Abstract
Insecticide-incorporated knitted fabrics are used to provide long-term efficacy against mosquitoes. This article described the method for making insecticide-incorporated filaments by using bi-component-spinning technique having core-sheath morphology whereas deltamethrin has been incorporated in the sheath-containing high-density polyethylene. The concentration of the deltamethrin is varied from 0.25% to 0.75% to assess its impact on bioassay efficacy. The developed filaments subsequently converted into knitted structure and possess excellent insecticidal efficiency even after 20 wash cycles and therefore can be effectively used for the fabrication of knitted mosquito nets. The residual deltamethrin content was also evaluated after successive wash cycles.
Introduction
Mosquitoes are considered the deadliest insects on the Earth. Mosquitoes mainly fall under three genera, i.e. Aedes, Anopheles and Culex responsible for transmitting vector borne diseases such as dengue, malaria, west nile fever and filaria among human beings. Vector borne diseases are usually associated with tropical and sub tropical regions of the world like Africa, India, Sri Lanka and South America. Several means of mosquito repellents already exist globally such as mosquito-repellent coils and sprays mats; however, all these repellents provide protection only in closed environment. Specifically for outdoor purposes, insecticide-treated textile articles such as bed nets, shelters, tents, curtains, uniforms, military clothing and various garniture materials provide protection against vectors. These articles can be made of various materials, e.g. cotton, nylon, polyester and polyethylene. In order to ensure protection against vector, various chemicals have been tried to impart insecticidal efficiency in textile articles [1–3]. Pyrethroids, a family of insecticides that includes permethrin, α-cypermethrin and deltamethrin (DM) are highly effective in killing mosquitoes, by disabling their nervous systems. WHO sanction the use of selected set of these pyrethroids [4]. DM is a type II synthetic pyrethroid-based insecticide. It contains a cyano group bonded to the hydroxyl-containing carbon atom of the alcohol moiety of pyrethroids as depicted in Figure 1, which provides additional biochemical function by inhibiting the neuro transmitter GABA in mosquito and may inhibit chlorine ion channel as well [5]. It has a much longer duration as well as stronger insecticidal activity than other commonly available insecticides; therefore, the doses needed for protection against mosquitoes are extremely low. So, it is efficiently economical. DM is also advantageous because its activity persists over a wide range of temperature.
Chemical structure of DM. DM: deltamethrin.
Various insecticide formulation-containing binders have been used to impart insecticidal efficiency on knitted fabric by exhaust/pad method. However, these fabrics could not sustain their insecticidal efficacy after repeated laundry washes [6]. Thus, these conventional-treated knitted fabrics need to be repeatedly retreated to remain effective [7]. In order to achieve durability, insecticides have been incorporated into polymer matrix prior to melt spinning and knitting. Commercial examples of this category are the Olyset® Net (Sumitomo Chemicals Co. Ltd, Tokyo, Japan), DuraNet (Clarke Products, Roselle IL USA), LifeNet™ (Bayer Crop Science, Lyons, France), MAGNET (VKA Polymers Tamil Nadu, India) and the Netprotect (Bestnet Europe, London, UK). In all the nets mentioned above, polyethylene is used as a fibre-forming polymer. Loading of insecticides per square metre of mosquito-repellent fabric also has its regulation limits. The filament will be less efficient, if additive is distributed in entire cross-section. Therefore, concentrating the insecticide in surface region by employing bi-component spinning improves the efficiency.
Due to several technical and operational reasons, olefins polymers (such as high-density polyethylene (HDPE)) have been found most suitable for the incorporation of pyrethroid insecticides. But HDPE has several drawbacks such as plastic like feel, difficulty in dyeing (only dope dyeing) and low tenacity as compared to polyester, whereas polyester has several advantages such as low cost, easy availability and high tenacity as compared to olefins [8]. This imparts the need of such a system that has advantage of both the above-said polymers. Thus, we adopted bi-component filament-spinning route having sheath-core structure where HDPE is in the sheath as a carrier for insecticides, and polyester is in the core to impart strength.
The novelty of this work is that it is a unique process for making insecticide-incorporated bi-component filament whereas insecticide distributed in the sheath with long-lasting efficiency.
