A study of the ferromagnetic microwires retention in cellulose matrix in the security papers

Abstract

Prepared by the Taylor – Ulitovsky technique, the glass-coated microwires are formed from a metallic core, with the diameter 3 to 50 μm, surrounded by an insulating layer from glass, with the thickness of 1 to 20 μm. Embedded in the cellulose matrix, the ferromagnetic glasscoated microwires allow their use as security element for the authentication of valuable papers in the electronic validation process. The authentication of the security paper is realised with a special detector, by “YES” or “NO” answer. This paper can be used as anti-shoplifting or validating elements to identify the counterfeit products. The paper presents the experimental results related to the retention of ferromagnetic microwires in the cellulose matrix, a complex process characterised by specific features, primarily due to the shape and diameter/length ratio of the microwires. The ferromagnetic retention yield was η = 65 – 90%, for the prepared papers with basis weight more than 50 g/m. Introduction Faced with increasing of goods counterfeiting, a wide range of methods are currently used to protect consumer goods, bank, state and commercial documents. Thanks to impossibility to produce security elements without proper equipment and under special conditions imposed by the very high degree of accuracy, the advanced technologies offer the solution, ensuring a high degree of protection against falsification. Investment and research efforts are being made to diversify the field of high security elements. The moment of launching the technology for glass-coated microwires (GMW) fabrication [1,2] has become revolutionary on the high-tech technology market, opening up the gates of a large variety of technological benefits for the existing applications and also setting the foundation for new applications [3-9]. The advantages of ferromagnetic GMWs securing [10] were: possibility of identifying at distance; stable magnetic properties even at high temperatures and corrosive media; wide range of functional temperatures; stability at shielding – the codes shielded by metallic panels can be read; stability at the mechanical action; small sizes and low consumption and, for the microfibbers from the last generation, with special properties, allowing the possibility to the information magnetic encoding): very large amount of the generated codes; the information can be read both from a stationary source and from a source in motion; the encoding is impossible to destroy, both in the Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 1-10 doi: http://dx.doi.org/10.21741/9781945291999-1 2 continuously and in variable magnetic field, (reliable encoding); possibility to read the information from any code randomly oriented in space. The structure of the paper consists of vegetable fibers (wood or non-wood), in which auxiliary materials, such as fillers, gluing agents, pigments, additives etc. are incorporated. Depending on the application field of paper, some structural features are imposed to the network: number of fiber-fiber contacts and of sizes of interfibrillar spaces, density and roughness of the surfaces. These properties are depending on the fibbers nature, on their processing degree, on the amount and properties of the auxiliary, and also on the processing technique used for forming and finishing of network. The term of filler defines any non-fibrous material added to the paper pulp to improve the optical properties of the paper, but also other features such as porosity, smoothness, printing ability etc. By incorporating of pigments into the paper pulp, the papers optical inhomogeneity increases, the amount of reflected and refracted light in the paper sheet increases, and the whiteness and opacity is improved. At the same time, the pigment particles retained in the sheet structure increase the interfibrillar spaces and reduce the possibility to set-up interfibrillar bonds having negative effects on the paper resistance indices. The fillers retention into the paper sheet is realised mainly by filtration for the particles with large sizes and by colloidal phenomena for fine particles. The introduction of filling is primarily determined by technical considerations, since certain characteristics of the paper, particularly optical indices and printing ability, are limited if only fibrous materials are used. Currently fillers can also serve as partial substitutes for fibrous materials in some cases, thereby helping to reduce the production costs. Developed by the Taylor-Ulitovsky process, the GMWs consist of a cylindrical metal core that is covered with a glass-insulating layer, the diameter of the metal core is 3 50 μm, and the thickness of the glass insulation is 1 20 μm. The length of such microwires, under laboratory conditions, reaches approx. 