Note: Descriptions are shown in the official language in which they were submitted.
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TAMPER nND~CATTNG ~rUL17LAYER SHEET
Field of the Invention
The invention relates to pressure-sensitive adhesive coated multilayer
sheet useful as removable stickers for application to smooth and rigid surfaces such as
glass or painted metal surfaces whereby the stickers preferably exhibit tamper-proof
characteristics. The invention furthermore relates to pressure-sensitive adhesive
materials which are useful in these multilayer sheets.
Background ofthe Tnvention
Multilayer sheets useful for tamper-proof labels and stickers are known.
WO 95/29474 describes, for example, a sticker for attachment to the inside of a vehicle
window. DE 44 05 945 also describes a tamper-proof sticker for application to a
transparent surface such as a windshield. DE 43 14 579 describes a tamper-proof
l 5 windshield sticker containing a hologram. Tamper-proof stickers known in the art are
designed, to have very high adhesion to smooth surfaces such as glass and to give
permanent bonding, and they can be removed only with the aid of solvents and/or
mechanical SC-~pil-g.
In some cases, transparent polymeric foils or sheets such as, f'or example,
polyurethane foils are laminated onto the inner surface of windshields. Having to use
solvents and/or mechanical tools for temporary stickers in inconvenient, and polymeric
sheets or foils can easily be damaged by solvents and scraping. Thus there is a need for
providing a multilayer sheet useful, for example, as a sticker, which is readily removable
from smooth and rigid surface and preferably furthermore exhibits tamper-proof
characteristics.
Multilayer sheets which are useful for the p- epar~lion of a sticker which
has high enough adhesion to render it tamper-proof, yet at the same removable without
the aid of solvents or mechanical scraping are not available in the prior art.
Brief Description of the Drawin~s
The present invention refers to a multilayer sheet (1) comprising a
flexible backing (2) and a pressure-sensitive adhesive layer (3) for attaching the
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multilayer sheet to a smooth and rigid surface (5), wherein the pressure-sensitive
adhesive is readily removable from the surface (S) and is selected to exhibit for an
adhesive layer with a thickness of 300 ~Im a 90~ peel adhesion of between 20 and 85
N/inch from glass after a dwell time of 3 days at a temperature of 70~C. In a pl ~rt;l I ed
S embodiment, the flexible backing (2) comprises at least one d~m~g~hle layer (4) and
shows intralayer failure when peeling offthe multilayer sheet (1) from the surface (5).
The present invention furtherrnore refers to a pressure-sensitive adhesive
which is obtainable by polymerizing a precursor COlllpl ;sh~g
a) a monomer component which contains one or more alkyl acrylates,
the alkyl groups of which have an average of 4-14 C atoms and 2-8 phr of at least one
copolymerizable monomer having a polar group,
b) 5-15 phr of hydrophobic silica,
c) one or more polymerization initiators, and
d) one or more crosslinker compounds in a concentration resulting in a
crosslink densily obtainable by using hexanedioldiacrylate as a reference crosslinker
compound in a concentration between 0.06 and 0.14 phr.
The present invention fi~rthermore refers to a pressure-sensitive adhesive
which is obtainable by polymerizing a precursor comprising
a) a monomer component which contains one or more alkyl acrylates,
the alkyl groups of which have an average of 4-14 C atoms, and more than 8-20 phr of
at least one copolymerizable monomer having a polar group,
b) 5-15phrofhydrophobicsilica,
c) one or more pol~.l,e.i~tion initiators, and
d) one or more crosslinker compounds in a concenl~lion resulting in a
crosslink density obtainable by using hexanedioldiacrylate as a . ere~ ~nce crosslinker
compound in a concentration between 0.06 and 0.10 phr.
Detailed Description of the Invention
The multilayer sheet ( I ) of the present invention comprises a flexible
substrate (3) and a layer of pressure-sensitive adhesive (3) for ~tt~ehing the film to a
smooth and rigid surface (5) such as, for example, glass, metal or painted metal surfaces.
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When trying to remove the multilayer sheet (1) from the surface, the
flexible substrate (2) preferably shows intralayer failure thus imparting tamper-proof
features to the multilayer sheet (1). The flexible substrate (2) can comprise one or more
layers and pr~fe. ably at least two layers. The flexible substrate (2) preferably comprises
S at least one d~m~g~ble layer (4) which is designed to be the weakest point of the
multilayer sheet ( 1) of the present invention and hence the predetermined failure point
when peeling offthe multilayer sheet (I) from the surface (5). The ~m~g~ble layer (4)
preft,~bly is an inner, i.e., not exposed layer ofthe multilayer sheet (I) and has 2
adjacent layers of the multilayer sheet. The flexible substrate can comprises further
layers such as, for examplet a cover layer (6) (not shown), or a lclrol~llective layer (7).
The multilayer sheet (1) of the present invention can be readily removed
from the surface (5) without any residue. The term "readily removable" means that the
pressure-sensitive adhesive can be manually removed from the surface (5) after the
period of use without using chemical agents such as organic solvents or mechanical tools
such as knives or scrapers.
Due to the unique combination of tamper-proof features and ready
removability from surfaces such as glass or painted metal, the multilayer films of the
present invention comprising a d~m~g~ble layer (4), are especially suited for the
preparation of temporary labels such as toll vignettes on vehicles or anticounterfeiting
labels on goods.
The pressure-sensitive adhesive useful for the multilayer sheet (1) ofthe
present invention is selected to exhibit a high adhesion to the flexible substrate (2), i.e.,
to the layer of the flexible substrate (2) which is a-~jacent to the pressure-sensitive
adhesive layer (3). In multilayer sheets according to the present invention comprising a
d~m~g~ble layer (4), this adhesion must be higher than the internal or cohesive strength
of the damagable layer (4) in order to avoid that the flexible substrate can be clearly
peeled from the adhesive layer without destroying the flexible su~strate (2). The
adhesion of the pressure-sensitive adhesive to the smooth and rigid surface (5) must also
be higher than the internal ~ cnglh of the d~m~g~ble layer (4) in order to avoid that th
multilayer sheet (1) is peeled offfrom the surface without destroying it. The adhesive
must not only be resistant to easy peeling at ambient tempe, al~lre, but must also adhere
well and resist attempts to remove the multilayer sheet (I) intact at elevated
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temperature. On the other hand, the adhesion between the pressurc-3en~ re adhesive
layer and the smooth and rigid surface must not be so high as to result in pel,.,ane.,
bonding of the pressure-sensitive adhesive layer to the surface (5).
It was found that pressure-sensitive adhesives which are usefu} in the
multilayer sheets (1) of the present invention exhibit, for a layer of the pressure-sensitive
adhesive in a thir~ness of 300 microns, a 90~ peel adhesion ~om glass after a dwell time
of 3 days at 70~C as measured by the test method specified below, of between 20 and 85
N/inch, and, especially, of between 30 and 75 N/inch, a very particularly of between 30
and 70 N/in.. The pressure-sensitive adhesive materials identified by this test method are
useful for glass surfaces and other smooth and rigid surfaces, such as metal or painted
metal surfaces. The pressure-sensitive adhesive materials are especially useful for glass
surfaces.
