Glow discharge treatments for the modification of technical textiles

Due to increasing requirements on the finishing of textile fabrics, increasing use of technical textiles with synthetic fibers, as well as the market and society demand for textiles that have been processed by environmentally sound methods, new innovative production techniques are demanded. In this field, the plasma technology shows distinct advantages because it is environmentally friendly, and even surface properties of inert materials can be changed easily.

Plasma technology can be used not only for textile finishing, but also for the optimisation of textile machines, for example, with hard coatings.

It has been known for at least 60 years that plasma could effect desirable changes in the surface properties of materials. However, the practical application of plasma required the development of commercially available, reliable, and large plasma systems. Such systems are now available (mostly in research laboratories) and the application of plasma to industrial problems has been increasing rapidly for the past 10 years.

The physical definition of a "plasma" (glow-discharge) is an ionised gas with an essentially equal density of positive and negative charges. It can exist over an extremely wide range of temperature and pressure. The solar corona, a lightening bolt, a flame and a "neon" sign are all examples of plasma. For the purposes of textile modification the low pressure (0,01 to 1 mbar) plasma, such as found in the "neon" sign or fluorescent lightbulb are used. In the above mentioned applications the desired result is to produce light. However, for the plasma treatment of polymeric substrates, the extremely energetic chemical environment of the plasma is utilised.

The plasma atmosphere consists of free electrons, radicals, ions, UV-radiation and a lot of different excited particles in dependence of the used gas. The figure above describes the principle of the plasma treatment.

Different reactive species in the plasma chamber interact with the substrate surface. Cleaning, modification or coating occur dependent of the used parameter.

Furthermore, the plasma process can be carried out in different manners.

1. The substrate can be treated directly in the plasma zone.
2. The substrates can be positioned outside the plasma; this process is called remote process.
3. The substrate can be activated in the plasma followed by a subsequent grafting.
4. The substrate can be treated with a polymer solution or gas which will be fixed or polymerised by a subsequent plasma treatment.

According to requirements the materials to be processed processing (foils, membranes, textiles, polymers,...) will be treated for seconds or some minutes with the plasma. Essentially, four main effects can be obtained depending on the treatment conditions.

1. The cleaning effect is mostly combined with changes in the wettability and the surface texture (see Point 2.). This leads for example to an increase of quality printing, painting, dye-uptake, adhesion an so forth.
2. Increase of microroughness. This effects, for example, an anti-pilling finishing of wool.
3. Generation of radicals. The presence of free radicals induce secondary reactions like cross linking. Furthermore, graft polymerisation can be carried out as well as reaction with oxygen to generate hydrophilic surfaces.
4. Plasma polymerisation. It enables the deposition of solid polymeric materials with desired properties onto the substrates.

The advantage of such a treatment is, that the modification is restricted to the uppermost layers of the substrate, thus not affecting the overall desirable bulk properties of the substrate adherent.

The figure below shows a block diagram of a typical plasma system. It consists of 5 modules or functions: vacuum system, power supply, matching network, reactor center, and controller.

• Vacuum system: Low pressure plasma systems operate at 0,1 mbar to 1 mbar with a continuous gas flow into the reactor. In some cases it is necessary to reduce the base pressure in front of the treatment below 0,1 mbar.
• Power supply: This furnishes the electrical power necessary to generate the plasma. The power required ranges from 10 to 5000 watts, depending on the size of the reactor an the desired treatment.
• Plasma reactors have been built utilising a wide range of frequencies, from DC to microwave.
• Controller: It controls all the process variables: type of gas, pressure, gas flow rate, power level, and processing time.
• Reactor center: This is the "heart" of the plasma system. It can be adapted to the process. The material for processing can be treated as batch, semicontinuous or air-to-air. The last is very expensive due to the necessity of vacuum transfer systems.

An example for a semi-continuous plasma plant is shown in figure above: semi-continuous apparatus at Fraunhofer IGB. More examples are described in the following lectures.

Physical parameters

• Frequency: 13.56 MHz
• Plasma Power: 10-1000 W
• Base Pressure: 10^-3 mbar
• Working Pressure: 0.01-1 mbar
• Speed: 0,2-20 m/min
• Maximum width: 18 cm

The plasma treatment of polymeric material has a lot of benefits compared with classical wet chemistry finishing. The main advantages are:

• Applicable to all substrates suitable for vacuum processes, i.e. almost free choice of substrate materials.
• Optimisation of surface properties of materials without alteration of bulk characteristics.
• on polymers, which are unable or very difficult to modify with wet chemicals, the surface properties can also easily be changed.
• The consumption of chemicals is very low due to the physical process
• The process is performed in a dry, closed system, and excels in high reliability and safety
• Environmentally friendly

Applications

The plasma modification of polymeric materials used as textiles, membranes, foils, non wovens, composites, and so on, is able to optimise a lot of interesting properties. Due to the great extend of literature results only a few are summarised exemplary in the following.

