Light Stability of Polyolefins

After absorbing light quanta, the compounds can give off the energy of electronic excitation in the form of radiation of the same frequency (fluorescence) in this case no light-stabilizing action is observed. 

Excited light-absorber molecules can also transfer energy through collision. A shorter time is required for this than for fluorescence. This type of energy transfer leads to a sensitizing influence of light absorbers, and hence to an increase in the rate of destruction of the polymer. 

Certain antioxidants - secondary amines, in particular, phenyl-/3-naphthylamine [85] - absorb light in a region close to the absorption of carbonyl groups. However, they intensify photooxidation. This is explained by the fact that the energy of electronic excitation of the amine molecule is expended on a chemical reaction of the type  

Tests for a large number of different compoimds capable of absorbing ultraviolet have shown that a tendency for sensitization is also possessed by certain compounds that possess no labile hydrogen in the molecule. They include, for example, benzophenone, which is characterized by localization of the absorbed light energy in flie carbonyl group. This 

TABLE 11. Effective Light Stabilizers Based on Benzophenone 

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The 2-imino-4-thiazolines may be used as ultraviolet-light stabilizers of polyolefin compositions (1026). 2-Aminothiazole improves adhesive properties of wood to wood glue (271). Cbmpound 428 exhibits antioxidant properties (Scheme 242) (1027). Ammonium N-(2-thiazolyl)dithio-carbamate (429) is a bactericide and fungicide used in industrial products such as lumber, paint, plastics, and textiles (1037). Compound 430 is reported (1038) to form an excellent volume of foam coating in aluminum pans when ignited with propane. 

The earliest data on solution of stabilizer permanency problems by means of polymerization of functionalized monomers having double bonds C=C were published in the late 1960s. They deal with results concerning stabilization of rubber [50-53] (experience from the chemical modification of rubber were successfully exploited in this case) and with light stabilization of polyolefins [54]. 

Glaser A, et al. Update of latest developments in light stabilization of polyolefins. International Polyolefins, conference proceedings. Society of Plastics Engineers 2006. 

These esters have a good efficiency to the UV light stabilization of polyolefin. 

If, for example, a physical or chemical process in the singlet state is very fast, the excited triplet state of carbonyl groups could also be practically absent. As will be seen, this fact would considerably influence the mechanism of photod radation and consequently the conditions of light stabilization of polyolefins. 

Uvasorb HA77 DF is highly effective and recommended for the light stabilization of polyolefins, olefin copolymers, PP elastomer blends, styrenic polymers, polyacetals and polyamides. 

Sesamex [51-14-9] (Sesoxane) (30) is a synergist oflow toxicity, acute oral LD q (rat) = 2000 2270 mg/kg, for pyrethrins and allethrin. 6,7-Dihydroxy 4-methylcoumarin has been offered as an antioxidant for phenoHcs and polymers, and as an anthelmintic. 2,4,5-Trihydroxybutyrophenone has been available as an antioxidant and light stabilizer for polyolefins, waxes, and foods. Isoflavones, eg (31), have been patented as components of antioxidant compositions for foods and cosmetics (qv) (97). 

The hindered-amine light stabilizers for polymers are piperidine derivatives. An example of these products is bis-(2.2,6,6-tetramethyl-4-pipendinyl)sebaeale, tradenained Tinuviii 770, useful as a light stabilizer for polyolefins and styrenics. 

Here we report the use of a readily prepared polymer immobilised TEMPO as a catalyst for alcohol oxidations.15 It was derived from a commercially available oligomeric, sterically hindered amine, poly[[6-[(l,l,3,3-tetramethylbutyl)amino]-l,3,5-triazine-2,4-diyl] [2,2,6,6-teramethyl-4-piperidinyl)-imino]-1,6-hexane-diyl[(2,2,6,6-tetramethyl-4-piperidinylimino]], better known as Chimassorb 944 (MW 3000 see figure 3 for structure). This compound is used as an antioxidant and a light stabiliser for plastics. It contributes significantly to the long-term heat stability of polyolefins and has broad approval for use in polyolefin food packaging.16... 

