Digital inkjet functionalization of water-repellent textile for smart textile application
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Digital inkjet printing is a production technology with high potential in resource efficient processes, which features both flexibility and productivity. In this research, waterborne, fluorocarbon-free ink containing polysiloxane in the form of micro-emulsion is formulated for the application of water-repellent sports- and work wear. The physicochemical properties of the ink such as surface tension, rheological properties and particle size are characterized, and thereafter inkjet printed as solid square pattern (10 × 10 cm) on polyester and polyamide 66 fabrics. The water contact angle (WCA) of the functional surfaces is increased from < 90° to ca. 140° after 10 inkjet printing passes. Moreover, the functional surface shows resistance to wash and abrasion. The WCA of functional surfaces is between 130° and 140° after 10 wash cycles, and is ca. 140° after 20000 revolutions of rubbing. The differences in construction of the textile as well as ink–filament interaction attribute to the different transportation behaviors of the ink on the textile, reflected in the durability of the functional layer on the textile. The functionalized textile preserves its key textile feature such as softness and breathability. Inkjet printing shows large potential in high-end applications such as customized functionalization of textiles in the domain of smart textiles.
In the past decades, inkjet printing was recognized as an emerging production technology because of its manufacturing capabilities. Inkjet printing is applied in various applications such as micro-manufacturing, photovoltaics, electrochemical sensors and ceramic tile [1, 2, 3, 4]. Gradual development of the inkjet printing technology  has now come to a stage where it is accurate and fast enough to compete as an alternative method for rapid printing, overall coating and periodic micro-patterning.
Inkjet printing targeting various functional applications has been investigated intensively on non-absorbent substrates, such as glass, silicon wafer and polymer film . Selective and mask-free deposition of functional materials is particularly important since the price of functional material is often high and the positioning of the material is critical in the field of microelectronics [6, 7, 8, 9, 10]. For example, inkjet printing was applied to manufacture conductive electrodes [6, 7], solar cell components , and to deposit precursor particles . The selective deposition of functional fluids and the digitalized process made inkjet printing a very versatile and flexible fabrication technology driven by the application demands . Furthermore, nanomaterials and/or functional materials that are compatible with the ink formulation can boost the performance of functional fluids with properties such as super-hydrophobicity/self-cleaning [11, 12], anti-bacterial and electrical conductivity . Nanoparticles of SiO2 , TiO2, ZnO , and Ag [13, 16], functional materials such as polysiloxane [12, 14], polysilane  and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)  are commonly involved. The efficient material usage, high precision and flexible production with high productivity made inkjet printing a potential method for resource efficient, high-end smart textile manufacturing. Moreover, inkjet printing could be beneficial to maintain the original characteristics of the carrying porous material, e.g., the softness in hand feeling and breathability of textile due to deposition of materials in pL range .
Water repellency, or sometimes extended to super-hydrophobicity, is an important function on technical textile to obtain self-cleaning and stain-repellent properties [11, 14, 19], especially for sports- and work wear. This function can be achieved by applying low surface energy compounds and introducing surface roughness via dip-coating, padding, knife coating or spray coating, etc. Liu et al.  investigated a single-side super-hydrophobic finishing on cotton by blade-coater with fluoropolymer foam. A water contact angle (WCA) higher than 150° and hysteresis lower than 15° were achieved. Zeng et al.  reported a one-pot coating solution consisting of SU-8 (a negative photoresist), fluoroalkyl silane and silica nanoparticles to tune WCA of cotton from < 90° to 160°. The coating is cured by UV light and is durable to withstand 100 laundry cycles. In other researches, surface roughness is combined with hydrophobic substrates or with hydrophobic top-coatings to achieve hydrophobic/super-hydrophobic surfaces without fluorocarbon. Karim et al.  inkjet-printed hydroxyl functional polystyrene nanoparticles (emulsion) on polyester in order to introduce surface roughness, which boost the WCA of the surface from < 90° to 143°. Liu et al.  used polydopamine@octadecylamine (PDA@ODA) nanocapsules to functionalize polyester fabric. The functionalized surface has WCA of 145°, and the water repellency can be healed by heating. Even though fluorocarbon (C x F y ) in general shows excellent water and oil repellency among known chemical compounds, the bio-accumulative and persistence of fluorocarbon in man and everywhere else in nature make it unacceptable toward the environmental and sustainable development of modern society . Perfluorooctanoic acid (PFOA), its salts and PFOA-related substances, one type of fluorocarbon (so-called C8), is restricted in EU . The industry thereafter moved toward relatively short chain C6 and C4. However, the performance of C6 and C4 is worse than C8, and the bioaccumulation of persistence organic compound remains .
