Nitrile rubber known as NBR, Buna-N, acrylonitrile butadiene rubber, is a synthetic rubber copolymer of acrylonitrile and butadiene. Trade names include Perbunan, Nipol and Europrene. Nitrile butadiene rubber is a family of unsaturated copolymers of 2-propenenitrile and various butadiene monomers. Although its physical and chemical properties vary depending on the polymer’s composition of nitrile, this form of synthetic rubber is unusual in being resistant to oil and other chemicals, it is used in the automotive and aeronautical industry to make fuel and oil handling hoses, seals and self-sealing fuel tanks, since ordinary rubbers cannot be used. It is used in the nuclear industry to make protective gloves. NBR's ability to withstand a range of temperatures from −40 to 108 °C makes it an ideal material for aeronautical applications. Nitrile butadiene is used to create moulded goods, adhesives, sponges, expanded foams, floor mats, its resilience makes NBR a useful material for disposable lab and examination gloves.
Nitrile rubber is more resistant than natural rubber to oils and acids, has superior strength, but has inferior flexibility. Nitrile gloves are therefore more puncture-resistant than natural rubber gloves if the latter are degraded by exposure to chemicals or ozone. Nitrile rubber is less to cause an allergic reaction than natural rubber. Nitrile rubber is resistant to aliphatic hydrocarbons. Nitrile, like natural rubber, can be attacked by ozone, ketones and aldehydes. Emulsifier, 2-propenenitrile, various butadiene monomers, radical generating activators, a catalyst are added to polymerization vessels in the production of hot NBR. Water serves as the reaction medium within the vessel; the tanks are heated to 30–40 °C to facilitate the polymerization reaction and to promote branch formation in the polymer. Because several monomers capable of propagating the reaction are involved in the production of nitrile rubber the composition of each polymer can vary. One repeating unit found throughout the entire polymer may not exist.
For this reason there is no IUPAC name for the general polymer. The reaction for one possible portion of the polymer is shown below: 1,3-butadiene + 1,3-butadiene + 2-propenenitrile + 1,3-butadiene + 1,2-butadiene → nitrile butadiene rubberMonomers are permitted to react for 5 to 12 hours. Polymerization is allowed to proceed to ~70% conversion before a “shortstop” agent is added to react with the remaining free radicals. Once the resultant latex has “shortstopped”, the unreacted monomers are removed through a steam in a slurry stripper. Recovery of unreacted monomers is close to 100%. After monomer recovery, latex is sent through a series of filters to remove unwanted solids and sent to the blending tanks where it is stabilized with an antioxidant; the yielded polymer latex is coagulated using calcium nitrate, aluminium sulfate, other coagulating agents in an aluminium tank. The coagulated substance is washed and dried into crumb rubber; the process for the production of cold NBR is similar to that of hot NBR.
Polymerization tanks are heated to 5–15 °C instead of 30–40 °C. Under lower temperature conditions, less branching will form on polymers; the raw material is yellow, though it can be red tinted, depending on the manufacturer. Its elongation at break is ≥ 300% and possesses a tensile strength of ≥ 10 N/mm2. NBR has good resistance to mineral oils, vegetable oils, benzene/petrol, ordinary diluted acids and alkalines. An important factor in the properties of NBR is the ratio of acrylonitrile groups to butadiene groups in the polymer backbone, referred to as the ACN content; the lower the ACN content, the lower the glass transition temperature. Most applications requiring both solvent resistance and low temperature flexibility require an ACN content of 33%; the uses of nitrile rubber include disposable non-latex gloves, automotive transmission belts, hoses, O-rings, oil seals, V belts, synthetic leather, printer's form rollers, as cable jacketing. Unlike polymers meant for ingestion, where small inconsistencies in chemical composition/structure can have a pronounced effect on the body, the general properties of NBR are not altered by minor structural/compositional differences.
The production process. The necessary apparatus is easy to obtain. For these reasons, the substance is produced in poorer countries where labor is cheap. Among the highest producers of NBR are mainland Taiwan. In January 2008, the European Commission imposed fines totaling €34,230,000 on the Bayer and Zeon groups for fixing prices for nitrile butadiene rubber, in violation of the EU ban on cartels and restrictive business practices. Nitric acid penetrates nitrile gloves in a few minutes. Hydrogenated nitrile butadiene rubber k
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity can be conceptualized as quantifying the frictional force that arises between adjacent layers of fluid that are in relative motion. For instance, when a fluid is forced through a tube, it flows more near the tube's axis than near its walls. In such a case, experiments show; this is because a force is required to overcome the friction between the layers of the fluid which are in relative motion: the strength of this force is proportional to the viscosity. A fluid that has no resistance to shear stress is known as an inviscid fluid. Zero viscosity is observed only at low temperatures in superfluids. Otherwise, the second law of thermodynamics requires all fluids to have positive viscosity. A fluid with a high viscosity, such as pitch, may appear to be a solid; the word "viscosity" is derived from the Latin "viscum", meaning mistletoe and a viscous glue made from mistletoe berries.
