Stabilization against Thermal-Oxidative Degradation

Because of the variety of degradation reactions, POM is stabilized using multi-component systems consisting of antioxidants and compounds that bind the secondary products (e.g., formaldehyde, protons) of autoxidation. The stabilizers allow numerous combinations that often have synergetic effectiveness. Sterically hindered phenols are mostly used as antioxidants here, polynuclear phenols are preferred because of their low migration tendency. In addition, sterically hindered amines are used as radical scavengers. 

A variety of compounds provide effective stabilization against the secondary products of autoxidation. They chemically bind cleaved formaldehyde and thus prevent its potential oxidation to formic acid and/or act as proton acceptors. Worth 

The effectiveness of various UV absorbers increases from benzophenone derivatives to benzotriazole derivatives to combinations of UV absorbers with HALS. Typical stabilizer loads range from 0.25 to 0.5%. The best light stability in POM is achieved using small particles of active carbon black, Table 3.6 [86]. 

Time to reduction of elongation at break to 40 % of initial value 

Light stabilizer Xeno test 450 (h) Open-air weatherin 

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Methylphenol is converted to 6-/ f2 -butyl-2-methylphenol [2219-82-1] by alkylation with isobutylene under aluminum catalysis. A number of phenoHc anti-oxidants used to stabilize mbber and plastics against thermal oxidative degradation are based on this compound. The condensation of 6-/ f2 -butyl-2-methylphenol with formaldehyde yields 4,4 -methylenebis(2-methyl-6-/ f2 butylphenol) [96-65-17, reaction with sulfur dichloride yields 4,4 -thiobis(2-methyl-6-/ f2 butylphenol) [96-66-2] and reaction with methyl acrylate under base catalysis yields the corresponding hydrocinnamate. Transesterification of the hydrocinnamate with triethylene glycol yields triethylene glycol-bis[3-(3-/ f2 -butyl-5-methyl-4-hydroxyphenyl)propionate] [36443-68-2] (39). 2-Methylphenol is also a component of cresyHc acids, blends of phenol, cresols, and xylenols. CresyHc acids are used as solvents in a number of coating appHcations (see Table 3). 

Defining a specific oxidative induction temperature as a quality criterion for the stability of plastics against thermal oxidative degradation - as demanded by GRl-GM13 [176] for PE-HD geo-membranes — is arbitrary and not very informative. However, it has been used in industry for several years. The fact that a material has an oxidative Induction time of 100 minutes says nothing about the quality of the material In service and its service life [177]. 

In the stabilization of plastics, we generally distinguish between stabilization against thermal-oxidative and photo-oxidative or light-induced degradation. Various... 

Effective thermai-oxidative stabilization is a prerequisite for good light stability. Thus, no disadvantageous material changes were observed over the years in polyamides stabiiized against thermal-oxidative degradation as long as extreme irradiation, heat and weather influences were exciuded [86]. 

Furthermore, stabilization against photooxidation is also obtained with stabilizers used against thermal-oxidative degradation (Cu-salts or antioxidants). Clear improvement in stabilizing effect is obtained by the following combinations ... 

A combined use of primary and secondary antioxidants is state-of-the-art in research and practical applications for stabilizing elastomers against thermal-oxidative degradation - just as in thermoplastics. Mainly phenolic antioxidants or phenols with sulfur compounds are used. 

Even at elevated temperatures (80 °C), stabilized PEAs are stable over long exposure times. For special application in the food industry, molding compounds without UV stabilizers are available antioxidants against thermal-oxidative degradation are generally required. Depending on the stabilization, the temperature indices according to UL 746 B are ... 

E. Richaud, X. Colin, C. Monchy-Leroy, L. Audouin, J. Verdu. Polyethylene stabilization against thermal oxidation by a trimethylquinoleine oligomer. Polymer Degradation and Stability 9A(3), 410-420 (2009). 

Stabilizers. Antioxidants provide protection against UV light and thermal oxidative degradation. Hindered phenols are the most common nonstaining antioxidants. For SBR latices, two additional type of stabilizers must be used. 

The complexes [Cu(S2CNEt2]2] and [Cu S2P(OPr )2 2] have been shown to be extremely effective scavengers for peroxy radicals and can be used to inhibit the autoxidation of hydrocarbons.99 Poly(2,6-dimethyl-1,4-phenylene oxide) can be effectively stabilized against thermal degradation by the bistriazene complex (41).100 The stabilizing action is thought to involve quenching of thermally excited states and the decomposition of hydroperoxides by the complex. 

Other Polymers. Various other polymers frequently require stabilization against thermal or thermo-oxidative degradation. Poly(vinylidene chloride) is stabilized by epoxides and sodium pyrophosphate. 

The most widely used antioxidants are sterically hindered phenols and bisphe-nols other additives are combined with phenols mostly in synergistic mixtures. The most recommended structures for stabilization of polyolefins against thermal oxidation and degradation are listed in Table 12.1. 

In another vein, secondary amines prepared by the reductive amination of appropriate carbonyl compounds with methyl 12,14-dinitrodehydroabietate (Fig. 4.22) gave promising results when used as stabilizers against thermal, photochemical and oxidative degradation of low-density polyethylene and ethylene-propylene elastomers [114—116]. This active role is not surprising given the well-known fact that aromatic amines are excellent free radical traps. 

Commercial products are additionally stabilized against thermal degradation with polyamides, hydrazine, urea, or thiourea. All of these additives react with formaldehyde or its reaction products, such as formic acid. Secondary and tertiary amines are also added to improve the resistance to oxidation, and carbon black or special UV stabilizers are used to increase light stability. 

Since the oxidation process is initiated by free chemical radicals, which are only very rarely formed under normal conditions, it starts only extremely slowly. However, the initiation of oxidation process is greatly accelerated by the influence of UV radiation or high temperatures, e.g. those used at polymer processing. One speaks of photo-oxidative degradation or thermal-oxidative degradation. Addition of antioxidants, which are effective even at low concentrations, stabilizes plastics against oxidative degradation as discussed in Chap. 2. 

The basic stabilization of PP (i.e., antioxidants and processing additions) is usually carried out by the polymer producer, immediately after polymerization, before separation, drying, and storage to minimize degradation of the polymer in the molten state at temperatures between 200 and 300°C. Finally PO needs a convenient stabilization against thermal and photo-oxidative degradation for the application in mind. 

Base stock functions that fall into the preservation area include volatility, pour point, solvency, colour, smell, toxicity, and stability against thermal stress, hydrolysis and shear. Biostability or bio-degradability may be required for different applications. Additive functions that fall under preservation include oxidation inhibition, dispersancy, detergency, demulsification, seal swelling, anti-foam and corrosion inhibition. 

M.M. Abdel-Aziz, H.A. Youssef, F. Yoshii, K. Makuuchi, A.A. El Miligy. Stabilization of radiation-vulcanized SBR against thermal oxidation. Polymer Degradation and Stability 46 2), 143-150 (1994). 

Protection of polymers against thermal and photo-oxidative degradation is achieved with appropriate stabilizers that ensure the desirable polymer properties throughout the entire service life of the polymer. Compatible and polymeric stabilizers usually give the best protection. In order to avoid migration and evaporation, polymeric stabilizers are used. 

The well-known susceptibility of vitamin C to thermal and oxidative degradation has focused interest in derivatives of increased stability. In general, partial modification of the enediol system leads to two isomers, both of which have markedly lower reducing power and are therefore stabilized against oxidation. However, vitamin C activities tend to decrease in these substituted derivatives. 

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