Evidence for migration

The suspicion surrounding epoxy-phenolic and polyvinyl chloride lacquers of metal packaging, as well as the weight and energetic cost of recychng glass, favors plastic materials. They are used for applications including sterihzed food. 

The solubility diffusion of food pigments is the phenomena responsible for this coloration. It is important to highlight that in the opposite direction, from the packaging to food, substances from the material, of approximately the same size of P-carotene or smaller, can also migrate provided that they are not covalently bonded to the polymer matrix. This contamination of food is usually invisible, without being smelled or tasted, but occurs insidiously. 

At the end of life, the material may be polluted by environmental substances (e.g. mixing of waste), by secondary use of the material (e.g. a water bottle used to stock a pain solvent or petrol) or even during pick-up (e.g. mixing of shampoo bottles and PET bottles containing frait juice). These pollutions affect the reuse potential of these materials even after decontamination (solvent washing and vacuum drying of pellets) for subseqirent contact with food. Currerrtly, these practices are orrfy authorized for PET [EFS 11, EUR 08], 

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Evidence for migration of the arylthio group across the double bond comes from C-14 labeling studies on ester 213 and product studies from unsymmetrical vinyl sulfonates 214 (181). Complete scrambling of the C-14... 

Arita Y. and Wada H. (1990) Stable isotopic evidence for migration of metamorphic fluids along grain boundaries of marbles. Geochem. J. 24, 173-186. 

Recently Bi2Al409 was investigated in a search for exciton migrationThe use of several activators (Cr , Eu ", Tb3+) did not yield any evidence for migration. The situation for Bii2Si02o and Bii2Ge02o is still unclear It is clear that in these types of compounds still lots of work remain to be done. For this it is not only necessary to apply modern spectroscopical techniques, but also to have available well-characterized materials without high defect concentrations. 

Experimental evidence for migration of energy has been given in many papers recently. Weber (42) investigated energy transfer between europium and chromium in phosphate glasses and analysed his data in view of the Yokota-Tanimoto theory. He verified that in... 

With a glassy solution of poly-1-vinylnaphthalene, the delayed emission spectrum has been shown to consist of an emission having a mean lifetime of approximately 80 ms at the normal fluorescence wavelength, in addition to the phosphorescence having a mean lifetime of about 2 s [159]. The delayed fluorescence did not appear in the spectrum of 1-ethylnaphthalene. With the polymer it was found to be inhibited by piperylene, a well-known triplet quencher. These results have been explained by mutual annihilation of two excited triplet states produced by the absorption of two photons by the same polymer molecule. They are considered as strong evidence for migration of the excited triplet state in poly-1-vinylnaphthalene. In polyacenaphthalene, however, which is chemically very similar to poly-1-vinylnaphthalene (see p. 409), no delayed fluorescence could be detected in the same experimental conditions [155]. 

An experimental value of R0 can be obtained by plotting f as a function of the logarithm of the acceptor concentration and fitting the theoretical curve calculated according to Forster s theory to the experimental results. An experimental value larger than the calculated one may be considered as evidence for migration of excitation prior to the transfer. Agreement was obtained between the experimental results and Forster s theory for the system polystyrene—tetraphenylbutadiene [165]. 

On the other hand, the promoting effect of Ti02 for CO hydrogenation may not be directly related to the presence of TiO, species on the metal surface because the enhancement in activity relative to other supports is observed even after reduction at low temperatures where there is no evidence for migration. However, there is evidence that deposition of... 

The suggestion that chevrons might cause accidents to migrate downstream was investigated, and no evidence for migration has been found. 

The picture which evolved from stationary state experiments complemented by a few early time-resolved measurements invoked a modified Birks kinetic scheme. This notion was accepted virtually without question in studies prior to 1980. Further, the majority of authors cited the occurrence of singlet energy migration and subsequent sampling of potential excimer sites in the absence of concrete evidence for migration except for that afforded by emission depolarization [66,67]... 

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