Organic peroxide, crosslinking

Waritz RS Toxikology of organic peroxide crosslinking agents for elastomers... 

Lower Temperature Organic Peroxide Crosslinking Agents, Organic Peroxides, Atofina Chemicals, Inc., Philadelphia, Pa. 

The crosslinking of ethylene-propylene copolymer rubber (EPR) in the presence of organic peroxides has been investigated by Natta and/or his coworkers (1-3) and others (4,5). Co-agents such as sulfur (3,4) and unsaturated monomers (6), including maleic anhydride (MAH)(3,7) have been utilized in an effort to increase the crosslinking efficiency in the EPR-peroxide system. 

Secondary processes are normally employed to crosslink chain growth polymers. In one example a linear thermoplastic, such as polyethylene, is compounded with an organic peroxide that is thermally stable at standard processing temperatures but decomposes to chemically react with the polymer chain at higher temperatures creating crosslinks. 

We can also produce direct crosslinks by the action of peroxy radicals, as shown in Fig, 18.8. In this process, we blend an organic peroxide, such as dicumyl peroxide, into molten polyethylene at a temperature below that at which the peroxide decomposes. Once we have formed the molten blend into the required shape, we increase its temperature until the peroxide decomposes into peroxy radicals, as shown in Fig, 18.8 a). The peroxy radicals abstract hydrogen atoms from the polyethylene chains to create free radicals, as shown in Fig. 18.8 b). Crosslinking takes place when two radicals react to form a covalent bond, which is shown in Fig. 18.8 c). 

A breaker an enzyme (at T<140°F), strong oxidizing agent, or an acid, is used to depolymerize polysaccharides and break crosslinks such that viscosity declines at a controlled rate so that the proppant may be deposited in the fracture. Too rapid proppant dropout would cause a premature "sand-out" which prevents future extension of the fracture. Peroxydisulfates are the most frequently used breakers. Less reactive organic peroxides may be preferred for high temperature formations (85). 

Organic peroxides have been used to crosslink elastomers and plastics for over 50 years. The organic peroxides utilised by the rubber industry reactvery predictably. Most are stable at room temperature and will decompose based on their half-life temperature curves. They can represent a severe hazard, however, if they are stored or used improperly. These issues are reviewed in detail. 4 refs. USA... 

Nitrogen and oxygen can be Incorporated Into the backbone such that they are surrounded by different atom types. For example, organic peroxides contain two covalently bonded oxygen atoms that form the peroxide linkage. These molecules are Inherently unstable. Two covalently bonded nitrogen atoms are also similarly unstable. These unstable structures decompose to form smaller unstable molecules that are used to start the polymerization for some types of monomers. Thus, to be incorporated implies that the molecules are found only singularly in the backbone chain. Sulfur and silicon are considered to be chain formers. They can be found in the backbone in multiple units connected covalently to molecules of the same type or with carbon. Complete molecules with a silicon backbone are possible, and molecules with multiple sulfur links incorporated into the system are common, particularly in sulfur-crosslinked rubber. 

Crosslinking can be achieved by the addition of organic peroxides, which improves the wear resistance. In addition, crosslinking occurs by the treatment with energy rich radiation. 

The upper temperature limit for safe operation depends on the onset decomposition temperatures of peroxides and blowing agents employed. Preferred chemical crosslinking agents are organic peroxides, such as dicumyl peroxide (8). 

M 9. —, and J. Scanlan Determination of degree of crosslinking in natural rubber vulcanizates. Part VI. Evidence for chain scission during the cross-linking of natural rubber with organic peroxides. J. Polymer Sci. 43, 23 (1960). 

The fact that these graft copolymers contain polyethylene chains makes them crosslinkable by organic peroxides. One of them has already found industrial application as a peroxide crosslinkable material in cable insulation. 

