Ultraviolet protection performance of cotton fabric modified by ionic liquid iron coordination complex

Abstract

In order to endow cotton fabric with the full wave band (290–410 nm) ultraviolet (UV) protection property, ionic liquid iron coordination complex (ILICC) was applied to modify cotton fabric. ILICC was prepared by in situ reaction on the surface of the cotton fiber directly. The Fourier transform infrared spectra, thermogravimetric and differential scanning calorimetry curves indicated that the strong interaction between ILICC and cellulose macromolecules of cotton fiber occurred through the O-Fe bond. Scanning electron microscopy images and particle size analysis indicated that the surface of the modified cotton fiber was covered by the agminate ILICC particles with diameters of 700–1600 nm. Investigation on the UV protection performances indicated that the UV transmittances (T%) of the modified cotton fabric were less than 1.3% for the full wave band (UVA and UVB) UV. The washing durability results showed that the UV protection factor values were more than 95 when the modified cotton fabric was washed with 30 st washing cycles. The reason for the excellent UV protection performance of the modified cotton fabric was absorption and shielding of ILICC particles to UV light.

Keywords

ionic liquid iron coordination complex ultraviolet protection performance chemical modification cotton fabric

Cotton fabric is the most common textile fabric because of its excellent performances, such as alkali resistance, hygroscopicity, moisture retention and so on. However, the ultraviolet (UV) transmittance (T%) of cotton fabric is more than 15% and the UV protection factor (UPF) is only 10–20, limiting the application of cotton fabric in the field of UV protection textiles.

In order to endow cotton fabric with UV protection property, organic UV absorbers and UV shield agents have been applied to modify cotton fabric.^1–5 The benzophenone derivatives^6,7 and benzotriazole derivatives^8,9 as the conventional organic UV absorbers show an excellent anti-UV property because of the UV absorbing groups. Nano-scaled inorganics, such as ZnO, TiO₂ and SiO₂, as UV shield agents also show an excellent UV protection property because of the shielding function.^10–16 Fabrics with TiO₂ nanoparticles are produced by the melt spinning method to endow fabric with varied properties. Fabrics are modified by coating ZnO nanoparticles to obtain an excellent UV protection property; however, without the interaction reaction, the washing or rubbing fastness of the modified fabric was still a potential problem. Meanwhile, the existing UV shield agents are difficult to absorb or cannot provide a shield from full wave band UV light, because UV absorber agents only absorb a certain wave band UV light and the UV shield agents only shield UV light with the wavelength comparable to the particle size of the shield agents. In order to resist the full wave band UV light, anti-UV agents with the double functions of absorption and shielding are needed.

As an environmentally friendly solvent, ionic liquid has been applied to dissolve cotton cellulose;^17,18 however, ionic liquid as an anti-UV agent is not satisfactory for resisting the full wave band UV function because the absorption wavelengths of ionic liquid are 340–400 nm.¹⁹ Ionic liquid iron coordination complex (ILICC) with an Fe central atom has been successfully employed as a solid phase absorbent to adsorb dyes in our previous work.^20,21 The imidazolyl groups of ILICC, shown in Figure 1, include the plentiful C = C and C = N bands, which are expected to have an UV absorption capacity similar to conventional UV absorbers agents; meanwhile, the diameter of ILICC particles is near nanometers, which indicates that ILICC could shield UV light with the same wavelength.

Figure 1.

The molecular structure of the ionic liquid iron coordination complex.

In this paper ILICC as an anti-UV agent was applied to modify cotton fabric by in situ reaction. The in situ reaction between ionic liquid and FeSO₄ occurred on the surface and the interior amorphous region of cotton fabric. The Fe central ion coordinated with imidazolyl ionic liquid groups through the coordination bond between Fe and N atoms. The positive Fe ion reacted with negative ions such as O⁻, SO₄²⁻, Cl⁻ and so on.^20,21 Cotton fabric treated by NaOH was negative, which indicated that cotton fabric could react with ILICC by electrostatic interaction.^22,23 Cotton fabric was immersed into the mixture solution with ionic liquid 1-butyl-3-methylimizolium bromide (BmimBr) and FeSO₄. ILICC simultaneously generated and interacted with the cellulose molecule of cotton fabric through electrostatic interaction. UV protection performance and washing resistance of the modified cotton fabric were determined and excellent UV protection performance and washing resistance were obtained.

