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Nanoindentation (NI) is a contact method to investigate localized micromechanical properties of materials, whereby NI of semi-crystalline polymers is challenging. The influence of morphological str...

The ageing behavior of ethylene vinyl acetate as encapsulant material in photovoltaic modules has been extensively studied over the years. This study aims to make a step forward in understanding how environmental conditions influence polymer behavior, how different module components interact with each other and finally how (and if) it is possible to correlate changes between material properties and power output. In this work, performances of laminated mini-modules subjected to different artificial ageing tests are compared and the degradation behavior of EVA is investigated. In particular, qualitative analysis of additives, color changes as well as changes in chemical structure and thermal stability of the encapsulant are analyzed.

This study presents aging of backsheet materials that were stored for 17 years in the dark at room temperature. The material properties were measured and compared to the results from right after production. After 17 years of storage the strain at break values decreased significantly for all materials. As chemical aging phenomena were excluded, a disentanglement of the polymer chains in the amorphous phase would explain the observed changes. Especially for PVDF the change in the mechanical behavior is considered as very critical and possibly would lead to cracking during operation.

The main focus of this work is to investigate the degradation behavior of two newly developed encapsulants for photovoltaic applications (thermoplastic polyolefin (TPO) and polyolefin elastomer (POE)), compared to the most widely used Ethylene Vinyl Acetate (EVA) upon exposure to two different artificial ageing tests (with and without ultraviolet (UV) irradiation). Additive composition, optical and thermal properties and chemical structure (investigated by means of Thermal Desorption Gas Chromatography coupled to Mass Spectrometry, UV-Visible-Near Infrared spectroscopy, Differential Scanning Calorimetry, Thermogravimetric Analysis and Fourier Transform-Infrared spectroscopy, respectively) of the analyzed polymers were monitored throughout the exposure to artificial ageing tests. Relevant signs of photo-oxidation were detectable for TPO after the UV test, as well as a depletion of material&rsquo, s stabilizers. Signs of degradation for EVA and POE were detected when the UV dose applied was equal to 200 kW h m&minus, 2. A novel approach is presented to derive information of oxidation induction time/dose from thermogravimetric measurements that correlate well with results obtained by using oxidation indices.

The very strict purity requirements of polymer parts that are used in the semiconductor manufacturing industry lead to an excessively low yield rate. To decrease the costs in the production chain, detailed knowledge about possible impurities during each processing step is essential. This work provides an overview of non-destructive testing techniques that are capable of either detecting or even identifying these impurities. The most promising techniques were then combined in a semi-automatized laboratory demonstrator that can be used as an in-line diagnostics tool to prevent processing of faulty parts. Results from these measurements are presented and discussed.

To contribute to the targeted 10 million tons per year of recycled plastic in Europe by 2025 and to improve the mechanical sorting degree of polyethylene (PE) products, density prediction models were developed from Fourier transform infrared-attenuated total reflectance (FTIR-ATR) and Raman spectroscopic data. State-of-the-art sorting in mechanical recycling provides separated polymer classes, however an improved classification with specific chemical and physical features such as density or melt flow rate has not been developed yet. Applying multivariate data analysis (MVDA) on the spectral datasets of 10 different PE materials, one FTIR-ATR and two Raman spectra based partial least square (PLS) density models were developed. However, whereas all three models are applicable to predict PE density accurately, the Raman models have shown some advantages. Firstly, less principle components (PC) are needed and secondly the density can be assessed with higher accuracy, due to the more robust cross-validated PLS model. Moreover, the obtained PC-s indicate that in the FTIR-ATR model the CH3/CH2 ratio, while in the Raman model the CH2, CH and the crystalline C–C bands can be correlated with the PE density. The most accurate PLS model was obtained from the 1500-1000 cm−1 Raman shift region. The developed models could improve the density based mechanical separation of PE and consequently increase the quality of recycled and reprocessed PE products.

