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A PRESSURE-SENSITIYE ADNESIYE LAMINATE,A METHOD AND A D~YICE FOR MODIFAYING AN
INITIAL RELEASE FORC~ IN SUCH A LAMINATE
The invention relates to a pressure-sensitive adhesive
laminate comprising an adhesive layer and a release layer contacting
each other.
Pressure-sensitive adhesive (PSA) laminates are well-
known in the art and commonly used as tapes, decals tabels and the like.
PSA laminates made with radiation-curable silicones are described in the
article of R.P. Eckberg "Chemistry and te~:hnology of radiation curable
silicone release coatings" published in Advances in Pressure Sensitive
Adhesive Technology-1 edited by Donata;s Satas, 1992, pages 50-76.
This article describes how during manufacturing of the pressure-
sensitive adhesive laminate an ultra-violet radiation is used in order to
cure the release layer and to give stable properties to the PSA
laminates. In such a manner, the release force between the adhesive
layer and the release layer is set and remains constant.
A drawback of the known PS,A laminates is that once the
initial release force is set, the latter can not be changed at a later stage.
Moreover, if laminates with different n~lease forces have to be
manufactured, this can not be realised in a .continuous manner. For each
value of the release force, a dedicate laminate with dedicated release
layer composition has to be prepared, thus necessitating to stop the
production and change the formula of the release layer.
It is an object of the present invention to provide a solution
to the aforementioned problem.
For this purpose, a PSA laminate comprising a release
layer and an adhesive layer contacting each other is provided, said PSA
laminate being characterised in that said adhesive layer comprises a
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first compound that is reactive on application of an ionising radiation or
an actinic radiation, in order to modify an initial release force between
said adhesive layer and said release layer. The addition of the first
compound in the adhesive Layer enables to obtain a laminate that can be
irradiated after manufacturing in order to modify the initial release force.
By providing a laminate whose release force can be modified after
production, a continuous production process becomes possible as it is
no longer necessary to establish the final release force during
manufacturing.
Instead of having the reactive compound in the adhesive
layer it is also possible to have the reactive compound in the release
.. layer or in both layers. When the .pressure-sensitive adhesive laminate ..
comprises a carrier, the use of a reactive compound in the release layer
not only enables to modify the release force between the release layer
and the adhesive layer, but also between the carrier and the release
layer. in such a manner, the adhesiveness of the laminate can be
mod~ed by modifying the release force at one or both interfaces of the
release layer .
Preferably, the reactive compound is chosen among : free
radical initiators, cationic initiators, acetophenone and derivatives,
benzophenone and derivatives, benzoin and derivatives, quinone and
derivatives, xanthones, acridones, titanocenes, polymer-bound
photoinitiators, dyelcoinitiator systems, 4,4'-bis(N,N-di-n-butylamino)-E-
stilbene, biphotonic photoinitiators, acetylacetonate of Co or Cr or Mn,
organometallic compounds based on at least one element of the groups
4,5,6,7,8 or 10; or a mixture of them. Those comDOUnds can easily be
mixed with the materials of the release layer, generally comprising
silicone, and with the materials of the adhesive layer, generally
comprising acrylic polymer or rubber.
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The invention also relates to a method for modifying an
initial release force between an adhesive layer and a release layer of a
pressure-sensitive adhesive laminate, wherein the adhesive layer
comprises a first reactive compound, having first predetermined reactive
properties.
The method according to the invention is characterised in that a
final release force to be obtained between said adhesive layer and said
release layer is set, and in that a radiation dose is detem~ined on the
basis of the final release force and the reactive properties of the first
compound, said radiation dose being then applied on said laminate by
means of an ionising radiation or an .actinic radiation. Since the
pressure-sensitive adhesive laminate according to the invention enables ..
to modify the initiat release force, the reqmested final release force can
be set in function of the client's requirements. Once the final release
force is set, and taking the reactive properties of the used compound into
account, the radiation dose requested to obtain that fins! release force
can be determined. After determination of this dose, the laminate is
irradiated in accordance with that dose. upon receiving the radiation, the
compound will react in order to modify thE~ release force between the
adhesive layer and the release layer, between the release layer and the
carrier, or both. As the dose is determined, the requested final release
' force will be obtained.
According to a first preferred embodiment of the present
invention, a predetermined segment of the pressure-sensitive adhesive
laminate is masked in order to prevent .said radiation to reach the
masked segment of the laminate. By masking a predetermined segment
of the laminate, the release force can be modified only in the unmasked
segments of the laminate. This is for example of interest for laminates
where only a segment of the front layer, for example that segment
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containing a particular item, has to be removed: By changing the release
force of the fatter, the delamination force is modified in order to make
deiamination easier.
According to a second preferred embodiment of a method
according to the invention, a topography of locations of the laminate to
be irradiated is determined, and a radiation is applied on said locations.
By determining a topography it becomes possible to irradiate only a
certain number of locations and thus to have on a same PSA laminate
locations with a different release force. This method is an alternative to
the use of masks and creates more flexibility in the determination of the
locations to be irradiated as there is no need to manufacture a mask.
According to a third preferred, embodiment of a method
according to the invention, at least one segment of said laminate is
selected and for said segment a radiation gradient is determined, said
radiation dose being applied on said segment according to said radiation
gradient. The determination of a radiation gradient and the radiation
according to that gradient enables to gradually change the release force
in the irradiated segment.
