Note: Descriptions are shown in the official language in which they were submitted.
CA 02461583 2004-04-08
. as
TAMPER EVIDENT TAPE AND LABEL
This application is a divisional application of
co-pending application 2,196,606, filed
August 2, 1995.
Field of the Invention
This invention relates generally to
tamper-evident closures and, more particularly, a
closure in the form of a tape or label that indicates a
forced opening, cooling below a particular temperature,
and/or heating above a particular temperature.
Background of the Invention
Tamper-evident closures for containers such
as bags, envelopes, packages, ete. and tamper-evident
tapes and labels for use with bags, envelopes, and
other packages have been available for several years.
Generally, if these existing closures are forced open,
the visual appearance of the closure changes so as to
provide an indication that the contents of the
container have been accessed. Containers fabricated
having tamper-evident closures are commonly used in
industries in which the contents of the containers must
be maintained in tight security, for example, in the
CA 02461583 2004-04-08
_2_
banking industry. In certain instances package
manufacturers employ prefabricated closures in the form
of a tape or label to provide a tamper-evident closure
on their packaging. Tamper-evident closures in the
form of tapes or labels are also often used by everyday
consumers who want to ensure that their packages are
securely sealed and will evidence tampering if it
occurs.
One form of container commonly used in
banking and other industries is a bag, pouch or
envelope (generally referred to herein as a bag) formed
of a plastic material such as polyethylene. The opening
in this type of bag is commonly closed with a
pressure-sensitive adhesive located on one side of the
bag. To close the bag, a peelback strip covering the
free side of the adhesive is removed, and the exposed
surface of the adhesive is then pressed against the
opposite side of the bag. Generally, if a bag of this
type is later forced open, the pressure-sensitive
adhesive and/or other parts of the bag will distort and
break apart, so as to provide an indication that the
bag has been opened, possibly without authorization.
Closures for a bag of this type have been formed with
layers in addition to the pressure-sensitive adhesive
to provide a clearer indication of when the closure is
forced open. For example, U.S. Patent No. 5,060,848 to
F.R. Ewan describes a tamper evident seal that uses a
layer of nitrocellulose or acrylic ink that breaks
apart in a selected pattern when the seal is forced
open. The layer of ink is adhered to a polyester panel
that is part of the seal. Before applying the ink the
plastic panel is masked with a desired pattern of a
silicon oil releasant material, which normally causes
the ink to break apart in the masked pattern when the
seal is forced open. Also, to ensure that the ink
layer adheres to the silicon oil releasant material and
polyester panel, a primer is applied over the silicon
CA 02461583 2004-04-08
-3-
oil releasant, and the ink is then applied over the
primer.
Unfortunately, bags of this type are
generally expensive to manufacture, use hazardous
materials, and are not easily recycled. For example,
the polyester panel disclosed in U.S. Patent No.
5,060,848 to F.R Ewan is not easily recycled, and the
primer required is hazardous. Furthermore, thieves
have devised a scheme to gain access to the contents of
bags of this type without detection. The scheme
devised involves spraying a liquid refrigerant onto the
adhesive to freeze the adhesive down to its glass
transition temperature, generally at about -10°F. At
this temperature, the adhesive becomes brittle and
loses its adhesive qualities, i.e., tack. The thief is
then able to open the bag and remove certain contents.
The thief allows the adhesive to warm back to room
temperature, at which point the adhesive regains its
tack and then simply recloses the bag by applying
pressure, all without any evidence of tampering.
Thieves have also devised a scheme to gain
access to the contents of bags of this type without
detection by heating the adhesive to a temperature at
which the adhesive softens. The thief is then able to
open the bag and remove certain contents. The thief
can reseal the bag while the adhesive is still soft by
applying pressure, all without any evidence of
tampering.
Tamper-evident closures for plastic bags have
been formed to combat the problem of refrigerant
tampering. Such closures provide an indication that
the bag was opened, whether or not a refrigerant is
first applied. These closures include multiple
adhesive and nonadhesive layers that have differing
strengths so that when the closure is forced open, one
or more of the layers is permanently altered, even if a
refrigerant is first applied. For example, U.S. Patent
CA 02461583 2004-04-08
-4-
No. 4,834,552 to K.R. Makowka describes a ,
tamper-evident seal for a plastic envelope. The
tamper-evident seal comprises two paper layers and an
adhesive layer. One of the paper layers is bonded to a
closure flap on the back wall of the envelope, the
other paper layer is bonded to the front wall of the
envelope, and the adhesive layer is applied to the free
side of one of the paper layers. To close the
envelope, the closure flap is folded over the envelope
opening, and the adhesive layer is pressed onto the
paper layer that has a free side. The adhesive seeps
into the interstices of the paper layers to form a
mechanical-type lock with the paper layers. The
strength of this mechanical-type lock is apparently
greater than the internal strength of the paper layers,
even if a refrigerant is first applied, so that the
paper layers break apart when the seal is forced open.
Unfortunately, closures such as the seal
disclosed in the Makowka patent have several
shortcomings. These closures do not provide any
evidence of refrigerant tampering unless the closure is
actually forced open. Thus, if a thief begins to
attempt to open a bag by applying a refrigerant, but
his efforts are somehow thwarted before he is able to
force the bag open, the thief's tampering will go
undetected. Even if these closures are farced open,
they do not always satisfactorily provide evidence of
such tampering. Any delamination of one of the paper
layers can only be detected by close inspection; the
delamination is not bold and distinct as would be
desired. Once the closure is forced open, it is
possible to use additional adhesive/glue to reclose the
closure, without any readily visible evidence that the
closure was ever opened. In addition, these closures
generally have high production costs. For example, in
addition to an adhesive layer as is commonly used to
close plastic bags, the closure described in the
CA 02461583 2004-04-08
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Makowka patent requires two paper layers, which must
both be bonded to the envelope during its production..
Furthermore, the bag and closure taught by the Makowka
patent is not readily recyclable.
To overcome these shortcomings in the
existing technology, what is needed is a closure in the
form of a tape or label that provides evidence of
forced opening of a seal created by the tape or label
regardless of whether a refrigerant is applied, and
additionally, provides evidence of refrigerant
tampering, regardless of whether the closure is
actually opened. Further, closures which provide
evidence of heat tampering are also needed. The tape
or label should be easy to close, and the evidence of
tampering provided should be readily visible, i.e.,
bold and distinct. Furthermore, the tape or label
providing these features should also be relatively
inexpensive and easy to recycle and include only
nonhazardous materials. As explained in the following,
the present invention provides a tape and label that
meets these criteria.
