The measurement and use of oxygen index

COMBUST~O.NANO FLAME 23, I-9 (1~'74)

The Measurernent and

1

of Oxygen Index

C. ABE;OTT The Steetley Co. L~d., Berk ~emicals Man~:tfactun~l! D/l,ision, Strarfbrd England

!. Introdncfion New test methods fo~ the m,!~asurement ef the relative flammability el materials are continual]ly behag devised. This pro~,feration of procedures causes great problems to those invoh,ed in their evaluation and use. Irt would seem that almost all the combinations and perm~,tations of specimen position and shape, si~z and type of ignition source and time of i~:~tion 'have been examined. A further criticism is :hat m nst of the tests do not quantify the results ,~.M terms such ;~s non.burning, self-extingui~ing, Ga~s 1 and Class I 1, SEn to UL94 are: familiar des:dptions. For many years the~:e has been a need for test methods which would give quantitative results. The Oxygen Index te~,~tdes~:,~ribedby Fent~more and Martin in 1966 [~] seemed to fulfil this requirement. This type of test procedure was orighaally developed by H. G. Wolfhard in 19'57 for the comparison of fl:a,'nmabflity of gaseous and liquid polymers [2]. Initial reports on the test [3] showed how it eo'.tld be used '~o classify polymeric materials in order of decreasing flammability. A typical set of results :is shown in Table 1. Results of round-r~bin tests proved the reproducibility [3] and rite publication of ASTM D2863-1970 establish ~d the test, however, there were do,tbts expressed as to 1:he. alue of the number obtained. As more information becomes avail. able, it is clear that the Oxygen Index test does represent a significant advance itn the comparative measurement of flammability properties. This paper desclSbes some methods of measuring Oxygen index and 'typical us¢o o[' thee test in the study of the burning eharacter~sties of materials. The results show how one factor particularly, that is, the flame retardant used in the composition, influences the Oxygen i;ndex. The results described have

beent el.'.l,,ainet, o~:er a 2-year period ofimermittent workirli! on Oxygen Index. They are not meant to repr,es,~ :'t a corap.rehensive examination of the test, I~,~ more t..~point ou~ areas of interest wl~eh have ~1rill;ca dul:'il~gth~s tlnle. 2. [de:~:~uremealtof Oxygelrl Index Sinc,~ ,Ihe origi nzi dew,~lopraent, the measurement of Ox'!,'gen Indic x has been carried out in a number ofvcay:~. These are summarized bdow: Pressm:~gauges The ASTM D2.863 recrJmmends thi3 method based on e~J;ibrated j,ewelled orifices, precision pressure regul'~tnag devices, and precision gas pressure gauges. Flows'r~uters This ]:~,consider cd equally satisfactory provide d the flowraetels :ire accurate to 1%. Parama~etic A:Lullyser This i~; a relati,¢ety new method based on a paramagnt~tic Oxy;~t~o analyser and has the advantage of a direct rea t-out of Oxygen Index. 2.1. A Simple t;lowmeter Appoxatus The most common syster~s are based on flowmeter! and thL~I:olim~:ng comments are based on exb~erienc¢ c ; using a typical flowmeter type instru:menl:. A s¢: ~ematic v [,,~wof the apparatus is ~hown in Fig. 1, (a) Fl,:~wmetezs The z~::~urz.~y¢',~!In'starters recommended in the , ~ T N D2863 ts 1% and such instruments are very expen~ive. ~t [; ,:!uesl:ionable if it is necessary for Copyri~Ix © 1974 by "fhoCombuztion Institute Publf,shv~dby Arce~c~n iEl~e~ierPublishing Company, Inc.

C ABBO'FF

TABLE 1 Thermoplastic

C~ygea h|dex %

Therraosets

Polyethylev ' Polymethyt methactYlate Polypropylene Polyvinyl chloride 9olystyrene Polycazbonate Pol,.,'tet:tafluoroe.thylene ABS copolymer

17.4 17.3 17.4 45.0 18.1 27.0 95.0 18.0

Polyester (GRP) Polyester (GEP) S.E. grade Fpoxy (GR P) EOlyurethal te foam P/F wood f~lled I~;/F mineral filled IP/F paper lltminate

