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INFO PHYS TECH PLASMA SUPPORTED SHRINKPROOFING OF WOOL
Fig. 1: Scale structure of the untreated wool fibre surface
Felting is a special property of wool that occurs when wool is subjected to mechanical agitation in water. Though this behaviour is required for creating felts or fabrics with special optical appearance, the well-known irreversible shrinkage during washing is one of the principal drawback’s of textiles made of wool. Felting and shrinkage is the result of movements of individual fibres within the textile always taking place towards the fibre root. The phenomenon of unidirectional fibre movement primarily is related to the unique scale structure (from the root to the tip) of the wool fibre surface (Fig. 1) which is responsible for a different coefficient of friction in the with-scale and against-scale direction. To prevent wool against felting different types of treatments are used industrially. For mild shrinkproofing, polymers are deposited on the fibre surface to coat the fibre or to produce spot-welding of the fibre junctions in fabrics (additive treatment) or the scales are smoothened by oxidative degradation (oxidative treatment). To obtain full machine washability however, a combination of oxidative and additive treatment is necessary. Today nearly 75 % of the overall production of machine washable wool is carried out at the stage of wool top being the base material for yarn production. State of the art so far is the Chlorine/Hercosett treatment, a continuously operating combined process using a chlorination as a preliminary step to polymer coating of the fibre with a polyaminoamide (Hercosett). The disadvantage of this process is its contribution to the overall discharge of AOX (Adsorbable Organic Halides) in effluents. Chlorinated organic substances are recognised as being mostly toxic and a number of countries established maximum concentration levels for these compounds in effluents. Since the AOX generation during conventional shrinkproofing of wool exceeds the permitted levels by far, the development of chlorine-free (combined oxidative and additive) treatments for imparting full machine washability is urgently demanded.
VDI-Technologiezentrum Physikalische Technologien
The aim of a joint research project funded by the Federal Ministry of Education and Research (BMBF) was the development of a plasma-supported shrinkproofing treatment for wool. As a result of interdisciplinary work between chemical, electronic and textile industry as well as research institutions specialised in electronic engineering, plasma physics and textile chemistry, a new continuous process and the corresponding machinery have been developed guaranteeing the production of fully machine washable wool. The process is based on a plasma treatment of tops followed by the application of a new water-dispersible polymer on isocyanate basis allowing a permanent coating of the plasma-treated wool fibres. This opens up the exciting new possibility to the wool industry of having a Zero-AOX shrinkproofing process for wool in the near future at its disposal.
FUNDAMENTALS Plasmas are „gases“ whose molecules are dissociated to a large extent into atoms, radicals, ions and electrons. Characteristic of low-temperature plasmas is the high temperature (high kinetic energy) of the electrons which is for several orders of magnitude higher than the gas temperature. The plasma-generated electrons have sufficient energy to cause the rupture of molecular bonds in surfaces and to induce subsequent reactions with the other plasma particles. The latter are also able to interfere directly with the surface to be treated. In dependency on the plasma gas used, the surface characteristics of textile fibres can be modified by etching, grafting and/or polymerisation. Therefore, low-temperature plasmas offer a virtually unlimited potential for modifying the surface characteristics of a textile material in accordance with specific requirements. For shrinkproofing of wool a plasma-induced surface oxidation as well as direct coating of the fibres by plasma polymerisation is known to reduce but not to eliminate felting. A further enhancement requires an additional resin treatment allowing a complete coating of the fibre. Since plasma polymers deposited on wool fibres are known to decrease the fibre’s dyeability, a plasma polymerisation does not meet the requirements for a treatment on the stage of top. Therefore the research was focused on the development of a plasma-induced surface oxidation followed by wet chemical application of a new environmentally acceptable resin tailored for the plasma treated surface. To replace the chlorination stage in wool shrinkproofing, barrier and glow discharges were investigated for their capability in oxidising the fibre surface without affecting the fibre quality. Barrier discharges (BD) can be generated by application of high voltages at atmospheric conditions in gas gaps between two electrodes where at least one electrode is covered with a dielectric barrier. BD consist of transient micro-discharges with low current discharge whose distribution (homogenous or filamented = joined micro-discharges) can be controlled by matched voltage wave forms. In contrast to BD glow discharges (GD) are generated under reduced pressure at lower voltage inputs and are characterised by a higher current discharge. As a consequence of the low pressure the mean free pathways of reactive plasma species are higher when compared to those generated at atmospheric conditions enabling a faster and deeper surface modification.
