FIELD

The disclosure is directed to a chemically strengthened glass having antimicrobial properties and method of making such glass. In particular, the disclosure is directed to a colorless transparent chemically strengthened glass with antimicrobial properties and with a functional coating on the glass that does not interfere with the antimicrobial properties of the glass.

BACKGROUND

The “health properties” of metallic silver (that is, its antiseptic, antibacterial, antiviral, antimicrobial effects) was known for centuries before knowledge of bacteria, viruses and microbes generally even existed. In antiquity (Hippocrates of Cos and Galen of Pergamum, both physicians), and well through the Middle Ages, silver vessels were used in the care of those who were ill and wounded. For example, E. Bradford in “The Great Siege[Malta, 1565]” (New York, Harcourt, Brace & World Inc., 1961, pp.190-191) describes the practices of the Knights of the Order of St. John, who were hospitalers, at Malta as:“Simple though their surgery was and ignorant though they were in many ways, they did at least understand the rudiments of hygiene. In the hospital, where under normal conditions both rich and poor, Knight and commoner, were served off silver plate—to increase “the decorum of the hospital and the cleanliness of the sick”—some attempt was made, even during the siege, to look after the patients properly.” [Emphasis added. Internal quotation from Francisco Balbi de Correggio, “La verdadera relación de todo lo que el ano de MDLXV ha succedido en la Isla de Malta,' (Barcelona, 1568)].

It was later discovered that the actual bactericide was not the metallic silver, but the silver ions on the surface of the bulk metallic silver. Thus, the beneficial effect of using silver vessels may be attributable to the migration of silver ions from the bulk silver to the food and drink that was consumed. With the knowledge that silver ions were are actual “antiseptic,” solutions of certain silver salts (e.g. silver nitrate) that are readily soluble in water were used as an antiseptic and bactericide for many decades. Until the 1980s, dilute silver nitrate eye drops were placed in the eyes of newborns to prevent neonatal conjunctivitis, which could lead to blindness in the newborn, which may occur after birth. (This practice has generally been discontinued in favor of the use of erythromycin ointment).

Recently, the use both metallic silver particles and silver salts have been described in the patent and technical literature as a means for imparting antibacterial properties to a variety of materials such as yarns, fabrics, glass and other materials. Yahoo Lv et al. (Polymers for Advanced Technologies, Vol. 19 (1008), pages 1455-1460) have shown that silver (Ag0) nano-particles coated on a glass slide have antibacterial/antiviral properties resulting from close attachment of the nano-particles with the bacterial cells. The principle activity of silver (0) particles is due to the production of silver ions within an aqueous matrix containing the bacteria. Di Nunzio et al in WO 2006/058906 describe prosthetic devices or implants having at least a surface layer or a part thereof (that is, an element of the prosthetic device or implant) made of a glass, glass-ceramic or ceramic material containing ions exchangeable with silver ions, and subjecting the device “to a silver ion containing aqueous solution ion-exchange process,” [id at the Abstract]. Di Nunzio et al., J. European Ceramic Society Vol. 24 (2004), pages 2935-2942, discuss bioactive glasses containing silver on their surface that were produced by ion-exchange from dilute silver nitrate melts. Verne et al, J. Material Science; Materials Medicine Vol. 20 (2009), pages 733-740, present test results of the surface doping of biocompatible glass that was described in WO 2006/058906. U.S. Pat. No. 7,232,777 describes yarns and fabrics having an antimicrobial silver particulate finish.

Silver ions do not possess a single mode of action. They interact with a wide range of molecular processes within microorganisms resulting in a range of effects from inhibition of growth and loss of infectivity to cell death (cytotoxicity). The mechanism depends on both the concentration of silver ions that are present and the sensitivity of the microbial species to the silver ions. Contact time, temperature, pH and the presence of free water all impact on both the rate and extent of antimicrobial activity. However, the spectrum of activity is very wide and the development of resistance relatively low, especially in clinical situations. Silver ions are known to be effective against some 650 types of bacteria.

The prevalence of “touch screens” in contemporary society gives rise to many surfaces that can harbor microbes, bacteria and viruses, and these microbes can be transferred from person to person. The present disclosure is directed to glass surfaces and to the application of the antimicrobial properties of silver ions to glasses such as the cover glasses used on many modern devices, for example, ATMs, touch screen computers, cellphones, electronic book readers, and similar devices.

SUMMARY

This disclosure is directed to a making a chemically strengthened glass having antimicrobial properties and to methods of making such glass. In one embodiment the disclosure is directed to a colorless transparent chemically strengthened glass with both antimicrobial properties and a functional coating on the glass that does not interfere with the properties of the glass. The functional coating is used to impart easy cleanability to the surface and is generally a hydrophobic coating. The task of creating a chemically strengthened glass having antimicrobial properties and having a functional coating on the glass is not straightforward due to the fact that there are numerous other required properties of the cover glass that must be maintained while trying to incorporate the antimicrobial property. It should also be understood that the interaction of the Ag-ion with the bacteria is restricted to the surface of the glass, so the pertinent characterization of the concentration needs to be expressed as per unit area. The reporting of the volume concentration at some interior point away from the surface, for example, >5 nm is of little consequence since the mobility of Ag+1at room temperature is negligible.

