1. Field of the Invention
The present invention relates to a pressure-sensitive recording paper, and more particularly, to an improvement in the mark formation and printability of the pressure-sensitive recording paper.
2. Prior Art
Generally, pressure-sensitive recording paper consists of a top sheet (CB) of which the back side is coated with microcapsules containing colorless or pale colored electron donative leuco dyes dissolved in an organic solvent (capsule oil) and a bottom sheet (CF) which has a color developing layer containing electron acceptive color developing agent on its front side.
When the top sheet is laid on the bottom sheet and local pressure is applied to them by handwriting with a ball-point pen or the like, the capsules to which pressure is applied are ruptured and the oil in the capsules containing leuco dyes is transferred to the color developing layer; and a printed mark is formed by color generating reaction of the leuco dyes and electron acceptive color developing agent.
The color developing sheet mentioned in this invention is a sheet which has the above color developing layer, including a middle paper (CFB) which is coated with a electron acceptive color developing agent on the front side and capsules on the back side in addition to a bottom sheet (CF) described above.
In recent years, the demand for pressure-sensitive recording paper has increased very rapidly with the systemization of offices, expansion of the information industry and the spread of computers.
As a result, the applications for paper have been diversified and many properties are needed with regard to the diversified applications for paper.
Not only is it required that the paper record images but improvement of the ink setting of the color developing sheets on which the mark forms is also required in order to adapt to high speed printing of normal ink for characters or lines and desensitizing ink.
It is necessary for the color developing layer to absorb and set the normal inks or the desensitizing ink in a very short time in order to adapt to high speed printing.
Also, stickiness of the printed surface or setting off of printed ink to another paper surface can be prevented by improvements of the ink setting speed of the color developing sheets; as a result, high speed printing becomes possible.
Especially, in the case of desensitizing printing, it is required to apply a larger amount of ink than for normal ink printing. Therefore, not only is it required to increase the ink absorbing speed of the color developing layer but also it is required to increase the ink absorbing amount in order to accelerate the ink setting.
Usually, the coating color of the color developing sheet contains electron acceptive coreactant with inorganic pigments such as kaoline, talc and calcium carbonate or organic pigments such as urea-formaldehyde resins as fillers.
Moreover, latexes and one or more sorts of natural or synthetic aqueous polymers are added in order to fix the above-mentioned materials on the sheet.
Furthermore, if desired, viscosity adjusting agents and pH adjusting agents and so on can be added.
The above-described coating color is coated on the base paper by a coater and dried.
For the purposes of advancing the color developing ability and improving the ink absorption, many methods for transferring capsule oil efficiently to the color developing sheet have been studied such as by adding some amount of fillers which absorb the oil very much to the color developing layer. With regard to these fillers, they are disadvantages in that the adhesion of the fillers to the sheet decreases as the amount of fillers increases. Therefore, filler is picked off from the paper surface during printing and the filler adheres to the rubber blankets and scum the printing plate. As a result, practical products cannot be obtained.
One method is to use relatively fine calcium carbonate of which 55 percent of the particle size distribution is less than 2.mu.. This method is described in Japanese Laid Open Patent Application No. 28857/1980. However, these finer fillers have higher specific surface; therefore, the obtained strength will be insufficient if the usual amount of binder is used and it becomes impossible to fix the fillers and other materials to the sheet sufficiently. As a result, the coating products are easily picked off and the surface strength of the sheet is insufficient for commercial printing.
Also, as the surface strength becomes high, the mark formability becomes low. If a larger amount of binder is added as a countermeasure, the desired result cannot be obtained. On the other hand, since latexes when used as a binder, in comparason with water soluble polymers as a binder, give a coating color with high concentration and low viscosity, they have advantages that decreased drier load can be obtained and various coaters can be used and hence high-speed coating etc. is possible. Furthermore, the resultant color developing sheet has excellent water-resistance.