June 23, 2014
First things first. If you’re looking for a perfectly hydrophobic coating that has solid all-around functionality, it’s hard to do better than pure PFA or FEP. These coatings have such high surface tension that water falls right off. We once coated a cylinder in PFA which was dipped in an acid solution, and the part came out of the solution perfectly dry. PFA, in particular, also offers high service temperatures, good slipperiness and release properties, decent mechanical toughness, and good chemical resistance.
However, there are times when the substrate or the use demands more than pure PFA or FEP can offer. In those cases, here’s a bit of guidance to get you thinking along the right track:
If you need significantly improved durability:
At a certain point, if you want to increase durability, you’ll have to sacrifice something of the hydrophobic properties. But there are still good solutions out there!
Xylan 1331 by Whitford Worldwide is a PPS / PTFE blend that is really designed to withstand serious wear, while delivering hydrophobic and nonstick properties. A great off-the-shelf solution.
We’ve been able to create our own custom PPS / PTFE blends that finish the cure cycle with a micron-thin layer of PTFE floating atop a layer of PPS (Ryton®) – which is one of the best solutions for hydrophobic + wear that we’ve come across, because you get all the toughness of PPS with the unadulterated hydrophobic properties of the PTFE.
Layering PPS, ceramic and PTFE is a common solution for cookwear and other consumer goods.
If you need a very low coefficient of friction:
This depends, in part, if you need side-to-side slipperiness, good release properties, or both. FEP will offer excellent up and down release properties, while PFA offers that plus great side to side slipperiness. If you still need better mechanical toughness than either of those two offer, custom PPS / PTFE blends or Xylan 1010 are the way to go. And if none of these are right, you might also consider the old-school Molykote solution. With excellent lubriciousness (but not great release properties), Molykote has been relied upon for decades in the automotive and defense industries. (And a plus – it can be mixed with epoxy resins.)
If you need a hydrophobic coating that can be autoclaved:
Here, you’re in luck – up to a point. Most hydrophobic coatings can be autoclaved, once or a few times before use. However, if you’re looking to coat a part to be repeatedly autoclaved over a long period of time, remember that PTFE can react to radiation. PTFE is a very popular coating for medical instruments, but if it will be exposed to repeated autoclaving, you’ll need to look at some of the other hydrophobic coatings we’ve discussed.