How Small Can You Go?

Advances in powder-based processing long have been focused on reducing particle size and improving the particle uniformity. Nanosized powders ," particles at the nanoscale, or approximately 1 nanometer (nm) to 100 nm in size ," are becoming increasingly critical to innovations in numerous applications, including catalysis, coatings, cosmetics, electronics, sensors and drug delivery.

Nanopowders offer controlled functionality, increased reactivity and a number of other advantages over existing materials. Moreover, new powder production and synthesis methods promise to further improve particle capabilities and push particle sizes to new lows.

Xerox has reinvented toner production through its emulsion aggregation (EA) process, which creates 1 micron (m) to 15 m particles from smaller nanometer-sized particles. The company also said the technology has potential applications in biotech, personal-hygiene and related applications.

Pat Burns, manager of the composite and nanostructure materials area at the Xerox Research Centre of Canada in Mississauga, Ontario, says the technology sprung out of the company's desire to make smaller toner particles in an economical manufacturing process. Xerox wanted to control particle size and shape to the greatest extent possible.

"The smaller particle size will give you improved image resolution, and you'll end up using about 40 percent less toner on a page," explains Burns. "We wanted to control structure ," to be able to put different things inside, put wax particles inside, not on the surface."

Xerox also wanted to be able to switch to lower-melt polymers, notes Burns. "With conventional processes, you're limited to certain resins," she says. "They have to be brittle enough to break in the grinding process."

The environmentally friendly EA process starts with monomers, says Burns, and uses emulsion polymerization to grow tiny polymer particles in an aqueous environment. The toner particles then are combined with pigment particles and wax particles, which are also in water and very small. See Figure 1.

Source: Xerox.

"Basically, we pour them together," Burns explains. "It's not very viscous; [it's] like water. Then we add a reagent to cause the particles to flocculate. Basically, most of the particles are stabilized with a negative charge, so we put in a positively charged material. That causes destabilization, and it causes the particles to flocculate together."

By controlling the physical chemistry of the mixture, says Burns, Xerox can determine the manner in which the particles form. "Basically, we control the temperature, time and stirring," she says, "and obviously the amount of flocculent we're adding. That causes the particles to come together in a controlled fashion."

After halting particle growth using an anionic stabilizer, Xerox ends up with a mixture that has a very narrow particle size distribution. That mixture then is heated, says Burns, so the polymer resins start to flow and form one solid particle. The material then is cooled down.

"What we end up with is toner particles that are in water," Burns explains. "There are some reagents that we use to make the particles in the water with them ," then we have to isolate them. What we do is washing, essentially just filtration. You end up with toner particles that are kind of the consistency of wet sand. Then you have to dry [them]."

The types of polymers used in the EA process allow the presence of functional groups on the surface of the resulting microspheres for potential ligand attachment or chemical modification, notes Xerox. The process permits magnetite, colorants and other materials to be incorporated within the microspheres and also enables the creation of layered structures.

Xerox introduced a toner product based on the new process about four months ago in Spain and plans to bring it to the United States as well. The company currently is offering the technology to parties interested in licensing it for use in biomedical, personal-hygiene and other applications. For more information, contact Ed Francis, licensing executive for Xerox, at ed.francis@usa.xerox.com.

Dr. Samy El-Shall, a professor of physical chemistry in Virginia Commonwealth University's (VCU) Department of Chemistry, says VCU researchers have been perfecting processes that use laser vaporization to synthesize nanoparticles of controlled sizes. See Figure 2.

Source: Dr. Samy El-Shall, Virginia Commonwealth University.

Researchers start with a bulk alloy material, says El-Shall, or make a specific target based on the composition desired. "These are large particles, 100 m," explains El-Shall. "We take the different composition we want to make from this powder, mix it and compress it under high pressure to form a target."