GB2168368A - Purifying dye solutions for use in the ink-jet process - Google Patents

Abstract

Aqueous dye solutions are purified by ultrafiltration, precision filtration or reverse osmosis using (i) a membrane or filter having a pore diameter slightly larger than the dye molecules to remove dispersants, surfactants and colloidal and polymeric substances, and (ii) a membrane having a pore diameter (slightly) smaller than the dye molecules to remove metal ions and their counterions. Inks made from the purified dyes have long-term stability and may be used in the ink-jet process.

Description

SPECIFICATION Method for purification of dyes Background of the invention Field of the invention This invention relates to a method for purifying aqueous dye solutions. More particularly, the present invention relates to a method for purifying to a high purity, water-soluble dyes which are useful coloring components of recording liquids used in ink jet recording and in writing pens (hereinafter these recording liquids are referred to as inks.) Description of the prior art Water-soluble dyes such as direct dyes, acid dyes, basic dyes, reactive dyestuffs and the like have conventionally been used widely for dyeing of fibers, woven cloths, etc.Further, these dyes are recently in wide use as coloring components of inks jet recording wherein an ink is discharged by any of various discharging means to form ink droplets and the ink droplets are allowed to deposit on a recording medium such as a paper or the like in accordance with image signals. Furthermore, the water-soluble dyes are in wide use as coloring components of inks used in writing pens such as fountain pen, felt pen, ball pen and the like.

As dyes as coloring components of inks, particularly inks used in ink jet recording, there are generally used dyes produced for dyeing of fibers. These dyes contain large amounts of additives or impurities.

Therefore, it is known that when these dyes of commercial availability are used in -inks for ink jet recording, the inks cause various troubles. The biggest trouble is clogging of ink-discharging orifice which is the heart of ink jet recording equipment and, in the ink jet recording method utilizing a heat energy for discharging of ink, is deposition of impurities on the heat-generating head. In order to avoid such troubles, purification of dyes used in inks have heretofore been proposed and carried out, and some improvements have been achieved and problems caused by inks have been reduced by using dyes of higher purity. However, there still exists a phenomenon that inks free from any problem immediately after production, when delivered from ink manufacturers and stored for several months or years, frequently cause troubles not anticipated right after production.These troubles are striking particularly in the ink jet recording method utilizing a heat energy for discharging of ink.

Summary of the invention The main object of the present invention is to provide a dye capable of giving an ink which has solved the above mentioned drawbacks of the prior art, causes even under a high dye concentration no clogging at the nozzle, orifice, etc. of ink jet recording equipment during use and also during long term storage, forms no deposit on the heat-generating head of ink jet recording equipment utilizing a heat energy, and thus has excellent stability.

In order to attain the above object the present inventors made an extensive research. Commercially available dyes contain various impurities (e.g. various inorganic and organic substances such as inorganic salts, dispersing agents and leveling agents and the above mentioned problems are caused mainly by these impurities. Hence, the adverse effects of the impurities were investigated.As a result, it was found that the biggest cause for clogging of nozzle and orifice, for formation of precipitates during ink storage and for deposition of impurities on heat-generating head in the ink jet recording method utilizing a heat energy is the presence in ink of Fe, Si, P, polyvalent metal ions, their counter ions (e.g. halogen ions, SO,, PO4-) and colloidal substances comprising these compounds and that these harmful substances can be efficiently removed from dyes used in inks by using a particular method. Based on this finding, the present invention has been completed.

According to one aspect of the present invention, there is provided a method for purifying a dye solution, characterized by dissolving a dye in a solvent and then treating the resulting dye solution with a membrane A having pores of diameters smaller than the diameter of the moledules or associated molecules of said dye and a membrane B having pores of diameters larger than the diameter of the molecules or associated molecules of said dye.

According to another aspect of the present invention, there is provided a method for puritying a dye solution, characterized by dissolving a dye in a solvent and then filtering the resulting dye solution through a filter having pores of diameters larger than the diameter of the molecules or associated molecules of said dye but smaller than the diameters of the colloidal substances present in the dye solution as impurities.

According to further aspect of the present invention, there is provided a method for producing an ink by utilizing the above method for dye purification.

