22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Atmospheric pressure plasma reactors for fibre processing
G. Borcia
IPARC, Faculty of Physics, Alexandru Ioan Cuza University, Iasi, Romania
Abstract: A study is conducted on the surface modification of synthetic fiber-based
materials, using atmospheric-pressure plasma, for controlled adhesion during subsequent
processing or direct applications. The plasma outcomes for controlling the material-fluid
interface by increasing the adsorption on fibers are analyzed.
Keywords: atmospheric-pressure plasma, surface processing, fibers, adhesion
1. Introduction
Polymeric fibres are materials with remarkable
mechanical properties, entering many fields of application,
under various forms, as monofilaments, polyfilaments, or
woven materials, conducting one step further the largely
acknowledged polymer functions. The various uses of
fibres encompass all types of fabrics, and also reinforced
composites, construction insulations, surgery fibres and
meshes, tissue engineering, membranes, etc. [1-6].
Although polymeric materials are recognized for their
bulk properties, with emphasis on their mechanical
behaviour, many polymer properties rest on the surface
chemical composition, structure and surface orientation of
specific chemical functionalities, all intrinsically related,
and defining the material interaction with its environment.
In this respect, adhesion is a crucial concern for most
polymer applications and this is particularly paramount
for fibres, due to their significant intrinsic surface-tovolume ratio. Therefore, the use of polymers, and
particularly fibres, requires surface processing in most
applications.
Fibres used in practice encompass 10 µm to 1-2 mm in
diameter, conducting to specific issues related to their
surface processing. Thus, complementary to the usual
requirements for tuning the physico-chemical properties,
their cylindrical surface and the curvature radius are to be
explicitly considered both in the processing technology
and the surface analysis method used. The processing
technology should provide uniform surface modification,
in respect to both surface functionalization and
deposition, on the entire fibre surface, avoiding the socalled “shadow effect”, whereas surface analysis should
yield reliable information, without artefacts and
measurement errors due to the curvature of the surface.
For example, the contact angle, which is the usual method
to monitor the hydrophilic/hydrophobic character of the
surface, directly related to adhesion properties, cannot be
compared on plan and cylindrical surfaces, the
measurement yielding, usually, a difference of at least
10°.
Also, the AFM imaging should correct the
roughness calculation taking into account the surface
curvature, whereas XPS or XRD are difficult to operate
on cylindrical surfaces, especially those of small radius.
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The development of specific surface treatments,
applicable to fibre-based materials, constitutes an
important research field, mainly in the textile-related
science and industry, due to the costs involved in this
market. The concern on adhesion of textiles is motivated
by the very beginning, with all synthetic textiles surfaces
containing manufacturing residuals (sizes and spinning
oils, monomers, etc.), which conducts to insufficient
adhesion due to weak boundary layers onto the surface.
This interest is also motivated for fibres used in the
biomedical field, for example surgical material as meshes,
where the main requested properties are the elasticity and
weight (characterizing the bulk material), also the pore
size and the hydrophilic/hydrophobic behaviour of the
surface, the adjustment of the latter being carried out by
surface processing.
Plasma treatments, both for adhesion promotion and
deposition, has developed markedly during the past two
decades, due to its potential environmental and energy
conservation benefits, in developing high-performance
fibre-based materials [7-14]. Plasma treatment on natural
and synthetic fibres or filaments can lead to processes
such as polymerization, grafting, cross-linking, etc. with
concomitant effects on wetting and wicking, dyeing,
printing, surface adhesion, electrical conductivity,
diffusion and other characteristics of interest in practice.
Since adhesion is a surface-dependent property, mediated
at a molecular scale, plasma technology can effectively
achieve modification of this near-surface region without
affecting the bulk properties of the materials of interest.
Atmospheric pressure plasma is particularly suitable for
treating fibres, either as individual filaments or woven,
with specific advantages as short treatment time, room
temperature operation, versatile geometry, flexibility with
respect to the type of the material, its dimensions and
shape.
