INK FOR CONTINUOUS INK JET PRINTING

Abstract: An ink which is to be used for high resolution continuous ink jet printing may also contain, in contrast to current knowledge, water insoluble colouring agents in dispersed form. The known problems associated with such inks stem from the additives which are necessary for the stabilisation of the ink, and from a badly adapted size distribution of the colouring agent particles. Thus, additives should be preferred whose average molecular weight is less than 20,000. With substances that are hydrogen bridge active, in particular that contain hydroxy or amino groups, this limit must be still lower, i.e. about 1,000. The particles of the colouring agent should have a size distribution wherein the colouring agent's total volume proportion of particles greater than 200 nm is less than about 10 %, and that of particles greater than 300 nm less than about 3 %.

Description: Ink for continuous ink jet printing Background of the Invention The present invention is related to an ink which is appropriate and destined for the continuous ink jet printing and which contains water as the major liquid component. Ink jet printing with continuous generation of droplets, the so-called continuous ink jet printing process, is used in a wide extent for producing professional sample printings. The advantage of this method consists mainly in the high resolution at a sufficient speed which results from the high rates of droplets which can be generated per second. Common are rates up to 100'000 Hz, i. e. 100'000 droplets per second. In this process, a continuous stream of droplets is generated from which droplets are deflected in a controlled manner and made to impinge onto a surface to be printed, and/or the remaining droplets are deflected on a wall surface from where they flow to a collecting vessel. The ink is pressed through a fine nozzle which is mechanically coupled to a piezoelectric element oscillating at the desired droplet frequency. As a result, the ink stream emerging from the nozzle becomes separated into droplets in a controlled manner. A voltage is applied to the nozzle so that the droplets are electrically charged, and their trajectory can then be influenced in any desired manner by the effects of an electric field through which the droplets are passing. Two particular, specific requirements are made to an ink suitable for this printing method: The droplets which are generated must be capable of being sufficiently /02599 PCMB95/00567 - 2 - electrically charged, and their flight path and speed must be able to be predicted in a sufficiently precise manner over a predetermined path length starting from their point of generation. The first requirement is generally only fulfilled by inks on an aqueous base. Since the printer is controlled in the simplest case only by putting the electric field on and off, the droplets being directed on the target, i.e. the substrate to be printed, when the field is switched off, and on the retaining shield screen when the field is switched on, it will be sufficient to guarantee a certain minimal effect of the electric field, i.e. a certain minimal charge of the droplets, as well as a sufficiently exact flight path when the field is off; the high dipole moment of water presents an advantage. It is necessary to be able to precisely predict the position of the droplets on their flight path since the printing position will for example meander over the target, each droplet being assigned to a certain predetermined destination point already at the moment of its formation. Thus, in order to generate or prevent, respectively, a printing dot at a predetermined location, it will be necessary to pilot each individual droplet precisely onto the target or, respectively, to deflect it in the direction of the wall or screen. It has been found during the development of this printing technique that water generally fulfills this requirement too. Consequently, inks have been developed on an aqueous base which contain dissolved dyestuffs. However,, such inks present the drawback that the choice of water soluble dyestuffs is very limited, and a great many of applications have requirements which can only be fulfilled by dyestuffs or pigments which are water insoluble. Such dyestuffs and pigments are designed in the following as "insoluble colouring agents". Especially for a high resolution (colour) printing where, inter alia, a high droplet rate is used, inaccuracies in the control of the droplets produce distinct deficiencies in the printing result. Faults are particularly apparent when the colour depth of a surface is controlled by a more or less thinned dot screen, each screen dot being produced by one sole droplet. In this case, clearly visible blots are produced if the droplet control is not perfect. Summary of the Invention A major object of the present invention is to provide a printing ink, appropriate for high resolution continuous ink jet printing, which contains water as a major liquid component, at least one dispersed, particulate colouring agent selected from pigments and water insoluble dyestuffs, and additives for stabilizing and/or adjusting the physical properties of the ink, in particular its rheology. The ink according to the invention is characterized by the fact that substantially all these additives have a weight average molecular weight below about 20,000. The ink contains first additives wherein hydrogen atoms that are capable of forming hydrogen bridges (especially hydroxy or amino hydrogen) represent more than about 5 % of the number of atoms in the molecule, have a weight average molecular weight below about 1,000. Furthermore, the particles of said colouring agents have such a size distribution that less than 10 % of the total number of the particles is greater than 