Scalable graphene-based nanocomposite coatings for flexible and washable conductive textiles

Abstract

Graphene has recently become one of the preferred conductive materials for smart textiles. The most widely employed strategy to integrate graphene into textiles consists of coating the surface of the target substrate. However, this approach presents limitations related to the preparation of the coating material and its performance during work. Herein we introduce a new concept for the coating of conductive carbon-based materials on textiles by using a nanocomposite ink composed of graphene and an elastomer, in a low boiling point solvent. It allows coating large areas of textiles in a short time and the covering layer presents good elasticity making it durable under strong mechanical deformation. In addition, the apolar nature of the coating confers washing resistance to the textiles. Furthermore, the approach is broad in scope as the coating behaves similarly on both synthetic (Nylon, polyester, acrylic) or natural (cotton, cellulose) textiles.

Introduction

Smart textiles are fabricated by the incorporation of functional components into conventional textiles providing them the ability to monitor, for example, mechanical, electrical, thermal, optical, or magnetic stimuli [1,2]. Of these functionalities, electrical conductivity is especially important because it permits the incorporation of electronic or energy storage devices directly on the textile [1,3]. Metal coatings have been preferred for many years as they provide excellent electrical conductivity and low contact resistance [4,5]. However, they also present some drawbacks such as limited mechanical compliance to stress [1], toxicity especially when the fabric is in contact with the skin, high weight that affects comfort and complicated or expensive techniques are usually required for adequate coating with metals. Conducting polymers (CP) have also been proposed as conductive elements in smart textiles due to their high solution processability, light weight, and closer mechanical properties to textile polymers than metals [[6], [7], [8], [9]]. However, CPs also present limitations, such as lower charge-transport than metals, instability under ambient conditions and degradation with usage due to de-doping [1,4,5].

Carbon-based materials emerge as ideal candidates for conductive coatings as they can combine electrical properties with lightness, non or limited toxicity and chemical inertness. In particular, graphene, a monoatomic carbon layer that presents excellent electrical and thermal conductivity as well as high mechanical strength and some flexibility, has become one of the most promising candidates for electronic components in conductive textiles, adding insignificant weight increase, and maintaining performance under mechanical perturbations [[10], [11], [12]]. The preferred method to incorporate graphene into textiles consists of the impregnation or coating of the surface of commercial fibres or textiles with graphene or its derivatives from appropriate dispersions or inks [3,[13], [14], [15], [16]]. This route is essentially simple and easy to implement, involving low-temperature processes, is economically more feasible at an industrial scale, and it is based on technologies already employed in the industry. In this methodology, adhesion between graphene and the target textiles plays a key role in most of the applications. Furthermore, it has been proposed that the electronic properties of graphene can be affected by elastic deformation caused by adhesion of graphene to its substrate [17].

The mechanical properties are of paramount importance in this type of materials that combine conducting elements with soft polymers and the main issue here resides in the mismatch between the mechanical properties of the covering layer and the substrate [1]. This is much more important when the materials are submitted to mechanical deformation, i.e. twisting, folding, etc., as the high tensile strength of graphene results in its fracture at low strain. In other words, graphene is much stiffer than textile substrates and is much more sensitive to mechanical deformations than the textile substrate, producing cracks that lead to an important deterioration of the conductive properties and to short-term utility.

Regarding graphene sources, graphene oxide (GO) has been initially employed [10,14,18]. However, covering textiles with graphene oxide requires a posterior step of reduction to recover electrical conductivity, where harsh chemical or thermal treatments can lead to degradation, chemical modification and/or hydrolysis of the substrate, worsening the textiles properties. The use of previously reduced graphene oxide (rGO) dispersions has also been reported [19,20]. But rGO dispersions are less concentrated resulting in low conductivity and several coating cycles are required, which is time consuming and more expensive.

In order to avoid GO and rGO and to obtain stable and relatively concentrated dispersions, the use of inks of pristine graphene with stabilizing polymers has been described [[21], [22], [23]]. However, these stabilizers need to be thermally decomposed after printing/coating to achieve high conductivity, and this treatment may harm the textile substrates.

In summary, the methods used to cover textiles with graphene present two main limitations related to the preparation methods employed, especially related with the graphene source, and to the performance during work. In this manuscript we report on the development of a nanocomposite conductive ink composed of graphene and an elastomer, poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS) to produce mechanically stable and washable coatings. Here, the polymer is not removed after coating, but is maintained so that it fulfils a key function during use: that of conferring flexibility to the conductive coating. In this system the modulus of graphene dispersed into the elastomer is expected to be smaller and more similar to that of the substrate, thus avoiding crack formation, or substantially delaying its appearance[24]. Our results show that electrical conductivity of the coated textiles is maintained for more than one thousand folding cycles. In addition, the hydrophobic nature of the coating makes them stable to several washing cycles. This behaviour is independent of the nature of the textile substrate, whether synthetic (nylon, polyester, acrylic) or natural (cotton, cellulose) fabrics, demonstrating a broad applicability of the proposed strategy.