Improving the functionality of clothing through novel pesticide protection
Chemical protection is needed to reduce occupational chemical exposure for individuals involved with mixing, loading and applying pesticide. Protection is provided by chemical protective materials that function by means of barrier (i.e. no permeation or penetration), repellency, or adsorption mechanisms, or a combination of these approaches. Basic mechanisms have been used to define novel approaches for pesticide-protective clothing including microporous and nanofibrous membranes as well as self-decontamination materials based upon N-halamines, metal oxides and polyoxometalates.
Chemicals may enter the body through a variety of routes including ingestion of food or in drinking water, inhalation and absorption through the skin. Although food safety issues have gained high visibility, occupational exposure for people involved with mixing, loading or application of pesticides is also a major health issue. Dermal absorption, rather than inhalation or ingestion, is the primary route for occupational chemical exposure.
Pesticide exposure levels are influenced by a variety of factors such as wind, type of activity, method and rate of application, duration of exposure and worker hygiene. Distribution patterns on worker’s garments vary with method of application, equipment used and environmental conditions. Studies of garments after pesticide application with air-blast sprayers showed a general trend of higher exposure levels on the upper body (forearm, shoulder, chest, neck) rather than the lower body (DeJonge et al., 1985). Coffman et al. (1999) observed the highest concentration on the neck, upper arm and shoulder in a diagonal pattern from upper right shoulder (spraying arm) to lower left torso. The use of a hooded sprayer with high volume, low concentration output produced considerably less contamination than an air-assist sprayer. Nigg et al. (1990) found higher deposition on the thigh rather than chest when a canopied tractor was used to pull the air-blast sprayer.
A wide variety of health hazards related to common pesticides are known. Local effects of pesticides include irritation, allergic contact dermatitis, photo irritation, photo allergic contact dermatitis and contact urticaria. Systemic effects include seizures, aplastic anemia, various neurological symptoms, cognitive and psychomotor dysfunction, sterility and some rare fatalities (Kamel and Hoppin, 2004; Tripp et al., 2007).
Over 16 000 pesticide products are used in the United States (NIOSH, 2009). Pesticides such as methyl parathion, O,O-dimethyl O-4-nitrophenyl phosphorothioate, can cause severe poisoning effects in humans and is classified as extremely hazardous, or Class IA (Tomlin, 1997; WHO, 2002). Therefore its use has been restricted in the United States since 1978; it can now be applied only as an agricultural insecticide by a certified individual (Abou-Donia, 1994). Restriction is necessary because methyl parathion and other pesticides inhibit the acetyl cholinesterase enzyme by blocking the binding site on the enzyme (Levin and Rodnitzky, 1976; Tafuri and Roberts, 1987). Blocked acetyl cholinesterase enzyme binding sites leads to high levels of acetylcholine, which can result in death from asphyxiation. Aside from the neurotoxic effects, methyl parathion is also toxic to several organs, such as the liver, and to cardiovascular and muscular systems (Garcia et al., 2003). Additionally, methyl parathion exposure can lead to lowered concentration, slower information processing time, memory and speech impairment, depression and anxiety (Levin and Rodnitzky, 1976). It is important to find better personal protective equipment (PPE) and strategies to limit human exposures to such pesticides.
Agencies such as the Environmental Protection Agency (EPA) and United States Department of Agriculture (USDA) develop mitigation strategy systems to reduce occupational pesticide exposure. Although specific guidelines vary with individual chemicals and situations, these measures combine several closely related approaches to create practical and enforceable procedural modifications. General criteria include: use of PPE, engineering controls, limit of exposure time or reduction of active ingredient, and establishment of buffer zones. Garment materials can range from highly specialized protective suits to conventional woven cotton/polyester work clothing.
The use of PPE directly limits exposure. However, clothing that limits pesticide exposure also limits water vapor transmission and contributes to discomfort or heat stress. In order to allow increased ventilation, workers wearing more occlusive garments inadvertently decrease their personal protection by not closing them properly. For such reasons, some workers prefer more traditional types of garments such as denim coveralls.
US EPA guidelines suggest the use of basis PPE that include coveralls, apron, broad-brimmed waterproof hat, boots, rubber gloves, goggles, face shields or respirators. These items are constructed of a variety of material types from highly specialized selectively permeable membranes to everyday clothing fabrics. They can be disposable or reusable, each with benefits and drawbacks depending on the situation, working environment, user groups and toxicity of the pesticide governing the specific choices of PPE. The Center for Disease Control and Prevention (CDC) recommends that people working as mixers, loaders and applicators of pesticides wear protective clothing according to the EPA guidelines (CDC, 2005).
Performance specifications for chemical protective body garments, ASTM F2669–09, were development by the American Society for Testing and Materials (ASTM, 2009). Materials are categorized into three different levels based on average penetration values for a given challenge chemical. Properties are material and seam resistance to penetration by liquid under pressure, resistance to permeation (in the absence of applied pressure), breaking and tearing strength.
