Time-dependent properties of isotactic polypropylene fibers.

Abstract

Many physical properties of polymers exhibit appreciable changes with the time. Some of these changes are due to the phenomenon reported as “physical aging.” Physical aging of polymers, depending only on time, occurs in amorphous polymers stored at a temperature below the glass transition temperature (Tg): when the system is rapidly cooled through Tg, it contains an excess of free volume and is not in thermodynamic equilibrium. The achievement of equilibrium in the glassy state is hindered by kinetic phenomena. The free volume excess decreases with time, with consequent decrease of mobility, which produces a slowing down of the overall process that is therefore defined as “self retarding.” The slow approaching of the thermodynamic equilibrium determines the change of the physical properties of the glassy polymers.1 In the case of semicrystalline polymers, either unoriented or cold drawn, changes of properties can be observed even in systems stored at temperatures above the Tg, due to a multiplicity of mechanisms. Stiffening occurs, for example, in fibers of low-density polyethylene and polypropylene, during the time the sample is left at room temperature after the mechanical treatment.2 The changes occurring in a material with aging can affect its application, performance, and lifetime. In this paper we analyze samples of isotactic polypropylene (iPP) drawn at 110°C and stored at room temperature up to two months: in this period of time, we followed many properties, to correlate change of structural organization and physical properties of the drawn samples. Fibers of isotactic polypropylene are widely used for many applications, and a study of the aging phenomenon, occurring after drawing, can help improve their applications and lifetime.