Rapid hydrophilic modification of poly(ethylene terephthalate) surface by using deep eutectic solvent and microwave irradiation

Abstract

Rapid hydrophilic modification of poly(ethylene terephthalate) (PET) fabric was carried out by deep eutectic solvent, ethylene glycol-choline chloride (EGC), under microwave irradiation. EGC is an inexpensive eco-friendly solvent, which is easy to handle along with low toxicity. Results showed that alkali concentration and microwave irradiation time were critical factors in determining surface characteristics of PET. The EGC-treated PET fabric showed highly hydrophobic surface with long wicking time (>2000 s) and high contact angle (135°) at 60 s of microwave irradiation time. However, by merely adding 0.5% sodium hydroxide, the EGC-treated PET fabric surface was drastically changed to highly hydrophilic surface with instant wicking time. The structure of modified PET was studied by various instrumental analyses such as Fourier transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and scanning electron microscopy. Tensile strength and methylene blue staining tests were also carried out to determine characteristics of the modified PET fabrics.

Keywords

deep eutectic solvent ethylene glycol choline chloride hydrophilicity poly(ethylene terephthalate)

Poly(ethylene terephthalate) (PET) has an excellent strength along with high chemical resistance, clarity, and processability. These properties make it one of the most important polymeric materials for both apparel and non-fibrous end uses.^1–3 However, water absorption of PET according to the ASTM method is commonly less than 0.2%, showing very low surface hydrophilicity. Such a low affinity toward water results in discomfort, high propensity toward pilling, and electrostatic charge during wearing.^1–5 Therefore, PET modification to improve its surface hydrophilicity has been examined for many years by various researchers.^4–6 But none of the methods is completely satisfactory and the continued research is needed.

The development and utilization of eco-friendly solvents is an active area of research, and ionic liquids have been at the forefront of this work due in large part to their low or negligible vapor pressures.⁷ Ionic liquids are a class of chemicals composed entirely of ions with melting points below 100℃. Due to their unique physicochemical properties, they have rapidly gaining in interest as greener replacements of traditional volatile organic solvents. Other main benefits of ionic liquids are their large liquid state range, favorable solvation behavior, low melting temperature, stability in air, easy recyclability, and high ionic conductivity along with low vapor pressures.^8,9 Imidazolium-based ionic liquids, such as 1-butyl-3-methylimidazolinium chloride, are most commonly used ionic liquids.^7–9

On the other hand, deep eutectic solvents are a new class of ionic liquid on the basis of eutectic mixtures of two compounds; mainly, choline chloride with metal chlorides (ZnCl₂, SnCl₂ etc.),¹⁰ or hydrogen bond donor species such as amides, acids, and alcohols.^11,12 By forming complexes with these compounds through hydrogen bonding, these deep eutectic solvent will produce a homogenous liquid of ions with significant decreased freezing points.¹² Unlike conventional imidazolium-type ionic liquids, choline chloride-based deep eutectic solvents can be easily prepared at low cost with high purity and low toxicity.¹¹ In particular, choline chloride is biodegradable and biocompatible which is also being widely used as feed additive for livestock.^13,14 It has also been approved as a food additive by the Food and Drug Administration in USA and it is thus environmentally-benign component.¹⁵ Deep eutectic solvents have very interesting solvent properties and have known to dissolve various compounds such as carbohydrates, enzymes, synthetic polymers, and metals.^{13,14,16–18} Nevertheless, a literature survey showed that little study has been carried out on the use of deep eutectic solvent to modify surface of one of the most common textile fabrics, PET.

Microwave-assisted organic synthesis moved to the forefront of chemical research.³ Use of microwave irradiation provides a number of advantages over conventional heating, such as noncontact, instantaneous, and rapid heating with high specificity.^3,19 It also renders a distributed manner inside the material, allowing more uniform and faster heating than conventional conduction or convection heating.¹⁹ Furthermore, the literature indicates that the microwave irradiation is more energy efficient by 60–70% than conventional heating methods.¹⁹

Therefore, our main objective was to investigate effect of deep eutectic solvent treatment on surface of PET. This is also the first use of deep eutectic solvent under microwave irradiation to initiate a rapid hydrophilic modification of PET surface. Effects of alkali concentration and microwave irradiation time during the treatment on structure of the treated PET were also investigated. These changes on modified PET were evaluated by Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and tensile strength measurement. Characteristics of hydrophilic surface were determined by wicking test and contact angle measurement, and dye staining test.

