In imaging methods as e.g. electro(photo)graphy, magnetography, ionography, etc. a latent image is formed that is developed by attraction of so called toner particles. Afterwards the developed latent image (toner image) is transferred to a final substrate and fused to this substrate. In DEP the so called toner particles are imagewise deposited directly on a final substrate and fused to this substrate.
Toner particles are basically polymeric particles comprising a polymeric resin as main component and various ingredients mixed with said toner resin. Apart from colourless toners, which are used e.g. for finishing function, the toner particles comprise at least one black and/or colouring substances, e.g., coloured pigment.
In the different imaging methods, described above, the toner particles can be present in a liquid or in a dry developer composition.
In most cases the use of dry developer compositions is preferred. The main advantage of using a dry developer composition resides in the absence of the need to eliminate the liquid phase after development. The avoidance of the need to evacuate (mainly organic) liquids is desirable both from an economical standpoint and from an ecological standpoint.
However, in all techniques using dry particulate material to form an image, the images are built up by application of particulate marking elements in multiple, superimposed layers onto the substrate. The problems associated with multiple, superimposed layers of particulate marking particles that are in one way or another fixed on a substrate are manifold, not only with respect to image quality but also with respect to image stability and with respect to mechanical issues.
In, e.g. EP-A 471 894, EP-A 554 981, U.S. Pat. No. 4,828,950 and U.S. Pat. No. 4,885,603, it has been disclosed to apply a layer of transparent toner particles on top of the toner image to provide better resistance to physical damage.
In, e.g., U.S. Pat. No. 3,723,114 the problem of storage properties of fused toner images is addressed, the main problem being the fact that the toner images can, depending on the storage conditions, become tacky after storage. The problem is solved by using in the toner resin a substantial portion of thermosetting polymers.
The use of photo-curable toners has been suggested in, e.g., U.S. Pat. No. 5,470,683 to produce toner images having better weather resistance. In that application, a capsule toner is provided having a core comprising a polymerizable compound, a polymerization initiator and other normal toner ingredients. The core is surrounded by a hard shell that breaks during the fixing step. After the fixing step the polymerizable compound is polymerized, in this particular disclosure, by low energy visible light. Although following the teachings of these disclosures leads to the production of toner layers that are not easily damaged, the nature of the solutions itself limits the variety of resins that can be used in the manufacturing of the toner. Therefore further improvements along the lines of the disclosures referred to above are desirable.