Radiation imagery chemistry: process – composition – or product th – Erasable imaging
Reexamination Certificate
2000-07-19
2001-09-18
McPherson, John A. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Erasable imaging
C430S945000, C430S964000, C503S200000, C503S216000
Reexamination Certificate
active
06291117
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reversible heat-sensitive recording material in which images are formable and erasable by controlling heat energy.
2. Prior Art
A heat-sensitive recording material generally has a support and a heat-sensitive recording layer which is composed mainly of normally colorless or slightly colored electron donating dye precursor and an electron accepting color developer and formed on the support. The dye precursor and the color developer instantaneously react upon application of heat by a thermal head, a thermal pen, laser beams, or the like, to form an image. Such heat-sensitive recording materials are disclosed in Japanese Patent Publications Nos. 43-4160 and 45-14039.
In the above heat-sensitive recording materials, generally, when an image is once formed, it is impossible to erase the image to restore an image portion to its original state. For recording further information, therefore, it is only possible to make recording in a portion where no image is formed. When the area for heat-sensitive recording is limited, there is therefore caused a problem that information that can be recorded is limited, so that not all of the necessary information can be recorded.
For overcoming the above problem, reversible heat-sensitive recording materials capable of repetitive formation and erasure of images have been proposed in recent years. For example, JP-A-54-119377, JP-A-63-39377 and JP-A-63-41186 disclose heat-sensitive recording materials comprising a matrix resin and an organic low-molecular compound dispersed in the matrix resin. In these heat-sensitive recording materials, however, the transparency thereof is reversibly changed, so that the contrast between an imaged portion and an non-imaged portion is insufficient.
There is also proposed a color-forming and color-erasing reversible heat-sensitive recording material on which color formation and color erasure can be repeated. JP-A-2-188293, JP-A-2-188294 and International Patent Publication WO90/11898 disclose reversible heat-sensitive recording materials comprising an electron-donating dye precursor and a color developing and decloring agent for causing the electron-donating dye precursor to form a color upon heating and erasing the color, i.e., a reversible color developer in the present invention.
JP-A-63-173684 and JP-A-4-247985 disclose a reversible heat-sensitive recording material comprising a combination of an electron-donating dye precursor and an ascorbic acid derivative, or a combination of an electron-donating dye precursor and an organic sulfonic acid compound or the like.
The above reversible heat-sensitive recording materials containing an electron-donating dye precursor have various problems caused by the photodecomposition of the electron donating dye precursor itself. That is, when exposed to light such as sunlight or fluorescent lamp light, a non-image portion decreases in whiteness, and the formed color phase of an image portion deteriorates. Further, there is another problem that when an image portion is erased, a non-erasable residue takes place.
In general heat-sensitive recording materials containing a dye precursor, it is general practice to incorporate an ultraviolet absorbent into a protective layer. In reversible heat-sensitive recording materials, however, it produces almost no effect to incorporate an ultraviolet absorbent. That is because general ultraviolet absorbents which undergo no crosslinking with a binder component in a protective layer have a problem that they are crystallized and bleed out after stored for a long period of time and cannot exhibit the capability of ultraviolet absorption. Particularly, in reversible heat-sensitive recording materials on which printing and erasing are repeated, further, an ultraviolet absorbent gradually shifts out of a protective layer, which causes a problem that the capability of ultraviolet absorption decreases and that the reversible heat-sensitive recording layer decreases in printing performance. Further, when a large amount of an ultraviolet absorbent is incorporated for compensating the crystallization and bleed-out of the absorbent, there are caused problems that an image is debased, and that the strength of a layer decreases during repeated printing and erasing due to an increase in the amount of the melting component. In JP-A-9-207437, further, an attempt is made to improve light resistance by adding an ultraviolet-absorbing polymer. However, this polymer has poor compatibility with other components, and the effect of absorbing ultraviolet light is insufficient.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a reversible heat-sensitive recording material that is capable of the formation and erasure of an image with a good contrast and capable of retaining an image stable with the passage of time under usual living environments, more specifically, a reversible heat-sensitive recording material which is improved in light resistance, which is almost free from discoloration caused by deterioration when exposed to light for a long period of time and erasing failure, and which has high layer strength and has durability against printing and erasure repeated many times.
According to the present invention, the above object of the present invention is achieved by a reversible heat-sensitive recording material having a support, a reversible heat-sensitive recording layer formed on the support and at least one protective layer formed thereon, the reversible heat-sensitive recording layer containing a normally colorless or slightly colored dye precursor and a reversible color developer capable of causing a reversible change in color density of the dye precursor on the basis of a difference in cooling rate after heating, wherein at least one of the said layers contains an isocyanate compound and an ultraviolet absorbent capable of reacting with the isocyanate compound to undergo crosslinking.
The above reversible heat-sensitive recording material is excellent in light resistance and is almost free from discoloration in exposure to light and erasing failure, and the present invention is accordingly completed.
DETAILED DESCRIPTION OF THE INVENTION
The light resistance in the present invention will be explained below. Most reversible heat-sensitive recording materials containing a dye precursor have very low strength against the exposure to visible light or ultraviolet light, and when the reversible heat-sensitive recording materials are exposed to light for a long period of time, an unreacted portion (to be referred to as “ground portion” hereinafter) or an erased portion where a color formed by a reaction is erased (to be referred to as “erased portion” hereinafter) is deteriorated from a colorlessness or a slightly colored state to a brown or other state of a lowered whiteness. Further, a portion where a reaction with a reversible developer has taken place, i.e., a color-formed portion is sometimes converted to a color-formed state different from an original color density, or when the portion is erased after exposed to light, an erased state is incomplete or is deteriorated to have a color density greatly different from the color of the ground portion. The above phenomenon is mainly caused by the decomposition and deterioration of the dye precursor exposed to light. In the reversible heat-sensitive recording material containing such a dye precursor, therefore, it is desired to minimize the above deterioration, i.e., to improve the reversible heat-sensitive recording material in light resistance.
The reversible heat-sensitive recording material referred to in the present invention has a reversible heat-sensitive recording layer composed of at least two components such as a dye precursor and a reversible color developer and has one or more protective layers formed thereon. In the present invention, an ultraviolet absorbent reactive with an isocyanate compound (to be referred to as “reactive ultraviolet absorbent” hereinafter) can be added to any one l
Hosaka Kenichi
Maruyama Jun
McPherson John A.
Mitsubishi Paper Mills Ltd.
Nixon & Vanderhye
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