Abstract:
Water based magnetic colloidal fluids, useable as inks, when prepared by coating chemically precipitated magnetite (Fe 3  O 4 ) with an adsorption site providing coating agent including certain organic anions, such as sulfates, sulfonates or amino carboxilates, and then dispersing the coated product with non-ionic, anionic or cationic surfactants may exhibit selectably cationic, anionic or non-ionic charge responsiveness.

Description:
BACKGROUND OF THE INVENTION 
     Magnetic colloidal fluids are essentially made of a solid magnetic material throughly dispersed in a liquid carrier. The magnetic material, usually magnetite (Fe 3  O 4 ) in extremely divided form is submicron in particle size produced by chemical precipitation, and the particles are subsequently coated with a layer of a surfactant retaining material, the coated particles are then permanently suspended in a carrier vehicle such as water mixed with a desired cationic or non-ionic surfactant. The resulting mixture is a colloidal magnetic fluid having several uses one of which is as a magnetic ink. A dispersing agent is usually included in the colloidal magnetic fluid formulation to prevent aggregation of the magnetic particles which could otherwise lead to flocculation and separation of the particles from the fluid. 
     In the development of the art of such magnetic colloidal fluids a limitation has been encountered in that the properties of the surfactant retaining coating which is usually a fatty acid such as oleate operates to limit flexibility in the properties of the surfactant so that particles receiving a certain coating are then useable only to produce a fluid with a certain charge characteristic. 
     Therefore, it is a general object of this invention to provide a technique of coating magnetic particles so that the properties of a resulting colloidal magnetic fluid containing them can be changed, by changing the net electrical charge applied to the magnetic particles. 
     Another object of this invention is to provide an aqueous based magnetic ink, by dispersing the so-coated Fe 3  O 4  into exclusively non-ionic surfactants, giving a non-ionic ink, in which particles have no net electrical charge. 
     A further object of this invention is to form a magnetic ink in which the so coated Fe 3  O 4  is dispersed in an anionic surfactant yielding an anionic ink, in which the particles have a net negative charge. 
     Another object is to form a magnetic ink by dispersing the so-coated Fe 3  O 4  in a mixture of cationic and non-anionic surfactants. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the present invention, finely divided magnetic particles are synthesized by chemical means and rendered dispersible into a colloidal water based vehicle system. 
     One general process for chemical synthesization involves forming separate aqueous solutions of ferrous chloride and ferric chloride. The solutions are mixed to maintain a molar ratio of Fe +3  /Fe +2  slightly under 2.0. Magnetite (Fe 3  O 4 ) can be formed by chemical precipitation of the ferrous/ferric mixture with a base such as ammonium hydroxide. In order to form a high rate of magnetite nucleation with a base such as ammonium hydroxide, growth temperatures of 5° C to 30° C at a pH of about 9.5 is used. 
     The Fe 3  O 4  particles are preferably 70 to 200A in diameter and are, in accordance with the invention, precoated with an adsorption site providing coating agent that serves the function of providing a coating on the particles that in turn is susceptible to combination with a selectable charge directing surfactant. Such an adsorption site providing coating may be an organic anion such as a sulfate, for example, sodium lauryl sulfate, it may be a sulfonate, such as sodium alkyl polyether sulfonate or it may be amino carboxilate, for example, N-alkyl mono or disodium amino propionate. 
     The adsorption site providing coating agent coats with the object of preventing agglomeration during particle preparation and initial vehicle dispersion and then serves to make possible combination with a surfactant with a selectable charge. 
