Magnetic Pigments Research Articles

Magnetic particles derived from iron nitride

Due to its high saturation magnetization ( sigma /sub s/(0K) approximately=208 emu/g) and to the ease of obtaining it as a particulate material, Fe/sub 4/N could be a suitable magnetic material for replacing highly reactive metallic iron pigments ( sigma /sub s/(0K) approximately=218 emu/g) in high-density recording. Throughout a nitriding process of properly selected precursors Fe/sub 4/N can be obtained in either of two morphologies: acicular or isotropic. Further protection of the magnetic particles can be ensured through coating of the precursor particles prior to thermal decomposition in NH/sub 3//H/sub 2/ atmosphere or through substitutions with selected elements. The obtained passivated nitrides are characterized by high magnetic properties (H/sub c/ approximately=600 Oe; sigma /sub s/ approximately=150 emu/g) and chemical stability well adapted for use as magnetic pigments for high-density recording.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Magnetic pigments for recording information

The magnetic pigments used for longitudinal audio, video and data recording have undergone steady development from the invention of particulate magnetic tape in the 1930s to the present day. The current types are acicular γ-Fe 2O 3, Co-γ-Fe 2O 3, CrO 2 and α-Fe magnetic pigments, and they differ from their predecessors, i.e. the comparatively coarse spherical carbonyl Fe and cubic ferrite powders, in that they have only one magnetic axis, are very finely divided and have a narrow particle-size distribution. Finely divided, magnetic, multiaxial solids for combined longitudinal/vertical recording attract interest for future high-density mass storage systems. In addition, finely divided, plate-like shaped Ba ferrite pigments hold out prospects for increasing the current values for the storage density by a substantial factor. This applied particularly to vertical recording.

Rapid measurement of magnetic pigment particle size by transient magnetic birefringence methods

The particles in magnetic pigment dispersions pose special problems in their characterization. This is due to their anisometric shape and to the long-range interparticle magnetic attraction that causes aggregation of magnetic particles to be more rapid than in other pigment types. Many of the normally employed particle-sizing techniques are not well suited for magnetics because of the shape factor and/or the sample change during the measurement. Here we describe the development of apparatus and methodology to employ a rapid relaxation technique, transient magnetically induced birefringence (TMB), for the characterization of the particle-size distribution of model colloid particles and commercial magnetic pigments. The particle-size distributions of magnetic pigments during small media milling procedures have been studied by rapid transient magnetic birefringence techniques and by electron microscopy. It is shown that during extended milling of a low mill base viscosity dispersion particle fracture can occur; this is observed by a shift in the size distributions measured by both methods. This result suggests that overmilled pigment dispersions are likely to contain shorter particles and fines or other fragments.

Uniaxial compaction behavior of filled polymer powders

AbstractA simple method is described to obtain rheological data on filled polymeric materials in the form of powders. The powder is compacted in a cylindrical chamber by a plunger driven by the cross‐head in an Instron testing machine and the load‐displacement curve is recorded. Further information is obtained by compressing the powder to a fixed load and measuring the load decay with time (stress‐relaxation). The tests are illustrated by application to “monocomponent toner” powders used in dry copying or nonimpact printing processes. It is shown that compaction and stress‐relaxation data are able to differentiate between different toners and facilitate the prediction of their relative performances in terms of pressure fusing. A mechanical spring‐dashpot‐slider model is effective in describing the rheological behavior of these powders and its dependence on the loading of a hard filler (magnetic pigment). The latter affects the “slider” yield stress in the model but has no influence on the relaxation times.

Effect of a Side Chain Length of Polymer on Both the Adhesion and Dispersibility of γ-Fe2O3

Abstract Acrylic ester-acrylic acid copolymers were synthesized as model binder of magnetic paints. The dispersibility of γ-Fe2O3 was investigated as a function of the content of-COOH groups and the aliphatic side chain length. The following results were obtained. The saturated absorbance of copolymers on γ-Fe2O3 increased with the increase in content of-COOH groups, and then the dispersibility of γ-Fe2O3 was improved. However, when the content of-COOH group was over the 2 mol%, the saturated adsorbance of polymers was constant and the dispersibility of γ-Fe2O3 decreased with increasing number of carbon atoms in the acrylic ester side groups. However, the dispersibility of γ-Fe2O3 was explained by the difference in conformation of adsorbed polymers.

