A tensile test is a relatively straightforward process for the most part. The purpose of the test is to apply a tensile, or pulling, load to a specimen of the material and observe both the load at which the material breaks and the elongation of the specimen at the point of breaking. A slightly more advanced testing equipment will also give you the Young’s Modulus – also known as the Modulus of Elasticity – which takes into account the extent to which incremental loads create incremental elongations in the material.

Most mechanical labs will have the equipment needed to test for tensile properties. However, while materials such as steel and rubber have well documented and easily accessed standards on how the test must be performed, polymers – most specifically high-performance plastics – can be a different story altogether.

We have observed, on more than one occasion, that the values obtained from a lab differ significantly from those on the material datasheet. Since the materials we buy are from reputed suppliers with their own in-house labs, this creates some confusion as to whether the properties are indeed not meeting the specifications, or whether there is a way to test the materials that the external labs may not always be aware of.

A few factors are at play here and need to be properly understood before sending any polymer to a lab for testing:

1. Shape to be tested

Most standards will specify a certain set of shapes that need to be made for testing. Usually, these are dumbbell in shape, with a thickness of 3mm to 5mm. The dumbbell shape is wider at the ends, where the test sample will be gripped by the machine. The cross-section area of the thin section in the middle is where the load is applied when the dumbbell is stretched lengthwise.

While most standards offer a general suggestion on how to make the dumbbell shape, a lot of thought needs to be put on the exact shape as well as the thickness. Care also needs to be given on deciding how the part should be cut to make the dumbbell, as we shall see below

2. Method of processing – polymers such as PEEK, PTFE, Polyimide, and even PVDF can be processed in a number of ways. PTFE, for example, lends itself to compression moulding, ram extrusion, isostatic moulding, and paste extrusion. Similarly, PEEK can be compression moulded, extruded, or injection moulded. The varying process methods would accordingly cause the internal grain structure of the polymer to be similarly varied.

In the case of PTFE, a tensile test can be performed on a compression moulded part that has then been skived into a 3mm thick tape. A dumbbell can then be punched out of this tape. However, because of the nature of compression moulding, the tensile strength will vary depending on whether the dumbbell is cut along the length of the tape or across it. For this reason, many datasheets for PTFE will give different values for LD (longitudinal direction) and TD (transverse direction). If a lab offers to punch the material into the dumbbell, the manufacturer will still need to specify which the longitudinal direction is so that the best result can be obtained. Similarly, ram extruded tubes would generally exhibit lower tensile properties when compared with paste extruded tubes. This is because ram extrusion is an additive process, whereas paste extrusion is continuous. Hence, there are weak spots in a ram extruded tube that will bring the value of the tensile strength down.

With PEEK, extruded rods would similarly exhibit better tensile properties than compression moulded materials. An added issue with PEEK is that machining the dumbbell shape places a lot of internal stress on the material and the dumbbells themselves need to be properly annealed before any tests are done.

3. Testing speeds

Even if the above precautions are taken, polymers are extremely sensitive to testing speeds. Again, standards will prescribe a range of anywhere from 1mm per minute to 50mm per minute for the speed of the testing, but it is the manufacturer that needs to often take trials to determine which speeds offer values that match the materials base properties and then use this as the benchmark. PTFE generally benefits from being pulled faster, so we test PTFE at 5mm/minute. PEEK, on the other hand, does not do well under high speeds and will break very easily if tested at 5mm/minute. Hence, 1mm/minute is the maximum speed at which PEEK should be tested.

Beyond testing, it is also useful to note that some parts simply cannot be tested. Machined components, for example, cannot be subjected to a tensile test as prescribed in the standards, since the components are shaped in such a way that a dumbbell shape cannot be extracted from it. In such cases, it is up to the manufacture and OEM to decide between them what tests can be done and what tests – for example a test on a moulded part that acts as a proxy to the component – would suffice as representative of the final component.

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ePTFE (expanded PTFE) is one of the most versatile sub-products of PTFE. Like so many products in the high-performance polymer space, ePTFE needs to be processed in its own special way. Unlike regular PTFE – which itself is very painstaking to manufacture due to its unique properties – ePTFE is made via a combination of paste extrusion, followed by stretching on special equipment. The resulting polymer structure is useful for a number of reasons:

1. The softness of ePTFE allows it to be used as a gasket material, creating very effective seals even with minimal torque

2. The insulative properties of ePTFE allow thin tapes to be used in cable wrapping, offering high dielectric properties.

