DEVELOPMENT OF NEW, FREE OF FLUORINE WHITE ENAMELS
Peter Hellmold - DEV, Germany
Bernd Rödicker - Ferro Techniek BV, Netherlands
Karsten Hartmann - TU Ilmenau, Germany

Abstract
On the basis of a current used fluoride containing titanium white enamel a free of fluorine enamel with comparable and partial improved optical (brightness, bluish shade), thermomechanical (flowability, workability) and chemical (acid resistance) properties was developed by versatile variations of the oxidic enamel components to which reasons can be given.

Introduction
For enamel application a basic condition is the adjustment of optical, chemical and thermomechanical properties depending on the application purpose.
Today white enamels are produced on the basis of partial TiO₂-crystallization during the firing process. The optical properties are mainly determined by the TiO₂-modifications anatase and rutile which are originated from the TiO₂ crystallization.
To improve frit melting (lowering the melting temperature, improve contacting of raw material particles) and flowing properties during enamel firing (viscosity) fluorides are added to the composition of raw materials. This fluoride addition causes during melting and firing fluorine emission.
The environmentaly damaging effect of fluorine can only be prevented by using complex absorption arrangements.
The structural implementation of fluorides within the enamel frit causes a umber of effects which depend on each other like effects on chemical and mechanical resistance and particulare due to viscosity lowering effect on the crystallization and transformation behaviour (TiO₂) and therefore on the optical properties of the titanium white enamel.
Due to the complex influence of fluorides on enamel properties fluoride containing raw materials can not be simple substituted by oxidic components.
For this reason currently well-known white enamels free of fluorine have to be developed in a novel manner.
The target of the development of free of fluorine white titanium enamels was a fundamental transformation of a present used fluorine containing Titanium white enamel into a free of fluorine titanium white enamel with at least the same optical and chemical properties and firing behaviour in comparison with the original fluoride containing Titanium white enamel.
In all, an optimised solution concerning to the technically relevant and consumer orientated properties (flowability, corrosion resistance, brightness and bluish shade) had to be found.
The single results had to be discussed with respect to general conclusions.
The original enamel for the systematic investigations was a fluorine containing white enamel frit (chemical composition see Table 1).
The oxidic components had to be varied single or complex to substitute fluorine to achieve unchanged or improved enamel properties.

Methods of characterisation
The enamels which had to be characterised were melted at 1200 °C, cooled down in water, wet milled with mill additions, sprayed and fired at 820 °C. The measurement of the thermomechanical properties was carried out at frit powder.
Instead of viscosity measurements the determination of the flow behaviour of enamels was examined on the base of DIN ISO 4534.
Here the melting time of the sample until the shape of a semi ball and the length of the enamel flow were measured.
To characterise the processes of TiO₂-crystallization and anatase/rutile transformation differential thermal analysis and X-ray methods were applied.
The chemical enamel properties were checked by use of a test procedure to determine the resistance against boiling citric acid (DIN ISO 2742).
To investigate the optical enamel properties (colour and gloss) both UV-Vis spectroscopy and colour measurement on the base of CIELAB system were applied.

Thermomechanical, chemical and optical enamel properties
At first the properties of the free of fluorine and the fluorine containing white enamels had to be compared to each other when the fluor component Na2SiF6 was substituted by Na₂O and SiO₂.
The result was an obvious change in the thermomechanical and opticalenamel properties due to the lack of fluorine.
Owing to substitution of fluoride ions by oxygen ions the number of disconnecting points in the silicate network decreased.
That means an improvement of the network structure and a strengthed glass structure. With this an increase of viscosity was connected which led to an increase of anatase/rutile crystallization temperature by about 70-80 K (figure 1).
The hampered TiOv-crystallization causes a poor crystal content and for this reason a smaller diffuse reflection.
Furthermore, the blue colour content decreased since the implementation of impurity ions (especial Cr³⁺) into the dominant rutile of crystallized TiO₂ caused an increase of the yellow colour content (1).
Figure 2 shows the considerable difference between the crystal phase situation of both enamels which can be recognized by the main interferences as results of X-ray investigations.
In the fluorine containing enamel both a clear crystallization of anatase and a lower crystallization of rutile occured.
However, in the free of fluorine enamel a smaller amount of anatase and a higher amount of rutile crystallized. Table 2 shows the most important property differences between these two enamels: the free of fluorine enamel had worse flow values, increased crystallization temperatures and decreased brightness and

bluish shade owing to the obvious decrease of the anatase/rutile ratio. In comparison with the fluorine containing enamel only the acid resistance of the free of fluorine enamel was improved by the strengthed network.

