Note: Descriptions are shown in the official language in which they were submitted.
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Process for obtaining a composite material naving controlled electro-
magnetic properties and the material obtained.
DRSCRIPTION.
The present invention relates to a process for obtaining a composite
material having controlled electromagnetic properties, in which
the fibres are filled prior to their weaving for cons~ituting the
fibrous reinforcement of the sought composite material.
A composite material is constituted by a fibrous reinforcement and
a binder ensuring the rigidity of the material. The reinforcement
is essentially obtained from very strong textile fibres such as
fibres of glass, silica, carbon, silicon carbide, alumina, alumino-
silicate, polyamide and other fibres having in combination the elem-
ents Si, N and C. The binder can be an organic resin, a metal or
a refractory product.
The fibres of the reinforcement can be oriented in two or more direc-
tions in space. They can be oriented in a random manner (D risk~,
in a manner organized in two directions {2D) or three directions
(3D or 3D evo).
These composite material can be used in numerous industrial fields
and in particular in the space, aeronautical and nautical fields
requiring the production of lightweight mechanical parts.
The invention applies to so-called woYen 2D reinforcements produced
on machines known as "looms"~ which have emanated from the textile
industry and which have been adapted to the needs of composi~e mate-
rials. For example, laminates belong to this category. The fibrescan be woven in 2,3 or more directions in the same plane. The inven-
tion more particularly applies to the fibrous reinforcement described
in FR-A-2 610 951 and which is referred to as 2.5D.
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The invention also applies to so-called 3D reinforcements constit-
uted by an arrangement of threads in three directions in space
giving parts with all shapes (bloc~, cylinder, cone, more complex
reinforcement) and also using a weaving method.
S For example, the invention applies to the three-dimensional struc-
tures described in FR-A-2 612 950 and FR-A-2 486 0~7.
Contrary to the situation in other fields, the aim is to adapt
composite materials to the final applications in a very precise
manner. At present, technologies make it possible to inter alia
orient the reinforcing fibres in the directions of the stresses
to which these materials are exposed and to give them excellent
mechanical properties.
Moreover, the diversity of uses of these materials obliges manu-
facturers to control other properties such as physicochemical prop-
erties (heat resistance, o~idation resistance) and also electro-
magnetical properties (antistatic material, electromagnetic shield-
ing of comple~ electronic circuits, microwave absorbers).
The invention more particularly applies to obtaining a composite
material having a controlled electrical conductivity and/or magnetic
~ 20 permeability.
:~ In these composite materials, the binder can be deposited in the
fibrous reinforcement either by the gaseous route, or by the liquid
route.
In the case of the gaseous route, the reinforcement is placed in
an enclosure at ~i~ed temperature and pressure and is subjec~ to
a gaseous ~low, whose molecules decompose in contact with the fib-
res, said process being called chemical vapour infiltration (CVI).
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In the liquid route a liquid impregnating agent is made to penetrate
the reinforcement which, by a subsequent treatment, is transformed
into a composite material ha~ing the requisite mechanical character-
istics.
At present two groups of processes for introducing charges or fil-
lers into a composite material are known. The first group consists
of introducing the fillers into the fibrous s~ructure during the
rigidification of the reinforcement by the binder or just prior
to said rigidification and the second group consists of the fillers
being carried by the fibres prior to the production of the reinforc-
ement.
According to the first group, the introduction of the charges can
take place by gaseous route infiltration into the reinforcement~
- The principle is the same as that described hereinbefore for putting
the binder into place in the reinforcement. Following CVI deposi-
tion o the fillers in the fibrous reinforcement, the thus filled
reinforcement is densified by the liquid route.
The major disadvantage of this process is that the fillers are
only deposited on the preferred chemical sites, which is contrary
to the obtaining of a homogeneously filled material. Moreoqer,
this process cannot be applied to all types of fibres. Thus, the
deposition temperature of the fillers by W I mu~t be below that
of the fibre deterioration temperature.
