aerosol propellants.pdf

7/30/2019 aerosol propellants.pdf

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402 JOURNALOF THE SOCIETY OF COSMETICCHEMISTSAEROSOL PROPELLANTS

P. DYSON, B.A., B.Sc.*Presented t the Symposium n "Aerosols",organised y the Societyat

Southport, ancs.,on 25th April 1965.The major lroleries, advantages and disadvantagesof the establishedand the lesser used lrolellants are reviewed. Some detail is also given ofwork done on the thermal decomlositionof halocarbons, which has a bearingon their use in aerosols.

INTRODUCTIONMucI oF rIE information n this paper is well-known; the object has beento collect t into one place for easy reference. In addition, it is hoped hatthe paper may put into perspective omeof the pros and cons hat haveoften been brandished ecklesslyby the protagonistsof one or other classof propellant. Vinyl chloridehas anaestheticproperties,but not at con-centrations emotely involved with its use of aerosols. The chloro- andchlorofluorocarbonsan producehighly toxic productson decompositionnnaked flamesand on red hot surfaces, ut it is virtually impossible y usingan aerosol o createconditionswhere his vouldpresenta hazard o the user.Although t is my personalopinion hat the uncontrolled se of flammablepropellants oulddo nothingbut irreparable arm to the industry, feel ustas strongly hat there is no logicalcasewhatever for a completeban on theuse of a flammablegas as a constituentof an aerosol.Commercialaerosolswere first produced n the U.S.A. in 1946, and inEurope and elsewhererom about 1951. Today, some1,300 million con-tainersare beingpackedevery year and there s no indication,even n theU.S.A., that the market is becoming aturated. Hair sprayshave been thebest seller in the North American market for some years; in the last yearfor which figureswere published,1961, some 150 million units accountedfor nearly 20% of American aerosolproduction. In the same year, cos-meticsas a class-hairsprays, having reams, olognes,erfumes nd otherpersonalproducts-comprisedbout one-third of U.S.A. aerosols, nd asimilar pattern is rapidly developing n Europe.The majority of the world's aerosols ontain chlorofiuorocarbonsndthe safe properties nd high standardof purity of thesepropellants avehad much o do with the publicacceptance f aerosols s safeand efficientcommodities. Indeed,when he pioneers f aerosols hosehe chlorofluoro-carbons s propellants,hey did so only after consideringhe propertiesofmany other liquefiedgases;they decided hat they alone had propertiesapproachingclosely o those of the ideal propellant.

*General ChemicalsDivision, Imperial Chemical ndustries Ltd., Runcorn, Ches.


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AEROSOL PROPELLANTS 403

However, n sucha progressive nd competitive ndustry, attention willalwaysbe directed o the useof alternativepropellants nd diluents,eitherfor technical reasons or to reduce the cost of the propellant element inaerosols.

LIQUEFIED GAS SYSTEMSA liquefied or iquefiable) as s definedofficially n the United Kingdomas one which has a boiling point below 30C but a more liberal descriptionis "a gas which is gaseous t ordinary temperaturebut can readily beliquefiedby the applicationof pressure". Thus a single iquefiedgas n aclosedcontainerexerts a pressurewhich dependsonly on temperatureandis independent f the quantity of liquid present. A mixtureof two liquefiedgasesexerts a pressurentermediatebetween he pressures f the twocomponentst anytemperature. For "ideal" systems, aoult'sLaw definesthe total pressure f such mixtures as:

