Volume 1 Number 5 Dennis R. Dinger 1 March 2003
An Update
This issue comes to you again from the good old US of A. (Although I must say, I had no problems distributing the February issue, even though I was half way around the world when I did so.) It's good to be home again. The rest of the world pays little attention to the political games in Washington, so it's refreshing to be out of touch for that reason. But the rest of the world is focused on our threatened aggressions in the Middle East. Where I visit, everyone is still quite friendly towards Americans, but the editorial pages of the English newspapers suggest that isn't universal among the people. So it's nice to be home again.
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Please send suggestions for future topics. The two articles in this issue were requested in this way. I would much rather address issues of concern to the readership than have to sit back, scratch my head, and decide what topics you might like to hear.
Why Do The Properties
of Bodies
Containing Recycled Scrap Materials
Always Differ from Those of The Virgin Bodies?
Along with this question, the comment was made that properties usually vary with the amount of recycled scrap used in a body. With respect to spray dried bodies, bulk densities of granules were generally found to be higher in bodies with recycled scrap than in bodies without the recycled materials.
Several different phenomena combine to produce these results, so we will approach this question from several points of view. We will consider the processing history of the recycled material, additive chemistries, particle physics, and dispersion intensities.
Processing History of the Recycled Material
The first consideration is the processing history of the recycled material. In particular, how much heat treatment, if any, has the material seen? Is the recycled material high solids content slip, casting scraps, or dry scrap? Does the scrap consist of waste granules, or dry, pressed-ware scraps? How will the new body containing the recycled material be processed? Will it again be subjected to blunging, HID, ball milling, aging tanks, etc.?
As scraps are recycled from further and further along in the process, more changes can be expected to have occurred to the body. Waste slip should be very similar to virgin slip. The interparticle fluid in green casting scraps will be altered from the virgin slip because casting scraps have been subjected to partial dewatering. Although few particles will escape during casting, interparticle fluids, including ions and additives, can be removed. Dry scraps should be chemically similar to green scraps, but the dry scraps have usually been through a drying (heating) operation. Depending on the drier schedule and temperature, interparticle chemicals may be affected (i.e., altered).
All scrap from spray drying processes has been subjected to slip processing, followed by heat treatment to form granules. The nature of the dewatering process in a spray drier doesn't allow interparticle chemicals to escape the granules, but the heat treatment process is generally an intense high temperature environment, so organic chemicals can be damaged, burned, and/or altered. If scraps are included from the pressing operation, particles and long-chain polymeric additives can all be broken and changed from their original states.
Experience has shown that low solids content milling preferentially reduces the particle sizes of the coarsest particles. They are the largest, slowest moving, and easiest to hit. But really high solids content comminution preferentially reduces the particle sizes of the particles that don't pack well. This applies to milling, but particularly to really high solids content processes such as calendaring and dry pressing. In these two operations, particles can't move easily and particle/particle contacts are high. Structures are formed as particles come into contact as pressures build. The particles that contact others and form the structures are the ones that will see the compressive stresses and will be broken. Particles that fit into pores and are not part of this structure will not see the stresses and will not be broken. As particles break and shift, the worst packing particles will always be the ones that transmit the compressive stresses through the compacts. If the goal of an operation is to produce the best possible packing distribution of particles, subject the system to dry pressing or to calendaring operations, and then redisperse the product. In one result from years ago, a poor PSD was subjected to calendaring, and the product, when redispersed, had almost a perfect dense packing PSD.
But consider also what this does to polymeric additives. If the polymeric additives are present as coatings on particle surfaces, and the particles break, the attached additives can break as well. Shorter versions of long chain polymers usually work similarly to, but they are never quite the same as, additives of the original length.
Finally, how much of the total processing will the recycled materials be subjected to again? Low intensity blunging, aging tanks, and even high intensity dispersion may not alter particle sizes or break chemicals, but milling can do both. Subjecting recycled materials to mills again will reduce particle sizes from those produced by the original milling operation.
All of these questions need to be considered if bodies containing recycled scrap differ in properties from the virgin bodies.