The front layer of a pressure-sensitive adhesive according
to the invention can be formed by an image can-ier on which an image
field can be printed. For example, when the front layer comprises a
publicity for a certain product, an image of the product is printed on the
front layer. When only the image has to be removed, the method
according to the present invention provides an elegant solution. For this
purpose, the method is characterised in that a cutting is applied along a
border line and in that a final release force, to be obtained between said
layers, is determined for at least one of the fiet~s, in function of said
predetermined third reactive properties, a radiation dose being
determined on the basis of the final release force, said radiation dose
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being applied on said field for which said radiation dose was determined,
by means of an ionising radiation or an actinic radiation. By cutting along
the border line the image field is dissociated from the rest of the image
carrier. The application of a radiation on either the image field or the
non-image field, or even on both, provided) in the latter case the dose is
different, will modify the release force in the irradiated fields) so as to
enable an easier removal of the image or non-image freld. Preferably, an
adhesive sheet is applied on the side onwhich the borderline was cut.
The use of an adhesive sheet, also called transfer film, facilitates the
0 removal of the image or non-image field.
The present invention further relates to a device for the
.. application of the method described herein before. Preferably, said
device comprises a radiation source provided for emitting an actinic
radiation or an ionising radiation onto said laminate and is characterised
in that it further comprises a cutting tool having an input for supplying a
predetermined cutting profile and compri:>ing a first transport member
provided for moving said cutting tool along said profile over said
laminate, said radiation source having a further input for receiving a
predetermined value indicating a radiation dose to be applied, and said
radiation source being mounted on a second transport member provided
for moving a radiation beam emitted) by said source over a
predetermined area of said laminate to be irradiated. By using a
transport member, the radiation beam is moved over the area to be
irradiated, which enables to access all areas to be irradiated.
A further problem of pressure-sensitive adhesive laminates
that is solved by the present invention, is the bleeding of the adhesive
Layer. Bleeding signifies that the adhesive substance, present in the
adhesive layer, shows a tendency to flow towards the edges of the
laminate. In particular when the laminates are stacked or wound into a
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roll, the pressure applied on the adhesive layer due to stacking can
make the adhesive substance flow. In order to remedy this problem the
method of the present invention is characterised in that a radiation dose
for increasing the initial release force between said adhesive and said
release layer is determined and thereafter said dose is applied on said
edge by means of an actinic radiation or an ionising radiation. By
irradiating the edge, the release force between the adhesive layer and
the release layer is increased, causing the adhesive substance to better
stick to the release layer at the edges and thus decreasing the bleeding.
When the adhesive layer comprises a compound reactive upon
irradiation, that adhesive layer could also be hardened by irradiation.
-. Another problem of pressure-sensitive adhesive laminates
that is solved by the present invention, is the shrinkage of the front
sheet. For printing or other processing purposes, the laminate is
9 5 submitted to high temperatures. As the front sheet generally comprises
a polymer, the latter tends to shrink upon cooling down: Since printing is
generally a multiple-step method, that shrinkage causes alignment
problems between the different printed colours. In order to reduce this
shrinkage, the present invention proposes a method wherein a radiation
dose for increasing the release force between said adhesive and said
release layer is determined and thereafter said dose is applied on at
least two edges of the laminate by means of an actinic or ionising
radiation. By irradiating the layers on at least two edges, the release
force between the adhesive layer and the release layer is increased, and
thus the release force between the front sheet and the carrier is
increased at those edges, causing a stronger binding between the front
sheet and the carrier and thus decreasing the shrinkage of the front
sheet.
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Delamination is often mechaniically realised, for instance in
order to apply labels onto objects. A problem that occurs during
mechanics! delamination is that the release force is too high on the
impact border, which is the border at which delamination is initiated.
Reducing the overall release force is not an appropriate solution as this
would cause an insufficient adhesion. The solution provided by the
present invention is characterised in that sin ionising or actinic radiation
dose is applied before delamination on said impact border andlor on a
further border opposite to said impact border for modifying the release
9 0 force on said irradiated bonier. 8y application of a radiation, the
release
force is modified on the irradiated border in order to facilitate
delamination at the impact border without affecting too much the overall
release force of the laminate.
The invention also relates to a device for applying on an
object a label which is part of a pressure-sensitive laminate comprising
an adhesive layer and a release layer contacting each other, wherein a
side of said release layer opposite to the one contacting said adhesive
layer is applied on a carrier, and wherein a side of said adhesive layer
opposite to the one contacting said release layer is applied on a front
sheet, said device comprising a supply station provided for supplying
said labels, said device further comprising a delamination station
downstream of said supply station and. provided for delaminating said
labels. Such a device is characterised in that said delamination station
comprises a first selection unit provided for selecting among said
supplied labels a first series of labels which have to be delaminated, and
in that said device also comprises a radliation station coupled to a
second selection unit provided for selecting among said supplied labels
a second series of labels which have to be. at least partially irradiated,
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said radiation station being provided for applying a radiation dose on
said second series of labels.
