Fluorine sheath

The size of the fluorine atom allows the formation of a uniform and continuous sheath around the carbon-carbon bonds and protects them from attack, thus imparting chemical resistance and stability to the molecule. The fluorine sheath is also responsible for the low surface energy (18 dynes/cm)[ i and low coefficient of friction (0.05-0.08, static)[ i of PTFE. Another attribute of the imiform fluorine sheath is the electrical inertness (or non-polarity) of the PTFE molecule. Electrical fields impart only slight polarization to this molecule, so volume and surface resistivity are high. Table 1.1 summarizes the fundamental properties of PTFE, which represents the ultimate polymer among all fluoroplastics. 

Fluorine Sheath - An analogy comparing the molecule of (PTFE) polytetrafluoroethylene with a wire. Fluorine atoms form a sheath around the carbon backbone of PTFE, rendering it impervious to chemicals, resembling the function of an insulation around a conductor. 

Steric Factors. Initially, most of the coUisions of fluorine molecules with saturated or aromatic hydrocarbons occur at a hydrogen site or at a TT-bond (unsaturated) site. When coUision occurs at the TT-bond, the double bond disappears but the single bond remains because the energy released in initiation (eq. 4) is insufficient to fracture the carbon—carbon single bond. Once carbon—fluorine bonds have begun to form on the carbon skeleton of either an unsaturated or alkane system, the carbon skeleton is somewhat stericaUy protected by the sheath of fluorine atoms. Figure 2, which shows the crowded hehcal arrangement of fluorine around the carbon backbone of polytetrafluoroethylene (PTFE), is an example of an extreme case of steric protection of carbon—carbon bonds (29). 

The nonbonding electron clouds of the attached fluorine atoms tend to repel the oncoming fluorine molecules as they approach the carbon skeleton. This reduces the number of effective coUisions, making it possible to increase the total number of coUisions and stiU not accelerate the reaction rate as the reaction proceeds toward completion. This protective sheath of fluorine atoms provides the inertness of Teflon and other fluorocarbons. It also explains the fact that greater success in direct fluorination processes has been reported when the hydrocarbon to be fluorinated had already been partiaUy fluorinated by some other process or was prechlorinated, ie, the protective sheath of halogens reduced the number of reactive coUisions and aUowed reactions to occur without excessive cleavage of carbon—carbon bonds or mnaway exothermic processes. 

Polytetrafluoroethylene and fluorinated ethylene-propylene are the only resins composed wholly of fluorine and carbon. The polymer consists of fluorine atoms surrounding the carbon chain as a sheath, giving a chemically inert and relatively dense product from the strong carbon-fluorine bonds. Polytetrafluoroethylene must be molded at high pressure. Fluorinated ethylene-propylene c.m be injection molded and extruded as thin fdm. Both plastics have exceptional heat resistance... 

The great kinetic stability of sulfur hexafluoride 73>74>, like that of carbon tetrafluoride 73>, particularly toward nucleophilic reagents, may be viewed as arising from the presence about the central atom s kernel (and about the kernels of the fluorine atoms) of a nearly complete, protective sheath of electrons with no pockets 52> of sufficient depth (orbitals of sufficiently low energy) to permit effective coordination with the unshared electrons of an entering nucleophile. The possibility remains, however, of attack by electrophilic reagents, e.g., by strong Lewis acids, such as sulfur trioxide 74>. 

Perfluoropolymers derive their chemical resistance from an extremely strong carbon-fluorine bond and an impermeable sheath of fluorine atoms surrounding the carbon-carbon chain. Relatively high crystalline content renders these polymers insoluble in solvents. 

Since HF is formed as a by-product of the reaction, glass should not come into contact with the reaction mixture, and the reaction vessel, fluorine delivery tube, thermocouple sheath, and filtration apparatus should all be made of Teflon. The delivery tube is made from /4-in. o.d. x yie-in. i.d. FEP tubing [Zeus]. One end of the tube should be heated and drawn to a narrow tip (ca. 1 mm i.d.). 

The PTFE chain adopts a slowly twisting hehx with an outer sheath of fluorine atoms encompassing a carbon-based core. There is mutual repulsion of the fluorine atoms. These cylinders can slip past each other and this can lead to cold flow or creep [32]. Creep is permanent deformation of a plastic due to the prolonged application of stress. An example is a plastic strap that is holding a weight and over time permanently stretches and is elongated even after the weight is removed. PTFE is more susceptible to creep than many other polymers. 

The helical chains present a sheath of fluorine atoms to neighbouring molecules. The low polarizability of the sheath means that the interactions with adjacent molecules will be very weak hence the non-stick properties. 

H The sheath of surrounding fluorine atoms, which gives the molecule a low polarizability and hence weakens intermolecular forces (Section 4.5). 

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