ε-Caprolactam: Properties, Reactions and Uses

caprolactam Structure

Caprolactam, also known as 2-oxohexamethylenimine or hexahydro-1H-azepin-2-one, is a cyclic amide derived from caproic acid with the formula C6H11NO. It exists as a white, hygroscopic solid with a slight odor. It is used to produce Nylon 6 and the global demand for caprolactam surpasses 5 million tons annually.

ε-Caprolactam discovery dates back to the 19th century, with independent syntheses reported by S. Gabriel and T.A. Maas in 1899 via cyclization of ε-aminocaproic acid, and by O. Wallach through Beckmann rearrangement of cyclohexanone oxime.

However, the commercial potential of ε-caprolactam remained largely untapped until 1938, when P. Schlack of IG Farbenindustrie achieved a breakthrough by polycondensing it into the first spinnable polymer, Nylon 6. This discovery increased the production volume of ε-caprolactam and its industrial interest.

For a detailed article on the industrial production of caprolactam, consult the following link →

Table of Contents

1. Physical Properties of Caprolactam

ε-Caprolactam (CAS: 105-60-2) is a white, hygroscopic, crystalline solid with a characteristic odor. Its physical properties are summarized in the following table:

Table 1: Physical Properties of Caprolactam
Property Value
molar mass 113.16
melting point, °C 69.2
boiling point, °C (101.3 kPa) 268.5
boiling point, °C (6.7 kPa) 174
boiling point, °C (1.3 kPa) 134
boiling point, °C (0.4 kPa) 111
Density, kg/L (120°C) 0.9829
Density, kg/L (100°C) 0.9983
Density, kg/L (80°C) 1.0135
Viscosity, mPa·s (120°C) 2.93
Viscosity, mPa·s (100°C) 4.87
Viscosity, mPa·s (80°C) 8.82
Specific heat, kJ kg⁻¹ K⁻¹ (150°C) 2.345
Specific heat, kJ kg⁻¹ K⁻¹ (80°C) 2.135
Heat of fusion, kJ/kg 123.5
Heat of polycondensation, kJ/kg -138
Heat of vaporization, kJ/kg (268°C) 481
Heat of vaporization, kJ/kg (168°C) 574
Heat of vaporization, kJ/kg (105°C) 628
Vapor pressure, kPa (268°C) 101.3
Vapor pressure, kPa (168°C) 5.3
Vapor pressure, kPa (105°C) 0.25
Flash point, °C 139.5
Ignition temperature, °C 375
Lower explosion limit (135°C), vol% 1.4
Upper explosion limit (180.5°C), vol% 8.0
Thermal conductivity coefficient (76–183°C), kJ m⁻¹ h⁻¹ K⁻¹ 0.5
Coefficient of volume expansion (80–90°C), K⁻¹ 0.00104

ε-Caprolactam has high solubility in polar and aromatic solvents, while its affinity for high molecular weight aliphatic hydrocarbons is limited. The detailed solubility data for various solvents are found in Table 2.

Table 2: Solubility of ε-Caprolactam (wt%)
Solvent 20°C 30°C 40°C 50°C
Water 82 86.5 90 93.5
Toluene 26 36.5 51 66.5
Ethyl acetate 24.2 33.3 48.5 66.2
Methyl ethyl ketone 34.6 45.7 59.2 72.9
Cyclohexanone 34.6 42.2 54.5 68.2
Cyclohexane 2 2.5 7 18.5

2. Chemical Reactions of Caprolactam

The importat characteristic of ε-caprolactam is its ability to polymerize to Nylon 6. In the presence of water (260–270°C), ring hydrolysis and subsequent polycondensation yield linear polymer chains. This equilibrium reaction yiels a 90% conversion to polymer under industrial conditions.

Alternatively, direct polyaddition reactions with existing polymer chains contribute to further polymerization.

Low moisture content (<100 ppm) enables anionic polymerization with a catalyst and cocatalyst system. This method operates at lower temperatures compared to hydrolytic polymerization.

