sodium silicate versus Darvan 7
Posted 09 March 2013 - 01:37 AM
What are the relative merits and demerits of sodium silicate versus darvan 7 as deflocculants?
Polymer deflocculant, de-flocculant
Alternate Names: Darvan 811, Darvan No. 7, Darvan 821A, Darvan C
Darvan is a deflocculant and used to disperse ceramic suspensions to minimize their water content. It is a liquid alternative to the long popular sodium silicate. About twice as much is required typically (0.4-0.5%) however Darvan does offer a number of advantages. Typically soda ash is not needed as a complement and Darvan does not attack plaster molds. In addition slurries are much less sensitive to over deflocculation and are more stable. It is thus easier to reprocess scrap. However a number of engineers still prefer using a sodium silicate:soda ash mix to control thixotropic properties better, especially if little scrap is being added.
There are a number of different varieties of Darvan:
Darvan 811 and Darvan 812 are low molecular weight short chain polymers for use in vitreous and semivitreous bodies and glazes. In comparison to the conventional soda ash-sodium silicate system, these polyelectrolytes produce slips with longer casting range, higher solids content, improved viscosity stability, fewer "soda" or "hard spots", and significantly increased mold life. Slips also tend to reclaim better without the need for constant adjustments with more deflocculant.
Darvan No. 7 is a high molecular weight, long chain polymer that has been used successfully as a general purpose dispersing agent for both ceramic bodies and glazes. Like 811 and 812, this poly-electrolyte shows the advantages mentioned above. Slips prepared with Darvan No. 7 show little tendency to thicken on standing (thus this version is considered better for glazes).
Darvan 811-D is a dry granular product with great potential for low moisture castables and in other refractory products, where a dispersing agent in the powdered form is preferred.
Darvan 821-A and C are ammonium types for electronic and specialty ceramic products. They have a low ash contents and work well when prolonged ball milling or shear mixing are necessary.
The active agent in Darvan is polyacrylic acid. Its molecules are negatively charged along their length. They attach to clay particles and cause them to repel each other.
There are two cautions with this material:
-It has a shelf life of two years, thus you should only buy material that has a manufactured date on the label.
-Some types cannot go below 40 degrees F without detrimental effects on their performance. Darvan definitely cannot be frozen. Companies who buy is in drum lots should roll the drum around to mix it up in case low temperatures have encourage any components to settle out.
In either of these cases, it will simply not disperse your slurry as expected.
Posted 09 March 2013 - 07:56 AM
I can't say that Sodium Silicate lasts that long.
Marcia Selsor, Professor Emerita,
Montana State University-Billings
Marcia Selsor Studio in Brownsville, Texas.
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