Rapid measurement of magnetic pigment particle size by transient magnetic birefringence methods

The particles in magnetic pigment dispersions pose special problems in their characterization. This is due to their anisometric shape and to the long-range interparticle magnetic attraction that causes aggregation of magnetic particles to be more rapid than in other pigment types. Many of the normally employed particle-sizing techniques are not well suited for magnetics because of the shape factor and/or the sample change during the measurement. Here we describe the development of apparatus and methodology to employ a rapid relaxation technique, transient magnetically induced birefringence (TMB), for the characterization of the particle-size distribution of model colloid particles and commercial magnetic pigments. The particle-size distributions of magnetic pigments during small media milling procedures have been studied by rapid transient magnetic birefringence techniques and by electron microscopy. It is shown that during extended milling of a low mill base viscosity dispersion particle fracture can occur; this is observed by a shift in the size distributions measured by both methods. This result suggests that overmilled pigment dispersions are likely to contain shorter particles and fines or other fragments.

Determination of the particle size distribution of magnetic pigments

Characterization of the particle size distribution (PSD) of a magnetic pigment enables the evaluation of the contributions to the noise power spectrum arising from the finite particle volume and the spread in particle size [1], The determination of the particle size distribution will be described for a γ-Fe 2 O 3 , a CrO 2 and a CoZn ferrite pigment using transmission electron microscopy (TEM) after sample preparation. Examination of the micrographs simultaneously yields information concerning particle morphology and formation of clusters. Sample preparation for TEM is described in detail. Measurements of the particle dimensions are obtained from TEM micrographs with a digitizer. The PSD is log normal distributed. Results are compared with BET measurements and discussed with respect to error and reproducibility. Some comments on the effect of the particle size distribution on the noise power spectrum are given.

Anorganische Pigmente: Disperse Festkörper mit technisch verwertbaren optischen und magnetischen Eigenschaften

AbstractThe great importance of inorganic pigments arises from the fields of information, safety technology, surface protection, and creation of a colourful environment, whereas the economic reach is underlined by a world production of approx. 4 million to per year.—For a controlled synthesis of technologically relevant pigmentary properties it is necessary, starting from solid matters selected by physico‐chemical criteria, to take preparative measures which interfere in the chemistry of solid matters and of the pigment/binder interface and allow—by controlling the particle size and particle size distribution—the desired optical and magnetic properties to be optimized. Such measures have to be supported by fixing an optimum degree of dispersion in the binder.—Among the numerous pigments presently being used, TiO2 has been chosen as a representative of scattering white pigments. Scattering and absorbing colour pigments are exemplified by cadmium sulpho‐selenides, oxide pigments and mixed oxide pigments. Moreover the paper gives an insight into the production and action of magnetic pigments as well as a survey of further pigment groups.

