GB2615057A - Inkjet ink set - Google Patents

Abstract

An aqueous inkjet ink set comprises four or more aqueous inkjet inks, each ink comprising a pigment and water, wherein the inkjet ink set includes a blue and/or cyan ink, a yellow ink, a magenta ink, and a black ink. The total amount of copper present in the inks of the ink set combined is ≤10000 ppm. Typically, the combined inks of the ink set comprise ≤3000 ppm nickel, ≤10000 ppm zinc, ≤5000 ppm chromium, ≤5000 ppm lead, ≤100 ppm molybdenum, ≤100 ppm cadmium, ≤100 ppm mercury, ≤100 ppm selenium, and ≤100 ppm arsenic. The inks of the ink set may be free of heavy metals. Each inkjet ink preferably comprises an organic solvent, e.g. a biodegradable solvent selected from glycerol, (poly)ethylene glycol, and (poly)propylene glycol. A method of inkjet printing the inkjet ink set onto a substrate and drying the ink is also disclosed. The substrate may be compostable.

Description

Inkjet ink set The present invention relates to a printing ink set and in particular to an aqueous compostable inkjet ink set.

The compostability of packaging items is now an important consideration resulting in a British Standard for compostability of packaging, BS EN 13432. Packaging products that conform to the compostable criteria of this standard are suitable for compositing. BS EN 13432 provides five compostable criteria, where each test is undertaken according to the test methods specified in BS EN 13432. Only if a material passes every test requirement is it deemed compostable. The five compostable criteria of BS EN 13432 are as follows: 1. Disintegration -the packaging sample is mixed with organic waste and maintained under test scale composting conditions for 12 weeks after which time no more than 10% of material fragments are allowed to be larger than 2 mm.

2. Biodegradability -a measure of the actual metabolic, microbial conversion, under composting conditions, of the packaging sample into water, carbon dioxide and new cell biomass. Within a maximum of 6 months, biodegradation of the test sample must generate an amount of carbon dioxide that is at least 90% as much as the carbon dioxide given off from the control/reference material.

3. Absence of any negative effect on the composting process.

4. Low levels of heavy metals and no adverse effect of the quality of compost produced. Upper limits, in mg/kg (ppm) of dry sample, are: zinc 150, copper 50, nickel 25, cadmium 0.5, lead 50, mercury 0.5, chromium 50, molybdenum 1, selenium 0.75, arsenic 5 and fluoride 100.

5. The composted packaging material must not have adverse effect on the bulk density, pH, salinity, volatile solids, total nitrogen, total phosphorus, total magnesium, total potassium and ammonium nitrogen characteristics of the compost.

The substrate is typically the main focus when considering compostability of packaging as the substrate is the largest component of the item. The present inventors have now considered however the effect of the printed ink layer on compostability. The printed ink layer has minimal effect on the physical degradation of the packaging as it contributes a small amount to the overall mass of the item. The present inventors have found however that the printed ink layer can have an effect on the levels of heavy metals in the produced compost.

There is therefore a need in the art for a compostable aqueous inkjet ink set for packaging.

The present inventors considered the components of aqueous inkjet inks and found that the choice of pigment can have an effect on the compostability of the printed article having the inkjet ink set printed thereon.

In this regard, during the drying process, pigments remain in the dried ink film. Pigments can be a source of heavy metals, particularly copper, and hence can be the cause of an accumulation of heavy metals in the resulting compost. Heavy metals of course need to be restricted according to BS EN 13432.

Accordingly, the present invention provides an aqueous inkjet ink set comprising four or more aqueous inkjet inks, each aqueous inkjet ink comprising a pigment and water, wherein the inkjet ink set includes a blue and/or cyan ink, a yellow ink, a magenta ink and a black ink, and wherein the total amount of copper present in the inks of the ink set combined is 10,000 ppm or less.

The present inventors have found that the aqueous inkjet ink set of the present invention, which comprises inkjet inks having a restricted amount of copper present therein, does not compromise the compostability of the printed item.

The aqueous inkjet ink set of the present invention comprises four or more aqueous inks. In this regard, the inkjet ink set includes a blue and/or cyan ink, a magenta ink, a yellow ink and a black ink. This is a multi-chromatic inkjet ink set. An ink set containing a blue and/or cyan ink, a magenta ink, a yellow ink and a black ink are often termed a so-called trichromatic set. The inks in a trichromatic set can be used to produce a wide range of colours and tones. A blue ink contains a blue pigment, a cyan ink contains a cyan pigment, a magenta ink contains a magenta pigment, a yellow ink contains a yellow pigment and a black ink contains a black pigment.

The inks can be categorised on the CIELAB (L"a"b") colour space system. The lightness, L*, represents the darkest black at U=0, and the brightest white at L"=100. The colour channels, a" and b", represents true neutral grey values at a*=0 and b*=0. The red/green opponent colours are represented along the a* axis, with green at negative a*values and red at positive a* values. The yellow/blue opponent colours are represented along the It axis, with blue at negative b* values and yellow at positive It values.