Detailed description of the preferred embodiments The present invention will be described in more detail below. The main features of the present invention lies in that purification of water-soluble dyes is conducted by the use of (a) a membrane having pores of diameters smaller than the diameter of the molecules or associated molecules of a dye to be purified in a solution form and (b) a membrane having pores of diameters larger than the diameter of the molecules or associated molecules of said dye, or by the use of a filter having pores of diameters larger than the diameter of the molecules or associated molecules of a dye to be purified in a solution form but smaller than the diameters of the colloidal substances present in the dye solution as impurities.The dyes to which the present invention method is applicable are water-soluble dyes conventionally used mainly for dyeing of various fibers, such as direct dyes, acid dyes, basic dyes, reactive dyestuffs and the like. As dyes which are preferably used in inks for ink jet recording and satisfy such requirements as distinctness, water solubility, stability, light resistance and the like, there are mentioned, for example, C.l. Direct Black 17, 19, 32, 51, 71, 108 and 146; C.l. Direct Blue 6,22, 25, 71, 86, 90, 106 and 199; C.l. Direct Red 1, 4, 17, 28 and 83; C.l. Direct Yellow, 12, 24, 26 and 98; C.l. Direct Orange 34, 39, 44, 46 and 60; C.l.Direct Violet 47 and 48; Cl. Direct Brown 109; Cl. Direct Green 59; Cl. Acid Black 2, 7, 24, 26, 31, 52, 63, 112 and 118; Cl. Acid Blue, 9, 22, 40, 59, 93, 102, 104, 113, 117, 120, 167, 229 and 234; Cl. Acid Red 1, 6, 32, 35, 37, 51, 52, 80, 85, 87, 92, 94, 115, 180, 256,317 and 315; Cl. Acid Yellow, 11, 17, 23, 25, 29, 42, 61 and 71; Cl.

Acid Orange 7 and 19; C.l. Acid Violet 49; C.l. Basic Black 2; C.l. Basic Blue 1, 3, 5, 7, 9, 24, 25, 26, 28 and 29; C.l. Basic Red 1, 2, 9, 12, 13, 14 and 37; C.l. Basic Violet 7, 14 and 27; C.l. Food Black 2; etc. These dyes are very preferably used in the present invention. However, the present invention is not restricted to such dyes.

The water-soluble dyes mentioned above generally are easily available in the market. These commercially available products contain large amounts of NaCI, Na2SO4 and other inorganic salts and further contain inorganic substances such as alkaline earth metal salts, various metal comounds and the like as well as many organic substances such as various surfactants, dispersing agents, levelling agents and the like.

Since the above mentioned additives contained in the conventional water-soluble dyes are also watersoluble, purification of these dyes is not so easy. However, when such dyes are thoroughly purified by the use of a conventionally known purification method such as, for example, filtration, salting-out, ion exchange or the like and are used as inks for ink jet recording, such inks show excellent performances as intended, right after their preparation. When stored for several months or longer after their preparation, however, these inks cause troubles as mentioned previously. These troubles caused by inks stored over a long period of time after their preparation occur, in many cases, after the inks have been diffused in the market and it is very difficult to recover the inks from the market. In order to solve this serious problem, the present inventor made an extensive research.As a result, it was found that (1) the troubles caused by inks stored over a long period of time can not be eliminated by applying to the inks an ordinary filtration method, an ion exchange method, etc., (2) the main cause of the troubles lies in colloidal substances which are present in inks with a relatively good stability but loses their stability when the inks encounter environmental changes such as, for example, temperature change, concentration change at discharging nozzle, thermal shock at heat-generating head and the like, as well as in various anions which are also present in the inks as counter ions of various metals, such as halogen ions, SOk, PO4-- and the like, and (3) said troubles can be completely solved by sufficiently removing the colloidal substances and the anions, prior to or during preparation of ink, with a fitter having a particular pore diameter or with two kinds of permeable membranes, each having different pore diameter.

