METHOD FOR PREPARING SURFACE OXIDISED CARBON BLACK NANOPARTICLES AND DISPERSIONS COMPRISING THEM

Abstract

A method for the surface oxidation of carbon black comprises continuously mixing a solution or slurry of the carbon black in an organic or other solvent with a liquid carrier containing a reagent for surface oxidation of the carbon black in a counter current mixing reactor whereby to obtain reaction of the carbon black with the reagent and the formation of a surface oxidised carbon black as a dispersion of nanoparticles in the liquid carrier and solvent mixture.

Claims

1. A method for the surface oxidation of carbon black, the method comprising continuously mixing: a solution or slurry of the carbon black in an organic or other solvent with a liquid carrier containing a reagent for surface oxidation of the carbon black in a counter current mixing reactor whereby to obtain reaction of the carbon black with the reagent and the formation of a surface oxidised carbon black as a dispersion of nanoparticles in the liquid carrier and solvent mixture.

2. A method according to claim 1, further comprising removing unreacted reagent and/or by-product from the dispersion when present.

3. A method according to claim 1, further comprising concentrating the dispersion.

4. A method according to claim 1, wherein the method provides a dispersion of nanoparticles of surface oxidised carbon black having mean diameter between 1 nm and 500 nm.

5. A method according to claim 1, wherein the method provides a nanoparticle dispersion of surface oxidised carbon black having unimodal polydispersity.

6. A method according to claim 1, wherein the method provides a nanoparticle dispersion of surface oxidised carbon black having a dynamic light scattering (DLS) polydispersity index between 0.1 and 3.0.

7. A method according to claim 1, wherein the method provides a nanoparticle dispersion of a surface oxidised carbon black wherein the nanoparticles have a volatile content higher than that of nanoparticles of a surface oxidised carbon black produced by a corresponding batch oxidation of the carbon black.

8. A method according to claim 1, wherein the method provides a nanoparticle dispersion of surface oxidised carbon black having a concentration of nanoparticles between 5 wt/wt % and 50 wt/wt %.

9. A method according to claim 1, wherein the method provides a nanoparticle dispersion of surface oxidised carbon black having a zeta potential of magnitude less than about 63.9 mV in deionised water.

10. A method according to claim 1, wherein the method provides a nanoparticle dispersion of surface oxidised carbon black substantially free from a wetting agent and/or dispersant.

11. A method according to claim 1, wherein the method is carried at a reactor temperature between 20° C. and 120° C.

12. A method according to claim 1, wherein the method is carried at a pressure of between 1 bar (0.1 MPa) and 250 bar (25 MPa).

13. A method according to claim 1, wherein the residence time of the carbon black in the counter current mixing reactor is between 5 seconds and 60 minutes.

14. A method according to claim 1, wherein the ratio of flow rates of the liquid carrier to the solution or slurry of carbon black is between 1:10 and 10:1.

15. A method according to claim 1, wherein the reagent comprises basic hypochlorite and each of the organic or other solvent and the liquid carrier comprise water.

16. A dispersion of nanoparticles of a surface oxidised carbon black in a liquid carrier, obtainable or obtained by the method of claim 1.

17. A dispersion of nanoparticles of a surface oxidised carbon black having mean diameter between 1 nm and 500 nm wherein the dispersion has unimodal polydispersity and dynamic light scattering (DLS) polydispersity index between 0.1 and 3.0.

18. A dispersion according to claim 16, having a zeta potential of magnitude less than about 63.9 mV.

19. A dispersion according to claim 16, which is substantially free from wetting agent and/or dispersant.

20. An ink concentrate for digital inkjet printing, comprising the dispersion of claim 16.

21. Nanoparticles of a surface oxidised carbon black obtainable or obtained by the method of claim 1.

22. Nanoparticles of a surfaced oxidised carbon black having mean diameter between 1 nm and 500 nm and capable of providing a dispersion having unimodal polydispersity and a polydispersity index between 0.1 and 3.0.

23. Nanoparticles according to claim 21, capable of providing a dispersion in deionised water having a zeta potential of magnitude less than about 63.9 mV.

24. Nanoparticles according to claim 22, capable of providing a dispersion in deionised water having a zeta potential of magnitude less than about 63.9 mV.

