Development of Mosquito-Repellent Lyocell Fiber

Title:
Development of Mosquito-Repellent Lyocell Fiber

Authors:
Zhang Feiyan, Zhang Xiaoli, Yao Lirong, Wang Luolan
(College of Textile and Clothing, Nantong University, Nantong 226019, China)

Abstract:
Citronellal nanocapsules were prepared using citronella oil and caprylic triglyceride as the core materials, isophorone diisocyanate (IPDI) as the wall material, and ethylene glycol as the porogenic agent through interfacial polymerization. The average particle size of the prepared citronellal nanocapsules is about 220.5 nm, with a mosquito repellency rate of over 90%, and the inhibition rates against both Escherichia coli and Staphylococcus aureus reach 99.99%. Mosquito-repellent Lyocell fibers were prepared by blending the citronellal nanocapsules with the Lyocell fiber spinning stock solution in a wet spinning process. The mosquito-repellent effect of the functional Lyocell fiber reaches Grade B, and it has a good inhibitory effect against Staphylococcus aureus.

Keywords:
Functional Lyocell Fibers; Mosquito Repellent Effect; Citronellal Nanocapsules; Interfacial Polymerization Method

1. Experimental Section

1.1 Materials, Reagents, and Instruments
Materials: Lyocell fiber (Lenzing Group, Austria)
Reagents: Citronella essential oil (industrial grade, Ji’an Qianyuan District Luyuan Natural Fragrance Oil Refinery), Caprylic triglyceride (GTCC, 99%, Shandong Yousuo Chemical Technology Co., Ltd.), Isophorone diisocyanate (IPDI, industrial grade), Sodium dodecyl sulfate (SDS, analytical pure), Tetramethylethylenediamine (analytical pure), Ethylene glycol (analytical pure) (Shanghai Runjie Chemical Reagent Co., Ltd.), Polyethylene glycol octyl phenyl ether (OP emulsifier, chemical pure), Anhydrous ethanol (analytical pure) (Jiangsu Qiangsheng Functional Chemical Co., Ltd.), Nutrient broth, Nutrient agar, Phosphate buffer solution (biological reagents, Hangzhou Baisibio Biotechnology Co., Ltd.), Escherichia coli (ATCC25922), Staphylococcus aureus [CMCC(B)26003] (Shanghai Luwei Technology Co., Ltd.), Deionized water (self-made)

1.2 Preparation of Citronellal Nanocapsules
The citronellal nanocapsules were encapsulated using the interfacial polymerization method as described in Section 1.2, and the preparation process is shown in Figure 1.

Figure 1 Flowchart of the preparation of citronellal nanocapsules

The total mass of the system was controlled at 50 g. 10 g of a 10% SDS solution was weighed, and an appropriate amount of deionized water and a certain amount of the porogenic agent ethylene glycol were added, stirred evenly to serve as the aqueous phase. 15 g of the core material (citronella oil/caprylic triglyceride) was weighed, and an appropriate amount of IPDI was added and stirred evenly to serve as the oil phase. After mixing the aqueous and oil phases, they were placed in an ice bath and emulsified for 20-30 minutes under the action of an ultrasonic cell disruptor to obtain an O/W type nano-emulsion. The nano-emulsion was placed in a constant temperature water bath and mechanically stirred with a magnetic stirrer, and an appropriate amount of the catalyst tetramethylethane diamine was added dropwise and reacted at 60-65°C for 3-4 hours. After the reaction was completed, the OP emulsifier was added and stirred evenly.

1.3 Mosquito-Repellent Finishing of Lyocell Fiber
The prepared citronellal nanocapsules were added to the spinning solution of the Lyocell fiber, and the fibers with mosquito-repellent effects were obtained through the wet spinning process (this process was completed at the Beijing Institute of Textile Science). In this process, the mass fraction of citronella oil was 5%, and the mass fraction of the nanocapsules mixed in the spinning solution was 22%.