The invention further provides a solution for modifying an
adhesiveness of a pressure-sensitive laminate. Sometimes it is
necessary to modify the adhesiveness .of the laminate, for example in
function of the support on which the laminate will be stuck For this
purpose, a method according to the invention is characterised in that at
least one segment of said laminate wherein said adhesiveness has to be
modified is determined, said segment being divided in a first set and
second set of mutually exclusive fields, a radiation dose being
determined on the basis of the reactive properties of the third reactive
compound and the adhesiveness value to be obtained, said radiation
dose being applied by means of an actinic or an ionising radiation on the
fields for which the adhesiveness value is determined.
The invention will now be described in more detail with
respect to the drawings showing preferred embodiments. In the drawings
figure 1 shows a cross-section through a pressure-sensitive
adhesive laminate according to the invention;
figure 2 illustrates the method and the device for irradiating
a laminate according to the invention;
figure 3, 4 and 5 show schematically the effect of an
irradiation of a laminate;
figure fi shows examples of laminates where only segments
are irradiated; and
figure 7 shows schematically a device for automatically
selecting and delaminating labels.
In the drawings a same reference sign has been assigned
to a same or analogous element.
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Figure 7 shows a cross-section through an elementary
embodiment of a pressure-sensitive adhesive (PSA) laminate 1
according to the present invention: The laminate comprises a carrier 2
generally made of paper or plastic on which a release layer 3 is applied.
A first side or underside of the release I<ayer 3 is in contact with the
carrier, whereas a second opposite side or' upper-side is in contact with
an adhesive layer 4. A front layer or front street 5 contacts the upper side
of the adhesive layer 4. The front layer is also made for example of
paper or plastic and can be used as an image carrier when an image has
9 0 to be printed on the PSA laminate.
In figure 1, only one adhesivE: and only one release layer
are shown. However several adhesive .and release layers :.can be
provided and the layers can be of different compositions depending on
the required properties. So, for example i;he adhesive layer could be
sandwiched between two release layers.
The release layer preferably comprises a substance chosen
among silicone, acrylated silicone, silicome comprising ethylenically
unsaturated ' groups, urethane, polysilane, polysilylether or
polyphtalaldehyde: The release layer can be heat-curable or radiation-
curable. The pressure-sensitive adhesive Ilayer comprises for instance
an acrylic- or rubber-based polymer.
According to the invention, a f rst compound is added to the
adhesive layer and a second compound is .added to the release layer. It
is however not necessary to add the compound to each layer and the
addition of a single compound to either the <adhesive or the release layer
could be sufficient for application of the present invention. Generally
spoken, one could consider that at least onf: third compound is added to
at least one of the adhesive or release layers. For the sake of clarity and
in particular with reference to the figures 1 and 2, there will however be
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considered that the adhesive layer comprises a first reactive compound
and the release layer a second reactive compound. The compound is
reactive on application of an ionising radiation andlor an actinic
radiation. The ionising radiation can be formed by a beam of electrons,
of neutrons or of other physical particles whereas the actinic radiation,
can be formed by a light or photon beam comprising either a well defined
wavelength or a range of wavelength for example between 100 nm and
1000 nm, preferably between 150 nm and 750 nm.
The purpose of adding a compound in the release andtor
adhesive layer is to be able to modify the initial release force after
manufacturing the PSA laminate. Contrary to the teaching of the prior art,
.. where the release force is established during manufacturing of the
laminate, the present invention proposes to add during manufacturing a
compound that is reactive to ionising or actinic radiation. fn such a
manner, the initial release force can be changed, in a controlled manner,
after manufacturing of the PSA laminate, by irradiation with a suitable
radiation dose.
The radiation applied on the PSA laminate to which the
compound was added will modify the initial release force at one or more
layers, because it will modify the physical andlor chemical interactions of
that or those layers which comprise the reactive compound. The result
will be either an increased or a decreased release force. The release
force is defined as the force, expressed in Newton, required for
separating the carrier with the release layer from the adhesive front
sheet according to a predetermined angle of 180° and at a
predetermined separation speed, such as set forth by the FINAT method
(see for example Technical Manual FINAT, 4'" edition 1995 and
published by FINAT, LAAN COPES VAN CA'(TENBURCH 74, NL - 2585
EW THE HAGUE).Generally, the release force is measured by
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separating the release layer from the adheaive Layer, although in some
cases in the present invention, the release force is measured by
separating the release layer from the carrier.
The degree at which the release force will be changed will
depend on the radiation dose and on thE: reactive properties of the .
compound, and generally also on the composition of the layers)
contacting the radiation-activable layer. In order to obtain a set final
release force, a radiation dose has to be determined taking into account
the set final release force and the reactive properties of the reactive
compound used. Once the radiation dose is applied, the final release
force will remain stable.
Alternatively, the use of an adlhesive which has radiation- ..
sensitive properties could also be considered, i.e. an adhesive or release
layer which already has a reactive compound in its composition. For
example the adhesive Acronal 342~~ and 3458 of BASF
(LUDWtGSHAFEN, D) have such properties. The viscosity of the
adhesive is reduced whereas the elasticity is increased by ultraviolet
irradiation. The irradiation causes the chemical andlor physical bondings
between the polymer chains to modify and to harden or soften the
adhesive, depending on the radiation and the substance used. Figure 2
illustrates schematically a device and a method according to the
invention. The device comprises one or two radiation sources 9, 10, of
which one (10) is placed under the laminate 1 and the other one above
the laminate. It will be clear that only one raidiation source is necessary
and that the radiation beam emitted by the source can be oriented by
means of a transport member, for example adjustable mirrors in the case
of actinic radiation or magnetic andlor electrical fields in the case of
ionising radiation. The transport member is preferably controlled by a
computer 11 in order to accurately control the movement of the beam.
