Sealing with TPV

Rubber World, Oct, 2008 by Ward E. Narhi

Santoprene TPV exhibits properties well suited to sealing applications and is commonly used to replace conventional thermoset rubber. It is currently used in many applications where sealing is a critical factor. Some of these are:

* Building profiles;

* automotive clean air ducts;

* automotive distributor wire boots;

* automotive radiator seals;

* automotive steering bellows;

* automotive weatherseals;

* automotive transmission bellows;

* pipe seals; and

* medical gaskets.

Most specifications related to sealing include a short term compression set (CS) performance requirement. Sometimes, but more often in the last 10 years, the stress relaxation method is considered in sealing applications. More of these applications could be developed with TPV, provided the fight test method is used for comparing its respective properties versus thermoset rubbers like EPDM, SBR, NBR and ECO. Combining design with material capability paves the way to success.

Two main areas for sealing are found in the marketplace (e.g., static and semi-dynamic applications). This document reviews these two areas in terms of material specifications and performance, and then demonstrates how the design of the part can improve its sealing performance.

Static sealing

Static sealing means a seal is put under strain for a definite period of time, generally for its lifetime. The seal is permanently maintained at the same level of strain and can be submitted to temperature changes. This is a situation currently met with most pipe seals in the fluid delivery market and in window glazing seals.

In such a sealing application, the most critical parameter is the variation of the sealing force with time. The design engineer should be in a position to determine the initial sealing force of a part, which can be determined via calculation, experiment or finite element analysis (FEA). This initial sealing force is then modified to account for stress relaxation so that over a specified lifetime, the seal maintains functionality. Along with an initial decay from 0 to 30 minutes, the ISO 3384 compression stress-relaxation testing methods help model force decay to some extent, as shown below.

Measure of compression force decay from 0 to 30 minutes

Table 1 shows an estimate of the force retention at 23[degrees]C (73[degrees]F) between the initial time of compression (t = 0) and 30 minutes of compression (t = 30) for the standard Santoprene TPV grades when the injection molded buttons are loaded at a speed of 2.54 mm (0.10") per minute.

It must be emphasized that these are compression force retention data obtained from buttons prepared by injection molding, a method that can induce high residual stress levels compared to extrusion and compression molding. These data can, however, be used by the design engineer as a good guideline to design a seal with Santoprene TPV. The percent retention factor is applied to the initial sealing force to determine the relaxation from t = 0 to t = 30 minutes.

Compression stress relaxation method ISO 3384(a)

The amount of force decay from t = 30 minutes to a desired time is generally measured in accordance with the compression stress relaxation test, ISO 3384(A). Here, a 13 mm (0.51") diameter, 6.2 mm (0.24") thick cylindrical button is compressed by 25% and the decrease of the initial compression force is then measured with time.

The results from the testing are presented from the following relationship:

[R.sub.t] (%) = 100 x [F.sub.t]/[F.sub.0] = A - B x log (t/[t.sub.0]) where:

[R.sub.t] (%) = compression force retention at time "t";

A ~ 100;

B = compression force retention at time "t"; also called "loss per decade";

[F.sub.t] = force at time t;

[F.sub.0] = force at time to;

t = elapsed time; and

[t.sub.0] = 30 minutes.

Using the compression stress-relaxation equipment from Wallace, we built a database with many commercial thermoplastic rubbers. Continuous stress relaxation equipment from Elastocon was acquired and our database is progressively upgraded from the results of the tests on this equipment.

In tables 2 through 4, data on compression stress relaxation are given from measurements made after aging at 23, 70 and 100[degrees]C (73, 158 and 212[degrees]F). The grade of choice for a particular static sealing application will be the softest grade that still provides the expected sealing force level during the part lifetime. A rigid Santoprene TPV grade such as 103-40 might be seriously considered when the sealing force cannot be reached

with a soft grade such as 101-64, provided that:

* The mating surface can support the original compression stress without significant deformation.

* The mounting force does not exceed specifications;

* tightness can be ensured on a smooth mating surface; and

* the sealing force does not decrease below a critical threshold, ensuring tightness with time.

Such is the case with many commercial automotive clean air ducts and steering bellows produced from Santoprene TPV 101-87 or 103-40. They are clamped on metal or reinforced plastic supports and ensure tightness to air or water and/or lubricant.