Background of the Invention
[0001] This invention relates to sheet material, especially a base sheet obscured by an
opaque but transparentizable microporous, diffusely light-reflective layer.
[0002] For centuries paper has been one of the most versatile substances made by man. Formed
from commonly available cellulosic materials, it can be made stiff or flexible, rough
or smooth, thick or thin, and provided with any desired color. After it has served
its intended purpose, it can often be repulped and used again. In recent years, however,
the demands for paper have increased to the extent .that it has finally been recognized
that the sources of cellulosic raw materials are not inexhaustible. Further, the energy
required to manufacture paper is a significant consideration in a world becoming increasingly
aware that supplies of energy are also finite. It has also become recognized that,
where paper is used as a carrier for indicia, it can generally be used only once,
it being impossible or impractical to remove indicia which are no longer needed or
desired. There has thus arisen a desire for a substitute for paper, especially one
which can be repeatedly and easily reused; even a substitute which was more expensive
to manufacture would be less expensive in the long run if it could be reused a sufficient
number of times.
[0003] Several U.S. Patents (e.g., Nos. 2,299,991, 3,031,328 and 3,508,344) disclose composite
sheet material wherein a light-colored opaque blushed lacquer layer is coated over
a base sheet which is either dark-colored or imprinted with dark-colored indicia.
The opacity and light color of the blushed lacquer coating are due to the inclusion
of numerous microvoids; the local application of (1) heat or pressure (either of which
irreversibly collapses the microvoids) or (2) a non-solvent liquid having substantially
the same refractive index as the lacquer (which fills the microvoids), causes the
coating to become selectively transparent and the underlying dark backing to become
visible. A liquid employed to impart transparency to the opaque microporous layer
can subsequently be evaporated to restore the original appearance.
[0004] U.S. Patent No. 2,854,350 describes structures which are functionally similar to
those just described, except that the blushed lacquer coatings are replaced by a microporous
layer of finely divided calcium carbonate in an organic binder. Transparency is imparted
by locally applying pressure or treating selected areas with a wax, oil or grease
having a refractive index similar to that of the calcium carbonate; see U.S. Patent
2,854,350. Other pigments may be incorporated in a microporous highly plasticized
resin binder; see U.S. Patent 3,247,006.
[0005] It is sometimes desirable to have microvoid-containing sheet material which can be
transparentized by applying a liquid, but which cannot readily be transparentized
by the application of heat or pressure. In such circumstances, a microvoid-containing
layer of the type described in copending U.S. patent application Ser. No. 94,645 is
preferred. This earlier U.S. application discloses a structure in which the microvoid-containing
layer consists essentially of particles held in pseudo-sintered juxtaposition by a
thermoset binder and has a cohesion value of at least 400 grams force
*.
[0006] To a greater or lesser extent, each of the products described in the preceding paragraphs
suffers from the disadvantage that localized application of a transparentizing liquid
results in an image which does not maintain its original sharp outlines with the passage
of time. In other words, there is a tendency for the marking liquid not only to penetrate
the microvoids perpendicularly to the surface but also to wick laterally. As a result,
the longer the transparentizing liquid remains in contact with the microvoid-containing
layer, the less distinct the original image. In some instances, it becomes difficult
to distinguish similar numerals (e.g., 6, 8 and 9) or letters (e.g., 1, i and t) from
each other. Prior to the present invention, no way of combatting this problem was
known.
Brief Description
[0007] The present invention provides an improved sheet material of the type wherein a base
sheet is coated on at least one face with an opaque white or
* The cohesion value is determined by knife-coating a dispersion of a putative composition
on a cleaned gray cold rolled steel panel, drying and curing as appropriate for the
composition, to provide a coating 50-60 micrometers thick. Using a "Balance Beam Scrape-Adhesion
and Mar Tester", sold by Gardner Laboratories, Inc., Bethesda, Maryland, a sapphire-tipped
stylus is lowered into contact with the test panel and held in fixed position while
a ball bearing-supported platform moves the panel. The minimum grams-force required
to form a 50-micrometer deep scratch in the coating in a single pass is determined
at a magnification of 40X and reported as cohesive value.
