Selection of dispersants for the dispersion of carbon black in organic medium
Introduction
The coating industry faces increasing problems of high production costs and high demand in terms of quality that requires the implementation of new methodologies in production plants.
In particular, the use of concentrated pigment slurries constitutes one of the solutions. Their first advantage is the duality of application. They can be useful either as “tinters” in order to adjust the hue of a paint formulation that had not the expected color, or to enter the formulation of coatings by mixing with a transparent varnish. The second advantage is the realization of better outputs. Evidently, a reduction of costs and an improvement of the use result from these two advantages [1].
Pigment concentrates can improve the performance of a coating system. For this purpose, fine and homogeneous dispersions are needed and dispersant additives are often used. The interaction between dispersant and pigment (dye) in the paint is of major importance and its optimization permits to ensure a good quality of the finished product. The presence of the polymer dispersant allows avoiding anomalies susceptible to generate flocculation, sedimentation, fall of gloss ….
The literature is rich in works dealing with this alliance of pigments with dispersants [2], [3]. In particular, interesting results in industrial applications have been obtained with dispersants bearing carboxylic functions [4], [5], [6].
The carbon black is among pigments that are widely used in the industry of paints, it is one of the most finely divided materials [7]. Its high specific area and oil absorption ability make it very difficult to disperse [8]. Thus, it is difficult to prepare a stable and fine dispersion of the carbon black pigment by means of grinding in an organic medium. Additives are always used to solve problems related to bad pigment dispersion, viscosity increase on storage and thixotropy. Indeed, the experience shows that carbon black concentrates have a high tendency to gel on storage if an adequate dispersant is not used or when its dose is not optimized. The consequences are usually low tinting strength that requires consuming larger amounts of concentrate for tinting. On the other hand, the increase of viscosity causes troubles in automatic tinting.
Thus, the polymer dispersants are added to obtain homogeneous dispersion of the pigment in the liquid phase that leads to weak viscosity and allows high pigment loading and high tinting strength. The stabilization of the dispersed state is also ensured [9], [10], [11].
In organic media, polymer dispersants usually adsorb by means of hydrogen bonding, ionic interactions, van der Waals forces and acid–base interactions [12], [13]. After adsorption at the surface of the particles, the polymer dispersant creates repulsive forces between the particles. These forces are essentially of a steric nature in non polar medium [14], [15], [16].
The influence of the surface chemistry of carbon black in the interactions with polymer dispersants in paint applications has not been studied in detail. However, interactions between carbon black surface groups and polymers highly contribute in determining the final state of paints. The functional groups presented on carbon black (carboxylic acid, phenolic, quinonic and lactone groups) are believed to be attached to the edges of the graphitic layers, which make up the carbon black structure. These groups can interact physically and chemically with polymers as well as additives [17], [18], [19].
In the present work, the dispersion of the carbon black pigment in non-aqueous medium has been studied in the presence of two dispersants having carboxylic functions. The conditions of dispersion and stabilization of this pigment in relation to the two dispersants have been studied and optimised. In a first part, the adsorption of such dispersants has been compared and the adsorption mechanism of the most efficient dispersant has been investigated by spectroscopic measurements. Rheological studies allowed optimising the formulation and lastly, the consequences of the improved dispersion on the color properties were evaluated.
Section snippets
Material
The carbon black pigment in powder form was supplied by Degussa. This powder was characterized by an average diameter of 13 nm and a specific area (BET) of 460 m2 g−1 according to information from the supplier.
Two carboxylic dispersants commercialised by Byk Chemie have been used in this work. Their characteristics are shown in Table 1. A medium oil alkyd Setal 196 supplied by Akzo Nobel resins has been used as a grinding resin; xylene has been used as a solvent.
Rheological measurements
For the rheological measurements,
Adsorption isotherms
The adsorption isotherms of the two dispersants have been determined by the depletion method. Isotherms presented in Fig. 2 showed a progressive increase of the amount of the adsorbed dispersant until an adsorption plateau corresponding to the saturation of the surface of the carbon black pigment was reached. The shape of the isotherms showed that, independently of the mass and the chemical composition of the dispersant, the adsorption was of a monolayer type.
The adsorbed dispersant density at
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