Hindered amine-type light stabilizers

The latest development in the field of light stabilizers for plastics is represented by sterically hindered amine-type light stabilizers (HALS). A typical such compound is bis-(2,2,6,6-tetramethyl-4-piperidyl)-sebacate (XXV). Since it does not absorb any light above 250 nm, it cannot be considered a UV absorber or a quencher of excited states. This has been confirmed in polypropylene through luminescence measurements. 

Free radical scavengers act by free radical scavenging reactions. By this mechanism is explained the stabilization due to the addition of -butylamine-nickel-2.2 -thiobis(4-ier -phenolate), nickel-bis-3.5-di-ieri-butyl-4-hydrophosphonic acid mono butyl ester, 2-hydroxy-4-dodecyl oxybenzophenone. The hindered amine type light stabilizers (HALS) have the highest effectiveness. 

Most polypropylene uses a radical trapping stabilizer of the hindered amine type. These hindered amine light stabilizers are often referred to as HALS. Figure... 

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). 

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] ... 

The stabilizers chosen for evaluation include different types of heat and light stabilizers selected to represent different mechanisms of action as well as chemical compositions (ArJi). Types of stabilizers evaluated include benzotriazole and benzophenone light stabilizers [ultraviolet (UV) light absorbers], hindered amine light stabilizers (HALS, catalytic radical scavengers), hindered phenol heat stabilizers (antioxidant radical scavengers), and thioester heat stabilizers (antioxidant hydroperoxide decomposers). 

Set B Based on the results of set A discussed below, a third set. Set B, was prepared (Table IV). Cyasorb 531 (a benzophenone UV light absorber) was found to be the most effective stabilizer in Set A. It was therefore rerun at an even lower add-on in Set B, and since it was the only UV light absorber included in Set A, a second type of UV absorber, Tinuvin 327 (a benzotriazole) was included in Set B. Two new non-polymeric hindered amine light stabilizers (HALS) Tinuvin 770 and Tinuvin 765 were also evaluated in Set B with the hope that they might be more effective than the polymeric HALS (Chimassorb 944LD) included in Set A which was quite detrimental to the heat stability. Combinations of these four additives were also applied in Set B. The three new compounds evaluated have been reported to be effective light stabilizers for polyester and polyamide fibers (Capocci, G., Ciba Geigy Corporation, Ardsley, NY, personal communication, 1986). 

The most common UV absorbers used commercially are hindered amine light stabilizers, which can also function as heat stabilizers in some plastics. Other UV absorbers include benzotriazoles and benzophenones, salicylate esters, cyanoacrylates, malonates and benzilidenes. Factors such as the nature of the resin, type and level of pigments being used, and the presence of crosslinkers and catalysts govern the selection of UV absorber to be used. 

Most hindered amine light stabilizers have evolved from the discovery (4) that compounds containing a 2,2,6,6-tetramethylpiperidine moiety can stabilize polymers against photodegradation and this moiety has been incorporated into HALS of various types (Figure 1). Much has been published on the mechanism of action of HALS and the literature in this area has recently been critically reviewed (5,6). While the complete mechanism of action has not been fully elucidated, the high performance of HALS is generally attributed to the ability of their oxidation products to... 

To make polymers useful for the above-mentioned applications, stabilizers of different types can be added to the polymers. As a result the lifetimes in the outdoor environments becomes long enough to make that polymers can be applied. In fact, without stabilizers the use of polymers in an outdoor environment would be very limited. Especially the discovery of hindered amine light stabilizer (HALS) led to a boost of the applicability of polymers in outdoor environments. 

The most important classes of UV stabilizers are UV absorbers (U VAs), quenchers and hindered amine light stabilizers. In this chapter an overview is given about the types of UV stabilizers in use, their mechanisms of action as well as possible synergistic and antagonistic effects. In the last part for a selected number of polymers the effectiveness of different stabilizers is shown. The chemicals structures of all stabilizers mentioned in this chapter are shown in Appendix 17.9. 

Chemical re-stabilization of recycled material against the thermal and light-induced degradation is essential. Addition of 0.1-0.5 wt% of a sterically hindered phenol and a phosphite at a ratio varying from 10 1 to 1 10 is recommended [Pauquet et al., 1994]. For outdoor applications, hindered amine light stabilizers with UV-absorbers of the benzotriazole type are to be used [Herbst et al., 1995,1998]. Examples of blends used for polymer recycling are listed in Table 1.81. For more details see Chapter 16 in this book. 

Ultraviolet-light stabilizers are of two types those that preferentially absorb UV, thus shielding the polymer structure from UV light and those that protect by quickly scavenging the free radicals as they form. The latter belong to a group of compounds known as hindered amine light stabilizers (HALS). 

Metal acetylacetonates and thio-organic complexes show a strong interaction during processing as they yield dark colored samples. These materials are not acceptable as UV stabilizers. Hindered amine light stabilizers (HALS)s are commonly used polymer photostabilizers. HALSs are ineffective in imparting photostability to the polymer. Successful photostabilization of PPS can be achieved by using UV absorbers rather than quencher-type additives. 

Obviously, in a PA melt, phenols undergo reactions and the reaction products are unable to react with the amide peroxy radical. On the other hand, antioxidants of the hindered amine light stabilizer type interrupt the oxidation reaction of PA6 after an induction period. During this induction period, they are oxidized to the N-oxyl form, which is essentially reactive... 

Hindered amine light stabilizers (HALS). Hindered amine light stabilizers are the newest type of UV light stabilizer. They were introduced in 1975 by Ciba and Sankyo. HALS do not screen ultraviolet light, but stabilize the resin via free-radical termination. HALS are used at lower levels than benzophenones and benzotriazoles, and are widely used in polyolefins for their cost-effectiveness and performance. The successful growth of HALS has been directly related to their substitution for benzophenones and benzotriazoles in many applications as well as their blending with benzophenones. 

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