U.S. patent application number 13/945688 was filed with the patent office on 2013-11-14 for compositions, thermally-insulating layers, and direct thermally imaging members containing the same.
The applicant listed for this patent is ZINK IMAGING, INC.. Invention is credited to John C. Day, John Hardin, Yulin Hardin, Fariza B. Hasan, Stephen Telfer, William T. Vetterling.
Application Number | 20130303670 13/945688 |
Document ID | / |
Family ID | 43544603 |
Filed Date | 2013-11-14 |
United States Patent
Application |
20130303670 |
Kind Code |
A1 |
Day; John C. ; et
al. |
November 14, 2013 |
COMPOSITIONS, THERMALLY-INSULATING LAYERS, AND DIRECT THERMALLY
IMAGING MEMBERS CONTAINING THE SAME
Abstract
Multicolor thermal imaging members are described that comprise
color-forming layers that are separated by thermally-insulating
layers and can be addressed with a thermal printing head in contact
with a surface to form an image. The thermally-insulating layers
are designed to be as thin as possible consistent with at least
partially independent addressing of the color-forming layers, and
are formulated so as not to lead to instabilities either before or
after printing or to give rise to dimensional changes of a thermal
imaging member when it is subjected to changes in temperature or
humidity. Coating compositions for manufacturing such
thermally-insulating layers are also provided.
Inventors: |
Day; John C.; (Andover,
MA) ; Hardin; John; (Hopkinton, MA) ; Hardin;
Yulin; (Hopkinton, MA) ; Hasan; Fariza B.;
(Waltham, MA) ; Telfer; Stephen; (Arlington,
MA) ; Vetterling; William T.; (Lexington,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ZINK IMAGING, INC. |
Bedford |
MA |
US |
|
|
Family ID: |
43544603 |
Appl. No.: |
13/945688 |
Filed: |
July 18, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13326584 |
Dec 15, 2011 |
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13945688 |
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12462421 |
Aug 3, 2009 |
8377844 |
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13326584 |
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11397251 |
Apr 3, 2006 |
7635660 |
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12462421 |
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10806749 |
Mar 23, 2004 |
7166558 |
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11397251 |
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10151432 |
May 20, 2002 |
6801233 |
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10806749 |
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60294486 |
May 30, 2001 |
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60364198 |
Mar 13, 2002 |
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Current U.S.
Class: |
524/101 ;
524/158; 524/350; 524/446; 524/503 |
Current CPC
Class: |
B41M 2205/38 20130101;
B41M 2205/04 20130101; C08L 21/02 20130101; B41M 5/44 20130101;
B41M 2205/42 20130101; B41M 5/423 20130101; B41M 5/34 20130101;
B41M 2205/28 20130101 |
Class at
Publication: |
524/101 ;
524/503; 524/350; 524/158; 524/446 |
International
Class: |
C08L 21/02 20060101
C08L021/02 |
Claims
1-13. (canceled)
14. An apparatus comprising a coating composition wherein the
coating composition comprises at least 20% by weight of water-borne
latex polymeric material, less than 20% by weight of a
hydrophobically-modified poly(vinyl alcohol), and percentages by
weight are relative to the total weight of the coating
composition.
15. The apparatus of claim 14, wherein the coating composition
comprises at least 30% by weight of water-borne latex polymeric
material and less than 10% by weight of a hydrophobically-modified
poly(vinyl alcohol).
16. The apparatus of claim 14, wherein the coating composition
further comprises an organic material that has a glass transition
temperature of at least 80.degree. C. in an amount that is at least
10% of the weight of the polymeric latex material, and wherein the
organic material comprises phenolic materials having a molecular
weight below 2,000.
17. The apparatus of claim 16, wherein the organic material is
chosen from the group consisting of
1,3,5-tris(2,6-dimethyl-3-hydroxy-4-tert-butylbenzyl)isocyanurate
and 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane.
18. The apparatus of claim 14, wherein the apparatus comprises a
thermal imaging member.
19. The apparatus of claim 14, wherein the viscosity of the coating
composition measured at 1000 s.sup.-1 is more than 50 mPas, the
flow behavior index n is within the range of 0.8 to 1, and the
water-borne latex polymeric material has a Tg in the range of about
15-35.degree. C.
20. The apparatus of claim 16, wherein the coating composition
further comprises an additive selected from the group consisting of
an anionic surfactant, an ethoxylated non-ionic fluorinated
surfactant, dispersed inorganic particles, a crosslinking agent,
and combinations thereof.
21. The apparatus of claim 20, wherein the anionic surfactant is
triisopropyl naphthalene sulfonate.
22. The apparatus of claim 20, wherein the crosslinking agent is
selected from the group consisting of an epoxide, aziridine,
isocyanate, anhydride, aldehyde and combinations thereof.
23. The apparatus of claim 20, wherein the dispersed inorganic
particles comprise clay materials.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of copending U.S.
patent application Ser. No. 11/397,251, filed on Jul. 23, 2002,
entitled "Imaging System", which is a continuation of U.S. patent
application Ser. No. 10/806,749, filed on Mar. 23, 2004 issued as
U.S. Pat. No. 7,166,558 which is a divisional of U.S. patent
application Ser. No. 10/157,432, filed on May 20, 2002, issued as
U.S. Pat. No. 6,801,233, which claims the benefit of prior U.S.
provisional patent application Ser. Nos. 60/294,486, filed on May
30, 2001, and 60/364,198, filed on Mar. 13, 2002 the disclosures
all of which are hereby incorporated by reference herein in their
entirety.
[0002] This application is related to the following commonly
assigned United States patent applications and patents, the
disclosures of all of which are hereby incorporated by reference
herein in their entirety:
[0003] U.S. Pat. No. 7,008,759 B2 which describes and claims
color-forming compositions for use in the present invention;
[0004] U.S. Pat. No. 7,176,161 B2 which describes and claims
color-forming compositions for use in the present invention;
[0005] U.S. Pat. No. 7,282,317 B2 which describes and claims
color-forming compositions for use in the present invention;
[0006] U.S. patent application Ser. No. 11/400,734, filed Apr. 6,
2006, which describes and claims an imaging method for use in the
present invention;
[0007] U.S. Pat. No. 7,408,563, which describes and claims an
imaging method for use in the present invention;
[0008] U.S. patent application Ser. No. 12/022,955, filed Jan. 30,
2008, entitled "Printhead pulsing techniques for multicolor
printers";
[0009] U.S. patent application Ser. No. 12/022,969, filed Jan. 30,
2008, entitled "Thermal Imaging Members and Methods";
[0010] U.S. patent application Ser. No. 12/343,234, filed Dec. 23,
2008, entitled "Novel Color-forming Compounds and Use Thereof in
Imaging Members and Methods"; and
[0011] International patent application serial no.
PCT/US2009/______, filed on even date herewith, entitled "Optical
Disc with Thermally-Printable Surface and Compression-Resistant
Layer".
FIELD OF THE INVENTION
[0012] The present invention relates generally to thermal imaging
and, more particularly, to thermally-insulating layers for
controlling the rate of diffusion of heat within a multicolor
thermal imaging member, and to imaging members comprising such
thermally-insulating layers.
BACKGROUND OF THE INVENTION
[0013] Direct thermal imaging is a technique in which a substrate
bearing at least one color-forming layer, which is typically
initially colorless, is heated by contact with a thermal printing
head to form an image. In direct thermal imaging there is no need
for ink, toner, or thermal transfer ribbon. Rather, the chemistry
required to form an image is present in the imaging member itself.
Direct thermal imaging is commonly used to make black-and-white
images, and is often employed for the printing of, for example,
labels and purchase receipts. There have been described in the
prior art numerous attempts to achieve multicolor direct thermal
printing. A discussion of various color direct thermal imaging
methods is provided in U.S. Pat. No. 6,801,233.
[0014] A preferred direct thermal imaging member described in the
above-mentioned patent comprises three color-forming layers, each
affording one of the subtractive primary colors, and is designed to
be printed with a single thermal printing head. The topmost
color-forming layer develops color in a relatively short period of
time when the surface of the imaging member is heated to a
relatively high temperature; the intermediate color-forming layer
develops color in an intermediate length of time when the surface
of the imaging member is heated to an intermediate temperature; and
the lowest color-forming layer develops color in a relatively long
period of time when the surface of the imaging member is heated to
a relatively low temperature. Separating the color-forming layers
are thermally-insulating layers whose thickness, thermal
conductivity and heat capacity are selected so that temperatures
reached within the color-forming layers may be controlled to
provide the desired color by appropriate choices of heating
conditions of the surface of the imaging member.
[0015] The composition of the thermally-insulating layers is
ideally chosen so as neither to compromise the chemistry
responsible for formation of color in the color-forming layers nor
to degrade the stability of the final image.
[0016] Each color-forming layer typically comprises a dye precursor
that is colorless in the crystalline form but colored in an
amorphous form. Materials such as thermal solvents or developers
may be incorporated into the color-forming layer to adjust the
temperature at which color is formed or the degree of coloration
that is achieved.
[0017] During heating of the thermal imaging member as it is
printed to form an image, or during prolonged storage of the
imaging member before or after an image is formed, it is possible
that components initially incorporated within a color-forming layer
may migrate from that layer into adjoining layers. Such migration
of components may produce problems such as unwanted coloration in
unprinted regions or changes in the activation temperature or
degree of coloration of a color-forming layer, as is discussed in
more detail below. It is also possible that materials in adjoining
layers may migrate into a color-forming layer and degrade its
performance.
