Patent Grant
7115221
The present invention relates to a process for the production of graphite powders of increased bulk density. The present invention relates in particular to an autogenous surface treatment of any pulverulent graphitic materials, their bulk density and tamped density being markedly increased and other important material properties being advantageously modified as a result of the mutual physical-mechanical action of the individual powder particles.
Graphitic materials, especially those with a high graphite content, are known per se and are used in industry in a variety of ways. High-purity graphitic carbons have xylene densities (also called single-crystal densities or real densities) ranging from 1.80 to 2.27 g·cm−3 and a crystal structure which can be characterized by a c/2 value of 0.3354 to 0.3360 nm and an Lc value of more than 40 nm (Lc>40 nm). These materials are obtained from natural sources, enriched and purified or produced synthetically from amorphous carbon products in a high temperature process. Subsequent grinding processes produce pulverulent materials with different mean particle sizes in each case. A given particle size for a powder is normally always a mean value of a specific particle size distribution. The particle size distribution to be used for a particular purpose depends especially on the composition of the graphitic material and the associated properties, as well as on the intended use.
The particle shape is always platelet-like, the anisotropy of the particles being the more pronounced the higher the xylene density and Lc values. The Scott density (also referred to as bulk density) of such materials, for example with particle sizes smaller than 100 micron (particle size<100 μm, determined by laser diffraction analysis), is normally below 0.25 g·cm−3, the Scott density being the lower the smaller the particle size. Comminution of the particles by grinding generally results in a lowering of the Scott density. The Scott density can be somewhat increased by an optimized particle size distribution. Thus, for example, Scott densities up to max. 0.3 g·cm−3 are achieved by an optimized composition of fine and coarse fractions for such materials with particle sizes below 100 micron.
The tamped density, the compressibility and the absorption capacity for polymeric binder materials and liquids such as oils, and for organic solvents and aqueous systems, are equally important properties of graphite powders. These properties correlate with the composition of the graphite powders and especially with the particle size distribution.
It has now been found that, surprisingly, the values of the Scott density for a particular graphite powder of any particle size distribution is considerably increased when the graphite powder is subjected to an autogenous surface treatment in which the particles impact with one another at an appropriate speed and for a sufficient length of time. The impacts and the associated mutual physical-mechanical action change the structure or surface of the graphite particle in such a way as to result in a considerable increase in the Scott density. The other properties mentioned above are also modified to a considerable extent.
Under the electron microscope, the crude, ground, platelet-like graphite particle has an irregular shape and sharp edges. The irregular particle contours are abraded and the edges rounded off by the treatment according to the invention. If the energy dose is appropriately optimized, the grinding effect which occurs with other mechanical treatments, leading to a noticeable lowering of the bulk density, is considerably reduced or minimized. Although the abrasion of the particles creates dust, which, together with a minimal grinding effect, leads to a slight reduction in particle size and Scott density (bulk density), this particle size effect is far outweighed by the surprisingly large total increase in Scott density, and the change in the other properties, caused by the treatment according to the invention. The present invention can be at least partly explained by the observed changes in the particle contours, but the invention is not bound to this explanation.
The present invention is defined in the Claims. In particular, the present invention relates to a process for increasing the Scott density of graphite powders of any particle size distribution, characterized in that the graphite powder is subjected to an autogenous surface treatment.
The process of autogenous surface treatment consists in allowing the individual powder particles to impact with one another at a measured speed so that, as a result of the associated mutual physical-mechanical action of the individual particles, their surface structure changes but the individual particle remains substantially unbroken, i.e. no substantial grinding effect occurs. This change in the particle contour or surface structure of the individual particle gives rise to the increase in Scott density according to the invention. The autogenous surface treatment is carried out, and the individual particles are allowed to act on one another, until the desired Scott density is achieved. The measured speed means that the speed or energy with which the individual particles are charged is adjusted so that the particles do not disintegrate on impact or collision, thereby practically avoiding a grinding effect. This adjustment is a question of process optimization and does not present a problem to those skilled in the art.
The Scott density achievable by means of the optimized grinding effect for a graphite powder of any particle size distribution can be increased in each case by at least about 10% to about 100%, preferably by about 20% to 80%, by the autogenous surface treatment according to the invention. Hitherto unattained Scott densities of 0.45 g/cm3 or more are thus achieved for graphitic materials.
