Wettability and spectroscopic characterization of silylated wood samples

POLYMERS FOR ADVANCED TECHNOLOGIES

Polym. Adv. Technol. 2006; 17: 932–939

terscience.wiley.com). DOI: 10.1002/pat.778

Wettability and spectroscopic characterization

of silylated wood samplesy

Ildiko Mohammed-Ziegler1,2*, Zoltan Horvolgyi3, Andras Toth4, Willis Forsling2 and

Allan Holmgren2

1Arany J. u. 28., H-2521 Csolnok, Hungary2Division of Chemistry, Lulea University of Technology, SE-97187 Lulea, Sweden3Department of Physical Chemistry, Budapest University of Technology and Economics, Budafoki ut 8., H-1521 Budapest, Hungary4Research Laboratory of Materials and Environmental Chemistry, Chemical Research Center of the Hungarian Academy of Sciences,

Pusztaszeri ut 59-67, H-1025 Budapest, Hungary

Received 2 December 2005; Revised 17 February 2006; Accepted 2 March 2006

*CorrespoQuality CHungaryE-mail: my8th InteTechnolo2005, Par

Surfaces of three European wood species (namely, English oak, Hungarian oak, and Scots pine) and

two tropical wood species (namely, teak and cloves) were silylated with dichlorodimethylsilane

(DDS), dichlorodiphenylsilane (DPS) and octadecyltrichlorosilane (OTS) in the first step, and with

chlorotrimethylsilane (CTMS) as a monofunctional reagent in a second step. Attenuated total

reflection-Fourier transform infrared (ATR-FT-IR) and electron spectroscopy for chemical analysis

(ESCA) measurements were performed in order to characterize the surface composition of the treated

samples. The progress of surface modification was also monitored by contact angle measurements of

different test liquids (water, formamide and diiodomethane). The water contact angles were found to

be in the range of 808 to 1458. The most hydrophobic surface was obtained by two-step silylation by

OTS and CTMS (water contact angles were between 1318 and 1408). Interestingly, the hydrophobicityof DDS-treated samples showed a significant decrease after additional silylation by CTMS. The

surface free energy values were evaluated in terms of the Lifshitz-van der Waals/acid-base theory.

The results obtained by the surface analytical methods (i.e. FT-IR and ESCA measurements) were

compared with the contact angle data. These results suggest that in the case of DDS-treated samples,

CTMS molecules substitute partly the DDS molecules, thus only physisorption of the silylating

agents is supposed at room temperature. Copyright # 2006 John Wiley & Sons, Ltd.

KEYWORDS: wood; silylation; ESCA/XPS; FT-IR

It is well-known that water is essential for life, including the

growth of micro-organisms, and that wood, an abundant

natural material, is ideal to feed many of these microbes. The

metabolism of these micro-organisms, however, leads to the

degradation of wood. Such a decay of wooden objects is not

desirable from an economical point of view. Therefore, the

application of various surface treatments against microbial

infection of wood has become more and more common. One

possibility to inhibit or slow down wood decay is to render

the wood surface hydrophobic, e.g. by silylation in order to

hinder the multiplication of fungi and bacteria. Increased

hydrophobicity decreases the amount of adsorbed water and

diminishes the adhesion of the microbial cells.

Various analytical methods are used to follow chemical

changes of wood and wood containing composites. FT-IR

spectroscopy is an efficient tool for the detection of changes

ndence to: I. Mohammed-Ziegler, Gedeon Richter Ltd,ontrol Department, Esztergomi ut 27, H-2510 Dorog,.ohazihu@yahoo.comrnational Symposium on Polymers for Advancedgies 2005 (PAT 2005), Budapest, 13–16 September,t 2.

at the molecular level caused by mechanical effects,

irradiation by light, chemical modification or other treat-

ments of wood.1–17 X-ray photoelectron spectroscopy (XPS)/

electron spectroscopy for chemical analysis (ESCA) is also a

worldwide spread method to follow changes in surface

composition at the atomic level (see, for example, Refs. 1, 5, 6,

18–23). The results, obtained by analytical methods on

surface treated wood samples, often can be directly

correlated with the wetting properties of wood, as has been

established by many researchers.8,18,23–27 The mentioned

analytical techniques are also suitable to follow the process

and extent of silylation reaction, which is well-known and a

very efficient way of surface modification (see, for example,

Refs. 28–30). However, to the best of the authors’ knowledge

only a few papers have been reported on silylation of wood

so far.1,3,31–33

In a previous work, six European and six tropical wood

species were treated by three different silylating agents using

Copyright # 2006 John Wiley & Sons, Ltd.

