Accelerant
Encyclopedia
Accelerants play a major role in chemistry. Most chemical reactions can be hastened with an accelerant. Accelerants alter a chemical bond, speed up a chemical process, or bring organisms back to homeostasis. Accelerants are not necessarily catalysts as they may be consumed by the process. 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.
, the term accelerant is used in a very broad, if not incorrect usage that very generally includes any substance or mixture that "accelerates" the development of fire
, not necessarily a fuel. Accelerants are often said to be used to commit arson
. Some fire investigators mistakenly use the term "accelerant" to mean any substance that initiates and promotes a fire without differentiating between an accelerant and a fuel. The terms are not, in the truest sense of chemical science, interchangeable. To a chemical engineer, "gasoline" is not at all considered an "accelerant", it is more accurately considered a "fuel", but usage by laymen is considered popularized if incorrect.
A fire is a self sustaining, exothermic
oxidation reaction that emits heat and light. When a fire is accelerated with a true accelerant like oxygen bearing liquids and gases (Like NO2) it can produce more heat, consume the actual fuels more quickly, and increase the spread of the fire. Fires involving liquid accelerants, like gasoline, burn more quickly, but at the same temperature as fires involving ordinary fuels.
erroneously known as "accelerant detection canines" trained to smell ignitable liquids. Well-trained dogs 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
.
-based fuel
s, sometimes more realistically referred to as petroleum
distillates
: gasoline
, diesel fuel, kerosene
, turpentine
, butane
, and various other flammable solvent
s. These accelerants are also known as ignitable liquids. Ignitable liquids can leave behind irregular patterns on the surface of a floor. These irregular burn patterns can indicate the presence of an ignitable liquid in a fire and can indicate the point of origin of the fire. Note, however, that irregular patterns may be found in fires involving no accelerant. This is particularly true in cases of full room involvement.
The properties of some ignitable liquids make them dangerous fuels. Many ignitable liquids have high vapor pressure
s, low flash point
s 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.
In chemistry, true "accelerants" are compounds or gases that promote the fuel, not the fuel itself.
. Whether available combustible materials constitute an accelerant depends on the intent of the person responsible for their use.
reaction to 80-125kJ/Mole from 210kJ/Mole which is necessary if we use ‘Sulphur’ alone. Accelerators and activators break sulphur chains. Accelerated sulphur vulcanization systems require only 5-15 sulphur atoms per cross-link as compared to 40-45 S atoms/crosslink for a non-accelerated sulphur vulcanization.
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, meaning 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.
like cyclohexylamine
or tert-Butylamine
.
Secondary amines like di-cyclohexyl-amine http://web2.cas.usf.edu/garey/BIO%20Department%20MSDS/D/Dicyclohexylamine.pdf can be used also but result in much slower accelerators. Such a slow accelerator is required 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 rubbers
), in combination with mixtures of ultra-accelerators.
In the vulcanization of neoprene
or polychloroprene rubber (CR rubber) the choice of accelerator is governed by different rules to other diene rubbers. Most conventionally used accelerators are problematic when CR rubbers are cured and the most important accelerator has been found to be ethylene thiourea (ETU) which, although being an excellent and proven accelerator for polychloroprene, has been classified as reprotoxic. The European rubber industry has started a research project SafeRubber to develop a safer alternative to the use of ETU.
s. In vulcanization of tire compounds they are used as small addition to sulfenamides to boost the speed and state of cure.
They have a very fast 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 °C)and therefore need an inherently rapid accelerator.
The major thiurams used are TMTD (tetramethylthiuram disulfide) and TETD(tetraethylthiuram disulfide http://potency.berkeley.edu/chempages/TETRAETHYLTHIURAM%20DISULFIDE.html), They are produced by the reaction between dimethylamine
or diethylamine
and carbon disulfide
.
The major dithiocarbamates http://web.due.uci.edu/urop/symp/2005_spring/38601_version3.doc are the zinc salts ZDEC (zinc diethyldithiocarbamate) and ZDBC (zinc dibutyldithiocarbamate).
