Phenols, hindered, discoloration

Primary antioxidants for plastics are often hindered phenols. Hindered amines are used in rubbers, where the discoloration they sometimes cause is less of a concern than it is with plastics. (Even the hindered phenolics can cause discoloration in certain circmnstances, imless combined with phosphites, which are discussed ftuther below.)... 

Use of a good antioxidant is recommended for almost all neoprene compounds where color is not of importance, a staining antioxidant can be used. The substituted or hindered phenols, eg, Naugawhite (Uniroyal) and Antioxidant 2246 (American Cyanamid), are used where a minimum of product discoloration or staining is requited. 

New developments are hydroxylamines and lactones (for processing stability), which operate at an earlier stage during stabilisation. Lactone (benzofuranone) chemistry has been identified as commercially viable, and marks a revolutionary advance in comparison to hindered phenols and phosphites [18]. New lactone chemistry (Figure 10.1) provides enhanced additive compatibility, reduced taste and odour (organoleptics), resistance to irradiation-induced oxidation, and inhibition of gas fade discoloration. The commercial introduction of fundamentally new types of stabilisers for commodity and engineering polymers is not expected in the near future. 

The tendency of aliphatic ethers toward oxidation requires the use of antioxidants such as hindered phenolics (eg, BHT), secondary aromatic amines, and phosphites. This is especially true in polyether polyols used in making polyurethanes (PUR) because they may become discolored and the increase in acid number affects PUR production. The antioxidants also reduce oxidation during PUR production where the temperature could reach 230°C. A number of new antioxidant products and combinations have become available (115,120,124—139) (see Antioxidants). 

Fig. 4 Effect of hindered phenol concentration on extent of discoloration of extruded PP (Reproduced with kind permission of Polym Degrad Stab, 1999, 64 145)...

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... 

Cterically hindered phenols are used widely today as effective non- discoloring, nonstaining antioxidants for hydrocarbon systems, particularly rubbers and plastics. These compounds may be grouped in two broad classifications—monocyclic phenols represented by a compound such as 2,6-di-fer -butyl-4-methylphenol (I) and polycyclic phenols such as 2,2 -methylenebis(4-methyl-6-terf-butylphenol) (II). (Throughout this chapter, X represents tert-butyl). 

Hindered Phenols. Because of the relationship between photooxidation and discoloration, we became interested in the effect of hindered phenols which are well-known oxidation inhibitors. A preliminary study was carried out to determine the relative effectiveness of a variety of such compounds, many of which are commercially available. The additives were incorporated into polystyrene at 0.1% concentration, and 50-mil molded plaques were exposed to a carbon-arc Fade-ometer. 

Hindered phenols are used preferentially for processing and long-term heat aging of PO where their very low-discoloring properties are advantageous (Pospisil, 1998 Zweifel, 1998 Pospisil, 1993). Phenolic antioxidants are effective also for thermal stabilization of styrene-based polymers. Low amounts are used in PET and aliphatic PA. Common concentrations in plastics range between 0.025 and 0.3 %. 

Oxidative transformations of hindered synthetic phenols and a-tocopherol are analogous to the oxidation of various mono- and dihydric phenols of plant origin. Natural phenols present in fruits or green tea leaves are oxidized on contact with air and/or during fermentation (a process characteristic of tea leaves) and are transformed into dark colored quinoid systems, not harmful for human beings. It may be extrapolated that trace amounts of discoloring quinones or quinone methides arising from phenolic antioxidants in plastics are harmless as well. 

Non-discoloring, metal deactivator and antioxidant, acts as a hindered phenolic antioxidant. Typical end use applications include wire and cable insulation, film and sheet manufacture as well as automotive parts. 

Alkyl and aryl phosphite esters are also effective melt stabilizers. They are often used in combination with hindered phenols to give highly efficient melt stabilizing systems and to reduce discoloration of the polymer because of the oxidation products of the phenols present. Phosphites (particularly those derived from aliphatic alcohols and unhindered phenols) are, however, generally susceptible to hydrolysis. Consequently, moisture-sensitive phosphites affect adversely the handling characteristics (i.e., flow properties) of the additive package and are a source of other problems corrosion of metal surfaces, formation of dark colored spots, and gel formation. In practice, hydrolysis-resistant phosphites based on sterically hindered phenols are used, e.g., AOs 17 and 18, Table 1. 

Phenols are stronger AO/AR than HAS. Contrary to the transformation of phenols into quinone methides, discoloration-free products are formed from HAS. Combinations HAS/phenol are, therefore, used to obtain protection at minimum discoloration. The efficient protection of PP or of a block copolymer of propylene with 1.5% ethylene irradiated with a dose of 5 Mrad was reached with 0.1% 28, R = H and 0.05% Irganox 1076. Moreover, it was found that hindered phenolic AO prevent radiolysis of the ester type HAS (e.g. 28) [226]. Combinations of HAS with phenolic AO may, therefore, be recommended for commercial stabilization of y-irradiated hydrocarbon polymers [227]. 

