije in je omogočila homogeno porazde- litev nano kovinskih oksidov v polimerni osnovi. Nato smo proučili učinkovitost nanodelcev CeO 2 nanodelcev za foto-stabilizacijo površin lesa kavču- kovca (Hevea brasiliensis). Pripravili smo disperzijo površinsko funkcionaliziranih nanodelcev v izopro- panolu ali brezbarvni transparentni poliuretanski premaz z nanodelci (koncentracija od (0,5 – 6) %). 69 Les/Wood, Vol. 69, No. 1, June 2020 Srinivasa, K., Pandey, K. K., & Petrič, M.: Photostabilization of rubberwood using cerium oxide nanoparticles. Part 1: Characterization and colour changes Pripravka smo nanesli na les. Površinsko obdelane in neobdelane preskušance smo v komori za umetno pospešeno staranje za 500 ur in 1000 ur izpostavili svetlobi tipa UV A (340 nm). Zaradi izpostavljenosti UV sevanju se je spremenila barva in spremembe le-te smo analizirali v rednih časovnih presledkih. Disperziji nanodelcev sta učinkovito omejili barvne spremembe in foto-degradacijo lesnih polimerov, zlasti pri koncentraciji nanodelcev, višjih od 2 %. 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