Latex-based aqueous UV-curable inks

EP 2 960 306 A1
Aqueous radiation curable inkjet inks
Agfa Graphics NV

UV curable ink jet inks lacking solvents have been used extensively in industrial applications, because of their reliability and because they adhere to a multitude of substrates. One disadvantage of these “100%” solids UV curable ink jet inks is that relatively thick layers are printed, compared to aqueous ink jet inks which generally contain less than 20% – and usually less than 10% – solids. Thin layers generally have better flexibility than thick layers and so a low solids UV curable formulation would potentially be advantageous. However in most other aspects, low solids aqueous latex ink jet inks tend to exhibit an inferior performance compared to UV curable inkjet inks.

A latex is a stable colloidal dispersion of natural or synthetic polymer particles in an aqueous medium. Synthetic latexes can be made by polymerising monomers, generally (meth)acrylates, that have been emulsified with surfactants, and such materials have been frequently used in ink jet inks. Acrylate based polymeric particles often have a relatively high minimum film formation temperature (MFT), which prevents the use of temperature sensitive substrates for latex ink jet inks. The MFT tends to be very closely related to the glass transition temperature (Tg) of the latex polymer and so thermosensitive substrates could feasibly be used if the Tg of the latex ink was low, below 0C for example. Unfortunately, such low Tg latexes tend to show poor jetting properties and blocked nozzles as the latex can easily film form in the nozzle.

This patent suggests that low MFT latex dispersions can be jetted successfully if fully water insoluble (meth)acrylate monomers are mixed into this latex. It is believed that this liquid polymerisable compound rapidly migrates into the surface of the hydrophobic polymer particles, forming a barrier and protecting them against agglomerating and fusing together. This is suggested to be particularly advantageous for low MFT (<0C) polyurethane particles which, without the exemplified monomers, can only be reliably used in ink jet inks at very low levels when high levels (>10 wt%) would need to be used to confer the required physical properties to the final printed film.

The experimental section discusses this approach in detail. One example took 100g of a 42 wt% solids polyurethane dispersion (Bayhydrol UV XP 2689) and added this to 100g of water. To this was added 5.3g of the monomer dipropyleneglycol diacrylate (DPPA) and stirred for 20 minutes at 800 rpm using a Disperlux mixer. This mixture was then filtered and used in an ink jet ink at 23-35 wt% along with 20 wt% of a colour pigment dispersion (Diamond D75C – a 15 wt% dispersion of C.I. Pigment Blue 15:3 in water), humectant (10 wt% 2-pyrrolidone and 20 wt% 1,2-hexane diol), photo initiator (1.1 wt% Irgacure 500) and water. Comparative inks were also formulated using only the polyurethane dispersion without the addition of the UV curable monomer or photo initiator.

These inks were evaluated on a Dimatix DMP2831 ink jet printer (10pl head) with a cartridge temperature set to 24C, at a firing frequency of 5 kHz and a firing voltage of 20-25V. The results show that at the higher levels of polyurethane dispersion, only the exemplified monomermodified dispersions show good ink jet performance, whereas the unmodified inks would not jet.

Further inks were formulated using this dispersion mix with an aim to investigate various photo initiators. Three groups of photo initiator were tested, water soluble, water insoluble and acrylated photo initiators. These inks were tested by applying them on polyester film, drying at 60C for 3 minutes then curing using a Fusion VPS/1600 lamp at 20 m/min. HPLC was used to test for the presence of any extractable photo initiator from the cure films. The results clearly show that the reactive acrylated photo initiators resulted in far less PI leaching from the cured film. No data is presented regarding mechanical properties.

Mirror image surfaces for décor applications

EP 2 871 062 A1
Production method of recording material, and recording material
Seiko Epson Corporation

This extensive patent (84 pages) is, like the following one, concerned with the production of a glossy or mirror-finished surface, however in this context it is applied to the production of hard surfaced décor panels and similar articles. The method can be used as an alternative to metallic plating or foil stamping and is beneficial in that it can be applied to a curved surface, and also that patterning and gradation in the metal tone can be achieved.

May-15In essence, two UV curable layers are sequentially printed on the substrate surface. The first layer is cured prior to printing the second layer so that it creates a regular dimpled texture with a surface roughness of 3 to100 μm; the metal-powder-containing upper layer has a planarising effect. Thus the time elapsing between the landing of any drop and curing is critical. In the case of the first layer (2), it is cured within a second of printing while a longer time (5-60 seconds) can elapse between deposition and cure of the top layer (3). The elapsed time can have significant influence on the appearance of the printed material.

