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Wettability of modified wood
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. SP Technical Research Institute of Sweden.ORCID iD: 0000-0003-0016-3398
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Despite many excellent properties of wood which make it suitable for many applications, it suffers from a number of disadvantages limiting its use. For instance, modification is needed to reduce water sorption and to improve decay resistance, dimensional stability and weathering performance. In addition, wood/liquid interaction such as water wettability on wood plays an important role in design and characteristics of many processes and phenomena such as adhesion, coating, waterproofing, wood chemical modification, and weathering. This thesis focuses on enhancing the understanding of wetting of wood, with emphasis on modified wood. The influence of surface chemical composition of wood and its microstructural characteristics on wetting and swelling properties has also been studied.

A multicycle Wilhelmy plate technique has been developed to evaluate wetting properties of porous materials, such as wood, in which the samples were subjected to repeated immersions and withdrawals in a swelling liquid (water) and in a non-swelling liquid (octane). This method was utilized to dynamically investigate contact angle, sorption and swelling properties, as well as dimensional stability of unmodified, chemically and surface modified wood samples. Scots pine sapwood and heartwood samples were utilized to establish the principles of the technique. Acetylated and furfurylated wood samples with different level of modification were thereafter examined utilizing the developed technique for wetting measurements. A perimeter model based on a linear combination of the measured force and final change in sample perimeter was suggested to evaluate the dynamic dimensional stability of wood veneers. The feasibility of this method for studying dynamic wettability was investigated by measuring the changes of advancing and receding contact angles over repeated cycles on surface modified wood samples, created by combining liquid flame spray and plasma polymerisation methods. X-ray photoelectron spectroscopy (XPS) and X-ray computed tomography (XCT) were employed to study the surface chemical composition and microstructural properties of the samples, respectively.

Three different kinetic regimes were observed in the wetting measurements: i) fast wetting and spreading of the liquid on the wood surface, ii) void filling and wicking and iii) swelling, which was the slowest of the three. The multicycle Wilhelmy plate method was found to be suitable for studying liquid penetration, sorption, and dimensional stability of swelling materials. The results demonstrate that the wetting properties of wood are highly affected by surface chemistry and microstructure. It was shown that using both swelling and non-swelling liquids in wetting measurements allow to distinguish between capillary liquid uptake and swelling. Based on this, for chemically modified samples, it was demonstrated that acetylation mostly reduces swelling, while furfurylation reduces both swelling and capillary uptake. This is in line with the microstructural study with X-ray computed tomography where a significant change in the porosity was found as a result of furfurylation, conversely acetylation left the total porosity values unchanged. Wetting results for hydrophobised wood samples demonstrate that the multi-scale roughness obtained by combination of nanoparticle coating and plasma polymerization increased both the hydrophobicity and the forced wetting durability compared to the micro-scale roughness found on wood modified with plasma polymerisation alone.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , 89 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2015:56
Keyword [en]
Wood, dimensional stability, dynamic wettability, surface chemistry, microstructure, swelling, multicycle Wilhelmy plate method, contact angle, sorption, acetylation, furfurylation, surface modification
National Category
Materials Chemistry
Research subject
Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-175875ISBN: 978-91-7595-707-4 (print)OAI: oai:DiVA.org:kth-175875DiVA: diva2:863723
Public defence
2015-11-20, Konferencerummet nr 3, SP AB, Drottning Kristinasväg 45, Stockholm, 10:00 (English)
Opponent
Supervisors
Projects
Sustainable wood modification
Note

QC 20151029

Available from: 2015-10-29 Created: 2015-10-23 Last updated: 2015-11-18Bibliographically approved
List of papers
1. Wettability and liquid sorption of wood investigated by Wilhelmy plate method
Open this publication in new window or tab >>Wettability and liquid sorption of wood investigated by Wilhelmy plate method
2014 (English)In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 48, no 1, 161-176 p.Article in journal (Refereed) Published
Abstract [en]