Fire "accelerant"
In fire protectionFire protection
Fire protection is the study and practice of mitigating the unwanted effects of fires. It involves the study of the behaviour, compartmentalisation, suppression and investigation of fire and its related emergencies, as well as the research and development, production, testing and application of...
, the term accelerant is used in a very broad, if not incorrect usage that very generally includes any substance or mixture that "accelerates" the development of fire
Fire
Fire is the rapid oxidation of a material in the chemical process of combustion, releasing heat, light, and various reaction products. Slower oxidative processes like rusting or digestion are not included by this definition....
, not necessarily a fuel. Accelerants are often said to be used to commit arson
Arson
Arson is the crime of intentionally or maliciously setting fire to structures or wildland areas. It may be distinguished from other causes such as spontaneous combustion and natural wildfires...
. Some fire investigators mistakenly use the term "accelerant" to mean any substance that initiates and promotes a fire without differentiating between an accelerant and a fuel. The terms are not, in the truest sense of chemical science, interchangeable. To a chemical engineer, "gasoline" is not at all considered an "accelerant", it is more accurately considered a "fuel", but usage by laymen is considered popularized if incorrect.
A fire is a self sustaining, exothermic
Exothermic reaction
An exothermic reaction is a chemical reaction that releases energy in the form of light or heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation:-Overview:...
oxidation reaction that emits heat and light. When a fire is accelerated with a true accelerant like oxygen bearing liquids and gases (Like NO2) it can produce more heat, consume the actual fuels more quickly, and increase the spread of the fire. Fires involving liquid accelerants, like gasoline, burn more quickly, but at the same temperature as fires involving ordinary fuels.
Fire investigation
Indicators of an incendiary fire or arson can lead fire investigators to look for the presence of fuel traces in fire debris. Burning compounds and liquids can leave behind evidence of their presence and use. Fuels present in areas they aren't typically found be can indicate an incendiary fire or arson. Investigators often use special dogsPolice dog
A police dog, often referred to as a "K-9 dog" in some areas , is a dog that is trained specifically to assist police and other law-enforcement personnel in their work...
erroneously known as "accelerant detection canines" trained to smell ignitable liquids. Well-trained dogs can pinpoint areas for the investigator to collect samples. Fire debris submitted to forensic laboratories employ sensitive analytical instruments with GC-MS
Gas chromatography-mass spectrometry
Gas chromatography–mass spectrometry is a method that combines the features of gas-liquid chromatography and mass spectrometry to identify different substances within a test sample. Applications of GC-MS include drug detection, fire investigation, environmental analysis, explosives investigation,...
capabilities for forensic chemical analysis
Forensic chemistry
Forensic chemistry is the application of chemistry to law enforcement or the failure of products or processes. Many different analytical methods may be used to reveal what chemical changes occurred during an incident, and so help reconstruct the sequence of events...
.
Types of accelerants
Many so-called accelerants are hydrocarbonHydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
-based fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...
s, sometimes more realistically referred to as petroleum
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
distillates
Distillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
: gasoline
Gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...
, diesel fuel, kerosene
Kerosene
Kerosene, sometimes spelled kerosine in scientific and industrial usage, also known as paraffin or paraffin oil in the United Kingdom, Hong Kong, Ireland and South Africa, is a combustible hydrocarbon liquid. The name is derived from Greek keros...
, turpentine
Turpentine
Turpentine is a fluid obtained by the distillation of resin obtained from trees, mainly pine trees. It is composed of terpenes, mainly the monoterpenes alpha-pinene and beta-pinene...
, butane
Butane
Butane is a gas with the formula C4H10 that is an alkane with four carbon atoms. The term may refer to any of two structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, butane refers only to the unbranched n-butane isomer; the other one being called "methylpropane" or...
