Weathering characteristics of wood treated with water glass, siloxane or DMDHEU

Specimens of Scots pine sapwood (Pinus sylvestris L.) and beech wood (Fagus sylvatica L.) free of knots and cracks were prepared with a size of 150×74×18 mm³ (longitudinal × tangential × radial). The modification chemicals that were used in this study are described in Table 1.

Impregnation of wood specimens was carried out by applying a vacuum of 60 mbar (30 min) and a subsequent pressure of 12 bar (2 h). All treatments were carried out in a laboratory scale process. After impregnation, siloxane impregnated specimens were pre-dried at 40°C (4 d). Curing of the siloxane was subsequently performed at 103°C (24 h). The water glass treated specimens were stored for three weeks in a desiccator under carbon dioxide atmosphere, which was established by floating the desiccator in regular steps with CO₂ from a gas bomb. DMDHEU impregnated specimens were cured in a hot steam dryer. The weight percent gain (WPG) of the specimens was determined from the dry masses before and after treatment.

The specimens of Scots pine sapwood and beech wood were placed and fixed on weathering racks with a 45° slope direction towards south west. The weathering racks were located at the field of the University of Göttingen. Eight samples per treatment were used. Weathering was performed from August 2006 to August 2008.

Within the initial 3 months of outside exposure, the lightness of all specimens decreased clearly, except for water glass treated specimens particularly in the case of Scots pine sapwood specimens (Figs. 1 and 2). Since the initial lightness was slightly decreased for all treated specimens, decline of lightness within the initial 3 months was lower compared to untreated specimens. During the subsequent exposure time the lightness of treated and untreated Scots pine sapwood specimens remained almost constant for up to 15 months. The lightness of untreated and siloxane treated specimens was lower than that of DMDHEU and water glass treated specimens. After 24 months outside exposure, all specimens reached approximately the same level of lightness except for the water glass treated specimens. The values of beech wood varied over the whole evaluation period. During 9 months of outside exposure all treated specimens displayed the lowest values of lightness. Subsequently the lightness increased and the untreated specimens showed the lowest values. After 15 months of outside exposure the lightness increased again particularly in the case of beech wood and water glass treated Scots pine. The variation of lightness is influenced by various factors such as wood moisture content and reflectance of light on the wood surface.

The dependence on different wood moisture contents is due to the effect of free water in the cells on the wood colour especially the L-values (Hon and Minemura 2001). The influence of wood moisture content could be reduced because the wood specimens were stored in a climatised room (20°C/65%RH) for three days to reach a climatised wood surface without any cluster of moisture. Furthermore the specimens were not evaluated after a rainfall period to prevent an evaluation of wet specimen surfaces.

Furthermore the surface layer of the specimens which is rich in cellulose fibres after lignin degradation reflects light non-uniformly which may result in a variability of lightness. An increase in lightness during weathering was also observed in previous investigations by Hon and Chang (1984), who reported regained brightness of some wood species such as Redwood, Southern yellow pine and Douglas fir during outside weathering.

Scots pine sapwood exhibited a rapid change in colour during 6 months of outside exposure (Fig. 3). Siloxane treated and untreated specimens displayed the same level of colour change during the whole evaluation period. The DMDHEU and water glass treated specimens showed reduced change of colour.

The untreated specimens of beech wood displayed an extensively changed colour within 3 months as well as between 9 and 18 months of outside exposure (Fig. 4). An increase of the overall colour change ΔE was observed in previous investigations during the initial stage of natural and accelerated weathering conditions (Feist and Hon 1984; Hon and Feist 1986). The extensive change of untreated beech wood after 9 months of exposure time can be explained by an increased growth of blue stain on the wood surface (pictures not shown). Between 15 and 18 months the surface of the specimens became greyer and the colour changed at decreased rate again.

The treated specimens showed fewer discolouration during 9 and 24 months of outside exposure.

After three months of outside exposure the untreated specimens displayed an infestation of staining fungi (Figs. 5 and 6) visible as dark coloured spots on the weathered surface. The water glass treated specimens showed no signs of surface discoloration. The DMDHEU and siloxane treated specimens displayed the typical colour from light to dark grey usually found on exposed wood surfaces, but compared to untreated specimens, no dark coloured spots or streaks were visible. After 12 months of outside exposure all specimens showed visible surface discolouration which is attributable to a combination of fungal growth and photo degradation (Figs. 5 and 6). Various studies conclude that this grey surface discolouration of wood is a combined effect of photo degradation and the growth of fungi on the surface of the wood (Feist 1982; Sell 1975).

