The
Ozone factor
SMC
Pneumatics enumerates the solutions for controlling the adverse effects
caused when ozone enters and corrupts a pneumatic system
Ozone
is an allotrope of oxygen, and small quantities of it are present in
air. It is formed by electrical discharges in dry gaseous oxygen or
air. It is also formed through the heating of fluorine, water, oxygen,
and through the exposure of air to ultraviolet irradiation or X-rays.
A light-blue gas with a distinctive odour, it is highly oxidative and
is used for sterilisation, bleaching, and oxidation purposes. It can
cause rubber materials (mostly NBR) used in pneumatic equipment to crack,
and thereby lead to air leakage and malfunction.
Rubber materials
The time taken In most cases, there is a clear-cut relationship between
ozone concentration (C) and the time it takes for ozone to generate
cracks (t).
t . Cn = Const.
t: Time it takes for ozone to generate cracks (hrs)
C: Ozone concentration (ppm)
n: Constant that varies depending on the rubber material
To establish a linear relationship between the variables mentioned above
a logarithm was taken and the equation was rearranged as follows:
logt = K - nlogC
K: Constant
Linear relationships were obtained when plotting the logarithms of the
experiments SMC conducted on ozone concentration and the time it takes
for ozone to generate cracks. What became evident is that as ozone concentration
increases, the time it takes for ozone to generate cracks decreases.
These tests were conducted under conditions that are much more severe
than those of a normal operating environment, and were likewise evaluated
more stringently. Changes in ozone concentration greatly affect the
time it takes for ozone to generate cracks.
Double
bond
Deterioration caused by ozone occurs when ozone acts on the NBR (Nitrile
Butadiene Rubber) double bond to break the rubber molecules apart. On
the other hand, HNBR’s basic structure, which is becoming more
and more standard, eliminates the double bond of NBR by adding hydrogen.
Although a minute amount of double bond remains in the structure of
HNBR(Hydrogenated Nitrile Butadiene Rubber) to allow it to maintain
its physicality as a rubber, its structure has an excellent ozone resistance.
Ozone resistance
An experiment was performed under test conditions conforming to JIS
K6259, and using air containing 1ppm of ozone. The test conditions (ozone
concentration, temperature, and tensile strain) were much more severe
than those in a normal operating environment; therefore, cracks were
evident in NBR in just 1-25 hours of exposure to ozonic air. On the
other hand, when the same stringent conditions were applied to HNBR,
cracks did not appear even after 1000 hours of exposure.
Grease coat protection effect
A coating of grease or other lubricants is an effective way to shield
NBR parts from an ozone attack. When the protective effects of grease
were tested and measured, it was found that cracks began to appear after
1 to 25 hours of ozone exposure on NBR components that had not been
given a grease coating. On the other hand, cracks did not appear on
the
....CONTD
