Note: Descriptions are shown in the official language in which they were submitted.
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THERMOCHROMIC SYSTEMS WITH CONTROLLED HYSTERESIS
FIELD
[0001] This disclosure pertains to the field of thermochromic dye
systems. More
particularly, these are reversible thermochromic dye systems having a
controllable color
transition range across a hysteresis window of the system.
BACKGROUND
[0002] Dyes that change color over a range of temperatures are known as
thermochromic dyes. Thermochromic dyes can be manufactured to have a color
change that
is reversible or irreversible. Formulated as inks or dyes, they are used in a
variety of
applications such as plastic masterbatch, paper, textiles, coatings, offset
ink, metal decorating
inks and coatings, ultraviolet radiation curable inks and coatings, solvent
based inks and
coatings, screen inks and coatings, gravure inks and coatings, paints,
security printing, brand
protection, smart packaging, marketing and novelty printing, among other uses.
[0003] Thermochromic dyes use colorants that are either liquid crystals or
leuco
dyes. Liquid crystals are used less frequently than leuco dyes because they
are very difficult
to work with and require highly specialized printing and handling techniques.
[0004] Thermochromic dyes are a system of interacting parts. The parts of the
system are leuco dyes acting as colorants, weak organic acids acting as color
developers and
solvents that variably interact with components of the system according to the
temperature of
the system. Thermochromic dye systems are microencapsulated in a protective
coating to
protect the contents from undesired effects from the environment. Each
microcapsule is self-
contained, having all of the components of the entire system required to
reproduce the color
change. The components of the system interact with one another differently at
different
temperatures. Generally, the system is ordered and colored below a temperature
corresponding to the full color point. The system becomes increasingly
unordered and starts
to lose its color at a temperature corresponding to a predetermined activation
temperature.
[0005] Below the activation temperature, the system is colored and above the
activation temperature they are clear or lightly colored. The activation
temperature
corresponds to a range of temperatures at which the transition is taking place
between the full
color point and the clearing point. Generally, the activation temperature is
defined as the
temperature at which the human eye can perceive that the system is starting to
lose color, or
alternatively, starting to gain color. Presently, thermochromic systems are
designed to have
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activation temperatures over a broad range, from about -20 C to about 80 C
or more. With
heating, the system becomes increasingly unordered and continues to lose its
color until it
reaches a level of disorder at a temperature corresponding to a clearing
point. At the clearing
point, the system lacks any recognizable color.
[0006] Specific thermochromic ink formulations are known in the
art. See, for
example, United States Patents 4,720,301, 5,219,625 5,558,700, 5,591,255,
5,997,849,
6,139,779, 6,494,950 and 7,494,537, all of which are expressly incorporated
herein by
reference to the same extent as though fully replicated herein. These
thermochromic inks are
known to use various components in their formulations, and are generally
reversible in their
color change. Thermochromic inks are available in various colors, with various
activation
temperatures, clearing points and full color points. Thermochromic inks may be
printed by
offset litho, dry offset, letterpress, gravure, flexo and screen processes,
amongst others.
Thermochromic inks containing leuco dyes are available for all major ink types
such as
water-based, ultraviolet cured and epoxy. The properties of these inks differ
from process
inks. For example, most thermochromic inks contain the thermochromic systems
as
microcapsules, which are not inert and insoluble as are ordinary process
pigments. The size
of the microcapsules containing the thermochromic systems ranges typically
between 3-51.1m
which is more than 10-times larger than regular pigment particles found in
most inks. The
post-print functionality of thermochromic inks can be adversely affected by
ultraviolet light,
temperatures in excess of 140 C and aggressive solvents. The lifetime of
these inks is
sometimes very limited because of the degradation caused by exposure to
ultraviolet light
from sunshine. Thus, there is a need in the art for thermochromic systems in
inks and
coatings having resistance to degradation from exposure to ultraviolet light.
[0007] Temperature changes in thermochromic systems are associated with color
changes. If this change is plotted on a graph having axes of temperature and
color, the curves
do not align and are offset between the heating cycle and the cooling cycle.
The entire color
versus temperature curve has the form of a loop. See generally FIG.1A where
the extent of
color change presents a gap 100a that differs between color change that occurs
upon heating
102 versus cooing 103. FIG. 1B presents a relatively larger gap 100b. Such a
result shows
that the color of a thermochromic system does not depend only on temperature,
but also on
the thermal history, i.e. whether the particular color was reached during
heating or during
cooling. This phenomenon is generally referred to as a hysteresis cycle and
specifically
referred to herein as color hysteresis or the hysteresis window. Decreasing
the width of this
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hysteresis window to approximately zero would allow for a single value for the
full color
point and a single value for the clearing point. This would allow for a
reliable color transition
to be observed regardless of whether the system is being heated or cooled.
Nonetheless, the
concept decreasing separation across the hysteresis window is elusive in
practice. The extent
of the respective gaps 100a, 100b may be controlled according to the
instrumentalities
described herein.
[0008] Prior art reveals that the color transition range of
microencapsulated
thermochromic systems may be adjusted by shifting the full color point upward
toward the
clearing point, or shifting the clearing point downward toward the full color
point, as
explained in U.S. patent 6,494,950. These shifts are accomplished by adding
high melting
point materials to increase the full color point or, alternatively, by adding
low melting point
materials to the system to decrease the clearing point. Thus, the full color
point or clearing
point may be lowered or raised, but the overall temperature range between the
two points
remains unchanged because the amount of separation or width across the
hysteresis window
is left largely unaffected.
SUMMARY
[0009] Thermochromic inks having a reduced susceptibility to UV degradation
and/or a reduced hysteresis window in the sense of a narrowed color transition
range from the
full color point to the clearing point in comparison to existing thermochromic
systems are
described herein. The thermochromic inks with a controlled color transition
range
advantageously allow for new applications of thermochromic inks in products
that require a
display of color at a precise temperature. Alternatively, inks and coatings
with wide
hysteresis for new applications provide unique properties that may be utilized
for particular
applications from user interactivity to semi-irreversible behavior.
[0010] Thermochromic inks contain microcapsules, which encapsulate a
thermochromic system mixed with a solvent. The thermochromic system has a
material
property of a thermally conditional hysteresis window that presents a thermal
separation.
These inks may be altered according to the instrumentalities described herein
by using a co-
solvent that is combined with the thermochromic system and selected from the
group
consisting of derivatives of myristic acid, derivatives of behenyl acid,
derivatives of palmytic
acid and combinations thereof. By way of example, this material may be
provided in an
effective amount to reduce the thermal separation in the overall ink to a
level less than eighty
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percent of separation that would otherwise occur if the material were not
added. This
effective amount may range, for example from the 12% to 15% by weight of the
composition.
[0011] Especially preferred co-solvents include isopropyl
myristate, isopropyl
palmitate, methyl palmitate, methyl stearate, myristyl, myristate, cetyl
alcohol, stearyl
alcohol, behenyl alcohol, stearyl behenate, and stearamide. These co-solvents
may be
combined in any manner or proportion to achieve the effective amount.
[0012] The thermochromic system may contain, for example, at least one
chromatic organic compound selected from the group consisting of
diphenylmethane
phthalide derivatives, phenylindolylphthalide derivatives, indolylphthalide
derivatives,
diphenylmethane azaphthalide derivatives, phenylindolylazaphthalide
derivatives, fluoran
derivatives, styrynoquinoline derivatives, and diaza-rhodamine lactone
derivatives. Light
stabilizers may also be added to protect against the deleterious effects of
ultraviolet radiation.