25. A dispersion according to claim 17, having a zeta potential of magnitude less than about 63.9 mV.

Description

[0095] The present invention will now be described in more detail with reference to the following non-limiting Example and the accompanying drawings in which:

[0096] FIG. 1 is a schematic illustration of a counter current reactor, described in International Patent Application WO 2005/077505 A2, which is suitable for carrying out the method of the present invention;

[0097] FIG. 2 is a schematic illustration of surface oxidation of carbon black in accordance with one embodiment of the present invention;

[0098] FIG. 3 is a graph showing the FT-IR absorbance spectra of surface oxidised carbon black in accordance with several embodiments of the present invention;

[0099] FIG. 4 shows graphs showing FT-IR absorbance spectra of a surface oxidised carbon black in accordance with one embodiment of the present invention as compared with the FT-IR absorbance spectrum of a surface oxidised carbon black manufactured by a corresponding batch process (SDP-100);

[0100] FIG. 5 shows photographs highlighting the stability of dispersions of a surface oxidised carbon black obtained in accordance with the present invention as compared to the stability of commercially available carbon black (NIPex® 160 IQ);

[0101] FIG. 6 shows graphs obtained by dynamic light scattering showing nanoparticle size distributions of a dispersion of surface oxidised carbon black obtained in accordance with the present invention and a dispersion of a surface oxidised carbon black manufactured by a corresponding batch process (SDP-100);

[0102] FIG. 7 shows graphs obtained by single particle size analysis showing nanoparticle size distributions of a dispersion of surface oxidised carbon black obtained in accordance with the present invention and a dispersion of a surface oxidised carbon black manufactured by a corresponding batch process (SDP-100);

[0103] FIG. 8 are graphs showing the zeta potential of the dispersion of a surface oxidised carbon black obtained in accordance with the present invention and the zeta potential of a dispersion of a surface oxidation carbon black obtained by a corresponding batch process (SDP-100); and

[0104] FIG. 9 are graphs showing thermal desorption of nanoparticles of surface oxidised carbon blacks obtained in accordance with the present invention and nanoparticles of a surface oxidised carbon black manufactured by a corresponding batch process (SDP-100).

[0105] Referring now to FIG. 1, a counter current mixing reactor, generally designated 10, comprises a first inlet 11 and an outlet 12 in which a second inlet 13 is diametrically opposed to the first inlet 11 and disposed in the first inlet 11. The first inlet 11 and the second inlet 13 are co-axial with one another and the second inlet 12 provides a nozzle 14 in the shape of a conical funnel 15.

[0106] Note that the reactor is associated with a pumping system including respective pumps providing an up-flow to the first inlet 11 and a down-flow to the second inlet 13 (not shown).

EXAMPLES

Surface Oxidation of Carbon Black

[0107] A study of the surface oxidation of a commercially available carbon black (NIPex® 160 IQ from Grolman Ltd, UK) in the counter mixing reactor described in international patent application WO 2005/077505 A2 was undertaken.

[0108] Note that, for this study, the material of the pipe providing the first inlet 11 and the outlet 12 and the material of the pipe providing the second inlet 13 and the nozzle 14 is polytetrafluoroethylene (PTFE).

[0109] The study commenced with a series of small scale batch tests to determine the most appropriate oxidising agent and suitable amounts of carbon black and oxidising agent enabling temperature control.

[0110] In these tests, a known quantity of carbon black (1 g, 2 g or 3 g) was vigorously stirred with an excess of the oxidising agent for a period of about 8 hours. After quenching of the reaction (by dilution), the surface oxidised carbon black was collected by centrifugation, washed with water and oven dried to a powder.

[0111] The Fourier Transform Infra-Red (FT-IR) absorbance spectrum for each powder was examined—and compared with the absorbance spectrum of a commercially available surface oxidised carbon black (SensiJet Black; SDP-100; Sensient Colours (UK) Ltd) manufactured by a batch process.

[0112] A Bruker® Tensor 27 FT-IR spectrometer having an attenuated total reflectance (ATR) attachment with a diamond/ZnSe crystal plate was used.

[0113] Although the quality of the spectrum in each case was poor (because of the similar refractive index of carbon black to the ATR diamond), the FR-IR spectrum of SDP100 exhibited clear differences over the spectrum of carbon black—most obviously in the presence of a broad peak centred around 1555 cm.sup.−1.

