Note: Descriptions are shown in the official language in which they were submitted.
200351:~
An unfoamed foil su.itable as an adheslve.
The invention relates to an unfoamed foil
of a thermoplastic resin capable of serving as an adhesive
when softened and adapted for use in the manufacture
of laminates.
Such foils are known for adhesive purposes.
They belong to the so-called hot melt adhesives. This
~ type of glued joints is important in a great many fields
- of the art, now as a direct joint and then again as
a further protection for a screwed or bolted joint or
for fillingly or structurally sealing a riveted or spot -
welded plate jolnt.
In many fields, e.g., in the construction
of trucks and in the ship- and aircraft-building industries,
plating structures, assemblies of plates, shells, ribs,
frames and stiffeners are realized in a laborious and
labor-intensive manner by means of riveting or spot-welding.
These discrete stiffenings and reinforcements can be
thought of as expanded in the form of sandwich plating
structures, two relatively thin plates of fiber reinforced
plastic or metal with an intermediate light core material
of foam or honeycomb material. Consequently, attempts
have also been made to replace the plating structures
by this type of laminates. This technique is often applied
in the aircraft-building industry. For assembling purposes
this requires massive edges. With these plates manufactured
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by means of an autoclave or a hot press, thls generally
occurs by applying separate thick laminates ln the lami-
nating phase. According to another technique pieces
of impregnated fabric were manually put on a table and
cut out, followed by applying the honeycomb as well
as a covering layer thereon and filling the edges. This
is of course extremely labor-intensive and must further
be followed by a more difficult treatment cycle at elevated
temperature and pressure. A more recent and improved
development is that the honeycomb is coated with an
adhesive, followed by assembling the whole. It is difficult,
however, to obtain a homogeneous coating of adhesive.
As far as a solvent is used, the coating must be dried.
European patent application 212,970 disclosès
an adhesive tape comprising an elastomer, a crosslinker
and optionally a blowing agent. This tape is especially
useful for isolation, such as sonic isolation.
It is an object of the invention to provide
a new type of unfoamed foil suitable for use as an adhesive
when softened (hot melt adhesive).
The invention relates to an unfoamed foil
of at least one thermoplastic resin, said foil being
capable of serving as an adhesive when softened and
having included therein at least one blowing agent.
Surprisingly, it has been found that the foil
according to the invention is eminently suited for the
manufacture of all kinds of articles. As a result of
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the action of the foamlng agent durlng softening there
is formed a layer of foam having an adhesive effect,
so that the materials to be joined are properly anchored
to each other.
The foil according to the invention is mainly
an unfoamed or substantially unfoamed foil of a thermo-
plastic resin. The nature of this thermoplastic resin
may be very different. This nature is codetermined by
the materials to be glued and by the conditions at which
the glued joint must stand up, as well as by the require-
ments imposed on the final product. In general, it is
preferred to use a thermoplastic resin, the strength
of which under conditions of service is equal to or
greater than the strength of the weakest of the materials
15 to be glued. Of course, this is not possible with all
applications, because the strength of many plastics
is less than the strength of metals. Anyhow, the nature
- of the materials to be glued is to be considered when
selecting the material. In view of the usual conditions
20 at which many glued joints must stand up, it is preferred
to apply a thermoplastic resin having a Tg of at least
50C, more in particular at least 100C.
In principle, any thermoplastic resin may
serve as a base resin for the foil according to the
25 invention. It is preferred, however, to select this
resin from the group consisting of polyetherimide, poly-
carbonate, acrylate polymers, styrene polymers, polyether
;
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sulfone, polyether ketone, polyether ether ketone, poly-
phenyleneoxide and mixtures of two more of these resins.
More in particular, preference is given to polyetherimide,
in connection with the excellent properties of this
s material.
The second component of the foil according
to the invention is the blowing agent. This blowing
-:
agent may be selected from the group consisting of solvents
for the thermoplastic resin, physical blowing agents,
chemical blowing agents, water and mixtures of two or
more of these blowing agents, more in particular, preference
is given to the use of a blowing agent which is also
a thermoplastic resin solvent, which in this connection
also comprises a gelling agent. The reason for this
preference first of all resides in that this selection
of materials leads to a surprisingly good result, while
further the manufacture of the foil can be very easy,
certainly when the preparation of the thermoplastic
resin or the foil prepared therefrom takes place from
a solution. Anyhow, this has the advantage that no additional
foreign components are introduced into the system. Moreover,
the foil can be easily manufactured from a solution,
with the removal of the solvent being discontinued at
the moment when the desired content of solvent is present.
Examples of blowing agents to be used are,
among others, di-, tri- and tetrachloromethane, trichloro-
:
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ethene, 1,2-dichloroethane, lower hydrocarbons, such
as butane, different pentanes, hexanes and heptanes
etc., which also comprise the different isomers thereof,
cyclic aromatic and aliphatic hydrocarbons etc., but
also lower alkanols, ethers and ketones. It is also
possible to apply physical blowing agents based on the
chloro-fluorohydrocarbons. In connection with the injurious
effect thereof, however, these are clearly not preferred.
Other examples of blowing agents are the chemical
blowing agents, such as sodium bicarbonate, azodicarbon-
amide, azobisisobutyronitrile, diazoaminobenzene, p-toluene-
sulfonyl hydrazide, benzenesulfonyl hydrazide, dinitroso-
pentamethylenetetramine, oxybis(p-benzenesulfonyl)hydrazide,
N,N'-dinitroso N,N'-dimethylterephthalamide, p-toluene-
sulfonyl semicarbazide, 5-phenyltetrazole, etc. These
compounds can be used as such or in combination with
an activator. Examples of activators are zincoid, metal
salts of sulfonated compounds, activated urea, stearic
acid, polyethylene glycol etc.
According to the simplest embodiment of the
foil according to the invention it comprises two components,
namely the resin and the blowing agent and optionally
the activator for the blowing agent. It is not impossible,
however, that additives may be included in the foil.
`- 25 Examples of such additives are selected, among others,
from the group consisting of stabilizers, antioxidants,
germinators, fillers, such as particulate fillers (as
20~5~
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sand, talc, sillca, clay etc.) fibers and/or li~uid
crystalline polymers, colorants, flame extinguishing
additives, other inert additives and mixtures of two
or more of the components. Further, a softener may be
included in the system, which softener is for the purpose
of reducing the temperature at which softening occurs.
In practice, such a softener is only useful if no softening
of the thermoplastic resin occurs owing to the presence
of a thermoplastic resin solvent, which is also active
as a softener.
Suitable softeners are, among others, the
fatty acids and their metal soaps. Further, APP, poly-
butylene, bitumen, or extender oil, such as naphthenic
and paraffinic oil, may be used. It is also possible
to suitably use waxes, such as microcrystalline wax.
In certain cases it can be advantageous to
include fibers in the foil. When softening and foaming
same, these can orient themselves and perform a reinforcing
function. In this connection, however, it is preferable
to start from relatively short and thin fibers, e.g.,
having a thickness in the order of a few ~m, e.g., 0.5
to 5 ~m, and a length of maximally a few mm, e.g., from
0.1 to 3 mm.
According to another embodiment of the invention
a fiber reinforcement can also be included in the foil,
e.g., a "woven fabric" of glass, carbon and/or aramide
fibers.
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The foil according to the invention can be
manufactured in different ways. According to a first
method the starting product is an already existing,
extruded or pressed foil, which is then provided with
liquid physical blowing agent by means of impregnation
of the foil with the liquid or by exposure to the vapor
thereof.
- Thus the foil can be easily passed through
a space in which the atmosphere is saturated with vapor
from the blowing agent. Velocity and degree of saturation
are then a function of the temperature, the pressure
and the time. When the foil is removed from the vapor-
saturated chamber, a portion of the blowing agent of
course evaporates again from the outer regions of the
foil, but measurements have shown that in the inside
of the foil the content of blowing agent remains practically
constant for a prolonged period of time (weeks or months),
so that the foil can be stored for a sufficient time,
if so desired. The treatment of the foil with blowing
agent is eminently suited for a continuous process.
- However, it is also possible to manufacture
a foil by extruding a mixture of the thermoplastic resin
and the other components or shaping it in a different
way. According to a third method the foil can be manufac-
tured by means of the method of "solvent casting", inwhich a solution of the thermoplastic resin is shaped
to a foil, followed by removing the solvent carefully.
2o~sl2
In the manufacture of the foil according to the invention
the same technique can be used, but it will suffice
to remove the solvent in part.
