CHEMICAL DATA NOTEBOOK SERIES #77: DICHLOROSILANE
HAZARDOUS MATERIALS
Dichlorosilane is a toxic, pyrophoric, flammable, corrosive, water-reactive, irritating gas with a very repulsive odor. The gas is colorless but forms a fuming white mist when exposed to air. Its major use is in the manufacture of electronic devices. It may be shipped in containers that have no safety relief devices; its usual shipping container is a cylinder with a maximum water capacity of 250 pounds. Its molecular formula is H2Cl sometimes written H or SiH Dichlorosilicane is a synonym.
PROPERTIES
Dichlorosilane has a flash point of — 35°F. While gases do not ordinarily have flash points, many resources list one for dichlorosilane because of its high boiling point of 46.8°F. Dichlorosilane is a liquid at temperatures below 46.8°F; under many conditions, therefore —including storage and transportation —dichlorosilane will be found in the liquid state. It has an ignition temperature of from 1,56°F to 212°F and an extremely wide flammable range of 4.1 to 98.8 percent in air. Ilie liquid has a specific gravity of 1.26, a vapor density of 3.48, and a molecular weight of 101. It freezes at – 187.6°F.
Dichlorosilane’s molecular structure is analogous to that of dichloromethane; one atom of silicon is substituted for the one atom of carbon. Methane (CH4) is an analog of silane (Sill.*). The reason for this is that carbon (C) and silicon (Si) have similar chemical properties; the answer lies in the position of both elements on the Periodic Table of the Elements. Elements within certain groups (columns or families) on the Periodic Table display remarkably similar chemical properties. But, while this is true for the elements, it cannot be said that the compounds have similar chemical properties. While both methane and dichlorosilane are gases and are flammable, the similarities between the two end there. Methane is a nontoxic, noncorrosive, odorless gas, and its only hazard is as a flammable gas. Dichlorosilane is a much more hazardous substance; its hazards are discussed below’.
HAZARDS
Once dichlorosilane has been released from its container, it reacts quickly with the moisture in the air, forming silics, silicon oxyhydride, and hydrogen chloride. Hydrogen chloride, in turn, dissolves in any moisture present to form hydrochloric acid. The hazards enumerated below are for dichlorosilane itself, unless hydrogen chloride and/or hydrochloric acid specifically are mentioned. Be aware, however, that dichlorosilane is very reactive and that it reacts rapidly with water.
Dichlorosilane is an extremely toxic gas. No TLV-TWA (threshold limit value-time weighted average) or STEL (short-term exposure limit) has been established for this gas because dichlorosilane reacts rapidly with the moisture in the air to form hydrogen chloride. The gas, nevertheless, is still deadly if released in quantities sufficient to remove all the moisture from the surrounding air or released in an atmosphere of low humidity. The formation of large quantities of hydrogen chloride is extremely dangerous (see “Hydrogen Chloride,” Fire Kngineering, February 1988). When released in large quantities, the dichlorosilane removes moisture from the air by reacting with it, and the additional dichlorosilane following it will remain in its original form until more moisture, with which it will react, becomes available. It, therefore, is possible to encounter high concentrations of dichlorosilane—with all the related hazards.
If dichlorosilane gas contacts the eyes, they will become severely and painfully irritated, as the gas will react with the water in the eyes to form hydrogen chloride, then heat, and finally hydrochloric acid. Prolonged contact with high concentrations of the gas or liquid will cause severe irritation, lachrymation, and possible irreversible damage to the eyes. If high concentrations of the gas or liquid contact the skin, tissue will be inflamed and possibly severely burned. The liquid may be absorbed through the skin and produce poisoning symptoms similar to those caused by ingesting the liquid.
Inhaling dichlorosilane gas will cause burning, chest pains, choking, labored breathing, pneumonia, pulmonary edema (possibly delayed), and death. The mouth, esophagus, and stomach will be burned if dichlorosilane is ingested. Symptoms include nausea, pain, and possibly death.
