CA2074489A1 - Method of manufacturing a composite material - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method of manufacturing a composite material comprising an intimate and homogeneous mixture of at least two components, the method being characterized in that said mixture is caused to vibrate in a receptacle at a frequency close to the mean of the resonant frequencies of each of the components or of the receptacle so as to degas the components, after which the components are bonded together so that the resulting composite material has a structure that is substantially free from included air, is compact, and is dense.

Description

A METHOD OF MANUFACTURING A COMPOSITE MATERIAL
The present invention relates to a method of manufacturing a composite material.
In general, the preparation of a composite material S requires at least two components that may be of different forms to be mixed together.
When it is desired to make a material comprising reinforcing components such as fibers and a matrix component such as a liquid resin, it is necessary to impregnate the fibers with the liquid resin. This operation is always difficult because of occlusions of air within the fibers impeding penetration of the resin. Such impregnation also becomes that much more difficult when the material includes a plurality of layers, wefts, or sheets of fibers in the form of draping. If impregnation is not complete, the resulting material does not have the compact and dense structure that is required for some utilizations. The smaller the amount of occlusions of air, the better the general impermeability and mechanical strength of the material.
~R-A-2 516 4~1 describes a composite material comprising at least one reinforcing component impregnated with at least one matrix component, which components are bonded together in such a manner that the assembly is substantially free from any inclusion of air.
The technical problem of degassing is that an important obstacle in making a composite material of very high quality.
Another problem lies in the statistical orientation and distribution of the filler materials in a liquid matrix.
The final qualities of the composite material depend closely on the arrangement in three dimensions of the various components. Thus, for example, blending short fibers in a liquid resin is a fundamental operation in obtaining a composite material that is strong.
Another problem lies in the reactional instability of certain components which react violently when subjected to movements that are too sudden or when brought into contact with other components, and therefore cannot be mixed or combined using traditional techniques.

An object of the present invention is to solve these technical problems in satisfactory manner.
This object is achieved by means of a method of manufacturing a composite material comprising an intimate and homogeneous mixture of at least two components, the method being characterized in that said mixture is caused to vibrate in a receptacle at a frequency close to the mean of the resonant frequencies of each of the components or of the receptacle so as to degas the components, after which the components are bonded together so that the resulting composite material has a structure that is substantially free from included air, is compact, and is dense.
According to another characteristic of the invention, the frequency and amplitude of the vibration to be used are determined by previously recording and analyzing the mechanical behavior of each of the components and of the receptacle taken in isolatlon in response to a vibration to which they are subjected and whose frequency is varied continuously so as to select the frequency that leads to resonance.
In an implementation of the invention, at least one re-inforcing component and at least one matrix component are used.
In a particular implementation of the invention, a polymerizable matrix component is used, and bonding between the two components is obtained by polymerization.
In another implementation, a thermoplastic matrix component is used, and bonding between the reinforcing components and the matrix components is achieved by heating to a temperature that is high enough to melt the matrix components.
Because the invention makes it possible to mix powders homogeneously regardless of their density or grain size and without weakening one of the components, the invention makes it possible to manufacture thermoplastic foams.
These foams are obtained by mixing fillers (microbeads of glass, carbon, phenolic material, acrilonitrile, acrylic material, polyvinyl chloride, ...) and a thermoplastic matrix in powder form such as polypropylene, polyamide, copolyamide, . . . ) .

The mixture is then heated to the melting temperature of the matrix, and this is done under pressure.
The relative density of the final product may lie in the range 0.2 to 0.6 without limit on size.
The invention thus makes it possible to mix powders of any kind with the same advantages as a fluidized bed, i.e. each particle travels the same distance. In addition, they all have the same contact with the walls of the receptacle. Electro-static charge can thus be transferred from the walls of the receptacle to the powders, thereby making it possible to invert charge, if necessary. It is thus possible to mix together substances having different electrostatic charges.
In variant implementations, said vibration is transmitted to the mixture via the matrix components or via the reinforcing components or else via the receptacles.
The invention also provides a composite material comprising at least one reinforcing component and at least one matrix component, which components are bonded together, the material being characterized in that the reinforcing component is embedded in the matrix component by being impregnated therewith so that the assembly is substantially free from any inclusion of air, is compact, and is dense.
The composite material of the invention is intended, in particular, for use in ship building, in building aircraft or spacecraft, and in making tanks, casings, cabins, or fairings in all types of industry.
The method of the invention makes it possible to obtain bubble removal, wetting, homogenization, impregnation, mixing or segregation, and/or a modification of the interface between the various components, depending on circumstances. As a result the reinforcing component is embedded in the matrix component in such a manner that the mixture is substantially free from included air, is compact, and is dense.
Cbnsequently, the mechanical properties of the resulting solid material are improved thereby.
The present invention will be better understood on reading the following description accompanied by Photographs 1 to 27 performed using an electron microscope.