Oxygen index % 20.5 40.0 24.0 18.0 28.1 39.5 26.9

P-phenol, F-formaldehyde, H-raelamme., GRP-.glass reinf)reed plastic. The results vary from 17A% for the highly eorabusuble ~olyolefin,, ¢o 95.0% for the almost non-flammable P.T.F.E. pxy~len /Nitrc;a~

Glass

2-! k-I

./ch'o.e,

IHCql II III

iir LHiHr i

Gas cylinders

Needl. valve,a

"

I~lUZe

~ Flowmelers

Fig. 1. Schematic view of a flowmeter apparatus of Ihe type used in "the oxygen index test. most purposes to read to such accuracy and normar rotameters, such as *.he Rotometer Seri~=s 1100 accurate to 2% o f i~=aicated flow, are ]?rob~bly adequate. The choice o f range of fiowmeters may depend upon the uses o f the apparatus. In general line Oxygen Index values of between 15% to 30% ~,xygen ale most frequently measured. Rather than usarg a single flowmeter to cover the entire range of oxygen levels t~om 0 to 100% it may be more suitable to select a narrower range instrument lo provide results within values of 15% to 30%. The same principle could also be applied to the nitrogen flowranter. To accurately calibrate the fiowmeters f,:,r oxygen and nitrogen it is necessary to use' a v, et drum volumetric flowmeter. This needs to be c-'trriefl out once before use and is recommended to be done at six monthly intervals. This latter proce-

,dureh ~,in fact r~trely carried out in pracl:!ce, and measurement of standard reference samples is a ratisfactory way of monitoring the equipment. Co) Nel~lle Valves These are critical parts o f fire apparatus and ide:ally some form of calibrated needle valves are most =mitable. (c) Oxygen Index Chamber Tire ASTM D2863 specifies a chamber of up to 9.0 cm diana, however, a 7.$-cm chimn~y is most satisfactory as this requires smaller quaqtities of :gases. (d) l~oeedum Cazr experience has shown that the method o's'scribed ASTM D2863 is satisfactory ex :ept for the following points: Ignition It is recommended that a 12-ram fiame, natural gas or propane, is applied to the specimen for 15 see or less if the sample is well lit ort the top :;urface. In curtain eases o f highly filled eomposi~r/ons an ignition trine o f up to I min rr, ay be required. In all cases the details of ignition, flame, and lti2ne, should be recorded. The Oxygen Index Definition The Index is suitably defined as the ],rightest Oxygen value at which the specimen doe.¢ not b u m for raore than 3 rain, or over a length of 50 mm. A ~¢pieal set of results is shown bek~w:

THE MEASUREMENT AND USE OF OXYGEN II~DEX

3

I

2O0

°°I 50

g

.'oo

i /f

1 10o o ¢o~swal~, r01.~L oA; F'LCU

Fig. 2. Total flow ~1~1"1floplvilteter reading.;.

Oxygen2 ~

Flame beha~ iour

19.0 18.7 18.3 18.0

Burat more that: 3 min~tes B u ~ t more, tha~ 3 min, mtcs Bursar more than 3 mint~tes Burnt less titan 2 minutes •". Oxy,gen Index H ~.2.

In this t :~ries, because ot" the difficull y o f setting the floats of the f;owmeters, it was nor possible" to exactly pin]point the Oxygen Index v,,lue.

Flo',v Rates The Oxygen Index does not signif?caatly alter between t~te flow rates 3 [o 5 cm/sec, and a rate of 4 cm/s~c is recommended. In order to maintain the total flow constant, the settings o f the flowmeters are ~uitably obtained by the use o f Fig. 2 and Oxygen Index values can be read from a gzaph o f Index against Oxygen scale reading. 2.2. Patamagnetie Oxygen Analyser A schematic view of the apparatus is shown in Fig. 3. in contrast to the complexities of the above flowmeter type of apparatus the paramagnetie oxygen analyser, as made by Stanton Redcroft, is a great advance. When connected to a

d,i~lJ,;al voltmeter,, a c ~rect reading in Oxygen Index call ',~e obtained This enables results to be obtai~,~rl fa~ter, rerlucin~ running costs, and with a gr..*'l~l~er accuracy. ~!~Iypical set i,.f re:suits is shown below:

Oz ~.,,i:'en % .~?.8 !I(1.9 )-J. 1 2L2

Flame behaviour Extinction after 1 rain 35 see Extinction after 2 rain 6 see Extinction after 2 rain 30 see Burnt for more than 3 min .'. Oxygen Index 21.2%.

