Depth profile of UV-induced wood surface degradation
V Jirous-Rajkovic,1 H Turkulin 1 and E R Miller 2
1
Faculty of Forestry, University of Zagreb, Svetosimunska 25, 10 000 Zagreb, Croatia
2
Miller Associates Ltd, 22 Hamilton Close, Bricket Wood, St Albans, AL2 3NA, UK
Tel:
*+385 1 235 2485
Fax:
+*385 1 235 2531
Email:jirous@sumfak.hr
Summaries
The depth profile of weathering effects on wood was studied after the laboratory ultraviolet
exposure of fir-wood. The ‘thin strip’ method (ie the measurement of changes in tensile
properties of microtomed wood sections) was modified in terms of exposure of batches of
strips packed in bundles and exposed in a QUV apparatus. The bundles of strips were exposed
untreated, covered with surface strips that had been impregnated with chromium trioxide or
Fe(III) nitrate, or protected with free films of clear stain with photostabilisers and without
photostabilisers.
It has been shown that ultraviolet (UV) light penetrates the wood surface to a depth greater
than 70m, resulting in changes in the tensile strength of the strips to a depth of 70 to 140m.
Strength changes in lower levels (down to 280m from the surface) could not be clearly
attributed to a predominant effect of UV light, and were probably controlled by hydrolytic
processes. 100 hours of exposure enabled the effectiveness of several treatments to be
established. The clear stain, enriched with a UV absorber and a HALS compound type, was
shown to offer the best protection. Impregnation with aqueous solutions of chromium (VI)
trioxide and Fe(III) nitrate proved ineffective in photostabilising the wood surface.
Keywords: clear wood stain, photostabilisers, thin strip method, UV exposure, wood surface
degradation
Introduction
A long and functional service life of timber (wood building) components that have been
designed for exterior applications may be ensured only by means of adequate protection
against weathering agencies, particularly sunlight and water. Different finishes give varying
degrees of protection from weather. Generally, the greater the pigment concentration (that is,
the greater the opacity), the greater is the protective role of the coating.1 Clear coating systems
have become popular due to their ability to enhance the aesthetic properties of the wood.
However, unlike opaque paints, they are unable to provide protection from the ultraviolet
1
(UV) component of sunlight that penetrates through the coating to the wood surface, resulting
in photodegradation and eventually delamination of the coating.2
Besides the UV portion of the solar spectrum, its violet/blue region significantly contributes
to the degradation of wood.3 This means that two competitive demands are made on the
optical characteristics of clear coatings because these finishing materials exhibit high levels of
transmittance in the visible part of the spectrum in order to sustain the natural appearance of
wood. This is the main reason for certain reservations about the efficacy of the transparent
finishes in the outdoor applications, unless a greater degree of photochemical stability is
imparted to the wood substrate.
During the last two decades numerous research studies have been focused on the
improvement of the photostability of wood. There is some evidence that treatment of wood
with chromium salts and iron salts are effective in protecting wood against weathering.4–8
Unfortunately, the majority of these treatments alter the natural colour of wood. Penetrating
treatments based on poly(ethylene glycol) and other organic compounds have been found to
improve the colour stability of irradiated wood.9
Organic UV absorbers of the benzophenone type have also been shown to have a
photostabilising effect on wood, especially when they are chemically bound to the wood
substrate.10 The problem with UV absorbers is that their efficacy diminishes with time.
Moreover, the level of their protection depends on the concentration in the finished
formulation and on the film thickness, thus rendering them unsuitable for very thin clear
coatings.
Considerable effort has been given to extending the useful life of clear coatings on wood by
the addition of organic UV absorbers and HALS compounds (hindered amine light
stabilisers).11–14 Some success has been recorded.
The aim of this work was to investigate the effectiveness of chemical treatments and clear
wood coatings in preventing wood deterioration during weathering. This was assessed by
monitoring the changes in the loss of zero-span tensile strength of treated thin wood strips.
