Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DURABLE SHEETS FOR PRINTING
FIELD OF THE INVENTION
This invention relates to durable printing sheets, such
as laser and thermally transfer printable multiple layer
sheets, as well as those printed by other printing processes
such rotogravure, lithography, screen, letterpress and
flexographic techniques. The sheets are of balanced structure
and remain curl free under the normal temperature, pressure,
tension and environmental conditions of the varied printing
processes.
BACKGROUND OF THE INVENTION
Laser printing of label materials is a relatively new
development and employs apparatus such as shown and described
in Gretchev, U.S. 4,494,129. The high pressures and
temperatures employed in laser printing can be accommodated
by simple, non-composite paper and synthetic sheets . They will
not curl and become di f f icult to feed and stack . I f , however,
dual-layer and multi-layer composite sheets, which are
necessary components of label or tag sheets, are fed to laser
printers, special methods are needed to prevent the composite
sheets from curling during and after printing. In one such
method, Rutkowski, U.S. 4,913,926, discloses laser printing
a dual-layer label sheet having a continuous film of pressure
sensitive adhesive between the layers. The resultant sheet
is shown to be curled up after passing through the printer.
I f , however, the adhesive is patterned into geometric f figures,
such as hexagons and diamonds, during deposition to leave a
number of gaps between the patterns, and then the sheets are
laser printed, no curling is observed because the melted and
squeezed adhesive layer flows into and fills the gaps
relieving any stresses developed by heat and pressure. In the
method of Rutkowski, special equipment will be necessary for
patterned printing and the high degree quality control
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necessary should be labor intensive. Another approach to
providing adhesive paper labels having curling resistance and
applicable to electrostatic and magnetic copying in disclosed
by Fuji Xerox Corp in Japanese Patent Publication No.
J59149970-A, August 28, 1984. Such a composite comprises two
sheets, a surface paper to receive the printing and a release
paper having a ratio of surface paper to release paper
elongation and/or contraction of 0.7-1.0:1Ø Furthermore,
the release paper has an elongation and/or contraction of
below 0.90. If such a composite is used to make laser
printed labels comprising wood pulp paper, curling is
distinctly not a problem, but the labels do not weather well
and cannot be used outdoors, such as to mark lumber, because
rain, wind and snow will make them difficult to read and
maintain. If, on the other hand, such labels are made of
synthetic paper, such as polyvinyl chloride paper, curling is
not a problem because the elongation/contraction requirements
are met, but the vinyl paper, like wood pulp paper, doesn't
weather well and tears readily during application, and has a
tendency to give off dangerous gases if burned. Improved
weatherability, equivalent printability, high tear resistance
and no tendency to elaborate noxious gases during heating can
be achieved, if instead of vinyl, tear-resistant synthetic
papers are used, such as those based on thermoplastic
polyesters and polyolefins, e.g., polypropylene, and the like.
However, the substitution of these for wood pulp paper and
vinyl paper in the composite dual-layer label sheets of the
Rutkowski and the Japanese Publication, above-mentioned, lead
to serious and substantial curling problems during and after
laser printing.
It has now been discovered that if a composite sheet is
provided having at least three layers and if, further, the top
and bottom layers are selected to have the same or
substantially the same thermal expansion and contraction
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characteristics, then non-vinyl, tear-resistant plastic papers
' such as polyester and polyolefins and the like can be used to
provide non-curling sheets, with none of the above-mentioned
disadvantages of wood pulp paper and vinyl composite sheets
and labels. This result is unexpected in view of the art
because the base layer of this invention can have an
expansion/contraction ratio substantially different than
either the printable top layer and the protective backing
layer or coating, whereas the Japanese Patent Publication
would teach otherwise.
Accordingly, a principal object of the present invention
is to provide printable sheets, e.g., of labels or tags
mounted on backing sheets, without the curling problem
discussed above. It is a further object of the invention to
provide a method for printing sheets without curling. It is
a further object to provide printed sheets of such structure
which will remain curl free throughout the useful life of the
printed piece under varied conditions of temperature and
pressure. It is still another object of the invention to
provide articles labeled with curl-free printed labels or
tagged with curl free printed tags.
These and other objects of the invention will become
apparent from the present specification and drawing.
