Inkable sheets

Details Inkable sheets Inkable sheets

1999-09-24

2001-04-10

Chen, Vivian (Department: 1773)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Physical dimension specified

C428S195100, C428S339000, C428S423700, C428S480000, C428S481000, C428S483000, C428S532000, C524S284000, C524S300000, C524S315000, C524S322000, C524S321000

Reexamination Certificate

active

06214459

ABSTRACT:

FIELD OF INVENTION
The present invention relates to inkable sheets and concerns inkable sheets and their production.
BACKGROUND TO THE INVENTION
With the recent proliferation of microcomputers and colour monitors, there has been a massive growth in the amount of information available for display in colour. Presentation of such information has created a demand for hard copy, for example on paper sheets, on opaque plastics films (which can be more robust than paper sheets) and also on transparent sheets, which are suitable for viewing in transmission mode, e.g., by overhead projection.
Ink jet printing is well established as a technique for printing such information including multi-colour graphics. In ink jet printing, an ink droplet is projected on to an ink receptive sheet at high velocity (e.g., up to 20 m/s). Movement of the ink jet may be computer controlled, and characters may be formed and printed rapidly. To derive advantage from this high speed operating capability requires an ink receptive sheet, which will rapidly absorb the high velocity ink droplet without blotting or bleeding.
To improve image resolution, ink jet printers have been developed which are capable of providing a greater density of ink droplets, for example up to about 1440 dots per inch (dpi). For a given droplet size, the increased ‘dpi’ of such printers as compared with that of lower resolution printers has the effect of increasing the volume per unit area of ink to be absorbed.
Generally, an inkable sheet suitable for use with ink jet printers comprises a substrate carrying an ink absorbent layer. For use in overhead projection, the substrate must, of course, be transparent and transparent polyethylene terephthalate or transparent polyvinyl chloride films are commonly used. In the case of opaque plastics films, opaque polyethylene terephthalate or opaque polyvinyl chloride films are commonly used. The ink absorbent layer typically comprises a polymer or a mixture of polymers and combinations of cellulosic polymers such as nitrocellulose, carboxymethyl cellulose and especially hydroxyethyl cellulose; gelatins; vinyl polymers such as polyvinyl acetate and polyvinyl pyrrolidone; and acrylic polymers such as polyacrylic acid are described in EP-A-0156532, EP-A-0232040 and EP-A-023 3703.
A further consideration is that the use of aqueous based inks having a high water content (possibly up to 95%) is becoming more common. Such inks used for multicolour printing (i.e. cyan, magenta, yellow and black) generally consist of an aqueous solution of an appropriate dye. However, recently there has been a trend to the use of black ink consisting of an aqueous dispersion of a pigment together with a polymer whose function is to hold the pigment together when the ink has dried. Under certain circumstances, the presence of such a polymer in the ink can cause problems, in that if the ink polymer and the polymer in the absorbing layer of an inkable sheet become intermixed, swelling and subsequent shrinking of the ink dot can result, leading to cracking of the dried dot with a consequent reduction of the optical density. This is particularly serious in the transmission mode, where the optical density can be reduced by up to 50% due to light passing through the cracks.
The present invention aims to alleviate the above problems associated with the prior art.
SUMMARY OF THE INVENTION
In its broadest aspect, the present invention provides an inkable sheet comprising a substrate having on at least one surface thereof an ink absorbent layer comprising a cellulose material and an acid functional resin.
The solubility of the ink absorbent layer in an ink applied thereto may be reduced by including materials in an absorbent layer which are insoluble in cold water of neutral or acidic pH. Alternatively, the solubility of the ink absorbent layer can be limited by including at least one cross linking agent in the absorbent layer.
In another aspect the present invention provides an inkable sheet comprising a substrate having on at least one surface thereof an ink absorbent layer comprising a cellulose material, an acid functional resin and a weak organic acid or salt thereof.
Suitably, the weak organic acid comprises citric acid, tartaric acid, succinic acid, acetic acid or malic acid. The salt is conveniently an ammonium salt of such acids. Preferably, the weak organic acid comprises citric acid, or an ammonium salt thereof. Mixtures of acids and/or salts may be used.
With tri-ammonium citrate it is believed that ammonia is lost during product manufacture (typically involving heating to a temperature of 130° C.) and that the presence of the citrate ions reduces the pH of the coating and reduces the solubility of the resins. Indeed, it has been found that citric acid is an effective alternative to tri-ammonium citrate, although ammonia may have to be simultaneously added to the formulation to maintain the solubility of the acid functional resins if they are not present as a dispersion. Other weak organic acids such as tartaric acid, succinic acid, acetic and malic acid are also effective in achieving good performance with pigmented inks.
In a particular aspect, the invention provides an inkable sheet comprising a substrate having on at least one surface thereof an ink absorbent layer comprising hydroxypropylmethyl cellulose, an acrylic polymer and a citrate.
According to a further aspect of the present invention, there is provided an inkable sheet comprising a substrate having on at least one surface thereof an ink absorbent layer comprising a cellulose material, an acid functional resin and at least one cross linking agent.
In a further particular aspect of the invention, there is provided an inkable sheet comprising a substrate having on at least one surface thereof an ink absorbent layer comprising hydroxypropylmethyl cellulose, an acrylic polymer and a cross linking agent.
Suitable cross linking agents for use in the present invention include melamine formaldehyde resins, polyethylene imines, urea formaldehyde or blocked isocyanates.
The cross linking agent acts to cross link at least the acid functional resin thus reducing solubility in ink, hence improving performance. The cross linking agent may additionally act to cross link the cellulose material. The cross linking agent should thus be effective in cross linking at least the acid functional resin and possibly also the cellulose material.
The cellulose material employed in the absorbent layer of an inkable sheet according to the present invention conveniently comprises hydroxypropylmethyl cellulose or carboxymethyl cellulose, with hydroxypropylmethyl cellulose currently being preferred.
Preferably, the acid functional resin comprises an acid functional polyurethane resin or an acid functional acrylic resin such as a polyacrylate/acrylic acid copolymer. In general, the higher the molecular weight of the resin, the better the performance. Further, the more insoluble the resin is in cold water at neutral or acidic pH, the better the performance. The acid functional resin (possibly after being cross linked) is thus preferably insoluble in water of neutral or acidic pH at room temperature. Suitable acid functional resins are commercially available and are generally supplied in solid form, in solution in appropriate solvents, or as aqueous dispersions.
It may be advantageous if the absorbent layer further comprises an agent which acts to reduce crystallisation of coloured (i.e. cyan, magenta or yellow) dyes when applied to the inkable sheet. The absorbent layer may therefore preferably further comprise hydroxyethyl cellulose, as the latter can be advantageous in reducing any tendency of coloured dyes to crystallise as described above.
It may also be advantageous for the absorbent layer further to comprise an agent which acts to reduce curl of an inkable sheet according to the present invention, in particular when the latter is placed on a hot surface, for example a platen of an overhead projector or the like. The absorbent layer preferably therefore further comprises a suitable an

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