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