In technical applications, sealing elements are used in order to prevent mass transfer between two auxiliary chambers or components. Achievement of the desired property profile is ensured mainly via a range of design options.
Apart from the polymer and necessary additives, which filler is used also has a significant part to play in establishing a sealing element’s characteristics – such as thermal resistance, chemical resistance and compressive strength.
The sealing elements endure constant changes in environmental and operating conditions. It is necessary to replace them after a period of time, as they are subjected to natural, mechanical, or thermo-oxidative aging processes.
In order to guarantee cost efficiency, a sealing gasket must be utilized for the duration of its service life. This ensures that the sealing element is not replaced too early, thereby saving on unnecessary acquisition costs. Nor is it replaced too late, thus preventing damage from leakages.
A number of control microsystems can be integrated in order to detect the development of damage in sealing elements. The majority of these give rise to a great degree of complexity in the overall structure and are associated with high costs.
Figure 1. DMA GABO DIPLEXOR®
A Seal Monitors its Own Wear
A simpler solution can be attained by using intelligent monitoring systems.
Reinforcing filler is a necessary part of any technical elastomer composite, and it can also be electrically conductive. The sealing element becomes electrically conductive above a system-specific percolation threshold when electrically conductive fillers are mixed into the rubber matrix and an electrical voltage is applied.
The present alterations in dielectric conductivity are in accordance with the state of its filler network, and consequently there is damage in the sealing element.
A styrene butadiene rubber (SBR) filled with 70 phr carbon black (N 234) was prepared in order to demonstrate the simultaneous dielectric and mechanical behavior of a sealing material, as well as how the progression of mechanical damage is able to be characterized simultaneously.
The rubber matrix acts as an insulator. As the surface area of the N 234 carbon black has a graphitic nano-crystallite structure, it is electrically conductive. The figure of 70 phr for the carbon black is higher than the percolation threshold – a necessary prerequisite in order to develop a closed filler network which provides the required conductive paths.
NETZSCH’s dynamic mechanical analyzer DMA GABO EPLEXOR was used to perform the simultaneous dielectric and mechanical measurements, as shown in Figure 1. This can be equipped with a dielectric controller in compression mode at room temperature and special sample holders. The dielectric controller is fitted with a broadband dielectric spectrometer (BDS) supplied by Novocontrol GmBH.
The device is also known as DIPLEXOR in this combination. The compression clamps operate as electrodes, and are electrically isolated from the remainder of the instrument in order to guarantee that the SBR sample’s dielectric properties are the only aspect being measured.
The samples constituted 2 mm thick cylinders which had a 10 mm diameter. As a means of improving contact with the electrodes and consequently reducing the stray field, a very thin silver was used to coat the sample.
A frequency range between 1 Hz and 105 Hz was used to record dielectric spectra. The static force was raised from 20 N to 40 N in 5-N steps.
If a defined static force is used to compress the SBR sample, its thickness changes accordingly. The sample thickness can be further reduced by an increase in the static load amplitude. Figure 2 illustrates this behavior.
Figure 2. Thickness variation of the SBR sample filled with 70 phr N 234 due to an increasing static load amplitude.
An alteration of as much as 30% in thickness as a result of mechanical loading correlates strongly with installation procedures for seals in real applications. The internal friction within the SBR sample can be increased by increasing the mechanic loading. This is a result of diffusion processes, in addition to orientation or displacement of filler particles in the direction of compression.
The sample stiffness decreases and the filler network is progressively destroyed. Consequently, there is an association between the gradual decrease in the density of the conduction paths within the sample and damage progression.
An electric current is generated within the SBR sample by an additional application of an alternating electric field, E(ω). This is because the ability to move along the surface of carbon black clusters is gained by the free electrical charge carriers. These clusters then form continuous conduction paths from one side to the other.
The electric current density, J(ω), is proportional to the electric field applied, as given by the following formula:
J(ω) = σ* · E(ω)
where ω=2πf is the angular frequency and σ· is the complex dielectric conductivity. The complex conductivity, σ*, represents a measure of the transported charge per unit of time.
Figure 3 displays variation in the real part of the complex dielectric conductivity, σ*, as a result of an increase in a static load. σ* is frequency-independent at frequencies up to 2000 Hz, and it reaches a plateau value called DC-conductivity.
Figure 3. Variation in the real part of the complex dielectric conductivity, σ*, of the SBR sample due to a varying static load in a frequency range between 1 Hz and 107 Hz at room temperature.
σ* becomes frequency-dependent at higher frequencies. As the variation in the electric field is not associated with an immediate change in the sample polarization, this area is known as dielectric dispersion.
It is clear that as the static force is increased, the real aspect of the complex dielectric conductivity, σ*, decreases over the entire frequency range. This is a result of the progressive deterioration of the filler network.
This fact is correlated to a reduction in the density of the conduction path, which occurs throughout the totality of the SBR sample as a result of mechanical destruction processes caused by the applied static load. Consequently, a clever method of monitoring the actual damage state is to monitor the variation in σ* during the operational life of an elastomeric sealing material.
When the variation in the real part of the complex dielectric conductivity, σ* - which is a result of varying static load – is analyzed at a given dielectric frequency, fel, this behavior becomes clearer.
This dependence at a dielectric frequency, f, of 10 Hz, is displayed in Figure 4.
The relationship between the increasing static load and decreasing complex dielectric conductivity is confirmed by Figure 4. This is the result of the decrease in density in the conduction paths within the SBR sample. This allows for the actual state of damage in the filler network to be monitored.
Figure 4. Variation in the real part of the complext dielectric conductivity, σ’, of the SBR sample filled with 70 phr N 234 as a function of the static force at a dielectric frequency, fel, of 10 Hz.
The primary quality control system for technical products under mechanical load is dynamic mechanical analysis (DMA). The development process for technical products is further supported by dielectric analysis (DEA).
DEA has an extremely large available frequency range compared to DMA, which enables an in-depth molecular understanding of the internal dynamics.
With little effort, conclusions can be drawn from this important insight into a material’s microstructure. These relate to the actual state of damage of a finished technical product during active operation when electrically conductive fillers are used.
It was demonstrated that current changes in dielectric conductivity are in accordance with the state of its filler network, and consequently the damage in the sealing element.
A unique benefit is offered by the DIPLEXOR 500 N. Characterization of the dielectric properties of sealing elements under high mechanical load is permitted, enabling the determination of both their properties and their actual operational performance.
This information has been sourced, reviewed and adapted from materials provided by NETZSCH-Gerätebau GmbH.
For more information on this source, please visit NETZSCH-Gerätebau GmbH.