U.S. patent application number 13/518182 was filed with the patent office on 2013-01-03 for heat-activated, glueable surface elements.
This patent application is currently assigned to Tesa SE. Invention is credited to Hans Karl Engeldinger, Judith Grunauer, Klaus Keite-Telgenbuscher.
Application Number | 20130000811 13/518182 |
Document ID | / |
Family ID | 43920023 |
Filed Date | 2013-01-03 |
United States Patent
Application |
20130000811 |
Kind Code |
A1 |
Engeldinger; Hans Karl ; et
al. |
January 3, 2013 |
Heat-activated, Glueable Surface Elements
Abstract
A heat-activated, glueable sheetlike element having at least one
heat-activatable adhesive, at least one inductively heatable
material, and at least one thermally conductive filler, wherein
material of the thermally conductive filler has a thermal
conductivity of at least 0.5 W/(m*K)
Inventors: |
Engeldinger; Hans Karl;
(Quickborn, DE) ; Grunauer; Judith; (Hamburg,
DE) ; Keite-Telgenbuscher; Klaus; (Hamburg,
DE) |
Assignee: |
Tesa SE
Hamburg
DE
|
Family ID: |
43920023 |
Appl. No.: |
13/518182 |
Filed: |
December 7, 2010 |
PCT Filed: |
December 7, 2010 |
PCT NO: |
PCT/EP2010/069057 |
371 Date: |
August 28, 2012 |
Current U.S.
Class: |
156/60 ; 523/400;
523/440; 523/457; 524/430; 524/502; 524/599; 524/606 |
Current CPC
Class: |
Y10T 156/10 20150115;
C09J 11/04 20130101; C09J 5/06 20130101 |
Class at
Publication: |
156/60 ; 524/502;
523/400; 524/606; 524/599; 524/430; 523/457; 523/440 |
International
Class: |
C09J 109/02 20060101
C09J109/02; B32B 37/12 20060101 B32B037/12; C09J 167/00 20060101
C09J167/00; C08K 3/22 20060101 C08K003/22; C09J 121/00 20060101
C09J121/00; C09J 177/00 20060101 C09J177/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 21, 2009 |
DE |
10 2009 055 099.2 |
Claims
1. A sheetlike element comprising: at least one heat-activatable
adhesive at least one inductively heatable material; and at least
one thermally conductive filler, wherein material of the thermally
conductive filler has a thermal conductivity of at least 0.5
W/(m*K).
2. The sheetlike element according to claim 1, wherein the material
of the thermally conductive filler is electrically
nonconducting.
3. The sheetlike element according to claim 1, wherein the
thermally conductive filler is used in a form of filler particles
and/or comprises filler particles.
4. The sheetlike element according to claim 3, wherein the filler
particles are aluminum oxide particles which consist to a
proportion of more than 95% by weight of alpha-aluminum oxide.
5. The sheetlike element according to claim 4, wherein the
proportion of the aluminum oxide particles, based on the
heat-activatable adhesive with the thermally conductive filler, is
in the range from 20% to 90% by weight.
6. The sheetlike element according to claim 3, wherein the filler
particles are primary particles having a specific surface area per
unit mass of 1.3 m.sup.2 or less.
7. The sheetlike element according to claim 6, wherein a proportion
of the primary particles, based on the heat-activatable adhesive
with the filler, is in the range from 5% to 70% by volume.
8. . The sheetlike element according to claim 3, wherein the filler
particles have an average diameter of at least 1 .mu.m.
9. The sheetlike element according to claim 4, wherein the
proportion of the aluminum oxide particles, based on the
heat-activatable adhesive with the thermally conductive filler, is
in the range from 40% to 80% by weight.
10. The sheetlike element according to claims 3, wherein the filler
particles are primary particles having a specific surface area per
unit mass of 1.0 m.sup.2 or less.
11. The sheetlike element according to claim 6, wherein a
proportion of the primary particles, based on the heat-activatable
adhesive with the filler, is in the range from 15% to 50% by
volume.
12. The sheetlike element according to claim 3, wherein the filler
particles have an average diameter between 2 .mu.m and 500
.mu.m.
13. The sheetlike element according to claim 3, wherein the filler
particles have an average diameter between 2 and 200 .mu.m.
14. The sheetlike element according to claim 3, wherein the filler
particles have an average diameter between 40 .mu.m and 150
.mu.m.
15. A method comprising: bonding the sheetlike element according to
claim 1 to one or more bonding substrates.
16. The method according to claim 15, wherein the sheetlike element
is heated at a heating rate of more than 50.degree. C. to bond the
sheet like element to one or more bonding substrates.
17. The method according to claim 15, wherein the sheetlike element
is heated at a heating rate of more than 100.degree. C. to bond the
sheet like element to one or more bonding substrates.
Description
[0001] This is a 371 of PCT/EP2010/069057 filed 7 Dec. 2010
(international filing date), and claims the priority of German
Application No. 10 2009 055 099.2, filed on 21 Dec. 2009.
[0002] The invention relates to heat-activatedly bondable sheetlike
elements more particularly having high bonding forces for
plastic/plastic bonds and also to a method for such bonds.
BACKGROUND
[0003] Heat-activatedly bondable sheetlike elements
(heat-activatable sheetlike elements) are used in order to obtain
high-strength connections between adherends. Especially suitable
are sheetlike elements of this kind for achieving, in the case of a
relatively thin bondline, strengths comparable with or higher than
those possible with sheetlike elements which contain exclusively
pressure-sensitive adhesive systems. Such high-strength bonds are
important particularly in light of the ongoing miniaturization of
electronic devices, in the consumer electronics, entertainment
electronics or communications electronics segment, for instance, as
for example for cell phones. PDAs, laptops and other computers,
digital cameras, and display devices such as displays and digital
readers, for instance.
[0004] The requirements in terms of processability and stability of
adhesive bonds are increasing particularly in portable consumer
electronics articles. One reason for this is that the dimensions of
such articles are becoming ever smaller, and so the area that can
be utilized for an adhesive bond is also reduced. Another reason is
that an adhesive bond in such devices must be particularly stable,
since portable articles are required to withstand severe mechanical
loads such as impacts or drops, for instance, and, moreover, are to
be used across a broad temperature range.
[0005] In products of these kinds, therefore, it is preferred to
use heat-activatedly bondable sheetlike elements which have
heat-activatedly bonding adhesives, i.e., adhesives which at room
temperature have no inherent tack, or at best a slight inherent
tack, but which, when exposed to heat, develop the bond strength
needed for a bond to the respective bonding substrates (adherends,
adhesion base). At room temperature, heat-activatedly bonding
adhesives of these kinds are frequently in solid form, but in the
course of bonding, as a result of temperature exposure, are
converted either reversibly or irreversibly into a state of high
bond strength. Reversibly heat-activatedly bonding adhesives are,
for example, adhesives based on thermoplastic polymers, whereas
irreversibly heat-activatedly bonding adhesives are, for instance,
reactive adhesives, in which thermal activation triggers chemical
reactions such as crosslinking reactions, for example, thereby
making these adhesives particularly suitable for permanent
high-strength bonds.
[0006] In this context there is a requirement more particularly for
increasingly thin adhesive tapes, with no reduction in the strength
requirements. Heat-activatable films are presently available in a
very wide thickness range--thus thicknesses of 30 to 250 .mu.m are
not unusual.
