WO2009034394A1

WO2009034394A1 - Inkjet ink composition

Info

Publication number: WO2009034394A1
Application number: PCT/GB2008/050823
Authority: WO
Inventors: Muhammad Karim Ali
Original assignee: Xennia Technology Limited
Priority date: 2007-09-13
Filing date: 2008-09-15
Publication date: 2009-03-19
Also published as: GB0717856D0

Abstract

There is provided an ink composition suitable for use in inkjet printing, particularlyin inkjet printingon textile substrates, comprising: a polymer binder, preferably a self-crosslinking acrylic polymer binder; a liquid vehicle comprising one or more solvents having a boiling pointin excess of 100°C, preferably in excess of 150°C, in which the binder is dissolved, e.g. diethylene glycol mono-n-butyl ether and γ-butyrolactone; and pigment. The invention can providean inkjet ink with goodprint reliability and excellent decap performance that permits printed textiles to be obtained having excellent wash-fastness and crock-fastness characteristics.

Description

InkJet Ink Composition

Field of the Invention

The present invention relates to inkjet inks, and has particular, but not exclusive, application to inkjet inks adapted for use in textile or fabric applications. The terms textile and fabric are used synonymously herein.

Background of the Invention

Inks for printing on textiles are commonly applied using a screen printing process. These inks are in general relatively viscous, and can include a wide variety of pigment types, including those of relatively large particle size. The ink is usually present in the form of thick deposits on the material. Because of the need for the printed fabrics to endure, for example, high temperature washing and dry cleaning, it is desirable for the inks to have good high temperature resistance and chemical resistance, having good wash- fastness. A further requirement is resistance to rubbing, often referred to as crock-fastness.

The use of inkjet printing for printing upon textiles has been employed. InkJet printing is a nonimpact method in which small droplets of ink are directed from a nozzle onto a printable porous or non-porous substrate.

InkJet printing processes fall into two main types: continuous processes and drop-on-demand (DOD) processes. Continuous processes use electrically conductive inks to produce a stream of electrically-charged ink drops that are deflected by an electric field to an appropriate location on a substrate. On the other hand, in DOD processes, individual drops of ink are expelled from the nozzle of a printhead either by vibration of a piezoelectric actuator (in piezoelectric inkjet printing) or by heating the ink to form a bubble (in thermal inkjet printing, also known as bubblejet printing). Jet velocity, separation length of the droplets, drop size and stream stability are all greatly affected by the surface tension and the viscosity of the ink. Pigment-based ink jet inks suitable for use with inkjet printing systems should typically have a surface tension in the range of about 20 dynes/cm to about 70 dynes/cm at 25⁰C, and more preferably, in the range 25 dynes/cm to about 45 dynes/cm at 25⁰C, and have a viscosity preferably no greater than 30 mPa-s, and preferably in the range of about 5.0 mPa-s to about 10 mPa-s at jetting temperature. In contrast to screen printing, inks for use in inkjet printing are required to have a relatively low viscosity and small particle size in order to have satisfactory jetting characteristics.

InkJet printing upon textile materials has been successfully accomplished on fabrics using dye- based inks. In many cases, however, the enduring light-fastness that is required in the use of the printed textile materials is not achieved or is not satisfactorily achieved with such inks. In some cases, satisfactory light-fastness is achievable only with the use of certain fabric pre-treatments and/or post-treatments.

Pigment-based inks can also be used in inkjet printing, for printing textiles using digital technology. Such inks include pigment particles and a polymer binder, with the binder acting to bind the pigment particles to a substrate after fixing, typically by exposure to heat to cause cross- linking of the binder. Pigments are intrinsically more light-fast and wash- fast than dyes, and are often less expensive. However, pigment-based inks tend to flow less well than dye-based inks, which is of relevance when delivering ink through an inkjet printhead. Advances in pigment dispersion techniques and printheads have assisted in mitigating this problem. Furthermore, technological advances in digital printing have led to improvements in the way that pigment- based inks adhere to the surface of the fabric. Pigment particles can be coated with advanced surfactants in order to improve dispersion stability, and the same technology is used to help the inks adhere properly to the fabric. Pigments have a number of advantages over dyes in inkjet printing, and these benefits have contributed to the growth in demand for pigment-based inks. For example, pigment-based inks can be printed on to a broad variety of fibres and fabrics, whereas dye-based inks are restricted to specific types of fibres and fabrics. In addition, pigment-based inks are more robust than dye-based inks. Furthermore, pigment based inks do not need to go through a fixation process using steam to ensure that the ink adheres to the fabric. This makes pigment-based printing more economical in terms of running costs. Moreover, textiles which have been coloured with pigment-based inks have higher levels of weather- fastness and light-fastness than those coloured with dye-based inks. Consequently, they are particularly suitable for large fabrics which are printed for outdoor use.

