Annals of Warsaw University of Life Sciences – SGGW Forestry and Technology № 100, 2017: 153-162 (Ann. WULS-SGGW, Forestry and Wood Technology 100, 2017)

The effect of wood surface finish on its hygroscopic properties

EWA DOBROWOLSKA, HANNA CHMIELEWSKA, AGNIESZKA MIELNIK Department of Wood Sciences and Wood Protection, Faculty of Wood Technology, SGGW

Abstract: The effect of wood surface finish on its hygroscopic properties. This work determines hygroscopic properties, such as humidity and swelling, of wood (Pinus sylvestris) wood (Quercus L.), wenge (Millettia laurentii) and merbau ( bijuga) with surfaces coated with a polyurethane lacquer. The testes wood samples were moistened for 30 days in a climate with a temperature of 40ºC and air relative humidity of 95%. As a result of performed analyses it was concluded that the changes of moisture content increase and tangential and axial swelling of lacquered wood were significantly lower, particularly during the first day of testing, than those of non-lacquered wood samples. At the same time, after a longer time the moisture content and swelling of both types of samples were becoming equal, regardless of the wood species. The tests showed that the applied coats of polyurethane lacquer protect wood against moisture only during a short period of time, i.e. up to several hours.

Keywords: hygroscopic properties of wood, solid wood, lacquered surface.

INTRODUCTION Cabinet making uses solid wood, particle boards and MDF boards enriched with high- quality natural veneers or modified boards [Bagnucka, 2007] with different types of surface coatings. They serve two very important functions, i.e. protective (the surface is protected against abiotic factors) and decorative (brings out the colour and grain of the wood as well as its anatomic structure, it also enhances the smoothness of the surface) [Proszyk, 1995]. One of the most important abiotic factors affecting the characteristics of solid wood is the moisture in the air. During use, wood material constantly adjusts its moisture content to the environment conditions. The time which is required for wood to achieve hygroscopic balance to a high degree depends on its structure, density and species. As a result of intense climatic fluctuations, there is a greater danger of wood tissue breaking and changing its dimensions and shape. The unfavourable impact of climate may be limited by using various protective coatings. They limit moisture circulation in the zones bordering with the timber surface and the surrounding air. Then the time of achieving hygroscopic balance is longer and, as a result, wood moisture content fluctuations and wood dimensions changes are reduced [Kozakiewicz et al. 2006]. At the same time, the amount and type of used coating and the technology of its application, determine the aesthetics and the quality of the finished piece of furniture [Krzoska-Adamczak, 2007]. The purpose of this work was to test the impact of decorative-protective coatings on the hygroscopic properties of solid wood most commonly used in cabinet making. The tests were performed using two most popular native wood species: pine (Pinus sylvestris) and oak (Quercus L.), as well as species from other climate zones, including wenge (Millettia laurentii) and merbau (Intsia bijuga). The surfaces of the solid wood were coated with polyurethane lacquer. Because Polish furniture are exported to countries with different climate conditions, the tests were carried out in a higher temperature (40°C) and in the air relative humidity of 95%.

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TEST MATERIAL 20 samples were made of each of the solid pieces of oak, pine (wide-grained and narrow-grained), wenge and merbau; the samples were 30 mm × 30 mm × 10 mm and they were cut in accordance with the anatomical directions (the last dimension was cut with the grain). The selected test material was characterised by determining its density and the width of annual growth rings (table 1). The sample surfaces were polished twice in commercial conditions, covered with a single coat of polyurethane primer and then with a lacquer which allowed to retain wood structure with partially uncovered lumina.

Table 1. Density and width of annual growth rings of the test wood species – pine, oak, merbau and wenge. Wood species Density [kg/m3] Width of annual growth rings [mm] Oak 590 2.5 V [%] 2.0 1.0 Pine (narrow-grained) 620 2.0 V [%] 0.9 0.8 Pine (wide-grained) 440 4,0 V [%] 1.3 1.2 Wenge 870 invisible V [%] 1.1 Merbau 680 4.0 V [%] 1.9 1.0

Determination of hygroscopic properties was performed in accordance with norm PN- D-04120:1960 Physical and mechanical properties of wood – Determination of hygroscopic properties.