Preferably therefore, the cyan ink has a* value from -60 to -10 and b* value from -70 to -20, preferably at value from -55 to -15 and b" value from -65 to -25, more preferably a value from -50 to -20 and b* value from -60 to -30. The 1_4-value will depend on the lightness of the cyan ink. The cyan ink may have L* value from 35 to 69, preferably 40 to 67, more preferably 42 to 65. Or the cyan ink may be a lighter cyan and have ILt value from 71 to 105, preferably 73 to 100, more preferably 75 to 95. The cyan ink preferably contains the dispersible pigments PB16, PB27, PB29 or PB79.

Preferably, the blue ink has a* value from -40 to 35 and b* value from -75 to -20, preferably at value from -35 to 30 and b* value from -70 to -25, more preferably at value from -30 to 25 and b* value from -65 to -30. The L" value will depend on the lightness of the blue ink. The blue ink may have ILt value from 5 to 69, preferably 10 to 67, more preferably 15 to 65. Or the blue ink may be a lighter blue and have L" value from 71 to 105, preferably 73 to 100, more preferably 75 to 95. The blue ink preferably contains the dispersible pigments PB25, PB56, PB60 or PB61.

Preferably, the magenta ink has a" value from 55 to 105 and b" value from -40 to 1 0, preferably a" value from 60 to 100 and b" value from -35 to 5, more preferably a" value from 65 to 95 and b" value from -30 to 0, most preferably a* value from 70 to 90 and b* value from -25 to -5. The L* value will depend on the lightness of the magenta ink. The magenta ink may have L* value from 30 to 64, preferably 35 to 62, more preferably 40 to 60. Or the magenta ink may be a lighter magenta and have L* value from 66 to 100, preferably 68 to 95, more preferably 70 to 90. The magenta ink preferably contains the dispersible pigments PR122, PR57.1 or PV19.

Preferably, the yellow ink has a* value from -40 to 10 and b" value from 70 to 120, preferably a" value from -35 to Sand b* value from 75 to 115, more preferably a* value from -30 to 0 and b* value from 80 to 110, most preferably a* value from -25 to -5 and b* value from 85 to 105. The L* value will depend on the lightness of the yellow ink. The yellow ink may have L" value from 70 to 104, preferably 75 to 102, more preferably 80 to 100. Or the yellow ink may be a lighter yellow and have L" value from 106 to 140, preferably 108 to 135, more preferably 110 to 130. The yellow ink preferably contains the dispersible pigments PY74, PY120, PY151, PY155 or PY180.

Preferably, the black ink has a" value from -25 to 25 and b* value from -20 to 30, preferably a* value from -20 to 20 and b* value from -15 to 25, more preferably a* value from -15 to 15 and b* value from -10 to 20, most preferably a* value from -10 to 10 and b* value from -5 to 15. The L* value will depend on the lightness of the black ink. The black ink may have a ILt value from 6 to 30, preferably 8 to 25, more preferably 10 to 20. Or the black ink may be a lighter black and have L" value from 35 to 75, preferably 40 to 70, more preferably 45 to 65. The black ink preferably contains the dispersible pigment PBk7.

The aqueous inkjet inks of the aqueous inkjet ink set of the present invention each comprise a pigment. The pigment is dispersed in the liquid medium of the ink. The pigment is typically a water-based pigment dispersion, which is combined with the other components of the ink by mixing.

The present inventors have found that the choice of pigment can greatly affect the compostability of the printed article having the inkjet ink set printed thereon. In this regard, during the drying process, pigments remain in the dried ink film. Pigments can be a source of heavy metals, and hence can be the cause of an accumulation of heavy metals in the resulting compost. Heavy metals of course need to be restricted according to BS EN 13432. Particularly problematic pigments include phthalocyanine pigments, which are commonly employed for blue, cyan and green inks, owing to the presence of copper, and pigment yellow 150 which contains nickel. Other pigments that are commonly used in the market, while not being based on heavy metals, may also contain trace amounts of heavy metals that will contribute to the total amount of heavy metal content in the printed item.

Accordingly, in the present invention, the total amount of copper present in the inks of the ink set combined is 10,000 ppm or less. In other words, the total amount of copper in all of the inks of the ink set combined is 10,000 ppm or less. Preferably, the total amount of copper present in the inks of the ink set combined is 5,000 ppm or less, most preferably 1,000 ppm or less. In other words, the total amount of copper in all of the inks of the ink set combined is preferably 5,000 ppm or less, most preferably 1,000 ppm or less.

Copper-based pthalocyanine pigments are hence preferably absent in the inks of the ink set of the present invention. Thus, in a preferred embodiment, the inks of the ink set are free of copper-based phthalocyanine pigments In a preferred embodiment, the total amount of nickel in the inks of the ink set is also restricted. Thus, in a preferred embodiment, the total amount of nickel in the inks of the ink set combined is 3,000 ppm or less. Preferably, the total amount of nickel present in the inks of the ink set combined is 2,000 ppm or less, most preferably 1,000 ppm or less. In other words, the total amount of nickel in all of the inks of the ink set combined is preferably 2,000 ppm or less, most preferably 1,000 ppm or less.