記録用磁性材料及び複合磁性材料に関する研究 ＸＶ アクリル酸エステル‐アクリル酸共重合体‐磁性粒子間相互作用と磁性塗料中における粒子の分散性

記録用磁性材料及び複合磁性材料に関する研究 ＸＶ アクリル酸エステル‐アクリル酸共重合体‐磁性粒子間相互作用と磁性塗料中における粒子の分散性

PREPARATION AND PROPERTIES OF PARTICLES FOR MAGNETIC RECORDING

Selected contributions in the fields of iron oxide, metal and CrO2 magnetic pigments are presented. Several wet chemical synthesis of undoped and cobalt bulk doped isometric Fe3O4-particles from Fe(OH)2 and iron(III) compounds are compared with regard to the influence of composition and geometrical dimensions of the ferric coreactant on the particle size and the magnetic properties of the product. Oxidation of cobalt bulk doped magnetite particles leads to pigments with multiaxial magnetic behaviour. The structure of passivation layers on metal pigments is analyzed by using different techniques. Thicknesses of 4 - 5 nm are measured 0.5 to 1 nm of which are due to surface coatings with phosphate compounds which are used to prevent sintering of the particles. Several reduction methods for the preparation of elongated metal pigment are compared. Details of the reaction mechanism of the hydrothermal synthesis of needle shaped CrO2 in the presence of antimony are discussed. CrO2 pigments qualitatively show the same behaviour with regard to the dependence of coercivity on particle size as described earlier for other materials. Maximum coercivity of 60 kA/m for Sb-Fe-doped CrO2 is obtained with crystallite sizes (220) of 23 nm. Chemically stabilized CrO2 pigments are protected by layers of 0.7 to 1.3 nm thickness only. For use in magnetic recording media both, metal pigment and Cr2O2 can thus be described by a core-shell-structure.

Measurement of the critical pigment volume concentration by means of mercury porosimetry

In this paper a relation is derived which shows that, above the critical pigment volume concentration (CPCV), the porosity of a tape, expressed as pore volume divided by the mass of the coating layer, is a linear function of the weight fraction of magnetic pigment. From this equation, the CPVC and the density of the binder material can be obtained if the density of the pigment is known. The validity of the equation is demonstrated by measuring the porosity of magnetic tape as a function of pigment loading by means of mercury intrusion porosimetry. Furthermore, it has been established for uncalendered coatings that above the CPVC the magnetic saturation moment remains constant, indicating a constant pigment volume fraction per unit of coating volume.

Transverse magnetic field stability of magnetic pigments

A simple screening method is presented, which allows the prediction of the time-dependent properties of a magnetic tape like erasability and print-through by using static magnetic and Mossbauer measurements. This method is applied to tapes coated with Co-modified iron oxide particles which are known sometimes to show problems with erasability and print-through. The reasons will be discussed within the framework of a theoretical explanation.

Magnetic layer design considerations for highly durable recording tape

AbstractAmong magnetic recording tapes for such applications as mass storage systems, the antiabrasion properties of the magnetic layer along with storage life are major factors determining magnetic tape life. This paper examines the effect of magnetic layer structural parameters which affect durability and the signal‐to‐noise ratio. Several magnetic tape layer design conditions aimed at improving magnetic tape reliability and recording density are then presented. That is, in order to effect a highly durable binder structure, it is necessary to achieve appropriate mechanical characteristics (tensile strength ≥1 kg/mm2 and elongation at break ≥40%) by crosslinking a large‐matrix, high‐molecular weight polyurethane as well as carefully adjusting the balance of each additive. On the one hand, lapping treatment is effective in improving the SN ratio, with a decrease of up to two‐thirds in surface roughness possible. At the same time, it is found that magnetic pigment content is an important factor limiting durability and that it is a sensitive structural parameter which must be determined based on its inverse relation to the SN ratio. Using a specially developed binder, we demonstrate that a highly durable magnetic tape capable of withstanding more than 108 head scan passes is possible, that the use of lapping treatment can improve the SN ratio by 4 dB, and that a suitable value for the magnetic pigment content is on the order of 68‐72%.

Radiation curable polymer blends for magnetic media

Binder resins in magnetic coating formulations must fulfill a diverse set of requirements. These polymers must have the ability to accept high pigment loadings while maintaining good abrasion resistance, substrate adhesion, inherent lubricity and resistance to temperature and humidity effects. In addition, they must act as grinding aids in the dispersion of the magnetic pigment. In the thermally converted coatings now in use, these requirements are ususally met by combining several polymers and crosslinking agents into an optimized blend. This approach is also effective in designing radiation curable binder systems. An overview of the methods employed to acshieve such optimized blends will be discussed.

EB curable wetting resins for magnetic media coatings

The magnetic media industry is studying means to improve the recording density, durability, product uniformity and production efficiency and to reduce wetting agent migration in the magnetic film. The use of electron beam curable resin binders for magnetic coatings is one of the approaches being studied for this. This paper compares the wetting efficiencies of several electron beam curable systems with a conventional resin and a conventional wetting agent. In this study it has been demonstrated that EB resins can be designed to effect proper magnetic pigment dispersion.

