FIELD OF THE INVENTION
This invention relates to improved laser-induced thermal transfer
imaging using overcoated donor elements.
BACKGROUND OF THE INVENTION
Laser-induced thermal transfer processes are well-known in applications
such as color proofing and lithography. Such laser-induced processes include, for
example, dye sublimation, dye transfer, melt transfer, and ablative material transfer.
These processes have been described in, for example, Baldock, U.K. Patent 2,083,726;
DeBoer, U.S. Patent 4,942,141; Kellogg, U.S. Patent 5,019,549; Evans, U.S. Patent
4,948,776; Foley et al., U.S. Patent 5,156,938; Ellis et al., U.S. Patent 5,171,650;
and Koshizuka et al., U.S. Patent 4,643,917.
Laser-induced processes use a laserable assemblage comprising (a)
a donor element that contains a thermally imageable coating in contact with a receiver
element. The laserable assemblage is imagewise exposed by a laser, usually an infrared
laser, resulting in transfer of exposed areas of the thermally imageable coating,
also referred to as material, from the donor element to the receiver element The
(imagewise) exposure takes place only in a small, selected region of the laserable
assemblage at one time, so that transfer of material from the donor element to the
receiver element can be built up one pixel at a time. Computer control produces
transfer with high resolution and at high speed. The laserable assemblage, upon
imagewise exposure to a laser as described supra, is henceforth termed an imaged
Known donor elements tend to lack high durability; that is, they can
be scratched, tend to block and can inadvertently adhere to many surfaces. Defects
resulting from the lack of durability can transfer to the final image resulting
in an unacceptable appearance.
Consequently, a need exists for an improved donor element that has
improved surface properties such as durability, antiblocking, rub and mar resistance,
adhesion and water and humidity resistance.
EP 0. 381 297 A1 discloses a heat transfer sheet for use in heat transfer
printing comprising a base film, a hot melt ink layer laminated on one surface of
the base film, and a filling layer laminated on the hot melt ink layer, said filling
layer comprising one or more waxes or resins which impart filling to the printed
areas of a receiving sheet during image transfer.
EP 0 955 183 A2 discloses a thermal transfer tape with a resin-bonded
release layer containing a wax-soluble polymer and a wax-bonded layer of thermal
transfer ink containing a narrow-cut wax with melting and solidification points
close together plus less then 8 weight % wax soluble polymer.
U.S. Patent 4,792,495 discloses a fusible ink sheet having a top layer
of carnauba wax and ethylene vinyl acetate copolymer on a color layer. The carnauba
wax may be used in combination with specific montan wax or paraffin wax.
U.S. Patent 5,045,383 discloses a thermosensitive image transfer recording
medium having a support, a release layer formed thereon as main components an unvulcanised
rubber and a thermofusible wax component, and a thermofusible ink layer containing
a coloring agent and a thermofusible resin component, with addition of a thermofusible
wax component thereto when necessary, formed on the release layer.
IR absorbers which are used to facilitate image transfer have been
found to negatively impact color purity when added to the thermally imageable layer
of the donor element. Thus, a need exists for an IR absorber layer in the donor
element separate from the thermally imageable layer.
SUMMARY OF THE INVENTION
Methods for laser induced thermal imaging are disclosed herein.
In a first aspect, this invention relates to a method for making an
BRIEF DESCRIPTION OF THE DRAWINGS
- (1) imagewise exposing to laser radiation a laserable assemblage comprising
whereby the exposed areas of the thermally imageable coating and overcoat layer
are transferred to the receiver element to form an image.
- (A) the donor element comprising
- (a) a thermally imageable coating having a coatable surface, and
- (b) an overcoat layer on the coatable surface of the thermally imageable coating
comprising a wax having a melting point in the range of about 30°C to about 350°C;
and a heating layer for absorbing the laser radiation and converting the radiation
into heat, wherein the thermally imageable layer, the overcoat layer or an optional
third layer functions as the heating layer; and
- (B) a receiver element in contact with the overcoat layer of the donor element;
the receiver element preferably comprising:
- (a) an image receiving layer; and
- (b) a receiver support;
A further aspect of the invention comprises:
- (2) separating the donor element (A) from the receiver element (B), thereby
revealing the image on the image receiving layer of the receiver element.
This so revealed image may then be transferred to a permanent substrate
by contacting the receiver element with the permanent substrate, with the image
receiving layer bearing the revealed image adjacent the.permanent substrate.
In a further aspect, the invention also relates to a method further
comprising, after step (2):
- (3) contacting the image on the image receiving layer of the receiver element
with an image rigidification element comprising:
the image being adjacent the thermoplastic polymer layer during said contacting,
whereby the image is encased between the thermoplastic polymer layer and the image
receiving layer of the receiving element;
- (a) a support, and
- (b) a thermoplastic polymer layer releaseably applied to the support,
- (4) removing the support thereby revealing the thermoplastic polymer layer;
- (5) contacting the revealed thermoplastic polymer layer from step (4) with a
permanent substrate. Typically, the donor element is formed by applying a thermally
imageable coating, usually comprising a colorant, to a base element, followed by
application of the overcoat layer.
In the second aspect, this invention relates to a method for making
a color image, further comprising after step (5):
- (6) removing the receiver support.
DETAILED DESCRIPTION OF THE INVENTION
- Figure 1 is a simplified schematic diagram showing a cross-section of a donor
- Figure 2 is a simplified schematic diagram showing a cross-section of a receiver
- Figure 3 is a simplified schematic diagram showing a cross-section of an image
- Figures 4 to 8 are a simplified schematic diagrams showing in cross-section
the subsequent processing steps employing the donor element, the receiver element
and the image rigidification element, and the final product obtained.
Processes and products for laser induced thermal transfer imaging
are disclosed wherein the donor element has durability, resistance to blocking,
rubs and mars, adhesion, water and humidity. In one embodiment, the donor element
of this invention also produces imaged products with better color purity when an
IR absorber is in an overcoat layer of the donor element.
The donor element comprises a support, a thermally imageable coating,
an overcoat layer and a heating layer, wherein the thermally imageable layer, the
overcoat layer or an optional third layer functions as the heating layer.
