CHEMICAL DATA NOTEBOOK SERIES #89: XYLENE
HAZARDOUS MATERIALS
Xylene is a flammable, moderately toxic, mildly irritating, clear, colorless, liquid hydrocarbon that belongs to the aromatic family that has an odor similar to benzene. It is used as a solvent in many paints, coatings, cements, adhesives, inks, and dyes and as a cleaning fluid, as a fuel component, and in the manufacture of other chemicals, such as dyes, fibers, insecticides, insect repellants, perfumes, and resins.
In its commercial form, xylene is a mixture of three isomers and therefore is not a pure compound. Even though the properties of each of the isomers are slightly different, when only “xylene” is mentioned, the properties of all three compounds are similar.
PROPERTIES
“Xylene” is really a mixture of three isomers—o-xylene, m-xylene, and pxylene. Therefore, when you encounter a release of the material, consider the lowest flash point, the widest flammable range, and the lowest ignition temperature listed in the table below as the material’s property values. In laboratories and other facilities where the proper isomer is required for the process, the xylene will be identified on the label of the container as “o-xylene,” “m-xylene,” or “pxylene.” In cases where the label just reads “xylene,” the material is a mixture of the isomers.
The molecular fomula, the molecular weights, and the vapor densities of all three isomers are the same. The differences in some properties may seem insignificant; but in an emergency situation, they might mean the difference between a devastating explosion and the safe conclusion of a hazardous-materials incident.
Xylene’s molecular formula is CCH02The structures of the isomers differ. Xylene is simply benzene with two methyl radicals substituted for two of the hydrogen atoms on the benzene “ring” (see “Benzene,” Fire Engineering, October 1988). A methyl radical is a molecular fragment that can be thought of as a molecule of methane with one hydrogen atom removed Whenever a hydrogen atom hits been removed from any hydrocarbon (or hydrocarbon derivative), a very’ reactive site will form on the remaining structure. If two methyl radicals are brought to a benzene ring that has two hydrogen atoms removed, the methyl radicals will occupy the spaces where the hydrogens were once attached, and the resulting compound would be xylene.
Ortho-xylene, meta-xylene, and para-xylene are isomers of each other. The terms ortho (o-), meta (m-), and para (p-) are derived from the places on the benzene ring where the methyl radicals have attached themselves. An isomer is defined as one of two or more chemical compounds with the same molecular formula but a different structural formula. Each of the three xylene isomers contains exactly the same number of carbon and hydrogen atoms as the others, but they are arranged slightly differently. These differences are enough to provide some differences in the properties of o-xylene, m-xylene, and pxylene (see box on page 107).
The xylene isomers are known as aromatic hydrocarbons, since they each contain a benzene “ring,” and benzene is the simplest of the aromatic hydrocarbons. Aromatic hydrocarbons consist of at least one benzene “ring” made of carbon atoms bound to each other in a closed, hexagonal (six-sided) structure, where each carbon atom is covalently bonded to two other carbon atoms.
HAZARDS
The principal hazard of all three xylenes is flammability, since the chemicals’ flash points are so low that vapors sufficient to form an ignitable mixture with the air near the surface of a spill of any of the xylenes will be present at all ambient temperatures of 63°E or lower. Their ignition temperatures can be reached by any common ignition source. This combination of low flash point and low ignition temperature means ignition of released vapors is an almost certainty. The vapor density of 3.66 (all three isomers) means that the vapors, whether released from a leaking container or from a released pool of liquid, will sink to the ground upon release, stay together in an invisible vapor cloud, and flow (vapors are fluids) down any low spots in the terrain until they encounter an ignition source. The result will be an explosion of the vapors, with the explosion and flames “flashing back” to the source of the vapors.
The xylenes are not generally considered highly toxic. Their TLV-TWA (threshold limit value-time weighted average) of 100 ppm (parts per million in air), and STEL (short-term exposure limit) of 150 ppm are moderate w hen compared with truly toxic materials.
Repeated contact of any of the xylenes with the skin will cause reddening, irritation, and cracking due to the defatting action the material has as it acts as a solvent for body fat. Contact with the eyes can result in moderate irritation. Exposure to high concentrations of xylene isomers will cause breathing difficulties, coughing, irritation of the nose and throat, and, eventually, unconsciousness. Avoid repeated exposure to high concentrations of the vapors.
Ingesting the xylenes will cause depression, headache, nausea, cramps, abdominal pain, vomiting, and possibly death. Xylene ingested into the lungs can cause lung irritation and pulmonary edema on a delayed basis.
The xylenes are considered stable chemicals, but they will react violently with all strong oxidizers. The xylenes are not corrosive to metals, but they may attack plastics, rubber, and some other polymeric coatings because they are good solvents of these materials.
NONFIRE RELEASE
All standard precautions for flammable-liquid releases must be taken when xylene is released. Approach by emergency responders must be from upwind and uphill, and all ignition sources must be eliminated. Unauthorized personnel should be restricted from the area, and emergency responders who may contact the material must wear respiratory, skin, and eye protection. The community’s emergency response plan must be activated if the release is large enough to endanger more than a small, uninhabited area.
Xylene, like other flammable liquids and hazardous substances, must be prevented from entering sewers and waterways. This can be accomplished by damming catch basins and other entryways to sewers w-ith nonabsorbent materials and by containing the liquid in containment ponds or pits. Ponds can be constructed by pushing up dikes of soil, sand, clay, peat, or other materials with earthmoving equipment. Digging containment pits and trenches can lead the liquid to the pit. In all cases, use sparkproof tools and explosionproof equipment to avoid igniting the vapors.
