PATENT ABSTRACT
A method of forming a seal for a subterranean drilling tool employs irradiation. The seal is formed of a single homogeneous elastomeric material. The seal has an exterior surface with at least one portion that is in sliding engagement with part of a seal gland of the drilling tool. A property gradient is formed in the elastomeric material on at least this portion. The property gradient is formed by exposing the seal to an environment to define a property that changes from a first level to a second level without essentially changing the composition of the elastomeric material. The step of exposing is done by irradiating the seal with an electron beam.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority to provisional application 60/586,785 filed Jul. 9, 2004. 

   FIELD OF THE INVENTION 
   This invention relates in general to manufacturing a seal for an earth-boring bit that has a property gradient portion formed on at least a portion of the seal. 
   BACKGROUND OF THE INVENTION 
   One type of earth boring bit has a bit body with three rotatable cones. Each cone has cutting implements for disintegrating earth formations. The cones are mounted on bearing pins that depend from the bit body. A lubricant cavity within the bit body supplies lubricant to the bearing spaces between the cones and bearing pins. A seal or seal assembly is located at the mouth of the cone to prevent loss of the lubricant. 
   One type of seal is an elastomeric annular member having an inner diameter and an outer diameter. The cross-sectional configuration is rounded, such as circular or elliptical. One of the diameter portions forms a dynamic seal. The dynamic seal portion is in sliding engagement and normally the other side of diameter portion forms a static seal. Sometimes both diameter portions encounter dynamic sealing engagement. The sliding engagement generates heat due to friction, thus causing wear over time. 
   Some drill bit seals are formed of a single homogeneous elastomeric material. The hardness of the elastomeric material is uniform throughout, including at both the inner and outer diameter portions. Another type of drill bit seal is formed of two or more different types of elastomer that are co-cured. One type has a greater hardness than the other type for serving as the dynamic seal portion. The softer portion provides the desired amount of force due to the squeeze on the seal when installed. It has also been proposed to plasma treat the surface of the seal with an inert gas containing a reactive gas species such as chlorine or fluorine. This treatment provides a chlorinated or fluorinated molecules at the surface. Various other techniques to change the surface of an earth boring bit seal are disclosed in the patented art. 
   SUMMARY OF THE INVENTION 
   In the method of this invention, a property gradient is formed in at least a portion of the elastomeric material. Preferably, the property gradient is formed by exposing the seal to an environment to create gradual change in at least one property of the elastomeric material, such as hardness, elastic modulus or bulk modulus. In one embodiment, an electron beam is used to irradiate the seal, increasing cross-linking of the polymer chains within the property gradient. The elastomeric material within the property gradient is essentially chemically the same as the remaining portion of the seal. Preferably, one property comprises a decreasing hardness in the gradient from the exterior surface for a selected depth. A portion of the exterior surface may be free of any property gradient. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view of an earth-boring bit having a seal constructed in accordance with this invention. 
       FIG. 2  is an enlarged sectional view of the seal of  FIG. 1 , shown removed from the bit. 
       FIG. 3  is a further enlarged sectional view of a portion of the seal of  FIG. 1 , shown installed within the bit. 
       FIG. 4  is a graph illustrating for the seal of  FIG. 2  the hardness versus the depth from the dynamic surface of the seal. 
       FIG. 5  is a sectional view of an alternate embodiment of a seal for an earth-boring bit of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , drill bit  11  has a bit body with at least one bit leg  13 , and typically three legs. Each bit leg  13  has a depending bearing pin  15 . A cone  17  rotatably mounts over each bearing pin  15 . A seal  19  seals lubricant within a cavity of cone  17  surrounding bearing pin  15 . A compensator  21  is in communication with the lubricant for equalizing the pressure of the lubricant with the drilling fluid hydrostatic pressure on the exterior of drill bit  11 . Drill bit  11  could also be a type utilized with compressed air as the drilling fluid. 
   Referring to  FIG. 2 , seal  19  in this embodiment is an O-ring having a circular transverse cross-section. Seal  19  has an inner diameter  23  that engages bearing pin  15  in a dynamic sliding contact as cone  17  rotates. Seal  19  has an outer diameter  25  that typically statically engages the cavity in cone  17 . Seal  19  in this embodiment thus typically rotates with cone  17 . Seal  19  is preferably formed as a single piece member of a single homogeneous polymer material, such as the following: nitrile butadiene rubber (NBR); hydrogenated nitrile butadiene rubber (HNBR); carboxylated nitrile butadiene rubber (XNBR); or hydrogenated carboxylated nitrile butadiene rubber (XHNBR). The material of seal  19  may also contain reinforcing materials, such as carbon black, silica or fibers. 
