Лако-красочные материалы — производство

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

therefore have no effect on the incident radiation. The mechanism of protection by HALSs is still being investigated [6,7,9], but the primary mechanism involves the HALS acting as an extremely efficient hydroperoxide decomposer and free-radical scavenger, involving a cyclic mechanism that regenerates the active HALS species (Fig. 6).

Referring back to Fig. 1, the HALS functions not by blocking the offending UV 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,

by attacking a second pathway, the addition of a HALS to a polymer can provide addi­tional stability beyond what can be achieved with a UVA alone. More important, since HALS efficacy is not dependent on competitive absorption of light, HALSs protect the surface layer of a polymer and can protect thin films. Again this is demonstrated in Fig. 4, where even the exposed surface of the polymer is protected from degradation. The term hindered amine light stabilizer is a somewhat limiting misnomer, as this class of additive is also finding wide use as a thermal stabilizer. Unfortunately, HALSs, functioning as anti­oxidants, cannot protect aromatic polymers from the nonoxidative photolytic processes described above.