Sealing with TPV

Rubber World, Oct, 2008 by Ward E. Narhi

It is quite clear from table 5 that once the temperature is increased, thermoset rubber compounds exhibit stress-relaxation data (e.g., force retention) inferior to Santoprene TPV. The higher the temperature, up to 100[degrees]C (212[degrees]F) in the table, the poorer the performance of the thermoset rubber compounds which exhibit a so-called "chemical" relaxation or degradation. Santoprene TPV does not show such a "chemical" degradation under the same conditions of testing.

This difference in performance at elevated temperatures is exhibited in figure 1, where the stress relaxation properties of Santoprene TPV are compared to one sulfur-cured EPDM compound (table 7).

On the basis of the information developed using the compression stress relaxation method, ISO 3884(A), the design engineer is, however, still not able to create the right geometry for a static seal because all measurements are made at the temperature of the aging.

To address this, ISO Method 3384(B) was developed.

Compression stress relaxation, ISO 3384(B)

While the compression stress relaxation method ISO 3384(A) data are quite good to compare the potential sealing properties of different materials, data from method ISO 3384(B) are more useful to the design engineer, in combination with force decay data between 0 and 30 minutes compression. All measurements are made at 23[degrees]C (73[degrees]F) after an aging period at a higher temperature, and this approach is more in line with sealing problems encountered in the field.

Many applications see higher temperatures for periods of time, but return to room temperature, which tends to accelerate relaxation. ISO 3384(B) attempts to isolate temperature effects on relaxation. The usage of ISO 3384(B) is similar to the method in example 1, but uses data from table 6. An example of how this method is used is shown in example 2.

This level of residual compression force may sometimes seem low, but the key parameter in sealing is the absolute value of the residual stress under load. It should be high enough to ensure tightness during the lifetime of an assembly. In other words, the initial sealing force should be high enough to compensate for the anticipated force decay with time and temperature, and thus maintain a sufficient sealing force. The maximum strain level should not be over 25%, even though 45% might be acceptable if required by tolerances.

As mentioned above, a harder Santoprene TPV grade may have to be chosen to reach the correct initial sealing force level to meet the recommended local constraint level for Santoprene TPV. The grades recommended for static sealing applications exhibit the best sealing performance when the nominal compression is limited to about 25%; up to 45% strain is acceptable from a tolerance viewpoint, as indicated from internal measurements on compression stress-relaxation.

Dynamic sealing

Dynamic sealing is when the seal is put under strain for a definite period of time, after which the strain is relieved for some time, and then applied again. This loading can be at a specific frequency or random.