Volume 5  Number 5                            Dennis R. Dinger                                1 March 2007

Updates

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"... for Ceramists" Series Books

The paperback version of Characterization Techniques for Ceramists is available on the Books and Downloads page at the web site!    Retail price is $29.95 plus shipping and handling. The book has 256 pages and it covers 34 different characterization techniques that are commonly used by ceramists.  Order your copy NOW!

The book sets on the web site have also been revised to include this new book.  A 3-book set of paperbacks, including one each of Particle Calculations for Ceramists, Rheology for Ceramists, and Characterization Techniques for Ceramists, is now available for $64.85 plus shipping and handling.  This is a $10 saving off the total retail price of the 3 paperback books.  A 3-book set of downloads is also available for $52.85.  This, too, represents a $10 saving off the total retail price of the 3 downloadable books.  

The E-Book version of Characterization Techniques for Ceramists is available for downloading at the Books and Downloads page of the website for $24.95.  The download is a 2.889 Mb self-extracting Zip® file for the Windows® environment which unzips to the 2.998 Mb book in PDF file format.  Those of you who order the downloadable book will want to know that the PDF book is formatted to print on 5.5" X 8.5" paper (i.e., 8.5" X 11" sheets cut in half.)

The other two books, Rheology for Ceramists and Particle Calculations for Ceramists, continue to be available for purchase as downloadable E-books and as paperback books at the Books and Downloads page of the web site.

Introduction

In this article, we will consider another fundamental question, "Who cares about solids content anyway???!!!"  Most companies have very tight controls over the solids contents of their production slips and bodies.  They frequently have less rigid controls over particle size distribution, but solids content MUST be exactly a certain value.  Let's look at this particular control.

When production slips and bodies are aqueous suspensions and high solids plastic forming bodies, solids content is one of the major controls in most processes.  First and foremost, solids content is easy to measure and to control.  If additive flocculants and deflocculants (and the corresponding states of flocculation and deflocculation) are also tightly controlled, it is a good thing to add the chemicals into bodies that have well-controlled solids contents.

In many cases, process viscosities are the most important slip and body parameter to control.  Thus, if solids content is slightly off, additive chemicals can adjust the body to achieve the desired process viscosity.  Usually, solids content is controlled first -- then additives are used to set the viscosity.

To be more precise, solids contents are easy to control when they are high -- which means they can be corrected by adding water.  In these cases, simply adding the right amount of water to a batch can lower the solids content to the right value.  When solids contents are low (which means too much water has already been added), they are difficult to control.  After a body has been batched and mixed, the only way to raise solids content is to add more powder of the proper batch ratio.  Adding more powders of the proper batch ratio may seem to be easy correction to make, but mixing the new powders into the batch formulation can be difficult -- and doing so homogeneously is nigh unto impossible.  

To test solids contents of slips, one can simply measure the density of the slip.  A container designed to hold a repeatable constant volume of slip can be filled with slip and weighed to find its mass, and its corresponding density.  When the density tests high, the appropriate amount of water can then be added to the batch to reduce the solids content.  When the density tests low, the appropriate amount of powders can be added to raise the solids content.  This correction is seldom, if ever, implemented.  All the way through batching and blunging or mixing, the goal is to be close to the desired solids content, but if an error is to be made, it should be made to the high side.  That way, water can be added later to correct the solids content.

To test solids contents of plastic forming bodies, a small sample can be weighed, dried in a drier, and weighed again.  In this case, the goal is to evaporate the water and to determine how much water was present.  Then, if the solids content is high, water can be added.  If the solids content is low, dry powders or higher solids scrap can be added.

Mixing to Change Solids Contents

It may be easy and simple to measure solids content, but proper mixing of the additional water or powder into the batch should be of great importance.  In many cases, batches have been completed, sent to a holding tank, or sent to the filter press before solids contents are measured.  When solids contents are then (finally) measured, the bodies are not necessarily in the appropriate locations to best accomplish the mixing required to homogeneously raise or lower solids contents.  Note, too, that in such cases as this, measuring solids contents is little more than a check to see how close to the desired specifications one actually is.  If the batch is 'complete' before solids contents are measured -- the test is a CHECK, not a CONTROL.

