Author Topic: ABS vs PBT Coefficient of Friction  (Read 8481 times)

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Offline Hypersphere

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ABS vs PBT Coefficient of Friction
« on: Sat, 27 July 2013, 14:16:54 »
Today I was comparing the feel of PBT vs ABS keycaps, and I came up with a simple qualitative test for sliding friction. The test consists of placing your fingertips on the keycaps on the home row, pressing down gently, and pulling your fingertips toward you. Doing this, I found that keyboards with PBT keycaps had very little friction, but those with ABS keycaps had very noticeable "drag". In fact, my fingertips on ABS would produce a slight "shudder" or "chatter" when pulled across the ABS keycaps.

My test boards and keycaps were a Filco TKL with Originative double-shot ABS keycaps or Filco ninja ABS keycaps, a Leopold FC660M with original PBT keycaps or Originative PBT keycaps, and an IBM SSK with original PBT keycaps.

This qualitative difference between ABS and PBT keycaps agrees with quantitative data on coefficients of static and dynamic friction, as depicted in the graph shown below (from Biron, Thermoplastics and Thermoplastic Composites, 2nd Ed., Elsevier, 2012). In any event, whether or not the difference in typing feel between ABS and PBT is due, at least in part, to the differences in static and sliding friction, I prefer the feel of PBT keycaps over that of ABS keycaps.

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Offline SpAmRaY

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Re: ABS vs PBT Coefficient of Friction
« Reply #1 on: Sat, 27 July 2013, 15:47:17 »
Maybe the ABS just needs typed on for a little while to smooth it out.

Offline damorgue

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Re: ABS vs PBT Coefficient of Friction
« Reply #2 on: Sat, 27 July 2013, 15:50:59 »
The ABS and PBT key caps I have encountered have had different surface textures. That could play a significant role in feel, as well as in that test of yours.

Offline Grim Fandango

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Re: ABS vs PBT Coefficient of Friction
« Reply #3 on: Sat, 27 July 2013, 17:04:50 »
The ABS and PBT key caps I have encountered have had different surface textures. That could play a significant role in feel, as well as in that test of yours.

I often see people say that PBT has a rougher surface texture, but I wonder how much this has to do with the material and how much this has to do with the actual production process. I have used ABS and PBT keycaps with different types of "grain" or "texture" at the surface of the keycaps. The IMSTO PBT caps have a pretty smooth surface. Much more smooth than the KBC and Vortex ones that I tried.

I also remember reading this on WASD's blog

With PBT, you can expect a much more durable finish on the texture of the key. ABS plastic has poorer wear properties which is why they are known to "shine" after heavy use. PBT isn't indestructible, but has better wear properties and will resist "shine" much longer than ABS. Contrary to popular belief, the "roughness" or texture of the key has nothing to do with the material used. Texture is created by the injection mold used to create the product.

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I am also not completely convinced that keycaps with a more grainy surface texture do actually provide more grip, even though this seems to be what people are claiming.  When going back and forth between ABS and PBT, I honestly do not feel like my old shiny ABS is any worse in terms of grip. If anything, my fingers actually feel more secure on the smooth surface of my ABS keys than on the more grainy, almost sandy surface of a "new" PBT set.
« Last Edit: Sat, 27 July 2013, 17:13:47 by Grim Fandango »
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Offline Danule

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Re: ABS vs PBT Coefficient of Friction
« Reply #4 on: Sat, 27 July 2013, 17:55:21 »
I find that ABS grips more than PBT but it does depend on the texture of the keycap.
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Offline dorkvader

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Re: ABS vs PBT Coefficient of Friction
« Reply #5 on: Sat, 27 July 2013, 18:20:34 »
as with the above, I suspect this has more to do with the surface texture from the mold than the inherent frictive properties of the material. Nevertheless, it is, perhaps, more important to measure that, so people will know what to expect when their new keycaps arrive.

I suppose to quantify the coefficient of friction (or even qualify it more accurately) one should cast a block of it, then machine it to a standard mass and surface area (of the sliding surface) to get as uniform a surface as possible.

Some polymers, such as POM (delrin) absolutely have lower coefficients of friction, which is why I prefer POM spacebars. Nevertheless, I am certainly inclined to believe the graphic posted in your textbook.

Now friction is a rather difficult thing: you see it's based on a number of factors (there's some sort of inherent baseline, then there's surface roughness, and porosity, etc.) I want to see the porosity of PBT compared to ABS, as I suspect PBT to be more porous, and hence better for dye sublimation.

Come to think of it, it makes sense that PBT has lower friction. Has anyone felt the side of a unicomp keycap? The sides are smooth, and it really shows.