Silylated wood samples 933

chloroform solutions of trimethylsilyl N,N-dimethylcarba-

mate, chlorotrimethylsilane (CTMS) and octadecyltrichlor-

osilane (OTS)31 and characterized by contact angle

measurements. The surface free energies were also deter-

mined analyzing the measured contact angles in order to

characterize the wettability of the samples. Further charac-

terization on selected samples was carried out by attenuated

total reflection-Fourier transform infrared (ATR-FT-IR)

spectroscopy and ESCA to obtain information about the

molecular composition of the treated surfaces and the

efficiency of the chemical modifications.1

In an attempt to increase the hydrophobicity of the

samples in the present study, additional silylating agents

were tested individually and in combination with each other,

using n-hexane as solvent and applying different time of

silylation. The surface modified samples were studied by

contact angle measurements, ATR-FT-IR and ESCA spectro-

scopic methods. In order to obtain a deeper understanding of

the wetting results, the surface free energies were also

determined by analyzing the contact angles in terms of the

Lifshitz-van der Waals/acid-base approach (LW-AB). The

results obtained in the present work were compared with

those of previous studies.1,31

EXPERIMENTAL

MaterialsThree European and two tropical wood samples were

studied. English oak (Quercus robur L.), and Scots pine

(Pinus sylvestris L.) were purchased from Sweden. Hungarian

oak (Quercus frainetto Ten. ) was obtained from Hungary.

Furthermore, two exotic wood samples, teak (Factona grandis)

and cloves tree (Eugenia aromatica) originating from Zanzi-

bar, Tanzania, were investigated. The African samples were

air dried for 6 weeks in Tanzania during the dry season and

posted to Hungary. Before the examinations they were

further air dried for at least 2 months. In order to ensure

comparability, the wood samples under study are identical

with earlier ones, reported in previous work.1–3,31 The wood

samples were cut into ca. 2mm thick plates parallel with the

fibres. Finally, each plate was successively polished in order

to obtain as smooth surface as possible using, in the last

stage, MX P320 waterproof silicon carbide polish paper

supplied by Struers.

The following chemicals were used. Silylating agents

included CTMS (Merck, �99%), OTS (Merck, �98%),

dichlorodiphenylsilane (DPS) (Fluka, �98.5%) and dichlor-

odimethylsilane (DDS) (Fluka, �98%). Wettability test

liquids included formamide (Aldrich, �99%), diiodo-

methane (Sigma, �99%), n-hexane (Merck, �99%), and ion

exchanged water (Millipore, 18.2MVcm).

Silanization in solutionThe treatment was adapted from the literature.34 Thus 1 vol%

n-hexane solutions of DDS, DPS and OTS were prepared.

Each wood sample (with an area of about 15 cm2) was kept in

the silanizing solution of 20ml under continuous stirring at

room temperature (in a fumehood) for 5min. Then the

samples were rinsed with pure n-hexane and air-dried. Some

of the samples were silanized again with the 1 vol% n-hexane

Copyright # 2006 John Wiley & Sons, Ltd.

solution of CTMS under identical conditions in order to

improve the silylation by silylating the (possibly polymer-

ized) precursors and the unreacted ligand groups.

ATR-FT-IR measurementsThese measurements were performed in order to obtain

information about the surface reaction and to assess semi-

qualitatively the amount of adsorbed reagents at the wood

surface. For the experiments a Perkin–Elmer System 2000 FT-

IR spectrometer equipped with a Specac (ATR) accessory

was used. Themeasurements were carried out at a resolution

of 4 cm�1, 128 scanswere accumulated using aKRS-5 internal

reflection element. The spectra were processed using Nicolet

Omnic software.

Contact angle measurementsDuring the sessile drop measurements, the wood plate was

placed horizontally in a chamber and monitored by a video-

microscope. About a 4-ml-droplet of the test liquid was

placed on thewood surface using amicro-syringe. Thewhole

process was recorded as a function of time and the contact

angles were determined by analyzing the drop contour. The

height and the maximum diameter of the droplet were

measured in steady-state circumstances at about the fifth

second of drop formation. The measurements were per-

formed only along the wood grains. Average contact angles

were calculated from the measurements of 5–10 droplets.