The pressure-sensitive adhesive according to the present invention
preferably exhibits a high optical transmission so that indicia (4d) such as printed
patterns which form part of the flexible substrate (2), can be viewed through the
adhesive layer (3). Pressure-sensitive adhesives exhibiting a transmission of visible light
of at least 80% and preferably of at least 90% are p. ererl ed. The pressure-sel,;,;li~fe
adhesives to be used in the multilayer films of the present invention are furthermore
preferably optically clear, i.e., not colored.
lt was found that a specific class of pressure-sensitive adhesives which
fulfill the above requirements is obtainable by polymerizing a precursor comprising
a) a monomer component which contains one or more alkyl acrylates,
the alkyl groups of which have an average of 4-14 C atoms, and 2-8 phr of at least one
copolymerizable monomer having a polar group,
b) 5-15 phr of hydrophobic silica,
c) one or more polymerization initiators,
d) and one or more crosslinker compounds in a conct;--L~Iion resulting
in a crosslink density obtainable by using hf~ne~iioldiacrylate as a ~eÇ~.t;nce crosslinker
compound in a concenl~lion of between 0.06 and 0.14 phr.
These pressure-sensitive adhesives are new and they are subject matter of
the present invention. Pressure-sensitive adhesive tapes cont~inin~ hydrophobic silica are
generally described in US 4,749, 590. The tapes are said to be initially repositionable,
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S
"but within a few days they normally cannot be removed" (US 4,749,590, col. 4, Ins. 49-
50). US 4,89S,745 describes dark acrylic pressure-sensitive adhesives comprisinghydrophobic silica and pigments such as, for example, carbon black pigments which
cannot be used, for example, for front face type stickers described below which require
highly transmissive pressure-sensitive adhesives.
The term average of 4-14C atoms means that the average number of C
atoms of the alkyl acrylate compounds, wei~hed by their respective perctnlage byweight with respect to the mass ofthe alkyl acrylate component, is bet~, en 4-14 and, in
particular, between 4-12 C atoms.
Useful alkyl acrylates (i.e., acrylic acid alkyl ester monomers) include
linear or branched monofunctional unsaturated acrylates or methacrylates of non-tertiary
alkyl alcohols, the alkyl groups of which have from 4 to 14 and, in particular, from 4 to
12 carbon atoms. Examples of these lower alkyl acrylates used in the invention include
but are not limited to, n-butyl acrylate, isobutyl acrylate, hexyl acrylate, 2-ethylhefxyl
acrylate, isooc~yl acrylate, n-octyl acrylate, n-octyl methacrylate, 2-methylbutyl acrylate,
isononyl acrylate, n-nonyl acrylate, isoamylacrylate, n-decyl acrylate, isodecyl acrylate,
isodecyl methacrylate, isobornyl acrylate, 4-methyl-2-pentyl acrylate and dodecyl
acrylate. Preferred lower acrylate and methacrylate esters include isooctyl acrylate, n-
butyl acrylate, 2-ethylhexyl acrylate, isononyl acrylate and dodecyl acrylate.
The precursor preferably contains up to 5 and, in particular, 1-4 alkyl
acrylates. The average number of carbon atoms in the alkyl groups of the alkyl acrylates
as defined above, preferably is between 4-14, especially between 4-12 and very
particularly between 5-10. The conce"~l ~lion of the alkyl acrylate component with
respect to the mass of the precursor of the pressurc-sel,s;live adhesive preferably is at
least 75 wt.%, and, in particular, at least 85 wt.%.
The precursor of the PSA can contain alkyl esters of unsaturated
aliphatic carboAylic acids other than acrylic acid such as, for eA~llll~lC, alkyl ~alf~Z.tec and
alkyl fumarates (based, r~:s~,e.,lively, on maleic and fumaric acid). In this regard, dibutyl
m~le~e, dioctyl m~ieate, dibutyl fumarate and dioctyl fumarate, are preferred. The
amount of ester compounds of unsaturated aliphatic carboxylic acids other than acrylic
acid pref~lably is not too high and, in particular, does not exceed 25 wt.% with respect
to the mass of the alkyl acrylate componcnl.
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. The terrn polar monomers includes both moderately polar and strongly
polar monomers. Polarity (i.e., hydrogen-bonding ability) is frequently described by the
use ofterms such as 'strongly', 'moderately', and 'pooriy'. References describing these
and other solubility terms include 'Solvents", Paint Testing Manual, 3rd ed., G.G.
Seward, Ed., American Society for Testing and Materials, Philadelphia, Pennsylvania,
and 'A Three-Dimensional Approach to Solubility', Joumal of Paint Technology, Vol.
38, No. 496,pp. 269-280. Exdmples for strongly polar copolymerizable monomers are
acrylic acid, methacrylic acid and acrylamides while N-vinyl lactams such as, for
example, N-vinyl pyrrolidone, N-vinyl caprolactam, acrylonitrile and dimethyl amino-
propyl methacrylate are typical e,~a""~les of moderately polar monomers.
The precursor contains between 2 and 8 phr of one or more
copolymerizable polar monomers. If the amount of the copolymerizable polar monomers
exceeds 8 phr, the 90~ peel adhesion of the pressure-sensitive adhesive from glass is too
high and the pressure-sensitive adhesive becomes permanently bonded. If the amount of
the copolymerizable polar monomers is less than 2 phr, the 90~ peel adhesion from glass
is too low to ensure that the multilayer sheet ( I ) cannot be removed intact from a glass
surface when applying for example, high temperatures.
The amount of the copolymerizable polar monomer preferably is from 2-
7 phr and especially preferably from 3-6 phr.
The pressure-sensitive adhesive to be used in the multilayer films of the
present invention furthermore comprises 5- 15 phr of hydrophobic silica. In an especially
prerc;,,t;d embodiment, the amount ofthe hydrophobic silica is between 7.5 and 15 phr
and especially between 10 and 15 phr.
Hydrophobic silica is commercially available, for example, from Degussa,
as "Aerosil" R972, R974 or R9976. According to a Degussa bulletin entitled "Product
Il~u~lllalion'' of June 4, 1984, the hydrophobic silicas "Aerosil" R972, R974 and R976
are prepa.~d from hydrophilic silicas denoted as "Aerosil" 130, 200 and 300,
peclively, which exhibit a surface area of 130, 200 and 300 m2/g, les~ecli./ely.Hydrophobic silica is furthermore co.. cn~ially available, for example, as TS-720 from
Cabot Cab-O-Sil Division, Tuscola, Illinois, USA In US 2,859,198 (Sears et al.) it is
- proposed that the surface of finely-divided inorganic solid silicon-co.. ~ ;ng materials,
such as silica, can be rendered hydrophobic by ll cating the material with an organo-
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siloxane material. According to US 4,136,081 (Schultz), silica "can be treated with
organosilicon materials such as chiorosilanes, si~7~nes, alkosysilanes and cyclic
siloxanes to produce hydrophobic surfaces" (col. 6, Ins. 47-52).
The enumeration of l,~drophobic silica materials given above, and the
description of some selected methods for preparing hydrophobic silica materials is to be
understood as illustrative and by no means limitative. The hydrophobic silica used to
prepare the PSA materials according to the present invention, p-ef~ bly exhibits a
surface area of at least 10 m2/g and especially of at least 50 m2/g. The surface area ofthe
hydrophobic silica çsrec~ y prerel~bly is between 50 to 400 mZ/g (B.E.T. surface area).
