Technical Field

The present invention relates to a method of preparing a liquid pigment dispersion suitable for forming an ink suitable for inkjet printing onto a substrate, such as a wall covering and particularly wallpaper. Background and Prior Art

In inkjet printing tiny drops of ink fluid are projected directly onto an ink receiver surface without physical contact between the printing device and the ink receiver.

The printing device stores the printing data electronically and controls a mechanism for ejecting the ink drops onto the ink receiver. Printing can be accomplished by moving a print head across the ink receiver or vice versa.

The jetting of the ink droplets can be performed in several different ways. In a first type of process called continuous inkjet printing, the ink stream jetted from an orifice of the print head is broken up, by applying a pressure wave pattern to this orifice, into ink droplets of uniform size and spacing, which can be electrostatically charged or not as desired.

According to a second process the ink droplets can be created by a“drop on demand” method. A drop-on-demand device ejects ink droplets only when they are needed for imaging on the ink receiver, thereby avoiding the complexity of drop charging, deflection hardware and ink collection. In drop-on-demand inkjet printing the ink droplet can be formed by means of a pressure wave created by a mechanical motion of a piezoelectric transducer, or by means of discrete thermal pushes.

Inkjet can be used to print onto a variety of substrates, some of which may be difficult or impractical to print onto by contact printing methods such as those which have fibrous or undulating surfaces such as embossing. One area where inkjet printing could be employed is in printing onto wall coverings such as wallpaper. However, it has been found that printing onto such surfaces can result in a washed out appearance and the colour printed do not have an acceptable vibrancy.

One way in which tis can be addressed is to increase the amount of pigment in the ink. However it has been found that, with conventional ink preparation methods, the resulting inks provide unacceptable wear of the printheads, which is believed to be due to the increases in solids content and conductivity.

Inkjet inks that are based on pigments for the colourant are typically made by obtaining a pigment“chip” which is a 50:50 blend of pigment and a dispersant. The pigment chip is then stirred with cyclohexanone or a derivative thereof. It is believed that the cyclohexanone provides a particularly effective stabilising effect on the pigment particles via chemical mechanism, believed to be akin to the formation of a complex of cyclohexanone molecules around a pigment particle. However, cyclohexanone is undesirable for other reasons such as its relatively high toxicity and flammability rating.

An example of an ink prepared in this way is given in US 7,950,794. The present invention provides improvements in this area.

Detailed Description of the Invention

The invention relates to the preparation of inks having a higher than usual pigment concentration that are stable and produce good image quality on a range of substrates, especially wall coverings such as wallpaper, and do not cause an increase in conventionally acceptable printhead wear rates. Furthermore they do not rely on the use of cyclohexanone and are prepared by a two-stage process, via the formation of a liquid pigment dispersion.

Thus, in a first aspect, the invention relates to a method of preparing a liquid pigment dispersion suitable for forming an ink suitable for inkjet printing, the method involving the steps of: (1) preparation of a liquid pigment dispersion by forming a liquid pre-mix comprising from 5 to 20 wt% pigment and and 80 to 95 wt% of a liquid carrier, the liquid carrier comprising at least 50 wt% butyl glycol acetate, followed by

(2) milling the liquid pre-mix in a bead mill to produce the liquid pigmented dispersion.

Without wishing to be bound by theory, it is believed that the butyl glycol acetate (BGA) stabilises the dispersion of pigment particles. It is believed that the process of milling forces the BGA between pigment particles so that there is intimate contact therebetween, forming the surprisingly stable pigmented dispersion. The resulting pigmented dispersion can then be formed into a usable ink by the addition of further liquid components, such as further solvent, wetting agents and/or polymers as desired. Additionally, the conductivity of the resulting inks is lower due to the use of the BGA as solvent. The liquid pre-mix can be prepared by any suitable mixing method, although care must be taken to blend the typically powder pigment material into the liquid carrier. Once an initial blend of materials has been formed it is generally desirable to carry out a high shear mixing step, in order to ensure that the pigment particles are well dispersed in the liquid carrier. Care should also be taken to monitor the temperature of the pre-mix as the blending process is typically exothermic.

