Note: Descriptions are shown in the official language in which they were submitted.
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MULTILAYER ELEMENT COMPRISING A REINFORCING MATERIAL
COMBINED WITH A SUPPORT LAYER BY MEANS OF AN
ELECTROSTATIC LINK
The present invention relates to the technical field
of reinforcing materials suitable for making composite
parts. More
precisely, the invention relates to
associating a reinforcing material with a support layer
by electrostatic bonding.
Composite parts or articles, i.e. that comprise
firstly one or more fiber sheets or pieces of fiber
reinforcement and secondly a matrix (which is usually
mainly of thermosetting type and may include
thermoplastics), may be fabricated, for example, by a
method that is said to be "direct" also known as liquid
composite molding (LCM). A direct method is defined by
the fact that one or more pieces of fiber reinforcement
are used in the "dry" state (i.e. without the final
matrix), with the resin or matrix being provided
subsequently, e.g. by injection into a mold containing
the fiber reinforcement (a method known as resin transfer
molding (RTM)), by infusion through the thickness of the
fiber reinforcement (the liquid resin infusion (LRI)
method or the resin film infusion (RFI) method), or
indeed by manually coating and/or impregnating by means
of a roller or a paintbrush each of the individual layers
of the fiber reinforcement, which layers are applied in
succession onto a shaper.
Indirect methods make use of reinforcing materials
of preimpregnated type, including the quantity of resin
that is needed for making the final part.
Various reinforcing materials are available for use
in such methods. Such
materials may be of woven, non-
woven, or unidirectional type, they may include one or
more layers, and they may optionally include a large
amount of thermoplastic or thermosetting binder. The
Applicant has in particular made proposals for
intermediate materials comprising a sheet of
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unidirectional fibers, in particular carbon fibers, that
is associated by adhesive on each of its faces with a
non-woven fabric of thermoplastic fibers, referred to
below as webbed UD. These
reinforcing materials are
described in prior patent applications WO 2010/046609 and
WO 2010/061114.
When possible, such reinforcing materials are
delivered on their own in the form of a reel or roll,
without being associated with a support layer that might
make them easier to manipulate. This makes it possible
to reduce waste and to simplify the laying devices that
are used since they do not need to have devices for
separating the support layer and for rewinding it.
Nevertheless, in certain specific applications, the
Applicant has found that there continues to be a need to
have a support layer. This
applies in particular for
wide strips of reinforcing material that are for cutting
out. Present-day machines that lay wide sheets generally
manage cutting by using an ultrasonic knife. Such
machines use the support film as a cutting anvil. Such
machines can produce straight cuts (at 90 to the fibers
if they are unidirectional) without a support film, but
arbitrary (zigzag) cuts are not possible. This is
unacceptable for making use on an industrial scale of
reinforcing materials that include wide unsupported
unidirectional sheets, since waste management is of great
importance economically and is directly associated with
managing cutting.
Certain machines that are designed exclusively for
managing preimpregnated unidirectional sheets can
nevertheless accommodate unidirectional sheets that are
said to be "dry" (i.e. that have no more than 10% by
weight of binder), providing they are delivered with a
support film.
In this context, the Applicant began by attempting
to laminate its webbed UD material with various types of
support film by applying heat, so as to use the adhesive
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nature of thermoplastic webs when hot. The
Applicant
then encountered difficulties associated with adjusting
the heating temperature during lamination. If the
temperature used is too high, it then becomes impossible
to separate the reinforcing material from the support
film. In
contrast, if the temperature used is too low,
then no bonding occurs.
Furthermore, the Applicant has observed that
lamination by heating presents certain major drawbacks:
The original reinforcing material is modified.
The conditions used during lamination (heating
temperature, pressure, cooling) do not necessarily
correspond to requirements for producing the original
material. For
example, while it is cooling, the
reinforcing material is in contact with the surface of
the support film, which will therefore make its imprint
on the surface of the reinforcing material so that it
ends up with a structure that is different from its
original structure.
= The heating temperatures associated with the
binder used for lamination can be very high, and thus
incompatible with numerous support films. The risk
of
the reinforcing material being polluted by components
coming from the support is then considerable, thereby
greatly reducing the range of films that are suitable.
Those difficulties clearly reveal the need to find a
substitute for lamination by heating, when using
reinforcing material that includes a binder, or more
generally the advantage of proposing a novel method of
associating reinforcing material with a support layer
that is easy to implement and that does not lead to any
degradation of the initial reinforcing material, while
serving to facilitate manipulating and cutting the
reinforcing material.
