Atmospheric-pressure plasma treatment of nonwovens using surface dielectric
barrier discharges
M. Šimor a), J. Ráhel a), D. Kovácik a), A. Záhoranová a), M. Mazúr b) and
M. Cernák* a)
a)
Faculty of Mathematics, Physics and Informatics, Comenius University, Mlynská dolina F2,
842 48 Bratislava, Slovak Republic
b)
Faculty of Chemical and Food Technology, Slovak Technical University, 812 37 Bratislava,
Slovakia
*Corresponding author (cernak@fmph.uniba.sk)
Abstract:
Preliminary results are presented on hydrophilization, grafting, and metal plating of PP
nonwovens using novel types of atmospheric-pressure low-temperature plasma sources, namely
the "Surface Discharge Induced Plasma Chemical Processing" source and the plasma source
based on a coplanar diffuse surface discharge. The plasma sources generate a thin ( ~ 0.3 mm)
surface layer of plasma and are capable of meeting the basic on-line production requirements for
surface activation and permanent hydrophilization of light-weight nonwovens.
Introduction
Uses of thermally bonded polypropylene (PP) nonwovens in baby diapers and similar
disposable hygiene products result in a markedly increased use of PP. PP and polyester (PES)
nonwoven fabrics are widely used also in filtration media, battery separators, geotextiles, oil
adsorbents, biomedical textiles, etc. The most common way to produce nonwovens is
spunbonding, where standard production lines for the lightweight fabrics are capable to achieve
production speeds up to 550 m/min, and the lines with the speed up to 1000 m/min are under
development.
In most instances the nonwovens for the above mentioned applications need to be wettable by
water or other water-based liquids. However, the low surface energy of PP and PES makes them
inherently hydrophobic, and their wettability is to be achieved by a hydrophilic surface treatment.
Traditionally, the surface treatment is performed by using aqueous solutions of surfactants either
by lick-up, or by spraying. The use of surfactants, however, introduces some problems such as
contamination and clean up problems, toxicity, and added cost. Moreover, the surfactants tend to
be washed off upon repeated exposure to liquids. Thus, when used as a topsheet in an absorbent
article or filter medium, the surfactant-treated nonwovens lose their ability to transport liquid
after repeated wettings.
Plasma surface activation of nonwovens, where polar molecular fragments affecting the
wettability are formed on the fabric fiber surfaces, has the potential to become an attractive
alternative to the use of surfactants. However, once treated, plasma-activated nonwovens may not
be stable. There are often changes in the wettability and surface chemistry as a function of the
5.3 - 1
storage time after treatment [1, 2]. Aging phenomena can be critically important in the practical
use of plasma-activated nonwovens [3].
A conventional method used for imparting the desired permanent hydrophilicity to PP and
PES is chemical graft polymerization requiring the use of organic solvents that, however, presents
both health and environmental concerns. An environmentally attractive alternative to the use of
organic solvents is a combination of the plasma surface activation with post plasma grafting using
aqueous solutions. This means that organic solvents are not required for the grafting, thereby
reducing the environmental pollution caused by the production process and rendering superfluous
the need for explosion prevention measures.
Apparently, the crux of plasma finishing techniques for nonwovens is the availability of robust,
reliable and cost-effective sources of low-temperature atmospheric-pressure plasma, capable to
operate in tandem with existing high-speed production lines, where the plasma exposure time
should be less than 1 second.
In this paper we summarize some of our preliminary results on hydrophilization, grafting, and
metal plating of PP nonwovens using novel types of atmospheric-pressure low-temperature
plasma sources, namely the "Surface Discharge Induced Plasma Chemical Processing" (SPCP)
source and the plasma source based on a coplanar diffuse surface discharge (CDSD). The results
obtained indicate that SPCP, and in particular CDSD plasma source, have the capability of
meeting the basic requirements for on- line high-speed surface plasma treatment of lightweight
nonwovens.
