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Why use antioxidants ?
Plastic has become an essential material in virtually every aspect of modern day life. Recent advances in plastic manufacturing and processing have led to ever more applications in which plastics replace other materials such as glass, metal, paper and wood.
Plastic generally ages rapidly under the effects of light, oxygen and heat, leading to: 
•Loss of strength, stiffness of flexibility
•Discoloration
•Scratching and loss of gloss
Antioxidants, Light Stabilizers and Fluorescent Whitening Agents can all help to combat these effects.
Most commercial plastics are manufactured by processes involving chain polymerization, polyaddition, or polycondensation reactions. These processes are generally controlled to produce individual polymer molecules with defined
•Molecular weight (or molecular weight distribution)
•Degree of branching, and
•Composition
Once the initial product of these processes is exposed to further shear stress, heat, light, air, water, radiation or mechanical loading, chemical reactions start in the polymer which have the net result of changing the chemical composition and the molecular weight of the polymer.
These reactions, in turn, lead to a change in the physical and optical properties of the polymer.
In practice, any change of the polymer properties relative to the initial, desirable properties is called degradation. In this sense, "degradation" is a generic term for any number of reactions which are possible in a polymer.
Autoxidation of many polymers is often modeled with a scheme originally developed for natural rubber:
The important aspect of this scheme is that once oxidation starts-which it always will-it sets off a chain reaction which accelerates degradation unless stabilizers are used to interrupt the oxidation cycle.
Exposure to sunlight and some artificial lights can have adverse effects on the useful life of plastic products. UV radiation can break down the chemical bonds in a polymer. This process is called photodegradation and ultimately causes cracking, chalking, color changes and the loss of physical properties
Photodegradation, once started, essentially follows the same scheme as shown above. Since photodegradation generally involves sunlight, thermal oxidation takes place in parallel with photooxidation.
Photodegradation 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. Trace amounts of other compounds within the polymer, such as degradation products or catalyst residues, can however absorb UV. For this reason, effective thermal and processing stabilization is a prerequisite for effective long-term light stabilization.
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