Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

PTFE Heat Exchangers: A Universal Solution for Corrosive Environments

The inert nature of PTFE (Teflon) has naturally endured it to applications involving corrosive chemicals. Not only does PTFE stay non-reactive to almost all chemicals (notable exceptions being sodium and alkalis at elevated temperatures), it also exhibits its properties all the way up to temperatures of 250°C. This seemingly invincible nature allows fabricators and systems designers to incorporate PTFE without worrying which chemicals may or may not present within a system.

Many chemical baths are designed to hold a host of different chemicals. The need to uniformly heat or cool these chemicals is met by the use of heat exchangers. In its most basic design, the heat exchanger consists of two adaptors, connected to one another by many lengths of tubes. The adaptors are round blocks with multiple holes through them, each meant to house one end of tube. The adaptor in turn is connected with a device that pumps out fluids, which then pass through the tube and out the other adaptor. The fluid may be heated or cooled to accordingly heat or cool the chemical bath, respectively. The tubes – which may be many meters in length – are submerged in the chemical bath, allowing for the heat transfer to take place between the chemical and the fluid within the tubes. The size, length and quantities of these tubes will define the volume of fluids than can be passed through the heat exchanger. Furthermore, the wall thickness of the tubes use will add to the efficiency of the heat transfer.

PTFE heat exchangers follow this design, with both the adaptors and the tubes being made from pure virgin PTFE. However, the fabrication process can be tricky. For one, as PTFE does not easily join even itself, the fusing of the tubes with the adaptor needs to be done in one of two ways:

1. Bonding – the outer diameter of the tube is chemically treated as are the inner holes of the adaptor. Bonding can be done using an industrial grade adhesive, which would also need to show resistance to the chemicals in the bath. The advantage of bonding is that it is much easier to fabricate such an assembly. The disadvantage is that the bond may not hold in the face of high temperatures and in the event that an unexpected chemical enters the mix

2. Welding – welding PTFE is very tricky and is as much an art as it is a science. PTFE welding can only be done if specially modified grades of PTFE are used, which allow themselves to be welding. Grades such as Chemour’s NXT and Inoflon’s M490 are examples of modified grades that can be used to make the adaptors. The tubes too need to be made accordingly. Modified grades of tubing are needed to ensure that both the tube and the adaptors are able to fuse with one another under high-temperature conditions

Aside from temperature and chemical resistance, it is also vital that the heat exchanger assembly is able to withstand the pressure build-up due to the passage of fluid. In the event of higher pressures, both the fused/bonded assembly as well as the tube itself would need to be capable of holding the same. Because of this, the wall thickness of the PTFE plays a dual role. On the one hand, because PTFE is a poor conductor, a lower wall thickness ensures that the heat transfer is more efficient. However, a thinner tube means more complications in bonding and/or welding, while also a lower capacity to withstand higher pressures.

Apart from the use of PTFE in the heat exchanger assembly, expanded PTFE (ePTFE) also finds significant use in this application. Like virgin PTFE, ePTFE also exhibits superior chemical and heat resistant properties. At the same time, the sealing properties of ePTFE ensure that it forms an effective gasket material in any portion of the assembly that may require clamping and effective sealing for the fluids.

Overall, it seems unavoidable that for certain chemical baths, PTFE is the only viable option for a heat exchanger assembly. With the proper fabrication and right materials, it offers a very effective, durable, and versatile solution for all industries where heat exchangers are needed.

General notes on Bonding PTFE Sheets to Metals

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-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.

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: 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 sheets 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.

It should be noted that even with etching, PTFE remains a material resistant to bonding. While the etching process allows for a reasonably strong bond to metals, there is a limit to the strength of this bond. Even well bonded surfaces offer a bond strength of only 4-5 Mpa (40-50Kg/cm2). As such, in areas where the shear load is expected to be higher, the PTFE sheet may need to be supported by clamping or bolting.

Polymers in Ventilators

Few crises have galvanised the industrial world in the way that the current Covid-19 pandemic has done. As more was known about the illness, countries across the world became aware that their capacities for and inventories of key medical equipment was sorely lacking. While face masks and PPE equipment have been relatively easy to scale up quickly, the more high-end and complex medical equipment has required an even larger push to ensure we have the numbers needed to confront the situation at hand.

With urgency being the need of the hour, the manufacture of ventilators has spiked across the world. In India, the government has approached various private contract manufacturers to divert capacity to the manufacture of ventilators. A plan to build a moving, railway hospital with 20,000 beds is said to be materialising, while key states have set up Covid-centric hospitals to ensure that they have sufficient capacity. Considering that roughly 5% of cases end up requiring hospitalisation, the demand for ventilators is, at present, insatiable. 

High-performance polymers have always been at the forefront of medical device manufacturing. Recently, both PEEK and PPSU have gained some attention for their application on ventilator manufacture.

Here we look at some of the key properties of these polymers and how they lend themselves to the manufacture of ventilators and other medical devices.

  1. Biocompatibility – different readily available grades in PEEK and PPSU offer biocompatibility. However, it should be noted that biocompatibility varies. Some medical grades are acceptable for use only in equipment and devices that will not come in contact with human skin. Still more specialised grades are suitable for human contact, while the most high-end grades might be used inside the body.

  2. FAD approval – most graded of PEEK, PPSU, PTFE, and even PVDF come pre-approved by the FDA. This means that for some medical applications, no further certification may even be needed. It is beneficial in a time like this, where the need to fast-track development is key.

  3. Low particle discharge – one of the key requirements of a material that is used in the medical space is that it needs to exhibit minimal particle discharge. It is common for many filled grades of polymers to exhibit high levels of discharge. This discharge can settle in other areas of the equipment and cause both mechanical and electrical problems within the device. Furthermore, for respirator devices, the chance of particles entering the human lung would be very detrimental indeed!
    Both PEEK and PPSU are known for being very stable in this regard. Data released by NASA shows that the total percentage of mass lost over the lifetime of the part is only 1.1% and 0.3% for PPSU and PEEK, respectively.

  4. Flammability – most polymers, including PEEK and PPSU conform to at least a V-0 rating on flammability, meaning that even with the application of a flame, the material will self-extinguish within 10 seconds once the flame is removed. This is a critical feature from the point of view of safety.

  5. Mechanical strength – aside from the chemical and medical compatibility, both PEEK and PPSU exhibit superior mechanical strength. PEEK has one of the highest tensile strengths among polymers. As a result, the loads and wear that the parts are able to take are unparalleled and yield a long-lasting solution.

  6. Machinability – once of the key issues during the pandemic has been time-to-market. There is very little time to engage in extensive R&D and even trials need to be conducted with a very quick turnaround in mind. The fact that PEEK and PPSU can be machined, moulded, and even 3D printed makes their application that much more beneficial to the current climate. While initial prototyping can be done using 3D printing or machining, the production can gradually shift to moulding once the dimensions are frozen and the mould is developed. Since mould development can sometimes take weeks, the machinability of PEEK and PPSU serves as an intermediary measure to ensure that parts are supplied in the short term.

There are a host of polymer solutions available in addition to PEEK and PPSU. As things proceed, there is no doubt these too will find their uses. Such is the nature of the pandemic that new applications, advantages, and properties are constantly being discovered.

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