AU690066B2 - Azeotrope-like compositions of pentafluoroethane and 1,1,1-trifluoroethane - Google Patents

Description

OPI DATE 12/12/94 AOJP DATE 19/01/95 APPLN. ID 40423/93 PCT NUMBER PCT/US93/04577 AU9340423 (51) International Patent Classification 5 (11) International Publication Number: WO 94/26836 CO9K 504 Al (43) International Publication Date: 24 November 1994 (24.11.94) (21) Interational Application Number: PCTIUS93/04577 (81) Designated States: AU, CA, JP, KR, European patent (AT, BE, CH, DE, ES, FR, GB, GR, IE, IT, LU, MC, NL, (22) International Filing Date: 13 May 1993 (13.05.93) PT, SE), (71) Applicant: ALLIED-SIGNAL INC. [US/US]; 101 Columbia Published Road, P.O. Box 2245, Morrstown, NJ 07962-2245 Wi1th international search report.

(72) Inventors: SI{ANKLAND, Ian, 200 Forest Hill Drive, Willianmsville, NY 14221 ATWOOD, Theodore; 1520 North Forest Road, Williamnsville, NY 14221 LUND, Earl, 404 Reserve Road, West Seneca, NY 14224 PHAM, Hang, Thanh;, 96 Sundridge Drive No.1, North Tonawanda, NY 14120 (US).

(74) Agent: BLEEKER, Ronald, Allied-Signal Inc., Law Dept.

McNally), 101 Columbia Road, P.O. Box 2245, Morristown, NJ 07962-2245 (US).

(54) Title: AZEOTROPE-LIKE COMPOSIMONS OF PENTAFLUOROETHANE AND 1,1,1-TRIFLUOROETHANE (57) Abstract Azeotrope-like compositions comprising pentafluoroetbane and 1,1,1-trifluoroethane are stable and have utility as refigerants for heating and cooling.

WO 94/26836 PCT/US93/04577 -1-

DESCRIPTION

AZEOTROPE-LIKE COMPOSITIONS OF PENTAFLUOROETHANE AND 1,1.-TRIFLUOROETHANE Field of the Invention This invention relates to azeotrope-like or essentially constant-boiling mixtures of pentafluoroethane and 1,1,1trifluoroethane. These mixtures are useful as refrigerants for heating and cooling.

CROSS-REFERENCE TO RELATED APPLICATION Commonly assigned U.S. patent No. 4,944,388 discloses azeotrope-like mixtures of pentafluoroethane; 1,1,1trifluoroethane; and chlorodifluoromethane.

BACKGROUND OF THE INVENTION Fluorocarbon based fluids have found widespread use in industry for refrigeration applications such as air conditioning and heat pump applications.

Vapor compression is one type of refrigeration. In its simplest form, vapor compression involves changing the refrigerant from the liquid to the vapor phase through heat absorption at a low pressure and then from the vapor to the liquid phase through heat removal at an elevated pressure. First, the refrigerant is vaporized in the evaporator which is in contact with the body to be cooled. The pressure in the evaporator is such that the boiling point of the refrigerant is below the temperature of the body to be cooled. Thus, heat flows from the body to the refrigerant and causes vaporization. The formed vapor is then removed by means of a compressor in order to WO 94/26836 PCT/US93/04577 2 maintain the low pressure in the evaporator. The temperature and pressure of the vapor are then raised through the addition of mechanical energy by the compressor. The high-pressure vapor then passes to the condenser whereupon heat exchange with a cooler medium, the sensible and latent heats are removed with subsequent condensation. The hot liquid refrigerant then passes to the expansion valve and is ready to cycle again.

While the primary purpose of refrigeration is to remove energy at low temperature, the primary purpose of a heat pump is to add energy at higher temperature. Heat pumps are considered reverse cycle systems because for heating, the operation of the condenser is interchanged with that of the refrigeration evaporator.

Certain chlorofluoromethane and chlorofluoroethane derivatives have gained widespread use in refrigeration applications including air conditioning and heat pump applications owing to their unique combination of chemical and physical properties.

The majority of refrigerants utilized in vapor compression systems are either single components fluids or azeotropic mixtures. The use of azeotropic mixtures as refrigerants is known in the art; for example, see R. C.

Downing, FLUOROCARBONS REFRIGERANTS HANDBOOK, Prentice- Hall, 1988 and U.S. Patents 2.101,993 and 2,641,579.