The addition of hydrophobic silica to the precursor of the pressure-
sensitive in the specified amount imparts a sufficient cohesive or internal strength to the
pressure-sensitive adhesive so that the adhesive has a tough and rubbery character. This
allows the adhesive residues to be removed, for example, from glass surfaces, bygrasping the edge of the adhesive residue and pulling it to effect complete removal from
the glass substrate. Entire sections of adhesive residue or adhesive-coated sticker residue
can then be removed manually. If the concentration of hydrophobic silica is chosen to be
below 5 phr, the pressure-sensitive adhesive was found to break upon attempts toremove large sections of the adhesive mass, thus necessitating the use of mechanical
tools and/or chemical agents such as organic solvents. Increasing the conce"l-~lion of
the hyd~ ophobic silica above 15 phr adversely affects the 90~ peel adhesion prope, lies
from glass.
The addition of hydrophobic silica in a properly selected amount to the
precursor of the pressure sensitive adhesives the~ efo. e is essenti~l to establish the
desired removability of the pressure-sensitive adhesive (3) from the surface (5). Other
fillers such as, for eA~ ple, hydrophilic silica or polysaccharide fillers were found to
adversely affect optical properties such as clarity and/or ",echal-ical properties such as
me51-~-;G~I s~ ,lh ofthe pressure-sensitive adhesive.
The precursor of the pressure-sensitive adhesive useful in the multilayer
films of the present invention furtherrnore contains one or more cro~slinker compounds
to increase the cohesive strength and the tensile strength of the resulting PSA material.
Useful crosslinkers include benzaldehyde, acetaldehyde, anthraquinone,
various benzophenone-type and vinyl-halomethyl-s-triazine type compounds such as, for
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example, 2,4-bis(trichlorul ,.e~ 6-p-methoxystyryl-s-triazine. I~l eÇc~ l ~d arepolyacrylic-functional monomers such as, for example, trimethylolpropane triacrylate,
pentaerythritol tetraacrylate, 1,2-ethylene glycol diacrylate, tripropyleneglycoldiacrylate,
1,6-hP.~ne~iol diacrylate or 1,12-dodec~nediol diacrylate. The compounds listed above,
which can be s~lbstituted or lln~ubstit~ltefl, sre int~nded to be illustrative and by no
means limitative.
The cros~linl~ing conlpori~;nl to be used in the present invention
preferably cor.lains 1-5, especially between 1-3 and very particular 1-2 clossli.lker
compounds. F.speci~ily pre{~,.ed crosslinker compounds are 1,6-hex~nedioldiacrylate
and tripropyleneglycoldiacrylate.
The degree of crosslinking (crosslin~ density) which can be expressed as
the number of cross-links per gram or per unit volume of the polymer, can be es~im~ted
theoretically and determined CA~JC;I in~e:nl.~llly for example, by swelling measure"len~s or
via stress strain curves (see Encyclopedia of Polymer Science and Engineering, 2nd. ed.,
New York 1988, vol. 4, p. 3 55-357). A detailed description of swelling measurements
which are prere.,~;d, is given in P.J. Flory, Principles of Polymer Chemistry, Cornell
University Press, Ithaca and London, 1953, pg. 579. When reacting two different
pre~,u~o,~ which di~er with respect to the crosslinker component but are otherwise
identic~l under identic~l external reaction conditions (pol~,."eli~alion method such as
bulk, solution, emulsion or suspension polymerization, ten")c.at~lre, pressure, irradiation
in case of W-polymerization, etc.), the crosslink density obtained depends on the
chemical nature of the crosslinkers used, their functionality and their respective
concel,lt~tions. In the present application, the crosslink density is reported with respect
to the crosslink density which is obtainable by using variable amounts of h~ ~Anediol
diacrylate (HDDA) as a ~eîclence crosslinker component under standardized reaction
conditions (bulk photopolymerization, photoinitiator: Irgacure 651, supplied by Ciba
Geigy, in a concentration of 0.24 phr; W irradiation with an exposure of 900-1500
mJ/cm2 from an W lamp, 90% of the emissions of which are between 300 and 400 nm,with a maximum at 351 nm; room te".pe,~ re; normal pressure; exclusion of oxygen).
The precursor to be used in the multilayer sheets according to the present
invention prc;l;,. ~bly cor.lains one or more cro.s~iinlcer compounds in a concentration to
give a crosslink density obtainable by using HDDA in a concentration between 0.06 and
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_9_
0.14 phr. The conce.nlalion of HDDA preferably is between 0.08 and 0.14 phr, andespecially preferably between 0.09 and 0.13 phr.
If the crosslinker compounds are used in concentration to give a crosslink
density obtainable by using HDDA in a concenl~tion of less than 0.06 phr, the cohesive
strength and the tensile strength of the resulting pressure-sensitive adhesive is too low.
In this case, the adhesive layer was found to be cuttable, for example, with a razor blade,
which allows intact removal of the multilayer film from the surface, without destruction,
and permits readhesion of the multilayer sheet to another surface. If the crosslinker
compounds are used in a concentration to give a crosslink density obtainable by using
HDDA in a concentration of more than 0.14 phr, the 90~ peel adhesion from the surface
(5) was too low to ensure destruction of the flexible backing ~2) comprising a damagable
layer (4) when peeling off the multilayer film from the substrate.
The pressure-sensitive adhesive materials to be used in the multilayer
sheets of the present invention can be obtained by applying generally known
polymerization methods such as bulk, solution, emulsion or suspension polymerization.
Due to environmental reasons bulk pol~"le, ;~lion is oP[en preferred in order to avoid
using organic solvents.
The polymerization reaction is preferably started by means of a
pol~ ion initiator and preferably proceeds via a radical polymerization l"eclla"isn,.
Useful examples of polymerization initiators include photoactivatable initiators such as,
for example, benzoin ethers (e.g., benzoin methyl ether, benzoin isopropyl ether,
sl~bstituted benzoin ethers such as anisoin methyl ether), acetophenones (e.g., 2,2-
diethoxyacetophenone) or alpha-ketols (e.g., 2-methyl-2-hydroy-propiophenone), and/or
thermally activatable initiators such as, for example, organic peroxides (e.g., benzoyl
peroxide and lauryl peroxide) and 2,2'-azobis(isobutyronitrile). Photopolymerization and
the addition of photoactivatable initiators are p,~Çe--ed. The initiator col..ponenl
preferably co,~""ises between 1-3 and, in particular, between 1-2 initiator compounds;
especially preferred are initiator components containing only one photoinitiator. The
initiator component is preferably present in an amount of 0.01-2.00 phr, in particular,
between 0.05-1.00 phr and very specifically between 0.1-0.5 phr.