However, pigment-based inkjet inks incorporating a polymer binder system have a tendency to dry out and suffer from the problem of latency or "decap". Decap time is defined as the amount of time a printhead can be left uncapped and idle and still fire a drop properly without loss of velocity or misdirection. Loss of performance recoverable by "servicing" of a printhead is known as "soft failure", while loss of performance not recoverable by "servicing" is known as "hard failure". In the case of film formation, this results in permanent nozzle loss i.e. hard failure. A longer decap time is therefore desirable because it allows greater productivity by reducing the need for "servicing" the printhead. Decap is at least in significant part caused by ink vehicle evaporation which leaves behind a deposit of non- volatile ingredients that are detrimental to jetting performance. This is particularly noticeable when highly coloured and chromatic images are required, since more colourant must be incorporated into the ink. High colourant concentrations can lead to problems with print reliability and decap as the ink dries in the nozzle and the pigment molecules aggregate blocking the nozzle or causing misdirected drops. This problem is further exacerbated by the small diameter of the printhead nozzles designed to deliver small volume drops. The smaller nozzles can clog more easily and, in addition, require a greater force to clear any high viscosity blockage.

One way to improve decap times is to add large concentrations of non- volatile humectants to the ink to retard drying in the nozzles. However, this also has the undesirable effect of slowing the drying of the ink on the substrate such that heating by exposure to a suitable stimulus e.g. heat (air flow), infra red heating, heat press, or steam is required. Moreover, the addition of very high levels of humectant tends to make the ink too viscous for most inkjet applications. Formulating inks containing pigment and binder that are suitable for inkjet printing present considerable practical difficulties.

US 6341856 discloses an inkjet ink including a dye or pigment, that may include a binder. The ink is in the form of an emulsion or emulsion-like system. The document does not include any specific examples, and does not address the problem of formulating useful inkjet inks containing pigment, particularly problems of decap time.

WO 2005/113692 discloses an inkjet ink including polymer binder, preferably a polyether polyurethane, pigment and solvent. The preferred solvent is water, and no other solvents are mentioned. All of the examples are aqueous ink formulations. The document does not address problems of decap time. EP 1724311 discloses an aqueous inkjet ink comprising resin emulsion and pigment. The inks are stated to have good decap performance. The exemplified inks contain at least 40% by weight water.

US 2005/0039634 discloses a substantially non-aqueous inkjet ink comprising non-aqueous solvent, colourant (dye or pigment) and non-ionic surfactant to improve decap times. A very small amount of binder may be included for printing on paper. The document does not address the problem of printing on textiles, where larger amounts of binder are required.

US 2005/0065234 discloses an inkjet ink comprising an emulsion polymer binder, e.g. acrylic butadiene and cross-linking agent, with pigment and a liquid medium, generally water, with the water content of the ink being up to 95% by weight. The document does not address the problem of decap time.

Summary of the invention

According to the present invention there is provided an inkjet ink comprising polymer binder; a liquid vehicle comprising one or more primary solvents having a boiling point in excess of 100⁰C in which the binder is dissolved; and pigment.

The binder is dissolved, fully or partially, in the primary solvent or solvents of the liquid vehicle. Where the binder is fully dissolved this produces a clear solution. Where the binder is partially dissolved this produces a cloudy, translucent or opalescent solution, with the binder in a state of dynamic equilibrium between fully dissolved and partially dissolved states. In contrast, where the binder is insoluble, this produces a turbid or opaque liquid. Solubility is determined at 25°C unless otherwise specified. The pigment is in the form of fine particles, which are carried by the binder solution.

In use, the ink of the invention is applied to a substrate, particularly a textile, by inkjet printing, particularly drop-on-demand printing, especially piezoelectric printing. The binder is allowed or caused to cure, typically by exposure to heat in the case of a thermally curable binder resulting in cross-linking of the binder. A typical curing regime is exposure to a temperature in the range 180-200⁰C for a time in the range 90-180 seconds. The binder acts to bind the pigment particles to the substrate. The printhead operating temperature is typically higher than 25°C, varying with different pieces of equipment, so that the binder is also soluble in the liquid vehicle (possibly to a greater extent) at the printhead operating temperature.

By using a non-aqueous liquid vehicle comprising one or more primary solvents having a boiling point in excess of 100⁰C, the invention can provide inkjet inks that can have good jetting reliability and excellent decap properties.

An ink in accordance with the invention can produce images on textiles with very good or excellent wash-fastness and crock-fastness properties, comparable to those achieved using a conventional screen ink applied by screen printing.

The present invention also provides an ink composition suitable for use in inkjet printing on textile substrates, comprising:

(a) a polymer binder;

(b) a liquid vehicle; and

(c) a colourant, particularly a pigment, wherein the polymer binder is partially or fully solubilized in the liquid vehicle, where, mixing with the colourant provides an inkjet ink with excellent decap performance that permits printed textiles to be obtained having excellent wash- fastness and crock-fastness characteristics.

Preferably, the polymer binder is a self-crosslinking polymer binder, more preferably a self- crosslinking polymer binder selected from the group including the following: acrylics, styrene- acrylic ester copolymers, styrene-butadiene copolymers, vinyl acrylic and vinyl acetate copolymers, and ethylene vinyl acetate copolymers. The polymer binder is preferably a self- crosslinking acrylic binder. The term self-crosslinking polymer binder means a binder capable of undergoing reaction with itself to crosslink, typically an exposure to heat, without requiring an external crosslinking agent.

