Note: Descriptions are shown in the official language in which they were submitted.
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Adhesive Film
The present invention relates to a multilayer adhesive film and also
to its use for the bonding of load-bearing metal parts, in particular
in the vehicle industry.
Hitherto load-bearing metal parts have been welded. The advantages of
bonding load-bearing metal parts compared with welding are obvious: in
addition to a reduction in weight as a consequence of increased energy
absorption by the bonded parts and their improved energy transmission,
investment in expensive welding robots is unnecessary. The change in
the metal resulting from the welding spots, for example in the case of
steel, and the increased susceptibility, associated therewith, of the
metal to corrosion no longer take place. In addition, the bridging of
gaps by bonded joints is better and the susceptibility of such joints
to corrosion is therefore again reduced.
Previous attempts to employ adhesives to join load-bearing metal
parts, for instance with epoxy resin pastes, did not yield a com-
pletely satisfactory solution. Extremely high requirements are
imposed today on such parts, in particular in relation to corrosion
resistance, high peel strength and particularly high impact strength.
It has hitherto been the strength under impact conditions in partic-
ular which has presented a problem only capable of an unsatisfactory
solution by means of adhesives, and this explains why adhesives have
been used in vehicle construction only to join non-load-bearing parts.
This applies to liquid and paste-like adhesives, and also to adhesive
films.
Multi layer adhesive films for use in the vehicle industry, in par-
ticular in motor car construction, have also been described. Thus,
for example, double-sided adhesive films with two thermosetting formu-
lations as adhesives and a woven textile base are known as repair aids
from German Offenlegungsschrift 3,125,393. The French Publication
No. 2,201,184 describes adhesive films consisting of a highly fluor-
inated elastomer and two hot-curing epoxy resin layers for joining
materials with unequal coefficients of thermal expansion, for instance
-
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metal and glass.
The present invention relates to a multilayer structural
adhesive film consisting of an elastomeric base film of styrene-
butadiene (SBR) or acrylonitrile-butadiene rubber (NBR) or a
polyurethane elastomer which is coated on both sides with a
thermosetting constructional adhesive film, said adhesive being a
polyaddition, a polycondensation or a polymerization adhesive.
Suitable elastomeric base films are materials which
combine the application characteristics of elastomers with those
of thermoplastic materials. Typical examples thereof are styrene-
butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), but
also polyurethane elastomers such as thermoplastic polyurethane
elastomers (TPU). These elastomers are well known to those
skilled in the art and are commercially available in various
modifications and compositions. In these cases the thickness
of the film is 0.05 to 1.5 mm, preferably 0.15 to 1.5 mm.
Particularly good results can be achieved if the surface of the
elastomeric film is textured.
In a particularly preferred embodiment, the elastomeric
base film is perforated. The perforations may take the form of
punched holes or the form of incisions or slits. This achieves
the result that the thermosetting materials come into direct
contact at various selected points. Surprisingly, the impact
strength is further increased as a result. In addition, the
behaviour under dynamic and static load, and the fatigue limit are
improved. This is apparent particularly at elevated temperature,
without the excellent low-temperature characteristics being
impaired. The proportion of the perforation referred to the total
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area of the elastomeric base film may constitute up to 70%.
preferably it is 10 to 60%, and in particular 10-30%. The
diameter of the perforation is, for example, 0.5 to 5 mm, in
particular 1.0 to 3.0 mm.
Suitable thermosetting constructional adhesives are a
polyaddition, a polycondensation or a polymerization adhesive.
The thermosetting layer has a thickness of 0.05 to 1 mmr
preferably 0.15 to 0.5 mm.