     The Fe 3  O 4  particles, coated in accordance with the invention, are then capable of being used in a colloidal fluid to which can be imparted a selectable charge characteristic by the addition of a suitable surfactant that serves a bridge between the magnetite and the dispersion medium while also directing the electrical charge conditions of the particles. 
     When the coated Fe 3  O 4  particles are dispersed with use of a non-ionic surfactant agent the result is a non-ionic colloidal fluid. In this case, the particles do not possess a net electrical charge, but are maintained in stable dispersion by the effect of entropic repulsion forces provided by the non-ionic surfactant. The dispersion is also enhanced by a large hydration layer where the dispersion medium or vehicle is water. 
     Further, in accordance with the invention, precoated Fe 3  O 4  particles may be dispersed into fluids by using a combination of non-ionic surfactants and + or - charge directing agents. The appropriate + or - charge directing agents may now yield positive or negatively charged particles. The anionic (-) or cationic (+) charge directing agents operate to impart a relatively large charge on the magnetic particles, for example, producing a zeta potential of -30 to 100 mV or +30 to +100 mV respectively. This increases the colloidal fluid stability by electrostatic repulsion. 
     One of the applications of colloidal magnetic fluids is for magnetic ink jet printing where a stream of ink is supplied under pressure and periodically pulsed to produce droplets which impinge upon a sheet of moving paper. The droplets of magnetic ink emerging from a nozzle have to be of uniform size, uniformly spaced from each other and emerging at a rate which can be of the order of 10 4  to 10 5  drops per second. Water is usually used as the vehicle because of its favorable viscosity and volatility properties. 
     In order to prepare an aqueous based magnetic printing ink, it is necessary to have available magnetic particles of small size characterized by high magnetic moment. Using magnetite (Fe 3  O 4 ) as an example, the particle size range should be about 50 to b 300A, preferably about 75 to 200A. Magnetic moment for aqueous magnetic inks should desirably be within the range of about 65 to 89 emu./gram of dried, magnetic material in air or vacuum with a magnetic moment of preferably about 70 emu/gram at room temperature. 
     Ferric chloride and ferrous chloride or ferrous sulfate are dissolved in separate bodies of water to form solutions thereof. The solutions are mixed in amounts to obtain the molar ratio Fe +3  /Fe +2  slightly under the value of 2.0. Magnetite, (Fe 3  O 4 ), can be formed by chemical precipitation of the ferrous-ferric mixture with a base, such as ammonium hydroxide. In order to favor a high rate of magnetite nucleation coupled with a slow rate of particle growth, chemical precipitation can be carried out preferably in an ultrasonic bath maintained at about 25° C. The pH of the mixture during chemical precipitation for deposition of magnetite may be maintained between 8.9 and 10.2, with the preferred pH being 9.5. 
     After precipitation is complete, the adsorption site providing coating agent is applied to the particles. A solution of organic anions such as sodium-lauryl sulfate or N-alkyl mono or disodium amino propionate is added. The coating material should be added very slowly, over a period of 10-45 minutes in order to achieve better distribution around the magnetic particles. The slow addition with agitation of the coating agent provides more uniform coverage of the Fe 3  O 4  particles. This in turn increases the ink yield since the non-coated Fe 3  O 4  does not remain suspended and hence is not transformed into ink. 
     In accordance with the invention the coating of the Fe 3  O 4  particles not only accomplishes the prevention of agglomeration of the particles as done heretofore in the art but it also provides &#34;adsorption sites&#34; for the attachment of the molecules of the non-ionic surfactant that in turn provides the wettability of the Fe 3  O 4  by the water medium. 
     Among the materials found suitable to provide the initial Fe 3  O 4  coatings are: 
     organic sulfates of the general formula; ##STR1## where R = C 12  or larger 
     for example, sodium-lauryl sulfate 
     
