Primary and Secondary Antioxidants

The term primary antioxidant is used to denote those additives that suppress oxidation over the lifetime of the product, whereas the main function of secondary antioxidants is to protect the polymer for the much shorter period when they are being processed. Nevertheless secondary antioxidants do have a lesser function during service life, as mentioned below. Primary antioxidants are also called chain-breaking antioxidants because they break the ehain of events that lead to oxidation. 

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.) 

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  

Some phenolic compoimds act as metal deactivators, preventing degradation from being accelerated by the copper carboxylates that tend to form at the interface between copper-containing alloys and polyolefins in wire and cable insulation. Oxalyl bis (benzylidene hydrazide) is also used in polyolefins contacting copper or brass. 

Secondary antioxidants work by preventing the formation of free radicals. Some of them will decompose hydroperoxides by a safe reaction before they get the chance to generate fiee radicals. Hydroperoxide decomposers fall into two categories some act by a catalytic mechanism. These include the sulfur-containing acids that are formed by the oxidation of thiodipropionate esters or metal dialkyldithiocarbamates. The last-mentioned, if they contain a transition metal, are also ultraviolet light absorbers. An alternative type of hydroperoxide decomposer acts by a stoichiometric mechanism, namely the phosphite esters. 

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When two antioxidants are used together, a synergistic improvement in activity usually results. Synergism can arise from three combinations (1) homosynergism — two chemically similar antioxidants (for instance, two hindered phenols) (2) autosynergism — two different antioxidants functions that are present in the same molecule (3) heterosynergism — the cooperative effect between mechanistically different classes of antioxidants, such as the combined effect of primary and secondary antioxidants. Thus, combinations of phenols and phosphites are widely used to stabilize synthetic rubbers. 

Very often, antioxidants are used in combinations to ensure maximum activity and typically, a commercial additive system may comprise both a primary and secondary antioxidant species, although the total concentration remains <1 wt%. Scheme 2.10 schematically shows... 

R + R. OH - RH + R. o flow a combination of primary and secondary antioxidants functions in a SCHEME2.8 Stabilizing polyolefin matrix.82 Some metal-chelate scavengers may also be based on activity of chain-breaking, a tertiary phenolic structure, thereby introducing two antioxidant properprimary antioxidants. ties into the same molecule. 

SCHEME 2.10 Combined stabilizing activity of primary and secondary antioxidants. 

Use of both primary and secondary antioxidants usually provides a synergistic effect, where the combined effect of two or more stabilisers is greater than the sum of the effects of the individual stabilisers. It is common practice to include both a phosphite, such as tris(t-butylphenyl)phosphite and a hindered phenol, such as octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyljpropionate to provide improved heat stabilisation in polyolefin formulations. 

The protection of polymers against high doses (20 - 1000 kGy) requires efficient additives preventing and/or stopping chain reaction type oxidative degradation. Primary and secondary antioxidants work well here in synergy. Commercial raw materials are available for radiation-sterilizable medical devices made out of polyolefins and other thermoplastics. Similarly, polymer compounds of suitable formulae are offered commercially for high-dose applications of polymers in nuclear installations. 

It is an important fact, that there is an explicit synergistic effect observed by combining primary and secondary antioxidants. A well known example of such synergistic mixtures is the use of thiodipropionates together with sterically hindered phenols for long-term stabilization of polyolefins. Of course, the different polymers and different applications require laborious optimization of each case. 

Because primary and secondary antioxidants differ in their mechanism of attack to prevent oxidative degradation, in practice both type of antioxidants are often used together to obtain the best results. Often such combinations provide a synergistic effect, where the combined antioxidant package provides greater protection than the sum of the two alone. However, antioxidants can also interact in an antagonistic fashion, so proper pairing is needed. 

Sulfoxides themselves yield, on further oxidation, even more powerful hydroperoxide decomposers than the original sulfides, in that they are able to destroy several equivalents of hydroperoxides. This catalytic effect is explained by the intermediate occurrence of sulfenic acids and sulfur dioxide. The fact that the phenomenon of synergism, which is defined as a cooperative action such that the total effect is greater than the sum of two or more individual effects taken independently, is often observed when primary and secondary antioxidants are combined ahs been explained with the concept of the simultaneous occurrence of the radical reactions (e.g.. Equation 1.72) and the nonradical hydroperoxide decomposition (e.g.. Equation 1.74 and Equation 1.75). 

As mentioned above extra stabilization is needed in some POs, particularly PP, to prevent induced oxidation and aging from sterilization treatments. Radiation-stabilized materials require a carefiil mix of primary and secondary antioxidants for optimum stabilization. However, given the multiple factors affecting sterilization degradation and the compoimds specialized nature, suppliers often conceal their stabilization formulations as proprietary information. 

Table 4.3 lists the chemical types of primary and secondary antioxidants and their major resin applications. Through the remainder of this chapter, antioxidants will be addressed by type based on overall chemistry. The class of antioxidant merely describes its mode of stabihzation. 

