Bernhard Möginger

Determination of the weighting function g(βi,r,vf) for fibre orientation analysis of short fibre-reinforced composites

Abstract Physical properties of fibre-reinforced composites depend on the fibre orientation distribution with respect to the measuring direction. The fibre orientation distribution is determined by the fibre inclination of a sectioning plane. However, the probability of sectioning a fibre depends on inclination angle and aspect ratio. A fibre parallel to a sectioning plane will be cut with a smaller probability than that of a fibre oriented perpendicular to the plane. In order to compensate for different sectioning probabilities the orientation vectors have to be weighted inclination and aspect ratio dependently. With the assumptions of constant aspect ratio, parallel and equidistant fibres, it is possible to derive a weighting function depending only on inclination, aspect ratio and fibre volume content. In the case of short fibre-reinforced composites the aspect ratio has to be substituted by the mean aspect ratio.

Curing kinetics of visible light curing dental resin composites investigated by dielectric analysis (DEA)

During the curing process of light curing dental composites the mobility of molecules and molecule segments is reduced leading to a significant increase of the viscosity as well as the ion viscosity. Thus, the kinetics of the curing behavior of 6 different composites was derived from dielectric analysis (DEA) using especially redesigned flat sensors with interdigit comb electrodes allowing for irradiation at the top side and measuring the ion viscosity at the bottom side. As the ion viscosities of dental composites change 1-3 orders of magnitude during the curing process, DEA provides a sensitive approach to evaluate their curing behavior, especially in the phase of undisturbed chain growth. In order to determine quantitative kinetic parameters a kinetic model is presented and examined for the evaluation of the ion viscosity curves. From the obtained results it is seen that DEA might be employed in the investigation of the primary curing process, the quality assurance of ingredients as well as the control of processing stability of the light curing dental composites.

Mechanical Properties of Isotactic Polypropylene with Oriented and Cross-hatched Lamellae Structure

Abstract The mechanical properties of injection moulded unreinforced isotactic, polypropylene depend on the crystalline structure and lamellae orientation. Push-pull processing is an injection moulding technique that allows the user to produce parts with oriented structures. Mostly applied for short fibre reinforced thermoplastics in order to align fibres parallel to the flow direction, this technique can also be used to orient the polymeric matrix of LCP and polypropylene. A comparison of mechanical properties as well as investigations on morphology between the samples produced using conventional injection moulding and push-pull processing are presented trying to improve the knowledge about this so far rather unknown technique.

Photopolymerization of highly filled dimethacrylate-based composites using Type I or Type II photoinitiators and varying co-monomer ratios

OBJECTIVES The use of a Type I photoinitiator (monoacylphosphine oxide, MAPO) was described as advantageous in a model formulation, as compared to the conventional Type II photoinitiator (Camphorquinone, CQ). The aim of the present work was to study the kinetics of polymerization of various composite mixtures (20-40-60-80 mol%) of bisphenol A glycidyl dimethacrylate/triethylene glycol dimethacrylate (BisGMA/TegDMA) containing either CQ or MAPO, based on real-time measurements and on the characterization of various post-cure characteristics. METHODS Polymerization kinetics were monitored by Fourier-transform near-infrared spectroscopy (FT-NIRS) and dielectric analysis (DEA). A range of postcure properties was also investigated. RESULTS FT-NIRS and DEA proved complementary to follow the fast kinetics observed with both systems. Autodecceleration occurred after ≈1 s irradiation for MAPO-composites and ≈5-10 s for CQ-composites. Conversion decreased with increasing initial viscosity for both photoinitiating systems. However despite shorter light exposure (3s for MAPO vs 20s for CQ-composites), MAPO-composites yielded higher conversions for all co-monomer mixtures, except at 20 mol% BisGMA, the less viscous material. MAPO systems were associated with increased amounts of trapped free radicals, improved flexural strength and modulus, and reduced free monomer release for all co-monomer ratios, except at 20 mol% BisGMA. SIGNIFICANCE This work confirms the major influence of the initiation system both on the conversion and network cross-linking of highly-filled composites, and further highlights the advantages of using MAPO photoinitiating systems in highly-filled dimethacrylate-based composites provided that sufficient BisGMA content (>40 mol%) and adapted light spectrum are used.

