A slip-ring configuration for electric motors and generators is provided, in which brushes made of carbon materials and slip rings of a slip-ring body are electrically conductively connected to each other. The slip rings include metallic rings of standard construction as a slip-ring base and an electrically conductive sliding layer made of a graphite material. The sliding layer has a thickness which amounts to a maximum of 11% of a radius of the slip ring and is electrically conductively fastened to the periphery of the metallic slip-ring base by gluing. A method for retooling slip-ring bodies having metallic slip rings includes machining the metallic slip rings and gluing on a sliding layer.

Field of the Invention
The invention relates to slip-ring configurations for electrical machines, such as electric motors and generators, having brushes made of carbon materials and slip-ring bodies, wherein the brushes are electrically conductively connected to slip rings of the slip-ring bodies. The invention also relates to a slip-ring body and a method for retooling slip-ring bodies.
Electric motors and generators with which electrical energy is converted into rotational energy or, conversely, rotational energy is converted into electrical energy, require a current supply to a rotatably disposed coil, which is connected in a force-locking or form-locking manner to a rotating shaft. A form-locking connection is one which connects two elements together due to the shape of the elements themselves, as opposed to a force-locking connection, which locks the elements together by force external to the elements. That usually takes place by way of slip rings which are connected to the rotating shaft, concentric therewith and conductively connected to stationary brushes, or by way of pairing brushes with so-called commutators or collectors. In addition to producing an electrical connection between the stationary part and the rotating part of the electrical machine, the commutators or collectors also effect commutation (in direct-current machines).
Usually, the slip rings and commutators are formed of metals such as copper or copper alloys such as, for example, bronze, tin bronzes, nickel bronze, silver or steel. The slip rings are connected by insulating fastenings to a hub (rotating shaft) to form slip-ring bodies, which are insulated with respect to the hub and with respect to each other. Electrically conductive brushes are disposed stationarily along the circumference of the slip rings and are held in contact with the surface of the slip rings by spring force. In the case of alternating-current motors and generators, slip rings are required individually or plurally for each phase.
The sliding contacts (brushes) generally are formed of carbon materials, possibly in combination with metals, for example metal graphite. In order to produce metal graphite, mixtures of metal powders, in particular copper, tin or lead, are pressed with graphite, in particular natural graphite, and subsequently hardened by calcining or sintering.
In the case of all of those material pairings, wear results from reciprocal movement as well as from transmission of somewhat high currents, in which case dust can form from abrasion. On one hand, that can lead to shortening of a creepage path because of dirt accumulation and thus to arcing. On the other hand, an eroding of contacting layers results. There is a necessity for replacing the brushes and subsequent treatment of a surface of the slip rings (machining or stripping off of defective spots such as grooves or the like). In that connection, additional maintenance intervals result, which are shorter than maintenance intervals of (roller) bearings, something which causes substantially increased maintenance costs, above all as a result of additional down-times.
It is therefore desirable to keep the abrasion as low as possible and thus to lessen the frequency of the maintenance work caused as a result, or to make it at most the same as the frequency of the maintenance work for the bearings and/or other wearing parts.
It is known from East German Patent DD 258 687 A1 and from a publication entitled VEM Journal 1975, pages 15 ff, that wear is very low in the case of a pairing of graphite brushes with slip rings made of graphite. However, that system has the disadvantage of only permitting small currents to be conducted through the graphite body of the slip rings because of its specific resistance, which is relatively high in comparison with metals. When high currents are conducted, the ohmic heat is unacceptably high. That can lead to damage to the system. In a slip ring, the introduction or removal of the current takes place by way of a metal conductor which extends parallel to the axis of rotation in such a manner that it is laterally offset with respect thereto and is electrically conductively connected to the body of the slip ring. The resistance inside a graphite slip ring is of a similar magnitude to the contact resistance between a slip ring and a brush. In the case of a constant induced current in the coil, that leads to periodic voltage fluctuations in a generator. In a motor, it leads to an uneven torque, depending on the path length of the current and thus the active resistance in the slip ring.
Another construction is known from East German Patent DD 248 909 A1. A slip ring having a metallic slip-ring base and a carbon sliding ring soldered onto it is described therein. The slip-ring base is provided with hollow spaces in order to be able to remove waste heat by ventilation on all sides. The side of the carbon sliding ring that faces the metallic slip-ring base has to be metallized in order to ensure a low contact resistance and permit a soldered joint. Thermal stresses occur as a result of the strong heating of the structure by the ohmic dissipated energy, as well as during soldering. An outer portion of the metallic slip-ring base is therefore preferably provided with recesses for compensation of thermal stresses.
It is accordingly an object of the invention to provide a slip-ring configuration in electric motors and generators, a slip-ring body and a method for retooling slip-ring bodies, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type, which on one hand lead to as little wear as possible and which on the other hand allow a sufficiently high current load, in order to permit systems of that kind to also be used in the high-current range without an occurrence of strong temperature rises known from the prior art. A further object is to be able to retrofit existing machines having metallic slip rings, in such a way that wear becomes less, with as few parts as possible needing to be replaced.