Working to an zccuraey of 0.1% with some materials can incre~:e the time spent on measuring Oxygen Index. This occurs particularly with thermoplastic materials which either flow or deveiop random char structures. It is often quicker and just as meani ngful to work to an accuracy of between 0.2 and 0.3% for such materials. 3. Applications of Oxygen Index 3.L Comparison w~th other Tes=Methods

3.L1. Test Methods (a) A S T M D633-68 Flammabilizy o f Self-

4,

C.. ABE!~OTT

Fig. 3. Oxygen analyser based apparatus (Stanton Redcroft).

Supporting Plastics. This is a we~l e~ablish-2d !aboratory test which involves applying 30-s~c ignition with a bunsen ,'lame ~o a bar specimen held ~n a horizontal position, l:f a sample is nonburning, that is, ff the fiame does not extend be. yond 25 mm then a further 30-see ignition is ~pplied. A wire gauze is placed under the sample and with certain formulations this serves to collect d.dps which would otherwise fall from the specimen, removing heat and often causing flame extinction.

(b) BS4735-1971 !gnitability and Self-Extingeishing Characteristics of Plastics and Rubber Cellular Materials. In this test for foam plastic the sample, 150 m m long X'50 mm wide X 25 mm thick, is placed in a horizontal position on at. L-shaped gauze. It iz ignited using a 38-ram high flame from a wing-topped burner. The ignition is for 60 see and the burning distance and time are recorded. The method specifies the use of a test chamber 600 nun X 300 wan × 760 ram; this was not usc.d in our tests and samples tested were 12.7 rnm in thiclmess.

(c) flnderwriter~_Laboratories Inc. Test. Sub]ect 94 IIUL94). This verb.teat test is mainly applied to thermoplastics such as abs and polyole f'ms to assess the hazard due to flaming droplets. The bar sample is held in a vertical position with the lower edge 12" above a small piece of cotton wool. The sample is subjected to two 10.'ec ignitions and the, extinction time measured. Glow time on the sec~M ignition is also assessed, and the remits are broadly classified as below. Th,~ aetuai classification is based on 5 specimens; Table 3 below is only a general guide.

(d) BS476 Part 71972-Surface Spread of Flame TestforMaterials. Thisstandardis based on the rr~easurement of the surface spread o~ flame across a sample held at right angles to a ladhnt gas furnace: Two methods are in use, one based or~ a 300 m m square furnace ar~:l a sample 300 m m :< 100 ram, and a larger one based on a 90f~ m m square fi~rnace and a sample 9120 mm X 23Q mm. In order to obtain an official certificate of performancz to BS476 Part 1, the test has to be C~Lrfled out on the large apparatus. The small scale test is tufty recommended as a guide to performance on the large scale. In our work, a small scale apparatus has been used which has been found to correlate satisfactorily with a large scale furnace. The t~st is considered very severe, as c~ltnbe seen from the temperature gradient a~t riti~at an['~es to the furnace. Distance from hot end of holder (mm) Approx. temperature (°C) My from 4ise. thermo couples

50

100

150

200

420

345

272

240

31

23

19

14

3.1.2. Results and Discussions Inttexlue~ion The results comparing the test method were obtained on the following polymer systemsacrylonitrile butadiene styrene copolymar (abs), polyether flexible polyurethane foam (pu), gla.,;s reinforced unsaturated polyester resin (GRP), and high impact polystyrene (ps).

THE MEASUREMENT AND USE OF OXYGEN INDEX

5

TABLE 2 Comparison of Test Methods Used UL94

BS476 Part 7

Sample (mra) I X w X t (app)a

127X6X3

127X 1 3 X 3

150X 5 0 × 13

1 2 7 x 13X 3

3 0 0 x 100x 3

Sample Position

Vertical

Horizontal axis 45 °

Horizontal

Vertical

Horizontal L to furnace

No. of s]mples

3

10

5

5

6

I[gn. source #

13 tara flame

25 rare flame

38 rnm flame

19 rare flame

50 mm flame

Ignition time

Until rod is alight

()no 30 sac

One 60 sac

Two 10 see

One 60 see

Flame travel

Vertically down

Horizontal

Horizontal

Vertically upwards

Along lace of panel

Gauze

None

9 tara under sample

Sample on gauze

None

None

Measurements

Vary O2/N2 ratio. Find lowest 0 2 level to support

E~t. time (s) ET Dis. burn (ins) DB Burn rate (in./m)

