The polymer precursor method has been mainly developed in the field of inorganic fibers. Carbon fibers with high strength were developed in the early years, and silicon carbide fibers were invented in 1970s [1,2]. The high heat resistance of SiC fibers, even in an oxidative atmosphere, promoted the synthesis chemistry of various ceramic precursors, like polycarbosilanes, polysilazanes and the recently successful polyborosilazanes [3,4]. On the other hand, the high temperature resistance of ceramic fibers derived from polycarbosilanes is being continuously improved, even at present [5,6,7,8]. Such advanced SiC based fibers are mainly used as reinforcements in ceramic matrix composites with extremely high heat resistance.
On the other hand, sol-gel methods for oxide base ceramic synthesis have been developed over the past few decades. Various alkoxides are now commercially available, and papers on the theme of sol-gel science and technology are being published at a rate of at least 4,000 per year.
When we contemplate such a situation, it is evident that the combination of carbide base ceramics and oxide base ceramics by some chemical technique will be a great issue, which will be final target of the organic–inorganic hybridization process. We, however, remember simultaneously that such hybridization processes have already been accomplished with great success many years ago. Just around the time of World War II, various silicone polymers, the polysiloxanes, were synthesized on a large scale and widely commercialized. They are available as electric insulator coatings, surface treatments for glass materials, heat resistant oils and chemically stable elastomers. Now such products are highly sophisticated industrial commodities, and we often forget the chemical background of the various commercialized silicone resins. It is a shame that the role of silicones in ceramic technology is reduced to that of being somewhat muddy additives for shaping the starting materials before the sintering process. Here, I tried to shed light again on the classic silicone polymer science, which is quite fundamental in organometallic chemistry and high temperature organic - inorganic reactions. Recent activity in the field and renewed interests in Si-O-C materials correctly indicate the great importance of the modern silicones as ceramic precursors.
2. Historical Background of Silicone Resin Production [9,10]
Silicon is industrially produced at present on a tremendous scale. The origin of silicon is high purity mineral silica sand. Such silica sand is typically reduced by carbon in an electric arc furnace at 3,000 °C. The silicon obtained, of 98–99% purity, is called “metal-grade silicon”. Besides the uses in the electronics and silicone polymer industry, such metal-grade silicon is used as an important component in various alloys of Fe, Al or Mg.
For the uses in electronics, the metal grade silicon is reacted with SiCl4 and hydrogen to yield HSiCl3. This compound is a transparent liquid with a boiling point of 31.8 °C. The liquid nature of HSiCl3 means that the compound can be distilled for purification. After the distillation, reduction by hydrogen and the Czochralski process, a single crystal of silicon with extreme high purity is obtained. The unique properties of the pure silicon, such as its semi conductive nature, acceptance of doping and formation of insulating silica layers on the surface during oxidation, are presently well known. The application of Si for solar cells is increasing its importance in recent times.
SiCl4 + C2H5MgBr → Si(C2H5)Cl3 + MgBrCl
Of course, the Grignard reagent attacks not only the Si-Cl bonds in SiCl4, but also the Si-Cl in Si(C2H5)Cl3 and Si(C2H5)2Cl2. Therefore, the products derived from SiCl4-C2H5MgBr combinations are often mixtures of various Si(C2H5)xCl4-x species. Each compound must be isolated from the mixture by fractional distillation.
SiR2Cl2 +2H2O → SiR2(OH)2 + 2HCl
nSiR2(OH)2 → (-SiR2-O-)n + nHCl
Some part of the silanols remains as terminal groups, and some oligosiloxanes with ring structures can also be obtained in the resulting mixtures. The first silicone resin, reported by Kipping as a “glue-like” product, did not however attract any kind of industrial attention in those days.
On the other hand, the polymeric nature of these “glue-like” silicones attracted the attention of Hyde and related groups at Corning Glass Works. The company developed the industrial process for silicone resin production on the basis of the SiCl4—Grignard reagent combination, and opened the doors to silicone resin commercialization. The silicone resins were found to be highly compatible with glass materials. Utilization as binders for glass fibers and scratch resistant coatings on glass plates was promoted. The main silicone investigated by Hyde was a kind of polyethylphenylsiloxane (PEPhS).
The synthesis of PEPhS with using Grignard reagent is, however, a multi-step process. Grignard reagents are highly flammable, and the synthesis requires a large amount of metallic magnesium. The silicone thus obtained is rather special and a little far from conventional plastic, like polyethylene, polyamide or phenolic resins, widely produced from petroleum industry raw materials.
In order to overcome such economical and industrial problems, Rochow developed at General Electric a direct synthesis process for organosilicon monomers without the aid of Mg in 1940. This was essential progress in the silicone industry.