[0008] pastel layer comprising a film-forming polymer containing interconnected microvoids.
The sheet material can be made to display contrasting indicia by applying to the exposed
surface a desired pattern of a colorless, transparent marking liquid which has a refractive
index approximating that of the constituents of the layer, thereby rendering the marked
portions transparent. The marking liquid for these conventional products is selected
on the basis that it is unreactive with the constituents of the layer and has a volatility
suited to the desired end use.
[0009] The improvement provided by the.present invention comprises, incorporated into the
microvoid-containing layer, a substance (typically an organic polymer) which significantly
increases the viscosity of the marking liquid. (For convenience, this thickening action
is referred to herein as "jellification.") When applied to the surface of the microvoid
layer the marking liquid penetrates into the layer and then thickens, or gels, when
it contacts the jellifying substance, its viscosity becoming so high that it retards
passage of the marking liquid through the microvoids, i.e., it inhibits lateral wicking.
While it might be anticipated that vertical penetration of the microvoids would likewise
be inhibited, so that transparentization would not occur, such is surprisingly not
the case. As a result, indicia can be readily generated but nevertheless maintain
substantially the same dimensions throughout the time that the layer is locally transparentized.
[0010] The jellifying substance employed in the practice of the invention can be either
natural or synthetic but is characterized by the property of jellifying the marking
liquid employed. A simple test for determining whether a given substance is suitable
for use with a specific marking liquid involves placing 10 grams of the putative jellifying
substance and-90 grams of marking liquid in a 500-cc glass jar, tightly capping the
jar, and tumbling it for 24 hours. If the substance and .marking liquid have formed
a homogeneous gelatinous ball with no liquid remaining, the combination is deemed
suitable for use in practice of the invention.
[0011] To determine whether a given jellifying substance-marking liquid combination will
probably be effective in practicing the invention, it has been found useful to consider
their respective solubility parameters., o
f(measured in hildebrands).
* Generally speaking, if the solubility parameters of the jellifying substance and
the marking liquid differ by approximately 2 hildebrands, the combination is likely
to be effective in practice of the present invention; smaller differences tend to
result in lower solution viscosities, and greater differences tend to result in insufficient
gelling to inhibit lateral wicking.
Presently Preferred Embodiments
[0012] Understanding of the invention will be further enhanced by referring to the following
illustrative but non-limitative examples, in which all parts, ratios and percentages
are by weight unless otherwise noted.
EXAMPLE 1
[0013] Following the general procedure described in U.S. Patent No. 2,854,350, a control
was prepared by placing 39.6 grams of water, 0.4 gram sodium alginate and
* Detailed discussions of solubility parameters, their measurement and calculation
are found in (1) Encyclopedia of Polymer Science and Technology, Interscience, New
York (1965), Vol. 3, page 833 et seq., and (2) Encyclopedia of Chemical Technology,
Interscience, New York, (1971), Supplement Vol., page 889 et seq.) 5 grams precipitated
calcium carbonate in a 4-ounce (approximately 125-cc) jar and ball milling for several
days. The resulting composition was knife-coated, at a thickness of approximately
200 micrometers, onto the surface of black 60-micrometer greaseproof paper and allowed
to dry at room temperature overnight; the dried coating was approximately 25 micrometers
thick.
[0014] In accordance with the invention, a sample was prepared which was identical to the
control except that 0.2 gram cellulose acetate butyrate (CB 500-1 available from Eastman
Chemical Products, Inc.) was included in the composition. Using a felt-tipped pen
filled with diethylphthalate, a 1.5-mm line was stroked on the surface of each of
the two products. The table below shows the width of the stroked lines, measured after
various time intervals.