With the foregoing and other objects in view there is provided, in accordance with the invention, a slip-ring configuration for electric motors and generators, comprising a slip-ring body having slip rings to be electrically conductively connected to brushes made of carbon materials. The slip rings each have a given radius and a metallic ring of standard construction acting as a slip-ring base with a periphery. There is at least one electrically conductive sliding layer made of a graphite material. The at least one sliding layer has a thickness amounting to a maximum of 11% of the given radius and each of the at least one sliding layer is electrically conductively fastened to the periphery of the metallic slip-ring base of a respective one of the slip rings by gluing.
Accordingly, this object is achieved by a slip-ring construction which includes a metallic ring of standard construction as a slip-ring base and a sliding layer glued onto this slip-ring base. The sliding layer preferably is formed of a carbon material. If a carbon material is used, it is advantageous to use a graphite material, particularly preferably an isostatically pressed graphite material. Furthermore, the flexural strength of the graphite material should preferably amount to at least 30 MPa (=30 N/mm2) in order to ensure that the layer thickness of the carbon material can be kept sufficiently small. The result of this construction is, on one hand, that the contact-surface pairing has minimal wear, because the material of the friction partner of the brushes can be chosen in such a way that the abrasion between these materials which are moved against each other is considerably lower than that between a pairing of metals or a pairing with metal and carbon material for the brushes. On the other hand, as a result of this construction, the contact resistance between the metallic base of the slip ring and the sliding layer is centrosymmetrical.
In accordance with another feature of the invention, not all of the slip rings of the slip-ring body are provided with the sliding layer.
As is conventional to a person skilled in the art, the configuration formed of the hub, the insulator (preferably the insulating covering in the form of a lateral cylinder surface) and the slip rings, which in the case of the invention are made up of the metallic slip-ring base and the sliding layer, is referred to herein as the slip-ring body.
The thickness of the sliding layer is upwardly limited by its conductivity (the thicker the sliding layer, which is poorly conductive in comparison with metals, the higher the resistance between the terminal lead, which is conductively connected to the metallic slip-ring base, and the connecting lead at the brushes). It has proven advantageous to ensure that the thickness of the sliding layer is not greater than 11% of the radius of the outer cylindrical or shell surface of the sliding layer.
The metallic slip-ring base is usually a squat cylindrical supporting ring which can be constructed in such a way that it is solid, with (mainly circular) recesses, or a spoked wheel. It is also possible, and preferred, for the width of the slip-ring base in the vicinity of the outer cylindrical or shell surface to be greater in this region than in the rest of the ring. The slip-ring base is thus given the appearance of a flat ring (which can also have recesses), on the periphery of which a wide (in the direction parallel to the axis) lateral cylinder surface like a collar is preferably formed. A sliding layer with a constant thickness is electrically conductively fastened on the (outer) lateral surface of this slip-ring base. This fastening is preferably produced by conductive gluing. The advantage of gluing is that the electrical connection has a contact area which is as large as possible. That lowers the contact resistance and divides the force between the two materials onto an area which is as large as possible. Heating to temperatures at which solder melts, which is otherwise required in the case of the production of a soldered joint, is dispensed with by gluing. When soldering, particular safety measures, such as dismantling or putting on a thermal shield, are namely required in order to avoid damage to the slip-ring base.
In accordance with a further feature of the invention, the sliding layer is formed of an electrically conductive graphite material. Preferably, a graphite material having a flexural strength of at least 30 MPa is used as the material for the sliding layer. Furthermore, isostatically pressed graphite material is preferably used. The thickness of the sliding layer should be kept as low as possible because of the specific resistance which is higher in comparison with the metallic slip-ring base. In this connection, however, it is to be taken into account that, on one hand, the mechanical stability of the sliding layer decreases with smaller thickness. On the other hand, the abrasion in connection with the brushes (that are usually and preferably formed of carbon materials) is to be determined by the suitable selection of the material and its thickness in such a way that maintenance intervals, which become necessary because of the renewing of the sliding layer, are equal to or greater than the average rolling-bearing lifetime. Therefore, the thickness of the sliding layer should not amount to more than 11% of the outer radius of the slip ring (i.e. of the outer radius of the sliding layer). Preferably, the thickness of the sliding layer is 10% or less of this radius, in particular 8% or less, with proportions of 6% and below or 4% and below being particularly preferred.
Conductive adhesives are used in order to glue the sliding layer and the metallic slip-ring base together. These adhesives are preferably to be chosen in such a way that their temperature stability is so great that a firm gluing of the sliding layer onto the metallic slip-ring base is also ensured at the temperatures of the slip ring that occur during the operation of the slip-ring configuration. Preferably, however, adhesives which do not have a suitable inherent conductivity, although to which a metal powder, preferably copper powder, is added, are also used. It is particularly preferable if after the depositing of the adhesive layer, the metal powder is scattered over the coated surfaces in order to obtain an electrically conductive adhesive connection. The metal powders being used preferably have a granulation of 0.01 mm to 0.2 mm. In particular, epoxy-resin adhesives, phenolic-resin adhesives, cyanate-ester-resin adhesives as well as adhesives based on polyurethane resins, polyester resins and amine resins, are counted among the adhesives being used. It is particularly preferable if phenolic-resin adhesive is used for the slip rings in accordance with the invention. The layer thickness of the adhesive on the metal surface of the slip-ring base or on the inside surface of the sliding layer preferably amounts to between 0.02 mm and 0.2 mm, particularly preferably between 0.05 mm and 0.1 mm. In the gluing process, sliding-layer segments are placed precisely onto the supporting slip-ring base and pressed on with even pressure. In this connection, a gap width between individual segments of the sliding layer is to be kept as small as possible.
