Polyethylene terephthalate (PET), one of the most widely used polymers in the global textile industry [1], is a semi-crystalline thermoplastic polymer made from the condensation of terephthalic acid and ethylene glycol to produce polyester materials with excellent tensile strength, chemical resistance and high thermal stability. However, their high hydrophobicity and limited flexibility lead to drawbacks in processing and wearing comfort. Hydrophilic modification of PET fabric is essential in order to increase their added value and improve wearing comfort.
Strong alkaline solutions are currently used to improve the hydrophilicity of PET fabric. However, alkaline hydrolysis treatment leads to severe weight loss of the fibres. Surface modification of polyester fibres can also be used to enhance the hydrophilicity of PET fabric through techniques such as surface coating or the use of cross-linking agents to attach hydrophilic groups to the fibre surface, as well as surface oxidation, and surface grafting modifications including chemically-initiated grafting modification and radiation grafting modification [2–6]. However, these methods require the use of harsh chemicals or high energy inputs, and the treatment process is not environmentally friendly and cumbersome. The modification effect is time-sensitive and affects the mechanical properties of fabrics.
In the last decade, biotechnological interest in biodegradation and modification of polyester is playing a key role. Since PET is a solid and its composition is hydrophobic, PET enzymatic hydrolysis has been described as a non-homogeneous reaction system. In this reaction system, the biocatalyst catalyses insoluble PET in an aqueous environment, which contains crystalline and amorphous regions, the latter being the more accessible region. Thus, PET fabric erosion may be similar to cellulose degradation. This phenomenon involves chain breakage and loosening, making individual substrate molecules more accessible and available for interaction with enzymes [7]. Through the breaking of ester bonds, new free hydroxyl and carboxyl groups are generated on the polymer surface, which leads to an increase in the hydrophilicity of PET fabric [8]. According to Kawai et al [9], hydrolytic enzymes were used to attack the polymer surface, thereby increasing its surface hydrophilicity with little effect on the polymer morphology. A US patent has reported the use of enzymes to treat a composite polyester fibre, which is a composite of an aromatic polyvinyl acetate inner core and an aliphatic polycool outer layer. When treated with enzyme, the outer layer of aliphatic polycool can be easily degraded, thus forming a kind of fibre with good appearance and feel and excellent performance. The above study lays a theoretical foundation for the enzyme treatment or modification of polyester in the future.
Esterases, lipases, and cutinases have been reported to act as biocatalysts for hydrolyzing polyesters [10, 11]. The open, surface-exposed, active site of cutinases makes them more suitable than lipases for applications such as textile bioprocessing, polyester hydrolysis, and plastic degradation [12].
Unfortunately, PET used for packaging has a low crystallinity of about 8%. However, most of the PET used to make bottles and textiles has a high crystallinity of 30–40% [13]. Enzymatic hydrolysis is improving for amorphous PET films (mainly packaging materials), while establishing enzymatic hydrolysis for highly crystalline and bottle-grade PET materials remains challenging. Compared to chemical method, the modification of PET fabric using cutinase still suffers from the drawbacks of a lengthy modification process, limited hydrolysis yield, and inadequate modification effectiveness. In addition to the high crystallinity of PET major challenges include enzyme activity, thermal stability, and and substrate accessibilityof cutinase [14]. Therefore, we need to find a green and efficient method to enhance enzymatic hydrolysis of high crystallinity PET.
Deep eutectic solvents (DES) are first reported by Abbott et al. in 2003 [15]. DES are primarily eutectic mixtures formed by combining and heating hydrogen bond donors (HBD) and hydrogen bond acceptors (HBA) at specific molar ratios. The most commonly used HBA are primarily quaternary ammonium salts (e.g., choline chloride), amino acids, or amphoteric ions (e.g., betaines). The conventioanlly applied HBD include urea, sugars, carboxylic acids (organic acids), and polyols [16–19]. It not only has similar properties to ionic liquids but also offers the advantages of easy production, non-toxicity, good biocompatibility, low cost, and biodegradability [20].
In recent years, DES, with the environment friendly nature, have been investigated as a potential medium that can create an optimal natural environment for enzymes. They can activate or inhibit enzymes depending on the composition and concentration of the monomer [21, 22]. Tan et al. reported the first instance of small molecule activation of cutinase, with a low concentration (10% v/v) of DES (choline chloride: glycerol; 1:1), causing a significant activation (more than 2-fold) of AmCut [23]. In addition, DES can also help to maintain protein activity under extreme substrate conditions. Angela et al. investigated the thermal stability of HiC in the presence of DES. Xyl and ChDHC were selected as the optimal hydrogen bond donor and acceptor molecules, and the effects of various ratios and concentrations wereexplored. The results indicated that the enzyme's half-life at 90°C tripled when 20% ChDHC:Xyl (1:1) was utilized [24].
To address the issues of poor efficiency and lengthy reaction time in the current technology for enzyme-catalyzed hydrolysis, we construct a new hydrolysis system by combining HiC with several deep eutectic solvents for PET hydrolysis. The impact of DES type, molar ratio, and concentration on enzymatic hydrolysis were investigated. The results showed that a significant increase of HiC-catalyzed hydrolysis yield was observed in the presence of 20% v/v cholinium DES (betaine:sorbitol; 1:2 molar ratio). Additionally, the surface morphology and chemical elements of the modified fabrics were characterized using field emission and XPS. The impact of DES on the enzyme structure was analyzed using circular dichroism and fluorescence spectroscopy. This study aims not only to develop an efficient and sustainable method to improve HiC hydrolysis of PET, but also open up new opportunity for comprehensive utilization of DES in textile area.