It will be observed that the control product suffered from lateral wicking which more
than doubled the width of the initial line; in contrast, the EXAMPLE 1 product maintained
a substantially constant line width, the diethylphthalate gelling when it contacted
the cellulose acetate butyrate, and thus preventing lateral wicking.
EXAMPLE 2
[0015] Following the general procedure described in U.S. Patent No. 3,508,344, a control
was prepared by placing. 75.1 grams acetone, 6.6 grams polymethyl methacrylate ("Elvacite"
2041 available from E. I. duPont de Nemours & Company), 1.6 grams diethylphthalate,
1.1 grams polyacrylate ("Rhoplex" B-15, available from Rohm & Haas Company) and 11.0
grams of water in an 8-ounce (approximately 250-cc) jar and ball milling overnight.
The resulting composition was knife-coated onto the black greaseproof paper used in
EXAMPLE 1 and allowed to dry at room temperature to leave a coating approximately
50 micrometers thick.
[0016] In accordance with the present invention, a sample was prepared which was identical
to the control except that 1.0 gram methyl cellulose ("Methocel" MC 4000 cp, available
from the Dow Chemical Company) was incorporated in the composition prior to ball milling.
Using a felt-tipped pen filled with 1,2-propanediol, 0.9-mm lines were drawn on the
surface of each product and measured after various time intervals. Results are shown
below:

While not so striking as the results shown in EXAMPLE 1, the results of this example
nevertheless show that the construction of the present invention, where the 1,2-propanediol
was jellified by the methyl cellulose and prevented lateral wicking, maintained a
significantly sharper image than did the control.
EXAMPLE 3
[0017] Following the procedure described in more detail in the aforementioned U.S. Patent
Application Ser. No. 94,645, a control coating composition was prepared by mixing
16 parts xylene, 16 parts diisobutyl ketone, 8 parts heptane, 11.39 parts thermosetting
acrylic resin (G-CURE 868 RX-60, available from Henkel Corporation), 0.2 part di(dioctyl
pyrophosphato) ethylene titanate (KR-2385, available from Kenrich Petrochemicals Inc.),
100 parts calcium carbonate having a 0.5-15 micrometer particle size, and 2.17 parts
"Desmodur" N-75 (75% solid solution in 1:1 xylene:2-ethoxyethyl acetate of the high
molecular weight biuret of 1,6-hexamethylene diisocyanate, having an equivalent weight
of 195, available from Mobay Chemical Corporation). The composition was coated.on
the black greaseproof paper of EXAMPLE 1 and cured 30 minutes at approximately 90°C
to leave a dried coating approximately 25 micrometers thick.
[0018] In accordance with the present invention a product was prepared which was identical
to that of the control except that 4 parts of carboxypolymethylene ("Carbopol" 941,
available from B. F. Goodrich Chemical Co.) was included in the coating composition.
A felt-tipped pen filled with tetraethylene glycol was then used to mark a 1.0-mm
line on the surface of each sheet. After 30 minutes, the line width on the control
product was 1.9 mm, while the line width on the product of this EXAMPLE 3 was only
1.1 mm.
EXAMPLE 4
[0019] A microvoid-forming control coating composition was prepared by mixing together 20
grams xylene, 7.6 grams methylisobutyl ketone, 7.6 grams ethylene glycol monoethylether
acetate, 0.2 gram di(dioctylpyrophosphato) ethylene titanate, 13.0 grams thermosetting
acrylic resin, 100.0 grams 0.5-15 micrometer calcium carbonate, and 2.5 grams "Desmodur"
N-75. The composition was coated on the black greaseproof paper of EXAMPLE 1 and cured
by heating 45 minutes at approximately 90°C.
[0020] In accordance with the present invention, a coating composition was prepared which
was identical to that of the control except that 2 grams of "Kraton" 1107 (isoprene:styrene:isoprene
block copolymer, having a 25
% toluene solution viscosity at 23°C of 1.6 Pa.s) was included in the coating composition.
