HK1069140A - High pressure media mill - Google Patents

Description

High-pressure medium mill

Technical Field

The present invention relates to a High Pressure Media Mill (HPMM) and methods of use thereof.

Background

Slurry media milling is an important unit operation in various industries for the fine and ultra-fine milling of minerals, paints, inks, pigments, microorganisms, food and agricultural products, and pharmaceuticals. In these mills, the feed particles are reduced in size between a large number of small grinding media, typically sand, plastic beads, glass, steel or ceramic beads. Finer particles of submicron or nanoparticle dispersed products can be produced due to the effect of the very small grinding media and liquid media (water-based, non-water-based or mixtures thereof) that are internally agitated, which was previously not possible with conventional grinding.

Supercritical fluid (SCF) processing technology has many uses in the food, nutraceutical, and chemical industries, and is currently emerging as an alternative technology in the pharmaceutical industry, involving particle formation, microencapsulation, coating, extraction, and purification. For pharmaceutical use, carbon dioxide is the most widely used SCF, although other hydrocarbon gases such as ethane, propane, butane, and ethylene, water, nitrous oxide, ammonia, and trifluoromethane have been reported for other uses. Three SCF processes have been disclosed. They are:

1) the Rapid Expansion of Supercritical Solution (RESS) method,

2) an anti-solvent process, and

3) method for obtaining particles from gas saturated solution (PGSS)

The RESS process is limited to SCF soluble compounds because it involves dissolving the compound in SCF, followed by rapid expansion through a nozzle to form particles. Most of the drug compounds are in SCF, especially supercritical CO₂Of these, the solubility is very low.

The antisolvent method uses SCF as an antisolvent to precipitate particles from a pre-dissolved solvent solution using the principle of antisolvent crystallization. The method developed by the university of Bradford in US 5,108,109 combined with antisolvent and nozzle expansion to control particle formation. A limitation of the anti-solvent process is that a soluble solvent must be used for a given compound.

Weidner (us patent 6,056,791) discloses a method,i.e. dissolving CO in a liquid or molten drug or polymer₂To form a gas saturated solution, and then depressurised to form particles. Some obvious disadvantages of this approach are: the need for higher temperatures to melt the compound can degrade the compound and the high viscosity of the melt can limit the particle size of the product.

U.S.5,854,311 discloses the use of 10-40 micron particles in powder coating applications. The disclosed process operates at no greater than 30 psig.

U.S.5,500,331 discloses crushing material with small particle abrasive material. U.S.5,145,684 discloses surface-modified drug nanoparticles. The techniques disclosed in these patents relate to milled slurries, rather than dried flowable nanoparticles, because a liquid medium is used in the process.

Tan and Suresh Borsadia inFormation of particles using supercritical fluid: pharmaceutical useExp. opin. ther. patents (2001)11(5), along with along disclosures ltd. some process concepts for controlled particle formation using supercritical fluid (SCF) processing methods are reviewed. However, this article does not describe a milling apparatus that uses SCF to produce dry flowable micro-sized powders.

The present invention, a High Pressure Media Milling (HPMM) process, combines slurry media milling with supercritical fluid (SCF) technology or with volatile gases as milling media to produce micro-or nanoparticles in the form of a dry free-flowing powder without limitation of solubility and without the need for organic solvents or high temperatures. Volatile gases may also include those gases that cool to a liquid state, such as liquid CO₂. The method has utility for a number of materials including heat sensitive bioactive materials and environmentally sensitive electronic materials.

Disclosure of Invention

The invention relates to a grinding method, which comprises the following steps: a) adding grinding media and materials to be ground into a high-pressure media mill; b) exhausting the mill to create a vacuum; c) adding a supercritical fluid or a volatile gas to the mill; d) pressurizing and maintaining pressure in the mill; and e) operating the mill such that the product particle size is reduced.

The method further comprises the additional steps of: adding a liquid or solid material to step (a) for coating the product particles.

