licentiate thesis in civil and architectural engineering licenta ...1499276/... · 2020. 11. 8. ·...

Licentiate Thesis in Civil and Architectural Engineering

Surface-modified wood based on silicone nanofilaments for improved liquid repellence HAIYAN YIN

Stockholm, Sweden 2020www.kth.se

ISBN 978-91-7873-692-8TRITA-ABE-DLT-2038

kth royal institute of technology

Haiyan yin Surface-m

odified wood based on silicone nanofilam

ents for improved liquid repellence

KTH

2020

Surface-modified wood based on silicone nanofilaments for improved liquid repellence

HAIYAN YIN

Academic Dissertation which, with due permission of the KTH Royal Institue of Technology,

is submitted for public defence for the Degree of Licentiate of Engineering on Wednesday the

9th December 2020, at 10:00 a.m. in U1, Brinellvägen 28A, Stockholm.

Licentiate Thesis in Civil and Architectural Engineering

with specialization in Building Materials

KTH Royal Institute of Technology

Stockholm, Sweden 2020

© Haiyan Yin

ISBN: 978-91-7873-692-8

TRITA-ABE-DLT-2038

Cover page photo: Maziar Sedighi Moghaddam, Haiyan Yin

Printed by: Universitetsservice US-AB, Sweden 2020

i

Abstract

The increasing awareness of sustainability motivates the development of

building materials from renewable resources. The requirements of wood-

based products with improved durability, for example, an enhanced liquid

repellence, is still a challenge. The aim of this thesis is to develop and study

concepts to functionalize wood surfaces to obtain superhydrophobicity or

superamphiphobicity, i.e. extreme liquid repellence of both water and oils.

Birch and acetylated birch veneer samples were surface-modified by

hydrophobized silicone nanofilaments. Specifically, birch samples surface-

modified by fluorinated silicone nanofilaments (F-SMB) showed

superamphiphobicity, which repelled water, ethylene glycol and

hexadecane with static contact angles greater than 150° and roll-off angles

lower than 10°. Birch and acetylated birch samples surface-modified by

non-fluorinated silicone nanofilaments (SMB and SMAB) showed

superhydrophobicity with static contact angles greater than 160° towards

water, even for samples prepared using the shortest silicone nanofilaments

reaction time of 1 h.

In liquid uptake measurements submerging the F-SMB in water,

ethylene glycol and hexadecane, a superamphiphobic plastron effect was

observed which indicates that the wood surface was in Cassie-Baxter state.

The plastron reduced the liquid uptake rate and extent depending on the

interactions (diffusion and solubility) between the liquid and the silicone

nanofilaments. The F-SMB showed good self-cleaning properties towards

water and hexadecane.

In multicycle Wilhelmy plate measurements, the SMB showed a lower

water uptake than that of the acetylated samples, while the SMAB showed

the lowest water uptake, i.e. a pronounced increased water resistance, due

to a combined effect of acetylation and surface modification.

ii

In addition, the SMB exhibited more color change than the SMAB,

which was caused by the release of hydrochloric acid during the surface

modification process.

Keywords

Wood, acetylated wood, surface modification, superamphiphobicity,

superhydrophobicity, plastron, silicone nanofilaments, multicycle

Wilhelmy plate method.

iii

Sammanfattning

Den ökande medvetenheten kring hållbar samhällsutveckling motiverar

utveckling av byggmaterial från förnybara resurser. Kraven på träbaserade

produkter med förbättrad beständighet, exempelvis en nödvändig

vätskeavvisande förmåga, är fortfarande en utmaning. Syftet med denna

avhandling är att utveckla och studera koncept för att funktionalisera

träytor för att uppnå superhydrofobicitet och superamfifobicitet, dvs en

extrem vätskerepellerande egenskap för både vatten och oljor.

Björk- och acetylerade björkfanérprover ytmodifierades med

hydrofobiserad silikon-nanofilament. Specifikt visade björk ytmodifierade

med fluorerade silikon-nanofilament (F-SMB) superamfifobicitet, som

repellerade vatten, etylenglykol och hexadekan med kontaktvinklar större

än 150° och avrullningsvinklar lägre än 10°. Björk- och acetylerade

björkprover ytmodifierade med icke-fluorerade silikon-nanofilament

(SMB och SMAB) visade superhydrofobicitet med kontaktvinklar större än

160° för vatten, även för prover framställda med kortast reaktionstid på 1

timme.

Vid vätskeupptagningsmätningar genom att sänka F-SMB i vatten,

etylenglykol och hexadekan observerades en plastroneffekt som indikerade

att träytan var i Cassie-Baxter-tillstånd. Plastronen minskade F-SMB

vätskeupptagningshastighet och -nivå beroende på växelverkan (diffusion

och löslighet) mellan vätskan och silikon-nanofilament. F-SMB uppvisade

goda självrengörande egenskaper för vatten och hexadekan.

Vid multicykel Wilhelmy-mätningar visade SMB ett lägre vattenupptag

än det acetylerade träet, medan SMAB visade den lägsta

vattenupptagningen, det vill säga en mycket märkbar ökad

vattenavvisning, tack vare av en kombinerad effekt av acetylering och

ytmodifiering.

iv

Dessutom uppvisade SMB en större färgförändring än SMAB, orsakad

av frisättningen av saltsyra under ytmodifieringsprocessen.

Nyckelord

Trä, acetylerat trä, ytmodifiering, superamfifobicitet, superhydrofob,

plastron, silikon-nanofilament, multicykel Wilhelmy-metod

v

Preface

The work presented in this licentiate thesis was carried out at KTH

Department of Civil and Architectural Engineering, Division of Building

Materials and RISE Research Institutes of Sweden, Division Bioeconomy

and Health, Department of Material and Surface Design. The work has

been financed by the Swedish Research Council FORMAS (grant no. 2016-

01362), which is greatly acknowledged. The project was also supported by

the Vinnova project 2017-02712 “Bärande utomhusträ” within the

BioInnovation program.

First of all, I would like to sincerely thank my supervisor, Prof. Agne

Swerin, for this opportunity and the guidance and support in the research.

I am also very grateful for my main supervisor, Prof. Magnus Wålinder, for

leading me to the wood science world and for his contribution to my study

at KTH and my research work.

I would like to express my gratitude to my co-supervisors at RISE, Dr.

Maziar Sedighi Moghaddam and Dr. Mikko Tuominen, for all the help in

labs and all the valuable scientific discussions. My co-supervisor, Dr. Andra

Dėdinaitė is thanked for her support of the project.

I am also grateful for my colleagues at RISE and KTH, for their supports

and help. At last, I would like to thank all my friends and family members,

especially my husband Xinfeng, for their supports.

Stockholm, October 2020

Haiyan Yin

vii

List of appended papers

This doctoral thesis is based upon the following scientific articles:

Paper І

Yin, H., Sedighi Moghaddam, M., Tuominen, M., Eriksson, M., Järn, M.,

Dėdinaitė, A., Wålinder, M., Swerin, A. (2020) Superamphiphobic

plastrons on wood and their effects on liquid repellence. Materials &

Design. 195:108974.

Paper ІІ

Yin, H., Sedighi Moghaddam, M., Tuominen, M., Dėdinaitė, A., Wålinder,

M., Swerin, A. Non-fluorine surface modification of acetylated wood for

improved water repellence. (2020). Submitted manuscript.

Contribution to the appended papers

Paper І: First author. Major part of the planning, performed the

experiments and data analysis except for sessile drop measurement (by

Sedighi Moghaddam), wrote major part of the manuscript.

Paper ІІ: First author. All the planning, experimental work and data

analysis, wrote the manuscript with the help of all co-authors.

Nomenclature

Abbreviations

CA Contact angle

CAH Contact angle hysteresis

CIE Commission internationale de L’Eclairage

EDS Energy-dispersive X-ray spectroscopy

F-SMB Fluorine surface-modified birch

FTIR Fourier-transform infrared spectroscopy

PFDTS 1H,1H,2H,2H-Perfluorodecyltrichlorosilane

RH Relative humidity

ROA Roll-off angle

SEM Scanning electron microscopy

SMAB Surface-modified acetylated birch

SMB Surface-modified birch

TCMS Trichloromethylsilane

TCOS Trichloro(octyl)silane

UV Ultraviolet

WPG Weight percentage gain

SiO2 Silicon dioxide

TiO2 Titanium dioxide

ZnO Zinc oxide

Latin and Greek Symbols

a* Redness

b* Yellowness

L* Lightness

ΔE* Total color change

Sq Root mean square average roughness [ μm ]

θa Advancing contact angle [ o ]

θr Receding contact angle [ o ]

m0 Initial mass of wood before sealing [ kg ]

m0＇ Mass of wood after sealing [ kg ]

mi Mass of wood after submersion in liquid [ kg ]

F Force [ N ]

P Perimeter [ m ]

γ Surface tension [ N m-1 ]

θ Contact angle [ o ]

ρ Density [ kg m-3 ]

A Area [ m2 ]

h Immersion depth [ m ]

g Gravitational acceleration [ m s-2 ]

Fw(t) Time-dependent force [ N ]

Fa,n Advancing force at cycle n [ N ]

θa,n Advancing contact angle at cycle n [ o ]

Ff Final force [ N ]

Ff,n Final force at cycle n [ N ]

Fr,n Receding force at cycle n [ N ]

θr,n Receding contact angle at cycle n [ o ]

Wn Water uptake at cycle n [ wt% ]

Contents

1. Introduction ............................................................................. 1

1.1. Context .................................................................................................. 1

1.2. Acetylation ............................................................................................ 1

1.3. Surface modification ............................................................................ 2

1.4. Silicone nanofilaments ......................................................................... 5

1.5. Aims and Objectives ............................................................................ 6

2. Experimental ............................................................................ 7

2.1. Materials ................................................................................................ 7

2.2. Surface modifications .......................................................................... 7 2.2.1. Surface modification in Paper І ................................................................. 7 2.2.2. Surface modification in Paper ІІ ................................................................ 8

2.3. Characterizations ................................................................................ 10 2.3.1. Surface morphology ................................................................................ 10 2.3.2. Chemical compositions ........................................................................... 11 2.3.1. Color change ........................................................................................... 11 2.3.2. Sessile drop measurement/Static contact angles ................................... 11 2.3.3. Liquid uptake ........................................................................................... 12 2.3.4. Multicycle Wilhelmy plate method ........................................................... 13 2.3.5. Self-cleaning ........................................................................................... 15

3. Results and discussion......................................................... 16

3.1. Morphology ......................................................................................... 16 3.1.1. Silicone nanofilaments on fluorine surface-modified birch (F-SMB) ........ 16 3.1.2. Silicone nanofilaments on fluorine-free surface-modified birch and

acetylated birch (SMB and SMAB) ...................................................................... 17

3.2. Surface chemistry ............................................................................... 21

3.3. Color changes ..................................................................................... 22

3.4. Wettability ............................................................................................ 23 3.4.1. Sessile drop measurement ..................................................................... 24 3.4.2. Sample submersion ................................................................................ 27 3.4.3. Wilhelmy plate measurement .................................................................. 31

3.5. Self-cleaning tests .............................................................................. 34

4. Conclusions ........................................................................... 35

5. Future work ............................................................................ 36

References ................................................................................. 37

INTRODUCTION | 1

1. Introduction

1.1. Context

Wood has a long history of being used as a building material due to its good

processability and accessibility, renewability, high strength-to-weight ratio

and aesthetic appearance. It is widely used both in structural applications

such as bridges and building frameworks and in non-load carrying

applications such as façades and window frames. Due to the cellular

structure and hygroscopic and solvoscopic (the ability of a substance to

attract and retain solvent molecules) nature of the wood components,

wood is prone to water and other liquid sorption and swelling, which leads

to dimensional instability and accelerated biological degradation,

especially in outdoor applications [1-7].

