Organic UV absorbers

The UV absorber must be capable of absorbing light at or near to the point in the spectrum at which the polymer in question absorbs harmful radiation most easily, i.e., in the range 260-400 nm (a tendency to imdergo absorption in the visible region will colour the polymer). 

The most important UVAs are derivatives of 2-hydroxybenzophenone and 2-hydroxybenzotriazole. Hydroxyphenyltriazines are also used. Less common UVAs include the oxanilides and cyanoacrylates. 

Benzophenones are moderately priced, but colom stability is not ideal and yellowing can occur. They are therefore rarely used alone, and are combined with hindered amines or with 

Benzotriazoles and benzophenones are popular UV absorbers in PVC pond liners, window trim and greenhouse fdm, although their effectiveness diminishes with time, limiting the period of protection. 

Benzotriazole derivatives such as 2 (2H-benzotriazole-2-yl) 4-methylphenyl can protect styrenics, thermoplastic polyesters and polycarbonate, providing good colour stability, and the protection lasts longer than with some of the competition - but they are more expensive than the benzophenones. 

Other secondary additives can be used to react with the hydroperoxides ae-ated in the UV-induced degradation cycle. Hydroperoxides create free radicals when exposed to light, resulting in unwanted changes in the polyolefin molecular skeleton. They are deactivated by agents referred to in Chapter 3 as secondary AOs—organophosphorous compounds that decompose hydroperoxides into harmless alcohols. 

4 HALS FREE-RADICAL SCAVENGERS 4.4.1 Overview of HALS 

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. 

HAS are relatively cosdy but fortunately, are effective at low concentrations ( 1%). This efficiency is due to a still not fully understood regenerative process cited by experts to explain HALS UV stabilization abilities. In short their mechanism depends on the initial oxidation of the HALS into a form that interferes with the propagation stage of degradation it may be summarized as follows  

This cyclic process, along with the fact that HALS are compatible with and relatively nonextractable from the resin, makes them valuable for imparting long-term UV resistance. They effectively delay the degradation of the properties of an exposed polyolefin, and a critical measure of their effectiveness is the speed with which a HALS compound is oxidized into its useful nitroxyl radical form. However, HALS can also react with other stabilizers, flame retardants, or other additives, as discussed in Section 4.4.2. These reactions can lead to color and property changes [1-1, 4-7, 4-12], 

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Zenker, A., Schutz, H., Fent, K. Simultaneous trace determination of nine organic UV-absorbing compounds (UV filters) in environmental samples. J. Chromatogr. A 1202, 64-74 (2008)... 

The ionisation principle is based on the soft desorption of the solid sample molecules into the vacuum and subsequent ionisation. First, the sample is cocrystallised with a 1,000-10,000 excess of a suitable matrix on a metallic plate. Small, organic, UV-absorbing molecules like sinapinic acid are used as matrix materials (Table 4.1). An electric field is applied between the sample plate and the entrance to the time-of-flight analyser (Fig. 4.2). A pulsed laser beam is then... 

With rigid PVC products used in exterior building applications, approximately 9-13% of rutile titanium dioxide is often nsed as an opacifier pigment. The pigment absorbs incident UV radiation shielding the bulk polymer from exposure. Organic UV absorbers such as benzophenone or benzotriazoles compete with the chromo-phores in the plastic in absorbing radiation. For instance, benzophenones reversibly isomerize in the process. 

Several approaches to stabilisation can be used. Light stabilisers will be discussed under four broad categories light-absorbing fillers organic UV absorbing compounds excited state quenchers and photo-antioxidants. 

Sunscreens, especially those with high sun protection factor (SPF), may contain considerable amounts of UV filters. E.g. for an SPF of 50, a UV filter content of 25% (w/w) can be necessary. Although this is achieved using a cocktail of several UV filters, the solubility of organic UV absorbers either in the oil or in the water phase of the emulsion should be at least about 10% as a rule of thumb. This is necessary, as the respective phase will be diluted upon emulsification, and the UV filter concentration in the final formulation will be correspondingly lower. 

Besides organic UV absorbers already presented, inorganic compounds such as Ti02, Ce02, ZnO, could be used as UV absorbers [33-35]. 

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