IRG/WP 05-40302

THE INTERNATIONAL RESEARCH GROUP ON WOOD PROTECTION

Section 4 Processes and properties EFFECTS OF ARTIFICIAL UV WEATHERING AND SOFT ROT DECAY

ON HEAT TREATED WOOD

M D Hale, S C Ghosh, M J Spear School of Agricultural & Forest Sciences, University of Wales, Bangor, UK

Paper prepared for the 36th Annual Meeting Bangalore, India 24-28 April 2005

IRG SECRETARIAT SE-100 44 Stockholm

Sweden www.irg-wp.com

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EFFECTS OF ARTIFICIAL UV WEATHERING AND SOFT ROT DECAY

ON HEAT TREATED WOOD

M D Hale, S C Ghosh, M J Spear

School of Agricultural & Forest Sciences, University of Wales, Bangor, LL57 2UW, UK, m.d.hale@bangor.ac.uk

ABSTRACT Oil and inert gas oven heat treated pine wood strips 100 µm thick were mildly heat treated (200°C, 30 mins linseed oil, 120 mins oven). Following treatment, specimens were exposed to UV weathering (300 hours) and decay by the soft rot fungus, Chaetomium globosum. The effects of each treatment were assessed by zero span tensile testing, microscopy and by FTIR. Tensile testing of heat treated strips showed greater strength losses in the oil treated samples (73%) than in the oven treated samples (26% loss). Heat treatment darkened the wood strips but this was photo-bleached on exposure to UV artificial weathering. The UV exposure caused some loss in tensile strength (35%) but the additional effects of UV on the tensile strength were less in heat treated wood (oil 15%, oven 18%). The combined effects of UV and fungi on the tensile strengths were less severe for treated wood than in untreated wood, (oil 4%, oven 18%) thus some protection was afforded by the heat tretaments. Exposure of untreated wood strips to the soft rot fungus caused some loss in tensile strength (32%), but heat treated wood showed small but not statistically significant gains in tensile strength after fungal exposure (oil, gain 30%, oven, gain 3%). Microscopic examination confirmed that a soft rot was minimal in heat treated samples, i.e. it was mainly confined to wood cell wall penetration. The combined effects of UV and fungi caused further losses of tensile strength in both oven and oil heat treated samples, but this loss was less severe than that which occurred in untreated controls. However, due to the severity of the initial losses caused by the oil heat treatment, the highest losses overall were in those oil heat treated samples exposed to UV (82%) and UV and decay (78%). Microscopic examination revealed that decay was more advanced in UV exposed samples. The FTIR spectra of oven heat treated samples did not show any change, while the oil heat treated samples increased absorbance at 1737cm-1 and 1450cm-1 relating to the presence of oil. Following fungal decay, no changes were observed, this corresponds with the cavity attack by soft rot fungi, which leave adjacent portions of cell wall material untouched. The ultra violet exposure affected the spectra of all specimens, increasing the absorbance at 1737cm-1. This relates to photo-degradation of the hemicelluloses, forming new carbonyl groups. In the UV exposed untreated wood there were also increases in absorbance at 1510 and 1160cm-1 relating to alterations in lignin structure. Keywords: Heat treatment, linseed oil, weathering, soft rot decay. 1. INTRODUCTION Heat treatment of wood is usually considered as an environmentally favourable wood protection technique (Rapp and Sailer, 2000; Sailer et al., 2000) because no preservative chemicals are