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~,' 4 207~489 I - with material made by draping, i.e. having a plurality of plies of fabric made of oriented fibers (roving) or of randomly-distributed fibers (mats)~ or indeed tufts disposed in strata, vibrations are generated which cause the llquid matrix (resin) previously deposited beneath the various plies to rise towards the free surface of the draping.
The vibration thus serves to degas the fibers and to superpose them in the bottom of the receptacle or mold containing them before they are bonded together by polymerization of the matrix. Suction applied to the free surface of the draping establishes a vacuum sufficient for removing the degassed air.
The vibration of the draping thus makes it possible to impregnate fabrics that have the reputation of being difficult to impregnate, such as very thick fabrics, multidirectional fabrics, or knitted structures which also have the reputation of being very difficult to impregnate between warp and weft.
For reinforcing components constituted, for example, by glass or carbon fibers and for matrix components such as a polyester resin or an epoxy resin, the vibration frequency is close to the mean of the resonant frequencies of each of the components, and for example it is 65 Hz + 25 Hz. The amplitude to be adopted is a function of the weight of the fiber fabric.
II - The method of the invention is also applicable for the purpose of obtaining a composite material in which the reinforcing components are constituted by short fibers. The short fibers are, for example, mixed with matrices that are thermoplastic or thermosetting.
Under such circumstances, the vibration serves not only to impregnate the fibers with the liquid matrix, but also to blend them or deflocculate them, thus obtaining uniform orientation and distribution of the fibers throughout the volume of the matrix.
By way of example, the vibration frequency for deflocculating very short cellulose fibers in an aqueous matrix is advantageously 180 Hz + 15 Hz which corresponds substantially to the mean of the resonant frequencies of the various components.

For short fibers of glass or of carbon in resins of modifled polyurethanes or epoxy, the vibration frequency is 60 Hz + lO Hz and the amplitude is about 3 mm.
III - When the matrix components are in solid form, as for powder matrices, and when the reinforcing components are fibers, vibration causes transverse expansion and separation of the fibrils constituting the lengths of fiber, thereby enabling the grains of powder to be received in the spaces opened out in this way. Dry, preimpregnated structures are thus provided.
When a thermoplastic powder is used, bonding between the fibers and the thermoplastic matrix component is performed after preimpregnation by heating the structure to a temperature that is high enough to cause the grains of thermoplastic powder to melt.
By way of example, the vibration frequency used to impregnate long carbon fibers with a thermoplastic powder is advantageously llO Hz + 15 Hz, and its amplitude is matched to the grain size of the powder.
IV - The method of the invention is also applicable to impregnating filler materials such as microbeads of inorganic material (silica~ sand, glass, carbon, ...~, microbeads of organic material (acrylic material, polyvinyl chloride, acrilo-nitrile, phenolic material, polystyrene, ...)~ ceramic fibers or crystals, silicon nitride or carbon fibers, or macrobeads (any thermoplastic, glass, and maize cobs ...).
As an example: the manufacture of syntactic foams is described. Syntactic foams are compression materials of very low relative density (0.3 to 0.6) made by mixing hollow glass microbeads (see Photo 13) having a grain size in the range 10 ~m to 150 ~m with a lightweight matrix ~epoxy or polyester resin). Mechanical kneading or conventional mixing generally breaks some of the beads which have a wall thickness of about 0.5 microns to 2 microns, thereby increasing inclusions of air.
The various existing non-vibrated foams suffer from numerous random defects, which can be seen in Photos 14 to 18 in which numerous cavities appear that are not filled with smallest diameter microbeads.