R-502 is an azeotropic blend which consists of monochlorodifluoromethane(R-22) and chloropentafluoroethane(R-115), a fully halogenated chlorofluorocarbon.

R-502 has been routinely used for medium to low temperature refrigeration applications.

WO 94/26836 PCT/US93/04577 3 Azeotropic or azeotrope-like compositions are desired because they do not fractionate upon boiling.

This behavior is desirable because in the previously described vapor compression equipment with which these refrigerants are employed, condensed material is generated in preparation for cooling or for heating purposes.

Unless the refrigerant composition exhibits a constant boiling point, i.e. is azeotrope-like, fractionation and segregation will occur upon evaporation and condensation and undesirable refrigerant distribution may act to upset the cooling or heating.

Non-azeotropic mixtures have been disclosed as refrigerants for example in U. S. Patent 4,303,536 but have not found widespread use in commercial applications.

The use of non-azeotropic mixtures which fractionate duriig the refrigeration cycle introduces additional complexity into the system which necessitates hardware changes. The use of non-azootropic refrigerants has been avoided primarily due to the added difficulty in charging and servicing refrigeration equipent and the situation is further complicated if an inadvertent leak in the system occurs during use or during service. The composition of the mixture could change affecting system pressures and system performance. If one component of the nonazeotropic mixture is flammable, then fractionation could shift the composition into the flammable region with potential adverse consequences.

The art is continually seeking new fluorocarbon based azootrope-like mixtures which offer alternatives for refrigeration and heat pump applications. Currently, of particular interest, are fluorocarbon based azeotrope-like mixtures which are considered to be environmentally safe substitutes for the presently used fully halogenated WO 9426836 PCT/US93/04577 -4chlorofluorocarbons(CFC's). The latter are suspected of causing environmental problems in connection with the earth's protective ozone layer.

The substitute materials must also possess those properties unique to the CFC's including chemical stability, low toxicity, non-flammability, and efficiency in-use. The latter characteristic is important in refrigeration and air-conditioning especially where a loss in refrigerant thermodynamic performance or energy efficiency may have secondary environmental impacts through increased fossil fuel usage arising from an increased demand for electrical energy. Furthermore, the ideal CFC refrigerant substitute would not require major engineering changes to conventional vapor compression technology currently used with CFC refrigerants.

Mathematical models have substantiated that hydrofluorocarbons, such as pentafluoroethane(HFC-125) and 1.1,1trifluoroethane(HFC-143a) will not adversely affect atmospheric chemistry, being negligible contributors to ozone depletion and to green-house global warming in comparison to the fully halogenated species.

Because HFC-143a is as efficient as R-502 and provides a modest increase in refrigeration capacity, HFC-143a might be considered a good refrigerant substitute for R-502. However, a disadvantage of HFC-143a as a refrigerant is that the vapor of HFC-143a is flammable.

As a result, the shipping, handling, and use of HFC-143a have to be carefully caotrolled due to the potential flammability.

Because HFC-125 is nonflammable and provides a modest increase in refrigeration capacity compared with R-502. HFC-125 might be considered a good refrigerant P'\O nRK\MJ h 23 93047 182lt substitute for R-502. However, a disadvantage of HFC-125 is that HFC-125 is about 5% less efficient than R-502.

It is a preferred object of this invention to provide novel azeotrope-like compositions based on pentafluorethane and 1,1,1trifluoroethane which will not fractionate under normal cooling or heating conditions.

Another preferred object of the invention is to provide novel environmentally acceptable refrigerants for use in the aforementioned applications.

Other objects and advantages of the invention will become apparent from the following description.

DESCRIPTION OF THE INVENTION In accordance with the invention, novel azeotrope-like 15 compositions have been discovered comprising pentafluoroethane and 1,1,1-trifluoroethane.

Accordingly the present invention provides the use of an environmentally acceptable material as a substitute for R502 as refrigerant in a heat pump or refrigeration system, 20 characterised in that the environmentally acceptable material is a binary azeotrope-like composition which comprises 49.7 to weight percent pentafluoroethane and 50.3 to 40 weight percent 1,1,1-trifluoroethane. These compositions are azeotrope-like because they are constant-boiling, i.e. exhibit essentially constant-vapor pressure versus composition and essentially identical liquid and vapor compositions over the aforementioned compositional ranges.

