Polysilazanes
Polysilazanes are polymeric compounds in which silicon and nitrogen atoms in an alternating arrangement form the basic chemical structure. Often each silicon atom is bonded to two nitrogen atoms and each nitrogen atom to two silicon atoms, so that molecular chains and rings of the formula [R 1 R 2 Si — NR 3 ] n are preferably formed. R 1 -R 3 can be hydrogen atoms or organic radicals . If only hydrogen atoms are present as substituents , the polymer is called perhydropolysilazane ([H 2 Si – NH] n , also polyperhydridosilazane, inorganic polysilazane). If hydrocarbon residues are bound to silicon, one speaks of organopolysilazanes . In their molecular structure, the polysilazanes [R 1 R 2 Si – NH] n are closely related to the polysiloxanes [R 1 R 2 Si – O] n ( silicones ), with which they are isoelectronic .
History
The synthesis of polyorganosilazanes was first described in 1964 by Krüger and Rochow. They initially produced trimeric or tetrameric cyclosilazanes by reacting ammonia with chlorosilanes ( ammonolysis ) , 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. The large-scale production of chlorosilanes according to the Müller-Rochow process laid the foundation for the development of silazane chemistry in 1940. Attempts to convert organosilicon polymers into quasi-ceramic materials were described for the first time in the 1960s . Suitable (so-called pre-ceramic) polymers were heated to approx. 1000 ° C or higher, whereby, with the splitting off of organic groups and hydrogen and rearrangement of the atoms, initially amorphous inorganic networks are formed, which have a unique profile of properties both chemically and physically exhibit. With the help of polymer-derived ceramics (PDC), new areas of application can be opened up, especially in the field of high-performance materials. The most important preceramic polymers are polysilanes [R 1 R 2 Si-R 1 R 2 Si] n , polycarbosilanes [R 1 R 2 Si-CH 2 ] n , polysiloxanes [R 1 R 2 Si-O] n and polysilazanes [R 1 R 2 Si-NR 3 ] n should be mentioned.
structure
Like all polymers, polysilazanes are made up of one or more basic units, the monomers . By lining up these basic units, chains, rings and three-dimensionally cross-linked macromolecules with a more or less broad molar mass distribution are formed . The monomer unit also serves to describe the chemical composition and the linkage of the atoms (coordination sphere), but without making any statements about the macromolecular structure.
In polysilazanes, each silicon atom is bonded to two nitrogen atoms and each nitrogen atom is bonded to at least two silicon atoms (there can also be three). When all other valences are satisfied by hydrogen atoms, the perhydropolysilazane [H 2 Si – NH] n is formed , the idealized structure of which is shown on the right. In the case of organopolysilazanes, at least one organic radical is bonded to the silicon. The number and type of residues has a significant influence on the macromolecular structure of these polysilazanes.
Silazane copolymers are usually produced by ammonolysis of chlorosilane mixtures. In this chemical conversion, the different chlorosilanes usually react with similar speed. The monomer units are therefore randomly distributed in these copolymers. The designations M, D, T and Q used to describe the structure of silicones are rarely used in the case of polysilazanes.
Manufacturing
The inexpensive chemicals ammonia and chlorosilanes, which are available on a large industrial scale, are usually used as starting materials in polysilazane production. The ammonolysis reaction produces large amounts of ammonium chloride that have to be separated from the product.
- R 2 SiCl 2 + 3 NH 3 → 1 / n [R 2 Si-NH] n + 2 NH 4 Cl
On a laboratory scale, this reaction is carried out in a dry organic solvent (the silazanes decompose in the presence of water) in which ammonium chloride does not dissolve, and then it is filtered. Since the filtration step is very time-consuming and costly, manufacturing methods have been developed in which no solids are produced in the final synthesis step.
The liquid ammonia process for polysilazane synthesis was developed by Commodore / KiON. The chlorosilane or chlorosilane mixtures are dosed into an excess of liquid ammonia. The resulting ammonium chloride dissolves in the ammonia and forms a second liquid phase in addition to the silazane. The two liquids can then be separated from one another at the interface. This patented process is used by AZ Electronic Materials (now part of Merck KGaA ) to manufacture polysilazanes.
The products VT 50 and ET 70 formerly sold by Hoechst AG are polysilsesquiazane solutions. Production took place in two stages: first, a trichlorosilane was reacted with dimethylamine and the resulting monomeric aminosilane was separated from the dimethylammonium chloride . The subsequent reaction of the aminosilane with ammonia produces a salt-free polymer.
