Production of Polyamide 6

The ketone - alcohol mixture (KA) produced from benzene summary is first converted to pure cyclohexanone, which is then used to make caprolactam, the monomer used to make PA 6.

chemical equations: caprolactam from "KA"

Making Polyamide 6
Caprolactam has typically been converted to PA 6 in a batch process, heating it with water and ethanoic acid (to regulate the length of polymer chain produced) for 12 hours at about 350°C.

Continuous production is always preferable, however, and this is also possible. The reaction vessels used are designed to allow more precise temperature control, with heat being removed and recycled. This means the operating temperature does not exceed 280°C, but a higher percentage of caprolactam is polymerised during the 16-20 hours spent in this stage of the process.

Unreacted caprolactam is extracted and re-fed into the reactor, along with waste polymer from later processes. This constant recycling helps keep waste to a minimum.

Polymerisation
The reaction is catalysed by water, which begins with the opening of the caprolactam ring.

chemical equation: caprolactum
The linear molecule produced will react with more caprolactam, producing a dimer. Reaction will continue between the growing polymer and more caprolactam.

chemical equation: more caprolactam

Modifications
The polymerisation may be carried out in two stages, with an increased rate of ring opening in the first stage. This makes the polymerisation quicker, and increases the output - fewer reactors are therefore required. A single stage reactor will typically have an output up to 130 tonnes per day, whereas a two-stage process can increase this to 300 tonnes per day.


diagram: polyamide 6 production


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