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The computer 11 is further connected to an input unit 12, for example a
keyboard enabling the user to enter data such as the set final release
force and the characteristics of the compound present in the adhesive
andlor release layer. The computer 11 is preferably charged with a
program suitable to calculate the radiation dose based on the set final
release force and the reactive properties. The computer is further
programmed to control the radiation once the dose has been calculated.
The beam of radiation is usually normal to the surface of the PSA
laminate. However for particular applications, such as for instance for
irradiating the edges of the laminate in order to decrease the bleeding of
the adhesive or the shrinkage of the front layer, as will be described
.. hereinafter, the beam will be oriented in parallel to the layers of the
laminate. The beam can also be inclined under an angle of less than
90°,
if for example only a segment has to be irradiated.
The radiation is preferably applied on a laminate that is
supplied by a roll 7 and wound on a further roll 8 after irradiation. Of
course the side that is irradiated has to be at least partially transparent
for the applied radiation in such ~ manner that the beam reaches the
activable layers) to be irradiated.
The radiation can be applied over the whole surface of the
layer to be irradiated or only on one or more segments of the surface
depending on the locations at which the release force has to be
modified. Of course, the intended use of the laminate will determine
which locations have to be irradiated. Figure 3 shows an example where
the PSA laminate is formed by a series of labels 1-a, 1-b, 1-c and 1-d.
Only the borders of two subsequent m~els are irradiated as indicated by
the shade on the labels. In that figu~ the front or the back border are
irradiated. it is however also possible to irradiate both the front and the
back border, or to irradiate both borders with a different dose in order to
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obtain a different release force between the font and the back edge. The
latter application is for example advantageous for a mechanical
application of labels on products. The front side or impact border needs
a weak release force in order to easily initiate the delamination, whereas
the back side needs a stronger release force in order to avoid that the
label would insufficiently adhere to the relestse layer.
Another application of the labf:l pictured in figure 3 is the
ability to delaminate and to apply selectively Label 1-a and 1-b when the
band is going from the left to the right and to apply selectively the labels
1-c and 1-d when the band is going from the right to the left, because the
labels are not easily delaminated when the release force at their impact
border is high. ..
Figure 4a + 4c show an example where the radiation is only
applied on a central part c of the laminate ao that upon delamination a
part 3-c of the release layer remains adhered to the adhesive layer 4. In
figure 4b; a different dose is applied on the part c with respect to the
parts a an b, so that in part c the adhesive layer 4 remains adhered to
the release layer 3, whereas in part a and b" the adhesive layer remains
adhered to the front layer 5.
Figure 5a + 5b show a self wcwnd laminate with adhesive
layers 4 and 4' and release layers 3 and 3'.. In this example a radiation
dose A is applied on the parts d and b, wherE:as on the parts a, c and f, a
radiation dose B is applied. In such a manner, only parts of the adhesive
Layers 4 remain on top of the release layer :3, and parts of the adhesive
layer 4' remains on the bottom of the front sheet 5'. Such an application
is for example of interest with PSA's having an adhesive layer on both
sides but with different adhesive properties, for example one side for
adhering to glass, the other to paper. The adhesiveness of the laminate
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at the top and bottom extem~l surfaces can thus be changed in mutual
exclusive fields of the laminate by application of a radiation dose.
The active ionising or actinic radiation that can be used for the
mod~cation of the release force can be chosen in the fvliowing, non-
iimitative list
electron beam, neutrons, x-rays, gamma rays, beta rays, alpha particles,
electron corona discharge, actinic light.
The actinic light can be provided for instance by mercury lamp, xenon
lamp, deuterium lamp, pulsed lamp, excimer laser, excimer lamp,
microwave driven lamps, laser or pulsed laser.
The ultraviolet wavelength range is considered to be between 200 nm
and 400 nm. w
Some examples of the method according to the invention
will now be described.
EXAMPLE 1.
A release layer, nearly one micron thick, of a silicone mixture of Tego RC
726, 70 parts and Tego RC 750, 30 parts (Th. Goldschmidt AG, Essen,
Germany, in parts by weight) is applied onto a 50-micron BOPP
transparent carrier film. Under a nitrogen blanketing, with less than 50
ppm oxygen in the irradiation zone, the silicone layer is cured, at a speed
of 100mlmin, by irradiating the siliconised BOPP with ultraviolet light
emitted by two medium-pressure mercury lamps ( 2x 80WIcm) with
reflectors. An adhesive layer of Acronal V210 ( BASF, Ludwigshafen,
Germany) is applied onto the silicone layer and dried in a thermal oven.
The coating weight of adhesive after drying is 20g/mz. A 100-microns
polyethylene film is applied onto the adhesive layer. When ultraviolet
light is applied onto the BOPP carrier of the laminate, the release force
increases as shown in the following table. The speed of the samples
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under the UV lamp is 20 mlmin, the power of the lamp is 80 Wlcm, the
lamp is a medium-pressure mercury lamp., parallel to the sample and
perpendicular to the transport direction of the laminate. The samples
were irradiated 1,2,3,4, or 5 times succE;ssively, the UV light being
applied onto the BOPP. The release values of the samples are given in
the table, measured just after UV irradiation and after six-months storage
in the dark. FTM 3, speed 300 mmlmin; FTM 4, speed 80 m/min, all
values are in N/ 25mm.
