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European Journal of Applied Sciences – Vol. 13, No. 1
Publication Date: February 25, 2025
DOI:10.14738/aivp.131.18177.
Sahin, C. K., & Merdan, R. (2025). Outdoor Exposure Effects on Different Formulated Clear Coats of Wood as Sustainable Urban
Design Material. European Journal of Applied Sciences, Vol - 13(1). 110-121.
Services for Science and Education – United Kingdom
Outdoor Exposure Effects on Different Formulated Clear Coats of
Wood as Sustainable Urban Design Material
Candan Kus Sahin
Suleyman Demirel University, Architecture Faculty,
Department of Landscape Architecture Isparta, Turkiye
Rahim Merdan
Isparta University of Aplied Sciences, Keçiborlu Vocational School,
Department of Interior Design, Isparta, Turkiye
ABSTRACT
When wood is exposed to the outdoors, degradation can occur either in the coatings
or the wood surface, or in both materials. However, interaction between exposure
radiation and the wood surface produces complex photooxidation reactions that
degrade the clear film and appear as discoloration. There are very complicated
variations found for the changes in all three-color coordinates in terms of outdoor
exposure impact on modifications of seven different coatings and exposure
directions. However, the similar surface coatings on tangent and radial surfaces
respond differently when exposed to outdoor. The varnish+teak oil and plant
oil+teak oil (ΔEEr: 3.47 vs. ΔEFr: 8.65) coated samples show higher radial surface
discolorations than counterpart tangent surfaces, while tangent surfaces show
higher discoloration than radial surfaces for the other five transparent coated
samples. The most stable surface against outdoor exposure was found with a radial
surface of varnish+enamel treatment of ΔECr: 1.71 (metric), followed by radial and
tangent surface of varnish+teak oil treatments (ΔEEr: 2.78 and ΔEEt: 3.47). It is
notable that all discolorations were perceptible to the human eye (ΔE:>1.0 unit)
when look carefully. There is no trend observed for chroma and hues of samples.
Solely varnish coated (hBt) and plant oil+varnish coated (hGt) samples
show considerably shifts of color for tangent surfaces and solely varnish coated
(hBr) and plant oil+ varnish coated (hFr) samples for radial surfaces (Δh ± >5
degree), while all other samples do not show noticeable differences between radial
and tangent surfaces of coated samples, either weathered or not, and those changes
were not perceptible by the human eye.
Keywords: Transparent coating, CIEL*a*b*, discoloration, weathering.
INTRODUCTION
Because wood is a natural material that has its own characteristic color and appearance, it has
become a popular sustainable urban design element that is expected to have a long service
life [1-3]. However, this specific aesthetic value can quickly be lost if left unprotected from
outdoor exposure. Therefore, to prolong the service life of wood products, enhanced resistance
against natural and artificial fading is necessary. In this regard, numerous bulk and surface
protective methods have been developed to slow down degradation processes [4-6]. Some of
those methods (e.g., thermal, surface, physical, chemical, impregnation, etc.,) have shown good
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Sahin, C. K., & Merdan, R. (2025). Outdoor Exposure Effects on Different Formulated Clear Coats of Wood as Sustainable Urban Design Material.
European Journal of Applied Sciences, Vol - 13(1). 110-121.
URL: http://dx.doi.org/10.14738/aivp.131.18177
potential for increasing abiotic and biological resistance and enhancing the natural
performance of wood [5-7].
Research in the field of open space design represents a source of technological evolution for
urban furniture objects. However, one of the considerations is to obtain resistive materials
against light (UV/Vis), be flexible in design places, and have an aesthetic appearance with
acceptable costs [2, 3]. Thus, wood is one of the best materials for urban furniture
manufacturing. It is noticeable that well-designed elements should be flexible and bring an
interesting practice [1-3]. Due to the variety of age groups accessing open spaces and
interacting with urban furniture, they could be accessible to the aged, the disabled and also
children [2].
One of the simplest and most cost-effective approaches is applying protective transparent
finishes or opaque coatings to wood surfaces. In this way, clear finishes display the natural
surface color and texture of the wood [8-10]. However, specially formulated surface coatings
could enhance performance by reducing the water uptake and increasing dimensional stability,
resistance to weathering, and insert color stability [6, 11, 12]. The formulations of surface
coating agents are primarily considered to be well penetrated and polymerized, resulting in cell
wall bulking, but a major proportion of the agents also remain located in the cell lumens [4, 10].