The best example of this is a production slip that has been blunged in the main production blunger and pumped to the holding tank before the solids content is checked.  Or, another case ... the slip has been in the holding tank for several days when the solids content is tested AGAIN.  If adjustments are required, the holding tank is not the proper place to do the mixing.  But pumping to another tank to do the mixing, followed by returning the slip to the holding tank is out of the question.  So the holding tank agitator will have to do.  To see that the holding tank is the wrong place to do adjustments, one needs only ask, "Is the agitator in the holding tank adequate to homogeneously mix adjusting water, powders, or chemicals?"  Most holding tank agitators are designed to prevent sedimentation.  They are totally different designs which produce much lower levels of agitation than production blungers.  They are not meant for mixing, but that's where we usually perform solids content adjustments.

Plastic forming bodies have similar, but even more difficult mixing problems.  Usually, plastic forming bodies WILL PASS solids content checks, unless something is really, dramatically, far out in left field, wrong.  Associated process problems will follow.

High Solids Contents

What happens when solids contents drift higher than desired, but no correcting adjustments are made?  What properties are associated with higher solids contents?

     Viscosities

As solids contents increase, suspended particles move closer together and viscosities increase.  As slips and bodies are stored prior to use in production processes, moisture can evaporate -- and the first, most noticeable change is that the viscosity of the slip or body will increase.  

     Particle/Particle Contacts

As moisture evaporates from slips and bodies, viscosities increase because particles are closer to one another.  There is less water present and all particles will be slightly closer to their neighbors.  During processing and flow, the fact that particles are closer together translates into increased numbers of particle/particle contacts and collisions.     

     Rheologies

Particle/particle collisions during shear flow and processing affect non-Newtonian rheological properties.  Specifically, increased numbers of particle/particle collisions will increase the dilatant character of the slip or body.  Increasing the dilatant character of a slip or forming body is generally bad -- but it depends what the next scheduled process will be.  In only a few cases, increased dilatant character can be good.  In most cases, increased dilatant character is bad.

     Dispersant Concentrations

When solids contents increase, viscosities also increase, as do particle/particle collisions, and dilatant character.  In this case, one has three choices:  (1) add water to correct the solids content;  (2) ignore the incorrect solids content and add dispersants to correct the higher viscosity;  or (3) ignore the problem completely and continue without making any adjustments.

All related problems could disappear if water is added to correct the solids content.  Some of the problems could disappear if dispersants are added to correct the rise in viscosity.  All problems remain if no corrections are made.

When solids contents increase without correction, dispersants are frequently added to address the major symptom:  the higher viscosity.  This solves one problem and exacerbates another:  Adding dispersants should reduce the high viscosity.  But more dispersants can and usually does exacerbate dilatancy problems.  To reduce dilatancy, one should increase the level of flocculation (which raises viscosities.)  Adding dispersants to a body that is already more dilatant because particles are closer together, will only move the rheology towards extreme dilatancy that much faster.  Dispersants are the wrong correction to reduce dilatancy, even though they are the right correction to reduce viscosity.

     High Intensity Dispersion (HID)

The one unit process that actually benefits from higher solids contents, higher viscosities, more particle collisions, more dilatant viscous character, and higher dispersant concentrations is high intensity dispersion (HID).  HID is designed to mix and deagglomerate at high shear rates and it benefits from all of these changes.  Few if any other process unit operations, however, benefit from these changes.  

Notice that dispersants are sometimes added in lieu of adding water to correct the solids content.  When this is done, the decision has been made to accept the faulty solids content and correct the high viscosity.  After all, when one pulls a sample of a suspension and measures its viscosity, without checking its solids content, one has accepted the solids content (wherever it is) and is considering whether or not the viscosity must be adjusted.  

By controlling solids content, one is primarily trying to control the distance between particles.  Viscosity, rheology, and dispersant/flocculant chemistries can all still be corrected with further adjustments.  All of these later corrections are related to the proper setting of the correct, design solids content.  When one passes up the original, fundamental design parameter (solids content) to treat other downstream properties (viscosity and rheology), one is willingly accepting a different value of the solids content in favor of controlling the other properties.  After all, it is easier to make one process adjustment rather than two consecutive adjustments.  

Low Solids Contents

What happens when solids contents drift lower than desired, but no correcting adjustments are made?  What properties are associated with too much carrier fluid and lower-than-desired solids contents?