figure 1 - TiO2 - crystallization of containing fluorine and free of fluorine enamels

figure 2 - X-ray investigations of the crystal phase situation of enamels

The result was that a direct substitute of fluorine by one or two oxydic components (Na₂O, SiO₂) does not reach the achieved enamel properties.
Hence, for systematic variations of the oxydic components following single aims were intentioned:

First of all for the improvement of the flow properties the entire amount of alkali metal oxides had to be increased.
These investigations were focussed on Li₂O because of the small ion diameter (78 pm) in contrast with the other alkali metals (Na⁺: 99 pm, K⁺: 133 pm) (2) in order to reduce the lowering of the network stability and to limit the decrease of the chemical resistance. Table 3 shows the results.
The flow behaviour, was, as expected, improved with increasing Li₂O content since the increase of the number of disconnected points in the glass network.
The optical properties got worse that means both a brightness reduction and a weakened bluish shade.
The reason is the increase of crystal grow in the enamel due to the lowered viscosity connected with a disadvantageous change of the anatase/rutile ratio. The chemical enamel resistance decreased with the increasing Li₂O content due to lowering of the network stability.

Since the increase of the entire content of alkali metals by adding Li₂O did not achieve the wanted results, apart from the improvement of the flow behaviour, the original concentration of K₂O had to be partial substituted by Li₂O.
At the variation of the alkali metal oxides the mixed alkali effect had to be used to improve the chemical enamel resistance (3).
On the basis of the enamel with 0,9% Li₂O (Table 4) a partial molar substitute of K₂O by additional Li₂O leaded to an expected increase of the chemical enamel resistance.

The reasons are the very low diameter of the Li⁺ ion and the well-known nonstatistical Li⁺ distribution in the glass structure.
The clearly different diameters of the cations (Li⁺, K⁺) cause at a special Li₂O/K₂O ratio (1,8% / 0,4%) a stable occupation of empty rooms in the glass structure, so that the corrosion which is determined by the ion exchange is inhibited. This is the well-known mixed alkali effect.

figure 3 - Dependence of the colour value b and the relation anatase/rutile of enamels on the firing time

The increase of the Li₂O/K₂O ratio caused further both an improvement of the flow behaviour and an increase of brightness and bluish shade. The change of the colour values corresponded with the increased anatase/rutile ratio.
Figure 3 shows the dependence of this correlation on the firing time. A high anatase/rutile ratio and the with this ratio connected wanted bluish shade of the enamel is achievable by the at relatively low temperatures occuring anatase crystallisation in the enamel with a convenient Li₂O/K₂O ratio. To achieve the enamel resistance of <4 g · m^-2 investigations by additional oxidic compound variations were continued with the enamel with the best Li₂O/K₂O ratio. Since the positive influence of Li₂O on the chemical resistance we have assumed a comparable action of small amounts of MgO due to the same ion diameter of Mg²⁺ ion (78 pm).
Additionally, the twofold charged Mg²⁺ ion in small MgO concentrations can act as a network builder and therefore contribute to strengthening of the glass network. For this reason the MgO concentration in the best enamel was increased by 50% in comparison with the original frit. Table 5 shows the results.

With an increase of the MgO concentration the expected improvement of the chemical resistance was achieved.
The flowability and the crystallization temperature of anatase and rutile changed only slightly but an obvious change in the bluish shade was observed which is caused by the strong decrease of the anatase/rutile ratio.
The brightness achieved the highest level. The chemical resistance of the enamels with the varied content of alkali metal oxides can be improved by addition of MgO but also by addition of
resistance increasing glass components like SiO₂ as a basic network builder and Al₂O₃ with a higher network builder action in contrast to MgO.
To reduce the well-known corrosive action of aqueous citric acid solutions on the enamel surface due to the development of very stable chelat complexes (4) the investigations were carried out with low concentrations of Al₂O₃.

