Degradation mechanisms of Fluorosilicone

Poly(methyl-3,3,3 trifluoropropylene-siloxane) has a Tg of -75°C. Unlike PDMS it does not exhibit a low crystallisation temperature, due to the inability of the polymer chains to pack into a crystalline lattice. Because of the low Tg, and absence of a low temperature crystallisation, the fluorosilicone remains quite flexible at very low temperatures [14].

The role of fluorine and carbon-fluorine bond in achieving a high degree of solvent resistance and stability is well known in organic polymer systems. The two properties were combined in silicone polymers by incorporation of fluorine into polyalkylsiloxane systems. Incorporation of the carbon-fluorine bond rather than the silicone-fluorine bond is a principal factor influencing the choice of structure for polyalkylsiloxane systems. The latter, although of high thermal stability, is subject to hydrolysis and therefore, is less useful than the former. The second structural consideration is the location of fluorine relative to silicone in the alkyl substituent. Because of the great electronegativity and strong inductive effect of fluorine, the position alpha to silicone, e.g., CF3SiRR'R'', suffers a major disadvantage: hydrolytic cleavage of the silicon-carbon bond. In addition, thermal rearrangement is possible with the formation of silicon-fluorine bonds as shown in Figure 4.23 [15]:

Chapter 4: Overview of Degradation and Stabilisation Mechanisms in General Figure 4.23: Thermal rearrangement with formation of silicone fluorine bonds [15].

This rearrangement appears to be similar to the alpha elimination mechanism proposed for one mode of carbon formation. The position beta to silicon, CF3CH2SiRR'R'', also suffers the same disadvantage, hydrolytic instability and thermal rearrangement. In the latter, the side chain is eliminated as an olefin (Figure 4.24) [15]:

R''

Figure 4.24: Hydrolytic instability and thermal rearrangement of silicon in beta position [15].

Therefore, the gamma position, CF3CH2CH2SiRR'R'', is the obvious choice for maximum stability and ease of preparation. Positions beyond gamma are also suitable, but their usefulness is limited by the oxidation of the CH bonds in the alkyl group [15].

4.5.1 Thermal degradation of Fluorosilicone

Fluorosilicones are in an interesting thermodynamics state where they are actually in equilibrium with their cyclic trimer and tetramers (Figure 4.25).

Si

Figure 4.25:Equilibrium between linear and cyclic oligomers Fluorosilicone [14].

However, it is the oligomers, not the polymer, which is thermodynamically favoured. Thermal degradation of fluorosilicones can occur by a reversion mechanism where heat shifts the equilibrium towards the tetramers. Thus, the polymer breaks down to form the cyclic tetramers (a thermodynamically stable compound). Basic compounds such as KOH accelerate this reaction. This degradation pathway is more dominant in the absence of oxygen since oxidative crosslinking becomes competitive in oxygen containing atmosphere.

The hydrocarbon spacer provided by two methyl groups gives optimum thermal stability and copolymerising with 3,3,3-trifluoropropylene provides the γ fluorosubstituent.

Silicones in general are known for their excellent retention of properties at elevated temperatures. Fluorosilicone elastomers are no exception although they have slightly reduced high temperature stability compared PDMS [14].

4.5.2 Chemical resistance of Fluorosilicone

Fluorosilicone resists deterioration by solvents, acids, chlorides and other severe chemicals as well as low-pressure steam and condensate [16]. Fluorosilicone also perform well with low volume swells in alcohol/fuel blends, once the solvent is removed the physical properties return nearly to the original non-swollen state. Coupled with its resistance to mineral oils, fuels and solvents, fluorosilicone rubbers most striking properties are its heat resistance. It is more resistant than most plastics and rubbers with purely organic bases. Generally, it may be assumed to be able to withstand long-term exposure to temperatures of 210 to 230^oC.

One striking property of fluorosilicone rubber is its flexibility at very low temperatures (down to about -60^oC) [17].