Wood is a porous material, therefore, the wetting process

can be divided into three periods.35 (1) The first stage is

characterized by an increase of the droplet diameter and

simultaneously the height of the droplet reduces which

results in a significant decrease of the contact angle. At the

end of this stage, temporary stabilization takes place and

the diameter becomes constant. (2) During the second

stage, the diameter of the droplet remains constant although

the height of the drop still decreases. Therefore, the contact

angle diminishes. Liptakova and Kudelka35 supposed that at

the end of this period all pores and holes in the surface layer

of wood is filled with liquid. (3) Finally, the liquid gradually

penetrates into the wood due to capillary forces and

consequently the diameter of the drop and the contact angle

decreases. Therefore, the contact angles are measured in the

stationary period of elapsed time (at the end of the first stage)

that corresponds to about the fifth seconds in the present

system. However, the contact angles should remain constant

or almost constant in higher contact angle range (>908)during the whole measurement, due to the hindered

imbibtion of the liquid.

The evaluation of the data is based on the LW-AB model

(for details see Refs. 30, 36, 37). The relations used for

calculations are the following:

ð1þ cos uÞg lv ¼ 2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffigLWsv gLWlv

qþ

ffiffiffiffiffiffiffiffiffiffiffiffiffigþsvg

�lv

qþ

ffiffiffiffiffiffiffiffiffiffiffiffiffig�svg

þlv

q� �(1)

gLWsv ¼ 0:25gLW

jv 1þ cos uj� �2

(2)

where g is the surface tension and u is the contact angle. The

subscripts s, l and v denote solid, liquid and vapour phases,

respectively. LW symbolizes the Lifshitz-van der Waals

component, gþ denotes the acidic (electron acceptor)

Polym. Adv. Technol. 2006; 17: 932–939

DOI: 10.1002/pat

Table 1. Physical properties and surface free energy components of test liquids used at 208C31

Surface free energy (mJm�2) (LW-AB approach)

Liquid Density (kgm�3) Viscosity (mPa sec) gLWlv gþlv g�lv gABlv glv

Water 1000 1.00 21.8 25.5 25.5 51.0 72.8Formamide 799 1.02 39 2.28 39.6 19 58Diiodomethane 3325 2.8 50.8 0 0 0 50.8

934 I. Mohammed-Ziegler et al.

parameter and g� is the basic (electron donor) parameter of

the surface free energy. The subscript j denotes diido-

methane, the apolar test liquid. If the contact angles of at least

three test liquids (with known surface free energy com-

ponents, see Table 1) are measured on the same solid surface,

the surface free energy components of the solid surface can

be calculated by using eqns. (1) and (2). As a reference, the

measured contact angle values of untreated wood samples

and their calculated surface free energy components are

summarized in Table 2.

ESCA experimentsMeasurements by XPS/ESCA were performed using a

Kratos XSAM800 spectrometer using Mg-Ka1,2radiation.

Data acquisition and processing were done by the Kratos

Vision 2 software.

RESULTS AND DISCUSSION

ATR-FT-IR measurementsFigures 1–4 show the regions of hydroxyl and C-H stretching

vibrations. In general, the intensity of n(CH) is linearly

increasing with the number/surface concentration of the

CH2 or CH3 units.

As illustrated in Fig. 1, OTS-silylation can be readily

detected due to the strong IR absorption of the methylene

groups. The asymmetric stretching vibration of the C–H

appears at 2927 cm�1, whereas the symmetric n(CH2) is

revealed at 2845 cm�1. The intensity of these bands reflects

the fact that different wood species can be silylated to

different extents. For comparison, the ATR-FT-IR spectra of

raw cloves and teak are also shown on the graph. As

apparent from the spectra (Fig. 1), Scots pine, teak and cloves

samples are silylated to a relatively remarkable extent,

whereas English oak bonds noticeably less OTS on its surface

than the other studied species. The orientation of the long

Table 2. Contact angles of different test liquids obtained on untre

calculated on the basis of the LW-AB model31

Contact angle (deg)

Wood samples Water Formamide Diiodo

Scots pine 46� 4 11� 3 20English oak 81� 4 32� 1 33Hungarian oaka 69� 3 28� 2 21Cloves tree 91� 1 42� 2 37Teak 18� 4 24� 2 34

aMeasured in this study.

Copyright # 2006 John Wiley & Sons, Ltd.

alkyl chains cannot be estimated from these measurements,

however, it is reasonable to assume that the octadecyl chains

are oriented randomly, since the surface of wood is porous

and inhomogeneous.

