Thioether antioxidants

The pressurised dissolution/cooling procedure of Macko el al. [490], which uses a UV-transparent low-boiling point solvent, is fast and simple as no additional evaporation of the solvent, preconcentration or redissolution of the additive is necessary. Macko el al. [491] have given an extensive listing of HPLC analyses of aromatic antioxidants and UVAs which can be separated with n-heptane and n-hexane as the main component of the mobile phase. The method was also used for HPLC quantification of thioether antioxidants (Santonox R, Chimox 14 and Irganox PS 802) in MDPE [612],... 

Macko et al. [35] quantified thioether antioxidants (including Santonox R) in PE by HPLC, after extraction under pressure in an autoclave in 100 mL of n-heptane IPA (97 3 v/v). The temperature was ramped up to 160 °C over 15 min and this temperature was then held for 75 min. Only about 80% of the... 

HPLC QUANTIFICATION OF THIOETHER ANTIOXIDANTS IN POLYETHYLENE AFTER DISSOLUTION OF POLYMER UNDER PRESSURE IN ALIPHATIC SOLVENT... 

Unwanted degradation and oxidation processes can be avoided or at least suppressed for some time either by structural modiflcation of the polymer or by special additives. In practice, the addition of so-called antioxidants is particularly effective. Chemical substances that slow down oxidations and the following aging phenomena serve for this purpose. Antioxidants are sufficiently effective even in concentrations below 1 wt% and are added as early as possible to the polymer to be protected, e.g., already during the drying of powdery polymeric materials or during the preparation of granulates. Some of the most important so-called primary antioxidants are sterically hindered phenols and secondary aromatic amines secondary antioxidants are thioethers as well as phosphites and phosphonites. 

Flow and fusion promoters Thixotropic agents Antiaging Antioxidants PMMA and acrylate ester copolymers, MBS Fumed silica, clays Sterically hindered phenols, sec-aromatic amines, phosphates, thioethers... 

Secondary antioxidants or hydroperoxide decomposers (see Scheme 2.1) are typified by organosulfur species having reducing properties such as sulfides and thioethers. Tertiary phosphites also fall into this category (see Scheme 2.9). 

Another class of secondary antioxidants used in food contact plastics is thioether. The most common examples used in polypropylene, polystyrene and PVC are thiodipropanoic acid, didodecyl ester (DLTDP) and thiodipropanoic acid, dioctadecyl ester (DSTDP). Thioethers react with hydroperoxides to form sulphoxides as shown in Fig. 10.8. 

Sulfur compounds are known as catalytic hydroperoxide decomposers (PD-C) one antioxidant molecule destroys several hydroperoxide molecules by the action of intermediate sulfur acid moieties.Thioethers and esters of thiodipropionic acid and metal dithiolates are examples of commercial significance (see Table 1, AOs 19-24). 

Thioethers increase long-term stability in conjunction with phenolic antioxidants. Their use is limited to applications where possible effect on odour or taste and negative interaction with HALS (hindered amine light stabilizers) is not important. 

At first glance, the data seem a little inconsistent because the chemiluminescence was measured (at 25°C or 150°C) from samples which had stood at ambient temperature after irradiation in each case. Apparently the irradiation and ambient-temperature storage left the thioether ester essentially unchanged, so that upon heating to 150°C after various storage periods, nearly the full complement of antioxidant became available to retard autoxidation. The resulting chemiluminescence data correlate therefore with aging results at 60°C, at which temperature the thioether ester is an active antioxidant. ... 

In many stabilizer formulations, different types of stabilizers are used. As processing stabilizer phenolic antioxidants and phosphites are applied, for long-term heat stability phenolic antioxidants and thioethers are used and for UV stability, combinations of HALS with other types of UV stabilizers can be applied. HALS stabilizers show interactions with these types of stabilizers that can lead to synergisms as well as antagonisms. 

Phenyl-substituted phenols constitute a novel class of chain-breaking antioxidants (4). Their action with and without / -activated thioethers has been investigated in polypropylene (PP) (5,6,7). 

Antioxidant Activity of Some Phenyl-Substituted Phenols (VII) in PP at 180°C (Oxygen-Uptake Test). In Table I the results obtained with some phenyl-substituted phenols are shown and compared with some well-known ferf-butyl-substituted phenols. A general conclusion from this table is the marked synergism between phenyl-substituted phenols and -activated thioethers compared with the ferf-butyl-substituted analog. Since the measurements are done in a closed tube, the influence of the volatility of the thiodipropionates is not reflected in the induction periods. An explanation for the anomalous behavior of phenyl-substituted phenols in synergistic mixtures can be found in the results of the model reactions, carried out with phenoxyl radicals and sulfides or sulfoxides (see "Model Reactions ). 

From the preceding section it could be concluded that among the variety of phenyl-substituted phenols, 4-stearoxy-2,6-diphenylphenol (Vile) proved to be the most powerful chain-breaking antioxidant when used in combination with -activated thioethers. 4-Stearoxy-2,6-diphenylphenol is used now as the chain-breaking antioxidant and tested with a variety of -activated thioethers. 