P=m p d- m2 P2where m and m2 are the mole fractionsof the two components andp2 are the vapourpressures f the two componentst the temperatureconsidered.Mixtures of the liquefiedgasesused as propellants ollow Raoult's Lawquite closelyover the temperature ange nvolved n filling and in use.If then a liquefiedgas, or a mixture of two or more suchgases, s dis-charged s liquid from a container, he pressuren the container and thecomposition f the mixture) will remain virtually constantuntil all theliquid has been discharged.(a) In two phaseaerosolshe liquefiedgas propellant s completelymisciblewith the rest of the formulation,e.g. a space nsecticide onsistingof propellant 2/11mixtureand a concentrate asedon odourless istillate.The liquid contents re driven out by the pressuren the gasphase. Thecomposition f the propellant/concentrate ixture and the internal pressureof the aerosol emain virtually the same until all the contentshave been

discharged, nsuring uniformspray performancehroughout he life ofthe container.When the valve is opened he liquefiedgas,as it leaves he spray head,expands apidly o produce 00 or 300 times ts volumeof gas. This effect,coupledwith the actionof the valveorifices, tc.,atomiseshe productbeingdispensed.The degree f atomization nd hence he coarsenessr particlesize of the spray s determinedboth by the composition f the propellantand by its proportionn the formulation, s well as by the structureof thevalve used. Temperature s, of course,an important consideration;a

product ormulatedo give a suitable prayat 50-70Fwouldnot dispense


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404 JOURNAL OF THE SOCIETY OF COSMETICCHEMISTSsatisfactorilyat temperaturesbelow freezingand similarly it might developan undesirablyhigh pressure or its container n the tropical sun.Most spacesprays, hairlacquers,colognes nd surface coatingsare dis-pensed y the two-phase ystem,employing rom say85% down o 30-40%by weight liquefiedgas prope!lant n the formulation.

(b) Threephaseaerosols ontaining wo liquid and one gaseous hasearemost commonly water-based products. Water is immiscible with all thefluorinatedhydrocarbonand hydrocarbonpropellantsused n aerosols, utit can be emulsifiedwith them to varying degreesby using emulsifyingagents and by shaking the container before use. Shaving creams, foamsand shampoos an virtually be regarded s two phasesprayscontainingustsufficientpropellant o expel the contentsand expand the emulsion nto afoam. Water-based urniture polishes nd starchsprayson the other handcontain a small proportionof prope!lant o expel the contentsand rely toa large extent on the assistance f a mechanica!break-up valve to producea coarsespray. Foams and water-basedpolishesof these types usuallycontain only 5-15% of liquefied gas propellant.

(c) More recently spacesprayssuch as air fresheners ave been formu-lated with a different water-basedsystem which uses a higher liquefiedgas propellantcontent of the order of 30-40%. Emulsifiersand shakingprovide mixing of the aqueousand propellant phases. The dip tube issometimes estricted to limit the rate of dischargeand an orifice in thevalve housing nside he vapour phaseallowsgaseous ropellant o join theliquid phaseand assistatomization. The valve is of the mechanicalbreak-up type.

(d) Powderaerosols re a!so hree phasesystems,with the finely dividedsolid phasesuspendedn the liquid propellantphase,e.g. talcs.COMPRESSED GAS SYSTEMS

In this type of aerosol, he propellant s almost entirely present as gasin the head space, usual!y about one-third of the tota! volume. As thecontentsare discharged he pressure al!s progressively. Higher pressuresare thus required than with liquefied gases-for example a typical packwou!d be charged nitially with nitrogen gas to a pressureof 90-100 p.s.i.g.to ensure hat sufficientpressure emained o expel he last of the contents.Viscousproducts,suchas toothpastes nd handcreams, re dispensedn thisway with nitrogen,where the main object is to expel the contentswithoutappreciablechange. The cost of the propellant s negligible.Where a degreeof solubility of the propel/ant n the formulationcan beachieved,compressedasesare used or foam productsor, with the aid ofmechanicalbreak-up valves, for very coarsesprays. The outstandinguse


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(largely in the U.S.A.) is of nitrous oxide/carbondioxide mixtures for dis-pensingwhippedcream oppings. Other food useshave been slow o follow.Nitrous oxide is occasionallyused in shaving creams, and carbon dioxidein de-icingand similar coarsesprays; both have been used n motor starterspraysn the U.S.A. but it is unlikely hat the useof compressedaseswillincreaseor applications hereany real degreeof atomizations required.Some"mother-and-daughter"acksusenitrogen o boost he pressureof the largerunit and so facilitate ransferof its contentso the handbagcontainer.