In a still preferred embodiment, the total amount of zinc, chromium, lead, molybdenum, cadmium, mercury, selenium and arsenic is also restricted. Thus, in a preferred embodiment: the total amount of zinc in the inks of the ink set combined is 10,000 ppm or less, preferably 5,000 ppm or less and most preferably 1,000 ppm or less; the total amount of chromium in the inks of the ink set combined is 5,000 ppm or less, preferably 2,000 ppm or less and most preferably 1,000 ppm or less; the total amount of lead in the inks of the ink set combined is 5,000 ppm or less, preferably 2,000 ppm or less and most preferably 1,000 ppm or less; the total amount of molybdenum in the inks of the ink set combined is 100 ppm or less, preferably 50 ppm or less, most preferably 25 ppm or less; the total amount of cadmium in the inks of the ink set combined is 100 ppm or less, preferably 50 ppm or less, most preferably 25 ppm or less; the total amount of mercury in the inks of the ink set combined is 100 ppm or less, preferably 50 ppm or less, most preferably 25 ppm or less; the total amount of selenium in the inks of the ink set combined is 100 ppm or less, preferably 50 ppm or less, most preferably 25 ppm or less; and the total amount of arsenic in the inks of the ink set combined is 100 ppm or less, preferably 50 ppm or less, most preferably 25 ppm or less. Preferably, the total amount of molybdenum, cadmium, mercury, selenium and arsenic in the inks of the ink set combined is 100 ppm or less, preferably 50 ppm or less and most preferably 25 ppm or less.

Preferably, the total amount of heavy metals per se is restricted and in a preferred embodiment, the inks of the ink set are free of heavy metals.

The aqueous inkjet ink set as claimed, which comprises inkjet inks having a restricted amount of copper present therein, preferably a restricted amount of copper and nickel, and more preferably a restricted heavy metal content, does not compromise the compostability of the printed item.

The amount of each heavy metal present in each pigment and in the ink as a whole is determined by inductively coupled plasma atomic emission spectroscopy (ICP). This is a well-known analytical technique for the detection of chemical elements. ICP uses inductively coupled plasma to produce excited atoms or ions that emit electromagnetic radiation at wavelengths characteristic of a particular element. The intensity of the emissions from various wavelengths of light are proportional to the concentrations of the elements within the sample.

The total heavy metal content present in each pigment and hence the ink as a whole can be determined by the following method: Before commencing the analysis, the powder pigments are digested in a mixture of hydrochloric and nitric acids to ensure that the total metal content, rather than the free metal levels in the samples are correctly determined. This is achieved by gently heating a mixture of the pigment with a 33%v/v solution of HCI and HNO3 (ratio of acids 1:3) on a hotplate until the pigment is dissolved. To obtain quantitative data for the metal levels, a calibration standard is used for each metal. Suitable certified ICP-OES metal standards can be obtained from Sigma. It is necessary to take into account the various interference bands of each metal and ensure the concentration range brackets the concentrations relevant to the application. A known amount of the prepared pigment sample is combined with a known amount of deionised water to create a solution that will position the concentration of metals present roughly into the centre of the calibration range. The analysis can then be conducted on a suitable ICP unit, for examples a Thermo ICS-5000 ICP-OES. Both the dual radial and axial detectors are used.

First, the calibration is run, followed by a period of blank deionised water to flush the system, the samples are then run in duplicate. Using the calibration, it is then possible to calculate the concentration of metals present.

Any suitable pigments can be used provided that the ink set as whole has the restricted amount of copper present therein.

Dispersible pigments are well-known in the art and are commercially available, for example, under the tradenames Paliotol (available from BASF plc), Cinquasia, lrgalite (both available from Ciba Speciality Chemicals) and Hostaperm (available from Clariant UK). Aqueous pigment dispersions are commercially available for example, under the tradenames of Projet ADP (Fujifilm FFIC), Hostajet (Clariant) and SP (Fuji pigments).

In a preferred embodiment, the yellow pigment is an azo pigment, such as pigment yellow 151, pigment yellow 155, pigment yellow 74, pigment yellow 13, pigment yellow 120 and pigment yellow 83.

In a preferred embodiment, the magenta pigment is a quinacridone pigment, such as Pigment violet 19, Pigment red 122 or a mixed crystal quinacridone, such as Cromophtal Jet magenta 2BC and Cinquasia RT.

In a preferred embodiment, the black pigment is a carbon black pigment, such as pigment black 7.

In a preferred embodiment, the cyan pigment is a non-copper-based phtalocyanine pigment, such as PB16 and PB79.