1.4 Performance Testing
1.4.1 Morphology and Particle Size of Nanocapsules
The DN-10B biological microscope was used to observe the distribution and morphology of the emulsion particles of the citronellal nanocapsules at different magnifications. The citronellal nanocapsules emulsion was diluted with deionized water by a certain multiple, and the particle size and distribution of the citronellal nanocapsules were measured using the 90plus Zeta nano-laser particle size meter, taking the average of three measurements.

1.4.2 Mosquito Repellent Performance Test
The samples were sent to the Parasitic Disease Control Institute of the Chinese Center for Disease Control and Prevention for testing. According to GB/T 28408—2012 “Protective Clothing Insect-Proof Protective Clothing” and GB/T 30126—2013 “Textiles – Detection and Evaluation of Mosquito Repellent Performance”, female adult mosquitoes (about 60) were placed in a mosquito cage (33 cm× 33 cm×33 cm), and the test samples were attached to the human body or blood feeder, and the number of mosquitoes landing on the test samples and control samples within a specified time was calculated. The repeated test data were calculated according to formula (1) to determine the repellency rate.

P = (n0 – n1)/n0 × 100% (1)

In the formula: P – Repellency rate, %; n1 – The total number of mosquitoes landing on the mosquito-proof treated fabric; n0 – The total number of mosquitoes landing on the control fabric.

The mosquito-proof performance of the fabric is evaluated based on the repellency rate: P>70% is rated as Grade A (has a very strong repellent effect); 50%<P<70% is rated as Grade B (has a good repellent effect); 30%<P<50% is rated as Grade C (has a repellent effect).

1.4.3 Fiber Performance
(1) Tensile Performance The YG001D single fiber strength tester was used to test the tensile performance of ordinary Lyocell fiber and functional Lyocell fiber. Due to the small diameter of the fibers, this test was conducted with 15 fibers as a bundle, and the average value was taken after multiple tests.
(2) Microscopic Morphology of Fibers The Gemini SEM 300 field emission scanning electron microscope was used to observe the transverse and longitudinal morphology of ordinary Lyocell fiber and functional Lyocell fiber at different magnifications under an acceleration voltage of 5 kV.
(3) Fourier Transform Infrared Spectroscopy (ATR-FTIR) The Nicolet iS50 infrared spectrometer was used to scan the functional Lyocell fiber in the range of 4,000 to 500 cm-1.
(4) Simultaneous Thermal Analysis (STA) The STA 449 F5 simultaneous thermal analyzer was used to analyze the thermal decomposition of the functional Lyocell fiber, with high-purity nitrogen as the gas flow, a heating rate of 10°C/min, and a temperature range of 30 to 800°C.

1.4.4 Antibacterial Performance Test
The antibacterial performance of the citronellal nanocapsules was tested according to the improved GB/T 20944.3—2008 “Textiles – Evaluation of Antibacterial Properties Part 3: Oscillation Method” [12]. Escherichia coli and Staphylococcus aureus were selected as test strains. The inhibition rate was calculated according to formula (2).
X =(n3 – n2)/n3 × 100% (2)

The section starting from 1.4.4 in the provided document discusses the antibacterial performance test and the results and analysis of the mosquito-repellent lyocell fiber development. Below is the translation of this part:

1.4.4 Antibacterial Performance Test
The antibacterial performance of the citronellal nanocapsules was tested according to the improved GB/T 20944.3—2008 “Textile Antibacterial Performance Evaluation Part 3: Oscillation Method” [12]. Escherichia coli and Staphylococcus aureus were selected as test bacterial strains. The inhibition rate was calculated using formula (2).
[ X = \frac{(n3 – n2)}{n3} \times 100\% \quad (2) ]
In the formula: X — Inhibition rate, %; n2 — The number of colonies in the test sample petri dish; n3 — The number of colonies in the control sample petri dish.