Due to the general price pressure PV modules experienced in the last decade, a variety of alternative polymer materials and new backsheet designs were developed and introduced into the market, amongst others also extruded backsheets based on polypropylene (PP). The production of co-extruded films represents a backward integration of backsheet manufacturers, allowing for easy material and design modifications regarding additive formulation, fillers and geometry. The main objective of this paper was to describe the influence of the special properties of PP-based backsheets on the performance and reliability of PV modules. The higher reflectance of the PP backsheet compared to a reference polyester (PET) based laminate led to an increase in power output between 1.5 and 2.5%. Test modules using PP and PET backsheets were exposed to 3000 h of damp heat testing. No changes in power output for either of the test modules was observed. However, the selective permeation properties of PP backsheets prevent humidity from entering the module (similarly to PET based backsheets) but allow for significantly higher diffusion of acetic acid out of the PV module. Therefore, in contrast to test modules using Ethylene Vinyl Acetate (EVA) encapsulants and PET backsheets, no silver grid corrosion was observed for modules using PP backsheets. Co-extruded backsheets based on PP show great potential to be a valid replacement of standard PET based backsheets in PV modules. On the one hand, the PP backsheet so far proved excellent stability, exhibiting no severe material degradation after extended exposure to temperature, humidity and irradiation. On the other hand, its improved functional properties (optical properties; selective permeability) lead to increased performance and improved long-term stability of the tested PV modules.

This paper describes the development of a novel high-performance thermoset based on epoxidized linseed oil and pure crystalline citric acid (CA). Fully bio-based polymer networks are generated and CA is shown to be a very efficient hardener for epoxidized plant oils. Comprehensive processing-structure-property relationships are established and optimum curing parameters are derived. The thermoset's polymer physical property profile (incl. high stiffness and strength) reveals its great potential to substitute petrochemical based resins.

This work focused on the technology of luminescent down shift (LDS), with a primary aim to identify and investigate a methodology to introduce the luminescent organic dye into PVB polymer encapsulant as emergent material for photovoltaic application. For this goal, we propose to study the feasibility to implement the LDS functionality and to identify suitability of available luminescent to be incorporated into the host polymer encapsulant material. The first step to this direction was through a comprehensive optical study of Violet 570 (V) organic dye in ethanol solvent. The methodology and experimental conditions such as laboratory polymer preparation and luminescence dye concentration were presented. Also, the emergent polymer encapsulant sheets were characterized by using optical and thermal analysis techniques. The absorption spectrum of the prepared PVB material shifts towards longer wavelengths, with increasing organic dye concentration.

This work focused on the technology of luminescent down shift (LDS), with a primary aim to identify and investigate a methodology to introduce the luminescent organic dye into PVB polymer encapsulant as emergent material for photovoltaic application. For this goal, we propose to study the feasibility to implement the LDS functionality and to identify suitability of available luminescent to be incorporated into the host polymer encapsulant material. The first step to this direction was through a comprehensive optical study of Violet 570 (V) organic dye in ethanol solvent. The methodology and experimental conditions such as laboratory polymer preparation and luminescence dye concentration were presented. Also, the emergent polymer encapsulant sheets were characterized by using optical and thermal analysis techniques. The absorption spectrum of the prepared PVB material shifts towards longer wavelengths, with increasing organic dye concentration.

The influence of the type of backsheet on the electrical performance of test modules was evaluated before and after increasing time of accelerated ageing (damp heat [DH] exposure). Besides the measurement of the electrical power of the modules and the performance of the cells by electroluminescence, the ageing-induced changes within the polymeric encapsulate and backsheets were investigated by means of vibrational spectroscopy and by thermo analytical methods. In addition, the permeability of the backsheets in the original and aged state was determined. This wide set of test parameters and methods allowed for the detection of correlations between (i) physical and chemical properties as well as their ageing-induced changes of the materials and (ii) the module performance. A clear dependence of the relative loss in power output upon exposure under DH conditions for 2000 h could be observed for a set of identical test modules varied in composition only in the type of back cover used. While the modules containing gas-tight backsheets and glass experienced only little loss in the relative power output, some modules with permeable backsheets showed a significant relative decrease in the power output and fill factor in dependence of the backsheet type used. Cell degradation could be visualised by recording electroluminescence images before and after the accelerated ageing test. The permeation properties of the backsheet used and their ageing-induced changes seem to have an influence on the module performance. However, the absolute values neither of the water vapour transmission rate (WVTR) nor of the oxygen transmission rate (OTR) are directly linked to the loss in power output upon accelerated ageing under DH conditions. It could be shown that the ageing-induced changes (relative transmission rates) between WVTR and OTR can be correlated with the module performance. These ageing-induced changes in the permeation behaviour of the backsheets can be explained by (i) physical changes (e.g. post-crystallisation, changes in the crystal structure or the crystalline microstructure) and (ii) chemical ageing effects such as a decrease in the molecular mass of the polyester (PET) polymer chains because of hydrolytic polymer degradation leading to a change in the crystallisation behaviour of PET. Hydrolytic degradation (= chemical ageing) of the PET core layer was observed (with varying extent) for all PET-based backsheets and can, thus, not be directly correlated with the loss in performance of the corresponding test modules. The physical ageing effects, however, were detected only for those backsheets showing (i) strong deviating changes in the relative permeation rates for oxygen and water vapour upon accelerated ageing and (ii) a clear loss in electrical performance. Copyright © 2015 John Wiley & Sons, Ltd.

