Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

PTFE Membranes - Variants and Typical Uses

Membranes involving PTFE have gained prominence over the past decade.  When we are approached for this product, however, it usually involves a lot of discussion and deliberation, as OEM clients are aware that they require PTFE membranes, but are not fully sure which type of membrane they require. In our own experience, there are four variants of PTFE membranes. There may be many more – but these are the variants we most frequently encounter and together they encompass most of the properties that a membrane would need. The manufacturing process of the microporous ptfe is very lengthy is expensive.

Before we delve into the variants, we need to first understand that both pure PTFE and expanded PTFE are used in membranes. We have earlier posted a piece on expanded PTFE, but for the sake of brevity, we will say that it involves a processing technique which effectively pushes air into PTFE, making it softer and lighter than pure PTFE and giving it a spongy texture.We also need to understand that with membranes, 2 properties define the product itself and need to be looked at during product development and manufacture.

  1. Pore size: this is the size (or range of sizes) of the individual pores or holes within the material. As we will see, controlling for pore size is an integral part of the process of making a membrane
  2. Porosity: this is the overall extent to which the PTFE is permeated by the pores. Typically, this is easy to control and calculate, as the final weight of the membrane compared with the weight for pure PTFE of the same volume will tell us to what extent the membrane is porous

 

  1. Variant 1: Pure PTFE MembraneIn truth, this should be called a “filter” rather than a membrane, but it is referred to as both. This is the simplest form of membrane, comprising a PTFE sheet of 0.5mm – 5mm thickness (maybe more) into which holes are drilled/ punched.  The process for making the sheet is the same as for any PTFE sheet: ie: skiving or moulding. The size and quantity of the holes can be altered based on the client requirement.Typical uses of this membrane would be in separating large particles/ lumps from a liquid suspension. It finds uses in biotech, chemicals and even food processing – where the food grade and inert nature of PTFE makes it a suitable material to come in contact with chemicals/ food products and not react/ affect the materials passing through it.

    Both porosity and pore size are easily controlled and measure here – as it is a machined item and the pore size is defined by the holes being drilled/ punched and the porosity is defined by the number of holes.
     

  2. Variant 2: Porous PTFE membranePorous PTFE is made in the same way as pure PTFE ie: the material is molded or skived. The difference is that the resin is compounded with a substance, which would sublimate (move directly from solid to gas) at the temperatures at which PTFE is sintered. Thus, the material – which is molded along with the PTFE, is evacuated during sintering, leaving behind voids in the PTFE. The material would also make fissures within the PTFE as the sublimated gas charts a path through the PTFE during its exit.Porous PTFE is the most inexact of the membranes as it involves a foreign substance whose behavior cannot be predicted entirely. For one, the compounding process is unlikely to be 100% uniform – so you may have some amount of agglomeration of the substance implying that the porosity (and pore size) in one section of the PTFE, may be more than in another. Secondly, while pore size can be somewhat controlled by ensuring that the particles of the foreign substance are all within a fixed range (say 1-2 microns) – the fissures themselves are not possible to control, so 2 fissures may joint at some point to create a larger pore size than required. Overall porosity is controlled by limiting the ratio of PTFE to the substance – but as mentioned before, there will be some variance in porosity within the membrane due to the non-uniformity of compounding.

    Porous PTFE membranes do not have a huge demand in comparison to the other variants. Its typical uses are in automotives and chemical plants, where the particle sizes are in the range of 30-100 microns.
     

  3. Variant 3: Plain expanded PTFE membraneExpanded PTFE is used in cases where a much finer filtration is required. Pore sizes here can be as low as 0.1 micron – since the pores are formed by effectively incorporating air into PTFE and can thus be controlled by limiting the force and volume of air being used. Similarly, limiting the ratio of air to PTFE during the process also easily controls porosity.The key feature of an expanded PTFE membrane is the property of “breathability”. This means that it is possible to control the pore size to an extent where air is able to pass through the membrane, but liquid vapors are not.