In this context, the present invention proposes a
multilayer element comprising reinforcing material
adapted to making composite parts, and a support layer.
81791935
3a
In one aspect, the present invention provides a multilayer
element comprising a reinforcing material adapted to making
composite parts, at least one face of the reinforcing material
associated with a support layer, the reinforcing material
comprising one or more woven, non-woven, or unidirectional
material fabrics made of glass reinforcing fibers, carbon
reinforcing fibers, aramid reinforcing fibers, or ceramic
reinforcing fibers, and the reinforcing material and the
support layer being associated with each other by electrostatic
forces.
In another aspect, the present invention provides a method
of fabricating a multilayer element as described herein,
wherein the reinforcing material and the support layer are
associated together by subjecting the multilayer element to a
static electricity generator.
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In the context of the invention, the reinforcing material
is associated on at least one of its faces, and in
particular on only one of its faces, with a support
layer, which association may be provided by electrostatic
forces.
Bonding in this way between the reinforcing material
and the support layer makes it possible to preserve the
integrity of the original reinforcing material and does
nothing to degrade its initial properties. Under
the
action of electrostatic charges, in the context of the
invention, an attraction bond is created between the
reinforcing material and the support layer, giving rise
to electrostatic bonds. This
bond is strong enough to
hold the support layer in position on the reinforcing
material, in particular during manipulation and cutting
operations, while subsequently making it easy to peel the
two components apart so as to be able to position the
reinforcing material when subsequently making a composite
part. Bonding
in this way, which does not make use of
any heating, makes it possible to widen the range of
support layers that can be used, by eliminating
constraints in terms of temperature stability and risks
of polluting the reinforcing material.
The invention is particularly advantageous when the
reinforcing material does not present an adhesive nature
over temperatures in the range 18 C to 25 C, thus making
it impossible to associate it with the support layer by
adhesion at ambient temperature by making use of residual
tackiness. Also,
and in preferred manner, the
reinforcing material does not include thermosetting
material or it includes thermosetting material
representing no more than 10% of the total weight of the
reinforcing material. In
particular, the reinforcing
material is constituted by reinforcing fibers only, or by
reinforcing fibers and a thermoplastic material, with the
weight of the thermoplastic material then representing no
more than 10% of the total weight of the reinforcing
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material and preferably representing 0.5% to 10% of the
total weight of the reinforcing material, and more
preferably 2% to 6% of the total weight of the
reinforcing material. It is
nevertheless possible for
5 the invention to be applied to reinforcing materials of
the preimpregnated type that present a larger quantity of
thermoplastic material or of thermosetting material. By
avoiding adhesion by heating, the invention makes it
possible to preserve the integrity of the original
material without leading to remelting of any
thermoplastic binder(s) that may be present.
In particular, in the context of the invention, the
reinforcing material may comprise one or more woven, non-
woven, or unidirectional material fabrics. In
particular, the reinforcing material may comprise one or
more woven, non-woven, or unidirectional material fabrics
made of reinforcing fibers, and in particular of carbon
fibers. The
element of the invention is said to be
"multilayer" since it comprises a layer of reinforcing
material and a support layer. It is also possible for
the reinforcing material itself to comprise one or more
layers that are bonded together by any appropriate means.
The invention is applicable to any type of
reinforcing material suitable for making composite parts.
Such materials are based in particular on fibers made of
glass, carbon, aramid, or ceramics.
The invention is particularly adapted to reinforcing
materials constituted by sheets of unidirectional carbon
fibers that are bonded on both of their faces to
thermoplastic binders, in particular of the non-woven
fabric type made of thermoplastic fibers. Such
reinforcing materials are described in particular in the
following documents: EP 1 125 728; US 6 828
016;
WO 00/58083; WO 2007/015706; WO 2006/121961; and
US 6 503 856; and in the following patent applications in
the name of the Applicant: WO 2010/046609 and
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WO 2010/061114, to which reference may be made for
further details.
In the context of the invention, the support layer
is preferably made of an electrically insulating
material. In
particular, the support layer presents
resistivity lying in the range 108 ohm meters (Q.m) to
1017 Q.m, and
preferably in the range 1010110.m to
1017 S-2.m. Such
measurements are performed in particular
at 20 C with 0% relative humidity, preferably in
compliance with IEC standard 60093: 1980. The greater the
resistivity, the stronger and more durable the adhesion
between the support layer and the reinforcing material.
In particular, the support layer may be a polymer,
preferably selected from thermoplastic polymers such as
polyamide, e.g. polyethylene terephthalate, copolyamides,
polyesters, copolyesters, or cellulose, cotton, natural
silk, or artificial fibers.
