Horseshoe - Patent Review 4496002

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BACKGROUND OF THE INVENTION

The present invention relates to shoes suitable for horses, or any animal requiring hoof protection, prepared from ultra-high molecular weight polyethylene.

One of the major causes of lameness in horses is the direct result of horseshoes that restrict the natural flexing (i.e. expansion and contraction) of the horse's foot, maximize concussion, cause corns, enhance fatigue from their heavy weight, inflict severe cuts and bruises, and provide inadequate protection. Whether used for pleasure, work, or competition, horses are used on a wide variety of surfaces, natural and artificial, which can cause overstressing and damage tendons, cartilage, and bone. It has been suggested that fatigue failure is a major cause of lameness, and lameness is directly related to the hardness of the surface on which horses are used.sup.1. Data show a linear, direct relationship between lameness on California racetracks and the hardness of the tracks. The peak shock level on a horse's hoof is about 2000 pounds of force when galloping across pavement or hard dirt, and 800-1000 pounds on grass. This shock can be greatly reduced by using a shoe that has good shock resistant and impact resistant properties. When shock and concussion are reduced on the horse's feet and legs, lameness is reduced.

.sup.1 "Foreleg Fatigue Fracture," Cheney, J. A.; Lion, S. Y.; Shen, C. K.; Wheat, J. D., Thoroughbread of California, November 1971.

With the increasing number of pleasure horses, work horses (metropolitan mounted police), and draft horses being used on paved surfaces, there is concern being given to the damage hard metal shoes cause to the surfaces. Indeed, several articles have been written expressing this concern and the subsequent increased costs of street and road maintenance and repair. Horse owners, including trail riders, Amish draft horse owners, and municipalities, are encouraged to replace hard metal shoes with more resilient, less damaging shoes. Similarly, the destruction and enhanced wear on private barn floors, walkways, and surfaced paths resulting from hard metal horseshoes is significant.

Another concern to the horse owner that results from the use of hard metal shoes is the restricted "natural" action of the hoof. When a horse is in motion, the natural movement of the hoof is to expand slightly under weight and to contract when the animal's weight is shifted. Rigid metal shoes restrict this natural hoof action, which further contributes to fatigue and lameness..sup.2 A further disadvantage to the use of hard, rigid shoes can be found in that, when this natural hoof flexing is restricted by the shoe, there is a tendency to place considerable stress on the nails holding the rigid shoe to the hoof that normally expands and contracts, resulting in the loosening of the nails or pulling the nails through the hoof and eventually throwing the shoe. A heavy, hard metal shoe that comes loose or is thrown can inflict severe cuts or bruises to the animal.

.sup.2 The complex action of the parts of the hoof has been described in several articles in the literature. A comprehensive study of the structure and behavior of horses' feet in action may be found in the August 1977 issue of Saddle Action, published by Saddle Action, Inc., Paso Robles, Calif., pages 18-21, 50, written by K. D. Butler, Jr.

Accordingly, there exists a need for a light weight, shock absorbant, abrasion resistant, flexible horseshoe that will provide therapeutic advantages to the animal and minimize damage to the surfaces on which the animal is used.

Many attempts have been made in prior art to provide a protective device for the horse's foot incorporating the objectives mentioned above. For example, the hoof coverings described in U.S. Pat. Nos. 4,235,292 and 4,206,811 consist of an integrally designed steel yoke in a plastic body attached with screws. U.S. Pat. Nos. 4,189,004; 3,703,209; and 3,732,929 teach that polyurethane rubber boot-type horseshoes have desirable characteristics. A hard rubber sheet containing small metal fragments is described in U.S. Pat. No. 3,513,915. Indeed, cast polyurethane horseshoes appear to be the preferred material for flexible horseshoes, as described in U.S. Pat. Nos. 3,494,422; 3,490,536; and 3,469,631. The types of shoes as described in these references overcome some, but not all of the recognized shortcomings of metal horseshoes.