Another procedure for infiltrating fillers into ~he reinforcement
consists of impregnating the latter with a liquid resin to which
the fillers have been added. This process described e.g. in
EP-A-0 307 968 suffers from several disadvantages.
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The quantity of fillers in the resin conditions the viscosity of
the impregnating resin. Thus, the more filled or charged with
powder the resin, the more it is viscous and the more difficult its
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introduction into the reinforcement. Moreover, in the case of
a large filler grain size, the final material is not homogeneous
and has at the surface a high filler level, which tends to decrease
on approaching the centre of the part.
The infiltration of the fillers into the reinforcement can also
be brought about by a filter press, as described in EP-A- 130 105.
In this case, the reinforce~ent is placed in an enclosure, in which
the fillers flow suspended in a solvent. The reinforcement, placed
in front of the filter, is traversed by said solution before depos-
iting the fillers thereon.
The disadvantage of this procedure is that it is very difficultto control the q~antity of fillers deposited in the reinforcement.
Moreover, the powder grain size is dependent on the size of the
cavities between the fibres of the reinforcement and conditions
the infiltration quality.
These filler infiltration processes lead to composite materials,
whose physicochemical and/or electromagnetic properties do not
always correspond to those which are sought.
According to the second gro~p which consists of the fillers being
carried by the fibres of the reinforcement prior to the production
thereof, reference can be made to that described in FR-A-2 566
324, which teaches the production of a fibre-metal or fibre-mineral
prepreg for producing a high performance, composite material object.
The strands of preimpregnated fibres are covered with a sheath
permitting the retention of the metallic or mineral powder and
can then be woven.
FR-~-2 562 467 describes a flexible composite ma~erial constituted
by a thermoplastic sheath con~aining a strand of fibres covered
with a thermoplastic powder.
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The disadvantage of these methods based on the fibres carrying
fillers is that the protect~ve sheath around the thread increases
the apparent cross-section thereof. This sheath must be eliminated
by a subsequent treatment, which makes the production o composite
materials somewhat more difficult. Moreover, this leads to an
excessive porosity of the materials obtained and to a non-optimum
fibre filling, which is prejudicial to a good mechanical behaYiour.
The principle of inserting fillers on a fibre is also known in
the case of wound composite materials, i.e. produced by filamentary
winding onto a mandrel. This method consists of impregnating the
basic thread with a solution containing fillers, a liquid resin
serYing as the binder and a solvent.
The special feature of this impregnation is that it is necessary
to deposit on the fibre the entire resin (binder) quantity necessary
for the final material during this stage.
Following the deposition of the solution on the basic thread, the
latter traverses a spinneret or die making it possible to retain
a clearly defined solution quantity and then flows into an oven
in order to transform the resin from the liquid state to the gelled
state and eliminate the solvent. The thread obtained is called
a prepreg or filled prepreg and is sticky on contact wi~h the hand.
This thread cannot be woven. The sticky thread is then deposited
; on a mandrel by winding in successive layers.
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The finished part is obtained after placing in an oven with a view
to rigidifying or hardening the resin by polymerization. This
; final operation ca~ be followed by pyrolysis transforming the resin
into carbon. It is clear that the composite material obtained
under these conditions with layer-type fibre winding is consequently
subject to a delamination risk.
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The invention relates to a process for obtaining a composite mate-
rial having controlled electromagnetic properties in which the
fibres are previously charged or filled making it possible to
obviate the disadvantages referred to hereinbefore. It makes it
possible to increase the homogeneity of the com?osi~e material,
increase the weavability of the fibres facilitating the production
of the reinforcement, as well as an excellent control of the elect-
romagnetic and/or physicochemical properties of the material obtai-
ned.
More specifically, the inYention relates to a process for obtaining
a composite material having controlled electromagnetic properties
and with a fibre reinforcement, essentially comprising:
a) - impregnating each fibre with a solution containing pulverulent
fillers having electromagnetic properties, a solvent and a first
binder soluble in the solvent,
b) - evaporating the solvent from the filled fibres,
c) ~ weaving the filled fibres obtained in b) in order to form
the reinforcement and
d) - rigidifying the reinforcement obtained in c) by a second binder.
