Hindered amine light stabilizer based

Journal of Applied Medical Polymers 5, No.2, Autumn 2001, p.77-9 NOVEL TECHNIQUE FOR THE DETECTION OF A HINDERED AMINE LIGHT STABILIZER, BASED ON SILICON TECHNOLOGY Narayan S Lee R E Hallberg D Malatesta V Great Lakes Chemical Corp. Great Lakes Chemical Italia... 

Grelier etal. (1997) synthesized a variety of UV stabilizers based upon hydroxy-phenylbenzotriazoles incorporating an isocyanate functionality. These were reacted with wood using microwave energy. The modified woods were exposed to UV irradiation and the photoinduced discolouration determined. It was found that the best protection was obtained when the UV stabilizer was used in conjunction with polyethylene glycol, or hindered amine light stabilizers. 

Siloxane-based dendritic hindered amine light stabilizers (HALS) such as 271 are synthesized through hydrosilylation of methacrylates containing hindered amines with polyhydrosiloxanes (equation 101)265. 

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). 

As alluded to above, a second class of light stabilizers, the hindered amine light stabilizers (HALSs), provide additional stability to polymers and can function even in thin film or in sample surfaces. The chemistry of HALSs is based on the 2,2,6,6-tetramethylpiperidine derivatives (Fig. 5). Unlike UVAs, HALSs have no significant UV absorption and... 

Many studies on the identification of polymer additives by Py-GC/MS have been published recently for example, an innovative method based on direct analysis of polymers containing polymeric hindered amine light stabilizers (HALS) by using pyrolysis coupled to GC/MS was applied successfully for fast and straightforward identification of these additives. Each of the HALS showed different pyrolysis gas chromatograms containing characteristic pyrolysis products. As a result, HALS additives with very similar chemical structures, e.g., Chimassorb 944 and Chimassorb 2020, could be distinguished. 

Figure 4.42 Great Lakes Chemical Corporation high-molecular-weight silicone-based hindered amine light stabilizer, Uvasil 299, CAS no. 182935-99-0.

Three different classes of compounds form the major and most important commercial categories of photostabilizers for PP. These are based on nickel complexes (those containing sulfur ligands function primarily as peroxide decomposers, PD), UV-absorbers (UVA), e.g. based on 2-hydroxybenzophenone and 2-hydroxyphenylbenztriazole, and sterically hindered amine light stabilizers (HALS). 

The latest class of photostabilizers developed commercially is based on hindered amines (known as HALS, hindered amine light stabilizers)... 

Of the various chemical classes of UV stabilizers that have been developed, the benzophenone and benzotriazole UV absorbers, the hindered amine light stabilizers (HALS), and their combinations continue to satisfy the UV stabilization requirements of most of the large-volume outdoor polymer applications. In addition, many new stabilizer chemistries, such as UV absorbers based on hydrox-yphenyl triazines, have been developed for more demanding applications or to satisfy specific market needs. Light stabilizers together with other additives (qv), such as antioxidants (qv), impact modifiers, colorants (qv), fillers, heat stabilizers (qv) and plasticizers (qv), have enhanced the properties and extended the service life of polymeric materials, resulting in the replacement of traditional materials in a multitude of outdoor weatherable applications. 

It is known that the lone pair electrons of amines are capable of capturing a carbon radical long enough to convert it to a nonradical species, as is the case with hindered amine light stabilizers (HALS) [70], It stands to reason that species with available lone pairs (Lewis bases) stabilize the TFE biradicaloid, allowing the end opposite to the catalyst to exist as a full radical and subsequently react. Barabanov s [23] observation of polymer formation is evidence for this terminal radical. Other perfluorinated species capable of forming biradicaloids would undergo similar radical stabilization. 

This harmless dissipation of the UV energy stabihzes the polymer. Another type of UV stabilizer is a class known as hindered amine light stabilizers (HALS). Many of them are based upon tetramethylpiperidines. 

A major development in the usefulness of polyolefins in outdoor service was the introduction of the hindered amine light stabilizers (HALS) (Gugumus, 1979). These compounds are based upon the sterically-hindered tetramethylpiperidine nucleus. 

Effective quenchers are based on nickel complexes (phenolate, dithiocarbamate, di-thiophosphate) and are used almost exclusively in polyolefins at a concentration of 0.2-0.5%. Nickel complexes impart a green coloration to plastic articles and their importance was significantly reduced with the introduction of hindered amine light stabilizers (HALS). 

Sterically hindered amine light stabilizers (HALS) constitute another class of light stabilizers that efficiently reduce UV-induced degradation. They are compounds that intercept chemically highly reactive radicals formed hy photochemical reactions and transform them into harmless reaction products. Fig. 3.8. Their effectiveness is not based on a competing physical absorption effect, therefore no minimum layer thickness is required for HALS effectiveness, unlike for the UV absorbers. That makes them suitable for applications such as films and fibers (527). In particular, they provide surface protection, which plays a special role in visible automotive parts. Another essential difference to UV absorbers is the fact that photooxidation is not prevented, but after initial damage its propagation is either reduced or suppressed [510],... 

Optimum photostability, for example in automotive clear topcoats, is obtained with synergistic combinations of UV absorbers and hindered amine light stabilizers (HALS) based on 2,2,6,6-tetramethylpiperidine. The UV absorber reduces the rate of generation of free radicals, while the light stabilizer reduces the rate of oxidative degradation by free radicals. 

In terms of resin chemistry, the clearcoats are based on a hydroxy-functional polyester and aciylic-resin blend. The polyester is responsible for providing the high flexibility at low temperature. The hardener is based on hexamethy-lene diisocyanate (HDI) trimer. Ultraviolet absorbers (UVA), and hindered amine light stabilizers (HALS) are additives added to absorb UV light, protect the basecoat pigments and to quench free radicals that could deteriorate and decompose the backbone resins. 

---