The polymer binder is preferably present in the ink in an amount of 1 % to 20 % by weight, more preferably, 2 % to 10 % by weight, based on the total weight of the ink. A mixture of polymer binders may be used.

The polymer binder will be discussed in more detail below.

The pigment is preferably a self-dispersible pigment, dispersible in an aqueous or non-aqueous medium.

A mixture of pigments may be used.

The weight ratio (based on active material, excluding carrier liquids etc.) of polymer binder, e.g. self-crosslinking polymer binder, to pigment is preferably in the range 1:1 to 2: 1. If there is insufficient binder the pigment is likely to be insufficiently bound to the substrate, leading to poor crock-fastness. If there is too mucy binder, the ink is more likely to dry out in the printer nozzles, leading to poor decap times.

The pigment will be discussed further below.

The liquid vehicle comprises one or more primary solvents having a boiling point in excess of 100⁰C. The primary solvents are preferably selected from the following classes: aliphatic alcohols, glycol ethers, lactones and pyrrolidones, with the solvents of all four classes being capable of solubilizing the polymer binder. The liquid vehicle conveniently comprises a mixture of primary solvents, preferably from more than one of the specified classes.

One or more aliphatic alcohols are suitably present in an amount in the range 1 to 40%, more preferably in the range 1 % to 35 %, yet more preferably 25 to 35% by weight, based on the total weight of the ink.

One or more glycol ethers are suitably present in an amount in the range 1.0 % to 20.0 %, more preferably in the range 2.0 % to 15.0 %, yet more preferably 10.0 % to 15 % by weight, based on the total weight of the ink. Preferably, the one or more glycol ethers comprise diethylene glycol mono-n-butyl ether (DEGMBE). One or more lactones are suitably present in an amount in the range 5 % to 25 %, more preferably in the range 10 % to 20 % by weight, based on the total weight of the ink. Preferably, the one or more lactones comprise γ-butyro lactone (GBL).

One or more pyrrolidones are suitably present in an amount in the range 1 to 5% by weight, based on the total weight of the ink. Preferably the one or more pyrrolidones comprise n-methyl- 2-pyrrolidone (NMP).

The primary solvent or solvents of the liquid vehicle are selected having regard to the polymer binder, particularly with respect to solubility properties, to produce an ink having desired properties and performance.

The liquid vehicle will be discussed in further detail below.

The ink of the present invention may include one or more optional additional ingredients such as humectants, surfactants, defoamers, dispersants, biocides, and thickeners etc. as is well known in the art. Humectants are used to retard drying of the ink and so improve decap times, and humectants and co-solvents may be used to adjust ink properties such as viscosity and surface tension. In this way an ink may be produced having appropriate and desired properties, such as a viscosity of not more than 30 mPa-s at jetting temperature, a surface tension in the range 25 to 45 dynes/cm, and a pH in the range of 4 to 9, more preferably in the range of 6 to 8.

Such optional additives will be discussed further below.

In another aspect of the present invention, there is provided an inkjet ink for use in a process of inkjet printing upon a textile material substrate, comprising a self-crosslinking binder polymer, a liquid vehicle, and a colourant, particularly a pigment, the ink having a viscosity of 2 to 30 mPa-s, as measured at the normal operating temperature of the nozzle, and a surface tension on deposition between 25 and 45 dynes/cm.

In a yet further aspect of the present invention, there is provided a method for inkjet printing onto textiles, comprising the steps of:

(a) providing an inkjet printer that is responsive to digital data signals; (b) loading the printer with a textile to be printed;

(c) loading the printer with an inkjet ink in accordance with the invention; and

(d) printing onto the textile using the inkjet ink in response to the digital data signals.

The ink composition of the present invention can be prepared by partially or fully solubilizing the polymer binder in the primary solvent or solvents, adding a pigment, and preferably adding one or more humectants. Additional optional ingredients such as surfactant etc. can be added if desired. An ink can be produced with physical properties compatible with a wide range of ejecting conditions, i.e. driving voltage and pulse-width for thermal inkjet printing devices, driving frequency of the piezo element for either a drop-on-demand device or a continuous device, and the shape and size of the nozzle. The inks have excellent storage stability for a long period and do not clog in an inkjet apparatus. Further, the ink does not corrode parts of the inkjet printing device it comes into contact with, and it is essentially odourless and non-toxic.

Although not restricted to any particular viscosity range or printhead, the ink composition of the present invention is particularly suited to lower viscosity applications such as those required by printheads that jet small droplet volumes, e.g. less than 10 pL. Therefore, in order for the ink to be discharged properly from the nozzle of an inkjet printhead without clogging, the quantities of the various components are adjusted so that the viscosity of the ink will range from 2 mPa-s to 30 mPa-s, more preferably ranging from 6 mPa-s to 15 mPa-s at jetting temperature. The ink of the present invention is specifically suitable to jet reliably at frequencies up to 50 kHz, more preferably, at frequencies in the range of 5 kHz to 35 kHz.

Liquid Vehicle

The term "liquid vehicle" refers to a liquid that is substantially comprised of solvent, or a mixture of solvents, where the solvent can be either polar or nonpolar. Solvents suitable for use in the inks of the present invention include but are not limited to: aliphatic alcohols, glycol ethers, lactones and pyrrolidones.