[0007] A feature common to all heat-activatebly bonding adhesive
systems is that for bonding they must be heated. Particularly in
the case of bonds where the adhesive systems are hidden from the
outside over their full area by the bonding substrates, it is
particularly important for the heat necessary for melting or for
activating the adhesive to be transported toward the bonding area
quickly. If one of the bonding substrates here is a good thermal
conductor, then it is possible to heat that bonding substrate by
means of an external heat source, as for example through a direct
heat transfer medium, an infrared heater or the like.
[0008] In the case of such direct heating or contact heating,
however, the short heating time that is needed for rapid,
homogeneous heating of the known adhesive can be realized only for
a large temperature gradient between the heat source and the
bonding substrate. Consequently, the bonding substrate that is to
be heated ought itself to be insensitive with respect to
temperatures which in some cases may even be considerably higher
than actually necessary for the melting or activating of the
adhesive. Accordingly, the use of heat-activatable adhesive films
is problematic for plastic/plastic bonds, Plastics used in
particular in consumer electronics include, for example, polyvinyl
chloride (PVC), acrylonitrile-butadiene-styrene copolymers (ABS),
polycarbonates (PC), polypropylene (PP) or blends based on these
plastics.
[0009] The situation is different, then, if none of the bonding
substrates is a sufficiently good thermal conductor or if the
bonding substrates are sensitive toward higher temperatures, as is
the case, for example, with many plastics, but also with electronic
components such as semiconductor components or liquid-crystal
modules, for instance. For the bonding of bonding substrates made
from materials of low thermal conductivity or from heat-sensitive
materials, therefore, it is appropriate to equip the
heat-activatedly bondable sheetlike element itself with an
intrinsic mechanism for heating, so that the heat required for
bonding need not be introduced from the outside, but is instead
generated directly in the interior of the sheetlike element itself.
In the prior art there are various mechanisms known that allow such
internal heating to be realized, in the form, for instance, of
heating by means of an electrical resistance heater, through
magnetic induction or by interaction with microwave radiation.
[0010] Heating in an alternating magnetic field is achieved on the
one hand through induced eddy currents in electrically conductive
receptors and on the other hand--to give a model-based
explanation--through hysteresis losses by the surrounding
elementary magnets in the alternating field. For eddy currents to
develop, however, the conductive domains are required to have a
certain minimum size. The lower the frequency of the alternating
field, the greater this minimum size is. Depending on the receptor
material, both effects occur in unison (e.g., magnetic metals) or
only one effect occurs in each case (e.g., eddy currents only in
the case of aluminum; hysteresis only in the case of iron oxide
particles).
[0011] In principle, a variety of heating devices for inductive
heating are known; one of the parameters which can be used to
distinguish them is the frequencies possessed by the alternating
magnetic field generated using the heating device in question. For
instance, induction heating may be accomplished using a magnetic
field whose frequency is situated in the frequency range from about
100 Hz to about 200 kHz (the so-called medium frequencies, MF) or
else in the frequency range from about 300 kHz to about 100 MHz
(the so-called high frequencies: HF). In addition, as a special
case, there are also heating devices known whose magnetic field
possess a frequency from the microwave range, such as the standard
microwave frequency of 2.45 GHz, for example.
[0012] Rising in line with the frequency of the alternating field
used is the technical cost and complexity involved in generating
the alternating field, and hence the costs of the heating device.
Whereas middle-frequency systems are already currently available at
a market price of around 5000 euros, the outlay for high-frequency
systems is at least 25 000 euros. Also rising in line with the
frequency, furthermore, are the safety requirements concerning the
heating system, and so, for high-frequency systems, it is regularly
necessary to add, to the higher acquisition costs, higher costs for
the installation of such technology as well.
[0013] Where high frequencies are used for the adhesive bonding of
components in electronic devices, it is possible, furthermore, for
unwanted damage to occur to electronic components in these devices
in the course of their exposure to the alternating electromagnetic
field.
[0014] Example applications that may be given for induction heating
include manufacturing operations from the areas of bonding, seam
sealing, curing, tempering, and the like. The usual technique here
is to employ those methods where the inductors surround components
completely or partially and heat them uniformly over the entire
extent or, when required, deliberately nonuniformly, in accordance,
for example, with EP 1 056 312 A2 or with DE 20 2007 003 450
U1.
[0015] DE 20 2007 003 450 U1 sets out for example, inter alia, a
method for fusing a container opening with a sealing film, in which
the metallic inlay of a sealing film is heated by induction and a
sealing adhesive is melted by conduction of heat. The containers
are closed with a screw-on or snap-in lid, which comprises a metal
foil and an adjacent polymeric sealing film. Using the induction
coil, eddy currents are generated in the metal foil, and heat the
metal foil. As a result of the contact between metal foil and
sealing film, the sealing film is also heated and is thereby fused
with the container opening. Induction coils in tunnel form have the
advantage over flat coils that they can be used as well to seal
containers having a large distance between the metal foil and the
top edge of the lid, since the coil acts on the metal foil from the
side.
[0016] A disadvantage of this method is that a substantially
greater part of the component volume than the pure adhesive volume
and the metal foil is passed through the electromagnetic field and
hence, in the case of an electronic component, instances of damage
are not ruled out, since heating may occur at unwanted locations. A
further disadvantage is that the entire lid film is heated, whereas
only the edge region in contact with the container would be
sufficient for bonding. Hence there is a large ratio of heated area
to bonding area, which for typical beverage bottles having an
opening diameter of 25 mm and a bonding width of 2 mm is
approximately 6.5. For larger container diameters, the ratio goes
up in the case of the usually constant bonding width.
[0017] In recent years, for the inductive heating particularly in
the bonding of plastic on plastic, inductively heatable
heat-activatable adhesive films (HAFs) have moved back beneath the
spotlight. The reason for this is to be found in the
nanoparticulate systems that are now available, such as
MagSilica.TM. (Evonik AG), for example, which can be incorporated
into the material of the body to he heated and which thus allow
heating of the body throughout its volume, without any attendant
significant detriment to its mechanical stability.
[0018] Because of the small size of these nanoscopic systems,
however, it is not possible to bring about efficient heating of
such products in alternating magnetic fields with frequencies from
the medium frequency range. For the innovative systems, instead,
frequencies from the high frequency range are required. It is at
these frequencies in particular, however, that the problem of
damage to electronic components in an alternating magnetic field is
manifested to a particularly severe extent. Generating alternating
magnetic fields with frequencies in the high frequency range,
moreover, requires increased cost and complexity of apparatus, and
is therefore unfavorable economically. Furthermore, the use of
nanoparticulate fillers is a problem from the standpoint of the
environment as well, since these fillers are not easily separated
from the surrounding materials on subsequent recycling. It is
difficult, furthermore, to use these particles in very thin films,
since the strong tendency of nanoparticulate systems to form
agglomerates means that the films produced therewith are usually
very inhomogeneous.
