CHEMICAL DATA NOTEBOOK SERIES #81: ISOBUTYLENE
HAZARDOUS MATERIALS
Isobutylene is a flammable, polymerizable, reactive, colorless gas with a faint petroleum-like odor. Used as a monomer for polymers such as polyisobutylene and butyl rubber, it also is used to make aviation gasoline, antioxidants, and other chemicals. Isobutylene is easily liquefiable and may be used by itself or be included in a mixture of propane, propylene, butane, and butylene gases and may be referred to as LPG, or liquefied petroleum gas. It almost always is transported and stored in liquefied form.
PROPERTIES
As a gas, isobutylene has no flash point. Some references, however, may give — 122°F as the flash point, arguing that isobutylene usually is stored as a liquid and that at — 122°F the liquefied gas produces vapors sufficient to form an ignitablc mixture with the air near the surface of the liquid. Since this temperature is so far below any temperature at which humans live, work, or would be involved in a hazardous-materials incident involving isobutylene, the flash point for this gas (and other easily liquefiable flammable gases) is meaningless. Gases already are in rcady-to-burn form, and enough fuel is released from the liquefied state to cause an explosion at all ambient temperatures.
Isobutylene’s flammable range is from 1.8 to 8.8 percent in air, and its ignition temperature is 869°F. The liquefied gas has a specific gravity of 0.63, a molecular weight of 56, and a vapor density of 1.93. It boils at 19.6°F, freezes at 220.5°F, and is not soluble in water. Its molecular formula is C4H8, sometimes written CH2G(CH3)2. Isobutylene is an alkene, which means it is a hydrocarbon compound containing only one double covalent bond It is an isomer of I butylene (1-butene), whose molecular formula also is C.4ll8, and 2-butylene (2-butene), whose molecular formula is C4H8.
The table below illustrates how the different structures of these isomers affect their respective properties. In all three isomers, the molecular formulas, the molecular weights, and the vapor densities are the same. The differences in some of the other properties may seem unimportant, but in an incident involving isomers with crucial differences, it could mean the difference between a devastating explosion and the safe conclusion of an incident.
HAZARDS
Flammability is isobutylene’s principal hazard. At all ambient temperatures, the gas will be ready to explode as soon as it reaches its ignition temperature of 869°F. This relatively low ignition temperature can be reached by all common ignition sources. The fairly narrow flammable range of only seven percent makes it somewhat safer than gases with wider flammable ranges, but the flammable range still will be found somewhat near the source of the gas.
Isobutylene’s vapor density of 1.96 means that the gas will “hang together” and (low along low spots in the terrain until it finds an ignition source. ‘Hie exploding gas then will “flash back” to the source of the gas.
A monomer for the plastic polyisobutylene, isobutylene (as shown in its structural formula) contains one double covalent bond, making it a very reactive molecule. All short-chain alkenes are monomers, which means they have the capability of reacting with themselves to form long-chain molecules called polymers through the chemical reaction of polymerization. This reaction is exothermic, often evolving enough heat to cause some small portion of the molecule to reach its ignition temperature, thus producing an explosion and/or a fire.
To prevent this chemical reaction from occurring during transportation and storage, a stabilizing chemical, called an inhibitor, is added to the isobutylene. When the product reaches its destination and is placed into the polymerization reactor, another chemical, called an initiator, is added to overcome the inhibiting effect of the stabilizer; the polymerization reaction then begins under carehilly controlled conditions. If the inhibitor is allowed to escape during transportation or storage and the reaction begins in the container, the resulting “runaway” or uncontrolled polymerization can produce an explosion resembling a BLEVE (boilingliquid, expanding-vapor explosion).
The double bond in isobutylene’s structure makes it a very reactive material. The double bond between carbon atoms is the weakest site in any hydrocarbon or hydrocarbon-derived molecule, and any reaction will occur at this point. Isobutylene reacts violently with acids, aldehydes, cyanohydrins, halogens, halogenated hydrocarbons, reactive metals, other monomers, and strong oxidizers. It essentially is nontoxic; breathing high concentrations of it, however, may irritate the mucous membranes, may be somewhat anesthetic, and can act as an asphyxiant. If the liquid or escaping gas contacts the skin, frostbite can occur.
NONFIRE RELEASE
Any incident involving a large container or large number of containers of isobutylene is serious enough to activate the community’s emergency response plan. Consider first evacuating all in the area who could be injured by an explosion (from one to two miles from the release of the product or stressed containers), and then institute other life-rescue operations. Approach the incident ground from upwind, and eliminate immediately all possible ignition sources.
Gas may be escaping under pressure from an opening in the container, or liquid may be leaking. Do not attempt to stop the leak unless there is no danger that the gas will ignite. After safety, the considerations that will determine if plugging efforts should be made are the amount of pressure behind the leak and the availability of the proper personnel and materials to plug it. Be aware that the friction caused by gas escaping under pressure can produce a static electricity discharge that can cause an explosion or, at a minimum, ignite the gas. The resulting explosion and flames will be deadly for any emergency responders nearby.
