Light sensitive polyolefins

In this paper we report on the identification of the luminescent impurity carbonyl species in two of the most light-sensitive polyolefins, polypropylene and poly(4-methylpent-l-ene) and examine the behavior of these groups during irradiation under sunlight-simulated conditions. 

Our results in Figure 6 show that during photooxidation of the polymers the ,/ -unsaturated carbonyl groups are converted into saturated ketonic/aldehydic groups that can themselves be converted subsequently to nonluminescent products by Norrish Type I and II processes, e.g., carboxylic acids (2). For the two light-sensitive polyolefins considered here, Reaction 1 is likely to be the more important, because the concentration of the species is very low. 

Luminescence and Photooxidation of Light-Sensitive Commercial Polyolefins... 

TABLE 14 Thermal stability and light sensitivity ranges of radicals formed in some polyolefins [435]... 

Stanlight P can also be used in conjunction with Stanlight 770 to provide optimum protection to polyolefins in thick section, especially when pigmented with light-sensitive colorants. In addition, most coatings systems benefit from a combination of UV absorber and HALS. 

In addition, there are many surface modification processes that use triplet sensitizers to permit oxidation reactions. In a typical process, polyisocyanate is applied on a polyolefin together with a sensitizer such as benzo-phenone and then irradiated with UV light. As shown in Eq. (15) the sensitizer has an oxidizing effect to produce hydroxyl groups over the polymer surface. These hydroxyl groups finally react with isocyanate to provide a functional polymer [56,57]. 

Abstract The oxidation of polymers such as polypropylene and polyethylene is accompanied by weak chemiluminescence. The development of sensitive photon counting systems has made it comparatively easy to measure faint light emissions and polymer chemiluminescence has become an important method to follow the initial stages in the oxidative degradation of polymers. Alternatively, chemiluminescence is used to determine the amount of hydroperoxides accumulated in a pre-oxidised polymer. Chemiluminescence has also been applied to study how irradiation or mechanical stress affects the rate of polymer oxidation. In recent years, imaging chemiluminescence has been established as a most valuable technique offering both spatial and temporal resolution of oxidation in polymers. This technique has disclosed that oxidation in polyolefins is non-uniformly distributed and proceeds by spreading. 

Polyolefins - An Arrhenius model has been developed for lifetime prediction of the light stability of polypropylene . Photooxidation processes in blends of polypropylene with poly(butylene terephthalate) (PBT) are complicated by the thermal sensitivity of the polypropylene and the screening effect of the terephthalate ester . This effect is shown in Scheme 1. 

As all polyolefins, polyethylene is sensitive to UV radiation, although less than polypropylene. For outdoor use polyethylene needs special stabilization against UV light. The light stabilizers for polyethylene are in principle the same as for polypropylene. On accelerated weathering, HALS show much better performance in HDPE tapes than UV absorbers, despite the latter being used in much higher concentrations. The comparison between HALS is, however, in favor of the polymeric HALS-III, which has the same performance when added at a concentration of 0.05% as HALS-I and HALS-II at 0.1%. 

Hydroperoxides formed during processing of polymers have a profoundly deleterious effect on the long-term performance of products made from them. This is a consequence of the very great sensitivity of hydroperoxides to light which leads to the rapid deterioration of the polyolefins, PVC and rubber-modified polymers in the outdoor environment. Their formation is of crucial importance to the weathering of industrial polymers since these highly reactive free radicals produced are initiators for the photooxidation of polymers. 

Polyethylene is sensitive to UV radiations, less than polypropylene. Polyolefin-based pipes need stabilization to combat UV light and heat. 

Polyolefins In addition to stabilization of polyolefins against thermal oxidation to reduce the sensitivity to light, stabilization against exposure to light is required for articles to be used outdoors as well as those intended for indoor use [93]. Light stabilizers include UV absorbers of the benzotriazole and the benzophenone types (except for thin sections), HALS, and nickel-containing stabilizers. The latter are used for thin sections such as tapes and films and for surface protection. The type of stabilizer is dictated by the type of polyolefin, its thickness, application, and desired lifetime of the article [20]. 

Pressure- sensitive Tacky adhesives used in a variety of commercial applications. Often used with polyolefins. Flexible. Bonds not very strong. Generally applied by spray with bonding effected by light pressure. 

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