CA1279033C

CA1279033C - Producing composite materials from high voltage electrostatically charged fibres by impregnation

Info

Publication number: CA1279033C
Application number: CA000491615A
Authority: CA
Inventors: Michel Berger
Original assignee: Pradom Ltd
Current assignee: MATERIALS TECHNICS
Priority date: 1984-09-26
Filing date: 1985-09-26
Publication date: 1991-01-15
Anticipated expiration: 2008-01-15
Also published as: ATE39079T1; JPS6184210A; PT81185B; AU578740B2; FR2570646B1; FR2610922B2; ZA857143B; FR2611086B2; FR2570646A1; FR2609934B2; FR2609934A2; DK162334B; DK434585D0; AU4791985A; FR2610922A2; DE3566632D1; IE852335L; BR8504704A; EP0179688A1; EP0179688B1

Abstract

Canada APPLICANT: MICHEL BERGER

INVENTOR: MICHEL BERGER

TITLE: PROCESS FOR PREPARING COMPOSITE
MATERIALS AND PRODUCTS OBTAINED WITH
SAID PROCESS

ABSTRACT OF THE DISCLOSURE

The present invention relates to a process for the preparation of a composite material wherein the element used to reinforce the composite material is subjected to an electrostatic field induced by a high voltage electric current, said element being then impregnated with a liquid matrix material or precursor of matrix, while still under the influence of said field, and to the composite materials obtained by carrying out said process.

Description

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The present lnvention relates to a process for preparing composite materials; it also relates to the intermediate or finished products which can be obtained with sa$d process.

Composite materials are materials comprised of reinforcing elements (mostly fibers - or filaments - such as glass fibers, carbon fibers, boron or polyamide fibers, etc...) and of a matrix (constituted either by a resin or a resistant material such as metal or ceramics).

The properties of composite materials are particularly dependent, as we know, on:
- the orientation of the reinforcing elements:
~ - the good distribution of the matrix throughout the volume ; between the reinforcing elements:
- and of any bonds which may be induced between said reinforcing elements and said matrix.

It is therefore an advantage to use a technique wherein the above parameters can be worked in such a way as to optimize the properties of the product as a function of the aim in view, and this is precisely the ob~ect of the present invention.
~, According to one of its aspects, the present invention provides a process for the preparation of a composite material that includes fibers used to reinforce the composite material comprising the steps of:
~ sub~ecting the fibers to an electrostatic field induced ;~ by a high-voltage alternating electric current for a time interval sufficient to cause a modification of the surface of the , ~ ; fibers; and .~ :
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la impregnating the fibers after the elapse of the time interval with a llquid matrix material or precursor of matrix, while the $ibers are electrostatically charged.

According to another of its aspects, the present invention provides a process for the preparation of a composite material that includes fibers used to reinforce the composite material comprising the steps of:
subjecting the fibers to a first electrostatic field that is induced by a first high-voltage electric current for a first time interval:
sub;ecting the fibers to a second electrostatic field different from the first electrostatic field that is induced by a second h~gh-voltage electric current for a second time interval after sub~ecting the fibers to the first electrostatic field; one of the first and second electrostatic fields being induced by a high-voltage alternating electric current for a time interval sufficient to cause a modification of the surface of the fibers.

Thus, it has been found that the reinforcing elements (i.e. the fibers) may be advantageously sub~ected to an electrostatic field induced by a very high voltage current, and then impregnated with the liquid matrix, using the known techniques, while the elements are electrostatically charged.
Thus, the present process encompasses (i) the case where the electrostatic field has been stopped or removed but the fibers are still charged, and (ii) the case where the electrostatic field is maintained during impregnation.

