Abstract
Exposure to ultraviolet (UV) radiation may cause the significant degradation of many materials. UV radiation causes photooxidative degradation which results in breaking of the polymer chains, produces free radical and reduces the molecular weight, causing deterioration of mechanical properties and leading to useless materials, after an unpredictable time. Polystyrene (PS), one of the most important material in the modern plastic industry, has been used all over the world, due to its excellent physical properties and low-cost. When polystyrene is subjected to UV irradiation in the presence of air, it undergoes a rapid yellowing and a gradual embrittlement. The mechanism of PS photolysis in the solid state (film) depends on the mobility of free radicals in the polymer matrix and their bimolecular recombination. Free hydrogen radicals diffuse very easily through the polymer matrix and combine in pairs or abstract hydrogen atoms from polymer molecule. Phenyl radical has limited mobility. They may abstract hydrogen from the near surrounding or combine with a polymer radical or with hydrogen radicals. Almost all synthetic polymers require stabilization against adverse environmental effects. It is necessary to find a means to reduce or prevent damage induced by environmental components such as heat, light or oxygen. The photostabilization of polymers may be achieved in many ways. The following stabilizing systems have been developed, which depend on the action of stabilizer: (1) light screeners, (2) UV absorbers, (3) excited-state quenchers, (4) peroxide decomposers, and (5) free radical scavengers; of these, it is generally believed that excited-state quenchers, peroxide decomposers, and free radical scavengers are the most effective. Research into degradation and ageing of polymers is extremely intensive and new materials are being synthesized with a pre-programmed lifetime. New stabilizers are becoming commercially available although their modes of action are sometimes not thoroughly elucidated. They target the many possible ways of polymer degradation: thermolysis, thermooxidation, photolysis, photooxidation, radiolysis etc. With the goal to increase lifetime of a particular polymeric material, two aspects of degradation are of particular importance: Storage conditions, and Addition of appropriate stabilizers. A profound knowledge of degradation mechanisms is needed to achieve the goal.Electronic supplementary materialThe online version of this article (doi:10.1186/2193-1801-2-398) contains supplementary material, which is available to authorized users.
Highlights
Exposure to ultraviolet (UV) radiation may cause significant degradation of many materials
The following stabilizing systems have been developed, which depend on the action of stabilizer: (1) light screeners, (2) UV absorbers, (3) excited-state quenchers, (4) peroxide decomposers, and (5) radical scavengers; of these, it is generally believed that excited-state quenchers, peroxide decomposers, and radical scavengers are the most effective Yousif et al (2013)
The use of polymeric materials has rapidly increased but it is well established that rapid photodegradation of these materials is possible when they are exposed to natural weathering (Guillet 1985; Hamid 2000; Rabek 1996; Bottino et al 2003)
Summary
Exposure to ultraviolet (UV) radiation may cause significant degradation of many materials. These steps are illustrated in the Figure 2: Photoinitiation step Internal and/or external chromophoric groups absorb light and produce low molecular weight radicals (R.) and /or polymeric macro radicals (P.) as follows: This reaction may be initiated by physical factors such as: UV radiation, heat, ionization, ultrasonic or mechanical effects or by chemical factors These ketone groups absorb photons of appropriate energy, break carbon–carbon bonds, and scission the polymer backbone (Rabek 1995; Feldman 2002) as shown in the Figures 3 and 4 (Grassie and Scott 1985): Propagation step This step can be divided into Rabek (1987) six different steps: A) Subsequent reaction, of low molecular radicals (R.) and polymer alkyl radical (P.) in a chain process similar to the abstraction of hydrogen from the polymer molecule:.
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