In accordance with this disclosure, a bag
closure that provides evidence of refrigerant tampering
is provided. The. bag closure is sensitive to being
cooled below a particular "breakdown temperature," so
that if a refrigerant is applied, th.e visual appearance
of the closure permanently changes, regardless of
whether the closure is opened. In one preferred
embodiment of the invention, the closure includes an
adhesive layer anal a delaminating layer that visibly
delaminates when the closure is cooled below the
breakdown temperature. The bag includes a back and
front wall that are joined at their peripheral side and
bottom edges. The opening of the bag is formed by the
CA 02461583 2004-04-08
-6-
upper ends of the back and front walls, and the closure
is located at this opening.
In accordance with further aspects of the
invention, one surface of the delami.nating layer is
bonded to a closure flap formed at the upper end of the
bag front wall. When the closure is closed, the
opposite surface of the delaminating~ layer is adhered
- to one surface of the adhesive layer, and the opposite
surface of the adhesive layer is adhered to the bag
' 10 back wall. Further, in one preferred embodiment, the
bag is formed so that one surface of the adhesive layer
is adhered to the bag back wall and the opposite
surface of the adhesive layer is covered with a
peel-back strip. To close the closure, the peel-back
strip is removed and the free surface of the adhesive
layer is pressed against the delaminating layer.
In accordance with still further aspects of
the invention, as the closure is cooled, e.g., with a
refrigerant, the delaminating layer delaminates from
the front wall prior to the adhesive: layer losing its
tack which occurs when the adhesive layer reaches its
glass transition temperature. Thus, an indication of
refrigerant tampering is provided before the
refrigerant allows the closure to be easily opened.
Furthermore, the delaminating layer delaminates
regardless of whether the closure is actually opened
so that the closure provides evidence of the mere
application of a refrigerant. Also, regardless of
whether a refrigerant is first applied, the
delaminating layer delaminates whenever the closure is
opened, so as to provide an indication that the
contents of the bag have been accessed.
In accordance with still further aspects of
the invention, the delaminating layer comprises a layer
of ink that is applied to the closure flap on the bag
f rout wall. Furthermore, the adhesive layer comprises
a pressure-sensitive adhesive and the closure flap
CA 02461583 2004-04-08
_7_
comprises a plastic material. The closure flap, ink
and pressure-sensitive adhesive are chosen so that when
the temperature of the closure is above the glass
transition temperature of the adhesive, the bond (i.e.,.
affinity) between the ink layer and the adhesive layer
is at least as strong as (and preferably stronger than)
the bond between the ink layer and the closure flap. As
a result, the ink layer delaminates when the closure is
forced open. Also, the adhesive layer and the closure
flap are chosen such that, when cooled, the closure
flap and the adhesive layer shrink at different rates.
As a result, the ink layer - which is sandwiched
between the adhesive layer and closure flap -
delaminates as the closure is cooled below the
breakdown temperature. Preferably, both the ink and
closure flap have contrasting colors so that
delamination of the ink can be easily seen. Further,
in one preferred embodiment, the closure flap comprises
a polyethylene plastic and the ink is water based. In
one preferred embodiment, the ink is in direct contact
with the closure flap without any intervening primer or
releasant, and the closure flap is not pretreated
(e.g., by corona discharge). A process for
manufacturing this bag closure is also provided by the
invention.
In a second preferred embodiment of the
invention, the previously described delaminating layer
of the bag closure includes two layers of ink. A
patterned layer of clear ink is applied directly to the
closure flap without any intervening primer or
releasant, or pretreating of the closure flap. For
example, the clear ink can be applied in a pattern to
form a series of "stop signs." After the pattern of
clear ink is applied, the closure flap is treated,
e.g., with a corona discharge process, so that the ink
more readily adheres to the closure flap. Then a
uniform layer of colored ink is applied over the
CA 02461583 2004-04-08
_8_
patterned layer of clear ink and ink-free portions of
the closure flap. when the closure flap is closed, the
two ink layers are sandwiched between the adhesive and
the closure flap, which is preferably a polyethylene
plastic. If the closure flap is quickly "frozen" or
forced open, the clear ink, and the colored ink
positioned over the clear ink delaminates from the
closure flap. The colored ink in between the clear ink
pattern remains an the closure flap, so that the
pattern of the clear ink appears.
In accordance with another aspect of the
present invention, a tamper evident tape or label
formed similarly to the tamper-evident closure
described above is provided. In one preferred
embodiment of this aspect of the present invention, a
polyethylene plastic layer is used to form the tape or
label backing. A layer of ink is applied to the
plastic layer, and a layer of pressure-sensitive
adhesive is then applied over the ink. The tape or
label is secured by pressing the adhesive layer onto
the desired surface. As with the closures described
above, if a thief or other unauthorized person applies
a refrigerant to the tape and/or forces the tape open,
the tape provides permanent evidence of such tampering
by visible delamination of the ink layer.
In accordance with yet another embodiment of
this invention, a bag closure that provides evidence of
heat tampering is also provided. The bag closure is
sensitive to being heated above a particular activation
temperature, so that if heat is applied, the visual
appearance of the closure permanently changes,
regardless of whether the closure is opened.
In one preferred embodiment of the invention,
the closure includes a thermally activatable or
thermochromic ink layer that visibly changes when the
closure is heated at or above the activation
temperature of the ink. The thermally activatable ink
CA 02461583 2004-04-08
_g_
layer can be applied uniformly or in a pattern, for
example, so that upon activation, the ink forms a
series of "stop signs." Alternatively, a uniform
coating of a standard ink can be used to provide a
background contrast, and a layer of the thermally
activatable ink can be applied in a pattern, such as
the above-referenced series of "stop signs" over the
layer of standard ink. In this embodiment of the
invention, when d.ry, the standard in,k and the thermally
activatable ink are the same color. However, if the
closure is heated to a temperature at or above the
activation temperature of the thermally activatable ink
layer, the thermally activatable ink visibly changes so
as to form a series of contrasting '°stop signs°' against
the standard ink layer. In a particularly advantageous
embodiment of the invention, the thermally activatable
ink layer is used in combination with any of the tamper
evident closures described above.
As.will be appreciated from the foregoing
brief summary, this disclosure provides a tape or label
that can be used as a bag closure that provides
evidence of the mere application of a refrigerant,
and/or the mere application of heat, regardless of
whether the closure is actually forced open.