, and various other flammable solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
s. These accelerants are also known as ignitable liquids. Ignitable liquids can leave behind irregular patterns on the surface of a floor. These irregular burn patterns can indicate the presence of an ignitable liquid in a fire and can indicate the point of origin of the fire. Note, however, that irregular patterns may be found in fires involving no accelerant. This is particularly true in cases of full room involvement.
The properties of some ignitable liquids make them dangerous fuels. Many ignitable liquids have high vapor pressure
Vapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...
s, low flash point
Flash point
The flash point of a volatile material is the lowest temperature at which it can vaporize to form an ignitable mixture in air. Measuring a flash point requires an ignition source...
s and a relatively wide range between their upper and lower explosive limit. This allows ignitable liquids to ignite
Ignite
Ignite is a program for gifted and talented young people in South Australia, created in 1998 and formerly known as SHIP . It is administered by the South Australian Department of Education and Children's Services....
easily, and when mixed in a proper air-fuel ratio
Air-fuel ratio
Air–fuel ratio is the mass ratio of air to fuel present in an internal combustion engine. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture, often abbreviated to stoich...
, readily explode. Many arsonists who use generous amounts of gasoline have been seriously burned or killed igniting their fire.
In chemistry, true "accelerants" are compounds or gases that promote the fuel, not the fuel itself.
Available combustibles
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 sometimes 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 arsonArson
Arson is the crime of intentionally or maliciously setting fire to structures or wildland areas. It may be distinguished from other causes such as spontaneous combustion and natural wildfires...
. Whether available combustible materials constitute an accelerant depends on the intent of the person responsible for their use.
Accelerators for rubber vulcanization
The use of accelerators and activators lowers the activation energy of vulcanizationVulcanization
Vulcanization or vulcanisation is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulfur or other equivalent "curatives." These additives modify the polymer by forming crosslinks between individual polymer chains. Vulcanized material is...
reaction to 80-125kJ/Mole from 210kJ/Mole which is necessary if we use ‘Sulphur’ alone. Accelerators and activators break sulphur chains. Accelerated sulphur vulcanization systems require only 5-15 sulphur atoms per cross-link as compared to 40-45 S atoms/crosslink for a non-accelerated sulphur vulcanization.
There are many accelerators available for the vulcanization
Vulcanization
Vulcanization or vulcanisation is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulfur or other equivalent "curatives." These additives modify the polymer by forming crosslinks between individual polymer chains. Vulcanized material is...
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
Styrene-butadiene
Styrene-Butadiene or Styrene-Butadiene-Rubber is a synthetic rubber copolymer consisting of styrene and butadiene. It has good abrasion resistance and good aging stability when protected by additives, and is widely used in car tires, where it may be blended with natural rubber...
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, meaning 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
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
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
Adhesion
Adhesion is any attraction process between dissimilar molecular species that can potentially bring them in close contact. By contrast, cohesion takes place between similar molecules....
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
Butyl
In organic chemistry, butyl is a four-carbon alkyl radical or substituent group with general chemical formula -C4H9, derived from either of the two isomers of butane....
rubber (IIR
IIR
IIR may refer to* IIR Holdings, a human capital development company acquired by Informa* Indo-Iranian languages* Infinite impulse response...
)
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
Zinc oxide
Zinc oxide is an inorganic compound with the formula ZnO. It is a white powder that is insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber , lubricants, paints, ointments, adhesives, sealants,...
and stearic acid
Stearic acid
Stearic acid is the saturated fatty acid with an 18 carbon chain and has the IUPAC name octadecanoic acid. It is a waxy solid, and its chemical formula is CH316CO2H. Its name comes from the Greek word στέαρ "stéatos", which means tallow. The salts and esters of stearic acid are called stearates...
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
Vulcanization
Vulcanization or vulcanisation is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulfur or other equivalent "curatives." These additives modify the polymer by forming crosslinks between individual polymer chains. Vulcanized material is...
