Differentiation of magnetic composites in terms of their nanostructural organization

Abstract

Important direction of technological progress is creation of substances, materials, and articles with controlled properties. Development of nanotechnologies will result in mass production of substances, materials, and articles containing nanoparticles. This poses new problems to forensics: identification and differentiation of nanomaterials by their properties. Determination of specific features of structure, composition, and other physicochemical properties of objects produced by nanotechnologies may form the basis for solving diagnostic and identification problems by forensic experts. The purpose of this work was to test a number of methods for studying nanostructures for forensic examination by the example of identification of signs of difference between documents printed by xerography. Such an expert problem takes place if it is necessary to establish the fact of substitution of pages of a contract or insertion of a text into a ready document. The objects of investigation were magnetic toners used in laser and multifunction printers (more than 50% sales in the market). Modern magnetic toners are sophisticated composites the base of which is ~5- µ m particles of a thermoplastic (e.g., a styrene‐acrylate copolymer). The particle surface is modified by aerosil, alumina, or titanium dioxide particles [1] to impart fluidity to toner powders. Among other compounds that are in the bulk of the particles and determine the stability of electrostatic, magnetic, and other properties of toner are magnetite nanoparticles (10‐15 vol %). In making a print, toner from a hopper is attracted to the surface of a roller under the action of a magnet from which the roller core is made. As the roller rotates, the toner on its surface passes through a narrow slit between a metering blade and the roller, contacts a photoconductive drum, and is attracted to the drum surface at places where a negative charge was neutralized by a scanning laser beam. Toner particles are transferred as an image from the photoconductive drum to paper because of electrostatic interaction between the drum and the charged paper surface and stuck to the paper owing to the thermoplastic properties of the polymer base of toner particles. Thus, a text made by xerography is small amounts (micrograms) of toner affixed by heat treatment to paper as printed characters. Modern printers use several hundreds of types of toners of similar compositions. In this work, we studied mainly toners produced by Hewlett-Packard (HP) and Canon and also samples of characters printed by printers of these companies. The samples were the characters “n” of font size 14 that were cut off from the body text as 3 × 3-mm paper squares. Morphological features of the surface of the text characters were investigated by electron microscopy with a Leo Supra 50 VP scanning electron microscope (Carl Zeiss, Germany) with a field-emission source at an accelerating voltage of 3‐10 kV using an InLens secondary electron detector. The chemical composition of toners and the element distribution over the character surface was were found by electron probe X-ray microanalysis with the same instrument using an INCA Energy+ energy-dispersive X-ray spectrometer (Oxford Instruments, UK) at an accelerating voltage of 20 kV within a selected range.