CONVEYOR BELT RUBBER AGING MEASURED BY DIELECTRIC SPECTROSCOPY

11th International Multidisciplinary Scientific GeoConference SGEM2011, www.sgem.org, SGEM2011 Conference Proceedings/ ISSN 1314-2704, June 20-25, 2011, Vol. 1, 751-758 pp

ABSTRACT

Rubbers used in conveyor belt transport undergo an aging caused by the aggressive
character of external environment during their operation. The main changes of conveyor
belts due to the aging process appear firstly on their surface being in an immediate
contact with the cargo transported and/or with an ambient air. Chemical changes caused
by the aging are reflected in changes of physical properties of belt components such as
network density, glass transition temperature Tg and other. Determination of physical
properties of conveyor belt rubbers is commonly realized by experimental methods –
mainly by mechanical and dielectric spectroscopy. Both noticed methods are suitable
because the changes of network density and Tg temperature are visible on the
mechanical and dielectric frequency and/or temperature spectra. Classical mechanical
and dielectric measurements are usually performed on bulk samples. Such laboratory
measurements require obtained samples of conveyor belt rubbers which mean that the
cutting is not possible during the course of operation. While in the mechanical case it is
for the time being necessary to use the mentioned geometry of sample, in the dielectric
one we have an opportunity to use a system of planar so called interdigital system of
(comb) electrodes.

Dielectric spectroscopy is a well established method that is used many years for characterization of non- or only low-conductive materials. As mentioned above, classical dielectric measurements require disc-like samples placed between the parallel plates of a measuring condenser. The main advantage of this arrangement consists in the
fact that the outer excitation electric field applied to the condenser plates is homogenous and enabling an accurate evaluation of measured material permittivity. On the other
hand, there are some limitations of this classical approach as e.g. necessity of cutting samples (non in situ non-destructive measurements), insensitivity to the spatial
inhomogenities (material heterogeneity). In the last years a new experimental method so called interdigital dielectrometry has been developed which is based on a planar
configuration of the electrode system. This new method is used more often for nondestructive determination of material characteristics, such as monitoring of humidity, density, porosity, cure processes, aging. To its main features belong a single sided
access to the measured material and the possibility to measure its spatial profiling by means of sensors with different spatial periodicity. Some limitation, that is balanced by a possibility to measure in situ during operation of conveyor belt transport, mirror the
inhomogenity of electric field between driven and sensing electrodes and therefore the uncertainty in determination of material permittivity. To obtain and optimize the maximum information about measured material it is necessary to design the geometry of
coplanar electrode system for any concrete application, in our case for the monitoring of an aging process in conveyor belt rubbers. The requisite information mainly the information about the behaviour of conveyor belt rubbers in different distances from surface Therefore we have fixated to study the effect of the sequential changes: the
geometry of used sensors, their total size, the amplitude of outer electric field, the air gap between the electrode system and the surface of measured belts, the pressure of planar electrode system to the surface during measurements without an air gap.

Keywords: conveyor belt, non-destructive testing, dielectric spectroscopy, rubber aging