Regular Article
Electrically conductive cotton fabric coatings developed by silica sol-gel precursors doped with surfactant-aided dispersion of vertically aligned carbon nanotubes fillers in organic solvent-free aqueous solution

https://doi.org/10.1016/j.jcis.2020.10.076Get rights and content

Highlights

  • Conductive coatings were prepared by using VACNT with DBSA in a sol-gel solution.

  • Sol-gel based VACNT cotton fabrics show long-lasting electroconductive properties.

  • Flexible humidity-sensitive cotton was fabricated by “knife-over-roll” technique.

  • Coated conductive layers showed an excellent adhesion onto cellulose fabrics.

  • A prototype of a textile sensor with switch point from 50% to 70% RH was realized.

Abstract

Hypothesis

From the end of the twentieth century, the growing interest in a new generation of wearable electronics with attractive application for military, medical and smart textiles fields has led to a wide investigation of chemical finishes for the production of electronic textiles (e-textiles).

Experiments

Herein, a novel method to turn insulating cotton fabrics in electrically conductive by the deposition of three-dimensional hierarchical vertically aligned carbon nanotubes (VACNT) is proposed. Two VACNT samples with different length were synthesized and then dispersed in 4-dodecylbenzenesulfonic acid combined with silica-based sol-gel precursors. The dispersed VACNT were separately compounded with a polyurethane thickener to obtain homogeneous spreadable pastes, finally coated onto cotton surfaces by the “knife-over-roll” technique.

Findings

Shorter VACNT-based composite showed the best electrical conductivity, with a sheet resistance value less than 4.0 · 104 ± 6.7 · 103 Ω/sq. As demonstrated, developed e-textiles are suitable for application as humidity sensing materials in wearable smart textiles by exhibiting adequate response time for end-users and repeatability at several exposure cycles, still maintaining excellent flexibility. The proposed environmentally-friendly and cost-effective method can be easily widened to the scalable production of CNT-containing conductive flexible coatings, providing additional support to the development of real integration between electronics and textiles.

Introduction

Recently, due to the growth of market requirement, significant progress in the textile industry has been achieved towards the development of technical and high-performance materials. Indeed, compared to other materials, textiles show several intrinsic benefits, such as flexibility and softness as well as lightness and strength that allow them to be easily accommodated to a large range of end-use demands. Consequently, many chemicals have been applied to improve the performance of textile goods [1], [2], [3], [4], [5], using different techniques [6], [7], [8], [9], [10], [11]. Thus, in this scenario, from the end of the twentieth century, chemical finishes for producing electronic fabrics (e-textiles) [12], [13] have been extensively investigated to develop the next generation of wearable electronics, with attractive application possibilities for smart textiles with built-in electronic functions, portable military equipment, and medical monitoring devices or implants [14], [15]. Since textiles are dielectric materials, different approaches have been made to overcome the difficulties of balancing the processing technologies, the mechanical properties, and the conductive characteristics of materials, opening up a new field of scientific research [16], [17], [18]. Conductive properties of textile fabrics have been reported in the literature by the insertion of electrically conductive carbon or metal fibers [19], through threads or fabrics metallization [20], [21], or using transparent organic metal oxides, metal particles, carbon as well as intrinsically conductive polymers (ICPs) [22] with a varying degree of success. Experimental results confirmed that applications which involve the electromagnetic interference (EMI) shielding [23], [24], heating textiles, and the transport of electrical data show relatively low resistance levels (less than 103 Ω/sq). On the other side, materials involved in the development of electrostatic dissipative protective clothing show resistance values of around 109 Ω/sq [25]. In the last years, in view of the development of conductive textiles, many reports have shown integration of functional nanomaterials with textile fabrics. Carbon nanotubes (CNTs) seem to be up-and-coming nanomaterials for developing textile-based wearable devices thanks to their unique electrical and mechanical properties [26]. CNTs are allotropes of carbon with a cylindrical structure, and their walls are graphite-like sheets rolled-up on themselves, characterized by strong covalent sp2 bonds. Moreover, better thermal conductivity compared to other materials, except for pure diamond and graphene [27], as well as the ability to transmit higher currents than copper, despite similar electrical conductivity properties, are characteristics that make CNTs very attractive fillers to develop innovative composites. Indeed, the interaction between carbon nanotubes, featured by van der Waals attraction energy of about 500 eV/μm [28], besides promoting the typical tendency to form aggregates, inspires their applications in several fields, such as reinforced coatings, conductive polymers, or electronic components [29]. Examples of CNTs deposited onto textile fabrics through dipping-drying and spinning process [30], [31] as well as by incorporation into coatings [4], [32] have been widely described in other research studies. To improve CNT interfacial adhesion and dispersion into solvents or polymers, the surface functionalization of nanotubes was proposed as a common solution [4]. Thanks to a large number of sp2‐hybridized CNT frameworks, various functional groups can be introduced particularly into the multi-walled structure, reducing the size of CNT bundle aggregates [33]. In this perspective, CNTs could be incorporated in a sol-gel matrix, as inspired by Berguiga et al. [34]. Recently, the sol-gel technique has been intensively investigated for textile finishing because of its tunable properties and very low health hazards compared to conventional systems [35]. Usually, metal-organic compounds or inorganic metal salts are used as starting materials able to promote a phase transition from a liquid “sol” (mostly colloidal) to a solid “gel” [36], [37], [38]. Moreover, this technique has been extensively investigated for innovative textile applications such as flame retardancy [39], self-cleaning [40], water repellency [41], sensing properties [42], and hydrogen production by water photo splitting [43]. Unfortunately, some disadvantages, such as the strong attractive force between CNTs (van der Waals), the weak interaction between CNTs and a silica network, and the high viscosity of the colloid solution, lead to inhomogeneous structures and make it challenging to avoid CNT agglomeration in the sol-gel based network. Furthermore, the CNT dimensions influence their uniform dispersion since the attractive force between aggregates increases proportionally with the surface area of the CNT [44]. Therefore, reduction of self-assembled bundles and ropes to individual CNTs in a matrix is a difficult task to plan their use utilization in textile finishing. To the best of our knowledge, research on dispersing non-functionalized CNTs in a solvent-free and water-borne sol-gel matrix for textile finishing have not been extensively carried out. In our previous research [32], the possibility to realize an homogeneous distribution of CNTs in a conductive coating applied onto cotton fabric was demonstrated, simultaneously using EDAES (N'-(3-triethoxysilylpropyl)ethane-1,2-diamine) and a thermo-degradable surfactant, without affecting CTTS (charge-transfer-to-solvent) electronic structure and properties. This study complements the previous investigation about millimetre-long non-functionalized carbon nanotubes by studying their dispersion and deposition onto cotton fabrics by a sol-gel based aqueous system, able to preserve their intrinsic properties. Indeed, this method regards the adsorption of surfactant molecules onto CNT surface (according to π–π stacking interaction or coulomb attraction) in a non-aggressive way [45], thus maintaining the π-electron moiety of nanoparticles and, consequently, their electrical properties [44], [46]. Since CNTs result negatively charged in aqueous media [47], Coulombic attractions between the solid surface (negatively charged) and the head group of surfactants (positively charged) are established, which represent the driving force for the adsorption of surfactants onto the nanotube surfaces. Consequently, the choice of surfactant is very critical due to its nature, its type of interaction with nanoparticles and its concentration too. Among many surfactants which are able to enhance CNT dispersion [48], [49], [50], 4-dodecylbenzenesulfonic acid (DBSA) was selected for this research since it possesses a molecular structure featured by a 12-carbon alkyl chain (hydrophobic segment), a benzene ring and a charged head group (hydrophilic segment), SO3-. The use of DBSA has allowed common organic solvents, such as acetonitrile, dichloromethane, DMF, hexan-1-ol, oxolane, and toluene to be avoided. Even if they are effective in dispersing CNT, most of them are toxic or poorly acceptable in textile processing due to their chemical-physical properties. To finalize a real, very cost-effectively feasible and simple application method for large scale realization of e-textiles, the DBSA-aided CNT-containing colloidal sol-gel solutions were combined with a suitable thickener agent to obtain viscous electro-conductive pastes, which were found to be useful in applying on cotton fabrics by knife-over-roll technique. Only few papers are reported in the literature regarding the treatment of textile materials with CNTs by conventional finishing techniques [51]. This emphasizes the challenging need of developing wearable electronics in a real system application. In this research, a polyurethane thickener was selected because of its versatility and unique properties for textile applications [52], such as flexibility, abrasion resistance, high tear strength and elasticity [53], [54]. Untreated and treated textiles were characterized by Fourier Transform infrared spectroscopy in Attenuated Total Reflection mode (ATR-FTIR) and Scanning Electron Microscopy (SEM), as well as by electrical resistance measurements.

