PLASMA TECHNOLOGY FOR TEXTILES: WHERE ARE WE?

J. VERSCHUREN,  P. KIEKENS

Department of Textiles, Ghent University, BELÇİKA

1. Introduction

This paper deals with the current situation of plasma technology as applied for textile treatments.  It gives an overview of factors which can help the textile industry to get a clear look at this technology which has enormous potential but so far remains "promising".  From this, it will be obvious that plasma technology is to be treated like any other contemporary textile treatment technology.  The paper will end with a short description of the plasma-related research which is currently done at the Department of Textiles (Belgium).

2. Types of plasma currently used in reactors with claimed industrial use:

One of the possible definitions of a plasma is: a partially ionised gas containing ions, electrons, neutral species and UV/visible radiation.  It is mainly the highly energetic electrons in the plasma which (in)directly induce chemical changes at the fibre surface of the treated textile.  Contrary to hot plasmas like the one created in the sun, the plasmas described further do their job at room temperature; hence the name "cold plasma".  This is due to the fact that the energy of the plasma is mainly confined to the energy of low mass electrons.  There are many different ways to induce the ionisation of gases.  Figure 1 shows a schematic priciple of different plasma sources.   

Glow discharge: is the oldest type of plasma; it is produced at reduced pressure and assures the highest possible uniformity and flexibility of any plasma treatment.  The plasma is formed by applying a DC, low frequency (50 Hz) or radio frequency (40 kHz, 13.56 MHz) voltage over a pair or a series of electrodes (fig. 1a-c).  Alternativley, a vacuum glow discharge can be made by using microwave (GHz) power supply (fig. 1d).
Corona discharge: is formed at atmospheric pressure by applying a low frequency or pulsed high voltage over an electrode pair, the configuration of which can be one of many types.  Typically, both electrodes have a large difference in size (fig. 1e).  The corona consists of a series of small lightning-type discharges; their inhomogeneity and the high local energy levels make the classical corona treatment of textiles problematic in many cases.
Dielectric barrier discharge (DBD): is formed by applying a pulsed voltage over an electrode pair of which at least one is covered by a dielectric material (fig. 1f).  Though also here lightning-type discharges are created, a major advantage over corona discharges is the improved textile treatment uniformity.

Legend :

3. Possible effects of a plasma treatment (limited list):

·        Improved wettability

·        Induced chemical reactivity of the fibre surface

·        Improved adhesion to coatings and to polymer matrices

·        Induced hydrophobic properties

·        Fibre surface cleaning

4. Possible advantages over wet processing (limited list):

·        Plasma processing requires no water; the treatment is done in the gas phase

·        Only a small amount of chemicals is needed

·        There is virtually no waste production

·        The treatment is confined to the fibre surface

·        Plasma processing is very energy-efficient

·        Some special textile properties can only be obtained via plasma processing

5. Current situation:

The application of plasma technology on textiles started in Russia in the sixties.  It got a revival in the West from the eighties onwards, where numerous studies have been published as a result of experiments in vacuum reactors designed for the treatment of inorganic (micro)electronic materials.  In the meantime - from the seventies onwards - Russian researchers developed a full industrial scale roll-to-roll vacuum reactor.  Also in the West such reactors have been built, but the Russian one remains the only large scale (up to 3,4 m fabric width) textile related plasma reactor  with a significant industrial "experience".  Such reactors can - in priciple - treat any type of fabric, and are only limited in the amount of material that can be charged in one batch; i.e. the fabric roll diameter is limited to e.g. 70 cm diameter.

While the development of vacuum plasma technology for textile surface modification has come to a virtual standstill - it is still the most perfected plasma technology available - literature regularly reports new designs of plasma sources working at atmospheric pressure.  Most of these are based on, or are a combination of the abovementioned corona and dielectric barrier discharges.  Their main aim is to further improve treatment uniformity and increase size and energy density of the discharge; these improvements are needed to give atmospheric plasmas a competitive industrial usability.  Another current limit for atmospheric reactors - which is due to the fact that the textile is treated in between the electrodes - is their apparent ability to successfully treat only thin, light weight textiles with an open structure.  It is ironic that especially for such thin webs the amount of fabric that can be treated in  a single batch of a vacuum reactor is the highest possible; i.e. the most productive.

The driving force behind the development of atmospheric plasma sources is the much heard - but not necessarily relevant - complaint from the textile industry that vacuum plasma technology is non-continuous.  Much hope was raised - in the mid eighties - from the availability of corona discharges at atmospheric pressure and - from de mid nineties - of atmospheric dielectric barrier discharge reactors, both of which enable an in-line continuous application.  In spite of increasing treatment uniformity and energy density, plasma technology for the treatment of textiles remains "promising" - as it has been for decades - without the textile industry picking in on its really enormous potential.  The question is: Why?

