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Liquid Na2O3Si: what causes solidification at temperature?

  1. Jul 6, 2015 #1
    I have a refractory adhesive based upon liquid Na2O3Si and other unknown constituents.
    It remains a liquid until a specific temperature..... when it boils, or gasses.
    At this point, if the temperature is removed (or increased), the liquid becomes a solid.

    It would appear (perhaps incorrectly) that the 'boiling' is connected to the water content....... the remaining 'solid' is puffed up with air pockets.
    Worse..... in joints, the gassing causes ejection of the liquid (from the joint), just prior to setting.

    The immediate thought would be, to raise the pressure to a point where the water does not boil, yet the desired temperature is achieved.

    Hence the question (stated differently):
    Would the liquid turn to a solid, simply as a result of hitting the temperature, or is the gasification of water, fundamental to solidification?

    Notes:
    1. The reaction occurs in the 115 - 120C range - exactness may perhaps be related to heat conduction through the components.
    2. Maintaining temperatures lower than the 'boil point' produces no solidification.
    3. As yet I have no ideal pressure test equipment..... my next step will be to test with a pressure cooker (clearly a 'dry steam' oven might produce conclusive results)........ hence why I figured that a chemist might be able to state what is actually happening.
    4. I'm concerned that the pressure cooker will not build up pressure until the water (in the cooker) is actually boiling, and at 1 bar, boiling point will be 120C...... so test results could be inconclusive, and anyway spoiled by the presence of vapour.

    Does anybody know whether we can achieve 'solidification' without gassing?
     
  2. jcsd
  3. Jul 6, 2015 #2

    Borek

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    Staff: Mentor

    Loss of water is what counts. If the water is lost by boiling, you see what you see. If you can allow the liquid just to dry, results should be better.
     
  4. Jul 6, 2015 #3
    The problem is that it skins, and the liquid remains, as if it has been kept in it's tube (in the short to medium term.

    Are you saying, that if it was left to dry out.... the solid would be as hard as if it had not been subject to temperature?
     
  5. Jul 6, 2015 #4

    Borek

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    Staff: Mentor

    I am not 100% sure, but that's what I expect.
     
  6. Jul 7, 2015 #5
    I can say we can't solve your problem without additional information.

    1. The solubility of sodium silicates is strongly influenced by temperature, concentration of silicate and concentration of other ions.
    2. There are two hydrate forms of Disodiummetasilicate, but these are not the stable ones and the anhydrous form has an unsoluble chain anion.

    I think the problem is the starting polymerization of the silicate anions influenced by your temperature protocol, the concentration and the unknown compound remaining in the solution. I don't have enough time to look it up but i found literature. But it also can be that the loss of water increases the concentration of silicate leading to supersaturation and the following nucleation and precipitation.

    Iler, R.K. The Chemistry of Silica, Solubility, Polymerization, Colloid and Surface Properties
    and Biochemistry; John Wiley-InterScience Publisher: New York, NY, USA, 1979.

    http://www.mdpi.com/1996-1073/7/2/568

    These are two very good references for the chemistry of silicates, but don't be dissappointed if you can't find a theory for the explanation of precipitation because the mechanisms are not fully understood.
     
  7. Jul 7, 2015 #6
    Fine.... thanks for the opinion.
    For myself, I'm thinking that temperature has its part to play, but I will leave a sample exposed to ambient temperature, as a control.
    I'll post the result.

    FoxOne's post seems to support the temperature theory...... the links make interesting reading, and do throw up some potentially pertinent points, that may directly relate to the problem of 'ejection'..... I'll highlight this after quoting FoxOne:

    While the above linked paper relates to a different scenario..... the fundamentals may apply.
    Here are some interesting quotes:

    2. Theory
    The detailed mechanism of silicate gelation is not fully understood.
    The polymerization rate of Na-silicate is affected by the pH, silicate concentration, temperature and concentration of the divalent cations.
    2.1. Effect of pH and Silicate Content
    Acidic gels are considerably stronger than alkaline gels. Since their gelation time is normally short and difficult to control
    (at temperature polymerization is quick and difficult to control... and as it is intended as a cement, we can presume the liquid is acidic in nature)
    2.4. Syneresis
    After gelation time, the gel network continues to reform, causing the gel strength to increase gradually. As the system approaches equilibrium, the gel shrinks and expels the liquid it contains

    the more rapidly the gel sets, the larger are the initial syneresis rate and the volume of expelled liquid (clearly..... this is the rub)

    However in my scenario, it is the 'liquid cement' that is ejected, just before it sets.
    How annoying.

    The water boils.... turns to a gas.... ejecting the cement, while it is still in its liquid state.
    This gassing is universal, as the final solid is made up of tiny air bubbles.

    Looking at the above notes, it seems that the key could be to increase the gelation time. IE. A slower set.

    Perhaps increasing the silicate content would help.... effectively thickening the liquid, though changing the proportions of its constituents. (I do have some sodium silicate in granular form).

    I wonder what would happen under compression?
    It may be that if the water does not boil off, the polymerization will not occur

    What about if the pressure is high, but the temperature is elevated to achieve boiling.
    The resultant bubbles would be much smaller, as the pressure would be compressing the liquid, while we hope it would polymerize into a stronger form

    Any thoughts?
     
    Last edited: Jul 7, 2015
  8. Jul 7, 2015 #7
    I don't know how the pressure influences your process, but i can told you that water solutions of the silicates are produced at 150°C and 3-5 bar or so. So it maybe can suppress the gelation.
     
  9. Jul 7, 2015 #8
    That would be useful, if it proves to be the case.

    Compression might be the controlling factor.
    IE. just sufficient pressure, and just sufficient temperature, to allow the reaction to kick off.

    The problem is that it takes place behind a metal door.
    Ideally the reaction needs to be witnessed, to establish when it occurs.

    Either way, it seems like the only way forward is testing.

    The difficulty will be in accessing the kit.
    A dry steam oven wouldn't go amiss....... but they tend to be standardised to 120C, for sterilisation purposes.
     
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