Water-free dewaxing of grey cotton fabric using supercritical carbon dioxide

Highlights

• Co-solvent and surfactant in scCO₂ aided elimination of impurities in grey cotton.

• Pectin was removed in scCO₂ containing methanol or tetrahydrofuran.

• Dewaxing dominantly improved the water wettability of grey cotton fabric.

• Water wettability was more compared to alkali water scoring, with less fabric loss.

Abstract

Grey cotton shows low water wettability owing to contamination of the fibre surface with hydrophobic non-cellulosic impurities, such as waxes. This study investigated the efficiency of supercritical carbon dioxide (scCO₂) water-free treatment for dewaxing grey cotton pile fabric to improve its hydrophilicity. The wettability of the virgin grey fabric was initially 14,400 s, and this value increased to 10,800 s upon treatment in pure scCO₂ compared to 1200 in scCO₂ containing acetone. The addition of polyoxyethylene lauryl ether to scCO₂ containing acetone dramatically enhanced the water wettability to 6 s. Fourier-transform infrared (FT-IR) analysis of the change in the surface chemical composition of the fabric suggested the removal of wax by treatment in scCO₂ containing acetone and polyoxyethylene lauryl ether. The study revealed that scCO₂ is an alternative waterless method for improving the wettability of grey cotton fabrics.

Introduction

Cotton is a highly acclaimed natural fibre. It consists of pure cellulose (88–96.5%) and other non-cellulosic impurities such as wax, pectin, hemicellulose, and protein [1], [2]. Wax, which comprises ~0.4–1% of cotton, mainly exists in the outermost layer of grey cotton fibres [3], [4] and is the material with the greatest influence on the hydrophilicity of cotton. A process called scouring is generally performed on grey cotton fibres to remove natural hydrophobic impurities and increase the hydrophilicity in manufacturing [5]. Conventional scouring, in which alkali and water are used, suffers from environmental problems such as wastewater generation, usage of harmful chemicals, and excessive time and energy consumption. Water scoring also causes fabric shrinkage, that is, a change in the physio-mechanical properties of fabric with a certain strength loss [6]. A great deal of research has been undertaken on the application of environmentally friendly technologies with lower waste potential [7].

Supercritical fluids (SCFs) are unconventional media with unique physicochemical properties that have driven their use in both physical and chemical processing [8]. Recently, SCFs, particularly carbon dioxide (scCO₂), have been extensively applied in extraction, separation, and purification in the food and drug industry. SCF technology is also expanding to the areas of petrochemicals and textiles [9], [10], [11], [12], [13], [14]. scCO₂ behaves as a non-polar solvent; thus, the use of pure scCO₂ is less effective for extracting polar solutes and non-polar solutes with high molecular weights [15]. This limitation can be overcome by adding co-solvents, which are usually polar organic solvents such as short-chain alcohols, esters, and ketones [12], [14], [16], [17], [18], [19], to enhance the solubility of scCO₂. Ethanol is a potential co-solvent because of its low toxicity and miscibility with scCO₂ [15], [20].

Surfactants play an important role in SCF technology [16]. The weak intermolecular interactions between CO₂ and solutes that limit the scCO₂ process can be controlled by adding surfactants. However, very few commercial surfactants such as fluorocarbon compounds dissolve in scCO₂ [21]. Thus, extensive efforts have been made to reduce fluorocarbon use owing to the high cost and environmental issues associated with these compounds [22]. Promising routes employing media based on micelles or emulsions in the scCO₂ phase have been investigated for extraction, reaction, and particle design [15], [22], [23], [24], [25]. Consan et al. [26] reported that non-ionic surfactants, such as polyoxyethylene lauryl ether, showed low solubility in scCO₂. The removal of a wide variety of contaminants via scCO₂ elimination has been successfully achieved with additives such as co-solvents, surfactants, dispersants, and chelating agents [16].

ScCO₂ water-free technology in the textile sector is currently of particular interest for achieving cleaner and more sustainable textile processing. The use of scCO₂ generates no wastewater because it is a waterless technology, and consequently requires no drying treatment of fabrics [27]. Previous studies on scCO₂ proved that it is a promising medium for many fabric treatment processes such as dyeing and finishing. This includes pre-treatments such as sizing, desizing, and bleaching of cotton in an eco-friendly frame, as well as the degumming of silk fibres and scouring of polyester, nylon, flax, and wool [28], [29], [30], [31], [32], [33]. Shi et al. and Liu et al. successfully carried out a bio-pretreatment including bio-desizing and bio-scouring of cotton fibres with enzymes in scCO₂ [23], [34]. However, the scouring/dewaxing of grey cotton fabric via extraction of wax and other hydrophobic impurities using scCO₂ medium has not been extensively studied.

The aim of the present study is to provide an applicable method for scouring grey cotton fabric by extracting hydrophobic impurities such as wax and pectin using waterless scCO₂ medium, thereby conferring high water-absorbency to the fabric.