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.

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.

Ink jet make-up applicator

WO 2015/097618 A2
Transfer makeup process and related device

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.

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.

Intermediate transfer technology from Xerox

US 2015/0022605 A1
System and method for transfixing an aqueous ink in an image transfer system
EP 2 826 633 A1
System and method for transfixing an aqueous ink in an image transfer system
Xerox Corporation

Interest in intermediate transfer systems is high at present, no doubt as a result of the Landa Digital demonstrations at Drupa 2012. Xerox has for many years sold printers using intermediate transfer methods with phase change inks. But the goal here is to find a way of doing the same with liquid inks. It is also desirable to not involve heat for the transfer process if possible, as excessive temperature of the transfer substrate will shorten its life.

The intermediate transfer substrate needs to have properties that will allow the release of the ink film and its transfer to the substrate, yet the ink must wet it sufficiently on landing. Insufficient wetting results in ‘drawback’; at impact the drop is flattened, but then surface tension pulls the ink back into a drop.

The proposed solution is to coat the intermediate transfer substrate with a powder layer including an aggregation treatment agent capable of precipitating colorants, latex or resin in the ink drops.

Jan-15The particles are very small, 1-10 microns in diameter, and need to be coated uniformly and at uniform density on the substrate. Coverage of 5-40% is sufficient providing the particle distribution is uniform, so that many particles will be impacted by an individual drop.

Not surprisingly, considering Xerox’ background, methods of coating the powder include electrostatically biased rollers, brushes, and cloud development used in electrophotography. The particles contained metal salts, such as iron sulphate and copper sulphate. These are combined with other materials to form the powder.

As the drops impact the powder the colorant or pigment begins to precipitate out of the ink due to the aggregation agent in the particles. As the process continues a coating of pigment and resin is formed on the substrate surface. This causes further diffusion of the aggregation agent into the drop, and also prevents the drawback of the ink. Examples shown and discussed in the patent use 14 pl drops printed on to a silicon plate as the substrate.

Improved printing of white backgrounds

US 2014/0354726 A1
Multi-layer printing on non-white backgrounds
Electronics for Imaging, Inc.

When printing on to a clear or coloured substrate it is normal to print a white layer, then print the image on top. However printing on to a bright white can require extra coloured ink to achieve full image density. Unfortunately the extra colour ink may lead to gloss banding effects.

Dec-14The proposed solution is to print with the white layer a proportion of the coloured image 31, then the remaining part of the colour image in the second pass 32. For instance 20% of the colour image may be printed with the white layer. Where colour ink will be printed less white is deposited, so that the total amount of ink laid down in the first pass is constant. The colour ink is printed into the white layer while it is still wet, and then the first layer is cured. The image is not particularly visible in the white layer at this stage. However, when the main part of the colour image is printed on top the colour density is higher than it would be if printed on a plain white layer, without loss of detail.

So instead of having to increase the amount of colour ink in areas of high image density to increase the saturation when printing on a uniform white layer, the amount of white ink is reduced in the areas of high image density.

There are other embodiments proposed. Instead of part of the colour image being printed with the white layer, only black or light black ink is used. This saves ink as only black is used, but the saturation is not quite so high. It works well in dense shadow or dark areas of the image.

The amount of the colour image printed with the white layer can also be varied according to the image or part of image. For instance where a photo appears, 60% of the colour image is printed with white, where a graphic appears 40%, and with text 0%.

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.