2. Plasma sources for nonwoven surface treatments
Over the past two decades the plasma treatment technique has been extensively reported,
including the surface treatment of PP and PES nonwo vens [5-11, 14-27]. Also, there is a limited
amount of information on plasma treatment of biodegradable PLA nonwovens [12,13]. The vast
majority of applications were made at reduced pressures on the order of 10-5 - 10 Torr (see, for
example, Refs. 5-13), where the spatially homogeneous low-temperature plasma can easily be
generated and brought into direct contact with the outer fabric surface. However, the use of
expensive vacuum systems that force batch processing has discouraged this application of low
pressure plasmas in larger industrial scale. Additional disadvantages of the technique are high
power consumption, long processing times, and difficulty to scale-up of an experimental set- up to
a large production reactor. Thus it is apparent that such low-pressure plasma treaters cannot be
used in line with standard production lines processing nonwoven fabric several meters wide at
speeds on the order of 100 m/min.
Moreover, to generate a low-pressure plasma inside of a textile to treat the inner textile
surfaces, an average fabric pore size must be larger than is the distance over which an electric
charge imbalance (so-called Debye length) can exist. This, however is for the pore size of some
10 – 100 µm fulfilled only at near-atmospheric pressures.
In a contrast, atmospheric-pressure plasma processes offer different advantages for the
finishing of nonwoven fabrics [1-5,14-26, 28-31]: costly vacuum equipment is unnecessary,
processing times are reduced, and the plasma finishing is simpler in an in- line process. Currently,
there are several atmospheric-pressure plasma systems for in- line nonwoven finishing currently at
the development:
5.3 - 2
Barrier (industrial corona ) discharge systems
The most common systems tested are modifications of barrier ("silent" or "industrial-corona")
discharge treaters widely used for the treatment of polymeric films [14,15]. In these cases the
discharge is generated by applying a high frequency, high voltage signal to an electrode separated
from a grounded plane by a discharge gap and a dielectric barrier. The nonwoven fabric treated is
localized on the dielectric barrier surface. The main drawback of the barrier discharge devices
used for nonwoven fabrics treatment, as well as of a similar discharge system by Nippon Paint
Co. [16], is that the useful plasma conditions are achieved only in small volume plasma channels
termed "streamers" developing perpendicularly to the fabric fibers. As a consequence, the plasma
is in a very limited contact with the fabric fiber surfaces, which results in low processing speeds,
typically on the order of ~1 m/min. Moreover, because the plasma channels are perpendicular to
the fabric fibers, and arcing is an intrinsic phenomenon associated with this discharge type,
localized arcing results in the formation of pinholes in the material being treated.
Atmospheric-pressure glow discharge systems
The developments of novel types of electrode systems and a fuller understanding of the
underlying discharge physics has is now made possible to construct atmospheric-pressure plasma
reactors operating in a glow discharge mode, i.e., with a homogenous volumetric plasma structure
similar to that of low pressure plasmas [1]. Such plasma reactors appear very promising to
perform at atmospheric pressure almost any plasma processing operation, which can be
performed at low pressures.
Even when a number of attempts have been made to apply such plasma reactors to nonwovens
finishing [17-24], only system brought to at least partial commercial viability is the APPS
(Atmospheric Pressure Plasma System) developed by Plasma Ireland in the framework of the
Plasmatex project supported by the EC-Brite programme [20]. Apparently, the main drawback
that has discouraged the use of APPS system in larger scale applications for nonwovens finishing
are the processing speed less than some 100 m/min.[20], and problems with the discharge
stability in reactive gases.
A special atmospheric-pressure glow discharge type is the microhollow cathode discharge,
which is generated using a metallic hollow cathode with holes size of 0.1 – 0.25 mm, an arbitrary
shaped metallic anode, and insulating ceramic between [25]. Recently an application of the
microhollow cathode discharge for surface treatment of nonwoven fibers has been suggested [26].
It is apparent, however, that in this specific application the microhollow discharge is at a
disadvantage with respect to APPS and OAUGDP because of the limited life-time of the smallscale metallic electrodes due to erosion by a direct contact with reactive discharge plasma.
