Photo initiators peroxides

Common thermosets are cured by a free radical addition mechanism. These types of composites are cured by heat initiators, such as peroxides, or by photo initiators, such as a-diketones. A characteristic of cured acrylates is large shrinkage in the course of polymerization, which is undesirable for many uses. Another undesirable characteristic of acrylates is the formation of an oxygen-inhibited layer on the surface upon curing. 

As initiators, common peroxides, such as dicumyl peroxide, di-benzoylperoxide can be used, but also photo initiators have been reported. Photoinitiating systems are the combinations of benzo-phenone with amines, such as n-butylamine, trimethylamine and triethylamine (43). 

It was previously noted that no evidence for direct photolysis for PFOS or PFOA has been observed experimentally. In aqueous solutions alone and in the presence of hydrogen peroxide (H2O2), iron oxide (Fe203) or humic material, PFOS has been observed to undergo some indirect photolysis [28] whereas PFOA did not undergo indirect photolysis [29]. Using an iron oxide photo-initiator matrix model, the indirect photolytic half-life for PFOS was estimated to be > 3.7 years at 25 °C. The half-life of PFOA was estimated to be > 349 d. 

UV oxidation of organic water pollutants in the presence of hydrogen peroxide, ozone or both with powerful medium-pressure mercury lamps is performed on industrial scales (Gottschalk et al., 2000). The synergistic combination of ozone and UV is especially suited for water sanitation, i.e. treatment of swimming pool and spa water (Rice, 1997). Many full-scale remediation applications of photo-initiated AOPs using hydrogen peroxide or ozone are already in operation (Chem-viron Carbon, 1997, Freeman and Harris, 1995). 

However, the comparison of the data collected in Tab. 6-4 offers a realistic estimate of process efficiencies. The quantum yield of OH radical formation on Ti02 is only 4 to 10% at best. This is the major reason why Ti02 processes have, in general, not been commercially successful. Pure economic analysis (favored by Bolton) results in an estimate that hydrogen peroxide based photo-initiated AOPs are 50 to 100 times more efficient in their use of electricity than Ti02 photocata-... 

Di(iso-propyl) -peroxide, (CH a.CH—OO—CHf013)2 mw 118H7, O 27.08% liquid, bp 94° at 760mm 55—58° at 233mm press w/o expln not exploded by impact undergoes photo-initiated, aut o-inhibited chain decompn at 269(Ref )... 

In view of the conclusion that hydroperoxides are important photo-initiators it is important to evaluate the relative contribution of uv screening, peroxide decomposition, triplet quenching and kinetic chain-breaking to the mechanism of action of typical nickel complex steibilisers. Nickel dibutyl dithiocarbamate (III, R=n Bu, M = Ni) was selected as representative of the peroxide decomposers... 

Photo- and Peroxide-Initiated Catalysis by Metal Complexes... 

Studies of model compounds have confirmed that both unsaturation and peroxides are important in the photo-initiation process. Figure 16 shows that diamylethylene( 1) autoxidizes in the presence of UV light in... 

Among other two eomponent photo initiating systems, eombinations of photoinitiators with peroxy eompounds, like with pyrilliuim peresters, or peroxides with Miehler s ketone were reported to be effeetive. The same is true of eombinations of aromatie ketones with dyes. Further diseussions of the use of dyes in eombination with ketones ean be found in seetions 2.2.4 and 2.2.7. 

Preparation of a novel cationic photoinitiator [Cp-Fe-diphenylether] PFe was reported. It was claimed that polymerizations of epoxy resins and eyeloaliphatic epoxides can be photo initiated by this compound effectively. Addition of benzoyl peroxide to this photoinitiator promotes the... 

Photo- and Peroxide-Initiated Catalysis by Metal Complexes. Photogenerated catalysts obtained by the light-induced generation of a ground-state catalyst obtained from a catalytically inactive precursor have become a topic of current interest in catalysis by transition-metal complexes. Such methods have also been applied in hydrosilylation. 

Photo-initiation for A -vinylcarbazole with azoisobutyronitrile or benzoyl peroxide leads to simultaneous radical and cationic polymerizations, as when the peroxide is used as a thermal initiator with this monomer. Triarylsulphonium salts can initiate both types of polymerization in some systems. The use of sources of radicals for indirect initiation of cationic polymerization has been reviewed. ... 

The reaction sequence (5)-(8) leads to the rapid accumulation of hydroperoxides and the attainment of a stationary peroxide concentration. Soluble Cu, Mn, Fe, Cr and Co compounds are the most effective promoters of peroxidation whereas Ni, Ce, V, Ti and Zn salts are less effective [16]. Transition metal ions are also photo-initiators of... 

When treating acne vulgaris, topical and systemic therapies (if indicated) are initiated 2 to 4 weeks prior to peeling. Topical antibiotics and benzoyl peroxide based products can be used daily and discontinued 1 or 2 days prior to peeling. However, unless a deeper peel is desired, retinoids should be discontinued 7-10 days prior to salicylic acid peeling. Broad-spectrum sunscreens (UVA and UVB) should be worn daily (see Photo damage. Sunscreen section). 

Several carbonyl-containing peroxide additives have been shown to increase the initial rate of the nonoxidative photo-dehydrochlorination of PVC (54). In studies with polymeric ketones unrelated structurally to PVC, the excited singlet and triplet states of the carbonyl groups in these polymers were found to sensitize 0-0 homolysis at rates approaching diffusion control (55). Similar reactions may well occur in oxidized vinyl chloride polymers. 

Molsidomine and pirsidomine, are both stable as solids at room temperature in the absence oflight [137]. However the ring opened metabolites SIN-1A and C87-3786 are both photo labile and sensitive to an oxidising environment resulting ultimately in the release of superoxide and NO in stoichiometric quantities [138]. Generation of these two species is an obvious problem due to the resulting formation of peroxynitrite and the generation of OH, which may initiate lipid peroxidation [139-141] (see Eq. (19)). Such concerns over the formation of peroxynitrite from SIN-1A or C87-3786 are warranted since their cytotoxic effects show close consistency with cellular studies doped with neat peroxynitrite [142, 143]. 

Both of these general anaesthetics have long been known to oxidize in the presence of air and sunlight. Chloroform was shown to photo-oxidize to a peroxide of uncertain structure which collapsed to the poison gas phosgene. Anaesthetic chloroform is protected by the addition of ethanol, which intercepts the initial free radical and prevents the photodegradation [17]. Diethyl ether... 

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