Polymers in Low Friction Applications: Reducing Wear and Tear and Keeping it Smooth.

The development of faster, more durable equipment usually calls for efficiency in energy utilisation and components that can sustain either rotary or linear motion over a very long product life cycle. This problem always boils down to the management of friction. Moving parts will typically experience wear and tear due to friction, leading to both part failure and an unnecessary build up of heat (and therefore a loss of energy).

Advancements in polymer science have allowed a significant number of metal parts to be replaced with specific, high-performance plastics that combine a low coefficient of friction with a high wear rate (also called the Pressure x Velocity, or PV value). These polymers, often when combined with specific fillers, are able to perform for far longer, minimising replacement costs and boosting energy efficiency.

One of the primary advantages of polymers in low friction applications is their innate lubricating properties. Unlike traditional lubricants that require constant replenishment, polymers can provide a durable and long-lasting solution. Polymeric materials, such as polyethylene and polytetrafluoroethylene (PTFE or Teflon), have self-lubricating properties, reducing the need for external lubricants and minimizing maintenance efforts. In the case of PTFE (Teflon) and UHMWPE, the static and dynamic coefficients of friction are so low that when sliding against certain materials (for example: polished stainless steel) the coefficient could fall to as little as 0.03. In layman’s terms: it would take only 30grams of horizonal push to move a 1Kg block across the surface of the PTFE. This is something we also call ‘near rolling friction’.

In the case of PTFE, the addition of specific fillers – such as bronze, glass, carbon, or MoS2 – can further enhance the wear properties of the material, making it more robust in certain industrial applications. PTFE can itself be used as a filler in other polymers, including PEEK, POM (Delrin), PPS (Ryton) or even Nylons. The addition of PTFE micro powders into these polymers – usually in a concentration of 5-25% - gives an appreciable boost to the low-friction properties of the base polymer, while allowing the polymer to retain its other characteristics.

In addition to their lubricating properties, polymers offer excellent resistance to wear and corrosion. When used in bearings, gears, or sliding components, polymers can withstand harsh conditions and maintain their integrity over time. This resilience contributes to the longevity of the components and reduces the frequency of replacements, ultimately leading to cost savings for industries. PEEK is highly sought after in gears. The hardness of PEEK ensures that the part will not wear out over time, while PEEK’s low density (specific gravity of 1.3) gives the added benefit of weight saving in the system. 

Many polymeric materials excel in low friction applications due to their lightweight nature. In industries where weight is a critical factor, such as aerospace and automotive, using polymers can lead to significant fuel savings. With specific gravities as low as 0.9, the weight saving over a metal component can be as high as 90%. Especially in aerospace applications, this is a benefit that creates immense savings for the end users. The reduced weight contributes to improved fuel efficiency and overall performance, making polymers both an eco-friendly and economically viable choice.

Medical devices also benefit greatly from the incorporation of polymers in low friction applications. Prosthetic joints, for example, often utilize polymer components to mimic the natural lubrication of human joints. The biocompatibility of certain polymers ensures that they can be safely used within the human body, providing low friction solutions for a wide range of medical applications. Similarly, PTFE tubes (usually with radiopaque fillers) are used in medical applications that require the tube to slide in and out of the patient’s body. Amplatz sheaths, for example, are used in urology wherein the tube is pushed in to make a channel through which a guidewire can be passed. The smoothness of the PTFE minimises the discomfort to the patient.

In conclusion, the use of polymers in low friction applications has ushered in a new era of efficiency, durability, and sustainability. Their innate lubricating properties, resistance to wear, and versatility make them indispensable in various industries. As technology advances and the demand for high-performance materials grows, polymers are likely to play an even more significant role in shaping the future of low friction applications.

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Read More

1. PTFE Extrusion - Ram vs Paste Extruded - A comparison of features

2. Exploring the Versatile World of PVDF

3. Air Permeability Testing and Water Entry Pressure Testing in Expanded PTFE Membranes

Extruding a material like PTFE is never straightforward. The nature of the polymer necessitates that any standard methods of processing – such as melt extrusion, injection moulding, or even thermoforming – fail to work with PTFE. The reason for this boils down to the fact that PTFE has no melt flow. Even when taken to its ‘melting point’ of about 380°C, PTFE will merely attain what is known as a ‘gel state’, wherein it goes from opaque to transparent, but stays very much in the same form. This means that the material cannot be injected into a mould to make complex shapes and that it cannot be otherwise shaped or drawn into any kind of final shape. As a result, different techniques have been devised to bring PTFE into a final shape as needed.

PTFE extrusion is one such method used for making tubes and hollow profiles. Rather than melt the material and draw it through a die, the process of PTFE extrusion can be done in one of two ways.

1. Ram extrusion - where subsequent charges of PTFE resin (powder) are layered and compressed one above the other inside a die and the die is heated from the outside to allow for the PTFE resin to fuse into a single piece.

2. Paste extrusion - where a special type of resin (fine powder resin) is blended with an extrusion aid and then squeezed at high pressure through a die at room temperature. The resulting ‘extrudate’ is then heated in ovens to fuse the material.