The liquid vehicle includes one or more primary solvents which function as a solvent for the polymer binder in the inkjet ink composition. The amount of primary solvent employed is sufficient to solubilise or at least partially solubilize the polymer binder. The liquid vehicle can be non-polar, and thus designed to be miscible with other non-polar components, or polar, and therefore miscible with more polar components. Preferably, the primary solvents are water-miscible, so that the ink can incorporate aqueous formulations of binder and pigment.

Preferably the one or more primary solvents have a boiling point in excess of 150⁰C, more preferably in excess of 200⁰C, for improved decap performance.

The liquid vehicle employed in the ink compositions of the present invention preferably comprises one or more glycol ethers, as these solvents have low viscosities and high boiling points. Further, glycol ethers may also function as humectants, preventing drying in the printhead nozzles. Suitable glycol ethers include, but are not limited to, the following: diethylene glycol mono-n-butyl ether (DEGMBE), propylene glycol n-butyl ether, propylene glycol methyl ether, propylene glycol ethyl ether, dipropylene glycol methyl ether, dipropylene glycol n-butyl ether, propylene glycol methyl ether acetate, dipropylene glycol methyl ether acetate, propylene glycol n-propyl ether acetate, dipropylene glycol n-propyl ether, ethylene glycol n-butyl ether (or ethylene glycol monobutyl ether (EGMBE)), diethylene glycol n-butyl ether, triethylene glycol butyl ether, diethylene glycol methyl ether, tripropylene glycol monomethyl ether (TPGME), tripropylene glycol n-butyl ether (TPGBE), and triethylene glycol methyl ether. Glycol ethers may also function as humectants, preventing drying in the printhead nozzles, and can be used either as dual function materials (primary solvent and humectant), as a primary solvent only or as a humectant only, depending on the particular glycol ether and binder combination.

The liquid vehicle may additionally or alternatively comprise one or more aliphatic alcohols, in particular aliphatic alcohols having one, two or three hydroxyl groups. Suitable aliphatic alcohols include, but are not limited to, the following: ethylene glycol (EG), propylene glycol (PG), dipropylene glycol (DG), tripropylene glycol (TG), 1,3-butanediol, 1,4-butanediol, diethylene glycol (DEG), triethylene glycol (TEG), tetra-ethylene glycol (TTEG), 1-butanol, and glycerol. These aliphatic alcohols may also function as humectants, and so can be used as dual function materials (primary solvent and humectant), as primary solvent only or as a humectant only, depending on the particular alcohol and binder combination. The liquid vehicle may additionally or alternatively comprise one or more lactones and/or one or more pyrrolidones, as these solvents have low viscosities and high boiling points. Suitable lactones and pyrrolidones include, but are not limited to, the following: caprolactone, priopio lactone, γ-butyro lactone (GBL), and substituted compounds thereof (3- and 5-methylated γ-butyrolactone, ethylated γ-butyro lactone, propylated γ-butyrolactone), 2-pyrrolidone, N- methyl-2-pyrrolidone (NMP), and substituted compounds thereof.

Details of various materials suitable for use as solvents (solvents for the binder, and possibly also as humectants) in inks in accordance with the invention are given in the table below.

The relative evaporation rate (ER) of solvents is determined relative to a standard, n-butyl acetate, which is assigned a value of 1.0. Solvents that evaporate slower than this standard receive an ER less than 1.0. Preferred solvents have a value of ER between about 0.004 and about 1.0, with solvents having higher values of ER being useful especially when combined in a liquid vehicle with solvents of lower ER.

Good results have been obtained using a mixture of solvents from different classes, to give a good overall balance of properties.

In particular, good results have been obtained using diethylene glycol mono-n-butyl ether (DEGMBE) and γ-butyro lactone (GBL), separately or together, as primary solvents. Both of these materials have high boiling points and good solvency properties.

Good results have been obtained using a mixture of polyethylene glycol (PEG), ethylene glycol (EG), tetra-ethylene glycol (TTEG), 1,3-butanediol and glycerol as humectants.

The one or more solvents (excluding solvents which also function as humectants) are preferably present in an amount of at least 30% by weight, based on the total weight of the ink.

The ink preferably has a total content of solvent having a boiling point in excess of 100⁰C, preferably in excess of 150⁰C. of at least 70% by weight, based on the total weight of the ink.

Polymer Binder

The polymer binder is a film-forming substance made up of long-chain macro molecules which, when applied to a textile, together with the pigment during a suitable fixation process, produces a three-dimensionally linked network. Crosslinking of the polymer binder serves to provide elasticity and improved adhesion of the polymer binder film containing the pigment. The crosslinking reaction must produce covalent bonds which are insensitive to hydrolysing agents such as detergents, body sweat and industrial atmospheres.

