Cracks can be formed in many different elastomers by ozone attack, the characteristic form of attack of vulnerable rubbers is known as ozone cracking. The problem was very common in tires, but is now seen in those products owing to preventive measures. However, it does occur in many other safety-critical items such as fuel lines and rubber seals, such as gaskets and O-rings, where ozone attack is considered unlikely. Only a trace amount of the gas is needed to initiate cracking, so these items can succumb to the problem. Tiny traces of ozone in the air will attack double bonds in rubber chains, with natural rubber, styrene-butadiene rubber and nitrile rubber being most sensitive to degradation; every repeat unit in the first three materials has a double bond, so every unit can be degraded by ozone. Nitrile rubber is a copolymer of butadiene and acrylonitrile units, but the proportion of acrylonitrile is lower than butadiene, so attack occurs. Butyl rubber is more resistant but still has a small number of double bonds in its chains, so attack is possible.
Exposed surfaces are attacked first, the density of cracks varying with ozone gas concentration. The higher the concentration, the greater the number of cracks formed. Ozone-resistant elastomers include EPDM, fluoroelastomers like Viton and polychloroprene rubbers like Neoprene. Attack is less because double bonds form a small proportion of the chains, with the latter, the chlorination reduces the electron density in the double bonds, therefore lowering their propensity to react with ozone. Silicone rubber and polyurethanes are ozone-resistant. Ozone cracks form in products under tension, but the critical strain is small; the cracks are always oriented at right angles to the strain axis, so will form around the circumference in a rubber tube bent over. Such cracks are dangerous when they occur in fuel pipes because the cracks will grow from the outside exposed surfaces into the bore of the pipe, so fuel leakage and fire may follow. Seals are susceptible to attack, such as diaphragm seals in air lines.
Such seals are critical for the operation of pneumatic controls, if a crack penetrates the seal, all functions of the system can be lost. Nitrile rubber seals are used in pneumatic systems because of its oil resistance. However, if ozone gas is present, cracking will occur in the seals unless preventative measures are taken. Ozone attack will occur at the most sensitive zones in a seal sharp corners where the strain is greatest when the seal is flexing in use; the corners represent stress concentrations, so the tension is at a maximum when the diaphragm of the seal is bent under air pressure. The seal shown at left failed from traces of ozone at circa 1 ppm, once cracking had started, it continued as long as the gas was present; this particular failure led to loss of production on a semi-conductor fabrication line. The problem was solved by adding effective filters in the air line and by modifying the design to eliminate the sharp corners. An ozone-resistant elastomer such as Viton was considered as a replacement for the Nitrile rubber.
The pictures were taken using ESEM for maximum resolution. The reaction occurring between double bonds and ozone is known as ozonolysis when one molecule of the gas reacts with the double bond: The immediate result is formation of an ozonide, which decomposes so that the double bond is cleaved; this is the critical step in chain breakage. The strength of polymers depends on the chain molecular weight or degree of polymerization, the higher the chain length, the greater the mechanical strength. By cleaving the chain, the molecular weight drops and there comes a point when it has little strength whatsoever, a crack forms. Further attack occurs in the freshly exposed crack surfaces and the crack grows until it completes a circuit and the product separates or fails. In the case of a seal or a tube, failure occurs; the carbonyl end groups which are formed are aldehydes or ketones, which can oxidise further to carboxylic acids. The net result is a high concentration of elemental oxygen on the crack surfaces, which can be detected using Energy-dispersive X-ray spectroscopy in the environmental SEM, or ESEM.
The spectrum at left shows the high oxygen peak compared with a constant sulfur peak. The spectrum at right shows the unaffected elastomer surface spectrum, with a low oxygen peak compared with the sulfur peak; the problem can be prevented by adding antiozonants to the rubber before vulcanization. Ozone cracks were seen in automobile tire sidewalls, but are now seen thanks to the use of these additives. A common and low cost antiozonant is a wax which bleeds to the surface and forms a protective layer, but other specialist chemicals are widely used. On the other hand, the problem does recur in unprotected products such as rubber tubing and seals, where ozone attack is thought to be impossible. Traces of ozone can turn up in the most unexpected situations. Using ozone-resistant rubbers is another way of inhibiting cracking. EPDM rubber and butyl rubber are ozone resistant, for example. For high value equipment where loss of function can cause serious problems, low cost seals may be replaced at frequent intervals so as to preclude failure.