Another embodiment the disclosure is directed to a method of making a chemically strengthened glass having silver ions (Ag+1) on the surface of the glass. There have been proposals to utilize a one-step (1-step) method in which an ion-exchange bath containing both a silver compound and an ion-exchanging alkali metal compound whose ions are larger than the alkali metal ions in the article into which they be exchanged, and using such bath exchange both Ag ions and the larger ions into the glass in a single step to form a chemically strengthened, silver containing glass having a compressive stress. For example, in addition to the silver ions, the alkali metal ions that are exchanged into the glass are potassium ions (K+1). Prior art and our experiments show that the 1-step ion-exchanging bath consisting of potassium nitrate containing silver nitrate in an amount in the range of 0.01 wt % to 5 wt % where the ion exchange temperatures are typically at 300-500° C. with ion exchange times of >3 hours exhibits distinct disadvantages. The most significant disadvantage is that to enhance the “kill” rate of bacteria and other microbes it is necessary that there be a sufficiently a high concentration of Ag-ion on the surface which is not achieved through the use a one-step process; and further when the 1-step method is used significant color is produced as a consequence. This color makes the glass unsuitable for use in electronic devices by altering the display, for example, by making it less clear or altering the colors.

Disclosed herein is a two-step (2-step) method that avoids the disadvantages of the 1-step method. The 2-step method uses a first ion-exchange bath containing an ion-exchanging alkali metal compound whose ions are larger than the alkali metal ions in the article into which they will be exchanged (the standard strengthening method on chemical strengthening), followed by ion-exchange using a second ion-exchange bath containing silver, the second silver-containing ion-exchange bath being used for a much shorter time than the first bath. The second ion-exchange bath contains a high concentration of both silver ions and an ion-exchanging alkali metal compound whose ions are larger than the alkali metal ions in the article into which they be exchanged, the silver ion concentration in the second bath being higher than that used in the in the silver-containing bath of the 1-step method. In the first step, the larger alkali metal ions are exchanged into the glass to form a glass article having a compressive stress in the usual manner for a time greater than 3 hours at a temperature in the range of 300-500° C.; for example, by ion-exchange at 420° C. for 5-6 hours. In the second step, the higher concentration of silver ions are exchanged into the surface of the glass and replace the alkali metal ions in the glass to a shallow enough depth without significant loss of compressive stress. In one embodiment of the 2-step method, the same alkali metal compound can be used in both the first and second ion-exchange baths. In certain embodiments of the 2-step method the silver compound and alkali metal compound used in the ion-exchange bath are both comprised of nitrates. Furthermore, the potassium ions are typically ion-exchanged for sodium and lithium alkali metal ions in the glass.

Generally, the 2-step method comprises providing a glass article containing ion-exchangeable alkali metal ions, providing a first ion-exchange bath containing ion-exchanging alkali metal compound whose ions are larger than the alkali metal ions in the article, and, as a first step, ion-exchanging the alkali metal ions in the glass for the alkali metal ions in the ion-exchange bath. As a second step, the method further comprises providing a second ion-exchange bath containing silver nitrate in an amount in the range of 1-10 wt %, the remainder of the bath being the nitrate of the an alkali metal ion having ion larger than the alkali metal ions at least equal in size to the alkali metal ions of the first ion-exchange bath. In one embodiment, the duration of the second bath exposure, that is, the ion-exchange time using the second bath, is less than 30 minutes in order to produce a shallow depth of exchange of less than 20 μm. In another embodiment the ion-exchange time is greater than zero and less than or equal to 20 minutes. In an additional embodiment, the ion-exchange time is greater than zero and less than or equal to 10 minutes. In all embodiments, the ion-exchange is carried out at a temperature in the range of 370° C. to 450° C. using a selected ion exchange time as described herein. In one embodiment, the ion exchange is carried out at a temperature in the range of 400° C. to 440° C. using a selected ion exchange time as described herein. The distinct advantage of the 2-step method is that since only the surface concentration of Ag+1is important for the antimicrobial effect, having the silver penetrate to greater depths as is achieved through the 1-step method adds nothing to the antimicrobial or antibacterial effect since silver ions do not move or migrate within the glass at temperatures at which electronic devices may be used; temperatures which typically range from below 0° C. to as high as 45° C. or higher. The 2-step method thus allows one to obtain a significantly higher concentration of Ag+1ions on the surface of the glass which results in a commensurate decrease in the “kill” time (the time to reach a log 3 reduction) while not producing any undesirable yellow color and therefore achieving the desirable transmittance characteristics as is shown inFIG. 1.

