Title:  Antimicrobial vulcanized EPDM rubber articles

United States Patent:  6,555,599

Issued:  April 29, 2003

Inventors:  Lever; John G. (Spartanburg, SC); Haas; Geoffrey R. (Spartanburg, SC); Patel; Bhawan (Bolton, GB)

Assignee:  Milliken & Company (Spartanburg, SC)

Appl. No.:  815483

Filed:  March 26, 2001

Ethylene-propylene diene monomer (EPDM) rubber-containing articles that exhibit highly desirable long-term effective antimicrobial characteristics are provided. Such articles are in either solid or blown (foam or sponge) state (or combinations of both in multilayered forms) that 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 control of antimicrobial efficacy of the rubber article as well.

OBJECT OF THE INVENTION

It is therefore an object of this invention to provide an antimicrobial vulcanized EPDM rubber-containing article exhibiting 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 EPDM rubber article comprising silver-based antimicrobial compounds which include curing agents which do not deleteriously effect the antimicrobial activity of the finished vulcanized EPDM article (and thus is essentially free from sulfur-based curing agents and accelerators). Yet another object of this invention is to provide a vulcanized EPDM 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 EPDM 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 EPDM rubber-containing article.

Accordingly, this invention encompasses a dimensionally stable vulcanized EPDM rubber-containing article exhibiting 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 EPDM 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 30^oC. and at greater than 90% humidity. Furthermore this invention encompasses such a vulcanized EPDM 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 EPDM 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 150^oC. and at least at a pressure of 3 bars, wherein said rubber formulation is substantially free from sulfur curing agent and accelerator.

The term "dimensionally stable" 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 EPDM 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 "silver ion release control additives".

The term EPDM rubber, as noted above, is intended to cover any standard rubber which possesses at least a majority by weight of EPDM rubber and which must be vulcanized to provide a dimensionally stable rubber article. It is intended that such vulcanization or other processing be performed in an environment that is inexpensive to provide and thus should be undertaken in an oxygen-rich atmosphere (as opposed to an anaerobic environment which is generally difficult to provide). EPDM rubber has been utilized previously within the rubber industry for a variety of applications and is generally well known and taught throughout the prior art. Such inventive rubber articles 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 non-silicone rubber component or components of the inventive rubber article is therefore a majority of EPDM and other types of possible rubber (in order to provide different strengths, flexibilities, or other properties) such as those, without limitation, selected from the group consisting of nitrile rubber [such as acrylonitrile-butadiene (NBR)], styrene-butadiene rubber (SBR), natural rubber, chloroprene, ethylene propylene rubber, natural rubber, polyurethane rubber, butyl rubbers, isoprene, halobutyl rubbers, fluoroelastomers, epichlorohydrin rubber, polyacrylate rubber, and chlorinated polyethylene rubber, hydrogenated SBR, hydrogenated NBR, and carboxylated NBR. 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. 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. 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 EPDM rubber-containing articles. 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 & Company, under the trade name ALPHASAN.RTM.. 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.RTM., or a silver-substituted glass available from Ishizuka Glass under the tradename IONPURE.RTM., 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.RTM., available from DuPont, as well as JMAC.RTM., 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.RTM. brand antimicrobials available from Milliken & Company, (U.S. Pat. Nos. 5,926,238, 5,441,717, 5,698,229 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 (>1000^oC.), 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 "Assessment of Antimicrobial Properties on hydrophobic Textiles and Solid Substrates" 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 EPDM 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 and mixtures of fungi including Aspergillus niger ATCC 6275, Paecilomyces variotii ATCC 18502, Trichoderma virens ATCC 9645, in accordance with Test Method ISO 486. 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, it is believed, without intending to be bound to any specific scientific theory, that 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 "appreciable amount" 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 form 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 present within the inventive vulcanized rubber article 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 antibacterial and antifungal efficacy is provided upon washing the finished inventive article. Abrading the surface of such an article also permits increases in such characteristics due to an increase in Ag+release; 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 antibacterial and antifungal activity than one that has been washed one, two, three, and up to at least 20 times (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 "fresh" 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 EPDM rubber-containing 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 EPDM rubber-containing article. 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.

Claim 1 of 18 Claims

What is claimed is:

1. A dimensionally stable vulcanized rubber article comprising at least a majority of EPDM rubber and at least one silver-based antimicrobial agent, wherein said rubber article exhibits log kill rates in accordance with the AATCC Draft Method entitled "Assessment of Antimicrobial Properties on Hydrophobic Textiles and Solid Substrates" for Staphylococcus aureus and Klebsiella pneumoniae of at least 1.0 each after 24 hours exposure at room temperature, and wherein said article optionally comprises at least one silver ion release control additive, and at least one antifungal additive other than said silver-based antimicrobial compound.

 

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