Field of the Invention The present invention is directed to charge generation layers which comprise a charge generation compound and microspheres. The invention is also directed to photoconductors including such charge generation layers. The invention is further directed to methods for forming charge transport layers.

Background of the Invention In electrophotography, a latent image is created on the surface of an image member which is a photoconducting material by first uniformly charging the surface and selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between those areas on the surface which are exposed to light and those areas on the surface which are not exposed to light. The latent electrostatic image is developed into a visible image by electrostatic toners. The toners are selectively attracted to either the exposed or unexposed portions of the photoconductor surface, depending on the relative electrostatic charges on the photoconductor surface, the development electrode and the toner. Electrophotographic photoconductors may be a single layer or a laminate formed from two or more layers (multi-layer type and configuration). Typically, a dual layer electrophotographic photoconductor comprises a substrate such as a metal ground plane member on which a charge generation layer (CGL) and a charge transport layer (CTL) are coated. The charge transport layer contains a charge transport material which comprises a hole transport material or an electron transport material. For simplicity, the following discussions herein are directed to the use of a charge transport layer which comprises a hole transport material as a charge transport compound. One skilled in the art will appreciate that if the charge transport layer contains an electron transport material rather than the hole transport material, the charge placed on the photoconductor surface will be opposite that described herein.

When the charge transport layer containing a hole transport material is formed on the charge generation layer, a negative charge is typically placed on the photoconductor surface. Conversely, when the charge generation layer is formed on the charge transport layer, a positive charge is typically placed on the photoconductor surface.

Conventionally, the charge generation layer comprises a charge generation compound or molecule alone and/or in combination with a binder. A charge transport layer typically comprises a polymeric binder containing the charge transport compound or molecule.

The charge generation compounds within the charge generation layer are sensitive to image-forming radiation and photogenerate electron hole pairs therein as a result of absorbing such radiation. The charge transport layer is usually non-absorbent of the image-forming radiation and the charge transport compounds serve to transport holes to the surface of the negatively charged photoconductors. Photoconductors of this type are disclosed in the Adley et al U. S. Patent No. 5, 130, 215 and the Balthis et al U. S. Patent No. 5, 545, 499.

Typically, the charge generation layer comprises a pigment or dye (phthalocyanines, azo compounds, squaraines, etc.), with or without a polymeric binder.

Since the pigment or dye in the charge generation layer typically does not have the capability of binding or adhering effectively to a metal substrate, the polymer binder is usually inert to the electrophotographic process, but forms a stable dispersion with the pigment/dye and has good adhesive properties to the metal substrate. The electrical sensitivity associated with the charge generation layer can be affected by the nature of polymeric binder used. The polymeric binder, while forming a good dispersion, should have a greater interaction with the metal substrate rather than the pigment.

Similarly, the charge transport layer typically consists of a charge transport molecule (CTM), typically selected from arylamines, hydrazones, stilbenes, pyrazolines, and other known in the art in a polymeric binder. The polymeric binder is typically a polycarbonate such as polycarbonate-A, polycarbonate-Z, etc. which provides good mechanical properties to the photoconductor.

The photoconductor (conventionally in drum, web or belt form) is often subjected to several modes of abrasion by paper, cleaner, toner, end-seals, and the like. Therefore, it is imperative that the wear on the photoconductor be minimal for the photoconductor to have an extended long life in a printer cartridge. Increased wear on a photoconductor

surface may lead to arcing of the charge roll, increased fatigue, scratches on the paper area, delamination, and the like, resulting in defects and decreased photoconductor life in the cartridge.

There have been several approaches to improving the sensitivity of the CGL in a photoconductor. Sensitivity may be improved by the use of certain pigments (e. g. Type- IV titanyl phthalocyanine instead of Type-I titanyl phthalocyanine or squarylium pigment), increasing the pigment concentration with respect to the polymeric binder, or through the use of polymeric blends in the charge generation layer.

Additionally, there have been several approaches to reduce the wear on photoconductor surfaces. The use of polycarbonate-Z has been known to exhibit improved wear resistance over polycarbonate-A. In addition, the use of polymeric blends, overcoats, organic additives (fluoropolymers, silicone oils, etc.), and inorganic additives have been known to improve the wear on the photoconductor surface. These approaches have varying effects on photoelectric properties of the photoconductors.