6 2074~9 1st method: High technology foam A frequency is sought and then locked onto that serves to expand the space taken up by the microbeads. The resonant frequency of the microbeads in the mold is thus used, with the beads in phase opposition to the mold. This produces expansion giving rise to an ordered arrangement of the microbeads.
Vibration is maintained throughout the time the mold is filled (Photos 19 and 20, filling at a frequency of 84 Hz). Very clear densification is obtained compared with a non-vibrated sample. The microbeads ordered in this way are then compacted by vibrating them in phase with the mold (e.g. 30 Hz + 5 Hz) (Photos 21-23). While the resin is injected from below, and while a vacuum is established by suction above the mixture in order to encourage resin migration, continuing the vibration has no effect on the integrity of the beads, nor on the resin-bead interface.
The compression strength of the resulting material having 35% beads by weight is 70 MPa to 130 MPa.
This material withstands chemical agents and moisture better than the resin on its own.
The following physical characteristics are obtained:
uniaxial compression strength: 3 to 3.6 daN/mm2 traction strength: 1.2 to 1.3 daN/mm2 shear strength: 1.8 daN/mm2 modulus of elasticity in compression: 180 to 200 daN/mm2 modulus of elasticity in traction: 90 daN/mm2 2nd method: High technology foam Microbeads and resin are brought together in a receptacle which, when put into vibration, sets up internal flows of matter that give rise to a mixture that is very homogeneous.
Under such circumstances, vibration enables very good wetting of the microbeads to be obtained and also complete control over filler ratio.
Comparative results are illustrated by Photos 24 to 27.
V - The mixing of components such as powders in the dry state may advantageously be performed by the method of the invention.

7 207~8g Thus, different powders of various densities and grain sizes are put into a receptacle and frequencies are scanned through in order to determine the resonant frequency and thereby obtain extremely intimate mixing. The mixing is the result of a flow generated by the particles bouncing off one another. The flow may be created from a vertical upwards movement in the center spreading towards the walls of the receptacle, or vice versa. It is also possible, from certain settings, to generate reciprocating movements enabling certain components to be filtered, sorted, chosen, or selected.
A particularly advantageous application of the method of the invention is to ensure that components can be provided in extremely accurately measured out small quantities when implementing mixtures on a very large industrial scale.
Intimate and homogeneous mixing of components may also be achieved advantageouslv by the method of the invention, particularly when one of the components is a substance that on its own or on coming into contact with other components is unstable or likely to give rise to a violent reaction such as an explosion on being sub~ected to motion that is too sudden.
The sensitivity of components can be allowed for by adjusting and controlling the frequency and the amplitude of the vibration used for mixing purposes.
This method is also applicable to manufacturing low technology foams. The fillers used may be maize ccbs, or large diameter (5 mm to 10 mm) balls of plastic or polystyrene. The strength obtained in this way is relatively low, from 10 MPa to 100 MPa, and this is done in a manner that enables large volumes to be obtained.
VI - The invention provides for transmitting vibration to the mixture via the receptacle or via the matrix components, or else via the reinforcing components.
When the receptacle is directly connected to the vibration source, it may enter resonance and deform at its base or at its side walls, transmitting vibratory motion to the components it contains, or else it does not deform, in which case it transmits the vibration directly to the mixture.

207~489 The effect of the receptacle deforming is to establish bulk flows of matter in the form of central or peripheral upwards motion, which on being repeated cause the components being degassed. Advantageously a vacuum is established inside the receptacle by suction, thereby causing air inclusions to expand and evacuating the extracted air after degassing. In this manner, after bonding by a physico-chemical treatment (polymerization~ ...), a composite material of very good quality is obtained in which the reinforcing components are in direct contact over their entire surface areas with the matrix components.
In another implementation, the vibration is generated by a probe in direct contact with the matrix or reinforcing components. The probe produces a palm-tree effect which gives rise, as described above, to motion and degassing of the components.
In an undeformable receptacle, accelerations of the order of several ~ are communicated to the mixture. Each of lts components responds differently to such accelerations depending on its own inertia, but at the end of a certain length of time the mixture becomes homogeneous and responds uniformly to accelerations.
In some implementation cycles, it is appropriate to transfer the mixture obtained in this way without damaging it, and this requires the transfer means (e.g. chutes, conveyors, tubes, pipes, ...) to be caused to vibrate as well. The vibratory state of the mixture is conserved during transfer.
In another implementation, the transfer means are made to vibrate independently of the receptacle, as applies, for example, to cellulose where the transfer means is a conveyor under tension and is the only part to be vibrated since the quality is obtained during the time re~uired for transfer. In this example, vibration is for deflocculation purposes and for keeping the fibers completely separated, thereby obtaining a mixture having very good homogeneity.
VII - The advantages of the method of the invention are illustrated by the results of the following tests described with reference to Photographs 1 to 27.