In materials science and engineering, one is interested in understanding the forces, or stresses, involved in the deformation of a material. For instance, if the material were a simple spring, the answer would be given by Hooke's law, which says that the force experienced by a spring is proportional to the distance displaced from equilibrium. Stresses which can be attributed to the deformation of a material from some rest state are called elastic stresses. In other materials, stresses are present which can be attributed to the rate of change of the deformation over time; these are called. For instance, in a fluid such as water the stresses which arise from shearing the fluid do not depend on the distance the fluid has been sheared. Viscosity is the material property which relates the viscous stresses in a material to the rate of change of a deformation. Although it applies to general flows, it is easy to visualize and define in a simple shearing flow, such as a planar Couette flow. In the Couette flow, a fluid is trapped between two infinitely large plates, one fixed and one in parallel motion at constant speed u.
If the speed of the top plate is low enough in steady state the fluid particles move parallel to it, their speed varies from 0 at the bottom to u at the top. Each layer of fluid moves faster than the one just below it, friction between them gives rise to a force resisting their relative motion. In particular, the fluid applies on the top plate a force in the direction opposite to its motion, an equal but opposite force on the bottom plate. An external force is therefore required in order to keep the top plate moving at constant speed. In many fluids, the flow velocity is observed to vary linearly from zero at the bottom to u at the top. Moreover, the magnitude F of the force acting on the top plate is found to be proportional to the speed u and the area A of each plate, inversely proportional to their separation y: F = μ A u y; the proportionality factor μ is the viscosity of the fluid, with units of Pa ⋅ s. The ratio u / y is called the rate of shear deformation or shear velocity, is the derivative of the fluid speed in the direction perpendicular to the plates.
If the velocity does not vary linearly with y the appropriate generalization is τ = μ ∂ u ∂ y, where τ = F / A, ∂ u / ∂ y is the local shear velocity. This expression is referred to as Newton's law of viscosity. In shearing flows with planar symmetry, it is what defines μ, it is a special case of the general definition of viscosity, which can be expressed in coordinate-free form. Use of the Greek letter mu for the viscosity is common among mechanical and chemical engineers, as well as physicists. However, the Greek letter eta is used by chemists and the IUPAC; the viscosity μ is sometimes referred to as the shear viscosity. However, at least one author discourages the use of this terminology, noting that μ can appear in nonshearing flows in addition to shearing flows. In general terms, the viscous stresses in a fluid are defined as those resulting from the relative velocity of different fluid particles; as such, the viscous stresses. If the velocity gradients are small to a first approximation the v
Halogenation is a chemical reaction that involves the addition of one or more halogens to a compound or material. The pathway and stoichiometry of halogenation depends on the structural features and functional groups of the organic substrate, as well as on the specific halogen. Inorganic compounds such as metals undergo halogenation. Several pathways exist for the halogenation of organic compounds, including free radical halogenation, ketone halogenation, electrophilic halogenation, halogen addition reaction; the structure of the substrate is one factor. Saturated hydrocarbons do not add halogens but undergo free radical halogenation, involving substitution of hydrogen atoms by halogen; the regiochemistry of the halogenation of alkanes is determined by the relative weakness of the available C–H bonds. The preference for reaction at tertiary and secondary positions results from greater stability of the corresponding free radicals and the transition state leading to them. Free radical halogenation is used for the industrial production of chlorinated methanes: CH4 + Cl2 → CH3Cl + HClRearrangement accompany such free radical reactions.