The tamped density achievable by means of the optimized grinding effect for a graphite powder of any particle size distribution can also be increased by at least about 10% to 100%, preferably by about 20% to 80%, by the process according to the invention. Hitherto unattained tamped densities of at least 0.90 g/cm3 are thus achieved for graphite powders.
In the case of particle sizes of <100 μm, the autogenous surface treatment according to the invention is preferably carried out by fluidizing or dispersing the graphite powder particles in an inert carrier gas and accelerating the particles with the aid of the carrier gas, as described below. The intensity of this treatment is determined by the carbon type and the mass of the particles, their speed and the amount of material used per treatment, i.e. the concentration of the fluidized particles dispersed in the gas. The intensity of the treatment increases with the softness of the graphitic carbon used, the mass of the particles, their speed and the amount used. For particle sizes of <300 μm, the dispersion and acceleration of the particles are preferably effected by means of rotating mechanical tools, for example in the present process by means of a turbine or directly by means of a rotating disk.
However, the grinding effect which occurs also increases simultaneously with increasing intensity of the treatment. Thus, to achieve the maximum bulk density of a material, there is a maximum intensity which results from the optimized parameters of particle speed, particle mass and amount used. The formation of agglomerates due to the agglutination of smaller particles, which would also lead to a sustained increase in the Scott density, has not been observed. Treated particles larger than the untreated particles used did not appear in any of the experiments performed. Analyses of the treated materials by scanning electron microscopy also showed no such agglomeration.
The treatment according to the invention not only increases the Scott density but also improves the compressibility properties of the graphite powders and reduces their absorption capacity for polymeric binder materials and liquids such as oils, organic solvents and aqueous systems. The crystallinity of the graphitic carbon particles, on the other hand, remains unaffected by the mechanical surface treatment. The structural parameters and the xylene density also remain unchanged compared with the untreated particles.
The process according to the invention also increases the pressed density achievable by the optimized grinding effect for a graphite powder of any particle size distribution by at least about 0.5% to 10%, preferably by about 1% to 8%. If the powders treated according to the invention are used to produce mouldings by compression under a pressure of 2.5 to/cm2, markedly higher pressed densities can be achieved compared with the untreated materials.
Furthermore, the powders treated according to the invention exhibit a markedly reduced oil absorption capacity and binder uptake ranging from about 10% to 50% and especially from an average of about 20% to 45%, values in excess of 50% also being obtainable. This effect is achieved by the treatment according to the invention because the porosity (pore structure) of the particles is not affected by the treatment, as can be demonstrated by the fact that the nitrogen adsorption properties and xylene densities hardly change.
Said markedly reduced absorption properties also result in markedly lower viscosities of dispersions of the graphite powders treated according to the invention in liquid media, so dispersions with a correspondingly increased solids content can be prepared with the graphite powders treated according to the invention. The solids content of liquid carbon dispersions can be increased by more than 5% to over 30% by using graphite powders treated according to the invention.
Graphite powders suitable for the use according to the invention are especially those with a high graphite content in the particle, and particularly so-called high-purity graphites, preferably with xylene densities ranging from 1.80 to 2.27 g·cm−3 and a crystal structure characterized by a c/2 value of 0.3354 to 0.3360 nm and an Lc value of more than 40 nm (Lc>40 nm). The powders can be obtained from natural sources or prepared synthetically from amorphous carbon products and can have any mean particle size and particle size distribution. Preferred pulverulent graphitic materials are those with a mean particle size of up to 150 μm, preferably of 1 μm to 50 μm, and especially high-purity pulverulent graphites. Such graphites are known per se.
The process according to the invention is preferably carried out in such a way that the graphite powder particles to be treated are dispersed and fluidized in a gas. This can be done using any method of fluidization technology known per se in which the particles impact with one another in the fluidized state and thereby change their surface contours and surface structures, as is the case e.g. in a fluidized bed. However, to carry out the process according to the invention, the fluidized particles are preferably provided with higher speeds so that the particles fluidized in this way are accelerated with higher energies. Preferably, the fluidized particles are continuously concentrated and diluted again in the gaseous environment. The resulting collisions between the particles set in rotation, and the friction between them, result in surface abrasion of the particles, the energy transferred to the particles being adjusted so that the collisions and friction cause substantially no disintegration of the particles.
