Ageing Effect of Plasma Treated Wool

ABSTRACT

Atmospheric pressure plasma treatment of wool fabric, with a relatively short exposure time,

effectively removed the covalently-bonded lipid layer from the wool surface. The plasma-

treated fabric showed increased wettability and the fibres showed greater roughness. XPS

analysis showed a much more hydrophilic surface with significant increases in oxygen and

nitrogen concentrations and a decrease in carbon concentration. Adhesion, as measured by

scanning probe microscopy (SPM) force volume analysis, also increased, consistent with the

more hydrophilic surface leading to a greater meniscus force on the SPM probe.

The ageing of fibres from the plasma-treated fabric was assessed over a period of 28 days.

Whilst no physical changes were observed, the chemical nature of the surface changed

significantly. XPS showed a decrease in the hydrophilic nature of the surface with time,

consistent with the measured decrease in wettability. This change is proposed to be due to the

reorientation of proteolipid chains. SPM adhesion studies also showed the surface to be

changing with time.

After ageing for 28 days, the plasma-treated surface was relatively stable and still

dramatically different from the untreated fibre, suggesting that the oxidation of the surface

and modification or removal of the lipid layer were permanent.

Keywords: Plasma, surface treatment, ageing, adhesion, hydrophilic/hydrophobic, wool

1

INTRODUCTION

The outermost layer of the wool fibre surface is covered by an external thin membrane called

the epicuticle, which is a thin band of proteinaceous material containing a high concentration

of the amino acid, cystine (Bradbury, 1973; Leeder & Bradbury, 1968), and a significant

proportion of bound lipid layer (Evans et al., 1985). A number of workers have studied the

surface of wool fibre and a wool surface model has been developed (Breakspear et al., 2005;

Evans et al., 2002; Huson et al., 2008; Leeder & Bradbury, 1968; Leeder & Bradbury, 1971;

McLaughlin & Pope, 1982; Negri et al., 1993a; Negri et al., 1996).

The model proposed by Evans et al (Evans et al., 1985) describes the epicuticle as an

outermost fatty acid monolayer (F- Layer) and a protein matrix. The major lipid component

of the fatty layer was identified as a methyl-branched, 21-carbon fatty acid (Evans et al.,

1985). Negri et al. (1993a) proposed a model in which the entire covalently-bound lipid

originated from the epicuticle. It was suggested that the intact lipid membrane is attached to

the protein through thioester bonds (Negri et al., 1993a; Negri et al., 1993b); the thickness

was estimated to be about 2.8-3.0 nm (Negri et al., 1993a). Scanning probe microscopy

results supported Negri’s estimation (Crossley et al., 2000).

Time of flight secondary ion mass spectrometry (ToF-SIMS) analysis of untreated wool has

identified a fragment at 325 amu as 18-methyleicosanoic acid (18- MEA) (C21H41O2) (Shao

et al., 1997; Volooj et al., 2000; Ward et al., 1993). Negri et al (Negri et al., 1993a)

described this lipid as oriented away from the fibre and thus forming a hydrophobic barrier at

the surface of each cuticle cell. Negri’s model also explains the presence of a high proportion

of carbon in the outer 3-5 nm of the cuticle as shown by XPS (Carr et al., 1986; Ward et al.,

1993).

Recently, Huson et al. (2008) and Maxwell & Huson (2005) argued that the lipid is an

integral part of the surface, rather than a discrete surface layer, and that the surface is capable

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of altering its structure in response to different environments. It was postulated that under dry

conditions, the surface of the keratin fibres is rich in lipid and is hydrophobic, while in high

relative humidity (RH) or water, it is rich in protein and hydrophilic (Maxwell & Huson,

2005).

It has been shown that the chemical treatment using alkali, oxidants and reductants can

change the surface of wool from hydrophobic to hydrophilic (Leeder et al., 1985; Negri et al.,

1993b; Shao et al., 1997). Treatments with alcoholic KOH remove the 18-MEA from the

surface (Negri et al., 1996), while hydroxylamine treatment selectively removes 18-MEA

without producing any other chemical alteration to the fibre (Breakspear et al., 2005). In

addition, it has been shown that bleaching and permanent wave treatments with alkaline

peroxide remove some 18-MEA from the surface of hair fibre and oxidise cystine residue

(Robbins, 1988).

