Exploring the Cutting Process of Coaxial Phase Change Fibers under Optical Characterization Tests

Abstract

Urban heat islands (UHI) are a growing issue due to urbanization, causing citizens to suffer from the inadequate thermal properties of building materials. Therefore, the need for climate-resistant infrastructure is crucial for quality of life. Phase change materials (PCMs) offer a solution by being incorporated into construction materials for thermoregulation. PCMs store and release heat as latent heat, adjusting temperatures through phase changes. Polymeric phase change fibers (PCFs) are an innovative technology for encapsulating PCMs and preventing leaks. This study produced PCFs via wet-spinning, using commercial cellulose acetate (CA, Mn 50,000) as the sheath and polyethylene glycol (PEG 2000) as the core. The PCFs were cut using a hot-cutting method at three different temperatures and washed with distilled water. Morphological analysis was conducted with a bright-field microscope, and chemical analysis was performed using Fourier transform infrared spectroscopy (FTIR) before and after controlled washing. Additionally, the washing baths were analyzed by UV-visible spectroscopy to detect PEG. The PCFs displayed a well-defined core-shell structure. Although some PEG 2000 leakage occurred in unsuccessful cuts, cuts at 50 °C showed sealed ends and less material in the baths, making it viable for civil engineering materials.

1. Introduction

The need for infrastructures that are resilient to climate change has become a growing challenge for improving the quality of urban life [1]. Due to global warming, the demand for innovative thermal regulation strategies is increasing, as urban heat islands (UHI) [2] and the lack of thermal comfort represent significant challenges as urban cities expand [3]. The heating of asphalt pavements contributes to the formation of UHI, resulting in substantial adverse impacts on the quality of health and life of citizens, thus requiring effective approaches to mitigate temperature extremes [4]. It is, therefore, necessary for the scientific and academic community to aim to develop sustainable and innovative solutions (Figure 1).

Amongst the different promising solutions, latent heat thermal energy storage materials (LHTS) [5], commonly called phase change materials (PCMs), stand out among researchers since PCMs take advantage of their latent heat capacity to regulate the temperatures of civil engineering materials [6].

PCM can absorb heat during the hottest hours and release it at night, thus contributing to the thermal regulation of urban environments and improving thermal comfort [7,8]. Wang et al. highlighted that this technology is especially relevant for applications in asphalt pavements, for example, to mitigate the phenomenon of UHI [9]. In contrast, Wang et al. discussed a crucial issue regarding the incorporation of PCMs into civil engineering materials that using traditional methods without any encapsulation method can face negative aspects such as leakage and instability [10].

In response to the various challenges faced by traditional methods with the direct incorporation of PCM [11], the production of co-axial polymeric phase change fibers (PCFs) [12] using the wet-spinning technique [13,14] emerges as an innovative solution to prevent leakage (Figure 2). Li et al. [15] studied the production of these co-axial fibers, also known as thermoregulatory fibers or PCF, where they have a well-formed sheath-core structure designed to effectively encapsulate PCM in building materials, storing thermal energy during the phase change process and consequently controlling the temperature.

A crucial step after producing PCFs is how to cut and seal these fibers so that the PCMs, which are incorporated into the core, do not suffer from leakage and loss of PCF thermal properties. This issue is scarce in the literature and requires attention from researchers, as it is a process that depends on the appropriate cutting procedure and materials to guarantee good protection and performance of PCFs applied in engineering. Cutting Techniques using laser [16], knife [17] and hot cutting [18] are covered in the literature. However, the researchers do not detail the processes nor present leakage results. Law et al. [18] studied the latter process, called hot cutting, which makes it possible to obtain a higher-performance bond, where the cut is more precise without compromising the integrity of the fiber. In addition, this process incorporates innovative mechanisms that save material and reduce costs. However, it should be noted that the literature does little to address the main limitations and complexities associated with cutting and sealing PCFs. These include concerns about structural integrity, the possible leakage of material from the core with risks of evaporation, material compatibility and impacts on mechanical properties [12]. A major challenge is to check for possible leakage of PCMs from inside PCFs [19]. Laboratory tests such as Fourier transform infrared spectroscopy (FTIR) by calculating the area index of the functional groups, UV-visible spectroscopy and scanning electron microscopy (SEM) are highlighted in the literature to assess PCM leakage.

