DURABLE ANTIMICROBIAL COATING AND PREPARATION THEREOF

Abstract

A process for the preparation of kinetically stable semiconductor/polymer nanodispersion whereby the aqueous stability of the dispersed photocatalysts and polymer latex particles are ensured by the high surface charge. Consequently, the thin photocatalytically active antimicrobial coating prepared from this electrostatically stabilized aqueous dispersion by spray-coating method contains well-dispersed and evenly distributed surface photocatalyst particles immobilized by polymer and thus it provides homogeneous, transparent and mechanically stable photoreactive and antimicrobial thin film on arbitrary surfaces.

Claims

1. A surface charge stabilized aqueous dispersion suitable for the preparation of photocatalytically active antimicrobial coating comprising: a) one or more variety of semiconductor photocatalyst particles and b) and at least one variety of polymeric binder material.

2. The aqueous photocatalyst dispersion as claimed in claim 1, wherein the semiconductor photocatalyst particles are TiO.sub.2, ZnO or a combination thereof and the pH of the aqueous dispersion is higher than the point of zero charge (PZC) of the particles.

3. The aqueous photocatalyst dispersion as claimed in claim 1, wherein the primary particle size of the photocatalyst is below 500 nm

4. The aqueous photocatalyst dispersion as claimed in claim 1, wherein polymeric binder materials are anionic polyelectrolyte latex particles, linear macromolecules or a combination thereof.

5. The polymeric binder materials as claimed in claim 4, wherein anionic macromolecules comprise ˜100 nm latex particles.

6. The aqueous photocatalyst dispersion as claimed in claim 1, wherein the total concentration of semiconductor photocatalyst particles and polymer binder material is below 2.5% and the pH of the dispersion is between 8 and 12.

7. A process for producing the aqueous photocatalyst dispersion as claimed in claim 1, wherein the following steps are taken: i) dispersing 0.01-0.5% of photocatalyst particles in distilled water then adding 0.04-2% of poly(MMA-MAA) latex particles dispersion; ii) optionally stirring the dispersion obtained in step i); and, ii) setting the pH of the dispersion obtained to 8-12, preferably 8-11.

8. The aqueous photocatalyst dispersion as claimed in claim 1, wherein the dispersion is suitable for the preparation of transparent and wear resistant antimicrobial coating with evenly distributed photocatalyst particles.

9. The coating as claimed in claim 8, in the form of a thin film deposited by spray-coating the aqueous dispersion on a substrate (˜1 L/15 m.sup.2) from a distance of 15-30 cm using a spray gun at an operating pressure of 3-5 bar.

Description

III. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention may take physical form in certain parts and arrangement of parts, some embodiments of which will be described in the specification and illustrated in accompanying drawings which form a part hereof, wherein, when referring to the drawings, the inventor identifies the following components thereto,

[0017] FIG. 1 provides TiO2 (0.016%)/ZnO (0.004%)/and anionic polyacrylate (0.08%) containing aqueous dispersion at different pH values;

[0018] FIG. 2 is a graph showing zeta potential values of 0.01% aqueous TiO2 dispersion as a function of pH;

[0019] FIG. 3A are examples of TEM aggregated Degussa P25 TiO2 photocatalyst particles at pH=6;

[0020] FIG. 3B are examples of TEM aggregated Degussa P25 TiO2 particles at pH=9;

[0021] FIG. 4 is a graph showing the zeta potential values of 0.01% aqueous ZnO dispersion as a function of pH;

[0022] FIG. 5 is a graph showing the zeta potential values of 0.01% aqueous polymer latex particle dispersion as a function of pH;

[0023] FIG. 6 shows the chemical structure of poly(methyl methacrylate-co-methacrylic acid) polymer binder material and the TEM picture of the polymer latex particles;

[0024] FIG. 7 is a graph showing the effect of pH on the particle size of 0.01% aqueous TiO.sub.2 dispersion (determined by DLS method);

[0025] FIG. 8 is a graph showing the effect of pH on the particle size of 0.01% aqueous ZnO dispersion (determined by DLS method);

