Biaxially Drawn Behavior of α- and β-Polypropylene Films under Simultaneous Stretching

Abstract

This study investigated the biaxially drawn behavior of polypropylene (PP) containing different crystalline structure in between of either α-monoclinic or β-hexagonal crystal form or a combination of both. The precursor sheet with α-crystalline PP was prepared from neat resin using the sheet extrusion process. The high content of β-crystalline phase (K-value of about 84%) was developed in the polypropylene sheet using a proprietary type and concentration of β-crystal nucleator. Biaxial film stretching was carried out using a laboratory stretching machine at the temperature of 145°C. Stretching speed and stretch ratio were kept relatively constant at 400 %/sec and 5.5 x 5.5, respectively. β-PP sample demonstrated a distinct biaxially drawn behavior as compared to that of the neat-PP sample, in particular, biaxial yield stress of neat PP (α-PP) was 3.55 MPa, where a much lower biaxial yield stress value was apparent for β-PP (~1.69 MPa). Interestingly, the total calculated area under stretching curves of the β-nucleated PP sample was about 34.8 % lower than that of the neat PP sample. These findings indicate the less energy required for stretching process of β-nucleated PP. Moreover, the drawn β-PP films also showed comparable mechanical properties to the α-PP sample as well as a good optical property, evidenced by low haze values of <1% which was close to that for the normal BOPP films.

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

“…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 . However, except few studies 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.…”

“…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.…”