Plastic has become an essential material in virtually every aspect of modern day life replacing other materials such as glass, metal, paper and wood. There are many advantages in using plastic though it generally ages rapidly under the effects of light, oxygen and heat, leading to:
- Loss of strength, stiffness or flexibility
- Discoloration and loss of gloss
Addivant's Light stabilizers and Antioxidants can all help to combat these effects.
This Light Stabilizers Center is designed to help you learn more about the degradation of plastic due to oxygen and heat - and how to combat this by using Addivant LOWILITE® UV absorbers, hindered amine light stabilizers (HALS) or nickel quenchers.
How do UV absorbers, HALS and nickel quenchers work?
UV absorbers and HALS
Most commercial plastics are manufactured by processes involving chain polymerization, polyaddition, or polycondensation reactions, generally controlled to produce individual polymer molecules with defined composition and molecular weight. This composition and molecular weight starts to change when the polymer is exposed to further shear stress, heat, light, air, water, radiation or mechanical loading. Exposure to such conditions starts chemical reactions (degradation) in the polymer leading to a modification of its physical and optical properties.
Exposure to sunlight and some artificial lights can also have adverse effects on the useful life of plastic products. A process called photodegradation takes place when UV radiation breaks down the chemical bonds in a polymer, ultimately causing cracking, chalking, color changes and the loss of physical properties. Addivant LOWILITE® UV absorbers and HALS are used to avoid this issue.
Photoxidation of many polymers is often modelled with a scheme described in Figure 1:
Figure 1: Photoxidation of many polymers
Since photodegradation generally involves sunlight, thermal oxidation takes place in parallel to photoxidation. Photoxidation differs from thermal oxidation in that it can be started by absorption of UV light. Most pure polymers are theoretically incapable of absorbing UV light directly but trace amounts of other compounds within the polymer, such as degradation products or catalyst residues, can however absorb UV. For this reason, Addivant's effective thermal and processing stabilization is a prerequisite for effective long-term light stabilization.
Addivant's LOWILITE® UV absorbers function by preferentially absorbing harmful ultraviolet radiation and dissipating it as thermal energy. Such stabilizers function according to the Beer Lambert law, which specifies that the amount of UV radiation absorbed is a function of both sample thickness and stabilizer concentration.
In practice, high concentrations of UV absorbers and sufficient thickness of the polymer are required before enough absorption takes place to effectively retard photodegradation.
Addivant's benzophenone and benzotriazole LOWILITE® UV absorbers are two classes of stabilizer in broad industrial use.
Hindered Amine Light Stabilizers (HALS)
Addivant's LOWILITE® HALS (Hindered Amine Light Stabilizers) are extremely efficient stabilizers against light-induced degradation of most polymers. They do not absorb UV radiation, but as shown in Figure 1, they form nitroxyl radicals which act as radical scavengers to inhibit degradation of the polymer. Significant levels of stabilization are achieved at relatively low concentrations. HALS' high efficiency and longevity are due to a cyclic process wherein the HALS are regenerated rather than consumed during the stabilization process. Because of the regenerative nature of this process, as well as the typically high molecular weights of these stabilizers, Addivant's LOWILITE® HALS are capable of providing extreme long-term thermal and light stability.
Addivant's LOWILITE® nickel quenchers are light stabilizers that are able to take over the energy absorbed by the chromophores and dissipate it either as heat or as fluorescent or phosphorescent radiation to prevent degradation. Nickel quenchers, unlike stabilization packages based on HALS, are unlikely to interact with pesticides, sulphur, halogen or phosphorous based acidic substances. They are mainly used for agricultural film applications (greenhouse and mulch films) where they offer the best balance between UV protection and interaction with pesticides. Addivant's LOWILITE® nickel quenchers work synergistically with UV absorbers and are often combined to take advantage of this.