Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Understanding PTFE Impellers: Revolutionizing Fluid Dynamics

Among the various properties of PTFE, that of its chemical resistance is one that industries are increasingly leaning on for newer applications. PTFE is resistant to almost all known chemicals, with the exception of some alkali metals (molten or in solution) and rare fluorinated compounds at high temperatures and/or pressures. This uniqueness means that PTFE is one of the few materials that can be left inside the human body indefinitely with no adverse effects.

Polytetrafluoroethylene (PTFE) impellers have become an essential component in various industrial applications, especially in fluid dynamics, due to their unique properties and versatile applications. Known for their exceptional chemical resistance, non-stick surface, and durability, PTFE impellers offer significant advantages over traditional materials.

Why PTFE Impellers?

Impellers are rotating components designed to move fluids by mechanical means in pumps and other fluid-handling equipment. The choice of material for impellers is crucial, as they are often exposed to harsh conditions. PTFE impellers stand out due to several key characteristics:

  • Chemical Resistance: PTFE is virtually inert to almost all chemicals, making PTFE impellers suitable for handling aggressive substances, including acids, bases, and solvents. This resistance ensures long-lasting performance and reduces the risk of corrosion.

  • Non-Stick Properties: The non-stick nature of PTFE prevents the build-up of materials on the impeller surface. This is particularly beneficial in processes involving sticky or viscous fluids, as it minimizes maintenance and cleaning requirements.

  • Thermal Stability: PTFE can withstand a wide temperature range, from -200°C to 260°C. This makes PTFE impellers ideal for applications that involve extreme temperatures, ensuring consistent performance without degradation.

  • Low Friction: PTFE has one of the lowest coefficients of friction of any solid material. This property reduces energy consumption and wear, leading to improved efficiency and longer service life of the impeller.

Applications of PTFE Impellers

PTFE impellers are used across various industries due to their robust characteristics. Some notable applications include:

  • Chemical Processing: In chemical plants, PTFE impellers are used in pumps that handle corrosive chemicals. Their resistance to chemical attack ensures the integrity and reliability of the pumping system.

  • Pharmaceutical Manufacturing: The non-reactive and non-stick properties of PTFE impellers are ideal for pharmaceutical processes, where contamination must be avoided. They help maintain the purity of products and prevent cross-contamination.

  • Food and Beverage Industry: PTFE impellers are used in food processing equipment to handle ingredients and products that may be sticky or prone to contamination. Their easy-to-clean nature ensures hygiene and compliance with food safety standards.

  • Water Treatment: In water treatment plants, PTFE impellers are employed in pumps that handle various chemicals used in water purification. Their durability and resistance to chemical wear enhance the reliability of the water treatment process.

Advantages Over Traditional Materials

Traditional impeller materials such as metals or ceramics may suffer from corrosion, wear, or chemical attack, leading to frequent replacements and maintenance. PTFE impellers, on the other hand, offer a longer lifespan and reduced downtime. This translates into cost savings and increased operational efficiency.

Furthermore, PTFE impellers contribute to energy savings due to their low friction properties. In high-speed applications, this can result in significant reductions in power consumption and operational costs.

Challenges in Development

One of the notable drawbacks of PTFE is that it cannot be moulded using traditional methods. As a result, the blades of a PTFE impeller need to be made by moulding a block of PTFE and machining the impeller from this. This poses two problems. The first is that it is very time consuming. A single impeller might take anywhere from 3-4 hours to machine, making it expensive. However, this is not a huge problem since these are fairly specialised items and the need for using PTFE negates any commercial hurdles that might present. More problematic, however, is the shape itself. The impeller blade has specific angles, and these can often not be machined using simple equipment. Usually, a 5-axis machine is needed to adequately make the final shape, with special CAM programming to allow the tool to finish the part to the desired dimensions. Even if the equipment is available, PTFE is extremely difficult to machine and it requires both a machining knowledge and an in-depth understanding of the polymer to get the right result.

Conclusion

PTFE impellers represent a significant advancement in the field of fluid dynamics, offering unmatched chemical resistance, non-stick properties, thermal stability, and low friction. Their application across various industries underscores their versatility and effectiveness. As technology advances, the use of PTFE impellers is likely to expand, driving improvements in efficiency and reliability in fluid-handling systems. Embracing PTFE impellers can lead to enhanced performance, reduced maintenance, and long-term cost savings, making them a valuable investment for any industry dealing with challenging fluid environments.

As 5-axis capabilities develop – as it the case at Poly Fluoro – these vital components can be handled with ease and with the precision needed.

Read More

1. Tensile Testing of High-Performance Plastics

2. Expanded PTFE (ePTFE) Gasket Tapes - Challenges in Testing 

3. Polymers in Metal Replacement

Tensile Testing of High-Performance Plastics

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.


Read More

1. Expanded PTFE (ePTFE) Gasket Tapes - Challenges in Testing 

2. Polymers in Metal Replacement

3. Advancements in Aerospace - High-Performance Plastics Take Flight

Expanded PTFE (ePTFE) Gasket Tapes - Challenges in Testing

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.


Read More

1. Polymers in Metal Replacement

2. Advancements in Aerospace - High-Performance Plastics Take Flight

3. Enhancing Electrical Systems: The Versatility of PTFE Busbar Supports