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PEEK is a critical component in high-end coffee machines Its ability to withstand high temperatures, remain dimensionally stable, and be suitable for food contact makes it an irreplaceable part of any machine that wishes to churn out a worthy brew!

In the ever-evolving world of coffee culture, enthusiasts and professionals alike constantly seek innovations to enhance the brewing experience. One such technological marvel making waves in the coffee industry is the use of PEEK (polyether ether ketone) valves in coffee machines. These valves, though small in size, play a significant role in ensuring a superior and consistent cup of coffee.

Our first introduction to this unusual application of PEEK happened about ten years ago. A manufacturer of high-end coffee equipment came to us with a dilemma. They had been using aluminium valves in their equipment for a while and had never faced any problems. However, as an increasing number of Indians travelled abroad and experienced the flavours of western-brewed coffee, the complaints had started to come in. The issue: the coffee tasted metallic.

The client had done their own research and found out that the Italian coffee machines had replaced aluminium with PEEK.

PEEK is a high-performance thermoplastic known for its exceptional mechanical and chemical properties. Its use in coffee machines brings a range of benefits that contribute to the overall efficiency and quality of the brewing process.

First and foremost, PEEK valves excel in temperature resistance, making them ideal for the hot and demanding environment of coffee machines. Unlike traditional materials that may degrade or lose integrity under high temperatures, PEEK valves maintain their structural integrity, ensuring a reliable and durable component in the coffee brewing system. This resistance to heat is crucial for consistent coffee extraction and flavour preservation. PEEK has a service temperature of 275°C and is therefore more than capable of withstanding the heat within the equipment.

Another notable feature of PEEK valves is their resistance to chemicals and corrosion. Coffee machines often come into contact with various substances, including minerals in water and coffee residues. PEEK's resistance to corrosion ensures that the valves remain unaffected by these elements, leading to a longer lifespan for the coffee machine and reduced maintenance requirements. This not only benefits coffee enthusiasts by providing a more reliable machine but also contributes to sustainability by reducing the need for frequent replacements.

Precision is paramount in the world of specialty coffee, where every parameter matters. PEEK valves offer a high level of precision in controlling the flow of water and steam in coffee machines. This precision allows for fine-tuning of the brewing process, enabling baristas and coffee enthusiasts to achieve the desired extraction profiles. The ability to control water flow with accuracy contributes to the consistency of flavour and aroma in each cup of coffee, a key factor in the pursuit of brewing excellence.

In addition to their mechanical properties, PEEK valves are preferred for their biocompatibility. This characteristic is particularly important in the food and beverage industry, where materials that come into contact with consumables must meet stringent safety standards. PEEK's biocompatibility ensures that it poses no risk of contaminating the coffee with harmful substances, meeting the highest hygiene and safety standards.

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.

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.

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

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.

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.

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.

Read More

1. Exploring the Versatile World of PVDF

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

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.

Of all the properties of PTFE/Teflon, the one that people tend to know best is that it is non-stick. While this characteristic is usually attributed to the now increasingly discontinued application of PTFE in non-stick cookware, it does have wider applications in surface protection, sliding elements, and self-lubricating bearing materials.

However, there exist many applications where PTFE is required to be bonded to other surfaces. Most notable among these would be in structural, sliding bearings, where a PTFE sheet must be bonded on one side to a metal surface, while the other side is exposed as a sliding element. In such an arrangement, the PTFE sheet is bonded to a metal plate and a stainless-steel plate is placed on top of the PTFE sheet. The low coefficient of friction between PTFE and polished stainless-steel means that the stainless-steel sheet can slide freely along the PTFE sheet’s surface. The stainless-steel plate is itself welded to another metal surface and a vertical load is applied to it. These PTFE sliding bearings are used in infrastructure to accommodate both loads and movement. However, the high vertical loads also mean that shear loads exist, which act directly on the bond between the PTFE and the metal, making it essential that the bonding process is done with the utmost care and technical understanding.

Here we look at some of the factors that affect the bonding and throw light on the precautions and preparations needed to ensure a strong, reliable bond.

Appearance: Virgin PTFE is white in color and does not bond to surfaces unless it is chemically treated (etched) using a special process. The formulation of the chemical etchant is proprietary, with each processor using a method that suits them best. Once etched, the surface of the PTFE changes color to brown. This brown surface can be bonded easily using standard industrial grade adhesives.

Surface preparation: The metal surface to be mounted with PTFE can be prepared by the normal machining methods such as grinding, milling, shaping, and planning. The surface roughness of all forms of preparation should be preferably between Ra = 1.6 µm and Ra = 3µm and not more than Ra = 6µm. Once roughened the surfaces can be cleaned with trichloroethylene, perchloroethylene or acetone. As with any bonding process, we would need to ensure the surface of the metal is free from grit and debris.

Bonding PTFE: For bonding of PTFE the following resin adhesive can be used: Araldite - Hardener - HV 953U; Araldite AW106. The Araldite should be applied both to metal and PTFE sheet and be spread as uniformly as possible by means of a serrated spatula. To obtain the best dispersion of the adhesive, when spreading on the surface brush in the longitudinal direction; when spreading on the metal, brush in the transverse direction. The total quantity of bonding should be approximately 200gm per sq. mt.

Other bonding agents can also be looked into, but usually a good industrial grade agent would be recommended.

Hardening: After mounting the PTFE a clamping pressure of between 10-15 Kg/cm2 is recommended. It is important to keep the pressure constant during the hardening process. Due to the differences in the thermal expansion coefficient of the materials, maximum curing temperature should not exceed 40°C. The hardening times for various temperatures are 20°C min 15 hours; 25°C min 12 hours; 40°C min 5 hours.

Finishing: After curing of the adhesive, the PTFE can be machined by conventional means – if required. The choice depends on the machinery available viz.: grinding; grindstone.

Grinding: For grinding of PTFE use the same speed as grinding cast iron, taking care that sufficient cooling is used with an ‘open stone’. The grindstone should be preferably silicon carbide based with rubber or polyurethane binding; grain size 80-30. Alternatively, aluminum oxide with rubber bonding may also be used for soft, fine grinding action, pre-polishing and pre-mating treatment.  

Oil Grooves: PTFE pads can be machined with oil grooves using the same methods and cutting data as used for cast iron. The form and depth of the oil grooves are optional. However, the oil grooves should never pierce through the PTFE. Oil grooves should be away from the edges by 6mm.

Maintenance: Bonded PTFE must be maintained, as the strength of the bond can be impacted by adverse environmental factors such as excessive sunlight, corrosion, and heat. Temperatures around the bonded areas should not exceed 120°C, while any presence of corrosive elements – such as sea-air and/or chemical fumes can – affect the metal surface and eat into the bond. Usually, when exposed to adverse elements, the bond strength can get affected along the edges and any corrosion along the metal can slowly eat its way into the middle.

In such a case, the bond can be reapplied along the edge of the sheet after cleaning any debris/rust from the affected area. However, care must be taken to apply a protective coating around the bonded area to ensure long-term functionality.