A comprehensive ccountof compressed as propellants or non-foodproducts s given by Webster .CItLOROFLUOROCARBON PROPELLANTS

The useof dichlorodifluoromethanes a hazard-freeefrigerantwas irstdevelopedn the U.S.A. in 1931. Other fiuorinatedhydrocarbonsollowedas the refrigeration nd air-conditioningndustries xpanded. Thesecom-pounds are now made in countries hroughout the world to the extent ofover 300,000 tons a year and for some years the demand for use in aerosolshas exceeded hat for refrigeration. They are manufactured n the UnitedKingdom under the trade names "Arcton" and "Isceon".The outstandingdvantagesf this classof liquefied aspropel!ant retheir freedom rom hazardand their high degreeof stability and chemicalinertness. The liquids are colourless, iving colourlessapourswith faintand not unpleasantodours. They do not damage urnishings r fabrics.Someof the physical roperties f the threepropellantsn mostcommon se(12, 11 and 114) are given in Table 1.The versatilityof these hreepropellantss almostunlimited. A pro-pellant can be madeto measure or almostevery non-food pplication yselectinghe right compound r mixture. The mostwidely usedsourceofpressures propeRant8; it exertsa pressure f about70 p.s.i.g.at 21C(70F)and is thus only usedalonewhere he overallpressures reduced ythe other componentsf the formulation,e.g. in aerosol aints. Morecommonlyt is mixedwith propeRant1 to reduce he pressure;his alsoincreaseshe solvent owerof the propel!antwithout he risk of damage ogaskets,etc. Throughout he world the most widely suppliedmixture ispropellant12/11 50/50--equalweightsof 12 and 11--with a pressure fabout37 p.s.i.g.at 21C 70F). It is the basisof many spacensecticides,air freshcriers nd hair lacquers.Although he chlorofiuorocarbonsre extremelystable, he most asym-metric moleculeof the three, propellant 11 (CClaF) is lessresistant ohydrolysis han the others and can also react under certain conditionswiththe loweralcohols. n addition, t can affectcertainclasses f perfume


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406 JOURNALOF THE SOCIETYOF COSMETICCHEMISTS


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adversely. Thus for water-basedproductsand a number of cosmetics, uchas colognes nd perfumes,propellant 12 is used alone or in mixtures withpropellant114 which is characterizedby its outstanding stability and ispractically odourless. Propellant 12/114 mixtures in proportionssuch as10/90, 20/80 and 40/60 are commonlyused.The gaugepressures f a range of 12/11 and 12/114 mixtures are givenin Table 2.

Table 2Vapour pressures of propellant mixtures at 21C (70F.)

Propellant Gauge pressure Propellant Gauge pressure12/11 mixtures p.s.i.g. 12/114 mixtures p.s.i.g.35/65 27 10/90 2040/60 30 20/80 2750/50 37 30/70 3460/40 44 40/60 4065/35 47 50/50 46

FlammabilityThe propellants are non-flammable and have flame suppressantpro-perties; indeed, t is sometimes ossibleo formulatenon-flammable roducts(judged by, say, the flame-projection est) containing flammable solvents,by incorporatingchlorofluorocarbon ropel/ants. The propel/ants do notform explosivemixtures with air in any proportion.ToxicityThe (U.S.) Underwriters LaboratoriesClassificationsor the vapours ofpropellants12, 11 and 114 are Group 6, 5a and 6 respectively. Group 6 isdefined as "Gasesor vapours which in concentrationsup to at least about20% by volume for durationsof exposureof the order of 2 hours do not

appearto produce njury", i.e. virtually free from toxicity under all normalconditionsof use. For comparison urposes, arbondioxide is classifiednGroup5a and hasan MAC (maximumallowableconcentrationor continuousworking) of 5,000 ppm (0'55o) by volume.Thermal StabilityPropel/ants11, 12 and 114 begin to decomposen contact with openflamesor red-hot surfaces o form acidic productsand sometimes races ofphosgene,which would be hazardous if inhaled in sufficient quantity.

