Silks have been used for textile production for millennia, since the silk of silkworms (Bombyx mori) can easily be collected from their cocoons. Silkworms can be cultivated in large scale, and the silk can be widely used in textile industry because of its smooth touch and the inherent shine , , .
Spider silk materials, in contrast, show additional properties such as extraordinary toughness. However, due to the cannibalistic behavior of most spider species, sericulture or farming is not possible to access this fascinating material. Recombinant production of silk proteins enabled for the first time to produce sufficient amounts of the materials and to develop applications thereof , , , , , , , .
Furniture textiles need an extraordinary quality and long life-time. Manufacturing of yarn and fabric for furniture has been optimized over several centuries. Single filaments or fibers are first processed into yarns, which are then arranged via stapling, layering or ring assembly to yield a fabric . In order to achieve the desired surface properties, protective coatings for textiles are applied. Water and dirt repellence, flame retardancy or abrasion resistance are implemented to increase the textiles life range . Coating methods using solid phases (lining/lamination, melt extrusion, calendaring) or liquid phases [dimethylformamide (DMF) coagulation/wet processing, roller coating, doctor blading, dip coating/Foulard-process, pressure-/spray coating] are typically used. In contrast to solid phase coatings, which mainly affect the templates surface, wet coatings produces, e.g. by the Foulard-process offer homogeneous coatings throughout the fabric, but in this case large amounts of coating solution and, therefore, drying temperatures and time are required to remove the excess of solvent. Spray coating methods reduce the energy consumption at the expense of coating efficiency but generally affect the textile surface only . Foam coating allows homogeneous coatings with little excess of solvent and, therefore, low drying expense. Foaming solutions and parameters are necessary to provide instable foams with low bubble sizes , . Foam coatings have been applied to improve textile properties for decades due to a high coating efficiency and economic factors. However, basic coating substances usually comprise harmful substances like hydrofluorocarbons.
Here, we developed a green, sustainable coating process of textiles by using spider silk protein foam. Spider silk coatings improved the stability and fraying behavior of yarn materials, and their pilling behavior was significantly reduced.
All chemicals were purchased from Carl Roth (Karlsruhe, Germany) or Sigma-Aldrich (St. Louis, MO, USA) at p.a. quality, if not stated otherwise. Textile fabrics (Fig. 1 and Fig. S1) as well as yarn reels in different fineness (1 tex=1 g km−1) were obtained from Rohleder GmbH, (Konradsreuth, Germany) comprising linen (LI, staple fiber yarn, Art. 927, 71 tex), rayon/linen (CV/LI, 70/30, staple fiber yarn, Art. 69, 111 tex), polyester (PES, filament yarn, Art. 66, Nm 48 tex) and poly(acrylonitrile) (PAN, staple fiber yarn, Art. 355, Nm 31 tex) .
Foam processing from spider silk solutions
Engineered Araneus diadematus fibroin 4 [eADF4(C16)] was purchased from AMSilk GmbH (Munich, Germany) . The non-ionic foaming agent (FA) Ultravon JUN (Huntsman Textile Effects GmbH, Langweid, Germany) was derived from Rohleder GmbH (Konradsreuth, Germany). eADF4(C16) silk protein was dissolved in 6 M guanidinium thiocyanate (GdmSCN, βsilk = 75 g L−1), and dialyzed against aqueous 50 mM Tris/HCl (pH 7.5, 100 mM NaCl), followed by a centrifugation step (20k rpm, 30 min), and then dissolved in ultra- pure water (MilliQ, Merck Millipore, Darmstadt, Germany). The protein solutions were placed in a glass beaker (1000 mL) with four tubes (Ø = 3 mm) for aeration with a pressure of 1 barg and at a flow rate of 3000 mL h−1. Foaming agent was added at a concentration of 30 g L−1. The solutions were stirred at 1000 rpm for 5 min each using a Heidolph RZR 2020 (Heidolph Instruments GmbH & Co. KG, Schwabach, Germany) with a three bladed propeller stirrer (PR 30) and a diameter of 58 mm, a shaft diameter of 8 mm and a length of 400 mm. The resulting foam was transferred into the foam application chamber.
Processing of fluorescently active spider silk protein solutions
Fluorescently labeled silk protein was produced by incubating eADF4(C16) silk protein with NHS-fluorescein (46409, Thermo Fischer Scientific Inc., Dreieich, Germany, βNHS = 1 μg mL−1) for 2 h, followed by dialysis against 50 mM Tris/HCl (pH 8) dissolved in ultra-pure water. The labeling efficiency was 0.6 mol*molsilk−1. For fluorescent coatings a dope solution was prepared by mixing not-labeled and labeled silk protein solutions at equimolar concentrations and at a ratio of 200:1.