Si (s) + 3HCl (g) → SiHCl3 (g) + H2 (g)
Excess CH3Cl (g) + Si (s) → Si(CH3)2Cl2 (L) + Si(CH3)Cl3 (L) + SiHCl3 (g, L) + SiH(CH3)Cl2 (L) + Si(CH3)3Cl (L) + SiCl4 (L)
This is intrinsically a gas-solid reaction. Thus, continuous operation is possible by adjusting the rate of introduction of CH3Cl gas, Si, and Cu powders into a reactor. By fractional distillation, each compound can be isolated with high purity. Si(CH3)2Cl2 has the highest boiling point, while Si(CH3)4 has the lowest boiling point, except for the starting CH3Cl (Table 1). Si(CH3)2Cl2 is the most valuable component in the obtained mixture, because it forms linear Si-O-Si chains after the hydrolysis. Any kind of methylchrolosilanes are, however, useful for tailoring silicone resins, oils, greases, rubbers and varnishes.
Table 1. Boiling points of methylchlorosilanes found in the Rochow process product.
|Compound||Boiling Point (°C)|
On the other hand, phenyl chlorosilanes are also useful monomers in the silicone industry. It is possible to synthesize phenyl chlorosilanes by direct reaction of chlorobenzene (C6H5Cl) and the Si-Cu alloy. Higher temperature (400–500 °C) and a larger amount of Cu content (30 mass %) are however required.
C6H6 + HSiCl3 → C6H5SiCl3 + H2
CH2=CHCl + HSiCl3 → CH2=CHSiCl3 + HCl
The introduced HSiCl3 is obtained by the fractional distillation of the product of Rochow process, or is obtained from the products in the pure silicon industry. From a cursory glance at such chemical processes, we can get a sense of how polymers so unique as the silicones have been widely produced at relatively low cost, and what kind of silicone is more popular from the viewpoint of the industry.
3. Thermal Degradation of Linear Silicones
Silicone is superior in heat and chemical resistance as compared with ordinary polymers. Silicone oils are necessary component in high vacuum systems, and we often see elastic silicone rubber materials in medical and chemical uses. In most cases, linear polysiloxane or partly cross-linked polysiloxane were used in commercialized products. Si-O bonds in siloxane chain are flexible as compared with C-C bonds, and silicones intrinsically maintain their liquid nature over a wide temperature region. For example, a glass transition temperatures of polydimethylsiloxane (PDMS) or polymethylphenylsiloxane (PMPhS) are −127 °C and –86 °C, respectively [11,12].
Thermal degradation of PDMS with complete linear structure proceeds at 290–600 °C with formation of cyclic oligomers. In an inert atmosphere or vacuum, a trimer (Si3O3(CH3)6) and tetramer (Si4O4(CH3)8) are the major components in the decomposition gas. Chemical species with higher molecular weight, like a hexamer and an octamer, are also found as components. Thomas et al. proposed an intramolecular cyclization process for the thermal degradation of PDMS . The low activation energy, 40 kcal/mol, suggests the existence of a stable transition state, which promotes the degradation of the silicone resin. The linear siloxane chains can easily make intramolecular contact because of the flexible nature of the chains and the Si d-orbital interactions. Thus, the degradation proceeds by simultaneous rearrangement of Si-O bonds with expulsion of cyclic oligomers (Figure 1).
Figure 1. Cyclic oligomer (trimer) expulsion mechanism from a polymethylsiloxane chain during thermal decomposition process.
Figure 1. Cyclic oligomer (trimer) expulsion mechanism from a polymethylsiloxane chain during thermal decomposition process.
It is interesting that PDMS is obtained not only from hydrolysis of (CH3)2SiCl2 but also from ring opening polymerization of the cyclic tetramer (Si4O4(CH3)8). In other words, the decomposition process of PDMS is a kind of depolymerization.
In the presence of oxygen, the degradation process of linear silicones becomes complex . In a relatively low temperature region, oxygen acts as a catalyst promoting the scission and rearrangement processes of the Si-O bonds. Thus, the depolymerization process and removal of volatile oligomers occurs at a relatively low temperature (290 °C) as compared with that in an inert atmosphere (400 °C). The residual siloxane is, however, condensed by oxidative cross-linking, which reduces the mass loss rate in the last stage of the decomposition. Such oxidation cross-linking is possibly triggered by the formation of radicals on side groups, which trap oxygen and form peroxides, which sometimes accelerate the cross-linking and sometimes accelerate the depolymerization process. In an oxidative atmosphere, ca. 10 mass % of silica is obtained as a result of the competition between oxidative cross-linking and volatilization of oligomers.
