Peroxides terms Links

For peroxide cross-linking, organic peroxides, such as dicumyl, di-t-butyl, and benzoyl peroxides, are used in amounts 1 to 3 phr (parts per hundred parts of rubber). Typical cure cycles are 5 to 10 min at temperatures 115 to 170°C (239 to 338°F), depending on the type of peroxide used. Each peroxide has a specific use. A postcure is recommended to complete the cross-linking reaction and to remove the residues from the decomposition of peroxide. This improves the long-term heat aging properties.62... 

Cross-linking polyethylene enhances its heat resistance (in terms of resistance to melt flow) since the network persists even about the crystalline melting point of the uncross-linked material. Cross-Knked polyethylene thus finds application in the cable industry as a dielectric and as a sheathing material. Three main approaches used for cross-linking polyethylene are (1) radiation cross-linking, (2) peroxide cross-linking, and (3) vinyl silane cross-linking. 

Muratoglu OK, Biggs SA, Bragdon CR, O Connor DO, Merrill EW, Premnath V, et al. Long term stability of radiation and peroxide cross-linked UHMWPE. 23rd Annual Meeting of Society for Biomaterials. New Orleans, LA 1997. 

During the vulcanization, the volatile species formed are by-products of the peroxide. Typical cure cycles are 3—8 min at 115—170°C, depending on the choice of peroxide. With most fluorosihcones (as well as other fluoroelastomers), a postcure of 4—24 h at 150—200°C is recommended to maximize long-term aging properties. This post-cure completes reactions of the side groups and results in an increased tensile strength, a higher cross-link density, and much lower compression set. 

This qualitative interpretation of structural and electronic similarity has also been employed to rationalize the fact that the quantum yield for the dioxetane derivative 6, in which the phenoxy substituent is directly linked to the peroxidic ring, is two orders of magnitude higher than for the dioxetane 7, in which the trigger function is separated by a methylene bridge. Furthermore, the different quantum yields were rationalized in terms of a competition between the intramolecular (pathway A) and intermolecular back-electron transfer (pathway B) in the decomposition of 7, whereas the intramolecular back-electron transfer was believed to occur exclusively in the decomposition of 6, due to the higher stability of the radical anion of the benzaldehyde derivative, as compared with the radical anion of acetone (Scheme 14). 

The dynamic cross-linking process is used to produce thermoplastic elastomers from mixtures of crystallizable polyolefins and various rubbers. Variations of basically the same method are employed to produce novel, stable polymer alloys by performing chemical reactions during extrusion of such mixtures. In that case, the cunent industrial term is reactive extrusion. Such processes are used, for example, to improve processability of LLDPE s into tubular film (by introducing long chain branches during extrusion with low levels of peroxides) or to... 

It has been concluded from a study of the optical and e.p.r. spectra of Co —Cu bovine superoxide dismutase, in which zinc has been replaced by cobalt, that the cobalt site reactivity should be described in terms of reaction of the Co-imidazolate-Cu system as a whole the crystal structure reported last year indicated that the metals were linked by a common histidine residue. There is an exchange interaction between the cobalt and copper however, this is abolished when the linking imidazole is protonated. Further evidence for the close proximity and interactive dependence of the zinc and copper binding sites was obtained from a study of the 4 Cu protein a two-fold enhancement of the activity of 2 Cu dismutase was observed upon occupation of the zinc sites by the Cu ". On the basis of C1 n.m.r. studies, Fee and Ward have suggested that one co-ordination position of Cu in superoxide dismutase is normally occupied by water they further suggest that superoxide can displace the solvent to form a cupric peroxide complex. 

The terms NEFA, unesterified fatty acids, and free fatty acids have been used synonymously in the literature, although these fatty acids are mainly bound to plasma proteins. Enzymatic assays are convenient with NEFA in the presence of added acyl-coenzyme A (CoA) synthetase forming acyl CoA esters, which are then oxidized by acyl CoA oxidase to yield hydrogen peroxide. The hydrogen peroxide produced is measured by a peroxidase-linked colorimetric detection reaction. Plasma samples should be separated promptly and stored frozen to prevent in vitro lipolysis (Zambon, Hashimoto, and Brunzell 1993). 

Numerous lists are available in the literature that give the decomposition temperatures or the half-lives at certain elevated temperatures of many initiators. Decompositions of peroxides may proceed via concerted mechanisms and the rates are structure-dependent. This can be illustrated on benzoyl peroxide. The benzoyl groups, the two halves of the molecule, are dipoles. They are attached, yet they repel each other. Rupture of the peroxide link releases the electrostatic repulsion between the two dipoles. Presence of electron donating groups in the para position increases the repulsion, lowers the decomposition temperature, and increases the decomposition rate. The opposite can be expected from electron attracting groups in the same position. The effect of substituents on the rate of spontaneous cleavage of dibenzoyl peroxide was expressed in terms of the Hammett equation, log K/Ko) = pa. This is shown in Table 2.3. 

In general, reactive extrusion is the term applied to chemical modification of polymers in the presence of an initiator such as peroxides. During chemical modification, there can be other undesired side reactions taking place, for example, cross-linking, degradation into low MW fractions and so on. Good process control is an essential requisite for attaining a balance of desired properties. 

The second-stage cross-linking (cure) reaction is initiated by organic peroxides MEK peroxide for room-temperature cure, and benzoyl peroxide or t-butyl perbenzoate or other stabler peroxides for higher-temperature cure processes. Peroxide action may be speeded by heat and/or activators such as cobalt soaps and tertiary amines. (Nonchemists are apt to use the terms catalyst and activator ratber loosely, which can he confusing or even dangerous in practice.)... 

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