Viscose and rayon are soft, absorbent and luxurious fabric materials and often imitate the qualities of silk but they both have a major drawback. Viscose and Rayon are man-made(semi-synthetic) fibres made up of cellulose artificially. They become weaker by 50–70% especially in wet condition, they may tear easily. It is somewhat surprising when they have the same polymer which is present in cotton called cellulose. The reason is the different morphology of the fibres, The water acts lubricating and weakening of the fibres as there are more random amorphous areas than crystalline ( more in-line) areas in viscose and rayon, but in cotton, the fibre consists of longer cellulose chain that too with more crystalline areas.
Synthetic fibres Polyester, nylon etc are mostly crystalline and hydrophobic in nature and stronger also.