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Why is nitrile the new choice of the medical community? The easy answer is that nitrile has no natural rubber proteins and therefore eliminates the well-documented risks associated with natural rubber latex. However, the simple elimination of natural rubber proteins does not completely address the hand protection needs of the healthcare professional. It is the history of nitrile's use in harsh environments, where it exhibits excellent barrier properties and durability, which helps to explain more completely why this material is becoming an alternative choice for healthcare professionals concerned with effective protection.
Let's begin by briefly discussing the history of nitrile. The first development of synthetic rubber materials was driven by the need to supplement the tight supply of natural rubber latex during World War I and World War II. The first commercial production of Acrylonitrile Butadiene (Nitrile) rubber was in 1930. In addition to providing good strength properties, these early nitrile elastomers out-performed ordinary rubbers in oil and gasoline resistance, abrasion resistance, gas permeability, and thermal stability.1 Reichhold and its predecessor companies were among the first to develop and market nitrile for gloves on a commercial scale in 1969. At that time, nitrile polymers were designed specifically for gloves with two essential properties chemical resistance and abrasion resistance. Further developments in vulcanization and polymerization have helped to evolve nitrile for the production of the thin, soft exam gloves, which have become extremely popular today. In current times, the need to replace natural rubber latex for medical devices has again focused attention on the further development of nitrile.
So what exactly is nitrile rubber? Nitrile is a synthetic polymer that exhibits rubber-like characteristics when vulcanized. The polymer is made in the form of latex, an emulsion, and can be processed much like natural rubber latex. However, some differences make this polymer unique. Unlike natural latex, which is polyisoprene, the backbone of the nitrile polymer is composed of three monomers (mono meaning one and mer meaning unit). These monomer ingredients are acrylonitrile, butadiene and carboxylic acid. The term terpolymer is sometimes used to describe a combination of three different monomers. The word polymer implies "many units" put together in unique ways to form a large molecule. The term nitrile is used to describe these polymers since many of the distinguishing features of these products are due to the use of the monomer acrylonitrile.
The properties of these polymers, or large molecules, are dependent on composition and the way in which the individual units or monomers are joined together. Each monomer in the composition performs a unique role and contributes to the overall balance of properties, The presence of acrylonitrile provides permeation resistance characteristics to a wide variety of fluids. Unlike natural rubber, nitrile polymers are especially resistant to hydrocarbon oils and fats.3 The butadiene component in the polymer contributes to the softness, flexibility, and feel of the glove. Butadiene is also responsible for participating in the vulcanization process, thereby enhancing the rubber-like or elastic quality of the glove.
So how effective are nitriles at providing barriers for the protection of the healthcare worker? Webster's Dictionary defines "barrier" as "a fence, etc., that bars advance or access, an obstacle, an obstruction, block, or impediment." The healthcare professional might define it further as an impediment to various toxic chemicals or as an obstacle to bloodborne pathogens. One might believe it provides an obstruction or hindrance from cuts or punctures. All of these definitions should be considered when addressing barrier properties. Numerous articles and various publications discuss barrier properties of nitrile gloves and film with regard to chemical permeation.2,3,4,5 Permeation resistance, as used here, is defined as the ability of gloves or films to block or impede a chemical from passing through the membrane on a molecular basis. The most prominent method is the American Society of Test Methods Designation F739-91 for the permeation of chemicals through chemical protective clothing. This testing protocol determines the Permeation Breakthrough Time, the time in minutes from when the initial contact occurred until the chemical is first detected on the inside of the glove, and the Steady State Permeation Rate, which is the constant rate of permeation that occurs after breakthrough when the chemical contact is continuous and all forces affecting permeation have reached an equilibrium.6 This rate is expressed in concentration of micrograms per square centimeter per minute.
How can we begin to understand the barrier properties of nitrile rubber? One manner may be a consideration of the selection of proper clothing for protection of a worker. Such a selection frequently requires the use of a chemical analogy approach or other decision model for evaluating chemical resistance. Simply put, similar chemicals would be expected to permeate the same barrier in a like manner. Using this chemical analogy model, a published guide to permeation that is arranged by chemical class can be used to estimate the performance of barriers to chemicals not tested.2 For example, nitrile is generally a good barrier against oils and greases while natural rubber is not.3 Still, it is important to recognize that chemicals are not abstract materials. In the case of water, natural rubber and nitrile rubber are affected to different degrees. Breakthrough time for water is greater than eight hours for nitrile rubber, making it a recommended barrier. With a breakthrough time of less than one hour and the possibility that degradation may occur,5 natural rubber is not recommended.
Many studies have been conducted on natural rubber latex and vinyl, the traditional glove materials of the medical community. The most common studies investigate defect rates (durability) and the effects of fluid saturation.7,8 Other studies have gone so far as to suggest that defects in the film structure are larger than the size of a viral organism based upon the porous nature of natural rubber latex.9,10
Why are these studies so important? Industrial nitrile gloves are typically chosen to replace other forms of gloves for two main reasons. The first is nitrile's superior barrier properties to a wide range of chemicals. The second reason, no less important, is nitrile's durability. The superior performance of nitrile in fats and oils, as well as in water, should suggest its relevance for medical professionals. The chemical analogy approach may suggest that nitrile would be the appropriate choice when dealing with the risks associated with bodily fluid contact; the analogy may also explain why other studies have questioned the adequacy of natural rubber as a medical barrier.
The durability of nitrile is most easily represented by its historical position in industrial applications. Nitrile was able to replace heavy leather gloves mainly because of its durability, excellent abrasion resistance, and cut and puncture resistance. More recently, nitrile gloves have begun replacing natural rubber latex in some of the world's high-tech cleanrooms. One of the major reasons for this switch is that nitrile glove surfaces do not degrade or shed, qualities that should also be of interest to the medical professional. The surface structures of the two polymer films can be seen below:
The visible difference at this magnification begins to highlight the potential for improved barrier performance of nitrile films or gloves.
These photomicrographs, along with the chemical permeation data, begin to help us understand that nitrile can provide an equal barrier to bloodborne pathogens and body fluids, as well as improve cut, puncture, and abrasion resistance. Research and experience to date clearly suggest that in the future nitrile may well be a superior product, not only in industrial markets, but also in newer markets such as high technology cleanrooms and the medical community. It is critical to continue our research to fully characterize the ability of properly vulcanized nitrile gloves to provide healthcare and other workers with the most effective hand protection possible.
REFERENCES
- J. Purdon, Nitrile Elastomers, Vanderbuilt Rubber Handbook, 13th Edition (1990)
- S.Z. Mansdorf, American Industrial Hygiene Association Journal 58:110-115 (1997)
- C. Nelson Schlatter, Occupational Health & Safety, Vol. 58(2), 24(1989)
- M.D. Morris, Journal of Natural Rubber Research, Vol. 9(4), 241-252 (1994)
- K. Forsberg, Quick Selection Guide to Chemical Protective Clothing, 2nd Edition, 1993
- ASTM Designation F 739-91, Pub., April 1992
- D. Korniewicz, Journal of Clinical Microbiology, April 1990, p, 787-788
- D. Korniewicz, Am. Ind. Hyg. Assoc. J. 54(1): 22-26 (1993)
- C. Roland, Naval Research Laboratory, Rubber World, June 1993, p. 15-18
- W. Cappuccio, Infection Control and Hospital Epidemiology, June 1997, 18:423-425
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