Ozone gas is produced during electric discharge by corona discharge for example. Static electricity can build up within machines like compressors with moving parts constructed from insulating materials. If those compressors feed pressurised air into a closed pneumatic system all seals in the system may be at risk from ozon
A thickening agent or thickener is a substance which can increase the viscosity of a liquid without changing its other properties. Edible thickeners are used to thicken sauces and puddings without altering their taste. Thickeners may improve the suspension of other ingredients or emulsions which increases the stability of the product. Thickening agents are regulated as food additives and as cosmetics and personal hygiene product ingredients; some thickening agents are gelling agents, forming a gel, dissolving in the liquid phase as a colloid mixture that forms a weakly cohesive internal structure. Others act as mechanical thixotropic additives with discrete particles adhering or interlocking to resist strain. Thickening agents can be used when a medical condition such as dysphagia causes difficulty in swallowing. Thickened liquids play a vital role in reducing risk of aspiration for dysphagia patients. Food thickeners are based on either polysaccharides, or proteins. A flavorless powdered starch used for this purpose is a fecula.
This category includes starches as arrowroot, katakuri starch, potato starch, sago and their starch derivatives. Microbial and Vegetable gums used as food thickeners include alginin, guar gum, locust bean gum, xanthan gum. Proteins used as food thickeners include collagen, egg whites, gelatin. Sugar polymers include agar, carboxymethyl cellulose and carrageenan. Other thickening agents act on the proteins present in a food. One example is sodium pyrophosphate, which acts on casein in milk during the preparation of instant pudding. Different thickeners may be more or less suitable in a given application, due to differences in taste and their responses to chemical and physical conditions. For example, for acidic foods, arrowroot is a better choice than cornstarch, which loses thickening potency in acidic mixtures. At pH levels below 4.5, guar gum has reduced aqueous solubility, thus reducing its thickening capability. If the food is to be frozen, tapioca or arrowroot are preferable over cornstarch, which becomes spongy when frozen.
Many other food ingredients are used as thickeners in the final stages of preparation of specific foods. These thickeners are not markedly stable, thus are not suitable for general use. However, they are convenient and effective, hence are used. Functional flours are produced from specific cereal variety conjugated to specific heat treatment able to increase stability and general functionalities; these functional flours are resistant to industrial stresses such as acidic pH, freeze conditions, can help food industries to formulate with natural ingredients. For the final consumer, these ingredients are more accepted because they are shown as "flour" in the ingredient list. Flour is used for thickening gravies and stews, it must be cooked in to avoid the taste of uncooked flour. Roux, a mixture of flour and fat cooked into a paste, is used for gravies and stews. Cereal grains are used to thicken soups. Yogurt is popular in Middle East for thickening soups. Soups can be thickened by adding grated starchy vegetables before cooking, though these will add their own flavour.
Tomato puree adds thickness as well as flavour. Egg yolks are a traditional sauce thickener in professional cooking. Overheating ruins such a sauce, which can make egg yolk difficult to use as a thickener for amateur cooks. Other thickeners used by cooks are glaces made of meat or fish. Many thickening agents require extra care in cooking; some starches lose their thickening quality when cooked for at too high a temperature. Higher viscosity causes foods to burn more during cooking; as an alternative to adding more thickener, recipes may call for reduction of the food's water content by lengthy simmering. When cooking, it is better to add thickener cautiously. Gelling agents are food additives used to thicken and stabilize various foods, like jellies and candies; the agents provide the foods with texture through formation of a gel. Some stabilizers and thickening agents are gelling agents. Typical gelling agents include natural gums, pectins, agar-agar and gelatin, they are based on polysaccharides or proteins.