METHODOLOGY The tests on the hygroscopicity of wood with raw surfaces and lacquered surfaces were performed at a temperature of 40ºC and relative air humidity of 95%. During hydration process, until the maximum equilibrium moisture content, the dynamics of moisture content changes and samples dimensions changes were determined. Tests of hygroscopic properties were performed in a climate chamber INSTRON WK 340. Prior to the tests, the samples were dried at a temperature of 40°C to the state of absolute dryness in a low-pressure dryer. The mass and dimensions of the samples in the absolute dry state were assumed as zero point in further analyses. After placing the samples in the climate chamber, the measurements of mass and dimensions changes were performed after 1, 4, 8 and 24 hours and then after 2, 3, 4, 7, 10, 12, 14, 17, 21 and 30 days. Before every measurement, each sample was placed in a plastic zip bag immediately after it was taken out of the climate chamber in order to minimise the sole of moisture during the measurement. After every measurement the absolute moisture content in every sample was determined using the following formula:

mk  mo Wo  100 [%] mo mo – the mass of absolutely dry sample [g] mk – the mass of sample after moistening [g]

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On the basis on the changes of dimensions of the samples, tangential and axial swelling was calculated:

g 2( t ,r )  g 1( t ,r ) G( t ,r )  100 [%] g 1( t ,r ) g1 (t, r) – dimensions of the sample in absolute dry state [mm] g2 (t, r) – dimensions of the sample after testing [mm]

Moreover, during testing all the samples underwent organoleptic inspection which was to reveal any damages on the sample surface. Mainly discolouration and shine changes were considered here. Observations were performed in accordance with norm PN-EN 12720:2000 Furniture – Assessment of the surface resistance to cold liquids.

TEST RESULTS Fluctuations of the parameters of the ambient air cause wood to absorb or to give up moisture, which results in an increase of wood moisture content and its dimensions, mostly in axial and tangential directions. The moisture of wood with non-lacquered and lacquered surfaces depending on the exposure time to climate with a temperature of 40C and relative air moisture of 95% was presented in figures 1-3. An analysis of changes in the moisture content of narrow-grained and wide-grained pine wood with non-lacquered surfaced (pic. 1) shows that the biggest increase takes place within the first hours of the moistening process.

Figure. 1. Changes in the moisture content of narrow-grained and wide-grained pine wood with non- lacquered surfaces (Pine-s, Pine-w) and lacquered (L-Pine-s, L-Pine-w) resulting from moistening in a climate with φ = 95%, t = 40° C.

The moisture contents of wide-grained and narrow-grained pine wood after the first hour of moistening were very comparable and were 10% and 11% respectively. Within the next eight days it increased to 19% and was stabilized after two days at a level of about 23%. Samples of wide-grained and narrow-grained pine wood with lacquered surfaces Po After the first hour, their humidity content increased only to 0.3%, after 8 hours to 1.5% and after one day of moistening it reached about 4%. When analysing the curves which present the course of changes in the humidity content of the oak wood samples (Fig. 2), it was observed that the greatest increase to about 7% occurs after the first hour of moistening. After the next 4 hours the moisture content was

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doubled and it reached 14%. The maximum moisture content of 23% was achieved after 7 days. A slight increase of up to 0.4% of humidity of the oak wood sample was observed within the first hour and after another eight hours the humidity rose to 1.4%. After the first day, humidity content reached almost 9% and the maximum value of 23% which remained stable until the end of the test was reached only after 14 days. On the basis of the obtained test results for this group of samples it was concluded that they were characterised with very similar moisture content growth dynamics. The wide- grained pine wood reached the highest equilibrium moisture content. A considerably lower hygroscopic balance was showed by wide-grained pine wood. This could be caused by a large percentage of late wood which means slower absorption of steam. The moisture content of all the tested native wood species, i. e. wide and narrow-grained pine and oak, equalled the mean equivalent humidity obtained in a climate with φ = 95%, t = 40°C presented in other publications [Krzysik, 1975].

Figure 2. A change of moisture content in oak wood samples with: non-lacquered surfaces Oak and lacquered surfaces L-Oak resulting from moistening in a climate with φ = 95%, t = 40° C.