Optional additional layers such as a heating layer or an intermediate
layer selected from the group consisting of a subbing layer or an ejection layer
or both may also be present.
An example of a suitable donor element is shown in Figure 1. The donor
element comprises an overcoat layer (15), and a thermally imageable layer (14) which
is prepared from a thermally imageable coating typically comprising a colorant.
Optionally, the donor element comprises an intermediate layer (12), a separate heating
layer (13), and a donor support (11). Typically, the heating layer (13) is located
directly on the support (11).
Typically, the donor support is a thick (400 guage) coextruded polyethylene
terephthalate film. Alternately, the donor support is a polyester film, specifically
polyethylene terephthalate that has usually been plasma treated to accept the heating
layer. When the donor support is plasma treated, an intermediate layer is usually
not provided on the donor support. Backing layers may optionally be provided on
the side of the donor support opposite the side of the support with the thermally
imageable coating. These backing layers may contain fillers to provide a roughened
surface on the back side of the donor support. Alternately, the donor support itself
may contain fillers, such as silica, to provide a roughened surface on the back
surface of the support.
The optional intermediate layer (12), as shown in Figure 1, is the
layer that may provide additional force to effect transfer of the thermally imageable
coating to the receiver element in the exposed areas.
If the laserable assemblage is imaged through the intermediate layer,
the intermediate layer should be capable of transmitting the laser radiation, and
not be adversely affected by this radiation.
The intermediate layer may be an ejection layer which, when heated,
decomposes into gaseous molecules providing the necessary pressure to propel or
eject the exposed areas of the thermally imageable coating onto the receiver element.
This is accomplished by using a polymer having a relatively low decomposition temperature
(less than about 350°C, preferably less than about 325°C, and more preferably less
than about 280°C). In the case of polymers having more than one decomposition temperature,
the first decomposition temperature should be lower than about 350°C. In a typical
embodiment, the ejection layer is flexible. In order for the ejection layer to have
suitably high flexibility and conformability, it should have a tensile modulus that
is less than or equal to about 2.5 Gigapascals (GPa), preferably less than about
1.5 GPa, and more preferably less than about 1 GPa. It has been found beneficial
if the polymer chosen is dimensionally stable.
When the intermediate layer functions as an ejection layer, examples
of suitable polymers include (a) polycarbonates having low decomposition temperatures
(Td), such as polypropylene carbonate; (b) substituted styrene polymers having low
decomposition temperatures, such as poly(alphamethylstyrene); (c) polyacrylate and
polymethacrylate esters, such as polymethylmethacrylate and polybutylmethacrylate;
(d) cellulosic materials having low decomposition temperatures (Td), such as cellulose
acetate butyrate and nitrocellulose; and (e) other polymers such as polyvinyl chloride;
poly(chlorovinyl chloride) polyacetals; polyvinylidene chloride; polyurethanes with
low Td; polyesters; polyorthoesters; acrylonitrile and substituted acrylonitrile
polymers; maleic acid resins; and copolymers of the above. Mixtures of polymers
can also be used. Additional examples of polymers having low decomposition temperatures
can be found in Foley et al., U.S. Patent 5,156,938. These include polymers which
undergo acid-catalyzed decomposition. For these polymers, it is frequently desirable
to include one or more hydrogen donors with the polymer.
Preferred polymers for the ejection layer are polyacrylate and polymethacrylate
esters, low Td polycarbonates, nitrocellulose, poly(vinyl chloride) (PVC), and chlorinated
poly(vinyl chloride) (CPVC). Most preferred are poly(vinyl chloride) and chlorinated
Other materials can be present as additives in the intermediate layer
as long as they do not interfere with the essential function of the layer. Examples
of such additives include coating aids, flow additives, slip agents, antihalation
agents, plasticizers, antistatic agents, surfactants, and others which are known
to be used in the formulation of coatings.
The intermediate layer may also be a subbing layer (12) to provide
a donor element having in order at least one subbing layer (12), optionally, a heating
layer (13), and at least one thermally imageable coating(14) and an overcoat layer.
When the intermediate layer is a subbing layer, it is characterized
by an ability to adhere to an adjacent layer of the donor element, such as the heating
layer or the donor support. Examples of suitable materials for the subbing layer
include polyurethanes, polyvinyl chloride, cellulosic materials, acrylate or methacrylate
homopolymers and copolymers, and mixtures thereof. Other custom made decomposable
polymers may also be useful in the subbing layer. Typically useful as subbing layers
for polyester, specifically polyethylene terephthalate, are acrylic subbing layers.
Typically, the subbing layer has a thickness of about 100 Ångstroms to about 1000Ångstroms.
The heating layer (13) of the base element, as shown in Figure 1,
is usually deposited on the optional intermediate layer (12). More typically, the
heating layer (13) is deposited directly on the support (11). The function of the
heating layer is to absorb the laser radiation and convert the radiation into heat.
Materials suitable for the heating layer can be inorganic or organic and can inherently
absorb the laser radiation or include additional laser-radiation absorbing compounds.
Examples of suitable inorganic materials are transition metal elements
and metallic elements of Groups IIIA, IVA, VA, VIA, VIIIA, IIIB, and VB, their alloys
with each other, and their alloys with the elements of Groups IA and IIA of the
Periodic Table of the Elements in Lange's Handbook of Chemistry, 13th
edition, John A. Dean, 1985. Tungsten (W) is an example of a Group VIA metal that
is suitable and which can be utilized. Carbon (a Group IVB nonmetallic element)
can also be used. Preferred metals include Al, Cr, Sb, Ti, Bi, Zr, Ni, In, Zn, and
their alloys and oxides; carbon is a preferred nonmetal. More preferred metals and
nonmetals include Al, Ni, Cr, Zr and C. Most preferred metals are Al, Ni, Cr, and
Zr. A useful metal oxide is TiO2.
The thickness of the heating layer is generally about 20 Ångstroms
to about 0.1 micrometer, preferably about 40 to about 100 Ångstroms.
Although it is preferred to have a single heating layer, it is also
possible to have more than one heating layer, and the different layers can have
the same or different compositions, as long as they all function as described above.