     Viscosities

As solids contents decrease, suspended particles move farther apart and viscosities decrease.  This does not usually happen when bodies are stored prior to use in the process.  Water is not usually gained, but lost, in holding tanks.  Lower-than-desired solids contents could result, however, from over-corrections of high solids conditions.  The addition of too much water to reduce solids content can produce a low solids content condition accompanied by lower viscosities.

     Particle/Particle Contacts

When solids contents are too low, too much carrier fluid is present, particles are further apart than desired, and particle/particle contacts are reduced.  This can be a good thing, because particle/particle contacts produce dilatant character -- and anything that can reduce dilatant effects is GOOD.  Lower solids contents, however, can cause settling problems.  When solids contents are extremely low, particles will be too are apart and viscosities will be especially low.  In such cases, large particles can settle, and settling is a stability problem.

     Rheologies

We noted above that particle/particle collisions affect non-Newtonian rheological properties.  Decreased numbers of particle/particle collisions will reduce dilatant character of a slip or body.  This is generally good -- reduced dilatant character is good for all types of ceramic processing.   

     Flocculant Concentrations

When solids contents decrease causing viscosity to decrease, particle collisions decrease, dilatant character decreases, and once again one has three choices:  (1) add ingredient powders or high solids scrap to correct the solids content;  (2) add flocculants to correct the lower viscosity;  or (3) continue without making any adjustments.

Lowering solids contents may seem to be a good thing in all cases, but obviously, one needs sufficient solids content to make ceramic processing possible.  If lowering solids contents was always good, then we could successfully make all of our ceramics out of 100% water and 0% particles of any kind.  Obviously, that is ridiculous -- but it is the end point if you carry this suggestion to its extreme.  

Each company will determine the optimum solids content for their particular process.  In this article, we are discussing variations about these optimum, design-specification levels.  

     High Intensity Dispersion (HID)

We mentioned above that HID benefits from higher solids contents.  That's true!  But it is also true that HID doesn't work well when solids contents are too low.  HID needs a sufficient level of particle/particle collisions to function properly.  When solids contents are too low, particles don't collide in sufficient numbers to produce acceptable HID action.  

Notice once again that flocculants are sometimes added in lieu of correcting solids contents.  It is easier to homogeneously mix a liter of flocculant chemicals into a tank of suspension to raise the viscosity, than to homogeneously mix a ton of dry powder or scrap into that tank to raise the solids content.  When the decision against fixing the solids content is made, once again the decision has been made for accepting the faulty solids content and correcting the low viscosity.  

The Problem with Out-of-Control Solids Contents

Why does it matter?  When solids contents are not controlled, the distances between particles are not constant from batch to batch.  Additive chemistries and rheologies then vary.  To finish this discussion, consider a scenario in which solids contents are not controlled, but the final process viscosity IS controlled with dispersant and flocculant additives.

     When Solids Contents Are at the Design Level

On days when solids contents meet design specifications, the body will generally have whatever rheological characteristics were designed into the final production body.  This will generally be slightly shear-thinning (but it may vary by body and process).

     When Solids Contents Decrease

On the days when solids contents are extremely below the design specs, lots of flocculants must be added to raise the viscosity to reach the design viscosity.  When this happens, the body will generally be over-flocculated and extremely shear-thinning.  It will generally produce very open structures with lots of dry and fired shrinkages.

     When Solids Contents Increase

On the days when solids contents are extremely far above the design specs, lots of dispersants must be added to lower the viscosity to reach the design viscosity.  If it's possible to reach the design-spec viscosity (and it may not be), the body will be extremely dilatant.  High solids enhance particle/particle collisions, and increased dispersant concentrations enhance them even more.  Such bodies will then tend to produce very tight structures (if they cooperate at all with forming techniques) with low dry and fired shrinkages.

Each design parameter has usually been specified to produce optimum processing performances.  When solids content variations are ignored and viscosities are corrected with additive chemicals, bodies can swing from extremely dilatant one day to extremely shear-thinning the next -- all while the final design viscosity is within design specifications.  

Solids content is one of those parameters that appears to be easy to control and correct, but which is sometimes overlooked in favor of correcting other related symptoms.  The fact that it is easier to correct viscosity with additives than to correct solids contents with water or ingredient powders, does not mean one should use the easier method.

The next article will continue this discussion by considering the subject of InterParticle Spacing (IPS).  If one is really wanting consistency from batch to batch, one may need to precisely control both PSD AND solids contents which allows one to control IPS.  IPS varies with both PSD and solids content.  IPS will be the subject of our next article.