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Pregled bibliografske jedinice broj: 531705

Activation spectra in photodegradation of wood


Živković, Vjekoslav
Activation spectra in photodegradation of wood, 2011., doktorska disertacija, Šumarski fakultet, Zagreb


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Naslov
Activation spectra in photodegradation of wood

Autori
Živković, Vjekoslav

Vrsta, podvrsta i kategorija rada
Ocjenski radovi, doktorska disertacija

Fakultet
Šumarski fakultet

Mjesto
Zagreb

Datum
14.10

Godina
2011

Stranica
XV + 198

Mentor
Turkulin, Hrvoje

Ključne riječi
wood photodegradation; activation spectra; colour; microtensile strength; chemical changes

Sažetak
The photodegradation of wood is essentially a surface phenomenon and even though it has very small – if any - effect on the mechanical properties of wood, it has a great influence on its aesthetical properties and maintenance requirements. Wood protected with clear or lightly pigmented coatings in general has poor surface stability under outdoor weather exposure. Studies of wood weathering phenomena are numerous, and principle breakdown processes during exposures are generally known. However, results obtained using different methods are incoherent and dependent on the particularities of the testing method applied. In order to better understand the processes which happen on and under the wood surface during weathering as well as to identify the most influential wavebands of light causing photodegradation of wood, several weathering tests (artificial and natural) were performed. Batches of thin strips of fir wood, assembled in packs of matching three strips (thus forming a 240 m thick surface layer), were exposed to irradiation behind a set of cut-on glass filters that transmitted selected regions of the UV and visible spectrum. Weathering regimes included different light sources (xenon light, UV fluorescent light and natural sunlight) and various levels of climatic conditions (temperature and relative humidity of air). Disassembling the pack of three strips after withdrawals from exposure enabled monitoring of the relevant changes in each particular strip i.e. in the depth of the wood surface during weathering. After weathering the strips were suscepted to measurements of colour, microtensile strength and chemical changes using FTIR spectroscopy. Microtensile testing was executed at zero span of grips (reflecting cellulose changes) and at 10 mm span (reflecting properties of the lignin-rich matrix). Studies of changes of colour, mechanical integrity and chemical composition of wood were shown to follow different patterns of order of appearance and intensity. Depth profiles of those changes develop in time and are specific to observed properties. Studies of activation spectra of wood indicated relatively narrow waveband of most active wavelengths in causing greatest proportion of all recorded damages (360 – 435 nm). Visible light of wavelengths up to 510 nm was shown to contribute significantly to surface damaging. With the duration of exposure the colour changes shift ever more into the depth and into the visible region of the spectrum. Strength and chemical changes are more regularly depth-profiled than colour changes. The amount of strength loss is evident below 80 μm, and the changes in deeper layers exhibit gradual reduction. The mechanical disintegration at prolonged exposures can also be attributed to the wavelengths in the visible region, but to a very small extent. The whole spectrum of wavelengths employed in this work actively contributes to degradation of wood surface, particularly to discoloration. In the spectral region below 335 nm the contribution of wavebands to the overall colour changes is very small. This is an important finding assuming that the former habitual knowledge implicitely included that the actinic effect of light progressively increased with reduction in wavelength. Shorter wavelengths (295 to 317 nm) seem to contribute to both zero and finite span strength losses during the complete exposure. However, since there is very little fluctuation of strength values below 360 nm, this region is obviously not predominantly influential for mechanical properties. The spectral region that contributes the most to strength changes is defined by inclusion of light between 360 and 435 nm. Longer wavelengths appear to contribute to strength losses of top strips, but to a little extent and only in final stages of exposure. All the methods employed in studies of wood photodegradation have their limitations and give different indications as to the development of degrading processes. Only the comparisons of different methods give a complete image of the process. Colour and strength develop their changes in different chronological orders and different depth profiles. If exposed to full spectrum of sunlight top strips rapidly change colour at the beginning of exposure, while the underlaying strips develop discoloration later in the process. Strength changes, on contrary, start with some delay in the process but subsequently increase in continuous manner during exposure. The high emission of QUV lamps in most active wavelength bands causes intensive acceleration of development of damage of top surface layers. However, the low emission in visible spectrum of the QUV lamps resulted in small changes in deeper layers (middle and bottom strips). This will render a Xenotest generally better, though slower and more expensive accelerating weathering machine than QUV. Efficient protection of wood from light would demand screening not only UV light, but also some violet and blue light wavebands. The UV protection must be efficient over prolonged periods of time in order to secure long-lasting stability of wood surface.

Izvorni jezik
Engleski

Znanstvena područja
Drvna tehnologija, Biotehnologija



POVEZANOST RADA


Projekti:
068-0682109-2096 - Oplemenjivanje i modifikacija površine drva (Jirouš-Rajković, Vlatka, MZOS ) ( POIROT)

Ustanove:
Šumarski fakultet, Zagreb

Profili:

Avatar Url Vjekoslav Živković (autor)

Avatar Url Hrvoje Turkulin (mentor)

Citiraj ovu publikaciju

Živković, Vjekoslav
Activation spectra in photodegradation of wood, 2011., doktorska disertacija, Šumarski fakultet, Zagreb
Živković, V. (2011) 'Activation spectra in photodegradation of wood', doktorska disertacija, Šumarski fakultet, Zagreb.
@phdthesis{phdthesis, author = {\v{Z}ivkovi\'{c}, V.}, year = {2011}, pages = {XV + 198}, keywords = {wood photodegradation, activation spectra, colour, microtensile strength, chemical changes}, title = {Activation spectra in photodegradation of wood}, keyword = {wood photodegradation, activation spectra, colour, microtensile strength, chemical changes}, publisherplace = {Zagreb} }




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