The chemical changes particularly lignin degradation was investigated by FTIR-spectroscopy. The lignin absorption of untreated Scots pine decreased with increasing exposure time, visible at the lignin absorption at 1510 cm⁻¹ (stretch vibration in aromatic ring), 1452 cm⁻¹ (CH₂-deformation) and 1264 cm⁻¹ (guaiacyl nuclei) (Schultz and Glasser 1986; Pandey and Theagarajan 1997). These absorptions were absent after 6 months of outside exposure. Absorptions at 1370 cm⁻¹, 1315 cm⁻¹ and 1162 cm⁻¹ which are assigned to cellulosic constituents (Pandey and Theagarajan 1997; Chang and Chang 2001) did not change significantly (Fig. 7). The lignin absorption of untreated beech wood was visible at 1507 cm⁻¹ (stretch vibration in aromatic ring), 1459 cm⁻¹ (CH₂-deformation) and 1235 cm⁻¹ (syringyl nuclei). The lignin absorption decreased with increasing exposure time (Fig. 8).

Infrared spectra of water glass treated wood showed absorption at 1030 cm⁻¹ for the Si–O stretching of poly-silicate and silica gel. But this absorption is partly overlaid by the C–O absorption bands in cellulose and hemicelluloses of wood. The spectra of water glass treated wood also revealed a reduced intensity of the lignin bands which is related to the lignin bands of untreated wood during 12 months of outside exposure.

The characteristic peaks of siloxane treated wood at 1229 cm⁻¹ (Scots pine) and 1231 cm⁻¹ (beech) assigned to the C–N vibration and at 1657 cm⁻¹ (Scots pine) and 1658 cm⁻¹ (beech) caused by NH₂-bending vibration (Gottwald and Wachter 1997; Bruker 2002) were not clearly visible. These bands were overlaid by absorption bands in untreated wood.

Infrared spectra of DMDHEU treated Scots pine and beech showed an increase in the carbonyl content (1709 and 1726 cm⁻¹) caused by carbonyl groups in DMDHEU (Petersen 1967; Schultz and Glasser 1986; Xie et al. 2005). This strong carbonyl band overlaid native carbonyl group absorptions in untreated wood. Additionally treated specimens displayed an absorbance maxima at 1237 cm⁻¹ (Scots pine) and 1232 cm⁻¹ (beech) for the C–O stretch vibration in the N-methylol group of DMDHEU.

Generally, the treatments did not prevent lignin from degradation during long-term outside weathering.

Irrespective of the treatments, absorptions of cellulosic constituents did not change significantly as a result of weathering.

Colour measurements and FTIR spectroscopy showed distinct surface discolouration and lignin degradation during the initial exposure time (3–6 months). The degradation processes of lignin are accompanied by various changes in colour, depending on wood species, time of exposure and band width of the irradiation source (Hon and Minemura 2001). The grey surface colour is a result of the leaching of decay products of lignin (Feist 1983; Sell and Leukens 1971). The surface discolouration was a combination of fungal growth and photo degradation of lignin. The fungal infestation of all treated samples was retarded while the lignin degradation was not. Therefore the lignin degradation did not influence the initial fungal infestation on the treated wood surfaces.

Fungal infestation mostly resulted from wetting the wood surface with liquid water. Free water in the lumens of wood cells over longer periods is essential for fungal growth (Grosser 1985; Eaton and Hale 1993).

In the case of water glass treated specimens the inhibition of fungal growth is not influenced by reduced wood moisture content because the treatment resulted in a high hygroscopicity of silicate and sodium salts in the cell lumens (Furuno et al. 1991, 1992; Furuno and Imamura 1998). Rather the water glass treated specimens showed a highly alkaline pH-value before and after storage under carbon dioxide atmosphere. Highly alkaline pH values can influence spore germination, mycelia growth and fruit body formation (Schmidt 2006; Reiß 1997). Previous investigations also reported high resistance of water glass treated specimens against wood destroying basidiomycetes, because of the high pH-values and the insoluble silicates in the cell lumens (Furuno et al. 1992; Dellith 2006).

