Functional assessment of biodegradable cotton nonwoven substrates permeated with spatial insect repellants for disposable applications

Abstract

Arboviral diseases and malaria transmitted by mosquitoes are major health concerns worldwide responsible for millions of deaths annually. Spatial repellents have been used as preventative measures via direct application to the skin or disposable devices worn outside of clothing. Naturally derived, plant-based repellents are safe, effective alternatives to synthetic repellents and may be applied to disposable substrates including adhesive patches. Many disposables are composed of synthetic petrochemical-based polyester and polypropylene nonwoven fabrics, contributing to growing concern surrounding microplastic pollution. Sustainable and biodegradable substrates including those made from cotton are cost effective and environmentally friendly. Therefore, we explored the utilization of cotton-based substrates for the application of natural spatial and synthetic contact insect repellents. Cotton fibers used in the study were commercially available greige cotton, scoured and bleached greige cotton and reginned greige cotton motes, a value-added byproduct of the ginning process. Synthetic polyester and polypropylene were used for comparison. Thermogravimetric/derivative thermogravimetric analysis (TG/DTG) in combination with modulated differential scanning calorimetry (mDSC) were used to discern vaporization properties of repellents from the substrates. Retention times of repellents on the substrates were analyzed using a Fourier-transform infrared spectroscopy–focal plane array detector and compared with the TG/DTG and mDSC data. Mosquito landing assays confirmed the feasibility of using cotton-based substrates treated with plant-based spatial repellents as an effective and environmentally friendly alternative to synthetic materials.

Keywords

coatings,cotton,environmental sustainability,essential oils,hydroentanglement,nonwovens,spatial repellent

Mosquitoes belonging to the Aedes, Anopheles and Culex genera cause millions of deaths annually and affect potentially billions more through the transmission of epidemiologically important pathogens including malaria, yellow fever, dengue fever, Rift Valley fever, West Nile virus and Zika virus.^1–4 Recently, outbreaks of arboviral diseases such as yellow fever in Angola in 2015, chikungunya in India (1.38 million cases in 2006) and the Americas (1.21 million cases in 2016) and Zika viruses in the Americas in 2015 have gained global attention. In fact, the Center for Disease Control (CDC) has included yellow fever, chikungunya and the Zika virus on the list of top potential disease threats.⁵ While a vaccine is available for yellow fever, no specific vaccine or treatment is currently available for chikungunya or the Zika virus.⁶ For this reason, the prevention of arbovirus transmission is critical for residents and visitors of at-risk areas as well as first responders or other emergency personnel.

Traditionally, synthetic and active ingredient repellents such as N, N-diethyl-3-methylbenzamide (DEET), 3-(N-acetyl-N-butyl)-aminopropionic acid ethyl ester (IR3535), 2-(2-hydroxyethyl)-1-methylpropylester (Icaridin) or p-menthane-3,8-diol (PMD)⁷ have been used topically applied to skin to repel disease-carrying vectors. In fact, DEET is often heralded as the “gold standard” of insect repellency.⁸ Recently there has been growing concern over the use of synthetic repellents due to their persistence in the environment and risk of toxicity.^9–11 Efforts have increased to identify natural, biodegradable substances which are less harmful to the environment, and essential oils present a generally safe, biodegradable¹⁰ and eco-friendly alternative.

Essential oils derived from Helichrysum italicum (immortelle),⁷,¹² Syzygium aromaticum (clove)^13–15 and Thymus vulgaris (thyme)¹³,¹⁴,¹⁶ have previously demonstrated spatial repellency against a variety of mosquitoes including those belonging to the Aedes, Anopheles and Culex genera. Unfortunately the direct application of essential oils to the epidermis may result in undesirable side effects through transdermal absorption.¹⁰ However, this potential drawback is circumvented by application of the essential oil to a substrate, such as an adhesive fabric patch, which can act as a wearable, mosquito-repellent device. Textile substrates might include popular synthetics such as polyester and polypropylene, or naturally derived fibers such as cotton. Synthetic polyester and polypropylene commonly used for disposable applications are not biodegradable¹⁷,¹⁸ and exacerbate the growing problem of microplastic pollution in terrestrial and marine environments.¹⁹,²⁰ In this regard, cotton is superior as it is both biodegradable²¹,²² and sustainable.²²,²³ Textiles intended for single-use applications are often produced by nonwoven methods, such as hydroentanglement, which has an advantage in terms of ease as well as the overall simplicity of production.²⁴,²⁵ Therefore, our goal was to compare the volatilization of spatially repellent H. italicum, S. aromaticum and T. vulgaris essential oils from both synthetic and naturally derived nonwoven substrates. Nonwoven substrates included greige cotton (GC), scoured and bleached greige cotton (SBGC), commercially scoured and bleached cotton (CSBC), reginned greige cotton motes (RGGM), polyester (PET) and polypropylene (PP). As the mode of action for spatial repellents involves the generation of a vapor phase to create a barrier,²⁶ the vaporization of the oils was compared using thermogravimetric/derivative thermogravimetric analysis (TG/DTG) and conventional modulated differential scanning calorimetry (mDSC) to calculate the energy of activation and enthalpy of vaporization for each essential oil and fabric combination. Fourier-transform infrared spectroscopy (FTIR) focal plane array data were collected to visualize the retention of essential oils on the nonwoven substrates as well. Mosquito landing experiments were also conducted to assay the practical correlation between the volatilization of essential oils as a function of thermoanalytical parameters and spatial repellency.

