Plasma treatment of textiles

Since the mid 80ties a plant has been running in Russia where low pressure plasma is utilized to oxidize fiber surfaces for improved dying. This is one of the very rare examples of plasma application in textile industry. What are the reasons that prevent a broader application of this new and promising technology?

It is only rather recently that the textile industry and plasma technology took notice from each other and started looking for fields that could take advantage from plasmas. On both sides there are some specific demands. Plasma is based on low pressure and needs vacuum equipment which is not common at all in textile industry. On the other hand, textiles have a rather large specific surface area and contain usually a substantial amount of water. Both facts ask for a good vacuum system, a better one than it is needed for the treatment of films or other solid parts. The role of water in plasma treatments is an important issue that needs to be investigated in more detail for non-oxidative processes.

The surface properties alterations obtained by a plasma treatment are complex. Particle induced reactions take usually place in the upper ten nanometers of a surface. Short wavelength UV-radiation as it is emitted by low pressure plasmas initiates reactions in a thicker layer (about 100 nm). The relation between the two and the extent of both can be controlled by the process gas and other process parameters. The outermost surface, only some atom layers, sometimes less than 1 nm, determines the interaction with other media. The chemical composition of this part of a fiber is responsible for good or bad adhesion in laminates or whether the fabric is suitable for impregnation or not. Exactly this part of a fiber can be modified by a plasma. For the success of the treatment not only the process parameters contribute, but also the original surface is crucial. Trace amounts of sizings, for example, can modify the reaction condition substantially and have to be taken into consideration for almost every process.

 

Technology

Many different types of treatments have been done so far in batch experiments. This is a legitimate way to demonstrate that a certain treatment can create a desired effect. However, for an industrial application a continuous process is inevitable. There are basically two types of vacuum equipment to realize a continuous web treatment:

• In an air-to-air system the material passes several differential pumping stages before it reaches the actual process chamber and after it had left it. This allows a real continuous treatment and is particularly suitable if one material is treated in one process for a long period of time without changes. The metalization of polymer films is done with this technology.
• For the treatment of different materials, a semi-continuous or roll-to-roll technique is usually more reliable. Deviations of the materials properties as for example thickness and water content can be controlled easier than in a continuous treatment. In particular for technological investigations this is the way to go.

Our roll-to-roll plasma reactor
Considering the things that were discussed in the last paragraphs, we set up an apparatus for technological investigations on semi-continuous roll-to-roll treatments of webs.

 

Application examples from our laboratory

Most of the problems that arise when textiles are treated with plasma do not apply when the surface has to be oxidized in the plasma for example to create polar and reactive functional groups. In that case the background pressure in the reactor is not so important and also the water from the webs can be utilized in the treatment. The only real danger is an overtreatment. In a prolonged plasma exposure, polymers form low molecular weight substances which can deteriorate the surface properties, can be washed off and expose a surface which is treated to a lower extent.

More sophisticated treatments have a much narrower range of treatment conditions and need much more care in process control. The hydrophobation in a fluorocarbon plasma is an example for such a type of treatment.

 

Example 1: Water repellent polyaramide fabric

The data in the table demonstrates that a fluorocarbon plasma treatment can reduce the soaking of fabrics in a similar way like a traditional impregnation. However, in contrast to the wet treatment, the fabric retains its flexibility after the plasma treatment.

 

Example 2: Water repellent cotton, hemp

Cotton or hemp fabric usually absorbs water immediately.

Applying a low-pressure plasma process, the fiber's surface can be altered to make it repell water. After the treatment, drops run freely over the surface while mechanical properties, the visual appearance, and the permeability for water vapor remain unchanged. The surface modification is limited to a very thin layer. A treatment as short as 2 seconds can be sufficient to achieve this effect in a batch process. Continuous treatments with a speed of more than 20 m/min are conceivable.

The stability of the modification can be seen in intermitted washing cycles of fluorocarbon treated cotton fabric. After an initial drop, the finishing remains stable for at least two hours at 95°C. The quality of the repellent effect is evaluated by putting water drops to the fabric surface. A value of 1 means that the drops run freely over the surface and do not penetrate into the material while at a value of 3 the water does not penetrate but it needs vibrations to move the drop. Obviously this evaluation depends also on the nature of the fabric.

Example 3: Wettability improvement

In an oxygen plasma the number of functional groups at the surface can be increased. The increased polarity makes the material more wettable which can be used to improve dying and sizing.

In the table we summarized examples where various polyamide fabrics were oxidized. The effect of the treatment was checked by a water rise test, i.e. a strip of the fabric was put into water end the time was measured until the water rise up 3 cm.

The test was repeated a certain time after the treatment. The results show a good stability of the treatment.

Water rise time (s):

 

Example 4: Adhesion improvement in laminates and composites

In an oxygen plasma the number of functional groups at the surface can be increased which can improve the adhesion to other material. The results are stronger laminates and better composite materials.

As an example, there are results of lamintation tests with polyester fabric. (PES)