Field of Invention
This invention provides a novel pressure-sensitive structural adhesive
construction which is normally tacky and forms, at room temperature, a pressure-sensitive
adhesive bond which is supplanted by a structural adhesive bond upon heat activation.
Background of the Invention
Structural adhesive materials such as cyanoacrylate epoxy resins
and the like are well-known for providing strong and permanent bonds. However,
before curing, such adhesive materials do not normally provide aggressive adhesive
qualities and, therefore, require external aids such as clamping devices to hold
the substrates to be bonded together until cure has been completed and a structural
Tacky, pressure-sensitive adhesives have been known for many years
and have been used in various bonding and fastening applications. They provide
a flexible bond and are used in a wide range of applications including adhering
labels, decals, and bonding automotive trim parts and name plates to various substrates.
However, they show substantially lower strength characteristics compared to a structural
adhesive like cured epoxy or cyanoacrylates.
There is a need in the industry to find a composition which exhibits
pressure-sensitive adhesive characteristics useful during assembly and which may
then be converted by subsequent treatment to a structural adhesive. It is preferred
that the subsequent treatment be by thermal means and could even be cured by electromagnetic
induction if desired.
In use, such adhesive could be provided between two release layers
or as a single tape with differential release, preferably using a release layer
which is differentially releasable from an adhesive layer bonded to a backing
which may also be a release surface. The adhesive could then be conveniently applied
between the surfaces to be bonded. Sufficiently firm contact or light pressure
established between the surfaces will cause sufficient adhesion to temporarily
hold the assembly. Thereafter, heat is applied to the composite structure to convert
the intermediate adhesive layer into a structural adhesive bond.
Applications for such tapes include "hem-flange" bonding and weld
reinforcement in automotive, reinforcing materials for strengthening thin sheet
metal and plastics by forming integral structures with the original substrate,
and bonding of plastics, precoated metals, SMC, and other advanced materials.
Several attempts were made in the past to come up with a pressure-sensitive
adhesive which changes into a structural adhesive after heat activation.
U.S.-A-3,326,741 (1967) to Olson discloses a tacky pressure-sensitive
adhesive which on heat activation achieves a strong permanent bond. The patent
discloses a nitrile rubber/epoxy resin blend with curing agent such as dicyandiamide.
However, the films did not show good cohesive strength at room temperature which
is typically required for a pressure-sensitive adhesive.
U.S.-A-3,639,500 (1972) to Muny and assigned to Avery Dennison Corporation
discloses a curable pressure-sensitive adhesive composition containing a polyepoxide,
a carboxylated diene polymer, and an acrylic ester tackifier which on heat activation
provides a structural bond.
U.S.-A-4,404,246 (1983) to Charbonneau et. al discloses an alkoxylated
amino formaldehyde condensate in an acrylic pressure-sensitive adhesive composition
as a latent crosslinking agent to improve the cohesive strength after heat activation.
However, the material behaves as a highly crosslinked pressure-sensitive adhesive
after heat activation and cannot be used for structural bonding.
U.S.-A-4,452,955 to Boeder discloses an adhesive composition consisting
of a polymer dissolved in polymerizable monomer, an accelerator such as organic
sulfimides and perfluoroalkylsulfonanilides, and an inhibitor. The adhesive shows
pressure-sensitive adhesive properties, and after heat activation, the adhesive
demonstrates properties similar to a structural adhesive. However, the properties
as detailed in the examples do not show properties of true structural type adhesives,
especially in lap shears.
U.S.-A-4,404,345 (1983) to Janssen also discloses a similar adhesive
composition consisting of an adhesive base as the first part and an initiator
portion as the second part. Bonding methods are also described. None of the prior
art described above teaches or suggests a pressure-sensitive adhesive that can
be heat cured to form the strong and permanent bond of a structural adhesive.