The total thickness of all the heating layers should be in the range given above,
i.e., about 20 Ångstroms to about 0.1 micrometer.
The heating layer(s) can be applied using any of the well-known techniques
for providing thin metal layers, such as sputtering, chemical vapor deposition,
and electron beam.
The thermally imageable layer (14) of the donor element is formed
by applying a binder composition on one side of the donor support. The thermally
imageable layer may comprise a polymeric binder which is different from the polymer
in the intermediate layer.
The binder for the thermally imageable coating is usually a polymeric
material having a decomposition temperature that is greater than about 300°C and
preferably greater than about 350°C. The binder should be film forming from solution
or from a dispersion. Binders having melting points less than about 250°C or plasticized
to such an extent that the glass transition temperature is less than about 70°C
are preferred. However, heat-fusible binders, such as waxes should be avoided as
the sole binder since such binders may not be as durable, although they are useful
as cobinders in decreasing the melting point of the top layer.
It is preferred that the binder does not self-oxidize, decompose or
degrade at the temperature achieved during the laser exposure so that the exposed
areas of the thermally imageable layer comprising a colorant and a binder, are transferred
intact for improved durability. Examples of suitable binders comprise an acrylate,
methacrylate, acrylonitrile, acrylic acid, methacrylic acid, C1-C4
olefin acrylate such as butyl acrylate, C1-C4 methacrylate
such as methyl methacrylate or butyl methacrylate. Other suitable binders include
copolymers of styrene and (meth)acrylate esters, such as styrene/methacrylate copolymer,
styrene/methylmethacrylate copolymer; copolymer of styrene and olefin monomers,
typically containing about 1 to about 4 carbon atoms, such as styrene/ethylene/butylene;
copolymers of styrene and acrylonitrile; fluoropolymers; copolymers of (meth)acrylate
esters with ethylene and carbon monoxide; polycarbonates having decomposition temperatures
higher than 300 °C, typically 280C; (meth)acrylate homopolymers and copolymers;
polysulfones; polyurethanes; polyesters. The monomers for the above polymers can
be substituted or unsubstituted. Mixtures of polymers can also be used.
Typically polymers for the thermally imageable layer include, but
are not limited to, acrylate homopolymers, copolymers and terpolymers; methacrylate
homopolymers, copolymers and terpolymers; (meth)acrylate block copolymers; and (meth)acrylate
copolymers containing other comonomers, such as acrylonitrile and styrene. Some
specific examples include a copolymer of methyl methacrylate and butyl methacrylate
and a terpolymer of butyl acrylate, acrylonitrile and methacrylic acid such as an
acrylic latex copolymer of 74% methyl methacrylate and 24% butyl methacrylate, and
a latex (47% solids) comprising a mixture of butyl acrylate/acrylonitrile/methacrylic
acid copolymer (60/35/5).
A plasticizer may also be included which, typically is a low glass
transition temperature polymer, that acts as a softener for the binder as may be
needed when the polymer of the binder has a high glass transition temperature. An
example of a suitable plasticizer is polyethylene glycol.
The binder is generally used in a concentration of about 15 to about
50% by weight, based on the total weight of the thermally imageable layer, typically
about 30 to about 40% by weight based on the total weight of the thermally imageable
When the thermally imageable layer imparts a color image, e.g, in
color proofing or color filter manufacturing, the colorant of the thermally imageable
layer can be a pigment or a dye, typically a non-sublimable dye. Typically pigments
are used as the colorant for stability and for color density, and also for the high
decomposition temperature. Examples of suitable inorganic pigments include carbon
black and graphite. Examples of suitable organic pigments include Rubine F6B (C.I.
No. Pigment 184); Cromophthal® Yellow 3G (C.I. No. Pigment Yellow 93); Hostaperm®
Yellow 3G (C.I. No. Pigment Yellow 154); Monastral® Violet R (C.I. No. Pigment
Violet 19); 2,9-dimethylquinacridone (C.I. No. Pigment Red 122); Indofast® Brilliant
Scarlet R6300 (C.I. No. Pigment Red 123); Quindo Magenta RV 6803; Monastral®
Blue G (C.I. No. Pigment Blue 15); Monastral®Blue BT 383D (C.I. No. Pigment
Blue 15); Monastral® Blue G BT 284D (C.I. No. Pigment Blue 15); and Monastral®
Green GT 751D (C.I. No. Pigment Green 7). Combinations of pigments and/or dyes can
also be used. For color filter array applications, high transparency pigments (that
is at least about 80% of light transmits through the pigment) are preferred, having
small particle size (that is about 100 nanometers).
In some embodiments of this invention, a pigment, such as carbon black,
is present in a single layer, termed the top thermally imageable layer. This type
of pigment functions as both a heat absorber and a colorant, and thus the top thermally
imageable layer has a dual function of being both a heating layer and a thermally
imageable layer. The characteristics of the top thermally imageable layer are the
same as those given for the thermally imageable layer. A preferred colorant/heat
absorber is carbon black.
In accordance with principles well known to those skilled in the art,
the concentration of colorant will be chosen to achieve the optical density desired
in the final image. The amount of colorant will depend on the thickness of the active
coating and the absorption of the colorant. Optical densities greater than 1.3 at
the wavelength of maximum absorption are typically required. Even higher densities
are preferred. Optical densities adequate for a particular application can be achievable
with application of this invention.
A dispersant is usually present when the colorant is a pigment.
The colorant dispersant is generally an organic polymeric compound
and is used to separate the fine pigment particles and avoid flocculation and agglomeration.
A wide range of colorant dispersants are commercially available. A colorant dispersant
will be selected according to the characteristics of the pigment surface and other
components in the composition as practiced by those skilled in the art. However,
one class of colorant dispersant suitable for practicing the invention is that of
the AB dispersants. The A segment of the dispersant adsorbs onto the surface of
the pigment. The B segment extends into the solvent into which the pigment is dispersed.
The B segment provides a barrier between pigment particles to counteract the attractive
forces of the particles, and thus to prevent agglomeration. The B segment should
have good compatibility with the solvent used. The AB dispersants of choice are
generally described in US 5,085,698. Conventional pigment dispersing techniques,
such as ball milling, sand milling, etc., can be employed.