SUMMARY OF THE INVENTION
According to this invention, in one of its major aspects,
there are provided durable composite sheets for printing which
lay flat and do not curl through out the printing process
where temperature, pressure and/or tension can cause curl,
i.e., generally all types of printing, the sheets including
at least three layers comprised of:
A. at least one base layer comprising a paper or a
synthetic paper or a coated film;
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B. at least one print receiving layer comprising a paper
or a synthetic paper or a coated film adhered to the top face
of base layer or layers A; and
C. at least one backing layer comprising a paper or a
synthetic paper or a coated film adhered to the bottom face
of base layer or layers A, the top layer B and the backing C
having the same or substantially the same thermal elongation
or contraction characteristics.
Special mention is made of a preferred aspect of the
invention which comprises a composite sheet which does not
curl when heated to temperatures normal 1y encountered in . laser
printing, the sheet including at least three layers comprised
of
A. at least one base layer comprising a tear resistant
synthetic paper having a pressure sensitive adhesive on the
bottom face thereof;
B. a print receiving tear resistant synthetic paper
layer permanently adhered to the top face of base layer or
layers A; and
C. a protective backing comprising a paper layer or a
coated film releasably adhered to said pressure sensitive
adhesive on the bottom face of base layer or layers A, said
top layer B and said protective backing C having the same or
substantially the same thermal elongation or contraction
characteristics.
In preferred features, the invention contemplates
printing sheets as defined above wherein layers A and B
comprise all or subdivided portions thereof; those, wherein
layer A comprises a thermoplastic polyester, and layer B and
protective backing C comprise a polyolefin; and such sheets
wherein layer A comprises a thermoplastic polyester, layer B
comprises a polyolefin and backing C comprises a thermoplastic
resin coating. Also contemplated are sheets as defined above
which also include a printing enhancing coating on the print
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receiving face of layer B, especially those wherein the
printing enhancing coating comprises an acrylic, polyester or
urethane resin filled with finely divided clay, silica,
titanium dioxide, zinc oxide, other fillers or blends of
fillers, and the like ; those wherein layer B is adhered to
layer or layers A through an adhesive layer having high
cohesive strength and low shear strength whereby layers B and
A can move transversely under the influence of heat without
parting. Further preferred embodiments comprise label sheets
as defined above wherein backing C can be releasably adhered
to the pressure sensitive adhesive on base layer or layers A
through a release coating, especially those wherein the
release coating comprises a silicone and the pressure
sensitive adhesive layer has high cohesion and low shear
strength whereby layers C and A can move transversely under
the influence of heat without parting. In addition, the
invention includes sheets as defined above wherein layer or
layers A include an effective amount of conductive filler for
dissipating static changes developed during laser printing
whereby sheet feeding and delivery problems are minimized,
special mention being made of such sheets wherein the
conductive filler comprises carbon black. Best results appear
to be obtained with sheets as defined above wherein layer B
has a ratio of thermal elongation or contraction in the range
of from about 0.7 to about 1.3 with respect to 1.0 for
protective backing C, especially those wherein said layer B
has a ratio or thermal elongation or contraction of about 1.0
with respect to 1.0 for protective backing C. Also
contemplated as preferred embodiments are sheets for printing
as defined above wherein the outwardly facing major face of
layer B, layer C, or layers B and C are provided with a print
receiving area or area, and sheets in which the print
receiving areas are provided in whole or part with a print
enhancing coating. Also contemplated are sheets as above
CA 02160442 2004-12-23
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defined coated in one or both major faces with an antistatic
composition so as to avoid jamming in subsequent high speed
production printing processes. Such antistatic compositions
comprise, for example, amines, quaternary ammonium compounds,
anionic or cationic surface active agents, or mixtures of any
of the foregoing, and it is preferred to apply them by
deposition from an aqueous medium. In an especially preferred
embodiment, the invention contemplates also sheets as above
defined which, before or after printing, are cut into a
plurality of pieces of substantially the same size.
In a broad aspect, then, the present invention relates to
a durable sheet for printing, the sheet including at least
three layers comprised of: layer A, which comprises at least
one base layer consisting of a paper, a synthetic paper or a
coated film; layer B, which comprises at least one print
receiving layer consisting of a paper, a synthetic paper or a
coated film, wherein layer B is adhered to the top face of
layer A; and layer C, which comprises at least one backing
layer consisting of a paper, a synthetic paper or a coated
film, wherein layer C is adhered to the bottom face of layer
A; ~ and wherein layer B and the backing C have the same or
substantially the same thermal elongation or contraction
characteristics; and wherein the top surface of layer A
comprises a release agent, the bottom surface of layer B
comprises a pressure sensitive adhesive and layer C is
permanently adhered to layer A.