The primary motive for this thesis is to develop durable water and oil

repellent wood via smart surface modifications and to study the wettability

of the surface-modified wood. Hydrophobized silicone nanofilaments were

coated on birch and acetylated birch wood surfaces. The wettability of the

silicone nanofilaments coated wood, viz., the static contact angle, the

dynamic contact angles and liquid uptake of wood, was studied using

sessile drop technique, multicycle Wilhelmy plate technique and sample

submersion tests.

1.2. Acetylation

The common method for improving the water resistance, i.e. lowering the

hygroscopicity, of wood is by modification of the bulk wood, which alters

the properties of wood by intervening the cell wall level. Three wood

modification methods used commercially are chemical and thermal

modification, and impregnation with preservatives [2, 8-10]. Chemical

modification of bulk wood via acetylation is one of the most widely used

methods for improving the properties of wood [9, 11-15].

Acetylation is a single-addition chemical reaction, in which the acetic

anhydride reacts with the accessible hydroxyl groups in wood and forms

2 | INTRODUCTION

an ester bond, which covalently bonded to the wood cell walls as shown in

Figure 1 [16].

Figure 1. Schematic acetylation of wood by classical acetic anhydride method.

The acetylation is regarded as a non-biocidal treatment for wood since no

biocides are involved in the process, in contrast to wood protection using

traditional wood preservatives such as copper-based salts. Furthermore,

the acetylated wood shows good moisture/water resistance due to the

blocking of the hydrophilic moieties (the accessible hydroxyl groups) in

wood [17-22]. It is reported that the equilibrium moisture content of

acetylated pine (20.4% weight percentage gain (WPG)) is reduced to 8.4%

at 90% RH, which is 2.6 times lower than that of non-acetylated pine [19].

Moreover, it exhibits improved physical, mechanical and biological

properties [8, 12, 18, 23-26]. For example, Chai et al. [24] report that

acetylated pine (WPG of around 20%) shows an anti-swelling efficiency of

60%-70% under all the selected relative humidity conditions. Bongers et

al. [23] investigated the performance of acetylated pine in marine and fresh

water exposure in Denmark. The result shows that the acetylated wood

exhibits no evidence of attack even after nine years of exposure. In

addition, the acetylated sheet piling is still almost completely intact after

20 years of exposure in fresh water, with only erosion on top of the surface.

Due to these benefits brought by acetylation, interest in using acetylated

wood for improving the performance of wood has grown [11, 14, 23, 25, 27-

30].

1.3. Surface modification

Superhydrophobic surface modification of wood is another solution for

improving the water repellence property of wood. Surface modification is

INTRODUCTION | 3

defined as the act of modifying the surface of a material by bringing

different physical, chemical or biological properties to its original surface

properties (or by bringing new functions on its surfaces) [2]. The most

widely used wood surface modification methods are varnish/paint coating

[31, 32]. However, such surface modification methods can bring undesired

color changes to the products, which affects the aesthetic pleasure of wood-

based products.

Superhydrophobicity has become a well-known research topic in the

past three decades as the functionalized surface can bring potential

applications such as self-cleaning, stain-resistant, anti-icing, anti-fogging,

anti-biofouling, and anti-corrosion coatings, etc. [33-38]. The preparation

of the artificial superhydrophobic surfaces is inspired by the

superhydrophobic and superoleophobic surfaces in nature, e.g., lotus

leaves (Nelumbo nucifera) and springtails (Orthonychiurus stachianus

and Tetrodontophora bielanensis) [34, 39-41]. Figure 2 shows some

superhydrophobic and superoleophobic surfaces in nature and their

relevant microstructures on the surfaces.

Figure 2. Superhydrophobic and superoleophobic surfaces in nature and their relative surface structures in microscale. (a) Lotus leaves show self-cleaning properties, note that dust is accumulated in the water droplet at the center of the leaves, copyright: © 2008 Zhang et al. [34], reproduced with permission from the Royal Society of Chemistry; springtails (b) Tetrodontophora bielanensis in water and (c) Orthonychiurus stachianus immersed in ethanol, copyright: © 2011 Helbig et al. [39], reproduced with permission from PLOS.

4 | INTRODUCTION

It is well established that superhydrophobic and superoleophobic surfaces

can be prepared by combining a low surface energy material with a nano-

and microscale structure or creating a nano- and microscale structure on a

surface with low surface energy chemistry on top [42-44]. The

hydrophobized nano- and microscale structure can trap air/vapor pockets

between the solid substrate, leading to a Cassie-Baxter wetting state on the

surface [45-47]. Such a superhydrophobic/superoleophobic surface

usually shows high static contact angles (>150°) and low roll-off angles

(<10°).

With the rapid development of the research field related to wettability,

various definitions and terminology have been developed regarding the

specific wetting state [48-50]. In this thesis, the superhydrophobic and the

superoleophobic surface refers to a surface that shows static contact angles

greater than 150° towards water and organic liquids with low surface

tension, respectively. The term superamphiphobic refers to a surface that

is both superhydrophobic and superoleophobic.

Wood surfaces show an intrinsic roughness at the microscale due to

their anatomical structures and porosity but are hydrophilic by nature.

Various methods, which combines wood surfaces with nanoparticles like

SiO2, ZnO and TiO2 and low surface energy chemicals, have been developed

for creating superhydrophobicity or superamphiphobicity on wood

surfaces [51-61]. For example, Guo et al. [51] report a superhydrophobic

coating on wood by applying a solution-based surface seeding of ZnO

crystals with a fluorinated organic silane on top. Tu et al. [52] fabricated a

mechanically robust superhydrophobic wood surface by a primer coating

of transparent epoxy resin followed by a fluorinated SiO2 nanoparticle

layer. Tuominen et al. [53] report an overhang structured coating on a

wood surface by depositing TiO2 nanoparticles on the surface using a

thermal spray method followed by plasma polymerization of

perfluorohexane, which shows liquid repellence against water, ethylene

glycol, diiodomethane and olive oil. However, in most of the reported

means for surface modification of wood, the preparation techniques

usually involve complex procedures, expensive and harmful reagents, or

INTRODUCTION | 5

harsh conditions, which limit the possibility to scale up the techniques and

are not applicable from the perspective of sustainable development.

1.4. Silicone nanofilaments

Superhydrophobic silicone nanofilaments coating, since it was first

reported in 2006 [62-64], has been applied on various substrates including

glass slides, silicon wafers, cellulose, polymers and metals, etc. [65-80].

The utilization of silicone nanofilaments to achieve superhydrophobicity

shows many advantages as the reaction can be performed via a simple

solution or gas-phase deposition process at ambient temperature, and the

silicone nanofilaments layer is thermally and chemically stable [81]. Thus,

this approach is promising for preparing a durable and superhydrophobic

wood surface without affecting its aesthetic appearance.

The basic chemistry of the formation of silicone nanofilaments is the

synthesis of polysiloxanes [79, 81, 82]. Reactive organosilanes (RSiX3) with

a hydrolyzable group, e.g., hydride, chloride or alkoxide are used as the

precursor in the reactions. In the synthesis of silicone nanofilaments, the

silane starts by hydrolysis with the presence of water (refer to the relative

humidity in the gas-phase reaction) and forms a trisilanol (Eq. 1). The

trisilanols condensate subsequently and yield a polysiloxane (Eq. 2), which

forms the nanofilamental network. The exact sequence and kinetics of all

possible hydrolysis and condensation reactions is unclear, e.g. homo and

heterofunctional condensation may occur in the synthesis of silicone

nanofilaments [81]. The reactions assuming complete condensation are

summarized:

RSiX3 + 3H2O → RSi(OH)3 + 3HX, (1) RSi(OH)3 → RSiO3/2 + 3/2H2O, (2)

where R is an organic residue (e.g., vinyl or methyl groups), and X is the

hydrolyzable group.

In the synthesis of silicone nanofilaments on substrates like glass slide

and silicon wafer, a pretreatment to activate hydroxyl groups on the

surfaces is needed so that the silicone nanofilaments can be anchored on

6 | INTRODUCTION

the surfaces [64, 83, 84]. The incorporation of silicone nanofilaments

should also be possible on a wood substrate since there are abundant

hydroxyl groups present on the surfaces, which enable the polysiloxanes to

anchor on its surfaces directly. The silicone nanofilaments layer, on the one

hand, can create a nano- and microscale roughness structure on the wood

surfaces; on the other hand, it can replace the hydroxyl groups on the wood

surfaces during the synthesis of the silicone nanofilaments, which

contributes to the hydrophobicity of the surfaces.

1.5. Aims and Objectives

The aim of this thesis is to develop and study means of surface modification

of wood, i.e. to functionalize wood surfaces, primarily by applying

superhydrophobic layers on wood for enhanced water and liquid

repellence. An ambitious aim is to only involve environmentally friendly

components in the surface modification process and to assess how such

modifications can be used in outdoor wood usage, such as for façades or

furniture. Specifically, a surface modification method based on silicone

nanofilaments coating was applied on birch wood surfaces and acetylated

birch wood surfaces.

The specific objectives were:

• Evaluate how the surface micro- and nanostructure and surface

chemistry influence the wetting properties.

• Assess the combination of surface modification and bulk wood

modification for improved liquid repellence and resistance.

• Determine the non-wetting properties of the surface-modified

wood, e.g., the static contact angles, the multicycle Wilhelmy curves

and the liquid uptake.

• Determine the relation between diffusivity and solubility in the

combined permeability in liquid repellent surface modification.

EXPERIMENTAL | 7

2. Experimental

2.1. Materials

The birch (Betula pendula) veneers in Paper І originated from Finland. The

prepared wood samples were in dimensions of approximately 40 × 30 × 1

mm3 in the longitudinal, tangential and radial directions (L × T × R),

respectively. The average moisture content was ca 7%.