1 km. The ferromagnetic glass-coated microwires, cut at ca. 7 mm lengths, is included in the paper composition also as filling material, but in the paper pulp and in the paper sheet structure, the microwire segments have a certain behaviour that differentiates them from the classical materials of filling. Unlike these materials, the ferromagnetic microwires introduced into the paper in very small amounts do not significantly influence the rheological characteristics of the paste and the paper resistance characteristics. The appearance of wires, the diameter, length and the microwires density are also specific characteristics that differentiate the materials currently used to fill the paper. The importance of retention efficiency in the case of ferromagnetic microwires is primarily due to the need to achieve a certain microwires density in the paper sheet, in order to ensure its security without affecting the paper quality and functionality. 2. Experimentals The Taylor-Ulitovsky technique for GMWs preparation consists in placing in a high-frequency inductor of a glass tube with a metallic rod inside (Fig. 1). Under the influence of the generated electromagnetic field, the metal melts, forming a drop. In contact with the molten metal, a part of the glass tube softens and a coating is formed from the glass covering the drop. For a particular working regime [11], this glass soaked by pulling also trains the metal, leading to microwires formation, which is collected on the spool. Different metal core structures can be obtained: polycrystalline crystals of different sizes (microcrystalline, nanocrystalline) or amorphous. For experimental research were used Fe77B13Si10 GMWs, which are structurally, by X-ray diffraction and magnetically, by vibrating sample magnetometry, characterized. Achieving certain density of GMW in the paper sheet structure, as in the fillers case, depends on the action of factors with a particular influence on intelligent material retention in the papermaking process. Knowing and controlling these influences will ultimately allow finally reaching the density that is sufficient for paper securing. In this respect, have been experimented Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 1-10 doi: http://dx.doi.org/10.21741/9781945291999-1 3 several programs in which the basic recipe for realisation of the GMWs secured paper has been supplemented with several variables specific to each influenced factor studied. Fig. 1. Aspect during the Fe77B13Si10 ferromagnetic glass-coated microwires drawing. Table 1. The studied parameters and the experimented recipes used to for paper preparation Parameter S/H ratio, [wt. %] S/H Schopper – Riegler degree, [SR] Microwires amount, [g] Retentor amount, [%] Paper weight, [g/m] Nature of the fibrous materials 100 S or 100 H 30 0.005; 0.007; 0.009 Schopper – Riegler degree SR of the cellulosic material 60/40 30/20; 40/30; 50/40; 60/50 0.009 GMWs amount 60/40 30/20; 40/30; 50/40; 60/50 0,005; 0,007; 0,009; 0,011 Softwood / hardwood cellulosic pulp ratio from the fibrous composition of paper 20/80; 30/70; 40/60; 50/50 45/30 0.007 Amount of retention emulsion, dosed in the paper manufacturing receipts 60/40 30/20 0; 0.2; 0.4; 0.6; 0.8 Paper weight 60/40 30/20 50; 70; 90; 110 The GMWs with 7 mm lengths are embedded as filler into the cellulose matrix (the pulp), the main receipt of the mixture, in wt.%, being: bleached cellulose sulphate from softwood (S) pulp (different amounts: 30 100%; Schopper – Riegler degree: 30 60 SR); bleached cellulose sulphate from hardwood (H) pulp (different amounts: 30 100%; Schopper – Riegler degree 20 50 SR); paper filling material: 15% calcium carbonate; gluing emulsion: 1,5% alkyl–dimercetene (AKD); retention additive: 0,5% polyamide–amine and different amounts of GMW (for 10 sheets with paper weight q = 75 g/m) – 0.005g; 0.007g; 0.009g and 0.011g. The particularities of the experimented recipes for paper sheets preparation are chosen to highlight the influence of different Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 1-10 doi: http://dx.doi.org/10.21741/9781945291999-1 4 process parameters (Table 1). The Schopper-Riegler test provides a measure of the rate at which a dilute suspension of pulp may be dewatered. It has been shown that the drainability is correlated to the surface conditions and swelling of the fibbers, and constitutes a useful index of the amount of mechanical treatment to which the pulp has been subjected. The retention efficiency (η) was expressed as the ratio of the amount of GMW initially used to prepare the cellulosic paste and the remaining GMW amount in the laboratory prepared sheet (in each experiment, the retention yield was determined for 10 sheets of paper). 3. Results and discussions 3.1 Structural characterization for Fe77B13Si10 ferromagnetic glass-coated microwires After preparation, the GMWs were structurally characterized by X-ray diffraction investigations. The glass-coating was