[0114] Oxygen labelling studies (reported in the literature) suggest that this broad peak results from overlap of carbonyl stretching modes with aromatic ring breathing modes. The breathing modes are enhanced by conjugation to the carbonyl group and the stretching frequencies of the carbonyl group is red-shifted (to about 1680 cm.sup.−1) by conjugation to carbon black.

[0115] The intensity of the broad peak is an indication of the degree of surface oxidation of carbon black—although care must be taken given the poor data quality. But a comparison of intensity in this peak between the different spectra can be made with reference to a carbonyl stretching band at around 1725 cm.sup.−1 which remains relatively unchanged by the oxidation.

[0116] Note, however, that an accurate comparison must take some account of the sloping base line—it being observed that different baseline corrections have a marked effect on the size and position of the broad peak.

[0117] The FT-IR spectra revealed that a 5% or 10% v/v aqueous solution of bleach containing 1 mass equivalent (for carbon black) of sodium hydroxide was most efficient for surface oxidation of carbon black as compared to 5% or 10% v/v aqueous solution of bleach alone, an aqueous solution 30% v/v hydrogen peroxide, an aqueous solution of 5% v/v sulphuric acid and (NH.sub.4).sub.2S.sub.2O.sub.8 and an aqueous solution of 5% v/v nitric acid.

[0118] The temperature during reaction of a 10% v/v aqueous solution of bleach containing 1 mass equivalent of sodium hydroxide with known quantities of carbon black (1 g, 2 g, 3 g) was determined (over about 10 minutes) by a thermocouple and temperature logging system to be within appropriate safety limits.

[0119] The mass equivalent of sodium hydroxide in the 10% v/v aqueous bleach solution was varied (between 1 and 5 times as compared to the mass of carbon black) during further tests—but tended to result in the appearance of a broad absorption peak at around 1400 cm.sup.−1 rather than the broad absorption peak at 1550 cm.sup.−1 (suggesting other, alcohol oxidation products).

[0120] The initial flow test concentration parameters were taken to be about 1 g of carbon black and 1 g of sodium hydroxide per 10 ml of water and 10% v/v aqueous bleach solution.

[0121] In these flow tests, an aqueous bleach solution containing sodium hydroxide was pumped in a downflow to the second inlet 12 and the slurry of carbon black in water was pumped in an up-flow to the first inlet 11. The relative flow rate varied between 1:1 and 1:2 (carbon black slurry to aqueous bleach solution). Table 1 outlines the concentration parameters for the initial flow tests using the counter current mixing reactor.

[0122] In each case, the reaction was quenched (by dilution) immediately the flow exited the apparatus. The product was collected by centrifugation, washed with water and oven dried to a powder.

TABLE-US-00001 TABLE 1 Sample No. Reagent Slurry Se-013A 5% v/v aqueous bleach solution 25 g/L C black containing 25 g/L NaOH in water Se-013C 10% v/v aqueous bleach solution 50 g/L C black containing 50 g/L NaOH in water Se-013D 10% v/v aqueous bleach solution 50 g/L C black containing 25 g/L NaOH in water

[0123] The FT-IR absorption spectra of the collected surface oxidised carbon black are compared in FIG. 3. As may be seen, the surface oxidation of carbon black appears to be substantially less when the concentration for the aqueous bleach solution is 5% v/v as compared to the surface oxidation when the concentration of aqueous bleach solutions is 10% v/v.

[0124] The ratio of flow rates of the carbon black slurry and the aqueous bleach solution containing sodium hydroxide were varied between 1 and 6—but appeared to have no appreciable effect on the peak height ratios shown in FIG. 3.

[0125] Further, the pre-heating of the aqueous bleach solution containing sodium hydroxide to 40° C. appeared to have no appreciable effect on the peak height ratios shown in FIG. 3 (even when the flow rate of the aqueous bleach solution containing sodium hydroxide was twice that of the aqueous slurry of carbon black).

[0126] The lesser degree of surface oxidation in the flow tests is consistent with a shorter reaction time in the flow tests as compared to the batch tests.

[0127] The reactor was modified to extend the residence time (by adjusting the up-flow and down-flow pressures) to several minutes and at different temperatures. Table 2 sets outlines the different temperatures.

