IE914416A1

IE914416A1 - Crosslinked polysilazanes and process for obtaining them

Info

Publication number: IE914416A1
Application number: IE441691A
Authority: IE
Original assignee: Atochem
Priority date: 1990-12-19
Filing date: 1991-12-18
Publication date: 1992-07-01
Also published as: CN1063291A; KR920012192A; CA2057944A1; AU8984891A; FR2670788A1; EP0495325A1; IL100394A0; NO914998L; FR2670788B1; JPH04335033A; NO914998D0

Abstract

The invention relates to the field of ceramics and more precisely of crosslinked precursors. These precursors are silazanes crosslinked by means of an aluminium derivative. These crosslinked products exhibit a high yield of ceramic after pyrolysis.

Description

This invention relates to the field of ceramics precursors and in particular to polysilazanes crosslinked by means of an organometallic crosslinking agent.

US Patent 4,730,026 describes polymers containing 5 the repeat units I— R —I I --si - N-*— R R -1 a and at least one bridge of formula -MR'n-, the said bridge or bridges being attached to the nitrogen atoms of the repeat units, and the symbol M representing a metal chosen from groups III A, II Β, IV B and II A of the Periodic Table. The polymers used in the preparation of these crosslinked polymers are organosilazanes and, by way of example, the patent specification mentions poly(1,1dimethylsilazane), (1,2-dimethylsilazane)/(1-methylsilazane) copolymers or a polymerised silazane derived from cyclic methylsilazane. In general, these various polymers are prepared from a halosilane and ammonia or an amine. According to this patent specification the symbol R' in the formula of the metallic reactant represents a hydrogen atom, an alkyl group containing from 1 to 6 carbon atoms or a mono- or di-aryl- or alkylamino, alkylphenyl or alkylaryl group.

According to the present invention, there are - 3 provided crosslinked polysilazanes comprising a plurality of units of formula I and at least one bridge of formula =A1X between nitrogen 5 atoms of the said units, in which each of the available valencies of the silicon or nitrogen atoms bears a hydrogen atom, an alkyl radical containing from 1 to 6 carbon atoms, a vinyl or allyl radical, a cyclohexyl radical or an aryl, alkylaryl or aralkyl radical containing up to 18 carbon atoms, and X represents an -NHZ group, in which Z represents a hydrogen atom or a metal or metalloid and preferably silicon, or X represents Si=.

In the present description the expression available valencies denotes the valencies of the silicon and nitrogen atoms which are not employed to form the linear or cyclic chain sequences of the polysilazanes or the abovementioned bridges.

The invention also provides a process for the preparation of a crosslinked polysilazane according to the invention, which process comprises reacting a compound containing a plurality of units of formula I with an aminoorganoaluminane of formula II - 4 H2NA1R1R2 (II) in which formula represents an alkyl radical containing 1 to 12 carbon atoms or a phenyl, alkyl(Cj-C^phenyl or phenylalkyl() radical and R2 represents H or a radical denoted by Rlf under conditions permitting the formation of an =A1X bridge.

When the silicon of the units of formula I carries a hydrogen atom, according to an alternative form or additionally, the reaction may result in an additional crosslinking by reaction of SiH with =NH, accompanied by release of H2 and/or NH3 and/or of hydrocarbons corresponding to R3 and R2.

The invention further provides a crosslinkable composition comprising a mixture of a polysilazane containing a plurality of units of formula I and a compound of formula II, suitably in an Si/Al molar ratio of 1 to 100.

Among the atoms or radicals which may be carried by the available valencies of the silicon or nitrogen atoms (as defined above) there may be mentioned in particular hydrogen, a methyl radical, a vinyl radical or a phenyl radical. Preferred products are those containing units of formula I, in which the nitrogen atom carries a hydrogen atom and the silicon atom carries a methyl or vinyl radical and/or a hydrogen atom. - 5 Clearly starting from mixtures of reactants it is possible to obtain polymers of units of formula I in which the substituents of the silicon and nitrogen atoms differ from one unit to another.

Uncrosslinked polysilazanes containing the units of formula I are known and do not form a subject of the invention. For their preparation reference may be made, for example, to European Patent Application published under No. 209,472.

The reaction resulting in the formation of the =A1X bridges can generally be carried out at room temperature.

It is also possible to operate at a temperature of between -40 and 200°C.

The reaction is advantageously conducted in a solvent medium.

By way of illustration of the solvents which can be employed there may be mentioned especially halogenated compounds such as methylene chloride, dialkyl ethers such as diethyl ether, cyclic ethers such as tetrahydrofuran, tetrahydropyran and 1,4-dioxane, aliphatic hydrocarbons such as pentane or hexane, and aromatic hydrocarbons such as benzene, toluene or xylene.