The colorant is usually present in an amount of from about 25 to about
95% by weight, typically about 35 to about 65% by weight, based on the total weight
of the thermally imageable layer.
The thermally imageable layer is usually applied by coating from a
dispersion. Any suitable solvent can be used as a coating solvent, as long as it
does not deleteriously affect the properties of the assemblage. The thermally imageable
layer can be applied to the base element of the donor element using conventional
coating techniques or printing techniques, for example, gravure printing. A preferred
solvent is water. A thermally imageable layer may be applied by a Waterproof®
Color Versatility Coater sold by DuPont, Wilmington, DE. Coating of the thermally
imageable layer can thus be done shortly before the exposure step. This also allows
for the mixing of various basic colors together to fabricate a wide variety of colors
to match the Pantene® color guide currently used as one of the standards in
the proofing industry.
The thermally imageable layer generally has a thickness in the range
of about 0.1 to about 5 micrometers, preferably in the range of about 0.1 to about
1.5 micrometers. Thickness greater than about 5 micrometers are generally not preferred
as they require excessive energy in order to be effectively transferred to the receiver.
Although it is preferred to have a single thermally imageable layer,
it is also possible to have more than one thermally imageable layer, and the different
layers can have the same or different compositions, as long as they all function
as described above. The total thickness of the combined thermally imageable layers
should be in the range given above.
Other materials can be present as additives in the thermally imageable
layer as long as they do not interfere with the essential function of the layer.
Examples of such additives include layer aids, plasticizers, flow additives, slip
agents, antihalation agents, antistatic agents, surfactants, and others which are
known to be used in the formulation of coatings. However, it is preferred to minimize
the amount of additional materials in this layer, as they may deleteriously affect
the final product after transfer. Additives may add unwanted color for color proofing
applications, or they may decrease durability and print life in lithographic printing
The overcoat layer (15), as shown in Figure 1, provides surface properties
of durability, resistance to blocking, rubs, mars, adhesion, water and humidity.
This layer comprises a wax having a melting point in the range of about 30°C to
about 350°C, typically about 45°C to about 300°C. The wax may be selected from both
natural and synthetic waxes. Usually, the natural wax consists of any vegetable
wax having a melting point (mp.) in the range of about 80°C to about 88°C, such
as carnauba (mp 83-86°C); any mineral wax having a melting point in the range of
about 45°C to about 100°C, such as paraffin (highly refined petroleum, mp 48°C-74°C),
montan (from lignite, mp 79°C-89°C), and microcrystalline (high MW petroleum distillate,
mp 73°C-94°C); synthetic wax having a melting point in the range of about 30°C to
about 350°C, typically about 85°C to 150°C, such as Fischer-Tropsch wax (from coal
gasification, mp approx. 99°C), polyolefin glycol (mp solids from room temperature
to approximately 65°C), high density polyethylene (mp 85-141°C), low density polyethylene,
(mp 30-141°C), polyethyleneacrylic acid (mp 75-80°C), polypropylene (mp 135-160°C),
polytetraflouroethylene (mp 320°C). Some useful synthetic wax come in an oxidized
form such as oxidized high density polyethylene. Typically, these waxes are solid
at ambient temperature.
Specific commercial waxes that are supplied either as neat solids
or in aqueous emulsions or dispersions are oxidized high density polyethylene waxes
such as A-C waxes from Allied Signal ; polyolefin wax such as Epolene® from
Eastman Chemical; ethylene acrylic acid wax such as Primacor® from Dow Chemical;
polyolefin glycol wax such as Carbowax® from Union Carbide and Pluracol®
from BASF ; stearate wax; amide wax; petrolatum wax such as paraffin wax and microcrystalline;
silicone wax; mineral wax such as montan wax, polypropylene wax; carnauba wax; and
fluorocarbon wax such as polytetrafluoro ethylene wax all supplied by Michelman.
Inc. under the trade name Michem®. A specific example of a useful wax is Zinpol®
20 which is an aqueous polyethylene wax.
Typically, the wax may be present in the amount of about 3 % to about
100 % by weight, more typically in the amount of about 30 % to about 70 % by weight,
based on the total weight of the overcoat layer.
Optionally, this overcoat layer may contain acrylic and methacrylic
polymers. A suitable acrylic polymer includes Carboset® GA-33 from B. F. Goodrich.
Typically, the acrylic polymer is present in the amount of about 5 to about 97 %
by weight, more typically in the amount of about 30% to about 70% by weight, based
on the total weight of the layer.
Other additives may be present in the layer imparting roughening or
texture to improve film handling and image quality. Some suitable additives include
inorganic fillers such as silica and alumina. Other additives may be present in
the layer to improve image transfer such as a thermal amplification additive such
as an NIR absorber. Typical examples include a cyanine dye or carbon black.
An overcoat layer comprising a wax permits a textured surface to be
imparted to the donor element. Textured overcoat layers may be achieved by any method
known in the art but use of a wax coating material which contains wax particles
of a size greater than the overall thickness of the was overcoat layer, typically
at least about 0.1 microns in size and more typically about 0.2 to about 1 micron
in size, will result in the overcoat layer having a texture.
The above identified overcoat layer provide a vehicle for the introduction
of the thermal amplification additive. A thermal amplification additive may also
optionally be present in the ejection layer(s), subbing layer or the thermally imageable
layer. It can also be present in all of these layers.
The function of the additive is to amplify the effect of the heat
generated in the heating layer and thus to further increase sensitivity. The additive
should be stable at room temperature. The additive can be (1) a compound which,
when heated, decomposes to form gaseous byproducts(s), (2) a dye which absorbs the
incident laser radiation, or (3) a compound which undergoes a thermally induced
unimolecular rearrangement which is exothermic. Combinations of these types of additives
may also be used.
