Carbon photostabilization

The hindered secondary amines can be highly effective photostabilizers for various polymers (]+.,5.,.6) Various hindered amines have been shown to retard oxidation, but most share the common feature of being secondary or tertiary amines with the a-carbons fully substituted. The most widely exploited representatives of this class are based on 2,2,6,6-tetramethylpiperidine either in the form of relatively simple low molecular weight compounds, or more recently as backbone or pendant groups on quite high molecular weight additives ( i.,5.,6). The more successful commercial hindered amines contain two or more piperidine groups per molecule. Photo-protection by tetra-methylpiperidines (near UV transparent) must result from the interruption of one or more of the reactions 1 to 3. Relatively recent results from our own laboratories, and in the open literature will be outlined in this context. 

UV protection can be obtained with cheap carbon blacks (but colour is grey to black) or with expensive photostabilizers. 

PAHs adsorbed on particles of carbon black were also photostabilized (Behymer and Hites, 1988). However, Barofsky and Baum (1976) demonstrated that BaP, anthracene, BaA, and pyrene deposited on carbon microneedle field desorption emitters and exposed to UV radiation were all photooxidized to carbonyl compounds. Similarly, PAHs can photodegrade efficiently in air when adsorbed to substrates of silica gel, alumina, or glass plates (e.g., see Lane and Katz, 1977 Kormacher et al., 1980 Behymer and Hites, 1985 Yokely et al., 1986). 

Cresol-formaldehyde, phenol-formaldehyde, and chlorinated cresol-formaldehyde novolac resins all undergo photo-oxidation upon UV-irradiation in air. The change of the carbon core level signals is shown in Fig. 8. The new IR band at about 1720 cm-, corresponding to formation of a carbonyl group, is also found after photo-oxidation of novolac resin. These oxidized layers of the novolac film are limited to a very shallow superficial surface only about 500 A thick, even after long deep UV irradiation, as discussed below in reference to photostabilization of resist images. 

The photostabilizing efficiency of A-methyl HAS pNCH3, e.g. 28,29,31, R = CH3,32) is within experimental error comparable to that of secondary HAS I NH [43,111,173,174], The simplest and most logical explanation includes a transformation of NCH3 into NH. Some studies contributed to the mechanism of this conversion. Model A-substituted tetramethylpiperidines having an H-atom on the a-carbon in the A-substituent were found to be photo-oxidized more easily than the corresponding secondary HAS [173]. Besides, the tertiary HAS decomposed /ert-butylhydroperoxide more rapidly than did NH. The reaction rates with terf-butylhydroperoxide at 132 °C were as follows ... 

Hindered Amine Stabilizers as Sources of Markers of the Heterogeneous Photooxidation / Photostabilization of Carbon Chain Polymers... 

The radical A is a stable unreactive free radical which is not capable of propagating the chain reaction. However, a disadvantage for hindered phenols and aromatic amines is that they do not trap carbon free radicals. Carbon free radical trapping reduces hydroperoxide formation to a great extent and is preferable to peroxy radical trapping. Hindered amines, which are well known as photostabilizers, can trap alkyl radicals and constitute a new class of radical trapping antioxidants. 

The products intended for use exposed to sunlight may be formulated with photochemical stabilizers (UV stabilizers) whenever it is possible, 1-2% of furnace grade carbon black (with particle diameter of 20-25 nm) represent the least expensive and most efficient stabilizer. In light colored formulations, benzophenone and benztriazole derivatives have been the most popular photostabilizers over the past period at present, the most powerful class of such stabilizers is represented by HALS, which has a typical structure (III) ... 

Maia, D.R.J., Balbinot, L., Popi, R.J., De Paoli, M.A. Effect of conducting carbon black on the photostabilization of injection molded poly(propylene-co-ethylene) containing Ti02. Polym. Degrad. Stab. 82, 89-98 (2003)... 

V. Other Applications. Bis(dithiocarbamate) complexes show a number of other potential applications. Some (R =Et R2 = C4Hg, C5H10) have been used as photostabilizers in the photodegradation of poly(vinylchloride) (PVC). The initial photoproduct is believed to be [Cu(S2CNR2)] , which is then rapidly oxidized by the carbon-chlorine bonds to produce [CuCl(S2CNR2)]2 and a cross-linked polymer (1790,1791). 

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