Fortunately, the sharp, acrid odour of the acidic productsgives sufficient


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408 JOURNAL OF THE SOCIETY OF COSMETICCHEMISTSwarningof their presence. Nevertheless, ork areasnearfillingplant shouldbe adequatelyventilated when, for example, welding s in progress ndheating installations should avoid red hot surfacesor naked flames.

From the aerosol serpoint of view, it is consideredhat the quantitiesinvolvedmake the possibilityof hazard from suchdecompositionxtremelyslight and this is borneout by the trouble-free ecordof fluorocarbon ro-pellants and refrigerants n this respect over many years. A number ofcaseswherepropellant12 was cited as a causeof poisoning ue to thermaldecomposition ere discussed y Downing and Madinabeitia n 1960. Theypointed out that the possibility of decompositionof the fluorinated com-poundscouldnot be ignored,and that there was no intentionof minimizingthe oxicityof some f theproductsormed; ut theyconcludedhat therewas insufficientevidence o implicate propellant 12.

Quite independently,work has been carried out in our laboratories osimulate the use of three types of domestic convectorheater in a smallunventilatedroom containing he vapour of a number of halocarbons. Anatmospheremaintainedat a constant omposition as fed to the intake portsof the heatersand the effluent air analysed or breakdownproducts. Theconvection rate of each heater was measured. The results obtained withan atmospherecontaining 1,000 ppm (0.1%) by volume have been used toestimate he concentrations f toxic breakdownproductswhich couldresultafter 10 minutes n a smallsealed oomof 1,000 t. a (28 ma). Theseestimatesare given in Table 3.

Consider first the estimated concentrations from the most toxic de-compositionproduct, phosgene,which has a maximum allowable concen-tration (MAC) for continuousworking of 1 ppm by volume.*

(i) With one exception,phosgenewas not detected n the effluentgasesfrom the electric and gas convectorheaters,but the paraffin heaterfavoured its formation. Only with methylene chloride was theM.A.C. reached or exceeded.(ii) With the paraffin heater, 5.7 oz of propellant 11 in the atmosphereproduced about the same concentrationof phosgeneas 2.6 oz ofvinyl chloride. Propellant 12 producednone and propellant 114a negligible amount.

*The M.A.C. is the maximum average atmospheric concentration of contaminants to which persons may beexposed or an 8-hour working day without injury to health. These values are basedon the best available infor-mation from industrial experience, from experimental studies and when possible, rom a combination of the two.They should be used as guides n the control of health hazards and should not be regarded as fine lines betweensafe and dangerousconcentrations. They represent only conditionsunder which it is felt that workers may berepeatedly exposed,day after day, without adverseeffect on their health. The figures isted refer to weightedaverage concentrations of an 8-hour working period rather than a maximum which is not to be exceeded evenmomentarily. M.A.C. figuresare publishedby the American Conferenceof Governmental ndustrial Hygienistsand are reviewed annually. They are usedby the International Labour Office,and have alsobeen usedsince1960as a basis for the "maximum permissibleconcentrations"published n the United Kingdom by the Ministry ofLabour.


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410 JOURNAL OF THE SOCIETY OF COSMETICCHEMISTS(iii) In all cases where phosgenewas produced, appreciably greaterconcentrations f the acidic gaseswere formed, confirming thatthese would have given adequate warning.All the compoundsested have been used n aerosols nd in most cases

the quantities involved could conceivably be discharged from standardaerosol ontainersn the home. Under certain circumstancesoxic productsare undoubtedly produced with all the compounds,but considerationof(i)-(iii) above,and the fact that deliberatelyunfavourableconditionswerecreated, suggestshat this would not constitute a hazard to health.CHLOROCARBON PROPELLANTS

