Ink-jet recording is recording of images or letters through ejecting fine ink droplets based on various operating principles and allowing them to be adhered onto a recording sheet, giving advantages such as relatively high speed printing, low noise and being readily accessible to multi-color printing. This system has overcome problems such as ink clogging of nozzles and maintenance in view of both inks and apparatuses, and has spread widely in the field of printers, facsimiles and computer end-items. Details thereof are described in "Trends in Ink-Jet Recording Techniques" (edited by Takaichi Nakamura and published by Nihon Kagakujoho Corp., Mar. 31, 1995).
Employed as an ink-jet recording sheet are a variety of recording sheets, including plain paper, various types of coated paper provided with a layer comprising a hydrophilic binder and an inorganic pigment (e.g., art paper, coated paper, cast-coated paper, etc.), various paper laminated, on both sides, with plastic resin, and transparent or opaque plastic resin film supports having thereon an ink-absorbing layer as a recording layer.
The ink-absorbing layer described above is classified into a swelling type, one mainly comprised of a hydrophilic binder and a void-type, having internal voids.
The void-type ink absorbing layer holds the ink within the voids formed in the layer, and the voids can be formed by allowing various inorganic solid particles or organic solid particles to be contained in the layer.
Ink-jet recording sheets requires high performance such that printing dots have a high density, exhibit light, vivid image color, and absorb ink so quickly that the ink does not flow or penetrate even when the printing dots overlap, and lateral diffusion of the printing dots is not excessive so that peripheries of the dots are smooth and not blurred. In cases where the ink absorbing speed of the layer is low, when two or more different colored ink droplets overlap, the droplets cause repellency on the recording sheet or penetration in the boundary region of different colors, leading to a deteriorated image quality, necessitating that the ink-jet recording sheet requires high ink absorptivity.
To overcome these problems, there have been proposed a number of techniques; for example, JP-A 52-53012 (hereinafter, the term, JP-A means unexamined, published Japanese Patent Application) discloses a recording sheet comprising low-sizing raw paper coated with a paint for surface treatment; JP-A 55-5830 discloses a recording sheet comprising a support coated with an ink absorbing layer; JP-A 56-157 discloses a recording sheet containing, as pigment, non-colloidal silica powder in a coating layer; JP-A 57-107878 discloses a recording sheet employing an inorganic pigment and an organic pigment in combination; JP-A 58-110287 discloses a recording sheet having void distribution with two peaks; JP-A 62-111782 discloses a recording sheet comprising two upper and lower porous layers; JP-A 59-68292, 59-123696 and 60-135786 disclose a recording sheet having amorphous cracking; JP-A 61-135786, 61-148092 and 62-149475 disclose a recording sheet having a fine powder layer; JP-A 63-252779, 1-108083, 2-136279, 3-65376 and 3-27976 disclose a recording sheet containing a pigment or fine silica particles with specified physical property parameters; JP-A 57-14091, 60-219083, 60-210984, 61-20797, 61-188183, 5-278324, 6-92011, 6-183134, 7-137431 and 7-276789 disclose a recording sheet containing fine silica particles, such as colloidal silica; JP-A 2-276671, 3-67684, 3-215082, 3-251488, 4-67986, 4-263983 and 5-16517 disclose a recording sheet containing hydrated alumina fine particles.
In cases where a support itself is water absorptive, the ink-jet recording sheet exhibits high absorbing volume and ink absorptivity, which are preferable in terms of ink absorption. However, disadvantages result such that after ink-jet recording, the support becomes wavy, wrinkling occurs on images or a part of the dyes penetrates into the support, which lowers the density.
In cases where the support is water-nonadsorptive, on the other hand, the above disadvantages were not produced and vivid images with high density were obtained. However, there was still a problem such that the total amount of ink absorbing voids formed in the ink absorbing layer were limited. In the case of an ink absorbing layer with a dry thickness of 40 .mu.m, when solid component(s) are presumed to form a layer with a thickness of 22 .mu.m, the total volume of the voids is to be only 18 ml per m.sup.2 of the ink-jet recording sheet, producing problems such that, though depending on the recording system, the volume of the voids for absorbing the ink becomes insufficient in the vicinity of the maximum ink quantity. Although an increase of a coating layer thickness enhances the void volume, such layers become brittle, resulting in cracking at low humidity or lowering adhesion onto the support.
To solve the problems described above, the Applicants of the present invention proposed a method of improving the layer-forming properties and brittleness of the layer through incorporating a hydrophilic binder capable of forming a void structure and a hardening agent (Japanese Patent Application No. 8-283636). A void-type ink-jet recording sheet in which a void-type ink absorbing layer is formed using polyvinyl alcohol as a binder and boric acid or its salt as a hardening agent is the specifically preferred one. However, when recorded on the ink-jet recording sheet containing polyvinyl alcohol as a hydrophilic binder and boric acid or its salt as a hardening agent by using a water-based ink containing as a high boiling solvent, a compound containing a hydroxy group, it was proved that other problems were produced such as partial cracking occurring in the layer during high temperature and high humidity storage of the recorded sheet.
As a result of a study of the causes, it was proved that the layer was deteriorated due to the action of a hydroxy group containing compound which was incorporated in the water-based recording ink. Furthermore, taking account of the fact that when the content of fine inorganic particles used for forming voids is less than 2, based on the weight of polyvinyl alcohol, cracking hardly occurs, it is contemplated that the use of the fine inorganic particles in large amounts used to form the voids, lowers layer forming ability of polyvinyl alcohol in printing portions.