The effect of heat setting on the structure and mechanical properties of poly(ethylene terephthalate) fiber. I. Structural changes

Abstract We report structural changes in commercial multifilament poly(ethylene terephthalate) (PET) yarn when it is heat set in silicone oil over a range of temperatures between 100 and 220°C for times ranging from 1 min to 1 hr, while the yarn was (1) free to relax, and (2) held taut at constant length. In one case, viz., a sample heat treated for 1 hr, the cooling time was also varied to study the effect of rate of cooling. The free‐annealed and taut‐annealed samples showed considerable differences. The predominant role of the temperature of heat setting on structure is discussed in detail. There are considerable changes in the amount and orientation of the amorphous phase, and these will be shown to have important influence on mechanical properties in subsequent reports.