EFFECTS OF MOLECULAR WEIGHT AND MOLECULAR WEIGHT DISTRIBUTION ON STRETCHING PROCESS OF POLYPROPYENE FILAMENT

Abstract

This present paper intends to investigate necking phenomena occuring in stretching polypropylene filament and to study on stress-birefringence relations during stretching taking into consideration of the effects of molecular weight and molecular weight distribution.Six kinds of polymer fractions ranging in molecular weight of 4.24×104 to 25.1×104 and unfractioned sample of molecular weight 18.0×104 were used. Samples were meltspun under same conditions except spinning temperature to get 30 denier filament.Results obtaind are summerized as follows:(1) Necking is manifested very sharply with decrease of molecular weight and of stretching temperature.(2) Transverse lines in the stretched filament are observed when stretching is sufficiently large almost close to its breaking point. The distance between neighbouring lines becomes far apart with rise in stretching temperature. Transverse lines becomes not clear with increase in molecular weight.(3) Relationship between stress and birefringence strongly depends on molecular weight even under same stretching conditions. The force for molecular orientation increases remarkably with increase in molecular weight.(4) Birefringence of filament increases with stretching stress up to the assymptotic value, (Δn)max. (Δn)max for stretched filament at room temperature, at first, rapidly increases then decreases with increasing molecular weight. On the other hand, (Δn)max at 120°C continuously increases with molecular weight. It holds that (Δn)max at room temperature <(Δn)max at 120°C for filament of same molecular weight.(5) After stretching birefiringence in filament was found to be reasonably constant over the time investigated in spite of fast relaxation of stress when the ends of filamentis fixed at constant distance.(6) Increase in molecular weight gives more pronounced degree of shrinkage in length when the stretched filament is kept at natural length.