As such, there is a continuing need for photoconductors increase exhibiting photoconductor sensitivity and enhance resistance to wear.

Summary of the Invention Accordingly, it is the object of the present invention to provide novel photoconductors and/or novel charge generation layers which overcome one or more disadvantages of the prior art. It is a more specific object of the invention to provide charge generation layers which improve electrical sensitivity and improve the wear resistance of photoconductors. Another object of the invention is to provide methods for forming charge transport layers, which methods allow a reduction in the amount of charge transport compound utilized as compared to conventional charge transport layers.

These and additional objects are provided by the charge generation layers, photoconductors including the same and the methods for forming charge transport layers of the present invention.

In one aspect of the present invention, the charge generation layer is comprised of a charge generation compound, a polymeric binder and microspheres. Another embodiment of the present invention is directed to a photoconductor comprising a

substrate, a charge generation layer and a charge transport layer, wherein the charge generation layer comprises a charge generation compound, a polymeric binder and microspheres.

Another embodiment of the present invention is directed to a charge generation layer which comprises a charge generation compound, a binder, and an effective amount of microspheres to improve at least one electrophotographic property.

Another embodiment of the present invention is directed to a method for forming a charge transport layer. The method comprises the steps of providing a mixture of solvent, binder, charge transport compound, surfactant and microspheres at an elevated temperature ; cooling the mixture ; and coating the mixture on a charge generation layer of a photoconductor.

These and additional objects and advantages will be more readily apparent in view of the following detailed description.

Detailed Description The charge transport layers and charge generation layers according to the present invention are suitable for use in single or multi-layer photoconductors. Dual layer photoconductors generally comprise a substrate, a charge generation layer and a charge transport layer. While various embodiments of the invention discussed herein refer to a charge generation layer as being formed on the substrate, with a charge transport layer formed on the charge generation layer, it is equally within the scope of the present invention for the charge transport layer to be formed on the substrate with a charge generation layer formed at the charge transport layer.

The present invention is directed to charge generation layers and photoconductors containing microspheres and to methods for forming charge transport layers.

In one embodiment of the present invention, a charge generation layer comprises a charge generation compound, a binder and microspheres. Various microspheres are commercially available and suitable for use in the invention. Preferably, the microspheres comprise silicone microspheres.

Yet another embodiment of the present invention is a charge generation layer comprising a charge generation compound, a binder and an effective amount of microspheres to improve at least one electrophotographic property such as lower dark decay, lower residual voltage and higher sensitivity. In a preferred embodiment, the

charge generation layer comprises the microspheres and the binder in a weight ratio of from about 1 : 50 to about 1 : 1 ; more preferably from about 1 : 20 to about 1 : 1 ; and most preferably from about 1 : 10 to about 1 : 1. Preferably, the silicone microspheres have a particle size of less than about 2 microns, more preferably less than about 1 micron, and even more preferably, less than about 0. 5 microns.

In another preferred embodiment of the present invention, the charge generation layer comprises from about 5 to about 80 weight percent of the charge generation compound, from about 10 to about 80 weight percent of the microspheres and from about 10 to about 85 weight percent of the binder. More preferably, the charge generation layer comprises from about 15 to about 60 weight percent of the charge generation compound, from about 10 to about 45 weight percent of the microspheres, and from about 20 to about 75 weight percent of the binder. Even more preferred, the charge generation layer comprises from about 10 to about 40 weight percent of the charge generation compound, from about 20 to about 40 weight percent of the microspheres and from about 30 to about 70 weight percent of the binder.

In a preferred embodiment of the invention, the charge generation layer comprises silicone microspheres and the binder in a weight ratio from about 1 : 50 to about 1 : 1 ; more preferably from about 1 : 20 to about 1 : 1 and most preferably from about 1 : 10 to about 1 : 1.

In another preferred embodiment, the charge generation layer comprises from about 5 to about 50 weight percent of the charge generation compound, from about 30 to about 40 weight percent of the silicone microspheres and from about 20 to about 65 weight percent of the binder.

Various charge generation compounds are known in the art and are suitable for use in the present charge generation layers. Examples of suitable charge generation compounds include inorganic and organic pigments and dyes, for example, squarylium compounds, perylene compounds, phthalocyanines (both metal and metal-free), and the like. In a preferred embodiment, the charge generation compound comprises a metal or metal-free phthalocyanine. More preferably, the charge generation compound comprises a metal phthalocyanine, particularly titanyl phthalocyanine.

