June 25, 2008
Chemical, Biological, Radiological and Nuclear Protection
By Seed, Mark Anand, Subhash; Kandola, Baljinder; Fulford, Robert
Mark Seed, Subhash Anand, Baljinder Kandola and Robert Fulford review the use of impermeable, permeable and semi-permeable ensembles for military and police forces high-threat situations, with particular emphasis on permeable systems, discussing the selection of fibres, fabrics and laminates, as well as processes. The armed forces and emergency services face an array of extreme conditions that necessitates they use protective clothing. Broadly defined these include threats from chemical, biological, radiological and nuclear (CBRN) materials. Moreover, in a hostile situation the clothing may also be expected to protect against ballistic impacts, shrapnel and slashing.
All the above mentioned types of attack are carried out with an aim to intentionally maim or kill. However, particularly in military operations, perhaps the most important and prevalent hazards to troops on the ground are extreme weather conditions such as cold, heat, rain, snow, dust, wind and adverse terrain. These pose a significant risk to the efficient functioning of troops(i), and their clothing ideally will provide assistance.
Many different types of CBRN protective clothing have been developed to deal with different situations; for instance, impermeable outfits made from butyl rubber, polyvinyl chloride (PVC) and other rubberized materials are used for their extreme resistance to chemicals. However, this style of clothing can only be used for short periods of time as it is extremely uncomfortable to wear, preventing the evaporation of sweat and radiation of heat from wearers to their environment, which can ultimately lead to heat stress.
In situations where the user is required to work for extended periods in contaminated areas permeable and semi-permeable ensembles are required that allow more breathability. Although the materials used are designed to be as resistant as possible to toxic agents, their inherently permeable nature means that these garments are not completely impervious to chemical and biological attacks. In general, it is fair to say that there is a trade-off between the thermophysiological factors and the level of protection offered by an ensemble.
Usually an active carbon layer provides a second level of protection adsorbing any contaminants that permeate the outer shell. There may also be a skin contact layer used to provide antimicrobial properties, heat and moisture regulation or both. Due to the number of layers required, these semi-permeable assemblies can be relatively heavy.
Figure I: Typical configuration of layers within a semi- permeable ensemble.
Taking into consideration all the characteristics the garment must possess, ensembles used for long-term use in CBRN situations, where the garment may need to be used for days without doffing, must still be relatively low in weight, particularly where the user may have other heavy equipment to carry. Moreover, it must be easy to clean and maintain, have relatively good drape and comfort, and have a storage life of 10-20 years.
Solutions to these requirements can be grouped in three categories:
* permeable; and
* semi-permeable ensembles.
Impermeable systems can be made from butyl rubber, halogenated butyl rubber, neoprene and other elastomers. Fully encapsulating impermeable systems with microclimate cooling systems offer the highest level of protection against chemical and biological agents. However, they are relatively heavy and the cooling systems usually provide a relatively comfortable environment for only a few hours. For impermeable systems that cover only parts of the body there are other solutions.
The design of the ensemble is of critical importance and it should provide suitable openings for heat and moisture to escape. The underwear chosen must be carefully selected too; for instance, it should wick liquid sweat laterally towards the regions of the body not covered by the protective clothing so compensating for the lack of permeability of the ensemble(ii).
An example of an impermeable system used in chemical/biological situations is the Improved Toxicological Agent Protective (ITAP) ensemble, which is a multi-laminate fabric with fluoropolymer and aramid layers. This allover impermeable assembly has a separate air supply and is used in conjunction with a Personal Ice Cooling System (PICS). PICS works by pumping ice-cold water from a twolitre bottle carried by the individual through a cooling garment. The ice-water containers need to be changed periodically as they lose their cooling effect. As a result some user groups, such as pilots, are unable to exploit this system because they can not re-supply the ice water(iii).
Another example is the Self-contained Toxic Environment Protective Outfit (STEPO). This is a totally encapsulating ensemble used by the military. It comprises a self-contained breathing supply and battery powered cooling system and provides up to four hours of self-contained breathing and cooling. As with ITAP, these ensembles can only be used in highly contaminated areas where lack of mobility is not a major concern.