[0026] FIG. 9 is a graph showing the zeta potential values of 0.01% aqueous poly(methyl methacrylate-co-methacrylic acid) polymer latex dispersion as a function of pH;

[0027] FIG. 10 shows the effect pH of aqueous dispersion on the film structure and homogeneity of photoreactive composite thin film;

[0028] FIG. 11 is a schematic representation of the photoreactive composite coating prepared by spray-coating method from aqueous dispersion;

[0029] FIG. 12 is a schematic representation of the photocatalytic degradation of ethanol vapour (c0=0.35 mM) on pure TiO2 photocatalyst film and polymer containing composite layer (TiO2/polymer ratio=60/40 wt. %);

[0030] FIG. 13 is a graph showing the effect of light intensity on the surface ROS concentration produced under irradiance of the photoreactive layer. The inserted figure shows the distance dependence of light intensity (here the dashed line is guide to eyes);

[0031] FIG. 14 is a graph showing the measured weight loss values as a function of abrasion cycle applied on photoreactive layers with and without polymer binder;

[0032] FIG. 15 are examples of photoreactive layers before and after of the abrasive test;

[0033] FIG. 16 are examples of before and after wiping photoreactive coating created on glass plate;

[0034] FIG. 17 are examples of SEM and EDX for carbon and titania content of the initial and wiped photoreactive surface (60 wt % TiO2/40% polymer); and,

[0035] FIG. 18 is a schematic representation of antibacterial efficiency of photoreactive coating measured by ISO 27447:2009 standard against E. coli test bacteria under visible light illumination with statistical analysis: *p<0.05 vs. blank.

IV. DETAILED DESCRIPTION OF THE INVENTION

[0036] As mentioned before, stability of semiconductor nanoparticles in aqueous media depends on several factors, e.g. pH, ionic strength or particle surface chemistry. Thus, the effect of the pH strongly influences the zeta potential of the particles, which is also affects the stability of the suspension.

[0037] FIG. 1 represents the photos of the TiO2 (0.016%)/ZnO (0.004%)/and anionic polyacrylate (0.08%) containing aqueous dispersion at different pH values. The difference is conspicuous: while at acidic (=3.0) or closely neutral pH (=6.3) the suspensions show unstable condition with clearly visible white sediment, at alkaline pH (=9.0) homogeneous sample was obtained. The reason for this is can be found in the colloidal stability of the dispersion. Repulsive electrostatic double-layer forces are responsible for the stabilization of charged colloidal particles. Stability of nanoparticles (NPs) in suspensions depends on several factors, e.g. pH, ionic strength or particle surface chemistry (F. M. Omar et al., 2014). The effect of the pH strongly influences the zeta potential of the NPs, i.e. it affects the stability of the suspension. Thus, determining the zeta potential is a key factor in the study of colloidal stability.

[0038] The particle size and zeta potential values of the synthetized latex nanoparticles and photocatalyst particles (0.01% aqueous dispersion) were determined by dynamic light scattering (DLS) at different pH1 values with a Zetasizer Nano ZS ZEN 4003 apparatus (Malvem Ins., UK) equipped with a He—Ne laser (λ=633 nm). The measurements were performed at 25±0.1° C. Size distribution measurements by DLS were carried out in triplicate, and average values are reported. Error bars refer to the standard deviation. FIG. 2 shows the zeta potential values of 0.01% aqueous TiO.sub.2 dispersion as a function of pH. When pH<5, TiO.sub.2 surface charge is found strongly positive and exhibit a constant ζ potential value equal to +50.0±3.1 mV. By increasing the pH, the point of zero charge (PZC) is reached with a value found here equal to 6.3±0.1. By further increasing the pH, TiO2 NPs exhibit a negative surface charge which stabilizes at a y potential value of −50.0±10.6 mV for a pH ranging from 7 to 10. Thus, it has been discovered that when pH<5 and pH>8, the TiO.sub.2 NPs are found stabilized against aggregation.

