MICROENCAPSULATION OF FRICTION MODIFIER ADDITIVES AND OTHER ADDITIVES FOR PREFORMANCE ENHANCEMENT IN AUTOMOTIVE AND INDUSTRIAL APPLICATIONS

Abstract

The present disclosure relates to microencapsulated friction modifiers additives used in lubricants or other solutions, to their preparation, and to the use thereof to improve fuel economy of the engines and machines by enhancing friction reduction and prolonging the friction reduction time period.

Claims

1. A method for microencapsulating friction modifiers, comprising: (a) preparing an aqueous suspension of an emulsifier; (b) preparing a solution comprising (i) at least one friction modifier; (ii) a polymer; and (iii) an organic solvent; (c) emulsifying the solution of step (b) by adding the aqueous suspension of step (a) to the solution of step (b) and stirring; (d) heating the emulsified solution of step (c); (e) cooling the emulsified solution; (f) forming microcapsules by diluting the emulsified solution of step (e) with an organic solvent; and (g) isolating the microcapsules resulting from step (f).

2. The method of claim 1, further comprising the step of: (h) washing the microcapsules.

3. The method of claim 2, further comprising the step of: (i) drying the microcapsules.

4. (canceled)

5. The method of claim 1, wherein the emulsifier is selected from the group consisting of ionic emulsifiers, non-ionic emulsifiers, and any mixture thereof.

6. The method of claim 1, wherein the polymer in step (b) is pre-formed.

7. The method of claim 1, wherein the polymer in step (b) is formed in-situ.

8. The method of claim 1, wherein the concentration of the emulsifier is in a range of about 0.05 to about 2 wt. %.

9. The method of claim 1, wherein stirring speed in step (c) is in a range of about 500 to about 20,000 rpm.

10. The method of claim 1, wherein the organic solvent is selected from the group consisting of alcohols, ethers, ketones, esters, hydrocarbons, halogenated hydrocarbons, and aromatic hydrocarbons, and any combination thereof.

11. The method of claim 1, wherein the emulsified solution in step (d) is heated to a temperature of about 20 to about 100° C.

12. The method of claim 1, wherein the emulsified solution is cooled to room temperature in step (e).

13. The method of claim 1, wherein the solution of step (b) comprises two or more friction modifiers.

14. The method of claim 5, wherein the ionic emulsifier is selected from the group consisting of sodium dodecylsulfonate, sodium dodecyl benzene sulfonate, dioctyl sulfosuccinate sodium, hexadecyltrimethylammonium bromide, poly(ethylene-alt-maleic anhydride), and any combination thereof.

15. The method of claim 5, wherein the non-ionic emulsifier is selected from the group consisting of gum arabic, polyvinyl alcohol, poly styrene-co-maleic anhydride, polyethylene glycol, polypropylene glycol, polyoxyethylene octyl phenyl ether, polyoxyethylene (20) sorbitan monolaurate, polyoxyethylene (40) sorbitan monolaurate, polyoxyethylene (60) sorbitan monolaurate, polyoxyethylene (80) sorbitan monolaurate, sorbitan monolaurate 20, sorbitan monolaurate 60, sorbitan monolaurate 65, and any combination thereof.

16. The method of claim 1, wherein the polymer in step (b) is selected from the group consisting of polymethacrylate, poly methyl methacrylate, polystyrene, poly urea formaldehyde, poly melamine formaldehyde, cellulose, polylactide, poly(lactide-co-glycoside), and any combination thereof

17. A microcapsule comprising: (i) at least one friction modifier; and (ii) at least one polymer shell encapsulating the at least one friction modifier, wherein (i) the surface of the microcapsule is smooth, (ii) a size of the microcapsule is between about 1 μm and about 40 μm, and (iii) a thickness of the polymer shell is between about 0.1 μm and about 2 μm.

18. The microcapsule of claim 17, wherein the microcapsule is shear resistant.

19. The microcapsule of claim 17, wherein the microcapsule releases the encapsulated additive upon changing temperature, pH, or stress.

20. The microcapsule of claim 19, wherein the temperature is changed to ring zone conditions.

21. The microcapsule of claim 19, wherein the temperature is changed to a range between about 180° C. and about 250° C.

22. The microcapsule of claim 19, wherein the temperature is changed to a range between about 250° C. and about 400° C.

23. A friction modifier prepared by the method of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0143] FIG. 1 is an illustration of an acorn-like particle that results from the encapsulation of an organic polar friction modifier (FM) using conventional methods.

[0144] FIG. 2 is an illustration of a failed encapsulation process of an organic polar FM using a conventional interfacial polymerization recipe.

[0145] FIG. 3 is an illustration of a failed encapsulation process of a polar FM additive using a conventional radical polymerization recipe.

[0146] FIG. 4 is an illustration of a failed encapsulation process of a polar friction modifier additive by emulsion polymerization using conventional methods.

[0147] FIG. 5 shows an optical microscope image of microcapsules prepared in accordance with the methods described herein with friction modifiers, showing the initial microcapsule size distribution.

[0148] FIG. 6 shows a scanning electron microscopy (SEM) image of microcapsules containing friction modifiers prepared in accordance with the methods described herein, showing the initial size distribution of the microcapsules.

[0149] FIG. 7 shows a scanning electron microscopy (SEM) image of the surface morphology of microcapsules prepared in accordance with the methods described herein with a polar friction modifier (FM) as the core.

[0150] FIG. 8 shows a scanning electron microscopy (SEM) image of microcapsules prepared in accordance with the methods described herein with a mixture of friction modifiers and supplemental additives in the microcapsule core.

[0151] FIG. 9 shows a scanning electron microscopy (SEM) image of the cut microcapsule exposing the shell wall, with a thickness of about 0.45 μm.

[0152] FIG. 10 shows a scanning electron microscopy (SEM) image of a novel mother ship microcapsule according to one of the aspect of the present invention, containing three separate smaller capsules, each houses either the same FM or different FMs, enabling a single capsule of the normal microcapsule size to deliver the combined effect of up to three separate additives when the capsule was triggered to release the additives.

[0153] FIG. 11 shows a scanning electron microscopy (SEM) image of a microcapsule with clear through holes of controlled hole size to allow continuous release of the additive or the core content in a unique aspect of the present invention.

