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.
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.
- 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
- 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
- Variant 1: Pure PTFE Membrane
In 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.
- Variant 2: Porous PTFE membrane
Porous 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.
- Variant 3: Plain expanded PTFE membrane
Expanded 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.
- Variant 4: Laminated expanded PTFE membrane
This 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 breathability ensures 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. 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.