Peroxide-substituted polysilazanes

Abstract: The preparation of an uncrosslinked polysilazane containing chemically bound peroxide groups by reacting a polysilazane and a hydroperoxide at a temperature below the decomposition temperature of the hydroperoxide is disclosed. A thermoset polymer is formed by heating a peroxide-substituted silazane that is also substituted by alkenyl groups or alkynyl groups. The chemically bound peroxide decomposes to yield a free radical, thereby initiating crosslinking of the unsaturated groups. The peroxide-substituted polysilazanes can be pyrolyzed to form ceramic articles.

Description: This invention relates to the preparation of polysilazanes. The crosslinking of polysilazanes with organic peroxides is known. For example, U.S. Patent No. 4,722,988, H. Porte et al., discloses an organopolysilazane composition that can be crosslinked by an energy input. The composition comprises an organopolysilazane and a free radical generator. U.S. Patent No. 5,021,533, J. M. Schwark, discloses crosslinkable poly(thio)ureasilazane compositions containing a free radical generator such as an organic peroxide. However, these organopolysilazanes are not self-thermosetting polymers. The peroxide must be mixed into the polymer before it can be thermoset. U.S. Patent No. 3,843,703, R. L. Ostrozynski, teaches a process for preparing a silicon peroxide compound by reacting a silazane with a hydroperoxide. However, the silyl peroxides that are produced do not contain any Si-N bonds as in the polysilazanes of the present invention. During the silyl peroxide synthesis, the amine groups are lost as free amine and no ammonium halide salt is formed. The process for preparing the polysilazanes of the present invention is characterized by reacting a polymeric silazane with a hydroperoxide having the formula ROOH, where R is selected from H, substituted or unsubstituted 1-10 carbon alkyl, 2-10 carbon alkenyl, 2-10 carbon alkynyl, aryl, a carboxylic acid or a silyl group, under conditions and for a time effective to produce an uncrosslinked polysilazane having chemically bound peroxide groups. Also according to the invention, an uncrosslinked, peroxide-substituted polysilazane that is also substituted with alkenyl or alkynyl groups can be heated to decompose the peroxide groups, initiate a crosslinking reaction and form a thermoset polymer. Also according to the invention, the peroxide-substituted polysilazane, which can be filled with a metal or ceramic powder, can be pyrolyzed to form a ceramic article. The peroxide-substituted polysilazanes of this invention have several advantages over systems in which the peroxide is simply mixed with, but not reacted onto, the polysilazane. Since the peroxide is attached to the backbone of the polymer, segregation of the peroxide upon storage cannot occur. In addition, the curing agent is distributed throughout the polymer on a molecular level. This is particularly advantageous for solid polysilazanes, since it is difficult to obtain a homogeneous distribution of a peroxide in such polymer. The term "polysilazane" is meant to include any polysilazane or modified polysilazane, such as the isocyanate-modified polysilazanes disclosed in U.S. Patent No. 4,929,704. Preferably, such polysilazanes include alkenyl or alkynyl groups. The silazanes can be liquids or solids, provided they are miscible with the hydroperoxide or soluble in a solvent compatible with the hydroperoxide. The preferred polysilazanes for use in the present invention can be prepared by reacting ammonia, or a mixture of ammonia and a substituted or unsubstituted 1-4 carbon alkyl or aryl amine, with a halogenated silicon compound selected from the group consisting of RSiX 3 , RR'SiX 2 and mixtures thereof, including mixtures where more than one compound having the formula RSiX 3 or RR'SiX 2 is used. Optionally, RR'R''SiX, SiX 