The birch veneers in Paper ІІ were provided by the plywood

manufacturer Koskisen Oy in Finland. Half of the received birch veneers

were acetylated by Accsys in the Netherlands according to the process

described on their web page [85]. The acetyl content of the acetylated birch

veneers was ca 20%. The dimensions of the birch and acetylated birch

wood samples used in this work were approximately 30 × 20 × 1 mm3

(L×T×R). The average moisture content was ca 5 and 1.4 %, respectively,

for the unmodified birch and acetylated birch.

2.2. Surface modifications

The surface modification method in Paper І and ІІ follows a similar

protocol, namely the synthesis of silicone nanofilaments on wood surfaces

in gas phase followed by a gas-phase based post-silanization. In Paper І,

the commonly used silicone nanofilaments preparation method [62, 67, 79,

86], which is based on the controlling of relative humidity (RH) conditions,

was utilized. The silicone nanofilaments coated wood samples were further

hydrophobized by a long-chain fluorinated silane. In Paper ІІ, the silicone

nanofilaments were synthesized by adding liquid water at reduced

pressure. The hydrophobization process used a long-chain non-fluorinated

silane. The detailed surface modification processes are described below.

2.2.1. Surface modification in Paper І

The synthesis of silicone nanofilaments was performed in a lab with

controlled RH conditions at room temperature (42 ± 2% RH, 20 ± 1 °C).

Specifically, a vial containing 300 μL of trichloromethylsilane (TCMS,

8 | EXPERIMENTAL

99%, Sigma-Aldrich) was placed in a sealed desiccator (reaction chamber)

with 5 pieces of wood veneers at 42–43% RH. The water content, which

refers to RH in the gas-phase based reaction, affects the formation of

polysiloxane nano- and microstructures. The different preparation

conditions lead to the formation of different structures, e.g.,

nanofilaments, microrods and mixtures of them [87, 88]. The RH at

around 43% is ideal for the synthesis of silicone nanofilaments [65].

The samples were removed from the desiccator after 24 h and then

plasma-activated in an 18 W air plasma cleaner (PDC-3XG, Harrick) for

5 min. In this process, the hydrophobic methyl groups of the silicone

nanofilaments on the surfaces were converted into hydrophilic hydroxyl

groups, which facilitated the bonding of the long-chain silane molecules on

the surface. In the second step, the wood samples were placed together

with a vial containing 80 μL of 1H,1H,2H,2H-

Perfluorodecyltrichlorosilane (PFDTS, Alfa Aesar, 96%) for 2 h at 5 kPa.

The surface-modified birch wood by silicone nanofilaments and PFDTS

silanization was denoted F-SMB.

2.2.2. Surface modification in Paper ІІ

The synthesis of silicone nanofilaments was modified by using liquid water

at reduced pressure. Figure 3a presents the schematic illustration of the

reaction setup for preparing silicone nanofilaments. Specifically, the wood

samples (a total surface area of around 70 cm2 for each type) were placed

together with a vial containing 300 µL of TCMS and a vial containing 75 µL

of ultrapure water (Type 1, prepared using a Milli-Q RO unit) in a sealed

desiccator (reaction chamber, ca. 5.7 L), which was attached with a rotary

vane vacuum pump (Edwards). The reaction chamber was evacuated down

to 5 kPa before the reaction started. Samples with a reaction time of 1, 2, 5

and 20 h were prepared.

During the synthesis of silicone nanofilaments, the water started by

evaporating, which was accompanied by an increase of the RH in the

chamber. This process can produce a higher RH in the chamber quickly,

thus facilitating the growth of silicone nanofilaments. It should be noticed

that the moisture absorption capacity for acetylated birch wood and non-

EXPERIMENTAL | 9

acetylated birch wood is different. To ensure the same RH changes in the

synthesis of the silicone nanofilaments, acetylated and non-acetylated

wood samples were placed in the same chamber. The RH change in the

reaction chamber, which was measured by placing a hygrometer (Testo

174) together with the water in the chamber, is shown in Figure 3b. As seen,

the RH at the reaction starting point was around 15% RH. It increased to

20% RH in a few minutes which was suitable for the growth of silicone

nanofilaments [81]. In 30 to 60 min, the RH reached a plateau value of

around 40% RH, which was ideal for the formation of the silicone

nanofilaments [81, 89].

Figure 3. (a) Schematic illustration of the reaction setup for preparing silicone nanofilaments in gas phase; (b) relative humidity versus time in the reaction chamber with wood samples.

The silicone nanofilaments coated samples were hydrophobized by

Trichloro(octyl)silane (TCOS, 97%, Sigma-Aldrich) for 2 h in a second step.

The surface-modified birch and acetylated birch samples are denoted as

SMB and SMAB, respectively.

The description of the surface-modified wood samples discussed in the

thesis is summarized in Table 1.

10 | EXPERIMENTAL

Table 1. Description of the surface-modified wood samples in the thesis.

Sample

name Full name

Surface modification In

paper Step 1 Step 2

F-SMB

Fluorine

surface-

modified

birch

Silicone

nanofilaments

coating (RH

based gas

phase

reaction)

Post-

silanization

by PFDTS

І

SMB

Surface-

modified

birch

Silicone

nanofilaments

coating

(liquid water

based gas

phase

reaction)

Post-

silanization

by TCOS

ІІ

SMAB

Surface-

modified

acetylated

birch

Silicone

nanofilaments

coating

(liquid water

based gas

phase

reaction)

Post-

silanization

by TCOS

ІІ

2.3. Characterizations

2.3.1. Surface morphology

Scanning electron microscopy (SEM) (FEI Quanta FEG 250) equipped

with energy-dispersive X-ray spectroscopy (EDS) (Oxford Instruments)

was utilized for imaging of the wood surfaces. The SEM characterization

was performed at an accelerating voltage of 10 kV with a working distance

of 10 mm in a low vacuum mode (LV-SEM) of 7 kPa. The EDS mapping was

EXPERIMENTAL | 11

scanned at the same conditions as the SEM characterization. No surface

precoating was applied in either case. The diameter distribution of the

silicone nanofilaments was calculated by analyzing the SEM images of the

SMB wood using ImageJ software. A Bruker Dektak XT stylus profilometer

equipped with a 2 μm radius tip was used to profile the wood surface

topography.

2.3.2. Chemical compositions

Fourier-transform infrared spectroscopy (FTIR) (Spectrum One, Perkin

Elmer) equipped with an attenuated total reflection was used to study the

chemical compositions on the wood surfaces. All spectra were recorded

between 4000 and 400 cm−1 at a resolution of 4 cm−1 and 16 scans per

sample.

2.3.1. Color change

The color of the wood samples was measured using an optical

spectrophotometer X-Rite SP60 D65/10° illumination. The CIE LAB color

space coordinate system was used to evaluate the color changes of the wood

samples after the surface modification. Three parameters, viz., L*, a* and

b* were recorded, where L* represents lightness (L* = 100 for pure white,

L* = 0 for total blackness); +a* the redness while -a* for green; +b* the

yellowness while -b* for blue. The total color change (ΔE*) was calculated

according to the equation:

∆𝐸∗ = √∆𝐿∗2 + ∆𝑎∗2 + ∆𝑏∗2, (3)

where ΔL*, Δa*, and Δb* are the differences between the color of the wood

samples before and after the surface modification. At least 8 spots were

evaluated and calculated for each sample.

2.3.2. Sessile drop measurement/Static contact angles

An optical contact angle goniometer (OCA40, DataPhysics) equipped with

a high-speed CCD camera (max 2200 images s−1) was used to capture the

profile of drop. Measurements were performed in a climate-controlled

room of 50 ± 3% RH at 23 ± 1 °C. The static contact angle (CA) and roll-off

angle (ROA) of water (ultrapure water), ethylene glycol (≥ 99.5%, Sigma-

12 | EXPERIMENTAL

Aldrich), hexadecane (99%, Sigma-Aldrich), decane (≥ 99%, Sigma-

Aldrich) and ethanol (95%, Solveco) were determined by the SCA 20

(DataPhysics) software. The surface tension of the liquids was listed in

Table 2. The advancing contact angle (θa), receding contact angle (θr) were

determined from the rolling droplets in the same software. The droplet

volume was 5 and 10 μL, respectively, for Paper І and ІІ. The tilting speed

of the roll-off angle measurement was 0.96° s−1 in Paper І and 1.58 s−1 in

Paper ІІ. At least 10 droplets were analyzed in each case.

Table 2. The surface tension of liquids at room temperature.

Liquid Surface tension (mN m−1)

Water 72.8

Ethylene glycol 48.3

Olive oil 32.1

Hexadecane 27.6

Decane 23.9

Ethanol 23.8

2.3.3. Liquid uptake

Liquid uptake in Paper І was measured by submerging the birch wood and

F-SMB wood samples separately into a beaker with 150 mL liquid (Milli-Q

water, hexadecane (95%, Alfa Aesar), ethylene glycol and ethanol). The

cross and radial sections of the wood samples were sealed by a

polyurethane lacquer in order to prevent end-grain sorption during

submersion. The excess liquid that adhered to the sample surface was

wiped off gently with tissue before weighing and the sample was put

immediately back into the liquid after weighing for the next point in a time

series. At least 3 samples were measured in water and hexadecane. One

sample was measured in ethylene glycol and ethanol. The liquid uptake was

determined as follows:

EXPERIMENTAL | 13

𝐿𝑖𝑞𝑢𝑖𝑑 𝑢𝑝𝑡𝑎𝑘𝑒 (%) = (𝑚𝑖 − 𝑚0′ )/𝑚0 × 100, (4)

where mi is the sample weight after submerging in the liquid for a certain

time; m0＇is the sample weight after sealing with polyurethane lacquer and

m0 is the original sample weight.

2.3.4. Multicycle Wilhelmy plate method

A Sigma 70 tensiometer from KSV Instruments (now Biolin Scientific) was

used for measuring the dynamic wetting properties and water uptake of

SMB and SMAB samples in Paper ІІ. The measurements were following a

previously developed method [22, 90]. Figure 4 shows the schematic

illustration of the multicycle Wilhelmy plate method and a two-cycle

Wilhelmy curve with a wood sample. In this work, a 100-cycle Wilhelmy

plate measurement was performed with the wood veneer in ultrapure

water. The immersion and withdrawal velocity of the veneer was 12 mm

min-1. The immersion depth was 10 mm and the withdrawing position was

5 mm above the water. To prevent end-grain water penetration and

longitudinal liquid transportation during measurement, the cross section

of the tested wood veneer was coated by polyurethane lacquer before the

measurement. Based on the immersion depth and the immersion and

withdrawal velocity of the veneer, the wood veneer contacted with the

water for a total of ca. 167 min in the 100 cycles.

Figure 4. (a) Schematic illustration of the multicycle Wilhelmy plate method. The sample is intermittently immersed in and withdrawn from the probe liquid. The veneer dimension is increased during the experiment as a consequence of swelling; (b) a two-cycle Wilhelmy plate experiment with a wood sample. Copyright © 2013, Sedighi Moghaddam et al. [91], reproduced with permission from the American Chemical Society.

