Лако-красочные материалы — производство

The simplest and most direct method to reduce photooxidation is to block the UV radiation from reaching the polymer, much as a sunscreen is used to prevent sunburn.

Fillers (carbon black, pigment, talc, TiO2, etc.) can potentially provide an improvement in UV stability by this mechanism, but may cause other complications (e. g., additive adsorption or deactivation, metal impurities, etc.). Chemical UVAs are used routinely to prevent the photooxidation of polymers. There are several major requirements for chemicals to function as UVAs: (1) they must absorb strongly in the UV region (290 to 400 nm) but must have a sharp cutoff in the visible region (>400 nm) so as not to contribute color to the polymer, (2) they must be photostable, and (3) they must dissipate the photoexcitation energy in a harmless way. Representative structures of two common classes of UVA that fulfill these requirements, the 2-hydroxybenzophenones and the 2-hydroxyphenylbenzotriazoles, are shown in Fig. 2.

The absorption of light follows Beer’s law, which says that the absorption of light equals a constant (extinction coefficient) multiplied by the UVA concentration multiplied by the path length:

absorption = const. x concentration x path length in which

absorption = log

where I0 is the intensity of incident light and I is the intensity of light having passed through the sample. This relationship implies that both the type and concentration of the absorbing species are important as well as the sample thickness.

To illustrate the function and limitations of these UVAs, we will look at the optical properties of a typical 2-hydroxyphenylbenzotriazole (BTZ). The UV absorption proper­ties are shown in Fig. 3 for several concentrations of a solution of BTZ-2. Although the

actual amount of light absorbed is a function of wavelength, for this discussion we can use an approximate value of 90% UV light absorption at a BTZ concentration of 0.0025% and a path length (sample thickness) of 1.00 cm. Beer’s law tells us that if 90% of the light is absorbed by a 0.0025% BTZ solution in 10 mm (0.4 in.), 90% of the light is also absorbed by a 0.025% BTZ solution in 1.0 mm (0.04 in.), and 90% of the light will be absorbed by a 0.25% BTZ solution in 0.10 mm (0.004 in., 4 mils). A BTZ concentration of 0.25 to 1.0% is typical for protection of a polymer.

Carrying this further, if at this concentration of 0.25% BTZ, 90% of the light is absorbed in the first 0.10 mm, then 99% of the light is absorbed in the first 0.20 mm and 99.9% is absorbed in the first 0.30 mm of the sample. This explains why UV absorbers are very effective at protecting all polymers of sufficient thickness but are not effective at protecting sample surfaces or very thin films. This is demonstrated clearly in Fig. 4, where a sample of polypropylene exposed to UV radiation from sunlight is microtomed and analyzed for degradation (loss of molecular weight and hence viscosity) [8]. With no light stabilizer, UV radiation is at a high intensity throughout the transparent sample. As oxygen diffuses in from both sample surfaces, the UV light accelerates oxidation. In the presence of a UVA, the bulk of the sample is protected except for the surface exposed to the UV light. (The effect of the HALS is explained below.) This dependence on sample thickness also explains why UV absorbers are widely used in sealant application but have limited efficacy in adhesive films, where typical thickness may be about 0.025 mm (1 mil).