Thermal amplification additives which decompose upon heating include
those which decompose to form nitrogen, such as diazo alkyls, diazonium salts, and
azido (-N3) compounds; ammonium salts; oxides which decompose to form oxygen; carbonates;
peroxides. Mixtures of additives can also be used. Preferred thermal amplification
additives of this type are diazo compounds such as 4-diazo-N,N' diethyl-aniline
When the thermal amplification additive is a dye whose function is
to absorb the incident radiation and convert this into heat, leading to more efficient
heating for image transfer. It is preferred that the dye absorb in the infrared
region. For imaging applications, it is also preferred that the dye have very low
absorption in the visible region. Examples of suitable NIR (near infrared absorbing)
dyes which can be used alone or in combination include poly(substituted) phthalocyanine
compounds and metal-containing phthalocyanine compounds; cyanine dyes; squarylium
dyes; chalcogenopyryioacrylidene dyes; croconium dyes; metal thiolate dyes; bis(chalcogenopyrylo)
polymethine dyes; oxyindolizine dyes; bis(aminoaryl) polymethine dyes; merocyanine
dyes; and quinoid dyes.
Infrared absorbing materials disclosed in U.S. Patent Nos. 4,778,128;
4,942,141; 4,948,778; 4,950,639; 5,019,549; 4,948,776; 4,948,777; 4,952,552, 5,550,884;
5,440,042; 5,932,740; 5,777,127; 5,576,443 and 5,440,042 may also be suitable herein.
The weight percentage of the thermal amplification additive, versus, for example,
the total solid weight composition of the layer, e.g. the overcoat layer may range
from about 0 to about 20%. When present in the thermally imageable layer, the thermal
amplification additive weight percentage is generally at a level of about 0.95 to
about 11.5%. When present in the intermediate layer, the thermal amplification additive
weight percentage is generally at a level of about 0-20%. The percentage can range
up to about 25% of the total weight percentage in the thermally imageable layer
or overcoat layer. These percentages are non-limiting and one of ordinary skill
in the art can vary them depending upon the particular composition of the ejection
layer or colored layer.
The donor element may have additional layers. For example, an antihalation
layer may be used on the side of the optional intermediate layer opposite the thermally
imageable layer. Materials which can be used as antihalation agents are well known
in the art. Other anchoring or subbing layers can be present on either side of the
intermediate layer and are also well known in the art.
Other donor elements may comprise alternate thermally imageable layer
or layers on a support. Additional layers may be present depending of the specific
process used for imagewise exposure and transfer of the formed images. Some suitable
thermally imageable layers over which the overcoat described above may be applied
are disclosed in US 5,773,188, US 5,622,795, US 5,593,808, US 5,334,573, US 5,156,938,
US 5,256,506, US 5,427,847, US 5,171,650 and US 5,681,681.
The receiver element (20), shown in Figure 2, is the second part of
the laserable assemblage, to which the exposed areas of the thermally imageable
layer, comprising binder and colorant, are transferred. In most cases, the exposed
areas of the thermally imageable layer will not be removed from the donor element
in the absence of a receiver element. That is, exposure of the donor element alone
to laser radiation does not cause material to be removed, or transferred. The exposed
areas of the thermally imageable layer, are removed from the donor element only
when it is exposed to laser radiation and the donor element is in contact with the
receiver element. In the preferred embodiment, the donor element actually touches
the receiver element.
The receiver element (20) may be non-photosensitive or photosensitive.
The non-photosensitive receiver element preferably comprises a receiver support
(21) and an image receiving layer (22). The receiver support (21) comprises a dimensionally
stable sheet material. The assemblage can be imaged through the receiver support
if that support is transparent. Examples of transparent films for receiver supports
include, for example polyethylene terephthalate, polyether sulfone, a polyimide,
a poly(vinyl alcohol-co-acetal), polyethylene, or a cellulose ester, such as cellulose
acetate. Examples of opaque support materials include, for example, polyethylene
terephthalate filled with a white pigment such as titanium dioxide, ivory paper,
or synthetic paper, such as Tyvek® spunbonded polyolefin. Paper supports are
typical and are preferred for proofing applications, while a polyester support,
such as poly(ethylene terephthalate) is typical and is preferred for a medical hardcopy
and color filter array applications. Roughened supports may also be used in the
The image-receiving layer (22) may be a coating of, for example, a
polycarbonate; a polyurethane; a polyester; polyvinyl chloride; styrene/acrylonitrile
copolymer; poly(caprolactone); vinylacetate copolymers with ethylene and/or vinyl
chloride; (meth)acrylate homopolymers (such as butylmethacrylate) and copolymers;
polycaprolactone; polyesters; and mixtures thereof. Typically, the image receiving
layer is a crystalline polymer layer, polyester or mixture thereof. The image receiving
layer polymer preferably has a melting point in the range of 50 to 64 °C, more preferably
56 to 64°C, and most preferably 58 to 62°C. Blends made from 5-40% Capa® 650
(melt range 58-60°C) and Tone® P-300 (melt range 58-62°C), both polycaprolactones,
are useful in this invention. Typically, 100% Tone P-300 is used. Useful receiver
elements are also disclosed in US Patent 5,534,387 issued on July 9, 1996. One additional
example is the WaterProof® Transfer Sheet sold by DuPont. Typically, it has
an ethylene/vinyl acetate copolymer in the surface layer comprising more ethylene
than the vinyl acetate.
This image-receiving layer may be present in any amount effective
for the intended purpose. In general, good results have been obtained at coating
weights of range of about 10 to about 150 mg/dm2, typically about 40
to about 60 mg/dm2.
In addition to the image-receiving layer, the receiver element may
optionally include one or more other layers (not shown) between the receiver support
and the image receiving layer. An additional layer between the image-receiving layer
and the support can be a release layer. The receiver support alone or the combination
of receiver support and release layer may also be referred to as a first temporary
carrier. The release layer can provide the desired adhesion balance to the receiver
support so that the image-receiving layer adheres to the receiver support during
exposure and separation from the donor element, but promotes the separation of the
image receiving layer from the receiver support upon transfer, for example by lamination,
of the image receiving layer to a permanent substrate or support. Examples of materials
suitable for use as the release layer include polyamides, silicones, vinyl chloride
polymers and copolymers, vinyl acetate polymers and copolymers and plasticized polyvinyl
alcohols. The release layer can have a thickness in the range of 1 to 50 microns.
