Several recent questions concerned gelation phenomena in suspensions.  So it is time to review the phenomena known as 'gelation' as it applies to particle/fluid suspensions. 

Some chemical reactions (polymerization and hydration, for example) appear outwardly similar to gelation, but they are not the same.  Epoxies and putties are shipped and stored in two parts:  the main ingredient and an activator.  When they are mixed, a chemical reaction occurs which causes the mixture to set and harden.  This is an irreversible reaction.  When water is added to cement, a hydration reaction occurs which ties all particles into one large mass.  This, too, is an irreversible reaction.  Neither of these phenomena, however, are characteristic of gelation as it occurs in ceramic suspensions -- primarily because gelation is not an irreversible chemical reaction.  Even though all three systems, with time, combine particles and fluid into one large mass, the fundamental phenomena behind the three are different.

Gelation Is Produced By The Attraction of Particles

Gelation in particulate suspensions results from van der Waals forces of attraction between suspended particles.  Gelation also can be produced by the electrostatic attractions of charged particle surfaces.  In both cases, suspended particles are attracted to one another to form 3-dimensional chains of particles.  The bonds that hold gel structures together are relatively weak so particles can easily be released to travel once again as individuals.  That is, gelation is a reversible phenomenon.

The gelation process draws particles together to form increasingly larger flocs and eventually, large gel structures.  Particles are pulled into these structures by attractive forces, and they are separated again from the structures by shear.  Stirring, mixing, and pipe flow all provide sufficient shear to release particles from gel structures and flocs.

          Flocs

Dust balls (a.k.a. 'dust bunnies') which frequently form and reside under beds and furniture are examples of flocs.  In fact, when the author thinks of a floc, he pictures a dust ball.  Jim Funk drew the parallel between a floc of particles and a flock of sheep.  His example of flocculation (actually, he defined it as "flock-ulation") was to imagine a hillside covered randomly with sheep.  When the shepherd calls his sheep, they 'flockulate' around him to form a flock of sheep.  Then, of course, if the shepherd is not present, but a wolf jumps into the center of the flock, the sheep will "de-flock-ulate."  Once you have this picture in your mind, you'll hardly ever forget the meanings of flocculation and deflocculation -- because particles behave similarly.

Transfer this 2-dimensional picture of a flock of sheep to its 3-dimensional counterpart and you'll see a floc of particles.  They are similar.  A flock can travel together as a relatively large entity, but individual sheep are free to wander off and go astray.  A floc of particles can travel together as an entity, but individual particles are free to be sheared off and separated from the floc to travel again as free, individual particles (which can also go astray.)

In the water purification process, flocculation ponds (that is, clarification ponds) are designed to allow individual impurity particles (which are too small to settle easily on their own) to come together to form flocs which are large enough to settle.  In this way, flocculation allows particulate impurities to be removed from water.

          Gel Structures

Flocs are usually associated with low solids suspensions, and with the very early stages of the gelation process in higher solids suspensions.  Large gel structures are formed as individual particles come together to form flocs, and then the flocs and more free particles continue to come together to form large networks of particles.  In typical high solids suspensions used in ceramic processing, the gel structure will fill the whole container and hold all particles in a single structure.  The container in this example could be a small beaker, or a 40,000 gallon holding tank.  In the absence of agitation, the gel structure will fill the whole container.  If the gel structure has sufficient strength, particles in the structure (and especially, the larger particles) will be held in place where they are not free to settle.  Many gel structures are weak, however, so they cannot prevent large particles from settling.  For this reason, most large holding tanks contain mild agitation to provide upward circulation which keeps the larger particles in suspension.  Even in the presence of agitation, some gel structure will have formed and particles will be circulating in flocs.  The size of the flocs is inversely proportional to the intensity of the agitation.  Intense agitation can cause all particles to circulate as individuals.  Mild agitation allows the buildup and circulation of floc structures.  Zero agitation allows a single complete gel structure to form in the container.  The strength of the gel structure determines whether or not large particles will be incorporated into the structure, or whether they will settle.

Gel structures are reversible in the sense that shear (stirring, mixing, pipe flow, etc.) frees flocs and individual particles from the gel structure to flow independently again.  When shear is removed, however, individual particles and flocs are again pulled into the gel structure.  Gelation is reversible between these two conditions (shear and no shear.)

When shear is sufficiently strong to cause particle physics changes (particle breakage, deagglomeration, etc.), suspensions and suspension properties will slowly change.  At relatively low rates of shear and in the absence of particle physics changes, particles will be pulled into the gel structure, released by shear, pulled into the gel structure, etc., over and over again without major changes to suspension properties.

