Heat-sensitive imaging material

The present invention provides a heat-sensitive layer prepared by radiation polymerizing a monomer composition comprising: PA1 (i) from about 30 to about 70 parts by weight of methacrylic acid, and PA1 (ii) correspondingly, from about 70 to about 30 parts by weight of a urethane (meth)acrylate compound having a urethane backbone and at least 2 (meth)acryloyl groups in a molecule, and PA1 (iii) up to about 40 parts of a viscosity controlling compound having at least one (meth)acryloyl group wherein the percentage of the urethane (meth)acrylate and the viscosity controlling agent totals no more than 70 parts, then stretching said polymer from at least a yield point to a break point.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a heat-sensitive imaging material, in particular, 
a heat-sensitive material suitable as an OHP film with which a reverse 
image to an original image can be easily formed there on by irradiating 
the imaging material placed on the original. 
This invention also relates to a transparent imaging sheet wherein a stripe 
of the heat-sensitive composition is coated or attached thereon, either 
before or after the imaging sheet has been stretched. This stripe is 
capable of becoming transparent upon the application of heat. 
2. Description of the Art 
Photothermographic material has long been known as a heat-sensitive 
material useful for a transparent film, suitable for use with overhead 
projectors (OHPs), see, e.g., Japanese Patent Kokai Publication No. 
52-31728. 
One presently used commercial embodiment, Thermofax.TM., from Minnesota 
Mining and Manufacturing Company (3M,) provides a black image using a 
combination of a silver salt of an organic acid and a reducing agent, 
through a reaction of the silver salt. The image is formed in black on a 
bright background, and an OHP image is projected in black on a white 
screen. When another color image is desired, a coloring reaction between 
an electron donating dye and an electron acceptor can be used. 
For the formation of the OHP image, a contact exposure method is used in 
which, infrared (IR) light is irradiated on the heat-sensitive film placed 
on the original, which causes carbon black therein to absorb heat and heat 
a part of the heat-sensitive material. 
When the projected image is viewed, bright color characters or figures in a 
dark background not only reduce exhaustion of the eyes, but also make it 
possible to color the characters or figures by placing a color film on the 
OHP film. 
To create a reverse image on the imaging material film, several products 
and methods are known, i.e.: 
(1) Use of a laminated two-sheet composite consisting of a thermal transfer 
ink donor sheet and a thermal transfer ink accepter sheet, in which the 
donor sheet transfers part of the ink to an acceptor sheet, which can be 
used as an OHP film. See, e.g., Japanese Patent Kokai Publication Nos. 
2-118986, 2-128897, 2-128898 and 59-106996. 
(2) Infrared copying processing of translucent fusible films which form 
negative photography due to perforation of the films on the original. See. 
e.g, Japanese Patent Kokai Publication Nos. 62-51492 and 61-31294). 
(3) Film comprising a resin matrix having a low molecular weight organic 
material dispersed therein, which changes from transparent to opaque when 
single crystals are converted to a polycrystal. See, e.g., Japanese Patent 
Kokai Publication Nos. 62-225392 and 63-31790, and 
(4) A mixture of an electron donor dye precursor with a developer and a 
particulate discoloration agent which is coated on a transparent support. 
See, e.g., Japanese Patent Kokai Publication No. 3-99881. 
However, each of (1) to (4) has some disadvantages. 
None of the materials exhibit gradation. That is, intermediate tones 
between the dark and bright of the image cannot be reproduced. This means 
that such imaging materials cannot be used to copy photographs and the 
like. 
In addition, materials (1) and (2) have low resolution and an upper limit 
of a heating temperature. Contrast between the bright and dark parts of 
the image is insufficient. 
Transparent sheets are used for imaging through printing or copying 
devices. Such devices are usually equipped with optical sheet detectors. 
It is necessary that each transparent sheet has an indicator, such as a 
white stripe, affixed thereon in order for the sheet to be detected by the 
optical detectors. Generally, the white stripe is opaque, and thus 
projects on an overhead projector as a black stripe. When this occurs, 
image information printed on the transparent sheet in the area of the 
white stripe will not be visible upon projection and this projected black 
stripe is also not aesthetically pleasing. 
Thus there is a need for a transparent sheet capable of being detected 
optically in the imaging devices but which becomes transparent after 
imaging through the devices. 
