Hindered Amine Light Stabilisers HALS

Recently Kimura and co-workers [77] have developed a novel method to determine an oligomeric HALS in polypropylene materials using reactive thermal desorption-GC (RTD-GC) in the presence of an organic alkali, tetramethylammonium hydroxide [(CH3)4NOH], This technique allowed the rapid and highly sensitive determination of the oligomeric HALS in polypropylene. 

Krishen [78] pointed out that one of the major drawbacks of previously published gas chromatographic techniques for the identification and/or determination of ester plasticisers of the type used in PVC is that, commonly, isothermal column conditions had previously been used, since relative retention data were available for these conditions. Only limited use had been made of temperature programming by earlier workers. 

One of the major drawbacks of isothermal column operation is the necessity for using either two different columns or two different temperatures for the identification of both the original plasticiser and the products obtained from it by hydrolysis and esterification. The procedure, described next, overcomes this drawback. 

Krishen [78] used a dual flame ionisation chromatograph equipped with a 1 mV recorder. Two stainless steel columns each 1.8 m x 13 mm od packed with 10% UCW-98 on 60-80 mesh Diatoport S were employed in the dual operation mode. The initial column oven temperature was 100 °C and after 4 minutes of isothermal operation, the temperature was programmed at a rate of 8 C per minute to a maximum of 330 C. The final temperature was held constant for 8 minutes. The injection block and the detector were maintained at 270 °C. The helium, hydrogen, and air pressures were 0.4 (Flowrator 0.8), 0.09, and 0.2 MPa, respectively. 

Samples of the plasticisers were dissolved in tetrahydrofuran and then injected into the gas chromatograph. When the plasticisers were present in PVC, the polymer sample was dissolved in tetrahydrofuran, insoluble components were allowed to settle out, and a sample of the clear solution was injected into the gas chromatograph. A 1 % solution of the polymer was suitable for this purpose when the plasticiser content of the polymer was between 10 and 40%. 

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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. 

Hsiao et al. [11] have studied the use of MALDI ionisation for the detection of antioxidants and hindered amine light stabilisers (HALS) in polyethylene extracts. Using 2,5-dihydroxybenzoic acid as matrix, diagnostic spectra were obtained on standards, but the presence of soluble low molecular weight polyethylene in extract solutions caused some problems with ionisation suppression. 

A worthwhile survey of this field was given by De Jonge and Hope (1980). d. Hindered Amine Light Stabilisers (HALS)... 

Light stabilisers also prevent photodegradation. UV absorbers such as hydroxybenzophenone or hydroxyphenyl triazole types, operate by absorbing and dissipating UV radiation prior to potential degradation of the polymer. No permanent chemical change occurs, so activity is retained. Hindered amine light stabilisers (HALS) are also used but their activity is not particularly well understood (80). 

Two groups of anti-oxidants are of practical importance hindered phenol light stabilisers (HPLS) and hindered amine light stabilisers (HALS). Examples are shown in Fig. 13.7 and the mechanism of the radical trapping reaction is shown in Fig. 13.8. They are often combined with UV absorbents (for partly synergistic effects) and mostly used for the fibre protection. 

When UV is absorbed by a polymer, free radicals may be formed. These can be scavenged by hindered amine light stabilisers (HALS) before they cause chain scission or oxidation. An example is... 

Hindered amine light stabilisers (HALS) are the main stabiliser type (as a scavenger to inhibit free radical chain propagation), while organo-nickel compoimds (as a quencher to prevent initiation of polymer degradation) are used for UV stabilisation. 

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]. 

Polymer is degraded by heat, energy, UV or residues of catalyst and generates alkyl radicals. This alkyl radical reacts with oxygen and form peroxy radicals. These peroxy radicals abstract hydrogen from other polymer and forms alkyl radicals and hydroperoxide. The decomposition of hydroperoxide to alkoxy and hydroxyl radicals induces additional decomposition of the polymer chain. In order to stop the radical chain reaction of degradation, stabilisers such as phenolic antioxidant, phosphites, thioether and hindered amine light stabilisers (HALS) are added. 

The early examples were variants or derivatives of piperidine, and have long been used as antioxidants, but it was foimd that some of them were also effective UV stabilisers. Today there are 35 or 40 different hindered amine light stabilisers (HALS) including monomeric, oligomeric and polymeric variants. They are used alone in protecting polyolefins, being particularly effective in polypropylene and HOPE and, although LDPE has provided some difficulties, appropriate HALS stabilisers can still be used. EVA copolymers can use HALS, but styrene polymers often need support from other types of stabiliser such as a benzotriazole, and flexible PVC also benefits from auxiliary stabilisation. 

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. 

UV absorbers are used in elear unpigmented top eoats to enhance the durability of the coating and provide protection to the base coat in clear over base systems. The UV absorber system is made up of a UV absorber, normally of the benzotiiazole or oxalanalide type and a hindered amine light stabiliser (HALS). The most commonly used commercial UV absorbers are the Tinuvin range of products from Ciba Geigy, the Sanduvor range from Sandoz, and Cyasorb from American Cyanamid. 

UV absorbers are an essential component of clear lacquers for external applications. Recent studies suggest their use should be extended to pigmented top coats to enhance exterior durability. The UV absorber system normally consists of a UV absorber, such as the benzotriazoles or oxalanilide types, and a radical scavenger, such as hindered amine light stabilisers (HALS). The HALS is required to quench free radicals which are byproducts of the mechanism of absorption by the UV absorber. 

The photografting kinetics of a hindered amine light stabiliser (HALS) containing a polymerisable double bond in low density polyethylene (LDPE) films was studied under natural weathering conditions. The HALS (PBH-3, chemical structure shown) was added to the polymer at a concentration of 0.3 wt.%. The photografting kinetics... 

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