[0013] Within the encapsulated thermochromic systems, complexes form between
the dye and the weak acid developer that allow the lactone ring structure of
the leuco dye to
be opened. The nature of the complex is such that the hydroxyl groups of the
phenolic
developer interact with the lactone ring structure forming a supra-molecular
structure that
orders the dyes and developers such that a color is formed. Color forms from
this supra-
molecular structure because the dye molecule in the ring open structure is
cationic in nature
and the molecule has extended its conjugation allowing absorption in the
visible spectrum
thus producing a colored species. The color that is perceived by the eye is
what visible light
is not absorbed by the complex. The extent of the dye/developer complex
depends on the
molar ratio of dye and developer. The stability of the colored complex is
determined by
numerous factors including the affinity of the solvent for itself, the
developer or the
dye/developer complex. In a solid state, below the full color point, the
dye/developer
complex is stable. In the molten state, the solvent destabilizes the
dye/developer complex
and the equilibrium is more favorably shifted towards a developer/solvent
complex. This
happens at temperatures above the full color point because the dye/developer
complex is
disrupted and the extended conjugation of the Tr cloud electrons that allow
for the absorption
of visible light is reversibly broken.
[0014] The melting and crystallization profile of the solvent
system determines the
nature of the thermochromic system. The full color point of the system occurs
when the
maximum amount of dye is developed. In a crystallized solvent state, the
dye/developer
complex is favored where the dye and developer exist in a unique crystallized
structure, often
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intercalating with one another to create an extended conjugated it system. In
the molten state,
the solvent(s), in excess, have enough kinetic energy to disrupt the stability
of the
dye/developer complex, and the thermochromic system becomes decolorized.
[0015] The addition of a co-solvent with a significantly higher
melting point than
the other dramatically changes the melting properties of both the solvents. By
mixing two
solvents that have certain properties, a blend can be achieved that possesses
a eutectic
melting point. The melting point of a eutectic blend is lower than the melting
point of either
of the co-solvents alone and the melting point is sharper, occurring over a
smaller range of
temperatures. The degree of the destabilization of the dye/developer complex
can be
determined by the choice of solvents. By creating unique eutectic blends, both
the clearing
point and the fall color point can be altered simultaneously. The degree of
hysteresis is then
shifted in both directions simultaneously as the sharpness of the melting
point is increased.
[0016] Preferred properties of at least one of the co-solvents used
in the present
disclosure include having a long fatty tail of between 12 and 24 carbons and
possessing a
melting point that is about 70 C to about 200 C greater than the co-solvent
partner. The co-
solvents are preferably also completely miscible at any ratio.
[0017] It is an object of the present disclosure to provide
thermochromic systems
with a reduced hysteresis window achieved by shifting both the full color
point and the
clearing point.
[0018] It is a further object of this disclosure to provide solvent
and co-solvent
systems that act as light stabilizers and temperature control regulators that
modify the
temperature profile of the thermochromic system while at the same time
improving the
stability of the thermochromic system to exposure to ultraviolet light.
[0019] It is also an object of the present disclosure to provide
ink formulations and
a method of correcting formulations that normally destroy the color changing
properties of
thermochromic systems possessing narrower color hysteresis windows and
narrower color
transition temperature ranges when compared to pre-existing thermochromic
systems.
Definitions
[0020] Thermochromic system - A mixture of dyes, developers, solvents, and
additives (encapsulated or non-encapsulated) that can undergo reversible or
semi-irreversible
color change in response to temperature changes.
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[0021] Full color point - The temperature at which a thermochromic system has
achieved maximum color density upon cooling and appears to gain no further
color density if
cooled to a lower temperature.
[0022] Activation temperature - The temperature above which the ink has almost
achieved its final clear or light color endpoint. The color starts to fade at
approximately 4 C
below the activation temperature and will be in between colors within the
activation
temperature range.
[0023] Clearing point - The temperature at which the color of a thermochromic
system is diminished to a minimal amount and appears to lose no further color
density upon
further heating.
[0024] Hysteresis - The difference in the temperature profile of a thermo
chromic
system when heated from the system when cooled.
[0025] Hysteresis window - The temperature difference in terms of degrees that
a
thermochromic system is shifted as measured between the derivative plot of
chroma of a
spectrophotometer reading between the cooling curve and the heating curve.
[0026] Leuco dye - A leuco dye is a dye whose molecules can acquire two forms,
one of which is colorless.
[0027] Eutectic system - A eutectic system is a mixture of at least
two solvents
having one composition of the at least two solvents having a freezing point at
a lower
temperature than any other mixture of the solvents.
[0028] Eutectic temperature - The temperature at which the eutectic system
freezes
is known as the eutectic temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIGURE 1 shows generally the effect of controlling color
hysteresis in a
thermochromic system where Fig. 1A has a narrower hysteresis gap than does
FIG. 1B;
[0030] FIG. 2 is a plot of color change versus temperature for a
reversible
thermochromic dye; and
[0031] FIG. 3 is a plot of color change versus temperature for a
reversible
thermochromic dye.
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DETAILED DESCRIPTION
[0032] Thermochromic systems are prepared by combining a color forming
molecule or molecules such as leuco dyes that are capable of extended
conjugation by proton
gain or electron donation; a color developer or developers that donate a
proton or accept an
electron; and a single solvent, or a blend of co-solvents. The solvent or
blend of co-solvents
are chosen based on melting point and establish the thermochromic temperature
range of the
system. These formulations are then microencapsulated within a polymeric
shell.
Leuco Dyes
[0033] Leuco dyes most commonly used as color formers in thermochromic
systems of the present disclosure include, but are not limited to, generally;
spirolactones,
fluorans, spiropyrans, and fulgides; and more specifically; diphenylmethane
phthalide
derivatives, phenylindolylphthalide derivatives, indolylphthalide derivatives,
diphenylmethane azaphthalide derivatives, phenylindolylazaphthalide
derivatives, fluoran
derivatives, styrynoquinoline derivatives, and diaza-rhodamine lactone
derivatives which can
include: 3,3-bis(p-dimethylaminopheny1)-6-dimethylaminophthalide; 3-(4-
diethylaminopheny1)-3-(1-ethyl-2-methylindol-3-y1) phthalide; 3,3-bis(1-n-
buty1-2-
methylindo1-3-yl)phthalide; 3,3-bis(2-ethoxy-4-diethylaminopheny1)-4-
azaphthalide; 342-
ethoxy-4-(N-ethylanilino)pheny1]-3-(1-ethyl-2-methylindol-3-y1)-4-
azaphthalide; 3,6-
dimethoxyfluoran; 3,6-di-n-butoxyfluoran; 2-methyl-6-(N-ethyl-N-p-
tolylamino)fluoran; 3-
chloro-6-cyclohexylaminofluoran; 2-methyl-6-cyclohexylaminofluoran; 2-(2-
chloroanilino)-
6-di-n-butylamino fluoran; 2-(3-trifluoromethylanilino)-6-diethylaminofluoran;
2-(N-
methylanilino)-6-(N-ethyl-N-p-tolylamino) fluoran, 1,3-dimethy1-6-
diethylaminofluoran; 2-
chloro-3-methy1-6-diethylamino fluoran; 2-anilino-3-methyl-6-
diethylaminofluoran; 2-
anilino-3-methy1-6-di-n-butylamino fluoran; 2-xylidino-3-methyl-6-
diethylaminofluoran; 1,2-
benzo-6-diethylaminofluoran; 1,2-benzo-6-(N-ethyl-N-isobutylamino)fluoran,1,2-
benzo-6-(N-
ethyl-N-isoamylamino)fluoran; 2-(3-methoxy-4-dodecoxystyryl)quinoline;
spiro[5H-(1)
benzopyrano(2,3-d)pyrimidine-5,43'H)isobenzofuran]-3'-one; 2-(diethylamino)-8-
(diethylamino)-4-methyl-spiro[5H- (1)benzopyrano(2,3-d)pyrimidine-
5,43'H)isobenzofuran]-
3'-one; 2-(di-n-butylamino)-8-(di-n-butylamino)-4-methyl-spiro[5H-
(1)benzopyrano(2,3-
d)pyrimidine-5,1'(3'H)isobenzofuran]-3'-one; 2-(di-n-butylamino)-8-
(diethylamino)-4-methyl-
spiro[5H-(1)benzopyrano(2,3-d)pyrimidine-5,1'(3'H)isobenzofuran]-3'-one; 2-(di-
n-
butylamino)-8(N-ethyl-N-isoamylamino)-4-methyl-spiro[5H-(1)benzopyrano(2,3-
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d)pyrimidine- 5,1.(3'H)isobenzofuran]-3'-one; and 2-(di-n-butylamino)-8-(di-n-
butylamino)-4-
phenyl and trisubstituted pyridines.
