The correlation of stress relaxation with elastic recovery in polypropylene film filaments

Abstract

A method has been developed for forecasting stress relaxation in polypropylene film filaments, which is based on linear viscoelasticity. The elastic recovery has been examined for polypropylene (PP) film filaments, which is related to stress relaxation processes of various durations. The recovery curves in semilogarithmic coordinates are characterized by initial and final asymptotic levels, and also by an S-shaped elastic component of the process, an important characteristic of which is the relaxation time. The recovery parameters are complicated and are nonlinearly dependent on the given level of strain and the duration of the direct relaxation. A method is proposed for the analytic description and forecasting of elastic recovery in PP film filaments on the basis of the parameters of the preceding stress relaxation: in accordance with the duration of the direct relaxation and the given level of stretching.Polypropylene (PP) film filaments are widely used for making fabrics for container products (bags and containers), and they have replaced more expensive packing materials [1]. In response to mechanical treatments, the fibers show relaxation: creep, stress relaxation, and elastic recovery. As such processes lead to considerable changes in deformation characteristics, they must be considered in the technology of processing and using the fibers. The relationship has been established [2][3][4] between the strain characteristics and the structural changes caused by relaxation in PP film fibers. Infrared spectroscopy and Raman scattering have shown that a consequence of stress relaxation is ordering in the structure of the amorphous regions, which makes itself felt in an increase in the deformation rigidity, while molecular destruction processes occur from the initial stages of stretching and continue up to the point where the filaments break. The structuring processes in deformed PP film filaments should be reflected also in the relaxation processes occurring in this material. Results have been given [5] from long researches on stress relaxation in these filaments over a wide range in strain and for various temperatures. However, in this research on relaxation, it is no less important to examine the regularities in the recovery of the filament dimensions after partial or complete removal of the external mechanical forces. The elastic recovery has not been extensively examined after the filaments have been kept for a certain time or for certain values of the strain in the deformed state, although they often occur in applications.This research has been concerned with stress relaxation and elastic recovery following stress relaxation in polypropylene film fibers, and also the development of methods of describing and forecasting those processes for long periods.We used PP film filament (made by Tver′khimvolokno Company) with drawing factor λ = 6, linear density T = 130 tex, failure load σ f = 580 MPa, and elongation at failure ε f = 22%. The mechanical properties were examined with an Instron-1122 instrument. We varied ...

Effect of strain level on stress relaxation and recovery behaviors of isotactic biaxially oriented polypropylene films

“…2,3 Among the oriented PP films, the biaxially oriented polypropylene (boPP) film is used in a variety of applications such as packaging for biscuits, crisps, baked goods, sweets; cigarette wrap, shrink wrap; industrial films and due to being less expensive it has replaced more expensive packing materials. [3][4][5] A well-known type of boPP film, that is, isotactic biaxially oriented polypropylene (i-boPP) film is greatly produced and the world production of this material is quite high and about 2 million tons per year. 5 When compared with other commonly used polymers, the isotactic boPP film has a range of remarkable properties such as good water barrier properties, 1 good elastic properties under short-time action, low specific density ($0.92 g/cm 3 ), high chemical resistance, almost zero hygroscopicity, and good dielectric properties.…”

“…4,11,14 Influence of stretching and drawing on the surface and structure of the PP films were discussed in some studies which involved generally the effect of the stretching conditions in the production stages of boPP films. [15][16][17][18] However, except few studies 19,20 which dealt with the effect of the level of preliminary deformation on the stiffness of the oriented PP fibers there is not any detailed study investigating the effect of preliminary extensions following stress relaxation on the uniaxial tensile properties of boPP films and the strain recovery process in long time.…”

“…In order to improve the properties of PP film, which has greater applications in industry and daily life than those in other forms, such as mechanical properties (tensile strength and Young's modulus), optical (haze, gloss) and barrier properties, biaxial orientation which means that stretching is applied in both directions, i.e., the machine direction (MD) and the transverse direction (TD), is seen to be a well‐known method in production processes . Among the oriented PP films, the biaxially oriented polypropylene (boPP) film is used in a variety of applications such as packaging for biscuits, crisps, baked goods, sweets; cigarette wrap, shrink wrap; industrial films and due to being less expensive it has replaced more expensive packing materials . A well‐known type of boPP film, that is, isotactic biaxially oriented polypropylene (i‐boPP) film is greatly produced and the world production of this material is quite high and about 2 million tons per year .…”

“…In the literature, the stress‐relaxation behaviors of PP at different forms such as thin films, PP‐based composites, fibers, and sutures at different temperatures and the effect of stress‐relaxation processes on their deformation properties were discussed in some studies and some studies were devoted to propose and discuss the mathematical description of the stress‐relaxation process and viscoelasticity and viscoplasticity of isotactic PP . Influence of stretching and drawing on the surface and structure of the PP films were discussed in some studies which involved generally the effect of the stretching conditions in the production stages of boPP films .…”