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used. It reduces the hygroscopicity of wood, improves dimensional stability and increases resistance to different types of biodegradation (Welzbacher and Rapp, 2002). Currently in Europe there is widespread interest in heat treatments (Evans, 2003), and a variety of commercial systems have been developed, e.g. Thermo Wood, Royal process, OHT wood, New Option Wood, PLATO wood and Stellac wood. These systems vary from each other in terms of treatment conditions e.g. temperature, time, stages, presence or absence of oxygen, use of inert gases, steaming, wet or dry process, use of oils and therefore the properties of wood from each system differ subtly. Wood exposed outdoors undergoes rapid surface degradation due to a complex combination of environmental factors, such as solar radiation (especially UV light), water, temperature, atmospheric gases and fungal decay, collectively known as weathering. Among different wood weathering factors, solar radiation is considered as the most damaging (Kataoka and Kiguchi, 2001). Even though weathering of wood is a surface phenomenon, it has important implications for the application and maintenance of exterior finishes like paints and varnishes and is associated with a general increase in moisture content which may lead to dimensional movement, splitting and colonisation by decay organisms (Derbyshire and Miller, 1981). Every year a considerable amount of money is spent to protect wood surfaces that are exposed to severe climatic conditions or in order to attempt to restore wood that is subjected to weathering influences (Anderson et al., 1991). Much effort has been put into the development of protective systems for wood to prevent photo-degradation during outdoor weathering such as surface treatment of wood with inorganic chemicals (Williams and Feist, 1988). Before widespread commercialisation of heat treated wood it is necessary to examine the weathering effects and fungal decay of heat treated wood. Heat treated wood shows marginally improved weathering characteristics. Photo-bleaching of the brown colour to a grey colour occurs during the first 6 months of exterior weathering but the improved dimensional stability leads to a reduction in the swelling of the wood and fewer shakes result (Mayes and Oksanen, 2002). It has been shown that the colour change which occurs during heat treatment of wood related to condensation reactions of compounds based on lignin, and that these compounds are less susceptible to weathering and photodegradation than the lignins in unmodified wood (Ayadi et al., 2003). When heat treated softwoods from the Thermowood process were exposed to weathering for 7 years, the phenolic content of the wood remained high, whereas the untreated control specimens had a predominantly cellulosic composition (Nuopponen et al., 2004). 2. MATERIALS AND METHODS Corsican pine sapwood thin strips were heat treated in an oven or in linseed oil and were exposed to UV weathering and / or soft rot attack as summarised in Table 1. Tensile test strips were prepared from defect free straight grained sapwood blocks of Corsican pine (Pinus nigra var. maritima) of dimensions 100 x 30 x 10 mm (L x T x R) by microtoming 100 µm sections from the radial surface. Sections were cut 10-20° from true radial orientation to avoid rays running directly across the specimens which would have given lines of weakness. After cutting, the strips were air dried, and thereafter kept flat in batches of 10-15 specimens in Perspex jigs in a conditioning room, temperature 20°C, relative humidity 65% ± 1%. After

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drying the microtomed sections were stored in batches of 10-15 in sealed envelops and shielded from light. Table 1. Experimental matrix.

Untreated controls Oven heat treatment Oil heat treatment Heat treated

- - - - + + + + + + + +

UV exposed

+ + + + + +

Fungus exposed

+ + + + + +

Replication 8 8 8 8 8 8 8 8 8 8 8 8 Code U UF UUV UUVF O OF OUV OUVF Oil OilF OilUV OilUVF For the heat treatment of wood specimens two methods were followed, an oven method (O) and an oil immersion method (Oil). For the oven method, 32 strips were treated at 200°C in a nitrogen atmosphere with a pre-heated vacuum oven for 2 hours. A vacuum was established to remove air, and then a steady slow flow of nitrogen gas was maintained. For the oil heat treatment 32 strips were immersed in linseed oil at 200°C for 30 minutes in the presence of an inert gas (nitrogen). A set of 48 strips (comprising 20 untreated, 20 oven heat treated and 20 oil heat treated) were exposed to UV radiation in the artificial weathering chamber (Suntest CPS/CPS+) for 300 hours. The temperature was maintained at 35°C with the intensity of irradiation from the xenon lamp at 375 w/m2. During exposure to UV irradiation the samples were kept horizontal and the surface which was directly exposed to UV irradiation was marked. Sets of 8 thin strips, of three different treatments (untreated, oven heat treated and oil heat treated), both before and after UV exposure (Table 1), were exposed to soft rot fungus, Chaetomium globosum. The strips were laid over a mineral nutrients agar in large square petri-dishes (245 x 245 mm, Corning) containing Oxoid No.3 agar: 20 g/l, ammonium nitrate: 6 gl/, magnesium sulphate heptahydrate: 4.0 g/l, potassium dihydrogen phosphate: 0.25 g/l , di-potassium phosphate: 0.2 g/l and deionised water. The agar was inoculated with a spore suspension immediately prior to planting the strips and planting was done by a random selection method. Sterilised glass microscope slides were placed over the ends of the wood strips to keep them in flat contact with the agar. The test assemblies were sealed with Parafilm and incubated at 29°C for 2 weeks. Sterile control groups were incubated under sterile conditions. The wood strips were wiped clean following their removal from the petri-dishes to remove any adhering mycelium and fungal spores. After mechanical testing, sections were examined by light microscopy in a high refractive index medium (benzyl benzoate) for soft rot decay features. In the case of oil heat treated wood strips, the treatment oil was removed from the samples by Soxhlet extraction with 4:1:1 solution of toluene, methanol and acetone for 5 hours. This was done to improve the clarity of the images of fungal attack when examined by light microscopy. Following incubation the wood strips were zero span tested in a wet condition using a Pulmac Z-span 1000. Once broken some strip halves were examined by microscopy while others were examined by Fourier Transform Infra-red (FTIR) spectroscopy, using a Nicolet 750, series II instrument. The thin strips were mounted across the aperture of a Specacard for spectroscopy. The FTIR peak assignments for each treatment were compared.