Changes in the tensile strength of thin wood strips have been used previously to assess the
quantitative physical deterioration of untreated wood during artificial weathering.15–22
The principle of the method is based on the fact that chemical photodegradation (initially
delignification, and eventually cellulose degradation) is rapidly reflected in changes of
longitudinal tensile properties of wood. These changes may be precisely recorded if 50 to
80µm thin microtomed strips, representing the affected wood surface layer, are exposed to
weathering and withdrawn at intervals for tensile testing. The rates of strength loss with time
2
of exposure may be calculated to yield various parameters for the interpretation of
photodegradation phenomena.23
This technique has been used by Evans and Schmalzl7 to assess the behaviour of treated wood
during weathering, and by Kiguchi et al24 to assess whether grafting of UV absorbers to wood
is effective in increasing the photostability of wood.
The work presented here utilises for the first time multiple-strip exposure. This is an attempt
to use a set of strips, packed and exposed in a bundle, for recording a depth profile of surface
degradation induced by weathering.
Experimental
Materials
Fir wood (Abies alba Mill.) was used in this work. The average density at 12% MC was
0.394g/cm3. The average growth ring width was 4.02mm with a latewood proportion of
25.8%. Wood blocks were microtomed in their tangential planes to give strips of wholly
earlywood, known to be most sensitive to ultraviolet degradation. Turkulin22 has shown that
earlywood possesses a lower initial strength than latewood, and also degrades at a faster rate.
Using earlywood strips enabled the duration of the trials to be significantly shortened due to
the faster degradation rates.
Preparation of thin strips
Material was selected to be straight-grained and free of all visible defects, and was converted
into blocks 120mm (longitudinal) x 10mm (tangential) x 35mm (radial) in size. The blocks
were smooth-planed. Tangential strips of earlywood approximately 70m thickness were cut
from the tangential face of each block using a sliding microtome, after the wood had been
vacuum-impregnated with distilled water. The clearance angle of the knife was 16 and the
draw angle was 22. A mixture of 10% ethanol in distilled water was used to lubricate the
surface of the block during cutting.
The choice of position within a growth ring from which the characteristic earlywood strips
are to be taken is very important in the preparation of tangential strips.22,25 In order to
minimise variations in density in the earlywood strips, samples from the earlywood/latewood
transition zone and from the zones at the borders of the ring were rejected. The earlywood
strips were taken from the zone with a radial position of approximately 10 to 40% in the ring
width. Strips were allowed to dry naturally overnight, stretched on polyethylene sheets, and
were then checked for uniformity of thickness using a Mercer electronic thickness gauge
which gave an accuracy  0.1m. The strips were pulled under the spherical gauge tip so that
3
the thickness was measured along the entire strip length. Any strip yielding a value outside
the range of 68 to 74m was rejected. The values measured on five points of every tenth strip
were recorded, and the average thickness of 70.44m and standard deviation  =1.53 were
calculated.
Tensile strength measurements
Tensile tests were carried out using a Pulmac short-span tensile testing unit (Pulmac
Instruments International, USA)In zero-span tests, the jaws that clamp the specimen are
initially set in contact, and the test is essentially a measurement of the tensile strength of the
cellulose fibrils. This testing unit operates pneumatically using compressed air to provide the
load and the clamping pressure. The failure load in Newtons is calculated from the pressure
required to produce failure and from the known machine constant. In order to establish the
variability of testing results, a group of 20 strips was randomly selected from the experimental
material and tested in tension. This was designed to give the initial breaking load value. The
usual coefficient of variance obtained on radial strips ranged between 8 and 14%.19 However,
the variability of initial breaking loads of tangential, pure earlywood strips was much greater,
regardless of the narrow thickness range of specimens. In order to avoid the effect of such
variations on subsequent results, it proved necessary to measure the strength of end-portions
of individual earlywood strips. All the strips were then grouped according to these initial
breaking load values. The additional time that is consumed in the selection of the strips is
obviously a shortcoming of the method. However, the reliability of the results increased in
such a way that later proved advantageous.