The permeable membranes used in the present invention comprise a membrane A having pore diameters smaller than the diameter of the molecules or associated molecules of a dye to be purified but larger than the diameters of inorganic ions, etc. and a membrane B having pore diameters larger than the diameter of the molecules or associated molecules of the dye but smaller than the diameters of high molecular substances and colloidal substances. In general, the diameters of the molecules or associated molecules of the dye are 0.2 IL or less, and those of the colloidal substances and high molecular substances are 0.5 i or more. However, the molecular diameters of dyes differ by the type and association condition of these dyes, and therefore selection of optimum membranes A and B is made experimentally.

Filtration of a dye solution through the membrane A can remove from the solution, metal ions and their counter ions, all having diameters smaller than the molecule diameters of the dye dissolved in the solution. Treatment of the resulting dye solution with the membrane B can remove high molecular substances (.e.g. intermediate molecular substances (dispersing agents, surfactants, etc.), other polymers) and colloidal substances, al! having diameters larger than the molecule diameters of the dye. Needless to say, the same effect can be obtained by conducting the above procedure in a reversed order, namely, by conducting the treatment with the membrane B at first.

Dyes such as direct dyes tend to cause association of molecules in their concentrated solution. For such dyes, the membranes A and B are similarly selected to as mentioned above so as to respond to the diameter of the associated molecules. It is also possible that a concentrated dye solution be used in the treatment with the membrane A to make easier the removal of small molecules such as ions and the like and in the treatment with the membrane B, the dye solution is diluted to make easier the removal from intermediate to high molecular substances and colloidal substances.

The another filter used in the present invention is a filter having pore diameters larger than the diameter of the molecules or associated molecules of a dye to be purified but smaller than the diameters of high molecular substances and colloidal substances. In general, the molecule diameters of dyes differ by the type of these dyes. Therefore, it is preferable to use such a filter through which dyes can pass easily but the majority of colloidal substances can not pass, namely, a filter having an average pore diameter of 0.001 to 0.5 wim. Selection of a filter best suited can be made experimentally.

Filtration by a filter whose pore diameter is in the above range can remove from a dye solution high molecular substances [e.g. intermediate molecular substances (dispersing agents, surfactants, etc.) and polymers] and colloidal substances, all having diameters larger than the molecule diameter of the dye.

For dyes such as direct dyes which tend to cause association of molecules in a concentrated solution, the above filter is similarly selected as mentioned above so as to respond to the diameter of the associated molecules.

With respect to the membranes A and B and the filter used in the present invention as described above, various membranes of various diameters are easily available commercially under the general names of precision filter membrane, ultrafilter membrane and reverse osmosis membrane. These membranes can be loaded in precision filtration equipment, ultrafiltration equipment and reverse osmosis equipment which are all known.The filter having particular pore diameter and the membranes A and B used in the present invention can be selected from various filter membranes of different pore diameter such as, for example, Dia Filter supplied by Ulvac Service Corp., Toyo Membrane Filter supplied by Toyo Roshi (K.K.), Fluoropore supplied by Sumitomo Electric Industries, Ltd., NTR series, NTU series and NTF series supplied by Nitto Denko (K.K.), Nuclepore UF Membrane supplied by Nuclepore Corp., Sartorius Membrane Filter supplied by Sartorius Corp., and DRS series and DUY series supplied by Daicel Chemical Industries, Ltd.

The method for purification of dyes according to the present invention can be achieved as follows. A dye to be purified is dissolved in water or a mixed solvent consisting of water and a water-soluble organic solvent, preferably in water in a concentration of 0.1 to 10% by weight, preferably 1 to 5% by weight; the resulting aqueous dye solution is treated by the use of a filtration equipment provided with the membrane A, for example, an ultrafiltration equipment to filter impurities having diameters smaller than the molecule diameter of the dye and to obtain a concentrate containing the dye and impurities having diameters larger than the molecule diameter of the dye; then the concentrate is treated with the membrane B in the same manner to allow only the dye molecules to pass through the membrane B and to obtain a dye solution free not only from the impurities having diameters smaller than the molecule diameter of the dye but also from molecules and colloidal substances having diameters larger than the molecule diameter of the dye. Through such treatments, there are removed from the dye, the majority of metal ions, their counter ions, high molecular substances, stable colloidal substances, etc., all present in the dye, whereby an intended purified dye solution can be obtained.