Typical representatives of polyaddition adhesives are
epoxy resins. Hot-curing epoxy resin adhesives are suitable for
the application
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envisaged. Typical reactants are carboxylic acid anhydrides, phenols
or polyamines. As hardeners or hardenin~ accelerators, mention may be
made here, for example, of the following substances: cyanamide, di-
cyandiamide, hydrazides, Friedel-Crafts catalysts such as boron tri-
fluoride and boron trichloride and their complexes and chelates which
are obtained by reacting boron trihalides with, for example, 1,3-di-
ketones. Mention may also furthermore be made of imidazoles such as
l-methylimidazole. The nature and quantity of hardeners or hardening
catalysts are familiar to those skilled in the art and permit them to
match adhesive systems to the special requirements in relation to
adhesive layer characteristics and curing conditions. The processing
conditions, especially the pot life, are also determined to a decisive
extent by the particular formulations. The single-component epoxy
resin adhesives may be highlighted here as particularly preferred.
Since the reaction mixture consisting of the two components may be
regarded in this connection as one component from the process engin-
eering point of view, such systems are also described as single-
component thermo-setting adhesives. The single-component thermo-
setting adhesives have the advantage that dispensing errors during use
are eliminated since the two components are already mixed at the
premises of the adhesive manufacturer.
Elastomer-modified epoxy resins are also very suitable. These are
known under the designation of toughened epoxy resin adhesives. In
this case up to 10% by weight of a rubber, for example acrylonitrile-
butadiene rubber are added to the epoxy resin. The rubber particles
become dispersed in the polymer and impart increased toughness without
substantially reducing the cohesive strength.
Another category of adhesives which fall within the definition of
poly-addition adhesives are single-component polyurethane adhesives.
Since the simple, low-molecular polyisocyanates form relatively hard
and brittle adhesive layers with low strength values on reaction with
moisture, the starting point in the single-component systems is pre-
ferably pre-crosslinked polymers, so-called prepolymers. These
compounds are known and are produced from higher-molecular polyols
with a stoichiometric excess of isocyanate. In this manner compounds
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are produced which already have urethane formations but, on the other
hand, still have reactive isocyanate groups which are accessible to
the reaction with moisture.
In addition to these single-component polyurethane adhesives, therm-
ally activatable polyurethane adhesives, i.e. compounds in which the
isocyanate group is masked or blocked and the isocyanate group can
only be removed at elevated temperature, are also suitable for the
application according to the invention, as are reactive polyurethane
hotmelt adhesives which, as is known, can be produced using higher-
molecular crystallizing and fusible diol and isocyanate components.
The cross-linking of the fusible prepolymers again takes place by
additional exposure to moisture which acts on the reactive isocyanate
groups still present and the crosslinking takes place via urea bonds
to the adhesive layer polymer.
Within the scope of the present invention, the term "polycondensation
adhesive" is understood to mean formaldehyde condensates, polyimides
and polysulfones. Of the formaldehyde condensates, phenol-formalde-
hyde resin adhesives and cresol-/resorcinol-formaldehyde resin adhe-
sives are particularly preferred. It is known to those skilled in the
art that, of the phenol-formaldehyde resins, only the so-called resols
are suitable for use as an adhesive. These are thermosetting phenolic
resins which, although they are soluble and fusible in the initial
stage, can be converted in the adhesive joint by heat or catalytic
action into the insoluble, nonfusible state, the so-called resites,
with a high degree of crosslinking. Because of the often troublesome
brittleness, pure phenol-formaldehyde resins are modified with further
compounds by copolymerization or cocondensation with suitable monomers
which predominantly yield thermoplastic polymers, for example with
polyvinylformal, polyvinyl butyral, polyamides, elastomers such as
polychloroprene, nitrile rubber, and also epoxy resins and similar
materials.
As a rule the cresol- and the resorcinol-formaldehyde adhesives also
require suitable modification. These are generally notable for a
higher rate of hardening and greater resistance to weathering
1339 ~66
effects.
Polyimide adhesives are notable particularly for their high heat
resistance. The production of industrially usable polyimides is
carried out by reacting the anydrides of tetra-basic acids, for
example pyromellitic anhydride with aromatic diamines, for example
diaminodiphenyl oxide. m e use takes place in the form of precon-
densate films.
