Fluorinated repellents structure

The theory of repellency is discussed in Chapter 11. The relationship between repellency and the structure of the fluorinated repellent is in agreement with the critical surface tension concept developed by Zisman [319]. Shafrin and Zis-man [320] determined the critical surface tensions of fu-perfluroalkyl-substituted -heptadecanoic acids and the wettabilities of their monolayers. The wettabilities... 

If only one of the orbitals <4, dyM, dZM is occupied, the structure no longer has regular octahedral symmetry. If three are occupied, as in CrFa, the regular symmetry is retained. The fourth 3d electron in CrF2 can be described as occupying the dxx orbital, and repelling the two fluorine atoms along +z and — z. 

The same effect is also observed in the bicyclo[2.2.2]octyl system 175f In this system, the effect has been attributed to a substituent induced structural perturbation. The electronegative fluorine and substituent repel each other resulting in enhanced p-character of the endocyclic C-C bonds. The C-F bond order may, therefore, be enhanced as discussed above for the unsubstituted polycyclic fluorides 17S>. 

Polytetrafluoroethylene (PTFE), also known by its generic DuPont tradename of Teflon, is a linear polymer that has a molecular structure similar to PE, but with all the hydrogen-atoms replaced by fluorine atoms (CE2-CE2) . Since fluorine-atoms prefer contact with fluorine-atoms, while repelling all others, this repulsion reduces the friction coefficient of the system and produces a non-adhesive surface (with self-lubricating properties). The strong C-C and C-F bonds make PTFE chemically inert, with excellent chemical resistance. 

Boron is an element in group 13 of the periodic table. Therefore, it has three valence electrons. The Lewis structure (Figure 4-45) of the boron trifluoride molecule, BF3, shows there are only three electron pairs (three electron domains) in the valence shell of the boron atom. These three bonding pairs repel each other equally, with the result that the boron trifluoride molecule is a trigonal planar (flat) molecule. The three boron-fluorine bonds point towards the three corners of an equilateral triangle. The bond angles are all equal at 120°. 

Representations of deformation from Oh symmetry consistent with electron diffraction data for gaseous XeFg. The lone pair of elections is shown to be repelling the fluorine atoms, to give structures with Csv and C2v symmetry. Redrawn with permission from [2]. Copyright 1968, American Institute of Physics. 

The repellency of fluorocarbon finishes depends on the structures of the fluorocarbon segment, the nonfluorinated segment of the molecule, the orientation of the fluorocarbon tail, the distribution and the amount of the fluorocarbon moiety on fibers, and the composition and geometry of the fabric [101]. The relationship between repellency and the structure of the fluorocarbon segment is in accord with the critical surface tension concept developed by Zisman and co-workers (see Chapter 11). Shafrin and Zisman [102] determined the wettabilities and critical surface tensions of -perfluoroalkyl substituted 77-heptadecanoic acids synthesized by Brace [103]. Once the seven outmost carbon atoms are fully fluorinated x = 7), the wettability of monolayers of the acids F(CF2)a(CH2)i6COOH approaches that of the perfluorocarboxylic acid F(CF2).vCOOH (Fig. 12.2). This suggests that a terminal perfluoroalkyl chain of seven carbon atoms is sufficiently... 

The patent literature on fluorocarbon repellents is voluminous. However, most patients disclose variations of nonfluorinated structural features of the repellent molecule. Structural variations of the fluorinated segment are limited by the small number of practical chemical routes to fluorinated intermediates. The most important commercial processes for producing fluorinated intermediates are based on either electrochemical fluorination or telomerization of tetrafluoroethylene (see Chapter 2). 

---