Fluoropolymers surface fluorination

The direct fluorination of inorganic,1,2 organometallic,3 5 and organic compounds,6-8 employing the LaMar9,10 and Exfluor-Lagow" methods, has impacted the synthesis of fluorinated compounds over the past 25 years. Among the most important applications of direct fluorination are the synthesis of fluoropolymers from hydrocarbon polymers and the conversion of the surface of the hydrocarbon polymers to fluoropolymer surfaces.12,13 The direct fluorination process is an excellent approach to the synthesis of fluoropolymers. 

We are going to discuss the syntheses of fluoropolymers, poly(carbon monofluoride), perfluoropolyethers, perfluorinated nitrogen-containing ladder polymers, and surface fluorination of polymers by direct fluorination. 

However, one should not forget that apart from the complexity of the synthesis fluoropolymers are very expensive. For example, the price of fluoro-rubber is more than 30-fold that of an ordinary rubber such as butadiene-styrene (SBR) or ethylene-propylene (EPDM). Cost was one of the factors that gave impetus to research polymer surface fluorination, with the object of imparting the properties of fluoropolymers to the surfaces of less expensive polymers without changing their bulk properties. 

Although we made no attempt to elucidate the mechanism of friction decreases in rubbers after surface fluorination, it seems to us that apart from the substitution of H atoms to F in the polymer macromolecule, which forms a fluoropolymer on the surface, there is another phenomenon that makes a significant contribution to the friction decreases, i.e., fluorination of carbon black, which is used in rubber recipes for reinforcement. It appears that when the carbon black in the surface of the rubber is fluorinated it produces a lubricating effect, followed by blooming on the surface of the treated rubber while it is under a friction load. So, in our opinion, two effects contribute to friction decrease of carbon-filled rubbers fluorination of the rubber macromolecules and fluorination of the carbon black rubbers that do not contain carbon black show a much smaller decrease in friction after XeF2 treatment. 

If solid polymer objects are fluorinated or polymer particles much larger than 100 mesh are used, only surface conversion to fluoropolymer results. Penetration of fluorine and conversion of the hydrocarbon to fluoropolymers to depths of at least 0.1 mm is a result routinely obtained and this assures nearly complete conversion of finely powdered polymers. These fluorocarbon coatings appear to... 

Firstly from a comparison of the integrated area ratios for the F ls and Cls levels and secondly from the individual components of the Cis levels. This readily establishes that the materials are copolymers of ethylene and tetrafluoroethylene which are largely alternating in character and that the outermost surface sampled by ESCA is identical in composition to the bulk. This is shown in Table 4 where a comparison is drawn with compositions determined by standard microanalysis (carbon by combustion, fluorine by potassium fusion). ESCA is highly competitive as a routine means of establishing compositions for fluoropolymers in particular, in terms of accuracy, nondestructive nature and speed. 

Extremely low interaction forces in monomolecular adsorption layers on liquids, Langmuir Blodgett films or in solid interfaces are the precondition for low energy surfaces. Perfluorocarbons or terminal fluorinated parts in amphiphilic molecules and fluoropolymers meet these demands. 

With values in the range of about 10-18 mN m 1 perfluorinated liquids have the lowest surface tensions among the known organic liquids, and will completely wet any solid surface. Increasing amounts of hydrogen in the molecule increase the surface tension. Fluorinated solid surfaces, e.g. fluoropolymers, possess very low critical surface tensions yc, which relates to their antistick and low frictional properties, whereas hydrocarbon polymers have substantially higher values (PTFE yc = 16.0 mN m-1 PE yc = 31.0 mN m-1).7... 

Etching has a profound effect on the surface chemistry of the fluoropolymers. Electron spectroscopy for chemical analysis (ESCA) also known as x-ray photoelectron spectroscopy (XPS) is a technique that has been used to study the chemical composition of surfaces. ESCA does not detect hydrogen, and elemental compositions exclude this element. Results of surface chemical composition of fluoropolymers have been summarized in Table 7.1. The consistent changes in surface composition of fluoropolymers because of treatment are a reduction in fluorine and/or chlorine content, and an increase in carbon and oxygen content. The treated PTFE surface is virtually comprised of carbon and oxygen and a small amount of fluorine. 

Table 7.2 shows the effect of sodium etching on several fluoropolymers by Tetra-Etch on the surface composition and lap shear bond strength. In general, the data for various fluoropolymers indicate an increase in the adhesive bond strength with increasing fluorine and chlorine content. Kinetics of treatment is more favorable to perfluorinated PTFE than PVF that contains one fluorine per monomer unit according to the data in Table 7.2. 

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