Moreover, environmental concerns associated with the use of conventional preservatives have
drawn attention to the wood modification approach, which has also altered some material
properties [5, 6, 13, 14]. Recent studies have reported the combined coating of wood with
various types of synthetic and natural agents to enhance the aesthetic quality of the wood
substrate [15, 16]. Šimunková et al. (2018) proposed that UV aging affected high-gloss
polyurethane lacquer and traditional shellac varnish-coated wood samples differently. For
evaluating these variations, they measured the coated wood's surface resistance to cold liquids,
adherence to the wood surface, surface hardness, and gloss properties [17]. It had also been
reported that shellac and polyurethane finishes were similarly effective in protecting gloss
following UV exposure in an accelerated lab test, but there was a marked decrease in the water
resistance of shellac finishes [17]. Silva et al., (2007) evaluated the photodecomposition
process of five tropical wood species and the protection effect of two wood finishing
products under artificial weathering [11]. It was suggested that changes in the natural color of
wood indicative of the photodegradation process, and the protection degree of a transparent
poliuretanic varnish and a semitransparent stain be monitored by spectrocolorimetry. It was
explained that the protective properties of varnish and stain are different for certain wood
species [11].
There have been numerous reports regarding urban furniture properties prepared from wood.
However, the majority of these investigations are related to wood’s drawbacks during outdoor
exposure and protective treatments against those detrimental factors in order to improve the
serviceability of wooden products. In this regard, this study aims to evaluate specially selected
seven different formulated surface transparent (clear) coating agents on two different surfaces
of the same Calabrian pine (Pinus brutia) wood, one of the most common wood species for
urban furniture manufacturing in Turkiye. Therefore, the aging of those transparent coatings
has been investigated by general color measurement assessments.
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European Journal of Applied Sciences (EJAS) Vol. 13, Issue 1, February-2025
MATERIAL AND METHODS
For the conduction of this work, the Calabrian pine (Pinus brutia) wood was used. The small
samples in the dimensions of 50 mm x 50 mm x 10 mm were prepared from a planed flat
(tangent surfae)- and quarter-sawn (radial surface) boards that were purchased from a local
timber company. The prepared samples were climatized at 20 °C and 65% relative humidity
(RH) until equilibrium moisture content was reached. There were five commercially
available, ready-to-use agents supplied from local markets to prepare desired coating
formulations in laboratory conditions. These are: teak oil, oil-modified alkyd-based
varnish, synthetic alkyd-based impregnation agent, wax-based synthetic wood enamel, and
natural plant oil (sesame oil). From those agents, as given above, the following seven coating
formulations were prepared and applied to wood specimens to evaluate their stability against
outdoor exposure. These are: A: Solely teak oil coatings; B: Solely varnish coatings; C: Varnish
and then surface enamel coatings; D: Impregnated and then varnish coatings; E: Varnish and
then teak oil coatings; F: Plant oil and then teak oil coatings; and G: Plant oil and then varnish
coatings. The dipping procedure in one minute were applied to all transparent coating
applications. At the end of the dipping, the samples were placed horizontally on a laboratory
table to dry for at least 72 hours. The both uncoated (controls) and seven different transparent
coated of tangent and radial surfaces of the wood samples were placed horizontally on the south
side of a height(Sobu height)to facilitate free distortion and exposed outdoors for the period May
2024 to September 2024 in Isparta, Turkiye. Concerning discoloration determination, three
measurements from each group were taken to analyze the performance of wood coatings. The
natural weathering aging of the coatings and tangent and radial surfaces of the same wood
species were equally prepared, and their surface discolorations were compared after periods
of weathering. In total, 48 wood samples were outdoor exposed.
The determination of the color coordinates was carried out by using an X-rite SP68
spectrophotometer with a circular measuring area with a diameter of 10 mm. Color evaluation
was done using the CIE L*a*b* C*, h° color system, the color parameters L* (lightness), a*
(redness), and b* (yellowness) as well as the total color changes (ΔΕ*) were determined after
weathering. The total color changes (ΔΕ*) were calculated using the following equation:
2 2 2 1/2 ΔΕ* = [(ΔL*) + (Δa*) + (Δb*) ]
(1)
Where ΔL*, Δa* and Δb* are the changes of the colour coordinates L*, a* and b*.
However, L*C*h° values are obtained by converting L*a*b* values. The L*C*h° color space of
lightness (L*), color saturation (C*) and hue angle (h°) were measured.
RESULTS AND DISCUSSIONS
Wooden elements are usually preferred for architectural purposes due to their naturalness and
aesthetic properties. However, natural or modified wood can be degraded, especially those left
outdoors for a long time, such as that shown in Figure 1 (a-i). The photos in Figure 1 clearly
show the degradation, discoloration or considerable changes from their original appearances
of some wooden urban elements. When those elements were analyzed visually, the outer layers
appeared to be destroyed, and a visually unpleasant form was established. As a result of those
changes, the wooden elements began to exhibit extensive graying on their surfaces after