14 | EXPERIMENTAL

For porous and hygroscopic samples such as wood, the measured force F

can be written as [92]:

𝐹 (ℎ, 𝑡) = 𝑃𝛾 cos 𝜃 + 𝐹𝑤(𝑡) − 𝜌𝐴ℎ𝑔, (5)

where 𝑃𝛾 cos 𝜃 equals to the wetting force and P is the wetted perimeter of

the wood veneer, γ the surface tension of the probe liquid, and θ the liquid-

solid-air contact angle; ρAhg corresponds to the buoyancy force and ρ is

the probe liquid density, A the cross-sectional area of the veneer, h the

immersion depth, and g the gravitational constant; and Fw(t) is the force

due to sorption of the liquid at time t. The θa and θr at cycle n can be

obtained by the linear regression of the corresponding curves extrapolated

to zero depth (h = 0) in the multicycle Wilhelmy plate plot (Figure 4b),

assuming that P and γ are constant, and Eq. 5 can be simplified to:

𝐹𝑎,𝑛 = 𝑃𝛾 cos 𝜃𝑎,𝑛 + 𝐹𝑓,𝑛−1, (6)

𝐹𝑟,𝑛 = 𝑃𝛾 cos 𝜃𝑟,𝑛 + 𝐹𝑓,𝑛, (7)

where Ff,n-1 and Ff,n are the final forces after cycle n-1 and n, respectively,

which are caused by the sorption. The Ff,n-1 and Ff,n are the same as the term

Fw(t) in Eq. 5. The two contact angles at cycle n, θa,n and θr,n, can be

calculated as:

cos 𝜃𝑎,𝑛 =𝐹𝑎,𝑛− 𝐹𝑓,𝑛−1

𝑃𝛾, (8)

cos 𝜃𝑟,𝑛 =𝐹𝑟,𝑛− 𝐹𝑓,𝑛

𝑃𝛾. (9)

Sorption is determined by linear regression of the final force, Ff, to zero

depth at each cycle.

The liquid uptake of wood veneer after cycle n, Wn is calculated as:

𝑊𝑛 =𝐹𝑓,𝑛

𝑚0𝑔× 100%, (10)

where m0 is the initial mass of the wood sample before sealing.

EXPERIMENTAL | 15

2.3.5. Self-cleaning

The self-cleaning test was performed by dispensing droplets on the tilted

F-SMB wood surface (30°). The wood surfaces were covered with a sand

layer (50-70 mesh size SiO2 particles, Sigma-Aldrich). Water and

hexadecane droplets of 20 μL were dropped respectively, from a height of

200 mm onto the F-SMB wood surface. The droplets rolled off from the

longitudinal direction of the wood veneers. The videos were recorded using

a CCD camera (Canon 550d).

16 | RESULTS AND DISCUSSION

3. Results and discussion

3.1. Morphology

3.1.1. Silicone nanofilaments on fluorine surface-modified birch (F-

SMB)

The silicone nanofilaments were successfully synthesized on the birch

wood surfaces as evidenced by the observed silicone nanofilaments with a

large length-to-diameter ratio were on the wood vessel walls and the

concave lumen surfaces after the surface modification (Figures 5b-c).

Moreover, silicone nanofilaments with a few micrometers in length were

observed from the cross-section (Figure 5d).

Figure 5. Top-view SEM images of the (a) birch wood, (b-c) F-SMB and side-view SEM image of the (d) F-SMB.

Figure 6 shows the topography maps of birch and F-SMB wood surfaces.

The unmodified birch wood surface exhibited an RMS average roughness

(Sq) of 16.7 ± 1.2 μm while the F-SMB surface showed an Sq of 19.9 ± 1.9

μm, suggesting that the silicone nanofilaments layer increases the surface

roughness. Therefore, combined with the microscale structure of the wood

RESULTS AND DISCUSSION | 17

surface itself, a nano- and microscale structure was in this way obtained by

the silicone nanofilaments.

Figure 6. 3D topographic maps of (a) the birch and (b) the F-SMB wood surfaces.

3.1.2. Silicone nanofilaments on fluorine-free surface-modified birch

and acetylated birch (SMB and SMAB)

The synthesis of silicone nanofilaments was modified in Paper ІІ by

introducing liquid water at reduced pressure. The effect of reaction time on

the formation of silicone nanofilaments on birch and acetylated birch wood

was studied. Figures 7 and 8 show the morphology of SMB and SMAB

prepared at different times at large and small magnifications.


Figure 7. Top-view SEM images of SMB samples at the reaction time of (a) 1, (b) 2, (c) 5 and (d) 20 h and SMAB samples at the reaction time of (e) 1, (f) 2, (g) 5 and (h) 20 h at large magnification. The scale bar indicated in Figure 7h applies to all images.


Figure 8. Top-view SEM images of SMB samples at the reaction time of (a) 1, (b) 2, (c) 5 and (d) 20 h and SMAB samples at the reaction time of (e) 1, (f) 2, (g) 5 and (h) 20 h at small magnification. The scale bar indicated in Figure 8h applies to all images.

As shown, the silicone nanofilaments were synthesized on both SMB and

SMAB wood surfaces at reduced pressure even at short reaction times (1

and 2 h). Some microrods with relatively large diameters were also formed

as a result of the relatively low RH at the beginning of synthesis. The


microrods, which were an important structure on the wood surfaces at

shorter reaction times, mainly located on the wood cell walls and

distributed sparsely. Most of these structures completed growth at the first

two reaction hours as they became less important at longer reaction times.

The normal silicone nanofilament structure became the dominant

structure when the reaction time was increased to 5 h and longer, and the

coverage of the silicone nanofilaments on the wood surfaces increased

accordingly. A dense silicone nanofilamental network was formed on both

SMB and SMAB wood surfaces. However, no obvious coverage change was

observed between 5 and 20 h.

Figure 9 shows that the average diameter of silicone nanofilaments on

both SMB and SMAB followed a similar change, i.e., it decreased with

increasing reaction time. But the changes between 5 and 20 h were small.

From the morphology and the average diameter changes of silicone

nanofilaments, it is concluded that a reaction time of 5 h is sufficient for

getting a proper silicone nanofilaments coverage on the wood surfaces

when preparing the silicone nanofilaments at reduced pressure.

Figure 9. The number-frequency per 25 nm size class and normal distribution curve (fitted to the data) of the silicone nanofilaments on the SMB surfaces at the reaction time of (a) 1, (b) 2, (c) 5 and (d) 20 h, and on the SMAB surfaces at the reaction time of (e) 1, (f) 2, (g) 5 and (h) 20 h. The corresponding average diameter with standard deviation and the median values of the polysiloxane structures are given.

The hydroxyl groups, which are presented on the substrate, are usually

responsible for the bonding of silicone nanofilaments on the surface [72,

81, 83]. As known, the freely accessible hydroxyl groups in the acetylated

wood substance are reduced [2, 9, 20, 93]. However, no obvious changes


were observed in silicone nanofilaments distribution on the surfaces

comparing SMB and SMAB samples prepared at the same time, suggesting

that the decrease of the accessible hydroxyl groups in the acetylated wood

did not hinder the synthesis of the silicone nanofilaments. On the contrary,

the silicone nanofilaments with larger average diameters were observed on

the SMAB at longer preparation times (5 and 20 h), indicating that the

decrease of the accessible hydroxyl groups in wood can lead to the growth

of the silicone nanofilaments in diameter.

3.2. Surface chemistry

Figure 10 shows the FTIR spectra of the unmodified wood and surface-

modified wood samples. The SMB and SMAB showed two additional

absorption peaks at 780 and 1270 cm−1, corresponding to the stretching

vibrations of Si-C bond and -CH3 deformation vibrations of the siloxane

compounds, respectively [21, 94, 95]. Furthermore, a peak which was

assigned to the C-H band from Si-CH3 was also observed at 2969 cm−1 for

the SMB and SMAB wood. The appearance of these peaks confirmed the

formation of silicone nanofilaments on the wood surface.

Figure 10. FTIR spectra of the birch (B), acetylated birch (AB) and SMB and SMAB samples prepared at 5 h.


Figure 11 shows that carbon, oxygen, fluorine, silicon, and chlorine were

detected on the F-SMB wood surface. The silicon, which was either from

the silicone nanofilaments layer or the hydrophobization process using

PFDTS, appeared densely distributed on the vessel walls but was also

detected on the concave lumen surfaces from the EDS mapping, indicating

that the silicone nanofilaments cover quite uniformly the whole wood

surface. A small amount of chlorine was also detected because the

byproduct in the surface modification process was hydrochloric acid.

Figure 11. EDS mapping of (a) carbon, (b) oxygen, (c) fluorine, (d) silicon and (e) chlorine of the F-SMB wood; (f) the corresponding SEM image. The number in (a-e) is the average atomic element percentage. The scale bar is 10 μm and applies to all images.

3.3. Color changes

Figure 12 shows the lightness and color changes of the SMB and SMAB

wood samples at different reaction times. As shown, the ΔL*, Δa* and Δb*

of SMAB wood varied in a small range from 1 to 20 h. And the ΔE* was

smaller than 2 at all reaction times, which reveals that the color change was

invisible by the naked eye [96]. On the contrary, the ΔL*, Δa* and Δb* of

SMB wood varied in a relatively large range, which results in a large ΔE*

value especially when the reaction time was increased to 2 h and longer.

Therefore, the surface modification makes more color changes on the non-

acetylated wood than for the acetylated wood.


Figure 12. Changes in (a) lightness ΔL*, (b) redness Δa*, and (c) yellowness Δb*, and total color change ΔE*of SMB and SMAB wood samples after the surface modification at different silicone nanofilaments reaction times. The original color (OC) and final color (FC) of wood samples prepared at 1 and 20 h are given in Figure 12d.

The color change of wood samples is caused by the release of hydrochloric

acid during the surface-modification process. Acids can catalyze the

condensation and degradation reactions in lignin structures and

contribute to the discoloration of wood [97-102]. Since more lignin

components in the wood are modified in the acetylation process [2, 16, 93],

the color change of acetylated wood in the surface modification is less

pronounced.

3.4. Wettability

The wettability of F-SMB (Paper І) was measured by the sessile drop

technique, where static CA, θa, θr, contact angle hysteresis (CAH) and ROA

were obtained, and liquid uptake (by submerging the wood samples) in

different liquids. The wettability of SMB and SMAB (Paper ІІ) was

measured using sessile drop measurement as well as the multicycle

Wilhelmy method, where θa, θr, and water uptake of samples during the

repeated wetting cycles in water were obtained.


