FRANK L. FIRE is vice president of marketing for Americhem Inc. in Cuyahoga Falls, Ohio. He’s an instructor of hazardous-materials chemistry’ at the University of Akron as well as an adjunct instructor of haz mats at the National Fire Academy. Fire is the author of The Common Sense Approach to Hazardous Materials and an accompanying study guide and the coauthor of SARA Title III, published by FireEngineering Books. He is on the editorial advisory board of Fire Engineering.

Isoprene is a reactive, polymerizing, flammable, narcotic, irritating, colorless liquid that has an odor somewhat similar to petroleum. The monomeric unit of natural rubber, isoprene was synthesized by man to form a synthetic rubber and is used as a monomer for some rubbers and plastics and as a raw material for the manufacture of other chemicals.


Isoprene has a flash point of — 165°F, an ignition temperature of 743°F (one source lists it as high as 802°F; another lists it as low as 428°F), and a flammable range of from 1.5 to 9 0 percent in air. It has a specific gravity of 0.68, a molecular weight of 68, and a vapor density of 2.35. It freezes at -230.7°F, boils at 93.4°F, and is insoluble in water. Isoprene’s molecular formula is CH = C(CHy)CH = CH» (sometimes written CH).


Although isoprene is extremely flammable, its major hazard is its reactivity. As a monomer, isoprene is capable of reacting with itself to form a long-chain molecule called a polymer. This capability is relatively rare among the more than 12 million identified and registered chemicals Polymerization is possible among shortchain, unsaturated hydrocarbons that possess at least one multiple bond. The isoprene molecule contains two double bonds and, therefore, has two very active sites for a chemical reaction to occur.

Polymerization is a special chemical reaction normally carried out very carefully in sealed vats (reactors) by chemical engineers. The polymerization reaction usually is highly exothermic, and the excess heat must be removed from the reaction as it is generated. If this is not done, the reaction may get out of control or “run away.” The polymerization reactor, therefore, usually has a double wall (or “jacket”), and cold water is run within the jacket to remove the heat and control the speed of the reaction. It is a rule of thumb in chemistry that for every 10°; (I8°F) rise in the heat of the material undergoing a chemical reaction, the speed of that reaction doubles. Should the heat generated by the polymerization reaction be allowed to increase the heat of the reactants in an uncontrolled fashion, the resulting rate of chemical reaction will double many times, creating so much heat energy so fast that the reactor vessel may blow apart. This explosion is called a runaway or uncontrolled polymerization explosion.

To protect isoprene (and other monomers) from polymerizing during transportation and/or storage, the manufacturer adds a chemical called an inhibitor (or stabilizer), designed to prevent premature polymerization of the monomer during relatively normal situations and at normal ambient temperatures. The inhibiting or stabilizing action of the chemical, however, may be overcome if the temperature of the liquid is allowed to rise very much. The radiated heat from a fire, for example, may be sufficient to provide enough energy to overcome the inhibitor and start the polymerization reaction. The resulting increase in heat energy (generated by the polymerizing material) may cause the container to fail catastrophically, with the unreacted monomer igniting as it is released. This explosion resembles and may be mistaken for a BLEVE (boiling-liquid, expanding-vapor explosion).

The manufacturer in the factory (or the chemist in a laboratory) might add a chemical called an initiator to the reactor to overcome the inhibitor’s stabilizing action and start the polymerization reaction.

Isoprene is such a reactive material that many chemicals can act as an initiator or catalyst and begin the polymerization reaction. These chemicals include barium, beryllium, calcium, cesium, chlorosulfonic acid, cresols. hydrazine, iron oxide (the rust formed by iron ), lithium, magnesium, nitric acid, oleum (fuming sulfuric acid), ozone in the presence of air, phenol, potassium, rubidium, sodium, strontium, and vinylamine. Certain classes of chemicals also will cause polymerization on contact with isoprene. They include acids, azo compounds, cyanides, caustics, diazo compounds, hydrazines, nitrides, peroxides, phenols, and sulfides. Isoprene also will react explosively with aldehydes, dithiocarbamates. explosives, and certain metals in addition to those mentioned above. The list is so long that it is almost certain that accidentally released isoprene will come in contact with one of these materials, resulting in a runaway polymerization or some other explosive reaction.