In a p, efe. ~ ~d method of p- ~a- alion~ a part of the initiator con,ponent is
added to the alkyl acrylate component which is partly poly~.~e.;~ed to a degree of
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typically 2-30% to form a syrup of coatable viscosity of, for example, 300-20,000 cps
(Brookfield) at ordinary room ten~ t-lres. The viscosity of the syrup is preferably
adjusted to the amount of hydl ophobic silica to be added. For high loadings with
hydrophobic silica of, for example, 12- 15 phr, the viscosity of the syrup prert~bly is not
more than 1,000 cps and, in particular, bet~. cen 250 and 1,00 cps. For lower loadings
with hydrophobic silica, the viscosity preferably is not less than 1,500 cps and, in
particular, 1750 cps or more. The viscosity of the precursor can also be adjusted by
adding a small amount of typically less than 5 phr of a polymeric additive whichpreferably is a photopolymerizable polyacrylate as is described, for example, inWO 94/00,052. The polymerization preferably proceeds as photopol~.,.el ization which
is described, for example, in US 4,182,752. In a preferred embodiment, the
polymerization is carried out with W black lights having over 60 percent, and
preferably over 75 percent of their emission spectra between 280 to 400 nm, with an
intensity between about 0. l to about 25 mW/cm2. The exposure is typically between
900- 1,500 mJ/cm2. The polymerization may be stopped either by removal of the
radiation or heat source andtor introduction of, for example, radical scavenging oxygen.
The filler component essenti~lly comprising hydrophobic silica, is
subsequently added to the prepolymerized syrup. When the amount of the hydrophobic
silica exceeds about 8 phr, it has been found necessary to employ a high-shear mixer
such as a paint mill to obtain uniforrn dispersions. By doing so and by properly adjusting
the viscosity of the pre-polymerized syrup, e~.~enti~lly uniform dispersions can be
obtained for loadings as high as about 25 phr. The pressure-sensitive adhesive used in
the multilayer sheets of the present invention co--.p. ises up to 15 phr of hydrophobic
silica.
The dispersion obtained is mixed with the rem~ining part of the initiator
co..,~)one.,l and, optionally, with other adjuvants such as, for example, chain transfer
agents, polymer additives like, for eAample, those described in EP 0,349,216 or EP
0,352,901, solvents, fire retardants, odor masking agents and/or other adjuvants known
in the tape art. The addition of pigments and colorants is not preÇcl . cd because the
pressure-sensitive adhesives useful for the multilayer sheets of the present invention
- pl ~r~, ~bly exhibit a high optical transmission and clarity. If pigmentslcolorants and/or
paints are added, these are preferably selected to impart color to the resulting PSA
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materials without adversely affecting their ll ansyal ency and clarity. An example of a
suitable paint is Yellow Permalink CTL-Druckfarbe (printing paint) which is obtainable
from SICPA Druckfarben, R~n~n~
To produce PSA films useful in the multilayer sheets of the present
invention, the above dispersion or mixture obtained is coated onto a bac~ing, a carrier
web or a release liner and polymerized in an inert, i.e. oxygen free atmosphere, for
example a nitrogen atmosphere. Above and below, the term film is used to describe a
structure whose thickness is substantially less than either its length or width and which
has two, ec.~çnti~lly parallel opposed surfaces. As used herein, the term films in~ludes,
for example, sheets, ribbons, tapes and discs.
Damagable layers (4) for tamper-proof multilayer sheets ( 1 ) which are
destroyed or irreversibly deformed when peeling o~the multilayer sheet (1), have been
described in the literature.
The incorporation of a damagable layer or damagable layers (4) into the
multilayer sheets (1) ofthe present invention is optional but preferred. The present
invention also comprises multilayer sheets comprising a flexible backing (2) and a
pressure-sensitive adhesive layer (3) for attaching the multilayer sheet to a smooth and
rigid surface (5) colllpli~;llg no d~m~g~ble layer (4).
The d~m~g~ble layer (4) can comprise, for example, a pattern of
adhesion-re~ ting material (4a) which can be applied onto the inner surface of the
pressure-sensitive adhesive layer and/or of the other layers of the multilayer sheet ( I ) in
a non-contim.ous fashion. The adhesion-regul~ting material covers only a part of the
inner surface of one or more of these layers, thus creating surface areas with different
adhesion properties. The adhesion-re~ ting material may be either an adhesion-
reducing substance such as a release agent or, alternatively, an adhesion-promoting
subst~nce such as a primer.
An adhesion-reducing pattern (4a) may be applied, for example, onto the
inner surface ofthe pressure-sensitive adhesive layer (3). While the adhesion ofthe
pressure-se"si~ e adhesive layer (3) to the adjacent layer of the flexible substrate is high
in areas where the adhesion-reducing material is absent, adhesion between the pressure-
- sensitive adhesive (3) and the adhesion reducing material is very low in areas where the
adhesion-reducing material is present. In area where the adhesion-reducing material is
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present, there is often practically no adhesion between the pressure-sensitive adhesive
layer (3) and the adjacent layer of the flexible substrate.
Several materials can be used as adhesion-reducing or adhesive-repellent
subsl~nces in order to create a pattem (4a). Examples of these materials includecdlb~ es like polyvinyl octadecyl ca,l.a~.ate, available under the name "Kalle release
coat K" from Hoechst AG, Gemmany, or "Escoat P20" from Aderson Development
Company, USA. Further suitable materials include silicone systems like W-curablefree-radical silicone acrylate release coatingc, silicone addition systems, silicone
condensation systems and cationically-curing silicone systems.
. A more complete description of both the silicone and non-silicone based
adhesion-reducing materials and their use to create patterns can be found, for example,
in US 5,061,535.
The adhesion-rç~ ting material used in the multilayer sheets (I ) of the
present invention may also be an adhesion-promoting substance. Layers on either side of
the pattern of an adhesion-promoting substance exhibit increased adhesion in areas
where the adhesion-promoting subst~nce is present. In areas where the adhesion-
promoting substance is absent, there is limited adhesion or occasionally even noadhesion between the adjacent layers.
Several types of materials are useful as adhesion-promoting substances.
These must be tailored specifically to the two adjacent layers, in that the adhesion-
promoting substance must have good adhesion to both adjacent layers which may bewidely different in chemical nature. Both polyurethane based primers and ethylene-
acrylic acid based primers have been found suitable for use in the present invention.
The type of pattem used for application of the adhesion-re~ ting
substance in the present invention is not critical.
The adhesion-reg~ ting pattem (4a) is prere.~bly and adhesion-
promoting substance and is preferably applied using a gravure printing methorl
The adhesion-re~ tin~ pattern (4a) provides areas with dilIelcn
adhesion properties for the adj~cent layers of the flexible backing (2) resulting in
defommation and/or destruction of the ~ cent layer or layers. In this sense, theadhesion-re~ ting pattem (4a) is temmed as a damagable layer in the multilayer sheets
of the present invention.
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The d~m~ble layer (4) can also comprise certain frangible optical
ele-n~nt~ or layers such as a kil.egl alll (4b) or a hologram (4b) as is described, for
example, in EP 0,253,089; EP 0,401,466 and US 5,066,041. During an attempt at
tall.?elil.g, the hologram would be destroyed. Kin~lallls which are conullercially
S available from I,andis & Gyr, Zug, Switzerland, and others, and holograms are very
attractive from an aesthetic point of view and they can be used to incG- ~ol ale user-
specific graphics or inforrnation into the films. Kinegrams or holograms can be used in
restricted areas of the multilayer film only, but it is also possible to use extended
holographic layers which comprises a structured layer and an optional reflective layer.