Once formed, the pre-mix is then passed through a bead mill. Bead mills are widely used in the paints and coating industries, and are typically used for grinding down the particl size of dispersed particles in a liquid medium. A suitable bead mill is the Buhler™ PML 2 containing beads of 0.5mm Zr0 ₂ . The bead mill provides a particular form of high shear and compression environment, which brings the butyl glycol acetate and pigment particles into intimate contact with each other. This intimate contact is believed to provide the resulting stability of the pigment dispersion that results.

For present purposes a‘liquid’ is liquid at 20°C and 760mm Hg pressure.

Thus, in a second aspect, the present invention relates to a liquid pigment dispersion comprising from 5 to 20 wt% pigment and 80 to 95 wt% of a liquid carrier, the liquid carrier comprising at least 50 wt% butyl glycol acetate. Preferably the liquid carrier comprises at least 70 wt% butyl glycol acetate, more preferably at least 85 wt% butyl glycol acetate.

The pigment dispersion can then be stored for weeks or even possibly months, and subsequently used to prepare an ink suitable for inkjet printing. However it can be expected that some settling of pigment may occur over time, and so some agitation of the dispersion is desirable prior to use of the dispersion to prepare an ink.

The pigments used in the present invention are preferably highly transparent (i.e. have a mean particle diameter of 1 pm or less). Preferably the pigments have a mean particle diameter of 500nm or less, and more preferably 300nm or less, e.g. a mean diameter as measured on a Malvern™ Mastersizer.

Thus, in a third aspect, the invention relates to a method of preparing an ink suitable for inkjet printing, the method comprising obtaining a pigment dispersion as described herein and blending with a liquid diluent in sufficient quantity to form the ink wherein the ink comprises 3 to 10 wt% pigment.

As a result, in a fourth aspect, the present invention relates to an ink suitable for inkjet printing comprising 3 to 10 wt% pigment, and at least 80 wt% liquid carrier, the liquid carrier comprising at least 30 wt% butyl glycol acetate.

Preferably the liquid diluent comprises at least 50 wt% butyl glycol acetate, more preferably at least 70 wt% butyl glycol acetate, most preferably at least 85 wt% butyl glycol acetate.

Additional compounds which can usefully form a component of the liquid carrier and/or the liquid diluent include ethylglycol acetate, cyclohexanone, Ethoxy Propyl Acetate, Diglyme, cyclohexanol, n-pentyl propionate, diacetonalcohol, diisobutyl Ketone, UCAR ester EEP, ethylene glycol n-butyl ether, propylene glycol n-butyl ether, butylglycol, cyclohexyl acetate, Proglyme, dipropylene glycol dimethyl ether, Diisobutyl Carbinol, Isopentanoic acid, 2-ethylhexanol, Valeric acid, butyl lactate, Polypropylene, Dipropylene glycol methyl ether, Ethyl Diglyme™, ethylene glycol diacetate, propylene glycol diacetate, butylglycol acetate, ethylene glycol n-butyl ether acetate, diethylene glycol methyl ether, methyldiglycol, Hexyleneglycol, ethylene glycol, 2-ethylhexyl acetate, diethylene glycol ethyl ether, ethyldiglycol, Gamma Butyrolactone, ethylene glycol hexyl ether, dipropylene glycol methyl ether acetate, dipropylene glycol n-propyl ether, Isophorone, Triglyme™, ethyl diglycol acetate, trimethylnonanol, 2-ethylhexoic acid, dipropylene glycol n-butyl ether, diethylene glycol n-butyl ether, butyldiglycol, Dipropyleneglycol, Tripropylene glycol methyl ether, propylene glycol phenyl ether, ethylene glycol phenyl ether, diethylene glycol n-butyl ether acetate, Diethyleneglycol, butyl diglycol acetate, triethylene glycol methyl ether, Butyl Diglyme™, ethylene glycol phenyl ether (major), triethylene glycol ethyl ether, diethylene glycol hexyl ether, tripropylene glycol n-butyl ether,

Polyglyme™ (High mol weight glymes), Higlyme™, Tetraglyme™, triethylene glycol n- butyl ether, dipropylene glycol phenyl ether, diethylene glycol phenyl ether, Poly(oxy- 1 ,2-ethanediyl), alpha-phenyl-omega-hydroxy.

