TIRE RESISTANCE TO OZONE

OZONE

We may not know it but Ozone is present in our atmosphere in very small quantities. Even if the amount of ozone is contained, this gas can cause cracks in rubber articles that are subjected to tensile stress and the consequence is the loss of the properties of the material. It is therefore necessary to test the resistance of the tires to exposure to ozone.

Due to the uncertainties of natural exposure, tests for ozone resistance of tires are normally performed in laboratories using specially designed ozone chambers.

The resistance test is based on the principle of exposing the specimens in most cases under static tensile stress, but they can also be tested under continuous dynamic stress, or under alternating periods of dynamic and static stress, in a closed chamber at a specific temperature, at high or unspecified humidity and in an atmosphere containing a fixed concentration of ozone.

The specimens are examined periodically or at the end of the test to verify surface cracking (ozone cracking phenomenon).

ACTION OF OZONE ON TIRES

Many rubber compounds have carbon-carbon double bonds that react easily with O3, particularly if they are in a tensile state. The consequence is the breakage of the elastomer chains and the presence of cracks/breaks at the surface level.

Ozone breaks the C=C double bond creating a very unstable compound called Molozonide which decomposes rapidly and tends to split to form a structure called Zwitterion and an aldehyde or ketone:

Fig.1 – Breaking of double bonds by ozone.

Some elastomers are intrinsically resistant to the action of ozone precisely because they do not have C=C double bonds in their structure, for example EPDM.

For tires that are sensitive to the action of ozone, there are suitable anti-ozonation agents. These additives, which slow down or completely block the above reactions, provide protection as they are more reactive than the surface of the rubber, forming a superficial shield film between the material and the atmosphere.

THE OZONE RESISTANCE TEST

At an international level there are regulations that regulate the requirements relating to instruments and recommend the parameters to be used for the test, for example:

% of Ozone, Temperature and Elongation to be imposed.

Obviously it will depend on the relevant specifications and therefore on the specific type of material and application to determine the precise conditions.

Generally the following parameters are used:

Ozone Concentration = 50 ± 5 pphm (50 parts of ozone per 100 million parts of air)

Temperature = 40 ±2°C

Elongation = 20 ± 2%

Time = 72h

As a type of specimen, either “strips” or dog bone specimens (dumbbell) are often used, which are used for tensile tests. The test pieces are die-cut directly from the finished article, or from the plates if you want to test a mix.

Speaking of resistance to ozone with the static method, these are inserted into appropriate clamps to impose the indicated elongation on them. Clamps should be made of a material (e.g. aluminum) that does not readily decompose ozone. The specimen holders are mounted on a rotating support (the specimens rotate both on themselves and around the chamber).

Fig.2 – Example of dumbbell and clamp specimens.

Strength results are generally expressed with the absence or presence of cracks.

Sometimes an evaluation is provided with an index of merit relating to the visibility of cracks, for example:

*MERIT INDEX*	*FOR SURFACE DEFECTS*	*CLASS UNI*
6	Cracks not visible at 6x 00 magnification	00
2	Cracks not yet visible at 2x 0 magnification	0
1	Cracks not visible at 1x 1 magnification	1
0	Small cracks visible at 1x 2 magnification	2
00	Medium and large cracks visible at 1x 3 magnification	3

Below is an example of a cracking phenomenon present at the end of aging (50pphm, 40°C, 20% elongation, 72h):

Fig.3 – Example of Ozone cracking.

Finished components are often also tested. According to the customer, they are for example stressed as in their application (in trim).

Generally the pipes can also be wound on mandrels with a diameter defined according to the minimum bending radius indicated on the drawing.

Do you have a rubber item or a compound and would like to know if it is appropriately additived with anti-ozonants? Send an email to [email protected] !

In our laboratories we can carry out the tests indicated and many others, we leave you the link to the laboratory tests!

A special thank you goes to our colleague Dr. Pasquariello, an expert in the rubber sector, who raises our awareness of this issue so as to provide us with new content for these #laboratory pills !

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