is this ABS or PBT?

[phx]

a friend bought these from China, claimed to be PBT.
Never seen a PBT cap before so I'm not sure...

http://imgur.com/a/mejnw#OKiT8

I know it's probably hard to tell from the pictures, how can I judge it myself?

[sth]

there's no way to tell the difference between plastics when you take a picture with a potato.

besides there is practically no visible difference between the two anyway.

look at the bottom to see if you can see PBT or ABS imprinted.

[phx]

I'm sitting here staring at this and trying to compare it to an ABS key.
the white one feels firmer and less shiny

i can almost break the ABS cap by squeezing it, it caves in a lot, but the white key is a lot harder to squeeze, overall feels better quality.

[sth]

well then put em on your board - if they get shiny in a month or 2 they're ABS

[Halverson]

Could always sacrifice one. Put it in some boiling water, pbt should be fine, abs should warp.

[kmiller8]

Could always sacrifice one. Put it in some boiling water, pbt should be fine, abs should warp.

Boil 'em, mash 'em, stick 'em in a stew!

Oh wait, keycaps not potatoes.

[phx]

Could always sacrifice one. Put it in some boiling water, pbt should be fine, abs should warp.

i was planning on selling them, >.>

[phx]

Could always sacrifice one. Put it in some boiling water, pbt should be fine, abs should warp.

ok I cooked it along with 2 other ABS keys.
http://imgur.com/M5it2
they all warped, so the conclusion is it's ABS.

[Halverson]

Could always sacrifice one. Put it in some boiling water, pbt should be fine, abs should warp.

ok I cooked it along with 2 other ABS keys.
http://imgur.com/M5it2
they all warped, so the conclusion is it's ABS.

Looks just like the abs cap I put in with my pbt caps I dyed. Warp right at the sprue mark.

[samwisekoi]

Drop them in water. PBT sinks, ABS floats.

 - Ron| samwisekoi

[Octane]

^^ always try this before you boil them.

[Halverson]

Oh my, I feel like an idiot now. Had no idea that worked. Went and tried, low and behold it does. A whole cap set I thought was abs turned out to be pbt.

[rowdy]

They were dropped in water - boiling water.

[samwisekoi]

They were dropped in water - boiling water.

I am not sure that affects specific gravity, but it certainly affects the keycaps!

Any chemists know if water temp invalidates the test?  I suppose I could just ask my Middle School student son.

 - Ron | samwisekoi

[BLJ Consulting]

ABS has a SG of about 1.04 and PBT has an SG of about 1.3.  SG of 1 would provide neutral buoyancy.  Polyolifins almost always have SG around 0.9 so they always float.  ABS often floats or at least sinks very slowly and PBT almost always sinks depending on geometry. 
There is a few catches with molded parts -
1-some materials have fillers which often change the density,
2-the molding process can use very high pressures and those pressures can change the true final density of the part
3- part geometry, especially thick sections, can lead to voids in a part (trapped air) again changing the net density.
4- you may be in some different physical reality than others and the laws of physics can be bent according to your imagination.

[clickclack]

If you also have some solvent (e.g., acetone) you could test a small area and if it makes the plastic tacky or color comes off it would likely be ABS instead of PBT. You can also cut/scrape or rub the plastic quickly and ABS will give off a sharper odor, though you need to be familiar with plastics alread or have known PBT available to compare. You can also scrape ABS to dust or flake easier than PBT, ABS sands cleaner and quicker than PBT and tends to dust/chip more and more quickly than PBT when using any number of rotational burrs. Shavings of ABS will also tend to curl a good bit more than PBT (even with just a tiny razor blade cut)

Any who back to the water and its specific gravity and density. It's a miniscule amount lower than 1 g/cm^3 and you need not worry too much about part buoyancy as water has a mean delta of (I think) a tad less than 3% through it's tempurature ranges and that is marked as a decrease not increase, so standard ABS and PBT will always sink (even if slow, excluding trapped air and surface tension). It's density would be marked as a parabola not a linear increase or deacrease. Hydrometer FTW!

Another thing to think about when checking the relative buoyance of a keycap is the infill (if any) has a higher specific gravity than water, on average it's between ABS and PBT (closer to PBT... 1.1, 1.2 something). Metallized plastics will be denser as well and you have to look to see if there are any tiny air bubbles hanging on to the keycap, that will make it float or very, very slow to sink. And two piece keys (not 2-shot) will likely trap air even if just by surface tension.

