ONLINE EXCLUSIVE: Understanding Ceramic Slips, Part 4
At first glance, sodium phosphates might seem to have the same type of potential for polymerization to a gel system as sodium silicates. However, the extra electron in phosphorus requires one double-bonded oxygen, which decreases the tendency for P-O-P bonds to form, so that polymerization in three dimensions is limited. The normal phosphate systems used in ceramics are tri- or tetra-polyphosphates, which are short chains (or oligomers), and these have relatively short shelf lives before they revert to the orthophosphate. They are not used as often in whiteware ceramic production.
Like organic chemistry in general, organic deflocculants offer many possibilities. A number of types of organic-based deflocculants are available, and they range from anionic and nonionic to cationic; from phosphates and carboxylates to ethylene oxide adducts and quaternary ammonium salts; from monomer to polymer; and from completely natural compounds and modified natural compounds to completely synthesized systems.
For industrial ceramics via aqueous processing, the preferred type of organic deflocculant is the polymerized carboxylate. This seems to be related to the steric effect and the water-holding capacity of such compounds, which contribute additional plasticity beyond that natural to the raw materials. Simple monomeric carboxylic acid-based soaps, or phosphate or sulfonate-based detergents, though effective in deflocculation, provide only an interfacial monolayer between particles and the surrounding fluid, without giving improved plastic properties more characteristic of polymers. They are also expensive.
Except for the lignosulfonates, most polymeric organic deflocculants are much more expensive than silicate-based deflocculants. Humic acids are natural plant/coal-based polymeric deflocculants of variable effectiveness. Modified byproducts of the paper pulping process, lignosulfonates are also plant-based polymers and hence can be relatively inexpensive, but they might also contain high levels of neutral salts that can cause problems in processing and may also affect the fired color of the product. Many lignosulfonates are useful for reducing water content in concrete and coal-water slurries, which is a deflocculation function. The sulfonation makes them very soluble and effective as deflocculants, and their polymeric nature also makes them effective as binders in ceramics.
Condensation polymers of naphthalene sulfonic acids with formaldehyde are also used for water content reduction in concrete and coal-slurries, but are not as useful in ceramics. This may be due somewhat to their incompatibility with negatively charged particle surfaces, but also to their low molecular weight (MW) of 750-1750, which limits their formation into protective colloids with steric repulsive properties. They are usually termed oligomers due to their relatively low MW. Condensation polymers of this type are more difficult and expensive to produce at high molecular weight.
Polycarboxylates like polyacrylates and polymethacrylates (or copolymers of these with maleic acid, anhydrides or other monomers) offer a range of molecular weights and levels of hydrolysis and neutralization (ionization with base), and, consequently, ranges of solubility and viscosity. They are addition polymers and can be made with a wide range of molecular weight without difficulty.
A series of polyacrylates or polymethacrylates with varying molecular weight will produce a minimum slip viscosity at an intermediate molecular weight. This has been shown for polyacrylic acid without pH adjustment,1 with the minimum viscosity in the 5000-10,000 MW range. At full deflocculation, however, a more pseudoplastic slip was produced by the lower molecular weights, and for the same viscosity, the pseudoplasticity was higher before full deflocculation than after. Viscosity with pH adjusted higher would likely decrease further, and more pseudoplasticity would be lost. In general, this supports the idea of minimizing the amount of deflocculant to give ideal casting properties while avoiding the crowding of pore water by an extra polymer or extra ions, both of which tie up too much water for optimum flow and optimum stability properties.
In industry, polyacrylates usually require only one-third the quantity that sodium silicate does to effect deflocculation, and they have been promoted as providing better slip stability as well. One reason for this is that they are not easily retired (insolubilized) by multivalent cations like calcium or magnesium.
The other reason relates to adsorption behavior. Polyacrylic acid begins to ionize at pH 3.1 Therefore, at pH near neutral its anion preferably adsorbs on the gibbsite (alumina-like) layer of clay platelets, which are positively charged at pH<9, but not on the silica-like layer, which is negatively charged at pH>3.5. Sodium silicate is more compatible with the silica-like layer, and therefore can adsorb on both sides of the clay platelet. In order for it to behave more like the higher molecular weight polyacrylate, though, the silica:soda ratio should be kept high and the pH of the slip, thereby, relatively low.