Experimental

Materials

Dacron 54 (plain weave, weight 171 g/m², Type 755) purchased from Testfabrics, Inc., Korea was used as PET substrate throughout the study. Choline chloride (ChCl, 99%), anhydrous ethylene glycol (EG, 99.8%), sodium hydroxide (NaOH), and glacial acetic acid were obtained from Sigma-Aldrich, Korea. All the chemicals were reagent grades and used without further purification. Methylene blue (C.I. Basic Blue 9) was also purchased from Aldrich.

Preparation of deep eutectic solvent

The deep eutectic solvent was prepared according to the method described in the previous study as follows:^11,12 2 mol of EG and 1 mol of ChCl was mixed at 80℃ during stirring until reaching homogeneous liquid phase (hereinafter, the deep eutectic solvent prepared by 2:1 mol of EG and ChCl is referred as EGC). The EGC prepared was stable for several weeks at room temperature with a proper protection from moisture.

Fabric treatment by EGC under microwave irradiation

PET fabric was firstly purified to remove any impurities by using 1% sodium carbonate solution, rinsed in the running water and deionized water several times, and finally air-dried. It was then placed in an Erlenmeyer flask containing EGC (liquor-to-fabric ratio 60 to 1, unless otherwise noted) with or without NaOH. Treatments were carried out in a commercial 700 W microwave oven (Daewoo Electronics, KR-V209Q, 20L, Korea) during stirring at times between 10 s and 300 s. The bath was stirred by a mechanical stirrer installed through the hole made on top of the microwave oven. The fabric was rinsed by 1% aqueous acetic acid solution and then deionized water. Rinsing was repeated several times to ensure a complete removal of EGC from the fabric surface followed by atmospheric drying. The specimen was stored in the standardized chamber (21±1℃, 65±2% relative humidity) for further analysis. Weight loss of the PET fabric after the treatment was calculated as follows:

Weight loss (%) = (W_{i} - W_{g}) / W_{i} \times 100

(1)

where W_i is the initial weight of the fabric and W_g is the weight of fabric after treatment.

Characterization of the modified PET

Surface characteristics of the treated PET fabric related to hydrophilicity (or hydrophobicity) were evaluated in two ways: area wicking “spot” test (a modified method of AATCC Method 39-1977, evaluation of wettability) and contact angle meaurement.³ Wicking time was used as an indication of surface hydrophilicity. As sorption of water drop occurred within one second, then it was considered as an “instant” wicking. At least three replications in different areas were made in the measurements for wicking time. Contact angle for the PET fabrics was evaluated by the sessile drop method with 2 μL water using a Contact Angle Analyzer DSA 100 (Krüss, Germany).

The presence of carboxylic acid groups within the treated PET was also analyzed by the methylene blue staining from a bath (liquor ratio 50:1) of 0.1 g/L dye solution with 1 g/L ammonia at 60℃ for 20 min. Staining was carried out in a stainless tube of IR dyeing machine (Daelim Starlet Co, Ltd., Korea). After staining process and its rinsing in the deionized water, the reflectance of the fabric was measured by Minolta Color Eye CM-512M3 (Japan) and the K/S value was calculated according to the Kubelka–Munk equation:³

K / S = (1 - R) 2 / 2 R

(2)

where R is the reflectance of the sample at the wavelength of maximum absorption.

Other analyses of the treated PET

Tensile strength was measured to evaluate strengths of the PET fabrics by using a Universal Testing Machine (H10KS, Hounsfield, UK) according to ASTM D5035-11 (strip method). At least three replications were used and the average value was calculated. FTIR spectroscope with an attached ATR (Bruker, Vertex 70, USA) was used to analyze the PET fabrics in the spectral region of 4000–600 cm⁻¹ with 64 scans at 4 cm⁻¹ resolution. SEM (COXEM, CX-100S, Korea) was used to study the effect of treatment on topological characteristics of the PET fabrics.