In this context, we are presenting here results on the
surface modification of synthetic fibre-based materials,
using atmospheric-pressure plasma, for controlled
adhesion during subsequent processing or direct
applications. Two types of applications are envisaged:
textiles processing for improved dyeing of fabrics and
surface coating for biocompatibility tuning on surgical
1
fibres and meshes.
2. Experimental
Two experimental arrangements are tested, in the
dielectric barrier discharge (DBD) configuration, i.e.,
plan-parallel electrode and plasma jet set-up, using the
same HV supply, run in helium, offering different
approaches related to the exposure of the substrate and the
active particles flow to the treated surface. In both
arrangements, the electrodes are positioned horizontally
and the sample is placed on the bottom electrode, which is
grounded and covered with a dielectric layer. The
plan-parallel configuration ensures uniform plasma
exposure of a rectangular 2.5 cm × 4 cm area, whereas the
plasma jet allows uniform treatment of a circular area
3 cm in diameter (due to spreading of the jet, by dielectric
effect, on the isolator covering the ground electrode),
implying comparable treated area.
The major difference between the two reactors is the
gas supply to the discharge. Thus, in symmetrical
electrode configuration, the DBD system is enclosed in a
chamber and the treatment is in static conditions, both
from point of view of the substrate exposure and the
gaseous environment, whereas in jet configuration the gas
is continuously fed to the interelectrode gap and the
substrate can be moved, at controlled speed, under the jet.
The surface processing is carried out for various
parameters of the applied HV pulse and variable exposure
time 10 - 60 s, on one side or both sides of the fabric.
The treated fabrics are several types of commercial
polyester and polyamide woven, presented as raw
materials, with different weaving parameters, intending to
establish the relation between the process components and
their characteristics, i.e., fibre (physical and chemical
properties), fabric (weaving characteristics, “directionspecific” properties, “application-specific” properties),
plasma (constituents, energies, mechanisms), coating
(physical and chemical properties).
The substrates tested for biomedical applications are
surgical fibres and meshes, coated with hydrophobic
layers, in order to adapt their biocompatibility and tissue
integration.
The DBD parameters are established by electrical
measurement and optical emission spectroscopy, whereas
the treated materials are analysed by wettability/
wickability measurement, optical microscopy, SEM, XPS,
also evaluation of colour changes on dyed fabrics ∆E* in
CIELAB colour space.
Since dyeing of structures permeable to fluids is
governed by the adhesion of the fibres and the diffusion
of the dye in the material, the plasma treatment influence
on the mechanisms at the interface between the material
and a test liquid is established by a diffusion method,
aiming to separate adsorption, absorption and diffusion,
which are simultaneously occurring at the contact
between the permeable sample and the fluid. The
measurement is carried out with the woven placed
between two cells, where the “source” cell (cell 1) is filled
2
with dye solution and the “acceptor” cell (cell 2) with the
same amount of distilled water.
The absorption measurement is performed at given time
intervals, until equilibrium is reached in both cells, at
fixed
wavelength
(535
nm)
with
UV-VIS
spectrophotometer. The calibration of the data shows that
the measured absorption is proportional to the dye
concentration, for all experimental conditions tested here.
This evaluation is valid both for fabrics, aimed to
exhibit enhanced dyeing performance, and coated surgical
meshes, which should not significantly perturb the flow
across of biological fluid components. One should note
here that coating of permeable materials requires caution,
since the deposited layer is modifying both the surface
chemistry and the porosity, so affecting the diffusion
process. The fluid flow across the mesh is tested with
BSA protein solution.
3. Results and discussion
The comparison of the two experimental arrangements
is showing obvious processing efficiency of the plasma jet
arrangement, compared to the plan-parallel one, although
the power of the discharge is higher for the first one.
For comparable HV pulse amplitude, of 3-5 kV, the
discharge energy is two orders of magnitude higher for
the symmetrical configuration.
Yet, the surface
modification, tested on simple polymer films, is
measurable only for the plasma jet exposure. This is due
to the gas being continuously fed to the interelectrode gap
in jet configuration, which ensures a continuous flux of
energy carrying particles to the surface, triggering surface
modification at high rate.
