Cross-Linking of Silicones

Platinum compounds Hydrosilation cross-linking of silicone polymers Hydrogenation, isomerization and hydroformylation of alkenes Automobile exhaust catalyst Sensitization dermatitis... 

Figure 18.4. Cross-linking of silicone rubber via hydrosilylation...

Atomic spectroscopy (including atomic absorption spectrometry, atomic emission spectrometry, and atomic fluorescence spectrometry) is of use across the span of reactive adhesive technologies. For example, the cure of anaerobic adhesives on non-reactive surfaces is usually assisted by the use of an active metal-based primer. Similarly, the cross-linking of silicone adhesives is promoted by the use of organometallic salts of cobalt, tin, iron, lead, and platinum. In the case of polyurethane adhesives, the key condensation reactions are catalyzed by tin salts (e.g., dibutyl tin dilaurate and stannous octoate). 

FIGURE 3.37 Peroxide cross-linking of silicone rubber. 

Baquey, G. Moine, L. Degueil-Castaing, M. Lartigue, J.-C. Maillard, B., Decomposition of Di-tert-butyl Peroxide in Siloxane An Approach of the Free Radical Cross-Linking of Silicones. Macromolecules 2005, 38, 9571-9583. 

Hydrosilylation cross-linking of silicone rubber has been increasingly used in rubber manufacture because of many technological and economic advantages (7). The composition and proportion of both polysiloxane components and the catalyst used determine the quality of the vulcanizate and in particular allow obtaining materials of given properties and for special applications. 

Muller, U., New insight in the oxygen influence on the photo-cross-linking of silicone-acrylates (pp. 663-667), Organosilicone Chem., 2000. 

FIGURE 13.5 Addition cross-linking of silicone rubber achieved by incorporating vinyl groups in the pre-polymer, which is reacted with hydrogen in a silane. 

The cross-linking of silicones is obtained through two methods, depending on whether the process is carried out by either step or chain polymerization. 

Silicones are a large group of compounds that include large polymers containing silicon. Depending on the formula and the degree of polymerization and cross-linking of the polymers, they may be slippery liquids, waxes, or rubbers. 

Dibutyltin diacetate, dilaurate, and di-(2-ethylhexanoate) are used as homogeneous catalysts for room-temperature-vulcanizing (RTV) silicones. The dialkyltin compounds bring about the cross-linking of the oligomeric siloxanes, to produce flexible, silicone rubbers having a host of different uses, such as electrical insulators and dental-impression molds. Recent work has also shown (560) that various dibutyltin dicar-boxylates catalyze both the hydrolysis and gelation of ethyl silicate under neutral conditions. 

Chlornitrofen and nitrofen conditions for GC/MS column, cross-linked methyl silicone capillary (12 m x 0.22-mm i.d., 0.33- am film thickness) column temperature, 60 °C (1 min), 18 °C min to 265 °C inlet, transfer line and ion source temperature, 260, 200 and 200 °C, respectively He gas column head pressure, 7.5 psi injection method, splitless mode solvent delay, 3 min electron ionization voltage, 70 eV scan rate, 0.62 s per scan cycle scanned mass range, m/z 100-400. The retention times for chlornitrofen and nitrofen were 11.8 and 11.3 min, respectively. The main ions of the mass spectrum of chlornitrofen were at m/z 317, 319 and 236. Nitrofen presented a fragmentation pattern with the main ions at m/z 283, 202 and 285. ... 

Gas chromatographic analysis of the product (Hewlett-Packard fused silica, cross-linked methyl silicone capillary column, 25 m x 20 mm, column temperature 100-270°C, injection temperature 250°C) shows that the product is over 99% chemically and isomerically pure, (Z,E)-l-Phenyl-l,3-octadiene shows the following spectral properties IR (neat) cm 1640, 1595, 1490, 985 ... 

Capillary GLC analysis (12 m, cross-linked methyl silicone, programmed, 45°C, 3°C/min, retention time of ethyl (Z)-crotonate, 2.5 min). Ethyl (E)-crotonate has a retention time of 2.95 min under the same conditions. Careful quantitative analysis reveals that the ratio of 1 and E isomers is reproducibly in the range 58 1 to 59 1. 

The presence of residual THF in the final product is easily detected by the characteristic NMR signals at 6 1.85 (4 H) and 3.75 (4 H). The checkers found GLC to be more convenient than 1H NMR for analysis at this point. Either a 6 x 2 mm glass column of 3% 0V-101 on MHP 80/100 with 30 nt/min He or a 15 m x 0.25 mm fused silica capillary column of DB-5 (cross-linked phenyl silicone) with 1 mL/min H2 proved sufficient to resolve butane, trimethylsilylacetylene, butyltrimethylsilane, THF, 1-chlorobutane, and b1s trimethylsilyl)acetylene. 

For some uses it is important to form bonds that link different polysilane chains, to transform soluble, meltable polysilanes into insoluble resins. This process is vital if the polysilanes are to be used as precursors to silicon carbide ceramics, since, if cross-linking is not carried out, most of the polymer is volatilized before thermolysis to silicon carbide can take place. Several methods have therefore been developed to bring about cross-linking of polysilanes.109 110... 

Cross-linking of fluorosilicones is done by essentially the same methods as conventional silicones. A comprehensive review of this subject is in Reference 65. Currently, there are three methods of cross-linking used in industrial practice ... 

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