In a preferred embodiment of the invention, the azeotropelike compositions of the invention comprise 53.7 weight percent 4 weight percent pentafluoroethane and 46.3 weight percent 1,,1-trifluoroethane 4 weight percent. Vapor phase compositions containing in 6 excess of about 38.4 weight percent pentafluoroethane were determined to be nonflammable in air at ambient conditions using the ASTM E-681 method as specified in the American Society of Heating, Refrigerating, and Air-Conditioning Engineecs(ASHRAE) Standard 34.

In a most preferred embodiment of the invention, the azeotrope-like compositions of the invention comprise 50 weight percent pentafluoroethane and 50 weight percent 1,1,1trifluoroethane. These compositions are constant-boiling, nonsegregating, and nonflammable.

These most preferred azeotrope-like compositions of the invention have a vapor pressure of about 167 psia (1113 kPa) 6 psia (27kPa) at 70 0 F(21.1 0

C).

0* A blend of HFC-125 and HFC-143a wae disclosed as 2 having utility as a refrigerant in RESEARCH DISCLOSURE 20 15402. February 1977 but this disclosure implied that such a blend was non-azeotropic, i.e. would fractionate upon 00#0 evaporation or condensation, and stated that the blend was disadvantageous because it was flammable. Contrary to this teaching, we have discovered that the blends of HFC- 25 125 and HFC-143a as recited above are both constantboiling, i.e. azdotrope-like. and are nonflammable.

The term "azeotrope-like" is used herein for the o, mixtures of the invention because in the claimed 30 proportions, the compositions of pentafluoroethane and 11,1,-trifluoroathane are constant-boiling or essentially constant-boiling.

All compositions within the indicated ranges, as well as certain compositions outside the indicated ranges, are azeotrope-like, as defined more particularly below.

WO 94/26836 PCTIUS93/04577 7 From fundamental principles, the thermodynamic state of a fluid is defined by four variables: pressure, temperature, liquid composition and vapor composition, or P-T-X-Y, respectively. An azeotrope is a unique characteristic of a system of two or more components where X and Y aLe equal at the stated P and T. In practice.

this means that the components of a mixture cannot be separated during a phase change, and therefore are useful in the cooling and heating applications as described above.

For the purpose of this discussion, azeotrope-like composition is intended to mean that the composition behaves like an azeotrope, i.e. has constant-boiling characteristics or a tendency not to fractionate upon boiling or evaporation. Thus, in such compositions, the composition of the vapor formed during boiling or evaporation is identical or substantially identical to the original liquid composition. Hence, during boiling or evaporation, the liquid composition, if it changes at all, changes only to a minivmal or negligible extent. This is to be contrasted with non-azeotrope-like compositions in which during boiling or evaporation, the liquid composition changes to a substantial degree.

Thus, one way to determine whether a candidate mixture is "azeotrope-like" within the meaning of this invention, is to distill a sample thereof under conditions resolution number of plates) which would be expected to separate the mixture into its separate components. If the mixture is non-azeotropic or nonazeotrope-like, the mixture will fractionate. i.e.

separate into its various components with the lowest boiling component distilling off first, and so on. If the mixture is azeotrope-like, some finite amount of a first distillation cut will be obtained which contains all of WO 94/26836 PCT/US93/04577 8 the mixture components and which is constant-boiling or behaves as a single substance. This phenomenon cannot occur if the mixture is not azeotrope-like it is not part of an azeotropic system.

It follows from the above that another characteristic of azeotrope-like compositions is thdt there is a range of compositions containing the same components in varying proportions which are azeotrope-like or constant-boiling. All such compositions are intended to be covered by the term azeotrope-like or constantboiling as used herein. As an example, it is well known that at differing pressures, the composition of a given azeotrope will vary at least slightly as does the boiling point of the composition. Thus, an azeotrope of A and B represents a unique type of relationship but with a variable composition depending on temperature and/or pressure. As is readily understood by persons skilled in the art, the boiling point of the azeotrope will vary with the pressure.

In one process embodiment of the invention, the azeotrope-like compositions of the invention may be used in a method for producing refrigeration which comprises condensing a refrigerant comprising the azeotrope-like compositions and thereafter evaporating the refrigerant in the vicinity of a body to be cooled.

In another process embodiment of the invention, the azeotrope-like compositions of the invention may be used in a method for producing heating which comprises condensing a refrigerant comprising the azeotrope-like compositions in the vicinity of a body to be heated and thereafter evaporating the refrigerant.

WO 94/26836 PCT/US93/04577 9 The pentafluo-roethane and 1.l,1-trifluoroethane of the novel azeotrope-like compositions of the invention are known materials. Preferably, the materials should be used in sufficiently high purity so as to avoid the introduction of adverse influences upon the cooling or heating properties or constant-boiling properties of the system.

