Magnetic Ink Character Recognition (MICR) technology is well-known. MICR toners contain a magnetic pigment or a magnetic component in an amount sufficient to generate a magnetic signal strong enough to be readable via MICR. Generally, the toner is used to print all or a portion of a document, such as checks, bonds, security cards, etc. For example, most checks exhibit an identification code area, usually at the bottom of the check. The characters of this identification code are usually MICR encoded.
The document may be printed with a combination of MICR-readable toner and non-MICR-readable toner, or with just MICR-readable toner. The document thus printed is then exposed to an appropriate source or field of magnetization, at which time the magnetic particles become aligned as they accept and retain a magnetic signal. The document can then be authenticated by passing it through a reader device, which detects or "reads" the magnetic signal of the MICR imprinted characters, in order to authenticate or validate the document.
Xerox inventors developed a more secure toner that includes stabilized magnetic single-crystal nanoparticles. The value of magnetic anisotropy of Xerox magnetic nanoparticles is greater than or equal to 2.times 104 J/m3. The toner includes single crystal magnetic nanoparticles ranging in size from about 10 nm to about 300 nm their strongly retain their magnetic strength overtime, a property known as remanence.
According to Xerox inventors Richard P. Veregin, Karen A Moffat, Marcel P. Breton, Peter M Kazmaier, Patricia A. Burns And Paul F. Smith, the magnetic material exhibits sufficient remanence once exposed to a source of magnetization, in order to generate a MICR-readable signal and have the capability to retain the signal over time.
Generally, an acceptable level of charge, as set by industry standards, is between 50 and 200 Signal Level Units, with 100 being the nominal value, which is defined from a standard developed by ANSI (the American National Standards Institute). A lesser signal may not be detected by the MICR reading device, and a greater signal may also not give an accurate reading. Because the documents being read employ the MICR printed characters as a means of authenticating or validating the presented documents, it is important that the MICR characters or other indicia be accurately read, without skipping or misreading any characters.
For purposes of MICR toner, remanence of the magnetic material should be at least a minimum of 20 emu/g to enable sufficient magnetization of the toner for MICR without use of excessively high pigment loadings in the toner. High pigment loadings in the toner poses difficulties in the toner preparation process and may negatively impact toner performance, and therefore high pigment loadings are undesirable. A higher remanence value in the toner corresponds to a stronger readable signal from the toner image.
Remanence tends to increase as a function of particle size and the density of the magnetic pigment coating. Accordingly, when the magnetic particle size decreases, the magnetic particles tend to experience a corresponding reduction in remanence. Achieving sufficient signal strength thus becomes increasingly difficult as the magnetic particle size diminishes and the practical limits on percent content of magnetic particles in the toner composition are reached. A higher remanence value will require less total percent magnetic particles in the toner formula, improve suspension properties, and reduce the likelihood of settling as compared to a toner formula with higher percent magnetic particle content. Xerox researchers were able to increase the signal strength while reducing particle size.
Remanence tends to increase as a function of particle size and the density of the magnetic pigment coating. Accordingly, when the magnetic particle size decreases, the magnetic particles tend to experience a corresponding reduction in remanence. Achieving sufficient signal strength thus becomes increasingly difficult as the magnetic particle size diminishes and the practical limits on percent content of magnetic particles in the toner composition are reached. A higher remanence value will require less total percent magnetic particles in the toner formula, improve suspension properties, and reduce the likelihood of settling as compared to a toner formula with higher percent magnetic particle content. Xerox researchers were able to increase the signal strength while reducing particle size.
According to U.S. Patent Application 20090325098, the magnetic nanoparticle may be a ferromagnetic nanoparticle, such as FePt. The toner includes a magnetic material that minimizes the size of the particle, resulting in excellent magnetic pigment dispersion and dispersion stability, particularly in emulsion/aggregation toner processes. The smaller sized magnetic particles of the toner also maintain excellent magnetic properties, thereby reducing the amount of magnetic particle loading required in the toner.
Such single crystal ferromagnetic nanoparticles, including the smaller size non-acicular particles, have very high magnetic shape anisotropy. Accordingly, these single crystal ferromagnetic nanoparticles demonstrate the requisite high remanance and coercivity suitable for MICR toner applications as well as non MICR application
The magnetic nanoparticles may be prepared by any method known in the art, including ball-milling attrition of larger particles (a common method used in nano-sized pigment production), followed by annealing. The annealing is generally necessary because ball milling often produces amorphous nanoparticles, which desirably are subsequently crystallized into the single crystal form. The nanoparticles can also be made directly by RF plasma. Appropriate large-scale RF plasma reactors are available from, for example, Tekna Plasma Systems. The nanoparticles can also be made by a number of in situ methods in solvents, including water.
The magnetic single crystal nanoparticles are comprised of bimetallic or trimetallic particles. The magnetic single crystal nanoparticles may be comprised of Fe, Mn and Co metallic particles. The magnetic single crystal nanoparticles may also be made from, FePt, Fe Co, FeCo, CoOFe2O3, CoPt, BaO6Fe2O3, MnAl, MnBi, and mixtures thereof.
The magnetic nanoparticles may be of any shape, including cubical and hexagonal. Additional exemplary shapes of the magnetic nanoparticles include needle-shape, granular, globular, amorphous shapes, and the like.
The magnetic single crystal nanoparticles have a loading of about 0.5 weight percent to about 15 weight percent. The magnetic nanoparticles have a coercivity of about 300 Oersteds to about 50,000 Oersteds. The magnetic nanoparticles have a magnetic saturation moment of from about 20 emu/g to 70 emu/g.
Magnetite (iron oxide, Fe2O3) is currently a commonly used magnetic material in MICR toners. Because of the low overall anisotropy, both low shape anisotropy and low magnetocrystalline anisotropy, spherical or cubic magnetite have lower magnetic remanence and coercivity, and loadings higher than 40 weight percent of the total toner weight are often needed to provide magnetic performance.
While spherical and cubic magnetite have the desired smaller particle size of less than 200 nm in all dimensions, the much higher loading requirement also makes them very difficult to disperse and maintain a stable dispersion. Moreover, such high loadings of the inert, non-melting magnetic material interfere with other toner properties, such as adhesion to the substrate and scratch resistance. Consequently, this worsens the suitability of magnetites for MICR toners.
Xerox’s stabilized magnetic single-crystal nanoparticles overcome the problems associated with magnetites in MICR toners.