By high voltage current-induced electrostatic field is meant a field at least equal to the filed obtained by applying between two electrodes 20 mm apart, a voltage equal to at least 20,000 volts in alternating current and to at least 40,000 volts in direct current. The reinforcing elements, and in particular the fibers, fibrils or roves used, are then positioned . ..
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between the electrodes subjected to the very high voltage current.
According to the invention, any type of fibers can be used as reinforcing elements, but they must be in a dielectric material, namely a material which, when under the effect of the field, becomes electrically charged and remains charged for a certain time. This is the case for example with polyami~e fibers (of NYLON or KEVLAR type), glassfibers, fibers in ce~rtain metallic oxides, fibers in complex materials (metaloxide) and with carbon fibers. On the contrary, conducting fibers, such as for example metallic fibers or surface-metallized fibers are-more aifficult to use in the process according to the invention.
The reinforcing fibers are placed between the electrodes, and the very high voltage current is applied between said electrodes for a period long enough to charge said fibers, then, the charged fibers, taken out of the field, are impregnated with the matrix material or with a precursor of the matrix material, which is in liquid form.
The charged fibers having a tendency to push one another back, a bed of fibers is obtained at the output of the field, of which the thickness is - ;25 between two and four times the thickness of the bed of ~-fibers initially introduced between the electrodes, and `~;it is when the fibers are in that "swollen" state that they should be impregnated.
Any one ofthe currently known and used matrix materials is suitable for the process according to the invention, for example resins ~epoxy or polyamide resins or hardened carbon mixtures) or silica-based mixtures capable of forming ceramics, and metals.
When the fibers have been impregnated - ~35 by the liquid matrix material (or its liquid precursor), ,~the resulting product can either be sold as is (normally ,"~
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It has been found that with the process according to the invention, the reinforcing elements (fibers) become thoroughly impregnated by the matrix.
sut it is also possible to bring to the process according to the invention certain particular-ly advantageous alterations.
If the electrostatic field is produced with a direct current, it is noted that, besides the swelling action of the bundle of initial fibers, there occurs a complementary orientation of said fibers.
This orientation will permit the preparation of a compo-site material having specific properties.
It is àlso possible, as we know, to ob-tain that same orientation for certain fibers, by the slmultaneous or prlor use of another field such as for example a magnetic field.
If the electrostatic field is produced with an alternating current, it is noted that besides the swelling action of the bundle of fibers described hereinabove, localized discharges occur between the ~ ~ fibrils, causing, principally in the presence of oxygen, ~ - 25 a modification of the surface of the fibers. This modification (which is probably an oxidation), stimulates the properties of the final material insofar as it makes it possible to obtain consolidated bonding between the fiber and the matrix.
~ It is conceivably possible, according to the invention, to use successively an A.C. electro-static field (swelling and surface treatment) and a D.C.
electrostatic field (swelling and orientation).
The invention will be more readily 35~ understood on reading the following description of a non-:, ., :. , , ~ ~
~ restrictlve xample, with reference to the accompanying Figures 1 to 9, in which:
Figure 1 represents a schematic view of anelectrostatic field inducing apparatus through whi~h the reinforcing element bundle is charged;
Figure 2 represents a fragmentary view of a bundle of elements prior to electrostatic char~e;
Figure 3 represents a fragmentary view similar to Figure 2 but showing the bundle after electrostatic charge;
Figure 4 represents a microscopic fragmentary view of a fibril prior to treatment;
Figure 5 represents a microscopic fragmentary view of a fibril after treatment;
Figure 6 illustrates a disorderly fibril bundle prior to treatment;
Figure 7 illustrates the bundle shown in Figure 6 but after treatment;
Figure 8 illustrates another form of fibril bundle prior to treatment; anà
Figure 9 represents a schematic view of means for treating the bundle shown in Figure 8 with a high voltage field.
Referring first to Figure 1, this shows a casing in insulating material 1 resting on insulating support members 2, and containing, in position between wedge members 3 and resting on an insulating base 4 : a first plate-shaped lower electrode 5, a first dielectric 6, a gap 7, a second dielectric 8 and a second, equally plate-shaped electrode 9. The fibrous bundle 10 is placed between the two dielectrics. The two electrodes 5 and 9 are connected to a generator of direct current of voltage about 100,000 volts. The assembly is charged for about 10 mins. for fibrils of between 5 and 6 mm thickness. Figure 2 shows the bundle before being charged, and Figure 3 shows the bundle after a 10-minute charging treatment.
It is found after successive experiments that the volume has virtually doubled, hence, doubling the , .
volume between the fibrils, the actual volume of the fibrils remaining unchanged.
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Figure 4 shows a microscopic view of a fibril before the treatment, and Figure S shows the same fibril as ground after the treatment.
From a practical standpoint, it has been found that the fact of subjecting the whole bundle of fibrils to a first A.C. field in order to obtain a more efficient etching with alternating current, and then subjecting it to a D.C. field in order to create an expansion, greatly con~ributes to obtaining a ground, expanded and tidy bundle. Indeed, a third effect noted is that a rather disorderly bundle, such as illustrated in Figure 6, becomes perfectly orderly after a treatment in a high voltage D.C.electrostatic field, as illustrated in Figure 7.
Another application, this time using A.C.
voltage, consists in injecting short fibers between the ` two electrodes, as illustrated in Figure`8;-and subjècting them to a high voltage A.C. field, as illustrated in Figure 9. It is found then that a bundle of short fibers is obtained in which the fibers are arranged somewhat random~y but homogeneously, which is very a~dvantageous in the case of short fiber composites, since sequencing always gives breaking points, hence weak points.
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Claims

1. Process for the preparation of a composite material that includes fibers used to reinforce the composite material comprising the steps of:

subjecting the fibers to an electrostatic field induced by a high-voltage alternating electric current for a time interval sufficient to cause a modification of the surface of said fibers; and impregnating said fibers after the elapse of said time interval with a liquid matrix material or precursor of matrix, while the fibers are electrostatically charged.

2. A process for the preparation of a composite material that includes fibers used to reinforce the composite material comprising the steps of:

subjecting the fibers to a first electrostatic field that is induced by a first high-voltage electric current for a first time interval;

subjecting the fibers to a second eleotrostatic field different from said first electrostatic field that is induced by a second high-voltage electric current for a second time interval after subjecting the fibers to said first electrostatic field;
one of said first and second electrostatic fields being induced by a high-voltage alternating electric current for a time interval sufficient to cause a modification of the surface of said fibers; and impregnating said fibers with a liquid matrix or precursor of matrix, after the elapse of said two time intervals, while the fibers are electrostatically charged.

3. The process as claimed in claim 2, wherein the other of said first and second fields is induced by a D.C. electric current.

4. The process as claimed in claim 3, wherein said D.C.
electric current is at a voltage equal to at least 20,000 volts.

5. The process as claimed in claim 3, wherein said fibers are a dielectric material selected from the group consisting of polyamide fibers, glass fibers, metallic oxide fibers, carbon fibers, and combinations thereof.

6. Composite materials obtained by carrying out the process as claim 1.