Fluorosilicones, commercially available

Fluorosilicones consist of PDMS backbones with some degree of fluoro-aliphatic side chains. The fluorinated group can be trifluoropropyl, nonafluorohexylmethyl, or fluorinated ether side group [78,28,79]. These polymers differ not only in substituent group, but also in the amount of fluoro-substitution relative to PDMS, the overall molecular weight and crosslink density, and the amount of branching. In most commercially available cases, these polymers are addition cure systems and the reactions are those discussed previously for silicone networks. 

In this context, by the term fluorosilicone are meant polymers containing C-F bonds and Si-O bonds with hydrocarbon entities between them. Thus, the repeating structure may be generally written as [RfX (CH J (CH,)vSiOz, where Rf is the fluorocarbon group.59 Commercially available fluorosilicones are based on polymethyltri-fluoropropylsiloxane (PMTFPS), or more accurately poly[methyl (3,3,3-trifluoropropyl)siloxane]. In some cases PMTFPS is copolymerized with polydim-ethyl siloxane (PDMS) for cost/benefit balance.60 The manufacture of monomers for fluorosilicones is discussed in some detail in Reference 60. Fluorosilicone elastomers... 

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. 

The reactivity of cyclosiloxane with the 3,3,4,4,5,5, 6,6,6-nonafluorohexyl group and its application to fluorosilicone synthesis was carefully studied by Furukawa et al. [27]. In contrast to polymerization of commercially available l,3,5-tra(3,3,3-trifluoropropyl)-1,3,5-trimethylcyclotrisiloxane (CF3-D3), polymerization of l,3,5-tra(3,3,4,4,5,5,6,6,6-nonafluorohexyl)- 1,3,5 -trimethylcyclotrisiloxane (C4F9-D3) with sodium hydroxide yielded 1,3,5,7-tetrakis(3,3,4,4,5,5,6,6,6-nonafluorohexyl)-l,3,5,7-tetramethylcyclotetrasiloxane [C4F9CH2CH2(CH3) SiO]4 (C4F9-D4) as the major product. On the other hand, it was reported that polymerization of C4F9-D3 with trifluoromethanesulfonic acid proceeded in... 

Fluorosilicone elastomers are commercially available and contain 7-trifluoro-propylmethylsiloxane units ... 

Let us take, for example, a comparison of fluorocarbon rubbers with fluorosilicone on this basis. Commercially available fluorosilicone and fluorocarbon rubbers were tested first at 25 °C and then at elevated temperatures by the use of a temperature conditioning cabinet fitted to a tensometer. Tensile strength and elongation at break were measured. The results are illustrated graphically in Figs 4 and 5. 

A classification by chemical type is given in Table 1. It does not attempt to be either rigorous or complete. Clearly, some materials could appear in more than one of these classifications, eg, polyethylene waxes [9002-884] can be classified in both synthetic waxes and polyolefins, and fluorosilicones in silicones and fluoropolymers. The broad classes of release materials available are given in the chemical class column, the principal types in the chemical subdivision column, and one or two important selections in the specific examples column. Many commercial products are difficult to place in any classification scheme. Some are of proprietary composition and many are mixtures. For example, metallic soaps are often used in combination with hydrocarbon waxes to produce finely dispersed suspensions. Many products also contain formulating aids such as solvents, emulsifiers, and biocides. 

The original and by far the most widely available fluorosilicone since its introduction in the 1950s is polymethyltrifluoropropylsiloxane (PMTFPS), more rigorously known as poly[methyl(3,3,3-trifluoropropyl)siloxane] or poly[methyl(lH,lH, 2H,2H-trifluoropropyl)siloxane]. Unless specifically mentioned, the unfluorinated carbons are those nearest to silicon, i.e., poly(methylnon-afluorohexyl)siloxane (PMNFHS) is poly[methyl(lH,lH,2H,2H-nonafluoro-hexyl)siloxane]. Currently, only copolymers of PMTFPS and PMNFHS with poly siloxane (PDMS) are available commercially.79... 

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... 

Fluorosilicone materials have always had the potential of being amongst the lowest known surface energy materials because they combine the low intermolecular forces between aliphatic fluorocarbons with the high flexibility of the siloxane backbone. Historically, commercial emphasis was on minimally fluorinated fluorosilicones such as poly (3,3,3-trifluoropropyl) methylsiloxane (PTFPMS) and it is only recently that more highly fluorinated materials have become available and this potential realized. ... 

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