A synergistic antimicrobial composition

Abstract: An antimicrobial agent and its use together with an acid provides a composition that possesses synergistic effects. More particularly, there is provided a synergistic antimicrobial composition in the form of a quaternary ammonium compound and an acid such as boric, citric and malic.

Description: This invention relates to an antimicrobial agent and its use together with an acid to provide a composition that possesses synergistic effects. More particularly, the invention provides a synergistic antimicrobial composition in the form of a quaternary ammonium compound and boric or citric acid. Examples of prior art are JP-A-60 185 866, JP-A-61 282 474 and WO-A-8706470. Antimicrobial agents are chemical compositions that are used to prevent microbiological contamination and deterioration of products, materials and systems. Particular areas of application of antimicrobial agents and compositions are, for example, cosmetics, disinfectants, sanitizers, wood preservation, food, animal feed, cooling water, metalworking fluids, hospital and medical uses, plastics and resins, petroleum, pulp and paper, textiles, latex, adhesives, leather and hides and paint slurries. Of the diverse categories of antimicrobial agents and compositions, quaternary ammonium compounds represent one of the largest of the classes of agents in use. At low concentrations, quaternary ammonium type antimicrobial agents are bacteriostatic, fungistatic, algistatic, sporostatic and tuberculostatic. At medium concentrations they are bactericidal, fungicidal, algicidal and viricidal against lipophilic viruses. Silicone containing quaternary ammonium compounds are well known. What is new and not believed to be taught in the prior art is the concept of combining acids with the particular silanes of the present invention to kill germs synergistically. In accordance with the present invention, it has been found that antimicrobial properties of silicone containing quaternary ammonium compounds can be improved by mixing the antimicrobial agent with an acid to produce a composition that acts synergistically in preventing microbiological contamination and deterioration of products, materials and systems. For example, 3-(trimethoxysilyl)-propyldimethyloctadecylammonium chloride is an effective antimicrobial agent in which the active ingredient hydrolyzes in water and reacts with substrates with which it is brought into contact. These substrates demonstrate nonleaching broad spectrum antimicrobial activity. By including an acid component in the antimicrobial composition, such activity of this particular quaternary ammonium compound has been increased substantially against a mixed fungal culture where neither the acid nor the quaternary ammonium compound was effective alone. Hence, the compositions set forth in the present invention possess advantages over existing antimicrobial treating agents and provide improved results thereover, since the combinations disclosed herein of an antimicrobial agent together with an acid provide better results than when either of the individual components are employed separately. Thus, the disadvantages of the prior art are overcome with the present invention wherein improved antimicrobial agents are provided. The present invention provides a synergistic antimicrobial composition comprising: (a) an organosilane having the general formula selected from wherein, in each formula, R b is R or OR where each R is an alkyl radical of 1 to 4 carbons or hydrogen; a has a value of 0, 1 or 2; R' is a methyl or ethyl radical; R'' is an alkylene group of 1 to 4 carbon atoms; R''', R'''', and R v are each independently selected from alkyl radicals of 1 to 18 carbon atoms, -CH₂C₆H₅, -CH₂CH₂OH, -CH₂OH, and -(CH₂) x NHC(O)R vi where x has a value of from 2 to 10 and R vi is a perfluoroalkyl radical having from 1 to 12 carbon atoms; and X is chloride, bromide, fluoride, iodide, acetate, or tosylate; and (b) an acid selected from boric acid and citric acid, and where the mole ratio of (a) to (b) is between about 1:1 and about 1:10. The most desirable mole ratio of (a) to (b) is preferably about 1.5. Specific silanes are, for example, (CH₃O)₃Si(CH₂)₃N⁺(CH₃)₂C₁₈H₃₇C1⁻ and (CH₃O)₃Si(CH₂)₃N⁺(C₁₀H₂₁)₂(CH₃)C1⁻. The invention also relates to a method of inhibiting the proliferation of potentially destructive microorganisms on a substrate by treating the substrate with an effective amount of a synergistic antimicrobial composition