[0019] Furthermore, and in order to avoid the above problems, it is
possible for heat-activatable films (HAFs) which are intended to be
inductively heatable to be filled with sheetlike metallic or
metallized structures. This is very efficient in the context of the
use of full-area metal foils, even in the medium frequency range;
high heating rates can be achieved, and so induction times of
between 0.05 and 10 s can be realized. Also possible in this
context is the use of very thin conductive films of between 0.25
.mu.m and 75 .mu.m.
[0020] Also known is the use of perforated metal foils, wire
meshes, expanded metal, metal webs or fibers, through which the
matrix material of the HAF is able to penetrate, thereby improving
the cohesion of the assembly. The efficiency of heating, however,
goes down as a result.
[0021] For adhesive bonds within mobile electronic devices, the
product Duolplocoll RCD from Lohmann is known, this product being
equipped with inductively heatable nanoparticles. This product can
be heated in a technically utilizable way exclusively in the high
frequency range. The disadvantages described above with the use of
particles and high-frequency alternating fields apply to this
product as well,
SUMMARY
[0022] It is an object of the invention to provide a
heat-activatable sheetlike element with which bonds, more
particularly plastic/plastic bonds, can be realized with a very
good bonding strength. The sheetlike elements are advantageously to
be bondable by means of induction heating with high cycle rates,
more particularly while avoiding the disadvantages of the prior
art,
[0023] Especially for use in electronic devices, the sheetlike
element ought to possess a high breakdown resistance in the
direction perpendicular to the plane of bonding, in other words
perpendicularly to the (average) areal extent of the sheetlike
element.
BRIEF DESCRIPTION OF THE DRAWING
[0024] FIG. 1 illustrates a system in an embodiment of the present
disclosure.
DETAILED DESCRIPTION
[0025] As an achievement of the object, a heat-activatable adhesive
tape is provided, more particularly for high-strength bonds of
plastic on plastic, comprising at least one inductively heatable
material and also at least one heat-activatable adhesive, the
heat-activatable adhesive having a high thermal conductivity in the
direction perpendicular to the (average) areal extent of the
sheetlike element (also identified below as "z-direction"), it is
very advantageous if the electrical conductivity in the z-direction
is zero or at least negligibly small, in order to maintain the
breakdown resistance.
[0026] The object is achieved in accordance with the invention more
particularly by means of a heat-activatedly bondable sheetlike
element comprising at least one heat-activatable adhesive, at least
one inductively heatable material, and at least one thermally
conductive filler (also identified as "thermally conductive
additive"), the material of the filler having a thermal
conductivity of at least 0.5 W(m*K)
[0027] The thermally conductive filler is an additive which induces
thermal conductivity within the layer of the heat-activatable
pressure-sensitive adhesive, more particularly in the z-direction.
Advantageously, the thermally conductive filler is composed wholly
or at least partly of a material which has good thermal
conductivity.
[0028] The heat-activatedly bondable sheetlike element
advantageously has a thermal conductivity in the z-direction of at
least 0.4 W/(m*K), more particularly of more than 0.8 W/(m*K). In
one advantageous procedure, the thermally conductive filler is
selected such that and/or added in an amount such that the
heat-activatable adhesive has a thermal conductivity in the
z-direction of at least 0.4 W/(m*K), more particularly of more than
0.8 W/(m*K),
[0029] The material of the thermally conductive filler
advantageously has a thermal conductivity of more than 0.5 W/(m*K),
preferably of more than 5 W/(m*K) more preferably of more than 10
W/(m*K). With increasing thermal conductivity on the part of the
additive, it is possible to reduce the amount added in order to
obtain a particular heat, thereby having less of a detrimental
effect on the adhesiveness of the adhesive.
[0030] With great preference the thermally conductive additive is
selected such that it is electrically nonconducting or only very
slightly electrically conducting. In this way, high thermal
conductivity is obtained for the sheetlike element, with the
breakdown resistance maintained at the same time.
[0031] With particular advantage the sheetlike element is a
double-sidedly heat-activatedly bondable sheetlike element.
[0032] Surprisingly it has been found that the bonding strengths of
the sheetlike elements of the invention are greater than for
comparable sheetlike elements which have no additives and/or do not
satisfy the thermal conductivity coefficient. As a result of good
thermal dissipation by conduction on the part of the inductively
heatable material, more particularly on the part of the
electrically conducting layer, the risk of instances of local
overheating is smaller, and greater heating rates can be used.
[0033] For adhesives having a thermal conductivity in the range
from 0.4 to 0.8 W/(m*K), a good compromise between bond strength
and required filler content has been found. In the case of
adhesives having thermal conductivities of 0.8 W/(m*K) or more, the
high thermal conductivity results in particularly good heat
distribution, allowing the disadvantages described above, more
particularly owing to local heating, to be avoided here to
particularly good effect.
[0034] Heat-Activatable Adhesives
[0035] As the at least one heat-activatedly bonding adhesive it is
possible in principle to employ all customary heat-activatedly
bonding adhesive systems. Heat-activatedly bonding adhesives can be
divided in principle into two categories: thermoplastic
heat-activatedly bonding adhesives (hotmelt adhesives), and
reactive heat-activatedly bonding adhesives (reactive adhesives).
This subdivision also includes those adhesives which can be
assigned to both categories, namely reactive thermoplastic
heat-activatedly bonding adhesives (reactive hotmelt
adhesives).
[0036] Thermoplastic adhesives are based on polymers which soften
reversibly on heating and solidify again in the course of cooling.
In contrast to these, reactive heat-activatedly bonding adhesives
comprise reactive components. The latter constituents are also
referred to as "reactive resins", in which heating initiates a
crosslinking process which, after the end of the crosslinking
reaction, ensures a permanent stable bond even under pressure.
Thermoplastic adhesives of this kind preferably also comprise
elastic components, examples being synthetic nitrile rubbers. Such
elastic components give the heat-activatedly bonding adhesive a
particularly high dimensional stability even under pressure, on
account of their high flow viscosity.
[0037] Described below, purely by way of example, are a number of
typical systems of heat-activatedly bonding adhesives which have
emerged as being particularly advantageous in connection with the
present invention.
[0038] A thermoplastic heat-activatedly bonding adhesive, then,
comprises a thermoplastic base polymer. This polymer has good flow
behavior even under low applied pressure, and so the ultimate bond
strength that is relevant for the durability of a permanent bond
comes about within a short applied-pressure time, and, therefore,
rapid bonding is possible even to a rough or otherwise critical
substrate. As thermoplastic heat-activatedly bonding adhesives it
is possible to use all of the thermoplastic adhesives known from
the prior art.
[0039] Suitability is possessed for example by those
heat-activatable adhesives of the kind described in DE 10 2006 042
816 A1, without wishing these details to impose any
restriction.
[0040] Exemplary compositions are described in EP 1 475 424 A1, for
instance. Hence the thermoplastic adhesive may comprise, or even
consist of, for example, one or more of the following components:
polyolefins, ethylene-vinyl acetate copolymers, ethylene-ethyl
acrylate copolymers, polyamides, polyesters, polyurethanes or
butadiene-styrene block copolymers. Employed preferably, for
instance, are the thermoplastic adhesives listed in paragraph
[0027] of EP 1 475 424 A1. Further thermoplastic adhesives
particularly suitable especially for specific fields of use such as
the bonding of glass bond substrates, for example, are described in
EP 1 95 60 63 A2. It is preferred to use thermoplastic adhesives
whose melt viscosity has been raised by rheological additives, as
for example through addition of fumed silicas, carbon black, carbon
nanotubes and/or further polymers as blend components.
