EFFICIENCY OF CUSTOM-MADE PDMS-PEG COPOLYMERS AS A DURABLE, HYDROPHILIC SOFTENER Andrea Körner, Ramona Ronge, Xiaomin Zhu, Martin Möller DWI an der RWTH e.v., Aachen (Germany) Abstract The extraordinary combination of hydrophobic properties with low glass transition temperature, high elasticity and chain flexibility distinguishes silicones significantly from other synthetic polymers. As a consequence silicon surfaces feature low surface tension, low adhesion and high dewetting potential. Silicones are widely used in the textile industry to improve the garments performance and to enhance soft handle, silk like gloss, soft and graceful flow and water repellency. On the one hand classical silicone softeners provide soft handle but on the other hand they increase the water repellency of the surface and impair the moisture management of the garments. In this project we developed commercial and custom-made, hydrophilic polydimethylsiloxanes (PDMS) modified with reactive groups to achieve a durable attachment to the surfaces of cotton and polyester fibres. The used PDMS feature a comb like structure bearing hydrophilic ethylene oxide side chains to implement both hydrophilic and soft handle properties. To attach the modified silicones covalently to cotton fibres triethoxysilane was used as an end-capped linking group. For interaction with polyester fibres the silicones were decorated with imidazolidinone to form multiple hydrogen bonds with the polyester surface. The optimized PDMS of both concepts feature a homogenous and durable finishing after washing with an anionic detergent. The thickness of the silicone film was between 4-10 nm depending on the selected PDMS. The water absorbency of the treated fabrics was used as a measure for the hydrophilicity. Modified silicones on polyester fibres led to an increase in hydrophilicity, for treated cotton fibres the inherent hydrophilicity was retained with selected silicones. Furthermore, the soft handle of the fabrics was clearly enhanced after treatment with the modified silicones. This effect was particularly visible for the shear-, tensile- and bending properties. Introduction Silicones differ significantly from other synthetic polymers due to their extraordinary combination of hydrophobic properties with low glass temperature, high elasticity and chain flexibility. They provide surfaces with low surface tension, low adhesion and high dewetting potential. For this reason silicones are widely used in the textile industry to improve the garments performance and to enhance soft handle, silk like gloss, soft and graceful flow and water repellency. Classical silicone softeners provide soft handle but they may also contribute to the water repellency of the surface and impair the moisture management of the garments. The textile finisher wishes for a single system which is simultaneously effective for both hydrophilic as well as hydrophobic fibre materials, e.g. cotton and polyester, and applicable from aqueous solution with existing machinery. Hydrophilicity and moisture management
should not be affected, and an even, permanent finishing effect surviving several washing cycles is aimed for. Aim of this work therefore was to design hydrophilic polydimethylsiloxanes (PDMS) modified with reactive groups to provide a durable attachment to the surfaces of cotton and polyester fibres. Results and discussion Commercially available and tailor-made, hydrophilic polydimethylsiloxanes (PDMS) were modified with reactive groups to achieve a durable attachment to both the surfaces of cotton and polyester fibres. The tailor-made PDMS have a comb like structure with hydrophilic ethylene oxide side chain grafts; by systematically varying the substitution pattern in the side chains of the PDMS-PEG backbone tailor-made, optimized functionalised PDMS-PEGs to implement both hydrophilic and soft handle properties were generated. The resulting PDMS- PEGs were fitted with reactive groups designed to bind covalently to the surface of cotton fibers or to bind to PES via multiple hydrogen bonds, polyelectrolyte complex formation and hydrophobic interactions (Figure 1). Figure 1 Project Strategies A range of commercially available PDMS compounds (Figure 2) was checked for their suitability as educts for derivatives with the designed anchor groups and for the proof of the concepts. In the course of the project it became visible that a more systematic approach was preferable and a series of tailor-made hydrophilic PDMS-PEGs was synthesized in laboratory scale (Table 1).
