Hi Jonathan Here. Happy New Year, the winter is here. It seems the extreme weather in the US freezes up many states. California has soaked in a couple rain storm last couple weeks, finally, we got sunshine.
For those who are not familiar with us. My name is Jonathan, I am the founder of Ascend Sealing Technology and your host in this channel.
I believe most of you have the experience to select proper material for your design.
Today, we are going to talk about chemical compatibility, particularly on Rubber Swell.
In the last episode, we discussed about Fluoro carbon elastomer FKM. I will use FKM as an example to explain chemical compatibility. To start with all the boring talk, let me show you what is swell.
To replicate this experiment you will require:
Couple Fluorocarbon elastomer O-ring
Other Optional accessories includes:
A pair of Glove
A pair of tweezer
Some Kim wipe
Ventilated Room, such as outside of our office
Solvent and Chemical Respirator
And we are ready to perform the test.
So firstly we can measured the original weight of these sample.
And the cross section thickness.
Then we pour some acetone into the beaker.
Dip the O-ring into the beaker.
Start the timer and wait for 5 minutes, we should be able to see the difference.
Now we can take out the O-ring and have a measurement.
It is quite obvious that the O-ring has swell a lot, and compare to Original O-ring side by side.
So we showed you the swell phenomenon. And now you will ask me why and how this happended?
Now it's the time to talk about the interesting science behind the theme.
In Last episode we have discussed the FKM type, we will use the most common FKM type A-type as the example. A type contained around 40% VDF and 60% of HFP, randomly mixed in the polymer chain.
VDF and Acetone both are polar molecule, that polar solvent will diffuse in polar and can be easily mix together.
You may ask why? Rubber is a solid object, how does Acetone dissolve into FKM.
Firstly, these monomer before polymerization they are Fluid.
Then after polymerization the molecule get heavier but still has fluidic property.
And Then, With Curing or crosslinking, the polymer chain formed a 3D network that become a highly viscus fluid but they are still fluid.
Let's bring up the definition of "fluid": a substance that has no fixed shape and yields easily to external pressure; Such as a gas or a liquid.
So take your O-ring apply pressure on it, yes, squeeze it. The Rubber is just a viscoelastic material by definition. We will explain viscoelastic later. Just noted rubber still remain the fluidic characteristic is good for now.
They remain the fluidic property and they will thermodynamically mix with other fluid.
So the solvent such as the experiment Acetone will diffuse into the FKM rubber 3D network.
Because of Acetone is a polar solvent and the 40% of FKM is polar monomer VDF. Polar solvent will dissolve in polar rubber such as A-type FKM. Non-polar solvent will not dissolve in polar rubber such as A-type FKM.
Well … in a simple way to explain it.
Consider this Soda water(Polar) as our FKM rubber, I mixed with an whiskey(Acetone). Due to alcohol is soluble(miscible) to water. My rubber volume is increased in to a cocktail volume. And I got a super extravagant highball cocktail.
"Your friends at Ascend Sealing remind you to drink responsibly"
On the other case, the soda water is our rubber. I pour some Italian extra virgin olive oil in to my soda water. Due to oil is in-soluble or immiscible, my rubber repel the oil and the volume remain the same. The olive oil can be fuel, gasoline, natural gases.
Even though the O-ring swelled a lot, a fluid diffusion is a physical mixing, there is no chemical reaction involve.
Thus, drying out the FKM O-ring let the acetone evaporate, the O-ring will roughly return to its original size.
However, the dimension back to original does not means everything is the same. The rubber was molded in a high temperature to increase the polymer chain randomness. We swell the O-ring and let solvent evaporate in room temperature will change the polymer conformation and 3D structure. Typically, this will result into slightly change of the physical property become slightly harder.
In summary, Rubber components are often exposed to a liquid that has some degree of thermodynamic compatibility or partial solubility. When two liquids are mixed together, they can be totally soluble, partially soluble, or totally insoluble. We avoid elastomer selections based on totally soluble unless we want to make a rubber solution. We prefer totally insoluble, but it is often cost too high or impossible. We must therefore learn to design with partial soluble.
We listed some common synthetic rubber's polarity for your reference.
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