Fig. 2: Different types of SLAN microwave plasma sources a) µSLAN, open version, 40 mm diameter b) SLAN II, downstream version, 160 mm diameter c) SLAN II, open version, 670 mm diameter Single fibre tenacity (wet) in daN/mm 16 14 12 10 8 6 4 2 0
Fig. 4: Supercontracted and burst fibre area after exposition of wool in continuous barrier discharges (filamented) Sreening metal sheet
Initial studies on the application of both plasma types were carried out with static treatments being the base experiments for corresponding dynamic treatments and the subsequent development of a continuous operating process and machinery. Consequently, the general treatment conditions for permitting a sufficient treatment and preventing fibre damage were determined. In the case of GD the capability of low pressure SLAN type microwave plasma sources of different diameters (SLAN I and SLAN II; Fig. 2) were investigated using a continuous power input and the water vapour desorbed from wool during treatment for plasma generation. For BD-treatments continuous and non-continuous (Homoplas) power input were used for ionising synthetic air at various double barrier (ceramic coated) electrode configurations. The high power density in the SLAN I (inner diameter: 160 mm; maximum power: 2 kW) results in partially brown-coloured wool indicating an overheating of the fibres. This problem was completely eliminated by corresponding treatments in the SLAN II which is related to the lower power density of this source (670 mm, 6 kW). For BD treatments a bar electrode/backing roll or roll electrode/backing roll electrode system was preferred. SEM studies and fibre strength measurements on differently plasma treated wool show that the application of GD generated in the SLAN II as well as of BD generated by Homoplas power input does not alter the fibre’s quality (Figs. 1, 3). However, when continuous sinusoidal BD are applied, a thermal damage of wool is detected by a strong decreasing fibre strength (Fig. 3) and supercontracted and burst fibre areas (Fig. 4). This is related to a filamentation of discharges and demonstrates the necessity of a Homoplas power input for treating wool with BD.
Fig. 3: Influence of continuous and Homoplas barrier discharges (BD) as well as of microwaveinduced glow discharges (GD) on the single fibre tenactiy of wool
Moving wool top
Fig. 5: Schematic set-up for a dynamic treatment in the SLAN II-low pressure plant (GD principle)
Fig. 6: Schematic set-up for a dynamic treatment at atmospheric conditions (BD principle)
To obtain an even modification of the individual fibres within the top (prerequisite for shrinkproofing of wool) the application of BD also requires a reduction of the original top weight by drafting as well as a double-sided treatment. Due to the higher mean free pathways of reactive plasma species, however, this is not essential for GD-treatments.
Contact angle in degree 80 70 60 50 40 30 20 10 0 Untreated
Fig. 7: Influence of exposing wool to glow resp. barrier discharges (GD resp. BD) on the fibre wettability according to Wilhelmy measurements
On the basis of these results two lab-scale stations allowing a continuous top treatment with BD resp. GD were constructed. As presented schematically in Fig. 5 for dynamic GD-treatments, a large vacuum chamber was developed for transporting the wool by two pairs of rolls from a screened starting depot through the plasma chamber to a screened end depot. For corresponding BD-treatments a system consisting of a spreading unit for wool top, a treatment station including generator and transformer and a wool top container for storage of the treated material was developed (Fig. 6). For the demanded double-sided treatment, the treatment station is fitted with two (exchangeable bar or roller) electrode units and motor driven backing roll systems. Both electrode systems are equipped with gas injection systems. With the construction of both lab stations larger quantities of plasma treated tops were provided for investigations on the plasma-induced surface changes of the wool fibre and especially for the development of resins for imparting machine washability.
