Additives ultra-violet stabilizers

Since acetal resins are degraded by ultra violet light, additives may be included to improve the resistance of the polymer. Carbon black is effective but as in the case of polyethylene it must be well dispersed in the polymer. The finer the particle size the better the ultra violet stability of the polymer but the poorer the heat stability. About 1.5% is generally recommended. For white compounds and those with pastel colours titanium dioxide is as good in polyacetals as most transparent ultraviolet absorbers, such as the benzophenone derivatives and other materials discussed in Chapter 7. Such ultraviolet absorbers may be used for compounds that are neither black, white nor pastel shade in colour. 

Figure 1 Notched impact strength of polycarbonate in (a) hot, dry and (b) hot, wet climates, with and without ultra-violet stabilizing additives. Solid circles - controls squares - sheet triangles - -heat stabilized crosses - ultra-violet stabilized injection moulding grades. From Davis, A. and Sims, D. (1983) Weathering of polymers. Applied Science Publishers, London. Available from Chapman Hall, London.

Exposure to solar irradiation (sunlight) may alter their chemical integrity and in due course some physical properties such as mechanical strength (e.g. brittleness) or color (e.g. fading) as well. Additives like industrial ultra violet (UV)-absorbers, hindered amine light stabilizers, radical scavengers and antioxidants are incorporated or applied in these products to effectively reduce or delay such radiation induced deteriorations. 

Thus, at nonsensibilized photooxidation of PA formation of hydroperoxides does not influence greatly the process of photooxidation and hydroperoxides are not main products radical Pi plays the main role as all other radicals may be formed from it, including peroxides. Hence, while stabilizing PA against action of ultra-violet radiation we may use additives which interact not only with peroxides but with other radicals. 

In order to elicit the light stabilizing mechanism of the examined additions, modified CDA- films were dissolved in acetonitrile before and after being radiated. The obtained solutions were spectrographed in Ultra-violet (Figure 2.28-2.30). 

Studying ultra-violet spectrums of indan derivatives let us make a supposition about their "shield" mechanism, which causes light stabilizing effect, as indan sulphuryl amide additions (as carbazolsulphonamides) absorb radiation in the same wave length interval as cellulose diacetate. 

Irradiation of PETP - fibres was carried out in the air for evaluation of dyes effect on light stability of these fibres modified by hexaazocyclanes additives. Relative conservation of initial specific viscosity was taken as criterion of ultra-violet irradiation effect. 

Thus, investigation of initial and stabilized PETP samples shows that additive HC-2 effectively stabilizes PETP at ultra-violet irradiation by the light with wave length 254 nm and 313 nm, and additive HC-1 acts as sensibilizer, speeding the process of PETP photodestruction at these wave lengths. 

The compound has functional groups that support dimerization type crosslinking and cationic polymerization upon UV exposure (A, = 300—360 nm).. Photodimerization of the chalcone-epoxy compound was confirmed by UV-visible and IR absorbance changes of the C=C double bond of the chalcone unit. Additions of small amounts of onium salts will also photoinitiate cationic polymerization of the epoxy groups present in the above chalcone-epoxy compound by exposine to UV. This ultra-violet light cured chalcone-epoxy compound was reported to possess excellent thermal stability and compares well with conventional UV-cured Bisphenol A type epoxy resins. (see Chapter 3)... 

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