Introduction

There are two major categories of processing problems in ceramic production companies:  those caused by chemistry and those caused by particle physics.  By chemistry, we refer to the additives in interparticle fluids that are used to adjust slip and plastic forming body viscosities and rheologies.  Many plastic forming bodies are mixed and adjusted as slips before dewatering to final processing consistencies in filter presses.  By particle physics, we refer to all physical properties of particles and their interactions (density, shape, surface area, porosity, etc.)  Degrees of non-sphericity, surface roughnesses, and interactions during flow directly relate to potential processing problems.

We learned early during our coal slurry project days (more than two decades ago) that both chemistry and particle physics had to be correct for suspension viscosities and rheologies to be good.  If either or both were bad, good suspensions could not be produced.  This is also the case within ceramic process systems -- both chemistry and particle physics must both be well-controlled to achieve desired processing properties.

In Part I of this two-part series, we will cover the major problem resulting from chemistry: syneresis.  In Part II of this series, we will cover the major culprit resulting from particle physics: dilatancy.

What Is Syneresis?

Most slips that we prepare and use in ceramic processes are slightly flocculated so some gelation occurs.  Gelation produces shear-thinning rheologies, stability, yield stresses, and the capability of freshly formed ware to hold their shapes. 

The goal when mixing suspensions is to cause each solid particle and chemical additive molecule to report as individuals and to be homogeneously dispersed throughout the batch.  Then, as mixing shear is removed, all particles and molecules can take their equilibrium, homogeneous positions within the batch.  The goal of partial flocculation is to allow the formation of a gel structure as mixing shear is removed.  Gelation causes particles to form flocs and then flocs form chains until all particles in suspension are members of a giant, 3-dimensional network of particles (the gel structure) that extends throughout the batch.  Shear can break the gel structure, but removal of shear allows it to reform.  During forming, shear breaks the gel structure, but removal of shear allows the gel to reform and the newly formed wares to hold their shapes.

Syneresis occurs in the presence of excess flocculation.  When syneresis occurs, the gel structure can again be broken during shear, but upon removal of shear, the gel reforms, newly formed wares hold their shapes, but with time, excess flocculation causes the gel structure to densify further than normal, expelling water.  This can show itself as random cracks that occur in ware, filter press cakes, and extrusion columns.  Syneresis occurs with time.  The normal gel structure can form quickly after removal of shear, but with time (minutes to tens of minutes), the gel structure will further densify, interparticle fluids will be expelled, and the ware or body will crack. 

Syneresis Is A Chemical Problem

Note that syneresis is a chemical problem.  It cannot be corrected by adjusting particle physics -- it can only be corrected by chemical adjustments.  Syneresis usually occurs with time, but the time required for syneresis to show itself is much less than the time required to dry the ware (so you can't ignore it and try to dry the ware quickly).  For this reason, when syneresis is present, the body must be chemically adjusted to a state of less flocculation.

Sometimes, the nature of the chemicals used can be the problem.  Sodium cations are weak flocculating cations, although they are large enough that they usually act as deflocculating cations.  Calcium and magnesium cations are moderate flocculating cations.  Calcium and magnesium ions, in the form of sulfates and other soluble forms, find frequent use as suspension flocculants.  Some may think that since sodium is a weak flocculating cation, and calcium and magnesium are moderate flocculating cations, then aluminum cations would be an even better, strong flocculating cation.  Aluminum cations, however, produce severe flocculation and should not be used as adjusting chemicals.  Aluminum cations are strong flocculants.  They will adsorb tightly to particles and may not easily be freed for removal as insoluble silicates -- so once they are present in a body, they may remain a problem.

Sometimes, the concentration of the chemicals used is the problem.  Some engineers think that if a little is okay, and more is good, then a lot will be the best.  Impatience during mixing can be a reason for use of too much additive.  When I prepared casting slips for labs at school, I frequently added too much flocculant too fast and the slip would go from low viscosity to high viscosity very quickly.  Then, I would need to add deflocculant to bring the viscosity back down -- and if slow additions and mixing took too long, I would frequently miss again by adding too much deflocculant too quickly.  Then, I would have to add more flocculant and repeat the process.  Eventually, the slip would achieve the desired viscosity.  I did this using a reasonably fast impellor mixer, and it was too slow for me.  How many of you adjust your suspensions in holding tanks?  Remember, impellors in holding tanks are usually designed to prevent settling, not to perform mixing -- so it will take much, much longer to achieve equilibrium when adjusting suspensions in holding tanks.  It might take several hours to determine the new viscosity after each flocculant addition.  If possible, use a good blunger tank and impellor to tune suspension -- and then, be patient (unlike this author). 