Experimental details
Materials and methods
PET and HDPE pellets were taken from M/s Reliance Industries and were used as a raw material for filament core and sheath part, respectively. DM was obtained from Fluka. Untreated polyester net was taken from M/s RHT Fashion, Amritsar, India. Technical specifications for the above are mentioned in Table 3.
PET and HDPE were spun simultaneously using a bi-component melt-spinning machine. The melt compounding of insecticide additive with resin results in loss or degradation of insecticide. Master batch of insecticide was used as active insecticide ingredient. The conc. of DM was varied from 0.25 to 0.75% on the weight of material. Sheath/core bi-component filaments were produced by extruding the melt of HDPE as the sheath and general purpose PET as the core through an annular spinneret using two different extrusion systems. Each system consisted of an extruder and a gear pump. The constant temperature maintained for sheath and core extruder is 170℃ and 265℃, respectively. The spin finish was given to the filaments and subsequently wound by a winding device. The filament thus obtained was drawn to draw ratio of 4–5. The schematic illustration of the spinning setup is presented in Figure 2. Furthermore, the filaments were wrapped on conventional direct filament warpers using creel and converted into knitted net fabric by 28 gauge two-bar tricot warp-knitting m/c.
Bi-component melt-spinning setup.
Performance evaluation and testing
Cross-sectional analysis
The cross section of filaments was obtained by embedding the bundles of filament in a wax medium and cutting the filament with a microtome. The resulted slices were observed under optical microscope and photographed.
Thermal analysis
The thermal behaviour of PET/HDPE filament was investigated by using differential scanning calorimetry (DSC Q200, TA Instruments). It was measured with 1.4000 mg of the filament samples at the heating rate of 10℃/min from ambient to 320℃. The thermal degradation behaviour of DM was determined with thermogravimetric analysis (TGA) performed on TGA Q500, TA Instruments. Data were obtained during heating run from ambient to 300℃ at a scan rate of 5℃/min.
Long-lasting insecticidal efficacy (cone bioassay test)
The insecticidal efficacy of insecticide-incorporated knitted fabric samples before and after repeated wash cycles has been evaluated by WHO cone bioassay test as given in WHO/CDS/WHOPES/GCDPP/2005.11. In WHO cone bioassay test [9], 10 non-blood fed, 2–5-days-old Anopheles species mosquitoes were exposed to netting materials (25 cm × 25 cm) for 3 min under standard WHO cones (Figure 3), after which they were held for 24 h with access to sugar solution. Knockdown (KD) was measured after 60 min post exposure and mortality after 24 h. On an average, 50 mosquitoes have been exposed to each test specimen. Untreated polyester-knitted fabric samples were used as control. Bioassays test was carried out at 25℃ ± 2℃ and 75% ± 10% RH. WHO cone bioassay test was performed on nets washed 0, 1, 5, 10, 15 and 20 times (less or more as necessary) to measure the efficacy of the insecticide in the knitted fabric after repeated washing.
WHO cone bioassay test apparatus.
Durability of the insecticide-incorporated knitted fabric was determined by WHO standard washing procedure mentioned in WHO/CDS/WHOPES/GCDPP/2005.11. Knitted fabric samples (25 cm × 25 cm) were individually introduced into 1-l beakers containing 0.5 litres deionized water, with 2 g/l soap yellow cakes (pH 10–11) added just before and fully dissolved. Beakers were immediately introduced into a water bath at 30℃ and shaken for 10 min at 155 movements per minute. The samples were then removed and rinsed twice for 10 min in clean, deionized water in the same shaking conditions as stated above. Knitted fabric samples were dried at room temperature and stored at 30℃ between two consecutive washes. Seven-day time interval was kept between two consecutive washes. Samples were then wrapped in aluminium foil and stored until tests were performed.
Chemical analysis
The initial concentration of DM incorporated into the knitted fabric and residual DM content remained in the samples after washing was determined by gas chromatography (GC) technique. The sample of DM-incorporated knitted fabric weighing approximately 2 g was refluxed with 40 ml xylene for 1 h at 140℃ and cooled to room temperature. Then, solution was filtered through GF/C filter paper. The filtrate was made up to 50 ml with xylene, and 2.0 µl of sample solution was injected in gas chromatograph column for determination. Stock solution of DM was prepared by dissolving 25 mg of pure pre-dried DM powder in 250 ml of xylene.
