Res. Chem. Intermed.
, Vol. 28, No. 2,3, pp. 89 –100 (2002)
Also available online - www.vsppub.com
The mechanism of action of crosslinking additives
in polyethylene vulcanisation: an EPR investigation
, A. BUTTAFAVA
, L. MARTINOTTO
, F. PERUZZOTTI
G. M. GHISONI
and A. FAUCITANO
Dipartimento di Chimica Generale, Universita’ di Pavia V. le Taramelli 12, 27100 Pavia, Italy
Centro Ricerche Pirelli Cavi e Sistemi, Milano, Italy
Received 3 December 2001; accepted 22 December 2001
Abstract—The nature and kinetics of the free radicals intermediates in the peroxide induced thermal
vulcanisation of low density polyethylene and the mechanism of action of two crosslinking co-agents
with the structure of alkenyl aromatic hydrocarbons have been elucidated by EPR spectroscopy. At
low peroxide concentration, the crosslinking mechanism in the pure low density polyethylene (LDPE)
matrix is based, as expected, on the H abstraction from the chains and the coupling of the resulting
macroalkyl radicals. The latter reaction takes place in competition with the disproportionationcausing
the formation of chain unsaturations that accumulate with the reaction time. At relatively large (>2%)
peroxide concentrations, such unsaturations reach high enough levels to scavenge all the initiating and
macroalkyl radicals giving macroallyl radicals as the dominant intermediatesand determining changes
in the structure of the crosslinks during the course of the reaction.
At the concentration used, the activities of the co-agents 1,3-diisopropenylbenzene and 2,4-
diphenyl-4-methyl-1-pentene are suf cient to scavenge all the initiating and macroalkyl radicals by
double bond addition; this mechanism, when involving macroalkyl radicals, leads to the attachment
of saturated and unsaturated pendants on the LDPE chains respectively. The unsaturated pendants are
thought to be responsible for the crosslinking promotion effect by the 1,3 diisopropenylbenzene co-
agent. The lack of the second unsaturation causes 2,4-diphenyl-4-methyl-1-pentene to act essentially
as a mild antiscorch agent.
: Vulcanization; EPR; crosslinking mechanism; cure boosters.
The most common methods for the crosslinking of polyole nes used as insulants
in electric cable manufacture make use of chemical initiators, such as peroxides
or silanes, or ionising radiations. The general processing scheme involves the
following stages: (a) Compounding of the polymer with a ller (kaolin, CaCO
To whom correspondence should be addressed.