[0154] FIG. 12 shows an optical microscope image illustrating another embodiment of a microcapsule of the present invention where the shell wall is a transparent membrane thin wall with many smaller thin-wall capsules containing FMs.

[0155] FIG. 13A shows an optical microscope image illustrating the microcapsule releasing additives.

[0156] FIG. 13B shows a scanning electron microscopy image of the dried broken capsules after the additive release.

[0157] FIG. 14 shows a scanning electron microscopy (SEM) image illustrating the effect of adding non-wetting agents to create varying degree of porosity for continuous release, in accordance with the present invention. FIGS. 14A-14E show the porosity as a function of the non-wetting agents used in creating porous holes in the shell, FIG. 14A is zero non-wetting agent used. FIG. 14E is 3% non-wetting agent used. FIGS. 14B-14D are various concentrations in between.

[0158] FIG. 15 shows a scanning electron microscopy (SEM) image illustrating the porous microcapsules with strong mechanical strengths in accordance with the present invention. These capsules were submitted for engine tests. The left-hand picture is the original picture taken of the microcapsule. The right-hand picture is the magnified picture of the square area to show the porous holes in the shell.

[0159] FIG. 16 shows a scanning electron microscopy (SEM) image of microcapsules illustrating another embodiment of the present invention to modify and control the surface charges of the microcapsules to either negative charge, positive charge or neutral charge to avoid entanglement or aggregation with other additives, or another medium such as charged filter elements. The left-hand picture shows the original magnification. The right-hand picture shows a larger magnification to show the details of the nano silica particles attached to the shell wall.

[0160] FIG. 17 shows the results of a friction test at 20 Kg, 3000 RPM for 30 minutes at room temperature for oil containing friction modifiers at 1.0% concentration.

[0161] FIG. 18 shows the results of a friction test at 20 Kg, 3000 RPM for 30 minutes at room temperature for oil containing friction modifiers at 1.5% concentration.

[0162] FIG. 19 shows the results of a friction test with oil containing capsules of friction modifiers and heated to 170° C. at 20 Kg, 3000 RPM for 30 minutes at room temperature at 1.0% concentration.

[0163] FIG. 20 shows the results of a friction test with oil containing capsules of friction modifiers and heated to 170° C. at 20 Kg, 3000 RPM for 30 minutes at room temperature at 1.5% concentration.

[0164] FIG. 21 shows the results of a friction test with oil containing capsules of friction modifiers at 5-30 Kg, 3000-600 RPM for 5 minutes at each load at 80° C. for oil containing friction modifiers at 0.5% concentration of friction modifier+0.5% concentration of capsules.

[0165] FIG. 22 is a table showing engine chassis dynamometer test results using microcapsulated FMs running in a 5 sets of daily driving cycles using the EPA fuel economy sequences.

DETAILED DESCRIPTION OF THE INVENTION

[0166] As used herein the following definitions shall apply unless otherwise indicated.

[0167] As used herein, the term “room temperature” refers to a temperature in the range from about 15 to about 30° C., preferably from about 20 to about 25° C.

[0168] As used herein, the term “shear resistant” refers to the ability of a microcapsule to withstand a shear force (such as that experienced in an engine) without rupture.

[0169] As used herein, the term “ring zone conditions” or “ring zone temperature” refers to the conditions or temperature due to the piston ring rubbing the cylinder liner under engine operating conditions.

[0170] As used herein, the term “through hole” refers to a clear hole through a capsule wall that connect the inside of the capsule to the surrounding environment

[0171] As used herein, the term “microcapsules” means hollow microcapsules comprising a solid or liquid core and a shell or membrane (typically polymeric) enclosing the solid or liquid core. The microcapsules contain one or more lubricant chemical additives, or combinations of additives, to be protected and to be released in controlled manner One embodiment of the present invention is a method for microencapsulating friction modifiers additives (FMs) to achieve friction reduction beyond normal life time of friction modifiers blended into the lubricant.

[0172] In addition, present invention also provides novel microencapsulation methods to include FMs, friction reduction enhancers and inhibitors to prevent the degradation of the friction reducing lubricating films in a single capsules.

[0173] In another embodiment, the methods described herein may be used to encapsulate a combination of additives, including, for example, the FMs, antioxidants, antiwear additives together with enhancers and protectors in a single capsule, including but not limited in terms of other additional additives, to be encapsulated as a mini package of additives in a formulation. The present invention applies, e.g., both to lubricants used in vehicle engines lubricants used in industrial machineries.

[0174] The microcapsules described herein may be added as a minor component to the non-polar lubricating base stock oil. The microcapsules described herein include a core material containing a solvent and polar/non-polar or combinations of the lubricant additive, more precisely friction modifiers, having solubility in the polar solvent and a polymeric shell or a membrane enclosing the core. The solubility of the polar lubricant additive in the non-polar lubricating oil base stock is enhanced by microencapsulating the additive in the core-shell form. The microencapsulated additive is released into the lubricant based on triggers such as i) a set temperature above which the capsule wall would collapse; ii) a set shear level above which the capsule would open; and iii) a set normal pressure when the capsule is in a sliding interface above the pressure capsule would open. FIGS. 13A and 13B illustrate additive release of microcapsule prepared according to the present invention. The microencapsulation of lubricant additives provides a means to enhance the life of the additives in the engine by protecting them against the harsh conditions that exist in the engine.

[0175] Friction modifying (“FM”) additives are chemicals that (i) reduce the coefficient of friction (i.e., the ratio of the tangential force divided by the normal force), thereby achieving lower friction and smoother sliding under applicable loads, or (ii) increase the coefficient of friction, slower the sliding speed up to stopping the sliding. Usually, FM additives form adsorbed surface films on both sliding surfaces, thus achieving friction modification under sliding interfaces.

[0176] FM additives that reduce the coefficient of friction conserve energy and are mostly applied in engine oils and automotive engine drive-train gear oils. These additives can provide 2-15% friction reduction in various laboratory bench tests, leading to the potential of improvements in engine fuel economy.