A cushion layer which is a deformable layer may also be present in
the receiver element, typically between the release layer and the receiver support.
The cushion layer may be present to increase the contact between the receiver element
and the donor element when assembled. Examples of suitable materials for use as
the cushion layer include copolymers of styrene and olefin monomers such as styrene/ethylene/butylene/styrene,
styrene/butylene/styrene block copolymers, and other elastomers useful as binders
in flexographic plate applications.
The receiver element is an intermediate element in the process of
the invention because the laser imaging step is normally followed by one or more
transfer steps by which the exposed areas of the thermally imageable layer are eventually
transferred to the permanent substrate.
The image rigidification element (30), shown in Figure 3, comprises
a releasable support (32) having a release surface (33), and a thermoplastic polymer
The support having a release surface or second temporary carrier (31)
may comprise a support (32) and a surface layer (33) which may be a release layer.
If the material used as the support, has a release surface, e.g., polyethylene or
a fluoropolymer, no additional surface layer is needed. The surface or release layer
(33) should have sufficient adhesion to the support (32) to remain affixed to the
support throughout the processing steps of the invention. Almost any material that
has reasonable stiffness and dimensional stability is useful as the support. Some
examples of useful supports include polymeric films such as polyesters, including
polyethylene terephthalate and polyethylene naphthanate; polyamides; polycarbonates;
fluoropolymers; polyacetals; polyolefins, etc. The support may also be a thin metal
sheet or a natural of synthetic paper substrate. The support may be transparent,
translucent or opaque. It may be colored and may have incorporated therein additives
such as fillers to aid in the movement of the image rigidification element through
the lamination device during its lamination to the color image containing receiver
The support may have antistatic layers coated on one or both sides.
This may be useful in reducing static when the support is removed from the thermoplastic
polymer layer during the process of the invention. It is generally preferred to
have antistatic layers coated on the back side of the support, i.e., the side of
the support away from the thermoplastic polymer layer. Materials which can be used
as antistatic materials are well known in the art. Optionally, the support may also
have a matte texture to aid in transport and handling of the image rigidification
The support typically has a thickness of about 20 µm to about 250
µm. A preferred thickness is about 55 to 200 µm.
The release surface of the support may be provided by a surface layer
(33). Release layers are generally very thin layers which promote the separation
of layers. Materials useful as release layers are well known in the art and include,
for example, silicones; melamine acrylic resins; vinyl chloride polymers and copolymers;
vinyl acetate polymers and copolymers; plasticized polyvinyl alcohols; ethylene
and propylene polymers and copolymers; etc. When a separate release layer is coated
onto the support, the layer generally has a thickness in the range of 0.5 to 10
The release layer (33) may also include materials such as antistats,
colorants, antihalation dyes, optical brighteners, surfactants, plasticizers, coating
aids, matting agents, and the like.
Thermoplastic polymers useful in the thermoplastic polymer layer are
preferably amorphous, i.e., non-crystalline, in character, have high softening points,
moderate to high molecular weight and compatibility with the components of the image
receiving polymer layer, e.g., polycaprolactone. Additionally, flexibility without
cracking and possessing the capability to be attached to many different permanent
substrates is advantageous. The polymer is preferably solvent soluble, has good
solvent and light stability and is a good film former.
There are many useful thermoplastic polymer materials. Preferred for
use in this invention are thermoplastic polymers having Tgs (glass transition temperatures)
in the range of about 27 to 150°C, preferably 40 to 70°C, and more preferably 45
to 55°C, a relatively high softening points, e.g., Tg of 47°C, melt flow of 142°C),
low elongations at break as determined by ASTM D822A of e.g., 3, and moderate weight
average molecular weight (Mw), e.g., in the area of 67,000. Polyester polymers,
e.g., having a Tg of about 47°C, are preferred because good compatibility is achieved
between the image receiving polymer, e.g., crystalline polycaprolactone and the
polyester polymer in the image rigidification layer. However, other suitable polymers
have been shown to give acceptable results. Some suitable materials include methacrylate/acrylate,
polyvinylacetate, polyvinylbutyral, polyvinylformal, styrene-isoprene-styrene and
styrene-ethylene-butylene-styrene polymers, etc.
The thermoplastic polymer is present in the amount of about 60 to
90% by weight, typically about 70 to 85% by weight, based on the total weight of
the thermoplastic polymer layer components.
The thermoplastic polymer layer and image receiving layer relate to
each other in that the colored image is encased between them so that it does not
move significantly during lamination to the permanent substrate, e.g., paper, and
cooling. This significantly reduces halftone dot movement, swath boundary cracking
and banding compared to similar processes not employing a thermoplastic polymer
layer in this manner, i.e., an image rigidification element, and renders them barely
perceptible or substantially eliminated.
The use of the thermoplastic polymer layer in the processes and products
of this invention results in an increase in lamination throughput speeds from 200mm/min
to approximately 600-800 mm/min (3-4 fold increase) without the introduction of
defects, and provides lamination process latitude to allow image transfer to many
different types of permanent substrates.
The thermoplastic polymer layer also provides a vehicle or mechanism
for the introduction of bleaching chemistry to reduce the impact on final color
associated with the NIR dye in the transferred color image to the permanent substrate.
The thermoplastic polymer layer may also contain additives as long
as they do not interfere with the functioning of this layer. For example, additives
such as plasticizers, other modifying polymers, coating aids, surfactants can be
used. Some useful plasticizers include polyethylene glycols, polypropylene glycols,
phthalate esters, dibutyl phthalate and glycerine derivatives such triacetin. Typically,
the plasticizer is present in the amount of about 1 to 20% by weight, most typically
5 to 15% by weight, based on the total weight of the thermoplastic polymer layer
As noted above, the thermoplastic polymer layer also preferably contains
dye bleaching agents for bleaching the thermal amplification additive, such as an
NIR dye, which may be present in the donor element and/or the receiver element.