4 or mixtures thereof can also be present in the reaction mixture. X can be Cl, Br or I; however, Cl is preferred. R, R', R'' can be the same or different and are selected from the group consisting of H, substituted or unsubstituted 1-6 carbon alkyl, aryl, 2-6 carbon alkenyl and 2-6 carbon alkynyl groups. Preferably, the reaction mixture also contains at least one halogenated silicon compound having an alkenyl or alkynyl group. Examples of halogenated silicon compounds suitable for use in the process of this invention include, for example, methyldichlorosilane, vinylmethyldichlorosilane, tetrachlorosilane, tetrabromosilane, trichlorosilane, vinyltrichlorosilane, methyltrichlorosilane, phenyltrichlorosilane, ethyltrichlorosilane, propyltrichlorosilane, butyltrichlorosilane, methyltribromosilane, dimethyldichlorosilane, phenylmethyldichlorosilane, dimethyldibromosilane, trimethylchlorosilane, dimethylchlorosilane, dimethylvinylchlorosilane, and trimethylbromosilane. Hydroperoxides of the formula ROOH can be used. Suitable hydroperoxides include, for example, hydrogen peroxide, methyl hydroperoxide, ethyl hydroperoxide, propyl hydroperoxide, isopropyl hydroperoxide, n-butyl hydroperoxide, dimethylbenzyl hydroperoxide, t-butyl hydroperoxide, n-octyl hydroperoxide, 2,4-dihydroperoxy-2,4-dimethylpentane, 2,5-dihydroperoxy-2,5-dimethylhexane, cumyl hydroperoxide, p-chlorocumyl hydroperoxide, allyl hydroperoxide, 1,1,2-trimethylallyl hydroperoxide, 1,1-dimethylprop-2-ynyl hydroperoxide, peroxyacetic acid, diperoxyterephthalic acid, peroxybenzoic acid, p-methylperoxybenzoic acid, triphenylsilyl hydroperoxide, tribenzylsilyl hydroperoxide, and diphenylmethyl hydroperoxide. Hydroperoxide levels of from 0.01-10.0 wt% (based on the weight of the polysilazane) can be used. The preferred hydroperoxide range is 0.01-5.0 wt%. The more preferred hydroperoxide range is from 0.01-2.0 wt%. The most preferred hydroperoxide range is from 0.03-1.0 wt.%. The following explanation is given to provide a better understanding of the present invention: Hydroperoxides are compounds with the general formula ROOH, and like alcohols and carboxylic acids, are reactive, protic compounds. Such compounds react with a silazane at a Si-N bond to produce Si-OOR and N-H groups. Pike and Shaffer ( Chemistry and Industry , 1957, p1294) showed that silyl peroxides could be prepared by the reaction of a silylamine with a hydroperoxide (Eq. 1). Me 3 SiNHR + t BuOOH → Me 3 SiOO t Bu + RNH 2 It has been discovered that when a polysilazane is used in place of a silylamine, the reaction produces a peroxide group bound to the polymer backbone. For polysilazanes containing a plurality of NH groups, ammonia is also a by-product of the reaction. For example, when a cyclic polysilazane of the formula (R 2 SiNH) x is reacted with a hydroperoxide, a ring-opening reaction occurs at a Si-N bond to place a silyl peroxide end group on the Si and form an NH 2 group (Eq.2). The SiNH 2 group can remain in the peroxide-substituted polysilazane if the substituents on the Si atom are bulky, e.g., ethyl or t-butyl. When the substituents on the Si atom are not bulky, e.g., hydrogen or methyl, several additional reactions can occur at this site (Scheme 1). While not wishing to be bound by any particular theory, self-condensation of two Si-NH 2 groups could occur to produce ammonia and a longer chain polysilazane (Path A). Alternatively, the Si-NH 2 group might react with another equivalent of hydroperoxide to produce ammonia and another Si-OOR' group (Path B). It is believed that Path B is often the preferred mode of reaction because the Si-NH 2 group created in the initial ring-opening reaction with the hydroperoxide is the most reactive Si-N bond in the reaction mixture. Both Path A and Path B produce peroxide-substituted polysilazanes which, if they contain alkenyl or alkynyl