However, the inks of the present invention are preferably substantially free of cyclohexanone and derivatives thereof. This is because this material represents an unacceptably high level of toxicity and flammability, rendering it unacceptable for many uses, especially in the printing of material which may come into contact with people or children such as wallpaper.

By“substantially free” is meant that the ink comprises less than 2 wt%

cyclohexanone or derivatives thereof, preferably less than 1 wt% and more preferably is completely free of any cyclohexanone or derivatives thereof.

Desirably the total solids content of the ink is in the range 4.0 to 20.0 wt%, more preferably in the range 4.0 to 16.0 wt%, and most preferably in the range 4.0 to 10.0 Wt%.

The solid component of the ink, in addition to the pigment, may comprise for example a resin or other binder substance. Suitable binders for use in the ink include, but are not limited to, methyl methacrylate/n-butyl methacrylate copolymer (e.g. available under the trade name Elvacite® 2614, molecular weight 56,000).

Other suitable acrylate resins are MMA resins; MMA copolymers; EMA resins; and BMA resins.

Other suitable binders comprise vinyl copolymers such as copolymers of vinyl chloride and vinyl acetate; terpolymers of vinyl chloride, vinyl acetate and

dicarbolyxic acids; copolymers of vinyl chloride & hydroxy acrylate; terpolymers of vinyl chloride, hydroxy acrylate and dicarboxylic acid ester; and terpolymers of vinyl chloride, vinyl acetate and vinyl alcohol. Another suitable binder is cellulose acetate butyrate. The ink of the invention may further comprise a dispersant. The function of the dispersant is to improve the homogeneity of the ink (e.g. by improving the separation of the particles and to inhibit settling). Generally dispersants attach to the surface of particles of the pigment. Typically, commercially available pigments are supplied by the manufacturer with an amount of dispersant already present. The ratio of dispersant to pigment (wt) is typically about 50:50, but could be higher than this. For example an acceptable ratio of dispersant to pigment could be in the range 1 :1 to 6:1. Any conventional dispersant may be used in the ink of the invention, and suitable examples are well known to those skilled in the art. A typical dispersant comprises a vinyl acetate/vinyl chloride (“VAVC”) copolymer, or acrylic. Other suitable dispersants may comprise a copolymer of styrene acrylic acid and methacrylic acid or styrene maleic acid, or a styrene-acrylic polymer. The dispersant can advantageously be selected for compatibility with the intended substrate, e.g. acrylic is suitable for printing onto an ABS substrate. The physical properties of the ink will desirably be such that the ink is suitable for use in inkjet printing. For example, the ink will typically have a viscosity in the range 7-20cP (as measured at 25°C, at 30 RPM using a Brookfield DV 11+ viscometer).

Examples

Inkjet inks were prepared in a two-stage process according to the invention. The first step is the preparation of a pigment dispersion and the second step is the production of the ink. The pigment dispersion formed in the first step is a standalone product in its own right.

Yellow Dispersion

40kg of butyl glycol acetate was mixed with 10kg of yellow pigment powder

(Hostaprint Yellow HG32 (Pigment Yellow 180)) in an ATEX rated propeller mixer to form an initial blend. The blend was then passed to a high shear mixer (Greaves VGM3) for 60 minutes to further break up the pigment particles so as to ensure a well-mixed blend. Care is taken to avoid overheating as the blending step is exothermic.

Once prepared the blend is then transferred to a bead mill (Buhler™ PML 2) in pass to pass mode containing 0.5mm Zr0 ₂ beads. The chiller was set to 5°C and the air pump was turned on to ensure a flow of blend flows slowly out the outlet pipe. The first 0.5kg is discarded.

The rotor is then turned on and the speed is slowly increased to 2000rpm. Once at 2000rpm the air pump is turned up to 50% and the resulting dispersion is collected in a separate container. This dispersion is passed through the mill as many times as is necessary to reach the quality control parameters, such as viscosity and particle size of the pigments. This resulted in 27.2kg of yellow dispersion which had a viscosity of 5.95 cP (as measured at 25°C, at 30 RPM using a Brookfield DV 11+ viscometer) and the particle size was less than 290 nm. It was stable at room temperature.

The pigment powder was provided as a 1 :1 w/w mixture with a dispersant. Thus, the dispersion comprised 10 wt% of pigment, 10 wt% dispersant and 80 wt% BGA.