Surface analysis of plasma-treated wool has shown that plasma treatments modify the surface

of the wool fibre (Dai et al., 2001; Kan et al., 2004; Molina et al., 2005; Molina et al., 2002;

Naebe et al., 2010a; Naebe et al., 2010b; Ryu et al., 1991; Wakida et al., 1993). Regardless

of the treatment conditions used, the impact of plasma treatment is attributed to several

changes on the wool surface such as partial or total removal of the fatty acid layer, formation

of hydrophilic groups and an etching effect (Erra et al., 2002; Rakowski, 1997; Ryu et al.,

1987).

Although a number of workers studied the surface morphology of wool fibres using SPM,

their focus has been on estimating the thickness of the surface lipid layer and measuring the

mechanical properties of the fibre surface (Crossley et al., 2000; Gibson et al., 2001;

Maxwell & Huson, 2005). Few obtained the local physical information such as roughness and

adhesion. In addition, plasma treatment has been widely studied for its effect on the surface

of the wool fibre, but the ageing effect (changes on the plasma-treated wool fibre surface over

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time) has received little attention. The objective of this study is to investigate the effect of

plasma treatment on the wool fibre surface, and especially the effects of ageing for a period

of 28 days after the treatment.

EXPERIMENTAL

1. Materials, plasma treatment, XPS and water uptake

The base fabric was a plain-weave pure wool fabric of 222 g/m2 manufactured from 20 µm

fibre, and cleaned by soxhlet extraction as described previously (Naebe et al., 2010a). A

plasma treatment with an exposure time of 30 sec was carried out using an Atmospheric

Plasma Treatment System from Sigma Technologies International (Arizona, USA) (APC

2000), operated at ambient temperature with helium as the plasma gas, as described previously

(Naebe et al., 2010a).

XPS analysis was conducted using an Axis Ultra spectrometer (Kratos Analytical Ltd., UK

equipped with a monochromatic X-ray source (Al Kα, hν = 1486.6 eV), operating at 150 W.

Water uptake by the fabric was conducted by weighing a vertically-clamped fabric sample

immersed in water as described previously (Naebe et al., 2010a) by weighing a vertically-

clamped fabric sample immersed in water. The equilibrium water uptake was defined as the

point where sample weight reached a constant value for 15 minutes. Samples were aged under

standard conditions and tested for wettability after 3 hours, and after 1, 2, 3, 7, 14, 21 and 28

days. A fresh sample was used for each measurement.

2. Scanning Probe Microscopy

Yarns were withdrawn from the fabric, cleaned by Soxhlet extraction and then treated with

plasma. Fibres were removed from the plasma-treated yarn and mounted on a microscope

slide using double-sided adhesive tape. The experiments were carried out in air under

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ambient conditions. A Digital Instrument Dimension 3000 SPM was operated in contact

mode using a silicon nitride probe with a nominal spring constant of 0.58 N/m. Contaminants

were removed from the tip by treatment in a PSD-UV Ultra-Violet/Ozone Probe and Surface

Decontamination unit manufactured by Novascan Technologies (USA). During the

measurements, humidity was monitored by a TESTO Model 610 humidity and temperature

meter.

Height and deflection images were captured simultaneously using a scan rate of 1Hz.

Adhesion measurements were undertaken, using the force-volume mode, to collect a 16×16

array of force-distance curves over a 5×5 µm2 area of the wool fibre surface. The adhesion

data was collected from 5 to 7 areas on each of two fibres over a period of 28 days. Care was

taken to return the probe to exactly the same area on the fibre each time, as described

previously (Maxwell & Huson, 2005). Adhesion values are reported in nm of deflection;

however, an indication of the force can be obtained by multiplying the value by the cantilever

spring constant. Prior to examination of the samples, the position-sensitive detector was

calibrated by conducting a force measurement on a hard material (glass). The root-mean-

squared (rms) roughness values (Rq) were determined from height images that had previously

been flattened (second order). To isolate the analysis to specific areas and avoid unwanted

peripheral features, roughness analyses were carried out using a box curser to analyse only

selected portions of the image.

3. Scanning electron microscopy

Surface morphology of the samples was evaluated using a Field Emission Scanning Electron

Microscope (FE-SEM, Hitachi S4300 SE/N). Fabric samples were sputter-coated with gold

using a Bal-tec SCD50 sputter coater. The images were taken at a working distance of 15 mm

and an accelerating voltage of 1.2 kV.