To the best of our knowledge, this is one of the first studies to investigate in detail the leakage of a phase change material incorporated into the core of a specific PCF structure using the cutting and heat-sealing process. In this work, PCFs containing cellulose acetate (CA) in the sheath and polyethylene glycol (PEG 2000) in the core were cut and heat-sealed. The evaluation of these cuts, seals and possible leakage of the core material was performed through optical and chemical tests. It is worth mentioning that this process is simple, low cost and suitable for applications in Civil Engineering that require large quantities of cut PCFs.

2. Materials and Methods

2.1. Materials

The materials used for this work were (i) commercial cellulose acetate powder (CA, Sig-ma-Aldrich, St. Louis, MO, USA), average Mn = 50,000, with an acetyl content of 39.8 wt.%, (ii) N,N-Dimethylformamide (DMF, 99.8%, Sigma-Aldrich, St. Louis, MO, USA), (iii) Polyethylene glycol 2000 H(OCH2CH2)nOH, MP = 53–55 °C, (PEG 2000, Thermo Fisher Scientific, Waltham, MA, USA) and (iv) deionized water (dH₂O).

CA, derived from cellulose and cellulose acetate ester, was chosen as the material for the protective sheath of co-axial fibers as it presents non-toxic and natural characteristics [20]. PEG, among the different PCMs found in the literature and market, was selected as the material for the core of co-axial fibers because this is one of the most used PCMs for Civil Engineering applications and presents chemical stability, high storage capacity and non-toxic characteristics [4].

2.2. Preparation of Solutions

The solutions of CA (10 wt.%) and PEG 2000 a (50 wt.%) were dissolved overnight in DMF and dH₂O, respectively, both stirred continuously at 50 °C using two magnetic stirring plates (Velp Scientifica, New York, NY, USA) (Figure 3). Before starting the production of PCFs, the respective solutions were left to stand at room temperature (RT) for 1 h to remove any air bubbles.

2.3. Fibers Production

For the PCFs production, the configuration of the wet-spinning system consisted of a NE-1000 syringe pump (New Era Pump Systems Inc., Farmingdale, NY 11735, USA), a dH₂O coagulation bath at a temperature of 20 °C, an automated collector and a co-axial needle with 11 a 21 G of internal and external, respectively (with two syringes connected to the needle and syringe pumps). Figure 4 shows a representation of the wet-spinning process used. To facilitate the removal of the PCFs from the automated collector, an aluminum foil was wrapped around this collector, and a moderate velocity of 5 rpm was set. The co-axial needle was submerged 5 cm inside the coagulation bath, and the distance between the automated collector and the co-axial needle was 1.00 m [12]. The ejection velocity for CA was 0.165 mL/min, while for PEG 2000, it varied from 0.120, 0.130, 0.140, to 0.150 mL/min [12].

2.4. Nomenclature of the Samples

The PCF samples were named using the following generic string: PCF_a/x_y. The letter a indicates the molecular weight of the CA (Mn 50,000); the letter x indicates the PEG 2000 concentration (50 wt.%); and finally, the letter y represents the PEG 2000 ejection velocities (0.120, 0.130, 0.140, 0.150 mL/min).

If we represent an example in PCF_50/50_120, i.e., PCF stands for phase change fiber; the letter a is 50, indicating the molecular weight of CA (Mn 50,000); the letter x is 50, representing a concentration of 50% PEG 2000; and finally, the letter y is 120, indicating the ejection velocity of 0.120 mL/min of PEG 2000.