As a preliminary step in the manufacture of unsaturated polyester thermoset plastic one uses low molecular weight linear polyester (Mr 10,000) obtained by a polycondensation of polyglycols with saturated and unsaturated dicarboxylic acids. The precondensate can then be dissolved and stored in the stabilized comonomer, e.g. styrene, with which it will be crosslinked later. The crosslinking polymerization reaction between the polyester chains and the styrene bridges is initiated with the help of organic peroxides which are added dispersed in plasticizers. The reaction begins at 60-90 °C and then proceeds exothermally. In addition to this a cold hardening reaction can also be carried out. For this reaction cold accelerators are necessary, e.g. tertiary amines or cobalt naphthenate. 

It comes as a liquid that typically contains low molar-mass, unsaturated polyester and styrene. The unsaturated polyester contains carbon-carbon double bonds. The polymerization is catalyzed with a hardener, an organic peroxide dissolved in an organic solvent. The peroxide forms free radicals that initiate polymerization of the styrene and crosslinking of the polyester. 

The moderate random chlorination of polyethylene suppresses crystallinity and yields chlorinated polyethylene elastomer (CPE), a rubber-like material that can be crosslinked with organic peroxides. The chlorine (Cl) content is in the range of 36 to 42%, compared to 56.8% for PVC. Such elastomer has good heat and oil resistance. It is also used as a plasticizer for PVC. They provide a very wide range of properties from soft/elastomeric too hard. They have inherent oxygen and ozone resistance, resist plasticizers, volatility, weathering, and compared to PEs have improved resistance to chemical extraction. Products do not fog at high temperatures as do PVCs and can be made flame retardant. 

The crosslinking reaction rate may be too slow for some commercial processes and the reaction may exceed the oxidation resistance time for the elastomer compound. In such cases, curing accelerators are used with the sulfur-curing process. Zinc oxide is a commonly used accelerator, however thioureas, hexamethylenetetramine, and others are effective. For organic peroxides, the cure rate can be greatly increased with an increase in applied temperature, though oxides of zinc,... 

Crosslinking of polyethylen and its copolymers EPM, EPDM, and EVA is mainly carried out by radicals generated from organic peroxides. Other crosslinking methods like electron-beam radiation and grafting of vinylsUanes assume a smaller role in this process. 

Cadox . [Akzo] Organic peroxide compds. initiator to curing polyester resins crosslinking agent to curing silicone rubbers. 

Luperox. [Atochem N. Am.] Organic peroxide compds. initiator for poly-meiizaticHi, polymer modification, dier-moplastic crosslinking, curing elastomers, high-tenq>. cure ci polyester resins. 

In general, polyester resins are synthesized by the reaction between carboxylic acids and alcohols, with three or more reactive groups. Recently, unsaturated polyesters were incorporated in various ways to produce terminal, pendant, and internal double bonds [57-59]. In the case of unsaturated polyesters, maleic anhydride is most commonly used to produce internal unsaturation. The double bond present on unsaturated polyester reacts with a vinyl monomer, mainly styrene, resulting in a 3D crosslinked structure. This structure acts as a thermoset. The crosslinking is initiated through an exothermic reaction involving an organic peroxide, such as methyl ethyl ketone peroxide or benzoyl peroxide (Fig. 3.18). 

As an alternative to organic peroxides, carbon-carbon (CC) initiators, for example, dimethyl diphenylbutane, hydroxylamine esters [51], or unsymmetric azo compounds [47] can be used for crosslinking reactions especially at higher processing temperatures. 

Within the different types of epoxies, are found epoxy diacrylates or vinyl ester resins, used to produce specific corrosion and chemical resistant composite systems. Vinyl ester resins are produced by either reacting epoxy resins of glycidyl derivatives with methacrylic acid, or from BPA and glycidyl methacrylates, where an active monomer (usually styrene) as crosslinker, hardener (usually organic peroxides), accelerators (cobalt) are added to the system. In the thermoset epoxy systems, there are also the mould releasers , which can be either internal such as, lecithin, or stearates of zinc and calcium, certain organic phosphates that are mixed in the resin, or, external - such as, fluorocarbons, silicone oil, and certain waxes, that are directly laid on the mould. 

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