Some useful bleaching agents include amines; azo compounds; carbonyl compounds;
hydantoin compounds selected from the dichlorodimethyl derivatives, dibromodimethyl
derivatives and cholorobromodimethyl derivatives; organometallic compounds; and
carbanions. Useful oxidants include peroxides, diacyl peroxides, peroxy acids, hydroperoxides,
persulfates, and halogen compounds. Typical dye bleaching agents for polymethine
type NIR dyes are those selected from the group consisting of hydrogen peroxide,
organic peroxides, hydantoin compounds, hexaaryl biimidazoles, halogenated organic
compounds, persulfates, perborates, perphosphates, hypochlorites and azo compounds.
Dye bleaching agents are present in the amount of about 1 to 20% by
weight, typically about 5 to 15% by weight, based on the total weight of the thermoplastic
One advantage of the process of this invention is that the permanent
substrate for receiving the colored image, can be chosen from almost any sheet material
desired. For most proofmg applications a paper support is used, preferably the same
paper on which the image will ultimately be printed. Any paper stock can be used.
Other materials which can be used as the permanent substrate include cloth, wood,
glass, china, most polymeric films, synthetic papers, thin metal sheets or foils,
etc. Almost any material which will adhere to the thermoplastic polymer layer (34),
can be used as the permanent substrate.
The first step in the process of the invention is imagewise exposing
the laserable assemblage, e.g., as shown in Figure 4, to laser radiation. The exposure
step is preferably effected at a laser fluence of about 600 mJ/cm2 or
less, most preferably about 250 to 440 mJ/cm2. The laserable assemblage
comprises the donor element and the receiver element, described above.
The assemblage is normally prepared following removal of coversheet(s),
if present, by placing the donor element in contact with the receiver element such
that overcoat layer actually touches the image-receiving layer on the receiver element.
This is represented in Figure 4. Vacuum and/or pressure can be used to hold the
two elements together. Alternately, the donor and receiver elements may be spaced
slightly apart using spacer particles in the overcoat layer or the image receiving
layer. As one alternative, the donor and receiver elements can be held together
by fusion of layers at the periphery. As another alternative, the donor and receiver
elements can be taped together and taped to the imaging apparatus, or a pin/clamping
system can be used. As yet another alternative, the donor element can be laminated
to the receiver element to afford a laserable assemblage. The laserable assemblage
can be conveniently mounted on a drum to facilitate laser imaging.
Various types of lasers can be used to expose the laserable assemblage.
The laser is preferably one emitting in the infrared, near-infrared or visible region.
Particularly advantageous are diode lasers emitting in the region of about 750 to
870 nm which offer a substantial advantage in terms of their small size, low cost,
stability, reliability, ruggedness and ease of modulation. Diode lasers emitting
in the range of about 780 to 850 nm are most preferred. Such lasers are available
from, for example, Spectra Diode Laboratories (San Jose, CA).
The exposure can take place through the flexible ejection layer or
subbing layer of the donor element or through the receiver element, provided that
these are substantially transparent to the laser radiation. In most cases, the donor
flexible ejection layer or subbing layer will be a film which is transparent to
infrared radiation and the exposure is conveniently carried out through the flexible
ejection or subbing layer. However, if the receiver element is substantially transparent
to infrared radiation, the process of the invention can also be carried out by imagewise
exposing the receiver element to infrared laser radiation.
The laserable assemblage is exposed imagewise so that the exposed
areas of the thermally imageable layer are transferred to the receiver element in
a pattern. The pattern itself can be, for example, in the form of dots or line work
generated by a computer, in a form obtained by scanning artwork to be copied, in
the form of a digitized image taken from original artwork, or a combination of any
of these forms which can be electronically combined on a computer prior to laser
exposure. The laser beam and the laserable assemblage are in constant motion with
respect to each other, such that each minute area of the assemblage, i.e., "pixel"
is individually addressed by the laser. This is generally accomplished by mounting
the laserable assemblage on a rotatable drum. A flat bed recorder can also be used.
The next step in the process of the invention is separating the donor
element from the receiver element. Usually this is done by simply peeling the two
elements apart. This generally requires very little peel force, and is accomplished
by simply separating the donor support from the receiver element. This can be done
using any conventional separation technique and can be manual or automatic without
As shown in Figure 5, separation results in a laser generated color
image preferably a halftone dot image, comprising the transferred exposed areas
of the thermally imageable layer and overcoat layer, being revealed on the image
receiving layer of the receiver element. Preferably the color image formed by the
exposure and separation steps is a laser generated halftone dot color image formed
on a crystalline polymer containing layer, the crystalline polymer containing layer
being located on a first temporary carrier.
The image rigidification element is then brought into contact with,
preferably laminated to, the image receiver element with the color image in contact
with the thermoplastic polymer layer of the image rigidification element resulting
in the thermoplastic polymer layer of the rigidification element and the image receiving
layer of the receiver element encasing the color image. This is best seen in Figure
6. A WaterProof® Laminator, manufactured by DuPont is preferably used to accomplish
the lamination. However, other conventional means may be used to accomplish contact
of the image carrying receiver element with the thermoplastic polymer layer of the
rigidification element. It is important that the adhesion of the rigidfication element
support having a release surface (31), also known as the second temporary carrier,
to the thermoplastic polymer layer (34) be less than the adhesion between any other
layers in the sandwich. The novel assemblage or sandwich, e.g., as illustrated by
Figure 6, is highly useful, e.g., as an improved image proofing system.
The support (32) having a release surface (33) (or second temporary
carrier) is then removed, preferably by peeling off, to reveal the thermoplastic
film as seen in Figure 6a. The color image on the receiver element is then transferred
to the permanent substrate by contacting the permanent substrate with, preferably
laminating it to, the revealed thermoplastic polymer layer of the sandwich structure
shown in Figure 6a. Again a WaterProof® Laminator, manufactured by DuPont, is
preferably used to accomplish the lamination. However, other conventional means
may be used to accomplish this contact which results in the sandwich structure shown
in Figure 7.