Interestingly, the stability, reproducibility and
efficiency of the discharge are better in presence of the
woven material between the electrodes.
The typical waveforms of the plasma jet voltage and
current (Fig. 1) show two discharges, so-called primary
and secondary discharge, respectively, associated to the
HV pulse rise and fall. The current profile is smooth,
indicating homogeneous discharge regime, without
filamentary microdischarges.
Fig. 1. Plasma jet voltage and current waveforms, in
presence of the fabric between the electrodes.
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22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
The calculation of the energy applied to the discharge
shows measurably higher values in presence of the fabric
between the electrodes (Table 1).
Table 1. Plasma jet energy (3.5 kV pulse).
primary
discharge
without
fabric
with fabric
E (µJ/pulse)
secondary
discharge
total
49
9
58
55
19
74
Also, monitoring of the current amplitude, for the
primary and secondary discharge, for extended intervals,
in absence and in presence of the fabric between
electrodes, shows that the woven substrate reduces the
variations of the current peak, contributing to discharge
stability (Table 2).
Table 2. Mean value and standard deviation of the
current amplitude.
I (mA)
without
fabric
with fabric
I (mA)
seconda
primary
ry
discharg
discharg
e
e
primary
discharg
e
seconda
ry
discharg
e
0.0096
0.0065
0.0021
0.0020
0.0109
0.0134
0.0011
0.0010
This behaviour is related to the particular nature of the
sample, which has heterogeneous structure at
macroscopic level, from both mechanical and electrical
point of view. The woven allows the flow of the gas
through, presenting also complex dielectric structure, with
insulating and conducting regions. Thus, on one side, the
fabric changes the speed of gas flow in a significant
region of the discharge, and, on the other side, it
represents a second dielectric layer in the discharge,
besides the compact dielectric film placed on the ground
electrode, with patterned dielectric characteristics.
Thus, the spatially heterogeneous substrate improves
imparting the discharge energy and enhances the stability.
The emission spectra of the discharge show, as
expected, excited species of helium and other atoms and
molecules due to atmospheric air, as N 2 , N 2 +, O*, OH and
NO. The presence of NO is noticeable here, because this
radical is not usually observed, even in atmospheric
pressure discharges. The temperature calculation from
the rotational structure of the 391 nm N 2 + band shows a
value close to the room temperature, which implies no
supplementary thermal treatment of the polymeric fibres.
From discharge diagnosis it results, thus, that the
presence of a permeable substrate between the electrodes
is advantageous, compared to the case when there is only
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dielectric film on the grounded electrode, for setting the
discharge parameters and controlling the treatment.
The physical and chemical modification of the fabrics
by plasma exposure is difficult here to render to evidence,
because both polyester and polyamide are highly polar
polymers, with an oxygen-rich chemical structure,
therefore the degree of oxidation induced by plasma,
measured by XPS, is inherently limited, due to the
maximum level of functionalization attainable [15].
Then, due to the woven nature of the material, the fabrics
are hydrophilic, in that these absorb fast aqueous
solutions, so, again, the modification in the
wettability/wickability is rather within error bars.
Nonetheless, the enhanced adhesion properties of the
plasma-treated samples are suggested by SEM images and
colour analysis on plasma-treated and dyed samples.
SEM shows that for untreated fibres, the dye forms, to
some extent, a film on the fibre surface, whereas the
treated fibres are smooth and uniform after dyeing,
indicating better absorption of the dye in the fibre (Fig. 2).
Also, the performance of the finished product is proven
by increased colour variation observed on plasma-treated
samples, compared to untreated ones, thus associated to
more intense colour (Table 3).
a)
b)
Fig. 2. SEM images of dyed fibres; a) untreated, b)
plasma-treated.
Table 3. Colour variation on untreated and plasmatreated dyed fabrics, various samples.
Sample
S1
S2
S3
S4
S5
S6
∆E* (untreated)
0.3
1.2
2.3
0.5
1.5
1.7
∆E* (treated)
0.7
2.6
2.8
2.5
3.5
3.0
These results demonstrate an effect of the plasma on the
adhesion properties, which, yet, cannot be explicitly
assigned to the modification of the surface chemistry. In
addition, the roughness modification may also be reduced,
due to the mild plasma conditions here, due to inert gas
environment, also to the short treatment time.