Experimental details

Materials

Cotton fabric (40 s × 40 s, grams per square meter of the woven gray fabric was 80 g/m²) was purchased from Textile Factories of Dandong (Dandong, China). 1-bromobutane (Beijing Chemicals, China) and 1-methylimidazole (Kaile Chemicals, China) were used as received. Ionic liquid 1-butyl-3-methylimizolium bromide was prepared as previously reported.²⁴ Other chemicals employed were at least of analytical reagent grade and were used without further purification. Deionized water was used throughout.

Modification of cotton fabric by in situ reaction

A total of 10 ml 1-butyl-3-methylimizolium bromide, 5.0 g NaOH and 12 g FeSO₄.7H₂O were added into 190 ml aqueous solution and then 10 g cotton fabric was immersed into the mixture solution. This cotton fabric and mixture solution was reacted at 40–100℃ for 2 h. The modified cotton fabric was washed with tap water three times and then dried at 90℃ for 1 h.

Characterization of the modified cotton fabric

The cotton fabric was modified at 90℃ for 2 h, and then was washed with tap water three times and then dried at 90℃ for 1 h. The cotton fabrics before and after modification were characterized by Fourier transform infrared (FT-IR) spectra, scanning electron microscopy (SEM) images and thermogravimetric (TG) and differential scanning calorimetry (DSC).

FT-IR spectra were recorded on a Nicolet 5700 FT-IR spectrophotometer by using a KBr disk with a resolution of 1.0 cm⁻¹ and 32 scans. The measurements were performed at 20℃ and a relative humidity of 65%. SEM images of the unmodified and modified cotton fabrics were evaluated with different magnifications by a scanning electron microscope (Hitachi H–600, Japan, 120 kV, 0.26 nm point resolution). The magnifications for the unmodified, modified cotton fabrics and ILICC particles were 5000, 5000 and 1000, respectively. The samples were mounted and gold sputtered to give the samples electronic conductivity under vacuum prior to observation. A Zetasizer Nano ZS particle size analyzer (Malvern, England) was used to monitor the size of the ILICC particles. TG and DSC curves of the unmodified and modified cotton fabric were determined by using a STA 449 F3 Jupiter simultaneous thermal analyzer (DSC/DTA-TG; Netzsch, Germany).

UV protection performances of the modified cotton fabrics

The UV protection performances of ILICC modified cotton fabric were recorded by a Labsphere UV-2000F Ultraviolet Transmittance Analyzer (Labsphere, USA). The spectral transmittances (T%) and UPF of the modified cotton fabric across the wavelength range of 290–410 nm were determined and obtained, which included the UVB and UVA. The UPF calculations used the GB/T 18830-2009 solar irradiance profile.

Washing resistance of the modified cotton fabric

Washing resistance of the modified cotton fabric against repeated launderings was evaluated based on the weight gain rate with different washing times. Before and after being modified by ILICC, cotton fabrics were washed according to AATCC test method 124-1996. The modified cotton fabrics were dried at 90℃ in a vacuum drying oven for 1 h and then weighed by electronic balance with accuracy for 0.0001 g. The weight of the unmodified cotton fabric was W₀ and the weights of the modified cotton fabric with different washing times were W₁. The weight gain rate W_G was calculated based on equation (1)

W_{G} = \frac{W_{1} - W_{0}}{W_{0}} \times 100 %

(1)

Results and discussion

In situ reaction between ILICC and cotton fabric

Because FeCl₃ reacted with Cl⁻ to form FeCl₄⁻, FeSO₄ was used to react with the Bmim⁺ group of ionic liquid BmimBr. Fe²⁺ ions reacted with N atoms of Bmim⁺ groups through the coordination bond to form ILICC particles, while Fe²⁺ ions were oxidized to Fe³⁺ ions.^20,21 The in situ reaction of ILICC on cotton fabric is shown in Scheme 1. The -OH groups of cotton fabric became negative under the alkaline condition, and reacted with Fe²⁺ ions by electrostatic interaction in the aqueous solution. Bmim⁺ cations coordinated with Fe²⁺ ions to produce ILICC on the surface of cotton fabric. As a result, cotton fabric was modified by ILICC through in situ reaction.

Scheme 1.

In situ reaction process of ionic liquid iron coordination complex (ILICC) on cotton fabric.