If the container in which isoprene is transported or stored is not insulated in some manner and is exposed to the energy of the sun (whenever the ambient temperature is relatively high), enough heat energy may be absorbed by the liquid to “drive out” the inhibitor or overcome its stabilizing action. This, of course, w ill allow the polymerization reaction to begin, generating its own heat energy. Once this reaction starts, it may be impossible to stop, and a very hot fire or explosion may result

Isoprene is so reactive that if it is exposed to air, partial oxidation will begin to occur. If allowed to continue, this reaction may cause unstable and explosive organic peroxides to form (in the same manner ethers form organic peroxides). Should a peroxide form and be heated by friction or some other heat source, it will begin to burn uncontrollably. Any reaction by such peroxides may be sufficient to start a runaway polymerization reaction within the container. These peroxides also will begin to self-react if the temperature rises to its SADT (self-accelerating decomposition temperature). (See “Benzoyl Peroxide,” Fire Engineering, November 1988 and “Ethyl Ether,” Fire Engineering, April 1991.)

In addition to its reactivity and ability to polymerize, isoprene is extremely flammable. Its flash point of — 65°F means that if isoprene is accidentally released into the atmosphere, an ignitable mixture of isoprene with the air will exist at all normal ambient temperatures at which it might be transported, stored, and used. Its ignition temperature of 743°E (or 428°F) means that any common ignition source can ignite isoprene vapors. Its flammable range of 1.5 to 9.0 percent in air may be relatively narrow compared with some other flammable liquids and gases, but it is wide enough to ensure that the isoprene vapors will be within the flammable range somewhere near the surface of the liquid and for some distance dow nwind.

Isoprene has no TLV-TWA (threshold limit value-time weighted average) listed by the ACG1H (American Conference of Governmental Industrial Hygienists). Concentrations somewhat above 2.0 percent will have a narcotic effect on anyone without respiratory protection. Inhaling the vapors may cause dizziness, headache, loss of some motor controls, unconsciousness, and even death. Isoprene can be detected by smell at concentrations of less than 0.01 ppm (parts per million in air), while concentrations as high as 5.0 percent (50,000 ppm) may be fatal.

The liquid’s contact with the skin will cause the skin to crack and become irritated by the liquid’s defatting action. The skin’s repeated exposure to high vapor concentrations in the air also will cause irritation.

If liquid isoprene contacts the eyes, severe irritation will result. Eye irritation also can be caused by unprotected exposure to high concentrations of neoprene vapors in the air.

Since isoprene is not considered a highly toxic material, ingesting the liquid is unlikely to cause death. It will, however, irritate the mouth, esophagus, and stomach.


Implement the community emergency plan required by SARA Title III (the Superfund Amendments and Reauthorization Act of 1986) whenever a significant quantity of any highly flammable liquid such as isoprene is released or is threatening to be released. The severe danger of a devastating explosion posed by the release of a material with such a low’ flash point, low ignition temperature, and high-vapor density requires that all emergency personnel be activated. The local environmental experts will be invaluable in their advice, as will all other experts and resource personnel activated by the plan.

Handle an isoprene release as any other highly flammable liquid spill: Approach from upwind, provide for sudden wind changes, and eliminate all ignition sources.

If equipment and tools are brought in to help contain the spill by constructing dikes for containment ponds or digging containment pits, they all must be sparkproof and otherwise incapable of producing energy sufficient to ignite the vapors. Once the liquid is confined, professionals may salvage it by vacuuming or pumping it into secure containers. The professional salvage firm w ill use explosionproof motors on all pumps; absorb any remaining liquid with cement powder, clay, fly ash, peat moss, sand, sawdust, straw, or soil; and dispose of all contaminated sorbents in accordance with federal, state, and local regulations. Emergency responders never should be involved in salvage or cleanup operations at a hazardousmaterials incident.

Emergency responders must see that the area around the spill is secured and begin evacuation for at least a half-mile around the spill (farther for very large containers) and for one to two miles downwind. Isoprene’s vapor density of 2.35 causes its vapors to “hang together” for a long time (if no strong breeze is present) and to travel great distances, looking for an ignition source. Once the vapors are ignited, they will “flash back” to the vapor source, often producing a tremendous explosion.

High-pressure spray or fog patterns will help disperse the vapors, but all runoff water must be contained. Even though isoprene is not soluble in water, any vapors condensed out of the air by using finely divided water will carry the isoprene to the ground, where it will float on the water.