In the above purification method, the order of treatments by the membrane A and the membrane B can be reversed and the same effect can be obtained.

In the above method, impurities having the same size as the molecule diameter of the dye can not be removed. However, since the amount of such impurities is extremely small, the object of the present invention is achieved sufficiently. When it is required to remove even these impurities present in a trace amout, there can be applied, prior to or after the above purification method, a method such as saltingout, fractional precipitation with a water-soluble oragnic solvent, ion exchange, exposure to air, softening with lime, electrolysis or the like.When the dye to be purified tends to cause association, the impurities in the dye having a molecular weight of about the same those of the dye can alternatively be removed by preparing a concenrated solution of the dye wherein the dye molecules are in an association state and subjecting this concentrated solution to the above membrane treatments.

The method for purification of dyes according to the present invention can also be achieved as follows.

A dye to be purified is dissolved in water or a mixed solvent comprising water and a water-soluble organic solvent, preferably in water in a concetration of 0.1 to 10% by weight, preferably 1 to 5% by weight and the resulting aqueous dye solution is treated by the use of an equipment provided with the above mentioned filter, for example, a precision filtration equipment to obtain a dye solution free from molecules and colloidal substances, all having diameters larger than the molecule diameter of the dye.

Through such treatment, there are removed from the dye, the majority of high molecular substances, stable colloidal substances, etc. all present in the dye, whereby an intended purified dye solution can be obtained.

In the above method, impurities having diameters smaller than the molecule diameter of the dye can not be removed. However, since the amount of such impurities is extremely small, the object of the present invention is achieved sufficiently.When it required to remove even these impurities present in a trace amount, there can be applied, prior to or after the above purification method, a method such as saltingout, fractional precipitation with a water-soluble organic solvent, ion exchange, exposure to air, softening with lime, electrolysis or the like.

The purified aqueous dye solution thus obtained can be made into an ink as it is or after adding thereto necessary additives such as an water-soluble organic solvent and the like, or can be made into a powder by removing the water present in the solution. The amount of impurities remaining in the purified aqueous dye solution or in the purified powder can easily be determined by the use of ion chromatography, atomic absorption spectrometry, inductively coupled plasma emission spectral analysis or the like.It is preferable that the concentration of impurities in the purified dye of the present invention is about 1% or below (halogen ions), 0.5% or below (SO4--,) about 200 ppm or below (each of Fe, Mg and P), 250 ppm or below (Si), about 2% by weight or below (total inorganic substances) and/or about 1,000 ppm (total colloidal substances including these metal compounds), all based on powder dye. A purified dye containing impurities so as not to exceed the above limits can provide an ink useful for use particularly in the ink jet recording method using a heat energy. Use of such ink does not result in the previously mentioned troubles even when the ink has been stored for a long period of time after its preparation.

Hereinafter the present invention will be described more specifically by way of Examples. Parts or % in the following refer to parts or % by weight.