Even in an inert atmosphere, an inorganic residue is sometimes obtained after pyrolysis with very high heating rates . Perhaps, condensation of low molecular weight oligomers, which cannot be diffused out, takes place. The obtained glassy black product is thought to possess Si-O-C composition.
The decomposition processes of various linear polymers composed of -SiR1R2-O-, -Si(CH3)2-CH2-Si(CH3)2-O- or -Si(CH3)2-CH2-CH2-Si(CH3)2-O- units were also investigated by Thomas et al. . Thermal degradation proceeds via the depolymerization reaction and the mechanism is similar to that of the usual linear polysiloxanes like PDMS. The major gaseous products are cyclic trimers and tetramers.
Introduction of phenyl groups in silicone polymers usually increases the onset temperature of mass loss of polysiloxanes [15,16,17]. The residual mass at 1,000 °C, however, does not increase, because of evolution of benzene, toluene and cyclic siloxane oligomers during higher temperature degradation processes at 400–600 °C. Siloxane oligomers with phenyl groups are absent in the gaseous products. Perhaps, phenyl side groups are decomposed by the radical reaction, and the cyclic oligomer expulsion follows such side group decomposition processes. When the chain is flexible, anyway, it is likely difficult to avoid the cyclic oligomer expulsion caused by the Si-O and Si-C bond rearrangements during the thermolysis.
4. Increased Ceramic Yield in Cross-Linked Silicones
In order to increase the ceramic yield of silicones, a dense cross-linked structure, which prevents the bond rearrangement process during heating, is necessary. As compared with the C-O (351.5 kJ mol−1), C-C bond (347.7 kJ mol−1) and C-Si bonds (290.0 kJ mol−1), the high energy of the Si-O bond (369.0 kJ mol−1) is promising for increasing the ceramic yield of silicones. Introduction of vinyl, phenyl groups or acetylene linkages in silicone molecular structure may be also effective for increasing the apparent ceramic yields. The efficiency of such side groups, however, often depends on behavior of “radicals”, which sometimes decompose side groups to gaseous product. There is also the concern that during pyrolysis such side groups are converted to free carbon domains, which are not directly incorporated into the inorganic Si-O-C networks.
Zhou et al. introduced T units in siloxane chains in order to investigate the effect of Si-O cross-linking on the thermal stability . As the content of T units increases, the resulting ceramic yield increases. On the other hand, Mantz et al. investigated thermolysis of polyhedral oligometric silsesquioxane (POSS)–siloxane copolymers . Loss of the cyclic dimethylsiloxane oligomers proceeds at 400 °C, and loss of the silsesquioxane “cage” structure proceeds at 450–650 °C. This means that the incorporated cage structure, composed of complete T units, is not simply maintained during the heating, and the thermal degradation process in this case is complex, which possibly corresponds to some steric hindrance effect of the cage structure on the main chain rearrangement process.
SiOxCy → SiC + xCO + (y-x-1)C
Residual masses after the carbothermic reduction process are 35–50 mass %. The yield tends to increase with the increase in the carbon content in the starting Si-O-C. The amount of excess carbon after pyrolysis at 1,800 °C also increases. SiC nanocrystallites formed in situ and excess carbon are expected to act as sintering aids for the loaded coarse SiC grains during high temperature heat treatment.
Hurwitz et al. carried out systematic studies on the pyrolysis process of polyphenyl–polymethyl–silsesquioxane co-polymers, which were synthesized by hydrolytic condensations of PhSi(OMe)3 and MeSi(OMe)3 . The ceramic yields of the co-polymers decreases as the phenyl content in the co-polymer increases, while the ceramic yield of polyphenylsilsesquioxne (PPSQ) is ca. 60%, and that of poymethylsilsesquioxane (PMSQ) is ca. 80%. The onset temperatures of mass losses of PPSQ and the co-polymers is 500 °C, while that of PMSQ, with 100% methyl side groups, is 750 °C.
Oxidation resistance of Si-O-C materials derived from co-polymers of 30P, 50P and 70P (P means a phenyl molar content to phenyl + methyl groups in the co-polymer) were also investigated . Carbon burning usually proceeds in the temperature range of 600–1,000 °C. As the phenyl group content decreases, however, the oxidation resistance of the Si-O-C materials increases, which is indicated by slow mass loss rate in a TG curve.
Brewer et al. (Dow Corning) investigated the oxidation resistance of Si-O-C materials derived from PhSi(OMe)3, MeSi(OMe)3
Мы его не украли, - искренне удивилась Росио. - Человек умирал, и у него было одно желание. Мы просто исполнили его последнюю волю. Беккер смягчился. В конце концов, Росио права, он сам, наверное, поступил бы точно так .