Fluorosilicone thermal stability

Based on the extensive work, Ghosh and De have concluded that fluoroelastomer and silicone mbber form technologically compatible blends of micro-heterogeneous stmcture with thermal stability between those for the blend components. The blend could be used as a replacement for fluorosilicone mbber. 

Medium-molecular-weight PMTFPS with vinyl or hydroxyl end blocks are used for adhesives and sealants. They are cured either at ambient temperature (RTV-room temperature vulcanization) or at elevated temperature. One-part moisture-activated RTV sealants have been available commercially for many years. Because of then-very high resistance to jet engine fuels, excellent flexibility at very low temperatures, and high thermal stability, they have been used in both military and civilian aerospace applications.78 Two-part, heat-cured fluorosilicone sealants have been used in military aircraft applications and for sealing automotive fuel systems.79 Special class of fluorosilicone sealants are channel sealants or groove injection sealants, sticky, puttylike compounds, which do not cure. They are used to seal fuel tanks of military aircraft and missiles.75... 

Conrad, M. P. C. Shoichet, M. S., Synthesis and Thermal Stability of Hybrid Fluorosilicone Polymers. Polymer 2007,48, 5233-5240. 

Fluorosilicones (FLS) are a class of polymers generally composed of siloxane backbone polymers and fluorocarbon pendant groups. Fluorosilicone materials are familiar because of their excellent properties such as high thermal stability, good chemical and environmental resistance, flame resistance, and surface characteristics. Currently, these materials are extensively used in a wide range of applications such as in the electronic, automotive, dairy, medical, and aerospace industries [1,2]. The primary and most commonly used commercially available fluorosilicone is poly(3,3,3-trifluoropropyl methylsiloxane (PTF-PMS). This polymer was discovered by Dow Coming Company [3] in 1950 and was given the trade name Silastic . It is prepared from l,3,5-trimethyl-l,3,5-tra(3, 3, 3 -trifluoropropyl)cyclotrisiloxane and has the repeat unit sfructure presented in Scheme 6.1. 

Over the years many fluoroelastomers have been prepared in addition to the materials described earlier in this chapter. These include fluorinated polyurethanes, fluorinated polyepoxides, hexafluoro-acetone/propylene oxide copolymers and polyfluorals. Many of these materials are thermally unstable, a fact which stresses the point that the presence of C—F bonds with their high bond strength is no guarantee of polymer thermal stability. One particular type of fluoroelastomer which is of technical importance, the fluorosilicone rubber family, are however of good thermal stability and are considered together with the silicone rubbers in a later chapter. 

Reversion and ring formation have been partially overcome through placement of chain-stiffening units in the silicone-backbone. Thus linear D2-m-carborane-siloxanes (11—14) with one to three trifluoropropyl moieties per repeating unit) exhibit better thermal and oxidative stability than silicones and fluorosilicones (J ). Initial degradation occurs in air about 300 to 350 C, almost 100 above that typically experienced for siloxanes and fluorosilicones. The carborane-siloxanes exhibit T s from -50 to 0 C ( 1 ). The carborane moiety also acts to inhibit formation of six-membered rings because of its size. 

In a general article about fluorosilicone elastomers [41], Kim analyzed the properties of classical fluorosilicones - [(R)(RF)SiO] - that are an excellent resistance to solvents, a good thermal and oxidative stability, an outstanding flexibility at low temperature. He concluded that fluorosilicones are superior to fluorocarbon elastomers, but they were not very good at high temperatures (above 450 C). Conventional polydimethylsiloxanes, and classical fluorosilicones, present the drawback to give reversion or depolymerization at high temperature, which deteriorates the physical properties. 

---