A - Weiqht savina bv usinq less resin for the same quantity of fabric thickness weight normally worked item 8.5 to 10 mm 14.42 kg/m2 item vibrated by the method 7.5 mm to 8 mm 11.8 kg/m2 of the invention saving 2 mm 2.62 kg/m2 The cloth/resin ratio of the material can thus be selected to obtain a saving of resin.
B - Increase in mechanical strenqths Tests have been performed in traction, in bending, and in delamination during bending, using test pieces taken from a common plate, of thickness 9.5 mm to 10 mm for a non-vibrated material and of thickness 7.5 mm to 8 mm for a material ~ibrated using the method of the invention.
B.l - Traction testing Conditions: rectangular test piece length 250 mm, several widths traction speed 5 mm/min Results: improvement in breaking stress of about 22 B.2 - Delamination testing on bending Conditions: rectangular test piece various lengths distance between supports 50 mm speed of descent 1 mm/min Results: identical breaking stresses for both materials. If the distance between the supports is reduced, then the vibrated item shows better resistance to delamination.
30 B.3 - Bending tests Conditions: rectangular test piece length 250 mm, several widths speed of descent 10 mm/min Results: improvement of about 23% in the breaking stress. It should be observed that the distance between supports of 150 mm penalizes the vibrated item.

C - Better constitution and homoaeneity of the resultina comPosite material The structures of composite materials obtained using conventional techniques and of materials obtained using the method of the invention are compared on the basis of Photographs l to 27 obtained using an electron microscope.
Photograph l is a detail view obtained using a scanning electron microscope (SEM) with backscattered electrons (BSE) showing a non-vibrated sample in cross-section through a bundle of fibers cut transversely. The abundance of voids in the structure can be seen.
Photograph 2 shows a detail of the preceding photograph in which the fibers are disjoint and locally free of any resin.
Photograph 3 is a detail view obtained using an SEM with BSE showing a non-vibrated sample in cross-section through a bundle of fibers cut transversely. Delamination can be seen on the left (white arrow) as can voids in the bundles of fibers.
Photograph 4 shows a center detail of the preceding photograph.
Photograph 5 is a detail view obtained using an SEM with BSE showing a cross-section through a vibrated sample. The resin appears gray and the cross-cut fibers appear white.
There is no visible separation at the fiber-resin lnterface.
Photograph 6 is a detail view obtained using an SEM with BSE showing a different field of a vibrated sample in cross-section. S~me of the fibers show interstices between the fibers and the resin (arrows).
Photograph 7 is a detail view obtained using an SEM with BSE showing a non-vibrated sample in cross-section. The resin appears gray while the cross-cut fibers appear white. Gaps can be seen in all of the fiber-resin interfaces (black margins around the fibers).
Photograph 8 is a detail view obtained using an SEM with BSE showing a longitudinal section of a non-vibrated sample.
The resin appears gray and the cross-cut fibers appear white.
As in the preceding case, all of the fibers are marked by resin shrinkage (black margin around the fibers).

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Photograph 9 is a detail view obtained using an SEM with BSE showing a non-vibrated sample in longitudinal section. The resin appears gray and the cross-cut fibers appear white. As in the preceding case, all of the fibers are marked by resin shrinkage (black margin around the fibers). The arrow indicates the field of Photograph 10.
Photograph 10 is a detail view of the zone marked by the arrow in the preceding photograph. The granular texture of the resin and the gaps in the fiber-resin interface can be seen.
Photograph 11 is a detail view obtained using an SEM with BSE showing a non-vibrated sample in longitudinal section. The resin appears gray and the cross-cut fibers appear white.
Delamination can be observed in the plane perpendicular to the plane of stratification.
Photograph 12 is a detail view obtained using an SEM with BSE, sho~ing a non-vibrated sample in longitudinal section.
The resin appears gray and the cross-cut fibers appear white.
This photograph shows another example of delamination, outside the plane of stratification.
Photograph 13 shows a sample of microbeads without resin.
Photographs 14 to 18 show samples of syntactic foams without vibration (G = 250 and G = 100).
Photographs 19 and 20 show a sample of microbeads vibrated during mold filling.
Photographs 21 to 23 show the compacting of a sample of microbeads obtained by vibration that is in phase with the mold.
Photographs 24 to 27 are comparisons on polished sections of vibrated and non-vibrated samples of syntactic foams.
Analysis of the photographs shows that the splits are on average larger in size (wider and longer) in the non-vibrated sample where they may be as much as one millimeter long, than they are in the vibrated sample.
Quantitative analysis also shows that these faults are two to four times more abundant in the non-vibrated sample than in the vibrated sample. The greatest difference is to be observed in the cross-section.

