All about Chemical Protective Clothing
Various hazards emerged in the atmosphere and surrounding areas due to developments in the science and technology, which is also affected to the worker, farmer, soldier or a common man in day to day life. There is a demanding concern observed related to health and safety of people in various sectors of industry, as well as in agriculture and defense. Protective clothing is utilized mainly to avoid exposure in a range of hazards and its dangerous effects in various activities. These hazards can be categorized as chemical, thermal, nuclear, radiation (X-rays, UV) and biological hazards. This discussion mainly concentrates on the protection against chemical hazards, selection criteria for chemical protective clothing and assessment of barrier properties of the same.

Chemists, agriculturists, horticulturist, emergency response team members, hazardous waste clean-up crews, propellant handling off-shore petroleum workers and soldiers are the people who may get affected by dangerous chemicals. The skin contact by hazardous materials is possibly avoided by applying some chemical resistance clothing. Following factors have to be measured while choosing chemical protective clothing (CPC).
.Feasible effects of skin contact with chemical
.Exposure time duration
.Contact area of body
.Cost factor.
.Properties of hazardous material (particulate / liquid / gas)
.Permeation or penetration possibility of CPC
.Additive or synergic effects of other routes of exposure like inhalation/ingestion etc.
.Other physical properties required such as flexibility, abrasion resistance, thermal resistance, etc.

Industrial workers' chemical protective clothing
With various toxicological properties, thousands of chemical products are used throughout the world. In last decades, the chemical industry had been experienced with an ever increasing degree of regularity control, intended to make sure that chemicals utilized /liberated /disposed by an industry do not cause any hazards to the human health or environment. According to the Occupational Safety and Hazard Association (OSHA), a worker is responsible to find out when personal protection equipment (PPE) are required and also identify to what type of personal protection equipment the job circumstances calls for.

Hazards from liquids, gases or dust arising in the industry require wearing of clothing that is resistant to hazardous liquids, fumes or dust. Some of these exposures can be controlled by applying a range of engineering controls. Others can be managed by replacing it with various materials or executing alternative work practices. If the exposure cannot be managed by any of these tools, chemical protective clothing should be taken as final line of defense. Others believe that the application of CPC is cost effective than applying complex engineering controls which may not be controlled at every time.

Though, it is significant to realize that no single material gives protection against all chemicals. As a substitute, protective clothing is made from various materials, each of which provides varying degrees of resistance to different substances.

Chemical Protective Clothing for Agricultural workers

In the last decades there has been a constant rise in the utilization of a variety of pesticides for the control of pests in agriculture and public health sector. Pesticides may perform a significant role in prevention of food losses, but at the same time it gives hazardous effects on non-targeted environment. Sometimes farmer's clothing absorbs the chemical residues during various activities such as mixing, handling and application. Through inhalation, eye splash and skin contact pesticides can enter the body. Reports have showed that among dermal exposure is accountable for about 87 percent of the total pesticides entering the body. Dermal exposure can be decreased considerably by applying proper protective clothing which is resistant to pesticides.

Though disposable protective clothing is offered in the market, agricultural workers are not utilizing it because of lack of knowledge or higher cost included. But they can not utilize the normal infected clothing frequently with or without cleaning. Even employees in the pesticide manufacturing industry are continually exposed to these dangerous chemicals during various activities.

Materials for Chemical Protective Clothing

The chemical protective clothing utilized in industrial scenario requires protecting the related workers from particulate contamination and/or hazardous liquids and/or toxic gases and fumes. Generally these type of clothing is made by a film or coating in combination with substrate fabric. The film may possibly place on one side or on both side of the fabric. Normally, film or coating gives barrier properties and substrate fabric gives mechanical strength and supports. Table 1 illustrated various materials applied in Chemical Protective Clothing along with their merits and demerits.