Furthermore, if the closure is forced open, whether or
not a refrigerant and/or heat is first applied, the
tape or label provides a permanent indication that the
closure was opened. As will be further appreciated
from the foregoing brief summary, the tapes and labels
and corresponding manufacturing process provided by
this disclosure present a cost savings over existing
tapes and labels since they include a. minimal number of
layers which are inexpensive to form. The materials
used are inexpensive and nonhazardous. Furthermore,
because the tape and labels are preferably formed of a
polyethylene plastic, they are easily recycled.
CA 02461583 2004-04-08
-10-
Brief Description of the Drawings
The foregoing aspects and many of the
attendant advantages of this invention will become more
readily appreciated as the same becomes better
understood by reference to the following detailed
description of preferred embodiments, when taken in
conjunction with the accompanying drawings, wherein:
FIGURES lA and 1B are pictorial views of a
bag including a tamper-evident seal;
FIGURE 2A is a side cross-sectional view of
the bag shown in FIGURES 1A and 1B, and FIGURE 2B is a
side crass-sectional view of the bag with the seal
closed;
FIGURE 3 is a front view of the bag
illustrating the visual appearance of the seal when
closed;
FIGURE 4A is a~pictorial view of the seal
illustrating how the seal visually distorts if the seal
is forced open, a:nd FIGURE 4B is a pictorial view
illustrating how the visual distortion remains, even if
the seal is reclosed;
FIGURE 5 is a front view of the bag
illustrating the visual distortion of. the seal that
occurs when the seal is cooled below a particular
temperature;
FIGURES &A-6D are top views of a plastic
strip formed in accordance with a further embodiment of
the present invention, and FIGURE 6E is a side
cross-sectional view of the plastic strip shown in
FIGURES 6A-6D;
FIGURE 7 is a side cross-sectional view of a
bag with a tamper-evident seal including the plastic
strip shown in FIGURES 6A-6E;
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FIGURE 8A is a front view of the bag shown in
FIGURE 7, illustrating the visual appearance of the
seal when a portion of the seal is forced open;
FIGURE 8B is an end cross-sectional view of
the seal in FIGURE 8A, illustrating the delamination of
the portion of the seal that is forced open; and
FIGURE 9A is a side cross-sectional view of a
bag including a tamper-evident tape formed in
accordance with a further aspect of the present
invention, and FIGURE 9B is a side cross-sectional view
of a bag with the tamper-evident tape sealing the bag
closed;
FIGURE 10 is a pictorial view of a roll of
tamper-evident tape; and
FIGURE 11 is a side view of a section of a
roll of tamper-evident tape.
Detailed Description of the Preferred Embodiments
FIGURES lA and 1B illustrate a bag 10 that
incorporates a seal 12 formed at the opening 14 of bag
10. The bag includes a front wall 1E and a back wall
18 that are joined together at the bottom and side
edges to form an enclosure having opening 14 at the
upper ends of front wall 16 and back wall 18. Seal 12
is included to close opening 14 and to provide visual
evidence of any forced opening of seal 12.
Furthermore, seal 12 will visually distort if the
opening of the bag is cooled below a particular
"breakdown temperature," e.g., by the: application of a
refrigerant .
Seal 12 includes a plastic strip 20, a layer
of ink 22, and a layer of adhesive 24. The bottom end
of plastic strip 20 is attached to the inner surface of
the upper end of front wall 16. Ink layer 22 is
printed on the inner surface of plastic strip 20.
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-12-
While ink layer 22 is represented by a grid of lines in
the figures, ink layer 22 is preferably a uniform layer
of ink. As shown in FIGURES lA and :LB, adhesive layer
24 is preferably applied to the inner surface of the
upper end of back wall 18. The free surface of
adhesive layer 24 is covered with a peel-back strip 26.
To seal the opening of the bag closed, peel-back strip
26 is removed from adhesive layer 24, and plastic strip
20 is pressed onto adhesive layer 2~6, which is a
pressure sensitive adhesive. Thus, when seal 12 is
closed, ink layer 22 is sandwiched between adhesive
layer 24 and plastic strip 20.
The location of the various layers of seal 12
can be seen more definitely in the side cross-sectional
views in FIGURES 2A and 2B. Plastic: strip 20 is
attached at its Lower end 28 to the inner surface of
the upper end of front wall 16, and ink layer 22 is
printed on the inner surface of plastic strip 20.
Adhesive layer 24 is applied to the inner surface of
the upper end of back wall 18, and the free surface of
adhesive layer 24 is covered with peel-back strip 26.
FIGURE 2B illustrates the alignment of seal 12 after
peel-back strip 26 is removed and the seal is pressed
closed. Plastic strip 20 is pressed onto adhesive layer
24 so that ink layer 22 adheres to adhesive layer 24.
As shown in FIGURES 2A and 2B, adhesive layer
24 preferably extends approximately an eighth of an
inch below the bottom edge of plastic strip 20, so that
when seal 12 is closed, a portion of adhesive layer 24
adheres directly to front wall 16. This helps prevent
loose contents within the bag from partially opening
seal 12 as the contents bump against the seal. Without
a portion of adhesive layer 24 adhering to front wall
16, contents within the bag could falsely activate the
tamper evidencing means of seal 12.
Preferably, back wall 18 includes a
detachable identification tab 30 formed by perforating
CA 02461583 2004-04-08
-13-
the upper end of back wall 18. As shown in FIGURES lA
and 1B, the perforations allow identification tab 30 to
be easily removed. Preferably, identifying text or
numbers are printed on identification tab 30 and
matching identifying text or numbers are printed on
either front wall 1& or back wall 18 of the bag. When
the bag is sealed closed, identification tab 30 can be
removed and used as a receipt.
After being closed, if seal 12 is forced
open, the seal visually distorts. 7?lastic strip 20 is
partially transparent so that ink layer 22 can be seen
from the outer side of plastic strip 20 before seal 12
is closed, as shown in FIGURE lA, and after seal 12 is
closed, as shown in FIGURE 3, which is a front view of
the bag with seal 12 closed. In particular, objects
that are in directs contact with the inner surface of
plastic strip 20 can be seen from the outer side of
plastic strip 20. In contrast, if an object is
positioned near the inner surface side of plastic strip
20, but not in direct contact with the inner surface of
plastic strip 20, the object can barely be seen, if at
all, from the outer side of plastic strip 20.
Accordingly, when ink layer 22 is printed on the inner
surface of plastic strip 20, the ink can be seen from
the outer surface of plastic strip 20, as shown in
FIGURE lA and FIGURE 3. However, as illustrated in
FIGURE 4A, if.seal 12 is forced open, a substantial
portion of the ink remains adhered to adhesive layer 24
and accordingly delaminates from plastic strip 20.
When this occurs, the ink is no longer visible from the
outer side of plastic strip 20.