[0177] FM additives which reduce the coefficient of friction are usually polar molecules soluble in lubricating base oils. They are molecules/compounds containing oxygen, sulfur, nitrogen, molybdenum, copper, carbon, etc. These coefficient-of-friction-reducing FM additives adsorb on the surface or antiwear films and tend to increase oil film strength by physical and chemical adsorption on both surfaces, thereby creating film to film sliding contacts, reducing friction. The film thickness is influenced by the length of the alkyl chains of the additive molecules. Thus, as FM additives, long chain compounds, such as, but not limited to, fatty acids, fatty alcohols, fatty esters, fatty amines/amides and glycerides, are used. Some FM additives also contain sulfur containing compounds, e.g., sulfurized olefins, sulfurized fats, oil soluble molybdenum sulfur compounds, molybdenum disulfide and graphite.

[0178] Organic FM additives adsorb on the antiwear film formed on the metal surface of engines and remain in place to separate mating surfaces. They are effective when the normal loading is not too excessive, i.e., where penetration of the oil film by surface asperities is not significant. Under boundary lubrication conditions (i.e., conditions wherein the load is supported by the contacting surfaces and wear is possible), the coefficient of friction is reduced by a tribochemically formed lubricating layer which has layer structure to reduce friction similar to the use of solid lubricants such as MoS.sub.2 or graphites. Oil soluble molybdenum-containing FMs are also commonly used.

[0179] One aspect of the present invention is a microcapsule comprising at least one friction modifier additive and at least one polymer shell. FIG. 7 provides a SEM image of microcapsules with a polar additive. FIGS. 8 and 10 illustrate multicore microcapsules. The multicore capsules can be fabricated to contain separate different additive(s) as well as the same additives. The fabrication of different additives in the core mini-capsules is a new process described herein where different capsules are fabricated beforehand with the capsule surface charge modified so that the intended capsules tend to be attracted to the other two. When interfacial polymerization emulsion is carried out, they tend to group together in the emulsion. Then they form compound capsules. This aspect of the present invention is especially useful when used in synergistic additive combinations as well as in self-repair applications.

[0180] In one embodiment of any of the microcapsules described herein, the surface of the microcapsule is smooth. FIGS. 6, 7 and 8 provide illustrations of the surface morphology of a microcapsule prepared as described herein, with a polar additive core or a mixed additives core according to the present invention.

[0181] In another embodiment, for transportation applications, the size of any of the microcapsules described herein is tailored to from about 1 μm to about 30 μm diameter. The size will change somewhat depending on engine designs, filtration systems, and duty cycles. FIG. 5 shows the initial microcapsule size distribution, which is bi-modal with one dominant larger size and a smaller uniform size. These sizes are primarily determined by the stirrer size and shape and the power input in the interfacial polymerization process. The microcapsules after washing, and drying, can go through a sieving process to separate the sizes. The selected size range is to ensure that the microcapsules can pass through the pores of filters in various engines. The size distribution of microcapsules can be controlled through processing conditions and the degree of mixing or evaporation temperature or sieving.

[0182] In another embodiment of any of the microcapsules described herein, the thickness of the microcapsule shell wall is between about 0.3 μm and about 1.5 μm. FIG. 9 provides an illustration of the shell thickness of a microcapsule prepared according to the methods described herein. A preferred shell thickness is 0.3 μm to 0.6 μm to achieve optimum strength and stability.

[0183] The thickness and the effectiveness of the adsorbed film of FM additives that reduce the coefficient of friction is a function of the following variables:

[0184] i) Polar group: the stronger the polarity, the greater the adsorption strength and higher tenacity of the adsorbed film.

[0185] ii) Alkyl Chain length: the longer the chain, the thicker the adsorbed film.

[0186] iii) Configuration: straight chain and branched chains allow closer packing and higher cohesion when cross-linked.

[0187] iv) Base oil: solubility of additive depends on aromatic and polar content of the molecules.

[0188] v) Concentration: the higher the concentration, the higher is the FM additive effect, up to a point; then it could become uneconomical or tend to produce results opposite those desired.

[0189] vi) Surface composition: metals have free electrons providing active adsorption sites, ceramics and insulators only have dangling bonds, much smaller active sites, therefore much less adsorption sites and bonding strength when these materials are used for the same FM.

[0190] Linear short-chain organic acids have been encapsulated using emulsion/interfacial, radical polymerization with a polymeric shell wall formed from PMMA (poly methyl methacrylate), polystyrene, PUF (poly urea formaldehyde), PMF (poly melamine formaldehyde) (see, e.g., Mahdavian, et al., Macromolecules, 2011, 44, 7405-7414 and Loxley et al., J. Colloid and Interface Science, 1998, 208, 49-62). These methods use in-situ polymerization of methyl-methacrylate monomer to form the PMMA polymer. These conventional methods work to encapsulate non-polar molecules, but the encapsulation of polar molecules (e.g., octanol) gave ‘acorn’ type particles, as shown in FIG. 1. In this case, the key determining factor was the low interfacial tension between octanol and the aqueous phase. Surfactants that reduce the oil-water interfacial tension too much were unsuitable as emulsifiers for forming core/shell microcapsules by these methods, yielding “acorn” shape particles. Failed encapsulation of polar lubricant additives using conventional methods is illustrated in FIGS. 2-4.

[0191] One example of the microencapsulation process described herein is illustrated for the encapsulation of polar and non-polar molecules (tricresylphosphate, crodamide O, paraffin wax, etc.) were performed. Paraffin wax formed microcapsules, but polar organic friction modifiers such as crodamide O, amine O, mono-, di-, triglycerides, and antiwear additive such as tricresylphosphate all failed to form successful microcapsules with defined capsule shapes.

[0192] Unsuccessful microencapsulation of the polar additives suggests that the conventional microencapsulation methods/recipes were not able to overcome the polar long chain limitation to allow direct microencapsulation of such molecules, including some organic friction modifiers.

[0193] For most of the encapsulation processes, emulsifiers are used to create stable emulsions. Emulsifiers are mostly long chain polar molecules with a hydrophilic end group at one end of the molecule and an oleophilic end group on the other end. When a encapsulate target molecule is also a polar long chain molecule (e.g., glycerol mono-oleate) as the core material, the emulsifier may entangle/mix with the encapsulate forming stable emulsions or mixtures, thus the process will fail to produce the desired microcapsules containing the desired core molecules.