Microporous films with pore size of 0.01 to 10 �m are sometimes applied in protective clothing for 'breathability' in terms of water vapor transmissions. In recent times developed microporous fabrics based on microporous or hydrophilic membranes unite at certain level of chemical protection with better water vapor permeability. This decreases the making of moisture vapor inside the clothing, which is likely to give a feeling of discomfort to the wearer. There is increasing application of adsorptive type protective materials which applies activated carbon particles to decrease diffusion of toxic gases and fumes. With active carbon microporous polymeric matrices set throughout pore structure has been developed. These matrices cover hollow and solid textile fibers like polyolefin fibers including carbon loaded films in the form of composites. In the form of composites, normally these textiles are used.

On account of their superior dimensional stability, substrate fabrics are normally made from woven fabrics. But nonwovens are also applied in these particular end uses, Nylon, polyester, nomex and fiberglass fabrics in case of woven fabrics are more common. In nonwovens, polyester, polypropylene and polyethylene are utilized; the procedures which need heat protection, apart from chemical protection, there is a need of the CPC to be made up of heat resistant materials such asfiberglass, nomex, teflon film. Nonwoven fabrics are either spunbonded or may comprise composite structure like SMS (spunbond-melblown-spunbond). Even tightly built wool felts are reported to offer good chemical protection.

Chemical protective clothing material's chemical composition, thickness, interaction times with challenge liquid and its concentration can affect the degree of protection. Sometimes these fabrics are provided with finishing treatments like oil and water repellant finishes with fluorocarbons. Such finishing improves the barrier properties and also offers mechanical and hygiene properties. Fabrics completed from wool are provided with acid resistant finish. Conditions which include danger of explosion because of static charge generation need antistatic finish to the protective clothing.

Assessment of barrier properties of protective clothing

Many national and international organizations like American Society for Testing and Materials (ASTM), International Standards Organization (ISO), German Standards (DIN) are suggesting various test methods for evaluating the barrier properties of protective clothing. ASTM committee F-23 on protective clothing, committee D-11 on rubber, committee D-13 on textiles and committee D-20 on plastics are upgrading a variety of test methods for protective clothing. Beside this, extensive research is still continued to develop new test methods which can overcome the problem of very less applications of standard methods.
There are three types of tests for evaluating the performance characteristics of chemical protective clothing.
.Degradation resistance
.Penetration resistance
.Permeation resistance.
Here only degradation resistance test can be applied as a screening test which can prove clear impossibility of applying specific material for particular chemicals. While, barrier properties of CPC can be find out by penetration and permeation tests.

Hydrostatic based tests

It includes pressurization liquid behind or below the specimen. Majority of these trials apply water as a challenge liquid. Water is bringing in above the clamped material specimen at a pressure controlled by water in rising column. A mirror is set below the specimen to permit the worker to look at the underside of specimen for emergence of water droplets. In many other tests (like ASTM D 7 51, ASTM D3393), the hydraulic pressure is raised until leakage comes about below the specimen.

Penetration resistance tests' limitations

.Visual observations included in the test leads to bias results
.Liquid penetration in most cases introduces in the form of fine droplets which is difficult to be observed.
.Water is utilized as a challenge liquid which has different nature than actual chemicals.
.Sometimes even degradation may cause penetration of chemical.
The above mentioned problem had been overcome by utilization of dyes to increase visual detection.

Permeation resistance
Permeation is a method of chemical penetration into chemical protective clothing at molecular level. In this method, chemical is first absorbed on exposure side of material, and then diffused through material and in the end it desorbs from the other surface. Resistance of the material to permeation is denoted in terms of breakthrough time and permeation rate. Breakthrough detection time is described as the elapsed time calculated from the beginning of the test to the sampling time that straight away precedes the sampling time at which the test chemical is first detected the analytical instrument.

Break-through time is accepted significantly while choosing CPC materials with similar degradation characteristics, whereas, permeation rate is a value of mass flux through a unit area of material for a unit time.