3.4.1. Sessile drop measurement

3.4.1.1. Fluorine surface-modified birch (F-SMB)

The birch wood was hydrophilic and showed a CA of 57 ± 11°. On the other

hand, the water drop rolled off immediately from the F-SMB wood, which

indicates the surface was superhydrophobic. Moreover, the F-SMB wood

showed CAs greater than 150° towards ethylene glycol and hexadecane

(Figure 13), which were accompanied by low ROAs, high θas (> 154°), θrs

(> 130°), and CAHs of 23° or less as indicated in Table 3. The results

suggest that a superamphiphobic wood surface was fabricated by the

fluorinated silicone nanofilaments. The image of the F-SMB with some

organic liquids of low surface tension is presented in Figure 14.

Figure 13. Static contact angles with standard deviations of F-SMB towards different liquids; the profile of the corresponding liquid drop is presented on top of each column. No water drop is given as the water drops roll off immediately from the F-SMB wood surface once dispensed (The CA was assumed to be close to 180° towards water).

Figure 14. Superamphiphobic F-SMB wood surfaces with different liquids (water drop rolled off immediately from the surfaces after dispensing; the ethylene glycol and olive oil were dyed).


Table 3. Advancing angle (θa), receding angle (θr), contact angle hysteresis (CAH) and ROA of the F-SMB wood with standard deviations.

Liquid a θa (°) θr (°) CAH (°) ROA (°)

Ethylene glycol 160 ± 5 140 ± 12 20 ± 9 4 ± 3

Hexadecane 154 ± 5 130 ± 15 23 ± 13 7 ± 4

Decane 150 ± 4 83 ± 10 67 ± 11 39 ± 10

aValues for water droplets could not be evaluated on the F-SMB surface as the droplets rolled

off once dispensed.

3.4.1.2. Fluorine-free surface-modified birch and acetylated birch (SMB

and SMAB)

Table 4 shows the static CA and ROA of SMB and SMAB prepared at

different silicone nanofilaments reaction times. It should be mentioned

that the results for SMB and SMAB samples can only partially represent

the surfaces as only droplets that can be analyzed were considered. Due to

the heterogeneity of wood surfaces, some drops rolled off immediately

after dispensing while other drops pinned on the surfaces.

The CA for untreated birch and acetylated birch wood was 59 ± 7° and

64 ± 8°, respectively. The CA increased significantly after surface

modification by the non-fluorinated silicone nanofilaments, as revealed by

the CA of 164 ± 6° and 163 ± 5°, respectively, for the SMB and SMAB

samples prepared at 1 h. The CAs varying from 162° to 164° were obtained

for both the SMB and SMAB samples prepared at longer times. The ROA

of the SMAB sample decreased from 26 ± 19° at 1 h to 11 ± 10° at 20 h, but

this decreasing trend was not observed for the SMB sample. Given the fact

that wood is heterogeneous, we conclude from high static CA and low ROA

for the longer reaction time that the modified wood is superhydrophobic.

The stated CAs underestimate the true CA because of the fact that several

drops could not be measured and those most likely have the highest CA

values.


Table 4. Static contact angle (CA) and roll-off angle (ROA) of the water drops on birch, acetylated birch, SMB and SMAB at different silicone nanofilaments reaction times.b

Reaction

time (h)

Sample

name CA (°) ROA (°)

Sample

name CA (°) ROA (°)

1

SMB

164 ± 6 34 ± 20

SMAB

163 ± 5 26 ± 19

2 162 ± 5 44 ± 17 162 ± 4 26 ± 7

5 163 ± 6 26 ± 16 164 ± 4 14 ± 5

20 163 ± 3 42 163 ± 5 11 ± 10

Birch 59 ± 7 n/a c Acetylated

birch 64 ± 8 n/a

bDue to the heterogeneity of wood surfaces, some drops rolled off immediately after dispensing while some other drops pinned on the surfaces. The static contact angles were calculated based on the drops that did not roll off immediately after dispensing and the roll-off angles were calculated based on the drops that did not roll off immediately but rolled off at certain degrees. The specific information: ⅰ) 1 h, 1 of 13 of the dispensed drops rolled off immediately while 2 of 13 of the dispensed drops pinned on the SMB surfaces; 1 of 11 of the dispensed drops rolled off immediately while 3 of 11 of the dispensed drops pinned on the SMAB surfaces; ⅱ) 2 h, 9 of 13 of the dispensed drops pinned on the SMB surfaces; all dispensed drops rolled off at certain degrees on the SMAB surfaces; ⅲ) 5 h, 5 of 16 of the dispensed drops rolled off immediately while 1 of 16 of the dispensed drops pinned on the SMB surfaces; all dispensed drops rolled off at certain degrees on the SMAB surfaces; ⅳ) 20 h, only one dispensed drop rolled off on the SMB surfaces; 1 of 7 of the dispensed drops rolled off immediately while no dispensed drops pinned on the SMAB surfaces. c n/a means not available.

3.4.1.3. Effect of the silane used in the post-silanization

It is well established that the wettability of a solid surface is determined by

the chemical composition and surface roughness [42, 103-111]. For the F-

SMB, SMB, and SMAB, they show a similar surface roughness structure,

i.e., silicone nanofilaments on the rough wood surfaces. Even though the

roughness values change among samples, e.g., the SMB wood samples

prepared at different silicone nanofilaments reaction time, only a small

difference was observed concerning the static water CAs (all SMB wood

samples are regarded as superhydrophobic). However, the F-SMB sample

was superoleophobic while the SMB and SMAB were not. This indicates

that the fluorine chemical used in the post-silanization plays an important

role for the superoleophobicity of the silicone nanofilaments coated wood

surfaces.


3.4.2. Sample submersion

The liquid uptake of F-SMB wood was examined by submerging the

samples in water, hexadecane, ethylene glycol and ethanol for up to 24 h.

The samples were withdrawn at certain times for observation and weighing

to record the liquid uptake.

3.4.2.1. Water uptake

As shown in Figure 15c, the F-SMB wood appeared unwetted even after 24

h in water. Most importantly, a sheath with a silvery shine (known as

plastron) [45, 67, 89, 112] was observed at the interface when the sample

was submerged in water (Figures 15b-c). The plastron was caused by the

hydrophobized silicone nanofilaments layer, which could trap air/vapor

pockets between the solid substrate and the contacting liquid and lead to a

Cassie-Baxter wetting state on the interface. Moreover, the plastron acted

as a physical barrier and reduced the water uptake rate. As a result, the

water uptake of F-SMB wood was slow at the beginning (Figure 15d).

However, the plastron layer did not prevent water from penetrating into

the wood after long-time submersion in water, resulting in final water

uptake of 71 ± 4 wt%.

Figure 15. Images of the (a) birch, (b) F-SMB in water at the first minute and the surfaces after withdrawing from the water, (c) F-SMB in water at 24 h and the surfaces after


withdrawing from the water and (d) water uptake of the birch and F-SMB wood samples with standard deviations as a function of time.

3.4.2.2. Hexadecane uptake

The wood samples were further submerged in a low surface tension liquid

hexadecane. As seen in Figure 16b, the F-SMB was not wetted after one

minute in hexadecane, but it appeared wetted upon further submersion.

An interesting phenomenon was observed when withdrawing the sample

at 5 and 10 min. As shown in the snapshots obtained from the movie in

Figure 17, a thin homogeneous liquid film was formed on the F-SMB wood

surface at the beginning. This thin film broke up quickly and the F-SMB

sample became dewetted in seconds. A possible explanation for this

phenomenon is that cohesion maintains an adhered liquid film even

though the surface is liquid-repellent but dewets rapidly at a critical liquid

film thickness as a result of draining.

Figure 16. Images of the (a) birch, (b) F-SMB in hexadecane at the first minute and the surfaces after withdrawing from the hexadecane, (c) F-SMB in hexadecane at 24 h and the surfaces after withdrawing from the hexadecane and (d) hexadecane uptake of the birch and F-SMB wood samples with standard deviations as a function of time.

As shown in Figures 16b-c, the plastron was also observed when the F-SMB

wood sample was submerged in hexadecane. Due to the plastron effect, the


hexadecane uptake of the F-SMB wood increased slowly at the beginning

and it leveled off gradually, reaching 3 ± 1 wt% at 24 h, which was much

less as compared to the final hexadecane uptake of the birch wood (Figure

16d).

Figure 17. The dewetting process of the F-SMB wood in hexadecane after withdrawing the sample at 10 min, snapshots obtained from the video.

3.4.2.3. Liquid uptake in ethylene glycol and ethanol

The F-SMB samples were further submerged in two low surface tension

and polar liquids, ethylene glycol and ethanol. As can be seen in Figure 18,

the plastron was observed in ethylene glycol but not in ethanol. A relatively

low liquid uptake of F-SMB in ethylene glycol and a high liquid uptake of

F-SMB in ethanol were obtained after longer time submersion.

Figure 18. (a) Image of F-SMB wood in ethylene glycol, (b) ethylene glycol uptake of the birch and F-SMB wood samples as a function of time, (c) Image of F-SMB wood in ethanol, and (d) ethanol uptake of the birch and F-SMB wood samples as a function of time (18 h was the maximum time recorded in ethanol).


3.4.2.4. Permeability, diffusion and solubility

The liquid uptake of F-SMB varied in different liquids as the permeability

of the liquid differs. The overall permeability of a liquid through the air

(plastron) layer and uptake into the wood is common for describing barrier

functionality and is dependent on factors linked to diffusivity and

solubility. The permeability P can be expressed as the product of diffusivity

D and solubility S [113, 114]:

𝑃𝑖𝑗 = 𝐷𝑖𝑗𝑆𝑖𝑗, (11)

in which Dij and Sij are the sums of individual contributions of liquid (or

vapor) i in material j. In our case of silicone nanofilament based plastron

layer on wood, three layers viz., air, silicone (sil) and wood are considered.

The total permeability P can be further described as:

𝑃 = (𝐷𝑎𝑖𝑟 + 𝐷𝑠𝑖𝑙 + 𝐷𝑤𝑜𝑜𝑑)(𝑆𝑎𝑖𝑟 + 𝑆𝑠𝑖𝑙 + 𝑆𝑤𝑜𝑜𝑑) (12)

where Sair is the vapor pressure of the liquid.

According to the liquid uptake results presented in Figures 15-16 and

18, the permeability of the F-SMB samples follows the order of water >

ethylene glycol > hexadecane > ethanol. Specifically, the permeability in

water is two orders of magnitude larger than that in hexadecane. The

estimated diffusion in the plastron is also in the orders of water > ethylene

glycol > hexadecane (ethanol is excluded from the discussion since no

plastron was observed). Particularly, the water diffusion is five orders of

magnitude larger than that of hexadecane (for detailed information, see

supporting information in the appended paper І). The large difference

between water and hexadecane in terms of the permeability and diffusion

suggests that the solubility of hexadecane in the plastron layer contributes

significantly to the liquid uptake of wood. This result is also supported by

the silicone swelling ratio, which is 1.14 for hexadecane but is much smaller

or negligible for the other liquids [115].

