An organic peroxide is any organic (carbon-containing) compound having two oxygen atoms joined together (-O-O-). This chemical group is called a "peroxy" group. Organic peroxides can be severe fire and explosion hazards. This question-and-answer document summarizes these and other hazards; another document provides information on how to work safely with organic peroxides.

The plastics and rubber industries are the heaviest users of organic peroxides. Organic peroxides and mixtures containing an organic peroxide are used as accelerators, activators, catalysts, cross-linking agents, curing agents, hardeners, initiators and promoters. Organic peroxides and mixtures containing an organic peroxide are often referred to by these terms. However, using terms like accelerator, activator, etc. to mean "organic peroxide" can be misleading since they can also refer to materials that do not contain organic peroxides. This can cause confusion and a serious accident could result if these substances were mixed with organic peroxides.

Organic peroxides are available as solids (usually fine powders), liquids or pastes. Some materials, such as water, odourless mineral spirits, and some phthalate esters do not react with organic peroxides and are often used to dilute them. The diluted mixtures or formulations are less likely to explode when exposed to heat or physical shock than the undiluted organic peroxide. Dilution makes the unstable peroxides safer to produce, handle, and use. We use the term "organic peroxide" to refer to both undiluted and diluted organic peroxides, unless otherwise specified. Check the supplier labels on chemical product containers.

The Canadian WHMIS (Workplace Hazardous Materials Information System) classifies organic peroxides as oxidizing materials. WHMIS also classifies many other materials that are not organic peroxides as oxidizing materials. Other hazard symbols may also be present, depending on the particular material.

It is wise to treat any unknown material as very hazardous until it is positively identified.

The main hazard related to organic peroxides are their fire and explosion hazards. Organic peroxides may also be toxic or corrosive. Depending on the material, route of exposure (inhalation, eye or skin contact, or swallowing) and dose or amount of exposure, they could harm the body. Corrosive organic peroxides can also attack and destroy metals.

It is the double oxygen of the "peroxy" group that makes organic peroxides both useful and hazardous. The peroxy group is chemically unstable. It can easily decompose, giving off heat at a rate that increases as the temperature rises. Many organic peroxides give off flammable vapours when they decompose. These vapours can easily catch fire.

Most undiluted organic peroxides can catch fire easily and burn very rapidly and intensely. This is because they combine both fuel (carbon) and oxygen in the same compound. Some organic peroxides are dangerously reactive. They can decompose very rapidly or explosively if they are exposed to only slight heat, friction, mechanical shock or contamination with incompatible materials.

Organic peroxides can also be strong oxidizing agents. Combustible materials contaminated with most organic peroxides can catch fire very easily and burn very intensely (i.e., deflagrate). This means that the burn rate is very fast: it can vary from 1 m/sec to hundreds of metres per second. Also the combustion rate increases as the pressure increases and the combustion (or reaction) zone can travel through air or a gaseous medium faster than the speed of sound. However, the speed of combustion in a solid medium does not exceed the speed of sound.

This is one characteristic that distinguishes deflagration from detonation. We mention these two terms because they are used in classifying organic peroxide formulations (see next question). Deflagrations and detonations are similar chemical reactions except that in detonations the burn rate in a solid medium is faster than the speed of sound. This supersonic speed results in a shock wave being produced. They can transmit the shock wave at speeds of about 2,000 to 9,000 m/sec and is not dependent on the surrounding pressure. This is another difference between detonation and deflagration: deflagration rates increase as the pressure becomes greater.

Explosive decomposition is a rapid chemical reaction resulting in almost instantaneous release of energy. This term includes both deflagration and detonation.

Organic peroxides may also have a self accelerating decomposition temperature (SADT). SADT represents the lowest temperature in which that particular organic peroxide formulation in its commercial packaging will undergo self-accelerating decomposition (begin the chemical process that leads to explosion). The SADT value will vary with each organic peroxide formulation and the size and shape of its packaging. Storage requirements will generally be 10 to 20 degress below the SADT.