Yellow Ink

A yellow ink was prepared by mixing together 27.2kg of yellow dispersion as prepared above, 36.416kg of butyl glycol acetate, 320g of a 10wt% solution of

Dynol™ 607 in butyl glycol acetate and 64g of a 10wt% solution of Tego Glide 450 in butyl glycol acetate.

27.2kg of dispersion was added to a shearing mixer (Silverson 0.75kW Multipurpose Batch Mixer) and the mixer was set to around 30% of maximum mixing speed.

36.416kg of butyl glycol acetate was slowly (5lg/minute) added while mixing. Then 320g of the Dynol™ solution was added and the speed of the mixer was turned up to 100% for 60 minutes. Then 64g of the Tego Glide™ was added. Care was taken not to allow the temperature to exceed 35°C. The resulting mix was then filtered to provide the final yellow ink which had a viscosity of 8.5cP (as measured at 25°C, at 30 RPM using a Brookfield DV I In- viscometer) and a surface tension of 29 dyn/cm.

The resulting ink had a pigment concentration of 4.25 wt%.

Cyan Dispersion 80kg of butylglycol acetate was mixed with 20kg of cyan pigment powder (Hostaprint Blue B2G 34-CN VP5306) in an ATEX rated propeller mixer to form an initial blend. The blend was then passed to a high shear mixer (Greaves VGM3) for 60 minutes to further break up the pigment particles so as to ensure a well-mixed blend. Care is taken to avoid overheating as the blending step is exothermic.

Once prepared the blend is then transferred to a bead mill (Buhler™ PML 2) in pass to pass mode containing 0.5mm Zr0 ₂ beads. The chiller was set to 5°C and the air pump was turned on to ensure a flow of blend flows slowly out the outlet pipe. The first 0.5kg is discarded.

The rotor is then turned on and the speed is slowly increased to 2000rpm. Once at 2000rpm the air pump is turned up to 50% and the resulting dispersion is collected in a separate container. This dispersion is passed through the mill as many times as is necessary to reach the quality control parameters, such as viscosity and particle size of the pigments.

This resulted in 44.625kg cyan dispersion which had a viscosity of 5.55 cP (as measured at 25°C, at 30 RPM using a Brookfield DV 11+ viscometer) and the particle size was less than 200 nm. It was stable at room temperature.

The pigment powder was provided as a 1 :1 w/w mixture with a dispersant. Thus, the dispersion comprised 10 wt% of pigment, 10 wt% dispersant and 80 wt% BGA. Cyan Ink

A cyan ink was prepared by mixing together 44.625kg of cyan dispersion as prepared above, 58.0125kg of butyl glycol acetate, 1.8375kg of Elvacite™ 2614 and 525g of 10% Dynol™ 607 in butyl glycol acetate.

58.0125kg of butyl glycol acetate was added to a shearing mixer (Silverson 0.75kW Multipurpose Batch Mixer). Then 525g of the Dynol™ in BGA was added and the mixed for two hours and allowing the resin to swell overnight. The following day the mixture was mixed further for 60 minutes to ensure complete dissolution of the Dynol™ resin.

44.625kg of cyan dispersion was slowly added and the mixer ran for 10 minutes to blend the materials together. Then the 1.8375kg of Elvacite™ in BGA is added and mixing continued for 60 minutes. Throughout, care is taken to prevent the

temperature exceeding 35°C.

The resulting mix was then filtered to provide the final cyan ink which had a viscosity of 1 0cP (as measured at 25°C, at 30 RPM using a Brookfield DV II+ viscometer) and a surface tension of 30 dyn/cm.

The resulting ink had a pigment concentration of 4.25 wt%.

Magenta Dispersion

40kg of butylglycol acetate was mixed with 10kg of magenta pigment powder (Hostaprint Magenta (Pigment Violet 19)) in an ATEX rated propeller mixer to form a liquid pre-mix. The pre-mix was then passed to a high shear mixer (Greaves VGM3) for 60 minutes to further break up the pigment particles so as to ensure that the pigment particles were well dispersed in the liquid carrier. Care is taken to avoid overheating as the blending step is exothermic.

