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Article in the magazine Varnishes aluminum oxyhydroxides

Aluminum oxides of varying degrees of hydration have long attracted the attention of chemical technologists specializing in the production of filled composite materials for various purposes. This is due to the great diversity of the properties of these chemical compounds and, accordingly, different, often opposite effects on the performance properties of the filled systems. This diversity is determined by the fact that the aluminum oxides and their hydrates are polymorphic (crystallized in orthorhombic, monoclinic, triclinic syngonies and their varieties, different in their packing and the nature of the bonds between the crystallites) and, depending on the conditions of production can be characterized by different deviations of the chemical composition from stoichiometry, manifested in a different content of hydroxyl groups and impurity ions. The impurity ions included in the crystal lattice very often determine the entire set of properties of the particle surface. As early as at the beginning of the XX century, when hydrated aluminum oxides were used as dispersants of organic pigments and dyes, it was noted that fillers obtained by precipitation from sodium aluminate solution and fillers precipitated from aluminum sulfate or acetate solutions have opposite adsorption properties with respect to acidic and basic dyes. This is determined both by the different acid-base nature of the particle surface and by the different sign of the charge acquired in the aqueous medium due to the formation of an electrical double layer. The constant tendency in the industry to increase as much as possible the dispersion degree of pigments and fillers naturally aggravates the influence of the surface of the dispersed phase on the properties of the filled material.
Hydroxides, oxyhydroxides and aluminum oxides are amphoteric - exhibit the properties of both solid acids and solid bases. In particular, aluminum oxyhydroxide (produced by "Functional Materials" LLC under "TS 101" trademark) crystallizes in the bemite modification of rhombic syngony and consists of crystals in which the Al3+ ion is surrounded octahedrically by five O2- ions and one OH- ion. The octahedrons combine to form layers. Hydroxyl groups on the crystal surface have different local charge densities and are characterized by different acid-base properties depending on the surroundings. The hydroxyl group surrounded by four O2- ions has the largest negative charge and exhibits basic properties. The hydroxyl group that is not surrounded by O2- ions is the acid center. The incompletely coordinated aluminum atoms on the surface are Lewis-type acid centers characterized by the strength (Gammet function values) H0≤+3.3, H0≤+1.5 and H0≤-3. During hydration these centers are transformed into acidic centers of the Branstedov type. The hydroxyl groups adsorbed on the surface of the oxyhydroxide, which are capable of exchange, are naturally the main centers. They determine the alkaline character of the aqueous extract of this filler (the value of the hydrogen index of the aqueous extract or aqueous suspension can reach pH = 10).
As the transition from gibbsite to bemite and further to γ-aluminum oxide, the refractive index increases. The average value of the refractive index for gibbsite is 1.58, for bemite - 1.65 and for γ-oxide - 1.73. As the content of hydroxyl groups in the filler composition decreases, the scattering ability of its dispersions in the polymer matrix increases.
Fillers play an important role in pigmented paint materials. While the main purpose of pigments is to provide hiding power and color of paints, fillers, being cheaper components than pigments, are added to paints mainly for volumetric concentration of mineral particles in the binder. The share of fillers in the mineral part of paints can be more than half. Most fillers are only inert materials that provide the physical and mechanical properties of the paints. However, some fillers have additional functional properties that improve hiding power and some other properties. In this case, the filler/pigment ratio can be increased without impairing the hiding power of the paint and is in fact the same as replacing part of the pigment by the filler.
Such fillers allowing to reduce consumption of pigments, mainly titanium dioxide, are conditionally called "active". Due to a particular structure, particle size and chemical activity, the particles of the active filler interact with the particles of titanium dioxide, which prevents the convergence and sticking of the latter (several titanium dioxide particles stick together and refract light as one particle). This increases the number of pigment particles participating in the light scattering and consequently improves the hiding power of the paint. An increase in the effective hiding power of pigments is only possible if the filler particles are smaller than the pigment particles and they interact with each other

Note: Paint No. 1 contains 25 % TiO2 ; Paint No. 2 - TiO2 content is reduced to 20 % without replacement with a filler, which gives significant deterioration of hiding power. If 20% TiO2 is replaced by TS 101 (paint No.3), hiding power and other properties are the same as for paint No.1 with more TiO2.

