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ABSTRACT
The developments on long-term protection of rubber against aerobic aging are reviewed. Although conventional anlidegradants such as N-isopropyl-N'-phenyl-p-phenylenediamine (IPPD) and N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) are still the most widely used antidegradants in rubber, there is a trend and demand for longer-lasting and non-staining products. The relatively low molecular weight (MW) antioxidants have undergone an evolutionary change towards higher molecular weight products with the objective to achieve permanence in the rubber polymer, without loss of antioxidant activity. In the last two decades, several approaches have been evaluated in order to achieve this objective: attachment of hydrocarbon chains to conventional antioxidants in order to increase the MW and compatibility with the rubber matrix; oligomeric or polymeric antioxidants; and polymer bound or covulcanizable antioxidants. The disadvantage of polymer bound antioxidants was tackled by grafting antioxidants on low MW polysiloxanes, which are compatible with many polymers. New developments on antiozonants have focused on non-staining and slow migrating products, which last longer in rubber compounds. Several new types of non-staining antiozonants have been developed, but none of them appeared to be as efficient as the chemically substituted p-phenylenediamines. The most prevalent approach to achieve non-staining ozone protection of rubber compounds is to use an inherently ozone-resistant, saturated backbone polymer in blends with a diene rubber. The disadvantage of this approach, however, is the complicated mixing procedure needed to ensure that the required small polymer domain size is achieved.
I. INTRODUCTION
Rubber compounds can be degraded by reactions with oxygen, ozone, light, metal ions and heat. Antidegradants protect rubber against aerobic aging (oxygen) and ozone attack. They are of prime importance and play a vital role in rubber products to maintain the properties at service conditions. Protection of rubbers or stabilization of crosslinked networks against anaerobic aging can be achieved via other approaches: employing an EV-curing system, application of 1,3Bis(citraconimidomethyl)benzene, Hexamethylene-1,6-bis(thiosulphate) disodium salt dihydrate, Hexamethylene-l,6-bis(dibenzylthiuram disulfide), Zn-soaps, etc.1
Degradation by oxygen and ozone proceeds via different chemical mechanisms2-13 and results in different effects on physical properties of rubber. Ozone degradation results in discoloration and eventual cracking of samples. Ozone degradation is primarily a surface phenomenon. Oxygen degradation results in hardening or softening (depending on the base polymer) throughout the rubber article. For example, vulcanizates that are based on natural rubber...