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Description  |
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This invention relates to a laser recording element, and more particularly
to a single-sheet laser recording element wherein the support is a
microvoided composite film.
In recent years, thermal transfer systems have been developed to obtain
prints from pictures which have been generated electronically from a color
video camera. According to one way of obtaining such prints, an electronic
picture is first subjected to color separation by color filters. The
respective color-separated images are then converted into electrical
signals. These signals are then operated on to produce cyan, magenta and
yellow electrical signals. These signals are then transmitted to a thermal
printer. To obtain the print, a cyan, magenta or yellow dye-donor element
is placed face-to-face with a dye-receiving element. The two are then
inserted between a thermal printing head and a platen roller. A line-type
thermal printing head is used to apply heat from the back of the dye-donor
sheet. The thermal printing head has many heating elements and is heated
up sequentially in response to the cyan, magenta and yellow signals. The
process is then repeated for the other two colors. A color hard copy is
thus obtained which corresponds to the original picture viewed on a
screen.
U.S. Pat. No. 5,244,861 relates a receiving element useful in the
above-described thermal dye transfer process which contains a microvoided
composite film as the support. There is no disclosure in this patent that
the support would be useful in other thermal systems.
Another way to thermally obtain a print using the electronic signals
described above is to use a laser instead of a thermal printing head. In
such a laser transfer system, the donor sheet includes a material which
strongly absorbs at the wavelength of the laser. When the donor is
irradiated, this absorbing material converts light energy to thermal
energy and transfers the heat to the dye in the immediate vicinity,
thereby heating the dye to its vaporization temperature for transfer to a
receiver. The absorbing material may be present in a layer beneath the dye
and/or it may be admixed with the dye. The laser beam is modulated by
electronic signals which are representative of the shape and-color of the
original image, so that each dye is heated to cause volatilization only in
those areas in which its presence is required on the receiver to
reconstruct the color of the original object. Further details of this
process are found in GB 2,083,726A, the disclosure of which is hereby
incorporated by reference.
In another mode of imaging using a laser beam, a laser recording element
with a dye layer composition comprising an image dye, an
infrared-absorbing material, and a binder coated onto a substrate is
imaged from the dye side. The energy provided by the laser drives off the
image dye and other components of the dye layer at the spot where the
laser beam impinges upon the element. In "laser removal" imaging, the
laser radiation causes rapid local changes in the imaging layer, thereby
causing the material to be removed from the layer. Usefulness of such a
laser recording element is largely determined by the efficiency at which
the imaging dye can be removed on laser exposure. The transmission Dmin
value is a quantitative measure of dye clean-out: the lower its value at
the recording spot, the more complete is the attained dye removal.
U.S. Pat. No. 5,330,876 relates to a dye ablative recording element as
described above. The element comprises a support having thereon a dye
layer containing an image dye, IR-absorbing dye and binder. The element is
imagewise-exposed by a laser and portions of the dye layer are ablated
away to produce a dye image. The support for this element is the
conventional supports used in this field.
There is a problem with the above dye ablative recording elements in that
the minimum density and speed are not as good as is desired.
It is an object of this invention to provide a dye ablative recording
element wherein the minimum density and speed are improved over that of
the prior art. It is another object of this invention to provide a process
for forming an image using this recording element.
These and other objects are achieved in accordance with the invention which
relates to a laser recording element comprising a base having thereon a
dye layer comprising an image dye dispersed in a polymeric binder, the dye
layer having an infrared-absorbing material associated therewith, the base
comprising a composite film laminated to at least one side of a support,
the dye layer being on the composite film side of the base, and the
composite film, comprising a microvoided thermoplastic core layer and at
least one substantially void-free thermoplastic surface layer.
Any visible image dye can be used in the laser recording element employed
in the invention provided it can be removed by the action of the laser.
