Nylon 6

A polyamide that can be drawn into fishing line.

Also known as
PA6, polyamide 6, polycaprolactam

In everyday products

Structure and behaviour

[–NH–(CH₂)₅–C(=O)–]ₙ

The ring-opened caprolactam unit contains six carbons, including the amide carbonyl. The amide N–H and C=O groups support hydrogen bonding; each repeat supplies both groups without a separate diamine–diacid pair.

  • A single filament can be made continuously by extrusion; drawing aligns chains and changes strength along its length.
  • Fine filaments bend more readily than thick ones, which helps distinguish cleaning bristles from a solid nylon part.
  • Resistance to wear is useful at contact surfaces, but a knot still introduces a local stress concentration.
  • Water can enter the polyamide and change stiffness, toughness and dimensions; wet and dry performance differ.
  • PA6 and PA66 share amide chemistry, but material choice still requires grade-specific data rather than assuming identical properties.

Preparation routes

Caprolactam route

Hydrolytic ring-opening polymerisation. Opening caprolactam rings and linking the units forms nylon 6 chains. Unlike nylon 6,6, this route does not require a separate diamine–diacid pair. Ring opening retains the monomer’s amide-derived atoms in each six-carbon repeat. It is not the same feedstock reaction as forming nylon 6,6 from two molecules.

Water participates in initiation and the reaction equilibria. Later extraction and drying remove residual small molecules and prepare the polymer for processing. Drawing line or brush filaments aligns existing chains rather than continuing the ring-opening reaction.

  1. Polyamides: nylon 6 and nylon 6,6Chemical Industry Education Centre, University of York
  2. Reference document on best available techniques in the production of polymersEuropean Commission, Joint Research Centre · 2007

From cyclohexanone through an oxime to caprolactam

  1. Cyclohexanone + Ammonia + Hydrogen peroxide → Caprolactam

    In the selected ammoximation route, cyclohexanone, ammonia and hydrogen peroxide produce cyclohexanone oxime. A separate vapour-phase Beckmann rearrangement converts that oxime into the cyclic amide caprolactam. Nitrogen enters the ring in the rearrangement; the six-carbon skeleton becomes a seven-membered lactam ring, ready for a later nylon 6 polymerisation.

  1. New technology to manufacture caprolactam: oxime formation and Beckmann rearrangementSumitomo Chemical
  2. Caprolactam process feedstocks, Investors Handbook 2012, section 5-5Sumitomo Chemical
  3. Vapour-phase Beckmann rearrangement technology transfer, November 2024Sumitomo Chemical

Material limits

Fishing-line performance depends on polymer, coating, diameter, drawing, knots and moisture. Separate brush bodies and metal anchors by material.

Sources

  1. Polyamides: nylon 6 and nylon 6,6Chemical Industry Education Centre, University of York
  2. Polyamides for monofilaments and fishing lineBASF
  3. Ultramid polyamides: processing, moisture and conditioningBASF
  4. Sustainable plastics: the role of chemistryRoyal Society of Chemistry