The blowing agent is preferably integrally
present in the foil, that is to say the content is roughly
equal throughout the thickness of the foil. However,
it may sometimes be advantageous to have a gradient
of the concentration of blowing agent. In particular,
it may be interesting to provide that the blowing agent
is present substantially on one side. This can be achieved,
e.g., by impregnating on one side with vapor or by removing
only on one side.
The content of blowing agent may vary within
broad limits. The lower limit is determined by the fact
that a certain degree of foaming must be present for
a proper adhesion. Of course, this remains dependent
on the specific combination of material, but a proper
lower limit is formed by a content of at least 0.5 wt.%
blowing agent, based on the weight of the thermoplastic
resin and the blowing agent together. Usual contents
range from 1 to 50 wt.%. The upper limit is substantially
determined by the fact that the foil must still be manageable
and may not be too weak, at least in the case of using
a thermoplastic resin solvent as a blowing agent.
Another measure of the amount of blowing agent
is the vol.% of pores. This is preferably in excess
of 10 vol.%.
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The thickness of the foil may vary within
broad limits, depending on its use. This range is from
- 10 ~m to, e.g., 2 mm. A thicker foil is hardly useful,
because it is easier to apply two layers than to properly
and regularly foam a thicker foil.
After the manufacture of the foil it is brought
to the required size, if desired, and further processed
or stored. When volatile blowing agents are used, it
is preferred that the foil be provided with a protective
foil of another material, which is preferably not or
only slightly permeable to the blowing agent. Materials
suitable therefor are the known barrier materials, such
as PVA and coextruded multilayer foils, e.g., provided
with at least one layer of a polyamide. According to
a preferred embodiment of the invention the foil is
coiled after manufacture with an intermediate layer
of another resin foil which is not affected by the blowing
agent.
The invention also provides a process for
adhering two materials together, which comprises interposing
a foil according to the invention between these materials
and then heating at least the foil. There are à number
of possibilities for this heating. The total assembly
of materials to be adhered can be heated, but it is
also possible to heat only one side or even to heat
only the foil.
Some embodiments will hereinbelow be discussed
20~)~51~
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in more detail.
In case two plates with an intermediate core
material, preferably a honeycomb structure are to be
adhered together, it is normally extremely difficult
S to apply a uniform adhesive layer on the thin sides
of the cells of the honeycomb. Furthermore, there is
only formed a very local glued joint. By means of the
invention it is easy to apply an adhesive foil, preferably
polyetherimide, provided with a blowing agent between
10 plate and honeycomb and then to heat the whole, while
at the same time the thermoplastic resin is going to
function as an adhesive and foams. Consequently, anchorage
to the honeycomb takes place and a better and more reliable
adhesion is obtained in a much easier way. Of course,
15 the honeycomb can be laminated either successively or
in one operation on both sides.
Further advantages of the novel gluing method
are that a greater impact strength and a greater pressure
strength are obtained, for the free collapsing length
20 of the honeycomb is reduced because the honeycomb absorbs
the foam. This collapsing length depends on
the square of the length, so that a shortening has a
square effect. Furthermore, better thermal and acoustic
isolation is obtained with the combination of honeycomb
25 and foam.
A special application of the above is the
protection against ballistic missiles by means of a
~00351~
covering layer of ceramic material. According to the
present invention the honeycomb can be suitably filled
with a mixture of ceramic globules and foaming thermo-
plastic resin. The globules and the propellant are together
incorporated in the honeycomb when the adhesive is foamed,
so that a homogeneous filling having a desired content
of globules can be formed in the entire honeycomb.
Another possibility for creating a barrier
against ballistic missiles using the present invention
is formed by the layer-by-layer application of fibrous
- mat and foil according to the invention, which structure
is then heated, so that the foll foams and adheres to
the fibrous mat, but does not impregnate it. Such a
structure with fibers that are not or anyway not completely
impregnated being applied between layers of foam forms
a good barrier against ballistic missiles.
If it is wished to connect a honeycomb structure
to another part, a complete edge finish is required.
