Abstract
Individual and societal expectations of apparel performance are surpassing the three key requirements of physical, psychological, and social protection. Users are increasingly envisaging integrated technologies to facilitate daily life and specialist applications. Electrical conductivity and sensor performance is an integral component of this and can be achieved by applying functionalising treatments. Of increasing interest are graphene-based treatments (i.e. reduced graphene oxide, graphene ink) applied to apparel fabrics based on high conductivity, strength, and light weight of the two-dimensional honey comb structure. Knits manufactured of natural fibres are most suited for apparel applications, especially those worn next to the skin. Research has focussed on functionalised cotton fabrics with less focus on wool fabrics. Exploration into knit constructions has been limited. Performance of fabrics manufactured from wool and cotton fibres are often linked to hydrophilicity. A pre-treatment may be required to reduce hydrophobicity caused by presence of oils or waxes remaining on finished wool or cotton fabrics respectively, or application of additional, or lack thereof, of processing treatments (e.g. chlorination, scouring, mercerisation).
Single jersey 100% merino wool and 100% cotton were selected as trial fabrics. Hydrophobic properties were determined using wettability tests (water absorbency time, contact angle, liquid absorptive capacity). A chemical pre-treatment was developed and optimised for concentration of the active ingredient and treatment time for each knit, potassium hydroxide for wool and for cotton, sodium hydroxide. One concentration and time was selected for knit constructed of wool (0.05mol/L, 15min) and cotton (2mol/L, 30min). Wettability increased based on decreased water absorbency time, contact angle; and increased liquid absorptive capacity and moisture regain with no evidence of degradation to the fabric, yarns, or fibres assumed from visual appearance (photographs, microscope images), surface chemistry based on infrared spectra, and fabric structural properties of mass, thickness, and stitch density.
Subsequently each knit was functionalised with one of two treatments, reduced graphene oxide stabilised with poly(sodium 4-styrenesulfonate) or graphene ink. Several functionalisation process variables were investigated where appropriate (e.g. prior pre-treatment, graphene concentration, treatment time, curing temperature, and curing time). The small-scale experiment involved investigation of deposits (photographs, microscopic images), conductivity, and durability to wash. The most critical parameters for the functionalisation treatments were identified, and levels selected that may confer desirable performance based on the aforementioned properties. Further examination was undertaken to optimise treatment time and curing temperature of reduced graphene oxide functionalisation on wool (8min, 24min; 110°C, 120°C) and cotton (20min, 40min; 70°C, 120°C). Graphene ink was applied with one optimum treatment time to the wool (16min) and cotton (40min). Properties investigated included visual evidence of graphene deposits based on photographs, optical microscope images, image analysis, scanning electron microscope, and colour coordinates (L*, a*, b*). Differences in surface chemistry were also identified based on infrared and Raman spectra. Electrical conductivity was calculated from measurements of electrical resistance. Changes to fabric structure were determined from measurements of mass, thickness, stitch density and performance properties included flexural rigidity, water absorbency time, contact angle, liquid absorptive capacity, regain, permeability to water vapour, permeability to air, and tactile acceptability. Durability to wash, abrasion, and storage were determined based on change or lack thereof in electrical conductivity and
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appearance (photographs, optical microscope, and colour coordinates) with exposure to sequential cycles.
Deposits of graphene treatments were assumed based on visual change from cream to dark grey and presence of nonfibrous deposits. Pixel intensity mean, minimum, and mode were lower compared to the non-treated knits. Maximum pixel intensity was similar for fabrics graphene treated and not. Fibre width and area of the region of interest were least affected by the graphene treatments. Lightness (L*) was smallest of fabrics graphene treated although a* and b* were not affected. Changes occurred to surface chemistry based on infrared and Raman spectral analysis indicative of the carbon composition of the graphene treatments. FTIR provided limited evidence with low merits for understanding deposits of the graphene treatments while Raman provided more definitive evidence of graphene presence.
