Phenolic and amine antioxidants

VasiTeva et al described a method for the determination of lonol and 4,4 -isopropylidenediphenol in PVC by anodic voltammetry. The sample is dissolved in 

Phenolic (and amine) antioxidants in extracts of PVC have been titrated electrometrically with lithium aluminium hydride, using platinum or silver electrodes. Small amounts of water in the sample or analysis solvent have an influence on the results obtained by these procedures. 

Electrophoresis is a technique worthy of further consideration for the analysis of antioxidants in PVC. Sawada et al report success l separations by coupling the antioxidants with p-diazobenzene sulphonic acid before electrophoresis. Amine antioxidants are coupled in acetic acid and phenolic antioxidants in sodium hydroxide-ethanol. Electrophoresis was carried out in 1% w/v methanolic sodium borate. 

Procedures involving solvent extraction followed by thin-layer chromatography have been described for the determination of phenolic and amine types of antioxidants and optical whiteners in PVC. 

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GC is extensively used to determine phenolic and amine antioxidants, UV light absorbers, stabilisers and organic peroxide residues, in particular in polyolefins, polystyrene and rubbers (cf. Table 61 of Crompton [158]). Ostromow [159] has described the quantitative determination of stabilisers and AOs in acetone or methanol extracts of rubbers and elastomers by means of GC. The method is restricted to analytes which volatilise between 160 °C and 300 °C without decomposition. A selection of 47 reports on GC analysis of AOs in elastomers (period 1959-1982) has been published... 

A collection of UV spectra of plasticisers, fluorescent whitening agents (optical brighteners), UV absorbers, as well as of phenolic and aminic antioxidants was published by Hummel and Scholl [21]. UV absorbance data for isolated chromophores are listed elsewhere [22]. A general UV atlas of organic compounds is available [23]. 

There have been some examples of the use of LDMS applied to the analysis of compounds separated via TLC, although not specifically dealing with polymer additives [852]. Dewey and Finney [838] have described direct TLC-spectroscopy and TLC-LMMS as applied to the analysis of lubricating oil additives (phenolic and amine antioxidants, detergents, dispersants, viscosity index improvers, corrosion inhibitors and metal deactivators). Also a series of general organics and ionic surfactants were analysed by means of direct normal-phase HPTLC-LMMS [837]. Novak and Hercules [858] have... 

Crompton [21] has reviewed the use of electrochemical methods in the determination of phenolic and amine antioxidants, organic peroxides, organotin heat stabilisers, metallic stearates and some inorganic anions (such as bromide, iodide and thiocyanate) in the 1950s/1960s (Table 8.75). The electrochemical detector is generally operated in tandem with a universal, nonselective detector, so that a more general sample analysis can be obtained than is possible with the electrochemical detector alone. 

The most widely used types of radical scavengers are phenolic and aminic antioxidants. The use of organometallic complexes of transition metals as antioxidants is becoming more prevalent, especially in engine oils organomolybdenum compounds are of particular importance. 

The above result, that the antioxidant response in two-stage hydroprocessed oils is clearly better than in solvent-refined base stocks, has recently been confirmed [76]. It was demonstrated that phenolic-based formulations, blended into two-stage hydroprocessed oils, gave the greatest oxidative stability when total and polynuclear aromatics were lowest. Most fully formulated turbine oils make use of the synergistic interaction between phenolic and aminic antioxidants, see Reaction (4.70). Thus, depending on the performance requirements and the base oil composition, phenol/amino ratios of 1 1 to 4 1 are used [77],... 

TLC has been used extensively for the determination of phenolic and amine antioxidants and UV stabilisers in plastics and rubber [17, 32, 36-58]. 

The principal classes of primary antioxidants for rubber remain the pheno-lics (nonblack articles) and amines (black-filled articles). Among the amine t5q>es are the substituted diphenylamines,para-phenylenediamines (PPD, (10), where R,R = alkyl or aryl), and polymeric trimethyl-dihydroquinoline (TMQ). Representative phenolic antioxidants are given in Table 14, and selected amine antidegradants are given in Table 15. The majority of tire and mechanical goods made from general-purpose rubbers are protected by the phenolic and amine antioxidants. 

METHOD 11 - DETERMINATION OF PHENOLIC AND AMINE ANTIOXIDANTS, ULTRAVIOLET ABSORBERS AND ORGANOTIN STABILIZERS IN POLYALKENES AND OTHER POLYMERS. THIN LAYER CHROMATOGRAPHY. ... 

Determination of phenolic and amine antioxidants, ultra-violet... 

It was known that the reaction of phenolic and amine antioxidants with peroxide radicals proceeds in the presence of hydroxy and amine groups. The fact that the reaction products of sulphur-containing antioxidants studied with peroxide radicals do not catalytically decompose CHP indicates that participation of hydroxy and amine groups in the formation of an effective catalyst for the decomposition of hydroperoxides. 

Examination of the history of antioxidants such as hindered phenols and amines shows a move from low-MW products to higher-MW products. Specifically, polymer industries have abandoned the use of, e.g., butylated hydroxy toluene (BHT) in favor of tetrakismethylene (3,5-di-f-butyl-4-hydroxydrocinnamate)methane (see Figure 15.9). Likewise, polymeric HALS, like poly-methylpropyl-3-oxy-(4(2,2,6,6-tetramethyl)piperidinyl) siloxane, replaced the low-MW hindered amine Lowilite 77 (see Figure 15.10). The next obvious step was to produce a new class of stabilizers. 