A comparison of the changes in moisture content in the lacquered samples shows that the tempo of their moisture content increase is different than that of non-lacquered wood. Lacquered samples are characterised with slow moisture content increase, particularly during the first day of moistening. The moisture content of lacquered pine wood, both narrow- grained and wide-grained, after a day of moistening was 4% and it constituted half the moisture content increase in non-lacquered samples in the first hour, similarly in case of lacquered oak wood samples, whose moisture content reached that of non-lacquered samples only after 24 hours of moistening. Picture 3 presents changes in the moisture content of wenge and merbau wood samples. On their basis it was concluded that the most intensive moisture absorption by these two wood species with non-lacquered surfaces occurs, just like for the native wood species, in the first day of moistening. The moisture content of merbau after first hour was 8% and after 4 hours nearly 13%. After that it was systematically rising until stabilising at 19% after 30 days. Moisture absorption by wenge wood samples was less intensive than by merbau wood samples. After the first hour of moistening their moisture content reached about 4% and after 4 hours about 7%. After the next two days their moisture content was 16% and it kept rising systematically by about 1% until the end of the test.

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Comparing the results for merbau and wenge we can see that the highest moisture content was achieved by non-lacquered merbau wood. A lower susceptibility of wenge to moistening is likely to be the result of a higher content on non-structural substances, including wax, which lower wood hydrophility. Maximum moisture content obtained by merbau wood after 14 days was 19% and by wenge 17%.

Figure 3. Changes in moisture content for merbau and wenge wood: with non-lacquered surfaces Merbau, Wenge and lacquered surfaces L-Merbau, L-Wenge, after moistening in a climate with φ = 95%, t = 40° C.

Lacquered surfaces helped to significantly lower the intensity of moisture absorption. In the first day of moistening, the moisture content of merbau wood samples was twice lower, and wenge moisture content was four times lower, than that of the non-lacquered samples. After the first hour or moistening, the moisture content of the lacquered samples of merbau was only 1% and after 4 hours 2%. After the first day it reached 8%. The moisture content of wenge wood samples with lacquered surfaces was 0,3% after the first hour and reached almost 4% after the first day. After 30 days of tests it turned out that the moisture content of lacquered samples was 16% and it was identical to that of the non-lacquered samples.

Figure 4. Change of tangential and axial swelling of narrow-grained pine wood with Non-lacquered and lacquered (L) surfaces, moistened in a climate with φ = 95%, t = 40° C.

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The tested samples of exotic wood species are characterised with much lower moisture content than native wood species. During the first day the moisture content in the native wood samples reached about 22% and in the exotic wood nearly 16%. Later during testing, it also showed a lower increase of moisture content which after 14 and 17 days was 4% for merbau and nearly 7% for wenge.

Figure 5. Change of tangential and axial swelling of wide-grained pine wood with non-lacquered and lacquered (L) surfaces, moistened in a climate with φ = 95%, t = 40° C.

Comparing the intensity of moisture absorption by non-lacquered and lacquered samples it was concluded that lacquering lowered greatly the moistening dynamics, similarly as with the native wood species. Merbau wood with lacquered surfaces had a moisture content of 8% after the first day of moistening, which is close to the value for lacquered oak wood samples. The amount of absorbed moisture by lacquered wenge wood was about 4%, just like in the case of lacquered pine wood. After 30 days, both lacquered merbau and wenge showed a steady increase of moisture and the tests had to be continued in order to determine their equivalent humidity.

Figure 6. Change of tangential and axial swelling of oak wood with non-lacquered and lacquered (L) surfaces, moistened in a climate with φ = 95%, t = 40° C.

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A change of the tangential and axial swelling of narrow-grained and wide-grained pine wood samples with lacquered and non-lacquered surfaces was presented in pictures 4 and 5. The analysis presented in picture 4 shows that tangential swelling of narrow-grained- pine wood increased after the first day of moistening to about 8%.increased after the first day of moistening to about 8%. Within the next day there was only a slight change of about 1%. Axial swelling after 8 hours of testing reached the level of 5% and increased to 6% during further moistening. Covering the surface of the samples with lacquer slowed down moisture absorption and changes in dimensions. Tangential and axial swelling of narrow-grained pine wood samples with lacquered surfaces increased to 8% and 5% respectively after 12 days of moistening. Further moistening caused a slight increase of both values by over 1%. The swelling of wide-grained pine was almost twice lower than the swelling of narrow-grained pine whose density (due to the annual growth rings) was higher by 200 kg/m3 from the wide-grained pine (table 1). After the first day of moistening tangential and axial swelling of wide-grained pine wood reached the maximum value of 6% and 3% respectively. Further moistening only slightly increased these values. While lacquering of the samples greatly slowed down the process of moisture absorption. Their tangential swelling after the first day was about 1% and the axial swelling was over 0,5%. During further moistening, both tangential and axial swellings were increasing gradually and reached the level of swelling of non-lacquered wood samples.