Figure 2 Commercial PDMS-PEGs used in this study Table 1 Tailor-made, hydrophilic PDMS-PEGs *Momentive Performance Materials
Examples for the reactions to generate the desired anchor groups for the approaches I and II are shown in Figure 1. Figure 3 Examples for chemical approaches I and II Application of silicone derivatives to cotton and PES The tailor-made hydrophilic PDMS-PEGs derivatives were applied to cotton and PES fabrics by padding from acetone to generate a fixed initial amount of 4% silicone derivative on the fabric. The fabrics were then scoured in the presence of the detergent Ecolable LD. The elemental composition of the fibre surface was then determined by X-ray Photoelectron Spectroscopy (XPS). The thickness of the silicone films after washing was estimated by calculation from the elemental composition (Figure 4, Figure 5). Furthermore the amount of silicium on the fabrics after scouring was determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and the actual amount of silicone in the fabrics calculated from the structures (Figure 5, Figure 6). For cotton film thicknesses of 8 nm on average (with a range of 4-10 nm) were observed after scouring, depending on the structure of the triethoxysilane functionalized silicones. Likewise were film thicknesses of 4-7 nm obtained on the surface of PES, again depending on the structure of the imidazolidinone-functionalized PDMS derivatives. The results in Figure 4, Figure 5 and Figure 6 show that in spite of the different nature of the cotton and PES surface comparable amounts of bound silicone are present on the fibre. This resistance to scouring is taken as a proof of principle for the design of the anchoring units. In analogy to the silicone amounts, also the thickness of the films is in the same range.
Figure 4 Permanency of tailor-made PDMS-PEGs: Influence of functionalisation on film thickness (XPS) on cotton after scouring with Ecolable LD Figure 5 Permanency of tailor-made PDMS-PEGs on cotton and PES: Film thickness (XPS) and silicone content (ICP-AES) after scouring with Ecolable LD
Figure 6 Permanency of tailor-made PDMS-PEGs on Cotton and PES: Cotton and PES: silicone content after scouring with Ecolable LD (ICP-AES) Figure 7 Eveness of silicone distribution on PES
Due to the weak linkages of conventional silicones to the smooth, locked PES surface amino silicones and also polyquaternary polysiloxanes are supposed to adsorb only partially and with uneven distribution. This is especially the case for blends of cotton/polyester, where the bulk of the softener will adsorb to the cotton while the PES fibres will stay largely free of silicone (1,2). For this reason the distribution of the imidazolidinone-modified silicone compounds 100/4-, 87/12 and 91/8 on the surface of the PES fabrics after scouring with the surfactant Ecolabel LD was checked by staining with the anionic dye Lanasol blue 3R (Ciba Geigy) at acidic ph. Thus the amino groups in the imidazolidinone anchor will be protonated and form ionic bonds with the sulphonic aicd groups of the dye. As shown in Figure 7 an even dye distribution is obtained immediately after application for all PDMS derivatives. The patchiness after scouring and the loss in colour intensity indicate the irregular release of silicone material. With PDMS 87/12-imidazolinone, however, an even dye/silicone distribution is achieved also after five washing cycles and the loss in color intensity is low. This compound, equipped with a high number of anchor groups and long PDMS loops, obviously allows for high permanency on PES without the need of surface acitivation e.g. by plasma. The high affinity and durableness of a comparable commercial PDMS-PEG-PPG functionalized with imidazolidinone becomes evident when the film thicknesses on cotton and PES are determined after their simultaneous finishing in competition mode from one and the same application liquor (Figure 8). It is visible that the affinity of this silicone to PES is markedly higher than to cotton and that the functionalization will add to the permanency and resistance to scouring. Figure 8 Affinity to CO and PES / Permanency: film thickness of functionalized PDMS- PEG after application from aqueous solution ( competition mode )
Treatment of cotton with conventional PDMS is known to cause loss in moisture uptake due to hydrophobisation of the fabric. The influence of the modified PDMS-PEGs on the moisture up take and hydrophilicity of cotton and PES fabrics was checked by means of the so called rewetting test for hydrophilic softeners. Figure 9 Hydrophilicity: Influence of functionalisation on the rewetting of Cotton Figure 10 Hydrophilicity: Influence of functionalisation on the rewetting of PES