In accordance with an added feature of the invention, graphite brushes are used as the sliding partner for the sliding layer of the slip rings, i.e. brushes made of carbon materials with a graphitic character. In particular, electrographite and burnt carbon materials which contain natural graphite are counted among these materials.
The fact that the sliding layer, which preferably is formed of the above-mentioned rigid carbon material, can be renewed without difficulty when necessary, is to be mentioned as a further advantage of this construction. In order to do this, it is only necessary to machine or strip off the remaining sliding layer and the adhesive layer down to the metal, whereupon a new sliding layer can then be applied. Changes in the brush position during this overhaul are not required in this case. In the case of a pure metal embodiment, the slip ring has to be reworked when worn, without going below a minimum diameter, or the entire slip ring has to be exchanged, in which case the brushes also have to be renewed.
The partial or complete retrofitting of existing machines having purely metallic slip rings is to be carried out without difficulty in such a way that the metallic contact layer on the outer cylindrical or shell surface of the existing slip rings in the slip-ring body is prepared, preferably worn down, particularly preferably by machining or stripping off. This is done in such a way that the sliding layer can be applied in the required thickness and connected to the remaining metallic slip-ring base by gluing. The sliding layer can then be reworked if necessary in order to remove surface irregularities, for example by stripping off or grinding. The advantage of the embodiment in accordance with the invention emerges in particular in the case of this retrofitting, because the thickness (in the radial direction) of the gliding layer of metallic slip rings is usually great enough to be stripped off or machined to the required diameter without a loss of stability. This applies in particular to metallic slip rings which have two layers in the radial direction, a metallic supporting layer and a separate outer gliding layer.
It is of particular advantage to prepare the metallic slip rings of an existing machine (for example by grinding, turning or milling) in such a way that at least at one of the edges of the outer cylindrical or shell surface of the remaining metallic slip-ring base, a projection (in the direction of the increasing radius) remains in each case which is preferably 0.5 mm to 5 mm wide, in particular 1 mm to 3 mm wide, and 0.5 mm to 3 mm high, preferably 1 to 2 mm high. The sliding layer is glued into the cylindrical groove which comes about in this way, in such a way that the sliding layer ends at the projections or preferably projects above them by up to 5 mm, in particular up to 3 mm.
In the configuration in accordance with the invention, the entire slip-ring body can be clamped for overhaul or renewal of the sliding layer, the slip rings are stripped off or machined down to the metallic base, and the sliding layer can be replaced (simultaneously with one or more slip rings).
In accordance with an additional feature of the invention, the sliding ring can be formed of a closed ring. However, it is preferred for the sliding layer to be made up of a plurality of segments, which are cut from one or more graphite rings. In that case they are applied to the carrier in at least two segments, particularly preferably in at least three segments.
In accordance with yet another feature of the invention, in this connection, the joint between two adjoining sliding-layer segments is not made parallel to the axis of rotation (i.e. at right angles to the tangent), but instead at an angle to the tangent of a maximum of 75xc2x0, preferably a maximum of 60xc2x0, and particularly preferably up to 45xc2x0. The word xe2x80x9ctangentxe2x80x9d is defined and will be used as follows: xe2x80x9cA tangent is that straight line which borders on the outer cylindrical or shell surface of the slip-ring and passes perpendicular to the rotational axis of the electrical machine.xe2x80x9d
Therefore, in accordance with a concomitant feature of the invention, the sliding layer is applied in one piece in the form of a ring and the latter is slit circumferentially at an angle xcex2 with respect to the tangent. That angle is preferably sized in such a way that the slit extends at least once around the entire circumference of the sliding layer. If the sliding layer is applied in more than one segment, it is advantageous for these segments not to be sized with the same (arc) length. Instead, the (arc) length of the longest segment should be at least 110% of the length of the other (or second-longest) segment. The thickness of the sliding layer amounts to up to 11% of the outer radius of the slip ring, preferably a maximum of 5 mm and in particular 4 mm and less.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a slip-ring configuration in electric motors and generators, a slip-ring body and a method for retooling slip-ring bodies, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

Slip-ring configuration in electric motors and generators, slip-ring body and method for retooling slip-ring bodies


Certain butadiene and/or natural rubber articles, such as acrylonitrile-butadiene rubber (NBR), natural rubber, and derivatives thereof, that exhibit highly desirable long-term effective antimicrobial characteristics. Such formulations are intended to be vulcanized to provide solid or foam rubber articles which can be utilized in a variety of different applications. Preferably such butadiene rubber formulations comprise silver-based antimicrobial compounds. As such silver-based compounds are deleteriously affected by utilization of standard non-silicone-rubber curing agents, such as sulfur-based catalysts, the ability to provide such an effective antimicrobial vulcanized rubber article is rather difficult. However, the invention encompasses the utilization of different catalyst species that permit vulcanization and silver stability for long-term antimicrobial performance of the silver-based compounds. Articles and specific vulcanizing procedures in accordance with such a novel antimicrobial vulcanized rubber are also included within the scope of this invention.