[0194] In contrast to conventional methods, one superior aspect of the present invention is the ability to encapsulate polar long chain organic molecules, which make up one key part of the friction modifier family. Long chain polar molecules intrinsically carry a net electrostatic surface charge (dipole moment). In a solution of polar molecules, like charges repulse one another, and opposite charges attract. In creating stable emulsions, there must be a balance of surface charges. If the charge of a compound/molecule can be changed by coupling with another molecule (surrogate molecules, not necessarily carrying FM characteristics), either opposite charged molecule or neutral molecule, the overall combination of the two molecular clusters will exhibit an overall net charge which is different from the individual starting compound/molecule, and in sometimes, a near neutral charge may result. So, if the polar long chain molecule can be coupled with a surrogate molecule/compound, the resulting aggregate will be easily emulsified to form stable emulsion, hence, to be capsulated. The charge intensity of the combined molecules/compound can be manipulated by varying the ratio of the two molecules. In this way, two or more lubricant additives can be combined to change the polarity/charge on the resultant molecular compound to enable successful microencapsulation. This use of multiple additive molecules or the use of a non-functioning surrogate molecule to control the charge is novel and new, solving one of the long standing barriers of polar additive encapsulation.

[0195] For successful microencapsulation of chemicals/additives, each microencapsulation process should include the following four processing steps:

[0196] i) Microencapsulation of the selected lubricant additive;

[0197] ii) Quantitative recovery of the additive from the microcapsules;

[0198] iii) Functionality tests be immediately performed on the recovered additive to ensure that the intended function had not changed; and

[0199] iv) The microencapsulated additive(s) should be blended into a formulation to be tested to make sure that the microencapsulated additive(s) can perform key application-specific performance tests showing intended advantage.

[0200] One embodiment of the present invention is a method for microencapsulating friction modifiers additives, comprising the steps of:

[0201] (a) preparing an aqueous suspension of an emulsifier,

[0202] (b) preparing a solution comprising one or more friction modifier additives,

[0203] (c) optionally adding (depending on application specific requirements) (i) one or more antioxidants, surface deactivators, or a combination thereof, (iii) a polymer; (iv) an organic solvent, or any combination of any of any of the foregoing to the solution of step (b);

[0204] (d) emulsifying the solution of step (b) or step (c) (if performed) by mixing in the aqueous suspension of step (a);

[0205] (e) heating the emulsified solution of step (d);

[0206] (f) cooling the heated emulsified solution of step (d);

[0207] (g) diluting the cooled emulsified solution with one or more solvents (e.g., one or more organic solvents) to form microcapsules; and

[0208] (h) optionally isolating, washing, and drying the resulting microcapsules.

[0209] Emulsifiers used in any of the methods described herein include ionic emulsifiers and non-ionic emulsifiers, and any combination thereof.

[0210] Suitable ionic emulsifiers that may be used in any of the methods described herein include, but are not limited to, sodium dodecylsulfonate, sodium dodecyl benzene sulfonate, dioctyl sulfosuccinate sodium, hexadecyltrimethylammonium bromide, poly(ethylene-alt-maleic anhydride), cetrimonium bromide, and any combination thereof.

[0211] Suitable non-ionic emulsifiers that may be used in any of the methods described herein may include, but are not limited to, gum arabic, polyvinyl alcohol, poly styrene-co-maleic anhydride, polyethylene glycol, polypropylene glycol, polyoxyethylene octylphenylether, polymethacrylic acid, TWEEN 20, TWEEN 40, TWEEN 60, TWEEN 80, Span 20, Span 60, Span 65, and any combination thereof.

[0212] Suitable polymers that may be used in any of the methods described herein include, but are not limited to, polymethacrylate, polystyrene, poly urea formaldehyde, poly melamine formaldehyde, cellulose, polylactide, poly(lactide-co-glycoside), and any combination thereof.

[0213] Suitable organic solvents that may be used in any of the methods described herein include, but are not limited to, alcohols, ethers, ketones, esters, hydrocarbons, halogenated hydrocarbons, aromatic hydrocarbons, and any combination thereof.

[0214] Low molecular weight hydrocarbon base oils may also be used as the solvent(s)

[0215] Examples of alcohols that may be used in any of the methods described herein include, but are not limited to, methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, and tert-butanol.

[0216] Examples of ethers that may be used in any of the methods described herein include, but are not limited to, diethyl ether and tetrahydrofuran.

[0217] Examples of ketones that may be used in any of the methods described herein include, but are not limited to, acetone and 2-butanone.

[0218] Examples of esters that may be used in any of the methods described herein include, but are not limited to, methyl formate, methyl acetate, methyl propionate, ethylformate, ethyl acetate, ethyl propionate, ethyl butyrate, propyl acetate, propyl propanoate, isopropyl acetate, n-butyl acetate, sec-butyl acetate, and tert-butyl acetate.

[0219] Examples of hydrocarbons that may be used in any of the methods described herein include, but are not limited to, C.sub.5-C.sub.2 straight or branched-chain alkanes.

[0220] Examples of halogenated hydrocarbons that may be used in any of the methods described herein include, but are not limited to, chloroform, methylene chloride, carbon tetrachloride, and 1,2-dichloroethane.

[0221] Examples of aromatic hydrocarbons that may be used in any of the methods described herein include, but are not limited to, benzene, toluene, o-xylene, m-xylene, and p-xylene.

Interfacial Polymerization Process

[0222] Another embodiment of the present invention is a interfacial polymerization method for producing any of the microcapsules described herein.

