in the Chemical laboratory Edited by NORMAN V. STEERE, 140 Melbourne Ave., S.E. Minneapolis, Minn., 55414
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XLV. Some Aspects of Hazard Evaluation Testing* ROBERT D. COFFEE, Ph.D., Technical Safety Laboratory, Eastmon Kodak Company, Rochester, New York 14603 INTRODUCTION Nowadays, many of the major chemical laboratories are equipped to run the simpler chemical hazard evaluation tests such as: flash paints, explosive limits, and autoignition temperature. A few may have facilities for running differential thermal analyses and others vapor pressure determinations. As a consequence, the literature contains numerous tabulations of test results. Because there are few (if any) redly basic standard procedures or standard test apparatus, numerous discrepancies may he found. Moreover, these discrepsncies have a habit of pmliferating from generation to generation. This is both oonfnsing and disturbing to industrial safety groups who must use these data. to establish the conditions under which these chemicals may be safely shipped, stored, and/or produced. Although various groups, such a5 the recently established ASTM Committee E-27, are attempting to remedy the situ* t,ion, what can be done in the interim7 In the first place, one must make fuller use of the information that is already available. The reliability of a given value
can often be checked by approximation calculations or by the use of related data and an understanding of general trends. Fnrthermore, one must be constantly aware of the limitations of many of the test methods and how misleading information may be obtained by misapplication. The fallowing examples will illustrate some of the ways test data, may be interpreted and some of the pitfalls to he avoided.
Interrelationships-Flash Lower Explosive Limit, Vapor Pressure
Points,
"The FLASH POINT of a liquid is the temperature at which it gives off vapor sufficient to form an ignitible mixture with the arr near the surface of the liqnid or within the vessel used." "The LOWER EXPLOSIVE LIMIT (LEL) is the minimum concentration of vapor in air below which propagation of flame does not occur on contact with a source of ignition." Ideally, the two values shordd be equivalent but in general the LEL is equivalent to a temperature lower than the Tag Closed Cup (TCC) flash point which in turn is lower than the Tag Open Cup
MONOETHYL ETHER
50
100 1 50 200 TEMPERATURE. ('F)
250
300
Figure 1.
*Presented a t the 1967 National Safety Congress, and printed with permission of the National Safety Co~mcil.
(TOG) value. Moreover, the TOC value is normally lower thsn the Cleveland Open Cup (COC) value. These relationships are shown in Figure 1 where they are oom-
Robert D. Coffee g~.atl~,:ilc20 min for cyclopnrsfins. Thus, the 5-min observation time specified by D-2155 is often inadequate for obtaining minimum ignition temperatnrea. Following ASTM D-"5.5 procedores, delay times prior to ignition of 2 0 4 0 minutes have been observed for some mater i d s , especially organic perehlorates. Fortunately, whether delay times are short or long, a plot of ignition temperature versus delay time will appear as in Figure 4 from which the minimum ignition temperature under the conditions of test may be readily est,imated. Npither D-286 nor D-2155 mention
"cool" flames or establish conditions under which such flames may be noted. I n order to follow "cool" flame reactions i t is necessary t,o operate in a t,otdly derkened room. A thermocouple s~lspendedwithin the flask can be quite helpful in following the reaction. "Cool" flames differ from normal flames in t h a t the visible light i5 very faint, the tempemture rise is relatively small, and the pressure developed in a confined spaee is also low. Nevertheless, some cool flame reactions can proceed to a normal or hot flame reaction and thus there is a definite potential hazard involved. Much more work is necessary before the oonditiorrs for transition can be defined. I n the meantime, both "hot" and "cool" flame ignition temperatures should be reported. Table 2 lists some (Continued on page A M ) I G N I T I O N TEMPERATURE.'F
NFPA 325M METHYL 150-BUTYL KETONE
860
METHYL ETHYL KETONE
960
150-PROPYL ALCOHOL
750
ISO-BUTYL ACETATE
793
n-BUTYL ACETATE
790
METHYL ISO-AMYL KETONE
(790)
TABLE 2
"COOL" FLAMEIEK CO)
Safety
...