Trap released liquid by building a containment pond or digging a pit, using only sparkproof tools and equipment. The liquefied gas will boil and evaporate quickly, producing tremendous amounts of gas. A high-pressure water spray or fog will break up this collection of gas. Any water reaching the liquid pool will speed up evaporation. Contain all runoff water.
Isobutylene boils rapidly and probably will foil attempts to salvage any liquefied gas caught by the pond or pit. Firefighting foam may be used to slow the evolution of gas even though its initial application may speed it up.
Confinement techniques also should prevent the liquid from flowing into a sewer or waterway. Should some of the liquid enter a river, stream, pond, or lake, it rapidly will boil away. TnT to disperse these vapors and although isobutylene essentially is insoluble in water, alert all downstream users of the water to the spill and any subsequent contamination.
Should liquefied gas enter a sewer, the liquid will boil and produce gas that will be trapped within the pipes, posing a serious explosion hazard all along the system. Even though the gas is heavier than air and will flow downhill, the slope of the sewer system usually is very slight, and the expanding gas could force itself up the gentle slope, as long as it is confined. Dikemanholes and catchbasins near the release, and protect all openings near any entry into the system from ignition sources that may be following the sewer line.
IDENTIFICATION NUMBERS AND RATINGS
CAS
(Chemical Abstract Services)
115-11-7
STCC
(Standard Transportation Commodity Code)
4905748-isobutylene 4905752 —liquefied petroleum gas
RTECS
(Registry of Toxic Effects of Chemical Substances)
UD0890000
UN/NA
(United Nations/North America)
1055—isobutylene
1075—liquefied petroleum gas
CHRIS
(Chemical Hazard Response Information System)
IBL
DOT
(IJ.S. Department of Transportation)
flammable gas
NFPA 704 Rating
1-4-0
IMO
(International Maritime Organization)
2.1, flammable gas
FIRE SCENARIO
If at all possible, keep the fire from approaching containers of isobutylene. Keep the containers as cool as possible by applying water with unmanned appliances from as far away as possible. If the containers are stressed by impinging flames or from radiant heat, do not approach them from any angle. Never get between the fire and the containers of isobutylene or any flammable gas or flammable or combustible liquid.
If the container is venting or leaking and the escaping gas has been ignited, do not extinguish it unless the flow of fuel can be stopped immediately after extinguishment.
The classic BLEVE occurs when a container holding a liquid is subjected to intense radiant heat or impinging flame. The pressure inside the container rises to a point where the safety relief device (usually a spring-loaded valve) operates to reduce internal pressure. Most times, the gas escaping through the vent ignites, producing more heat to act on the metal skin. If the pressure rises too fast for the vent to relieve and the internal pressure rises above the design strength of the container, the container will fail catastrophically, releasing a massive fireball (the liquid is above its boiling point, producing the maximum amount of vapor). The container (railroad tank car or tank truck, for example) may be blown apart, propelling the car some distance and producing deadly shrapnel (as if the fireball isn’t deadly enough).
Flames impinging on the vapor space will weaken the metal of the tank, causing a tear in the container. This situation also will cause a fireball to be released and the tank to come apart. In both situations, a visible increase in the length of the flame coming from the vent and an audible increase in the noise the escaping gas is making may be detected. These may’ be the only warnings that an explosion is imminent, and they may be inadequate as far as allowing firefighters to withdraw. The danger of BLEVE is so great that the safest mitigation technique might be to protect exposures and allow the incident to run while applying cooling water with unmanned appliances from as far away as possible.
Extinguish pools of burning isobutylene with carbon dioxide or dry7 chemical, depending on the amount burning, weather conditions, and the amount of extinguisher available. Water may not be effective on the fire, and any water contacting the liquid will increase evaporation.
An extinguished pool of burning isobutylene may be explosively reignited if the heat of the fire heats an exposed piece of metal to a temperature above isobutylene’s relatively low ignition temperature. If the fire is extinguished and the metal has not been cooled below 869°F, the extinguished isobutylene vapors will reignite.
SYNONYMS
gamma-butylene
isobutene
liquefied petroleum gas LPG
2-methylpropene
uns-dimethylethylene
PROTECTIVE CLOTHING AND EQUIPMENT
Emergency responders should use protective clothing and equipment that is impervious to isobutylene, that will not absorb it, and that prevents the liquid’s contacting the skin or eyes. Nitrile rubber gloves and boots, chemical splashproof goggles, and face shields may provide adequate protection. The coldness of the liquid may affect compatible materials. Consult the manufacturers of protective clothing and of isobutylene. Positivepressure, self-contained breathing apparatus is required to provide respiratory protection whenever high concentrations of isobutylene gas are present.
FIRST AID
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 such action might expose the first-aid giver to the material in the victim’s lungs and/or vomit. Seek immediate medical attention.
Eye contact. Flush the eyes immediately for at least 15 minutes, lifting the eyelids occasionally. Medical attention is needed if irritation persists after flushing.
Skin contact. Wash the affected body areas with large amounts of soap and water. Do not rub skin that may have been exposed to the liquid.
Ingestion. Ingestion of the liquefied gas is not very likely.