[0039] To study the morphology and aggregation state of the photocatalyst nanoparticles, transmission electron microscopy (TEM) measurements were performed using a FEI Tecnai G2 20 X-TWIN microscope with a tungsten cathode operated at 200 kV. For TEM measurements, 10 μL of aqueous nanodispersion was dropped on a grid (carbon film with 200 Mesh coper grids (CF200-Cu, Electron Microscopy Sciences, USA) and dried at room temperature. The above presented results were also supported by the results shown in FIG. 3. TEM show that at pH=6 highly aggregated TiO.sub.2 particles were obtained while at pH=9 separated, 10-20 nm Degussa P25 TiO2 particles can be observed.

[0040] Similarly. the corresponding zeta-potential values of ZnO (FIG. 4) and aqueous poly(methyl methacrylate-co-methacrylic acid) polymer latex particle dispersion (FIG. 5) were also determined as a function of pH. In the case of ZnO (FIG. 4) the pH of zero point charge (PZC) of ZnO nanoparticles was 9.5±0.6, it was discovered that for pH values lower than 10.1 the surface of the ZnO nanoparticles was positively charged while above this pH negatively charged. As regarding the polymer latex particles (FIG. 5) it can be seen that until around pH=5-6 the measured zeta potential values were continuously decreased and reached the ca. −75 mV value. This is due to the deprotonation of methacrylic acid functional groups of polymer as a function of pH (FIG. 6).

[0041] The above presented surface charge and colloidal stability was also affected the particle size of the semiconductor photocatalysts and polymer dispersion. FIGS. 7 and 8. show the particle size of TiO.sub.2 and ZnO. In both cases the highest values were measured at pH=PZC, since at this point the particle size of both TiO2 and ZnO was almost 500 nm at pHP=˜6) and ˜9, respectively. This is obviously due to the pH induced neutral charge of the particles. However, it also can be seen that at alkaline pH (>9-10) the measured size values were significantly decreased until about 200 nm indicating the disaggregation of the particles. Moreover, at this pH the polymer latex particles were also shown negatively charged surface (FIG. 5) with a particle size of ˜100 nm (FIG. 9). The foregoing confirms that by the proper adjustment of the pH (>9) the stability of the aqueous dispersion can be significantly improved trough the electrostatic repulsion of the dispersed particles. This also manifested in the film properties prepared from the aqueous dispersions with different pH: if the pH was neutral (pH=7), a very inhomogeneous, stained thin film was obtained, while at alkaline pH the film was homogenous and smooth (FIG. 10).

[0042] FIG. 11. shows the schematic drawing of the photoreactive composite coating prepared by spray coating method and it consists of two main components: the semiconductor photocatalyst particles responsible for the light induced photocatalytic effect while the role of the polymer binder is the immobilization of the particles on the surfaces. The chemical structure of the polymer can be seen on FIG. 6. It was synthesized by soapless emulsion polymerizations from methyl methacrylate and methacrylic acid monomers. Due to the emulsion polymerization polymeric nanoparticles were obtained. The thin films can be prepared by spray coating method, however, it was found that the pH of the aqueous suspension is highly affected both the stability of the suspension (FIG. 1) and the homogeneity of the thin films obtained (FIG. 10).

[0043] The next important issue is how the presence of polymer in the composite layer affects the photocatalytic properties of the TiO.sub.2 and ZnO particles. FIG. 12 represents the photocatalytic degradation of ethanol vapour (c.sub.0=0.36 mM) on pure TiO.sub.2 photocatalyst film and polymer containing composite layer (TiO.sub.2/polymer ratio=60/40 wt. %). The composition of ethanol vapor was analyzed by gas chromato-graph (Shimadzu GC-14B) equipped with a thermal conductivity (TCD) and a flame ionization detector (FID). The flow rate of the gas mixture in the photoreactor system was 375 mL×min.sup.−1. The initial concentration of the ethanol was 0.36±0.018 mmol L.sup.−1 atrelative humidity of ˜70%. The light source (LightTech light source, Hungary) was fixed at 50 mm distance from the films. After injection of ethanol and water vapour, the system was left to stand 30 min for the establishment of adsorption equilibrium on the surface of films. During the measurements, the c/c0 values were determined as a function of illumination time, where c is the concentration of ethanol at time (t) and co is the initial concentration (c0=0.36±0.018 mmol L). In the case of pure TiO.sub.2 without polymer binder, 80% of the initial ethanol was photodegraded under 60 min illumination time.