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RESULTS AND DISCUSSION

1. Surface Change after Treatment

1.1. Surface morphology

The SEM images for untreated and plasma-treated fibres are shown in Figure 1. There was no

apparent morphological change to the surface or cuticle edge of the treated wool fibre. In

contrast, SPM images showed significant microscopic changes on the fibre surface,

consistent with an etching effect of the plasma treatment, as previously reported (Cai & Qiu,

2008; Canal et al., 2007). This can be due to the difference between the two techniques in

interpreting subtle differences in height. SEM image were collected in vacuum on surfaces

coated with gold. In addition, changes in slope can result in an increase in electron reflection

from the sample surface, producing a higher intensity in the image. However, it can

sometimes be difficult to determine whether the feature is sloping up or down. On the other

hand, AFM images were collected in air on uncoated samples and yield direct height

information, determining whether a feature is a bump or pit is straightforward (Nessler,1999).

SPM images of the untreated fibres show more longitudinal surface striations compared to

the plasma-treated fibre (Figure 2). The three-dimensional nature of the SPM image was used

to calculate changes in roughness and surface area variations.

An increase in roughness (Rq), which leads to an increase in the surface area, of the plasma-

treated fibre relative to the untreated fibre can be seen in Figure 2. The Rq of the surface of

the untreated fibre varied between 25 and 43 nm (with an average of 33.4 nm and a standard

deviation of 6.3 nm) whereas that of the plasma-treated fibre varied from 70 to 93 nm (with

an average of 85.2 nm and a standard deviation of 6.1 nm). An increase in the average

roughness value of the treated wool surface under different plasma conditions has been

reported previously (Klausen et al., 1995; Phillips, 1995).

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(a)

(b)

Figure 1 SEM images of wool fibres: (a) untreated, (b) plasma-treated

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Figure 2 SPM analysis of wool fibres (3D surface plot, 5×5µm2): (upper image) untreated fibre, (lower image) plasma-treated fibre

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1.2 Surface hydrophilicity immediately after treatment

Water uptake is defined here as the total movement of a liquid through the fabric driven by a

difference in surface energy between the fibre and the liquid and absorption of the liquid by

the fibre. The water uptake of untreated fabric and the fabric that had been treated with

plasma for 30 seconds was evaluated as soon as possible after treatment, usually within 4

hours. With the treated fabric, equilibrium uptake was achieved in less than 1 minute and it

showed very high (>300 %) initial water uptake at equilibrium. The untreated fabric showed

very little water uptake (0.03 %) after 15 minutes equilibration (Figure 3). A 30-second

plasma exposure time was judged to be sufficient to produce a significant change in surface

energy.

1.3. Surface analysis by XPS

Table 1 shows the surface composition of untreated and plasma-treated wool analysed by

XPS. A significant increase in oxygen and nitrogen concentration and a decrease in carbon

concentration were observed for the plasma-treated fabric compared to the untreated fabric.

The C/N ratio (an indicator of the level of covalently bound surface lipid) for plasma-treated

wool was consistent with values determined by chemical analysis for low-sulphur protein

fractions (4.4) and whole extract of wool (3.9) (Carr et al., 1986; Ward et al., 1993). A C/N

ratio of 3.4 has also been reported for the epicuticle (King & Bradbury, 1968). High

resolution XPS from a previous study (Naebe et al., 2010a) has shown that the plasma

treatment conditions used cause significant oxidation of the underlying protein matrix, in

particular the disulfide bonds, with the formation of cysteic acid residues.

A previous study (Naebe et al., 2010a) confirmed that plasma treatment under the conditions

used in the present study removed the 18-MEA, the major component of the lipid layer,

which is covalently bound through thioester linkages to the surface of the epicuticle (Negri et

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al., 1993a). This resulted in exposure of the underlying, hydrophilic protein of the epicuticle,

thereby increasing wettability of the wool.

Table 1 Relative atomic concentration of untreated and plasma treated fabric

Elements Carbon (C1s) Oxygen (O1s) Nitrogen (N1s) Sulphur (S2P) C1s/ N1s

Binding Energy (eV) 284.0 530.0 399.0 167.0

Untreated 73.8 13.8 9.9 2.4 7.5

Plasma Treated 54.2 26.9 15.3 2.6 3.5

1.4. Surface analysis by SPM

SPM adhesion studies showed significant variability on both plasma-treated fibres and

untreated control fibres, as assessed from measurements taken at different parts of each fibre.

Nevertheless, the average adhesion on the plasma-treated fibre was significantly higher than

that of the untreated fibre. Table 2 shows the variation of average adhesion values for six

different areas on one untreated fibre and one plasma-treated fibre after 24 hrs conditioning.