If hexamethyldisilazane (HMDS) is used as the source of nitrogen instead of ammonia , transamination takes place. The chlorine atoms released from the chlorosilane are bound to the trimethylsilyl groups of the HMDS, so that no solid containing chlorine is formed. This process was used by Dow Corning to produce the hydridopolysilazane HPZ.
Numerous other methods for building polymeric SiN frameworks have been described in the literature (e.g. dehydrogenative coupling between Si – H and N – H, ring-opening polymerizations), but have not been used on a large scale.
For the industrial production of perhydropolysilazane [H 2 Si – NH] n , ammonolysis in a solvent is still used. The resulting higher price is accepted as a coating material in the electronics industry because of the special properties (including insulating effect with a thin layer). The product is available as an approx. 20% solution.
nomenclature
Silicon-nitrogen compounds with alternating silicon (“sila”) and nitrogen (“aza”) atoms are known as silazanes. Simple representatives of the silazanes are the disilazane H 3 Si – NH – SiH 3 and the hexamethyldisilazane (H 3 C) 3 Si – NH – Si (CH 3 ) 3 . If only one silicon atom is bound to the nitrogen atom, one speaks of silylamines or aminosilanes (e.g. triethylsilylamine (H 5 C 2 ) 3 Si – NH 2 ). If three skeleton-forming nitrogen atoms are arranged around the silicon atom, the compounds are called silsesquiazanes. Small ring-shaped molecules with a basic structure of Si-N are known as cyclosilazanes (e.g. cyclotrisilazane [H 2 Si – NH] 3 ). Polysilazanes, on the other hand, are polymeric silazanes that are made up of chains and rings of different sizes and have a molar mass distribution. A polymer with the general formula (CH 3 ) 3 Si - NH - [(CH 3 ) 2 Si - NH] n - Si (CH 3 ) 3 is referred to as poly (dimethylsilazane). According to the IUPAC rules for naming linear organic polymers, the compound should actually be called poly [aza (dimethylsilylene)], and according to the provisional rules for inorganic macromolecules, catena-poly [(dimethylsilicon) -m-aza].
properties
Polysilazanes are colorless to yellow liquids or solids. Due to the manufacturing process, the liquids often contain dissolved ammonia, which dominates the smell. The mean molecular mass can be from a few thousand to approx. 100,000 g / mol, while the density is usually around 1 g / cm 3 . The physical state and the viscosity are dependent on both the molecular mass and the molecular macrostructure. Solid polysilazanes are produced from liquid through chemical reactions (linking of smaller molecules to form larger ones, cross-linking). The solids can be fusible or infusible and soluble or insoluble in organic solvents. As a rule, these are thermosets , but in some cases thermoplastic processing is possible.
After the synthesis, an aging process often takes place in which dissolved ammonia plays an important role. The R 3 Si – NH 2 groups formed during ammonolysis form silazane units with elimination of ammonia. If the ammonia cannot escape, the silazane units are split up again into R 3 Si – NH 2 groups. Frequent gas changes over the liquids can lead to an increase in the molecular mass (removal of ammonia). Even functional groups that are not directly integrated into the basic structure can react with one another under suitable conditions (e.g. Si – H with N – H groups), which leads to an increasing crosslinking of the rings and chains. An increase in molecular mass can also be observed after prolonged storage at elevated temperatures or in sunlight.
Polysilazanes decompose more or less quickly on contact with water or (air) moisture. The water molecules attack the Si atom and the Si – N bond is broken. R 3 Si — NH — SiR 3 initially results in R 3 Si — OH and H 2 N — SiR 3 , which then react further (condensation), which ultimately results in R 3 Si — O — SiR 3 units (siloxanes). The rate of reaction with water (or other OH-containing compounds such as alcohols) depends on the molecular structure of the polysilazanes and the substituents . For example, the perhydropolysilazane [H 2 Si – NH] n can decompose on contact with water in a very rapid, strongly exothermic reaction, while polysilazanes with voluminous side groups only react slowly.