Rheological characterization of dopes
Silk protein solutions were rheologically characterized using a rotational rheometer (AR-G2, TA Instruments Ltd., Crawley, Great Britain) and a cone-plate system (diameter: 40 mm, 0.5°) with a flow procedure (logarithmic shear ramp from 2.86 s−1 to 286.4 s−1) and 10 collection points per decade at 25°C (see Fig. S2).
Foam coating of fibers and fabrics
Textile fabrics and single fiber yarns were placed on a transportation foil with a diameter of 150 mm. The velocity was set to 0.1 m min−1. Then the coating was performed. To cut the foam at a height of 3 mm above the textile, a doctor blade was adjusted accordingly. Due to the linear motion and using a sufficient amount of silk foam all samples were coated homogeneously. The samples were placed on a perforated cylinder, and applied vacuum caused the collapse of the foam and the suction of the liquid through the textile. Remaining liquid could be recycled and reused. Each coated textile was dried using infrared lamps at a distance of 100 mm for 30 min using a temperature of 40°C.
Analysis of coating stability and efficiency
The morphology of the coated yarn or textile surface was analyzed using scanning electron microscopy at a voltage of 3 kV (SEM, LEO1530, Carl Zeiss Microscopy GmbH, Jena, Germany). All samples were sputtered with a 2 nm thick platinum layer by vacuum evaporation for 2 min. The stability of the coating was tested with fluorescently labeled silk protein coated yarn or fabric. All samples were flushed three times with 1 mL ultra-pure water or ethanol and drying steps for 30 min at 40°C. The samples were analyzed using fluorescence microscopy with an excitation wavelength of 475 nm (DMI3000B, Leica Microsystems GmbH, Wetzlar, Germany). To examine the amount of silk protein applied to the textile fabric, five samples of 25 cm2 surface area were incubated in 8 M urea for 48 h. The solution was centrifuged at 20k rpm for 30 min to remove textile residues. The amount of dissolved protein was analyzed using UV/Vis spectroscopy at a wavelength of 280 nm (Nanophotometer P330, Serva Electrophoresis GmbH, Heidelberg, Germany).
Characterization of yarn to fabric friction (capstan test)
Coated yarns were characterized using a testing standard for analyzing the friction coefficient modified after capstan , , . Briefly a standardized cotton fabric (SM25, Rohleder GmbH, Konradsreuth, Germany) was attached to a wooden cylinder with a diameter of 22 mm, which was fixed in a traverse. The yarn was connected to a weight (10 mN tex−1) on one end and to a 2.49 N load cell of an ElectroForce 3220 tensile testing device (Bose Corporation, Eden Prairie, MN, USA) on the other end. The yarn was then placed perpendicularly on the standard fabric and wooden cylinder. Alternating vertical movements (6 mm, 0.33 mm s−1, n = 10) caused pills and knots on the yarn surface. Visual imaging using light microscopy (DMI3000B, Leica Microsystems GmbH, Wetzlar, Germany) and computational transfer into black and white pictures (Photoshop CS3, Adobe Systems GmbH, München, Germany) led to a subsequent profile plot (ImageJ 1.47 v, Wayne Rasband, Bathesda, MD, USA). The implementation of the hairiness index (H/μm2 cmyarn−2), describing the total black pixel surface area in μm per centimeter of the strained yarn allows the numerical comparison of fraying reduction .
Fabric characterization – abrasion resistance (pilling)
Pilling was tested for coated textile fabrics modified after Martindale abrasion resistance in accordance to norm EN ISO 12945-2 using a Martindale NU testing device (James H. Heal & Co. Ltd., Halifax, GB) at Rohleder GmbH (Konradsreuth, Germany) . A standard cotton textile (SM25) was attached to a stamp and pressed onto the textile sample (Ø = 140 mm) with a contact force of 3.8 N. Pilling of the fabric was visually evaluated after defined revolutions (500, 1000, 2000, 5000) in five main and eight sub quality categories .
Results and discussion
Foam coating of selected textile fabrics using spider silk foams resulted in smoothing of the initial surface of partially natural (N) and polymeric textiles (P1) (Fig. 2a2, a3 and b2), as well as the polymeric textile (P2) (Fig. 2c2 and c3) with a clearly visible coating (Fig. 2a2, c3) and visible fiber interconnections (Fig. 2a3, c2). The fluorescence microscopy images indicated that the coating was homogeneously distributed on the yarn and throughout the textile types (Fig. 2a1, b1, c1). Herein, the resulting thickness of the protective coating was increased upon an additional coating step (Fig. 2b2, b3). Since the coated fibers were better interconnected, the stability of the yarns was increased.
Yarn improvement and coating stability
Fraying tests of yarns showed a lower fraying behavior upon silk coating as exemplarily depicted in Fig. 3 (linen yarn) and Table 1 in comparison to uncoated yarn (Fig. 3a); less fibers were ripped out of the yarn (Fig. 3b). Black/white pixel ratio analysis revealed a larger volume of the uncoated versus the spider silk coated yarn after fraying (Fig. 3c–e). Spider silk foam coating significantly reduced the volume increase of uncoated yarns upon the fraying test.