Protective garments function primarily by barrier (no permeation or penetration), repellency, adsorption or a combination of these mechanisms. Thus, chemicals are kept away from the skin by retention in the fabric structure or rolling off the outside layer without penetration. Lee and Obendorf (2007a) show the relationship between protective properties and air permeability for materials including typical woven work fabrics, non-wovens, microporous membranes and laminated fabrics (see Fig. 19.1).
19.1 Protection/comfort model (Lee and Obendorf, 2007a).
Textile structures have highly complex three-dimensional lattices with overall material properties derived from a combination of fiber chemistry and geometry, degree of open spaces, and tightness in yarn and weave. Pesticide penetration is governed by properties of the material (content and structure) as well as the physical and chemical properties of the challenge chemical. Contamination may occur through a combination of permeation, penetration and sorption. Structural and chemical characteristics of the material determine which mechanism will dominate when exposed to a specific liquid challenge chemical. Many different materials are used for PPE including monolithic polymeric films for glove, non-woven disposable fabrics, woven fabrics, and laminated or coated fabrics.
Protective garments, such as gloves, act primarily as barriers to chemical permeation. Permeation involves the sequential absorption of molecules, diffusion of absorbed molecules followed by desorption. From Fick’s Second Law, the diffusion rate of a given chemical through a material is proportional to the concentration difference through the material:
where φ is concentration, t is time, D is diffusion coefficient, and x is diffusion distance. Chemically resistant gloves or suits designed for high levels of exposure are commonly made from monolithic film or use a film or coating as part of a system. Common materials for reusable suits and gloves include: butyl rubber, nitrile, neoprene and chlorinated polyethylene (Raheel, 1994, p. 262). For these materials, the type and concentration of the carrier solvent is the most important determinant of permeation of pesticides (Schwope et al., 1992).
Permeation predictive models have been developed to estimate the movement of solvents through polymeric materials used for chemical protective clothing (Evans and Hardy, 2004; Evans et al., 2008; Goydan et al., 1988; Zellers, 1993; Zellers and Zhang, 1993). The relative affinity of a polymer and solvent can be assessed using Hansen three-dimensional solubility parameters; thus researchers studied their correlation with permeation data (Zellers, 1993; Zellers and Zhang, 1993). The model for solubility and permeation of organic solvents in polymeric glove material was improved using a combination of Hansen solubility parameters and the polymer solution theory of Flory–Rehner (Evans and Hardy, 2004; Evans et al., 2008; Zellers, 1993):
where φm is the volume fraction of polymer in the polymer/solvent system, χs is the Flory Higgins polymer interaction parameter (a constant often set between 0.3 and 0.4), Vm is the molar volume of solvent, and A is defined for a specific polymer (solute) and solvent:
where subscripts 1 and 2 stand for the solute and solvent respectively, a and b are weighting factors, δ is the individual solubility parameters where d represents dispersion, p polar, h hydrogen bonding. High correlations are observed to the solvent-polymer interactive term χ φp2 for steady-state permeation rates, breakthrough times, and lag times for butyl gloves (Evans et al., 2008) and elastomeric Viton gloves (Evans and Hardy, 2004).
Monolithic materials such as nitrile glove materials are usually impermeable in both directions. The type of carrier solvent and concentration are the primary factors to impact permeation of the active ingredient. There are no universally effective glove barrier materials against active ingredients in solvents such as alcohols, ketones, aliphatic and aromatic petroleum distillates. However, in general nitrile, butyl and Silver Shield® materials were more effective than polyvinyl chloride and natural rubber (Schwope, 1986; Schwope et al., 1992).
Upon contact with chemicals, various processes can occur such as chemical degradation and penetration, as well as permeation. Degradation can result from dissolution of the material, chemical reaction with the material, or chemical leaching. Cracking, shrinking or loss in mechanical properties can occur. Penetration is the passage of liquid chemical through pores or over openings such as holes, punctures, cracks or seams.
Interaction between the textile surface and the liquid challenge chemical is governed by characteristics including chemical composition, surface configuration and fiber roughness, pore geometry of the textile, and liquid parameters such as surface tension and viscosity. In order to understand the penetration of pesticides through protective materials, it is necessary to understand the mechanisms of interaction between challenge liquids and textile surfaces (see Fig. 19.2). The processes of surface wetting and wicking form an important foundation for understanding the two major mechanisms for protective clothing materials, sorption and repellency.
Penetration into or through the fabric occurs if the material is ‘wettable’. In wetting, the fiber–air interface is displaced by a fiber–liquid interface. Spontaneous wetting is the flow of a liquid over a solid surface toward thermodynamic equilibrium in the absence of external forces. The displacement of a fiber–air interface with a fiber–liquid interface is characterized by the contact angle θ formed between the liquid and solid and their surface energies. The Young-Dupré equation describes this equilibrium at the solid–liquid interface:
where γ is interfacial tension; subscripts S, L, and V denote solid, liquid, and vapor phases; θ is the equilibrium contact angle. Wettability increases with decreasing contact angle, or increasing cos θ; The surface tension at the maximum value for cos θ is the critical surface tension of a solid (γc), a constant property of a given solid. Thus, spontaneous spreading and wetting occurs when the liquid surface tension is less than or equal to the critical surface tension of the solid. If wetting does occur, capillary force-driven wicking allows the flow of liquid through a porous material (Hsieh, 1995; Kissa, 1996; Miller, 1977; Miller and Tyomkin, 1994).