Example 7A (dyepurffication) 10 g of Direct Blue 199 (commercially available product) was dissolved in 490 me of water. The resulting aqueous solution was treated using an ultrafiltration apparatus provided as the membrane A, with a membrane filter of 0.01 pm in average pore diameter manufactured by Sartorious Corp. For each 250 me of the aqueous solution permeated through the filter, the same volume of pure water was added to the original solution. This procedure was repeated 8 times, whereby a concentrate (residual solution) containing the dye and particles larger than the dye was obtained.Subsequently, in the above apparatus was set, as the membrane B, a membrane filter of 0.2 ,am in average pore diameter manufactured by the same company, and the above concentrate was allowed to permeate through this filter to obtain a permeated solution containing only the dye.The permeated solution was subjected to ion chromatography and inductively coupled plasma emission spectral analysis for impurity determination.The results are a follows. (The numerals show ratios to dye.) Cl p/oJ SO4 { / ) Fe (p pm) Si (p pm) Before 18 11 318 205 purification After 0.34 0.22 102 56 purification Example 2A (dye purification) Food Black 2 (commercially available product) was purified in the same manner as in Example 1A (dye purification) except that there were used, as the membrane A, an ultrafilter membrane having a fractionation molecular weight of 5,000 manufactured by Daicel Chemical Industries, Ltd. and, as the membrane B, an ultrafilter membrane having a fractionation molecular weight of 40,000 manufactured by the same company. The permeated solution was subjected to the same analyses as in Example 1A (dye purification).The results are as follows. (The numerals show ratios to dye.) Cl {o/o) S04 (%) Fe (ppm) Si (p pm) Before 13 ~0 292 142 purification After 0.27 ~0 61 32 purification Example 3A (dye purification) 550 me of an aqueous solution containing 2% of Acid Red 35 (commercially available product) was treated using an ultrafiltration apparatus provided, as the membrane B, with an ultrafilter membrane having a fractionation molecular weight of 20,000 manufactured by Nuclepore Corp., whereby 500 me of a permeated solution containing the dye and particles smaller than the dye was obtained Subsequently, an ultrafilter membrane having a fractionation molecular weight of 5,000 manufactured by the same company was provided as the membrane A in the above apparatus and, using this apparatus, the permeated solution was treated. For each 250 me of the solution permeated through the membrane A, the same volume of pure water was added to the original solution. This procedure was repeated 10 times, whereby a concentrate (residual solution) containing only the dye was obtained. The concentrate was subjected to the same analyses as in Example 1A (dye purification). The results are as follows.

(The numerals show ratios to dye.) Cl {Q/o) 504 (%) Fe (p pm) Si (pom) Before 21 13 362 210 purification After 0.42 0.33 65 58 purification Example 4A (dye purification) Acid Yellow 23 (commercially available product) was purified in the same manner as in Example 3A and subjected to the same analyses as in Example 1A (dye purification). The results are as follows.

(The numerals show ratios to dye.) Cm (%) S04(%) Fe (ppm) Si (ppm) Before 11 4 525 360 purification After 0.51 0.16 77 42 purification Example 1A' (ink evaluation) Using the purified dye obtained in Example 1A (dye purification), there was prepared an ink for ink jet recording having the following composition.

Dye 2 parts Diethylene glycol 40 parts Water 60 parts This ink was subjected to the following T1 to T5 evaluation tests, using a recording apparatus having an on-demand type multi-head (discharging orifice diameter: 35 it, resistance of heat-generating resistor: 150 Q; driving voltage: 30 V, frequency: 2 kHz) wherein the ink stored in the recording head was given a heat energy to generate ink droplets for execution of recording. The ink gave good results in all the tests.

(T1) Long term storage stability: The ink was sealed in a plastic film bag and stored for 6 months at -30 C and 60"C, separately. No insoluble matter appeared and further there was no change in ink properties and color.

(T2) Discharging stability: The ink was subjected to continuous discharging of 24 hrs in atmospheres of room temperature, 5"C and 40"C, separately. Constantly stable and high quality recording could be conducted in all the conditions.

(T3) Discharging response characteristic: Two cases of intermittent discharging of 2 sec and discharging after 2 month standing were examined. In each case, there was no clogging at the orifice tip and there was obtained stable and uniform recording.

(T4) Quality of recorded image: Using the ink, recording was conducted on the recording media listed in Table 1A which appears later. Each recorded image had a high density and a clarity. When each image was exposed to an indoor light for 6 months, the reduction percentage of its density was 1% or less.

(T5) Fixing property on various recording media: Using the ink, printing was conducted on the recording media of Table 1A which appears later. After 15 sec, the printed portion on each recording medium was rubbed with fingers to see image aberration and spreading. In all the cases, there was neither image aberration nor image spreading, and fixing was excellent.

Examples 2A' to 4A' (ink evaluation) Using the purified dyes obtained in Examples 2A to 4A (dye purification) according to the present invention method, inks were prepared in the same manner as in Example 1A' (ink evaluation). Each ink was subjected to the same T1 to T5 evaluation tests as in Example 1A (ink evaluation). All the inks showed same excellent results as in Example 1A' (ink evaluation).