Morphological investigations of injection moulded fibre-reinforced thermoplastic polymers

Abstract Fibres affect the matrix morphology in fibre-reinforced composites. Especially in semi-crystalline melts the fibres can act as nucleation rods causing a structure known as transcrystallinity. Transcrystalline structures are also found in injection moulded parts. They affect the structure of fracture surfaces especially in the case of long fibre-reinforced polymers with good fibre/matrix adhesion. Transcrystalline structures are usually generated and investigated in a microscope with a hot stage. For injection moulded parts this is an inadequate method. The morphological results presented here were obtained from plasma-etched inner surfaces and thin sections by means of light and contrast interference microscopy accompanied by scanning electron microscopy of fracture surfaces. Plasma etching is a well suited preparation technique to reveal both morphological superstructures and damage such as voids, debonding and fibre cracks in composites.

ABS — Sprödbruch-Untersuchungen der Morphologie-Versagens-Beziehung

Die Kunststoffe aus der Gruppe der Acryl-Nitril-Butadien-Styrol-Copolymerisate (ABS) sind schlagzahe Modifikationen des Styrol-Acrylnitril-Copolymerisats (SAN). Die Schlagzahmodifikation geschieht dadurch, das in eine koharente, hartsprode SAN-Matrix submikroskopisch kleine Elastomerteilchen — meist aus Butadien — eingebettet werden.

Morphological studies of deformed polybutylene terephthalate (PBT)

PBT is a semi-crystalline thermoplastic polymer whose deformation behavior highly depends on processing parameters. This makes it a model polymer for investigating morphological changes caused by deformation on the spherulitic and lamellar level. In the neck region all states of deformation of the spherulites are observed. Even in the fibrillar phase the borders of the spherulites remain visible. The spherulitic structure is not totally destructed in the neck.The lamellar structure of the fibrillar phase significantly differs from that of the spherulitic region. The lamellae are orientated with respect to the direction of deformation and the lamellae heights are reduced distinctly. Scanning electron microscopy of fracture surfaces reveals for some samples a sharp frontier between spherulitic and fibrillar region. This leads to the conclusion that the necking process may be a phase transition between an isotropic and a highly orientated phase, as predicted for a Van der Waals network.

Quasi Static Indentation Measurements: A Tool for Micromechanical Investigations of Interfaces in Polymer Materials

Abstract The ongoing use of miniaturization, multi layer structure parts, and hybrid parts requires methods to determine mechanical properties on a micro scale. However, there is a gap in measuring techniques. On one hand there are the classical methods to measure hardness, e.g., Vickers, Rockwell, Universal, and IRHD, having resolutions typically above 100 µm. On the other hand, there are well-developed AFM methods that allow for the determination of mechanical properties in the nanometer range. This article describes an indentation technique that yields data of mechanical properties in the micrometer range between typically 5 and 50 µm. The measuring device and the data evaluation are presented. Results of micro-mechanical mapping are shown for NR-SBR rubber interfaces, a fuel tank, and a part manufactured by two-component injection molding. Finally, the measured micro-mechanical stiffness is compared to the Youngs modulus of the corresponding materials.

Creep Behaviour of Short Glass Fiber Reinforced and Virgin Polybutylene Terephthalate (PBT) - a Comparison of Experimental Data to Modeled Data Using The Elementary Volume Concept (EVC)

The mechanical behaviour of short fiber reinforced composites depends on the mechanical properties of both matrix and fiber (e. g. Young’s moduli), the fiber volume content, the fiber length distribution or at least the mean fiber length, the fiber matrix adhesion, and the fiber orientation distribution in a quite complicated way. In the Elementary Volume Concept (EVC) it is assumed that the fundamental mechanical properties of a composite are completely determined by the mechanical properties of a square column shaped Elementary Volume (EV) containing a single fiber of length 〈L〉 in its center, Fig. 1.

The Influence of Processing Conditions on Mechanical Properties of Unreinforced and Discontinuous Glass Fibre Reinforced Thermoplastics

Conventionally injection moulded short glass fibre reinforced thermoplastics show the well-known symmetrical morphology of skin-, shell- and corelayer. The fibre orientation in each layer depends on the dominance of either shear or elongational flow. In Push-Pull processed glass fibre reinforced thermoplastics, morphology and fibre orientation distribution differ distinctly from conventionally processed ones. This processing technique yields to a much higher anisotropy of the elastic properties. The influence of the modified morphology and fibre orientation distribution on the mechanical properties are investigated in tensile tests, tensile creep tests and DMA for isotactic polypropylene (Vestolen P 7000, P 7032 G, Daplen K2 Xmod) and two different grades of discontinuous glass fibre reinforced polyamide 66 (Verton RF 70010, Ultramid A3EG10).