as defined above. The substrate is prefrably paper. The invention further relates to a cellulosic fiber substrate having a surface that inhibits the proliferation of potentially destructive microorganisms present on the substrate by treating the substrate with an effective amount of a synergistic antimicrobial composition as defined above. In order to illustrate the concept of the present invention, tests were conducted as set forth hereinbelow. Results from the tests are set forth in the form of tabular data in order to facilitate an understanding of the invention. Varied samples were prepared some of which acted as control samples, while others contained varied compositions of components selected such that the data could be better interpreted. For example, in Table I, four samples were prepared containing the ingredients shown and in the amounts specified. The sole sample containing the acid component of the invention was Sample No. 4. These four samples were tested one against the other along with a control sample for the prevention of fungal overgrowth and the results of these tests as well as the comparison between the various samples can be seen in Table II. Table II shows that the sole sample providing a zero percent fungal overgrowth was Sample No. 4 which is equivalent to the identically numbered sample in Table I, which is the acid containing composition of the present invention. Thus, it should be apparent that the inclusion of citric acid, for example, along with the antimicrobial quaternary ammonium compound indicated as "B" in the Tables produces a result not obtainable with either the acid component alone or the antimicrobial quaternary ammonium compound when used separately. For example, when used separately in Sample No. 2 in Table I, the quaternary ammonium compound "B" allowed 30% fungal overgrowth as shown in Table II. Sample No. 1 containing a cyclic antimicrobial quaternary ammonium compound of 3-(trimethoxysilyl)propyl-octadecyldimethyl ammonium chloride and 3-chloropropyltrimethoxysilane in methanol, was not as effective as the acid containing Sample No. 4, allowing a 75% fungal overgrowth. Sample No. 4 out performed a mixture of two antimicrobial agents in Sample No. 3 where the antimicrobial compound "B" was combined with another quaternary ammonium salt ARQUAD® T2, manufactured by Armour Hess Chemicals, allowing a fungal overgrowth of 75%. The blank control sample in Table II was ineffective allowing a 100% fungal overgrowth. In view of the data obtained and set forth in Tables I and II, additional tests were conducted with samples of citric and boric acid to confirm the results of Table II. Thus, in Table III, varied samples were prepared and tested for the prevention of fungal overgrowth over a two to three week period. It can be seen from Table IV that the citric and boric acid compositions containing the antimicrobial compound "B" again out performed both the acids when employed alone, as well as the quaternary ammonium compound "B", when each were employed individually. Thus, over a two week period, there was no fungal overgrowth with both the mixtures of compound "B" with either citric or boric acid. Over the three week period, the mixture of boric acid and compound "B" outperformed the mixture of compound "B" with citric acid by 20%. In order to demonstrate the durability and the substantivity of the compounds of the present invention, it should be noted that the anion of an aqueous sodium salt of bromphenol blue can be complexed with the cation of a polymerized silane of this invention while it is on a substrate. The blue colored complex, substantive to a water rinse, is qualitatively indicative of the presence of the cation on the substrate thus indicating the extent of antimicrobial agent on a given substrate. A comparison of the intensity of retained blue color to a color standard is used as a check to determine if the treatment has been applied properly. The method consists of preparing a 0.02 to 0.04 weight percent solution of bromphenol blue in distilled water. This solution is made alkaline using a few drops of saturated Na₂CO₃ solution per 100 milliliters of the solution. Two to three drops of this solution are placed on the treated substrate and allowed to stand for two minutes. The substrate is then rinsed with copious amounts of tap water and the substrate is observed for a blue stain and it is compared to a color standard. For a spectrophotometric determination, the following test is