Hindered phenol antioxidants structures

One of the present authors (31) has developed a series of additives which combine the features of both free radical inhibitors and flame retardants of the tetrabromophthalimide or chlorendic imide type with hindered phenol antioxidant structures such as the following compounds ... 

Figures 10 and 11 show the structure of the hindered phenolic antioxidant Irganox 1010 (Ciba) and its negative ion APCI mass spectra, respectively. Separation was achieved under the following LC conditions Column Aqua Cl 8 (Phenomenex) 3 pm, 150x2.00 mm, 15% carbon loading, proprietary end capping. Column Temp 50°C. Injection volume 5 pi.

A hindered phenol commonly used as an antioxidant is 2,6-di-terf-butyl-4-meth-ylphenol (also known as butylated hydroxy toluene or "BHT"). Structures of BHT and other hindered phenol antioxidants are shown in Figure 8.3. Many of these complex structures have lengthy lUPAC names and are frequently called by trade names assigned by manufacturers, e.g., Irganox 1135 from Ciba (now BASF). 

Figure 8.3 Structures of hindered phenol antioxidants. (Reproduced with permission from J. Fink, A Concise Introduction to Additives for Thermoplastic Polymers, Wiley-Scrivener Publishing, Salem, MA, 2010).

The molecular structures of hindered phenols are often complex. They typically contain one tertiary butyl group and one methyl, or two tertiary butyl, groups in positions 2 and 6 of the benzene ring. Examples of commercial hindered phenol antioxidants include ... 

The hydrogen-bonded intermediate complex of a hindered phenolic antioxidant with a stable aminoxyl radical (TEMPO), used as a model for a hydrocarbon oxidant, has been isolated and its structure determined, as confirmation of the radical scavenging mechanism (79). 

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. 

Chemical structures of several phenolic antioxidants are presented in Scheme 1. Hindered phenolic antioxidants (AO) are the chemically most diversified group of stabilizers. Phenolic moities also constitute an important part of UV light absorbers and metal chelators. 

The evolution of HALS technology to meet the requirements of emerging polyolefin markets and applications is a story of continuous improvement and structure/property optimisation [19-21], Current trends in the industry are toward low-volatility, extraction-resistant, hindered phenolic AOs, such as oligomeric polysiloxane based, high-MW antioxidants... 

We have recently evaluated the chlorendic imide/hindered phenol for its effect on the oxygen index of polyethylene, and we found only a miniscule increase, not considred statistically significant, in comparison to the same loading of chlorine as chlorendic anhydride. We believe that if the antioxidant approach to flame retardancy is to be successful, special high temperature antioxidant structures must be designed for this purpose. 

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. 

Mesitylene. One of the principal derivatives of mesitylene is the sterically hindered phenol of the structure shown in Figure 4. Its trade name is Ethanox 330 and it is produced by Albemade Corporation (formedy Ethyl Corporation) (31). Ethanox 330 is an important noncoloring antioxidant and thermal stabilizer for plastics, adhesives, rubber, and waxes (qv) (32,33) (see Antioxidants). The oral toxicity of Antioxidant 330 is extremely low (oral 1D. in rats > 15 g/kg) since its large size, Cc4H7 0, effectively eliminates absorption from the gastrointestinal tract. 

Similarly, the antioxidant activity of vitamin E is centreed on its chainbreaking donor activity in-vitro rate studies on a-tocopherol have shown that it is one of the most efficient alkylperoxyl radical traps, far better than commercial hindered phenols such as BHT, 2,6-di- ferf.butyl-4-methylphe-nol. Its efficiency was attributed [30, 31] to the highly stabilised structure of tocopheroxyl radical (which is formed during the rate-limiting step, reaction 3) because of favourable overlap between the p-orbitals on the two oxygen atoms. 

Figure 2. The chemical structures of the two different types of antioxidants used in gasoline are phenylenediamines (PDA) and hindered phenols (such as BHT).

Unsaturated and Vulcanized Rubbers. Oxidation occurs most readily at polymers with structural double bonds, such as natural rubber, polybutadiene, or polyisoprene. Aromatic amines and sterically hindered phenols are effective antioxidants. From the rubber antioxidants, 96.8 million pounds were amines, and 20 million pounds were phenols. Amines act also as antiozonants whereas phenols are not effective. Furukawa shows that amines have a lower oxidation potential which is a prerequisite for antiozonant action. 

In the 1960s, the Emanuel Institute of Bioehemical Physics, Russian Academy of Sciences, initiated studies in the new field - chemistry and biology of antioxidants. The scientists of the Institute had to solve an important task - to find out whether the biological activity of antioxidants as inhibitors of radical reactions depends on their properties. For this purpose, nontoxic different-structure antioxidants were synthesized derivatives of hindered phenols and heterocyclic hydrocarbon hydroxy compoimds [12, 13]. The existence of homologous arrays of antioxidant derivatives made it possible to determine the structure-activity dependence and select the most efficient and least toxic compoimds. 

Phenolic and polyphenolic compounds are the most active dietary antioxidants (14). The structural variation of phenolic antioxidants directly influences their physical properties, resulting in differences in their antioxidant activity. BHA and BHT are examples of phenols, in which the aromatic ring contains alkyl groups (hindered phenols), which are extremely effective as antioxidants (11). 

This structure has many resonance forms that result in a highly stabilized radical. In eq 8.6, a phosphite is oxidized to a phosphate and the hydroperoxide is reduced to an alcohol. Phosphites are often used in combination with hindered phenols. (Please see references 4-6 for details on functioning of antioxidants.)... 

The main function of metal deactivators (MD) is to retard efficiently metal-catalyzed oxidation of polymers. Polymer contact with metals occur widely, for example, when certain fillers, reinforcements, and pigments are added to polymers, and, more importantly when polymers, such as polyolefins and PVC, are used as insulation materials for copper wires and power cables (copper is a pro-oxidant since it accelerates the decomposition of hydroperoxides to free radicals, which initiate polymer oxidation). The deactivators are normally poly functional chelating compounds with ligands containing atoms like N, O, S, and P (e.g., see Table 1, AOs 33 and 34) that can chelate with metals and decrease their catalytic activity. Depending on their chemical structures, many metal deactivators also function by other antioxidant mechanisms, e.g., AO 33 contains the hindered phenol moiety and would also function as CB-D antioxidants. 

FIGURE 1.41 Structural formulas [45] of important sterically hindered phenols used as antioxidants for thermoplastics. 

An important goal of antioxidant research has been to provide poly-phenols of high molecular weight and low volatility. Most of the commercial sterically hindered phenols have molecular weights in the range 300-600 and above 600. Figure 1.41 depicts structural formulas [45] of the important sterically hindered phenols used as antioxidants. 

---