         CH.sub.3 --(CH.sub.2).sub.11 -- O . SO.sub.3 . Na 
    
     amino carboxilate anions, of the general formula: ##STR2## where R = C 12  to C 18   
     for example, sodium lauryl amino propionate 
     
         C.sub.12 H.sub.25 NH -- CH.sub.2 CH.sub.2 COO -- Na.sup.+ 
    
     and others of similar composition. 
     The coated magnetic particles in aqueous suspension are then heated to about 100° C, to increase the magnetic moment thereof and to cause precipitation of the coated magnetite particles for cleaning. The particles are then rinsed with distilled water to remove NH 4  salts. Final separation can be carried out in any conventional manner, such as by use of a centrifuge. 
     Once the adsorption site coated magnetite particles (Fe 3  O 4 ) of the preferred 75 to 200A size have been obtained or prepared as above, the particles are placed in a colloidal suspension by dispersing with an appropriate charge directed surfactant. The coated particles may be dispersed with only a non-ionic surfactant yielding an ink with no charge; they can be dispersed with non ionic + cationic surfactants yielding a cationic ink with a net positive charge on the particles; or they can be dispersed with the other combination of non-ionic + anionic surfactants yielding an ink in which the particles have a net negative charge. 
     The charge directing surfactant always has as an essential component a non-ionic component that serves as a wetting agent and is selected to provide a bridge between the lyophobic disperse phase, i.e., magnetite, and the dispersion medium of water. 
     Other non-ionic surfactants may also be used such as polyoxyethylene. Similarly alkylarylpolyether alcohols or alkylphenol ethers of polyethylene glycol wherein the alkyl chain is C 8  to C 24  and containing from 8 to 15 oxyethylene units can be employed. For example, compounds of the formula below may be used: ##STR3## where R is an alkyl chain and x designates the number of oxyethylene units and ##STR4## is a benzene ring. 
     The octyl or nonyl compounds wherein x is 9 or 10, are preferred due to their excellent water solubility and reasonable viscosity, for example, nonyl phenol or tertiary octyl phenol polyoxyethylenated with 9 to 10 moles of ethylene oxide. 
     FLUIDS WITH NON-CHARGED PARTICLES 
     The amount and type of non-ionic surfactant agent is selected to provide an interfacial tension between magnetite particles and water of about 24 to 36 dynes/cm, preferably about 30 to 34 dynes per cm. Generally, about 5 to 10 weight percent based on magnetite of non-ionic surfactant will be sufficient, preferably about 7%. 
     Water based inks prepared only with a non-ionic surfactant maintain their stability due to entropic repulsion forces and to the large hydration layer. They are isotropic and free of magnetic remanence. 
     FLUIDS WITH POSITIVE CHARGE PARTICLES 
     In water based inks employing the non-ionic surfactant + cationic agent combination, the type and amount of cationic surface active agent is selected to impart a zeta potential of about +30 to +100 millivolts, thus providing particle stability due to electrostatic repulsion between the positive charged particles. 
     Usable cationic agents are the quaternary ammonium compounds, of the general formula, ##STR5## where R and R&#39; are the same or different branched or chain Alkyl or Benzyl between 8 and 24 carbon atoms, preferably C 12  to C 18 , and 
     X -   is an anion such as chloride. 
     The concentration of the cationic agent used is of the order of 2 to 10% by weight, based on the magnetite, which in turn is preferably 4 to 8% by weight/100 grs. of Fe 3  O 4 . A preferred type of cationic agent used is one in which the (CH 3 ) groups of the general formula above, are replaced by ethoxylated or methoxylated groups, which increase the compatibility of the agent with water, for example, the ethoxylated ammonium compound of the formula: ##STR6## where R is one of chains C 12  to C 18 . 
     FLUIDS WITH NEGATIVELY CHARGED PARTICLES 
     In the water based inks prepared by dispersing the coated Fe 3  O 4  with a combination of non-ionic surfactants + anionic agent, the anionic agent used can be selected from one of the groups of polyether sulfonates or long chain organic carboxylates, for example, sodium alkyl aryl polyether sulfonate: ##STR7## where R = C 12  to C 18   
     or, for example, alkyl beta amino sodium propionate. ##STR8## where R = C 12  to C 18   
     where the negative charge on the carboxylic or sulfonic groups provides electrostatic repulsion. 
     The amount of the anionic agent to be used is of the order of 2 to 12% based on the magnetite, which in turn is preferably 4 to 8 grs for 100 grs of Fe 3  O 4 . 
     Finally, for all inks, to complete the dispersion and to adjust the viscosity and other properties of the ink, a final addition of a mixture of glycerol and polyetheylene glycol is carried out, in the proportion of 15% by weight of polyethylene glycol 200 and 10% by weight of glycerol, based on amount of Fe 3  O 4 . 
    
    
     EXAMPLES 
     The following examples illustrate the preparation of magnetic inks, in accordance with the present invention. 
     EXAMPLE 1 
     Step A -- Coating of Fe 3  O 4  with an organic sulfate 
     378 of FeCl 3  6 H 2  O and 160 grs FeCl 2  . 4 H 2  O are dissolved together in 2 liters of distilled water. In this case it is essential to use FeCl 2  instead of FeSO 4  in the precipitation of the magnetite because sulfate ions interfere with the coating properties of the organic sulfate. To the mixture, maintained at room temperature and under mechanical and ultrasonic stirring, 500 cc of NH 4  OH solution is added. One minute after precipitation, start adding a solution of 16 g of sodium lauryl sulfate dissolved in 250 cc of water. This addition is done very slowly at a rate of about 5 cc/minute. 
     Once precipitation and Fe 3  O 4  coating is completed, the beaker containing the coated magnetite is next heated to boiling and then set aside to settle. The excess liquid is decanted and the coated Fe 3  O 4  is rinsed 6 times with about 2 liters of hot distilled water, removing each time after settling the excess water by decantation. After final wash, the suspension is centrifuged for 1/2 hour. The product at this stage is typically a wet mass of about 450 grs. 
     Step B -- Making of non-ionic ink 
     The wet coated magnetite (about 450 grs) is added to a suitable mixing apparatus such as an attritor. Thereafter, a solution of non-ionic surfactant is added thereto. This solution is prepared by first dissolving 30 grs of a polyoxyethylenated nonyl phenol in 100 ml of water. Next, the dispersing agent of 20 grs of glycerol and 26 grs of polyethylene glycol (Mol. weight 200) is added thereto. The attritor is started slowly and then run at full speed for 2 hours. Following 2 hours in the attritor, the mixture is transferred to a beaker and heated in a boiling bath of water for 3 hours, with stirring, after which it is cooled to room temperature and centrifuged for 45 minutes at 3000 rpm. The fluid remaining after decantation is usable as magnetic ink, with the following characteristics: 
     
