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%.

3D printing using Memjet printheads

US 2014/0345521 A1
Printing system for forming three dimensional objects
3D Systems, Inc.

This is a continuation patent dating back to 2011 and filed by Kia Silverbrook. However the assignee is 3D Systems, the leading 3D printing machine manufacturer. The basic idea is to use stationary Memjet printheads to deposit layers of material to build up structures. The printhead technology proposed is not the bubble jet system which has been commercialised, but a moveable element system. Nov-14aFor instance Silverbrook has demonstrated a moving nozzle prototype in the past. A wide range of materials can be deposited up to a viscosity of 10 centipoise. The printhead is heated so that low-melting point metals such as indium (156C) and alloys of indium and gallium can be used, polymers with melting points of 120-180C, and sacrificial waxes with melting points above 80C.

Printheads 402 and 404 deposit drops onto a moving conveyor. Multiple printheads are used to deposit separate materials. In the figure 4 printheads 402a-402d are being used to deposit 4 different materials on the first layer. Some materials may need some form of processing, for instance forced cooling, evaporative drying, UV curing, precipitation reactions etc. Here the first two materials are processed by curing station 406. The second two materials have a different curing requirement and so a second curing system 408 is used. The second layer is built using seven diff erent materials by printheads 404a-404m. In this case 3 different curing systems are used.

Nov-14bNote also that the layers are off set slightly from each other. This enables small cavities to be constructed that are self-supporting and do not need a sacrificial support material. This avoids the extra processing and complication of removing the support material later.

The pitch of drop separation and the height of a layer is 10 microns. Larger cavities can be created to allow the insertion of dies such as integrated circuits, memory, LED and so on.

The process speed envisaged is 208 mm/sec. and the print width 295 mm. Up to 1,000 layers are envisaged allowing products to be printed up to 10 mm thick. This requires a minimum of 1,000 printheads, but if multiple materials per layer are used something like 8,000 printheads may be needed.

The patent application talks of applications such as flat panel TVs and PDAs, perhaps dating the document and also demonstrating how fast product development has been. Production speeds are 0.37 and 432 per second respectively for those two examples.

Although this process sounds incredible and optimistic, remember the patent application has been acquired by a market leader. It will be interesting to see what comes of it.

New silicon-MEMS printhead from STM

US 2014/0313264 A1
Method for manufacturing a fluid ejection device and fluid ejection device
STMicroelectronics S.r.l.

STM is one of the largest manufacturers of printheads in the world, making a large proportion of the dies for HP. In 2014 they announced projects to develop piezo MEMS devices, including page wide array ink jet printheads.

The manufacturing processes for a thin film piezo printhead is shown below. Previous designs have used 4 wafers but this design only uses 3, with subsequent cost savings and reduced manufacturing complexity as now only two alignment and bonding steps are required.

Oct-14Here the printhead is shown with the piezo actuator deflecting into the actuator chamber 232. The components made from the three wafers are designated 100, 200 and 300. The central wafer 200 is made from SOI (silicon on insulator) type. The top layer is process to form the diaphragm,piezo actuator and the bottom part the actuator chamber. Layer 300 can also be made from a SOI wafer, and forms the structure of the printhead with ink feed channels 316. The third wafer forms both the nozzle plate and the sealed chamber for the actuators. The full manufacturing process is shown in the patent.

Oct-14b

Compared to previous designs, the saving of a wafer is achieved by processing the nozzle layer and actuator capping structure in one piece. This is then brought into contact with the actuator wafer in the step shown. Bonding occurs due to the adhesive layer 230 coated onto the contact parts of the top structure.

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.

Ink jet wallpaper media from HP

WO 2014/120149 A1
Printable medium
Hewlett-packard Development Company L.P.

Wallpaper and other wall covering papers are used both for decorative and display purposes. One function of the paper will be to conceal cracks and other imperfections in the underlying surface, so it is important that it should have a high degree of opacity. This is achieved by using a laminated structure in which an adhesive is tinted with a dark-coloured opaque pigment or dye and is sandwiched between two sheets of paper. In fact, several layers can be laminated in this way if necessary. The laminated structure is shown in the diagram.