Diagram 1. Dispersion curves of alkyd paint containing:
№1 — 25% (TiO2); № 2 — 20% (TiO2) + 5% (TS 101)
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High whiteness, sufficiently high refractive index, easy dispersibility of "TS 101" suggest the possibility of its use in enamels of different colors and purposes to improve their performance and decorative properties, not only as a substitute for expensive white pigments.
The presence of hydroxyl groups on the surface of aluminum oxyhydroxide particles, and especially their presence on the edges and corners, leads to the formation of flocculation and coagulation spatial structures, mainly due to hydrogen bonds formed by hydroxyl groups of filler particles either directly with each other or with functional groups of pigments. Formation of flocculation structure, including water molecules and molecules of film formers, surfactants, drying agents and other components of disperse system is possible. Formation of spatial reversible mesh structure in disperse system increases its stability and gives it thixotropic properties (Fig.1-2 water-based paste, Fig. 3-4 paste based on pentaphthalic oligomer).

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Figure 1. Dependences of viscosity on velocity gradient. 76 hours after dispersion. 48 hours of rest.
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Figure 2. Flow curves. 76 hours after dispersion. 48 hours of rest.
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Fig. 3. Dependences of viscosity on shear rate for paste №1.

  • Curve 1 - 24 hours after dispersion.
  • Curve 2 -26 hours after dispersion. 2 hours of rest.
  • Curve 3 - 50 hours after dispersion. 24 hours of rest.

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Figure 4. Flow curves for paste №1.

  • Curve 1 - one day after dispersion.
  • Curve 2 - 2 hours after the first measurement.
  • Curve 3 - after 24 hours of rest.

Thixotropy is desirable for many paintwork materials, primarily to increase their sedimentation stability, to prevent the formation of non-dispersible dilatant precipitates. Thixotropic materials enable application of a thick coating in one painting operation. Naturally, the high chemical activity of "TS 101" opens up the possibility to use it as a structuring additive in various kinds of film-forming substances, taking into account their resistance to bases.
High-base nature of hydrated aluminum oxides, including oxyhydroxide, suggests the possibility of their use in the composition of anticorrosive materials to strengthen the inhibiting effect of traditional anticorrosive pigments. High basicity and adsorption ability stipulate binding of corrosive agents penetrating into coatings and binding of degradation products of film formers. The high complexing ability of aluminum, the presence on its surface, and especially on the edges and corners of the particles of coordination-unsaturated metal atoms can also positively affect the corrosion resistant properties of coatings having hydrated aluminum oxide in their composition. The influence of hydrated aluminum oxides on the protective properties of anticorrosive materials requires detailed study.
About thirty years ago the prospects of using hydrated aluminum oxides as cheap flame retardants for filled polymeric materials were shown. When using gibbsite as a flame retardant flame retardant is achieved by intense release of water vapor in the temperature range 180 - 300oC. Thermal transition of bemite into γ-aluminum oxide is also accompanied by water release and, naturally, energy absorption, but at 300 - 500oC.
It should be noted that the knowledge of all the properties of "TS 101", such as high chemical activity and adsorption capacity of hydrated aluminum oxides makes it possible to scientifically justify their use in the composition of paintwork materials to purposefully improve the properties of these materials. In particular, we can assume mechanochemical interaction of these compounds under conditions of high shear stresses developed in modern dispersing equipment. Based on the fact that oxygen-containing aluminum compounds are the main surface modifiers of a number of pigments to improve their lightfastness, we can confidently expect the effect of improving the indicator from the presence of hydrated aluminum oxides in the dispersion system due to mechanochemical modification. Studies have confirmed an improvement in lightfastness and weathering resistance of paints prepared with the use of "TS 101".
The residual content of hydroxyl groups in the oxihydroxide has a considerable influence on the properties of bemite, especially in the case of using this filler in water-dispersion materials. The value of the isoelectric point of the filler can be determined by precise control of such an indicator as loss on ignition. In the transition from gibbsite through bemite to γ-oxide, the isoelectric point passes through a maximum (pHi = 9.45 for bemite, pHi = 9.2 for gibbsite and pHi = 8 for γ-oxide). The isoelectric point of fillers and pigments is currently unrated, but it determines the compatibility of fillers, pigments and polymer dispersions. Very sensitive to deviation from stoichiometry in either direction is the ζ-potential of filler particles. Moreover, this parameter related to the surface charge is also important for non-aqueous media.
In conclusion it should be noted that the TS 101 filler pigment based on hydrated aluminum oxides possessing a set of properties essentially improving the quality of pigmented paintwork materials must take its due place in the technology of these materials.
Use of the TS 101 active filler enables paint manufacturers to save up to 25% of titanium dioxide or other pigments in formulations due to a better distribution of pigment particles in the paint. TS 101 acts as an effective dispersant in the manufacture of inks, as well as an anti-settling agent and viscosity adjuster. "TS 101 offers these advantages in both waterborne and solventborne paints.

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