[0044] Incorporation of the photocatalysts particles into the polymer matrix resulted in a ˜50% decrease in the photocatalytic reaction rates, compared to pure TiO.sub.2, since the polymer partially covered the photocatalyst particles. Next it was also studied how many radicals are formed on the photoreactive surface depending on the light intensity since considering the potential antimicrobial application of the photoreactive coating, it is also an important question.

[0045] The amount of hydroxyl radicals was measured from the reaction of luminol and hydrogen peroxide. The results were calculated from the chemiluminescence (CL) data with Sirius L Single Tube luminometer (Berthold Detection Systems, Hungary). Six milligrams of luminol was diluted in 1 mL of sodium hydroxide (0.1 M) and filled out to 20 mL with distilled water. The nanohybrid films were immersed in 40 mL of distilled water, then illuminated and shaken continuously during the experiment using a magnetic stirrer. Samples were taken after 60 min of illumination, 100 μL of the samples was added to 100 μL of luminol solution, and the intensity of the chemiluminescence was measured immediately with the luminometer (Hirakawa, T., and Nosaka, Y. (2002). Properties of O2-And OH Formed in TiO2 Aqueous Suspensions by Photocatalytic Reaction and the Influence of H2O2 and Some Ions. Langmuir 18, 3247-3254. doi:10.1021/la015685a). Based on the previously determined calibration curve (0-5 mM), the concentration of OH radicals is directly proportional to the measured RLU values as follows: CH.sub.2O.sub.2 (mM)=measured RLU value/41866, R.sup.2=0.9977. For quantitative characterization of the free radical concentration from the RLU data, the calculated equivalent concentration of H.sub.2O.sub.2 (mM) is displayed as a function of illumination time with the used light source (15 W low pressure mercury lamp (LightTech, Hungary) with characteristic emission wavelength at λ.sub.max=435 nm) at 25.0±0.5° C. The distance of the light source from the nanohybrid films was systematically changed in order to determine how the surface reactive oxygen species concentration changes with increasing distance from the light source. As it can be seen the measured light intensity is inversely proportional to the square of the distance from the source (FIG. 13 inserted graph). In parallel, the reactive oxygen species (ROS) concentration values were also measured at given distances via luminometric measurements. FIG. 13 also shows that the measured ROS concentration values (expressed as H.sub.2O.sub.2 equivalent) increases almost linearly up to ˜13 W/m.sup.2 light intensity, then a constant value (˜80 mM/m.sup.2 H.sub.2O.sub.2 equivalent) is taken. For comparison, the average solar irradiance value is about 1000 W/m.sup.2, however, —according to our measurement—the light intensity values experienced in indoor environment are also sufficient for the generation of reactive species on the photocatalytic coating material. Thus, it can be concluded that the application of polymer matrix was reduced the photocatalytic activity of the embedded TiO.sub.2 particles, however, even at relatively low light intensities, a sufficient amount of radicals is formed on the irradiated photoreactive surface. Furthermore, the mechanical stability and wear resistance of the composite layer was also significantly improved due to the presence of the polymer binder (FIGS. 14-15). To evaluate the abrasion resistance of coatings the taber abraser test is used. The abrasion tests were carried out with a 418 type manual Taber Abraser (United States). During the measurement the pure TiO.sub.2 and TiO.sub.2/polyacrylate (=60:40 wt %) photocatalyt layer with 1 mg/cm.sup.2 specific surface mass was abraded and the percentage weight loss of the tested surfaces were measured as a function of abrasion cycle.