The following commercially-available pigments classified according to Colour Index International according to the following tradenames are preferred non-limiting examples that can be used in the ink of the present invention: Blue pigments: PB22, PB24, PB25, PB28, PB50, PB56, PB60, PB61, PB61:1, PB66, PB73, PB75, PB80, PB81, PB82, PB84 and PB86. Such pigments are not copper based. Preferred examples include P825, P856, P860 and P861. In contrast, pigments, including P815.4 and P1315.3, are not suitable pigments for use in the inkjet ink set of the present invention as these pigments are phthalocyanine-based pigments and have a high copper content.

Cyan pigments: PB16, PB27, PB29 and PB79. Such pigments are not copper based.

Yellow pigments: FYI, PY1:1, PY2, PY3, PY4, PY5, PY6, PY9, PY10, PY12, PY13, PY14, PY16, PY17, PY21, PY42, PY43, PY44, PY45, PY46, PY47, PY48, PY55, PY61, PY62, PY62:1, PY63 PY65, PY73, PY74, PY75, PY77, PY81 PY83, PY87, PY93, PY94, PY95, PY97, PY98, PY100, PY101, PY104, PY105, PY108, PY109, PY110, PY111, PY112, PY113, PY115, PY120, PY126, PY127, PY127:1 PY128, PY130, PY133, PY134, PY136, PY137, PY138, PY139, PY147, PY 148, PY151, PY152, PY153 PY154, PY155, PY156, PY172, PY173, PY174 PY175, PY176, PY180, PY181, PY182, PY183, PY185, PY188, PY190, PY191, PY191:1, PY192, PY193, PY194, PY200, PY203, PY204, PY207, PY213, PY223, PY224 and PY226. Preferred examples include PY74, PY120, PY151, PY155 and PY180.

Pigment yellow 150 has a high nickel content and as such is preferably absent in the inkjet sets of the present invention.

Magenta pigments: PR122, PR57.1 and PV19.

Black pigments: PBk6 and PBk7.

Pigment particles dispersed in the ink should be sufficiently small to allow the ink to pass through an inkjet nozzle, typically having a particle size less than 8 pm, preferably less than 5 pm, more preferably less than 1 pm and particularly preferably less than 0.5 pm.

The pigment is preferably present in each inkjet ink in an amount 0.5 to 15% by weight, more preferably from 0.5 to 5% by weight, based on the total weight of the ink. A lighter colour ink will contain less pigment than a darker colour ink.

The inkjet ink set may further comprise additional colour inks, including a white ink, an orange ink, a green ink, a violet ink, a red ink and mixtures thereof.

Preferably, the orange ink has at value from 25 to 75 and b" value from 55 to 105, preferably at value from 30 to 70 and b* value from 60 to 100, more preferably at value from 35 to 65 and b* value from 65 to 95, most preferably at value from 40 to 60 and b* value from 70 to 90. And preferably, the orange ink has L* value from 50 to 90, preferably 55 to 85, more preferably 60 to 80. The orange ink preferably contains the dispersible pigment orange 43 or dispersible pigment orange 36, more preferably Kenalake Orange HPRLO.

Preferably, the green ink has at value from -105 to -55 and b" value from -15 to 35, preferably at value from -100 to -60 and b* value from -10 to 30, more preferably at value from -95 to -65 and b* value from -5 to 25, most preferably at value from -90 to -70 and b* value from 0 to 20. And preferably, the green ink has L* value from 40 to 80, preferably 45 to 75, more preferably 50 to 70. The green ink preferably contains dispersible pigment green 8 or dispersible pigment green 54. Preferably, the green pigment is an algae-based pigment or an inorganic pigment, such as Green earth or Verona green. Green earth is known commercially as PG23 and is composed of ferrous and ferric silicates of potassium, manganese and aluminium. Verona green is an inorganic pigment derived from the minerals celadonite and glauconite, which has the chemical formula KRAI,Fe3+),(Fe2+,MgRAISi3,Si4)O1o(OH)2.

Preferably, the violet ink has a* value from 35 to 85 and b* value from -90 to -40, preferably a* value from 40 to 80 and If value from -85 to -45, more preferably at value from 45 to 75 and b" value from -80 to -50, most preferably at value from 50 to 70 and b* value from -75 to -55. And preferably, the violet ink has L" value from 10 to 50, preferably 15 to 45, more preferably 20 to 40. The violet ink preferably contains the dispersible pigment violet 23, more preferably Hostaperm Violet RL-NF. Other pigments that could be used include pigment violet 22 and pigment violet 122.

Preferably, the white ink has at value from -25 to 25 and b* value from -20 to 30, preferably at value from -20 to 20 and b" value from -15 to 25, more preferably at value from -15 to 15 and b" value from - 10 to 20, most preferably a" value from -10 to 10 and b" value from -5 to 15. Preferably, the white ink has L" value from 81 to 99, preferably 83 to 97, more preferably 85 to 95. The white ink preferably contains the dispersible pigment white 6, more preferably Tipaque CR-60-2.