Polymer binder properties, particularly film softness, flexibility, tensile strength and durability, are related to the glass transition temperature (Tg) of the monomer unit selected to form the polymer. Generally, the lower the (Tg) of the monomer units, the softer is the resulting polymer. When the inks of the present invention are printed on textiles, the Tg of the binder therefore affects the stiffness and feel to the touch of the printed fabric, which can be classified by so- called textile hand ranges. It is preferred to use a binder having a Tg < 40⁰C more preferably < 25°C, yet more preferably < 0⁰C. A binder can be selected having regard to the intended use of the ink, e.g. those for printing on clothing fabrics where a soft feel is required should have a low Tg, e.g. < 20⁰C, while those intended for printing on furnishing fabrics should have a higher Tg for greater stiffness and durability. A wide range of polymer binders are commercially available having a range of Tgs. The binders are generally available in liquid form, as solutions, dispersions and emulsions, commonly in water but also in other solvents and carrier liquids. Polymer binders based on acrylic materials offer the greatest durability, colour stability, and dry/wet performance. Acrylic binders have the widest range of fabric hand properties and can be formulated to vary from very soft (Tg = - 40⁰C) to extremely hard (Tg = 105⁰C). These binders can be used in virtually all non-woven textile applications, where they can be made to crosslink with substantial improvement in durability. Styrenated acrylic binders are tough, hydrophobic binders which resulting textile hand ranges from soft-to-firm (Tg varies from -20⁰C to +105⁰C). These binders can be used in applications where there is a need for some wet strength without crosslinking. Vinyl acetate binders are firm (Tg = +30⁰C to +40⁰C) and offer good dry strength and toughness, but are somewhat hydrophilic and have a tendency to yellow when subjected to heat. Vinyl acrylic binders are more hydrophobic than the straight vinyl acetate binders. They provide excellent toughness, flexibility, and better colour stability. The hand range is limited to intermediate softness (Tg = -10⁰C) to a firm hand (Tg = +30⁰C). Ethylene vinyl acetate binders have a (Tg range of-20°C to +115°C), which is equivalent to soft ranging to an intermediate textile hand. They exhibit high wet strength, coupled with excellent absorbency. In general, they are less costly than acrylics. They are used primarily in wipes, air- laid pulp fabrics and similar applications. Styrene-butadiene binders have an excellent combination of flexibility and toughness, and range in hardness from very soft (Tg = -30⁰C) to very firm (Tg = +80⁰C). However, the Tg of a styrene-butadiene binder is not strictly comparable to other classes of non-woven binders. The styrene-to -butadiene ratio (S/B ratio) is the most common method for describing the relative hand resulting from the use of these binders. When cross-linked, this class of binder is very hydrophobic and durable.

As noted above, it is preferred to use a self-crosslinking binder, particularly a self-crosslinking acrylic binder. The self-crosslinking binder preferably has a minimum film forming temperature (MFFT) below 50⁰C, more preferably below 25°C.

Suitable commercially available self-crosslinking binders include: LYOPRINT PBA (Huntsman Textile Effects (Germany) GmbH), RHOPLEX E-2780 (Rohm & Haas), RHOPLEX TR-407 (Rohm & Haas), TEXICRYL 13-216 (Scott Bader), TEXICRYL 13-737 (Scott Bader), TEXICRYL 13-326 (Scott Bader), TEXICRYL 13-297 (Scott Bader), TEXICRYL 13-205 (Scott Bader), TEXICRYL 13-291 (Scott Bader), STYCAR SB-1168 (Lubrizol Corp.), STYCAR SB- 0706 (Lubrizol Corp.), VYCAR VA-0450 (Lubrizol Corp.), LURAPRET D 420 (BASF), LURAPRET D 471 (BASF), LURAPRET D 579 (BASF), LURAPRET D 2337 (BASF) and LURAPRET D 2373 (BASF). (LYOPRINT, RHOPLEX, TEXICRYL, STYCAR, VYCAR and LURAPRET are Trade Marks.) All of these materials have a MFFT of less than 25°C.

Details of some of these materials are given below.

The binder (binder actives excluding carrier liquid) is typically present in the ink in an amount up to 20% by weight, based on the total weight of the ink, with the amount of binder being determined having regard to the pigment content of the ink so that the weight ratio of the binder to pigment is preferably in the range 1:1 to 2: 1 as discussed above.

The currently preferred binder material is LYOPRINT PBA (Huntsman Textile Effects (Germany) GmbH). LYOPRINT PBA is an aqueous emulsion of self-crosslinking acrylic binder (32% binder) and has the advantages of low Tg (-27⁰C), low viscosity, good stability in aqueous solutions with pH values between 4 and 10, and good adhesion on textiles following thermal treatment to activate self-crosslinking of the polymer binder and binding of the colourant to the substrate. Good results have been obtained using LYOPRINT PBA, suitably present in an amount to provide binder actives at an amount in the range 1 % to 15 % by weight, more preferably in the range 2 % to 8 % by weight, based on the total weight of the ink, dependent on the pigment content. The following Table indicates the solubilities at 25°C of the polymer binder LYOPRINT PBA used in some of the preferred solvents (1:5 weight ratio of binder actives to solvent, which represents a realistic likely maximum concentration of binder in primary solvent).