An existing need remains for an economical, light weight horseshoe which will protect the horse's hooves and be simple to attach to and remove from the horse's hooves.

SUMMARY OF THE PRESENT INVENTION

In accordance with the present invention, a lightweight, abrasion resistant, shock absorbant horseshoe is provided that essentially eliminates the known shortcomings of conventional horseshoes. The shoes are fabricated in conventional shapes from an ultra-high molecular weight ethylene polymer (UHMW PE). They can be fabricated from neat resin or resin formulations containing additives to provide various colors, metallic appearances, or special effects which include fluorescent or phosphorescent additives. Standard techniques commonly used by farriers can be used to attach the shoe to the hoof. Surprisingly, horseshoes made from UHMW PE can be cold formed to precisely fit the individual hoof using routine hammer-anvil techniques. Regular horseshoe nails can be used to attach the shoe, which has unusually high resistance to nail pull-through.

The UHMW PE shoes weight only one eighth the weight of steel shoes of like volume and have twenty times the abrasion resistance of steel. The excellent shock absorbant, impact resistant, and sound dampening properties of these shoes indicate that concussion and shock to the horse's feet and legs on hard surfaces can be reduced seventy-five to eighty percent as compared with steel shoes. The somewhat rigid but flexible properties of the UHMW PE shoes allow for the natural action (expansion and contraction) of the hoof thereby reducing fatigue, corns, and potential damage to the tendons and ligaments. The abrasion resistant, self-lubricating properties of the UHMW PE shoe, coupled with the tensile strength and stiffness properties minimize wear and damage on hard surfaces while still providing good grip and allowing controlled action for normal leg action and breakover.

Actual wear data obtained from shoes of the present invention worn by horses indicate a useful life of the shoe equal to or possibly greater than that obtained with steel or other types of shoes such as hard rubber or urethane rubber shoes.

Various design features and surface contours can be readily incorporated into the shoes, including accurately positioned nail holes, toe bars or studs for traction, corrective hoof shape and thickness, and many other features so as to provide the optimum service and therapeutic requirements for the animal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The ultra-high molecular weight (UHMW) ethylene polymers employed in the manufacture of the horseshoes of the invention constitute a well-known class of thermoplastic ethylene polymers. Such polymers can be homopolymers of ethylene or copolymers thereof having minor amounts, e.g., up to about 10 vol %, of a second hydrocarbon monomer such as butene-1 or the like copolymerized therewith. The ethylene polymers will have a molecular weight of at least about 1 million (1.0.times.10.sup.6). The properties of the horseshoes improve somewhat with the ethylene polymer's molecular weight. For ease of fabrication combined with properties of the finished shoes, it is preferred to employ ethylene polymers having molecular weights in the range of about 1-6 million and preferably about 3-6 million. As used herein, molecular weight values are determined by solution viscosity methods known and reported in the art. For convenience of description, the term "UHMW PE" is used in this application to refer to any ethylene polymer having the structure and composition referred to above.

Two UHMW PE's which have been employed successfully in the practice of the invention are Hifax 1900.RTM. produced by Hercules, Inc., Wilmington, Del., described as "an ultrahigh molecular weight, high-density polyethylene resin having a molecular weight range between 3 and 6 million"; and Hostalen GUR.RTM. UHMW Polymer, produced by American Hoechst Plastics, Somerville, N.J., and described as "a high density polyethylene of very high molecular weight, prepared by the Ziegler low pressure polymerization process," with the molecular weight as measured by the solution viscosity method reported as being in the region of 3.5 to 6.times.10.sup.6.

The properties of the UHMW PE resins which render them uniquely suitable in the fabrication of the horseshoes of the present invention are excellent abrasion resistance, impact resistance, high tensile strength, self-lubricating properties, and the desired stiffness. The combination of these properties provides a horseshoe with ideally suited toughness and overall properties, including the ability to forge or cold-form to the precise fit desired. The high Vicat softening point of 135.degree.-138.degree. C. along with the excellent low temperature impact resistance provide a useful temperature range for the products from -65.degree. F. to 200.degree. F. Typical properties of UHMW PE resin are shown in Table I.

