# Coaxial Fibres Incorporated with Phase Change Materials for Thermoregulation Applications
## 1 Introduction
[25] explained that the wet spinning technique is a widely used commercial process that allows the manufacture of mono- or multi-structured fibres using polymers dissolved in solvents that are extruded through small holes, forming continuous filaments in a liquid medium (coagulation bath), i.e., as the extruded filaments pass through the coagulation bath, the polymer solidifies, resulting in the formation of fibres with a uniform morphology. According to [PERSON] et al. [26], the wet spinning technique was first used to produce rayon fibres and is considered the oldest fibre spinning process. Due to its simple configuration, the wet spinning technique [27; 28; 29] has recently attracted attention as an alternative encapsulation method that allows for the stable incorporation of PCMs. The main advantage of this technique is that it does not involve heat during production, reducing the risk of possible thermal degradation. Another point is that it can produce fibres with different diameters, shapes and cross-sectional sizes. It is also a continuous process and can produce any type of polymer, which makes it a very efficient [26]. Through wet coaxial spinning, hollow or single-core fibres can be produced for different purposes, such as electronic textiles, sensors and controlled release applications [30]. Phase change fibres (PCFs) are considered intelligent materials because, in addition to containing PCMs, PCFs react to environmental changes without any human action required. On their own, PCFs recognize changes and work to maintain the standard for which they were programmed [31]. The reproducibility and the performance of coaxial fibres will depend on the rigorous selection and maintenance of wet spinning parameters [26]. Among the different existing parameters, it is necessary to take into account the evaluation of the process parameters, from solution to production, in order to obtain fibres with a well-defined core and sheath to maximise the performance of the coaxial fibres in the final application [32]. Although several studies are exploring potential applications to produce PCFs, there needs to be more research in the literature on this subject in order to broaden the range of innovation. In a study carried out by [PERSON] et al. [33] where PCFs were developed using the electrospinning technique, containing cellulose acetate (CA) as the support material and polyethylene glycol (PEG) as the PCM, it was observed that the thermal properties of PEG were different to those of virgin PEG due to the presence of CA. [PERSON] et al. [34] studied the production of fibres, via wet spinning, consisting of CA/PEG in the same solution. As a result, they observed that the viscosity of the solutions and the dH\({}_{2}\)O used as a coagulation bath induced a more solid structure in the fibres. The addition of PEG to the CA solution considerably increased the mechanical properties of the fibres. To the best of the authors' knowledge, this is one of the first research works detailing the manufacturing process of wet spinning using this specific structure of PCF (coaxial fibres with CA and specially recycled CA from cotton fabrics (CAt) in the protective sheath and PEG in the core). Therefore, different manufacturing parameters were studied: (i) CA concentration and molecular weight; (ii) CA source (conventional versus recycled); and (iii) PEG concentration, molecular weight, and ejection. PCFs were analysed for their morphological, chemical, thermal, and mechanical properties. Morphological characteristics were examined using both bright-field microscopy and scanning electron microscopy (SEM). Chemical properties were assessed through Fourier transform infrared spectroscopy (ATR-FTIR), while thermal properties were evaluated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Mechanical properties were tested with a dynamometer. The PCFs were then compared to their analogous uniaxial (Fu) and hollow (Fh) fibres.