Surface discharge systems
The first surface discharge system tested for the nonwoven surface treatment is based on a
discharge with a high density of streamers generated on a dielectric surface between metallic strip
electrodes. Such a discharge system was invented by Masuda and his coworkers [27] and named
by its inventors "Surface Discharge Induced Plasma Chemical Processing" (SPCP). SPCT has
been found to be very effective in producing high-density ozone and in decomposition of gaseous
organic contaminants in air. The surface discharge systems applied to treat nonwoven fabrics
differ from the barrier discharge treaters chiefly in that the plasma streamers are parallel with the
5.3 - 3
nonwoven fabric surface. In this way, the dense streamer plasma is in a better contact with the
fibers, which reduces the treatment time significantly. An important construction advantage is
that the plasma is generated only in a small volume that roughly equals to the volume of the
fabric to be treated, resulting in reduced power consumption.
The first use of SPCP for nonwoven surface treatment was reported in Ref. [28]. This patent,
however, covers only the treatment of a thin skin layer of outer nonwoven fabric surface. At the
Institute of Physics, Comenius University a SPCP plasma source was designed and constructed
for the purpose of the surface hydrophilization both outer and inner nonwoven surfaces [2, 2931]. Based on the results obtained including those presented below, this type of reactor appears
particularly promising for permanent hydrophilization of lightweight (less than 50 gms)
nonwovens. The SPCP system, however, is of limited value for industrial implementation
because of a limited life-time (on the order of 102 -103 hours) of the discharge electrodes that are
in a direct contact with the discharge plasma.
To remedy this limitation a novel surface discharge type (the coplanar diffuse surface
discharge – CDS D) has been developed [4, 5], whose major advantage combined with otherwise
identical properties, is that the discharge plasma is generated without any direct contact with
electrodes, which protects the electrodes erosion.
3. Experimental
3.2. Plasma treatment
The SPCP discharge element
The SPCP element used in our experiments consists of two electrodes separated by a 100 x 100mm alumina dielectric plate. The thickness of alumina plate is 0.5 mm. The alumina plate has a
discharge electrodes system on its upper surface and an induction electrode on its lower surface.
The discharge electrode consisted of 21 interconnected 1-mm wide, 10-µm thick, 80- mm long,
and 3- mm strip-to-strip distance strips made from TiN using a magnetron sputtering method. The
induction electrode (90 x 90- mm, 10-µm, molybdenium) on the lower surface of the dielectric
barrier served to produce a tangential electric field component on the upper dielectric barrier
surface. The electrode configuration of SPCT element is illustrated in Fig. 1(a). The electrode
system was energized by a sinusoidal high voltage with peak-to-peak voltage of 8 kV and two
different frequency values of 2.5 kHz and 6 kHz. This produced a stable surface discharge
starting from both edges of each strip electrode and covering uniformly the ceramic surface.
The CDSD discharge element
A schematic diagram of the discharge electrode system is shown in Figure 1 (b). Two systems of
parallel striplike electrodes (1- mm wide, 50-µm thick, 150 mm long, 0.5- mm strip-to-strip;
molybdenum) were embedded in 96% alumina using a green tape technique. The thickness of the
ceramic layer between the plasma and electrodes was 0.4 mm. A sinusoidal high- frequency high
voltage (1 - 15 kHz, up to 10 kV peak) was applied between both electrode systems. Such a
discharge electrode arrangement and energization were found to generate visually almost uniform
plasmas of some 0.3-mm thickness in nitrogen and ambient air at atmospheric pressure. The
reader is referred to Ref. [5] for the pictures illustrating the visual discharge appearance and the
results concerning its basic plasma parameters and physical mechanism.
5.3 - 4
Fig. 1: Schematics of a) SPCP discharge element and b) CDSD discharge element.
