Oxyfluorination

Assignee South African Nuclear Energy Corporation, Ltd. (ZA) 

Polypropylene has been oxyfluorinated using a gas mixture consisting of fluorine, nitrogen, and water. When cured with mortar slurry, oxyfluorinated surfaces had 20% greater shear bond strength than fluorinated surfaces. 

Monofilament polypropylene fibres were prepared by direct extrusion having a rectangular cross section of 0.5 x 1.3 mm, a length of 40 mm, a specific gravity of 0.91, a tensile strength of 120 MPa, and an elongation at break of 14%. The fibers were placed under 18% humidity at ambient temperature air and then treated with 20% F2 and 80% N2 at apressure of 45 kPa at 38°C for 2.5 hours. Thereafter the reaction vessel was evacuated, and the product was isolated. 

TABLE 1. Shear bond testing of surface modified polypropylene after oxyflnorination curing in mortar slurry. 

Note The Control sample was prepared under anhydrous conditions. 

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Other hand, oxyfluorination is a technique offering unique possibiUties for the functionalization of fluorocarbons and for the preparation of functional fluorocarbon membranes (39). 

The surface activation method, especially the gas mixture used, determines the chemistry of the surface and can thus promote the adhesion properties with epoxy adhesives. The following example illustrates the importance of the correct activation gas mixture. As is clearly illustrated by Table 16.6, oxyfluorination induces better adhesive properties than fluorination on PE with Pro-Struct 30/71. 

The lap shear test involves measuring the adhesive shear strength between two surface fluorinated polyolefin sheet tokens that are adhesively secured with a reinforcement resin. The tokens are individually reinforced with steel backing plates to eliminate flexural distortion in the shear joint. Lap shear tests carried out with various reinforcing polyester-type resins, contrasting fluorination and oxyfluorination as surface treatment, are shown in Table 16.8. 

From the above it can be deduced that, in general, oxyfluorinated PP surfaces give slightly improved adhesion over oxyfluorinated HDPE surfaces. This can be attributed to the greater ease with which PP forms the acyl fluoride functionality during treatment.28... 

Our experience indicates that fluorination generally results in decreased adhesion strength compared to oxyfluorination (see Tables 16.6 and 16.8). This is in contrast to reported findings in the literature where epoxy adhesives were used.7,29 This discrepancy can be attributed to the surface energy implications of the increased acyl fluoride to the C—H—F functionalkation ratio (compare Figures 16.3 and 16.4). However, as indicated by our data, certain resin formula-... 

Polyester resin oxyfluorinated HDPE (MPa) Oxyfluorinated pp (MPa) Fluorinated HDPE (MPa)... 

Coaxial push-out tests (DIN 53769-A) are typically performed on production run pipe samples for quality-controlpurposes. Typical results using oxyfluorinated HDPE as liner material are shown in Table 16.11 below. 

A typical oxyfluorination treatment that is sufficient to activate polyolefin surfaces with respect to polyester adhesion includes the following ... 

Figure 16.4. Effect of oxyFluorination time on the surface tension of PE (10% F2/O2, 30°C, constant of 20 cm /min).

The oxyfluorinated polymers would be expected to react with a variety of organic, inorganic, and organometallic compounds. Derivatives of each functionalized material have been prepared with CH3OH, C2HsOH, n-BuOH, and aniline. The efficiency of the derivativization process ranged from 44 to 98%. 

The fact that functionalization of polymers and small molecules is observed to occur predominately on terminal (methyl) carbon atoms does not imply that the oxyfluorination reaction is truly selective. Although the reaction mechanism has not been studied in detail, it is undoubtedly a free-radical process. Molecular oxygen reacts spontaneously with the fluorocarbon—hydrogen radicals generated by fluorine during the fluorination process. Acid fluorides are retained on terminal carbon atoms because they are stable in 1 atm of elemental fluorine. Hypofluorites, which may be short-lived intermediates of oxygen reactions with methylene radical sites along the carbon chain, are not observed in the functionalized polymers. It is probable that, if they are intermediates, they are cleaved and removed by the excess elemental fluorine. 

Aromatic oxyfluorination using HF recyclable inorganic fluorides 211... 

As already mentioned, most of the syntheses use hydrothermal synthesis. The reaction takes place in the autoclave, which acts as a black box for the understanding of the mechanism of formation of the solids which are finally isolated. Nothing was known about the interaction between organic and inorganic species. This needed to choose appropriate systems for the study of the pertinent chemical parameters, to carefully characterize the compounds and use in situ measurements to collect informations able to shed some light on the steps of formation of these solids. Such an approach has been initiated only on oxyfluorinated species, and shall be developped in the following. 

The improvement of the understanding of the formation of oxyfluorinated compounds with an open structure needed the systematic study including chemistry, structures and NMR characterisitics described above on the ULM-n (University of Le Mans) system M203 (M = Al,Ga)-P205-HF-amine-H2O. 

If it becomes easy now to obtain open structures with di and trivalent transition metals, the litterature is very poor concerning tetravalent cations. However, very recently, two zirconium [72,73] and two titanium [74,75] fluorophosphates were evidenced. The titanium family provides the first example [74], of a mixed valence compound, X iIIITilvF(P04)2, 2 H20 (Fig. 13) in the series of oxyfluorinated solids with an open framework. 

Finally, a question obviously arises at the end of this chapter on oxyfluorinated open frameworks. Does open frameworks exist with only fluoride anions The recent discovery of two-dimensional uranium fluorides templated by alkyldiamines by O Hare et al. [81,82] is a first answer to the question, and no doubt that in a few years, the discovery of purely fluorinated solids with an open framework will be claimed. 

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