[0128] The FT-IR spectra for Se-016 B to D were similar but displayed a ratio of intensity of peak at 1725 cm.sup.−1 over peak at 1555 cm.sup.−1 less than that of SDP-100 (viz., less than 1:2). The FT-IR spectrum for SE-016A showed no peak at 1723 cm.sup.−1—suggesting some reaction other than the desired oxidation.

TABLE-US-00002 TABLE 2 Sample No. Temperature/° C. Se-016A 30 Se-016B 40 Se-016C 50 Se-016D 60

[0129] Referring now to FIG. 4, the fitting of an absorption spectrum obtained for Se-016D to a linear baseline leads to a carboxylate band absorption intensity that is substantially similar to that found for the surface oxidised carbon black (SDP-100) obtained by conventional oxidation of carbon black.

[0130] However, the fitting is not so good in the other areas of the spectrum. In these areas, the fitting to an exponential baseline (log) leads to better agreement but substantially reduces the intensity of the carboxylate band absorption.

[0131] The dispersions for Se-016B to Se-016D displayed a surprising stability as compared to a dispersion of SFP-100 in water—requiring extensive centrifugation (4500 rpm for 3 hours) to obtain a pellet sufficient for FR-IR analysis.

[0132] Referring now to FIG. 5, there are shown photographs in which the dispersion of a sample of the surface oxidised carbon black (Se-016D, designated B) in water is compared with a dispersion of the untreated carbon black (NIPex® 160 IQ, designated A) in water.

[0133] As may be seen, the dispersion of the surface oxidised carbon black (B) is relatively stable as compared to the dispersion of the untreated carbon black (A).

Characterisation of Surface Oxidised Carbon Black

[0134] Samples of the dispersion Se-016D obtained by the flow tests were examined, after concentrating and decanting any sediment, by Dynamic Light Scattering (DLS).

[0135] The samples were obtained by rotary evaporation of the crude dispersion at room temperature followed by rotatory evaporation at 45° until a concentrate having a surface oxidised carbon black loading of about 10 to 15 wt/wt % was obtained. The concentrates were prepared for examination by dilution of 1 mL of the supernatant fluid in 20 mL of deionised water.

[0136] The samples were analysed at 25° C. in a 10 mm cuvette using a Malvern Instruments Nano ZS particle sizer fitted with a back-scattering detector at 173° with an incident laser source (He—Ne laser with wavelength 632.8 nm).

[0137] A CONTIN algorithm was used to deconvolute the scattered light signal and give a size distribution. The analysis assumed a continuous phase of pure water (viscosity=0.8872 cP; refractive index=1.330) for the measurement settings. The Z-average size of the nanoparticles was taken from the raw cumulants data fit from the DLS instrument.

[0138] FIG. 6 shows graphs in which the nanoparticle size distribution of the samples Se-016D are compared with the nanoparticle size distribution of samples of SDP-100 in deionised water.

[0139] As may be seen, the Z-average particle size of the nanoparticles of the Se-016D and SDP-100 samples are respectively about 152.5 nm and about 128.1 nm. The DLS polydispersity index of the samples Se-016D and SDP-100 were determined to be 0.250 and 0.146 respectively.

[0140] FIG. 7 shows graphs obtained by single particle optical analysis (SPOS) of a sample of the dispersion obtained from the flow test Se-016D and a sample of a dispersion of SDP-100 in deionised water. The analyses were undertaken using an Accusizer® 780 autodilution particle sizing instrument (from Particle Sizing Systems, USA) with particle counter and LE-400-05 SUM sensor.

[0141] As may be seen, the graphs show that particles having diameter over 1000 nm are not present in either dispersion—and that the minor peak in the DLS spectrum of the Se-016D sample is an artefact. The dispersion Se-016D is, therefore, shown to be unimodal.

[0142] A study of the zeta potential of the respective dispersions was also undertaken using a Malvern Instruments Nano ZS particle sizer fitted with zeta potential cell.

[0143] The zeta potential is a measure of the surface charge of the nanoparticles in solution. The electric potential at the boundary between the nanoparticle and a thin layer of ions of opposite charge is measured.

[0144] FIG. 8 shows that the zeta potential in deionised water of the surface oxidised carbon black Se-016D is—49.0 mV and that the zeta potential of SDP-100 in deionised water is—69.3 mV.

[0145] These zeta potential measurements suggest that the surface oxidised carbon black obtained by the continuous process has a degree of surface oxidation which is 76.7% of that of the surface oxidised carbon black obtained by the batch process.

