Aromatic retardants

The effects of added species. The rate of nitration of benzene, according to a rate law kinetically of the first order in the concentration of aromatic, was reduced by sodium nitrate, a concentration of io mol 1 of the latter retarding nitration by a factor of about Lithium nitrate... 

Another circumstance which could change the most commonly observed characteristics of the two-stage process of substitution has already been mentioned it is that in which the step in which the proton is lost is retarded because of a low concentration of base. Such an effect has not been observed in aromatic nitration ( 6.2.2), but it is interesting to note that it occurs in A -nitration. The A -nitration of A -methyl-2,4,6-trinitroaniline does not show a deuterium isotope effect in dilute sulphuric acid but does so in more concentrated solutions (> 60 % sulphuric acid kjj/kjj = 4 8). ... 

A nitro group behaves the same way m both reactions it attracts electrons Reaction is retarded when electrons flow from the aromatic ring to the attacking species (electrophilic aromatic substitution) Reaction is facilitated when electrons flow from the attacking species to the aromatic ring (nucleophilic aromatic substitution) By being aware of the connection between reactivity and substituent effects you will sharpen your appreciation of how chemical reactions occur... 

Bis(bexacbIorocycIopentadieno)cycIooctane. The di-Diels-Alder adduct of hexachlorocyclopentadiene [77 7 ] and cyclooctadiene (44) is a flame retardant having unusually good thermal stabiUty for a chlotinated aUphatic. In fact, this compound is comparable ia thermal stabiUty to brominated aromatics ia some appHcations. Bis(hexachlorocyclopentadieno)cyclooctane is usedia several polymers, especially polyamides (45) and polyolefins (46) for wire and cable appHcations. Its principal drawback is the relatively high use levels required compared to some brominated flame retardants. 

Tetrachlorphthalic Anhydride and Tetrachlorphthalic Acid. Tetrachlorphthalic anhydride [117-08-8] (TCPA) is manufactured by the ferric chloride catalyzed chlorination of phthalic anhydride. The relatively low chlorine content and the lower flame retardant efficiency of the aromatic chlorides limit use to unsaturated polyester resin formulations that do not requite a high degree of flame retardancy. 

Chloromethylation of the aromatic nucleus occurs readily with alkyl and alkoxy substituents accelerating the reaction and halo, chloromethyl, carboxyl, and nitro groups retarding it. 

The first HFIP-based polycarbonate was synthesi2ed from bisphenol AF with a nonfluorkiated aromatic diol (bisphenol A) and phosgene (121,122). Incorporation of about 2—6% of bisphenol AF and bisphenol A polycarbonate improved the dimensional stabkity and heat-distortion properties over bisphenol A homopolycarbonate. Later developments in this area concern the flame-retardant properties of these polymers (123,124). 

Polycarbonates are prepared commercially by two processes Schotten-Baumaim reaction of phosgene (qv) and an aromatic diol in an amine-cataly2ed interfacial condensation reaction or via base-cataly2ed transesterification of a bisphenol with a monomeric carbonate. Important products are also based on polycarbonate in blends with other materials, copolymers, branched resins, flame-retardant compositions, foams (qv), and other materials (see Flame retardants). Polycarbonate is produced globally by several companies. Total manufacture is over 1 million tons aimuaHy. Polycarbonate is also the object of academic research studies, owing to its widespread utiUty and unusual properties. Interest in polycarbonates has steadily increased since 1984. Over 4500 pubflcations and over 9000 patents have appeared on polycarbonate. Japan has issued 5654 polycarbonate patents since 1984 Europe, 1348 United States, 777 Germany, 623 France, 30 and other countries, 231. 

Another commercially available retarder for sulfur vulcanization is based on an aromatic sulfenamide. Like CTP, this product is most effective ki sulfenamide cure systems, but it also works well ki thiazole systems. Performance properties are generally not affected except for a slight modulus kicrease. In some cases this feature allows for the use of lower levels of accelerator to achieve the desked modulus with the added potential benefits of further scorch delay and lower cost cure system (23). 

Styrene is a colorless Hquid with an aromatic odor. Important physical properties of styrene are shown in Table 1 (1). Styrene is infinitely soluble in acetone, carbon tetrachloride, benzene, ether, / -heptane, and ethanol. Nearly all of the commercial styrene is consumed in polymerization and copolymerization processes. Common methods in plastics technology such as mass, suspension, solution, and emulsion polymerization can be used to manufacture polystyrene and styrene copolymers with different physical characteristics, but processes relating to the first two methods account for most of the styrene polymers currendy (ca 1996) being manufactured (2—8). Polymerization generally takes place by free-radical reactions initiated thermally or catalyticaHy. Polymerization occurs slowly even at ambient temperatures. It can be retarded by inhibitors. 

The spontaneous polymerization of styrene was studied in the presence of various acid catalysts (123) to see if the postulated reactive intermediate DH could be intentionally aromatized to form inactive DA. The results showed that the rate of polymerization of styrene is significantly retarded by acids, eg, camphorsulfonic acid, accompanied by increases in the formation of DA. This finding gave further confirmation of the intermediacy of DH because acids would have Httie effect on the cyclobutane dimer intermediate in the Flory mechanism. 

Aramid Fibers. Aromatic polyamide fibers exhibiting a range of mechanical properties are available from several manufacturers, perhaps the best known being Du Pont s proprietary fiber Kevlar. These fibers possess many unique properties, such as high specific tensile strength and modulus (see Fig. 4). Aramid fibers have good chemical resistance to water, hydrocarbons, and solvents. They also show excellent flame retardant characteristics (see High PERFORMANCE fibers Polyamdes). 

Substitutions that displace electrons toward the carboxyl group of aromatic acids diminish the rate of the reaction (16). The substitution of fluoromethoxy or ethoxy groups in the ortho position has an accelerating action, whereas iodo, bromo, nitro, or methyl groups produce retardation. The influence of groups in the meta and para positions is not nearly so marked (17). 

Through a study of the influence of thiophene and other aromatic compounds on the retardation and chain transfer on the polymerization of styrene by stannic chloride, the relative rates of attack of a carbonium-ion pair could be obtained. It was found that thiophene in this reaction was about 100 times more reactive than p-xylene and somewhat less reactive than anisole. ... 

Organic polymers provide one of the most versatile groups of materials and have widespread uses. Due to some inherent deficiencies, mainly poor heat and flame resistance, these materials suffer from limitations in certain areas of application. The resistance of polymers to high temperatures and flame may be increased by the incorporation of both aromatic rings and certain chemical elements in the polymer chain. It has been found that phosphorus, present either as a constituent in the polymer chain or incorporated as an additive in the form of a phosphorus compound to the polymer system, can make polymers flame retardant [109]. 

Phosphorus containing poly(maleimide-amines) were synthesized from N,N -bisdichloromaleimido-3,3 -diphenyl alkylphosphine oxides and aromatic diamines or piperazine [144]. The polymers prepared from piperazine are soluble in DMF, DM AC, DMSO, etc., but have poor thermal stability and flame retardancy. 

Polysulfone It is a high performance amorphous plastic that is tough, highly heat resistant, strong and stiff. Products are transparent and slightly clouded amber in color. Material exhibits notch sensitivity and is attacked by ketones, esters, and aromatic hydrocarbons. Other similar types in this group include polyethersulfone, polyphenyl-sulfone, and polyarylsulfone. Use includes medical equipment, solar-heating applications and other performance applications where flame retardance, autoclavability and transparency are needed. 

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