Unsaturated compounds alkenes and alkynes, add halogens: RCH=CHR′ + X2 → RCHX–CHXR′The addition of halogens to alkenes proceeds via intermediate halonium ions. In special cases, such intermediates have been isolated. Aromatic compounds are subject to electrophilic halogenation: RC6H5 + X2 → HX + RC6H4XThis reaction works only for chlorine and bromine and is carried in the presence of a Lewis acid such as FeX3; the role of the Lewis acid is to polarize the halogen-halogen bond, making the halogen molecule more electrophilic. Industrially, this is done by treating the aromatic compound with X2 in the presence of iron metal; when the halogen is pumped into the reaction vessel, it reacts with iron, generating FeX3 in catalytic amounts. The reaction mechanism can be represented as follows: Because fluorine is reactive, the protocol described above would not be efficient as the aromatic molecule would react destructively with F2. Therefore, other methods, such as the Balz–Schiemann reaction, must be used to prepare fluorinated aromatic compounds.
For iodine, oxidising conditions must be used in order to perform iodination. Because iodination is a reversible process, the products have to be removed from the reaction medium in order to drive the reaction forward, see Le Chatelier's principle; this can be done by conducting the reaction in the presence of an oxidising agent that oxidises HI to I2, thus removing HI from the reaction and generating more iodine that can further react. The reaction steps involved in iodination are the following: Another method to obtain aromatic iodides is the Sandmeyer reaction. In the Hunsdiecker reaction, from carboxylic acids are converted to the chain-shortened halide; the carboxylic acid is first converted to its silver salt, oxidized with halogen: RCO2Ag + Br2 → RBr + CO2 + AgBrThe Sandmeyer reaction is used to give aryl halides from diazonium salts, which are obtained from anilines. In the Hell–Volhard–Zelinsky halogenation, carboxylic acids are alpha-halogenated. In oxychlorination, the combination of hydrogen chloride and oxygen serves as the equivalent of chlorine, as illustrated by this route to dichloroethane: 2 HCl + CH2=CH2 + 1⁄2 O2 → ClCH2CH2Cl + H2O The facility of halogenation is influenced by the halogen.
Fluorine and chlorine are more aggressive halogenating agents. Bromine is a weaker halogenating agent than both fluorine and chlorine, while iodine is the least reactive of them all; the facility of dehydrohalogenation follows the reverse trend: iodine is most removed from organic compounds, organofluorine compounds are stable. Organic compounds and unsaturated alike, react usually explosively, with fluorine. Fluorination with elemental fluorine requires specialised conditions and apparatus. Many commercially important organic compounds are fluorinated electrochemically using hydrogen fluoride as the source of fluorine; the method is called electrochemical fluorination. Aside from F2 and its electrochemically generated equivalent, a variety of fluorinating reagents are known such as xenon difluoride and cobalt fluoride. See also: PhotochlorinationChlorination is highly exothermic. Both saturated and unsaturated compounds react directly with chlorine, the former requiring UV light to initiate homolysis of chlorine.
Chlorination is conducted on a large scale industrially. Bromination is more selective than chlorination. Most bromination is conducted by the addition of Br2 to alkenes. An example of bromination is the organic synthesis of the anesthetic halothane from trichloroethylene: Organobromine compounds are the most common organohalides in nature, their formation is catalyzed by the enzyme bromoperoxidase which utilizes bromide in combination with oxygen as an oxidant. The oceans are estimated to release 1–2 million tons of bromoform and 56,000 tons of bromomethane annually. Iodine is reluctant to react with most organic compounds; the addition of iodine to alkenes is the basis of the analytical method called the iodine number, a measure of the degree of unsaturation for fats. The iodoform reaction involves degradation of methyl ketones. All elements aside from argon and helium form fluorides by direct reaction with fluorine. Chlorine is more selective, but still reacts with most metals and heavier nonmetals.
Following the usual trend, bromine is less reactive and iodine leas
Plastic is material consisting of any of a wide range of synthetic or semi-synthetic organic compounds that are malleable and so can be molded into solid objects. Plasticity is the general property of all materials which can deform irreversibly without breaking but, in the class of moldable polymers, this occurs to such a degree that their actual name derives from this specific ability. Plastics are organic polymers of high molecular mass and contain other substances, they are synthetic, most derived from petrochemicals, however, an array of variants are made from renewable materials such as polylactic acid from corn or cellulosics from cotton linters. Due to their low cost, ease of manufacture and imperviousness to water, plastics are used in a multitude of products of different scale, including paper clips and spacecraft, they have prevailed over traditional materials, such as wood, stone and bone, metal and ceramic, in some products left to natural materials. In developed economies, about a third of plastic is used in packaging and the same in buildings in applications such as piping, plumbing or vinyl siding.