Materials and methods

Nonwovens fabric production

Staple fibers used in this study for in-house nonwoven fabric production are shown in Table 1 and included: mechanically cleaned greige cotton fibers (True Cotton, TJ Beall Company, Greenwood, MS); commercially scoured and bleached cotton fibers (HighQ, Barnhardt Manufacturing Company, Charlotte, NC); polyester fibers (Consolidated Fibers Incorporated, Charlotte, NC); polypropylene fibers (FiberVisions, Duluth, GA) and reginned greige cotton motes (TJ Beall Company, Greenwood, MS). Staple fibers were chute fed to a 101.6 cm-wide textile card fitted with Cardmaster plates (Saco Lowell), followed by feeding the card web into a commercial crosslapper and needlepunch (NP) machine (Technoplants srl., Pistoia, Italy). The NP processing parameters were as previously described.²⁷ The NP fabrics were converted into hydroentangled (H-E) nonwoven fabrics on a 1m-wide Fleissner pilot-scale hydroentanglement system (Trützschler Nonwovens GmbH, Dülmen, Germany) running at a constant production speed of 5 m/min. The H-E system utilized three pressure heads: one low pressure for fabric wet-out maintained at a constant pressure of 3 MPa during fabric production; and two high-pressure heads both maintained at 7.5 MPa during fabric production. Each strip on the pressure heads consisted of 16 orifices per centimeter with an orifice pore size of 120 µm. The water used for the H-E fabric production was ambient temperature, 25°C. Following H-E, the fabrics were fed directly through a gas-fired fabric drying oven (Trützschler Nonwovens GmbH) at 170°C and wound into rolls. The 100% scoured and bleached mechanically cleaned cotton was produced from the mechanically cleaned greige cotton that was subsequently scoured and bleached in-house using a JFO overflow jet dyeing system (Werner Mathis AG, Oberhasli, Switzerland) as previously described.²⁸

Molecular weight estimation of essential oils

Clove bud (Syzygium aromaticum, Indonesia), Immortelle (Helichrysum italicum, Bosnia and Herzegovina) and Thyme (Thymus vulgaris, Spain) essential oils were obtained from Plant Therapy Essential Oils (Twin Falls, ID). The molecular weight of each essential oil was determined using the quantitative GC-MS profiling certificate of analysis provided by the manufacturer of the essential oils. The molecular weight of each component was multiplied by the molar ratio indicated in the GC-MS spectrum to provide an accurate estimation of the molecular weight of each essential oil. Using these data, the molecular weights of the following essential oils were calculated as: S. aromaticum: 165.65 g/mol (97.1%); H. italicum: 182.78 g/mol (96.41%); and T. vulgaris: 134.31 g/mol (92.4%).

Thermoanalytical analyses

Samples subjected to thermoanalytical analyses included GC, SBGC, CSBC, RGGM, PET and PP nonwovens as described above. S. aromaticum, H. italicum and T. vulgaris essential oils were applied to each nonwoven sample. Analytical grade (3-phenoxyphenyl) methyl 3-(2, 2-dichloroethenyl)-2, 2-dimethylcyclopropane-1-carboxylate (Permethrin, Sigma-Aldrich, St. Louis, MO) was also included for comparison, as it served as the positive control for the mosquito landing/biting assays.

Thermogravimetric and derivative thermogravimetric analysis

TG/DTG experiments were carried out on a TA Instruments Q 500 Thermogravimetric Analyzer (TA Instruments, New Castle, DE) under nitrogen. Samples consisted of 7.0 µl of the essential oil (or Permethrin) applied to a 3.0 (±0.2) mg sample of nonwoven fabric and were subjected to rising temperature programs from 30°C to 300°C at a heating rate of 10°C/min. DTG curves plotting the derivative mass percent change over time as a function of temperature were generated using TA Instruments Universal Analysis software.

Modulated differential scanning calorimetry

Conventional mDSC was carried out using a TA Instruments Q100 Differential Scanning Calorimeter (TA Instruments, New Castle, DE) under nitrogen. Samples consisted of 3.5 µl of the essential oil (or Permethrin) on a 1.5 (±0.2) mg sample of fabric and were incubated at 30°C for 5 minutes before being heated to 300°C at a rate of 5°C/min with an amplitude of +0.250°C and a period of modulation of 20 seconds. The enthalpy of vaporization for the vaporization of the essential oil (or Permethrin) from the fabric was found by integration of the endotherm of the non-reversible heat flow component²⁹ using TA Instruments Universal Analysis software.

Activation energy calculation

The derivation of kinetic parameters for various chemical processes has been the subject of extensive research.^30–33 The Arrhenius Equation (1) is most often the starting point to determine kinetic parameters such as the activation energy (E_a), coefficient of evaporation (k) and the pre-exponential factor (A) and is expressed as:

k = A e^{(- \frac{E a}{R T})}

(1)

Where R is the gas constant and T is the temperature in Kelvin. Taking the natural log of both sides of the equation yields:

ln k = ln A - (\frac{E a}{R T})

(2)

As evaporation occurs, k is related to the fraction of the sample evaporated (α) over time (t) by the expression:

α = k t

(3)

The fraction of sample evaporated is simply the difference between the initial mass and the mass at a given time, t, divided by the difference of the initial mass and the final mass. The rate of change of α over the rate of change of t is expressed as k as a function of α and written as:

\frac{d α}{d t} = k f (α)

(4)

And is related to the change in temperature by the expression:

\frac{d α}{d T} = (\frac{d α}{d t}) (\frac{d t}{d T})

(5)

Where dt/dT is the inverse of the heating rate, β, in degrees Celsius per second.