In this context, nonwovens generally perform better than woven fabrics(iv). A flashspun polyethylene nonwoven (DuPont'sTyvek(R)) and a spunbond-meltblown-spunbond (SMS) polypropylene fabric Kimberly-Clark's (Kleenguard(R)) are two examples of nonwovens used in the chemical industry for their excellent liquid- and vapour- repellent characteristics. Their microfibre structures provide a superior filter for dry particulates and many water-based liquids when compared with even the most tightly woven fabrics'"'. The fibres are also inherently hydrophobic. For improved barrier properties coated or laminated nonwovens can be used; for instance.Tyvek can be laminated with Dow Chemical Co's Saranex(TM)- a coextruded multilayered film containing a layer of polyvinylidene chloride (PVDC) resin - Saran(TM) -integrally sandwiched between outer polyolefin layers.
In industries where the user is exposed to chemical/biological agents and requires limited physical activity, such as during laboratory work, the light nonwovens described above will be the most suitable choice principally due to their excellent filtration properties. However, although there is a level of air permeability associated with these types of nonwoven, it is not sufficient for applications where there is sustained physical exertion. Also, chemical and biological resistance are not necessarily the only criteria to be considered; flame resistance and puncture resistance are also important in the military and first response arenas and if so nonwovens will not be suitable.
Heavy woven fabrics of twill construction are commonly used in military and police applications and have reasonable chemical/ biological resistance(v). Probably the first effective waterproof and breathable fabric to be developed for military purposes, in the 1940s, was Ventile(R) (from Ventile Fabrics). This is made from long staple cotton fibre, using only the finest types of cotton in order to fill the smallest gaps in the fabric. The cotton yarn is plied to give high regularity and then densely woven in a plain weave structure. When wetted the fibres swell to fill any gaps and render the fabric waterproof.
Modern woven waterproof fabrics can be made from synthetic polyester or polyamide yarns. Microfilaments with diameters of 10 [mu]m or less would usually be used to engineer the smallest pores possible and the Ventile range has expanded to include such fibres. The resistance against liquid penetration can be improved by the application of a silicone or fluorocarbon finish(vi). However, even the most densely woven fabrics will freely allow gas and vapour penetration, as well as some liquid penetration under pressure, and therefore additional sorptive underlayers are needed.
When designing permeable and semi-permeable garments for CBRN applications it becomes more critical to create an adequate barrier against toxic chemical liquids and gases. Consequently, in order for the product to be both air permeable and still provide protection against chemical and biological agents, a layer of activated carbon is usually used as a sub-layer in the clothing to adsorb any chemical or biological agents that penetrate the permeable or semipermeable outer shell.
Activated carbon works by adsorption-gases and vapours are attracted to and then condense on or bond to the surface of the carbon. This contrast with absorption where molecules penetrate the bulk phase of the solid(vii). Foreign atoms at the surface of the activated carbon have imbalanced forces, as compared with those within the bulk, and the resulting net force attracts foreign molecules to the surface(viii). These type of attractions are known as London dispersion forces, or Van de Waals forces, and the process is classed as physisorption. It is also possible for electrons to transfer between the contaminant and carbon surfaces, causing the formation of covalent bonds, a process termed chemisorption, although this is far less common.
The discovery of active charcoal's adsorptive power is generally attributed to Scheele(ix) who, in 1773, described experiments on gases exposed to carbon. Through the 1800s, development of activated carbon was generally related to sugar refineries and the decolourization of liquids. The event that led to extensive research in active carbon, and the first recorded use of active carbon for military application, occurred in 1915. In World War I, at a stage where the war became very defensive with armies landlocked in trenches, the German command ordered the release of chlorine gas over a four mile front. The Allies were defenceless against this attack and the line was breached. However, the Germans themselves were not protected against the poisonous gas and so turned to other methods of attack. This interval gave the Allies the chance to develop protection in the form of gas masks containing activated carbon.
Modern day activated carbon is usually produced in a twostage process that consists of:
* carbonization; and
* activation by means of oxidation.
Carbonization is usually conducted in the absence of air at below 600[degrees]C. During carbonization the source is heated to produce a char. The char is then activated - typically in steam, air or carbon dioxide - at 600-900[degrees]C(ix).