Because of its relatively high vapor density of 3.48, dichlorosilane “hangs together” for a long time (if not disturbed by a strong breeze or wind) and flows along low spots in the terrain. Wherever it gathers, in low spots or confined spaces, its toxicity (if it has not reacted with moisture and been converted to hydrogen chloride) will endanger the lives of anyone entering the atmosphere without adequate respiratory protection. If the dichlorosilane has converted to hydrogen chloride, the atmosphere still will be hazardous, since hydrogen chloride dissolves in any available moisture and forms hydrochloric acid.
Hydrogen chloride is designated as an upper respiratory irritant, but its high solubility in water (forming hydrochloric acid) causes it to dissolve in the moisture of the nose, mouth, and throat and result in having some of the material reach the lungs, producing a coughing reaction. If an unprotected individual remains in an atmosphere where the concentration of hydrogen chloride is high, enough of the gas may be inhaled to cause some lung damage. Some manufacturers of dichlorosilane, therefore, recommend using the absolute ceiling limit of hydrogen chloride (five parts per million of air) for safe exposure to dichlorosilane.
Dichlorosilane is very flammable. Since its listed flash point is — 35°F, any release of dichlorosilane will produce an explosive atmosphere at all ambient temperatures above — 35°F once the lower flammable limit of 4.1 percent is reached. Its flammable range is unusually wide, stretching from 4.1 to 98.8 percent in air. This means that it is virtually impossible to get a concentration of dichlorosilane in air above its upper flammable limit (too rich to burn) except inside its ow n container. However, if any air is present in the container when it is filled, it is possible to have dichlorosilane burn or detonate within the I container.
Some references list dichlorosilane as pyrophoric, but its listed autoignition temperature usually is given as 136°F. It also reacts instantly in the air to convert to hydrogen chloride, hydrochloric acid, and amorphous silica dust. However, enough cases of the spontaneous ignition of dichlorosilane on exposure to air have occurred to qualify spontaneous ignition as a legitimate hazard. What probably occurs is that the released dichlorosilane reacts instantly with the moisture in the air to form hydrogen chloride. This exothermic reaction releases enough heat energy to easily raise the temperature of the air around the reaction to more than 136°F. The constant reaction and buildup of heat are sufficient to ignite additional quantities of dichlorosilane so that it appears to ignite instantaneously in air, which is the accepted definition of a pyrophoric material. So even if chemical purists insist that dichlorosilane technically is not pyrophoric, its behavior to the contrary should warn you to treat it as such.
Dichlorosilane not only is incompatible with water, but it also oxidizes all types of agents. Dichlorosilane is very flammable in air, so the presence of any additional oxidizer could cause it to become explosive.
Dichlorosilane, with no water present, may not be considered a very corrosive material. Since, however, in almost every release of the material there will be moisture contacted in one form or another, hydrogen chloride (and, almost instantaneously, hydrochloric acid) will be formed. The formation of hydrochloric acid is the main corrosive hazard of dichlorosilane.
NONFIRE RELEASE
Any sizable release of dichlorosilane will trigger the community’s emergency response plan, as mandated by SARA (Superfund Amendments and Reauthorization Act of 1986). All emergency responders must conform to this plan and work as a team to bring the incident to a safe and successful conclusion. The presence of environmental and other experts will make the incident commander’s job a lot easier, as they can offer expert advice.
Consider any release of dichlorosilane extremely hazardous, and immediately consider evacuation. Evacuation distances may range from onehalf mile to two miles in diameter around the release site, depending on the amount released, the form of the material leaking (gas or liquid), and the conditions at the site of the release. Evacuation distances downwind always must be at least two to four times the diameter of the evacuation zone around the release of the material.
Emergency vehicles always must approach from uphill and upwind, and they should have plans for withdrawal should the wind suddenly shift. Eliminate all ignition sources immediately, and take all other precautions in handling a release of a flammable and toxic gas such as dichlorosilane. Do not attempt to plug the leak unless it can be done safely.