Thus, equation (4) can be written as:

\frac{d α}{d T} = (\frac{d α}{d t}) β^{- 1} = k f (α)

(6)

For zero-order processes, f (α) is a constant equal to 1. Substituting the value of 1 for f (α) and rearranging the equation to solve for k gives:

k = (\frac{d α}{d T}) β

(7)

Substituting this expression into the Arrhenius Equation (equation (2)) yields:

ln [(\frac{d α}{d T}) β] = ln A - (\frac{E a}{R T})

(8)

Which can be written in the form of a linear equation as:

ln [(\frac{d α}{d T}) β] = - (\frac{E a}{R}) (\frac{1}{T}) + ln A

(9)

Using this expression, a series of points may be plotted as $ln [(\frac{d α}{d T}) β]$ vs $\frac{1}{T}$ . The line through these points found by using the least square regression analysis has a slope equal to the value of –(E_a/R), meaning that the slope multiplied by –R gives E_a. Additionally, the pre-exponential factor A is given by the y-intercept.³⁴ Using this approach, the energy of activation and pre-exponential factor may be found.

Scanning electron microscopy (SEM)

The sample fabrics were mounted on standard aluminum stubs using carbon adhesive tabs. The SEM mounts were coated using a custom-made 4-gun sputtering tool with a gold target to a thickness of 100 nm. The specimens were examined in a KLA-Tencor (Amray) 3600FE field emission microscope (SEMTech Solutions, Inc., North Billerica, MA) at an accelerating voltage of 15 kV under high-vacuum conditions. SEM images of each fabric sample were taken at magnifications of 50×, 100×, 200× and 500× to observe the morphology of the fabrics.

Focal plane array–FTIR

Samples were prepared by treating a 5 cm ×10 cm nonwoven fabric sample, placed in a 2 dram vial with a solution containing 225 mg of the essential oil of interest (or Permethrin) dissolved in 3 ml of 200 proof ethanol for 10 minutes. The samples were then placed in the hood and the ethanol allowed to evaporate at ambient temperature for 1 hour. The samples were analyzed 0 minutes, 6 hours and 24 hours after fabric substrate treatment with the repellent of interest. Treated samples were examined with an IMAC macro sampling chamber (Bruker Optics, Billerica, MA). The sampling chamber was outfitted with a Mid-IR focal plane array detector and a FastIR single reflection ATR unit (Harrick Scientific Products, Pleasantville, NY). Samples were placed atop the ATR crystal, and secured with a metal clamp and metal plate. All FTIR data were collected in reflectance mode. Samples were examined with 32 scans with a resolution of 8 cm⁻¹ (3800 cm⁻¹ to 900 cm⁻¹). Pixels were grouped (co-addition of adjacent pixels) into 16 × 16 groups. An air background was acquired prior to each examination. Spectra for each sample were corrected for atmospheric CO₂ and baseline-corrected using the OPUS 3D spectroscopy software version 6.5 (Bruker Optics, Billerica, MA).

Landing assays

Female Aedes aegypti mosquitoes (Orlando strain, 1952) from a colony maintained at the USDA Center for Medical, Agricultural and Veterinary Entomology (CMAVE) in Gainesville, Florida were used to conduct landing/biting assays on three human volunteers. Newly emerged adult mosquitoes were received 10% sugar water and were kept in laboratory cages with a temperature of 28 + 10°C and relative humidity of 35–60%. Non-fed female mosquitoes were pre-selected from stock cages using a hand-draw box and trapped in a collection trap.³⁵ Approximately 500 females were placed in a 45 cm × 37.5 cm × 35 cm test cage and acclimatized for 20 minutes before testing commenced.³⁶ A 50 cm² (5 cm × 10 cm) sample of the nonwoven cloth was placed in a 2 dram vial containing 75 mg of the candidate repellent in 1 ml of 200 proof ethanol to give an applied rate of 1.5 mg/cm². The vial was sealed to allow the solution to saturate the test fabric. The saturated cloth was then removed and each end of the cloth attached to a 5 cm × 2.5 cm file card and the card/cloth assembly allowed to dry for 15 minutes to evaporate the ethanol. To prepare the arm for testing, the volunteer placed a latex glove extended length beyond the wrist over the hand and arm and then pulled a nylon hose stocking over the hand and arm up past the elbow. A thick plastic sleeve with a 4 cm × 8 cm window was fastened around the arm. The cloth card frame was taped onto the forearm at a position overlaps the window opening of the sleeve, allowing attractive human odors to escape through the opened area of the sleeve. Once the cloth/card assembly was on the arm, the arm was placed inside an insect cage for one minute. If five bites or more are received during a test, this indicates the failure of the chemical as a repellent. If fewer than five bites were received, then the chemical is considered to pass as a mosquito repellent. In addition to the essential oils of interest, permethrin was included as a positive control.

Results and discussion

Scanning electron microscopy

SEM was used to compare the morphology of fabric substrates utilized in this study to probe any correlations between the evaporative properties of the treated fabric samples and their surface features. As shown in Figure 1(a) and Figure 1(b), all substrates exhibit identical morphologies. Therefore, any differences in the vaporization behavior of essential oils from these substrates is most likely a function of the constituent fiber’s properties. Fiber characteristics which may affect their interaction with substances such as essential oils include their surface area and lipophilic status.

