Dispersions

A dispersion is defined as a system consisting of fine insoluble or only slightly soluble particles distributed throughout a liquid continuous phase. The particles distributed within the liquid continuous phase constitute the dispersed phase. The distributed particles are usually solid (i.e., sol), but can also be liquid droplets (i.e., emulsions). If the size regime of the dispersed phase is between 0.1 and 10 microns, the system is described as a colloidal dispersion.  If the dimensions of the dispersed phase happen to be < 0.1 microns, the materials are described as nanoparticle dispersions. Currently, there is significant interest throughout science and technology in colloidal dispersions (0.1‑10 microns) as well as nanoparticle dispersions (0.01 – 0.1 microns). Examples of such products and processes include: inkjet, chemical mechanical polishing, paints, emulsions, agrochemicals, foodstuff, paper, pharmaceuticals, nutraceuticals, cements, high tech ceramics, adhesives, chromatography, catalysis, detergency, filtration, grinding, oil recovery, water treatment, polymer colloids, etc. The methods used to prepare, stabilize, process and characterize such materials are the basis of Colloid and Interfacial Science and Technology.

AVEKA, Inc has the research and development infrastructure and production capabilities to formulate, design, and produce a dispersion to meet your market needs. (Select our Dispersion Brochure.) The optimized parameters of interest might include: particle size distribution, high % solids, color, opacity, stability, proper rheology, etc. Critical to successful dispersion processing is the control and manipulation of the interparticle forces generated between particles in this sub micron size range. High quality dispersions typically possess high stability with a long shelf life, have a high solids content and possess the proper rheology.

Choosing the correct surfactants and dispersants and the proper process to make dispersions is a core competency of AVEKA. Several technology platforms useful in the processing and scale up of high quality dispersions are available, including: high energy bead milling, high pressure processing, high shear mixing, dry ball milling, pigment surface treatment technologies and compounding. AVEKA has a full range of analytical tools to measure and understand the behavior of dispersions (particle size measurement, particle charge by zeta potential measurement, complex rheology, surface area, etc).

Many dispersions are water-based, although AVEKA has the capability to prepare solvent-based and/or 100% curable dispersions. AVEKA has prepared stable dispersions of inks, agriculture additives, polishing solutions, high tech slurries for electronic markets, carbon particles, cosmetic lotions, ceramic slurries and more.

Dispersions are divided into two types: solid in liquid and liquid in liquid (called emulsions). Each of these systems can be applied throughout a wide range of industries. The solid in liquid dispersion is one in which the solids are suspended in the fluid. These dispersions are created for a wide variety of applications with some of the most common uses being paint, ink, microabrasives, ceramics and cosmetics. An emulsion is a homogenized mixture of two or more immiscible liquids. Droplets of one of the liquids will be suspended in the other. A common example of this is milk. Current state-of-the-art applications include enhanced oil recovery and drug delivery. For our customers, we have also generated triple emulsions of water in oil in water and /or solids suspended in oil which is then emulsified in water.

Methods of Making Dispersions
Aveka has three primary methods of generating dispersions: bead milling, high pressure processing and high shear mixing. Each of these has specific strengths and can be employed to make an ultra-fine dispersion of most any material, including pigments, ceramics, pharmaceuticals, pesticides, and even polymers.

State-of-the-art bead milling is often the best choice for preparing and scaling up high quality dispersions. Bead milling employs high density, high strength grinding media and a high speed rotating shaft enclosed in a grinding chamber. A starting millbase suspension is pumped through the grinding chamber to allow for size reduction and deagglomeration.  In the grinding chamber the grinding media beads travel at speeds in excess of 20 ft/s, constantly colliding with the dispersed materials. The high kinetic energy generated during this process allows materials to be deagglomerated often reducing the average size from 100 microns to submicron within minutes. Typical final parameters for dispersions processed in this manner are particle size from 0.1 to 5 microns, % solids of 20-75 wt%, and a viscosity from 100-10,000 cp.

High pressure processing is used to make fine emulsions and to generate ultra-fine colloidal dispersions as well. This method is often called medialess milling as there is no high energy impaction and grinding. Using a pump, the starting millbase suspension is pressurized to 5000-25000 psi and forced through a series of small orifices and nozzles. The dispersion experiences extremely high shear forces and explosive expansion forces causing micro cavitations which rend the materials into ultra-fine particles or emulsions. Typical particle sizes for this type of processing range from 0.1 to 5 micron.

High shear mixing is the least energetic method of dispersing particles. This batch method is commonly used to break apart loose agglomerates (e.g. soft materials like those commonly found in pigments), introduce dry powers initially into liquids, and create coarse emulsions. This process often uses a Cowles blade dissolver or a high speed rotor stator type of mixing head. Most often we use this method as a first step process prior to high energy bead milling and high pressure processing. Alternately, we will use this method to add additional ingredients, dispersants, defoamers, co solvents, rheological additives or other special ingredients critical for the end application.

The following figure shows the types of particles and particle groupings that pertain to any discussion of dispersions. An Ideal dispersion will be a stable suspension of a fairly high concentration of aggregate or primary particles.

Characterization
Many state of the art methods are used to characterize and optimize dispersion formulations, including the ones listed below. Additional information about these methods is provided on our Characterization Services web page

 

Dispersions Processing and Development Equipment Available at AVEKA, Inc.