Once prepared the blend is then transferred to a bead mill (Buhler™ PML 2) in pass to pass mode containing 0.5mm Zr0 ₂ beads. The chiller was set to 5°C and the air pump was turned on to ensure a flow of blend flows slowly out the outlet pipe. The first 0.5kg is discarded. The rotor is then turned on and the speed is slowly increased to 2000rpm. Once at 2000rpm the air pump is turned up to 50% and the resulting dispersion is collected in a separate container. This dispersion is passed through the mill as many times as is necessary to reach the quality control parameters, such as viscosity and particle size of the pigments.

This resulted in 27.2kg of magenta dispersion which had a viscosity of 8.5 cP (as measured at 25°C, at 30 RPM using a Brookfield DV 11+ viscometer) and the particle size was less than 250 nm. It was stable at room temperature.

The pigment powder was provided as a 1 :1 w/w mixture with a dispersant. Thus, the dispersion comprised 10 wt% of pigment, 10 wt% dispersant and 80 wt% BGA.

Magenta Ink

A magenta ink was prepared by mixing together 36.0kg of magenta dispersion as prepared above, 80.64kg of butyl glycol acetate, 3.0kg of Elvacite™ 2614 and 360g of a 10wt% solution of Dynol™ 607 in butyl glycol acetate. 80.64kg of butyl glycol acetate was added to a shearing mixer (Silverson 0.75kW Multipurpose Batch Mixer). Then 3.0kg of Elvacite™ was added and then mixed for two hours and allowing the resin to swell overnight. The following day the mixture was mixed further for 60 minutes. 36.0kg of magenta dispersion was slowly added and the mixer ran for 10 minutes to blend the materials together. Then the 360g of Dynol™ is added and mixing continued for 15 minutes. Throughout, care is taken to prevent the temperature exceeding 35°C. The resulting mix was then filtered to provide the final magenta ink which had a viscosity of 9.5cP at 30rpm (as measured at 25°C, at 30 RPM using a Brookfield DV II+ viscometer) and a surface tension of 30 dyn/cm.

The resulting ink had a pigment concentration of 3.0 wt%. Black Dispersion

42.5kg of butylglycol acetate was mixed with 7.5kg of black pigment powder (Microlith 0066K (Pigment Black 7)) in an ATEX rated propeller mixer to form an initial blend. The blend was then passed to a high shear mixer (Greaves VGM3) for 60 minutes to further break up the pigment particles so as to ensure a well-mixed blend. Care is taken to avoid overheating as the blending step is exothermic.

Once prepared the blend is then transferred to a bead mill (Buhler™ PML 2) in pass to pass mode containing 0.5mm Zr0 ₂ beads. The chiller was set to 5°C and the air pump was turned on to ensure a flow of blend flows slowly out the outlet pipe. The first 0.5kg is discarded.

The rotor is then turned on and the speed is slowly increased to 2000rpm. Once at 2000rpm the air pump is turned up to 50% and the resulting dispersion is collected in a separate container. This dispersion is passed through the mill as many times as is necessary to reach the quality control parameters, such as viscosity and particle size of the pigments. This resulted in 38.4kg of black dispersion which had a viscosity of 7.3 cP (as measured at 25°C, at 30 RPM using a Brookfield DV 11+ viscometer) and the particle size was less than 230 nm. It was stable at room temperature.

The pigment powder was provided as a 1 :1 w/w mixture with a dispersant. Thus, the dispersion comprised 7.5 wt% of pigment, 7.5 wt% dispersant and 85 wt% BGA.

Black Ink

A black ink was prepared by mixing together 38.4kg of black dispersion as prepared above, 25.28kg of butyl glycol acetate and 320g of a 10 wt% solution of Dynol™ 607 in butyl glycol acetate.

38.4kg of dispersion was added to a shearing mixer (Silverson 0.75kW Multipurpose Batch Mixer) and the mixer was set to around 30% of maximum mixing speed. 25.28kg of butyl glycol acetate was slowly (5kg/minute) added while mixing. Then 320g of the Dynol™ solution was added and the speed of the mixer was turned up to 100% for 60 minutes. Care was taken not to allow the temperature to exceed 35°C. The resulting mix was then filtered to provide the final black ink which had a viscosity of 9.25cP (as measured at 25°C, at 30 RPM using a Brookfield DV II+ viscometer) and a surface tension of 31 dyn/cm.

The resulting ink had a pigment concentration of 4.5 wt%.