Activated carbons have a vast network of pores of varying sizes. The International Union of Pure and Applied Chemistry (IUPAC) defines pores in the following way(x):
Figure 2: Pore structure of activated carbon showing the typical relationship between mocropores, mesopores and micropores.
* macropores have a diameter of more than 50 nm;
* mesopores have a diameter of 2-50nm;and
* micropores have diameters of less than 2 nm.
And in the network, macropores are generally connected to mesopores and micropores in a manner similar to the root structure of a plant.
With a surface area in the region of 1200 m^sup 2^.g^sup -1^, potentially up to 3000 m^sup 2^.g^sup -1^, activated carbons have a huge capacity for adsorbing unwanted chemical and biological agents. The effectiveness of activated carbon has not yet been superseded, which is why permeable and semi-permeable CBRN systems still use activated carbon liners(xi-xii).
For many years the UK military has used a disposable twolayer NBC suit comprising an inner nonwoven sprayed with powdered activated charcoal in a carrier/binder; subsequently, an oil- and water- repellent fluorocarbon finish is applied. The outer layer is a woven twill fabric comprising a polyamide filament warp with a modacrylic weft, which carries a water-repellent finish(xiii).
This type of ensemble can be used for longer periods than rubber- based impermeable suits and still provides good protection against chemical and biological warfare agents (CWA and BWA) thanks to its activated carbon layer. Usually these activated charcoal-based CBRN systems comprise of a water-resistant treated outer layer that prevents liquid entering the middle activated charcoal layer.
Finally, there would usually be a skin contact layer that is mainly used for comfort - it keeps the abrasive charcoal layer away from the skin - but may also include antibacterial agents. Modern fibres and fabric structures used as a base layer are designed to remove sweat and heat from the skin, and transport it to the outer layers and eventually to the environment. One such system has recently been patented in the UK specifically for use by the police and armed forces(xiv).
Coatings, membranes and films
Where the traditional permeable shell and activated carbon layer combination does not provide sufficient protection, permeable membranes can be used. These increase the level of protection against chemical and biological agents, while maintaining a level of breathability.
There are two main categories of permeable membranes:
* porous; and
* solution-diffusion membranes.
Moreover, depending on their pore size, porous membranes can be:
* microporous; or
Macroporous membranes allow a convective flow of such as air, aerosols and vapours through their large pores and no separation occurs.
The extruded polytetrafluoroethylene (PTFE) film (GoreTex from W.L. Gore and Associates) is an example of a microporous membrane(xv). When material is expanded during processing tiny holes are produced; Gore claims as many as 1.4 billion holes.cm^sup -2^. The holes are much smaller than the smallest raindrops (2-3 pm compared with 100 [mu]m), yet are 700 times larger than water vapour molecules(vi,xv). Further, the hydrophobic nature of microporous polymers means that liquids are efficiently repelled and high pressure is needed to cause penetration.
However, Gore-tex is expensive and tends to be used only in specialist applications(xvi). Other microporous coatings and films can be used that are not as expensive. For instance, microporous coatings and films based on polyurethane chemistry are available such as Toray's Entrant(R)-a microporous membrane made by a coagulation process. In some cases these membranes or coatings incorporate a top coat of hydrophilic polymer, to resist contamination of the pores by sweat residues and penetration by liquids with low surface tensions(xvii).
An ultraporous membrane is similar to a microporous membrane in that it contains many small holes. However, the holes in an ultraporous membrane are small enough to allow small molecules through but exclude larger molecules.
Solution diffusion membranes, which have also been called nonporous or monolithic membranes, allow Fickian permeation. The membrane contains amorphous regions, socalled "intermolecular pores" that allow water molecules to pass through but prevent the penetration of liquid water owing to their solid nature(vi,xiii).
Figure 4: Diagrams showing the separation mechanisms of the pore- flow (left) and solution-diffusion (right) models.
These membranes are commonly used in the filtration industry for applications such as reverse osmosis purification of water supplies and gas separation. However, the materials that these membranes are made from are not renowned for their chemical resistance and so far microporous membranes give the best chemical resistances.
PROTECTION FROM FIREAND HEAT
Fire and heat threats towards the military and the police are like no other in that the events are initiated on purpose and with an intention to kill/maim. As a result, attacks such as those with burning fuel bombs and exploding munitions can be extremely severe and targeted on an individual.
