Antiozonants, rubber protection

Stabilizing activity is improved in mixtures of aromatic amines varying in their structures. This is due to different contributions to antioxidant, antifatigue or antiozonant effects. Combinations of DPA 9a with PD 11c, d, PNA with PD, ternary mixtures PD/DPA/PNA, PD lib with DHQ 16b or 16c, oligomeric 21 with 11 or 16 (R = NHCftHj), condensate 22 with lib, PT 20 with 9 or lib are examples of combinations effective in rubber protection. Mixtures containing... 

Jackson, A. T., Jennings, K. R., and Scrivens, J. H., Analysis of a fivepolymer additives by means of high energy mass spectrometry and tandem mass spectrometry. Rapid Commun. Mass Spectrom., 10,1449, 1996. Lattimer, R. P., Layer, R. W., and Rhee, C. K., Mechanisms of antiozonant protection Antiozonant-rubber reactions during ozone exposure. Rubber Chem. Technol, 57, 1023, 1984. 

Toxicology LD50 (oral, rat) 750 mg/kg, (oral, mouse) 1700 mg/kg LDLo (IP, rat) 800 mg/kg mod. toxic by ing., IP routes TSCA listed Hazardous Decomp. Prods. Heated to decomp., emits toxic fumes of NOx Uses Antiozonant/antioxidant protecting natural rubber and syn. rubbers for dynamically stressed goods, tires styrene polymerization inhibitor gasoline antioxidant and sweetener Manuf./Distrib. ChemService http //www. chemservice. com,... 

Lattimer, R.P., et al., Mechanisms of Antiozonant Protection Antiozonant — Rubber Reactions During Ozone Exposure, The B.F. Goodrich Research and Development Center, presented at the Rubber Division, A.C.S. Meeting in Indianapolis, May 8-11, 1984. 

The effect of ozone is complicated in so far as its effect is largely at or near the surface and is of greatest consequence in lightly stressed rubbers. Cracks are formed with an axis perpendicular to the applied stress and the number of cracks increases with the extent of stress. The greatest effect occurs when there are only a few cracks which grow in size without the interference of neighbouring cracks and this may lead to catastrophic failure. Under static conditions of service the use of hydrocarbon waxes which bloom to the surface because of their crystalline nature give some protection but where dynamic conditions are encountered the saturated hydrocarbon waxes are usually used in conjunction with an antiozonant. To date the most effective of these are secondary alkyl-aryl-p-phenylenediamines such as /V-isopropyl-jV-phenyl-p-phenylenediamine (IPPD). 

An antiozonant should have 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. Poor solubility in rubber may result in excessive bloom. 

Rubber is protected against ozone attack by addition of physical and/or chemical antiozonants. Hydrocarbon waxes are the most common type of physical antiozonants, and p-phenylenediamine derivatives are the prevalent chemical antiozonants. Waxes bloom to the rubber surface and form a protective barrier. 

Since this bloom is brittle, it is broken by flexing. Therefore, waxes only protect under static conditions. For serving conditions which involve continuous flexing, /j-phenylenediamines (A, A -alkyl-aryl derivatives) can be added. These chemical antiozonants scavenge the ozone before it reacts with the rubber. A barrier of ozonized products is created which protects both the rubber and antiozonant from further attack. However, p-phenylenediamines are staining compounds. Whenever colour is an important concern, blends of elastomers can be used elastomers loading should be higher than 30 phr to provide sufficient effectiveness. 

Rubbers can be protected against ozone by use of chemical antiozonants and via several physical methods. The chemical antiozonants protect rubber under both static and dynamic conditions, whereas the physical methods are more related towards protection under static conditions. 

Chemical antiozonants have been developed to protect rubber against ozone under such dynamic conditions. Several mechanisms have been proposed to explain how chemical antiozonants protect rubber. The scavenging mechanism, the protective film mechanism, or a combination of both are nowadays the most accepted mechanisms. 

The most effective antiozonants are the substituted PPDs. Their mechanism of protection against ozone is based on the scavenger-protective film mechanism [68-70]. The reaction of ozone with the antiozonant is much faster than the reaction with the C=C bond of the rubber on the rubber surface [56]. The rubber is protected from the ozone attack tUl the surface antiozonant is depleted. As the antiozonant is continuously consumed through its reaction with ozone at the mbber surface, diffusion of the antiozonant from the inner parts to the surface replenishes the surface concentration to provide the continuous protection against ozone. A thin flexible film developed from the antiozonant/ozone reaction products on the mbber surface also offers protection. 

Materials used to protect vulcanisates from deterioration, including surface coatings as well as the ingredients added to the rubber at the mixing stage. See Antioxidant, Antiozonant, Paraffin Wax. 

Benzofuran derivatives and enolethers are offered as chemical antiozonants for light coloured rubber compounds. Benzofuran derivatives are used in CR and its blends with other rubbers and give ageing protection in addition to ozone protection. 

The solubility of wax in vulcanised rubbers is low (of the order of 0.5% for NR) but enough wax has to be added to a rubber compound to ensure that once the compound has been vulcanised and the rubber cools, the rate of migrational movement of the wax from the rubber mass to the surface of the rubber is rapid. Dependant upon the application, the addition level of wax can be up to about 10 phr. Migration of the wax to the rubber surface will also carry other ingredients such as antioxidants, antiozonants and other materials (e.g., vulcanisation residuals), to enhance the surface protection. 