Figure 19 shows the schematics of the liquid uptake of F-SMB wood in

water, hexadecane, ethanol and ethylene glycol. As known, no plastron was

observed in ethanol; ethanol can wet the F-SMB wood immediately. Due to


the plastron effect, water, hexadecane and ethylene glycol can only diffuse

through the air layer into the wood by vapor. As the saturated water vapor

pressure at the interface is high, the water vapor diffusion to the wood

surface is fast. Furthermore, only hexadecane can interact with the silicone

and swell the polysiloxane layer, thus hexadecane will also interact with the

surface layer, which leads to the very low hexadecane uptake of F-SMB

wood.

Figure 19. Schematics of the liquid uptake of F-SMB wood in water, hexadecane, ethanol and ethylene glycol.

3.4.3. Wilhelmy plate measurement

The dynamic wettability of SMB and SMAB was studied by the multicycle

Wilhelmy plate method. As seen in Figure 20, the initial θa of the birch and


acetylated birch wood sample was 93° and 75°, respectively. The θr in all

cycles and the θa in all subsequent cycles were zero, suggesting the birch

and acetylated birch wood surfaces were completely wetted in water.

Figure 20. Force per perimeter length as a function of immersion depth sample position for 100-cycle Wilhelmy plate measurements for (a) birch, (b) acetylated birch and (c) SMB and (d) SMAB prepared at 5 h.

On the other hand, the initial θa of the SMB and SMAB was 122 ± 3° and

154 ± 8°, respectively, which is accompanied by an initial θr of 70 ± 11° and

89 ± 7°, respectively (Table 5). The θa and θr showed a decreasing trend

with increasing cycles and the largest decrease was observed in the first 10

cycles for both the SMB and SMAB samples. Furthermore, the SMAB

showed higher θas and θrs than those of the SMB. Meanwhile, it showed

lower CAHs. Therefore, the SMAB shows a better liquid-repellent ability

than the SMB.


Table 5. Advancing angle (θa) and receding angle (θr) and contact angle hysteresis (CAH) with standard deviations for SMB and SMAB samples prepared at 5 h at specific cycles.

Cycle No.. SMB SMAB

θa(°) θ

r(°) CAH (°) θ

a(°) θ

r(°) CAH (°)

1 122 ± 3 70 ± 11 52 ± 11 154 ± 8 89 ± 7 65 ± 9

10 93 ± 23 48 ± 22 45 ± 6 109 ± 12 72 ± 16 38 ± 14

50 89 ± 27 36 ± 32 53 ± 5 90 ± 17 67 ± 21 23 ± 4

100 87 ± 27 37 ± 33 51 ± 6 85 ± 15 63 ± 22 22 ± 7

Figure 21 shows the water uptake versus the immersion cycle obtained

from the 100-cycle Wilhelmy curves. The water uptake of birch increased

dramatically with increasing immersion cycle, approaching 86 ± 6 wt%

after 100 cycles. The acetylated birch showed a much lower water uptake

rate, leading to a final water uptake of 15 ± 3 wt%. The water uptake

reduction of the acetylated birch is mainly due to the reduction of primary

sorption sites and the decrease of the volume in the wood cell walls which

is available to water [20].

Figure 21. Water uptake with standard deviations as a function of cycle number for birch (B), acetylated birch (AB) and SMB and SMAB prepared at 5 h.


The SMB exhibited an even slower water uptake behavior as a result of the

superhydrophobic coating, ending at 10 ± 1 wt% after 100 cycles. This

finding suggests that a superhydrophobic coating on the wood can be more

efficient for improving the water resistance of the wood than the

acetylation approach. The SMAB wood showed the lowest water uptake of

all samples with a result of 3 ±1 wt% after 100 cycles, which was 29, 5 and

about 3 times lower than that of the birch, acetylated birch and SMB,

respectively. To conclude, the combination of the acetylated wood with the

superhydrophobic coating can result in remarkably reduced water uptake

of wood.

3.5. Self-cleaning tests

Figure 22 shows the self-cleaning properties of the F-SMB towards water

and hexadecane. As shown, the water droplets rolled off quickly from the

F-SMB wood surface. Meanwhile, the rolling water droplets took away the

sand particles that they contacted, leading to a clean surface. When the

water was changed to hexadecane, the droplets could also roll off and

removed the sand from the surface. The finding reveals that the prepared

superamphiphobic wood surface has very good self-cleaning properties

against both water and oils.

Figure 22. Self-cleaning tests of the F-SMB wood samples towards (a) water and (b) hexadecane in snapshots obtained from the videos. The liquid droplets are marked with dashed circles.

FUTURE WORK| 35

4. Conclusions

Hydrophobized silicone nanofilaments coating on wood can effectively

improve the liquid repellence of wood. The silicone nanofilaments layer

improved the nano- and microscale roughness of wood, which together

with the low surface energy chemistry of silanes, resulted in the

superhydrophobicity and superamphiphobicity on wood.

The fluorinated silicone nanofilaments on birch wood (F-SMB) led to

superamphiphobic surfaces with high static contact angles and low roll-off

angles towards water, ethylene glycol and hexadecane; the non-fluorinated

silicone nanofilaments on birch wood (SMB) and acetylated birch (SMAB)

wood led to superhydrophobic surfaces. The silicone nanofilaments

coating effectively resulted in superhydrophobicity of the wood surfaces

even at the shortest reaction time of 1 h.

In the submersion test, a plastron was observed when the F-SMB wood

was in water, hexadecane and ethylene, which indicates that the surface

was in Cassie-Baxter wetting state. The plastron reduced the liquid uptake

of the F-SMB in hexadecane and ethylene glycol but not in water under

long-time submersion. The diffusion in the plastron accounts for the major

difference in the liquid uptake of F-SMB. But the interaction between the

liquid and the surface layer/solubility of the liquid in the surface layer

contributes significantly to the liquid uptake in hexadecane.

In the multicycle Wilhelmy plate measurement, both SMB and SMAB

wood samples showed high water repellence and resulting high θa and θr

in the repeating wetting cycles. Moreover, the SMB wood showed lower

water uptake than that of the acetylated birch wood, which suggests that

the surface modification of wood by the hydrophobized silicone

nanofilaments is more efficient than the acetylation of wood in repelling

water. However, the SMAB shows the lowest water uptake of all wood

samples due to the combined effects of the superhydrophobic coating and

the acetylation. The surface modification of acetylated wood opens a new

way of achieving high water resistance of wood.

Color changes are observed on both SMB and SMAB surfaces due to the

release of hydrochloric acid in the modification process (both in the

36 | CONCLUSIONS

synthesis of silicone nanofilaments and post-silanization). It is, however,

less a problem for acetylated wood than for non-acetylated wood as the

color change of the acetylated wood is less pronounced.

5. Future work

In the present study, hydrophobized silicone nanofilaments were coated on

birch and acetylated birch wood surfaces, which showed

superhydrophobicity and even superamphiphobicity on the wood surfaces.

The water resistance of wood was significantly improved. Future research

is suggested to include the durability study of the silicone nanofilaments

surface-modified wood, for example, how the superhydrophobicity

changes under UV irradiation. Other effects related to the UV irradiation

test, such as the color changes of wood, the surface chemistry changes in

terms of the degradation of the polysiloxane bonds and the lignin bonds,

could be included. Moreover, other non-fluorinated surface modification

approaches on wood should be investigated so that methodologies that are

potentially scalable and viable for outdoor wood usages, such as façade or

furniture materials can be developed.

In addition, it is important to develop the methodologies on the

superhydrophobic or superamphiphobic surfaces and study the liquid

repellence of the surfaces based on the permeability (diffusion and

solubility) effect.

REFERENCES | 37

References

[1] E. Sjöström, Wood chemistry: Fundamentals and applications, Gulf professional publishing (1993). [2] C.A. Hill, Wood modification: Chemical, thermal and other processes, John Wiley & Sons (2006). [3] R.M. Rowell, Handbook of wood chemistry and wood composites, CRC press (2012). [4] M. Yalinkilic, Ft-ir studies of the effects of outdoor exposure on varnish-coated wood pretreated with ccb, Proceedings of the fourth Pacific Rim bio-based composites symposium, Nov 2-5, Bogor, Indonesia, (1998), pp. 345-357. [5] A. Cogulet, P. Blanchet, V. Landry, Wood degradation under uv irradiation: A lignin characterization, J Photochem Photobiol B 158 (2016) 184-91. [6] I. Kubovský, E. Oberhofnerová, F. Kačík, M. Pánek, Surface changes of selected hardwoods due to weather conditions, Forests 9(9) (2018) 557. [7] M. Žlahtič-Zupanc, B. Lesar, M. Humar, Changes in moisture performance of wood after weathering, Construction and Building Materials 193 (2018) 529-538. [8] C.A. Hill, Wood modification: Chemical, thermal and other processes, John Wiley & Sons(2007). [9] S. Himmel, C. Mai, Effects of acetylation and formalization on the dynamic water vapor sorption behavior of wood, Holzforschung 69(5) (2015) 633-643. [10] A. Temiz, N. Terziev, M. Eikenes, J. Hafren, Effect of accelerated weathering on surface chemistry of modified wood, Applied Surface Science 253(12) (2007) 5355-5362. [11] R. Ringman, A. Pilgård, K. Richter, Brown rot gene expression and regulation in acetylated and furfurylated wood: A complex picture, Holzforschung 74(4) (2020) 391-399. [12] G. Beck, E.E. Thybring, L.G. Thygesen, Brown-rot fungal degradation and de-acetylation of acetylated wood, International Biodeterioration & Biodegradation 135 (2018) 62-70. [13] K. Laine, K. Segerholm, M. Wålinder, L. Rautkari, M. Hughes, C. Lankveld, Surface densification of acetylated wood, European Journal of Wood and Wood Products 74(6) (2016) 829-835. [14] S. Anderberg, Mechanical properties of chemical modified wood of load-bearing constructions, TVBK-5257 (2016). [15] M. Pries, R. Wagner, K.-H. Kaesler, H. Militz, C. Mai, Acetylation of wood in combination with polysiloxanes to improve water-related and mechanical properties of wood, Wood science and technology 47(4) (2013) 685-699.

