Self-Sealing Porous Plastic
Have you heard the term “self-sealing” porous plastic used as a design option for a venting of filtering application? The material is used in a variety of applications from pipette tip filters, to catheter vents, to hospital suction canister valves. Yet, in all of these applications – the pores that are located in the porous plastic that never actually seal. So why are these parts referenced with the name “self-sealing” porous plastic? Well, we’ll answer this question and more in this five minutes segment.
Hi, my name is Ken Milam. I’m an application engineer here at ThermoPore. Welcome to Thermo.TV™. In this series we’ll take a look at the material science behind self-sealing porous plastic and explain in detail why these materials are often referred as “seal-sealing” porous plastic. Our discussion will involve three parts. First, we’ll discuss raw materials and raw material blends. Then, we’ll describe the additive that enables the self-sealing functionality. Lastly, we’ll describe what happens when a fluid comes into contact with a self-sealing porous plastic part. In the end – you’ll be able to determine if the term “self-sealing” porous plastic is appropriate.
Okay, so our discussion begins with the fact that the raw materials used to make porous plastic vary depending on the part specification. In some cases, a single raw material is used to make a part. In other cases, multiple raw materials might be blended together to make a part.
In the case with our self sealing porous plastic - a blend might is constructed from a plastic raw material and a functional additive. The plastic fulfills two roles. First, the plastic serves as the foundation for the part’s porosity. It creates the internal open-cell structure of the part. Second, the plastic functions as a binder to maintain the additives presence in the part’s matrix.
A blend of this type can be easily demonstrated by mixing two materials together. Imagine the white material as our plastic raw material. Imagine the green material as our functional additive. By blending these two materials together – we ensure a homogenous raw material blend. After subjecting this blend to a thermal cycle, the functional additive is trapped and incorporated into the part’s porous structure.
So you might be wondering by now – what’s the additive? Well, it goes by the name Carboxyl Methyl Cellulose but fortunately for us it’s commonly referred to as CMC. While there are a number of different grades of CMC (industrial grade, food grade, pharmaceutical grade, etc.) they all accomplish one important role. CMC increases the viscosity of a fluid in which it’s dissolved. Let me say this again. Once CMC goes into solution, it thickens or increases a liquid’s viscosity.
Now, let’s imagine a part design that requires a filter or a vent that should function until – until liquid makes contact with the porous part. This scenario is typical of the previous applications that we mentioned – pipette tip filters, catheter vents, to hospital suction canister valves. In all of these cases, the porous plastic component acts as a filter/vent until the part comes into contact with a liquid. As the fluid starts its attempt to navigate through the porous matrix, the CMC is dissolved in the liquid and in a nearly instantaneous fashion, the liquid’s viscosity located in the porous plastic part increases.
Fortunately, there’s a direct correlation between the amount of force that is required to push a fluid through a porous media and the liquid’s viscosity. As one goes up, so does the other. When the viscosity of the fluid increases appreciably, the fluid flow through the part discontinues. This gives the appearance, emphasis on the word appearance, of a part that magically seals, hence, the terminology self-sealing porous plastic – even though the porous structure of the porous plastic remain virtually unchanged.
So now, back to our first question: is the term self-sealing porous plastic accurate? I think the bet way to answer that question is by stating that if it looks like self-sealing porous plastic and if it behaves like self sealing porous plastic, then it’s probably contains CMC.
With that, I think I’ll conclude this edition of Thermo.TV™. Stay on the look out for additional videos by signing up for our RSS feed and as always, if you have any additional questions or if there are some topics that you’d like to see added to the Thermo.TV™ channel – give us a call or drop us a line. For now, I’m Ken Milam saying thanks for watching this installment of Themro.TV™ – we’ll see you next time.