SEM image and particle size analysis of the agminate ILICC particles

The SEM image of the agminate ILICC particles under 5000× magnification, shown in Figure 2(a), indicated the shape of ILICC particles was spherical and the particle size was less than 2 µm. In addition, there were some flocculent microcrystals on the surface of the ILICC particles. The reason for this was that the in situ reaction between cotton fabric and ILICC resulted in ILICC being adsorbed on the surface of the cotton fabric. The rough surface micromorphology was attributed to the aggregation of ILICC particles. Figure 2(b) showed that the primary size range of the aggregated ILICC particles in the dispersion solution was 700–1600 nm, which indicated that the ILICC particles were easy to aggregate.

Figure 2.

Scanning electron microscopy image (a) and particle size (b) of the agminate ionic liquid iron coordination complex particles.

FT-IR spectra of the modified cotton fabric

The FT-IR spectra (Figure 3) showed the main absorption bands of the unmodified and modified cotton fabric. Before modification, the band at 3428 cm⁻¹ was assigned to the O-H stretching vibration; the bands at 2901 and 1060 cm⁻¹ corresponded to the C–H and C–O stretching vibration bands, respectively. After modification, the main absorption bands occurred at 3423, 2901, 1632, 1429 and 1060 cm⁻¹, indicating that the main structure of cotton fabric was almost unchanged. The reason for the intense OH band could be due to unreacted OH groups in the fabric and the presence of absorbed moisture arising from washing. However, the O–H stretching vibration band varied from 3428 to 3423 cm⁻¹, meaning electrostatic interaction between O atoms in cellulose macromolecules of cotton fabric and Fe ions in ILICC molecules occurred.

Figure 3.

Fourier transform infrared spectra of the unmodified and modified cotton fabric.

SEM images of the modified cotton fabric

The SEM images in Figure 4 show an obvious different morphology between the unmodified and modified cotton fabric. The surface of the unmodified cotton fabric was smooth without any particles (Figure 4(a)). However, the surface of the modified cotton fabric was covered by ILICC particles, and a mass of ILICC particles was deposited in the lacunas of the modified cotton fiber (red circle in Figure 4(b)).

Figure 4.

Scanning electron microscopy images of the unmodified cotton fabric (a) and the modified cotton fabric (b). (Color online only.)

TG and DSC curves

TG and DSC curves of the pure ILICC and the unmodified and modified cotton fabric are shown in Figures 5 and 6. TG curves indicated the obvious difference between unmodified and modified cotton fabric. The mass change of the modified cotton fabric was less than that of the unmodified cotton fabric.

Figure 5.

Thermogravimetric (TG) curves of the unmodified cotton fabric (a) and the modified cotton fabric (b).

Figure 6.

Differential scanning calorimetry (DSC) curves of the pure ionic liquid iron coordination complex (a), the unmodified cotton fabric (b) and the modified cotton fabric (c).

DSC curves in Figure 6 indicate that the decomposition temperatures at 350℃ and 356℃ for the unmodified cotton fabric were higher than that of the modified cotton fabric at 341℃ and 349℃. The reason for this was that after modification cotton fabric interacted with ILICC, while the agminate ILICC particles enhanced the heat conducting property of the modified cotton fabric.

A decomposition temperature of 325℃ was recorded for the modified cotton fabric, which was different to that of pure ILICC particles at 299℃. This was because electrostatic interaction between ILICC and cotton fabric resulted in an increment of the decomposition temperature of ILICC.

UV protection performances of the modified cotton fabric

The UV transmittances (T%) of the unmodified cotton fabric (Figure 7(a)) showed that the T-UVB% was 5.3–6.8%, and T-UVA% was 4.6–18.4%. The T-UVA% obviously increased with the increment of the UV wavelength. However, Figure 7(b) shows the UV transmittances (T%) of the modified cotton fabric were 0.7–1.3% across the wavelength range of 290–410 nm. The T-UVA% of the modified cotton fabric did not increase with increment of the UV wavelength. The reason for this was the absorption and shielding of ILICC particles to UV light. The Bmim groups in ILICC molecules could absorb the 340–400 nm wavelengths light,¹⁹ and the agminate ILICC particles could shield the different wave bands of UV light.

Figure 7.

Ultraviolet transmittance of the unmodified cotton fabric (a) and the modified cotton fabric (b).