In general, a pit is more desirable than a containment pond because the pit probably will have a smaller surface area than a pond. One of the variables affecting the evaporation of volatile liquids is the surface area of the liquid interfacing with the atmosphere. The larger the surface area, the higher the rate of evaporation, provided all other factors (temperature, pressure, etc.) are equal. When the surface area is smaller, mitigation techniques, such as covering with a tarpaulin or blanketing with foam, are more feasible.
Firefighting foam is fairly effective in reducing vapor evolution, particularly with a flammable liquid like xylene. However, the foam may break down and reapplication may be necessary. As the foam breaks down, it will add to the volume of the contained liquid and cause runoff, which also will have to be contained. Use of a fine water spray or fog may accelerate the dispersal of vapors immediately downwind of the spill. Since this water may be contaminated with xylene, it must be contained. Spray used in dispersal may add to the volume of the contained product.
IDENTIFICATION NUMBERS AND RATINGS
CAS
(Chemical Abstract Services)
o-xylene: 95-47-6 m-xylene: 108-38-3 p-xylene: 106-42-3
STCC
(Standard Transportation Commodity Code)
p-xylene: 4909351 mixture: 4909350
HTECS
(Registry of Toxic Effects of Chemical Substances)
o-xylene: ZE245000 m-xylene: ZE2275000 p-xylene: ZE2625000
UN/NA
(United Nations/North America)
1307
CHRIS
(Chemical Hazard Response Information System)
o-xylene: XLO m-xylene: XLM p-xylene: XLP
RCRA
(Resource Conservation and Recovery Act)
U239
DOT
(U.S. Department of Transportation) flammable liquid
NFPA 704 Rating
2–3-0
IMO
(International Maritime Organization)
3.1, flammable liquid
An accidental release of a large volume of xylene, w hether contained or not, requires the evacuation of the immediate downwind areas. If the liquid enters a sewer or waterway and it is not possible to contain the waterway through damming or diking, all downstream users of the water and operators of sewage treatment facilities must be notified. Intake of xylenecontaminated water into an industrial operation could cause disastrous explosions.
Because of its specific gravity of 0.86 and its insolubility in water, xylene will float on the water and therefore may be skimmed off the top of a lake or slow-moving stream by conventional floating booms and suctioned into secure containers. One mitigation technique is to dam or dike a waterway, diverting the xylene into a containment area, where it may be removed. Again, all tools and equipment must be sparkproof and explosionproof.
Once in containment ponds and pits, xylene will tend to percolate into the soil, spreading contamination. At the first indication of a spill of xylene or any other hazardous material, notifythe proper environmental authorities. They have the final word on cleanup and how far contamination may have spread. Cleanup at hazardous materials incidents must be performed byqualified professionals— not the fire department —unless, of course, fire department members have been properly educated, trained, and equipped for the task. Cleanup may be as hazardous as mitigation, and most fire departments are not qualified to do it.
The spilled liquid may be absorbed using clay, soil, sand, fly ash, cement powder, peat moss, straw, or commercial sorbents. Even after the material has been absorbed, it is still a hazardous, flammable liquid; use the same care in handling the absorbed materials as you would the spilled product. It should be disposed of by qualified professionals.
FIRE SCENARIO
In a case where a tank truck or other container of xylene is not involved in fire but is threatened by flames or radiant heat, the container
must be kept as cool as possible w ith water applied from as far away as possible, with as little exposure of firefighters as possible. As a container is heated, catastrophic failure is possible if excess pressure is not vented away.
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First-hand accounts and analysis of the bombing incident that took place on February 26, 1993—one of the largest and most dramatic emergency response operations in recent history, and one destined to provide lessons learned for the fire and emergency services for years to come.
Xylene tank trucks ordinarily may not subject to a BLEVE (boiling-liquid, expanding-vapor explosion), since the emergency pressure-relief system will function as designed. However, if the conditions are just “right” (“wrong” for firefighters), a container may rupture, and the resulting explosion and fire could be as devastating as a BLEVE.
Most departments have responded to xylene or other hydrocarbon flammable liquid fires and have experience in this type of emergency. Great strides in technology have made it possible to effectively use water, which at one time was forbidden because it displaced the burning xylene or other hydrocarbon liquids, carrying them beyond the fire scene. New nozzles that create a fine water
spray or fog, for example, have allowed new’ tactics to be used to control the burning liquid vapors. Carbon dioxide, dry chemical, and foam also will be effective on xylene fires, depending on the volume released, the weather, and the terrain.
SYNONYMS
1.2-dimethylbenzene
1.3-dimethylbenzene
1.4-dimethylbenzene
meta-xylene
m-xylene
ortho-xylene
o-xylene
para-xylene
xylol
PROTECTIVE CLOTHING AND EQUIPMENT
Protective clothing and equipment used by emergency responders should prevent the material from contacting the skin or eyes. Nitrile rubber gloves and boots, chemical spashproof goggles, and face shields that are impervious to and will not absorb xylene will suffice. Compatible materials may include polyvinyl alcohol, polyurethane, and Viton™. One reference adds Teflon ™ and polyethylene/ polyvinyl alcohol composite to the list. Consult manufacturers of protective clothing regarding compatibility. Respiratory protection in the form of positive-pressure, self-contained breathing apparatus (SCBA) is required.
FIRST AID
Inhalation. Move the victim to fresh air, keeping 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. If irritation persists after flushing, seek medical attention.
Skin contact. Wash the affected body areas with large amounts of soap and w ater. If irritation continues after washing, seek medical attention.
Ingestion. Do not induce vomiting. Keep the victim warm and calm. Seek immediate medical attention.