Examples are: Alginic acid, sodium alginate, potassium alginate, ammonium alginate, calcium alginate - polysaccharides from brown algae Agar Carrageenan Locust bean gum Pectin Gelatin Commercial jellies used in East Asian cuisines include the glucomannan polysaccharide gum used to make "lychee cups" from the konjac plants, aiyu or ice jelly from the Ficus pumila climbing fig plant. Food thickening can be important for people facing medical issues w
Chemistry is the scientific discipline involved with elements and compounds composed of atoms and ions: their composition, properties and the changes they undergo during a reaction with other substances. In the scope of its subject, chemistry occupies an intermediate position between physics and biology, it is sometimes called the central science because it provides a foundation for understanding both basic and applied scientific disciplines at a fundamental level. For example, chemistry explains aspects of plant chemistry, the formation of igneous rocks, how atmospheric ozone is formed and how environmental pollutants are degraded, the properties of the soil on the moon, how medications work, how to collect DNA evidence at a crime scene. Chemistry addresses topics such as how atoms and molecules interact via chemical bonds to form new chemical compounds. There are four types of chemical bonds: covalent bonds, in which compounds share one or more electron; the word chemistry comes from alchemy, which referred to an earlier set of practices that encompassed elements of chemistry, philosophy, astronomy and medicine.
It is seen as linked to the quest to turn lead or another common starting material into gold, though in ancient times the study encompassed many of the questions of modern chemistry being defined as the study of the composition of waters, growth, disembodying, drawing the spirits from bodies and bonding the spirits within bodies by the early 4th century Greek-Egyptian alchemist Zosimos. An alchemist was called a'chemist' in popular speech, the suffix "-ry" was added to this to describe the art of the chemist as "chemistry"; the modern word alchemy in turn is derived from the Arabic word al-kīmīā. In origin, the term is borrowed from the Greek χημία or χημεία; this may have Egyptian origins since al-kīmīā is derived from the Greek χημία, in turn derived from the word Kemet, the ancient name of Egypt in the Egyptian language. Alternately, al-kīmīā may derive from χημεία, meaning "cast together"; the current model of atomic structure is the quantum mechanical model. Traditional chemistry starts with the study of elementary particles, molecules, metals and other aggregates of matter.
This matter can be studied in isolation or in combination. The interactions and transformations that are studied in chemistry are the result of interactions between atoms, leading to rearrangements of the chemical bonds which hold atoms together; such behaviors are studied in a chemistry laboratory. The chemistry laboratory stereotypically uses various forms of laboratory glassware; however glassware is not central to chemistry, a great deal of experimental chemistry is done without it. A chemical reaction is a transformation of some substances into one or more different substances; the basis of such a chemical transformation is the rearrangement of electrons in the chemical bonds between atoms. It can be symbolically depicted through a chemical equation, which involves atoms as subjects; the number of atoms on the left and the right in the equation for a chemical transformation is equal. The type of chemical reactions a substance may undergo and the energy changes that may accompany it are constrained by certain basic rules, known as chemical laws.
Energy and entropy considerations are invariably important in all chemical studies. Chemical substances are classified in terms of their structure, phase, as well as their chemical compositions, they can be analyzed using the tools of e.g. spectroscopy and chromatography. Scientists engaged in chemical research are known as chemists. Most chemists specialize in one or more sub-disciplines. Several concepts are essential for the study of chemistry; the particles that make up matter have rest mass as well – not all particles have rest mass, such as the photon. Matter can be a mixture of substances; the atom is the basic unit of chemistry. It consists of a dense core called the atomic nucleus surrounded by a space occupied by an electron cloud; the nucleus is made up of positively charged protons and uncharged neutrons, while the electron cloud consists of negatively charged electrons which orbit the nucleus. In a neutral atom, the negatively charged electrons balance out the positive charge of the protons.
The nucleus is dense. The atom is the smallest entity that can be envisaged to retain the chemical properties of the element, such as electronegativity, ionization potential, preferred oxidation state, coordination number, preferred types of bonds to form. A chemical element is a pure substance, composed of a single type of atom, characterized by its particular number of protons in the nuclei of its atoms, known as the atomic number and represented by the symbol Z; the mass number is the sum of the number of neutrons in a nucleus. Although all the nuclei of all atoms belonging to one element will have the same
In chemistry a phosphite ester or organophosphite refers to an organophosphorous compound with the formula P3. They can be considered as esters of an unobserved tautomer phosphorous acid, H3PO3, with the simplest example being trimethylphosphite, P3; some phosphites can be considered esters of the dominant tautomer of phosphorous acid. The simplest representative is dimethylphosphite with the formula HP2. Both classes of phosphites are colorless liquids. From PCl3Phosphite esters are prepared by treating phosphorus trichloride with an alcohol. Depending on the synthetic details, this alcoholysis can give the diorganophosphites: PCl3 + 3 C2H5OH → 2PH + 2 HCl + C2H5ClAlternatively, when the alcoholysis is conducted in the presence of proton acceptors, one obtains the C3-symmetric trialkoxy derivatives: PCl3 + 3 C2H5OH + 3 R3N → 3P + 3 R3NHClNumerous derivatives have been prepared for both types of phosphites. By transesterificationPhosphite esters can be prepared by transesterification, as they undergo alcohol exchange upon heating with other alcohols.