Thermal characteristics of pristine and treated PET fabrics were appraised by TGA (Mettler, USA) under nitrogen atmosphere. DSC (Perkin Elmer, USA) was also used to study thermal characteristics of the PET fabrics with a 20 cm³/min flow of nitrogen at heating rate of 10℃/min. The PET fabric was heated up to 290℃, cooled to −30℃, and reheated to 290℃ again. Only the second heating curve was analyzed. With heat of melting and cold crystallization data obtained from the second heating DSC curves, % crystallinity of PET fabric was calculated as follows:

Crystallinity (%) = [(Δ H_{m} - Δ H_{c}) / Δ H_{m}^{o}] \times 100

(3)

where ΔH_m is a heat of melting of PET, ΔH_c is a heat of cold crystallization of PET (if any), and

Δ H_{m}^{o}

is a heat of melting of PET single crystal (122 J/g).³

Results and discussion

Effect of EGC in the absence of alkali

Among various choline-based deep eutectic solvents, EGC was selected as a treatment medium because it contained EG that could cause glycolysis reaction simultaneously.³ Results showed that the EGC treatment resulted in very drastic effects on the PET fabrics under microwave irradiation even in the absence of NaOH. Except for 60 s, EGC-treated PET fabrics at 180 and 300 s of microwave irradiations exhibited severe degradation in fabric structure and thereby wicking time and contact angle as well as tensile strength were unable to determine. It should be noted that the temperature of the system was not controlled due to the limitation of this study, but different exposure time could easily change the temperature during microwave irradiation. This degradation was also corroborated by SEM analyses, revealing molten surface of the EGC-treated PET fabrics at these longer microwave irradiations such as 180 and 300 s in the absence of NaOH (Figure 1). This suggested that EGC was especially a powerful solvent system under microwave irradiation probably due to its high electronic conductivity compared with water.^11,21

Figure 1.

SEM micrographs of EGC-treated PET fabrics under microwave irradiation (no NaOH): (a) pristine PET; (b) 60 s; (c) 180 s; (d) 300 s.

On the other hand, surface of the PET fabric treated by EGC for 60 s of irradiation demonstrated a greater hydrophobic surface indicated by longer wicking time (>2000 s) and higher contact angle (135°) than those of the pristine PET (177 s and 94.4°, respectively). Therefore, the treatment with proper control can initiate a highly hydrophobic surface of PET fabric. We believe that high hydrophobicity of the EGC-treated PET surface was most likely due to higher % crystallinity (see later) along with greater surface roughness initiated by EGC. Similar increase in hydrophobicity was also observed with water-treated PET surface in the absence of NaOH.³

Effect of EGC on structure of PET fabric was further investigated by TGA analyses, as shown in Figure 2. Onset temperature (T_o) as the starting temperature for degradation in TGA curve (Figure 2(a)) and peak temperature (T_p) as the temperature for maximum degradation in first derivative curve (Figure 2(b)) were mainly determined along with % residue at 700℃. Results indicated that both T_o and T_p of the EGC-treated PET fabrics at different microwave irradiation times were resembled to those of pristine PET, despite of great level of degradation as shown above in Figure 1. However, the treated PET fabrics demonstrated somewhat higher % residue at 700℃ and much greater rate at T_p.

Figure 2.

TGA curves of EGC-treated PET fabrics under microwave irradiation (no NaOH).

DSC curves of the EGC-treated PET fabrics (Figure 3) display relatively sharp melting endotherms between 250℃ and 254℃, which slightly shifted to higher temperature with increase in microwave irradiation time. In general, no definite glass transition or cold crystalline peak was observed in the EGC-treated PET fabrics, resulting in much greater % crystallinity than that of pristine PET, i.e. 33.6–38.3 % for the EGC-treated PET versus 15.4% for the pristine. This indicated that degradation mainly occurred on the PET surface and reorganization of PET crystalline structure resulted in higher % crystallinity of the EGC-modified PET under microwave irradiation. Even though microwave generally rendered more uniform and faster heating of the materials,¹⁹ modification of PET with EGC was largely on surface, probably because of too short microwave irradiation.

Figure 3.

DSC curves of EGC-treated PET fabrics at various microwave irradiation times (no NaOH).

Effect of alkali-containing EGC

We also examined effect of NaOH during EGC treatment to obtain hydrophilic PET surface, as shown in Figure 4. Drastic increase in weight loss of EGC-treated PET fabrics was observed by addition of NaOH, resulting in more than 75% at 3% NaOH and 180 s of microwave irradiation (Figure 4(a)). Such a great weight loss was presumably due to enhanced glycolysis–hydrolysis reaction caused by EG with NaOH. The presence of ChCl also facilitated the reaction rate, procreating extremely high level of weight loss. It has been known that alkali hydrolysis reaction was markedly promoted by addition of quaternary ammonium compound.²² The EGC-treated PET fabric at 5% NaOH for 180 s of microwave irradiation showed 90% weight loss with almost complete degradation or decomposition, leaving barely visible fabric structure. A research is currently undertaken to utilize this reaction for recycling of PET waste products.

