GB2410954A - Assessing the quality of an azo colorant prepared in a micro-reactor - Google Patents

Abstract

A method for assessing the quality of an azo colorant or of an image formed by an azo colorant on a substrate which comprises the following steps: <SL> <LI>(i) preparing an azo colorant using a micro-reactor; <LI>(ii) forming a liquid composition from the azo colorant and/or applying the azo colorant from step (i) to a substrate to form a printed image; and <LI>(iii) applying a predetermined test to either the liquid composition or the printed image from step (ii) to determine whether the composition or the printed image meets pre-selected criteria. </SL> The method can be used for assessing the quality of an ink suitable for ink jet printing e.g. by applying the ink to the substrate directly from the micro-reactor using electrospray. Also a micro-reactor process for the preparation of azo compounds wherein the azo compounds are purified by means of their electrophoretic migration is described.

Description

24 1 0954

PROCESS AND SYSTEM

This invention relates to a rapid throughput testing system for azo dyes, especially azo dyes for ink jet printing, and also to a means of purifying an azo dye in a micro reactor.

Inkjet printing is a non-impact printing technique in which droplets of ink are ejected through a fine nozzle onto a substrate without bringing the nozzle into contact with the substrate. The set of inks used in this technique typically comprise yellow, magenta, cyan and black inks.

Azo dyes and pigments are the most widely utilised class of colorants and have been used in the full range of colour applications over many years. Thus, thousands of chemicals comprising this group have been synthesized. However, colorants for inkjet printing must meet a range of demanding technical requirements. For example, there are the contradictory requirements of providing ink colorants which are soluble in the ink medium and yet do not run or smudge excessively when printed on paper. The inks need to dry quickly to avoid sheets sticking together after they have been printed, but they should not form a crust over the tiny nozzle used in the printer. Storage stability in the ink is also important to avoid particle formation that could block the tiny nozzles used in the printer. Furthermore, the resultant images desirably do not fade rapidly on exposure to light or common oxidising gases such as ozone.

While it is possible to predict to a certain extent how the chemical nature of an azo colorant will influence its properties in an ink formulation the properties of the azo colorant when inkjet printed onto a substrate are much more difficult to predict and can vary significantly with the nature of the substrate.

Thus the selection of the best azo colorants for use in inkjet printing inks requires the screening of prints of a range of compounds on different media for properties such as; chrome, hue, wet-fastness, light- fastness and ozone fastness.

The key step in the selection operation is the synthesis of the various azo compounds. Using conventional synthetic procedures azo dyes may be prepared, depending on their complexity, over the course of several hours via reactions which require cooling. Wooten et al. (Lab on a Chip, 2002, 2, 5-7) have demonstrated that simple azo compounds may be prepared rapidly and safely in a microreactor, albeit with a low conversion.

The present invention is an integrated system by which small amounts of complex azo compounds may be prepared in high conversion and isolated over the course of a few minutes at room temperature and then optionally applied to a substrate, thus enabling a rapid throughput screening.

According to the present invention there is provided a method for assessing the quality of an azo colorant liquid composition or of an image formed by an azo colorant on a substrate which comprises the following steps: .e .e e - . ...

(i) preparing an azo colorant using a micro-reactor; (ii) optionally forming a liquid composition from the azo colorant and/or applying the azo colorant from step (i) to a substrate to form a printed image; and (iii) applying a pre-determined test to either the liquid composition or the printed image from step (ii) to determine whether said liquid composition or printed image meets pre selected criteria.

Preferably the azo colorant is an azo dye more preferably an azo dye with an appreciable water solubility, preferably greater than 2.5% by weight.

The term azo colorant incorporates all forms of azo colorant including monoazo, disazo and trisazo colorants. Azo colorants may exist in tautomeric forms and these tautomers are included within the scope of the present invention. The present invention also incorporates salts of azo dyes.

The present invention also incorporates azo compound metal complexes.

