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.

Detecting drops by measuring temperature of actuator chamber

US 2015/0321470 A1
Base, liquid discharge head, printing apparatus, and method for determining liquid discharge status
Canon Kabushiki Kaisha

With increased interest in high resolution page arrays, a major issue is maintaining print quality in the event of nozzle failures. Nozzles may fail for a variety of reasons — clogging due to debris or the ink viscosity increasing locally. Air bubbles may get trapped inside the actuator chamber, or the nozzle plate can become contaminated. Many of these failures can be overcome by performing some nozzle maintenance, while firing adjacent nozzles can compensate for more permanent failures.

While it is possible to regularly perform nozzle maintenance, this can be problematic with single pass page array printing. It is possible to spit drops onto the substrate either in between pages, or even in the images or background areas. but any more extensive maintenance, for instance cleaning the nozzle plate or purging ink from the nozzles, requires a halt to printing while the maintenance station and printhead are brought together.

Therefore it is becoming more desirable to know the status of all nozzles all of the time. With this knowledge nozzle maintenance can be carried out when required, rather than at regular intervals. In addition missing nozzle compensation algorithms can be used to maintain high print quality in the event of nozzle failures.

Popular techniques for determining missing nozzles are optical systems to either detect drops in flight or dots on the substrate after printing. Neither of these processes are easy to implement.

470-4a470-4b

Here Canon explores building detection circuits under the heaters in each actuator chamber to detect partial or full drop generation failures. The detection circuits track temperature to determine if the drops are fired or not. In the figure heater 104 is generating a bubble within an actuator chamber. Underneath the heater is the temperature detection circuit 105 which records the temperature rise and decay. If no drop is generated then the ink remains in the chamber and the heater is completely uncovered by the bubble. If a drop is generated then some ink is left above the heater. These different conditions can be detected thermally.

470-6b

On the left the temperature detection circuits are shown. These are underneath the heater, separated by an insulating film. There are two circuits, 105a under the centre of the heater and 105b under the periphery of the heater. Both circuits have serpentine-shaped tracks to increase the circuit length and therefore the resistance, making it easier to measure variations in voltage in the circuit. The connections to the circuits 110 are taken out by vias to tracks on a layer underneath the detection circuits, so they are less affected by the temperature changes.

The two circuits for each heater 105a and 105b are connected in a detection circuit. The outputs of differentiators 121 and 122 feed the comparator 123 resulting in Vout. If there is a drop discharge failure there is always a bubble on the surface of the actuator chamber above the heater, so the temperature gradually lowers. If a drop is generated then heat is removed from the chamber by the ink drop, so the temperature of the surface of the chamber decreases much more rapidly.

470-9

The graph shows the output voltage Vout from the comparator. If V2≥V1 then the output is high. If V1>V2 then the output is low. If no drop is generated then V1>V2 is always true so the output stays low. The time for the measurement Tk is chosen to be when the ink fragment left from the discharge should remain on the heater, and before the refill process.

The detection circuits are integrated with the silicon substrate of the printhead die. The detection process is very sensitive yet accurate. no complicated signal processing is required, so detection is very fast.

Ricoh recirculating ink printhead

US 2015/0306875 A1
WO 2015/163487 A1
Inkjet head that circulates ink
Ricoh Printing Systems America, Inc.

The benefits of circulating ink through the actuator chambers of ink jet printheads is now well known. The chambers are easily and quickly primed, air bubbles can be displaced, inks with a tendency for pigment settling or separation can be used, and fresh ink is brought to the nozzles reducing the requirements for nozzle maintenance.

Oct-15aInk passes from an inlet manifold 302 through passageway 225 into the actuator chamber 221. In this design a second passageway 223 close to the nozzle allows ink to return to a separate manifold 304. Ink being supplied to the printhead is fed under a positive pressure, and a negative pressure is applied to the return path.

Oct-15bOverall though, the average pressure is slightly negative by arranging the negative pressure to be higher than the positive. Therefore there is enough differential pressure to achieve the continuous flow through the actuator chambers and a net negative pressure at the meniscus.

The ink flow paths and pressure chambers within the printhead are made up from a stack of etched stainless steel plates that are bonded together. Note that as the inlet and outlet manifolds are both the same side of the actuator chambers, then the overall width of the printhead is minimised.

