Abstract

The term antiozonant, in its broadest sense, denotes any additive that protects an elastomer against ozone deterioration. Most frequently, the protective effect results from a reaction with ozone, in which case the term used is chemical antiozonant. Ozone is generated naturally by electrical discharge and also by solar radiation in the stratosphere. Ozone is also produced in urban centers by ultraviolet photolysis. Only a few pphm ozone in air can cause rubber cracking, which may destroy the usefulness of elastomer products. Ozone attacks any elastomer with backbone unsaturation. Degradation results from the reaction of ozone with rubber double bonds. Commercial antiozonants have been available since the early 1950s. A physical antiozonant must provide an effective barrier against the penetration of ozone at the rubber surface. A chemical antiozonant, on the other hand, must first of all be extremely reactive with ozone. The antiozonant should possess adequate solubility and diffusivity characteristics. Since ozone attack is a surface phenomenon, the antiozonant must migrate to the surface of the rubber to provide protection. The antiozonant should have no adverse effects on the rubber processing characteristics, eg, mixing, fabrication, vulcanization, or physical properties. The antiozonant should have a low toxicity and should be nonmutagenic, and the antiozonant should be acceptable economically. Waxes are one of the two general classes of commercial antiozonants. Waxes are derived from petroleum and are of two common types, paraffin and microcrystalline. Chemical antiozonants comprise the second general class of commercial antiozonants. Of the many compounds reported to be chemical antiozonants, nearly all contain nitrogen. The most effective antiozonants, however, are derivatives of p-phenylenediamine (p-PDA). The commercial materials fall into three classes: N,N¢-dialkyl-p-PDAs, N-alkyl-N¢-aryl-p-PDAs, and N,N¢-diaryl-p-PDAs. The principal objection to p-PDA antiozonants is their staining characteristics. The lack of suitable alternative antiozonants for light-colored diene rubber articles is one of the outstanding problems in rubber technology. Several theories have appeared in the literature regarding the mechanism of protection by p-PDA antiozonant, which the scavenger theory, states that the antiozonant diffuses to the surface and preferentially reacts with ozone; the protective film theory in which the ozone–antiozonant reaction products form a film on the surface that prevents attack; the relinking theory which states that the antiozonant prevents scission of the ozonized rubber or recombines severed double bonds; and a fourth theory which states that the antiozonant reacts with the ozonized rubber or carbonyl oxide to give an inert self-healing film on the surface. The literature suggests that more than one mechanism may be operative for a given antiozonant. All of the evidence, however, indicates that a combined scavenger–protective film mechanism is the most important. The N,N¢-dialkyl-p-PDAs are manufactured by reductively alkylating p-PDA with ketones. Alternatively, these compounds can be prepared from the ketone and p-nitroaniline with catalytic hydrogenation. Waxes act as antiozonants for diene rubbers under conditions requiring little or no flexing. Current higher molecular weight N,N¢-dialkyl or N-alkyl-N¢-aryl derivatives are not primary skin irritants. A notable exception is N-(1-methylethyl)-N¢-phenyl-p-PDA, which causes dermatitis. However, since some individuals are more sensitive than others, antiozonants should always be handled with care. In case of eye contact, flush well with water. Inhalation of rubber chemicals should be avoided. Chemical antiozonants are routinely used to protect diene rubbers (NR, IR, BR, SBR, NBR) against atmospheric ozone for extended periods of time. Large volumes are used in tire, belt, and hose applications.