Ultraviolet FTM 3 FTM 3 FTM 4 FTM 4
dose 6 months 6 months
no in-adiation0.2 0.25 0.2 ~ 0.27
1x 20 mlmin 0.6 0.7 0.3 0.35
2 1.5 1.5 0.6 0.7
3 3 3.3 1 1.1
4 >10 >10 2 2.3
5 x 20m/min > 10 > 10 4 4.5
EXAMPLE 2.
The release layer is for example made of the: following products
Tego 450, Tego 711, Tego 726 and Tego 750 are products from Th.
Goldschmidt AG, Essen, Germany.
Darocur 1173 is from Ciba Specialities.
BDBS is 4,4'-bis(N,N-di-n-butylamino)-E-stilbene
A combination can be used such as given below wherein all values are
expressed in parts in weight.
Formula 1.
A mixture of Tego 726 : Tego 711: Darocur 1173 of about 70:30:2.
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Formula 2.
A mixture of Tego 726 : Tego 711: Darocur 1173 : BDBS of about
70:30:2:1.
Formula 3.
A mixture of Tego 450: Tego 726 : Tego 711: Darocur 1173 : BDBS of
about 30:30:20:2:1.
Formula 4 to 6. In another series of experiments, 1000 ppm of Cr (3)
acetylacetonate and 1000 ppm of Mn (3) acetylacetonate were added in
the formulas 1 to 3, in oFder to make formulas 4 to 6. --
About 1 glmz of each formula is coated on a 50-microns BOPP film which
serves as carrier. Each coated siliconised liner is cured with ultraviolet
light under nitrogen (less than 50 ppm oxygen) at 50 mlmin, the power of
the medium-pressure mercury lamps being 80 Wlcm. An adhesive, made
of about 82.5 weight percent Acronal V 205 (BASF) and about 17.5
weight percent Snowtack 352 A (Akzo-Nobel ) is then coated on the
siliconised BOPP and dried in a thermal oven in order to form the
adhesive layer. The coated weight of dried adhesive is about 20 glm~. A
100-micron polyethylene film is then applied on the adhesive layer.
The laminates are stored for one month, in absence of light, before
subsequent irradiations.
The pressure-sensitive laminates formed with the silicones formulas 1 to
6 are called Laminates 1 to 6. The initial release force (test FTM 3) is
between about 0.1 NI25mm and 0.3 NI25 mm for the laminates 1 to 6.
After removal of the siliconised BOPP, a high tack silicone tape is
applied on the siliconised side of the BOPP, for each of the sample 1 to
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6, and the laminates are stored for one week. The release force between
the BOPP and the silicone release layer is then measured for the fi
samples.
Increase of the release force between the release layer and the adhesive
lever.
1. Irradiation by ultraviolet light.
One sample of eacta of the six laminates (laminates 1 to 6) is irradiated
with ultraviolet light on the 50-micron BOPP liner, by applying a 1 Wlcmz
or 5 Wlcmz dose, with a medium-pressure mercury lamp. After
irradiation, the release force (FTM3) of the laminates is increased by
more than 20 °~ for each sample and each dose, compared to the values
before ultraviolet irradiation.
2. Irradiation by visible light.
One sample of each of the six laminates is irradiated with a pulsed Laser
(600 nm) on the 50-micron BOPP liner.
The global irradiation dose is around 1 WIcm2 or 5 WIcmZ. After
irradiation, the release force ( FTM 3) of the laminates is increased by
more than 20 °~ for each sample and each dose, compared to the values
before laser irradiation.
3. Irradiation by an electron beam.
One sample of each of the six laminates i:; irradiated with an electron
beam, on the 50-micron BOPP liner, with a 1-Mrad dose or a 10-Mrad
dose (200 Key. After irradiation, the release force (FTM3) of the
laminates is increased by more than 20 % for each sample and each
dose, compared to the values before electron irradiation.
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increase of the release force between the release layer and the BOPP
carrier.
Measurement of the release force between the release layer and the.
BOPP carrier.
Another series of samples of the laminates 1 to 6 is delaminated, and the
silicone side of their UV-cured liner is applied on the adhesive side of a
tape bearing a high-tack silicone pressure-sensitive adhesive, in order
to measure the release force between the 50-micron BOPP and the
cured silicone. After 10 days, the tape is separated from the liner,
" dragging off the silicone release layer. The release force between the
W-cured silicone layer and the BOPP liner is measured by cleavage at
the interface between the BOPP carrier and the UV-cured release layer.
Some other series of samples 1 to 6 are used in the following tests.
l.In-adiation by ultraviolet light.
One sample of each of the six laminates (laminates 1 to 6} is irradiated
with ultraviolet light on the 50-micron BOPP, by applying a 1 Wlcm2 or 5
Wlcmz dose, with a medium-pressure mercury lamp.
After irradiation, the release force of the Laminate is measured. After
delamination, the release layer stays anchored on the adhesive layer.