substituents, are crosslinked when heated to temperatures at which the silyl peroxide undergoes decomposition. For example, a liquid poly(methylvinyl)silazane, [(MeSiHNH) 0.8 (MeSiCH=CH 2 NH) 0.2 ] x , was reacted with 5 mol% t-butyl hydroperoxide under conditions detailed in Example 1 to produce a liquid peroxide-substituted polysilazane. The reaction of the polysilazane with the hydroperoxide can be carried out over a wide range of conditions and temperatures, provided the reaction mixture is not heated to a temperature at which the hydroperoxide decomposes. The reaction temperature range can be from -78°C to 175°C. The more preferred reaction temperature range is from -78°C to 30°C. The most preferred reaction temperature range is from 0°C to 30°C. The reaction is preferably conducted using a solvent, such as pentane, hexane, heptane, octane, benzene and toluene, although any solvent compatible with the hydroperoxide can be used. Optionally, the reaction can be conducted without a solvent if the polysilazane is a liquid. Although not wishing to be bound by any particular theory, upon heating, it is believed that the peroxide moiety will undergo decomposition to give peroxide radicals that initiate crosslinking reactions to produce a crosslinked silazane polymer. When the peroxide-substituted polysilazane contains alkenyl or alkynyl groups, the crosslinking reaction produces a thermoset, crosslinked silazane polymer. The crosslinking reaction is conducted at a temperature at which a significant fraction of the peroxide has decomposed to form radical species. This temperature will depend upon the particular peroxide moiety that is bound to the polysilazane backbone and can be readily determined by one skilled in the art. Thus, a liquid silazane can be thermoset to a solid. A solid polysilazane can also be thermoset so that it will not melt upon heating. The polysilazanes of the present invention can contain ceramic or metal fillers. Suitable fillers include, for example, SiC, Si 3 N 4 , SiO 2 , BN, AlN, Al 2 O 3 , TiN, TiC, Si, Ti, Zr, Hf, ZrC, and B 4 C in the form of powders, whiskers or platelets. The filled or unfilled, peroxide-substituted polysilazanes can be shaped by processes, including, for example, dry pressing, isostatic pressing, slip casting, tape casting, extrusion and injection molding. Solid or liquid polymers can be used. In addition, the polymers of the present invention can be used to form coatings, foams, infiltrate preform structures, or as fiber precursors. The shaped body can be thermoset by heating using conventional treatment parameters. A ceramic article can then be produced by pyrolysis and sintering of the shaped article in a non-reactive atmosphere such as argon, helium or hydrogen, or in a reactive atmosphere such as ammonia. In the following examples, all reactions were performed in an Ar-filled dry box or under nitrogen using standard inert atmosphere techniques. t BuOOH was obtained from Aldrich as a 3.0 M solution in isooctane and used as received. Hexane was dried using 3Å and 13X Linde molecular sieves. The poly(methylvinyl)silazane, [(MeSiHNH) 0.8 (MeSiCH=CH 2 NH) 0.2 ] x , was prepared by standard ammonolysis procedures such as those described in U.S. Patent No. 4,929,704, and used without further purification. Thermogravimetric analyses (TGA) were performed at 20°C/min from 25-1000°C in nitrogen. Differential scanning calorimetry (DSC) was performed at 10°C/min under nitrogen from 40-320°C. EXAMPLE 1 A 50 ml, two-necked, round-bottomed flask was oven-dried and equipped with a stir bar and septum, and sparged with nitrogen. The flask was charged with 5.00 g (77.7 mmol) of a liquid poly(methylvinyl)silazane, [(MeSiHNH) 0.8 (MeSiViNH) 0.2 ] x , and 10.0 ml hexane by syringe. A 3.0 M solution of t BuOOH (1.30 ml, 3.89 mmol) in isooctane was added dropwise by syringe over 10 