FR2649356A1 - Cellular composite material having a high acoustic insulation capacity and method for obtaining it - Google Patents

Abstract

Cellular composite material comprising substantially contiguous hollow microspheres 1 and a solid binder forming meniscuses 3 in the areas of contact between the microspheres and ensuring their mutual connectivity, but leaving the rest of the volume of the interstices between the microspheres free. Such a material has a substantially greater acoustic insulation capacity than when the interstices are filled with binder, while still maintaining good compressive strength.

Description

Honeycomb composite material with high acoustic insulation power and method for obtaining it
The invention relates to cellular composite materials.

Such materials are known comprising substantially contiguous hollow microspheres and a solid binder or matrix, generally constituted by a polymer, filling the interstices between the microspheres. These composite materials, called "syntactic foams", are very light, like expanded foams, and are distinguished from these by a very good resistance to compression due to the presence of microspheres.

For example, the syntactic foam marketed by the company EUROSHORE under the reference FM 100.1, has a density of 0.56 and a pressure! implosion rate of 450 kg / cm-l ..

When immersed in water, it attenuates an 80 kHz frequency wave by 1.75 dB cm'l.

Even lower densities can be obtained by using larger diameter spheres. Thus, the EUROSHORE FSA-B.1200 "super-light" syntactic foam, which can withstand an isostatic pressure of 120 kg / cm'2, has a density of 0.44. It contains hollow spheres whose maximum diameter is 50 mm, and has the disadvantage of being able to be used only in layers with a thickness greater than 150 mm.

The object of the invention is to modify known syntactic foams so as to optimize their acoustic behavior, even at low thicknesses, while retaining sufficient hydrostatic compression resistance.

The invention relates to a cellular composite material comprising hollow microspheres substantially contiguous and a solid binder present in the interstices between the microspheres, ensuring their mutual joining, characterized in that the binder occupies only a minority fraction of the volume of said interstices, the remainder of this volume remaining free.

Preferably, the quantity of binder is substantially limited to the formation of menisci in the areas of contact between the microspheres.

The composite material according to the invention can comprise, in addition to the hollow microspheres, solid microspheres.

According to an advantageous characteristic of the invention, the microspheres are made of a material having hydroxyl groups on the surface, and the binder is capable of forming hydrogen bonds with these hydroxyl groups.

The binder is in most cases an organic polymer.

By way of example, the microspheres can be made of at least one material chosen from glass, silica and phenolic resins, the binder being able to be a phenolic, epoxy, polyester, vinyl resin for example.

The invention also relates to a block of the composite material as defined above, the residual interstices of which are filled on the surface with an impregnation resin when the block is to be made waterproof.

Another subject of the invention is a process for obtaining such a composite material, in which hollow microspheres and, where appropriate, full microspheres are stacked, the interstices of the stack are filled with a liquid capable of supplying the binder by chemical transformation after removal of any solvent, it is wrung to remove the excess liquid and the residual liquid is treated to form binder particles between the microspheres.

According to particular methods of the process - the stacking of the microspheres is carried out under vibration; - spin drying takes place under vacuum; and the liquid is an organic resin solution, the treatment comprising the evaporation of the solvent.

Other characteristics and advantages of the invention will become apparent from the detailed description below and from the appended drawings in which - FIG. 1 schematically represents four successive stages in the manufacture of a composite material according to the invention; - Figure 2 schematically shows an apparatus for introducing the liquid into the interstices between the microspheres and for spinning; - Figures 3 and 4 are diagrams relating to the properties of a composite material according to the invention.

FIG. 1 schematically illustrates, under the letters a, b, c and d, four states through which the composite material according to the invention passes successively during its manufacture, the first three states also applying to known syntactic foams. State a corresponds to the starting hollow microspheres, indicated by the reference 1. In state b, microspheres 1 are brought close to one another until they are joined, each of them being practically tangent to several others , so as to form a substantially incompressible stack. In I1 state c, the precursor liquid of the binder fills the interstices 2 between the microspheres. In the case of a conventional syntactic foam, this precursor is generally a liquid resin which is then polymerized or crosslinked to form a solid matrix occupying also substantially the total volume of the interstices.

