To determine a polymer’s spectral response of photo degradation, two main methods have been established. The so-called “filter technique” uses polychromatic radiation, which is filtered by a set of various sharp cut-on filters. This procedure provides separated reading points of an activation spectrum of the investigated material. With the "spectrographic technique” a specimen is irradiated with pectrally dispersed radiation, locally quasi monochromatically, and the local property changes.are related to the radiant exposure at the respective wavelengths. A third method, the irradiation of specimens with quasi monochromatic radiation behind interference filters, is rarely used.
These methods have advantages and disadvantages and therefore, there are limits for conclusions from these spectral data. Concerning the quasi monochromatic Irradiation of specimens the development of new UV radiation sources looks promising.
With the rapid development of inexpensive UV Light-emitting diodes (LEDs), new : sources of nearly monochromatic radiation in the UV ränge are available, even down to about 250 nm. To determine the spectral response, the use of such UV-LEDs in TO-18 ) and TO-39 metal cases combines the advantages of low cost and energy efficiency, stable spectral irradiance, temperature.and humidity control, larger specimen areas, and an easy and stable analysis.
Combinations of different UV LEDs are also discussed as replacement for xenon arc ' radiation sources, see patent application EP01528388A1.
Radiation in the spectral ranges of UV and VIS are environmental impact factors that can cause ageing of many materials or products. The reason for this is that especially organic materials, such as food or many pharmaceutical, are subject to photochemical degradation. Of course, suited transparent packaging material may give protection against such impact for radiation sensitive fillings. But for this, it is necessary to know about the spectral range of the fillings sensitivity as well as about the radiation impact of the radiation sources that are relevant during the life time of the product.
The spectral irradiance for characterizing the emission of various radiation sources is easily measured by means of a spectroradiometer. The spectral sensitivity of a filling's property can be determined by spectrally dispersed irradiation, where the positions on a sample are related to the different wavelengths. Thus, the damaging effect of the different wavelengths can be directly evaluated. A lateral measurement of the relevant property change shows the activation spectrum, which is the product of the spectral radiant exposure and the spectral sensitivity. By measuring the spectral irradiance for each sample position, the spectral sensitivity of the ageing property can be calculated.
Comparing the fillings spectral sensitivity and the spectral irradiance of a potential radiation source during later transport and storage, conclusions can be drawn about necessary spectral absorption of a (partly) transparent packaging to give sufficient protection.
The setup for such investigation will be shown and several illustrating samples from daily food experience will be given.
Exposure response function for a quantitative prediction of weathering caused aging of polyethylene
(2019)
The exposure response function of the carbonyl formation over the bulk has been determined for a high-density polyethylene of a thickness of 200 μm, which was used as a weathering reference material according to ISO TR 19032. To this end, spectral sensitivity was studied by local measurement of the effect of spectrally dispersed irradiation. Both the exposure device and the methodology of determination are described.
The temperature dependency of photooxidation was determined by UV exposure at various temperatures between 23 and 80 °C. Deviations from linearity and thus reciprocity below 40 °C are discussed and assumed to be related to diffusion limitations. An Arrhenius approach –based on data of linear carbonyl formation – has been incorporated into the exposure response function. Using this exposure response function, aging in terms of the distribution of a quantitative property change over a plastic component can be predicted for a specific outdoor location with real chronologic weather data as input for the exposure. Thus, artificial and natural weathering can be linked and compared. The established exposure response function has been validated by outdoor exposure results from the literature. If an estimated diffusion limitation is taken into consideration, calculations and published data are in good agreement.