Beta-modification of isotactic polypropylene

Abstract

Commercial grades of isotactic polypropylene (iPP) crystallize essentially into α-modification (α-iPP) with sporadical occurrence of the β-phase (β-iPP). Crystallization in temperature gradient or in sheared melt encourages the development of the β-phase in commercial, non-nucleated iPP [1]. For preparation of samples rich in β-modification or of pure β-iPP, the introduction of selective β-nucleants is the most reliable method [1]. The known β-nucleating agents are collected in Tables 1 and 2, indicating their advantages and drawbacks. The most widespread high active β-nucleating agent is a γ-quinacridone red pigment. Some two-component compounds obtained by the reactions of certain organic acids with CaCO₃, also possess a very high β-nucleating activity. Different calcium and zinc salts of aliphatic and aromatic dicarboxylic acids having high thermal stability, belong to the selective β-nucleants, as well [1]. The β-content of iPP samples — and so the efficiency of β-nucleants and the influence of thermal and mechanical conditions of the crystallization on the polymorphic composition - can be characterized by the k-value determined from wide angle X-ray scattering (WAXS), by differential scanning calorimetry (DSC) on non-recooled samples or by polarized light microscopy (PLM) using thin sections of the samples [1]. It should be pointed out that the melting curves of iPP samples cooled to room temperature do not give correct information about the β-content. This statement is due to the βα-recrystallization which takes place during heating of the sample and overlaps with the melting process of the β-phase [1, 2]. Unfortunately, the efficiency of β-nucleants was characterized by DSC on cooled samples in many studies. Therefore, the quantitative conclusions of these studies should be treated with caution. Studies on β-nucleated iPP revealed that the formation of pure β-iPP has an upper (T(βα) = 140°C) and a lower limit temperature (T(aβ) = 100-110°C) [1]. The crystal structure, morphology, melting and recrystallization behavior of β-iPP are discussed in detail in a later chapter’ spherulitic crystallization and structure’. This section deals with the preparation, properties and application of β-iPP.