In order to explain the mechanisms of dyeing and
diffusion through the pores of the woven heterogeneous
sample, we studied the parameters of the kinetics of the
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flow across the material, which can be discussed by
considering three mechanisms, the adsorption, absorption
and diffusion, respectively, which are simultaneously
occurring at the contact between a permeable substrate
and a fluid.
Of course, adsorption is the first necessary condition for
absorption.
The diffusion and adsorption/absorption mechanisms
operate at different levels in the substance, i.e.,
macroscopic and nanolevel, respectively. Each of these
mechanisms is controlled, therefore, by different
parameters. Thus, adsorption relates to surface properties,
absorption relates to surface and bulk properties, whereas
diffusion relates to the permeability of the woven and the
fluid features (concentration, viscosity, temperature).
Under constant conditions for the fluid phase, there are
two variables in the process, associated to the solid phase,
i.e., the weaving parameters of the fabric, which influence
the diffusion, and the surface properties of fibres, which
influence both diffusion and adsorption.
The fluid flow in this two-phase system conducts to
similar monotonous variation of the concentration in both
diffusion cells, c 1 and c 2 , respectively, with fast evolution
during the first few hours, followed by a slow tendency to
level out (Fig. 3). Yet, the dye concentration does not
reach the same equilibrium value for cell 1 and cell 2.
Thus, the time interval required to reach equilibrium τ eq
and the difference between the equilibrium concentrations
Δc eq yield information on the shift of the flow process.
components of the flow, adsorption and diffusion,
respectively.
K 1 and K 2 are changing for plasma-treated samples, due
to the surface properties of fibres influencing both
processes.
Experiments performed at variable
temperature of the dye solution will allow assigning K 1
and K 2 to adsorption and diffusion processes, as
temperature affects only the diffusion.
A similar analysis on adsorption and diffusion was
performed on surgical mesh coated with polystyrene,
optimizing the plasma polymerization parameters as to
obtain the most convenient biological fluid flow across
the biomaterial.
4. Conclusion
The treatment of fibre-based materials raises particular
issues, for both plasma diagnosis and surface analysis,
related to the presence of an inhomogeneous dielectric
layer between the electrodes.
Thus, the spatially heterogeneous substrate improves
imparting the discharge energy and enhances the stability.
In a context where plasma effects on polymeric surfaces
are considered to be combined cleaning - chemistry roughness modification, our study is suggesting a
pronounced physical effect of the plasma on the treated
material, attributed to removal of the molecules and small
fragments, including gas and vapours, adsorbed in the
entire woven matrix. This mechanical cleaning improves
the dyeing process of fabrics.
An experimental model for kinetics analysis of the flow
across the fabric sample renders to evidence two major
processes, adsorption/absorption and diffusion, controlled
by different parameters of the solid/fluid interface.
The atmospheric pressure plasma can be used to control
the interface processes, related to the adhesion properties
of the fibres.
5. Acknowledgement
This work has been carried out in the CASPIA project,
funded by the Executive Agency for Higher Education
Research Development and Innovation, Romania, PN-IIPT-PCCA-2013 programme, grant 254/2014.
Fig. 3. Linear fit of the time evolution of concentration
during the flow across fabric.
τ eq represents the time interval for the active sites on the
material to saturate with dye molecules and Δc eq relates to
the amount of dye adsorbed on the fibres. τ eq and Δc eq are
changing for plasma-treated samples, consistent to plasma
increasing the adsorption on fibres.
The evolution of the concentration obeys a variation
law of type eKt, where K is the process rate (s-1). In this
respect, Fig. 3 shows the graph representation of the
function ln[(c 1 − c 2 )/(c 1,0 − c 2.,0 )] versus time (where c 1,0
and c 2,0 are the initial values), which can be fitted, for the
interval before saturation, with two slopes, K 1 and K 2 .
These represent the rates associated to the two
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22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
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