LOW-PILL STAPLE POLYESTER

PET staple blends with wool and cotton were highly successful from the very first introduction
of PET in the 1950s. However, consumers soon noticed an annoying problem. It was the
formation of small fuzzy balls (called ‘‘pills’’) on the surface of fabrics. This phenomenon is
Polyester Fibers 19
known as ‘‘pilling’’ and it is common to all staple fibers, particularly if the level of yarn twist is
low, so that the fiber has many loose ends. The pills rub off harmlessly with wool because
wool is a weak fiber. However, PET is a strong fiber (tenacity ca. 5 g=decitex) and therefore
pills do not rub off; instead, they cling and have a negative impact on fabric esthetics. To
reduce pilling, the IV of the polyester is reduced to make weaker fibers. These do not pill so
obviously because the pills break away. A polymer of IV¼0.42 was selected as the best
compromise for a low-pill PET staple fiber, but it caused many problems. The melt viscosity
was so low and the molten polymer so fluid that the process became unstable. A method had
to be found to raise the effective melt viscosity of the polymer while maintaining the low-pill
properties to give an acceptable melt-spinning process. The method adopted was to introduce
branching points into the polymer chain by adding a multifunctional component
(either a polyacid or a polyhydric alcohol) so as to produce a star-branched polymer.
Such polymers are known to have higher melt viscosities for the same (nominal) polymer
IV. The branching agent added (ca. 1 mol%) was usually pentaerythritol. Too much
additive would lead to gel formation by forming cross-linked networks, but this is not a
problem at low levels [2].