Solvent Flammability Basics

8/6/2019 Solvent Flammability Basics

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Solvent Flammability Basics

The concept of flammability may seem elementary on the surface, when inactuality, flammability and flash point measurements must be performed usingthe correct standardized methods. You may be surprised when you do.

by Ronald L. Shubkin and Barbara F. Kanegsberg

There are a number of sources of confusion regarding flammability. One problemis the application of test methods conducted outside of the range of parametersfor which the test is designed. Finally, the vapors of certain halogenated solventscan burn within a narrow concentration range in air even though they exhibit noflash point. They are self-extinguishing. All of these factors have led to incorrectflammability data. As with other physical and chemical parameters, it is crucial formanufacturers to have access to flash point data based on the appropriateconsensus standard for the material in question and performed by independenttest facilities.

Flammability Definitions and Standards The Occupational Safety and Health Administration (OSHA) defines “Liquid,Flammable” as “any liquid having a flashpoint below 100 °F (37.8 °C), except anymixture having components with flashpoints of 100 °F (37.8 °C) or higher, thetotal of which make up 99 percent or more of the total volume of the mixture.” i The U.S. Department of Transportation (DOT) defines Class 3 Flammable Liquids ii for transportation purposes on essentially the same basis iii. Both regulatoryagencies define “Flash Point” as the minimum temperature at which a liquid givesoff a vapor in sufficient concentration to ignite when tested by specific methodsas set forth in the regulations iv . Both DOT and OSHA specify appropriate testmethods from the American Society for Testing and Materials (ASTM), specificallyASTM D56, ASTM D93 and ASTM D3278. DOT also allows ASTM D3828.

The National Fire Protection Agency (NFPA) Code 30 is a definitive consensusdocument for determining flammability. This document clarifies whichflammability tests are considered valid for particular compounds or mixtures,referring to essentially the same ASTM test methods, as does OSHA. In Code 30,the characteristics of a flash point are defined in section 1.7.2.2 as the minimumtemperature of a liquid at which sufficient vapor is given off to form an ignitablemixture with the air, near the surface of the liquid or within the vessel used, asdetermined by the appropriate test procedure and apparatus specified in 1.7.4.Note that both the appropriate apparatus and the test procedure depend on thematerial being tested. The use of an inappropriate approach for the chemicalunder investigation, or the employment of an incorrect apparatus, may lead to anincorrect result.

Choosing the Flash P oint Test Method A variety of Flash Point test methods have been developed to determine theflammability of liquids. Certain of these have become industry standards and arecorrect in a technical sense. However, these various methodologies are valid onlyover specified temperature ranges and for liquids with viscosities within specifiedranges.

If an incorrect technique is used for the material in question, a result is obtainedthat is not valid. For example, nPB has a relatively low viscosity, and independentlaboratories, using the accepted techniques for low viscosity materials, havedetermined that n-propyl bromide does not have a flash point. v,vi Contrarily, a

method using the Pensky-Martens Closed Cup procedure (ISO 1523) amethodology designed for a viscous fluid and valid only at temperatures above+10°C obtained a flash point of –10°C for nPB. vii Following that report, an



independent testing laboratory, Chilworth Technology Ltd., determined if thismethodology gave results that differed from what was reported for other well-established non-flammable liquids. viii,ix Tests were performed using ASTM D93, aconsensus standard that employs the Pensky-Martens Closed Cup apparatus, thesame equipment specified in ISO 1523. In addition, results obtained using ASTMD93 were compared to those obtained using ASTM D56 (Tag Closed Cup). UnlikeASTM D93, ASTM D56 is specified to be used for low viscosity fluids and may beused at low temperatures. The results for nPB and for five widely accepted non-flammable liquids are given in Table I.

Compound

Flash PointRun W ithin TestRangesASTM D56

Flash P oint, °CRun Outside TestRangesASTM D93

n-propyl bromide None - 5

Trichloroethylene None + 24

Methylenechloride None - 5.5

HFE-72DE None - 9

HFE-71 DE None - 16

HFE-71 DA None - 14

Table 1. Flash Points by Appropriate and I nappropriate Methods: The flashpoint method of choice depends on the compound to be measured. If an incorrectmethod for the compound being studied is employed, even solvents accepted andused for decades as having no flash point can erroneously appear to be

flammable.

Accepting the results of a seemingly more stringent flash point determination,which is not an industry standard, is not more conservative or protective of workers. The reason is that the flash point apparatus is designed to start firesunder specific conditions. The use of an apparatus and a method outside therange that it was designed for can give a flash point under conditions that are notrelevant to real-world conditions.

As indicated, a definitive, consensus document is NFPA Code 30. The relevantmethod for low viscosity liquids, is that described in section 1.7.4.1, which is forliquids with a relatively low viscosity, in other words, below 5.5 cSt at 104°F(40°C) or 9.5 cSt at 77°F (25°C). The appropriate technique is ASTM D56,

“Standard Method of Test for Flash Point by the Tag Closed Cup Tester”. Forexample, the viscosity of nPB is 0.36 cSt at 25°C., more than an order of magnitude below the dividing line between the ASTM D56 technique (lowviscosity) and the ASTM D93 technique (Pensky-Martens Closed Cup for highviscosity liquids). ASTM D 3278 for small-scale testing is also permitted undercertain conditions as indicated in Section 1.7.4.4.

