Flouropolymers are used in the chemical processing industry principally for outstanding resistance to almost all chemicals their ability to withstand high temperatures also plays an important part in their functional role. The low mechanical Properties and high permeation rates of flouropolymers at high temperature limit their use of linings to about 175 Degree Celsius, although they remain chemical inert up to 260 Degree Celsius.
Selection of polymers is an integral part of the overall material selection process. This implies that all the available material such as metals, ceramics and plastics are considered candidates for an application. The End user then considers these materials against established criteria such as required life, mean time between inspection, ease of fabrication, frequency of inspection ,extent of maintence and capital cost.
More often than not it is the initial capital cost rather than life cycle cost of the equipment that affects the decision made during the material selection step. However the most important piece of data in the medium under consideration over the life of the equipment.
Corrosion of Metals
Metals corrode by an electrochemical reaction where metal loss occurs as oxidation at the anode sites and plating out occurs at the cathode. There are two types of Corrosion known as General Corrosion and Localized Corrosion. General Corrosion refers to a uniform loss of the metal thickness due to the interaction of the metal and the chemical to which it is exposed. This degradation can be measured as corrosion rate in the thickness per year. Localized Corrosion can be of various types such as pitting, Intergranular corrosion and Crevice Corrosion.
Pitting Corrosion is the localized corrosion of a metal surface confined to a point or small area that takes the form of cavities. Pitting is one of the most damaging forms of corrosion.
Intergranular corrosion is sometimes also called "intercrystalline corrosion" or "interdendritic corrosion". In the presence of tensile stress, cracking may occur along grain boundaries and this type of corrosion is frequently called "interranular stress corrosion cracking (IGSCC)" or simply "intergranular corrosion cracking".
"Intergranular" or 'intercrystalline" means between grains or crystals. As the name suggests, this is a form of corrosive attack that progresses preferentially along interdendritic paths (the grain boundaries). Positive identification of this type of corrosion usually requires microstructure examination under a microscopy although sometimes it is visually recognizable as in the case of weld decay.
Crevice Corrosion refers to the localized attack on a metal surface at, or immediately adjacent to, the gap or crevice between two joining surfaces. The gap or crevice can be formed between two metals or a metal and non-metallic material. Outside the gap or without the gap, both metals are resistant to corrosion.
The damage is normally confined to one metal at localized area within or close to the joining surfaces.
Corrosion of Polymers
Polymers do not corrode by an electrochemical process. The chemical Degradation and Chemical attack are used to describe polymer/chemical interaction. The following descriptions are often used cracking, blistering, chalking, swelling, discoloration. It is difficult to measure the corrosion rate for polymers.
Corrosion of metallic materials takes place via an electro-chemical reaction at a specific corrosion rate. Consequently, the life of a metallic material in a particular corrosive environment can be accurately predicted. This is not the case with polymeric materials. Polymeric materials do not experience specific corrosion rates. They are usually completely resistant to a specific corrodent (within specific Temperature ranges) or they deteriorate rapidly. Polymers are attacked either by chemical reaction or solvation. Solvation is the penetration of the polymer by a corrodent that causes swelling, softening, and ultimate failure.
Corrosion of plastics can be classified in the following ways as to attack Mechanism:
- Disintegration or degradation of a physical nature because of absorption, permeation, solvent action, or other factors
- Oxidation, where chemical bonds are attacked
- Hydrolysis, where ester linkages are attacked
- Thermal degradation involving depolymerization and possibly repolymerization
- Dehydration (rather uncommon)
- Any combination of the above
Results of such attacks will appear in the form of softening, charring, crazing, delamination, embrittlement, discoloration, dissolving, or swelling. The corrosion of polymer matrix composites is also affected by two other factors: the nature of the laminate and in the case of thermoset resins, the cure. Improper or insufficient cure will adversely affect the corrosion resistance, whereas proper cure time and procedures will generally improve corrosion resistance.Polymeric materials in outdoor applications are exposed to weather extremes that can be extremely deleterious to the material. The most harmful weather component, exposure to ultraviolet (UV) radiation, can cause embrittlement fading, surface cracking, and chalking. After exposure to direct sunlightfor a period of years, most polymers exhibit reduced impact resistance, lower overall mechanical performance, and a change in appearance. The electromagnetic energy from the sunlight is normally divided into ultraviolet light, visible light, and infrared energy. Infrared energy consists of wavelengths longer than visible red wavelengths and starts above 760 nm. Visible light is defined as radiation between 400 and 760 nm. Ultraviolet light consists of radiation below 400 nm. The UV portion of the spectrum is further subdivided into UV-A, UV-B, and UV-C. Because UV is easily filtered by air masses, cloud cover, pollution, and other factors, the amount and spectrum of natural UVexposure is extremely variable. Because the sun is lower in the sky during the winter months, it is filtered through a greater air mass. This creates two important differences between summer and winter sunlight: changes in the intensity of the light and in the spectrum. During winter months, much of the damaging short wavelength UV light is filtered out. For example, the intensity of UV at 320 nm changes about 8 to 1 from summer to winter.