Fig. 8: Appearance of a plasma treated wool fibre after resin application
0,18 without Resin with Resin
Felting density in g/cm3
0,16 0,14 0,12
The application of GD and BD results first of all in an increase in primary amino, carboxylic and hydroxylic groups of the wool fibre surface and is accompanied by an increase in wettability (Fig. 7), being the condition of subsequent coating of the fibre with the new AOX-free resins. As shown in Fig. 8 the resin is able to spread homogeneously over the fibre surface resulting in a complete coating of the pre-treated fibre, a result which counts for both plasma treatments investigated. As a consequence of the coating of the plasma treated fibres the scale heights of wool are significantly reduced resulting in a nearly equalised with- and against-scale friction which is also accompanied by a general increase in friction values. Both effects lead to a reduced felting behaviour of wool. As demonstrated in Fig. 9 the felting tendency of loose wool is diminished by the action of plasma to an extent which does not meet the corresponding value of chlorinated wool. Nevertheless, the felting tendency of plasma treated wool is significantly reduced by subsequent resin application to an extent which is comparable to the corresponding value determined for Chlorine/Hercosett treated wool. Differences in the degree of felting observed after different plasma-only treatments are nearly equalised by the resin application. A comparison of the values determined for resin-only treated wool with the corresponding data of wool after plasma and resin treatment clearly indicates the necessity of plasma-induced surface oxidation for generating machine washability.
0,1 0,08 0,06 0,04 0,02 0 Untreated
Fig. 9: Felting densities (IWTO 20-69) of differently plasma treated wool before and after resin application in comparison to the corresponding data for the untreated and Chlorine/Hercosett treated material
The determination of the felting density of loose wool can be carried out using smaller quantities of sample and was therefore used as a first indicator for evaluating the shrinkproofing efficiency of the treatment. To determine the optimal plasma treatment and to ensure that the combined plasma/resin treatment is able to give a satisfactory performance for the consumer (prerequisite for scaling-up of one of the lab-scale plants), however, the specifications set by the Woolmark Company in the “Technical Method 31” (TM 31) have to be fulfilled. This means that the area felting shrinkage of knitted fabrics (produced from tops by spinning and subsequent knitting) must not exceed an area felting shrinkage of 10 % after 50 simulated washing cycles in a domestic washing machine. Though both plasma treatments meet the requirements of TM 31 after resin application, the scaling-up of the plant using atmospheric conditions was decided for economical reasons. The scaling-up was carried out on the basis of the experiences and results collected with plant 1 indicating that the electrode roll/backing roll electrode configuration should be preferred. Fig. 10 shows treatment station 2 with a treatment capacity of 12 kg top/h. The most essential components are two cooled and motor driven ceramic coated backing rolls and four units with adjustable ceramic coated electrode rolls which are separately connected with the generator. Wool top coming out of the spreading unit enters the station, passes maximum four treatment sections and following extraction section before leaving the station for storage. Transport of wool top (up to 20 m/min) through the station is supported by the driven backing rolls, a combination of driven guide rolls with nip roll and guidance components. To insert the wool the treatment sections can be opened hydraulically.
Fig. 10: Up-scaled treatment plant for treating wool tops in barrier discharges (plant 2)
Tab. 1: Area felting shrinkage of knitted fabrics made from plasma treated top with and without resin treatment after 50 simulated washing cycles in a domestic washing machine (TM 31) in comparison to the corresponding data for the untreated and Chlorine/Hercosett treated material
The treatment capacity of this plant allows larger-scaled trials for the resin treatment at industrial lines which are essential for a final assessment of the feasibility of substituting the industrially established Chlorine/Hercosett process. Table 1 summarises the results of the final assessments. The comparison of the values for area felting shrinkage determined (according to TM 31) for the plasma-/resin- and Chlorine/Hercosett treated samples clearly indicate that the new AOX-free process completely fulfils the demanded requirements.
OUTLOOK AND FUTURE PROSPECTS Creativity, flexibility and compatibility with the environment are increasingly required to secure the long-term capacity of the German textile industry. In addition to the development of new products as well as of economically-ecologically optimised production processes, a permanent competitive advantage can be created in particular by the introduction of new technologies. For this purpose the new AOX-free plasma-/resin-treatment of wool provides a promising starting point for a successful substitution of the Chlorine/Hercosett process which has no future due to the resulting effluent load with AOX (Fig. 11). The process is without chemical pre-treatment which is accomplished without the addition of water and thus saving resources.
