transparent analogue of PTFE or MACOR?

[runcyclexcski]

Hi all,

I am looking for HSS-machinnable, solvent-resistant, transparent (>50% in 400-900 nm) material to make a top plate for microscopy, about 2 x 2 x 0.25 inches. Is there a transparent analogue of PTFE or MACOR? Scratch-resistance is less important, b.c. I can cut a window in the middle and insert a glass slide in there. Currently, I am using these materials, but all of them have limitations

-- borosilicate glass: only machinnable with diamond tools, this ruins my mill and takes forever.
-- acrylic: attacked by solvents; scratches easily
-- PTFE: non-transparent.
-- MACOR: non-transparent.
-- polystyrene: even worse than acrylic.
-- polycarb: attacked by solvents; scratches easily

I came across ALON while looking on this forum, but it appears to have glass-like properties in terms of machinning. Have there been any other developments in this area?

Thank you in advance!

 

[btrueblood]

PFA
ECTFE, fairly soft stuff.
low-density polyethylene, if thin enough, can be transparent
Polysulfone, though it has a slight yellow tint.
there are transparent pvc's, though it is not resistant to all solvents
PVDF can be transparent if thin enough.
PET and PETG are the stuff plastic water/soda bottles are made from, may not be resistant to all solvents.

Like everything else, there are tradeoffs. Polysulfone is pretty brittle, which makes it difficult to machine, while PFA is quite rubbery which makes it hard to machine. Solvent resistant is a spectrum, not a property, you need to be specific about the solvent and what type of damage and rate of change you are willing to accept.

 

[Pud]

You might find this COC useful. I have no idea if you can get it in the form you want though. Might give these a call: Topas

www.tynevalleyplastics.co.uk

Politicians like to panic, they need activity. It is their substitute for achievement.

 

[runcyclexcski]

Thank you, all. Indeed, I should have been more specific about solvents. I suppose there is little chance to get the resistance and high as that of PTFE and glass while retaining transparency and machinnability? My favourite cleaning reagents are acidic piranha (H2O2 + H2SO4), KOH, and chloroform. I could drop any one of the two. Only glass and PTFE are resistant to all three.

polyethylene swells in chloroforom, not good. So does polypro (and both are not very transparent at 5 mm thickness; I need the 5 mm thickness).
Ditto on PVDF and polysulfone. PET: if you leave a soda bottle in chloroform, it will leak out b.c. the bottle swells.

Spincoating machines have lids made of a PTFE-like materials that are transparent. I have a Laurel spincoater, and its lid appears made of acrylic, but it was never damaged by toluene or acetone, so it might be the material of choice. Upon reading the specs, the material is this stuff, but appears to be discontinued:

"Flametec® Halar® ECTFE - Clear 453"

 

[moltenmetal]

Glass is not resistant to strong KOH solution.

ECTFE is still made, but like the other fluorocarbons it is much more translucent than transparent.

 

[moltenmetal]

To clarify- you can expose glass briefly to a wipe-down with KOH solution, just like you could swish out a PE or PP container with chloroform and have no ill effects. But you can't immerse glass in strong KOH and expect it not to etch and dissolve.

Perhaps if we can understand what you're trying to clean we can help you select a transparent material that can also be machined and cleaned. I think you've overspecified the problem right now such that no good total solution exists.

 

[runcyclexcski]

Hi moltenmetal,

I use 5M KOH to clean glass all the time; this, indeed, relies on the etching properties of KOH towards glass. However, the rate of the etching is very slow. In my experience (and depending on the type of glass of course) the etching is less then 1 micron per hour. Soda lyme is more susceptible than borosilicicate. I have borosilicate substrates that have been cleaned with KOH at least 20 times, 1 hour at a time, room temperature, with sonication. They still appear 'as good as new'. In fact, there are apertures in these glasses, and, as far as I can tell, the diameter of the apertures has not increased by an amount measurable with calipers (10-25 microns). I would not store KOH in them long-term, but it's not as unsafe as, say, trying to store acetone in an acrylic jar (the latter will dissolve), or chloroform in a polypro bottle (the latter will swell by about 25%).

"Halar® ECTFE - Clear 453" seems to be the material I am after, but it does not seem to be manufactured anymore. Spincoating company 'Laurel' makes their chamber lids out of the material. The lid is about 5 mm thick, and is clear (i.e. as clear as one would expect from a piece of acrylic or polycarb). The lid has seen massive abuse with toluene and vapours of sulfuric acid, and still appears as good as new.

For now, I have resorted to machinning thin glass, and assembling larger pieces from multiple pieces of glass glued with JB Cold Weld. This resists short-tern exposure to acetone. I was still hoping that some other material might fit the constraints " HSS-machinnable, solvent-resistant, and transparent "

 

[btrueblood]

Have you looked at PFA?

 

[runcyclexcski]

I have dealt with small PFA tubes in fluidics (wall 0.5-1mm), they were cloudy/translucent. My application involves making a mechanically strong lid (say, 5 mm thick) to clamp down on a chamber, and I imagine at 5 mm thickness PFA would be pretty much non-transparent.

 

[runcyclexcski]

I have an update on this, may be someone will find this useful. Folks at Laurel the spincoating company where kind enough to send me a cracked lid from one of their units, made from clear Halar 453 (according to Laurel). The material is 5 mm thick, and has the transparency/appearance of polycarbonate, and the softness of PTFE. I put two 1/4" chunks of the lid into chloroform at 37C in a teflon jar, and left shaking for 2 hrs. In parallel, I also challenged chunks of polycarbobate with chloforom, same conditions. By the end of this, polycarbonate was entirely gone, dissolved. The Halar was as good as new. I have enough Halar material for now, but it would be nice to have access to thicker stock (>5 mm thick). Sadly, the stuff does not appear to be produced anymore, at least not by this name.

 

[moltenmetal]

ECTFE has another trade name at least- Kel F. Yes, chloroform is a good solvent for polycarbonate and acrylic and not a solvent for the fluoropolymers. But you will be on a fun search of grades of material to get optical transparency, since most such polymers are translucent rather than fully transparent.

 

[runcyclexcski]

Thank you! For ECTFEs, it appears that most of them are defined as 'translucent' (and appear as such in pictures), and only that Halar 453 stuff is defined as 'clear'. I beleive it's used in visors for hazmat helmets (acrylic/polycarb would be a problem, and glass might shatter). For my application, I need just enough transparency to manipulate small parts (by hand/by eye) with fine tweezers behind a screen at <10 inch distance. I.e. I do not need 'transparent at infinity' (not for telescopes etc ).

I hope I can find whether that Kel F product exists as clear.

P.S. Update: Laurel has indicated that they've got all the remaining stock of Halar 453.

 

[runcyclexcski]

I have yet another update on Halar/ECTFE behaviour in chloroform. After 18 hrs at 37C in Chloroform, it got swollen, and become rubbery. Transparency was still there. After I dried it off for several days, the rubbery consistency has remained, so it got affected irreversibly. So, although ECTFE much more resistant to harsh solvents than polycarbonate, it is not as resistant as PTFE long-term (PTFE remains unaffected after weeks of exposure to chloroform at 37C, in my lab).

 

[moltenmetal]

Face it- you've over-specified the problem. There isn't a material that meets all of your needs. PFA will meet the chemical resistance but is translucent rather than transparent. Transparent, resistant materials aren't even fully resistant to your full suite of materials (try glass immersion in 50% NaOH or KOH at 37 C for 18 hrs- weigh it before and after and report back), and the glasses don't meet your machinability requirement. Figure out what is most important to you and compromise on what isn't.