[0034] Particlarly preferred materials for use as chromatic organic compounds
are
of diphenylmethane phthalide derivatives, phenylindolylphthalide derivatives,
indolylphthalide derivatives, diphenylmethane azaphthalide derivatives,
phenylindolylazaphthalide derivatives, fluoran derivatives, styrynoquinoline
derivatives,
2,4,6, trisubstituted pyridines, quinazolines, bis-quinazolines, and diaza-
rhodamine lactone
derivatives, in any combination.
[0035] Specific examples of 2,4,6 trisubstituted pyridine dyes are
described in
detail in copending United States patent application serial number 61/542,738
filed October
3,2011, which is hereby incorporated by reference to the same extent as though
fully
replicated herein. Compounds 1-45 below are dyes that exemplify these
materials and may
be used in any combination.
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0
L
0 lei 0
0 0 101
I I I
0 Nr * 0 N * * N 0
1 2 3
0
J
NH
0 0 0
I I I
0 Nr 0 CI O Nr 6 CI 0 N S
4 5 6
LN J =N 0 0N 0
* 0 0
lb Nr 6 0 N- 0
I I I I
7 8 9
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el el L J
N N
* 0 *
0.--- ---
1 '=- 0 I
I I
0 *
0 N 0 N * N *
0 0 F F
I I 11
10 12
J LN J L J
N
N
0 0
=
1 I
0 N 0 CI is N-, 0 O. CI I
0110 N
CI CI
13 14 15
J J L J
N
N N
0 1101 101
I 1
r I
0 1N 0
1 N \ * N $
I
OMe
N
I
N-
16 17 18
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11
LNJ L J
N L J
N
101 0 *
I I I
5N 0
N 1110 10 N 5
HO Me0 HO
19 20 21
* If¨ 4. V-----
0 1411 41111
I I
5N5 I
* r\I 1110
0 IN
OMe Me0 CO
22 23 24
N --..
0
140 I.
I
(110 .1µ1 '= I I
I '-. 0 N
N 0 ,-- I I
N .-
N /
25 26 27
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-,--
L NJ -. ..-
N
4111
140 0
I
,.
I
N
'',- N
1 I
.`-
1 N I
'-,,
I
N N I
N ,'
28 29 30
OH .
OH 0
0 OMe
0 OMe
0111
¨I
I
N I
N
1 N I
N ''N.1 I '-. i
N I ,,N N
31 32 33
0 0 0
411:1 4111 I.
I 1 I
0 'IV -.'' 110 1\1
I I :isi N I I
N N,
.- N
34 35 36
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,---
L NJ
lµr 1\r
0
0 0
I ,
---- N . S , S
S S
37 38 39
OH =
OH 0
0 OMe
0 OMe
1
.
. s 1 1
\ SN
----- N
0 S
40 41 42
di
0 0
S 0 lel
I I I
\ N 0 . S , 0
---- ---- N ----- N
43 44 45
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4,4'-dialky1-2,2'-biphenol,
4,4'-dichloro, difluoro, dibromo, diiodo-2,2'-biphenol,
4,4'-dicarboalkoxy-2,2'-biphenol, and
4,4'-diacetyl, dibenzoy1-2,2'-biphenol and 5-alkyl-salicylic acid.
Developers
[0036] Weak acids that can be used as color developers act as proton donors,
changing the dye molecule between its leuco form and its protonated colored
form; stronger
acids make the change irreversible. Examples of developers used in the present
disclosure
include but are not limited to: bisphenol A; bisphenol F; tetrabromobisphenol
A; 1'-
methylenedi-2-naphthol; 1,1 ,1 -tris(4-hydroxyphenypethane; 1,1 -bis(3 -
cyclohexy1-4-
hydroxyphenyl)cyclohexane; 1,1-bis(4-hydroxy-3-methylphenyl)cyclohexane; 1,1-
bis(4-
hydroxyphenyl)cyclohexane; 1,3-bis[2-(4-hydroxypheny1)-2-propyl]benzene; 1-
naphthol; 2-
naphthol; 2,2 bis(2-hydroxy-5-biphenylyl)propane; 2,2-bis(3-cyclohexy1-4-
hydroxy)propane;
2,2-bis(3-sec-butyl-4-hydroxyphenyl)propane; 2,2-bis(4-hydroxy-3-
isopropylphenyl)propane; 2,2-bis(4-hydroxy-3-methylphenyl)propane; 2,2-bis(4-
hydroxyphenyl)propane; 2,3,4-trihydroxydiphenylmethane;
Dimethylbutylidene)diphenol; 4,4'-(2-Ethylidene)diphenol;
hydroxybenzylidene)bis(2,3,6-trimethylphenol); 4,4'-biphenol; 4,4'-
dihydroxydiphenyl ether;
4,4'-dihydroxydiphenylmethane; 4,4'-methylidenebis(2-methylphenol); 4-(1,1,3,3-
tetramethylbutyl)phenol; 4-phenylphenol; 4-tert-butylphenol; 9,9-bis(4-
hydroxyphenyl)fluorine; 4,4'-(ethane-1,1-diy1)diphenol; alpha,alpha'-bis(4-
hydroxypheny1)-
1,4-diisopropylbenzene; alpha,alpha,alpha' -tris(4-hydroxypheny1)- 1 -ethy1-4-
isopropylbenzene; benzyl 4-hydroxybenzoate; bis(4-hydroxyphenyl)sulfide; bis(4-
hydroxyphenyl)sulfone; propyl 4-hydroxybenzoate; methyl 4-hydroxybenzoate;
resorcinol; 4-
tert-butyl-catechol; 4-tert-butyl-benzoic acid; 1,P-methylenedi-2-naphthol
1,1,1-tris(4-
hydroxyphenypethane; 1,1-bis(3-cyclohexy1-4-hydroxyphenyl)cyclohexane; 1,1-
bis(4-hydroxy-
3-methylphenyl)cyclohexane; 1,1-bis(4-hydroxyphenyl)cyclohexane; 1,3-bis[2-(4-
hydroxypheny1)-2-propyl]benzene; 1- naphthol 2,2'-biphenol; 2,2- bis(2-hydroxy-
5-
biphenylyl)propane; 2,2-bis(3-cyclohexy1-4-hydroxyphenyl)propane; 2,2-bis(3-
sec-buty1-4-
hydroxyphenyl)propane; 2,2-bis(4-hydroxy-3-isopropylphenyl)propane; 2,2-bis(4-
hydroxy-3-
methylphenyl)propane; 2,2-bis(4-hydroxyphenyl)propane; 2,3,4-
trihydroxydiphenylmethane;
2- naphthol; 4,4'-(1,3-dimethylbutylidene)diphenol; 4,4'-(2-
ethylhexylidene)diphenol 4,4'-(2-
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hydroxybenzylidene)bis(2,3,6-trimethylphenol); 4,4'-biphenol; 4,4'-
dihydroxydiphenyl ether;
4,4'-dihydroxydiphenylmethane; 4,4'-ethylidenebisphenol; 4,4'-methylenebis(2-
methylphenol); 4-(1,1,3,3-tetramethylbutyl)phenol; 4-phenylphenol; 4-tert-
butylphenol; 9,9-
bis(4-hydroxyphenyl)fluorine; alpha,alpha'-bis(4-hydroxypheny1)-1,4-
diisopropylbenzene;
a,a,a-tris(4-hydroxypheny1)-1-ethyl-4-isopropylbenzene; benzyl 4-
hydroxybenzoate; bis(4-
hydroxyphenyl) sulfidem; bis(4-hydroxyphenyl) sulfone methyl 4-
hydroxybenzoate;
resorcinol; tetrabromobisphenol A; derivative salts of salicylic acid such as
3,5-di-tertbutyl-
salicylic acid; zinc 3,5-di-tertbutylsalicylate; 3-phenyl-salicylic acid; 5-
tertbutyl-salicylic
acid; 5-n-octyl-salicylic acid; 2,2'-biphenol; 4,4'-di-tertbuty1-2,2'-
biphenol; 4,4'-di-n-alky1-
2,2'-biphenol; and 4,4'-di-halo-2,2'-biphenol, wherein the halo is chloro,
fluor , bromo, or
iodo.
[0037] Specific examples of known leuco dye developers are shown below:
Bisphenol A
OH
= OH
Bisphenol F
10 OH
HO
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Tetrabromobisphenol A
Br
Br . O
OH
HO
Br Br
1'-Methylenedi-2-naphthol
OH
OH
O 110
. el
1,1,1-Tris(4-hydroxyphenypethane
OH
HO is, _OH
1,1-Bis(3-cyclohexy1-4-hydroxyphenypeyclohexane
=
= 411 OH
ii 0
OH
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1,1-Bis(4-hydroxy-3-methylphenyl)cyclohexane
HO
11104
HO 40 .
1,1-Bis(4-hydroxyphenyl)cyclohexane
HO
0
HO 40 ill
1,3-Bis[2-(4-hydroxypheny1)-2-propylibenzene
. ISI
411 O
HO H
1-Naphthol
ablel
MP OH
2-naphthol
100
HO
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2,2-Bis(2-hydroxy-5-biphenylyl)propane
Si,
0 OH
2,2-Bis(3-cyclohexy1-4-hydroxy)propane
HO . Me, OH
Me
2,2-Bis(3-sec-butyl-4-hydroxyphenyl)propane
HO SI =
OH
2,2-Bis(4-hydroxy-3-isopropylphenyl)propane
HO . iiOH
2,2-Bis(4-hydroxy-3-methylphenyl)propane
li iht
O
HO H
2,2-Bis(4-hydroxyphenyl)propane
= O
HO OH
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2,3,4-Trihydroxydiphenylmethane
OH
ip is OH
OH
4,4'-(1,3-Dimethylbutylidene)diphenol
OH
afr OH
4,4'-(2-Ethylidene)diphenol
1.1Si
HO OH
4,4'-(2-hydroxybenzylidene)bis(2,3,6-trimethylphenol)
HO le lei OH
HO
4,4'-Biphenol
HO 41k 40 OH
4,4'-Dihydroxydiphenyl Ether
40 0
HO OH
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4,4'-Dihydroxydiphenylmethane
I I
HO
4,4'-Ethylidenebisphenol
HO OH
4,4'-Methylidenebis(2-methylphenol)
140
HO OH
4-(1,1,3,3-Tetramethylbutyl)phenol
OH
4-Phenylphenol
HO 4.=
4-tert-Butylphenol
=OH
9,9-Bis(4-hydroxyphenyl)fluorine
11/
O
HO H
Alpha,alpha'-Bis(4-hydroxypheny1)-1,4-diisopropylbenzene
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HO 0 =
lel OH
Alpha,alpha,alpha'-Tris(4-hydroxypheny1)-1-ethyl-4-isopropylbenzene
OH
go. OH
S
OH
Benzyl 4-Hydroxybenzoate
HO, 0 1.
o
Bis(4-hydroxyphenyl)Sulfide
0 s 0
HO OH
Bis(4-hydroxyphenyl)sulfone
0
HO . 4. OH
I I
0
Propyl 4-Hydroxybenzoate
HO = 0-/
0
Methyl 4-Hydroxybenzoate
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\o
HO*
0
Resorcinol
HO el OH
4-Tert-butyl-catechol
HO
HO 411
4-Tert-butyl-benzoic acid
. COON
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[0038] The leuco dyes are combined with leuco dye developers for making
thermochromic compositions. These materials are found to generate absorption
densities
from the leuco dyes when formulated with a carrier that contains one or more
fatty ester, fatty
alcohol, and fatty amide. The combination of leuco dyes, developers and
carrier materials
may be used in any combination to achieve the listed functionalities. By way
of example,
this combination of molecules includes any combination of the following
molecules:
bipyridyl and terpyridine leuco dyes of the type 2{2-pyridy1]-6-phenyl- 4-
dialkylamino-
pyridine, 2[2-pyridy1]-6-pheny1-4-diarylamino-pyridine, 242-pyridy1]-6-pheny1-
4-hydroxy-
pyridine, 2[2-pyridy11-612-pyridy1]-4-dialkylamino-pyridine, 242-pyridy1]-642-
pyridy1]-4-
diarylamino-pyridine, 2[2-pyridy1]-642-pyridy1]-4-hydroxy-pyridine, molecules
from Figure
3 including at least the following; 26,27, 29, 30, 31, 32, 33, 34, 35, 36, 38,
39, 41, 42, and
43; also 2,6-dipheny1-4-dialkylamino-pyridines, 2,6-dipheny1-4-diarylamino-
pyridines, 2,6-
dipheny1-4-hydroxy-pyridines, 2,6-dipheny1-4-alkoxy-pyridines, 2,6-dipheny1-4-
aryloxy-
pyridines, molecules from Figure 3 including at least the following; 1, 3, 5,
6, 7, 8, 9, 10, 13,
17, 19, 20, 21, 22, 23, 24; and 4,4'-dialky1-2,2'-biphenol, 4,4'-dichloro,
difluoro, dibromo,
diiodo-2,2'-biphenol, 4,4'-dicarboalkoxy-2,2'-biphenol, 4,4'-diacetyl,
dibenzoy1-2,2'-
biphenol as well as salicylic acids including at least 5-alkyl-salicylic acid.
[0039] Furthermore the composition so obtained may be encapsulated in a
separate
composition, such as a melamine-formaldehyde resin, to produce absorption
changing
pigments designed for use in formulated ink and coating products as well as
plastic pellet
concentrates for injection molded or extruded plastic products.
[0040] Some materials function as both leuco dyes and light
absorbers:
Visible Range absorbers (400 nm to 700 nm):
4-(4'-dimethylamino-phenyl)-2,6-diphenyl-pyridine (dye 11)
4-(4'-diphenylamino-phenyl)-2,6-diphenyl-pyridine (dye 3)
Near UVA Range absorbers:
4-(4-ethoxy-phenyl)-2,6-diphenyl-pyridine (dye 1).
4-(4-phenoxy-phenyl)-2,6-diphenyl-pyridine (dye 3).
[0041] These developers are particularly preferred for use with the
2,4,6 tri-
subsittuted pyridine dyes
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3,5-di-tertbutyl-salicylic acid
OH
0 OH
Zn 3,5-di-tertbutylsalicylate
0
171
0
\
0
0
3-phenyl-salicylic acid
OH OH 411)
Os
5-tertbutyl-salicylic acid
0
OH
OH
5-n-octyl-salicylic acid
0
OH
OH
2,2'-biphenol
OH
=
HO
4,4'-di-tertbuty1-2,2'-biphenol
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OH
41,
HO
4,4'-di-n-alkyl-2,2'-biphenol
n-alkyl
OH 40,
40 OH
n-alkyl
4,4'-di-halo-2,2'-biphenol, halo= chloro, fluoro, bromo, iodo
go OH
X X
HO
X = CI, F, Br, I
Solvents
[0042] The best solvents to use within the thermochromic system are those that
have low reactivity, have a relatively large molecular weight (i.e. over 100),
and which are
relatively non-polar. Ketones, diols and aromatic compounds should not be used
as solvents
within the thermochromic system.
[0043] Solvents and/or co-solvents used in thermochromic generally
may include,
but are not limited to: aldehydes, thiols, sulfides, ethers, ketones, esters,
alcohols, and acid
amides. These solvents can be used alone or in mixtures of 2 or more. Examples
of the
sulfides include, but are not limited to: di-n-octyl sulfide; di-n-nonyl
sulfide; di-n-decyl
sulfide; di-n-dodecyl sulfide; di-n-tetradecyl sulfide; di-n-hexadecyl
sulfide; di-n-octadecyl
sulfide; octyl dodecyl sulfide; diphenyl sulfide; dibenzyl sulfide; ditolyl
sulfide;
diethylphenyl sulfide; dinaphthyl sulfide; 4,4'-dichlorodiphenyl sulfide; and
2,4,5,4'tetrachlorodiphenyl sulfide. Examples of the ethers include, but are
not limited to:
aliphatic ethers having 10 or more carbon atoms, such as dipentyl ether,
dihexyl ether,
diheptyl ether, dioctyl ether, dinonyl ether, didecyl ether, diundecyl ether,
didodecyl ether,
ditridecyl ether, ditetradecyl ether, dipentadecyl ether, dihexadecyl ether,
dioctadecyl ether,
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decanediol dimethyl ether, undecanediol dimethyl ether, dodecanediol dimethyl
ether,
tridecanediol dimethyl ether, decanediol diethyl ether, and undecanediol
diethyl ether;
alicyclic ethers such as s-trioxane; and aromatic ethers such as phenylether,
benzyl phenyl
ether, dibenzyl ether, di-p-tolyl ether, 1-methoxynaphthalene, and
3,4,5trimethoxytoluene.
[0044] Examples of ketone solvents include, but are not limited to:
aliphatic
ketones having 10 or more carbon atoms, such as 2-decanone, 3-decanone, 4-
decanone, 2-
undecanone, 3-undecanone, 4-undecanone, 5-undecanone, 6-undecanone, 2-
dodecanone, 3-
dodecanone, 4-dodecanone, 5-dodecanone, 2-tridecanone, 3-tridecanone, 2-
tetradecanone, 2-
pentadecanone, 8-pentadecanone, 2-hexadecanone, 3-hexadecanone, 9-
heptadecanone, 2-
pentadecanone, 2-octadecanone, 2-nonadecanone, 10-nonadecanone, 2-eicosanone,
11-
eicosanone, 2-heneicosanone, 2-docosanone, laurone, and stearone; aryl alkyl
ketones having
12 to 24 carbon atoms, such as n-octadecanophenone, n-heptadecanophenone, n-
hexadecanophenone, n-pentadecanophenone, n-tetradecanophenone, 4-n-
dodecaacetophenone, n-tridecanophenone, 4-n-undecanoacetophenone, n-
laurophenone, 4-n-
decanoacetophenone, n-undecanophenone, 4-n-nonylacetophenone, n-decanophenone,
4-n-
octylacetophenone, n-nonanophenone, 4-n-heptylacetophenone, n-octanophenone, 4-
n-
hexylacetophenone, 4-n-cyclohexylacetophenone, 4-tert-butylpropiophenone, n-
heptaphenone, 4-n-pentylacetophenone, cyclohexyl phenyl ketone, benzyl n-butyl
ketone, 4-
n-butylacetophenone, n-hexanophenone, 4-isobutylacetophenone, 1-
acetonaphthone, 2-
acetonaphthone, and cyclopentyl phenyl ketone; aryl aryl ketones such as
benzophenone,
benzyl phenyl ketone, and dibenzyl ketone; and alicyclic ketones such as
cyclooctanone,
cyclododecanone, cyclopentadecanone, and 4-tert-butylcyclohexanone, ethyl
caprylate, octyl
caprylate, stearyl caprylate, myristyl caprate, stearyl caprate, docosyl
caprate, 2-ethylhexyl
laurate, n-decyl laurate, 3-methylbutyl myristate, cetyl myristate, isopropyl
palmitate,
neopentyl palmitate, nonyl palmitate, cyclohexyl palmitate, n-butyl stearate,
2-methylbutyl
stearate, stearyl behenate 3,5,5-trimethylhexyl stearate, n-undecyl stearate,
pentadecyl
stearate, stearyl stearate, cyclohexylmethyl stearate, isopropyl behenate,
hexyl behenate,
lauryl behenate, behenyl behenate, cetyl benzoate, stearyl p-tert-
butylbenzoate, dimyristyl
phthalate, distearyl phthalate, dimyristyl oxalate, dicetyl oxalate, dicetyl
malonate, dilauryl
succinate, dilauryl glutarate, diundecyl adipate, dilauryl azelate, di-n-nonyl
sebacate, 1,18-
dineopentyloctadecylmethylenedicarboxylate, ethylene glycol dimyristate,
propylene glycol
dilaurate, propylene glycol distearate, hexylene glycol dipalmitate, 1,5-
pentanediol
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dimyristate, 1,2,6-hexanetriol trimyristate, 1,4-cyclohexanediol didecanoate,
1,4-
cyclohexanedimethanol dimyristate, xylene glycol dicaprate, and xylene glycol
distearate.
[0045] Without limitatoin, ester solvents may be selected from
esters of a saturated
fatty acid with a branched aliphatic alcohol, esters of an unsaturated fatty
acid or a saturated
fatty acid having one or more branches or substituents with an aliphatic
alcohol having one or
more branches or 16 or more carbon atoms, cetyl butyrate, stearyl butyrate,
and behenyl
butyrate including 2-ethylhexyl butyrate, 2-ethylhexyl behenate, 2-ethylhexyl
myristate, 2-
ethylhexyl caprate, 3,5,5-trimethylhexyl laurate, 3,5,5-trimethylhexyl
palmitate, 3,5,5-
trimethylhexyl stearate, 2-methylbutyl caproate, 2-methylbutyl caprylate, 2-
methylbutyl
caprate, 1-ethylpropyl palmitate, 1-ethylpropyl stearate, 1-ethylpropyl
behenate, 1-ethylhexyl
laurate, 1-ethylhexyl myristate, 1-ethylhexyl palmitate, 2-methylpentyl
caproate, 2-
methylpentyl caprylate, 2-methylpentyl caprate, 2-methylpentyl laurate, 2-
methylbutyl
stearate, 2-methylbutyl stearate, 3-methylbutyl stearate, 2-methylheptyl
stearate, 2-
methylbutyl behenate, 3-methylbutyl behenate, 1-methylheptyl stearate, 1-
methylheptyl
behenate, 1-ethylpentyl caproate, 1-ethylpentyl palmitate, 1-methylpropyl
stearate, 1-
methyloctyl stearate, 1-methylhexyl stearate, 1,1dimethylpropyl laurate, 1-
methylpentyl
caprate, 2-methylhexyl palmitate, 2-methylhexyl stearate, 2-methylhexyl
behenate, 3,7-
dimethyloctyl laurate, 3,7-dimethyloctyl myristate, 3,7-dimethyloctyl
palmitate, 3,7-
dimethyloctyl stearate, 3,7-dimethyloctyl behenate, stearyl oleate, behenyl
oleate, stearyl
linoleate, behenyl linoleate, 3,7-dimethyloctyl erucate, stearyl erucate,
isostearyl erucate,
cetyl isostearate, stearyl isostearate, 2-methylpentyl 12-hydroxystearate, 2-
ethylhexyl 18-
bromostearate, isostearyl 2-ketomyristate, 2-ethylhexy1-2-fluoromyristate,
cetyl butyrate,
stearyl butyrate, and behenyl butyrate.
[0046] Examples of the alcohol solvents include, without limitation,
monohydric
aliphatic saturated alcohols such as decyl alcohol, undecyl alcohol, dodecyl
alcohol, tridecyl
alcohol, tetradecyl alcohol, pentadecyl alcohol, hexadecyl alcohol, heptadecyl
alcohol,
octadecyl alcohol, eicosyl alcohol, behenyl alcohol and docosyl alcohol;
aliphatic unsaturated
alcohols such as ally' alcohol and oleyl alcohol, alicyclic alcohols such as
cyclopentanol,
cyclohexanol, cyclooctanol, cyclododecanol, and 4-tert-butylcyclohexanol;
aromatic alcohols
such as 4-methylbenzyl alcohol and benzhydrol; and polyhydric alcohols such as
polyethylene glycol. Examples of the acid amides include, but are not limited
to: acetamide,
propionamide, butyramide, capronamide, caprylamide, capric amide, lauramide,
myristamide,
palmitamide, stearamide, behenamide, oleamide, erucamide, benzamide,
capronanilide,
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caprylanilide, capric anilide, lauranilide, myristanilide, palmitanilide,
stearanilide,
behenanilide, oleanilide, erucanilide, N-methylcapronamide, N-
methylcaprylamide, N-methyl
(capric amide), N-methyllauramide, N-methylmyristamide, N-methylpalmitamide, N-
methylstearamide, N-methylbehenamide, N-methyloleamide, N-methylerucamide, N-
ethyllauramide, N-ethylmyristamide, N-ethylpalmitamide, N-ethylstearamide, N-
ethyloleamide, N-butyllauramide, N-butylmyristamide, N-butylpalmitamide, N-
butylstearamide, N-butyloleamide, N-octyllauramide, N-octylmyristamide, N-
octylpalmitamide, N-octylstearamide, N-octyloleamide, N-dodecyllauramide, N-
dodecylmyristamide, N-dodecylpalmitamide, N-dodecylstearamide, N-
dodecyloleamide,
dilauroylamine, dimyristoylamine, dipalmitoylamine, distearoylamine,
dioleoylamine,
trilauroylamine, trimyristoylamine, tripalmitoylamine, tristearoylamine,
trioleoylamine,
succinamide, adipamide, glutaramide, malonamide, azelamide, maleamide, N-
methylsuccinamide, N-methyladip amide, N-methylglutaramide, N-
methylmalonamide, N-
methylazelamide, N-ethylsuccinamide, N-ethyladipamide, N-ethylglutaramide, N-
ethylmalonamide, N-ethylazelamide, N-butylsuccinamide, N-butyladipamide, N-
butylglutaramide, N-butylmalonamide, N-octyladipamide, and N-dodecyladipamide.
[0047] To our surprise, among these solvents, it has been
discovered that certain
solvents reduce the hysteresis window. The solvent may be material combined
with the
thermochromic system, for example, to reduce thermal separation across the
hysteresis
window to a level demonstrating 80%, 70%, 50%, 40%, 30% or less of the thermal
separation
that would exist if the co-solvent were not present. The co-solvent is
selected from the group
consisting of derivatives of mysristic acid, derivatives of behenyl acid,
derivatives of
palmytic acid and combinations thereof. Generally, these materials include
myristates,
palmitates, behenates, together with myristyl, stearyl, and behenyl materials
and certain
alcohols, hi one aspect, these materials are preferably solvents and co-
solvents from the
group including isopropyl myristate, isopropyl palmitate, methyl palmitate,
methyl stearate,
myristyl myristate, cetyl alcohol, stearyl alcohol, behenyl alcohol, stearyl
behenate, and
stearamide. These co-solvents are added to the encapsulated thermochromic
system in an
amount that, for example, ranges from 9% to 18% by weight of the thermochromic
system as
encapsulated, i.e., excluding the weight of the capsule. This range is more
preferably from
about 12% to about 15% by weight.
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Light Stabilizers
[0048] In other instances, additives used to fortify the
encapsulated thermochromic
systems by imparting a resistance to degradation by ultraviolet light by have
a dual
functionality of also reducing the with of separation over the hysteresis
window. Light
stabilizers are additives which prevent degradation of a product due to
exposure to ultraviolet
radiation. These compounds may include blocked phenols, singlet oxygen
quenchers,
UVA/B absorbers, borotriazoles, and hindered amino light stabilizers (HALS).
Specific
examples of light stabilizers used in thermochromic systems of the present
disclosure and
which may also influence the hysteresis window include but are not limited to:
avobenzone,
bisdisulizole disodium , diethylaminohydroxybenzoyl hexyl benzoate, Ecamsule,
methyl
anthranilate, 4-aminobenzoic acid, Cinoxate, ethylhexyl triazone, homosalate,
4-
methylbenzylidene camphor, octyl methoxycirmamate, octyl salicylate, Padimate
0,
phenylbenzimidazole sulfonic acid, polysilicone-15, trolamine salicylate,
bemotrizinol,
benzophenones 1-12, dioxybenzone, drometrizole trisiloxane, iscotrizinol,
octocrylene,
tetrakis-(methylene-(3,5-di-(tert)-butyl-4-hydrocinnamate)) methane,
oxybenzone,
sulisobenzone , bisoctrizole, titanium dioxide, zinc oxide, and sterically
hindrered phenols
such as pentaerythritol tetralds(3-(3,5-di-tert-butyl-4-hydroxyphenyl)
propionate sold as
Irganox 1010 by Ciba Specialty Chemicals Inc. of Tarrytown New York.
Encapsulation Process
[0049] Nearly all thermochromic systems require encapsulation for protection.
As
is known in the art, the most common process for encapsulation is interfacial
polymerization,
although encapsulation is not limited to interfacial polymerization. . During
interfacial
polymerization the internal phase (material inside the capsule), external
phase (wall material
of the capsule) and water are combined under homogenization through high-speed
mixing.
By controlling all the temperature, pH, concentrations, and mixing speed
precisely, the
external phase will surround the internal phase droplet while crosslinking
with itself. Usually
the capsules are between 3-5 gm or smaller. Such small sizes of capsules are
referred to as
microcapsules and the thermochromic system within the microcapsules are
microencapsulated. Microencapsulation allows thermochromic systems to be used
in wide
range of materials and products. The size of the microcapsules requires some
adjustments to
suit particular printing and manufacturing processes.
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[0050] The size distribution of preferred microcapsules can range from as much
as
0.2gm to 100 gm. Further example techniques of physical microencapsulation
include but
are not limited to pan coating, air suspension coating, centrifugal extrusion,
vibration nozzle,
and spray drying. Examples of chemical microencapsulation techniques include
but are not
limited to interfacial polymerization, in-situ polymerization, and matrix
polymerization.
Example polymers used in the preferred chemical microencapsulation include but
are not
limited to polyester, polyurethane, polyureas, urea-formaldehyde, epoxy,
melamine-
formaldehyde, polyethylene, polyisocyanates, polystyrene, polyamides, and
polysilanes.
[0051] The capsule isolates the thermochromic system from the environment, but
the barrier that the capsule provides is itself soluble to certain solvents.
Therefore, the
microcapsule constituents interact with the environment to some extent. The
solubility
parameter describes how much a material will swell in the presence of
different solvents.
This swelling will directly impact the characteristics of the reaction
potential within the
capsule, as well as potentially making the capsule more permeable, both of
which will likely
adversely affect the thermochromic system. Solvents in which the microcapsules
are exposed
to are chosen so as not to destroy, or affect, the thermochromic system
within.
[0052] The capsule is hard, thermally stable and relatively
impermeable. The
infiltration of compounds through the capsule are stopped or slowed to the
point that the
characteristics of the dye are not affected. The pollution of the
thermochromic system within
the capsule by solvents from the environment affects the shelf life of the
thermochromic
system. Therefore, the formulation of the applied thermochromic system, as an
ink for
example, should be carefully considered.
[0053] In an embodiment of the present disclosure, capsules are made from
melamine formaldehyde. One technique used to produce the encapsulated
thermochromic
systems is to combine water, dye, oil, and melamine formaldehyde and agitate
to create a
very fine emulsification. Because of the properties of the compounds, the oil
and dye end up
on the inside of the capsule and the water ends up on the outside, with the
melamine
formaldehyde making up the capsule itself. The capsule can then be thermo-set,
similar to
other resins, such as formica. The thermo-set substance is very hard and will
not break down,
even at temperatures higher than the encapsulated thermochromic system is
designed to be
exposed to. The melamine formaldehyde capsule is almost entirely insoluble in
most
solvents, but it is permeable to certain solvents that might destroy the
ability of the
thermochromic system to color and decolorize throughout a temperature range.
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[0054] The extent to which capsules will react with their environment is
influenced
by the pH of the surrounding medium, the permeability of the capsule, the
polarity and
reactivity of compounds in the medium, and the solubility of the capsule.
Preferred media
used in formulating encapsulated thermochromic system are engineered to reduce
the
reactivity between that medium and the capsules to a low enough level that the
reactivity will
not influence the characteristics of the dye for an extended period of time.
[0055] Highly polar solvent molecules, with the exception of water,
often interact
more with the leuco dye than with the capsule shell and other non-polar
molecules of the
thermochromic system. Therefore, polar solvents that are able to cross the
capsule barrier
should, in general, be eliminated from the medium within which the
encapsulated
thermochromic system is formulated.
[0056] Ideally, aqueous media that the encapsulated thermochromic systems are
placed within should have a narrow pH range from about 6.5 to about 7.5. When
an
encapsulated thermochromic system is added to a formulation that has a pH
outside this
range, often the thermochromic properties of the system are destroyed. This is
an irreversible
effect.
[0057] One aspect of the present disclosure is for a method of improving the
formulations of the thermochromic system by removing any aldehydes, ketones,
and diols
and replacing them with solvents which do not adversely effect the
thermochromic system.
Solvents having a large molecular weight (i.e. greater than 100) generally are
compatible
with the thermochromic systems. The acid content of the system is preferably
adjusted to an
acid number below 20 or preferably adjusted to be neutral, about 6.5 - 7.5.
Implementing
these solvent parameters for use in the thermochromic system will preserve the
reversible
coloration ability of the leuco dyes.
[0058] Formulations for thermochromic systems are engineered with all the
considerations previously mentioned. The examples below describe a
thermochromic system
with excellent color density, low residual color, narrow temperature ranges
between full color
and clearing point, and a narrow hysteresis window. The full color point and
the clearing
point are determined by visual inspection of the thermochromic system at a
range of
temperatures. The difference in temperature between the maxima of color change
during the
cooling cycle and the heating cycle is used to calculate hysteresis.
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Adjusting the Acid Content
[0059] Water-base inks are pH adjusted prior to addition of thermochromic
pigment. As mentioned above, the pH should be neutral unless observation
indicates that a
different pH is required. To achieve the correct pH, one uses a good proton
donor or
acceptor, depending on whether the pH is to be adjusted up or down. To lower
the pH, HC1 is
used, to raise it, the best proton acceptor so far is KOH. These two chemicals
are very
effective and do not seem to impart undesirable characteristics to the medium.
(In other
words, K+ and Cl- do not seem to harm the thermochromic pigment.) Use pH paper
to
determine the pH. Remember not to add pigment before the pH and all other
characteristics
for that matter are correct in the ink itself. The most effective pH has been
7.0, however,
some tolerance has been noted between 6.0 and 8Ø A pH below 6.0 and above
8.0 frequently
destroys the pigment
[0060] The acid value is defined as the number of milligrams of a 0.1 N KOH
solution required to neutralize the alkali reactive groups in 1 gram of
material under the
conditions of ASTM Test Method D-1639-70. It is not yet fully understood how
non-aqueous
substances containing acid affect the thermochromic, but high acid number
substances have
inactivated the thermochromic pigments. Generally, the lower the acid number
the better. To
date ink formulations with an acid value below 20 and not including the
harmful solvents
described above have worked well. Some higher acid value formulations may be
possible but
generally it is best to use vehicle ingredients with low acid numbers or to
adjust the acid
value by adding an alkali substance. The greatest benefit of a neutral or low
acid value
vehicle will be increased shelf life.
[0061] Buffers have been used historically in offset ink
formulations to minimize
the effects of the fountain solution on pigment particles. This is one
possible solution to the
potential acidity problem of the varnishes. One ingredient often used as a
buffer is cream of
tartar. A dispersion of cream of tartar and linseed oil can be incorporated
into the ink. The net
effect is that the pigments in the ink are protected from the acidic fountain
solution.
Alternatively, the salt of a weak acid may be used as a buffer.
Mixing
[0062] Thermochromic leuco dyes formulated as inks are sold as a dry powder or
a
water based slurry. Mixing systems have been developed for both slurry and
powder that will
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allow for consistent and well dispersed pigment.
Drying Technique
[0063] The aqueous slurry may be used to make solvent based ink formulations
by
drying the slurry first. In traditional ink manufacturing, there is a
technique known as
flushing. Many traditional pigments come in slurry form, similar to that of
the thermochromic
capsules. "Flushing" in traditional manufacturing, means to press most of the
water out of the
slurry to form what is called a press cake which is then "flushed" into a
mixing varnish. The
press cake is about 25-40% solids. Because of the hydrophobic properties of
the pigment and
the varnish, the pigment is mixed into the varnish and away from the water.
The water
separates from the varnish and is left behind. Flushing with the thermochromic
capsules does
not work. All of the water stays in the varnish rather than separating. This
may happen
because of the water's attraction to the surface of the capsule.
Ink Formulations
[0064] The encapsulated thermochromic systems of the present disclosure may be
referred to as pigments. In an embodiment of the present disclosure, the
pigments are used in
formulating thermochromic dyes. In order to add normal pigment to ink, dye, or
lacquer, the
pigment itself is ground into the base. This disperses the pigment throughout
the base. Since
the pigment is usually a solid crystal with a diameter no larger than 1.0
microns this grinding
is not difficult to do. The eye cannot see particles that size, so the pigment
will give the base
a solid color. The addition of more pigment intensifies the color. Since the
pigment has a
very intense color only about 10% of the final ink is made up of normal
pigments.
[0065] A base for an ink formulation using encapsulated thermochromic systems
of the present disclosure may be developed using off the shelf ingredients.
The ink will
incorporate, where possible, and be compatible with different ink types and
solvents with
molecular weights larger than 100 while avoiding aldehydes, diols, ketones,
and, in general,
aromatic compounds. Important considerations with respect to the ingredients
within the ink
vehicle are the reactivity of the ingredients with the encapsulated
thermochromic system.
[0066] An example of unwanted interactions between media and the encapsulated
thermochromic systems can occur between compounds found in ink formulations.
The long
alkyl chains of many of the compounds found in ink vehicles may have reactive
portions that
can fit through the pores of the capsule and interact with the inner phase and
denature it
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through this interaction. Since the behavior of the thermochromic system is
related to the
shape and the location of its molecules at given temperatures, disrupting
these structures
could have a large impact on the characteristics of the thermochromic system.
Even
molecules that cannot fit through the capsule pores may have reactive portions
that could
protrude into the capsule and thereby influence the color transition of the
thermochromic
system within the capsule. Therefore, mineral spirits, ketones, diols, and
aldehydes are
preferably minimized in any medium in which the encapsulated are also
preferably avoided.
If these compounds are substantially reduced or eliminated the thermochromic
systems will
perform better and have a longer shelf life.
[0067] Another important step in using the encapsulated thermochromic systems
of the present disclosure in ink formulations is to adjust the pH or lower the
acid value of the
ink base before the thermochromic system is added. This can be done by
ensuring that each
individual component of the base is at the correct pH or acid value or by
simply adding a
proton donor or proton acceptor to the base itself prior to adding the
thermochromic system.
The appropriate specific pH is generally neutral, or 7Ø The pH will vary
between 6.0 and
8.0 depending on the ink type and the color and batch of the thermochromic
system.
[0068] Once a slurry and the base have been properly prepared, they are
combined.
The method of stirring should be low speed with non-metal stir blades and
other
manufactur9ng equipment known to those skilled in the art of ink making. Other
additives
may be incorporated to keep the thermochromic system suspended. The ink should
be stored
at room temperature.
[0069] Most thermochromic inks undergo a color change from a specific color to
colorless. Therefore, layers of background colors can be provided under
thermochromic
layers that will only be seen when the thermochromic layer changes to
colorless. If an
undercoat of yellow is applied to the substrate and then a layer containing
blue
thermochromic dye is applied the color will appear to change from green to
yellow, when
what is really happening is that the blue is changing to colorless.
[0070] In an embodiment of the present disclosure an encapsulated
thermochromic
system may be formulated as an ink by placing the slurry in a forced air dryer
where the
temperature is maintained at between 100 and 150 degrees F. When the slurry
reaches the
"stiff clay" stage, at about 80% to 95% solids, the slurry is removed and
incorporated into a
varnish. The varnish is mixed until smooth and the remaining ingredients are
than added to
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this mixture. This mixture is then run over the mill, making the final
product. A press cake
which has 80 to 95 percent solids does not alter the properties of the ink too
severely.
[0071] If the ink requires powder to make it, there may be a problem with
dispersion because, in the drying process, the capsules form aggregates that
are very difficult
to break up. Over stiffing or the wrong type of stirring will damage or
denature the dye. The
technique that has been developed to solve this problem is simple, effective
and inexpensive.
The first step is to add the powder to an appropriate solvent as listed above.
The solids
content of this mixture should be about the same as for the water base slurry
of about 50%
solids. Once the solvent and the powder are combined, the container with the
mixture is
submerged in an ultrasound bath. The vibration breaks up the aggregates and
also conditions
the capsule for its addition to the rest of the formulation medium.
[0072] The substrates that the thermochromic inks are printed upon should be
neutral in pH, and should not impart any chemicals to the capsule that will
have a deleterious
effect on it.
[0073] An example of a specific formulation of thermochromic systems are
provided below using the principles and techniques taught above.
[0074] An aqueous slurry of thermochromic pigment containing approximately
50% pigment solids is dried in an oven at 100-150 F to achieve a solids
concentration of
80% -95% by weight of thermochromic system. Solid levels below 80% introduce
excess
water into finished ink formulations and make it difficult to properly
disperse the
thermochromic system in the ink vehicle, and generally solids concentrations
above 90% are
preferable. Solids greater than 95% result in strong agglomeration of the
thermochromic
system particles and make dispersion difficult, however, drying to solids
concentrations up to
98% has worked. The consistency of the dried thermochromic system slurry will
vary
between that of wet clay and nearly dry kernels and flakes. This material is
then combined
with a grinding/mixing varnish formulated for the dispersion of dry
thermochromic system or
press cake, which typically is high in tack and viscosity, may contain a
significant proportion
of alkyd resin, and have an acid value not to exceed 15.
[0075] In an embodiment, thermochromic systems of the present disclosure
formulated as dyes or inks may be used for the printing of identification or
forgery detection
marks or patterns on security documents. These inks may also be used
simultaneously with
conventional printing inks and also may be used with pre-existing printers by
substitution
with one of the normally used printing inks.
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[0076] The various embodiments shown below are nonlimiting in nature, teaching
by way of example and not by limitation.
Example 1
[0077] A microencapsulated thermochromic system that is unimproved with
methods of formulation from the present disclosure has the following
composition and
thermochromic characteristics:
IP Solvent Dye(s) Developer Temp.
Modifier
Methyl Black XV BPA None
Palmitate GN-2
82% 7% 11%
Full Color Point: 12 C
Clearing Point: 26 C
Hysteresis Window: 10.1 C
[0078] Figure 2 depicts the wide (-10 C) hysteresis window of the
microencapsulated thermochromic system of example 1 and the broad color
transition range
of ¨14 C.
[0079] The thermochromic systems of the present disclosure improve upon
existing thermochromic systems by narrowing the temperature transition range
and by
decreasing the hysteresis window. The hysteresis window is narrowed at both
the full color
point and the clearing point to less than 5 C and preferably to less than 1
C.
[0080] A thermochromic dye system having an improved hysteresis window and a
narrower color transition range between the full color point and the clearing
point can be
made by adding a co-solvent into a known thermochromic system. The co-solvent
can be
added in amounts varying from about 12% to about 15% of the weight of the
thermochromic
system, excluding the capsule. The solvent can be chosen from the following
list of solvents
including singly or in combination: isopropyl myristate, isopropyl palmitate,
methyl
palmitate, methyl stearate, myristyl, myristate, cetyl alcohol, stearyl
alcohol, behenyl alcohol,
stearyl behenate, and stearamide. In another embodiment of the present
disclosure, a more
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preferred co-solvent list consists of stearamide, behenyl alcohol and stearyl
behenate from
about 12% to about 15% by weight.
[0081] The co-solvent should also have the attribute of having a
long alkyl chain,
preferably between 12 and 24 carbons as well as having a melting point that is
about 70 C to
about 200 C above the other co-solvent of the thermochromic system. Example 2
shows a
preferred embodiment of the present disclosure using behenyl alcohol as both a
temperature
modifier and co-solvent. The co-solvent system has an improved hysteresis
window and a
narrower color transition range between the full color point and the clearing
point over
Example 1, a pre-existing thermochromic system.
Example 2
[0082] A microencapsulated thermochromic system that is improved with methods
of and formulations from the present disclosure has the following composition
and
thermochromic characteristics:
IP Solvent Dye(s) Developer(s) Temp.
Modifier
Methyl Black XV, BPA Behenyl
Palmitate GN-2 alcohol
69.7% 7% 11% 12.3%
Full Color Point: 16 C
Clearing Point: 23.5 C
Hysteresis Window: 3.6 C
[0083] Figure 3 shows that the hysteresis window of the microencapsulated
thermochromic system of example 2 has been narrowed by increasing the full
color point 4
C and decreasing the clearing point 2.5 C so that the color transition range
is ¨7 C. The
hysteresis window is now less than 5 C.
[0084] Accordingly, it is to be understood that the embodiments of
the disclosure
herein described are merely illustrative of the application of the principles
of the disclosure.
Reference herein to details of the illustrated embodiments is not intended to
limit the scope of
the claims, which themselves recite those features regarded as essential to
the disclosure.