[0041] A reactive heat-activatedly bonding adhesive, in contrast,
advantageously comprises an elastomeric base polymer and a modifier
resin, the modifier resin comprising a tackifier resin and/or a
reactive resin. Through the use of an elastomeric base polymer it
is possible to obtain adhesive layers having outstanding
dimensional stability. As reactive heat-activatedly bonding
adhesives it is possible, in line with the specific applications in
each case, to use all of the heat-activatedly bonding adhesives
that are known from the prior art.
[0042] Also included here, for example, are reactive
heat-activatedly bonding films based on nitrile rubbers or
derivatives thereof such as, for instance, nitrile-butadiene
rubbers or mixtures (blends) of these base polymers, additionally
comprising reactive resins such as phenolic resins, for instance;
one such product is available commercially under the name tesa
8401, for instance. On account of its high flow viscosity, the
nitrile rubber gives the heat-activatedly bonding film a pronounced
dimensional stability, allowing high bond strengths to be realized
on plastics surfaces after a crosslinking reaction has been carried
out.
[0043] Naturally, other reactive heat-activatedly bonding adhesives
can be used as well, such as, for instance, adhesives comprising a
mass fraction of 50% to 95% by weight of a bondable polymer and a
mass fraction of 5% to 50% by weight of an epoxy resin or a mixture
of two or more epoxy resins. The bondable polymer in this case
comprises advantageously 40% to 94% by weight of acrylic acid
compounds and/or methacrylic acid compounds of the general formula
CH.sub.2.dbd.C(R.sup.1)(COOR.sup.2) (R.sup.1 here represents a
radical selected from the group encompassing H and CH.sub.3, and
R.sup.2 represents a radical selected from the group encompassing H
and linear or branched alkyl chains having 1 to 30 carbon atoms),
5% to 30% by weight of a first copolymerizable vinyl monomer which
has at least one acid group, more particularly a carboxylic acid
group and/or suifonic acid group and/or phosphonic acid group, 1%
to 10% by weight of a second copolymerizable vinyl monomer which
has at least one epoxide group or an acid anhydride function, and
0% to 20% by weight of a third copolymerizable vinyl monomer which
has at least one functional group different from the functional
group of the first copolymerizable vinyl monomer and from the
functional group of the second copolymerizable vinyl monomer. An
adhesive of this kind allows bonding with rapid activation, the
ultimate bond strength being achieved within just a very short
time, with the result, overall, that an effectively adhering
connection to a nonpolar substrate is ensured.
[0044] A further reactive heat-activatedly bonding adhesive which
can be used, and which affords particular advantages, comprises 40%
to 98% by weight of an acrylate-containing block copolymer, 2% to
50% by weight of a resin component, and 0% to 10% by weight of a
hardener component. The resin component comprises one or more
resins selected from the group encompassing bond strength enhancing
(tackifying) epoxy resins, novolak resins, and phenolic resins. The
hardener component is used for crosslinking the resins from the
resin component. On account of the strong physical crosslinking
within the polymer, a formulation of this kind affords the
particular advantage that adhesive layers having a greater overall
thickness can be obtained, without detriment overall to the
robustness of the bond. As a result, these adhesive layers are
particularly suitable for compensating unevenesses in the
substrate. Moreover, an adhesive of this kind features good aging
resistance and exhibits only a low level of outgassing, a
particularly desirable feature for many bonds in the electronics
segment.
[0045] As already mentioned above, however, apart from these
particularly advantageous adhesives, it is also possible in
principle to select and use all other heat-activatedly bonding
adhesives in line with the particular profile of requirements for
the adhesive bond.
Thermally Conductive Filler
[0046] The thermally conductive filler is added preferably in a
modification which allows effective distribution within the
adhesive, i.e., more particularly, in the form of (especially
finely divided) particles ("filler particles") or bodies.
[0047] As thermally conductive filler it is possible accordingly,
for example, to use carbon fibers, more particularly those of the
kind described by EP 456 428 A2. Carbon fibers of this kind can be
used advantageously in an amount of 20% to 60% by weight, based on
the adhesive with fibers.
[0048] As thermally conductive filer it is preferred to use a filer
which comprises particles or consists of particles that are
composed of primary particles and that have a specific surface area
per unit mass of 1.3 m.sup.2/g or less. It has been observed,
particularly in the case of such particulate additives having
specific surface areas of less than 1.3 m.sup.2/g, that they result
in a significantly higher thermal conductivity in the
heat-activatable adhesive than do particulate additives made from
the same material but having a greater surface area.
[0049] The normal expectation, in contrast, would have been that
the thermal conductivity of the adhesive increases in line with the
specific surface area of the thermally conductive additive, since a
larger surface area ought to mean a larger area for heat transfer,
with the consequent assumption of an improved thermal transfer from
the matrix polymer to the thermal conduction additive.
[0050] Tests have shown, however, that a highly thermally
conductive thermal conduction composition of this kind possesses a
sufficiently high internal cohesion only when the individual
additive particles are additionally formed as accumulations of
individual primary particles and hence have an irregularly shaped
surface which is not smooth. Only in the case of such a
three-dimensional structure on the part of the particles are these
particles anchored structurally in the polymer matrix in a manner
sufficiently firm that the resultant thermal conduction composition
overall has a high level of cohesion and does not lose this
cohesion even at relatively high temperatures under mechanical
load.
[0051] It has proven particularly advantageous in this context if
the primary particles have an average diameter of at least 1 .mu.m
or even of more than 2 .mu.m, since in this way adhesives of high
thermal conductivity are obtained whose cohesion is still high
enough, even at high temperatures, at which the viscosity of the
polymer matrix goes down, in order to ensure a stable cohesion
overall.
[0052] In this context it is possible to achieve particularly high
thermal conductivities on the part of the adhesive if the particles
of the thermally conductive additive have an even lower specific
surface area per unit mass, of not more than 1.0 m.sup.2/g.
[0053] In one advantageous embodiment, the thermally conductive
additive comprises at least substantially aluminum oxide particles
and/or boron nitride particles or consists thereof.
[0054] Particular preference is given to using those aluminum oxide
particles and/or boron nitride particles which are composed of
primary particles and which have a specific surface area per unit
mass of 1.3 m.sup.2/g or less.
[0055] By virtue of the use of such inert additives such as
aluminum oxide particles and/or boron nitride particles, adhesives
are obtained which have a high chemicals resistance and which,
moreover, are advantageous from both an economic and an
environmental standpoint, since these materials are readily
available and at the same time are not toxic, and offer a good
compromise, relative to other possible additives, in terms of high
thermal conductivity in tandem with low costs.