U.S. Pat. No. 5,156,709 discloses a coating composition composed of a 
polymer pigment combined with an emulsion or latex binder to form a white 
stripe on a transparency. The polymer pigment particles preferably has a 
glass transition temperature of between 35.degree.-80.degree. C., and are 
dispersed homogeneously within a suitable binder such as an emulsion 
binder. The white stripe becomes transparent upon fusing. 
The present invention provides a heat-sensitive imaging material suitable 
for an OHP film which can easily form a reverse image to an original 
image, and has gradation and good resolution. 
The present invention also provides a transparent sheet having a pattern of 
a heat-sensitive composition coated or attached thereon, such that the 
pattern is detectable by any optical detector in an imaging device and is 
capable of becoming transparent upon the application of heat. 
SUMMARY OF THE INVENTION 
Specifically, the present invention provides a heat-sensitive layer 
prepared by 
a) radiation polymerizing a monomer composition comprising: 
(i) from about 30 to about 70 parts by weight of methacrylic acid, and 
(ii) correspondingly, from about 70 to about 30 parts by weight of a 
urethane (meth)acrylate compound having a urethane backbone and at least 2 
(meth)acryloyl groups in a molecule, and 
(iii) up to about 40 parts of a viscosity controlling compound having at 
least one (meth)acryloyl group wherein the percentage of the urethane 
(meth)acrylate and the viscosity controlling agent totals no more than 70 
parts, and 
(b) stretching said polymer under a condition from at least a yield point 
to a break point. 
The invention also provides a transparent imaging sheet comprising a 
transparent support having a pattern of the heat-sensitive composition 
described above coated or attached on at least a portion of one major 
surface thereof. 
As used herein, the term "heat-sensitive film" means that the film is made 
opaque by stretching and returns to a transparent state when heated. 
All parts, ratios and percents herein are by weight unless specifically 
noted otherwise.

DETAILED DESCRIPTION OF THE INVENTION 
The cured polymer can be stretched by any method insofar as the above 
condition is met. For example, a liquid monomer composition can be 
polymerized and stretched and then laminated on a support. Alternatively, 
the liquid monomer composition can be applied onto the support and 
polymerized and then the composite comprising the polymer film and the 
support are stretched. 
This transparent imaging sheet comprises a transparent support or 
substrate, which can be also coated with a layer of image-receptive 
coating. Specifically, a pattern of the heat-sensitive composition is 
coated on one major surface of a transparent sheet wherein the 
heat-sensitive composition is capable of becoming transparent upon the 
application of heat. The transparent sheet is suitable for use in a 
copying or printing device, which permits sheet detection capabilities 
without blocking information or interfering with information copied or 
printed on the transparent sheet thereof. 
The heat-sensitive monomer composition of the invention can be polymerized 
by a per se conventional method. For example, a photoinitiator may be 
added to the composition which is then irradiated with UV light. 
Alternatively, the composition may be cured by the use of ionic radiation 
such as an electron beam generated by an electron accelerator, without the 
use of photoinitiator. 
When a liquid mixture of methacrylic acid and urethane acrylate is 
polymerized and cured by the irradiation of UV light, a cured material is 
formed consisting of a crystalline phase which is mainly derived from 
methacrylic acid and an amorphous phase which is mainly derived from 
urethane acrylate. In this case, the two phases form a microphase 
separation structure. 
A weight ratio of methacrylic acid to urethane acrylate should be from 
about 30:70 to about 70:30. As explained above, 0 to 40 parts by weight of 
the weight urethane acrylate can be replaced with the viscosity adjusting 
component (c). 
Useful urethane acrylates are not limited, with any of those conventionally 
known ones being useful, and a commercially available one may be used as 
shown in Examples. 
The unstretched polymer is transparent and has a glass transition 
temperature higher than room temperature and also a yield point. When the 
polymer is stretched uniaxially or biaxially under the condition from the 
yield point to the break point, the stretched film becomes opaque and 
blocks about 90% or more of the visible light. 
Without wishing to be bound by theory, it is believed that the reason for 
this may be that cavitations could be formed by stretching at an interface 
between the crystal phase and the amorphous phase in the above microphase 
separation structure. 
The thickness of the stretched film is from about 0.02 mm to about 2.0 mm. 
The glass transition temperature of the opaque film depends on the precise 
monomer formulation used. When at least a part of the opaque film is 
heated at a temperature higher than its glass transition temperature, it 
returns to a transparent state. 