This invention relates to certain non-silicone vulcanized rubber articles made from at least a majority by weight of nitrile butadiene rubber and/or natural rubber that include silver-based compounds to provide highly desirable long-term antimicrobial characteristics within the cured rubber articles. Such articles are in either solid or blown (foam or sponge) state (or combinations of both in multilayered forms) which can be utilized in a variety of different applications. As silver-based compounds are deleteriously affected by utilization of standard curing agents and curing accelerators, such as sulfur-based compounds and/or systems, the ability to provide such an effective antimicrobial vulcanized rubber article is rather difficult. However, this invention encompasses the presence of different non-sulfur-based curing systems and agents, such as peroxides, as one example, that permit vulcanization and do not irreversibly bind silver ions thereto, thereby resulting in long-term antimicrobial performance of the ultimate rubber article itself. The rubber articles must also comprise fillers and may also include plasticizers to provide desired characteristics of dimensional stability, stiffness, flexural modulus, tensile strength, abrasion resistance, elongation, and the like, for the ultimate rubber article, while simultaneously and surprisingly enhancing the antimicrobial efficacy of the rubber article as well.
All U.S. Patents listed below are herein entirely incorporated by reference.
There has been a great deal of attention in recent years given to the hazards of bacterial contamination from potential everyday exposure. Noteworthy examples of such concerns include the fatal consequences of food poisoning due to certain strains of Eschericia coli being found within undercooked beef in fast food restaurants; Salmonella enteritidis contamination causing sicknesses from undercooked and unwashed poultry food products; and illnesses and skin infections attributed to Staphylococcus aureus, Klebsiella pneumoniae, yeast (Candida albicans), and other unicellular organisms. With such an increased consumer interest in this area, manufacturers have begun introducing antimicrobial agents within various everyday products and articles. For instance, certain brands of cutting boards, shoe soles, shoe inserts, medical devices and implements, liquid soaps, etc., all contain antimicrobial compounds. The most popular antimicrobial for such articles is triclosan. Although the incorporation of such a compound within liquid or certain polymeric media has been relatively simple, other substrates, specifically vulcanized rubber and surfaces thereof, have proven less accessible. Furthermore, such triclosan additives have proven to be difficult in use or ineffective for certain bacteria. For instance, triclosan itself migrates easily within and out of certain polymeric substrates and/or matrices (and thus is not very durable), lacks thermal stability, and does not provide a wide range of bacterial kill (for instance does not exhibit any kill for Pseudomonas aeruginosa).
Antimicrobial rubber formulations are certainly highly desired for the production of vulcanized rubber articles and compositions to provide not only antibacterial benefits, but also antifungal, antimildew, antistaining, and odor control properties. Rubber articles are utilized in many different applications, from automobiles (hoses, tires, bumpers, etc.), to household items (toys, sink washers, gaskets, appliances, floor mats, door mats, carpeted rubber mats, gloves, and the like), and other areas in which bacterial growth is a potential problem. There thus remains a long-felt need to provide an effective, durable, reliable antibacterial vulcanized rubber formulation which will provide such long-term antibacterial, etc., effects within the final vulcanized article. Unfortunately, such a highly desired antimicrobial rubber formulation and/or vulcanized article containing silver-based antimicrobial agents has heretofore not been provided by the pertinent prior art.
The closest art includes Japanese Patent Application 1997-342076 which discloses the production of unvulcanized rubber formulations and articles exhibiting antibacterial properties due to the presence of silver complexes. Such formulations are formed through high temperature kneading in an oxygen-free atmosphere and are used as parts in a water disinfection system. Again, no vulcanized rubber is taught or obtained within or through this disclosure. Antimicrobial rubber bands have been taught in Japanese Patent Application 1997-140034 in vulcanized form with silver antimicrobials therein. However, such compounds are rather limited in use and the vulcanization step must include a sulfur curing agent to effectuate the final vulcanized arrangement of the subject rubber. Such sulfur curing agents have a remarkably deleterious effect on certain silver-based antimicrobials such that the sulfur reacts with the silver ion to form silver sulfide, therby rendering it ineffective as a bactericide. As such, the utilization of such specific rubber band formulations for and within large-scale antimicrobial articles is basically unworkable.
Certain types of antimicrobial peroxide-catalyst vulcanized rubber formulations have been produced in the past; however, such peroxide-cured rubbers are all silicone-based. It is well understood and accepted that silicone rubbers cannot be vulcanized by typical sulfur-based catalysts. Thus, the antimicrobial rubber formulations of Japanese Patent Applications 1997-026273 and 1995-065149 as well as U.S. Pat. No. 5,466,726 are standard vulcanized silicone rubber formulations and articles which also include certain antimicrobial compounds.
Furthermore, rubber latexes (non-vulcanized) comprising antimicrobials have been disclosed (U.S. Pat. No. 5,736,591, for example), as have floor mats having silver-based antimicrobials incorporated within pile fiber components and which have non-antimicrobial rubber backings cured through peroxide-catalyzed vulcanization to protect the pile fiber antimicrobial compounds from attack by any sulfur compounds (as in Japanese Patent Applications 1993-3555168 and 1995-38991). Again, however, to date there have been no disclosures or suggestions of producing a vulcanized non-silicone rubber formulation exhibiting excellent antimicrobial properties through the long-term effective utilization of silver-based antibacterial compounds. This invention fills such a void.