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3. RESULTS AND DISCUSSION 3.1 Tensile strength It can be seen in Figure 1 that both the oven heat treatment (O, 26% loss) and the oil heat treatment (Oil, 73% loss) caused significant losses of tensile strength (p=0.002 and p=0.000 respectively, by one-way ANOVA at α = 0.05). It is well known that heat treatment causes a loss of strength, e.g. a loss of to 18% has been recorded for PLATO wood (Tjeerdsma et al., 1998), also a loss of 30 to 50% in tensile strength of softwoods (Syrjänen and Kangas, 2000) and 26% for heat treated Eucalyptus globulus (Santos, 2000). The strength loss shown by oven treated wood in this study is therefore acceptable. The high strength loss for oil heat treated wood is likely to relate to the greater efficiency of heat transfer by the hot oil method, and in specimens of this thickness (100µm) the treatment duration should have been further reduced. Oil heat treated timber of commonly used dimensions has shown a lower strength loss than air heat treated timber (Buro 1954, Sailer et al., 2000).

92

63 60

34

68 70

55

43

2532

21 24

0

20

40

60

80

100

120

Tens

ile s

treng

th (M

Pa)

0 32 35 63 26 24 40 53 73 65 77 74

U UF UUV UUVF

O OF OUV OUVF

OIL OILF OILUV

OILUVF

% loss vs. U control% loss vs. own control 0 32 35 63 0 -3 18 36 0 -30 15 4

92

63 60

34

68 70

55

43

2532

21 24

0

20

40

60

80

100

120

Tens

ile s

treng

th (M

Pa)

0 32 35 63 26 24 40 53 73 65 77 74

U UF UUV UUVF

O OF OUV OUVF

OIL OILF OILUV

OILUVF

% loss vs. U control% loss vs. own control 0 32 35 63 0 -3 18 36 0 -30 15 4

Figure 1. Average tensile strength in MPa of different treatment samples with standard error.

The exposure of the untreated samples to UV radiation caused a loss of tensile strength (UUV, 35% loss, p=0.000), as did the exposure to the soft rot fungus C. globosum (UF, 32% loss, p=0.000). When fungal exposure followed the UV exposure, the strength loss was increased significantly (UUVF, 63% loss with respect to control, p=0.000). Derbyshire et al. (1995) reported that UV irradiation reduces strength due to degradation of lignin. It is possible that this exposes the holocellulose, and thus facilitates subsequent decay of the cellulose by soft rot fungi. The strength loss of UUVF strips relative to the UV treated strips was 44% (p=0.000). Two-way ANOVA of this set of data (U, UF, UUV, UUVF) indicated that there was no significant interaction between the fungal and UV exposure effects (p=0.652).