Figure 4.

Effect of NaOH concentration on characteristics of EGC-treated PET fabrics under microwave irradiation (the samples at 60 s also showed all instant wicking): (a) weight loss; (b) wicking time.

Surprisingly, the presence of small amount of NaOH (0.5%) in the EGC treatment bath dramatically changed surface properties of the PET fabrics from highly hydrophobic to highly hydrophilic surface, i.e. decrease in wicking time from >2000 to 6 s at 60 s of microwave irradiation, as shown in Figure 4(b). As mentioned above, the surface of PET fabric was melted at 180 s of microwave irradiation without NaOH. A further increase in NaOH concentration consistently resulted in instant wicking at both 60 and 180 s of microwave irradiation. These results clearly implied that shifting of highly hydrophobic to highly hydrophilic surface could be obtained by just adding a small amount of NaOH (0.5%) in EGC solution within 60 s of microwave irradiation.

To understand effect of NaOH, we measured conductivity of EGC solution with or without NaOH (Table 1). These solutions were not exposed to microwave irradiation. Conductivity represents amounts of ion within the solution. High conductivity of EGC revealed a great level of ionized states with hydrogen bonds between hydrogen donor (EG) and hydrogen acceptor (ChCl). In addition, the conductivity of the EGC diminished with increase in NaOH concentration as shown in Table 1. This attributed to expedited formation of hydrogen bond by adding OH⁻, producing extra hydrogen bonds between donor (EG) and OH⁻, in addition to original hydrogen bond between EG and Cl⁻ in EGC.

Table 1.

Conductivity and pH measurement of EGC solution (no microwave irradiation)

NaOH conc. (%)	Conductivity (mS/cm)	pH
0	13.22	6.1
5	7.14	11.3
10	4.11	12.1

Decrease in conductivity due to addition of NaOH was also substantiated by FTIR analysis, as shown in Figure 5. FTIR spectra for EGC at different NaOH concentrations were varied in a couple of ways: firstly, OH stretching peaks shifted to lower energy region with increase in NaOH concentration, from 3289 cm⁻¹ at 0% to 3284 cm⁻¹ at 5% and to 3273 cm⁻¹ at 10% of NaOH, presumably due to the presence of more hydrogen bonds. Secondly, intensity of OH peak was substantially decreased with increase in NaOH concentrations which was confirmed by increase in pH of the EGC solution (Table 1). This considerable increase in pH suggested that some of EG molecules could have deprotonated into ethoxide molecules. Addition of NaOH in EGC system thereby created two opposite effects in modifying PET surface to more hydrophilic: firstly, enhancing effect by increasing potential hydrolysis and glycolysis reaction at more alkaline conditions, and secondly, impeding effect by lowering conductivity of EGC causing slow heating under microwave irradiation. In here, we believed that the first effect was pre-dominant, accounting for high hydrophilic surface by merely adding 0.5% NaOH.

Figure 5.

FTIR spectra of EGC solutions in the presence of NaOH (0, 5, and 10% NaOH).

Thermal analyses of alkali-containing EGC-treated PET

Figure 6 shows TGA curves of EGC-treated PET at 5% NaOH with 60 and 180 s of microwave irradiation time. Data for the specimen with 300 s was not presented due to complete degradation. Results in TGA curves represented that weight loss curve followed quite different path at 180 s. Unlike TGA curve of pristine or EGC-treated PET for 60 s, which showed a levelling of weight loss at around 480℃, the EGC-treated PETs for 180 s did not show the levelling and their weight residues were continuously decreased with a second step starting at around 460℃. This was further appeared as a secondary small peak after T_p in first derivative curve as shown in Figure 6(b).

Figure 6.

TGA curves of EGC-treated PET fabrics at 5% of NaOH under microwave irradiation (severe degradation for 300 sec).

