Retardants of Radiation Cross-Linking

The above-mentioned radical acceptors are not the only effective protective agents against radiation. Studies performed with benzene and nitrobenzene in natural rubber and styrene butadiene rubber (SBR) with N-phenyl-p-naphtylamine demonstrated radiation protection of the respective compounds. 

Many of the studied radical-accepting compounds were found to reduce the yield of scissions. - The antirads tested proved effective not only in the presence of air but also in its absence (see Table 5.3). 

Different pol)uneric materials respond to irradiation by electron beam in different ways. A large number of them will be modified by the formation of a cross-linked network, by changing their surface properties or structure, and some will be degraded. Another field applicable to polymeric systems is polymerization and grafting. Electron beam can also be used for pol5unerization and cross-linking of oligomers and monomers, i.e., in conversion of liquids to solids. 

---

The promotion of cross-linking of polypropylene51 and ethylene-propylene copolymers52 has been confirmed. However, other bases such as amines, were not found effective — in some cases in fact, they acted as retarders of radiation cross-linking.53... 

FLAME RETARDANCY OF RADIATION CROSS-LINKED POLY (VINYL CHLORIDE)(PVC) USED AS AN INSULATING MATERIAL FOR WIRE AND CABLE... 

In contrast to metals and semiconductors, the valence electrons in polymers are localized in covalent bonds.The small current that flows through polymers upon the application of an electric field arises mainly from structural defects and impurities. Additives, such as fillers, antioxidants, plasticizers, and processing aids of flame retardants, cause an increase of charge carriers, which results in a decrease of their volume resistivity. In radiation cross-linking electrons may produce radiation defects in the material the higher the absorbed dose, the greater the number of defects. As a result, the resistivity of a radiation cross-linked polymer may decrease. Volume resistivities and dielectric constants of some polymers used as insulations are in Table 8.3. It can be seen that the values of dielectric constants of cross-linked polymers are slightly lower than those of polymers not cross-linked. 

When enhanced temperature resistance is required, polyvinylidene fluoride or other fluoropolymers are used. Fluoropolymers have the advantage of being oil resistant and flame retardant, but are also more expensive base materials. PVDF is one of the materials that is very easy to cross-link by EB radiation. 

Balabanovich AI, Zevaco TA, Schnabel W. Fire retardance in polyfbutylene terephthalate). The effects of red phosphorus and radiation-induced cross-links. Macromol. Mater. Eng. 2004 289 181-190. 

---