Biaxially Drawn Behavior of α- and β-Polypropylene Films under Simultaneous Stretching
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.
Zhong Cao, Xueqiang Cao, Lixian Sun, Yinghe He
N. Kerddonfag et al., "Biaxially Drawn Behavior of α- and β-Polypropylene Films under Simultaneous Stretching", Advanced Materials Research, Vols. 239-242, pp. 1939-1943, 2011