Antiozonants, amines

Environmental Impact of Ambient Ozone. Ozone can be toxic to plants, animals, and fish. The lethal dose, LD q, for albino mice is 3.8 ppmv for a 4-h exposure (156) the 96-h LC q for striped bass, channel catfish, and rainbow trout is 80, 30, and 9.3 ppb, respectively. Small, natural, and anthropogenic atmospheric ozone concentrations can increase the weathering and aging of materials such as plastics, paint, textiles, and mbber. For example, mbber is degraded by reaction of ozone with carbon—carbon double bonds of the mbber polymer, requiring the addition of aromatic amines as ozone scavengers (see Antioxidants Antiozonants). An ozone decomposing polymer (noXon) has been developed that destroys ozone in air or water (157). 

Antidegradants. Amine-type antioxidants (qv) or antiozonants (qv) such as the phenylenediamines (ppd) can significantly decrease scorch time. This is particulady tme in metal oxide curing of polychloroprene or in cases where the ppd had suffered premature degradation prior to cure. 

The hterature suggests that more than one mechanism may be operative for a given antiozonant, and that different mechanisms may be appHcable to different types of antiozonants. All of the evidence, however, indicates that the scavenger mechanism is the most important. All antiozonants react with ozone at a much higher rate than does the mbber which they protect. The extremely high reactivity with ozone of/)-phenylenediamines, compared to other amines, is best explained by their unique abiUty to react ftee-tadicaHy. The chemistry of ozone—/)-PDA reactions is known in some detail (30,31). The first step is beheved to be the formation of an ozone—/)-PDA adduct (32), or in some cases a radical ion. Pour competing fates for dissociation of the initial adduct have been described amine oxide formation, side-chain oxidation, nitroxide radical formation, and amino radical formation. 

Plasticiser/oil in rubber is usually determined by solvent extraction (ISO 1407) and FTIR identification [57] TGA can usually provide good quantifications of plasticiser contents. Antidegradants in rubber compounds may be determined by HS-GC-MS for volatile species (e.g. BHT, IPPD), but usually solvent extraction is required, followed by GC-MS, HPLC, UV or DP-MS analysis. Since cross-linked rubbers are insoluble, more complex extraction procedures must be carried out. The determination of antioxidants in rubbers by means of HPLC and TLC has been reviewed [58], The TLC technique for antidegradants in rubbers is described in ASTM D 3156 and ISO 4645.2 (1984). Direct probe EIMS was also used to analyse antioxidants (hindered phenols and aromatic amines) in rubber extracts [59]. ISO 11089 (1997) deals with the determination of /V-phenyl-/9-naphthylamine and poly-2,2,4-trimethyl-1,2-dihydroquinoline (TMDQ) as well as other generic types of antiozonants such as IV-alkyl-AL-phenyl-p-phenylenediamines (e.g. IPPD and 6PPD) and A-aryl-AL-aryl-p-phenylenediamines (e.g. DPPD), by means of HPLC. 

LDPE or HDPE extracts has been determined colorimet-rically at 430 nm by oxidation with H202 in the presence of H2S04 [66]. p-Phenylenediamine derivatives such as Flexzone 3C, used as antiozonants in rubber products, have been determined colorimetrically after oxidation to the corresponding Wurster salts [67]. A wide range of amine AOs in polyolefins has been determined by the p-nitroaniline spectrophotometric procedure [68]. Monoethanolamine (MEA) in a slip agent in PE film has been determined as a salicylaldehyde derivative by spectrophotometric quantification at 385 nm [69]. Table 5.6 contains additional examples of the use of 1JV/VIS spectrophotometry for the determination of additives in polymers. 

FD-MS is also an effective analytical method for direct analysis of many rubber and plastic additives. Lattimer and Welch [113,114] showed that FD-MS gives excellent molecular ion spectra for a variety of polymer additives, including rubber accelerators (dithiocar-bamates, guanidines, benzothiazyl, and thiuram derivatives), antioxidants (hindered phenols, aromatic amines), p-phcnylenediamine-based antiozonants, processing oils and phthalate plasticisers. Alkylphenol ethoxylate surfactants have been characterised by FD-MS [115]. Jack-son et al. [116] analysed some plastic additives (hindered phenol AOs and benzotriazole UVA) by FD-MS. Reaction products of a p-phenylenediaminc antiozonant and d.v-9-lricoscnc (a model olefin) were assessed by FD-MS [117],... 

Unsaturated and Vulcanized Rubbers. Oxidation occurs most readily at polymers with structural double bonds, such as natural rubber, polybutadiene, or polyisoprene. Aromatic amines and sterically hindered phenols are effective antioxidants. From the rubber antioxidants, 96.8 million pounds were amines, and 20 million pounds were phenols. Amines act also as antiozonants whereas phenols are not effective. Furukawa shows that amines have a lower oxidation potential which is a prerequisite for antiozonant action. 

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-... 

Antiozonant is regarded as a scavenger for ozone (II, 21) or its reaction product (4). Amine may react with ozone more rapidly than does rubber to give rise to amine-jty-oxide (12). A nucleophilic attack of pyridine on ozone was also proposed by Slomp and Johnson (22). 

This paper shows that Fukui s theory is plausible for interpreting the presence of the optimum oxidation potentials for amines to be effective as antioxidants or antiozonants. 

Amines as Antioxidants or Antiozonants for Rubber (13). Oxidative degradation of vulcanized rubber is evaluated from the depression in the tensile properties during aging in the Geer oven. 

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. 

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. 

On the other hand, antiozonization for the NR vulcanizate in Figure 11 and SBR solution in Figure 12 seems to be correlated better with the nucleophilic index Sn than with the radical index Sr. The direct reactivity of amines toward ozone is also associated with Sn as shown in Figure 13. 

This reference documents the chromatographic properties of over 100 rubber-related amine and phenolic antioxidants, antiozonants, guanidines, accelerators, and amine hydrochlorides. This reference examined the chromatographic characteristics of the cited additives and, thus, did not characterize acmal polymers. 

The influence of molecular structures and substituents on the antiozonant properties of a series of related aromatic diamine compounds was studied. The relative effectiveness of the compounds was determined by viscometric techniques and by comparison of the rate of degradation of protected vuicanizates. Results indicate that unsymmetrical p-phenylenediamine derivatives are less effective than analogous symmetrical compounds as antiozonants. The protective capacity of the antiozonants decreases as the size or number of the N-hydrogen substituents, or the distance between the amine groups, increases. The comparative stability of the free radicals of aryl diamines, in terms of the theory of resonance, is utilized to explain the relative inhibiting properties of the chemicals examined. 

Influence of Amine-Group Substituents. Figure 6 shows the influence of the number of amine-group substituents on the protective capacity of aromatic diamine chemicals. An increase in the number of AT-phenyl-group sustituents produces a corresponding decrease in the antiozonant properties of the diamine. 

Influence of Ring Substituents. The influence of para ring substituents on the antiozonant characteristics of aromatic diamine chemicals is shown in Figure 7. Data indicate that the protective capacity of the additives decreases as the distance between the amine groups of the aromatic diamines increases. 

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. 

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