This process can be used to produce mixed alkyl phosphites. Alternatively, if the phosphite of a volatile alcohol is used, such as trimethyl phosphite the by product can be removed by distillation, allowing the reaction to be driven to completion. Phosphites are oxidized to phosphate esters: P3 + → OP3This reaction underpins the commercial use of some phosphite esters as stabilizers in polymers. Alkyl phosphite esters are used in the Perkow reaction for the formation of vinyl phosphonates, in the Michaelis–Arbuzov reaction to form phosphonates. Aryl phosphite esters may not undergo these reactions and hence are used as stabilizers in halogen-bearing polymers such as PVC. Phosphite esters may be used as reducing agents in more specialised cases. For example, triethylphosphite is known to reduce certain hydroperoxides to alcohols formed by autoxidation. In this process the phosphite is converted to a phosphate ester; this reaction type is utilized in the Wender Taxol total synthesis. Phosphite esters hence can form coordination complexes with various metal ions.
Representative phosphite ligands include trimethylphosphite, triethylphosphite, trimethylolpropane phosphite, triphenylphosphite. In contrast to phosphine ligands, phosphites exhibit a smaller ligand cone angles, making them appealing as ligands, they remain somewhat less important. Diorganophosphites are derivatives of phosphorus and can be viewed as the di-esters of phosphorous acid, they exhibit tautomerism, however the equilibrium overwhelmingly favours the right-hand form: 2POH ⇌ 2PHThe P-H bond is the site of high reactivity in these compounds, whereas in tri-organophosphites the lone pair on phosphorus is the site of high reactivity. Diorganophosphites do however undergo transesterification. Phosphinite PR2 Phosphonite P2R Ortho ester CH3 Borate ester B3
Ultraviolet designates a band of the electromagnetic spectrum with wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays. UV radiation is present in sunlight, contributes about 10% of the total light output of the Sun, it is produced by electric arcs and specialized lights, such as mercury-vapor lamps, tanning lamps, black lights. Although long-wavelength ultraviolet is not considered an ionizing radiation because its photons lack the energy to ionize atoms, it can cause chemical reactions and causes many substances to glow or fluoresce; the chemical and biological effects of UV are greater than simple heating effects, many practical applications of UV radiation derive from its interactions with organic molecules. Suntan and sunburn are familiar effects of over-exposure of the skin to UV, along with higher risk of skin cancer. Living things on dry land would be damaged by ultraviolet radiation from the Sun if most of it were not filtered out by the Earth's atmosphere.
More energetic, shorter-wavelength "extreme" UV below 121 nm ionizes air so that it is absorbed before it reaches the ground. Ultraviolet is responsible for the formation of bone-strengthening vitamin D in most land vertebrates, including humans; the UV spectrum thus has effects both harmful to human health. The lower wavelength limit of human vision is conventionally taken as 400 nm, so ultraviolet rays are invisible to humans, although some people can perceive light at shorter wavelengths than this. Insects and some mammals can see near-UV. Ultraviolet rays are invisible to most humans; the lens of the human eye blocks most radiation in the wavelength range of 300–400 nm. Humans lack color receptor adaptations for ultraviolet rays; the photoreceptors of the retina are sensitive to near-UV, people lacking a lens perceive near-UV as whitish-blue or whitish-violet. Under some conditions and young adults can see ultraviolet down to wavelengths of about 310 nm. Near-UV radiation is visible to insects, some mammals, birds.