Summary of the Invention
This invention provides an adhesive that has the ease of application
of a pressure-sensitive adhesive, yet upon cure by application of heat, forms the
strong and permanent bond of a structural adhesive. This adhesive is made up of
a core layer of a curable permanent structural adhesive having opposed surfaces
with a thin skin of a pressure-sensitive adhesive on one or both surfaces. The
core layer is preferably made up of either a partially cured B stage structural
adhesive or a blend of curable polymeric material such as an epoxy with an acrylate
resin and hardener which upon cure will form a structural adhesive. It is this
core layer that forms the strong and permanent bond after final cure. The preferred
embodiment also includes an impact improving elastomer to make the bond more impact
resistant. In the most preferred embodiment, an impact absorbing agent is included
in the core layer.
The skin of pressure-sensitive adhesive that is applied to one or
both opposing surfaces of the core layer is inherently tacky and provides a temporary
bond between the adhesive and a substrate at room temperature. This temporary
bond created by the pressure-sensitive adhesive provides a means for holding two
substrates together during the cure of the core layer structural adhesive. The
core layer structural adhesive is cured by heating the entire assembly made up
of the two substrates and the adhesive. Once the heat-cure process has been completed,
a strong and permanent bond results. During cure, the skin or skins of pressure-sensitive
adhesive are absorbed, presumably blending into the core layer structural adhesive.
Such an adhesive is useful in that it eliminates the need to clamp
or otherwise hold substrates to be bonded together during heat cure. The skin or
skins of pressure-sensitive adhesive act in place of clamps by providing a temporary
bond that lasts long enough to enable the structural adhesive to be cured.
Brief Description of the Drawings
- FIG. 1 is a cross section of one embodiment of the invention.
- FIG. 2 is a cross section of a second embodiment of the invention.
- FIG. 3 is a cross section of a third embodiment of the invention.
This invention provides a novel pressure-sensitive adhesive construction
which is normally tacky and forms a pressure-sensitive adhesive bond at room temperature
and which on heat activation is supplanted by a structural adhesive bond. This
is achieved by distributing on one or both sides of a core layer of structural
adhesive a thin skin of a pressure-sensitive adhesive. If the pressure-sensitive
adhesive is applied to only one side of the core layer, the other side of the
core layer can be prebonded to a backing or face stock. The skin or skins of pressure-sensitive
adhesive can be either continuous or discontinuous layers. Various different pressure-sensitive
adhesives can be used for the skin layer or layers. For example, either acrylic-
or rubber-based pressure-sensitive adhesive may be used. Additionally, the pressure-sensitive
adhesive can be either one that is inherently tacky or one that requires addition
of a tackifier prior to bonding. The core layer is made up of various different
structural adhesives such as a partially cured B stage structural adhesive or a
blend of epoxy with an acrylate ester resin and hardener. While the skin or skins
of pressure-sensitive adhesive provide the initial tack, these layers are absorbed
into the core layer by heat activation. This unique feature provides a pressure-sensitive
adhesive with structural adhesive properties after heat activation.
Referring to the figures, three different embodiments of the invention
are illustrated in FIGS. 1-3. In FIG. 1, a cross section of the adhesive is shown
in which a core layer of structural adhesive 10 is sandwiched between two continuous
skin layers of pressure-sensitive adhesive 12 and 14. The illustration also shows
a suitable release liner 16 which protects the pressure-sensitive adhesive and
prevents the adhesive from inadvertent bonding prior to use.
FIG. 2 is a cross section of a second embodiment of the invention
in which the core layer of structural adhesive 18 is sandwiched between two discontinuous
skin layers of pressure-sensitive adhesive 20 and 22. These discontinuous skin
layers can take the form of stripes, dots or various other patterns of pressure-sensitive
adhesive. The release liner is illustrated as 24.
FIG. 3 is a cross section of a third embodiment of the invention.
In this embodiment, the core layer of structural adhesive 26 is prebonded to substrate
28. A skin layer of pressure-sensitive adhesive 30 is found on the opposite side
of the core layer. Release liner 32 protects the adhesive and prevents inadvertent
bonding prior to use.