The structured layer can be formed by several methods that are well
known in the art, disclosed in US 4,856,857. It may be made of materials such aspolymethyl methacrylate, nitrocellulose, and polystyrene. The structured layer includes a
microstructured relief pattern of holographic or diffraction grating images in the form of
logos or patterns that reflect light. In one embodiment, an embossed microstructured
layer may be formed by contacting the material from which the structured layer will be
madc with a non-deformablc embossing plate havin~ a microstructurcd rclicf pattcrn,
and applying heat and pressure. Alternatively, the structured layer may be made by any
other suitable process, such as radiation curing, and may be made of materials such as
urethane, epoxy, polyester, and acrylate monomers and oligomers, which are formnl~ted
with photoinitiators, cast on a non-deformable tool having a microstructured relief
pattern, and radiation cured.
The optional reflective layer is coated on the structured layer of the
holographic layer (4b) either before or after embossing. The reflective layer preferably
has a higher refractive index than the structured layer. In a pitrtl,~d embodiment, the
reflective layer is subst~nti~lly ll ~n~,are~l and colorless. Illustrative examples of suitable
reflective layer materials include but are not limited to bismuth trioxide, zinc sulfide,
titanium dioxide, and zirconium oxide, which are desc~ibed in US 4,856,857. Lesstransparent materials such as thin ~hlnlinllnl or silver, or patterned reflectors can also be
used. The reflective layer enhances the reflection of light through the structured layer
due to the di~rcnce in refractive index between the structured and reflective layers.
Thus, the structured holographic pattern is more readily visible to the lln~ided eye once
the reflective layer is coated on the structured layer, and the pressure-sensitive adhesive
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can be directly applied to the structured layer without r~;...ini~ g the visibility of the
structured pattern.
The l,anspale..l multilayer film described in DE 44 24 148.8 comprises a
destructible or irrcve. ~il ly def~"nable layer (4c) COl~.p~ ;sh~g, for example, a hot-melt
adhesive or a p,~,ss.lre-sensitive adhesive, which also can be used in the multilayer sheets
(1) of the present invention. The destructible or irreversibly d~m~g~hle layer (4c) may
also comprise, for example a paper layer whereby the internal strength and the thickness
of the paper layer are selected in a manner so that the paper layer is a predetermined
breaking point ofthe multilayer sheet (1).
I 0 The d~m~gable layer can also comprise indicia (4d) which can be applied
by any number of processes, including transfer processes or direct printing processes.
Indicia are preferably applied by standard printing processes, such as screen printing.
Screen printing inks commonly used for this purpose include 3M screen-printing inks
(800 series, 900 series), dried in a forced air oven at 45~C for 30 min. (800 series) or
for 45 sec. at 65~C. (900 series). ~nks should preferably by selected which can be dried
or cured at relatively low tcmperatures for short pcriods of time, to facilitatc rapid
manufacturing and prevent damage of thermally sensitive layers which may be present.
Printed patterns and other indicia may operate as damagable layers only when used as an
inner layer having two adjacent layers of the multilayer sheet. Top-printings, for
example, usually do not operate as damagable layers.
The use of printed patterns as a damagable layer is described, for
example in US 4,121,003 and US 4,184,701.
The specific embodiments of the d~m~ga~-le layer described above are
int~n-led to be illustrative and not limiting.
The term "d~ pahle layer" is used in the present invention to
characterize readily d~m~ble, i.e. destructible and/or inelastically defo~ able layers
which can be continuous or discontinuous. The d~mag~ble layer exhibits a low internal
or cohesive sli ~,ngtll and is the predetermined breaking point of the multilayer sheet
upon removing or attempts at tampering. It should be noted, however, that a multitude
of failure patterns is observed for the multilayer sheet of the present invention in
- practice. The failure process is relatively complex and is influenced by various
pa, anlelers such as the elasticity or brittleness, res~,ec1i~rely, of the di~rel1l layers, the
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tensile sll ~nglh of the materials used and the adhesion between the a~jacent layers, the
thickness of the di~rtrcl,l layers, the respective pattern of discontinuous layers if present,
the peel direction and peel angle, the peel strength and peel speed and the te,npel~l~Jre
during peeling. The failure pattern may change when varying one or more of the
pal~ ers. On tal.. p~. ;"g, the internal strength of d~ma~ble layers of type (4c), for
example, may increase in some cases as a result of stretching and simUlt~rleous
orientation of macromolecules in the d~m~g~ le layer so that one or more other areas of
the multilayer sheet have similar or even less internal strength and/or adhesion with
respect to ~djacent layer or layers than the damagable layer. This means in practice a
layer or layers of the multilayer sheet other than the r~m~hle layer may be deformed
and/or irreversibly destructed prior to the damagable layer and/or in addition and/or
instead of the r~m~g~hle layer. multilayer sheets (I ) with one or more damagable layers
(4) are subject-matter of the present invention irrespective of the actual failure mode, i.e.
irrespective of whether the failure pattern relies on damaging of the d~ hle layer or
layers and/or of other layers.
The multilayer sheet (I ) can furthermore comprise a cover layer (6) (not
shown) which forms the exposed layer opposite to the adhesive layer.
The cover layer (6) (not shown) of the multilayer sheet according to the
invention is typically a polymeric film which is preferably dimensionally stable, wear-
resistant, tear resistant, abrasion resistant and/or solvent resistant. The cover layer may
be corona-treated, flame-treated, plasma-treated or treated with a chemical primer.
Methods for the ll G~llllenl of surfaces are described, for example, in G.
Habenicht et al., Adhesion, 1992(5), pp. 21-36 and in K.W. Gerstenberg, Coating, 1990,
pp. 260-263.
A material which is p~cr~led for this purpose is, for example, a
polyethylene terephthalate (PET) polymeric film. The dimensionally stable, wear-r~;cl~lt, tear resistant, abrasion resistant and/or solvent resistant cover layer protects
the underlying layers of the multilayer sheet ~1) from wear, dirt, moisture and other
undesirable influences during its lifetime.
The cover layer plcrel~bly has a thickness of from 10 to 200 microns and
more pr~ Ç~.~bly from 10 to 150 microns. Especially preferred is a thickness from about
10 to about 75 microns. In order to improve adhesion to the underlying layer, the film is
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pl ~fetably treated with corona discharge or with a chemical primer. The primer can be
applied in the customary manner, for example with a coating rod, at a useful thickness of
up to about 1 micron.
The multilayer sheet (1) accor~ g to the invention can furthermore
CO,ll~.. ise a reflective layer (7) which rnh~nces its visibility. IncGl~,olation of a reflective
layer (7) is optional but prefwled. The reflective layer can be a metal layer such as a thin
metal foil, for example, or it can colllp.ise one or more types of rGLrorenective materials,
including microsphere-type ~ olt;llective materials and cube corner-type lelrorenective
materials. The, e~l orenective layer disclosed in US 2,407,680 may comprise an enclosed
monolayer of glass microspheres which are coated in a spacing resin co,.. pri~;ng, for
example, polyvinyl butyral or polyester. The spacing resin conforms to the microspheres.