Various binder resins are known for use in charge generation layers and are suitable for use in the present invention. In another preferred embodiment of the present invention, the binder in the charge generation layer comprises polyvinylbutyral.

Another embodiment of the present invention is directed to a photoconductor comprising a substrate, a charge generation layer and a charge transport layer, wherein the charge generation layer comprises a charge generation compound, a binder and microspheres. Preferably, the microspheres in the charge generation layer of the photoconductor comprise silicone microspheres. In a preferred embodiment, the charge generation layer of the photoconductor comprises the microspheres and the binder in a weight ratio from 1 : 50 to about 1 : 1 ; more preferably from about 1 : 20 to about 1 : 1 and most preferably from about 1 : 10 to about 1 : 1. In another preferred embodiment, the charge generation layer of the photoconductor comprises from about 5 to about 80 weight percent of the charge generation compound, from about 10 to about 50 weight percent of the microspheres, and from about 20 to about 95 weight percent of the binder.

The charge transport layer comprises a charge transport compound and a binder.

In a preferred embodiment, the charge transport compound of the photoconductor comprises a hydrazone charge transport compound. In another preferred embodiment, the charge transport compound of the photoconductor comprises N, N-diethylamino benzaldehyde-1, 1-diphenylhydrazone. In yet another preferred embodiment, the charge transport compound of the photoconductor comprises a diamine charge transport compound, more preferably, the charge transport compound comprises N, N'-bis (3- methylphenyl)-N, N'-bisphenylbenzidine. In another preferred embodiment of the present invention, the binder in the charge transport layer of the photoconductor comprises polycarbonate. More preferably, the binder of the charge transport layer of the photoconductor comprises a polycarbonate polymer or copolymer, and even more preferably comprises polycarbonate-A.

Preferably, the silicone microspheres comprise Tospearl. Tospearl is a silicone polymer having a complex silicon structure formed of organic and inorganic silicone compounds which provide a network structure with siloxane bonds extending in three dimensions. Tospearl has a spherical appearance and has a mean particle diameter preferably ranging from about 0. 1 microns to about 12. 0 microns. Preferably, Tospearl has a moisture content of less than 5% by weight at 105 C. Preferably, the bulk density ranges from about 0. 1 to about 0. 5 and the specific surface area ranges from about 15 m2/gram to about 90 m2/gram. Tospearl preferably has a pH of about 7. 5. Tospearl is commercially available from D-D Chemical Company of Northridge, California under the

tradenames Tospearl 120A, Tospearl 130A and Tospearl 145A. Tospearl is additionally commercially available from GE Silicones of New York under the tradenames Tospearl 105, Tospearl 108, Tospearl 120, Tospearl 130, Tospearl 145, Tospearl 3120 and Tospearl 240.

Another embodiment of the present invention is directed to a method for forming a charge transport layer. The method comprises the steps of : providing a mixture of solvent, binder, charge transport compound, surfactant and microspheres in an elevated temperature ; cooling the mixture ; and coating the mixture on a charge generation layer of a photoconductor. Any organic solvent suitable for dissolving or dispersing the charge transport layer components may be employed. Preferably, the solvent solution comprises an aliphatic ether, alicyclic ether, or a mixture thereof. More preferably, the solvent solution comprises tetrahydrofuran, 1, 4-dioxane, or a mixture thereof. Preferably, the surfactants comprise polydimethylsiloxane. More preferably, the surfactant comprises DC-200 from Dow Chemical. DC-200 is a nonionic polydimethylsiloxane.

The mixture is heated to a temperature above ambient temperature, but lower than the solvent boiling point. In a preferred embodiment, the elevated temperature comprises a temperature of from about 30°C to about 65 C. More preferably, the elevated temperature comprises a temperature of from about 40°C to about 60 C, and most preferably comprises a temperature from about 45 C to about 60°C.

Preferably, a polycarbonate binder is dissolved in the solvent system (which preferably comprises about 75 weight percent tetrahydrofuran and about 25 weight percent 1, 4-dioxane), followed by dissolution of the charge transport compound and addition of a surfactant (DC-200) and the silicone microspheres. Preferably, the mixture is heated for at least 20 minutes to a temperature of not greater than 60 C. The mixture is then cooled to ambient temperature to allow for more uniform coating of the photoconductor.