Figure 1.

SEM images of nonwoven samples used in the study. (a) All images were acquired at 50× magnification. (b) All images were acquired at 200× magnification. The white scale bar shown in the lower right of all panels is 100 µm. GC: greige cotton; SBGC: scoured and bleached greige cotton; CSBC: commercially scoured and bleached cotton; RGGM: reginned greige cotton motes; PET: polyester; PP: polypropylene.

The GC sample at 200x magnification (Figure 1(b)) reveals fibers with the quintessential twists and crimped shape associated with cotton fibers. This sample is also more lipophilic, as the fibers retain some degree of their epicuticular waxes. The fibers in the RGGM sample are also very similar to that of the GC sample except that the RGGM fibers are shorter and may contain various impurities such as seed coat or leaf fragments. The SBGC and CSBC also exhibit the same twists and shape as that of the GC fibers; however, these samples have increased hydrophilicity, due to the scouring and bleaching process, which removes lipophilic waxes and other contaminants. The images of the two synthetic samples, PES and PP, show fibers which are smooth and tubular in appearance, with virtually no irregularities. PES and PP fibers are also lipophilic, a reflection of the chemical composition of their monomeric subunits.

It would be expected that samples composed of fibers with more surface area, such as the cotton-based samples, may support enhanced oil absorption compared with smoother, extruded fibers which lack surface features such as PES and PP. The lipophilicity of the fiber may play a role as well, with more lipophilic fibers such as PES, PP and GC displaying a greater affinity for essential oil than less lipophilic fibers such as SBGC and CSBC.

Activation energy and enthalpy of vaporization

Essential oils are composed of mixtures of volatile organic compounds whose vapor phase acts as a spatial repellent to obstruct the feeding behavior of hematophagous insects such as mosquitoes.¹¹,¹⁵ Therefore, the efficacy of essential oils as spatial repellents depends to some degree on how easily they vaporize from liquid to vapor, creating a protective barrier. The volatility of essential oils may be described using kinetic parameters including the activation energy and the enthalpy of vaporization. Unfortunately, little literature exists on this topic due to the variability of these substances, which poses unique challenges. Although essential oils are primarily composed of monoterpenes, sesquiterpenes and phenylpropanoids, these components can have over 100 possible configurations.¹⁰ Another factor contributing to the variability of essential oils is their status as secondary metabolites. The practical implication of this is that a number of factors such as the stage of harvest, temperature, soil acidity, photoperiod, humidity and geographic location play a role in the chemical profile of the resultant essential oil.¹⁰ Thus, the analysis of essential oils can be complicated, as they are multi-component systems whose molecular weights can vary. However, an estimation of the average molecular weight of an essential oil may be found if the molar composition of the sample is determined, usually by quantitative GC-MS. Using these data in conjunction with the information provided by TG/DTG and mDSC thermograms, it is possible to describe the kinetic parameters of activation energy and enthalpy of vaporization for the volatilization of essential oils from various nonwoven fabric substrates.

TG curves for all samples (Figure 2) demonstrated the characteristic reverse sigmoidal curve indicative of a single-step process. All nonwovens treated with the essential oils of interest demonstrated a 70% weight loss, corresponding to the evaporation of 7.0 µl of the essential oil which was applied to the 3.0 mg fabric sample. The repellent-only samples (i.e. neat) showed 100% mass loss due to total evaporation. In order for the process to be classified as evaporation, it must follow zero-order kinetics, which can be verified from the shape of the corresponding DTG curve depicting derivative percent mass over time versus temperature. For a zero-order process, the DTG curve exhibits an abrupt, nearly vertical decline from the maximum value to the baseline.³⁰,³¹,³³ DTG curves for the vaporization of the essential oils and permethrin (Figure 3) from fabric substrates were indicative of a zero-order process. The shape of both the TG and DTG curves is also reflective of the complexity of the essential oils. S. aromaticum essential oil, which is 80% 4-allyl-2-methoxyphenol (eugenol), shows a single DTG peak. T. vulgaris essential oil, which is 46% 2-isopropyl-5-methylphenol (thymol), shows one major peak with a small shoulder, due to the presence of other minority constituents. H. italicum essential oil, whose major component is 2,6,6-trimethylbicyclo[3.1.1]hept-2-ene (α-pinene) (21%) also shows a major peak with overlapping minor peaks corresponding to the remaining 80% of its constituents preceding it. The DTG curve for the permethrin-treated substrates demonstrates the characteristic sharp decline from the maximum found in zero-order processes. However, the PET and PP permethrin-treated samples did not exhibit the signature zero-order process curve shape due to overlapping peaks generated by the decomposition of PET and PP, which decompose at lower temperatures than the cotton-based samples, which must be exposed to temperatures exceeding 300°C.

Figure 2.

TG thermograms depicting percent mass loss versus temperature for (a) S. aromaticum, (b) H. italicum, and (c) T. vulgaris essential oils as well as permethrin (d) applied to various nonwoven fabric substrates. GC: greige cotton; SBGC: scoured and bleached greige cotton; CSBC: commercially scoured and bleached cotton; RGGM: reginned greige cotton motes; PET: polyester; PP: polypropylene.

Figure 3.

DTG thermograms depicting derivative mass over time versus temperature for (a) S. aromaticum, (b) H. italicum, and (c) T. vulgaris essential oils as well as permethrin (d) applied to various nonwoven fabric substrates. GC: greige cotton; SBGC: scoured and bleached greige cotton; CSBC: commercially scoured and bleached cotton; RGGM: reginned greige cotton motes; PET: polyester; PP: polypropylene.