200 nm, and less than 3 % of said total number is greater than 300 nm. It has already been tried to formulate inks for the continuous ink jet printing having therein dispersed insoluble colouring agents; however, the results have been absolutely unsatisfactory. The ink droplets showed a very 96/02599 PCMB95/00567 - 4 - irregular velocity distribution, and their position on the trajectory were thus not sufficiently precisely predictable for the deflection, and this fact produced at least blurred, if not fully unusable, printing images. From this findings, it has been concluded that the addition of insoluble substances in dispersed form disturbs the splitting up of the ink jet into droplets in such a severe manner that such inks are in principle not suited for the continuous ink jet printing. However, research efforts made by the inventors have shown that the problem is not primarily based on the presence of insoluble, dispersed particles; to the contrary, basic compositions for dispersion inks, i.e. for inks having colour pigments or dyestuffs dispersed therein, already show such an unsatisfactory behaviour prior to the addition of the colouring agent to be dispersed. It has been concluded therefrom that the problem is based on the different additives necessary or usual in dispersion printing inks. Further investigations have shown that the usually added surface active substances (surfactants), the polymers used for adjusting the viscosity, and the antifoaming agents are responsible for the negative behaviour of the inks. Starting from this knowledge, further research has shown that, on one hand, polymeric additives having a molecular weight of more than about 10,000 seriously impair the properties of a basic composition, and they render such compositions virtually unusable at a molecular weight of more than about 20,000. Furthermore, groups capable of forming hydrogen bridges, particularly hydroxy and amino groups, will strongly shift this limit to lower molecular weights. It has also been found that substances which contain hydrogen bridge forming hydrogen atoms in an appreciable extent may only be present as low molecular entities, for example up to a molecular weight of about 1,000 in order to not impair too much the usability. In particular, molecules that contain hydrogen bridge active hydrogen atoms should only make up not more than about 3 % by weight of the ink, preferably not more than 1.5 %, if their molecular weight is higher than about 15,000, in order to maintain the usefulness of the ink for a high resolution printing. Lower proportions of these substances having a high molecular weight down to 0 % yield a continuous decrease of the detrimental influence of the hydrogen bridge active substances. A decrease of the molecular weights given above as reference points to lower values also improves the advantageous behavior. Since, on one hand, glycol and, on the other hand, polymers having an average molecular weight of about 1,000 and with two terminal hydroxy or carboxy groups have been proven to be acceptable additives, and supposing that the hydrogen bridge active hydrogen atoms stem exclusively from hydroxy groups, a general rule for the expression "appreciable quantity of hydroxy groups" as used above can be given, namely a value of more than 5 % of the total number of the atoms in a molecule. Additives wherein more than 5 % of all atoms are hydrogen atoms that are capable of forming hydrogen bridges must therefore not have a molecular weight of more than 1,000, preferably of more than 500. It is also advantageous if this limitation is also observed for substances having a lower proportion of hydrogen bridge active hydrogen atoms. A certain intermediate position is taken by substances which do not contain hydrogen bridge forming hydrogen atoms but contain positions which may participate in hydrogen bridging, for example ether linkages. The presence of the following usual ink additives has proven particularly disadvantageous: polyvinyl alcohol, hydroxylalkylcelluloses, e.g. hydroxylpropylcellulose, and polyvinyl pyrrolidone. Another parameter of great importance is the size of the particles. Since the colouring agents are milled, a mixture of particles of the most different sizes is obtained; the mixture may be characterized by an average particle size and a distribution of the particle sizes around an average value. As a general rule, an approximately binomial distribution is obtained. In the following, the averages and the distribution widths are given by volume if not otherwise indicated. It has now been found that particles having a diameter of more than 200 nm are disadvantageous, and particles having a diameter exceeding 300 nm must only be present in very small amounts. The average particle size should preferably not exceed 100 nm and should better be smaller. As a general guideline for a good ink, average particle sizes of from 68 to 81 nm in connection with distribution widths of 61 to 63 nm should be used. These values are determined as described below. The invention will now be explained in further detail by way of the following description of particular exemplary embodiments and of the Figures in the drawing. Brief Description of the Drawings In the drawing: Fig. 1 shows the determination of the broadening proportion R = d|| /di_, Fig. 2 and 3 show stroboscope photographs of an ink droplet jet, and Fig. 4 to 6 show size distributions of colouring agent particles. Detailed Description of the Invention The property of a base composition optimized according to the above described findings is shown in Fig. 1. For the establishment of the outlined data, a currently employed continuous ink jet printing apparatus was used. The ink jet is pressed through a nozzle which is coupled to a piezoelectric oscillator. The