TABLE I ______________________________________ Typical Properties of UHMW PE ______________________________________ Density, g/cm.sup.3 ASTM D792 0.93-0.94 VICAT Softening Point, .degree.C. D1525B 135-138 Hardness Rockwell R Scale D785 64 Shore "D" D2240 65-67 Tensile Properties (1) Max. Strength, psi 6,300 Yield strength, psi 3,100-3,400 Elongation at break, % greater than 350 Flexural Modulus, D790B 150,000-170,000 psi, 1% secant Izod impact, ft-lbs/in, D256A notched 23.degree. C. No Break -40.degree. C. No Break ______________________________________ (1) Determined by ASTM 638 employing a cross head speed of two inches per minute.

The horseshoes are easily fabricated by compression molding of fine powdered UHMW PE. This molding technique provides shoes remarkably free of strains and with a remarkably good ability to hold shoe nails as described infra. Other molding techniques possibly can be employed, but may require special mold designs and molding conditions.

The following examples are set forth to illustrate the principles and practice of the invention to those skilled in the art.

EXAMPLE 1

An aluminum mold is prepared by milling a cavity of the approximate width and shape of a standard Diamond DS-00 steel shoe and to a depth of about 15/16ths inch. A male part then is cut from aluminum sheet stock of the same size and shape, but with sufficient clearance on the cavity walls so as to provide compression on the resin particles. Approximately 40 grams of 1900 UHMW Polymer (Hercules) is placed in the cavity and the male part is placed on top of the resin. The filled mold then is placed in a platen press that has been heated to an indicated 400.degree. F. The resin is compressed to about 3/8ths inch and this thickness maintained, under pressure, for twenty minutes. The mold then is removed from the heated platen press and placed in a second press to cool, while maintaining 500 psi pressure. When cooled to room temperature, the mold is opened and the horseshoe removed.

EXAMPLE 2

To illustrate special aesthetic effects easily obtainable by the present invention, two UHMW PE resin powders (Hercules 1900 and Hostalen GUR 413) were dry blended with pigments to prepare the formulations shown in Table II. Horseshoes were molded therefrom as described in Example 1.

TABLE II ______________________________________ Resin Additive Type Wt. % Effect ______________________________________ Hercules Carbon Black 1.00 Black horseshoes 1900 (Phillips) Hostalen Acetylene Black 0.75 Black horseshoes GUR 415 (Gulf) Hostalen Evergold Richpale 2.00 Metallic GUR 413 #1575 (1) appearance Hostalen Eterna Copper 2.00 Metallic GUR 413 #120 (1) appearance Hostalen Aluminum Brilliant 2.00 Metallic GUR 413 #40NL (1) appearance Hostalen Fluorescent Yellow 4.40 Fluorescence GUR 413 #2266 (2) Hostalen Phosphorescent 4.00 Phosphorescence GUR 413 Excite #2479 (2) Hercules Phosphorescent Red 2.00 Phosphorescence 1900 #2304 (2) Hercules Phosphorescent Red 4.00 Phosphorescence 1900 #2304 (2) Hostalen Phosphorescent Red 4.00 Phosphorescence GUR 413 #2304 (2) Hercules Fluoral Green Gold 4.00 Green-gold color 1900 #084 (3) Hercules Fluoral Yellow 4.00 Yellow color 1900 #088 (3) ______________________________________ (1) Atlantic Powdered Metals, Inc., NY, NY (2) USR Optinox, Hackettstown, NJ (3) BASF Wyandotte Corp., Parsippany, NJ

As will be appreciated by those skilled in the art, any number of colorants, optical brighteners, and fluorescent or phosphorescent additives can be used to provide a wide variety of colors and special effects in the horseshoes. This offers particular advantage to show horses or those in competition, where it is common practice to match the color of the horseshoe with the hoof, which is usually accomplished with paint or enamel. Painted surfaces, especially metal, are subject to chipping or peeling, whereas the colored horseshoe of the subject invention is uniformly colored throughout, thereby offering a unique advantage.