The conversion of ultimate breaking load to breaking stress is difficult for several reasons.
Firstly, the load-bearing area of the cell walls of various thicknesses and the proportion of
half-split cells on strip surfaces may differ significantly between strips of the same
macroscopic cross-sectional dimensions. Secondly, the loss of wood substance during
weathering causes thinning of the cell walls23 and irregular contractions in the strip crosssection. The authors, therefore, decided to express the tensile properties of strips in terms of
ultimate break load and its proportional retention during exposure (%).
Materials and suppliers
Chemical treatments
Aqueous solutions of chromium trioxide (CrO3) and Fe(III) nitrate (Fe (NO3) 3 x 9H2O were
prepared to give a solution with 2.5% metal ion concentration. A solution of chromium
trioxide was prepared by dissolving 26.3g of CrO3 in 500cm3 of distilled water. A solution of
Fe(III) nitrate was prepared by dissolving 94g of Fe (NO3) 3 x 9H2O in 500cm3 of distilled
4
water. These agents were applied to the specimens by immersion of thin strips into their
respective solutions for ten seconds. A brush was used to remove excess surface liquid.
Chemical uptake was determined by weighing each batch of five strips before treatment and
after treatment. The treated strips were placed in a conditioning room at 20 1 and 60  5%
relative humidity for seven days. Subsequently, the strength tests were carried out using the
Pulmac tester.
Preparation of free films of coatings
Two variants of a medium-build water-borne exterior wood transparent stain (Ecol DS, by
Color Medvode, Slovenia) were used in the investigation. The binder of the stain was a
modified acrylic co-polymer emulsion with a 45% solid content. The binder was combined
with the polyurethane dispersion in order to improve the surface resistance and to reduce the
effect of blocking. The coating was essentially a high-quality commercial product for exterior
wood.
The first type was a stain without photostabiliser (mark L), and the second type (mark F2L)
was the same stain enriched with 1% of the Tinuvin 1130 UV absorber and 1% of the HALS
Tinuvin 292 (weight proportions in respect to the final formulation). The photo-protective
chemicals are commercially-available products from CIBA Specialities Ltd.
The stains were spread over a firm, flat, smooth polyethylene plate and were left to dry in
room conditions for 24 hours. Dry films of uniform thickness of approximately 100µm were
peeled off the plates and stored in the dark before further handling.
Composing the bundles of thin strips
The strips were laid one over another to form packs of three or four strips (total pack
thickness amounted to 200to 280µm). Each strip in the bundle was labelled and its initial
strength was recorded.
Strips were exposed in metal frames for a standard exposure in a QUV weathering machine,
tightly packed between backing plates and front covers. Strips were backed with PE sheets
laid over aluminium plates. Front covers were made of opaque, 0.6mm-thick thermostable
PVC plastic sheets with rectangular openings at regular distances along the length of each
pack of strips (see Figure 1). This procedure was intended to enable sampling of those
portions of each strip bundle that were exposed to light, heat and condensation, and of those
portions that were exposed under the same conditions, but were masked from the access of
UV light.
Two sets of three strip packs were assembled for each treatment, one set for 100 hours of
artificial weathering and another set for 500 hours of exposure (see Table 1).
5
Artificial weathering
Exposure of testing materials was performed in a QUV artificial weathering tester with Solar
Eye Irradiance Control (The Q-Panel Company, Clevelend Ohio, 44145 U.S.A.). Eight
fluorescent lamps served as the source of ultraviolet radiation with little output in the visible
region. In these investigations, the lamps used were Type 340 A, having a spectral output
between 300 and 400nm with a peak intensity at 340nm. The Solar Eye system of continuous
irradiance controll of the QUV cabinet maintained the recommended irradiation dose of 0.77
W/m2 /nm at black panel temperature of 60 °C. The 24 hours of exposure consisted of two
periods of eight-hour intervals of UV radiation, followed by four hours of condensation.
During UV radiation, the conditions within the chamber were 30 + 5% relative humidity and
60 +2°C.