The term fillers having electromagne~ic properties is understood
to mean fillers ha~ing magnetic, conducting or semiconducting prop-
erties.
The process according to the invention makes it possible to give
the reinforcement of ~he composite material special properties
without modifying the basic mechanical characteristics of the mater-
ial. In particular, it makes it possible to place electromagnetic
fillers in a very thick material in a perfectly homogeneous manner,
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both in the core of the material and on the surface, no matter
- what the shape i.e. complex or simple of the fibrous reinforcement.
As a function of the envisaged application of the filled composite
material, it is possible to use fillers having magnetic properties
such as iron, ferrite, nickel or cobalt powder, filler~ having
electricity conducting properties such as platinum, silver, copper,
nickel or carbon powder, or fillers having semiconducting properties
such as silicon, germaniu~ or silicon carbide properties.
The fillers are in the form of a powder or a mixture of powders
having a grain size of approximately 1 micrometre or a submicronic
grain size. The fibres of the reinforcement are in particular
those described hereinbefore.
The process of the invention more particularly makes it possible
to ob$ain a woven, rigidified reinforcemen~ in which ~he electrical
conduction is controlled in a ~ery precise manner. Thus, starting
with a basic fibre behaving electrically like a dielectric (or
insulant), it is possible ~o introduce conductive charges or fillers
onto it in a very precise quantity in order to make the final ma~er-
ial assume a state intermedia~e between a conducti~e and a non-
conductive material.
This very considerable con~rol of the electrical conductivity makesit possible to envisage a varia~ion thereof as a func~ion of the
material thickness. Thus, independently of the homogeneitY of
the material, it is possible by means of the present invention
to produce a material having a controlled conductivity, which is
dependent on the thickness.
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The reinforcement is then constituted by layers of superimposed
threads or fibres having different filler levels between the indi-
vidual layers and/or fillers having different conductivities.
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Thus, it is possible to produce a conductivity gradient in the
thickness of the material and e.g. produce a ma~eri~l, whose condu-
- ctivity decreases progressively from its centre to its surface.
Thus, the invention is more particularly applied to fibres which
do not conduct electricity, such as glass, silica, alumina, silicon
carbide, aluminosilicate or polyamide fibres. In addition, the
invention more particularly applies ~o a dielectric ri8idification
binder.
The solution containing the fillers, the solvents and the first
binder is ~laced in and on each fibre forming a smooth, sliding
sheath around the sa~e and serving in part as a size. This makes
it possible to significalltly reduce the friction coefficient of
~he fibre with the ~urrounding elements and thus improve its weav-
ability.
However, no matter whether the prepreg, which may or may not be
- filled, used for the filamentary winding is not weavable as a result
of the stickiness due to the gelled state of the resin during the
elimination of the solvent.
The fibre generally used in composite materials is constituted
by parallel fibrils. The cross-section of each of them does not,
according to the prior art, give the flbre a maximum composite
filling level. Moreover, by using the process according to the
invention, it i~ possible to fill the cavities between the fibrils
by fillers and thus significantly increase the material filling
level without changin8 the cross-section of the fibre.
Following the weaving of the fibres, it is necessary to rigidify
the reinforce~ent by a binder. In the case of thermostructural
composites of the ceramic-vitroceramic, ceramic-ceramic and carbon-
ceramic type, said rlgidification stage can be long and costly.
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In addition, by depositing the fillers on the fibre before weaving
the reinforcement, there is a reduction in the densification time
and therefore in the production costs of the composite material
parts. This densification time reduction is considerable in the
case of densification by CVI, the time gain being 20 to 60%.
Thus, the reinforcement according to the invention, prior to densi-
fication, has a larger quantity of products than a reinforcement
without a filler. Therefore, in order to terminate densification,
less rigidification binder has to be deposited.