After wetting of fibers assembled with capillary spaces between them occurs, capillary forces drive the spontaneous flow of the liquid through the porous substrate, in the wicking process. Pesticide penetration is defined as the flow of a chemical through pores, or other discontinuities in the material such as closures or other imperfections. The flow of a liquid through a fibrous material may be described using a capillary bundle model with the Laplace and Poiseuille equations. The Laplace equation gives the pressure of fluid in a capillary:
where P is pressure, γ is liquid surface tension, θ is solid/liquid contact angle, and r is pore radius. As described by the Laplace equation, positive capillary pressure (and thus liquid flow) occurs when cos θ is positive; the contact angle is between 0° and 90°.
where ρb is fabric density and ρs is fiber density. It is an important material property for sorption, flow of liquids, and vapor transmission. Fabric density (solid volume fraction) can be calculated by:
Sorption by a material is governed by inter- and intra-fiber spaces where liquids are retained through capillary forces. When fabrics have the same weave structure, yet different fiber contents, the adsorption capacity can be also quite different due to different fiber pore structures. Pore geometry and connectivity, however, is not easily described (Hsieh, 1995). Pore sizes for woven fabrics commonly have a bimodal distribution. Large sizes represent the inter-yarn spaces, whereas the inter-fiber spaces are reflected by smaller spaces (Lee and Obendorf, 2007a; Miller and Tyomkin, 1994). For woven (Tencel) and non-woven (polyethylene) fabrics with similar solid volume fraction (0.386, 0.399 respectively), Lee and Obendorf (2007a) observed different water vapor transmission rates (20 and 15 g/h m2) and through-pore size distributions. The woven fabric showed a range of pore diameters of 6.5–114.4 μm, whereas the non-woven ranged from 0.3 to 6.2 µm, all pores smaller than the lower bound of the woven distribution. They demonstrated that a property such as water vapor transmission rate cannot be linked solely to a characteristic such as solid volume fraction but must also include pore size and fiber hydrophobicity. Researchers have created statistical models to predict qualities of comfort and protection based on basic physical properties of fabrics and challenge liquids (Lee and Obendorf, 2001, 2005).
Liquid properties such as surface tension and viscosity play an important role in penetration and may be more influential in penetration than chemical composition of the active ingredient. In commercially produced pesticide formulations, ingredients (adjuvants) are often added to alter the liquid properties of the solution and increase wettability in the target environment.
Models for performance of non-woven fabrics have been developed. The penetration of repellent finished fabrics is driven by one main property, surface tension difference between the fiber substrate and the challenge liquid (γdiff = γS – γL), whereas that of untreated fabrics appears to be affected by various liquid/fabric properties (Lee and Obendorf, 2001). Repellent finished fabrics with low surface energies function through a repellency mechanism regardless of the fiber type or fabric porosity. On the other hand, untreated fabrics go through repellency, wicking and absorbency processes that are affected by not only liquid–medium surface interaction but also other fabric/liquid parameters. For fabrics with a surface tension difference (γS – γL) below − 13 mN/m, no penetration was observed for a series of non-woven fabrics; this may be an empirical diverging point determining penetration mechanisms of individual fabrics when no pressure is applied. When γS is very small, for instance, due to repellent finish, thus γdiff is equal to or less than − 13 mN/m, fabric performance is governed by repellency. The process is driven mainly by one factor, liquid-medium surface interaction (wetting and wicking). Thus, a model for pesticide penetration is as follows:
When γS is large, thus γdiff is greater than − 13 mN/m, fabric performance is governed by a combination of repellency, wicking and absorbency mechanisms, which are affected by various additional fabric/liquid parameters. For untreated (no fluorocarbon finish) materials, pesticide penetration increased with increased surface tension difference, decreased solid volume fraction, and decreased fabric thickness (Lee and Obendorf, 2001). An empirical model has been presented to predict pesticide penetration for untreated non-woven fabrics. The regression equation is:
where γdiff is the surface tension difference; ρb is the solid volume fraction in equation [19.8]; t is the fabric thickness. Pesticide penetration increases as the difference in surface tension between fabric and pesticide mixture gets larger, whereas penetration increases with decreased solid volume fraction and fabric thickness.
