Polysilazane

Polysilazanes are polymeric compounds in which form the chemical backbone of silicon and nitrogen atoms in an alternating arrangement. While each silicon atom is often bound to two nitrogen atoms and are preferably formed so that each nitrogen atom to two silicon atoms, molecular chains and rings of the formula [ R1R2Si NR3 - ] n. R1 -R3 can thereby be hydrogen atoms or organic radicals. If only H atoms present as substituents, referred to the polymer as perhydropolysilazane ( [ H2Si - NH] n, also Polyperhydridosilazan inorganic Polysilazane ). Are hydrocarbon radicals bonded to the silicon, one speaks of organo-polysilazanes. In their molecular structure, the Polysilazanes [ R1R2Si - NH] n are closely related with the polysiloxanes [ R1R2Si -O ] n ( silicones), with which they are isoelectronic.

History

The synthesis of Polyorganosilazanen was first described in 1964 by Kruger and Rochow. They placed first by reaction of ammonia with chlorosilanes ( ammonolysis ) trimeric or tetrameric cyclosilazanes ago, which then react under the influence of a catalyst at high temperatures to form high molecular weight polymers. The ammonolysis of chlorosilanes is still the most important method for the synthesis of (poly ) silazanes represents the large-scale production of chlorosilanes by the Müller -Rochow process thus laid in 1940 also laid the foundation for the development of Silazanchemie. In the 1960s, the first attempts for the conversion of the organosilicon polymer in a quasi- ceramic materials have been described. Process, suitable ( so-called preceramic ) polymers were heated to about 1000 ° C or higher, which initially form with elimination of organic groups and hydrogen, and rearrangement of the atoms amorphous inorganic networks, both in chemical and in physical point of view a unique property profile have. With the help of polymer- derived ceramics (English polymer -derived ceramic, PDC ) may in particular new applications be developed in the area of ​​high performance materials. As the main preceramic polymers polysilanes [ - R1R2Si R1R2Si ] n, polycarbosilanes [ R1R2Si - CH2 ] n, polysiloxanes [ R1R2Si -O ] n and polysilazanes [ - R1R2Si NR3 ] n may be mentioned.

Structure

All such polymers are polysilazanes, composed of one or more repeat units, the monomers. By using an inanimate basic units are formed differently sized chains, rings and three-dimensionally crosslinked macromolecules with a more or less broad molecular weight distribution. The monomer unit is also used to describe the chemical composition and the linking of atoms ( coordination sphere ), but without making statements about the macromolecular structure.

Polysilazanes in each silicon atom is bonded to two nitrogen atoms each nitrogen atom and at least two silicon atoms ( it may be also three). If all other valences are saturated by hydrogen atoms, the generated perhydropolysilazane [ H2Si - NH] n, the idealized structure shown on the right. Wherein the at least one organo-polysilazanes organic radical bonded to the silicon. The number and type of radicals has significant influence on the macromolecular structure of these Polysilazanes.

Silazane copolymers are generally prepared by ammonolysis of chlorosilane mixtures. In this chemical reaction, the different chlorosilanes react quickly usually similar. Therefore, the monomer units are randomly distributed in these copolymers. The usual case of silicones designations M, D, T and Q to describe the structure in Polysilazanes hardly use.

Production

As starting materials in the Polysilazanherstellung one usually uses the industrially available and inexpensive chemicals ammonia and chlorosilanes. In the ammonolysis reaction produces large amounts of ammonium chloride, which must be separated from the product.

On a laboratory scale, this reaction is carried out in a dry organic solvent ( silazanes decompose in the presence of water), in which ammonium chloride does not dissolve, and then filtered. Since the filtration step is very time -consuming and costly, preparation methods were developed in which the last step of the synthesis are no solids.

The liquid ammonia process for Polysilazane synthesis was developed by Commodore / KiON. Here, the chlorosilane or chlorosilane mixtures is metered into an excess of liquid ammonia. The resulting ammonium chloride is dissolved in the ammonia and forms in addition to the silazane a second liquid phase. The two liquids can then be separated from each other at the interface. This patented process is now used by the firm AZ Electronic Materials for the production of Polysilazanes.