MethyleneChloride CH2Cl2)Methylene chloride s a non-flammable iquid boiling at 40.2C. It isone of the least toxic of the chlorinatedsolvents MAC=500 ppm v/v,Underwriters Laboratories Group 4-5). Although it has an anaestheticeffect f breathedat high concentrations,he quantities nvolved n aerosolsdo not constitutea real user hazard in this respect.It is used n aerosolsn its own right as a so!vent, o depress he flam-mability of other solvents,as a co-solvent o increase he solubility of someingredients n propellantsand as a cheapersubstitute for part of the pro-pellant 11 in 12/11 mixtures. As a pressuredepressant or propellant 12its effect is very similar to that of propellant 11.It has, however, a number of technical disadvantageswhich limit itsuse in aerosols. It hydrolysesmore readily than propel!ant 11 which canincreasecorrosion roblems. It has a more definedsmell and this, coupledwith the higherhydrolysis ate, can seriouslyaffect fragrancesand perfumesused n aerosols. It has a marked solvent and swellingeffect on elastomers ,and resistant materials must be used in valves, etc. Its strong solventaction also occasions caution when used in household aerosols; we havefound that the misuseof an aerosolcontainingmore than 15% methylenechloride,by applying from a short distance,can damage synthetic fibressuch as acetate and triacetate rayons.

Although these disadvantages imit its range of applications and theproportionwhich may be used,methylenechloridehas already establisheditself as a valuable raw material in the aerosol ndustry.1.1.1-TrichloroethaneCCl.CH)Stabilisedgradesof this non-flammable solvent are sold under the tradenames Genklene" ICI) and "Chlorothene"NU (Dow). 1.1.1-trichloroethanehasa similar ow oxicity o that of methylene hlorideMAC=500 ppm,v/v)but its higherboilingpoint (ca.76C)makes t a rather safersolvent or coldcleaning. It is believed, in general, to have less effect on the elastomers


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used n valves and little or no effect on textiles, but this apart, it shares hedisadvantagesof methylene chloride isted above. Its pressurecharacter-istics n mixtures with propellant 12 are very close o those of propellant 11.It is used as a solvent n aerosols e.g. for cold cleaning, spotting) and toreduce the flammability of other solvents. Appreciably more costly thanmethylene chloride, t is not likely to be considered olely as a cheapersubstitute for propellant 11.Vinyl Chloride (CH:CHC1)Vinyl chloride s a liquefiedgas,boiling at -13.9C. Its vapour pressureis very similar to that of propellant 12/11 mixture. It has an MAC of 500ppm v/v and, althoughnot classified y the UnderwritersLaboratories,tstoxicity would appear to place it between Groups 4 and 5. At higherconcentrations t has an anaestheticeffect but the quantities which mightbe used in aerosolsare never likely to create a user hazard in this respect.Vinyl chloride is cheaper than the fluorocarbonpropellants, s a strongersolvent and has a higher expansionratio on evaporation. On the debitside, it is highly flammable and forms explosivemixtures with air in therange 4-22% by volume. It has a marked swellingeffect on elastomersand possesses pronounced dour. It is manufacturedalmost exclusivelyfor polymerization o PVC and for the productionof allied co-polymers;publishedwork6 has recommendedhat it shouldcontain a stabiliser f usedin aerosols.

To the best of my knowledgevinyl chloride has not been used com-mercially n aerosolsn the United Kingdom. In a few Europeancountries,and in Japan, t hasbeenusedasa primary propellantand two manufacturersin the U.S.A. offer blendsof propellant 12 or 12/11 and VC. One of thesefirms6 reports that vapour mixtures of vinyl chloride and propellant 12/1150/50 are non-flammablef the VC content s below45% v/v, i.e. 29.4% ona liquid weight basis; becauseof fractionation the actual amount of VCthat can be tolerated s 22/0w/w, hence heir blend s offeredas 78% 12/1150/50+22% VC.HYDROCARBON PROPELLANTS