Other kinetic parameters including the energy of activation may be found by applying the Arrhenius Equation using equations (1) to (9) to the TG data. The enthalpy of vaporization may also be found using mDSC, the results of which are tabulated in Table 2. mDSC is useful in this case because it enables the non-reversible heat flow, which is attributed to vaporization, to be isolated and the enthalpy of vaporization to be elucidated by integration of the endotherm. mDSC thermograms for the vaporization of S. aromaticum essential oil (Figure 4(a)) and permethrin (Figure 4(d)) both alone and from fabric substrates show the presence of one major endotherm corresponding to the vaporization of S. aromaticum essential oil and permethrin, respectively. The curves for samples treated with H. italicum (Figure 4(b)) and T. vulgaris (Figure 4(c)) essential oils also reveal a major peak, although it is less distinct. In this case, the major peaks are less intense and less defined that that of the S. aromaticum essential oil and permethrin-treated samples, due to their more varied compositions. In addition, in all PET and PP samples, the mDSC curves exhibit additional small endotherms in the regions of 155°C and 247°C, which correspond to their respective melting points as reported in the literature.³⁷,³⁸ Endotherms related to the decomposition of the cotton nonwoven samples were not observed, as cotton decomposes at temperatures above 300°C.

Table 1.

Summary of staple fibers used for in-house production of nonwovens roll goods converted into wipes materials

Fiber Type	Manufacturer	Staple Length (mm)	Fineness (Denier)
Polyester	Consolidated Fibers Inc., Charlotte, NC	38.00	0.90
Greige Cotton	TJ Beall Co., Greenwood, MS	27.94	1.50
Reginned Greige Cotton Motes	TJ Beall Co., Greenwood, MS	14.73	1.24
Conventionally Scoured and Bleached Cotton	Barnhardt Manufacturing Co., Charlotte, NC	24.60	1.64
Polypropylene	Fiber Visions, Duluth, GA	38.10	1.50

Table 2.

Kinetic data obtained for the vaporization of S. aromaticum, H. italicum and T. vulgaris essential oils as well as permethrin from various nonwoven fabric substrates. E_a = energy of activation; ΔH_vap = enthalpy of vaporization; GC: greige cotton; SBGC: scoured and bleached greige cotton; CSBC: commercially scoured and bleached cotton; RGGM: reginned greige cotton motes; PET: polyester; PP: polypropylene.

	E_a (kJ/mol)	R²	ΔH_vap (kJ/mol)
S. aromaticum
No substrate	36.18	0.99	40.75
GC	38.39	0.99	29.06
SBGC	42.23	0.99	24.95
CSBC	40.43	0.99	27.35
RGGM	38.87	0.99	28.61
PET	39.07	0.99	28.77
PP	40.00	0.99	33.13
H. italicum
No substrate	12.18	0.90	16.96
GC	13.61	0.90	11.96
SBGC	11.49	0.92	11.84
CSBC	11.80	0.91	12.57
RGGM	11.76	0.94	13.85
PET	11.72	0.94	11.42
PP	12.24	0.92	15.26
T. vulgaris
No substrate	7.875	0.99	14.00
GC	11.21	0.99	13.47
SBGC	15.48	0.99	13.38
CSBC	11.44	0.99	11.29
RGGM	12.80	0.99	12.31
PET	15.66	0.99	15.35
PP	12.78	0.99	15.47
Permethrin
No substrate	53.57	0.99	79.47
GC	57.53	0.98	37.25
SBGC	58.62	0.98	31.67
CSBC	62.29	0.99	32.57
RGGM	64.50	0.99	32.68
PET	56.62	0.98	29.88
PP	56.50	0.99	34.92

Figure 4.

mDSC thermograms depicting non-reversible heat flow versus temperature for (a) S. aromaticum, (b) H. italicum, and (c) T. vulgaris essential oils in addition to permethrin (d) applied to various nonwoven fabric substrates. GC: greige cotton; SBGC: scoured and bleached greige cotton; CSBC: commercially scoured and bleached cotton; RGGM: reginned greige cotton motes; PET: polyester; PP: polypropylene.

Table 3.

Mosquito (Aedes aegypti) landing assay results for spatially repellent essential oils and contact repellent permethrin on various substrates. The concentration of the repellent on each substrate was 1.5 mg/cm². Pass = 0–4 bites; Fail = ≥5 bites. GC: greige cotton; SBGC: scoured and bleached greige cotton; CSBC: commercially scoured and bleached cotton; RGGM: reginned greige cotton motes; PET: polyester; PP: polypropylene.

Substrate	Control	T. vulgaris	S. aromaticum	H. italicum	Permethrin
GC	F	P	P	P	P
SBGC	F	P	P	P	P
CSBC	F	P	P	P	P
RGGM	F	P	P	P	P
PET	F	P	P	P	P
PP	F	P	P	P	P

S. aromaticum

The activation energy and enthalpy of vaporization for the vaporization of S. aromaticum essential oil, shown in Table 2, were 36.18 kJ/mol and 40.75 kJ/mol and were in good agreement with previously reported literature values.³⁹ The activation energy and enthalpy of vaporization values did not vary greatly with the type of fabric, having a standard deviation of 1.58 and 2.67, respectively, for both sets of data. Of the treated fabric samples, the GC samples had the lowest activation energy with GC having an activation energy of 38.39 kJ/mol and the RGGM having an activation energy of 38.97 kJ/mol. The treated fabric with the highest activation energy was SBGC, with an activation energy of 42.23 kJ/mol. This slight difference may be attributed to the ability of the majority component of S. aromaticum, 4-allyl-2-methoxyphenol (eugenol) shown in Figure 5(a), to hydrogen bond with the fabric through its hydroxyl group, which can act as both a hydrogen bond donor and acceptor and its methoxy group, which can act as a hydrogen bond acceptor. Examples of nonwoven fabrics which possess the ability to hydrogen bond include cotton and PET. The cellulose present in cotton may hydrogen bond through its hydroxyl and ether linkages, while PET has the potential to hydrogen bond through its ester group oxygens, which can act as hydrogen bond acceptors. In SBGC in particular, the cellulose chains are more accessible as extraneous waxes, pectin and other components have been removed. This increased accessibility and dynamic interactions via hydrogen bonding results in a slightly higher affinity of S. aromaticum essential oil for SBGC than the other treated fabrics, even CSBC. The CSBC sample, although it is also cotton which has been scoured and bleached, has been treated with an oil finish to provide lubricity and facilitate fiber processing. Therefore, the hydroxyl groups of CSBC are less accessible than those of SBGC, which lacks a finish. Interestingly, the neat essential oil sample has a higher enthalpy of vaporization than the treated fabric samples, with an enthalpy of vaporization of 40.75 kJ/mol. This is most likely because the cohesive intermolecular forces present within the essential oil mixture require more energy to disrupt than when the essential oil is applied to the nonwoven samples, in which case the essential oil is dispersed along the surface of the fabric and the cohesive intermolecular forces are strained. Therefore more energy, in the form of heat, is required to disrupt the forces holding the essential oil together as a neat liquid, than when the molecules are more diffuse and intermolecular forces are weakened such as when the essential oil has been applied to a nonwoven fabric.

Figure 5.

FTIR spectra of greige cotton treated with (a) S. aromaticum, (b) H. italicum and (c) T. vulgaris essential oils and (d) permethrin. Chemical structures denote the primary components of each repellent.

H. italicum

The H. italicum essential oil-treated SBGC has the lowest activation energy at 11.49 kJ/mol, while GC has the highest activation energy at 13.61 kJ/mol. Although the essential oils studied here are implicitly hydrophobic, some are slightly more hydrophobic than others. S. aromaticum essential oil, whose chief component eugenol, shown in Figure 5(b), permits hydrogen bonding, favors nonwoven fabrics whose fibers afford maximum hydrogen bonding opportunities, such as SBGC. In the case of H. italicum, the inverse is true. The GC-MS reveals that H. italicum essential is composed of more lipophilic components than the essential oils S. aromaticum or T. vulgaris. Thus, this essential oil binds well to similarly lipophilic fibers such as GC. Although the RGGM sample is also greige cotton, it has a lower activation energy which may be due to the shorter, thinner fibers (Table 1) present in the sample. Thinner, shorter fibers mean that less surface area is available to interact with the essential oil, leading to lower affinity of the oil for the smaller fibers in the RGGM.

It may initially appear contradictory that the H. italicum oil has a greater affinity for GC (E_a = 13.61 kJ/mol) than PP (E_a = 12.24 kJ/mol). However, the slightly higher activation energy of GC over PP is likely a result of the GC and PET fiber morphologies. Although the PET fiber is slightly longer than the GC fiber and the two fibers have identical fineness, GC fibers have additional features which promote liquid uptake through capillary action. PP fibers are formed by melt extrusion to yield fibers with uniform and well-defined geometries. In the case of the PET fibers used for the nonwovens in this experiment, the fibers are smooth and uniform. Unlike cotton fibers, these PET fibers do not possess a lumen, providing virtually no surface character and decreasing the amount available fiber surface area to interact with the environment. As is the case with S. aromaticum essential oil, the neat H. italicum essential oil has the highest enthalpy of vaporization at 16.96 kJ/mol. However, the enthalpy of vaporization for PP is very close at 15.26 kJ/mol, a result of the oil’s affinity for PP. One explanation as to why GC has the highest activation energy, yet PP has the highest enthalpy of vaporization of the treated fabrics, is that the energy needed to activate the vaporization process is different than the enthalpy of vaporization. The energy of activation in the case of evaporation may be thought of as analogous to specific heat, or the amount of energy in the form of heat required to raise the temperature of a system to its boiling point. Once the system reaches the boiling point, the energy required to complete the phase change from liquid to vapor is described by the enthalpy of vaporization. In this regard, GC requires more energy to activate the vaporization process whereas PP needs more energy to complete the phase change from liquid to vapor.

T. vulgaris

The neat T. vulgaris essential oil has the lowest activation energy of the T. vulgaris essential oil-treated samples at 7.88 kJ/mol. As mentioned previously, this is likely due to the close proximity of the oil molecules to one another, as opposed to being distributed along nonwoven fabrics; heat is transferred more efficiently and less heat is needed to activate the vaporization process. The treated PET sample had the highest activation energy of the samples at 15.66 kJ/mol and SBGC has the second highest activation energy at 15.48 kJ/mol. In this case, T. vulgaris essential oil, whose major component 2-isopropyl-5-methylphenol (thymol) shown in Figure 5(c), acts as both a hydrogen bond donor and acceptor through its hydroxyl group, has a slightly higher affinity for fibers which also have accessible hydrogen bonding such as SBGC and PET. However, in terms of the enthalpy of vaporization, the treated PET and PP have the highest values at 15.35 kJ/mol and 15.35 kJ/mol, respectively. This observation may be explained by the enhanced role lipophilicity plays in the vaporization process. As the essential oil adheres more strongly with the lipophilic fibers, it is difficult to interrupt the interaction between the liquid oil and the nonwoven fibers to achieve complete vaporization.

