Indian 10urnal of Fibre & Textile Research Vo1.3 1 , March 2006, pp. 99- 1 1 5

Plasma polymerization and its applications i n textiles

Dirk Hegemann"

EMPA - Materials Science & Technology, Functional Fibers and Textiles, Lerchenfeldstrasse 5 , 90 1 4 St.Gallen, Switzerland

Plasma polymerization enables the deposit ion of thin coatings on all k inds of substrates using electrical monomer discharges. This paper reviews plasma polymerization processes as surface modification (finishing) for textile applications. The dry and ecofriendly plasma technology aims at replacing wet-chemical process steps and adding new values to textile products. Characteristics of hydrocarbon, organosilicon, fluorocarbon, hydrophilic functional, monofunctional and ceramic coatings have been discussed to demonstrate their potential for textiles and fibers. Plasma technology requires adequate reactors for the continuous treatment of fabrics and fibers. To enable the optimization of plasma polymerization on batch reactors, questions of up-scal i ng are addressed to demonstrate the transfer to an industrial level. Both atmospheric and low pressure plasmas are considered regardi ng their effectiveness and efficiency. Examples for applications in textiles, such as hydrophobic, oleophobic and permanent hydrophi l ic treatments, have also been reported.

Keywords: Fluorocarbon coating, Hydrocarbon coating, Hydrophobic treatment, Hydrophi l ic treatment, Oleophobic treatment, Plasma polymerization

IPC Code: Int. CI.8

D06M 10100, H05H

1 Introduction The challenges the European textil e i ndustry is

facing today are enormous. Therefore, the need for a reorientation is strong. In order to survive, the European textile industry is taking the trans ition towards a knowledge-based economy focusing on h igh-value added products. Plasma technology is offering an attractive way to add new functional ities, such as water repellency, hydrophi l icity, dyeabil i ty, conductivity and b iocompatibil i ty , due to the nanoscaled modification of textiles and fibers. At the same time, the bulk properties of textiles and fibers remain unaffected. Moreover, due to a low material and energy input, while avoiding wet-chemical processes, the plasma technology provides the realization of environmental ly sound processes. I

The plasma state consists of an equal part of nega­tively and positively charged particles (quasi­neutrality), excited states, radicals, metastables, and vacuum ultraviolet (VUV) radiation. Surface treat­ments of materials can be performed by non­equil ibrium plasmas which are excited by electric fields. S ince the energy coupling i s conducted by electrons which achieve a mean temperature of seve­ral electron volts (correspondi ng up to 1 00,000 K),

"E-mail : Dirk.Hegemann@empa.ch

the main part of the plasma-activated gas remains close to room temperature. The electrons, however, gain just the right energies to excite, dissociate and ionize atoms and molecules. The degree of ionization typically l ies between l OA and 10-6. The earliest plasma treatments on textiles and fibers date back to the 1 960s, focusing on the improvement of wettabi l i ty, shrinking resistance, tWIsting and desizing? Low pressure plasmas operating between 0. 1 Pa and 1 00 Pa were usually considered, which can be activated by direct current (DC), alternate current (Ae), radio frequency (RF) or microwave (MW). Plasma polymerization is performed using different kinds of plasma-polymerizable gases (monomers) .3

These gases might not undergo polymerization by conventional activation (e.g. methane), showing a main difference between plasma and conventional polymerization. A plasma polymer typ ically results from a rivaling etching and deposition process depending on the plasma species present during film growth yielding a more or less cross-l inked structure.

Atmospheric pressure plasmas, such as corona or dielectric barrier .discharges (DBD), however might be of special i nterest for the textil e industry due to an easier processab i lity.4 During the past decade, considerable efforts have been made to generate stable atmospheric pressure plasmas.5 While they

1 00 INDIAN J . FIBRE TEXT. RES., MARCH 2006

show some use for activation and hydrophil ization of textiles6-8, it is rather difficult to obtain high quality plasma-polymerized coatings on textiles. Thus, low pressure plasma is a serious alternative for multifunctional coatings for texti le applications .9 First attempts to deposit plasma polymers on natural and synthetic fibers within a continuous low pressure plasma reactor, where the fibers are processed air-to­air, were performed in the early 1 980s. ' 0 Phosphorus containing monomers were used to impart flame retardancy to cel lu losic fibers and acrylamide plasmas to improve the physico-mechanical properties. In recent years, plasma polymerization on textiles and fibers was mainly investigated for hydrophobization, permanent hydrophil ization and adhesion improvement in fiber reinforced composites. I I

Although, within the textile industry up to now merely special applications and niche products have been performed with the help of plasma polymerization, especially low pressure plasma polymerization is on the way to stimulate the texti le sector.

2 Plasma Polymerization When adverting of plasma polymerization, a

radical-dominated plasma chemical vapor deposi tion (plasma CYD) process is thought of, resulting in macromolecule formation, i .e. mainly amorphous, more or less cross-linked structures. The underlying growth mechanism is known as Rapid Step-Growth Polymerization (RSGP). ' 2 As schematically shown in Fig. 1 , the recombination of reactive species and

� . - . - . - . - . - . - . - . - . - . - . - . - . - . - . - . - . - . - . - . - . - . - . , I

, I I I I I I

I i Plasma ! boundary I - - ,1- . _ . _ . 1

I I I I !

Fig. 1- Plasma polymerization via Rapid Step-Growth Polymerization (RSGP) i nvolving an activation and a recombination zone (concept of chemical quasi-equil ibria)

reactivation of reaction products determine the plasma polymerization. The reactive species are mainly radicals , where cycle 1 consists of reactions of reac tive species with a single reactive site, and cycle 2 is based on divalent reactive species. Cross-cycle reactions from 2 to 1 can occur. Furthermore, the surface takes part in plasma polymerization by third­body reactions and etching processes that lead to ablation and re-deposition.

The concept of chemical quasi-equi libria enables a macroscopic approach to plasma polymerization. It i s assumed that the gas particles (e.g. monomer) first enter an active zone, where excitation and dissociation processes are taking place, and then travel through a passive zone yielding recombination and stable products, such as deposition on substrate, electrode or wall . 13 Following this macroscopic approach, the reaction parameter power input per gas flow (WIF), which represents the energy invested per particle within the active plasma zone, determines the mass deposition rate (Rill) using the following relationship:

. . . ( 1 )

where G is the reactor depending geometrical factor; W. the power input; F, the gas flow; and Ea, the activation energy corresponding to the used monomer. 14. 1 5

I f Eq. ( 1 ) holds, a l inear fit is obtained using an Arrhenius-type plot for the mass deposition rate per gas flow depending on the (inverse) energy input as shown by the straight line in Fig. 2 around the

I I I

lon·induced : ,Deposition I \, I . I

I I I

I I I I I I I I I I

Oligomers taking part � - _ _ in deposition - - - - - - - -: Activation energy = : Slope of linear fit

(Energy input)"'

Fig. 2- Dependence of deposition rate per gas flow on energy i nput

HEGEMANN : PLASMA POL YMERIZA nON AND ITS APPLICATIONS IN TEXTILES 1 0 1

activation energy. 1 6 The negati ve slope of the l inear fit represents the activation energy, which is found to be the minimum energy required to initiate the plasma polymerization process and to obtain stable cross­l inked plasma polymers. 1 7 Deviations from this straight line might be found at low specific energies due to oligomerization and at high energies due to etching, sputtering or temperature effects . 1 8 At rather energetic plasma conditions or long durations, an increas ing temperature during plasma polymerization has to be taken into account that might strongly influence the deposition rate depending on the type of monomer used.