If an attempt is made to re-close seal 12,
the visual distortion of the ink is still visibly
apparent, as illustrated in FIGURE 4B, because the ink
does not re-adhere to plastic strip 20. The ink is
printed onto plastic strip 20 while wet, i.e., in a
liquid state. The ink then dries. Once dried, if the
CA 02461583 2004-04-08
-14-
ink is delaminated from plastic strip 20, the ink no
longer adheres to the inner surface of plastic strip
20. As a result, even when plastic strip 20 is
re-pressed against adhesive layer 24, the ink remains
sufficiently separated from plastic strip 20 so that
the ink cannot be clearly seen through the outer side
of plastic strip 20. In the illustration shown in
FIGURES 4A and 4B, a portion 32 of seal 12 is forced
open and then re-closed. As illustrated in FIGURE 4B,
portion 32 that was forced open is visibly distorted,
in sharp contrast to the portion 34 of seal 12 that was
not opened.
In order for ink layer 22 to delaminate from
plastic strip 20 when the seal is forced open, it is
necessary that the bond between the ink and plastic
strip 20 be relatively weak i.e., weak relative to the
bond between adhesive layer 24 and ink layer 22.
Because the bond between the ink and plastic strip 20
is relatively weak, if adhesive layer 24 merely adhered
to ink layer 22, the overall strength of seal 12 would
be relatively weak. Adhesive layer :Z4 would simply
release from plastic strip 20 whenever ink layer 22
delaminated from plastic strip 20. This could cause
the seal to inadvertently open during handling and
shipping of the bag, which would be highly undesirable.
To avoid this potential problem, ink layer 22 is
sufficiently thin so that there are ,voids in ink layer
22. As a result, when seal 12 is pressed closed,
portions of adhesive layer 24 adhere to the inner
surface of plastic strip 20 through the voids in the
ink. The affinity between the adhesive and the plastic
strip is sufficiently high so that the strength of seal
12 is acceptably strong. Thus, seal 12 generally does
not open unless it is intentionally forced open by
pulling plastic strip 20 away from back wall 18.
In addition to distorting when forced open,
seal 12 is sensitive to cooling so that if the seal is
CA 02461583 2004-04-08
-15-
cooled below a particular "breakdown temperature,"
e.g., -10°F, the seal visibly distorts in a manner
similar to when the seal is forced open. In
particular, as illustrated in FIGURE 5, when the seal
is cooled below a breakdown temperature, ink layer 22
delaminates from plastic strip 20 so that the ink can
no longer be clearly seen when viewing the outer side
of plastic strip 20. The mechanism for the
delamination of the ink is differential rates of
shrinking of adhesive layer 24 and plastic strip 20.
In particular, plastic strip 20 and adhesive layer 24
are chosen so that they shrink at different rates when
cooled. In a preferred embodiment, the adhesive
shrinks mare and at a greater rate than plastic strip
20. Because the ink has a strong affinity to the
adhesive, as the adhesive and the plastic strip shrink
at different rates, ink layer 22, which is sandwiched
between the plastic strip and the adhesive, is pulled
away from the plastic strip. As a result, the ink is
na longer clearly visible through the outer side of
plastic strip 20.
The ability of seal 22 to provide visual
evidence of cooling is important because a common
technique used by thieves to gain access to plastic
bags sealed with a pressure sensitive adhesive is to
"freeze" the bag with a refrigerant, as previously
described herein. Seals that combat this form of
tampering have been introduced. However, as previously
described herein, these prior art seals do not provide
evidence of mere "freezing." Rather, the prior art
seals simply provide evidence of a forced opening of
the bag, whether or not the bag is first "frozen.°° In
sharp contrast, seal 12 provided by the present
invention provides a permanent visual indication if the
seal is cooled below the breakdown temperature,
regardless of whether or not the seal is actually
forced open.
CA 02461583 2004-04-08
a
-16-
Furthermore, seal 12 provided by the present
invention cannot be opened without ink layer 22 visibly
delaminating, whether or not the seal is first
"frozen.°' This is ensured by choosing a pressure
sensitive adhesive for adhesive layer 24 that has a
relatively low glass transition temperature. When a
pressure sensitive adhesive is cooled to its glass
transition temperature, the adhesive loses its adhesive
properties, i.e., its adhesive tack. The adhesive is
chosen so that its glass transition temperature is
lower than the breakdown temperature of seal 12, at
which temperature ink layer 22 delaminates from plastic
strip 20. As a result, as the seal. is progressively
cooled, ink layer 22 at least partially delaminates
from plastic strip 20 before the glass transition
temperature of the pressure sensitive adhesive is
reached. Thus, the ink delaminates before the seal is
sufficiently "frozen" to allow the seal to be opened
without any significant force.
It is important that the breakdown
temperature of seal 12 be greater, i.e., at a higher
temperature, than the glass transition temperature of
the pressure sensitive adhesive, to ensure that the
seal cannot be opened without detection. If, in
contrast, the glass transition temperature is above the
seal's breakdown temperature, a refrigerant could be
used to cool the seal to the adhesive°s glass
transition temperature, at which point the adhesive
would lose its adhesive tack and release from the upper
end of back wall 18 and/or ink layer 22 and plastic
strip 20. The bag could then be opened, and then after
warming to room temperature be reclosed. As long as
the temperature of the bag is kept above the breakdown
temperature, no visual indication of tampering would
exist.
Adhesive layer 24 and plastic Strip 20 are
also preferably chosen so that they expand at different
CA 02461583 2004-04-08
relative rates when warmed, i.e., the plastic strip and
adhesive layer have different thermal coefficients of
expansion. In one preferred embodiment, the adhesive
expands more and at a greater rate i~han plastic strip
20: As a result,. if, after the seal is "frozen" below
the breakdown temperature, a portion of ink layer 22
has not delaminated from plastic strip 20, the ink will
further delaminate upon warming of seal 12. This
further ensures that the delamination is sufficient to
provide a significant visual indication of "freezing."
The embodiment of seal 12 shown in FIGURES 1
and 2 is one preferred embodiment of-_ the invention.
FIGURES 9A and 9E3 illustrate a second embodiment. The
second embodiment includes many of t_he same component
parts as the first preferred embodiment; accordingly,
like components are referred to with the same reference
numerals, except that the reference numerals are double
primed. In the second embodiment, adhesive layer 24"
is applied to the inner surface of p.nk layer 22"
instead of to the inner surface of t=he back wall 18".
The free surface of adhesive layer 24" is covered with
peel-back strip 26". The other structural aspects of
seal 12" are the same as seal 12 in FIGURES 1 and 2.