          Van der Waals Forces of Attraction

Van der Waals forces are weak, natural forces of attraction between all atoms.  In the absence of repulsive forces, most of which are stronger than van der Waals forces, the van der Waals forces of attraction will pull particles together.  Mechanical shear forces can overpower van der Waals forces and pull flocs apart.  Other forces of repulsion such as electrostatic and hydrophobic forces can also overpower van der Waals forces and prevent gelation.  When mechanical shear forces are removed, and repulsive forces are not present, van der Waals forces will again pull particles together to form flocs and gel structures.

When particles are in close proximity, van der Waals forces can hold the particles together.  Although van der Waals forces of attraction are fundamentally different from magnetism, van der Waals forces holding a small particle to a large particle behave very similarly to a magnet holding a piece of steel.  When a piece of steel is held by a magnet, it is not permanently bonded to the magnet surface, but it will be held there until some other force is applied to separate the two.  Overpower the magnet's force, and the steel can be pulled away from the magnet.  Release the steel in close proximity to the magnet, and it will be pulled in again.   Note:  van der Waals forces of attraction are NOT magnetic!!  I use this example only because everyone knows how magnets work and the two phenomena, from the viewer's perspective, behave similarly.  Van der Waals forces are weak, non-magnetic forces (weaker than magnetic forces.)  When a small particle is held to a larger particle by van der Waals forces, the small particle is not permanently bonded to the large particle, but it will be held there until some other force is applied.  Overpower the van der Waals force, and the small particle can be pulled away from the large particle.  Release the particle in close proximity to the larger particle, and it will be pulled in again. 

Since Van der Waals forces are weak, it does not take much to overpower them.  But when all other forces of repulsion have been removed, van der Waals forces (which are always present) will take over and pull particles together.

           Electrostatic Forces of Attraction

Opposite charges attract.  Like charges repel.  So to have electrostatic forces of attraction, opposite charges are required.  Opposite charges can appear on certain particles, such as kaolinite platelets, which frequently have positive charges on their edges while their surfaces are negatively charged.  Relatively large 'card house' stacks can occur in such systems:  positive edges of platelets are attracted to the negative surfaces of other platelets.  Large, open structures can form in this way.

Most ceramic bodies contain several different ingredient particles.  At any particular pH, particles of different species will have different electrostatic surface charges.  When different particle species are oppositely charged, the oppositely charged particles will be attracted to one another.  When particles of species A are negatively charged, and particles of species B are positively charged, particles of the two species will be attracted to each other.  The attraction, however, doesn't usually only bring a single A & B together, but it usually produces A-B-A-B-A-B-etc. chains which grow into flocs.

Electrostatic forces are stronger than van der Waals forces, so when electrostatic surface charges are non-zero, they (not the van der Waals forces) will control suspension properties.  When electrostatic surface charges are zero (and in the absence of any other repulsive forces), van der Waals forces of attraction will control suspension properties by causing gelation to occur. 

Controlling Gelation Behavior

This background into gelation phenomena provides the information we need to control gelation behavior in suspensions.  To enhance gelation, we must enhance the level of electrostatic surface charges of oppositely charged particles in suspensions.  When this has been achieved, the attraction between oppositely charged particles will be enhanced.  The other way to enhance gelation is to cancel all electrostatic surface charges and eliminate all other repulsive forces so van der Waals forces dominate.

          pH

Each particle species has a natural electrostatic surface charge at each suspension pH.  The zeta potential of a particle is indicative of its electrostatic surface charge.  As the pH of a suspension is varied from 2 to 12, zeta potentials and electrostatic surface charges will vary from positive at low pH to negative at high pH.  The isoelectric point (IEP) of each particle species indicates the pH at which the natural surface charge is zero.  Clay particles, for example, have IEPs in the 2-3 pH range.  Alumina particles have IEPs in the 9-10 pH range.  So when clay and alumina particles are in the same suspension, like charges on all particles will only occur below the IEP of the clays or above the IEP of the alumina, producing all positive and all negative surface charges, respectively.  At all pH values in the normal processing range between these two extremes, some particles will be negatively charged (the clays) while other particles are positively charged (the aluminas.)

pH is the first control parameter one should use to control electrostatic surface charges, the rheology of the suspension, and gelation behavior.

          Additives

A variety of chemical additives are available to control the effective surface charges of particles.  Many of these are long-chain polymers with ions at the end of each of many side chains.  For example, many deflocculants are anionic polyelectrolytes.  They can be pictured as millipedes with negative charges at the ends of each of their feet.  When the polymeric backbones of such additives are hydrophobic (which is frequently the case), the backbones are forced onto particle surfaces by the interparticle fluid (the water), as the legs with their negative charges stick out into the water.  When this occurs, the backbones act like paint that coats particles and hides the natural surface charges, while the charged legs dangling out into the interparticle fluid control the effective surface charges of the particles.  Hydrophobic forces are stronger than electrostatic forces, so negatively charged anionic polyelectrolytes can coat electrostaticly negative surfaces.  Like charges may repel, but hydrophobic forces trump electrostatic forces.