Effect of reaction temperature on UV protection performances

The T-UVB (%), T-UVA (%) and UPF values of the modified cotton fabric under different reaction temperatures are recorded in Table 1. The results indicated that the weight gain rates increased with the increment of reaction temperature. The reason for this was that high temperature was favorable to the formation of ILICC particles. The T-UVB (%) and T-UVA (%) of the modified cotton fabric were reduced with the increment of reaction temperature. T-UVB (%) was about 0.8–0.9% and T-UVA (%) was about 0.9–1.3% when the modification temperatures were more than 60℃. Compared with that of the unmodified cotton fabric, the T-UVA (%) of the modified cotton fabric was obviously reduced. UPF values indicated that UV protection performances of the modified cotton fabric were excellent when the modification temperatures were more than 40℃.

Table 1.

Ultraviolet (UV) protection performances of the modified cotton fabric under different reaction temperatures with washing time for 30 st (determination number = 5).

Sample	Weight gain rate %	T-UVB (%) 290–315 nm	T-UVA (%) 315–410 nm	UPF degree	UVR protection category
The unmodified cotton fabric	0	5.3–6.8	4.6–18.4	16	Ordinary
The modified cotton fabric at 40℃	0.34 ± 0.02	0.9–1.3	1.3–1.8	65	Excellent
The modified cotton fabric at 60℃	0.54 ± 0.02	0.8–0.9	0.9–1.3	95	Excellent
The modified cotton fabric at 80℃	0.84 ± 0.02	0.8–0.9	0.9–1.3	97	Excellent
The modified cotton fabric at 90℃	1.12 ± 0.02	0.8–0.9	0.9–1.3	98	Excellent

UPF: UV protection factor.

Washing resistance of the modified cotton fabric

Washing resistance of the modified cotton fabric against repeated launderings was evaluated and is shown in Table 2. The results about weight gain rate indicated that the agminate ILICC particles on the surface of the modified cotton fabric were easily washed off. The reason for this was that the agminate ILICC particles only adsorbed on the surface of the modified cotton fabric. The weight gain rate with 40 st washing cycles was about 0.83%, indicating that ILICC interacted with cotton fabric and was not washed off. The UPF values of the modified cotton fabric were more than 95 even when the washing times were 40 st, indicating that the modified cotton fabric had excellent washing resistance.

Table 2.

Washing resistance of the modified cotton fabric under 80℃ with different washing cycles (determination number = 5).

Sample	Weight gain rate %	T-UVB (%) 290–315 nm	T-UVA (%) 315–410 nm	UPF degree average	UVR protection category
The unmodified cotton fabric	0	5.3–6.8	4.6–18.4	16	Ordinary
The modified cotton fabric without washing cycle	6.8 ± 0.02	0.4–0.8	0.8–1.0	165	Excellent
The modified cotton fabric with 3 st washing cycles	5.8 ± 0.02	0.5–0.8	0.8–1.0	145	Excellent
The modified cotton fabric with 10 st washing cycles	1.41 ± 0.02	0.8–0.9	0.9–1.3	101	Excellent
The modified cotton fabric with 20 st washing cycles	0.98 ± 0.02	0.8–0.9	0.9–1.3	98	Excellent
The modified cotton fabric with 30 st washing cycles	0.84 ± 0.02	0.8–0.9	0.9–1.3	97	Excellent
The modified cotton fabric with 40 st washing cycles	0.83 ± 0.02	0.8–0.9	0.9–1.3	96	Excellent
The modified cotton fabric with 50 st washing cycles	0.83 ± 0.02	0.8–0.9	0.9–1.3	96	Excellent

UPF: UV protection factor.

Conclusions

ILICC as an UV agent was synthesized by in situ reaction and then modified cotton fabric. In the in situ reaction process, Fe²⁺ ions interacted with cotton fabric by electrostatic interaction and simultaneously coordinated with ionic liquid BmimBr to form the ILICC particles on the surface of cotton fabric. Electrostatic interaction between ILICC and cotton fabric did not change the molecular structure of the cotton fabric.

SEM images and particles size analysis indicated that the particle size of the ILICC particles was less than 2.0 µm. The absorption of Bmim groups and the shield of ILICC particles resulted in the excellent UV protection performance of the modified cotton fabric.

The transmittances of the modified cotton fabric were less than 1.3% and UPF values were more than 95, indicating that ILICC was an excellent UV protection agent for the all (UVA and UVB) UV wave bands.

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 Programs of the Natural Science Foundation of China (No. 51343002), Liaoning BaiQianWan Talents, Liaoning Provincial Key Laboratory of Functional Textile Materials and Liaoning Excellent Talents in University and Science.

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