This invention is useful for various applications where a strong
bond between two substrates or between a backing and a substrate is desirable but
difficult to achieve because the substrates are difficult to hold together while
the adhesive cures. Applications for such adhesives are numerous. In the automobile
manufacturing industry, such adhesives would be useful for "hem-flange" bonding
and weld reinforcement by providing a temporary bond which would become permanent
during the paint-bake step in manufacturing. The adhesive would also be useful
in the aerospace and other industries by providing a means of easily forming integral
bonds between plastics, metals and various advanced materials.
The core layer can be comprised of an epoxy resin, a polymer resin
and a hardener. Typical examples of epoxy resins include Epon 828, Epon 826, Epon
836 and the like available from Shell Chemical Co., and are characterized by the
presence of epoxide functionality. The resins at room temperature could be a liquid
of low molecular weight or solid resins which are higher in molecular weight.
Blends of several epoxy resins of different structures, molecular weight and epoxy
functionality could be used to achieve the desired balance of properties for the
The polymer resin is preferably an acrylic ester resin like polymethyl
methacrylate (PMMA) and PMMA copolymers and the like. For example Acryloid Resin
A21, B 66T etc. manufactured by Rohm and Haas can be used for this application.
The acrylic ester resins could be based on methyl methacrylate, butyl acrylate,
and isobutyl methacrylate and the like. Other polymers or polymer blends can also
Typical examples of hardeners include boron trifluoride or trichloride
amine complexes like boron trifluoride: monoethyl amine, and blocked amines like
HT 9506 (produced by Ciba Geigy), or Dicyandiamide (Dicy). Mixtures of a blend
of hardeners could also be used. Other types of epoxy hardeners could also be used
if they provide the desired stability and efficient cure during the heat activation.
The hardeners would preferably be incompatible with the resin or inactive at room
temperature but if compatible, the hardeners could also be encapsulated in a heat
or pressure-sensitive polymer shell.
The core layer can also be comprised of a partially cured B stage
epoxy resin. A typical example of such a resin is the FM-73 type manufactured by
American Cyanamide. Additional fillers, modifying additives, fibers, and the like
could be added to the core layer to improve the strength or modify the properties
of the core layer. Due to brittleness of resins without fillers, the preferred
embodiment includes fillers. Low density additives like microballoons could be
incorporated if desired. Electromagnetic materials, such as particulate magnetizable
iron, cobalt, nickel, alloys of nickel and iron, alloys of nickel and chromium,
inorganic oxides of iron, inorganic oxides of nickel and the like could be included
to make the adhesive induction curable. The material could also be formulated for
UHF, radio frequency, or microwave curing if desired.
The skin layer or layers of pressure-sensitive adhesive can be comprised
of an acrylic pressure-sensitive adhesive such as Polytex 7000 which is produced
by Avery Chemical, Division of Avery Dennison Corporation. This is a high performance
pressure-sensitive adhesive within the scope of the claims of U.S.-A-4,812,541
to Mallya et al., incorporated herein by reference. This high performance pressure-sensitive
adhesive provides unusually high adhesion to high energy surfaces such as aluminum
and stainless steel due to synergistic combination of a glycidyl monomer and an
N-vinyl lactam monomer.
The skin layer or layers of pressure-sensitive adhesive can also
be comprised of an elastomeric pressure-sensitive adhesive. Curable elastomeric
pressure-sensitive adhesives are disclosed in U.S.-A-4,948,825 to Sasaki, incorporated
herein by reference. These curable elastomeric pressure-sensitive adhesives incorporate
organic additives to reduce the energy requirements of a pressure-sensitive adhesive
that is cured by actinic radiation.
Various other pressure-sensitive adhesives with different properties
can be used for the skin layer as well. A permanent pressure-sensitive adhesive
can be used, or if repositioning of the substrates to be bonded is desired prior
to final cure, a removable pressure-sensitive adhesive can be used. A repositionable
pressure-sensitive adhesive is disclosed in U.S. Patent Application Serial No.