The method of permeation of chemical through protective clothing can be represented by applying combination of theory for solubility and theory of diffusion. The solubility parameter is calculated by evaluating Gibbs Free Energy of Mixing. The diffusion of permeant is associated with the ease with which a permeant molecule fits into the lattice of polymer chain segments and capable to transfer from place to place within the lattice. The formula is given by Fick's law which explains diffusion as:

where, J is rate of transfer per unit area, C is concentration of the diffusion substance, X is distance into the material, and D is proportionality constant, called the diffusion coefficient.

If it is presume that chemical permeation through protective clothing material follows Fick's law, the diffusion equation can be solved to get the permeation rate and chemical mass permeated as a function of time.

Tests for permeation resistance
The tests for permeation resistance are divided in two types.
.Vapor transmission tests
.Liquid permeation tests.

Vapor transmission is dealing with diffusion of gases or vapor through the material. The basic process in permeation test is to charge chemical into the challenge side of the test cell and to determine the concentration of test chemical in the collection side of the test cell as a function of time. Breakthrough time and permeation rate is considered importantly in these tests.

Variables in permeation test

Test cell diameter

ASTM F739 and ASTM FI383 suggests 51 mm diameter test cell. While a substituted test cell of diameter 25.4 mm is applied from time to time. This is mainly done to utilize less chemical and reduce disposal problems while testing hazardous chemicals. A few numbers of workers had observed the difference between these two types on cell, which observed slightly dissimilar results. In one case, somewhat longer breakthrough times were seen for smaller test cell. While other workers observed no statistically noteworthy dissimilarity between two test cells.

Closed-loop system presents to a testing mode in which the collection medium volume is set. While, open loop system suggests to a testing mode in which fresh collection medium flows constantly through the collection chamber of the test cell. In closed loop system (Fig 2), permeation rate is estimated as follows:
(Cn - Cn - 1) x V

The choice of open or closed loop system is done by properties of the challenge chemical and detector utilized. Researchers have observed the difference between open and closed loop system and arrived at the result with no difference in the both. But the former is chosen for volatile chemicals and second one is for inorganic substances.

Chemical contact method

The contact of challenge chemical kept either continuous (ASTM F739) or intermittent (ASTM F1383). In the first method the chemical is directly charged in challenge side of the cell and placed with the specimen for the chosen period. In case of intermittent contact, the time of material specimen exposure to chemical is changed in regular interval. This provides the conditions of splash-like exposure.

Form of collection media

Collection method is selected by either liquid or gas medium that does not influence the calculated permeation and in which the test chemical is without restraint soluble or adsorbed to a saturation concentration, greater than 0.5 weights or volume percent. The collection medium is evaluated quantitatively for dilution of the chemical and thus the amount of the chemical that has permeated the barrier as a function of time after its primary contact with the material.

Collection medium must selected to high capacity for permeating chemical of interest, permit ease of mixing and should be promptly analyzed and should not have any impression on clothing material under the test. As a rule of thumb, it has been suggested that the concentration of the permeant in the collection medium be maintained below 20 percent of saturation. The common collection media are air, nitrogen, helium and water. A number of chemicals show low water solubility and vapor pressure. For such chemicals solid collection media has been applied which applies highly absorbent film, absolutely against specimen. But swelling of the specimen avoids uniform contact between specimen and collection medium.

Analytical technique

Analytical technique is a process whereby the concentration of the test chemical in a collection medium is quantitatively measured. These processes are habitually an identifiable to chemical and collection medium blends. Applicable techniques cover UV (ultraviolet) and IR (infrared) spectrophotometry, gas and liquid chromatography, colorimetry, length-of-stain detector tubes, and radio nuclide tagging/detection counting/ radiochemical labeling. Inorganic chemicals are measured by pH meters in water collection media, ion-specific electrodes, and atomic absorption or ion chromatography methods.

