Polymer-bound antioxidants

Nonbound and polymer-bound hindered phenolic antioxidants... 

It has been observed that complete immobilisation of the stabiliser through a graft leads to deactivation. However, proper selection of the ratio of phenolic to graftable groups leads to a polymer-bound product which retains sufficient mobility to provide a high level of antioxidant activity. An n/m ratio of 5-10 provides an optimal balance of graftability and antioxidant activity [144]. 

Kim el al. [582] have described maleimide-based antioxidants melt grafted onto low-MW PE. IR spectroscopic methods and titration were used for the quantitative determination of the extent of grafting of the monomeric antioxidant. Smedberg el al. [583] have characterised polymer-bound stabilisers by FTIR and NMR. The binding of antioxidants and photostabilisers to polyurethanes was verified by tJV/VIS spectroscopy [584]. 

Great Lakes has reported that functionalisation with graftable moieties results in a product which can be chemically bound to a polysiloxane backbone, e.g. Silanox MD. Functionalisation of polysiloxanes with HALS (polymer-bound HALS, P-HALS) and phenolic antioxidants has been described [22]. Functionalised polysiloxanes (Figure 3.23) exhibit high stabilisation activity in critical applications such as PP fibres and PE cables [58]. 

Polymer-bound antioxidants must be molecularly dispersed (i.e. infinitely soluble) and cannot be physically lost from the substrate. High-MW phenolic AOs are preferred for applications requiring FDA approval, minimal discoloration, and long service life at high temperatures. Antioxidants are used for protection of polymers, plastics, elastomers, foods, fuels and lubricants. 

Another approach was developed by Scott in the 1970 s (7.8) which utilises the same mechanochemistry used previously by Watson to initiate the Kharacsh-type addition of substituted alkyl mercaptans and disulphides to olefinic double bonds in unsaturated polymers. More recently, this approach was used to react a variety of additives (both antioxidants and modifiers) other than sulphur-containing compounds with saturated hydrocarbon polymers in the melt. In this method, mechanochemically formed alkyl radicals during the processing operation are utilised to produce polymer-bound functions which can either improve the additive performance and/or modify polymer properties (Al-Malaika, S., Quinn, N., and Scott, 6 Al-Malaika, S., Ibrahim, A., and Scott, 6., Aston University, Birmingham, unpublished work). This has provided a potential solution to the problem of loss of antioxidants by volatilisation or extraction since such antioxidants can only be removed by breaking chemical bonds. It can also provide substantial improvement to polymer properties, for example, in composites, under aggresive environments. 

The mechanoradical produced will react with the small amount of oxygen to form hydroperoxides these are subsequently utilised as radical generators in the second stage. The resulting hydroxyl radical (from hydroperoxide decomposition) abstracts a hydrogen from the substrate to form macroradical which, in turn, will react with more of the thiyl radical to form more bound antioxidant. The polymer bound antioxidant made in this way is very much more resistant to solvent leaching and volatilisation when compared to commercial additives (13). see Figure 2. 

Radiation chemistry in polymer research, 168-169 Reactive macroalkyl radicals, formation, 409 Reactive modifiers addition of reactive antioxidants on rubbers, 417 adhesion, 420,422 demanding applications, 414,416 improving additive performance during melt processing, 412 polymer bound antioxidant, 418-419/ Reduced poly(vinyl chloride),... 

This obvious drawback can be circumvented by adding polymer bound antioxidants. Actually, polymer bound antioxidants are highly resistant to volatilization and leaching. However, the main disadvantage polymer bound antioxidants is their restricted mobility. On the other hand, if the antioxidant is bound to a low molecular weight polymer this problem is not as serious. 

An example for the second type of polymer bound antioxidant is the bounding of p-phenylene diamine to a low molecular weight chlorinated PIB (71). 

Reactivity With Nitroso Compounds. Functionalization of diene based rubbers with aromatic nitroso compounds bearing aminic or phenolic moieties 174, like with iV,A-diethyl-4-nitrosoaniline, 4-nitrosodiphenylamine, 4-nitrosodiphenylhy-droxylamine or 4-nitrosophenol represents an effective way for the synthesis of polymer-bound antioxidants [233], The respective nitroso compound can be mixed with rubbers during compounding or with concentrated rubber latexes. The chemical attachement of stabilizing active moieties takes place during subsequent... 

Antioxidants are classified as primary or secondary, depending upon how they react. Hindered phenols are primary antioxidants and function by donating a hydrogen to convert a peroxy radical to a hydroperoxide. Phosphites are among what are called secondary antioxidants and function as hydroperoxide decomposers. The ultimate outcome of these reactions is to convert the polymer bound radical to derivatives that are less destructive to the polymer. 

Scott, G. Macromolecular and polymer-bound antioxidants. In Atmospheric Oxidation and Antioxidants, Scott, G., Ed. Elsevier Applied Science London, 1993 Vol. 2, 279-326. 

Al-Malaika, S. Ibrahim, A.Q. Rao, J. Scott, G. Mechanisms of antioxidant action photoantioxidant activity of polymer-bound hindered amines. II. bis acrylates. J. Appl. Polym. Sci. 1992, 44, 1287-1296. 

The reasons for the present interest in polymer-bound antioxidants is discussed, and some of the more important approaches to the chemical attachment of antioxidants and stabilizers to polymer molecules are briefly reviewed. 

It is concluded that the modification of rubbers after manufacture with chemically reactive antioxidants offers the most promising procedure for producing concentrates of polymer-bound antioxidants that can be used as conventional additives. 

However, if migration is important to the function of an antioxidant, then polymer-bound antioxidants should not be effective. In the following sections, data will provide at least a partial answer to the last point. 

Approaches to Polymer-Bound Antioxidants. Three basic methods of... 

Figure 2. Residual antioxidant remaining during solvent (pet ether/toluene at 20 °C) and detergent (1% Tide at 100 °C) leaching of a typical bisphenol (WSP) and a polymer-bound phenol (BHBM).

Reactions of Antioxidants with Polymers During Processing. One of the earliest polymer-bound antioxidants was obtained by reaction of nitroso antioxidants (ANO) with rubbers during vulcanization (29). The chemistry of this process is complex, but its discoverers proposed an "ene" reaction with the unsaturation in the polymer. 

Table 12. Yields of Polymer-Bound Antioxidants in EPOM...

In the last few years, it has become fully appreciated that polymeric antioxidants are effective in retarding the thermal and autooxidation. Such polymer-bound stabilizers are similar in efficiency to the low molecular weight stabilizer incorporated into the polymer by blending. The polymer-bound stabilizer should have a flexible spacer between the point of attachment to the polymer and the functional group of the phenolic antioxidants. 

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