To close the seal, peel-back strip 26" is removed and
the free surface of adhesive layer 24" is pressed
against the inner_ surface of back wall 18". If the
seal is forced open or "frozen," inlc layer 22" visually
delaminates, as described with respect to the first
embodiment shown in FIGURES 1 and 2. While the second
embodiment shown in FIGURES 9A and 9B is an alternative
embodiment, the first embodiment shown in FIGURES 1 and
2 is preferred because adhesive 24 adheres more
strongly to back wall 18 when applied hot, as described
in more detail below, as opposed to adhesive layer 24'°
of the second embodiment that is pressed against back
wall 18" to close seal 12".
CA 02461583 2004-04-08
Q
s
-1a-
While ane preferred embodiment of a bag
incorporating a seal formed in accordance with the
present invention has been shown so far, various other
bag structures can be formed. For example, with
respect to FIGURE 2A, if front wall 16 is formed of the
same material as plastic strip 30, front wall 16 can be
extended to the same height as back wall 18. Plastic
strip 30 would then be eliminated and ink layer 22
would be printed on the inner surface of the upper end
of front wall 16.. As a further alternative, front wall
16 could be extended beyond the height of back wall 18,
so that a fold-over closure flap is formed by the upper
end of front wall 16. Adhesive layer 26 would then be
applied to the outer surface of back wall 18, and the
closure flap would be folded over the opening of the
bag onto the adhesive on the outer surface of back wall
18.
The seal provided by the present invention
can be formed as a tape or label 40, as shown in
FIGURES 10 and 11. The structure of tape 40 is
essentially the same as the structure of seal 12" of
the second embodiment shown in FIGURES 9A and 9B. Tape
40 includes a flexible, plastic backing 42, a layer of
ink 44 printed on plastic backing 42, and a layer of
adhesive 46 applied to the free surface of ink layer
44. Plastic backing 42 is analogous to plastic strip
20" in FIGURES 9A and 9B, and ink layer 44 is
sandwiched between plastic backing 42 and adhesive
layer 46. The free surface of adhesive layer 46 is
covered with a peel-back strip 48. To apply tape 40 to
an object, peel-back strip 48 is removed from a section
of the tape then adhesive layer 48 is pressed onto the
object to form a seal. As desc~.~ibed with respect to
bag 10 and 10", if tape 40 is forced off the object or
if the tape is frozen below its breakdown temperature,
ink layer 44 delaminates from plastic backing 42, to
provide a visual indication of tampering.
CA 02461583 2004-10-08
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Roll of tape 40 can be used in various
applications to form seals on surfaces of containers
such as bags and envelopes. For example, tape 40 could
be used to seal an envelope. The tape could also be
used to close and seal the opening of a bag as previously
described herein. For example, bag 10 shown in FIGURE
2A could be formed without seal 12, so that the bag
simply consists of back wall 18 and front wall 16.
Front wall 16 could be extended up beyond back wall 18,
so that the upper end of front wall 16 forms a closure
flap that can be folded over opening 14 onto the outer
surface of back wall 18. The closure flap could then
be secured onto back wall 18 with tape 40 by
overlapping the tape over the closure flap and back
wall 18. Alternatively, bag 10 shown in FIGURE 2A
could be formed without seal 12 and then the upper
portion of back wall 18 could be folded over the upper
end of front wall 16 to contact the outer surface of
front wall 16. The closure flap could then be secured
onto front wall 16 with tape 40 by overlapping the tape
over the closure flap and front wall 16.
In addition to providing a tape for forming
seals on containers, tape 40 can be printed on, for
example, on plastic backing 42 to provide a label or
other type of indicating means.
The seal provided by the present invention is
preferably constructed of relatively simple,
inexpensive, and nonhazardous materials. The seal is
preferably manufactured on a continuous line system,
using conventional equipment including printers and
handling machines. With respect to seal 12 shown in
FIGURE 2A, plastic strip 20 is preferably formed of a
high density polyethylene or other polyolefin such as
polypropylene. Plastic strips 20 are preferably formed
of polyethylene, as opposed to some other plastic such
as polyester, so that the strips can be easily
recycled. Preferably, plastic strip 20 is colored so
CA 02461583 2004-04-08
s
-20-
as to contrast with the color of the ink of ink layer
22. For example, if the ink is blue, the plastic strip
could be yellow. The plastic strip can be formed using
an extrusion process as is commonly done in the
plastics industry. Typically, to extrude polyethylene
sheets, polyethylene pellets are melted and then
extruded. To form colored plastic strips, color
pigment, e.g., yellow pigment, is preferably added to
the melted polyethylene, e.g., at a ratio of ten
percent (10%) of the total mixture. In one preferred
embodiment, the polyethylene sheets are formed of a
thickness of approximately 2.3 mils. The polyethylene
sheets are cut into approximately 1.125 inch strips,
and then cut to length, to form plastic strips 20.
I5 The ink of ink layer 22 is preferably a
water-based ink that has a Ph of 7.5 to 8.2, such as
Universal Reflex Blue*sold by CPI Inks, Inc. However,
other inks such as a solvent-based ink could be used.
The ink is printed on the inner surface of plastic
strip 20. Actually, the ink is preferably printed on
the polyethylene sheets before the sheets are cut to
form plastic strips 20. The pattern of ink does not
have to be very exact; in fact, in one preferred
embodiment the ink is printed as a uniform layer.
Thus, inexpensive printing techniques can be used. For
example, a flexographic press that includes a photo
polymer print roller can be used to print the ink even
if a particular pattern is desired. The advantage of
using a flexographic press is that the photo polymer
roller for creating the print pattern is relatively
inexpensive to manufacture. In contrast, a rotogravure
printer, which includes steel printing plates and is
much more expensive, has generally been used to hold
more exact registration and produce more precise
characters on the walls of plastic bags.
As previously described herein, while the
precision of the pattern of ink is not critical, the
* trade-mark
CA 02461583 2004-04-08
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thickness of the ink layer is important. Ink layer 22
must be sufficiently thin so that there are microscopic
voids in the ink through which adhesive layer 24 can
migrate and therefore adhere to plastic strip 20. If
ink layer 22 is too thick, adhesive layer 24 will not
migrate through the ink layer and adhere to the inner
surface of plastic strip 20. Instead, the adhesive
layer will only adhere to ink layer 22, which would
result in a seal that is toa weak. It is also
important that the ink have a relatively weak bond or
adhesion to the inner surface of plastic strip 20, so
that ink layer 22 delaminates from plastic strip 20 if
the seal is forced open or "frozen."