How To Measure Syneresis

The easiest way to measure syneresis is to measure the gelation behavior using a rotating cylinder viscometer at constant low rpm for 10-20 minutes.  We routinely used a form of our gelation test (see the Ceramic Processing E-zine, Volume 1, Number 7) to determine this.  Figure 1 shows a sample 20 minute test.

Figure 1.  Twenty Minute Constant RPM Gelation Test

Tests like this can be run for a 20 minute duration at 10 or 6 rpm (or even lower rpm) using the appropriate spindle on (for example) a Brookfield viscometer.  The idea is to use a low viscometer rpm which allows measurement of the viscosity as the gel structure builds.  Three gelation curves are shown in this figure.  The "poor" curve hardly qualifies as a "gelation curve", but this type of curve actually occurs.  When the shape of the curve matches the "poor" example, gelation is hardly occurring at all.  It occurs only verrryyyy ssllloooowwwwwllllyyy.

As one adds more and more flocculant to the slip, the rate of gelation increases from a state like the "poor" curve to a state like the "good" curve.  The difference between the initial shapes of these curves indicates the state of flocculation and the amount of flocculant added.  As more and more flocculant is added, the slope of the gelation curve will gradually rise (from the "poor" curve towards the "good" curve) indicating that gelation is occurring more and more quickly.

Our experience has shown that as more and more flocculant is added, the curve will eventually gel so quickly that the viscosity will quickly pass the steady state final viscosity, reach a peak value, and then it will drop back down to the stead-state value.  This is demonstrated by the "syneretic" curve in the figure.  When this begins to occur, it is an indication that too much flocculant has been added and syneretic behavior is taking place.  The explanation is that the viscosity quickly exceeds the steady-state final viscosity, water is expelled from flocs as they densify, and the viscosity drops slightly to the steady state value.

Gelation is desirable in ceramic bodies.  Flocculant additions cause gelation behavior to increase.  Too much gelation, however, causes the problem known as syneresis.  Too much of a good thing can produce later processing problems.

Remember also that when slip is being prepared for filter pressing and later extrusion or plastic forming, the slip should NOT be adjusted right up to the maximum gelation behavior prior to the onset of syneresis.  The reason for this is that after filter pressing, the particles are much closer together and the optimum chemistry in the slip may not be the optimum chemistry for the plastic forming body.  It is difficult to measure the viscosity and rheology of plastic forming bodies without expensive rheometers, so one must learn the level of flocculation in the slip that corresponds to acceptable behavior in the plastic forming body.  Without a rheometer to directly measure viscosities of plastic forming bodies, one must rely on process experience.

Summary

Syneresis can be a major problem in ceramic processes such as slip casting and plastic forming.  Syneresis is the result of excessive use of chemical additives in the batch.  It cannot be corrected by particle physics adjustments -- it is a chemistry problem.  To correct for syneretic conditions, the body or suspension must be adjusted to a lower state of flocculation.

Syneresis can be measured using a 10-20 minute gel test in which the viscosity of the suspension is monitored continuously at low rpm.  The goal is to set the rpm low enough so the viscosity can be monitored as gelation occurs.  An ideally adjusted suspension will gel quickly and the viscosity will level off at a steady-state value without overshoot of the final viscosity in the first few minutes of the test.  A syneretic condition occurs when the measured viscosity quickly overshoots the final steady-state viscosity, reaches a peak viscosity, and then slowly drops back to the steady state condition.  When this occurs, optimum flocculation has been passed and a reduction in flocculant is necessary.

When a suspension is filter pressed to produce a plastic forming body, one can measure the gelation behavior of the suspension and correlate it to the processing properties of the plastic forming body.  If syneresis problems occur in the plastic forming body, one can adjust the suspension to a slightly lower state of flocculation.

Miscellany

Suggested topics for future issues of this E-zine .... Please continue to send your ideas or questions for future topics.  Thanks.  Until next time ...

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Copyright © 2006  Dennis R Dinger

103 Augusta Rd, Clemson, SC 29631   (864) 654-5731

consulting@DingerCeramics.com

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