Further silylation by CTMS results in vibrational bands at

2962, 2918 and 2850 cm�1, as shown in Fig. 2, where the IR

spectra of OTS and CTMS silylated English oak, Hungarian

oak, Scots pine and teak samples are presented. The band at

2962 cm�1 is quite weak and it is tentatively assigned to the C–

H stretching mode of the methyl groups of the attached CTMS

[n(CH3), see, for example, Ref. 38]. The other two vibrations at

lower frequencies correspond to the attached OTS.

As can be seen in Fig. 3, silylation by DDS and both DDS

and CTMS can also be followed by ATR-FT-IR experiments.

A characteristic n(CH3) band appears at 2958 cm�1 in the

spectra of both Scots pine and English oak that corresponds

to n(CH3) of the reagents. Silylation with DDS results in a

new band at 2959 cm�1 (as reference, the IR spectra of the

reactants were used from Ref. 38). It is important to note that

this vibration is distinct and can be unambiguously

differentiated from the n(CH3) of CTMS that appears slightly

shifted at 2962 cm�1. The other two bands at 2919 and

2851 cm�1 correspond to asymmetric and symmetric C–H

stretchings of CH2, respectively, probably originating from

hexane residue attached onto the wood surface together with

CTMS (it differs somewhat from the locations of analogous

vibrations of OTS).

However, the n(CH3) of DDS cannot be obtained by curve-

fitting from the measured envelop, therefore, it can be

assumed from the intensity ratios of the different CH3

stretching modes that post-silylation by CTMS results in a

partial exchange of the attached DDS by CTMS. Hence, it is

reasonable to assume that the DDS molecules are only

physisorbed on the wood surface that is why exchange of

reagents, DDS and CTMS, can occur. However, it is not clear

from the spectra if CTMS reacted with unreacted hydroxyl

ated wood samples and the surface free energy components

Surface free energy (mJm�2)

methane gLWsv gþsv g�sv gtot

� 1 47.6 1.2 24.2 58.5� 1 42.9 3.5 0.3 45.1� 1 47.4 1.7 5.0 56.1� 2 41.2 3.1 0.2 42.8� 1 42.6 0.2 59.3 50.3

Polym. Adv. Technol. 2006; 17: 932–939

DOI: 10.1002/pat

Figure 1. ATR-FT-IR spectra of OTS-silylated wood samples

(spectra are off-set for clarity).

Figure 3. ATR-FT-IR spectra of DDS and DDSþCTMS-sily-

lated Scots pine and English oak (spectra are off-set for

clarity).

Silylated wood samples 935

groups of the wood surface or CTMS simply substituted the

physisorbed DDS in part.

ATR-FT-IR spectra of some DPS-treated wood samples are

shown in Fig. 4. Aromatic C–H stretchings are located at

3072, 3049, 3023 and 2959 cm�1 (in good agreement with the

reference spectrum of neat DPS).38 The most characteristic

aromatic C–H stretching bands are marked by arrows in

Fig. 4. As apparent from the spectra, the different wood

species can be silylated by DPS to different extents. Solvent

residues are also detected on these wood samples, and the

locations of the aliphatic n(CH2) are at 2920 and 2851 cm�1.

Contact angle measurements

The role of silylation timeThe effect of the duration of silylation on surface properties

of wood has already been studied in the case of OTS and

CTMS.31 Therefore, only surface modification with DDS was

studied in this work. The water contact angle and the total

surface free energy data of the treated Scots pine sapwood

samples are shown in Fig. 5. The calculation is based on the

LW-AB theory. It is interesting to note that water contact

angles do not differ significantly if the sample is silylated for

Figure 2. ATR-FT-IR spectra of OTSþCTMS-silylated wood

samples (spectra are off-set for clarity).

Copyright # 2006 John Wiley & Sons, Ltd.

5min or for 15min, whereas, those increase remarkably after

a 30min or 60min silylation. The calculated surface free

energies, however, do not reflect this behavior. These values

decrease with time of silylation as expected but, surprisingly,

reach a minimum after a 30min reaction time and increase if

the sample is treated for an hour. Though it is not understood

clearly but one can suppose that extractable silylated (more

or less hydrophobic) small molecules migrate from the wood

sample into the organic test liquid decreasing the contact

angles significantly (see, for example, the extraordinary low

contact angles of diiodomethane in Tables 3 and 4). Hence,

the uncertainty of the determination of surface free energies

also becomes larger if the test liquid extracts some

components from the observed solid. Therefore, conclusions

must be drawn cautiously from contact angle measurements.

