Polyethylene is a volume leader in the global plastic
industry with numerous applications. However, its
low upper service temperature makes ordinary polyethylene
unsuitable for the applications requiring continuous
use under stresses at temperatures up to 100ºC
(for example in hot water pipes) or occasional
increases in temperature above a certain limit during
extrusion and temporary electrical overloads on
cables. By cross-linking of polyethylene some important
properties would be drastically improved (Table
1) [1-5]. Some applications include heat shrinkable
products (in cable installation), heat-resistant foodstuff
packaging (up to 200ºC), foams for thermal insulation,
and chemical-resistant seals [6-9]. Moreover,
cross-linked polyethylene is more resistant to water
treeing and tracking which are undesirable in cable
jacketing industry. Besides the capability of crosslinked
polyethylene in withstanding higher electric
loading, its ability to tolerate thermal shocks, and the
small thickness required for insulation, makes it a
suitable candidate for cable jacketing industry.
Polyethylene has the melting point of 100-130ºC,
however, after cross-linking no flow would be noticed
even at 150ºC [10] where elastic behaviour prevails.
In addition to the advantages associated with crosslinked
polyethylene, it is also capable of absorbing
high loadings of fillers (e.g., carbon black) compared
to uncross-linked polyethylene which becomes brittle
on incorporation of fillers. The reason is that by formation
of cross-links, the particles are bonded and
trapped within the polymer matrix. As a result, levels
of filler that are disadvantageous and make the polymer
brittle would impart reinforcement in crosslinked polyethylene [11]. Some advantages of using
cross-linked polyethylene pipes instead of metal pipes
include: lower installation costs, long service life,
easy replacement of damaged parts, good chemical
and abrasion resistance, etc. [12-14]. Table 1 briefly
presents some properties of polyethylene which are
subjected to change by cross-linking.
There are different methods of cross-linking of
polyethylene. However, despite being the most important
cross-linking technology in production of cables
and pipes, there is not yet a comprehensive review
study on silane method, particularly on the “two-step”
Sioplas® process which is the subject of this study.