The above process includes an embodiment wherein the median product particle size is less than 200 microns, preferably less than 100 microns, more preferably less than 1 micron. It is preferred that the product be free of residual grinding liquid or gas.

The invention also includes a mill comprising: a) a grinding chamber capable of holding material at a pressure of up to 2000 psig; b) a magnetically driven stir bar in the chamber; and c) magnetic drive.

The invention also includes the mill as described above, further comprising: d) one or more ports leading into the grinding chamber are used for loading and unloading grinding media, material to be ground and high pressure fluid.

Drawings

FIG. 1 depicts a general design of an SC media mill.

FIG. 2 depicts a sketch of a media mill experimental setup.

FIG. 3 shows CO in SC Medium Mill₂PT curve of (2).

Fig. 4 depicts calculated values of a pressure density curve.

FIG. 5(a) shows a graph for HNO₃KNO titrated with KOH₃Supercritical milled TiO of (III)₂。

Fig. 5(b) shows a scanning electron microscope image of the product.

FIG. 6(a) shows a microscopic image of the NaCl raw material.

Fig. 6(b) shows a microscopic image of the same material after grinding.

Fig. 7(a) and (b) show optical micrographs of the processed material in 19 a.

Fig. 7(c) shows an SEM photograph of ibuprofen on 19 a.

FIG. 8 shows an SEM of ibuprofen under kv acceleration of test 19 (c).

Detailed Description

The slurry media mills described herein are capable of micron and nanoparticle slurry production and can be widely used in the chemical industry for large scale operations. SCF is used herein as a low viscosity liquid medium to facilitate better dispersion and energy transfer during milling. When the SCF is released after the milling process, a dispersed, dry, free-flowing powder is obtained as the product. The method is also not limited to the use of SCF. At T_c(or T)_{Critical point of}Critical temperature) and P_c(or P)_{Critical point of}Critical pressure) of liquid CO₂Or other volatile gases may be used as the grinding media.

This method offers significant advantages over existing micronization techniques, particularly for pharmaceutical applications. These advantages include: producing dried micro-and nanoparticles that are difficult or impossible to produce by micronization and other existing methods; coating or encapsulation is integrated during the grinding process; dry fine particles for direct inhalation formulations, including dry powder inhalation and metered dose inhalation as well as oral and parenteral formulations; and the integrated destruction and extraction of active ingredients from solid particles, cells, plants, etc.

The High Pressure Media Mill (HPMM) apparatus described herein and shown in fig. 1 is a media mill milling chamber (4) which is pressurized with a supercritical gas, such as carbon dioxide. The energy required for size reduction, deagglomeration and dispersion of the product particles is derived from a mechanical agitator (5), the mechanical agitator (5) controlling a set of agitator discs (17) which move the grinding bead media (27) in the grinding chamber (4). The grinding chamber (4) of the mill has a lower part (20) and an upper part (19). Product particles are trapped between the stirring discs (17) and exposed to the colliding grinding bead media (27). The drive belt (28) is connected to a motor (29), the motor (29) having a speed sensor (30) and a torque sensor (31).

The mill is in a fluid (in most cases CO)₂) Although any compressible gas may be used, including but not limited to hydrocarbons (HFC's) and alternatives thereto, propane, methane, and the like. The choice of pressure and temperature allows control of the viscosity and density of the fluid, which has a significant effect on the flow pattern within the grinding chamber and therefore on heat and mass transfer.

HPMM is particularly useful for the production of submicron particles in dry form. It is possible to produce a dry, well-dispersed powder because the supercritical fluid is vented after processing. The use of water is not required (e.g., certain materials such as proteins are unstable in water) and the drying step is eliminated. The production flow is simplified and integrated (e.g. surface treatment and dispersion of nanocrystalline material; grinding, destruction and extraction of biological components of cells occur simultaneously without exposure to air/oxygen) and is therefore generally cheaper than other methods of dispersion and grinding.

