HALS-type

Monomeric HALS have been determined by HPLC [470,663]. Excellent separation was achieved for HALS-type samples (Tinuvin 770 and Chimassorb 944) with NPLC-PDA (230 nm) using an amino column with acetonitrile/water as the mobile phase RPLC using C or cyano columns was not effective [664]. 

The origin of mass-dependent isotopic fractionation in FUN CAIs is commonly (and somewhat casually) assumed to be the result of Rayleigh-type distillation, while the inclusions were molten. It is true that a strong case for distillation has been made in the case of the so-called HAL-type hibonites (see Section 1.08.7), based on trace element and isotopic properties (Lee et al, 1979, 1980 Davis et al, 1982 Ireland et al, 1992 Floss et al, 1996). Such an origin is problematic for other FUN CAIs, however, especially those that are otherwise identical in bulk composition to non-FUN CAIs. Most notably this is true of the FoBs that also happen to have F or FUN properties (Clayton et al, 1984 Davis et al, 1991). These objects are magnesium-rich relative to other CAIs, yet distillation experiments conducted on chondritic starting materials consistently show that... 

For forsterite-bearing FUN CAIs and other FUN CAIs that are not HAL-type hibonites, we still do not understand whether their isotopic fractionation is the result of melt evaporation or... 

Floss C., El Goresy A., Zinner E., Kransel G., Ratrrmensee W., and Palme H. (1996) Elemental and isotopic fractionations produced through evaporation of the Allende CV chondrite implications for the origin of HAL-type hibonite inclusions. Geochim. Cosmochim. Acta 60, 1975-1997. 

Figure 12. Light stability of a two-coat high-solid paint (silver metallic) with a benzotriazole and a HALS type stabilizer. Exposure Florida, 5 south, black box, unheated.

Nowadays, there are many HALS stabilizers commercial of which the majority is based on 2,2,6,6-tetra-methyl-4-piperidinyl moieties. The first commercial type (LMW-HALS-1) is relatively low in molecular weight, which caused that due to its high volatility it is not suitable for thin applications. To overcome this problem oligomeric HALS types were developed. One of the drawbacks of the piperidinyl moiety of HALS stabilizers is that it is basic causing that it can react with acids and forms a not stabilizing salt. Consequently, the effectiveness of HALS in systems where acids are present or can be formed is limited. To beat this problem less basic HALS types as, for example, N-O-R types were developed. 

Although, for the photolytically degrading halogenated flame-retardants combinations of absorbers and HALS might give sufficient stability [108], in combination with a flame-retardant several other UV stabilizer types can outperform HALS. The development of less basic NOR HALS types can change this situation. These types are less deactivated, causing that in combination with flame-retardants they can outperform all other stabilizers [103,108]. 

Comparison Different UV Stabilizers To increase the stability of PP different UVAs can be applied. From Table 17.8 it is clear that the HALS types are most effective and that the benzoates are more effective than the UVAs. Above it already was shown that especially in thicker applications LMW-HALS is more effective than HMW-HALS (Fig. 17.3). 

The sterically inhibited amines (HALS) were the most important new development in the field of photoprotectants of the past 10 years. The low molecular types have now been supplemented by highly effective polymeric types. They are particularly effective with very oxidation-sensitive PPs and other polyolefins - even in products with minimum wall thicknesses (foils, fibers). The polymeric HALS types have proven highly resistant to extraction and migration. 

Two different types of photostabilizers are typically added to antomotive clearcoats, ultraviolet light absorbers (UVA) and HALS. Clearcoat UVA acts to reduce the intensity of light inside the paint system. That is, it reduces clearcoat PIR and screens light sensitive coating layers underlying the clearcoat (e.g., basecoat) from UV light. As a result, clearcoat UVA also acts to extend HALS longevity. Clearcoat UVA performance is not discussed further in this chapter. Typical HALS types that are used in clearcoats are shown below. " ... 

The key issue for HALS stabilization in coatings is choosing the correct HALS type and concentration to achieve effectiveness and longevity. In CToss-Iinked coatings. 

Because of its excellent weathering resistance, PMMA is used as weathering protection in the form of co-extruded layers or laminates for PVC and ABS products. HALS-type UV-absorbers and light stabilizers in concentrations up to 2% in thin PMMA layers offer not only effective surface protection, but also light protection, e.g., for PVC-window profiles or phenol resin panels in exterior applications. 

Stabilizers such as sterically hindered phenols and amines, in particular piperidine derivates, increase thermal stability. Hydroxyphenyl propionates and HALS-type stabilizers have proven to be highly effective. 

IPP stabilized with some 2, 4 -dihydroxy phenyl ketones [173, 174] oxidizes at smaller rates under UV irradiation, compared with the unstabilized IPP. Changes in structure and properties of LDPE films containing spirooxazine as stabilizers [175] showed that this one acts as an UV absorber and shows a synergetic effect with HALS-type stabilizer. 

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