Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From silicon reactant having at least one...
Reexamination Certificate
1999-11-12
2001-12-11
Dawson, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From silicon reactant having at least one...
C528S028000, C528S031000, C528S038000, C556S412000
Reexamination Certificate
active
06329487
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to the preparation of ammonolysis products and more particularly to the synthesis of novel silazane and/or polysilazane compounds, including monomers, oligomers and polymers containing the Si—N structure in the molecule.
BACKGROUND OF THE INVENTION
Silazanes, which have a Si—N—Si bond configuration, are increasingly important because they can be pyrolyzed to yield ceramic materials, such as silicon carbide and silicon nitride.
Silazanes are usually synthesized by an ammonolysis process wherein ammonia or a primary amine is reacted with a halide substituted silane. The ammonolysis of organohalosilanes is a complex process consisting of several concurrent reactions as shown below. These formulas carry no structural implication, they merely are average formulations to illustrate the reactions such as:
and Homo- and heterofunctional condensation.
The preparation of silazanes by ammonolysis has been described in several U.S. patents. For instance, U.S. Pat. No. 4,395,460, issued to Gaul, describes a process for the preparation of polysilazanes in which gaseous ammonia is introduced to a solution of chlorodisilanes that have been dissolved in an inert solvent. However, during the reaction NH
4
Cl is precipitated concurrently with the formation of the ammonolysis products. The precipitated NH
4
Cl greatly increases the viscosity of the reaction mixture and interferes with the progress of the reaction. To overcome this problem, additional inert solvent must be added to the reaction mixture to facilitate agitation of the mixture. Furthermore, to recover a purified ammonolysis product several constituents of the reaction product mixture have to be removed. The precipitated NH
4
Cl formed during the reaction and intermixed with the ammonolysis products has to be removed by filtration and the filter cake washed with additional solvent for complete product recovery. Subsequently the inert solvent which is used for dissolving the chlorodisilanes, for reducing the viscosity of the reaction mixture, and for washing the filtered crystals must be removed from the preferred products.
U.S. Pat. No. 4,954,596, issued to Takeda et al, describes preparation of organosilazanes by introducing gaseous ammonia into a reaction mixture comprising organochlorosilanes dissolved in an organic solvent. However, the added organic solvent must be removed by distillation to isolate the silazane products. Likewise in U.S. Pat. No. 2,564,674, organochlorosilanes are dissolved in ether before the ammonolysis process and additional ether is added during the process to dissolve the silicon compounds and prevent their gelation. Again, purification of the final product requires several steps.
U.S. Pat. No. 4,255,549, issued to Christophliemk et al., describes reacting organohalosilanes, dissolved in an inert solvent, with liquid ammonia to form ammonolysis products. To maintain the reaction course and to prevent overheating due to a high heat of reaction and/or heat of crystallization of precipitating ammonium halide salt, an inert solvent is added to the reaction vessel. As a result of this addition, the solvent has to be evaporated under controlled conditions to produce the polymer films.
As apparent from the foregoing description, preparing silazane products by known ammonolysis methods leads to unwanted co-products, such as NH
4
Cl precipitates, that prompts the need for increased additions of inert solvent to the reaction mixture. The addition of the solvent is required to decrease the viscosity and improve agitation of the reaction slurry. Furthermore, an inert solvent is needed to reduce the heat of reaction and/or heat of crystallization due to precipitating ammonium halide salts. However, the NH
4
Cl precipitates must be filtered from the reaction slurry and the inert solvent removed from the final ammonolysis product.
Another problem encountered during the production of silazanes is the formation of a high proportion of low molecular weight species. These low molecular weight silazanes can evaporate during pyrolysis resulting in a reduced weight yield of the ceramic product relative to the starting silazane material. British patent, 737,229, issued to Midland Silicones Limited, describes a method for producing silazanes wherein organohalosilanes, completely substituted with organic groups and/or halogen atoms and dissolved in an inert solvent, are added simultaneously to ammonia under pressure. However, the majority of prepared organocyclosilazanes are limited by the starting compounds to only 3-4 Si—N linkage units and a low yield of polysilazanes. As such, the prepared silazanes are volatile and difficult to pyrolize to ceramic material.
Accordingly, there is a need for novel silazanes and/or polysilazanes having an increased number of Si—N units and for improved methods for preparing silazanes, and/or polysilazanes that provide a means to easily separate desired products from any unwanted co-products generated in the reaction, that do not require large quantities of inert solvent to be introduced into the reaction mixture, that moderate the reaction exotherm for quick and efficient ammonolysis and provide polysilazanes having an increased number of Si—N linkages.
SUMMARY OF THE INVENTION
For purposes of this invention, the terms and expressions appearing in the specification and claims, are intended to have the following meanings:
“Silazane” as used herein means monomers, oligomers, cyclic and linear polymers having one to four Si—N repeating units in the compound.
“Polysilazane” as used herein means oligomers, cyclic, polycyclic, linear polymers or resinous polymers having at least five Si—N repeating units in the compound.
“Ammonolysis products” as used herein is at least one member selected from the group including silazanes, polysilazanes, aminosilanes, organosilazanes, organopolysilazanes and mixtures thereof.
“Si—H starting compounds” as used herein is at least one member selected from the group including halosilanes, organohalosilanes, silazanes and/or polysilazanes, all of which have at least one Si—H bond.
“Anhydrous liquid ammonia” as used herein means anhydrous ammonia containing less water than an amount that will cause unwanted hydrolysis of the product.
It is an object of the present invention to provide novel compounds containing at least one Si—N unit.
It is another object of the present invention to provide improved methods of preparing both known and novel compounds containing at least one Si—N unit from starting compounds containing at least one Si—H bond.
Yet another object of the present invention is to provide novel liquid and solid compounds containing the Si—N unit having modifiable viscosity.
A further object of the present invention is to provide methods to catalytically polymerize novel and/or known silazanes and/or further polymerize polysilazanes.
A still further object is to provide a method for catalytically synthesizing known and/or novel silazanes and/or polysilazanes wherein a small amount of an acid catalyst is added to initiate the synthesis and thereafter the effective catalyst is generated in the ammonolysis reaction.
Another object of the present invention is to provide an improved method for preparing known and/or novel silazanes and/or polysilazanes wherein the prepared ammonolysis products are easily separated from the reaction mixture and do not require extensive purification for removal of unwanted by-products.
Yet another object of the present invention is to provide a method for preparing known and/or novel silazanes and/or polysilazanes without requiring the addition of inert solvents to dissolve the reactants, reduce increasing viscosity of the reaction mixture during ammonolysis or to moderate the heat of reaction and/or heat of crystallization of formed ammonium salts.
Still another object of the present invention is to provide methods for preparing known and/or novel silazanes and/or polysilazanes having viscosities ranging from liquid to solid. The silazanes and/or polysilazanes having at least one struc
Abel Albert E.
Knasiak Gary J.
Kruger Tracy A.
Mouk Robert W.
Dawson Robert
Ellis Howard M.
Fuierer Marianne
Kion Corporation
Zimmer Marc S.
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