Antioxidants limitations

Antioxidant Limitation or restriction Maximum permitted level (mg kg-1)... 

Two principal classes of antioxidant are effective in thermal oxidation. Chainbreaking or primary antioxidants limit the rate of the chain propagation steps (Eqs. 3-2 and 3-3) by trapping carbon- or oxygen-centered free radicals. Hydroperoxide decomposing or secondary antioxidants prevent chain initiation by interfering with ROOH. Photoantioxidants protect plastics exposed to photo-oxidation. 

Dittmar, T B. Ondruschka J. Haupt M. Lauterbach. Improvement of the oxidative stability of fatty acid methyl esters with antioxidants—limitations of the rancimat test. Chemie Ing. Techn. 

The most popular natural antioxidants on the market are rosemary extracts and tocopherols. Natural antioxidants have several drawbacks which limit use. Tocopherols are not as effective ia vegetable fats and oils as they are ia animal fats. Herb extracts often impart undesirable colors or flavors ia the products where used. In addition, natural antioxidants cost considerably more than synthetic ones. Despite this, the pubHc s uncertainty of the safety of synthetic antioxidants continues to fuel the demand for natural ones (21). 

Applications. These materials are stiU in developmental infancy. Current production is limited to one commercial process in Europe and a demonstration-scale process in North America. The lignins produced in these processes have potential appHcation in wood adhesives, as flame retardants (qv), as slow-release agents for agricultural and pharmaceutical products, as surfactants (qv), as antioxidants (qv), as asphalt extenders, and as a raw material source for lignin-derived chemicals. 

Diarylamiaes are of the greatest iadustrial importance as stabili2ers and antioxidants (qv) for polymers, stabili2ers for explosives, polymeri2ation inhibitors, and ia dyes. Today, the use of these materials as antioxidants is essentially limited to derivatives of diphenylamine siace /V-phenyl-2-naphthylamine is ao longer used. 

Diarylamiaes fuactioa as mbber antioxidants by breaking the peroxidative chain reactions leading to mbber deterioration. Nearly all commercial synthetic mbbers (see Elastomers, synthetic), including neoprene, butyl, styrene—butadiene, and the acrylonitrile—butadiene mbbers, can be protected with about 1—2% of an alkylated diphenylamine. DPA itself is not used as a mbber antioxidant. An objectionable feature of these antioxidants is that they cause discoloration and staining which limits their use to applications where this is not important. 

The major use of this compound is in the production of mordant and acid dyes. 2-Amino-4-nitropheno1 also has found limited use as an antioxidant and light stabilizer in butyl mbbers and as a catalyst in the manufacture of hexadiene. The compound has been shown to be a skin irritant and continuous exposure should be avoided. Toxicological studies indicate that it is nonaccumulative (162). 

Solubility. Another desirable property of a degradant is its high solubihty in mbber but poor solubihty in water and solvents that come in contact with mbber. Poor solubihty in the mbber means that only small quantities of antioxidants can be dissolved without producing a bloom. As an example, N,lSf-diphenyl- phenylenediamine (DPPD) has limited use because of its poor solubihty in mbber. On the other hand, phenohc and phosphite antioxidants have high solubihty and bloom is not a problem. 

We have shown that known reaction of luminol with peroxydisulphate at low luminol concentrations takes place in the regime of controlled generation of SO ion-radicals at spontaneous destruction of peroxydisulphate. The detection limit for various types of antioxidants in water using this reaction is varied from 10 to 10 M. It is possible also to determine some polluting admixtures present in the atmosphere. The reagent used is the mixture of the luminol, base and K S O, which, once prepai ed, could be used during a working day. 

The detection limit per chromatogram zone is 50—200ng for lipids [11], 200— 400 ng for antioxidants [3] and several ng for ascorbic acid. 

The prepolymerization reactor for HIPS is similar ( ). A solution of rubber and styrene monomer is charged to the reactor along with catalysts, antioxidants, and other additives, and the temperature program is carried out until the desired conversion is reached. This is usually close to the point where increasing viscosity seriously limits mixing and temperature control. 

Limited compounding flexibility due to the reaction of peroxides with other compounding ingredients for example, with antioxidants, plasticizers, and resins... 

The changes in properties observed on aging of different elastomers and their vulcanizates, and of many other polymeric materials, are well known. Antiozonants and antioxidants are employed to limit these changes. However, the most effective antioxidant for one material may be ineffective. 

The rate of peroxide decomposition and the resultant rate of oxidation are markedly increased by the presence of ions of metals such as iron, copper, manganese, and cobalt [13]. This catalytic decomposition is based on a redox mechanism, as in Figure 15.2. Consequently, it is important to control and limit the amounts of metal impurities in raw rubber. The influence of antioxidants against these rubber poisons depends at least partially on a complex formation (chelation) of the damaging ion. In favor of this theory is the fact that simple chelating agents that have no aging-protective activity, like ethylene diamine tetracetic acid (EDTA), act as copper protectors. 

In the case of carbohydrates blue chromatogram zones are produced on a yellow background that slowly fades [2]. Steroids, vitamins, antioxidants, phenols and aromatic amines yield, sometimes even at room temperature, variously colored chromatogram zones [5]. -Blockers and laxatives also acquire various colors [7, 10]. The detection limits are in the nanogram to microgram range [S]. 

This work demonstrated that an airlift system was suitable for the cultivation of Haematococcus pluvialis, one of the most effective microorganisms that could produce high potential antioxidant carotenoid, astaxanthin. Aeration was shown to be crucial for a proper growth of the alga in the airlift bioreactor, but it must be maintained at low level, and the most appropriate superficial velocity was found to be at the lower limit of the pump, i.e. 0.4 cm s". ... 

---