Especially good results have been obtained with dyes such as anthraquinone
dyes, e.g., Sumikaron Violet RS.RTM. (product of Sumitomo Chemical Co.,
Ltd.), Dianix Fast Violet 3R-FS.RTM. (product of Mitsubishi Chemical
Industries, Ltd.), and Kayalon Polyol Brilliant Blue N-BGM.RTM. and KST
Black 146.RTM. (products of Nippon Kayaku Co., Ltd.); azo dyes such as
Kayalon Polyol Brilliant Blue BM.RTM., Kayalon Polyol Dark Blue 2BM.RTM.,
(products of Nippon Kayaku Co., Ltd.); direct dyes such as Direct Dark
Green B.RTM. (product of Mitsubishi Chemical Industries, Ltd.) and Direct
Brown M.RTM. (product of Nippon Kayaku Co. Ltd.); acid dyes such as
Kayanol Milling Cyanine 5R.RTM. (product of Nippon Kayaku Co. Ltd.); basic
dyes such as Sumiacryl Blue 6G.RTM. (product of Sumitomo Chemical Co.,
Ltd.), and Aizen Malachite Green.RTM. (product of Hodogaya Chemical Co.,
Ltd.);
##STR1##
or any of the dyes disclosed in U.S. Pat. Nos. 4,541,830, 4,698,651,
4,695,287, 4,701,439, 4,757,046, 4,743,582, 4,769,360, and 4,753,922, the
disclosures of which are hereby incorporated by reference. The above dyes
may be employed singly or in combination. The dyes may be used at a
coverage of from about 0.05 to about 1 g/m.sup.2 and are preferably
hydrophobic.
Another embodiment of the invention relates to a process of forming a dye
image comprising imagewise-heating, by means of a laser, the recording
element described above, the laser exposure taking place through the dye
side of the element and causing dye to be removed imagewise to obtain the
dye image in the recording element.
In still another preferred embodiment of the invention, dye is removed
imagewise by means of an air stream, vacuum and filter system.
The laser recording elements of this invention can be used to obtain
medical images, reprographic masks, printing masks, etc. The image
obtained can be a positive or a negative image. The dye removal process
can generate either continuous (photographic-like) or halftone images.
Any polymeric material may be used as the binder in the recording element
employed in the invention. For example, there may be used cellulosic
derivatives, e.g., cellulose nitrate, cellulose acetate hydrogen
phthalate, cellulose acetate, cellulose acetate propionate, cellulose
acetate butyrate, cellulose triacetate, a hydroxypropyl cellulose ether,
an ethyl cellulose ether, etc., polycarbonates; polyurethanes; polyesters;
poly(vinyl acetate); polystyrene; poly(styrene-co-acrylonitrile); a
polysulfone; a poly(phenylene oxide); a poly(ethylene oxide); a poly(vinyl
alcohol-co-acetal) such as poly(vinyl acetal), poly(vinyl
alcohol-co-butyral) or poly(vinyl benzal); or mixtures or copolymers
thereof. The binder may be used at a coverage of from about 0.1 to about 5
g/m.sup.2.
In a preferred embodiment, the polymeric binder used in the recording
element employed in the process of the invention has a polystyrene
equivalent molecular weight of at least 100,000 as measured by size
exclusion chromatography, as described in U.S. Pat. No. 5,330,876, the
disclosure of which is hereby incorporated by reference.
A barrier layer may be employed in the laser recording element of the
invention if desired, as described in copending U.S. Ser. No. 321,282,
filed Oct. 11, 1994, and entitled BARRIER LAYER FOR LASER ABLATIVE
IMAGING, the disclosure of which is hereby incorporated by reference.
To obtain a laser-induced image according to the invention, a diode laser
is preferably employed, such as an infrared diode laser, since it offers
substantial advantages in terms of its small size, low cost, stability,
reliability, ruggedness, and ease of modulation. In practice, before an
infrared laser can be used to heat a recording element, the element must
contain an infrared-absorbing material, such as cyanine infrared-absorbing
dyes as described in U.S. Ser. No. 099,969, filed Jul. 30, 1993, and
entitled, "INFRARED-ABSORBING CYANINE DYES FOR LASER ABLATIVE IMAGING" or
other materials as described in the following U.S. Pat. No.: 4,948,777,
4,950,640, 4,950,639, 4,948,776, 4,948,778, 4,942,141, 4,952,552,
5,036,040, and 4,912,083, the disclosures of which are hereby incorporated
by reference. The laser radiation is then absorbed into the dye layer and
converted to heat by a molecular process known as internal conversion.