[Goodfella]

So if my new poker keyboard, which I paid for abs caps (was $20 less for them), and I tried the water test (not boiling water), my poker key sits at the bottom when I get the air out of the stem, and my signature plastic cap floats when I get the air out of the stem. Is this water test typically correct?

[kmiller8]

So if my new poker keyboard, which I paid for abs caps (was $20 less for them), and I tried the water test (not boiling water), my poker key sits at the bottom when I get the air out of the stem, and my signature plastic cap floats when I get the air out of the stem. Is this water test typically correct?

No, the only true way to tell difference between PBT and ABS is science. As mentioned, acetone will 'melt' ABS while PBT will be fine.

[samwisekoi]

So if my new poker keyboard, which I paid for abs caps (was $20 less for them), and I tried the water test (not boiling water), my poker key sits at the bottom when I get the air out of the stem, and my signature plastic cap floats when I get the air out of the stem. Is this water test typically correct?

It has always been definitive for me.  I've just tried six tests in tepid water:

1. Imsto thick PBT dye-sub - sank like a stone.
2. Hazard dye-sub thinner PBT (SP?) - sank like a stone.
3. IBM Model M dye-sub PBT one-piece keycap - sank like a stone.
3. Filco red escape key that I thought was dye-sub PBT, but apparently is lasered ABS - won't sink no matter what.
5. WASD lasered ABS - floated, then wafted gently downward and then bobbed along the bottom.
6. Double-shot ABS biohazard key (SP?) - same as the WASD, but slightly higher above the bottom.

The immediate reaction was obvious.  Five minutes later it is less so.  (Except for #3; I can't get it to sink no matter what I do.)

Oh, #5 has floated back up to the surface now.

How is that for anecdotal data?

PBT: 3 straight to the very bottom, where they stayed put.
ABS: 1 can't be sunk, 2 floated, then bobbed gently down (and one came back up.)

Oh, as a control (or for entertainment value) I also dropped in a 2x2 Lego piece.  It is floating at the top with the Filco key.

 - Ron | samwisekoi

p.s.  kmiller8; I'll drop some acetone on the back of that Filco key tomorrow.

[kmiller8]

samwisekoi, try this (I just read it on the internet so it must be true!)

Add 1 tablespoon of salt to 2 cups of water, then throw all your keys in, apparently that should help with the ABS keys that like to sink

[samwisekoi]

samwisekoi, try this (I just read it on the internet so it must be true!)

Add 1 tablespoon of salt to 2 cups of water, then throw all your keys in, apparently that should help with the ABS keys that like to sink

Better than throwing all my keys into acetone! 

 - Ron | samwisekoi

p.s.  You're not K Miller from Cincinnati, are you?

[kmiller8]

Better than throwing all my keys into acetone! 

 - Ron | samwisekoi

p.s.  You're not K Miller from Cincinnati, are you?

no, you're looking for kmiller3

[samwisekoi]

So apparently we need to be floating them in Ethylene Dichloride, a.k.a. Freon 150.  It has a specific gravity of 1253.00 kg/cu.m at 20c.

Unfortunately, NIOSH considers ethylene dichloride to be a potential occupational carcinogen.  And no alcoholic beverage seems to be better than water for this test.  Mogen-David, perhaps.

Perhaps I'll try the salt water thing first.

 - Ron | samwisekoi

[deegaf]

lol potato...

[Zehkul]

3- part geometry, especially thick sections, can lead to voids in a part (trapped air) again changing the net density.

This is pretty much it. I’ve never seen an ABS key that didn’t sink after making sure the air in the stem got replaced by water. Sometimes really takes some work to get it out though.

The water test is really easy and reliable, as the falling speed difference is huge. And PBT would need quite the amount of air to float. ^^

[esoomenona]

ABS vs PBT guide

[Goodfella]

Need to see this for myself, ordering some PBT's for my new poker. Are they more resistant to shine as well?

[TheQsanity]

Are blank Das III PBT or ABS. They seem to sink.

[kmiller8]

Are blank Das III PBT or ABS. They seem to sink.

Aceeeetoooooneeeeeeee

[TheQsanity]

hmmm, no.

I would not use that on any of my keyboards. Even if I was planning to burn it.

prolly pbt.

[esoomenona]

If I had to bet, I'd put money on them being ABS.

[dorkvader]

I always thought PBT/ABS was easy to differentiate with acetone (PBT is resistant to acetone). I have heard of the "density challenge" before, but I don't like it (especially with easy-to-get acetone).

