Patent Number: 044143397
Section: description

The following examples are given to illustrate the invention and should not be construed as limiting its scope. Unless otherwise indicated, all parts and percentages are by weight. EXAMPLE 1 A. Preparation of Aqueous Dispersion of Fe.sub.3 O.sub.4 (ELM Absorber) An aqueous dispersion of magnetic iron oxide (Fe.sub.3 O.sub.4) (ELM absorber) is prepared by mixing aqueous solutions of ferric and ferrous salts in amounts to maintain the Fe.sup.+3 /Fe.sup.+2 molar ratio at .about.2:1. Magnetic iron oxide is then precipitated at 0.degree.-10.degree. C. by rapid addition of 1 N NH.sub.4 OH and vigorous agitation until a pH of 9-10 is reached. Immediately thereafter, the dispersant is introduced with agitation to the aqueous medium containing the precipitated iron oxide and the mixture is heated at 90.degree. C. for one hour. During this period, hydrochloric acid is added until the pH of the mixture reaches 7.5. The particles of precipitated iron oxide are washed with deionized water and redispersed in deionized water containing .about.0.5 g of a potassium salt of a functionalized oligomer (Polywet KX-4 sold by Uniroyal Chemical) per gram of precipitated iron oxide, by using an ultrasonic probe. Magnetization of the dispersed iron oxide is measured by a Collpits oscillator circuit technique. B. Preparation of Magnetic Latex (Dielectric/ELM Absorber) To a 3-neck flask equipped with a stirrer, two addition funnels and a condenser is added a mixture of 507 g of the 28.5 percent solids dispersion of Fe.sub.3 O.sub.4 (200 gauss and average particle size of less than 0.08 micrometer) and 203 g of deionized water. The mixture is then heated under nitrogen atmosphere to 90.degree. C. while stirring the mixture. At this temperature of 90.degree. C., a monomer stream and an aqueous surfactant stream are separately introduced via the two addition funnels into the flask, each stream being introduced at the rate of .about.6 ml/min over a period of 65 minutes. The monomer stream consists of 64 g of styrene, 16 g of butyl acrylate and 3 g of t-butyl hydroperoxide. The aqueous stream consists of 110 g of deionized water, 2.9 g of the potassium salt of a functionalized oligomer ("Polywet KX-4") and 2 g of sodium formaldehyde hydrosulfite. The resulting reaction mixture is stirred and maintained under nitrogen at 90.degree. C. for an additional half hour. The resulting 25 percent solids latex is concentrated by distillation under vacuum to a 30.3 percent solids latex (dielectric/ELM absorber) having dispersed particles with a polymeric as well as magnetic characteristic. The particles of this latex have a narrow particle size distribution and an average particle diameter of 0.11 micrometer as determined by hydrodynamic chromatography. The latex remains stable in an applied magnetic field of 1800 gauss and exhibits properties common to magnetic colloids. For example, such magnetic colloids are magnetizable liquids that are instantly demagnetized upon removal of a magnetic field and levitate an object upon application of a magnetic field. Magnetization of the latex by a Collpits oscillator circuit technique, described by E. A. Peterson et al. in the Journal of Colloidal and Interfacial Science, 70, 3 (1977), is estimated to be 135 gauss. The particles of the latex are recovered by freeze drying the latex at -80.degree. C. under vacuum at 0.5 mm Hg. C. Preparation of ELM Composition (Dielectric/ELM Absorber/ELM Attenuator) One ELM composition (Sample No. 1) is prepared by dry blending 50.3 g of a dry powder of the aforementioned latex (55.4 percent dielectric/44.6 percent Fe.sub.3 O.sub.4) with 33.5 g of carbonyl iron (ELM attenuator) having an average portion size of 3-4 micrometers and sold by GAF Corporation under the trade name Super Fine Special. The blending is carried out on a Brabender mixing apparatus and the resultant blend is then compression molded into flat plates (0.8 cm thickness.times.2.6 cm diameter) at 2000 pounds of positive pressure and 230.degree. C. for 2 minutes. The sample is cooled to room temperature and the pressure on the sample is released. The resultant plate of the ELM composition is machined into two flat disks having a diameter of 2.54 cm and a thickness of 0.64 cm and 0.32 cm, respectively. A second ELM composition (Sample No. 2) is prepared following the foregoing procedure using 56.5 g of the dry powder of the latex and 18.8 g of the carbonyl iron. The sample is similarly blended, molded and fabricated into disks. For purposes of comparison, a third sample (Sample No. C) of dry particles of the latex is molded and fabricated into disks by the foregoing procedure. All of the foregoing samples are tested for ELM absorption and the results are reported in Table I. TABLE I __________________________________________________________________________ ELM Absorption (3) Sample Components, (1) % Density (2), Frequency Magnetic Attenuation No. Fe.sub.3 O.sub.4 Fe Polymer g/ml gHz Permeability Tangent M dB/cm __________________________________________________________________________ 1 26.8 40 33.2 2.57 0.3 2.662 0.173 0.262 1 2.244 0.393 1.658 2.4 1.382 0.586 4.548 5 1.198 0.246 3.899 8.5 1.330 0.154 4.540 2 33.5 25 41.5 2.16 0.3 2.210 0.177 0.224 1 1.741 0.391 1.329 2.4 1.230 0.533 3.606 5 0.991 0.190 2.610 8.5 1.139 0.102 2.810 C* 44.7 0 55.3 1.75 0.3 1.683 0.173 0.191 1 1.509 0.305 0.957 2.4 1.068 0.437 2.689 5 0.849 0.176 2.197 8.5 0.955 0.056 1.650 __________________________________________________________________________ *Not an example of the invention (1) Components of composition given in weight percent based on the weight of the composition. Polymer is a copolymer of styrene and butyl acrylate as described hereinbefore. (2) Density of the compositions on a dry weight basis. (3) The ELM absorption characteristics are measured by the procedures described in Dielectric Materials and Applications edited by Arthur R. Vo Hippel and published by the M.I.T. Press, Massachusetts Institute of Technology, Cambridge, Massachusetts, March, 1966. As evidenced by the data in Table I, the compositions of the present invention (Sample Nos. 1 and 2) exhibit significantly better attenuation at a given frequency than does the composition of Sample No. C.