Weathering Evan

After leaching of UV degradation products, underlying cell layers are exposed and further eroded Feist , Williams and Feist , Reinprecht Erosion is more rapid in the less dense early-wood than in latewood, which leads to an uneven and rougher surface Williams et al. Formation of checks, split and cracks caused by moisture and temperature changes also leads to tangible surface roughness Feist , Ozgenc et al. A lot of studies were aimed at evaluating the properties of wood treated or modified with some protective compound after natural weathering Kropf et al.

In these studies, the surface became rougher and darker after weathering in comparison with the initial stages. Total colour change increased with increasing exposure times in natural weathering. Despite contemporary ecological trends, significantly less studies deal with the weathering characteristics of untreated wood Hon and Feist , Tolvaj and Faix , Gonzalez de Cademartori et al.

Untreated wood specimens exhibited higher colour changes in the shorter time than the treated specimens Saei et al. The colour changes were reported strictly as a surface phenomenon, they occurred to a depth of only 0,,5 mm Feist , Reinprecht The trend of increasing roughness value was observed in these studies. The rate of degradation is usually related to the intensity of light Kataoka et al. This study is aimed on the evaluation and comparison of surface degradation of selected wood species during twelve months of natural weathering. The information about behaviour of untreated wood, both softwoods and hardwoods, exposed to outdoor climatic conditions is given.

The change of colour, depth of these changes, change of gloss, roughness and formation of cracks and mould were investigated. The information about the depth of these changes could serve for those who is not satisfied with the grey shade of the wood and decides to sand the wood element to initial colour. Above mentioned information, important for a maintenance and service life of wood, must be provided for further effective use of wood in the exterior. The experiment was carried out using three samples, all of them with dimensions x78x20 mm LxTxR , from nine wood species table 1.

In the case of heartwood species, only the heartwood was tested. Test samples were sanded with the sandpaper grit of Initial characteristics of wood samples before weathering are shown in table 1. Mean values and Standard deviations in parenthesis were determined from 24 measurements of colour and gloss and from 12 measurements of roughness for each wood specie. The natural durability against fungi was determined according to , value , while 1 signifies the highest natural durability.

Overview of the climatic conditions during 12 months of weathering can be seen in table 2. Overview of the climatic conditions at the test site during the exposure period. The colour and gloss characteristics of the wood were measured using spectrophotometer d Konica Minolta, Japan according to CIE Measurement points for the analysis of the colour changes and their depths.

The total colour changes were calculated according to CIE using the following equation: The depth of colour changes was determined after 3, 6 and 12 months of exposure. The depth of these changes was measured by electronic gauge Mitutoyo, Japan at the same initial points for each sample figure 1.

The measurement was carried out in four traversing lengths oriented perpendicularly to the length of the samples over the tangential surface. Visual observation of samples with 10x magnifying glass was done during the period of natural weathering test to observe the formation of mould, cracks and other visual defects. The statistical evaluation was carried out in the software Statistica 12 Statsoft Ltd. The visual appearance of the samples is given in figure 6.

The trend of gloss mean values did not show any systematic trend during exposure. Generally, the gently increase was observed for the most cases after 6 months and then large decrease after 12 months of natural weathering test table 3. The highest change of gloss was observed for pine, the lowest one for oak wood.

Mean values and Standard deviations in parenthesis were determined from 24 measurements. If the wood contains a high amount of extractives, the chemical processes of the colour changes quickly take place. Regarding the wood species with low extractive content, the photo-degradation of lignin soon begins with slow but continuous colour change speed. The final decrease of yellowness may be attributed to leaching of decomposed lignin and extractives by water Turkoglu et al.

The results confirmed that in the early stages of weathering, dark wood trend to become light and light woods become dark or turn into the silver grey colour Dawson et al. Colour changes of softwoods left and hardwoods right samples during 12 months of natural weathering. The depth of colour change was relatively similar for softwood species.

For hardwoods, it varied more figure 3. The highest value after 12 months of weathering was observed for larch 1,26 mm and the lowest one for oak 0,50 mm.

