ABSTRACT
Environmental stress cracking (ESC) is one of the most prominent failure mechanisms for polymer components. The high sensitivity of plastics in regard to environmental influences has always meant that plastics as materials have been viewed very critically in outdoor applications. Recently, the massive occurrence of microplastics in the environment means that questions about the long-term stability of plastic parts and the studies of plastic fragmentation are of great scientific interest. ESC behavior also plays an important role in connection with the formation of microplastics. In this work, the influence of two different sample wetting methods on ESC behavior was investigated. In case A, the sample was in situ wetted with the medium during the measurement by using a sponge. In case B, the sample was wetted by storage in the medium prior to measurement. Different stress cracking agents (SCA) were examined for polymethylmethacrylate (PMMA). Fracture-mechanical fatigue crack propagation (FCP) tests were carried out to quantitatively determine the sensitivity to ESC. Correlations between the absorption behavior and the ESC behavior of the SCA and the resulting morphological phenomena were established. Depending on the wetting method, significant differences in FCP were observed. The in situ wetting of the samples (case A) during the FCP measurement with ethylene glycol (EG) and with deionized water (DI) led to a significant shift in the crack propagation curves to higher ∆K-compared to the PMMA reference. In the case of n-heptane (NH), a more brittle crack propagation behavior was observed due to the chemical interaction with PMMA. The previously immersed samples (case B) give different results. Storage in NH and EG showed no influence on the crack propagation behavior. Samples immersed in DI showed a completely different course of crack growth. At a certain load, a sudden deceleration of the crack propagation and thus a horizontal curve could be seen. Above a certain ∆K value, crack growth began again. Depending on the immersion time (14, 30, or 60 days), this so-called stepped behavior shifted to lower da/dN values.
ABSTRACT
The incorporation of nanoscale fillers into a polymer can lead to a considerable improvement of mechanical properties, i.e. stiffness and toughness of a material can be enhanced simultaneously by the insertion of nanofillers. Thereby, the crucial difference between conventional microscale fillers and nanofillers is the high specific surface of the latter. In order that this surface can interact with the matrix material a good dispersion, i.e. a good separation and a homogeneous distribution of the nanoparticles into the polymer, is required. In the present study ultrasonic waves generated by an ultrasonic horn were used to disperse titanium dioxide nanoparticles into epoxy resin. The process parameters, e.g. the ultrasonic amplitude, the dispersion time and the material's volume, were varied systematically with the aim of achieving an optimum dispersion process. A dispersion model for bead mills was adapted to the ultrasonic process and compared to a second dispersion model in order to find an adequate mathematical expression to correlate the ultrasonic process parameters to the particle sizes in the material and to allow predictions for further experiments.
ABSTRACT
Environmental stress cracking (ESC) is a phenomenon associated with disposable plastic products. This article describes new methods to determine ESC in luer fittings. The findings of two experimental studies are reported.