KEY: + clear, i.e. soluble; o opalescent, cloudy, translucent, i.e. partially soluble; - turbid, i.e. insoluble

Pigment

The pigment comprises fine particles of coloured materials. Preferably, the pigment particle size is as small as possible to enable a stable dispersion of the particles in the liquid vehicle and to prevent clogging of the ink channels or nozzle when the ink is used in an inkjet printer. Preferred particle average diameters are generally from about 0.001 to about 0.3 micron, although the particle size can be outside this range in specific embodiments. Preferably, at least 70% of the pigment particles should have an average particle diameter of less than about 150 nm for carbon blacks and less than about 120 nm for colour pigments. For incorporation in the ink of the invention, the pigment particles are formulated as a dispersion. It is preferred to use an aqueous dispersion, as this can assist interaction of the pigment with fabric, but other dispersing liquids may also be used, particularly liquids capable of solubilising the binder, particularly diethylene glycol mono-n-butyl ether (DEGMBE) or tripropylene glycol n-butyl ether (TPGBE). Suitable pigments, in a side range of colours, are well known and are commercially available either in powder or press cake form, e.g. from BASF Corporation, Engelhard Corporation and Sun Chemical Corporation, or as dispersions.

Pigments, traditionally, are stabilized to dispersion by dispersing agents, especially polymeric dispersants. More recently though, so-called "self-dispersible" or "self-dispersing" pigments (hereafter "SDP") have been developed. As the name would imply, the term "self-dispersing" as it is generally understood in the art and used herein, means a pigment having stabilizing groups covalently attached to the surface of the pigment particles such that the pigment forms a stable aqueous dispersion in the absence of any additional dispersing agents. It is preferred to use SDP in the present invention .Examples of typical SDPs include, but are not limited to, those sold under the trade names: Aery Jet Extreme Black 170 (Rohm & Haas), Hostajet (Clariant), Bonjet CW-2 (Orient Corp.), Cab-O-Jet 300 and Cab-O-Jet 200 (Cabot Corp.). Cab-O-Jet 200 has sulfonated groups at the surface of the pigment and Cab-O-Jet 300 has carboxylated groups at the surface of the pigment for dispersion purposes. AcryJet, Bonjet, Hostajet and Cab-O-Jet are Trade Marks.

The self-dispersible pigments in the commercial pigment dispersions having carboxylated functional groups disposed about their outer surface are preferred.

Pigment dispersions sold under the trade name Cab-O-Jet (Cabot Corp.), are currently favoured. These comprise aqueous dispersions of self dispersing pigment particles, with pigment being present at a level of 10% or 15% by weight. The Cab-O-Jet dispersions are conveniently used in the inks in an amount in the range 10% to 35% by weight based on the total weight of the ink, i.e. 1.0% to 3.5% by weight pigment for 10% dispersions and 1.5% to 5.25% by weight pigment for 15% dispersions.

Humectants

The ink desirably includes one or more humectants. Humectants are materials that act to retard drying of the ink and so improve decap times.

Humectants typically comprise non-aqueous solvents, and some of the primary solvents listed above can also function as humectants. A preferred class of humectants comprises aliphatic alcohols as listed above, particularly glycols, such as polyethylene glycol (PEG), ethylene glycol (EG), tetra-ethylene glycol (TTEG), glycerol and polyethylene glycol (PEG) e.g. PEG 200.

It is preferred to use a mixture of different humectant materials, desirably a mixture of at least 3 humectants, and good results have been obtained with a mixture of glycols, particularly polyethylene glycol, ethylene glycol, tetra-ethylene glycol, 1,3 butanediol and glycerol as humectants.

One or more humectants are desirably present in an amount in the range 10 to 50%, preferably 40 to 50% by weight, based on the total weight of the ink.

A preferred feature of the invention is to use a mixture of at least 3 humectants (a humectant set) in the form of solvents having different boiling points, the higher the boiling point of the solvent the lower the quantity of the solvent. A preferred humectant set includes at least one first humectant having a boiling point in the range 160-220⁰C in an amount in the range 10-40%, at least one second humectant having a boiling point in the range 221-270⁰C in an amount in the range 5-20%, and at least one third humectant having a boiling point in the range 271-330⁰C in an amount in the range 1-10%, with the amounts being by weight based on the total weight of the ink and not including any primary solvent that also functions as humectant. Examples of first humectants include 1,3 butanediol, ethylene glycol, and propylene glycol. Examples of second humectants include 1,4 butanediol and diethylene glycol. Examples of third humectants include glycerol, triethylene glycol and tetraethylene glycol.

Additives

Additives may optionally be formulated into the ink of this invention, to the extent that they do not adversely affect the jettability, viscosity and other desirable properties of the ink. Such additives are generally well known in the art and include one or more of: pH adjusting agents, antifoaming agents, rust preventives, fungicides, antioxidants, evaporation accelerators, chelating agents, and water-soluble polymers other than the above-described components, may be added to the inks used in the present invention to improve various properties or function of the ink composition as needed. The amount of each additive must be properly determined, but is typically in the range of about 0.1 to about 15.0 % by weight, and more typically about 0.2 to about 10.0 % by weight, based on the total weight of the ink. Biocides may be used to inhibit growth of micro-organisms.

A biocide (0.01-1.0% by weight) may also be added to prevent unwanted microbial growth which may occur in the ink over time. Suitable biocides include, but are not limited to: NUOSEPT 44 (ISP), and PROXEL GXL (Zeneca Colours Co.). A preferred biocide for the inks employed in the present invention is NUOSEPT 44 (ISP) at a concentration of 0.05-0.5 % by weight. PROXEL and NUOSEPT are Trade Marks.