A felt-tipped pen containing a mixture of saturated branched chain hydrocarbons, having
a distillation range of 244°-286°C, was then used to mark lines on both the control
and EXAMPLE 4 products, the width being measured after various time intervals. Results
are tabulated below:

EXAMPLE 5
[0021] A microvoid-forming control composition was prepared by mixing together 8.0 parts
of an oil-free thermosetting alkyd resin having an equivalent weight of 400 ("Aroplaz"
6022, available from Ashland Chemical Company), 56.0 parts ethylene glycol monoethylether
acetate, 100.0 parts isopropyl triisostearoyl titanate pigment, and 3.8 parts "Desmodur"N-75.
The composition was coated on 64-micrometer dark brown greaseproof paper and cured
6 minutes at 230°C to leave a dry-coating approximately 25 micrometers thick.
[0022] In accordance with the present invention, a product was prepared which was identical
to the control except that the coating composition also contained 2.0 parts cellulose
acetate butyrate and 2.0 parts carboxypolymethylene. A felt-tipped pen containing
dibutyl phthalate was then used to mark a 1.5-mm line on each coated.product and the
width measured after various time intervals. Results are tabulated below:

Similar results were obtained using propylene glycol as a marking liquid, except that
the marks on both products disappeared by evaporation of the liquid before 1,440 minutes
had elapsed.
[0023] The preceding examples illustrate the incorporation of a jellifying substance directly
into a microvoid-forming coating composition. It is also feasible to post-treat a
microvoid coating to introduce a jellifying material into the microvoids, as the following
examples illustrate.
EXAMPLE 6
[0024] A sample was prepared in substantially the same manner as the control sheet of EXAMPLE
1. In accordance with the present invention, a portion of this control was coated
with a 5% solution of cellulose acetate butyrate in methylisobutyl ketone and all
excess wiped from the surface, after which the solvent was allowed to evaporate at
room temperature. A felt-tipped pen containing diethylphthalate was stroked across
the treated and untreated portions to produce a line which was initially 2 mm wide.
After 30 minutes, the line on the untreated portion was 5 mm wide, while the line
on the treated portion was still only 2 mm wide. In a similar manner, a felt-tipped
pen containing dioctylphthalate was stroked across the treated and untreated portions
to produce a line which was initially 1 mm wide. After 13 days, the line on the untreated
portion had become so wide that it could no longer be distinguished, while the line
on the treated portion was still only 1 mm wide and easily recognized.
EXAMPLE 7
[0025] A control sheet substantially identical to the control sheet of EXAMPLE 2, was prepared.
In accordance with the present invention, a portion of this control was then treated
with a 2% aqueous solution of methyl cellulose ("Methocel" MC Standard 4000 cps),
the excess solution wiped from the surface, and the sample allowed to dry at room
temperature. A felt-tipped pen containing 1,2-propanediol was then used to mark a
0.7-mm line on the surface of both the control and the treated material of this EXAMPLE
7, line width on each being measured after various time intervals. Results are tabulated
below:

EXAMPLE 8
[0026] Following the procedure described in more detail in copending U.S. Patent Application
Ser. No. 94,465, a control sheet was prepared by mixing 17.5 parts xylene, 17.5 parts
methyl ethyl ketone, 100 parts dry ground calcium carbonate having a 0.5-15 micrometer
particle size, and 13 parts thermosetting acrylic resin and ball milling the mixture
for 2 days. Then 2.5 parts of the high molecular weight biuret of 1,6-hexamethylene
diisocyanate was added and ball milling continued for an additional 4 hours. The dispersion
was knife-coated onto the black greaseproof paper of EXAMPLE 1, dried 1 minute at
90°C andcured 45 minutes at the same temperature to leave a dried coating about 50
micrometers thick. To a portion of the control sheet coated surface there was applied
a 5% methyl ethyl ketone solution of cellulose acetate butyrate (Eastman CAB 500-1)
and, after the solution had penetrated the surface, the excess was wiped off and the
sample allowed to dry at room temperature. A felt-tipped pen containing diethylphthalate
was then used to mark a 1.2-mm line on the surface of both the control and the treated
material of this EXAMPLE 8, line width on each being measured after various time intervals.