[0018] In the prior state of the art it has been necessary to
provide barrier layers to impede the migration of components from
the color-forming layers into the thermally-insulating layers or
from the thermally-insulating layers into the color-forming layers.
Such additional barrier layers introduce complexity into the
process for manufacturing the thermal imaging member, and may also
contribute undesirable physical properties to the final article, as
discussed in detail below. The need for additional barrier layers
would be obviated by a thermally-insulating layer with improved
properties, such that migration of components between the
thermally-insulating layer and adjacent layers would be impeded or
such that, if such migration were to occur, no objectionable
consequence would result.
SUMMARY OF THE INVENTION
[0019] It is therefore an object of this invention to provide a
multicolor thermal imaging member comprising at least two
color-forming layers, separated by a thermally-insulating layer,
that can be addressed with a thermal printing head to form an
image.
[0020] Another object of the invention is to provide such a
multicolor thermal imaging system wherein each color can be printed
alone or in selectable proportion with the other color(s).
[0021] In one embodiment, a thermal imaging member includes (A) a
substrate having first and second opposed surfaces; and
(B) first and second color-forming layers carried by the first
surface of the substrate. The first color-forming layer is closer
to the first surface of the substrate than the second color-forming
layer. The thermal imaging member also includes (C) a
thermally-insulating layer between the first and second
color-forming layers. The thermally-insulating layer includes at
least 50% by weight of a polymeric latex material and at least 5%
by weight of an organic material with a glass transition
temperature of at least 80.degree. C.
[0022] In another embodiment, a coating composition for the
manufacture of a thermally-insulating layer includes at least 20%
by weight of water-borne latex polymeric material and less than 20%
by weight of a hydrophobically-modified poly(vinyl alcohol). The
viscosity of the coating composition measured at 1000 s.sup.-1 is
more than 50 mPas and the flow behavior index n is in the range
0.8-1.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a partially schematic, side sectional view of an
embodiment of a multicolor thermal imaging member; and
[0024] FIG. 2 is a graphical illustration showing the relative
times and temperatures of heating required to address the separate
colors of an embodiment of a multicolor thermal imaging member;
and
[0025] FIG. 3 is a partially schematic, side sectional view of an
embodiment of a multicolor thermal imaging member;
[0026] FIG. 4 is a partially schematic, side sectional view of an
embodiment of a laminar structure as it is distorted by dimensional
changes of its constituent layers; and
[0027] FIG. 5 is a partially schematic, side sectional view of a
preferred embodiment of a multicolor thermal imaging member.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The structure and method of printing of thermal imaging
members of the present invention will now be discussed in
sufficient detail that the role of the thermally-insulating layers
may be understood and the requirements for their properties fully
appreciated.
[0029] Referring now to FIG. 1, there is seen a thermal imaging
member 100 that includes a substrate 102, that can be transparent,
absorptive, or reflective, and three color-forming layers 104, 108,
and 112, that when heated produce cyan, magenta and yellow
coloration, respectively; thermally-insulating layers 106 and 110;
and an overcoat layer 114 that protects the surface of the imaging
member and provides lubrication during the printing process.
[0030] Each color-forming layer can change color, e.g., from
initially colorless to colored, where it is heated to a particular
temperature referred to herein as its activating temperature.
[0031] Any order of the colors of the color-forming layers can be
chosen. One preferred color order is as described above. Another
preferred order is one in which the three color-forming layers 104,
108, and 112 provide yellow, magenta and cyan, respectively.
[0032] All the layers disposed on the substrate 102 are
substantially transparent before color formation. When the
substrate 102 is reflective (e.g., white), the colored image formed
on imaging member 100 is viewed through the overcoat 114 against
the reflecting background provided by the substrate 102. The
transparency of the layers disposed on the substrate ensures that
combinations of the colors printed in each of the color-forming
layers may be viewed.
[0033] In the preferred embodiments of the invention where the
thermal imaging member includes at least three color-forming
layers, all the color-forming layers may be arranged on the same
side of a substrate, or two or more of the color-forming layers may
be arranged on one side of a substrate with one or more
color-forming layers being arranged on the opposite side of the
substrate.
[0034] The color-forming layers are addressed at least partially
independently by variation of two adjustable parameters, namely,
the temperature at the surface of the thermal imaging member and
the time or duration of heating at that temperature. These
parameters can be controlled by adjusting the magnitude and
duration of the supply of electrical power to the resistive heating
elements of a thermal printing head that is in thermal contact with
the imaging member 100, as discussed in detail in U.S. patent
application Ser. No. 12/022,955. In this way, each color of the
multicolor imaging member can be printed alone or in selectable
proportion with the other colors.
[0035] Depending upon the printing time, available printing power,
and other factors, various degrees of independence in the
addressing of the color-forming layers can be achieved. The term
"independently" is used to refer to instances in which the printing
of one color-forming layer typically results in a very small, but
not generally visible change of optical density (e.g.,
density<0.05) in the other color-forming layer(s). In the same
manner, the term "substantially independent" color printing is used
to refer to instances in which inadvertent or unintentional change
in coloration of another color-forming layer or layers results in a
visible density change which is at a level typical of interimage
coloration in multicolor photography (e.g., density<0.2). The
term "partially independent" addressing of the color-forming layers
is used to refer to instances in which the printing of maximum
density in the layer being addressed results in a coloration change
of another color-forming layer or layers in a density amount higher
than 0.2 but not higher than 1.0. The phrase "at least partially
independently" is inclusive of all of the degrees of independence
described above.
[0036] The color-forming layers of the thermal imaging member
undergo a change in color to provide the desired image in the
imaging member. The change in color may be from colorless to
colored, from colored to colorless, or from one color to another.
The term "color-forming layer" as used throughout the application,
including in the claims, includes all such embodiments. In the case
where the change in color is from colorless to colored, an image
having different levels of optical density (i.e., different "gray
levels") of that color may be obtained by varying the amount of
color in each pixel of the image from a minimum density, Dmin,
which is substantially colorless, to a maximum density, Dmax, in
which the maximum amount of color is formed. In the case where the
change in color is from colored to colorless, different levels are
obtained by reducing the amount of color in a given pixel from Dmax
to Dmin, where ideally Dmin is substantially colorless.
[0037] According to a preferred embodiment of the invention, each
of the color-forming layers 104, 108 and 112 is independently
addressed by application of heat with a thermal printing head in
contact with a single surface, such as the topmost layer of the
member, optional overcoat layer 114 in the member illustrated in
FIG. 1. The activating temperature (Ta3) of the third color-forming
layer 112 (as counted from the substrate 102, i.e., the
color-forming layer closest to the surface of the thermal imaging
member) is greater than the activating temperature (Ta2) of the
second color-forming layer 108, which in turn is greater than the
activating temperature (Ta1) of the first color-forming layer 104.
Delays in heating and subsequent cooling of color-forming layers at
greater distances from the thermal printing head are provided by
the time required for heat to diffuse through the
thermally-insulating layers 106 and 110. The lower temperature of
layers below the thermally-insulating layers permits the
color-forming layers closer to the thermal printing head to be
heated to above their activating temperatures without activating
the color-forming layer (or layers) further from the thermal
printing head, even though the activating temperatures of the
color-forming layers closer to the thermal printing head can be
substantially higher than the activating temperatures of the
color-forming layers that are further away from the thermal
printing head. Thus, when addressing the uppermost color-forming
layer 112 the thermal printing head is heated to a relatively high
temperature, but for a short time, such that insufficient heat is
transferred to the other color-forming layers of the imaging member
to provide image information to either of color-forming layers 108
and 104. The combination of thickness and thermal diffusivity of
thermally-insulating layers 106 and 110 is chosen so as to satisfy
this requirement, as discussed in more detail below.
[0038] The heating of the lower color-forming layers, i.e., those
closer to the substrate 102 (in this case color-forming layers 108
and 104), is accomplished by maintaining the thermal printing head
at temperatures such that the upper color-forming layer(s) remain
below their activating temperatures for sufficient periods of time
to allow heat to diffuse through them to reach the lower
color-forming layer(s). In this way, coloration of the upper
color-forming layer(s) is avoided when the lower color-forming
layer(s) are being imaged.
[0039] Thermal printing heads used in the method of the present
invention typically include a substantially linear array of
resistors that extends across the entire width of the image to be
printed.
[0040] The imaging member is typically imaged while being
transported in a direction perpendicular to the line of resistors
on the printing head while pulses of heat are provided by supplying
electrical current to these resistors. The time period during which
heat can be applied to thermal imaging member 10 by a thermal
printing head is typically in the range of about 0.001 to about 100
milliseconds per line of the image. The lower limit may be defined
by the constraints of the electronic circuitry or by the thermal
response time of the system comprising the thermal imaging member
and thermal printing head, while the upper limit is set by the need
to print an image in a reasonable length of time. The spacing of
the dots that make up the image is generally in the range of
100-600 per inch in directions both parallel and transverse to the
direction of motion, and is not necessarily the same in each of
these directions.
[0041] The heating of the color-forming layers according to the
invention may be accomplished by a single printing head in a single
pass, or by more than one pass of a single thermal printing head,
or by a single pass of each of more than one thermal printing head,
as is described in detail in U.S. Pat. No. 7,408,563.