In fig. 1, the design of the media mill itself is known, as described above. The grinding chamber is a pressure vessel (4) which consists of a lower part (20) and an upper part (19). As shown in fig. 2, the HPMM experimental setup herein consists of connecting 4 stirring disks (17) to a shaft controlled by a magnetic stirrer (5) in the upper part (19) of the assembly. The lower part (20) of the container is connected to the upper part by sealing means. The sealing means may be mechanical, magnetic or a combination thereof. The bolt may be used with or as part of the sealing means. Connections for heating and cooling (21 and 22) the jacket around the vessel are also connected.

The conduits of the rupture disk (10) to the brake drum (11) and the drum exhaust (25) are connected for safety.

The filling opening (13) in the upper part is unplugged and a funnel is used to fill the grinding media and the solids to be processed. Any other liquid or solid components used to coat the particles are then added through the same ports. The port is closed with a plug and is ready to be filled with the supercritical fluid to be used.

All valves of the supercritical medium mill are closed and valves (14, 15 and 16) from the vacuum pump (7) through the product collection filter (6) are opened to draw all air out of the system before processing begins. The vacuum is broken with the SC fluid (1) on the balance (24) to be used in the process, this is done by closing the valve to the vacuum pump (16) and opening the valve to the SC fluid cylinder (2) to be used. This evacuation and purging process was repeated three times before starting the charge.

When the last evacuation of the container is complete, the weight from the cylinder balance (24) is recorded. The jacketed cooling water (9) is opened, the vessel is then filled with a specified weight of SC fluid from the cylinder (1) and valve (2), either by pipeline or using a pump (3), and the valve of the cylinder (2) is then closed. The fluid weight was recorded. The container is isolated by closing the valves (14 and 15).

The motor (5) is turned on to a set speed, the cooling water (8) is turned off and heating (9) is started. For the designed equipment to be operated, the heating is set at a specific temperature. Data, including RPM, torque, temperature, pressure and flow to the jacket (indicated by GPM) are recorded on the monitoring and control system (12) until the desired test time is complete.

Heating is then stopped (9) and cooling is started (8), and when the temperature of the vessel is below 25 ℃, the motor drive is stopped (5). When cooling is complete, the valve (15) is opened to collect product in the collection filter (6). Recovering the material from the filter for use.

The lower part (20) of the mill was removed, any excess material remaining in the vessel and on the blades was recovered, the unit was cleaned and reassembled for further testing.

Starting conditions

Initial testing involved loading and unloading of the mill, product collection, temperature and power control, and data acquisition.

In loading SC CO₂Previously, the mill required air extraction in a vacuum cycle. The vacuum cycle should be repeated at least 3 times to remove entrained air. Monitoring of pressure and temperature is necessary because small changes may result in large pressure increases. The monitoring allows the localization of the SC zone in the phase diagram (fig. 4). Furthermore, after the initial test, the following results were noted:

note that TiO₂Fast dispersion in SC mill. The primary particle size was obtained within 10 minutes. Polymer bead collisions were sufficient to destroy the TiO₂And (3) agglomeration. The polymer beads have reduced wear rates compared to SEPRs.

The loading of the mill was measured with a balance to achieve a preferred loading of 0.65-O.7g/cc.

The operating conditions follow the phase diagram shown in fig. 4. Heat exchange/mixing is poor at lower RPM.

Acceptable results were obtained at bead loadings of 50-70 vol%. Good circulation of the contents of the mill was also noted.