minutes. An exotherm from 24.7°C to 29.7°C occurred and gas evolution was observed. The gas was basic when tested with pH paper. The reaction mixture was stirred for an additional 30 minutes at 25°C. Gas evolution was still evident so the reaction mixture was stirred overnight (16 hours) at 25°C. The hexane was removed in vacuo to give a peroxide-substituted polysilazane as a colorless oil. DSC (10°C/min, 40-320°C): Heat of reaction, 105.4 cal/g; Peak decomposition temperature, 155.3°C. A 17 ml vial equipped with a septum and thermocouple was sparged with nitrogen and charged with a 1 ml sample of the peroxide-substituted polysilazane by syringe. The sample was placed in a room temperature oil bath and slowly heated. When the oil bath temperature was 124°C, the reaction mixture exothermed to 203.7°C and thermoset to a hard solid. Gas evolution was visible and the mixture foamed slightly. TGA (20°C/min, 25-1000°C): 75.9 wt%. EXAMPLE 2 A 50 ml, three-necked, round-bottomed flask was oven-dried, equipped with a stir bar and septa, and sparged with nitrogen. The flask was charged with 20 ml hexane and 10.0 g (155.4 mmol) of poly(methylvinyl)silazane, [(MeSiHNH) 0.8 (MeSiViNH) 0.2 ] x . The appropriate amount, shown below, of a 3.0 M solution of t BuOOH in isooctane was added by syringe. Each reaction was started at 21.5°C. The maximum temperature reached during the hydroperoxide addition is listed below: 1) 12 µl (0.036 mmol, 0.032 wt%); Max. temperature (T) = 21.5°C 2) 60 µl (0.18 mmol, 0.16 wt%); Max. T=21.9°C. 3) 120 µl (0.36 mmol, 0.032 wt%); Max. T=21.9°C. 4) 600 µl (1.80 mmol, 1.62 wt%); Max. T=23.9°C. For purposes of this data, Wt.% is based on the weight of the polysilazane. Gas evolution was observed in each reaction. The reaction mixtures were stirred for 1.5 hours until the gas evolution had ceased. The hexane was removed in vacuo to give the peroxide-substituted polysilazane as a clear oil in quantitative yield. Each modified polysilazane was thermoset using the following procedure. A 17 ml vial equipped with a septum and thermocouple was sparged with nitrogen and charged with a 1 ml sample of the peroxide-substituted polysilazane by syringe. The sample was placed in a preheated 160°C oil bath. As soon as the sample was placed in the bath, a timer was started and the timer was stopped when the maximum exotherm temperature was reached. Each modified polysilazane thermoset to a solid. The exotherm temperature attained, the time to reach this temperature, and the TGA yield of each thermoset product is presented in Tables 1 and 2. Peroxide-substituted Polysilazane TGA Yields and Quantity Initiator Used Radical Generator Generator Level (wt. %) Maximum Temp. (°C) TGA Yield (Wt. %) t BuOOH 0.032 216.8 70.07 0.16 206.4 75.1 0.32 209.7 73.4 1.60 241.2 75.2 Cure Time and Initiator Level Peroxide-substituted Polysilazane Radical Generator Generator Level (wt. %) Cure Time (Minutes) Maximum Temp.°C t BuOOH 0.032 6.55 216.8 0.16 4.63 206.4 0.32 5.15 209.7 1.60 2.50 241.2 EXAMPLE 3 A peroxide-modified polysilazane was prepared by the method described in Example 2. A 250 ml Schlenk flask was equipped with a stir bar and a septum and sparged with nitrogen. The flask was charged with poly(methylvinyl)silazane (50.0 g) and 100 ml hexane by syringe. The hydroperoxide (0.60 ml of a 3.0 M solution of t-butyl hydroperoxide in isooctane) was added dropwise via a syringe over 5 minutes. Gas evolution was observed. The reaction mixture was stirred for 1.5 hours and the hexane was removed in vacuo to give a peroxide-substituted polysilazane as a clear oil in quantitative yield. A 10.0 g sample of the peroxide-substituted polysilazane was mixed, by hand, with 14.22 g of a Starck Grade S silicon nitride powder. The fluid mix was poured into a test tube which was then placed in a 160°C oil bath. The sample was thermoset and then cooled to room