Other additives are simply inorganic ionic species in which the highly charged cations are attracted to negative particle surfaces to cancel the negative surface charges.

In the example given above where clay particles are negatively charged and alumina are positively charged, very minor additions of appropriate anionic polyelectrolytes can coat positively charged alumina particles to give them net negative surface charges, at the same time as they enhance the negative surface charges of the already negatively charged clay particle surfaces.  This means that low concentrations of appropriate additives can render all particles in suspension to have like surface charges (negative charges in this case.)

          Enhancing Or Moderating Gelation Behavior

When gelation is too strong, deflocculating additives will moderate the gelation behavior.  When gelation is too weak, flocculating additives will strengthen the gelation behavior.  In each case, the question that must be answered is:  Which additives are good flocculants and good deflocculants for this particular body?

This can be a tricky question to answer because there are many possible additives, most bodies contain multiple ingredients, and the order of application of mixtures of additives affects the final outcome.  When an addition of A, followed by an addition of B, produces different results than an addition of B, followed by an addition of A, the system is definitely complex.  And this happens frequently when tuning ceramic slips and forming bodies.  These are definitely complex systems.

Note also that repeated flocculation/deflocculation adjustments are not reversible.  If a body is repeatedly flocculated with one additive, deflocculated with another additive, flocculated with more of the first, deflocculated with more of the second, and on and on and on, the system will be slowly and irreversibly changing with each successive addition.  The ionic conductivity of the interparticle fluid will increase with each subsequent addition of flocculant or deflocculant, and ultimately, all further additions will have no affect and the body will be flocculated.  Further additions of deflocculant will flocculate.  Further additions of flocculant will flocculate.  It won't matter which is added -- and when that point is crossed -- the body will be essentially ruined. 

Consider a specific example.  Sodium silicate is the deflocculant and calcium chloride is the flocculant.  The silicate ion is the ion that deflocculates and the calcium ion is the ion that flocculates.  The silicate actually combines with excess calcium ions to precipate calcium silicate as an insoluble salt.  The remaining ions, however, are the ones that irreversibly change the suspension.  The sodium from the silicate and the chloride from the CaCl₂ will remain in solution even though the other two ions combine, precipitate, and neutralize one another.  Repeated flocculation and deflocculation, that is, repeated adjustments back and forth, will increase the salt (NaCl) concentration in the interparticle fluid.  There is no easy way to remove excess NaCl from the fluid, so they continue to pile up.  Eventually, the suspension's behavior will be destroyed due to the high NaCl content in the interparticle fluid.  So keep this in mind -- up to a point, adjustments in both directions can be made -- but eventually, the interparticle fluid will contain such concentrations of soluble salts that further adjustment controls will be impossible.  It is best, therefore, to use as little deflocculant and flocculant as possible in any suspension.

          Other Considerations

     1  --  Another consideration is that all additives are not necessarily compatible with one another.  For example, a deflocculant was added to one body to reduce its viscosity prior to the addition of a binder which would provide dry strength while also raising the viscosity of the body.  Strength didn't increase as much as expected when the binder was added, so more deflocculant and more binder were added.  This was repeated several times, but desired strengths were never produced.  The final determination was that the two additives were incompatible with one another so neither was behaving as expected.  Actually, in the presence of the deflocculant, the binder was balling up and not coating the particles as expected.

     2  --  Some additives only work well in certain specific pH ranges.

     3  --  Some additives have short shelf lives.

     4  --  When bodies are mixed, some phenomena which appear to be gelation may actually be chemical reactions that are irreversibly changing the body.  (Beware!)

Summary

Gelation in particulate suspensions is a reversible phenomenon that is controlled by attractive/repulsive interparticle forces.  Flocs and gel structures, which form in quiescent systems, can be broken down using shear.  Upon removal of shear, gelation will again occur and rebuild floc/gel structures.

Electrostatic surface charges and van der Waals forces of attraction control the strength and rate of gelation processes.  When both are present, electrostatic forces prevail.  In the absence of electrostatic forces, van der Waals forces prevail.

pH and a variety of additives can be used to enhance or moderate gelation.  Numerous tests must be performed to identify the most effective additives for each body ingredient.  The order of additions affects final body properties.  The nature of additives (polymeric, inorganic salts, etc.) affects final body properties.  Body pH affects the behavior of each additive and each ingredient particle.  Shelf life affects the effectiveness of some additives.  Many additives are incompatible with one another. 

Finally, some phenomena, such as chemical and polymeric reactions which are irreversible, may appear at first glance to be gelation, but they are not.

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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