The discharge elements were used as atmospheric-pressure plasma sources in two types of
reactors shown in Fig.2: The first reactor shown in Fig.2(a) was used for ”batch” treatment of 8 x
8 cm samples in well-defined gas atmosphere. After the stabilization of the discharge current
(approximately 5 s) the sample was brought into contact with the discharge plasma layer using a
revolving sample carrier. The treatment time was measured as the contact time of the sample with
the plasma by an electronic stopwatch. The second reactor shown in Fig.2 (b) was designed to
simulate conditions of a continuous plasma treatment of nonwovens. The reactor was operated in
flowing nitrogen or dry air gas, at 100 sccm. Operating the reactor in this regime it was not
possible to avoid of a penetration of ambient air into the nitrogen plasma. In such a case the
content of oxygen in nitrogen reached some 1–5 %, which was measured using mass
spectrometry analysis of the gas samples taken from the reactor.
Fig. 2: Reactors used for atmospheric-pressure plasma treatment of nonwovens
a) reactor for batch plasma treatment
b) small bench-scale reactor simulating continuous plasma treatment.
5.3 - 5
3.2. Materials and chemical surface modification methods used
Industrial spunbounded polypropylene nonwoven fabrics (17 gsm, and 50 gsm with a
thickness of 280 µm, average pore size 38 µm, and porosity of 78%) supplied by PEGAS
Company, (Czech Republic) were used in all plasma treatment experiments described below.
Technical purity nitrogen and technical dry air were employed as plasma gases.
In electroless plating of the plasma-activated nonwovens a conventional two-step process
using commercial metallization baths was used to deposit a thin Ni layer on the fabric fibers
surface: Chemical activation of the fabric fibers surface was accomplished by immersion for 5
minutes at 40o C in aqueous bath containing a catalytically active Pd/Sn-compound. Then the
samples were immersed in an acidic solution to remove tin from the surface and rinsed with
deionized water. Finally, the electroless deposition of nickel was accomplished by immersion for
5 minutes in a nickel plating bath at 50o C.
In the dying experiments, the PP nonwoven sample was immersed in a 0.1 wt.-% aqueous
solution of methylene blue and stirred at 80°C for 1 hour. The samples were rinsed with water
and stirred in order to remove the dyes that had not bonded with the nonwoven fibers surfaces,
and then were air-dried.
In the grafting experiments commercial stabilized acrylic acid (AAc) purified by vacuum
distillation was used. The grafting method used was identical to that described by Seto et al. [14]:
The samples were immersed into a 10 wt.-% aqueous solution of AAc in a glass ampoule. After
deareating by nitrogen bubbling through, the ampoule was sealed and the grafting was carried out
for 1 h at a temperature of 60o C. PAAc homopolymers were removed by washing in distilled hot
water and air-dried.
3.3. Testing and surface analysis methods used
Surface energy measurements
Surface energy of the plasma treated samples was measured using the ”Critical Wetting Surface
Tension” (CWSW) method described in detail in [32]: A series of test solutions having a range of
surface tension was used. The measurement was carried out by placing 10 standard-sized drops of
a test liquid on the surface of the nonwoven. Ten minutes later the test drops were observed for
letting and absorption into the nonwoven. If at least nine of 10 drops are absorbed, it is concluded
that that the test solution wets the fabric, i.e. the nonwoven surface energy is higher than the test
solution surface tension.
Strike-through time measurements
As a measure of the hydrophilicity of inner fibers surfaces of the treated nonwoven
hydrophilicity, the strike-through time of test liquid was measured using a standard ETR 150.396 method. Tested sample was placed on the top of five layers of filter paper (ERT FF3 strike
through/wetback filter paper supplied by Hollongsworth & Vose Ltd.) and weighted by strikethrough plate. Subsequently, the cylindrical hole in strike-through plate was filled by 5 ml of
water solution of NaCl (9 g/l) with surface tension of 70±2 mN/m. The time required for
permeating the liquid through the nonwoven fabric to the filter paper was measured with an
accuracy of 0.01 s. An average value from 10 samples measurement with its standard deviation
gives the value of liquid strike-through time. The minimum of the strike-through time is given by
the strike-through time of stock of five filter papers that is, according to the manufacturer,
(3.0±0.5) s for 5 ml of the liquid passing through 85 mm2 area of the nozzle.
