02/23/2014 from 10/4/2012
Patents with Abstracts
1. “Polymeric substrates or polymeric materials, primarily those of an "organic" nature, are susceptible to degradation from high energy radiation. Ultraviolet (UV) radiation from sunlight is the most common initiator of polymeric photo-oxidation.
UV light when absorbed by organic polymer systems generates "free-radicals." These free-radicals (peroxides and molecular fragments with an unpaired electron) cause the exposed material to degrade via various oxidation reactions resulting in color change and loss of physical and chemical properties.
Basically two main classes of additives are commonly used for efficient photo stabilization of organic polymers such as plastics, elastomer-modified plastics, fibres and coatings: UV absorbers and photo-antioxidants of the hindered amine stabilizers (HAS) type. The additives interfere with chemical and physical processes induced by solar and artificial radiation. Structures of photo stabilizers have to be adjusted to a particular polymer and environmental conditions of its application.
The UV absorbers, sometimes also referred to as UV light stabilizers, are a still growing class of compounds. These compounds act as interceptors to ultraviolet light absorbing, as it attacks and prevent it from starting the destabilizing reactions which can damage cells and destroy plastics. By adding UV absorbers to these products a significantly prolonged useful lifetime can be achieved.
The practical approaches to prevent or at least retard photo-oxidative degradation in susceptible polymeric systems are: a) Reduction of the absorption of UV light by the matrix of the organic substrate b) Reduction of the light induced energy absorbed by adding certain molecular groups in the polymer ("quenching") c) Antioxidant action
Reducing the absorption of UV light by the polymeric matrix can be achieved by the "blocking" of UV absorption using various UV opaque pigments or by employing UV absorbers which selectively absorb the harmful radiation and re-emit it in a less harmful wavelength, mainly as heat.
Commercially known UV-absorbers are o-hydroxy-benzophenones or o-hydroxyphenyl-benzotriazoles or benzoates, cinnamates, oxanilides or salicylates. Most hindered amine stabilizers are the well known derivatives of amino-tetra-alkyl-piperidines.
All these compounds are described e.g. in the "Plastics Additives Handbook", 5.sup.th edition, editor H. Zweifel, Hanser Publishers Munic, Germany (2001) and Hanser Gardener Publications, Cincinnati, USA (2000), chapter 184.108.40.206, pp. 11-13.
Often both UV-opaque pigments and UV absorbers are used jointly for maximum protection. However, because of esthetics and other considerations, many times UV-opaque pigments cannot be used.
Commercially, the main usage of UV absorbers is split between "benzophenones" (alpha hydroxy benzophenones), "benzotriazoles" (alpha hydroxy benzotriazoles) and hydroxyphenyltriazines. To a lesser extent compounds such as benzoates, cinnamates, oxanilides, and salicylates are used.
UV absorbers having inherent photo stability in the 290-400 nm wavelength regime are used for protection of polymers against their photo degradation. UV-absorbers with hydroxyphenyl groups act usually by intramolecular proton transfer mechanism taking place in the excited state.
Commodity and engineering polymers are not per se considered as photostable materials. Catalytic impurities such as residues of polymerization catalysts and traces of accidental metallic contaminants and sensitizers, also some pigments, are candidates to trigger photo degradation by atmospheric aging. Moreover, some transformation products of phenols may trigger photo oxidation. Stabilization of polymers exposed to actinic solar radiation is therefore mandatory.
In practice, various types of additives used are in combinations for UV protection. Combinations of additives have to be miscible in the polymeric matrix and have to show minimal migration out of the polymeric matrix. Also other side effects like nucleation have to be minimal in order to maintain the general physical properties of the organic substrate. Furthermore the additives have to be as efficient as possible in order to use as low concentrations as possible.
[Avar and Kroehnke, US Patent 8,277,689 (10/4/2012)]
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Roger D. Corneliussen
Maro Polymer Links
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Copyright 2012 by Roger D. Corneliussen.
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* Date of latest addition; date of first entry is10/4/2012.