1. Field of the Invention
The present invention relates to a high gloss paper having a superior printability and to a process for the production thereof.
2. Prior Art
Paper having a coating composed of pigment and binder are used as high grade printing paper when the surface gloss is an important factor, in addition to printability, including ink absorbency, coated layer strength, etc. For enhancing the gloss, however, smoothing the coated layer with a press causes the destruction of voids therein, thereby lowering the ink absorbency. For enhancing the gloss, the use of a large amount of water soluble or dispersible polymer, such as polymeric latex, which is used as the binder for pigment, increases strength and gloss of the coated layer, but lowers its ink absorbency by decreasing the voids. The type and amount of pigment and binder, the amount of coating material, the degree of smoothing treatment and the like are determined based on a consideration of an appropriate balance of gloss and printability. Therefore, other techniques are required for the production of a high gloss paper having a superior printability.
The gloss value of the coated printing paper is generally increased in the following order: slightly coated paper, coated paper, art paper, superart paper and cast coated paper. The term "high gloss" as used herein means a higher gloss value than that of superart papers. Accordingly, "a high gloss paper" means a coated printing paper having a higher gloss value than that of superart paper. Conventionally, a cast coater is used for the production of high gloss papers. The cast coater applies a wet layer composed of pigment and binder by press contacting the paper with a cast drum having a mirror finish. The coated paper is dried by heating. This method has disadvantages including a remarkably slower production speed compared with methods used for the production of conventional art papers, coated papers, and slightly coated papers.
Further, a method using a heated calender without using cast drums is well known. For example, Japanese Patent Laid Open Application No. 56-68188, Japanese Patent Publication Nos. 64-10638 and 64-11758 disclose a method for coating a mixture of pigment and polymeric latex or water soluble polymer, drying the resultant coated layer, and further treating the coated layer with a heated calender. In this case, the polymeric latex having a glass transition temperature of a least 5.degree. C. or at least 38.degree. C. is used as the latex, and the temperature of a heated calender is set at a temperature higher than the glass transition temperature of the latex. Since this method uses a calender treatment of a latex, it is simplified and superior in productivity, but it has as a defect an insufficient gloss. This method does not provide a higher gloss than that of superart papers, and, therefore, it does not provide the same gloss as that of cast coated papers.
Another paper coating is disclosed in Japanese Patent Laid Open Application No. 59-22683. This method comprises coating a combination of at least two polymeric latexes having various minimum film-forming temperatures on an uncoated sheet, or on a coated sheet, drying the coated sheet, and optionally smoothing the sheet with a calender. In this case, drying of combined latexes having various minimum film forming temperatures causes fine cracks on the surface of the coated paper, thereby resulting in a superior ink absorbency without impairing the gloss. The important feature of the above technique is in causing fine cracks on the surface of the coated sheet, wherein special care must be exercised in the drying step. That is, the drying condition must be set so as to completely melt the latex having a higher minimum film forming temperature and, only partly melt the latex having a lower minimum film forming temperature. However, as is well known, the drying conditions are easily varied by many factors. Considering industrial application of this technique, it is practically impossible to keep the drying conditions uniform and constant over an entire production system. Therefore, it is very difficult to maintain a constant stable quality.