Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

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

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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1. Polyimide - The Ultimate Champion Among Polymers

2. Injection Moulding High-Performance Polymers

3. PTFE Machined Conduits

Polyimide - The Ultimate Champion Among Polymers

When we first started processing high-performance plastics, it seemed impossible for anything to match the versatility and high-performance capabilities of PTFE. However, when we developed PEEK, we realized that while PTFE does have a significant range of unmatched characteristics, PEEK surpassed it on out-and-out strength parameters. For a while, it did seem like PEEK was a good as it got with regards to robust polymers capable of withstanding immense loads at high temperatures.

When we explored further along high-performance polymers, however, we found that the universe of such materials is constantly expanding, yielding newer polymers that slowly become more mainstream. Standing out among these, is polyimide.

Not to be confused with polyamide – which is the name for nylon – polyimide is more commonly known by its trade names, such as Vespel and Kapton. Polyimide, a versatile polymer, has gained significant attention in various industries due to its exceptional properties and wide-ranging applications. 

This article delves into the processing methods employed for polyimide, explores its key properties, and highlights the advantages that make it a sought-after material in numerous fields.

Processing Methods:

Polyimide can be processed using several techniques, including:

Solution Casting: This method involves dissolving the polyimide in a solvent to create a solution that can be cast into various shapes. Solution casting offers flexibility in producing thin films, coatings, and fibres, making it suitable for applications in electronics, aerospace, and flexible displays.

Compression Moulding: Compression moulding involves placing polyimide granules or powder into a mould and applying heat and pressure to create the desired shape. This technique is commonly used in the production of high-strength, high-temperature-resistant components such as gears, seals, and electrical insulation. A similar process is employed in making PEEK; however, the exception is that by using higher pressures (in excess of 3000 Bar), the polyimide powders can be compressed like PTFE and sintered in batches in an oven. This significantly increased productivity as compared with PEEK, where a single bush or rod can take anywhere from three to five hours to mould.

Film Extrusion: Polyimide pellets or powder are melted and extruded through a die to form continuous sheets or films. Film extrusion enables the production of thin, uniform polyimide films, widely utilized in the electronics industry for flexible circuits, insulating layers, and display components.

Injection Moulding: This method involves melting the polyimide and injecting it into a mould under high pressure. Injection moulding allows for the production of intricate and complex shapes, making it ideal for manufacturing components like connectors, switches, and automotive parts. However, the processing conditions for polyimide require high temperatures and the release of corrosive gases. Hence, care needs to be taken on the mould development, the equipment used for moulding, and the overall handling of the material.

Properties of Polyimide:

Thermal Stability: Polyimide possesses excellent thermal stability, retaining its mechanical and electrical properties at high temperatures, typically up to 500°C (932°F). This property makes it suitable for applications in aerospace, automotive, and electronics industries.

Chemical Resistance: Polyimide is highly resistant to a wide range of chemicals, including solvents, acids, and bases. This resistance enables its use in harsh environments, such as chemical processing, where resistance to corrosive substances is crucial.

Mechanical Strength: Polyimide exhibits exceptional mechanical strength and rigidity, making it resistant to deformation and capable of withstanding high loads. Its strength allows for the production of lightweight yet durable components.

Electrical Insulation: With its excellent electrical insulation properties, polyimide serves as an effective dielectric material in applications requiring insulation against high voltages and frequencies, such as electrical motors, transformers, and circuit boards.

Advantages of Polyimide:

High Temperature Resistance: Polyimide maintains its stability and mechanical integrity at elevated temperatures, enabling its use in demanding applications where other materials would degrade or fail.

Low Outgassing: Polyimide exhibits low outgassing properties, making it suitable for applications in the aerospace industry, where off-gassing could affect the performance of sensitive equipment.

Lightweight: Polyimide's high strength-to-weight ratio makes it an attractive choice for industries that require lightweight materials without compromising on mechanical performance.

Dimensional Stability: Polyimide demonstrates minimal dimensional changes under varying conditions, ensuring consistent performance and reliability over time.

Versatility: Polyimide can be formulated into various forms, including films, fibres, coatings, and adhesives, enabling its use in a wide range of applications, from electronics and aerospace to medical devices and automotive components.

A key issue with polyimide – as with so many high-performance polymers – is that it is expensive. Even compared with PEEK – which costs between 12-50X what a regular polymer like polypropylene or Nylon 6 might cost – polyimide is pricey. Its price can range from 10-15X the price of PEEK. This premium is the reason that it has remained a niche polymer, restricted to thin films and smaller components. However, like PEEK, PTFE, and so many other high-performance plastics, applications that call for polyimide can use nothing else. It is likely that as time goes on and adoption continues, the price of polyimide will gradually reduce, making more suitable for more mainstream applications.


