Curative

Polymer compounds often contain curatives or crosslinking agents. This is especially true for rubber compounds. The most important curatives are sulfur and sulfur compoimds. Elementary sulfur exists in a number of aUotropic forms [ 55 ]. Ordinary sulfur is a yellow solid substance, which forms crystals with orthorhombic symmetry. It is called orthorhombic (or rhombic, a-) sulfur. At 112.8 °C orthorhombic sulfur melts to form a straw-colored liquid, which consists of staggered ring Sg molecules and has a low viscosity. If this melt is cooled down, it crystallizes into a second polymorphic form monoclinic sulfur (y-sulfur). Orthorhombic sulfur (a-sulfur) is the most stable form at low temperatures, but above 95.5 C the most stable form is monoclinic sulfur (P-sulfur), which is stable between 94.5 and 120 C. This melts at 120 °C to form an Sg straw-colored melt. When molten sulfur is heated, it becomes more viscous because of its polymerization. It becomes darker in color and turns red. 

Sulfur is insoluble in water. It is, however, soluble in carbon disulfide, benzene, and toluene. Industry requires a sulfur that is insoluble in carbon disulfide and a sulfur that does not migrate to compoimd surfaces. Thus, a polymerized sulfur with different functionalities is used to avoid these difficulties. 

Selenium and tellurium, which occur below sulfur in periodic table, are both capable of producing crosslinking. They are, however, both more toxic and more expensive. They are relatively little used. 

For sulfur vulcanization one may also use various compoimds that degrade to liberate sulfur at the vulcanization temperature. These include dithiodimorpholine (DTDM) 

Another method of crosslinking polymers is to use peroxides, which have the structure O O 

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Malic acid crystallizes in colourless needles m.p. lOO C. It o- curs in many acid fruits, such as grapes, apples and gooseberries. It can be prepared by microbiological processes using various moulds or from ( + )-bromosuccinic acid by the action of NaOH. 

The accurate and absolute measurement of the distance, D, between the surfaces is central to the SFA teclmique. In a typical experiment, the SFA controls the base position, z, of the spring and simultaneously measures D, while the spring constant, k, is a known quantity. Ideally, the simple relationship A F(D) = IcA (D-z ) applies. Since surface forces are of limited range, one can set F(D = go) = 0 to obtain an absolute scale for the force. Furthennore, SF(D = cc)/8D 0 so that one can readily obtain a calibration of the distance control at large distances relying on an accurate measurement of D. Therefore, D and F are obtained at high accuracy to yield F(D), the so-called force versus distance cur >e. 

It is easy to see that the full shape of the orbital is better represented by the sum of these two Gaussians, especially at the tail of the cur ve where chemical bonding takes place, than it is by one Gaussian. When we run an STO-2G ab initio calculation on the hydrogen atom using the GAUSSIAN stored parameters rather than supplying oirr own, the input file is... 

M. Klobukowski, S. Huzinaga, Y. Sakai, Computational Chemutr.y Review., of Cur,vent Trend. Volume 3 49, J. Leszczynski, Ed., World Scientific, Singapore (1999). 

When we consider sources of methane we have to add old methane methane that was formed millions of years ago but became trapped beneath the earth s surface to the new methane just de scribed Firedamp an explosion hazard to miners oc curs in layers of coal and is mostly methane Petroleum deposits formed by microbial decomposi tion of plant material under anaerobic conditions are always accompanied by pockets of natural gas which IS mostly methane... 

The same kind of spontaneous racemization oc curs for any as 1 2 disubstituted cyclohexane in which both substituents are the same Because such compounds are chiral it is incorrect to speak of them as meso compounds which are achiral by definition Rapid chair-chair interconversion however converts them to a 1 1 mixture of enantiomers and this mix ture IS optically inactive... 

The predominant clinical use of corticosteroids is a result of their associated antiinflammatory properties. These are commonly used as topicals for the suppression of symptoms, including inflammation, occurring in a particular disease state these compounds are rarely considered curative in their usage. Many other disease states do, however, respond well symptomatically to treatment with corticosteroid therapy. Some of these (11) are Hsted below. 

Natural-mbber-based pressure-sensitive adhesives can be cured by standard mbber curatives, eg, sulfur plus an accelerator (see Rubber, natural) ... 

The most ancient uses of spices appear to be therapeutic in nature. The use of spices was common in China but tittle, if any, authentic Chinese records exist to confirm this. According to Chinese myths and legends, Shen Nung, the Divine Cultivator, founded Chinese medicine and discovered the curative powers of many herbs. He is said to have described more than 100 plants in a treatise reportedly written in 2700 BC. It has been shown, however, that no written language was available in China at that time. Although some of the herbal uses in the treatise go back several centuries BC, the work seems to have been produced by unknown authors in the first century AD. Other records on the use of cassia and ginger are known to have been written in the fifth and fourth centuries BC, in the latter case by Confucius. 