Observing a Flash Poin t Some of the confusion in reporting the results of flash point testing lies inunderstanding the definition of a flash point. A flash point test consists of passinga pilot light over the surface of the liquid being tested. If the vapors of a liquidare flammable, in other words, capable of sustaining combustion, the flame front

will propagate from the pilot light to the liquid surface. The lowest temperature atwhich this flame propagation happens is called the flash point. The NFPA gives a



number of definitions of flashpoint in its Glossary of Terms, but they all includethe phrase, “The minimum temperature at which a liquid gives off vapor sufficientto form an ignitable mixture in air near the surface of the liquid .” In lay terms,the flame front must move from the pilot light to near the surface of the liquid. n-Propyl bromide does not exhibit this behavior under the conditions of the ASTMD56 or the Cleveland Open Cup methods. 7

A flash point test consists of moving a pilot light over an open cup of liquid (opencup tests) or opening the cover of a closed container when a pilot light ispositioned over the cover (closed cup tests). Non-flammable liquids that will notburn at the temperature of the pilot light will not affect the pilot light flame.

One source of erroneous reports of flash points consistent with inaccurateobservations is that the vapors of certain halogenated compounds, can becombusted in an external source of ignition. The result of this characteristic in theperformance of a flash point test is that the pilot light may be briefly enhancedbefore it is extinguished. In this case, the enhanced flame is above the pilot lightand not near the surface of the liquid as required by the various NFPA definitions.

Enhancement of the pilot light is typical of hydrochlorocarbons that are classifiedas non-flammable (i.e., methylene chloride, trichloroethylene, etc.). The mistakeninterpretation of pilot light enhancement for flashpoint led to early, erroneousidentification of n-propyl bromide as a flammable liquid.

Flammable liquids behave quite differently in flash point tests than do non-flammable, halogenated liquids. Specifically, in contrast with halogenatedsolvents and other non-flammable liquids, the “flash” is readily apparent as itspreads almost explosively through the flash point cup and above it.

Flammability Limits Most hydrochlorocarbons, some hydrofluoroethers, some

hydrochlorofluorocarbons and nPB will burn when the vapor concentration in air iswithin narrow limits. When such vapors begin to burn, several factors rapidlychange the vapor concentration so that it is no longer within the flammable limits.The concentration in the vapor state is depleted by both consumption in theoxidation process and by expansion of the vapor caused by the exothermiccombustion. The combustion rapidly self-extinguishes.

Flammability limits are usually reported as the Lower Explosive Limit (LEL) andUpper Explosive Limit (UEL). The units are commonly the volume percent of thevapor in air. The flammability limits of some well-known, “non-flammable” liquidsare given in Table II.

Table 2. Flammability Limits of Repre sentative Non-Flammable Liquids a

Compound LELx UEL 12

n-propyl bromide 4.0 vol.% 7.8 vol.%

methylene chloride 13.0 23.0

trichloroethylene 8.0 10.5

1,1,1-trichloroethane 7.0 13.0

HCFC-141b 7.6 17.7

HFE 7200 b 210 g/m 3 1070 g/m 3



a. Some non-flammable liquids can burn within narrow concentration ranges.However, the combustion rapidly self-extinguishes.b. Data from 3M Technical Data Sheet. The flammability range is approximately2-10 vol.%.

When an organic solvent is the appropriate selection, an understanding of flammability characteristics is imperative. As with other issues concerningperformance, compatibility, safety, and environment, it is important to criticallyevaluate all flammability characteristics. Further, to be meaningful, availableconsensus standards and methods are the ones to use.

The bottom line is safety. The use of any organic solvent, or any blend containingvolatile organic vapors, must include an analysis of potential flammability,including the flash point. A number of additional considerations, including processspecifics and other processes performed in the vicinity must also be taken intoaccount.

Thanks to Battalion Chief Michael Bean of the Warminster Fire Department andFire Marshal’s office for reviewing the NFPA sections of this article.

About the Authors Ronald L. Shubkin is Manager of Technical Services for Poly Systems USA, Inc, aprovider of nPB. He holds thirty U.S. patents with ten dealing with formulationsand applications for nPB-based solvents in critical cleaning. He can be reached [email protected] .

Barbara Kanegsberg is President of BFK Solutions, LLC, an independentconsulting company specializing in critical cleaning issues. She is a chemist withover eighteen years of experience in contamination control issues. She may bereached at [email protected].

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i Code of Federal Regulations, 29 CFR 1910.1200(c).ii Code of Federal Regulations, 49 CFR 173.2.iii Code of Federal Regulations, 49 CFR 173.120(a).iv Code of Federal Regulations, 49 CFR 173.120(c)(1) and 29 CFR 1910.1200(c).v Joe Miller, Document No. OAR-2002-0064-0040, “Albemarle Corporation – Flash Point Data for n-Propyl Bromide”, submitted to the USEPA on 8/4/003.vi Richard G. Morford, Document No. OAR-2002-0064-0030 and –0031, “Enviro Tech Comment re:Section IVD Flammability”, submitted to the USEPA on 7/25/2003.vii Report by the German National Flammables Laboratory to the European Chemicas Bureau (ECB).For a discussion of this report see reference no. 8, document –0031, Exhibit G.viii Study done under contract to the International Brominated Solvents Association, IBSA.ix Stephen M. Rowe (Chilworth Technology, Ltd., England), Reports No. 09802 (3/26/02) and No.15302 (5/10/2002). These may be viewed at reference 8, document –0031, Exhibit H, or may beobtained from R. Morford, Enviro Tech International, Inc.x Ronald L. Shubkin, “ normal -Propyl Bromide”, CRC Handbook for Critical Cleaning (B. Kanegsbergand E. Kanegsberg, eds.), CRC Press, 2001, Chapter 1.7, pp. 111-131.

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Solvent Flammability Basics