In this study, we investigated an alternative solution that stresses a resource efficient process, inkjet printing and environment-friendly polysiloxane ink, to replace conventional large run machineries with fluorocarbon chemistry. Polysiloxane used in this work is a linear-type polysiloxane (estimated as Mw ≈ 70000 and n ≈ 900, defined as middle-high molecular weight polysiloxane by Mojsiewicz-Pieńkowska et al. ) which does not pose threat to organisms via the respiratory system (when n > 4) and therefore is favorable for textile application. Furthermore, middle-high molecular weight polysiloxane features low toxicity and does not bio-accumulate once it enters the environment according to existing studies .
Despite desired functionalities, the fastness properties of functionalized textile toward wash and abrasion are a fundamental challenge . Zhou et al.  demonstrated a durable super-hydrophobic SiO2/PDMS nanocomposite coating on polyester fabrics by dip-coating. The WCA of the functional layer maintained almost the same (~ 170°) after 500 wash cycles or after abrasion testing (Martindale method) of 28000 cycles under 12-kPa load. Liu et al.  introduced a cross-linking agent, and therefore, the covalently bonded functional layer is capable to stand 20 wash cycles without significant changes in WCA (decreases from 153° to 148°).
In this research, we used inkjet printing to deposit water-repellent ink on polyester and polyamide 66, which is commonly used in the application of functional (technical) textile. There is a lack of publication concerning hydrophobic functionalization by inkjet printing on a porous and absorbent (textile) substrate. We focus on the inkjet formulation, the inkjet printing process and ink–substrate interaction for the application of functional textiles in sports- and work wear.
Materials and methods
Raw polysiloxane micro-emulsion dispersion in water (Dow Corning Corporation) was kindly supplied by Univar Inc. The substrates used for inkjet printing were 100% plain woven polyester fabric (PET, FOV Fabrics AB, Sweden) and 100% plain woven polyamide 66 fabric (PA, FOV Fabrics AB, Sweden). The PET fabric has a zero twist two ply yarn warp (336 dtex) and weft of 368 dtex and a weight of 145 g m−2. The weft/warp count is 22/20 cm−1 for PET. The PA fabric has a warp and weft of 188 dtex and a weight of 118 g m−2. The weft/warp count is 27/39 cm−1 for PA. The fabrics were received as washed and preset from the producer and thereafter soaked and rinsed by deionized water and iron dried before inkjet printing. Reference PET and PA samples with the same construction but finished with fluorocarbon by exhaustion were provided by FOV Fabrics AB and used to compare with the sample functionalized by inkjet printing. The average pore size of PET and PA was 17.7 and 11.2 µm, respectively, which are measured by a liquid–liquid type porosimeter (PSM 165, Topas GmbH).
The polysiloxane dispersion is chosen based on the molecular weight and architecture of polysiloxane, the carrying fluid and the rheological property of the dispersion. A waterborne dispersion containing linear polysiloxane with viscosity from a few to a few tens of mPa s which is close to the inkjet printing profile of print head is preferred to formulate the ink. Two types of polysiloxane dispersion, one contains amino functional dimethyl polysiloxane (Mw: 70000, CAS 71750-79-3) and the other contains methylamino siloxane with glycidyl trimethylammonium chloride (CAS 495403-02-6), are evaluated (Supplementary information, SI). It is convenient to formulate inkjet inks using existing polysiloxane dispersion which is available with industry-scale amount. The pre-examination showed that the inkjet-printed amino functional dimethyl polysiloxane dispersion had higher WCA than methylamino siloxane with glycidyl trimethylammonium chloride dispersion (SI, Fig. S1d). Therefore, amino functional dimethyl polysiloxane dispersion (so-called the functional ink later on) is chosen as the focus of hydrophobic component in this study. The amino functional dimethyl polysiloxane dispersion was used as it is after inkjet profile characterization.
The rheological properties of the functional ink were measured by a rheometer (Physica MCR500, Paar Physica) with a double gap cylindrical cell. The ink formulation was measured: (a) on heating from 15 to 40 °C at a constant shear rate of 10000 s−1; and (b) at a shear rate increasing from 0 to 10000 s−1 at 25 °C. The viscosity was acquired at highest measurable shear rate of the instrument at 10000 s−1. The estimated shear rate at the nozzle tip (ε) of Dimatix print head could reach ~ 400000 s−1 by using ε = v/D  (drop velocity, v, is 8 m s−1 and diameter of a nozzle, D, is 21 μm).