Ext. time(s) Dis. bt~m (ram) Bum rate rara/s

Ext. time (s} ET Glow time (S)GT Flame drops FD

Flame ;pread (FS) (ins) .,vi~L time

Non-burning NB Self-Ext. SE Burning B

SEO-ET< 10 SEI-ET < 30 No FD SEII-ET< 30 FD SB-ET > 30

Class I F S < Y ' Class 2 FS < 5.5" Class 3 FS < 9.0" Class 4 FS > 9.0"

'rest ble~thod

ASTM D2863

BS4735

ASTM D635

B I~

combustion

Results e Good T Poor

95% Oxygen T Concen. t.ration 18%

Non-burning NB Self.Ext. SE Bur ailag B

a I X w X t-length X width X thiekno!:s (approximate). bNatoral gas used, CBroad classificationonly. NB-immediate extinction. SE- Ext. not complete burning. TABLE 3

Oxygen indexdnd

3oL.

General Requirements Glow time Flaming drops

UL94

Ext. t~me

Oass

(see)

(see)

which ignite wool

SEO SEI SEll SB

< 10 <30 <30 7,3113

< 30 <60 <60 >60

None None Yes Yes or ~ample burns completely

T h e flame retardaut chemicals examined were H a m m e x B l 0 - d e c a b r o m o d i p h e n y l , Flammex T 2 3 P tris 2,= , d i b r o m o p r o p y l phosphate, and Flammex 5BT pentabromotoluene, The samples were prepared by standard laboratory techniques. The abs and ps were compounded w i t h the flame retardants on a heate,d t w o roll mill and compression moulded into sheets. The pu

28 •

ASTM D~335

"-~

.

;2g

~ =

\/?oo

,/~04g:-

o

,310 0 2 6 g 12 15 AO 0 2 3 4 5 r~,ts by built.red resin (phr)

Fig, 4. Oxygen inaex ~::,: ASTM D635. T:zc base mix is as follows (pa:ts by weight polymer): ABS: 100; l'abricant: 2; PVC resin: 10; stabilizer: 3. Flammex (310) as shown. Antimony trioxide (AO) as shown. Key to graph ASTM . . . . . Oxygen i n d e x - - .

(2. ABBOTT

foam was prepared using a smnple batch process; stirring the ingredients at ,high speed prior to foltming in a cardboard mould. The GRP samples were based on a cold press mo~ding system to give smooth sheets.

Oxygen index end 854735

3011

/ ~ ~2s

o

28F I ,~ 26 )l

/ /

100~

(a) Oxygen Index and ASTM D635 (See Fig. 4). The results of the ASTM D635 test could only 'be ¢xpressed on a graph using an arbitrary scale wilh two scale changes. The sudden change in slope of ~ e curve between 8 phr and 12 phi' of additive would no doubt lead to difi:.cnlties of reproduc.. ~ility if formulations were examined in this region. In contrast, the Oxygen Index results show a linear change which is def'med by the followinll equations: Oxygen Index n = 0.45 (FR) + 19.6

t8 L.

25 I 5 2o Level of T23P Z by wt. foam

Fig. 5. Oxygen index and BS4735. The base mix is as follows: Polyol and ca;~alyst,etc: 101.7; wa~et: 3.6; Hylene TM: 47.0; Flummox T23P: as shown. Foam density 32 kg/m3. Key togtaph ×-X oxyganindex;

o- - - o

FR = concentration of flame retardant.

Oxygen ~ndex n = 0.62 (FR) + 18.5.

(c) Oxygen Index and Flame Spread (See Fig. ,5}. The results follow a similar pattern to those de-

BS4735.