Table 2 The average adhesion values of untreated and plasma-treated fibres from six different parts of the surface of each fibre

Sample Area

Mean STDEV 1 2 3 4 5 6

Untreated 83.3 37.1 81.2 37.8 84.2 76.3 66.7 22.8

Plasma Treated 127.3 146.7 135.8 146.1 156.2 121.8 133.8 13.0 The higher adhesion on the plasma-treated sample was consistent with the results on a micro-

patterned model surface used by Huson et al (Huson et al., 2008), which showed that

adhesion on a hydrophilic carboxylic acid surface was stronger than that on a hydrophobic

methyl surface.

The surface of the plasma-treated fibre was hydrophilic due to removal of the lipid layer. The

resultant higher water affinity increases the probability of a water bridge forming between the

probe tip and fibre specimen, the increased meniscus force in turn causing greater adhesion

between the tip and the sample. In addition, the higher water affinity is likely to affect the

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real area of contact between the tip and sample (Bhushan & Jung, 2008), further increasing

the adhesion.

Unlike the results presented above, previous studies on hair (Bhushan & Chen, 2006;

Breakspear et al., 2005; LaTorre & Bhushan, 2005; 2006) and wool (Huson et al., 2008)

have shown that chemical treatment known to remove lipids from the surface generally

reduce adhesion. Huson et al acknowledged that this behaviour was unusual and speculated

that it could be due to the surface being able to absorb moisture, thereby affecting the

thickness of the adsorbed layer, and the possibility of ions or organic molecules leaching

from the bulk of the fibre and contaminating the adsorbed layer.

It is also possible that in the plasma-treated fibres used in the present study, oxidation of the

fibre surface might play a role in the increased adhesion. High resolution XPS analysis

showed significant oxidation of sulphur to sulphonic acid as a consequence of the plasma

treatment.

2 Surface Change with Ageing

2.1 Surface morphology

Changes in the surface morphology of the untreated and plasma-treated fibres were examined

by SPM. While the variation in roughness was observed between different areas of a sample,

no significant changes in surface roughness were observed on untreated and plasma-treated

fibres over a period of 28 days.

2.2 Surface hydrophiliciy changes with time

Untreated fabric and the fabric that had been treated with plasma for 30 seconds were stored

at 20+2 0C and 65+2 % RH and tested over a period of 28 days. The rate of water uptake was

used to determine the changes in the wetting properties of wool fabrics with ageing time.

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Untreated

Figure 3 shows the percentage of water uptake at equilibrium versus ageing time. The

untreated fabric took up very little water and showed no change over the 28 days. In contrast,

the plasma-treated sample showed significantly higher initial water uptake, the equilibrium

value decreasing significantly over the first 3 days, and subsequently more slowly. After 28

days, although the plasma-treated fabric had lost some of its initial wettability, it was still

considerably more hydrophilic than the untreated fabric.

Figure 3 Equilibrium water uptake as a function of time; plasma-treated (■), untreated (▲). Note two different y-axis scales.

2.3 Surface analysis by XPS

Table 3 shows the surface composition of plasma-treated wool analysed by XPS for different

ageing times from one day to 28 days. An increase in carbon concentration and a decrease in

oxygen and nitrogen concentrations were observed. The sulphur content remained almost

constant over the entire period. The relative atomic concentration of carbon in the surface of

the plasma-treated sample increased rapidly and then reached a constant value (after

approximately 14 days) that was significantly less than that of the untreated surface (73.8 %).

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Nitrogen concentrations decreased to an almost constant level over the same period. The C 1s

/N 1s ratio remained unchanged (~4.1) after 14 days of ageing.

Table 3 Surface analysis of plasma treated fabric over a period of one month Elements Binding Energy

(eV) Relative Atomic Concentration (%)

Storage time after plasma treatment (days) 1 2 3 7 14 21 28

Carbon (C1s) 284.0 54.2 54.5 54.8 55.9 57.0 57.3 57.3

Oxygen (O1s) 530.0 26.9 26.8 26.6 26.1 25.2 25.0 24.6

Nitrogen (N1s) 399.0 15.3 15.1 15.1 14.6 13.8 13.9 13.9

Sulphur (S2P) 167.0 2.6 2.6 2.5 2.5 2.6 2.6 2.6

C1s/ N1s 3.5 3.6 3.6 3.8 4.1 4.1 4.1

High resolution XPS sulphur spectra from plasma-treated wool samples after two, three and

seven days ageing are shown in Figure 4. The S 2P spectra consisted of two peaks at 164 eV

and 168 eV. The Peak at 164.0 eV corresponds to the presence of sulphur as a disulphide

linkage. The peak at higher bonding energy (168.0 eV) is attributed to the presence of

oxidised sulphur species (S6+) in the protein matrix after plasma treatment (Brack et al., 1996;