oscillator is excited with the desired droplet frequency and puts the nozzle in oscillations which dissolve the ink jet leaving the nozzle into droplets. The droplets are observed by means of a stroboscope and a video camera. The stroboscobe produces light flashes having the droplet fequency with which the ink jet is illuminated (M. Ronay, J. of Colloid and Interface Science 66/1 (1978), 55). Depending on the particular ink, droplets with sharp outlines are observed, and the droplets appear to get more and more broadened in the direction of flow. This broadening is expressed as the relation R between the width d|| and the height d_ι_ (Fig. 1). The measuring is made in a predetermined distance dø s fro the exit opening of the nozzle. In the frame of this specification, the readings are taken at a distance d 0 b s = 8 mm. A value of R = 1 is the optimum in all cases, which is reached by, e.g., water. Under the above mentioned measuring conditions (a distance of 8 mm from the nozzle), an ink showing values up to R = 1.1 is to be considered as very good, i.e. even at droplet rates of more than 100,000 s ~1 up to 1' 000,000 s" 1 and nozzle diameters of less than the usual 75 μm, in particular smaller than 50 μm and even about 20 μm. Stroboscope photographs are shown in Fig. 2 for R=1 and in Fig. 3 for R=1.4 The determination of the particle sizes was made by means of PCS (photon correlation spectroscopy) . The measuring apparatus was a MALVERN MASTERSIZER LOC (MALVERN). The distribution of the particle sizes was determined according to the method of Pike-Ostrowsky. By this measuring apparatus, the volume fraction of the particles in dependence of their size is determined. The integration of the thereby obtained distribution of the particle sizes yields the total volume of particles in the test sample. Accordingly, the following data as to the fraction of particles of a certain size refer to the total volume of particles. The following Table shows the dependence of R on the average diameter d of the particles and the distribution width: Fig. d distribution R [nm] width [nm] 4 68 61 1 5 81 63 1.1 6 139 150 1.3 Table 1 : Influence of particle size and distribution on R It follows from the Table that not more than 10 % of the colouring agent particles relative to the total volume of colouring agent particles may be greater than 200 nm, and not more than 3 % may be greater than 300 nm. Preferably, not more than 5 % are greater than 200 nm, and not more than 1 % are greater than 300 nm. Particularly preferred axe maximal proportions of 2 % greater than 200 nm, and a nearly insignificant proportion (< 0.5 %) greater than 300 nm, or even narrower distributions. The average size of the particles is preferably situated below 100 nm. Further improvements are possible by a further reduction to below 80 nm, especially in the range of from 60 to 70 nm or lower, if possible with parallel reduction of the distribution range of the particle sizes to preferably less than 100 nm, particularly to less than 70 nm. Table 2 shows the influence of some possible additives to continuous jet inks as the change of the relation R due to these additives. As to the indicated molecular weight averages, a distribution can be supposed where at least 95 % are situated in the range of ±10 % of the average value. Broader distributions may render compulsory the use of an additive having a lower weight average molecular weight, especially when the proportion of molecules having a higher molecular weight increases due to the greater variation range. additive c R (weight %) hydroxypropyl cellulose (MW = 60,000) 3 >3 polyvinyl alcohol (MW = 17,000) 3 1.3 (MW = 37,000) 3 1.7 (MW = 61,000) 3 >3 polyvinyl pyrrolidone (MW = 55,000) 3 1.7 Table 2: c: concentration of the additive, related to the total amount; MW: weight average molecular weight The hydroxyl group containing additives hydroxylpropylcellulose and polyvinyl alcohol have a strong influence on R already at low concentrations. Polyvinyl pyrrolidone behaves simularly; it does not contain hydrogen atoms obviously capable of forming hydrogen bridges, but it is able, by its ring nitrogen atom and the keto group, to participate in a hydrogen bridge. The effect seems thus to be smaller than with polyvinyl alcohol for example. The effect is less pronounced with additives preferably containing ether bridges, for example polyethylene glycol and nonionic surfactive agents. The compounds glycerol and d-fructose could be added until an amount of 10 % without an essential influence on R although they contain hydroxyl groups. As a result, it can be stated that hydroxyl group containing compounds and also amines and similar compounds should preferably only be added as low molecular additives and especially in unimolecular form. Compounds containing hydrogen bridge acceptors (e.g., oxygen in ether bridges) show a smaller effect on the visibility length and can be added as polymers of lower to moderate molecular weight. If the hydrogen bridge acceptor effect is weak, polymers having increasing molecular weights can be used. The invention will now be explained further by way of examples. These examples are not construed to limit the invention thereto, and the one skilled in the art will be aware of modifications and extensions without leaving the scope of the invention. Example 1 As a general example, an ink which contains an insoluble coulouring agent in dispersed form and which can be used for the continuous ink jet process, is composed as follows: Dispersed dyestuff and/or pigment 4 % glycol 4 % nonionic tenside 5.5 % antifoaming agent 0.03 % water remainder to 100 %. It has been possible, starting from the basic ink compositions of this invention, to produce inks which are well suited for the use in the continuous ink jet process, having