Fluorosilicone elastomers are especially suited for applications involving exposure to fuels, oils, hydraulic fluids and various other chemicals. Fluid resistance is excellent to almost all solvents with a few exceptions, e.g., some esters and ketones due to higher swells. Even dilute caustic solutions, nitric acid, hydrochloric acid, and sulphuric acid have little effect on fluorosilicone rubbers [15].

4.5.3 Chemical resistance of Fluorosilicone to chlorine

A study of the given polymer structure given for the Fluorosilicone (from the supplier Silicone Specialty Fabricators, USA) the polymer chain is vinyl terminated with C-C double bounds (Figure 4.26). Double bonds are reactive towards various chemicals. In chlorine environment the addition of chlorine atoms would take place to saturate the double bonds (Figure 4.26). This will lead "new" polymer after the chlorination reaction, which might be stable against further chlorination.

Chapter 4: Overview of Degradation and Stabilisation Mechanisms in General

Figure 4.26: Cl2 addition of vinyl terminated Fluorosilicone

References to Chapter 4

1. D. J. Carlsson, D. W. Wiles; Degradation in Encyclopaedia of Polymer Science and Engineering, vol. 4, p. 630-696, John Wiley & Sons Inc., USA 1986.

2. W. Scnable; Polymer Degradation - Principles and Practical Applications, Hanser Publishers, Germany 1992.

3. N. Grassie and I. G. Macfarlane; The Thermal Degradation of Polysiloxanes-I Polydimethylsiloxane, Eur. Polym. J., vol. 14, p. 875-884, 1978.

4. A. Holmström, E. M. Sörvik; Thermal Degradation of Polyethylene in a Nitrogen Atmosphere of Low Oxygen Content. III. Structural Changes Occurring in Low-Density Polyethylene at Oxygen Content Below 1.2%, J. Appl. Polym. Sci., vol 18, p. 3153-3178, 1974.

5. W. L. Hawkins; Stabilisation in Encyclopaedia of Polymer Science and Engineering, vol. 4, p. 630-696, 1996.

6. S. J. Clarson, J. A. Semlyen; Siloxane Polymers, Ellis Horwood-PTR Prentice Hall, USA 1993.

7. P. R. Dvornic, R.W. Lenz; High temperature Siloxane Elastomer, Hüthig & Wepf Verlag, Germany 1990.

8. P. R. Dvornic; Some Recent Advances in the Silicone Containing Polymers, Materials of Sci. Forum, vol. 214, p. 131-138, Transtec Publications, Switzerland 1996.

9. R. Vera-Graziano, F. Hernandez, J. V. Cauich-Rodriguez; Study of Crosslinking, Density in Polydimethylsiloxane Networks by DSC, J. Appl.Polym. Sci., vol. 55, p.1317-1327, 1995.

10. K.A. Andrianov; Metalorganic Polymers, Interscience, p.50, New York 1965.

11. Yu. V. Moiseev and G.E. Zaikov; Chemical Resistance of polymers in aggressive media, Consultants Bureau, New York 1987.

12. M.E.Hodgson; Silicone Rubber Membranes, Filter Media-Development and Innovation Filtration Society Symposium, p. 418-419, Manchester, October 3rd 1972.

13. M. A. Brook, S. Balduzzi, M. Mohamed, R. Stan; Report 2 to Norsk Hydro-Tel-Tek, Acid Lability of Silicone Membranes (confidential), Chemistry Department McMaster University, Canada, Feb.1999.

14. J. Scheirs (Editor), M.T. Maxson, A.W. Norris, M. J. Owen; Modern Fluoropolymers:

high performance polymer for diverse applications, Chapter 20, John Wiley & Sons, Great Britain 1997.

15. Y. K. Kim; Poly(fluorosilicones) in Kirk-Othmer Encyclopaedia of Chemical Technology, 3rd Ed., vol.11, p. 74-81, USA 1978.

16. K. Parker; Harsh chemicals can't beat fluorosilicone lubricant, Eng. & Maintenance, October 1991.

17. D. Klages, U. Raupbach; Fluorosilicone Rubber - a modern material, Gummi Fasern Kunststoffe, International Polymer Sci.and Tech., vol 22, No.5, p. T/11-T/13,1995.