Preferably, the red ink has at value from 50 to 100 and b* value from 20 to 75, preferably at value from 60 to 90 and b* value from 30 to 65, more preferably at value from 65 to 80 and b* value from 40 to 60.

Preferably, the red ink has L* value from 30 to 65, preferably 35 to 62, more preferably 40 to 60.

The following commercially-available pigments classified according to Colour Index International according to the following tradenames are preferred non-limiting examples that can be used in the ink of the present invention: Green pigments: PG8, PG23, PG24 and PG54.

Orange pigments: P01, P02, P03, P05, P013, P015, P016, P022, P034, P036, P038, P040, P041, P043, P046, P047, P048, P049, P051, P052, P053, P060, P061, P062, P064, PO 66, P067, P069, P071, P072, P073, P074, P075, P077, P078, P079, P080, P081, P082, P085 and P084.

Red pigments: PRI, PR2, PR3, PR4, PR5, PRO, PR7, PR8, PRO, PR12, PRI3, PRI4, PR15, PR17, PR19, PR2I, PR22, PR23, PR3I, PR32, PR38, PR47, PR48, PR48:2, PR48:3, PR48:4, PR49, PR49:1, PR49:2, PR52:1, PR52:2, PR53:1, PR57, PR57:2, PR58:4, PR60:1, PR61, PR62, PR63, PR63:1, PR69, PR81:2, PR81:3, PR81:4, PR83, PR83:1, PR83:3, PR85, PR88, PR89, PR90:1, PR101, PR101:1, PR102, PR112, PR114, PR119, PR120, PR123, PRI39, PR144, PR146, PR147, PR148, PR149, PR150, PR160, PR166, PRI68, PR170, PR170:1, PR171, PR172, PR173, PR174, PR175, PR176, PR177, PR178, PR179, PR1880, PR181, PR13, PR184, PR185, PR1887, PR188, PR190, PR192, PR193, PR194, PRI97, PR200, PR202, PR204, PR206, PR207, PR208, PR209, PR210, PR211, PR212, PR213, PR214, PR216, PR220, PR221, PR223, PR224, PR226, PR231, PR232, PR238, PR239, PR242, PR243, PR245, PR251, PR252, PR253, PR254, PR255, PR256,--PR258, PR259, PR260, PR262, PR264, PR265, PR266, PR268, PR269, PR270, PR272, PR273, PR274, PR275, PR276, PR279, PR282, PR286, PR287 and PR288 Violet pigments: PV3:3, PV5, PV5:1, PV7, PV13, PV15, PV18, PV22, PV23, PV25, PV29, PV31, PV32, PV36, PV37, PV42, PV44, PV50, PV55, PV58, PVI22 and PV171.

White pigments: PW6, PW6:1, PWI2, PW24, PW25, PW26, PW27, PW28, PW30, PW32 and PW33.

In a preferred embodiment, the white pigment is a titanium dioxide pigment.

The aqueous inkjet inks of the aqueous inkjet ink set of the present invention each comprise water.

During the ink drying process, water is evaporated from the ink and as such does not affect compostability. And in any case, the presence of water in the printed film does not negatively affect compostability.

The total amount of water present in each inkjet ink is preferably 40 to 80%, more preferably 60 to 75% by weight, based on the total weight of the ink.

The aqueous inkjet inks of the aqueous inkjet ink set of the present invention each preferably comprise an organic solvent. The organic solvent is in the form of a liquid at ambient temperatures and is capable of acting as a carrier for the components of the ink. The organic solvent may be a single solvent or a mixture of two or more solvents. As with known solvent-based inkjet inks, the organic solvent is required to evaporate from the printed ink, typically on heating, in order to allow the ink to dry. The solvent can be selected from any solvent commonly used in the printing industry, such as glycol ethers, glycol ether esters, alcohols, glycols, ketones and esters.

During the ink drying process, the majority of the organic solvents are evaporated from the ink film.

Traces of the solvents can remain in the ink film after drying however. Preferably therefore, the solvents are biodegradable solvents and so do not negatively affect compostability.

Examples of co-solvents suitable for the present invention are water-soluble polyols or glycols. For example glycerol, (poly)ethylene glycol, (poly)propylene glycol and the like.

The organic solvent is preferably present in each inkjet ink in an amount of 10% to 40% by weight based on the total weight of the ink, more preferably 15% to 30% by weight, based on the total weight of the ink.

The aqueous inkjet inks of the inkjet ink set of the present invention each preferably comprise a resin.

The resin preferably has a weight-average molecular weight of 20-200 KDa and preferably 20-60 KDa, as determined by GPO with polystyrene standards. The resin is preferably in the form of an aqueous dispersion and is thus preferably a water-based resin. It is preferably soluble in the liquid medium of the ink (water and, when present, additionally the solvent).

The resin, when present, is preferably present in each inkjet ink at 1 to 20% by weight of the active polymer content, based on the total weight of the ink.

