Flourocarbons

Certainly the most versatile and important group of thermoplastics for use in the process industries are the fluorocarbons. Most of these products will handle extremely corrosive environments that no metal can tolerate. The fluorocarbons are seldom attacked by solvents The extreme solvent resistance of the materials allows their almost universal application to chemical problems. As with other products, when such inertness is obtained in the material, certain other properties must be sacrificed. In this case, the fluorocarbon materials are more difficult to work in any manner and are much more limited in design and application than are other thermoplastic materials. The materials are porous. As indicated below, the permeation of specific compounds must be considered rather than making blanket assumptions regarding the porosity that is not necessarily of the same relative value for the other products. When using these fluorocarbons, this factor should be considered and discussed thoroughly with the vendor to assure the proper selection of the proper fluorocarbon for the intended service. One mode of deterioration for these inert materials has been adsorption of a component from a stream, followed either by reaction with another subsequent component in situ in the matrix of the thermoplastic or by polymerization of the product within the matrix of the plastic. When this occurs, surface degradation appears to develop, and in many instances blistering will occur. Also, as with other thermoplastic materials, absorption of high vapor pressure materials by the plastic under pressure followed by a sudden pressure release can cause deterioration of the plastic. The materials do have a definite heat limitation. Overheating should be avoided.

The excellent properties of these materials are such that a person tends to believe they will serve under any circumstances. Vacuum service must be carefully appraised. Lined pipe must be chosen which is suitable for such service. Collapse of the liners has occurred in many instances, not only because of inattention to design vacuum but also because of vacuum formed by liquid head in the piping systems. Thermal cycling can cause fatigue of the materials. The fluorocarbons in general are highly crystalline and fail in many modes associated with the metals. Pipe liners of a given design when exposed to thermal cycling will fatigue near the flange from expansion and contraction of the mass repeatedly over a period of time. Cold flow of the resins is well known to most persons. Consequently, the design and use of the product should be such that excessive compressive stresses are not imposed to create a cold flow condition.

The materials have been designed into a number of solid items for chemical service, such as impellers, mixers, spargers, packing, smaller containers, and a few more intricate shapes. However, the vast proportion of the use of fluorocarbons in the process industry is as linings in steel or ductile iron. All shapes of lined pipe can be obtained. In addition, certain of the thermoplastic can be cut and jointed in the field using appropriate tools. Lined pumps and valves are available. The smaller and simpler designed reactors and other closed vessels can be lined with FEP, PVF, and presumably the new fluorocarbons. Considerable attention needs to be given such a design to assure success of the application.