Figure 7. Change of tangential and axial swelling of merbau wood with non-lacquered and lacquered (L) surfaces, moistened in a climate with φ = 95%, t = 40° C.

Changes of swelling of lacquered and non-lacquered oak wood are presented in picture 6. It shows that tangential and axial swelling after the first day of testing was nearly 6%. Tangential swelling of 3% was reached after 8 hours. For both swelling directions 30 days of moistening caused an increase of swelling to 7% and 4% respectively. Lacquering the surface of oak wood samples slowed down the intensity of dimensions changes. Tangential swelling of 6% was obtained after 12 days of moistening. Axial swelling of 3% was obtained only after 7 days. In the final stage of the test both types of samples showed identical levels of tangential and axial swelling. The results of tangential and axial swelling of wenge and merbau wood samples with lacquered and non-lacquered surfaces were presented in pictures 7 and 8.

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The swelling of merbau (pic. 7) which occurred within the first 24 hours was very intense and constituted 6% in the tangential direction and 3.5% in the axial direction. After 10 days of moistening it increased slightly and settled at a level of 7% and 4% respectively. Lacquering of the surface of merbau wood significantly lowered the swelling, which after the first day of moistening was 1% in both directions, tangential and axial. The maximum change of dimensions was obtained after 10 days of moistening; tangential swelling was 6% and axial swelling was 3% and was comparable to non-lacquered wood samples. Further moistening caused only a slight increase of the swelling of lacquered wood samples which levelled with the swelling of non-lacquered wood samples. Picture 8 presents tangential and axial swelling of wenge wood with non-lacquered and lacquered surfaces. In case of non-lacquered samples, the swelling after the first day of moistening was 7% for tangential direction and about 4% for the axial direction. Next, the tangential swelling increased slightly to reach 8% and remained at this level until the end of the test.

Figure 8. Change of tangential and axial swelling of wenge wood with non-lacquered and lacquered (L) surfaces, moistened in a climate with φ = 95%, t = 40° C.

As a result of covering the surface of the samples with lacquer, the dynamics of the changes of wenge wood dimensions were significantly reduced. After 24 hours of moistening, tangential swelling was 1.5% and axial swelling was nearly 1%. During further testing, an increase of tangential swelling occurred which after 10 days was 7% and increased by 0,5% at the end of the test. Axial swelling after 12 days was 4% and did not change despite of further moistening. On the basis of the test results it was concluded that merbau wood was characterised with better dimensional stability in the tangential rather than in axial direction. The swelling of wenge wood, with density almost 200 kg/m3 higher, was almost double of that of merbau wood. Covering the surface of the samples with lacquer significantly reduced the dimensional changeability of both wood species, especially during a short time of their exposure to moisture. An analysis of the obtained values of the swelling of pine wood (narrow and wide- grained), oak and merbau and wenge suggests that the differences in the swelling of lacquered and non-lacquered samples occur most of all in the first period of moistening. By reducing the exposure to moisture, a lacquer coat significantly reduces the increase of moisture content and

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swelling. After a sufficiently long time, the moisture content and swelling of wood samples with lacquered surfaces reach the level of non-lacquered wood. Thus, the used layers of polyurethane lacquer protect wood against the adverse effects of moisture only during a short period of time.

SUMMARY On the basis of the performed tests it was concluded that: 1. The highest dynamics of moisture content increase occurred in non-lacquered wood during the first day of moistening. At the same time, coating the wood samples with a polyurethane lacquer allowed to considerably limit the amount of absorbed moisture. In this time, the non-lacquered samples of wide-grained and narrow-grained pine wood and oak reached the maximum moisture content value of about 21%. The lacquered samples of both types of pine wood reached the moisture content level of 4% and oak wood 9%. The moisture content levels of the non-lacquered samples of wenge and merbau were 15% and 16% respectively and their lacquered versions 4% and 8% respectively. 2. The equivalent humidity was: 23% for narrow and wide-grained pine wood after 2 days and for oak wood after 7 days of moistening. The lacquered versions of these wood species samples reached hygroscopic balance at this level after 21 days of moistening. A hygroscopic balance of 19% was reached by merbau wood after 14 days and the hygroscopic balance of 16% for wenge wood was achieved after 17 days. The equivalent moisture of the lacquered samples was not stabilized within 30 days and required longer testing. 3. The lacquered samples displayed a much lower swelling dynamics compared to the non-lacquered samples. At the same time after a sufficiently long time there were no significant differences in the swelling of non-lacquered and lacquered wood samples. 4. A lacquer coat applied to narrow and wide-grained pine, oak, merbau and wenge did not show any signs of damage during the entire testing period.