While the actual interaction of plasma particles and plasma-induced radiation influence the absolute deposition rates depending on the reactor geometry described by the geometrical factor in Eq. ( 1 ) , the activation energy merely depends on the plasma chemistry of the considered monomer and is thus independent of the reactor design. To demonstrate the general meani ng of the acti vation energy, the true energy consumed within the active plasma zone yielding the measured deposition rate (concept of chemical quasi -equi libria) has to be known . 1 8 Beside the knowledge of the absorbed power and the gas flow, a geometrical consideration enables the finding of the general activation energy corresponding to the used monomer through the simi larity parameter (S), as shown below:

S = W duc,Vxus , F dgu.,VcliS

(2)

where dart is the length of active plasma zone; dga" the (mean) distance between gas inlet and deposition area; Vgas, the volume occupied by the gas ; and Vdi.\ , the volume occupied by the gas discharge. 16. 1 9 Eq. (2) helps to find out the specific energy consumed within the active plasma zone which determines the measured deposition rate. Replacing WIF by S within Eq. ( 1 ), the general activation energy of a certain monomer gas can be derived.

At an energy input below the activation energy, oligomers can take part in the fi lm growth leading to an increased deposition rate compared to Eq. ( 1 ) . High energy inputs, on the other hand, might also yield a deviation from the straight line due to etching, sputtering or temperature-induced effects (Fig. 2) . However, the macroscopic approach helps to identify the range of (basic) plasma polymerization and gives hints to optimize the fil m properties. In a low pressure

plasma these conditions are given for different monomers and gas mixtures within a broad parameter range, even when additional gases are added to obtain plasma co-polymerization. 1 8 In the latter situation the total gas flow is given by adding the different gases considering a flow factor that weights its contribution to the plasma polymerization process using the following relationship:

. . . (3 )

where Fill is the monomer gas; Fe. the non­polymerizable or carrier gas ; and a, the reaction cross­section (flow factor) which is smaller than 1 . Eqs ( 1 )­(3) enable the up-scal ing of plasma polymerization processes. The situation in atmospheric plasma processes, which appears to be less defined, has not yet been examined using th is macroscopic approach.

For plasma polymerization, in principle, all types of materials might be treated. The plasma treatment of different types of texti les and fibers has already been reported. 1 .8, 1 1 .20,2 1 Ho�ever, it has to be considered that the film growth depends on al l particles entering the active plasma zone. Beside the selected gas flow, which is applied externally, particles might also arrive from the inside of the plasma reactor, i .e. desorption from walls or from the substrates as well as etching or sputtering products. Thus, the gas composition using a textile as substrate might substantially differ from the one without the substrate. Under certain ablation conditions, in particular with porous substrates, plasma polymer layers can be formed without external addition of polymerizable gases (redeposi tion) . Especially for textiles, their water content, additives and manufacturing residuals (such as sizes) have to be taken i nto account that might strongly influence the obtained film properties . Therefore, the textile samples have to be cleaned prior to the coating process to obtain a suitable adhesion. Recently, it could be demonstrated that RF-excited low pressure plasmas yield an excellent cleaning (desizing) and h 'd h ' I 22 23 t us avO! e wet-c emlca process steps . . .

Furthermore, the materials brought into the plasma disturb the quasi-neutral plasma state by implying electric fields which avoid the direct contact to material surfaces . Therefore, active particles always travel through passive zones (concept of chemical quasi-equilibria) which might be micrometer to centimeter wide. The type of active particles contributing to the deposi tion depends on the mean free path (pressure), the lifetime (reaction probabil ity)

1 02 INDIAN J . FIBRE TEXT. RES., MARCH 2006

and the width of the passive zone (plasma sheath) . Most of all, with low pressure RF plasmas a high voltage drop might be observed across the plasma sheath resulting in high energetic particle interactions. In case of textiles, this situation becomes complicated due to the structure of the considered textile which shows several dimensions, filament distance, interfiber (yarn) distance as well as contacting areas or protruding fiber ends (as observed for staple yarn) . Mainly by the variation in pressure, the plasma process can be optimized for different textile structures. At high pressures (atmospheric plasma), the plasma can be ignited within smaller volumes (due to the Paschen law)?4 It is possible to have the plasma burning inside a textile structure (at least i nside the mesh openings), yielding plasma polymer growth. An atmospheric plasma generated outside the textile as usually done, on the other hand, hardly yields a uniform coating since active particles are just able to reach the outside lying fiber sUIfaces. Uncoated areas are sti l l taking part in external interactions. Moreover, a high pressure plasma is fi lamentary in nature, thus leading to moving hot spots. Therefore, i t i s difficult to obtain controlled deposition conditions and the effects that can be achieved by plasma polymerization at atmospheric pressures on textiles are l imited. Moderate pressure plasmas ( 1 00- \ 000 Pa) were found to give optimum cleaning and activation conditions with non-polymerizable gases since a high number of acti ve particles contribute to chemical etching.22.25

Plasma polymerization final ly can best be controlled using low pressure plasmas ( 1 - 1 00 Pa) due to well­defined plasma zones . In this pressure range, the mean free path lengths are high enough to allow the penetration of the textile structure by energetic particles and long-living radicals . Good conditions for plasma polymerization are thus enabled on textile fibers up to several fiber layers in depth so that the external media interact with the treated fibers. Therefore, at EMPA, mainly the low pressure plasmas are considered to obtain plasma-polymerized coatings on textiles.

To build up a reactor for plasma polymerization on textiles, a vacuum chamber is required. First note that no ultra high vacuum conditions must be fulfi l led regarding the outgassing of textiles26, and secondly the atmospheric plasma chambers also require a surrounding box to allow for defined gases. I n vacuum i t i s easier, a t least when different types of textile fabrics, nonwovens, membranes and papers

should be treated, to have the winding inside the chamber enabling a semi-continuous treatment. Obviously, this kind of process cannot be integrated directly i nto a production l ine. However, when looking at achievable process velocities for plasma polymerization on plane substrates, a principle incompatibility might occur, as discussed below.

Large reactors for low pressure plasma polymerization on textiles are available in the market.27.28 Plasma excitation in the kilohertz range (typical ly 40 kHz) is mainly used for industrial purposes due to lower costs. RF or MW sources are also applicable if required by the process. The reactors are typically designed for the textile widths up to 1 20 cm, l arger widths of up to 4 m are also possible. The reactors are loaded with textile packings, closed, evacuated and run in a semi­continuous process. The textiles are led through the plasma regions adapted to the required process, which comprises usual ly a cleaning and a plasma deposition step.