McCutcheon's Emulsifers and Detergents and McCutcheons Functional Materials are good references for these and many other types of chemistry for industry.
AgingParticularly in ceramics, the aging process fulfills at least three important functions: the solubilization of alkalis and alkali earths from minerals, the development of humates from wood/coal residues in clays, and the volatilization of leftover materials from beneficiation. Each of these can have significant effects on the deflocculation state.
Alkalis are soluble from feldspars and, to a lesser degree, alkali earths from a number of minerals. Mixing increases the rate of solution due to increased temperature, particularly for alkalis, as a function of time and amount of shear. Alkalis deflocculate and alkali earth flocculates, but alkalis act as simple deflocculants and can be detrimental to plasticity if they act alone. Both effects are often present simultaneously. Alkali earths are sometimes deliberately added to counteract the effect of increasing alkali with aging, but too much of both increases the neutral salt content and conductivity of the slip excessively, and with excess conductivity it is possible to eventually prevent deflocculation completely.
Ball clays were often deposited between layers of woody residues that coalified over time, and in most such cases the clays will also contain organic residue. Higher amounts of oxygen in low-grade coals (lignites) make them more water soluble. The majority of the soluble fraction is humic acid, which is a general name for a mixture of aromatic and aliphatic polymeric compounds of variable molecular weight containing carboxylic acid groups.
Further oxidation of the lignite solubilizes more humic acid, and alkali solubilized from feldspars turns it into an effective humate deflocculant having both steric and electrostatic properties-that is, protective colloid. If the type and quantity of humate is correct, other deflocculants can be almost eliminated. This is also the source of a significant amount of plasticity in ball clays, as they are polymeric and increase water retention in the form of bound water. The Methylene Blue Index (MBI) of clays increases over time with the generation of additional humic acids from lignite.
Coarse lignites are normally screened from ball clays, but finer lignites keep reacting slowly. It is good practice to accelerate the slip aging process with high-intensity, high-temperature mixing, so that subsequent auto-deflocculation from solubilized humic acid and alkali is minimized and the slip is stable in the long term. It is also good practice to minimize the alkali added as Na2CO3 or NaOH to just what is necessary to activate the humic acid; if the feldspar is fine, none should be necessary. Also keep in mind that water glass, particularly water glass with high soda:silica ratio, contains extra alkali to keep it in solution, and perhaps enough to activate the humic acid. The intent should be to keep slip pH near neutral for good gelation and plasticity while adding the polymeric deflocculant for electrostatic and steric dispersion, as well as a strong cast body. Also, if the coarse lignite is screened out of the system and the fine lignite is solubilized, firing problems will decrease and fired strength will increase.
Some beneficiation processes use volatile substances to treat materials and dissolve away unwanted contaminants. Mineral processing uses this method heavily. For example, silicon carbide as-produced includes silica and metallic residues that are removed by treatment with volatile acids before further processing, particularly if the process involves solid state sintering. These acids, with the dissolved contaminants, must then be removed or deflocculation cannot occur due to high conductivity. A small amount of the acid remains adsorbed on the particles even after the washing process, and if the slip is left in a closed aqueous environment the acid will act as a flocculant.
Silicon carbide has no natural alkali to cause auto-deflocculation. Aeration or evacuation of the system can volatilize the remaining acids and reduce viscosity. In this case, aging can decrease the amount of deflocculant necessary to achieve a given viscosity by 30% or more, but while this may also allow a lower deflocculated viscosity, some flocculation should be maintained for the stability of the slip and a good casting rate. This aging effect has caused disastrous results in industry due to overdeflocculation.
Similar effects have been seen in aged coal while making high-density ground slurries. Pyrite degrades in coal that is stockpiled too long, producing acids (pH 3) and soluble iron salts that are both flocculants. The combination completely prevented deflocculation, but normal properties could be achieved if the coarse coal was washed with tap water before slurrying. Some clay slips become flocculated over time from pyrite degradation. In some of these cases, the raw clay must be stockpiled until most of the degradation is complete, and then either treated with barium salts while slurrying, or slurried and filter-pressed to reduce soluble sulfates and allow good deflocculation.