Propane (Calla),n-and isobutane CH0)are liquefiedgases. They arevirtually non-toxic, being classifiedby the Underwriters Laboratories nGroup5b. Commercial radesvary in composition ccording o source,butmixtures of two or more of the hydrocarbons rovide pressures uitable foraerosol ispensing. For example, mixtureof 50% nbutane,25% isobutaneand 25% propanegivesabout the samepressure s propellant12/11 50/50.The hydrocarbonpropellants re low-priced. They are immisciblewith,and have a goodstability n, water and, unlike the fluorocarbon ropellants,


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412 JOURNAL OF THE SOCIETY OF COSMETICCHEMISTShave densitiesess han 1. Theseproperties ll favour the hydrocarbonsfor water-based pplications uchas iurniture polishes,wheresmall pro-portionsof propellantare used n three phasesystems.

The majordisadvantagef the hydrocarbonss their high lammabilityand the fact that they form explosivemixtureswith air at quite low levels(


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AEROSOL PROPELLANTS 413REFERENCES

Webster, R.C. Aerosol Age 6 20 (June 1961)Nord. Hyg. ridskr. 39 11 (1958)Downing, R. C. and Madinabeitia, D. AerosolAge $ 25 (September1960)Toxic Substancesn Factory AtmospheresMinistry of Labour, Safety, Health andWelfare New Series, No. 8 (March 1960) (H.M. Stationery Office, London)Brown, J. A. Proc. Chem. Specialties3lifTs.Assoc.46th Annual Meeting 80 (1959)Scott, R. J. and Terrill, R. R. Aerosol Age 7 18 (January 1962)Jones,G. W. and Scott, F.E. ReportRI $908 (1946) (U.S. Bureauof Mines)I)ISCUSSION

MR.C. BLOOM:Propellants142b and 152a have been claimed o be goodsolvents or certain drugs and they might, therefore, serve a quite usefulrole in the cosmeticand pharmaceutical ields. I)o they have any seriousdisadvantages nd what progress, f any, has been made towards theircommercialexploitation?THE LECTURER:Propellants152a (CH3.CHF2) and 142b (CH3.CC1F2)boil at -24C. and -9.4C. respectivelyand have the relatively low densitiesof 0.91 and 1.12 g/cc at 21C. I believe hat 152awas first manufacturednthe U.S.A. for use as a refrigerant n an azeotropicmixture with 12, design-ated refrigerant500. To the best of my knowledgepropellants152a and142b are only offered n commercialquantities by one American supplierwho claims that 152a has unique solvent characteristics nd recommends142b for cosmetics uchas perfumes,either alone or in mixtures with 114.It is noteworthy hat both propellants an form flammablemixtures ll air(152a: 5.1-17.1%, 142b: 9.0-14.8% v/v). There has beenno demand orthesepropellantsn the United Kingdomand the high cost associated ithsmall scaleproductionwould only warrant their adoption where they hadoutstanding advantages.MR. A. I-IERZKA: S it likely that your companywill supply, n the nearfuture, propellantblendssimilar to the two listed at the end of your paper?THE LECTURER: NO.MR. A. HERZKA:You refer to odour problemswhen usinghydrocarbonpropel!ants. Is it not possibleor suchmaterials o be supplied ree fromobnoxiousodours, as is the case n the U.S.A.?THE LECTURER:My company does not supply, or use, unstenchedhydrocarbonsor aerosols, ut it is my understandingrom fillers hat suchhydrocarbonsuppliedon the United Kingdom market still have someresidual odour. Perhaps another participant would care to comment

further.MR. I). S. RANDALL: In destenchedbutane there is a residual odour, but we

consider ur products o be almostcomparablewith Americanand European


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414 JOURNALOF THE SOCIETY OF COSMETICCHEMISTShydrocarbons, hichwe have ested. Work continues n this problem,andwe hope o have thoroughly cceptable roducts n the near future.

