Crosslinking and post‐crosslinking of ethylene vinyl acetate in photovoltaic modules

Ethylene vinyl acetate (EVA) is the dominating material for the encapsulation of solar cells. A better understanding of the crosslinking reaction progress during PV module lamination could lead to promising approaches for shortening of PV module lamination times but also for optimization of the EVA formulation. Therefore, the main aim of this study is to investigate the crosslinking behavior of EVA but also for optimization potentials of the EVA formulation. Currently, a degree of crosslinking higher than 70% obtained from Soxhlet extraction, is used as quality control standard in PV industry. Thermomechanical properties of the investigated EVA films demonstrate a sufficient state of crosslinking already after 5 min, which corresponds to a Soxhlet value of around 50%. Nevertheless, the effect of the remaining, still reactive peroxide crosslinker under service relevant conditions cannot be neglected. Therefore, the behavior of mini‐modules manufactured at different lamination times and stored under various aging conditions is investigated. EVA not fully cured during lamination are undergoing postlamination crosslinking. At the same time, remaining active crosslinker causes discoloration at soldering ribbons after accelerated aging. The crosslinking time in the lamination process may be reduced to ≥6 min, compromising between high throughput in production and the need of avoiding degradation. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44912.     

Studies on the concentration of oxidized components of abused fats and the application of HPLC to their separation

The efficiency of five separation techniques at removing unaltered compounds from the methyl esters of oxidized fats was compared, as were concentrations of oxidized products. A batch type distribution method using acetonitrile/hexane was the most effective in concentrating the polar products and removal of palmitate and stearate from abused fats which had been hydrogenated. The level of high molecular weight material present in samples was measured indirectly by determination of the percent elutable material via gas chromatography. A high performance liquid chromatography system developed to partially separate or “profile” the oxidized products used an octadecyl bonded phase column and a linear gradient from 50% aqueous acetonitrile to 85% aqueous acetonitrile, at a rate of 5%/min. The resultant chromatograms can be useful in assessing the quality of used fats.     

Estimation of reinforcement in compatibilized polypropylene nanocomposites by extensional rheology

Structural characterization in polymer nanocomposites is usually performed using X‐ray scattering and microscopic techniques, whereas the improvements in processing and mechanical properties are commonly investigated by rotational rheometry and tensile testing. However, all of these techniques are time consuming and require quite expensive scientific equipment. It has been shown that a fast and efficient way of estimating the level of reinforcement in polymer nanocomposites can be performed by melt extensional rheology, because it is possible to correlate the level of melt strength with mechanical properties, which reflect both the 3D network formed by the clay platelets/polymer chains as well as final molecular structure in the filled system. The physical network made of silicate filler and polymer matrix has been evaluated by X‐ray diffraction and transmission electron microscopy. Extensional rheometry and tensile testing have been used to measure efficiency of the compatibilizer amount in a polypropylene‐nanoclay system. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Improved bounds on sorting by length-weighted reversals

We study the problem of sorting binary sequences and permutations by length-weighted reversals. We consider a wide class of cost functions, namely f(ℓ)=ℓ^α for all α0, where ℓ is the length of the reversed subsequence. We present tight or nearly tight upper and lower bounds on the worst-case cost of sorting by reversals. Then we develop algorithms to approximate the optimal cost to sort a given input. Furthermore, we give polynomial-time algorithms to determine the optimal reversal sequence for a restricted but interesting class of sequences and cost functions. Our results have direct application in computational biology to the field of comparative genomics.     

Degradation of photovoltaic backsheets: Comparison of the aging induced changes on module and component level

In reliability testing of components for PV modules an always remaining question is about material (in)compatibilities and synergistic effects and thus, how results of singly tested materials correlate with materials aged within PV modules. Testing of single materials would simplify sample preparation, reduce costs and offer more testing options. Therefore the main objective of this study was to compare the aging behavior of single backsheets with that of backsheets incorporated within PV modules. Four different types of backsheets were chosen, all of them comprising of polyethylene terephthalate (PET) core layers, but differing outer protection layers. Test modules using identical components, varying only in the type of backsheet used were produced and damp heat aged (85°C/85% RH ≤2000 h). The results revealed no influence of the PV module lamination on the thermal characteristics of the polymeric backsheets. Even after DH aging, differences between single and module laminated backsheets were negligible. Degradation effects of PET could be detected for all aged sheets by thermal analysis and were confirmed by tensile tests and rheological measurements. Thus, it can be stated that testing of single PET based backsheets under DH aging conditions is a practicable way to investigate the applicability of a new backsheet. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42093.     

Diurnal Patterns of Wheat Spectral Reflectances

Spectral reflectances of Produra wheat were measured at 13 different times of the day at Phoenix, Arizona, on April 14, 1979 using a nadir-oriented hand-held 4-band radiometer which had bandpass characteristics similar to those on current LANDSAT satellites. Different sun altitude and azimuth angles caused significant diurnal changes in radiant return in both visible and near-IR regions of the spectrum and in several vegetation indices derived from them. The magnitude of these changes were related to different canopy architecture, percent cover, and green leaf area conditions. Spectral measurements taken at each time period were well correlated with green leaf area index but the nature of the relationship changed significantly with time of day. Thus a significant bias in the estimation of green leaf area index (GLAI) from remotely sensed spectral data could occur if sun angles are not properly accounted for.     

Correlation study of damp heat and pressure cooker testing on backsheets

For photovoltaic (PV) modules, high‐quality backsheets are essential. To verify these qualities, hot and humid tests of up to 1000 h are standard in the PV industry. Damp heat (DH) tests are useful for evaluating material quality and infant failures, especially for materials that are sensitive to hydrolysis. However, shorter test periods are needed. Hence, the objective of this study was to investigate and correlate the aging behavior of hydrolysis‐sensitive polyester [poly(ethylene terephthalate) (PET)] based backsheets after exposure to DH and pressure cooker conditions. The material properties, such as thermal and mechanical parameters, correlated well between the pressure cooker at 120 °C and the DH test, with the qualification that differences in the failure mechanism were seen after 96 and 2000 h, respectively. Nevertheless, it should be acceptable to use pressure cooker tests (PCTs) for fast material testing. The PCT is proposed to significantly reduce the testing time for infant failures and material quality to around 4 days for PET‐based backsheets. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44230.     

A numerical methodology for lifetime estimation of HDPE pressure pipes

Using linear elastic fracture mechanics a new methodology for a lifetime assessment of internally pressurized polymer pipes was developed. The concept is based on the numerical calculation of stress intensity factors for pipes under different loading conditions in combination with experimentally determined creep crack growth kinetics. Comparison of simulated lifetimes with experimental data from tests of internally pressurized pipes proved the principal applicability of the concept suggested. Major parameters affecting the pipe lifetime are identified and discussed. Combination of the creep crack growth tests and numerical simulations can be a very powerful tool for lifetime estimation of plastic pipes under different loading conditions.