In the formerly marketed by Hoechst AG products VT 50 and ET ​​70 is Polysilsesquiazan solutions. The preparation was carried out in two stages: first a trichlorosilane with dimethylamine was implemented and separated the resulting monomeric aminosilane from dimethyl ammonium chloride. In the subsequent reaction of the aminosilane with ammonia salt-free results in a polymer.

Is used as the source of nitrogen instead of ammonia, hexamethyldisilazane (HMDS ), a transamination takes place. The freed from the chlorosilane chlorine atoms are bound to the trimethylsilyl groups of the HMDS, so no chlorine-containing solids formed. This method was used by Dow Corning for the preparation of Hydridopolysilazans HPZ.

Numerous other methods for the synthesis of polymeric SiN scaffolds have been described in the literature ( for example, dehydrating coupling between Si -H and N- H, ring-opening polymerization ), but have not been used on a large scale.

For industrial production of perhydropolysilazane [ H2Si - NH] n, use is made to be the ammonolysis in a solvent. The resulting higher price is (including insulating effect at low film thickness) taken in the electronics industry because of the special properties as a coating material in purchasing. The product is an approximate 20% solution available.

Nomenclature

Silicon-nitrogen compounds having alternately arranged silicon ( " Sila " ) and nitrogen atoms ( " TFI " ) are referred to as a silazane. Simple representative of Silazanes are the disilazane H3Si NH- SiH3 and hexamethyldisilazane ( H3C ) 3Si -NH -Si ( CH3 ) 3 If only one silicon atom to the nitrogen atom bound, one speaks of silylamines or amino silanes (eg Triethylsilylamin ( H5C2 ) 3Si - NH 2). Are three structure-forming nitrogen atoms arranged around the silicon atom, the compounds hot Silsesquiazane. Small ring-shaped molecules with a backbone of Si -N is called cyclosilazanes (eg Cyclotrisilazan [ H2Si - NH] 3). Polysilazanes, however, are polymeric silazanes, which are composed of different sizes of rings and chains and have a molecular weight distribution. A polymer having the general formula ( CH3) 3Si -NH- [(CH3) 2Si - NH] n- Si (CH3) 3 is referred to as poly ( dimethylsilazane ). According to the IUPAC rules for naming linear organic polymers, the connection should really be called poly [ aza ( dimethylsilylene ) ], according to preliminary rules for inorganic macromolecules catena- poly [-m -aza ( dimethylsilicium ) ].

Properties

Polysilazanes are colorless to yellow liquids or solids. Manufacturing processes often contain liquids dissolved ammonia, which dominates the smell. The average molecular weight can be from a few thousand to about 100,000 g / mol, while the density is usually about 1 g/cm3. The state of aggregation and the viscosity are dependent on both the molecular weight and of the molecular macrostructure. Fixed Polysilazanes are formed by chemical reactions of liquid produced (linking smaller molecules into larger, cross-linking). The solids may be fusible or infusible and insoluble and soluble in organic solvents. In general, there are thermosets, in some cases, however thermoplastic processing possible.

After the synthesis is often takes place an aging process, in which dissolved ammonia plays an important role. The costs incurred in the ammonolysis R3Si - NH2 groups form with elimination of ammonia silazane units. Can not escape the ammonia silazane units are again split into R3Si - NH2 groups. Thus, a common gas exchange to the liquids to increase the molecular mass of lead (removal of ammonia). Functional groups that are not directly involved in the backbone, can react with each other under appropriate conditions ( eg, Si -H - N -H groups ), creating a growing network of rings and chains occurs. An increase in the molecular weight can be observed even after prolonged storage at elevated temperature or in the sunlight.

On contact with water or (air ) humidity is Polysilazanes decompose more or less quickly. In this case, the water molecules attack on the Si atom, and the Si-N bond is broken. From R3Si -NH- SiR3 initially arise R3Si -OH and H 2 N- SiR 3, which then react further (condensation), which at the end R3Si -O- SiR 3 moieties ( siloxanes ) are formed. The speed of the reaction with water (or other OH-containing compounds such as alcohols ) will depend on the molecular structure of the polysilazane and the substituents. Thus the perhydropolysilazane [ H2Si - NH] n may decompose on contact with water in a very fast, highly exothermic reaction while Polysilazanes react only slowly with bulky side groups.

