Gernot M. Wallner

Nanometer scale characterization of polymer films by atomic-force microscopy

Atomic-force microscopy was applied to perform a comprehensive surface roughness characterization of commercially available, highly transparent polymer films for transparent insulation applications. The morphological characterization included evaluation of the root-mean-square roughness, the lateral correlation length of roughness as well as the roughness exponent. In addition, high-resolution scans have been recorded yielding morphological information on a length scale of only a few ten nm. These measurements revealed the correlation between surface nanostructure and fabrication technique. For an impact-modified polymethylmethacrylate film with rubber inclusions phase images clearly uncovered the core-shell structure of these inclusions.

Damp heat induced physical aging of PV encapsulation materials

Apart from its chemical structure, mechanical properties of polymers depend mainly on polymer morphology, which is determined by the processing conditions. Exposure of the materials at elevated temperatures lead to changes in polymer morphology. The main objective of this study was to investigate the change in thermo-mechanical properties of different encapsulation materials (EVA, Ionomer) after damp heat testing. Therefore the materials were exposed for 1000h to three different temperature and humidity levels and the thermo-mechanical properties of unaged and aged materials were measured by dynamic mechanical analysis (DMA) and by differential scanning calorimetry (DSC). At temperatures below −20°C, EVA and Ionomer showed high elastic modulus values around 1000MPa, which decrease significantly to below 10MPa at temperatures higher than 20°C. All films showed an increase in elastic modulus values over the whole temperature range due to damp heat exposure. Using DSC, recrystallization was found to be the dominant physical aging mechanism. Higher temperature levels during exposure caused greater changes in the polymer morphology, which led to stiffening of the polymer. The significant increase in film stiffness under application relevant temperatures could cause some severe problems during the service life time of a PV module, starting with delamination and a reduced ability to withstand mismatches in thermal expansion and eventually cracking of the cell or the wiring.

Optical properties of polymer films for transparent insulation

Commercially available, highly transparent polymer films for transparent insulation applications were investigated systematically as to their relevant optical properties in the solar and infrared wavelength range. The photometric characterisation in the solar range and the calculation of non-spectral, solar optical film properties using models for scatteringabsorbing media have shown, that the solar extinction is dominated by scattering occuring mainly at the surface. For various amorphous and semicrystalline films the root-mean-square surface roughness correlated well with the solar optical thickness. Regarding high infrared absorptance in the wavelength range of about 10 μm the carbon-oxygen single bond is highly effective for commercial materials with maximum service temperatures of about 100 °C. For 50 μm thick films of different polymer types with carbon-oxygen single bonds in the molecular structure a good correlation between the concentration of the functional corbon-oxygen group and the non-spectral, infrared optical thickness was found.

ALL POLYMERIC FLAT-PLATE COLLECTOR - POTENTIAL OF THERMOTROPIC LAYERS TO PREVENT OVERHEATING

The investigations showed that the maximum absorber temperatures can be limited by thermotropic glazings. In general the impact of the thermotropic layer on collector efficiency is insignificant, as long as the solar transmission is above 85% in the clear state. Collectors with twin-wall sheet covers were found to be best suited in combination with thermotropic layers. As to thermotropic layer design for this collector configuration a residual hemispherical solar transmittance ranging from 25 to 60% in opaque state is effectual to control the stagnation temperatures in the range from 80 to 130°C. For maximum working efficiency a rapid and steep switching process and switching temperatures between 50 and 60°C are required.

Morphology of polyethylene ski base materials.

Abstract We used high-resolution Raman spectroscopy and differential scanning calorimetry for a comprehensive analysis of carbon black-filled polyethylene ski base grades at processing stages from the raw material to the structured ski base. Based on Raman mapping, we assessed the applicability of an advanced evaluation procedure for amorphous, disordered, and crystalline phase fractions of polyethylene for polyethylene extrusion and sinter grades. For sinter grades, a sufficient segregation between carbon black and polyethylene was confirmed, allowing for a comprehensive Raman spectroscopic morphological analysis. Significant morphological changes in polyethylene due to processing from the raw material to the semi-finished film and to the structured ski base were identified. Throughout the processing chain, we observed a decrease in crystallinity and an increase in the amorphous phase fraction. Although the raw material and the sintered semi-finished film exhibited a different but uniform polyethylene morphology, the morphological changes due to structuring of the ski base are limited to the top surface layer. The highest amorphous phase fractions were detected in the surface of the structured ski bases.