For the relationship between fabric barrier performance and thermal comfort properties, research shows that, in general, a negative relationship exists between fabric protection performance and air permeability for untreated non-woven fabrics (Lee and Obendorf, 2001). A statistical model predicting air permeability based on fabric thickness and solid volume fraction was developed. The regression equation was:
where t is the fabric thickness; ρb is the solid volume fraction. This could be useful in estimating thermal comfort based on simple measurements of fabric parameters. An empirical model that related air permeability of fabric and fabric protection performance was presented:
Modeling of liquid penetration through woven fabrics has used fabric cover factor, yarn twist factor and yarn packing factor as possible predictor variables, in an attempt to describe the complex capillary geometry of woven textiles with parameters that can easily be derived from basic fabric characteristics (Lee and Obendorf, 2005). Cover factor is a measure of fabric tightness and describes compactness of the weaving of a given yarn system, which could represent the relative magnitude of inter-yarn space of a given fabric. Cover factor was calculated from fabric counts and yarn diameters using the following equation:
where C represents cover factor of the fabric; e is number of warp yarns over 1 in of fabric width; p is number of filling yarns over 1 in of fabric width; d1 is diameter of the warp yarn (in); d2 is diameter of the filling yarn (in).
Relative magnitude of inter-fiber space could be quantified by textile parameters such as yarn twist factor or yarn packing factor. Twist factor is a measure of ‘twist hardness’ of yarn and describes compactness in yarns of the same size. Twist factor was calculated using the following equation:
where tw represents twist factor of the yarn; tpi is the twist in turns per inch; Ne is yarn number in the cotton system. Average twist factors of warp and filling yarns for each specimen ranged from 2.53 to 5.30 (Lee and Obendorf, 2005).
Wicking height was measured for each combination of pesticide mixtures and woven fabrics to reflect the complex fabric–liquid interactions. Average wicking heights of warp and filling direction for each combination ranged from 0.2 to 8.8 cm (Lee and Obendorf, 2005).
Surface energy of the solid, γS; is a property that reflects the chemical nature of the solid surface, and such inherent fiber characteristics contribute to wetting and liquid transport properties. In this study, critical surface tension of fiber was used as a predictor variable to represent surface free energy of the solid (Lee and Obendorf, 2005).
To develop a statistical model for pesticide penetration through woven fabrics, regression analyses were performed using those fabric/liquid parameters (Lee and Obendorf, 2005). The final model selected to predict pesticide penetration through woven fabrics was a polynomial model with linear terms of cover factor, twist factor, critical surface tension of fiber, viscosity of pesticide mixture, and wicking, and quadratic terms of cover factor, twist factor, critical surface tension of fiber, and wicking:
where C is fabric cover factor; tW is yarn twist factor; η is viscosity of pesticide mixture (mPa s); γS is critical surface tension of fiber (mN/m); w is wicking height (cm). Influence of surface tension of liquid, solid volume fraction, and yarn packing factor were shown to be insignificant at the 5% significance level for these experimental conditions. Fiber swelling from absorption of liquid or the complex interactions of fibers with liquid causes shifting of fibers and changes of the pore structure, which results in an increase in penetration with increased viscosity of the challenge liquid (Miller and Schwartz, 2001; Rajagopalan et al., 2001).
Fabric thickness was one of the highly influential factors affecting liquid penetration of woven fabric as a singular parameter (Lee and Obendorf, 2005). Fabrics with thickness above 0.8 mm showed very little or no penetration regardless of other fabric/liquid parameters for the experimental conditions (see Fig. 19.3); therefore, fabric thickness as a single factor is a dominant factor in the penetration phenomenon of woven work clothing fabrics. However, further statistical modeling processing to find a model with multiple variables for a better fit revealed that influence of fabric thickness decreases once other fabric parameters are entered into the model, which indicates that fabric parameters are interrelated. Consequently, influence of fabric thickness was insignificant at the 5% significance level when other textile parameters are present, thus replaced by a combination of fabric cover factor and yarn twist factor in the final model.
19.3 Relationship between fabric thickness and pesticide penetration (Lee and Obendorf, 2005).
Lee and Obendorf (2005) found cover factor and twist factor were better parameters in describing the geometry of woven fabrics than solid volume fraction. Pesticide penetration increases as fabric cover factor and yarn twist factor decrease.
Fabric air permeability is a characteristic closely related to comfort performance of fabric. To develop a statistical model predicting air permeability from basic textile parameters, regression analyses were performed using fabric thickness, fabric cover factor, yarn twist factor, yarn packing factor and solid volume fraction as independent variables (Lee and Obendorf, 2005). The final model selected to predict air permeability of fabric was a polynomial model with linear terms of fabric thickness, cover factor, twist factor, and packing factor, and quadratic terms of fabric thickness, cover factor, and packing factor:
where C is fabric cover factor in equation [19.14]; t is fabric thickness (mm); tW is yarn twist factor; ϕ is yarn packing factor. Yarn packing factor ϕ is the ratio of total fiber area to actual yarn area in the cross-section of a multifilament yarn:
A fabric that displays repellency has a critical surface tension that is lower than that of the liquid, creating a bead on the surface, which can roll off rather than penetrate the internal fabric structure. Finishes such as fluorocarbons are applied to fabric surfaces to lower the surface tension thus reducing wetting; these finishes reduce penetration for pesticide-protective clothing (Laughlin et al., 1986). Nanotechnology is being used by the textile industry to upgrade chemical finishing. Electrospraying processes provide control of agglomeration of nanoparticles improving performance (Güneolu et al., 2010). Layer-by-layer deposition has been used to self-assemble nanoparticles on the surfaces of fibers to develop functional textiles of protective clothing (Hyde et al., 2005).