Water contact angles are more reliable since they do not

exhibit so pronounced changes during the measurements

and cannot dissolve easily the silylated organic wood

components.

The result of the one-step silylationAfter the time-dependence study, 5min was chosen as the

duration of silylation for further experiments in this study.

The observed contact angles and the calculated surface free

Figure 4. ATR-FT-IR spectra of DPS-silylated Scots pine,

English oak and cloves (spectra are off-set for clarity).

Polym. Adv. Technol. 2006; 17: 932–939

DOI: 10.1002/pat

Figure 5. Water contact angle (&) and total surface free

energy (*) on Scots pine sapwood samples treated with

DDS for different times obtained by the LW-AB approach.

936 I. Mohammed-Ziegler et al.

energy values are summarized in Table 3. As can be seen

from the water contact angle data, OTS renders the wood

surfaces more hydrophobic than DDS in line with expec-

tations, in most cases. The long alkyl chain of OTS can

certainly render the surface hydrophobic more effectively.

This also meets expectations even if polymerization of

precursors can occur.

Cloves wood differs from other wood samples. It was less

efficiently silylated by DDS than other samples. In line with

this, it exhibits relatively high surface free energy after the

treatment with DDS as compared with other studied

samples, mainly due to the extraordinary low diiodo-

methane contact angle. If cloves tree is treated with OTS,

the effect changes in an opposite way: the water contact angle

is somewhat lower than in the case of the other species,

whereas the surface free energy is lower than those of the

other OTS-treated wood samples under study. Con-

sequently, it is reasonable to assume that cloves mainly

have extractable small molecules by diiodomethane in case

of using DDS for silylation. This assumption is in good

agreement with earlier observations when chloroform

solutions of the silylating agents were applied for cloves

tree.31 The OTS-treated teak also show a strange behaviour

Table 3. Contact angles of different test liquids obtained on woo

components calculated on the basis of the LW-AB model

Contact angle (deg)

Wood samples Water Formamide Diiodomet

1% DDSScots pine 129� 3 104� 1 66� 2English oak 108� 3 92� 3 70� 3Hungarian oak 125� 2 101� 2 71� 1Cloves tree 124� 1 87� 2 38� 2Teak 129� 2 108� 2 72� 11% OTSScots pine 134� 2 113� 2 68� 2English oak 133� 2 97� 2 73� 1Hungarian oak 135� 1 102� 2 61� 1Cloves tree 127� 1 100� 2 81� 2Teak 132� 1 113� 1 25� 2

Copyright # 2006 John Wiley & Sons, Ltd.

concerning the surface free energy, which can also be

attributed to diiodomethane contact angles: its hydrophobi-

city is similar to that of the other samples but its calculated

surface free energy is remarkably higher than that of the

other samples (one step silylation) due to the extraordinary

low diiodomethane contact angles. This behavior cannot be

explained at present. Further investigation is necessary to

reveal the reason for the observed low diiodomethane

contact angles on this sample. This fact also underlines the

importance of surface free energy calculations. Apparently a

single contact angle value does not characterize the surface

properties and behavior unambiguously.

When DPS was used as silylating reagent, the diiodo-

methane contact angles were surprisingly low and practi-

cally complete wetting was observed in the case of

Hungarian oak. Therefore only water contact angles are

presented (Fig. 6). As evident from the data, silylation with

DPS results in moderate increase of hydrophobicity, with the

exception of Scots pine, which exhibits the average water

contact angle of 1458. That is comparable with values

obtained by silylation with OTS.

The result of the two-step silylationThe most important result here is that the consecutive

silylation with the different agents leads to an improved

water-repellent property of wood surfaces applying OTS in

the first step (Table 4). However, the post-silylation by

CTMS, in the case of DDS, deteriorates the hydrophobicity

that was reached in the first step. The resultant water contact

angles in this case are slightly higher than those obtained

only with silylation by CTMS31 (Fig. 7). The fact that DDS

could be partly exchanged with CTMS also suggests that the

silylating agents do not chemisorb on the wood surface. It

seems that at room temperature only physisorption occurs,

in similarity to silylation of silica.39 It is reasonable to assume

that OTS has higher affinity to the wood surface than DDS.