5.3 - 6
Vertical wicking rate measurements
Five test nonwoven samples having 8 cm x 2.5 cm size were prepared. A lower end of the test
sample was vertically dipped by 5 mm into an aqueous solution of potassium hydroxide of 31%
concentration. Two and five minutes after, a rising height of the solution due to capillarity was
measured as a measure of a wicking rate.
SEM surface analysis
The microstructure of PP nonwoven before and after plasma treatment, grafting and electroless
metal plating were characterized by SEM using a Tesla BS 300 microscope with digital
microscopy imaging TESCAN.
ESR spectrometry measurements
The first derivative ESR spectra were recorded on a Bruker ER 200 D SRC with an Aspect
computer X-band (≈9.6 GHz) EPR spectrometer with a single TE102 rectangular cavity (ER
4102 ST) at room temperature. The 100 kHz modulation technique was used to record the signal.
The ESR spectra were measured before and after the plasma treatment of the polypropylene
textile sample. The planar sample geometry was used for ESR experiment as follows: The small
rectangle of the PP nonwoven was cut, connected to the ESR silent planar sample holder, and
vertically inserted into the microwave cavity. Two orientations of the textile planar sample in the
microwave cavity were investigated in thant the plane of the planar sample was: (i) parallel, (δ =
0o), and (ii) perpendicular, (δ = 90o) to the direction of the ESR spectrometer static magnetic
field, Bo. The ESR spectra accumulation technique was used to improve the signal-to- noise ratio.
The g-factor values were obtained using a standard sample and were verified by a computer
simulation of ESR spectra.
XPS measurements
XPS measurements were carried out using a Kratos Axis 165" spectrometer (Shimazu
Corporation) with monochromatic Al Kα excitation source (1486.7 eV). The electron take-off
angle was 90° with respect to the sample surface. Spectra were referenced with respect to the
258.0 eV carbon 1s level (C 1s) observed for hydrocarbon.
Results and discussion
4.1 Plasma hydrophilization
Preliminary comparisons of the plasma hydrophilization in dry air and nitrogen plasmas
indicated significantly higher efficiency of the nitrogen plasma. This is in agreement with the
results of Massines et al. [34], who have found that in the volume dielectric barrier discharge
conditions, the specificity of N2 atmosphere compared to an atmosphere containing oxygen it that
it allows very extensive surface transformation and a greated increase of the PP surfaces
wettability. As a consequence the data on the plasma hydrophilization to be reported here were
confined to those in the atmospheric-pressure nitrogen plasma.
A series of plasma-activation experiments was aimed to find roughly a minimal treatment
time necessary to reach average strike-through times less than the value of (3.5±0.5) s
5.3 - 7
corresponding to a satisfactory hydrophilization of 17 gms PP nonwoven samples for application
as a topsheet in disposable hygiene products.
Table 1 compares the strike-through times of 17 g/m2 PP nonwoven samples treated using
both SPCP and CDSD discharge elements operating at two different voltage frequencies of 2.5
kHz and 6 kHz at two different treatment times of 3 s and 1 s, respectively. These conditions
correspond to approximately the same exposure energy density of 1.5 Ws per 1 cm2 of the
sample. The results of two sets of experiments using both “batch-treatment” (SPCP, CDSD) and
“continuous-treatment” (SPCP*, CDSD*) reactors (see Fig.2) are presented. Table 1 also
illustrates the effect of aging the treated specimen in ambient air.
The results shown in Table 1 indicate somewhat higher efficiency of the CDSD comparing to
that of SPCP. The slightly worse results obtained using “continuous-treatment” reactor can be
explained by the higher content of air in the plasma gas. The striking feature of the results shown
in Table 1 is the fact that at approximately the same exposure energy level, better results and
more stable hydrophilization was obtained operating both plasma sources at the higher frequency
of 6 kHz. The energy efficiency computed for the most efficient treatment, i.e., the treatment
using the CDSD element operating at 6 kHz in the “continuous-treatment” reactor, was 0.245
kWh per 1 kg. Multiple strike-through measurements aimed to determine the wash-off stability of
the hydrophilized samples did not reveal any wash-out effect for the aged samples. This is in
contrast to, for example, the properties of a corona-treated PP film in conditions representative
those use in industrial coating applications, where a washable low- molecular-weight oxidized
material is created on the plasma-treated surface [35].
Surprisingly, as indicated by the results in Table 1, the strike-through time values were not too
dramatically affected by the aging. We tentatively explain this from practical point of view very
important finding as follows: Upon aging, hydrophilic molecular segments created by the
nitrogen plasma treatment reorient and embed themselves and expose hydrophobic segments at
the PP polymer surface. If a crosslinking is induced at the surface, the crosslinking tends to limit
polymer chain mobility, thus significantly slowing the “hydrophobic recovery” process and
therefore preserving polymer surface wettability over time. Since in our conditions of the
atmospheric-pressure nitrogen plasma exposition can result in a significant chemical crosslinking
a durably wettable liquid pervious PP non-wovens can be produced.
In order to test potential applicability of our plasma treatment technique for hydrophilization of
PP nonwovens used as battery separators a limited number of measurements was made using 50
gsm PP nonwovens. In these experiments a complete hydrophilization corresponding to the
strike-through time of 2.38±0.21 s and CWST values above 103.8 mN/m measured 30 s after the
treatment were obtained by 18 s exposure of both sides of the 50 gsm nonwoven samples. The
corresponding vertical wicking values measured were (55.2±3.6) mm/2min and (63±1.8)
mm/5min. After 76 weeks the strike-through time and CWST values were changed to 5.64±1.34
s and 76.85 mN/m respectively. The measured energy consumption was of 2 kWh per 1 kg that is
nearly one order higher that in the case of the lightweight 17 gsm samples.
No changes in FTIR spectra due to the plasma treatment were observed. Preliminary results of
XPS analysis measured several minutes after the nitrogen plasma treatment, which will be
published and discussed in detail in [30], indicate an increase in oxygen content and presence of
nitrogen functional groups.
Figure 3 compares the SEM micrographs of 17 gsm PP nonwoven samples before and after the
CDSD nitrogen plasma treatment for 1 Ws/cm2 energy exposure level. A homogeneous surface
roughening due to the plasma treatment can be seen in Fig. 3(b).
5.3 - 8
Table 1: The strike -through times of 17 g/m2 PP nonwoven samples.
2.5 kHz power
source,
treatment time
of 3 s
6 kHz power
source,
treatment time
of 1 s
after 1 min
After 24 hours
After 7 days
After 20 days
After 3 months
SPCP
SPCP*
CDSD
CDSD*
SPCP
SPCP*
CDSD
CDSD*
SPCP 4.5±0.5
SPCP* 4.4±0.7
CDSD 3.8±0.4
CDSD* 4.0±0.5
SPCP 3.9±0.8
SPCP* 4.1±0.7
CDSD 3.8±0.2
CDSD* 3.6±0.7
SPCP 5.1±0.6
SPCP* 5.5±0.9
CDSD 4.2±0.6
CDSD* 4.8±0.3
SPCP 3.8±0.4
SPCP* 4.2±0.7
CDSD 3.7±0.5
CDSD* 3.9±0.3
SPCP 5.4±0.6
SPCP* 5.5 ±0.8
CDSD 4.5±0.5
CDSD* 5.2±0.8
SPCP 3.9±0.5
SPCP* 4.2±0.6
CDSD 3.9±0.5
CDSD* 3.5±0.8
SPCP 5.5±0.6
SPCP* 5.6±0.7
CDSD 4.2±0.6
CDSD* 5.0±0.2
SPCP 3.9±0.3
SPCP* 4.0±0.8
CDSD 3.8±0.5
CDSD* 3.9±0.7
3.3±0.4
3.5±0.5
3.3±0.3
3.4±0.5
3.2±0.2
3.4±0.4
3.3±0.5
3.3±0.6
It is well known that the peroxy radicals are formed after the surface activation of PP by
nitrogen plasma and their exposition to laboratory air [34]. As a consequence we have proposed
that the free radicals formed in the PP nonwovens after our plasma hydrophilization could be the
peroxy type. Therefore, the possible generation of the peroxy radicals was verified by ESR
spectroscopy.