Further, it will also be obvious to those skilled in the art that a wide variety of other additives can be incorporated into the basic UHMW PE polymer, as long as they do not substantially detract from the desirable properties of the shoes as described. Such additives include thermal stabilizers, UV stabilizers, antioxidants, lubricants, flow modifiers, and various processing aids, which can be used in amounts up to 20 weight % and include waxes, polymers, copolymers, terpolymers, and the like, and even RF absorbants used to preheat the resin.

Since it is known in the art that fillers, extenders, reinforcing agents, coupling agents, and the like can affect such properties as tensile strength, stiffness, elongation, impact, and abrasion resistance, it is included within the scope of the present invention to allow formulations of the UHMW PE to include these materials as long as the desirable properties of the shoes are retained.

In that the normal and customary practice for attaching horseshoes to the animal's hoof is to use nails, several horseshoes of the subject invention prepared by the compression molding technique described in the previous examples were subjected to a nail pull test. The procedure involved cutting the horseshoe into six segments of approximately equal length and numerically identifying each segment with respect to its relative position in the horseshoe. A pilot hole of 1/16th inch in diameter was drilled through the center of each test specimen, verticle to the plane of the shoe. A No. 4 horseshoe nail was forced into the pilot hole and the specimen mounted on an Instron tensile tester, such that the instrument measured and recorded the amount of force required to pull the nail completely through the specimen. The Instron crosshead speed was two inches per minute. The results are shown in Table III and indicate not only excellent nail pull strength, but a very uniform composition of the horseshoe.

TABLE III ______________________________________ Thickness Max. Force Composition Specimen.sup.1 in inches in lbs. ______________________________________ Hercules 1900 1 0.3675 245 2 0.3795 262 3 specimen damaged before test 4 0.370 261 5 0.370 265 6 0.3795 257 Average.sup.2 0.3733 .+-. 0.007 258 .+-. 9 Hostalen 1 0.392 245 GUR 415 2 0.392 261 3 0.395 257 4 0.394 263 5 0.393 266 6 0.392 267 Average.sup.2 0.393 .+-. 0.001 259 .+-. 8 Hercules 1900 1 0.362 241 containing 0.75% 2 0.362 247 carbon black 3 0.364 248 4 0.363 240 5 0.364 242 6 0.361 240 Average.sup.2 0.3627 .+-. 0.001 243 .+-. 4 GUR 413 1 specimen damaged before test containing 0.75% 2 0.300 239 carbon black 3 0.290 238 4 0.300 240 5 0.300 242 6 0.300 241 Average.sup.2 0.298 .+-. 0.005 240 .+-. 2 ______________________________________ .sup.1 Specimens numbered clockwise from the heel of horseshoe. .sup.2 Standard deviation shown at 95% level of confidence.

For some special purposes, e.g., for use on icy roads, it may be mandatory to provide a metal shoe with special road gripping surfaces. In this event, many of the advantages of the present invention can be obtained by providing a laminated shoe including a foot contacting liner lamina fabricated from an UHMW PE and a metal lamina for contact with the walking surface. Such laminated shoes are easily fabricated by simply gluing the two laminae together. The laminated shoes can be attached to the hoof by nailing.

A further advantage of the shoes of the invention is that an owner's identifying brand or indicia of ownership can be incorporated into or on the shoe. A unique chemical not identifiable by visual inspection can be incorporated into the UHMW PE. Such chemical, however, can be identified by a simple chemical test to identify a horse believed to have been stolen. As such a chemical can be employed in a small concentration, the manufacturer can keep a log of specific chemicals employed in each resin batch to readily identify the date of shoe manufacture and the date the shoes were shipped to particular customers. Such data can be useful to law enforcement agencies. Alternatively, a brand mark can be easily written on the shoe (preferably on the underside) with indelible ink felt tip markers or the like.

While the articles herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise articles, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

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