Thermodynamics of force

The PT curves of the different experiments are shown in figure 3. The effect of the exhaust is clearly seen. It is clear that if the test is started with too high a density (larger CO in SC Mill)₂Mass, series 5, 6), the pressure must be increased to 4000psig to reach the supercritical isotherm (T_{Critical point of}＝31.1℃)。

Series 1: heating only

Series 2: heating and stirring

Series 3: heating + stirring test 2

Series 4: heating, stirring and TiO₂ 50g

Series 5: heating, stirring and TiO₂ 150g

Series 6: heating, stirring and TiO₂150g exhaust 1

Series 7: heating, stirring and TiO₂150g exhaust 2

FIG. 4 is a "calculated pressure-density curve" and shows calculated values (10, 27, 31, 35, 50℃.) for different operating temperatures. Constant volume grinding chamber is charged with a known mass of CO₂. Therefore, CO₂The density was at a constant level during the test. This graph is used to predict the pressure in the SC grinding chamber for different operating temperatures and can demonstrate that SC conditions are achieved.

Definition of

The following definitions are used herein:

SC: supercritical fluid

SC CO₂：MG Industries，Malvern，PA

Fungicides: famoxadone

SEPR: ceramic grinding media available from s.e. firestone assoc., Russell Finex inc., Charlotte, NC

YTZ: ceramic grinding media available from s.e. firestone assoc., Russell Finex inc., Charlotte, NC

Poly-Sty: polystyrene grinding media available from s.e. firestone assoc., Russell Finex inc., Charlotte, NC

Nylon: nylon powder, Dupont Co., Wilmington, DE

Silver: silver particles for silver-containing conductors, DuPont Company, Wilmingtond

Unless otherwise indicated, all chemicals and reagents were used as received from aldrich chemical co.

Examples

Examples 1 to 19

The following experiments were conducted with HPMM to explore the operating range (motor speed, pressure level, run time) and study the effect of media loading, media type and additives. The experimental conditions are listed in table 1, "experimental conditions". The following (organic and inorganic) materials were tested.

0 inorganic-insoluble in Water (TiO)₂)

0 organic-soluble in Water (glucose, Acetaminophen, ibuprofen)

0 organic-insoluble in Water (famoxadone)

0 inorganic-soluble in Water (NaCl)

In addition, silver-containing conductive pastes were tested. These are thick film compositions that are applied to ceramic substrates and dielectric compositions by screen printing. These substrates are then fired in an oxidizing atmosphere (air) in a conveyor oven to form interconnected lines and pads in single and multi-layer microcircuits. Silver-containing conductor solder joints are commonly used for the attachment of passive SMT components with low temperature eutectic Sn/Pb solder or conductive epoxy adhesives.

Acetaminophen (Paracetamol) was tested with HPMM to produce 1-5 micron particles for inhaler use.

Examples 19a, 19b and 19c

Ibuprofen with HPMM

Table 1 lists the conditions for experiments with ibuprofen on HPMM. Ibuprofen is available from Spectrum Chemicals. The fluid used for the test was CO₂。

Test 19 a: ibuprofen in supercritical CO₂Medium grinding in

In test 19a, the temperature was maintained at 35 ℃. The pressure in the grinding chamber was 1550 psi. The total test time was 2 hours. The product was collected after depressurization using a vibrating screen.

Table 2 lists the resulting median particle diameters (D50). The particle size distribution was measured with a forward light scattering device (Malvern Mastersizer 2000). The size distribution drifts to the right, indicating that the particles grow due to agglomeration and aggregation of the fine product particles. This was confirmed by light microscopy and SEM photographs. Fig. 7a and b show optical micrographs (NikonOptiphot) of ibuprofen as received. FIG. 7c shows a photograph of the processed material (trial #19 a).

Figure 7c shows an SEM photograph of ibuprofen for test 19a, with particles as small as 30 nm. The operating temperature (35 ℃) of test 19a was higher than the softening temperature of ibuprofen, which resulted in fusion/aggregation of the particles.

