IRRADIATION CROSSLINKING

Table II. Chemical Reactions Occurring during Irradiation Crosslinking and Scission (Soluble Fraction-Gel Fraction Study)...

Samples for irradiation crosslinking were prepared by milling in the antioxidant, followed by compression molding. Sample dimensions were 6 X 7 X 0.083 inch. Details of the milling and compression molding procedures are given in Table II. 

A pseudo solid-like behavior of the T2 relaxation is also observed in i) high Mn fractionated linear polydimethylsiloxanes (PDMS), ii) crosslinked PDMS networks, with a single FID and the line shape follows the Weibull function (p = 1.5)88> and iii) in uncrosslinked c/.s-polyisoprenes with Mn > 30000, when the presence of entanglements produces a transient network structure. Irradiation crosslinking of polyisoprenes having smaller Mn leads to a similar effect91 . The non-Lorentzian free-induction decay can be a consequence of a) anisotropic molecular motion or b) residual dipolar interactions in the viscoelastic state. 

The new absorptions in the spectra of crosslinked rubber are assigned on the basis of 13C solution NMR chemical shifts for a variety of model compounds, such as pentenes and mono-, di- and tri-sulfidic compounds, by using the 13C chemical shift substituent effect. From the calculated values for particular structural units, the experimental spectra of a sulfur vulcanized natural rubber 194,195,106), natural rubber cured by accelerated sulfur vulcanization 197 y-irradiation crosslinked natural rubber198 and peroxide crosslinked natural rubber and cis-polybutadiene 193 1991 are assigned. 

C. Jiao, Z. Wang, X. Chen, B. Yu, and Y. Hu, Irradiation crosslinking and halogen-free flame retardation of EVA using hydrotalcite and red phosphorus, Radiat. Phys. Chem., 2006, 75 557-563. 

The formation of oxygen anion radicals and molecules of ozone also should be counted with at the ionization initiation of oxidation [26]. Initiation reaction caused by oxygen anion radicals may play an important role within the polymer bulk while the effect of ozone forming in the surrounding air atmosphere will include only the formation of radicals on the polymer surface. The latent effect of ionization initiation on polymer oxidation which is very distinct may be documented on a relatively fast increase of concentration of carbonyl groups, observed over 1 year after irradiation crosslinking of polyethylene [27]. 

Peroxide crosslinked ionomer mixtures made from the abovementioned raw materials may be used in contact with fatty foods under certain limited conditions, according to their raw materials and additives. Vinyl chloride and vinylidene chloride should not be used in materials having either ionic (ionomer) or physical (e.g. through electron beam irradiation) crosslinks. Crosslinked polyethylene can be used for the manufacture of food contact materials, e.g. for drinking water pipes and fittings. The cross-linking can be done using either peroxide or electron beam irradiation. 

Irradiation crosslinking of PVC-U, carried out using electron beam, has shown a marked increase in glass transition temperature in the presence of a triacrylate, with minimal thermal degradation (216). Electron beam irradiation has also been investigated on a PVC-P wire coating formulation in the presence of different reactive monomers (185). Flame retardancy has also been improved with the incorporation of appropriate flame retardants (118). The effect of UV irradiation on formulations, incorporating iron chloride or cobalt chloride, has also been studied (67). 

Meola, C. Nele, L. Giuliani, M. Suriano, P. Chemical and irradiation crosslinking of polyethylene. Technological performance over costs. Polym. Plast. Technol. Eng. 2004, 43 (3), 629-646. 

Chemical evidence for trapped free radicals has been obtained by the observation of various reactions initiated by these radicals after irradiation. These are those of grafting, oxidation, isotope exchange and post-irradiation crosslinking. 

The most beneficial properties of irradiation are thus obtained at high temperatures for crosslinking polymers. Above their softening point, in fact, irradiated crosslinked polymers behave like rubber instead of flowing like viscous liquids. This has been observed for polyethylene and polyvinylchloride. 

Irradiation by UV Irradiation crosslinking Irradiation degradation Irrotational flow field Isocyanate-containing species Isopropenyl oxazoline (IPO)... 

A positive effect of fillers may be observed during irradiation crosslinking. It was found that the yield of radicals in polyethylene was increased 50% when a small amount (0.05 %) of aerosil was added [21]. It has been assumed that a higher production of radicals takes place at the interphase aerosil-polyethylene, where macromolecules can be in the nonequilibrium state cf uncompensated strains. With a higher content of a filler, a transfer of energy from the filler to the polymer j se may occur and thus contribute to a higher yidd of free radicals. Combination of irradiation with reactive admixtures may, moreover, affect a localization of crosslinks along the poly-mo chain. 

Jurkin, T. and Pudc, 1.2006. Post-irradiation crosslinking of partially cured unsaturated polyester resin. Radiat. Phys. Chem. 75 1060-1068. 

Wang (2001) found that irradiation crosslinked UHMWPE had a significantly lower wear rate than un-crosslinked material. The radiation dose must be high to obtain the optimum effect (Fig. 15.19). Rieker et al. (2003) showed that the wear surfaces of highly crosslinked UHMWPE implants after 18 months in vivo, consisted of folds (Fig. 15.20). Such folds are also found in conventional UHMWPE, but fatigue leads to their detachment from the surface. The folds on the surface of the crosslinked polymer appear to stay in place. Crosslinking leads to a reduction in crystallinity, hence a... 

Low voltage electrical wire with irradiation crosslinked outer coating. 

The wire is a low voltage wire for telecommunications equipment. It has a total diameter of 1 mm. The conductor is around 0.6 mm diameter. The outer layer compound is manufactured by conventional processes, for example, dry blending, pellet manufacture, extrusion, and then irradiation up to 10 MRad dose level. The irradiation crosslinked outer layer offers sufficient abrasion resistance to protect the inner layer (polyethylene) and the conductor in usage for a wire in switchboard or main frame wiring. It also gives the required degree of non-flammability. 

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