This time gain is due to a reduction in the porosity of the fibrous
structure and more particularly a modification of the architecture
of the gaps between the fibres. The fillers deposited on the fibres
create bridges, which aid the grafting of the matrix.
The binder of the solution can be a liquid organic resin or a mix-
ture of liquid organic resins polymerizable by ionizing radiation
or thermally. They can be thermosetting or thermoplastic. These
resins can be silicone, phenolic, epoxy, metha(acrylic), vinyl
and similar resins and in more general terms resins having ethylene
unsaturation.
When the binder of the solution containing the fillers is a therm-
ally polymerizable resin, it is necessary to evaporate the solvent
from the solution at a temperature below the gel point of the binder
in the solvent (the gel point of a binder in solution e~ceeds that
of the binder alone). It is also preferable to use as the solvent,
solven~s having a low vapour tension and a low boiling point.
Said solvent generally has a boiling point at the most equal to
100C and a vapour tension exceeding 92 kPa at 20C.
For e~ample, it is possible to use a lower alcoho~ such as ethanol,
n-propanol, isopropanol, n-butanol or lower halogenated alkanes
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such as dichloromethane, chloroform, dichloroethane or acetone,
methyl ethyl ketone, ethyl acetate and tetrahydrofuran.
As a function of the envisaged application, the binder of the solu-
tion containing the fillers and the rigidification binder of the
fibrous reinforcement can be of the same type, i.e. of the same
composition, or of a different type, provided that they are chemi-
cally compatible.
The rigidification binder can be an oxide (borosilicate or alumino-
silicate) glass, a vitroceramic (lithium aluminosilicate) a refrac-
tory product (a dielectric ceramic, more particular]y silica, alum-
ina, boron nitride or silicon nitride) or a dielectric organic
resin. The resin is more particularly one of those referred to
hereinbefore.
Moreover, the fillers can be of the same or different types compared
with that of the fibres. They can also be of the same or different
type to that of the binder used for the rigidification.
In order to only retain a precise solution quantity, there is advan-
tageously a calibration or sizing stage. This stage consists of
making the fibre covered with the solution containing the fillers
; 20 pass through a spinneret. The fibre impregnation level is below
5% and in particular approximately 3~.
In case of need, it is possible to carry out a complementary heat
treatment following the evaporation of the solvent with a view
to stabilizing the state of the binder and ensure the maintenance
of the fillers on thç fibre. The stabilization temperature must
be between the gel point of the binder and the polymerization temp-
erature of said binder in order to permit good weavability.
The invention is described in greater detail hereinafter relative
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to non-limitative examples and with reference to the attached draw-
ings, wherein show:
Fig. 1 diagrammatically the different stages of the process accord-
in8 to the invention.
Fig. 2 the fibre filling stage according to the invention.
Fig. 3 a composite material having an electrical conductivity grad-
ient according to the invention.
With reference to figs. 1 and 2, the first stage 10 of the process
according to the invention consists of depositing conductive, semi-
conducting or magnetic pulverulent fillers on each dielectric fibre12 for forming the composite material reinforcement. For this
purpose impregnation takes place of each fibre 12 with a solution
14 containing pulverulent fillers, a polymerizable liquid organic
binder and an organic solvent at a boiling point below 100C and
a vapour tension above 92 kPa at 20C. This impregnation takes
place by the transfer to the fibre 12 of the solution 14 contained
in a tank 15.
By weight, the solution contains 5 to 13% fillers, 9 to 17% binder
and 78 to 86% solvent. The powder has a grain size equal to or
~ 20 below 1 micrometre.
; After depositing the solution on the fibre, the la~ter passes thro-
ugh a spinner~t 18 enabling it to only retain an impregnation level
of approximately 3%. This calibration stage i9 symbolized at 20
in fig. 1.