The process according to the invention can be put into optimum effect e.g. in the device shown in
The graphite powders treated according to the invention can advantageously be used as pigments in aqueous or solvent-based dispersions, thereby achieving higher solids contents than with untreated powders. The viscosity of liquid dispersions of materials treated according to the invention is markedly lower for the same solids content. Also, when dispersions according to the invention are applied to substrates and dried, coatings with markedly lower porosity values are obtained because the content of liquid phase is markedly lower. The higher solids content also means that smaller binder/carbon ratios are needed to stabilize a dried carbon coating on a substrate. The low polymeric binder contents result in a marked increase in the electrical and thermal conductivities of such carbon layers.
Dispersions containing mixtures of synthetic and/or natural graphitic carbons treated according to the invention and a polymeric binder in an aqueous or solvent-based medium can be applied to metal foils and dried to give stable coatings (for thicknesses of 10 to 5000 μm) with an increased graphite/binder ratio and hence also increased electrical and thermal conductivities. The porosities of the dried films are normally below 50% and are thus appreciably lower than those of films formed of conventional graphites. Such dispersions can therefore advantageously also be used for gas-tight and liquid-tight coatings on metal substrates, which can be used as electrically conducting anticorrosive films on metal foils and plates.
The dried coatings formed by the graphites treated according to the invention can be compressed by a calender without the graphite film delaminating from the metal foil. This delamination from the metal foil is frequently observed with untreated graphites. The calendering of graphite films produced from graphite powders treated according to the invention affords coatings with porosities below 30% without altering the texture or particle structure of the graphite powders used. Such film coatings on metal foils, characterized by porosities below 30% and stabilized with lower binder/carbon ratios, can be used in lithium ion batteries as negative electrodes with charge densities above 550 Ah/l. The current-carrying capacity of such electrodes is markedly higher than that of electrodes made of conventional graphite powders. Such negative electrodes can thus be used very advantageously for lithium ion cells with a high power density.
The high packing density of the synthetic or natural graphites treated according to the invention, combined with the relatively low polymeric binder absorption capacity, is advantageous in the production of graphite/polymer composites which can be compressed to gas-tight graphite plates of high electrical conductivity. Such plates are advantageously used as bipolar plates in polymer electrolyte fuel cell technology.
Mixtures of polymers with synthetic or natural graphites or graphitic carbons treated according to the invention form thermoplastic or thermosetting composites with a higher proportion of carbon filler and a lower processing viscosity. Thermoplastic polymer/graphite composite materials with graphites treated according to the invention have higher (and hence improved) values in respect of their isotropic, mechanical, thermal and electrical properties and behave more isotropically than composites with untreated graphitic carbons.
Metallic non-ferrous sintered materials which have been produced with synthetic or natural graphitic carbons treated according to the invention, or contain such carbons, have improved isotropic, mechanical and tribological properties.
The Examples which follow describe the invention.
Examples 1 to 5 show the material properties of various graphites before and after the autogenous surface treatment according to the invention. The experiments were performed in the device described in the above section. The rotating disk used had a periphery of 0.75 m and a speed of rotation of 800 rpm.
Examples 1–5 were carried out under the experimental conditions given in Table 1.
Examples 1 to 5 show a marked increase in Scott density (bulk density) and tamped density for the powders treated according to the invention. The treated powders exhibited no agglomerates whatsoever. The resulting change in particle size distribution is indicative of a small grinding effect. The slight lowering of d values, however, is caused especially by the dust produced by the abrasion of the particles. The pore structure of the treated particles is not affected by the surface treatment. It is assumed that the dust produced by the treatment and the slight decrease in particle size distribution are the main reason for the slight lowering of the Lc values and the xylene densities. The elastic recovery of the compressed treated materials drops sharply. The pressed density of mouldings produced from the treated materials under a pressure of 2.5 to/cm2 increases sharply. Although the BET values are increased somewhat, the oil absorption and binder absorption of the particles treated according to the invention decrease markedly. Dispersions of treated carbon particles in liquid media exhibit markedly lower viscosities than dispersions of untreated carbon particles. The solids content of liquid carbon dispersions can be increased by more than 5% by using carbon particles according to the invention. The electrical resistance of the carbons treated according to the invention decreases. The changes in surface contours of the individual particles which result from the treatment of powders according to the invention can be clearly seen from scanning electron micrographs.
