New Insights into Crystallization of Heterophasic Isotactic Polypropylene by Fast Scanning Chip Calorimetry.

Daniela Mileva,Jingbo Wang,René Androsch,Katalee Jariyavidyanont,Markus Gahleitner

doi:10.3390/polym12081683

Daniela Mileva, Jingbo Wang + Show 3 more

Open Access

https://doi.org/10.3390/polym12081683

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Abstract

The crystallization kinetics of metallocene-catalyzed heterophasic isotactic polypropylene composed of a matrix of isotactic polypropylene (iPP) and rubbery particles made of random ethylene–propylene copolymers (EPC), often denoted as heterophasic iPP copolymers, was analyzed as a function of the cooling rate and supercooling in nonisothermal and isothermal crystallization experiments, respectively. Fast scanning chip calorimetry (FSC) allowed assessing crystallization at processing-relevant conditions, and variation of the content (0–39 wt %) and composition (0–35 wt % propylene counits) of the EPC particles revealed qualitatively new insight about mechanisms of heterogeneous crystal nucleation. For neat iPP homopolymer, the characteristic bimodal temperature dependence of the crystallization rate due to predominance of heterogeneous and homogeneous crystal nucleation at high and low temperatures, respectively, is reconfirmed. At high temperatures, in heterophasic iPP, the here studied ethylene-(C2)-rich EPC particles accelerate crystallization of the iPP-matrix, with the acceleration or nucleation efficacy correlating with the EPC-particle content. The crystallization time reduces by more than half in presence of 39 wt % EPC particles. An additional nucleating effect of the EPC particles on iPP-matrix crystallization is detected after their crystallization, suggesting that liquid/rubbery particles are less effective than solid/semicrystalline particles in affecting crystallization of the surrounding iPP-matrix. At low temperature, homogeneous crystal nucleation in the iPP-matrix outpaces all heterogeneous nucleation effects, and the matrix-crystallization rate is independent of the sample composition. The obtained results lead to the conclusion that the crystallization kinetics of iPP can be affected significantly by the content and composition of EPC particles, even towards superfast crystallizing iPP grades.

Highlights

Since the introduction of isotactic polypropylene in the mid-1950’s [1,2], this polymer found numerous fields of application as commodity thermoplastic and for many engineering purposes [3,4]
Regarding the sample PP/ethylene–propylene copolymers (EPC)-0-0, that is neat isotactic polypropylene (iPP), slow cooling at rates up to around 200 K/s allows the formation of α-crystals, with the crystallization temperature decreasing with cooling rate
The experimental observations obtained on neat iPP further confirm fast scanning chip calorimetry (FSC) studies available in the literature [50,62,63,64], they are still shown here for comparison with the nonisothermal crystallization behavior of heterophasic iPP, synthesized using similar conditions

Summary

Introduction

Since the introduction of isotactic polypropylene (iPP) in the mid-1950’s [1,2], this polymer found numerous fields of application as commodity thermoplastic and for many engineering purposes [3,4]. Many of the favorable properties like stiffness, tensile strength, or heat resistance relate to the presence of a rather large fraction of crystalline phase; on the other hand, simultaneously, the material is characterized by relatively low ductility, toughness, or impact strength, in particular at low temperatures [5,6,7] Reasons for such deficiency may be the phenomena of so-called cross-hatching of crystal lamellae within the semicrystalline spherulitic superstructure [8,9,10,11,12], the crystallization-induced. This is achieved with the addition of 1-alkenes at random position into the propylene chain, simultaneously leading to a decrease of the glass transition temperature depending on the type and concentration of the counit [24,25,26,27,28,29]

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