Results are tabulated below:

[0027] As previously indicated, maintaining a sharp, distinct image is highly desirable
when the applied indicia are to be read visually. It is perhaps even more critical,
however, that the indicia maintain their . original dimensions when they are to be
scanned by optical character recognition (OCR) or optical mark reading (OMR) devices.
[0028] Whether incorporated in the microvoid-forming coating composition or subsequently
applied to a microvoid-containing coating, it is preferred that the jellifying substance
occupy a minimum of the void-volume. The incorporation of an excessive amount of jellifying
substance will reduce the size of the voids to the point where they are unable to
scatter white light effectively. While the jellifying substance can occupy up to 50%
of the void volume, it is preferred that it occupy 1-10%.
[0029] As a general guide to the selection of jellifying agents and marking liquids, which
work effectively together, attention is directed to the following illustrative table:

[0030] In most instances, a marking liquid having a desired volatility will be chosen, after
which an appropriate jellifying agent will be selected.
[0031] The invention having been described and exemplified as above, it will be recognized
that numerous variations of coating compositions, marking liquids, etc. are within
the ordinary skill of the art.
[0032] The trade names "Aroplaz", "Carbopol", "Desmodur","Elvacite","Methocel", and"Rhoplex"
are believed to be registered trademarks.
1. Sheet material of the type wherein a base sheet is coated on at least one face
with a layer comprising a film-forming polymer containing interconnected microvoids,
said layer being locally transparentizable to display contrasting indicia when there
is applied to its exposed surface a transparent, colorless liquid which has a refractive
index similar to that of the solid constituents of the layer, characterized in that
there is incorporated in said layer a substance which jellifies the liquid,
whereby the indicia maintain substantially the same dimensions throughout the time
that the layer is locally transparentized.
2. The invention of claim 1 further characterized in that the layer consists essentially
of particles held in pseudo-sintered juxtaposition by a thermoset binder.
3. The invention of claim 1 or 2 further characterized in that the jellifying substance.comprises
cellulose acetate butyrate.
4. The invention of claim 1 or 2 further characterized in that the jellifying substance
comprises methyl cellulose.
5. The invention of claim 1 or 2 further characterized in that the jellifying substance
comprises carboxypolymethylene..
6. The invention of claim 1 or 2 further characterized in that the jellifying substance
comprises a styrene:isoprene:styrene block copolymer.
7. Sheet material according to any preceding claim further characterized in that the
sheet material comprises in combination:
a. a self-supporting base sheet,
b. bonded over at least one side of the base sheet, a reflective opaque white to pastel
layer having a cohesive value of at least 400 grams and consisting essentially of
particles held in pseudo-sintered juxtaposition by a thermoset binder so that interconnected
microvoids are present throughout the layer, and
c. a jellifying substance other than the binder partially filling said microvoids,
whereby, when there are applied to the exposed surface of said layer desired indicia
of a marking liquid which has a refractive index approximately that of the particles
and which is jellified by the jellifying substance, the liquid not only penetrates
the microvoids and is jellified by the jellifying substance, thereby reducing the
reflectivity of the layer in the vicinity of the liquid-penetrated microvoids to impart
transparency and maintaining substantially constant dimensions of the indicia throughout
the time that the layer is locally transparentized, but also is inhibited from lateral
wicking.
8. A method of making the sheet material of any preceding claim characterized by incorporating,
in an opaque microvoid-containing layer which is locally transparentizable when contacted
with a liquid having a refractive index similar to that of the solid constituents
of said layer, a substance which jellifies said liquid.
9. The method of claim 8 further characterized in that, prior to formation of said
layer, the jellifying substance is incorporated in the composition which is used to
form the layer.
10. The method of claim 8 further characterized in that, after said layer has been
formed, the jellifying substance is incorporated in said layer.