    Such membranes find uses in medical equipments and also apparels – where many applications require the material to only allow the passage of air and not other substances.
     

  4. Variant 4: Laminated expanded PTFE membraneThis is the most popular variant as per our experience. The drawback of plain EPTFE membranes is that due to its spongy texture, it does have a tendency to absorb some amount of moisture over time. Furthermore, EPTFE is very soft and light and thin membranes tend to cling to themselves, making handling difficult.The lamination of the membranes is usually done with polypropylene or polyethylene. The benefit is that the membrane is easier to handle and also limits the long-term seepage of moisture. The limitation is that the laminate would not be nearly as effective as PTFE in withstanding harsh chemicals (although this is easily remedied by ensuring that the side facing the chemicals is the pure PTFE side). Furthermore, the membrane will not be able to withstand high temperatures.

    We see a lot of applications of this membrane in filters for medical devices. There is also some use in the automotive segment – where the membrane acts as a filter to evacuate air from oil. The breathabilityensures that only air is sucked through the filter and not oil.
     

In summary, one must point out that PTFE membranes are expensive due to the lengthy process involved in making them and the cost of the material itself. Hence they are sparingly used only in applications where only PTFE will suffice. PTFE Materials offer excellent control over pore size, porosity, permeability, water intrusion pressure and thickness and they can be used as organic solvents. Nonetheless, the range of options they offer – inertness, food grade, temperature resistance and breathability – make them unmatched by any other material in the area of membranes.

The Effect of Low Temperatures on PTFE Component Dimensions

One of the most challenging elements of machining PTFE components for export markets is factoring the effects of temperature on the material when it moves into colder climates.

Since we do all our machining in Bangalore, India, where the room temperature varies between 22-32 Degrees Celsius (on average), we need to be constantly mindful of the dimensional shrinkage that would happen when machined parts are shipped to colder countries.

In many cases, this problem is not a huge one – since the tolerance on the part may be high enough that even after shrinkage it would still fall within the acceptable band for that dimension. A tolerance of +/-0.25mm, for example, could be machined with a plus side bias of 0.15-0.2mm – which is easily maintained on a CNC machine. After shrinkage, even if the dimension reduces by 0.2-0.3mm (not unheard of as we will later demonstrate), the part would still be acceptable when inspected at the client’s works prior to assembly.

The real challenge lies in accommodating much closer tolerances. In our own experience, we encourage customers to design the part keeping in mind a tolerance of +/-0.05mm at most. Often, as the customer may have dealt mainly with metal parts, they expect that PTFE would also conform to the same dimensional yardsticks as metal (which can be machined to tolerances as fine as 1 micron). In reality – PTFE is a much softer material, which undergoes the following changes during machining, affecting it’s ability to be attain very close tolerances:

  • Stress build up in material due to tool hardness
  • Deformation of material due to heat from machining
  • Burrs forming on part which may need to be manually removed, affecting tolerance

The closest tolerance we have managed to maintain on PTFE has been +/-0.012mm – which was done on a component made from PTFE+15% glass fiber, having an outer diameter of 19.04mm.  We did this by experimenting with different combinations of tools, RPM, feed rates and programs, until the dimension was consistently within the tolerance range. However, when the part was shipped to the client in Canada, the trial lots failed due to being undersize. Eventually, through trial and error, it was found that a dimension of 19.08-19.10 was needed in India, in order for the part to be within tolerance in Canada.

While this worked out well eventually, in many cases, customers are not willing to experiment with trial lots – especially if their requirement is urgent. This has led us to seriously consider the practical implications of shrinkage and how we can make an educated guess on dimensions so as to avoid rejection/ rework and/or minimize trials.