Temperature

Temperature is another factor that can influence the calculation of solvent permeation through chemical protective clothing. Many observations have identified that the breakthrough time reduces and the permeation rate rises exponentially with rising temperature. ASTM F739 gives some direction on controlling and monitoring temperature. For nonambient temperatures, the procedure confirms that the test should be carried in a constant temperature chamber or bath that can be managed to 1 DC. Though, for ambient tests there is no requirement for controlling temperature. The outcome of low temperature on the glove materials had been observed by Raheel and Dapo. In this observation the glove materials were put at -3DC for ten days. Noteworthy changes in the physical properties such as flexural rigidity, puncture resistance were measured which influence the level of protection the material can offer.

Alternative test cells

Bromwich suggested a number of limitations of standard ASTM cell such as,
1.The cell is breakable as it is made of glass;
2.It does not particularly direct the flow of the incoming collecting medium to interrupt any boundary layer of permeant that may develop on the collection side of the test sample;
3.It is made for fluids and cannot test solid chemicals;
4.It needs about 60 ml of challenge chemical;
5.It is also slow and tedious to utilize;
6.Though the ASTM cell was made to test finished objects, it is normally applied to test the samples of CPC materials. Though, it is large and applies a test sample of 68-mm diameter, preventing the taking of samples from the finger; of gloves, and the utilization of multiple bolts avoids the testing of intact garments.

Patton tried to confirm the proprietary Radian Microcell against the ASTM cell and found a 1985 draft of an ASTM Standard Practice for Determining Equivalency of Optional Chemical Permeation Cells to that of the ASTM Cell was not enough to find out equivalency.

Bromwich had established the new cell to conquer some of the limitations of the ASTM cell covering ruggedness, low dead space, ease and speed of application and small sample size. The working of the new cell was acquired to be within the considerable criteria for the normalized breakthrough time and steady state permeation rate.

The suggested flow rate in ASTM F739 was decreased to 0.050 to 0.150 I/min in the 1991 amended from 0.050 I/min in the 1985 version and maintained within this range in the 1996 version of ASTM F739. But increased flow rates may be needed for permeates with low solubilities in the collection medium (i.e., low vapor pressures in the case of gaseous collection media) or with high permeation rates. Anna et al observed permeation tests done on 151 glove solvent combinations and suggest that the flow rates recommended in ASTM F739 (0.050 to 0.150 l/min) are not sufficient for the majority of circumstances to precisely calculate the permeation rates.

A few set of data was gathered to identify the effect of the collection stream flow rate on the normalized breakthrough time measurement. Though there was an obvious trend toward reducing BT with rising flow rate, it was neither statistically nor practically considerable.

Current status of research activities in protective clothing

Extensive research is still carried on in the field of protective clothing particularly in protective clothing against pesticides. Many workers have observed effect of water repellent finishes. Statistical structure has also being established to forecast pesticide penetration through woven as well as nonwoven chemical protective clothing.

Shaw et al examined the three commonly utilized test methods to calculate repellency, retention and penetration of liquid pesticides through protective clothing.

Fabric properties such as fiber content, type of fabric, fabric construction and finishing type can influence the barrier properties of protective clothing. Amongst these, finishing and coating have many opportunities. Experiments done by many workers prove that the absorption of pesticides by finished fabric is less than that of unfinished fabrics.
Obendorf observed the effects of liquid/fabric surface tension difference, solid volume fraction of fabrics, thickness of fabrics and viscosity of pesticide mixture on pesticide penetration of nonwoven fabrics. A predictive, statistical structure, which can be helpful in giving suggestion on the selection of chemical protective clothing, was built up to measure chemical penetration through nonwoven fabrics using basic chemical and fabric parameters. Multivariant analysis was applied to build up an empirical model to forecast pesticide penetration for untreated nonwoven fabrics.

The result of layered clothing on penetration of pesticides was observed. It have been suggested that single layers of heavyweight denim worked as a trap permitting little pesticide to pass through, while the thinner shirt weight fabric was without difficulty penetrated. In two-layer systems that applied shirt weight print cloth for the outer layer, penetration was decreased by the existence of the second fabric layer, while air permeability stayed unchanged. Other observation noted to assess different materials and finishes for their barrier effectiveness against normally used pesticides.