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

The development of expanded PTFE (ePTFE) membranes unleashed a material that was unmatched in terms of properties. The breadth of applications that have been – and continued to be – found to benefit from ePTFE membranes is significant. With membranes, ePTFE’s exceptional properties such as chemical resistance, thermal stability, and high tensile strength were combined with micro-porosity to find applications in a wide range of fields, including filtration, medical devices, textiles, automotives, chemical plants, and many more.

The overarching tagline for ePTFE membranes is this: they repel dust and water but allow vapours and moisture to pass. This seemingly simple property can be further fine tuned to block not only water, but oils and mineral spirits as well, turning them from plain hydrophobic membranes to oleophobic membranes.

However, to understand how the membrane functions, and indeed to test the efficacy of the membrane, two fundamental parameters need to be considered.

The two important parameters that characterize the performance of ePTFE membranes are air permeability and water entry pressure. In this article, we will delve into the significance of air permeability testing and water entry pressure testing in expanded PTFE membranes.

Understanding Expanded PTFE Membranes

Expanded PTFE (ePTFE) is a remarkable material made by rapidly stretching extruded PTFE under high temperatures. The stretching process – which may be either uni-axial or bi-axial – imparts a micro-porosity to the material. These pores are much smaller than a molecule of water, but far larger than a molecule of steam. As a result, the membranes can act as a sort of valve, allowing only vapours through, but no liquids or dust. This feature – often called breathability – is somewhat unique to ePTFE and when combined with the ability to withstand high temperatures and corrosive chemicals, makes the material very useful.

However, to truly develop a tailored solution, one must be able to tweak the parameters to suit the end-application. While air and moisture can pass through the material, an application will have specific requirements as to the volume of air or moisture that should pass at a given pressure. Similarly, while water will be repelled by the membrane, certain applications would also have requirements pertaining to the pressure at which water might pass through the membrane.

Air Permeability Testing

Air permeability, also known as air porosity, is a crucial property of ePTFE membranes that influences their performance in applications where gas flow is involved. It refers to the ease with which air can pass through the membrane's pores. Air permeability testing is essential to assess the suitability of a membrane for specific applications, particularly in industries such as outdoor clothing, medical devices, and industrial filtration systems.

The air permeability test involves measuring the volume of air that passes through a unit area of the membrane under a defined pressure difference. The unit of measurement is usually cubic feet per minute per square foot (CFM/ft²) or litres per minute per square meter (LPM/m²). The test apparatus typically consists of a controlled air source, a sample holder, and a differential pressure gauge. The sample is securely clamped between two chambers, and air is allowed to flow through one side of the membrane while the pressure drop across it is measured. The standard ASTM D737 is used to test the material for air permeability.

The air permeability value provides valuable information about the membrane's breathability, which is crucial in applications like outdoor clothing and medical textiles. Membranes with higher air permeability values allow for better moisture vapor transmission while maintaining water resistance.

Water Entry Pressure Testing

Water entry pressure, also known as bubble point pressure or hydro testing, is another essential parameter in assessing the performance of ePTFE membranes, particularly in applications involving liquid filtration and moisture management. It determines the pressure at which liquid begins to pass through the pores of the membrane. Water entry pressure testing helps determine the membrane's resistance to liquid penetration and is crucial in applications like medical filtration devices, fuel filtration, and waterproof textiles.

To conduct a water entry pressure test, the ePTFE membrane is subjected to increasing pressure on one side while the other side is exposed to a wetting liquid, often water. As the pressure increases, the point at which liquid starts to penetrate through the membrane's pores is the water entry pressure. This pressure is usually measured in psi or bar. The standard ASTM D751 is used to test the material for water entry pressure.

A higher water entry pressure indicates that the membrane can withstand greater hydrostatic pressure before allowing liquid to pass through, making it suitable for applications that require liquid resistance. Conversely, membranes with lower water entry pressures are ideal for applications that require controlled liquid transport.

Importance of Testing

Air permeability and water entry pressure testing are vital for ensuring that ePTFE membranes meet the specific requirements of their intended applications. Different industries have different needs, and these tests help manufacturers and engineers select the most appropriate membrane for their products. 

For example, membranes used in automotive vents are required to have a very high-water entry pressure, as these vents protect the circuitry. Even under very harsh weather – including events where a vehicle might find itself partially submerged – the membrane cannot succumb to the pressure. Similarly, in case water does find a way into the circuitry, the heat within the system should be sufficient to turn the water into vapour so it can escape through the vent.

In medical applications, such as wound dressings or drug delivery devices, a balance between air permeability and water entry pressure is crucial to ensure optimal patient comfort and controlled liquid absorption. In industrial filtration, membranes with high air permeability and suitable water entry pressure values are chosen to maximize filtration efficiency and minimize pressure drop.

Conclusion

Expanded PTFE membranes have revolutionized various industries due to their exceptional properties. Understanding their air permeability and water entry pressure characteristics is crucial for their successful deployment. The ability to manufacture, test, and fine tune membranes is something that Poly Fluoro has been developing over the past few years. With in-house testing facilities for air permeability and water entry pressure, our engineering can pinpoint the exact process needed to ensure the end-properties are exactly what are needed by the end-user.

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Read More

1. Polyimide - The Ultimate Champion Among Polymers

2. Injection Moulding High-Performance Polymers

3. PTFE Machined Conduits