It should be understood that the present compositions may include additional components so as to form new azeotrope-like compositions. Any such compositions are considered to be within the scope of the present invention as long as the compositions are constantboiling or essentially constant-boiling and contain all of the essential components described herein.

The present invention is more fully illustrated by the following non-limiting Examples.

EXAMPLE 1 This example shows that certain compositions of pentafluoroethane and l,1,1-trifluoroethane are azeotropelike. i.e. exhibit essentially identical liquid and vapor compositions, and are constant-boiling, i.e. exhibit essentially constant vapor pressure versus composition within this range.

Vapor liquid equilibrium experiments were performed by preparing mixtures of HFC-125 and HFC-143a in an approximately 150 cubic centimeter vessel. The vessel, equipped with a magnetically driven stirrer and a 0-300 psia(0-2068 kPa) pressure transducer accurate to was submerged in a constant temperature bath controlled to within ±0.05*F(0.03*C). Once thermal equilibrium was WO 94/26836 PCTIUS931/04577 10 attained, as determined by constant vapor pressure readings, vapor and liquid samples were withdrawn from the vessel and analyzed by standard gas chromatographic techniques. This procedure was repeated at three nominal compositions of about 25, 50, and 75 weight percent HFC-125 in HFC-143a, and at three temperatures of 70 0 F(21.1 0 and 112.5*F(44.70C). Table I summarizes the results of these experiments. In Table I. the compositions are in weight percent HFC-125 in HFC-143a.

TABLE I VAPOR LIQUID EQUILIBRIA DATA Temperature -10.0(-23.3) -10.0(-23.3) -10.0(-23.3) -10.0(-23.3) -10.0(-23.3) Liquid Composition (Weight Percentage HFC-125) 0.0 22.4 5102 76.1 100.0 Vapor Composition (Weight Percentage HFC-125) 0.0 22.1 52.0 77.0 100.0 Vapor Pressure psi&(kPa 40.2(268) 40.1(267) 40.1(267) 41.0(273) 43.5(290) 70.0(21.1) 70.0(21.1) 70.0(21.1) 70.0(21.1) 70.0(21.1) 112.5(44.7) 112.5(44.7) 112.5(44.7) 112.5(44.7) 112.5(44.7) 0.0 22.5 51.0 76.0 100.0 0.0 22.4 50.9 76.2 100.0 0.0 22.8 52.3 77.7 100.0 0.0 23.1 50.6 78.0 100.0 165.2(1101) 165.2(1101) 167.1(1114) 171.6(1144) 180.0(1200) 297.9(1986) 299.2(1995) 300.9(2006) 309.9(2066) 326.0(2173) The data shown in Table I indicate that the vapor and liquid compositions are essentially identical within the experimental uncertainty of ±1.0 weight percent unit associated with the chromatographic analysis. The vapor pressures of the blends are essentially constant to within over the composition range from about I to about weight percent HFC-125 and from about 99 to about weight percent HFC-143a, i.e. these blends are constant-boiling or azeotrope-like.

WO 94/26836 PCT[US93/4577 11i EXAMPLE 2 This example shows that certain HFC-125/HFC-143a blends are nonflammable.

Flammability measurements were performed using the ASTM E-681 technique modified according to ASHRAE Standard 34. Briefly, this technique involves preparing fluorocarbon/air gas mixtures to a total pressure of one atmosphere(lOlkPa) in a 5-liter spherical glass vessel, stirring the mixture with a magnetically driven propeller to ensure a uniform composition, and then attempting to ignite the mixture using an electrically activated kitchen match head as the ignition source. A ternary flammability diagram was mapped by preparing mixtures of HFC-125, HFC-143a, and air by the method of partial pressures and then-determining whether or not a flame would propagate as defined by ASTM E-681. The critical flammability composition, i.e. the composition of the HFC-125/HFC-143a blend which contains the maximum proportion of the flammable HFC-143a but does not exhibit flame limits in air, was determined in a graphical manner similar to that described in Haenni et al, Industrial and Engineering Chemistry 685(1959). The critical flammability composition was found to be 61.6 weight percent HFC-143a and about 38.4 weight percent HPC-125. In other words.

blends of HFC-125 and HFC-143a containing 38.4 or more weight percent HFC-125 are nonflammable in all proportions in air at ambient conditions.