Between these processes PTFE paste extrusion is considered more complex and it is usually employed when the final profile has a thin cross section. Hence, PTFE thin-walled tubes, or spaghetti tubes are made in this fashion. Ram extrusion is usually used when the cross section is thicker and when the requirements are for a single solid rod.

Comparing properties

1. Material grades - both ram and paste extrusions need to be made with specific grades. For paste extrusion, fine powder resins are used. These resins mix easily with extrusions aids (usually a mineral spirit like naphtha) and once blended, they will form fibrils when pressed between the fingers. In some sense, the extrusion aid forces the PTFE to behave somewhat like a liquid at high pressures, so that the material can be easily passed through a die. Typically, virgin grades are used in paste extrusion, although mixing pigments and even some quantity of additives is done easily. However, when blending fillers (such as glass fibre or carbon fibre) in excess of 5%, there can be complications, as the distribution of the filler is not always uniform. In contrast, for ram extrusion, pre-sintered or free flowing resins are used. Typically, virgin materials would be pre-sintered resins – as they flow easily and can be uniformly distributed in the die without issue. For filled grades, any grade can be used provided it is a free flow grade.

2. Mechanical strength - the difference in the two processes rests largely on the way in which the extrusion is done. For ram extrusion, the final form is built up by adding subsequent charges of PTFE one on top of the other. It is therefore an additive process. As a result, the material will have higher strength in the radial direction and less strength longitudinally (weak points would result in the joint between subsequent charges). For the same reason, since paste extruded resins are formed by passing the resin lengthwise through a die (think of how penne pasta would be made), the strength lies in the longitudinal direction. Un-sintered tube has a tendency to crack or split along its length if it is handled even slightly roughly before being cured in the oven. It is for this reason that paste extruded tubes are preferred in applications where there is likely to be a high tensile load on the tube. On the other hand, ram extruded tubes are used in lining applications, as radial force needs to be applied to the tube when inserting it into a metal pipe to line it from the inside.
   Ram extruded tubes also tend to be much stiffer and lack the flexibility of paste extruded tubes.

3. Sizes - as mentioned earlier, paste extruded tubes would typically be used where the wall thickness was small. Usually, anything within a wall thickness of 2.5mm would be paste extruded, whereas higher diameters and wall thicknesses are ram extruded. Another limitation with ram extrusion is that because the tube is stiff, there is a cap on the length that can be obtained before the finished tube reaches the ground and needs to be cut. With paste extrusion, due to the tube’s flexibility, material can be collected in a coil, allowing for bundles of many of hundreds of meters in a single length. The same flexibility allows the paste extruded tubes to be easily bent and shaped to suit the final requirement.

Fundamentally, both ram and paste extruded tubes are made with the same material. However, the processing techniques dictate that the end properties are different. Care needs to be taken to understand the end application and to choose the tube that best suits this.

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Read More

1. Exploring the Versatile World of PVDF

2. Air Permeability Testing and Water Entry Pressure Testing in Expanded PTFE Membranes

3. Polyimide - The Ultimate Champion Among Polymers

Apart from PTFE, very few polymers command the respect of PVDF when it comes to chemical applications. However, Polyvinylidene fluoride, commonly known as PVDF, is a high-performance polymer with a remarkable combination of properties that make it invaluable in numerous different industries. This versatile material possesses unique characteristics, including excellent chemical resistance, high thermal stability, piezoelectric properties, and remarkable electrical insulation capabilities. 

While we at Poly Fluoro have always worked with PVDF by machining parts from extruded rods, we were recently presented with the opportunity to develop compression moulded and injection moulded parts using this amazing polymer. The results were more than satisfactory and put us in an exclusive league of companies that can make both large, machined components using compression moulding, as well as smaller intricate parts using our specially developed, high-temperature injection moulding press.

Before we delve any deeper into processing, let us look at some of the properties and applications of PVDF.

Properties of PVDF

1. Chemical Resistance

PVDF exhibits exceptional resistance to a wide range of chemicals, including acids, bases, and solvents. This property makes it ideal for use in corrosive environments, such as chemical processing plants and laboratories. It can withstand exposure to harsh chemicals without degrading or losing its integrity, ensuring long-term reliability.

2. High Thermal Stability

PVDF can operate at elevated temperatures without losing its mechanical properties. It has a high melting point, typically around 177°C (350°F), making it suitable for applications that involve exposure to heat. This thermal stability is particularly important in industries like aerospace, where materials must endure extreme conditions.

3. Piezoelectricity

One of PVDF's most intriguing properties is its piezoelectricity. When subjected to mechanical stress or pressure, PVDF generates an electrical charge. This property is crucial in various applications, such as sensors, transducers, and actuators, where the conversion of mechanical energy into electrical signals is necessary.

4. Excellent Electrical Insulation

PVDF is an outstanding electrical insulator, making it essential in the electronics industry. Its high dielectric strength and low dielectric constant allow it to insulate and protect electronic components from electrical interference and damage. This property is invaluable in the manufacturing of cables, capacitors, and electrical connectors. 

