Radiation imagery chemistry: process – composition – or product th – Liquid crystal process – composition – or product
Reexamination Certificate
1998-05-06
2001-03-06
Wu, Shean C. (Department: 2753)
Radiation imagery chemistry: process, composition, or product th
Liquid crystal process, composition, or product
C430S019000, C430S042100, C349S002000, C359S003000, C369S275200, C428S001100
Reexamination Certificate
active
06197460
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a rewritable heat sensitive, color image recording medium and to a method of recording an image using the recording medium.
There is a known rewritable, thermosensitive recording medium having a thermosensitive layer containing a derivative of a long chain alkyl carboxylic acid, such as a behinic acid. The thermosensitive layer can assume a white color state and a transparent state depending upon thermal hysteresis thereof. Thus, when the thermosensitive layer backed by a colored substrate is heated imagewise, a white image is obtainable with the color of the substrate as a background. No rewritable, thermosensitive recording media capable of giving colored images are known.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a rewritable, heat sensitive recording medium adapted for giving a color image.
Another object of the present invention is to provide a method of forming a color image using the above recording medium.
In accomplishing the foregoing objects, there is provided in accordance with one aspect of the present invention a rewritable, heat sensitive, color image recording medium comprising a pair of opposed substrate sheets at least one of which is transparent, and a heat sensitive layer interposed between said substrate sheets and comprising a cholesteric liquid crystal compound having a molecular weight of not greater than 2,000 and a glass transition point of at least 35° C.
In another aspect, the present invention provides a method of recording a color image, comprising the steps of:
(a) providing a heat sensitive, color image recording medium comprising a pair of opposed substrate sheets at least one of which is transparent, and a heat sensitive layer interposed between said substrate sheets and comprising a cholesteric liquid crystal compound having a molecular weight of not greater than 2,000 and a glass transition point of a least 35° C.;
(b) varying imagewise the temperature of said heat sensitive layer from a first temperature to a second temperature such that said cholesteric liquid crystal compound assumes a cholesteric liquid crystal phase at at least one of said first and second temperatures, thereby forming an image; and
(c) then rapidly cooling said recording medium obtained in step (b) to a temperature lower than said glass transition point to fix said image.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
The recording medium according to the present invention has a pair of opposed substrates each generally in the form a sheet, a film or a plate, and a heat sensitive layer interposed between the substrates. Each of the substrates may be flat or curved and may be rigid or flexible. It is important that at least one of the substrate should be transparent so that the heat sensitive layer can see through the transparent substrate. The transparent substrate may be formed of, for example, a plastic material or a glass. The opaque substrate may be made of, for example, a paper, a metal or a plastic material. If desired, a light or heat reflecting layer or a light (laser beam) absorbing layer may be interposed between the heat sensitive layer and one of the substrates that need not be transparent.
The heat sensitive layer comprises a cholesteric liquid crystal compound having a molecular weight of not greater than 2,000, preferably 500-1,500, more preferably 700-1,200 and a glass transition point of at least 35° C., preferably at least 50° C., more preferably 70-110° C.
The cholosteric liquid crystal compound represented by the formula:
Z—O—CO—R—CO—O—Y
wherein Z and Y are each independently selected from the group consisting of a cholesteryl group, a hydrogen atom and an alkyl group with the proviso that at least one of Z and Y stands for a cholesteryl group, and R stands for a divalent hydrocarbyl group having 2-20 carbon atoms may be suitably used for the purpose of the present invention. The divalent hydrocarbyl group R may be, for example, one which is represented by the formula:
—(CH
2
)
m
—C≡C—C≡C—(CH
2
)
n
—
wherein m and n are each independently an integer of 1 or more with the proviso that the total of m and n does not exceed 16.
Illustrative of cholesteric liquid crystal compounds are dicholesteryl 10,12-docosadiynedioate, dicholesteryl eicosadioate, cholesteryl 10,12-pentacosadiynoate, dicholesteryl dodecadicate and dicholesteryl 12,14-hexacosadiynedioate.
The heat sensitive layer may contain one or more additives such as a pigment, a dye and an oxidation resisting agent. The content of the cholesteric liquid crystal compound in the heat sensitive layer is preferably at least 90% by weight. The thickness of the sensitive layer is not specifically limited, but is generally in the range of 1-100 &mgr;m.
The recording medium may be prepared by, for example, forming a molten cholesteric liquid crystal compound layer on one of the substrates, followed by placement of the other substrate on the molten cholesteric liquid crystal compound layer. Alternatively, a molten cholesteric liquid crystal compound may be filled in a thin gap between opposed substrates using capillary action or suction force.
The recording medium thus constructed gives a color image when the temperature of the heat sensitive layer is changed imagewise from a first temperature to a second temperature such that the cholesteric liquid crystal compound assumes a cholesteric liquid crystal phase at at least one of the first and second temperature.
The cholesteric liquid crystal compound is colorless or transparent in a non-cholesteric liquid phase (such as a glass state, a crystal phase or an isotropic phase), but is colored in a cholesteric liquid crystal phase. The color of the compound in the cholesteric liquid crystal phase varies depending upon the temperature due to a change in the molecular arrangement.
Thus, when the temperature of the heat sensitive layer is changed such that the phase of the cholesteric liquid crystal compound is converted from a cholesteric liquid crystal phase to a non-cholesteric liquid crystal phase or vice versa, the color thereof changes from a colored state (e.g. blue) to a colorless state (e.g. white) or vice versa. Further, when the temperature of the heat sensitive layer is changed while maintaining the cholesteric liquid crystal compound in the cholesteric liquid crystal phase, the color thereof changes (e.g. from blue to red) depending upon the temperature.
The temperature of the heat sensitive layer may be changed imagewise from the first temperature to the second temperature by any desired method such as by heating the recording medium with a laser beam, a thermal head, a heated roll or a heated stamp or by cooling the recording medium with a cold roll or cooling with a cold stamp.
The recording medium in which an image has been formed by changing imagewise the heat sensitive layer from the first temperature to the second temperature is then rapidly cooled to a temperature lower than the glass transition point of the cholesteric liquid crystal compound to fix the image. The fixation of the color of heat sensitive layer is considered to be due to the phase conversion from the cholesteric liquid crystal phase to a glass cholesteric liquid crystal phase. The rapid cooling may be performed by any suitable method such as by immersing the medium in a cold liquid or a cold gas environment or by contacting the medium with a cold gas blow or a cold solid surface.
The color fixation by the cooling of the recording medium is preferably carried out at a rate of at least 1° C./second, more preferably at least 10° C./second, most preferably at least 50° C./second. Too low a cooling rate below 1° C./second might cause a color change during the fixation. For the same reasons, it is advisable to initiate the color fixation as soon as the color development has been completed. Namely, when the colored recording medium is allowed to stand for a certain period of time, e.g. more than 10 seconds at room temperature, the color thereof changes b
Masaki Atsushi
Matsuda Hiro
Parfenov Alexander
Tamaoki Nobuyuki
Director--General of Agency of Industrial Science and Technology
Lorusso & Loud
Wu Shean C.
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