Samples from numerous companies have been treated in both experimental and claimed industrial reactors, without these efforts leading to any acquisition of plasma technology by the industry.  Apart from the fact that treatment results did not meet expectations in many cases, interested parties hardly got objective information neither on plasma technology in comparison with wet chemistry, nor on atmospheric plasma in comparison with vacuum technology.  In following paragraphs different ways to look at the potential avantages of plasma technology for the treatment of textiles will be given.  It will become clear that - as is the case with any other technology - it is all a matter of making choices after a complete and objective comparison of available technology.

6. Ways of looking at plasma technology:

The potential of plasma-for-textiles can be looked upon from more different engineering, textile, financial and environmental aspects than any other textile related technology.  Everything depends on what a textile company want to accomplish with the acquisition of a plasma reactor system.  It is therefore essential for the textile industry to get acquainted with all these aspects before decisions on the application of plasma in their company can be made.  The list below does not pretend to be complete …

6.1. Aspect 1: Plasma treatment as a step in the total textile production cycle:

·        A plasma treatment could replace an existing wet processing step.  Examples are treatments for improved wetting and adhesion properties, such as advantageous for dyeing, coating and making composite materials.  Saving environment-related costs is a priority.

·        The plasma treatment can be a final step in creating a textile with novel properties.  Here textiles can be produced with properties that cannnot be induced via wet processing.  Added value is a priority.

6.2. Aspect 2: Plasma treatment as a means to save water, materials and energy

This environmental aspect is without doubt the most exposed advantage of plasma technology.  It wins in importance everyday due to ever increasing environment-related costs.  However, its relevance depends on abovementioned cases.  The largest advantage will be experienced when a plasma treatment can replace an existing wet process completely.  Even though such cases are not common, and in view of a continuing cost increase for both water extraction and discharge, plasma technology becomes relatively more cost effective every day.  Other, affiliated cost savers are:

·        Reduction of the amount of chemicals needed in wet treatment following the plasma treatment; better exhaustion of chemicals from the bath; reduced BOD/COD of discharged processing water.

·        Shortening of the wet processing time; this compensates for the possible extra time required for the plasma treatment.

·        Reduction of the needed wet processing temperature; saving heating energy.  This adds to the efficient use of energy during the plasma treatment.

Extra advantages can be that the finished textile shows better performance and improved fastness properties, i.e. has an extra added value.

6.3. Aspect 3: Plasma treatment as a means to create unique textile properties

Though currently not very relevant in produced amounts (square metres), this type of high-performance textiles will certainly grow in economic importance.  Due to their high added value even small textile batches can be produced at high profit, though perfect process control is absolutely necessary.  Typically, textiles for medical applications or used in the sector of biotechnology are expected to increase in importance.  Applications are special selective filtrations, biocompatibility, growing of biological tissues, etc.  Especially in this case, high investment costs have a fast pay-off.

6.4. Aspect 4: Continuous (i.e. atmospheric) versus batch (i.e. vacuum) plasma processing

Most of the current atmospheric plasma reactor development is done because - supposedly - the textile industry can only use a plasma reactor when it can be integrated in a continuous production/finishing line.  Extra requirements are an ability of full width textile treatment (at least 2 m wide) and high processing speed (at least 20 m.min^-1).  For this reason, vacuum technology is regarded as being  noncompetitive.

A thorough review of literature shows that currently available industrial vacuum reactors treat a 2000 m full width fabric roll in all-including batches of 1 hour duration.  On the other hand, some publications that are keen on stressing the continuous aspect of atmospheric treatments are proud to announce a processing speed of 2 m.min^-1; i.e. treating 2000 m of fabric would take more than 16 hours!  This extreme difference in processing speed it far from general, but the example makes clear that a complete and objective comparison is desirable in order to come to the right conclusions regarding which technology to choose.

More, it must be stressed that - in comparison with atmospheric discharges - plasmas created at low pressure remain superior in application flexibility, treatment homogeneity and stability of treatment results.  This is due to the inherent difference between physics at reduced pressure and physics at atmospheric pressure.  These inherent differences will not become smaller in the future ….  Research groups are continuously finding solutions that reduce the limits of atmospheric reactors, but it can be expected that this will come with increased hardware complexity and costs.

On the side of upscaling, an advantage of vacuum reactors is their ability to incorporate a long (e.g. 20 m) plasma treatment zone in a compact manner, similar to textile dryers or steamers.  Similar solutions have not been proposed for atmospheric reactors due to the vertical space needed for the electrode and dicharge control system.  When long treatment durations should prove necessary, an unpractically long reactor or slow processing speed are currently the options.

