Cross-linking peroxides

In contrast, antioxidants can have an opposite effect when peroxide curing. Because peroxide cross-linking involves a free-radical mechanism, and antioxidants are designed to scavenge free radicals, it is obvious that peroxide efficiency can be compromised by the addition of antioxidants. Thus the decomposition products of the ppds were acting as accelerators (29). 

Peroxide Cross-Linking of EPM Using Polar Co-Agents.404... 

Cross-linking with peroxides has been known since 1915 when Ostromyslenski [46] disclosed that NR could be transformed into a cross-linked state with dibenzoyl peroxide. However, little interest in peroxide cross-linking evolved until the development of fully saturated ethylene-propylene copolymers in the early 1970s. 

Peroxides cross-linking of the more highly unsaturated polymers is more efficient due to higher concentration of allylic hydrogen. These are readily abstracted and efficiently converted to cross-link. 

Ethylene-propylene and silicone rubbers are crosslinked by compounding with a peroxide such as dicumyl peroxide or di-t-butyl peroxide and then heating the mixture. Peroxide cross-linking involves the formation of polymer radicals via hydrogen abstraction by the peroxy radicals formed from the decomposition of the peroxide. Crosslinks are formed by coupling of the polymer radicals... 

Figure 4. X-ray diffraction pattern of peroxide-cross-linked SE-BR. Gum stock,...

When polypropylene is exposed to high energy radiation or gets heated with a peroxide, cross-linking takes places in polypropylene to give a useful material. 

Ethylene/propylene products reign supreme among the copolymers. They are elastomers. Plastics containing about 20% or more propylene perform like natural rubber and can be cured by peroxide cross-linking. They are faster to chemical and to ageing than other types of natural rubber. 

Properties of peroxide cross-linked polyethylene foams manufactured by a nitrogen solution process, were examined for thermal conductivity, cellular structure and matrix polymer morphology. Theoretical models were used to determine the relative contributions of each heat transfer mechanism to the total thermal conductivity. Thermal radiation was found to contribute some 22-34% of the total and this was related to the foam s mean cell structure and the presence of any carbon black filler. There was no clear trend of thermal conductivity with density, but mainly by cell size. 27 refs. 

The most common compound for peroxide cross-linking of PE in the wire and cable industry is dicumyl peroxide (DCP), which decomposes at 120-125°C (248-257°F), generating free radicals needed for the process plus by-products (carbinol, acetophenone, and methane). The production line for continuous vulcanization of wire and cables by peroxide must be about 200 m (61 ft) long. 2 The plant space required for radiation cross-linking using... 

Comparison of Radiation and Peroxide Cross-Linking of Wire and Cable Insulation Materials... 

Property Radiation Cross-Linking Peroxide Cross-Linking... 

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... 

Fig. 32. First derivatwe ESR spectrum at 123 A from peroxide cross linked cis-polyisoprene, tensile tested at 77 K in an oxygen free nitrogen atmosphere...

A high-temperature cyclic peroxyketal peroxide cross-linking agent for polyethylene has been found to function as a blowing agent as well. This is another example of dual-function additives. Activated by thiodipropionate antioxidants, it evolves COj and should be useful in making crosslinked polyethylene foams. 

The peroxide cross-linking reaction occurs in three steps ... 

Smedberg, A. Hjertberg, T. Gustafsson, B. Effect of molecular structure and topology on network formation in peroxide cross-linked polyethylene. Polymer 2003, 44, 3395-3405. 

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