Surface modification of solid wood using different techniques
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School of Chemical Technology |
Doctoral thesis (article-based)
| Defence date: 2012-02-24
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Author
Date
2012
Major/Subject
Mcode
Degree programme
Language
en
Pages
126
Series
Aalto University publication series DOCTORAL DISSERTATIONS, 4/2012, 14/2012
Abstract
The rising demand for consistent, high quality, as well as high density wood species has raised interest in different wood modification techniques. One modification method is wood densification, where solid bulk wood can be compressed, following softening, until its density reaches that of the cell wall (1.50 g/cm3). It is well known that many of the properties of solid wood correlate with its density and as such can be enhanced by increasing the density. By targeting the compression to the surface of solid wood only, the surface properties can be enhanced. The main aim of this thesis was to evaluate the effect of different wood surface densification methods on surface properties. Three methods were evaluated; (i) a lamination technique, where low density wood was laminated to a high density compressed wood surface, (ii) surface modification where densification was accomplished using a single sided heated press and (iii) with a single sided heated press assisted by frictional heating. With the lamination technique, it proved easy to generate the desired hard surface, but an adhesive is needed. The difference between compression with and without friction is that when using friction the process temperature is rather difficult to control, but an even layer of extractives are formed on the surface. Without friction, however, extractive spots are seen on the surface. The results show that the process parameters used in surface densification have a significant influence on the vertical density profiles, which in turn have an influence on surface hardness. On the other hand hardness correlates with the degree of densification and the thickness of the densified surface. Moreover, wettability decreased significantly, caused by the closure of lumens, a smooth surface and an extractive layer on the surface. The extractives on the surface were analysed by FTIR-ATR spectroscopy and X-ray photoelectron spectroscopy. Using FTIR-ATR spectroscopy there was no trace of extractives, but with X-ray photoelectron spectroscopy an extractive layer was identified, this was most probably due to the penetration depth of the IR-beam, which is rather high, compared to X-ray photoelectron spectroscopy, which is highly surface sensitive. The extractive layer could provide a natural coating and reduce the need for further coating. Furthermore, a potential measure error was found when measuring the vertical density profile of composites with large differences in densities, caused by the calibration process.Tasa- ja korkealaatuisen sekä tiheän puumateriaalin kasvava kysyntä on lisännyt kiinnostusta puun eri modifiointitekniikoihin. Eräs modifiointimenetelmistä on puun puristaminen, jossa pehmennettyä massiivipuuta voidaan puristaa kasaan kunnes soluseinän tiheys (1.50 g/cm3) on saavutettu. Monet massiivipuun ominaisuudet ovat yhteydessä tiheyteen ja siksi niitä voidaan parantaa kasvattamalla tiheyttä. Kohdentamalla puristus vain massiivipuun pintakerrokseen voidaan pinnan ominaisuuksia parantaa. Työn päätavoitteena oli arvioida erilaisia puun pinnan puristusmenetelmiä ja niiden vaikutuksia pinnan ominaisuuksiin. Kolme eri menetelmää arvioitiin: (i) laminointitekniikka, jossa puristettu puu laminoitiin alhaisen tiheyden omaavan puun pinnalle, (ii) pinnan modifiointi yhdeltä puolelta lämmitetyllä puristimella ja (iii) yhdeltä puolelta lämmitetyn puristimen ja kitkan avulla. Laminointitekniikalla saatiin halutunlainen kova pinta, mutta liimaa tarvitaan. Kitkan käyttö puristuksessa eroaa siten, että kitkaa käytettäessä prosessilämpötilaa on melko vaikea hallita, mutta pinnalle muodostuu tasainen uuteainekerros. Puristettaessa ilman kitkaa pinnalla näkyy uuteainetäpliä. Tulokset osoittavat, että pintapuristuksessa prosessiparametreilla on merkittävä vaikutus tiheysprofiiliin, joka taas vaikuttaa pinnan kovuuteen. Toisaalta kovuus korreloi puristusasteen ja tiivistetyn pinnan paksuuden kanssa. Lisäksi pinnan kostuvuus vähentyi merkittävästi, johtuen suljetuista soluonteloista, sileästä pinnasta ja pinnalle muodostuneesta uuteainekerroksesta. Uuteainekerrosta analysoitiin FTIR-ATR spektroskopialla ja röntgenfotoelektronispektroskopialla. FTIR-ATR spektroskopialla ei havaittu uuteaineita, mutta röntgenfotoelektronispektroskopialla uuteainekerros havaittiin, mikä hyvin todennäköisesti johtui IR-säteen mittaussyvyydestä, joka on melko korkea verrattuna erittäin pintaherkkään röntgenfotoelektronispektroskopiaan. Muodostunut uuteainekerros voisi tarjota luonnollisen pinnoitteen, joten muuta pintakäsittelyä voisi vähentää. Lisäksi havaittiin mahdollinen laitteen kalibroinnista johtuva mittausvirhe, kun mitattiin tiheysprofiilia korkeita tiheyseroja omaavasta komposiitista.Description
Supervising professor
Hughes, Mark, Professor, Aalto University, FinlandThesis advisor
Hughes, Mark, Professor, Aalto University, FinlandKamke, Frederick, Professor, Oregon State University, USA
Keywords
densification, surface densification, surface modification, wood modification, puun modifiointi, pinnan modifiointi, puun puristaminen
Other note
Parts
- [Publication 1]: Lauri Rautkari, Frederick A. Kamke, and Mark Hughes. 2011. Density profile relation to hardness of viscoelastic thermal compressed (VTC) wood composite. Wood Science and Technology, volume 45, number 4, pages 693-705.
- [Publication 2]: Lauri Rautkari, Frederick A. Kamke, and Mark Hughes. 2011. Potential error in density profile measurements for wood composites. European Journal of Wood and Wood Products, volume 69, number 1, pages 167-169.
- [Publication 3]: Lauri Rautkari, Kristiina Laine, Nick Laflin, and Mark Hughes. 2011. Surface modification of Scots pine: the effect of process parameters on the through thickness density profile. Journal of Materials Science, volume 46, number 14, pages 4780-4786.
- [Publication 4]: Lauri Rautkari, Milena Properzi, Frédéric Pichelin, and Mark Hughes. 2009. Surface modification of wood using friction. Wood Science and Technology, volume 43, numbers 3-4, pages 291-299.
- [Publication 5]: Lauri Rautkari, Milena Properzi, Frédéric Pichelin, and Mark Hughes. 2010. Properties and set-recovery of surface densified Norway spruce and European beech. Wood Science and Technology, volume 44, number 4, pages 679-691.
- [Publication 6]: Lauri Rautkari, Tuomas Hänninen, Leena-Sisko Johansson, and Mark Hughes. 2012. A study by X-ray photoelectron spectroscopy (XPS) of the chemistry of the surface of Scots pine (Pinus sylvestris L.) modified by friction. Holzforschung, volume 66, number 1, pages 93-96.