Thus, the construction of a useful dye layer will depend not only on the
hue, transferability and intensity of the image dyes, but also on the
ability of the dye layer to absorb the radiation and convert it to heat.
The infrared-absorbing dye may be contained in the dye layer itself or in
a separate layer associated therewith, i.e., above or below the dye layer.
Preferably, the laser exposure in the process of the invention takes place
through the dye side of the recording element, which enables this process
to be a single-sheet process, i.e., a separate receiving element is not
required.
Lasers which can be used in the invention are available commercially. There
can be employed, for example, Laser Model SDL-2420-H2 from Spectra Diode
Labs, or Laser Model SLD 304 V/W from Sony Corp.
The dye layer of the laser recording element of the invention may be coated
on the support or printed thereon by a printing technique such as a
gravure process or a hopper coater.
Due to their relatively low cost and good appearance, composite films are
generally used and referred to in the trade as "packaging films." The
support may include cellulose paper, a polymeric film or a synthetic
paper.
Unlike synthetic paper materials, microvoided packaging films can be
laminated to one side of most supports and still show excellent curl
performance. Curl performance can be controlled by the beam strength of
the support. As the thickness of a support decreases, so does the beam
strength. These films can be laminated on one side of supports of fairly
low thickness/beam strength and still exhibit only minimal curl.
Microvoided composite packaging films are conveniently manufactured by
coextrusion of the core and surface layers, followed by biaxial
orientation, whereby voids are formed around void-initiating material
contained in the core layer. Such composite films are disclosed in, for
example, U.S. Pat. No. 5,244,861, the disclosure of which is incorporated
by reference.
The core of the composite film should be from 15 to 95% of the total
thickness of the film, preferably from 30 to 85% of the total thickness.
The nonvoided skin(s) should thus be from 5 to 85% of the film, preferably
from 15 to 70% of the thickness. The density (specific gravity) of the
composite film should be between 0.2 and 1.0 g/cm.sup.3, preferably
between 0.3 and 0.7 g/cm.sup.3. As the core thickness becomes less than
30% or as the specific gravity is increased above 0.7 g/cm.sup.3, the
composite film starts to lose useful compressibility and thermal
insulating properties. As the core thickness is increased above 85% or as
the specific gravity becomes less than 0.3 g/cm.sup.3, the composite film
becomes less manufacturable due to a drop in tensile strength and it
becomes more susceptible to physical damage. The total thickness of the
composite film can range from 20 to 150 .mu.m, preferably from 30 to 70
.mu.m. Below 30 .mu.m, the microvoided films may not be thick enough to
minimize any inherent non-planarity in the support and would be more
difficult to manufacture. At thicknesses higher than 70 .mu.m, little
improvement in either print uniformity or thermal efficiency is seen, and
so there is not much justification for the further increase in cost for
extra materials.
Suitable classes of thermoplastic polymers for the core matrix-polymer of
the composite film include polyolefins, polyesters, polyamides,
polycarbonates, cellulosic esters, polystyrene, polyvinyl resins,
polysulfonamides, polyethers, polyimides, poly(vinylidene fluoride),
polyurethanes, poly(phenylene sulfides), polytetrafluoroethylene,
polyacetals, polysulfonates, polyester ionomers, and polyolefin ionomers.
Copolymers and/or mixtures of these polymers can be used.
Suitable polyolefins include polypropylene, polyethylene,
polymethylpentene,.sup.I and mixtures thereof. Polyolefin copolymers,
including copolymers of ethylene and propylene are also useful.
The composite film can be made with skin(s) of the same polymeric material
as the core matrix, or it can be made with skin(s) of polymeric
composition different from that of the core matrix. For compatibility, an
auxiliary layer can be used to promote adhesion of the skin layer to the
core.