2.5. Hot Cutting and Controlled Washing

Once produced, the PCFs were cut and heat-sealed into 10 cm sizes. The respective cut and sealed parts were subjected to a controlled wash for around 15 min; afterward, the wash liquid was analyzed by the controlled release test in the spectrophotometer [21] to identify possible PEG leaks resulting from unsuccessful cuts.

A Corning PC-420D digital heating plate (Corning, NY, USA) and a spatula heated to three different temperatures (room temperature—RT, 50 °C, and 100 °C) were used to perform cutting and heat sealing (Figure 5). For the post-cutting phase, called controlled washing (Figure 6), the sealed pieces were placed in Beckers containing 25 mL of dH₂O for 15 min.

2.6. Bright-Field Microscope

The PCFs were characterized under their morphology after production to visualize a co-axial structure and after cutting to analyze the cutting sections using the Leica DM IL LED bright-field microscope (Leica Microsystems, Weetzlar, Germany). The images were taken using a magnification of 5× (Figure 7).

2.7. Fourier-Transform Infrared Spectroscopy (FTIR) and Index of Functional Groups

2.7.1. FTIR

The chemical Fourier transform infrared spectroscopy (ATR-FTIR) test was employed to evaluate the chemical composition of the PCFs before and after hot cutting, as well as to investigate structural changes of the PEG 2000, with a specific focus on the calculation of the functional group area index as a standard approach to estimate possible PEG leakage from the inside of the PCFs.

The chemical and surface compositions of PCFs pre and post-controlled washing were analyzed using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) IRAffinity-1S (Shimadzu, Kyoto, Japan), coupled to a HATR 10 accessory with a diamond crystal. The transmittance spectra were obtained in a wavenumber range of 400–4000 cm⁻¹, with a scanning velocity of 200 scans and a resolution of 2 cm⁻¹ (Figure 8). The chemical peaks were identified to check the presence of PEG, CA, and DMF in each PCF.

2.7.2. Index Calculation

To calculate the area index, it was necessary to perform (i) identification of FTIR peaks, (ii) establishment of a relationship between chemical groups of PCFs (pre and post-controlled washing) and (iii) comparison of indices calculated before and after controlled washing. Structural and functional indices are calculated from the areas of the valley-to-valley strip. The analysis focused on areas rather than band heights because vibrations of the same type can exist simultaneously (e.g., CA has a C-O-C stretching bond that extends up to 1030 cm⁻¹, while PEG also exhibits strong absorption bands for C-O-C stretching vibrations in the range 1000–1150 cm⁻¹) [12]. The use of the index calculation type with the band area was defined [22]. The chemical structure of the materials (CA, DMF and PEG) was analyzed using the FTIR index (I). Equations (1)–(3) present I, which is calculated by the ratio between the peak area of the identified band and the total spectrum area ($\sum A)$. Each peak is assigned to a functional group that remains unchanged during useful life, but also to groups responsible for the presence of polymeric materials.

$$I_{Aliphatic structures \left(PEG\right):}={\displaystyle \frac{A_{(1465,1368)}}{\sum A}}$$

(1)

$$I_{Carbonyl \left(CA\right):}={\displaystyle \frac{A_{\left(1745\right)}}{\sum A}}$$

(2)

$$I_{Carbonyl \left(DMF\right):}={\displaystyle \frac{A_{\left(1657\right)}}{\sum A}}$$

(3)

where,

$$\begin{array}{c}\sum A=A_{2885}+A_{1745}+A_{1657}+A_{1465}+A_{1376}+A_{1368}+A_{1328}+A_{1278}+\\ A_{1148}+A_{1105}+A_{1047}+A_{939}+A_{901}+A_{835}+A_{600}\end{array}$$

2.8. UV-Visible Spectroscopy

The optical UV-visible spectroscopy test was carried out on the baths of the controlled washes of the sealed PCFs for potential PEG detection. Precision in each of these stages, from the preparation of the cut to the final tests, is essential to guarantee the functionality, durability and excellence of the PCFs in their expected final applications.