Azo compounds are usually formed by the coupling of an aromatic diazonium cation with an aromatic system (usually referred to as a "coupling component") which is highly activated to electrophilic attack. The most commonly encountered electron releasing activating groups are the hydroxy and amino groups. Thus, most coupling components are phenols, napthols or aromatic amines.

Coupling of the diazonium salt to the coupling component is highly dependent on pH and the optimum pH will vary with the nature of the coupling component, for example phenols are usually coupled under mildly alkali conditions while aromatic amines are coupled under mildly acid to neutral conditions.

Aromatic diazonium salts are formed by the reaction of an aromatic amine with nitrous acid. Nitrous acid is unstable and so the diazonium salt is usually formed by reacting the aromatic amine with sodium nitrite in the presence of a strong acid, usually HCI.

The diazonium intermediate in the synthesis of azo compounds is thermally unstable. Thus, the micro-reactor is preferably manufactured from a material which allows heat to rapidly dissipate so there is no need for the reactor to be cooled, as is usual with diazonium chemistry, and the reactor may be used a room temperature.

The micro-reactor may be of any suitable design but preferably comprises a bottom plate with reaction channels etched into it and a top plate with holes which correspond to the desired position for reagent addition on the reaction channels. These holes may be filled with reagent and act as reservoirs. The top and bottom plate of the microreactor may be made from the same or from a different material and the reservoir holes may be lined with a different material Preferably the micro-reactor comprises: glass, especially borosilicate glass; a plastic, especially PDMS; or a metal, especially stainless steel.

ë e .e e . e e.. e e e e e e . ...

Preferably the micro-reactor comprises borosilicate glass.

The reagents may be driven through the reactor by physical or electrokinetic means, preferably the reagents are driven through the reactor by pressure driven flow.

In a preferred embodiment the azo colorant formed in the micro-reactor is purified by electrophoretic migration. Thus the colorant may move at a different rate to the bulk fluid or even in the opposite direction to the bulk fluid allowing ready purification of the colorant.

Electrophoretic migration may be achieved within the micro-reactor by applying a voltage to the micro-reactor. This may be either a voltage used to drive a micro-reactor under electrokinetic control or a separate voltage during or at the end of a reaction carried out under electrokinetic or non-electrical control, for example pressure driven flow.

When the colorant is purified by electrophoretic migration it is particularly preferred that the micro-reactor comprises reagent reservoirs which are coated with polydimethylsiloxane (PDMS). In such a system the uncoated glass channels can support electro-driven flow whilst the PDMS polymer coating limits unwanted pressure driven effects that would otherwise occur from hydrodynamic flow.

The direction and extent of the electrophoretic migration will vary depending on the nature of the azo colorant and will be highly dependent on the properties of the solvent system used, especially with regard to factors such as the nature of the solvents, the ionic strength and especially the pH of the system.

The direction and extent of the electrophoretic migration of the azo colorant will also be influenced by factors such as the strength and direction of the applied voltage and the design of the micro-reactor.

The azo colorant formed in the micro-reactor may be collected by any suitable means such as pumping. However it is preferred that it is collected and, optionally, at the same time applied to the substrate by means of electrospray.

The method by which the azo colorant from step (i) is applied to the substrate is highly dependent on the substrate. Preferably the azo colorant is applied as an ink either by electrospray or by means of a printer, especially an ink jet printer.

Preferably the substrate is paper, plastic, a textile, metal or glass, more preferably paper, an overhead projector slide or a textile material, especially paper.

A preferred aspect of the invention provides a method for assessing the quality of an ink suitable for ink jet printing or a printed image formed on a substrate from said ink which comprises the following steps: (a) preparing an azo colorant using a micro-reactor; (b) preparing an ink by incorporating the azo colorant from step (a) into an ink medium; (c) applying the ink from step (b) to a substrate to form a printed image; and .e e.-e . . . . e c e e ..

. . . (d) applying a pre-determined test to either the ink from step (b) or the printed image from step (c) to determine whether said ink or said printed image meets pre-selected criteria.