741,556 to Mallya et al., incorporated herein by reference. This application discloses
a repositionable pressure-sensitive adhesive that has adhesive characteristics
that vary depending on the application pressure. Furthermore, an inherently tacky
pressure-sensitive adhesive can be used, or one that requires the addition of
a tackifier prior to bonding.
The impact resistance of the structural adhesives disclosed in this
specification tends to be low. Impact resistance can be improved by the addition
of fillers, modifying additives, fibers or the like. These additives are included
in the core layer at levels which provide satisfactory impact resistance and reduced
brittleness. Since the skin layer of pressure-sensitive adhesive is absorbed into
the core layer during cure, the impact modifier can also be added to the pressure-sensitive
adhesive as well as the core layer with equivalent results.
While numerous options are available for the skin layers of pressure-sensitive
adhesive that make up this pressure-sensitive structural adhesive, the preferred
embodiment uses an acrylic pressure-sensitive adhesive that includes a compatible
Various release layers are available which may be applied to the
adhesive and are useful in protecting the skin layer or layers of pressure-sensitive
adhesive from inadvertently bonding prior to use. Suitable release layers are
described in some detail in Chapter 23 of the Handbook of Pressure Sensitive
Adhesive Technology, 2d Ed., edited by Donatas Satus, and incorporated herein
by reference. If skin layers of pressure-sensitive adhesive are used on both sides
of the core layer of structural adhesive, then release layers can be applied to
both sides of the adhesive. These two release layers would preferably be differentially
releasable from the adhesive layer to provide additional convenience in application.
Following are examples which more specifically illustrate the invention.
In these examples, the 180° peel adhesion is measured as described in PSTC-1, using
2 mil Mylar as face material and stainless steel substrate. The shear adhesion
failure temperature (SAFT) is measured by applying the adhesive to a 0.051 mm (2
mil) aluminum strip and bonding it to stainless steel with a 2.54 cm2
overlap. A 1.0 kg load is attached on one end and the temperature is raised at
0.5°C per minute until the adhesive fails in shear. The temperature at which the
adhesive fails is regarded as the SAFT temperature.
Lap shear is determined by applying the adhesive film to a 0.64 to
1.01 mm thick by 2.54 cm strip of steel that is subsequently bonded to a second
strip of steel with the same dimensions. A 1.27 cm overlap is maintained between
the steel strips for a total bond area of 1.27 cm by 2.54 cm. A 0.5 mm bond thickness
for the structural adhesive is maintained by using 0.5 mm spacer bars placed between
the steel strips. An external force is applied to pull the strips apart, and the
lap shear is measured as the force at which the bond breaks divided by the bond
area. Lap shear is reported in pressure units as MPa. Since the bonded steel strips
are offset in nature, the measured bond failure is not purely due to shear stress,
but includes cleavage and peel stresses as well. This determination of lap shear
follows general engineering standards of the automobile industry.
A core layer was prepared by mixing 48 parts of Acryloid B66T (51%
in toluene) a methyl methacrylate dissolved in toluene, 16 part of Epon 828 and
8 parts of HT9506. The mixture was coated onto a Teflon FEP film to give a coat
weight of 75 g/m2. This was first dried for 15-20 minutes at room temperature
and then at 70°C for 15 minutes.
A skin layer of an acrylic pressure-sensitive adhesive was prepared
by solvent coating 15 g/m2 of Polytex 7000 onto a Teflon FEP film and
drying at 70°C for 15 minutes. The skin layer was then laminated on both sides
of the core layer to prepare a sandwich construction.
This film gave a 180° peel adhesion of 526 N/m, shear adhesion of
441 minutes, shear adhesion failure temperature of 60°C (140°F) and lap shear of
1.3 MPa (189 psi). After baking this film at 200°C for 30 minutes, it gave a lap
shear of 6.89 MPa (1000 psi).