Renewable and clean energy sources are getting more important. Therefore, a resource-saving alternative to conventional solar concentrators has been designed. This pneumatic pre-stressed concentrator is completely made of polymer films. Due to its pre-stressed construction the mechanical properties of the transparent film have a high influence on the lifetime of the concentrator. The dimension stability of the film significantly affects the effectiveness of the systems due to the shift of the focus point of the reflected radiation after deformation. Therefore, special requirements concerning thermal and mechanical properties in the application relevant temperature range have to be fulfilled. The aim of this work is to characterise fluoropolymer films as transparent layers in a pneumatic pre-stressed sun concentrator. Fluoropolymers have been chosen due to their excellent optical and mechanical properties and their extraordinary stability against ultraviolet radiation. A total of five different fluoropolymer films have been selected and their thermal, mechanical and thermo-mechanical properties at application relevant temperatures were characterised. Even though no material complies with the requirements, an ethylene-tetrafluorethylene-copolymer (ETFE) is suggested, with the recommendation to adapt and adjust the film thickness and the production process to influence the morphology and subsequently the thermo-mechanical behaviour of the material.

This paper presents an approach enabling localization and identification of foreign bodies in polymer materials applying a combined approach of x-ray imaging, imaging microscopy, optical coherence tomography and Raman imaging spectroscopy. The reliable detection of even small foreign bodies in polymer materials and parts designated for use in semiconductor manufacturing and processing machines is essential. Foreign bodies can in particular be metals, burnt particles of the polymer of the work piece, or intact or degenerated foreign polymers. In either case, all surfaces of e.g. a handling equipment that get in contact with the semiconductor material or process solutions have to be free of foreign bodies to ensure the integrity of the manufacturing process. Size, localization and material of the foreign body are main parameters that decide if a work piece has to be rejected. Current inspection systems may enable the localization of the foreign body, but are not capable of identifying the material and structure of the foreign body; many components with inclusions are therefore rejected as a precaution. This work aims towards the development of a combined sensor approach as part of an automatic quality assurance procedure which can be integrated in the fabrication process. X-ray imaging is used to identify metal foreign bodies. Imaging microscopy is used to detect foreign bodies on the surface of the polymer parts. Optical coherence tomography is used to measure the three-dimensional position and size of the foreign bodies. Raman imaging spectroscopy is used to identify the composition of the foreign bodies if they are located on the surface.

A total of 16 analytical methods, spanning from classical solvent extraction over different thermo-analytic and mechanical approaches to acoustic and optical spectroscopy, have been evaluated as to their ability to determine the crosslinking state of ethylene vinyl acetate (EVA), the prevailing encapsulant for photovoltaics applications. The key objective of this work was to create a systematic and comprehensive comparison, using a unified set of traceable test samples covering the full range of realistically occurring degrees of EVA crosslinking. A majority number of these tested methods proved fundamentally suitable for detecting changes in the polymer properties during crosslinking based on the effect e.g. its mechanical properties or its crystallinity. Interestingly, when investigated in detail, most of the methods showed mutually different dependencies on the lamination time, indicating a complex range of effects of the chemical crosslinking on the properties and behaviour of the material. Furthermore, Raman spectroscopy could be identified as a potential new method for measuring the degree of crosslinking in-line in the PV module manufacturing process, thus providing an interesting approach for improving process control in PV module processing.