Additive Chemistries
Slip casting can remove ions and some chemicals from the interparticle environment into the mold. Driers and spray drying do not. But the nature of the additives has to be taken into account. Polymeric chemicals that are added to spray drying slips are generally known to ceramists as binders. The rest of the world knows these same organic materials as glues. How easily will a glue that has not only dried, but been subjected to a heat treatment, redissolve? Will it redissolve? Will it come back to its original molecular form, or will it have been altered during this process that it will have a slightly different chemical composition upon reconstitution?
If the scrap materials are dry pressed scraps, have the chemicals been broken during the pressing process? If so, and they can be redissolved, how will the new, smaller pieces behave compared to the original molecular weight additive? When the scraps are added to the new body slips, are allowances made for the fact that additives are already present in the scraps? If scraps are from casting processes, which ions and other additives have been removed during the dewatering process? Does the reconstitution process allow those particular ions and additives to be replaced?
The answers to most of these questions are process specific and material specific. For this reason, I can ask lots of questions without providing answers. If slips containing recycled materials are causing problems in your process, you can run through these questions to determine which, if any, apply.
Particle Physics
The comments above concerning comminution at really high solids contents are important to many systems. I don't believe this is well known, but it will certainly affect the properties of bodies containing recycled materials. Excellent particle size distributions, from a packing point of view, are produced during really high solids operations such as dry pressing and calendaring. I realize that dry pressing and calendaring are not normally expected to produce comminution, but when pressures and compressive stresses are high, both operations can break particles. When this happens, recycled materials can contain very different PSDs compared to those of the virgin materials.
Using ball mills to deagglomerate during reprocessing operations can be expected to break particles. After all, ball mills are mills. They are designed to break particles. Can they (the ball mills) distinguish between milling and deagglomerating? No. Will they do both? Yes. Will they perform more milling or more deagglomeration on any given body? Yes, they will! The answer to this depends on a lot of things. How are you running the mill? How has the scrap material been processed? Which are stronger -- the particles, the agglomerates, or the binders? A good wood glue is supposed to be stronger than the two pieces of wood that it joins. How strong are ceramic binders after they've been dried and heat treated? Actually, almost all of the questions raised in this article apply here.
Aging tanks produce rather gentle agitation that most people would assume is inconsequential. But there are slip systems which are agitated gently for days to gain the benefits of attrition through long duration particle/particle contacts. Sharp corners on the particles are broken off and the particles become more spherical during such operations. Each time particles are slurried and agitated again as slips, attrition and aging can take place. So even though aging tanks are low shear systems, particle physics properties of the slips can change during storage and aging.
Dispersion Intensity
The main question here is this: Are you using sufficient dispersion intensity to redisperse the scrap into a slip again? The story comes to mind of a silicon carbide suspension that Jim Funk tested about 20 years ago at Alfred University. The supplier had sent him a sample of suspension, as well as the ingredients and precise instructions to make more suspension. He made more suspension and compared it with the original produced by the supplier. The two suspensions had very different viscosities. They seemed like two very different suspensions. So he tested the two suspensions with every test he had available. He tested particle size distributions, zeta potential, specific surface areas, etc. He looked at them in the SEM and he exhausted all other possible tests. Every test showed the two suspensions to be IDENTICAL. He was at the point of pulling out his hair. (Well, no, he wasn't doing that because he never had any hair to speak of. But you get the idea.) Anyway, as he told it, he was driving to the airport one day, counting deer in the fields and thinking about this suspension problem, when he had an idea. He stopped at a phone (this was back in the dark ages before cell phones), called the technician, and told him to use our high shear rotor/stator mixer on both suspensions. The viscosities of both suspensions decreased when subjected to HID. Surprisingly, the viscosities decreased to values that were IDENTICAL. The whole answer appeared to be that the two suspensions were in two different states of partial agglomeration and when they were subjected to HID, they were both deagglomerated sufficiently that their viscosities were then identical as were all their other properties.
