FR2646432A1 - Heat-stable syntactic foam - Google Patents

Abstract

The invention relates to a novel heat-stable syntactic foam comprising hollow microspheres and a binder, characterised in that the binder is based on a polystyrylpyridine or polystyrylpyrazine resin. The microspheres are made of glass, silica, ceramic or a synthetic material. The foam is prepared by dispersing the microspheres in the resin and hardening the whole by baking. A pre-bake may be carried out before the baking. The foam obtained can withstand high temperatures in continuous operation without losing its mechanical properties.

Description

Thermostable syntactic foam.

The invention relates to a new thermostable syntactic foam, in particular a new syntactic foam capable of being used at high temperature.

Syntactic foams are solid materials made up of rigid, hollow microspheres, mixed with a binder.

Known syntactic foams, constituted by a binder such as an organic epoxy or polyester resin for example, have characteristics of use which are limited. Indeed their mechanical resistance decreases with temperature. When the syntactic foam is used at a raised temperature, there may be melting of the resin when it is thermoplastic or irreversible modification of the structure when it is thermosetting, which leads to the degradation of the characteristics.

When the damage is not immediate, there is always a rapid aging effect.

If syntactic foams are to be used as structuring materials, they must be able to withstand high temperatures and have good mechanical properties.

Known syntactic foams do not meet these requirements.

There was therefore a need for a syntactic foam usable at high temperature, in continuous regime, whose properties and in particular the mechanical properties do not significantly decrease. The present invention aims to meet this need, by proposing a new syntactic foam.

The new syntactic foam according to the invention, comprising hollow microspheres and a binder, is characterized in that the binder is based on polystyrylpyridine or polystyrylpyrazine resin.

The combination of this binder and microspheres makes it possible to obtain foams which have excellent mechanical strength at high temperature, for example of the order of 150 'to 200 ° C, and in continuous regime. In addition, these foams are light, their density is generally less than 0.6 and they have good fire resistance.

The pol ystyrylpyridine or pol ystyrylpyrazine resins used as binder for producing the syntactic foam according to the invention are resins generally obtained by polycondensation of alkylated derivatives of pyridine or pyrazine and of aromatic dialdehydes or hydroxyaldehydes or of their derivatives, optionally modified, in particular the resins as described in U.S. Patents 3,994,862, 4,163,740, 4,362,860, 4,526,925, 4,500,690, 4,515,938, 4,540,745, 4,736,035, 4,578,439, French
No. 2,261,297 and Europeans No. 136,226, 136,227, 135,443.

They are generally resins in solution or solid products whose viscosity can be lowered by melting.

dans lequel X est un substituant méthyle ou éthyle ou un motif du même type, comme décrites dans le brevet américain N' 3 994 862 et celles décrites dans les brevets américains Ns4 163 740, 4 515 938, 4 736 035, français Ns 2 261 297 et européens N 136 226, 136 227 et 135 443.It is preferred to use polystyrylpyridine resins, modified or not, the main unit of which comprises at least radicals derived from a di- or tri-alkylated pyridine, in particular di- or trimethylated, and from an aromatic dialdehyde or an aromatic hydroxyaldehyde. By way of example, mention may be made of resins comprising patterns of the type

in which X is a methyl or ethyl substituent or a motif of the same type, as described in American patent N '3,994,862 and those described in American patents Ns 4,163,740, 4,515,938, 4,736,035, French Ns 2,261 297 and Europeans N 136 226, 136 227 and 135 443.

The hollow microspheres suitable for the production of syntactic foams according to the invention are microspheres made of glass, silica, ceramic, synthetic material such as acrylic resin, polyethylene or phenolic.

Their average diameter is generally between 15 m and 250 μm and preferably between 150 μm and 200 μm. Microspheres of different sizes can be used, the smaller ones generally being between the larger ones.

The volume proportion of these microspheres in the syntactic foam is between approximately 40% and 85% relative to the foam, advantageously between 50% and 80%.

The preferred fillers according to the invention are hollow glass microspheres.

The syntactic foam can also contain another filler, such as silica or glass, in granular or short fiber form. The proportion of this charge can be up to 50% by volume relative to the resin.

One or more coupling agents can be added to improve the bond between the fillers and the binder. Suitable coupling agents are, for example, silanes, alkoxytitanates and epoxy resins. The amount of coupling agents is at most 5% by mass relative to the combined microspheres and any other charge.

Among the known processes for preparing syntactic foams, those described by the publication "Syntactic Foams I. Preparations,
Structure and Propertiesw of Puterman et al., In the journal
Journal of Cellular Plastics, July-August 1980, pages 223-229 can be used.

A process which is very suitable for the production of a syntactic foam in accordance with the invention consists in dispersing the microspheres and possibly the other fillers in the resin and then hardening the whole by baking, preferably in a mold under moderate pressure.

To effect the dispersion of the microspheres, the resin can be brought to a sufficiently low viscosity, either by melting or by dissolving. In the latter case, when the mixing is complete, the solvent is removed by conventional methods, for example by heating and / or under partial vacuum. A solid which is brittle is obtained, which can be reduced into small blocks to facilitate introduction into a mold.

More particularly, according to the invention, a precooking step is carried out before introduction into the mold or after it, at a temperature between 160 'and 300 ° C, preferably between 180 and 270-C, during a duration generally less than one hour.

Cooking takes place at a temperature between 180 'and 350'C, preferably between 240' and 300'C for several hours under a pressure between 0.1 MPa and 5 MPa, preferably between 0.5 and 2 MPa.

The implementation of the foam obtained is possible with all conventional techniques. In particular, it can be easily cut and machined by removing material.