Beyond polymerization, ε-caprolactam exhibits diverse reactivity, readily participating in various cyclic amide reactions, including:

  • Oxidation: Molten caprolactam absorbs atmospheric oxygen, forming small amounts of peroxides at temperatures above 75°C and adipic acid imide above 100°C, the latter reaction is catalyzed by trace heavy metals. In the absence of oxygen, it demonstrates good thermal stability.
  • Hydrolysis: Quantitative conversion to ε-aminocaproic acid is achieved by hydrolysis with aqueous acids or alkalis.
  • N-alkylation: Reaction with gaseous methanol in the presence of a dehydrating catalyst yields N-methyl-ε-caprolactam, a valuable solvent.
  • Reactions with phosgene, nitric acid, and subsequent reduction provide chloroformic acid ε-caprolactim ester, nitrocaprolactam, and aminocaprolactam, respectively.
  • Resolution: L-lysine is commercially produced from ε-caprolactam via hydrolysis and resolution of the resulting racemate.

While numerous additional chemical reactions are possible with ε-caprolactam, the polymerization to Nylon 6 is the only industrially important.

3. Uses of Caprolactam

Nearly all ε-caprolactam is converted to Polyamide 6, with applications including: engineering resins (40%), textile fibers (40%), high-tenacity yarn (tires, cables) (12%), bulked carpet yarn (6%) and staple yarns (1.5%).

Application distribution varies geographically, with Asian consumption focusing on fibers and mature economies favoring engineering resins.

Caprolactam also finds applications in filaments for fishing lines and 3D printing, films for food packaging, and membranes for filtration processes.

In 2016, global ε-caprolactam consumption reached ~5.5 million tons, driven primarily by rising demand for Polyamide 6 (PA6) fiber applications in China, which dominates the market, followed by other Eastern Asian countries, Western Europe, and the USA.

4. Toxicology of Caprolactam

Acute Toxicity:

  • ε-Caprolactam exhibits relatively low acute oral toxicity in rats (LD50: 1155-1660 mg/kg).
  • Tonoclonic convulsions are observed as the primary symptom of acute intoxication.
  • Rabbits and cats are more susceptible compared to rats.

Skin and Eye Irritation:

  • Repeated skin contact (50% solution) and intradermal injection (0.1%) did not cause local irritation or sensitization in guinea pigs.
  • However, some animal studies suggest weak skin sensitization potential.
  • Eye exposure to 5-10% solutions in rabbits did not induce irritation.

Inhalation and Chronic Effects:

  • Workplace exposure to ε-caprolactam occurs primarily through inhalation of dust or vapor.
  • Repeated inhalation of dust (118-261 mg/m³ ) by guinea pigs was tolerated with minimal effects.
  • Subchronic inhalation studies in rats at higher concentrations (500 mg/m³ vapor, 120-150 mg/m³ aerosol) showed prolonged estrus cycle and altered ovarian function in females.

Developmental and Reproductive Toxicity:

  • ε-Caprolactam did not exhibit teratogenicity in rats orally administered 1/10 the LD50 dose during gestation.
  • Effects on the kidneys are considered the most concerning potential chronic toxicity.

Mutagenicity and Carcinogenicity:

  • Short-term genotoxicity tests (Ames, CHO, cell transformation) were negative for ε-caprolactam.
  • Long-term feeding studies in mice and rats at high doses (up to 15000 ppm) revealed no carcinogenicity, only dose-dependent weight gain reduction.

Workplace Safety and Exposure Limits:

  • Handling ε-caprolactam can cause skin and eye irritation, though skin sensitization is rare.
  • Some reports suggest menstrual irregularities and pregnancy complications in female workers exposed to high concentrations (“a multiple of 10 mg/m³”).
  • The US TLV and REL for ε-caprolactam (both dust and vapor) are 5 and 1 mg/m³, respectively.

Reference