Antioxidants aromatic amine type

Rubber antioxidants are commonly of an aromatic amine type, such as dibeta-naphthyl-para-phenylenediamine and phenyl-beta-naphthylamine. Usually, only a small fraction of a percent affords adequate protection. Some antioxidants arc substitute phenolic compounds (butylatcd hydro -vamsole, di-tert-butyl-para-cresol, and propyl gallate). 

Aromatic amine-type antioxidants do not usually affect mechanical properties of polymers processed up to 250°C. They are used for dark-colored materials intended only for technical applications. Their efficiency is increased by the presence of OH groups [1]. The most commonly used are secondary amines... 

Process 4, conversion of peroxy radicals to hydroperoxides can be interrupted by traditional primary antioxidants (see Fig. 16). The fastest reacting primary antioxidants are the aromatic amines (e.g. Naugard 445). However, these materials yellow upon exposure to UV light which restricts their applieations. More common in adhesives are the hindered phenol types of which numerous types are available, with Irganox 1010 the most common choice for adhesives. 

The rate constant of Reaction 8.1 is much greater than the rate constant of Reaction 8.2, which means that antioxidants of this type can be used in very low concentrations with good effect. A typical thermoplastic would contain only 0.01-0.5% by mass of such an antioxidant. Typical compounds which work by this mechanism include substituted phenols and secondary aromatic amines. 

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. 

In an acetone extract from a neoprene/SBR hose compound, Lattimer et al. [92] distinguished dioctylph-thalate (m/z 390), di(r-octyl)diphenylamine (m/z 393), 1,3,5-tris(3,5-di-f-butyl-4-hydroxybenzyl)-isocyanurate m/z 783), hydrocarbon oil and a paraffin wax (numerous molecular ions in the m/z range of 200-500) by means of FD-MS. Since cross-linked rubbers are insoluble, more complex extraction procedures must be carried out (Chapter 2). The method of Dinsmore and Smith [257], or a modification thereof, is normally used. Mass spectrometry (and other analytical techniques) is then used to characterise the various rubber fractions. The mass-spectral identification of numerous antioxidants (hindered phenols and aromatic amines, e.g. phenyl-/ -naphthyl-amine, 6-dodecyl-2,2,4-trimethyl-l,2-dihydroquinoline, butylated bisphenol-A, HPPD, poly-TMDQ, di-(t-octyl)diphenylamine) in rubber extracts by means of direct probe EI-MS with programmed heating, has been reported [252]. The main problem reported consisted of the numerous ions arising from hydrocarbon oil in the recipe. In older work, mass spectrometry has been used to qualitatively identify volatile AOs in sheet samples of SBR and rubber-type vulcanisates after extraction of the polymer with acetone [51,246]. 

Four main types of antioxidants are commonly used in polypropylene stabilizer systems although many other types of chemical compounds have been suggested. These types include hindered phenolics, thiodi-propionate esters, aryl phosphites, and ultraviolet absorbers such as the hydroxybenzophenones and benzotriazoles. Other chemicals which have been reported include aromatic amines such as p-phenylenediamine, hydrocarbon borates, aminophenols, Zn and other metal dithiocarbamates, thiophosphates, and thiophosphites, mercaptals, chromium salt complexes, tin-sulfur compounds, triazoles, silicone polymers, carbon black, nickel phenolates, thiurams, oxamides, metal stearates, Cu, Zn, Cd, and Pb salts of benzimidazoles, succinic acid anhydride, and others. The polymeric phenolic phosphites described here are another type. 

The discovery that exposure to exogenous chemicals could lead to cancer in humans was first made in the late 18th century, when Percival Pott demonstrated the relationship between cancer of the scrotum and the occupation of chimney sweepers exposed to coal tar/soot. Other examples noted later were scrotal cancers in cotton spinners exposed to unrefined mineral oils, and cancers of the urinary bladder in men who worked in textile dye and rubber industries due to their exposure to certain aromatic amines used as antioxidants. Experimental induction of cancer by chemicals was first reported in detail by Yamagiwa and Ichikawa in 1918, when repeated application of coal tar to the ear of rabbits resulted in skin carcinomas. Over the next few years, Kennaway and Leitch confirmed this finding and demonstrated similar effects in mice and rabbits from the application of soot extracts, other types of tar (e.g., acetylene or isoprene), and some heated mineral oils. These researchers also observed skin irritation sometimes accompanied by ulcers at the site of application of the test material. Irritation was thought to be an important factor in skin tumor development. However, not all irritants (e.g., acridine) induced skin cancer in mice and conversely, some purified chemicals isolated from these crude materials... 

