Spinning polyester and nylon

Section 3 of 5

Polyester and nylon are the condensation polymers you met earlier: two monomers taking turns along each chain, joined by ester links in polyester and amide links in nylon. A factory turns them into fibres by melting them, spinning them and lining up their chains.

PET, the polyester of drinks bottles, melts at about 260 °C, and nylon-6,6 at about 265 °C. The molten polymer is pushed through a spinneret, a metal plate with tiny holes like a shower head, and each jet cools in a stream of air into a thin filament. A freshly spun filament is weak and stretchy, because its chains lie tangled at random, just as they were in the melt.

Next comes drawing. The filament passes over one roller and on to a second that turns about four times as fast, so it is stretched to about four times its length. The pull straightens the chains and lines them up along the fibre, where they pack close and pull on their neighbours along their whole length. The drawn fibre is thinner and several times stronger. A strip of carrier bag turns thin and white as you pull it for the same reason: its chains are lining up.

Drawing a fibre lines its chains up Top: a short, thick piece of freshly spun fibre, its chains tangled at random inside it, with one chain in amber. An arrow points down, labelled stretched to 4 times its length. Bottom: the same fibre drawn out long and thin, its chains now lying straight and close together along its length. The amber chain lies straight among them. freshly spun chains tangled stretched to 4 times its length drawn out chains lined up along the fibre
Drawing a fibre. In the freshly spun filament the chains lie tangled. Stretched to four times its length, the fibre grows thin, and its chains lie straight along it. Follow the amber chain.

Nylon and polyester side by side

Nylon's amide links carry the same N–⁠H and C=O pair as silk and wool. In a drawn nylon fibre, the N–⁠H groups of each chain make hydrogen bonds with the C=O groups of the chains beside it, so nylon is strong and tough: ropes, fishing line, tights and parachutes. PET's chains are benzene rings and CH₂ groups joined through ester links, with no N–⁠H or O–⁠H groups along them, so they hold each other with weaker pulls, helped by the flat rings stacking close together.

Groups that make hydrogen bonds hold on to water too, so they decide how much water a fibre soaks up from damp air. Silk has an amide link at every amino acid, and nylon has runs of four and six CH₂ groups between its links, so nylon takes up less. PET's chains carry C=O groups but no N–⁠H or O–⁠H, and their flat rings pack tightly and keep water out, so polyester stays light when you sweat and dries fast, which suits sports clothes and fleeces.

Water a fibre takes up from air at 20 °C and 65% humidity, as a share of its dry weight: typical values
FibreGroups that hold waterWater
woolN–⁠H, C=O and moreabout 15%
silkN–⁠H, C=Oabout 11%
cotton-⁠OHabout 8%
nylonN–⁠H, C=Oabout 4%
polyesterC=O onlyabout 0.4%

Quick check

A freshly spun nylon filament is stretched to about four times its length. What does the stretching do?