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While the oven heat treated strips were less strong than the control strips (67.7MPa compared to 92.1MPa), there was no significant further loss on exposure to the soft rot fungus C. globosum (2.8% increase, p=0.733, Figure 2). In fact, the fungal exposed oven treated wood (69.7MPa) is marginally stronger than the fungal exposed untreated wood (62.8MPa) although this difference is not statistically significant (p=0.158). Oven treatment appears to have a beneficial effect against soft rot attack by C. globosum. Ultra violet exposure of oven heat treated wood strips resulted in a significant (p=0.027) strength loss of 18.2% from 67.7MPa to 55.4MPa. The strips which were exposed to UV radiation and then fungi (OUVF) showed a further loss of strength to 43.1MPa (p=0.000). However, these strips are significantly stronger than the untreated ultra violet with fungal exposed (UUVF) strips (33.8MPa, p=0.019), i.e. the initial strength loss has been compensated for by the protective effect against soft rot attack, even after UV weathering.

-505

10152025303540

OF OUV OUVF

Stre

ngth

loss

relat

ive to

O (%

)

Figure 2. A comparison between relative strength losses of the oven treated samples, where O = oven treated, OF = + fungal exposure, OUV = + UV and OUVF was oven treated, UV irradiated the fungal exposed. Oil heat treatment had a seriously detrimental effect on strength, possibly indicating that a shorter treatment time should have been used on these thin specimens. The strength of oil heat treated wood (Oil) was reduced to 24.9MPa before exposure to either UV or fungi. Following fungal exposure (OilF) the tensile strength apparently increased to 32.5MPa (Figure 3), but this difference was not statistically significant (p=0.119). Similarly, following UV exposure (OilUV) the strength loss (21.2MPa) was not significant, and nor was the small loss following combined UV-fungal exposure (OilUVF, 23.9MPa). Comparisons of fungal and UV results to their equivalent untreated control results were not made, due to the high strength loss introduced by the treatment, and the negligible influence of the fungus on oil heat treated specimens.

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-35-30-25-20-15-10

-505

101520

OilF OilUV OilUVFSt

reng

th lo

ss re

lative

to O

il (%

)

Figure 3. A comparison between relative strength losses of the oil treated samples, where Oil = oil heat treated, OilF = + fungal exposure, OilUV = + ultra violet irradiated and OilUVF was oil heat treated, UV irradiated the fungal exposed.

3.2 Observation of wood veneers following exposure to UV irradiation The most noticeable effect of exposure of wood strips of different treatment groups to UV irradiation was the change of colour. Untreated wood strips turned grey after 300 hours of UV exposure. Oven heat treatment and oil heat treatment of wood strips produced a dark brown colour, but this colouration was not stable on exposure to UV irradiation. Photo-bleaching for UV irradiated wood subjected to thermal treatment has been reported by Mitsui et al. (2001). Retified pine showed a lower colour change than untreated wood after UV exposure in quantitative study (Ayadi et al., 2003).

3.3 Microscopic observation of wood veneers exposed to fungi

Dark staining was clearly observed in the latewood of untreated wood strips following two weeks exposure to C. globosum. These samples had also undergone severe cavity attack which was visible by light microscopy (Figure 4). The latewood was more severely attacked than the earlywood. Orientated chains of biconical or diamond shaped cavities were seen throughout the untreated wood strips.

Figure 4. Fungal decay in untreated wood strips showing heavy decay in latewood (a) and cavities in earlywood (b).

Figure 5. Soft rot decay in untreated samples following exposure to UV irradiation.

a

b

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In the untreated samples exposed to UV, the soft rot decay was found to be more severe than in unexposed strips (Figure 5). The staining and decay was so severe that it was difficult to obtain clear micrographs of fungal decay. Chains of adjacent soft rot cavities had started to coalesce, in a manner typical of the late stages of soft rot attack, indicating extensive destruction of the S2 layer (Eaton and Hale, 1993). Soft rot decay has been enhanced by the UV degradation before the strips were exposed to decay. Eaton and Hale (1993) report that UV irradiation is one of several factors which make wood more likely to suffer from soft rot.