The following examples demonstrate various embodiments and advantages of the charge generation layers and/or charge transport layers and photoconductors according to the present invention. In the examples and throughout the present specification, parts and percentages are by weight unless otherwise indicated.

Example 1 In this example, photoconductors according to the present invention and comparative photoconductors were prepared using charge generation layers according to the present invention and conventional charge generation layers, respectively. Each of the photoconductors described in this example was prepared by dip-coating a charge generation layer dispersion on an aluminum substrate, followed by dip-coating a charge transport layer on the charge generation layer. In each of the photoconductors, the charge generation layer comprised about 30 weight percent N, N'-bis (3-methylphenyl)-N, N'- diphenylbenzidine (TPD) and about 70 weight percent polycarbonate binder (Makrolon- 5208 from Bayer).

The charge generation layers of the respective photoconductors according to this example comprised polymeric binder (polyvinylbutyral, BX-55Z from Sekisui Chemical Co.) and a charge generation compound. As described in Table 1, compositions 1B, 1C, 1E and 1F additionally contained silicone microspheres (Tospearl from Toshiba/GE Silicones). The charge generation compound of this example was a type IV polymorph of titanyl phthalocyanine (TiOPc).

As will be apparent from Table 1, photoconductors 1A, 1B, 1C and 1D are comparative photoconductors, whereas photoconductors 1E and 1F are photoconductors containing charge generation layers according to the present invention comprising a charge generation compound in combination with silicone microspheres.

The charge generation layers of photoconductors lA-1F were prepared by dissolving a binder in a solvent mixture. The silicone microspheres were added to this mixture and the mixture was stirred. In the case of photoconductor 1F, a solution containing the binder and silicone microspheres was heated to about 50 C and cooled to room temperature. For photoconductors lA-lE, the binder and silicone microspheres were stirred in the solvent mixture at ambient temperature. The binder/Tospearl dispersion prepared with or without heat was then agitated in a paint-shaker along with phthalocyanine pigment. The resulting dispersions were used to prepare photoconductors lA-1F.

Table 1 : CGL Photocon Weight Weight Percent Silicone ductor Percent Polyvinylbutyral Microspheres, Type IV TiOPc Wt. % lA 45 55 0 1B 45 52. 25 2. 75 1C 45 49.5 5.5 1D 35 65 lE 35 52 13 1F* 35 52 13 * CGL prepared with heat Optical density and various electrical characteristics of the photoconductors described in this example were examined. Specifically, drum optical density was measured using a MacBeth TR 524 sensitometer. Sensitivity measurements were made using an electrostatic sensitometer, fitted with electrostatic probes to measure voltage as a function of light energy shining on the photoconductor surface using a 780nm laser. The drum was charged by a Corona and the exposed-to-develop time for all measurements was 76 milliseconds. Photosensitivity was measured at a discharge voltage on the photoconductor drum previously charged to about-850 volts and measured at a light energy of 0. 21, uJ/cm2. The results of the described measurements are set forth in Table 2.

Table 2 Photoconductor Optical Density Charge Voltage Voltage @ 0. 21 Residual Voltage (-V) IlJ/cm2 (-V) 1A 1. 53 848 219 89 1B 1. 56 846 213 92 1C 1. 49 850 216 92 1D 1. 5 847 352 163 1E 1. 54 851 305 121 1F* 1.52 846 296 126 * CGL prepared with heat

The results set forth in Table 2 demonstrate that the photoconductors 1E and IF comprising charge generation layers according to present invention and containing 13 weight percent silicone microspheres in the charge transport layer exhibited enhanced sensitivity over the comparative photoconductor 1D. The addition of silicone microspheres at about 5 weight percent or less in the charge generation layer of photoconductors 1B and 1C resulted in minimal differences from the comparative photoconductor 1A containing no silicone microspheres. The results also indicate the effect of heat in preparing the binder-silicone microspheres dispersion of photoconductor IF was relatively insignificant to the sensitivity of the photoconductor when compared to the similar photoconductor 1 E in which the binder-silicone micropsheres dispersion was prepared with no heat added.