While the substrate 1 can be more or less anything and may be absorbing or non-absorbing, a non-absorbing polyester-based substrate for which the contact angle of the ink droplets is at least 10 degrees is preferred and plastics such as PET are particularly suitable.

The first layer is formed from an ink comprising 50-88% of at least one monomer with an alicyclic structure, as this gives good adhesion to the polymer substrate. If at least some (15-75%) of this content is a monofunctional monomer with a heteroatom in the alicyclic structure, such as tris (2-(meth)acryloxyethyl) isocyanurate, then curing shrinkage can be minimised and furthermore, the long term jetting stability is good. The remainder of the monomer content need not have an alicyclic structure and can enhance some aspects of the film, for example inclusion of 2-(vinyloxyethoxy) ethyl acrylic acid improves the cure speed while inclusion of phenoxyethyl acrylate gives rise to a relatively flexible film. Inclusion of a colourant or metal powder in this layer can influence the colour tone or the opacity of the finished article.

The second, and metallic ink, will be deposited at a volume of 80 to 200% of the deposition volume of the first ink, in order to ensure full coverage and a glossy mirror finish. The greater print to cure interval also enables a smooth surface. More or less any metal powder (31) can be dispersed in this ink at a content up to 10% w/w. Ideally, the powder is formed by CVD and subsequently pulverised to give a flake structure which may additionally take the form of curved flakes for additional effects. The thickness of the flakes is preferably in the 20 to 80 nm range while the diameter is between 500 nm to 3 μm. Treatment of the flakes with a fluorinated compound, such as a silane, disperses the particles in the ink and enhances its storage stability, as well as improving the abrasion resistance and gloss of the printed film. In common with the first ink, the bulk of this ink is also made up of alicyclic monomers which will result in good inter-film adhesion, while the inclusion of minor quantities of other monomers will improve dispersion stability of the metal flakes, enhance cure rate and endow the final article with specific properties such as hardness and flexibility.

Low migration UV-curable ink for packaging films

US 2014/0285568 A1
Curable liquids and inkjet inks for food packaging applications
Agfa Graphics NV

The use of radiation curable ink jet inks is preferred for ink jet printing onto non-absorbing surface due to the rapid drop solidification and subsequent removal of the drying stage needed with solvent inks. Due to the demand for increased print speeds and image quality, UV curable ink jet inks are necessarily of relatively low viscosity, thus limiting the materials available. In addition, low viscosity curable liquids tend to be either toxic, irritating or both. This patent discusses the use of very low viscosity and relatively benign monomers containing both acrylate and vinyl ether functionality (discussed in a previous Agfa patent application EP 0097508 A), as applied to food packaging.

Even when using the safest of UV materials, there is some resistance to residual monomers coming in contact with food. In addition, photo initiators, UV stabilisers and colorants also present issues if allowed to move into foodstuffs. Extractable monomers can cause problems in two separate ways; set-off and migration. Set-off occurs in roll-to-roll printing where the printing front-side of a packaging material comes in contact with the unprinted back-side and unreacted monomers are “set-off” onto the back-side intended for food contact. Migration occurs when the unreacted monomers move through the packaging film to the covered foodstuff.

The solution to the former issue is to ensure the formulation cures fast enough and to a sufficient degree and hardness such that transfer set-off is impossible. This is also discussed in the previous Agfa patent above. The migration issue is also partially solved by this approach, but the first step in the ink jet printing process is the deposition of uncured monomers onto the packaging film. It is this monomer migration into the film, in the period between monomer/film contact and cure, that this patent addresses.

Popular packaging materials that show issues with monomer migration tend to be olefin base polymers such as polypropylene film. Due to the very low viscosity of some of the exemplified inks, the patent suggests that the monomers can easily penetrate into the substrate before they can be effectively cured. The proposed solution to this is the application of a primer coating that acts as a barrier to monomer migration. It is claimed that the key to this is the combination of a free-radical polymerisable monomer or oligomer, at least one diffusion hindered acetalisation catalyst and a diffusion hindered hydroxyl containing compound.

The diffusion hindered materials are simply low molecular weight species covalently bonded to high molecular weight polymer or fragments. The experimental describes the preparation of such a diffusion hindered UV sensitiser, shown below. This molecule has an additional benefit of having two acrylate groups available to react the UV active moiety into the cured network, ensuring zero migration.