Therefore, the attached OTS cannot be exchanged or washed

off easily from the treated wood surface. It is also possible in

the case of pre-silylation with OTS that the long alkyl chains

sterically hinder the migration of CTMS molecules onto the

wood surface, and therefore OTS cannot be squeezed out

from the active sites of the surface. In Table 4, the

d samples treated in one step and the surface free energy

Surface free energy (mJm�2)

hane gLWsv gþsv g�sv gtot

25.11 (�)1.75 (�)0.41 2622.86 (�)0.32 1.26 2222.30 (�)0.71 (�)0.20 2240.56 (�)1.15 2.69 4421.75 (�)1.89 (�)0.07 22

23.97 (�)3.84 (�)0.13 2421.20 (�)0.00 (�)3.62 2127.97 (�)1.49 (�)2.43 3216.97 (�)0.00 (�)0.76 1746.12 (�)15.51 0.00 46

Polym. Adv. Technol. 2006; 17: 932–939

DOI: 10.1002/pat

Figure 6. Water contact angles obtained on DPS-treated

Scots pine, English oak, Hungarian oak, cloves tree and teak.

Figure 7. Water contact angles obtained on Scots pine,

English oak, cloves and teak treated by DDS and

DDSþCTMS (in this study), and by CTMS.31

Silylated wood samples 937

corresponding diiodomethane contact angles are extremely

low. It reflects that diiodomethane may dissolve the

adsorbed silylating agent.

Due to the very low hydrophobicity of the DPS treated

wood samples, post-silylation of these samples was not

carried out.

It is also necessary to mention the appearance of high

contact angle values (due to the surface roughness) and

anomalous surface free energy components obtained for

water-repellent surfaces in this study. These results confirm

the observations of other authors (see, for example, Refs. 36,

40). In the present study, the suggestion of de Meijer et al.36

has been applied, i.e. if a negative value was obtained during

the calculation of the surface free energy components, the

component was taken into account as a negative contribution

to the total surface free energy of the solid surface. This

assumption is necessary to use because the solution of the

linear equation system [derived from eqns. (1) and (2)]

results in square roots of the surface free energy components.

Theoretically, the surface free energy value can only be

positive (see, for example, Ref. 37), thus its square root can

also be only positive. In ref. 36 the appearance of the negative

Table 4. Contact angles of different test liquids obtained on woo

components calculated on the basis of the LW-AB model

Contact angle (deg)

Wood samples Water Formamide Diiodometh

1% DDSþ 1% CTMSScots pine 95� 3 42� 2 26� 1English oak 80� 3 28� 2 32� 1Hungarian oak 101� 4 34� 2 32� 1Cloves tree 107� 1 70� 2 38� 1Teak 103� 2 44� 2 34� 11% OTSþ 1% CTMSScots pine 140� 2 128� 3 94� 2English oak 131� 2 117� 2 67� 3Hungarian oak 135� 4 126� 2 81� 2Cloves tree 136� 1 118� 1 80� 2Teak 141� 2 125� 2 77� 1

Copyright # 2006 John Wiley & Sons, Ltd.

square roots was explained by the observed very high water

contact angle values obtained on the hydrophobic wood

samples. The high contact angle is connected to the surface

roughness of these samples. The application of ‘‘negative

contributions’’ results in realistic surface free energy values

for the treated wood surfaces. However, this phenomenon is

evenmore complex in the present case for samples with even

more hydrophobic character, e.g. in the case of the

consecutive application of OTS and CTMS. In these cases

even the application of ‘‘negative contributions’’ does not

make it possible to calculate the total surface free energy of

the solid surface.

It is necessary to underline the fact that in the case of wood

samples, the possible dissolution of some components in one

of the test liquids, as was mentioned earlier, may cause

extraordinary low contact angles in certain cases. In these

cases, eqns. (1) and (2) should not be used, since the dissolved

components modify the surface tension values summarized

in Table 1. However, finding non-polar test liquids that do

not extract components from the wood samples seems

impossible due to the great variety of wood extractives.

Hence, the basic reason for the found controversy is very

d samples treated in two steps and the surface free energy

Surface free energy (mJm�2)

ane gLWsv gþsv g�sv gtot

45.76 2.57 (�)1.44 4243.34 4.17 0.20 4443.34 (�)12.32 11.12 ?40.56 3.06 (�)0.60 3942.46 4.16 (�)5.77 ?

