AGNIESZKA JANKOWSKA, PAWE KOZAKIEWICZ, MAGDALENA SZCZ SNA Department of Wood Science and Wood Protection, Warsaw University of Live Science SGGW

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1 Annals of Warsaw University of Live Science SGGW Forestry and Wood Technology No 71, 010: (Ann. WULS-SGGW, For. And Wood Technol., 71, 010) Discoloration of bilinga (Nauclea diderrichii (De Wild. & Th.Dur.) Merr.) and iroko (Milicia excelsa (Welw.) C.C.Berg.) wood, caused by coatings and light aging. AGNIESZKA JANKOWSKA, PAWE KOZAKIEWICZ, MAGDALENA SZCZSNA Department of Wood Science and Wood Protection, Warsaw University of Live Science SGGW Abstract: Discoloration of bilinga (Nauclea diderrichii (De Wild. & Th.Dur.) Merr.) and iroko (Milicia excelsa (Welw.) C.C.Berg.) wood, caused by coatings and light aging. Last years, interest of exotic wood species increases, caused by its specific properties. Aesthetics of the material is most important, especially color. Unfortunately, wood is prone to serious discoloration because of coating process or sunlight exposure. Following work describes influence of these factors on color stability of African bilinga and iroko wood, used mainly for flooring. Keywords: exotic wood, Nauclea diderrichii (De Wild. & Th.Dur.) Merr., Milicia excelsa (Welw.) C.C.Berg., wood color, light aging, wood coatings INTRODUCTION Last years increasing popularity of exotic wood species is visible (over a dozen fold - Kozakiewicz 006). Floors made of exotic wood species change its color over external factors (Kozakiewicz 005) especially UV. This creates serious problems especially in case where floor is partially covered or blinded. Color is a reaction of sight on nm light inciding on eye retina (visible light). Wood color can be affected with suitable coatings, like lacquers or oils, which can totally cover natural wood color, but create protective film against external threats. Transparent lacquering increases natural color intensiveness, but emphasizes all pattern and color defects. Despite exotic wood color descriptions, professional literature lacks unbiased color change data, not organoleptic but made with colorimeter. Aim of the following work was to determine coated bilinga and iroko wood discoloration under light aging process. MATERIALS AND METHODS Two popular in Poland African wood species were selected for the work (nomenclature in accordance to PN-EN 13556:005) standard): bilinga (Nauclea diderrichii (De Wild. & Th.Dur.) Merr.) and iroko (Milicia excelsa (Welw.) C.C.Berg.), often used for flooring. Detailed description and characteristics are provided by Kozakiewicz and Szkarat (004, 005). Before tests samples were planed and sanded. Radial section of more uniform pattern was selected, in opposite to coarser tangential section. Wood moisture content ranged from 8% up to 1 %. Boards were cut into test samples of 70x70x10 mm dimensions. Samples were sorted into groups, in every group surfaces were finished with various coatings produced by various manufacturers: group A one-component polyurethane lacquer group B two-component water-based polyurethane lacquer group C polyurethane lacquer group D nitrocellulose lacquer group E - wax group F shellac lacquer. Materials were used in accordance to producers suggestions, Coated samples were exposed indoors to direct sunlight for one year. 70