According to the invention, the product is drained from state c to obtain state d in which the liquid precursor no longer forms anything but menisci 3 in the zones of tangency of the microspheres therebetween. In addition, this liquid precursor can be one. solution of a polymerizable or crosslinkable resin, the evaporation of the solvent further reducing the volume occupied by the final binder.

The composite material according to the invention has very strongly increased sound insulation properties compared to known syntactic foams, and behaves as an acoustic impedance breaker, that is to say that it does not transmit acoustic waves. , because it has a high acoustic reflection power. In the case where one does not want a break in acoustic impedance, but a neutrality of the material, one can adjust its acoustic behavior to that of the ambient environment, for example water in which the material is immersed, by partially replacing the hollow microspheres by solid microspheres of similar diameter, in a suitable proportion.

The mechanical resistance of the composite material according to the invention, although lower than that of a conventional syntactic foam obtained from the same microspheres and the same binder, is much higher than that of expansion foams owing to the sphericity of the fillers joined sockets.

Commercially available microspheres of silica, glass or phenolic resin with a diameter of a few tens of microns and a specific mass in the joined state (Figure l, b) between 110 and 190 kg / m '3 approximately.

Such microspheres can be used in the invention in combination with a binder consisting of a phenolic, epoxy, polyester, vinyl resin for example. The use of a binder comprising hydroxyl groups, for example a phenolic resin, improves adhesion by the formation of hydrogen bonds with the hydroxyl groups present on the surface of the microspheres. In the case where the microspheres are made of a hygroscopic material, for example a phenolic resin, they must be dried beforehand in order to reduce the formation of aggregates.

For applications at high temperatures which do not allow the use of organic binders, an inorganic binder can be used such as silica obtained for example by in situ hydrolysis of a silane.

To implement the method according to the invention, it is possible to use (FIG. 2) a rigid plate 4 of stainless steel, pierced with holes of 1 to 2 mm in diameter spaced from one another by 1 to 2 mm, associated with removable sidewalls 5 made of stainless steel covered with an aluminum film and polytetrafluoroethylene 6 facilitating demoulding.

 The microspheres are deposited in the mold thus formed, on a thick filter paper with a fine texture 7, resting on the plate 4 and pinched between the latter and the sides 5.

The microspheres are brought together (FIG. 1, state b) by means of a vibrating table until the apparent volume has stabilized. A second filter paper surmounted by a metal grid 8 is placed on the surface of the microsphere bed in order to preserve the flatness of the latter during the pouring of the impregnation liquid.

The mold is then placed on a Büchner funnel 9 with interposition at the periphery of a flexible rubber seal 10 ensuring lateral sealing, and the impregnation liquid is poured at atmospheric pressure until covering the microspheres, after which the vacuum is applied by means of a wringing flask 11 on which the funnel 9 is mounted with the interposition of a seal 12. To obtain the lowest possible quantity of binder, the vacuum is applied until at the end of the flow of the liquid.

The mold is then removed from the funnel 9 and placed in an oven for the evaporation of any solvent and the polymerization and crosslinking of the binder. The release before complete hardening prevents any cracking of the material and the obturation of the orifices of the metal plate 4.

When the composite material according to the invention must be used in pressurized water, its open pores are closed at the surface by impregnation using a rigid resin of good toughness and polymerizable at room temperature, for example an epoxy resin. The residual gaps are for example filled to a depth of the order of 0.2 to 0.3 mm. This impregnation is preferably carried out on a block previously machined.

Example 1
We start from hollow silica microspheres marketed by EMERSON & CUMING under the name ECCOSPHERE Si, with a diameter between 45 and 175 microns and an apparent density in the joined state of 0.14. Their resistance to isostatic compression is expressed by a fraction of 55% of non-imploded microspheres under 110 kg / cm'2
The impregnation liquid is a solution composed by weight of 50% of a phenolic resin marketed by RHONE
POULENC under the name RESOL ABLAPHENE RA 101 (density of the polymerized resin 1.29) and 50% methyl ethyl ketone. This solution is used in an amount of 60% of the apparent volume of the microspheres.

Spinning is carried out for 2 hours under a vacuum of 10 to 20 mm of mercury.