After evapora~ing ~he solvent in an oven 24 ~fig. 2) at at the
most 100C, a fibre i9 obtained which is constituted by its original
material, the binder and the filler~. This i9 referred to as a
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filled fibre. A heat treatment at the gel point of the binder
makes it possible to stabilize the latter and maintain the fillers
on the fibre. This s~age is symbolized at 30 in fig. 1. ~le fibre
can be a glass, silica, aramide, silicon carbide or alumina fibre.
This is followed by a weaYing 40 according to a known process of
the filled fibres in order to form a reinforcement of filled fibres
of type 2D, 2.5D or 3D, as described hereinbefore.
This is followed by a rigidification or densification 50 using
the liquid or gaseous route of the woven reinforcement, according
to the prior art, using a dielectric binder. By the liquid route,
the filled fibrous reinforcement is impregnated in ~acuo e.g. by
a dielectric resin, which is polymerized and then crosslinked
hot. These stages are performed seYeral times (generally 5).
In the case of the gaseous route the densification can be carried
out in the manner described in FR-A-2 611 198 and FR-A-2 643 898.
The final stage consists of machining 60 the part obtained.
~ E~am~le 1: Impregnation of the so-called "filled" thread in order
- to obtain a controlled conducti~ity composite material.
Carbon fillers are introduced onto silica fibres 12 by impregnating
each of them with the aid of a solution 14 containing carbon fillers
in th form of a submicronic grain si~e powder, the quantity of
fillers contained in the solution being proportional to the desired
conductivity and is chosen in the range 15 to 35 g, 240g of iso-
propanol and 29g of phenolic resin.
The basic thread is unwound from its reel 16 and impregnated in
the impregnation tank 15 containing ~he solution 14. After impre-
gnating the fibre 12 in the solution, it passes through a sizing
or calibrating spinnere~ or die 18 enabling it to retain a clearly
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defined solution quantity.
The impregnation level is appro~imately 3%, which makes it possible
to obtain a material having a controlled electrical conductivity.
In addition, this level is equivalent to that of a size.
The so-called filled thread is wound onto a support 22 after passing
through the oven 24 in order to eliminate the solvent~ The oven
temperature is appro~imately 100C.
The thread then undergoes thermal stabilization at 95 + 5C in order
to block the evolution of the phenolic resin and avoid any risk
of dilution of the mixture during subsequent treatments.
:,
This is followed by a weaving according to a 3D configuration and
the structure is then rigidified by a phenolic resin. This rigid-
ification consists of an impregnation of the structure by resin
followed by polymerization at 180C. The material is optionally
machined. I~ has a homogeneous filler distribution and therefore
a perfectly controlled electrical conduc~iYity.
~` Example 2: Producing a laminate.
. ~
In order to create an electrical conductivity gradient in the thick-
ness of a composite material, a reinforcement is produced in the
manner shown in figo 3 and which is constituted by a stack 1 of
fabrics 2,3,4,5 and 6 differing from one another by their filler
level. ~ach fabric is produced according to the 2D reinforcement
weaving process and uses a glass fibre sized according to the proc-
ess described in Example 1. These different fabrics are stacked
and impregnated with the epoxy resin which is hardened, thus forming
a laminate.
Thu9, the final reinforcement has a thickness-variable conduction,
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which is controlled as from the starting of the production process
and not as in the case when the fillers are deposited following
the production of the reinforcement.
It is in particular possible to use five different fibre types
having a different carbon filler level and gradually varying from
0.5 to 2.5% by volume. This corresponds to a conductivity respect-
ively varying from 2 to 20 ohms lm 1. It is then possible to form
a laminate 1, whose conductivity e.8. decreases from the upper
fabric 2 to the lower fabric 6.
E~amPle 3
This example differs from Example 1 by the use of a solution cont-
aining a polymerizable epoxy resin and methyl ethyl ketone in place
of the phenolic resin and ~he isopropanol.
Exam~e 4
This example differs from Example 2 by the use of a phenolic resin
in place of the epo~y resin.
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