By way of example, the support layer may be a film,
a paper, or a textile, or any type of layer that performs
the support role, i.e. that facilitates handling and
cutting.
Advantageously, the support layer presents
thickness lying in the range 10 micrometers (pm) to
500 pm.
Adhesion between the support layer and the
reinforcing material must be sufficient to hold those two
elements in position. In the
context of the invention,
the electrostatic forces serving to associate the
reinforcing material and the support layer preferably
correspond to a peeling force of 50 millinewtons (mN) to
1000 mN. The
electrostatic forces serving to associate
the reinforcing material and the support layer
correspond, in particular, to a residual charge of
voltage lying in the range 0.1 kilovolts (kV) to 3 kV.
The electrostatic forces also persist well over time.
Even though an electrostatic force reduction can be
observed immediately after charge has been generated at
the interface between the reinforcing material and the
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support layer, with the force going down to a value lying
in particular in the range 0.1 kV to 3 kV, thereafter,
there is no longer any significant reduction in the force
of adhesion. In
particular, excellent stability is then
observed while the multilayer element is being stored for
one month in the form of a roll or a reel. In contrast,
it is very easy to separate the two portions by a peeling
action. It is
also observed that they have a clear
tendency to reassociate after peeling, but with
attraction being reduced on each reassociation and/or
peeling cycle.
Advantageously, the multilayer element is in the
form of a strip of width greater than or equal to
50 millimeters (mm). The
invention is particularly
advantageous when such wide strips need to be cut, since
the support is then essential. Such
strips having a
length of several meters can be stored in particular in
the form of rolls. Multilayer elements of the invention
are entirely suitable for being used on laying machines
having means capable of performing complex cutting.
The present invention also relates to a method of
fabricating a multilayer element of the invention in
which the reinforcing material and the support layer are
associated by subjecting the multilayer element to a
static electricity generator.
For that purpose, a stack of the support layer and
the reinforcing material in contact with each other is
placed in an electric field, e.g. generated by applying a
voltage in the range 10 kV to 50 kV, and preferably in
the range 15 kV to 30 kV. In the context of the
invention, the support layer and the reinforcing material
are thus associated without applying heat or pressure.
In conventional manner, the electrostatic field is
generated between a conductive bar connected to a
positive voltage generator and a conductive bar connected
to ground. The stack may be positioned equally well with
the reinforcing material facing the conductive bar
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connected to the positive voltage generator or to the
conductive bar connected to ground.
Finally, the invention also provides a method of
fabricating a composite part made from at least one
reinforcing material obtained from a multilayer element
of the invention after removing the support layer.
Usually, the support layer is removed after performing a
cutting operation on the multilayer element, in
particular in a direction that is not parallel to its
width. Conventional stacking techniques are performed as
are conventional techniques of injecting or infusing
resin, in the event that the reinforcing material does
not contain a sufficient quantity of thermoplastic and/or
thermosetting material. For
further details on the
techniques that can be used, reference may be made to
patent application WO 2010/046609.
The examples given below with reference to the
accompanying figures, serve to illustrate the invention,
but they have no limiting character. Figure 1 shows the
results of voltage measurements and Figure 2 shows the
results of peeling force measurements.
EXAMPLES
Multilayer elements of the invention were fabricated
using:
= a reinforcing material constituted by a sheet of
unidirectional carbon fibers (sold by the supplier Hexcel
Corporation under the reference HT40 and presenting a
weight of 150 grams per square meter (g/m2)) bonded on
each of its faces to a web of copolyamide fibers having a
thickness of 118 pm and weighing 6 g/m2 (sold by the
supplier Protechnic, 41, avenue Montaigne, 68700 Cernay,
France, under the reference 1R8D06, at 3 g/m2). Bonding
was performed by heat using the adhesive nature of the
thermoplastic web when hot, and was performed in
compliance with the method described on pages 27 to 30 of
application WO 2010/046609;
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= a support layer constituted by a film of
polyethylene terephthalate (sold under the reference
PEPOLIT 150.8 by the supplier Effegidi International
S.p.A, Via Provinciale per Sacca, 55, 43052 Colorno
(Parma) Italy) having a thickness of 75 micrometers.
Charges generation, and thus obtaining association
with electrostatic force, were performed on samples of
150 mm x 150 mm constituted by such a reinforcing
material superposed with such a support layer.
For this purpose, two unwinders were used:
= one supporting the plastics film; and
= another supporting the reinforcing material.