To avoid some l imits of plasma polymerization on fabrics or other webs (huge reactor, semi-continuous process, contacting fibers), the direct treatment of fibers is also important. One advantage of fiber processing is the possibility to perform winding off and up in air by leading the fibers through suitable openings into the low pressure region using a differentially pumped sealing system. Flexible texti le fibers, yarns, etc. can be run several times through the plasma zones enhancing the plasma length and thus the process velocity. Furthermore, different treatment times that might be required for different process steps such as cleaning, deposition, and post-treatment can thus be adapted for a one-step processing. In-line production is achievable by process velocities as high as several hundred meters per minute.

Therefore, both a semi-continuous web coater (up to 63 cm in width) and a continuous fiber coater are present at EMPA (Fig. 3) beside several batch reactors. 1 9 While the flexible batch reactors, where different geometries are realized to allow for symmetric and asymmetric radio frequency excitation, microwave and magnetron sputtering as wel l as combinations thereof, are used for the optimization of plasma processes, the continuous pilot-plant reactors serve to demonstrate industrial scale-up.

3 Plasma-polymerized Coatings 3.1 Hydrocarbons

Unlike conventional polymerization, plasma poly-

HEGEMANN : PLASMA POLYMERIZATION AND ITS APPLICATIONS IN TEXTILES 1 03

Fig. 3-Web coater (a) and fiber coater (b) developed and used at EMPA

merization can be performed with any kind of hydrocarbon monomer (gaseous). For the ease of processing, mainly methane (CH4), ethylene (C2H4) or acetylene (C2H2) i s used. While the ach ievable film properties of the amorphous hydrocarbon layers mainly depend on the degree of cross-linking, which is rather independent of the used monomer, saturated and unsaturated monomers show a difference i n deposition rate and amount of unsaturated bonds left within the film structure. Most of all , the acetylene­derived plasma polymers appear to be yellowish by light adsorption at unsaturated bonds. Methane, on the other hand, shows noticeably reduced deposition rates (Fig. 4).

The deposition of plasma-polymerized hydrocarbon layers with defined permeation properties can be controlled by the energy i nput into the active plasma zone using the reaction parameter WIF and deriving the activation energy (Fig. 5 ) . These parameters control the polymeric character of the coatings by the residual hydrogen content. The figure

0 . 1

E <.) u f/) '"'E u co 'E 0.01 a, "' <:> ::. u... E a::

0.001

0 0.002

Q 0 o 0 0

"

0.004 0.006 0.008

(W/Fr' [(J/cmJr'l

n

0.01 0.0 1 2

Fig. 4-Mass deposition rates per monomer tlow for pure methane, ethylene and acetylene discharges, depending on the reaction parameter WIF within our web coater. The slope of the l inear fits represents the corresponding activation energy

� '" D "0 NE ",-E .£ a:: t-O

l 00 r----,�r_---------------------------,

10

0 . 1

0

I I 1 1 I �

q 1 I I : 0 0 0 :-o 0 _ _

- MW - C2H2 o RF - CH4

1 - -: C- .. 1 1 : E� I

2 3 4 C; [in units of E.l

o

Fig. 5- Oxygen transmission rates (OTR) for plasma­polymerized hydrocarbon layers

shows that the activation energy indicates the transition from weakly cross-linked to dense plasma polymers. While at an energy input below the activation energy no barrier properties can be obtained, the permeation can be reduced at higher specific energies both for RF and MW activated discharges.29 These coati ngs are of interest for the controlled drug release from textile substrates and for their hydrophobic properties.

At increasing energy input, the internal stress increases within the hydrocarbon network yielding diamond-like coatings (OLe). It could be shown that the fi lm properties nOw depend on the voltage drop across the plasma sheath (given mainly by the bias potential) and thus on the interaction of energetic part icles during fi lm growth, which favor the formation of a Sp3 hybridized network. OLC coatings

1 04 INDIAN 1. FIBRE TEXT. RES., MARCH 2006

on texti les and fibers are investigated for their mechanical, tribological and biological properties.30

Adhesion on flexible substrates can be enhanced by interfacial gradient layers.

The deposition of hydrocarbon plasma polymers was i nvestigated as adhesion improvement in fiber reinforced composites. Plasma polymerization of ethylene was found to increase the adhesion strength between PET fibers and a PE matrix from I N/mm to 2.5 N/mm7, while pYlTole or acetylene as monomer gases i ncreased the pul l-out force of aramid cords embedded in rubber by up to 90% with negl igible decrease in single fiber tensile strength.3 ! .32 Ooij et al. 32 used a semi-continuous DC plasma reactor to treat the aramid cords. Ultrathin « SO nm) plasma­polymerized acetylene layers were also found to increase the tensi le strength of carbon fibers due to their cross-l inked network of molecular chain structure.3

3.2 Organosilicones

Organosi l icones are the materials consisting of Si atoms bonded to organic hydrocarbon groups. Additional ly, they might contain oxygen or n itrogen bonds. In general, S i-containing fi lms can be plasma­polymerized using gaseous mixtures of silane (SiH4) or s i l icontetrachloride (SiCI4), mainly used to deposit amorphous sil icone (a-Si :H) layers for semi­conductor fabrication, and is also of interest for flexible solar cel ls on textiles. 34 However, for textile application, the organosi licon monomers show some advantages due to the possibility of low deposition 35 PI I " f' temperatures: asma po ymenzatlon 0 organosilicon compounds i s used to impart good dielectric properties, thermal stability, scratch resistance, lowered friction, flame retardance and baITier properties as well as to adjust the wettabi lity of textiles. Moreover, these films have been employed as gas filtering membranes36 or for UV protection of polymers.37

Hexamethyldisiloxane [HMDSO; (CH3hSi-O­Si(CH3hJ is the most common monomer, since the l iquid precursor has a high vapor pressure, is non­toxic and enables the deposition of siloxane coatings at low temperatures. While at low energy inputs [below the activation energy (� 1 2.8 eV)] 1 6. 1 7 rather oligomer-like coatings are deposited by condensation reactions within the gas phase, higher specific energies yield durable plasma coatings by cross­l inked Si-O-Si backbones with -CHn groups (n = 1 .. 3) . 1 5 ,38 By mixing with oxygen, the organic amount

retained within the plasma-polymerized coatings can be successively reduced to attain more inorganic quartz-like coatings (SiOx)' Thus, a wide range of different film properties can be adjusted by mainly varying the gas ratio using moderate energy inputs to keep the deposition temperature below about 60 °C. The organic/inorganic character that varies from polydimethylsiloxane (PDMS)-like to quartz-like (Fig. 6) directly affects the surface energy (adjustable between 20 mN/m and 68 mN/m), the permeability (down to oxygen transmission rates of 0. 1 cm3/m2 d bar), the density (between 1 .0 g/cm3 and 2.0 g/cm\ the mechanical properties (up to a hardness of about 6 GPa), the friction and the flame retardance.39-45

Films with desired properties can be designed depending on the energy input and the gas ratio (e.g. 02/HMDSO) using the activation energy for the plasma polymerization process derived from Eqs ( 1 ) - (3) .