used. The sodium salt of bromphenol blue is depleted from a standard solution by complexing with the cations on a treated substrate. The change in bromphenol blue concentration is determined spectrophotometrically or by comparison with color standards whereby the level of substrate treatment by the cationic silane is determinable. The method consists of preparing a 0.02 weight percent standard solution of bromphenol blue in distilled water. It is made alkaline with a few drops of saturated Na₂CO₃ solution per 100 milliliters of bromphenol blue solution. The color of this solution is purple. The blank solution is adjusted to yield a 10 to 12% transmittance reading when measured in 1 cm cells using a spectrophotometer set at 589 nm by the following method. Fill a container 3/4 full of distilled water and add 2 ml of the 0.02% standard bromphenol blue solution for every 50 ml of distilled water. Add 0.5 ml of a 1% Triton® X-100 surfactant (manufactured by Rohm and Haas, Philadelphia, PA, USA) aqueous solution for every 50 ml of water. Mix and, using the spectrophotometer, determine the maximum absorbance. Adjust the upper zero to 100% transmittance with distilled water. Check the percent transmittance of the working bromphenol blue solution at the maximum absorbance setting. Adjust the blank solution to 10 to 12% transmittance with either water or bromphenol blue standard solution as necessary. The samples of treated substrate are tested by placing 0.5 gram samples of the substrate standards in a flask large enough for substantial agitation of the sample and the test solution. Add 50 ml of the working solution. Agitate for 20 minutes on a wrist-action shaker. Fill the test curvette with the test solution. Centrifuge if particulate matter is present. Measure the percent transmittance at the wavelength set forth above. The transmittance is compared against a standard curve prepared by preparing several substrate samples of known concentration of the cationic silane. For example, samples containing a known amount of cationic silane at, for example, 0%, 0.25%, 0.50%, 0.75% and 1% are read spectrophotometrically and a curve is plotted. The data in Tables II and IV was generated using the following technique. The antimicrobial activity of a treated surface was evaluated by shaking a sample weighing 0.75 grams in a 750,000 to 1,500,000 count Klebsiella pneumoniae suspension for a one hour contact time. The suspension was serially diluted, both before and after contact and cultured. The number of viable organisms in the suspensions was determined. The percent reduction based on the original count was determined. The method was intended for those surfaces having a reduction capability of 75 to 100% for the specified contact time. The results are reported as the percent reduction. Media used in this test were nutrient broth, catalog No. 0003-01-6 and tryptone glucose extract agar, catalog No. 0002-01-7 both available from Difco Laboratories, Detroit, Michigan, U.S.A. The microorganism used was Klebsiella pneumoniae American Type Culture Collection; Rockville, Md., U.S.A., catalog No. 4352. The procedure used for determining the zero contact time counts was carried out by utilizing two sterile 250 ml. screw-cap Erlenmeyer flasks for each sample. To each flask was added 70 ml of sterile buffer solution. To each flask was added, aseptically, 5 ml of the organism inoculum. The flasks were capped and placed on a wrist action shaker. They were shaken at maximum speed for 1 minute. Each flask was considered to be at zero contact time and was immediately subsampled by transferring 1 ml of each solution to a separate test tube containing 9 ml of sterile buffer. The tubes were agitated with a vortex mixer and then 1 ml of each solution was transferred to a second test tube containing 9 ml of sterile buffer. Then, after agitation of the tubes, 1 ml of each tube was transferred to a separate sterile petri dish. Duplicates were also prepared. Sixteen ml of molten (42°C.) tryptone glucose extract agar was added to each dish. The dishes were each rotated ten times clockwise and ten times counterclockwise. The dishes were then incubated at 37°C. for 24 to 36 hours. The colonies were counted considering only those between 30 and 300 count as significant. Duplicate samples were averaged. The procedure used for determining the bacterial count after 1 hour was essentially the same as that used to determine the count at the zero contact time. The only difference was that