For this reason the SiO₂/Al₂O₃ ratio of the best enamel from the alkali metal oxide variation was increased (Table 6).
Due to a slight increase of the ratio which caused a higher SiO₂ content the chemical enamel resistance was obviously improved.
Owing to the constant anatase/rutil ratio the good colour values did not change.
In contrast to this the shorter flow length showed the worse enamel workability. In the case of a clear increase of the SiO₂/Al₂O₃ ratio due to a reduction of the Al₂O₃ concentration the improvement of the chemical resistance was not as good as before and the colour values got worse owing to the lowered anatase/rutile ratio.
The reason is the inhibition of the anatase crystallization within the enamel at the to low Al₂O₃ concentration as it is shown by DTA investigation results (figure 4).

figure 4 - TiO2 - crystallization in enamels with varying SiO2 and Al2O3 contents

The temperature of the anatase crystallization was not changed by the slightly increased SiO₂ concentration (B) in comparison with the original enamel (A). However, the enamel with the obviously lowered Al₂O₃ content (C) showed an increase of the anatase crystallization temperature of about 27 K.
Concerning to the optical properties (brightness, slight bluish shade) of the free of fluorine white enamel a stabilization of the anatase modification was very important. It is well-known from literature, that P₂O₅ contents in enamels cause a stabilization of the anatase modification and an inhibition of the rutile crystallization (5). structure come into existance, which lead to a reduction of the chemical enamel resistance (6).
These negative effects of the P₂O₅ content had to be compensated by the decrease of corrosion causing Al₂O₃ contents.
Table 7 shows the thermomechanical and optical enamel properties of the different (increasing) P₂O₅/Al₂O₃ ratios.

In contrast to the Al₂O₃ influence on anatase crystallization, P₂O₅ was stabilizing in all in our experiments used P₂O₅/Al₂O₃ ratios the crystallized anatase modification, which caused glossy enamels with a bluish shade, that means the colour values corresponded to the achieved targets.
The enamel flow values got worse slightly with increasing P₂O₅/Al₂O₃ ratio owing to the network building action of P₂O₅which can be attributed to the high DIETZEL electrostatic field force of the P⁵⁺ ion (7) and therefore to the strengthening of the glass network.

figure 5 - Dependence of the acid resistance of enamels on the relation P2O5 / Al2O3

With increasing P₂O₅/A₂O₃ratio mainly by the reduction of the Al₂O₃ content we have found until a quotient of 0.94 surprisingly high chemical resistance values (figure 5).
This can be explained by the lower A₂O₃ content and the therefore reduced network damage by complex developing acids during the corrosion.
Despite of the decrease of the chemical enamel resistance at a further increase of the P₂O₅/Al₂O₃ratio this by these variations recieved enamel has still a higher corrosion resistance than the original fluorine containing enamel.

Conclusions
The stressed results shall be shortly summerized. The interpretation of the results is partly very difficult since some effects with respect to the investigated property parameters overlapped what we took into account before making the
experiments.

References
1. Hellmold, P.; Stuchlik, U.: Einfluß der Anatas-Rutil-Umwandlung auf die Farbstabilität schmelzgefärbter Emails, Mitt. VDEfa 42 (1994) 1-9.
2. Petzold, A.: Physikalische Chemie der Silicate und nichtoxidischen Siliciumverbindungen, Deutscher Verlag für Grundstoffindustrie, Leipzig 1991.
3. Paul, A.: Chemistry of glasses, Chapman and Hall, London - New York 1990.
4. Hellmold, P., Saß, H.: Korrosion von Titanemails durch organische Säuren, Mitt. VDEfa 39 (1991) 132-140.
5. Eppler, R. A., Spencer-Strong, G. H.: Role of P2O5 in TiO2-opacified porcelain enamels, J. Amer. Ceram. Soc. 52 (1969) 263-266.
6. Dunken, H. H.: Physikalische Chemie der Glasoberfläche, VEB Deutscher Verlag für Grund-stoffindustrie, Leipzig 1981.
7. Dietzel, A.: Emaillierung, Springer-Verlag, Berlin Heidelberg NewYork 1981.

The Authors thank Bayer AG resp. Pemco, Email Brugge for sponsoring of this work.