6.5. Aspect 5: The capital investment and the processing costs

A full width (2,4 m) vacuum plasma reactor which has been designed for the treatment of textiles is considerably more expensive than the commercial atmospheric reactors currently available, though with the latter type the fabric width is currently limited to 1,7 m.  It will obviously depend on the application of the reactor how many years it will take for this difference in investment cost to be compensated.  As mentioned above two compensation mechanisms can be regarded: savings in environment-related costs, and extra profit from the sale of high added value products.

When the processing gas - the gas which is partly ionised in the plasma - is air the related processing costs are as favourable for atmospheric reactors as for vacuum reactors.  However, when the treatment requires the use of pure processing gases (ammonium, hydrogen) this condition can be reached with much less effort at reduced pressure.  Refreshing a given reactor volume with pure gas - as is necessary for a controlled treatment effect - requires about 800 times less material at reduced pressure than at atmosheric pressure.  Also the evacuation of "waste" gases and recovery of processing gases requires more engineering effort during atmospheric pressure processing.  All this leads to the feeling that the higher investment cost of a vacuum system is compensated with a relative technical simplicity and lower processing costs.

6.6. Aspect 6: Laboratory conditions vs. industrial reality

A large majority of publications which describe a novel application of plasma technology for textile treatment result from experiments experimental reactors.  Though such research must continue to be encouraged, interested parties from the textile industry should be aware of the plasma processing conditions and even more of the treatment the textile was given before plasma processing.  Often, for analytical purposes, samples are cleaned extensively with water and/or solvents.  Though under laboratory conditions such cleaning operations often lead to extremely interesting treatment results, it must be clear that they are hardly realistic under textile industrial conditions.  Apart from increasing the total processing costs, an extra industrial (solvent) cleaning would completely compensate for the positive environmental impact of plasma technology.

7 . Plasma research at the Textile Department

The research subjects and objectives presented here in short are closely related to what has been said in previous parts of this presentation.  Though one might get the impression that vacuum plasma is favoured to atmospheric treatments, our research wants to find answers to questions which are related to both treatment types.  This is done by looking deeper into the specific behaviour of fibrous samples during all stages of plasma processing.  The current and antiproductive research competition between vacuum and atmospheric plasma processing is no issue here.  In the following paragraphs three topics of research will be briefly presented.

7.1. The specific behaviour of textiles

·      Their three dimensional porous structure:

The textile structure often prevents the homogeneous treatment of the complete fibre surface.  Some applications, e.g. filtration, require that both the fibres at the outsides of the textile as the ones in the middle of it are modified in an identical and reproducible way.

·      Their large total fibre surface:

A typical nonwoven for gas filtration purposes contains no less than 0.2 m² of fibre surface per gram of fabric.  Removing unwanted chemicals (gases, liquids and solids) from this large surface can be complicated and often leads to non-reproducible plasma treatment results.

·      The occurrence of impurities on industrial textile products:

Impurities in textiles can interfere with plasma reactions.  Examples are water and additives for improved fibre processing.  As plasma processing aims at treating fibres rather than e.g. a spin finish, removal of impurities during the plasma treatment must be considered.

The relevance of above topics obviously depends on the expected result for a selected application.  The tougher the requirements for high (added value), lasting and reproducible performance the more important above topics will become.  These observations have resulted in the experimental reactor setup at the Department of Textiles.

7.2.  Reactor design

The system works at reduced pressure and treats samples in a homogeneous plasma which is created between identical electrodes (cfr. Fig 1 a).  The most special feature is that a homogeneous excited gas column is forced through the textile sample, thus favouring removal of adsorbed water and atmospheric gases, the treatment of the textiles’ complete fibre surface, and the removal of unwanted reaction products.  Plasma treatment effects can go from fibre surface “activation” (e.g. via oxygen or air plasma) to inducing hydrophobic properties (plasma polymersation of fluorocarbon monomer).  The reactor enables the study of the behaviour of textile products in a plasma.  It is hoped that it will to lead to a better understanding of the physical and chemical processes that occur inside the textile during plasma treatment.

7.3.  Analysis of treatment effects

Analysis methodology for the assessment of plasma effects on the complete textile volume is being developed.  Most methods are based on ultraviolet, visible, near infrared and mid-infrared spectroscopies.  Other methods are based on simple textile tests which can be easily used in the textile industry at reasonable cost.

8.  Final Remarks

This presentation has touched some aspects of textile plasma treatment that are rarely discussed in literature.  It is hoped that the public from the textile industry, when interested in the correctly much hailed potential of plasma technology, will realise that buying plasma equipment requires the same thinking as when buying other textile processing equipment.  This means that one must start with the question: which processing result(s) do we want to obtain on what type(s) of material.  Reading between the lines, getting complete information and making an objective comparison between available equipment will enable you make the right decision.