Use of a copper-based white alloy for producing articles with antibacterial properties, preferably intended to be employed in the health and/or food field, in which the desired white colour is imparted by adding manganese to the alloy; the latter and possibly other alloying elements, except for impurities, are present in an amount such as to determine machinability, colour and surface properties of the alloy, similar to those of stainless steel, and simultaneously intrinsic antibacterial properties of the articles on respective exposed surfaces thereof.
The present invention relates to the use of a copper-based white alloy for producing naturally antibacterial articles, which simultaneously have an excellent mechanical resistance, an easy machinability, a high cleaning ease and an appearance similar to that of stainless steel. The invention further relates to a method for producing these articles.
It has been known since ancient times (ancient Egypt) that copper has disinfectant properties. More recently, copper and its alloys (brasses) have been investigated for their bactericidal properties on some pathogen microorganisms [
However, copper and brasses have aesthetical and technological features which are generally considered inappropriate to validly replace stainless steel, for instance in the manufacturing of articles such as sanitaryware, sinks, tables, working surfaces for canteens or laboratories, basins for sanitary use, lockers, both because the mechanical features are typically much poorer than those of stainless steel, and because they are not white, but are instead of different colours such as red, pink and yellow. For all of these reasons, the use of copper and alloys thereof in the health field has been for the time being limited to components intended to remain hidden, for instance those of hydraulic or conditioning systems, such as finned heat exchangers.
There are also known, for producing jewels having an appearance similar to silver and for producing accessories for clothes, such as buttons and zips, white copper alloys, such as copper and nickel alloys, which however are nowadays increasingly less used due to the allergising effects of nickel, and the so-called "white brasses", which are alloys of copper and zinc with the addition of manganese and/or aluminium, which have progressively replaced the previous, such as for instance the alloys disclosed in
Although manganese may have a whitening effect on copper similar to that of nickel, the only known white alloys which do not contain nickel and are not brasses, are the so-called "manganins", used for manufacturing electrical resistors due to the low conductivity manganese imparts alloyed copper, already from a few percentage points.
It is an object of the present invention to therefore allow the replacement of stainless steel for producing articles intended to be employed in the health and/or food field or for which the intrinsic antibacterial properties are in any case important, with a material that has an appearance similar to that of stainless steel, has a low tendency to get dirty, may easily be cleaned with common cleaning products and, especially, has intrinsic antibacterial features, so as not to require frequent disinfection operations which are required instead by stainless steel when used in health and/or food applications.
It is also an object of the invention to provide a method for producing articles, in particular intended to be employed in the health and/or food field, which are naturally, i.e. intrinsically, provided with antibacterial abilities, so as not to require frequent disinfections of the exposed surfaces.
According to the present invention, the use is therefore provided of a copper-based white alloy for producing articles having antibacterial properties, preferably intended to be employed in the health and/or food field, in which the desired white colour is imparted by the addition of manganese in the alloy, according to claim 1.
In particular, the manganese, and possible other alloying elements, except for impurities, are present in an amount such as to determine machinability, colour and surface properties of the alloy similar to those of stainless steel and simultaneously intrinsic antibacterial properties of the articles on respective exposed surfaces thereof.
According to the invention, the employed alloy does not contain aluminium, except as an impurity and preferably, neither zinc nor other alloying elements, but exclusively manganese and copper. Tin, in case it is required, may also be contained as further alloying element, but again in very low percentages, preferably below 2% so as not to produce in use the segregation of precipitates or low-melting phases which impart fragility to the alloy.
In the preferred embodiment, said alloy exclusively contains, except for impurities, copper and manganese, copper being prevailing.
The content of manganese ranges, in any case, in varying percentages between 15% and up to 40% by weight and preferably in an amount such as to obtain a crystalline structure of the alloy, exclusively in alpha phase, at ambient temperature. The optimal nominal composition of the alloy according to the use of the invention is 80% copper and 20% manganese.
The composition of the alloy according to the use of the invention is in any case such as to display on the surface antibacterial properties directed at least against microorganisms selected from the group consisting of: E.coli, MSRA, Listeria monocytogenes.
The invention also relates to a method for producing articles, in particular intended to be employed in the health and/or food field, provided with intrinsic antibacterial properties, according to claim 9.
In particular, the method of the invention comprises the steps of:
According to the preferred embodiment, the alloy contains manganese in a varying weight percentage between 15% and 40% and does not contain aluminium, and preferably neither zinc, except as impurities.