Other uses include automobiles and toys. In the developing world, the applications of plastic may differ—42% of India's consumption is used in packaging. Plastics have many uses in the medical field as well, with the introduction of polymer implants and other medical devices derived at least from plastic; the field of plastic surgery is not named for use of plastic materials, but rather the meaning of the word plasticity, with regard to the reshaping of flesh. The world's first synthetic plastic was bakelite, invented in New York in 1907 by Leo Baekeland who coined the term'plastics'. Many chemists have contributed to the materials science of plastics, including Nobel laureate Hermann Staudinger, called "the father of polymer chemistry" and Herman Mark, known as "the father of polymer physics"; the success and dominance of plastics starting in the early 20th century led to environmental concerns regarding its slow decomposition rate after being discarded as trash due to its composition of large molecules.
Toward the end of the century, one approach to this problem was met with wide efforts toward recycling. The word plastic derives from the Greek πλαστικός meaning "capable of being shaped or molded" and, in turn, from πλαστός meaning "molded"; the plasticity, or malleability, of the material during manufacture allows it to be cast, pressed, or extruded into a variety of shapes, such as: films, plates, bottles, amongst many others. The common noun plastic should not be confused with the technical adjective plastic; the adjective is applicable to any material which undergoes a plastic deformation, or permanent change of shape, when strained beyond a certain point. For example, aluminum, stamped or forged exhibits plasticity in this sense, but is not plastic in the common sense. By contrast, some plastics will, in their finished forms, break before deforming and therefore are not plastic in the technical sense. Most plastics contain organic polymers; the vast majority of these polymers are formed from chains of carbon atoms,'pure' or with the addition of: oxygen, nitrogen, or sulfur.
The chains comprise many repeat units, formed from monomers. Each polymer chain will have several thousand repeating units; the backbone is the part of the chain, on the "main path", linking together a large number of repeat units. To customize the properties of a plastic, different molecular groups "hang" from this backbone; these pendant units are "hung" on the monomers, before the monomers themselves are linked together to form the polymer chain. It is the structure of these side chains; the molecular structure of the repeating unit can be fine tuned to influence specific properties in the polymer. Plastics are classified by: the chemical structure of the polymer's backbone and side chains. Plastics can be classified by: the chemical process used in their synthesis, such as: condensation and cross-linking. Plastics can be classified by: their various physical properties, such as: hardness, tensile strength, resistance to heat and glass transition temperature, by their chemical properties, such as the organic chemistry of the polymer and its resistance and reaction to various chemical products and processes, such as: organic solvents and ionizing radiation.
In particular, most plastics will melt upon heating to a few hundred degrees celsius. Other classifications are based on qualities that are relevant for product design. Examples of such qualities and classes are: thermoplastics and thermosets, conductive polymers, biodegradable plastics and engineering plastics and other plastics with particular structures, such as elastomers. One important classification of plastics is by the permanence or impermanence of their form, or whether they are: thermoplastics or thermosetting polymers. Thermoplastics are the plastics that, when heated, do not undergo chemical change in their composition and so can be molded again and again. Examples include: polyethylene, polypropylene and polyvinyl chloride. Common thermoplastics range from 20,000 to 500,000 amu, while thermosets are assumed to have infinite molecular weight. Thermosets, or thermosetting polymers, can melt and take shape only once: after they have solidified, they stay solid. In the thermosetting process, a chemical reaction occurs, irreversible.
Thermal insulation is the reduction of heat transfer between objects in thermal contact or in range of radiative influence. Thermal insulation can be achieved with specially engineered methods or processes, as well as with suitable object shapes and materials. Heat flow is an inevitable consequence of contact between objects of different temperature. Thermal insulation provides a region of insulation in which thermal conduction is reduced or thermal radiation is reflected rather than absorbed by the lower-temperature body; the insulating capability of a material is measured as the inverse of thermal conductivity. Low thermal conductivity is equivalent to high insulating capability. In thermal engineering, other important properties of insulating materials are product density and specific heat capacity. Thermal conductivity k is measured in watts-per-meter per kelvin; this is because heat transfer, measured as Power, has been found to be proportional to difference of temperature Δ T. For comparison purposes, conductivity under standard conditions is used.
For some materials, thermal conductivity may depend upon the direction of heat transfer. The act of insulation is accomplished by encasing an object with material of low thermal conductivity in high thickness. Decreasing the exposed surface area could lower heat transfer, but this quantity is fixed by the geometry of the object to be insulated. Multi-layer insulation is used where radiative loss dominates, or when the user is restricted in volume and weight of the insulation For insulated cylinders, a critical radius must be reached. Before the critical radius is reached any added insulation increases heat transfer; the convective thermal resistance is inversely proportional to the surface area and therefore the radius of the cylinder, while the thermal resistance of a cylindrical shell depends on the ratio between outside and inside radius, not on the radius itself. If the outside radius of a cylinder is increased by applying insulation, a fixed amount of conductive resistance is added. However, at the same time, the convective resistance is reduced.