The MSDSs and the container labels should explain all of the hazards of the organic peroxides with which you work. For example, the MSDS should describe the conditions that cause an organic peroxide formulation to undergo dangerous chemical reactions and cause explosions. They also should state if there are any special storage requirements. Some should be stored in a refrigerator to reduce the possibility of fire and the refrigerator should meet the electrical code requirements for the products being stored (e.g., should be "explosion-proof"). The MSDS should say if there is a minimum temperature under which the product should not be stored. If the temperature is too low, crystals of the peroxide may form. These crystals could be an explosion hazard since they can be very sensitive to shock.

An example of an organic peroxide is methyl ethyl ketone peroxide (also known as 2-butanone peroxide, ethyl methyl ketone peroxide, or MEKP). It is used as a polymerization catalyst in the manufacture of polyester and acrylic resins and as a hardening agent for fiberglass reinforced plastics. It is a colourless liquid with a characteristic odour. It is considered a combustible liquid and vapour. There is an extreme risk of an explosion from exposure to shock, friction, flame, or other sources of ignition. It is dangerously reactive and may decompose violently. Contact with water or moist air liberates irritating gases. Contents may develop pressure if exposed to water. It is also very toxic. It may be fatal if inhaled, absorbed through the skin or swallowed and it is corrosive to the eyes, skin and respiratory tract. It may cause lung injury although the effects may be delayed.

Yes, some chemicals can form explosive peroxides when they are stored (e.g., isopropyl ether, vinylidene chloride). Exposure to light and heat can increase the rate of peroxide formation. Others form peroxides that become hazardous when concentrated (e.g., by distillation). Some examples include ethyl ether, tetrahydrofuran (THF), p-dioxane, some secondary alcohols like 2-propanol and 2-butanol, and some unsaturated hydrocarbons like propyne (an acetylene compound), cyclohexene, and tetra-and deca-hydronaphthalenes.

Another kind of peroxide-forming compound are unsaturated monomers that, in the presence of a peroxide, can polymerize exothermically (i.e., produces heat when it reacts). For example, uninhibited styrene can form a peroxide that can cause the styrene to polymerize. It can occur explosively under certain conditions. Other examples of some unsaturated monomeric compounds are acrylic acid, acrylonitrile, butadiene, methyl methacrylate, and vinyl chloride.

To generalize, the kinds of chemicals that can form peroxides include aldehydes, ethers, and numerous unsaturated hydrocarbon compounds (i.e. hydrocarbon compounds having double or triple bonds). Examples in this group include allyl compounds, haloalkenes, dienes, monomeric vinyl compounds, vinylacetylenes, unsaturated cyclic hydrocarbons like tetrahydronaphthalene or dicyclopentadiene.

This is not a comprehensive list. The intention of mentioning these examples is to point out the importance of learning about the hazards of the chemicals you handle by reading the MSDSs and any relevant safety bulletins that the chemical producers provide. They should recommend how frequently they should be tested for the presence of peroxides (e.g., once every 3 months for diisopropyl ether or vinylidene chloride; once every 12 months for ethyl ether). Follow their directions regarding the safe disposal or repurifications procedures (if recommended).

So if, for example, you see crystals inside a bottle of a "pure" ether, suspect the presence of an ether peroxide. Do not handle the container. Do call your emergency response group. They should contact the local bomb squad after assessing the situation. Ether peroxides, like other peroxides, are very sensitive to shock and could explode if handled improperly - just like a bomb.

The U.S. National Fire Protection Association (NFPA) has developed a hazard classification system for typical organic peroxide formulations. The NFPA classification system describes the fire and explosion hazards of these formulations in their normal shipping and storage containers that have been approved by the Transport Canada or the U.S. Department of Transport (DOT).

If a formulation is transferred to a different container, the given hazard classification may no longer apply. See the NFPA 432 "Code for the Storage of Organic Peroxide Formulations" (2002) for details. In general:

• Class I formulations are capable of deflagration but not detonation.
• Class II formulations burn very rapidly and are a severe reactivity hazard.
• Class III formulations burn rapidly and have a moderate ractivity hazard.
• Class IV formulations burn in the same manner as ordinary combustibles and have a minimal reactivity hazard.
• Class V formulations burn with less intensity than ordinary combustibles or they do not support combustion and present no reactivity hazard.