[0223] In one embodiment, to encapsulate the lubricant additive(s), an aqueous suspension of emulsifier(s) was prepared. A lubricant additive or mixtures of lubricant additives and monomers of the polymer, a preformed polymer or monomer which may form a polymer in-situ, and, optionally, one or more organic solvent(s) was added to the aqueous suspension The solution was emulsified with stirring for 60 minutes at room temperature. The bath temperature was then raised to 40 to 80° over a period of 30 minutes and maintained at this temperature for a further 2-12 hours for the interfacial polymerization to form the microcapsules. The crosslinker(s) and initiator(s) were then added to initiate the polymerization. The reaction mass was then cooled and diluted with water. The microcapsules were isolated, washed with water, and dried. The microcapsules were produced having the core as a single additive or a mixture of two or more additives (polar and non-polar, polar and metal containing (metal coming from the additive such as the friction modifier), and/or non-polar and metal containing). The capsules surface was smooth and was formed with a size range of about 5 to 20 μm. The polymer shell thickness was 1±0.5 μm. The size of the microcapsules can be controlled by using ionic/non-ionic emulsifiers, and by changing the stirring speed (e.g., in a range of about 500 rpm to about 20,000 rpm), temperature (e.g., in a range of about 20° C. to about 100° C.), and concentration of the emulsifier (e.g., in a range of about 0.05 to about 2 wt. %).

[0224] The polymerization reactions are intrinsically exothermic, i.e., producing heat during the reactions and additional heat will raise the interfacial temperatures inside the water-oil interfacial layer. The interfacial layer thickness is controlled by many factors: emulsifiers used, reactor design, the presence of baffles, cooling mechanism, the power input from the agitation, the design and size of the propellers, etc. Conventional microencapsulation recipes simply state an agitation speed in terms of rpm, which does not define the processing conditions. Through judicious experimentation and capsule property monitoring, the present inventor has surprisingly developed microencapsulation processes that are able to produce consistent microcapsules defined by their properties as defined above.

Solvent Evaporation Process

[0225] Another embodiment of the present invention is a solvent evaporation method for producing any of the microcapsules described herein. This process is more economic and easier to scale up to produce large amount of the FM capsules. At the same time, the mechanical property of the microcapsules formed is in general about 20 to 40% weaker than the capsules produced by interfacial polymerization, due to the lack of cross-linking of the polymers formed. As discussed above, use of the microcapsules in automotive applications depends on the size of the engine, duty cycles, and design tolerance, or whether the engine is fortified by diamond-like coatings. It is envisioned that for some vehicles, the strength of the microcapsules should be tailored to various engines designs.

[0226] One embodiment of this invention is a method for preparing any of the microcapsules described herein by a solvent evaporation method In one embodiment, the solvent evaporation methods comprises the following steps:

[0227] i) The polymer used for the capsule shell material and the FM (or FMs or FM with several other additives) are dissolved in dichloromethane (DCM) or other organic solvents. An emulsifier is added, and the mixture is then emulsified in aqueous solution until uniform.

[0228] ii) The emulsion is then heated to evaporate the solvent forming the microcapsules.

[0229] Thus, in one embodiment, the present invention relates to a method for producing microcapsules, the method comprising:

[0230] i) dissolving (a) one or more friction modifiers, (b) one or more polymers (e.g., one or more polymers used to for the capsule shell), and (c) optionally one or more additional lubricant additives (e.g., antiwear additives, corrosion inhibitors, antioxidants and any combination thereof) in one or more solvent(s) (e.g., dichloromethane);

[0231] ii) adding one or more emulsifier(s);

[0232] iii) emulsifying the resulting mixture (e.g., by stirring);

[0233] iv) heating the product of step iii) to evaporate the one or more solvent(s) and form microcapsules.

[0234] The temperature and the stirring speed are controlled to evaporate the one or more solvent(s). Typical temperatures are in the range of about 50° C. to about 80° C. Typical stirring speeds are from about 500 rpm to about 800 rpm (to ensure smooth graduate solvent evaporation, depending which solvent or solvents to be evaporated). For example, for dichloromethane, a stirring speed of about 600 rpm is preferred.

[0235] In certain embodiments, the method further comprises

[0236] v) filtering the dispersion;

[0237] vi) washing the dispersion; and

[0238] vii) drying the microcapsules (e.g., at about 50° C. for about 12 hours).

[0239] In certain embodiments of any of the methods described herein, the ratio of core material (additives) to polymer (shell) is specified, in order to control the microcapsule shell wall thickness in the solvent evaporation method. It is desirable for a capsule to contain as much additives as possible while maintaining the shell wall thickness (strong capsules).

[0240] In certain embodiments, the ratio of core to shell (e.g., in the solvent evaporation process) ranges between about 3:1 to about 9:1. The preferred core to shell ratio for FMs or mixtures of additives is 4:1 or 3:1 or any ratio in between these ranges, depending on the specific FM molecular structures and molecular weight.

[0241] In certain embodiments, the evaporation temperature at a constant stirring rate can range from about 40° C. to about 80° C. (at high temperatures, nitrogen or argon atmosphere is required to prevent oxidation). The preferred temperatures in additive encapsulation are about 50° C. to about 60° C.

Encapsulation Efficiency and Additive Recovery Percent

[0242] In order to recover the additive from the microcapsules made from these two processes for formulation insertion as well as for quality control purposes, the microcapsules containing additives were broken using a variety of methods: shear, thermal, solvent extraction, and mechanical pressures to break open the capsules to allow recovery of the encapsulate. The use of high temperatures to release the encapsulated material has to be done under argon atmosphere to avoid oxidation. The lubricant additives were extracted in hexane and the polymer shell, being insoluble in hexane, was filtered. The hexane was evaporated, and the residue was weighed. Microcapsules prepared according to this invention have an encapsulation efficiency of about 50 to about 90%, with preferred recovery rates of about 70 to about 75% to balance shell thickness and additive content. Since the microcapsule is negatively charged at the surface, the interior of the capsule has to be positively charged and tend to attract the encapsulated additive or additives combination. Additive recovered from the capsules undergo performance tests compared with the virgin additive or additives.

