Process Breakthrough Makes Fumed Silica Easy-To-Disperse

Abstract

The field of coatings technology has utilized many forms of fumed silica in the last 70 years.   Due to highly adaptable surfaces and structure, numerous grades have been developed over the decades to provide functional solutions to many coating problems.  Successful use of technology is contingent on adequate dispersion, which often requires some type of milling technology.  Now, an innovative downstream process breakthrough has created an easy-to-disperse fumed silica design on an experimental scale to enable formulators to use technology without the need for high intensity milling.

Historically, there are many tailor-made designs that resulted from fumed silica’s adaptable character. These designs create performance attributes including rheological flow control, suspension, reinforcement, and scratch resistance using standard high-speed dispersion which replace more intense bead milling to achieve high efficiency.

This paper will compare performance using both forms of dispersion; standard high speed dissolver type compared to bead-mill type to demonstrate the easy-to-disperse innovation.

Introduction

Increasing customer focus to improve process efficiency has driven raw material innovation to reduce the complexity of the dispersion process.  Using a bead mill to create the perfect dispersion is the most technically challenging and time-consuming processing step in manufacturing paints and coatings. To overcome this, raw materials manufacturers offer dispersions, preparations, and pastes in which the dispersion of insoluble solids (silica, pigments, fillers, etc.) has been optimized. Dispersions such as these, however, may also contain undefined quantities of solvents, water, binders, or additives.  While formulators have no control over this, the effects of these components can have a significant influence to finished coating system.

It would be far simpler if manufacturers could use typical high-speed dispersion to process the solid raw materials for the coating. Unfortunately, the particle sizes that can be achieved by dispersing materials under typical dissolver-type dispersion conditions are routinely significantly above the maximum particle size accepted for use in paint and coating products.

Thanks to an innovation in the production process of fumed silica by Evonik Resource Efficiency GmbH, standard fumed silica technology routinely used by the coatings industry can now undergo wetting and dispersion processes in a single high speed dispersion step, when previously it had to be carried out in two separate steps (dissolver, bead mill) (Figure 1).

The challenge involved is not a trivial one. The formation of aggregates and agglomerates is an essential component of the flame hydrolysis process used for producing fumed silica - without them, fumed silica would not have its characteristic structure or, consequently, its effects.  The resulting particle spectrum extends beyond 200 µm.  However, to achieve favorable optical and application characteristics, manufacturers aim for aggregate sizes of < 10 µm.

The feasibility of the idea had already been confirmed two years ago when a laboratory-scale setup produced the first product samples, which were named “easy-to-disperse” (or “E2D” for short), a term that describes the unique properties of these products (i.e., readily dispersible in a dissolver).  A pilot plant constructed and commissioned at the Hanau site this year has now generated the first pilot-scale product samples.

The experimental program began with VP DDS, an E2D version of dimethyldichlorosilane (DDS) treated silica. Measurements of the particle size distribution revealed the critical difference (Figure 2). The particle spectrum of the standard version of DDS treated silica is split into a bimodal curve, whereas the new process yielded a unimodal distribution shifted far to the smaller size range. The elimination of coarse particles would allow paints and coatings manufacturers to avoid milling processes involving high shear forces (bead milling, for example).

To test its processing characteristics, rheological effects, and optical properties within a coating system, the standard product was processed using traditional milling procedures, while the experimental product was processed exclusively in the dissolver.  The results for viscosity, gloss, haze, and jetness (depth of color) were all similar, within the accuracy of measurement.

A significant, absolute reduction in processing time was observed as well, even as the degree of dispersion improved. Table 1 and Figure 3 highlight the comparative results between the standard material and the E2D variants.

The graphs highlighted in Figure 3 demonstrate that the rheological effectiveness is in-line with the standard technology.  Results show there is no reduction of thickening or recovery efficiency, because of the E2D processing step.  Jump curves also show that recovery in both test systems (2K clear solvent-based polyurethane and an epoxy modified alkyd primer) are in-line with the standard product, processed with a bead mill.  These jump curves are also predictive of vertical stability performance, once coating is spray applied.

Figures 4 and 5 highlight gloss retention of the test systems as determined by Crock meter via both wet and dry scratch methods.  These graphs compare the E2D variant to the original particle of the structure modified, hydrophobic standard fumed silica developed for scratch resistance. The results demonstrate a similar scratch resistance improvement in the variant with no demonstrated reduced performance. 

Once the VP DDS treated silica had been produced successfully, the pilot plant was used for developing easy-to-disperse versions of other fumed silica grades. To date, three other grades have been successfully produced:  VP HMDS (which is the E2D variant of the standard Hexamethyldisilazane (HDMS) treated silica, VP TMOS (which is the E2D variant of standard Tetramethoxysilane (TMOS) treated technology, and most recently, VP structure modified (SM) DDS which is the E2D variant of hydrophobic, structure modified technology for improving scratch resistance, while the other grades are targeted for rheology improvement, whether thickening, thixotropy or vertical sag stability. 

The use of easy-to-disperse silica decreases processing times, cleaning times, production losses, etc., while omitting the milling step reduces investment and maintenance costs—especially in clear coating production. Pigment manufacturers likewise offer products that can be dispersed with high-speed dispersion technology, opening the possibility of formulating pigmented coatings without the use of a media or bead mill.

In summary, paints and coatings manufacturers have been looking for ways of making dispersion processes less complex.  Pertaining to fumed silica technology, this innovation has met this demand in two ways.  First, by reducing the number of production steps needed for processing, and second, required batch cycle time has decreased to achieve the optimum degree of dispersion.  These combined efficiencies ultimately reduce production costs, while maintaining performance.