It is therefore an object of this invention to provide an antimicrobial vulcanized rubber-containing article, wherein said rubber is selected from the group consisting of nitrile butadiene rubber, natural rubber, and any mixtures thereof, wherein said article exhibits sufficient antimicrobial activity and structural integrity to withstand repeated use without losing an appreciable level of either antimicrobial power or modulus strength. Another object of the invention is to provide an antimicrobial vulcanized nitrile butadiene or natural rubber article comprising silver-based antimicrobial compounds which include curing agents which do not deleteriously effect the antimicrobial activity of the finished vulcanized nitrile butadiene or natural rubber article (and thus is essentially free from sulfur-based curing agents and accelerators). Yet another object of this invention is to provide a vulcanized nitrile butadiene or natural rubber-containing article that exhibits log kill rates for Staphylococcus aureus and Klebsiella pneumoniae (and/or other types of bacteria as well) of at least 1.0 after 24 hours exposure at room temperature as well as prevention of growth of certain fungi after at least 15 days of exposure. Still another object of this invention is to provide a vulcanized nitrile butadiene or natural rubber-containing article comprising structural integrity filler components and plasticizers (such as silica, metal salts, organic salts, pigments, such as carbon black, calcium carbonate, paraffinic oils, phthalate oils, metal oxides, and the like) that also provide enhancements in the control of antimicrobial efficacy of the article itself through regulated silver ion release to the article surface (e.g., exhibits higher log kill rates for Staphylococcus aureus and Klebsiella pneumoniae and prevention of growth of fungi such as Aspergillus niger). Still another object of the invention is to provide a finished article that exhibits increases in antimicrobial activity after industrial washing and/or abrasion. Yet another object is to provide a simple method of producing such an atimicrobial vulcanized nitrile butadiene or natural rubber-containing article.
Accordingly, this invention encompasses a dimensionally stable vulcanized rubber-containing article, wherein said rubber is selected from the group consisting of nitrile butadiene rubber, natural rubber, and any mixtures thereof, wherein said article exhibits log kill rates for Staphylococcus aureus and Klebsiella pneumoniae of at least 1.0 each after 24 hours exposure at room temperature. Also, this invention encompasses a dimensionally stable vulcanized nitrile butadiene or natural rubber-containing article exhibiting antifungal properties such that said article exhibits at least 70% inhibition in accordance with Test Method ISO 486, of Aspergillus niger ATCC 6275 for at least 15 days at 30xc2x0 C. and at greater than 90% humidity. Furthermore this invention encompasses such a vulcanized nitrile butadiene rubber or natural rubber-containing article comprising at least one silver ion control release additive, such as those selected selected from the group consisting of fillers (such as carbon black, calcium carbonate, inorganic salts, organic salts, silica, and mixtures thereof) and plasticizers (oils such as phthalate oils and paraffinic oils). Additionally, this invention encompasses a method of producing a vulcanized nitrile butadiene rubber or natural rubber-containing article comprising the steps of providing a rubber formulation comprising uncured rubber, at least one non-sulfur based curing agent, and at least one silver-based antimicrobial compound, and vulcanizing said rubber formulation at a temperature of at least 150xc2x0 C. and at least at a pressure of 3 bars, wherein said rubber formulation is substantially free from sulfur curing agent and accelerator.
The term xe2x80x9cdimensionally stablexe2x80x9d is intended to encompass a vulcanized rubber article that is structurally able to be handled without disintegrating into smaller portions. Thus, the article must exhibit some degree of structural integrity and, being a rubber, a certain degree of flexural modulus.
Such a specific antimicrobial vulcanized nitrile butadiene and/or natural rubber-containing article has not been taught nor fairly suggested within the rubber industry or prior art. As noted above, the avoidance of sulfur-based curing agents and accelerators to any appreciable degree thus permits the retention of silver antimicrobials within the final product in amounts sufficient to provide long-lasting log kill rates for Staphylococcus aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Escheria coli, at the very least. Furthermore, due primarily to high costs, non-sulfur curing agents have not been prevalent within vulcanized rubber formulations and articles. As such, there has been no teaching or fair suggestion of coupling non-sulfur curing agents (and most preferably peroxide curing agents) with silver-based antimicrobial agents within pre-vulcanized rubber formulations to form effectively antimicrobial vulcanized rubber articles.
Additionally, generally and preferably, though not necessarily, certain fillers and oils (such as silica, carbon black, stearates as fillers, and phthalate and paraffinic oils) are required to provide both flexural modulus and structural integrity to vulcanized rubber articles. The rubber component alone generally does not exhibit proper dimensional stability without such additives. Surprisingly, the presence of such additives also provides the ability to control silver-ion release at the target article surface. Without intending to be bound to any specific scientific theory, it appears that such fillers as silica and such oils as paraffinic oil (as some examples), act in such a way as to draw moisture into the article which then transports silver ions from within the article to the surface. In such a situation, then, the rubber article may exhibit enhanced silver release resulting in higher log kill rates for certain bacteria due to the presence of larger amounts of available surface silver ions. Other hydrophobic fillers, such as pigments (for example carbon black) and calcium carbonate (as some examples) appear to work in the opposite manner by keeping water out of the target article and thus prevent silver-ion migration to the article surface. Thus, the reduction of such silver-ion availability decreases the antibacterial efficacy of the rubber article. In effect, then, the actual antibacterial efficacy of the entire rubber article can be controlled through the presence of certain amounts of such generally required fillers and oils (some hydrophilic antistatic agents also appear to act in the same manner as silica as well). As a result, the necessary filler and/or oil constituents required to provide dimensional resiliency and/or flexural modulus (and thus actual usefulness) of the finished article serve a dual purpose heretofore unrecognized within the rubber industry. Rubber articles can be produced with specific end-uses in mind depending upon the duration of antimicrobial activity desired through the addition of specific amounts of such additives. Again, such a targeted duration antimicrobial vulcanized article and the benefits thereof have heretofore been unknown and unrecognized within the rubber industry. These rubber components are thus hereinafter referred to as xe2x80x9csilver ion release control additivesxe2x80x9d.