TGA curves of PET fabrics treated at different concentration of NaOH for 180 s (Figure 7) also showed the same trends. As long as alkali was added in the treatment system, they all showed similar two-steps weight loss curves (Figure 7(a)) along with the secondary small peaks after T_p (Figure 7(b)). Degradation of polymer chain could be plausible reason for these results. Moreover, in both first derivative curves in Figures 6(b) and 7(b), the rates at T_p was initially greater with EGC treatment in the mild conditions (60 s in Figure 6(b) or 0–3% NaOH in Figure 7(b)), but with further increase in microwave irradiation time or NaOH concentration, the rates at T_p decreased and were equal or less than that of pristine PET. This suggested occurrence of polymer chain reorganization at a short microwave irradiation time and/or low NaOH concentration.

Figure 7.

TGA curves of EGC-treated PET fabrics for 180 s of microwave irradiation at different NaOH concentrations.

DSC curves of the EGC-treated PET fabrics in the presence of NaOH at 60 and 180 s of microwave irradiation times are shown in Figure 8. These DSC curves were very similar to those of the EGC-treated PET without NaOH, as previously illustrated in Figure 3, i.e. exhibiting relatively sharp melting endotherms. The melting temperature was generally higher with NaOH than that of pristine (250℃). Furthermore, the EGC-treated PET fabrics with NaOH also demonstrated no definite glass transition or cold crystalline peak, resulting in much greater % crystallinity than that of pristine PET. As presented in Figure 9, percentage crystallinity of the EGC-treated PET fabrics was great at no or 0.5% NaOH, but did not show much difference at high concentrations of NaOH. This again confirmed that the reorganization of PET was occurred significantly at low concentration of NaOH as described above in TGA data, showing greater rate at T_p. This was quite different from the percentage crystallinity results obtained from glycolysis reaction, showing higher value at high NaOH concentration.³ Therefore, this revealed that effect of EGC treatment on crystalline structure of PET was not the same as glycolysis reaction in the presence of NaOH under microwave irradiation.

Figure 8.

DSC curves of EGC-treated PET fabrics in the presence of NaOH at different microwave irradiation times: (a) 60 s; (b) 180 s.

Figure 9.

Effect of NaOH concentrations on percentage crystallinity of EGC-treated PET fabrics under microwave irradiation (percentage crystallinity of pristine PET was 15.4).

Mechanical properties of EGC-treated PET

As illustrated in Figure 10, when the PET fabric was treated by EGC with various concentrations of NaOH at 60 s microwave irradiation, reduction in tensile strength was very substantial. In all the cases, the tensile strength decreased to less than 50% of that of pristine PET (58 MPa), regardless of NaOH concentration and the decrease was gradual with increase in NaOH concentrations at both microwave irradiation times. It should be noted that even without NaOH, the same reduction in tensile strength occurred but only at lesser level. Longer microwave irradiation facilitated tensile strength reduction. Therefore, at 3% NaOH with 180 s retention of tensile strength was only reached to 15% of pristine PET. High tensile strength reduction with EGC was believed to be due to glycolysis reaction of EG in the presence of NaOH and ChCl that could facilitate the reaction rate. These results revealed that EGC made of EG and ChCl could have acted as solvent and reactant (EG) as well as catalyst (ChCl) during the treatment of PET fabric. Therefore, simultaneous glycolysis and hydrolysis reactions in the presence of EGC and NaOH would have caused considerable reduction in degree of polymerization of PET polymers, resulting in a significant strength loss during the EGC treatment.

Figure 10.

Effect of NaOH concentrations on tensile strength of EGC-treated PET fabrics under microwave irradiation.

SEM analyses of EGC-treated PET

The SEM micrographs exhibited that surface characteristics of PET fabrics treated by EGC in the presence of NaOH was not practically different from that of pristine PET at short microwave irradiation time (60 s), as shown in Figure 11(a) and (b). With extended microwave irradiation at 180 s, however, appearance of PET fabric was markedly changed: fineness of PET fiber was drastically decreased from ca. 15 μm of pristine or EGC-treated PET at 60 s in Figure 11(a) and (b) to ca. 7.5 μm at 1% NaOH in Figure 11(c) and to ca. 6 μm at 3% NaOH in Figure 11(d). It should be pointed out that these four samples all displayed instant wicking.

Figure 11.

SEM micrographs of EGC-treated PET fabrics in the presence of NaOH under microwave irradiation: (a) 1% NaOH for 60 s (no weight loss); (b) 3% NaOH for 60 s (5.7% weight loss); (c) 1% NaOH for 180 s (37.2% weight loss); (d) 3% NaOH for 180 s (75.8% weight loss).