A reflector layer which underlies the spacing resin may comprise opaque materials such
as silver, aluminum, chromium, nickel, or magnesium, or trans~arenl high-index reflector
materials such as those described above for use on the holographic structured layer, such
as zinc sulfide, or multilayer reflectors as described in US 3,700,305. Thus, light that
enters the retroreflective layer is focused by the glass microspheres through the spacing
resin, and reflected by the reflector layer back through the spacing resin and glass
microspheres to an observer. Other retroreflective materials using glass microspheres are
described in US 3,801,183. Microsphere-type r~lor~nective materials are commercially
available, for example, from 3M, St. Paul, USA, as 3M~ SCOTCHLITEg) High
Intensity Vehicle Marking Sheeting Series No. 790, 3M~) SCOTCHLITE~ Validation
Security Sheeting Series No. 5300, 5350, 5390, 4250, and 4290; 3M~ SCOTCHLITE~
Label Sheeting Series No. 5330; and 3M~ SCHOTCHLITE~ Face Adhesive
Verification Sheeting Series No. 2500.
The r~ll orellective layer (7) may also include a multiplicity of cube-
corner r~trorellective elements, instead of glass microspheres and resin, that may be
made of materials such as vinyl, polycal l,onate, or acrylic polymers for embossed cube-
corner ~ olenective elements, and urethane, epoxy, polyester and acrylate oligomers
and monomers for radiation-cured cube-corner rel~ orenective elements. The cube-corner elements typically have three mutually perpendicular faces with surface areas of
between applu~ ately 1.9 x 10-3 mm2 to 0.1 mm2. Cube-corner rel,orenective
elements can be embossed by a master mold into a sheeting material under suitable
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tc~llpelalule and pressure. The ~ .c may also be created by coating a radiation-curable resin onto a master mold, l~min~ting an overlay film under sufficient ple;,;,.l~e,
and solidifying the resin by curing it with radiation.
The thickness of each of the various layers forming the flexible substrate
(2) are plt;felably selected to be within the range of 10 to 1000 microns, preferably 15-
500 microns, and most pref~lably 20-200 microns.
The materials follllh-g the layers ofthe multilayer sheet (I) ofthe
invention can be brought together and bonded to one another using a variety of known
coating and lamination processes. Typically the pressure-sensitive adhesive layer (3) is
laminated to the flexible substrate (2) by a transfer process using only pressure. Layers
of the flexible substrate (2) are commonly combined using lamination equipment at
either ambient or elevated temperatures.
In cases of non-continuous layers such as patterns forming indicia or
patterns forming layers of adhesion regulating substances, these pattems may be applied
I S by standard printing techniques such as gravure printing or screen printing. Non-
continuos layers may also be formed separately on a temporary substrate such as a
release liner, for example and transferred onto other component layers of the multilayer
film by lamination as well.
Figure I shows a multilayer sheet (I) according to the present invention,
comprising a pressure-sensitive adhesive layer (3) which is adhered to a destructible or
irreversibly deformable layer (4c) which has an indicia layer (4d) on its inner surface.
The destructible or irreversibly deformable layer (4c) can be, for example, a paper layer.
Upon attempts to remove the multilayer film from the substrate, the indicia layer (4d)
breaks, followed or accompanied by an irreversible deformation and/or destruction of
the layer (4c) and/or the adhesive layer (3).
When using a paper layer as the destructible or i~ .sibly defol~,.able
layer (4c), the type of paper and the thickness of the paper should be selected so that the
paper layer splits and is irreversibly damaged upon tampering.
~ If the destructible or irreversibly deformable layer (4c) is an opaque layer
like, for example, a paper layer, the multilayer sheet of Fig. Ia can be applied, for
example, onto a glass surface or on the surface of another tl~ns~are..l, smooth and rigid
material, like for e~arllplc~ polymethylmeth~crylate sheeting and viewed from the
~ .. . . .
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opposite side ofthe l,ansl)ale.,1 material through the 1l~r,~,arent adhesive. This
construction, which will be termed above and below as a front face type construction,
can be used, for example, for a windshield sticker which is applied to the windshield
from the inside of the vehicle and viewed from the outside.
Fig. I b shows a multilayer film similar to that of Fig. 1 a whereby the
sequence of layer (4c) and indicia layer (4d) have been reversed. This construction
which will be termed above and below as rear face type construction, can be used, for
example as a la,~ e. yl oof and removable label or sticker which is adhered via a
pressure-sensitive adhesive layer (3) to a non-transparent surface such as a painted metal
surface. ~n this case the viewer looks at the printed pattern or indicia layer (4d). ~n this
construction the printed pattern or indicia layer (4d) does not forrn a predetermined
breaking point, because this layer is applied on top of layer (4c) and does not form an
inner layer as in case of Fig. 1 a.
In another embodiment which is derived from the embodiments shown in
Fig. Ia and Ib but is not shown, the destructible or irreversibly deformable layer (4c) is
replaced with a retroreflective layer (7), and the tamper-proof properties are primarily
based on the indicia layer (4d).
Figs. 2a shows a multilayer sheet ( I ) according to the present invention
comprising a pressure-sensitive adhesive layer (3) bearing an indicia layer (4d). The
adhesive layer bonds to a ~,~ns~arel,1 or s~"~ nsparent holographic layer (4b) such as a
microstNctured ho!ographic layer. The transparent holographic layer is bonded to the
light-entrant surface of a re~l ol enective layer (7) using a pattern-coating of an adhesion-
promoting subst~nce (4a) such as a primer. The pattern coating of primer promotes
adhesion of the thin, frangible holographic layer in areas where it is present, thus causing
the irreversible destruction of the holographic layer (4b), and any indicia (4d) it might
bear, upon attempts at t~""~e.ing.
The embodiment of Fig. 2a is a front face type construction which is
viewed through the pressure-sensitive adhesive layer and the transparent material it is
adhered to.
Fig. 2b is a rear face type construction which corresponds to the
construction of Fig. 2a. In this construction the printed pattern or indicia (4d) does not
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form a predetermined breaking point because this layer is applied on top of layer (4b)
and does not form an inner layer as in case of Fig. 2a.
The failure modes described for the embodiment of Fig. I and 2 are
illustrative and not lin-itin,e, and further failure patterns which are, for example,
intermediate between the failure modes described are observed in practice.
It was furthermore found by the present inventors that a pressure-
sensitive adhesive which is obtainable by polymerizing a precursor comprising
a) a monomer component which contains one or more alkyl acrylates,
the alkyl groups of which have an average of 4-14 C atoms, and more than 8-20 phr of
at least one copolymerizable monomer having a polar group,
b) 5-15 phr of hydrophobic silica,
c) one or more polymerization initiators, and
d) one or more crosslinker compounds in a concentration resulting in a
crosslink density obtainable by using hexanedioldiacrylate (HDDA) as a referencecrosslinker compound in a concentration between 0.06 and 0.11 phr,
exhibits advantageous 90~ peel adhesion values on smooth and rigid
surfaces such as glass or painted metal surfaces, and these pressure-sensitive adhesive
materials are subject-matter of the present invention. The concentration of the reference
crosslinker HDDA preferably is from 0.06-0.10 and especially preferably from 0.06-
0.09.