FIG. 1compares the transmission in the 400-750 nm range of an antimicrobial glass ion-exchanged for 20 minutes using the 2-step method with an antimicrobial glass prepared by the 1-step method using the same ion-exchange bath for 5 hours. The glass of the 2-step method has a transmission, uncorrected for reflection losses, of greater than 88% over the entire range whereas the glass prepared by the 1-step method is less than 88% in the range of 400 mm to approximately 560 nm and equal to or greater than 88% only in the range of approximately 560 nm to 750 nm.

As mentioned above, the 2-step method, by restricting the second step to very short times, <30 minutes, one can produce a high concentration of Ag+1on the surface without imparting any color. In fact the silver ion concentration on the surface is larger for the short IOX time of the 2-step method compared to the longer IOX time (typically 4-6 hours) of the 1-step method. The volume concentration of Ag ion in the first 5 nm of the glass can be determined using XPS (x-ray photoluminescence-spectroscopy. Tables 1 and 2 show the XPS results for the Ag ion concentration in the first 5 nm of the glass.

TABLE 11-step Ion-ExchangeAg concentration,Sample NoAtomic %11.8521.5531.8641.94average1.8095% uncertainty0.28All samples ion-exchanged using the 1-step method and a 5 wt % AgNO3/95 wt % KNO3bath at 420° C. for 5.5 hours

TABLE 22-step Ion-ExchangeAg concentration,Sample NoAtomic %12.6821.9732.2243.22Average2.5295% uncertainty0.90All samples were ion-exchanged using the 2-step method and a second step 5 wt % AgNO3/95 wt % KNO3bath at 420° C. for 20 minutes

A comparison of the Tables 1 and 2 shows that the glass prepared by the 2-step method, 20 minute IOX time for the second bath, has a 40% increase in the Ag ion concentration in the first 5 nm relative to the Ag ion concentration in the glass produced by the 1-step, single bath process, 5% AgNO3/95% KNO3bath being used in both processes.

Another aspect the present disclosure is the ability to produce a glass that has an antimicrobial surface that is easily cleanable without the loss of the antimicrobial activity. A “low surface energy” coating is applied to the surface of the silver-containing, chemically strengthened antimicrobial glass to impart easy-to-clean properties to the glass surface. The low surface energy coating is a hydrophobic coating that facilitates the ease of cleanability of the surface. In one embodiment, the low surface energy coating has a thickness in the range of 0.5 nm to 20 nm. In another embodiment, the low surface energy coating is a continuous coating and has a thickness in the range of 0.5 nm to 10 nm. In a further embodiment, the film is continuous and has a thickness in the range of 0.5 nm to 5 nm. In an additional embodiment, the thickness is in the range of 1 nm to 5 m. In an additional embodiment, the film is continuous and has a thickness in the range of 1 nm to 5 nm. In another embodiment, the low surface energy coating can form a pattern or domains on the surface, for example without limitation, “islands-in-the-sea” where the silver-containing, chemically strengthened surface of the glass is the sea and the low surface energy coating forms the islands. Stated another way, the low surface energy coating is discontinuous and does not cover every portion of the glass surface, but does cover a sufficient portion of the surface such that the coating is effective for its intended antimicrobial use and for its use in facilitating ease of cleanability. Such discontinuous coating can also vary in thickness over the range of 0.5 nm to 20 nm. In one embodiment, the discontinuous coating has a thickness in the range of 0.5 nm to 10 nm. In an additional embodiment, the discontinuous coating has a thickness in the range of 1 nm to 5 nm. In a still further embodiment, the coating can be a continuous “peak-to-valley” (“PV”) coating where the coating “peak” is thicker than the coating “valley”. For example, within the range of 0.5 nm to 20 nm, the coating within the “peak” can be in the range of 15 nm to 20 nm and the coating within the “valley” areas can be, for example, in the range of 1 nm to 5 nm. The selected depth to which the Ag+1ions are deposited may vary according to the silver concentration in the IOX bath, the temperature of the bath and the time for which the ion-exchange of the silver ions into the glass was carried out.

In one embodiment, the surface of the glasses described herein having antimicrobial activity and exhibiting a surface Ag ion concentration of greater than zero to 0.05 μgAg/cm2, are not cytotoxic. In another embodiment, the non-cytotoxic glass has a surface Ag ion concentration in the range of 0.005 μgAg/cm2to 0.047 μgAg/cm2. In an addition embodiment the non-cytotoxic glass has a surface Ag ion concentration in the range of 0.005 μgAg/cm2to 0.035 μgAg/cm2. In a further embodiment, the non-cytotoxic glass exhibits a surface concentration in the range of 0.01 μg/cm2to 0.030 μg/cm2. In an additional embodiment the surface concentration of the non-cytotoxic glass is in the range of 0.015 μg/cm2to 0.03 μg/cm2.