The azo colorant may be added to the ink medium within the micro-reactor.

Alternately steps (a) and (b) may be combined by preparing the azo colorant in a solvent system which is able to act as an ink medium.

The azo colorant is as defined and as preferred above.

The ink medium preferably comprises: (a) from 0.01 to 30 parts of an azo compound; and (b) from 70 to 99.99 parts of a liquid medium; The number of parts of component (a) is preferably from 0.1 to 20, more preferably from 0.5 to 15, especially from 1 to 5 parts. The number of parts of component (b) is preferably from 99.9 to 80, more preferably from 99.5 to 85, especially from 99 to 95 parts.

The composition may of course contain further ingredients in addition to (a) and (b).

Preferably component (a) is completely dissolved in component (b). Preferably component (a) has a solubility in component (b) at 20 C of at least 10%. This reduces the chance of component (a) precipitating if evaporation of the liquid medium occurs during storage.

Preferred liquid media include water, a mixture of water and organic solvent and organic solvent free from water.

When the liquid medium comprises a mixture of water and organic solvent, the weight ratio of water to organic solvent is preferably from 99:1 to 1:99, more preferably from 99:1 to 50:50 and especially from 95:5 to 80:20.

It is preferred that the organic solvent present in the mixture of water and organic solvent is a water-miscible organic solvent or a mixture of such solvents. Preferred water miscible organic solvents include C,alkanols, preferably methanol, ethanol, n-propanol, isopropanol, nbutanol, sec-butanol, tert-butanol, n-pentanol, cyclopentanol and cyclohexanol; linear amides, preferably dimethylformamide or dimethylacetamide; ketones and ketone-alcohols, preferably acetone, methyl ether ketone, cyclohexanone and diacetone alcohol; water-miscible ethers, preferably tetrahydrofuran and dioxane; dials, preferably diols having from 2 to 12 carbon atoms, for example ethylene glycol, propylene glycol, butylene glycol, pentylene glycol, hexylene glycol and thiodiglycol and oligo- and poly-alkyleneglycols, preferably pentane-1,5- diol, diethylene glycol, triethylene glycol, polyethylene glycol and polypropylene glycol; triols, preferably glycerol and 1,2,6-hexanetriol; mono-C,4-alkyl ethers of diols, preferably mono-C,4- alkyl ethers of diols having 2 to 12 carbon atoms, especially 2- methoxyethanol, 2-(2-methoxyethoxy)ethanol, 2-(2-ethoxyethoxy)-ethanol, 2-[2-(2-methoxyethoxy)ethoxy]ethanol, 2-[2-(2ethoxyethoxy) ethoxy]-ethanol and ethyleneglycol monoallylether; cyclic amides, preferably 2-pyrrolidone, N-methyl-2-pyrrolidone, N-ethyl-2pyrrolidone, caprolactam and 1,3-dimethylimidazolidone; cyclic esters, preferably caprolactone; sulfoxides, preferably dimethyi sulfoxide and e cee eee e e . e .. e e e e .

e e .

sulfolane. Preferably the liquid medium comprises water and 2 or more, especially from 2 to 8, water-soluble organic solvents.

Especially preferred water-soluble organic solvents are cyclic amides, especially 2-pyrrolidone, N-methyl-pyrrolidone and N-ethyl-pyrrolidone; dials, especially pentane-1,5-diol, ethyleneglycol, thiodiglycol, diethyleneglycol and triethyleneglycol; and mono- C,4-alkyl and C,4-alkyl ethers of dials, more preferably mono- C'4-alkyl ethers of dials having 2 to 12 carbon atoms, especially 2-methoxy-2-ethoxy-2-ethoxyethanol.