LITERATURE 1. BAGNUCKA A. 2007: Płyty fornirowane. Gazeta Przemysłu Drzewnego nr 10, 17 – 18 2. DOBROWOLSKA E., DOMAŃSKI M., OSIPIUK J., STECZOWICZ M. 2008: Wybrane zagadnienia suszenia tarcicy. Wydawnictwo Szkoły Głównej Gospodarstwa Wiejskiego, Warszawa. 23, 3. DZIĘGIELEWSKI S., FABISIAK B. 2007: Wzornictwo a sprzedaż mebli. Przemysł Drzewny nr 4. 30. 4. KOLLMANN F. 1951: Technologie des Holzes und der Holzwerkstoffe. Springer – Verlag, Berlin – Göttingen – Heidelberg. 396 – 399. 5. KOZAKIEWICZ P. 2003: Fizyka drewna w teorii i zadaniach. Wybrane zagadnienia. Wydawnictwo Szkoły Głównej Gospodarstwa Wiejskiego, Warszawa. 23 – 25, 6. KOZAKIEWICZ P., MATEJAK M. 2006: Klimat a drewno zabytkowe. Wydawnictwo Szkoły Głównej Gospodarstwa Wiejskiego, Warszawa. 87, 93, 95, 128, 137, 141 – 142, 144, 149 – 151, 168, 7. KRZOSKA-ADAMCZAK Z. 2007: Powierzchnie płyt drewnopochodnych – metody uszlachetniania i ocena ich jakości. Biuletyn Informacyjny Ośrodka Badawczo- Rozwojowego Przemysłu Płyt Drewnopochodnych, Czarna Woda nr 1-2. 55 – 59. 8. KRZYSIK F. 1975: Nauka o drewnie. Państwowe Wydawnictwo Naukowe, Warszawa. 345, 346, 355, 386, 387, 655.

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9. KRYSTOFIAK T., LIS B. 2007: Wyroby lakierowe PUR i ich zastosowanie w drzewnictwie. Biuletyn Informacyjny Ośrodka Badawczo-Rozwojowego Przemysłu Płyt Drewnopochodnych, Czarna Woda nr 3-4. 141 – 143. 10. PROSZYK S. 1995: Technologia tworzyw drzewnych 2, Wykończanie powierzchni. Wydawnictwo Szkolne i Pedagogiczne, Warszawa. 11-12, 14, 11. ROGOZIŃSKI M. 2006: Jakość mebli w kontekście wymagań EN. Meblarstwo nr 8. 71 – 73 12. PN-D-04120:1960 Fizyczne i mechaniczne własności drewna – Oznaczanie higroskopijności

Streszczenie: Wpływ wykończenia powierzchni drewna na jego właściwości higroskopijne. W pracy określono właściwości higroskopijne, takie jak wilgotność i pęcznienie drewna sosnowego (Pinus sylvestris) i dębowego (Quercus L.), wenge (Millettia laurentii) i merbau (Intsia bijuga) o powierzchniach pokrytych lakierem poliuretanowym. Próbki drewna były nawilżane przez okres 30 dni w klimacie o temperaturze 40ºC i wilgotności względnej powietrza 95%. W wyniku przeprowadzonych analiz stwierdzono, że dynamika wzrostu wilgotności i jednostkowego pęcznienia w kierunku stycznym i promieniowym drewna o uszlachetnionych powierzchniach szczególnie w pierwszej dobie badania były znacząco mniejsze w porównaniu z próbkami nielakierowanymi. Jednocześnie po odpowiednio długim czasie następowało wyrównanie wysokości wilgotności i pęcznienia między próbkami o powierzchniach nieuszlachetnionych i uszlachetnionych bez względu na rodzaj badanego drewna. Badania wykazały, że użyte powłoki z lakieru poliuretanowego chronią drewno głównie w czasie krótkotrwałego do kilku godzin niekorzystnego oddziaływania wilgoci.

Corresponding author:

Ewa Dobrowolska Department of Wood Sciences and Wood Protection, Faculty of Wood Technology, Warsaw University of Life Sciences – SGGW, ul. Nowoursynowska 159, 02-776 Warsaw, Poland e-mail: [email protected]

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