[0056] Where the additive is or comprises aluminum oxide, it has
proven particularly favorable for the aluminum oxide particles to
consist of a proportion of more than 95% by weight of
alpha-aluminum oxide, more particularly in a proportion of 97% by
weight or more. In this way it is possible to prevent premature
crosslinking or gelling within the adhesive by polymer components
based on acrylic acid or methacrylic acid or esters thereof; such
crosslinking or gelling may occur in the mixing assembly itself and
results in a sharp rise in the viscosity. Where a high
alpha-aluminum oxide proportion is taken into account, the
resultant mixtures continue to remain outstanding processable. In
contrast to this, for polymers based on esters of acrylic acid or
methacrylic acid, it has been found that, when the proportion of
gamma-aluminum oxide or beta-aluminum oxide rises to at least 5% by
weight, there is gelling or crosslinking of the polymer right at
the stage where the additive is being incorporated into the melt,
meaning that the resultant adhesive can no longer be shaped or
applied as a homogeneous layer.
[0057] On the basis of the results from the experimental
investigations it is thought that the effect according to the
invention derives from a lower level of interaction of
alpha-aluminum oxide, relative to beta-aluminum oxide and
gamma-aluminum oxide, with the polymeric phase, meaning that there
is no formation of a supercordinate network composed of a plurality
of polymer molecules. With a mass fraction of gamma-aluminum oxide
(and/or, possibly, of beta-aluminum oxide) of less than 5% by
weight, based on the total mass of the aluminum oxide particles
(corresponding to an alpha-aluminum oxide content of more than 95%
by weight), it is not possible for a network to form that
percolates throughout the volume of the adhesive, and hence
complete gelling is prevented.
[0058] In this way it is possible to prevent premature crosslinking
or gelling, within the thermally conductive pressure-sensitive
adhesive, of polymer components based on acrylic acid or
methacrylic acid or their esters; such premature crosslinking or
gelling may occur in the mixing assembly itself, and results in a
sharp increase in the viscosity. When a high proportion of
alpha-aluminum oxide is taken into account, the resultant mixtures
continue to remain outstandingly processable.
[0059] In this context there are certain adhesive systems where the
problem of viscosity increase is particularly great, since in these
systems the gelling of the polymer matrix occurs with particular
ease. For this reason, for readily gelling adhesives of this kind,
the application of the inventive concept has emerged as being
particularly advantageous.
[0060] For instance, subsequent gelling is a problem especially
with polymers which contain free acid groups or free hydroxyl
groups, since in such polymers the interaction with the aluminum
oxide is particularly strong. The advantageous effect of the
present invention, therefore, is also particularly great in the
case of these systems.
[0061] Gelling frequently occurs when the polymer composition is
constructed of monomer units which are at least weakly acidic,
examples being acrylates, methacrylates, their esters and
derivatives of these, especially when these monomer units are
present in the polymer composition in a high proportion of at least
50% by weight, based on the mass of the polymeric fractions of the
adhesive. Polymer compositions of this kind are employed
principally when the aim is to produce adhesives having a
particularly high viscosity. Accordingly, the inventive concept is
also particularly favorable in respect of highly cohesive adhesives
with compositions of this kind.
[0062] Rapid gelling also occurs when the base polymer of the
polymer composition has a high average molecular mass of at least
500 000 g/mol, more particularly of more than 1 000 000 g/mol, and
so the present invention is likewise particularly meaningful in the
case of adhesives of this kind.
[0063] Furthermore, a thermal conduction composition is
particularly suitable if the material of the thermally conductive
additive has a thermal conductivity of more than 1 W/(m*K), more
particularly of more than 10 W/(m*K), favorably of more than 25
W/(m*K) or even of more than 100 W/(m*K). This ensures that the
thermal conduction composition allows a high level of heat transfer
even at a low level of additive included. The proportion of the
thermally conductive additives in the adhesive can therefore be
kept low, thus making it possible to realize highly cohesive
adhesives.
[0064] It is particularly useful in this context if the thermally
conductive additive is present in the adhesive in a proportion of
at least 5% by volume and not more than 70% by volume, more
particularly of at least 15% by volume and not more than 50% by
volume, based in each case on the volume of the thermally
conductive additive in the adhesive, In the case of aluminum oxide
particles it is very advantageous if they are present in the highly
cohesive, thermally conductive adhesive in a proportion of at least
20% by weight and not more than 90% by weight, based on the mass of
the aluminum oxide particles in the pressure-sensitive adhesive.
For aluminum oxide particles, an amount of 40% by weight to 80''/o
by weight represents a particularly good compromise.
[0065] With the aforementioned quantities of additive it is ensured
that the thermal conduction composition overall permits rapid heat
transport from the heat source to the heat sink.
[0066] This is attributable on the one hand to the high thermal
conductivity of such adhesives, but also, on the other hand, to a
level of internal cohesion of the polymer matrix that is
sufficiently high under these conditions, and that even under
mechanical load offers reliable thermal contact to the surfaces of
the heat source and of the heat sink.
[0067] In addition, however, there may also be advantage in those
thermally conductive adhesives which comprise aluminum oxide
particles in a proportion of at least 20% by weight and not more
than 40% by weight, specifically when adhesives with a particularly
high bonding performance are to be realized, or in those which
comprise aluminum oxide particles in a proportion of at least 80%
by weight and not more than 90% by weight, specifically when
particularly high thermal conductivity is required.
[0068] It is of advantage, furthermore, if the particles have an
average diameter from a range from 2 .mu.m to 500 .mu.m, more
particularly from a range from 2 .mu.m to 200 .mu.m or even from a
range from 40 .mu.m to 150 .mu.m. As a result of this design of the
additive, the thermal contact with the heat source and with the
heat sink is in fact improved still further, since the particles on
the one hand are sufficiently small to conform precisely to the
shape of the surface of the heat source and of the heat sink, but
on the other hand are sufficiently large to attain a high thermal
conductivity without adversely affecting, overall, the internal
cohesion of the thermal conduction composition.
[0069] Inductively Heatable Material
[0070] As inductively heatable material it is possible to use not
only sheetlike structures (more particularly electrically
conductive layers) but also particulate materials (particles), of
the kind known per se for this purpose from the prior art. In the
case of conductive particles, however, migration occurs again when
a field is generated for heating, with the consequence that the
problem of increased breakdown resistance becomes current
again.
[0071] An electrically conducting layer is considered to be any
layer of at least one material that has a conductivity (electrons
and/or holes) at 23.degree. C. of at least 1 mS/m, thus allowing an
electrical current flow within said material. Such materials are,
in particular, metals, semimetals, and also other metallic
materials, and possibly also semiconductors in which the electrical
resistance is low. Accordingly, the electrical resistance of the
electrically conducting layer is on the one hand high enough to
allow heating of the layer when an electrical current is flowing in
the layer, but on the other hand is also low enough for a current
flow to be actually established through the layer. Also considered
to be electrically conducting layers, as a special case, are layers
of materials which have a low magnetic resistance (and hence a high
magnetic conductivity or magnetic permeability), examples being
ferrites, although the latter frequently have a relatively high
electrical resistance given alternating currents of low
frequencies, and so heating here is frequently achieved only with
alternating magnetic field frequencies that tend to be relatively
high.
[0072] It is preferred to make use, for example, of electrically
conductive sheetlike materials (sheetlike structures), since these
materials can be heated with low frequencies, resulting in a higher
depth of penetration of the magnetic field and also in lower
equipment costs. These electrically conductive sheetlike structures
preferably have a thickness of less than 100 .mu.m, more
particularly less than 50 .mu.m, and especially less than 20 .mu.m,
since as the thickness of the electrically conducting sheetlike
structure goes down, the adhesive tapes become more flexible and,
more particularly at very low thicknesses, acquire a sufficient
breakdown resistance.