Sept-14The experimental discloses the composition of a full CMYK ink jet designed to show no migration when cured and so no set off, but the core of the patent is the primer. This material consists of propoxylated neopentyl glycol diacrylate (67 wt%), triglycerol diacrylate (15 wt%), a phosphate ester of propylene glycol monoacrylate (5 wt%) and 11 wt% of various polymeric photoinitiators.

How the glycol and the phosphate ester (acetalisation catalyst) helps prevent monomer migration is not mentioned, but the results are impressive. Without a primer layer, monomer extractables measured through the back of the printed polypropylene film are up at around a few thousand parts per billion (ppb). With a comparative primer without the acetalisation catalyst the measured extractables were down to a few hundred ppb, and with this catalyst the results were at <10 ppb.

Seiko-Epson’s drum-based label press with UV inks

US 2013/0127962 A1 
Image recording device, image recording method 
Seiko Epson Corporation

This is a design for a web-based printer using UV-curable inks.  Epson is concerned that as well as exposing the web to UV light, a lot of heat is also generated by curing systems.  As the temperature of the substrate will rise much more where ink has been printed due to better absorbtion, the substrate can experience distortion and wrinkling.  This can also lead to uneven glossiness of the image, and changes in the texture and even the colours printed.  Substrates including paper, PET, PP and the like are considered.

May13

The solution is to use a drum-based system with a cooling mechanism.  The substrate is fed from roll 20 and through rollers 31.  At the other side the web is taken up on roll 40.  Rollers 32 draw the web through the system, with rollers 31 also driven to adjust the tension around the drum.  As the web passes around the drum the image is printed by heads 51 with pinning of the image using UV-lamps 61.  A more powerful UV source 62 fully cures the image.

There is a further ink jet head 52 that can jet transparent ink on the substrate to adjust the gloss levels of the image or overall.  This ink is cured by UV source 63.

Heat is removed from the system by fan 81 which blows cool air on the region of the drum not covered by the web.

The patent applications cover other topics such as web tension, and moving the printheads away from the drum when the web tension is being adjusted.

Improved head and UV lamp layout

US 2012/0287214 A1
Image forming apparatus
Seiko Epson Corporation

One of the problems with printers using UV-curable inks is that the curing systems must be placed relatively close to the printheads.  However if stray UV-light reaches the ink in the nozzles it can cure the ink at the meniscus and stop the printhead from working.  Most of the light that can reach the printhead is reflected from the substrate, so the standard technique is to space the printhead and curing system as far apart as is practical and insert an optical trap in between.  An optical trap is normally a component with multiple walls painted black to intersect and absorb as much light as possible.

Nov12

The solution here is to angle both the lamps and printheads towards the paper feed directions as shown in the figure.  This reduces the reflected light and removes the need for the optical traps.  This enables the printheads to be mounted closer together.  When printing and curing multiple colours, the closer together the printheads are together the better the registration between colours is likely to be.

Emulsion-based thermal ink jet inks

WO 2012/105950 A1
Emulsion-based thermal inkjet inks
Hewlett-Packard Development Company

Thermal ink jet technology requires an ink to explosively boil when the relevant firing resistor is activated, and thus print a droplet.  This generally requires inks that use carrier liquids, such as water, that boil at relatively low temperatures.  There are a huge range of applications that require the jetting of fluids that do not boil, either because their boiling points are too high at atmospheric pressure, or they do not have a clearly defined liquid/gas phase transition.

So although piezoelectric printheads are more expensive to produce, they are able to jet a significantly larger range of ink jet ink chemistries than the cheaper, but more limited, thermal printheads.  Some progress has been made in which UV curable formulations have been jetted through thermal printheads, but these have used water soluble UV monomers as they still require water (or some similar organic solvent) to form the bubble.

Using water as a solvent for such aqueous soluble UV curable materials has its own problems, one being that as the concentration of monomers is increased and the proportion of water decreased, the first order phase transition to the gas phase can be smeared out and so the bubble production is impacted.  Ideally more typical hydrophobic monomers would be used in thermal printheads, but as these show very limited water solubility they have seen limited success.

This patent presents an approach to allowing a wide range of high boiling point, hydrophobic solvents, to be used in thermal printheads without resorting to low boiling co-solvents as the drive fluid.  The inventive step is to formulate an ink that is mainly a hydrophobic, high boiling liquid with water dispersed in this continuous phase as a stable water-in-oil emulsion.  It is suggested that other low boiling solvents could be used in this way, but water is the main target.