Many textile structures have the ability to retain liquids and other chemicals via sorption providing protection by trapping a contaminant within a fibrous matrix limiting dermal contact. This is the mechanism of activated carbon used in military garments. Washable woven garments made from cotton often blended with other fibers such as polyester are popular garments for pesticide applicators, especially those working in hot environments; this traditional work clothing functions by sorption to reduce dermal pesticide exposure (Obendorf et al., 2003; Welch and Obendorf, 1997). Many agricultural workers prefer traditional work clothing for its comfort, cost and availability (DeJonge et al., 1985). Although the National Institute for Occupational Safety and Health (NIOSH, 2009) reports that work clothing such as long-sleeved shirts and long pants can provide 90% protection, dermal pesticide exposure even on a comparatively small scale can have dramatic health effects (Thongsinthusak and Frank, 2007). Therefore, there is a continued effort to develop specialized barriers or selective membranes to provide higher levels of protection.
Treatment of traditional non-barrier textiles with chemical finishes such as starch or carboxymethyl cellulose increase sorption properties while also decreasing transfer by rubbing and enhance removal of contaminants by laundering (Csiszár et al., 1998; Obendorf et al., 1991; Obendorf and Ko, 1997). Increased fabric thickness/weight can also be used to trap addition liquids within the fabric structure thus reducing penetration (see Fig. 19.3). Following this reasoning, layering of clothing materials has been shown to offer increased protection (Crossmore and Obendorf, 1992; Laughlin et al., 1986).
Researchers have shown that textile treatments such as renewable starch finish or durable carboxymethylation of cotton increase the amount of pesticide adsorbed by the fabrics. Both treatments also are effective in enhancing the decontamination of cotton fabrics with laundering. The addition of enzymes such as amylase also improves decontamination (Csiszár et al., 1998; Ko and Obendorf, 1997). Cyclodextrins are also used to enhance sorption of toxins on fibrous structures (Martel et al., 2002).
An approach to achieving a balance between chemical protection and comfort is to limit pore size. Microporous membranes that allow vapor penetration but prevent liquid penetration alleviate the thermal discomfort often associated with traditional barrier materials while maintaining a high degree of chemical protection (Shaw and Hill, 1990). When laminated to conventional fabric structures, these materials were shown to have higher barrier properties than their unlaminated woven and non-woven counterparts, or the membrane alone (see Fig. 19.4) (Branson et al., 1986; Lee and Obendorf, 2007a). Although microporous membranes and laminates have very low air permeability, their water vapor transmission rate was comparable to most non-wovens (Lee and Obendorf, 2007a). Pore size and pore size distribution are important parameters determining water vapor transport. Engineering the pore size can provide increased thermal comfort while maintaining high protection for liquid chemical penetration.
19.4 Relationship between protection performance and air permeability for non-woven, woven, microporous membranes and laminates (Lee and Obendorf, 2007a). W14, a woven fabric, and NW1, a non-woven fabric that have similar protection and solid volume fraction but different air permeability and moisture vapor transport that was shown to be related to pore size and pore size distribution.
Although effective barriers to pesticide penetration for tasks with high risk of spilling or large volume contamination, full-body chemical suits of a barrier material limit water vapor transport causing heat stress and fatigue, unless a cooling device is included. Thus, they are often inappropriate for extended use outdoors at elevated temperatures. In Californian agriculture, full-body chemical resistant suits are not permitted at temperatures exceeding 30°C during the day (26°C night) (Thongsinthusak and Frank, 2007).
Membranes that have selective permeability have been developed that are permeable to water vapor and impermeable to more hydrophobic chemical challenges. Perfluoro-sulfonic polymers (e.g. Nafion®) and sulfonated polyaryles (e.g. sulfonated polyetherketones) show characteristic hydrophilic/hydrophobic nano-separations, especially in the presence of water. While the hydrophobic domain provides morphological stability, the hydrated hydrophilic domain is responsible for the transport of water of hydration. Such polyelectrolyte membranes were evaluated by Rivin et al. (2004) for use as permselective diffusion barriers in protective fabrics giving insight into the complex interactions between solvent and polymers.