When the liquid medium comprises organic solvent free from water, (i.e. less than 1% water by weight) the solvent preferably has a boiling point of from 30 to 200 C, more preferably of from 40 to 1 50 C, especially from 50 to 1 25 C. The organic solvent may be water-immiscible, water-miscible or a mixture of such solvents. Preferred water-miscible organic solvents are any of the hereinbefore described water-miscible organic solvents and mixtures thereof. Preferred water-immiscible solvents include, for example, aliphatic hydrocarbons; esters, preferably ethyl acetate; chlorinated hydrocarbons, preferably CH2CI2; and ethers, preferably diethyl ether; and mixtures thereof.

When the liquid medium comprises water-immiscible organic solvent, preferably a polar solvent is included because this enhances solubility of the dye in the liquid medium.

Examples of polar solvents include C,4-alcohols. In view of the foregoing preferences it is especially preferred that where the liquid medium is organic solvent free from water it comprises a ketone (especially methyl ethyl ketone) and/or an alcohol (especially a C,4 alkanol, more especially ethanol or propanol).

The organic solvent free from water may be a single organic solvent or a mixture of two or more organic solvents. It is preferred that when the medium is organic solvent free from water it is a mixture of 2 to 5 different organic solvents. This allows a medium to be selected which gives good control over the drying characteristics and storage stability of the composition.

Liquid media comprising organic solvent free from water are particularly useful where fast drying times are required and particularly when printing onto hydrophobic and non-absorbent substrates, for example plastics, metal and glass.

Since water soluble azo dyes are preferred the liquid media in the ink is preferably comprises water or a mixture of water and organic solvent.

In step (c) the ink may be applied to a substrate either directly from the micro-reactor by for example electrospray or via an inkjet printer.

Preferred inkjet printers for use in step (c) are piezoelectric inkjet printers and thermal inkjet printers. In thermal inkjet printers, programmed pulses of heat are applied to the composition in a reservoir by means of a resistor adjacent to the orifice, thereby causing the composition to be ejected in the form of small droplets directed towards the paper during relative movement between the substrate and the orifice. In piezoelectric inkjet printers the oscillation of a small crystal causes ejection of the composition from the orifice. Alternately the ink can be ejected by an electromechanical actuator connected to a moveable paddle or ë e ëe . .e see e :.e .e:. :e plunger, for example as described in WO 00/48938 and WO 00/55089.

It is particularly preferred that in step (c) the ink is applied to a substrate directly from the micro-reactor using electrospray.

Electrospray is a method of generating a very fine liquid aerosol through electrostatic charging. Thus, droplets may be generated by electrically charging the liquid at the terminal reactor channel to a very high voltage. The charged liquid becomes unstable as it is forced to hold more and more charge and eventually reaches a critical point, at which it can hold no more electrical charge when it forms a cloud of tiny, highly charged droplets.

The substrate in step (c) is preferably paper, plastic, a textile, metal or glass more preferably paper, an overhead projector slide or a textile material, especially paper.

Preferred papers are plain, coated or treated papers which may have an acid, alkaline or neutral character.

The pre-selected criteria which the ink or printed image must meet will vary depending on the nature of the azo colorant and purpose of the test. For example when a series of analogue dyes is being evaluated the preselected criteria will be to match or better the performance of a particular analogue in a particular test. Alternately when a new azo dye is being evaluated then the pre-selected criteria will be to match or better the performance of a target commercial dye in a particular test.

Tests which may be performed in step (d) include assessing the stability of the ink on storage. This may be assessed by placing the ink in an incubator where the temperatures varies (for example between -10 C to 25 C) so that the ink alternately freezes and thaws over a 24 hour cycle for period of, typically, one week. At the end of this time the ink may be evaluated by optical microscopy and the stability judged by the absence of any fine precipitates in the ink.

The potential of the ink to cause kogatation (nozzle blockage), both before and after storage, may be assessed by loading the ink into a cartridge and firing repeatedly.

Kogatation will lead to rapid loss print quality and the cartridge will after a short time cease to fire.

Other physical/chemical properties of the ink such as, for example, the viscosity and/or surface tension may also be determined.