Irradiation of PP in air leads to oxidative degradation, evidenced by discoloration and embrittlement. The extent of the degradation depends on crystallinity, MW, MWD, and chain mobility [Kadir et al., 1989 Kashiwabara and Seguchi, 1992 Williams, 1992]. Neat PP does not discolor on irradiation up to 100 kGy [Williams, 1992]. The antioxidants should be selected so as not to cause the discoloration. However, most commercial preparations containing phenolic antioxidants turn yellow on irradiation. Phenolic antioxidants produce stable phenoxyl radicals that convert into colored quinonoids. Other stabilizers and antioxidants are compounds that contain either phosphorous [Bentrude, 1965 de Paolo and Smith, 1968], sulfur [Jirackova and Pospisil, 1979], or hindered piperidine derivatives [Carlsson, et al., 1980 Felder et al., 1980 Allen et al., 1981]. A comprehensive list of stabilizers and their mode of action was given by Dexter [1992]. It is noteworthy that antioxidants and stabilizers are excluded from the crystalline regions [Winslow et al., 1966] thus they would provide protection only within the amorphous domains. 

The antioxidant used in thermoplastic polyurethane elastomers are hindered phenols, e.g., BHT, octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate, and pentaerythrityl-tetrakis-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate, and aromatic amines, e.g., 4,4 -di-tcrt-octyl-diphenylamine, as well as their combinations. Aromatic amines can, however, be employed only in very limited concentrations (250-550 ppm at most) because of their discoloring properties. 

Hindered phenols, like 2,6-di-rert-butyl-p-cresol, which in an oxidizing environment and at high concentrations transforms to stilbenequinone. This species absorbs visible light, and therefore adds to discoloration. It has also been found that hindered phenols, containing ortho or para methyl groups react with natural rubber latex in the presraice of free radicals. This bound antioxidant is much more efficient than conventional antioxidant [123]. 

While amines and some annular hydrocarbons are suitable chain terminators, hindered phenols such as di-l-butyl-p-cresol (alias butylated hydroxytoluene or BHT) are most popular because they avoid discolorization and they eliminate two free radicals per BHT molecule (Fig. 54.3). The resonance-stabilized aryloxy radical is protected by the bulky electron-releasing l-butyl groups in the 2 and 6 positions, so the hindered phenol can combine with a second peroxy radical but cannot combine readily with molecular oxygen or with another aryloxy radical nor abstract H atoms from the polymer to initiate a new firee-radical chain reaction. 

Ageing of polymers leads to drastic changes to their physical and mechanical properties. Hindered phenols are the most important antioxidants for polyolefins. However, some of the transformation products from these antioxidants can lead to discoloration during service life. In PP, mixtures of phosphites, hindered phenols and lactones are widely used [10-13]. Hindered phenols are very effective... 

The two major groups among the primary antioxidants are hindered phenolics and aromatic amines. The most widely used antioxidants in plastics are phenolics. The products generally resist staining or discoloration. However, they may form quinoid (colored) structures on oxidation. Phenolic antioxidants include simple phenolics, bisphenolics, polyphenolics, and thiobisphenolics. Hindered phenolics, such as butylated hydroxytoluene (BHT), high-molecular-weight phenolics, and thiobisphenolics, are the most popular of the primary antioxidants. 

Powerful synergism is achieved, for example, in the melt and thermal stabilisation of PP by using combinations of hindered phenols (CB-D) and phosphites and/or phosphonites (PD) and sulfur compounds (PD) in fact phosph(on)ites are seldom used alone. The latter enhance the melt stabilizing effect of hindered phenols (Table 1) and reduce discoloration of the polymer caused by phenol transformation products. The cooperative effect of hindered phenols (PhOH) and phosphites (P) occurs through two steps, whereby phenols scavenge alkylperoxyl radicals and phosphites decompose peroxides in a nonradical reaction which leads to enhanced melt stability of the polymer (Figure 7). Further interaction between the colored transformation products of phenol and the phosphite antioxidant, or its product(s), results in noncolored products, hence... 

Description Sterically hindered phenols (radical scavengers) good high-temperature, long-term thermal stabilizers good resistance to discoloration and extraction ... 

Stabilizers. Stabilizers are often added to slow down the rate of oxidation and uv aging. Heat stabilizers can be organic antioxidants (such as hindered phenols or aromatic amines), hydroperoxide decomposers, or metal salts. The latter are most commonly used in the form of copper halide mixtures. This system, which has the side effect of discoloring the nylon, acts as a regenerative free-radical suppressor. Above about 120°C, the copper halide system is by far the most effective... 

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