The invention will be better understood in the light of the illustrative examples below.

Example 1
To 75 parts by weight of a solution of polystyrylpyridine resin tPSP 6022M ", manufactured by SNPE, at 75% of dry extract in methyl ethyl ketone, which corresponds to 100 parts of resin without solvent, 75 parts by weight of methyl ethyl ketone are added. , then with a low shear 100 parts by weight of glass microspheres "28/750 '(density 0.28) sold by the company 3M. The mixture is dried in an oven under vacuum for approximately 3 hours and the solid obtained, containing 80 t by volume of microspheres and whose volatile content is less than 2 *, is cut into small blocks. These blocks are introduced into a compression mold. with a punch placed between the plates of a press. Pre-baking is carried out at 250 ° C for approximately 15 minutes, a pressure of 0.7 MPa is applied to the material and baking is carried out for 6 hours at 250 C. The syntactic foam plate, approximately 7 mm thick, is demolded and has the following properties
Density: 0.57 at 20'C
Compressive strength: 9.2 MPa at 20 "C
10 MPa at 150 C
Compression module: 480 MPa at 20 * C
190 MPa at 150'C
Example 2
100 parts by weight of a solution of polystyrylpyridine resin "PSP 6022N", manufactured by SNPE, are mixed with 75% of dry extract in methyl ethyl ketone with 100 parts by weight of glass microspheres "B 30 B" (density 0 , 25 to 0.30) sold by the company 3M.

After drying, the mixture in the form of a thick mastic is placed in a compression mold and baked at 1800C under low pressure for 16 hours.

We obtain a syntactic foam with a density of 0.27 at 23-C, not very brittle, whose compressive strength is 2.3 MPa at 23 C and whose limit oxygen index (IOL) is between 62% and 65 t.

Example 3
250 g (100 parts) of solid polystyrylpyridine resin "PSP 7022PL" manufactured by SNPE are dissolved in methyl ethyl ketone (133 parts by weight) with stirring at room temperature and the glass microspheres "28/750" (100 parts) are introduced. ) as in example 1.

The mixture obtained is dried in a vacuum oven, first at room temperature, then at 80 ° C. for 2 hours. The volatile content measured after 10 minutes at 160 ° C. is 1.4%. The solid mixture containing 80% by volume of microspheres is cut into small blocks which are introduced into a mold.

After 20 minutes precooking at 220 C, a pressure of 0.34
MPa is applied to the material and baking is carried out for 16 hours at 220 C. A plate whose thickness is approximately 9 mm is obtained after demolding.

The properties of the syntactic foam obtained are as follows
Density: 0.53 to 23 C,
Flexural strength (ratio d / e = 16): 23 MPa at 23 C
8 MPa at 200C
Flexural modulus: 4,200 MPa at 23 C
1,100 MPa at 200 C
Compressive strength: 16 MPa at 230C
2.9 MPa at 200 C
Compression module: 900 MPa at 230C
60 MPa at 200 C
Example 4
The mixing is carried out as above with 250 g (100 parts) of "HSP SL" resin, manufactured by ONERA (melting temperature 160 C, volatile content 10% at 250 C), 133 parts of methyl ethyl ketone and 133 parts of glass microspheres 28/750.

After a final drying of 4 hours at 80 ° C. under vacuum, a solid is obtained, the volatile content of which is less than 95% and which contains 73% by volume of microspheres.

The molding is carried out under the same conditions as above. The plate obtained has a thickness of approximately 11 mm.

The properties of the syntactic mouse are as follows
Density: 0.48 to 23 C
Flexural strength: 27 MPa at 23 C
19 MPa at 200'C
Flexural modulus: 3300 MPa at 23 C
2,690 MPa at 200 C
Compressive strength: 20 MPa at 23 C
14 MPa at 200 C
Compression module: # 1300 MPa at 23 C
# 400 MPa at 200 C.

Claims (10)

Claims 1. A thermostable syntactic foam comprising hollow microspheres and a binder, characterized in that the binder is based on polystyrylpyridine or polystyrylpyrazine resin. 2. syntactic foam according to claim 1 characterized in that the binder is a polystyrylpyridine resin, modified or not, the main unit of which comprises at least radicals derived from a di- or trialcylated pyridine and had an aromatic dialdehyde or a aromatic hydroxyaldehyde. 3. syntactic foam according to claim 1 or 2 characterized in that the microspheres are made of glass, silica, ceramic or synthetic material. 4. Syntactic foam according to any one of the preceding claims, characterized in that the microspheres have an average diameter between 15 μm and 250 μm, preferably between 150 Vm and 200 Vm. 5. syntactic foam according to any one of the preceding claims, characterized in that the volume proportion of the microspheres in the foam is between 40 and 85 t relative to the foam and preferably between 50 and 80%. 6. syntactic foam according to any one of the preceding claims, characterized in that it contains another filler, granular or in the form of short fibers. 7. syntactic foam according to any one of the preceding claims, characterized in that it contains one or more coupling agents improving the bond between the binder and the fillers. 8. A method of preparing a syntactic foam according to any one of claims 1 k 7 characterized in that it consists in dispersing the microspheres and possibly the other fillers in the resin and k then hardening the whole by cooking, preferably in a mold under moderate pressure. 9. Method according to the preceding claim characterized in that before cooking the mixture obtained is precooked at a temperature between 160 and 300-C, preferably between 180 and 170 C. 10. Method according to claim 8 or 9 characterized in that the cooking is carried out at a temperature between 180 and 350-C, preferably between 240 and 300C, under a pressure between 0.1 MPa and 5 MPa, preferably between 0.5 and 2 MPa.