Tests which may be performed on the printed image in step (d) include determining the optical density and colour properties of the printed image such as the CIE colour co-ordinates of the print (a, b, L, Chroma and hue). Other properties of the printed image which may be determined include determining the wet- fastness, light-fastness and fastness to oxidising gasses, such as ozone, of the print.

The CIE colour co-ordinates of each print (a, b, L, Chroma and hue) may be measured using a Spectrodensitometer with 0 /45 measuring geometry with a spectral range of 400-700nm at 20nm spectral intervals, using illuminant C with a 2 (CIE 1931) observer angle and a density operation of status T. e sea A . ate a te.

a a a ä äa a Water-fastness of the printed image may be assessed by suspending the print at 45 and running 0.5 ml of water down a section of print, preferably comprising a number of printed lines, and then assessing the visible ink run. Low ink run correlates to high water fastness. This test may be carried out at various times post printing to assess how fast the print dries to provide a water fast image. Alternately water fastness may be assessed by a simple wet rub-fastness test where a wet finger is rubbed against the printed image Light-fastness of the printed image may be assessed by fading the printed image in an Atlas Ci35 Weatherometer for 50 hours. The optical properties of the printed image before and after light exposure may be evaluated using a densitometer. The light fastness is evaluated as %OD loss, where the lower the %OD loss the greater the ozone fastness, and as degree of fade. The degree of fade is expressed as AE where a lower figure indicates higher light fastness. AE is defined as the overall change in the CIE colour co-ordinates L, a, b of the print and is expressed by the equation AE = (AL2 + a2 + Ab) . Ozone-fastness of the printed image may be assessed using an ozone test cabinet (such as those available from Hampden Test Equipment) and exposing the printed image, for twenty four hours at 40 C and 50% relative humidity, to 1 part per million of ozone.

Fastness of the printed image to ozone may then be judged by the difference in the optical density before and after exposure to ozone where the lower the %OD loss the greater the ozone fastness.

Other tests that may be carried out on the printed image include the "highlighter smear test" where a highlighter pen (preferably yellow) is used to draw a horizontal line across a series of printed vertical bars. The amount of ink smear between the vertical bars is then assessed visually against controls.

A preferred embodiment of the invention comprises synthesising a water soluble azo dye in an ink-medium in a micro-reactor, optionally purifying the azo dye by means of its electrophoretic migration, and applying the resultant ink directly from the micro-reactor to a substrate using electrospray. In this preferred embodiment the dye can be rapidly synthesized, purified and sprayed on to a substrate for further analysis using a single micro-reactor system.

Thus, preferably the invention provides a method for assessing the quality of an image formed on a substrate by an inkjet printer which comprises: (i) preparing a water soluble azo dye in an ink medium in a micro-reactor and optionally purifying the azo dye within the reactor by means of its electrophoretic migration; (ii) applying the ink from step (i) to a substrate by means of an inkjet printer to form a printed image; and cat..

, a . * ..

* J..

a.

(iii) applying a pre-determined test to the printed image from (ii) to determine whether said image meets pre-selected criteria.

More preferably the invention provides a method for assessing the quality of an image formed on a substrate which comprises: i) preparing a water soluble azo dye in a ink medium in a micro-reactor, and optionally purifying the azo dye within the reactor by means of its electrophoretic migration; (ii) applying the ink from step (i) directly from the micro-reactor to a substrate by means of electrospray to form a printed image; and (iii) applying a pre-determined test to the printed image from (ii) to determine whether said image meets pre-selected criteria.

Dyes, conditions, tests, criteria and preferences for these preferred embodiments are as described above.

A second aspect of the invention provides a micro-reactor process for the preparation of azo compounds wherein the azo compounds are purified by means of their electrophoretic migration.

In the second aspect of the invention the azo colorant moves within the micro reactor at a different rate to the bulk fluid, preferably the azo colorant moves in the opposite direction to the bulk fluid.