Antiozonents protect the rubber surface by the formation of a protection layer, the ozonides on the surface of rubber by reaction of the antiozonents with ozone. Certain polymers also provide good ozone protection. The use of 10-20 parts of EPDM, a low diene rubber, in natural rubber compound significantly increases ozone resistance. 

A number of amines and phenols are known to be effective stabilizers - for rubber (2, 20). They are capable of protecting unsaturated polymers from the attack of oxygen and ozone, but the effects of the stabilizers on antioxidation and antiozonization are not always the same —e.g., N,N -diphenyl-p-phenylenediamine (A) is an antioxidant, whereas jV,N -phenylcyclohexyl-p-phenylenediamine (B) is an antiozonant. 

Antidegmdents. This group of chemicals is added to prevent undesirable chemical reactions with the polymer network. The most important are the antioxidants, which trap free radicals and prevent chain scission and cross-linking. Antiozonants are added to prevent ozone attack on the rubber, which can lead to the formation and growth of cracks. Antiozonants function by diffusion of the material to the surface of the rubber, thereby providing a protective film. Certain antioxidants have this characteristic, and waxes also are used for this purpose. 

It is easier to dissolve a stabilizer than to evaporate it [27]. The physical loss of stabilizers due to the leaching from polymer surfa< layers into liquids which come into contact is therefore more serious than volatilization. Problems arise mainly in systems where the degradation process has been concentrated at the surface layer and therefore an efficient surface protection of a polymer is mandatory. This phenomenon takes place mainly in photostabilization of plastics or antiozonant protection of rubbers. The surface loss of stabilizers is extremely serious in very thin profiles or products having a very high surface/mass ratio. 

In amorphous systems, e.g. in rubbers, a better solubility of stabilizers and a more uniform distribution than in polyolefins can be reached. The equilibrium stabilizer concentration should not exceed the saturation state to prevent stabilizer blooming after 1 year storage at ambient temperature. Complications may arise with insoluble stabilizers, as with JV,h/ -diaryl-l,4-phenylenediamine in rubbers [16]. The limited solubility does not allow a sufficient antiozonant protection to be achieved. 

Over half of the remaining market for products used in the processing of rubber is made up of antioxidants, antiozonants and stabilizers, either amino compounds or phenols. Aniline is used to manufacture vulcanization accelerators, antioxidants and antidegradants. Of the latter, several are A-substituted derivatives of p-phenylenediamine and octyl dipheny-lamine. Diphenylamines terminate free-radical reactions by donating the amino hydrogen, and are used to protect a wide range of polymers and elastomers. Many synthetic rubbers incorporate alkylated diphenylamine antioxidants. Other antioxidants include aryl amine resinous products from, e.g. condensation of aniline and acetone in the presence of... 

Provides antiozonant and antioxidant properties with high temperature, fatigue and flex resistance to rubber compounds. Used in pneumatic tire components, solid tires, belts, hoses, cables, automotive mounts, bushings and general mechanical products that are exposed to continuous and intermittent dynamic operating conditions that require protection from ozonation. 

Two types of stabilizers inhibit crack growth in rubbers microcrystalline waxes and alkylated phenylene diamines. A small quantity of the wax milled into a rubber will gradually diffuse to the surface where it will serve as a barrier impervious to ozone. A combination of wax and alkylated phenylene dicunine antiozonant is generally used for optimum protection. The exact function of antiozonant is still obscure but it is possible that it accelerates scission processes on the polymer surface producing a protective film of viscous products. 

Secondary aromatic amines are effective antioxidants in the protection of saturated hydrocarbon polymers (polyolefins) against autooxidation. Their role in the stabilization of unsaturated hydrocarbon polymers (rubbers) is more complex depending on their structure, they impart protection against autooxidation, metal catalyzed oxidation, flex-cracking, and ozonation. The understanding of antioxidant, antiflex-cracking and antiozonant processes together with involved mechanistic relations are of both scientific and economic interest. 

Due to the high reactivity of ozone with unsaturated hydrocarbons moieties, surface cracking of stressed or flexed NR, BR, NBR, and SBR vulcanizates arises. Rubber goods designed for outdoor applications must therefore be stabilized against both C>2 and 0 attacks. Antioxidant protection mechanisms have been discussed in detail ( 1). Discussions dealing with antiozonant mechanism involve some contradictory experimental observations. 

An extensive screening of structure-activity relations revealed ( 1 ) the outstanding properties of N,N -disubstituted PD. It is generally accepted that the presence of N-sec.alkyls accounts for better antiozonant protection than that of N-prim. and N-tert.alkyls or N-aryls (21). This may be one of the clues to decipher the chemical pathways of the antiozonant mechanism. The final effect is moreover fully dependent on the composition of the vulcanizate. The structure of commercially used antiozonants is an optimum compromise of efficiency, physical properties and toxicity. N, N -Disec.alkyl-1-4-PD are used in the U.S.A., N-sec.alkyl-N -aryl-l,4-PD are preferred in Europe. N,N -Diaryl derivaties are not applied as antiozonants in NR, BR, IR, or SBR. One of the reasons may be their low solubility in rubber vulcanizates (22). It does not allow them to reach a concentration level in the rubber bulk which is able to act as a long-term operative store of a stabilizer ready to supply the rubber surface slowly but continuously with active compounds by migration and to maintain the protective effect without inefficient quick blooming of an incompatible PD. A chemical reason accounting for the minority antiozonant role of N,N -diaryl PD is discussed later. 

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