My question: Other than surface roughness, how can you tell PBT from POM?

[IvanIvanovich]

Tell difference PBT from POM? Touch it. At least I can tell that way, POM always feels more greasy and PBT really dry to me. Also some other things in science... http://www.makeitfrom.com/compare-materials/?A=Polybutylene-Terephthalate-PBT&B=Polyoxymethylene-POM-Acetal

[kmiller8]

Tell difference PBT from POM? Touch it. At least I can tell that way, POM always feels more greasy and PBT really dry to me. Also some other things in science... http://www.makeitfrom.com/compare-materials/?A=Polybutylene-Terephthalate-PBT&B=Polyoxymethylene-POM-Acetal

so it's simple, heat your cap to 200C, if it melts, its POM, if it still looks like a keycap, PBT!

[samwisekoi]

Tell difference PBT from POM? Touch it. At least I can tell that way, POM always feels more greasy and PBT really dry to me. Also some other things in science... http://www.makeitfrom.com/compare-materials/?A=Polybutylene-Terephthalate-PBT&B=Polyoxymethylene-POM-Acetal

so it's simple, heat your cap to 200C, if it melts, its POM, if it still looks like a keycap, PBT!

So first drop it in boiling water; if it is destroyed it was ABS.

Then drop it in acetone.  If it is destroyed it was PBT. 

Finally, put it in an oven set to 400 degrees.  If it is destroyed, it was POM!.

Excellent!

To be certain, we should always conduct these tests on the entire keycap set, correct?

 - Ron | samwisekoi

[kmiller8]

...
Then drop it in acetone.  If it is destroyed it was PBT. 
...

Then drop it in acetone.  If it is destroyed it was ABS.

PBT and POM are resistant to acetone.

Boiling water -> warped/unusuable ABS key

Acetone -> slightly thinner underside of ABS key (if done properly)

Oven -> POM puddle

[kmiller8]

Double post FOR SCIENCE!!

221° F (105° C)
Acrylonitrile butadiene styrene, Melting point

347° F (175° C)
Polyoxymethylene plastic, Melting point

433.4° F (223° C)
Polybutylene terephthalate, Melting point

So you could start with your oven around 250F, to melt ABS, then crank it to 400F to melt POM, then 450F to melt PBT!

[Rumudiez]

Best thread of the day, declared less than halfway through the day..

Really interesting methods for identifying plastics!

[BLJ Consulting]

i'm not an expert on material identification and characterization and it's never been a big part of my career but i can tell share a few of the basics.  1st keep in mind that there are literally tens of thousands and grades of resin that can be molded and more if you consider extruded, blown, cast, etc. so there are very sophisticated methods such as gas chromotography and mass spectrometry to make precise identifications.  There are also plastic alloys which are widely used to optimize cost and performance which makes identification even harder.

overall i think people have hit on the basics:
melt temperature - obviously works best with a calibrated and monitored probe but cruder methods are ok for large differentiation (for example POM has a very low melt temp and PBT has a relatively high melt temp).  you can google "vicat softening" for an ASTM standard and more info

chemical/solvent resistance - again you have figured out the acetone thing but there are many tests than include polar/nonpolar solvents and well as alkali for differentiation. 

burn tests are used as some materials release a very distinct smell or smoke, others are self extinguishing.  this is not used much for obvious reasons and also because complex fire retardants have been more prominently commercialized in the last 5-8 years which skews results.  for your POM question you can note that POM releases formaldehyde compounds at very low temperatures which have a very distinct smell (and are also carcinogenic) so a low tech ID form is to put a heated pin or paperclip into the surface and smell the result.   

density testing - i mentioned this before, it is a real thing.  In a lab the test liquids are calibrated for density/SG so they may include distilled water or various alcohol compounds and can be used to narrow down density pretty well.

physical properties - the quote from lysol is pretty accurate.  POM is well known for have excellent wear characteristics and is used almost exclusively in any plastics gears and cams

obviously there is a lot more to this but this is already  more time than i planned to invest.  hopefully that is helpful, maybe i can type up something better if i have time or if someone has specific questions

edit - wow you guys posted a lot while i was typing this.  i think most of it is still relevant

[IvanIvanovich]

I like this oven method. And of course using multiple keys like a full set is best to ensure reliable results while testing.

[samwisekoi]

I like this oven method. And of course using multiple keys like a full set is best to ensure reliable results while testing.

Or perhaps a whole mess of domed Windows keys could be turned into puddles -- for SCIENCE!

 - Ron | samwisekoi