Clothing used by these forces must be capable of withstanding flash flames and heat, at least enough to give the user time to react and evacuate the danger area. Usually textiles worn by the user are the first to ignite when exposed to flame and heat. Moreover, if volatile liquids and gases are produced by the textile during burning they will act as fuel for further combustion in a feedback mechanism(ixx). It is therefore essential that the fibres and materials used are retardant to flames and heat.
The criteria specific to clothing used in the protection of military and police forces against flame and heat threats are:
* prevention of the outer clothing catching fire (by using flame retardant, self-extinguishing fibres). At least 25% by mass of the original material should be preserved and shrinkage should not be more than 10%;
* prevention of conducted or radiant heat reaching the skin (by providing sufficient insulation in the form of additional layers or air gaps);
* when exposed to heat or flames the generation of toxic fumes should be minimized (by the selection of the appropriate materials);
* prevention of melting of the clothing in contact with the skin- by not using thermoplastic fibres such as polyamide, polyester, polyolefins and polyvinylidene chloride (PVDC)(xvii).
Most flame retardant textiles are designed to reduce the ease of ignition, usually to a small flame source (for instance, one designed to simulate a match), and reduce flame propagation rates. Heat resistant textiles should, in addition, offer a barrier to heat and flame penetration to underlying materials or surfaces. So, protective clothing must be sufficiently flame and heat resistant to prevent damage to underlying fillings, clothing layers and ultimately the wearer's skin.
The minimum concentration of oxygen in an atmosphere of oxygen and nitrogen that is required to sustain a flame after ignition is called the limiting oxygen index (LOI) and is a good indicator of the flammability of a fibre: the higher the LOI, the more retardant the fibre is to flame and heat. Typically, fibres with an LOI above 21%, which is roughly the ambient concentration of oxygen in the air, will be used for fire retardant materials. For fibres with an LOI above 21 % (including wool, modacrylic and aramid) the burning behaviour and tendency to propagate flame will be reduced or even zero after the removal of the ignition source in a standard atmosphere(xx). Fibres with an LOI lower than 21 (such as cotton and viscose) can be treated or chemically modified during processing to raise the LOI value and render them flame retardant.
Cotton can be chemically treated, for instance, to give adequate fire retardant properties. It is also common for inherently flame retardant fibres, such as wool, to be treated to enhance their natural characteristics. The chemical treatments used are usually in the form of fabric coatings or in some cases the chemical is mixed with the polymer dope during fibre formation. Fire retardant chemicals usually operate in one or both of the gas or condensed phases.
Halogens are the most common gas phase fire retardants, compounds of chlorine or bromine being the most usual forms. Fire retardants that work in the gas phase inhibit the production of the highly reactive hydrogen (H*) and hydroxyl (OH*) free radicals that are formed during combustion. This action is known as the radical trap. In addition to the radical trap, the dilution of the flame caused by the inert halogen and halide molecules decreases the concentration of combustible gases. It is also thought that the steady stream of bulky halogen and other non-fuel molecules emitted could retard the penetration of oxygen into the polymer and slow down the pyrolysis(xxi). The condensed phase mechanism relies on a chemical interaction between the fire retardant chemical and the polymer that is being protected. Phosphorous containing compounds are the most common fire retardants that work in this way. Phosphorous containing condensed phase fire retardants work by bonding with the fabric and then, on exposure to heat, converting to a protective carbonaceous char(xxi,xxii).
It is believed that char-forming chemicals are the most effective fire retardants, as the char also behaves as a carbonized replica of the original fabric, continuing to function as a thermal barrier; in contrast to flame retardant thermoplastic fibres, for instance. Char- forming flame retardants, therefore, act not only by resisting flames, but also offer thermal resistance to fabrics. As a result, they can compete with many of the so-called high performance flame- and heatresistant fibres such as the aramids(xxii).