38 | REFERENCES

[16] R. Rowell, R. Simonson, S. Hess, D. Plackett, D. Cronshaw, E. Dunningham, Acetyl distribution in acetylated whole wood and reactivity of isolated wood cell-wall components to acetic anhydride, Wood and fiber science 26(1) (1994) 11-18. [17] T. Yang, E.E. Thybring, M. Fredriksson, E. Ma, J. Cao, R. Digaitis, L.G. Thygesen, Effects of changes in biopolymer composition on moisture in acetylated wood, Forests 11(7) (2020) 719. [18] K. Forsman, E. Serrano, H. Danielsson, J. Engqvist, Fracture characteristics of acetylated young scots pine, European Journal of Wood and Wood Products 78(4) (2020) 693-703. [19] R. Rowell, Acetylation of wood–a review, International Journal of Lignocellulosic Products 1(1) (2014) 1-27. [20] C.-M. Popescu, C.a.S. Hill, S. Curling, G. Ormondroyd, Y. Xie, The water vapour sorption behaviour of acetylated birch wood: How acetylation affects the sorption isotherm and accessible hydroxyl content, Journal of Materials Science 49(5) (2013) 2362-2371. [21] I. Mohammed-Ziegler, I. Tánczos, Z. Hórvölgyi, B. Agoston, Water-repellent acylated and silylated wood samples and their surface analytical characterization, Colloids and Surfaces A: Physicochemical and Engineering Aspects 319(1-3) (2008) 204-212. [22] M. Sedighi Moghaddam, M.E.P. Wålinder, P.M. Claesson, A. Swerin, Wettability and swelling of acetylated and furfurylated wood analyzed by multicycle wilhelmy plate method, Holzforschung 70(1) (2016) 69-77. [23] F. Bongers, S. Uphill, Performance of acetylated wood in aquatic applications, International Wood Products Journal 10(3) (2019) 95-101. [24] Y. Chai, J. Liu, Z. Wang, Y. Zhao, Dimensional stability and mechanical properties of plantation poplar wood esterified using acetic anhydride, BioResources 12(1) (2017) 912-922. [25] P. Larsson Brelid, Benchmarking and state of the art for modified wood, (2013). [26] R.M. Rowell, R.E. Ibach, J. Mcsweeny, T. Nilsson, Understanding decay resistance, dimensional stability and strength changes in heat-treated and acetylated wood, Wood Material Science and Engineering 4(1-2) (2009) 14-22. [27] T. Joffre, K. Segerholm, C. Persson, S.L. Bardage, C.L.L. Hendriks, P. Isaksson, Characterization of interfacial stress transfer ability in acetylation-treated wood fibre composites using x-ray microtomography, Industrial Crops and Products 95 (2017) 43-49. [28] P. Fei, L. Liao, B. Cheng, J. Song, Quantitative analysis of cellulose acetate with a high degree of substitution by ftir and its application, Analytical Methods 9(43) (2017) 6194-6201. [29] K. Mitsui, Acetylation of wood causes photobleaching, J Photochem Photobiol B 101(3) (2010) 210-4.

REFERENCES | 39

[30] C. Hansmann, M. Schwanninger, B. Stefke, B. Hinterstoisser, W. Gindl, Uv-microscopic analysis of acetylated spruce and birch cell walls, Holzforschung 58(5) (2004) 483-488. [31] P. Evans, J. Haase, A. Seman, M. Kiguchi, The search for durable exterior clear coatings for wood, Coatings 5(4) (2015) 830-864. [32] C. Jourdain, J. Dwyer, K. Kersell, D. Mall, K. Mcclelland, R. Springate, S. Williams, Changing nature of wood products—what does it mean for coatings and finish performance?, Journal of Coatings Technology 71(890) (1999) 61-66. [33] G. Barati Darband, M. Aliofkhazraei, S. Khorsand, S. Sokhanvar, A. Kaboli, Science and engineering of superhydrophobic surfaces: Review of corrosion resistance, chemical and mechanical stability, Arabian Journal of Chemistry (2018). [34] X. Zhang, F. Shi, J. Niu, Y. Jiang, Z. Wang, Superhydrophobic surfaces: From structural control to functional application, J. Mater. Chem. 18(6) (2008) 621-633. [35] H. Teisala, M. Tuominen, J. Kuusipalo, Superhydrophobic coatings on cellulose-based materials: Fabrication, properties, and applications, Advanced Materials Interfaces 1(1) (2014) 1300026. [36] C. Chen, M. Liu, L. Zhang, Y. Hou, M. Yu, S. Fu, Mimicking from rose petal to lotus leaf: Biomimetic multiscale hierarchical particles with tunable water adhesion, ACS Appl Mater Interfaces 11(7) (2019) 7431-7440. [37] X. Zhao, B. Yu, J. Zhang, Transparent and durable superhydrophobic coatings for anti-bioadhesion, J Colloid Interface Sci 501 (2017) 222-230. [38] P.S. Brown, B. Bhushan, Mechanically durable, superoleophobic coatings prepared by layer-by-layer technique for anti-smudge and oil-water separation, Sci Rep 5 (2015) 8701. [39] R. Helbig, J. Nickerl, C. Neinhuis, C. Werner, Smart skin patterns protect springtails, PLoS One 6(9) (2011) e25105. [40] S. Wang, K. Liu, X. Yao, L. Jiang, Bioinspired surfaces with superwettability: New insight on theory, design, and applications, Chem Rev 115(16) (2015) 8230-93. [41] Y. Sun, Z. Guo, Recent advances of bioinspired functional materials with specific wettability: From nature and beyond nature, Nanoscale Horizons 4(1) (2019) 52-76. [42] H. Teisala, H.J. Butt, Hierarchical structures for superhydrophobic and superoleophobic surfaces, Langmuir (2018). [43] A.K. Kota, G. Kwon, A. Tuteja, The design and applications of superomniphobic surfaces, NPG Asia Materials 6(7) (2014) e109-e109. [44] T.L. Liu, C.J. Kim, Repellent surfaces. Turning a surface superrepellent even to completely wetting liquids, Science 346(6213) (2014) 1096-100.

40 | REFERENCES

[45] N.J. Shirtcliffe, G. Mchale, M.I. Newton, C.C. Perry, F.B. Pyatt, Plastron properties of a superhydrophobic surface, Applied Physics Letters 89(10) (2006) 104106. [46] R. Poetes, K. Holtzmann, K. Franze, U. Steiner, Metastable underwater superhydrophobicity, Phys Rev Lett 105(16) (2010) 166104. [47] A. Cassie, S. Baxter, Wettability of porous surfaces, Transactions of the Faraday society 40 (1944) 546-551. [48] A. Marmur, Hydro- hygro- oleo- omni-phobic? Terminology of wettability classification, Soft Matter 8(26) (2012) 6867. [49] A. Marmur, C. Della Volpe, S. Siboni, A. Amirfazli, J.W. Drelich, Contact angles and wettability: Towards common and accurate terminology, Surface Innovations 5(1) (2017) 3-8. [50] J. Yong, F. Chen, Q. Yang, J. Huo, X. Hou, Superoleophobic surfaces, Chem Soc Rev 46(14) (2017) 4168-4217. [51] H. Guo, P. Fuchs, K. Casdorff, B. Michen, M. Chanana, H. Hagendorfer, Y.E. Romanyuk, I. Burgert, Bio-inspired superhydrophobic and omniphobic wood surfaces, Advanced Materials Interfaces 4(1) (2017) 1600289. [52] K. Tu, X. Wang, L. Kong, H. Chang, J. Liu, Fabrication of robust, damage-tolerant superhydrophobic coatings on naturally micro-grooved wood surfaces, RSC Advances 6(1) (2016) 701-707. [53] M. Tuominen, H. Teisala, J. Haapanen, J.M. Mäkelä, M. Honkanen, M. Vippola, S. Bardage, M.E.P. Wålinder, A. Swerin, Superamphiphobic overhang structured coating on a biobased material, Applied Surface Science 389 (2016) 135-143. [54] S.K. Pandit, B.K. Tudu, I.M. Mishra, A. Kumar, Development of stain resistant, superhydrophobic and self-cleaning coating on wood surface, Progress in Organic Coatings 139 (2020) 105453. [55] Y. Wang, S. Vitas, I. Burgert, E. Cabane, Beech wood cross sections as natural templates to fabricate superhydrophobic surfaces, Wood Science and Technology 53(5) (2019) 985-999. [56] Q. Yao, C. Wang, B. Fan, H. Wang, Q. Sun, C. Jin, H. Zhang, One-step solvothermal deposition of zno nanorod arrays on a wood surface for robust superamphiphobic performance and superior ultraviolet resistance, Sci Rep 6 (2016) 35505. [57] S. Jia, M. Liu, Y. Wu, S. Luo, Y. Qing, H. Chen, Facile and scalable preparation of highly wear-resistance superhydrophobic surface on wood substrates using silica nanoparticles modified by vtes, Applied Surface Science 386 (2016) 115-124. [58] L. Gao, W. Gan, S. Xiao, X. Zhan, J. Li, A robust superhydrophobic antibacterial ag–tio2 composite film immobilized on wood substrate for photodegradation of phenol under visible-light illumination, Ceramics International 42(2) (2016) 2170-2179.

REFERENCES | 41

[59] B. Hui, Y. Li, Q. Huang, G. Li, J. Li, L. Cai, H. Yu, Fabrication of smart coatings based on wood substrates with photoresponsive behavior and hydrophobic performance, Materials & Design 84 (2015) 277-284. [60] M.A. Tshabalala, V. Yang, R. Libert, Surface modification of wood by alkoxysilane sol–gel deposition to create anti-mold and anti-fungal characteristics, Silanes and Other Coupling Agents 5 (2009) 135-147. [61] Z. Chu, S. Seeger, Robust superhydrophobic wood obtained by spraying silicone nanoparticles, RSC Advances 5(28) (2015) 21999-22004. [62] G.R.J. Artus, S. Jung, J. Zimmermann, H.P. Gautschi, K. Marquardt, S. Seeger, Silicone nanofilaments and their application as superhydrophobic coatings, Advanced Materials 18(20) (2006) 2758-2762. [63] L. Gao, T.J. Mccarthy, A perfectly hydrophobic surface (θa/θr= 180/180), Journal of the American Chemical Society 128(28) (2006) 9052-9053. [64] D.-a.E. Rollings, S. Tsoi, J.C. Sit, J.G. Veinot, Formation and aqueous surface wettability of polysiloxane nanofibers prepared via surface initiated, vapor-phase polymerization of organotrichlorosilanes, Langmuir 23(10) (2007) 5275-5278. [65] L. Gao, T.J. Mccarthy, (ch3) 3sicl/sicl4 azeotrope grows superhydrophobic nanofilaments, Langmuir 24(2) (2008) 362-364. [66] H.S. Khoo, F.G. Tseng, Engineering the 3d architecture and hydrophobicity of methyltrichlorosilane nanostructures, Nanotechnology 19(34) (2008) 345603. [67] J. Zhang, S. Seeger, Polyester materials with superwetting silicone nanofilaments for oil/water separation and selective oil absorption, Advanced Functional Materials 21(24) (2011) 4699-4704. [68] J.T. Korhonen, T. Huhtamäki, T. Verho, R.H.A. Ras, Hollow polysiloxane nanostructures based on pressure-induced film expansion, Surface Innovations 2(2) (2014) 116-126. [69] H. Sai, R. Fu, L. Xing, J. Xiang, Z. Li, F. Li, T. Zhang, Surface modification of bacterial cellulose aerogels' web-like skeleton for oil/water separation, ACS Appl Mater Interfaces 7(13) (2015) 7373-81. [70] E.G. Atici, E. Kasapgil, I. Anac, H.Y. Erbil, Methyltrichlorosilane polysiloxane filament growth on glass using low cost solvents and comparison with gas phase reactions, Thin Solid Films 616 (2016) 101-110. [71] M. Meier, V. Dubois, S. Seeger, Reduced bacterial colonisation on surfaces coated with silicone nanostructures, Applied Surface Science 459 (2018) 505-511. [72] M.A. Shirgholami, M. Shateri-Khalilabad, M.E. Yazdanshenas, Effect of reaction duration in the formation of superhydrophobic polymethylsilsesquioxane nanostructures on cotton fabric, Textile Research Journal 83(1) (2012) 100-110.