The surface tension of the ink was measured using an optical tensiometer (Attension Theta, Biolin Scientific). The surface tension of the inks was measured by pendant drop method with an ink drop volume of 4 μL and reported in average of three independent measurements. The prepared ink formulation was filtrated through a nylon syringe filter with a pore size of 0.45 and 0.2 μm, respectively, to qualitatively evaluate the particle size. Still, in order to eliminate agglomeration of particles in the orifice or the nozzle channel, the functional ink was filtered through a nylon syringe filter with a pore size of 0.45 μm before loading into the print head.
Inkjet printing was performed using a Xennia Carnelian 100 inkjet development and analysis system equipped with a piezoelectric type, Dimatix Sapphire QS-256/10 AAA print head. The print head features a fundamental printable drop size of 10 pL. Solid square pattern (10 × 10 cm) and custom-designed patterns were inkjet printed at a resolution of 300 dpi on PET and PA substrates by multi-pass. Thereafter, the inkjet-printed samples were heated in an oven at 150 °C for 5 min to dry the functional ink. The dried samples that could still be hydrophilic at the functional surface were immersed in excessive amount of water to remove the water-soluble component in the ink. Eventually the samples were air-dried, and the functional surface became hydrophobic.
WCA was measured by sessile drop method with the optical tensiometer (Attension Theta). Three to five random spots on a textile substrate were selected to represent the surface property and measured with a water drop volume of 3 μL. For a hydrophobic textile surface, the WCA was read 3 s after the water droplet was stabilized on the substrate. The observed WCA reduces with time for a hydrophilic textile material. Therefore, the WCA was read immediately once the water droplet landed on the substrate. All samples were air-dried in ambient condition with a temperature of 20 ± 2 °C and a relative humidity of 20–30 ± 3% for 24 h and ironed prior to measurement. ASTM D5725-99 suggests a repeatability of 7% of WCA within a laboratory on absorbent substrates; therefore, we only distinguish the result when the WCA difference is larger than 10° .
The wash fastness was tested according to ISO 6330, with type A machine and procedure 4 N. The abrasion test was performed using a Martindale 2000 abrasion tester (Cromocol Scandinavia AB) at standard climate with a temperature of 20 ± 2 °C and a relative humidity of 65 ± 2%. The tests were performed according to ISO 12947-2:1998/Cor.1:2002 but modified to investigate the hydrophobicity of the functional layer. The PET and PA samples were conditioned in standard climate for at least 18 h and subsequently rubbed at 3000, 5000, 10000 and 20000 revolutions, respectively, with a load of 12 kPa. The WCA of functional surface was measured at five random locations on each sample after wash or abrasion tests. Scanning electron microscope (SEM) was performed on JEOL JSM-6301F, and EDX analysis was conducted on a Zeiss Supra 60 VP SEM with EDX probe.
Results and discussion
The surface tension and particle size were analyzed by pendant drop method and filtration test, respectively. The functional ink has a surface tension of 23.9 mN m−1, which is slightly below but still around the typically acceptable surface tension for inkjet printing between 25 and 35 mN m−1 . The transparent functional ink containing polysiloxane in the form of a micro-emulsion should have a particle size distribution between 1 and 100 nm. The stated particle size (15 nm) is considerably below 200 nm, which is an experienced particle size limitation to avoid agglomeration of particles in the printing nozzle channel . The functional ink was able to filter through nylon syringe filters with pore sizes of 0.45 and 0.2 μm, respectively, which confirmed that the functional particles are compatible with the inkjet printing process. The results obtained from measurements of viscosity, surface tension and particle size suggested that the functional ink fulfilled the print head specifications.
SEM was applied to study the morphology of the functional layer. As shown in Fig. 5a, d the functional ink formed a thin layer wrapped around the textile filament. Besides, the functional ink formed uneven circular layers on the surface of PA filament (Fig. 5d), so-called coffee-stain effect after drying of the functional ink . The formation of coffee-stain effect on the PA, which, however, is less obvious on PET and indicates that the inkjet-printed ink was spread more evenly on PET. Energy-dispersive X-ray spectroscopy (EDX) analysis was performed on PA × 10 sample. As shown in EDX spectrum in Fig. 5h, O and Si were the main element detected which are the components of the functional ink (O could contribute from PA substrate as well but less likely due to detectable depth of EDX). Element mapping of Si by EDX was also performed locally on one filament of the PA × 10 (SI, Fig. S5a). Accumulation of Si was detected as layer wrapped around the filament. The above results support that the hydrophobicity of the PET and PA surface is introduced by covering the surface with a thin layer of polysiloxane that is the active compound in the ink formulation.