I don't know whether all binders used in spray driers can be reconstituted and redissolved to states similar to their original states, but I do know that if it can be done, HID should be able to do it. HID deagglomerates, but it does not normally break particles. Experience has shown that specific surface areas (SSA) don't increase during HID. If particle breakage is a normal function during HID, SSA values should increase, but they don't. Agglomerates must be really strong before HID won't do the job. If HID can't redisperse the particles in dry press scraps, it may be a sign that insufficient plasticizer was used with the binder, that the binder is too strong, or that the binder doesn't redissolve easily. If the problem is with binder/plasticizer ratio, or binder strength, pressing problems (such as the inability to deform granules during pressing) may also be evident. If HID can't redisperse systems properly, mills may then need to be used. But remember -- mills were designed for milling and they will break particles. They may also deagglomerate, but they can certainly be counted on to break particles. So using mills to reconstitute scraps can be expected to alter the particle physics properties to differ from the original properties.
Summary
Lots of questions need to be asked to solve this problem. Each process will have its unique characteristics that will affect slips and granules containing recycled scraps. The two major areas to examine are particle physics and additive chemistries. Particle size distributions can change dramatically during pressing operations. Additive molecules can be broken during pressing, and changed chemically during spray drying and ware drying operations. Changes may be sufficiently dramatic that the slips cannot easily be reconstituted to be identical to virgin systems. When this happens, one has to determine which of the possible changes accounts for the new properties of the reconstituted slip. This determination can then be used as a guide to replace particles and/or chemicals in the reconstituted slip, and to adjust the properties back towards their original values.
The other major area of consideration is to determine if the dispersion intensity is adequate to the task. Many processes could take advantage of more intense dispersion systems than are currently in use.
What are the biggest causes for change, in my opinion? I believe that the high pressures used in dry pressing operations frequently cause particles and polymeric additives to break. New size distributions and shorter additives will be fundamentally different from those used in the virgin materials. Neither new particle size distributions, which pack differently (and frequently more densely), nor new, shorter additives, which may behave similarly (but not quite the same), will allow slips to be easily reconstituted to exhibit properties consistent with those of the original suspension.
Clay Packing Densities
Another question was raised concerning whether or not clay packing densities play a role in casting and plastic forming bodies? The answer is: Yes and No.
Casting Bodies
Particle size distributions that pack well will help to contribute good rheological properties to casting bodies, but it's doubtful that they will actually produce greater bulk densities in the ware. The reason for this is that casting bodies are usually tuned to be flocculated (at least partially flocculated.) The forces of flocculation cause the particles to come together to build gel structures that are relatively open, porous structures. If you sprayed styrofoam balls with contact cement, allowed them to dry, and then dumped all the balls into a box, you would have a very low density, open structure defined by the balls. If you took solid plastic balls (like billiard balls) of the identical particle size distribution as the styrofoam balls, coated their surfaces with lubricant, and dumped them into the same size box, you would define a much higher density, lower porosity structure.
Flocculated suspensions take structures somewhat similar to the styrofoam balls/contact cement example. Even if the particle size distribution of the particles (the balls) was perfect, they would still produce an open, high porosity pack when flocculated. Flocculated systems behave like this. Even after cast ware are formed, internal porosities will be high, and there is no reason to think that as drying occurs, the particles will slide over one another to densify to higher levels. The open structures will mostly be locked into place.
Deflocculated suspensions take structures like the lubricated solid plastic balls. As they form, they pack and continue to slide over one another until they reach relatively dense states. Here is where you'll see the packing benefits of an excellent particle size distribution. But you wouldn't want to use deflocculated suspensions to form ware. Dilatancy (shear-thickening behavior where apparent viscosities increase as shear rate increases) is characteristic of deflocculated suspensions. Casting slips are not usually tuned to be deflocculated because they produce hard casts that crack easily and handle poorly.
To achieve maximum plasticity in a body, ease of handling, and desirable cast properties, flocculated suspensions are normally used. When this is the case, the packing benefits of an excellent particle size distribution will be masked and not readily apparent due to the state of flocculation. The additive chemistries that produce the flocculated states are the more powerful phenomenon. So playing with the particle size distribution of clays probably won't produce packing benefits in casting bodies.
