Hindered amine light stabilizers application

In the present study possible modes of action of a relatively new class of light stabilizers, the Hindered Amine Light Stabilizers (HALS, tetramethylpiperidine derivatives, TMP) will be discussed. Certain members of this group have a light stabilizing effect superior in many fields of application to that of previous additives. They are effective in both thick layers and fibers13 12. 

Thioethers increase long-term stability in conjunction with phenolic antioxidants. Their use is limited to applications where possible effect on odour or taste and negative interaction with HALS (hindered amine light stabilizers) is not important. 

To make polymers useful for the above-mentioned applications, stabilizers of different types can be added to the polymers. As a result the lifetimes in the outdoor environments becomes long enough to make that polymers can be applied. In fact, without stabilizers the use of polymers in an outdoor environment would be very limited. Especially the discovery of hindered amine light stabilizer (HALS) led to a boost of the applicability of polymers in outdoor environments. 

Chemical re-stabilization of recycled material against the thermal and light-induced degradation is essential. Addition of 0.1-0.5 wt% of a sterically hindered phenol and a phosphite at a ratio varying from 10 1 to 1 10 is recommended [Pauquet et al., 1994]. For outdoor applications, hindered amine light stabilizers with UV-absorbers of the benzotriazole type are to be used [Herbst et al., 1995,1998]. Examples of blends used for polymer recycling are listed in Table 1.81. For more details see Chapter 16 in this book. 

Hindered amine light stabilizers (HALS). Hindered amine light stabilizers are the newest type of UV light stabilizer. They were introduced in 1975 by Ciba and Sankyo. HALS do not screen ultraviolet light, but stabilize the resin via free-radical termination. HALS are used at lower levels than benzophenones and benzotriazoles, and are widely used in polyolefins for their cost-effectiveness and performance. The successful growth of HALS has been directly related to their substitution for benzophenones and benzotriazoles in many applications as well as their blending with benzophenones. 

Of the various chemical classes of UV stabilizers that have been developed, the benzophenone and benzotriazole UV absorbers, the hindered amine light stabilizers (HALS), and their combinations continue to satisfy the UV stabilization requirements of most of the large-volume outdoor polymer applications. In addition, many new stabilizer chemistries, such as UV absorbers based on hydrox-yphenyl triazines, have been developed for more demanding applications or to satisfy specific market needs. Light stabilizers together with other additives (qv), such as antioxidants (qv), impact modifiers, colorants (qv), fillers, heat stabilizers (qv) and plasticizers (qv), have enhanced the properties and extended the service life of polymeric materials, resulting in the replacement of traditional materials in a multitude of outdoor weatherable applications. 

Polymeric hindered amine light stabilizer is excellent for protection against ultraviolet degradation. Applications include polyolefins (polypropylene, polyethylene), olefin copolymers such as EVA as well as blends of polypropylene with elastomers. Also effective in polyacetals, polyamides, polyurethanes, flexible and rigid PVC, and PVC blends. 

Typically hindered amine light stabilizers are used up to 3 % by mass of coating, depending on other formulation parameters such as type of binder and pigments. A combination of UV absorbers and hindered amine light stabilizers are often used in demanding applications due to their synergistic effect. 

Sterically hindered amine light stabilizers (HALS) constitute another class of light stabilizers that efficiently reduce UV-induced degradation. They are compounds that intercept chemically highly reactive radicals formed hy photochemical reactions and transform them into harmless reaction products. Fig. 3.8. Their effectiveness is not based on a competing physical absorption effect, therefore no minimum layer thickness is required for HALS effectiveness, unlike for the UV absorbers. That makes them suitable for applications such as films and fibers (527). In particular, they provide surface protection, which plays a special role in visible automotive parts. Another essential difference to UV absorbers is the fact that photooxidation is not prevented, but after initial damage its propagation is either reduced or suppressed [510],... 

---