Membranes have also been developed by filling the pores of a porous substrate film with a polymer with different solubility (Yamaguchi et al., 1991). Due to the solubility differences the membrane has selective permeability. The pores of a hydrophobic host membrane such as polypropylene can be filled with a polyelectrolyte that is hydrophilic while it retards permeation by organic compounds. This forms a polymer composite membrane with the cross-linked gel filling the nanopores of the host hydrophobic membrane. Such membranes have been developed using porous polypropylene and polyelectrolytes such as poly(acrylic) acid, poly(methacrylic acid), poly(acrylamide) and poly (2-hydroxyethyl methacrylate) (Y. M. Lee et al., 1989; Yamaguchi et al., 1991; Yamaguchi et al., 1994). Using this approach, highly selective nanocomposite membranes have been developed that exhibit high water-transport rate and low toxic agent transport rate for breathable protective clothing. H. Chen et al. (2006, 2007) have shown that membranes with oriented polyelectrolyte nanodomains exhibit enhanced transport properties useful in chemical protective materials.
A selective membrane has been developed using a cross-linked lyotropic liquid crystal–butyl rubber composite membrane that appears to function by molecular size discrimination rather than differences in solubility (Lu et al., 2008). This new membrane was evaluated for reduction in vapor transport of dimethyl methylphosphonate (DMMP); it is believed that the membrane has an effective pore size of around 0.57 nm due to the nanostructure of the composite material.
Electrospun fiber webs and laminates have been developed to provide smaller pore size between fibers allowing enhanced protection while maintaining high water vapor transport (Lee and Obendorf, 2006, 2007c). Schreuder-Gibson et al., (2002) demonstrated the enhancement of aerosol protection using a fine layer of electrospun fibers that increased aerosol particle protection without significant change in the moisture vapor transport.
Electrospinning is an effective and promising technique for the production of fibers, with diameters from 40 to 5000 nm (Doshi and Reneker, 1995; Reneker and Chun, 1996). Nanofibers have great potential for application in filtration, membrane and protective clothing applications due to the large surface area and the small inter-fiber pore sizes (Lee and Obendorf, 2007b, 2007c).
Layered structures with electrospun nanofiber web gave protection performance lower than microporous materials but higher than most non-wovens (see Fig. 19.5). Air permeability of layered fabric systems was higher than microporous materials and many conventional non-wovens used for PPE. For moisture vapor transport, layered fabric systems gave a similar range as conventional non-wovens (see Fig. 19.6) (Lee and Obendorf, 2007c).
19.5 Comparison of SEM micrographs; (a) microporous membrane, (b) layered fabric system with electrospun polyurethane nanofiber web, and (c) spunbonded non-woven (Lee and Obendorf, 2007c).
19.6 Air permeability, water vapor transmission rate and protection performance against pesticide mixture (Prowl® 3.3 EC) of layered fabric systems with electrospun nanofiber web compared with existing PPE materials; (•) layered fabric system with 1.0 g/m2 web area density, () layered fabric system with 2.0 g/m2 web area density, (×) microporous membrane and laminated fabrics, () non-woven fabrics (Lee and Obendorf, 2007c).
Pesticides may contaminate clothing and skin directly during application in the field or by indirect contact with contaminated surfaces. Elevated temperatures with perspiration and other dermal secretions increase the potential for transfer of pesticides to clothing and skin (Nelson et al., 1993). Pesticides adsorbed on clothing can be transferred with friction (Obendorf et al., 1994). This is a critical factor when donning and doffing a contaminated garment since toxic chemicals may be transferred to people and their immediate environment. Studies of pesticide residues in homes confirm accumulation on surfaces especially on carpets with large fiber surface areas. Residues found as settled dust confirm that chemicals may be redistributed within a household (Obendorf et al., 2006).
Decontamination is the removal of chemicals from exposed clothing systems. For protective clothing worn by operators applying pesticides, this is most often accomplished by laundering (Laughlin, 1993). Removal of pesticide soils is a complex system involving material, chemical and structural factors. Formulation, active ingredients and concentration of pesticides, fiber type and washing conditions impact effectiveness. Pesticides from contaminated protective clothing have been found distributed on the surfaces of both cotton and polyester, as well as inside the cotton lumen (Obendorf and Solbrig, 1986). One laundering cycle removed most of the surface residue for the fiber surfaces of cotton and polyester (McQueen et al., 2000). Even properly used, garments may still contaminate the wearer during doffing. Thus, it may be useful to employ textile treatments that reduce the toxicity of contaminants through oxidation or destructive adsorption. These self-decontaminating properties could reduce the toxicity of chemicals even before the laundering process.
Self-decontaminating fabric treatments, which decompose pesticides on contact may provide enhanced dermal protection as well as limit garment mediated contamination. Materials with self-decontaminating treatments are a promising approach to comfortable yet protective clothing systems. This class of materials incorporates compounds with detoxifying properties (such as oxidation) onto protective textiles. By converting pesticides to potentially less harmful forms on contact, the efficacy of porous materials for limiting dermal pesticide contamination may be enhanced.