5. UV Resistance

PVDF is highly resistant to ultraviolet (UV) radiation, which can degrade many other materials over time. Its UV resistance makes it suitable for outdoor applications, such as solar panels, architectural cladding, and signage, where prolonged exposure to sunlight is inevitable.

6. Low Density

PVDF has a relatively low density compared to many other engineering plastics. It has a specific gravity of 1.8 compared to PTFE, which is at 2.25. This characteristic makes it lightweight, which is advantageous in industries like aerospace and automotive, where weight reduction is a critical factor in enhancing fuel efficiency and overall performance.

Applications of PVDF

1. Aerospace

In the aerospace industry, PVDF is used in a wide range of applications due to its exceptional properties. It is employed in aircraft components such as fuel lines, insulation for wiring, and lightweight structural parts. Its resistance to extreme temperatures and chemicals makes it suitable for aircraft exposed to harsh environments.

2. Electronics

PVDF's excellent electrical insulation properties make it a key material in the electronics sector. It is utilized in the manufacturing of cables, wire coatings, and printed circuit boards. Its piezoelectricity also finds application in sensors and transducers for detecting and measuring physical parameters.

3. Chemical Processing

PVDF's chemical resistance makes it a top choice for the chemical processing industry. It is used in the construction of pipes, valves, pumps, and storage tanks that transport and store corrosive chemicals safely. PVDF-lined equipment ensures the integrity of chemical processes and prevents contamination.

4. Renewable Energy

The solar energy sector benefits from PVDF's UV resistance. It is used as a protective material in solar panels, where it helps extend their lifespan and maintain their efficiency by shielding them from the harmful effects of UV radiation. PVDF's lightweight nature is also an advantage in solar panel design.

5. Medical Devices

In the medical field, PVDF is employed in various applications due to its biocompatibility and resistance to sterilization methods. It is used in medical tubing, catheters, and surgical instruments. Its piezoelectric properties are harnessed in ultrasound transducers for medical imaging.

6. Architectural Cladding

PVDF is a popular choice for architectural cladding materials in construction. It is used in the form of coatings on aluminium, steel, or other substrates to provide durable and aesthetically pleasing facades for buildings. Its UV resistance ensures that the cladding retains its colour and appearance over time.

7. Oil and Gas Industry

In the oil and gas sector, PVDF is used in applications that require resistance to harsh chemicals and high temperatures. It is utilized in the production of seals, gaskets, and liners for equipment used in drilling, refining, and transporting petroleum products.

8. Water Treatment

PVDF membranes are employed in water treatment processes, such as ultrafiltration and microfiltration. These membranes effectively remove contaminants and microorganisms from water sources, ensuring the production of clean and safe drinking water.

9. Automotive

PVDF's lightweight properties make it valuable in the automotive industry for reducing vehicle weight, thereby improving fuel efficiency and reducing emissions. It is used in components like fuel lines, engine components, and interior trim parts. The recent spike in electric vehicle manufacturing had put tremendous pressure on PDFV, as it happens to be ideal for use in a variety of areas essential to EV manufacturing.

Processing PVDF

PVDF can be extruded, compression moulded, or injection moulded. While it does require a higher temperature as compared with nylons, POM, or ethylenes, the temperature is still well below what it might take to process PEEK, PTFE, or PI (Vepel). Nonetheless, like all high-performance polymers, care needs to be taken on the selection of metals used in the moulding process, as these can very easily corrode when the polymer reaches its liquid state.

In its liquid state, PVDF is viscous enough that it tends not to leak out from a well-designed mould or die. At the same time, the viscosity is not so high that there are issues with cracks or blowholes. Provided enough pressure is given during the moulding (either compression or injection), the voids are easily removed, and the polymer generally behaves well. Unlike PEEK, which only melts at temperatures above 400°C and gives of effluents that need to be removed at high pressures of over 400Bar, PVDF melts at a far more sedate 200-250°C and at pressures of only 100Bar. Further, PVDF also has the option of being melted in a separate die and allows itself to be transferred to another die, where the pressure is applied to give it form. This makes it easier to process than PEEK, where it can sometimes take upwards of 4 hours to even mould a single piece.

Conclusion

Recent spikes in demand for PVDF (driven mainly by the EV boom globally), had pushed the price of raw materials up to the point where industrial applications were being priced out of the market. However, capacity expansions around the world have now brought the prices to more stable levels and the industry is once again looking poised for amazing things.

PVDF's unique combination of properties, including chemical resistance, high thermal stability, piezoelectricity, and excellent electrical insulation, means that it finds application in all corners of industry. From aerospace and electronics to renewable energy and medical devices, PVDF plays a pivotal role in enhancing the performance, durability, and reliability of numerous products and applications. As technology continues to advance, the versatility of PVDF will likely lead to even more innovative uses in the future, making it a material of enduring importance in the world of materials science and engineering.

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Read More

1. Air Permeability Testing and Water Entry Pressure Testing in Expanded PTFE Membranes

2. Polyimide - The Ultimate Champion Among Polymers

3. Injection Moulding High-Performance Polymers