TY - BOOK

T1 - Natural and artificial weathering tests of polymer films used in solar applications

AU - Ottersböck, Bettina

N1 - no embargo

PY - 2017

Y1 - 2017

N2 - Weathering stability is the key factor for the reliability of polymers in outdoor applications. All polymers age which can lead to deteriorations of material properties over the application time. Required lifetimes of more than 20 years with a limited loss in functionality of the component are a challenge for polymers, like it is in the photovoltaic (PV) industry. Synergisms and antagonisms of the stress factors, such as irradiation, temperature, precipitation, etc. can cause different and complex ageing mechanisms. No universally valid method exists to forecast the ageing behaviour and the corresponding changes in the properties in order to predict a long-term weathering performance. Although natural weathering is the most reliable method to examine long-term performance, its major drawback is the extremely long exposure time required to determine the service lifetime of the components. In order to significantly decrease the testing time it is necessary to use harsher artificially accelerated tests. A feasible correlation of different ageing tests is required in order to ensure similar ageing behaviour. Therefore, this thesis deals with the influence of ageing methods and the influence of the microclimate on the properties of polymeric materials used in solar applications. Components which are in the focus in this thesis are the encapsulant of the solar cells and the back side covering (polymeric multilayer backsheet) of a PV module. The results enhance the knowledge in the field of polymer degradation and give an overview of suitable characterisation methods useful to identify different ageing processes and the correlated changes in properties of polymeric films. It is shown that all polymeric materials exhibit deteriorations and changes in its physical and chemical properties after natural and artificial weathering. These changes are dependent on the intensities of the stress levels and the microclimate. A significant correlation of the crystallisation temperature gained by thermal analysis and molar mass characterised by gel permeation chromatography can be found for polyethylene terephthalate films. Even though some changes in infrared spectra are observable, differences dependent on ageing temperatures or humidity levels are hard to distinguish. Therefore, infrared spectroscopy in attenuated total reflection mode is not a suitable method to characterise chemical degradation via hydrolysis of polyesters. The ageing behaviour of polymeric multilayer backsheets after natural and artificial weathering is analysed. Natural and artificial weathering of polymeric backsheets show different effects on the properties of the materials. Obviously it is not feasible to conduct artificial weathering tests for accelerated lifetime predictions, unless the parameters and the test design are adapted. The right test design for artificial ageing tests is highly important, as well as the consideration of the degradation mechanisms when trying to simulate natural ageing conditions. A feasibility study of the correlation of accelerated damp heat (85°C, 2000 hours) and ultra-accelerated pressure cooker ageing of polyester-based backsheets is conducted. The results of mechanical and thermal analysis using unaged and aged samples reveal similar changes in properties independent of the ageing method. Thus, the pressure cooker test (120°C, 96 hours) can be used for fast quality testing to support rapid material development in case of polyester-based backsheets instead of damp heat testing. The use of the pressure cooker test leads to an acceleration of the speed of testing by a factor of 21. Film and laminated module samples have been aged in order to study the effect of microclimates on the ageing behaviour of polyolefin-based films. The results show a strong influence of the microclimate on the respective polymeric ageing behaviour. Differences are even detectable in the unaged state between film and module samples after the production process.

AB - Weathering stability is the key factor for the reliability of polymers in outdoor applications. All polymers age which can lead to deteriorations of material properties over the application time. Required lifetimes of more than 20 years with a limited loss in functionality of the component are a challenge for polymers, like it is in the photovoltaic (PV) industry. Synergisms and antagonisms of the stress factors, such as irradiation, temperature, precipitation, etc. can cause different and complex ageing mechanisms. No universally valid method exists to forecast the ageing behaviour and the corresponding changes in the properties in order to predict a long-term weathering performance. Although natural weathering is the most reliable method to examine long-term performance, its major drawback is the extremely long exposure time required to determine the service lifetime of the components. In order to significantly decrease the testing time it is necessary to use harsher artificially accelerated tests. A feasible correlation of different ageing tests is required in order to ensure similar ageing behaviour. Therefore, this thesis deals with the influence of ageing methods and the influence of the microclimate on the properties of polymeric materials used in solar applications. Components which are in the focus in this thesis are the encapsulant of the solar cells and the back side covering (polymeric multilayer backsheet) of a PV module. The results enhance the knowledge in the field of polymer degradation and give an overview of suitable characterisation methods useful to identify different ageing processes and the correlated changes in properties of polymeric films. It is shown that all polymeric materials exhibit deteriorations and changes in its physical and chemical properties after natural and artificial weathering. These changes are dependent on the intensities of the stress levels and the microclimate. A significant correlation of the crystallisation temperature gained by thermal analysis and molar mass characterised by gel permeation chromatography can be found for polyethylene terephthalate films. Even though some changes in infrared spectra are observable, differences dependent on ageing temperatures or humidity levels are hard to distinguish. Therefore, infrared spectroscopy in attenuated total reflection mode is not a suitable method to characterise chemical degradation via hydrolysis of polyesters. The ageing behaviour of polymeric multilayer backsheets after natural and artificial weathering is analysed. Natural and artificial weathering of polymeric backsheets show different effects on the properties of the materials. Obviously it is not feasible to conduct artificial weathering tests for accelerated lifetime predictions, unless the parameters and the test design are adapted. The right test design for artificial ageing tests is highly important, as well as the consideration of the degradation mechanisms when trying to simulate natural ageing conditions. A feasibility study of the correlation of accelerated damp heat (85°C, 2000 hours) and ultra-accelerated pressure cooker ageing of polyester-based backsheets is conducted. The results of mechanical and thermal analysis using unaged and aged samples reveal similar changes in properties independent of the ageing method. Thus, the pressure cooker test (120°C, 96 hours) can be used for fast quality testing to support rapid material development in case of polyester-based backsheets instead of damp heat testing. The use of the pressure cooker test leads to an acceleration of the speed of testing by a factor of 21. Film and laminated module samples have been aged in order to study the effect of microclimates on the ageing behaviour of polyolefin-based films. The results show a strong influence of the microclimate on the respective polymeric ageing behaviour. Differences are even detectable in the unaged state between film and module samples after the production process.

KW - polymer films

KW - natural weathering

KW - artificial weathering

KW - photovoltaic module

KW - backsheet

KW - encapsulation

KW - ageing behaviour

KW - Polymerfolie

KW - Natürliche Bewitterung

KW - Künstliche Bewitterung

KW - Photovoltaik Modul

KW - Backsheet

KW - Einkapselung

KW - Alterungsverhalten

M3 - Doctoral Thesis

ER -