EXAMPLE 3 This example shows that a blend of 50 weight percent HFC-125 and 50 weight percent HFC-143a undergoes essentially no fractionation and maintains a constant- WO 94/26836 PCT/US9.1/04S77 12 vapor pressure during a vapor leak which illustrates one advantage of a constant-boiling or azeotrope-like composition.

The vessel described in Example 1 was charged with approximately 105 grams of a 50/50 weight percent mixture of HFC-125 and HFC-143a. Vapor was allowed to leak from this container until about 14 weight percent of the original charge had dissipated, at which point the vapor pressure of the remaining liquid was measured and a vapor phase sample collected for analysis. The vapor leak was continued until 80% of the original charge had dissipated; additional vapor samples were taken for analysis at different stages during the leak. The temperature of the vessel was maintained at 70"F(21.1*C) during the leak.

Vapor pressure and composition data are reported in Table II. In Table II, the composition is in weight percent HFC-125 in HFC-143a.

TABLE II FHACTIONATION DATA Percent Vapor Preassre Vapor Composition Dissipation pi(U (Weight Percent HFC-125) 0.0 167.3(1115) 50.0 14.4 167.0(1113) 50.4 31.7 166.2(1108) 49.8 48.5 165.4(1103) 49.6 64.3 164.7(1098) 48.8 79.9 163.3(1089) 48.8 Residue liquid 48.6 Residue vapor 48.6 The data listed in Table II show that the composition of the mixture varies by no more than 1.2 weight percent and that the vapor pressure remains constant within 4psia (27 kPa) or about 2.5% of the total pressure.

WO 94126836 PCT/S93104577 13 EXAMPLE 4 This example shows that constant-boiling HFC-125/ HFC-143a blends have certain advantages when compared to other refrigerants which are currently used in certain refrigeration cycles.

The theoretical performance of a refrigerant at specific operating conditions can be estimated from the thermodynamic properties of the refrigerant using standard refrigeration cycle analysis techniques; see for example, R. C. Downing, FLUOROCARBONS REFRIGERANTS HANDBOOK, Chapter 3, Prentice-Hall. 1988. The coefficient of performance(COP) is a universally accepted measure, especially useful in representing the relative thermodynamic efficiency of a refrigerant in a specific heating or cooling cycle involving evaporation or condensation of the refrigerant. In refrigeration engineering, this term expresses the ratio of useful refrigeration to the energy applied by the compressor in compressing the vapor. The capacity of a refrigerant represents the volumetric effectiveness of the refrigerant. To a compressor engineer, this value expresses the capability of a compressor to pump quantities of heat for a given volumetric flow rate of refrigerant. In other words, given a specific compressor, a refrigerant with a higher capacity will deliver more cooling or heating power.

We have performed this type of calculation for a medium to low temperature refrigeration cycle where the condenser temperature is typically 100F(38*C) and the evaporator temperature is typically -40 to -10*F(-40 to We have further assumed isentropic compression and a compressor inlet temperature of 65eF(18.3*C). Such calculations were performed for a 60/40 by weight percent WO 94/26936 P'MUS93/04577 1.4 blend of HFC-125 and HFC-143a as well as for R-.502. Table III lists the COP of a 60/40 blend of HFC-125 and HFC-143a relative to that of R-502 over a range of evaporator temnperatures. In Table 111, the indicates that COP and capacity are given relative to R-502.

TABLE III THERMODYNAMIC PERFORMANCE R-502 125/14~3& [Evaporator Discharge Discharge Temp. Temp. Temp.

7(eC) COP* C"Aeaitz* M(C) F(C -40(-40) 1.01 1.08 235(113) 213(101) -30(-34) 1.00 1.07 220(104) 200(93) -20(-29) 0.99 1.07 205(96) 189(87) -10(-23) 0.99 1.06 192(89) 177(81) The data listed in Table III show that the HFC-125/HFC-143a blend provides essentially the same COP(within as that attainable with R-502, provides about a 7% increase in refrigeration capacity. and also produces lower discharge temperatures from the compressor, which contributes to compressor reliability. It has been recommended that compressor discharge temperatures be limited to about 22507(107.20C). This temperature is exceeded in the current example by R-502 at evaporator temperatures lover than the HFC-125/ HFC-143a blend can operate down to an evaporator temperature of -500F(-45.66C) before exceeding the 22SF1(107.2'C) discharge temperature limit. Even '.over evaporator temperatures could be accomplished by enriching the HFC-125 component up to 80% of the total mixture.

without significant impact on the performance.