For each sample and each dose, the release value measured between
the silicone layer and the BOPP is increased by more than 20 °r6
compared to the value before irradiation.
2.Irradiation by visible light.
°
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One sample of each of the six laminates is irradiated with a pulsed laser
(600 nm) on the 50-micron BOPP liner.
The global irradiation dose is around 1 W/crrlz or 10 Wlcmz.
For each sample and each dose, the release value measured between
the silicone layer and the BOPP is increased by more than 20 °~
compared to the value before laser irradiation.
3.Irradiation by an electron beam.
One sample of each of the six laminates is irradiated with an electron
beam, on the BOPP film, with a 1-Mrad dose or a 10-Mrad dose (200
KeV). For each sample and each dose, the release value measured
between the silicone layer and the BOPP i;> increased by more than 20
%, compared o the values before electron irradiation.
The release force. and/or the adhesive properties of the PSA laminates
can be mod~ed in selected areas and to a selected level, by irradiation
of the PSA laminate with a selected type and dose of actinic or ionising
radiation. The radiation can modify the interactions between an adhesive .
layer and a release layer andlor the interactiions between a carrier and a
release layer.
In order to modify the release properties in a specific zone of a PSA
laminate, the whole zone can be irradiated, or only specific areas in the
zone can be in-adiated, in order to modify they average release properties.
In order to modify the adhesive properties, the PSA laminate is irradiated
on .selected areas with a specific dose and type of irradiation. In a
preferred embodiment, the anchorage of the release layer on the
adhesive layer is higher than the anchorage: of the release layer on the
carrier in the irradiated areas, but lower in the non-irradiated areas, and,
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during the removal of the release liner, the release layer is transferred
onto the adhesive layer on the areas that were irradiated.
The percentage of the adhesive surface covered with a release layer
after removal of the release liner is generally directly related to the
decrease of adhesiveness (FTM1 and FTM9 values) of the adhesive
Payer.
EXAMPLE 3.
Other samples of the six PSA laminates ( not irradiated) described in the
example 2 are used in the following tests.
An aluminium (thickness 0.5 mm) mask with a gradient of holes (5 x10
rim) is applied on the BOPP liner of the PSA. laminates. A dose of
ultraviolet light, about 1 WJcmz or about 10 Wlcmz, is applied on the
BOPP liner of the laminates, through the aluminium mask. After
irradiation, the laminates are cut into stripes (25 mm width), the BOPP
liner is removed, and the adhesive side of the stripes of laminates are
applied on glass plates, in order to measure the adhesive properties. In
the zones that were irradiated with ultraviolet light, the anchorage of the
release liner is higher on the adhesive than on the BOPP liner_
Therefore, after removal of the liner, the adhesive surface of the PSA
laminate is partially covered with a silicone Layer in the zones that were
partially masked, the adhesive surface is totally covered with the release
layer in the zones that were totally irradiated, and the adhesive surface
is free of silicone in the areas that were not irradiated.
As can be seen in the following table, the adhesive properties of the
laminate are roughly proportional to the surface of adhesive that is not
covered with the silicone release layer. With this method, the adhesive
properties of a PSA laminate with a high adhesion (15N) can be
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decreased to a lower (7.4 N) value, to a removable (4N), to an ultra
removable (0.7 N) or even to a non adhesion value by irradiation.
All FTM values are in N/25 mm (Finat test methods).
~ adhesive 0 20 4p 50 75 90 95 100
covered with
silicone
~6 surface 100 80 60 50 25 10 5 0
of
free adhesive
FTM i 15 12.2 9.3 7.4 4 1. 0.7 0
5
FTM9 ~ 16 13 ~ ~ 4.2 1 0.7 0
~ 9.8 _5
..
!n the example 3, the release layer is not pressure-sensitive.
In another embodiment, the release layer, based on silicone, is a
pressure-sensitive adhesive, the layer contacting said release layer is a
pressure-sensitive adhesive based on an acrylic polymer. The laminate
is irradiated through a mask, in the same way as that described
previously in example 3. After removal of tie release liner, the adhesion
properties are those of the acrylic adhesive: in the areas that are free of
silicone, are substantially equivalent to those of the silicone in the zones
totally covered with the release layer, and are comprised between the
properties of the acrylic adhesive and those; of the (adhesive) silicone in
the adhesives zones that are partially covered with the release layer.
The silicone-based pressure-sensitive adhesives have high adhesion at
low temperatures, or when applied onto substrates having low surface
tension, however their price is much higher than that of acrylic- or
rubber-based adhesives. A PSA laminate having a PSA layer partially or
totally covered with a pressure-sensitive adlhesive silicone release layer
can be obtained with the method described in the present invention. This
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PSA laminate has new and interesting properties, that are intermediate
between the properties of the adhesive and the properties of the
pressure-sensitive adhesive release layer: good adhesion at low
temperatures, good adhesion on low-surface-tension substrates and a
cost much lower than that of a purely silicone-based adhesive. The
acrylic pressure-sensitive adhesive of the laminate can be replaced by a
rubber-based adhesive or an adhesive based on another polymer.
