MXPA00012202A - Method of inhibiting color change in a plastic article comprising silver-based antimicrobials - Google Patents

Abstract

This invention relates to improvements in inhibiting undesirable discoloring of plastic articles within which silver-based antimicrobials have been introduced. Such a method requires the utilization of very low amounts of acid scavengers or stabilizers such as aluminum-magnesium hydroxycarbonate, otherwise known as hydrotalcite (and not a zinc-based compound). Such hydrotalcites are very low in cost, easy to handle, and, utilized in very low levels in combination with a silver-based antimicrobial within a plastic composition, surprisingly substantially prohibits the generation of unwanted aesthetically displeasing colors.

Description

METHOD FOR INHIBITING THE CHANGE OF COLOR IN A PLASTIC ARTICLE COMPRISING ANTIMICROBIAL AGENTS BASED ON • SILVER FIELD OF THE INVENTION 5 This invention relates to improvements in the inhibition of undesirable discoloration of plastic articles into which agents have been introduced antimicrobials based on silver. This method requires the use of very large quantities ^ low fe of compound that acts as an acid remover and as stabilizer of silver ions. The preferred silver ion stabilizer / acid stabilizer compound is aluminum-magnesium hydroxycarbonate, also known as hydrotalcite. Such hydrotalcites have a very low cost, are easy to handle, and when used at very high levels low in combination with a silver-based antimicrobial agent within a plastic composition, prevent ^ substantially surprisingly the generation of aesthetically unpleasant unwanted colors. BACKGROUND OF THE INVENTION In recent years, great attention has been given to the dangers of bacterial contamination from a potential daily exposure. Notable examples of this concern include the fatal consequences of food poisoning caused by certain strains of Escherichia coli that are found inside beef that is not sufficiently cooked in fast food restaurants; contamination by Salmonella that causes diseases from poultry food products not sufficiently cooked or insufficiently washed; and diseases as well as 5 cutaneous infections attributed to Staphylococcus aureus, yeast and other unicellular organisms. With growing consumer interest in this area, manufacturers have begun to introduce antimicrobial agents into various products and household items. • 10 inorganic microbicides containing silver have recently been developed and used as antimicrobial agents on various substrates and surfaces. Particularly, such microbicides have been adapted for incorporation into plastic and fiber compositions for the purpose of providing household and consumer products that inherently exhibit antimicrobial characteristics. Even when such silver-based agents • provide excellent antimicrobial properties within plastic items, and the like, are found often aesthetic problems. It is believed that this is due to several causes, all based on the inherent photo-stability of silver ions. The formation of color species of silver metals causes discoloration of the plastic composition which, again from an aesthetic perspective, it is highly undesirable. Thus, there is a need to provide a method for introducing silver-based antimicrobial agents into plastic compositions that substantially reduces the degree of undesired discoloration within the resulting article due to contamination by silver metal. Previous methods have included the addition of benzatriazoles, as in U.S. Patent No. 5,405,644 to Ohsumi et al., And triazoles and stabilizers (such as metallic stearate acid removers), as in U.S. Patent No. 4,938,955 to Niira, et al. to the. However, these methods have proven to be costly (with the high cost of benzotriazoles, initially), especially since relatively high concentrations of costly stabilization compounds are required. Also, since these stabilizers do not have thermal stability, they introduce additional processing complications. There are no clear teachings or suggestions within the prior art that allow improvement in methods of inhibition of color change (discoloration) of plastic articles comprising antimicrobial agents based on silver using an aluminum-magnesium hydrotalcite in an amount of 0.001. to approximately 0.2% of the total plastic composition. OBJECTS OF THE INVENTION Accordingly, it is an object of the present invention to provide an improved method for inhibiting discoloration of plastic articles comprising antimicrobial agents. • based on silver. A further object of the invention is to provide a compound that acts as an acid remover and as a silver-based antimicrobial complex stabilizer within a plastic composition. Another object of the present invention is to provide an aesthetically pleasing plastic article having excellent anti-microbial properties. Another object of the present invention is to offer an economical method to inhibit the color change in a plastic composition due to the degradation of silver-based antimicrobial complexes. SUMMARY OF THE INVENTION Accordingly, this invention encompasses an article of A plastic comprising