Evaporation of the solvent and polymerization of the resin are carried out at atmospheric pressure, under the following conditions: heating 16 hours at 800C in the mold; mold release at room temperature; heating 2 hours at 1000C, then 2 hours at 1200C and finally 3 hours at 150oc.

In the composite material obtained, the binder represents 30% of the total mass and occupies 5% of the apparent volume.

The open porosity rate is 38%, and the density of 3.21.

The surface pores of a 45 x 35 x 20 mm sample were closed with an epoxy resin (LY 556 + system
HY 951 from CIBA). For this sample, a hydrostatic compression resistance of 50 kg / cm.sup.2 and an acoustic transmissibility in zero water at 80 kHz was observed.

Using a method using piezoelectric transducers, between 3 and 10 kHz, the transfer function of this sample, compared to that of a reference material, very little absorbent, polymethacrylate, was noted. methyl. It has been observed that the acoustic attenuation is of the order of 3 dB.cm'l for this frequency range (FIG. 4).

Example 2
The procedure is as in Example 1, bringing the concentration of the RESOL solution in methyl ethyl ketone to 70%, the duration of the spinning being 3 hours.

The mass and volume rates of the binder pass to 45% and 9% respectively, the rate of open porosity to 38% and the density to 0.26.

The resistance to hydrostatic compression under the same conditions as in Example 1 is 65 kg / cm # 2, and the acoustic transmissibility in water remains zero at 80 kHz
Example 3
The procedure is as in Example 2, bringing the concentration of the RESOL solution to 80%. The mass and volume rates drop to 48% and 12% respectively, the open porosity rate is reduced to 36% and the density is 0.30.

The hydrostatic compressive strength of a sample of dimensions 45 x 35 x 20 mm is 85 kg / cm # 2.

Example 4
The same microspheres are used as in the previous examples. The impregnating liquid is a solution composed by weight of 20% of a two-component epoxy resin marketed by REZOLIN under the reference EPO HT 688/1 and 80% of methyl ethyl ketone.

The spin time is 16 hours at room temperature.

The evaporation and polymerization treatment is carried out entirely after demolding, and comprises successive stages of 2 hours at 800C, 1 hour at 1000C, 2 hours at 1200C and 3 hours at 1500C.

The mass and volume rates of binder are 15% and 2.5% respectively.

The open porosity rate is 43%.

The density is 0.19.

The resistance to hydrostatic compression under the same conditions as in the previous examples is 30 kg / cm 2 measured as in example 1, the acoustic attenuation is of the order of 4 to 5 dB.cm # 1 between 3 and 10 kHz (Figure 4). -
Example 5
The procedure is as in Example 4, bringing the concentration of the epoxy resin solution to 50%.

The mass and volume rates are 33% and 6.8% respectively.

The open porosity rate is 40%.

The density is 0.24.

The resistance to hydrostatic compression under the same conditions as in the previous examples increases to 50 kg / cm # 2.

Example 6
We use insoluble glass microspheres, supplied by EMERSON ~ CUMING under the name ECCOSPHERE
FTD 202, with an average diameter of 65 microns and an apparent density at 11 contiguous state of 0.17. Their resistance to isostatic compression is expressed by an 80% fraction of non-imploded microspheres at 110 kg / cm # 2.

We then proceed as in Example 3 (phenolic binder).

The mass and volume rates of the binder are 47% and 11.5% respectively.

 The open porosity rate is 37%.

The density is 0.31.

The resistance to hydrostatic compression under the conditions of the preceding examples is 95 kg / cm 2, and is reduced to 50 kg / cm 2 for a sample of 100 x 100 x 20 mm.

Example 7
The procedure is as in Example 6, replacing the resin solution in methyl ethyl ketone with the same volume of undissolved resin.

The mass and volume rates of binder are increased to 49% and 12.5% respectively.

The open porosity rate is 33%.

The density drops to 0.34.

The resistance to hydrostatic compression is greater than 100 kg / cm'2 and equal to 65 kg / cm'2 respectively for the two sample sizes indicated in Example 6.

Example 8
We start with glass microspheres supplied by the company 3M under the reference SCOTCHLITE D32 / 4500, with a diameter of less than 70 microns and an apparent density in the joined state of 0.19 + 0.03. These microspheres have an isostatic compressive strength of 310 kg / cm # 2.

