compositions based on silicones. Three groups of silicon adhesives are distinguished, depending on use: (1) adhesives for metals and heat-resistant nonmetallic materials; (2) adhesives for heat-resistant rubbers and for bonding them to metals; and (3) adhesives for bonding thermal and acoustical insulation materials to steels and titanium alloys.
The first group includes mixtures of various silicones with fillers (asbestos and other inorganic materials) and hardeners (such as peroxides and amines). The adhesive bonds are serviceable at temperatures from —60° to 1000°C (for several hours), and are resistant to aging under various conditions and to the effects of exposure to fuels and lubricants. The bonding of metals by adhesives of this group yields strong but brittle bonds. The adhesive bond strength in joining glass-fiber-reinforced plastics, graphite, or asbestos cement is equal to or higher than that of the materials being bonded. Hardening of these adhesives occurs upon heating (up to 250°C), but modification of the materials with organic additives produces adhesives that may be be hardened at room temperature.
Silicone adhesives of the second group are usually produced from solutions of silicone rubbers with added quantities of various silicones, as well as heavy metal oxides or hydroxides. Adhesive bonds based on adhesives of this group withstand vibrational loads over a wide temperature range and are resistant to the action of transformer oil, kerosine, moisture, and atmospheric conditions. Adhesives of this group are used in a number of cases to bond glass, fabrics, polyethylene terephthalate, polytetra-fluoroethylene, and ceramics, and also as sealants in aircraft and rocket construction.
The third group includes mixtures of modified silicones with organic polymers in organic solvents with added hardeners, such as amines, and often with added active fillers (for example, ZnO). A particular feature of these materials is the possibility of bonding thermal and acoustic insulation materials without heating and pressure to give adhesive joints that are serviceable up to 300°–400°C. The active filler imparts rapid setting capability to the adhesive, but in this case, the pot life of the adhesives is limited to 45–60 min.
Compositions for the production of adhesive tapes make up a separate group of silicone adhesives. Such compositions usually contain silicone rubbers and oils, whose macromolecules contain terminal hydroxyl groups, as well as an organosilicon monomer, mineral filler, and organic additive. The composition is applied to the tape, which may be a polymer, or a glass fiber tape, and the finished product is used as electrical insulation tape.
REFERENCEKardashov, D. A. Sinteticheskie klei, 2nd ed. Moscow, 1968. Pages 213–32.
M. M. LEVITSKII