   Although a single homogenous material, at least one property of at least a portion of seal  19  differs from the exterior surface of seal  19  to its interior. A property gradient  27  extends inward a selected distance from the exterior of seal  19 . By way of example, property gradient  27  is shown in  FIG. 4  extending inward from inner diameter region  23 , however it could be alternately or also located on outer diameter region  25  or the entire exterior surface. Property gradient  27  has at least one property that changes from a first level, at the surface of seal  19 , to a second level within the interior of seal  19 . The property may be hardness, elastic modulus, bulk modulus, toughness, abrasion resistance, friction coefficient, strength and other characteristics. For example, property gradient  27  is harder at the surface of inner diameter region  23  and reduces in hardness in a direction toward the interior of seal  19 . 
   Property gradient  27  has a depth along an X-axis, which is a thickness of seal  19  from inner diameter  23  to outer diameter  25 , which is considerably less than about one-half the total distance from inner diameter  23  to outer diameter  25 . In the preferred embodiment, the depth of property gradient  27  in the X-direction is less than about 1/10 th  the transverse cross-sectional diameter of seal  19  along the X-axis. The depth of property gradient  27  in the X-direction is greater than about one percent of the transverse cross-sectional diameter of seal  19  along the X-axis. For example, for a seal having a transverse cross-sectional diameter along the X-axis of 0.250″, the depth of property gradient  27  is preferably only about 0.010–0.040″. 
   The Y-dimension  28  of property gradient  27  along the Y-axis, which is perpendicular to the X-axis, may be less than the total cross-sectional dimension of seal  19  along the Y-axis. For a circular cross-section, as shown, the cross-sectional dimension of seal  19  in the above example is 0.250″. The Y-dimension  28  of property gradient  27  in its natural state ( FIG. 2 ) prior to installation should be sufficient to present the property gradient  27  of seal  19  to bearing pin  15  when installed.  FIG. 3  shows seal  19  installed under a typical operational squeeze along the X-axis. Squeezing seal  19  along the X-axis increases the surface contact between inner diameter  23  and bearing pin  15 . The bulk of seal  19  will remain at a different property value than the property values in property gradient  27 , as indicated in  FIG. 4  for hardness. The surface area of property gradient  27  is less than the remaining portion of the exterior surface of seal  19  in this example. 
   In one embodiment, property gradient  27  has a hardness measured in terms of IRHD (International Rubber Hardness Degrees) utilizing a micro-hardness tester. The first level, at the exterior surface of seal  19  is preferably at least about 10 percent greater than the second level in property gradient  27 , which is the average value within the interior of seal  19 . In one example, the IRHD is in the range from 85 to 90 at the surface and 75 in the interior. These values correspond generally to Shore A hardness values. Conventional measuring instruments for measuring Shore A will not accurately resolve the difference in hardness within property gradient  27 . 
   Property gradient  27 , if in rotary sliding contact, wears less due to its properties than if inner diameter region  23  were of the same properties as the interior of seal  19 . One additional advantage of retaining the majority of seal  19  at a lower compressive modulus than that in property gradient  27  is that the force developed by the seal when compressed along the X-axis to its compressed position is less than if the seal were of a uniform higher stiffness. The lower compressive modulus in the majority of seal  19  thus results in reduced contact stress and consequently friction at the seal dynamic interface. 
   Seal  19  could be of different shapes. For example,  FIG. 5  shows a seal  55  that in its natural configuration is elliptical. Seal  55  has a greater cross-sectional thickness along its X-dimension than its Y-dimension, creating a major axis along in the X-direction and a minor axis in the Y-direction. Seal  55  has an inner diameter  61  and an outer diameter  63 . Property gradient  65  is shown located on the inner diameter region  61  in this embodiment, but could be on the outer diameter region  63  or the entire exterior surface. 
