factored by the B. F. Goodrich Co., for which a good solvent is acetone.
The other group of materials used herein is carbon in the form for example, of powdered lampblack, chimney black or channel black which is inert, inactive and/ or insoluble in the solvents aforementioned. The preferred embodiment of the invention contemplates coating thermoplastic resinous material 64 running in particle size from approximately 25 to 50 microns with carbonaceous material the particle size of which is on the order of from 8 to 25 millicrons.
Returning now to the operational description, the vaporous mixture 68 of air and solvent forced through the fluidized bed 66 attacks the thermoplastic material 64 causing it to soften and become slightly tacky in consistency and, as shown in FIG. 2, readily permits the smaller carbon particles 62 to adhere thereto. After a sufficient time the solvent is driven off and the carbon particles embed themselves in the surface portion of the resinous particles as shown in FIG. 3. The two powders thus become bonded together. Thereafter the fluidizing mass 66 is air dried as by closing valve 50 and continuing to vent air or gas therethrough. The fluid reactor keeps the particles separated thus preventing the formation of aggregates. Also, the insoluble powder i.e., carbon, acts as a parting agent should coated particles collide during processing. In this manner a dry powder product is produced. In the preferred embodiment this novel method results in an opaque powder having an excellent rich, black color and desirable resisitivity for good electrostatic printing. A small amount of a fine dust which might be carried to the top of the chamber 12 by the air column will be vented through the air outlet opening 30 in the lid of the container.
By virtue of the press-fitted relationship between the container 12 and the conduit portion 18 of the container may be removed to permit the decanting of the coated dry powder or conversely the coated dry powder may be scooped out of the container after the "bed" has been deactivated or shut-down.
In a typical example utilizing the disclosed method to produce a dry, black electrostatic printing ink having the desired degree of electrical conductivity and color the following formulation gave excellent results.
One gram by weight of powder lampblack was added to 10 grams by weight of polyvinyl acetate powder, dry mixed and placed in the reactor 10. After reacting approximately three minutes according to the hereinabove described method the powder product was removed from the reactor. Resistance measurements made on the power indicated an effective resistance of 20,000 ohms, a sufficiently high resistance for good electrostatic printing.
As applied to the particular case of producing an electrostatic ink, it is seen from the foregoing that carbon can be coated on various resin powders with the immediate advantage that a relatively small amount of carbon is required to obtain sufficient conductivity and black color saturation. The method is fast, and a single reactor can be used for the mixing, reacting and drying stages. The thermoplastic characteristic of the chosen resin is preserved since only the surface is carbon-loaded. The electrical resistance can be measured while the reaction is in progress, and the resistance measurement can
be used as a control of the reaction. Various resistances can be obtained from a single formulation by varying the reaction time accordingly. With suitably and properly chosen proportions of powders and reaction time, the
5 carbon will be completely absorbed by the resin surface, resulting in an ink with no fine carbon particles, producing virtually no smearing, of print. An additional important advantage of the present method is that if the two powders chosen for use are widely different in size,
jO the excess or "unreacted" fine particles can be removed easily by means of a relatively inefficient classifier, elutriator or winnower. In the preferred and illustrated embodiment of the present method the solvent is introduced into the reactor by means of a "bubbler." However,
15 sprays could be used for this operation. Or, the solvent could be vaporized by boiling. Temperature control of the bubbler, sprayer or vaporizer thus can be used as another means to control the quality of the end product. While the present invention is extremely useful in the
20 coating of thermoplastic resin powders with powdered carbon using solvents such as isopropyl alcohol or acetone, the process and method is not limited to these materials and has been used effectively with other and different materials. The present method is particularly
25 useful for intentionally forming aggregates and/or elutriating fines i.e., the smallest particles, as for example, where it is desirable to coat one type of powder material with particles of a similar powder material thus to form larger size powder particles.
30 What is claimed is:
The method of manufacturing electrostatic printing ing comprising the steps of:
(a) dry mixing ten grams by weight of polyvinyl acetate resin powder material of a particle size of from
35 25 to 50 microns with one gram by weight of inert lamp-black material of a particle size of from 8 to 25 millicrons, the latter having an opaque dense black consistency,
(b) introducing the mixture into a fluid bed reactor, 40 (c) passing pressurized dry air upwardly through said
mitxure to form a dense phased fluidized mass,
(d) passing an acetone vapor in which the resin is soluble through said dense fluidized mass whereby
r said resin powder is slightly softened and made relatively tacky so that particles of said lamp-black powder become partially imbedded in and bonded to the surfaces of said resin material, and
(e) air drying said fluidized mass with pressurized air 50 without the solvent to a powder consistency.
References Cited by th& Examiner
UNITED STATES PATENTS
2,638,416 5/53 Walkup 252—62.1
55 2,683,669 7/54 Coler 117—100
2,729,597 1/56 Garbo.
2,788,297 4/57 Louis 117—100
2,879,173 3/59 Yacoe 117—100
2,999,764 9/61 Rhoads 117—47
60 3,008,826 11/61 Mott et al. 3,036,338 5/62 Nack.
WILLIAM D. MARTIN, Primary Examiner. 65 J. GREENWALD, Examiner.