2 Discoloration analysis was based on international CIE L*a*b* model. Beginning from the samples preparation samples were isolated from sunlight in aim to preserve natural color. Color tests were made before coating, after coating and after sunlight exposition. X-Rite SP-60 spherical spectrophotometer was used for the test. Color indexes were calculated basing on the component values: total color difference: 1/ * [( L*) ( a*) ( b*) ], where E ab L * - lightness difference a * - red color difference (a>0) b * - yellow color difference (b>0) saturation from the formula: 1/ C * ab ( a * b * ) hue from the formula: H* arctg( b * / a*) From the obtained data (5 measurements on a single sample) averages were calculated. In consequence of the lacking standard on wood discoloration presented work bases on PN- ISO 774-1:003, PN-ISO 774-:003, PN-ISO 774-3:003 standards, regarding lacquer discoloration. RESULTS Obtained results for Nauclea diderrichii (De Wild. & Th. Dur.) Merr.) and Milicia excelsa (Welw.) C.C.Berg., wood are presented in table 1. Basing on the detailed analysis of the gathered data one may conclude that coated exotic wood changes its color over time, which is caused by exposure to sunlight. Lightness values drop in comparison to control samples, which means that wood becomes darker. Greatest luminance change of bilinga wood showed up in group C (polyurethane coated), lowest change was in the UV group, uncoated. In case of iroko wood greatest lightness change was in D group (nitrocellulose lacquered), smallest similarly like with bilinga wood in uncoated UV group. Analysis of the remaining two color parameters shows that saturation and hue of exotic wood change under long sunlight exposition. Saturation of bilinga wood for A, B, E and F coatings groups decreased, in case of iroko wood saturation increased for all groups. Tests show that hue values dropped for both species, basin on this parameter one may determine that bilinga wood changes hue from yellowish-orange down to orange, greatest change is shown by C group (49,40). In case of iroko wood, samples from yellow turn into darker yellow hue, greatest change is shown by B group (61,35). 71

3 Color parameters Wood species Lightness (L*) Saturation (C*) Hue (H*) L* min L* r L* max C* min C* r C* max H* min H* r H* max BILINGA (Nauclea diderrichii (De Wild. & Th. Dur.) Merr.) IROKO (Milicia excelsa (Welw.) C.C.Berg.) uncoated CS 5,3 54,09 57,35 33,74 35,0 36,75 60,8 61,95 63,56 wood UV 53,06 53,37 53,80 33,80 34, 34,93 61,64 6,15 6,40 A 48,41 49,33 49,91 35,07 36,33 36,87 59,45 59,84 60,40 B 46,96 48,9 49,18 36,35 37,09 38,16 58,36 59,84 60,91 coated wood after UV C 4,45 43,3 43,87 9,67 31,57 3,64 49,01 49,40 49,80 exposure D 44,66 45,19 46,10 33,37 34,80 35,71 55,55 55,69 55,84 E 47,34 49,8 49,99 35,15 36,78 37,3 57,33 59,03 59,70 F 43,77 45,11 47,40 35,3 36,61 38,81 54,0 55,07 57,45 uncoated CS 55,35 58,68 63,10 6,41 7,87 30,51 68,66 73,14 77,4 wood UV 51,44 5,76 53,8 6,73 7,1 7,69 65,75 66,70 67,4 A 50,01 5,71 54,37 31,3 3,06 3,85 63,14 65,19 66,44 B 46,73 48,55 50,84 31,4 3,50 33,08 59,80 61,35 63,03 coated wood after UV C 47,90 48,84 50,07 9,48 30,6 3,1 6,88 63,59 64,1 exposure D 46,9 47,43 48,84 9,98 31,4 33,30 63,08 63,84 64,4 E 49,57 50,14 50,7 7,91 8,80 9, 64,04 64,47 64,99 F 47,06 48,73 50,74 3,83 35,83 38,37 63,01 63,86 65,31 CS control samples UV samples after light exposure A one component polyurethane lacquer B two-component water-based lacquer C polyurethane lacquer D nitrocellulose lacquer E wax F shellac lacquer For comparison of different coatings, total color difference criterion (fig. 1-) and color stabilization was set. Color stability degrees were set on the basis of five step scale (Mielicki 1997) shown in table. Table. Color stability Color difference 0±0, 0,8±0, 1,7±0,3,5±0,35 3,4±0,4 4,8±0,5 6,8±0,6 9,6±0,7 13,6±1,0 E Color stability Basing on the presented results one may conclude that highest color stability was shown by A group, and lowest by C group, in both wood species tested. Change direction in both bilinga and iroko wood was the same. 7