   There are a number of ways to achieve the desired property gradient  27 . In one method, seal  19  (or seal  55 ) is molded conventionally to form a rubber compound with generally uniform properties throughout. During the conventional molding process, which utilizes pressure and temperature, cross links are formed between the polymer chains. The cross linking occurring in the conventional process is self-limiting and utilizes a cross-linking agent, such as an organic peroxide. The cross-linking stops when the cross-linking agent is consumed. Typically, a co-agent is employed along with the cross-linking agent. 
   Subsequently, seal  19  is treated to create property gradient portion  27  without changing the essential chemical composition of the material of seal  19 . In one technique, an electron beam process is employed to generate property gradient portion  27 . Seal  19  is placed in a chamber within an inert atmosphere, such as nitrogen. A focused electron beam is directed through a window into the chamber, striking at least a portion of the exterior surface of seal  19 . If seal gradient  27  is to be only on a fractional portion of seal  19 , a shield will be employed to mask the electron beam from the remaining portion. The depth of property gradient  27  is controlled by the accelerating voltage of the electron beam. The radiation dosage varies the properties within property gradient  27 . The radiation dosage is controlled by the time of exposure. In one embodiment, the electron beam has an accelerating voltage in the range from 70 kV to 250 kV. The radiation dose delivered by the electron beam is in the range from 10 kGy to 300 kGy. During the electron beam process, additional cross-linking of the polymer chains occurs to create property gradient  27 . 
   In a second method, a radiation cross-link promoter (prorad) is utilized to enhance the further cross-linking. The prorad is included into the polymeric formulation prior to molding. The prorad survives the molding process and may serve to facilitate the cross-linking of the elastomer during the electron beam irradiation process. Suitable prorads are commercially available for electron beam curing of polymers. The following lists the prorad by chemical name and in some instances, by trademark and manufacturer:
     diallyl maleate,   triallyl cyanurate (TAC)   triallyl isocyanurate (TAIC)   n,n′-(m-phenylene bismaleimide (HVA-2)   polyacrylates and polymethacrylates   trifunctional acrylate, e.g. Saret SR.519   trifunctional methacrylate, e.g. Saret SR-517   pentacrythritol tetraacrylate, e.g. Saret SR-295   dipentaerythritol pentaacrylate, e.g. Saret SR-399   trimethylolpropane trimethacrylate, e.g. Saret SR-350   liquid butadienes with 1,2-vinyl content 45% or higher (e.g. Ricon 100, Ricon 153, Ricon 154, Krasvl LB 2000, Krasvl LB 3000, Lithene AH, Lithene AX)   Methacrylated polybutadiene (e.g. Ricacryl 3100)   Saret, Ricon and Ricacryl are trademarks of Sartomer Company, Inc.   Krasvl LB is a trademark of Kancrik A.S.   Lithene is a trademark of Synthomer Ltd.   

   The elastomeric formulation may contain any of the above prorads alone or as a combination of two or more. The amount of prorad to be added to the elastomeric formulation is about 2 to 20 phr (parts per hundred parts of rubber). In some cases, the co-agent used during the conventional molding process can also function as a prorad during radiation cross-linking. 
   In some applications, the outer diameter region of a seal may be in dynamic, sliding engagement while the inner diameter region is in static engagement. In those cases, the property gradient may be only on the outer diameter region, if desired. In other cases, the inner and outer diameter regions could be alternatively in sliding contact, in which case, both the inner diameter and the outer diameter regions would have a property gradient. Another embodiment would be for a property gradient to exist completely around the surface of the seal such that any seal surface in sliding contact would have the property gradient. 
   Another embodiment would allow for the formation of a property gradient in non-axisymmetric areas of the seal. Such areas could comprise selected regions on a sector or multiple discontinuous regions or sectors of the seal. 
   In addition to seals for drill bits, seals for other subterranean applications are feasible, particularly for downhole well and mining tools. In addition to the materials for seal  19  mentioned above, other suitable materials for seals for downhole well tools include fluorocarbon elastomers, perfluorocarbon elastomers, and fluorocarbon/propylene copolymer elastomers. 
   The invention has significant advantages. The method provides a property gradient in desired areas without changing the chemical composition of the seal. The properties on the exterior provide better wear resistance for dynamic engagement. The different properties in the interior or bulk portion of the seal avoid excessive force being generated due to deformation when installed. 
   While the invention has been described in only a few of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the invention. For example, the method of forming the property gradient may be by gamma ray or x-ray processes.