The method described for increasing the release force can
be adapted for decreasing the release force. For instance, in order to
decrease the release of a pressure-sensitive adhesive laminate by
irradiation, a compound that is depolymerised by radiation can be
incorporated in the release layer, and treated similarly as described
previously. Such compounds can be selected among: polysilanes,
polyphtalaldehydes, silicones containing silylether groups or tertiary
polycarbonates groups in the main chain, silicones, cationic initiators, or
a mixture of them.
As already described, it is possible to irradiate either the
whole surface of the laminate or only segments thereof. Figure 6 shows
examples of labels made of a PSA laminate where only segments of
some labels, indicated by shade, are irradiated. When only segments
have to be irradiated, it is possible to mask those segments or portions
that need not be irradiated, for example by using an aluminium foil
wherein the portions to be irradiated are cut out; in this case the device
is provided with a mask-carrier member provided to carry the mask. The
mask-carrier member is placed in the path of the radiation beam in order
to prevent the beam from reaching the masked segments. Alternatively, it
is possible to determine a topography of locations on the laminate which
have to be irradiated. That topography is for example formed by sets of
co-ordinates identifying the location on the laminate. That set is then
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entered into the computer, which will control the radiation beam in such a
manner that only the predetermined locations are irradiated. The
release force andlor adhesiveness is modified in the irradiated
segments. When the release force between the release layer and the
adhesive layer is increased to a level hic,~her than the release force
between the release layer anti the carrier in a speck segment, the
release layer will remain on the adhesive after delamination, thus
modifying the adhesiveness of the PS/~ laminate. A gradient of
adhesiveness on a segment can thus be obtained.
Another problem is the printing and delamination of
pressure-sensitive labels applied on a can-ier roll: When- a roll of
pressure-sensitive laminate bearing die-cut !abets is printed, the release
force at the front edge of the labels must be: sufficiently high in order not
to have delamination of some labels in the printing machine. However,
when the labels are subsequently delaminsited, the release force at the
front edge of the labels must be fow in order to have an easy
delamination of the labels, especially with the automatic machines
delaminating the labels and applying them onto objects. The !abets
currently existing on the market generally (nave a medium release force
on their whole surface in order to be rather easily printed and
delaminated. In another embodiment of the present invention, the
release force at the front edges of the labels is increased by irradiation,
before printing, thus preventing the labels to be delaminated in the
printing machine. After printing the labels, the roll is wound up. The roll is
then unwound in the reverse direction and the high release edge; that
was the front edge, is now the back edge. ~~he present front edge has a
low release and the labels can easily be d~elaminated and applied onto
objects. The same advantage can be obtainE:d for sheets of PSA !abets.
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Sometimes, it is requested that the release force increases
or decreases gradually over a segment of the laminate. 1=or example in
mechanically applying labels a gradual change of the release force will
enable to have a weak release at the front that grows to a stronger one
thus enabling an easy release at the front and a more stronger towards
the end. By determining a gradient on the final release force, that
gradient can be applied on the radiation dose thus obtaining a changing
radiation dose and thus a changing release force. The gradient value
can be introduced by means of the input unit 12.
Pressure-sensitive adhesive laminates are often used for
advertising purposes and an image of the product is applied on the front
layer which is then made c~f a material that can be printed on and serves
as an image carrier. As the laminates are often originally rectangular
shaped or have a larger dimension than the image, and as the image is
not necessarily rectangularly shaped, the image-field, that is the field on
which the image of the product is applied, has to be separated from the
rest or non-image field. For this purpose a cutting tool is used, for
example made of a knife or a laser that is driven along a border line,
delimiting the image field from the non-image field. Preferably, the
cutting tool is controlled by a computer in order to enable an accurate
cutting operation either through the whole laminate or at least through
the front sheet . The cutting tool enables to separate the image field from
the non-image field. As generally only the image field needs to be
delaminated, the latter is irradiated in order to modify the initial release
force and enable an easier delamination. As the irradiation can be
applied only on the image field or only on the non-image field after
printing the image, a more accurate process is possible. Of course, it
could also be possible to irradiate the non-image field or irradiate both
felds with a different dose. It is also possible to irradiate only part of the
i,
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image or non-image fields. The irradiation i,s done either before or after
cutting, but preferably after cutting as a more reliable radiation is
possible on the already cut-out image field.
Also when letters or numbers o~r other items are to be made
of a PSA laminate, the cutting of the letters and of all segments inside
the letters can be realised with the afcxementioned method, thus
avoiding manual separation.
In order to explain the separation of letters, numbers and
other signs from the non-image fields, i;he following example with
illustrative values of the release force will now be described. The PSA
laminate is the one described in the figure 'I , the release force FTM3 is
about 0:3 N125 mm before irradiation. Signs are cut in the whple
thickness of the front sheet of the faminat~e, the adhesive layer being
generally partially or totally cut. An in-adiation device similar to the one
of
figure 2; irradiates selectively parts of the :>urface or the whole surface
of the non-image fields of the laminate. After irradiation, .the release force
in the non-image fields that were irradiated its significantly increased, for
instance to 2NI 25mm. The adhesive surface of a transfer film is applied
with some pressure onto the whole surface of the front sheet of the PSA
laminate; the adhesion between the adhesive of the transfer film and the
front sheet of the PSA laminate is 1 NI25 mm~ (FTM1 ). The transfer film is
separated from the laminate. The irradiated non-image fields of the PSA
laminate have a release force (FTM3 :2 Nl2:i mm) that is higher than the
adhesion between the front sheet of the PSA laminate and the adhesive
present on the transfer film (FTM3: 1 N/ 25mm). The image fields have a
low release force (FTM3: 0.3NI 25 mm). Thins, in the image areas, the
cut pieces of the front sheet of the PSA laminate are transferred onto the
transfer film, while the non-image fields are remaining on the carrier. The
transfer film carrying the image fields cut in the front sheet is then
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applied with some pressure onto the object that has to be decorated, the
transfer film is removed from the object, leaving the object decorated with
the image fields of the PSA laminate. The advantage provided by the
method of the invention is that the non-image fields ( weed zones) are
automatically separated from the image fields. In the prior-art methods,
the non-image fields are manually separated, necessitating a long
manual work for the technician.
