TY - JOUR A1 - Gaal, Mate A1 - Bovtun, Viktor A1 - Stark, Wolfgang A1 - Erhard, Anton A1 - Yakymenko, Y. A1 - Kreutzbruck, Marc T1 - Viscoelastic properties of cellular polypropylene ferroelectrets N2 - 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. KW - Viscoelastic KW - Cellular polypropylene KW - Ferroelectret PY - 2016 U6 - https://doi.org/10.1063/1.4944798 SN - 0021-8979 SN - 1089-7550 SN - 1520-8850 VL - 119 SP - Article Number: 125101 PB - AIP Publishing AN - OPUS4-35956 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - McHugh, Jarlath A1 - Stark, Wolfgang T1 - Determination and interpretation of changes in thermophysical properties of a carbon-fibre prepreg during cure N2 - Epoxy composites used for high-end structural applications are typically cured under the influence of temperature and pressure causing a number of complex chemical and physical transformations. On heating a mould, temperature gradients will occur through the component which will depend largely on the thermophysical properties of the specific composite. The crosslinking reaction is exothermic leading to additional heat release, thus complicating heat transportation models. If such effects are not accounted for, it can lead to variations in resin flow, poor fibre wetting causing voiding and inhomogeneous cure, leading to shrinkage and unfavourable variations in moulded part geometry. Limited information is available for thermal models used in the manufacture of reinforced thermosets. Autoclave [1, 2] and laser or infra-red curing processes [3, 4] typically use constant values determined on fully cured parts. In this work, the variation in thermal conductivity (K) (W/(m K)), thermal diffusivity (a) (m(2)/s) and specific heat capacity at constant pressure (c(p)) (J/(g K) is determined for a carbon fibre prepreg during cure. It is the intention to improve understanding of how these parameters are related to chemical or physical transformations occurring during cure, and where estimates or shortcuts may be used for heat transfer models KW - Differential scanning calorimetry KW - Temerature KW - Epoxy system PY - 2016 U6 - https://doi.org/10.1016/j.polymertesting.2015.11.015 SN - 0142-9418 VL - 49 SP - 115 EP - 120 PB - Elsevier Science CY - Oxford AN - OPUS4-35788 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -