Note: Descriptions are shown in the official language in which they were submitted.
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Description
Hot Melt Inks for Colored Ink Jet Imaqes
Technical Field
This invention relates to hot melt inks for pro-
S ducing colored images by ink jet printing and, moreparticularly, to new and improved colored hot melt
inks providing improved image quality.
Backqround Art
To produce black-and-white images by projection,
it is only necessary for the transparency to attenuate
or prevent light from being projected to the screen in
the appropriate areas. For color projection images,
however, the ink must block or attenuate only selected
wavelengths of the light and must permit the other
wavelengths to be projected to the screen. ~here
water-based inks are utilized, the image on the trans-
parency is substantially flat so that the only factor
affecting the transmission is absorption of the appro-
priate colors of light. Where hot melt inks are used,
however, the ink drops form hemispherical lenslets
which, as described in the Fulton et al. Patent No
4,873,134, interfere with the proper reproduction of ~ lt! r~/2
projection images from a color transparency. As de- CG~
scribed in that patent, this problem may be overcome
by causing the ink drops to spread on the substrate to
reduce the curvature of the lenslets.
There are, however, additional problems encoun-
tered in the formation of color images with hot melt
inks. For example, in order to assure that all of the
ink drops are formed and projected in substantially
the same way, the various colored inks should have
certain similar physical properties and characteris-
tics even though they are not comprised of identical
components in identical proportions. For optimum
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quality, all of the colored inks must be substantially
transparent at the appropriate wavelengths even though
some of the inks may contain pigment while others
contain dye, and the vehicle for the ink should be
substantially colorless.
One problem encountered in hot melt inks is the
tendency for the ink vehicle to become substantially
crystalline upon solidification since a significant
crystalline content will interfere with the transpar-
ency of the ink. On the other hand, it is desirablethat the ink vehicle soften at as high a temperature
as possible so that it is not subject to softening in
extreme ambient conditions such as the trunk of an
automobile in summer or the platen of an overhead
projector. In contrast, however, it is also desirable
that hot melt inks melt at a temperature which is
sufficiently below the jetting temperature that the
inks have a relatively low viscosity, typically in the
range of 10-30 centipoise, at the temperature of ap-
plication. If the application temperature is too
high, such as above about 140C, the printhead mate-
rials will be subjected to undue thermal stresses and
the inks may age quickly. Moreover, as described in
the copending application of Spehrley, No. 07/202,488, ~ ,
filed June 3, 1988, the rheology of the hot melt inks !1i.,
above the melting point should be controlled so as to ~ '7iC~
facilitate good spreading of the ink drops on or into
the substrate.
Furthermore, when different colored ink drops are
maintained above their melting points on a substrate
to permit spreading in accordance with Patent No.
4,873,134, it is important that the rate of spreading
of the different colored drops be substantially uni-
form in order to assure not only the desired coverage
of different inks on the substrate, but to avoid unde-
sired intermixing of different-color drops, which can
cause severe image ~uality degradation.
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Finally, solid hot melt inks must have a compli-
ance which is compatible with that of the substrate on
which they are printed so that, when the substrate is
folded or bent, the inks will not crack or separate
frsm the substrate.
Disclosure of Invention
Accordingly, it is an object of the present in-
vention to provide new and improved hot melt color
inks which overcome the above-mentioned disadvantages
of the prior art.
Another object of the invention is to provide a
set of hot melt color inks having substantially
matched characteristics.
A further object of the invention is to provide
hot melt color ink images of high quality.
These and other objects of the invention are
attained by providing hot melt inks having relatively
narrow melting ranges, i.e., differences between
liquidus and solidus temperatures, while at the same
time having low crystallinity when solidified by rapid
cooling. Moreover, a set of different colored hot
melt inks according to the invention have surface
tensions which vary by no more than about 3 dynes per
centimeter, and preferably no more than 2 dynes per
centimeter at the temperature at which they are held
for spreading on the substrate. Finally, each of the
different colored hot melt inks of the invention has
an elongation of at least 3%, and preferably about 5-
10%, to provide a compliance or flexibility at room
temperature which permits bending of a substrate con-
taining the ink to a radius of 0.10 to about 0.03 inch
(2.54 to about 0.76mm) without causing fracture or
flaking of the ink.
Brief Description of Drawinas
Further objects and advantages of the invention
will be apparent from a reading of the following de-
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scription in conjunction with the accompanying draw-
ings in which:
Fig. lA is a graphical representation illustrat-
ing a differential scanning calorimeter curve of ab-
sorbed energy per degree versus temperature for a hotmelt ink having desired characteristics;
Fig. lB is a graphical representation showing
variation of viscosity with temperature for the ink of
Fig. lA;
Fig. 2A is a graphical representation of a dif-
ferential scanning calorimeter curve of absorbed en-
ergy per degree versus temperature for an ink showing
high crystallinity;
Fig. 2B is a graphical representation of the
viscosity versus temperature characteristic of the ink
of Fig. 2A;
Fig. 3A is a graphical representation showing a
differential scanning calorimeter curve for an ink
having relatively amorphous characteristics with low
crystallinity;
Fig. 3B is a graphical representation of the
variation of viscosity with temperature for the ink of
Fig. 3A;
Fig. 4A is a schematic illustration showing the
light transmission characteristics of an ink drop on a
transparent substrate;
Fig. 4B is a graphical representation showing the
attenuation of light as a function of thickness of ink
for an ink of different crystallinity;
Figs. 5A-SD illustrate successive stages in the
spreading of hot melt ink drops maintained on a sub-
strate at a temperature above their melting point;
Fig. 5E is an enlarged view illustrating the
overlap of two adjacent drops before spreading on the
substrate;
Figs. 6A-6E are schematic illustrations showing
the spreading and overlap of adjacent drops maintained
above their melting point on a substrate;
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Figs. 7A and 7B are enlarged views similar to
Figs. 6A and 6B illustrating the elimination of over-
lap of adjacent drops upon spreading on a substrate;
and
Fig. 8 is a fragmentary cross-sectional illustra-
tion showing an ink image on a transparent polyester
substrate;
Fig. 9 is a f ragmentary view showing the ink and
substrate of Fig. 8 when bent to provide a curvature,
Fig. 10 is a graphical representation showing the
maximum shear stress against the bending radius for
ink on a substrate of the type shown in Fig. 8;
Fig. 11 is an enlarged fragmentary view illus-
trating the contact angle at the edge of an ink region
on a substrate; and
Fig. 12 is an enlarged fragmentary view showing
an ink layer on a substrate subjected to bending in
which the ink layer has become torn to create a sharp~
edge.
Best Mode for Carryina Out the Invention
The melting and rheological characteristics of a
hot melt ink may be understood by reference to two
graphical relationships, i.e., the differential scan-
ning calorimeter ("DSC") curve of absorbed energy per
degree versus temperature and the curve of viscosity
versus temperature. In Fig. lA, the curve 1 illus-
trates representative DSC data for a hot melt ink of
the type which normally has moderate crystallinity.
Conventionally, this curve is simplified by fitting
the data with four straight lines, a line 2a which
indicates the change in specific heat with temperature
below the melting range, a line 2b which indicates the
change in specific heat with temperature above the
melting range, a line 3 which indicates the change of
specific heat with temperature from the maximum TM to
the liquidus temperature TL where the line 3 inter-
sects the line 2b, and a line 4 representing the
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change of specific heat with temperature between the
maximum temperature TM and the solidus temperature TS
at which the line 4 intersects the line 2a. The en-
tire range from TS to TL is designated as the melting
range, and the melting point TM is represented as the
temperature at the intersection of the lines 3 and 4.
While all hot melt inks soften with increasing
temperature as shown by the curve in Fig. lA, the
typical hot melt ink softens very rapidly above the
solidus temperature TS up to the melting point TM and,
above the temperature TM, there is a liquid fraction
but the ink may not pour or have a viscosity of the
conventional type until the temperature is substanti-
ally above TM. At temperatures above the liquidus
temperature TL, the ink is completely liquid (except
for pigments) and has a viscosity versus temperature
relationship represented by the line 6 in Fig. lB.
Hot melt inks are jetted at a temperature TJ, a
temperature which is substantially above the liquidus
temperature, where the viscosity of the ink is in the
range of, for example, 10-30cps and the slope of the
viscosity line 6 is low. When hot melt inks deposited
on a substrate are cooled, the viscosity of the inks
increases slowly in the direction of decreasing tem-
perature along the line 6 and then more rapidly alonga line 7 and finally reaches a point where the viscos-
ity is unmeasurable by conventional techniques in a
region designated 8.
As described in the copending application of
Spehrley, No. 07/202,488, filed June 3, 1988, once the
viscosity of a hot melt ink has reached a level above
about 200cps upon cooling, the ink spreads on or pene-
trates a substrate at such a low rate as to be essen-
tially immobile. This spread-limiting temperature is
designated TSL in Fig. lB, and the difference between
the temperature TSL and the jetting temperature TJ,
denoted ~T and called the spreading temperature,
determines the enthalpy available in each ink droplet
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to provide heat so as to cause self-spreading of the
ink on or into the substrate. Since it is desirable
for the ink to spread to a relatively large extent on
or into the substrate, it is important to have a large
~T between the jetting and the spread-limiting temper-
ature, and for the viscosity to remain as low as pos-
sible throughout this temperature range. Because of
printhead material limitations, however, it is not
practical to achieve such large ~T by increasing
printhead temperatures significantly above about
125C.
In order to obtain the highest softening point
and the largest spread between the jetting temperature
TJ and the temperature TSL at which spread-limiting `
viscosity is reached, an ink having a DSC curve of the
type illustrated in Fig. 2A would be desirable. Such
inks have narrow melting ranges, i.e., TL ~ TS~ but
they tend to be more crystalline and therefore less
transparent after solidification than inks having a
broader melting range, while the energy available to
spread the ink as indicated by ~Tl in Fig. lB is
larger than that of the ink shown in Fig. lB.
Hot melt inks which are amorphous in character
and therefore more transparent after solidification
have a broad melting range of the type illustrated in
Fig. 3A. Such inks start to soften at a relatively
low temperature, making them less durable at mildly
elevated temperatures. Moreover, the jetting tempera-
tures for such inks may have to be increased to levels
above the maximum desirable printhead temperatures in
order to obtain a low enough jetting viscosity. In
addition, the enthalpy available in each drop for
self-heated spreading, represented by ~T2, is normally
significantly less for inks of this type than for the
inks of Figs. lA and 2A.
From the foregoing, it should be apparent that,
considering the combined aspects of ink durability,
reasonable jetting temperature and enthalpy available
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for self-spreading of ink, a very narrow melting range
ink material is desirable. For example, a hot melt
ink having a melting point of at least 75C, a melting
range of no more than about 40C, and a spreading
temperature range of at least 30C is preferable.
However, such narrow-range materials are normally more
highly crystalline than broader melting range inks and
crystallinity renders the inks unsuitable for use in
transparencies because of the poor ink transparency in
the solid state.
Crystallinity of solidified hot melt inks results
in two detrimental effects, i.e., surface losses and
bulk transmission losses. Microscopic examination of
a crystallized hot me]t ink drop shows a surface which
looks "frosty" and which produces significant scatter-
ing of light passing through the drop. Internal crys-
tallinity reduces the bulk transmission coefficient of
the drop by causing scattering of light within the
interior of the drop.
The importance of reducing such detrimental ef-
fects of crystallinity in inks will be understood by
reference to the schematic illustration of Fig. 4A
which shows a cross-section of a transparency with an
ink drop illuminated with light normal to the surface
of the transparency. The ink drop has a small contact
angle with the surface of the substrate as a result of
having been maintained above its melting point in the
manner described in Patent No. 4,873,134, and, because
of the small contact angle, it will be assumed in the
following discussion that light is transmitted in
substantially straight lines through the drop rather
than being deviated by refraction of the surface of
the drop.
The li~ht Io which is incident on the portion of
the transparency substrate having no ink drop is
transmitted with some intensity reduction due to index
of refraction mismatch to an observer but without any
variation in the spectral content of the light. In
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other words, the light Il will be perceived as "white"
by the observer. In contrast, the light passing
throuqh the ink drop is selectively attenuated, the
attenuation being a function of both the thickness of
the ink and the absorption of light of different
colors x,y,z by the ink. The light is also attenuated
by scattering IS as a result of any "frosting" on the
surface of the ink drop and crystallinity within the
drop. The ratio of the intensity I2 of the resulting
light beam passing through the drop to that of a light
beam Il which does not intersect the drop is a measure
of the optical properties of the ink drop. Each of
these attenuation mechanisms is explained in more
detail hereinafter.
Fig. 4B illustrates the measured attenuation of
light of a specific wavelength or color as a function
of thickness. Lambert's Law predicts that the loga-
rithm of the attenuation will be linear with thickness
and this is generally true for the low colorant load-
ings typical of transparent hot melt inks. The three
diagonal lines 10, 11 and 12 shown in Fig. 4B repre-
sent three different ink conditions. The lowest line
10 represents an ink that was maintained above its
melting point for a period of time and then caused to
cool slowly with a cooling or "quenching" rate on the
order of 0.02C per second. The middle line 11 and
the upper line 12 show that the attenuation of ink is
reduced by increasing the rate of quenching of the
ink, the middle line 11 indicating the attenuation
when the ink is cooled at a rate of 20C/sec. and the
upper line 12 indicating the attenuation when the ink
is quenched, for example, in ice water at a rate of
40,000C/sec. For high-quality color reproductions,
the attenuation of the ink at the desired wavelengths
should be less than about 25%, and preferably less
than about 10%, which can be achieved based on the
illustration in Fig. 4B if the ink is quenched at a
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rate of at least 500C/sec., and preferably at least
1,000C/sec.
By extrapolating the lines 10, 11 and 12 on Fig.
4B back to zero thickness, 1.e., the ordinate axis,
the respective intersections 13, 14 and 15 indicate
that there is some attenuation not dependent on the
thickness of the drop, which corresponds to the sur-
face losses resulting from a "frosty" crystallinity on
the surface rather than the bulk losses resulting from
crystallinity within the drop.
In order to provide a hot melt ink which has the
lowest possible attenuation at high quenching rates,
the ink vehicle should include a constituent which
tends to prevent the molecular chains in the vehicle
from moving into their preferred crystalline orienta-
tion during the quenching period. For this purpose,
about 1-25 weight percent of a vehicle crystallinity
modifier such as a branching pclymer or resin or a
crosslinked polymer or resin is added to the ink ve-
hicle. Alternatively, the vehicle crystallinity modi-
fier may be a small amount of a polymer or resin which
is greatly mismatched (l.e., by 50-100 times) in size
or effective volume with respect to the basic vehicle
material. Such a relatively small proportion of modi-
fiers such as branching polymers or crosslinked poly-
mers or mismatched material need not significantly
alter the DSC curve of the vehicle, but the presence
of high molecular weight polyamides such as those
having a molecular weight above 10,000 which are known
to be viscosity modifiers should be avoiaed.
Typical vehicle crystallinity modifiers added for
the purpose of providing reduced attenuation upon
rapid cooling or quenching include ester-modified
montan waxes, i.e~,
RCO2-(CH2)n-02C-R'
in which R' and R are alkyl chains with 25-35 carbon
atoms and n is an integer. Such materials are de-
scribed, for example, in Patent No. 4,851,045, but
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that patent does not suggest the use of such materials
for reducing crystallinity.
In addition, the tendency of the vehicle to crys-
talli2e may also be reduced by the addition of petro-
S leum ceresin waxes, as described, for example, at page252 in The Chemistry and Technoloqy of Waxes by A.H.
Warth (1947~. Also, based on the discussion at page
246 of Warth, it should be possible to use microcrys-
talline waxes such as the "pale yellow" microcrys-
talline wax melting at 85-87C to improve quenching so
as to reduce the light attenuation of rapidly quenched
hot melt inks resulting from crystallization. Other
materials discussed in Warth and in Bennett, Indus-
trial Waxes, Vol. I (1975), pp. 281-283, include ke-
tones such as stearone and laurone, which have a spa-
tial configuration similar to that of the microcrys-
talline waxes and which do not deteriorate on storage.
Another characteristic of hot melt inks which is^
necessary to provide high-quality color hot melt ink
images is the ability of the ink to spread to the
desired extent while it is maintained above its melt-
ing point, as described in Patent No. 4,873,134, with-
out causing interference with adjacent ink drops, for
example, by bleeding of one color ink drop into an
adjacent drop of a different color.
In Figs. 5A-5D, a series of illustrations shows
the sequence of hot melt ink drops 100 jetted onto a
nonporous substrate, solidified, and then later main-
tained above the melting point for a short period of
time. In Fig. 5A, the drops are illustrated within a
few milliseconds of application to the substrate. In
Fig. 5B, the condition of the drops within a few hun-
dred milliseconds is shown, indicating that the first
drops 100 have partially spread and solidified and
that a second set of drops 101 is deposited adjacent
to the first set of drops. In Fig. 5C, the condition
of all of the drops after several hundred milliseconds
when all of the drops have solidified is shown. In
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this illustration, an isolated droplet 101 has solid-
ified with a large contact angle which will cause
undesired refraction of the light passing through the
drop.
In Fig. 5D, the condition of the ink drops is
shown after they have been maintained above their
melting point for a selected time period in accordance
with Patent No. 4,873,134 to reduce the contact angle
of the drops with the substrate. According to a pre-
ferred embodiment, a 95-picoliter drop will have
spread to produce a drop diameter of about fl-9 mils
(0.2n-0.23mm) with a contact angle of about 5-7 after
being maintained for about 3 seconds at a temperature
of about 100C, which is about 10C above the liquidus
temperature of the ink, and for single drops, the
spread diameter is relatively insensitive to surface
tension in the range 27-30 dynes per centimeter. Al-
though the droplet has not spread to its equilibrium
diameter after that period of time, the rate of ink
spread is very slow and, if it were maintained at that
temperature for 30 seconds, the diameter would in-
crease only a few percent more and after several min-
utes at that temperature, there would be little fur-
ther noticeable spreading.
With a conventional polyester substrate having a
surface tension of about 35 dynes per centimeter and a
hot melt ink having a surface tension of about 27-30
dynes per centimeter, the average rate of spreading of
a drop of molten ink on the substrate during the first
three seconds is about 1 mil per second. Surpris-
ingly, however, the spreading velocity of ink at the
intersection of two coalesced groups of ink drops,
such as the fields 110 and 120 in Fig. SD, can be 10-
20 times greater than the spreading velocity of a
single drop. As a result, whereas a single drop will
spread within a desired maximum range while held at a
temperature above its melting point for a short period
such as 3 seconds, two adjacent ink fields 110 and 120
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may spread into each other to a much greater extent,
such as 10-25 mils (0.25-0.63mml or more, during the
same period of time, causing undesired bleeding and
intermixing of different colored inks.
There appear to be at least two sets of phenomena
which must be controlled to prevent such objectionable
intermixing at the boundaries of color fields. Inter-
mixing due to diffusion-like phenomena is one phenome-
non, and is related to miscibility and chemical con-
centration differences, and perhaps to pigment sizes.
Such diffusion-like intermixing appears to be limited
to dimensions on the order of 1-2 pixels (3-6 mils
(0.076-0.15mm)) during the times of interest (i.e.,
about 3 seconds) and are much less objectionable in
projection images than the second type.
The second type of intermixing occurs to a much
greater extent (about 10-25 mils (0.25-0.63mm)) and is
caused by surface tension mismatch effects, which
provide the driving force to create ink flows, the
resistance to such flow being produced by viscous
resistance which is a strong function of fluid thick-
ness, as will be further described.
Fig. 6A shows an enlarged view of the region 103
of Fig. 5C when the ink has not been maintained above
its melting point for a selected period of time such
as 3 seconds. In this case, ink from the coalesced
field 120, which has a surface tension of about 30
dynes per centimeter, is illustrated at the instant of
melting along with ink from an adjacent field 110
having a lower surface tension such as 25 dynes per
centimeter. Both fields are printed on a polyester
substrate which typically has a surface energy of
about 35 dynes per centimeter. In Fig. 6B, which
illustrates the condition after the inks have been
maintained above their melting point for a few hundred
milliseconds, it will be seen that the ink from the
field 110, which has a lower surface tension, wets and
flows over the ink from the field 120 having the
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higher relative surface tension. Fig. 6C shows the
overlap condition after the inks have been maintained
above their melting point for a longer period of time,
producing substantial intermixing of the inks which is
S detrimental to the image quality. Although these
figures illustrate the intermixing as being substanti-
ally two-dimensional, there are important three-dimen-
sional effects. In particular, the lower-surface-
tension ink 110 tends to be drawn more quickly into
the interstices between original droplets where the
curvatures are greater.
Prior efforts to prevent such bleeding and inter-
mixing of adjacent fields of molten ink have not been
successful. For example, orienting the substrate in
such a way that gravity inhibits flow of the low-
surface-tension ink in the direction toward the
higher-surface-tension ink has not avoided the prob-
lem.
In accordance with the invention, however, sur-
face tension modifiers are utilized so as to keep thesurface tensions of all of the inks applied to the
same substrate within less than 3 dynes per centimeter
and, preferably, within less than 1-2 dynes per centi-
meter at the temperature at which they are maintained
above their melting point for a selected period of
time. The result of such matching of surface tensions
is shown in Figs. 7A and 7B. Fig. 7A illustrates the
initial interface between two adjacent ink drops ap-
plied in succession, a drop 110 being applied after a
drop 120. Both inks have a surface tension of 30
dynes per centimeter and the substrate has a surface
energy of 35 dynes per centimeter. After being main-
tained above their melting point for a few seconds,
the drops have an interface in the region 113 which
may include some intermixing due to diffusion, but
which does not vary significantly from the original
interface, and neither drop has spread over the other
drop so as to cause undesired intermixing of the inks.
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Surface tension modifiers are known, for example,
from the Japanese Patent Publication of Ohta, No. 55-
54368. It should be noted, however, that the surface
tension modifiers used in hot melt inks for the pur-
pose of avoiding bleed and interminglin~ of ink on asubstrate in accordance with the invention may be
different from those which are appropriate for con-
trolling drop formation during ink ejection, which
occurs in a matter of microseconds. For the purposes
of the present invention, the surface tension modi-
fiers should be effective on a time scale of a few
seconds, such as 3-10 seconds. Such surface tension
modifiers are fatty acids or salts (soaps), resin
salts, salts of long-chain sulphonic acids, salts of
long-chain alkyl ester sulphonates and the like, and
they are characterized by being fairly similar in : -
chemical nature to the materials commonly used in
successful hot melt inks and by being ionic, although
some nonionic versions of these materials may be use-
ful in certain hot melt ink formulations. Also, the
addition of paraffinic materials up to C2S materials
may be expected to lower the surface tension of the
vehicle and the molecular weight of such materials
would tend to maintain the DSC melting point and dura-
bility of the ink vehicle. In fact, any soluble hy-
drocarbon with a surface tension less than that of the
vehicle will tend to lower the surface tension of the
ink.
Most importantly, the surface tensions of the
inks applied to the same substrate in accordance with
the invention must be matched at the temperature at
which the inks are maintained on the substrate for a
short period of time, as deQcribed in Patent No.
4,873,134. This may be different from the surface
tension at the jetting temperature, which is the sur-
face tension considered to be significant in prior art
disclosures such as that of the Ohta Japanese Patent
Publication No. 55-54368.
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Finally, it is also important that hot melt inks
used for preparing transparencies be essentially im-
mune to flaking, peeling or otherwise partially delam-
inating from the surface of the transparent substrate.
S The difficulty encountered with hot melt inks is en-
tirely different from that of water-based inks in
which the carrier evaporates, leaving dyes or pigments
dissolved into and chemically bound on a coating on
the surface of the substrate.
For hot melt inks, the ink carrier or vehicle
does not evaporate and the entire volume of the ink is
maintained on the top of the smooth surface of the
transparent substrate. Moreover, for subtractive
color printing, two or more layers of ink may be over-
printed to achieve a desired color and density. For
example, to print a dark blue, a first layer providing
complete coverage of a magenta ink may be completely
overprinted with a second layer of cyan ink. The
resulting ink film is relatively thick, being about 1
mil thick for a typical 95 picoliter droplet at the
300 by 300 spots per inch resolution typical of office
printing.
Hot melt ink images printed on Mylar (polyethy-
lene terephthalate) surfaces have been observed to
flake and peel upon even gentle handling with some ink
formulations. This problem is particularly associated
with large solid area coverage, whereas other ink-
related phenomena are associated with edges or lines
of single dots. Moreover, the failures do not neces-
sarily result in ink falling off the substrate sur-
face, but ink images which have become partially
delaminated from the substrate surface will project a
noticeably darker image due to the two extra inter-
faces (between the ink/air and the air/substrate) in a
delaminated region. Normally, one would expect to
improve durability and resistance to flaking by in-
creasing the bond strength between the ink and the
substrate. This might be possible if a special coat-
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ing for the substrate were provided which could
mechanically or chemically bond to the ink. For exam-
ple, the frosted surface of conventional polyester
drafting material has a microtextured layer which
effectively receives the ink and mechanically bonds to
it, providing images which cannot be flaked off under
even the most rigorous handling. Such surfaces, how-
ever, are too rough to be transparent and those images
can be viewed only by reflection.
It might also be possible to provide a coating on
the surface of the substrate which would enhance the
chemical bond strength. Such an approach is well
known, for example, since some commercially available
films designed for use in transparency projection
include special coatings of some sort to enhance the
feedability and/or adhesion of xerographic toners or
other liquid inks. Such approaches, however, require
a specially prepared transparency substrate, whereas ^
the present invention is capable of providing satis-
factory results on any commercially available trans-
parency substrate, such as, for example, Scotch Brand
Types 503 and 8803 from Minnesota Mining and Manufac-
turing Company, Types 364-01-01 from Arkright, Types
174, 574, 688, 154 and 570 from 3M, Type 505 from
ICI/Melinex and ordinary Mylar from Du Pont.
It has also been found that the flaking problem
associated with hot melt ink applied to a transparent
substrate cannot be overcome by increasing bond
qtrength within the known limits of adhesive (lap
shear) strengths of wax-like hot melt ink formula-
tions.
Typical shear or peel strength data of a range of
adhesive materials applied to transparent substrate
materials are given in Table I.
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:
-18- 2 ~?,~7
TABLE I
Adhesive Strenqths
Adhesive to Base Method Value
Desmocoll polyester to Mylar Shear 138 p5i
strength (9.7 kg/cm2)
Polyurethane to Mylar Peel 23 lb/in
strength(4.1 kg/cm)
Tektronics thermal transfer Lap shear 27 psi
ink to Tektronics film (1.6 kg/cm2)
10 Tektronics thermal transfer Lap shear 51 psi
ink to 3M ~688 Mylar (3.7 kg/cm2)
Spectra experimental ink Lap shear18-25 psi
#U1319 to Mylar (3M #688) (1.26-1.76
kg/cm2 )
15 Spectra experimental ink Lap shear100 psi
#U1249 to Mylar (3M #688) (7.03 kg/cm2)
Fig. 8 shows a cross-section of a typical hot
melt ink image stress condition, i.e., a large-area
solid dark blue ink field on a 4-mil (O.lmm) polyester
substrate 200 with an adjacent clear area. At 300-
spot-per-inch resolution and with 95-picoliter drops,
the dark blue area consists of a first layer of cyan
ink 201 which covers 1~0% of the pixel sites and a
second layer 202 of magenta ink covering 100% of the
pixel sites, providing a total ink thickness of about
1 mil (0.025mm). Since both inks are of similar chem-
ical composition and similar mechanical properties, it
i9 convenient to simplify this to consider a single
layer of ink 203 with uniform mechanical properties
coated on a region of the substrate 200.
Flaking of the ink is caused by local deformation
of the substrate to create an area oÇ local curvature
of radius R as illustrated in simplified form in
Fig. 9. For typical materials and conditions tf~ the
thickness of the substrate, is 4 mils (O.lmm), ti, the
ink thickness, is 1 mil (0.025mm) and, since Young's
.
., ,- ... . .
.
.
--1 9 ~ 3 L~
Modulus E of the ink (being about 50,000 to 100,000
psi (3,515-7,030 kg/cm~)) is small compared to that of
the polyester substrate (400,000-600,000 psi 28,120-
42,180 kg/cm2)~, the neutral axis of the ink-substrate
composite is very near the center of the substrate.
The shear stress at the interface 205 between the ink
and the substrate is nearly zero everywhere except
near the edge 204 where it rises to a peak value T
which is represented in the graphical illustration of
Fig. 10 for an abrupt edge. Por durable hot melt
inks, the Young's Modulus is about 50,000 psi (3,515
kg/cm2), and preferably 100,000 psi (7,030 kg/cm2) or
larger. During normal handling it has been found that
4-mil (O.lmm) transparencies are subjected to defor-
mation which produces local curvatures having a radiusof curvature as low as 0.10-0.03 inches (2.54-0.76mm)
and, for comparison, the radius at which "crease-
whitening" occurs is only slightly smaller, being
about 0.015 inch (0.38mm). Thinner sheets of 2-3 mil
(0.5-0.75mm) thickness produce whitening at smaller
curvatures, and thicker sheets produce whitening at
larger curvatures. Nevertheless, it being an objec-
tive of the present invention to provide durable
transparencies on commonly available substrate materi-
als, it is necessary for the hot melt ink to withstandthe stresses resulting from local curvatures as small
as 0.10-0.03 inch (2.54-0.76mm) radius, which causes a
shear stress of about 1,200 psi (84.4 kg/cm2) to 5,000
psi (351 kg/cm2), as illustrated in Fig. 10. Accord-
ing to Table I, however, materials suitable for hotmelt ink are far weaker than these values and any time
an abrupt edge such as shown at 204 in Fig. 9 occurs,
the ink can be expected to delaminate from the sub-
strate at small-radius curvatures, and the delamina-
tion can be expected to propagate until it reaches anarea where the film curvature has an increased radius.
Fortunately, in printing processes wherein the
inks are applied to a heated substrate in such a way
.. .. ..
,~
-20- ~ Q
that they can self-spread or where they are maintained
above their melting point for a period of time to
permit spreading, as described in Patent No.
4,873,134, the edges of the ink areas which have
S spread produce a feathered edge with a contact angle
208, as shown in Fig. ll, which is less than 45, and
preferably as low as 5-7, as described in Patent No.
4r873,134. Under these conditions, the shear stresses
at the ink-film interface are reduced by an order of
magnitude and the feathering is effective to prevent
delamination at the edges.
On the other hand, delamination at abrupt ink
edges such as indicated at 204 in Pig. 9 can be pre-
vented only if the ink layer remains intact. As indi-
cated in Fig. 9, the strain on the ink layer is givenby
~ = (ti + tf/2)/R
In order for the ink layer to remain intact, it
must be able to stretch under the same 0.10-0.03 inch
(2.54-0.76mm) substrate curvature R as specified
above, which requires elongation of at least 3%, and
preferably about 5-10%, for a l-mil-thick (0.025mm)
ink layer. If the elongation capability of the ink is
less than this, then it will tear and create a sharp
edge 209, as shown in Fig. 12, from which delamination
can propagate.
The capability of a layer of solid ink to elon-
gate and thereby avoid delamination and tearing can be
measured in bulk by common techniques such as
"Instron" pull-testers, and the delamination resist-
ance of such an ink correlates directly with that of
thin jetted and remelted films which are bent over a
test mandrel to produce specified curvatures.
In accordance with the invention, it has been
determined that an ink elongation capability of at
least 3%, and preferably 5-10~, is necessary to avoid
delamination and tearing and also that the ink elonga-
tion capability must be maintained at very high strain
. : . .. .. . .
- .. : , .
. :, ~ : .. .
-
. . .: .. . . . .. .. .
~ : '' ~ -
: .. : .. .. . . . .
rates, since casual transparency handling conditions
creates small creases and also produces strain rates
which are as high as 25% per second and which may be
as high as 100~ per second as a result of vigorous ~-
handling or distortion of the transparency substrate.
Certain additives may be used to improve the
elongation of hot melt inks. For example, acrylic
resins or polyamides will increase flexibility and
ethylene vinyl acetates provide good polymers to util-
ize at levels of about 4-20% because they tend to
increase adhecion strength as well as elongation.
These categories provide a wide range of specific
materials from which to choose for particular inks and
provide opportunities for obtaining good flexibility
lS without sacrificing the matching of inks within an ink
set with respect to viscosity and surface tension.
Where inks have been modified to achieve at least
3%, and preferably 5-10%, elongation before failure ih
accordance with the invention, we have found that
transparencies made with such inks which have been
maintained above their melting point in accordance
with Patent No. 4,873,134 successfully withstand rough
handling and avoid delamination even though the adhe-
sive strength of the ink to the polyester substrate
remains in the range of 20-100 psi (1.41-7.0 kg/cm2).
Although the invention has been described herein
with reference to specific embodiments, many modifica-
tions and variations therein will readily occur to
those skilled in the art. All such variations and
modifications are included within the intended scope
of the invention.
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