Hindered amines, determination

Oligomeric hindered amine light stabilisers, such as Tinuvin 622 and Chimassorb 944, resist satisfactory analysis by conventional HPLC and have required direct UV spectroscopic analysis of a polyolefin extract [596], PyGC of an extract [618,648], or SEC of an extract [649]. Freitag et al. [616] determined Tinuvin 622 in LDPE, HDPE and PP by saponification of the polymer dissolution in hot toluene via addition of an... 

SFC-FID is widely used for the analysis of (nonvolatile) textile finish components. An application of SFC in fuel product analysis is the determination of lubricating oil additives, which consist of complex mixtures of compounds such as zinc dialkylthiophosphates, organic sulfur compounds (e.g. nonylphenyl sulfides), hindered phenols (e.g. 2,6-di-f-butyl-4-methylphenol), hindered amines (e.g. dioctyldiphenylamines) and surfactants (sulfonic acid salts). Classical TLC, SEC and LC analysis are not satisfactory here because of the complexity of such mixtures of compounds, while their lability precludes GC determination. Both cSFC and pSFC enable analysis of most of these chemical classes [305]. Rather few examples have been reported of thermally unstable compounds analysed by SFC an example of thermally labile polymer additives are fire retardants [360]. pSFC has been used for the separation of a mixture of methylvinylsilicones and peroxides (thermally labile analytes) [361]. 

NMR spectroscopy is most effective in qualitative analysis when the samples examinated are substantially pure compounds and has been used to confirm the theoretically predicted low-energy conformations of the Af-acylated hindered amine light stabiliser Tinuvin 440 [210]. Trace amounts of PDMS (quantification limit 0.1 ppm) in plastic additives, dyes and pigments were determined by 111 NMR after Soxhlet extraction [211]. ll NMR was also used for the detection of octadecanol, an impurity in Irganox PS 802 (3,3 -dioctadecyl thiodipropionate). NMR has identified the nature of a supposedly UV stabiliser of empirical formula C17H18N3CIO [44] (Scheme 5.2). 

Three factors determine the effectiveness of antioxidants in polymers, namely (i) intrinsic molar activity (ii) substantivity in the polymer and (iii) solubility in the polymer. Multifunctional AOs combine multiple functions in one molecule. Sterically hindered amine stabilisers (HAS), such as Chimassorb 944, Tinuvin 622 and Tinuvin 783 are prime examples. 

Complex formation constants could also be determined directly from UV spectrophotometric measurements. Addition of tert.-butyl hydroperoxide to a solution of nitroxide I in heptane at RT causes a shift of the characteristic absorption band of NO at 460 nm to lower wavelengths (Fig. 9). This displacement allows calculation of a complex equilibrium constant of 5 1 1/Mol. Addition of amine II to the same solution causes reverse shift of theC NO" absorption band. From this one can estimate a complex formation constant for amine II and +00H of 12 5 1/Mol (23 2 1/Mol was obtained for tert.-butyl hydroperoxide and 2,2,6,6-tetramethylpipe-ridine in ref. 64b). Further confirmation for an interaction between hindered amines and hydroperoxides is supplied by NMR measurements. Figure 10a shows part of the +00H spectrum in toluene-dg (concentration 0.2 Mol/1) with the signal for the hydroperoxy proton at 6.7 ppm. Addition of as little as 0.002 Mol/1 of tetra-methylpiperidine to the same solution results in a displacement and marked broadening of the band (Fig. 10b). 

The anti-oxidant itself is often consumed in performing its function, although hindered amine light stabilisers (HALS) are self-regenerating. Depending on the polymer, their concentration in the polymer may vary from a few parts per million (e.g., in PE) to several per cent (e.g., in ABS, natural rubber). Because it is impossible to determine all the polymer down-stream end uses, nearly all have food contact approval. 

Finally, Persson et al. (1995) measured the UC/14C KIEs for the SN2 reactions between several amine nucleophiles and labelled methyl iodide in dimethoxyethane or acetonitrile at 15°C and 30°C, respectively, to determine how sterically hindered nucleophiles affects the transition state of a Menshut-kin reaction. The results in Table 25 show that all the fc11// 14-values for these reactions are large. In fact, they are all near the theoretical maximum value for these KIEs. Secondly, the KIE for the reaction with the more sterically hindered amine, 2,6-lutidine, is larger than that for the less sterically hindered... 

Grelier etal. (1997) synthesized a variety of UV stabilizers based upon hydroxy-phenylbenzotriazoles incorporating an isocyanate functionality. These were reacted with wood using microwave energy. The modified woods were exposed to UV irradiation and the photoinduced discolouration determined. It was found that the best protection was obtained when the UV stabilizer was used in conjunction with polyethylene glycol, or hindered amine light stabilizers. 

Secondary amines give the aminosilane, with the more hindered amines requiring heating under pressure. With primary amines, the aminosilane is formed initially, but the size of the substituents at nitrogen and silicon determines whether deamination subsequently occurs. Prolonged heating under reflux in the presence of ammonium sulfate, or the precipitated amine hydrochloride as acid catalyst yields the disilazane in good yield. 

The hindered-amine type stabilizer was ineffective, and the zinc acetate dihydrate moderately effective, in protecting the wool fabrics. Further study is needed to determine the mechanisms by which the latter compound imparts photostabilization. 

Hindered amine which formula is presented below is introduced into polymer. As a result of amine reaction with peroxide radicals formed during polymer oxidation process (Scheme 5) stable nitroxyl radicals are formed. Nitroxyl radicals may be formed only at those parts of polymers where there are peroxide radicals, i.e. process of thermo-oxidative (or photo-oxidative) destruction proceeds. Then one determines spatial distribution of nitroxyl radicals through the sample and so, it becomes possible to identify those regions of polymer in which oxidation reaction proceeds. 

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

Kimura, K., et al. Direct determination of a polymeric hindered amine light stabilizer in polyproplylene by thermal desorption-gas chromatography assisted by in-line chemical reaction, Analyst, 2000, 125, 465. 

Mixtures of diastereomers were formed, but the ratio of these was not determined. In generating R, the use of anilines or hindered amines should be avoided because subsequent aminoacylation may fail. 

The photo-oxidative degradation of polypropylene and stabilization by hindered amines has been reviewed. A study has appeared of the effect of P-carotene on the photoreactivity of anthracene in hexane solution and a kinetic scheme has been proposed to account for the photochemical and photophysical processes that occur on irradiation at 365 nm. Quenching rate constants have been determined between /S-carotene and singlet oxygen. Some characteristics have been communicated of the sensitized photo-oxidation of abietic acid contained in a vinyl butyl ether-butyl methacrylate-methacrylic acid copolymer. At 400 nm... 

Whereas the equilibrium constant K can be determined relatively readily for many primary amines, it is much more difficult for secondary or sterically hindered amines because of the slow reaction. This kinetic limitation can be overcome by associative substitution of Al(OBu03(Pr"NH2) with the hindered amine (Reaction R2.4), which proceeds much more rapidly ... 

This method was applied to the determination of organotin catalysts, e.g., dibutyl tin dilaurate, stablisers, e.g., Cyanox 1790, a hindered phenol antioxidant and Tinuvin 292, a hindered amine light stabiliser (HALS) and reactive diluents (such as aliphatic epoxides, epichlorohydrin, vernonial oil and so on) in acrylate resins. 

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