This implies that adding insulation below a certain critical radius increases the heat transfer. For insulated cylinders, the critical radius is given by the equation r c r i t i c a l = k h This equation shows that the critical radius depends only on the heat transfer coefficient and the thermal conductivity of the insulation. If the radius of the insulated cylinder is smaller than the critical radius for insulation, the addition of any amount of insulation will increase heat transfer. Gases possess poor thermal conduction properties compared to liquids and solids, thus makes a good insulation material if they can be trapped. In order to further augment the effectiveness of a gas it may be disrupted into small cells which cannot transfer heat by natural convection. Convection involves a larger bulk flow of gas driven by buoyancy and temperature differences, it does not work well in small cells where there is little density difference to drive it, the high surface-to-volume ratios of the small cells retards gas flow in them by means of viscous drag.
In order to accomplish small gas cell formation in man-made thermal insulation and polymer materials can be used to trap air in a foam-like structure. This principle is used industrially in building and piping insulation such as, rock wool, polystyrene foam, urethane foam, vermiculite and cork. Trapping air is the principle in all insulating clothing materials such as wool, down feathers and fleece; the air-trapping property is the insulation principle employed by homeothermic animals to stay warm, for example down feathers, insulating hair such as natural sheep's wool. In both cases the primary insulating material is air, the polymer used for trapping the air is natural keratin protein. Maintaining acceptable temperatures in buildings uses a large proportion of global energy consumption. Building insulations commonly use the principle of small trapped air-cells as explained above, e.g. fiberglass, rock wool, polystyrene foam, urethane foam, perlite, etc. For a period of time, Asbestos was used, however, it caused health problems.
When well insulated, a building: is energy-efficient, thus saving the owner money. Provides more uniform temperatures throughout the space. There is less temperature gradient both vertically and horizontally from exterior walls and windows to the interior walls, thus producing a more comfortable occupant environment when outside temperatures are cold or hot. Has minimal recurring expense. Unlike heating and cooling equipment, ins
EPDM rubber, is a type of synthetic rubber, which can be used in a wide range of applications. This is an M-Class rubber where the'M' in M-Class refers to its classification in ASTM standard D-1418. EPDM is made from ethylene, propylene and a diene comonomer that enables crosslinking via sulphur vulcanisation systems; the earlier relative of EPDM is EPR, ethylene-propylene rubber, that contains no diene units and can only be crosslinked using radical methods such as peroxides. Dienes used in the manufacture of EPDM rubbers are ethylidene norbornene, dicyclopentadiene, vinyl norbornene. EPDM is related to polyethylene, into which high amounts, from 45% to 85% by weight, of propylene have been copolymerised to reduce the formation of the typical polyethylene crystallinity. EPDM is a semi-crystalline material with ethylene-type crystal structures at higher ethylene contents, becoming amorphous at ethylene contents that approach 50 wt%. Rubbers with saturated polymer backbones, such as EPDM, have much better resistance to heat and ozone compared to unsaturated rubbers such as natural rubber, SBR or polychloroprene.
As such, EPDM can be formulated to be resistant to temperatures as high as 150°C, properly formulated, can be used outside for many years or decades without degradation. EPDM has good low temperature properties, with elastic properties to temperatures as low as -40°C depending on the grade and the formulation; as with most rubbers, EPDM is always used compounded with fillers such as carbon black and calcium carbonate, with plasticisers such as paraffinic oils, has useful rubbery properties only when crosslinked. Crosslinking takes place via vulcanisation with sulphur, but is accomplished with peroxides or with phenolic resins. High energy radiation such as from electron beams is sometimes used for producing foams and wire and cable. EPDM is compatible with polar substances, e.g. fireproof hydraulic fluids, ketones and cold water, alkalis. It is incompatible with most hydrocarbons, such as oils, aromatic, gasoline, as well as halogenated solvents. EPDM exhibits outstanding resistance to heat, ozone and weather.