Test 19 b: ibuprofen in liquid CO₂And media milling in surfactant (SDS)

The purpose of this test is to demonstrate that agglomeration can be avoided/reduced by low operating temperatures and surfactants. During test 19b, the temperature was maintained at 10 ℃ and the pressure in the grinding chamber was 600psi (see Table 2). The total run time was 30 minutes. 35% by weight of a surfactant (sodium lauryl sulfate, MW 288.38, supplied by ICN Biomedical inc.

The particle size was reduced from 33.85 microns (as received) to 1.805 microns. FIG. 8 shows an SEM of the product of run 19 b.

Test 19 c: ibuprofen in liquid CO₂And media milling in surfactant (SDS)

Ibuprofen in liquid CO as in test 19b₂But 2 wt% SDS surfactant was used. The results are shown in Table 2.

Table 1: test conditions

Examples	Bead	Bead	Bead	Product of	Product of	CO2	Additive agent
									Type (B)	％	Size (mm)	Keke (Chinese character of 'Keke')		Keke (Chinese character of 'Keke')	Name/gram
1	SEPR	80	.8-1.0	50	TiO2	410
								2	SEPR	76	.8-1.0	150	TiO2	410
3	SEPR	80	.8-1.0	150	TiO2	386
								4	SEPR	50	.8-1.0	150	TiO2	477
5	SEPR	70	.8-1.0	150	TiO2	410
								6	Poly-Sty	70	0.5	150	TiO2	363
7	Poly-Sty	50	0.5	150	TiO2	454
								8	Poly-Sty	70	0.5	150	Dextrose	295
9	Poly-Sty	70	0.5	150	Dextrose	318
								10	Poly-Sty	70	0.5	150	NaCl	340
11	SEPR	70	.8-1.0	150	Dextrose	363
								12	SEPR	70	.8-1.0	150	NaCl	363
13	Poly-Sty	70	.25/.15	150	Silver	431
								14	Poly-Sty	70	0.5	150	Silver	363
15	SEPR	70	.8-1.0	150	Famoxadone	363
								16	Nylon	70	.5/.88	150	TiO2	409
17	Poly-Sty	70	0.5	150	Silver	363	Stearic acid 0.75
								18	Poly-Sty	70	0.5	150	Silver	363	Stearic acid 0.75
19	YTZ	70	0.3	150	Acetominophen	370
								19a	SEPR	70	8-1.0	150	lbuprofen	370	—
19b	SEPR	70	8-1.0	110	lbuprofen	370	Sodium dodecyl sulfate
								19c	SEPR	70	8-1.0	150	lbuprofen	370	Sodium dodecyl sulfate

Process monitoring and product characterization

During each test, the temperature and pressure in the HPMM, the power absorbed by the mill, the stirrer speed were monitored. The product is characterized by its size, shape, surface morphology and reactivity/activity.

The particle size distributions of the feed and the product were measured with Microtrek UPA and Microtrek FRA from Leeds and Northrop (see Table 2). Scanning Electron Micrographs (SEM) were taken using Hitachi S-4700(Hitachi Instruments, San Jose, Calif.), powder samples were mounted on double sided tape, and X-ray powder diffraction was performed on many of the samples. The instrument used for the powder diffraction studies was a Philips X-ray diffractometer PW3040(Philips Analytical Instruments, Natick, MA). The technique used is powder X-ray diffraction using CuK alpha radiation. FIG. 6a shows the SEM of the NaCl of example 12 before milling and FIG. 6b shows the SEM of the same material after milling. A reduction in the size of the material can be noted.