3. The porous structure of ePTFE is highly unique, as is allows gases and vapour to pass through, but not liquids and dust, making it an idea material for vents

4. ePTFE is also safe for the human body, meaning medical devices frequently employ the material in stents and grafts that can be left in the body indefinitely without causing any harm.

Despite all this, there exist very few actual standards that define the properties of ePTFE and outline testing parameters. Because a lot of ePTFE applications are so specialised, companies that develop solutions in this material usually treat the testing parameters and methods as proprietary. So, while the end product – such as a medical device, for example – may be approved by the device manufacturer, the process and testing metrics are held on to by the manufacturer.

As a result, developing an ePTFE solution for an OEM can be tricky, since the OEM has no yardstick against which to judge the material supplied. In many cases, the OEM may use the material in the end-assembly and decide that it works well. However, they would still need the ePTFE manufacturer to provide a set of testing criteria to be applied to every batch such that the OEM is guaranteed a consistent product. In this endeavour, and in the absence of any suitable standards, the decision needs to be taken between the OEM and the manufacturer on what tests and what values count as acceptable.

An interesting case presented itself when an OEM approved our tapes for a very specialised application but felt that there was inconsistency in some of the batches. We wanted a quantifiable way to assess the same so that both parties could agree on the results. The key requirements were the following:

1. The tape needed to have a consistent density and softness throughout

2. The specific gravity of the tape needed to be as close to 0.4 as possible

3. The tape should not have any excess fibrils and ‘gatoring’

Before we get into the above points, it should be mentioned that there does exist one useful standard for ePTFE tapes. The AMS3255A is an aerospace standard but offers enough by way of density and tensile properties to be a good starting point.

Going back to the OEM, they were trying to find ways to determine whether the properties were as they needed, but the results were all over the place.

For starters, they were using a durometer to measure the hardness of the tape. While Shore A and Shore D durometers are frequently used for elastomers and polymers respectively, in the case of ePTFE the durometer does not work. Why? Well, for one, ePTFE is porous. The tip of the durometer can easily pierce the surface of the tape and since the pressure is applied by a human hand, there may be areas where the tip pierces through and other areas where it does not. The other issue is that unlike elastomers, ePTFE has no elasticity. Once compressed, the material stays as such until (in the case of a tape), you pull it lengthwise, at which point it retains its original dimension. Both these issues mean that a Shore A durometer will give a wildly inconsistent and inaccurate reading.

The other issue was around something we call ‘breath back’. ePTFE tape is made via a stretching process, wherein the tape is rapidly stretched at a high temperature. The resulting tape is a smooth, marshmallow-like material that is easily compressed by hand. However, unless the tape is sufficiently restrained after stretching, it will slowing begin to shrink lengthwise. The extent of this shrinkage could be as much as 10-20%. So, even if we were able to achieve the specific gravity requirement of 0.4, once the material reached the OEM, there was every chance that it could have gone up to 0.5. It should also be stated that most ePTFE tapes available for industrial applications have a specific gravity of around 0.75. By attempting 0.4, we were already stretching the tape to the maximum, meaning the tendency to breathe back would be even more pronounced.

In the absence of any global testing standards, we had to adopt customized methods to check the consistency of the tapes.

1. We ran the tape through a calendaring machine to compress it and then checked to see whether the readings were consistent across the length. If there were lumps or weak spots in the tape, the result of this exercise would be a tape with varying thickness

2. We cut the tape into even lengths and checked each lengths for specific gravity

3. We built a fixture wherein the tape would sit under a v-shaped weight that would compress it. Again, if the thickness of the compressed tape against a fixed weight was uniform across different points on the tape, the tape could be deemed consistent

4. Finally, we cut lengths of the tape and performed a simple tensile test to check tensile strength and elongation. Variations in tensile strength and/or sudden breaks in the tape during load application would suggest weak spots within the tape

5. To address the issue of ‘breathe back’, the tape was re-stretched at 150 Degrees C before spooling and then spooled tight before shipping. While this certainly helped keep the tape soft during transit, the client needed to be informed that once the spool was opened, the shrinkage was inevitable. However, even a manual pull on the tape lengthwise would bring the tape back to its earlier softness, so it was something the client would need to do prior to assembly.

Because the stretching process is mechanical and because the specific gravity required was so low, this tape was especially tricky. It would therefore be impossible to avoid micro-variations in density across the full length of tape. However, we were able to produce a product that was consistent enough to answer all the tests we conducted, and would therefore be deemed suitable to this specific end-application.

