HALS Free-radical scavengers

HALS FREE-RADICAL SCAVENGERS 4.4.1 Overview of HALS... 

Light Stabilizers Pigments (carbon black, iron oxides), UV absorbers (hydroxyphenones, benzotriazoles), excited-state quenchers (organic Ni complexes), free-radical scavengers Hindered amine light stabilizers [piperidines, hindered amine light stabilizers (HALS)]... 

Primary phenohcs (free-radical scavengers) or secondary phosphite (hydroperoxide decomposer) may be used as heat stabilizers for WPC, while light stabilizers commonly used in WPC include UV absorbers (e.g., benzotriazole or benzophenone), radical scavengers or hydroperoxide decomposers, and hindered amine light stabilizers (HALS). 

The mechanism is not fully understood. Hydroperoxide decomposition and free radical scavenging certainly play a part, as also does the regeneration of HALSs, where UV absorbers are frequently consumed as a result of their operation. There are several theories for how this works - possibly by energy transfer, free radical termination, or peroxide decomposition. Significant stabilization is achieved at relatively low concentrations and it appears that the HALS is actually regenerated by the stabilization process, rather than consumed by it. Theory suggests that the hindered amine oxidizes to form amine-ether, which is a non-radical species. 

New patented polyfunctional additives from Ciba are claimed to act as both primary and secondary anti-oxidants. Based on hydroxylamines, functioning as free radical scavengers, peroxide decomposers, and reducing agents, they have low colour and act synergistically with HALS UV stabilizers. Coded FS, they are designed to replace blends of phenolic, a phosphite, or thioester and a HALS, and are aimed initially at PP fibres, but early trials show similar benefits in PP films and thick-section parts. 

Based on the accumulated experimental evidence we can summarize by saying that HALS protect polymers by a combined mechanism where the principal activities are free radical scavenging, hydroperoxide deactivation, and formation of charge transfer complexes with oxygen. The stabilization action of HALS can be adversely affected by some other substances, like organic thioether costabilizers, halogen flame retardants, or an acidic environment. Several N-substituted HALS derivatives have... 

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. 

HALS HALS is kind of free radical scavengers and offers protection from two mechanisms. In the presence of oxygen, HALS mms into a nitroxyl radical... 

HALS was based on the discovery that the 2,2,6,6-tetramethyl-l-piperidinyloxy, free radical (TEMPO) (1)), which already was known as an effective radical scavenger [46,47], was a very effective UV stabilizer too [48,49]. However, due to its physical and chemical properties TEMPO itself did not led to practical use. TEMPO is colored and will impart color to the to be stabilized polymer, it is thermally unstable and volatile [49]. Furthermore, it reacts with phenolic antioxidants present in many polymers leading to a reduction of processing and/or long-term heat stability. The discovery that compounds in which the /V-oxyl functionality was replaced by a N—H functionality also showed good UV stabilization activity was the key finding that led to the development of HALS stabilizers [49]. 

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). 

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... 

Referring back to Fig. 1, the HALS functions not by blocking the offending LTV radiation but by destroying the unstable and radical-initiating hydroperoxides and by scavenging free radicals before they can be involved in the propagation reactions. Thus,... 

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. 

As with radical-scavenging AOs, HALS (or sometimes more concisely called hindered amine stabilizers, HAS) deactivate free radicals created by reactions with UV light. HALS has been a successful additive technology by any measure. 

The main difference in the various HALS is the substituent on the hindered nitrogen of the piperidine ring. Hindered amine light stabilizers reduce the photooxidation rate by scavenging free radicals and thus interfering with the oxidation cycle of R2-R3. The basic reactions that are important are summarized in reactions R6-R10.15... 

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