Electrophoretic migration may be achieved within the micro-reactor by applying a voltage to the micro-reactor. This may be either a voltage used to drive a micro-reactor under electrokinetic control or a separate voltage during or at the end of a reaction carried out under electrokinetic or non-electrical control, for example pressure driven flow.

The direction and extent of the electrophoretic migration will vary depending on the nature of the azo colorant and will be highly dependent on the properties of the solvent system used, especially with regard to factors such as ionic strength and especially pH.

The direction and extent of the electrophoretic migration of the azo colorant will also be influenced by factors such as the strength and direction of the applied voltage and the design of the micro-reactor.

Micro-reactors and azo-compounds in the second aspect of the reaction are as preferred in the first aspect of the invention.

The invention is further illustrated by the following Examples in which all parts and percentages are by weight unless otherwise stated.

e see c.

. e sa . e e 8 s 8 e e.

e äe e

Example 1

Preparation of: Cl HO15: Example 1 utilised a micro-reactor operating under electrokinetic control. A schematic for the synthesis of the title dye is shown below, H-acid is 1- hydroxy-3,6- disulpho-8 aminonaphthylene.

bee e.

e e a .

. ... . OH NH2 Cl OH HNgN'lC HO354-SO3H CIlN'lCl HO3S -SO3H laced Cyanunc Chlonde

N NH2 N

SO3H HNO3 '4'SO3H > > W SO3H '1 jib HO3S SO3H /J 5-amnosalcylc aced NH2 Cl SO3H IN 1 NH2<OH HO3S SO3H A simple borosilicate glass micro-reactor device was fabricated incorporating a T- shaped channel network; the scheme below shows the position of addition of the reagents. The channels in the micro-reactor were formed by photolithography and wet chemical etching of the base plate. The channels were approximately 1 50pm wide by 50pm deep. The final reactor was formed by bonding the top plate, with reservoir holes therein, to the base plate.

All reactions were performed on the 1 OmM scale and the diazo intermediate was preformed off-chip.

e eve e eee e e e e e e bee e eee e e e e e e e e e e e e e e e e eve e A B A = H-acid B = Cyanuric Chloride C = Preformed Diazo Intermediate | C D = 5-amino salicylic acid E = Distilled water for collection of the final dye product

D

The reactor system used did not contain a mechanism for the removal of hydrodynamic flow. Hydrodynamic flow was assessed by allowing the reactor system to remain static for 10 minutes whilst 40,u1 water was placed in reservoir B and 10,uL water was placed in reservoir A. After 10 minutes the volume changes were recorded and the hydrodynamic flow calculated. The method was performed in duplicate and repeated for all six reservoirs. The hydrodynamic flow for this system was determined as 1.1,uL min'.

The synthesis was carried out at both 5 C (to prevent decomposition of the diazo intermediate) and at room temperature. Table 1 shows a summary of the experimental conditions used and includes the field strengths applied during these experiments. The voltage was applied for 20 minutes.

Table 1

Reservoir Reagent Conc. / mM Volume / ,ul Field Strength / V cm' A H-acid 10 40 200 B Cyanuric chloride 10 40 200 C Diazo Intermediate 10 40 300 D 5-amino salicylic 10 40 200 acid E Water 40 Ground potential The contents of reservoir E were analysed using HPLC and it was seen that the title product had been produced in 20.3% yield when the reaction was carried out at 5 C and 20.2%. This shows the cooling to 5 C is unnecessary in this rapid microreactor system.

In order to investigate the possibility of separating the formed dye from the starting materials the reaction was monitored using an Axiovert S100 inverted microscope (Carl e see e see e e e e e e see e e e ee. e e e e e e e e e e e e e e ee e e e eve e Zeiss) coupled with fluorescence optics and filter at 550 nm. Imaging the reaction as it proceeded on-chip ascertained the point within the simple channel network at which the dye was formed, allowing its flow characteristics to be investigated. Observation of the flow characteristics of the title dye showed that it flowed via its own electrophoretic mobility and in the opposite direction to the net flow. This shows that it is possible to move an azo dye formed in a micro- reactor in an opposite direction to the bulk flow allowing it to be easily isolated and purified

Example 2

The following experiment was carried out to confirm the electrophoretic movement of the dye of Example 1, without the use of a microscope. A simple T-style channel manifold was used and the reactor system set up as shown below.