In this test the migration distance of water which migrates by capillary force vertically into the fabric serves as a measure to characterise the degree of hydrophilicity (see small graph in right upper corners Figure 9 and Figure 10). The migration is made visible by spreading of a marker dye and the migration distance is measured. The factor is then calculated from the ratios of the migration distances for the treated fabric and a blank reference quoted with 100 % rewetting. The higher the migration factor is the more hydrophilic is the tested softener. For all tailor-made PDMS species a decrease of the water migration distance was observed directly after application; reducing the amount by scouring to approx.. 2 % of silicone on the fabric, however, caused a significant recovery of the hydrophilicity. The optimized PDMS compounds of both species thus will yield good hydrophilicity when applied in thin films. Furthermore the functional groups designed for anchoring to the fibre surface also contribute to the improvement in hydrophilicity of the fabric. This is shown for cotton in Figure 9 and for PES in Figure 10. An aging effect on storage of the cotton fabrics was observed for the PDMS-PEGs with 7 EO grafts; this is thought to be the consequence of a rearrangement of the PDMS chains within the film, going along with reduced hydrophilicity. For the PDMS-PEGs with 17 EO units in the side chain grafts this effect was not significant; hence an increase in the hydrophilic portion of the EO-repeating units helps to avoid the aging. Objective measurement of hand properties An overview on the results of the objective measurements of hand properties is given in Table 2. A significant improvement after application of thin PDMS/ PDMS-PEGs films was obtained both for cotton and for PES. The effects were especially explicit for the silicones fitted with long PDMS loops; effected parameters were resilience, flexibility, formability, as well as friction and elasticity of the fabrics. PDMS-PEGs with longer PEG grafts (repeating units of 17 EO) provoked slightly higher values for the bending stiffness and resilience properties than obtained for the PDMS species with shorter grafts of 7 PEG units. Positive effects were likewise obtained for recovery properties, resilience, friction and elasticity, flexibility, bending stiffness and roughness of PDMS treated PES fabrics. Table 2 Objective measurement of hand properties PDMS- PDMS- Tool Tested property PEG 7 PEG 17 Effect Stretchability + + KES-FB1 Shear elastic properties + + KES-FB2 Bending properties + - KES-FB3* Compression not tested not tested KES-FB4 Surface properties + + *defective system
Influence of silicone finishing on the wear comfort The wear comfort note was measured by the Institute Hohenstein. The results for the most promising tailor-made PDMS /PDMS-PEG 87/12 applied to cotton and PES are shown in Figure 11. The study showed that finishing with these functionalized PDMS derivatives neither impairs the wear comfort notes of PES fabric nor of that of cotton significantly. For cotton fabric a marginal deterioration of the wear comfort and the thermophysiological comfort was detected. The skin sensorial comfort, however, was improved for both fibre species. Cotton fabric finished with functionalized PDMS-PEG 87/12 still meets the tight demands of the Norm DIN 10524:2004-05 Food hygiene - Work wear in food business. Figure 11 Wear comfort results for cotton and PES fbric finished with PDMS /PDMS- PEG 87/12 Acknowledgements We wish to thank the research association Forschungskuratorium Textil e.v., Reinhardtstraße 12-14, 10117 Berlin for the financial support of the research project IGF-No. 16499 N, which was provided within the promotion program of Industrielle Gemeinschaftsforschung und entwicklung (IGF) from budget funds of the Federal Ministry of Economics and Technology (BMWi) due to a resolution of the German Bundestag via Arbeitsgemeinschaft industrieller Forschungsvereinigungen e.v. (AiF). Thanks are further due to Coatema Coating Machinery GmbH, Dormagen Gebr. Röders AG, Soltau Evonik Industries AG, Hanau-Wolfgang B. Laufenberg GmbH, Krefeld Momentive Performance Materials GmbH, Leverkusen.
Literature 1. Habereder, P., Bereck A., Rev Prog Color 32, Part 2, 125 137 (2002) 2. Wagner, R., Lange, H., A.Hesse, H. Thoss, H. Höcker, unveröffentlichte Ergebnisse