While the pressure-sensitive adhesive material described above which is
useful for the preparation of the pressure-sensitive adhesive layer (3) of multilayer sheet
(I) comprises an amount ofthe copolymerizable monomer or mono,l~c.~ of between 2-8
phr, it was found that useful pressure-sensitive adhesive materials with high values of
90~ peel adhesion from glass are obtained when varying the amount of the
copolymerizable monomers having a polar group from more than 8-20 phr, preferably
from more than 8-15 phr and especi~lly preferably from more than 8-10 phr. The
pressure-sensitive adhesives are new and subject-matter of the present invention. In
these pressure-sensitive adhesive materials the alkyl acrylates, the copolymerizable
monomers having a polar group, the polymerization initiators and the crosslinkercompounds can be selected as is described above for the pressure-sensitive adhesive
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useful for the pressure-sensitive adhesive layer (3), and these materials can be pl epa~ ed
as indicated above.
The multilayer sheets (1) of the present invention are readily removable
from rigid surfaces such as glass or painted metal and they are especially useful for the
preparation of temporary labels and stickers such as toll vignettes which are to be
removed after a specific period of use. In a pl ~. . ed embodiment the multilayer sheets
(I) comprise one or more d~ma~ble layers (4) and exhibit a unique coll,lJination of
tamper-proof features, and complete and ready removability.
The present invention is funher explained by the following examples
which are illustrative, but not limitative. The following test methods are used to
characterize the materials used in the examples.
Test Methods
90~ peel adhesion from glass
The 90~ peel adhesion test is a modified version of PSTC-3 which is
available from the Pressure Sensitive Tape Council of Glenview, Illinois, USA A
flexible rell ol enective sheeting (3M~ Scotchlite g) 4780A without EAA top film,
available from 3M, St. Paul USA) was substituted for the polymeric film normally used
as a backing in this test.
The adhesive layer to be tested was produced by photopolymerizing the
prepolymerized precursor between two layers of siliconized biaxially oriented
polyethylene terephth~ e (PET) film having a thickness of 50 microns. Strips of the
adhesive material between the two polyester sheets were cut into widths of 2.54 cm and
lengths of 11 cm for us in this test.
One of the polyester sheets was removed and the exposed adhesive face
l~tnin~ted to the retroreflective side of a co""~lerl ially available ~ t;L- oreIlective sheeting
(see above) with a rubber-coated roller. The roller was passed over the sample 5 times
with a pressure that can be exerted by hand. The adhesive/reflective ~heetin~ l~min~te
thus p~epaled was then stored at 23~ C for 24 hours.
The second polyester sheet was then removed and the exposed adhesive
face applied to a glass plate (side not exposed to metal bath during production) which
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had been cleaned with isopropyl alcohol. The adhesive/~ oreilective ~heetine l~rnin~te
was pressed onto the glass surface using a conventional mec~qnical l~min~tor. The
lamination was pe.ro,l"ed with a rubber-coated roller weighing 6.8 kg which was passed
over the sample 4 times at a rate of 300 mm/min.
Samples were then stored at various conditions before the peel adhesion
measurements were made. Peel adhesion at 90~ peel angle were then made at 23~C with
a peel rate of 300 mm/min. using a tensile tester.
Three measure",e.lts were made for each sample. The values were then
averaged and recorded in N/in (N/2.54 cm).
Removablilty Test
Pressure-sensitive adhesive compositions were photopolymerized
between two siliconized transparent polyester sheets. One polyester film was removed
from the adhesive layer thus formed and the exposed adhesive surface applied to a
multilayer backing (2). The two materials were laminated together using a rubber-coated
roller. The roller was passed over the sample 5 times with a pressure that can be exerted
by hand. The adhesive/reflective sheeting l~min~te thus prepared was then stored at
23~C for 24 hours.
The remaining polyester sheet protecting the pressure-sensitive adhesive
layer was then removed. The final multilayer sheet (1) was then adhered to a glass
substrate using a conventional mechanical laminator. The lamination was performed with
a rubber-coated roller weighing 6.8 kg which was passed over the sample 4 times at a
rate of 300 mm/min.
The behavior ofthe multilayer sheet (1) was evaluated qualitatively
during attempts to peel the multilayer sheet from glass by hand.
Examples 1-5 and Co"~para~ e Examples 1-5
A mixture of 0.04 pph or Irgacure 651 (commercially available through
Ciba-Geigy) and 100 parts by weight of isooctylacrylate (IOA) was partially
polymerized by UV light to form a syrup of a coatable viscosity of about 2 000 mPas.
- Then as in(lir~ted in Table I various amounts of hydrophobic silica, hexanediol
diacrylate (HDDA) and, in each case 0.2 phr of lrgacure 651 (commercially available
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through Ciba-Geigy) were added to the syrup followed by mixing with a lab stirrer for
90 minutes. The coating and curing of the syrup were performed between two
transparent siliconized polyester films. The level of radiation dosage was about 1,300
mJ/cm2. The radiation was supplied from lamps having about 90% of the emissions
between 300 and 400 nm, and a peak emission at 351 nm. The thickness ofthe adhesive
samples was chosen to be 300 or 800 ~lm.
Table 1 Adhesive compositions used in the examples
Adhesive Isooctyl Acrylic Hydrophobic HDDA (2)
acrylate acid silica (I)
A 96.5 3.5 15 0.13
B 95 5 10 0.12
C 96.5 3.5 10 0.12
D 98 2 10 0.12
E 92 8 10 0.12
cA 100 0 15 0.3
cB 90 10 15 0.12
cC 90 10 12.5 0.12
cD 90 10 10 0.12
(1) Hydrophobic filmed silica, available as R-972 from Degussa.
(2) HDDA = hexanedioldiacrylate
Examples 1-5
90~ p~el adhesion was measured for the .. 'hesives of Examples 1-5 and
comparative Examp.~ 1-5 i~ describe above. It is evi~; 3~ that high adhesion values
were present initially for the adhesives of Examples I -5 and were maintained a~er aging
and adhesive bond under various conditions. The adhesive bond could still be broken if
sufficient force was applied and the adhesive could be removed completely.
Contrary to this pressure-sensitive adhesive compositions of Co,npar~live
20 Examples 1-5 exhibit adhesion to glass which is either too high (C2-C5) or too low (Cl)
to be useful for the particular application envisioned.
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Table 2 Results of peel adhesion tests
Ex. Adhesive Adhesive Glass, Glass, Glass, Paintcd mctal,
comp. thickness 20 min, 3 days, 23~C. 3 days, 70~C. 20 m~n, -10~C~,
(microns) 23~C (NAn.) (N/in), pccl (N/in.), pcclat (N/in.),
peel at 23~C at 23~C 23~C peel at -lOC~
A 800 76Ø 60.0 74.9
2 B 300 73.2 36.8 52.8
3 C 300 32.8 33.0 41.5 27.2
4 D 300 29.2 30.1 33.4 14.0
E 300 38.9 49.6 80.9
Cl cA 800 10.6 9.4 9.4 --
C2 cB 800 28.6 57.1 ~ --
C3 cC 800 47.1 64.9 ~ --
C4 cD 800 62.9 73.0 ~ --
C5 cD 300 57.9 63.4 ~ --
~ Conslmclion brcak, adhcsion loo high lo bc mcasurcd, adhcsivc not co,.")l cly rcmovablc without
solvcnts and/or ",r~h~ ! tools
Examples 6-8 and Comparative Example 6
The pressure-sensitive adhesive materials shown in Table 3 were
obtained by using the method of Examples 1-5.