Finally, developed conductive cellulose-based fabrics were studied to demonstrate the prospect for achievable performance in the field of wearable and flexible electronic systems. For this purpose, since the significant importance of the humidity sensors in different application fields (e.g. agriculture, medicine, automated systems, as well as climatology) [55], treated textiles have been investigated as sensors for environmental humidity detection. The numerous application fields have led to the study of different kind of humidity sensors based on the combination of physical, chemical and electronic properties with the aim of enhancing the sensing properties. However, compared to the mentioned sensors, the proposed one features the flexibility and low weight derived from cotton substrates, which make the designed fabric attractive over those fabricated in rigid materials, including plastic or metal.

Section snippets

Materials

Two types of vertically aligned carbon nanotubes (VACNT) with high aspect ratio were synthesized according to previously reported procedures [32] and here named as A-CNT and B-CNT. In this study, a bleached 100% plain-woven cotton fabric (areal density = 331 g/m2), kindly given by Mascioni S.p.A. (Cuvio, Italy) was used for all experiments. In order to clean it from impurities, cotton was laundered with a detergent (non-ionic) for 20 min by settling the temperature and the pH values of the

VACNT morphological characterization

According to HRTEM images (Fig. 4), it was possible to establish the morphology of both A-CNT and B-CNT that reveals their multi-wall (MW) structure. Indeed, from 2 to 4 walls can be observed for both CNT samples as the result of the two growth durations (30 min and 60 min, respectively). As expected from the longer growth duration, the higher outside diameter was measured for B-CNT (10 nm) compared to A-CNT (7 nm). In addition, HRTEM images reveal a high degree of crystallinity and lengths

Conclusion

In this paper, CNT-doped coatings were easily realized using the sol-gel technique for the development of textile humidity sensors. VACNT at high aspect ratio were synthesized using CVD and, by light ultrasonic treatment for a contained local share, dispersed in a sol-gel composite. The designed method has been shown to efficiently disperse carbon nanotubes in an aqueous-based paste for textile finishing. It could be a promising alternative to long-time or extreme sonication conditions [4], [75]

CRediT authorship contribution statement

Valentina Trovato: Investigation, Visualization, Writing - original draft. Eti Teblum: Methodology, Validation. Yulia Kostikov: Methodology, Validation. Andrea Pedrana: Methodology, Validation. Valerio Re: Investigation, Validation, Writing - original draft. Gilbert Daniel Nessim: Writing - review & editing, Supervision. Giuseppe Rosace: Writing - review & editing, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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