4 Total color difference ,71 3,38 5,34 1,81 8,94 6,6 11,17 0 Uncoated wood A B C D E F Fig. 1. Total color difference of Nauclea diderrichii (De Wild. & Th. Dur.) Merr.) wood, caused by coating and light aging ,46 14,39 Total color difference ,45 Uncoated wood 10,95 11,14 9,05 6,39 A B C D E F Fig. Total color difference of Milicia excelsa (Welw.) C.C.Berg. wood, caused by coating and light aging. CONCLUSION Tests performed on two African wood species, bilinga and iroko, based on color change determination under coating and light aging influence, allow to conclude as follows: 1. Wood coatings (lacquer, wax, shellac) and sunlight exposure cause wood discoloration. Color changes have same properties in both wood species tested, degree of the change is dependent on coating type. Bilinga wood from yellowish-orange turned closer to orange, Iroko from yellowish turned into darker shade of yellow.. Coated wood shows more discoloration than uncoated one. 3. After coating with lacquer, wax and shellac and light aging wood turned darker (lightness decreased) 73

5 4. Lacquering, waxing and shellac lacquering of wood does not protect it against discoloration, but causes color evening (wood color is more even on the whole surface). REFERENCES 1. KOZAKIEWICZ P., SZKARAT D., 004: Iroko [Milicia excelsa (Welw.) C.C.Berg] drewno egzotyczne z Afryki. Przemys Drzewny nr 7-8, s: KOZAKIEWICZ P., SZKARAT D., 005: Bilinga [Nauclea diderrichii De Wild.] drewno egzotyczne z Afryki. Przemys Drzewny nr 7-8, s: KOZAKIEWICZ P., 005: Drewno w budownictwie podogi. Przemys Drzewny nr 6, s KOZAKIEWICZ P., 006: Waciwoci i zastosowania drewna egzotycznego w Polsce. Uszlachetnianie powierzchni drewna, cz. I (Dodatek specjalny do czasopisma Lakiernictwo), s MIELICKI J., 1997: Zarys wiadomoci o barwie. Wyd. Fundacja Rozwoju Polskiej Kolorystyki. ód. 6. PN-EN 13556:005 Drewno okrge i tarcica. Terminologia stosowana w handlu drewnem w Europie. 7. PN-ISO 774-1:003 Farby i lakiery Kolorymetria Cz 1: Podstawy. 8. PN-ISO 774-:003 Farby i lakiery Kolorymetria Cz : Pomiar barwy. 9. PN-ISO 774-3:003 Farby i lakiery Kolorymetria Cz 3: Obliczanie rónic barwy. Streszczenie: Badanie zmian barwy drewna bilinga (Nauclea diderrichii (De Wild. & Th.Dur.) Merr.) i iroko (Milicia excelsa (Welw.) C.C.Berg.) pod wpywem dziaania lakierów i foto-starzenia. Na przestrzeni ostatnich lat obserwuje si wzrastajce zainteresowanie drewnem egzotycznym, ze wzgldu na jego cenne waciwoci. O rosncej popularnoci decyduj walory estetyczne tego materiau, a przede wszystkim barwa. Niestety, barwa drewna moe ulega drastycznym zmianom pod wpywem lakierowania i nasonecznienia (foto-starzenia). W niniejszej pracy zbadano wpyw tych czynników na barw popularnych w Polsce dwóch rodzajów drewna egzotycznego z Afryki (bilinga i iroko), stosowanego gównie na materia podogowy. Dowiedziono, e surowe drewno istotnie ciemnieje pod wpywem dziaania wiata i lakierowania. Lakierowanie drewna nie zabezpiecza go przed dalszymi zmianami barwy, jednak wyrównuje jego kolorystyk. Corresponding authors: Agnieszka Jankowska Pawe Kozakiewicz Magdalena Szczsna Department of Wood Sciences and Wood Protection, Faculty of Wood Technology, Warsaw University of Life Sciences SGGW, Ul. Nowoursynowska 159, Warsaw, Poland agnieszka_milewska@sggw.pl pawe_kozakiewicz@sggw.pl magdalena_szczesna@sggw.pl

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