Polymer Degradation and Stability 46 (1994 ) 251-257 Q 1994 Elsevier Science Limited Printed in Northern Ireland . All rights reserved ELSEVIER 0141 . 3910 ( 94)00064 .6 0141.3910/94/507.00 Studies on natural rubber-bound diphenylamine antioxidants S. Avirah & R. Joseph Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin 682 022, India (Received 31 January 1994 ; revised versions received 10 May and 30 June 1994 ; accepted 3 July 1994) Diphenylamine was chemically attached to depolymerised natural rubber by photochemical reaction. The rubber - bound diphenylamine was characterised by TLC , HNMR , IR and TGA. The efficiency and permanence of the bound diphenylamine was compared with conventional amine type antioxidant in natural rubber vulcanizates . The rubber -bound diphenylamine was found to be less volatile and less extractable compared to the conventional antioxidant. The vulcanizates showed improved ageing resistance in comparison to vulcani- zates containing conventional antioxidant . Also, the presence of liquid rubber - bound diphenylamine reduces the amount of plasticiser required for compounding. INTRODUCTION Service requirements placed on finished rubber products demand improved polymer stabilisation. Many amine and phenolic antioxidants are commercially available. Serious drawbacks of all antioxidants existing today are volatility and extractability. The decrease in concentration of the antioxidants impairs the resistance to degradation. One method to increase the persistance of the antioxidant in polymers is to chemically attach the antioxidant to the rubber. The potential advantages of these polymer-bound antioxidants are that they cannot migrate to the rubber surface and hence cannot be lost by volatilisation or solvent leaching.',' Cane et al.3 have shown that mobility of the antioxidant is not essential for heat ageing or oxidative ageing, but will become essential for protection against ozone attack or dynamic fatigue. Polymerisable monomeric antioxidants were described by Tamura and coworkers.'-' Prepara- tion of bound phenyl-(3-naphthylamine by the reaction of naphthylamine with rubber modified with dichlorocarbene was described by Kirpichev & Yakubchik.6 The most versatile method of preparation of bound antioxidant is by the direct reaction of conventional antioxidants with rub- ber. Scott et al. have demonstrated that simple hindered phenols which contain a methyl group in the ortho or para position, can react with natural rubber in the presence of oxidising free radicals to yield polymer-bound antioxidants.'-9 Antioxidants like styrenated phenol, diphenyla- mine, etc., bound to hydroxy-terminated liquid natural rubber by a modified Friedel-Craft's reaction, were also found to be effective in improving ageing resistance.10 This paper de- scribes chemical binding of diphenylamine to natural rubber by a photochemical reaction. This rubber-bound antioxidant was compared with conventional antioxidants in filled natural rubber compounds. EXPERIMENTAL Materials Natural rubber (NR) (ISNR-5, Mooney viscosity ML(1 + 4) 100°C-82) was supplied by the Rubber 251