Compositions: coating or plastic – Coating or plastic compositions – Marking
Reexamination Certificate
2000-09-28
2003-03-04
Klemanski, Helene (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Marking
C106S031250
Reexamination Certificate
active
06527842
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to emulsion inks, particularly for use in stencil duplicators and digital duplicators.
BACKGROUND OF THE INVENTION
Emulsion inks are widely used in stencil duplicators and digital duplicators. The preferred emulsion inks are water-in-oil (w/o) emulsion inks, comprising a water phase dispersed in an oil phase. A wide variety of these kinds of inks is known, some containing colouring agent in the continuous oil phase and others containing colouring agent in the dispersed water phase, depending on the materials used and the properties required in the final ink.
For instance, EP-A-0609076 discloses a w/o emulsion ink comprising a colouring agent in the continuous oil phase and an oil-in-water (o/w) type resin emulsion in the water phase. The minimum film-forming temperature of the o/w type resin emulsion is 40° C. or less. The resulting w/o emulsion ink is stated to have excellent drying properties, sticking tendency and stability.
JP-A-6128516 discloses a variety of emulsion inks comprising an oil phase and a water phase, and a hot-melting component (preferably having a softening temperature in the range 40 to 100° C.) or a thermosetting component in the oil phase and/or the water phase. Said inks are said to have improved drying and binding properties, thereby preventing set-off. The emulsion inks may optionally include a silicone oil in the oil phase and/or the water phase, to prevent offset. However, no information is given as to how the silicone oils may be incorporated into the water phase, and the examples are limited to its inclusion in the oil phase. A colouring agent may also be included in the oil phase.
U.S. Pat. No. 2,839,412 discloses w/o emulsion inks for use on stencil duplicators comprising a water phase dispersed in a continuous oil phase through the use of aluminium stearate as an emulsifying agent, and wherein the water phase comprises a colour pigment. Said inks are said to have good shelf-life and flow properties, amongst other qualities.
However, despite their long standing use, w/o emulsion inks still demonstrate the following shortcomings:
1. There are contradictory requirements regarding the proportion of water phase within the ink. Amongst other properties, the ratio of dispersed water phase to continuous oil phase determines the flow properties of the ink. It is of critical importance to optimise this property. Excessive flow will cause the ink to exude from under the trailing edge of the stencil during the printing process giving rise to unwanted ink marks on the printed sheet, a defect sometimes termed “tail flooding”.
In order to prevent this defect the water phase of commercially available w/o emulsion inks is generally greater than 50% of the total ink weight, sometimes greater than 70%. However, as the water content of the ink increases another printing defect becomes more evident. The water component causes swelling of the paper fibres in the printed sheet. This swelling results in a distortion of the paper surface and the raised points in the wet print areas promote transfer of ink to the back of superimposed prints in the receiving tray of the print machine, a defect known as “set-off”.
It can be appreciated therefore that, in order to avoid tail flooding problems, inks are formulated with a higher concentration of water than is desirable regarding set-off performance.
2. Inks containing pigments in the water phase can exhibit a variation in properties according to pigment content. One of the perceived advantages of incorporating pigments within the dispersed phase of w/o emulsion inks is the ability to achieve a range of inks with a common formulation in which only the pigment is varied and which consequently exhibit many common or closely similar properties.
For example, EP-A-0848046 describes a system for making a wide range of coloured inks based on this concept. The process entails the preparation of a basic range of aqueous pigment dispersions, and mixing them in different proportions to form components of the water phase of differently coloured inks. However, in such a system there are some properties that do vary with pigment content, particularly in relation to the overall concentration of pigment in the water phase.
For example changes in rheology are apparent as the concentration of a pigment is varied. Thus, as the concentration of a pigment is decreased the resultant prints, particularly those with large block areas, can exhibit an unevenness of ink laydown, typically lateral bands of increased print density. This defect is exacerbated as the ambient temperature increases. This change in rheology can be characterised by use of a rheometer capable of performing a creep test in which viscosity is measured with application of a boost shear stress. Inks which exhibit this defect of uneven ink distribution have significantly lower creep viscosities than inks with higher concentrations of pigment which do not give rise to the defect, even though viscosities measured under normal conditions show little difference.
3. Inks containing polar pigments, such as azo lakes, in the water phase can show poor stability. Thus EP-A-0791639, which is concerned with the formulation of coloured w/o emulsion inks with pigments in the water phase, specifically excludes so-called “soluble” pigments such as lakes on the grounds of poor ink stability. These “soluble” pigments are distinguished by the presence of polar functional groups within the pigment's molecular structure which confer solubility in the reaction medium in which they are synthesised.
4. Notwithstanding the advantages of formulating w/o emulsion inks with pigments in the water phase, there is often an economic advantage to oil phase pigmentation because of the lower cost of pigment dispersants for use in the oil phase. Performance benefits achieved with water phase pigmentation can therefore be compromised by economic constraints.
There is therefore a need for an emulsion ink which does not demonstrate these shortcomings.
SUMMARY OF THE INVENTION
According to the present invention, an emulsion ink comprises a continuous oil phase; two or more dispersed liquid phases, one of which is a dispersed oil phase; and a solid material dispersed in the dispersed oil phase.
Surprisingly, it has been found that such inks can be formulated to have rheological properties suitable for use on a stencil or digital duplicator without experiencing the conflicting problems of tail-flooding or set-off. In fact, set-off may be significantly reduced with the inks of the present invention. Furthermore, the inks of the present invention tend to be less susceptible to rheology changes associated with changes in pigment concentration, and hence are capable of achieving uniform print quality consistently. Furthermore, inks may be formulated which provide a stable environment for polar pigments.
DESCRIPTION OF THE INVENTION
According to a first aspect of the present invention, at least one of the dispersed liquid phases is dispersed in another of the dispersed liquid phases, in the form of an emulsion, which in turn is dispersed in the continuous oil phase.
According to a second aspect of the present invention, a plurality of dispersed liquid phases are directly in contact, or form an interface with, the continuous oil phase.
According to a third aspect of the present invention, an emulsion ink comprises a plurality of dispersed liquid phases that directly contact the continuous phase, with one or more of those dispersed liquid phases comprising, dispersed therein, yet another dispersed liquid phase.
In the following, the present invention will be described primarily in the context of the first aspect of the invention. However, unless otherwise stated, formulation considerations, such as the types of materials to be used and their amounts, apply equally to the other aspects of the present invention.
The continuous phase is an oil phase which may comprise any oil that is known in the art to form the continuous phase of w/o emulsions. Suitable materials include,
Adams John Christopher
Hamilton John
G R Advanced Materials Ltd.
Klemanski Helene
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