Another embodiment includes the additional step of removing, preferably
by peeling off, the receiver support (21) (also known as the first temporary carrier),
resulting in the assemblage or sandwich structure shown in Figure 8. In a preferred
embodiment, the assemblages illustrated in Figures 7 and 8 represent a printing
proof comprising a laser generated halftone dot color thermal, image formed on an
image receiving layer, and a thermoplastic polymer layer laminated on one surface
to said image receiving layer and laminated on the other surface to the permanent
substrate, whereby the color image is encased between the image receiving layer
and the thermoplastic polymer layer.
In proofing applications, the receiver element can be an intermediate
element onto which a multicolor image is built up. Some of the donor elements in
a proofing application do not require an overcoat layer for making multicolor images.
An overcoated donor element having a thermally imageable layer comprising a first
colorant and an overcoat layer thereon is exposed and separated as described above.
The receiver element has a color image formed with the first colorant, which is
preferably a laser generated halftone dot color thermal image. Thereafter, a second
overcoated donor element having a thermally imageable coating different than that
of the first overcoated thermally imageable element forms a laserable assemblage
with the receiver element having the colored image of the first colorant and is
imagewise exposed and separated as described above. The steps of (a) forming the
laserable assemblage with a donor element having a different colorant than that
used before and the previously imaged receiver element, (b) exposing, and (c) separating
are sequentially repeated as often as necessary in order to build the multicolored
image of a color proof on the receiver element.
The rigidification element is then brought into contact with, preferably
laminated to, the multiple colored images on the image receiving element with the
last colored image in contact with the thermoplastic polymer layer. The process
is then completed as described above.
These non-limiting examples demonstrate the processes and products
claimed and described herein. All temperatures throughout the specification are
in °C (degrees Centigrade) and all percentages are weight percentages unless indicated
The following black solution was made and coated to 12-14 mg/sq dm
using a #6 wire round rod onto 50% T Chrome (that is a chromium coating) on 4 mil
Melinex® 562 (DuPont):
1 is an acrylic latex copolymer of 74% methyl methacrylate
and 24% butyl methacrylate
2 is a latex (47% solids) comprising a mixture of butyl
acrylate/acrylonitrile/methacrylic acid copolymer (60/35/5)
3 is manufactured by Penn Color, PA.
4 is polyethylene glycol, MW 300
5 is a fluorocarbon surfactant
Black Donor Solution(100 g sol'n)
Penn Color 32B563
Zonyl® FSA(25%FC) 5
The following solutions were made of Carboset ® GA-33 (aqueous
acrylic polymer dispersion made by B. F. Goodrich) at 5% solids and Zinpol®
20 (aqueous polyethylene wax emulsion made by B. F. Goodrich possessing a melt point
of 138 °C) at 5% solids and then blended to make overcoat solutions. The overcoat
solutions were coated using a #4 wire rod to 2 mg/sq dm on top of the black film
and dried. Below are the solutions and blends that were made and tested:
Acrylic (GA-33) Solution
Wax (Zinpol® 20) Solution
Wax (Zinpol® 20)
Blended Overcoat Solutions
100% Wax (Zinpol® 20)
30/70 Acrylic (GA-33)/Wax (Zinpol® 20)
50/50 Acrylic (GA-33)/ Wax (Zinpol® 20)
70/30 Acrylic (GA-33)/Wax (Zinpol® 20)
Coated films tested were:
- Film # 1 - Black Control - no overcoat
- Film #2 -100% Wax (Zinpol 20) overcoated on black
- Film #3 - 30/70 Acrylic (GA-33)/Wax(Zinpol 20) overcoated on black
- Film #4 - 50/50 Acrylic (GA-33)/Wax (Zinpol 20) overcoated on black
- Film #5 - 70/30 Acrylic (GA-33)/Wax (Zinpol 20) overcoated on black
For the durability evaluation, each film was placed on a solid surface
with the coating face up. A 6-inch stroke applied to the Film #1 coating with either
a fingernail or a No. 2 pencil caused deep scratches to form, removing the coating
entirely, thus damaging the coating surface. The same test applied to Films #2-#5
produced no damage to the coating surface.
The following receiver element and image rigidifation elements were
used in making a color image:
Receiver Element 1:
A receiver element, comprised of 100% Tone P-300 (Polycaprolactone,
crystalline polymer, melt range 58-62 °C, Union Carbide) was made by coating a 15%
solids solution in tetrahydrofuran (THF) to a dried thickness of 53 mg/dm2
on 300 gauge EB-11 Mylar® polyester film, as a receiver support (or first temporary
carrier) having a release surface (sold by DuPont). The dried coating thickness
was 50-55 mg/dm2 and comprised the image receiving layer.
Image Rigidification Element 1:
An image rigidification layer incorporating a plasticizer and an NIR
dye bleaching agent was made by coating the following composition from a 20% solids
solution, with a #10 wire wound rod on slip treated Melinex® 377 polyester film,
as the support having a release surface, and dried thickness of 55 mg/dm2.
Dibutyl Phthalate (plasticizer) Dicyclohexylphthalate (plasticizer)
1,3-dichloro-5,5-dimethyl hydantoin (NIR dye bleaching agent)
Vitel® 2700B (thermoplastic polymer)
Each of the above identified black films #1-5 was placed in the cassette
of a Creo Spectrum Trendsetter and imaged to receiver element #1 at 12.5 watts,
170 rpm. The image formed was laminated to the image rigidification layer, of image
rigidification element 1. After peeling of the receiver support, the sandwich was
then laminated to a final permanent substrate, (Lustro Gloss #100 paper).
Laminations were done with the standard WaterProof® laminator
(DuPont) using the paper setting (120°C top roll, 115 °C bottom roll; 450#; 600
mm/min). After allowing the sandwich to cool (about 2 minutes), the receiver support
(first temporary carrier) was removed leaving behind a black halftone dot thermal
image on paper. Results with all 5 films, indicated that the image quality of the
halftone dot images were the same. Where coating surfaces had been damaged with
Film #1, the halftone image could not be produced. This demonstrates that relative
to the control Film #1, the overcoated films prevent handling damage and do not
adversely affect image quality.
Four color images may be prepared by repeating the above steps using
the receiver having the black image thereon and magenta, cyan, and yellow films,
respectively in the imaging step instead of the black film, and then repeating the
following steps to get a four color image on paper.