HALSs

Of available light stabilisers potentially operating via an antioxidant mechanism, the most interesting are the hindered amine light stabilisers (HALS). Initially developed in Japan, these are largely based on active sites comprising 2,2,6,6-tetraalkylpiperidine [147] and 1,2,2,6,6-pentaalkylpiperidine [148]. 

Many studies have been made to identify the mechanism(s) by which HALS provide such effective UV stabilisation, and these have been reviewed [149-151]. Most authors agree that the NH moiety is not, in fact, the stabilising species, but that NO stable radicals and NOR hydroxylamine ethers formed during photo-oxidation are the true antioxidant species. These are constantly regenerated the nitroxyl scavenging alkyl radicals and the hydroxylamine ethers intercepting hydroperoxy radicals and hydroperoxides. Hydroxylamines may also be formed, which are efficient photostabilisers in their own right. 

No systematic study of the use of HALS as UV stabilisers in aromatic polyesters has been found in the literature very few articles even touch on the subject, and these are limited to PBT [157-159]. Without comparisons with other stabiliser types, it is difficult to assess the true capability of the HALS tested by these authors in PBT. It was noted by Borukaev and co-workers [159] that poly[[6[(l,l,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl]-[(2,2,6,6-tetramethyl-4-piperidyl)imino]-l,6-hexamethylene[(2,2,6,6-tetramethyl-4-piperidyl)imino]] (Chimmasorb 944 Ciba) was a better UV stabiliser than the dimethylsuccinate polymer with 4-hydroxy-2,2,6,6-tetramethyl-l-piperidineethanol (Tinuvin 622 Ciba). 

A potential problem with HALS in aromatic polyesters is the very high basicity of these amines, which may result in reaction with the ester linkages and loss of molecular weight. There are indications that this may be a factor with the use of some HALS in fibre-grade polyesters [160]. 

The patent literature on HALS is voluminous, and some patents include a reference to the potential use of their claimed species in polyesters, although it is clear from reading the embodiment that other polymers are the true targets. Many fewer patents deal with HALS which are specifically claimed as being effective stabilisers for polyesters. 

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Carbocations usually generated from an alkyl halide and aluminum chloride attack the aromatic ring to yield alkylbenzenes The arene must be at least as reactive as a halobenzene Carbocation rearrangements can occur especially with primary alkyl hal ides... 

Preparation of alkanes using lithium di alkylcuprates (Section 14 11) Two alkyl groups may be coupled together to form an alkane by the reaction of an alkyl hal ide with a lithium dialkylcuprate Both alkyl groups must be primary (or meth yl) Aryl and vinyl halides may be used in place of alkyl halides... 

Alkylation of phthalimide The Gabriel synthesis (Section 22 8) The potassium salt of phthalimide reacts with alkyl hal ides to give N alkylphthalimide deriva fives Hydrolysis or hydrazinolysis of this derivative yields a primary alkylamine... 

ALKYLPHENOLS] (Vol 2) Hindered-amine light stabilizers (HALS)... 

Table 3. Stabilization of Polypropylene Fiber by Polymeric HALS ...

Test specimens were 0.5 tex (4.5 den) filaments containing 0.25% specified HALS (22). 

The most important appHcation of metal alkoxides in reactions of the Friedel-Crafts type is that of aluminum phenoxide as a catalyst in phenol alkylation (205). Phenol is sufficientiy acidic to react with aluminum with the formation of (CgH O)2Al. Aluminum phenoxide, when dissolved in phenol, greatiy increases the acidic strength. It is beheved that, similar to alkoxoacids (206) an aluminum phenoxoacid is formed, which is a strong conjugate acid of the type HAl(OCgH )4. This acid is then the catalyticaHy active species (see Alkoxides, metal). 

E. Hals and co-workers, in Prof, of 20th Sjmposium on Engineering Aspects of MHD, Irving, Calif.,June 1982, pp. 1.1.1—1.1.9. 

F. Hals, "Conceptual Design Study of Potential Eady Commercial MHD Power Plant," NH374 CK-165235, NASA Lewis Research Center, Cleveland, Ohio, Mar. 1981, pp. 3—60. 

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