[0042] Although the heating of imaging member 100 is preferably
carried out using a thermal printing head, any method providing
controlled heating of the thermal imaging member may be used in the
practice of the present invention. For example, a modulated source
of light (such as a laser) may be used. In this case, as is well
known in the art, an absorber for light of a wavelength emitted by
the laser must be provided in the thermal imaging member or in
contact with the surface of the imaging member.
[0043] When a thermal printing head (or other contact heating
element) is used to heat the thermal imaging member 100, heat
diffuses into the bulk of the thermal imaging member from the layer
in contact with the thermal printing head (typically, overcoat
layer 114). When a source of light is used for heating, the layer
or layers containing an absorber for the light will be heated as
light is converted to heat in these layers, and heat will diffuse
from these layers throughout the thermal imaging member. It is not
necessary that the light-absorbing layers be at the surface of the
imaging member, provided that the layers of the thermal imaging
member separating the source of light from the absorbing layers are
transparent to light of the wavelength to be absorbed. In the
discussion below it is assumed that the layer that is directly
heated is the overcoat layer 114, and that heat diffuses from this
layer into the thermal imaging member, but similar arguments apply
whichever layer or layers of the thermal imaging member 10 is (or
are) heated.
[0044] FIG. 2 is a graphical illustration showing the thermal
printing head temperatures and times of heating required to address
color-forming layers 104, 108 and 112, assuming that these layers
are all initially at ambient temperature. The axes of the graph in
FIG. 2 show the logarithm of the heating time and the reciprocal of
the absolute temperature at the surface of the imaging member 100
that is in contact with the thermal printing head. Region 200
(relatively high printing head temperature and relatively short
heating time) provides imaging of color-forming layer 112, region
202 (intermediate printing head temperature and intermediate
heating time) provides imaging of color-forming layer 108 and
region 204 (relatively low printing head temperature and relatively
long heating time) provides imaging of color-forming layer 104. The
time required for imaging color-forming layer 104 is substantially
longer than the time required for imaging color-forming layer
112.
[0045] The activating temperatures selected for the color-forming
layers are generally in the range of about 90.degree. C. to about
300.degree. C. The activating temperature (Ta1) of the first
color-forming layer 104 is preferably as low as possible consistent
with thermal stability of the imaging member during shipment and
storage and preferably is about 90.degree. C. or more. The
activating temperature (Ta3) of the third color-forming layer 112
is preferably as low as possible consistent with allowing the
activation of the second and third color-forming layers 108 and 104
by heating through this layer without activating it according to
the method of the invention, and preferably is about 200.degree. C.
or more. The activating temperature (Ta2) of the second
color-forming layer 108 is between Ta1 and Ta3 and is preferably
between about 140.degree. C. and about 180.degree. C.
[0046] In one embodiment of the present invention, the
color-forming layers comprise a material that is colorless in the
crystalline form and colored in an amorphous form (hereinafter
referred to as a "crystalline color-forming material") as described
in detail in U.S. Pat. No. 7,176,161. The third color-forming layer
112 preferably comprises no other fusible material except the
crystalline color-forming material, since it is important that in
this layer the activation temperature be as independent of the
heating time as possible, as described in the abovementioned U.S.
Pat. No. 7,176,161.
[0047] One or more thermal solvents, which are crystalline, fusible
materials, are incorporated into the first and second color-forming
layers in certain preferred embodiments of the thermal imaging
member. The crystalline thermal solvent(s), upon being heated, melt
and thereafter dissolve or liquefy the crystalline color-forming
material, thereby converting it to an amorphous form and providing
a color change (i.e., an image). Thermal solvents may be
advantageously used when it is required for a color-forming layer
to have an activation temperature that is lower than the melting
point of the crystalline color-forming material itself. The melting
point of the thermal solvent, rather than that of the crystalline
color-forming material, may in such a case establish the activation
temperature of the color-forming layer.
[0048] It will be clear to one of ordinary skill in the art that
the activation temperature of a color-forming layer that comprises
a mixture of crystalline materials may be different from the
melting points of any of the individual components. A eutectic
mixture of two crystalline components, for example, melts at a
lower temperature than either of the components in isolation.
Conversely, if the rate of solubilization of the crystalline
color-forming material in the molten thermal solvent is slow, the
activation temperature of the mixture may be higher than the
melting point of the thermal solvent. Recall that the activation
temperature of a mixture of a crystalline color-forming material
and a thermal solvent is the temperature at which the color of the
mixture changes, i.e., the temperature at which a sufficient amount
of the crystalline color-forming material dissolves in the molten
thermal solvent to provide a visible color change. It will be clear
from the above discussion that the activation temperature of a
mixture of a crystalline color-forming material and a thermal
solvent or solvents may be dependent upon the rate of heating. In
the design of thermal imaging members of the present invention,
therefore, determination of the actual activation temperature of a
composition is preferred to be carried out experimentally.
[0049] Any suitable thermal solvent may be incorporated into the
color-forming layers of the thermal imaging members of the
invention. Suitable thermal solvents include, for example, aromatic
and aliphatic ethers, diethers and polyethers, alkanols containing
at least about 12 carbon atoms, alkanediols containing at least
about 12 carbon atoms, monocarboxylic acids containing at least
about 12 carbon atoms, esters and amides of such acids, aryl
amides, especially benzanilides, aryl sulfonamides and
hydroxyalkyl-substituted arenes.
[0050] Specific preferred thermal solvents include:
1,2-diphenoxyethane, 1,2-bis(4-methylphenoxy)ethane,
tetradecan-1-ol, hexadecan-1-ol, octadecan-1-ol, dodecane-1,2-diol,
hexadecane-1,16-diol, myristic acid, palmitic acid, stearic acid,
methyl docosanoate, 1,4-bis(hydroxymethyl)benzene, diaryl sulfones
such as diphenylsulfone, 4,4'-dimethyldiphenylsulfone, phenyl
p-tolylsulfone and 4,4'-dichlorodiphenylsulfone, and
p-toluenesulfonamide.
[0051] Particularly preferred thermal solvents are ethers such as
1,2-bis(2,4-dimethylphenoxy)ethane,
1,4-bis(4-methylphenoxymethyl)benzene,
bis(4-phenoxyphenoxymethyl)benzene and
1,4-bis(benzyloxy)benzene.
[0052] It is possible that the dissolution of the crystalline
color-forming material by a thermal solvent may lead to an
amorphous form in which the amount of color that is formed is
different from the amount that would be present in an amorphous
form resulting from melting the crystalline color-forming material
alone (i.e., without interaction with the thermal solvent).
Typically, the crystalline color-forming materials of the present
invention are tautomeric compounds in which at least one tautomer
is colorless and at least another tautomer is colored. The
crystalline form comprises substantially the colorless tautomer,
whereas the colored form comprises both tautomers in proportions
that depend upon the structure of the particular color-forming
material and the environment in which it is located. The proportion
of the colored tautomer in the amorphous material may be enhanced
by use of hydrogen-bonding or acidic adjuvants. It is possible that
such materials may actually protonate the color-forming material to
produce a new, colored compound. Materials that increase the
proportion of the color-forming material that is in a colored form
are hereinafter referred to as "developers". It is possible that
the same compound may serve the function of thermal solvent and
developer. Preferred developers include phenols such as
4,4'-butylidenebis[2-(1,1-dimethylethyl)-5-methyl-phenol],
2,2'-methylenebis(6-tert-butyl-4-methylphenol),
2,2'-methylenebis(6-tert-butyl-4-ethylphenol),
2,2'-ethylidenebis(4,6-di-tert-butylphenol),
bis[2-hydroxy-5-methyl-3-(1-methylcyclohexyl)phenyl]methane,
1,3,5-tris(2,6-dimethyl-3-hydroxy-4-tert-butylbenzyl)isocyanurate,
2,6-bis[[3-(1,1-dimethylethyl)-2-hydroxy-5-methylphenyl]methyl]-4-methylp-
henol, 2,2'-butylidenebis[6-(1,1-dimethylethyl)-4-methylphenol,
2,2'-(3,5,5-trimethylhexylidene)bis[4,6-dimethyl-phenol],
2,2'-methylenebis[4,6-bis(1,1-dimethylethyl)-phenol,
2,2'-(2-methylpropylidene)bis[4,6-dimethyl-phenol],
1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane,
tris(3,5-di-t-butyl-4-hydroxybenzyl)isocyanurate,
2,2'-thiobis(4-tert-octylphenol), and
3-tert-butyl-4-hydroxy-5-methylphenyl sulfide.
[0053] In order for the image formed by the amorphous color-former
to be stable against recrystallization back to the crystalline
form, preferably the glass transition temperature (Tg) of the
amorphous mixture of the color-former and any thermal solvent
and/or developer should be higher than any temperature that the
final image must survive. Typically, it is preferred that the Tg of
the amorphous, colored material be at least about 50.degree. C.,
and ideally above about 60.degree. C. In order to ensure that the
Tg is sufficiently high for a stable image to be formed, additional
materials having a high Tg may be added to the color-forming
composition. Such materials, hereinafter referred to as
"stabilizers", when dissolved in the amorphous mixture of
color-former, optional thermal solvent, and optional developer,
serve to increase the thermal stability of the image.