During the condensation cycle the lamps go off. The temperature drops to 50°C and the
relative humidity increases to >90%. This cycle causes intensive condensation on the surfaces
of specimens. Duration of exposure, as mentioned later in this paper, encompasses the
duration of complete cycles.
Figure 1: Assembly of the packs of thin wood strips for exposure. Bundles of strips were
tightly packed between backing plates and covered with small frames for localised irradiation
of specimens.
Cutting of strips (or preparing the test specimens)
After weathering, the bundles of thin strips were removed from the frames. Twelve test
specimens were cut from irradiated parts of the strip and three test specimens were cut from
masked parts of the strip (see Figure 2).
Figure 2: Scheme of the sampling of zero-span tensile specimen from 120mm-long thin wood
strips.
Results and Discussion
Figure 3 presents the breaking load values for thin wood strips after 100 hours and 500 hours
of artificial UV exposure respectively. The depth profile of deterioration can be seen from the
strength values for particular positions of strips in the pack (strip no 1 at the surface, no 4 at
the bottom). Each point in a diagram represents an average of 36 readings for exposed
portions of the strips and of nine readings for masked (covered) portions.
100 hours of exposure did not show clear evidence that light had affected the strength
properties of the fourth strip in each pack. However, after 500 hours of exposure, all of the
specimens exhibited a drop in initial strength of the lowermost strips.
6
Such units could indicate that in moist conditions the UV light is able to initiate reactions at
greater depths than 200µm under the wood surface. On the other hand, it is unlikely that the
fourth strip may have been influenced by light. This finding is supported by other researchers
who have recorded a depth of light penetration below 200m.26,27 The strength reductions of
covered portions of these lowest strips (diagrams in the right column) were nearly identical to
those of samples in the exposed portions (left column). Because the colour of the masked
portions was not significantly altered, it seems more probable that the strength reduction in
the fourth strips was caused by hydrolytic reactions at elevated temperatures. This is in
accordance with the findings of Banks and Evans16 who measured a 20% tensile strength loss
in pine strips and lime strips after five days of exposure in the dark, immersed in water at 50
to 65C. These hydrolytic processes acted more intensively on wood that was impregnated
with Fe(III) nitrate than in other cases (see Figure 3c).
The third strips in the packs also failed to show noticeable differences between masked
regions and directly UV-exposed portions. Reductions in tensile strength were more obvious
in the third layer (ie at a depth of 140 to 200µm) than in the fourth layer. However, these
changes were not consistent for all treatments and cannot be attributed to the dominant action
of light.
Only the second layer of strips (70 to 140m from the surface of wood) exhibited marked
differences in strength of masked and exposed portions. This shows that UV light directly
influences the photochemical reactions at depths that are greater than 70µm, previously
believed to be the limit of UV light penetration into the wood surface.28
The first exposed strips markedly lost their strength after 100 hours, whether in direct
exposure or behind a film of transparent stain (see Figure 3d).
The actual depth of UV light penetration was not measured directly; it is estimated that direct
UV penetration amounts to 70 μm26. However, some other results indicate that the depth of
light-induced photooxidative reactions stretches deeper than 200 μm. The interest of this
research was the record of the depth of light-induced changes in wood surface. The character
of light spectrum at certain penetration depths, as well as the intensities of irradiation and
changes in various wood species, will be the object of further investigations.
The absence of changes (see Figure 3e) shows that the incorporation of a UV absorber (UVA)
and HALS into the stain formulation significantly blocked reactions with light, even after 500
hours.
7
The effect of a chemical treatment on the tensile strength of thin strips initially and during UV
exposure is presented in Table 2.