The term nitrile butadiene rubber is intended to cover any standard rubber which possessing at least some nitrile and butadiene moieties therein and which must be vulcanized to provide a dimensionally stable rubber article. Natural rubber is intended to include all rubbers harvested from a natural source (e.g., rubber trees) as well as such rubber compositions which originate from such a natural source but which may be processed in some manner before incorporation within the target article. The specific types are listed below and have been utilized for many years and are generally well known and taught throughout the prior art. Such inventive rubber formulations should also possess a chemical plasticizer which aids in the breakdown period of the elastomer during compounding and processing (and provides flexural modulus properties to the finished article) as well as fillers required for reinforcement (e.g. calcium carbonate, carbon black, silica, and clays). Optionally, to form a blown (foam or sponge) rubber type, a blowing agent may be added to the inventive formulation.
The rubber component or components of the inventive raw rubber formulation is therefore selected from the group consisting of nitrile rubber [such as acrylonitrile-butadiene (NBR)], and other types, including, as noted above, natural rubber, and any mixtures of such rubber types. Other types of rubber may also be mixed with the base butadiene types in order to provide different strengths, flexibilities, or other properties (such as ozone resistance with EPDM additions). Such other rubber components may then be, without limitation, chloroprene, ethylene propylene diene modified rubber (EPDM), polyurethane rubber, butyl rubbers, neoprene, isoprene, halobutyl rubbers, fluoroelastomers, epichlorohydrin rubber, polyacrylate rubber, and chlorinated polyethylene rubber, as long as the majority of the mixture is comprised of butadiene rubber. Modified rubbers which are potentially useful, though more expensive, include hydrogenated SBR, hydrogenated NBR, and carboxylated NBR and the like, and may make up the majority as these are butadiene-based. Although the presence of silicone-rubber is discouraged within the inventive formulation, there remains the possibility of adding certain low amounts of such specific unvulcanized rubber components without adversely affecting the overall antimicrobial rubber formulation itself. Thus, up to 25% by total weight of the formulation may be silicone-rubber; however, the vast majority of the rubber formulation must be non-silicone rubber. Thus, with a total number of parts between about 300 and 2,000 parts throughout the target vulcanized rubber article, the rubber constitutes from about 25 to about 70% of the percentage by parts of the entire article. The remainder comprises additives such as fillers, oils, curing agents, the desired antimicrobial agents, optional blowing agents, and the like (as discussed more thoroughly below).
Furthermore, the non-silicone rubber portion must not possess an appreciable amount of sulfur-based curing agent or residue (in the finished article) and thus must be vulcanized through curing with primarily non-sulfur-based compounds (such as peroxides and metal oxides, for example). The rubber component is present in amount of from about 10 to about 1,000 parts of the entire composition, more preferably from about 50 to about 500 parts, and most preferably from about 100 to about 200 parts.
The antimicrobial agent of the inventive raw rubber formulation may be of any standard silver-based compounds. Such compounds, in contrast with organic types, such as triclosan, for example, do not exhibit low thermal stability and thus remain within the target matrix or substrate at different temperatures. Thus, such an antimicrobial is more easily controlled, as discussed above, for surface release as desired. Such agents include, without limitation, silver salts, silver oxides, elemental silver, and, most preferably ion exchange, glass, and/or zeolite compounds. Of even greater preference are silver-based ion exchange compounds for this purpose due to the low levels of discoloration and enhanced durability in the final product provided by such compounds, the efficacy provided to the final formulation with such a compound, and the ease of manufacture permitted with such specific compounds. Thus, the antimicrobial agent of this invention may be any type which imparts the desired log kill rates as previously discussed to Staphylococcus aureus, Klebsiella pneumoniae, Escherichia coli, and Pseudomonas aeruginosa, as merely representative organisms. Furthermore, such antimicrobial compounds must be able to withstand elevated processing temperatures for successful incorporation within the target non-sulfur (peroxide, for example) cured rubber formulations. Again, such antimicrobial agents comprise, preferably, silver-containing ion exchange, glass, and/or zeolite compounds. Most preferably, such a compound is a silver-based ion-exchange compound and particularly does not include any added organic bactericide compounds (thereby not permitting a release of volatile organic compounds into the atmosphere during processing at high temperatures, etc.). The preferred silver-based ion exchange material is an antimicrobial silver zirconium phosphate available from Milliken and Company, under the trade name ALPHASAN(copyright). Such compounds are available in different silver ion concentrations as well as mixtures with zinc oxide. Thus, different compounds of from about 0.01 to 10% of silver ion concentration, preferably from about 3 to about 8%,, and most preferably amounts of about 3, 3.8, and 10% by total amount of components (e.g. of the total amount of silver ions and zirconium phosphate) are possible. Other potentially preferred silver-containing solid inorganic antimicrobials in this invention are silver-substituted zeolite available from Sinanen under the tradename ZEOMIC(copyright), or a silver-substituted glass available from Ishizuka Glass under the tradename IONPURE(copyright), which may be utilized either in addition to or as a substitute for the preferred species. Other possible compounds, again without limitation, are silver-based materials such as MICROFREE(copyright), available from DuPont, as well as JMAC(copyright), available from Johnson Mathey.