Permethrin

Compared with the nonwoven samples treated with essential oils, the permethrin-treated samples exhibited generally higher enthalpy of vaporization and energy of activation values. This observation is explained by the increased boiling point of permethrin, which is a structurally larger molecule than the major components found in the essential oils, shown in Figure 5(d). More energy is required to volatilize permethrin than eugenol, thymol or α-pinene. It is also evident in the TG, DTG and mDSC data shown in Figures 2(d), 3(d) and 4(d), respectively, that permethrin must be heated to higher temperatures than the essential oils studied here to achieve vaporization. Due to its low volatility, permethrin is considered to be a contact, rather than spatial, repellent. Permethrin acts as a repellent only when a mosquito comes into direct contact with it, whereby it inhibits neuronal function via sodium channels leading to mortality.⁴⁰ The volatility of permethrin likely contributes to its demonstrated extended residual protection from mosquitoes and other arthropods when applied to clothing such as military uniforms.⁴¹

Of the permethrin-treated nonwoven samples, PET and PP exhibited lower energy of activation values of 56.62 kJ/mol and 56.50 kJ/mol, respectively, compared with the cotton-based samples. Energy of activation values for the cotton-based samples ranged from a minimum of 57.53 kJ/mol for GC and a maximum of 64.50 kJ/mol for RGGM. This observation may be explained by the ability of permethrin to hydrogen bond with the cellulosic hydrogens through the carbonyl and ether oxygens which act as three hydrogen bond acceptors. The enthalpy of vaporization for neat permethrin was found to be 79.47 kJ/mol, which is in reasonable agreement with the literature value of 86.07 kJ/mol reported by the Joback Method.⁴² The enthalpy of vaporization for pure permethrin was also observed to be nearly twice as much as the enthalpy of vaporization for the permethrin-treated nonwovens. It was observed that the permethrin oil readily crystallized when pipetted into the TGA and DSC pans, which did not occur when the permethrin oil was applied to the nonwoven fabric substrates. The crystallized permethrin requires additional energy input to undergo an additional phase change from the solid state to the liquid state where it can then be evaporated. In the permethrin-treated samples, the permethrin is dispersed among the fibers and cannot crystallize as in the neat state. This additional phase change from solid to liquid therefore contributes to a higher enthalpy of vaporization for the neat permethrin. Similarly to the energy of activation results, the enthalpy of vaporization results for the permethrin-treated nonwovens showed that the synthetic PET had the lowest enthalpy of vaporization at 29.88 kJ/mol while GC had the highest enthalpy of vaporization at 37.25 kJ/mol. Among all of the permethrin-treated nonwovens, GC and RGGM displayed generally higher energy of activation and enthalpy of vaporization values than the synthetic PET and PP treated samples.

Focal plane array–FTIR

FTIR analysis of the essential oil-treated fabrics showed a decrease in peaks associated with the predominant component over a period of 24 hours. In the S. aromaticum essential oil-treated samples (Figure 5(a)) the depletion of the essential oil was monitored by following peaks at 1604 and 1512 cm⁻¹, corresponding to the Ar C=C stretches in eugenol. In the H. italicum essential oil-treated samples (Figure 5(b)) the depletion of the essential oil was monitored by following the peak at 1724 cm⁻¹, corresponding to the C=C stretch in α-pinene. In the T. vulgaris essential oil-treated samples (Figure 5(c)) the depletion of the essential oil was monitored by following peaks at 1616 and 1581 cm⁻¹, corresponding to the Ar C=C stretches in thymol.

In the permethrin-treated samples (Figure 5(d)) the relative amount of permethrin followed by observing the C=O stretch at 1724 cm⁻¹ for all samples except PET, due to the overlapping of peaks associated with PET. For the PET samples, the C=C stretch at 1581 cm⁻¹ was followed. The peaks of interest in the essential oil-treated samples are most intense at the initial 0 minute time point and gradually decrease until they are barely discernable at 24 hours. In the permethrin-treated samples, the peaks of interest at 1724 and 1581 cm⁻¹ remain unchanged in intensity, even after 24 hours. This observation is in agreement with previous literature which suggests that permethrin is a contact, rather than spatial repellent.

To compare the amounts of the essential oils retained on each fabric type, FTIR–focal plane array images were obtained. In the S. aromaticum essential oil-treated samples (Figure 6(a)), PET has a higher initial concentration than the other samples while SBGC has less than the other samples. The difference in the initial amount present on the fabric sample is most likely a function of oil absorption by the fabric. The SBGC absorbs less of the S. aromaticum essential oil because it is less lipophilic than the other samples. The CSBC, which is also scoured and bleached cotton, does not exhibit similar behavior because it has been treated with a finish to increase lubricity, which in turn increases the lipophilic character of the substrate and increases its affinity for the essential oil. After 6 hours PET retains the most essential oil while CSBC and RGGM also retain elevated amounts. After 24 hours, the majority of the S. aromaticum essential oil has evaporated from all of the nonwoven substrates except PET, which retains slightly more than the other samples. Such results are not unexpected, as all of the treated samples have reasonably close energy of activation and enthalpy of vaporization values, with standard deviations across all treated fabric samples of 1.39 and 2.67, respectively.

