IMPROVED DEVELOPED MASS COMPACTNESS BY POLYMER PRECIPITATION
Original Publication Date: 1990-Apr-30
Included in the Prior Art Database: 2004-Apr-05
Xerox Disclosure Journal
One problem with developing an image on a receiver and transferring it to a sheet is that of maintaining image integrity during transfer. An advantage of liquid toner materials is that they form a more stable image on the receiver and improve the efficiency of transfer. The particles comprising the wet image on a receiver in liquid toning are loosely flocculated. Often the particles only occupy about 10% by volume of the wet image with the remainder being the dispersion medium. One way of increasing the volume fraction of the toner and, therefor bringing the toner particles into a closer,and more coherent structure is to introduce to the image a liquid which is miscible with the dispersion medium but is not a solvent for the polymer used in the liquid developer. The addition of a nonsolvent to a polymer solution causes the polymer coils to collapse into a tightly packed cohesive structure. The structure is cohesive because the polymer essentially precipitates from the solution. One way of accomplishing this is to introduce the nonsolvent either as a vapor, or as a slight spray after the development zone and before the transfer station. Another way is to move a portion of the polymer/dispersion medium phase diagram where the polymer is no longer soluble. This may be achieved by changing the composition of the solvent, as previously described, or to change the temperature. Most nonaqueous polymer solutions have critical solution temperatures at which this occurs. This temperature change may be done in the transfer zone. If the polymer solution has a lower critical solution temperature, the transfer zone should be heated. If the polymer solution has an upper critical solution temperature, the transfer zone should be cooled. Another method of increasing the toner particle density in the wet image is to decrease the toner particle to particle repulsive forces by eliminating the electric charges on the toner particles. This can be accomplished by adding to the wet image a charging agent.