Any gas released from the container will form a visible vapor cloud as the dichlorosilane hydrolyzes in air and forms a mist of hydrogen chloride gas and condensed water vapor. The heat of the reaction may cause the water mist to evaporate quickly, so the vapor cloud may disappear a short distance from the leak. Do not assume that since the vapor cloud is gone, the danger is gone: An invisible cloud of a mixture of dichlorosilane and hydrogen chloride may be slowly drifting downwind and/or along low spots in the ground. Any accumulation of the flammable gas in low spots or confined areas could cause an explosion upon the introduction of any ignition source, or it could produce a toxic trap for anyone without respiratory protection.
Liquid released from a container will produce considerably more gas as the liquid boils away. Determining evacuation distances will be affected by whether the release is a liquid or gas. The high vapor density of dichlorosilane ensures that it will travel a great distance along low spots in the terrain, unless it is dispersed by a strong breeze or reacts with moisture along the way. If the latter occurs, the gas now will be hydrogen chloride. Exercise care around the release, since the gas can ignite at any time — due to the heat generated by any reaction occurring with moisture in the air.
Dichlorosilanc’s high water reactivity leads to an effective mitigation technique, assuming conditions are right. In the first case, a high-pressure water spray or fog can be used on gas leaking or being generated by a pool of liquid. Applying water will cause an instant reaction and convert the dichlorosilane to the less hazardous hydrogen chloride. Additional spray or fog will dissolve the hydrogen chloride from the air. Be careful to apply the spray or fog some distance from the leak so that heat generated by the chemical reaction will not ignite the dichlorosilane before it reacts with the water. And, of course, contain all runoff water.
If the amount of liquid released is relatively small, rapidly adding flooding amounts of water to the pool will dilute the dichlorosilane so that the generation of gas will be slowed considerably or stopped altogether. Do this quickly but with care. There is always the possibility of ignition, other violent reactions, or splashing of the dichlorosilane.
Prevent liquid dichlorosilane from entering sewers and waterways. Build containment dikes or dig containment pits to hold the liquid using sparkproof tools and equipment compatible with the product. The liquid may be boiling rapidly and may not be able to be salvaged, but containment ponds or pits will prevent the spread of contamination. Any seepage into the soil will necessitate the removal of the contaminated soil by professional cleanup firms under the supervision of environmental experts.
If the liquid enters a sewer, the worst-case scenario would be the explosion of dichlorosilane gas on ignition. If there is sufficient water in the sewer and no explosion occurs, the sewer lines will be filled with hydrogen chloride gas and a solution of hydrochloric acid. Warn all sewagetreatment facilities immediately.
If the liquid enters a waterway, an explosion is possible. If the waterway is a fast-moving river or stream, the dichlorosilane will be diluted and converted to hydrogen chloride and hydrochloric acid. Warn all downstream users of the water immediately. A neutralization agent, such as calcium carbonate (ground limestone), will react with the hydrochloric acid and liberate harmless carbon dioxide. If the liquid enters standing water, such as a pond or lake, it will produce highly contaminated water at the point of entry, in addition to posing the threat of ignition of the gas. This water also may be neutralized by adding calcium carbonate. No matter what waterway is affected, the environmental authorities must have the final word on when it is safe.
The salvage of the remaining liquefied dichlorosilane is unlikely due to its rapid evaporation, but if attempts are made, they must be carried out by a professional salvage firm using the proper equipment. Firefighters should not be involved in the salvage or cleanup of dichlorosilane.
Any material that has not evaporated, perhaps because of low temperatures, may be absorbed into inert materials such as cement powder, clay, diatomaceous earth, dry earth, fly ash, or dry sand. Dispose of all contaminated sorbents and contaminated earth in accordance with federal, state, and local laws.