The secondary antioxidants are usually sulfur compounds (mostly thioethers and esters of thiodipropionic acid) or trimesters of phosphorous acid (phosphates). A remedy for many types of discoloration in plastics is often the use of a phosphite or a thioether. Both have the ability to react with hydroperoxides, as those formed in Reaction 1.72, to yield noruadical products, following heterolytic mechanisms. The reaction of phosphites to phosphates as an example ... 

Poly(thioether)s should not be confused with poly(sulfide)s, in that the term poly refers directly to the sulfide linkage, i.e., —Sn—, but at the same time to a polymer. These types of polymers are used in a completely different field of application, e.g., additives for elastomers, antioxidants for lubricating oils, intermediates for the production of organic chemicals, insecticides, germicides, and as an additive to diesel fuels to improve the octane number and ignition qualities of these fuels. These polymeric types are not dealt with in this chapter. 

Uses Reducing agent in redox-catalyzed polymerization color stabilizer, modifier, catalyst for polyamides antioxidant intermediate for forming metallic salts used as stabilizers accelerator for org. peroxide catalysts improver of polysiloxane resins free radical promoter in emulsion polymerization promoter in polyester resin curing mfg. of self-extinguishing fibers (reacts with polyester resin and polyolefin) color/odor inhibitor, catalyst for some thermoplastics discoloration inhibitor in PE corrosion inhibitor on thin aluminum surfs. lubricity improver in polyphenyl thioether-based lubricants Manuf./Distrib. Akzo Nobel http //www.akzonobel.com, Ferro http //WWW. ferro. com... 

Polymer is degraded by heat, energy, UV or residues of catalyst and generates alkyl radicals. This alkyl radical reacts with oxygen and form peroxy radicals. These peroxy radicals abstract hydrogen from other polymer and forms alkyl radicals and hydroperoxide. The decomposition of hydroperoxide to alkoxy and hydroxyl radicals induces additional decomposition of the polymer chain. In order to stop the radical chain reaction of degradation, stabilisers such as phenolic antioxidant, phosphites, thioether and hindered amine light stabilisers (HALS) are added. 

Figure 6.2 shows some representative processes for degradation and stabilisation of polymers. Phenolic antioxidants react with peroxy radical and alkoxy radical by donation of hydrogen. Phosphites and thioether act as reducing agent for hydroperoxide, which is converted to alcohol. Hydroxylamine, which is a relatively new stabiliser, acts as hydrogen donor and hydroperoxide decomposer. 

Description Thioethers (long-term stability of polyolefins) phosphite/phosphonite (superior processing stability) both with phenolic antioxidants ... 

These antioxidants have many varieties, and the important products include 2, 6 er -butyl-4-methylphenol, bi-(3, 5 tert-butyl-4 hydroxyphenyl) thioether, and P-(3, 5-/cr/-butyl-4-hydrophenyl) propionate pentaerythritol tetraester. Such antioxidants are mainly used in plastics, synthetic fibers, latex, petroleum products, food, drugs, and cosmetics. The structure of hindered phenolic antioxidant is shown in Figure 4.1. 

Preventive antioxidants (sometimes referred to as secondary antioxidants), on the other hand, interrupt the second oxidative cycle by preventing or inhibiting the generation of free radicals. The most important preventive mechanism is the non-radical hydroperoxide decomposition, PD. Phosphite esters and sulphur-containing compounds, e.g. AO 12-18 in Table 1, are the most important classes of peroxide decomposers. The simple trialkyl phosphites (e.g. Table 1, AO 12) decompose hydroperoxides stoichiometrically (PD-S) to yield phosphates and alcohols, see reaction 4. Sulphur compounds, e.g. thioethers and esters of thiodipropionic acid and metal dithiolates (Table 1, AO 15-18, 31, 32), decompose hydroperoxides catalytically (PD-C) whereby one antioxidant molecule destroys several hydroperoxides through the intermediacy of sulphur acids, see reaction 5. References 1 and 2 give detailed discussion on antioxidant mechanisms. 

While HALS have been demonstrated to provide super light stabilization, co-additive interactions must also be carefully considered when formulating stabilization packages. Resins, phenohc antioxidants, thioethers, pigments, flame retardants, fillers and external pollutants all are known to interact with HALS under certain conditions. Here some... 

Thioethers such as DSTDP have been used in combination with primary antioxidants to provide extended lifetimes to polyolefins exposed to elevated temperatures. Unfortunately, the mechanism by which these thio-synergists function has been shown to produce sulfenic and sulfonic acids which are capable of further reacting with HALS. Figure 5 shows the effect of DSTDP on HALS performance. While both HALS-1 and HALS-3 show a strong negative interaction with DSTDP, HALS-5 is relatively imaffected. 

Alvinox P is utilized to protect polymers from thermal oxidation particularly during processing. It acts essentially as a decomposer of hydroperoxides. Optimum performance is obtained when the product is used in combination with antioxidants (hindered phenols and/or thioethers) which additionally protect the polymer against long-term heat degradation. 

Alvipack 122 is a highly efficient antioxidant blend of Alvinox FB (a primary phenolic antioxidant) and Alvlnox P (an organophosphite of low volatility and high hydrolytic stability) and DSTDP (a thioether)... 

---