Conductivity was conferred from graphene deposits with small effects on structural characteristics of the knits (mass, thickness, stitch density) and performance properties including stiffness, water absorbency time, liquid absorptive capacity, contact angle, moisture regain, permeability water vapour and to air, and tactile acceptability. Minimal changes to the fabric structure and performance properties suggest acceptability to be used in next-to-skin apparel. Durability to wash (20 cycles for reduced graphene oxide, 100 cycles for graphene ink), abrasion (50,000 for graphene ink), and storage (28 days for reduced graphene oxide, 365 days for graphene ink) were a challenge, where conductivity decreased, sometimes being lost from the fabrics functionalised with reduced graphene oxide. Similar trends occurred with both the wool and cotton knit following functionalisation with graphene treatments. Graphene ink functionalisation produced knits with superior functionality, especially in terms of electrical conductivity and effects of exposure to wash, abrasion, and storage. Graphene ink functionalised specimens were selected for further investigation of encapsulation to enhance performance, primarily increasing durability without compromising fabric structure or performance properties1.
Encapsulation of functionalised fabrics with substances impervious to external elements, that have potential to cause degradation of treatment deposits and electrical conductivity, can attenuate negative effects. Exposure to wash (100 cycles), abrasion (50,000 cycles), and storage (365 days) were considered critical, however, modification of fabric structure and performance caused by encapsulation can result in a product that is non-textile like, potentially undesirable for next-to-skin apparel. Three polydimethylsiloxane-based products were selected for encapsulating the two knits functionalised with graphene ink. Each encapsulant differed in effects on fabric structure and performance properties, as well as durability. The SYLGARDTM 184 Silicone Elastomer Kit resulted in a finished product that was most dissimilar to the original knits including changed visual properties (photographs, optical and scanning electron microscope images) and surface chemistry (infrared and Raman spectra); increased mass, thickness, stitch density, stiffness, water absorbency time, contact angle, regain, permeability to water vapour and to air; with decreased liquid absorptive capacity, and tactile acceptability. Superior durability to wash, abrasion, and storage was achieved in terms of maintenance of treatment deposits and electrical conductivity. Granger's® Clothing Repel and the polydimethylsiloxane polymer conferred durability to a lesser extent but maintained similar
1 Sourcing issues of reduced graphene oxide also restricted availability for further investigation. ii
fabric structural and performance properties to the original state of knits. Prioritisation for conferring durability or retaining conventional fabric structure and performance is required. For a small- confined patch, properties pertinent to next-to-skin performance may be compromised to ensure durability .
Sensor performance was based on measurable changes in conductivity with exposure to differences in air temperature and humidity (20°C, 65%RH to 30°C, 30%RH or 35°C, 23%RH), to wetting by immersion and droplet exposure, and to changing carbon dioxide air concentration. Each can provide information to support health of the user by providing measurements of garment microclimate temperature and humidity, wetting sourced from the body or external environment, and carbon dioxide concentration of breath or small confined environmental spaces. Conductivity of the wool knit reduced graphene oxide functionalised increased with increased temperature and decreased humidity, often not recovering to the initial conductivity. The same response was shown of graphene ink and encapsulated specimens except for those encapsulated with SYLGARDTM, where wool specimens had minimal change and cotton the opposite trend of decreased conductivity with exposure to increased temperature and decreased humidity. Exposure to wetting by full immersion and droplets evoked an increase in mass and conductivity of graphene ink functionalised and encapsulated wool and cotton knits. Conductivity decreased as the functionalised knits dried. After complete drying, conductivity was less than the initial measurement. Immersion resulted in greater change compared to droplet wetting. Small changes occurred for specimens encapsulated with SYLGARDTM. No change in conductivity of graphene ink functionalised wool or cotton knits arose with exposure to carbon dioxide. Thus, sensor performance was possible for moisture (vapour, liquid) but not carbon dioxide. Fluctuating carbon dioxide concentrations will not interfere with the moisture sensing function, ensuring validity in this respect.