In another broad aspect, then, the present invention
relates to a durable sheet for printing, the sheet including
at least three layers comprised of: layer A, which comprises
at least one base layer consisting of a paper, a synthetic
paper or a coated film; layer B, which comprises at least one
print receiving layer consisting of a paper, a synthetic paper
or a coated film, wherein layer B is adhered to the top face
of layer A, and layer C, which comprises at least one backing
CA 02160442 2004-12-23
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layer consisting of a paper, a synthetic paper or a coated
film, wherein layer C is adhered to the bottom face of layer
A; and wherein layer B and the backing C have the same or
substantially the same thermal elongation or contraction
characteristics; and wherein the top surface of layer C
comprises a release agent, the bottom surface of layer A
comprises a pressure sensitive adhesive and layer B is
permanently adhered to layer A.
In yet another broad aspect, then, the present invention
relates to a durable sheet for printing, the sheet including
at least three layers comprised of: layer A, which comprises
at least one base layer consisting of a paper, a synthetic
paper or a coated film; layer B, which comprises at least one
print receiving layer consisting of a paper, a synthetic paper
or a coated film, wherein layer B is adhered to the top face
of layer A; and layer C, which comprises at least one backing
layer consisting of a paper, a synthetic paper or a coated
film, wherein layer C is adhered to the bottom face of layer
A; and wherein layer B and the backing C have the same or
substantially the same thermal elongation or contraction
characteristics; and wherein layer A comprises a thermoplastic
resin, and layer B and layer C comprise a polyolefin.
In a further broad aspect, then, the present invention
relates to a durable sheet for printing, the sheet including
at least three layers comprised of: layer A, which comprises
at least one base layer consisting of a paper, a synthetic
paper or a coated film; layer B, which comprises at least one
print receiving layer consisting of a paper, a synthetic paper
or a coated film, wherein layer B is adhered to the top face
of layer A; and layer C, which comprises at least one backing
layer, consisting of a paper, a synthetic paper or a coated
film, wherein layer C is adhered to the bottom face of layer
A; and wherein layer B and the backing C have the same or
substantially the same thermal elongation or contraction
CA 02160442 2004-12-23
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characteristics; and wherein layer A comprises a thermoplastic
polyester, layer B comprises a polyolefin and layer C
comprises a thermoplastic resin coating.
In a still further broad aspect, then, the present
invention relates to a durable sheet for printing, the sheet
including at least three layers comprised of: layer A, which
comprises at least one base layer consisting of a paper, a
synthetic paper or a coated film; layer B, which comprises at
least one print receiving layer consisting of a paper, a
synthetic paper or a coated film, wherein layer B is adhered
to the top face of layer A; and layer C, which comprises at
least one backing layer consisting of a paper, a synthetic
paper or a coated film, wherein layer C is adhered to the
bottom face of layer A; and wherein layer B and the backing C
have the same or substantially the same thermal elongation or
contraction characteristics; and which also includes a
printing enhancing coating on the print receiving face of
layer B.
In another broad aspect, then, the present invention
relates to a sheet for printing, said sheet including at least
three layers comprised of: layer A, which comprises at least
one base layer consisting of a tear resistant synthetic paper
having a pressure sensitive adhesive on the bottom face
thereof; layer B, which comprises a print receiving tear
resistant synthetic paper layer permanently adhered to the top
face of layer A; and layer C, which comprises a protective
backing consisting of a paper layer or a coated film
releasably adhered to said pressure sensitive adhesive on the
bottom face of layer A, said layer B and said layer C having
the same or substantially the same thermal elongation or
contraction characteristics.
In yet another broad aspect, then, the present invention
relates to a printed sheet including at least three layers
comprised of : layer A, which comprises at least one base layer
CA 02160442 2004-12-23
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consisting of a tear resistant synthetic paper having a
pressure sensitive adhesive on the bottom face thereof; layer
B, which comprises a printed tear resistant synthetic paper
layer permanently adhered to the top face of layer A; and
layer C, which comprises a protective backing consisting of a
paper layer or a coated film releasably adhered to said
pressure sensitive adhesive on the bottom face of layer A,
said top layer B and said layer C having the same or
substantially the same thermal elongation or contraction
characteristics.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a three-layer sheet of
printed labels mounted on a backing sheet, a section of the
edge being enlarged in FIG. 1A to show detail:
FIG. 2 is a top view of one of the labels from the sheet
shown in FIG. 1;
FIG. 3 shows a three-layer sheet having balanced thermal
expansion/contraction characteristics in accordance with the
invention following printing through a printer, a section of
the edge being enlarged in FIG. 3A to show detail; and
FIG. 4 shows a three-layer sheet having unbalanced
thermal expansion/contraction characteristics not in
accordance with this invention, which has been curled up as a
result of being printed in a printer, a section of the edge
being enlarged in FIG. 4A to show detail.
DETAILED DESCRIPTION OF THE INVENTION
Referring more particularly to the drawings, Figure 1
shows a three-layer sheet 2 including an upper sheet made up
of a large number of labels 4 which have been die cut to be
separate from one another and which are mounted on backing
sheet 6. As will be seen from FIG. 1A, each label comprises
printable surface layer 8 alone or on top of base layer 10.
Figure 2 is a front view of one of the labels 4 which has
been removed from the backing sheet 6. More specifically, on
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the front of the label 4 as shown in Figure 2, is some
printing or advertising 12, and a bar code configuration 14.
On the back of label 4 .is a continuous layer of pressure
sensitive adhesive (not shown) of an entirely conventional
type well known to those skilled in the art. There is no need
whatsoever to pattern the layer of adhesive into geometric
forms as is done in some of the commercial labels in the
current state of the art.
Figs. 3 and 3A show a three-layer sheet 16 of the present
invention, comprising a top sheet 8 and protective backing 6
having the same thermal expansion/contraction characteristics,
in which the sheet 16, following printing, is entirely level
and flat. In Figs. 4 and 4A, however, it is shown that a
three-layer sheet 18 comprising a top sheet 8a and protective
backing 6a having substantially different and unbalanced
thermal expansion/contraction characteristics on either side
of base layer 10, and, following printing in a printer, it
curls up as indicated, particularly at corners 20. This
curling is found to be so significant that proper stacking of
the printed sheets is not practical, nor is further mechanical
processing of the sheets.
Laser printing apparatus, which are well known, are
described in U.S. 4,494,129 and 4,913,926, mentioned
hereinabove. In general, all such apparatus use
electrostatically chargeable drums to form an image and heated
rollers to apply moderately high levels of pressure, e.g.,
greater than about 100 pounds per square inch, and elevated
temperatures, e.g., greater than about 250° F, to the sheets
during the printing process. It is believed that these
relatively high pressures and temperatures produce the curled
adhesively bonded label sheets as shown in Figure 4 when no
particular attention is paid to selecting top and bottom
layers 8a and 6a so that they have the same or substantially
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the same thermal expansion/contraction characteristics as is
required by the present invention.
However, by using a balanced composite as shown in Figure
3A of the drawing, wherein layers 8 and 6 have the same or
substantially the same thermal expansion/contraction
characteristics, the tension or strain produced downstream in
any process where temperature, pressure, and tension can cause
curl and flat sheets for printing will be produced.
Suitable materials from the paper sheets comprise
thermoplastic polyester, e.g., poly(ethylene terephthalate),
poly(acrylonitrile), and the like, as well as polyolefins,
such as polypropylene, polyethylene, and the like. For the
base layer or layers, polyesters are preferred, and especially
polyethylene terephthalate) which is available from a number
of sources. For the top layer, monoaxially or biaxially
oriented polyolefins are preferred, especially polypropylene,
and such papers are available from a number of sources, such
as Mobil Chemical Company, Pittsford, N.Y. 14534, U.S.A.,
tradename "Oppalyte" TW and Toray Plastics, Inc., North
Kingston, RI, 02852, U.S.A., tradenames "Treafilms" and "Treax
Films".
Thermal elongation and/or contraction characteristics are
measured by standard test methods . The values are used to
select suitable substrates for use in-this invention. It is
important that the relative values rather than their magnitude
receive the most attention. It is known for example that
polypropylene paper has a shrinkage at 275°F. of -4.5~ in the
MD and -5.0~ in the TD. If a composite is prepared having
polypropylene on the top and the bottom and polyethylene
terephthalate) in the center, curl-free laser printing will
be achieved because the top and bottom layers will have the
same thermal contraction. If, however, a dual layer composite
of polypropylene on top of polyester or a three layer
comprising polypropylene (top), polyester(middle) and
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polyester (bottom) is prepared, the composites are unbalanced,
as explained above, and they will not print without curling.
