Polypropylene stress relaxation curve

In order to semi-quantitatively estimate the surface-energy-associated retractive stress, the stress relaxation curves were studied, curve 1 in Figure 2 is the stress relazation curve of HEPP at 0% elongation. It tends to level off after 6 min. When samples were soaked in liquids which can wet polypropylene surface, the stress dropped immediately as shown by curves 2 and 3 in Figure 2. After liquids were removed and evaporated from the fibre, the stress relaixation curves went back to their previous height. 

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