Process. Any standard precursor material can be used, but the preferred material is wet spun Courtaulds special acrylic fiber (SAF), oxidized by RK Carbon Fibers Co. to form 6K Panox B oxidized polyacrylonitrile (PAN) fiber (OPF). This OPF is treated ia a nitrogen atmosphere at 450—750°C, preferably 525—595°C, to give fibers having between 69—70% C, 19% N density less than 2.5 g/mL and a specific resistivity under 10 ° ohm-cm. If crimp is desired, the fibers are first knit iato a sock before heat treating and then de-knit. Controlled carbonization of precursor filaments results ia a linear Dow fiber (LDF), whereas controlled carbonization of knit precursor fibers results ia a curly carbonaceous fiber (EDF). At higher carbonizing temperatures of 1000—1400°C the fibers become electrically conductive (22). 

Hexamethylenetetramine. Hexa, a complex molecule with an adamantane-type stmcture, is prepared from formaldehyde and ammonia, and can be considered a latent source of formaldehyde. When used either as a catalyst or a curative, hexa contributes formaldehyde-residue-type units as well as benzylamines. Hexa [100-97-0] is an infusible powder that decomposes and sublimes above 275°C. It is highly soluble in water, up to ca 45 wt % with a small negative temperature solubiUty coefficient. The aqueous solutions are mildly alkaline at pH 8—8.5 and reasonably stable to reverse hydrolysis. 

The polyamide copolymer of dodecanoic acid with methylenedi(cyclohexylamine) (MDCHA, PACM) was sold as continuous filament yam fiber under the tradename QIANA. As late as 1981, over 145,000 t was produced using high percentages, typically 80%, of trans, trans MDCHA isomer. The low melting raffinate coproduct left after t,t isomer separation by fractional crystallisation was phosgenated to produce a Hquid aUphatic diisocyanate marketed by Du Pont as Hylene W. Upon terrnination of their QIANA commitment, Du Pont sold the urethane intermediate product rights to Mobay, who now markets the 20% trans, trans—50% cis, trans—30% cis, cis diisocyanate isomer mixture as Desmodur W. In addition to its use in polyamides and as an isocyanate precursor, methylenedi (cyclohexyl amine) is used directiy as an epoxy curative. The Hquid diamine mixture identified historically as PACM-20 is marketed as AMICURE PACM by Anchor Chemical for performance epoxies. 

Cyclohexanediamine is marketed by Milliken Chemical as an epoxy curative, Millamine 5260. It may be adducted with epichlorohydrin to generate solventiess low viscosity curatives for varnishes and surface coatings (76). Other cycloaHphatic diamines have long been modified as epoxy curatives to modify their reactivity profile (77). 

MCHD from ring reduction of I DA (60,78) has been cited as an epoxy curative (79) and is available from Air Products and Chemicals as a developmental cycloaHphatic diamine. Ring reduction of stericaHy hindered arylenediamines such as diethyltoluenediamine [68479-98-1J -ptovides slower-reacting alkylated 1,3-cyclohexanediamines for polyurethane, polyurea, and epoxy use (80). 

Menthanediamine has been effectively reacted to form polyamides (84) and is sold in metric tons per year volume as a premium (- SlS/kg) epoxy curative (85) by Rohm and Haas. 1-Adamantylamine hydrochloride [665-66-7] is a prophylactic against type A viral infections sold by Du Pont under the trade name Symmetrel. 

Only a few commercial uses for TDA per se have been found. In epoxy curing appHcations, 2,4- I DA has been used as a component of a eutectic mixture with short chain aUphatic glycidal ether resins (46) as well as by itself (46,47) TDA (46) and single isomers (47) are also used as amine curatives. TDA can be used as a chain extender in polyurethanes (48,49). TDA is cited as a monomer in making aromatic polymers with unique properties, eg, amorphous polyamides (50), powdered polyamides (51), and low melting, whoUy aromatic polyamides (52). 

Alkylated and alkenated toluenediamines are used as antioxidants (qv) in oils and elastomers (10,53,63—65), as chemical intermediates for polyamides, polyimides, and polyesterimides (53,1) and as epoxy curatives (53,58,66) (see Epoxy resins). 

Diamine curatives were the first cross-linking agents for fluorocarbon mbbers. They are corrosive to mild steel molds and have been replaced in many appHcations by the bisphenol or other more recent cure systems. Nevertheless, some diamines are stiU used for food-contact appHcations of fluorocarbon mbbers and in zinc-free cures of halobutyl mbbers for pharmaceutical stoppers. Methylene dianiline and triethylene tetramine are cross-linking agents for ethylene—acryflc elastomers. 

Amine Cross-Linking. Two commercially important, high performance elastomers which are not normally sulfur-cured are the fluoroelastomers (FKM) and the polyacrylates (ACM). Polyacrylates typically contain a small percent of a reactive monomer designed to react with amine curatives such as hexamethylene-diamine carbamate (Diak 1). Because the type and level of reactive monomer varies with ACM type, it is important to match the curative type to the particular ACM ia questioa. Sulfur and sulfur-beating materials can be used as cure retarders they also serve as age resistors (22). Fluoroelastomer cure systems typically utilize amines as the primary cross-linking agent and metal oxides as acid acceptors. 

Curatives and accelerators are combkied to achieve desired performance properties through cure system design. 

Process Oils, Plasticizers. Petroleum-based mbber process oils generally contain a mixture of paraffinic, naphthenic, and aromatic components. These oils vary in composition from grade to grade, but most contain some unsaturated moieties and this unsaturation can compete with the polymer for curatives. Therefore, state of cure can be decreased. This is not easily detected because oil softens the compound which masks the loss of state of cure. 

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