Without wishing to be bound by theory, it is believed that this may occur 
because of a shape-memory effect imparted by a three-dimensional 
crosslinking formed by the urethane acrylate. 
When the heat-sensitive imaging film of the invention is placed on an 
original and irradiated by IR light, carbon black on the original absorbs 
heat and is heated to a higher temperature than other parts so that a part 
of the imaging film corresponding to the carbon black part is made 
transparent through the above mechanism. Accordingly, when the 
heat-sensitive imaging film is used as an OHP film, a reverse image to the 
original image can be formed. 
On the OHP film consisting of the heat-sensitive imaging material of the 
present invention, an overcoat layer of, for example, a silicone resin may 
be formed to prevent transfer of the carbon black from the original to the 
surface of the OHP film, whereby a higher quality image can be obtained. 
The heat-sensitive imaging material can be used as a film for a thermal 
head. 
Prior to being stretched and cured, a film of this heat-sensitive polymer, 
either with or without an unstretched transparent support, can be coated 
with an adhesive. A portion of this adhesive-coated film is then attached 
to an unstretched or uniaxially stretched transparent imaging sheet, and 
the entire composite is stretched to form a biaxially oriented sheet. 
Alternately, a film of this heat-sensitive polymer can be coated directly 
onto an unstretched or uniaxially stretched transparent imaging sheet and 
then the entire composite undergoes biaxial orientation. 
This heat-sensitive imaging polymer film can also be coated on a 
transparent support, then stretched. After stretching, it can be attached 
with an adhesive layer to a transparent imaging sheet after the 
transparent sheet has been biaxially oriented. 
The heat-sensitive polymer film is preferably applied in a predetermined 
pattern on the transparent imaging sheet. The pattern is preferably in the 
form of a stripe, and can be located anywhere on the sheet, the location 
depending on the configuration of the optical detectors in the printing or 
copying device. Usually it is located along at least one leading edge of 
the sheet. When coated directly onto the transparent imaging sheet, it can 
be done in any known coating method, such as spraying, gravure coating, 
dip coating, or silk screen techniques. The stripe can be white or 
opaquely colored. 
Any colorant can be used, as long as it does not interfere with the 
function of the stripe and becomes transparent upon the application of 
heat. 
The heat-sensitive imaging material of the present invention will be 
illustrated in detail. The examples are for illustrative purposes only, 
and the scope of the invention is that which is defined by the claims. 
EXAMPLES 
Example 1 
This Example shows that methacrylic acid as the component (A) and the 
urethane acrylate compound (B) are essential for the preparation of the 
heat-sensitive imaging material of the present invention. 
A monomer composition was prepared by mixing the components with a stirrer 
at room temperature. The formulation of each monomer composition is 
expressed in terms of parts by weight of the components. 
As a photo radical initiator, "Darocure D1173" (MERK JAPAN Co., Ltd.) was 
used. The liquid composition was polymerized by irradiating it with UV 
light from a high pressure mercury lamp at 100 mW/CM.sup.2 for 40 seconds. 
The presence and absence of the heat sensitivity are expressed by symbols 
(0) and (X) respectively. 
Example 2 
This Example studies a range of ratios among the various components of the 
heat-sensitive composition. FIGS. 1 and 2 are ternary composition diagrams 
of methacrylic acid/UX4101/2-ethylhexyl acrylate and methacrylic 
acid/UV3000B/2-ethylhexyl acrylate, respectively. The ratios which impart 
the UV cured film having the heat sensitivity are marked "O" in FIGS. 1 
and 2. 
This Example shows superior resolution of the heat-sensitive imaging 
material of the present invention to commercially available heat-sensitive 
OHP film for negative image formation. 
A mixture of methacrylic acid, (UV3000B), 2-ethylhexyl acrylate and "D1173" 
photoinitiator in a ratio of 30/30/40/1 was coated with to a thickness of 
50 micrometers on a 100 micrometer thick polycarbonate film. The coating 
was cured by UV irradiation. 
The laminate film was then stretched 100% using TENSILON.TM. UTM-4-100, 
Tokyo Baldwin Co., Ltd. at a stretching rate of 200 mm/min. The stretched 
film became opaque. 
FIG. 3 shows a projected image of a negative image formed on the imaging 
film (FIG. 3b) together with the original image. 
FIG. 3a. The original image of FIG. 3a was copied by the infrared copying 
process using Transparency Marker.TM. commercially available from 3M. 