The ability of esters to form hydrogen bonds with polar reactants is especially important for the reactions of peroxyl radicals with antioxidants such as phenols and amines. Amines form hydrogen bonds with ester groups. The hydrogen bonding lowers the activity of antioxidants as acceptors of peroxyl radicals (see Chapters 14 and 15). 

The rare example of synergistic action of a binary mixture of 1-naphthyl-A-phcnylaminc and phenol (1-naphthol, 2-(l,l-dimethylethyl)hydroquinone) on the initiated oxidation of cholesterol esters was evidenced by Vardanyan [34]. The mixture of two antioxidants was proved to terminate more chains than both inhibitors can do separately ( > /[xj). For example, 1-naphtol in a concentration of 5 x 10 5 mol L-1 creates the induction period t=170s, 1 -naphthyl-A-phenylamine in a concentration of 1.0 x 10-4 mol L 1 creates the induction period t = 400s, and together both antioxidants create the induction period r = 770 s (oxidation of ester of pelargonic acid cholesterol at 7= 348 K with AIBN as initiator). Hence, the ratio fs/ZfjXi was found equal to 2.78. The formation of an efficient intermediate inhibitor as a result of interaction of intermediate free radicals formed from phenol and amine was postulated. This inhibitor was proved to be produced by the interaction of oxidation products of phenol and amine. 

The degradation of polymers is mostly promoted by autoxidation. The propagation of autoxidation can be inhibited by antioxidants (e.g. hindered phenols and amines). 

Commercially available antioxidants include phenols and amine derivatives the latter, though generally more effective, have the drawback of alteriiig the coloration of dyed products. These additives are necessary to prevent, to some extent, the process of thermal oxidation of rubbers, though it has to be borne in mind that the stability of rubbers is primarily determined by the chemical nature of the chains as well as by the cross-links that define network structure. 

Secondary antioxidants react with hydroperoxides to produce non-radical products and are therefore often termed hydroperoxide decomposers . They differ from priniaiy phenols and amines in that they are decomposed by reaction with hydroperoxide, rather than containing it. They are particularly useful in synergistic combinati(His with primary antioxidants.Systems that do not contain a phenolic... 

Oxidation Oxidation of the lubricant basestock leads to viscosity increase and consequent issues of pumpability, fuel economy and ultimately oil starvation. Oxidation products also include organic acids which can cause corrosion and deposits around the engine but particularly in the cooler parts of the engine, discussed previously in detail in Chapters 4 and 7. Dedicated antioxidants such as phenols and amines are used to control oil oxidation, and the antiwear agent ZDDP also acts as a powerful antioxidant. 

ZDDP functioning as an antioxidant can be replaced by ashless antioxidant components, such as hindered phenols and amines. Whilst effective, they are more expensive than ZDDP and can also compromise elastomer compatibility. Additionally, depending on the level of ashless antioxidant in the formulation, then-use may mean that product material safety labelling is more alarming to the end consumer. 

Both chain-terminating oxidation inhibitors, e.g. hindered phenols and amines, and peroxide-destroying inhibitors, e.g. dithiophosphate and dithiocarbamates, can be included in marine formulations. Mixtures of phenols and amines are often used for synergy but they must have good high-temperature performance. The sulphur-containing oxidation inhibitors also have extremely useful anti-wear properties. Oxidation inhibitors can be used advantageously in some base oils refined from low sulphur crudes and in synthetic basestocks. They compensate for the lack of natural antioxidant species. 

Due to the free-radical character of both the y-radiation induced transformation and oxidation in the post-irradiation phase, free-radical scavenging stabilizers - phenols and amines - effectively protect PO [11,226,227]. This was confirmed by excellent antioxidant/antiradiant effects of DP A, PNA and PD in particular in the post-y-irradiation phase of PO and elastomers [11]. Oligomeric DHQ 21 or its combination with 9b efficiently stabilized EPDM or X-LPE used as insulation materials for electric cables exposed to irradiation doses up to 2MGy at a dose rate 300 Gy h 1 [242]. 

AIBN-initiated oxidation of cumene and Tetralin in the presence of deuterated amines were unsuccessful. They proposed an alternative mechanism involving reversible formation of a complex of antioxidant with peroxy radical as the kinetically controlling process. We observed an isotope effect, dAh = 1.8, consistent with the hydrogen-donation mechanism in the retarded oxidation of SBR polymer with deuterated amines (7,8). Our results were confirmed by observation of significant isotope effects in the initial stage of oxidation of purified cfc-l,4-polyiso-prene with both hindered phenols and amines (9). Table I shows the effect of temperature and antioxidant concentration on the rates of oxidation and the observed deuterium isotope effects. 

Premature cure at processing tenqperature ("scorch at 121 0 is conveniently delayed or prevented by snail amounts of various phenolic and amine free-radical inhibitors and antioxidants. When the inhibitor concentration is properly chosen, the retarding effect disappears on the curing temperature (177 0. 

Among common antioxidants for plastics, there are phenolics and amines (primary antioxidants), and phosphates and thioesters (secondary antioxidants). 

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