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Gas Plasma Treatments

A book entitled “Plasma Technologies for Textiles” has just been published by Woodhead Publishing, Cambridge, UK.  Its publication is timely.  It is by far the most comprehensive review of the subject to date and highlights the huge potential of gas plasma treatments for textile manufacturing and processing.  And yet, although the versatility and the eco-friendliness of gas plasma treatments have been amply demonstrated, the acceptance of gas plasma technology by much of the textile industry has been grudging at best.  So what is gas plasma technology, why does it offer such commercially exciting prospects and how can the textile industry at large become convinced of its merits?

Gas plasma treatments of materials alter their surface character without affecting their bulk properties.  The depth of the surface treatment is only a few nanometres.  The surface of the material is roughened and surface chemical properties may also be changed.  So, for textiles, gas plasma treatment offers an alternative method of surface treatment to the coating technologies conventionally applied.  The idea of treating materials with gas plasma is by no means new (gas plasmas were introduced in the 1960s!), but it is only recently that it has become possible to treat textiles in this way on a commercial scale.

Gas plasmas are complex mixtures of positive and negative ions, electrons, free radicals, ultra-violet radiation and many different electronically excited molecules.  Their nature depends on the gas (or gas mixture) used for the desired plasma treatment.  There are striking everyday examples of gas plasmas: the solar corona, “neon” lights (which are not always neon!) and flashes of lightning.  The mixture of species comprising any particular gas plasma is complex, and all of them can interact with the textile surface.

The effectiveness of a plasma treatment is governed by a variety of factors: the composition of the gas, the type of textile, the pressure within the plasma chamber, the frequency and power of the electrical supply, and the temperature and duration of the treatment.  The treatment process is hence highly complex; even so, several effects can be readily highlighted despite these complexities.

In general, three main effects can be identified, depending on the type of gas plasma treatment applied:

Examples of the types of gas plasma applied and their effects are given in the table:

Gas PlasmaEffect
ArgonIncreased surface roughness
OxygenModification of surface chemical groups
Better wetting
FluorocarbonsPolymerisation
Improved water repellency
Ammonia, carbon dioxideModification of surface chemical groups

Thus, oxygen plasma treatments enable polypropylene fibres to be wetted by water (see figure below) and to be easier to glue or bond.  Moreover, the biocompatibility of the fibres is, consequently, improved, given that most body fluids are aqueous-based.  Cotton fabrics can be rendered water repellent by plasma treatment with a fluorocarbon.  This type of treatment could be useful for outdoor wear and for hospital theatre clothing.  It can be noted too that the internal structure of the fibres remains hydrophilic, and so can still allow wicking.  Such a fabric could therefore also be useful for sportswear, for example.

Two types of plasma treatment system are available: treatment at low pressure and treatment at atmospheric pressure.  Low pressure systems generally operate at 1–100 pascal, and a flow of gas is fed continuously into the plasma reactor chamber.  To generate the plasma, electrical power of up to 5 kilowatts may be necessary, depending on the treatment required and the size of the chamber.  To generate this power, a high voltage between a ground and positive electrode is applied, often in the radio-frequency (RF) range.  Low pressure plasma equipment is now available on a commercial scale, and textile fabrics greater than one metre in width can be successfully treated by this method.  It is often thought that this technology has to be a batch operation; it doesn’t, as the book explains!  It is possible to build continuous systems using low pressure plasma technology.

Plasma treatment at atmospheric pressure is perhaps less well advanced so far, giving less uniform results.  The technology is, however, advancing, not least because of its appeal for continuous processing.  As the book explains, there are a number of different forms: corona treatment, dielectric barrier discharge and glow discharge.  Whilst corona treatments are the most widely used overall, dielectric barrier discharges and atmospheric pressure glow discharges are arguably better suited to the constraints of treating textile fabrics, notably relatively low temperatures to prevent fabric melting or degradation.

There is still considerable debate about the relative merits and costs of installing and using low pressure and atmospheric pressure systems.  Many textile fabricators are concerned, for example, at the cost of operating a process at reduced pressure, yet in the book evidence is provided that the total cost is no higher than €0.05 per square metre (less than 4p).  Moreover, protagonists for low pressure treatments contend that far less gas is needed, an argument which becomes particularly cogent for expensive gases such as fluorocarbons.  However, atmospheric pressure treatments, even though still less developed, will surely be preferred eventually in applications where they can match cost and performance.

Gas plasma technology originally started in the microelectronics industries, but eventually extended to metals, plastics, biomaterials, and ceramics and other inorganic materials.  These industries have come to recognise that gas plasma treatments offer some real commercial advantages, and these advantages apply just as much to the textile industry.  Not only does gas plasma act on the surface of a textile whilst leaving its bulk properties unchanged, there is also no need for water or an organic solvent as a medium, as is required in many conventional coating processes.  Gas plasma treatment can therefore be considered a clean, dry technology.  In addition, the versatility of plasma treatments can lead to textile surface properties unobtainable by most conventional techniques.  Gas plasma treatment, therefore, has the potential to benefit many aspects of textile technology, ranging from workwear production to complex biomedical applications.

Perhaps the most doubt being voiced by textile processors about gas plasma treatments concerns the durability of the treatments.  Will a particular treatment withstand 20–30 (or even more!) wash cycles?  Techniques exist to enhance the stability of the treated surface. In one, known as CASING (crosslinking by activated species of inert gases), the surface of a polyolefin textile is pretreated with the plasma of a noble gas, such as argon.  The effect is to crosslink the polymer chains at the textile surface.  In addition, the introduction of suitable chemical groups to the textile surface allows polymers to be readily grafted onto the textile.  These added polymers can both stabilise the textile surface and, depending on the nature of the grafted polymer, confer other desired properties.

Those who are interested or sceptical about the benefits of gas plasma treatments on textiles should read this book.  They will learn about a clean, inexpensive and effective way to modify textile surfaces for enhanced performance in application.  

Enquiries about gas plasma treatments are always welcome on this website.  Please send an email to me, Robert Mather, at:

info@mathertechnologysolutions.com

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Last updated: Fri, December 06, 2013
Created: August 23, 2007

 
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