a dyestuff or pigment content, respectively, of up to 10 %, referred to the total weight of the ink. Experience shows that such inks should contain at least from 3 to 6 % of dyestuff, values which are reached and exceeded with the present inks. As nonionic surfactants in the above Example, those of the following list may be used. Tensides, antifoaming agents and also replacement substances for glycol can be found in the general knowledge on dispersion inks, the prejudice against the use of dispersed pigment dyestuffs in continuous ink jet inks having been surmounted by the present invention. Possible alternatives of the additives can for example be found in the publication by A. M. Wells, Printing inks: Recent developments, Noyes Data Corporation, 1976, p. 109 which is herewith incorporated by reference. A non-exhaustive list of components indicated by way of example which may be present in the ink of this invention, is now to follow. The designations which are given are in most cases product names of the manufacturers. Colouring agent: dyestuffs Lurafix Blue 590; Lurafix Blue 660; Lurafix Red 420; Lurafix Yellow 138: BASF, Ludwigshafen, Germany; Sublaprint Navy 70017, Sublaprint Orange 70007, Sublaprint Orange 70020, Sublaprint Orange 70040, Sublaprint Red 70011, Sublaprint Scarlet 70021, Sublaprint Yellow 70000, Sublaprint Yellow 70004, Sublaprint Yellow 70064, Sublaprint Yellow 70069, Sublaprint Blue 70013, Sublaprint Blue 70014, Sublaprint Blue 70032, Sublaprint Blue 70037, Sublaprint Blue 70044, Sublaprint Crimson 70010: Hollidays Dyes & Chemicals, Huddersfield, UK; Colouring agent: pigments Irgalite, Cromophtal: Ciba-Geigy AG, Basel, Switzerland; Hostaperm, Novoperm, Permanent: Hoechst AG, Frankfurt-αn-the-Main, Germany; Drying retarders Dipropylene glycol, 1 ,2-propylene glycol. Tensides Nonionic tensides of the type HO(CH 2 0CH2θ) x (CH(CH3)CH2θ) y (CH2CH2θ) z H; preferably x, y and z have values such that the molecular weight (weight average) of the substance is about 6,500; Imbentin product line, especially Imbentin-PAP, Imbentin-TAM (tallow amine ethoxylate) : Dr. W. Kolb, Hedingen, Switzerland; Surfynol: Air Products and Chemicals Inc., Allentown, Penn., USA; Atlox and Atlas product line: Imperial Chemical Industries PIC, London, UK. Anti-foaming agents Drewplus T4202, TS4384, W4311, W4510: Drew Ind. Division, Drew Chemical Corp., Boonlon, N.J., USA; Byk 032: Byk Mallinckrodt. In the following, particularly preferred formulations and production methods of the inks according to this invention are given. The percentages which are given here and in the following refer to the weight of the ink if not otherwise indicated. Example 2 Component amount Dyestuff DR 60 (Color Index) 3 % dipropylene glycol (drying retarder) 2.8 % emulsifier WNS (Bayer AG, Leverkusen, Germany) 4 % antifoaming agent RD (Rohm GmbH, Darm¬ stadt, Germany) 0.03 % water remainder to 100 % Example 3 Component amount Phthalocyanine pigment 5 % dipropylene glycol 4 % Imbentin PAP (nonionic tenside) (Dr. W. Kolb AG, Hedingen, Switzerland) 8 % antifoaming agent RD 0.03 % water remainder to 100 % The colouring agents are reduced to a powder having a particle size of less than about 200 nm and preferably less than 100 nm. Mills to be used for this comminution step are known, e.g. ball mills. /02599 PCMB95/00567 - 1 5 - The inks of this invention can also be manufactured by methods principally known in the technique of ink production. A method which can be used for example is the following: - Combining the ingredients and blending in adding a certain amount of water for adjusting the viscosity necessary for the following grinding; grinding until the desired fineness in a conventional mill, especially a ball mill; - transfer into a mixer and addition of water until the water content according to the formulation is reached. In order to obtain the preferred particle fineness of the colouring agent, it may be necessary to grind the mass several times on different mills so that the initial coarse comminution progresses to a fine grinding. It is further preferred to provide a particle size selection after the grinding process in order to ascertain a sufficient narrow distribution of the particle sizes and to reduce before all the proportion of too coarse particles. Methods known per se may be used in this step which therefore do not need to be described in detail. Generally, it can be expected that inks which contain the colouring agents or part of them in another form than dissolved, for the continuous ink jet method can be formulated when additives according to the invention are selected and/or by employing colouring agent particles of the proposed size distribution. Besides of hydroxy groups, amino groups are in particular to be considered as hydrogen bridge active. The inks of the invention can generally be used for the printing on all substrates appropriate for the continuous ink jet printing, especially also on textile materials and sheet and web substrates, e.g. of paper, and for the production of thermotransfer patterns and designs. Since the present inks offer the possibility to use them up to high and the highest droplet rates of more than 100,000 s ~1 , in particular up to 1 '000,000 s ~1 and more, and at small droplet diameters by the use of narrow nozzles of less than 50 μm, especially down to 20 μm, absolutely exact and precise printings having a high contour sharpness can be produced. It is also possible to control the intensity of the colour shade of the ink by a more or less dispersed but even and regular application of the ink droplets without rendering visible a screen or halftone by the naked eye at normal observation. This renders possible the production of high quality polychrome printings having nearly continuous coulour graduations and shades.

