(a) Technical Field of Invention
This invention relates to an inkable sheet, and, in particular, to a sheet suitable for use with an ink jet printer.
(b) Background of the Art
Ink jet printing is already established as a technique for printing variable information such as address labels, multi-colour graphics, and the like. A simple form of ink jet printer comprises a capillary tube coupled to an ink reservoir and a piezo-electric element which, on application of a voltage pulse, ejects an ink droplet from the capillary tube at high velocity (e.g. up to 20 ms.sup.-1) onto an ink-receptive sheet. Movement of the ink jet may be computer controlled, and new characters may therefore be formed and printed at electronic speeds. To derive advantage from this high speed operating capability requires the use of an ink-receptive sheet which will quickly absorb the high velocity ink droplet without blotting or bleeding. Although plastic sheets may be employed, these generally tend to exhibit inferior ink absorption and retention characteristics. In particular, drying of an applied ink pattern is slow, and immediate handling of a freshly imaged sheet is therefore prevented.
(c) The Prior Art
Various recording sheets have been proposed for use with ink jet printers. For example, British patent specification GB 2050866-A discloses an ink-jet recording sheet comprising a layer of a water-soluble coating polymer disposed on a support having a water absorptivity of not more than 30 gm.sup.-2 (JIS P8140). The support, which may be of paper, cloth, plastic film, metal sheet, wood board or glass sheet, should be sized, if necessary, to provide the specified water absorptivity level to prevent penetration of the water-soluble coating polymer into the support. The characteristic feature of the sheet is that the layer of water-soluble polymer, which desirably has a high viscosity, dissolves or swells in the water of a subsequently applied aqueous ink to increase the viscosity of the ink. Although such behaviour is said to provide an image of high density, high resolution and good colour reproduction without causing ink overflow, mixing or flying, a pattern applied to the sheet using an aqueous organic solvent-based ink is relatively slow to dry.
British patent specification GB 2116880-A relates to a material, used to bear writing or printing, comprising a substrate having a coating layer which is divided by micro-cracks of irregular form into lamellae. The width of each micro-crack is usually several microns, whereby the solvent medium of a subsequently applied ink passes through the micro-cracks and is quickly absorbed into the substrate which comprises a porous, liquid-absorbing material, such as paper. The dimensions of the micro-cracks are such that the product is opaque and therefore unsuitable for use in the production of transparencies for use in an overhead projector. Furthermore, the requirement for the solvent medium of the ink to pass through the micro-cracked structure into the porous substrate precludes the provision of an intermediate adhesive layer to promote adhesion between the substrate and coating layer.
We have now devised an inkable sheet which is particularly suitable as an ink jet recording sheet for use with an ink jet printer.