The structure and manufacturing process of
the seal embodiment shown in FIGURES 1 and 2 do not
involve any pretreating, such as with a corona
discharge or a silicon releasant, of the inner surface
of plastic strip 20. That is, the ink is applied
without any intervening adhesion promoting material or
pretreating, and without any intervening adhesion
suppression material. Accordingly, this embodiment of
the seal is very inexpensive to manufacture. For
example, with respect to FIGURE 2A, the inner surface
of plastic strip 20 is not pretreated with a corona
discharge process, because a relatively weak adhesion
between the ink and plastic strip is desired.
Furthermore, because a water-based ink is preferably
used, which has a relatively weak adhesion with
polyethylene, no releasants such as silicone are
required.
With' respect to FIGURE 2A, ink layer 22 is
preferably printed onto the inner surface of plastic
strip 20 in a single coat, which involves printing wet
ink onto plastic strip 20. The single coat of ink is
uniform, i.e., a flood coat. While it is important
that ink layer 22 is sufficiently thin so that there
are voids in ink layer 22, there is a lower limit on
CA 02461583 2004-04-08
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haw thin the ink should be. If ink layer 22 is
extremely thin, there will riot be a sufficient visual
color contrast in the seal when ink layer 22
delaminates from plastic strip 20. In one preferred
embodiment, an appropriate ink thickness is obtained by
using a water-based ink that has a viscosity reading of
20 to 30 seconds with a # 3 Zahn cup, and printing the
ink on plastic strip 20 with a flexographic printer.
After the wet ink is printed on~plastic strip 20, the
ink is thoroughly dried, for example, by using heat
guns.
It is also possible to print a particular
pattern of ink e.g., a grid pattern, onto plastic strip
20. For example, using a flexographic press, two coats
of ink could be printed onto plastic strip 20. The
first coat of ink would be a flood coat that is
uniform, and the second coat of ink would be applied in
a grid pattern over the flood coat.
Regardless of the ink pattern used, the
bottom end of plastic strip 20 is preferably secured to
the inner surface of front wall 16 by a heat seal,
e.g., a running heat sealer, as is commonly done to
seal together plastic materials such as polyethylene.
However, plastic strip 20 can also be attached in other
ways, e.g., using an adhesive or glue. Plastic strip
20 is preferably secured to the inner surface, as
opposed to the outer surface, of front wall 16 so that
it is relatively easy to visually detect any slits or
cuts made near plastic strip 20. While it is certainly
possible to adhere plastic strip 20 to the outer
surface of front wall 16, if this done, a slit, e.g.,
made using a razor blade, could be made under plastic
strip 20 where plastic strip 20 ~s sealed to front wall
16. It would be difficult to visually detect such
slits as the slits would be underneath the
substantially opaque plastic strip 20.
CA 02461583 2004-04-08
a
-23-
As previously described, front wall 16 and
back wall 18 are preferably formed of a plastic
material, such as polyethylene. Common extrusion
techniques can be used to form the back and front
walls. The side edges of the back and front walls are
preferably joined together by a heat_ seal. The bottom
ends of the back and front walls are>_ preferably formed
of a single sheet of plastic that is folded to form the
bottom end, as shown in FIGURE 2A. Alternatively, two
separate sheets of plastic could be used to form the
back and front walls, in which case, the bottom ends of
the walls would be joined together with a heat seal.
Because the walls of the bag and plastic strip 20 are
all preferably formed of polyethylene, the bag is
recyclable.
As previously described, adhesive layer 24 is
applied to the inner surface of back wall 18, as shown
in FIGURE 2A. The free surface of adhesive layer~24 is
covered with peel-back strip 2&. To close the seal 12,
peel-back strip 28 is removed and the free surface of
adhesive layer 24 is pressed against ink layer 22 as
shown in FIGURES 2A and 2B. Preferably, no intervening
materials are applied to the inner surface of back wall
18 or the inner surface of ink layer 22. As a result,
when the seal is closed, adhesive layer 24 is in direct
contact with the inner surface of bacrk wall 18 and the
inner surface of ink layer 22, as shown in FIGURES 2H.
In the preferred embodiment, adhesive layer
24 is formed of a pressure sensitive adhesive that is
rubber-based, has a relatively high liquid tactifier
content, and is applied as a hot melt using an
extrusion process. The adhesive must have a low glass
transition temperature and simultaneously a relatively
high internal cohesive strength. As previously
described, it is important that the adhesive have a
glass transition temperature that is below the
breakdown temperature of the seal. Preferably, the
CA 02461583 2004-04-08
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adhesive has a glass transition temperature that is
below -10°F. In addition to the requirement that the
glass transition temperature be below -10°F, the
adhesive preferably has the following characteristics:
180° peel strength of 7.9 lbs. (~0.4 lbs.) on steel;
viscosity of 10,500 cps at 300°F, 3,900 cps at 325°F,
2,000 cps at 350°F; a melting point of 181°F; a SAFT
reading of 500gm/sq. in. at 143°F; and an application
temperature of 300-325°F. In one preferred embodiment,
the adhesive is clear so that ink layer 22 can be seen
through balk wall 18 and adhesive layer 24 when seal 12
is closed. Various adhesive compositions exist in the
prior art. Based upon the preceding characteristics,
an appropriate pressure-sensitive adhesive can be
readily composed. Most likely, the :basic ingredients
of the adhesive include a rubber base of synthetic
block polymers with a liquid tactifier added to provide
the specified viscosity. An adhesive having the
preceding characteristics can be manufactured by
various adhesive producers, including Swift Adhesives
Co. and Ecomelt, Inc.
To apply the adhesive, the adhesive is melted
and extruded onto the inner surface of back wall 18.
Peel-back strip 28 is then placed over the free surface
of the adhesive. In one preferred embodiment, the
peel-back strip is formed of high density polyethylene,
and the surface of the peel-back strip that is in
contact with the adhesive is coated 'with silicon so
that the peel-back strip easily releases from the
adhesive. When the adhesive cools, a pressure
sensitive adhesive is formed.
While the materials and manufacturing process
were described in the context of a bag incorporating a
seal as provided by the present invention, it will be
readily appreciated that the materials and
manufacturing process of tape 40 shown in FIGURES 10
and 11 are basically the same. In particular, tape 40
CA 02461583 2004-04-08
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is formed of the same materials as in the same process
as seal 12~~ shown in FIGURES 9A and 9B.
FIGURES 6A-E illustrate an alternative ink
composition/pattern and manufacturing process for
plastic strips 20 and 20« shown in FIGURES 2A and 9A.