Both treatments reduced the initial strength of strips, but to a significantly lesser extent than
has previously been reported.7
The strength of treated strips during exposure changed at very similar rates to those of the
untreated strips. This would suggest that both chromium salt and Fe(III) ion treatments are not
distinctively effective in their light protection of wood pulp cellulose. Their efficiency in
lignin protection in early stages of photodegradation could be eventually checked by parallel
zero and 10mm-span tensile testing.20,21
Brent et al.29 recorded unexpected changes in carbonyl bands of FTIR spectra at the beginning
of UV exposures, and postulated that the initial degradation may not be dominated by a single
process, but that competing mechanisms are involved. Turkulin and Sell30 also showed that
the increase in mechanical integrity of surface at the beginning of exposures may explain the
initial increase in stability. Further investigations should focus on the character of light
spectrum at certain depths from the surface and consequent chemical changes, particularly in
initial weathering stages.
Figure 3: Breaking load values for strips exposed in packs during 100 and 500 hours of UV
radiation. Strength changes for directly exposed portions of strips (left diagrams) and masked
portions (right) are presented for each treatment. 1 to 4 are positions of strips in a pack,
number 1 being a surface strip.
Conclusions
The application of the thin strip technique to packs of 70m-thick microtomed wood strips
has enabled the depth profile of UV-induced changes during artificial weathering to be
analysed. Strips wholly from earlywood reacted readily to light exposure and showed very
sensitively the effects of photodegradation. Such samples require additional care on the part
of the operator during testing.
It has been shown that UV light penetrated wood surfaces deeper than 70µm, causing
distinctive changes in the tensile strength of strips at the depth of 70 to 140m. Strength
changes in the lower levels (down to 280m from the surface) could not be clearly attributed
to the predominant effect of UV light, and were probably controlled by hydrolytic processes.
Impregnation with weak aqueous solutions of chromium trioxide and Fe(III) nitrate proved
ineffective in photostabilising the wood surface, in much the same way as the film of clear
stain offered no protection from UV light. The addition of UV absorber and HALS compound
8
of Tinuvin type to a clear stain film effectively protected wood from photodegradation during
500 hours of UV radiation.
References
1.
Feist W C, Role of pigment concentration in the weathering of semi-transparent stains’,
Forest Prod J, 38, (2), 41–44, 1988
2.
MacLeod I T, A D Scully, K P Ghiggino, P J A Ritchie, O M Paravagna and B Leary,
‘Photodegradation at the wood-clearcoat interface’, Wood Sci Technol, 29, (3), 183–9, 1995
3.
Derbyshire H and E R Miller,The photodegradation of wood during irradiation. Part I:
Effects on structural integrity of thin wood strips’, Holz als Roh-und Werkstoff, 39, 341–50,
1981
4.
Feist W C, E A Mraz and J M Black, ‘Durability of exterior wood stains’, Forest Prod J
27, (1), 13–16, 1977
5.
Feist W C, ‘Protection of wood surfaces with chromium trioxide’, Research Paper FPL
339, US Department of Agriculture, Forest Service, Forest Products Laboratory, Madison,
WII, 1979
6.
Hon D N-S and S T Chang, ‘Photoprotection of wood surfaces by wood-ion complexes’,
Wood and Fiber Science, 17, (1), 92–100, 1985
7.
Evans P D and K J Schmalzl, ‘A quantitative weathering study of wood surfaces
modified by chromium VI and iron III compounds’, Holzforschung, 43, (5), 289–92, 1989
8.
Feist W C and R S Williams, ‘Weathering durability of chromium-treated southern pine’,
Forest Prod J, 41, (1), 8–14, 1991
9.
