RHEOLOGY OF CLAY-FREE ENAMEL SLIPS ON THE BASE MOLYBDENUM AND TUNGSTEN CONTAINING FRITS
Malkhaz T. Razmadze, A.V. Sarukhanishvili
Georgian Technical University, Tbilisi, Georgia

Introduction
Despite the widely spread development of electrostatic application of enamel powder to the metal, the classical use of wet enamel as a slip is still in general use and the subject of clay- free slips remains as important.
There are known a lot of methods of elimination of clay from a slip [1-4]. At the 15th International Enamellers Congress, in Prague, we proposed oxides of polyvalent elements for the formulation of special enamel frits, as a base for clay-free enamel slips without suspending components.
Firstly, manganese containing frits were investigated [5] as a material giving a stable colloidal suspension in a water medium, yielding dark black ground coats after firing.
To obtain light coloured coatings, molybdenum and tungsten containing enamels have been proposed, which provided good clay-free slips.
Some of the physical, chemical and technological properties of these frits are listed in Table 1.

Our recent work has generalized the results of investigations, and has revealed the basis of the formation of colloidal suspensions. The aim of the current work relates to the influence of various electrolytes on some of the rheological and technical properties of clay-free slips.

The investigation
The influence of molybdenum and tungsten oxides on the rheological and technical properties of clay-free slips have therefore been investigated. The data is shown in figures 1.1 - 1.3.

figure. 1.1. - Dependence of plastic viscosity on the contents of tungsten oxide

figure 1.2. - Dependence of statistical voltage of shift on the contents of tungsten oxide

figure 1.3. - Dependence of sedimentation stability on the contents of tungsten oxide. (The kind of dependence of the rheological data of molybdenum containing slips is actually identical totungsten containing slips)

figure 1.1-1.3 - Dependence of rheological parameters on the contents of a tungsten oxide at ~ 1.79 g/cm3 and quality of milling ~ 5-7 cm-1 according Lysenko's method

As seen from the figures, the optimum content of tungsten and molybdenum lies between 3-4 mass %. With further increased oxide content, the rheological and technical properties are reduced, apparently due to an increase in density of the enamel frit.
This, in turn, requires an increase in the inter micelle interaction forces. On melting of the enamel frit, RO₃ does not dissolve in the silica-borate skeleton of the glass by more than 6 mass% and macro stratification of the melt is observed. In order to test the possibility of colloidal suspension formation, the "frit - water" system (without the addition of electrolytes) was studied.
Clay-free slips were produced by manual milling in ceramic mortars (quality of the milling up to 5-8 cm^-1 according to Lysenko's method), but they all showed increased density (1.90 - 1.93 g/cm³), plastic viscosity (up to 0.9 - 1,1 mPa.s) and poor sedimentation stability (75 % and lower).
This process was also carried out in ball mills. During the first stages of milling, the balls were mechanically blocked by particles of frit, resulting in termination of the milling process.
The addition of small quantities of electrolytes to the milling facilitated the milling process and a colloidal suspension with a density between 1,75 - 1,86 g/cm³ was obtained with satisfactory technical parameters.
To confirm this, a qualitative investigation of the process of leaching of sodium and potassium ions during the milling of 1kg of frit (ration of frit : water = 1.1, see figure 2.1) has been carried out and established the dependence of the pH of a centrifuged dispersion media of frit on the milling time (figure 2.2)

figure. 2.1-2.2 - Dependence of leaching (figure 2.1) and pH of dispersing medium (figure 2.2) on milling time

The observations have shown that, for the frits investigated, the quantity of ions leaching (Na⁺ + K⁺) will reach up to 450 mg per 1 kg of frit after 7 hours milling time. This will not seriously affect the rheological and technological parameters of clay-free slips, including changes during ageing.
The process of ageing over 100 hours at constant density of the slip (~ 1,79 g/cm³, quality of milling ~ 5-7cm^-1 as per Lysenko's method) has been confirmed. The experimental data are shown in figures 3.1 - 3.3.
The same or only slightly modified behaviour is true for all clay-free slips of frits and systems which we have investigated.

figure 3.1. - Dependence of plastic viscosity on time of ageing

figure 3.2. - Dependence of static voltage of shift on time of ageing

figure 3.3. - Dependence of dynamic voltage of shift on time of ageing

figure 3.1-3.3. - Dependence of rheological parameters on process of ageing of slips

As may be seen from the figures, the rheological parameters during ageing in comparison with their initial values do not vary sharply,(a smooth change at intervals of 5°“20% is observed).
Such a distribution of properties does not actually influence the parameters of covering ability and sedimentation stability of the clay-free enamel slips.
As mentioned above, the addition of electrolytes to a milling in a ball mill facilitated the milling process. This is probably due to the participation of ions in the process of developing the structure of the colloidal particles of a clay-free enamel suspension a double electrical layer is formed around particles of enamel frit (of size 80 microns or less). This mechanism explains all systems for the formation of self suspending colloidal enamel suspensions.
A complex investigation of the "frit - electrolytes - water" system has been carried out to identify the fundamental nature of the effect of electrolytes on the rheological and technological properties of clay-free enamel slips. The variation of the rheological parameters, such as the plastic viscosity (ηpl, mPa.s), statistical voltage of shift (Pst, Pa), dynamic voltage of shift (Pdin, Pa), sedimentation stability (B, %), density of a slip (ρ, g/cm³) and covering ability of a slip (G, g/dm²), were studied in terms of the quantity and type of electrolytes in a ball milling.
It is known [6,7] that the following electrolytes (NH₄)₃PO₄·3H₂O, Na₃PO₄ and NaNO₂ are used as set adjustment materials for slips.
The results of their effect on the rheological properties of enamel slips are shown in figures 4.1 - 4.6.