The samples were analysed with Perkin Elmer GC model Clarus, equipped with BPX-5 (5% phenyl polysilphenylenesiloxane) capillary column (30 m × 0.25 mm i.d., 0.25 -µm film thickness). Nitrogen was used as a carrier gas and kept at constant pressure with a flow of 1.5 ml/min. The split ratio, air flow and hydrogen flow were, respectively, 1 : 2, 450 ml/min and 45 ml/min. The detector temperature was 320℃. The injection volume was 2.0 µl, and the column temperature was programmed as initial temperature 180℃ to 260℃ at 25℃/min and then 260℃ to 320℃ at 5℃/min, thus giving a total run time of 12 min.
Physical property evaluation
The tensile properties of single PET/HDPE bi-component filament were tested as per ASTM D3822 on a Favimat single fibre tester that had a load-cell capacity of 1200 cN provided by Textechno, Germany. The gauge length was kept 20 mm and test performed at crosshead speed of 20 mm/min. Fineness of the filament was calculated as per IS:1315. The elongation at break and tenacity were obtained by averaging at least 20 test specimens for each sample. The mass of the knitted net fabric was measured according to IS:1964. The mass of 20 mm × 20 mm samples was measured. Mass was expressed as grams per square metre (gsm). Bursting strength was measured according to IS:1966.
Results and discussion
Morphology of filaments
Cross-sectional view of filaments clearly shows the presence of core-sheath morphology of bi-component filaments, and all the filaments were found almost circular in shape. Core-sheath structure of bi-component filaments is also vividly depicted in Figure 4, which shows two concentric rings having polyester core in the inner circle and HDPE sheath in the outer circle. It is also observed from the figure that the central part has higher concentration compared to the outer part with respect to their area.
Cross-sectional view of polyester/HDPE filaments.
Thermal analysis
The TGA graph of pure DM powder sample as shown in Figure 5 clearly reflects that the onset degradation temperature for DM is 232℃ and almost completely degraded 98.99% up to 260℃. The thermal properties of DM clearly indicate that it can only be melt mixed with low-melting temperature polymer like HDPE. Incorporation of insecticide into textile articles also has its regulation limit, and if insecticide has been incorporated only into surface of filament instead of entire cross-section, then enhanced insecticidal efficiency will be resulted. Therefore, bi-component melt-spinning process has been adopted to make insecticide-incorporated filaments.
TGA curve of DM powder. DM: deltamethrin; TGA: thermogravimetric analysis.
The result of DSC for the insecticide incorporated filament is presented in Figure 6. The thermogram obtained for the insecticide-incorporated filament sample shows two distinct peak at 127.36℃ and 248.47 to 254.58℃, one very sharp peak at 127.36℃, which indicates the melting point of the sheath material i.e. HDPE and other one is within the range of 248℃ to 254℃, which is an acceptable melting point of polyester.
DSC thermogram of the insecticide-incorporated knitted fabric. DSC: differential scanning calorimetry.
Insecticidal efficiency (cone bioassay test)
Initial effectiveness against mosquitoes
Initial effectiveness against mosquito at varied DM content.
DM: deltamethrin; KD: knockdown.
Effect of washing and regeneration time on knitted net fabric
Figure 7 depicts the insecticidal efficiency of the 0.25% DM-incorporated knitted fabric after repeated wash cycles. Initially, unwashed fabrics show 90% KD and 100% mortality rate. However, after washing, sharp decrease in the effectiveness of the knitted fabric within three wash cycles was observed. Ten percent KD and 50% mortality were illustrated in column diagram of Figure 7. After 10th wash, bioassay results show 0% knock down and 50% mortality.
Cone bioassay test results after 3rd, 5th and 10th washes of knitted samples containing 0.25% DM. AR: after regeneration; BR: before regeneration.
Washing of the knitted fabric removes the insecticide from the surface, and the net becomes chemically inactive. After each wash, some of the insecticide is thought to be removed from the surface, but active ingredient just beneath the surface will diffuse from inside after a certain period of time. Diffusion of insecticides from inside the fibre of net to the surface is temperature dependant and can be expedited by heat treatment. In this work, the fibres were treated at 40℃ for 4 h in an oven to improve the diffusion of insecticide. Practically, sunlight helps the diffusion of insecticide onto the surface of insecticide-incorporated textile articles.