The most widely used flame-retardant material in the UK military forces is Proban(R)-treated cotton, alone or in blends with up to 30% polyester. Proban (from Rhodia) is a phosphorus containing coating compound based on a phosphonium salt complex tetrakis hydroxymethyl phosphonium chloride (THPC). With low fabric shrinkage when exposed to heat and/or flame, it costs little and is widely available, which makes it suitable for cheaper end garments. However, it is unable to be laundered in soap and hard water as flammable residues can be left in the fabric. It also liberates smoke and fumes, which excludes it from use in specialist navy and firefighters suits(xxiii).
Wool is another popular fibre used in fire retardant clothing for the armed forces. Without any treatment, wool is the most inherently non-flammable of all the so-called conventional fibres. Its ignition temperature of 570-600[degrees]C is a consequence of its high moisture regain, and high nitrogen and sulphur, but low hydrogen content.
Char-promoting phosphorus or halogen-containing retardants can give additional protection against heat and flame, although the most common used for wool is Benisek's Zirpro(R) system(xxii-xxiii). The Zirpro finish contains potassium hexafluorozirconate or a mixture of this and potassium hexafluorotitanate. Although its effectiveness is not fully understood from a mechanistic point of view, its ability to produce extremely effective flame and heat barriers is clearly associated with the char structure generated(xxii). Zirprotreated wool is used in the UK's Royal Navy and Royal Air Force (RAF) in firefighting suits.
Inherent flame retardance
There are fibres available that provide superior thermal and flame resistance without any additional treatment. The most commonly used is the poly(m-phenyl isophthalamide) or meta-aramid (such as DuPont's Nomex). Du Pont introduced Nomex in 1962 and its high degradation temperature (around 430[degrees]C) and high thermal resistance, owing to its highly oriented meta-aramid chemical structure, make it useful in protective apparel. The mode in which the polymer degrades is also of great benefit: rather than igniting into flames under intense heat or fire the aramid polymer degrades into a char, which itself provides a level of protection, and valuable extra seconds that may be long enough to save the life of the wearer(ixx).
Currently, for instance, the UK police's first responder ensemble contains an outer layer of woven meta-aramid laminated to a microporous membrane; there is also an activated carbon sub- layer(xxix).
Nomex fibres can be blended with cheaper materials to reduce costs or with other high performance materials, such as a para- aramid (including DuPont's Kevlar) to enhance the fabric's properties further. Other fire retardant fibres that have not been discussed, such asTrevira CS polyester, Lenzing FR viscose and modacrylic fibres,are also used in blends to enhance heat/flame resistance and/or to reduce cost.
It is evident that there is a range of systems that have been designed for different applications from the lowest to highest levels of risk. The reason for this is that there is no one ensemble that will completely protect against all environments while allowing the user to stay mobile, active and comfortable for prolonged periods. There must be a tradeoff when comparing the level of protection offered against the mobility and comfort properties of an ensemble. Therefore, scope for improvement persists.
In the USA, NanoSynTex Inc has developed advanced nonwoven composite structures(xxv). To date these nonwovens are around 25% lighter and stronger than conventional woven textiles used in military uniforms and, at the same time, can be made to exhibit more than three times the air permeability or breathability when compared with the currently used woven twill fabrics.
Further studies have now begun to try to incorporate fire retardant properties.
The most important limitations of nonwovens are the aesthetic properties, and elastic recovery and flexibility. A great deal of research is being conducted worldwide to enhance these characteristics of nonwovens to forward their suitability for use in apparel applications.
Fibres, materials and membranes
Advances in fibres, materials and membrane technologies have allowed systems to be designed that will inhibit a wide range of chemicals and biological agents entering, while still offering a degree of breathability; however, heavy activated carbon layers are still required to give adequate protection. Carbon-based fabrics rely on adsorption of toxic agents and provide relatively little protection from aerosols. Carbon fabrics are also generally required to be quite thick and, therefore, heavy when used as part of a permeable/semipermeable ensemble(xviii).
Currently, as a result, leading companies (such as W.L. Gore & Associates Inc and DuPont) are working in collaboration with the US Army Natick Soldier Centre to develop selectively permeable membranes, which will allow perspiration vapour to escape while preventing the ingress of toxic chemical warfare agents(xvi). The mechanism relies on a selective solution diffusion process.
The two membrane systems being investigated are amine-based and cellulose-based. Initial tests are promising and it is thought that selectively permeable membranes could eventually eliminate the need for activated carbon layers in CBRN protective clothing, thereby halving the weight of carbon-based systems(xxvi).