Decorating on coffee

US 2015/0251470 A1
Method and apparatus for applying designs on the surface of a beverage
Steam CC Ltd.

This patent application from an Israeli company is a machine for printing onto frothed milk on the top surface of premium coffees, such as café au lait, cappuccino, latte or the like. It is already carried out manually, by pouring coffee solution onto the frothed milk in a pattern, pouring frothed milk on the coffee while moving it, or by using a utensil dipped in coffee to trace a design.

Sept-15The printer consists of an ink jet printhead on an X-Y gantry so that the printhead can be moved across the top of the coffee cup 9 in both axes. The cup isn’t moved so as not to disturb the foamed milk and hence the image being printed. The only adjustment involving the cup is the height and hence spacing between the foamed milk surface and the printhead, adjusted by table 8.

The “ink” printed is a coffee solution. The system enables much more elaborate designs to be printed compared to manual methods, plus repeatability.

Low-cost moulded page array printhead from HP

WO 2015/116025 A1
Flexible carrier
Hewlett-Packard Development Company

This review is from a further batch of patent applications describing more details of HP’s moulded printheads. We discussed these in the September/October 2014 issue of Directions. HP introduced their low-cost page array printhead in the OfficeJet Pro X printer in 2013. The printhead used consists of a staggered array of four colour dies.

The new moulded printhead design has some important benefits. First of all it allows the use of much narrower silicon dies. Patent applications related to the first low-cost page array printhead design described the die size as 5 mm wide. The moulded printhead patent applications describe the dies as “slivers” 500 microns wide, so an order of magnitude narrower.

The use of moulded epoxy resin for the body of the printhead significantly reduces the cost of the substrate for the dies – considerably cheaper than others previously considered or used, such as silicon, glass, ceramic and so on. HP is also able to incorporate in-flight drop detection or image scanning into the moulding on the front face. In addition other patent applications describe ink property sensing and ink level detection.

The first of the patent applications we have selected for review describes the manufacturing processes for the moulded printhead, in particular the use of a flexible carrier sheet.

Aug-15aThe first stage of the manufacturing process is to fix a flex circuit 66 with conductors 22 on to a flexible carrier sheet 68 with a thermal release tape 70. The flexible carrier sheet used can be cured epoxy sheet or a high temperature plastic. It is also possible to use semi-rigid and rigid materials such as metal, carbon fibre, composites etc. by adding grooves so that in the final stage of manufacturing the substrate can be peeled away.

The printhead die 12 is then positioned over the gap 72 in the flex circuit. The die is 500 microns wide, 100 microns thick and 26 mm long. Dry etching is used to form the ink flow port 56. This is possible and viable due to the thin structure.

On the front face of the die the ink channels 54 and actuator chamber 50 are formed in the usual way in a spin-on photo-imageable resin, and the nozzle layer formed on top. The die contains not just the heaters for drop actuation, but addressing and drive circuits too. Electrical contacts 24 connect to the flex circuit tracks 22.

Aug-15bAfter the die is in position on the substrate, the body of the printhead14 is formed by transfer moulding with tool 74. The transfer moulding process has been adapted by standard techniques used in the semiconductor industry for device packaging. Preferably no release agent is used in the moulding process, the concern being contamination of the surfaces that will come into contact with the ink.

The stiffness of the moulding can be adjusted, depending on whether the print bar will be used directly or whether it needs to conform to a separate support structure.

Aug-15cThe final step is to strip away the flexible carrier 68 and thermal release tape 70 to leave the finished print bar. Note that with this design there is a single wide ink manifold slot 16 stretching along the full length of the die, with separate ink feeds 56 into each actuator chamber 50. As shown there are probably two rows of nozzles, one each side of the ink feed slot, and therefore using a single colour of ink. This would be a similar configuration to the new HDNA (High Definition Nozzle Architecture) printheads that will be field upgrades for HP’s high-speed web presses. These have 1,200 nozzles/inch in each row, and therefore 2,400 nozzles/ inch for each ink feed slot.

Aug-15dHere a wafer-scale view is shown. There are 5 print bars being made on one wafer, with each print bar having 4 rows of silicon die “slivers”. 10 dies are shown in a staggered arrangement across the width of the print bars, giving a print width of 230 mm. The width of each print bar is around 16mm. With a potential saving on manufacturing costs and printhead size, these printheads are likely to be very competitive with Memjet’s low-cost page wide arrays.