Small birds have a fourth color receptor for ultraviolet rays. "Ultraviolet" means "beyond violet", violet being the color of the highest frequencies of visible light. Ultraviolet has a higher frequency than violet light. UV radiation was discovered in 1801 when the German physicist Johann Wilhelm Ritter observed that invisible rays just beyond the violet end of the visible spectrum darkened silver chloride-soaked paper more than violet light itself, he called them "oxidizing rays" to emphasize chemical reactivity and to distinguish them from "heat rays", discovered the previous year at the other end of the visible spectrum. The simpler term "chemical rays" was adopted soon afterwards, remained popular throughout the 19th century, although some said that this radiation was different from light; the terms "chemical rays" and "heat rays" were dropped in favor of ultraviolet and infrared radiation, respectively. In 1878 the sterilizing effect of short-wavelength light by killing bacteria was discovered.
By 1903 it was known. In 1960, the effect of ultraviolet radiation on DNA was established; the discovery of the ultraviolet radiation with wavelengths below 200 nm, named "vacuum ultraviolet" because it is absorbed by the oxygen in air, was made in 1893 by the German physicist Victor Schumann. The electromagnetic spectrum of ultraviolet radiation, defined most broadly as 10–400 nanometers, can be subdivided into a number of ranges recommended by the ISO standard ISO-21348: A variety of solid-state and vacuum devices have been explored for use in different parts of the UV spectrum. Many approaches seek to adapt visible light-sensing devices, but these can suffer from unwanted response to visible light and various instabilities. Ultraviolet can be detected by suitable photodiodes and photocathodes, which can be tailored to be sensitive to different parts of the UV spectrum. Sensitive ultraviolet photomultipliers are available. Spectrometers and radiometers are made for measurement of UV radiation.
Silicon detectors are used across the spectrum. Vacuum UV, or VUV, wavelengths are absorbed by molecular oxygen in the air, though the longer wavelengths of about 150–200 nm can propagate through nitrogen. Scientific instruments can therefore utilize this spectral range by operating in an oxygen-free atmosphere, without the need for costly vacuum chambers. Significant examples include 193 nm photolithography equipment and circular dichroism spectrometers. Technology for VUV instrumentation was driven by solar astronomy for many decades. While optics can be used to remove unwanted visible light that contaminates the VUV, in general, detectors can be limited by their response to non-VUV radiation, the development of "solar-blind" devices has been an important area of research. Wide-gap solid-state devices or vacuum devices with high-cutoff photocathodes can be attractive compared to silicon diodes. Extreme UV is characterized by a transition in the physics of interaction with matter. Wavelengths longer than about 30 nm interact with the outer valence electrons of atoms, while wavelengths shorter than that interact with inner-shell electrons and nuclei.
The long end of the EUV spectrum is set by a prominent He+ spectr
In chemistry, a radical is an atom, molecule, or ion that has an unpaired valence electron. With some exceptions, these unpaired electrons make radicals chemically reactive. Many radicals spontaneously dimerize. Most organic radicals have short lifetimes. A notable example of a radical is the hydroxyl radical, a molecule that has one unpaired electron on the oxygen atom. Two other examples are triplet triplet carbene which have two unpaired electrons. Radicals may be generated in a number of ways. Ionizing radiation, electrical discharges, electrolysis are known to produce radicals. Radicals are intermediates in many chemical reactions, more so than is apparent from the balanced equations. Radicals are important in combustion, atmospheric chemistry, plasma chemistry and many other chemical processes. A large fraction of natural products is generated by radical-generating enzymes. In living organisms, the radicals superoxide and nitric oxide and their reaction products regulate many processes, such as control of vascular tone and thus blood pressure.
They play a key role in the intermediary metabolism of various biological compounds. Such radicals can be messengers in a process dubbed redox signaling. A radical may be otherwise bound. In chemical equations, radicals are denoted by a dot placed to the right of the atomic symbol or molecular formula as follows: C l 2 → U V 2 C l ⋅ Radical reaction mechanisms use single-headed arrows to depict the movement of single electrons: The homolytic cleavage of the breaking bond is drawn with a'fish-hook' arrow to distinguish from the usual movement of two electrons depicted by a standard curly arrow; the second electron of the breaking bond moves to pair up with the attacking radical electron. Radicals take part in radical addition and radical substitution as reactive intermediates. Chain reactions involving radicals can be divided into three distinct processes; these are initiation and termination. Initiation reactions are those, they may involve the formation of radicals from stable species as in Reaction 1 above or they may involve reactions of radicals with stable species to form more radicals.