Excellent writeup and I'm glad you posted.

Offline PointyFox

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Re: ABS vs PBT Coefficient of Friction
« Reply #6 on: Sat, 27 July 2013, 19:58:15 »
There is a lot more to it than this. 
You need to find the static and kinetic friction versus a human skin tissue analogue since your fingers are not made of steel.  In addition, surface roughness/patterning/asperities and even material hydrophobicity play a large role in friction.  You will need to prepare samples of the polymers with identical weight and roughness and use them with your analogue to determine friction via a sliding force gauge or inclined plane.  This will not provide an exact correlation due to differing amounts of sweat and oils on your fingers than the tissue analogue, but should get you close enough in developing your thesis.
« Last Edit: Sat, 27 July 2013, 20:00:54 by PointyFox »

Offline davkol

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Re: ABS vs PBT Coefficient of Friction
« Reply #7 on: Mon, 29 July 2013, 18:54:05 »
As others have suggested, it doesn't make much sense. For example, compare inner half of Unicomp keycaps (very smooth PBT) to SP's PBT DSA set (rough texture). Or use different kinds of sandpaper on same ABS keycaps. The real difference is probably mostly in resistance to aggressive sweat. One of my old Cherry keyboards has some of its lasered PBT keycaps shiny, it's the same kind of wear as in case of ABS, but it took about 15 years of use to get it to this point.

Offline Hypersphere

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Re: ABS vs PBT Coefficient of Friction
« Reply #8 on: Mon, 29 July 2013, 19:22:14 »
...since your fingers are not made of steel....

Speaking for yourself, of course. ;)

Points well taken, however. There are indeed many variables that need to be taken into account.

Offline Danule

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Re: ABS vs PBT Coefficient of Friction
« Reply #9 on: Tue, 30 July 2013, 07:43:05 »
After trying out both Cherry doubleshot ABS and Thick replica Cherry PBT keycaps I think I'm leaning towards likeing the doubleshot ABS more.  They are softer to the touch than the PBT.  But both are very nice I'm sure I will be switching between the two often.
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Offline TheSoulhunter

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Re: ABS vs PBT Coefficient of Friction
« Reply #10 on: Tue, 30 July 2013, 08:49:18 »
Both is important, texture and material... A rough texture will take longer to "level down" than one that is smooth to begin with (Look at Filco caps where the base texture is filled up with clear coat -> They get shiny really fast). Abrasion resistance, which results from base hardness and friction coefficient iirc (some time since material since class ^^;), as well as chemical stability to contacting substances (Sweat, Dorito seasoning, whatever...) will determine how surfaces with identical texture will compare to each other. What I read (an experienced) PBT, POM and PC will do much better than ABS in both categories. Worst vs Best case comparison: Filco Majestouch Spacebar (smooth black ABS) -> Got a shiny spot after 1 month, same on a second one... KBC black PBT Spacebar (rough'ish texture) -> Still not shiny after 1 year of use.

Offline pixel5

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Re: ABS vs PBT Coefficient of Friction
« Reply #11 on: Tue, 30 July 2013, 09:01:39 »
PBT has a less uniform surface, thus making less contact with your fingers. Linoleum vs Concrete, basically. One might be 'smoother', but grabs your skin more.
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Offline TheSoulhunter

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Re: ABS vs PBT Coefficient of Friction
« Reply #12 on: Tue, 30 July 2013, 12:34:28 »
I totally forgot about material pairing... Abrasion resistance (actually more the hardness) of the material that gets in contact with the base material is important as well. If a material is less resistant than your skin, your skin will wear it down, but if the material is more resistant it will wear down you skin. A material only slightly less resistant than your skin (even if it's just -10%) will wear down, a material only slightly more resistant (even if it's just +10%) could resist abrasion very long. Actually the Linoleum vs Concrete is a good example -> Linoleum might cause more friction, but as it is relatively soft, it won't cause much abrasion on your skin (tho the friction might cause enough heat to burn your skin), concrete on the other hand is much harder than you skin causing strong abrasion.

Offline COFFFEEEE

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Re: ABS vs PBT Coefficient of Friction
« Reply #13 on: Wed, 31 July 2013, 13:33:42 »
PBT is little more "cripsy" to me. The more important is of course the durability.

I will always buy a keyboard with PBT now, or buy both the same time.

Offline Danule

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Re: ABS vs PBT Coefficient of Friction
« Reply #14 on: Wed, 31 July 2013, 13:39:37 »
im finding that the ABS keycaps stick to my fingers more when they get sweaty.  I find that my grip on PBT stays pretty consistent regardless of the state of my fingers.
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