Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

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.

Solar Tracker Bearings - A Key Component in Renewable Energy

While the Covid 19 pandemic has caused deep pains across many industries, the world has been careful not to ignore the ever-present threat of global warming. Indeed, with the advent of the coronavirus, many nations have looked towards the renewable energy sector as a magic bullet. Not only do investments in renewable energy allow for a boost in economic activity, but the move away from fossil fuel-based power sources ensures that we continue to aim for the reduction in emissions that remain so vital to our planet’s future.

Some nations, such as Spain, have set aggressive targets for themselves, stating that by 2050 they will be 100% based on renewable energy. Others, such as the UK, have already started setting domestic records for the number of days in which coal-based power has not been required. India has also pledged to move towards solar energy, with many of the projects running well ahead of schedule. Recently, the Adani Group has been awarded the largest such contract on record, underlining the seriousness with which the government is approaching renewable energy.

At Poly Fluoro, we have delved deep into the renewables sector through our involvement in solar tracker bearings. We engaged in the first of such projects in mid-2018, when the industry was still ramping up and when new technologies were only just being understood. The learnings we have uncovered have allowed us to further invest and develop our own polymer technologies, with the aim that we are positioned to take on any project and offer the technical support and guidance needed by our clients.

Before we go into these learnings, let us first understand the purpose of the solar tracker bearing.

For all solar projects, the efficiency of the system is maximised when the installed panels are able to maximise their harvest of the sun’s radiation. Since the sun moves across the sky, the angle of the panels needs to alter as the day progresses. To facilitate this, a tracker is installed, which rotates the panels at the required rate. Since the tracker itself consumes power, it is vital to ensure that the energy expended in rotating the panels does not exceed the extra power generated by this system. It is for this reason that a smooth functioning bearing is essential. The solar tracker bearing clamps around the square tube on which the panels are fixed. The tracker rotates the square tube and the bearing’s job is to allow for this rotation with minimal friction.

Some of our key learnings are as follows:

  1. Size

    For the most part, a solar tracker bearing needs to match the size of standard square tubes. Since most projects would prefer to use off-the-shelf square tube dimensions, the bearings need to be designed accordingly.

    In India, the sizes of 75mm, 100mm, 120mm, and 150mm would be most common. Here too, the preference is for 100mm tubes, as this maximises the number of panels that can be installed on a single length of tube, but minimises on the weight of the tube itself.

    Our designs have focussed on creating a uniform bearing for the 100 square tube. In doing so, we have studied the dimensions of existing bearings to understand how much load and friction the part would be submitted to.

  2. Composition

    Our experience has shown that while erstwhile bearings demand the use of UHMWPE for solar tracker bearings, this polymer becomes problematic as volumes increase. The main reason is that UHMWPE is not injection mouldable. This means that parts need to be machined. Machining is time consuming, expensive, and it also results in a very heavy final component.

    Other candidates include POM (Polyacetal), HDPE, and Nylons. Each of these has advantages, although our preference has been towards POM, as it offers both strength and an ease of moulding. While HDPE is lightweight, it is not nearly as strong as POM. Nylon is the least preferred because it tends to swell when there is moisture. While Nylon bearings may have done well in projects where the climate is dry, the use of Nylon in more humid conditions would not be advisable.

    Other than the polymer, the key lies in what additives one incorporates. There are three primary factors to consider. The first is strength – which is managed by the choice of polymer. A typical solar tracker bearing of 100 square would need to accommodate at least 1000-1500 Kgs of vertical load. The second criterion is friction. The use of friction reducing additives is required in making solar tracker bearings, as it greatly enhances efficiency. Experience has shown us that there is a balance needed here. Too little friction can also be a bad thing, as it may cause the panels to rotate due to wind loads. Finally, the bearing needs to be UV resistant. Poly Fluoro has explored the impact of different fillers on UV resistance. While many manufacturers simply add carbon black and claim that the part is resistant to UV, this can be misleading. Our own research shows that the addition of black pigment can only withstand UV up to a point. In most cases, the bearing will begin to deteriorate within 2-3 years. The more expensive option is to use HALS (Hindered amine light stabilizers), which work by ensuring that the UV radiation is managed within the first 0.1mm of the part’s surface. HALS would allow the solar tracker bearing to have a life of 25-30 years, which is what is required when considering a solar project.

  3. Design

    While smaller projects may opt for machining, it is our belief that an injection moulded part is most suitable for solar tracker bearings.

    Using a special honeycomb or lattice structure, the bearings can be made lightweight, but strong enough to take heavy loads. Furthermore, with a cycle time of only under two minutes, the manufacture of a solar tracker bearing can be done in a fraction of the time it would take to machine the bearing from a rod or sheet.

    The combined impact of the weight reduction and lower cycle time results in a cost saving of over 50% in some cases. For large projects, where tens of thousands of bearings are needed, the machining route is both financially wasteful and unlikely to meet any stringent project timelines.

We hope the above information has been useful in better understanding the structure of the solar tracker bearing. As projects grow in size, we expect to come across more learnings. In the meantime, Poly Fluoro is proud to be one of the pioneers in this space. Our library of solar tracker bearings allows us to take on projects of different sizes and volumes, while offering both a quality product and a genuine academic understanding of the engineering behind it.

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