pour plating was performed at the 10⁰ and 10⁻¹ dilutions as well as at the 10⁻² dilution. "Percent reduction" was calculated by the formula where A is the count per milliliter for the flask containing the treated substrate; B is zero contact time count per milliliter for the flask used to determine "A" before the addition of the treated substrate and C is zero contact time count per milliliter for the untreated control substrate. SAMPLE I II III IV Water(gms.) ---- 539 539 533.5 Temperature(°C) 61 61 61 61 Time(min.) 30 30 30 30 Solution(gms.) 550 550 550 550 Paper(gms.) 5 5 5 5 Temp.Rise(°/min.) 2 2 2 2 Compound A B B B Compound(gms.) 550 11 5.5 5.5 Acid ---- ---- ---- Citric Acid(gms.) ---- ---- ---- 11 Additive ---- ---- C ---- Additive(gms.) ---- ---- 5.5 ---- A= Cyclic quaternary ammonium salt B= (CH₃O)₃Si(CH₂)₃N+(CH₃)₂C₁₈H₃₇Cl- C= ARQUAD® T2, a quaternary ammonium salt of Armour Hess Chemicals SAMPLE FUNGAL (% Overgrowth) Control 100 1 75 2 30 3 75 4 0 ACID ACID (gms.) WATER (gms.) B (gms.) Citric 11 550 5.5 Citric 11 539 ---- Boric 11 550 5.5 Boric 11 539 ---- ----- ---- 494 55 SAMPLE FUNGAL (% Overgrowth) TWO WEEKS THREE WEEKS Blank Control 100 100 Citric Acid Control 100 100 Citric Acid + B 0 20 Boric Acid Control 10 10 Boric Acid + B 0 0 Solution B 90 90 10% Solution B 90 90 The silanes useful in this invention also have the general formula It should be noted that generically, these materials are quaternary ammonium salts of silanes. Most of the silanes falling within the scope of this invention are known silanes and references disclosing such silanes are numerous. One such reference, United States Patent No. 4,259,103, issued to James R. Malek and John L. Speier, on March 31, 1981, discusses the use of such silanes to render the surfaces of certain substrates antimicrobial. Canadian Patent No. 1,010,782, issued to Charles A. Roth shows the use of fillers treated with certain silanes to be used in paints and the like to give antimicrobial effects. Numerous other publications have disclosed such silanes, namely, A. J. Isquith, E. A. Abbott and P. A. Walters, Applied Microbiology, December, 1972, pages 859-863; P. A. Walters, E. A. Abbott and A. J. Isquith, Applied Microbiology, 25, No. 2, p. 253-256, February 1973 and E. A. Abbott and A. J. Isquith, United States Patent No. 3,794,736 issued February 26, 1974, U.S. Patent 4,406,892, issued September 27, 1983, among others. For purposes of this invention, the silanes can be used neat or they can be used in solvent or aqueous-solvent solutions. When the silanes are used neat, the inventive process is preferably carried out in a system in which some small amount of water is present. If it is not possible to have a system with some small amount of water present, then a water soluble or water-dispersable, low molecular weight hydrolyzate of the silane may be used. What is important is the fact that the durability of any effect produced by the silane as part of a product requires that the silane molecule react with a surface to a certain extent. The most reactive species, as far as the silanes are concerned, is the ≡SiOH that is formed by hydrolysis of the alkoxy groups present on the silane. The ≡SiOH groups tend to react with the surface and bind the silanes to the surface. It is believed by the inventor even though the prime mode of coupling to the surface system is by the route described above, it is also believed by the inventor that the alkoxy groups on the silicon atom may also participate in their own right to bind to the surface. Preferred for this invention is a reactive surface containing some small amount of water. By "reactive", it is meant that the surface must contain some groups which will react with some of the silanols generated by hydrolysis of the silanes of this invention. R in the silanes of this invention are alkyl groups of 1 to 4 carbon atoms. Thus, useful as R in this invention are the methyl, ethyl, propyl and butyl radicals. RO in the above formulas can also be R. R can also be hydrogen thus indicating the silanol form, i.e. the hydrolyzate. The value of a is 0, 1 or 2 and R′ is a methyl or ethyl radical. R″ for purposes of this invention is an alkylene group of 1 to 4 carbon atoms. Thus, R″ can be alkylene groups such as methylene, ethylene, propylene and butylene. R′′′, R″″ and R v are each independently selected from a group which consists of alkyl radicals of 1 to 18 carbons, -CH₂C₆H₅ , -CH₂CH₂OH, -CH₂OH and -(CH₂) x NHC(O)R vi . x has a value of