Organic polymeric compounds with oxidative properties are promising candidates for detoxifying pesticides on protective clothing. N-halamine compounds, which derive their efficacy from disassociation of chloramines bonds (N-Cl) have demonstrated the ability to oxidize commonly used carbamate pesticides which contain sulfur bonds such as aldicarb and methomyl (Fei et al., 2006). Researchers have demonstrated their ability to convert alcohols to ketones, sulfides to sulfoxides and sulfones, and cyanides to carbon dioxide and water (Sun and Xu, 1998). These materials also have biocidal properties (Qian and Sun, 2005). N-halamine polymers can be grafted onto polyester/cotton and exhibit durable and rechargeable properties when reactivated with a chlorine treatment (Ko et al., 2000; Sun and Xu, 1998; Sun and Sun, 2001).
Three forms of chloramine bonds are imide, amide and amine halamines. The bond stability was found to be inversely related to reaction rate with aldicarb (imide halamine > amide halamine > amine halamine) (Fei et al., 2006) as well as biocidal properties (Qian and Sun, 2005). The imide bond which is found in 1,3 dimethyol-5,5-dimethylhydantoin (DMDMH) dissociates readily and reacts the fastest with aldicarb compared to the amide and amine bond types. Researchers have shown the decrease in aldicarb concentration with exposure to N-halamines and the oxidation of the thio bond to sulfoxide (− SO–) and later sulfone (− SO2.) (see Fig. 19.7). Researchers have developed both imide and amide N-halamine treatments for fabrics; DMDMH contains predominantly imide bonds, whereas the 3-methylol 2,2,5,5-tetramethyl imidozalidin 4-one (MTMIO) treated fabrics contain amine halamine structures. The imide halamines, though more reactive, are significantly less durable than the amines with repeated laundering.
19.7 Degradation of aldicarb by DMDMH- and MTMIO-treated fabrics (5 g of fabrics in 20 mL of 0.25 mMol and 0.025 mMol aldicarb at 25°C) (Fei et al., 2006).
Bleaching reactivates all three types of N-halamines. A mixture of highly reactive imide and stable amine compounds may provide the desired properties of both compounds. It is possible that the amine halamines may be able to recharge the imide halamines on the fabric surface (Qian and Sun, 2005).
Self-decontamination of polyacrylonitrile (PAN) electrospun fibers has been achieved using surface oximation (L. Chen et al., 2009). Using excess hydroxylamine, the PAN fibers were functionalized by forming polyacryl-amidoxime (PAAO). In the presence of water, these nucleophilic amidoxime groups hydrolyze organophosphate pesticides. Thus, these functionalized fibers are possible candidates for use in self-detoxifying fabrics.
Photocatalytic oxidation is one of the most effective ways to decompose alkenes and other volatile organic compounds. Titanium dioxide, TiO2, is a well known metal oxide with photocatalytic properties; it has been incorporated into non-woven filtration fabrics to aid in degradation of volatile organic compounds (Park et al., 2006). In this system, the metal oxides act as a photocatalyst in the presence of UV light. TiO2 has been incorporated in both textile fibers and finishes. Cotton fabrics that were treated with aqueous TiO2 in a silicone finishing solution aided in the decomposition of gaseous ammonia (Dong et al., 2006), and electrospun PAN fibers containing TiO2 nanoparticles degrade aldicarb (Woo and Obendorf, 2010; see Fig. 19.8). Titanium dioxide has been proven to degrade mixed pesticides including methyl parathion (Senthilnathan and Philip, 2009). In the outdoor environment, with the combination of elevated temperatures during the summer growing season, direct sunlight and moisture, these self-decontaminating properties of textiles may be effective in increasing personal protection.
19.8 Degradation of aldicarb solution with reaction time in the presence of UV and TiO2 nanofiber (Woo and Obendorf, 2010).
Nanocrystalline materials exhibit a wide array of unusual properties; one of these unusual features is enhanced surface chemical reactivity toward incoming adsorbates (Li and Klabunde, 1991). Nanocrystalline MgO, CaO and Al2O3 adsorb polar organics such as aldehydes and ketones in very high capacities (Khaleel et al., 1999). The chemical reactivity of adsorbates on the nanoparticles has been shown to follow a two-step decomposition mechanism. The first step is adsorption of the toxic agent on the surface by means of physisorption, and the second step is chemical decomposition. This two-step mechanism substantially enhances the detoxification abilities of the nanoparticles.
Magnesium oxide in nanocrystalline form (particle size ≤ 8 nm, aggregate size 3.3 μm) has a large reactive surface area due to polyhedral shapes and high proportion of corner/edge sites compared to typical polycrystalline material (Klabunde et al., 1996). The surface morphology of the MgO structure is very important; higher numbers of defects and corners of small nanoparticles provide high surface reactivity (Klabende et al., 1996). High surface area combined with high surface reactivity gives these materials great potential for use in decontamination of toxic substances by dissociative chemisorptions termed ‘destructive adsorption’.
