RTV Silicone (Room-Temperature-Vulcanizing silicone) is a type of silicone rubber made from a two-component system (base plus curative; A+B) available in a hardness range of very soft to medium--usually from 15 to 40 Shore A. RTV silicones can be cured with a catalyst consisting of either platinum or a tin compound such as dibutyltin dilaurate. Applications include low-temperature over-molding, making molds for reproducing, and lens applications for some optically clear grades.
To produce the material, the silicone rubber is mixed with the curing agent or vulcanizing agent. Usually, the mixing ratio is a few percent. In order for the RTV silicone to reproduce the surface texture, attention is paid to the cleanliness of the original. Vacuum de-airing removes entrained air bubbles from the mixed silicone and catalyst to ensure optimal tensile strength, which affects reproduction times. In casting and mold-making, RTV silicone rubber reproduces fine details and is suitable for a variety of industrial and art-related applications including prototypes, furniture, sculpture, and architectural elements. RTV silicone rubber can be used to cast materials including wax, gypsum, low melt alloys/metals and urethane, epoxy or polyester resins (without using a release agent). A more recent innovation is the ability to 3D Print RTV silicones. RTV silicones' industrial applications include aviation, aerospace, consumer electronics, and microelectronics. Some aviation and aerospace product applications are cockpit instruments, engine electronics potting, and engine gasketing. RTV silicones are used for their ability to withstand mechanical and thermal stress.
- Good characteristics of easy operation
- Light viscosity and good flow-ability
- Low shrinkage
- Favorable tension
- No deformation
- Favorable hardness
- High-temperature resistance, acid and alkali resistance, and ageing resistance
Advantages and disadvantages
RTV silicone rubber has excellent release properties compared to mold rubbers, which is especially an advantage when doing production casting of resins (polyurethane, polyester, and epoxy). No release agent is required, obviating post-production cleanup. Silicones also exhibit good chemical resistance and high-temperature resistance (205 °C, 400 °F and higher). For this reason, silicone molds are suitable for casting low-melt metals and alloys (e.g. zinc, tin, pewter, and Wood's metal).
RTV silicone rubbers are, however, generally expensive--especially platinum-cure. They are also sensitive to substances (sulfur-containing modelling clay such as Plastilina, for example) that may prevent the silicone from curing (referred to as cure inhibition). Silicones are usually very thick (high viscosity), and must be vacuum degassed prior to pouring, to minimize bubble entrapment. If making a brush-on rubber mold, the curing time factor between coats is long (longer than urethanes or polysulfides, shorter than latex). Silicone components (A+B) must be mixed accurately by weight (scale required) or they do not work. Tin catalyst silicones shrink somewhat and do not have a long shelf life. Certain types of RTV release acetic acid during the curing process, and this can attack solder joints, causing the solder to detach from the copper wire.
- Jorge Cervantes, Ramón Zárraga, Carmen Salazar-Hernández "Organotin catalysts in organosilicon chemistry" Appl. Organometal. Chem. 2012, vol. 26, 157–163. doi:10.1002/aoc.2832
- Hans-Heinrich Moretto, Manfred Schulze, Gebhard Wagner (2005) "Silicones" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim. doi:10.1002/14356007.a24_057
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