After the cutting procedure (RT, 50 °C and 100 °C), the twelve cut and sealed PCFs were subjected to controlled washing for 15 min in beakers containing 25 mL of dH₂O. As the focus was on the UV domain, these baths were analyzed by UV-VIS spectroscopy to identify the presence of PEG 2000 leakage in this UV region.

The absorption measurements of the baths from the controlled washing of the PCFs in the presence of PEG 2000 were recorded using a UV-VIS spectrophotometer (Sarspec, Porto, Portugal) in the 190–700 nm range, slow measurement, with a wavelength resolution level in its class (1 nm) and with a 1.0 cm quartz cuvette [23] (Figure 9). Avitzky–Golay smoothing preprocessing functions (digital filter) were applied after obtaining the spectra.

3. Results

3.1. Morphology

3.1.1. Morphological Structure of PCFs

Figure 10 illustrates the morphologies of the four different PCFs produced, where the only parameter varied was the velocity at which the PEG 2000 was ejected into the core of the PCFs. Direct observation revealed that PCFs_50 have a flatter shape. This can be explained by the concentration of CA (10 wt.%; Mn 50,000) being lower and less viscous compared to a fiber produced with a higher concentration and viscosity; for example, PCFs produced with CA (30 wt.%; Mn 30,000) [12].

Another point to consider is that the core with PEG 2000 appears to be smaller when lower ejection velocities are used for PEG 2000, compared to higher ejection velocities. This may be correlated with the solubility of PEG in dH₂O and the insolubility of CA in dH₂O. In other words, the higher the PEG ejection velocity, the greater the amount of material ejected into the core per unit time (mm/min), which results in less time for the CA to form its protective sheath.

3.1.2. Hot Cutting Analysis

Figure 11 shows the sealed ends of the different PCFs. Three temperatures for hot cutting were tested (RT, 50 °C and 100 °C). When analyzing Figure 11, for the cuts made at RT, larger chips and imperfections are noted since the temperature for cutting and sealing was low and insufficient to seal the ends of the PCFs. At a temperature of 50 °C, a smaller amount of chips was observed, with the PCFs produced with average ejection velocities for PEG (Figure 11c,d) having better appearances. Finally, for the PCFs cut and sealed at 100 °C, chips were again observed for the lowest and highest PEG ejection velocity (Figure 11a,d), but with a better appearance than those cut at RT.

As mentioned previously, the only variable parameter between the four PCFs was the PEG ejection velocity, keeping the CA molecular weight the same to verify the reactions in hot cuts at different temperatures. CA has a glass transition temperature (Tg) between 95–130 °C [24]. Furthermore, the sealing temperature of the PCFs must be maintained below 230 °C, as thermal degradation of the CA may occur above this temperature [12,24]. Therefore, the sealing temperature must be kept slightly below this limit. Regarding the material incorporated into the PCF core, PEG 2000 has a phase change of around 53–55 °C [12]; when it is incorporated into PCFs, it can reach temperatures slightly above its phase change, thus affecting the properties of PCFs and the integrity of cutting and sealing.

An important point that must be taken into consideration in this and future analyses is that when PEG is ejected into the core of the PCFs, part of the material can bind to the protective sheath, and when cuts are made at temperatures above its temperature change, phase, this may harm the integrity of the cutting and sealing.

In summary, the ideal cutting and sealing temperature needs to be around 50 °C, that is, slightly below the PEG phase change temperature and heated enough to cut and seal the CA sheath, which is in accordance with the literature [15]. Therefore, sealing temperatures must be below the CA thermal degradation point. The procedure must be carried out uniformly across the entire thickness of the material to avoid imperfections or chips. However, it is important to highlight that the ideal temperature for cutting and sealing PCFs probably depends on the initial production formulation and thickness.

3.2. Chemical Test

The ATR-FTIR test was used to investigate the characteristic absorption bands of PCFs composed of a CA protective sheath and a PEG 2000 core (Figure 12). The chemical characterization of PCFs has received attention in the literature, as it can present functional groups related to molecular interactions within these PCFs [12].