The study

Virgin PTFE theoretically experiences a 1.3% variation in dimension between 0 and 100 Degrees Celsius. Plotting this as a linear progression around the dimension of 19.04 would give us a chart like this:

In other words, given the room temperature in Canada being ~10-15 degrees cooler than in India, a shrinkage of 0.03mm could be expected when the parts reached Canada. We expected that for glass filled PTFE, this may not be quite as high – as glass itself may not be as susceptible to dimensional deviation based on temperature.

In order to check this, we machined identical components from different grades of PTFE, having the outer diameter of 19.07mm at room temperature and cooled them down to well below 0 degrees Celsius. We measured this dimension when the pieces were taken out of the sub-zero environment and then allowed them to sit at room temperature, measuring the outer diameters and temperatures at regular intervals to plot a curve of dimension versus temperature.

The grades we used were the following:

For each grade – 5 identical parts were machined and their dimensions were measured along the same point. The dimension considered was 19.07 +/- 0.02mm. The parts were first measured at room temperature (about 25 degrees Celsius) and then put into a sub-zero environment for 4-5 hours. Each part was then taken out individually and measured again at fixed intervals of 10 minutes. The aim was two –fold: (1) to observe the extent of shrinkage due to the cold and assess the rate of expansion as the part warmed up at room temperature (2) To gauge whether the part, when left at room temperature overnight, regained it’s original dimension.

The charts below show the results for each grade:

It was observed that the virgin material experienced the highest shrinkage (0.9% over a temperature range of 40 degrees Celsius). Both Glass and Bronze filled PTFE experienced lower shrinkage (0.5% over a temperature range of 30-32 degrees Celsius). It is also interesting to note that virgin PTFE reached a much lower minimum temperature. While the filled grades were recorded as having temperatures of  -4 to -5.3 degrees at their lowest, the virgin material was recorded with a minimum of -11 degrees – despite being subjected to the same sub-zero environment prior to measurement.

 

Additionally, all three grades reverted to within 0.01mm of their original dimensions when left overnight to warm under room temperature. This suggests that the dimensional change is linked purely to the ambient temperature and that there is no observable stress build-up in the material due to the cold which causes it’s original dimension to alter permanently (at least, not in the range of -15 to +30 degrees Celsius).

All three materials lend themselves to a high R-squared straight-line graph. While we accept that this is a fairly simplistic relation to assume, the R-squared changes only marginally when we try to introduce more complex equations. Furthermore, while the relation between the dimension and the temperature may not be a strictly linear one – for the purpose of practicality, we believe that it serves quite well. In other words, assuming a 0.2% shrinkage for every 10 degrees in temperature (for virgin PTFE) would imply that a 20mm dimension would need to accommodate a plus side tolerance of 0.4mm. This is in line with our own trial and error conclusions thus far, when exporting to colder countries.

Finally – it could be pointed out that the same experiment could be carried out at higher temperatures to gauge whether PTFE continues to expand the same way in the other direction. However, as the bulk of our export destinations are in fact colder countries, we have not looked at this right now. Perhaps the same could be taken up in a separate post.

Mapping the PTFE Price Increase

While much has been said about the causes and implications of the PTFE price escalation, we felt it necessary to go through our archives and chart out the exact extent to which the prices have changed.

The chart below shows the price per Kg in US$ for three standard grades – Virgin PTFE, Glass Filled PTFE (15%) and Bronze Filled PTFE (40%). In addition, we have included a table showing the total and monthly growth in prices.

Needless to day, the growth has been unprecedented. In Virgin PTFE, a nearly 8% increase in prices every month has put the industry in a state where there is no breathing time between processors getting new pricing information and passing on that information to the customers.

Most processors are well aware of the effect this has had on their businesses. The main issue has been convincing customers regarding the price increase and furthermore making them aware that the trend may be expected to continue. In addition to this, there is the impact on repeat business, as clients withhold contracts which would have otherwise spanned their requirements over a full year – since processors are unable to commit to prices for more than a one month horizon.