an antimicrobial agent based on silver and an aluminum-magnesium hydrotalcite wherein said hydrotalcite is present in an amount of about 0.001 to about 0.2% of the total weight of the plastic article. Likewise, this invention covers a A method for inhibiting the color change in a plastic composition or an article comprising a silver-based antimicrobial complex, said method comprising the step of introducing an aluminum-magnesium hydrotalcite into a melted thermoplastic resin composition and The molding of said resultant thermoplastic / hydrotalcite resin composition into a plastic article, wherein the concentration of said hydrotalcite within said plastic article is from about 0.001 to about 0.2% of the total weight of said plastic article. Nowhere in the prior art has there been clearly disclosed, employed or suggested a specific plastic article or method for making it to produce an antimicrobial article resistant to color change due to degradation of the silver-based antimicrobial complex. The closest technique, US Patent No. 5,750,069 to Nosu et al., Discloses an ultraviolet ray protecting agent for incorporation into various compositions such as films, fibers, cosmetics and the like. The protective agent in this patent is a zinc-based hydrotalcite that acts only as an ultraviolet ray absorber; it must be present in an amount of at least 1% of the total white composition; and it must contain a zinc component (which raises the cost of a compound of this type considerably compared to a simple aluminum magnesium compound). The hydrotalcite contemplated within this invention does not expressly include a zinc-based compound. In addition, said zinc compound must be present in high concentrations in order to be able to act effectively as an ultraviolet blocking agent; in lower proportions, there would not be sufficient amounts of this compound to • provide said desired results of UV absorption. Thus, there is no disclosure that clearly teaches or suggests the specific antimicrobial plastic items as well as the methods for making them that we discuss now. Any plastic in which an antimicrobial agent based on silver can be properly incorporated can • 10 used in this invention. For example, and without limitation, polyolefins, such as polyethylene, polypropylene, and polybutylene, halogenated polymers, such as polyvinyl chloride, polyesters, such as polyethylene terephthalate, polyamides, such as nylon 6 and nylon 6, 6, Polyurethanes, and the like may be employed within the scope of this invention. Preferably, the plastic is a thermoplastic that can be molded into different shapes and sizes by extruding an article molded with the antimicrobial compounds based on silver and hydrotalcite. Thus prefer polyolefins, particularly polypropylene, and polyesters, particularly polyethylene terephthalate. In addition, such plastics may preferably be colored to provide other aesthetic characteristics to the end user. Thus, the plastic can also comprise dyes, such as, for example, poly (oxyalkylenated) dyes, pigments, and the like. Other additives may also be present, including antistatic agents, gloss compounds, • nucleating agents, rinse agents, antioxidants, UV light stabilizers, fillers and the like. The preferred silver-based antimicrobial agent is a silver zirconium phosphate available from Toagasei Chemical Industry Co., Ltd., under the tradename NOVARON®, even when any silver-containing antimicrobial agent is present. susceptible to discoloration in the presence of silver metals The dissociation of a complex or acid scavengers can also be used within the plastic article of the present invention (for example, by way of example only, a silver-substituted zeolite available from Shingawa under the trade name ZEOMIC® AJ ). Usually, Said antimicrobial agent is added in an amount of about 0.01 to 10% of the total weight of the white plastic composition; more preferably, from about 0.05 to about 2.0%; and especially from about 0.1 to about 1.0%. The essential hydrotalcite compound, which must be a magnesium aluminum hydrotalcite, is available from Kyowa Chemical Industry Co. , Ltd., under the trade name DHT-4A. Said compound generally complies with the following formula (I) (I) [MgaAlb (OH) cC03-XH20] where a is from 3 to 8, b is from 1 to 4, c is from 10 to 20, and X is from 1 to 10. Preferably, a is approximately 4.5, b is fl approximately 2, c is approximately 13, and X is approximately 4. As will be seen below, the basic procedures followed in the production of the antimicrobial plastic article of the present invention comprise standard plastic forming techniques. There are two basic methods for incorporating additives (such as agents • 10 silver-based antimicrobials and magnesium aluminum hydrotalcites of the present invention, for example, within polymeric articles. One method is the dry mixing of a mixture of polymer, additives, antimicrobial agents, and hydrotalcites; the melting of the dry mix together in an extruder to form a melted composition which is then formed into pellets; and the melting and subsequent molding of these pellets in a plastic article.