The peak at 2885 cm⁻¹ is associated with the C–H stretching vibrations of the methyl (-CH3) and methylene (-CH2-) groups [25]. The peak of 1745 cm⁻¹ indicates the incorporation of the acetate structure, represented by the C=O stretching vibration of the acetate groups [26]. The 1657 cm⁻¹ peak corresponds to the C=O stretching vibrations originating from the residual DMF in the PCFs. Meanwhile, the 1465, 1376 and 1368 cm⁻¹ peaks are linked to the vibrations of the PEG 2000 and acetate groups, representing a deformation of the C-H bonds of the methylene group (-CH₂-) [27]. However, peaks 1278, 1238 and 1105 cm⁻¹ stretch C-O and C-O-C, i.e., ester bonds in cellulose acetate and ether bonds of PEG 2000 [28]. In addition, the peaks at 1047 cm⁻¹ are related to the stretching of the ether bond (C-O-C), while 939 and 901 cm⁻¹ represent the polysaccharide nature of PCFs, with the 901 cm⁻¹ peak attributed to the out-of-plane deformation of the C-O group of esters or other oxygenated groups, indicating the out-of-plane vibration of the glycosidic ring of cellulose structures [28]. Finally, the peak at 835 cm⁻¹ is attributed to C-O-C stretching vibrations in the ether bonds of the PEG 2000 molecule [25].

After carrying out the spectra of the four PCFs, they were subjected to hot cutting, sealing, controlled washing and drying. After this, the sections were subjected to the ATR-FTIR test to calculate the areas of the respective components [29], where optical absorption can be detected and used to identify different functional groups, and thus, it was expected to check possible PEG leaks.

To carry out the next step, the aliphatic functional groups (C-H) (1465 cm⁻¹ and 1368 cm⁻¹), characteristic of PEG 2000, the carbonyl groups (C=O) (1745 cm⁻¹ and 1657 cm⁻¹) and characteristic of CA and DMF, respectively, were represented in boxes in Figure 12. From these representations, calculations of the area index of these functional groups were carried out and presented in Figure 13.

The bar graphs represent the control group (CG) first, showing the PCF index before the controlled cutting and washing process. Next, the PCFs cut at three different temperatures are presented.

The objective of studying the functional group indices was to investigate the possibility of leakage of the PEG incorporated into the PCF core after cutting and controlled washing. When calculating the areas of the respective indices, three key points were observed and discussed. Firstly, the carbonyl group of CA presented a higher index compared to the others due to the abundant presence of CA in the protective sheath of PCFs. Secondly, there was a significant reduction in the presence of DMF, due to the decrease in carbonyl, a positive observation, due to its toxic properties affecting the mechanical properties of PCFs and causing undesirable environmental impacts [12]. Finally, interestingly, the areas of PEG aliphatic groups increased smoothly after the cutting and controlled washing process. This observation can be justified in two ways: firstly, by the formation of stable complexes between PEG and CA [30], resulting in a homogeneous distribution of PEG throughout the CA matrix and avoiding leaks; secondly, the increase can be attributed to the fact that residual DMF can mask the representative PEG bands, and its removal during controlled washing allows for better appearance and enhancement of these bands.

Regarding the initial objective of identifying PEG leaks, it was not possible to reach a definitive conclusion, as PEG interacts molecularly with CA, forming functional groups and being retained in the protective sheath of the PCFs, despite small evidence of chips in the post-cut micrographs.

3.3. Absorbance Measurements for Controlled Washing

The UV-VIS spectroscopy test was used to investigate the possible leakage of PEG 2000 through the absorption curves realized in the baths from the controlled washing of the PCFs after cutting (Figure 14).

Initially, the dissolutions of the base materials (CA, DMF and PEG) were subjected to UV-VIS spectroscopy to verify their presence and respective influences in the controlled washing baths. In the UV domain, CA dissolved in DMF showed a low absorbance in the UV range of around 276 nm [31]. DMF diluted in dH₂O, on the other hand, showed a high absorbance in the range of 235 nm [32]. Finally, as the literature shows, PEG 2000 dissolved in dH₂O exhibited a maximum absorbance in the range of 203 nm [33].

