Ingenieurwissenschaften und zugeordnete Tätigkeiten
Filtern
Erscheinungsjahr
- 2019 (5) (entfernen)
Dokumenttyp
- Zeitschriftenartikel (5) (entfernen)
Sprache
- Englisch (5)
Referierte Publikation
- ja (5)
Schlagworte
- Ageing (1)
- Biodiesel (1)
- Cavitation (1)
- Crystal Texture (1)
- DLO (1)
- Degradation (1)
- Dielectric spectroscopy (1)
- Diesel (1)
- Diesel Fuel (1)
- Electrical conductivity (1)
Organisationseinheit der BAM
- 7 Bauwerkssicherheit (5)
- 7.5 Technische Eigenschaften von Polymerwerkstoffen (5)
- 5 Werkstofftechnik (2)
- 5.3 Polymere Verbundwerkstoffe (2)
- 6 Materialchemie (2)
- 6.6 Physik und chemische Analytik der Polymere (2)
- 6.5 Synthese und Streuverfahren nanostrukturierter Materialien (1)
- 8 Zerstörungsfreie Prüfung (1)
- 8.5 Röntgenbildgebung (1)
Polymers of Intrinsic Microporosity (PIMs) of high performance have developed as materials with a wide application range in gas separation and other energy-related fields. Further optimization and long-term behavior of devices with PIMs require an understanding of the structure-property relationships including physical aging. In this context the glass transi-tion plays a central role, but with conventional thermal analysis a glass transition is usually not detectable for PIMs be-fore their thermal decomposition. Fast scanning calorimetry provides evidence of the glass transition for a series of PIMs, as the time scales responsible for thermal degradation and for the glass transition are decoupled by employing ultrafast heating rates of tens of thousands K s-1. The investigated PIMs were chosen considering the chain rigidity. The estimated glass transition temperatures follow the order of the rigidity of the backbone of the PIMs.
Elastomers are used in almost all areas of industrial applications, such as tires, engine mounts, bridge bearings, seals or coatings. During their use in operation, they are exposed to different environmental influences. These include, in particular, climatic factors such as air oxygen, high temperatures, light (UV radiation) and the influence of media (e.g. oils, fuels). A very important result of these factors is the chemical ageing of elastomers. In this case, the elastomer degenerates and changes its chemical structure in the aged regions, which leads to an irreversible change in the material properties in connection with the reduction in its usability. In this paper, chemical ageing of nitrile butadiene rubber (NBR) is investigated. Especially in case of thermo-oxidative ageing at elevated operating temperatures, the ageing processes run inhomogeneously. These effects are known as diffusion-limited oxidation (DLO) and are associated with the diffusion–reaction behaviour of atmospheric oxygen with the elastomer network. For these reasons, NBR samples are artificially aged in air and subjected to different experimental methods, which are presented and discussed. Additional results from inhomogeneous mechanical tests and permeation tests indicate the causes of the DLO-effect, show the influence of chemical ageing and are subsequently used for parameter identification in relation to the diffusion–reaction equation. A continuum-mechanical modelling approach is also presented here, which describes the finite hyperelasticity, diffusion–reaction processes as well as chemical degradation and reformation of the elastomer network. This multifield problem leads to a system of partial and ordinary differential equations and constitutive equations and is solved within the finite element method.
High-density polyethylene becomes optically transparent during tensile drawing when previously saturated with diesel fuel. This unusual phenomenon is investigated as it might allow conclusions with respect to the material behavior. Microscopy, differential scanning calorimetry, density measurements are applied together with two scanning X-ray scattering techniques: wide angle X-ray scattering (WAXS) and X-ray refraction, able to extract the spatially resolved crystal orientation and internal surface, respectively. The sorbed diesel softens the material and significantly alters the yielding characteristics. Although the crystallinity among stretched regions is similar, a virgin reference sample exhibits strain whitening during stretching, while the diesel-saturated sample becomes transparent. The WAXS results reveal a pronounced fiber texture in the tensile direction in the stretched region and an isotropic orientation in the unstretched region. This texture implies the formation of fibrils in the stretched region, while spherulites remain intact in the unstretched parts of the specimens. X-ray refraction reveals a preferred orientation of internal surfaces along the tensile direction in the stretched region of virgin samples, while the sample stretched in the diesel-saturated state shows no internal surfaces at all. Besides from stretching saturated samples, optical transparency is also obtained from sorbing samples in diesel after stretching.
Superglassy polymers with a large fractional free volume have emerged as novel materials with a broad range of applications, especially in the field of membrane separations. Highly permeable addition-type substituted polynorbornenes with high thermal resistance and chemical stability are among the most promising materials. The major obstacle for extending the practical membrane application is their strong tendency to physical aging, leading to a partial decline in their superior transport performance over time. In the present study, broadband dielectric spectroscopy with complementary X-ray scattering techniques were employed to reveal changes in microporous structure, molecular mobility, and conductivity by systematic
comparison of two polynorbornenes with different numbers of trimethylsilyl side groups. Their response upon heating (aging) was compared in terms of structure, dynamics, and charge transport behavior. Furthermore, a detailed analysis of the observed
Maxwell−Wagner−Sillars polarization at internal interfaces provides unique information about the microporous structure in the
solid films. The knowledge obtained from the experiments will guide and unlock potential in synthesizing addition-type polynorbornenes with versatile properties.
The physical and chemical effects of diesel and biodiesel fuels on two high-density polyethylene (PE-HD) types were investigated. Both semi-crystalline PE-HD are common thermoplastic materials for container and storage tank applications. Biodiesel, a composition of unsaturated fatty acid esters from renewable resources, was chosen as it is regarded a possible green alternative to fossil fuels. The study aims at identifying significant differences between biodiesel and conventional diesel fuels based on the differences in the chemical nature of the two. The physical effects of the fuels on the polymer at first comprises the sorption behavior, i.e. kinetics and final equilibrium concentration. Not only are both fuels absorbed by the amorphous phase of the semi-crystalline PE-HD, they also induce a plasticization effect that modifies the molecular mobility and therefore also the characteristic yielding properties, manifest in the obtained stress-strain curves. The chemical effects related to degradation phenomena is investigated by a long-term storage scenario using partially immersed tensile test specimens in diesel and biodiesel. We were able to confirm the proposed co-oxidation mechanism by Richaud et al. for polyethylene-unsaturated penetrant systems on a larger scale based on practical tensile tests. One of the investigated polyethylene grades subjected to tensile drawing showed a significant loss of plastic deformation and the onset of premature failure after 150 days of storage in biodiesel. Further biodiesel storage showed a systematically reduced elongation at break before necking. None of these effects were observed in diesel. Oxidation of fuels and polymer after progressing storage times were analyzed by the evolution of carbonyl species in FT-IR/ATR spectroscopy.