Figure 4 shows the ESR spectra of the peroxy radicals in polypropylene textile samples after
their nitrogen plasma treatment. No ESR signals of peroxy radicals were detected in the EPR
spectra of polypropylene nonwoven samples, which were recorded before the nitrogen plasma
treatment.
Fig. 3: SEM image of PP nonwovens a) untreated b) nitrogen plasma treated.
5.3 - 9
g1 = 2.034
(a)
EPR signal intensity [ arb. units ]
g = 2.007
2
θ = 90 o
g = 2.002
3
(b)
θ = 0o
3200
3300
3400
3500
B[
10-4
T]
Fig. 4: Room temperature ESR spectra of PP nonwoven fabric after the nitrogen plasma
treatment. The orientation of the textile planar sample in the microwave cavity was:
(a) perpendicullar, and (b) parallel to the direction of the spectrometer static magnetic field, Bo .
It is suggested that the composite ESR spectra of peroxy radicals in Figure 4 consist of two
individual spectral components: (i) The powder ESR spectrum due to randomly oriented peroxy
radicals in the amorphous regions of plasma treated polypropylene textile. (ii) The ESR spectra of
the rigid peroxy radicals, which are trapped in the crystalline regions, and which showed the
angular dependence. Also, the ESR spectra of the ambient-air plasma treated samples were
measured. Compared to the results for the nitrogen-plasma treatment, the measured radical
densities were significantly lower. A detailed analysis of our results that will be published in [31]
indicates that the peroxy radicals, which are formed by the nitrogen plasma treatment of the
polypropylene textile, can be located in the two different regions (amorphous and crystalline).
4.2. Electroless metal plating of the nitrogen plasma activated PP nonwovens
A series of experiments was made, where the SPCP atmospheric-pressure nitrogen plasma was
used to render a PP nonwoven fabric hydrophilic and facilitate absorption of a palladium catalyst
in order to provide a catalytic surface for the deposition of electroless nickel. At the voltage
frequency of 6 kHz the optimum quality nickel coating was obtained for a 0.5 sec. treatment time.
Treatment times in excess of 0.5 sec. resulted in a reduction of the nickel plating deposition rate,
uniformity and Ni- coating adhesion values.
5.3 - 10
treated by plasma (0.5 sec.)?
+
+?
untreated
Fig. 5: SEM image of 17 gsm PP nonwovens surface activated in SPCP atmospheric-pressure nitrogen plasma and
electrolessly nickel plated.
As illustrated by Fig.5, an advantage of the SPCP element is that it makes possible a patterned
plating of the nonwoven surface, where the metallized pattern follows exactly the shape of the
discharge plasma generated between the surface electrodes (see Fig. 1(a)). From Fig. 5 it can be
seen that the plating without plasma treatment was not possible because the fibers localised above
the discharge electrodes were not wetted by the plating solutions. The plasma treated and
subsequently Ni-plated fibers are dark in Fig.5. The Ni- plating exhibits very good adhesion to the
PP fibers since it canno t be removed by a conventional Scotch tape test. The reader is referred to
[29] for a detailed discussion of underlying chemical processes and potential applications of these
results.
4.3. Acryl acid grafting of the nitrogen plasma activated PP nonwovens
Based on the results obtained on hydrophilization of PP nonwovens using the nitrogen plasma
treatment we speculated that the nitrogen plasma treatment can make PP nonwovens dyeable and
printable with the environmentally friendly water soluble dyes and pigments. Surprisingly, our
attempts to colour the nitrogen plasma activated PP nowovens by the standard methylene blue
dye failed. This, together with an unsatisfactory stability of the nitrogen plasma hydrophilized 50
gsm PP nonwoven samples in the 30% KOH solution environment let us to perform some
experiments aimed on the grafting of polyacrylic acid (PAAc) on the plasma treated nonwovens.