The ability to make such narrow low-cost arrays of nozzles leads to the ability to offer extra rows for redundancy in case any nozzles become defective. However to be able to substitute for non-working nozzles you need to know which ones they are. In the second patent application two schemes for determining these are described, in both cases with all of the functionality within the printhead.

Ink jet make-up applicator

WO 2015/097618 A2
Transfer makeup process and related device
L’Oreal

This patent application describes a process that L’Oreal have used to demonstrate transfer printing of makeup onto a human body. If a transfer process is used for makeup then it is desirable that the materials are still moist at the time of transfer and shortly after, so that the transfer process is successful and that any retouching or blending can take place after transfer.

Jul-15aA Canon Pixma IP100 printer was modified so that instead of feeding paper the mechanism would rotate a transfer roller. Inks were prepared according to the table below.

The transfer roller had a diameter 60 mm and length of 80 mm with a blanket surface 2 mm thick, made from a silicone gel elastomer.

Jul-15bAfter printing the transfer roller 1 is rolled along the appropriate body part transferring the makeup or image. A variety of different profile rollers are proposed, flat for backs and other body areas, and concave for arms and legs. For smaller parts of the body such as lips and eyelids then pads using the transfer material would be used.

High solids content ink for intermediate transfer

US 2015/0175821 A1
EP 2 886 618 A1
US 2015/0175820 A1
Aqueous dispersible siloxane-containing polymer inks useful for printing
Xerox Corporation

Aqueous indirect printing technology, in which an intermediate transfer member (ITM) is used to transfer the ink jet printed image to the target media, is currently of great interest and is generating numerous patents by a number of key players.

These Xerox patents take an interesting approach in that they focus solely on the polymer chemistry of the binder and then its interaction with the ITM surface. Firstly, the patent suggests that aqueous inks are preferred not just from an environmental perspective, but because they do not attack the ITM hydrophobic surface. It seems that this is especially true if the hydrophobic belt is treated with a release fluid similar to the dampening fluid used in lithographic printing, in this case suggested to be octamethylcyclotetrasiloxane.

In particular, the patent proposes the synthesis of a siloxane containing polymer dispersion to enable the efficient wetting of such a hydrophobic surface without having to use separate wetting agents, which apparently could swell and interfere with the dampening fluid and/or belt surface, causing print transfer issues. The polymer of interest is a self dispersing sulfonated polyestersiloxane, the core of which is similar to the excellent Eastman Eastek series of polymer dispersions. The benefit of such self dispersing polymers is that they contain essentially no surfactant and their inherent stability generally results in low viscosity dispersions of very low particle size.

The exemplified polymer was prepared by taking dimethylterephthalate (313g), sodium dimethyl 5-sulfoisophthalate (38g) 1,2-propane diol (250g), diethyleneglycol (37.5g), polydimethylsiloxane carbomethoxy terminated and butyl tin oxide (1g) as catalyst and were charged into a suitable one litre reaction vessel and heated to 175C for 1 hour, increasing to 185C for a further 3 hours, all the while removing the methanol and water by-product from the mixture as the polymerisation progressed. The vessel was finally taken to 200C and put under reduced pressure for 2 hours to remove excess glycol. The resultant polymer was discharged whilst still molten to give a polymer with a Tg of 55.2C, a number average molecular weight (Mn) of 2237g/mol and a weight average molecular weight (Mw) of 3777 g/mol. The polymer structure is shown in the diagram below, with n:m:X:p ratios of 1.00 : 0.86 : 0.61 : 1.090.

June-15
To prepare the ink, first a cyan (15 wt% pigment) dispersion in water was prepared containing a Dowfax surfactant as dispersant. 100g of this was heated to 80C and15g of the above resin was added, stirred for 1 hour and then allowed to cool to room temperature. This resulted in a blue ink with total solids of 27% and with a suggested viscosity of <10cps.

No ink jet printing is discussed, but the manual coating on to a treated hydrophobic transfer surface and then on to the media is discussed and it is suggested that the siloxane component is critical in formulations without aggressive wetting agents. An additional benefit of these self dispersing polymer dispersions is suggested to be the exceptionally high solids loading achievable for ink jet printable formulations. This is important as little water is apparently needed to significantly raise the viscosity to levels suitable for transfer to the media. What this means for the printhead dwell time is not discussed.