The wettability of Scots pine veneers was investigated with different approaches using the Wilhelmy plate method. The probe liquids were water and octane, which differ; in that, water is able to swell the wood sample, whereas octane does not. Novel approaches based on the Wilhelmy plate method to study wettability, liquid penetration, and swelling behavior of wood veneers are introduced. First, immersion to constant depth was performed, and liquid uptake with time was evaluated. Different kinetic regimes, the fastest one associated with contact angle changes and the slowest regime associated with liquid sorption by capillary and diffusion, were observed. Two other approaches, imbibition at constant depth (with initial deeper immersion) and full immersion, were utilized in order to keep the contact angle constant during measurements. Dynamic wettability studies were done by a multi-cycle (10-20 cycles) Wilhelmy method. Based on this, the time-dependent swelling of wood and changes in sample perimeter could be obtained. Generally, water showed higher absorption than octane. In all wettability studies, and for both probe liquids, the penetration process starts with a fast initial sorption, which is followed by swelling in the case of water.

Keyword
Acid-Base Properties, Surface Wettability, Dynamic Wettability, Heat-Treatment, Part 1, Angle, Adhesion, Fibers, Energy, Panels
National Category
Wood Science
Identifiers
urn:nbn:se:kth:diva-142522 (URN)10.1007/s00226-013-0592-1 (DOI)000330626100015 ()2-s2.0-84893668704 (Scopus ID)
Note

QC 20140306

Available from: 2014-03-06 Created: 2014-03-06 Last updated: 2017-12-05Bibliographically approved
2. Multicycle wilhelmy plate method for wetting properties, swelling and liquid sorption of wood
Open this publication in new window or tab >>Multicycle wilhelmy plate method for wetting properties, swelling and liquid sorption of wood
2013 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 29, no 39, 12145-12153 p.Article in journal (Refereed) Published
Abstract [en]

A multicycle Wilhelmy plate method has been developed to investigate wetting properties, liquid sorption, and swelling of porous substrates such as wood. The use of the method is exemplified by studies of wood veneers of Scots pine sapwood and heartwood, which were subjected to repeated immersion and withdrawal in a swelling liquid (water) and in a nonswelling liquid (octane). The swelling liquid changes the sample dimensions during measurements, in particular its perimeter. This, in turn, influences the force registered. A model based on a linear combination of the measured force and final change in sample perimeter is suggested, and validated to elucidate the dynamic perimeter change of wood veneer samples. We show that pine heartwood and pine sapwood differ in several respects in their interaction with water. Pine heartwood showed (i) lower liquid uptake, (ii) lower swelling, (iii) higher contact angle, and (iv) lower level of dissolution of surface active components (extractives) than pine sapwood. We conclude that the method is also suitable for studying wetting properties of other porous and swellable materials. The wettability results were supported by surface chemical analysis using X-ray photoelectron spectroscopy, showing higher extractives and lignin content on heartwood than on sapwood surfaces.

Keyword
Lignin contents, Linear combinations, Liquid sorption, Porous substrates, Surface active components, Surface chemical analysis, Swelling liquid, Wetting property
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-133192 (URN)10.1021/la402605q (DOI)000330148800010 ()2-s2.0-84884998170 (Scopus ID)
Note

QC 20131028

Available from: 2013-10-28 Created: 2013-10-28 Last updated: 2017-12-06Bibliographically approved
3. Wettability and swelling of acetylated and furfurylated wood analyzed by multicycle Wilhelmy plate method
Open this publication in new window or tab >>Wettability and swelling of acetylated and furfurylated wood analyzed by multicycle Wilhelmy plate method
2016 (English)In: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, Vol. 70, no 1, 69-77 p.Article in journal (Refereed) Published
Abstract [en]

The wetting, dimensional stability and sorption properties of a range of modified wood samples obtained either by acetylation or furfurylation were compared with those of unmodified samples of the same wood species via a multicycle Wilhelmy plate method. Wettability measurements were performed with water and octane as the swelling and non-swelling liquids, respectively. It was found that acetylation reduces water uptake mainly by reducing the swelling. In comparison, furfurylation reduces both swelling and the void volume in the sample. To quantify the effect of the modification process of the wood properties, the parameters “liquid up-take reduction” and the “perimeter change reduction” were introduced, which were determined from multicycle Wilhelmy plate measurements. Compared with the acetylated wood, the furfurylated wood with a higher level of weight percent gain exhibited larger property changes on the surface and in terms of swelling and sorption properties.

