József Varga

β-MODIFICATION OF ISOTACTIC POLYPROPYLENE: PREPARATION, STRUCTURE, PROCESSING, PROPERTIES, AND APPLICATION

The methods of preparation and formation of supermolecular structures in quiescent and sheared melts and the properties of the β-modification of isotactic polypropylene (β-iPP) are reviewed. The introduction of selective β-nucleants is the most reliable method for preparation of samples rich in β-modification or of pure β-iPP. The advantages and drawbacks of the known β-nucleating agents are summarized. It is emphasized that pure β-iPP can be prepared under laboratory and processing conditions in the presence of highly active and selective β-nucleants. Nevertheless, there are no literature data—apart from that of the authors groups—which evidenced unambiguously the formation of pure β-iPP. It hints at the insufficient selectivity of β-nucleants used or at the inappropriate crystallization or melting conditions applied by other scientists. The structure formation during the high-temperature hedritic crystallization is discussed comprehensively and illustrated by polarized light microscopy and scanning electron microscopy micrographs. Some specific features of β-iPP, namely the high- and low-temperature growth transition, the restricted temperature range of the formation of pure β-iPP, and the unique melting and recrystallization characteristics (melting and annealing memory effect) are summarized. It was emphasized that impact strength and toughness of β-iPP markedly exceed those of α-iPP. Processing of β-nucleated iPP and application of β-nucleated iPP is described briefly.

β-Modification of polypropylene and its two-component systems

Abstractβ-modification and multi-component systems ofβ-polypropylene were prepared both under laboratory and processing conditions. Characteristic features of crystallization, melting, and annealing ofβ-PP are summarized. The very distinct memory effect in the melting and annealing ofβ-PP is also presented. The existence of a lower and an upper limit temperature ofβ-PP formation is demonstrated. The structural stability and the orientation-inducedβα-recrystallization ofβ-PP are analyzed. Preparation and properties of polymer blends and filled composites fromβ-PP are introduced, too.ZusammenfassungSowohl unter Laboratoriums- als auch unter Betriebsbedingungen wurden die β-Modifikation und Multikomponentensysteme von β-Polypropylen hergestellt. Hierbei wurden die charakteristischen Wesenszüge von Kristallisation, Schmelzen und Tempern von β-PP beschrieben. Es wird auch der sehr ausgeprägte Memory-Effekt von β-PP bei Schmelzen und Tempern dargestellt. Für die Formierung von β-PP wird die Existenz einer unteren und einer oberen Grenztemperatur verdeutlicht. Es wurde weiterhin die strukturelle Stabilität und die dehnungsinduzierte β-α- Rekristallisation von β-PP untersucht. Herstellung und Eigenschaften von Polymergemischen und Verbundstoffen von β-PP wurden ebenfalls beschrieben.РезюмеВ лабораторных и заво дских условиях получ еныβ-модификация полипр опилена (ПП) и многокомпонентные системы на его основе. Приведены особенности кристал лизации, плавления и термообр аботкиβ-ПП. Установл ен резко различный характер э ффекта памяти при плавлении и термообр аботкеβ-ПП. Показано наличие низшего и верхнего пр еделов образованияβ-ПП. Проанализирован ы структурная устойч ивость и ориентационно-навед еннаяβα-перекристаллизац ия. Приведены также по лучение и свойства полимерных смесей и наполненных композитов на основеβ-ПП.

Effects of β-α transformation on the static and dynamic tensile behavior of isotactic polypropylene

It was demonstrated that the mechanical stress-induced βα-transformation in isotactic polypropylene (iPP) is associated with considerable toughness enhancement. This toughness improvement depends on the test conditions (loading frequency). The toughness of β-iPP was superior to the α-iPP by 13% under static (characterized by a load frequency of ca. 5 × 10−4 Hz) and 70% under dynamic (tensile impact with a loading frequency in the range of ca. 3 × 102…103 Hz) conditions, respectively. By applying the essential work of fracture (EWF) concept to single-edge notched tensile (SEN-T) specimens it was shown that for the toughness upgrading observed, energy dissipation in the enlarged plastic zone is responsible. The occurrence of the βα-transformation was evidenced by differential scanning calorimetry (DSC). Based on DSC measurements it was found that the degree of βα-trans-formation depends on the local strain. At high strain values the βα-conversion is complete (at elogation at break in uniaxial static tensile test), while this transformation is only partial at lower strains (at tensile impact). In addition, in the plastic (or deformation) zone the βα-conversion changed locally, and can be used for mapping of this region.

Comparison of the fracture and failure behavior of injection‐molded α‐ and β‐polypropylene in high‐speed three‐point bending tests

The fracture and failure mode of α- and β-isotactic polypropylene (α-iPP and β-iPP, respectively) were studied in high speed (1 m/s) three-point bending tests on notched bars cut from injection-molded dumbbell specimens and compared. The fracture response of the notched Charpy-type specimens at room temperature (RT) and −40°C, respectively, was described by terms of the linear elastic fracture mechanics (LEFM), namely fracture toughness (Kc) and fracture energy (Gc). Kc values of both iPP modifications were similar, while Gc values of the β-iPP were approximately twofold of the reference α-iPP irrespective of the test temperature. It was demonstrated that β-iPP failed in a ductile and brittle-microductile manner at RT and −40°C, respectively. By contrast, brittle fracture dominated in α-iPP at both testing temperatures. Based on the fracture surface appearance, it was supposed that β-to-α (βα) transformation occurred in β-iPP. The superior fracture energy of β-iPP to α-iPP was attributed to a combined effect of the following terms: morphology, mechanical damping, and phase transformation. Results indicate that their relative contribution is a function of the test temperature.

