What is Thermal Ageing?
Ageing of the materials with
temperature could happen and differs for various materials. Important safety
systems/equipments need to be evaluated for the effect of temperature on the
materials used in the system. Even storage for long years shall also be
considered for some materials like polymers to estimate the ageing effect.
Ageing effect couldn’t be estimated
practically for operating conditions or storage conditions as the duration is very high (tens of years) and the qualification of product couldn’t happen.
Hence, Ageing effect due to temperature could be evaluated by exposing the unit
to higher temperature for accelerated time. This is called as Accelerated Thermal
ageing test.
How to calculate time required for Thermal Ageing of an instrument at
accelerated temperature?
There
are different methodologies to do Thermal Ageing test. Most commonly used
methodology is application of Arhenius equation. Usually, this methodology is
applicable for Polymer/Organic materials for which temperature effect is more.
Thermal
Ageing is due to the chemical process induced by the temperature, which could
lead to change the bulk properties of the material over a time.
The Arhenius methodology has been developed
to simulate the accelerated ageing mechanism. As per this methodology, deterioration
of materials in service is due to chemical reaction. These occur internally,
sometimes between components of the material, and sometimes with compounds in
the environment such as oxygen or water vapor. Chemical reactions occur more
rapidly at higher temperatures.
Arrhenius showed that temperature dependence
of chemical reactions follows an exponential equation. He postulated a
consistent correlation between the amount of physical change and chemical
reaction so that the time to reach a selected amount of physical change will
vary according to an equation. The rate of thermal aging is the slope of the graph
using the Arrhenius equation.
Accelerated Time calculation for Thermal Ageing Test
As
per Arhenius equation,
where,
‘E’
is activation energy for the material under consideration; in eV
‘K’
is Boltzman Constant i.e. 8.617 x 10-5 eV.K-1
‘t1’
is ageing time required at accelerated temperature ‘T1’.
‘t2’
is service life time operating temperature ‘T2’.
‘T1’
is accelerated temperature
‘T2’
is Operating temperature
For instance, let us consider the cross linked polyolefin cables (XLPO cables) being used for an application where the normal operating temperature is 50 oC and the expected service life is 40 years. Let us calculate the accelerated time required for thermal ageing test at accelerated temperature of 135 oC.
i) Activation energy (E) for XLPO cable is considered as 0.78 eV
[Ref: An Experimental Study on Dielectric Response of the Cross linked Polyolefin (XLPO) Insulation of the Unshielded Cable Under Electric Field and Heat Flux; Publisher: IEEE]
ii) t2 = 40 years
iii) T1 = 135 degC
Time required for accelerated Thermal Ageing test for accelerated temperature of 135 oC is ~43 days.
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