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1. Injection Moulding High-Performance Polymers

2. PTFE Machined Conduits

3. Expanded PTFE (ePTFE) Vents

Injection Moulding High-Performance Polymers

For the most part, injection moulding is seen as a well traversed process. It is a widely used manufacturing process that allows for the mass production of complex plastic parts with excellent dimensional accuracy and mechanical properties. Unlike machined components, injection moulded parts can be easily produced, with a single part taking only about a minute to make, where the same part may have required anywhere from a few minutes to a few hours to machine, depending on complexity. The downside for injection moulding is that the up-front costs are far higher. An injection moulding die might cost anywhere from a few thousand dollars to many tens of thousands, again depending on how complex the part is or how many cavities are needed. The trade-off is therefore made along the lines of cycle-time and consumption per part (both which tend to be lower with moulding), set against the mould cost, which needs to be amortized over a high volume of parts to offset the increase in the per part rate as against machining.

However, even if moulding is a viable option, there exist limitations depending on the type of polymer we are looking at. Regular injection moulding polymers would include polypropylene, poly-ethylenes (HDPE and LDPE), nylons, and polyacetal. Each of these has been extensively worked with such that any experienced moulder would know the methods of working with them. However, once we move into the realm of high-performance polymers, the matter becomes more complex.

Before we go further, let us look at the polymers in question. 

PEEK (Polyether Ether Ketone)

PEEK is a high-performance, semi-crystalline thermoplastic known for its excellent combination of mechanical properties, chemical resistance, and thermal stability. PEEK components offer exceptional strength, stiffness, and dimensional stability even at elevated temperatures. Its resistance to chemicals, including harsh acids and bases, makes it suitable for applications in the chemical processing, oil and gas, and medical industries. PEEK's inherent biocompatibility also makes it a valuable material in medical implants and devices.

PPS (Polyphenylene Sulfide aka Ryton)

PPS is a semi-crystalline, high-temperature thermoplastic renowned for its excellent chemical resistance, dimensional stability, and flame retardancy. PPS parts exhibit superior resistance to a wide range of chemicals, including solvents, fuels, and acids. Its low moisture absorption, excellent electrical properties, and high thermal resistance make it an ideal choice for electrical and electronic components, automotive parts, and industrial applications.

PVDF (Polyvinylidene Fluoride and Kynar)

PVDF is a highly versatile thermoplastic known for its excellent chemical resistance, UV stability, and fire retardancy. PVDF components exhibit exceptional resistance to harsh chemicals, such as acids, solvents, and chlorinated compounds, making it suitable for applications in the chemical processing, semiconductor, and electrical industries. Its inherent UV stability and flame-retardant properties also make it a popular choice for outdoor applications and fire-safe products.

PEI (Polyetherimide aka Ultem)

PEI is an amorphous, high-temperature thermoplastic that offers outstanding dimensional stability, excellent electrical properties, and inherent flame retardancy. PEI parts exhibit remarkable mechanical strength, stiffness, and resistance to creep, even at elevated temperatures. PEI's excellent electrical insulation properties make it suitable for electrical connectors, switches, and semiconductor components. It is also commonly used in aerospace, automotive, and healthcare applications due to its flame-retardant nature and biocompatibility.

Polyimide (PI aka Kapton)

Polyimide is a high-performance, semi-crystalline thermoplastic renowned for its exceptional thermal stability, excellent electrical insulation properties, and resistance to wear and chemicals. Polyimide components offer exceptional dimensional stability, high strength, and excellent resistance to creep, making them ideal for demanding applications in the aerospace, automotive, and electronics industries. Polyimide's ability to withstand extreme temperatures and harsh environments has made it a material of choice in applications such as insulation, aerospace composites, and high-temperature electronics.

PAI (Polyamide-Imide aka Trolon)

PAI is a high-performance thermoplastic known for its exceptional mechanical properties, high temperature resistance, and excellent chemical resistance. PAI parts exhibit exceptional dimensional stability, high strength, and excellent resistance to wear and creep, even at elevated temperatures. Its inherent flame retardancy, low outgassing properties, and resistance to a wide range of chemicals make it suitable for aerospace, automotive, and industrial applications.

The superior properties of these polymers make them vital in several industries. However, there are drawbacks to moulding these polymers. For one, high-performance polymers tend to be far more expensive than regular polymers, often in the range of 20-50X the price. This makes moulding a challenge, as a new mould usually requires some trial and error before it is production ready. The second issue is that these polymers only melt at elevated temperatures, usually in excess of 300°C. Adding to the complexity is that when in a molten state, these polymers will give of effluents that can be highly corrosive to regular injection moulding equipment. As a result, the entire injection moulding machine and the mould itself need to be made of higher-grade metals and have high-temperature heating elements to ensure that the equipment can handle the process.

Because of these challenges, there exist very few moulders that will take on parts in these polymers. However, as the demand for these polymers increases, so too will the demand for processors that have focused their efforts on developing these plastics.

In 2023, Poly Fluoro embarked on a venture to develop an injection moulding machine tailored to moulding high-performance plastics. The addition of this equipment will make Poly Fluoro one of the very few polymer companies world-wide that can take on high-volume injection moulded parts in these high-performance polymers. We look forward to yet again being pioneers in our field and moving to areas that others would otherwise hesitate to go.


Read More

1. PTFE Machined Conduits

2. Expanded PTFE (ePTFE) Vents

3. PTFE in Brake Cables