Oxygen index and BS476 Part 7

(b) Oxygen Index and BS4735 (See Fig. 3). The Oxygen Index tests and BS4735 tests were carried out on foam samples 150 m m X 150 toni X 13 ram. The samples tested for Oxygen Index were supported in a veitical U-shaped clamp. Tie tests were found to be reproducible, but excessive/low of the decomposition products caused blocking of the gauze at +he base of the glass col. umn; this was cleaned at intervals. The results again show the linear relationships of the Oxygeli I n d e x results c o m p a r e d t o the normal burning t a f t BS4735. It is particularly interesting to compar,n the results of formulations containing between 10-18%of T23P. The Oxygan Index results ~ar:ied from 24% to 30%, whereas the BS4735 results varied by only 10 man. This is probably due tot.ae limitation of the test method in not differentiati:~g between materials with a high resistance to bum, ing. The converse is also true if the results between 0-10% T23P are examined, the small chat=gas in level of T23P in this regiol: produced wide variation in the distance burnt. The line~Lr and regular pattern of results obtained by using the Oxygen Index test is again shown and the equation in this case is:

,~ A

4o

12 10 g~

32

'7- 28

~ 24 o 20

~,

2~ o 10 ' 20 ' 3'0 40 Level of 5BT/A(D1,'1phr

Fig. 6. Oxygen index and BS476 Part 7. The base mix is as follows: unsaturated polyes.tct resin: 100; Flammex (SBT): as shown; antimony el:ida (AO): as shown; continuous filament mat: 40% by weight. Key to graph ,D- - - O flame spread; X - X oxygen index.

scribed above. The slope of the flame spread curve changes only slightly as the flame retardant level increz~ses from 20--40 lphr. This is probably because in the low flame spread region the temperature is very high and it is difficult to prevent the organic material from burning in this area. In contrast to the BS4735 results, where it was suggested that f[ze similar change in slope was due to the inadequacy of the test method, in tiffs situation the flame spread test is more that= adequate and is possibly too severe. This raises the difficult question of the classification and rating of results, lit would

THE MEASUREMENTA~lDUSE OF OXYGENINDEX perhaps be interesting to investigate the history of the development of many fire tests to determine how the arbitrary class borders were decided. Whilst one can recognize the important reduction in hazard achieved by using a material which has a fla,~e spread of 4 in. compared to one of 9 in. The benefit of achieving a 3-in. flame spread by adding a further 100% of flame retardant is questionable. The equation for the Oxyg~.nIndex and flame retardant level is defined by: Oxygen Index n = 0.5 (FR) + 20. Discussion The linear relationships obtained above between oxygen Index and flame retardant concentration can be expressed by the general formula: Oxygen Index n = K (FR) + no. FR = eoneantration of flame retardant; n o = oxygen Index of polymer without flame retard~t; K = constant. The constant K may provide a measure of the ef~'ectivenessof the flame retardant, increasing towards unity with the greater efficiency of the flame retardant, however, this may depend on other factors which are described in the following section. Also this efficiency constant K is only relevant to the Oxygen Index test situation and is not necessarily a true guide to the overall efficiency of the flame retardant. Furthermore, recent work ~arded out has shown that the straight line relation~,_hipbetween Oxygen Index and coneentration of flame retardant is generally limited to Oxygen Index v~ues below 40%, and in some eases orgy occurs ore, a limited range of flame retardant concentrations. This work will be reported ha detail in due course.

(d) Oxygen lndex and UL94 Test [See Fig. 7). The UL94 test specifically attempts to assess flow properties as well as flame extinction behaviour. The results of a comparative series of tests ,~reshown in Fig. 7. The results in Fig. 4 show that, whereas the UL94 ratings change from SEO to SEII, the Oxygen Index results are generally similar. There is no real correlation because the UL94 ratings z,.e influenced by glow and flow of the material. How-

Base polymer-.polystyrene

,

25

231 2~ 810phr A0 phr

,

t P--~

,

\_J

o---o $8

SE1 7.9

sEo

t 11 10"7 10 914 7"2 2.8 3,6 5 6'3 8.4 10.7

Fig. 7. Oxygenindex ar,d UL94 test. Basepolymer: polystyrene;Flamro:x(BI0 phi) as shown. Antimony trP~xid¢ (AO phr) as shown. vet. even in this series, it is likely that the Oxy.;en Index needs to be above 24.0% to achieve a UL94 of SEll or SEO. 3. Oxygen Index and Flame Retardant Type During the course of our work with the Oxygen Index test and various f~ameretardant formulations, a trend has been shown hadicating that the Oxygen Index value is dependent on rite .type of flame retardant used. The result:; below illustrate thil point: Formulations 1,2, 3, and 4 thow how compound~ with similar Oxygen Index give ve:ry dif. ferent results on other test methods dependent on the fl~me retardant used. The results indicate that the Oxygen Index is related to the decomposition temperature of the flame retardant, such that a low decomposition temperature produces a hig~h Oxygen Index. This conclusion has been proposed befere as an explanation for results obtained by Learmonth et al. whi~.i~led to the Oxygen Inde× being considered as a test of low thermal stress [2]. Formulatiogs 5 ad 6 illustrate a further problem with materials which have high flow characteristics. This work has already been reported in detail recently [3, 5, 6]. A comparison of Formulations 1 and 6 show how Oxygen Index v~ues alone copld be misleading, as an SEII rath C means that flaming droplets are produced. It is important to consider the Ox-

8

~. ABBOTT Oxygen Index and Flame Retatdant Type Results Mix No.

Approx Decomp Temp.°C

Compound

I 2 3 4 5 6

360 360 250 200 200 200

BIO Dech C70 AC B60 HBCD

Base formation

Level phr

Nona drip phr

Antimony Oxide Sb2Oa

Oxygen Index %

ASTM D635 rating

LL94 Class

30 30 30 30

5 5 5 5

10 10 10 10

24.2 24..2 24.2 25.8

NB B SE NB SE NB

~,EO SB ~.EII SEO ~Eli SEll

4

6.5

-

2

24..2

2

28.0

Polypropylerte i00 p~s.

aNon-drip additive was based on a Bentone system [:!]. Bl0-Flammex BI0; Dech-Dechlorane plus; HBCD-.Hexabromocyelododecane; C70-70% Chlor. paraffin; B60-60% Bzom. paraffin; AC-Ammonium chloride. ygen Index test as useful addition, but not as a re, placement for normal burning tests. 3.3. Oxygen Index as a Quality Control lnstrmnent Quality control o f a particular formulation in which only production variables are likely to oc:l~t is a most suitable application for the Oxygen Index test. The results below show the variation in Index of different batches o f a flame retardant additive in polystyrene.

Oxygen Index ET See DB Ins

21.7 350 3.0

21.8 282 3.0

realistic assessment of burning properties timn other t,.~sts examined. (2) "]me results which relate high Oxygen Index values 1to the low deeoruposation tempe:rature of the flame retardant suggest that Oxygen Inde~ as a measure of the effectiveness of a flame retardant could be misleading. (3) ~ t e influence o f flow of thermoplastic:; on Oxygen Index should serve as a wa~ning against the use o f Oxygen Index in legislative control, as the possible spread o f t~re due to burning droplets m a y increase the fire hazard.

3

4

5

6

7

8

9

10

21.8 230 2.3

2!.ll 21~ 2..11

22.0 300 2.5

22.1 421 3.0

22.1 320 3.0

22.1 353 3.3

22.1 3:50 3,0

21.8 34C 2.7

ET-extinetion time. DB-distanee burnt on AST]d D635 test. The use of Oxygen Index is now more valuab:te with the availability of a direct read-out instrument which enables work to be carried out to a high degree of accuracy. Although the ASTM remits provide a ~ruide to the performance, the test is time consuming ancl by settLng an upper and lower limit of' Oxygen Index standard testing could be done very quickly.

4. Conclusions (1) The linear relationships between the Oxygell Index and flame retardant level provide a more

(4) The Oxygen Index test is most valuable in two specific areas: (a) For research in conjunction with other tests. (b) For quality control of standard formulations. (5) Tile relevance of the Oxygen Index to the real fire situation and other fire tests needs further ~.n~estigation. References 1 Fennhnom, C., P., and Martin, F. L Candle type lest for flamm~,bility of polymers, Modert, Plastics 44~ No. 3,141 (1966).

THE MEASUREMENTt,ND USE OF OXYGEN INDEX 2. Wolfhatd, B. G. et al. Combustionand Flame 1, 155 (1957). 3. lsaacs~.L L. The oxyg~'nindax flammabilitytest, J. Fire and Flammability I, .I6 (Jan. 1970). 4. Learmonth, G. S., and Thwaite, D.G. British Polymer J. 2, 104 (1970). 5. Abbott, C.J. Second FlammabilityConference 1972, C,zechoslovalda.

6. Gouialock, E. V. et al. J. Fire and Flam,'nability 2 (July 1971).

Received December 15, 1973; revised February 19, 1974