Of the polymerization adhesives, especially the acrylic resins or
methacrylic resins are of importance. The fact that the free-radical
chaln-growth polymerization of the MMA monomer involves reactions
which proceed very rapidly after the components - monomer, hardener
and accelerator - have been combined, has resulted in several
developments of a processing tailored to manufacturing needs for
practical application. These are known to those skilled in the art
as A-B processes and no-mix processes.
All the thermosetting constructional adhesives mentioned are basically
known adhesives. The choice of the particular system depends on the
particular requirements of a particular application. The advantages
which are basically inherent in a particular constructional adhesive
are known per se to those skilled in the art. It was not possible to
foresee, however, that the load-bearing parts fully bonded with the
claimed structural adhesive films are capable of absorbing equal, and
in some cases higher, energies than welded constructions. This
completely surprising behaviour, coupled with the advantages of the
bonded joints generally, such as, for example, the optimum corrosion
protection, make it possible to bond load-bearing parts in vehicle
construction cheaply, cleanly and durably and make the use of the
films according to the invention particularly attractive in mass-
production motor car construction.
The term "structural adhesive" or "constructional adhesive" are under-
stood to mean adhesives which, in the cured state, have mechanical
properties which have a particular minimum level, have fatigue
strength, can be varied reproducibly and can therefore be used for
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reliably designing and dimensioning adhesive bonds. In a wider sense,
if account is taken of the stress conditions to be required and if the
component construction is suitable, these adhesives make it possible
to produce a bond with the most economic utilization of the parts to
be joined possible. The terms chosen here and familiar to those
skilled in the art are descriptions which essentially serve as a
distinction from adhesive systems of lower adhesive layer strengths,
for example contact or pressure-sensitive adhesives.
Obviously, mixtures of the abovementioned thermosetting plastics are
also suitable as constructional adhesive films, i.e. the structural
adhesive films may be coated with constructional adhesive films which
are chemically different from each other. Preferred are epoxy resin
adhesives or mixtures of two different epoxy resins.
The production of the multilayer structural adhesive films according
to the invention is known. The procedure for doing this may, for
example, be such that a solution of the thermosetting plastic or
preferably a melt is applied to the elastomeric base film with a
doctor blade. This multilayer film produced in this manner is stored
at a temperature at which the curing does not as yet occur. A
particular advantage of adhesive film technology is prefabrication.
This is understood to mean the bringing together of a section of film
cut to size and the part to be joined. The adhesive layer then sets
after combination with the second piece to be joined under the
influence of energy. Depending on the starting monomers, the curing
reaction normally takes place at temperatures between 80~ and 250~C.
In this connection, it should be noted that in choosing the adhesives
for this type of combination bonding, similar time and temperature
curing parameters are a prerequisite for obtaining optimally cured
adhesive layers in all cases under the same conditions.
In order to achieve an optimum adhesion between elastomeric base film
and thermosetting adhesive film, in a preferred embodiment of the
present invention, the base film is pretreated. The polyurethane
elastomers are pretreated, as is known, by applying a so-called
primer, for example a silane adhesion promotor. In the case of the
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styrene-butadiene and acrylonitril~-butadiene rubbers, the surface is
preferably partially oxidized, for example by treatment with sulfuric
acid or with an oxygen plasma or by means of corona radiation. This
involves common techniques which have hitherto been carried out on the
numerous, commercially available elastomeric films.
The thermosetting constructional adhesive films which are applied to
both sides of the base film are preferably chemically identical; they
may, however, also be different in order to make allowance for the
different materials to be bonded.