Generally, such an antimicrobial compound is added to a rubber formulation in an amount of from about 0.1 to 10% by total weight of the particular total rubber formulation; preferably from about 0.1 to about 5%; more preferably from about 0.1 to about 2%; and most preferably from about 0.2 to about 2.0%.
Furthermore, with regard to silver-based inorganic antimicrobial materials, these particular antimicrobial rubber articles are shown to be particularly suitable for the desired high levels of efficacy and durability required of such articles. It has been found that certain silver-based ion exchange compounds, such as ALPHASAN(copyright) brand antimicrobials available from Milliken and Company, (U.S. Pat. Nos. 5926238, 5441717, 5698229 to Toagosei Chemical Industry Inc.), exhibit impressive bio-efficacy. After a period of time, alternative antimicrobial compounds (e.g. triclosan, microchek, OBPA, Zn-omadine) initially suffer from decomposition under the high processing temperatures, followed by depletion of the biocide through leaching into the surrounding environment and finally through depleted bactericidal activity. However, silver-containing ion exchange, glass, and/or zeolite compounds do not suffer from these shortcomings. Such antimicrobial agents exhibit high temperature stability ( greater than 1000xc2x0 C.), do not leach into the environment and provide substantial amounts of the oligodynamic silver ion to provide for the desired extensive durability.
The inventive antimicrobial articles should exhibit an acceptable log kill rate after 24 hours in accordance with the AATCC Draft Test Method entitled xe2x80x9cAssessment of Antimicrobial Properties on Hydrophobic Textiles and Solid Substratesxe2x80x9d as well as in accordance with Japanese Test Method JIS 2 2801. Such an acceptable level log kill rate is tested for Staphylococcus aureus or Klebsiella pneumoniae of at least 0.1 increase over baseline. Alternatively, an acceptable level will exist if the log kill rate is greater than the log kill rate for non-treated. (i.e., no solid inorganic antimicrobial added) rubber articles (such as about 0.5 log kill rate increase over control, antimicrobial-free vulcanized nitrile and/or natural rubber). Preferably these log kill rate baseline increases are at least 0.3 and 0.3, respectively for S. aureus and K. pneumoniae; more preferably these log kill rates are 0.5 and 0.5, respectively; and most preferably these are 1.0 and 1.0, respectively. Of course, the high end of such log kill rates are much higher than the baseline, on the magnitude of 5.0 (99.999% kill rate). Any rate in between is thus, of course, acceptable as well. However, log kill rates which are negative in number are also acceptable for this invention as long as such measurements are better than that recorded for correlated non-treated rubber articles. In such an instance, the antimicrobial material present within the rubber article at least exhibits a hindrance to microbe growth. Furthermore, such rubber articles should exhibit log kill rates of the same degree for other types of bacteria, such as, Psedumonas aeruginosa and Eschericia coli.
Of great surprise within this invention is the ability for the finished inventive articles to provide antifungal benefits as well as antibacterial characteristics. Such versatility is rare among antibacterial compounds; however, without intending to be limited to any particular scientific theory, it appears that the silver ions, and particularly the silver ions present at the article surface in great abundance, provide excellent antifungal properties. Thus, another embodiment of this inventive rubber formulation should provide fungal kill durability of at least 15 sequential days for such organisms as Aspergillus niger and mixtures of fungi including Aspergillus niger ATCC 6275, Paecilomyces variotii ATCC 18502, Trichoderma virens ATCC 9645. In order to provide a greater array of potential antifungal benefits, other compounds may be incorporated within the target pre-vulcanized rubber formulation (and subsequent article), such as zinc oxide, as one example.
Of great importance to the effectiveness of the inventive articles in terms of antimicrobial and antifungal activity is the omission of deleterious amounts of sulfur-based curing agents and accelerators from the rubber article. As noted above, sulfur reacts with the preferred silver-based antimicrobials and irreversibly binds the silver ions (as silver sulfides, for example) within the rubber composition and/or article itself. As such, the resultant silver sulfides, etc., are ineffective as antimicrobial agents and their presence thus renders the final product antimicrobially inactive. Thus, it has been necessary to produce a vulcanized rubber article lacking any appreciable amount of sulfur curing agents and accelerators therein. It should be appreciated that the term xe2x80x9cappreciable amountxe2x80x9d permits a small amount to be present. It has been found that, as a molar ratio, a 1:1 ratio (and above) between sulfur molar presence and silver molar presence results in a clear loss of antimicrobial activity within the desired ultimate vulcanized article. However, greater molar amounts of silver in relation to sulfur provide at least some antimicrobial properties to the desired article. A molar ratio range of from 0.25:1 to about 0.000000001:1 of sulfur to silver ions is thus at least acceptable. The primary curing agent, however, must be of non-sulfur nature (and is preferably, though not necessarily) a peroxide-based compound in order to provide the desired antimicrobial activity for the subject rubber. Although peroxide curing agents have been utilized for vulcanization of rubber previously, such a different type of curing agent is not widely utilized as a suitable vulcanization catalyst for rubber for a number of reasons. Foremost, such curing agents are much more costly than standard sulfur-based agents and thus the utilization of such peroxides, and the like, as a replacement for the sulfur-based compounds have been rather limited to mostly silicone-based rubbers or, at the very least, non-antibacterial rubber articles. However, due to the problems associated with antimicrobial activity when such compounds are reacted with sulfur-based curing agents, alternatives to such sulfur-based cured articles was to permit utilization of such effective antimicrobial compounds within raw and vulcanized rubber for long-term high log kill rate effects. Thus, although non-sulfur-based compounds are not readily utilized within the non-silicone industry as vulcanization curing agents, utilization of such curing agents was necessary to provide an effective, ultimate antimicrobial vulcanized rubber article.