2 Results and Analysis
2.1 Characterization of Citronellal Nanocapsules
2.1.1 Stability and Morphology of Citronellal Nanocapsule Emulsion
According to the process in section 1.2, the citronellal nanocapsule emulsion was prepared. The prepared emulsion is milky white and does not separate after being placed for a long time (over 30 days), indicating good stability of the emulsion system. Microscopic observation revealed that the prepared citronellal nanocapsules are relatively uniform in size and distribution, with small differences in particle diameter (see Figure 2).
Figure 2 Microscopic image of citronellal nanocapsules (×1,000)

2.1.2 Particle Size and Distribution of Citronellal Nanocapsules
The particle size and distribution of the prepared citronellal nanocapsules were measured using a nano particle size analyzer, and the results are shown in Figure 3.
Figure 3 Particle size distribution of citronellal nanocapsules

As can be seen from Figure 3, the particle size of the citronellal nanocapsules is normally distributed, mainly ranging from 100 to 500 nm, with an average particle size of 220.5 nm. The small particle size will not clog the spinneret, which is beneficial for the smooth progress of subsequent spinning.

2.2 Performance Characterization of Mosquito-Repellent Functional Lyocell Fiber
2.2.1 Fiber Tensile Performance
The average breaking strength and breaking elongation rate of ordinary lyocell fiber and functional lyocell fiber are shown in Table 1.
Table 1 Tensile strength of ordinary and functional lyocell fibers

As can be seen from Table 1, after the addition of citronellal nanocapsules, the tensile strength of the fiber decreased by 24.09%, and the breaking elongation rate decreased by 36.63%. This is due to the high mass fraction of nanocapsules in the spinning solution, which is 22%, and has a significant impact on the mechanical properties of the fiber. The addition of citronellal nanocapsules disrupts the regular arrangement of the macromolecules in the lyocell fiber, leading to some changes in the aggregated structure and a decrease in crystallinity. In addition, citronellal nanocapsules hinder the hydrogen bond combination between cellulose molecules, reducing the degree of cross-linking and resulting in a decrease in the tensile strength and breaking elongation rate of the fiber.

2.2.2 Fiber Microscopic Morphology Analysis
The longitudinal and transverse microscopic morphologies of ordinary lyocell fiber and mosquito-repellent functional lyocell fiber were observed using a scanning electron microscope, and the results are shown in Figure 4.
Figure 4 Electron microscopies of cross-section of ordinary and functional lyocell fibers

From Figure 4, it can be seen that in terms of fiber longitudinal morphology, the surface of the ordinary lyocell fiber is relatively smooth, while the surface of the functional lyocell fiber with added citronellal nanocapsules is slightly rough with fine particulate matter, which may be due to the mixing of nanocapsules with the spinning solution increasing the surface particulates; in terms of fiber transverse morphology, the cross-section of the ordinary lyocell fiber is relatively dense, while the cross-section of the functional lyocell fiber has nano-scale pores, which may be due to the nanocapsules breaking when making the Ha氏 slice, leaving nano-scale pores, indicating that citronellal nanocapsules have entered the interior of the fiber.

2.2.3 Fourier Transform Infrared Spectroscopy Analysis
The infrared spectrum of the functional lyocell fiber is shown in Figure 5.
Figure 5 Infrared spectrum of functional lyocell fiber

From Figure 5, a significant broad absorption peak can be observed at 3,357 cm-1, and a strong absorption peak at 1,017 cm-1, which are the stretching vibrations of —OH and C—O, respectively, indicating the presence of unreacted ethylene glycol. The absorption peak at 1,633 cm-1 is the stretching vibration absorption peak of the carbonyl group C=O, which is the amide bond (—CO—NH—) formed by the reaction of IPDI with water and tetramethyl ethylenediamine. However, due to the small amount of wall material, the stretching vibration of N—H is not obvious. In summary, this proves the presence of citronellal nanocapsules in the functional lyocell fiber.