Molina et al., 2002). These oxidised species have been shown to be sulphonic acid (-SO3-)

(Bradley et al., 1994; Shao et al., 1997) and (S-SO3H) (Shao et al., 1997)

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Figure 4 High resolution XPS sulphur spectra from plasma-treated wool after 2, 3 and 7 days ageing

The ratio of disulphide linkages to oxidised sulphur species (C-S/S-Oxidised) recorded from

S2p spectra of the plasma-treated fabrics after different ageing times has been summarised in

Table 4. The ratio increased significantly over the first 7 days before remaining constant,

suggesting that whilst the total sulphur had not changed (Table 3), the more oxidised sulphur

species had migrated away from the surface of the plasma-treated sample.

Table 4 Ratio of sulphur species of plasma treated fabric over a period of one month

Storage time after plasma treatment (days) 1 2 3 7 14 21 28

C-S/S-Oxidised 1.01 1.11 1.18 1.33 1.30 1.32 1.48

The results suggest that the surface of plasma-treated wool fibre is mobile and changes as the

fibre ages. The increase in carbon content can be attributed to the migration of the carbon

from the bulk of the fibre onto the surface. This migration would explain the decreased

wettability (Figure 3). The results also show a decrease in the polarity of the surface and a

14

decrease in oxygen (Table 1) and S6+ (Figure 4). This can be attributed to the orientation of

polar groups from the surface to the bulk phase, resulting in rearrangement of the surface of

the wool fibre.

2.4 Surface analysis by SPM

The effect of ageing plasma-treated fibres was studied by SPM adhesion measurements and

compared to an untreated control fibre. Figure 5 shows the average adhesion ratio of plasma-

treated to untreated samples over a period of 28 days. This ratio increased up to the ageing

time of 14 days and then the value remained relatively constant. After ageing for 28 days, the

surface showed three times more adhesion than that of the untreated fibre. For polymer

surfaces, Everaert et al. (1997) postulated that ageing or “hydrophobic recovery” changes the

hydrophilic nature of the surface created by a plasma treatment to a relatively or fully

hydrophobic surface over time. In the present study, ageing did not change the hydrophilic

surface of the plasma-treated wool to a completely hydrophobic surface. This would suggest

that the oxidation of the surface and modification or removal of the lipid layer were relatively

permanent and rearrangement of the surface would not return the fibre to its original

hydrophobic state.

Rearrangement on the surface was relatively rapid immediately after the plasma treatment,

and slowed with ageing time. The results agree with the model proposed by Huson et al.

(2008), suggesting that the disordered lipid layer is capable of rearranging in response to

changes in the environment, but the changes in adhesion measured by SPM are difficult to

explain. As the hydrophilic nature of the surface created by the plasma treatment changes to a

more hydrophobic surface over time, one would expect a reduction in surface adhesion. More

work is needed to expand the current understanding of the wool fibre surface to explain this

anomaly.

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Figure 5 Average adhesion ratio of plasma-treated to untreated fibre over a period of 28 days for all fibres and areas measured

CONCLUSION

Atmospheric plasma treatment of wool fabric resulted in increased wettability of the fabric

and fibres, with greater roughness and adhesion as measured by SPM analysis. XPS

suggested a much more hydrophilic surface with a significant increase in oxygen and

nitrogen concentrations and a decrease in carbon concentration. These changes are consistent

with the removal of the covalently-bound lipid from the surface of the fibre. The significant

oxidation of the underlying protein matrix, in particular the disulfide bonds, with the

formation of cysteic acid residue was shown.

When the plasma-treated wool fibre was allowed to age, no changes were seen in the

morphology of the surface. However, wettability studies, as well as surface analyses using

SPM and XPS, showed that the chemical nature of the surface was changing with time. An

increase in carbon concentration and a decrease in oxygen and nitrogen concentrations were

observed over 14 days. After that period, the concentrations remained relatively constant.

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Although the sulphur content was fairly stable over 28 days of ageing, the intensity ratio of

disulphide linkages to oxidised sulphur species (C-S/S-Oxidised) changed, showing that

(with time) the more oxidised sulphur species migrated away from the surface of the plasma-

treated sample. This increase in the hydrophobic nature of the surface was consistent with the

decrease in wettability with time, but at odds with the increased adhesion as measured by

SPM. After ageing for 28 days, the plasma-treated surface was relatively stable and still

dramatically different from the untreated fibre, suggesting that the oxidation of the surface

and modification or removal of the lipid layer were permanent. In spite of rearrangement of

the surface, the treated fibre remained considerably more hydrophilic than the untreated fibre.

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