Table 2: summary of test results (Whole milling test at 1750 rpm)

Examples	Total time of day	SC time	Net torque (zero-14.6)	Tile	Temperature of	Pressure of	(Energy)	Specific energy	Median particle diameter
										No.	Hrs.	Hrs.	n-lbs.		Degree centigrade	Psi	KWh.	KWh./Kg	D50[micron]
1	2.00	1.80	22.8	472	35	1400	0.94	18.89	0.34
										2	1.60	0.50	25.4	526	33	1500	0.84	5.61	0.27
3	1.08	0.50	45.4	940	38	1300	1.02	6.77	0.32
										4	0 83	0.50	10.6	220	37	1350	0.18	1.21	0.28
5	0.25	0.17	23.4	485	36	1550	0.12	0.81	0.28
										6	0.28	0.17	10.4	215	34	1320	0.06	0.40	0.35
7	0.33	0.10	2.4	50	32	1330	0.02	0.11	0.37
										8	0.25	0.17	12.4	257	35	1300	0.06	0.43	173
9	0.50	0 42	17.4	360	36	1400	0.18	1.20	185
										10	0.25	0.00	7.4	153	25	570	0.04	0.26	-
11	1.00	0.83	23.4	485	40	1580	0.48	3.23	58
										12	1.00	0.83	20.4	422	38	1530	0.42	2.82	4.3
13	1.00	0.75	0.9	19	35	1240	0.02	0.12	23
										14	1.00	0.83	2.7	56	37	1300	0.06	0.37	1.8
15	1.00	0.66	20.4	422	33	1550	0.42	2.82	5.4
										16	0.50	0.36	6.9	143	37	1375	0.07	0.48	0.33

Examples	Total time of day	SC time	Net torque (zero-14.6)	Tile	Temperature of	Pressure of	(Energy)	Specific energy	Median particle diameter
										No.	Hrs.	Hrs.	ln-lbs.		Degree centigrade	Psi	KWh.	KWh./Kg	D50[micron]
17	1.00	0.95	6.4	133	40	1400	0.13	0.88	1.78
										18	1.00	0.83	7.8	162	40	1400	0.16	1.08	28.31
19	4.00	3.70	27.4	583	46	1600	0.16	2.11	5.2
										19a	2.00	2.00	5.0	104	35	1550	0.21	1.38	44.68*
19b	0.50	0.50	3.2	66	10	600	0.03	0.30	1.805
										19c	2.00	2.00	3.1	61	10	600	0.13	1.11	4.106

^*Agglomerates

Examples 20 to 26: TiO2₂Dispersion of powders in SC Mills

Milling TiO Using HPMM described herein₂And with standard TiO₂(R900 from E.IduPont DE Nemours and Co., Wilmington, DE). To this end, a number of particle characterization techniques were used, as shown in table 3 below. The isoelectric point was determined using Matec MBS 8000(Matec applied sciences, MA). The isoelectric point is the pH at which the ESV is equal to 0, i.e. the point coinciding with the zero zeta potential. The isoelectric point was determined by instrumenting the Electrokinetic Sonic Amplitude (ESA) and titrating the suspension in a stirred vessel against nitric acid (lowering the pH) or potassium hydroxide (raising the pH), as shown in figure 5 a. Through the reaction of potassium nitrate 10^-3mol/dm³The suspension of SC product was prepared by mixing in solution and then dispersing in an ultrasonic bath for 30 seconds. The isoelectric point of the supercritical milled product was different from that of the starting material, indicating some difference in surface chemistry.

After supercritical milling, there was no discernible difference in particle size or surface properties between the starting material and the product.

The bulk density of the SC milled product was up to twice that of the original material. The fluidity is improved. Furthermore, these materials appear the same based on the SEM shown in fig. 5b and 5 c.

The power absorption and heating/cooling of the HPMM are interactive to maintain the system at a desired/selected temperature. Monitoring of the temperature is necessary because small changes can result in large pressure increases. Monitoring of temperature and pressure allows the positioning of the SC point in the phase diagram.

At the beginningAfter the initial test, it was concluded that TiO occurred rapidly₂Dispersion in SC mill. Polymer bead collisions were sufficient to destroy the TiO₂And (3) agglomeration. Within 10 minutes of milling, the primary particle size was reached. Using a balance, the loading of the mill can be reasonably accurate. The operating conditions follow a phase diagram. There is good circulation of the mill contents despite poor heat exchange and mixing at low mill speeds. Good grinding results were obtained with a 50-70 vol% loading of the grinding beads. The use of polymer beads reduces the wear rate compared to the use of ceramic pellets (SEPR).

