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.