Prevent the liquid from entering sewers and waterways. Dam all sewer entrances as well as all entryways into rivers, streams, lakes, or ponds. If a volatile material such as isoprene enters a sewer, it presents an explosion hazard everywhere along the system where vapors might be within the flammable range. Warn all processors of sewage of the potential entry of isoprene into their facilities.

Isoprene has a very low specific gravity (0.68) and therefore will float on any water that it contacts. As it sits on the water (or moves with moving water), it constantly generates explosive vapors. Since the vapors are more than twice as heavy as air, they will • hang close to the water’s surface and may be somewhat confined by the banks of the waterway, producing a great explosion hazard anywhere near the pond, lake, or flowing waterway. All downstream users of the water and anyone near the waterway may be within the danger zone of an explosion.

Once again, professional salvage firms are able to “skim” the product off the water using techniques perfected in many spills of petroleum and petroleum products. They also may be able to divert moving water into a “holding” area, where the isoprene may be salvaged, or they may use other techniques to remove the product from the water. These firms have the proper tools to do the job safely— as long as the areas around the contained (or moving) isoprene are secure from onlookers and all other unnecessary personnel. Environmental authorities will conduct tests to determine the spread of contamination and to decide when the contamination has been removed and/or when the water no longer is contaminated.


beta-nicthylbi vinyl

1,3-butadiene, 2-methyl






Even if isoprene is contained and thereby prevented front entering waterways, its vapors are capable of moving great distances with a gentle breeze. Even with no breeze at all, the vapor will flow like a liquid, following low spots in the terrain. These vapors may accumulate in low-lying areas or confined spaces, producing an explosion hazard a great distance from the original release. lacking an ignition source, an asphyxiation hazard will exist in all accumulations of high vapor concentrations.


Isoprene, which has the flammability properties previously described, is very easy to ignite. Small pools of burning isoprene vapors may be extinguished by using carbon dioxide, dry chemicals, foam, or water spray. The vapors very likely will reignite, however, because of isoprene’s low ignition temperature. Any metal exposed to the fire usually remains hot enough to be the reignition source.

Any isoprene container exposed to the radiated heat (or flames) of a fire must be kept as cool as possible by applying water with unmanned appliances from as far away as possible; all containers holding isoprene are subject to catastrophic failure and BLEVE.

Water curtains may be used to protect containers from radiated heat, but firefighters never should get caught between the containers and the fire. Heated containers will explode as soon as the pressure rises above the container’s design strength. Naturally, the larger the container, the greater the explosion and all other subsequent dangers.

If a container is leaking vapors and the vapors are burning, do not extinguish the flames unless the flow of fuel can be stopped immediately after extinguishment. Reignition of the vapors always is explosive. If the fireposes no danger to human life, the environment, or societal systems, perhaps the best action is to allow it to continue until all the fuel is consumed. In some cases, deliberately igniting the vapors may be the preferred mitigation technique rather than risking a massive explosion of the vapors later in the incident.



(Chemical Abstract Services) 78-79-5


(Standard Transportation Commodity Code) 4907230


(Registry of Toxic Effects of Chemical Substances) NT4037000


(United Nations/North America) 1218


(Chemical Hazard Response Information System) IFR


(U.S. Department of Transportation) flammable liquid

NFPA 704 Rating



(International Maritime Organization) 3.1, flammable liquid


Emergency responders should wear clothing that protects them from any contact with the liquid and exposure to the vapors. Any impervious turnout gear should offer some limited protection. If total encapsulating suits are worn, at least one source recommends polyvinyl alcohol and Viton®. Consult suit manufacturers for their recommendations.


Inhalation. Move the victim to fresh air and keep him/her calm and warm. If the victim’s breathing has stopped or becomes labored, administer artificial respiration, being aware that the first-aid giver might be exposed to the material in the victim’s lungs and/or vomit. Seek immediate medical attention.

Eye contact. Flush the eyes immediately for at least 1 5 minutes, lifting the eyelids occasionally. Seek immediate medical attention.

Skin contact. Wash the affected areas of the body with large amounts of soap and water. If irritation continues after washing, seek medical attention.

Ingestion. Do not induce vomiting. Keep the victim calm and warm. Seek immediate medical attention.

Next articleNEWS IN BRIEF

No posts to display