Compatibility testing format for lubricant exposure to plastic or rubber

Making sure a lubricant does not have any compatibility issues with a plastic or rubber is very important, especially if you are similar to our typical customer and are using one of our synthetic lubricants inside your own finished device that will be manufactured in large volumes; any deformation or degradation of plastic or rubber could cause big problems – see our introductory article on the subject of lubricant vs plastic/rubber compatibility.

To ensure there are no rubber or plastic compatibility problems due to the use of a lubricant, a compatibility test is recommended. It is always an option that a customer can perform their own test but for new and existing customers, we can arrange for a compatibility test to be carried out free of charge (commercial justification required of course).

So what is the format for a compatibility test? There are no strict rules and currently there is no DIN or ASTM exists that is appropriate. There is a typical format for the test, parameters of which can be altered depending on the type of plastics and grease to be used in the test. Details of a typical test follow:

Equipment used – Lab oven, petri dishes, measuring devices/equipment and a suitable standard of camera.

Overview – The necessary plastic/rubber pieces will be exposed to the relevant lubricant (grease or oil) at elevated temperatures using a laboratory oven, the higher temperatures and durations considered a type of accelerated ageing process. Duration of test would be agreed with the customer, normally maximum 168hrs.

Parts required from the customer – Typically, four pieces of each plastic/rubber material are required – two test pieces, two to act as a lubricant free control piece for the test. We can deal with less than four pieces but that would impact the test.

Before testing commences – All parts to be tested are measured, weighed and photographed. It is also possible (on request) to note the shore hardness and tensile strength of parts, but the hardness and strength measurements are not standard measurements taken plus there are issues with taking tensile strength measurements for unusual shaped parts.

Test temperature – Customer must advise the correct oven temperature for the test. Normally the test temperature is from around 70°C to 150°C. When selecting the test temperature it should be considered what the normal or upper operating temperature would be when the relevant plastic/rubber part is mounted in the finished device. Another consideration is obviously that the test temperature should not exceed the plastic/rubber part manufacturers specified maximum operating temperature.

The testing process:

• Control 24hrs – Part is placed in a petri dish and placed in the oven. Removed from the oven after 24hrs.
• Test 24hrs – Part is placed in a petri dish and is immersed in the chosen lubricant. Part placed in the oven. Removed from the oven after 24hrs.
• Control 168hrs – Part is placed in a petri dish and placed in the oven. Removed from the oven after 168hrs.
• Test 168hrs – Part is placed in a petri dish and is immersed in the chosen lubricant. Part placed in the oven. Removed from the oven after 168hrs.

After each part is removed from the oven, it is weighed, measured and photographs are taken. Weight, dimensions and image of tested parts are compared to the pre-test statistics. A conclusion can then be given.

Lubricant incompatibility can cause materials to swell, shrink, harden, soften, become more or less brittle and change colour. Dimensional changes of up to 5% are usually considered acceptable.

A written report of the test outcome is provided to the customer including photographs of the relevant parts and table of results. Some test image examples are below along with an associated example of a typical table of results, the format of which will vary slightly depending of the parts supplied and the technician performing the test.

Note – Tested parts can be returned to the customer for inspection and storage for future reference.

If you are interested in reading more about compatibility issues, we have a ‘traffic light’ guide to the base oil of lubricants and possible plastic or rubber compatibility issues.