The Chlorine-Hercosett-shrinkproofing treatment (state pf the art) leads to 40 mg “Adsorbable Organic Halides” (AOX) per liter effluent, originating from
Softener 5,6% Resin 17,3%
2. Rinse 2,6%
The Plasma-/Resin-shrinkresist treatment is a combination of physikally initiated surface oxidition followed by wet chemical resin application.
The process is > AOX-free and has a > significantly lowered water consumption
1. Rinse 11,7%
Fig. 11: Advantages of the new plasma-/resin-treatment for shrinkproofing of wool top over the established Chlorine/Hercosett treatment
The industrial implementation of the new process requires further research which is mainly related to the changed surface characteristics of the fully treated fibre material. This requires an adjustment of the individual finishing steps (spinning, dyeing, knitting) in the complete finishing line of the wool top. The aim of a new joint project between industrial enterprises and research institutions also funded by BMBF is to clarify the existing scientific gap by i) production of larger quantities of shrink-proofed tops, ii) detection of changes during finishing at industrial scale and iii) optimisation of the finishing performance possibly accompanied by the development of new tailor-made auxiliaries.
Das VDI-Technologiezentrum Physikalische Technologien ist im Auftrag und mit Unterstützung des Bundesministeriums für Bildung und Forschung (BMBF) als Projektträger für physikalische Technologien tätig. Das VDI-TZ betreut im Bereich ”Physikalische Technologien” die Fachthemen Schicht- und Oberflächentechnologien, Plasmatechnik, Supraleitung sowie Neue Gebiete. Das Faltblatt ” INFO PHYS TECH” will frühzeitig über Ergebnisse von Projekten, die das BMBF im Bereich ”Physikalische Technologien” fördert, und über technische Neuentwicklungen informieren. Ziel der Faltblattreihe ist, einen Beitrag zur Verbesserung der Innovationsfähigkeit von Unternehmen durch Einsatz neuer Technologien zu leisten. Die Faltblattreihe erscheint mehrmals jährlich. Die Ausgaben können kostenlos angefordert werden. Herausgeber des ”INFO PHYS TECH”: VDI-Technologiezentrum Physikalische Technologien, Postfach 10 11 39, 40002 Düsseldorf Fax: (02 11) 62 14-4 84, E-mail: [email protected]
Redaktion: Dr. Thorsten Schaefer, E-Mail: [email protected]
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INFO PHYS TECH
VDI-Technologiezentrum Physikalische Technologien
PROJECTORGANISATION ❍ Funding: Bundesministerium für Bildung und Forschung (BMBF) Heinemannstraße 2 53175 Bonn ❍ Project: Grundlagen zur plasmagestützten Antifilzausrüstung von Wolle ❍ Projectpartner: Bayer AG, Spezialprodukte Forschung, Leverkusen Bergische Universität GH Wuppertal, Forschungszentrum für Mikrostrukturtechnik BIAS-Bremer Institut für Angewandte Strahltechnik GmbH, Bremen Bremer Woll-Kämmerei AG, Bremen Deutsches Wollforschungsinstitut e.V. an der RWTH Aachen DyStar Textilfarben GmbH & Co. Deutschland KG, Frankfurt Institut für Niedertemperatur-Plasmaphysik e.V., Greifswald Technische Universität Braunschweig, Institut für Hochspannungstechnik Softal Elektronik Erik Blumenfeld GmbH & Co., Hamburg ❍ Projectmanagement: Dr. Ralf Fellenberg VDI-Technologiezentrum Physikalische Technologien P:O. Box: 10 11 39 40002 Düsseldorf Telefon +49 (0) 211 62 14-5 59 Fax +49 (0) 211 62 14-4 48 E-Mail [email protected]
❍ Contact Person: Dr. Helga Thomas Dr. Karl-Heinz Lehmann Deutsches Wollforschungsinstitut an der RWTH Aachen e.V. Veltmanplatz 8 52062 Aachen Telefon +49 (0) 241 44 69-1 22 oder 1 12 Fax +49 (0) 241 44 69-1 00 E-Mail [email protected] [email protected]