A seal incorporating the plastic strip 20' shown in
FIGURES 6A-E has the benefits of being more difficult
to force open and providing a more visible indication
of a forced opening than the previously described
embodiments. The drawback of the embodiment shown in
FIGURES 6A-E is that the resulting seal does not
evidence refrigerant tampering as effectively as the
previously described embodiments. The ink/plastic
strip embodiment shown in the top views in FIGURES 6A-D
and the side cross-sectional view in FIGURE 6E includes
a plastic strip 20', a patterned layer of clear ink 60
(e. g., ink extender?, and a uniform layer of colored
ink 62. Plastic strip 20' is preferably formed of a
high density polyethylene, as previously described
plastic strip 20. Preferably, plastic strip 20' is
colored, e.g., yellow.
Patterned layer of clear ink 60 is printed,
e.g., using a flexographic press, onto an untreated
surface of plastic strip 20'. Clear ink 60 should have
a weak affinity to untreated polyethylene and should be
able to withstand a corona discharge process as
described in the following. Preferably, ink 60 is a
water-based ink as previously described, except that
ink 60 contains no pigment so that the ink is clear.
For example, colorless, water-based ink extender
manufactured by CPI Inks, Inc. sold under the name
Universal Flex Extender~can be used. However, a
solvent-based ink extender could also be used. Ink 60
is printed in a selected pattern, so that the majority
of the surface of plastic strip 20' is free of clear
ink 60. As shown in FIGURE 6B, in one preferred
* trade-mark
CA 02461583 2004-10-08
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embodiment, clear ink 60 is printed as a series of
"stop signs."
After clear ink 60 is applied and dried,
e.g., using a heat gun, the surface of plastic strip
20' on which clear ink 60 is applied is treated with a
corona discharge process to roughen and increase the
surface energy of plastic strip 20', as illustrated
pictorially by line dashes 64. In one preferred
embodiment, a corona discharge treater set to a
strength of approximately 43 to 50 dynes is used. The
corona discharge process is used so that colored ink 62
adheres well to plastic strip 20'. In particular,
after plastic strip 20' is subjected to a corona
discharge, colored ink 62 is printed as a uniform layer
over plastic strip 20', so as to cover the entire
surface of plastic strip 20'. In place of using a
corona discharge process, the plastic strip 20' could
be subjected to plasma treatment, chemical treatment,
or flame treatment.
FIGURE 7 is a side cross-sectional view of a
bag 10' incorporating plastic strip 20' to form a seal
12'. When seal 12' is closed, colored ink layer 62 is
sandwiched between adhesive layer 24~and plastic strip
20'. Because plastic strip 20' is subjected to a
corona discharge process before the application of
colored ink 62, colored ink 62 forms a strong adhesion
with those portions of plastic strip 20' not covered
with clear ink 60. As a result, if an attempt is made
to force open seal 12', colored ink 62 will not
delaminate from plastic strip 20', except along the
pattern where clear ink 60 was applied, as shown in
FIGURE 8B. Colored ink 62 does not delaminate from
plastic strip 20' because colored ink 62 adheres more
strongly to the treated gortions of plastic strip 20'
than to adhesive 24: However, because clear ink 60 was
applied to the untreated surface of plastic strip 20',
clear ink 60 does not adhere very well to plastic strip
CA 02461583 2004-10-08
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20'. As a result, colored ink 62 and clear ink 60
delaminate from plastic strip 20' along the pattern of
clear ink 60 when an attempt is made to force the seal
open.
Colored ink 62 is chosen to have a color that
contrasts with the color of plastic strip 20'. In one
preferred embodiment, plastic strip 20' is yellow and
colored ink 62 is blue. Preferably, colored ink 62 is
water-based, as is the case for previously described
ink layer 22. Because the color of colored ink 62
contrasts with the color of plastic strip 20', the
pattern in which clear ink 60 was applied boldly
appears when an attempt is made to force open the seal.
For example, as indicated in FIGURE 8A, when a portion
66 of seal 12' is forced open, the "stop sign" pattern
appears, whereas an untampered portion 68 of seal 12'
appears uniformly opaque. The delamination of colored
ink 62 and clear ink 60 in portion 66 of seal 12' is
shown in FIGURE 8B, which is an end cross-sectional
view of portion 66 shown in FIGURE 8A.
Because colored ink 62 adheres much more
strongly to treated plastic strip 20' than ink layer 22
previously described with reference to FIGURE 2A, seal
12' illustrated in FIGURE 8A is much more difficult to
open. After being closed, if seal 12' is quickly
"frozen" with a refrigerant, some "stop signs" will
appear as a result of clear ink 60 delaminating from
plastic strip 20' due to the differential shrink rates
between plastic strip 20' and adhesive 24'. However,
because colored ink 62 adheres strongly to the majority
of the surface of plastic strip 20', colored ink 62
provides structural support that prevents the
differential shrink rates from delaminating clear ink
60 as readily as ink layer 22 in the previously
described embodiments.
Other than the differences specifically
described hereinabove, the manufacturing process and
CA 02461583 2004-04-08
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materials of seal 12' are the same as for previously
described seal 12 and 12" shown in FIGURES 2A and 9A.
For example, both clear ink 60 and colored ink 62 are
preferably water-based with a viscosity reading of 20
to 30 seconds with a #3 Zahn cup, and the inks are
preferably applied using a flexographic printer.
In accordance with yet another embodiment of
this invention, a bag closure that provides evidence of
heat tampering is also provided. The bag closure is
sensitive to being heated above a particular activation
temperature, so that if heat is applied, the visual
appearance of the closure permanently changes,
regardless of whether the closure is~ opened.
As discussed above, in addition to the use of
refrigerant cooling, thieves also ueoe heating to tamper
with bag closures. Typically, the adhesive layer of a
bag closure can be heated using, for example, a hair
dryer or other heating device. The adhesive is
softened and the bag can then be opened.
To indicate tampering by heat, a "heat
activatable," or "thermochromic," ink layer is used in
the closure of the invention. As used herein, the term
"heat activatable" or "thermochromic" ink refers to
inks which, upon the application of heat, exhibit a
visible, permanent color change.
Any of the types of thermochromic inks known
in the art can be used. In the present invention, it
is particularly advantageous to use a thermochromic ink
which permanently visually changes when heated to a
temperature of at least about 140°F, more preferably at
least about 150°F, or higher. This activation
temperature range is desirable because typically the
adhesives described above for use with the closures of
the invention soften sufficiently at a temperature
between about 140°F and about 180°F to enable a person
to open the closure. A particularly preferred ink is
CA 02461583 2004-04-08
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available from Midwest Coatings Inc. of St. Louis,
Missouri, and is described in more detail below.