The microscopic observation of oven heat treated samples showed that although soft rot cavity attack was much less prevalent than that in untreated control samples, the penetration attack (i.e. Corbett, 1965 active penetration) was intense in some areas (Figure 6). This is stain-type penetration, because the penetration hyphae and lumen hyphae traversed several cell wall pairs without apparently giving rise to cavities. Soft rot cavity attack was not intense; it was restricted to a few unwidened T-branches (Figure 7) and slightly more developed stages in some areas (Figure 8). It is probable that the changes in the cell wall are either masking the T-branch initiation sites or modifying the fungal behaviour to favour stain activity. The soft rot decay in UV exposed oven heat treated samples was more advanced than in the oven heat treated samples (Figure 8). Developed cavity attack was readily found (Figure 9); penetration attack alone and few early stages of decay (i.e. unwidened T-branches) were observed.

The oil heat treated samples were darkened by the presence of oxidised oil, making it difficult to get clear micrographs. Soxhlet extraction removed most of the oil, and better images resulted. Some oil globules within the cell lumen were still observed and the hydrophobic property imparted by the oil made it difficult to remove air bubbles, even after extraction. Minor fungal attack was seen, in the form of penetration of hyphae and infrequent T-branching (Figure 10). In the oil heat treated UV exposed samples, fungal decay was also found to be restricted to the development of penetration hyphae and infrequent T-branching (Figure 11).

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Figure 6. Soft rot decay in oven heat treated sample with numerous penetration hyphae (a) and active penetration (b).

Figure 7. Soft rot decay in oven heat treated sample as T-branching (arrowed).

ab

Figure 8. Soft rot decay with advanced stages of T-branching in oven heat treated sample (arrowed).

Figure 9. Different forms of soft rot cavities in UV exposed oven heat treated samples (arrowed).

Figure 10. T-branching of fungal hyphae in oil heat treated sample (a) and staining (b).

Figure 11. Soft rot decay in oil heat treated sample after exposure to UV irradiation with symptom of T-branching (a), penetration hyphae (b) and some air bubbles (c).

bb

c

a

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3.4 FTIR characterisation

3.4.1 Treatment effects FTIR spectra of oil and oven heat treated specimens were compared with the spectrum for untreated Corsican pine wood. A marked increase in absorption at 1737cm-1 was observed for oil heat treated pine, but not for oven heat treated pine. This absorption relates to the ester linkages which are present in both the holocellulose of wood and the triglyceride of linseed oil (Kuo et al., 1988, Fengel and Wegener, 1989, Silverstein et al., 1991). A spectrum for linseed oil, obtained by the NaCl disk method, showed an absorption at 1748cm-1 for the C=O stretch of the ester. It also shows absorption at 1160cm-1 for the C-(C=O)-O stretch and at 1026cm-1 for the O-C-O ester stretch. This was not visible for the oil treated wood because the 1000-1200cm-1 region of the spectra of heat treated wood were near the detection limit, due to the sample thickness. The oil treated wood also showed a slight increase in absorption at 1450cm-1 relating to the CH3 bend vibrations of the oil. The spectrum of oven heat treated wood showed no significant changes compared to that of untreated wood. Again the thickness of wood samples made accurate determinations in the 1000-1200cm-1 region difficult as the detection limit had been reached in some cases, due to the section method used. Weiland and Guyonnet (2003) reported that heat treatment of Pinus pinaster under nitrogen at temperatures below 250°C decreased the C=O bands at 1750 and 1650cm-1 as hemicelluloses were degraded. They also noted slight increases of the 1510 cm-1 (C=C in lignin) and 1160 cm-1 (C-O-C in holocellulose) bands in the heat treated wood. 3.4.2 Ultra-violet exposed wood The exposure of untreated wood to ultra violet radiation gave an increase in the ester absorption at 1737cm-1, probably relating to degradation of the cellulose and hemicellulose, with the formation of new ester, lactone and ketone functionalities. An increase in this portion of the spectrum after UV exposure was also observed by Mitsui et al. (2003) for Cryptomeria japonica, and by Tolvaj and Faix (1995) for five timber species. The UV treated pine samples also showed a slight reduction in the absorption at 1510cm-1, relating to the ring vibrations in the phenolic groups of lignin, which indicates some change in the lignin structure, most likely in the form of cross linking of aromatic rings which restricts these C=C ring vibrations. This is consistent with the reports of Derbyshire et al. (1995). Oil heat treated UV specimens retained the strong 1737cm-1 absorption of the oil, which masked any changes in hemicellulose content. When oil heat treated wood was subjected to UV exposure, this absorption appeared to be further strengthened. Oxidative cross linking of linseed oil proceeds by a free radical process, via hydroperoxide formation to form ether, ester and C-C linkages between the fatty acid chains (Mallegol et al. 1999). The ester linkages will also be detected in the region of 1740cm-1, and this increase after UV treatment indicates that cross linking was continuing during exposure, it is possible that the exclusion of air during oil heat treatment retarded complete cross linking, allowing this process to continue during the UV phase. It is known that UV light eventually degrades linseed oil via radical attack on the tertiary carbons of the C-C bonds between oil molecules (Mallegol et al. 2000). 3.4.3 Fungal exposure Exposure of untreated wood to soft rot fungi gave no significant alteration in the FTIR spectrum. It is likely that the cavity attack of the cell wall leaves areas of undegraded cellulose and hemicellulose outside the cavities, therefore spectroscopy does not detect any change in the chemical composition. The oil heat treated and oven heat treated specimens also showed no further changes in their spectra after fungal exposure.