Experimental Section
The particle size distribution of the materials was determined by laser diffraction analysis using a MALVERN Mastersizer. The structural parameters were obtained from X-ray diffraction experiments based on the CuKα1 line. The crystallographic cell constant in the c direction (c/2) was determined from the relative position of the (002) or (004) diffraction reflex. The maximum height of the single-crystal domains in a particle in the crystallographic c direction, Lc, and the resulting number of ideally stacked graphite planes were obtained from the (002) or (004) diffraction reflex according to the model of Scherrer and Jones (P. Scherrer, Göttinger Nachrichten 2 (1918) p. 98; F. W. Jones, Proc. Roy. Soc. (London) 166 A (1938) p. 16). The xylene density was determined according to DIN 51 901. Determination of the Scott density was based on ASTM B 329. The tamped density was determined according to AKK-19. The specific surface areas were determined by the method of Brunauer, Emmett and Teller using a Micromeritics ASAP 2010. To determine the elastic recovery, the material was placed under a pressure of 0.5 to/cm2. The recovery was obtained from the height of the moulding with and without applied pressure and is given in percent. The electrical resistance was measured according to DIN 51 911 using a moulding produced under a pressure of 2.5 to/cm2. The pressed density of this moulding is also given. The oil absorption was measured on the basis of DIN ISO 787 with initial weights of 0.5 g of material and 1.5 g of oil. The mixture was centrifuged in a Sigma 6–10 centrifuge for 90 min at a speed of 1500 rpm.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2165/99 | Nov 1999 | CH | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CH00/00514 | 9/22/2000 | WO | 00 | 7/15/2002 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO01/38220 | 5/31/2001 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 3333841 | Olstowski | Aug 1967 | A |
| 3553527 | Gutsche et al. | Jan 1971 | A |
| 3626149 | Carney et al. | Dec 1971 | A |
| 3642538 | Zito | Feb 1972 | A |
| 3684446 | Markel | Aug 1972 | A |
| 3807961 | Markel | Apr 1974 | A |
| 3852113 | Yokota et al. | Dec 1974 | A |
| 4041220 | Armand | Aug 1977 | A |
| 4388381 | Mennicke et al. | Jun 1983 | A |
| 4409073 | Goldberger | Oct 1983 | A |
| 4419455 | Shikeno et al. | Dec 1983 | A |
| 4435444 | Goldberger | Mar 1984 | A |
| 4543240 | Goldberger | Sep 1985 | A |
| 4547430 | Goldberger et al. | Oct 1985 | A |
| 4560409 | Goldberger et al. | Dec 1985 | A |
| 4634545 | Zaleski et al. | Jan 1987 | A |
| 4863818 | Yoshimoto et al. | Sep 1989 | A |
| 4895713 | Greinke et al. | Jan 1990 | A |
| 5082296 | Aizawa et al. | Jan 1992 | A |
| 5086022 | Roca et al. | Feb 1992 | A |
| 5094780 | von Bonin | Mar 1992 | A |
| 5103609 | Thoreson et al. | Apr 1992 | A |
| 5118576 | Imae et al. | Jun 1992 | A |
| 5134030 | Ueda et al. | Jul 1992 | A |
| 5149055 | Huber et al. | Sep 1992 | A |
| 5149518 | Mercuri et al. | Sep 1992 | A |
| 5169876 | Heitmann et al. | Dec 1992 | A |
| 5180459 | Bauer et al. | Jan 1993 | A |