Having described thro invention in detail and by reference to preferred embodiments thereof, it will be WO 94/26836 PCT/US93/04577 15 apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.

Claims (3)

1. The use of an environmentally acceptable material as a substitute for R502 as refrigerant in a heat pump or refrigeration system, characterised in that the environmentally acceptable material is a binary azeotrope-like composition which comprises 49.7 to 60 weight percent pentafluoroethane and 50.3 to 40 weight percent 1,1,1-trifluoroethane.

2. Use according to claim 1, wherein the composition comprises 10 53.7 weight percent 4 weight percent pentafluoroethane and

46.3 weight percent 4 weight percent l,l,l-trifluoroethane and which has a vapour pressure of 167 psia 6 psia at 70°F (1113 kPa 40 kPa at 21.10C). *e 3. A use according to claim 1 or claim 2 in which the composition comprises 50 weight percent pentafluoroethane and weight percent 1,1,1-trifluoroethane. S" 4. A use according to any of claims 1 to 3 as a substitute for R502 as a refrigerant in a refrigeration system. DATED this 16th day of February 1998. ALLIEDSIGNAL INC. by their Patent Attorneys DAVIES COLLISON CAVE I I 0 %NTrE RNATIONAL SEARCH ItEPORtT Intern iat AppltAtiori No PCT/US 93/04577 A. CLASSIFICATION OF SUBIJECT' MKITER IPO 5 C09K5/04 According to International Patent Classification (IPC) ct to both national classification and IPC B. F.LIS SEARCHED Minimum documentation searched (classification system followed by clasificaion symbols) IPC 5 C09K Documenitation searched other than mmum documentation to the extent that such documents are included in the fields searched Electronic data base consulted during the iternational search (namec of data base And, where practical, search terms used) C. DOCUMENTS CONSIDERED TOBE Category' Citation of document, with indication, where appropriate, of the relevant passage& Relevant to claim No, L,E US,A,5 211 867 (SHANKLAND) 18 May 1993 1-8 see the whole document X EP,A,0 526 745 (DATx IN INDUSTRIES) 10 1-8 February 1993 see abstract; claims 1,2; figures 1,2 Y US,A,4 943 388 (SHANKLAND) 24 July 1990 1-8 see abstract; claims 1-4 Y DATABASE VAPI 1-8 Section Ch, Week 9136, Derwent Publirations Ltd., London, GB; Class E16, AN 91-262356 JP,A,3 170 583 (MATSUSHIITA ELEC IND KK) 24 July 1991 see abstract Further documents are listed in the continuation of box C. Mv Patent family memrbas are listed in annex. Specal ateorie ofcitd doumets:'T later document published after the international filtig date A dcumnt efiingthegenra stte f te at wichIs otor priotity date and not in conflict with the appication but ''dcn to ef pth ca rerleae teatwic sntcted to understand the principle or theory underlying the consderd t beof artculr rlevnceinvention earlier document but published on or after the international *X document of particular relevance; the claimed invention filing date cannot be considered novel or cannot be considered to V document which may throw doubts on priority clsn(') or involve An inventive step when the document is taken alone which is cited to establish the publication date of another document of particular relevance; the claimed invention citation or other special meason (as specified) cannot be considered to involve an inventive step when the document refernrig to an oral disclosure, use, exhibition or document is combined with one or more other such docu- other means ments, such combination being obvious to a person skled document publishecd prior to the international filing date but in the art. later than the prionity date claimed document member of the same patent family Date of the acual completion of the international search Date of mailing of the internationail search report 21 December 1993 1 3, 01. 94 Name and mailing address of the ISA Authorized officer European Patent Office, P.D. 5818 Patentlai 2 NL 2280 liV Rsj.swlk Tel. (*31-70) 340-2040, Tx. 31651eponlJ, Ncls Paz 31-70) 340-3016 N c ls Form PCTIISA1210 (sacond shoot) fiuly 1992) 4 0 1 INTEliNA110NAL SEARCHI REOR IntL~r ,ial Applicatbon No tnfoimbon on p~tent fAily rrmeaI PCT/US 93/04577 Patent document Publication Patent faxnily Publication cited in search report date mcmber(s) date US-'A-5211867 18-05-93 NONE EP-A-0526745 10-02-93 NONE US-A-4943388 24-07-90 CA-A- 2057863 29-12-90 EP-A- 0479815 15-04-92 JP-T- 4502484 07-05-92 WO-A- 9100323 10-01-91 Form PCTflSA/210 (Pawnt UeiUY tonlfl (Muy 1#92)