Fortifying the Last Line of Defense
By learning more about chemical protective clothing selection, users can better protect themselves against chemical exposure and flashfire hazards
- By James P. Zeigler, Susan Lovasic
- Apr 01, 2005
What factors should you consider when faced with the dual hazards of chemical exposure and flash fire? The simple answer is barrier -- both chemical and thermal barrier. However, as much as we wish that personal protective equipment (PPE) selection could be a simple process, this one word '"barrier'" encompasses a number of both chemical-protective and flash-fire-protective properties that need to be considered during the protective clothing selection process.
Factors to Consider When Selecting Chemical Barrier Protection
Fabric, seam, and garment design combine to provide chemical protection. And the choice of fabric, seam, and garment style starts with an assessment of the chemical-exposure hazard. For each task or activity that might lead to chemical exposure, the assessment should address the likelihood of contact, the level of contact, and the length of contact. What is the likelihood or probability that the wearer will come in contact with any amount of the chemical? In defining the likelihood and level of contact, consider if the area is drenched in chemicals, whether there are active drips, splash or spray, or if the area is lightly contaminated. The length of contact depends on the task duration, procedures, and practices.
To illustrate how to apply likelihood, level, and length, consider the hazards encountered during a process line break and the hazards presented during a routine process patrol. If line break procedures and practices are effective and followed, there should be little likelihood of any substantial amount of material remaining in a production pipeline. However, if procedures and practices are not followed, if they are not effective, or if there are equipment failures, such as a leakage through one of the isolation valves on either side of the line break, then the potential level of exposure will be high. If the line break is overhead, the whole body will be exposed from head to toe. The length of exposure could be short or long, depending on the local rules and practices. The garment material should provide barrier for longer than the anticipated contact time. The design of the garment should protect the parts of the body likely to be exposed -- most likely the entire body.
Contrast the exposure scenario above, with an operator conducting a routine process patrol assignment that requires walking through a process area to visually observe that equipment and conditions are normal. The likelihood of contact is very low. During patrol, the products should be contained in the process equipment and the operator should only contact a residual level of the product. In this scenario, any contact time will be short.
The design of the garment must match the likelihood, level, and length of exposure. Totally encapsulating garments with attached gloves, socks, and multi-layer closure flaps provide much more extensive protection than hooded coveralls with open wrists, ankle openings, and simple closure assemblies.
Garment design also governs how well the skin is protected from chemical vapor contact. Level A garments certified to National Fire Protection Association (NFPA) 1991 and Class 1 of NFPA 1994 provide protection factors of over 5,000 (outside concentration divided by inside concentration). But a hooded coverall with elastic-lined wrist, ankle, and hood openings essentially provides no vapor protection due to these openings.
Sewn or bound seams are not liquid tight and are not vapor tight. Sewn seams are, at best, adequate for residual contamination. Any situation involving a detectable liquid volume requires sealed seams, either welded or taped. Taped seams are generally stronger than the surrounding fabric. In the case of our line break example, the potential for high levels of liquid contact confirm the need for sealed seams. For the process patrol scenario, where only residual contact is likely, a sewn-seam garment may suffice.
Testing Chemical Barrier Clothing
The choice of garment material is based on chemical barrier. The permeation test is the most sensitive measure of the chemical barrier against liquids and gases. The term permeation has a very specific meaning in chemical protective clothing. Permeation involves movement of the chemical through the barrier without flow through holes or voids. Permeation is a process that starts with the hazardous chemical absorbing or dissolving into the barrier. The chemical moves through the barrier and eventually saturates it. As the chemical builds up in the fabric, it also migrates through the fabric to the opposite, '"inner,'" surface. There it will begin to desorb or off-gas from the inner, or skin-side, of the material, creating a high potential for local skin exposure.
Permeation is different from penetration. Penetration involves movement of the chemical through pores or holes in a barrier. During penetration, the chemical does not change state -- a vapor flows through the pores as a vapor and emerges as a vapor, a liquid remains in the liquid state as it flows through the pores, and a particle penetrates a porous material as a solid particle. In contrast, permeation involves a change of state in which the liquid or gas dissolves into the solid barrier before migrating through and re-emerging from the opposite surface.
Some garment materials have been engineered with very small pores to allow vapors to penetrate while blocking liquids. These microporous materials have interconnecting submicron pores. Pressure is required to force liquids through these narrow pores. Many organic compounds and many aqueous corrosives, such as hydrochloric acid, sulfuric acid, nitric acid, and ammonium hydroxide, have high vapor pressures. Even though penetration of these liquids can be blocked by these narrow pores, the potentially harmful vapors from these liquids can easily diffuse through the pores. While there is no visible liquid penetration, the more sensitive permeation test will detect the passage of the vapors. With the known corrosive effects of these corrosive off-gases and unknown skin toxicity of many volatile organic compounds, the permeation test shows the potential vapor exposure even when there is no apparent liquid penetration. Hence, permeation has been accepted as the more sensitive and appropriate chemical barrier test for protective garments.
Thermal Barrier Clothing Criteria
Not all situations involving flammable materials create the risk of a flash fire. Flash-fire protective clothing is not worn when removing fingernail polish, even though fingernail polish remover generally consists of highly flammable acetone. A flash-fire ignition requires the right concentration of flammable materials (vapors, liquids, or dust), oxygen, and an ignition source.