42 | REFERENCES

[73] Z.H. Liu, X.Q. Pang, K.T. Wang, X.S. Lv, X.M. Cui, Superhydrophobic coatings prepared by the in situ growth of silicone nanofilaments on alkali-activated geopolymers surface, ACS Appl Mater Interfaces (2019). [74] E. Kasapgil, I. Anac, H.Y. Erbil, Transparent, fluorine-free, heat-resistant, water repellent coating by infusing slippery silicone oil on polysiloxane nanofilament layers prepared by gas phase reaction of n-propyltrichlorosilane and methyltrichlorosilane, Colloids and Surfaces A: Physicochemical and Engineering Aspects 560 (2019) 223-232. [75] J. Zhang, B. Yu, Z. Gao, B. Li, X. Zhao, Durable, transparent, and hot liquid repelling superamphiphobic coatings from polysiloxane-modified multiwalled carbon nanotubes, Langmuir 33(2) (2017) 510-518. [76] E. Kasapgil, E.G. Atici, R. Cicek, I. Anac, H.Y. Erbil, Superhydrophobic polysiloxane filament growth on non-activated polymer coatings, RSC Advances 6(78) (2016) 74921-74928. [77] J. Zhang, A. Wang, S. Seeger, Universal self-assembly of organosilanes with long alkyl groups into silicone nanofilaments, Polym. Chem. 5(4) (2014) 1132-1139. [78] G.R.J. Artus, S. Seeger, Scale-up of a reaction chamber for superhydrophobic coatings based on silicone nanofilaments, Industrial & Engineering Chemistry Research 51(6) (2012) 2631-2636. [79] D.-a.E. Rollings, J.G. Veinot, Polysiloxane nanofibers via surface initiated polymerization of vapor phase reagents: A mechanism of formation and variable wettability of fiber-bearing substrates, Langmuir 24(23) (2008) 13653-13662. [80] J. Zimmermann, M. Rabe, G.R.J. Artus, S. Seeger, Patterned superfunctional surfaces based on a silicone nanofilament coating, Soft Matter 4(3) (2008) 450. [81] G.R. Artus, S. Seeger, One-dimensional silicone nanofilaments, Adv Colloid Interface Sci 209 (2014) 144-62. [82] R. Chen, X. Zhang, Z. Su, R. Gong, X. Ge, H. Zhang, C. Wang, Perfectly hydrophobic silicone nanofiber coatings: Preparation from methyltrialkoxysilanes and use as water-collecting substrate, The Journal of Physical Chemistry C 113(19) (2009) 8350-8356. [83] J. Zhang, S. Seeger, Superoleophobic coatings with ultralow sliding angles based on silicone nanofilaments, Angew Chem Int Ed Engl 50(29) (2011) 6652-6. [84] J. Zimmermann, G.R.J. Artus, S. Seeger, Superhydrophobic silicone nanofilament coatings, Journal of Adhesion Science and Technology 22(3-4) (2008) 251-263. [85] Accsys, What is acetylation? How we enhance nature. https://www.accsysplc.com/products/what-is-acetylation/. (Accessed 28 September 2020).

https://www.accsysplc.com/products/what-is-acetylation/

REFERENCES | 43

[86] J. Zhang, S. Seeger, Silica/silicone nanofilament hybrid coatings with almost perfect superhydrophobicity, Chemphyschem 14(8) (2013) 1646-1651. [87] S. Olveira, A. Stojanovic, S. Seeger, Systematic parametric investigation on the cvd process of polysiloxane nano-and microstructures, Journal of Nanoparticle Research 20(11) (2018) 307. [88] G.R. Artus, S. Olveira, D. Patra, S. Seeger, Directed in situ shaping of complex nano- and microstructures during chemical synthesis, Macromol Rapid Commun 38(4) (2017). [89] J. Zimmermann, F.A. Reifler, G. Fortunato, L.-C. Gerhardt, S. Seeger, A simple, one-step approach to durable and robust superhydrophobic textiles, Advanced Functional Materials 18(22) (2008) 3662-3669. [90] M. Sedighi Moghaddam, G. Heydari, M. Tuominen, M. Fielden, J. Haapanen, J.M. Mäkelä, M.E.P. Wålinder, P.M. Claesson, A. Swerin, Hydrophobisation of wood surfaces by combining liquid flame spray (lfs) and plasma treatment: Dynamic wetting properties, Holzforschung 70(6) (2016). [91] M. Sedighi Moghaddam, M.E. Walinder, P.M. Claesson, A. Swerin, Multicycle wilhelmy plate method for wetting properties, swelling and liquid sorption of wood, Langmuir 29(39) (2013) 12145-53. [92] M.E. Wålinder, G. Ström, Measurement of wood wettability by the wilhelmy method. Part 2. Determination of apparent contact angles, Holzforschung 55(1) (2001) 33-41. [93] R. Rowell, F. Bongers, Coating acetylated wood, Coatings 5(4) (2015) 792-801. [94] A.G. Cunha, C. Freire, A. Silvestre, C. Pascoal Neto, A. Gandini, M.N. Belgacem, D. Chaussy, D. Beneventi, Preparation of highly hydrophobic and lipophobic cellulose fibers by a straightforward gas-solid reaction, J Colloid Interface Sci 344(2) (2010) 588-95. [95] Z. Tang, L. Xie, D.W. Hess, V. Breedveld, Fabrication of amphiphobic softwood and hardwood by treatment with non-fluorinated chemicals, Wood Science and Technology 51(1) (2016) 97-113. [96] Š. Barcík, M. Gašparík, E.Y. Razumov, Effect of temperature on the color changes of wood during thermal modification, Cellulose chemistry and technology 49(9-10) (2015) 789-798. [97] Y. Chen, Y. Fan, J. Gao, N.M. Stark, The effect of heat treatment on the chemical and color change of black locust (robinia pseudoacacia) wood flour, BioResources 7(1) (2012) 1157-1170. [98] K. Lundquist, Acid degradation of lignin, Acta Chem. Scand 24(3) (1970). [99] B. Sundqvist, Colour changes and acid formation in wood during heating, Luleå tekniska universitet, (2004). [100] S. Chen, Z. Ling, X. Zhang, Y.S. Kim, F. Xu, Towards a multi-scale understanding of dilute hydrochloric acid and mild 1-ethyl-3-

44 | REFERENCES

methylimidazolium acetate pretreatment for improving enzymatic hydrolysis of poplar wood, Industrial Crops and Products 114 (2018) 123-131. [101] V. Kučerová, R. Lagaňa, E. Výbohová, T. Hýrošová, The effect of chemical changes during heat treatment on the color and mechanical properties of fir wood, BioResources 11(4) (2016) 9079-9094. [102] B. Esteves, A. Velez Marques, I. Domingos, H. Pereira, Heat-induced colour changes of pine (pinus pinaster) and eucalypt (eucalyptus globulus) wood, Wood Science and Technology 42(5) (2007) 369-384. [103] M. Miwa, A. Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe, Effects of the surface roughness on sliding angles of water droplets on superhydrophobic surfaces, Langmuir 16(13) (2000) 5754-5760. [104] A. Tuteja, W. Choi, M. Ma, J.M. Mabry, S.A. Mazzella, G.C. Rutledge, G.H. Mckinley, R.E. Cohen, Designing superoleophobic surfaces, Science 318(5856) (2007) 1618-1622. [105] N. Michael, B. Bhushan, Hierarchical roughness makes superhydrophobic states stable, Microelectronic Engineering 84(3) (2007) 382-386. [106] H.-J. Butt, C. Semprebon, P. Papadopoulos, D. Vollmer, M. Brinkmann, M. Ciccotti, Design principles for superamphiphobic surfaces, Soft Matter 9(2) (2013) 418-428. [107] A.K. Kota, W. Choi, A. Tuteja, Superomniphobic surfaces: Design and durability, MRS Bulletin 38(05) (2013) 383-390. [108] H. Bellanger, T. Darmanin, E.T. De Givenchy, F. Guittard, Influence of long alkyl spacers in the elaboration of superoleophobic surfaces with short fluorinated chains, RSC Advances 3(16) (2013) 5556. [109] M. Psarski, J. Marczak, G. Celichowski, G. Sobieraj, K. Gumowski, F. Zhou, W. Liu, Hydrophobization of epoxy nanocomposite surface with 1h,1h,2h,2h-perfluorooctyltrichlorosilane for superhydrophobic properties, Open Physics 10(5) (2012). [110] K. Honda, M. Morita, H. Otsuka, A. Takahara, Molecular aggregation structure and surface properties of poly (fluoroalkyl acrylate) thin films, Macromolecules 38(13) (2005) 5699-5705. [111] K. Honda, M. Morita, O. Sakata, S. Sasaki, A. Takahara, Effect of surface molecular aggregation state and surface molecular motion on wetting behavior of water on poly(fluoroalkyl methacrylate) thin films, Macromolecules 43(1) (2010) 454-460. [112] H. Bellanger, T. Darmanin, E. Taffin De Givenchy, F. Guittard, Chemical and physical pathways for the preparation of superoleophobic surfaces and related wetting theories, Chem Rev 114(5) (2014) 2694-2716. [113] T. Merkel, V. Bondar, K. Nagai, B. Freeman, I. Pinnau, Gas sorption, diffusion, and permeation in poly (dimethylsiloxane), Journal of Polymer Science Part B: Polymer Physics 38(3) (2000) 415-434.

REFERENCES | 45

[114] E. Favre, P. Schaetzel, Q. Nguygen, R. Clement, J. Neel, Sorption, diffusion and vapor permeation of various penetrants through dense poly (dimethylsiloxane) membranes: A transport analysis, Journal of Membrane Science 92(2) (1994) 169-184. [115] Lee J N, Park C, W.G. M, Solvent compatibility of poly(dimethylsiloxane)-based microfluidic devices, Analytical chemistry 75(23) (2003) 6544-6554.

licentiate thesis in civil and architectural engineering licenta ...1499276/... · 2020. 11. 8. ·...

Documents