Wicking is influenced by various parameters such as surface roughness and porosity. Capillary pressure ( P _C) in wicking is inversely proportional to mean pore radius in horizontal absorption:²³

P_{c} = 2 γ \cos θ / r

(4)

where γ is the liquid surface tension, θ is the contact angle defined at the common interface of three phases (fiber-liquid-air), and r is the mean pore radius.²³ As shown in Figure 11(c) and (d), these EGC-treated PET fabrics could have significantly greater pore size, resulting in easy absorption of water, due to considerable weight loss and decrease in fiber fineness. The sessile drop method, however, failed to differentiate wettability of hydrophilic surface because liquid drop on the surface disappeared rapidly being absorbed into the pores of materials. At the same time, fabric softness evaluated by simple hand touch was increased considerable probably due to decrease in fiber fineness. In addition, there was no visible yellowing occurred after EGC treatment. Therefore, these results revealed that increase in microwave irradiation time provided greater effect on modification of PET than increase in NaOH concentration. While considering strength of the treated PET fabric for practical end-uses, an optimal condition for obtaining highly hydrophilic surface of PET could be the condition at 60 s of microwave irradiation with 0.5–1% NaOH with EGC.

K/S values of EGC-treated PET

K/S values of EGC-treated PET fabrics at different NaOH concentrations shown in Figure 12 were 0.19, 2.28, 2.29, 1.18, 1.08, 1.08, and 1.02 for 0, 0.5, 1, 2, 3, 4, and 5% of NaOH (pristine = 0.01), respectively. K/S value of the fabrics dyed by methylene blue indirectly estimates the number of carboxyl groups within PET. Therefore, these results suggested that hydrolysis reaction generating free carboxylic groups was prominent at low concentrations of NaOH. As stated above, the increase in alkali concentration increased pH of the system, but at the same time it decreased the conductivity of system, allowing additional reaction such as glycolysis.²⁴ Therefore, additional glycolysis reaction hindered generation of free carboxylic groups, resulting in less K/S values by cationic methylene blue staining. This implies that at high concentrations of NaOH (more than 1% NaOH) a potential dyeing problem caused by ionic charged surfaces could be minimized and very fine fibers along with soft touch could be obtained.

Figure 12.

Effect of NaOH concentration on K/S values of EGC-treated PET fabrics under 60 s of microwave irradiation followed by dyeing with methylene blue (λ_max = 660 nm).

Conclusion

Hydrophilic PET surface was successfully obtained by applying deep eutectic solvent, EGC, under microwave irradiation with NaOH. Results indicated that severe degradation in PET fabric structure was observed by the EGC treatment without NaOH. Drastic increase in weight loss of EGC-treated PET fabrics was shown by adding NaOH, presumably due to enhanced hydrolysis-glycolysis reaction caused by EG and ChCl. A small amount of NaOH (0.5%) in the EGC dramatically changed PET surface from highly hydrophobic to highly hydrophilic surface at 60 s of microwave irradiation. A further increase in NaOH concentration consistently resulted in instant wicking at both microwave irradiation times (60 and 180 s).

In the presence of NaOH, TGA thermograms all showed similar two-steps weight loss curves along with the secondary small peaks after T_p due to degradation of polymer chain. DSC curves of the EGC-treated PET fabrics with or without NaOH were similar each other, exhibiting no definite glass transition or cold crystalline peak, resulting in much greater percentage crystallinity than that of pristine PET. However, there was a considerable tensile strength reduction on the EGC-treated PET fabrics and much fine fibers were obtained by EGC treatment which was confirmed by SEM micrograph. K/S value investigation with Methylene Blue indicated that hydrolysis reaction was prominent at low concentrations of NaOH.

Therefore, rapid hydrophilic modification of PET surface was possible by using deep eutectic solvent such as EGC with 0.5% of NaOH under 60 s of microwave irradiation. Contrarily, the finer fiber with softer touch could be obtained with greater level of NaOH (i.e. 1%) during EGC treatment. While considering an inherent high strength of PET, these EGC-treated fabrics could be suitable in apparel applications as a new type of materials with definite advantage in great hydrophilicity along with soft touch and light fabric density due to finer fiber.

Footnotes

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Research Foundation of Korea (grant number NRF-2014R1A1A 3049867).

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