Antioxidants and Antiozonants

Lattimer et al have very successfully applied mass spectrometry to the determination and identification of organic additives (antioxidants and antiozonants) in rubber vulcanizates (Method 42). A wide variety of components are involved - polymers, fillers, solvents and organic and inorganic additives. Field desorption/ionization (FD/FI) is the most efficient for identifying typical organic additives in rubber vulcanizates. 

Antiozonant protection is required for most rubber products, which are based on diene rubbers. Ozone attack is different to oxidative ageing in that specific conditions are required for it to occur. These conditions are  

Many of the chemical antiozonants also provide powerful antioxidant protection. The most important of these are the paraphenylene diamines but other products also have commercial importance. 

Wax blends are also widely utilised for static ozone protection. The wax migrates to the rubber surface where it forms a protective film. This film cracks, however, when the product is flexed. The most comprehensive system uses a combination of both materials. The wax enhances the chemical system by carrying it to the surface more efficiently where it is required. 

Coupling agents are chemicals, which are able to form a strong, physical or chemical bond between a filler and the polymer in which it is used. By so doing, the properties of the composite can be significantly modified. Coupling agents are important technically, as they allow composite materials to be produced with enhanced performance characteristics. 

Coupling agents are discussed in detail in Chapter 4. In rubber systems, coupling agents can improve a wide range of properties, so much so that they are indispensible for high performance products such as power cable [9] and tyres [10,11]. 

---

Rubber Chemicals. Sodium nitrite is an important raw material in the manufacture of mbber processing chemicals. Accelerators, retarders, antioxidants (qv), and antiozonants (qv) are the types of compounds made using sodium nitrite. Accelerators, eg, thiuram [137-26-8J, greatly increase the rate of vulcaniza tion and lead to marked improvement in mbber quaUty. Retarders, on the other hand (eg, /V-nitrosodiphenylamine [156-10-5]) delay the onset of vulcanization but do not inhibit the subsequent process rate. Antioxidants and antiozonants, sometimes referred to as antidegradants, serve to slow the rate of oxidation by acting as chain stoppers, transfer agents, and peroxide decomposers. A commonly used antioxidant is A/,AT-disubstituted Nphenylenediamine which can employ sodium nitrite in its manufacture (see Rubber chemicals). 

Anorin-38 has also shown an interesting effect as a multifunctional additive (a single additive to replace many of the conventional additives) for natural rubber (NR). It showed excellent blending behavior and compatibility with NR. Aorin-38 enhances the tensile properties and percent elongation, decreases fatigue, acts as an antioxidant and antiozonant, and positively affects many of the other properties, apart from acting as a process aid and a cure enhancer [183-186]. 

In order to support and meet this demand, an all-around development has taken place on the material front too, be it an elastomer new-generation nanofiller, surface-modified or plasma-treated filler reinforcing materials like aramid, polyethylene naphthenate (PEN), and carbonfiber nitrosoamine-free vulcanization and vulcanizing agents antioxidants and antiozonents series of post-vulcanization stabUizers environment-friendly process oil, etc. 

Plastic and rubber additives are both commodity chemicals and specialties. The Handbook of Plastic and Rubber Additives [27] mentions over 13 000 products antioxidants and antiozonants amount to more than 1500 trade name products and chemicals [28], flame retardants to some 1000 chemicals [29] and antimicrobials to over 1200 products [30]. 

ASTM Standard D 3156-81, Thin-layer Chromatographic Analysis of Antidegradants (Stabilizers, Antioxidants and Antiozonants) in Raw and Vulcanized Rubbers, Annual Book of ASTM Standards, ASTM, Philadelphia, PA (1990). 

The deterioration of the physical properties of rubber products when exposed to service conditions also the controlled exposure of rubber samples to a variety of deteriorating influences in the evaluation of antioxidants and antiozonants. See Accelerated Ageing. 

The next step is the incorporation of various additives—compounding. Typical additives include fillers, processing aids, activators, age resistors, sulfur, antioxidants, and antiozone compounds, extenders, plasticizers, blowing agents, pigments, and accelerators. 

Almost all commercial polymers are susceptible to the attack of oxygen or traces of ozone in the atmosphere. Oxidation is the major cause of their discoloration, impairment of mechanical properties, and subsequent failure. It is accelerated by heat or sunlight. Antioxidants and antiozonants are added to the polymers to extend their useful temperature ranges and service lives and to allow outdoor application. Their 1967 consumption for various polymers is given in Table II. 

Aromatic amines, such as phenyl- -naphthylamine or condensation products of diphenylamine with acetone condensates, are excellent antioxidants and antiozonants but cause color development. From the sterically hindered phenols, monocyclic phenols, such as 2,6-di-teit-butyl-p-cresol, are less effective antioxidants but remain white and nontoxic during aging. They are, however, volatile and provide poor protection at elevated processing temperatures. Polycyclic phenols, such as 2,2 -methylene-bis (4-methyl-6-teit-butylphenol), are relatively nonvolatile, but become discolored by oxidation to a conjugated system. O Shea... 

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. 

In this work, the activities of amines as antioxidants and antiozonants are discussed in relation to their chemical structures. As a possible mecha-... 

As a measure of the activity of antioxidants and antiozonants, oxidation potential has been used (3, 15, 16, 19). Compounds having an oxidation potential lower than 0.44 volt seem to be effective as antiozonants (20), whereas good antioxidants are known to have the potentials between 0.7 and 0.9 volt (16). The Hiickel molecular orbital calculations... 

These results lead us to conclude that some amines are effective as both antioxidants and antiozonants for rubber, and their efficiencies depend on their oxidation potentials. The optimum potentials of amines exist at about 0.4 volt for antioxidants and 0.25 volt for antiozonants. 

As is shown in Figure 3 the highest occupied energy level is found to parallel well the oxidation potential in aniline and p-phenylenediamine derivatives. To elucidate the existence of the optimum oxidation potentials for antioxidation and antiozonization, the reactivity indices Sr and S for radical and nucleophilic reactions have been used. These are ... 

The relationships of oxidation potential to radical reactivity index Sr and nucleophilic reactivity index Sn illustrated in Figure 4 are very similar to those with antioxidation and antiozonization, where the maximum values were observed at 0.4 and 0.25 volt. Therefore, antioxidation seems to proceed by a radical mechanism in contrast to the nucleophilic type of antiozonization. Indeed, the antioxidation effect of amines toward NR, SBR, BR, and HR is well correlated with radical reactivity as shown in Figures 5-8. The protection of SBR solution by amines from oxidative degradation and the termination of chain reaction in the oxygen-Tetralin system are also shown as functions of Sr in Figures 9 and 10. 

Santoflex [Solutia]. TM for a series of rubber antioxidants and antiozonants. 

---