Aug12

The figure left demonstrates the theory behind this approach.  A surfactant 12 is added to the high boiling point solvent at an amount above its critical micelle concentration (CMC).  Then, in the absence of water, the surfactant will form micellar structures 14 in which the surfactant polar groups 12a are situated on the inside of a spherical structure.  Small additions of water will hydrate the core 14a of the micelles to give the structure 18, but at this point the water is effectively dissolved in the surfactant and so will not possess bulk water properties.  As the water content is increased, there is a gradual transition to a vesicular structure 20, where a water core 20a is surrounded by a surfactant shell.  In this state the water present possesses its bulk properties and if the vesicle size is small enough it can form a stable dispersion in the continuous oil phase.  Ideally this stable state is a micro-emulsion.

The patent suggests that it has been experimentally shown that 5% water in such a micro-emulsion is enough to jet a hydrocarbon oil-based ink or commercially available UV ink.  In one example 5 wt% of an anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and 7.7 wt% water were added to Isopar L and mixed to give a micro-emulsion of water-in-oil.  A magenta ink was then formulated by the addition of 2 wt% pigment and this was successfully jetted through a thermal printer, giving a drop weight of 10 pl and a drop speed of 7.5 m/s with a 25V/1.7µs drive pulse.

A similar example took an HP Scitex FB221 light magenta ink and added 8 wt% AOT, 8 wt% water, 8 wt% Isopar L and 15 wt% lauric acid and mixed to give a micro-emulsion.  The lauric acid was included as a viscosity modifier to bring the viscosity below 4 cps.  This formulation was again jetted through a thermal printhead giving a drop weight of around 10 pl and a drop velocity of 7.5 m/s using a drive pulse of 24 V at 3 µs and a precursor pulse of 0.5 µs.

Although these examples seem somewhat preliminary, they seem to prove an interesting point that complex inks, viscosity allowing, could be jetted through thermal printheads.  It will be interesting to see if more specific and complete patents will be published using this chemistry technology to allow the printing of other complex and functional inks through thermal printheads.

Fixed array printhead for higher viscosity inks

US 2012/0069104 A1
EP 2 436 520 A1
Liquid discharge head and recording device using same
Kyocera Corporation

Here Kyocera wishes to improve image quality when using higher viscosity inks, such as UV curable. These inks have a higher viscosity than aqueous inks – 8 cP is mentioned, and therefore the actuators must be driven to generate higher pressures to form drops at the same velocity. Unfortunately, at the higher pressures satellite drops are formed and image quality deteriorates.

Mar12a  Mar12b

The solution is to introduce a restriction in the descending passage leading to the nozzle. Here a section through the printhead is shown. Ink flows from manifold 5 into actuator chamber 10 via connecting passageway 6. The actuator consists of two piezo layers 21a and 21b, both around 20 microns thick. In between the piezo layers is a common electrode 34, and on the top the individual electrode 35. A stack of Fe-Ni plates 4, each separately etched, defines the ink chamber and channels. The plates are fixed together using a thermosetting resin with a curing temperature of 100-150C. The reason for the relatively low temperature curing is to reduce the chances of stresses being set up between the different materials on cooling.

<a href=”http://inkjetpatents.com/blog/wp-content/uploads/2012/03/Mar12b1.gif”><img class=”size-full wp-image-75 alignright” title=”Mar12b” src=”http://inkjetpatents.com/blog/wp-content/uploads/2012/03/Mar12b1.gif” alt=”” width=”297″ height=”218″ /></a>Drops are fired using a bi-polar waveform. To begin with the piezo actuator rises, increasing the chamber volume. The drive waveform is then reversed driving the actuator down and reducing the chamber volume. The pulse width between these two events is set to the acoustic length between the manifold 5 and the nozzle 8. This allows the positive reflected pressure wave from the expansion phase to be added to the drive phase so that a stronger overall pressure wave is generated. This is the well-known “fill before fire” technique. Drops generated are 5-7 pl.

Returning to the objective of these patent applications, that is to reduce the satellite drops being generated, the solution is to introduce a restriction in the descender. This increases the damping and hence pressure oscillation at the nozzle. The feature is defined by plate 28, where the descender diameter is reduced from Sd1 to Sd3 with a height Ld3. The distance Ld2 is also important. Ratios of Ld3/Ld0 of 10-15% and Ld2/Ld0 of 20-40% were effective at eliminating satellite drops during tests.

Mike Willis