It is an electrical insulator. Typical properties of EPDM vulcanizates are given below. EPDM can be compounded to meet specific properties to a limit, depending first on the EPDM polymers available the processing and curing method employed. EPDMs are available in a range of molecular weights, varying levels of ethylene, third monomer, oil content. A common use is in vehicles: door seals, window seals, trunk seals, sometimes hood seals; these seals are the source of noise due to movement of the door against the car body and the resulting friction between the EPDM rubber and the mating surface. The synthetic rubber membrane properties has been used for flat roofs because of its durability and low maintenance costs; this noise can be alleviated using specialty coatings that are applied at the time of manufacture of the weather seal. Such coatings can improve the chemical resistance of EPDM rubber; some vehicle manufacturers recommend a light application of silicone dielectric grease to weatherstripping to reduce noise.
Other uses in vehicles include cooling system circuit hoses where water pumps, thermostats, EGR valves, EGR coolers, oil coolers and degas bottles are connected with EPDM hoses, as well as charge air tubing on turbocharged engines to connect the cold side of the charge air cooler to the intake manifold. EPDM rubber is used in seals (for example, it is used in cold-room doors since it is an insulator, as well as in the face seals of industrial respirators in automotive paint spray environments. EPDM is used in glass-run channels, radiators and appliance hose, pond liners, belts, electrical insulation, vibrators, O-rings, solar panel heat collectors, speaker cone surrounds, it is used as a medium for water resistance in electrical cable-jointing, roofing membranes, rubber mechanical goods, plastic impact modification, thermoplastic and many other applications. Colored EPDM granules are mixed with polyurethane binders and troweled or sprayed onto concrete, screenings, interlocking brick, etc. to create a non-slip, porous safety surface for wet-deck areas such as pool decks and as safety surfacing under playground play equipment
Hydrogen is a chemical element with symbol H and atomic number 1. With a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table. Hydrogen is the most abundant chemical substance in the Universe, constituting 75% of all baryonic mass. Non-remnant stars are composed of hydrogen in the plasma state; the most common isotope of hydrogen, termed protium, has no neutrons. The universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, tasteless, non-toxic, nonmetallic combustible diatomic gas with the molecular formula H2. Since hydrogen forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays a important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a negative charge when it is known as a hydride, or as a positively charged species denoted by the symbol H+.
The hydrogen cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds are always more complex. As the only neutral atom for which the Schrödinger equation can be solved analytically, study of the energetics and bonding of the hydrogen atom has played a key role in the development of quantum mechanics. Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance, that it produces water when burned, the property for which it was named: in Greek, hydrogen means "water-former". Industrial production is from steam reforming natural gas, less from more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing and ammonia production for the fertilizer market. Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks.
Hydrogen gas is flammable and will burn in air at a wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion is −286 kJ/mol: 2 H2 + O2 → 2 H2O + 572 kJ Hydrogen gas forms explosive mixtures with air in concentrations from 4–74% and with chlorine at 5–95%; the explosive reactions may be triggered by heat, or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C. Pure hydrogen-oxygen flames emit ultraviolet light and with high oxygen mix are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine, compared to the visible plume of a Space Shuttle Solid Rocket Booster, which uses an ammonium perchlorate composite; the detection of a burning hydrogen leak may require a flame detector. Hydrogen flames in other conditions are blue; the destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a rich mixture of hydrogen to oxygen combined with carbon compounds from the airship skin.
H2 reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with chlorine and fluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are potentially dangerous acids; the ground state energy level of the electron in a hydrogen atom is −13.6 eV, equivalent to an ultraviolet photon of 91 nm wavelength. The energy levels of hydrogen can be calculated accurately using the Bohr model of the atom, which conceptualizes the electron as "orbiting" the proton in analogy to the Earth's orbit of the Sun. However, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity; because of the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, therefore only certain allowed energies. A more accurate description of the hydrogen atom comes from a purely quantum mechanical treatment that uses the Schrödinger equation, Dirac equation or the Feynman path integral formulation to calculate the probability density of the electron around the proton.
The most complicated treatments allow for the small effects of special relativity and vacuum polarization. In the quantum mechanical treatment, the electron in a ground state hydrogen atom has no angular momentum at all—illustrating how the "planetary orbit" differs from electron motion. There exist two different spin isomers of hydrogen diatomic molecules that differ by the relative spin of their nuclei. In the orthohydrogen form, the spins of the two protons are parallel and form a triplet state with a molecular spin quantum number of 1. At standard temperature and pressure, hydrogen gas contains about 25% of the para form and 75% of the ortho form known as the "normal form"; the equilibrium ratio of orthohydrogen to parahydrogen depends on temperature, but because the ortho form is an excited state and has a higher energy