Surprisingly, it has now been found that the inventive rubber articles listed above are available without such sulfur-based curing agents in any appreciable amounts; most importantly, with the introduction of certain additives, the structural integrity and/or flexural modulus of the rubber formulation is improved to an acceptable level and the efficacy of the antimicrobial components are can be controlled simultaneously.
Thus, the curing agent present within the raw rubber formulation to be vulcanized to from the inventive article must be at least a majority, and preferably at least about 75% by weight of a non-sulfur-based curing agent. As discussed above, traditional sulfur and sulfur-based catalysts will not work with the inventive antimicrobial formulations due to chemical reactions between the sulfur atoms and and the biocidal Ag+ion. However, non-sulfur-based catalysts, such as, for example, and without intending to being limited to peroxides, certain compounds provide effective curing for the inventive raw rubber formulations, such as organic peroxides, including dicumyl peroxide, 2,5-bis(t-butylperoxy)-2,5-dimethylhexane, di-(t-butyl-peroxy-isopropyl) benzene, di-(t-butyl-peroxy-trimethyl)-cyclohexane, and the like, and inorganic peroxides and oxides, including zinc oxide, and the like. Such a curing agent should be present in amount of from about 0.5 to about 100 parts per hundred parts of rubber (pphr); more preferably from about 1 to about 50 pphr; and most preferably from about 2 to about 10 pphr, all either as one curing agent alone, or as the combination of any number of different types.
Other additives to the rubber formulation include any of the aforementioned silver ion release control additives, accelerators, accelerator activators, antidegradants, softeners, abrasives, colorants, flame retardants, homogenizing agents, internal lubricants, and deodorants. Such components should be present, if at all, in rather low amounts, of from about 0. 1 to about 10 pphr.
It has further been unexpectedly determined that a substantial increase in the antimicrobial and antifungal efficacy is provided upon washing the finished inventive article. Abrading the surface of such an article also permits increases in such characteristics; however, industrial laundering of certain rubber products (mats, and the like) can be improved in antimicrobial, etc., efficacy through a simple washing. In fact, such an increase steadily improves with greater numbers of consistent washes such that it has been found that a rubber article as first vulcanized exhibits lower overall antimicrobial and antifungal activity than one that has been washed one, two, three, and up to at least 20 times (with a standard detergent such as a mixture of anionic surfactants, in a standard industrial rotary washing machine). Such a surprising benefit thus permits utilization of such rubber articles as floor coverings (mats, as one example, such as those with carpeted portions or those which are rubber alone; particularly foamed rubber mats for antifatigue properties and reduced specific gravity so as to reduce the chances of machinery damage during such industrial rotary launderings and dryings), and other articles which can be easily washed within standard laundry machines.
Furthermore, as alluded to above, friction with the subject rubber article surface can remove very slight layers of rubber from the article surface thereby permitting xe2x80x9cfreshxe2x80x9d silver-comprising crystallites to the surface to act as desired in their antibacterial and/or antifungal capacities. Basically, then, the inventive article produced from the inventive raw rubber formulation exhibits an even dispersion of antimicrobial particles throughout the entire rubber article. Such an even dispersion of the biocide throughout the rubber article thus provides a reservoir of fresh crystallites containing the biocidal metallic ion. As layers of the rubber are worn and abraded away, antimicrobial particles containing untapped silver ions become available.
The preferred peroxide cured rubber articles of this invention containing the antimicrobial agent can be processed into rubber articles which exhibit excellent antimicrobial qualities as well as antimicrobial efficiency throughout the rubber article""s lifetime. Examples of other such rubber articles encompassed within this invention include, but are not limited to hard rubber mats, static dissipative rubber mats, anti-fatigue rubber mats, rubber mats which include a face fiber, rubber link mats, rubber gaskets, rubber medical goods, rubber gloves, rubber medical devices, rubber conveyor belts, rubber belts and rubber wheels used in food processing, rubber clothing, rubber shoes, rubber boots, rubber tubing, and rubber automotive fuel hoses. Such inventive formulations may also be incorporated into a multilayered rubber article in which the antimicrobial agent can be incorporated into any surface layer and still provide the desired antimicrobial efficiency.
Of particular interest is the formation of multilayered rubber articles wherein at least one of such rubber layer exhibits the desired antimicrobial activity and thus is made from an inventive raw rubber formulation. Such layered articles may be adhered together through co-vulcanization, gluing, and the like. Furthermore, layers of other types of materials may be placed being rubber layers as well to provide, as one non-limiting property, better structural stability to the desired multilayered article.

Antimicrobial articles made from nitrile or natural rubber