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.

Hollow titanium oxide particles for white inks

WO 2015/047306 A1
White pigment dispersions
Hewlett-Packard Development Company

We have discussed the issues regarding white ink jet inks many times in these pages when we have described the many different approaches employed to get over the unavoidable issue of good white pigments being generally heavy, and the pigment size being necessarily large in order to scatter the light. This leads to settling, and potentially complex ink systems.

Some of the more promising white ink jet ink inventions have involved neutral density hollow resin particles – using low refractive index “bubbles” rather than high refractive index dense pigment particles. Others have used a combination of the two. This patent poses the question that if titanium dioxide is the best white pigment, but is too dense, then why not lower its density to give a pigment incorporating the best of both worlds.

The patent suggests two approaches to achieving this, both of which grow the titanium dioxide particles around a template, to give hollow pigment particles with an overall density less than the bulk. The first approach uses a carboxylic acid functional polymer dispersion, such as poly(methyl vinyl ether-alt-maleic acid), in the presence of titanium (IV) oxysulfate. The patent suggests that the carboxylic acid groups of the dispersion strongly coordinate with the Ti4+ groups, resulting in a titanium rich shell. Calcination of this mixture at 550C results in the inventive low density pigment.

The second approach uses an amazing sequence of chemical reactions to form the hollow titanium dioxide particles. A mixture of titanium (IV) oxysulfate is dissolved into water, hydrogen peroxide and urea. The hydrogen peroxide decomposes to water and oxygen, and onto these bubbles adsorbs the titanium oxo species, which are reduced in situ by ammonia (from the hydrolysed urea) to give titanium hydroxide. The resultant sludge is calcined at high temperature to yield the hollow titanium dioxide particles with a claimed 35% reduction in density over standard titanium dioxide.

Inks were made from these lower density pigments and successfully jetted through HP thermal printheads. Pigment settling was claimed to be undetectable during the 45 day experiment.

Ink jet dye discharge printing for fabrics

WO 2015/025310 A1
Dye discharge inkjet ink compositions
Kornit Digital Ltd.

Dye discharge printing is a method of forming a design on a dyed fabric by printing onto this surface a colour-destroying (dye discharging) agent to create a white or light pattern. This discharge ink can also include a colorant that is unaffected by the discharge chemistry, thereby allowing light, vibrant colours to be printed onto darkly dyed backgrounds, with perfect registration.

An alternative approach to dye discharge printing, often used for printing onto dark coloured T-shirts, is one in which a thick opaque white background ink is first applied to the fabric before the subsequent colours. Although this is a cost effective and relatively simple approach, the resultant fabric can have a compromised feel due to the quantity of materials deposited on the surface. In comparison, dye discharge printing “removes” colorants resulting in a fabric with a soft hand-feel in the printed areas and so is often used for more luxurious fabrics.

A common discharge agent is zinc formaldehyde sulfoxylate (ZFS) that, although effective, has a short pot life of around a working day. Solutions of this material tend to degrade into insoluble side products of zinc oxide and zinc sulphide which can go on to block ink jet printheads. A discharge ink suitable for ink jet printheads would have to be stable for months, and ideally a year, to be useful. This patent suggests that this can be achieved with the ZFS discharge agent if it is formatted with an excess of a complexing agent such as ethylene diamine tetra acetic acid (EDTA ).

Specifically the ratio of the ZFS:EDTA should be less than 0.6:1 to achieve lifetimes of greater than 1 month and, in some cases, up to one year. This combination was formulated into an ink jet ink formulation with 7 wt% ZFS, 18.9 wt% EDTA , 37.2 wt% various humectants, 0.2 wt% surfactants, enough acetic acid to neutralise/buffer and water to make up. This was loaded into a “Kornit Avalanche” T-shirt printer and used to print onto a black T-shirt at various levels from 30 to 100%. As the black T-shirt used was not pre-bleached before dying, the base colour of the cotton is a light brown colour, requiring a small amount of a white ink to bring the colour towards pure white.

The exemplified test print demonstrates that a good white can be achieved on black T-shirts at significantly lower pigmented white ink levels.