Figure 6.

FTIR–focal plane array images of nonwoven samples treated with (a) S. aromaticum, (b) H. italicum and (c) T. vulgaris essential oils after 0 minutes, 6 hours and 24 hours. GC: greige cotton; SBGC: scoured and bleached greige cotton; CSBC: commercially scoured and bleached cotton; RGGM: reginned greige cotton motes; PET: polyester; PP: polypropylene. Images to the right of each data set indicate 225 mg, 75 mg, 25 mg and 0 mg of the essential oil applied to greige cotton.

In the samples treated with H. italicum essential oil, shown in Figure 6(b), GC shows an increased concentration of the essential oil at the initial time point. After 6 hours, the H. italicum essential oil rapidly depletes from the GC, RGGM and PP samples, while depletion in the SBGC and CSBC samples is less evident. After 24 hours, the majority of the H. italicum essential oil has evaporated from the original amount of 225 mg to approximately 25 mg, as indicated on the side bar. As in the case with the other essential oils, the energy of activation and enthalpy of vaporization values for all of the treated samples do not differ significantly among different fabric samples, with the energy of activation for the H. italicum essential oil-treated fabric samples having a standard deviation of 0.77 and the enthalpies of vaporization having a standard deviation of 1.47.

Nonwovens treated with T. vulgaris essential oil shown in Figure 6(c) reveal that all samples except for PET show a similarly high concentration of T. vulgaris essential oil initially. After 6 hours at ambient conditions, GC and PET retain slightly more of the essential oil than the remaining samples, while the CSBC treated sample exhibits accelerated depletion. This finding is in agreement with the energy of activation and enthalpy of vaporization data for T. vulgaris-treated CSBC, which has the lowest energy of activation and enthalpy of vaporization values at 11.21 kJ/mol and 11.29 kJ/mol, respectively. After 24 hours, nearly all of the T. vulgaris essential oil has evaporated from all of the samples. It is not surprising that all of the fabrics are depleted after 24 hours, as the differences in the values obtained for energy of activation and enthalpy of vaporization treated samples are relatively small, having standard deviations of 1.93 and 1.65, respectively.

Mosquito landing/biting assays

As the mode of action of spatial repellents requires vaporization of a substance to effectively create a barrier of vapor to prevent insect bites, mosquito landing/biting assays were conducted to investigate if differences in the vaporization behavior of each repellent from different nonwoven substrates might be related to their efficacy as spatial repellents. Despite minor differences in the energy of activation, enthalpy of vaporization and FTIR–focal plane array results showed minor differences between the evaporation of the essential oils from the different nonwovens, mosquito landing assays revealed that all of the fabrics which were treated with the essential oils and permethrin as a positive control were equally repellent to Aedes aegypti mosquitoes (Table 3). In this instance, the small differences in evaporative behavior of repellents from the nonwoven fabrics did not affect their performance as substrates for spatial and contact repellents. Interestingly, it should also be noted that the untreated GC and SBGC samples exhibited characteristics with respect to the mosquito feeding that were different from other fabrics. On these two control samples, only a few mosquitoes initially landed on the sample. Typically, 20 or more mosquitoes immediately land on the sample when the arm is placed in the test cage. This observation suggests that the GC and SBGC may prevent the detection of attractive human odors by hematophagous mosquitoes. The mosquitoes which did exhibit hematophagous behavior were observed to probe the untreated GC and SBGC samples extensively before biting. It was only during the very end of the trial that several more mosquitoes landed and were able to bite through the untreated GC and SBGC samples.

The results of the landing/biting assays have exciting implications. First, the treated cotton samples performed comparably to the synthetic PET and PP treated samples. This means that cotton is suitable for applications such as mosquito-repellent adhesive patches, presenting an eco-friendly alternative to synthetics. Additionally, RGGM, a value-added product, may be used for this application. The use of RGGM not only decreases the waste generated by the cotton ginning process but also presents a cost-friendly alternative to other available nonwovens. Second, the observation that the untreated GC and SBGC control samples almost passed the landing/biting assay suggests that these fabrics may require a lesser quantity of repellent chemical to be effective than traditionally synthetic nonwovens, leading to decreased costs and increased environmental benefits.

Conclusions

The vaporization of three spatially repellent essential oils from various nonwoven fabric substrates was investigated using thermodynamic data such as energy of activation and enthalpy of vaporization. FTIR analysis of all of the treated fabric samples showed a clear decrease in key peaks corresponding to the functional groups present in the essential oils as they evaporated from the fabrics. FTIR–focal plane array images were also constructed to visually compare the vaporization of the essential oils from different fabric samples. Although the thermodynamic and FTIR–focal plane array data show some small differences among the fabric samples, these differences are not great enough to produce any noticeable results when the treated samples were subjected to mosquito landing/biting assays. These results suggest that cotton-based nonwovens, including reginned greige cotton motes, which can be utilized as a value-added product, perform comparably to popular synthetic nonwovens such as polyester and polypropylene, which contribute to microplastic pollution.

Footnotes

Acknowledgments

We thank I. Cuba, G. Allen and N. Sanscrainte for the expert conduct of the landing/biting trials, and T. Carney for production of host-seeking Aedes aegypti mosquitoes at the USDA CMAVE insectaries. This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the US Department of Agriculture (USDA). USDA is an equal opportunity provider and employer.

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 received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Doug J. Hinchliffe

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