Protection of Polymers from Photo-oxidation

A variety of additives are available for incorporation into polymers to act as antioxidants. A selection of these additives is listed in Table 8.2 together with the names of the polymers for which they are most suited. 

Whilst not an exhaustive list the compounds given in Table 8.2 do represent the major classes of antioxidant. One feature that is clear from this Table is that these antioxidants tend to be effective in many different polymers. 

Polyamides, poly(esters), poly(alkenes), PVC, polyurethanes, and rubbers 

Poly(esters) and rubbers Poly(alkenes) and rubbers Cellulosic polymers, ethylene-vinyl acetate copolymers, poly(alkenes), PVC, rubbers, and poly(styrene) Cellulosic polymers, poly(esters), poly(alkenes), polyurethanes, PVC, rubbers, and poly(styrene) 

The rate constant of Reaction 8.1 is much greater than the rate constant of Reaction 8.2, which means that antioxidants of this type can be used in very low concentrations with good effect. A typical thermoplastic would contain only 0.01-0.5% by mass of such an antioxidant. Typical compounds which work by this mechanism include substituted phenols and secondary aromatic amines. 

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An alternative mechanism by which additives may protect polymers from photo-oxidation is radical trapping. Additives which operate by this mechanism are strictly light stabilizers rather than antioxidants. The most common materials in this class are the hindered amines, which are the usual additives for the protection of poly (ethylene) and poly (propylene). The action of these stabilisers is outlined in Reactions 8.3-8.5. 

Polyolefins are intrinsically unsuitable for outdoor durable applications since the will easily photo-oxidize in exterior conditions. However, their combination of low cost, ease of processing, and recyclability continues to promote research into effective methods of photo stabilization of polyolefins. Adequate pigment levels will afford some protection from UV radiation to the host polyolefin. UV stabilizers and antioxidants, however, need to be added to the durable in order to ensure long term performance in outdoor environments. There is usually negligible to no protection of colorants from photo degradation with these same polymer additives. 

The ozone concentration in the troposphere during the daytime is typically about 1 pphm (parts per hundred million parts of air by volume) [20], Values up to 100 pphm were measured in some photochemical smog areas. The molecular mechanism of the ozone aging of diene based elastomers was studied in detail and is well understood [19,21], Products or intermediates different from those arising in autoxidation or photo-oxidation of polymers were identified ozonides (3), zwitterions (4), diperoxides (5), polyperoxides (6), polymeric ozonides (7) and terminal aldehydes (8). Reactivity of aminic antiozonants (AOZ) with these species accounts for the protection of rubbers against atmospheric 03. AOZ must also possess antioxidant properties, because the free radical processes are concerted with ozonation due to the permanent presence of oxygen. 

The hindered secondary amines can be highly effective photostabilizers for various polymers (]+.,5.,.6) Various hindered amines have been shown to retard oxidation, but most share the common feature of being secondary or tertiary amines with the a-carbons fully substituted. The most widely exploited representatives of this class are based on 2,2,6,6-tetramethylpiperidine either in the form of relatively simple low molecular weight compounds, or more recently as backbone or pendant groups on quite high molecular weight additives ( i.,5.,6). The more successful commercial hindered amines contain two or more piperidine groups per molecule. Photo-protection by tetra-methylpiperidines (near UV transparent) must result from the interruption of one or more of the reactions 1 to 3. Relatively recent results from our own laboratories, and in the open literature will be outlined in this context. 

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