[0073] With particular advantage, therefore, the heat-activatedly
heatable material is an electrically conductive sheetlike
structure, more particularly an electrically conductive layer. Such
structures are coated on at least one side, more particularly from
both sides, with the heat-activatable adhesive modified in
accordance with the invention. This produces an outstanding
breakdown resistance.
[0074] In one advantageous embodiment, the electrically conducting
layer of the heat-activatedly bondable sheetlike element has a
layer thickness of less than 20 .mu.m, more particularly of less
than 10 .mu.m, in order to limit its heating rate in a particularly
simple way. Furthermore, the sheetlike element may have a further
heat-activatedly bonding layer of adhesive. A sheetlike element of
such kind, as a double-sidedly bondable sheetlike element, is
particularly suitable for joining two bondind substrates to one
another.
[0075] At the same time, additionally, the electrically conducting
layer is preferably also magnetic, more particularly ferromagnetic
or paramagnetic. Although it was expected that in such materials,
in addition to the induction of eddy currents, there is also a
heating as a result of hysteresis losses, with an overall greater
heating rate as a result, the actual observation, in contrast, was
that even magnetic materials such as nickel or magnetic steels,
which are good conductors of electrical current, all have lower
heating rates than materials which, while conducting the electrical
current very well, are themselves not magnetic, such as copper or
aluminum, for example. It is therefore possible, by using magnetic
materials which conduct the electrical current, to control the
heating more easily and to lessen the occurrence of heating effects
away from the bondline.
[0076] It is favorable, furthermore, if the electrically conducting
layer has an electrical conductivity of more than 20 MS/m (which
can be achieved, for example, by using aluminum), more particularly
of more than 40 MS/m (which can be achieved, for example, by using
copper or silver), determined in each case for 300 K. In this way
it is possible to realize the sufficiently high bondline
temperatures that are required for producing high strengths in the
adhesive bond, and also to realize homogeneous through-heating,
even in very thin sheetlike elements. Surprisingly it has been
observed that heating increases with increasing conductivity, as a
result of eddy currents induced, and not, as expected, with
increasing electrical resistance,
[0077] Sheetlike Element Constructions
[0078] A sheetlike element is considered for the purposes of this
specification to encompass, in particular, all customary and
suitable structures having substantially sheetlike extent. These
structures allow two-dimensional bonding and may take various
forms, more particularly flexible, in the form of an adhesive
sheet, ;adhesive tape, adhesive label or shaped diecut. The
sheetlike element may be designed in the form of a cut-to-size
sheetlike element, the shape of which is adapted to the shape of
the bonding area, in order to reduce the risk of the bonding
substrate suffering thermal damage in the course of the inductive
heating.
[0079] Sheetlike elements in the sense of this specification each
have two side faces, a front face and a back face. The terms "front
face" and "back face" here refer to the two surfaces of the
sheetlike element parallel to its principal extent (two-dimensional
extent, principal plane of extent) and serve merely to distinguish
these two faces, disposed on opposite sides of the sheetlike
element, without the choice of the terms determining the absolute
three-dimensional arrangement of the two faces; accordingly, the
front face may also constitute that side face of the sheetlike
element that lies at the back three-dimensionally, and namely when,
accordingly, the back face forms the side face thereof that lies at
the front three-dimensionally.
[0080] This heat-activatedly bondable sheetlike element is to be
bonded to a bonding substrate. For this purpose, at least on one of
its two side faces, the sheetlike element has a heat-activatediy
bonding adhesive, preferably in fact on both side faces.
Heat-activatedly bonding adhesives are all adhesives which are
bonded hot at elevated temperatures and, after cooing, afford a
mechanically robust connection. The adhesive is present typically
in the form of a layer of adhesive.
[0081] A layer is more particularly a sheetlike arrangement of a
system of unitary functionality whose dimensions in one spatial
direction (thickness or height) are significantly smaller than in
the two other spatial directions (length and width), which define
the principal extent. A layer of this kind may be compact or else
perforated in form, and may consist of a single material or of
different materials, especially when these materials contribute to
the unitary functionality of said layer. A layer may have different
thicknesses or else a thickness which is constant over its entire
two-dimensional extent. Furthermore, of course, a layer may also
have more than one single functionality.
[0082] The sheetlike element to be bonded in the present context
advantageously comprises at least two different layers, these being
at least one electrically conducting layer and at least one
heat-activatedly bonding layer of adhesive.
[0083] The at least one electrically conducting layer may in
principle be of any suitable design--for example, a thin layer
which is compact over its full area or is perforated (in the form
of a lattice, for example). The thickness of the electrically
conducting layer is preferably less than 50 .mu.m, more
particularly less than 20 .mu.m or, indeed, less than 10 .mu.m. The
latter makes it possible to limit the heating rate at the upward
end in a relatively simple way.
[0084] The electrically conducting layer may consist of all
customary and suitable materials, such as of aluminum, copper,
gold, nickel. Mu-metal.sub.: alnico, permalloy, ferrite, carbon
nanotubes, graphenes, and the like. The electrically conducting
layer in this case is preferably also magnetic as well, more
particularly ferromagnetic or paramagnetic. The electrically
conducting layer in this case advantageously has an electrical
conductivity of more than 20 MS/m (corresponding to a specific
resistance of less than 50 m.OMEGA.mm.sup.2/m), more particularly
of more than 40 MS/m (corresponding to a specific resistance of
less than 25 m.OMEGA.mm.sup.2/m), determined in each case for 300
K.
[0085] In addition to the at least one electrically conducting
layer, the sheetlike element may of course also have further
electrically conducting layers; these layers may be identical to or
different from the at least one electrically conducting layer.
[0086] Overall, the heat-activatedly bondable sheetlike element may
be of any suitable design. Thus the sheetlike element, in addition
to the two layers described above, may comprise further layers,
examples being permanent carriers or temporary carriers.
Furthermore, the sheetlike element may be designed to be bondable
on only one of its two side faces or to be bondable on both side
faces, in the form, for instance, of a single-sidedly bondable or
doubie-sidediy bondable adhesive tape. In the latter case, the
sheetlike element has at least one further layer of adhesive, which
may be identical to or different from the at least one
heat-activatedly bonding adhesive. Accordingly, the further layer
of adhesive may comprise, for example, a heat-activatedly bonding
adhesive or even a pressure-sensitive adhesive.
[0087] In order to attain a sufficient breakdown resistance, the
layers of the heat-activatable adhesive ought advantageously to be
at least 10 .mu.m thick, preference being given to a thickness of
between 20 and 50 .mu.m, in order to guarantee sufficient breakdown
resistance with sufficient adhesive strength. For particularly
strong adhesive bonds, in contrast, a thickness of 50-200 .mu.m is
advantageous.
[0088] In one advantageous embodiment of the sheetlike element of
the invention, characteristically, [0089] the sheetlike element has
a thickness of less than 70 .mu.m, more particularly of less than
50 .mu.m, especially of less than 30 .mu.m, [0090] and/or the
electrically conducting sheetlike material has a thickness of less
than 30 .mu.m, more particularly less than 20 .mu.m, especially
less than 15 .mu.m, [0091] and preferably, after production of the
adhesive bond, by heating of the adhesive tape by means of magnetic
induction, the bonding strength in the static shear test to
polycarbonate is greater than 400 N/cm.sup.2.