Nanoparticles of metal oxides such as MgO exhibit unique properties that are related to particle size (Koper et al., 1993; Klabunde et al., 1996). Particle sizes are dependent on the formation methods. Chemists reported several methods for the preparation of metal oxides. The most popular one is a conventional preparation using boiling water and vacuuming treatments (CP), and the other is an aerogel method using hydrolysis and thermal treatments with autoclaving (AP). The unique characteristics between the particles made by the two methods are their surface area and crystal size. Aerogel-prepared (AP) nanoparticles have larger surface area and smaller crystals compared with conventional prepared (CP) ones; for example, CP-MgO has 150 m2/g of surface area and 8 nm of crystal size and AP-MgO has 400 m2/g and 4 nm (Rajagopalan et al., 2002).
Metal oxides have been shown to degrade various organic compounds including paraoxon and other pesticides (Klabunde et al., 1996; Li and Klabunde, 1991; Rajagopalan et al., 2002), and military agents such as GD, VX and HD (Decker and Klabunde, 1996; Rajagopalan et al., 2002; Wagner et al., 2000, 2001). Nanocrystalline MgO reacts faster and in higher capacity than activated carbon, a commonly used material for chemical and military protective clothing (Khaleel et al., 1999; Rajagopalan et al., 2002).
Magnesium oxide (MgO) has been shown to degrade organophosphate including DMMP a chemical agent stimulant (Li and Klabunde, 1991) and paraoxon (Rajagopalan et al., 2002) through a surface stoichiometric process rather than catalytic. It is believed to cleave the P–S bond or P–O bond of organophosphates. The mechanism involves –OH groups that are bound to the Mg metal on the surface of the MgO structure. First of all, oxygen from the MgO matrix attaches to the phosphorous of the DMMP. Surface –OH group results in methanol being released from the reaction. Methanol dissociates on the surface, and the adsorbed CH3O group is oxidized by DMMP thus releasing formic acid as the main volatile product. About two surface MgO moieties are used in the decomposition of one DMMP molecule. Water, formic acid, methanol, adsorbed –OH and the remaining portion of the DMMP molecule containing phosphorous adsorbed onto the surface of the MgO are the products of the degradation reaction (see Fig. 19.9). The water formed could possibly dissociate to provide more surface –OH groups to promote further reactions. It is believed that the presence of small amounts of water is beneficial for the decontamination process. The non-volatile product is immobilized on the MgO surface bound by two bridging oxygen atoms with the empirical formula of C2H6PO3MgO. This mechanism indicates that the MgO surface moieties then will be blocked from further participation in the reaction.
19.9 Overall balanced reaction of DMMP with surface MgO. The products of this reaction are methanol, water, non-volatile product, adsorbed –OH group, and two formic acid molecules (Li and Klabunde, 1991).
Polyoxometalates (POM) are negatively charged metalate anions in the form of nanoclusters. They consist of transition metal ions bonded to other ligands, generally oxygen atoms (Hill and Prosser-McCarth, 1995; Müller and Roy, 2003). POM are in their highest oxidation state with the general formula XM12O40 x−8 where X is Si4+, P5+, etc. and M can be tungsten, molybdenum and vanadium, etc. These transition metal–oxygen cluster complexes are relevant because of their oxidation-reduction chemistry and their relevance as environmentally benign catalysis. They have been known and used in the chemistry laboratories for nearly 200 years, but with the advances in materials science and nanotechnology, POM are beginning to be considered as unique chemical species that could turn from very special molecules to very useful materials. With sizes just one order of magnitude smaller than the smallest of living biological structures such as the Rhinovirus, i.e. ~ 20 nm, POMs are not colloids but soluble polynuclear species. They have structural and topological features in common with transition metal oxides and have similar reduction-oxidation reaction, electron transfer and ion transport behavior. In all these respects, POM can be generically considered as the perfect models for quantum-sized transition metal oxide nanoparticles. POMs are inexpensive, easy to synthesize, non-toxic, have catalytic nature, and can be incorporated on fabrics.
The Keggin-type POM H5PV2Mo10O40 has been deposited on cotton, polyacrylic and nylon fabrics (Xu et al., 2000). Acting as a redox-based, radical chain initiator, they catalyzed O2-based oxidation (aerobic) of representative air toxins, acetaldehyde and 1-propanethiol. These fabrics offer self-deodorizing and self-decontamination properties. POM can be cross-linked to cellulose thus to cotton fibers (see Fig. 19.10).
Nanohybrid membranes based on POM H5PV2Mo10O40 and a poly(vinyl alcohol)/polyethyleneimine (PVA/PEI) blend have been prepared as a chemical and biological protective, permeable membrane material (Wu et al., 2009). H5PV2Mo10O40 nanoparticles were incorporated on electrospun fibrous materials using toluene diisocynate as a cross-linker. Oxidation of 2-chloroethyl-ethyl sulfide (CEES) was observed with the significant color change from orange to blue. In addition to oxidation of CEES, these membranes exhibited antibacterial properties against both gram-negative and gram-positive bacteria.
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