Influence of structure gradients in injection moldings of isotactic polypropylene on their mechanical properties

Highlights

• The internal structure of injection molded model iPP components is studied.

• Changes in polymorphic state, crystallinity and spherulitic superstructure are quantified.

• The influence of structural features on mechanical properties is investigated.

• Storage modulus and toughness vary depending on the local structure.

Abstract

The internal structure of injection molded model components made from a commercial non-nucleated isotactic polypropylene grade is investigated by wide angle X-ray diffraction and polarized-light optical microscopy. Changes in the polymorphic state, degree of crystallinity and spherulitic superstructure are quantified depending on the distance from the outer surface $d$. Interrelations between structure and local cooling rate as obtained from numerical simulations are considered. The influence of structural features on mechanical properties is studied using thin quasi-homogenous films microtomed at different depth from the injection molded component. A significant variation of the storage part of the tensile modulus $E^{\prime }$ and the toughness $W_t$ depending on the local semi-crystalline state is observed. A pronounced maximum in $W_t$ is found about $200\mathrm{\mu }\mathrm{m}$ below the surface where a fine spherulitic superstructure occurs. The changes in $E^{\prime }$ are moderate (values ranging from 1.4 to 1.8 MPa) and show a maximum at intermediate depth although the degree of crystallinity is continuously increasing from $\approx$ 27% at the surface to $\approx$ 45% in the core region. Relations between structural features and mechanical properties measured at small and large deformation ($E^{\prime }$ and $W_t$) are discussed. General conclusion is that a deeper understanding of relations between structural state of semi-crystalline polymers determined by the processing conditions and mechanical parameters is of major importance for predicting the properties of injection molded components.

Introduction

The majority of plastic components – which are increasingly used in modern applications – is made from semi-crystalline thermoplastics and produced by injection molding[1]. Polypropylenes with different tacticity are often used due to a high variability of the structural states which can be achieved. It is well known that depending on the chosen processing conditions, tacticity and used nucleating agents, states ranging from practically amorphous to highly crystalline with $\alpha$ (monoclinic)[2], [3], $\beta$ (trigonal)[4], [5] or $\gamma$ (orthorhombic)[6] structure can be obtained. In addition, the formation of a mesophase has been reported for melt quenched isotactic polypropylene (iPP) samples, which are solidified at low temperatures[7], [8]. In case of injection molded components, the structural state is determined to a large extent by the processing conditions[2]. Parameters like cooling rate, shear flow as well as pressure influence the multiscale structure of semi-crystalline polymers[9], [10], [11], [12], [13], [14], [15], [16]. Although this is in principle known for a long time, the consequences of this structural heterogeneity of polymers depending on the processing conditions is usually neglected in state-of-the-art simulations of plastic components[17]. The semi-crystalline state of thermoplastics is usually not taken into account or considered only in a very qualitative manner. The mechanical properties of different structural states co-existing in a spatially heterogeneous injection molding are commonly not considered and not even known since not measured on structurally homogeneous reference samples. This is a serious limitation for the prediction of mechanical properties of components since the mechanical parameters of different structural states can vary significantly[18], [19], [20]. Earlier measurements of differently nucleated isotactic polypropylene (iPP) samples have demonstrated this clearly[21]. However, detailed mechanical data sets for individual structural states do exist only in exceptional cases[22], [23]. Moreover, it is basically unexplored in which way different features of the multiscale structure of a semi-crystalline polymer like iPP will influence the mechanical properties[24]. For a better prediction of the mechanical properties of thermoplastic-based components it seems to be therefore urgently required to understand and quantify systematically how structural parameters like degree of crystallinity, spherulite size or lamellae thickness and orientation influence mechanical parameters like Young’s modulus, strength or toughness. This is an important precondition to improve the quality of FEM simulations of the mechanical properties of complex injection molded components and other application-relevant effects like component distortion. In the present work we have studied process-structure–property relations based on model injection moldings made from a commercial iPP without nucleating agent. Structural states occurring in different regions of injection-molded iPP bars are characterized and the corresponding mechanical properties are quantified based on $50\mathrm{\mu }\mathrm{m}$ sections microtomed sequentially from the outer surface to the inner core of the bar.