Characteristics of the Stabilising Action of Phenolic Antioxidant 4,4'-Bis(2,6-Di-Tert-butylphenol) in the Ageing Processes of Rubbers

Abstract

T/41 4,4ʹ-Bis(2,6-di-tert-butylphenol) (BP) is of great interest to scientists and practitioners on account of its high stabilising effectiveness in processes of ageing of carbonchain polymers, primarily elastomers. In all probability it can be assumed that the high potential of the stabilising action of BP is connected with its ability readily to be oxidised to 3,3ʹ,5,5ʹ-tetratert-butyl-4,4ʹ-diphenoquinone (DPQ) under conditions of thermal oxidation of polymers with the formation of an equilibrium system and with subsequent participation of the products of transformation in the inhibition of the thermo-oxidative degradation of polymers [1]. With the aim of obtaining experimental proof of the proposed hypothesis, it was of interest to conduct a comparative study of the anti-oxidative activity of BP and DPQ, and also model mixtures based on BP and DPQ, under conditions of ageing of rubbers. Investigations were conducted under standard conditions adopted in factory practice at the Nizhnekamskneftekhim Open Joint Stock Company. Specimens of butyl rubber, SKI-3 polyisoprene rubber, and SKD-N polybutadiene rubber containing the investigated and industrial antioxidants were oxidised in hot air at 140°C for 90 min (for SKI-3) and 120 min (for SKD-N and butyl rubber). The indices of retention of the properties of the rubber were the values of the Mooney viscosity and elastic recovery 60 s after the viscometer rotor had stopped [2]. Testing of butyl rubber specimens under ageing conditions showed (Table 1) that the greatest effectiveness is exhibited by BP which in its stabilising properties is superior to industrial antioxidant 2,2ʹ-methylenebis(4methyl-6-tert-butylphenol) – AO-2246 – which is currently used at Nizhnekamskneftekhim. DPQ is markedly less effective than antioxidants BP and AO-2246 in stabilising the physicomechanical properties of butyl rubber. It was shown that, for rubber SKI-3 (Table 2), both BP and DPQ are effective stabilisers, and also mixtures based on them. These stabilisers ensure retention of the physicomechanical properties of SKI-3 at the level of the amine antioxidant Dusantox-1 widely used in industry, and they perform considerably better than phenolic antioxidant AO-2246. The index used in evaluating the stabilising properties of antioxidants in SKI-3, characterising the viscosity gradient over a prescribed period from the start of rotor rotation J, again indicates the effectiveness of the stabilisers investigated (see Table 2): reduction in this index corresponds to an increase in the antioxidative stability of the rubber [2]. Physicomechanical tests of SKD-N also showed the high effectiveness of the antioxidants investigated (Table 3). In the presence of individual DPQ, the Mooney viscosity of SKD-N rubber hardly changed under ageing conditions. Industrial AO-2246 proved to be considerably less effective in the stabilisation of SKD-N than DPQ, BP, or their mixtures. Kauchuk i Rezina, No. 3, 2006, pp. 17–19