| 5183273 | Dennys et al. | Feb 1993 | A |
| 5183491 | D'Iribarne et al. | Feb 1993 | A |
| 5188376 | Ueda et al. | Feb 1993 | A |
| 5192811 | Heitmann et al. | Mar 1993 | A |
| 5194198 | von Bonin et al. | Mar 1993 | A |
| 5221575 | Nakano et al. | Jun 1993 | A |
| 5222744 | Dennys | Jun 1993 | A |
| 5226662 | Justus | Jul 1993 | A |
| 5246638 | Goldberger | Sep 1993 | A |
| 5270902 | Bellar et al. | Dec 1993 | A |
| 5282975 | Maryasin et al. | Feb 1994 | A |
| 5283219 | Mauran et al. | Feb 1994 | A |
| 5288429 | von Bonin et al. | Feb 1994 | A |
| 5294300 | Kusuyama et al. | Mar 1994 | A |
| 5294382 | Goldberger | Mar 1994 | A |
| 5301960 | Meyer et al. | Apr 1994 | A |
| 5309690 | Symons | May 1994 | A |
| 5330680 | Sakawaki et al. | Jul 1994 | A |
| 5346650 | Toda et al. | Sep 1994 | A |
| 5348694 | Goldberger | Sep 1994 | A |
| 5362074 | Gallo et al. | Nov 1994 | A |
| 5370405 | Ueda | Dec 1994 | A |
| 5381818 | Nendzig et al. | Jan 1995 | A |
| 5382387 | von Bonin | Jan 1995 | A |
| 5395469 | Suggs et al. | Mar 1995 | A |
| 5397643 | von Bonin et al. | Mar 1995 | A |
| 5413359 | Latty | May 1995 | A |
| 5421594 | Becerra | Jun 1995 | A |
| 5431831 | Vincent | Jul 1995 | A |
| 5445748 | Holinski | Aug 1995 | A |
| 5452904 | Huber et al. | Sep 1995 | A |
| 5454397 | Miszczak | Oct 1995 | A |
| 5468003 | Staab et al. | Nov 1995 | A |
| 5482798 | Mototani et al. | Jan 1996 | A |
| 5499825 | Maeda et al. | Mar 1996 | A |
| 5501582 | Gautier et al. | Mar 1996 | A |
| 5503717 | Kang et al. | Apr 1996 | A |
| 5509993 | Hirschvogel | Apr 1996 | A |
| 5518189 | Grondin et al. | May 1996 | A |
| 5518519 | Kondoh et al. | May 1996 | A |
| 5522603 | Naitou et al. | Jun 1996 | A |
| 5531454 | Borneby | Jul 1996 | A |
| 5540277 | Gourlia et al. | Jul 1996 | A |
| 5544898 | Nendzig et al. | Aug 1996 | A |
| 5549306 | Ueda | Aug 1996 | A |
| 5554348 | Bauer | Sep 1996 | A |
| 5560892 | Bennett et al. | Oct 1996 | A |
| 5569534 | Lambotte et al. | Oct 1996 | A |
| 5570501 | Atkinson et al. | Nov 1996 | A |
| 5573253 | Naitou et al. | Nov 1996 | A |
| 5582781 | Hayward | Dec 1996 | A |
| 5605341 | Ueda | Feb 1997 | A |
| 5607889 | Prosdocimi et al. | Mar 1997 | A |
| 5612272 | Prosdocimi et al. | Mar 1997 | A |
| 5628520 | Ueda et al. | May 1997 | A |
| 5634645 | Seki et al. | Jun 1997 | A |
| 5656794 | Krone et al. | Aug 1997 | A |
| 5671929 | Pflug | Sep 1997 | A |
| 5672327 | Bennett et al. | Sep 1997 | A |
| 5683281 | Metter | Nov 1997 | A |
| 5683778 | Crosier | Nov 1997 | A |
| 5687974 | Wilkinson et al. | Nov 1997 | A |
| 5698088 | Kang et al. | Dec 1997 | A |
| 5706165 | Saito et al. | Jan 1998 | A |
| 5716055 | Wilkinson et al. | Feb 1998 | A |
| 5722670 | Novak et al. | Mar 1998 | A |
| 5765838 | Ueda et al. | Jun 1998 | A |
| 5772215 | West | Jun 1998 | A |
| 5776372 | Saito et al. | Jul 1998 | A |
| 5786555 | Saito et al. | Jul 1998 | A |
| 5788865 | Smirnov et al. | Aug 1998 | A |
| 5795518 | Parr | Aug 1998 | A |
| 5803464 | Ueda et al. | Sep 1998 | A |
| 5820788 | Smith | Oct 1998 | A |
| 5824923 | Kondoh et al. | Oct 1998 | A |