Place, publisher, year, edition, pages
De Gruyter, 2016
Keyword
acetylation, contact angle (CA), furfurylation, multicycle Wilhelmy plate method, swelling, wettability, wood
National Category
Wood Science
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-175870 (URN)10.1515/hf-2014-0196 (DOI)000367822500009 ()2-s2.0-84954284884 (Scopus ID)
Projects
Sustainable wood modification
Note

QC 20160203

Available from: 2015-10-23 Created: 2015-10-23 Last updated: 2017-12-01Bibliographically approved
4. Wetting hysteresis induced by temperature changes: supercooled water onhydrophobic surfaces
Open this publication in new window or tab >>Wetting hysteresis induced by temperature changes: supercooled water onhydrophobic surfaces
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The state and stability of supercooled water on (super)hydrophobic surfaces is crucial for low temperature applications and for obtaining anti-icing and de-icing properties. Surface characteristics such as topography and chemistry are expected to affect wetting hysteresis during temperature cycling experiments, and also the freezing delay of supercooled water. We utilized stochastically rough wood surfaces that were further modified to render them hydrophobic or superhydrophobic. Liquid flame spraying (LFS) was utilized to create a multi-scale roughness by depositing titaniumdioxide nanoparticles. The coating was subsequently made non-polar by applying a thin plasma polymer layer. As flat reference samples modified silica surfaces with similar chemistries were utilized. With these sets of surfaces we test the hypothesis that superhydrophobic surfaces also should retard ice formation. Wetting hysteresis was evaluated using contact angle measurements during a freeze-thaw cycle from room temperature to freezing occurrence at -7 °C, and then back to room temperature. Further, the delay in freezing of supercooled water droplets was studied at temperatures of -4 °C and -7 °C. The hysteresis in contact angle observed during a cooling-heating cycle is found to be small on flat hydrophobic surfaces. However, significant changes in contact angles during a cooling-heating cycle are observed on the rough surfaces, with a higher contact angle observed on cooling compared to during the subsequent heating. This hysteresis is lower for hydrophobic wood samples with multi-scale roughness compared to those with predominantly micro-scale features. Condensation and subsequent frost formation at sub-zero temperatures induce the hysteresis. The freezing delay data suggests that the multi-scale roughness reduces the penetration of supercooled water into surface depressions, and enhances the freezing delay at low degrees of supercooling. However, the flat surface is even more efficient in enhancing the freezing delay than the rougher surfaces, which can be rationalized considering heterogeneous nucleation theory. Thus, our data suggests that molecular flat surfaces, rather than rough superhydrophobic surfaces, are beneficial for retarding ice formation under conditions that allow condensation and frost formation to occur.

Keyword
wetting hysteresis, contact angle, supercooled water, morphology, hydrophobization, multi-scale roughness, wood, superhydrophobicity, liquid flame spray (LFS), plasma polymerization
National Category
Physical Chemistry
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-175872 (URN)
Note

QS 2015

Available from: 2015-10-23 Created: 2015-10-23 Last updated: 2015-12-09Bibliographically approved
5. Hydrophobisation of wood surfaces by combining liquid flame spray (LFS)and plasma treatment: dynamic wetting properties
Open this publication in new window or tab >>Hydrophobisation of wood surfaces by combining liquid flame spray (LFS)and plasma treatment: dynamic wetting properties
Show others...
2016 (English)In: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, Vol. 70, no 6, 527-537 p.Article in journal (Refereed) Published
Abstract [en]