Determining the hairiness index (H/μm2 cmyarn−2), describing the total black pixel area in μm per centimeter of yarn, allowed the numerical comparison of fraying reduction upon silk coating (Table 1a), which was 36±13% (Table 1) .
Depending on the surface texturization of the used textile fibers, the reduction of fraying differed. In case of smooth fibers like PES (6±1%) or CV (13±3%) the fraying reduction was lower than that in case of textured fiber materials like LI (36±13%) or PAN (51±21%). The rayon (CV) fibers feature OH-groups, which support the adsorption of polar molecules in the presence of aqueous solutions. Therefore, the smoothening effect was higher than for PES, which shows a negative surface charge likely electrostatically repelling the negatively charged eADF4(C16) , . Since the increased surface area of structured fiber materials showed more friction, the silk coating had a stronger impact thereon.
The stability of the spider silk foam coating (Table 1b) was tested in the presence of water and ethanol, which are regularly used in contact with furniture textile surfaces (e.g. upon cleaning). The spider silk coating was stable on all textile materials in both liquids. The amount of spider silk coating was apparently slightly reduced in case of the smooth surface materials such as PES due to the reasons depicted above (Fig. S3).
Abrasion test of spider silk coated fabrics
Abrasion resistance tests displayed the textile improvement and reduction of pilling after spider silk coating for all types of textiles. The uncoated partially natural textile fabric showed a low tendency to pill throughout all revolutions (Fig. 4a). The fabric contains rough linen fiber, but 53% of this fabric resembles smooth CV and PES, reducing the friction and therefore the tendency to pill. Additionally, the employed ring yarn offered a higher resistance to mechanical stress . Coated with spider silk, the textile integrity was slightly improved in terms of intermediate strain, and a double coating (the coating procedure was performed twice) further increased the long term resistance by about 17%.
Both polymeric fabrics contained smooth PAN-filaments and PES-staple yarn fibers with smooth surfaces and low friction. However, due to the use of combed yarn and a mere wet-stretching post treatment, fabric P1 provided an initial texturization which is slightly higher than expected (Fig. 1). The larger amount of exposed textile on the surface enhanced the tendency to pill, as depicted in Fig. 4b. Uncoated, the fabric integrity dropped with increasing revolutions. An initially low amount of pills at short term abrasion (500 rounds) evolved into an undesirable high pilling at long term usage (5000 rounds). The silk coating significantly increased the fabric quality after strain throughout all numbers of revolutions by up to 200% (long-term strain). Especially at long term abrasion (5000 rounds) the tendency to pill was severely reduced. A second coating step showed no effect on this textile. Regarding the rather smooth polymeric textile P2, the initial strain on uncoated textile had a similar effect compared to that on P1 regarding short term strain (Fig. 4 P2). The ring yarn and steam pressing post treatment yielded a compact fabric morphology and less surface texturization. The abrasion resistance significantly increased upon silk coating by more than one quality category (see Experimentals – fabric characterization) throughout all revolutions, and in particular it nearly doubled for long-term strain . Additional coating steps had no further impact (data not shown).
Three different fabrics, one partially based on the natural materials cotton and rayon (in total 66% of the fabric) as well as two completely polymeric fabrics comprising PES and PA were foam coated with recombinant spider silk proteins and analyzed regarding their sensitivity to friction and the resulting fraying. Homogeneous and stable coatings were successfully applied to yarns as well as complex fabrics by a foam coating method which was newly established for using spider silk proteins as such. Hearl described that single fibers in yarns are torn out of their matrix either by ripping or when individual fibers slip over each other . Here, fiber interconnections and therefore total stabilization of the yarns by the spider silk coating (Fig. 2) reduced the ripping of filaments out of the yarn, and therefore reduced pilling. A clear dependency of yarn fraying as well as pilling tendency was determined on the presence of spider silk coating, which was reduced significantly for all tested materials. Furthermore, the spider silk coating doubled the quality of fabric in short as well as long-term abrasion tests.
Funding was derived from Oberfrankenstiftung (“Equipment of furniture textiles using spider silk”, grant ID: 03892). We would like to thank Robert Gleuwitz for experimental assistance, Johannes Diehl and Claudia Stemmann for performing SEM as well as Elise DeSimone for proofreading. Furthermore we gratefully acknowledge the material supply and technical support of Peter Raich and Peter Haag (Rohleder GmbH) as well as the performance of abrasion resistance tests.
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The online version of this article offers supplementary material (https://doi.org/10.1515/pac-2017-0601).
About the article
Published Online: 2017-09-09
Published in Print: 2017-11-27
Citation Information: Pure and Applied Chemistry, Volume 89, Issue 12, Pages 1769–1776, ISSN (Online) 1365-3075, ISSN (Print) 0033-4545, DOI: https://doi.org/10.1515/pac-2017-0601.http://creativecommons.org/licenses/by-nc-nd/4.0/.