The results presented in Figure 14 showed that the PCF baths presented absorbances of 0.81 a.u., 0.75 a.u. and 0.78 a.u., with maximum peaks around 211 nm for the cuts at RT, 50 °C and 100 °C, respectively.

When comparing with PEG 2000, it was observed that all baths, in addition to exhibiting absorbances higher than the PEG 2000 value, showed a slight shift of the curves to the right. A possible initial explanation for the higher absorbance of the baths in the UV domain could be the absorption of water molecules trapped in the PEG polymeric matrix, which absorbs in this UV range [33]. Another explanation involves the influence of DMF: during the production of PCFs, DMF may not have completely evaporated, remaining retained in the PCFs [12,31]. During controlled washing, residual DMF may have been released along with PEG, resulting in curves with higher absorbance and a shift to the right. Furthermore, there may be an affinity between the polymeric complexes of PEG and CA [31], as in index calculations, where PEG can stick to the protective sheath of CA. Thus, CA and PEG particles can be removed simultaneously, causing the curve to shift, as the CA curve is more to the right.

Therefore, it can be concluded that even though the absorbance values are very similar for the three types of cuts, the 50 °C cut was the one that showed the lowest presence of residual materials in the baths since its absorbance value was the lowest among the three. Finally, knowing that the DMF and PEG curves are in the same UV domain, it is not possible to prove with certainty that there is only leakage from PEG.

4. Conclusions

In this study, an innovative approach was used to test the cutting and sealing of the ends of PCFs produced by the wet-spinning technique. These fibers were composed of cellulose acetate (CA, Mn 50,000) in the sheath, while the core was composed of PEG 2000; only the PEG ejection velocities varied. The main objective of the research was to evaluate the possible leakage of PEG from the PCF core using three different cutting and sealing temperatures. Optical and chemical tests were performed to investigate the presence and leakage of PEG 2000.

Initial results obtained by brightfield microscopy confirmed the presence of a well-formed nucleus in PCFs. Among the cutting temperatures tested, 50 °C showed the smallest chips and sealed ends, according to the literature. To highlight PEG leaks, the calculation of the FTIR indices of the respective functional groups of the materials was analyzed. The PEG interacts molecularly with the CA and forms groups that are retained in the protective sheath of the PCFs, the DMF is also removed, which makes the PEG areas more evident. UV-VIS spectroscopy analyses showed that, although the absorbances are quite similar for the three types of cuts, the cut at 50 °C presented the lowest presence of materials in the baths, evidenced by the lowest absorbance value among the three. However, due to the overlap of the absorbance curves of DMF and PEG in the same UV domain, it is not possible to confirm with certainty that the leakage exclusively involves PEG.

Knowing that the objective of this work was to evaluate the cutting and sealing of PCFs to prevent PEG leakage for applications in civil engineering materials, it is planned to encapsulate the PEG to improve the thermal properties without compromising the mechanical characteristics of the composite materials. In other words, the PCFs must undergo a more automated cutting process to obtain a large volume of material for an application in a real context. For example, the fibers could already be manufactured and simply applied to building materials, as is the case with various additives that are widely used. In relation to better visualization of the cuts and seals, the SEM test may be carried out. Furthermore, the presence of residual DMF complicates the analysis. In future work, a thermal test below the phase change temperature of PEG 2000 could be performed to evaporate the residual DMF and extend the pre-drying time. Another future analysis will be to test PEG 2000 combined with a fluorimeter to measure fluorescence or use PEG linked to modified chemical agents as a detection probe, potentially shifting the absorbance peak to wavelengths different from those coinciding with other components. Finally, it will be possible to study the degradation of PCFs to verify whether, over time, the CA will degrade to the point of contributing to PEG leakage. Therefore, studies using ultraviolet light, solutions with a more acidic pH and weathering can be carried out.