Example 5A' (ink evaluation) The yellow ink, magenta ink, cyan ink and black ink obtained in Examples lA' to 4A' (ink evaluation), respectively, were subjected to the same T1 to T5 evaluation tests as in Example 1A' (ink evaluation), using a recording apparatus having an on-demand type recording head (discharging orifice diameter: 50 ;, driving voltage for piezoelectric vibrator: 60 V, frequency: 4 kHz) wherein each ink was discharged by the piezoelectric vibrator. All the inks showed excellent results.

Example 6A' (ink evaluation) Using the yellow ink, magenta ink, cyan ink and black ink obtained in Examples 1A' to 4A' (ink evaluation) and the same ink jet recording apparatus as in Example 1A' (ink evaluation), a photograph of full color was reproduced. In the resulting image, each color was very clear and color reproduction was good.

Example 7A' (ink evaluation) Each of the inks of different color prepared in Examples 1A' to 4A' (ink evaluation) was charged into a felt pen Without the cap, each felt pen was left for 10 days. Then, recording was made on paper using these felt pens. There was no ink skipping and the recording was smooth.

Comparative Example A (ink evaluation) Using the unpurified dyes of Examples 1A to 4A (dye purification) and in the same manners as in Examples 1A' to 4A' (ink evaluation), inks for comparison were prepared.

These inks were subjected to the same T1 to T5 evaluation tests as in Example 1A' (ink evaluation).

When stored, each ink showed precipitation of insoluble matters in one month. In (T2), no discharging of ink was often seen, at which time the increase of driving voltage was necessary. Upon observation of the surface of the heat-generating head using a microscope, adherence of brown deposits was seen.

In (T3), clogging occurred at the orifice after standing for 1 month and discharign of ink was unstable.

Examples IB to 7B (dye purification) Aqueous solutions each containing 2% of one of the commercially available dyes shown in Table 1B which appears later were filtered through a pressure filter containing one of the microfilters shown in Table 1B. Each original solution and each filtrate were subjected to inductively coupled plasma emission spectral analysis for measurement of iron and silicon centents. The results are as shown in Table 1B.

Examples lB' (ink evaluation) Using the purified dye obtained in Example 1 B (dye purification), there was prepared an ink for ink jet recording having the following composition.

Dye 2 parts Diethelene glycol 46 parts Water 60 parts This ink was subjected to the same T1 to T5 evaluation tests as in Example 1A' (ink evaluation). The results were good in all the tests. In T4 and T5, the recording media of Table 1A were used.

Examples 2B' to 7B' (ink evaluation) Using the purified dyes obtained in Examples 2B to 7B (dye purification) according to the present invention method, inks were prepared in the same manner as in Example 1B' (ink evaluation). Each ink was subjected to the same T1 to T5 evaluations tests as in Example 1B' (ink evaluation). All the inks showed same excellent results as in Example 1B (ink evaluation).

Example 8B' (ink evaluation) The yellow ink, magenta ink, cyan ink and black ink selected from the inks of Examples 1 B' to 7B' (ink evaluation) were subjected to the same T1 to T5 evaluation tests as in Example 1 B' (ink evaluation), using a recording apparatus having an on-demand type recording head (discharging orifice diameter: 50 u, driving voltage for piezoelectric vibrator: 60 V, frequency: 4KHz) wherein each ink was discharged by the piezoelectric vibrator. All the inks showed excellent results.

Example 9B' (ink evaluation) Using the yellow ink, magenta ink, cyan ink and black ink selected from the inks of Examples 1B' to 7B' (ink evaluation) and the same ink jet recording apparatus as in Example 1B' (ink evaluation), a photograph of full color was reproduced. In the recorded image, each color was very clear and color reproduction was good.

Example IOB' (ink evaluation) Each of the inks of different color prepared in Examples 1B' to 7B' (ink evaluation) was charged into a felt pen. Without the cap, each flet pen was left standing for 10 days. Then, recording was made on paper using these felt pens. There was no ink skipping and the recording was smooth.

Comparative Example B' (ink evaluation) Using the unpurified dyes of Examples 1B to 7B (dye purification) and in the same manners as in Examples 1B' to 7B' (ink evaluation), inks for comparison were prepared.

These inks were subjected to the same T1 to T5 evaluation tests as in Example 1B' (ink evaluation).