Another advantage of the present invention is the possibility
to use the positive (image fietds) and negative (non-image fields) cut in
the front sheet of the same PSA laminate. In the preceding example,
after the removal of the transfer film with the image fields, the non-image
fields remain on the release layer because of their high release force
(FTM3 : 2 W25 mm). A transfer film with a high adhesion ( FTM1: 4 N125
mm) on the ftont sheet is applied onto the whole surface of the front
sheet of the PSA laminate, then the separation of the carr;er from the
transfer film leaves the whole negative image onto the transfer film. With
this method, a surface of PSA laminate can be used to decorate one
object with the positive and another object with the negative of the same
image cut in a PSA laminate.
For storage purposes, the PSA laminates are often either
stacked or wound into a roll. This however causes the adhesive layer to
be compressed due to the pressure of the stacked sheets or the wound
roll. Therefore this compression causes a bleeding of the adhesive
substance embedded into the adhesive iayer(s). Due to the applied
pressure, the adhesive substance thus tends to flow to the edges of the
laminate. Although the adhesiveness of the adhesive substance on the
ftont layer is generally high, the adhesive substance in contact with the
release layer, where the adhesion force is lower, tends to flow towards
the edges causing an accumulation of adhesive substance on the edges
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where a rim is then formed. This accumulai'.ion of adhesive substance at
the edges causes the different laminates to. stick together arid to disturb
unwinding or de-stacking. As the rim remains sometimes on the printing
machines, a cleaning of the latter is necessary. The present invention
offers a solution to this bleeding problem b;y irradiating the edges of the
Laminates. The irradiation causes the initial release force to be modified
so that a stronger release force is obtained at the edge, thus preventing
a bleeding at that edge. Indeed, by increasing the release force at the
edges, the adhesive substance will more si~rongly adhere to the release
'10 layer at the edges so that the tendency to flow under influence of the
applied pressure witl be reduced. Depending on the reactive properties
- of the reactive compound used. and on the final release force to be
obtained, the radiation dose applied on the edge can be determined.
Preferably, the irradiation is done on a width that is smaller than 7
millimetre so as not to modify the average release force of the laminate.
Another problem to which the present invention offers a
solution is the shrinkage of the front sheet. In particular, when the front
sheet is printed in order to apply a muliticolour image thereon, that
printing process is can-led out in several steps, comprising the printing of
the different colours, the heating and cooling of the laminate. As the front
sheet is often made of a polymer material, the latter tends to shrink after
heating. That shrinking then leads to positioning problems between the
printing of the different colours. In order to decrease this shrinkage, the
present invention proposes to irradiate at to<~st two edges of the laminate
by means of an actinic or ionising radiation. The two edges are
preferably opposite to each other and preferably extend lengthways. By
irradiating those edges, the release force increases at those edges,
causing an improved adhesion at them and thus decreasing the
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shrinkage as the front layer adheres more strongly to the carrier along
those edges.
Figure 7 shows schematically a device for delaminating
labels of a PSA laminate, in order to apply these labels onto objects in a
subsequent step. The PSA laminate is supplied from a supply station
formed by a roll 7 from which it is guided towards a first station 21
imposing an angle variation a = 90° to the laminate. This causes a
delamination at the front edges of those labels having a weak release
force at their front edge. In such a manner, the device selects at station
21 those labels with a weak release force and those labels can be
applied onto objects ( not shown). Subsequent stations 22 and 23 show
angles ~i = 105° and Y = 160 ° which causes the labels with a
higher
release force at their front or impact border and which passed station 21
to be delaminated. On roll 24, at the end, the carrier bearing the
remaining labels is then wound, which enables to save those unused
labels. The angles are only indicative for a better understanding of the
device. Preferably the irradiation device 9 is placed upstream of station
21, in order to irradiate selected labels. The irradiation device is
controlled by the computer ( such as shown in figure 2). This computer
comprises a first selection unit provided for selecting among the supplied
labels a first series of labels which have to be delaminated by one of the
stations 21, 22 or 23. The computer also comprises a second selection
unit provided for selecting among the supplied labels of the first series
those which have to be irradiated. So for example when labels have to
be delaminated by station 22, the computer will send a control signal to
the source 9 each time one of such labels of the second series has
reached the beam of the source. Under control of the latter signal the
source will emit a radiation burst towards the label in order to modify, the
required level, the release force at that location of the label. At the
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irradiated Location the label will thus have the required release force in
order to enable delamination at station 22. Of course the labels of the
second series can comprise all those of the first series, or some of them.