A "B" stage epoxy resin (FM-73) from American Cyanamide was used
as the middle core layer.
A skin layer of pressure-sensitive adhesive (Polytex 7000) was laminated
to each side of the core layer at 15 g/m2 coat weight.
This film gave a lap shear of 0.25 MPa (37 psi). After baking at
200°C, this film gave a lap shear of 6.89 MPa (1000 psi).
A core layer was prepared by mixing 66 parts of Acryloid A21 (30%
in toluene), 39 parts of Epon 828 and 12 parts of HT9506. Acryloid A21 is a methyl
methacrylate polymer. The mixture was coated onto a Teflon FEP film to give a
coat weight of 75 g/m2. This was first dried for 15-20 minutes at room
temperature and then at 70°C for 15 minutes.
A skin layer of an acrylic pressure-sensitive adhesive was prepared
by coating 15 g/m2 of Polytex 7000 onto a Teflon FEP film and drying
at 70°C for 15 minutes. The skin layer was then laminated on both sides of the
core layer to prepare a sandwich construction.
This film gave a 180° peel adhesion of 526 N/m, shear adhesion of
2000 minutes, shear adhesion failure temperature of 83.89°C (183°F) and lap shear
of 1.23 MPa (174 psi). After baking at 200°C for 30 minutes, this film gave a
lap shear of 6.89 MPa (1000 psi) typical of a structural adhesive.
A middle core layer was prepared by blending 227 parts of Araldite
GY 6010, an unmodified liquid epoxy resin (manufactured by CIBA GEIGY), and 95
parts of Acryloid B66, a methyl/butyl methacrylate copolymer, in a Brabender Plasticorder
using the roller mixer at 140° C until the mixture was homogenous. The temperature
of the mix was reduced to 45-50°C by cooling and 32 parts of Dicyanex 200-X, a
solid curing agent, (manufactured by American Cyanamide) was mixed into this mixture.
This blend was then extruded as a sheet, 0.076 to 0.152 mm (3 to 6 mil) in thickness
using the Brabender extruder. The die temperature was maintained at about 50 to
60°C. Two thin (15 g/m2) acrylic pressure-sensitive adhesive layers
were laminated on either side of this core.
This film laminated to 0.051 mm (2 mil) Mylar showed a peel adhesion
of 1400 N/m on stainless steel substrate, room temperature shear of 255 minutes
and SAFT of 45°C. The sample showed a lap shear of 0.72 MPa (105 psi). The lap
shear of the sample after baking at 200°C for 30 minutes increased to 19.29 MPa
A core layer was prepared by blending until homogeneous 300 parts
of Acryloid A-21, a methyl methacrylate polymer resin, (heated to 175°C) with
400 parts of Araldite GY-6010, an unmodified liquid epoxy resin, raising the temperature
to 215°C and adding 44 parts of glass fiber (965-57) and mixing for about 30 minutes.
The mixture was cooled to 90°C and 50 parts Hycar 1300 x 13, an acrylic copolymer,
were added then the blend was mixed for an additional 30 minutes. Finally 70 parts
of Dicyanex 200-X, a solid curing agent, were added and mixed for another 30 minutes.
The mixture was coated onto a Teflon FEP film to give a coat weight of 550 g/m2.
Two skin layers of Polytex 7000 adhesive film were laminated to either
side of the core layer to give a coat weight of 17 g/m2. This sample
gave a 180° peel adhesion of 1020 N/m. The lap shear after baking at 196°C for
30 minutes was 13.57 MPa (1970 psi).
A core layer was prepared as in Example 5 to a coat weight of 625
g/m2. Skin layers of I-406 were laminated to either side of the core
layer to give a coat weight of 10.6 g/m2.
The film gave a 180° peel adhesion of 875 N/m. After baking at 196°C
for 30 minutes, the sample gave a lap shear of 9.64 MPa (1400 psi).