Reservoir A 40,ul buffer solution +v 1 1 1 Outlet Reservoir B 30,ul dye solution zero V The buffer used was 10mM disodium tetraborate buffer, pH 9. The dye solution was a 1 OmM solution of the dye of the title compound of Example 1 in water.

The difference in volume of the two solutions resulted in a fluid height difference within the fluid reservoirs in the micro reactor. This results in a pressure difference that caused the buffer solution to hydrodynamically flow from reservoir A into reservoir B. An electric field was applied to reservoir A causing the buffer solution to move via electro osmotic flow as well as hydrodynamically.

On application of the electric field the dye solution immediately flowed from reservoir B towards reservoir A. That is, against the electro osmotic and hydrodynamic flow of the bulk liquid.

The ability of the dye to flow via electrophoretic migration may be exploited in order to separate and isolate the dye from any remaining starting materials present within the micro reactor system. This wouldthen allow the dye to be synthesised and then separated on-chip to produce a pure version of the final dye product for further analysis.

e eve e see e e e e e see e e see e e e e e e e e e e e e e e e ee e ee. e

Example 3

Synthesis of the title compound of Example 1 using a pressure driven flow glass micro reactor The glass micro-reactor was prepared by Micro Chemical Systems Ltd., (Hull, UK). The reactor was based on a borosilicate base plate and channels were etched into the plate using by photolithography and wet chemical etching. The channel dimensions (180 Am wide, 30 Am deep and 207 cm long) were designed to produce optimal mixing of the reagents whilst maintaining an acceptable level of back-pressure within the system. A borosilicate top plate containing seven inleVoutlet holes was then thermally bonded over the etched base plate. Standard fused silica capillary tubing was used to connect the inleVoutlet holes to the reagent syringes (1 ml Luer lock glass syringes) via capillary connectors (Upchurch Scientific, UK). The reagent syringes were driven using Baby Bee Syringe Pumps, MF-9090 (Bioanalytical Systems Inc.).

Solution A: H-acid (0.1707 9) was dissolved in 50 ml water and the pH of the solution was raised to pH 8 by addition of sodium hydroxide (2.0 M)) and added to a first reservoir.

Solution B: Cyanuric chloride (0.0922 9) was dissolved in 50 ml water.

Solution C: Aniline sulphonic acid (0.0866 9) was dissolved in 50 ml water and the pH of the solution was adjusted to pH 8 by addition of sodium hydroxide (2.0 M). Sodium nitrite (0.0345 9) was then added, followed, when it had dissolved by concentrated hydrochloric acid (0.2 ml). This solution was stored at 5 C until required when it was used within the micro reactor at room temperature.

Solution D: Sodium hydroxide (1 M).

Solution E: 5-Aminosalicylic acid (0.0766 9) was dissolved in 50 ml water.

: . A: . .: - : .

. ... . OH NH2 ICI N-N HO3S J[ 11 SO3H

A B C

1 1 1 1 OH D NaOH CO2H NH2

NAN

NSOaH Co,OH The reaction was carried out at room temperature. All reagent syringes were driven at a flow rate of 0.1 pi mind. The final dye product was analysed by HPLC-VIS detecting at 535 nm as no other reagents in the system absorbed in this region. A 71 % conversion was seen with a reaction time of only 165 s.