MR. H. F. FROST: assume, rom the nature of the results n Table $,that the gas convectorheater used n the experimentwas not the moderntype in which the combustion roductsgo up the flue and hot air only iscirculated,but the older free-standingype where all the combustion ro-ducts were released into the room. is this so?THE LECTURER: This is true. The convector heaters used in these

experimentswere standarddomesticapplianceswhich rely on convectionof air andwhichwerenot fitted with fluesor chimneys.DR. P. H. WITJENS: Are any quantitative data known on the formationof COC12rom CH2Ci vapour in the presenceof a radiant bathroom heater

(e.g. metal coated ceramicheating bar)?This problem is of significancen Holland as some new buildingshave small bathroomcubicles o heated, with poor ventilation,where hairsprayscontainingmethylenechloridemay be used.THE LECTURER: know of no data referring o thesespecific onditions.I would think, however, hat since he quantity of propel/ant,or methylenechloride, nvolved in one application o a woman's hair would be small, theconcentration f phosgene roducedwould be correspondingly inute.DR. H. K/JBLER:The toxicity values for vinyl chlorideand 1,1,1-tri-chloroethanen Table1 do not agree with those I have quoted. i mustadmit that I did not determine the value for the latter, but I know that thevinyl chloridevalue is proven.THE LECTURER: presumeyou are referring to the U.S. UnderwritersLaboratory's classificationgroups for vinyl chloride and 1,1,1-trichloro-ethane. To the best of my knoMedgeneither compound asbeenclassifiedby the Underwriters Laboratoriesand the figures quoted were estimatedfrom a review of available data.The data on vinyl chloride s somewhatconflicting,e.g. the findingsofTorkelson,which do not agreewith those of the Battelle Institute, quotedby you. Toxicity data on the 1,1,1-trichloroethanes limited. However,becauseof the anaestheticpropertiesof vinyl chlorideand bearing n mindthe fact that the current MAC figure for each compounds 500 ppm, as formethylenechloride, t seemed easonable o me to suggest hat both wouldfall within the sameUnderwritersLaboratoriesGroupas methylenechloride,i.e. Group4-5.DR. H. K/JBLER: You are quoting estimated concentrationsof decom-

positionproductsof halocarbons,n Table 3, whereas he figuresquoted by


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me are exact. I consider emperature o be decisive or decomposition,which must be observedonly above 500C.TaE LECTURER: I would like to assure Dr. Kiibler that the actual

measurement f the decomposition roducts eaving the convectorheaterswas carried out very preciselyand was not "estimated". Phosgene,orexample, was measuredby individually calibratedDrtger tubes and byabsorptionn aqueous niline o form diphenylurea. I have used he wordestimatedonly in so far as I have interpreted heseresults n terms of anatmospheren a sealed oom, .e. attempted o relate the scientific ests oextreme conditionsunder which an aerosolmight be discharged. Tempe-rature is indeedan important factor but other effectssuchas humidity andmetal surfaces re also mportant so far as decompositions concerned.

}V[R.J. C. THORNTON:With regard to the thermal decomposition fhalocarbons etailed n Table $ of your paper, were the temperaturesof theelements in the three different convector heaters measured?

TE LECTURER: O. The experimentswere carriedout with standarddomestic appliances o give realistic conditionsrather than to obtainadditional academic information.1VtR.H. F. FROST:Have you examined the decomposition f pro-pellantsdrawnthrougha lightedcigarette n the "domestic"experiments?TE LECTUre;R: No. We have, however, carried out work on the

problem of smoking cigarettes n atmospherescontaining the vapour ofchlorinatedhydrocarbonsnd the resultswere publishedby Little [Brit. J.Ind. Med. 12 304 (1955)]. In no casewas phosgeneound in the vicinity ofthe glowing ip of a cigarettesmoked n the contaminated tmospheres, ndevenwhena trace of phosgene asadded o the atmosphere,t wasdestroyedby passagehrough he cigarettebeing smoked.

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