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.

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.

Xerox’s inks with nano-diamond colorants

US 2013/0083134 A1
Ink jet inks containing nanodiamond black colorants
Xerox Corporation

Nano-diamond is a type of diamond material comprising a tetrahedral sp3 carbon diamond- like core and a fullerene sp2 graphite shell surrounding this core. The diamond core is generally around 5 nm and the overall particle size is about 20-50 nm with a BET surface area of 270-380 m2/g. The unique morphology of these materials results in a core with diamond-like hardness, with a chemically modifiable graphitic shell to aid dispersibility.

The method of manufacture results in a relatively impure graphite outer structure comprising carbon (~76%), oxygen (~6%) and nitrogen (~10%). However, this functionality is helpful when applying surface modifications in order to disperse the particles. The production method of these nano-diamond powders is similarly novel. Carbon particles are suspended in an oxygen depleted atmosphere in a pressure vessel and a high explosive is then detonated in this (hopefully sturdy) container.

These powders are highly conductive, both thermally and electrically, and show superb lubrication and wear characteristics. Importantly for this patent, the materials also show excellent light absorption and allegedly make for superior, more easily dispersible, black pigments.

The rest of the patent discusses the preparation of phase change inks and aqueous inks, offering up examples of each. The nano-diamond powder was obtained from Nanoblox and dispersed into a typical hot melt ink using a bead-mill. The aqueous inks were much simpler creations, using sonication to disperse the powder into a boilerplate ink jet ink formulation.

No description of the resultant print performance, in the printhead or on the media, is given, but it will be interesting to see if any more patents are forthcoming if these pigments are as special as is suggested.

Stretchable fluroelestomer ink from Xerox

EP 2 546 312 A1
Stretchable ink composition
Xerox Corporation

It is useful for an ink jet ink to be tolerant to a small amount of elongation in order to survive the flexing of the substrate media, but the ability to stretch a few hundred percent is another matter entirely.

This patent suggests that there is a demand for using ink jet to print onto flexible, stretchable materials such as latex gloves, tyres, tubes, cables etc.  The patent also suggests that these inks should be highly resistant to environmental factors in addition to a robustness comparable or better than the printed article itself.  All of which is a tough challenge.

To meet such stringent criteria, the patent suggests the use of fluroelastomer emulsions as the sole ink jet ink binders.  In more detail, 65 wt% of Technoflon TN latex was mixed with 25 wt% of a Pigment Green 7 dispersion (pre-dispersed in water using sodium dodecyl sulfate to give a 20% solids dispersion).  This mixture exhibited a particle size distribution from 40-300 nm with an average of 120 nm.

This mixture was then diluted down to 5 centipoise with water (no data given as to how much was needed), and then 10 wt% ethylene glycol was added as a humectant.  This ink was loaded into a Dimatix DMP-2800 ink jet printer equipped with 10 pl cartridges and printed onto a latex glove.  After printing, the ink was dried at 60C for 5 minutes to give an image that could stretch, repeatedly and robustly up to 500%, which was the limit of the substrate not the ink.

A second experiment substituted the latex glove for a silicone rubber sheet.  Silicones are exceptionally difficult materials to wet out and achieve any kind of  adhesion to, but surprisingly the ink adhered and demonstrated reliable stretching up to the limit of the substrate for hundreds of cycles.  Although this is a very simple ink formulation, the use of these fluoropolymer dispersions may be of use for some specialist applications, although the long term reliability in ink jet printheads would be a challenge.