• Alternatively, conventional resin pellets and a master batch concentrate containing The antimicrobial agent and the hydrotalcite additives can be molded in conventional molding equipment. The aforementioned molding steps can preferably be carried out with injection molding equipment; However, other plastic forming operations can also be used as for example, without limitation, blow molding, fiber extrusion, film forming, compression molding, rotary molding, and the like. These operations ^ Plastic article training alternatives are well understood by a person with knowledge normal of the technique. Preferably, for example, from about 0.01 to about 5% (by weight of the polymer composition) of a silver-based antimicrobial agent (NOVARON® from Toagasei, as mentioned above; preferably from about 0.1 to about 2% by weight) with the thermoplastic resin (such as, for example, preferably polypropylene) within an injection molding machine (such as for example an Arburg Molder). The preferred hydrotalcite is also added to the formulation of polymer / antimicrobial agent in an amount from about 0.001 to about 0.2% by weight of the entire formulation; preferably of approximately 0.01 to • approximately 0.1% by weight; especially from about 0.02 to about 0.06%. PREFERRED EMBODIMENTS OF THE INVENTION The following examples are indicative of the preferred embodiment of this invention: Color change analysis EXAMPLE 1 A plastic article was produced in the following manner: 993.5 grams of polypropylene homopolymer were mixed with 5.0 grams of an antimicrobial phosphate compound of • zirconium silver (NOVARON® AG300, from Toagasei), 0.8 gram of a phenolic antioxidant (Irganox® 1010, from Ciba-Geigy), 0.5 5 gram of a phosphorus antioxidant (Irganox® 168, from Ciba-Geigy), and 0.2 gram hydrotalcite (DHT, 4AÓ, from Kyowa Chemical), were melted and mixed together in a single screw extruder. These pellets were then ^^ subsequently fed by gravity to a machine Arburg injection molding, melted and molded into a plastic plate. EXAMPLE 2 (Comparative Example) A comparable plastic article was prepared in accordance with the method and composition of EXAMPLE 1, except that 992.9 grams of polypropylene homopolymer were used and the hydrotalcite was replaced by 0.8 gram of calcium stearate. The resulting article was also extruded in the form of a plate. These two plates of EXAMPLES 1 and 2 were after compared to determine the color change after 40 hours of exposure to a xenon arc light source. The color difference between the initial plastic formed and the plastic that was exposed to a xenon arc light source for 40 hours was calculated for each plate using the following equation: DE = ((L initial - L aging) "*" (initial 3 ~ aging) + l \ Ui- > initial - K u * aging ^) 2 \ I? • where DE * represents the color difference between the initial plate and the exposed plate, L #, a *, and b * are the color coordinates 5; where L * is a measurement of the clarity and darkness of a plate; a 'is a measurement of the red or green character of the plate; and b * is a measurement of the yellow or blue character of the plate. For an additional commentary and more in-depth explanation of the test procedure, see Billmeyer, F.W., • 10 et al., Principles of Color Technology, 2nd. edition, pages 62-64 and 101-104. Thus, the color change was measured as DE; a low measurement indicated a low color change (and therefore a low degree of unwanted discoloration). The results for the DE measurements of these compositions comparatives were 0.53 for the plate of EXAMPLE 1 and 11.02 for EXAMPLE 2. Thus, it can be clearly seen that the hydrotalcite of EXAMPLE 1 provided improved color change characteristics compared to the standard composition of EXAMPLE 2. 20 Yellowness analysis EXAMPLE 3 A plastic article was produced in accordance with the method presented above in EXAMPLE 1. 