Propagation reactions are those reactions involving radicals in which the total number of radicals remains the same. Termination reactions are those reactions resulting in a net decrease in the number of radicals. Two radicals combine to form a more stable species, for example: 2Cl·→ Cl2 Radicals can form by breaking of covalent bonds by homolysis; the homolytic bond dissociation energies abbreviated as "ΔH °" are a measure of bond strength. Splitting H2 into 2H•, for example, requires a ΔH ° of +435 kJ·mol-1, while splitting Cl2 into 2Cl• requires a ΔH ° of +243 kJ·mol-1. For weak bonds, homolysis can be induced thermally. Strong bonds require high energy photons or flames to induce homolysis. Radicals or charged species add to non-radicals to give new radicals; this process is the basis of the radical chain reaction. Being prevalent and a diradical, O2 reacts with many organic compounds to generate radicals together with the hydroperoxide radical; this process is related to rancidification of unsaturated fats.
Radicals may be formed by single-electron oxidation or reduction of an atom or molecule. These redox reactions occur in electrochemical cells and in ionization chambers of mass spectrometers. Although radicals are short-lived due to their reactivity, there are long-lived radicals; these are categorized as follows: The prime example of a stable radical is molecular dioxygen. Another common example is nitric oxide. Organic radicals can be long lived if they occur in a conjugated π system, such as the radical derived from α-tocopherol. There are hundreds of examples of thiazyl radicals, which show low reactivity and remarkable thermodynamic stability with only a limited extent of π resonance stabilization. Persistent radical compounds are those whose longevity is due to steric crowding around the radical center, which makes it physically difficult for the radical to react with another molecule. Examples of these include Gomberg's triphenylmethyl radical, Fremy's salt, such as TEMPO, TEMPOL, nitronyl nitroxides, azephenylenyls and radicals derived from PTM and TTM.
Persistent radicals are generated in great quantity during combustion, "may be responsible for the oxidative stress resulting in cardiopulmonary disease and cancer, attributed to exposure to airborne fine particles". Gomberg's free radical can be generated by following reaction in lab - 3C-Cl + Ag === 3C• + AgCl The reason for persistivity of free radicals is either the delocalisation of unpaired electron or the unavailability of unpaired electron to other species due to the screening of neighbouring atoms/groups. Diradicals are molecules containing two radical centers. Multiple radical centers can exist in a molecule. Atmospheric oxygen exists as a diradical in its ground state as triplet oxygen; the low reactivity of atmospheric oxygen is due to its diradical state. Non-radical states of dioxygen are less stable tha
Hindered amine light stabilizers
Hindered amine light stabilizers are chemical compounds containing an amine functional group that are used as stabilizers in plastics and polymers. These compounds are derivatives of tetramethylpiperidine and are used to protect the polymers from the effects of photo-oxidation, they are increasingly being used as thermal stabilizers for low and moderate level of heat, however during the high temperature processing of polymers they remain less effective than traditional phenolic antioxidants. HALS do not absorb UV radiation, but act to inhibit degradation of the polymer by continuously and cyclically removing free radicals that are produced by photo-oxidation of the polymer; the overall process is sometimes referred to as the Denisov cycle, after Evguenii T. Denisov and is exceedingly complex. Broadly, HALS react with the initial polymer peroxy radical and alkyl polymer radicals formed by the reaction of the polymer and oxygen, preventing further radical oxidation. By these reactions HALS are oxidised to their corresponding aminoxyl radicals, however they are able to return to their initial amine form via a series of additional radical reactions.
HALS's high efficiency and longevity are due to this cyclic process wherein the HALS are regenerated rather than consumed during the stabilization process. The use of a hindered amine possessing no alpha-hydrogens is key, as this prevents the HALS being converted into a nitrone species; this could either react with any alkene groups in the polymer to give inactive species. Though HALS are effective in polyolefins and polyurethane, they are ineffective in polyvinyl chloride, it is thought that their ability to form nitroxyl radicals is disrupted due them being protonated by HCl released by dehydrohalogenation of PVC. Hindered amine light stabilizers Eversorb® Light Stabilizers Gas treating