December 11, 2019, 11:49:28 AM
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Topic: polymer that acts like a rock  (Read 518 times)

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Offline jb_wa

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polymer that acts like a rock
« on: September 30, 2019, 11:49:34 AM »
Hello everyone

I am looking for a transparent polymer that acts likes rock, when looking at the rock at geological time scale. It is indeed for a fluid mechanics experiment about planet formation. What I am looking for is something like silly putty ; elastic, very brittle but able to flow. Problem is, silly putty is not transparent so it is tricky to see what happens in it and it is not brittle enough. According to my calculation I need a material with a yield strengh of about 10^3 Pa. I have had a few leads up until there but if you have any suggestion of other compounds, or of modification of the compounds that I will cite that woud render them more brittle ;

- polyvinyl alcohol maybe mixed with polyvinyl acetate  and with sodium tetraborate
- " superball " wich appears to be a skeleton of silicate with ethyl group on the side (https://webpages.uncc.edu/~jmconrad/hsed/mseg_7tm.pdf)  Maybe I could use an other alcohol ?
- Poly dimethylsiloxane linked with boric acid. It seems to be the primary component of the commercial silly putty but how can I can modifiy to make it more brittle ?

Thanks in advance for the suggestion, it could same me a lot of time as chemistry is not really my goal, I am a fluid mechanics/ geophysicist researcher


Offline Enthalpy

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Re: polymer that acts like a rock
« Reply #1 on: September 30, 2019, 08:52:16 PM »
Welcome, jb_wa!

Not the same price, but X rays can image opaque compounds. Or ultrasound scan.

Making a material brittle is the easier way. What about mixing some powder in your polymers? Like silica nanopowder for instance, maybe talcum.

Did you consider wax and paraffin? Thin parts are nearly transparent.

Can you adjust the temperature to act on the yield strength?

From wiki's
https://en.wikipedia.org/wiki/Silly_Putty
"The original coral-colored Silly Putty is composed of 65% dimethylsiloxane (hydroxy-terminated polymers with boric acid), 17% silica (crystalline quartz), 9% Thixatrol ST (castor oil derivative), 4% polydimethylsiloxane, 1% decamethyl cyclopentasiloxane, 1% glycerine, and 1% titanium dioxide."
I see some hope to make transparent silly putty.
Transparent silicone exists, I used it to glue protective glasses on satellite solar cells. Mineral charges can be transparent, like silica; if their refractive index matches silicone (TiO2 doesn't), or if the powder is fine enough, they won't diffuse light. I guess they serve to make silicone brittle. The rest is by(products of dimethylsiloxane, pigment. Remains castor oil (beware poison) and the banal glycerine.

To make extra-soft silicones, you can vulcanize silicone oil instead of the usual silicone resin.

You might ask the manufacturer of silly putty to provide a transparent one. It could be as simple are removing TiO2 and replacing the castor oil derivative with an other one.

Offline jb_wa

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Re: polymer that acts like a rock
« Reply #2 on: October 01, 2019, 07:10:59 AM »
thanks a lot !

putting silica particles is indeed a good idea, the thing is I need to have particle much thinner than the typical size of the flow I am expecting, so I need to find particle with a close enough refrative index and very thin, I figured it may involve more work to find that instead of just modifing the skeleton of the silicon.
Concerning parafin, I think it doesn't really flow once solify.
There is one thing I should precise ; because of constraints of imaging and controlling the boundary conditions, the experiment will have a typical width between 0.1 m and 1 m, so the solid in question must be transaprent within that length scale

Offline Enthalpy

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Re: polymer that acts like a rock
« Reply #3 on: October 01, 2019, 11:12:35 AM »
Nanopowders down to 30nm are usual, at least for graphite and MoS2 (as lubricants). Please protect your lungs.

I found quickly the refractive index of silicone oil: 1.402 to 1.404. For transparent rubbers, it must be available too. Pure silica has 1.46 at 550nm, rather good match. Unmodified castor oil has 1.47. Ask an optician if this match is good enough, or experiment with silicone oil and silica.
https://refractiveindex.info/
https://www.filmetrics.com/refractive-index-database
Fluorosilicates (and very few others) have a smaller index, glasses tend to have a bigger one, TiO2 and similar much bigger.

1m transparency is very difficult! For instance banal glass 0.3m thick is dark green. Silica and fluorosilicates are transparent over 1000km, PMMA is very transparent over 0.4m, for silicone I don't know. This constraint is extremely restrictive.

Several classes of silicone exist, even within the most common dimethylpolysiloxane. It depends on the processing path: prepolymer, solvent, cross-linker. For 1m transparency, every trace of remaining solvent and catalyst matters, so this choice can be made early.

Silicone rubbers have extremely varied stiffness, depending on the cross-linking and on the length of the resin oligomers. You have two adjustment possibilities here, which may be useful, as I suspect that your rheology doesn't boil down to a single parameter.

The silicon glue I used on my satellite was extremely expensive because the manufacturer made special small batches containing no silica powder (and because these customers are rich). I believe it was Dow Corning 93500. One European alternative existed then, even more expensive.

Would a liquid containing much powder resemble your needs? A thin powder won't sediment, as Indian ink shows. I fear the suspension will be brittle before you obtain a flowing force threshold. Little water in glycerol adjusts the viscosity and 0.1 is very transparent. Polyethylene glycol comes in any viscosity.

The paraffins I saw were translucent, not transparent over 1m. Mixing them from pure straight alkanes, you can adjust the melting point or melting range so they creep at your desired temperature. Octadecane melts at +29°C.

Offline Phalcone42

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Re: polymer that acts like a rock
« Reply #4 on: October 11, 2019, 03:36:32 PM »
Edit: I missed that Enthalpy mentioned this earlier, but their question was not addressed, so i'm keeping the post anyways.

Do you need both time and temperature fixed?

The flow behavior of polymers depends on both of these variables (time-temp superposition), as well as molecular weight.

If you are looking for a nice ratio for time, i.e 100my for rock  = 1 sec for polymer, then you can use any amorphous polymer, as long as you adjust the temperature and MW accordingly.

idk if rock is shear thickening, shear thinning, or newtonian at geographic timescales, but that information should be available. Also available should be the time-temp viscosity data / MW dependent viscosity data for common amorphous polymers like Polystyrene or PMMA.

Find a point where the shear rate coefficient of the polymer corresponds that of rock (noting the time conversion problem stated above). Its a bit of work/ calculation, but it is easy to acquire something like PS of a specific MW vs making a silica-silicone nanocomposite.

note: if you want brittle behavior, just stay below the Tg of whatever amorphous polymer you select, or alternatively, strain it really fast (TT superposition once again)

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