Notes
Darvan is a deflocculant used to disperse ceramic suspensions to minimize their water content. It is a liquid alternative to the long-popular sodium silicate. A typical proportion in an easy-to-deflocculate porcelain is about 0.35 Darvan to 100 clay powder. For an iron-oxide-containing stoneware up to double that might be needed. Darvan has advantages over sodium silicate. 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. They are also useful with high iron bodies. 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 properly 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 low ash content 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 40F without detrimental effects on their performance. Darvan definitely cannot be frozen or it will not work as expected. Low temperatures can encourage settling of certain components, if you buy it in drum lots consider rolling the drum around to mix it up if you suspect this has happened.
Although the #811 grade is recommended for high iron bodies, test first to see if it is actually any better than #7 in your application. Although it might be possible to achieve higher specific gravities with #811, these might not cast as well as a lower SG one made using No. 7.
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