Example 2 In this example, photoconductors according to the present invention and comparative photoconductors were prepared using charge generation layers according to the present invention and conventional charge generation layers, respectively. Each of the photoconductors described in this example was prepared by dip-coating a charge generation layer on an aluminum substrate, followed by dip-coating a charge transport layer dispersion on the charge generation layer. In each of the photoconductors, the charge transport layer comprised about 30 weight percent bis (N-N'-3-methylphenyl)- N, N'-diphenylbenzidine (TPD) and about 70 weight percent polycarbonate binder (Makrolon-5208 from Bayer).

The charge generation layers of the respective photoconductors according to this example comprised polymeric binder (polyvinylbutyral, BX-55Z from Sekisui Chemical Co.) and a charge generation compound. As described in Table 3, compositions 2B, 2C, 2E and 2F additionally contained silicone microspheres with varying particle sizes. The charge generation compound of this example was a type-I polymorph of titanyl phthalocyanine (TiOPc).

As will be apparent from Table 3, photoconductors 2A and 2D are comparative photoconductors, whereas photoconductors 2B, 2C, 2E and 2F are photoconductors containing charge generation layers according to the present invention comprising a charge generation compound in combination with silicone microspheres.

Table 3 : CGL Photoconductor Type I TiOPC, Polyvinylbutyral, Silicone Particle Size, Wt. % Wt. % Microspheres, Wt. % 2A* 45 55 0------ 2B 45 44 11 0. 3 2C 45 44 11 2 2D* 45 55 0------ 2E 45 44 11 0. 3 2F 45 44 11 2 *comparative photoconductor Optical density, dark decay and various electrical characteristics of the photoconductors were examined using the techniques as described in Example 1. The results of the described measurements are set forth in Table 4.

Table 4 Optical Charge Voltage Voltage Residual Dark Decay Photoconductor Density (-V) @0. 51) J7cmVoltage (-V) 2A* 1. 11 700 454 323 22 2B 1. 11 701 418 226 18 2C 1. 09 702 442 258 21 2D* 1. 28 699 375 237 47 2E 1. 27 698 354 171 39 2F 1. 25 699 364 188 39 *comparative photoconductor

The results set forth in Table 4 demonstrate that the photoconductors 2B, 2C, 2E and 2F comprising charge generation layers according to the present invention and containing eleven percent microspheres in the charge generation layer exhibited enhanced sensitivity over the comparative photoconductors 2A and 2D. The results also indicate that the smaller particle size silicone microspheres in photoconductors 2B and 2E lowered the residual voltage to a greater extent than photoconductors 2C and 2F which comprised larger particle size silicone microspheres. One possible explanation for the lower residual voltage could be that the formulations were prepared based on weighted material, therefore the smaller particle size material resulted in a greater quantity of silicone

microspheres dispersed in the photoconductors than in the photoconductors containing larger particle size silicone microspheres.

In addition, as set forth in Table 4, the dark decay was reduced in photoconductors containing the silicone microspheres in the charge generation layer according to the present invention.

Example 3 In this example, photoconductors according to the present invention and comparative photoconductors were prepared using charge transport layers according to the present invention and conventional charge transport layers, respectively. Each of the photoconductors described in this example was prepared by dip-coating a charge generation layer dispersion on an aluminum substrate, followed by dip-coating a charge transport layer dispersion on the charge generation layer. In each of the photoconductors, the charge generation layer comprised about 45 weight percent type-IV polymorph of titanyl phthalocyanine (TiOPc) and about 55 weight percent polymeric binder.

The charge transport layers of the respective photoconductors according to this example comprised a polymeric binder, silicone microspheres and a charge transport compound. As described in Table 5, compositions 5B and 5D were prepared with the addition of heat during formulation of the charge transport dispersions, whereas compositions 5A and 5C were formulated without the addition of heat to the charge transport dispersions. The charge transport compound of this example was N, N- diethylamino benzaldehyde-l, l-diphenylhydrazone (DEH). In addition, 4 drops of surfactant were added to the charge generation layer dispersions during formulation of the dispersions.

As will be apparent from Table 5, photoconductors 5A and 5C are comparative photoconductors, whereas photoconductors 5B and 5D are photoconductors containing charge transport layers formulated according to the present invention.