According to the state of the art filled edges are obtained
20 with so-called prepregs (massive laminate) or with a
preferably filled resin. According to the invention
there are different possibilities of obtaining filled
edges. Edges can simply be pressed, which is known per
se as core crushing. Thus a kind of finish is obtained
which, however, is not yet strong, for the honeycomb
is damaged. It is better to manufacture a laminate-with-
honeycomb to provide this with a slit and place therein
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s
200351~
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a foil according to the invention containing a blowing
agent, and to heat and thus fill the whole. Moreover,
the thus obtained edges can be pressed, resulting in
a structural edge which is also durable.
Another problems arises from the wish to have
a reinforcement in the middle of a laminate-with-honeycomb.
This is sometimes needed in component produc~ion. For
- this purpose it has hitherto been necessary to apply
a resin f.illing, i.e. a kind of encapsulation (potting).
This is laborious and difficult, for the honeycomb has
to be locally filled completely. Thus it is possible
to anchor or fix a metal part and to obtain a great
strength.
According to the invention a foil can be locally
superposed and heated under light pressure. The adhesive
foam, optionally with a filler, penetrates itself, resulting
in a structural filling. If required, there can also
be pressed on site.
The invention has been illustrated above mainly
on the basis of a honeycomb core material, because of
some special advantages resulting therefrom. The invention,
however, can also be applied to other core materials,
such as balsa wood and foams of resins, both thermoplastic
and thermosetting resins.
In general, the invention therefore also relates
to a process for manufacturing a laminate, which comprises
adhering together a core layer snd 9t least one other
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layer by means of the foil accordig to the invention.
The core layer generally comprlses the above-mentioned
materials, while the other layer may consist of wood,
metal or a resin plate optionally reinforced with fibers,
such as a woven or knit fabric.
With regard to the nature of the resin for
the core material and/or the other layer it is observed
that this fully depends on the use of the laminate.
For the more high-grade uses the same resins will generally
be applied as described for the foil, as far as thermo-
plastic resins are concerned. As thermosetting resins,
polyurethanes, epoxy resin based systems and unsaturated
polyesters are preferentially considered.
, Also the selection of the metal depends on
the use, while it stands to reason that for aeronautical
applications aluminum will soon be adopted.
The other layer, often a top layer, is reinforced
with fibers in order to obtain a high-grade laminate.
These may be all kinds of fibers, such as glass, carbon,
metal and polyamide (more in particular aramide) fibers.
The fibers may be applied on all sides, but
it-is preferred that these be applied either unidirection-
ally or in an arrangement, such as a woven or knit fabric.
It is observed that the blowing agent can
temporarily reduce the softening temperature of the
thermoplastic adhesives. This, too, can be determined
by means of simple pretests. Not only flat plates, but
-14-
also single- or double-curved plates can ~e laminated
according to the invention with the present foils. Curved
plates reinforced with continuous fibers are used for
this purpose. During the heating required in the process
according to the invention the curve is maintained.
In addition, during the heating before foaming,
a deformation of the skin may take place, if a thermoplastic
skin is used. On it, there can also be made articles
the top layers of which do not parallel each other.
The invention will now be illustrated by means
of the following, non-restrictive examples.
Example l
A foil having a thickness of 400 ym sf polyether-
imide (Ultem) was impregnated with dichloromethane by
passing the foil through a vapor of dichloromethane.
After impregnation the content of dichloromethane in
the foil was about 20 wt.%. The dichloromethane was
almost integrally distributed through the foil.
A laminate was built up from two layers of
l00 g glass fabric impregnated with polyetherimide (weight
ra~io l:l), having an intermediate layer of the impregnated
foil. This assembly was placed in a mould heated to
120C, which was closed for 2.5 s. There was thus obtained
a flat sandwich material having a thickness of 6.4 cm.
The properties of this sandwich material were
as follows:
2003512
Specific weight 110 kg/m'
E-modulus (pressure) 50 N/mm2
E-modulus (tension) 35 N/mm2
Shearing strength 1.5 N/mm2
5 Compressive strength 3 MPa
Tensile strength 5 MPa
Flexural strength 220 MPa
Flexural stiffness 3.2*106 N.mm2/cm
The above result indicates clearly that good
materials are obtainable using the invention.
Example 2
Two metal layers were adhered together at
a temperature of 120C, using a foil comparable to the
foil used in Example 1 but having a thickness of 50 ~m.
The strength of the adhesion between the metals corresponded
to the self-strength of the polyetherimide. From this
it may be concluded that the properties of the polyether-
imide are not adversely affected by the treatment with
a blowing agent.