New developments in the area of fire retardancy include the use of intumescents in combination with conventional heat and fire resistant textiles, which can significantly enhance the barrier properties(xxvii,xxviii). The use of polymer layered silicate nanocomposites in regenerated cellulosic and synthetic fibres is also a promising futuristic approach.
The next task is to develop the next generation of permeable systems that offer reduced weight and improved thermophysiological properties, while still maintaining the highest levels of chemical, biological and flame/heat resistance. It would appear to be a difficult challenge, but is now being addressed by the industry and academia in order to provide the ultimate protection for our Armed and Police forces.
Research at the University of Bolton
The University of Bolton itself is involved in a range of projects relating to protective clothing for Police and Military uses:
a bullet-impeding material is currently being developed by the University in conjunction with Eastern Michigan University and Signal Medical Corp. Initial test results have been encouraging and the invention is now covered with a provisional US Patent Application (number BProof-755, filed on I August 2007); moisture permeable fabric structures. One such project, for instance, relates to the development of comfortable base layers to be worn against the skin by those involved in strenuous activities for long periods. The research is fully sponsored by a UK company and the invention is covered by UK Patent Application (number 06053367 filed on 17 March 2006);and a three-year project to develop a two-layer chemical- and fireresistant material using the latest membranes and a lightweight knitted spacer material.
(i) W Wilmott H.P., World War Two, 1st edition, DK Publishing, London, UK, 2004.
(ii) Rossi R., Interactions between protection and thermal comfort. Scott R.A. (editor), Textiles for Protection, 1st edition, Woodhead Publishing Ltd, Cambridge, UK, 2005, pages 233-253.
(iii) Whitaker J., Advanced lightweight microclimate cooling equals relief from heat, http://www.natick.army.mil/about/pao/2001/ 0140.htm, US Army Soldier Systems Center-Natick, USA, 2001.
(iv) Holmes A.D., Textiles for survival, Horrocks A.R. and Anand S.C. (editors), Textile Institute Handbook of Technical Textiles, Woodhead Publishing Ltd, Cambridge, UK, 2000, pages 461-488.
(v) Stull J.O., Civillian protection and protection of workers from chemicals, Scott R.A. (editor), Textiles for Protection, 1st edition, Woodhead Publishing Ltd, Cambridge, UK, 2005, pages 295- 353.
(vi) Holmes A.D., Waterproof breathable fabrics, Horrocks A.R. and Anand S.C., (editors), Textile Institute Handbook of Technical Textiles, Woodhead Publishing Ltd. Cambridge, UK, 2000, pages 282- 314.
(vii) Fletcher A.J., Porosity and sorption behaviour, http:// www.staff.ncl.ac.Uk/a.j.fletcher/adsorption.htm#l, University of Newcastle upon Tyne, UK, 2006.
(viii) Manocha M.S., Porous carbons, Sadhano, 2003,28 (1/2), pages 335-348.
(ix) Hassler W.J., Activated Carbon, 2nd edition, Chemical Publishing Co Inc, London, UK, 1967.
(x) McCusker L.B., Nomenclature of microporous and mesoporous materials, IUPAC Pure Applied Chemistry, 2001;73(2), pages 381-394. (xi) Adanur S., Military and defence textiles, Adanur S., (editor), Wellington Sears Handbook of Industrial Textiles.Technomic Publishing Co Inc. Lancaster, USA, 1995, pages 359-378.
(xii) Anon, Basic concepts of adsorption on activated carbon, Chemviron Carbon, 1970.
(xiii) Scott A.R., Military protection, Scott R.A., (editor), Textiles for Protection, 1st edition, Woodhead Publishing Ltd. Cambridge, UK, 2005, pages 597-621.
(xiv) Anand S.C. Wetton S. and Fulford R., Moisture permeable fabric structures, UK Patent Application 0605336-7, 17 March 2006.
(xv) W.L. Gore & Associates, http://www.gore-tex.co.uk; see also, Technical Textiles International, January/February 2008, pages 37- 41, Gore shows the worth of collaborating along the whole value chain, http://www.technicaltextiles.net/htm/p20080203.1 10950.htm
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