Michael Gehde

Analysis of Dynamic Crack Propagation in Elastomers by Simultaneous Tensile- and Pure-Shear-Mode Testing

The present work proposes a new fracture mechanical testing concept for determination of dynamic crack propagation of rubber materials. This concept implements a method of simultaneous tensile- and pure-shear-mode testing. The present approach is based on an upgrade of the Tear Analyzer (Co. Coesfeld GmbH & Co. KG), on the fracture mechanics theory of dynamically loaded test specimens and on the definition of pure-shear states according to the test specimen’s geometry ratio.

Vibration Welding of Amorphous Thermoplastic Nanocomposites

Polycarbonate (PC) nanoclay composites and noryl nanoclay composites were prepared by melt blending technique both in presence and absence of compatibilizer. The compatibilizer used for dispersing nanoclay in PC was poly (caprolactone) (PCL). In case of noryl, polystyrene (PS) was used as a compatibilizer. The nanoclay used in this study is Cloisite 20 A. The main aim in utilizing the nanoclay was to improve the mechanical properties and reduce the viscosity of the amorphous thermoplastic nanocomposites. Vibration welding has been done on pure and nanoclay-filled PC and noryl polymer matrices. The dispersion of nanoclay in the polymer matrices was observed through high resolution transmission electron microscopy (HR-TEM). Both PC and noryl nanocomposites prepared in presence of compatibilizers showed better dispersion of nanoclay platelets in the polymer matrices compared to the nanocomposites system prepared in absence of compatibilizer. The nanoclay filled polymer systems show increase in percentage crystalinity compared to the pure ones, which may be due to the nucleating effect of nanoclay. The mechanical properties were tested for the pure and nanoclay-filled PC and noryl welded joints. The mechanical properties showed a drop for the polymer nanocomposites, which contains nanoclay dispersed directly in the polymer matrices. On the other hand, PC and noryl nanocomposites prepared in the presence of compatibilizer showed improved mechanical properties compared to pure one. The compatibilizer helps to improve the dispersion of nanoclay platelets in the base polymer matrix. This indeed improves the weld strength of the thermoplastic nanocomposites.

An Investigation on Vibration Welding of Amorphous and Semicrystalline Polymers

Vibration welding technique has been used to study the weld zone of thermoplastic polymers using ABS (amorphous), PC (amorphous), PMMA (amorphous), and PBT (semicrystalline). Polymers were welded using alike components and combinations of semicrystalline polymer with different amorphous polymers. Mechanical testing of welded polymers has proved that the tensile strength, elongation at break, and deformation was highest for PC–PC weld and least for ABS–ABS weld, when alike polymers were welded. However, welding of semicrystalline and amorphous polymer shows enormous reduction in its tensile strength as well as other tensile properties. Also, the tensile fracture of PBT with other amorphous polymers always occurred at weld zone which was not always in case of alike polymer welds. The weld strength of these polymers was observed to be dependent on the mechanical interlocking among the layers and not on interfacial bonding. This phenomenon may be due to the difference in glass transition temperatures of semicrystalline and amorphous polymers. XRD, FESEM, and AFM have been used in this study to observe the morphology of welded surfaces.

Ultrasonic Welding of Amorphous and Semi Crystalline Materials

Ultrasonic welding have been studied using Polycarbonate (amorphous) and Polypropylene (semi-crystalline) in presence and absence of polymeric compatibilizer. A solution phase compatibilization has been used using Polycaprolactone, Polymethylmethacrylate and Maleic anhydride grafted Polypropylene. Cylindrical samples were used in the near and far field ultrasonication in order to achieve the better weldability. The welding strength was increased while utilizing the compatibilizer than that of un-compatibilized samples. Mixture of Polycarbonate and Polymethylmethacrylate on the PC phase enhanced the weld strength tremendously. The XRD analysis revealed the increase in crystallinity during the ultrasonication process resulting the enhanced weldability. The phase fusion is quite prominent from the SEM study of the fractured surface and also from the SEM of the welded portion. The FTIR study indicated the phase interaction through the compatibilizer thus enhancing the ultrasonic weldability of the thermoplastics.

Effect of a thermal treatment on thermoplastic polyurethane– metal hybrids

The effect of a thermal treatment, i.e. annealing, on the adhesion of a thermoplastic polyurethane overmolded on a precoated steel substrate was evaluated. The hybrid part was subjected to three different annealing temperatures after the overmolding process. Adhesion of the thermoplastic elastomer was achieved by using an in-house-developed, two-step curable organic powder coating, which was selectively crosslinked to a polyallophanate state. The powder material was applied to the steel substrate by electrostatic charging followed by molecular crosslinking in an oven. The influence of the metal surface pretreatment was also evaluated regarding adhesion. The effects of the postthermal treatment were assessed through mechanical peel test, differential scanning calorimetry, atomic force microscopy, and scanning electron microscopy. Interesting results were obtained regarding strong adhesion between the precoated metal sheets and thermoplastic polyurethane after injection molding, especially in hybrids that were annealed at higher temperatures with the in-house adhesive coating.

Effects of post-curing on the thermo-mechanical behavior and the chemical structure of highly filled phenolic molding compounds

Abstract Due to their excellent thermomechanical properties, as well as their good media resistance, phenolic molding compounds are outstanding materials for high temperature applications, such as components for the engine compartment. An additional post-curing process, after injection molding, shifts the glass transition temperature towards high temperatures up to 300 °C. The objective of this work is to analyze the influence of the post-curing process on the chemical structure of the material. This structure is responsible for the mechanical and thermal properties of the material. Therefore tensile test bars were molded with a highly filled phenolic molding compound and different post-curing cycles under different atmospheres were done. The tensile properties and the glass transition temperatures (TMA) were measured. To get detailed information of the chemical structure ATR-FTIR and NMR analyses were done. The mechanical properties decreased up to 25 % with higher post-curing temperature (under air atmosphere) although the glass transition temperatures (TMA) and the grades of curing (NMR) increased. Furthermore, in the ATR-FTIR measurements, oxidation products were detected on the surface of the sample which were post-cured under air atmosphere. In comparison, no oxidation products could be detected on the samples post-cured under nitrogen atmosphere and the mechanical properties just decreased about 4 %. Oxidation processes could be provided by post-curing under nitrogen atmosphere which led to higher mechanical properties, in connection with a high grade of cure and a higher glass transition temperature in comparison with post-curing under air atmosphere.

A Study of Correlation between Crack Initiation during Dynamic Wear Process and Fatigue Crack Growth of Reinforced Rubber Materials

The aim of this study is concentrated on the experimental investigation of crack initiation during dynamic wear process and its correlation with fatigue crack growth of reinforced rubber materials. The analyzed rubber compounds suitable for applications such as treads for truck tires were based on natural rubber (NR) and polybutadiene rubber (BR). The dynamic wear behavior has been studied using an own developed testing equipment based on gravimetric determination of mass loss of test specimen. Fatigue crack growth (FCG) analysis was performed under pulse loading in accordance with real dynamic loading conditions of rolling tires using the Tear Analyser (TA). We show the crack initiation process during dynamic wear with respect to different impact energies and correlate the liability of crack initiation with FCG data at given tearing energy as a function of the rubber compositions. We demonstrate the higher crack initiation resistance of rubber blends with increased content of BR, while a predominant influence of NR improves the resistance against crack propagation especially at higher strain levels due to strain induced crystallization.