Oxidation of polyethylene

Oxidation of polyethylene with the formation of carbonyl groups can lead to a serious increase in power factor. Antioxidants are incorporated into compounds for electrical applications in order to reduce the effect. 

Figure lO.U. Oxidation of polyethylene in air at 105°C, Effect of adding 0.1% antioxidant on power factor. A, blank. B, /V,/V -diphenyl-p-phenylenediamine. C, 4,4 -thiobis-(6-butyl-m-cresol). D, Nonox WSP. E, N./V -di-pl-naphthyl-p-phenylenediamine... 

Oxidation of polyethylene by sulphuric acid and potassium chlorate [9,10] improves its adhesiveness. The free energy of adhesion of the polymer is found to increase linearly with the surface density of the hydrophyl-lic sites created by oxidation. 

If degradation is due to sequential or synergistic mechanisms, then a simple extrapolation may not be reasonable. For example, in accelerating the rate of oxidation of polyethylene it may be necessary to identify the induction time and subsequent degradation time separately and to produce Arrhenius diagrams for each. The total time to failure is the sum of the two times. Figure 9.3 shows an instance of where unknowing extrapolation of the short-term results in tests on polyaramid fibres could have led to overestimates of lifetime and premature failure. 

The products of thermal oxidation of polyethylene films can be characterized by C FTNMR furthermore, using the spin-lattice relaxation technique, quantitative estimates can be made of the oxidized functional groups. Observation of the development progress of the various functional groups leads to the postulation of hydroperoxides as the primary oxidation products, which undergo further transformations to the other derivatives in a complex scheme . 

Allyl Free Radicals. Ayscough and Evans (3) have recently studied, by ESR measurements, the types of allylic free radicals produced by gamma-irradiation of several monomeric olefins. In irradiated polyethylene the allyl free radical is quite stable, persisting for several months at room temperature (31). The presence of these allyl free radicals is most noticeable in the case of high density polyethylene, and this type of free radical is undoubtedly the cause of the slow oxidation of polyethylene at room temperature, which lasts for 40 or more days after irradiation (10). Williams and Dole (40) could observe little if any oxidation of low density polyethylene when it was exposed to air after irradiation. By oxidation we mean formation of carbonyl groups as detected by infrared absorption studies at 1725 cm"1. Parenthetically, it should be noted that adding an oxygen. molecule to a free radical produces initially another type of free radical, a peroxy free radical, but in this paper we shall not discuss free radicals of this or any other types except those of hydrocarbons. 

The initiation of thermo-oxidation of polyethylene films by transition metal catalysts during composting proceeded slowly compared to oxidation at the same temperature in an oven (60-70 °C) [56]. The starch-filled polyethylene bags exposed on the surface of the compost broke down into small pieces while the buried bags remained intact after 49 days of exposure [55,... 

The rate constant of a transfer reaction will therefore be the higher, the weaker C-H bond is attacked by a peroxyl radical. From this it is obvious that the maximum rate of oxidation of polyethylene will increase with increasing number of tertiary hydrogens in the polymer [13]. Since the process includes the interaction of a macroradical with a macromolecule which both are of restricted translational mobility, the maximum rate of oxidation does not depend on the low content of reactive allyl hydrogens in polyethylene. 

This reaction has been put forward to explain the observed fact that the number of chain scissions corresponds to the number of carboxyl groups formed in the oxidation of polyethylene. Activation energy of both processes is 140 kj/mol. The mechanism of such an elementary fragmentation reaction remains however uncertain. The reactions of a chain scission are likely to precede the isomerization of original secondary alkyl peroxy radicals. 

The chemical stability of PE is comparable to paraffin. It is not affected by mineral acids and alkalis. Nitric acid oxidizes PE and halogens react with it by substitution mechanisms. By chlorination in the presence of sulfur dioxide, chlorine groups and sulfonyl chloride are incorporated and an elastomer is formed. Oxidation of polyethylene which leads to structural changes can occur to a measurable extent at temperatures as low as 50 °C. Under the influence of ultraviolet light the reaction can occur at room temperature. 

Modifications of polymer surfaces by exposure to electrical plasmas and discharges have also been subjected to XPS examination in several recent articles (4, , 7). An example is the plasma oxidation of polyethylene, polypropylene and polystyrene in a radiofrequency inductively coupled system ( ). Figure 14 shows the Cls and 01s spectra of a polyethylene film after... 

The most Importeuit Initiation process involved in the early stages of the photo-oxidation of polyethylene is shown to be hydroperoxide photolysis eissociated with the decay of virylidene groups. This is followed by carbonyl photolysis occurring primarily by the Norrish type II process. 

Polyethylene. The action of ozone on polyethylene was studied in the temperature range from 25° to 109° C. The reaction was followed qualitatively and quantitatively by infrared spectra. The products appeared to be of the same nature as those of O2 oxidations of polyethylene 2)—i.e., the formation of aldehydic and ketonic groups as indicated by the appearance of a strong absorption band in the region of 5.9 microns, and the existence of hydroxyl groups as shown by the 2.9-micron band. Polyethylene is readily oxidized in the presence of ozone, as even short period ozonizations carried out at temperatures as low as 25° C. yielded considerable concentrations of carbonyl and hydroxyl groups. Pure O2 oxidations did not yield comparable results until the reaction temperature was raised approximately... 

Fig. 1 Carbonyl group development in polymer samples exposed to UV illumination. The materials were an unpigmented polyethylene sample and similar samples pigmented with 5phr Ti02 pigment in the form of (i) anatase and (ii) rutile. (Data from Jin, C.Q. Ph.D. thesis. University of Newcastle upon Tyne, 2004. See also Jin, C. Christensen, P.A. Egerton, T.A. White, J.R. Effect of anisotropy on photomechanical oxidation of polyethylene. Polymer 2003, 44, 5969-5981.)...

Herold DA, Keil K, Bruns DE (1989) Oxidation of polyethylene glycols by alcohol-dehydrogenase. Biochem Pharmacol 38( 1 ) 73—76... 

Long-Tenn Photo-and Thermal Oxidation of Polyethylene... 

This result is in agreement with those reported by M. Iring, et al (2 ) for polypropylene and polyethylene. The oxygen consumption data indicate the oxidation of polyethylene consists of a complex group of reactions beyond the induction period with no single, simple relationship between the number of oxygen molecules consumed and the number of chain scissions. 

Gugumus F. Re-examination of the thermal oxidation reactions of polymers 2. Thermal oxidation of polyethylene. Polym Degrad Stab 2002 76(2) 329-40. 

This reaction results in the formation of an aldehyde group at the end of a chain. The evidence for this mechanism was provided by the observation of luminescence in the oxidation of polyethylene and polypropylene [139,140] and by other methods [59, 110]. 

The origin of chemiluminescence in polyolefins has been profoundly analyzed and it has contributed to the better understanding of their complex mechanism of thermooxidation The thermal oxidation of polyethylenes with different manufacturing histories has been compared, which allowed to establish a relationship between CL and some structural characteristics of the polymers. Modification of their stability in the presence of antioxidants, or other additives such as the activity of nano- and micron particles of pigments has been evaluated. 

---