Table 5 : CTL PhotoconductorCharge TransportPolycarbonate-A, Silicone Compound Wt. % Microspheres, (DEH), Wt. % Wt. % SA 35 63. 05 1. 95 5B* 35 63. 05 1. 95 5C 30 67. 9 2. 1 5D* 30 67. 9 2. 1 * CTL prepared with heat Various electrostatic properties of the photoconductors were measured using the techniques as described in Example 1. The results of the measurements are set forth in Table 6.

Table 6 Photoconductor Coat Weight Charge Voltage Voltage @ Residual Voltage (mg/in2) (-V) 0. 2 J/cm2 (- 5A 17 849 376 221 5B* 16. 1 849 414 207 5C 19. 1 848 462 358 5D* 18 849 482 321 * CTL prepared with heat The results set forth in Table 6 demonstrate that the photoconductors 5B and 5D comprising charge transport layers formulated according to the present invention with the addition of heat exhibited enhanced sensitivity over the comparative photoconductors 5A and 5C.

Example 4 In this example, photoconductors according to the present invention and comparative photoconductors were prepared using charge transport layers according to the present invention and conventional charge transport layers, respectively. Each of the photoconductors described in this example was prepared by dip-coating a charge generation layer dispersion on an aluminum substrate, followed by dip-coating a charge transport layer dispersion on the charge generation layer. In each of the photoconductors, the charge generation layer comprised about 45 weight percent type-IV polymorph of titanyl phthalocyanine (TiOPc) and about 55 weight percent binder. The binder in the charge generation layer of photoconductors 7A-7C comprises about 100 weight percent polyvinylbutyral, BX-55Z from Sekisui Chemical. The binder in the charge generation layers of photoconductors 7D-7F comprises about 25 weight percent polyvinylbutyral (BX-55Z) and about 75 weight percent epoxy resin, Epon-1001 from Shell Chemical.

The charge transport layers of the respective photoconductors according to this example comprised a polymeric binder and a charge transport compound. As described in Table 7, compositions 7C and 7F additionally contained silicone microspheres. The charge transport compound of this example was N, N'- (bis-3-methylphenyl)-N, N-di phenylbenzidine (TPD).

As will be apparent from Table 7, photoconductors 7A, 7B, 7D and 7E are comparative photoconductors whereas photoconductors 7C and 7F are photoconductors containing charge transport layers according to the present invention.

Table 7 : CTL Photoconductor TPD, Wt. % Polycarbonate-A, Wt. % Silicone Microspheres, Wt. % 7At* 30 70 7B1* 25 75 ---- 7C'25 71. 25 3. 75 7D>* 30 70 7E2* 25 75 7F2 25 71.25 3.75 * comparative photoconductor 1 CGL (100 wt. % polyvinylbutyral) 2 CGL (25 wt. % polyvinylbutyral, 75 wt. % epoxy resin) Various electrostatic properties of the photoconductors as described in Example 1 were measured. The results of the measurements are set forth in Table 8.

Table 8 Photoconductor Coat Weight Charge Voltage Voltage @ Residual Voltage (mg/in2) (-V) 0. 21lJ/cm2 (-V) 7A'* 20. 8 847 331 177 7B'* 18. 7 854 351 236 7C1 19. 1 849 323 175 7D2* 19.7 851 215 153 7E2* 18. 1 854 222 203 7F2 18. 5 849 182 148 *comparative photoconductor 'CGL (100 wt. % polyvinylbutyral) 2CGL (25 wt. % polyvinylbutyral, 75 wt. % epoxy resin)

The results set forth in Table 8 demonstrate that photoconductors 7C and 7F comprising charge transport layers according to the present invention and formulated with the addition of heat exhibited similar sensitivity and electrostatic properties to photoconductors 7A and 7D respectively. Photoconductors 7C and 7F comprised 5 weight percent less of the charge transport compound but resulted in photoconductors having similar electrostatic properties. Thus, photoconductors comprising less charge transport material than typical photoconductors can exhibit similar properties at a cost savings due to the reduced amount of charge transport compound required therein.

Thus, these Examples demonstrate that the photoconductors according to the present invention exhibit good electrical characteristics. The various preferred embodiments and examples set forth herein are presented in order to further illustrate the claimed invention and are not intended to be limiting thereof. Additional embodiments and alternatives within the scope of the claimed invention will be apparent to those of ordinary skill in the art.