FIRE SCENARIO
Since there may be no safety relief device on the dichlorosilane container. any heat that reaches it may cause the container to fail by overpressurization. This danger exists with any gas or liquid in a container, but most containers do have some sort of device to relieve the pressure. This means that the danger of BLEVE (boiling-liquid. expanding-vapor explosion) is even greater with dichlorosi: lane. Therefore, cool all containers in I danger of being heated by applying water with unmanned appliances from as far away as possible. Never get between the fire and containers of dichlorosilane or any other container that can fail catastrophically due to overpressurization.
If leaking dichlorosilane is burning, do not extinguish the fire unless the flow of dichlorosilane can be stopped quickly and safely. Dichlorosilane’s low ignition temperature means reignition after extinguishment is very likely. If it is decided to extinguish the fire, extinguish small fires by applying carbon dioxide or dry chemical. Larger fires may require water or foam. Water spray may be ineffective because of dichlorosilane’s water reactivity. Flooding amounts of water may be effective. Prevent water from contacting the area of the leak, since the corrosion action of hydrochloric acid might enlarge the opening.
There is no agreement as to what type of foam should be used, so regular, all-purpose, and alcohol foams may be effective. Apply each of these foams carefully; base your decision on the degree of effectiveness. Again, reignition is a constant danger.
Emergency responders must have proper respiratory protection against the combustion products of dichlorosilane, which might include chlorine in addition to hydrogen chloride. These combustion products, just as the vapors of dichlorosilane, will be carried downwind.
Dichlorosilane is an extremely hazardous substance, and it represents a particularly dangerous firefighting problem. Its hazards of toxicity, flammability, reactivity, and corrosiveness are only reduced slightly after extinguishment, when you will have toxic, reactive, and corrosive combustion products. Everyone with possible exposure to the products and its combustion products is at great risk.
PROTECTIVE CLOTHING AND EQUIPMENT
Choose protective clothing and equipment to avoid any contact of dichlorosilane with the eyes or skin. Rubber gloves, aprons, and boots may offer some skin protection; wear splashproof chemical goggles to protect the eyes. Use positive-pressure, self-contained breathing apparatus for respiratory protection.
IDENTIFICATION NUMBERS AND RATINGS
CAS
(Chemical Abstract Services)
4109-96-0
STCC
(Standard Transportation Commodity Code)
4920176
RTECS
(Registry of Toxic Effects of Chemical Substances)
UU3040000
UN/NA
(United Nations/North America)
2189
DOT
(U.S. Department of Transportation)
poison gas (formerly flammable gas)
NFPA 704 Rating
3-4-2
IMO
(International Maritime Organization)
2.3, poison gas
Dichlorosilane poses a special problem, since the pure material represents one type of chemical reactivity; contact with water presents the formation of a new problem; and combustion products represent a third problem. Individual manufacturers of dichlorosilane have recommended using only total encapsulating suits made of neoprene as protection against all three situations. Suits made from natural rubber, nitrile rubber, polyethylene, and polyvinyl chloride may protect the wearer from dichlorosilane alone. Contact suit manufacturers concerning their specific tests against dichlorosilane, and also consult dichlorosilane manufacturers for their recommendations.
FIRST AID
Inhalation. Move the victim to fresh air. 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 you to the material in the victim’s lungs and/ or vomit. Seek immediate medical attention. Monitor the patient for signs of delayed pulmonary edema.
Eye contact. Flush the eyes immediately for at least 1 5 minutes, lifting the eyelids occasionally. Immediate medical attention is needed, especially if redness and irritation persist
Skin contact. Wash the affected areas of the body with drenching amounts of water. Apply the water rapidly —and in large amounts —to prevent possible ignition of dichlorosilane. If irritation continues after washing, seek medical attention.
Ingestion. If the victim is conscious, make him/her drink large quantities of water immediately, but do not induce vomiting. Never try to make an unconscious person vomit or drink anything. Seek immediate medical attention.