When heating Polysilazanes can not go into the gas phase high-molecular compounds, because the intermolecular forces are too large. Therefore, rather a further crosslinking of the molecules with the elimination of ammonia and hydrogen takes place at temperatures of 100-300 ° C. The polysilazane contains other functional groups such as vinyl units, additional reactions occur. Liquid compounds therefore be an increase in temperature in the rule. In the range of 400-700 ° C, the organic groups decompose by elimination of small hydrocarbon molecules, ammonia and hydrogen. Between 700 and 1200 ° C for a three-dimensional network of amorphous Si, C and N ( " SiCN ceramic " ) having a density of about 2 g/cm3. When the temperature increases, the amorphous material can crystallize, with silicon nitride, silicon carbide and carbon form. This so-called pyrolysis of the polysilazanes produced from low viscosity fluids, ceramic materials in a high yield, which can be in excess of 90 mass%. However, generally it is significantly lower ( 60-80 %), since the organic groups which reduce the ceramic yield, a good processability in the polymer state are necessary.

Application Examples

Though long the Polysilazanes are known and are at an early stage a large application potential has been certified, so far have reached market maturity only a few products. This is certainly justified in the high development costs of using these relatively "expensive" chemicals. The poor in the past availability of Polysilazanes is both a cause and a consequence of it. For some applications, the property profile of the compounds, however, has proved to be advantageous so that competitive polysilazane products are commercially available today.

The reactivity of polysilazanes with respect to moisture and polar surfaces is used, its use as a coating material. Is a thin film deposited on a substrate on the surface contains OH groups (for example many metals, glass, ceramics, plastics ), can be formed at the interface of Si -O bonds, the chemical for a ensure anchoring of the polysilazane on the substrate. This creates a very good substrate adhesion. The free surface of the coating can react with atmospheric moisture, in the case of organopolysilazanes form siloxane -like structures, which may have excellent easy-to- clean properties. For example, the German railway uses a organopolysilazane -based product with the trade name tutoProm ® for protection against graffiti and paint refresher on railway wagons. In addition organopolysilazanes in high temperature coatings and corrosion protection systems shall apply.

The inorganic perhydropolysilazane can be used in the same way. However, it offers the additional advantage that after the complete curing of air, a carbon - free SiOx network is created. The layers are characterized Although less flexible, but very smooth and very dense, which is why they exhibit excellent barrier effect (for example, oxygen or water vapor). Since such glassy layers are naturally also good insulators, the perhydropolysilazane found in both the electronics and in the solar industry.

Due to the chemical reactivity of the polysilazanes, the use is being investigated as a resin or as a resin - hardener. The application is not yet fully developed, but is aimed at the production of non-combustible composites. According pretreated moldings have been proven in the experimental stage for use in the critical temperature range between 400 and 600 ° C, fail in the other plastics in general.

The Polysilazanes have as preceramic polymers also potential for use in the ceramics industry. In ceramics, the production of complex shapes is very difficult or very expensive. With suitable organic binders can indeed be used to produce injection-moldable masses; but it follows generates a time-consuming Debinding, the fragile "white bodies ", and the shrinkage during sintering must be investigated and recorded. Preceramic polymers could replace the organic binder. Instead of debinding the green compact was pyrolyzed, resulting in a relatively dense form part finished (high ceramic yield of the polymers ) endkonturnäher could. At least in the civilian sector this application lies still in its infancy.

Due to their chemical variability to physico- chemical properties of the preceramic polymers can be tailored. Of which many testify conducted at research institutions and in industry attempts to produce ceramic fibers for composites. The SiC fiber prepared from polycarbosilane is playing a pioneering role. The production of Si3N4 fibers perhydropolysilazane was in the late 1980s of Tonen Corp.. described, Dow Corning use the modified HPZ polymer to produce SiCN fibers while the Hoechst AG conducted successful trials with VT50.