Plasma-polymerized SiOx coatings usually show excellent adhesion on flexible substrates due to an interface formation within the first steps of fi lm growth.46 Increased internal stresses which imply higher forces acting on the interface within thicker or more inorganic coatings, however, can be adapted by depositing a gradient layer on the interface.42 I n this case, the film character is changed from more organic to inorganic by variation of the gas ratio. The structure of the gradient layer can be optimized using the mass deposition rate [Eq. ( 1 ) and Eq. (3)] with a = 0 .6 for oxygen added to HMDSO. Therefore, SiOx coatings can be deposited on al l textile and fiber materials. Gradient l ayers can also be deposited within continuous processes by flowing the gases in

1 20 r-----------------------------------� - - - - - - - - - - - - - - - - - - - - - - - - - - - JPr.

C 1 00 PDMS • • • � g> 80 ro t5 60 ro C o u 40 ;>

• • SiO,Cy: H films •

• '0 <t: � quartz 20 � . - - - - - - - - - - - - - - - - - - -- - - - - - - - - - -

o 1 0 2 0 30 40

ReI. carbon concentration ( % I 50

Fig. 6-Control l ing the residual carbon content in plasma­polymerized SiO, coatings; the fi lm properties can be adjusted covering the range from PDMS-l ike to quartz-l ike

HEGEMANN: PLASMA POLYMERIZATION AND ITS APPLICATIONS IN TEXTILES 1 05

parallel to the transported texti le and not towards it, which is normally used to obtain homogeneous deposition conditions. Thereby, the residence times of the monomer molecules within the active plasma are varied and thus the organic/inorganic character of the SiOx deposit.47

HMDSO-derived plasma polymers are used for the hydrophobization of cotton showing water contact angles up to 1 30° (ref. 7). The influence of outgassed water vapor during the plasma treatment is rather small , since oxygen is i ncorporated into the backbone of plasma polymer (Si-O chains), which is supported by oxygen radicals. The water vapor transmission, on the other hand, is not influenced by the plasma polymerization. Gas-separating properties of membranes coated by HMDSO plasma polymers were i nvestigated by Li and Meichsner.4 1 Depending on the energy input different permeability coefficients could be obtained. Plasma-polymerized HMDSO coatings on PP fabrics (RF plasma at 1 0 Pa) were further applied to lower the water uptake values and increase the contact angles.48 Plasma polymerization of HMDSO/02 coatings on carbon fibers was found to increase both fracture toughness and strength of fiber­resin composite material .49

HMOS plasma polymers deposited on polyester textiles were investigated as selective membranes to separate H20 from hydrophobic organic compound.50

Organosi l icones such as hexamethyldisi lane (HMOS) and tris(trimethylsi lyloxy)vinylsilane (TTMSYS) were also used with atmospheric pressure plasmas (DBD, 1 5 kHz) to enhance the color intensity by forming an anti-reflective layer at the surface of PET fabrics.5 1 Both vacuum and atmospheric plasma treaters are distributed by different companies for the d

. . f ' l

. b 27 28 epos1tIOn 0 SI oxane coatIngs on we s. .

3.3 Fluorocarbons

The plasma treatment of surfaces with fluorocarbons is a rather complex process, since it i s generally a rivaling process between deposition and etching. For example, CF2 radicals are not j ust formed in the active plasma zone, but also arrive as etching products from the growing film.52 Therefore, Eg. ( 1 ) cannot be used i n a strict way, i .e. generally no l inear region in the Arrhenius-type plot of the deposition rates is obtained. However, the specific energy per molecular mass of the monomer might ·be used as a composite parameter to compare different fluorocarbons. 1 5 ,53 It is . also well known that for fluorocarbons with a F/C ratio > 3 .0 (e.g. CF4) etching

prevails. The ratio can be shifted by either adding hydrogen (towards more deposition) or oxygen (towards more etching) to the FC monomer and is also influenced by the bias potential, i.e. the interaction of energetic particles as depicted in Fig. 7 (refs 54,55) .

The obtained etching might lead to the formation of radical sites at surfaces which are oxidized in post plasma reactions, y ielding a strong hydrophil ization effect, or to the i ncorporation of CF groups and rather hydrophobic surfaces. If a polyester foi l (or fabric) i s brought into a CF4 discharge (at floating potential) within an asymmetric reactor set-up, one side can be rendered hydrophobic, while the other side becomes strongly hydrophilic. At the side facing, the RF electrode etching conditions prevail , whereas milder plasma conditions result in fluorination at the back side. These conditions were obtained using a ' web coater for low pressure plasma treatments .56 However, the treated polymers are prone to aging effects due to the formation of low molecular weight oxidized material (LMWOM) and no real plasma polymerization takes place.

Nevertheless, the surface energy can be adjusted over a wide range by controll ing the etching/ deposition reactions, for example using CHF3 under varying pulsed plasma conditions.57 This approach even enables the formation of domain structures on surfaces, which might find applications in wettability, friction, and cell growth on textiles. A mixture .of hexafluoroethane (C2F6) and hydrogen was also investigated as hydrophilic barrier against hydrolysis

-200 � � � :J III ,g - 100 '0 .!!1 Q. a. (Q III (Q 00

0

H2 addition ... -- • O2 addition C2F. C.F,o C2F6 CF.

- , \ \ \ Etching \ , , ,

- , , \ \ Polymerization \

\ \ \ I 1 I

1 2 3 4 F/C

Fig. 7- Transition of polymerization and etching conditions depending on FIC ratio of the monomer and bias potential (adapted from Coburn and Winters55)

1 06 INDIAN J. FIBRE TEXT. RES., MARCH 2006

of aramid fabrics such as Nomex®.7 The deposition of a thin layer leaves the fibers intact by immersion i nto a 85% H2S04 solution (20 h at room temperature), while a conventional fluorocarbon fin ishing shows a significant shrinkage of the fibers and loss of properties .7

The use of pure saturated fluorocarbons (CnF21l+2) might lead to hydrophobic surfaces merely by fluorination. A similar fluorination of textile fabrics such as silk can also be performed using SF6 plasmas without the deposition of a plasma-polymerized layer.58

Plasma polymerization, i .e . the deposition of fluorocarbon films with plasma duration, on the other hand, can just be obtained with unsaturated fluorocarbons, manely C3F6 and C4Fg or more complex structures such as carbon rings and perfluoroalkyls . ' 5,53 Hence, the possibility of cross­linking supports the plasma polymerization process. However, due to long-living radical sites left within the fluorocarbon film, these surfaces tend to get the aging effects by oxidation and reorientation, which limits the oleophobic properties of plasma­polymerized FC coatings . Moreover, using textiles as substrates an increased amount of unwanted addi tional gases (water vapor and additives) might strongly influence the fluorocarbon film growth with respect to adhesion and repellency.