Testing and Evaluation of the Microcapsules

[0243] To conduct the functionality test of the recovered additive from the microcapsules, fully formulated lubricating oil was used. The tests were conducted by two means: i) direct substation of the FM with microencapsulated FM at equal dosage level from the capsule recovery test results; and ii) adding to the formulated oil an additional microencapsulated FM or FM.sup.+ antioxidant additive to test for prolonged FM performance. Oils with an encapsulated mixture of friction modifiers at 1.0% and 1.5% concentration were tested. The conditions for the friction test were set by carrying out ball-on-three-flats tests using a four-ball wear tester. The tests were conducted under 20 kg loads, 3000 rpm, room temperature for 30 minutes. The set of test conditions were chosen to give a coefficient of friction in the range from 0.09 to 0.1. The friction modifiers used were at an equivalent amount of microencapsulated friction modifiers and non-encapsulated friction modifiers. At a concentration of 1.0% (FIG. 17) and 1.5% (FIG. 18), the microencapsulated FM capsules showed friction decrease but higher than the uncapsulated FM at equal concentration.

[0244] The test results suggested that the FM microcapsules release FM at a controlled rate under the test conditions. To test the friction level when all of the FM has been released, the oil containing the FM microcapsules was heated to 170° C. for 2 hours under a nitrogen atmosphere with agitation for the encapsulated friction modifiers to be released into the oil. The friction tests were then repeated with the same set of conditions (3000 rpm, 20 Kg, 30 minutes at room temperature). Two different concentrations (1.0% and 1.5%) were prepared and tested, as shown in FIGS. 19 and 20. At both the concentrations, 1.0% and 1.5%, the coefficient of friction is lower for the oil with microcapsules of friction modifiers as compared to the oil with the non-capsulated friction modifiers. The reason why capsulated FM outperforms the virgin FM is due to the fact that 100% of the virgin FM was subjected to degradation condition from the beginning during the wear tests, and the capsulated FM was protected from rapid degradation and therefore able to outperform the virgin FM.

[0245] With the above set of data proving that the encapsulated FM being protected from rapid degradation yields better performance than the non-encapsulated FM, the static Ball-on-three-flats test was conducted to test the rapid transient conditions similar to the engine operational conditions. Rapidly changing speed and load sliding conditions are historically the severe condition for FM performance. The rapid transient friction test was conducted on a four-ball wear tester with a ball-on-three flats contact geometry. The test sequence was divided into a three speed sequence: 3000 rpm, 1200 rpm, and 600 rpm and within each speed, the load was cycling from 5 kg load to 30 kg every five minutes with a 5 kg step increase. The test took 360 minutes to complete. The baseline lubricant was fully formulated oil with the FM removed. The top line (trace A) line in FIG. 21. This was compared to the same lubricant with 0.5% virgin FM added (trace B). The third lubricant (trace C) has 0.5% virgin FM plus 0.5% capsulated FM added. The cycling test has three zones, the first zone has 3000 rpm, low load, typically in the hydrodynamic lubrication regime. The second zone has 1200 rpm, medium load, simulating the mixed lubrication regime. The third cycle has 600 rpm with the same loading cycle, simulating the boundary lubrication regime. It is in this regime, after the previous two cycles, the FM alone lost its effectiveness and the microcapsulated FM begin to release additional FM to maintain low friction. So, it simulates the condition that when the 0.5% virgin FM lost its effectiveness during the third cycle, the microcapsulated FM continued to provide low friction, as shown in FIG. 21.

[0246] There are large variety of engines and engine system configurations in the transportation industries and there are also a variety of oil injector pumps and oil filter systems used in various applications. For microencapsulated additives to function effectively, additional requirements for the microcapsules are needed. There are no microcapsules that will meet all the requirements posed by all engines and all systems with all duty cycles. The present invention aims to provide solutions to tailor-made microcapsules containing lubricant additives to function effectively in such defined applications.

[0247] Another aspect of this invention is to provide a solution to meet the requirement of releasing additives using controlled continuous additive release in a somewhat static environment by creating through-holes in the microcapsule shells. The release rate can be controlled by various sizes of the through-holes, shell thickness, lengths of the hole, and surface charges within the hole walls. The release rate can be controlled from diffusion to controlled leakage, depending on the temperature, size, or molecular weight of the encapsulate, and vibration frequency of the vehicle. Since the volume of the oil circulation from the oil sump to the hot zone of the engine is relatively small (also depends on the pump injection rate, which depends on the duty cycle and engine size). Conventional porous microcapsules tend to be spongy and weak which will not work in this application.

[0248] To create through holes in the microcapsule wall while maintaining the smooth relatively strong shell wall, has, to the best of the inventor's knowledge, not been done before. Current art teaches to use phase separation materials to create porous layers to release small molecules or gaseous encapsulates. But this causes the capsules to form very porous layers as the capsule wall and become very weak capsules. From the broader perspective, non-wetting agents with the polymethacrylate polymer but oleophilic or neutral material such as carbon nanotubes, or organic-inorganic molecules such as silanes would create through holes in the microcapsule shell wall. Silanes such as aminopropyl-trimethoxysilane (APTMS), tetra-ethoxy orthosilicate (TEOS, trimethoxy octyl silane, etc. have been used to create holes capsule walls), high molecular weight alcohols such as oleyl-alcohol, can also be used. The creation of through holes, however, also depends on the concentration of the non-wetting agents used and also depends on the emulsifiers used in the process. An optimum combination of the non-wetting agents, the specific emulsifier used, and the processing conditions used.

[0249] Accordingly, in certain embodiments of any of the methods or microcapsules described herein, the microcapsule contains one or more through-holes.

[0250] The addition of non-wetting agents works for both the solvent evaporation and interfacial polymerization processes described herein with the addition of the agent at the proper concentration with corresponding emulsifiers used in each of the processes.

[0251] When adding non-wetting oleophilic materials to create holes in the microcapsule shell to facilitate controlled additive release, the emulsifiers used in the process are important. Non-polymeric emulsifiers such as sodium dodecylbenzene sulfonate are not suitable emulsifiers for this process. High molecular weight polymeric such as polyvinyl alcohol (MW 130.000 and up) are good emulsifiers for the non-wetting agents, which are typical small molecular weight and physically relatively small.

[0252] Suitable emulsifiers for this process are: polyvinyl alcohols with various molecular weights, sodium dodecyl-benzene, or other compatible emulsifiers, and any the emulsifiers listed in herein. For many of the emulsifiers, the ratio of emulsifier to non-wetting oleophilic materials is important in producing controlled clear-cut through-holes. In one embodiment, the amount of emulsifier(s) used is from about 0.1% to about 2% by weight, such as from about 0.25% to about 0.5% by weight, depending on the MW range of the emulsifier and the polymer used.