Furthermore, the first and second predetermined percentages of sectional reduction are on the order of 90% and the step of complete re-crystallization annealing of the cold semifinished article is performed at a temperature ranging between 550°C and 750°C and for about 4 hours, if performed in a static furnace or for a total time of about 45 minutes, if performed in a tunnel type furnace.
A step of abrasive brushing is performed so as to remove a surface layer of the cold annealed semifinished article having a thickness of about 4 microns.
Thereby, complex articles such as sinks, basins, tanks, working surfaces of tables, furniture tops, plates, bathroom details, as well as tubes, bars or wires, for instance to produce handrails, handles, knobs, sieves, may be obtained according to the invention and substantially without virtually any scraps, the articles having the appearance of stainless steel and being provided at the same time with very good natural or intrinsic antibacterial properties, which substantially make common sterilisation operations with disinfectants useless or at least redundant. Similar stainless steel articles are usually subjected to these disinfection operations, by the way with results which are always less effective as many bacterial strains have developed a considerable resistance to commonly used disinfectants and to antibiotics and have therefore become a continuous danger, in particular in a hospital environment, such as MRSA or E- coli O157:H7.
Furthermore, according to the results of the experiments performed by the Applicant, the antibacterial properties of copper alloys experimentally tested on many different bacterial strains, are probably due to the surface migration, and therefore on the exposed surfaces of articles made with these alloys, of copper atoms, a migration that is interrupted by the presence of aluminium in the alloy.
Therefore, according to the invention, the presence of aluminium in the alloy, unless the latter is present only as an impurity (the presence of impurities is in any case inevitable) must be avoided.
A preferred embodiment of the invention will now be disclosed by way of mere non-limitative example with reference to the figures of the attached drawings, wherein:
Further features of the invention will be apparent from the following examples for carrying out the invention.
Two 900 kg plates were prepared by casting, the plates having dimensions 1800 x 500 x 115 mm (length x width x thickness) and each having the mean composition shown in Table 1.
The plate was subjected to a hot rolling process bringing it to 740°C for 3.5 hours and was subsequently rolled thus obtaining in subsequent steps a reduction in the section of about 90%; the thickness was in particular reduced from 115 mm to 10 mm in 8 steps, thus reaching a final temperature of about 660-670°; the hot-rolled product was cooled down to about 350°C before forcedly cooling down to ambient temperature with water.
The rolled product was then examined and analysed to detect the depth and nature of the surface layers oxidised and depleted of alloying elements (manganese).
The results which were obtained are shown in tables 2 and 3, which follow.
Mn
Cu
Mn
Cu
Mn
Cu
Mn
Cu
As may be appreciated, the depth of the detected layers of oxidation and depletion of Mn were relatively high and discontinuous, but such that said layers could easily and totally be removed by the normal milling/scalping operations which are generally performed after hot-rolling.
Subsequently, the hot-rolled product, after the milling/scalping operation and subsequent removal of the oxidised layers and with depletion of manganese, was subjected to a cold-rolling operation, again obtaining a reduction in the section of about 90% (from 10 to 0.7 mm of thickness) in subsequent steps, with the consequent work hardening and increase of hardness. The cold-rolled product was then subjected to a heat annealing treatment in a static furnace in a reducing atmosphere at 600°C for 4 hours for the re-crystallization. This treatment has been selected on the basis of a series of laboratory tests at different temperatures, in order to identify the optimal treatment, as shown in table 4.
Test specimens of the rolled material were subjected to the common tests for identifying the mechanical properties, in comparison to reference materials (other copper alloys and stainless steel). The results which were obtained are shown in
(GPa)
(MPa)
(MPa)
(%)
(mm)
(%)
(%)
Finally, whether there was a surface depletion of manganese due to the heat annealing treatment was identified on the different sample specimens. The results which were obtained are shown in table 8.
As may be noted, the depletion involves a layer which is no more than 4-5 microns, which may easily be removed with a common operation of abrasive brushing after the heat treatment.
From the comparison among the various materials, it may also be noted that the tested CuMn20 alloy is suitable to replace the stainless steel in any application, such as the manufacture of furniture, tools, work surfaces, etc., having a considerable hardness and high mechanical resistance, well beyond those of brasses.
The antimicrobial properties of three different copper alloys were compared, and specifically the CuMn20 alloy (alloy 3) according to the invention, previously prepared according to the previous examples, a CuSn6Zn6 alloy (alloy 2) and a CuZn10 alloy (alloy 1), with two different reference materials and specifically DHP copper AISI 304 steel (stainless steel).
Three different pathogen microorganisms were used for the tests, specifically E. coli, MSRA and Listeria monocytogenes.
