Photostabilizing effect

Laser flash photolysis techniques offer the possibility of examining in detail the transient processes responsible for the photostabilizing effect discussed above. The triplet lifetimes are frequently too short, even for this technique however, they can still be estimated using as a probe the quenching by 1-methyl-naphthalene, which leads to the formation of its easily detectable triplet. The optical absorbance due to the 1-methylnaphthalene triplet (Aft) produced as a result of energy transfer is related to the Stern-Volmer slope by equation 5, where N stands for... 

UV absorption spectra, fluorescence emission spectra and photostabilization effect of 2-(2-hydroxy-4-acryloyloxyphenyl)-2/7-benzotriazole and of its polymer bound forms were studied in poly-CK-l,4-polybutadiene [337]. The following activity series was found copolymer with methyl methacrylate > homopolymer > monomer. It seems that chromophoric units incorporated into a macromolecule behave cooperatively (causing self-absorbance of the emitted radiation). 

Asker AF, Colbert DY. Influence of certain additives on the photostabilizing effect of acid for solutions of FD C Blue No. 2. Drug Dev Ind Pharm 1982 8(5) 759-773. 

The influence of variations in the film layer thickness on photo protection was investigated using uncoated nifedipine tablets. Samples of the tablets were completely covered with yellow transparent blister films of layer thicknesses ranging from 30-300 pm. The photostabilizing effect of the yellow blister film was found to increase with film layer thickness. After six hours irradiation the loss of drug content was 24% for the 30 pm films and only 6% for the 300 pm films, (Fig. 6) a substantial improvement in the photo protection of the product (21). 

Due to absorption of incident radiation in upper layers, the drug itself may have a photostabilizing effect. Photodegradation often leads to the production of colored products which themselves absorb and further shield the sample. Consequentially, degradation rates decrease with increasing concentrations as shown by the example of nifedipine solutions (Fig. 1) (3). 

The photoprotecting effect of opacifiers is based primarily upon their total absorption and/or reflection of incident radiation. Another approach to predict the photostabilizing effect of a film is therefore to determine its opacity. 

Other benzoic acid derivatives also function as effective photoprotective agents. Methylparaben has a photostabilizing effect on solutions of riboflavin (69). Ethyl aminobenzoate significantly enhanced the photostability of menadione in aqueous solutions (107). 

Sodium thiosulfate provides a photostabilizing effect for solutions of minoxidil (48), riboflavin (69), tetracycline hydrochloride (71), and phenobarbital (72). 

It is well known that HALS compounds (sterically hindered amine light stabilizers) exert excellent photostabilizing effect on polyolefins (1-3). Most of the authors agree that N-oxyl radicals formed from HALS compounds play an important role in the photostabilizing process (4-6). 

A photostabilizing effect by complex formation with cyclodextrins has been reported for a number of biologically active compounds (Zejtli et al., 1980 Uekama et al., 1983 Ammar and El-Nahhas, 1995 Chen et al., 1996 Lin et al., 2000 Bayomi et al., 2002, Tpnnesen et al., 2002). However, formation of an inclusion complex does not necessarily lead to a stabilization of the guest molecule. This is clearly... 

J. Lucki, J.F. Rabek, and B. Ranby, Photostabilizing effect of hindered piperidine compounds Interaction between hindered phenols and hindered piperidines, Polym. Photochem. 1984, 5, 351-354. 

Decahydroquinoxaline 19 (R = benzyl) is another excellent LS and AO for PP [39], The photostabilizing effect is lower than that of HAS [113], most probably due to the presence of the a-CH bond to the amino group enabling formation of mesomeric structures of the derived aminyl. The respective nitroxide may disproportionate similarly as shown with 18 in Eq. (13). A stable nitroxide 113 was prepared from 19 and perbenzoic acid [39]. It is not clear if 113 may also form with ROO ... 

Hydroxylamines are easily oxidized at ambient temperature with molecular oxygen and with ROOH (Scheme 21) [65,134]. The last reaction is very fast [127]. In spite of formation of alkoxy radicals RO , deactivation of ROOH contributes to the photostabilizing effect of NOH (formed RO are effectively scavenged by phenolic AO, mostly used in combination with HAS). 

Additives can also have a similar effect decabromo-diphenyl ether flame retardant in polypropylene was shown to enhance oxidation. Furthermore, where hindered-amine light stabilizers HALS were present in the compound, the degradation product HBr reacted with it to form the ammonium salt that has no photostabilizer effectiveness... 

Law, W.Y., Pan, J.Q. Photostabilizing effectiveness of new HALS produced by isocyanation of hindered piperidine derivatives. Polym. Degrad. Stabil. 60, 459 64 (1998)... 

Figure 11 Effect of mixing procedure (processing under mild conditions, closed mixer (CM) and under oxidative conditions, open mixer (OM) of an internal mixer, a RAPRA torque rheometer) on photooxidative stability of PP containing additives (2.5 X 10mol 100g ). Inset compares photostabilizing effectiveness of mildly processed (CM) thiophosphoryl disulfide...

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