[0253] It has been suggested the use of specific chemicals that will cause overall phase separation creating highly porous polymeric shell. These phase separation agents created puffy, porous, loose shell wall, weakening the mechanical strength, which is not suitable for automotive engine applications. Non-wetting agents, on the other hand, are localized to form holes associated with specific material and the size of the holes and the number of holes can be controlled to some extent, with much less chemical change of the materials. So, non-wetting agents, such as single and multiwalled carbon nanotubes, some inorganic species, silanes, etc., can be used.

[0254] Another aspect of the present invention is an optimization process for fabricating various types of porous microcapsules by adjusting the amount of non-wetting agents, interfacial polymerization conditions, and the temperatures between the phases to form continuous phase polymeric shells with through holes in the porous microcapsule shell walls. The use of non-wetting agents or the small amount of phase separation agents described above applies to both interfacial polymerization microcapsule fabrication method and the solvent evaporation microcapsule fabrication method. These two basic fabrication techniques produce the basic microcapsules. Additional refinements (porosity, charge change, charge modification steps) techniques can be applied to the microcapsules equally.

[0255] According, in one embodiment, the present invention relates to a method of optimizing any of the methods for preparing microcapsules described herein (e.g., to form microcapsules with continuous phase polymeric shells with through holes in the porous microcapsule shell walls), comprising:

[0256] i) adjusting the amount of non-wetting agents, or

[0257] ii) adjusting the interfacial polymerization conditions, or

[0258] iii) adjusting the temperatures between the phases, or

[0259] iv) any combination of i), ii) and/or iii)

[0260] to form continuous phase polymeric shells with through holes in the porous microcapsule shell walls.

[0261] They porous microcapsules have been tested in by using a liquid organic dye encapsulated by the two methods described previously and tested in oil solution at various temperatures from 25° C. to 80° C. and the rate of dye release was observed. Generally, the additive release rate increased with the rise of temperature, but depending on the formulations, the hole size, and total amount of holes on the capsules, the release rates can be controlled to allow slow release.

[0262] Another embodiment of the present invention is a method to fabricate microcapsules to pass through various filter media to stay in the lubricating oil. ASTM standard engine tests typically use a simple metal screen as the filter to facilitate the flow of the reaction products, increasing the severity of the tests. In vehicles, there are many types of filter media used but the materials used are primary cellulose (paper), blended with synthetic fibers such as glass fibers and polyester fibers. Resins are used to saturate the fibers to provide strength and stiffness. So, the filter primarily operates on depth filtration, even though they provide a nominal pore size diameter for the filter. There are also secondary filters which use magnetic medium, and all synthetic fibers with higher filtering efficiency. Taking into account of the pore size specification for most filters are 20 μm diameter to 40 μm diameter, but due to the depth filtration design, the microcapsule size should not exceed 10 μm to 12 μm in diameter.

[0263] Another aspect of the present invention is surface charge control. We tested the surface charge of the filtering media; they range from slightly positive charge to moderate positively charged. Since the microcapsules are negatively charged, the capsules will be filtered out regardless of the capsule diameter in relation to the nominal “filter pore size”. To adjust the friction modifier laden microcapsules surface charge, the present inventor developed a technique to convert the PMMA microcapsules with a negatively charged surface to positively charged surface in order to pass through the filters commonly used in cars and trucks.

[0264] The present inventor is unaware of any prior process to convert a polymeric microcapsule containing friction modifiers from negatively charged microcapsule surface to a positively charged surface without interfering with the porous through holes that the invention described. The general principle is to adsorb controlled monolayer or ultra-thin positively charged nanoparticles at the exterior microcapsule shell wall without impeding the porous through holes built-into the microcapsule by the methods described herein, and at the same time, not too thick a layer to interfere with the designed functions of the microcapsules.

[0265] There are several techniques that are commonly used to deposit a monolayer silica nanoparticles on the capsule surface using a self-assembled monolayer (SAM) technique. The SAM monolayer technique is well-known, putting the microcapsules in a pure water bath in a clean room environment at very dilute solution of positively charged silicate nanoparticles for 12-24 hours. The nanoparticles will adsorb on the microcapsule surface about a monolayer deep. The SAM method was chosen for its ability to control the amount of the silicate particles on the capsule surface. The purpose of the controlled deposition of silicate nanoparticles is to control the surface charge to slightly positive but not strongly positive. Silicate nanoparticle at a monolayer strength will produce a slightly positive charge to pass through the filter media but not strongly enough to attract other additives in the oil, which are predominately negative surface charged.

[0266] Another aspect of this invention is a method to convert normally negatively charged silicate nanoparticles to positively charged particles. In one embodiments, the method comprises fictionalizing the negatively charged silicate nanoparticles with i) sodium bis-(2 ethylhexyl) sulfosuccinate (AOT) dispersant, and/or ii) a silane, such as, but not limited to, aminopropyl trimethoxysilane (APPTMS) or 3-aminopropyl trimethoxysilane (APTES). The choice is based on reactivity and charge intensity. To stabilize the attachment process, 0.5 ml of APTMS functionalized SiO.sub.2 nanoparticles dispersion at a concentration of 0.02 g/ml was added to 50 ml of microcapsules dispersion concentration of 0.015 g/ml and stirred for 12 hours at room temperature. The ratio of the SiO.sub.2 to microcapsules can be varied by using increasing amount of SiO.sub.2 dispersion to increase the surface coverage of positively charged SiO.sub.2, hence increase the positive charge intensity. The zeta potential of the AOT stabilization microcapsules is about −25 mV, when converted, the zeta potential became +24 mV. The positively charged capsules with capsule diameters between 8-10 μm diameters were fed through a commercial filter used in the engine chassis dynamometer test stand, and most of the capsules successfully passed through the filter.

[0267] Another aspect of the present invention is creating through-hole microcapsules not only to facilitate the steady diffusion controlled release of the additives inside the capsules, but also if some capsules got caught up on the filter medium (the filtration including depth filtration, as time went on, some capsules will get caught by the filter), the additives will continue to be released into the oil passing through the filter, thus achieving the timed-release intent and design.