As will be appreciated by the skilled
artisan, the time required to effect the color change
can vary as heating conditions vary, i.e., the
activation temperature of the ink system used, the
temperature applied to the seal, how long heat is
applied, and the like. For example, when the seal is
heated to a temperature at the lower end of a range of
thermochromic ink activation temperatures, the visible
color change can take place more slowly than when
higher activating temperatures are used.
The thermochromic ink can be used in the
tamper-evident closures of the invention in various
Z5 ways. For example, in one preferred embodiment, the
thermochromic ink is applied to an outer surface of
plastic strip 20, on a surface opposite ink layer 22.
The plastic layer 20 is printed using standard
techniques, as described above, for example,.using a
flexographic press including a photo polymer roller
printed. The thermochromic ink can. be printed onto the
outer surface of plastic strip 20 before or after the
ink layer 22 is printed thereon. Further, the
thermochromic ink layer can be applied as a
substantially uniform coating on a surface of the
plastic strip 20, or applied in a pattern, for example,
so that upon activation, the ink forms a series of
"stop signs." The thermochromic ink is then dried
using conventional equipment, such as heat guns. Of
course, the thermochromic ink is dried at a temperature
below its activation temperature.
When dry, the thermochromic ink is a specific
color. However, when heated to its activation
temperature, dye compounds in the thermochromic ink can
undergo a chemical reaction. As the dye compounds
react, the color of the dye compounds changes, which
results in a change in the color of the ink.. In the
CA 02461583 2004-04-08
a
-~a-
example given below, the thermochromic ink initially
appears white, similar to a standard white ink which is
not thermally activatable. Once activated, the
thermochromic ink turns from white to a reddish-purple
color. The present invention is not, however, limited
to this particular color format. The thermochramic ink
can provide a permanent visual indication if the seal
is heated at or above the activation temperature,
regardless of whether or not the seal is actually
forced open.
Alternatively, a standard, non-thermochramic
ink, such as a conventional alcohol based ink, can be
applied in a uniform coating to the outer surface of
plastic strip 2a to provide a background contrast
I5 against which the color change is clearly visible. A
layer of thermachromic ink is then applied in a
pattern, such as the above-referenced series of "stop
signs" over the layer of standard ink. Preferably, the
standard, non-thermochramic ink and the thermochromic
ink are the same color initially, such as white. If
the closure is subsequently heated, the thermochromic
ink visibly changes so as to form a series of
contrasting reddish "stop signs" against the standard
ink layer. Preferably, an industry standard water
based clear overcoat layer is applied over at least a
portion of the strip to protect the thermochramic ink
from scuffing.
Preferably, as described above, the
thermochromic ink layer is used in combination with
seal 32, i.e., which includes an ink layer 22, and in
which the plastic strip 2a and the adhesive layer 24
exhibit differential shrinking and expansion rates. It
will be apparent, however, that the thermachromic ink
layer can be used for security purposes with various
other seals which include an adhesive bonding layer and
a plastic strip as a support far the thermochromic ink
layer which do not include ink layer 22. In this
CA 02461583 2004-04-08
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embodiment of the invention, the thermochromic ink
layer can be printed on either the inner or outer
surface of a plastic strip. The free surface of the
adhesive layer can be covered with a peel-back strip.
As described above, to seal the closure, the peel-back
strip is removed from the adhesive layer, and the
plastic strip is pressed onto adhesive layer. Further,
the thermochromic ink layer can be used without regard
to specific differential shrinking rates of the plastic
support strip and the adhesive layer upon cooling, or
differential expansion rates thereof upon warming.
Further, the thermochrornic ink layer can also
be provided as a component of a seal in the form of a
tape or label 40, as shown in FIGURES 10 and 11. The
structure of tape 40 which includes 'the thermochromic
ink layer is essentially the same as described above.
An exemplary thermochromic ink formulation is
as follows. First, Solution "A" is prepared as
follows. A carrier for the dye compounds is prepared
by mixing 85o water and 15% polyvinyl alcohol (PVOH).
This mixture can be strained, for example, using a 25
micron filter. A mixture is then prepared with the
following: 60% PVOH; 39.6% of a conventional leuco dye
(well known in the industry); 0.1% Ncpko-W dispersing
agent (industry standard); O.la Surfonal TG wetting
agent (industry standard); and 0.2% defoamer (industry
standard) .
Solution "B" is then prepared by first
formulating a carrier as described above including 85%
water and 15o PVOH. A mixture is then prepared
comprising: 60% PVOH; 39.60 bisphenol-A activator; O.lo
Nopka-W dispersing agent (industry standard); 0.1%
Surfonol TG wetting agent (industry standard); and 0.2%
defoamer (industry standard).
The final formulation is prepared by mixing 1
part Solution "A" to 4 parts Solution "B". Although
not wishing to be bound by any explanation of the
CA 02461583 2004-04-08
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invention, it is believed that when the ink is heated,
the bisphenol A activator can initiate a chemical
reaction within the structure of the leuco dye
compound, thus altering its structure and producing a
color change.
This particular formulation has an activation
temperature of abo~.a 155°F. The ink is initially white
when dry and changes color to a bright red or reddish
purple when heated to its activatior.~ temperature or
i0 higher to provide a permanent visible color change.
This, as noted above, is particularly advantageous for
use with adhesives having a softening point between
about 140°F and 180°F.
However, this formulation is subject to
various modifications. For example, higher or lower
percentages of each of the formulation components can
be used, and/or higher or lower "A" to "B" ratios, and
the like. Further, thermochromic dye compounds and
thermochromic dye compound activators, other than leuco
dyes and bisphenol A, respectively, can also be used in
accordance with this aspect of the invention. In
addition, polyvinyl alcohol is a preferred carrier
because its use can provide a desired degree of
flexibility to the ink layer, which is particularly
advantageous in continuous in-line application to
plastic substrates. However, other carriers can also
be used.
As will be appreciated by the skilled
artisan, preferably the particle size of the ink
formulation is sufficiently small to provide smooth
consistent application of the ink to a substrate
surface. Further, it is preferred that the
thermochromic ink formulation not be cut or mixed with
other inks or solutions, and that the equipment used to
produce the seals of the invention is clean.
While the preferred embodiments of the
invention have been illustrated and described, it will
CA 02461583 2004-04-08
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be appreciated that various changes can be made therein
without departing from the spirit and scope of the
invention. Accordingly, the scope of the invention is
not to be limited by the description of the preferred
embodiments, but instead should be determined by
reference to the claims that follow.
