Polymer antioxidant systems

The electrical properties of polypropylene are very similar to those of high-density polyethylenes. In particular the power factor is critically dependent on the amount of catalyst residues in the polymer. Some typical properties are given in Table 11.3 but it should be noted that these properties are dependent on the antioxidant system employed as well as on the catalyst residues. 

Interaction of acceptors of reactive free radicals and compounds that suppress the transfer reaction of an inhibitor radical with the substrate as it occurs in a system comprising antioxidants and polymer chain with conjugated system of double C = C bonds. 

A variety of methods for evaluating antioxidants in polypropylene has been developed during the past several years. Polymer producers, end-use manufacturers, additive suppliers, academicians, and others have developed widely disparate test methods, all of which presumably yield the same results—i.e., the test methods rate the antioxidants and antioxidant systems in the same relative order of effectiveness. Many of these test methods are useful tools in distinguishing unstabilized polymer, moderately stabilized polymer, and highly stabilized polymer systems. Today, all of the polypropylene producers offer highly stabilized polymers. Effective antioxidants are available from several additive suppliers. How does one select the best antioxidant or polymer formulation for a particular end use This paper compares the results obtained by various test methods used to evaluate the two basic types of oxidative stability, processing stability and end-use or environmental stability. The correlation or lack... 

A Modified Oven-Aging Technique for Studying Polymer Antioxidant Systems... 

Fig. 2 presents the analysis based on OIT data and the linear extrapolation of these data to longer times. The time to reach depletion of the antioxidant system can thus be predicted even after relatively short testing times (see insert figure in Fig. 2). Data by Hassinen et al. (//) for the antioxidant concentration profiles taken from high-density polyethylene pipes exposed to chlorinated water (3 ppm chlorine) at different temperatures between 25 and 105°C followed the Arrhenius equation with an activation energy of 85 kJ mol (0-0.1 mm beneath inner wall surface) and 80 kJ mol (0.35-0.45 mm beneath the inner wall surface). It is thus possible to make predictions about the time for antioxidant depletion at service temperatures (20-40°C) by extrapolation of high temperature data. However, there is currently not a sufficient set of data to reveal the kinetics of polymer degradation and crack growth that would allow reliable extrapolation to room temperature.

Reactive antioxidants grafted on polymer melts behave in a similar way to low molar mass conventional antioxidants, but offer many additional advantages. The polymer-linked antioxidants do not suffer from the problem of compatibility, volatility, and migration, i.e., they do not suffer physical loss even under highly aggressive and extractive environments. Such antioxidant systems would be much more riskfree and environmentally friendly. The ability to produce highly grafted antioxidant concentrates (master batches), which can be used in conventional (the same or different) polymers, as normal additives would extend the use of reactive antioxidants to new areas of application. 

Since the dehydrochlorlnation was conducted in nitrogen, the antioxidant system alone had no effect on the dehydrochlorlnation rate. However, the combined antioxidant and stabilizer system had an appreciable effect on peroxide crosslinked CPE, giving rise to an induction period of 105 hours when Dicup was used for crosslinking and 90 hours when Dlcup emd TMPT were combined to crosslink the polymer. The muimum rate of dehydrochlorlnation was also appreciably reduced from 55x10 to 3.1x10 mg. eq. HCl/g. polymer-hour due to the incorporation of the stcd>lllzer... 

Catalytically active metallic ions may play an important role in ageing of polymers. Their effect may become evident also in transformations of an antioxidation system. From this standpoint, data149 dealing with oxidative transformation of 2,6-di-tert-butyl-4-alkylphenols in the presence of Co(II) complexes of Schiff bases are interesting. The mixture of compounds which results in this case contains 2,6,2, 6 -tetra-tert-butyl-4,4 -dioxycyclohexadiene-l-one XCVIII and compounds formed by its cleavage, among them 2,6-di-tert-butyl-4-hydroxy-4-alkyl-2,5-cyclohexadiene-l-one of the type CX, 2,6-di-tert-butyl-l,4-benzoquinone XXII, and 3,5,3, 5 -tetra-tert-butyl-4,4 -diphenoquinone XX. 

A variety of factors, including compounding or processing conditions, end use, and expected performance, should influence the selection of an appropriate antioxidant system. These factors include compatibility with the polymer and other additives or components, antioxidant mobility and volatility, discoloration, resistance to hydrolysis, extraction resistance, radical trapping efficiency, toxicity, and cost-effectiveness. 

Stress relaxation experiments involve the measurement of the force required to maintain the deformation produced initially by an applied stress as a function of time. Stress relaxation tests are not performed as often as creep tests because many investigators believe they are less readily understood. The latter point is debatable, and it may only be that the practical aspects of creep measurements are simpler. As will be shown later, all the mechanical parameters are in theory interchangeable, and so all such measurements will contribute to the understanding of viscoelastic theory. Whereas stress relaxation measurements are useful in a general study of polymeric behavior, they are particularly useful in the evaluation of antioxidants in polymers, especially elastomers, because measurements on such systems are relatively easy to perform and are sensitive to bond rupture in the network. 

Uses Stabilizer, antioxidant for polymers, org. substrates, polyolefins, crosslinked PE systems esp. wire and cable, BR, SBR, NBR, SEBS, elastomer, petrol, prods., powd. and high-bake coatings, food pkg. food pkg. adhesives, paper, polymers, rubber Features Nondiscoloring, nonstaining... 

Natural rubber or synthetic elastomers can be blended vith a range of additional materials that can be defined as secondary polymer systems. This vould include such materials as resin systems, high molecular veight green strength promoters, and polymeric antioxidant systems. 

Nearly all polymeric materials require the addition of antioxidants to retain physical properties and to ensure an adequate service life. The selection of an antioxidant system is dependent upon the polymer and the anticipated end nse. 

Most thermal analysis methods for studying polymeric stabilizer systems are based on the antioxidant s ability to delay the oxidation process. Usually a sample is heated to a specified temperature and the induction time, or period of time before the onset of rapid thermal oxidation, is determined [see discussion of oxidative induction time (OIT) in Section 3.4.2 of this chapter]. The end of the induction period is marked by an abrupt increase in the sample s temperature, evolved heat, or mass and can be detected by DTA, DSC or TGA, respectively (Bair 1997). The effect of antioxidant structure and its concentration on prolonging a sample s induction period can be used to determine the most effective antioxidant system for a polymer such as polyethylene. Extensive data have shown that thermal information such as this can be used successfully to estimate the lifetime of polyethylene at processing temperatures (Bair 1997). 

The results in Table 1 illustrate that the proper choice of the antacid can improve the overall performance of antioxidant systems. However, the performance ranking of the individual acid scavengers (as shown in Table 1) is influenced, furthermore, by the type of phosphorus processing stabilizer, and therefore, the optimal polymer stabilization should preferably be achieved by adjusting the whole additive package and not only by the one component (e.g. acid scavenger) variation. 

Polymerizetble in emulsion, solution, and bulk/suspenslon systems. Functions in all free radical polymerizations. AZO initiators are preferred. May be incorporated at conventional levels in the total polymer, added as a masterbatch when polymerized at a high concentration, or grafted into polymers. Masterbatches may be used for dry rubber or latex compounding. Synergizes with secondary antioxidants. When polymer bound, it is non-volatile, non-extractable, non-migratory, and non-staining. 

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