Search results
Results from the WOW.Com Content Network
The type I reaction dominates, which cause chain scission at the carbonyl unit to give a range of products. [1] [38] Type II Norrish reactions are less common but give rise to acetaldehyde by way of vinyl alcohol esters. [36] This has an exceedingly low odour and taste threshold and can cause an off-taste in bottled water. [39]
For melanin this mechanism has developed later in the course of evolution. Melanin is such an efficient photoprotective substance that it dissipates more than 99.9% of the absorbed UV radiation as heat. [13] This means that less than 0.1% of the excited melanin molecules will undergo harmful chemical reactions or produce free radicals.
Photoexcitation is the first step in a photochemical process where the reactant is elevated to a state of higher energy, an excited state.The first law of photochemistry, known as the Grotthuss–Draper law (for chemists Theodor Grotthuss and John W. Draper), states that light must be absorbed by a chemical substance in order for a photochemical reaction to take place.
Photosensitizers are light absorbers that alter the course of a photochemical reaction. They usually are catalysts . [ 1 ] They can function by many mechanisms, sometimes they donate an electron to the substrate, sometimes they abstract a hydrogen atom from the substrate.
This reduces the absorption of UV rays by the polymer matrix and hence reduces the rate of weathering. Phenolic benzotriazoles (e.g. UV-360, UV-328) and hydroxyphenyl-triazines (e.g. Bemotrizinol) are used to stabilise polycarbonates and acrylics, [25] oxanilides are used for polyamides and polyurethanes, while benzophenones are used for PVC.
The proposed mechanism for cationic photopolymerization begins with the photoexcitation of the initiator. Once excited, both homolytic cleavage and dissociation of a counter anion takes place, generating a cationic radical (R), an aryl radical (R') and an unaltered counter anion (X).
2,4-Di-tert-butylphenol (2,4-DTBP) is a white solid with a phenolic odour.It is primarily used as a raw material for the production of several commercially important antioxidants and phenolic benzotriazole-type UV absorbers.
In chemistry, a photoinitiator is a molecule that creates reactive species (free radicals, cations or anions) when exposed to radiation (UV or visible). Synthetic photoinitiators are key components in photopolymers (for example, photo-curable coatings, adhesives and dental restoratives).