Hindered light stabilizers

NOR-HALS (Alkoxyamine Hindered Light Stabilizers) represent a new generation of sterically hindered amines. Thanks to their low basicity and high chemical resistance, they disrupt radically initiated processes faster than standard HALS stabilizers [527]. 

Horroeks, A.R., Liu, M.G. UV stabilizing synergies between carbon black and hindered light stabilizer in linear low density polyethylene. Macromol. Symp. 202, 199-219 (2003)... 

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

Hindered Amines. Hiadered amines are extremely effective ia protecting polyolefins and other polymeric materials against photodegradation. They usually are classified as light stabilizers rather than antioxidants. 

Most of the commercial hindered-amiae light stabilizers (HALS) are derivatives of 2,2,6,6-tetramethylpiperidine [768-66-1] (15). 

Oligomeric hindered amine light stabilizers are effective thermal antioxidants for polypropylene. Thus 0.1% of A[,Af-bis(2,2,6,6-tetramethyl-4-piperadinyl)-l,6-hexanediamine polymer, with 2,4,6-trichloro-l,3,5-triazine and 2,4,4-trimethyl-2-pentaneainine [70624-18-9] (35) (Fig. 5), protects polypropylene multifilaments against oxidation when exposed at 120°C in a forced-air oven (22) for 47 days. 3,5-Di-/ l -butyl-4-hydroxytoluene [128-37-0] (0.1%) affords protection for only 14 days. 

The exterior durabiHty of relatively stable coatings can be enhanced by use of additives. Ultraviolet absorbers reduce the absorption of uv by the resins and hence decrease the rate of photodegradation. Eurther improvements can be gained by also adding free-radical trap antioxidants (qv) such as hindered phenols and especially hindered amine light stabilizers (HALS). A discussion of various types of additives is available (113). 

Hindered Amine Light Stabilizer Standards Antioxidant Standards... 

Figure 13.21 shows the resolution of a dozen polymer additives at very high resolution using chloroform as the mobile phase. Tinuvin 622 will elute in pure chloroform whereas Chimassorb 944 and many other hindered amine light stabilizers (HALS) will not. With the addition of 1% triethyl amine to the chloroform, however, virtually all HALS will elute. 

The early recognition of the role of stable nitroxyl free radicals, e.g., 2,2,6,6-tetramethyl-4-oxopiperidine, and their hindered amine precursors, in polymer stabilization soon led to the development of the hindered amine light stabilizer (HALS) class of photoantioxidants. The first HALS, Tinuvin 770, AO-33, (commercialized in 1974) proved to offer much higher UV-stabil-ity to polymers than any conventional UV-stabilizer available at the time such as UV-absorbers, nickel compounds and benzoates. Table 3). 

UV absorbers have been found to be quite effective for stabilization of polymers and are very much in demand. They function by the absorption and harmless dissipation of the sunlight or UV-rich artificial radiation, which would have otherwise initiated degradation of a polymer material. Meyer and Geurhart reported, for the first time in 1945 [10], the use of UV absorber in a polymer. They found that the outdoor life of cellulose acetate film was greatly prolonged by adding phenyl salicylate (salol) [10]. After that, resorcinol monobenzoate, a much more effective absorber, was introduced in 1951 [11] for stabilization of PP, but salol continued to be the only important commercial stabilizer for several years. The 2,4-dihydroxybenzophenone was marketed in 1953, followed shortly by 2-hydroxy-4-methoxybenzophenone and other derivatives. Of the more commonly known UV absorbers, the 2-hydroxybenzophenones, 2-hy-droxy-phenyl-triazines, derivatives of phenol salicylates, its metal chelates, and hindered amine light stabilizers (HALS) are widely used in the polymer industry. 

A polymer-bound hindered amine light stabilizer [P-HALS] has been synthesized by terminating the living anionic polymerization of isoprene with 4(2,3-epoxy pro-poxy)-1,2,2,6,6-pentamethylpiperidine followed by hydrogenation of the resulting polymer to E-P copolymer using Zeigler type catalyst [40] ... 

An alternative mechanism by which additives may protect polymers from photo-oxidation is radical trapping. Additives which operate by this mechanism are strictly light stabilizers rather than antioxidants. The most common materials in this class are the hindered amines, which are the usual additives for the protection of poly (ethylene) and poly (propylene). The action of these stabilisers is outlined in Reactions 8.3-8.5. 

There are several claims in literature for the reaction of acetone and ammonia, at temperatures like 50°C, to give triacetoneamine (2,2,6,6-tetramethyl 4-piperidone), using Nafion and other CIERs (Eqn. (10)). This product is the starting material for the renowned hindered amine light stabilizers. 

Figure 3.22) [22]. Another typical example of a new, evolutionary high-MW hindered amine stabiliser (HMW HAS), with new levels of light stability and reduced interaction with pigments, is Chimassorb 2020 (Fig. 3.25e). 

Mechanisms for the action of hindered amine light stabilizers are reviewed briefly. On the basis of these considerations, two aspects of the mode of action of these substances were examined more closely. 

In the present study possible modes of action of a relatively new class of light stabilizers, the Hindered Amine Light Stabilizers (HALS, tetramethylpiperidine derivatives, TMP) will be discussed. Certain members of this group have a light stabilizing effect superior in many fields of application to that of previous additives. They are effective in both thick layers and fibers13 12. 

Three main possible modes of action of hindered amines as light stabilizers have recently come under discussion ... 

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