However, when the potential for a flash fire does exist, protection cannot be based solely on controlling the ignition source. An overlooked ignition source -- such as sparking tools, static from flowing liquids or powders, dissimilar materials rubbing together, or a hot surface -- may be sufficient to ignite the materials. No matter how scrupulous the efforts to eliminate possible ignition sources, the worker needs clothing that provides a thermal protection barrier should a flash fire occur.
Thermal barrier and '"FR'" are not synonymous. Whether interpreted as flame-retardant or flame-resistant, there are no universally accepted definitions or set of test methods for FR. Each industry has developed its own definition. And while these definitions and performance tests may be suitable for that industry, they may not be adequate for others. For example, the definition of FR used for draperies and wall hangings is not adequate when selecting flash-fire protective clothing.
NFPA 2112, "'Standard on Flame-Resistant Garments for Protection of Industrial Personnel Against Flash Fire,'" brought together a group of users and experts to develop an internationally recognized consensus standard for thermally protective garments in industrial situations. The scope statement of NFPA 2112 states: '"This standard shall specify the minimum design, performance, certification requirements, and test methods for flame-resistant garments for use in areas at risk from flash fires. NFPA 2112, 2001 Edition'".
NFPA 2112 does not rely upon a single FR criterion. The standard requires that a series of criteria must be met by the garment to qualify as a flash-fire protective garment -- vertical flammability, thermal shrinkage and heat resistance, thermal protective performance, and overall flash-fire protective performance. Multiple criteria are also common in the other NFPA thermal protective clothing standards for structural fire gear, proximity firefighting clothing, or wildlands firefighting clothing.
Testing Thermal Barrier Clothing
In the vertical flammability test, a strip of fabric is hung above an open flame for 12 seconds. This test determines whether the fabric ignites, if continues to burn, and if it melts and drips. NFPA 2112 requires less than two seconds of after-flame time, char length less than 4 inches, and no melting or dripping. This vertical flammability test is essentially the same used in NFPA 701, a standard that applies to wall coverings and draperies.
Thermal shrinkage and heat resistance is determined by exposing the garment material to 500 degrees Fahrenheit for five minutes. This test evaluates how the garment material reacts to the high heat that could occur during a flash fire. NFPA 2112 requires that garment materials, including components, can not shrink more than 10 percent; nor can they melt, drip, ignite, or separate; and they must remain functional after exposure. Additionally, thread used to sew the garment must not melt at 500 degrees Fahrenheit and must be made of an inherently flame-resistant fiber.
The thermal insulating capability of the garment material is evaluated with the thermal protective performance (TPP) test using a heat flux likely encountered in a flash-fire environment. The TPP test measures the time it takes for heat transfer through the fabric to cause the onset of a second-degree burn. NFPA 2112 requires that specimens have a '"contact'" TPP rating greater than 3 cal/cm2 and a '"spaced'" TPP rating greater than 6 cal/cm2. The contact rating simulates direct contact between the fabric and skin; the spaced rating simulates a 1/4 inch gap between the fabric and skin.
The overall thermal performance of the garment fabric is evaluated with an instrumented manikin using a simulated flash fire. The instrumented manikin is used to predict the percentage of total body surface area that would suffer second- or third-degree burns during a simulated flash fire. A standard coverall design is used to eliminate fit and design-related variables. The standard requires less than 50 percent predicted total body burn after a three-second exposure at 2 cal/cm2·sec, heat flux.
The thermal manikin test can also be used to uncover problems that are not detected during a simple fabric vertical flammability test. For example, after manikin flash-fire exposure tests on some commercial polyvinyl chloride (PVC) rainwear labeled as FR, the closures were welded shut; and in the case of jacket/pant ensembles, the overlap between the jacket and pants also was sealed together. Heavy shears were required to cut through these welded sections. In a real situation, burn injuries often occur after the initial flash fire due to prolonged contact with the hot garment. Difficulty in removing the garment prolongs the victim's contact time, delays treatment, and possibly increases burn injury level.
Note of Caution
While Superman's costume may have a red letter '"S'" on its chest, garments that provide chemical and thermal barriers do not. No item of PPE should be relied upon as the only means of protection. Chemical releases and flash fires are unpredictable events and even with specially designed and selected clothing, injuries can still occur. In any potentially hazardous situation, the options to eliminate or avoid the hazard, engineering controls and shielding, work practices,
and administration procedures should be considered before resorting to PPE, the last line of defense.
This article originally appeared in the 04/01/2005 issue of Environmental Protection.
Of his 22 years with DuPont, Jim Zeigler, PhD, has spent the past 10 working on fabrics for the protective apparel market. Located at the DuPont-Richmond, Va., site, he is the senior member of the market support and product development team for DuPontä Tyvek® protective apparel and DuPontä Tychem® chemical barrier fabrics, Richmond, Va. Dr. Zeigler is the inventor of many of DuPont's protective apparel fabrics, including Tychem® 7500, Tychem® 9400, Tychem® BR, Tychem® LV, Tychem® 10,000 and Tychem® TK. A member of the F23 Protective Clothing Committee of the American Society for Testing and Materials (ASTM), Zeigler also sits on the National Fire Protection Association (NFPA) Technical Committee for Hazardous Materials Protective Clothing and Equipment. He has particular knowledge in protective apparel technology; industry regulations and standards; and on matters involving weapons of mass destruction.
Susan L. Lovasic is a research associate with DuPont Personal Protection. A graduate of Penn State with a bachelor's degree in chemical engineering, she has worked for DuPont for 20 years. Her work experience has included both research and marketing responsibilities in several DuPont businesses. Lovasic is currently a lead researcher for clothing that protects workers from thermal burn injury hazards.