Plastic Forming Bodies
Plastic forming bodies tend to be flocculated, too, so particle size distribution control may again not help to achieve denser packed wares. But when clays are used in the body, another phenomenon comes into play. Clay platelets tend to align under shear. Extruded ware, roller formed ware, thrown pots, ram pressed ware, etc., will all have the platelets oriented to a certain extent with their faces parallel to the shear forces. The typical flow profile within a pipe is a parabolic shape with the highest flow rate at the center of the pipe and the lowest flow rates at the circumference of the pipe. The typical pipe velocity profile resembles the shape of the tip of a bullet. Clay platelets tend to take orientations with their face surfaces parallel to the parabolic flow profile. In an extruded column, the clay plates at the center of the column (at the tip of the bullet) will have their faces perpendicular to the axis of flow of the column, while clay plates near the outer surfaces of the column will have their faces lined up parallel to the outer surface of the column. Note that this represents a 90o change from the center to the circumference of an extruded column. During drying, stresses can build because aligned clay particles tend to shrink differently in the directions parallel to, and perpendicular to, their orientations. These stresses are the result of differential shrinkage. When the stresses form, cracks, delaminations, and warpage can also be produced during drying and firing.
In extruded columns from auger extruders, clay platelets will also tend to orient parallel to the auger surfaces. Unless the column has been intentionally and properly sheared between the end of the auger and the die (or as it passes through the die to remove this internal structure), the extruded columns can contain funny, screw-shaped internal remnant structures, stresses, cracks, etc.
It may be possible, in plastic forming systems, to play with the particle size distributions of the powders to achieve denser wares. In plastic forming systems, shear forces can overcome and dominate even in flocculated bodies. In my experience, however, the coarse particles have more influence on packing densities than the fines. One way to look at a plastic forming body is to consider it to be a system of coarse particles bound together by the fluid and fine clays. The fluid and fine clays control the rheological properties of the body, and the coarse particles contribute most other properties (including packing potential). Many bodies and glazes contain clays specifically to produce the desired rheological forming properties.
Yes, clays need to have their particle size distributions (PSDs) under tight control, but in my opinion, the coarser particles can better influence packing densities. To achieve higher packing densities, I think it would be more beneficial to adjust and control the PSD of the coarse fractions of the body rather than the fine fractions which include the clays.
Homogeneity
Keep in mind that any modifications made to the PSDs of bodies will be masked by inhomogeneities due to inadequate mixing. A much more uniform body can be produced by mixing the body as a slip, and filter pressing the slip to the plastic forming consistency, than by trying to mix the ingredients at the solids content of the plastic forming body. This may add steps to a process, but if homogeneity is desired, it is worth the extra steps. When a slip is the starting point for a body, High Intensity Dispersion (HID) can be used to achieve excellent homogeneity. Then, filter pressing can produce relatively uniform plastic forming bodies. Attempting to directly mix a body at the plastic forming consistency and achieve anything close to homogeneity is nigh unto impossible in my opinion. I know this is a standard processing method that many use successfully. I'm just questioning the possibility of successfully achieving high levels of homogeneity in such processes. The fact that many have been using such techniques successfully for years is not the issue. If warpage problems occur during firing, are they caused by inadequate packing densities, by inadequate homogeneity, by forming techniques and processes, or by some other phenomenon?
Summary
Flocculated casting bodies won't ever pack densely, regardless of the PSD of the powders, because the flocculated state of the body will control the structure (the internal particle/pore structure) of the system. Deflocculated casting bodies can produce dense packing with good distributions of powders, but deflocculated systems are not usually used due to other problems and properties consistent with deflocculated forming bodies.
Clay particles tend to align during plastic forming, which can cause differential shrinkage problems during drying and firing. The PSDs of the clay fractions may help to achieve better packed wares, but the coarser non-clay particles usually have greater influences on packing than the fines.
Finally, don't forget homogeneity. Improper, inadequate, and/or non-uniform mixing in otherwise 'perfect' formulations can always be detrimental to the final shape and properties in any ceramic system.
Miscellany
Don't forget -- please send questions and suggestions for topics for upcoming issues. Topics of interest to any one of you will be of great interest, applicability, and value to everyone else.
Thanks. See you next time.
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