2.2.4 Thermogravimetric Analysis
The functional lyocell fiber prepared needs to undergo multiple heat treatments in subsequent weaving and dyeing processes, so it is necessary to test the thermal stability of the natural citronellal in the fiber, and the results are shown in Figure 6.
Figure 6 Heat resistance of ordinary lyocell fiber and functional lyocell fiber

From Figure 6, it can be seen that 50~150 ℃ is the evaporation of moisture in the fiber. In this process, the mosquito-repellent functional lyocell fiber loses weight faster than the ordinary lyocell fiber, which may be due to the high temperature accelerating the volatilization of the core material of the nanocapsules and water, leading to weight loss. The ordinary lyocell fiber begins to lose weight rapidly at around 240 ℃, which is the process where the glycosidic bonds in the cellulose macromolecules begin to break under heat, with a weight loss of about 60%; DTG has two peaks, reflecting different weight loss steps; it tends to be stable at 550 ℃, and the fiber is carbonized. However, the functional lyocell fiber begins to thermally decompose at a slightly higher temperature than the ordinary lyocell fiber, starting to decompose at around 250 ℃, with a weight loss of about 55% in this process. This may be due to the presence of a cycloalkane structure in IPDI, which not only gives the shell of the citronellal nanocapsules a certain strength but also forms a curing network structure in the fiber, improving the heat resistance of the lyocell fiber; in addition, the decomposition rate is faster than that of the ordinary lyocell fiber, possibly due to the decomposition of the citronellal nanocapsules; the fiber is carbonized at around 400 ℃. Therefore, the prepared functional lyocell fiber has excellent heat resistance.

2.3 Mosquito Repellent Performance Test
The prepared citronellal nanocapsules were directly finished on the fabric through the after-treatment method, and it was found that the repellency rate against Aedes albopictus reached 93.58%, with a strong repellent effect, and the mosquito-proof rating was Grade A. The functional lyocell fiber, after the mosquito-proof test, showed a repellency rate of 52.83% against Aedes albopictus, with a good repellent effect, and the mosquito-proof rating was Grade B. Since some of the citronellal nanocapsules are encapsulated inside the fiber, the core material is released slowly, affecting the mosquito-proof effect, but it still has a good repellent effect.

2.4 Antibacterial Performance Analysis
The antibacterial performance test results of citronella essential oil, citronellal nanocapsules, and functional lyocell fibers are shown in Table 2.

Table 2 Antibacterial Rate of Different Samples

As can be seen from Table 2, both citronella essential oil and citronellal nanocapsules have an antibacterial rate of 99.99% against Escherichia coli and Staphylococcus aureus, demonstrating good antibacterial properties. The functional lyocell fiber does not exhibit a significant antibacterial effect against Escherichia coli, but it achieves an antibacterial rate of 97.72% against Staphylococcus aureus. This may be due to the lower content of citronellal nanocapsules in the functional lyocell fiber, with a mass fraction of only 5% of citronella oil, which only reaches the minimum inhibitory concentration (MIC) for Staphylococcus aureus, and has not yet reached the MIC for Escherichia coli. Therefore, it shows a significant antibacterial effect against Staphylococcus aureus but not against Escherichia coli.

3 Conclusion
This study synthesized citronellal nanocapsules with mosquito-repellent effects using the interfacial polymerization method. The prepared citronellal nanocapsules have an average particle size of about 220.5 nm, with a uniform particle size distribution, and the mosquito repellency reaches Grade A. Moreover, they exhibit an antibacterial rate of 99.99% against both Escherichia coli and Staphylococcus aureus, indicating good antibacterial activity. By adding 22% citronellal nanocapsules to the lyocell spinning solution and undergoing wet spinning, functional lyocell fibers with mosquito-repellent effects were obtained. These fibers have improved heat resistance compared to ordinary lyocell fibers, but their tensile strength and elongation at break have decreased due to the incorporation of nanocapsules, and the mosquito repellency reaches Grade B. Since the content of citronellal nanocapsules in the functional lyocell fiber is relatively low, with only a 5% mass fraction of citronella oil, it does not have a significant effect on Escherichia coli, but it still has a good inhibitory effect on Staphylococcus aureus.