[0268] One additional embodiment of the present invention is the extension of the previously discussed long polar molecule charge control and balance principles by pre-mixing several molecules to form a molecular cluster, the end combination of the molecular cluster will be either charge neutral, or slightly negative. All chemicals when in solution carry charges. When a long polar molecule is paired with rectangular (3D) molecule, the end result is much less polar. The case in point is the organic FM (long chain polar molecule) mixing with zinc dithiophosphate (rectangular neutral molecule) for example that have allowed us to encapsulate the organic FM. When investigated further, it was found possible to selectively pair some molecules first, then mix with other paired molecules, providing functionally synergistic functional enhancement and encapsulate the group. It has been demonstrated that up to 5 additive molecules can be combined together and encapsulate them into a single microcapsule.

[0269] In this embodiment, as an example, to encapsulate 3 antioxidants, one molybdenum-containing FM, one organic FM. When the microcapsules rupture, most of the original additives would have been degraded and rendered useless, so the lubricant would need a booster in antioxidation additives, and organic FM plus a moly-containing FM. For the antioxidants, an amine antioxidant, a phenolic antioxidant, and a high temperature phenol antioxidant are blended in ratios that would provide optimum synergistic performance. In this case, the amine and phenolic antioxidants will be paired first in a solution for 12 hours at 27° C. under an argon atmosphere, then add the high temperature phenolic antioxidant slowly with vigorous stirring for another 12 hours. The friction modifiers will then be blended in a ratio that will have synergism together at the same condition for 12 hours under an argon atmosphere. Then the two mixtures of the additives will be blended together at 27° C. overnight. The pre-pairing is important to make this method work. Then the final mixture will be microencapsulated according to normal microencapsulation procedures. Such combination of additive opens a new era of mini-packaging of inhibitors to be isolated from the overall formulation, when it is released, the effect on the lubricant is high impact.

[0270] Such mini-package also creates the concept of delivery of performance on demand, right to the location where it is needed because the additive release can be tailored to the specific location/conditions/environment at that location/condition.

[0271] The discovery of the concept of “charge neutralization” or charge optimization by mixing various molecules to create a stable aggregate cluster of several molecules that enable them to be encapsulated together is has, to the inventors best knowledge, never been carried out in microencapsulation of lubricant additives and applied to additive encapsulation. In this case, a “surrogate molecule” can be used to encapsulate a chemical that otherwise cannot be encapsulated due to interference with emulsion creation.

[0272] Another embodiment of this invention is the creation of an ultra-high performance package within a single capsule. For example, in applying friction modifiers to lubricants, molybdenum-containing FMs such as, but not limited to, molybdenum-dithiocarbamate (MoDTC) can form MoS.sub.2 in situ. Depending on the structures, the initiation of decomposition and availability of sulfur atoms in the immediate vicinity often require high temperatures to get the reaction to proceed to form MoS.sub.2. Once the MoS.sub.2 is formed, under boundary lubrication conditions, the MoS.sub.2 film often oxidizes, forming moly-trioxide on the surface of the film, losing the friction reduction properties. To make this additive overcome the initiation barrier, the MoDTC may be paired with sulfurized olefins, another friction modifier, making sulfur readily available. To prolong the useful life of the MoS.sub.2 film, surface oxidation inhibitors may also be added. Finally, the organic friction modifier is added into the mix to produce a microcapsule containing the four chemicals (FM, sulfurized olefin, surface oxidation inhibitor and organic friction inhibitor) with an optimized ratio of the four components. The resulting friction reduction properties are significantly enhanced, the initiation temperature is much lower, and the low friction duration is substantially prolonged.

[0273] The porous microcapsules containing friction modifiers without surface charge modification were blended in an experimental OW-16 lubricant (George Washington University), which lubricant has gone through extensive fuel economy tests in the same series. The engine tests were conducted on a Chassis engine dynamometer on a 5.3 L 350 hp modern engine equipped with up-to-date fuel-efficient technologies such as direct injection, active fuel management and variable valve timing with an advanced combustion system. To improve the test precision, a baseline oil was tested before and after the candidate oil. The fuel economy was determined by fuel metering, and tailpipe carbon analysis. The test procedure lasted for 5 days. For each oil, the engine test started by flushing the engine with the test oil. A daily test using cold start FTP (EPA city driving cycle) followed by a double FFE (EPA highway test cycle) for the day. The test cycles were the same test protocol used in the EPA CAFE tests.

[0274] Since one of the primary purposes is to use the microcapsulated additives to prolong the effective of friction modifiers, the test protocol was designed similarly. The initial phase is the fuel economy daily tests with a cold start, followed with an FTP (EPA city driving cycle), FFE (EPA highway driving cycle) and the combined driving cycle. Then the engine was shut down and restarted next day for five consecutive days. Since microencapsulated friction modifiers should prolong the low friction performance period, the test procedure would need to be modified to test the extended time period to observe the prolonged fuel economy benefits. The test was stopped, and vehicle was put on mileage accumulator for one week repeating the test cycles for 500 miles. Then the fuel economy testing would start again to see whether the fuel economy results changed over time. A total of three engine tests were conducted. The table in FIG. 22 shows the test results. The data represent the average of the 5 day test results and compared with the baseline data before and after the 5 day testing. The initial test showed that the microcapsules improved the city driving cycle in this engine by 1.2% but lost 0.8% in the highway cycles. After 500 miles in the test engine, the oil was tested again. In this case, both the city and highway cycles showed an improvement of 0.7% improvement in fuel economy.

[0275] Subsequent analysis showed the engine parts are clean without varnish or sludge or additional deposits. The microcapsules were collected by the filter media. There were no microcapsules in the oil, but the fuel economy improvement was noted.

[0276] Modified FM capsules have been developed with positive surface charge and it has been demonstrated that the improved microencapsulated FMs described herein will pass through the filter and function normally, leading to better fuel economy improvement.

[0277] The description of the present embodiments of the invention has been presented for purposes of illustration but is not intended to be exhaustive or to limit the invention to the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. As such, while the present invention has been disclosed in connection with an embodiment thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention. Patents and publications cited herein are incorporated by reference in their entirety.