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The measurement after 6 months of exposure were done for all the samples, but after 12 months of exposure some samples cannot be measured alder, poplar, maple due to the presence of blue-stain fungi, which caused deeper degradation of wood and influenced the colour changes caused only by weathering. The graphs at figure 4 show the correlation after 6 and 12 months of exposure to natural weathering.

This fact confirms that during early stage of weathering the total colour change of wood surfaces is mainly caused by light decomposition of lignin which in consequence enables radical depolymerization to penetrate deeper into the wood mass. The depth of colour changes of some species alder, poplar and maple could not be measured after 12 months of exposure by chosen methodology due to the presence of blue-stain fungi growth in the volume of tested specimens.

The penetration of UV radiation depends both on the wood density and the wavelengths distribution, denser wood is penetrated less by UV radiation and shorter wavelengths also penetrate less Rowell The correlation between total colour change and the depth of colour change during 6 months of natural weathering of nine wood species left ; and during 12 months of natural weathering of six wood species right. The measurements were done at the same points before and after the weathering process see figure 1.

The roughness measurements confirmed previous studies: Any increased roughness can be related to natural weathering and degradation of the wood polymers especially lignin and removed degrading materials due to leaching after rain, mist or dew Feist The relatively big differences in roughness values can be explained by the surface erosion occurring mainly in the early wood, due to its thin walled cells and low density Williams et al.

As the result, the early wood is more and faster degraded Tolvaj et al. The mould growth was first observed at maple, poplar and alder samples after the fourth month of weathering. After 6 months of weathering, wood samples mostly Douglas fir wood showed formation of numerous cracks which basically ran in the direction of wood fibres. An appearance of the samples after 12 months of outdoor exposure is demonstrated in figure 6.

Mould and blue-stain fungi growth was observed in those species that are not durable against fungal attack EN Faster attack of biotic factors was caused by higher mean temperature and the total precipitation after the fourth month of weathering table 2. Colour changes of wood samples during 12 months of exposure from up to down: The results of this experiment describe surface degradation process of softwoods and hardwoods during 12 months natural weathering in the Czech Republic Central Europe.

It was confirmed that the surface properties of wood such as roughness, colour and formation of cracks are affected by the specific climatic conditions already during initial stage of weathering. But structural differences of wood are still the most important factors which affect weathering characteristics as concluded in the study of other authors. All the colour changes were fast and visible in a very short time about 1 month for all tested species.

Also information about the depth of these colour changes of tested wood species, which can occur during stocking or longer production time of untreated construction, is given. The roughness values increased during outdoor exposure for all tested wood species, the highest was observed for spruce samples high difference between density of early wood and late wood. Mould and blue-stain fungi growth was observed in those wood species that are not durable against fungal attack maple, alder and poplar.

Douglas fir was determined as the most liable to formation of the cracks. A basis for service life prediction of wood and wood-based products. Wood Material Science and Engineering 1: Czech University of Life Scences.

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Determination of the density]. Enhancing exterior performance of clear coatings through photostabilization of wood. Journal of Coatings Technology and Research 5 2: Durability of wood and wood-based products. Natural durability of solid wood, Part 2: Guide to natural durability and treatability of selected wood species of importance in Europe. European Committee for Standardization, Brussels. Coating materials and coating system for exterior wood, Part 3: Geometrical product specifications GPS.

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Terms, definitions and surface texture parameters. International Organization for Standardization, Geneva. A parent experiences a feeling of competence and positive self-esteem. A child moves another step closer to development of a healthy sense of self. Life, not just childhood, is full of disappointments.

The good-enough mother, another Winnicott term, does not insulate and protect her child from life's struggles. Again quoting from my book: She reflects their experience and contains their distress in a manner appropriate to their level of development.

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She holds them in mind through the difficult times. In doing so she gives her children the tools of empathy, flexibility, and resilience, a secure base from which to become an effective adult. Originally published on the blog Child in Mind. Sorry, we could not find your e-mail or password.

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