Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Tensile Testing of PTFE

The tensile strength is among the most important properties of any polymer. It offers an insight into the ability of the polymer material to withstand loads – primarily pulling loads – and along with tensile elongation, tells us to what extent the polymer will yield before breaking. While other properties such as specific gravity, Young’s Modulus and Compressive Strength also feature as important, most manufacturers start with checking tensile strength and then move on from there.

For this reason, it is vital that a full understanding of the methods to check tensile strength are understood before conducting the same.

How is a tensile test done?

In its most basic form, the tensile test involves pulling a material apart until it breaks. You then look at the load that the material was able to sustain before breaking, as well as how far it elongated before it broke.

Different materials have different standards on how to ensure that this test is conducted uniformly and using the parameters defined.

1. In any tensile test, there are a few critical parameters to consider:

2. The speed of the test – namely the speed at which the material is pulled apart

3. The thickness of the test specimen - The shape of the specimen – usually a dumbbell shape, but the exact shape may vary across materials

Tensile test for PTFE

For PTFE, the ASTM standards form the basis for most of the testing done. Specifically, standards pertaining to PTFE include the following:

1. ASTM D4894 – used for virgin (unfilled) grades of PTFE

2. ASTM D4745 – used for PTFE compounds such as glass filled, bronze filled, and carbon filled PTFE

Both the above standards make reference to ASTM D698 with regards to tensile testing.

Because both standards refer to ASTM D698 for tensile testing, most test laboratories will test as per this standard, without verifying with the parent standard for specifics pertaining to PTFE.

As a material, PTFE tends to behave differently to other polymers. As such, both the ASTM D4894 and ASTM D4745 make specific references to the tensile test and outline the differences between the regular testing as per D698 (which makes no mention of PTFE) and the testing needed for PTFE.

The key differences can be found on all three parameters listed above. Namely:

1. Thickness

While ASTM D698 suggests thicknesses of 3mm and above, the PTFE specific standards recommend the PTFE tensile testing specimen should be no more that 1mm thick (+/-0.25mm)

2. Speed of testing

ASTM D698 recommends standard testing speeds of 5mm/minute, whereas PTFE specific standards give speeds of 50mm/minute. One of the properties of PTFE – as we have experienced during the manufacture of expanded PTFE – is that it behaves differently when stretched rapidly as against being stretched slowly. As a result, it is conceivable that the standards would make allowance for a higher stretch rate when considering the tensile properties of PTFE as a material.

3. Shape of the specimen

In our own experience, the shape of the specimen does not change the tensile testing results significantly. Nonetheless, the dumbbell shapes defined by ASTM D698 differ slightly from the shape recommended by PTFE specific standards on tensile testing.

It should also be remembered that equipment used for testing should be able to clamp the PTFE specimen adequately to ensure there is no slippage during testing. Given that PTFE has a low coefficient of friction, the risk of slippage can be high, and this can distort the final readings. At the same time, too aggressive a clamp can cause the material to fracture at the point of clamping itself. In such a case, the test would need to be repeated as the reading would be spurious.

Further, PTFE as a material can experience elongation in excess of 500%. In such a situation, the equipment would need to be capable of this much travel, as the ideal reading would be taken at the point when the material breaks. However, in the event that the required tensile and elongation properties are met, even the reading on a specimen that does not break can be considered acceptable.

Tensile properties of PTFE

In general, PTFE will exhibit tensile strengths ranging from a low of 10Mpa all the way up to 35-40Mpa. Much of this depends on the type of filler used, the moulding and sintering parameters as well as the purity and quality of the base resin. Elongation values will also range from a little as 150% to as much as 550%. Here again, the composition and quality of the material plays a key role. Although varying the parameters does not – in our opinion – greatly alter the final readings one obtains on tensile and elongation properties, it is nonetheless best practice to follow the same as per the standard. It is possible that using a different speed setting, thickness and specimen shape causes an early breakage and rejects the material that would otherwise have been accepted under settings prescribed by the standards.

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