The two sheets were guided and positioned one onto
the other. It is important to establish contact between
the two sheets as well as possible prior to entering the
zone in which charge is created and thus in which
electrostatic bonding occurs.
Description of procedures
Association by creating electrostatic charge
Use was made of a 0-30000V Fraser 7300P positive
voltage generator (suitable for supplying a voltage that
is adjustable over the range 0 to 30 kV at a current of
1 milliamps (mA)) and having a 7080 static electricity
generator bar with a length of 300 mm (supplier Boussey
Control). That bar
gives off electricity from the
generator in the form of a cloud of ions. The bar
was
positioned 25 mm above the sample. Beneath the sample, a
conductive plate (aluminum angle bar) having a length of
140 mm was positioned and connected to ground, which bar
extended parallel to the electricity generator bar. The
conductive plate was also situated at 25 mm from the
sample, which was thus at equal distances from the
electricity generator bar and the conductive plate. The
length of the conductive plate was selected so as to
avoid projecting beyond the width of the sample, in order
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to avoid creating a preferred flow of ions between the
bar and the plate.
The sample was supported on two very fine nylon
yarns, tensioned using a weight of 700 grams (g), so as
5 to be positioned parallel to the bar and to the
conductive plate. The conductive material could equally
well face the generator bar or the conductive plate.
The voltage selected for the generator was applied
continuously for 10 seconds (s). The bar created a cloud
10 of ions that was picked up by the outside face of the
plastic film (beside the generator bar). On the opposite
face (beside the reinforcement sample), a mirror image of
the charges was formed. The plastics film constituted a
barrier that retained the positive charge and that was
thus attracted by the negative charge of the mirror
image. The film was thus "stuck" against the reinforcing
material by attraction between the positive and negative
charges. Such attraction occurs once the applied voltage
is greater than or equal to 15 kV.
Measuring the residual charge voltage
The residual charge voltage on the sample was
measured using a Fraser 715 static voltage measuring
appliance. The measurements were performed in compliance
with the manufacturer's recommendations, with calibration
remote from a charged source, grounding, and then
pointing orthogonally relative to the sample at a
distance of 100 mm.
Measuring the peeling force
The sample was fastened on a plane support by means
of double-sided adhesive tape in contact with the
reinforcing material. A small rigid bar of width equal
to the width of the film was fastened to one end of the
plastics film in such a manner that the bar was
perpendicular to the direction of the unidirectional
fibers. A beaker was secured to the bar; water was
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poured progressively into the beaker using a pipette
until the film separated from the reinforcing material.
The unit comprising the bar, the beaker, and the water
was then weighed.
Results of measuring surface voltage
In order to perform the test, two series of six
samples were produced, one at 15 kV and the other at
30 kV.
The samples were all produced at the same time and
manipulated only once, so as to be positioned on two
tensioned nylon yarns providing support.
The residual voltages were measured at defined time
intervals.
Regularly, a sample was taken in order to subject it
to a peel strength test. Since that test is destructive,
the number of samples diminished over time.
Figure 1 shows the results of voltage measurements
on the samples averaged for each reading. It can thus be
seen that the size of the population diminished regularly
over time (from six individuals to one individual).
It can thus be seen that the surface voltage drops
considerably in the first few minutes after charging,
regardless of whether the applied voltage was 15 kV or
30 kV. Thereafter, the voltage stabilized asymptotically
around a value close to 0.3 kV, with this applying for
both initial charge values.
Results of surface voltage measurements
Certain measurements were performed on samples that
had aged for several hours, whereas others were taken a
few minutes after the charge generation step.
Figure 2 shows the various measurements taken: all
of the unfilled-in marks were measured immediately after
the charge generation step.
The initial charge voltage appears to have no
influence on peeling performance. A given residual
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surface voltage may correspond to various initial charge
voltages, given that surface voltage decreases and then
stabilizes over time. It is
therefore possible to
question the pertinence of measuring residual voltage in
the first few minutes after charging. The result
is
subjected to variations that are too great In that time
interval.
In conclusion, it can be seen that:
= the residual surface voltage decreases quickly in
the first few minutes after charging and stabilizes at a
low level, with this applying regardless of the initial
charge;
= the voltage of the initial charge, providing it is
at least 15 kV, turns out to have no influence on the
peeling performance, providing the peeling test is
performed several tens of minutes after charging; and
in the tests performed, the measured peeling force
was on average equal to 11 g (i.e. 107.9 mN) 30%.
Comparable results have been obtained with other
types of plastics film as the support layer, and in
particular with a polyester film of trademark Airtech
(reference: WL3800) having a thickness of 50 um.