Fluorocarbon plasmas were also i nvestigated by Vinogradov and Lunk59 under atmospheric pressure. They obtained no deposition rate with the saturated monomer C3FS unless hydrogen is added. Due to the formation of toxic byproducts, low process velocities and incorporation of oxygen, there are no real advantages compared to low pressure plasmas.

Hence, though the plasma polymerization is considered as an ecofriendly technique, the still improving wet-chemical treatments dominate the market for stain repellent textiles, which show a good permanence by cross-linking through annealing ( 1 20-1 60 °C) and envelopment of the single fibers. A plasma activation, however, might be used to enhance the capi llary transport of wet chemicals within the textile structure to improve the envelopment and thus reduce the amount of chemicals needed.60 Oil repellency of the maximum grade of 8 (according to AATCC 1 1 8- 1 972) can be attained.

Nevertheless, fluorocarbon treatments wi thin low pressure plasmas are industrially performed on selected textiles when a low film thickness « 1 00

nm) and good adhesion to the textile surface is required.6 1 Acrylic fabrics are coated with fluorocarbons within a semi-continuous low pressure plasma device for outdoor applications?3 While the plasma coatings show waterproof and abrasion characteristics comparable to conventional wet­chemical treatments, a better softness of the textiles is observed due to the nanoscaled coating. Oil repellency grades of 4-5 are thus achievable showing superior properties compared to Scotchgard-sprayed samples within short treatment times (30-60 S),z 1 .62 Moreover, the softness, feel , color, permeability, abrasion resistance, water retention and friction coefficient are found to be unaffected by the nanoscaled plasma coating. Low pressure plasma deposition of perfluorinated alkyl chains shows an oil repellency grade of up to 7 due to oriented -CF3 groups at the surface.2 1 A s imilar approach has been commercialized by P2i Limited, U K in 2004.

3.4 Hydrophilic functional coatings

Hydrophi lic treatments of textiles and fibers are of great importance for many applications in printing, dyeing, lamination, fiber reinforced composites, capil lary transport, etc . To avoid wet-chemical treatments, plasma activation with non-polymerizable gases shows some posi tive effects such as cleaning, etching, cross-linking, formation of radical sites and functional groups, which are able to enhance the binding in subsequent process steps. However, this type of plasma activation is prone to aging due to internal re-orientation effects as well as external influences. Aging can be strongly minimized by deposition of functional plasma polymers since internal re-orientation processes are hindered by the cross-linked film network. Moreover, a higher density of functional groups can be obtained by the film volume as long as they are accessible within a polymer matrix .

Thus, hydrocarbons can b e effectively mixed with non-polymerizable gases such as Nz, NH3, O2, H20, CO2 or others to obtain functional groups within a cross-l inked amorphous hydrocarbon network.3

Rival ing processes of etching and polymerization can be used to yield highly functionalized and permanent plasma polymers. We used RF plasmas at t o Pa of gaseous mixtures of acetyle!1e with water vapor, carbon dioxide and ammonia to obtain oxygen or n itrogen contain ing functional groups on fabrics (PP, PET, CA, cotton and others) within our web coater. Evaluation of the mass deposition rates compared to

HEGEMANN: PLASMA POLYMERIZATION AND ITS APPLICATIONS IN TEXTILES 1 07

pure C2H2 discharges shows that the admixture of H20 has no influence on the deposition rate for moderate specific energies W/ Fm related to the monomer flow (Fm) of C2H2 (Fig. 8). The reduced deposition rates compared to pure C2H2 indicates deviations in the growth mechanism. At higher energy inputs, however, a drop in the deposition rate indicates inhibition of plasma polymerization. Admixture of CO2, on the other hand, reveals a reduced deposition rate by chemical etching effects, while ion-induced and chemical etching effects can be observed with NH3 .

According to Yasudal 2, the excessive H20 added to a monomer acts as an efficient modifier of the growth mechanism, which shifts the major growth path from cycle 2 to cycle 1 (Fig. I ) , thus inhibiting plasma polymerization. Water vapor admixture is found to strongly reduce the concentration of dangling bonds in the plasma polymers. Nitrogen and carbon oxide, on the other hand, have similar electron structures as acetylene and evidently participate in the chemical reactions of cycle 2 (divalent reactive species). However, both 0 and N are etching gases, and energetic N2+ ions formed at higher energy inputs yield strong etching effects. 1 8

Compared to Corona treatments, low pressure plasma activation and plasma-polymerized SiOx coatings, these coatings show less aging effects under storage at ambient air (Fig. 9). It is found that the static contact angles i ncrease less for mixtures of acetylene with oxygen or nitrogen containing gases. Permanent hydrophilic treatments are of interest to improve the sweat management of clothing.

0. 1

E u u Vl N E 0.01 u c 'E 0, '" " 0.001 .-

E u.. E ex: 0.0001

o

o C2H2 o C2H21NH3

. c2H21H20

. C2H2IC02

��.8. _ _ _ A - ' - CL ' . �- - - - -3- -.' . .... . - . 'i" . - . - ...... . _-• , , •

/ I' .' /

0 ...... ""G- -O--�---/ 0 0 /6 0

Q l 0 2 Q3 Q4 CN/Fmrl [(W/sccmr11

0.5

Fig. 8- Mass deposition rates per monomer flow for pure acetylene discharges as wel l as C2H2!NH3, C2HiH20 and C2H21C02 mixtures depending on the reaction parameter WIF",

A mixture of acetylene and oxygen (2: 1 ) was used by Feih and Schwartz63 to deposit plasma polymers on PAN-based carbon fibers to increase the fiber-epoxy resin adhesion in composites. A 90% i mprovement in interfacial shear strength (IFSS) is found to be accompanied by no significant change in the tensile strength of the carbon fibers, which is attributed to carbonyl and hydroxyl groups on the surface as well as long-living free radicals entrapped within the plasma polymer. Dilsiz et al.64 used dioxane and xylene/air plasmas to treat carbon fibers. The plasma­polymerized layers are found to heal flaws on the fiber surfaces and increase their tensile strengths . Low pressure plasma co-polymerization of hydrocarbon monomers and oxygen or nitrogen containing reactive gases are carried out on an industrial level to obtain a permanent hydrophilic treatment of synthetic fabrics such as polyester and polyamide used for fi ltration.os

Using dichloromethane within a RF plasma ( 1 0 Pa), the dyeability with reactive dyes can be enhanced at short treatment times ( 1 0-45 s) on cotton and polyester fabrics.06 Functional C-O groups and free radicals are formed within a plasma-polymerized thin surface l ayer, whereas at longer treatment times etching processes are found to dominate. An increase in the diffusion coefficient of acid dyes into wool is reported by Wakida et al.67 using an atmospheric pressure plasma. Acetone/ Ar plasma polymerization produces a hydrophilic polymer which modifies the surface of the endocuticle or the cell membrane complex contributing to accelerated dye diffusion. Plasma-polymerized hydrophilic coatings of H20 or N2 mixed with acetylene, benzene and heteroaromatic amines are found to improve the characteristics of reverse osmosis membranes.68