During the ink drying process, the resin binder remains in the dried ink film, which can include polymeric material. The effect on compostability is minimal as only very small amounts of polymeric material are usually present.

The resin is preferably selected from polyurethane, polymethacrylate, polyvinyl, polybutadiene and copolymers and blends thereof. Copolymers may be methacrylate copolymer which includes copolymers with different alkyl substituents like methyl, ethyl, propyl and butyl methacrylates. Blends are mixtures of polymers or copolymers, like methacrylate copolymer and polyvinyl copolymer blend or polybutadiene and methacrylate copolymer blend. In a preferred embodiment, the resin is a (meth)acrylate copolymer resin because a (meth)acrylate copolymer system is regarded as being non-harmful to life.

The resin may optionally be radiation crosslinkable when exposed to UV radiation or thermally crosslinkable when exposed to thermal drying. However, in a preferred embodiment, the resin is a passive resin, in the sense that it does not undergo crosslinking when exposed to UV radiation or thermal drying.

In a preferred embodiment, the surface tension of each ink is controlled by the addition of one or more surface active materials such as commercially available surfactants. Therefore, each ink of the inkjet ink set of the present invention preferably further comprises a surfactant.

Surfactants are well-known in the art and a detailed description is not required. An example of a suitable surfactant is Surfynol 440 and Capstone FS-1. Adjustment of the surface tension of the inks allows control of the surface wetting of the inks on various substrates, for example, plastic substrates. Too high a surface tension can lead to ink pooling and/or a mottled appearance in high coverage areas of the print. Too low a surface tension can lead to excessive ink bleed between different coloured inks.

The surface tension is preferably in the range of 20-40 mNm-1 and more preferably 25-35 mNm-1.

Preferably, the surfactant is present in each inkjet ink in an amount of 0.01 to 5% by weight, based on the total weight of the ink.

The amounts by weight provided herein are based on the total weight of the ink.

The inkjet ink preferably exhibits a desirable low viscosity (100 mPas or less, more preferably 50 mPas or less and most preferably 35 mPas or less at 25°C). The ink most preferably has a viscosity of less than 20 mPas at 25°C. Viscosity may be measured using a rotational viscometer fitted with a thermostatically controlled cup and spindle arrangement, running at 20 rpm at 25°C.

Other components of types known in the art may be present in the ink to improve the properties or performance. These components may be, for example, pH buffers, humectants, defoamers, dispersants, synergists, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers.

The inks of the inkjet ink set may be prepared by known methods such as stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill.

The present invention also provides a method of inkjet printing comprising inkjet printing the inkjet ink set as defined herein onto a substrate and evaporating the water.

In the method of inkjet printing of the present invention, the inkjet ink set is inkjet printed onto a substrate. The printing is performed by inkjet printing, e.g. on a single-pass inkjet printer, for example for printing (directly) onto packaging, or a multiple-pass printer where the image is built up in print swathes. As discussed above, inkjet printing is well-known in the art and a detailed description is not required.

The ink is jetted from one or more printing heads through narrow nozzles on to a substrate to form a printed image.

Print heads account for a significant portion of the cost of an entry level printer and it is therefore desirable to keep the number of print heads (and therefore the number of inks in the ink set) low. Reducing the number of print heads can reduce print quality and productivity. It is therefore desirable to balance the number of print heads in order to minimise cost without compromising print quality and productivity.

The substrate is not limited but include those for packaging applications. Examples include substrates composed of polyvinyl chloride (PVC), polystyrene, polyester, polyethylene terephthalate (PET), polyethylene terephthalate glycol modified (PETG) and polyolefin (e.g. polyethylene, polypropylene or mixtures or copolymers thereof). Further substrates include all cellulosic materials such as paper and board, ortheir mixtures/blends with the aforementioned synthetic materials. In a preferred embodiment, the substrate is a compostable substrate. A substrate is compostable if it meets the criteria as set out in BS EN 13432. Examples of compostable substrates are commercially available, including Sustainex (supplied by Mondi as an alternative to PE packaging material) and Nativa (a compostable bioplasfic polylactic acid (BoPLA) derived from plant sugars, supplied by Taghleef). Nativa can be derived from any sugar, such as corn starch, cassava, sugar cane, or sugar beet.

When discussing the substrate, it is the surface which is most important, since it is the surface which is wetted by the ink. Thus, at least the surface of substrate is composed of the above-discussed material.

The inks are then dried. By drying, it is meant the removal of the water (and optional solvent) by evaporation. Evaporation of the water can occur simply by exposure of the inks to the atmosphere, but the inks may also be heated to accelerate evaporation.

In the embodiment where the inkjet ink comprises a radiation crosslinkable component, such as a radiation crosslinkable resin, the inks are also cured by exposing the inkjet ink to a curing source. By curing, it is meant the crosslinking of the crosslinkable component. The inks may be cured by any means known in the art, such as exposure to actinic radiation and low-energy electron beam radiation.

