Investigation of deformation and pore formation in isotactic polypropylene containing active nano-CaCO3

Abstract

In order to improve the β-lamellae distribution and properties of β-iPP membrane, 5% and 10% active nano-CaCO3 were added into β-iPP. The DSC, XRD and SEM results show that nano-CaCO3 would not reduce the content of β-crystal, but the thickness, lamellae thickness distribution and stability of β-lamellae decrease apparently. Then the tensile test was conducted at 25 °C and 90 °C. The results manifested that the second yield point, which has a strong negative correlation with lamellae thickness distribution, delays monotonously with adding nano-CaCO3 when stretched at 25 °C, indicating that nano-CaCO3 could narrow down the lamellae thickness distribution of β-iPP effectively. Furthermore, when stretched at 90 °C, the subdued yield peak, retarded necking down phenomenon and enhanced strain-hardening modulus demonstrated that the deformation stability improves gradually with introduction of nano-CaCO3, which has completely opposite law to the β-lamellae stability. Through further detailed characterization of morphological evolutions during stretching, we found that the interfacial debonding between nano-CaCO3 and β-iPP is triggered and abundant microviods can be formed, which can retard the rotation and slip of β-lamellae and make β-α transformation slow down in the initial stage of stretching, consequently make a better isotropic deformation. Moreover, nano-CaCO3 could restrain the formation of coarse fibrils efficiently, leading to more uniform pore size distribution within biaxial stretching microporous membrane. However, excessive nano-CaCO3 (10%) would cause the aggregation within the β-iPP cast film and finally resulted in larger pores and poor pore distribution in the membrane.