80 70

c 60 .S1 g> 50 '" � 40 c: 8 30 o -£ 20

10 o

v v-:-: 'P-" o 5

.... Corona treatment

� Arl02 plasma

..... SiOx coating

-0- a-C:H (O.N) coating

10 15 Aging [days]

..,

20 25

Fig. 9- Aging observed for polyester fabrics treated with different kinds of plasma activation and plasma polymerization

1 08 INDIAN 1. FIDRE TEXT. RES. , MARCH 2006

Silicontetrachloride (SiCI4) can also be used to deposit hydrophilic plasma coatings. While energetic plasma conditions lead to the formation of amorphous silicon layers, milder plasmas retain some SiClx (x < 3) molecular fragments that easily convert to Si-OH groups in the' presence of atmospheric moisture (post plasma reactions).69 Additionally, the roughened surface enhances the hydrophil icity of the treated PET fabric surface. These layers were also reported to improve the PET fabric dyeing properties.7o

3.5 Monofunctional Coatings

Plasma polymers with functional groups can also be obtained using suitable monomers which already contain the required functional group. Depending on the energy i nput into the active plasma zone, these functional groups can be retained during plasma polymerization. Thus, monofunctional coatings can be tailored containing a varyirig amount of distinct functional groups, such as carboxyl (-COOH), amino (-NH2), hydroxyl (-OH) or ethylene-oxide (EO) units (-CH2-CHrO-) .7 1-J7

Plasma polymerization of acrylic-like coatings on fabrics such as polyester or polyamide was investigated using low pressure plasmas with acrylic acid as monomer.78 Keeping the fabrics inside the plasma zone (at floating potential), good penetration of the fabrics is achieved by a both side treatment. Acrylic-like coatings are found to enhance wettability, dyeability (using basic dyes) and soi l resistance, while they show less aging compared to plasma treatments with non-polymerizable gases (02, H20, Ar, air).78

The color depth of dyed fabrics (polyester/cotton blend) increases with the acrylic-like film thickness.8

Stable plasma-deposited acrylic acid surfaces also enable improved cell adhesion for tissue . . 79 engmeenng.

Sarmadi et al. 80 reported the surface modification of PP fabri'cs by acrylonitrile cold plasma to deposit polyacrylonitrile-like (PAN) layers. The presence of nitrogen- and carbon-based unsaturated linkages and the formation of second generation =C=O groups led to improved water absorption and dyeing properties. According to the authors, short treatment times should make industrial application possible.

Plasma deposition of acrylamide (AAm) on silk fabrics (RF plasma, 650 Pa) was found to improve the elastic recovery of the fabrics. 8 I The retention of hydroxyl groups (-OH) can be performed using plasma polymerization of allyl alcohol, e.g. for

enzyme immobilization membranes. 82

on polysulphone

Allylamine plasma polymerization was investigated for the improvement of fiber-matrix adhesion in PE fiber reinforced epoxy resins83 or aramid fiber reinforced composites. 84 Monofunctional plasma coatings are industrially used for the regioselective treatment of separation membranes required for b lood dialysis .85 Therefore, the blood is led through a bundle of hollow fibers to separate blood cells from the blood plasma by suitable pore sizes.86 The membrane wall should be functionalized with primary amino groups to immobilize scavengers that are able to bind toxic components (endotoxines) sometimes found in the patient' s blood. However, NH2 groups are not allowed at the inner surface of the hollow fibers to avoid direct contact with the blood. Hence, wet-chemical treatments are excluded. Therefore, the fibers are continuously led through a sealing system from atmosphere to the low pressure region, where an allylamine discharge is generated which is not able to burn inside the hollow fibers.87

The reactive plasma species penetrates the fiber wall from the outside to about two third of the wall thickness forming amino groups within the porous wall but not at the inner surfaces .

Even polyaniline-l ike plasma polymers can be deposited from aniline within low pressure discharges (40-50 Pa) by retention of quinoid and benzenoid rings, showing a conductivity of 1 0- 1 1 rrl cm- I , which is noticeably higher than other plasma pOlymers.88

However, the quality of conductive plasma polymers is l imited due to cross-linking reactions, Conductive polypyrrole-coated polyester fabrics are produced by plasma polymerization of 1 -(3-hydroxypropyl)pyrrole resulting in pyrrole moieties covalently bonded to the fabrics.89 It is observed that the coated fabrics neither cause hemolysis nor they alter the blood coagulation properties. Hence, they are considered as biocompatible coatings in cardiovascular applications.

The retention of ethylene-oxide moieties within plasma polymer fi lms is known to impart non-fouling properties to surfaces by deposition of polyethylene­oxide (PEO)-like coatings.9o Using di-ethylene glycol dimethyl ether (DEGDME or diglyme) as monomer, the PEO character of the plasma-polymerized films (RF plasma, 50 Pa) can be varied depending on the power i nput and gas flow.77 The PEO character is commonly defined as the relative per cent of ether carbon functionalities (at 286.5 e V) of the C I s high-

HEGEMANN: PLASMA POL YMERIZA TION AND ITS APPLICATIONS IN TEXTILES 1 09

resolution scan usmg X-ray Photoelectron Spectroscopy (XPS). Figure 1 0 shows the evolution of PEO character, depending on the energy input (WIFm) with respect to the monomer flow. Higher amounts i ndicate a higher retention and less fragmentation during the plasma process.

Good non-fouling properties can be obtained for a PEO character exceeding 75%. Therefore, low specific energies, noticeably below the corresponding activation energy, are required to obtain swellable (hydrogel) coatings that effectively screen the surface by water adsorption to avoid biomaterial adhesion. The energy input can be further decreased using pulsed plasma polymerization. Wu et al.9 1 .92 used diethylene glycol vinyl ether (E02V) as well as cyclic ethers at 4- 1 0 Pa pressure and varying RF plasma cycles (on and off times) for power modulation. A high PEO character was reported for low specific energies, which can be further examined by evaluation of the mass deposition rates as shown in Fig. 1 1 . Non-fouling properties can be observed for energy inputs below the activation energy.

3.6 Ceramics

Titania (Ti02) surfaces are known for their super-. . . 93 94 T'O hydrophi licity and photocatalytIc actlVlty . · · I 2

can be plasma polymerized using tetra-isopropylorthotitanate (TTIP) and oxygen within a microwave ECR (Electron Cyclotron Resonance) discharge at low temperatures95 « 1 00°C). Compared to sol-gel coatings, their activity is found to be lower due to a higher amount of amorphous phase. Using titanium tetrachloride (TiCI4) as precursor compound, Ti02 films are deposited on cotton knitwear in the presence of oxygen within the RF plasma phase yielding a substantially enhanced bactericidal activity.96 Thin Ti02 films or microparticles might also be of interest for use as UV -absorbing layer for synthetic fibers.97 However, the Degradation effect of the photocatalytic Ti02 on polymers has to be taken into account for application on polymers.