994.8 grams of low density polyethylene (LDPE) was mixed with 5.0 grams of an antimicrobial compound of zirconium silver phosphate (NOVARON® AG300, from Toagasei), and 0.2 grams of hydrotalcite (DHT-4AÓ, from Kyowa Chemical), were melted and mixed together and then extruded to form a plate . 5 - EXAMPLE 4 A plastic article was produced in accordance with the method presented above in EXAMPLE 1. 994.8 grams of low density polyethylene (LDPE) was mixed with 5.0 grams of an antimicrobial compound of zeolite substituted with • 10 silver (ZEOMIC® AJ, from Shingawa), and 0.2 gram hydrotalcite (DHT-4AÓ, from Kyowa Chemical), were melted and mixed together and then extruded to form a plate. EXAMPLE 5 (Comparative) 15 A comparable plastic article was produced in accordance with the method presented above in example 1. 994.5 grams of low density polyethylene (LDPE) was mixed with 5.0 • grams of an antimicrobial compound of zirconium silver phosphate (NOVARON® AG300, from Toagasei), and 0.5 gram of stearate Sodium, they were melted and mixed together, and extruded to form a plate. EXAMPLE 6 (Comparative) A comparable plastic article was produced in accordance with the method presented above in EXAMPLE 1. 994.5 grams of low density polyethylene (LDPE) were mixed with 5.0 grams of an antimicrobial silver-substituted zeolite compound (ZEOMIC® AJ, from Shingawa), and • 0.5 gram of sodium stearate, melted together and extruded to form a plate. EXAMPLE 7 (Comparative) A plate was produced in accordance with the extrusion method indicated above in EXAMPLE 1, which comprised 1,000 grams of LDPE. The plates of EXAMPLES 3 to 7 were tested to measure the change in yellowness of each plate from the formation by exposure to the atmosphere for 20 hours. Yellowness is measured by changing the variable b * of the aforementioned DE formula (color change). The lower the registered b * value, the better the result in terms of discoloration. The results appear in the following table: TABLE EXAMPLE Yellowness 3 -1.04 4 -0.81 20 5 10.54 6 2.34 7 -1.54 Clearly, the best results are obtained through the use of LDPE alone, but there is no agent antimicrobial present to provide such properties to the plastic. Thus, by using the hydrotalcite-containing plastic of the present invention, in combination with a FLK silver-based antimicrobial agent, the overall lower degree of yellowness is obtained, thus offering the plastic article aesthetically pleasing from the perspective of non-aesthetics. yellowness Obviously, there are numerous alternative embodiments and modifications to the present invention that are within the spirit and scope of the appended claims. w 10 fifteen • twenty

Claims (4)

1. CLAIMS 1. A plastic article comprising an antimicrobial agent based on silver and a magnesium aluminum hydrotalcite where said hydrotalcite is found 5 present in an amount from about 0.001 to about 0.2% of the total weight of the plastic article.
2. 2. The plastic article according to claim 1 wherein said hydrotalcite is • 10 present in an amount from about 0.01 to about 0.1% of the total weight of the plastic article.
3. 3. The plastic article according to claim 2 wherein said hydrotalcite is 15 present in an amount from about 0.02 to about 0.06% of the total weight of the plastic article. •
4. 4. A method for inhibiting the color change in a plastic composition or in an article comprising A silver-based antimicrobial complex, said method comprises the step of introducing a magnesium aluminum hydrotalcite into a melted thermoplastic resin composition and molding said thermoplastic / hydrotalcite resin composition. 25 resulting in a plastic article, wherein the concentration of said hydrotalcite is from about 0.001 to about 0.1% of the total weight of said plastic article. The method according to claim 4 wherein said hydrotalcite is present in an amount of about 0.01 to about 0.1% of the total weight of the plastic article. The method according to claim 5 wherein said hydrotalcite is present in an amount of about 0.02 to about 0.06% of the total weight of the plastic article.