Gottfried W. Ehrenstein

Highly active thermally stable β-nucleating agents for isotactic polypropylene

Calcium salts of suberic (Ca-Sub) and pimelic (Ca-Pim) acids were synthesized and implemented as in different grades of isotactic polypropylene (iPP). Propylene homopolymer, as well as random and block copolymers containing these additives, crystallized iPP into pure or nearly pure β modification in the isothermal and nonisothermal crystallization experiments. Recently, Ca-Sub proved to be the most effective β-nucleating agent of iPP. The Ca-Sub nucleating agent widens the upper crystallization temperature range of pure β-iPP formation up to 140°C. In this study the effect of the these additives on the crystallization, melting characteristics, and structure of the PP were studied. The degree of crystallinity of β-iPP was markedly higher than that of α-iPP. A widening in the melting peak of the samples crystallized in a high temperature range was first observed and discussed in regard to literature results of the same phenomenon for α-iPP. The morphology of the β-iPP samples was revealed by scanning electron microscopy. Independent of the type of polymer or nucleating agent, hedritic structures were found in the early stages of growth of the β-spherulites.

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.

Features of the hedritic morphology of β-isotactic polypropylene studied by atomic force microscopy

The lamellar organization of melt-crystallized β-isotactic polypropylene was studied by atomic force microscopy (AFM) after permanganic etching. Hedritic objects grown at a high crystallization temperature (140-143 °C) were investigated. Essential features of the hedritic development were revealed by the characteristic projections exposed at the sample surface. A three-dimensional view of the morphology was obtained by AFM. Hedritic growth proceeded mainly by branching around screw dislocations resulting in new lamellae that further developed. Successive lamellar layers often diverged. Deviation from the planar lamellar habit was observed, varying with the position within the hedrite. Twisting of the lamellae also was observed occasionally in the vicinity of the screw dislocations.

Instrumented tensile and falling weight impact response of injection‐molded α‐ and β‐phase polypropylene homopolymers with various melt flow indices

In this study the instrumented tensile (ITI) and falling weight impact (IFWI) behavior of injection-molded α- and β-phase polypropylene (PP) homopolymers were compared at ambient temperature in a broad melt flow index (MFI = 0.7–13 dg/min) range. It was found that the toughness of β-PP is superior to the α-PP: the difference between them increased with decreasing MFI or increasing molecular weight (MW). As expected, the injection molding induced skin layer thickness increased with increasing MW. Effects of the skin-core morphology were deduced indirectly by considering the results achieved on specimens molded at low and high injection speeds (vinj = 6 and 150 mm/s), respectively. It was found that the effect of the skin-core structure is markedly stronger under uniaxial in-plane (i.e., ITI) than in biaxial out-of-plane type loading (i.e., IFWI).

Interdependence between the curing, structure, and the mechanical properties of phenolic resins

The interdependence between the curing conditions, structure, and the mechanical properties of tow neat phenolic resin systems was investigated. Changes of the distribution of the void diameters were characterized by light- an scanning electron microscope analyses. Tensile tests and dynamic mechanical thermo analysis were performed to determine the influence of the hardener concentration and the curing temperature on the mechanical and the thermomechanical properties. The study reveals that the hardener concentration predominately influenced the microscopic structure, and thus the mechanical properties of the phenolic resin systems. By varying the postcuring times, it can be shown that independent from the microstructure of the phenolic resin system, the degree of cure has a strong influence on the mechanical properties.

New aspects of process induced properties of microinjection moulded parts

Abstract The microinjection moulding process is subject to microspecific phenomena, such as rapid cooling or high shear rates, which greatly affect part properties. While the correlations between morphology, crystallinity and the mechanical properties are well known for parts of usual macroscopic dimensions, there is less information available for microparts. In this paper, these correlations are discussed, related to the dimensions of semicrystalline thermoplastic parts. Results indicate that, if submitted to rapid cooling, microparts exhibit a fine structure, with low crystallinity, low yield strength and low elastic modulus. Experimental investigations have shown the influence of process parameters to be negligible. More important are the materials rate and ability to crystallise, which allow for properties to be significantly enhanced. Another possibility to considerably improve the performance of microparts independent of the used polymer is processing with slow cooling in thermally conductive moulds.

Thermoanalytical investigations of self-reinforced polyethylene

The mechanical properties of the self-reinforced polyethylene increase due to the formation of a shish-kebab structure.Thermal analysis demonstrated that high-strength-PE has a heat deflection temperature 20‡C higher than normal PE. The DSC analyses showed that the presence of a crystalline structure exhibits high thermal stability. This structural form does not relax at high temperatures as is shown by the fact that upon re-crystallization there were enough stable nuclei to build a similar crystalline structure.ZusammenfassungDie Sseigerung der mechanischen Eigenschaften von eigenverstÄrktem Polyethylen (PE-HD) wird durch die Ausbildung einer Shish-Kebab-Struktur erreicht.Die durchgeführten thermo-analytischen Untersuchungen weisen für das hochfeste PEHD gegenüber normalen PE-HD eine um 20‡C höhere WÄrmeformbestÄndigkeit nach. Die DSC-Untersuchungen zeigen, da\ eine kristalline Struktur hoher thermischer StabilitÄt existiert. Diese morphologische Form relaxiert bei höheren Temperaturen nicht sofort; bei der Rekristallisation sind genügend stabile Keime vorhanden, um wieder eine Ähnliche Kristallstruktur zu bilden.

Evaluation and Modeling of Injection-Molded Rigid Polypropylene Integral Foam

Employing a modified injection-molding technology, where the mold is opened a short stroke after injection of the polymer melt, it is possible to manufacture plastic parts with a density reduction of 50% and more. For this processing approach, standard injection-molding equipment can be used. Together with an appropriate selection for the chemical foaming agent, most thermoplastic materials are suitable. The resulting moldings are a real lightweight design with compact skin layers and a foamed core fraction. To estimate the lightweight performance, bending specimens were investigated, and the gain in stiffness was visualized. By varying the thickness and the expansion degree, hints on the optimum foaming conditions were obtained. For engineering purpose, it is important to predict the performance of the integral foam molding during part design. A three- and a five-layer modeling approach were employed to represent the integral foam structure. Both ways were suitable to predict the stiffness enhancement, where already the three-layer representation is sufficient for primary design calculations. It became obvious that the foaming of thermoplastics with a breathing mold enables weight reduction up to 40% in flexion-loaded applications. Since the processing is still highly integrated, as it is the case with standard injection molding, and the cycle times do not increase significantly, the extra cost for the gain in stiffness almost reduces to the cost for foaming agent.