Peroxide and Radiation Crosslinking

Peroxide crosslinking involves the formation of polymer radicals via hydrogen abstraction by the peroxy radicals formed from the thermal decomposition of the peroxide. Cross-linking occurs by coupling of the polymer radicals 

The crosslinking efficiency of the peroxide process can be increased for some systems by incorporating small amounts of a comonomer containing vinyl groups into the polymer. This approach is used for polysiloxanes by copolymerization with small amounts of vinyltrimethylsilanol 

Peroxide crosslinking of the copolymer is more efficient than that of the homopolymer (Table 9-1). The process becomes a chain reaction (but with short kinetic chain length) involving polymerization of the pendant vinyl groups on the polysiloxane chains in combination with coupling of polymeric radicals. The crosslinking of EPDM rubbers is similarly more efficient when compared to EPM rubbers since the former contain double bonds in the polymer chain. 

TABLE 9-1 Efficiency of Crosslinking of Polydimethylsiloxane by Bis(2,4-dichlorobenzoyl) Peroxide  

Vinyl Comonomer Content (mol %) Number of Crosslinks per Peroxide Molecule Decomposed for Peroxide Concentration of  

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Uses Antioxidant, metal deactivator for polymers, PP, HDPE, LDPE, some thermoplastic elastomers used as primary insulation in wire/cable applies., EPDM, peroxide- and radiation-crosslinked PE, nylon, polyacetal, polybutene, PU, styrenic copolymers, unsat. rubber, PVC, PVB, hot-melt and sol n. adhesives, powd. coatings, metal coatings, rubber/plas-tic gaskets, plastic fabricated parts in contact with catalytic metals, oils/ lubricants in contact with metals, food pkg. 

Before the advent of suitably flame-retardant crosslinked EVA insulation compounds, peroxide and radiation-crosslinked blends of PVC/VA solution copolymers and EVA were used for switchboard wire and related applications. These compounds routinely passed 90°C long-term wet electrical testing and 7-day heat aging at 150 °C. This is an approach that might be reconsidered. 

The hydrogenated nitrile (HNBR) grades which contain the lowest level of residual double bonds can only be crosslinked by the use of peroxides and radiation, whilst those with a level of residual double bonds greater than about 3.5% can be cured by sulphur. 

The saturated main chain of the copolymer confers excellent resistance to oxygen, ozone and light, but means that these materials cannot be crosslinked by sulphur. Peroxides and radiation are the only methods by which crosslinking can be accomplished, and coagents are often required to achieve the required state of cure. 

It has already been mentioned in Sect. 2 that the simplest assumption, affine deformation of the tubes d = dgk, yields the Mooney-Rivlin equation (1). The value V = 1/2 was obtained by a microscopic model which is briefly discussed in Sect. 2. It is suitable for the description of moderately but almost completely cross-linked networks (e.g. sulphur-, peroxide-, or radiation-crosslinked NR, PB and PDMS chains of very high degree of polymerization). 

In the same example, if a partially crosslinked, medium-Mooney-viscosity NBR is used instead (e.g., Nipol 1411), melt viscosity will be considerably higher, but die swell will be cut dramatically. The resultant compound will have higher tensile strength and greatly improved abrasion and flex resistance. The compression set of both variations will be lower than the control. The major benefits to NBR levels higher than 30 phr are in abrasion and flex resistance. At ratios of 50/50 and 70/30 NBR/PVC, the product is best treated as an elastomer. Such blends arc routinely mixed in Banbury or similar mixers (generally for extmsion applications such as hose covers and cable jackets), and are vulcanized by the action of sulfur/accelerator systems or via peroxide or radiation crosslinking. 

NR can be crosslinked by the use of sulphur, sulphur donor systems, peroxides, isocyanate cures and radiation, although the use of sulphur is the most common method. 

Due to the absence of double bonds in the main chain, these materials can only be crosslinked by the action of peroxides or radiation. It is recommended that metal oxides are added to act as acid acceptors during vulcanisation, the oxides of magnesium and lead being used zinc oxide is not used as it decreases the stability of the polymer. 

Much research into radiation effects on polymers is done with samples sealed under vacuum. However, polymer materials may, in practical applications, be subjected to irradiation in air. The effect of irradiation is usually substantially different in air, with increased scission at the expense of crosslinking, and the formation of peroxides and other oxygen-containing structures. Diffusion rates control the access of oxygen to radicals produced by the radiation, and at high dose rates, as in electron beams, and with thick samples, the behaviour may be similar to irradiation in vacuum. Surface changes may be quite different from bulk due to the relative availability of oxygen. 

Recombination of macroradicals, which are generated either chemically or i ysi-cally, is one of the steps of the free radical medianism of crosslinking. Thermally unstable cmnpounds such as peroxides and azo compounds are usually used for free radical generation. The physical route of free radical generation in polyolefins includes mostly p-radiation (faetatrones) and gamma rays (radioactive isotopes). In some cases, a comlaned effect of microwave excitation and a suitable peroxide, ultraviolet light, and a photosensitizer may be used,... 

Besides the chemical structure of a network junction, the properties of a cross-linked polymer are also affected by the distribution of crosslinks in the polymer matrix. The difference in properties of the polymer crosslinked by thermally laUle compounds such as peroxides and/or ionization radiation may thus be evidoit. 

In radical polymerization (see Scheme 2), radicals are formed in a first stage determining the rate. Examples used industrially for radical sources, apart from electron radiation, are aroyl peroxides such as bis(2,4-dichlorobenzoyl) peroxide or bis(2,4-methylbenzoyl) peroxide and alkyl peroxides such as dicumyl peroxide or 2,5-dimethyl-2,5-di-/er/-butyl peroxyhexane. In the second stage, the actual crosslinking reaction, a radical addition based on the usual pattern for a radical chain reaction, takes place via the double bonds of the vinyl groups in the polymer. Radical polymerization is today used virtually exclusively for crosslinking solid silicones. 

In chemical crosslinking, one must either extend the polymerization process, or add suitable reactants and additives to induce the formation of chemical bonds between adjacent polymeric chains. One example of chemical crosslinking is rubber, which may be chemically crosslinked by addition of sulfur. With UHMWPE, peroxides such as dicumyl peroxide may be mixed with the resin powder to chemically crosslink the polymer during conversion to bulk rod or sheet (Shen, McKellop, and Salovey 1996). However, chemical crosslinking is not used to process UHMWPE for medical applications, and for this reason we will focus on radiation crosslinking for the remainder of this chapter. 

Various applications are targeted by crosslinked PP. Crosslinking to a gel content of 55% proved to be beneficial for cable insulation. Both crosslinking via grafting of silanes initiated by peroxide decomposition and radiation in the presence of polyfunctional monomers result in a substantial increase of the resistance to electrical breakage in high-voltage tests. 

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