Amine antioxidants and antiozonants

HPLC of polymer extracts has been used to determine dilauryl dithiopropionate, Irganox 1076 and distearyl-dithiopropionate in polyolefins [18], amine antioxidants and antiozonates in rubbers [16], Irganox 1076, Irganox 1010 and BHT in PE [19] and miscellaneous antioxidants, light stabilisers in polyolefins [20,21] and Santonox R, Ethanox 736, CAO-5, Irganox 1035, Irganox 259 and Topanol [22] in polyolefins and also the determination of antioxidants and accelerators in vulcanised rubber formulations [23-25]. 

Protivova and Pospisil [19] have reported on the behaviour of some amine antioxidants and antiozonants and some model substances (phenols, aromatic hydrocarbons and amines)... 

Figure 5.12 Separation of amine antioxidants and antiozonants by one-dimensional thin-layer chromatography. Solvent benzene - acetone - concentrated ammonium hydroxide (100 5 0.1) development distance 15 cm indicator 4% benzoyl peroxide in benzene. 1 = N-phenyl-l-naphthylamine 2 = N-phenyl-2-naphthylamine 3 = p-(p-tolylsulfonylamido)-diphenylamine 4 = nonylated diphenylamines 5 = octylated diphenylamines 6 = octylated diphenylamines 7 = octylated diphenylamines 8 = 4,4-dimethoxydiphenylamine 9 = 4-isopropoxydiphenylamine 10 = 4"-isopropyl-aminodiphenylamine 11 = N, N-diisopropyl-p-phenylenediamine 12 = N,N -6i-sec-h xtyi-p-p)Lieny emdizTmne. Reproduced from [119]...

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-... 

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. 

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. 

Staining antioxidants such as AJ-isopropyl-AT-phenyl-/>-phenylenediamine [101-72-4] (36) are preferred for the manufacture of tires (see also Amines, AROMATIC, PHEN YLENEDI amines). These potent antioxidants also have antiozonant activity and retard stress cracking of the vulcanized rubber. Carbon black (qv), used in tires for reinforcement, hides the color developed by the antioxidant. According to use requirements, up to 3% of an amine antioxidant having antiozonant activity is added prior to vulcanization. 

Vulcanised rubber has for many decades, been stabilised by p-phenylene diamines (PPD), as antioxidant and antiozonant. However, traditional PPD tend to migrate to the surface and discolour non-carbon black vulcanisates, as they are capable of extraction from the rubber and volatile. In general, all amine antidegradants are soluble in water and need special precautions and environmental labelling. 

Antioxidant and antiozonant types most commonly used are aromatic amines or phenolics, though others are also employed, and can be determined using a variety of techniques such as UV-visible spectrophotometry, FTIR, near-infrared spectroscopy, TEC, GC (if the material can be volatilized), supercritical fluid chromatography, and HPLC. Identification of unknown antioxidants requires a separation technique like chromatography followed by mass spectrometry, NMR, ETIR, X-ray crystallography, etc. Standardized TEC methods are given in ASTM D3156 and... 

Ward [119] has discussed in some detail the determination of phenolic and amine types of antioxidant and antiozonant in polymers by the chronopotentiometric techniqne, using a paraffin wax impregnated graphite indicating electrode and solntions of lithinm chloride and lithium perchlorate and acetonitrile in 95% ethanol as snpporting electrolytes. The precision obtainable for repeated chronopotentiometric runs in acetonitrile was fonnd to be better than 1.0% in cases in which electrode fouling did not occnr, and 1.7% when the electrode was fouled by electrolysis products. 

Lattimer and co-workers [25] have applied mass spectrometry (MS) to the determination of antioxidants and antiozonants in rubber vulcanisates. Direct thermal desorption was used with three different ionisation methods [electron impact (El), chemical ionisation (Cl), field ionisation (FI)]. The vulcanisates were also examined by direct fast atom bombardment mass spectrometry (FAB-MS) as a means for surface desorption/ionisation. Rubber extracts were examined directly by these four ionisation methods. Of the various vaporisation/ionisation methods, it appears that field ionisation is the most efficient for identifying organic additives in the rubber vulcanisates. Other ionisation methods may be required, however, for detection of specific types of additives. There was no clear advantage for direct analysis as compared to extract analysis. Antiozonants examined include aromatic amines and a hindered bisphenol. These compounds could be identified quite readily by either extraction or direct analysis and by use of any vaporisation/ionisation method. 

