I. Mechanism of Photodegradation
The degradation of plastics during natural weathering is caused by many factors. Sunlight, moisture and rain, air pollutants, heat, mechanical stress, and even microorganisms can all affect the color and the retention of mechanical properties. However, the most detrimental factor is usually the ultraviolet (UV) light radiation from sunlight. Figure 26.1 shows a typical spectrum of sunlight, including UV and visible light but excluding infrared (IR) light. Visible light has a wavelength between about 400 to 750 nm. The UV range is divided into UVA (315 to 400 nm), UVB (280 to 315 nm), and UVC (100 to 280 nm).
The photodegradation process involves the generation and reaction of free radicals and can be divided into four steps: chain initiation, chain propagation, chain branching, and chain termination. Figure 26.2 is a simplified illustration of the degradation process for polyolefins. The chain initiation in polyethylene is due to the interactions between UV light and impurities as mentioned above. The hydroperoxides formed during processing and storage are especially detrimental to light stability due to their high quantum yield; unstable and easily decomposed under light. Eventually, some polyethylene chains will break down into alkyl radicals. The chain initiation step is a step of generating free radicals. Alkyl radicals are very reactive and can quickly react with oxygen to form peroxy radicals, which is the reason why oxidation is always coupled with photodegradation at ambient conditions. The peroxy radical can then abstract a hydrogen from another polymer chain and turn itself into hydroperoxide. The polymer chain that donated the abstracted hydrogen forms a new alkyl radical. This step is chain propagation and is shown in Cycle I in Figure 26.2.
The regenerated hydroperoxides again quickly decompose into one or more free radicals under heat, UV light, or in the presence of catalyst residues; this is the chain branching step or Cycle II in Figure 26.2. More free radicals are generated in this step and the degradation process is thus accelerated via further chain propagation and branching. The chain termination step involves the reaction of two free radicals into non-radicalized species which can be either active for further reactions or relatively inactive.
II. Light Stabilizers
2.1. UV Absorbers
To prevent or retard the photodegradation of polymers, UV screeners and absorbers were first used to either physically screen UV radiation, or absorb UV energy and then release that energy as heat. UV absorbers can be inorganic or organic. Most common inorganic UV absorbers are titanium dioxide (TiO2) and carbon black. The effectiveness of TiO2 and carbon black as UV absorbers usually depends on the particle size and distribution, quality, as well as their surface chemistry. In general, silica- coated rutile TiO2 and carbon black with a small particle size can lead to better performance, assuming good dispersion in the polymer matrix. At high loadings of TiO2 or carbon black, the UV stability may be sufficient for non-demanding applications. However, other requirements, such as color and transparency, can limit the use of both materials. There are several different chemical categories of organic UV absorbers (UVA). The generic structures of three most popular ones, i.e., benzophenone, benzotriazole, and triazine, are illustrated in Figure 26.4.
2.2. Hindered Amine Light Stabilizers
Hindered amine light stabilizers (HALS) were invented in the 1970s. They act as free radical scavengers and thus overcome the weakness of UVA materials; i.e., path length dependent absorbance or performance. HALS are very effective in polyolefins, both near the surface and in the bulk, and have been enabling new applications ever since. The representative chemical structure of HALS and its stabilization mechanism are shown in Figure 26.8. The active group is the substituent R1 and tetramethylpiperidine group. R1 can be H, methyl group, or other groups. The substituent R is mainly for fine tuning the secondary properties of HALS. The structures of several commercially available HALS materials are provided in Figure 26.9.