When stored, each ink showed precipitation of insoluble matters in one month. In (T2), no discharging of ink was often seen, at which time the increase of driving voltage was necessary. Upon observation of the surface of the heat-generating head using a microscope, adherence of brown deposits was seen.

In (T3), clogging occurred at the orifice after standing for 1 month and discharging of ink was unstable.

TABLE 1A Recording medium (brand name) Classification Manufacturer Ginkan High quality Sanyo-kokusaku paper Pulp Co., Ltd.

Seven Star High quality HOKUETSU PAPER paper PAPER MILLS, LTD.

Hakubotan Intermediate Honshu Paper Co., quality paper Ltd.

Toyo Roshi No. 4 Non-sized paper Toyo Roshi K.K.

TABLE 1B Analytical value Micro filter (ppm based on dye) Example Commercially Average Original Permeated No. available dye pore Manufacturer solution solution diameter ('im) Fe Si Fe Si 1B Acid Yellow 23 0.01 Sartorius Corp. 525 360 81 52 2B Direct yellow 86 0.1 Sumitomo Electric 720 275 120 86 Industries, Ltd.

3B Acid red 35 0.05 Sartorius Corp. 362 210 75 71 4B Direct Blue 86 0.2 Spectra 416 191 115 57 5B Direct Blue 199 0.5 Spectra 318 205 122 68 6B Feed Black 2 0.1 Sumitomo Electric 292 142 68 37 Industries, Ltd.

7B Direct Black 19 0.3 Sumitomo Electric 660 316 136 82 Industries, Ltd.

Claims (19)

1. A method of preparing a purified dye solution, characterised by dissolving a dye in a solvent and then filtering the resulting dye solution through a filter having pores of diameters larger than the diameter of the molecules or associated molecules of said dye but smaller than the diameters of the colloidal substances present in the dye solution as impurities.

2. A method according to claim 1 wherein the filter has an average diameter of 0.01 to 0.5 Fm.

3. A method according to claim 1 wherein the filter is a precision filter membrane, an ultrafilter membrane or a reverse osmosis membrane

4. A method of preparing a purified dye solution, characterised by dissolving a dye in a solvent and then treating the resulting dye solution with a membrane A having pores of diameters smaller than the diameter of the molecules or associated molecules of said dye and a membrane B having pores of diameters larger than the diameter of the molecules or associated molecules of said dye but smaller than the diameters of the colloidal substances present in the dye solution as impurities.

5. A method according to claim 4 wherein the pore diameter of the membrane A is close to the diameter of the molecules or associated molecules of said dye.

6. A method according to claim 4 or claim 5 wherein the pore diameter of the membrane B is close to the diameter of the molecules or associated molecules of said dye.

7. A method according to any of claims 4 to 6 wherein the dye solution is treated firstly with the membrane A and then with th membrane B.

8. A method according to any of claims 4 to 7 wherein the dye solution is treated firstly with the membrane B and then with the membrane A.

9. A method according to any of claims 4 to 8 wherein the membranes A and B are independently a precision filter membrane, an ultrafilter membrane or a reverse osmosis membrane.

10. A method according to any preceding claim wherein the dye is a water-soluble dye.

11. A method according to any preceding claim wherein the solvent is water.

12. A method according to any of claims 1 to 10 wherein the solvent is a mixed solvent comprising water and a water-soluble organic solvent.

13. A method according to any preceding claim wherein the dye is dissolved in the solvent in the range of 0.1 to 10% by weight.

14. A method according to any preceding claim wherein the treated dye solution contains 2% by weight or less, based on dye weight, of inorganic substances.

15. A method according to any preceding claim wherein the filtered dye solution contains 1,000 ppm or less, based on dye weight, of the colloidal substances.

16. A method for producing an ink comprising preparing a purified dye solution by a method according to any preceding claim and then mixing the dye solution with water and/or a water-soluble organic solvent to prepare an ink.

17. A method of preparing a purified dye solution substantially as described herein with reference to any one of the Examples.

18. A method of preparing a purified ink for ink jet printing substantially as described herein with reference to any one of the Examples.

19. A method of ink jet printing in which there is utilised an ink prepared by a method according to claim 16 or claim 18.