from 2 to 10 and R vi is a perfluoroalkyl radical having from 1 to 12 carbon atoms. X is chloride, bromide, fluoride, iodide, acetate or tosylate. Ph is phenyl. Preferred for this invention are the silanes of the general formula wherein R is methyl or ethyl; a has a value of zero, 1 or 2; R″ is propylene; R‴ is methyl or ethyl; R″″ and R v are selected from alkyl groups containing 1 to 18 carbon atoms wherein at least one such group is larger than eight carbon atoms and x is either chloride, acetate or tosylate. Most preferred for this invention are those silanes having the formula (CH₃O)₃Si(CH₂)₃N ⊕ (CH₃)₂C₁₈H₃₇Cl⁻ and (CH₃O)₃Si(CH₂)₃N ⊕ CH₃(C₁₀H₂₁)₂Cl⁻. As indicated above, most of these silanes are known from the literature and methods for their preparation are known as well. See, for example, U.S. Patent 4,282,366, issued August 4, 1981; U.S. Patent 4,394,378, issued July 19, 1983, and U.S. Patent 3,661,963 issued May 9, 1972, among others. Specific silanes within the scope of the invention are represented by the formulae: (CH₃O)₃Si(CH₂)₃N⁺(CH₃)₂C₁₈H₃₇Cl⁻, (CH₃O)₃Si(CH₂)₃N⁺(CH₃)₂C₁₈H₃₇Br⁻, (CH₃O)₃Si(CH₂)₃N⁺(C₁₀H₂₁)₂CH₃Cl⁻, (CH₃O)₃Si(CH₂)₃N⁺(C₁₀H₂₁)₂CH₃Br⁻, (CH₃O)₃Si(CH₂)₃N⁺(CH₃)₃Cl⁻, (CH₃O)₃SiCH₂CH₂CH₂P⁺(C₆H₅)₃Cl⁻, (CH₃O)₃SiCH₂CH₂CH₂P⁺(C₆H₅)₃Br⁻, (CH₃O)₃SiCH₂CH₂CH₂P⁺(CH₃)₃Cl⁻, (CH₃O)₃SiCH₂CH₂CH₂P⁺(C₆H₁₃)₃Cl⁻, (CH₃)₃Si(CH₂)₃N⁺(CH₃)₂C₁₂H₂₅Cl⁻, (CH₃)₃Si(CH₂)₃N⁺(C₁₀H₂₁)₂CH₃Cl⁻, (CH₃)₃Si(CH₂)₃N⁺(CH₃)₂C₁₈H₃₇Cl⁻, (CH₃O)₃Si(CH₂)₃N⁺(CH₃)₂C₄H₉Cl⁻, (C₂H₅O)₃Si(CH₂)₃N⁺(CH₃)₂C₁₈H₃₇Cl⁻, (CH₃O)₃Si(CH₂)₃N⁺(CH₃)₂CH₂C₆H₅Cl⁻, (CH₃O)₃Si(CH₂)₃N⁺(CH₃)₂CH₂CH₂OHCl⁻, (HO)₃Si(CH₂)₃N⁺PhCl⁻, (CH₃O)₃Si(CH₂)₃N⁺PhCl⁻, (CH₃O)₃Si(CH₂)₃N⁺(CH₃)₂(CH₂)₃NHC(O)(CF₂)₆CF₃Cl⁻, (CH₃O)₃Si(CH₂)₃N⁺(C₂H₅)₃Cl⁻. It should be noted that carriers may be employed herein such as gels, powders, creams and lotions, emulsions, microemulsions and solvent solutions, of the active antimicrobial agents. Surfaces that can be treated include carpet; fabrics, for example socks, clothing, shoe inner liners, towels, bedding, upholstery, curtains and draperies; as well as hard surfaces, for example walls, tables, ceilings and furnishings. While the treated substrates of the present invention are of general application, they are especially useful in the preparation of papers employed in conjunction with foodstuffs. Thus, the treated papers and substrates are useful as milk containers and cartons, field transport containers for fruit and vegetables, cardboard fruit containers, consumer foodstuff containers and wrappings, wrapping paper for fruit, vegetables and meat and as wrappers for soap, for example. It will be apparent from the foregoing that many other variations and modifications may be made in the structures, compounds, compositions and methods described herein without departing substantially from the essential concepts of the present invention. Accordingly, it should be clearly understood that the forms of the invention described herein are exemplary only and are not intended as limitations on the scope of the present invention.

Claims: A synergistic antimicrobial composition comprising: (a) an organosilane having the general formula selected from wherein, in each formula, R b is R or OR where each R is an alkyl radical of 1 to 4 carbons or hydrogen; a has a value of 0, 1 or 2; R' is a methyl or ethyl radical; R'' is an alkylene group of 1 to 4 carbon atoms; R''', R'''', and R v are each independently selected from alkyl radicals of 1 to 18 carbon atoms, -CH₂C₆H₅, -CH₂CH₂OH, -CH₂OH, and -(CH₂) x NHC(O)R vi where x has a value of from 2 to 10 and R vi is a perfluoroalkyl radical having from 1 to 12 carbon atoms; and X is chloride, bromide, fluoride, iodide, acetate, or tosylate; and (b) an acid selected from boric acid and citric acid, and where the mole ratio of (a) to (b) is between about 1:1 and about 1:10. A method of inhibiting the proliferation of potentially destructive microorganisms on a substrate comprising treating the substrate with an effective amount of a synergistic antimicrobial composition as claimed in Claim 1. A cellulosic fibre substrate having a surface that inhibits the proliferation of potentially destructive microorganisms present on the substrate, the surface being formed by treating the substrate with an effective amount of a synergistic antimicrobial composition as claimed in Claim 1.

Inventors: BLANK LYNNE MARIE BLEHM, BLANK, LYNNE MARIE BLEHM
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1989
Main classification: A01N55
IPC codes: A61K8, A61K31, A01N55