Markus Gahleitner

Crystallinity and mechanical properties of PP-homopolymers as influenced by molecular structure and nucleation

The evolution of crystallinity and mechanical properties of two different series of PP-homopolymers (RE grades coming directly from the polymerization reactor and CR grades priorly subjected to a defined degradation process) as influenced by the molar mass and heterogeneous nucleation was investigated, including one highly isotactic material to check the tacticity influence. In principle, the effects seem explainable by differences in the number of nuclei and the spherulithic growth speed, which were determined separately. The nucleation effects are similar for all materials, but strongly dependent of the molar mass of the materials. Apart from the bulk material properties, also the development of shearinduced structures is strongly influenced by molar mass and nucleation, contributing additionally to mechanics.

Effects of the catalyst system on the crystallization of polypropylene

In search of a better understanding of catalyst effects on final product qualities of polypropylene, an attempt was made to establish a correlation between catalyst type, polymer chain structure of homopolymers, crystallization behavior, as well as final morphology and mechanical properties. Conventional Ziegler–Natta catalyst systems as well as novel metallocene catalysts were investigated, and influences of molar mass distribution and chain regularity were investigated as separate factors. Metallocene-based isotactic polypropylene shows some special effects regarding the nucleation density and the correlation between stereoregularity and mechanics that do not fit into the general picture for Ziegler–Natta catalyst based products.

Ageing of polypropylene: processes and consequences

Based on information available in the literature and new investigations, the ageing behaviour of polypropylene at room temperature and in annealing was studied. Two effects have to be discerned: changes in the amorphous and/or mesomorphic regions of the material at room temperature, which lead to an increase in density and modulus in connection with embrittlement on the one hand, and relaxation- and recrystallization-processes at higher temperatures, which positively influence heat deflection temperature and impact strength alongside with modulus and density on the other hand. Use of elevated temperatures to accelerate ageing for the determination of long-term properties is therefore not possible in the case of polypropylene.

Correlation between molecular structure and rheological behaviour of polypropylene

Abstract Using data from different types of polypropylene (reactor-grade, peroxide-degraded and technical types), various correlations between parameters of the molar mass distribution, the storage and loss modulus and the steady shear viscosity are investigated. Most assumptions turn out to oversimplify the real, complex relations between molecular structure and rheological properties. Nevertheless they appear to be useful for first estimations in screening tests and production control.

POST-CRYSTALLIZATION AND PHYSICAL AGING OF POLYPROPYLENE: MATERIAL AND PROCESSING EFFECTS

The application properties of plastic products are significantly influenced by the structural changes after conversion, resulting from a combination of post-crystallization and physical aging for semicrystalline materials. For three different types of polypropylene—homopolymer, ethylene/propylene random- and heterophasic copolymer—and two types of conversion—injection molding and cast film extrusion—the aging processes were studied extensively. The temperature level of the aging process plays a decisive role in determining the effects; atleast two important transitions can be identified with increasing temperature. Apart from that, the nature of the polymer and the type and conditions of conversion are also important. The cumulative effects of crystallization behavior of the polymer and cooling history determine the crystallinity and superstructure of the formed article, which in turn determines the aging behavior.

Influence of molecular structure on crystallization behaviour and mechanical properties of polypropylene

Abstract Two series of polypropylenes with different molar mass—reactor grade (RE) and peroxide-degraded (CR) types—were investigated with respect to mechanical and crystallization parameters. A significant influence of the production process on mechanical parameters was found, which can be attributed to differences in the crystallization behaviour, mainly to the number of nuclei per unit volume. At a comparable average molar mass, RE types crystallize faster and exhibit higher levels of stiffness, combined with lower levels of impact strength, than CR types.

Characterization of modified polypropylene by scanning electron microscopy

Characterizing the morphology of modified multiphasic polymer systems, as are often applied for improving the impact strength, is normally a complicated and tedious task. Nevertheless, knowledge about the volume fraction and particle-size distribution of the elastomer phase is important for the specific development of high-impact systems. Direct production in the reactor enables only indirect control of these two quantities. Computer-controlled scanning electron microscopy in combination with image processing allows an automated measurement of both all the necessary particle parameters (size distribution, shape, orientation, etc.) and the elastomer content of the material. Since bulk materials are used for the investigation, additionally, three-dimensional information about the structure of the material can be gained by simply varying the electron energy, without the necessity to resort to multiple slices. This information is especially important in the case of particles with extremely irregular shapes, as obtained, for example, by strong agglomeration of the modifier particles. The mathematical routines used for calculation of the particle-size distributions from the measured profile-size distributions cannot be applied in such cases. The method was tested for several materials with significantly different compositions, both immediately after molding and also after a subsequent thermal relaxation.

Effect of the Molecular Structure of the Polymer and Nucleation on the Optical Properties of Polypropylene Homo- and Copolymers

Two soluble nucleating agents were used to modify the optical properties of nine PP homo- and random copolymers. The ethylene content of the polymers changed between 0 and 5.3 wt %. Chain regularity was characterized by the stepwise isothermal segregation technique (SIST), while optical properties by the measurement of the haze of injection molded samples. Crystallization and melting characteristics were determined by differential scanning calorimetry (DSC). The analysis of the results proved that lamella thickness and change in crystallinity influence haze only slightly. A model was introduced which describes quantitatively the dependence of nucleation efficiency and haze on the concentration of the nucleating agent. The model assumes that the same factors influence the peak temperature of crystallization and optical properties. The analysis of the results proved that the assumption is valid under the same crystallization conditions. The parameters of the model depend on the molecular architecture of the polymer. Chain regularity determines supermolecular structure and thus the dependence of optical properties on nucleation.

Chain regularity of isotactic polypropylene determined by different thermal fractionation methods

The chain regularity of isotactic polypropylene (iPP) homopolymer and random copolymers was characterized by different thermal fractionation methods in this study. Different stepwise temperature programs were applied in a calorimeter (DSC), in order to develop a method which is significantly faster than stepwise isothermal segregation technique (SIST) and provides reliable information about the chain regularity of iPP. Our studies prove that self-seeding accelerates the crystallization process during annealing in SSA–DSC experiments (successive self-nucleation and annealing). Consequently, the time of isothermal steps can be shortened significantly in the SSA–DSC method. On the other hand, we found that step time should not be too short if the goal of the measurement is the determination of average chain regularity. Our results clearly indicate that both the experimental conditions and the evaluation technique influence the obtained results. A standard experimental procedure is proposed for reliably determining the average chain regularity of iPP. The length of the SSA–DSC temperature program developed in this study is much shorter compared to that of the conventional SIST measurements used recently for such experiments. The proposed SSA–DSC program makes the reliable characterization of a large number of samples on an acceptable timescale possible.

Influence of nucleants on the formation of shear-induced structures in polypropylene

The interaction between the type of nucleating agent and base polymer was investigated for a combination of three commercial nucleants and polypropylenes each. Both the crystallisation behavior under DSC conditions and the mechanical and optical properties were determined for all materials. For both the influences of the nucleant and of the base polymer, the present study has provided qualitative indications as to the triggered crystal morphology and shear-induced superstructure. Especially the latter can be related to the differences between optical and mechanical effects of the nucleation process.