Regeneration of the knitted samples shows improved % average knock down after third and fifth wash, i.e. 40% and 30%, respectively; however, no improvement after 10th wash cycle could be seen whereas the regeneration process improved the percentage mortality, i.e. 90–100% up to fifth wash as compared to 50% before regeneration. However, after 10 washes, it shows only 60% mortality after regeneration. Therefore, further washing of the samples containing 0.25% DM in the filament was not performed.
Bioassay test results (% mortality) of knitted samples containing 0.50% and 0.75% DM.
Mechanical properties of insecticide incorporated bi-component filaments and knitted net fabric.
DM: deltamethrin.
Incorporation of 0.75% DM in the yarn significantly improved the insecticidal efficiency i.e. 100% mortality was achieved after 3rd, 10th and 20th wash cycles as depicted in Table 2. The results show that after successive wash cycles up to 20 washes, residual DM conc. is still sufficient to impart insecticidal efficiency in case of 0.75% DM in the yarn.
To study the effect of washing on insecticidal chemical, residual DM conc. in the knitted fabric having 0.75% DM, after 3rd, 10th and 20th wash cycles, was evaluated by using GC. Figure 8 shows the effect of washing on DM content present into the bi-component yarn of knitted fabric. The unwashed knited fabric initially contained 0.75% DM. Results show that the DM conc. after third wash cycle decreased up to 0.707%, and after 20th wash, the residual conc. of DM was 0.252% (Figure 7). The residual active content of 0.75% DM in the yarn after 20 wash cycles is equivalent to the content of unwashed samples having 0.25% DM in the yarn. Both the samples are showing 100% mortality. The DM content loss in knitted fabric after 20 washing is around 66.7% but still the fabric provides 100% mortality.
Residual deltamethrin content after 3rd, 10th and 20th wash cycles in knitted fabric containing 0.75% deltamethrin in overall yarn.
Tensile properties
The measured tensile and other physical properties of developed insecticide-incorporated bi-component filaments and knitted fabric are summarized in Table 3. Untreated knitted fabric was mainly evaluated for bursting strength and mass. Fineness of the filaments was calculated by weighing the known length of filaments. Tensile property evaluation of 0.25%, 0.50% and 0.75% DM-incorporated bi-component filaments shows the tenacity of around 3.2 gpd to 3.0 gpd and elongation around 43%–40%. These values are comparable to the commodity textile fibres. No significant decrease in the tenacity and elongation of the bi-component filaments has been observed by increasing the content of DM from 0.25% to 0.75%. DM-incorporated net fabric shows excellent bursting strength more than 630 kPa against untreated polyester-knitted fabric. Untreated polyester-knitted fabric was outsourced and possessed different polyester grade as against polyester used in DM-incorporated knitted fabric. Due to the difference in molecular weight and processing conditions of untreated polyester-knitted fabric and DM-incorporated knitted fabric lead to different bursting strength results. This clearly indicates that incorporation of insecticide as well as bi-component structure of filament does not adversely affect the mechanical property of filaments and converted knitted fabric.
Conclusions
Bi-component melt-spinning technique has been utilized to develop long-lasting insecticidal yarn, and it was further converted into the knitted structure. Core-sheath morphology of yarn was used to incorporate DM only into the surface regime of the yarn as well as low-melting polymer, i.e. HDPE was used to disperse insecticide in order to overcome the degradation of DM at higher temperature. We have successfully achieved 100% mortality up to 20 wash cycles for long-lasting effect, incorporating 0.75% DM in the yarn. Polyester was chosen to impart strength to the yarn structure. The developed DM-containing yarn shows tenacity of around 3.1 gpd and elongation of around 40.02%. The insecticide-incorporated knitted fabric also possesses excellent bursting strength of 640 kPa, which was much higher than the untreated knitted fabric. Thus, the bi-component melt-spinning technique can be effectively used to manufacture insecticide-incorporated filaments having long-lasting efficacy that can be further converted into various textile stores such as long-lasting insecticidal net, tents, shelters and curtains.
Footnotes
Acknowledgements
The authors greatly appreciate the support given by BiswaRanjan Das during preparation of the manuscript.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The auth or(s) received no financial support for the research, authorship, and/or publication of this article.