temperature. The thermoset, solid plug was removed from the test tube mold and retained the shape and surface finish of the mold. The piece could not be broken by hand. The thermoset green body was then pyrolyzed under Ar from room temperature to 700°C at 0.5°C/min and from 700°C to 1200°C at 10°C/min. After cooling to room temperature, a black fired ceramic article having the same surface finish and shape as the mold was obtained. Likewise, a 6.89 g sample of the peroxide-substituted polysilazane was mixed, by hand, with 10.00 g of a Starck B 10 beta silicon carbide powder. The fluid mix was poured into a test tube which was then placed in a 160°C oil bath. The sample was thermoset and then cooled to room temperature. The thermoset, solid plug was removed from the test tube mold and retained the shape and surface finish of the mold. EXAMPLE 4 A 10.00 g sample of the peroxide-substituted polysilazane of Example 3 was placed in a nitrogen-sparged 29.6 ml (1 oz.) jar capped with a septum. The jar was placed in a 160°C oil bath and the sample was thermoset. After cooling to room temperature, the sample was broken into chunks and placed in a graphite boat in a tube furnace under Ar and heated from room temperature to 1600°C at 10°C/min. The sample was held at 1600°C for 6 hours and then cooled to room temperature. A black ceramic material was obtained in 52.7 wt% yield. The ceramic was not a powder; chunks present in the unfired sample were maintained in the fired sample.

Claims: A process for the preparation of a polysilazane, characterized by reacting a polymeric silazane with a hydroperoxide having the formula ROOH where R is selected from H, substituted or unsubstituted 1-10 carbon alkyl, 2-10 carbon alkenyl, 2-10 carbon alkynyl, aryl, a carboxylic acid or silyl group, under conditions and for a time effective to produce an uncrosslinked polysilazane having chemically bound peroxide groups. The process of claim 1, further characterized in that the hydroperoxide is t-butyl hydroperoxide. The process of claim 1, further characterized in that the hydroperoxide is present at levels of 0.01 to 10.0% by weight of the polysilazane. The process of claim 3, further characterized in that the levels of hydroperoxide are within the range of 0.03 to 1.0% by weight of the polysilazane. The process of any of the preceding claims characterized in that the polysilazane is substituted with alkenyl or alkynyl groups. The process of any of the preceding claims characterized in that the reaction temperature is less than the temperature of decomposition of the hydroperoxide. The process of claim 6, further characterized in that the temperature is within the range of -78°C to 30°C. The process of claim 6, further characterized in that the reaction is performed in the presence of a solvent. The process of any of the preceding claims characterized in that the peroxide-substituted polysilazane is a solid. The process of claim 5, further characterized by heating the uncrosslinked polymer substituted with alkenyl or alkynyl groups at temperatures sufficiently high to decompose the chemically bound peroxide to yield a free radical and continuing the heating to initiate a crosslinking reaction and form a thermoset polymer. An uncrosslinked polysilazane characterized in that it is substituted with chemically bound peroxide groups. The polysilazane of claim 11, further characterized in that it is additionally substituted with alkenyl or alkynyl groups. The polysilazane of claims 11 or 12, further characterized in that it is mixed with a ceramic or metal powder. Use of the polysilazane of claims 11-13 to make ceramic articles.

Inventors: SCHWARK JOANNE MARIE, SCHWARK, JOANNE MARIE
Applicants: LANXIDE TECHNOLOGY CO LTD, LANXIDE TECHNOLOGY COMPANY, LP
Application year: 1992
Main classification: C08G77
IPC codes: C08L83, C08G77, C01B15, C08K3, C04B35