Utilizing a voltammetric measurement technique, the RULER quantitatively analyses the relative concentrations of antioxidants (hindered phenolic and aromatic amine) in new and used oils. This data can be trended to determine the depletion rates of the antioxidant protection package in the oil provided the instrument has been calibrated for that oil type. From pre-established limits, proper oil change cycles, potential interval extension or timely antioxidant replenishments can be determined. 

Antioxidants, which prevent oxidation by reaction with peroxide radicals. Such additives often contain aromatic compounds in their structure with relatively weak O-H and N-H bonds. Examples of such compounds are phenols, naphthols, aromatic amines, aminephenols, and diamines. This type of additive has strong reducing properties and reacts quickly with peroxide radicals. 

The primary antioxidants are normally broken down further into the classes of chain-breaking donor (CB-D) and chain-breaking acceptor (CB-A). CB-D additives interact with peroxy radicals, and are by far the commonest class of antioxidant in general use. They are represented by such additives types as hindered phenols and secondary aromatic amines. CB-A additives interact with alkyl radicals but, due to the rapid oxidation of such radicals, these additives are really useful only under low oxygen availability. CB-A types are represented by aromatic nitro and nitroso compounds, and a few speciality stable free radicals. Some transformation products of CB-D antioxidants can also act as CB-A species. 

Hydroxylamines [25] have the advantage that they are almost completely colourless, unlike the aromatic amines which are coloured, and the hindered phenols which form highly coloured breakdown species. This class of additive is exemplified by N,N-di(hydrogenated tallow)hydroxylamine (Irgastab FS-042 Ciba). This type of additive appears to operate as a radical scavenger and as a peroxide decomposer. Nitrones formed by oxidation of the hydroxylamine are also said to be stabilisers. Patents claim these additives as useful in reducing aldehyde content in polyester [28, 29], but there does not appear to have been any systematic study of their antioxidant capabilities in aromatic polyesters. 

Such substances thus act as inhibitors by keeping the concentration of potential initiator low. Aromatic amines and phenols are also widely used as antioxidants. Generally, these types of substances are believed to function by undergoing hydrogen atom transfer with alkylperoxy radicals in preference to hydrocarbons. The resulting radicals are relatively stable and do not propagate a chain process, but instead dimerize or react in other ways to effect chain termination. 

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... 

In modern vulcanization processes, NR is generally compounded (either under high or low temperature) with 0.5 to 1 wt% of accelerators, different concentrations of CBs (which act as a filler) (up to 45 wt% for tyre manufacturing), low concentrations of aromatic amines and phenols for antioxidation purposes and 5-8 wt% of sulfur. These types of vuleanized NR are commonly known as NR composites (when the filler dimension is on the microscale) or NR nanocomposites (when the filler dimension is on the nanoscale). NR-based composites or nanocomposites will be discussed in more detail in the following sections. 

The usual antioxidants, also used for other polymers, of the type of aromatic amines and phenols, as well as certain special inhibitors, are recommended as organic stabilizers for polyamides. 

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. 

Secondary aromatic amines are very reactive antioxidants [91-94] and, as with the phenolic antioxidants, the primary radical can react with radicals, leading to various decomposition products. The main drawback of this type of antioxiiimt is that many of these decomposition products are colored. For this reason their application is mainly limited to carbon black-filled polymers. 

The antioxidants used in adhesive formulations are similar to those used in rubber compounding and include materials such as the aromatic amines, substituted phenols, and hy-droquinoes. Elastomer and resin manufacturers typically incorporate antioxidants (0.1-0.3 wt %) in their products for protection during storage and shipment. Adhesive formulators will usually add additional antioxidant to protect the adhesive during processing and use. It is not unusual for an adhesive formulation to contain as many as three or four different types of antioxidants. 

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