[0092] It is advantageous, furthermore, to coat electrically
conductive sheetlike material with the heat-activatEible adhesive
from both sides, in order to ensure a sufficient breakdown
resistance.
[0093] Method
[0094] The invention further provides a method for bonding a
heat-activate* bondable sheetlike element to a particular kind of
bonding substrates.
[0095] These bonding substrates may in principle be made of any
material which can be bonded by means of heat-activa(able adhesives
(hence, in particular, which withstands the temperatures employed
in such a method). The materials to be bonded may be the same or
different.
[0096] With particular advantage, the method of the invention is
used to bond two identical or two different plastics to one
another.
[0097] In a first version, the method is characterized in that a
sheetlike element of the invention is used.
[0098] It is advantageous to use the heat-activatable sheetlike
elements of the invention in a method wherein these sheetlike
elements are heated at a heating rate of more than 50.degree. C.,
more particularly of more than 100.degree. C. The adhesive tapes of
the invention are particularly suitable for high heating rates,
since on account of their thermal conductivity they are able to
conduct excess heat away, and so the risk of local overheating is
low (see above), As a result of the better thermal distribution,
therefore, greater heating rates can be realized.
[0099] Use
[0100] The sheetlike element of the invention is used preferably
for bonding subassemblies of electronic devices, as for instance
those from the consumer electronics, entertainment electronics or
communications electronics segments (for example, for cell phones.
PDAs, laptops and other computers, digital cameras and display
devices such as, for instance, displays, digital readers or organic
light-emitting diode displays (OLEDs), and also for solar cell
modules, such as, for instance, electrochemical dye solar cells,
organic solar cells or thin-film cells). Subassemblies in the
present context are all constituents and collections thereof that
are used in electronic devices, examples being electronic
components (discrete and integrated components), housing parts,
electronic modules, antennas, display arrays, protective screens,
populated and/or nonpopulated circuit boards, and the like.
[0101] The sheetlike element of the invention can be implemented
using an induction heating means (inductor) which is customary for
inductive heating. Induction heating means (inductors) contemplated
include all customary and suitable arrangements, in other words,
for instance, coils, conductor loops or conductors through which an
alternating electrical current flows, and which generate an
alternating magnetic field of appropriate strength as a result of
the current flowing through them. Accordingly, the magnetic field
strength necessary for heating may be provided by a coil
arrangement with an appropriate number of turns and length of coil,
through which a corresponding current is flowing, in the form of a
point inductor, for example. This point inductor may be designed
without a ferromagnetic core, or else may have a core, made of iron
or pressed ferrite powder, for example. The preliminary assembly
may be exposed directly to the magnetic field thus generated.
Alternatively, of course, it is also possible to arrange the above
coil arrangement as a primary winding on the primary side of a
magnetic field transformer, on whose seconder` side a secondary
winding provides a correspondingly higher current. As a result, the
actual excitation coil, arranged in the immediate vicinity of the
preliminary assembly, can have a lower number of turns, as a result
of the higher current, without the field strength of the
alternating magnetic field being reduced as a result.
[0102] Where the preliminary assembly is subjected to a pressing
pressure in the course of the inductive heating, there is an
additional requirement for a pressing device for this purpose. As a
pressing device it is possible to use all devices suitable for
exerting a pressing pressure, examples being discontinuously
operating pressing machines such as, for instance, a pneumatic
press or hydraulic press, an eccentric press, a crank press, a
toggle press, a spindle press or the like, or else continuously
operating pressing machines such as a pressing roll, for instance.
The device may be provided as a separate unit or else may be
present in conjunction with the inductor. It is preferred, for
instance, to employ a device which as a first pressing tool
comprises at least one press-ram element which additionally has an
induction heating means. As a result, the induction field can be
brought very close to the bond site to be formed, and thus also can
be limited three-dimensionally to the area of this bond site.
[0103] Result
[0104] With the sheetlike elements of the invention it is possible
outstandingly to realize bonds of two substrates to one another,
more particularly plastic/plastic bonds, by means of induction
heating, with high cycle rates, while avoiding the disadvantages of
the prior art. Surprisingly it has been found that the bonding
strengths of the sheetilke elements of the invention are greater
than with known bonds using adhesive tapes according to the prior
art. With the sheetlike elements of the invention it is possible to
conduct the heat (formed during inductive heating) more effectively
away from the electrically conducting sheetlike structure
(inductively heatable material) and/or from the thermally
conductive additives, thereby lowering the risk of local
overheating and allowing the realization of greater heating
rates.
[0105] Experimental Investigations
[0106] For determining the thermal conductivity of the adhesives
with the thermally conductive auxiliaries, a method according to
ISO draft 22007-2 was carried out (thickness of the test specimens:
10 mm on both sides of the sheetlike heating element).
[0107] The bonding strength was determined in a dynamic tensile
shear test in a method based on DIN 53283 at 23.degree. C. with a
testing speed of 1 mmimin.
[0108] The electrical breakdown resistance of the
pressure-sensitively adhesive sheetlike elements obtained with the
thermal conduction compositions was determined in accordance with
VDE 0100.