Claims: Claims 1. An ink for continuous ink jet printing, containing - water as a major liquid component, at least one dispersed colouring agent in particle form, selected from the group comprising pigments and water insoluble dyestuffs, and additives for stabilizing and/or adjusting the physical properties of the ink, especially its rheology, wherein substantially all these additives have a weight average molecular weight below about 20,000, - first additives wherein hydrogen atoms which are capable of forming hydrogen bridges, especially hydrogen atoms of hydroxyl or amino groups, represent more than about 5 % of the number of the atoms of the molecule, have a weight average molecular weight below about 1,000, and - the particles of said colouring agents have such a size distribution that relative to the total volume of the particles of the colouring agents, less than 10 % is greater than 200 nm and less than 3 % is greater than 300 nm. 2. The ink according to claim 1, wherein not more than 5 %, preferably not more than 2 %, of the total volume of the particles of the colouring agents is greater than 200 nm, and not more than 1 %, preferably not more than 0.5 %, is greater than 300 nm. 3. The ink according to claim 1 or 2, wherein the average particle size of the total of said colouring agents is less than or equal to about 100 nm and is preferably situated in the range of less than 80 nm, in particular in the range of from about 60 to about 70 nm. 4. The ink according to any one of claims 1 to 3, wherein the distribution width of the particle size of the total of said colouring agents is smaller than or equal to about 100 nm. 5. The ink according to claim 4, wherein said distribution width is smaller than about 70 nm. 6. The ink according to any one of claims 1 to 5, wherein the proportion of the total of colouring agents is about 1 to 10 % by weight and preferably about 3 to 6 % by weight of the ink. 7. The ink according to any one of claims 1 to 6, wherein the total of said first additives supply to the ink not more than 3 % by weight of molecules having a molecular weight greater than about 15,000 and not more than 1 % by weight of molecules having a molecular weight greater than about 40,000. 8. The ink according to claim 7, wherein the said first additives have a molecular weight of less than 1,000, preferably of less than 500. 9. The ink according to any one of claims 1 to 8, wherein the ink is free from polyvinyl alcohol, cellulose derivatives having free hydroxyl groups, and polyvinyl pyrrolidone. 10. The ink according to any one of claims 1 to 9, containing at least one antifoaming agent, a viscosity controlling additive, and an additive for stabilizing the distribution of the colouring agent within the ink. 11. The ink according to claim 10, containing a nonionic tenside in a concentration in the range of from 0.5 to 10 % by weight, preferably in the range of from 1 to 5 % by weight. 12. A process for the high resolution continuous ink jet printing comprising droplet rates of more than 100 kHz, particularly of from 10-^ to 10^ Hz, and/or nozzles for the generation of the ink jet, having a diameter of less than or equal to 50 μm, wherein an ink is used according to any one of claims 1 to 11. 13. The process according to claim 12 or 13, wherein textile substrates are printed. 14. The process according to claim 12 or 13, wherein multicolour thermotransfer patterns or direct multicolour printings are produced.

Inventors: AMON ALBERT, BOKSANYI LASZLO, GRUFFEL PASCAL, AMON, ALBERT, BOKSANYI, LASZLO, GRUFFEL, PASCAL
Applicants: AMON ALBERT, BOKSANYI LASZLO, GRUFFEL PASCAL, SICPA HOLDING SA, AMON, ALBERT, BOKSANYI, LASZLO, GRUFFEL, PASCAL, SICPA HOLDING S.A.
Application year: 1995
Main classification: C09D11
IPC codes: B41J2, B41M5, C09B67, C09D11, D06P5