If desired, to adjust the viscosity, reactive diluents such as, for
example, styrene oxide, butyl glycidyl ether, 2,2,4-trimethylpentyl
glycidyl ether, phenyl glycidyl ether, cresyl glycidyl ether or
glycidyl esters of synthetic, highly branched, mainly tertiary al-
phatic monocarboxylic acids may be added to the mixtures according
to the invention. As other common additives, the mixtures according to
the invention may furthermore contain plasticizers, extenders, fillers
and reinforcing agents such as, for example, coal tar, bitumen, tex-
tile fibres, glass fibres, asbestos fibres, boron fibres, carbon
fibres, mineral silicates, mica, silica flour, aluminium oxide hy-
drate, bentonites, kaolin, silicic acid aerogel or metal powder, for
example aluminiu~ powder or iron powder, and also pigments and dyes
such as soot, oxide colours and titanium dioxide, flame retardents,
thixotropic agents, flow control agents such as silicones, waxes and
stearates, which are also used in some cases as mould release agents,
adhesion promoters, antioxidants and light protection agents.
Example 1: To bond steel sheets, a film system of the following
composition was produced:
A) Elastomeric base film: 0.25 mm thick styrene-butadiene rubber
film (available commercially, Shore hardness 63-70, tensile
strength 15-20 MPa, surface textured).
B) Thermosetting constructional adhesive:
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20% by weight of liquid epoxy resin based on bisphenol A (epoxy
value 5.3 e q/ kg)
35% by weight of an epoxy resin based on bisphenol A (epoxy value
5.3 eqlkg) modified with a carboxyl-terminated butadiene rubber
(CTBN rubber) in the ratio 70: 30
20% by weight of sol id epoxy resin based on bisphenol A (epoxy
value 1.6 eq/kg)
20% by weight of chalk
2.5% by weight of dicyandiamide
1.0% by weight of monuron
1. 5% by weight of aerosil thixotropic agent
The epoxy resins of the thermosetting adhesive composition are homo-
geneously mixed in a divided trough kneader at 100-130~C in vacuo.
The filler, the accelerator, monuron and thixotropic agent were
successively added, also in vacuo, with the temperature of the mixture
being allowed to drop at the same time to 80~C.
The thermosetting constructional adhesive (A) thus produced is applied
to both sides of the elastomeric base film to a thickness of 0.1 mm in
each case on a doctor-blade coater at 80-90~C.
Example 2: The multilayer adhesive film according to Example 1 is
laid between two oiled steel sheets (steel 1403) and cured for 30 min
at 180~C. The bonded construction has the following properties:
(a) Shear strength according to DIN 53 283: 8-9 N/mm2
(b) Floating roller peel test according to DIN 53 289: 8-10 Nlmm
After 28 cycles (16 h at 80%, 99% humidity and 4 h at -30~C):
(c) Shear strength according to DIN 53 283: 6-8 N¦mm2
(d) Corrosion: none, cohesive fracture
Example 3: Analogously to Example 1, a film system of the same chemi-
cal composition was produced, in which, however, the textured elasto-
meric film A) had a thickness of 0.4 mm and the constructional
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adhesive layers had a thickness of 0. 5 mm each. This mul tilayer
adhesive film is laid between two oiled steel sheets (steel 1403) and
cured for 30 min at 180~C. me bonded construction has the following
properties:
(a) Shear strength according to DIN 53 283: 11 N/mm2
(b) ~loating roller peel strength according to DIN 53 289: 16 N/mm
(c) Impact strength: 37 Nm
Example 4: Analogously to Example 1, a film system of the same
chemical composition was produced in which the textured elastomeric
film A) had a thickness of 0.4 mm and uniformly distributed perfor-
at~ons having a diameter of 2 mm, and the constructional adhesive
layers were each 0.3 mm thick. This multilayer adhesive film with the
proportion of perforations mentioned below is laid between two oiled
steel sheets (steel 1403) and cured for 30 min at 180~C. The bonded
construction has the following properties:
Proportion of the area occupied by
the perforations in the elastomeric
film
1 0% 3 0% 60%
( a) Shear strength according 7-8 N/mm2 6-7N/mm2 6-7N/mm2
to DIN 53 2 83
(b) ~loating roller peel strength
according to DIN 53 289 17N/mm 16N/mm 14 N/mm