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As technological and industrial landscapes undergo transformations, more sustainable, efficient, and cost-effective materials are always emerging. Among these, polymers are emerging as frontrunners, challenging the traditional dominance of metals in various applications.

The shift towards polymers in applications traditionally dominated by metals is driven by several compelling advantages that polymers offer, including:

Weight Reduction: Polymers are significantly lighter than metals, which is a critical factor in automotive and aerospace industries where weight reduction translates to improved fuel efficiency and reduced emissions.

Corrosion Resistance: Unlike metals, polymers are inherently resistant to corrosion, reducing the need for protective coatings and maintenance.

Cost Efficiency: The production and processing of polymers can be less expensive than metals, especially when considering the lifecycle costs including maintenance and durability. Although pound for pound, polymers do tend to be far more expensive (especially when we consider plastics like PEEK and PTFE), the weight reduction combined with ease of manufacture can potentially lower costs.

Design Flexibility: Polymers can be easily moulded into complex shapes, allowing for more innovative design possibilities compared to metals.

Insulating Properties: Polymers are excellent insulators of electricity and heat, making them indispensable in electrical and electronic applications.

The benefits of using polymers are usually derived through a case-by-case analysis of the applications involved. However, some industries have naturally gravitated towards polymers, as a result of the above reasons.

Automotive and Aerospace

In the automotive and aerospace sectors, the drive for fuel efficiency and emission reduction is a powerful motivator for the adoption of polymers. Polymers are being used to fabricate components such as fuel tanks, bumpers, interior panels, and even structural components in aircraft. The Boeing 787 Dreamliner and Airbus A350, for instance, feature airframes that include significant amounts of carbon fiber-reinforced polymers, offering unmatched strength-to-weight ratios compared to metals.

Medical Devices

The biomedical field is another area where polymers are replacing metals, owing to their biocompatibility, flexibility, and the ability to be sterilized. Polymers are used in a wide array of medical devices, including catheters, implants, and enclosures for medical instruments, providing improved patient comfort and outcomes.

Electronics

The electronics industry has embraced polymers for their insulating properties and flexibility. Polymers are used in the insulation of cables, components of electronic devices, and as substrates for flexible electronics. Their lightweight nature and versatility also enable the development of innovative products such as wearable devices and foldable screens.

Construction

In construction, polymers are increasingly used in place of metals for applications such as piping, roofing, and insulation. Polymer-based materials offer advantages in terms of ease of installation, resistance to corrosion, and thermal insulation properties, contributing to more energy-efficient buildings.

While there do exist an number of polymers that would, in their own way, be suitable candidates for metal replacement in a given application, the below three are the ones we see the future developing around:

Polytetrafluoroethylene (PTFE)

PTFE, best known by the brand name Teflon, is a fluoropolymer with exceptional chemical resistance, low friction, and high-temperature tolerance. In industrial settings, PTFE is prized for its resistance to corrosive substances, making it an excellent choice for seals, gaskets, and linings in chemical processing equipment, where metal counterparts would suffer from corrosion. Furthermore, its low friction coefficient is beneficial in the manufacturing of bearings and gears, particularly in applications where lubrication is undesirable or impractical.

Polyether Ether Ketone (PEEK)

PEEK is a semi-crystalline thermoplastic with a unique combination of strength, heat resistance, and chemical stability. Its ability to retain mechanical properties at temperatures up to 250°C, coupled with its resistance to aggressive chemicals, makes PEEK an excellent metal substitute in harsh environments. In the aerospace industry, PEEK is used to manufacture components such as seals, bushings, and fasteners, contributing to weight reduction without compromising performance. The medical field also benefits from PEEK's biocompatibility, where it is used in the production of surgical instruments, spinal fusion devices, and dental implants, offering an alternative to metals that may cause allergic reactions or interfere with medical imaging.

Polyimide

Polyimide is a polymer known for its exceptional thermal stability, electrical insulation, and mechanical strength over a wide temperature range. These properties make polyimide an invaluable material in the electronics industry, where it is used in the fabrication of flexible printed circuits and insulation for high-temperature applications. Additionally, its resistance to radiation and vacuum compatibility makes it an ideal choice for space applications, including insulation for spacecraft and satellites. In the automotive sector, polyimide films are employed in sensors and components exposed to high temperatures, showcasing its versatility as a metal replacement.

Future Perspectives

The future of polymers is incredibly promising, with ongoing research focused on enhancing their properties and expanding their applications. Innovations in polymer science, such as the development of high-performance thermoplastics and bio-based polymers, are paving the way for polymers to replace metals in even more demanding applications. Moreover, the environmental benefits 

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