A protective sheath of titanium nitride was deposited on aramid fiber bundles introduced into a MW plasma to increase their durability.98 We were also able to deposit transparent conducting oxides such as ITO (indium tin oxide) on fabrics, which might serve as anti-static and heat-reflecting coatings.

Ceramic coatings in the system Si-B-C-N which are known to be hard and temperature resistant can be plasma polymerized from different single-source

80 70 60

* -;:: 50 CD U � 40 .s:: 0 @ 30 a. 20 1 0 0 0 1000 2000 3000 .aoo

WlFm [J/Cm31

5000 6000 7000

Fig. 1 0- Retention of ethylene-oxide units during plasma polymerization of DEGDME, given by the PEO character depending on energy input77

10

E 8 <II C E ) 01 'I' a :::::.. E u. E 0::

0. 1 0

• E02V o 1 2-crown-4 ether

f-- non-fouling properties ' . .

0.01

_ _ 0_ . _ .•. _ . _ . _ . _ . _ . _ . _ . _ . _ . _ . _ . o

0.02 0.03

Fig. 1 1- Mass deposition rates per monomer flow for E02V and 12-crown-4 ether discharges depending on the reaction parameter WIF1I/ 9 1 .92

precursors at substrate temperatures of around 250 °C (probably lower also).99 Interesting coatings are SiC, SiCN and SiBCN, which show reduced internal stresses compared to OLC, B N or BCN due to incorporation of four-fold Si atoms. IOO Especially, plasma-polymerized thin coatings of SiBCN show excellent oxidation resistance at temperatures exceeding 1 200°C (Fig. 1 2) 1 0 1 - 1 03 and can thus be used for the protection of carbon fibers. 1 04 Figure 1 2 shows that the annealing in argon or air at 1 200 °C merely results in an oxygen uptake at the surface (few hundred nanometers) leaving the bulk composition of SiBo.8C3.6N 1 .2 unaffected.

Using sputtering techniques, the deposition of further ceramic coatings on textiles and fibers were enabled such as Sn-doped In203 (ITO) used as

1 10 INDIAN 1. FIBRE TEXT. RES., MARCH 2006

50 .-------------------------------__ -,

_ 40 :i? o c o � 30

C � c 8 20

c Ql 0> >-0 10

-<>-As-deposited

..... A Iter annealing (Ar. 1200 .c)

.....-After annealing lAir. 1 2oo'C)

O ������������� __ �� __ � a 100 200 300 400

SiBCN film depth [nm) SOO 600

Fig. 1 2- Oxygen concentration at SiBCN fil m surfaces measured by XPS under Ar sputtering

transparent conductive oxide or piezoceramic lead zirconate titanate (PZT) coatings. 1 05. 1 06

3.7 Plasma Co-polymerization

By using proper experimental configurations, it i s possible to couple a sputtering or an evaporation process with plasma co-polymerization in a way that clusters of material (metal, ceramic, polymer) can be included in a plasma-deposited organic matrix. 107 Examples are Au-containing teflon-l ike and quartz­like coatings. 108. 109 Released Au nanoparticles are found to specifically bind to cancer cells pushing the field of nanobiotechnology. l l o Of special i nterest for textile application are si lver-containing PEO-like coatings, which couple the non-fouling properties of polyethylene-oxide (PEO) polymers with the anti­bacterial effects of silver. 1 1 1 Combining sputter conditions with plasma polymerization through a high excess of Ar added to the OEGOME monomer, Ag nanoparticles are incorporated in the growing film.77 The remaining PEO character (27 .5 %) and the anti­bacterial effects through the release of Ag + ions in wet conditions are found to completely prevent the adhesion of P. aeruginosa, which might be used for medical textiles such as wound bandages. Monovalent si lver ions specifically bind to bacteria via sulphydryl (-SH) groups at their cell membrane surface and h· d h ' f 1 1 2 III er t elr energy trans er system.

Certain toxic elements such as Cu, Ag and V can also be embedded in OLC coatings, which are released and cause toxic reactions when exposed to biological media. 1 1 3 This allows the preparation of surfaces with a tunable antibacterial effect. Metal­containing OLC coatings can be prepared using a

metal cathodic arc ignited in the presence of acetylene to form a dual plasmal 1 4 or by combining plasma polymerization of CHJ Ar with DC magnetron sputtering of several target materials. 1 1 5 Thus, metal nanoclusters or nanocrystall ine metall ic carbides can be embedded in the carbon network.

Barranco et al. 1 1 6. 1 17 examined the possibility of the incorporation of macromolecules such as dyestuff in porous columnar inorganic S iOx coatings using plasma co-polymerization. The films were deposited in a remote microwave reactor using tetramethylsilane (TMS) and tetraethylorthosi l icate (TEOS) as organosil icone precursors and dye sublimation, yielding a durable dyeing procedure.

4 Up-scaling The transfer of plasma polymerization processes

into the textile industry is a major concern of today' s plasma research activities. A s long as the effects that can be achieved by atmospheric plasmas are weak, low pressure plasma processes are sti l l state-of-the-art technology. Generally, these processes are optimized using batch reactors and then transferred to (semi­continuously operating pilot-scale reactors to demonstrate the feasibi l ity for an industrial up-scale. For an efficient up-scaling similarity parameters can be identified enabling the transfer of plasma polymerization processes to different reactor . 16 geometnes.

The deposition from highly fragmented monomers under the interaction of high energetic particles is mainly control led by the bias potential applied to the deposition electrode. The transfer to different reactor geometries can then simply be performed by maintammg bias potential and pressure. The deposition of DLC and ceramic coatings are examples. Nevertheless, the deposition rates should be adapted since the forces applied by the internal stresses of the hard coatings l inearly scale with the film thickness. For thicker films, gradient layers might be required to attain good adhesion on flexible substrates, which must also be controlled. In most cases, the deposition of plasma polymers obtained under moderate or weak interaction of energetic particles has to be transferred to different (larger) reactors. Thus, the identification of a similarity parameter strongly faci l i tates up-scaling.

As discussed earlier, the measurement of the mass deposition rates of plasma polymers depending on the energy input, which can easily be carried out by weighing the samples (e.g. thin glass s lides), is a

HEGEMANN: PLASMA POLYMERIZATION AND ITS APPLICATIONS IN TEXTILES I I I

convenient method to derive the corresponding activation energy according to Eq. ( 1 ) . This activation energy can be used to optimize the hydrophobic properties of plasma-polymerized siloxane coatings using HMDSO as monomer while obtaining highly durable surfaces. 1 5 If the geometrical factors used in Eq. (2) are not known for the considered plasma reactors, the measurement of the deposition rates is repeated for the new reactor to identify the corresponding reactor, depending on the activation energy that can then be used to scale the plasma process. Knowledge or an estimation of the geometrical factors help to speed up the transfer by reducing the required experiments due to a good starting range for the reaction parameter power input per gas flow (W/F). Figure 1 3 shows the water contact angles of plasma-polymerized siloxane coatings obtained using different plasma reactors with respect to the activation energy (E{/).

