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In the stress relaxation process of HEPP (Hard Elastic Polypropylene) fibre, the added liquids,e. g. ethanol and n-hexane, which can wet polypropylene, cause an abrupt decrease of stress. By measuring the contact angles and decreases of stress and evaluating according to Young-Dupre’s equation, it was found that about one-third of the initial stress is associated with the sur-face energy. When soaked in liquids mentioned above, HEPP fibre almost lost its elasticity. It isnot the result of plasticizing or ’solvent cracking’ but most likely comes from the changes of the sur-face energy.