An impregnation solution comprising 80% of resin from Example 1 in 20% of ethanol is left in the presence of the microspheres for 1 h 30 at atmospheric pressure.

Spinning takes place for 2 hours under vacuum
For the heat treatment, a temperature rise of 1200C per hour is carried out, interrupted by steps of 2 hours at 900C and 3 hours at 1500C. The product is then machined and then annealed for 10 hours at 1750C.

The mass and volume rates of binder are 48% and 12% respectively. The open porosity rate is 35% and the density 0.32. After closing the surface pores with an epoxy resin of the GEL COAT EPO 417 type supplied by
RESOLIN, the hydrostatic compressive strength of a 100 x 100 x 20 mm sample is 95 kg / cm # 2, and its compressibility (AV / v) is from 0.009 to 50 bar.

The low frequency acoustic properties of this material were measured, in the range from 0 to 25 kHz, for bars with a length of 148 mm and a mass of 8.5 g, density 0.32. We observe that the complex Young's modulus, which is real, has a value of 1.7 109 Pa. The loss angle (damping) is close to 0.5, ie practically zero.

The acoustic attenuation is 2.5 dB.cm # 1 between 3 and 10 kHz (Figure 4).

Six other composites were produced with the same microspheres and various dilutions of phenolic resin with ethyl alcohol to obtain volume levels of various binders, from 5 to 14%. The curve in FIG. 3 shows the variation in the hydrostatic pressure limit for the use of each of these composite materials as a function of the volume rate of binder.

 The comparison of examples 1 to 3 shows that the binder rates, the density and the hydrostatic compressive strength of a sample of given dimensions are an increasing function, and the open porosity rate a decreasing function, of the resin concentration of the liquid. impregnation.

The same observation follows from the comparison of Examples 4 and 5 on the one hand and 6 and 7 on the other. These last two examples also show that at equal thickness the resistance to hydrostatic compression depends on the lateral dimensions of the sample. Figure 6-illustrates the fact that the acoustic attenuation by reflection is a function of the porosity of the composite. Finally, Example 8 shows that good resistance to compression can be obtained for a relatively large sample by an appropriate choice of microspheres.

Claims (13)

Claims 1.- Cellular composite material comprising hollow microspheres substantially contiguous (1) and a solid binder present in the interstices between the microspheres, ensuring their mutual joining, characterized in that the binder occupies only a minority fraction of the volume of said interstices , the remainder of this volume remaining free. 2.- composite material according to claim 1, characterized in that the amount of binder is substantially limited to the formation of menisci (3) in the contact areas between the microspheres. 3.- composite material according to one of claims 1 and 2, characterized in that it comprises, in addition to the hollow microspheres, solid microspheres. 4.- composite material according to one of the preceding claims, characterized in that the binder is an organic polymer. 5. Composite material according to one of the preceding claims, characterized in that the microspheres are made of a material having hydroxyl groups on the surface, and in that the binder is capable of forming hydrogen bonds with these hydroxyl groups. 6.- composite material according to claim 5, characterized in that the microspheres are in at least one material chosen from glass, silica and phenolic resins, and in that the binder is a phenolic, epoxy, polyester, vinyl resin for example. 7. A composite material according to claim 5, characterized in that the microspheres are in at least one material chosen from glass, silica and phenolic resins and in that the binder is a silica obtained by hydrolysis of a silane. 8. A block of composite material according to one of the preceding claims, the residual interstices of which are filled on the surface with an impregnation resin. 9.- Method for obtaining a composite material according to one of claims 1 to 7, characterized in that stacked hollow microspheres and if necessary full microspheres, that fills the interstices of the stack of a liquid capable of providing the binder by chemical transformation after elimination of a possible solvent, which is wrung to remove the excess liquid and which the residual liquid is treated to form particles of binder between the microspheres.  10.- Method according to claim 9, characterized in that the stacking of the microspheres is carried out under vibration. 11.- Method according to one of claims 9 and 10, characterized in that the spin is carried out under vacuum. 12.- Method according to one of claims 9 to 11, characterized in that the liquid is an organic resin solution, and that the treatment comprises evaporation of the soil. 13.- Method according to one of claims 9 to 12, characterized in that the treatment comprises polymerization and / or crosslinking of the hot binder.