The highest contact angles are obtained at energy inputs c lose to Ea. A slight difference can be observed using either asymmetric or symmetric reactors. While the interaction of energetic partic les during plasma polymerization is weak within the symmetric reactor, the asymmetric set-up causes a stronger ion bombardment due to a higher sheath voltage, yielding a shift in the retention of methyl groups at the growing film surface. Nevertheless, the hydrophobic plasma treatments can be transferred using the demonstrated macroscopic approach. Recently, it has been demonstrated that in addition to pure monomer discharges, the mixture of different gasses can also be transferred [Eq. (3)] . I S

For industrial-scale processes also the efficiency of plasma polymerization processes is of great interest. For the treatment of textile fabrics, knits and nonwovens, a plasma length and width of around 2 m can be assumed; a l arger width might also be possible.6 1 High deposition rates can be achieved, e.g. with organosilicones to i nfluence wettability, friction, permeabi lity and mechanical properties of textiles. Considering that, for a typical treatment a film thickness of at least 10 nm is required to account for the textile structure and roughness, which can be deposited at a rate of 10 nrnls and the process velocities exceeding 1 00 rnlmin are thus achievable. However, many plasma polymerization processes take more than one second. The improvement of the dyeability of cotton is reported to be optimum within 1 2s, yielding a possible process velocity of 1 0 rnlmin

l IS 1 10

o -; 105 � til 100 ti til E 8 95 Q; iii 90 �

85 80 o

· · ·

� cIO • � y- . d �� - _ Receding • ." 0- _

i -�-����====��--o · · --Web coater --- Adv.lre<:.. · · -<>-Sym. small -<>-Adv.frec. · · _Sym. large --Adv.frec. ·

: Ea -O--Asym. large -0--Adv .frec.

50 100 1 50 200 250 300 S [J/cm1

350

Fig. 1 3- Water contact angles of HMDSO-derived plasma polymers using different batch reactors (asymmetric and symmetric set-ups of different sizes)

(ref. 66), whereas fluorocarbon l ayers can be deposited within 30s and 4 rnlmin respectively.62

Deposition rates for hard coatings (ceramics, DLC), on the other hand, might be around 10 nrnlmin. If rather thick coatings of around 200 nm are required, the process velocity might be reduced down to 0. 1 rnlmin. These considerations demonstrate that it is generally difficult to integrate a plasma polymerization process directly i nto a textile production l ine, even if atmospheric pressure plasma processes would be available. For S iOx deposition on PET fabrics within a dielectric barrier discharge a deposition rate of around 1 5 nrnlmin (for HMOS and TIMSVS) was reported5 1 , which is noticeably lower than low pressure plasma polymerization from organosilicones. Therefore, there is no real difference between low and atmospheric pressure plasmas with respect to the processability being a semi-continuous process.

Hence, it strongly depends on the application and the corresponding market situation, whether a plasma polymerization is economically suitable or not. The decisive add-on value, that allows the cost of product to be well above the cost of unprocessed product and/or that cannot be attained by other means, is the key factor in selecting plasma polymerization. I I S

Further advantages of plasma, which should be taken into account, are an environment-friendly dry process without requiring any additional manipulation such as wetting or drying, deposition of nanoscaled coatings, that do not significantly affect the original characteristics of fabrics, such as breathability, feel , softness, and mechanical strength, and the higher durability of the plasma-polymerized coatings

1 1 2 INDIAN 1. FIBRE TEXT. RES., MARCH 2006

compared to many wet-chemical processes. Yasuda1 1 S

has recently proven the efficiency of plasma polymerization processes for different commercial applications, even when the i nvestment of a large vacuum system has to be taken i nto account. Web coaters sui table for plasma polymerization processes on textiles can be purchased from different companies.

Process velocities can be enhanced if the treatment is performed on yarns or monofi lament fibers that can be processed in air and led several times through the (low pressure) plasma zone, thus improving the plasma length. Process velocities of several 1 00 mlmin can be achieved with a true continuous process, meeting the velocity of other textile processes and thus enabling the integration into a production line (e.g. for aramid fibers). Industrial implementation is currently under way.

5 Conclusions

The textile industry nowadays strongly requires . innovations and transformations to account for the challenging market situation and thus demands the inclusion of new technologies. Especially, plasma technology is offering an attractive way to add new functionalities such as water repellence, hydrophilicity, dyeability, conductivity and biocompatibility due to the nanoscaled and tailored modification of textiles and fibers. Plasma polymerization, which leads to the deposition of thin coatings via gas phase activation and plasma-substrate interactions, enables the design of more or less cross­l inked plasma polymers containing more or less functional groups covering the entire range from swell able hydrogel- like (PEO-like) up to diamond­like (DLC) coatings. General ly, these plasma polymers are more durable compared to other surface modification techniques (wet-chemistry, radiation or plasma activation), being surface sensitive and ecofriendly.

Different (low pressure) plasma polymerization processes are already transferred to an industrial level. S iloxane coatings derived from organosilicone discharges are used as fluorine-free hydrophobic coatings, whereas fluorocarbons are plasma­polymerized as stain repel lent coatings showing better durabil ity compared to common wet-chemical treatments. Moreover, textile properties, such as feel (touch), optics and mechanical strength, remain unaffected by the nanoscaled plasma polymers

covalently bonded to the textile surfaces, being an ecofriendly technique. The key factor for plasma polymerization, however, is given by the add-on value . outperforming other avai lable techniques. Examples are tailored plasma polymers containing functional groups to achieve permanent hydrophilic treatments for improved wicking and adhesion or specific binding of linker molecules, enabl ing regioselective treatments on hollow fibers used for blood dialy,sis.

The industrial scale-up is faci l itated by the identification of simi larity parameters, which enable the optimization of p lasma polymerization conditions on ( lab-scale) batch reactors. Both industrial-scale web and fiber coaters are already implem(�nted by different companies to produce high-value added textiles and related materials. However, a direct incorporation into an in-line production is difficult to attain due to the achievable process velocities. Moreover, high investment costs or the requirement of vacuum technology hold back many interested companies, although the new technology might be more efficient for the considered process. Hence, plasma polymerization is sti l l a process merely used for textile niche products . Comparing the development with other web-based products such as packagings, however, a high potential can be expected for the future. Therefore, research both on low and atmospheric pressure plasmas is assumed to be forward-looking and should be strongly supported to stimulate the textiie sector.

Plasma-modified textiles enable new and interesting application in different fields such as clothing, filtration, automotive industry, architecture, medical engineering and biotechnology.

Acknowledgement The author is thankful to all the co-workers at

EMPA for their contributions. Thanks are also due to the Commission of Technology and Innovation (CTI) , Switzerland, for granting funds for parts of the study.

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