Formation of β-modification of isotactic polypropylene in its late stage of crystallization

Abstract In the late stage of the crystallization of isotactic polypropylene (IPP), melt inclusions are encapsulated by the crystallized phase. The contraction caused by the proceeding crystallization within the inclusion leads to a reduced pressure accompanied by the appearance of vacuum bubbles. In the surface layer of the melt surrounding the vacuum bubbles, the polymer chains are subjected to extension during the development of bubbles, which leads to the formation of α-row nuclei. As has been observed during the shear-induced crystallization of IPP, the row-nuclei formed in situ can also induce the growth of the β-phase in this case. It was demonstrated that a possible reason for the α — β transition on the surface of growing α-spherulites is the local mechanical stress caused by the contraction. Based on experimental results, suggestions are made for the origin of the strongly birefringent phase observed by Duval et al. during the crystallization of blends of IPP and IPP grafted with maleic anhydride.

Crystallization and Melting of β-Nucleated Isotactic Polypropylene

Ca salts of suberic (Ca-Sub) and pimelic acid (Ca-Pim) were synthesized and used as β-nucleating agents in different grades of isotactic polypropylene (IPP). Propylene homo-, random- and block-copolymers containing these additives crystallize principally in pure β-modification as demonstrated in isothermal and non-isothermal crystallization experiments. Ca-Sub proved the most effective β-nucleating agent known, so far. It broadens the upper crystallization temperature range of pure β-IPP formation up to 140°C. The effect of the additives on the crystallization and melting characteristics of the polymers was studied. The degree of crystallinity of the β-modification was found to be markedly higher than that of α-IPP. High temperature melting peak broadening was first observed and discussed in literary results regarding the same phenomenon for α-IPP.

Crystallization, melting and structure of polypropylene/poly(vinylidene-fluoride) blends

Blends were prepared from isotactic polypropylene (iPP) along with its b-nucleated form and poly(vinylidene-fluoride) (PVDF). Melting, and crystallization characteristics as well as structure of the blends were studied by polarized light microscopy (PLM) and differential scanning calorimetry. According to PLM studies, the phase structure of these blends is heterogeneous in the molten state. The temperature range of crystallization of PVDF during cooling is higher than that of iPP. PVDF has a strong α-nucleating effect on iPP. The crystallization of iPP starts on the surface of dispersed PVDF droplets and an α-transcrystalline layer forms on the surface of the crystalline PVDF phase. The iPP matrix crystallizes predominantly in a-form in spite of the presence of a highly active b-nucleating agent.

Kinetics of competitive crystallization of β- and α-modifications in β-nucleated iPP studied by isothermal stepwise crystallization technique

Crystallization kinetics of β-nucleated isotactic polypropylene (β-iPP) under isothermal conditions were investigated by differential scanning calorimetry. iPP was nucleated by a trisamide derivative, namely tris-2,3-dimethyl-hexylamide of trimesic acid (TATA). In the presence of TATA possessing dual nucleating ability, the formation of the α- and β-form occurs simultaneously. An isothermal stepwise crystallization method is suggested in this study, which can separate the crystallization process of β- and α-iPP and consequently their crystallization kinetics can be evaluated separately. The results indicated that the mechanism of crystallization changes in temperature especially in the vicinity of the upper critical temperature of the formation of the β-phase. In addition, it was found that the ratio of the growth rates of β- and α-modification determines the characteristics of crystallization and influences the apparent rate constant of crystallization of both polymorphs.

Determination of the equilibrium melting point of the β-form of polypropylene

The melting behavior of the β-form of isotactic polypropylene (β-iPP) was investigated as a function of crystallization time and temperature. Calcium suberate, a selective β-nucleating agent was used to produce samples that consist entirely of β-form i-PP. The experimental melting points were recorded at different crystallization times and were extrapolated to the start of the crystallization process in order to eliminate the effect of lamellar thickening. Using the non-linear Hoffman—Weeks approach to correlate these extrapolated experimental melting temperatures with the corresponding crystallization temperatures, an equilibrium melting point of 209°C was obtained for β-iPP. The equilibrium melting point estimated through the non-linear Hoffman—Weeks analysis is about 30°C higher than that (Tm0=177°C) obtained on the basis of the linear extrapolation. These results are consistent with earlier claims that a linear extrapolation of Tm−Tc data leads to an underestimation of the equilibrium melting point. The results obtained for β-iPP exemplify the importance of accounting for both the isothermal lamellar thickening effects and the non-linearity in the Tm−Tc correlation, when the determination of an equilibrium melting point is carried out using a procedure based on the predictions of the Lauritzen—Hoffman secondary nucleation theory.

Beta-modification of isotactic polypropylene

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.