[0054] Preferred stabilizers have a Tg that is at least about
60.degree. C., and preferably above about 80.degree. C. Examples of
such stabilizers are the aforementioned
1,3,5-tris(2,6-dimethyl-3-hydroxy-4-tert-butylbenzyl) isocyanurate
(Tg 123.degree. C.) and
1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane (Tg
101.degree. C.). The stabilizer molecule may also serve as a
thermal solvent or as a developer.
[0055] For example, the color-forming material may itself have a
melting temperature above the desired temperature for imaging, and
a Tg (in the amorphous form) of about 60.degree. C. In order to
produce a color-forming composition melting at the desired
temperature, it may be combined with a thermal solvent that melts
at the desired temperature for imaging. The combination of thermal
solvent and color-forming material may, however, have a Tg that is
substantially lower than 60.degree. C., rendering the (amorphous)
image unstable. In this case, a stabilizer such as
1,3,5-tris(2,6-dimethyl-3-hydroxy-4-tert-butylbenzyl) isocyanurate
may be added, to raise the Tg of the amorphous material. In
addition, there may be provided a developer, for example, a
phenolic compound such as
2,2'-ethylidenebis(4,6-di-tert-butylphenol), in order to increase
the proportion of the color-forming material that is in the colored
form in the amorphous phase.
[0056] Preferably the color-forming compound of the present
invention, the (optional) thermal solvent, the (optional) developer
and the (optional) stabilizer are each predominantly in their
crystalline forms prior to imaging. By "predominantly" is meant at
least about 50% and preferably more than that. During imaging, at
least one of these materials melts and an amorphous mixture of the
materials is formed. As noted above, the amorphous mixture is
colored, whereas the crystalline starting materials are
colorless.
[0057] The temperature range over which melting (and therefore
coloration) occurs should be as narrow as possible, especially in
the case that the crystalline color-forming compounds are
incorporated into a thermal imaging member capable of forming
full-color images. It is preferred that the temperature range of
melting (as measured by differential scanning calorimetry) of a
color-forming composition comprising a crystalline color-forming
compound be less than 15.degree. C. as measured at the half height
of the peak, and preferably less than 10.degree. C. measured at
half height.
[0058] It is possible that one of the components in the amorphous,
colored mixture may recrystallize after the image has been formed.
It is desirable that such recrystallization not change the color of
the image. In the case that a color-former, thermal solvent,
developer and stabilizer are used, the thermal solvent may
typically recrystallize without greatly affecting the color of the
image.
[0059] Color-forming layers may comprise any of the image-forming
materials described above, or any other thermally-activated
colorants, and are typically from about 0.5 to about 4.0 .mu.m in
thickness. Color-forming layers may also comprise more than one
layer (hereinafter referred to as "sub-layers"), which may not have
identical composition. For example, a crystalline color-forming
material may be incorporated into one sub-layer while a thermal
solvent may be located in another. Other arrangements, including
sub-layers for control of the rates of chemical diffusion, will
occur to those of ordinary skill in the art. In such cases each of
the constituent sub-layers is typically from about 0.1 to about 3.0
.mu.m in thickness.
[0060] Color-forming layers may comprise dispersions of solid
materials, encapsulated liquid, amorphous or solid materials or
solutions of active materials in polymeric binders, or any
combinations of the above.
[0061] Preferred binder materials for use in color-forming layers
include water-soluble polymers such as poly(vinyl alcohol),
ethylene vinyl alcohol polymers, polyacrylamide, gelatin,
cellulosic materials, and salts of carboxylated polymers (for
example, ammonium salts of polymers containing acrylic acid units).
One disadvantage of the use of such water-soluble polymers,
however, is that they may be prone to hydration (and therefore
swelling) in humid environments, and conversely dehydration in dry
environments. Changes in hydration typically change the physical
and chemical properties of the binder material. For example,
dehydrated poly(vinyl alcohol) may have a Tg of about 75.degree.
C., whereas the hydrated material may have a Tg of 20.degree. C. or
less. Such gross changes in properties may affect the mixing of
components during imaging and therefore the activation temperature
of the color-forming layer.
[0062] In addition, the dimensional changes that occur during
hydration and dehydration, coupled with the change in Tg, may lead
to physical distortions of the thermal imaging member, such as
curling, as discussed in more detail below.
[0063] For this reason, it is preferred that the binder for the
color-forming layers comprise a material that is water-dispersed
(to allow it to be coated by conventional coating methods) but
that, after drying, is not susceptible to rehydration. Many
water-borne latex materials exhibit this desired property.
Preferred latex binders for use in the present invention have a Tg
as high as possible, consistent with a coalescence temperature that
can be attained during drying of the coating without activation of
the color-forming chemistry. A preferred Tg is at least about
25.degree. C. In addition, it is preferred that the latex binder
have a pH between about 6 and about 8. Materials falling outside
this range may affect the tautomeric equilibrium of the
color-forming materials. Finally, it is preferred that the latex
binder be free of small molecules such as surfactants or
cosolvents, since these materials also may affect the color-forming
chemistry. A preferred water-borne latex binder for use in the
present invention is styrene-butadiene rubber CP-655, available
from Dow Chemical Co., Midland, Mich.
[0064] For certain applications of the present invention it is
required that the thermal imaging member remain flat when subjected
to changes in temperature and/or humidity. For example, the
perceived quality of a photographic image is markedly reduced if
the image is curled, particularly if the imaging member curls
towards the printed side. As is well known in the art, curl that is
induced by changes in temperature or humidity may be corrected by
the application of a balancing layer to the opposite side of the
substrate. Such a balancing layer is designed to exhibit similar
dimensional changes to the layers applied to the image side of the
substrate under all environmental conditions to which the imaging
member is likely to be subjected. In certain labeling applications,
however, such a solution may not provide a practicable solution,
especially when the curling is caused by changes in humidity.
[0065] Referring now to FIG. 3, there is seen a thermal imaging
member of the present invention that is intended to produce a
printed label. Substrate 302 is coated on one side with all the
layers required to form an image (represented by layers 304). Layer
304 may, for example, comprise all the layers 104-114 shown in FIG.
1. On the opposite side of the substrate 302 are an optional
curl-balancing layer 306, an adhesive layer 308 and a removable
liner 310. For the curl-balancing layer 306 to be effective in
correcting curl induced by dimensional changes in layers 304 that
are caused by changes in environmental humidity, adhesive layer 308
and liner 310 must be permeable to water vapor. In practice, it may
be difficult to achieve sufficient water permeability of these
layers such that equilibration of layer 306 to changing humidity is
not substantially slower than equilibration of layers 304. In
addition, in normal use the label liner 310 is removed and adhesive
layer 308 is used to stick the label to a surface. If the surface
and the substrate 302 are impermeable to water there is no way that
curl-balancing layer 306 can equilibrate to the environment. It is
then possible that the curl forces caused by dimensional change of
layers 304 may overwhelm the strength of the adhesive bond between
the label and the surface, causing the label to peel off the
surface.
[0066] In one preferred application, disclosed in copending
International patent application serial no. PCT/US2009/______,
entitled "Optical Disc with Thermally-Printable Surface and
Compression-Resistant Layer", filed on even date herewith, the
label is applied to the surface of an optical disc. In one
embodiment the label is adhered to the disc prior to printing. In
such a case a curl-balancing layer such as layer 308 cannot be
effective, since it is located on the same side of the neutral axis
for bending of the labeled disc as the imaging layers 304. In such
a case it becomes necessary to design the imaging layers 304 such
that they do not exert a substantial curl force when subjected to
changing temperature and/or humidity.
[0067] As disclosed in copending International patent application
serial no. PCT/2009/______, filed on even date herewith, entitled
"Optical Disc with Thermally--Pintable Surface and
Compression-Resistant Layer" a label for an optical disc may
comprise a thermal imaging member similar to member 100 shown in
FIG. 1 in which substrate 102 is polycarbonate. When the thickness
of such a polycarbonate substrate 102 is 50 microns, it is
preferred that the curvature induced by changes in temperature
and/or humidity in thermal imaging member 100 be less than 120
m.sup.-1. If this criterion is not met, it is likely that the label
will exert sufficient curl force to warp an optical disc to which
it is affixed, so that the data stored in the disc will not be
reliably written to or read back by a laser addressing the face of
the disc opposite to that bearing the label. This problem is
discussed in detail in the aforementioned copending International
patent application serial no. PCT/2009/______ filed on even date
herewith. As is shown in that patent application, the maximum
tolerable coverage of a layer of fully-hydrolyzed poly(vinyl
alcohol) coated onto a polycarbonate base of 62.5 microns thickness
is 3 g/m.sup.2 if the disc warp criterion is to be met.
[0068] A discussion of the mechanism of curl induced by changes in
humidity will now be given in more detail. As is known in the art,
during drying from a fully hydrated state a layer comprising a
water-swellable polymer such as poly(vinyl alcohol) will exhibit
shrinkage due to loss of water. Such shrinkage may not cause
curling of the substrate onto which the layer is coated so long as
sufficient water is retained that viscous flow within the
water-swellable polymer layer is possible. As further water is
lost, however, elastic behavior (i.e., energy storage) may start to
dominate viscous flow and a curl force, caused by shrinkage due to
the loss of water, may be experienced by the substrate, provided
that the adhesion between the polymer layer and the substrate is
sufficiently strong that slippage does not occur at this interface.
