Hindered amine light stabiliser

The aminoxyls produced have similar light-stabilising activity to the TMPOs by the catalytic mechanisms shown in reactions (3.8) and (3.9) (A- = NO-). 

For further information on antioxidants and stabilisers, the reader is referred to Refs 1-4 and 11-14 at the end of this chapter. 

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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... 

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). 

Diketopyrrolopyrrole HALS Hindered amine light stabiliser... 

Figure III.7 Product sheet of the low-MW hindered amine light stabiliser di-(2,2,6,6-tetramethyl-4-piperidinyl) sebacate. After Gijsman [1], Reproduced by permission of P. Gijsman...

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. 

A hindered amine light stabiliser has been found to enhance the light stability of blends of low and linear low density polyethylene with the latter contributing linearly to the overall stability of the blend. In coatings hindered piperidine light stabilisers are also effective especially when used in conjunction with a benzotriazole absorber while surface protection of styrene copolymers with 2-(2-hydroxy-5-vinylphenyl)benzotriazole requires a small amount of a hindered piperidine stabiliser. Polymeric hindered piperidine compounds on the other hand have been found to inhibit the singlet oxygen attack on poly(butadiene). ... 

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. 

Allied Signal Incorporated [87] have patented cinnamamides of the general structure Ph[NH-(C=0)-C(CN)=CPh2]2, and have observed that these are especially effective stabilisers for aromatic polyesters. Clariant have noted that a combination of benzylidene bis-malonates with hindered amine light stabilisers showed excellent synergism in UV-stabilising polyalkylene terephthalates [88]. 

UV1164 has been shown to be particularly effective at protecting PEN [126]. It has also been observed that triazine-type UV absorbers form a particularly good synergistic combination with hindered amine light stabilisers in photostabilisation of a range of polymers [122]. 

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. 

Ciba Specialty Chemicals has been adapting an existing light stabiliser technology to offer a flame retardant functionality. One of the company s types of hindered amine light stabilisers shows some capability as a non-halogenated flame retardant, one example is Flamestab NOR 116. 

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