Plasma finishing of textiles with oil and water repellent properties

Background

For conventional textile finishing, water and oil repellent properties are achieved using wet-chemical treatment with perfluorinated organic compounds. Good oil repellency (oleophobicity) requires particularly long fluorocarbon chains. However, molecular fragments of the finishing chemicals may be released during the original treatment as well as the washing and re-impregnation stages. These fragments or their reaction products include perfluorooctanoic acid and perfluorohexanoic acid. These compounds are toxic environmental pollutants that are bioaccumulative and suspected of being carcinogenic.

It is therefore necessary to establish more efficient and more environmentally friendly finishing processes that release fewer pollutants and avoid fluorocarbon treatments as far as possible. They are not necessary in an outdoor environment, but oil repellency is still important for personal protective equipment (PPE) such as surgical textiles. Furthermore, coatings should be stably applied to the surface in order to avoid constant post-processing of the fabric.

Plasma technology for textile finishing

Plasma technology is a technique with which coatings can be stably bound to surfaces (with covalent chemical bonds) using minimal amounts of chemicals. One challenge is to achieve a good quality coating together with a high rate of application. In our studies, we have therefore used a new process that uses liquid compounds in addition to coating processes based on gaseous precursors (e.g. perfluorinated alkanes [1]). These make very high deposition rates possible (always based on a defined period) since the quantity of material applied is significantly larger than with conventional processes based entirely on gas-phase technology. The substances are sprayed in the form of a low-pressure plasma and polymerize on the surface [2].

© Fraunhofer IGB

Fig. 2: Water repellency of plasma-coated textiles finished according to AATCC 22.

Fig. 1 shows sample measurements for the water repellency of various different finished textiles. In the established test for hydrophobicity according to the AATCC-22 standard (American Association of Textile Chemists and Colorists), a value of 100 corresponds to maximum water repellency, while a value of 0 corresponds to complete wetting. Various coatings prepared from different starting compounds exhibit optimal water repellency (100 points). These include perfluorinated coatings as well as fluorine-free alternatives. Furthermore, both gaseous and liquid compounds can be used for coating, although liquid substances allow higher deposition rates. Oil repellency was also investigated according to AATCC 118. It was found that fluorine-free alternatives had no oil repellency at all, although a certain degree of repellency could be achieved with plasma-based perfluorinated coatings.

© Fraunhofer IGB

Fig. 2: Evaluation of the stability to washing of an octafluorocyclobutane finish according to ISO 105 C12.

After the coatings were subjected to a washing test, there was a tendency for the efficacy to be reduced. This effect is also found with wet-chemical finishing and is due to abrasion, adsorption of surfactants and reorientation of functional groups. An example of the effects of washing on plasma-treated textiles (according to ISO 105 C12) is shown in Fig. 2. This series of measurements shows that the stability of the finishing treatment illustrated depends on pH. Good durability of the coating is obviously possible. However, this clearly depends on the choice of surfactant, as is the case with the wet-chemical method.

Outlook

The high deposition rate is an outstanding feature of the new methods based on liquid compounds and plasma. Furthermore, additional coating functions can be provided by admixture of other substances (for example, an antibacterial effect can be achieved with silver compounds or chitosan). These methods are therefore of interest not just in the clothing sector, but also in other areas such as medical technology.

References

[1] Verfahren zur Herstellung von funktionalen Fluorkohlenstoffpolymerschichten mittels Plasmapolymerisation von Perfluorocycloalkanen, WO 2007012472 A8.
[2] Nieder- und Mitteldruckplasmaverfahren zur Oberflächenbeschichtung mittels Precursorzuführung ohne Trägergas, EP 14191221.2.

Funding

This work was carried out in cooperation with the Institute of Interfacial Process Engineering and Plasma Technology, University of Stuttgart. We would like to thank the German Federal Environmental Foundation (Deutsche Bundesstiftung Umwelt) for funding this work, reference no AZ 30276.

Project partner

Plasma Electronic GmbH, Neuenburg am Rhein, Germany