If slippage does occur, the polymer layer will be observed to have
shrunk relative to the substrate. In the case that the polymer is
poly(vinyl alcohol), measurements of weakly-adhered coatings have
indicated that the amount of shrinkage is about 2.5% in each
direction parallel to the surface of the substrate when a fully
hydrated coating is placed in an (dry) atmosphere of 5-10% relative
humidity (RH).
[0069] It is possible to estimate the amount of curl that a laminar
structure will exhibit when its constituent layers change in
volume, provided that the thickness and Young's modulus of each
layer is known. Referring now to FIG. 4 (a) there is seen a laminar
structure composed of four layers, 402-408, with thicknesses
T.sub.1, T.sub.2 . . . T.sub.4 and Young's modulus values of
.eta..sub.1, .eta..sub.2 . . . .eta..sub.4. FIG. 4 (b) shows the
situation in which the four layers have undergone a dimensional
change, such that each layer is now of a different length. Such a
situation may pertain, for example, in a thermal imaging member
comprising four layers each of which contains a different
proportion of a water-swellable polymer. In FIG. 4 (b) the
dimensional change of each layer is shown as it would be were the
layers not adhered together. In the thermal imaging members of the
present invention, however, the adhesive bonding between layers is
strong enough that the situation shown in FIG. 4 (c) obtains, in
which the entire structure is warped into a curvature that may be
predicted from the degree of dimensional change of each layer, its
starting thickness, and its Young's modulus using equation (1):
1 r 1 .apprxeq. 12 n = 1 N m = 1 N .alpha. n .alpha. m ( L n - L m
L 0 ) ( S n - S m ) n = 1 N m = 1 N .alpha. n .alpha. m ( T m 2 + T
n 2 + 12 ( S n - S m ) 2 ) where S n .ident. j = 1 n - 1 T j + T n
2 and .alpha. n .ident. .eta. n T n L n ( 1 ) ##EQU00001##
[0070] As noted above, the amount of shrinkage typically observed
for poly(vinyl alcohol) layers upon dehydration from a fully
hydrated state is about 2.5%. The Young's modulus of the dehydrated
polymer is in the range of 1 GPa (to the nearest order of
magnitude). When a layer comprises crystalline organic materials
(having Young's modulus estimated in the range of 10 GPa) in a
poly(vinyl alcohol) binder, the shrinkage of the layer is reduced
(because the crystalline materials do not tend to swell upon
hydration) but the Young's modulus of the composite is higher than
that of the pure polymer. The present inventors have found that
these two factors approximately cancel out, and the curl force
exerted by such a layer is roughly the same as would be exerted by
a layer of the pure polymer.
[0071] Those of ordinary skill in the art will be aware that the
grade of poly(vinyl alcohol) affects the degree of curl seen upon
dehydration. In general the curl force exerted upon dehydration is
lower when the molecular weight or the degree of hydrolysis of the
polymer is lower.
[0072] The present inventors have found that when it is necessary
to introduce a water-swellable polymer such as poly(vinyl alcohol)
it is preferable to do so as a component in a layer that also
contains a material that does not swell upon hydration and that has
a relatively low Young's modulus. Note that the values of Young's
modulus given above are approximate values that are cited for the
purpose of illustration only, and should not be taken to limit the
scope of the present invention in any way.
[0073] The structure and mechanism of color formation in the
thermal imaging member having been described, the requirements of
the thermally-insulating layers 106 and 110 will now be discussed
in more detail.
[0074] As noted above, the principal purpose of the
thermally-insulating layers is to protect underlying color-forming
layers from reaching their activation temperatures when overlying
color-forming layers are exposed to heat. Preferably, the thermal
insulation is achieved with the thinnest possible
thermally-insulating layer.
[0075] There are several reasons why thin thermally-insulating
layers are preferred. Thicker thermally-insulating layers may be
more difficult to manufacture than thinner layers and may introduce
physical problems such as a propensity to curl, as discussed above.
Also, the image formed in color-forming layer 112, for example,
must be viewed through both thermally-insulating layers 106 and
110. Any scattering of light within the thermally-insulating layers
will decrease the effective optical density of the image in the
color-forming layers below the thermally-insulating layers. The
transparency requirement of the material from which the
thermally-insulating layer is made becomes more stringent,
therefore, as a thermally-insulating layer becomes thicker.
[0076] Yet another requirement of a thermally-insulating layer is
that it must have sufficient dimensional stability that the layer
is not physically disrupted (for example, "plowed" by the thermal
printing head) during printing, during which the
thermally-insulating layer may be heated to a high temperature.
Such a physical disruption could cause unwanted visible marring of
the printed image. As discussed below, thermally-insulating layers
are typically composed of relatively soft materials. Relatively
thinner thermally-insulating layers may be less prone to physical
disruption during printing than relatively thicker
thermally-insulating layers.
[0077] In order to make the thermally-insulating layer as thin as
possible, its composition preferably has the highest possible
specific heat capacity and the lowest possible thermal
conductivity, for a given density.
[0078] Typical preferred values for the specific heat capacity of
the thermally-insulating layer are 1500 J/kgK or greater (as
measured at 25.degree. C.). Typical preferred values for the
thermal conductivity of the thermally-insulating layer are 0.2 W/mK
or less (as measured at 25.degree. C.).
[0079] Using materials with these thermal properties, having
densities in the range of 1-1.5 g/cm.sup.3, thermally-insulating
layer 106 of imaging members of the present invention typically has
a coverage of 6-30 g/m.sup.2 in imaging members of the present
invention, while the coverage of thermally-insulating layer 110 is
typically at least 3 times thinner, in the range of about 1-5
g/m.sup.2.
[0080] In general, the square of the thickness of
thermally-insulating layer 110 divided by its thermal diffusivity
(a quantity that relates to the time required for heat to diffuse
through the layer) should be at least four times greater than the
square of the thickness of thermally-insulating layer 106 divided
by its thermal diffusivity.
[0081] It is preferred that the thermally-insulating layers be
deposited by aqueous coating processes in the manufacture of the
thermal imaging members of the present invention. It will be
apparent to those of ordinary skill in the coating art that the
production of layers with such a relatively high dried coverage
requires the use of concentrated aqueous coating fluids if problems
with drying of the wet coating are to be avoided. Fluid
concentrations of at least about 20% solids by weight, and
preferably 30% solids or more by weight are preferred.
[0082] Yet another desired property of the thermally-insulating
layer is waterfastness of the dried coating, especially if the
image is intended for outdoor use.
[0083] The present inventors have found that certain water-borne
latex materials best meet the combination of required properties
discussed above, namely, high heat capacity, low thermal
conductivity and waterfastness of the dried material, together with
the property of being coatable onto a substrate from an aqueous
coating fluid having a solid content in the range of 20-50% by
weight.
[0084] Generally, the use of dispersions of inorganic particles,
for example, silica, insoluble metal oxides or insoluble metal
salts, is less preferred, since such materials, although available
as concentrated aqueous dispersions, typically exhibit lower
specific heat capacity and higher thermal conductivity than
dispersions of organic amorphous polymers. It may, however, be
advantageous to incorporate dispersed inorganic particles into the
composition of the thermally-insulating layer in order, for
example, to strengthen the composition or to provide chemical or
gas barrier properties. Clay materials such as Laponite,
Montmorillonite, Bentonite and Hectorite, for example, are
particularly useful in this regard.
[0085] When mixtures of components are used to make up the
thermally-insulating layer, it is preferred that their refractive
indices be as closely matched as possible, or that their particle
size be sufficiently small that light scattering is minimized and
the greatest possible transmission of light achieved.
[0086] It is important that water-borne latex materials for use in
the thermally-insulating layers of the present invention have a
glass transition temperature, Tg, in an appropriate range.
Materials with too high a Tg may fail to form a cohesive film when
dried. On the other hand, a number of problems are encountered when
materials with too low a Tg are used. Firstly, such materials tend
to flow when heated at temperatures that are attained during
thermal printing, and this can lead to disruption of the printed
image. Secondly, the rates of diffusion of small molecules in
amorphous materials (such as the latex polymers) are much higher
above the glass transition temperature than below it. Diffusion of
small molecules from the color-forming layers into the
thermally-insulating layers may lead to significant problems with
the stability of the printed image, or of the thermal imaging
member prior to printing. A particular problem that is commonly
encountered is diffusion of the color-forming material itself into
the thermally-insulating layer. When this occurs, it is possible
that unintended coloration of the color-forming material may be
seen. This is particularly a problem in areas of a picture that are
intended to be white (i.e., Dmin, the optical density in white
regions of the image, may be higher than is desired). It is an
object of the present invention to provide a composition for a
thermally-insulating layer in which minimal increase of Dmin is
observed during storage of the thermal imaging member either before
or after printing.
[0087] Preferred water-borne latex materials for use in the
thermally-insulating layers of the present invention have a Tg in
the range of about 15-35.degree. C. The highest possible Tg
consistent with film formation and coalescence of the latex during
drying of the coating is desired. Typically, the maximum
temperature used during drying of the thermally-sensitive coatings
is about 70.degree. C. Preferred minimum film-forming temperatures
are in the range of 20-60.degree. C.
[0088] As is well known in the art, core-shell latex materials are
known in which a core of high Tg is encapsulated by a shell of
lower Tg material. Coalescence of the film is achieved by flow of
the shell material, maintaining a higher average Tg in a cohesive
film than would be achievable otherwise. Such materials may be used
in the practice of the present invention, in which case the
material that comprises the shell of the particle should meet the
Tg requirements outlined above.