The grafting method used is described in Sec. 3.2.
The results of our preliminary experiments, which will be published in detail in Ref. [31], are
promising since the nitrogen plasma activation led to a very fast and homogenous grafting. The
SEM micrograph of the nitrogen plasma activated and subsequently grafted PP nonwovens are
shown in Fig. 6. The grafted PAAc layer of less than 1µm thickness was very homogeneous and
has an excellent adhesion to the PP fibers and the dye affinity. The 50-gsm samples grafted after
5.3 - 11
2 x10 s nitrogen plasma activation were permanently hydrophilic with a strike through time of 3.5
± 0.7 s, which is indicative of homogeneus grafting also on the inner fabric surface. The grafted
PAAc coating is evident from careful examination of SEM micrographs in Fig.6(b) on points
where dust particles on the fiber surface were coated by the PAA layer, and the projections
generated in this way are smoother that the original sharp dust particles. An indication of a good
grafting can be drawn also from the results of FTIR spectral analysis in Fig.7.
An interesting result of our preliminary experiments, corresponding well with the results of
ESR measurements mentioned in Chap. 4.1., is that much better grafting was observed using the
nitrogen plasma activation than using the air plasma activation. This is apparently due to the
higher density of the peroxy radicals, which initialite the AAc graft polymerization.
a)
b)
Fig. 6: SEM micrographs of (a) untreated and (b) grafted PP nonwovens
2,0
PP nonwoven
1700
C=O
Absorbance
1,5
1238
C-O
1533
grafted
1,0
0,5
0,0
3500
untreated
3250
3000
2750
2500
2250
2000
1750
1500
1250
1000
-1
Wavenumber [cm ]
Fig. 7: FTIR spectra taken from both untreated and nitrogen plasma activated and
subsequently grafted 50 gsm PP nonwoven samples.
(Note that at used exposure times the FTIR spectra of the plasma activated samples
were identical to those of untreated samples).
5.3 - 12
750
Conclusions
The presented results on surface activation of PP nonwovens, including the post plasma
grafting and electroless plating, show that the SPCT plasma source is capable of meeting the
basic on- line production requirements for surface activation and permanent hydrophilization of
light-weight nonwovens. It is apparent that in this and many other surface treatment applications
a thin ( ~ 0.3 mm) surface layer of plasma may be more useful than the relative large volumes of
plasma generated by the volume barrier and atmospheric-pressure glow discharges. Another
unique features that make this method attractive are a very short treatment time (less than 1 sec.
at the current stage of its development), its robustness and technical simplicity. Electric energy
requirements of some 0.3 kWh/kg for the activation of light-weight PP nonwovens is
significantly less than 2-3 kWh/kg that consumes a typical spunbonding process.
Our current efforts are focusing on reaching even shorter treatment times that will make
possible to scale up the technique to processing speeds above of 100 m/min. To this end a SPCT
element equipped with an oil cooling system, which will be capable to operate at frequencies up
to 50 Khz and at the energy density up to some 10 W/cm2 is under construction.
Also, we speculate that a very attractive area for significant applications of SPCT plasma
source is hydrophilization of lightweight biodegradable PLA-based nonwovens: PLA-based
nonwovens, which in the near future hold strong promise for the application in disposable
hygiene products, are naturally hydrophobic like PP nonwovens. The traditional surfactant-based
wet treatment, however, involves a certain critical moisture level that must be controlled before
packaging the PLA nonwoven since the biodegradation is induced by hydrolysis. Comparing to
PP nonwovens, this is a serious additional obstacle to applying the wet surface treatment to PLA
nonwovens. This, as we believe, offers unique opportunities for dry finishing of lightweight PLA
nonwove ns using atmospheric-pressure plasma and, in particular, the SPCT plasma source.
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