The ink optionally includes surfactant in small amount (say up to 10.0 % by weight) to improve wetting. Suitable surfactants can be selected having regard to the substrate on which the composition is to be printed. Suitable surfactants include, but are not limited to the following:

(BYK, Dynol, Easy Wet, Surfadone, Zonyl and Tego are Trade Marks).

The fluorosurfactant, Zonyl FSA, e.g. in an amount of 0.1 % to 10.0 % by weight, is currently favoured. The ink optionally includes defoaming agents in small amount (say up to 0.50 % by weight) to reduce foaming in the ink. Suitable defoaming agents include, but are not limited to the following:

Respumit and Surfynol are Trade Marks.

The defoamer, Respumit S, e.g. in an amount of 0.02 % to 0.50 % by weight, is currently favoured.

The ink optionally includes stabilisers in small amount (say up to 0.50 % by weight) to prevent solution polymerization in the ink. Suitable stabilisers include, but are not limited to the following: hydroquinone, methyl hydroquinone.

Further details of conventional and optional additives for printing inks and their use are given, e.g. in U.S. Pat. No. 6,294,592.

Although the solvents in the liquid vehicle are non-aqueous, the ink composition of the present invention may include some water, such as water present in a polymer binder dispersion and water present in a pigment dispersion. The water content of the ink of the present invention may be up to about 25% by weight, based on the total weight of the ink, and is preferably lower, with the water content desirably being minimised to improve decap performance and to produce versatile inks that may potentially be used in print heads that are sensitive to, or not compatible with, water.

The present invention is particularly advantageous for inkjet printing on porous and non-porous substrates, more particularly advantageous for inkjet printing on textiles. The term "textile" is intended to encompass all forms of textile article, including woven textiles, knitted textiles and non- woven textiles. A typical ink in accordance with the invention has the following composition (% by weight).

The solvent is typically composed of

The invention also includes within its scope a method of printing on a substrate, particularly a textile substrate, comprising depositing an ink in accordance with the invention on the substrate by inkjet printing, preferably piezoelectric printing, and causing or allowing the binder to cure to adhere the pigment to the substrate. The printed ink is preferably exposed to heat to cause curing of the binder.

The invention also includes within its scope a substrate, particularly a textile substrate, bearing a printed image produced by inkjet printing an ink in accordance with the present invention.

Examples

The invention will now be described, by way of illustration, with reference to the following non- limiting Examples. In the Examples all the quantities are given as % by weight unless otherwise specified.

The formulations of a preferred embodiment are listed in Table 1. The binder and pigment are both aqueous dispersions, and the specified quantities are for the weight of dispersion (active material and water). Table 1

* on Scale of 1-5, where 5 being excellent and 1 being very poor. Preparation of InkJet Ink Compositions

The exemplified inkjet inks of the present invention were prepared by mixing the polymer binder, DEGMBE and γ-butyro lactone. In all cases this produced a clear solution. The humectants, surfactant, defoamer and biocide were then added. The pigment dispersion was then added, with high shear mixing. Processing was carried out at room temperature (25°C).

The inkjet inks of the present invention can be easily modified if required, e.g. by adjusting properties such as viscosity by addition of one or more suitable further solvents selected in known manner, to print using any one of the inkjet techniques known in the art, for example, thermal or bubble jet printers, piezoelectric printers, continuous flow printers. The compositions of the invention are preferably printed using a piezoelectric inkjet printer equipped with a Xaar Omnidot 760 printhead (XAAR, Cambridge, UK) for single pass printing (higher resolution images (600 dpi) achieved in two passes). The inkjet printed ink is first pre-dried, and then cured, i.e., dried and crosslinked, at a selected time and temperature. In a preferred embodiment, the inkjet printed ink is first pre-dried in hot air for about 4 minutes at 12O⁰C, and then cured in hot air for about a further 4 minutes at 17O⁰C. It is understood that shorter cure times will ordinarily require higher temperatures to effect curing. The cure may be effected by combinations of thermal and radiation energy, such as microwave or infrared radiation.

Viscosity

Viscosity measurements were performed using a Brookfield DV-II+ viscometer equipped with a UL adapter spindle 0 operating with a rotational speed of 60 rpm at a temperature of 25⁰C. Briefly, 17.5 ml of ink was transferred to the chamber, to which a suitable spindle was then lowered into the chamber and left until the temperature stabilized. Measurements were taken every 30, 60, 120 and 300 seconds, until a reproducible viscosity reading could be obtained. [Units: 1 mPa-s ≡ 1 cP]

Surface Tension

Surface tension (Y) measurements were carried out using a White Electrical Instrument Co. torsion balance and a platinum ring. All measurements were carried out at 20-21⁰C. Solution temperatures were controlled by means of a thermostated water bath. Reproducibility was checked by frequent determination of the surface tension of deionised doubly-distilled water (72-73 dynes/cm). The inkjet ink compositions in Table 1 represent the currently preferred formulation of a CMYK ink set. As well as having excellent jetting reliability and good start-stop performance (i.e. where the printhead can be left idle after a period of continuous printing and on re-starting printing is readily recoverable), they also exhibit excellent decap performance of over 2000 seconds.