The layers can be simply heat-bonded with heat-activated
adhesive, but it is preferred to use an adhesive of a
permanent type and of a pressure sensitive type. Many
adhesives are suitable although it is preferred to select one
which has a high cohesive strength and low shear strength to
facilitate transverse movement between different layers during
heating while precluding parting.
In preferred embodiments, the backing C or layer A will
be adhered to the pressure sensitive adhesive through a
release coating, such as a poly(tetrafluoroethylene film) or
more preferably a silicone resin, as is known in the art, for
the labels . Layers B and C may be permanently adhered to layer
A by a laminating adhesive for the tags. A release coating
is not needed for the tags.
Among the preferred features of the invention are label
sheets of the type described wherein the base layer is
rendered electrically conductive by including an effective
amount of a conductive filler, e.g., a powder such as silver
and nickel powders or carbon powders. Conductive fillers can
also be put in adhesives instead of base layers and also in
printable coatings, without departing from the spirit or scope
of the invention. The polyethylene terephalate) base layer,
and/ or the other substrates, can for example include 5 to 40
percent by weight of carbon powder, and not only will the
sheets not curl during printing, but they also will dissipate
static changes developed during printing which can also
interfere with feeding, imaging and delivery. For best
results, when using polyolefin layers, composite volume
resistivity gives best printing at values equal to or less
than 1014 ohms-cm.
Concerning the backing, this can be the same or
different in terms of material from the top layer so long as
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the expansion/contraction characteristics are the same or
substantially the same, preferably from 0.7-1.3:1 and more
preferably 1:1. The backing can even comprise a film coating
instead of a sheet, and the backing can even comprise a wood
pulp paper sheet, such as a silicone-coated paper sheet,
instead of a synthetic paper sheet, without departing from the
scope of the invention.
The composites are assembled in conventional ways using
conventional equipment. The sheets are consolidated
continuously under moderate heat and pressure and cut to any
desired size and, if desired, the labels or tags are die-cut
into the sheets by means well known to those skilled in this
art.
The antistatic agents used herein are applied as surface
coatings or are incorporated as additives into print receiving
coatings, preferably from an aqueous medium. Such coatings
are conveniently applied by gravure, mayer rod, reverse roll,
or other techniques known to those skilled in this art. For
example, a thin coat of antistatic agent can be applied to one
surface of the film or anti-stat additive can be incorporated
into a clay coating. Suitable such coatings or additives can
be selected from the many commercially-available materials
known in this art, such as listed, for example in Modern
Plastics Encyclopedia, Mid-October Issue, 1987, "Antistatic
Agents" by J. L. Rogers, pages 130 and 132, as well as pages
579-581. Preferred for use herein are commercially-available
antistatic coating compositions available, for example, from
Akzo Chemie America, Chicago, IL, under the trade name or
designation Armostato Aqueous Ethoquad CY12, from Union
Carbide Corp., Danbury, CT, under the trade name or
designation Silweto L-77, a modified silicone, or from Process
Resources Corp., Thornwood, NY, under the tradename or
designation PD 945, a proprietary mixture which is gravure
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applicable and having the typical properties described in the
Table:
TABLE
Solids 4 ~
pH 8.5 - 9.5
Viscosity 10 - 50 CPSo2/20
RPM @ 77 F
Weight / gallon 8.5 LBS / GAL
Color Off White
Diluent Water
Clean-Up Water
Shelf-Life 90 Days
The antistatic coating can be applied as part of the
gravure printing process using the same techniques for
applying any printing enhancing layer and it may be applied
either before or after application of any adhesive layer.
Many variations of the present invention will suggest
themselves to those skilled in the art in light of the above
detailed description and accompanying drawing. For example,
instead of oriented polypropylene as the face film,
polyethylene terephthalate), cellulose paper, cellulose
acetate, polyethylene, polycarbonate, fluoropolymers and
polyimide films can be used. Instead of polypropylene as the
release film, paper or silicone coated paper can be used. A
printing enhancing coating such as an acrylic or polyester or
urethane resin containing finely divided clay, silica,
titanium dioxide, zinc oxide, or mixtures of any of them, can
be spread on the print receiving face of the top and/ or
bottom sheet(s).
All such obvious modifications are within the full intended
scope of the appended claims.