207~489 The differences between the stratified materlal made using the conventional method and that made by applying the method of the invention relate:
to defects in fiber wetting;
S to the volumes of resin shrinkages at the fiber-resin interface; and to the number and size of delaminations for which the vibrated sample shows an improvement by a factor lying in the range 2 to 10 relative to the non-vibrated sample.
In addition, the vibrated sample is distinguished by a more uniform distribution of fibers in the resin, by a higher density of fibers, and by substantially improved bubble removal.
A study on the take~p of water performed on samples of stratified composite material obtained using the method of the invention serves to provide a quantitative comparison of the wetting of the fibers by the resin, and thus of the lifetime of the stratified material.
Stratified 500 mm by 500 mm plates of glass or Kevlar and epoxy composite were made from from which bubbles were removed either manually, or by the method of the invention.
Samples having dimensions of 400 mm by 150 mm were cut from the centers of the plates.
Samples A: Rovimat 300 mm by 300 mm No. 1 epoxy resin having bubbles removed manually, ~ = 16C.
B: same as A with bubbles removed and vibrated, ~ = 16C.
C: Kevlar 48 glass 65 mat 200 (380 grams of fabric) No. 1 epoxy resin, bubbles removed manually, 0 = 16C.
D: same as C with bubbles removed and vibrated, 0 = 16C.
The quality of epoxy resins No. 1 and No. 2 is identical.
Resin No. 2 is of lower viscosity.
Results Test ~ resin C Thickness Po in g Pl in % P2 in %
A manual 16 5.9 453.5 1.87 3.74 B vibrated 16 6.3 456.0 0.43 1.64 C manual 16 6.4 470.3 3.12 5.14 D vibrated 16 6.1 455.6 0.52 0.76 207~48g Samples A, B, C, and D were made under identical conditions (~ = 16C) and may therefore be compared:
A very clear improvement provided by the method of the invention can be observed.
The significant water takeup of Kevlar (already observed on other samples) when prepared manually is considerably improved by vibration.

Claims (18)

14

1/ A method of manufacturing a composite material comprising an intimate and homogeneous mixture of at least two components, by causing said mixture to vibrate in a receptacle at a frequency close to the mean of the resonant frequencies of each of the components or that of the receptacle so as to degas the components, and then bonding the components together such that the resultant composite material has a structure that is substantially free from included air, is compact, and is dense.

2/ A method according to claim 1, characterized in that the frequency and amplitude of the vibration to be used are determined by previously recording and analyzing the mechanical behavior of each of the components and of the receptacle taken in isolation in response to a vibration to which they are subjected and whose frequency is varied continuously so as to select the frequency that leads to resonance.

3/ A method according to claim 1 or 2, characterized in that vibration is applied both to the mixture and to the receptacle, the two vibrations being in phase.

4/ A method according to any preceding claim, characterized in that a receptacle is used in which sufficient suction is established to remove the degassed air.

5/ A method according to any preceding claim, characterized in that at least one reinforcing component and at least one matrix component are used.

6/ A method according to any preceding claim, characterized in that at least one polymerizable matrix component is used.

7/ A method according to any preceding claim, characterized in that bonding between the components is obtained by polymerization.

8/ A method according to claim 5, characterized in that bonding between the reinforcing components and the matrix components is obtained by heating to a temperature high enough to melt the matrix components.

9/ A method according to any one of claims 1 to 7, characterized in that reinforcing components are used comprising microbeads of glass.

10/ A method according to any preceding claim, characterized in that reinforcing components are used comprising at least one sheet of fibers.

11/ A method according to claim 10, characterized in that reinforcing components are used comprising short fibers.

12/ A method according to any preceding claim, characterized in that matrix components are used constituted by a liquid resin.

13/ A method according to any one of claims 1 to 11, characterized in that matrix components are used constituted by a dry powder of thermoplastic material.

14/ A method according to any preceding claim, characterized in that said vibration is applied to the mixture via the matrix components.

15/ A method according to any one of claims 1 to 13, characterized in that said vibration is applied to the mixture via the reinforcing components.

16/ A method according to any one of claims 1 to 13, characterized in that said vibration is applied to the mixture via the receptacle.

17/ A method according to claim 1, characterized in that at least one component is used constituted by a product that is unstable and that is liable to produce an explosion on its own or on making contact with one of the other components.

18/ A method according to any preceding claim, characterized in that the vibration frequency lies in the range 30 Hz to 180 Hz.