The image of FIG. 3C was the projected image using a commercially available 
OHP film for negative image formation (3M). It can be seen that the 
heat-sensitive imaging material (FIG. 3b) has better resolution than the 
commercially sold OHP film (FIG. 3c). 
Example 4 
In this Example, copying of a photographic original was attempted using the 
heat-sensitive imaging material of the present invention. 
When the OHP film for the negative image formation was used, such copy was 
impossible, while when the imaging material of the present invention was 
used, a clear image was formed. 
Example 5 
A heat-sensitive stripe was prepared in the following manner: 
A mixture of 33 parts by weight methacrylic acid, 33 parts by weight 
urethane acrylate (available from Nippon Gosei Kagaku, as uv 3000B 
(SICOU), 33 parts by weight 2-ethylhexylacrylate and 1 part by weight 
photoinitiator (Darocure.TM. 1173, available from Merck), was sandwiched 
between 2 PET films, each film being 100 micrometers in thickness, and 
irradiated with UV light from a high pressure mercury lamp at 100 
mW/cm.sup.2 for 40 sec. 
After removing one of the PET films, the exposed surface of this cured 
layer was coated with a 50% solid solution of an adhesive, in 80/20 
toluene/isopropanol, at a coating thickness of about 50 micrometers, 
comprising 40 parts styrene-isoprene-styrene block copolymer, (Kraton.TM. 
1107, available from Shell Chemical Corp.), 100 parts total tackifier, 
including 50 parts Wingtack.TM. Plus, available from Goodyear Tire and 
Rubber Company, and 50 parts Wingtack.TM.10, also available from Goodyear, 
and 1 part Irganox.TM. 1076, from Ciba Geigy. 
The solvent was dried in a 65.degree. C. oven for 20 minutes. The resultant 
heat-sensitive adhesive film was subjected to a 40% stretching. The film 
was translucent both before and after stretching. 
The film was then adhered to a PP2500 transparency film (available from 3M) 
and fed through a copier. The translucent film became transparent. 
TABLE 1 
______________________________________ 
Evaluation of whether UV-cured films according to the 
invention become opaque by stretching. The films comprise 
various ingredients with the A B initiator ratio being 
40 60 1 in all cases. 
(A) (B) Evaluation 
______________________________________ 
Isooctyl acrylate UX4101 x 
n-Butyl acrylate UX4101 x 
2-Ethylhexyl acrylate 
UX4101 x 
2-Methoxyethyl acrylate 
UX4101 x 
Ethyl acrylate UX4101 x 
Methyl acrylate UX4101 x 
Tetrahydrofurfuryl acrylate 
UX4101 x 
2-Hydroxyethyl acrylate 
UX4101 x 
Benzyl acrylate UX4101 x 
Cyclohexyl acrylate UX4101 x 
Cyclohexyl methacrylate 
UX4101 x 
Vinyl acetate UX4101 x 
t-Butyl acrylate UX4101 x 
Isobornyl acrylate UX4101 x 
Acrylic acid UX4101 x 
Dicyclopentanyl acrylate 
UX4101 x 
Dicyclopentanyl methacrylate 
UX4101 x 
N-Vinyl pyrrolidone UX4101 x 
N-Isobutoxymethyl acrylamide 
UX4101 x 
1,6-Hexanediol diacrylate 
UX4101 x 
Tripropyleneglycol diacrylate 
UX4101 x 
Trimethylolpropane triacrylate 
UX4101 x 
Methacrylic acid UX4101 0 
Methacrylic acid UV3000B 0 
Methacrylic acid V4221 0 
Methacrylic acid V4350 0 
Methacrylic acid M6200 x 
Methacrylic acid 9G x 
Methacrylic acid ACR-210 x 
______________________________________ 
UX4101: Urethane acrylate (Nippon Kayaku, KAYARAD) 
UV3000B: Urethane acrylate (Nippon Gosei KK, SICOU) 
V4221: Urethane acrylate (DaiNippon Ink & KK, UNIDIC) 
V4350: Urethane acrylate (DaiNippon Ink & Kagaku, UNIDIC) 
M6200: Oligoester acrylate (Toagosei Kagaku, ARONIX) 
9G: Diester dimethacrylate (ShinNakamura Chemical, NKESTER) 
ACR210: Butadiene acrylate (Japan Hydrazine, Polybee)