Effect of heat treatment on morphology and dielectric properties of PE cable insulation material

Abstract

It is known that the heat treatment process in the
production of extruded cables, affects the electrical
properties of polymeric insulation materials; this effect
is via two main phenomena; first, it leads to diffusion
and removal of polar substances from the system;
second, it affects the polymer morphology by increasing
the polymer chain relaxation and creation of crystallites,
hence increasing the crystallinity of the material.
In this study, thick pressed samples with additive free
low density polyethylene (LDPE) insulation material
were heat treated at different conditions for different
periods of times. Differential scanning calorimetry
(DSC) measurements confirmed that heat treatment
leads to higher crystallinity and higher lamella thickness
in LDPE. Dielectric properties of the material was
studied by conduction current measurement and
dielectric spectroscopy.
Conduction current of samples with different heat
treatment times was measured at high electric fields
which shows that the conduction current of LDPE is
affected by heat treatment. Generally, the apparent
conductivity decreases by heat treatment.
Dielectric spectroscopy measurements show that heat
treatment does not have a significant effect on real and
imaginary permittivity of the polymer. Interestingly, an
increase in the sample thickness via heat treatment, was
observed both by sample thickness related to the
memory effects in the polymer which needs to be taken
into consideration for accurate capacitance
measurements.