[0089] It is preferred that a water-borne latex material for use in
the present invention have a pH in the range 4-8, and preferably in
the range 6.5-7.5. It is also preferred that the acid number of the
latex be moderate, i.e., in the range 18-35. As discussed above,
the color-forming materials of the present invention may exhibit
tautomerism in which at least one tautomer is colorless and at
least another is colored. As noted above, the relative proportion
of the colored and colorless tautomers depends upon the chemical
environment of the color-forming material. Any color-forming
material that migrates from the environment of the color-forming
layer (in which it is substantially in the colorless form) into the
thermally-insulating layer may experience an environment in which
it is more colored. It has been found that this may be a
significant problem if the pH or acid number of the latex material
fall outside the ranges outlined above.
[0090] It is also preferred that water-borne latex materials for
use in the present invention contain minimal amounts of small
molecules such as cosolvents or surfactants. Such materials may
migrate out of the thermally-insulating layer into a color-forming
layer and affect the imaging chemistry. If necessary, such small
molecules may be removed from the latex material by dialysis, as is
well known in the art.
[0091] Preferred water-borne latex materials for use in the present
invention include NEOCRYL A-6162, a styrene/acrylic material
available from DSM NeoResins, Waalwijk, The Netherlands, and
CP-655, a styrene/butadiene rubber latex material available from
Dow Chemical Co., Midland, Mich.
[0092] The mechanical properties of the water-borne latex materials
used in the present invention may be improved by crosslinking. As
is well known in the art, such crosslinking may be achieved by the
use of, for example, thermally-induced covalent bonding by means of
polyfunctional epoxides, aziridines, isocyanates, anhydrides, or
aldehydes, for example, or by use of reversible mechanisms such as
metal chelation. It is also possible that photochemical
crosslinking, via, for example, free radical or cationic mechanisms
may be used. Crosslinking may be achieved by introduction of a
separate reagent or by building the crosslinking functionality into
the latex material itself. Other methods for crosslinking, such as
the use of electron beams or other sources of radiation, will occur
to those of ordinary skill in the art.
[0093] The present inventors have found that even with the use of
the preferred latex materials, with or without additional
crosslinking, the thermal stability of the final image or of the
imaging member prior to imaging may still be inadequate for certain
demanding applications. The present inventors have discovered that
this situation may be remedied by the incorporation of certain
organic materials into the composition of the thermally-insulating
layers of the present invention. These materials are preferred to
have a Tg in the amorphous form above 80.degree. C. They may be
incorporated into the composition either in a crystalline or an
amorphous form.
[0094] Preferred organic materials for use in the invention are
phenolic materials having a molecular weight below 2000. These
materials may be crystalline solids with melting point below the
activation temperature of the layer overlying the
thermally-insulating layer, although this is not a requirement of
the invention.
[0095] One particularly preferred material for use in combination
with a water-borne latex material in compositions of the present
invention is 1,3,5-tris(2,6-dimethyl-3-hydroxy-4-tert-butylbenzyl)
isocyanurate, a material with a molecular weight of 700, a melting
point of 159.degree. C. and a Tg in the amorphous state of
123.degree. C. Another preferred material is
1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, having a Tg
in the amorphous state of 101.degree. C.
[0096] The organic phenolic material is ideally incorporated into
the thermally-insulating layer by preparing an aqueous dispersion
of the material in the crystalline state. This is done by reducing
the particle size of a slurry of the crystalline material in water,
in the presence of a dispersing aid, by means of a milling process
(using, for example, an attritor or a horizontal mill, as is well
known in the art). During the milling process, some of the organic
phenolic material may be converted from the crystalline to the
amorphous state, but preferably this is less than 50% of the
material.
[0097] When the organic phenolic material that is used is
1,3,5-tris(2,6-dimethyl-3-hydroxy-4-tert-butylbenzyl) isocyanurate,
preferably the dispersing aid is a styrene-maleic acid copolymer
such as SMA1000 MA, available from Sartomer Company, Inc., Exton
Pa., although other materials may also be employed (for example,
poly(vinyl alcohol) or small molecule surfactants, as is well known
in the art of making dispersions).
[0098] The mechanism by which the Dmin is controlled in the
compositions of the present invention comprising organic phenolic
materials is not fully understood. It is possible that migration of
the color-forming materials occurs, but that the chemical
environment of the thermally-insulating layer is such that
increased coloration does not occur. Alternatively, it is possible
that the diffusion rate of the color-forming material within the
thermally-insulating layer is decreased, such that less of the
material diffuses into the thermally-insulating layer in a given
amount of time. It is known that the Tg of the material from which
the thermally-insulating layer is made is increased by the addition
of the organic phenolic material.
[0099] The possible mechanisms discussed above for the efficacy of
the use of the organic phenolic materials of the present invention
are speculative and presented for illustrative purposes only, and
are not intended to limit the scope of the present invention in any
way.
[0100] As mentioned above, it is preferred that the
thermally-insulating layer of the present invention be coatable
from an aqueous composition having a solid content of at least
about 20% and preferably more than about 30%. The preferred coating
methods for manufacture of the thermal imaging members of the
present invention are curtain and slide-hopper (bead) coating, and
these techniques require that the viscosity of a coating fluid fall
within a certain range determined by the desired coating speed, wet
coverage, and geometry of the specific coating applicator, as is
well known to those skilled in the coating art.
[0101] The viscosity of a non-Newtonian fluid such as the coating
fluids used in the present invention may be approximated by the
Ostwald-de Waele power law:
.mu..sub.eff=K(S).sup.n-1 (2)
where .mu..sub.eff is the effective viscosity, S is the shear rate,
K is a parameter referred to as the flow consistency index and n is
a dimensionless quantity known as the flow behavior index that has
the value 1 for a Newtonian fluid. Typical coating fluids of the
present invention have values of n that are less than 1; i.e., they
are shear-thinning fluids. It is preferred in the practice of the
present invention that .mu..sub.eff lie within the range of 35-200
mPas at a shear rate of 1000 s.sup.-1, with n in the range 0.8-1,
when measured at the temperature of the fluid during coating (which
is typically in the range of 20-50.degree. C.). Fluids with
viscosity parameters outside these ranges may be difficult to coat
using conventional slide-hopper or curtain coating methods.
[0102] When the flow behavior index, n, has a value less than about
0.8 the shear thinning behavior of the fluid may make the
production of a coating of uniform cross-web thickness difficult.
This is because a typical coating applicator is a slot at least as
wide as the width of the coating that is fed by a cylindrical hose
through a fan-shaped channel. It is important that the flow rate of
the fluid be the same at the edges of the applicator as at the
center. When the viscosity of the fluid depends strongly upon the
shear rate, however, the fluid may exhibit "plug flow", such that
the flow rate at the center is greater than at the edges.
[0103] In order to formulate an aqueous coating fluid with a solid
content of 20-50% by weight of a water-borne latex material of the
present invention, having viscosity parameters in the ranges
discussed above, it may be necessary to incorporate a rheology
modifier in addition to the components that are required for
performance of the dried composition. Such a rheology modifier must
not, however, affect the performance of the dried composition with
respect to its thermal properties or its effect on the stability of
the thermal imaging member either prior to printing or in the final
image. Unfortunately, many commercially-available materials
designed for rheology modification do exhibit undesired effects on
the performance of the thermal imaging member. For example, many
polyether polyol or acrylate-based rheology modifiers affect the
activation temperature of the crystalline color-forming materials.
In particular, the activation temperature of the topmost
color-forming layer, layer 112 in FIG. 1, may be reduced or made
more dependent than it would otherwise be on heating rate
(presumably because of diffusion-limited dissolution of the
crystalline color-forming material in some of the rheology modifier
from the adjacent thermally-insulating layer).
[0104] The present inventors have found that the use of a
hydrophobically-modified, fully-hydrolyzed poly(vinyl alcohol)
polymer as a rheology modifier introduces minimal adverse effects
on the performance of the thermal imaging member while allowing the
preparation of a coating fluid for the thermally-insulating layer
of the present invention with viscosity parameters within the
preferred ranges described above.
[0105] It has been found by the present inventors that addition of
a poly(vinyl alcohol) polymer with hydrophobic modification allows
adjustment of .mu..sub.eff of a coating fluid containing a
water-borne latex of the present invention to higher values than
would be obtained in the absence of the polymer, but with less
reduction of the flow behavior index, n, than would be possible
with the addition of a non-hydrophobically modified poly(vinyl
alcohol) polymer or of certain inorganic rheology modifiers such as
clay-based materials.
[0106] Referring now to FIG. 5, a preferred thermal imaging member
500 according to the invention is shown in schematic form. All
layers are coated from aqueous fluids which contain small amounts
of a coating aid, Zonyl FSN, available from Dupont Co., Wilmington,
Del.
[0107] The substrate 502 may be a filled, white, oriented
polypropylene base of thickness from about 75 to about 200 microns.
Such materials are available, for example, from Yupo Corporation
America, Chesapeake, Va. 23320.
[0108] Other choices for the film base include paper substrates,
poly(ethylene terephthalate), polycarbonate, and other synthetic
substrates as are well known in the art. A particularly preferred
substrate for use in a label intended for an optical disc is a
filled, white polycarbonate with a thickness in the range 25-75
microns.
