Polyolefin radiation degradation

Degradation of polyolefins such as polyethylene, polypropylene, polybutylene, and polybutadiene promoted by metals and other oxidants occurs via an oxidation and a photo-oxidative mechanism, the two being difficult to separate in environmental degradation. The general mechanism common to all these reactions is that shown in equation 9. The reactant radical may be produced by any suitable mechanism from the interaction of air or oxygen with polyolefins (42) to form peroxides, which are subsequentiy decomposed by ultraviolet radiation. These reaction intermediates abstract more hydrogen atoms from the polymer backbone, which is ultimately converted into a polymer with ketone functionahties and degraded by the Norrish mechanisms (eq. 

PBS (Figure 30) is an alternating copolymer of sulfur dioxide and 1-butene. It undergoes efficient main chain scission upon exposure to electron beam radiation to produce, as major scission products, sulfur dioxide and the olefin monomer. Exposure results first in scission of the main chain carbon-sulfur bond, followed by depolymerization of the radical (and cationic) fragments to an extent that is temperature dependent and results in evolution of the volatile monomers species. The mechanism of the radiochemical degradation of polyolefin sulfones has been the subject of detailed studies by O Donnell et. al. (.41). 

The prime factors causing degradation of polymers are UV light and the heating effects of solar radiation. Additional relevant factors include ozone, atmospheric contaminants and induced radiation. It is the impurities present in the commercial polymers that initiate photochemical reactions because pure saturated polyolefins... 

This volume is including information about thermal and thermooxidative degradation of polyolefine nanocomposites, modeling of catalytic complexes in the oxidation reactions, modeling the kinetics of moisture adsorption by natural and synthetic polymers, new trends, achievements and developments on the effects of beam radiation, structural behaviour of composite materials, comparative evaluation of antioxidants properties, synthesis, properties and application of polymeric composites and nanocomposites, photodegradation and light stabilization of polymers, wear resistant composite polymeric materials, some macrokinetic phenomena, transport phenomena in polymer matrix, liquid crystals, flammability of polymeric materials and new flame retardants. 

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. 

The content of aldehyde groups in the final product of polypropylene photodegradation is 7%. In the case of polyethylene degradation these groups are completely absent [6]. The relatively lower amount of aldehyde groups in the products of polyolefine photo-oxidation is due to the fact that they absorb radiation and subsequently react as above [278]. 

The external effects of the environment on polymer blends are chemical in nature, and normally lead to degradation of the polymers. Chain scission, depolymerization and reactions on the side-chain substituents all contribute to overall deterioration of blend properties. These are described for blends containing polyvinyl chloride, polystyrene, acrylics and polyolefins mixed with a variety of other polymers. The general feamres of radiation damage and the detrimental effects of processing are reviewed. 

Radiation-induced Degradation.—There have been several reports on radiation effects in polymers,288 including single crystals,287 fluoropolymers,288 polyamides,289 polysiloxanes,270 polyethylene and its copolymers,271 polypropylene,272 polyolefins,273 polystyrene and its copolymers,274 poly(vinyl chloride) and related polymers,275 rubbers,278 polysulphones and other sulphur-containing polymers,277 polycarbonate,278 nylon,279 poly(vinylpyridines),280 and wool.281... 

In the presence of oxygen or ozone, as soon as free radicals form, oxygenation of the radicals gives rise to peroxy radicals, which through a complex series of reactions result in polymer degradation. Oxidative degradation may occur at moderate temperature (thermal oxidation) or under the influence of ultraviolet radiation (photooxidation). Unsaturated polyolefins are particularly susceptible to attack by oxygen or ozone (Equation 9.6). 

Subsequent reactions are strongly dependent on the chemical nature of the polymer. Recombination of radicals to form a new chemical bond is often observed and is the key process in radiation-induced cross-linking. Examples of polymers in which cross-linking is favored include polyolefins such as polyethylene (PE), natural rubber, or poly-dimethylsiloxane (PDMS). In other polymers, including most fluori-nated polymers, poly(methyl methacrylate) (PMMA), and natural polymers such as DNA and cellulose, chain scission is favored, leading to degradation of the polymer (for a more comprehensive list, see Drobny [2, p. 21]). 

Quenching agents do not absorb ultraviolet radiation but stabilize polymers by reacting with the free radicals generated by degrading polymers to stop the chain reaction. Hindered amine hght stabilizers (HALS) were introduced in 1975 and are the newest of the hght stabihzers. They have replaced benzo-phenones and benzotriazoles in polyolefins and are more cost-effective. 

Information about the degradation pathways occurring in polyolefins exposed to vacuum-ultraviolet radiation (VUV) were obtained by MALDI-TOF analysis of oligomers formed by VUV photolysis of linear C36-alkane. ° Both molar masses and end-groups of degradation product were studied. 

Zaharescu, T., Chipara, M., Postolache, M., Radiation processing of polyolefin blends. II. Mechanical properties of EPDM-PP blends. Polymer Degradation and... 

Thus the incorporation of polyfunctional monomers in polyolefins, followed by radiation polymerization, can improve properties, lower the dose for gelation, or minimize degradation, depending on the polymer. It should be pointed out that a major use of the materials described here is as insulators for electrical wires and cables (Nicholl, 1969). 

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