Ozone testing rubber automotive parts
Rubber components are particularly prone to ozone attack. This risk is increased in an automotive environment.
Materials used in automotive applications have to be sufficiently durable and reliable to withstand a wide range of environmental conditions. Some of these will be external such as weather and atmospheric conditions, whilst the vehicle itself may generate others – for example high temperatures in the engine compartment or around bearings and brakes.
For some materials common to automotive applications, one of the most damaging environmental factors is ozone. Ozone (O3), a toxic form of oxygen atoms, is commonly formed from regular oxygen (O2) by electrical discharges.
Some electrical equipment can generate significant levels of ozone. For example, electric motors using brushes can generate ozone from repeated sparking inside the unit.
In the home, outdoors, and in most industrial atmospheres, ozone concentrations are likely to be fairly low, and risk of ozone damage minimal. However, in some cases, particularly in an automotive environment, higher ozone concentrations may be present in the local atmosphere. Although automotive applications tend to use fairly low voltages (12 or 24 volts), a build-up of ozone-rich air can result where many electrical motors and other components are present.
Rubbers are particularly prone to ozone attack and exposure can cause a crazing effect on uncovered surfaces, eventually leading to cracking and fracture. Ozone attack can also bring additives to the surface and cause the rubber compounds to separate. Areas where rubber materials are flexed or moulded tend to fail first as these areas are already under high stress.
Many test methods exist for testing rubber in the form of ASTM, BS or ISO standards, or specific manufacturers’ specifications. The test methods most commonly used for general rubber applications are ISO 1431-1:2012 (general methods) and ISO 1431-3:2017 (determining ozone concentration). Materials are placed in a chamber containing an ozone-rich atmosphere for a period of time, after which they are assessed for damage or tested for physical properties such as strength.
Samples are often held in tension or flexed during the ozone exposure period. There are generally three types of test:
- static (strained) loading of samples of fixed dimension – typically a strip or dumb-bell test piece is either strained to a fixed elongation or bent around a mandrel to create surface stress
- dynamic testing of samples of fixed dimension – a rubber strip is mounted in a test machine that will continually flex the samples throughout the exposure period to replicate the dynamic stresses that will occur in use
- exposure of finished products – the whole rubber product is held in the test atmosphere to replicate the specific static stress points within the material, and to predict where any damage is likely to occur.
In some specialised cases it is also necessary to design a test in which the finished product is dynamically flexed during exposure, especially if the motion of the product causes different stress patterns in the material compared to static stresses alone.
There are few general requirements for the ozone resistance of a product, so it is difficult to define a performance level at which a rubber item can be labelled ozone resistant. However, performance tests can be defined for specific applications.
Testing for ozone resistance under controlled conditions is an accurate and consistent method for product development, product comparison or quality control. SATRA is able to carry out ozone testing (under static or dynamic strain conditions) over a range of concentrations up to 20 ppm (parts per million) under controlled conditions of temperature, humidity and gas flow rate.
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SATRA can carry out component testing for most automotive parts and is always willing to discuss testing for your particular applications. SATRA also builds and supplies ozone test chambers. Please email firstname.lastname@example.org for more information.