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- Biodiesel (2)
- Dynamic-mechanical analysis (2)
- Fuel sorption (2)
- Impact strength (2)
- PE-HD (2)
- Cellular polypropylene (1)
- Charpy impact test (1)
- Desorption (1)
- Ferroelectret (1)
- Relaxation (1)
- Viscoelastic (1)
Organisationseinheit der BAM
- 5 Werkstofftechnik (5) (entfernen)
Viscoelastic properties of cellular polypropylene ferroelectrets (PP FEs) were studied at low
frequencies (0.3–33 Hz) by dynamic mechanical analysis and at high frequencies (250 kHz) by laser Doppler vibrometry. Relaxation behavior of the in-plane Young’s modulus (Y´
11~1500 MPa at room temperature) was observed and attributed to the viscoelastic response of polypropylene matrix.
The out-of-plane Young’s modulus is very small (Y´33≈0.1 MPa) at low frequencies, frequency- and stress-dependent, evidencing nonlinear viscoelastic response of PP FEs. The highfrequency mechanical response of PP FEs is shown to be linear viscoelastic with Y´33≈0.8 MPa. It is described by thickness vibration mode and modeled as a damped harmonic oscillator with one degree of freedom. Frequency dependence of Y*33 in the large dynamic strain regime is described by the broad Cole-Cole relaxation with a mean frequency in kHz range attributed to the Dynamics of the air flow between partially closed air-filled voids in PP FEs. Switching-off the relaxation contribution causes dynamic crossover from the nonlinear viscoelastic regime at low frequencies to the linear viscoelastic regime at high frequencies. In the small strain regime, contribution of the air flow seems to be insignificant and the power-law response, attributed to the mechanics of polypropylene cell walls and closed air voids, dominates in a broad frequency range. Mechanical Relaxation caused by the air flow mechanism takes place in the sound and ultrasound frequency range (10 Hz–1MHz) and, therefore, should be taken into account in ultrasonic applications of the PP FEs deal with strong exciting or receiving signals.
In a previous paper we investigated the influence of sorbed biodiesel or diesel on mechanical properties of a typical polyethylene grade for tank applications. Besides the basic sorption and desorption behavior of these two fuels, the study addressed the concentration-dependent mechanical properties as revealed by a non-instrumented Charpy impact test and dynamic mechanical analysis (DMA). In the present paper we extend this investigation focusing on the temperature-dependent impact fracture behavior. Therefore, an instrumented Charpy impact test was employed, allowing a more detailed analysis of the fracture behavior. Furthermore, from the load-time-diagrams obtained from the instrumented impact test, corresponding fracture times can be calculated, allowing a clear correlation of the fuel sorption induced changes in fracture toughness with the enhanced ß-relaxation observed by DMA. As in the previous study, the fracture surfaces of the impact tested specimens were analyzed in order to confirm the brittle or ductile character of the fracture indicated by impact strength and the corresponding load-deflection diagrams.
The time-dependent sorption of biodiesel in a typical polyethylene for container applications is investigated in comparison to conventional diesel fuel at three different temperatures. In this context, the desorption behavior is also addressed. Subsequently, the effects of both penetrants on mechanical properties are characterized in terms of impact strength and dynamic-mechanical analysis. The discussion of property changes is firstly based on the sorption kinetics of biodiesel and diesel, which is determined by immersion experiments allowing for the calculation of respective diffusion coefficients. Changes in impact strength as determined by the Charpy method are further characterised in more detail by analyzing the fracture surfaces, and correlated with results of dynamic-mechanical analysis.