[0146] FIG. 9 shows of percentage weight loss as a function of temperature and the rate of weight loss at a given temperature obtained by a conventional thermogravimetric analysis of a sample Se-022A prepared as described at ambient pressure with reactor temperature of 60° C. and residence time of 4 minutes above as compared to NIPex® 160 IQ and SDP-100.

[0147] The thermogravimetric analysis was performed using a temperature programmed desorption in an inert atmosphere of nitrogen at heating rate 3° C. per minute to about 900° C.)

[0148] The weight loss up to 100° C. can be ignored in that it is largely due surface absorbed water. Differences between the surface oxidised carbon blacks Se-022A and SDP-100 are thought to result from the drying process rather than the oxidation process.

[0149] Both the surface oxidised carbon blacks show weight loss at around 300° C.—which is attributed in the literature to the decomposition of carboxylate functional groups. However, the starting temperature and temperature of peak weight loss for Se-022A (285° C. and 305° C. respectively) is different to that for SDP-100 (200° C. and 250° C.) as well as the overall weight loss here (2% and 5% respectively) suggests that a much greater proportion of carboxylic acid functional groups are present on the surface of SDP-100 as compared to Se-022A.

[0150] Both the surface oxidised carbon blacks show weight loss beginning at 580° C. and continuing until 700° C.—which is attributed in the literature to decomposition of anhydride and lactone functional groups. The weight loss of Se-022A (10%) over the range is greater than that of SDP-100 (7%)—suggesting that more anhydride and lactone functional groups are present on the surface of Se-022A as compared to SDP-100.

[0151] Only the surface oxidised carbon black Se-022A shows a weight loss beginning at 700° C. and continuing until 860° C.—which is attributed in the literature to decomposition of phenol, ether and carbonyl functional groups. The Se-022A loses another 14% of its starting weight. The absence of phenols, ethers and carbonyls in SDP-100 suggests a more complete oxidation as compared to Se-022A especially as phenol, ether and carbonyl species are observed as intermediate species for carboxylate in the oxidation of aromatic organic compounds.

[0152] Performance of Surface Oxidised Carbon Black

[0153] A sample Se-018 of a surface oxidised carbon black was prepared as described above for Se-016D except that the residence time was extended to about an hour. The sample was washed by repeated concentrated by ultrafiltration to ⅕.sup.th of starting volume and diluted to its original volume with deionised water. The washed sample was concentrated to ⅕.sup.th of its starting volume to give a dispersion containing 5 wt/wt % solids content (lower than SDP-100).

[0154] A draw-down test was performed on office printer paper to assess the colour of the dispersion as compared to similar dispersions (5 wt/wt % in deionised water) of SDP-100 and NIPex® 160 IQ. A 0.1 ml droplet of each dispersion was placed on the paper using a syringe and drawn down with the flat edge of a metal spatula.

[0155] The dispersion of NIPex® 160 IQ showed hardly any adherence to the paper (light grey track) whereas the dispersion of SDP-100 adhered better (dark track) and the Se-018 dispersion adhered best (darkest track).

[0156] These studies clearly show a continuous process for the surface oxidation of carbon black providing nanoparticle dispersions.

[0157] The surface oxidised carbon black obtained by the continuous process may be different from that obtained by the corresponding batch process and may exhibit higher stability and printing performance.

[0158] Accordingly, the present invention provides a single, continuous process for the preparation of stable dispersions of a surface oxidised carbon black with nanoparticle size suitable for ink jet printing.

[0159] The continuous process is easier to control as compared to the batch process and provides highly stable dispersions (for example, in water) without complex purification techniques or subsequent treatments.

[0160] The dispersions can be used directly in inkjet compositions or even in cosmetic compositions (requiring a high purity content).

[0161] Note that references herein to nanoparticles are references to particles having a mean diameter between 1 nm and 500 nm. References to ranges “between” a first value and a second value include the first value and the second value.

[0162] Note also that references herein to a reagent for the surface oxidation of carbon black include references to more than one reagent and, in particular, to mixtures of two or more chemical compounds which combine or otherwise provide a chemical species oxidising carbon black.

[0163] Note further that, except where context demands otherwise, references herein to a corresponding batch process are references to a batch process which uses the same reagent or reagents as the present method (but not necessarily the same concentrations, temperature, pressure, time, etc).