Chemical apparatus and process disinfecting – deodorizing – preser – Process disinfecting – preserving – deodorizing – or sterilizing – Using disinfecting or sterilizing substance
Reexamination Certificate
2000-05-11
2002-08-13
McKane, Elizabeth (Department: 1744)
Chemical apparatus and process disinfecting, deodorizing, preser
Process disinfecting, preserving, deodorizing, or sterilizing
Using disinfecting or sterilizing substance
C252S372000
Reexamination Certificate
active
06432357
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the field of sterilization and more particularly to sterilizing gas compositions comprising ethylene oxide, pentafluoroethane and heptafluoropropane.
BACKGROUND OF THE INVENTION
Sterilization by the application of boiling water or steam to the article to be sterilized has been carried out for many years. More recently the need to employ a different sterilant has arisen because certain articles, particularly those employed in the medical and aerospace industries, cannot withstand the temperatures or the moisture associated with steam sterilization.
Ethylene oxide (EO) has become widely used as a sterilant because it is highly effective and its residues are less likely to be absorbed by or adsorbed to the articles being sterilized because they volatilize quickly. By itself, ethylene oxide is an extremely flammable gas. It has a flash point less than −20° F., and forms explosive mixtures in air from about 3. 0 volume percent to 100 volume percent ethylene oxide. Thus, when ethylene oxide is used alone as a sterilizing gas, precautions such as explosion proof equipment are mandatory.
A preferable practice is to blend the ethylene oxide with an inert carrier gas or flame suppressant composition which serves to dilute the ethylene oxide and render the mixture as a whole, nonflammable. If the inert component is truly inert, i. e. it does not participate chemically in the combustion process, then the extinguishing efficiency of the inert species depends on such physical properties as its specific heat and thermal conductivity, see for example H. F. Coward and G. W. Jones “Limits of Flammability of Gases and Vapors”, Bulletin 503 p. 5 (1952). The physical extinction mechanism relies upon removal of the energy required to maintain combustion.
The flammability properties of ethylene oxide/halocarbon blends do not follow this simple physical correlation-rather, it is well known their extinctive properties stem from a chemical mechanism whereby the halogen species chemically participates in the combustion reaction, and interferes with or inhibits the combustion reaction. R. Hirst states in
Institution Of Fire Engineers Quarterly, vol.
25 (No. 59) Sep. 1965 p. 231-250, that the extinguishing ability of halogen species follows the order I>Br>Cl>F. Iodine containing halocarbons are generally known to be less chemically stable and more toxic than other members of the halocarbon family. The bromine containing species are known to possess a much greater ozone depletion potential than their chlorine containing analogs. For environmental reasons, potential halocarbon carrier gases are restricted to the hydrohalocarbons containing fluorine and/or chlorine. A hydro-substituted halocarbon possesses a much lower atmospheric lifetime than a fully halogenated chlorofluorocarbon. However, decreasing the halogen content of the carrier gas, by incorporating hydrogen in the molecule, would tend to reduce the flammability suppressant or extinctive properties of the carrier gas.
Over the last two decades the flame suppressant of choice for use with ethylene oxide in a sterilant mixture has been dichlorodifluoromethane (known in the industry as CFC-12). Recently, however, CFC-12 has come under scrutiny because it is one of the chlorofluorocarbons believed to cause significant damage to the ozone layer in the upper atmosphere. Accordingly, worldwide reduction and elimination of the use of CFC-12 is now underway.
Carbon dioxide is another flame suppressant known for use with ethylene oxide in a sterilant mixture. Because of the characteristics of carbon dioxide, however, a non-flammable ethylene oxide/carbon dioxide mixture contains less than 40 percent of the ethylene oxide per unit volume as an ethylene oxide/CFC-12 mixture. Thus, sterilization must be carried out either at higher pressures or for longer contact time. Furthermore, the large difference in the vapor pressures of ethylene oxide and carbon dioxide causes the mixture to separate upon withdrawal from the storage tank or cylinder, raising the danger of delivering a sterilant mixture rich in carbon dioxide, which will not sterilize, or rich in ethylene oxide, which is explosive.
A short term solution to the concern over the ozone depletion effects of CFC-12 has been to employ hydrochlorofluorocarbons (HCFCs), which have a reduced chlorine content. Known ethylene oxide sterilant gas mixtures include 1-chloro-1, 2, 2, 2-tetrafluoroethane (HCFC-124), chlorodifluoromethane (HCFC-22) and mixtures thereof. OXYFUME® 2002, commercially available from AlliedSignal Inc. , is a widely used sterilant mixture comprising HCFC-124 and HCFC-22. Due to the presence of chlorine, however, such compositions still have a potential for stratospheric ozone depletion.
Hydrofluorocarbons (BFCs) do not contain chlorine and have ozone depletion potentials (“ODPs”) of nearly zero. They are considered to be environmentally acceptable. However, not all hydrofluorocarbons are suitable as flame suppressants in ethylene oxide gas mixtures. Simple substitution of an arbitrary nonflammable gas does not necessary ensure a useful sterilizing gas mixture. For example, 1, 1, 1, 2-tetrafluoroethane (HFC-134
a
), generally accepted as the most likely replacement for CFC-12, is not an effective flame suppressant in ethylene oxide gas mixtures. It has been suggested that the additional hydrogen atoms may contribute to its flammability.
As a first consideration, the flammability properties of the gas mixture must be such that sufficient ethylene oxide is delivered by the mixture to effect the sterilization in an appropriate time. The Association for the Advancement of Medical Instrumentation (AAMI) recommends an absolute minimum ethylene oxide concentration of 400 mg/liter. If the carrier gas does not mask the flammability to a sufficient extent, a lower concentration of ethylene oxide must be used to ensure non-flammability. In such cases, either a longer exposure time is required to perform the sterilization, which affects productivity, or greater operating pressures are required to increase the effective ethylene oxide density in the sterilization chamber. Increasing the operating pressure is generally not a viable option because existing sterilization chambers may not be rated for the increased pressure. Furthermore, increased pressure can lead to swelling and rupture of the sealed plastic bags commonly used to package disposable medical devices. Ideally, the sterilization is performed using the highest safe concentration of ethylene oxide in order to minimize cycle time.
As an additional consideration, a candidate flame suppressant must also be miscible with ethylene oxide in the liquid phase and must not segregate from the ethylene oxide to any great extent during vaporization. Segregation or fractionation can lead to potentially flammable or explosive situations. The degree of segregation that may occur during evaporation is related to the relative volatility of the components of the mixture. The vapor pressure of ethylene oxide at 70° F. is 22 psia. A very large difference in volatility between ethylene oxide and the candidate flame suppressant increases susceptibility for the sterilant gas mixtures to fractionate.
As yet another consideration, the vapor pressure of a candidate hydrofluorocarbon may be too high and the resultant ethylene oxide gas mixture may not be suitable for use with conventional low pressure cylinders.
As can be appreciated from the above, there remains a continuing need in the art for sterilizing gas mixtures that are non-flammable yet contain sufficient ethylene oxide for effective, rapid sterilization; are miscible; are environmentally acceptable; provide sufficient vapor pressure to deliver the liquid mixture to the sterilization chamber but not too high a vapor pressure for standard shipping cylinders; are safer to use; and are cost effective.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
The invention provides sterilizing gas and methods for using the compositions.
I
Conviser Stephen Alan
Decaire Barbara Ruth
Richard Robert G.
Chess Deborah
Honeywell International , Inc.
McKane Elizabeth
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