In addition, the short wavelength solar cutoff shifts from approximately 295 nm in summer to approximately 310 nm in winter. As a result, materials sensitive to UV below 320 nm would degrade only slightly, if at all, during the winter months. Photochemical degradation is caused by photons or light breaking chemical bonds. For each type of chemical bond, there is a critical threshold wavelength of light with enough energy to cause a reaction. Light of any wavelength shorter than the threshold can break a bond, but longer wavelengths cannot break it. Therefore, the short wavelength cutoff of a light source is of critical importance. If a particular polymer is only sensitive to UV light below 290 nm (the solar cutoff point), it will never experience photochemical deterioration outdoors. The ability to withstand weathering varies with the polymer type and within grades of a particular resin. Many resin grades are available with UV-absorbing additives to improve weather-ability. However, the higher molecular weight grades of a resin generally exhibit better weather ability than the lower molecular weight grades with comparable additives. In addition, some colors tend to weather better than others. Many of the physical property and chemical resistance differences of polymers stem directly from the type and arrangement of atoms in the polymer chains. In the periodic table, the basic elements of nature are placed into classes with similar properties, i.e., elements and compound that exhibit similar behavior.
These classes are alkali metals, alkaline earth metals, transition metals, rare earth series, other metals, nonmetals, and noble (inert) gases of particular importance and interest for thermoplasts is the category known as halogens that are found in the nonmetal category. The elements included in this category are fluorine, chlorine, bromine, and iodine. Since these are the most electro-negative elements in the periodic table, they are the most likely to attract an electron from another element and become part of a stable structure. Of all the halogens, fluorine is the most electronegative, permitting it to strongly bond with carbon and hydrogen atoms, but not well with itself. The carbon–fluorine bond, the predominant bond in PVDF and PTFE that gives it such important properties, is among the strongest known organic compounds. The fluorine acts as a protective shield for other bonds of lesser strength within the main chain of the polymer. The carbon–hydrogen bond, that such plastics as PPE and PP are composed, is considerably weaker. This class of polymers is known as polyolefins. The carbon–chlorine bond, a key bond of PVC, is weaker yet. The arrangement of elements in the molecule, the symmetry of the structure, and the polymer chains' degree of branching are as important as the specific elements contained in the molecule. Polymers containing the carbon–hydrogen bonds such as polypropylene and polyethylene, and the carbon–chlorine bonds such as PVC and ethylene chlorotrifluoroethylene are different in the important property of chemical resistance from a fully fluorinated polymer such as polytetrafluoroethylene. The latter has a much wider range of corrosion resistance. The fluoroplastic materials are divided into two groups: fully fluorinated fluorocarbon polymers such as PTFE, FEP, and PPA called perfluoropolymers, and the partially fluorinated polymers such as ETFE, PVDF, and ECTFE that are called fluoropolymers.
Approaches for selection
Selection of any material will depend on identifying the degradation mechanism, its rate and criteria based on the expected life of the unit. The ability to fabricate the Fluoropolymers in the required shape is another consideration in its selection.
The ability to inspect and maintain the equipment or piping is also an important consideration in the selection process. Many operations are reluctant to shutdown equipment for reasons of downtime, safety and cost associated with the clean up.
Fluoropolymers are principally used for chemical resistance, nonstick, and Product parity applications.
Economics of selecting Fluoropolmers
Carbon steel, stain less steel, high nickel alloys and exotic metals (Ti and Ta) represent an upward progression in terms of corrosion resistance and cost. For Polymeric Materials polylefins, vinyls and some type of fiberglass represent the low end of spectrum. Some other forms of fiberglass and thermoplastics such as polypropylene are in the middle.Fluoropolymers are at the high end of the spectrum and they compete with the high nickel and exotic metals.