Caprolactam (CPL) is an organic compound with the formula (CH2)5C(O)NH. This colourless solid is a lactam (a cyclic amide) of caproic acid. Global demand for this compound is approximately five million tons per year, and the vast majority is used to make Nylon 6 filament, fiber, and plastics.[2]
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Caprolactam was first described in the late s when it was prepared by the cyclization of ε-aminocaproic acid, the product of the hydrolysis of caprolactam. World demand for caprolactam was estimated to reach five million tons per year for . 90% of caprolactam produced is used to make filament and fiber, 10% for plastics, and a small amount is used as a chemical intermediate.[2] Due to its commercial significance, many methods have been developed for the production of caprolactam. It was estimated that 90% of all caprolactam is synthesised from cyclohexanone (1), which is first converted to its oxime (2). Treatment of this oxime with acid induces the Beckmann rearrangement to give caprolactam (3):[2]
The immediate product of the acid-induced rearrangement is the bisulfate salt of caprolactam. This salt is neutralized with ammonia to release the free lactam and cogenerate ammonium sulfate. In optimizing the industrial practices, much attention is directed toward minimizing the production of ammonium salts.[2]
The other major industrial route involves formation of the oxime from cyclohexane using nitrosyl chloride, and this method accounts for 10% of world production.[2] The advantage of this method is that cyclohexane is less expensive than cyclohexanone.
Other paths to caprolactam include the depolymerization of waste Nylon 6, and the reaction of caprolactone with ammonia.[2] At bench scale, the reaction between cyclohexanone with hydrazoic acid to give caprolactam in the Schmidt reaction has been reported.[3]
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Almost all caprolactam produced goes into the manufacture of Nylon 6. The conversion entails a ring-opening polymerization:
Nylon 6 is widely used in fibers and plastics.
In situ anionic polymerization is employed for cast nylon production where conversion from ε-caprolactam to Nylon 6 takes place inside a mold. In conjunction with endless fiber processing the term thermoplastic resin transfer molding (T-RTM) is often used.
Caprolactam is also used in the synthesis of several pharmaceutical drugs including pentylenetetrazol, meptazinol, and laurocapram.
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Caprolactam is an irritant and is mildly toxic, with an LD50 of 1.1 g/kg (rat, oral). In , it was included on the list of hazardous air pollutants by the U.S. Clean Air Act of . It was subsequently removed from the list in at the request of the manufacturers.[4] In water, caprolactam hydrolyzes to aminocaproic acid, which is used medicinally.
As of caprolactam had the unusual status of being the only chemical in the International Agency for Research on Cancer's lowest hazard category, Group 4: "probably not carcinogenic to humans".[5]
Currently, there is no official permissible exposure limit set for workers handling caprolactam in the United States. The recommended exposure limit is set at 1 mg/m3 over an eight-hour work shift for caprolactam dusts and vapors. The short-term exposure limit is set at 3 mg/m3 for caprolactam dusts and vapors.[6]
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The production of caprolactam can produce nitrous oxide as a by-product, a highly potent greenhouse gas. Emissions differ significantly due to different production processes and inconsistent use of emission abatement technology. A study commissioned by the German Federal Ministry for Economic Affairs and Climate Action estimates emissions between 9 kg of nitrous oxide per ton of caprolactam and almost zero. [7]
Nitrous oxide emissions from caprolactam production are unregulated in most countries. Unlike other chemical production processes, nitrous oxide emissions from caprolactam production are not included in the European Union Emissions Trading System. [8]
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Nylon 6 or polycaprolactam is a polymer, in particular semicrystalline polyamide. Unlike most other nylons, nylon 6 is not a condensation polymer, but instead is formed by ring-opening polymerization; this makes it a special case in the comparison between condensation and addition polymers. Its competition with nylon 6,6 and the example it set have also shaped the economics of the synthetic fibre industry. It is sold under numerous trade names including Perlon (Germany), Dederon (former East Germany),[1] Nylatron, Capron, Ultramid, Akulon, Kapron (former Soviet Union and satellite states), Rugopa (Turkey) and Durethan.
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Polycaprolactam was developed by Paul Schlack at IG Farben in late s (first synthesized in ) to reproduce the properties of Nylon 66 without violating the patent on its production. (Around the same time, Kohei Hoshino at Toray also succeeded in synthesizing nylon 6.) It was marketed as Perlon, and industrial production with a capacity of 3,500 tons per year was established in Nazi Germany in , using phenol as a feedstock. At first, the polymer was used to produce coarse fiber for artificial bristle, then the fiber quality was improved, and Germans started making parachutes, cord for aircraft tires and towing cables for gliders.
The Soviet Union began its development of an analog in the s, while negotiating with Germany on building an IG Farben plant in Ukraine, basic scientific work was ongoing in . The production only started in in Klin, after USSR obtained the volumes of IG Farben, and 10,000 volumes of AEG technical documentation,[2] as a result of victory in the World War II.
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Nylon 6 can be modified using comonomers or stabilizers during polymerization to introduce new chain end or functional groups, which changes the reactivity and chemical properties. It is often done to change its dyeability or flame retardance.[3] Nylon 6 is synthesized by ring-opening polymerization of caprolactam. Caprolactam has 6 carbons, hence Nylon 6. When caprolactam is heated at about 533 K in an inert atmosphere of nitrogen for about 4&#;5 hours, the ring breaks and undergoes polymerization. Then the molten mass is passed through spinnerets to form fibres of nylon 6.
Polymerization of caprolactam to Nylon 6.During polymerization, the amide bond within each caprolactam molecule is broken, with the active groups on each side re-forming two new bonds as the monomer becomes part of the polymer backbone. Unlike nylon 6,6, in which the direction of the amide bond reverses at each bond, all nylon 6 amide bonds lie in the same direction (see figure: note the N to C orientation of each amide bond).
Nylon 6 (above) has a structure similar to Nylon 6,6 (below).[
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Nylon 6 fibres are tough, possessing high tensile strength, elasticity and lustre. They are wrinkleproof and highly resistant to abrasion and chemicals such as acids and alkalis. The fibres can absorb up to 2.4% of water, although this lowers tensile strength. The glass transition temperature of Nylon 6 is 47 °C.
As a synthetic fibre, Nylon 6 is generally white but can be dyed in a solution bath prior to production for different color results. Its tenacity is 6&#;8.5 gf/D with a density of 1.14 g/cm3. Its melting point is at 215 °C and can protect heat up to 150 °C on average.[4]
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Flavobacterium sp. [85] and Pseudomonas sp. (NK87) degrade oligomers of Nylon 6, but not polymers. Certain white rot fungal strains can also degrade Nylon 6 through oxidation. Compared to aliphatic polyesters, Nylon 6 has been said to have poor biodegradability. Strong interchain interactions from hydrogen bonds between molecular nylon chains is said to be the cause by some sources.[5] However, in a catalyst that rapidly breaks Nylon 6 down was reported. [6]
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At present, polyamide 6 is a significant construction material used in many industries, for instance in the automotive industry, aircraft industry, electronic and electrotechnical industry, clothing industry and medicine. Annual demand for polyamides in Europe amounts to a million tonnes. They are produced by all leading chemical companies.
The largest producers of polyamide 6 in Europe:[7]
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