Aug-14At the core of the laminate is an opacity enhancing adhesive layer (110) that can be from 2-50 μm in thickness. On either side of this is a paper layer (104, 108). This part of the structure forms the basic substrate, which will have 150 to 400 μm thickness and a weight of 100-500 gsm. On the top or outer surface of the paper adjacent to paper 104 is a printable image receiving layer (106) while a glue layer (118) may be positioned on the reverse surface adjacent to paper 108.

The paper layers are composed of a mixture of wood pulp and synthetic fibres and may also contain fillers, which in themselves will impart a certain degree of opacity. The paper will be dimensionally stable with a machine direction to cross direction tensile stiffness that is less than 2.5 and with hydroexpansibility of less than 1%.

The light blocking adhesive layer will contain up to 3% of a light absorbing pigment or dye for which the lightness (L*) will be around 30, giving a dark colour. So light will consequently be blocked from passing through the paper, giving an opacity defined as i(x)/io ≤ 0.05 where i(x) is the light intensity at distance x from the surface of the substrate where the light intensity is io. Any black or dark coloured pigment or dye is suitable, for example direct black dyes such as Pergasol black BTB or carbon black pigments such as irgalite black 2BL. The adhesive itself, which can be cross-linkable for improved hardness, may be an aqueous polymer emulsion such as polybutadiene or styrene-butadiene or it may be a solution polymer in which case the solvent can be aqueous or organic.

Finally, the ink receiving layer is coated on the surface of the substrate and comprises pigments and a polymeric binder. Provision is also made for a latex ink film-forming agent, such as a citrate of a glycol polymer.

An exemplary paper showed superior physical properties (tear and tensile strength, hydroexpansion and taber stiffness) in comparison to HP PVC free wallpaper.

Flight path compensation

US 2014/0184682 A1
Print gap compensation
Inca Digital Printers Limited

Variations in the print gap, the spacing between nozzles and substrate, can lead to a distortion of the image. If printing is being carried out bi-directionally then there is a possibility of double images, where the printing in each direction does not line up.

July-14The figure shows a printhead jetting drops to a substrate. In the first example the substrate 86a is spaced away from the printhead, which is moving to the right relative to the substrate. Drops ejected at point x1 reach the substrate at x2. In the second example the substrate 86b is closer to the printhead due to distortion of the substrate or bed. The flight path of the drops is shorter, so for the drops to land at x2 the release point needs to be delayed from x1 to x3.

Inca Digital manufactures flat-bed printers which use a large table on which the substrate is mounted. The table passes up and down underneath the full width printhead. A small number of multiple passes over the substrate are made to reach the full print resolution and to mask jetting errors. The problem of image distortion is greater if the table size is large. Even if the table is made to very tight manufacturing tolerances, distortions due to thermal expansion during operation may occur and be significant. For instance, a large UV curing lamp system is used and generates a significant amount of heat.

The solution is to determine variations in the table flatness under operating conditions, and then adjust the timing of the drop data to compensate for variations in the flight path. A test pattern consisting of short lines or dots is printed in two passes. The pattern is repeated over the table, although it has been found that just 5 positions are sufficient for a 3m long table. The spacing between the marks on the forwards and backwards passes depends on the print speed, jet velocity and the gap between the nozzles and the substrate. The pattern is printed twice, at two different print heights, with a small offset between each pattern so they do not overlap. Each test image is analysed using a scanner that may be mounted with the printhead.

Although the exact spacing of the printhead to substrate is not known, the difference between the two positions can be accurately determined. Assuming the jet velocity is constant, simultaneous equations can be used to find the jet velocity and the actual print gap for the printhead. This enables corrections to be determined to remove the distortion, delaying or advancing the release of drops.

Metallic effects from a thermal ink jet ink

US 2014/0170384 A1
Neutral gray reflective ink
Hewlett-Packard Development Company

Judging by the number of patents filed, it appears that the ink jet printing of expanded colours and appearances are sought after for home and office decorative printing. Reflective inks, in particular, have been disclosed with a particular emphasis on metal flakes – materials currently used in more traditional metal-effect printing processes. Thermal printheads have significant constraints regarding viscosity and particle size and so large metal flakes are a non-starter and a silver nano-particle approach would be expensive and questionable from a toxicological perspective.

