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FAQ: In Viscose-Lycra blended knit fabrics, after compaction and relaxation of one or two days, elongation occurs across the length of the fabric. Why does this happen and how can it be prevented?

There can be multiple causes for this phenomenon: 

a. Elongation of Viscose/blend fabrics during relaxation/storing is sometimes referred to as 'creeping'. In the case of Viscose blends with Lycra, the shrinkage/elongation problems could be compounded due to the Viscose component being vulnerable to easy deformation and slippage. Viscose can be partly stabilized with 6% caustic soda treatment (causticising). Causticising in the case of Viscose blends with Lycra can be carried out on a merceriser using 60 to 65 gpl caustic in the impregnation bath. Caustic concentration of 9% or higher will dissolve viscose and lead to consequent loss of weight. Hence, care needs to be taken not to exceed the recommended concentration. 

Short duration in the caustic bath would not affect Lycra, whereas the Viscose component would be better stabilized. During finishing, if a thermosetting resin is used, the Viscose blends shall become more dimensionally stable.

b. Mechanical shrinking beyond the threshold levels can also cause creeping as this strained state cannot sustain the shrinkage levels and hence can cause creeping on storage. 

c. Viscose from different sources shall have different shrinkage potentials under different chemical and physical processing. Every time a new batch of Viscose blend is used, it is necessary to take trials and fine-tune the processes including compacting. 

Thus, the real cause for creeping needs to be checked by repeated trials, and the dimensionally stable length that fabrics can reach after compacting needs to be determined. With a shrewd acumen for details, a processor can predict the behavior of a new blend and take precautions by conducting a few trials in the laboratory.