Accelerants play a major role in chemistry. Most chemical reactions can be hastened with an accelerant. Accelerants are catalysts which alter a chemical bond, speed up a chemical process, or bring organisms back to homeostasis. An accelerant can be any substance that can bond, mix, or disturb another substance and cause an increase in the speed of a natural, or artificial chemical process.
In fire protection, an accelerant is any substance or mixture that "accelerates" the development of fire. Accelerants are often used to commit arson, and some accelerants may cause an explosion. Some fire investigators use the term "accelerant" to mean any substance that initiates and promotes a fire without implying intent or malice.
A fire is a self sustaining, exothermic oxidation reaction that emits heat and light. When a fire is accelerated, it can produce more heat, consume the reactants more quickly, burn at a higher temperature, and increase the spread of the fire. An accelerated fire is said to have a higher "heat release rate," meaning it burns more quickly.
Indicators of an incendiary fire or arson can lead fire investigators to look for the presence of accelerants in fire debris. Accelerants can leave behind evidence of their presence and use. Accelerants present in areas they should not be can indicate an incendiary fire or arson. Investigators often use special dogs known as accelerant detection canines trained to smell ignitable liquids. The dog can pinpoint areas for the investigator to collect samples. Fire debris submitted to forensic laboratories employ sensitive analytical instruments with GC-MS capabilities for forensic chemical analysis.
Types of accelerantsEdit
Many accelerants are hydrocarbon-based fuels, sometimes referred to as petroleum distillates: gasoline, diesel fuel, kerosene, turpentine, butane, and various other flammable solvents. These accelerants are also known as ignitable liquids. Ignitable liquids can leave behind tell-tale marks in the fire debris. These irregular burn patterns can indicate the presence of an ignitable liquid in a fire.
The properties of some ignitable liquids make them dangerous accelerants. Many ignitable liquids have high vapor pressures, low flash points and a relatively wide range between their upper and lower explosive limit. This allows ignitable liquids to ignite easily, and when mixed in a proper air-fuel ratio, readily explode. Many arsonists who use generous amounts of gasoline have been seriously burned or killed igniting their fire.
Common household items and objects can accelerate a fire. Wicker and foam have high surface to mass ratios and favorable chemical compositions and thus burn easily and readily. Arsonists who use large amounts of available combustible material rather than ignitable liquids try to avoid detection. Using large fuel loads can increase the rate of fire growth as well as spread the fire over a larger area, thus increasing the amount of fire damage. Inappropriate amounts and types of fuel in a particular area can indicate arson. Whether available combustible materials constitute an accelerant depends on the intent of the person responsible for their use.
Sales of any accelerant are limited to the particular group allowed to purchase them for trainings and fire demolitions (to train new firefighters).
Accelerators for rubber vulcanizationEdit
There are many accelerators available for the vulcanization of rubber. That is because there is a wide range of rubber articles on the market with a wide variety of properties. For instance in a car tire alone there can be already up to eight different rubber compounds, each with specific properties. For instance the tread in a typical passenger car tire consists of a mixture of SBR (styrene-butadiene rubber) and BR (butadiene rubber). This rubber should have high abrasion resistance and high grip on both dry and wet roads. The side wall of the tire should have a high flexibility, that means that it should resist many flexings during the running of the tire, without cracking. It consists normally of a mixture of natural rubber and butadiene rubber. Inside the tire there is a rubber compound with as major function the adhesion between rubber and the steel cord of the belt. It typically consists of natural rubber with a very high sulfur level (up to 8 phr), to get a relatively stiff rubber, with sulfur promoting the adhesion with the steel cord. The basis of the tire is formed by the carcass, normally a mixture of NR (natural rubber), SBR and BR. It should have a very good adhesion to the polyester cord, used as reinforcement. And the inner side of the tire is formed by the inner liner, normally consisting of halogenated butyl rubber (IIR) For all these compounds with their different properties different accelerators and mixtures of accelerators have to be used to obtain the required properties. A vulcanization accelerator is typically used in combination with sulfur as the cross-linker, and with zinc oxide and stearic acid as activators. Other additives can be added too, but for the cross-linking reaction the abovementioned ones are the most important. The various types of rubber used in the various tire compounds all have different vulcanization characteristics, like speed of cure (cure is the crosslinking reaction) and extent of cure (the number of cross-links). A typical passenger car tire is vulcanized for 10 minutes at 170 degrees C. This means that all the different compounds have to be cured to their optimum state of cure in this same 10 minutes. This is the reason why a lot of different accelerators or mixtures thereof are used in the same tire.
Classification of acceleratorsEdit
There are two major classes of vulcanization accelerators, primary accelerators and secondary accelerators or ultra accelerators.
Of the primary accelerators the major group used in tire manufacture is formed by sulfenamides[].These are produced by an oxidative coupling reaction of mercaptobenzthiazole[] (MBT) with a primary amine like cyclohexylamine or tertiary butyl amine. Secondary amines like di-cyclohexyl-amine[] can be used also but result in much slower accelerators. Such a slow accelerator is rquired in the steel cord adhesion compound mentioned above, because for optimal adhesion a slow cure is required. Another important group of primary accelerators is formed by the thiazoles. The two main products are mercaptobenzthiazole (MBT) and mercaptobenzthiazole disulfide (MBTS), a product formed by oxidative coupling of two MBT molecules). The thiazoles are used for the vulcanization of thick articles, and as basic accelerator in EPDM compounds (ethylene-propylene-diene rubers), in combination with mixtures of ultra-accelerators.
Of the secondary or ultra-accelerators the main categories are the thiurams and the dithiocarbamates. In vulcanozation of tire compounds they are used as small addition to sulfenamides to boost the speed and state of cure. They have a very vulcanization speed and therefore, next to boosters in tire compounds they are used as main accelerator in EPDM compounds and in latex compounds. EPDM compounds have much less cure sites than natural rubber or SBR, and therefore need a rapid vulcanization system to have sufficient cure speed. Latex is cured at relatively low temperature (100- 120 degrees C)and therefore need an inherently rapid accelerator. The major thiurams used are TMTD (tetramethylthiuramdisulfide) and TETD( tetraethylthiuramdisulfide[]), They are produced by the reaction between dimethyl amine ar diethylamine and carbondisulfide. The major dithiocarbamates[] are the zinc salts ZDEC (zinc diethyldithiocarbamate) and ZDBC (zinc dibutyldithiocarbamate).