Table 3
Adhesive IOA AA ~ydrophobic HDDA
silica
F 90 10 15 0.06
G 90 10 15 0.08
H 90 10 15 0 10
cE 90 10 15 0.12
90~ peel adhesion was measured for the adhesives of Examples 6-8 and
Coml)a~ati~e Example 6 using the test method specified above. The results are
summarized in Table 4.
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Table 4
Example Adhesive Adh. thickness, Glass, 3 days, 23~C,
comp. microns (N/1.27 cm)
6 F 800 73.2
7 G 800 77.9
8 H 800 65.3
c6 cE 800 47.1
Example 9
A multilayer backing (2) was prepared by the following procedure:
1. A polyester film (= backing) was heat laminated onto the aluminum
side of a rel~ ul enective sheeting based on microspheres. The laminate
backing/retroreflective sheeting is available from 3M, St. Paul, USA as 3M~
Scotchlite g) FAVS sheeting 2005 E.
2. A pattern of ethylene-acrylic acid based adhesion-promofing primcr
70:30 wt.% mixture of Neocryl A45 (commercially available from Zenica, Netherlands)
and Adcoate 50 T 4990 (commercially available from Morton, USA) was then appliedto the front or face side of the retroreflective sheeting used above by a simplified hand
printing process to generate a regular array of spots of approximately 3 cm in diameter.
I S A sheet of the material thus prepared was cut into samples of approxi".ately 6 cm x 4
cm.
3. A thin transparent film bearing a hologram was then heat l~min~ted to
the front surface of the reflective sheeting bearing the patterned adhesion-promoting
substance. The holographic film which is commercially available from Crown Roll Leaf,
Inc of Peterson, New Jersey, was a thin ll ~n~pa~ e"l hologram transfer film, which
consisted of a polyester liner, a polymethyl methacrylate based structured layer, a high-
index zinc sulfide reflector, and a thin adhesive. Lamination was performed at
approxilllately 130~C. with a pressure of 2 bar and a speed of 5 cm per minute using a
cGml"el ,;ially available heat laminator (Sallmetal, Raalte, The Netherlands).
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4. An image was then screen-printed on top of the tl an~par,nL
holographic layer using a co.l,mel cially available screen-printing ink (3M screen-printing
ink 8~2 (red)). The ink was dried 30 min. at 45~C.
A 300 micron thick layer of p- .,s;,-lre-sensitive adhesive of FY~rnrle2
was applied by first removing one ofthe protective liners and then 15~".;na~ing the PSA to
the screen-printed surface using a rub~er-coated roller passed over the sample 5 times
using that pressure which could be exerted by hand, thus prc?a.il-g a front face type
multilayer sheet (I ) of the invention.
Effectiveness of the complete adhesive-coated multilayer sheet
construction was evaluated by first removing the second protective liner, adhering the
multilayer sheet to a glass test surface and qualitatively evPII1atin~ the removal behavior
as described under the "Removability test" method above. After multilayer sheet
removal, several areas were examined for the plcse"ce of residues: I) the glass
substrate; 2) areas of the I ~ll oreflective layer where adhesion-prollloting s~lbstance had
becn applied; and 3) areas of the retroreflective layer where no adhesion-promoting
su~stance was applied. Results of the removability test are given in Tables 6 and 7.
FX- nrle 10
Example 10 was plepalcd in an identical manner to Example 9, with the
exception that the pressure-sensitive layer of Example 3 was applied to the face of the
multilayer sheet at a thickness of 300 microns. Results of the removability test are given
in Tables 6 and 7.
Example I I
Example 11 was plel)aled in an identical manner to Example 9, with the
exception that a 300 micron layer ofthe PSA of Example 4 was applied to the face of
the multilayer sheet. Results of the removability test are given in Tables 6 and 7.
Example 12
Example 12 was plel)aled in an identic~l manner to Example 9, with the
exception that a different screen-printing ink was employed. The screen-pl h~ling in
applied to the pattern-coating of primer on the l e~rol ellective layer was 3M screen-
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p,i"lh~g ink 910 (blue), dried 45 sec. at 6S~C. Results ofthe removability test are given
in Tables 6 and 7.
Example 13
Example 13 employed a multilayer sheet as described in Example 12,
with the exception that the polyester film present on the ~IIlminurn-coated side of the
l~tlolenective sheeting was omitted. As in Example 12, a 300 micron layer ofthe
pressure-sensitive adhesive denoted B was present. The adhesive layer was l~min~ted to
the aluminum-coated side or rear side of the r el~ o, enective sheeting, however, rather
than the front face, thus creating a rear face type multilayer sheet suitable for application
to a painted metal surface such as a motorcycle body.
Ex~mrles 14-15
E~c~"~,les 14 and 15 were p,~,~aled in the same manner as Example 13,
with the exception that the pressure-sensitive adhesives denoted as C and D,
res~c~lively were employed at a thickness of 300 microns.
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Table 5 Construction of F.Y~m, les 9-15
(The order in the Table reflects the construction of the multilayer sheet.)
Ex. PSA, backing ~ ulen. primer hologram .print PSA, microns
~,ficl~ns ~heeting
9 x x x x ink 1 B, 300
x x x x ink 1 C, 300
11 . x x x x inkl D,300
12 x x x x ink 2 B, 300
13 B, .300 *) x x x ink 2
14 C,300 *) x x x ink2
D, 300 ~) x x x ink2
Ink I ~M screen-printing ink 882 (red), dried 30 min. at 45~C
Ink 2 3M screen-printing ink 9lO (blue), dried 45 sec. at 65~C
~) 3M~ Scotchlite~ FAVS sheeting 2005 E without polyester backing
Table 6 Results of removability tests (I hr. 23~C)
Example Residue on glass Residue on Residue on
primed area unprimed area
9 Adhesive, portionsof Hologramand No residue
hologram and ink ink
Adhesive, portionsof Hologram and No residue
hologram and ink ink
11 Adhesive, portionsof Hologramand No residue
hologram and ink ink
12 Adhesive, portions of Hologram and No residue
hologram and ink ink
Table 7
Adhered multilayer sheets of Examples 9-15 were stored under a variety
of conditions as inrlic~ted in the Table and then removed from glass. All failures
intlin~ted that the tamper-proof features of the multilayer sheets such as layer splitting
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were operational. Adhesive residues and/or backing residues on the glass substrate were
all cGr,.pl~lely removable by hand.
Sticker Glasstemp., Storageconditions, Removal conditions,
temp., ~C ~C time and temp, ~C. approx. temp, ~C.
23 23 1 hour, 23~ 23~
23 -30 2 days, -30~ -30~
23 -30 2 days, 40~ (90%r.h.) 40~
23 -30 I wk,-30~ -30~
-6 11 2 days, 23~ 23~
-6 11 2 days, -30~ -30~
-6 11 1 wk,-30~ -30~
-6 11 2 days, 40~(90%) 40~
-6 11 1 wk,23~ 23~