[0046] FIGS. 14 and 15 shows the photos and the measured weight loss values as a function of abrasion cycle applied on photoreactive layers with and without polymer binder. The vulnerability of the pure TiO.sub.2 layer is clearly visible on the photo since after the abrasion test the layer was completely destroyed. According to the percentage weight-loss measurement the layer mass was decreased very sharply, especially during the first few abrasion cycles. On contrast, if we applied polymer for the facilitation of photocatalyst particles immobilization, the mass loss of the composite film was negligible and the TiO.sub.2 particles (and the polymer) were completely covered the surface even after 1000 abrasion cycles. Thus, it can be conclude that the photoreactive layer presented here shows not only obvious photocatalytic properties but its mechanical durability also enables the potential practical use of the coating.

[0047] In order to mimic the effect of practical use and the wear resistance of the photoreactive coating the composite layer was also prepare on the surface of glass plate and the coating was wiped with a cloth (FIG. 16.). After this process the surface of the polymer based composite layer was examined by scanning electron microscope (SEM, Hitachi S-4700 microscope), applying a secondary electron detector and 5 kV acceleration voltage. Energy dispersive X-ray spectra were also measured using the Röntec EDX detector at 20 keV. The results on FIG. 17 shows that at this photocatalyst content (60 wt.) in the layer both the C of the polymer (red colour) and the Ti content of the photocatalyst (green colour) expressed on the surface. This dual presence of the components at optimal composition resulted surfaces with simultaneous photocatalytic and good mechanical properties. It can be also seen that after the abrasion (wiping) the photoreactive composite layer exhibited evenly and continuous distribution of the photocatalyst particles and the polymer indicating the wear-resistance behavior.

[0048] The antibacterial tests were carried out according to EN ISO 27447:2009 standard. For the evaluation of the surviving bacteria the washing technique was used, because the counting of the survival bacteria is more accurate with this method. Before the microbiological measurements nanohybrid films on glass samples were activated by UV-irradiation for an hour (lightsource: LightTech GCL307T5U/Cell lamp λ=250 nm) to increase the surface concentration of the photocatalyst particles in the surface region of the nanohybrid films. 1×10.sup.5-5×10.sup.5 cfu/mL bacterial suspensions were spread uniformly (0.1 mL) on the surface of the nanohybrid films (2.5×2.5 cm.sup.2) and covered with the top of Petri dish during the experiment, to avoid water vapour evaporation which can modify results. During the microbiological measurements the glass samples with the nanohybrid films were illuminated with visible-light (light source: LED lamp 7W; λ=405 nm), exposure times were 0, 4 and 24 h. During the experiments the distance of the light source from the nanohybrid films was 35 cm. The light intensity on the surface of the nanohybrid films was measured with a power meter (Thorlabs GmbH, Germany). After different illumination periods the inoculated nanohybrid films were placed into anew sterile Petri-dish by sterile tweezers and the inoculums were washed out from the activated nanohybrid films with 3 mL sterile physiological saline water to regain all surviving bacteria from the uneven surface of the samples. Bacterial suspensions with survival bacteria were streaked (0.1 mL) uniformly on the Mueller-Hinton (Oxoid, Hampshire, UK) media. After the incubation time (37° C.; 24 h) the antibacterial activity was evaluated by counting colony forming units (cfu/mL) with BZG40Colony Counter (WTWGmbH, Germany). The number of colony forming units were converted to the cell number of the survival bacteria per milliliter of the original inoculums on the nanohybrid films. The result on FIG. 18. represent that the photoreactive coating have obvious antibacterial effect: after 4 and 24 h irradiation time 100% of the initial 5×10.sup.5 cfu/ml E coli bacteria were inactivated on the surface of composite layer.

EXAMPLES

Example 1: Synthesis of poly(methyl methacrylate-co-methacrylic acid) Latex Particles

[0049] The poly(MMA-MAA) latex particles (FIG. 6.) were synthesized by soapless emulsion polymerizations. In this reaction, MMA (16.5 g) and MAA (5.5 g) were polymerized in the presence of H.sub.2O (180 g) using 0.1 wt. % potassium persulfate (KPS) as heat-initiator. The polymerization was carried out at 80° C. for 2 h under a nitrogen atmosphere and continuous agitation (˜600 rpm). After the reaction proceeded for 2 h, 20-40 nm poly(MMA-MAA) latex particles (FIG. 6.) were obtained and the conversion of monomers, which was measured by the method of weight analysis, was about 98.6%. Then the poly(MMA-MAA) latex particles were purified by centrifugation and washed with deionized water. After the purification process, poly(MMA-MAA) latex particles were redispersed in deionized water in a concentration of 30%.