| 5826669 | Zaleski et al. | Oct 1998 | A |
| 5842350 | Spinner et al. | Dec 1998 | A |
| 5856398 | Oizumi et al. | Jan 1999 | A |
| 5858486 | Metter et al. | Jan 1999 | A |
| 5861207 | Bou et al. | Jan 1999 | A |
| 5876042 | Graf et al. | Mar 1999 | A |
| 5882570 | Hayward | Mar 1999 | A |
| RE36206 | Borneby | May 1999 | E |
| 5972538 | Saito et al. | Oct 1999 | A |
| 5976727 | Mercuri et al. | Nov 1999 | A |
| 5981072 | Mercuri et al. | Nov 1999 | A |
| 6030913 | Heine et al. | Feb 2000 | A |
| 6149972 | Greinke | Nov 2000 | A |
| 6169059 | Skiles et al. | Jan 2001 | B1 |
| 6194358 | Lawrence et al. | Feb 2001 | B1 |
| 6231791 | Heine et al. | May 2001 | B1 |
| 6258457 | Ottinger et al. | Jul 2001 | B1 |
| 6268086 | Honbo et al. | Jul 2001 | B1 |
| 6287694 | Zaleski et al. | Sep 2001 | B1 |
| 6350722 | Skiles et al. | Feb 2002 | B1 |
| 6372376 | Fronk et al. | Apr 2002 | B1 |
| 6383467 | Honbo et al. | May 2002 | B1 |
| 6385956 | Ottinger et al. | May 2002 | B1 |
| 6406612 | Greinke et al. | Jun 2002 | B1 |
| 6413663 | Mercuri | Jul 2002 | B1 |
| 6416815 | Greinke et al. | Jul 2002 | B1 |
| 6494926 | Saito et al. | Dec 2002 | B1 |
| 6555223 | Kubo | Apr 2003 | B1 |
| 6558782 | Bakman et al. | May 2003 | B1 |
| 6620359 | Meza et al. | Sep 2003 | B1 |
| 20010001096 | Skiles et al. | May 2001 | A1 |
| 20010046557 | Greinke et al. | Nov 2001 | A1 |
| 20020045100 | Honbo et al. | Apr 2002 | A1 |
| 20020109125 | Krassowski et al. | Aug 2002 | A1 |
| 20020141933 | Kubo | Oct 2002 | A1 |
| 20020168526 | Mercuri et al. | Nov 2002 | A1 |
| 20020172855 | Mercuri et al. | Nov 2002 | A1 |
| 20020197476 | Mercuri et al. | Dec 2002 | A1 |
| 20030113542 | Norley et al. | Jun 2003 | A1 |
| 20030116753 | Norley et al. | Jun 2003 | A1 |
| Number | Date | Country |
|---|---|---|
| 2246953 | Mar 1998 | CA |
| 2246953 | Sep 1999 | CA |
| 1253130 | Oct 1967 | DE |
| 3505656 | Aug 1986 | DE |
| 3704537 | Aug 1988 | DE |
| 0044867 | May 1985 | EP |
| 0085121 | Jun 1985 | EP |
| 0228488 | Jul 1987 | EP |
| 0274165 | Jul 1988 | EP |
| 0305984 | Aug 1988 | EP |
| 0375469 | Jun 1990 | EP |
| 0375469 | Apr 1991 | EP |
| 0432944 | Jun 1991 | EP |
| 0228488 | Sep 1991 | EP |
| 0274165 | Mar 1992 | EP |
| 0557906 | Sep 1993 | EP |
| 0596801 | May 1994 | EP |
| 0675556 | Apr 1995 | EP |
| 0375469 | Apr 1996 | EP |
| 0784352 | Jul 1997 | EP |
| 0998984 | May 2000 | EP |
| 1094045 | Apr 2001 | EP |
| 1120378 | Aug 2001 | EP |
| 1120378 | Sep 2001 | EP |
| 1213272 | Jun 2002 | EP |
| 1221432 | Jul 2002 | EP |
| 1213272 | Apr 2003 | EP |
| 1221432 | Apr 2003 | EP |
| 2677975 | Dec 1992 | FR |
| 58156515 | Sep 1983 | JP |
| 60166211 | Aug 1985 | JP |
| 06100727 | Apr 1994 | JP |
| WO8102292 | Aug 1981 | WO |
| WO83001170 | Jan 1983 | WO |
| WO9722677 | Jun 1997 | WO |
| WO9901403 | Oct 1998 | WO |
| WO9845223 | Jan 1999 | WO |
| WO9946437 | Sep 1999 | WO |
| WO0021151 | Apr 2000 | WO |
| WO0179137 | Oct 2001 | WO |
| WO0189992 | Nov 2001 | WO |
| WO0211900 | Feb 2002 | WO |
| WO0240807 | May 2002 | WO |
| WO0244080 | Jun 2002 | WO |
| WO02071508 | Sep 2002 | WO |
| WO02083775 | Oct 2002 | WO |
