1. Field of the Invention
This invention relates to an image-receiving sheet. More particularly, it relates to an image-receiving sheet for an overhead projector.
2. Background Art
An overhead projector (OHP) is an information transmission means which is used extensively in lecture meetings, schools, etc. Handwriting with an oil-base ink, printing and electrophotographic copying have hitherto been used as means for forming an image on an image-receiving sheet for OHP (hereinafter referred to as an "OHP sheet"). In order to steadily record and hold thereon image information, such as lines, letters and pictures, using the above means, OHP sheets generally have an image-receiving layer on a transparent substrate sheet. Therefore, the side of the image-receiving layer in an OHP sheet, on which information is to be recorded, should be surely distinguished from the other side of the sheet. For this reason, a detection mark, for example, a white arrow, for identifying the side of the image-receiving layer, that is, distinguishing the two sides of an OHP sheet is put on the sheet surface. This marking also serves to mechanically distinguish an OHP sheet from other types of paper.
Further, some electrophotographic copying machines are designed to begin work upon detection of the position of the sheet within the machine at the time of copying, which needs OHP sheets with a white detection mark formed thereon.
The above detection mark becomes unnecessary upon the formation of an image. Rather, the presence of a detection mark after the formation of an image on an OHP sheet gives rise to the problem that when the OHP sheet is applied to an overhead projector, the detection mark is unfavorably projected together with the necessary image, so that the copresence of the unnecessary image on the projected image face deteriorates the quality of the projected image and sometimes makes it difficult to clearly see the contemplated image.
In view of the problem associated with the detection mark, Japanese Patent Laid-Open No. 170944/1991 teaches a detection mark for an OHP sheet, comprising an opaque porous resin layer that becomes transparent when heated at the time of forming an image by means of electrophotographic copying. Proposed methods for producing such a porous resin layer are 1) a method which comprises incorporating a foaming agent during or after coating of a hydrophobic resin, such as a polystyrene resin or a polyester resin, on a substrate and conducting foaming and 2) a method which comprises coating the above-described hydrophobic resin together with an extractable resin or solvent on a substrate and then rendering the resultant coating porous by carrying out a water or solvent extraction.
According to studies made by the present inventors, however, it has been found that the method 1) is disadvantageous in that not only the opacity of the detection mark is low but also the detection mark cannot be sufficiently rendered transparent by heating, and the method 2) has the drawbacks that the extraction step requires a considerable time and the extractant should be used in large amount.
On the other hand, OHP sheets for an electrophotographic copying machine raise the following problems particularly when a multi-color image is formed by using a multi-color copying machine.
Specifically, when a multi-color image is formed on an OHP sheet, toners of three or four colors are usually put on top of another and heat-fixed. This causes the thickness derived from the superimposition of toners to become larger than that in the case of formation of a monochromatic image, so that the surface of the print after heat fixing is likely to become uneven. In this case, at the time of projection, the incident light scatters in the uneven portions, which renders color reproduction of the projected image, particularly at highlight portions, unsatisfactory. That is, clouding (graying) of the image projected by OHP occurs.
Japanese Patent Laid-Open No. 198063/1991 proposes an image-receiving sheet comprising a coating of a material having a melting point above room temperature but below the fixing temperature of the toner and compatible with a binder resin for a color toner, and Japanese Patent Laid-Open No. 125567/1992 proposes a penetrable transfer medium comprising a toner-image-holding layer containing a thermoplastic resin having a softening point below that of a color toner. In these proposals, in order to solve the above-described problem, the softening point or melt viscosity of the image-receiving resin are specified so that the toner penetrates into the image-receiving layer to provide a print having a reduced surface unevenness. Further, Japanese Patent Laid-Open No. 47667/1990 proposes an OHP sheet comprising a porous surface layer comprised of a polyester resin.
An object of the present invention is to provide an OHP sheet having a detection mark capable of being rendered transparent upon heating, which OHP sheet is free from the problem of the prior art.
Another object of the present invention is to provide an image-receiving sheet capable of forming a high-quality multi-color image that can provide an image free from clouding (graying) when applied to OHP.