Polymer-reactive antioxidant reactions chains

In the case of BR or SBR, the efficiency can be much greater than 1.0, especially if all antioxidant materials are removed. A chain reaction is indicated here. It might be explained by steric considerations. In butadiene-based rubbers, double bonds are quite accessible. Radical addition to double bonds could give highly reactive radicals, which would be likely to add to other polymer double bonds. A chain of additions might be more likely in butadiene rubber than in the presence of hindering methyl groups in isoprene rubbers. 

The oxidation stability of the polymers is inferior to that of the bitumen. Most of the polymers which are used in the plastics industries are more or less adequately stabilized against oxidative attacks by the manufacturer. Polymers with reactive double bonds, which are for the most part used in the robber industry, should be protected against oxidation by those antioxidants which are in common use in that industry. Easily oxidizable polymers cannot protect bitumen against oxidative attacks. The radicals which form during the oxidation of the polymers, may also start chain reactions in the bitumen. Effective protection against oxidation may only be achieved using radical scavengers or suitable antioxidants, which terminate the radical chain reaction or deactivate the free radical starters. 

Unwanted reactivity with host polymer. Conversely, deliberate reactions can be indnced to attach antioxidants to the polymer chains, or to bnild them into chains. 

The proximity of the methyl group to the double bond in natural rubber results in the polymer being more reactive at both the double bond and at the a-methylenic position than polybutadiene, SBR and, particularly, polychlor-oprene. Consequently natural rubber is more subject to oxidation, and as in this case (c.f. polybutadiene and SBR) this leads to chain scission the rubber becomes softer and weaker. As already stated the oxidation reaction is considerably affected by the type of vulcanisation as well as by the use of antioxidants. 

Interaction of acceptors of reactive free radicals and compounds that suppress the transfer reaction of an inhibitor radical with the substrate as it occurs in a system comprising antioxidants and polymer chain with conjugated system of double C = C bonds. 

Antioxidants act so as to interrupt this chain reaction. Primary antioxidants, such as hindered phenol type antioxidants, function by reacting with free radical sites on the polymer chain. The free radical source is reduced because the reactive chain radical is eliminated and the antioxidant radical produced is stabilised by internal resonance. Secondary antioxidants decompose the hydroperoxide into harmless non-radical products. Where acidic decomposition products can themselves promote degradation, acid scavengers function by deactivating them. 

The effect of primary antioxidants is based on reactions with polymer radicals containing oxygen that are formed during auto-oxidative degradation of plastics. These very reactive radicals are transformed into inactive or less reactive compounds, thus causing the end of radical chain reaction. Fig. 1.13, Section 1.4.2. The antioxidants are consumed during this reaction. 

It is apparent that photo-oxidation of polymers is a complex and very important component of photodegradation of such materials. Detailed discussion of mechanism and the effects of these reactions is not part of this review. However, prevention of breakdown has a very obvious economic impact and a few generalizations on stabilization of polymers are relevant before the detailed discussion which follows in Volume 6, Chapter 19. Given that radiation produces free radicals, which become involved in a chain reaction, then any substance which preferentially absorbs the harmful radiation (a screen) or any substance which can efficiently remove radicals via non-reactive products (an antioxidant) will operate as a stabilizer. However, the planning and chemical design of stabilizers is not easy and Volume 6, Chapter 19 gives an excellent survey of just how complicated the process of stabilization can be. 

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