Caprolactam

Caprolactam (CPL) is an organic compound with the formula (CH₂)₅C(O)NH. This colourless solid is a lactam or a cyclic amide of caproic acid. Approximately 4.5 billion kilograms are produced annually. Caprolactam is the precursor to Nylon 6, a widely used synthetic polymer.

Synthesis and production

Caprolactam was first described in the late 1800s when it was prepared by the cyclization of ε-aminocaproic acid, the product of the hydrolysis of caprolactam. Given the commercial significance of Nylon-6, many methods have been developed for the production of caprolactam. Most of the 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):

The Beckmann Rearrangement

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.

The other major industrial route involves formation of the oxime from cyclohexane using nitrosyl chloride. The advantage of this method is that cyclohexane is less expensive than cyclohexanone. In earlier times, caprolactam was prepared by treatment of caprolactone with ammonia.

Uses

Almost all caprolactam produced goes into the manufacture of Nylon-6. The conversion entails a ring-opening polymerization:

n (CH₂)₅C(O)NH → [(CH₂)₅C(O)NH]_n

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.

Safety

Caprolactam is an irritant and is mildly toxic, with an LD₅₀ of 1.1 g/kg (rat, oral). In 1991, it was included on the list of hazardous air pollutants by the U.S. Clean Air Act of 1990. It was subsequently removed from the list in 1996. In water, caprolactam hydrolyzes to aminocaproic acid, which is used medicinally.

As of 2014 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".

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.