3.25 5.28 (�)5.08 ?(�)125.8 192.7 (�)1.00 ?(�)106.1 150.3 (�)0.35 ?(�)61.49 110.57 1.47 ?(�)120.7 174.44 (�)2.01 ?

Polym. Adv. Technol. 2006; 17: 932–939

DOI: 10.1002/pat

938 I. Mohammed-Ziegler et al.

likely the dissolution of some component(s) in some of the

test liquid(s).

Moreover, surface roughness, which can be the main

reason for high contact angles as was mentioned earlier, is

not taken into account by eqns. (1) and (2) that can also cause

the aforementioned controversies. From this point of view, it

would be necessary to use some additional equations to re-

calculate the surface tension values, determined on a rough

surface. For this calculation, however, it would be necessary

to know the ratio of the apparently wetted surface area to the

total surface area of the solid but this information is not

available presently.

It is worth mentioning, however, that the application of

LW-AB theory can also cause some uncertainties in the

analysis. Kwok,41 for instance, determined the surface free

energy of ideal solid surfaces [namely fluorocarbon,

polystyrene, and poly(methyl methacrylate)] by means of

the LW-AB theory and found that the results altered

significantly if the test liquid triplet changed. These

experiences can question the applicability of LW-AB theory

for the calculation of surface free energy components, though

it is widely used in the literature.

In any case, a more thorough explanation of the present

findings requires further experimental and theoretical efforts

in the future.

ESCA measurementsTable 5 reports the results of elemental analysis (in atom%)

for the variously silylated Scots pine and English oak

samples, together with the corresponding O/Si and C/Si

atomic ratios. Since Si is a marker element for the coating, the

Si-content and/or the Si/O and Si/C ratios could in principle

give indications of the efficiency of silylation. However, this

is complicated by the fact that the Si/C ratios of the silylating

agents themselves are different and also their numbers of

reactive functionalities are different. (The calculated Si/C

values for DDS, DPS, OTS and CTMS are 0.50, 0.08, 0.06 and

0.33, respectively, and the corresponding numbers of

reactive functional groups are 2, 2, 3 and 1.) The silylating

agent applied in the first step consumes at least one

functional group upon reacting with the substrate (or one

active site in case of physisorption). The remaining func-

tional group(s) hydrolyze, since no chlorine was detected by

ESCA. Table 5 reflects clearly that post-silylation by CTMS

changes the surface composition for both the Scots pine and

the English oak, regardless if CTMS is applied after DDS or

after OTS. Comparing the results obtained for samples

Table 5. Results of ESCA measurements on treated Scots

pine and English oak samples

Treatment

Scots pine English oak

O C Si Si/O Si/C O C Si Si/O Si/C

DDS 29.9 60.3 9.9 0.33 0.16 34.9 57.7 7.4 0.21 0.13DDSþCTMS 31.2 61.1 7.7 0.25 0.13 37.2 57.2 5.6 0.15 0.10DPS 19.7 73.8 6.6 0.34 0.09 22.2 69.5 8.3 0.37 0.12OTS 12.5 83.0 4.5 0.36 0.05 15.6 80.1 4.3 0.28 0.05OTSþCTMS 14.7 80.1 5.2 0.35 0.06 13.4 80.8 5.8 0.43 0.07

Copyright # 2006 John Wiley & Sons, Ltd.

treated by OTSþCTMS with those treated by OTS alone, it

can be seen that the Si-content increased and the Si/O ratio

either remained practically unchanged (Scots pine) or

increased (English oak). A similar comparison for samples

treated by DDSþCTMS with those treated by DDS alone,

however, shows a decrease in both the Si-content and the Si/

O ratio. This suggests that post-silylation by CTMS shows

basically different mechanisms after OTS or DDS: after OTS-

treatment the post-silylation proceeds mainly at the

unreacted sites of the substrate and/or of the attached

OTS, while after DDS the exchange of some DDS moieties

with CTMS occurs. Thus, the results obtained by ESCA are in

good agreement with those obtained by contact angle

measurements, according to which a consecutive silylation

byDDS andCTMS leads to a partial removal of DDS from the

wood surface.

AcknowledgmentsThis work was supported in part by the Hungarian National

Scientific Foundation for Research (OTKA T 049156 and

T 037643). The authors are grateful for the help of Zanzibar

Ministry of Agriculture in obtaining the tropical wood

samples. This project participates in the action of COST E41.

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