The present invention also provides a printed substrate obtainable by the method of inkjet printing according to the present invention. The printed substrate having the inkjet ink set of the present invention printed and dried thereon has a maximum amount of copper of 50 ppm. Preferably, the printed substrate having the inkjet ink set of the present invention printed and dried thereon has a maximum amount of copper of 50 ppm and nickel of 25 ppm. More preferably, the printed substrate having the inkjet ink set of the present invention printed and dried thereon has a maximum amount of copper of 50 ppm, nickel of 25 ppm, zinc of 150 ppm, cadmium of 0.5 ppm, lead of 50 ppm, mercury of 0.5 ppm, chromium of 50 ppm, molybdenum of 1 ppm, selenium of 0.75 ppm and arsenic of 5 ppm.

The present invention also provides a printed substrate having the inkjet ink set as defined herein printed thereon.

The substrate is preferably as discussed hereinabove. Preferably, the substrate is a compostable substrate. A substrate is compostable if it meets the criteria as set out in BS EN 13432.

The inkjet ink set is particularly suitable for composting. Accordingly, the present invention further provides a method of composting a printed substrate having the inkjet ink set as defined herein printed thereon comprising composting the printed substrate. The criteria for composting is set out in BS EN 13432.

The invention will now be described with reference to the following examples, which are not intended to be limiting.

Examples

Example 1

An ink was prepared according to the formulation set out in Table 1. The inkjet ink formulation was prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

Table 1

Component Supplier/material type Amount (weight %) Polyethylene glycol Sigma 6.61 (MW 20,000) Aldrich/Solvent Propylene glycol Sigma 3.31 Aldrich/Solvent Glycerol Sigma 6.95 Aldrich/Solvent Surfynol 440 Evonik/Surfactant 0.26 Proxel GXL Lonza/Biocide 0.075 Projet APD 1000 blue Fujifilm/Aqueous pigment 25.16 dispersion Rovene 4111 Mallard 7.41 Creek/Resin dispersion Water - 50.225 Total 100 Projet APD 1000 blue is an aqueous pigment dispersion and contains 15.9% PB60 pigment, based on the total weight of the pigment dispersion. The ink therefore contains 4% P860 pigment based on the total weight of the ink. The pigment dispersion was prepared by mixing the components in the given amounts and passing the mixture through a bead mill until the dispersion had a particle size of less than 0.3 microns.

Rovene 4111 is a styrene-butadiene resin dispersion and contains 54% solid polymer content, based on the total weight of the resin dispersion.

PB60 pigment has a heavy metal content as set out in Table 2, as determined by ICP. Specifically, the total heavy metal content was determined by the following method: Before commencing the analysis, the powder pigments are digested in a mixture of hydrochloric and nitric acids to ensure that the total metal content, rather than the free metal levels in the samples are correctly determined. This is achieved by gently heating a mixture of the pigment with a 33%v/v solution of HCI and HNO3 (ratio of acids 1:3) on a hotplate until the pigment is dissolved. To obtain quantitative data for the metal levels, a calibration standard is used for each metal. Suitable certified ICP-OES metal standards can be obtained from Sigma. It is necessary to take into account the various interference bands of each metal and ensure the concentration range brackets the concentrations relevant to the application. A known amount of the prepared pigment sample is combined with a known amount of deionised water to create a solution that will position the concentration of metals present roughly into the centre of the calibration range. The analysis can then be conducted on a suitable ICP unit, for examples a Thermo ICS-5000 ICP-OES. Both the dual radial and axial detectors are used. First, the calibration is run, followed by a period of blank deionised water to flush the system, the samples are then run in duplicate. Using the calibration, it is then possible to calculate the concentration of metals present.

Table 2

Heavy metal Ca Fe Si K Na Cr Ni Al Cu Mg Zn Zr Content (PPm) 31.24 109.1 3.15 3461.73 620.1 4.34 2.19 2.78 1.08 6.98 0.21 0.31 The copper content of a compostable substrate (1 m2 and 500 pm thick) having the ink of Table 1 printed thereon to produce a 5 pm dried film weight was calculated, where the substrate and dried ink film density was 1.1 g/cm3.

In order to calculate the copper content, the percentage pigment in the film after drying was first calculated. Based on the ink of Table 1, the percentage of film remaining after thermal drying is 14.61% (4% solid polymers from Rovene 4111, 4 % pigment and 6.61% PEG 20,000). The percentage pigment in the film is therefore 4/14.61 x 100 = 27.38%.

The compostable substrate weight was also calculated. The volume of the 1 m2 and 500 pm thick substrate is 100 x 100 x 0.05 = 500 cm3 which multiplied by the relative density of 1.1 g/cm3, provides a compostable substrate weight of 550 grams The weight of the pigment in the film, where the film is 1 m2 and 5 pm thick, was also calculated. In order to do so, the weight of the ink for the 5 pm film was calculated. In this regard, the volume of the 5 pm film was calculated as 100 x 100 x 0.0005 cm = 5 cm3, which multiplied by the relative density of 1.1 g/cm3, provides a weight of the ink film of 5.5 grams. The percentage pigment in the film is 27.38% and as such, the weight of the pigment in the film is 5.5 x 0.2738 = 1.506 grams.