The patent suggests that to look metallic, then only a certain threshold value for specular reflection needs to be achieved. It is suggested that 10% of the incident light must be specularly reflected, but greater than this is always desirable. With this specular reflectivity requirement relaxed, alternative non-metallic materials can be considered, such as magnetite. Magnetite (Fe3O4) can be made up into a stable nano-dispersion, suitable for thermal ink jet, and will give a metallic lustre when printed on glossy paper.

However, magnetite has an inherent yellow-orange hue and so to achieve a neutral silver effect a mixture of blue, cyan or magenta dyes must be added to counteract this. The exemplified ink jet formulations first prepared the magnetite dispersion in water by heavily bead milling a 5.6 wt% mixture of the oxide in water with a polyether alkoxysilane reactive dispersant at 0.5 wt%, relative to the pigment. This was then used (at 36.3 wt%) to make a simple thermal ink jet ink with 14 wt% humectant, 0.1 wt% neutralised styrene acrylic binder, 0.3 wt% surfactants, 5.4 wt% of a cyan and magenta dye mixture and water.

June-14aThis ink was loaded into a HP black cartridge from a Photosmart 8450 printer to print colour test patches. The chart left shows the colour coordinates of the neutral formulation above, compared against the non-colour corrected magnetite dispersion and a comparative silver nano-particle ink. It can be seen that the yellow tint has been fairly well counteracted in comparison to the metal ink samples.

The patent goes on to discuss the importance of the media in achieving a metallic effect and that the pore size of the surface of the media must be smaller than that of the magnetite pigment particles to ensure they remain on the surface as a contiguous layer. Fig 1 below shows this schematically with the dyes (16) absorbing into the media under the ceramic particles (14). There is an obvious issue to such a configuration in that the robustness of the printed image will be compromised if the pigment is left unprotected on the surface, but this is addressed in the patent application US 2014/0170395 A1.

June-14bAlthough it seems that the ultimate specular reflectivity of this approach is not quite that of a metallic silver ink, there do seem to be significant advantages in using such benign and low cost iron oxide materials.

Increase in bonding strength of décor ink using primer

WO 2014/084787 A1
Method of producing a building panel using digital printing
Ceraloc Innovation AB

In recent times, there has been a move towards digital decoration of architectural parts and panels. Radiation cure inks lend themselves well to this sort of application as they can be cocured with other primer and protective layers. Conversely, aqueous inks give problems in terms of inter-layer adhesion and image quality. However, environmental considerations make the use of aqueous inks an attractive proposition.

The application addressed in this instance is that of Luxury Vinyl Tiles and Planks (LVT) in which a thermoplastic décor layer, often PVC-based, is laminated to a core which may be a thermoplastic or a wood based material. A pattern such as a wood or natural stone effect is printed on the carrier, conventionally by rotogravure or direct printing, and a further protective coating bonded to it by a heating process to finish the décor film. At the lamination stage a texture such as a wood grain can be embossed. Neither of these printing methods lends itself well to alterations in the design, nor is a certain amount of repetition of the pattern avoidable.

The problems of print quality and bonding strength are addressed by applying and drying a primer to the thermoplastic foil, prior to printing with aqueous pigment-based inks. The primer principally consists of a concentrated metal salt solution, generally with the addition of a surfactant as a wetting agent. The deposited salt crystals act to fix the ink in place by causing agglomeration of the pigment in the ink dispersion on contact with the surface so that the ink is unable to spread unduly. The salt solution must have a concentration that is at least 50, and preferably 90% of the saturation concentration; and to obtain uniform wetting of the surface, it should have a contact angle with the carrier that is ideally less than 35 degrees. Addition of a wetting agent will ensure that this is the case. The salt can alternatively be applied as a dry powder, but if so, the crystals must be distributed homogeneously over the surface and adhere to it. To achieve this, the carrier surface can be electrically charged such that it attracts the salt powder, or the surface can be plasticised or porous so that the salt particles adhere to, or are absorbed by it.

May-14aA structure for a LVT to which a décor film has been laminated is illustrated. The carrier (3) is shown laminated to the core, which in this case is formed with shaped ends to allow an interlocking structure to be constructed. The image or pattern (10) is printed on the core and coated with a protective coating to which, in this case, an additional coating (5) is applied. This may be a UV cured material.

May-14b