Example 4

Electrospray of the product formed in the micro reactor An aqueous solution of the dye prepared in Example 1 was placed in reservoir A of the T-shaped channel network, described in Example 2, and 451 water was placed in reservoir B. A circular aluminium target plate was held at high voltage in the range 1 kV to 4kV, and placed 1 mm from the terminus of the channel. As described in Example 2 the difference in volume in the two reservoirs prevented the dye from flowing via pressure driven effects. The reservoirs were at ground potential. It was expected for the dye to flow via electrophoretic migration in the direction of the channel terminus, towards the highly positive aluminium electrode. All experiments were carried out in an insulated safety box.

Table 2 summarises the results seen as the applied voltage was varied.

: ë I: . : . * ** . . ...

Table 2

Applied voltage / V Observation 1000 No observed effect 1500 No observed effect 2000 No observed effect 2500 A small purple corona discharge speared between the channel terminus and the aluminium electrode 3000 A small purple corona discharge speared between the channel terminus and the aluminium electrode 3500 The electrical discharge was more intense and a small yellow spark was observed at the channel terminus. The dye slowly moved in the direction of the aluminium disc.

4000 The yellow sparking appeared to increase towards the T-channel intersection. The dye moved very quickly and sprayed from the channel terminus onto the surface of the aluminium disc.

These preliminary investigations illustrate the potential for the electrospray of the synthesised dye directly from the micro-reactor system to a substrate.

e e - e e e e e e e e e e e e

Claims (10)

1. A method for assessing the quality of an azo colorant liquid composition or of an image formed by an azo colorant on a substrate which comprises the following steps: (i) preparing an azo colorant using a micro-reactor; (ii) optionally forming a liquid composition from the azo colorant and/or applying the azo colorant from step (i) to a substrate to form a printed image; and (iii) applying a pre-determined test to either the liquid composition or the printed image from step (ii) to determine whether said liquid composition or printed image meets pre- selected criteria.

2. A method according to claim 1 for assessing the quality of an ink suitable for ink jet printing or a printed image formed on a substrate from said ink which comprises the following steps: (a) preparing an azo colorant using a micro-reactor; (b) preparing an ink by incorporating the azo colorant from step (a) into an ink medium; (c) applying the ink from step (b) to a substrate to form a printed image; and (d) applying a pre-determined test to either the ink from step (b) or the printed image from step (c) to determine whether said ink or said image meets pre-selected criteria.

3. A method according to claim 2 wherein steps (a) and (b) are combined by preparing the azo colorant in a solvent system which is able to act as an ink medium.

4. A method according to either claim 2 or claim 3 wherein step (c) the ink is applied to a substrate directly from the micro-reactor using electrospray.

5. A method according to any one of the preceding claims wherein the azo colorant is an azo dye with a water solubility greater than 2.5% by weight.

6. A method according to any one of the preceding claims wherein the azo colorant formed in the micro-reactor is purified by electrophoretic migration.

e. ...

a e e eve

7. A method according to any one of the preceding claims wherein the micro-reactor comprises reagenVproduct reservoirs which are coated with polydimethylsiloxane.

8. A method according to claim 1 or claim 2 for assessing the quality of an image formed on a substrate by an inkjet printer which comprises: (i) preparing a water soluble azo dye in a ink medium in a micro-reactor and optionally purifying the azo dye within the reactor by means of its electrophoretic migration; (ii) applying the ink from step (i) to a substrate by means of an inkjet printer to form a printed image; and (iii) applying a pre-determined test to the printed image from (ii) to determine whether said image meets pre-selected criteria.

9. A method according to claim 1 or claim 2 for assessing the quality of an image formed on a substrate which comprises: i) preparing a water soluble azo dye in a ink medium in a micro-reactor, and optionally purifying the azo dye within the reactor by means of its electrophoretic migration; (ii) applying the ink from step (i) directly from the micro-reactor to a substrate by means of electrospray to form a printed image; and (iii) applying a pre-determined test to the printed image from (ii) to determine whether said image meets pre-selected criteria.

10. A micro-reactor process for the preparation of azo compounds wherein the azo compounds are purified by means of their electrophoretic migration.

. a. . . - . , a ' -. . A.