Polyethylene hydroperoxides

Using the data of Chien (8), we calculated the half-life of polyethylene hydroperoxide to be 6.4 hours at 100°C. Thus, since our shortest induction period at 100°C was over 35 hours, it is reasonable to postulate the scissioning of polyethylene results from unimolecular homolytic dissociation of polyethylene hydroperoxide, a major oxidation product ... 

In spite of the numerous studies reported on photooxidation of polyolefins, the detailed mechanism of the complete process remains unresolved. The relative contribution by species involved in photoinitiation, the origins of the oxidative scission reaction, and the role played by morphology in the case of photoreactions in solid state are not completely understood. Primary initiator species in polyethylenes [123] and polypropylenes [124] are believed to be mainly ketones and hydroperoxides. During early oxidation hydroperoxides are the dominant initiator, particularly in polypropylene, and can be photolyzed by wavelengths in solar radiation [125]. Macro-oxy radicals from photolysis of polyethylene hydroperoxides undergo rapid conversion to nonradical oxy products as evidenced by ESR studies [126]. Some of the products formed are ketones susceptible to Norrish I and II reactions leading to chain scission [127,128]. Norrish II reactions predominate under ambient conditions [129]. Concurrent with chain scission, crosslinking, for instance via alkoxy macroradical combination [126], can take place with consequent gel formation [130,131]. 

Lacoste J., D.J. Carlsson, S. Falicki, and D M. Wiles. 1981. Polyethylene hydroperoxide decomposition products. Polym Degr Stab 34 309-323. 

Ethylene-styrene copolymer Ethylene-vinyl acetate copolymer Ethylene-vinylchloride copolymer Fourier transform infrared spectroscopy Gas chromatography High-density polyethylene Hydroperoxide decomposition Isobutylene... 

Iring M, Kelen T, Tiidos F, Laszlo-Hedvig S. Study of the thermal oxidation of polyolefins. Part IV formation, characterization, and decomposition of polyethylene hydroperoxide. J Polym Sci Polym Symp 1976 57 89-99. 

Figures 3.14 and 3.16 show that the maximum rate of initial carbonyl formation in polyethylene (LDPE) observed in Figure 3.15 is associated with a higher initial hydroperoxide concentration (Fig. 3.14) and a higher rate of hydroperoxide formation during subsequent thermal oxidation (Fig. 3.16). Figure 3.14 and 3.16 also show that the hydroperoxide concentration rises to a maximum and then decays with heating time both in the melt and the solid phase, and that the maximum concentration achieved increases with decreasing temperature. In the absence of oxygen, hydroperoxide concentration decayed to zero in less than 20 h at 110°C [414]. The half-life of polyethylene hydroperoxide is 6.4 h at 100 °C [989].

Acryhc stmctural adhesives have been modified by elastomers in order to obtain a phase-separated, toughened system. A significant contribution in this technology has been made in which acryhc adhesives were modified by the addition of chlorosulfonated polyethylene to obtain a phase-separated stmctural adhesive (11). Such adhesives also contain methyl methacrylate, glacial methacrylic acid, and cross-linkers such as ethylene glycol dimethacrylate [97-90-5]. The polymerization initiation system, which includes cumene hydroperoxide, N,1S7-dimethyl- -toluidine, and saccharin, can be apphed to the adherend surface as a primer, or it can be formulated as the second part of a two-part adhesive. Modification of cyanoacrylates using elastomers has also been attempted copolymers of acrylonitrile, butadiene, and styrene ethylene copolymers with methylacrylate or copolymers of methacrylates with butadiene and styrene have been used. However, because of the extreme reactivity of the monomer, modification of cyanoacrylate adhesives is very difficult and material purity is essential in order to be able to modify the cyanoacrylate without causing premature reaction. 

Experimental evidence is available to show that at room temperature diperoxides are formed in polyethylene, whereas polypropylene and polyvinyl chloride generate hydroperoxides. The temperature at which the peroxides decompose and initiate grafting depends on the type of polymer used. 

Thermal aging is another simple pretreatment process that can effectively improve adhesion properties of polymers. Polyethylene becomes wettable and bondable by exposing to a blast of hot ( 500°C) air [47]. Melt-extruded polyethylene gets oxidized and as a result, carbonyl, carboxyl, and hydroperoxide groups are introduced onto the surface [48]. 

The formation and role of hydroperoxide groups, particularly in the early stages of polymer oxidation is well discussed in the introduction to the next chapter and also features in many of the references cited in this chapter. Their detection and quantification is therefore important. Although this can be done directly or implicitly through many of the instrumentation techniques discussed in this chapter, there are several tests that have been developed, some of which are still widely used, that are based more on chemical methods, titration or staining. The majority have been applied to polyolefins, especially polyethylene. 

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 . 

Fourier transform NMR spectroscopy, polyethylene thermal oxidation, 695 Fourier transform-Raman spectroscopy hydroperoxides, 692 nitrile hydrolysis, 702 see also Raman spectroscopy Four-memhered peroxides, 164, 1212-13 FOX (Xylenol Orange-ferric complex) assay hydrogen peroxide determination, 628, 632, 657, 658... 

Polyester-melamine coatings, hydroperoxide formation, 683 Polyethylene y-radiation, 685... 

When irradiation is carried out at higher temperatures (60°C.), polyethylene may also become peroxidized by a chain reaction leading to hydroperoxides (li). 

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