[0109] Color-forming layer 504 may be in direct contact with
substrate 502 as shown in FIG. 5, or there may be optional
intervening adhesion-promoting or oxygen barrier layers (not
shown). Layer 504 is composed of a cyan color-forming compound, Dye
X of copending U.S. patent application Ser. No. 12/022,969 (7.72%
by weight), 1,4-bis(benzyloxy)benzene (a thermal solvent having
melting point 125.degree. C., coated as an aqueous dispersion of
crystals having average particle size under 1 micron, 48.6% by
weight), a phenolic antioxidant/developer (Anox 29, having melting
point 161-164.degree. C., available from Chemtura, Middlebury,
Conn., coated as an aqueous dispersion of crystals having average
particle size under 1 micron, 7.91% by weight), Lowinox 1790 (a
second phenolic antioxidant/stabilizer, available from Chemtura,
Middlebury, Conn., coated as an aqueous dispersion of crystals
having average particle size under 1 micron, 13.28% by weight), and
a binder (a water-borne latex, CP655, available from Dow Chemical
Co., Midland, Mich., 22.31% by weight). This layer has a coverage
of 2.53 g/m.sup.2.
[0110] Overlying the cyan color-forming layer 504 is a
thermally-insulating layer 506 composed of the above-mentioned
CP655 (69.9% by weight), the above-mentioned Lowinox 1790 (coated
as an aqueous dispersion of crystals having average particle size
under 1 micron, the dispersant of which is a styrene-maleic acid
copolymer, SMA 1000MA, available from Sartomer Company, Inc.,
Exton, Pa., 14.25% by weight), a hydrophobically-modified,
fully-hydrolyzed grade of poly(vinyl alcohol) POVAL MP103,
available from Kuraray America, Inc., Houston, Tex. (13.5% by
weight), an aziridine crosslinker, CX-100, available from DSM
NeoResins, Waalwijk, The Netherlands, 1.7% by weight, and a
surfactant, Alkanol OS, available from E. I. DuPont de Nemours,
Wilmington, Del., 0.3% by weight. This layer has a coverage of 18
g/m.sup.2.
[0111] Overlying the thermally-insulating layer 506 is a magenta
color-forming layer 508, composed of a magenta color-former, Dye 23
described in copending U.S. patent application Ser. No. 12/343,234,
8.93% by weight; a phenolic ether thermal solvent,
1,4-bis[(4-methylphenoxy)methyl]benzene, (melting point 172.degree.
C., coated as an aqueous dispersion of crystals having average
particle size under 1 micron, 46.74% by weight); a phenolic
antioxidant/developer (Lowinox 44B25, having melting point
210-211.degree. C., available from Chemtura, Middlebury, Conn.,
coated as an aqueous dispersion of crystals having average particle
size under 1 micron, 18.26% by weight), a second phenolic
antioxidant/stabilizer (Lowinox 1790, available from Chemtura,
Middlebury, Conn., coated as an aqueous dispersion of crystals
having average particle size under 1 micron, 5.13% by weight); and
a binder (poly(vinyl alcohol), Celvol 540, available from Celanese,
Dallas, Tex., 20.39% by weight). This layer has a coverage of 2.56
g/m.sup.2.
[0112] Overlying the magenta color-forming layer 508 is a second
thermally-insulating layer 510, having the same composition as
thermally-insulating layer 506. This layer has a coverage of 4
g/m.sup.2.
[0113] Overlying the second thermally-insulating layer 510 is a
yellow color-forming layer 512 composed of a yellow color-former
(Dye XI described in U.S. Pat. No. 7,279,264, having melting point
202-203.degree. C.), 61.3% by weight, a phenolic
antioxidant/stabilizer (Lowinox 1790, available from Chemtura,
Middlebury, Conn., coated as an aqueous dispersion of crystals
having average particle size under 1 micron, 6.13% by weight), a
rheology modifier, the above-mentioned POVAL MP103, 10% by weight,
and a binder, Carboset CR717 (a latex available from Lubrizol,
Cleveland, Ohio, 22.26% by weight). This layer has a coverage of
2.05 g/m.sup.2.
[0114] Deposited on the yellow color-forming layer 512 is an
ultra-violet blocking layer 514 composed of a nanoparticulate grade
of titanium dioxide (MS-7, available from Kobo Products Inc., South
Plainfield, N.J., 62% by weight), the above-mentioned POVAL MP103
(35% by weight) and glyoxal (3% by weight). This layer has a
coverage of 2 g/m.sup.2.
[0115] Deposited on the ultra-violet blocking layer 514 is an
overcoat 516 composed of Carboset 526 (a polymeric binder available
from Lubrizol, Cleveland, Ohio, 5 parts by weight), the
above-mentioned POVAL MP103 (2.12 parts by weight), NEOREZ R-989 (a
polyurethane latex, available from DSM NeoResins, Waalwijk, the
Netherlands, 4.34 parts by weight), Hidorin F-115P (a meltable
lubricant, available from Nagase America Corp., New York, N.Y., 5
parts by weight), Pinnacle 2530, a grade of erucamide, available
from Lubrizol Advanced Materials, Inc., Cleveland, Ohio, (1 part by
weight), and Ultraflon AD-10 (a poly(tetrafluoroethylene) lubricant
available from Laurel Products LLC, Elverson, Pa., 1.72 parts by
weight). This layer has a coverage of 1.2 g/m.sup.2.
[0116] The imaging member described above can be printed using
techniques such as those described in U.S. Pat. No. 6,801,233, U.S.
patent application Ser. No. 11/400,734, filed Apr. 6, 2006, U.S.
Pat. No. 7,408,563, and U.S. patent application Ser. No.
12/022,955, entitled "Print Head Pulsing Techniques for Multicolor
Printers", filed Jan. 30, 2008.
[0117] The invention will now be described further in detail with
respect to specific embodiments by way of Examples, it being
understood that these are intended to be illustrative only and the
invention is not limited to the materials, amounts, procedures and
process parameters, etc. recited herein. All parts and percentages
recited are by weight unless otherwise specified.
[0118] Reflection optical densities were measured using a
spectrophotometer from GretagMacbeth AG, Regensdorf,
Switzerland.
[0119] The following commercially-available materials were used to
prepare the Example coatings:
[0120] NEOCRYL A-6162, a styrene/acrylic water-borne latex
available from DSM NeoResins, Waalwijk, The Netherlands;
[0121] CP-655, a styrene/butadiene rubber latex with Tg 27.degree.
C. available from Dow Chemical Co., Midland, Mich.;
[0122] PB6692MNA, a styrene/butadiene rubber latex with Tg
1-5.degree. C. available from Dow Chemical Co., Midland, Mich.;
[0123] Carboset 526, an acrylic polymer available from Lubrizol,
Cleveland, Ohio, used as a solution in aqueous ammonia;
[0124] Zonyl FSN, a coating aid available from E. I. duPont de
Nenours, Inc., Wilmington, Del.; and
[0125] Alkanol XC, a surfactant available from E. I. duPont de
Nemours, Inc., Wilmington, Del.
[0126] NEOREZ R-989, a polyurethane latex, available from DSM
NeoResins, Waalwijk, the Netherlands;
[0127] Hidorin F-115P, a meltable lubricant, available from Nagase
America Corp., New York, N.Y.;
[0128] Ultraflon AD-10, a poly(tetrafluoroethylene) lubricant
available from Laurel Products LLC, Elverson, Pa.;
[0129] Pinnacle 2530, a grade of erucamide, available from Lubrizol
Advanced Materials, Inc., Cleveland, Ohio;
[0130] Aerosol 501, a surfactant available from Cytec Industries,
Inc., West Paterson, N.J.;
[0131] Mirataine H30, a surfactant available from Rhodia, Inc.
(USA), Cranbury, N.J.; and
[0132] Aziridine crosslinker CX-100, available from DSM NeoResins,
Waalwijk, the Netherlands.
[0133] The following dispersions of crystalline, solid materials
were used in the Examples.
[0134] Dispersion A
[0135] A slurry in water of 34% by weight 1,4-bis(benzyloxy)benzene
and 6% by weight poly(vinyl alcohol) POVAL 403, available from
Kuraray America, Inc., Houston, Tex., was prepared and subjected to
grinding in a horizontal mill until 95% of particles were reduced
in size to under 1 micrometer.
[0136] Dispersion B
[0137] A slurry in water of 18.75% by weight Lowinox 1790,
available from Chemtura, Middlebury, Conn., and 6.25% by weight of
the above-mentioned poly(vinyl alcohol) POVAL 403 was prepared and
subjected to grinding in a horizontal mill until 95% of particles
were reduced in size to under 1 micrometer.
[0138] Dispersion C
[0139] A slurry in water of 33.5% by weight Lowinox 1790, available
from Chemtura, Middlebury, Conn., and 5.9% by weight poly(vinyl
alcohol) Gohseran L3266, available from Nippon Gohsei, Japan, was
prepared and subjected to grinding in a horizontal mill until 95%
of particles were reduced in size to under 1 micrometer.
[0140] Dispersion D
[0141] A slurry in water of 32% Lowinox 1790, available from
Chemtura, Middlebury, Conn., and 8% by weight styrene-maleic
anhydride copolymer SMA 1000MA, available from Sartomer, Inc.,
Exton, Pa., was prepared and subjected to grinding in a horizontal
mill until 95% of particles were reduced in size to under 1
micrometer.
