1. Field of the Invention
This invention relates to a recording sheet for ink jet printer (hereinafter refered to as ink jet recording sheet).
2. Prior Art
In recent years, the demand for color printer is increased. Particularly, an ink jet printer, one of the non-impact recording systems, is highly increased since a comparatively rapid color recording is possible for its simplified system, since the noise is less than impact printers such as dot-printer, etc., and since the use in offices is easy, and the like.
In ink jet recording sheets, the use of synthetic amorphous silica has been proposed in various forms for the purpose of improving the color density, ink absorbency, ink drying ability, feathering of ink and flowing out of ink. For example, there are disclosed an ink jet recording sheet coated with a non-colloidal silica powder in Japanese Laid-Open Patent Application No. 55-51583, an ink jet recording sheet having a base paper coated with a mixture of a fine silicic acid and a water-soluble resin in Japanese Laid-Open Patent Application No. 56-148583, and an ink jet recording sheet obtained by at least double applications of the same coating color consisting of both synthetic silica and aqueous binder in total coating-amount of at least 10 g/m.sup.2 in Japanese Laid-Open Patent Application No. 57-107879.
As synthetic amorphous silica, there are silica manufactured by a wet precipitation process, by a wet gel process or by a humed process, i.e. a process of hydrolysing silicium tetrachloride in a oxyhydrogen flame. In conventional ink jet recording papers, silica manufactured by a wet precipitation process or by a wet gel process is mainly used, and it is disclosed in the above Laid-Open Patent Applications.
The use of silica manufactured by a humed process is disclosed in Laid-Open Patent Application 60-204390, which describes that an ink receptive layer comprises as the feature a synthetic ultra-fine silica manufactured by a vapor phase process. However, this Patent Application practically discloses a method for coating a ultra-fine silica on an ink receptive layer which is obtained by coating a synthetic amorphous silica in a coating amount of at least 10 g/m.sup.2 on a substrate, and it discloses a method for coating a mixture of an ultra-fine silica and a silica manufactured by a precipitation process in a certain mixed-ratio on a substrate in a coating amount of 15 g/m.sup.2. In any cases, this Laid-Open Patent Application discloses a coating amount of at least 10 g/m.sup.2. Accordingly, the technology in Japanese Laid-Open Patent Application No. 60-204390 is substantially equal to the technology disclosed in Japanese Laid-Open Patent Application Nos. 55-51583, 56-148583, 57-107879 and the like.
That is, the purpose of the above Japanese Laid-Open Patent Publications consists in the preparation of an ink jet recording sheet having both a good recording density and an excellent ink absorbency by applying a coating material of 10-20 g/m.sup.2 on a substrate.
As methods for preparing an ink jet recording sheet having an improved water-resistance, Japanese Laid-Open Patent Application No. 56-84992 discloses the coating of a polycation-polyelectiolyte on the surface of an ink receptive layer, and Japanese Laid-Open Patent Application No. 59-20696 discloses the use of an ink receptive layer containing polydimethyldiallyl ammonium chloride.
As the method for preparing a recording sheet having both a good light resistance and an excellent image-light resistance, there is wellknown various ink receptive layers containing each of metal oxide, UV-absorber, oxidation inhibitor, etc.
Japanese Laid-Open Patent Application No. 58-177390 discloses that an ink jet recording sheet suitable for an ink jet printer of electric field control type is obtained by coating an electric conductive agent of quaternary ammonium type on the surface of an ink receptive layer containing both a synthetic silica and an aqueous binder for the purpose of providing a normal printability, wherein the sheet is independent of environmental humiditic changes by means of preventing the electrostatic chargeability.
As described above, the subject of the hitherto ink jet recording sheets was to improve various properties, that is, the color density, the ink-absorbency, drying ability, feathering, flowing out, the water resistance and light resistance of a image and a recording sheet (printability of a recording sheet), and the like. Recently, a discoloration and a fading caused by indoor and outdoor gases have become a great problem.
As gases causing a discoloration or a fading, there are NO.sub.x -gas, sulfur dioxide gas, etc., in exhaust gases or factory smokes; ozone generated from many copying machines; smokes of cigarets; and the like.
Ozone is generated more and more, since many small or medium copying machines have been used increasingly in offices owing to the miniaturization and the simplification of many copying machines through the Office Automations. However, the discoloration or the fading caused by indoor and outdoor gases (oxidizing gases) forms a particularly great problem. It is wellknown that some of oxidizing gases cause an oxidizing excitation of acidic dye-molecules in an aqueous ink for an ink jet recording.
For the preparation of a sheet having both an appropriate recording density and a sufficient ink absorbency in an full color ink jet recording with high quality and density, it is required that an ink-receptive layer is provided in accordance with the maximum amount of ejected inks in an unit printing area. In this case, the conventional technique of coating a conventional synthetic amorphous silica such as a silica manufactured by a wet process or by a humed process, usually requires a coating amount of at least 10 g/m.sup.2 on a substrate.
In coating a filler such as silica etc. on a fiber-like substrate in an amount of circa 10 g/m.sup.2, the fibers are covered with powdered fillers completely, so that the paper-like figure and feeling are lost in a paper-base substrate. And owing to the less bulk density, the fine silica requires more binder than other fillers.
Since many vacant spaces in the coating layer are filled with the binder, the coating amount must be further increased for providing a good ink-absorbency, so that the paper-like figure and feeling are increasingly lost. In a small amount of binder, the recording sheet has a good ink absorbency, but it exhibits as the defects a weak surface strength and the troubles of paper-powder generation, of clogging the oriffices by the powders and the like, so that it is not used as a usual recording sheet.
The synthetic silica, which is used as a catalyst in oxidizing-reactions, has an excellent function as a catalyst. Accordingly, in the conventional recording sheets coated with a large amount of synthetic silica for improving both a recording density and an ink absorbency, there are following defects: With an increased amount of synthetic silica, the fading of the image is strengthen by oxidizing gases generated in the catalytic action, and the paper-like figure and feeling are further decreased.
Moreover, metal oxide, UV-absorber, oxidation-inhibitator, polycation-polyelectrolyte. etc., which are used for improving both a light resistance of a recording sheet and that of an image, have no ability of preventing a fading caused by oxidizing gases. And some of the above agents increase the fading.
As described above, there is unknown a method for preventing the fading of the ink jet recording-image by oxidizing gases, etc., under maintaining both a good recording density and a high ink absorbency with excellent paper-like figure and feeling.
Further, it has been found that such a method is not achieved by extending the conventional techniques.