Antioxidants and antiozonants examined include aromatic amines (HPPD, DOPPD, DODPA and poly-TMDQ) and a hindered bisphenol (AO 425). These compounds could be identified quite readily by either extraction or direct analysis and by use of any vaporization/ionization method. 

Figure 1 shows the correlation between the relative tensile strength (TS)/(TS)0 or the time, tc, for the crack formation of NR gum vulcanizates and the oxidation potential of the amines employed as the stabilizer. The antioxidant or antiozonant activity of amine first increases with increasing oxidation potential and reaches a maximum at about 0.4 or 0.25 volt, beyond which the ability decreases with the increase in the potential. 

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... 

Nitroxides and benzoquinonediimines are formed from aromatic amines and diamines respectively as a consequence of amine involvement in antioxidant and/or antiozonant processes. Their participation in antioxidant regenerative mechanisms is suggested. Features of phenylenediamine involvement in antiozonant processes are discussed in relation to contemporary theories. 

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. 

Antiozonant Properties. Aromatic secondary diamines are the only class of organic chemicals able to reduce efficiently the ozone crack growth of vulcanizates under dynamic conditions and be acceptable at the same time from both the technical and toxicological points of view. The presence of a secondary aromatic amine moiety itself in a molecule is not a sufficient condition to attain antiozonants efficiency. (E.g., secondary monomaines are only antioxidants and flex-crack inhibitors without appreciable antiozonant activity. On the other hand, all N,N -disubstituted PD antiozonants are also efficient antoxidants and most of them also act as flex-crack inhibitors (1). Both these stabilization activities have to be considered in the complex antiozonant mechanism, together with some metal deactivating activity. 

It has been known for a number of years that FD-MS is an effective analytical method for direct analysis of many rubber and plastic additives. Major components and impurities in commercial additives can be assessed quickly, and the FD-MS data can be used to help determine what (if any) additional analytical characterization is needed. Lattimer and Welch showed that FD-MS gives excellent molecular ion spectra for a number of polymer additives, including rubber accelerators (diWocarbamates, guanidines, benzothiazyl, and thiuram derivatives) antioxidants (hindered phenols, aromatic amines) p-phenylenediamine-based antiozonants, processing oils, and phthalate plasticizers. Zhu and Su characterized alkylphenol ethoxylate surfactants by FD-MS. Jackson et al. analyzed some plastic additives (hindered phenol antioxidants and a benzotriazole UV stabilizer) by FD-MS. ... 

The LD-MS of Compound 8, which contains the Wingstay 300 antiozonant and an aromatic antioxidant, has characteristic peaks at m/z 268, 211, and 183 representative of the antiozonant and new peaks present at m/z 352, 288, 274, and 260. These latter three peaks are thought to represent the three molecular ions of the components of the antioxidant mixture in Goodyear s Wingstay 100, an aromatic amine antioxidant. 

Amine-type antioxidants or antiozonants such as the p-phenylenediamines (PPD) can decrease scorch time. As the PPD degrades and liberates free amine, scorch time decreases and cure rate is increased. 

Halobutyl rubbers with almost completely saturated backbones usually do not require extra antioxidants or antiozonants the rubber producer adds traces of stabilizers. For severe service circumstances, particularly in blends with unsaturated rubbers, the addition of some antidegradant may be necessary. In the selection of any antidegradant care should be exercized, particularly regarding amine derivatives that may interfere in the crosslinking process, but with proper technology vulcanizates with enhanced stability may be obtained [103]. Ozone resistance and weathering behavior of halobutyl rubber vulcanizates are also dependent on the curing system, on the nature and amount of fillers and plasticizers. 

Table 7.20 - The Behaviour of Amine Antioxidants, Antiozonants and Model Compounds in Gel Permeation Chromatography...

DNPD is a member of the p-phenylenediamine family of AOs however, it is used more as an effective antioxidant and copper inhibitor (to prevent copper from degrading the cured rubber compound) than it is as an antiozonant (which is how most p-phenylenediamines are used). One to two parts per hundred rubber (phr) of DNPD will impart good heat and oxidative aging resistance with many different types of rubber. DNPD is relatively low staining compared to many other amine AOs. But DNPD can cause some discoloration with bright light exposure. 

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