Addenda may be added to the core matrix to improve the whiteness of these
films. This would include any process which is known in the art including
adding a white pigment, such as titanium dioxide, barium sulfate, clay, or
calcium carbonate. This would also include adding optical brighteners or
fluorescing agents which absorb energy in the UV region and emit light
largely in the blue region, or other additives which would improve the
physical properties of the film or the manufacturability of the film.
Coextrusion, quenching, orienting, and heat setting of these composite
films may be effected by any process which is known in the art for
producing oriented film, such as by a flat film process or by a bubble or
tubular process. The flat film process involves extruding the blend
through a slit die and rapidly quenching the extruded web upon a chilled
casting drum so that the core matrix polymer component of the film and the
skin components(s) are quenched below their glass transition temperatures
(Tg). The quenched film is then biaxially oriented by stretching in
mutually perpendicular directions at a temperature above the glass
transition temperature of the matrix polymers and the skin polymers. The
film may be stretched in one direction and then in a second direction or
may be simultaneously stretched in both directions. After the film has
been stretched it is heat set by heating to a temperature sufficient to
crystallize the polymers while restraining the film to some degree against
retraction in both directions of stretching.
By having at least one nonvoided skin on the microvoided core, the tensile
strength of the film is increased and makes it more manufacturable. It
allows the films to be made at wider widths and higher draw ratios than
when films are made with all layers voided. Coextruding the layers further
simplifies the manufacturing process.
The support to which the microvoided composite films are laminated for the
base of the recording element of the invention may be a polymeric,
synthetic paper, or cellulose fiber paper support, or laminates thereof.
Preferred cellulose fiber paper supports include those disclosed in U.S.
Pat. No. 5,250,496, the disclosure of which is incorporated by reference.
When using a cellulose fiber paper support, it is preferable to extrusion
laminate the microvoided composite films using a polyolefin resin. During
the lamination process, it is desirable to maintain minimal tension of the
microvoided packaging film in order to minimize curl in the resulting
laminated support. The backside of the paper support (i.e., the side
opposite to the microvoided composite film) may also be extrusion coated
with a polyolefin resin layer (e.g., from about 10 to 75 g/m.sup.2), and
may also include a backing layer such as those disclosed in U.S. Pat. Nos.
5,011,814 and 5,096,875, the disclosures of which are incorporated by
reference. For high humidity applications (>50% RH), it is desirable to
provide a backside resin coverage of from about 30 to about 75 g/m.sup.2,
more preferably from 35 to 50 g/m.sup.2, to keep curl to a minimum.
In one preferred embodiment, in order to produce recording elements with a
desirable photographic look and feel, it is preferable to use relatively
thick paper supports (e.g., at least 120 .mu.m thick, preferably from 120
to 250 .mu.m thick) and relatively thin microvoided composite packaging
films (e.g., less than 50 .mu.m thick, preferably from 20 to 50 .mu.m
thick, more preferably from 30 to 50 .mu.m thick).
In another embodiment of the invention, in order to form a recording
element which resembles plain paper, e.g. for inclusion in a printed
multiple page document, relatively thin paper or polymeric supports (e.g.,
less than 80 .mu.m, preferably from 25 to 80.mu.m thick) may be used in
combination with relatively thin microvoided composite packaging films
(e.g., less than 50 .mu.m thick, preferably from 20 to 50 .mu.m thick,
more preferably from 30 to 50 .mu.m thick).
The following example is provided to further illustrate the invention.
EXAMPLE
Preparation of the Microvoided Support-Support A
A commercially available packaging film (OPPalyte.RTM. 350 TW, Mobil
Chemical Co.) was laminated to a paper support. OPPalyte.RTM. 350 TW is a
composite film (38 .mu.m thick) (d=0.62) consisting of a microvoided and
oriented polypropylene core (approximately 73% of the total film
thickness), with a titanium dioxide pigmented, non-microvoided, oriented
polypropylene layer on each side; the void-initiating material is
poly(butylene terephthalate).