Fracture of poly(vinylidene fluoride): a combined synchrotron and laboratory in-situ X-ray scattering study

Semi-crystalline polymers show a complex fracture mechanism, which is controlled by the micro-mechanisms associated with formation and breakdown of a plastic deformation region. Such regions develop at notches, cracks or other stress-raising defects. In the present paper, we use time-resolved synchrotron X-ray scattering techniques during the deformation process in poly(vinylidene fluoride) to study the plastic zone formation and fracture processes at different strain rates. This gives new insight into the micro-mechanisms of cavitation, lamellae separation and fibril formation in this particular material.

Damp Heat Aging Behavior of a Polyamide-Based Backsheet for Photovoltaic Modules

This paper describes and evaluates accelerated aging of a titanium dioxide (TiO2) filled polyamide (PA) based backsheet film for photovoltaic (PV) modules. Damp heat exposure (85%RH, 85 °C) was carried out up to 2000 hrs. The backsheet was characterized using microscopic, spectroscopic, thermoanalytic, chromatographic, and mechanical methods. While Raman microscopy, infrared spectroscopy in attenuated total reflection mode (IR-ATR), scanning calorimetry (DSC), and thermal gravimetric analysis did not reveal aging-induced changes, significant yellowing was detected by ultraviolet visible near infrared (UV/VIS/NIR) spectroscopy. Depending on the stabilizer type (UV-absorbers, hindered amine light stabilizers (HALSs), and antioxidants), rather different consumption rates were ascertained by high-performance liquid chromatography (HPLC) and gas chromatography (GC). Although the ultimate mechanical properties decreased significantly, no full embrittlement was obtained after damp heat exposure of up to 2000 hrs. The observed physical and chemical aging mechanisms were classified as within the induction period without premature failure.

Advanced characterization of laminated electrical steel structures under shear loading

ABSTRACT The thermomechanical and mechanical behavior of electrical steel laminates was investigated by dynamic mechanical analysis (DMA) and a short beam test. These tests were implemented to evaluate the effect of various steel alloys on the adhesion properties of epoxy. DMA showed a dependency of curing state affected by the thermal conductivity of the steel alloy. Strain evaluation by digital image correlation showed that interlaminar shear failure occurred in a predominately cohesive manner in the epoxy layer at a shear angle of about 5°. This critical angle was not affected by the yield strength of the steel alloy used to create the laminate.

Effect of inter-plies on the short beam shear delamination of steel/composite hybrid laminates

ABSTRACT Interlaminar shear test methods (ILS) were implemented to characterize the delamination behavior of asymmetric steel/carbon fiber reinforced plastic (CFRP) hybrids. To improve the delamination behavior thermoplastic inter-plies were inserted between CFRP and steel. Supported by optical strain measurement the maximum shear stress (τMAX), the shear stress at interfacial delamination (τIF) and the shear stress at large-scale CFRP ply delamination τD were evaluated. The significant effect of inter-plies on the adhesion was best reflected by the shear stress value at interfacial delamination. Finite element analysis of the actual shear stress distribution in an asymmetric hybrid sample without inter-ply revealed that the calculated shear strength is just slightly overestimated compared to the standardized evaluation procedure.

3D small angle X-ray scattering (SAXS) on deformed PVDF foils

3D Small Angle X-ray scattering (3D – SAXS) was applied to study the microstructure and the deformation mechanism in PVDF – foils (polyvinyhdene fluoride). SAXS is a powerful tool to investigate structural changes in deformed polymers to reveal morphology at the nanometer scale. When PVDF is strained the structure changes from spherohtic isotropie to a highly anisotropie fiber bundle structure, which requires a full three-dimensional analysis of the SAXS signal.