surface reaction is d w to the presence of ferric oxide on the i m r and may be checked by dropping powdered iron oxide
EASTONE ORANGE 2 R
I 50
I
100
I
I
I
150 200 2W TEMPERATURE,
I 300
I
350
400
"C
Figure 5.
ignition values to illustrate the possible spread between normal and cool flame ignition temperatures. In some cases, the spread has been found to he as much as 450°F and in others, only 5-10°F. If the response of a, thermocouple suspended in the test flask is used to indicate ignition or follow the exothermic reactions, it must be remembered that some materials such a s iso-propyl alcohol will cause an iron-constantan thermocouple to glow red-hot s t flask temper* tures well below the ignition temperature. If the thermocouple is small, however, it will not serve as a source of ignition. The
0
50
Figure 60 and 6b.
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(FelOl) into the flask and observing the numerous glowing spots.
Differential Thermal Analysis I n recent years differential thermal analysis (DTA) has been widely employed by manylaboratories investigating the hmards of chemicals. The test has become very popular since a great deal of infarmation can be obtained in a short time from very small samples. Test samples me commonly of the order of 5 mg. When interpreting the results it is extremely (Continued on page A58)
150 200 250 TEMPERATURE, 'C
300
350
Safety
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important to remember that whereas endothermic responses due usually to p h m changes are not essentially rate dependent, exothermic responses are. Figure 5 shows the effect of heating rstes on t,he exothermic decomposit,ion of a pure dye, Eastone Orange 2R. Note that the endotherm corresponding to the melting point of this dye is not affected by heating rate. Both the amplitude and acuteness of the exotherm m e indicative of the degree of potential hazards. Should the DTA results indicate an eodotherm closely followed by an exot h e m as shown in Figure 6a, the,, additional tests should be run a t slower heating rates. Figure 6b shows what may be hidden by too high a heating rate. Similar results may be obtained by pressurizing the sample. I n 6a the sample melts around 60°C, starts to decompose exothermdlv around 250°C hot then st,arts r 1, ,il an, lntl .'*O°C. 111.111 limll\. llw WIIIlwtmic ~Iwamq,~,.i~i~m pwdowt~~ritt.. III (ill l l w 1.wline rate i.3 ?low er.lw&l>so t h k ~ the sample decomposes exothermally before the boiling point is reached. Just because DTA analysis may not indicate any exothermic reactions does not necessarily eliminate the possibility of a. potential hazard. I n the previous example the exotherm was almost missed. With propargyl bromide (6) no exothermic activity is noted because the sample boils itway a t 84°C. If heated under confinement, however, the material decomposes violently amwrd 100°C and probably detonates.
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Summary I n the preceding prtmgraphs it has been pointed out that althot~ghthe literature may contain numerous disc~qmncies,unreliable data may oft.en he disclosed through the use of simple plot,s. Examples are given of how misappliontion of current flash point tests to mixtures can produce some very misleading results. Finally, precautions are given in the use and interpretation of mtnignition and differential thermal analysis data.
REFERENCES (1) Z A ~ E T A KM. ~ ~ G., , "Flammability Characteristics of Combustible Gases and Vapors," Bureau of Mines Bulletin 627 (1965), page 21. (a) -, "Handbook of Chemistry and Physics," 4th Edition, pp. 2457250.5, Chemical ltohber Publishing Co. (3) -, "Fire-Hazard Properties of Flammable Liquids, G m s and Volatile Solids," NFPA No. 326M-1965. (4 ) -, "Ketones," Bulletiu F-4767-F, Union Carbide Co. (1964). ( 5 ) SETCHKIN, N. P., "Self-Ignition Temperatures of Combustible Liquids," J . Res. Nat. Bu. 01Slds., 53, No. 1, 49-66 (1954). (7) COFFEE,R. D. AND J. J. WHEELER, "The Enplosibility and Stabilimtion of Propargyl Bromide," AIChE Safety iu Loss Symposium, Feh. 20-22. 1967.
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