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 accelerators
There are two major classes of vulcanization accelerators, primary accelerators and secondary accelerators or ultra accelerators.Primary accelerators
Of the primary accelerators the major group used in tire manufacture is formed by sulfenamides http://www.iop.org/EJ/abstract/0036-021X/65/5/R02.These are produced by an oxidative coupling reaction of mercapto-benzthiazole http://palimpsest.stanford.edu/don/dt/dt2213.html (otherwise called mercaptobenzothiazole) (MBT) with a primary amineAmine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
like cyclohexylamine
Cyclohexylamine
Cyclohexylamine, also called hexahydroaniline, 1-aminocyclohexane, or aminohexahydrobenzene, is an organic chemical, an amine derived from cyclohexane. It is a clear to yellowish liquid with fishy odor, with melting point of −17.7 °C and boiling point 134.5 °C, miscible with water...
or tert-Butylamine
Tert-Butylamine
tert-Butylamine is an organic chemical compound with the formula 3CNH2, and occurs as a colorless liquid. tert-Butylamine is one of the four isomeric amines of butane, the others being n-butylamine, sec-butylamine and isobutylamine.-Preparation:tert-Butylamine is commercially available...
.
Secondary amines like di-cyclohexyl-amine http://web2.cas.usf.edu/garey/BIO%20Department%20MSDS/D/Dicyclohexylamine.pdf can be used also but result in much slower accelerators. Such a slow accelerator is required 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 rubbers
EPDM rubber
EPDM rubber , a type of synthetic rubber, is an elastomer which is characterized by a wide range of applications. The E refers to ethylene, P to propylene, D to diene and M refers to its classification in ASTM standard D-1418. The M class includes rubbers having a saturated chain of the...
), in combination with mixtures of ultra-accelerators.
In the vulcanization of neoprene
Neoprene
Neoprene or polychloroprene is a family of synthetic rubbers that are produced by polymerization of chloroprene. Neoprene in general has good chemical stability, and maintains flexibility over a wide temperature range...
or polychloroprene rubber (CR rubber) the choice of accelerator is governed by different rules to other diene rubbers. Most conventionally used accelerators are problematic when CR rubbers are cured and the most important accelerator has been found to be ethylene thiourea (ETU) which, although being an excellent and proven accelerator for polychloroprene, has been classified as reprotoxic. The European rubber industry has started a research project SafeRubber to develop a safer alternative to the use of ETU.
Secondary accelerators
Of the secondary or ultra-accelerators the main categories are the thiurams and the dithiocarbamateDithiocarbamate
A dithiocarbamate is a functional group in organic chemistry. It is the analog of a carbamate in which both oxygen atoms are replaced by sulfur atoms. Sodium diethyldithiocarbamate is a common ligand in inorganic chemistry....
s. In vulcanization of tire compounds they are used as small addition to sulfenamides to boost the speed and state of cure.
They have a very fast 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 °C)and therefore need an inherently rapid accelerator.
The major thiurams used are TMTD (tetramethylthiuram disulfide) and TETD(tetraethylthiuram disulfide http://potency.berkeley.edu/chempages/TETRAETHYLTHIURAM%20DISULFIDE.html), They are produced by the reaction between dimethylamine
Dimethylamine
Dimethylamine is an organic compound with the formula 2NH. This secondary amine is a colorless, flammable liquified gas with an ammonia-like odor. Dimethylamine is generally encountered as a solution in water at concentrations up to around 40%...
or diethylamine
Diethylamine
Diethylamine is a secondary amine with the molecular structure CH3CH2NHCH2CH3. It is a flammable, strongly alkaline liquid. It is miscible with water and ethanol. It is a colorless liquid which often appears brown due to impurities...
and carbon disulfide
Carbon disulfide
Carbon disulfide is a colorless volatile liquid with the formula CS2. The compound is used frequently as a building block in organic chemistry as well as an industrial and chemical non-polar solvent...
.
The major dithiocarbamates http://web.due.uci.edu/urop/symp/2005_spring/38601_version3.doc are the zinc salts ZDEC (zinc diethyldithiocarbamate) and ZDBC (zinc dibutyldithiocarbamate).