Example 2: Synthesis of Photoreactive Coating Material Consists of TiO.SUB.2., ZnO and Anionic Polyacrylate Binder

[0050] During the synthesis of the aqueous photoreactive coating material (FIG. 1C) suitable for the preparation of homogenous and mechanically stable composite thin films (FIG. 10B), first 0.16 g P25 TiO.sub.2 and 0.04 g ZnO was added to 996.8 mL of water. Next, 2.73 g 30% aqueous poly(MMA-MAA) latex particles dispersion (Example 1) was also added to the photocatalyst dispersion and the pH was set to >9, preferably 11 by the addition of NaOH. The obtained dispersion was sonicated and, if necessary, the pH was set again for >9, preferably 11.

Example 3: Synthesis of Photoreactive Coating Material Consists of TiO.SUB.2., and Anionic Polyacrylate Binder

[0051] 0.2 g P25 TiO.sub.2 was added to 996.8 mL of water. Next, 2.73 g 30% aqueous poly(MMA-MAA) latex particles dispersion (Example 1) was added to the photocatalyst dispersion and the pH was set to >9, preferably 11 by the addition of NaOH. The obtained dispersion was sonicated and, if necessary, the pH was set again for >9, preferably 11.

Example 4: Synthesis of Photoreactive Coating Material Consists of TiO.SUB.2., and Anionic Polyacrylate Binder

[0052] 0.2 g ZnO was added to 996.8 mL of water. Next, 2.73 g 30% aqueous poly(MMA-MAA) latex particles dispersion (Example 1) was added to the photocatalyst dispersion and the pH was set to >9, preferably 11 by the addition of NaOH. The obtained dispersion was sonicated and, if necessary, the pH was set again for >9, preferably 11.

Example 5: Synthesis of Photoreactive Coating Material Consists of TiO.SUB.2., ZnO and Anionic Polyacrylate Binder

[0053] Beside the anionic polyacrylate binder describe in Example 1 other negatively charged synthetic- or natural polyanions and their derivatives are also suitable for the preparation of photoreactive coating material such us polyacrylic acid, sodium-polyacrylate, anionic polyacrylamide, sodium-poly(styrene sulfonate), alginate, carboxymethyl cellulose, etc. During the synthesis of the aqueous photoreactive coating material suitable for the preparation of homogenous and mechanically stable composite thin films, first 0.16 g P25 TiO.sub.2 and 0.04 g ZnO was added to 996.8 mL of water. Next, negatively charged synthetic- or natural polyanions listed above was also added to the photocatalyst dispersion in an amount that the photocatalyst/polymer mass ratio will be from 0.2/0.8 to 0.8/0.2 and the pH was set to 11 by the addition of NaOH. The obtained dispersion was sonicated and, if necessary, the pH was set again for 11.

Example 6: Preparation of Photocatalyst Composite Thin Films

[0054] The hybrid layers consist of photocatalysts particles and polyacrylate binder was prepared by applying the spray-coating technique on the substrate surface. During the preparation process, the aqueous suspension obtained in Example 2-6 was evenly sprayed on the substrate surfaces (˜1 L/15 m.sup.2) from a distance of 15-30 cm using a R180 type Airbrush spray gun at an operating pressure of 3 bar.

Example 7: Preparation of Large Surface are Photocatalyst Composite Thin Films

[0055] During the surface coating process, the aqueous suspension obtained in Example 2-6 was evenly sprayed on the substrate surfaces (˜1 L/15 m.sup.2) from a distance of 15-30 cm using a Graco HVLP type air assisted spray gun at an operating pressure of 1-2 bar.