ASTM International is the curator of many industry standards throughout the industrial world. The ASTM D2000 standard governs the specification of rubber products in automotive applications. This particular standard has been adopted to cover many areas beyond the scope of the automotive industry and is a commonly used tool used to establish the minimum requirements for a rubber material in a given application. In the composition of an ASTM D2000 callout, the two most important factors in defining a material are the temperature at which it is tested and the rate the material swells in oil. These two factors become the base of a recipe for a material. Like flour and eggs in a cake, they provide the structure for the material formulation. Other factors and test requirement provide the flavor of the cake and create application specific requirements for the rubber product’s formula. Still, the base structure is the most critical aspect simply because above all else, temperature and swell determine the suitability of a rubber material for the application.
The temperature of an application can affect many different components of a system. For example, a high temperature system may require specialty fluids, different materials for wear components, or other considerations. It stands to reason that the material of the rubber components would also be affected. The test temperature element of the ASTM D2000 callout specifies the base temperature for testing of the rubber material and classifies it as the “Type”. Understanding the nature and limitations of the various rubber material families can make interpretation of the test temperature quite straightforward. When the testing is performed at 100˚C, the material options are quite broad. When testing is performed at 200˚ C, options become far more limited. Testing of the material for application suitability is ideally performed at worst case conditions for the sealing material to ensure the best possible performance of the rubber.
In most applications, the way a rubber seal actually performs its sealing function is through squeezing the material so it can create a compressive force that prevents fluid from getting past the seal. Consideration is made for accommodating the volume of the seal in this compressed state to prevent overfilling the gland and causing damage to the housing or seal component itself. That being the case, a rubber material that swells in the fluid it is sealing can potentially cause catastrophic problems for an application. The ASTM D2000 component specifying swell classifies swell as the “Class” of rubber material and is based upon the volume swell, as a percentage, of a given material in IRM 903 Oil after 70 hours at the temperature classified in the “Type” component of the callout. A material designed to function in hydraulic fluid might have a maximum volume swell allowed of 10%, while a material intended for use in water and incompatible with oil might have no requirement because it would swell over 300% when exposed to the test oil.
There are several other critical tests performed to ensure suitability of a rubber material’s usefulness for an application, including compression set values, ozone resistance, low-temperature capabilities, tear resistance, and many other specialty requirements which users require. Still, at the heart of a rubber product is how it handles at temperature and how it reacts to fluids. These two factors form the heart of a material specification and determine the base compatibility for most applications. With these two, key factors, the base material can be selected for an application, and the structure of the formula is determined. That “cake” can begin to mix and the specific flavor elements can be added to suit each unique need.