The functionalized PET showed better wash fastness property due to (a) the stronger ink–filament interaction with PET, and (b) the deeper transportation of the functional ink in PET. As shown in Fig. 4b, the WCA of PET × 1 was 132° and became 125° after 10 wash cycles. But the WCA of PA × 1 became < 90° after 2 wash cycles from an initial WCA of 134°. The PET × 1 shows better wash fastness than PA × 1 sample. This could be a result of weak interaction between the functional ink and PA filament. As shown in liquid absorption tests of untreated PET and PA (SI, Table S1), the untreated PA absorbed slightly more high surface tension liquid, e.g., water  (72.09 mN m−1) but less low surface tension liquid, e.g., acetonitrile and ethyl acetate, than untreated PET. The surface tension of the functional ink (23.9 mN m−1) is very similar to acetonitrile (27.76 mN m−1)  and ethyl acetate (24.11 mN m−1) . This means the PET might have stronger ink–filament interaction than PA due to better wettability with the functional ink.
The ink transportation and ink–filament interaction are crucial to understand the durability of the functional layer in abrasion. Two factors, porosity/effective pore size of the textile substrate and ink–filament interaction played a key role. As shown experimentally in capillary height and functional ink absorption measures in Fig. S1 and S2, as well as theoretical deduction in Eq. (1), the functional ink penetrates deeper in PET fabrics. This can be the reason to obtain better abrasion resistance with PET. The inkjet-printed ink first covered the top layer of the textile surface and thereafter transported deeper through inter- and intra-yarn spaces into the bulky textiles. The ink transported deeper into the textile could offer better resistance, whereas functional ink accumulated on the top of the surface is more vulnerable to abrasion. Furthermore, PET indicates stronger ink–filament interaction with the functional ink. The functional ink adhered better on PET × 1 and × 3 samples than PA × 1 and × 3 samples, which agreed with previous wash fastness results. Moreover, the filaments wrapped around with a thicker layer of functional material are more resistant to abrasion. As shown in Fig. 4d, the sample with less functional material (PET × 1, × 3 and PA × 1, × 3, < 1 g m−2) is more vulnerable to abrasion tests than PET × 10 and PA × 10 samples (1.6 g m−2). Overall, the inkjet-printed PET and PA surface with polysiloxane showed moderately high WCA, wash fastness and abrasive property. However, due to the nature of non-covalent interaction between the functional layer and textile substrate, the fastness properties are constrained. To improve water repellency, as well as fastness properties, it could be interesting to introduce covalent bonding between the functional layer and textile substrates in combination with nanoscale surface roughening.
Waterborne, fluorocarbon-free and water-repellent ink with polysiloxane micro-emulsion was formulated and inkjet printed on plain woven PET and PA textile substrates, in order to functionalize the textile with water repellency. The PET and PA have a WCA from < 90° to 140° after inkjet functionalization. The functional textile has a microstructure with polysiloxane wrapped around the filament which modified the surface property of the textiles. Furthermore, the functional surfaces show resistance to wash and abrasion. The 10-pass printed PET (PET × 10) preserved a WCA of 133° after 10 washes and a WCA of 140° after 20000 rubs. We found that the ink transportation in the textile structure as well as the ink–filament interaction plays a key role in the durability of the functional layer. The functional ink was transported faster and deeper in PET, and with stronger non-covalent ink–filament interaction with PET than with PA. Therefore, PET showed better wash fastness and abrasion resistance. The microscopic images showed that inkjet printing did not or had little impact on the tactile feeling of the textile, i.e., the hand feeling and softness of textile is well preserved. Inkjet printing demonstrated selective and efficient deposition of functional material on textiles, as a promising resource-efficient process in the applications of functional and smart textiles.
The authors are grateful for the support from CROSSTEXNET, Digifun project, KK Stiftelsen (The Knowledge Foundation) with Diarienummer: 20150040, TEKO (The Swedish Textile and Clothing Industries Association), SST Stiftelsen (The foundation for Swedish Textile Research), Myfab, Borås stad and Sparbanksstiftelsen Sjuhärad for enabling this research.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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