A great attention was paid to grafting of light stabilizers onto polyolefins. 2-Hydroxy-4-methacryloyloxybenzophenone was grafted onto LDPE, HDPE, PP, poly(4-methylpent-l-ene) or ABS in extruders in the presence of peroxides or onto LDPE or PP in the melt without peroxide. Grafting on PS was very efficient. 

Sevini, E. Marcato, B. Chromatographic determination of some hindered amine light stabilizers in polyolefins. J. Chromatogr. 1983, 260, 507-512. 

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. 

This volume is including information about thermal and thermooxidative degradation of polyolefine nanocomposites, modeling of catalytic complexes in the oxidation reactions, modeling the kinetics of moisture adsorption by natural and synthetic polymers, new trends, achievements and developments on the effects of beam radiation, structural behaviour of composite materials, comparative evaluation of antioxidants properties, synthesis, properties and application of polymeric composites and nanocomposites, photodegradation and light stabilization of polymers, wear resistant composite polymeric materials, some macrokinetic phenomena, transport phenomena in polymer matrix, liquid crystals, flammability of polymeric materials and new flame retardants. 

It has been shown that the efficiency of esters of 4-hydroxy-3,5-di-f-butylbenzoic acid as light stabilizers for polyolefins is not related to the yield of the photo-Fries rearrangement [132]... 

F. Gugumus, Possibilities and limits of synergism with light stabilizers in polyolefins 1. HALS in polyolefins, Polym. Degrad. Stab. 2002, 75, 295-308. 

The piperidines and and the nltroxlde are recognized as being very efficient light stabilizers for polyolefins (204). Whatever the reasons nay be for the activity of these UV-transparent, sterlcally hindered piperidines, let us nevertheless now consider their ability to quench singlet oxygen. 

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]. 

Gugumus F. Possibilities and hmits of synergism with light stabilizers in polyolefins. 2. UV absorbers in polyolefins. Polym Degrad Stab 2002b 75 309-320. 

Antioxidants are key ingredients in the compounding of polyethylene (PE) and polypropylene (PP) due to the limited stability of polyolefins to high temperatures and ultraviolet (UV) light [8]. 

RE Lee, 01 Kuvshinnikova, JM Zenner. Interactions of Pigments and Hindered Amine Light Stabilizers in Polyolefin Systems. Hilton Head, SC, March 2-4, 1998. 

Gugumus, F. Possibilities and limits of synergism with light stabilizers m polyolefins 2. UV absorbers m polyolefins. Polymer Degradation and Stability, 75 (2002), p. 309 - 320... 

Commercial light stabilizers for polyolefins of the 2-hydroxybenzophenone class ... 

Gugumus F, Lelli N. Light stabilization of metallocene polyolefins. Polym Degrad Stab 2001 72 407-21. 

On the other hand, dithiocarbamate nickel and zinc complexes are very effective light stabilizers for polyolefins. Using a combination of iron(pro-oxidant) and nickel or zinc (stabilizers) dithiocarbamates, it is possible to control the rate of photo-oxidative degradation very accurately [1931]. For long induction periods a combination of iron and nickel dithiocarbamates is recommended, whereas for shorter times it is more usual to combine iron and zinc dithiocarbamates [1932]. The combination of different metal ions decreases the induction period and increases the rate of photo-oxidation. At higher concentrations of stabilizing (Ni, Zn) dithiocarbamates, much longer lifetimes can be achieved. 

Eversorb 73 is a benzotriazole type absorber. It offers superior light protection of polyolefin substrates against the deterioration of UV radiation. Eversorb 73 is also efficient for the light stabilization of polyvinyl chloride, unsaturated polyester, polyurethane, styrenic polymers, coatings and lacquers. 

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