figure 4.1-4.6. - Dependence of rheological parameters on quantity of added electrolytes at slip density up to 1,79 g/cm3

As may be seen from the plotted graphs, the effect of these electrolytes is ambiguous and very specific.
In our understanding, this unequivocally indicates that there are different mechanisms of action for (NH₄)₃PO₄·3H₂O, Na₃PO₄ and NaNO₂ in the formation of the double electrical layer formed around the enamel particle.
It has been observed that the individual effect of electrolytes is unique. This, once again demonstrates that in the rheology of enamel slips, the important factor is the type and quantity of ions, supplemented by leaching during milling, discharging or during ageing.
However, it is important to consider one further factor - the factor of the surface activity of the finely ground enamel frit particles and their chemical composition (figure 1).
To carry out a study of the influence of electrolytes on the rheology of enamel slips requires the application of the general theoretical principles of colloid chemistry of suspensions.
We propose that the investigation and fundamental principles of electro-kinetic and adsorptive phenomena on the surface of a particle of enamel slip can explain the control of the rheological parameters of enamel slips.
Dependent on the nature of the structure, enamel slips are classified as heterogeneous colloidal suspensions, where the dispersed phase is the fine dispersed particles of enamel frit, clay and miscellaneous other solid components, and the dispersing medium is water with ions of electrolytes dissolved in it. Here the clay acts as an agent for formation of the structure, forming a finite lattice, contributing to slip sedimentation stability and the complex rheological and technological parameters.
Clay-free enamel slips possess the same system, but differ only that in their composition they have no clay. In their case the role of the structure former is taken over by the finely dispersed particles of an enamel frit. The process of structure formation within the slip progresses with the active interaction of the particles of the enamel and the ions of the dispersed phase. The high activity at the surface of the enamel particles promotes the completion of a process of
specific adsorption and electrostatic attraction of ions of the dispersing medium onto their surface, thus forming a well-established double electric (DEL) and diffusive (DL) layer around the enamel particles.

figure 5 - The scheme of structure of colloid micelle of clay-free eamels slip

Fine grinding of the enamel particles will break the valency bonds of surface activity of oxides such as MoO₃, WO₃, SiO₂, TiO₂, Al₂O₃ etc.
This leads to the production of an active inorganic structure on the surface of the enamel particles. A simple example for silicon oxide is shown in Figure 5, in which specific adsorption of OH^- and H⁺ ions occurs from the dispersing medium.

figure 6

During the adsorption of the OHions, the oxygen atom reforms the broken Si^-O bond with a consequent dissociation of the H⁺ of this active centre into the dispersing medium. Thus, a negatively charged surface of the enamel particle is formed, followed by surface electrostatic attraction of the enamel particles with the formation of DEL and DL.
The completion of these processes results in an equal balance to give a colloidal micelle, of which a diagrammatic representation is shown in figure 6.
The formation of well developed DEL and DL promotes an enhancement of the sedimentation stability of clay-free slips and gives them the necessary combination of rheological and processing properties. It describes well the characteristic of control of the rheological parameters of enamel slips. Work continues in the defining of the theoretical basics of the process described above.

References
1. Rodionoff G.P., Perminoff A.A., Smirnoff N.S. "Comparative Studies of Applicability of Natural and Synthetic ("Laponit") Clay for Preparing of Enamel Slips" (In Russia). Proceeding UralNIICherMet, 1978, v.33, p. 22-31.
2. Kazanoff I.K., Markina L.V., Savin L.S. " Substitution of Clay in Enamel Slips" (In Russia), Glass and Ceramics, 1985, 4, p.16-18.
3. Goncharoff S.I., Barinoff I.D., Kazanoff I.K. And other. "Titanium - magnesium Industrial Waste Utilization as a Enamel Slip Generating Element". Glass and Ceramics, 1981, 12, p.4-5.
4. Melnik M.T., Khodski L.G. "Studies of Influencing Suspending Components on Properties Clay-free Enamel Slips" (In Russia), Glass, glassceramics and silicates. 1978, 7, p.123-127.
5. Sarukhanishvili A.V., Gordeladze V.G., Razmadze M.T. Clay-free Enamelling Slip-casting Materials and their Practical Applications. J. The Vitreous Enameller, v. 41,No 2, p.43-47.
6. Petzold A., Pöschman G. Email und Emailliertechnik. Leipzig, 1986.
7. Vargin V.V. Enamel and Enamelling. Leipzig, 1972.
8. Tanabe K. Solid Acids and Bases. Tokyo, 1970.
9. Dukhin S.S., Deriagin B.V. Eectrophoresis. Moscow, 1976.
10. Iler R. The Chemistry of Silica. A Wiley-Interscience Publication, 1979.