Packaging films may be laminated in a variety of way (by extrusion,
pressure, or other means) to a paper support. In the present context, they
were extrusion-laminated as described below with pigmented polyolefin onto
a paper stock support. The pigmented polyolefin was polyethylene (12
g/m.sup.2) containing anatase (titanium dioxide) (12.5% by weight) and a
benzoxazole optical brightener (0.05% by weight).
The paper stock support was 137 .mu.m thick and made form a 1:1 blend of
Pontiac Maple 51 (a bleached maple hardwood kraft of 0.5 .mu.m length
weighted average fiber length), available from Consolidated Pontiac, Inc.,
and Alpha Hardwood Sulfite (a bleached red-alder hardwood sulfite of 0.69
.mu.m average fiber length), available form Weyerhauser Paper Co. The
backside of the paper stock support was coated with high-density
polyethylene (30 g/m.sup.2).
Preparation of the Non-micrivided Support-Support B (Control)
A non-microvoided support was prepared by extrusion-coating a pigmented
polyolefin unto a paper stock support. The pigmented polyolefin was
polyethylene (12 g/m.sup.2) containing anatase (titanium dioxide) (12.5%
by weight) and a benzoxazole optical brightener (0.05% by weight). The
paper stock support was the same as described above. The backside of the
paper stock support was coated with high-density polyethylene (30
g/m.sup.2).
Laser Dye Ablation Layer
The following mixture was prepared and stirred until dissolved:
12 g nitrocellulose (Hercules)
0.24 g IR-1
0.24 g of cyan-2 dye illustrated above
70 g acetone
##STR2##
Element 1
The above solution was coated at 34 g/m.sup.2, wet coverage, on paper
Support A as described above.
Control 1
This is similar to Element 1 except that Support B was used instead of
Support A.
Laser Exposure
After drying, the elements were exposed using Spectra Diode Labs Lasers
Model SDL-2432, with a maximum power of 600 mW per laser beam, at 830 nm,
1000 rev/min and a spot size of approximately 12 .mu.m .times. 25 .mu.m
using a lathe type printer with a drum circumference of 53 cm. The diode
laser beams were scanned across the surface of the element to achieve 945
lines per cm, or 2400 lines per in. An air stream was blown over the donor
surface along with a vacuum and a filter system to remove the ablated
material.
A step tablet image was printed by reducing the laser intensity linearly in
successive patches from the maximum to 0. Reflection Status A red
densities were measured using an X-Rite Model 310 reflection densitometer.
Readings were referenced to the uncoated paper support, respectively. The
results are given in the following Table. The closely matched D-max values
indicate that the coating thicknesses were the same within experimental
error as intended (step 21).
TABLE
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Laser Power Exposure Element Control
Step (mW) (mJ/cm.sup.2)
1 1
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1 600 643 0.12 0.21
2 585 627 0.12 0.21
3 570 611 0.12 0.21
4 555 595 0.12 0.21
5 540 578 0.12 0.21
6 525 562 0.13 0.23
7 510 546 0.13 0.24
8 495 530 0.13 0.25
9 480 514 0.14 0.26
10 465 498 0.15 0.26
11 450 482 0.15 0.30
12 435 466 0.16 0.32
13 420 450 0.17 0.35
14 405 434 0.19 0.38
15 390 418 0.22 0.40
16 375 402 0.25 0.44
17 360 386 0.31 0.50
18 345 370 0.34 0.55
19 330 354 0.43 0.59
20 315 337 0.53 0.67
21 0 0 0.76 0.77
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The above results show the element with the microvoided support (Element 1)
was more efficient, exhibiting a considerably lower D-min (Steps 1-5),
than Control 1 which used a non-microvoided support. The microvoided
support also gave a speed improvement which can be seen in the data by
defining a speed point as the exposure required to print to 0.03 above
D-min (step 10 for Element 1 and step 7 for Control 1). The speed point
for Element 1 was 498 mJ/cm.sup.2, while the speed point for Control 1 was
546 mJ/cm.sup.2. Thus, Element 1 requires 9% less exposure to achieve this
speed point.
The invention has been described in detail with particular reference to
preferred embodiments thereof, but it will be understood that variations
and modifications can be effected within the spirit and scope of the
invention.
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