Fluorocarbons construction materials

Fluorocarbons and fluoropolymers have been in commercial use for over half a century and have found their way into a diverse array of products. The members of this array are too numerous to hst but they include nonstick coatings for cookware, construction materials, carpets, textiles, paints, electronic materials, household cleaners and personal hygiene products. As fluorocarbons are expensive compared to their hydrocarbon analogues, they are chosen carefully and usually to fill a void in a product attribute that simply cannot be accommodated by another material. The attributes typically afforded by the use of fluorocarbons are repellency, lubricity, chemical and thermal inertness, and low dielectric constant, in regards to which fluorocarbons are unique among their hydrocarbon counterparts. 

Mild steel may be suitable as a material of construction for handling caustic soda at ambient temperature. At elevated temperatures, >60°C (140°F), corrosion may occur. Nickel and/or its alloys are most suitable for caustic handling at all temperatures and concentrations, including anhydrous molten caustic up to 480°C (896°F) (Leddy et al. 1978). Polypropylene, fluorocarbon plastics, and flberglass/vinyl ester resins are being used for many applications. Aluminum, tin, zinc, and other amphoteric metals should not be used in construction materials. 

Volatile metal halides, usually chlorides and fluorides, also form the heart of several processes used to produce surface layers, rich in aluminium, chromium, or silicon, or combinations of these. In these processes, the workpiece to be coated is buried in a powder bed and heated to reaction temperature. The bed consists of a mixture of inert alumina filler, a master alloy powder that contains the aluminium, etc., and an activator such as ammonium chloride. Basically, at about 630°C, the activator volatilizes and the aluminium chloride vapour reacts with the master alloy to produce a volatile aluminium chloride, which then reacts with the workpiece surface to deposit aluminium. The deposited aluminium proceeds to diffuse into the surface layers of the workpiece to produce a diffusion coating. The process is driven basically by the difference in aluminium activity between the master alloy and the worlqtiece. These processes are well documented in principle, but their execution to provide reproducible and reliable results still involves considerable experience, or rule of thumb. These processes will be described in detail in Chapter 10. Finally, a chlorination treatment is used to remove tin from tin-plated steel. This uses a normally deleterious reaction to advantage and profit in the recovery of both tin and steel for recycling. Fluorination is used in the manufacture of polymers and fluorocarbon consequently, materials suitable for construction of these plants must be resistant to fluorine attack. 

Polymers used for seat and plug seals and internal static seals include PTFE (polytetrafluoroeth ene) and other fluorocarbons, polyethylene, nylon, polyether-ether-ketone, and acetal. Fluorocarbons are often carbon or glass-filled to improve mechanical properties and heat resistance. Temperature and chemical compatibility with the process fluid are the key selec tion criteria. Polymer-lined bearings and guides are used to decrease fric tion, which lessens dead band and reduces actuator force requirements. See Sec. 28, Materials of Construction, for properties. 

Unless laboratory studies on material compatibility establish otherwise, it is recommended that equipment used to collect groundwater samples for pesticide analysis be constructed of metal, fluorocarbon polymer, or glass.However, for a water-supply well, inert well, pump, and plumbing materials are not likely to have been installed for all components. In this case, in-place well, pump type, and plumbing materials should be documented. 

The reservoir materials may be PVC, stainless steel, or a fluorocarbon polymer, and the porous cup may be constructed of ceramic, stainless steel, or fluorocarbon polymer. Ceramic cups have a smaller pore size, a greater bubble pressure (pressure under which the cup produces bubbles), and a greater operational suction range, and are preferred to other porous cup materials. All materials used for the construction of the suction lysimeter should be tested in the laboratory to determine if any bias in the sample analysis will result from their use. 

All of the unique properties imparted by fluorocarbons can be traced back to a single origin the nature of the C—F bond. These properties include low surface tension, excellent thermal and chemical stability, low coefficient of friction, and low dielectric constant. However, not all of these properties are possessed by the entire inventory of available fluorocarbons. The fluorocarbons can be assigned to two major categories (1) fluoropolymers, which are materials that are comprised mainly of C—F bonds and include such examples as PTFE, and (2) fluorochemicals (FA) based on the perfluoroalkyl group, which are materials that generally have fewer C—F bonds and often exist as derivatives of other classes of molecules (e.g., acrylates, alcohols, esters). In addition, the properties that dictate the uses of fluorocarbons can be classified into (1) bulk properties (e.g., thermal and chemical stability, dielectric constant) and (2) surface properties (e.g., low surface tension, low coefficient of friction). The types of materials available and properties imparted are not exclusive and overlap substantially. From this array of fluorocarbons and attributes, a large variety of unique materials can be constructed. 

Spinner assemblies (l.e., stator, holder, rotor) have been constructed from three materials Kel-F, Delrin, and machinable boron nitride (BN) (14). For observation of hydrocarbon materials at ambient and low temperatures, the Kel-F assembly is used. It displays suitable mechanical properties and does not interfere with the carbon spectrum since the resonances of the carbons in the Kel-F are >10 kHz in width due to the unremoved C-F dipolar interactions. To observe fluorocarbon materials at ambient and low temperature by C-F dipolar decoupling/CP/MAS, the Delrin assembly is used since unremoved C-H dipolar interactions broaden... 

Wet chlorine is very corrosive to all of the more common construction metals. At low pressures wet chlorine can be handled in chemical stoneware, glass or porcelain equipment and in certain alloys. Hard rubber, polyvinyl chloride (maximum of 41 kPa (6 psig)), fiberglass-reinforced polyester, polyvinylidene chloride or fluoride and frilly halogenated fluorocarbon resins have been used successfully. For maximum working pressure see Chlorine Pamphlet 6 [9]. All of these materials must be selected with care. For higher pressures, lined metallic or compatible metallic systems should be used. In the metallic systems, Hastelloy C, titanium and tantalum have been used. Within limits, titanium may be used with wet chlorine, but must not be used with dry chlorine under any circumstances because it bums spontaneously on contact. Tantalum is inert to wet and dry chlorine at temperatures up to 300°F(149°C). 

Tantalum has a unique place as a material of construction in the chemical industry since its first application in the 1940s. Its chemical inertness in highly corrosive chemical environments is remarkably similar to that of glass, yet it has a number of advantages in comparison to glass, as well as other nonmetallics such as graphite or fluorocarbons. 

Catalyst Research Corporation (USA) is a major manufacturer of lithium—iodine batteries used for pacemaker and other applications. The Catalyst Research Series 800 cells use the lithium envelope concept. The cell is constructed with a centrally located cathode current collector and a lithium envelope which surrounds and contains the iodine depolarizer material (Figure 24.14). This depolarizer material is corrosive to the stainless steel case and must be kept irxnn contacting the case for maximum cell life. A second barrier, formed from fluorocarbon plastics, surrounds the lithium envelope, insulates it from the case, and provides a second envelope for the containment of the depolarizer. The corrosive effect of iodine-containing depolarizer on stainless steel is lower than that of depolarizer made with more active halogens, such as bromine or chlorine. 

---