TABLE 3

Example numbering		Appearance of the product	SEM	Isoelectric point	Density measured by pycnometer method	With N2Measured pore volume distribution	BET surface area m2/g	d10，d50，d90，um	XRD, allTiO2. Crystal size
										20	Raw materials	Enantiomeric white powder	To carry out	7.5	4.22+/-0.01	Without micropores	6.2	0.32，0.60，1.12	2337
21	Raw materials	Fine gray powder	To carry out	not enough	3.875+/-0.005	Without micropores	40.2	0.20，0.34，0.88	610
										22	Raw materials	Fine gray powder	To carry out	5.5	4.086+/-0.008	Without micropores	8.9	0.16，0.27，0.45	899
23	Grinding media	Gray beads	To carry out	n/a	n/a	n/a	n/a	n/a	n/a
										24	Grinding media	Fine gray powder	To carry out	not enough	4.130+/-0.006	Without micropores	8.2	0.17，0.32，0.64	1636
25	Grinding media	Fine gray powder	To carry out	5.6	4.155+/-0.006	Without micropores	8.1	0.16，0.27，0.48	1603
										26	Grinding media	Fine gray powder	To carry out	5.9	4.176+/-0.004	Without micropores	8.3	0.16，0.28，0.44	1573
Comp.B	R900standardTiO2			9.0

Electrokinetic results:

the isoelectric point was determined using Matec MBS 8000, as described above. The isoelectric point is the pH at which ESA ═ 0. The isoelectric point of the supercritical milled product was different from that of the starting material, indicating some difference in surface chemistry.

There was no discernable difference in particle size or surface properties between the starting material and the product after supercritical milling. The bulk density of the SC milled product was up to twice that of the starting material. The flowability is improved relative to the starting material.

Examples 27 to 31

Dispersion of precipitated silver particles

The silver was milled using a high pressure media mill as described above. The product was characterized using a scanning electron microscope and evaluated for particle size distribution, shape, isoelectric point and wettability.

TABLE 4

Example numbering	All of the following experiments were with dry silver powder	PSD, d10, d50, d90, microns, determined by Microtrac	Scanning electron microscope, dry mounting	Surface tension of EtOH/Water mixture required to wet the powder
					27		1.15、3.09、6.21	Having agglomerates of spherical primary particles which occur at about 0.5 to 1 micron	Greater than 72.6 dynes/cm
28		0.81, 1.80, 106.8, bimodal distribution with a main peak at 2 microns and another peak at 100 microns	Less regular agglomeration, irregular primary particles of about 0.5 to 1 micron. Plate-like or hard-shell, also visible	< 43.7 dynePer centimeter
					29		0.20、0.33、0.59	Hard shell-like appearance, agglomerated spherical primary particles of about 0.3 microns and less	n/a
30		0.62, 1.78, 9.21, typically with 2 large shoulders	Irregular, slightly agglomerated primary particles of about 0.7 microns and less	< 33.6 dynes/cm
					31		1.17, 28.31, 260.1 with a main peak around 2, followed by many others at larger sizes	Agglomerated irregular particles with some hard shell. Particle size of 1 micron and less	< 33.6 dynes/cm

Only the "as received" samples wet into water and therefore the isoelectric point cannot be assessed. The stearic acid coating appears to be represented by the larger size of examples 30 and 31 compared to examples 28 and 29.

The wetting aspect of the silver powder further reveals the following properties. Different ethanol/water ratios produce solutions with different surface tensions. These will in turn wet or not wet the powder. The starting material of silver example 27 was easily wetted by all solutions, even up to 72.6 dynes/cm, as was the case for the comparative example C material. This is shown in table 5.