In this paper, 4- and 5-flux models were implemented and used for the determination of absorption and scattering coefficients of transparent polymer films by UV/vis/NIR spectroscopy. The models were applied for a systematic characterization of polar ethylene copolymer films in the solar radiation range. The investigated ethylene copolymer films were highly transparent in the solar range of radiation, with hemispheric transmittance values above 91% and hemispheric reflectance values of about 8%. Both models revealed a significant forward scattering behavior of the films. The 4-flux model fitted hemispheric and diffuse transmittance far better than the 5-flux model. Hemispheric reflectance values were overestimated by both models. Similar absorption coefficient values ranging from 0.4 to 0.6 cm−1 were obtained for all investigated films. While ethylene(acrylic acid/butylacrylate) terpolymer showed the lowest scattering coefficient (10.9 (5-flux) and 6.8 cm−1 (4-flux)), the highest scattering coefficient (26.2 (5-flux) and 35.2 cm−1 (4-flux)) was found for ethylene(butylacrylate) copolymer.

This paper focuses on the description of the ageing behaviour of commercially available ethylene vinyl acetate (EVA) greenhouse films due to accelerated weathering. Two commercially available multi-layer films were chosen as model materials. The degradation behaviour was analyzed by infrared (IR) spectroscopy in transmittance and attenuated total reflectance (ATR) mode, by ultra violet (UV) visible/near infrared-spectroscopy, differential scanning calorimetry (DSC) and by comprehensive mechanical characterization. Using IR spectroscopy, hydroxyl-, vinylidene- and acid-groups were identified as main degradation products. By IR and UV/VIS/NIR-spectroscopy the consumption of stabilizers was detected. Post-crystallization was detected by DSC measurements. The analytical results of the weathered films were correlated to ultimate mechanical properties determined by investigating unnotched and notched specimens. The results suggest that degradation of the investigated polymer films is strongly confined to the surface. While both films were of different layer structure, the ageing processes on the surface of the 200 μm thick film are not reflected by mechanical properties, in contrast to the 30 μm thick film. Nevertheless, tensile tests appeared to be the most versatile method for describing ageing phenomena. Both chemical ageing and physical ageing as well as local (initial) and global ageing effects are reflected by mechanical properties.

This work analyses samples of the widely used encapsulant of photovoltaics modules, ethylene vinyl acetate (EVA). The samples were cross-linked using a lamination technique for different curing times (0 - 20 minutes). The cross-linking characterization is done by determinating the material dispersion with the aid of a combined temporal- and spectral domain white-light interferometer. With the proposed technique it was possible to discriminate the differences in crosslinking for the given curing times. One important feature of this approach is the possibility to perform space resolved measurements of the crosslinking state with μm-resolution. Furthermore the paper discusses the mathematical analysis and processing of measurement data and shows a prototype solution for the fast and automated data acquisition for industrial application.

In this paper an understanding of the physical relationships between the material structure and the temperature dependent infrared optical properties of different transparent polymer films for solar applications is described. The infrared optical properties are relevant for the heat transport of e.g. greenhouse and transparent insulation structures. The properties were determined based on infrared transmittance measurements and the assumption of a constant index of refraction from the visible range. To establish structure-property-correlations molecular structure parameters such as the concentration of carbon-oxygen single bonds and carbon-hydroxyl groups were determined. For 50 μm thick films a good correlation between the concentration of the functional carbon-hydroxyl and the carbon-oxygen group and the infrared optical thickness as well as the hemispherical emittance was found. This correlation fits well for high and low infrared radiation absorbing polymeric materials consisting of carbon, hydrogen, and oxygen atoms. The carbon-hydroxyl group appears to be slightly more effective than the carbon-oxygen single bond. Interestingly, the correlation works for polymers with aromatic (PC, PET) and aliphatic (PMMA, CTA, ethylene copolymers) groups.

The aim of these investigations was to develop, implement and evaluate an appropriate procedure in order to qualify new polymeric components for the use in PV modules and to predict the lifetime of materials and modules. A test program concerning five accelerated artificial aging tests (varying UV, temperature and humidity levels) was set up and the degradation behavior of six polymers was investigated. Visual transmittance and strain-at-break were identified as most significant degradation indicators. A suitable material specific material degradation model was obtained, where changes of the material properties are accurately related to the applied environmental stresses. For the modeling of service life time the micro-climate, to which the encapsulation materials are exposed in a PV-module, was considered and reasonable end-of-life criteria were defined. Accelerated aging tests showed that effects of heat and humidity are dominating in optical properties, whereas changes in mechanical properties are equally effected by UV and humidity. An exact description of the micro-climate was found to be most critical for the accuracy of the material degradation model. For first qualification of the principal applicability of new materials both, a standard damp heat test and a UV test have to be done in order to validate all degradation effects properly.

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.