There are over 70 suppliers of antioxidants worldwide. Numerous suppliers offer both primary and secondary antioxidants to complete their product line. However, very few actually manufacture both primary and secondary antioxidants, since the products are based on different manufacturing routes, processes, and feedstock sources. As a result, it is quite common in this industry to resell products produced by another company. Table 4.4 displays selected suppliers of antioxidants by type. 

Unlike primary antioxidants, all secondary antioxidants work by decomposing reactive species like peroxides as shown in Fig. 4.9 for reactions (7) and (8). They do not have the ability to trap radicals initially (9). However, one of the most popular phosphite secondary antioxidants is made from three phenolic groups (Alkanox 240, Fig. 4.10). As those groups are made available, they have primary activity. Unfortunately, most phosphites typically do not become effective until 180-200°C. Therefore, they cannot work well by themselves. Primary and secondary antioxidants work by different mechanisms and often are synergistic. 

Antioxidants can be divided into two basic classifications primary and secondary antioxidants. Primary antioxidants interrupt oxidation degradation by tying up the free radicals. Secondary antioxidants destroy the unstable hydroperoxides that function as sources of free radicals during oxidative degradation. 

In this simple model description of antioxidant fimction, it can be seen, that a combination of primary and secondary antioxidants ensures a particularly effective stabilisation. Synergies resulting from mixing different antioxidants are still the subject of scientific investigation. Specific packages of stabilisers have been used as accepted standards in practice for a considerable time. A typical stabiliser package for polyethylene and polypropylene contains a phosphite and a phenolic antioxidant. (Zweifel 2001) reports on developments in antioxidants offered. 

Traditional antioxidants are classified as either primary or secondary types depending on their mode of action. Primary antioxidants act by trapping free radicals, usually hydroperoxy radicals, through donation of a labile hydrogen to the radical species. Secondary antioxidants interfere with the propagation steps of au-toxidation by decomposing hydroperoxides to form stable, nonradical species. It is quite common for a combination of primary and secondary antioxidants to be used to provide the maximum stabilization of a plastic. Use of antioxidants in plastics is ubiquitous, since nearly all resin types require some form of stabilization in order to provide useful and durable materials. 

Different types of primary and secondary antioxidants, or the same types of antioxidants with different molecular structures, have different functions and application effects and both have their own strengths. A composite antioxidant is compounded with two or more different types of antioxidants or with different varieties of the same type that are mutually complementary, thereby achieving optimum antithermal oxidation and aging effects with minimal amount of antioxidants and lowest cost. 

They tend to react preferentially, preventing the regeneration of new free radicals from the decomposition of the hydroperoxides. Primary and secondary antioxidants are often used together because they exhibit a synergism which provides an effective mechanism for polymer stabilization. 

Further studies have focused on novel ways to combat foam degradation. Co-additives can have both positive and negative influences on scorch development and discoloration. Secondary antioxidants such as phosphites and thioesters contribute to maintenance of pol3uner integrity and color stability. An understanding of co-additive mechanisms and interactions aids the successful development of stabilization packages specifically formulated for defined applications. The impact of both primary and secondary antioxidants on scorch inhibition will be reported in this work. 

The second type of UV stabilizer is hindered amine light stabilizers (HALS) that are an extremely efficient stabilizer against the photodegradation process. They are compounds that do not absorb UV radiation, but act to inhibit degradation. Relatively high levels of stabilization are achieved with relatively low levels of concentration because the HALS are not consumed during the stabilization process (unlike primary and secondary antioxidants). 

Both working principles can be combined in one chemical compound and combinations of primary and secondary antioxidants are state-of-the-art. The application concentration of antioxidants ranges between 0.03 and 0.3 wt.%, depending on the plastic in special cases the concentration may exceed 1 wt.% [525]. 

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. 

The best-known representative of the class of segmented polyether esters is the combination of polybutylene terephthalate as the hard component with polyether glycol as the soft component. The presence of polyether components causes a sensitivity to thermal-oxidative degradation in this class of materials. In non-stabilized form, these polymers cannot be processed. Their main oxidation product is formic acid. Moreover, re-formed monomers (terephthalic acid) of the polymer can collect on the surface and form a white, hard-to-remove deposit that changes the gloss level and color [512], Oxidative degradation reactions can be inhibited by primary and secondary antioxidants. Acidolysis caused by the formic acid can be controlled by adding acid acceptors that will bind either the precursor of formic acid, formaldehyde, or the acid itself. Acid amides, urethane, or urea are utilized as acid acceptors [86]. 

The products formed by hydrolysis are effective primary and secondary antioxidants. Polyfunctional antioxidants are formed during the application of cyclic arylene phosphites and various HD and chain breaking moieties which autosynergetically enhance their antioxidative activity. In addition to decomposing hydroperoxides, aromatic phosphites can also react with unsaturated (vinyl) groups in the polymer, coordinate with transition metal residues, help to preserve the hindered phenol, and prevent discoloration by reacting with quinoidal compounds. 

It is desirable to combine the effectiveness of primary and secondary antioxidants by mixing or by synthesis of compounds with two structural functions in one and the same molecule. 

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