Flat sheet sample specimens, made of the tested alloys and of the reference materials, each of a size equivalent to about 5.5 cm2 were treated with 10 microlitres of microbial cell suspension in water, having a concentration of about 108 cell units per millilitre.
Subsequently, the sample specimens were incubated at 37°C. The test was repeated 6 times, varying the time of contact in incubation. Each sample specimen was tested for the following times: 0, 5, 10, 20, 40 and 80 minutes.
After each incubation step was finished, the sample specimen was recovered and treated with an aqueous solution to remove the pathogens; the solution was then diluted and incorporated in agar and a microscopical count was finally carried out on the survived microbial cells.
The results which were obtained are shown in
As may be noted, the antimicrobial activity of the previously prepared and machined alloy, CuMn20 is substantially very similar to that of copper DHP against all of the tested pathogenic microorganisms, while stainless steel is substantially devoid thereof. The antimicrobial activity of the CuMn20 alloy is also comparable (or better), with the content of copper being the same, with respect to that of the other tested alloys, which however have a mechanical resistance and hardness that make them unsuitable to replace steel and, especially, do not have a colour comparable to that of steel, while the CuMn20 alloy according to the invention has a white metal colour comparable to that of steel.
Four cylindrical ingots were prepared having a diameter of 305 mm using standard copper ingot moulds with an inner graphite insert so as to render less drastic the cooling of the metal in the ingot mould and therefore avoid the formation of contraction cracks. The metal was taken to a temperature of 1140°C in the melting furnace and, after the analytical controls to verify the adherence to the nominal composition, was poured in the holding furnace; the casting started when the metal reached the temperature of 1100 °C.
The chemical composition resulting from the mean of the two samples of cast withdrawn respectively at the beginning and at the end of each casting is shown in table 9.
Subsequently, the double-hole extrusion of the ingots was performed so as to obtain wires with a diameter of 26 mm, with a 4000 ton press in a range of extrusion temperatures from 770 to 800 °C, spraying the outputted wires with water coming from appropriate sprayers to limit oxidation of the alloy and/or the depletion of Mn. In these conditions the power absorbed by the press, expressed in tons, varies from 2000 to 2300 due to the high mechanical features of the Cu80Mn20 alloy. From the metallographic point of view, the best results both as regards the mean diameter of the grain and as regards the homogeneity thereof, were obtained with an extrusion temperature equivalent to 780°C. Later, the extruded products were rolled to a diameter of 20 mm and an annealing was performed for 3 hours at 650°C in a reducing atmosphere. Different cold plastic deformation (drawing) operations were then performed from a semifinished product having a 20mm-diameter to a wire having a 2.08mm-diameter, performing an intermediate step of milling from 5.70 to 5.30 mm to remove the layers of oxidation and depletion of Mn, the depth of which was less than that identified for the corresponding layers of example 1 for the rolled products. Finally, after a complete re-crystallization annealing performed at 650°C for 3 hours in a reducing atmosphere, the wires were subjected to a final step of cold drawing to a 2mm- or 1.80mm-diameter, so as to obtain a work hardening of 7.4% or, respectively, of 25% and therefore give rise, with the same alloy, to a final wire material in a medium-hard or hard physical state. As a final step, the possible brushing ensured the absence of surface layers of oxide or of layers depleted of Mn.
The values of the mechanical features which may be obtained by means of the drawing operations, depending on the different possible strain hardening, are shown in table 10.
Billets of Cu80Mn20 alloy were extruded under water to tubes having a 100mm-diameter, 11mm-thickness wall, so as to limit the formation of surface oxides and/or layers depleted of Mn.
The extrusion of the billet in the form of a tube was performed in a range of extrusion temperatures in the range between 790 and 805°C. In these conditions, the power absorbed by the press, expressed in bars, was about 290, which is an acceptable value considering the high mechanical features of the Cu80Mn20 alloy.
Subsequently, a cold plastic deformation processing was performed using a pilgrim mill set at 70 hits/minute with a feed of 11.11 mm until tubes having a 45mm-diameter by a 2.25-wall thickness were obtained and later the semifinished products obtained thereby were subjected to a single operation of rectilinear cold drawing to a 35mm-diameter by 1.90 mm-thickness and a following annealing at 650°C for 4 hours in a reducing atmosphere. Subsequently, a series of coil drawing operations (to the spinners) were performed up to a 9.52mm-diameter by a 0.45mm-thickness as well as a final annealing at 650°C for 4 hours in a funnel type furnace with a reducing atmosphere. The possible final brushing ensured the absence of surface layers of oxide and/or of layers depleted of Mn.
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.