[0142] Dispersion E
[0143] A slurry in water of 34.2% by weight Anox 29, available from
Chemtura, Middlebury, Conn., and 5.8% by weight by weight of the
above-mentioned poly(vinyl alcohol) POVAL 403 was prepared and
subjected to grinding in a horizontal mill until 95% of particles
were reduced in size to under 1 micrometer.
[0144] Dispersion F
[0145] A slurry in water of 30.6% by weight Dye X of copending U.S.
patent application Ser. No. 12/022,969, 5.62% by weight methyl
acetate and 4.6% by weight of the above-mentioned poly(vinyl
alcohol) POVAL 403 was prepared and subjected to grinding in a
horizontal mill until 95% of particles were reduced in size to
under 1 micrometer.
[0146] Dispersion G
[0147] A slurry in water of 1.45% Aerosol 501, 5.24% Mirataine H30,
15.8% by weight Pinnacle 2530 and 1.6% by weight of the
above-mentioned poly(vinyl alcohol) POVAL 403 was prepared and
subjected to grinding in a horizontal mill until 95% of particles
were reduced in size to under 1 micrometer.
Example 1
[0148] This Example illustrates the thermal stability of an
unprinted thermal imaging member of the present invention
comprising a cyan color-forming layer in contact with a
thermally-insulating layer of the present invention.
[0149] A coating fluid for a cyan color-forming layer was prepared
by combining the materials shown in the table below in the
proportions indicated:
TABLE-US-00001 Ingredient % solids in fluid Carboset 526 2.61 Zonyl
FSN 0.04 Dispersion A 3.55 Dispersion E 0.59 Dispersion B 2.67
Dispersion F 0.58
[0150] A coating fluid for a thermally-insulating layer (TI-1) of
the present invention was prepared by combining the materials shown
in the table below in the proportions indicated:
TABLE-US-00002 Ingredient % solids in fluid NEOCRYL A-6162 10
Dispersion C 2.35
[0151] Coatings were prepared as follows:
[0152] Coating A: Cyan color-forming layer was coated onto a
filled, white, oriented polypropylene base of thickness 200 microns
available from Yupo Corporation America, Chesapeake, Va. to a dried
thickness of 4 g/m.sup.2.
[0153] Coating B: On top of and underneath a cyan color-forming
layer of dried thickness of 4 g/m.sup.2 on the above-mentioned
polypropylene base were provided layers of 1 g/m.sup.2 of NEOCRYL
A-6162.
[0154] Coating C: On top of and underneath a cyan color-forming
layer of dried thickness of 4 g/m.sup.2 on the above-mentioned
polypropylene base were provided layers of 1.28 g/m.sup.2 of
thermally-insulating layer TI-1 of the present invention. In order
to apply the uppermost layer of TI-1, 0.25% of the coating aid
Zonyl FSN was added to the coating fluid. The change in red
reflection density (Dmin) was recorded after coatings A, B and C
were subjected to environmental conditioning, as shown in the Table
below:
TABLE-US-00003 Condition Coating A Coating B Coating C 70.degree.
C., dry, 17 h 0.008 0.023 0.009 40.degree. C., 90% RH, 17 h 0.008
0.012 0.013 70.degree. C., dry, 17 h, then 0.015 0.057 0.023
40.degree. C., 90% RH, 17 h
[0155] It can be seen that coating C, in which the
thermally-insulating layer of the present invention was employed,
was more stable than coating B, with a control thermally-insulating
barrier containing no organic phenolic material, and almost as
stable as coating A, in which there was no thermally-insulating
barrier layer.
Example 2
[0156] This Example illustrates the thermal stability after
printing of a thermal imaging member of the present invention
comprising a cyan color-forming layer in contact with a
thermally-insulating layer of the present invention.
[0157] A coating fluid for a cyan color-forming layer was prepared
by combining the materials shown in the table below in the
proportions indicated:
TABLE-US-00004 Ingredient % solids in fluid CP-655 6.25 Zonyl FSN
0.056 Dispersion A 13.6 Dispersion B 3.72 Dispersion E 2.21
Dispersion F 2.16
[0158] Coating fluids for a thermally-insulating layers TI-2-TI-5
were prepared by combining the materials shown in the table below
in the fluid % solids indicated:
TABLE-US-00005 TI-2 TI-3 TI-4 TI-5 CP655 14.6 10.3 0 0 PB6692MNA 0
0 14.6 10.3 POVAL MP103 2.3 2.3 2.3 2.3 Dispersion D 0 4.3 0 4.3
Zonyl FSN 0.06 0.06 0.06 0.06 Alkanol XC 0.05 0.05 0.05 0.05
[0159] A coating fluid for a thermally-resistant overcoat was
prepared by combining the materials shown in the table below in the
proportions indicated:
TABLE-US-00006 Ingredient % solids in fluid Carboset 526 5.07 POVAL
MP103 2.12 Zonyl FSN 0.1 NEOREZ R989 4.3 Hidorin F115P 5.0
Ultraflon AD-10 1.72 Dispersion G 1.0
[0160] Coatings were prepared as follows:
[0161] Cyan color-forming layer was coated onto a filled, white,
oriented polypropylene base of thickness 95 microns available from
Yupo Corporation America, Chesapeake, Va. to a dried thickness of 3
g/m.sup.2.
[0162] On top of the cyan color-forming layer were coated
thermally-insulating layers TI-2-TI-5 (four separate coatings) to a
dried thickness of 3 g/m.sup.2.
[0163] On top of the thermally-insulating layers was coated a
thermally-resistant overcoat to a dried thickness of 1.7
g/m.sup.2.
[0164] Each coating was printed as described in U.S. Pat. No.
6,801,233 to give approximately equally-spaced cyan dye densities
varying from 0.07 (Dmin) to 2.00 (Dmax). These images were
subjected to environmental conditioning and the changes in dye
density that resulted are shown in the table below.
TABLE-US-00007 Condition TI-2 TI-3 TI-4 TI-5 60.degree. C., dry, 63
hr, 0.41 0.01 0.59 0.01 Dmin 60.degree. C., dry, 63 hr, 0.21 0.07
0.11 0.12 average delta OD 40.degree. C., 90% RH, 63 hr, 0.13 0.03
0.35 0.06 Dmin 40.degree. C., 90% RH, 63 hr, 0.12 0.06 0.11 0.02
average delta OD
[0165] It can be seen that coatings TI-3 and TI-5, in which
thermally-insulating layers of the present invention were employed,
were more stable (i.e., showed less change) than coatings TI-2 and
TI-4, the control thermally-insulating barrier containing no
organic phenolic material. Coating TI-3, in which the latex CP655,
having a Tg of 27.degree. C. was employed, was more stable than
coating TI-5, in which the latex PB6692MNA, having a Tg of
1-5.degree. C., was used.
Example 3
[0166] This example illustrates the use of a variety of grades of
poly(vinyl alcohol) as rheology modifiers for coating fluids
intended to produce thermally-insulating layers of the present
invention.
[0167] Six coating fluids (A-F) were prepared using six different
grades of poly(vinyl alcohol), each fluid having 33.5% solids
content in water. Each fluid was prepared by adding reagents in the
order indicated in the table below to water. In each fluid the
composition of the solids (and, therefore, of the dried coating)
was as follows:
TABLE-US-00008 Poly(vinyl alcohol) 13.5% CP655 69.91% Zonyl FSN
0.34% Alkanol XC 0.3% Dispersion D 14.25% Crosslinker CX-100
1.7%
[0168] The poly(vinyl alcohol) grades used were:
TABLE-US-00009 PVA Other Fluid grade % Hydrolysis Modification
Supplier A Poval 98%-99% Hydrophobically Kuraray MP103 modified
America, Houston, TX B Exceval 98%-99% High EVOH Kuraray 4104
content America, Houston, TX C Gohseran 86.5%-89%.sup. Anionically
Nippon Gohsei, L3266 modified Japan D PVA-403 78.5%-81.5% none
Kuraray America, Houston, TX E Mowiol .sup. 98%-98.8% none Kuraray
4-98 America, Houston, TX F Mowiol 86.7%-88.7% none Kuraray 4-88
America, Houston, TX
[0169] The viscosities of fluids A-F were measured using an AR
1000N rheometer, available from TA Instruments, Newcastle, Del.,
using a cone-and-plate geometry at shear rates from 10-1000
s.sup.-1 at a temperature of 25.degree. C. The results are shown in
the Table below:
TABLE-US-00010 Fluid .mu..sub.eff at 1000 s.sup.-1 n K A 72.2 0.90
142.5 B 68.5 0.41 3700 C 39.8 0.37 2814 D 48.2 0.46 1830 E 76.9
0.39 4822 F 63.2 0.45 2676
[0170] It can be seen that only in fluid A is .mu..sub.eff within
the range of 35-200 mPas at a shear rate of 1000 s.sup.-1, with n
in the range 0.8-1. Fluid A contains POVAL MP103, a
fully-hydrolyzed poly(vinyl alcohol) with a hydrophobic
modification. The other poly(vinyl alcohol) grades tested do not
have such a hydrophobic modification.
[0171] Although the invention has been described in detail with
respect to various preferred embodiments, it is not intended to be
limited thereto, but rather those skilled in the art will recognize
that variations and modifications are possible which are within the
spirit of the invention and the scope of the appended claims.
* * * * *