The copper content (ppm) of the print was then calculated. In this regard, the total weight for the 1 m2 printed substrate is 550 grams + 5.5 grams = 555.5 grams. In order to calculate the copper content of the print (ppm), the weight of the pigment in the film 1.506 g was divided by the total weight of the printed substrate of 555.5 g, and then multiplied by 1.08 ppm copper content in the pigment to provide a copper content in the printed substrate of 0.00293 ppm copper in printed substrate.

Comparative Example 2 A comparative ink was prepared according to the formulation set out in Table 3. The inkjet ink formulation was prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

Table 3

Component Supplier/material type Amount (weight %) Polyethylene glycol Sigma 6.61 (MW 20,000) Aldrich/Solvent Propylene glycol Sigma 3.31 Aldrich/Solvent Glycerol Sigma 6.95 Aldrich/Solvent Surfynol 440 Evonik/Surfactant 0.26 Proxel GXL Lonza/Biocide 0.075 Projet APD 1000 cyan Fujifilm/Aqueous pigment 28.57 dispersion Rovene 4111 Mallard 7.41 Creek/Resin dispersion Water 46.815 Total 100 Projet APD 1000 cyan is an aqueous pigment dispersion and contains 14% PB15.3 pigment, based on the total weight of the pigment dispersion. The ink therefore contains 4% PB15.3 pigment based on the total weight of the ink. The pigment dispersion was prepared by mixing the components in the given amounts and passing the mixture through a bead mill until the dispersion had a particle size of less than 0.3 microns.

Pigment PB 15.3 contains 11% Cu atoms, and as such contains 110,000 ppm Cu.

Using the same method and calculation as for Example 1, the total copper content (ppm) of the print was calculated.

In order to calculate the copper content of the print (ppm), the weight of the pigment in the film 1.506 g was divided by the total weight of the printed substrate of 555.5 g, and then multiplied by 110,000 ppm copper content in the pigment to provide a copper content in the printed substrate of 298.2 ppm copper in printed substrate.

It can be seen from the calculations of the copper content in the printed substrate for the inks of Example 1 and comparative Example 2 that the ink of the invention provides a 5 pm film which falls within the limits set out in BS EN 13432 for the copper content of 50 ppm, as it has a copper content in the printed film of 0.00293 ppm. This is in marked contrast to the comparative ink which provides a 5 pm film which falls outside the limits set out in BS EN 13432 for the copper content of 50 ppm, as it has a copper content in the printed film of 298.2 ppm.

Claims (11)

1. Claims 1. An aqueous inkjet ink set comprising four or more aqueous inkjet inks, each aqueous inkjet ink comprising a pigment and water, wherein the inkjet ink set includes a blue and/or cyan ink, a yellow ink, a magenta ink and a black ink, and wherein the total amount of copper present in the inks of the ink set combined is 10,000 ppm or less.
2. 2. An aqueous inkjet ink set as claimed in claim 1, wherein the total amount of nickel in the inks of the ink set combined is 3,000 ppm or less. 10
3. 3. An aqueous inkjet ink set as claimed in claim 2, wherein: the total amount of zinc in the inks of the ink set combined is 10,000 ppm or less; the total amount of chromium in the inks of the ink set combined is 5,000 ppm or less; the total amount of lead in the inks of the ink set combined is 5,000 ppm or less; the total amount of molybdenum in the inks of the ink set combined is 100 ppm or less: the total amount of cadmium in the inks of the ink set combined is 100 ppm or less; the total amount of mercury in the inks of the ink set combined is 100 ppm or less; the total amount of selenium in the inks of the ink set combined is 100 ppm or less; and the total amount of arsenic in the inks of the ink set combined is 100 ppm or less.
4. 4. An aqueous inkjet ink set as claimed in any preceding claim, wherein the inks of the ink set are free of heavy metals.
5. 5. An aqueous inkjet ink set as claimed in any preceding claim, wherein each aqueous inkjet ink comprises an organic solvent.
6. 6. An aqueous inkjet ink set as claimed in claim 5, wherein the organic solvent is a biodegradable solvent, preferably selected from glycerol, (poly)ethylene glycol, (poly)propylene glycol and mixtures thereof.
7. 7. A printed substrate having the inkjet ink set as claimed in any of claims 1 to 6 printed thereon.
8. 8. A printed substrate as claimed in claim 7, wherein the substrate is compostable.
9. 9. A method of inkjet printing comprising inkjet printing the inkjet ink set as claimed in any of claims 1 to 6 onto a substrate and drying the ink, preferably wherein the substrate is compostable.
10. 10. A printed substrate obtainable by the method of inkjet printing as claimed in claim 9.
11. 11. A method of composting a printed substrate according to any of claims 7-8 and 10 comprising composting the printed substrate.