[0109] The heat-activatable adhesives used were as follows:
TABLE-US-00001 Adhesive Type Description 1 tesa HAF 8400 Nitrile
rubber/phenolic resin 2 tesa HAF 8865/8860 Synthetic rubber/epoxy
resin 3 tesa HAF 8440 Copolyamide 4 tesa HAF 8464 Copolyester
[0110] The following fillers were selected for increasing the
thermal conductivity:
TABLE-US-00002 Manufacturer figures (Composition % figures in % by
Filler Type weight) 1 Nabalox 105 Al.sub.2O.sub.3 99.6%; SiO.sub.2
0.02%, Nabaltec AG, Schwandorf Na.sub.2O 0.3%, Fe.sub.2O.sub.3
0.03%, specif. surface area (BET) <1 m.sup.2/g,
.alpha.-Al.sub.2O.sub.3 content 98%, average particle diameter 80
.mu.m, primary crystal size 2 .mu.m, particle diameter <45
.mu.m: about 20%, net density 3.9 g/cm.sup.3, bulk density 950
g/m.sup.3, pouring angle 50.degree. 2 Nabalox 115-125
Al.sub.2O.sub.3 99.6%; SiO.sub.2 0.03%, Nabaltec AG, Schwandorf
Na.sub.2O 0.3%, Fe.sub.2O.sub.3 0.03%, specif. surface area (BET) 1
m.sup.2/g, .alpha.-Al.sub.2O.sub.3 content 98%, average particle
diameter 4 .mu.m, primary crystal size 2 .mu.m, net density 3.9
g/cm.sup.3, bulk density 800 g/m.sup.3 3 Nabalox 715-10
Al.sub.2O.sub.3 99.6%; SiO.sub.2 0.03%, Nabaltec AG, Schwandorf
Na.sub.2O 0.3%, Fe.sub.2O.sub.3 0.03%, specif. surface area (BET)
1.6 m.sup.2/g, .alpha.-Al.sub.2O.sub.3 content 98%, average
particle diameter 2.5 .mu.m, primary crystal size 2 .mu.m, bulk
density 800 g/m.sup.3, green density (100 Mpa, about 4% moisture)
2.35 g/cm.sup.3, sinter density 3.65 g/cm.sup.3, sinter temperature
1725.degree. C. linear contraction 13% 4 .alpha.-Aluminum oxide
nanoparticles IBU-tec advanced materials GmbH, Weimar 5 Boron
nitride Grade A 01 CeramTec AG, Plochingen 6 Aluminum powder, fine;
spherical particles, about 25 .mu.m TLS Technik GmbH & Co,
Bitterfeld
[0111] MagSilica 50-85: nanoparticles, from Evonik, Matrix:
SO.sub.2, magnetic domains iron oxide, domain content 80-92% by
weight, surface size 40-50 m.sup.2/g, diameter 82.+-.11 nm, density
3.72 g/cm.sup.3, volume fraction of magnetites in the particle: 40%
by volume
[0112] The following thermally conductive adhesives and
corresponding comparative examples were produced:
TABLE-US-00003 Proportion of filler [% Adhesive Filler by volume]
.lamda. [W/mk] Example 1 1 1 40 1.2 2 1 2 20 0.43 3 1 2 31.5 0.71 4
1 2 40 0.99 5 1 3 40 0.74 6 1 4 40 0.56 7 1 5 40 1.29 8 2 2 20 0.39
9 3 2 20 0.43 10 4 2 20 0.44 11 1 6 40 0.85 12 3 2 20 + 10% 0.45 by
weight MagSilica 50-85 (Degussa AG, Hanau) Comparative example C1 1
0 0.20 C2 2 0 0.18 C3 3 0 0.22 C4 4 0 0.23 C5 1 MagSilica 50-85 10%
by weight 0.28
[0113] For producing the thermally conductive adhesives, in the
case of adhesive types 1 and 2, the adhesive tape was dissolved at
room temperature in butanone and a solids content of 30% by weight
was set. The fillers were dispersed using a high-speed stirring
assembly of the Ultraturrax type. Films of adhesive were then
coated out using a doctor blade and dried, giving a film thickness
of approximately 100 .mu.m,
[0114] For example 12, in addition to the thermally conductive
filler, 10% by weight of an inductively heatable filler was
incorporated additionally.
[0115] In the case of adhesive types 3 and 4, the fillers were
incorporated into the melt at a temperature of 180.degree. C. in a
laboratory kneading apparatus from Haake. Sheetlike moldings having
a thickness of about 100 .mu.m were then produced at a temperature
of 150.degree. C. in a vacuum press.
[0116] The electrically conductive sheetlike structure used for
induction heating was an aluminum foil with a thickness of 36
.mu.m. For examples 1-11 and C1-C4, the metal foil was laminated
together on both sides in each case with the layers of adhesive at
a temperature of about 90-115.degree. C., depending on the adhesive
system. In the case of adhesives 1 and 2, the chemical crosslinking
reaction is not initiated here; instead, merely an adhesion is
induced.
[0117] Bonding substrates used for the inventive adhesive tape 1
were 2 polycarbonate sheets 2 20 mm in width, 100 mm in length, and
3 mm in thickness, which overlapped at the bondline 3 by 10 mm (cf.
FIG. 1). In this case, therefore, the bonding area encompassed a
rectangle with an edge length of 10.times.20 mm. In order to
investigate the differential separation of the adhesive tape, the
adherends selected were made from the same material. FIG. 1
additionally shows, schematically, the lower press-ram element 4,
the upper press-ram element 5, and the force F.
[0118] The bonding method was carried out, except in the case of
example 12, using a modified induction system of type EW5F from IFF
GmbH, ismaning (DE). Serving as the inductor for local provision of
the alternating magnetic field here is an induction field
transformer composed of just one water-cooled current-bearing
conductor, which is used as a secondary coil circuit in a
transformer-field transformer and which interacts in a coaxial
transformer with the transformer field generated on the primary
coil side. The induction field transformer was embedded into a
matrix of polyetheretherketone (PEEK) and the resultant arrangement
was used as the lower press-ram element 4 of a press device, which
also has an upper press-ram element 5. The applied pressure
resulting from the force F, which was applied to the preliminary
assembly between the lower press-ram element 4 and the upper
press-ram element 5, perpendicularly to the side faces of the
heat-activatedly bondable sheetlike element, was 2 MPa in each
case.
[0119] With the aid of the modified induction system, alternating
magnetic fields with a frequency of 30 kHz were produced with a
pulse width of 70% for the investigations. The pulse width
indicates the percentage fraction of the pulse duration (pulse
length) of the alternating magnetic field as a proportion of the
overall period duration (the sum of pulse duration and the duration
of the pauses between two successive pulses) of the alternating
magnetic field. The time for which the heat-activatedly bondable
sheetlike element was exposed to the pulsed alternating magnetic
field (i.e., the duration of inductive heating) lay within a range
from 3 at 9 s.
[0120] All experiments (apart from example 12) were carried out,
furthermore, with a subsequent pressing time of 8 s, within which
there was inductive afterheating in an alternating magnetic field
of the same frequency as for the thermal activation of the
adhesives, with a pulse width of 20% (corresponding to a ratio of
pulse duration to pause duration of 1:4).
[0121] For example 12, a high-frequency induction system from Odes
was used, which operated with a 3.5-turn plate inductor at a
frequency of 586 kHz and was run with a power consumption of 20 kW.
The samples were placed on the plate inductor, lined with a 0.25 mm
plate of glass fiber-reinforced plastic, and were loaded with a
weight force of approximately 20 N, which was guided on to the
joining site via a plastics rod.
[0122] Results:
[0123] The results show average values from five test bonds in each
case,
TABLE-US-00004 Induction Pressing Adhesive time pressure strength
Breakdown [s] [MPa] [N/cm.sup.2] resistance Example 1 9 2 652 pass
2 9 2 566 pass 3 9 2 585 pass 4 9 2 597 pass 5 9 2 572 pass 6 9 2
579 pass 7 9 2 636 pass 8 9 0.5 504 pass 9 3 0.2 444 pass 10 3 0.2
379 pass 11 9 2 611 fail 12 10 0.1 559 pass Comparative example C1:
tesa HAF 9 2 552 pass 8400 C2: tesa HAF 9 0.5 462 pass 8865/8860
C3: tesa HAF 2 0.2 403 pass 8440 C4: tesa HAF 2 0.2 345 pass 8464
C5: tesa HAF 531 pass 8400 + 10% by weight MagSilica
[0124] The results show that as a result of the increased thermal
conductivity, improved bonding strengths are achieved This is
unexpected, since to increase the thermal conductivity it is
necessary to add a high volume fraction of fillers, and so the
skilled person expects that the bonding strengths will fall.
[0125] The electrical breakdown resistance test in accordance with
VDE 0100 was likewise passed by all inventive samples with the
exception of experiment 11. From this it can be seen that thermal
conduction compositions of the invention can be made electrically
non-conducting and hence can be used even in those applications
where electrical insulation from components connected in a
thermally conducting way is required, such as in electronic
devices, for instance. Furthermore, the electrical breakdown
resistance is not adversely affected by the internal metal
foil.
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