TABLE 5

％EtOH	Corrected% EtOH	Example 27	Example 28	Example 29	Example 30	Example 31	Comparative example C	Surface, tensile dyne/cm measured
									100	70	0	0		2，f	2，f	0	26.7
50	35	0	0		1，f	1，f	0	29.1
									75	52.5	0	0	Without a sample	0，f	0，f	0	33.6
25	17.5	0	2，f		99，nw	99，nw	0	43.7
									10	7	1	99，f，nw		99，f，nw	99，f，nw	0	54.2
0	0	0	99，f，nw		99，f，nw	99，f，nw	0	72.6

Number wetting expressed as time, S (99 ═ no wetting during this time)

The precipitate is wetted

Non-wetting silver on a film formed on a surface

The product shown in example 28, which has no additives but has been treated in a supercritical mill, exhibits a well-defined hydrophobic character. A surface tension of less than 43.7 dynes/cm is required to wet the powder. Immersion wetting is fairly easy, probably due to the particular powder density, but the most significant problem is internal wetting of the powder agglomerates upon impregnation.

Silver that has been coated with stearic acid is even less wettable, requiring a surface tension of less than 33.6 dynes/cm to wet them. By using an additional EtOH/water mixture, more dissolution can be obtained.

Silver powders processed in HPMM do not change particle size significantly, but the surface appears to be modified to become more hydrophobic. Addition of stearic acid to a supercritical mill, followed by CO removal₂It appears that an effective coating of surfactant on the particles is left, which is less than that of the absenceThose treated in SC mill of stearic acid are more hydrophobic. This proves that the particles are actually coated in the process.

Claims (18)

1. A method of grinding comprising the steps of:

(a) adding grinding media and materials to be ground into a high-pressure media mill;

(b) evacuating the mill to create a vacuum;

(c) adding a supercritical fluid or a volatile gas to the mill;

(d) pressurizing and maintaining pressure in the mill; and

(e) the mill is operated such that the product particle size is reduced.

2. The method of claim 1, comprising the additional steps of:

adding a liquid or solid material to step (a) for coating the product particles.

3. The process of claim 1 or 2, wherein the process is continuous.

4. The process of claim 1 or 2, wherein the process is continuous, and wherein the supercritical fluid is selected from the group consisting of CO₂HFC or alternative, propane, methane, and combinations thereof.

5. The method of claim 1 or 2, wherein the grinding media is a ceramic, glass, steel, or polymeric material.

6. A process according to claim 1 or claim 2, wherein 95% of the product particles are no more than 1 micron.

7. The method of claim 4, wherein, CO₂Is in a liquid state.

8. A mill, comprising:

(a) a grinding chamber capable of holding material at a pressure of up to 2000 psig;

(b) a magnetic stirrer in the chamber; and

(c) and (4) magnetically driving.

9. The mill of claim 8, further comprising:

(a) one or more ports into the grinding chamber for loading and unloading grinding media, material to be ground and fluid under high pressure.

10. The mill of claim 8 or 9 wherein the grinding chamber comprises two or more sections joined together by sealing means.

11. The mill of claim 8, wherein said agitator comprises a disk attached to a shaft.

12. The mill of claim 8 or 9, wherein said milling chamber contains milling media selected from the group consisting of ceramics, glasses, metals, polymeric materials, and combinations thereof.

13. The mill of claim 8 or 9 wherein the high pressure fluid is selected from the group consisting of CO₂HFC and substitutes, propane, methane and combinations thereof.

14. The mill of claim 8, further comprising control means for controlling temperature, pressure and feed rate.

15. The process of claim 1 or 2 wherein the product median particle size is less than 200 microns.

16. The method of claim 15, wherein the median particle size is less than 100 microns.

17. The product of the process of claim 1, 2, 3, 4, 5,6, 15 or 16.

18. The product of claim 17, wherein the product is free of residual grinding fluid or gas.