In the preparation of the crosslinked polymers in accordance with the invention the reactants (compound containing a plurality of units of formula I and aluminum derivative of formula II) are suitably used in quantities - 6 such that the Si/Al molar ratio is 1 to 100 and preferably 3 to 40. The crosslinkable compositions comprising a polysilazane of formula I and an organoaluminane form a part of the invention.

The crosslinked polymers according to the invention can be in various physical states, ranging from viscous liquids to solids.

Pyrolysis of the crosslinked polymers according to the invention can be carried out at a temperature which is generally between 850°C and 1200°C and results in a yield (pyrolysis residue content) of ceramic which is markedly higher than that obtained from the compound solely containing the units of formula I.

In addition to its role as a crosslinking agent due to incorporation into the polysilazane chain (formation of =A1X bridges), it is thought that the aminoorganoaluminane (II) may also have a catalytic role in a crosslinking via SiH/NH bridges or in other crosslinking reactions such as those described, for example, in the abovementioned European Patent Application No. 209,472.

The following Example further illustrates the invention.

EXAMPLE The polysilazane employed corresponds to the formula - Ί CH η — I 3 --Si - NH-I *— H —1 n It has been prepared by reaction of ammonia and of methyldichlorosilane by following the conditions described in Example 1 of European Patent Application No. 209,472.

The aminodiethylaluminane was prepared from 5 triethylaluminium and ammonia (1/1 molar ratio) under the conditions described in US Patent 4,696,968 (Example 1).

A solution of aminodiethylaluminane [NH2A1(C2H5)2] in hexane is added to a solution of 1 g of polysilazane in methylene chloride (Si/Al molar ratio = 18). This operation is carried out at room temperature and the reaction is allowed to continue for 16 hours.

After removal of hexane and methylene chloride a hexane-soluble viscous liquid is collected.

After pyrolysis at 1000°C under argon the residue 15 content (degree of ceramisation) is 80 %, whereas it is only 40 % with the polysilazane by itself and 55 % with triethylaluminium (Si/Al molar ratio = 21).

The percentage compositions of the viscous liquid and of the ceramic obtained after pyrolysis are the following: C H N Al Si Viscous liquid 23.8 7.5 18.4 2.4 43.8 Ceramic 12.48 0.24 27.6 2.9 52.9

Claims

1. A crosslinked polysilazane comprising a plurality of units of formula I Si - N (I) 5 and at least one bridge of formula =A1X between nitrogen atoms of the units, in which each of the available valencies of the silicon or nitrogen atoms bears a hydrogen atom, an alkyl radical containing from 1 to 6 carbon atoms, a vinyl or allyl radical, a cyclohexyl radical or an aryl, 10 alkylaryl or aralkyl radical containing up to 18 carbon atoms, and X represents an -NHZ group, in which Z represents a hydrogen atom or a metal or metalloid, or X represents Si=.

2. A polysilazane according to Claim 1, in which, 15 in at least some of the units of formula I, the nitrogen atom carries a hydrogen atom and the silicon atom carries a hydrogen atom and/or a methyl or vinyl radical.

3. A polysilazine according to Claim 1 or 2, in which Z represents silicon. 20

4. A polysilazine according to Claim 1 substantially as described herein. - 9 5. A process for the preparation of a polysilazane as claimed in any preceding claim, which process comprises reacting a compound containing a plurality of units of formula I with a compound of formula

5. II h 2 nair 1 r 2 (II) in which R x represents an alkyl radical containing 1 to 12 carbon atoms or a phenyl, alkyl(¢^-04)phenyl or phenylalkyl (C-^—C 4 ) radical and R 2 represents H or a 10 radical denoted by R 1# under conditions permitting the formation of an =A1X bridge.

6. A process according to Claim 5, wherein the Si/Al molar ratio is 1 to 100.

7. A process according to Claim 6, wherein the 15 Si/Al molar ratio is 3 to 40.

8. A process according to any one of Claims 5 to 7 wherein the compound of formula II is aminodiethylaluminane [NH 2 Al(C 2 H 5 ) 2 ].

9. A process for the preparation of a 20 polysilazane according to Claim 1 substantially as described herein.

10. A process for the preparation of a crosslinked polysilazane substantially as described in the Example.

11. A polysilazane according to Claim 1 prepared 25 by the process claimed in any one of Claims 5 to 10. - 10

12. A crosslinkable polysilazane composition comprising a mixture of a polysilazane containing a plurality of units of formula I as defined in Claim 1 and a compound of formula II as defined in Claim 5.

13. A composition according to Claim 12 wherein the Si/Al molar ratio is 1 to 100.

14. Ceramic material obtained by pyrolysis of a crosslinked polysilazane as claimed in any one of Claims 1 to 4 and 11.

15. Ceramic material according to Claim 14 substantially as described in the Example.