Test Methods:

Jetting reliability

Jetting reliability was assessed by observing the print quality at regular intervals through the life of a fixed volume of ink. Wash- fastness was assessed by wiping the printed substrate with a wet cloth and examining the print for loss of ink. Crock-fastness was assessed by performing the standard AATCC test method 8-2005. Jetting reliability, wash- and crock- fastness were assessed on a scale of 1 to 5, with 5 being the highest and 1 being the lowest. For compositions to be of practical use, they should have a jetting reliability rating of at least 4. For applications where wash- and crock- fastness are important, the composition should have ratings of 4, respectively.

Decap time

Decap time was determined according to the following procedure. Just prior to the beginning of the test, the nozzles were primed and a nozzle check pattern was performed to ensure all nozzles were firing acceptably. No further servicing was conducted. The nozzles were then left exposed to the surrounding environment for varying lengths of time. After such time, the nozzles were then fired, and the corresponding print visually assessed for misfiring or mis-directed firing of the nozzles.

Print Quality

An image is printed directly to an untreated textile fabric. A wide array of textiles may be used e.g. cotton, polyester (blends thereof), silk, wool etc. The ink is not textile specific. The ink also prints to treated textiles; however the ink has the advantage of printing to cheaper untreated textiles.

Once the image has been inkjet printed, the ink is then fixed to the textile substrate by means of thermal curing. The thermal curing methods can take the form of dry heat (air flow), infra red heating, or heat press. The temperature can vary from between 100-200⁰C. Once cured, the textile confers excellent handle properties and performs excellently in wash- and crock-fastness testing.

Comparative Example 1

An ink was made up using water as the primary solvent for a water-soluble binder, Carboset 531. (Carboset is a Trade Mark). Carboset is a self-crosslinking acrylic dispersion. The ink had the following composition:

The resulting ink had a decap time of only 3 to 5 minutes and so was not suitable for inkjet printing. (Surfynol is a Trade Mark).

Claims

1. An inkjet ink comprising polymer binder; a liquid vehicle comprising one or more primary solvents having a boiling point in excess of 100⁰C in which the binder is dissolved; and pigment.

2. An inkjet ink according to claim 1, wherein the primary solvent or solvents constitute at least 30 % by weight of the ink.

3. An inkjet ink according to claim 1 or 2, wherein the one or more primary solvents have a boiling point in excess of 150⁰C.

4. An inkjet ink according to claim 1, 2 or 3, wherein the liquid vehicle comprises one or more materials selected from aliphatic alcohols, glycol ethers, lactones and pyrrolidones.

5. An inkjet ink according to claim 4, wherein the liquid vehicle comprises one or more glycol ethers in an amount in the range 1-20 % by weight.

6. An inkjet ink according to claim 4 or 5, wherein the liquid vehicle comprises diethylene glycol mono-n-butyl ether.

7. An inkjet ink according to claim 4, 5 or 6, wherein the liquid vehicle comprises one or more lactones in an amount in the range 5-25 % by weight.

8. An inkjet ink according to any one of claims 4 to 7, wherein the liquid vehicle comprises γ-butyro lactone.

9. An inkjet ink according to any one of the preceding claims, wherein the ink comprises one or more humectants.

10. An inkjet ink according to claim 9, wherein the one or more humectants are present in an amount in the range 10-50 % by weight.

11. An inkjet ink according to claim 9 or 10, wherein the humectant(s) comprise one or more aliphatic alcohols.

12. An inkjet ink according to claim 11, wherein the humectants comprise one or more of polyethylene glycol, ethylene glycol, tetra-ethylene glycol, 1,3-butanediol and glycerol.

13. An inkjet ink according to any one of claims 9 to 12, wherein the ink includes a mixture of at least 3 humectants in the form of materials having different boiling points, the higher the boiling point of the material the lower the quantity of the material.

14. An inkjet ink according to claim 13, wherein the ink includes at least one first humectant having a boiling point in the range 160-220⁰C in an amount in the range 10-40%, at least one second humectant having a boiling point in the range 221-270⁰C in an amount in the range 5-20 % and at least one third humectant having a boiling point in the range 271-330⁰C in an amount in the range 1-10 %, with the amounts being by weight based on the total weight of the ink.

15. An inkjet ink according to any one of the preceding claims, wherein the ink includes solvent or solvents having a boiling point in excess of 100⁰C in an amount of at least 70 % by weight.

16. An inkjet ink according to any one of the preceding claims, wherein the ink has a total water content of less than 25 % by weight.

17. An inkjet ink according to any one of the preceding claims, wherein the binder is a self- crosslinking binder.

18. An inkjet ink according to claim 14, wherein the binder comprises one or more acrylic polymers, styrene-acrylic ester copolymers, styrene-butadiene copolymers, vinyl acrylic copolymers, vinyl acetate copolymers and ethylene vinyl acetate copolymers.

19. A method of printing on a substrate, comprising depositing an ink in accordance with any one of the preceding claims on the substrate by inkjet printing, and causing or allowing the ink to cure to adhere the pigment to the substrate.

20. A substrate bearing an image produced by the method of claim 19.