Mechanical properties

• Material: e.g. cotton, other cellulose-based polymers
• Treatment: e.g. oxygen plasma
• Reduced felting
• Material: e.g. wool
• Treatment: e.g. oxygen plasma
• Crease-resistance
• Material: e.g. wool, cotton, silk
• Treatment: e.g. dipping in DMSO, subsequently N₂-plasma

Electrical properties

• Antistatic finish
• Material: e.g. rayon
• Treatment: e.g. plasma consisting of chlor-(chlormethyl)dimethylsilane

Wetting

• Improvement of wetting
• Material: e.g. PA, PE, PP, PET PTFE...
• Treatment: e.g. O₂-, air-, NH₃-plasma
• Hydrophilic treatment serves also as dirt-repellent and antistatic finish.
• Hydrophobic finish
• Material: e.g. cotton, cotton/PET,
• treatment: e.g. siloxan-, perfluorocarbon-plasma (figure on the left)

Oleophobic finish

• Material:cotton/polyester,
• Treatment:grafting of perfluoroacrylat (figure on the left)

Dyeing, printing

• Improvement of the capillarity
• Material: e.g. wool, cotton
• Treatment: e.g. oxygen-plasma
• Improved dyeing
• Material: e.g. polyester
• Treatment: SiCl₄-plasma
• Increase of dyeing depth
• Material: e.g. Polyamid
• Treatment: e.g. Ar-plasma

Other properties

• Bleaching
• Material: e.g. wool
• Treatment: e.g. oxygen-plasma
• UV-protection
• Material: e.g. dyed cotton/polyester
• Treatment: HMDSO-plasma
• Flame-retardancy
• Material: e.g. PAN, Rayon, cotton
• Treatment: e.g. phosphorus containing monomers

Metal-coated organic polymers

Metal-coated organic polymers are used for a variety of applications in many types of industries. If the metallized polymer is to fulfill it's function, it is usually essential that the metal adhere strongly to the polymer substrate. This can be obtained by a plasma pre-treatment of the polymer.

• Example: Oxygen plasma treatment of ABS before copper deposition by evaporation

Composites/laminates

Good adhesion between fibers and matrix (or laminates) depends upon the surface characteristics of fibres, matrix and the physico-chemical interactions taking place at the interface. A prerequisite condition of good adhesion between fibre and matrix remains the surface energy of fibers which must be higher or equal to the surface energy of the matrix. This can be achieved with plasma treatments.

Applications in biology and medicin

• Fabric favouring overgrowth with cells (possibly biodegradable) for
• cell culture tests
• fermentation
• implants (blood vessels, skin)
• Fabric not favouring overgrowth with cells for
• catheters
• membranes in fermenters
• Enzyme immobilisation
• Sterilisation

Applications in membrane and environmental technology

• Gas separation
• e.g. oxygen enrichment
• Solution-diffusion membranes
• e.g. alcohol enrichment
• UF/MF-membranes
• Improvement of selectivity
• antifouling finishing
• Hydrophilisation of pore walls
• Functionalized membranes
• Affinity membranes
• Charged membranes
• Bipolar membranes

The textile industry have to fullfill a lot of partly new technological requirements which have an impact to European and American markets. The following examples belong hereto.

• increasing environmental awareness
• demands on safety of the production process and working place
• increasing requirements as to the performance of the products
• aspiration for personal safety and comfort
• rising price pressure and increasing energy and raw material costs
• decrease of unit costs and at the same time improved quality
• tailor made products.

Here it is necessary to recognise chances and risks. New markets and additional sales potential can be found where products specifically respond to these trends. The greatest challenge has been and still is, the development and use of new technologies, for example, the plasma technology to achieve a better price-to-performance ratio. This calls for two options: First of all a broadening and a deepening of the knowledge about possibilities and chances of the plasma technology. Secondly, a strong partnership between research centres, developer of plasma-based plants, and companies from the textile sector. Both are certainly not totally new ideas, but we very often lack the understanding of the necessity which is prerequisite for success.

We think, that this symposium gives a lot of information concerning the plasma modification of polymeric materials, together with the possibilities and limits of these innovative technique.