When polysilazanes are heated, high molecular compounds cannot pass into the gas phase because the intermolecular forces are too great. At temperatures of 100 to 300 ° C, the molecules are more likely to become more cross-linked, with hydrogen and ammonia being split off. If the polysilazane contains other functional groups such as B. vinyl units , additional reactions occur. Liquid compounds therefore usually solidify when the temperature rises. In the range from 400 to 700 ° C, the organic groups decompose with the elimination of small hydrocarbon molecules, ammonia and hydrogen. Between 700 and 1200 ° C a three-dimensional amorphous network of Si, C and N (“SiCN ceramic ”) with a density of approx. 2 g / cm 3 is created . With a further increase in temperature, the amorphous material can crystallize, with silicon nitride , silicon carbide and carbon being formed. This so-called pyrolysis of the polysilazanes produces ceramic materials from low-viscosity liquids in high yield, which can be over 90% by mass. As a rule, however, it is significantly lower (60–80%), since the organic groups that reduce the ceramic yield are necessary for good processability in the polymer state.
Application examples
Although the polysilazanes have been known for a long time and they were recognized at an early stage as having great application potential, so far only a few products have reached the market. This is certainly also due to the high development effort involved in using these comparatively "expensive" chemicals. The poor availability of polysilazanes in the past is both a cause and a consequence of this. For some applications, however, the property profile of the compounds has proven so advantageous that competitive polysilazane products are commercially available today.
The reactivity of polysilazanes towards moisture and polar surfaces is exploited when they are used as coating material. If a thin film is applied to a substrate that contains OH groups on the surface (e.g. many metals, glass, ceramics, but also plastics), Si – O bonds can form at the boundary layer, which are essential for chemical Ensure anchoring of the polysilazane on the substrate. This creates very good substrate adhesion . The free surface of the coating can react with atmospheric moisture, and in the case of the organopolysilazanes, siloxane-like structures are formed which can have excellent easy-to-clean properties. For example, Deutsche Bahn uses an organopolysilazane-based product with the trade name tutoProm® to protect against graffiti and to refresh the paintwork on railway wagons. Organopolysilazanes are also used in high-temperature paints and anti-corrosion systems.
The inorganic perhydropolysilazane can be used in the same way. However, it also offers the advantage that after complete curing in air, a carbon-free SiO x network is created. The layers are less flexible, but very smooth and very dense, which is why they have an excellent barrier effect (for example for oxygen or water vapor). Since such glass-like layers are naturally also good insulators, the perhydropolysilazane is used both in the electronics and in the solar industry.
Due to the chemical reactivity of the polysilazanes, the use as synthetic resin or as synthetic resin hardener is also being investigated. The application is not yet fully developed but is aimed at the manufacture of non-combustible composites . Correspondingly pretreated molded parts have proven themselves in the experimental stage for use in the critical temperature range between 400 and 600 ° C, in which other plastics usually fail.
As preceramic polymers, the polysilazanes also have application potential in the ceramic industry . In ceramics, the production of complex shapes is very difficult or very expensive. With suitable organic binders Although be injection-moldable mass produced; however, a time-consuming debinding process follows, which creates fragile “ white bodies ”, and the shrinkage during sintering must be examined and recorded. Pre-ceramic polymers could replace the organic binders. Instead of debinding, the green compact would be pyrolysed, which would allow a comparatively dense molded part (high ceramic yield of the polymers) to be manufactured with a closer net shape. At least in the civil sector, this application is still in its infancy.
Due to their chemical variability, the physico-chemical properties of the preceramic polymers can be specifically adjusted. This is evidenced by many tests carried out at research institutions and in industry to manufacture ceramic fibers for composite materials. The SiC fiber made from polycarbosilanes plays a pioneering role. The production of Si 3 N 4 fibers from perhydropolysilazane was started by Tonen Corp. in the late 1980s. described that Dow Corning used the modified HPZ polymer to produce SiCN fibers, while Hoechst AG carried out successful tests with VT50.
literature
- CR Kruger, EG Rochow, J. Polym. Sci. Vol. A2, 1964, pp. 3179-3189
- Polymer Derived Ceramics, Eds. G. Soraru, R. Riedel, A. Kleebe, P. Colombo, DEStech publications, Inc. 2010
- R. Riedel, A. Gurlo, E. Ionescu: Synthesis methods for ceramic materials, high-tech materials. In: Chemistry in Our Time 44, No. 3, 2010, 208–227
- M. Mahn, F. Osterod, S. Brand, Farbe und Lack 114, 2008, 22-24
- S. Brand, M. Mahn, F. Osterod, Farbe und Lack 116, 2010, 25–29