         ______________________________________Non-ionic (uncharged particles) inkmagnetic moment        18 to 23 electromagnetic units (emu)weight of ink        400 to 460 gramsviscosity    8 to 13 centipoises measured at 60 rpm        with Brookfield Viscometersurface tension        29 to 32 dynes/cmpH           5.5 to 6.5resistivity  400 to 500 Ω cm______________________________________ 
    
     The ink will pass 3 micron filter mesh. 
     Making a positively charged particle ink 
     For the preparation of a + charged particle ink known in the art as a cationic ink, Fe 3  O 4  coated with organic sulfate, is prepared as in Step A. 
     Step B is also followed with the following exception: 
     To the Fe 3  O 4  in the attritor add to the mixture of 30 g of the non-ionic polyoxyethylenated nonyl phenol surfactant, 10 g of a cationic agent, ethoxylated quaternary ammonium, followed by the dispersing agent 20 grs of glycerol and 26 grs of polyethylene glycol M.W. 200. The rest of the procedure is the same as above, resulting in an ink with the following characteristics: 
     
         ______________________________________Cationic ink (Fe.sub.3 O.sub.4 having a net positive charge)magnetic moment   20 to 25 emu/grviscosity         10 to 15 cpspH                5.5 to 6.5resistivity       200 to 300 Ω cmsurface tension   28 to 32 dynes/cmweight of ink     450 to 500 grs______________________________________ 
    
     Making a negatively charged particle ink 
     For making negatively charged particle inks, known in the art as anionic inks, the organic sulfate-coated magnetite, prepared in step A is used. 
     In step B, to the magnetite in the attritor is now added to the surfactant mixture of 30 g of polyoxyethylenated nonyl phenol, an anionic agent, 8 g of disodium amino propionate dissolved in 100 ml of water. The glycerol and ethylene glycol dispersing agent being the same, the proedure as above will provide an ink with the following characteristics: 
     
         ______________________________________Anionic ink (negatively charged Fe.sub.3 O.sub.4 particles)magnetic moment   24 to 27 emu/grviscosity         20 to 30 cpspH                4.5 to 5.5surface tension   28 to 33 dynes/cmresistivity       150 to 250 Ω cmamount of ink     450 to 550 grs______________________________________ 
    
     EXAMPLE 2 
     Coating of Fe 3  O 4  with amino carboxylate 
     Step A -- 378 grs of FeCl 3  6 H 2  O + 160 grs of FeCl 2  4 H 2  O are dissolved in 2 liters of distilled water. In this case FeCl 2  can be replaced by FeSO 4  for precipitation of the magnetite. The grade of FeSO 4  can be below technical grade such as fertilizer grade. To the mixture, at room temperature, with ultrasonic and mechanical stirring, 500 cc of NH 4  OH is added. Soon after precipitation a solution of 26 g of N-alkyl disodium amino propionate is dissolved in 500 cc of water, is added slowly at a rate of 5-10 cc/min. After addition is complete, the suspension is heated to boiling and then set aside to decant. The rest of the washing and separation technique is the same as Example 1, step A. 
     Making a non charged particle (non-ionic) ink 
     Step B -- The centrifuged, coated Fe 3  O 4  (about 450 grs) is transferred to attritor to which is added the non ionic surfactant 35 grs of polyoxyethelenated nonyl phenol, and the dispersing agent 20 grs of glycerol + 37 grs of polyethylene glycol 200 and the procedure is the same as above in Example 1, step B. 
     
         ______________________________________Ink properties______________________________________magnetic moment     18 to 22 emu/grviscosity           8 to 12 cpspH                  5 to 7resistivity         300 to 500 Ω cmink yield           400 to 450 grssurface tension     28 to 32 dynes cm______________________________________ 
    
     In the light of the principles set forth above, a cationic ink can be produced by using in addition a cationic agent, 10 grams of an ethyoxlated quaternary ammonium compound to the non ionic surfactant. Similarly an anionic ink can be produced by using an anionic agent, 8 grams of N-alkyl disodium amino propionate to the non-ionic surfactant. 
     While the invention has been described in detail with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.