The hydrophilic nature of wood surfaces is a major cause for water uptake and subsequent biological degradation and dimensional changes. In the present paper, a thin transparent superhydrophobic layer on pine veneer surfaces has been created for controlling surface wettability and water repellency. This effect was achieved by means of the liquid flame spray (LFS) technique, in the course of which nanoparticulate titanium dioxide (TiO2) was brought to the surface, followed by plasma polymerisation. Plasma polymerised perfluorohexane (PFH) or hexamethyldisiloxane (HMDSO) were then deposited onto the LFS-treated wood surfaces. The same treatment systems were applied to silicon wafers so as to have well-defined reference surfaces. The dynamic wettability was studied by the multicycle Wilhelmy plate method, resulting in advancing and receding contact angles as well as sorption behaviour of the samples during repeated wetting cycles in water. Atomic force microscopy (AFM) and Xray photoelectron spectroscopy (XPS) were employed to characterise the topography and surface chemical compositions and to elucidate the question how the morphology of the nanoparticles and plasma affect the wetting behaviour. A multi-scale roughness (micro-nano roughness) was found and this enhanced the forced wetting durability via a superhydrophobic effect on the surface, which was stable even after repeated wetting cycles. The hydrophobic effect of this approach was higher compared to that of plasma modified surfaces with their micro-scale modification.

Place, publisher, year, edition, pages
Walter de Gruyter, 2016
Keyword
contact angle (CA); dynamic wetting, hydrophobisation; hexamethyldisiloxane (HMDSO); liquid flame spray (LFS); multi-scale roughness; nano-sized metal oxide (TiO2); perfluorohexane (PFH); plasma polymerisation; superhydrophobicity; Wilhelmy plate method; wood.
National Category
Wood Science
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-175873 (URN)10.1515/hf-2015-0148 (DOI)000376680300006 ()2-s2.0-84973442118 (Scopus ID)
Funder
Swedish Research Council Formas, 2014-172
Note

QC 20161101

Available from: 2015-10-23 Created: 2015-10-23 Last updated: 2017-12-01Bibliographically approved
6. Microstructure of chemically modified wood using X-ray computedtomography scanning in relation to wetting properties
Open this publication in new window or tab >>Microstructure of chemically modified wood using X-ray computedtomography scanning in relation to wetting properties
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

X-ray computed tomography was utilized to visualize and quantify the 2D and 3D microstructure of acetylated and furfurylated southern yellow pine (SYP) and maple samples. The total porosity and the porosity of different anatomical components, the cell wall thickness and the maximum opening of tracheid lumens were evaluated. The wetting properties (swelling and capillary uptake) were related to these microstructural characteristics. Our data show significant changes in the wood structure for furfurylated sapwood samples, including a change in tracheid shape and filling of tracheids by furan polymer. In contrast, no such changes were noted for acetylated samples at the resolution of the measurement (0.8 μm). The images obtained for furfurylated maple samples demonstrated that all ray cells and some vessel elements were filled with furan polymer while the fibres largely remained unchanged. Furfurylation significantly decreased the porosity of the sample, and this was observed in both earlywood and latewood regions in southern yellow pine softwood samples. In contrast, the total porosity of this softwood sample was hardly affected by acetylation. These findings are in line with wetting results demonstrating that furfurylation reduces both swelling and capillary uptake in contrast to acetylation which reduces mostly swelling. Cell wall thickness measurements revealed a significant increase after chemical modification especially at higher levels of furfurylation.

Keyword
acetylation, cell wall thickness, furfurylation, hardwood, microstructure, porosity, softwood, tracheid, wettability, sub-micron X-ray Computed Tomography
National Category
Wood Science
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-175874 (URN)
Note

QS 2015

Available from: 2015-10-23 Created: 2015-10-23 Last updated: 2015-10-29Bibliographically approved

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