Soft magnetic toners: A simple analytic model

Soft magnetic toners, made of magnetic pigments dispersed in a polymer matrix, are modeled. The pigments, roughly spherical in shape, are assumed multidomain and isotropic. The input parameters are the initial intrinsic susceptibility of the pigments, their volume fraction and the particle packing factor of the bulk powder. The analyzis proceeds through 3 steps : (a) microscopic analysis at pigment level, (b) macroscopic analysis at toner particle level, (c) gigascopic analysis at bulk powder level, which usually is the only source of measured data. Demagnetizing fields are taken into account as well as interaction fields which are synthetically modeled to match known limiting cases. A set of analytic formula is derived that, for each of the above levels, relates the apparent initial susceptibility to the input parameters. Applications and experimental verifications are described.

Measurement of the critical pigment volume concentration of particulate magnetic tape

In this paper a relation is derived, which shows that above the critical pigment volume concentration the porosity of a tape, expressed as pore volume divided by the mass of the coating layer, is a linear function of the weight fraction of magnetic pigment. From this equation the CPVC and the density of the binder material can be obtained if the density of the pigment is known. The validity of the equation is demonstrated by measuring the porosity of magnetic tape as a function of pigment loading by means of mercury intrusion porosimetry. Further it was found that above the CPVC the magnetic saturation moment stays constant indicating a constant pigment volume fraction per unit of coating volume.

Studies on adsorption of poly(vinyl butyral) and dispersibility of γ‐fe2o3 in magnetic paints

AbstractThe adsorption of poly(vinyl butyral) (PVB) at the γ‐Fe2O3–benzene interface and the air–water interface was investigated by means of the adsorbance on γ‐Fe2O3 and surface pressure (π)–area (A) curves. The adsorption isotherms of PVB on γ‐Fe2O3 from the various solvents are of the Langmuir type and can be explained by competitive adsorption between PVB and the solvents. From the π–A curves of monolayers and the adsorbance on γ‐Fe2O3, it was confirmed that the adsorbed PVB on γ‐Fe2O3 is in a train‐loop conformation with the hydroxyl groups and the butyral segments directed toward the solution phase. The packing density of γ‐Fe2O3, particles in the magnetic coatings preferentially depends on the conformation of polymers on the particle surface. The PVB–solvent system which gives the most compact train‐loop conformation gives the highest dispersion and packing of γ‐Fe2O3 in the magnetic coatings.

Performances of metal pigment tapes with 600-700 Oe coercivity

Audio or video recording performances of various magnetic tapes including Co-modified γ-Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> , metal and Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> N tapes having their coercivity range of 600-700 Oe were investigated. The tapes were prepared by the conventional method using the above magnetic pigments which were prepared to give their coercivity range of 600-800 Oe. The electromagnetic characteristics of the tapes were discussed in comparison with the commercial reference tape. The metal tapes were superior to the other two tapes on Sensitivity (S), Frequency Properties (F), Maximum Output Level (MOL) and Dynamic Range (D.R.) The Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> N tape has an excellent property on Distortion (THD).

Mercury porosimetry analysis of magnetic coatings

Mercury porosimetry can be used to characterize the structure of magnetic tape coatings but to get accurate information, compressibility corrections for the sample cell and basefilm must be made. Equations to make these corrections have been developed and published previously. These equations have been used to help interpret recent studies of tape coatings. These analyses have shown: 1. The Critical Pigment Volume Concentration (CPVC) of a magnetic pigment can be determined by measuring the mercury porosity of actual magnetic tapes. This CPVC can differ from the CPVC determined from tensile measurements of thick uncalendered coatings. 2. Postcuring of magnetic metal powder coatings was followed by measuring porosity and compressibility with mercury intrusion. The curing with isocyanate follows a first order reaction kinetics and is a relatively slow process, even after 7 days of storage the coating is not fully cured. 3. Multipass durability of (metal powder) audio tapes is directly correlated with porosity of the coating layer. For durable tapes porosity must be absent.