Treatments with siloxanes diminish the uptake of liquid water. The reduction in water uptake is caused by blocking the main penetration paths such as pits, ray cells and ray tracheids (Donath et al. 2006). However, the moisture content of the surface layer might still be high enough to allow fungal infestation, particularly after rain periods with liquid water on the sample surface.

DMDHEU treatment reduces the speed of liquid water uptake caused by the inclusion of the chemical in the ray cells, the major penetration pathways for water in untreated wood (Xie et al. 2005, 2008). Therefore, fungal infestation particularly during initial stages of outside weathering can be reduced. The decelerated initial infestation is not attributable to a biocidal effect of DMDHEU, because most of the DMDHEU in wood is fixed through covalent bonding to the cell wall or self condensation (Verma et al. 2005). Rather the changed chemical structure of wood modified with DMDHEU particularly lignin and its breakdown products might have an impact to fungal growth on weathered wood surfaces.

Furthermore, a shift of the peak maximum of the carbonyl band of DMDHEU treated Scots pine sapwood from 1707 cm⁻¹ to 1718 cm⁻¹ occurred during weathering. An explanation for this might be the removal of DMDHEU which was linked to lignin molecules and a resultant overlapping of carbonyl bands in DMDHEU and those present in wood. These results correspond with those of previous studies (Xie et al. 2005).

In addition to the investigations on fungal growth on the specimen surface the radial penetration of fungal hyphae was studied by SEM.

All specimens were infested by staining fungi after 24 months of outside exposure. Furthermore, all specimens displayed cracks during and after outside exposure. The different treatments could not inhibit the formation of cracks. Sapstaining fungi colonise wood tissues by spreading from cell to cell primarily through pits (Liese and Schmid 1961, 1964; Eaton and Hale 1993). The cross-sectional view of staining fungi penetration (Figs. 9, 10, 11) revealed a radial penetration of fungal hyphae in untreated wood specimens. The untreated specimens exhibited a radial penetration of fungal hyphae into the wood tissue (Fig. 12). The growth of fungal hyphae through the pits was clearly visible.

In the cross sectional view no differences of fungal penetration between the wood species were visible. Rather the fungal penetration was influenced by the different chemical treatments. SEM micrographs of wood were evaluated from specimens taken from the labelled area (see arrow in Fig. 9A). The SEM micrographs showed similar results. There were also no differences of fungal penetration depending on the wood species; rather the various treatments have an influence on fungal penetration into the wood tissue.

In water glass and siloxane treated specimens no radial penetration was visible. The cracks formed during outside weathering did not influence the penetration of fungal hyphae in the wood tissue. No penetration of hyphae was found in the crack areas (Fig. 11). The fungal penetration for untreated wood (Fig. 12), DMDHEU treated wood (Fig. 13), and siloxane and water glass treated wood (Figs. 14 and 15) are shown in the following SEM-micrographs. Only superficial infestation and no radial penetration of fungal hyphae were visible at SEM-micrographs for water glass and siloxane treated specimens, representatively shown in Fig. 14 for siloxane treated wood and Fig. 15 for water glass treated wood. Investigations on siloxane treated Scots pine sapwood and beech wood have shown that deposits of siloxane occur in the cell lumens of ray cells and pits (Donath et al. 2006). This might be inhibiting the radial penetration through these pathways. Investigations by Dellith (2006) showed penetration of water glass into cell wall areas and cell lumens of ray cells and deposits onto the pits. The penetration of water glass within the pits is visible in Fig. 15 (see arrow). Hence these penetration paths for fungal hyphae are partly blocked.

In DMDHEU treated specimens radial penetration was reduced. Infestation was visible along the area of radial cracks. The radial penetration depth of fungi starting from the exposed specimens’ surface was reduced in DMDHEU treated specimens. But hyphae growth was clearly visible near the weathered surface (Fig. 13, weathered surface in arrow direction). Any fungal infestation through the pits was not visible. The reduction of radial penetration on DMDHEU treated wood might be caused by blocking of the penetration pathways because of the inclusion of the chemical in the ray cells (Xie et al. 2008).

Based on these results, it was assumed that there is an infestation of fungi with different physiology in treated and untreated specimens. In the case of treated specimens fungi might mainly utilise sugars in the superficial wood tissue and lignin breakdown products on the wood surface as nutritional source. In the case of untreated specimens the fungi are able to grow through the wood tissue along the rays consuming available sugars in these cells and in the superficial wood tissue.