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In specimens which had been exposed to UV and then fungi, the 1737cm-1 absorption returned to a normal level, whereas the 1510cm-1 lignin absorption remained low. The reduction of the ester absorption is consistent with digestion of the degraded sugars by fungi or diffusion of these degradation products into the agar culture medium. The degradation of lignin appears to be irreversible, which is consistent with cross-linking by free radical formation and subsequent condensation reactions. When oil heat treated timber strips were exposed to fungi, either before or after UV irradiattion, there was a small reduction in absorption at 1737cm-1, indicating some loss of ester moieties. This is likely to relate to the diffusion of soluble degradation products of hemicelluloses, in a similar manner to that seen for the oven heat treated wood strips. However, this ester carbonyl absorption peak remained stronger than in untreated wood, because of the presence of oil. Oven heat treated timber also showed a small increase in the 1737cm-1 absorption on UV exposure, but after a subsequent fungal decay this absorption was reduced again in a similar manner to the untreated wood. 4. CONCLUSIONS Both oven heat treatment and oil heat treatment of thin wood veneers were shown to cause the loss of tensile strength (24% and 71% respectively). Tensile strength of wood was also reduced by UV degradation and fungal decay. Oven heat treated wood was shown to give better resistance to fungal decay. However, this benefit was reduced after UV exposure. Fungal exposure and UV irradiation of the oil heat treated wood had no statistically significant change on tensile strength. Microscopic studies on the heat treated and decayed wood sections confirmed that the decay was significantly reduced by both of the heat treatments. Although the heat treated wood was more decay resistant, stain activity (i.e. repetitive cell wall penetration) by the fungus was more intense, and most of the soft rot was confined to cell wall colonisation by T-branching / L-bending without further decay development. UV exposure enhanced decay by the soft rot fungus, C. globosum. This occurred in all specimens, but the extent of enhancement was only minor in the heat treated wood. FTIR investigations did not detect any chemical change resulting from fungal decay, due to the localised nature of soft rot cavity formation. However, for ultraviolet irradiation studies, the degradation of holocellulose and lignin was detected with increases in absorbance at 1737cm-1 and 1510cm-1 respectively. In all oil heat treated specimens there was an increase in the broad absorbance band around 1737cm-1, due to the presence of esters within the triglyceride, and esters formed during oxidative cross linking of the oil. There was also an increase in absorbance at 1450cm-1

for all specimens, due to the olefin portion of the fatty acids. Although more work needs to be done to examine the decay resistance of wood surfaces exposed to natural weathering in a range of localities (giving different environmental conditions) there is good cause for optimism regarding the use of these treatments for EHC 3 exposure.

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light-irradiated wood with heat treatment. Holz als Roh- und Werkstoff. 61: 82. Mitsui, K., Takada, H., Sugiyama, M. and Hasegawa, R. (2001). Changes in the properties of

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Sailer, M., Rapp, A. O. and Leithoff, H. (2000). Improved resistance of Scots pine and spruce by application of an oil-heat treatment. International research Group on Wood preservation. Document no. IRG/WP 00-40162.

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