Hon D N –S, S T Chang and W C Feist, ‘Protection of wood surfaces against
photodegradation’, J Appl Pol Sci, 30, 1429–48, 1985
10. Williams R S, ‘Effect of grafted UV stabilisers on wood surface erosion and clear coating
performance’, J Appl Pol Sci, 28, 2093–103, 1983
11. Kastien H, ‘Einfluß von UV-Absorbern auf die Wetterbeständigkeit farbloser
Acryldispersionen’, Farbe & Lack, 95, (1), 16–19, 1989
12. Böhnke H and E Hess, ‘Lichtschutzmittel in Lacken: Möglichkeiten und Grenzen’,
Farbe & Lack, 95, (10), 715–9 , October 1989
13. Heinskill J, ‘Möglichkeiten der UV-Stabilisierung farbloser Holz-Außenanstriche’,
Farbe & Lack, 100, (12), 1011–5, 1994
14. Rogez D and M Koehler, ‘Challenges and solutions for the 21st century’, Paper 24,
Second Woodcoatings Congress, The Paint Research Association, 8 Waldegrave Road,
Teddington Middlesex, TW11 8LD, UK, 2000
9
15. Raczkowski J, ‘Seasonal effects on the atmospheric corrosion of spruce micro-sections’,
Holz Roh-Werkst, 38, 231–4, 1980
16. Banks W B and P D Evans, ‘The degradation of wood surfaces by water’, The
international research group on wood preservation, Document No IRG/WP/3289, IRG
Secretariat, Sweden, 1984
17. Evans P D and W B Banks, ‘Degradation of wood surfaces by water. Changes in
mechanical properties of the thin wood strips’, Holz Roh-Werkstoff, 46, (11), 427–35, 1988
18. Evans P D and W B Banks, ‘Degradation of wood surfaces by water. Weight losses and
changes in ultrastructural and chemical composition’, Holz Roh-Werkstoff, 48, (3), 159–63,
1990
19. Derbyshire H, E R Miller, J Sell and H Turkulin, ‘Assessment of wood photodegradation
by microtensile testing’, Paper presented at the XX IUFRO World Congress, Tampere,
Finland, 6th to 12th August 1995
20. Derbyshire H E R Miller and H Turkulin, ‘Investigations into the photodegradation of
wood using microtensile testing. Part 1: The application of microtensile testing to
measurement of photodegradation rates’, Holz Roh Werkstoff, 53, (5), 339–45, 1995
21. Derbyshire H, E R Miller and H Turkulin, ‘Investigations into the photodegradation of
wood using microtensile testing. Part 2: An investigation of the changes in tensile strength of
different softwood species during natural weathering’, Holz Roh Werkstoff, 54, (1), 1–6, 1996
22. Turkulin H, ‘Photodegradation of exterior timber building components’, Dissertation,
Faculty of Forestry, Zagreb University, Zagreb, Croatia, 1996
23. Turkulin H and J Sell,’ Investigations into the photodegradation of wood using
microtensile testing, Part 4:Tensile properties and fractography of weathered wood’, Holz
Roh-Werkst, 60, 96–105, 2002
24. Kiguchi M, P Evans, J Eksted and S Williams, Challenges and solutions for the 21st
century’, Paper 23, Second Woodcoatings Congress, The Paint Research Association, 8
Waldegrave Road, Teddington Middlesex, TW11 8LD, UK, 2000
25. Ifju G, ‘Tensile strength behaviour as a function of cellulose in wood’, For Prod J, 21,
(8), 366–72, 1964
26. Hon D N-S and G Ifju, ‘Measuring penetration of light into wood by detection of
photoinduced free radicals’, Wood Science, 11, (2), 1978
27. Horn B A, J Qiu, N L Owen and W C Feist, ‘FT-IR studies of weathering effects in
western red cedar and southern pine’, Chemical Modification of Lignocellulosics, FRI Bull,
176, 7th to 8th November, Rotorua, New Zealand, 1992
10
28. Hon D N-S and S T Chang, ‘Surface degradation of wood by ultraviolet light’, Journal of
Polymer Science: Polymer Chemistry Edition, 22, 2227–41, 1984
29. Horn B A, Qiu J, Owen N.L., Feist W C, “FT-IR studies of weathering effects in western
red cedar and southern pine. In: Chemical modification of lignocellulosics. FRI Bull 176
1992, Nov. 7-8, Rotorua, New Zealand. Rotorua, New Zealand: New Zealand Forest Research
Institute: 67-76, 1992
30. Turkulin H, Sell J, Investigations into the photodegradation of wood using microtensile
testing. Part 4. Tensile properties and fractography of weathered wood. Holz Roh- Werkstoff 60
(2), 96 – 105, 2002
11