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I unnooll- Alternate Materials Phenolic Resins Phenolic resins have been used for a long time. They are highly crosslinked resins based on reaction between phenol and formaldehyde. Advantages include very good heat resistance as well as low smoke generation due to ablative or carbonizing properties. The ratio of phenol to formaldehyde primarily determines the properties. Novolac resins are based on a deficiency of formaldehyde and are supplied as solid powders typically used in reactive injection molding applications. They are then cured with hexa methylene tetramine, which provides a formaldehyde source. Resoles, on the other hand, are made with an excess of formaldehyde and are normally supplied as low viscosity liquids dissolved in water. They are normally cured by application of heat and catalysis by an acid. Composite applications employ the resole versions. A big disadvantage to resole resins is the out -gassing of water vapor which occurs during the cure. This leads to porosity and voids as well as odor problems during processing. These voids detract from composite properties including corrosion resistance. Glass wet -out is another problem. Quite often glass reinforcement commonly used in the composites industry is not compatible with phenolic resin. Since resoles are water soluble, corrosion resistance to water or aqueous based solutions can be very poor if the cure is not conducted properly. Care should also be taken to avoid contact of phenolic composites with carbon steel in the final application. Over time, the acid catalyst can leach out and severely corrode the steel. Rubber and Elastomers Rubberoften displays good chemical resistance, especially to sulfuric acid. It is sometimes used in FGD applications for lining of steel piping and process equipment. Rubber liners have also been used in various bleaching applications. Apart from corrosion resistance, rubber can offer good abrasion resistance. In the case of rubber linings, skilled and specialized installation is required, which tends to make them expensive. Many of the linings are difficult, if not impossible, to install around restrictive geometry. It is essential to obtain good bonding between the rubber and steel since any permeation or damage to the liner can cause the steel to quickly corrode. The low glass transition temperature of rubber restricts use to moderate temperatures. Some rubbers and elastomers can become embrittled if subjected to cyclic wet and dry conditions. Solvents present swelling problems, and water permeation can also be an important consideration. 1 Acid Resistant Brick and Refractories Both castable and mortar block chemically resistant refractories have been used extensively. A good example is in chimney construction, to withstand sulfuric acid dew point corrosion. Usually steel is used for structural support along with appropriate buckstays. Installation costs can be high. Castable products must be anchored to the steel structure by studs or Y -anchors. Refractories are not ductile and concerns involve thermal cycling and cracking. Block must be skillfully placed with proper acid resistant mortar. High weight is a factor as well as seismic considerations. The biggest problems involve operation of wet stacks in conjunction with flue gas desulfurization. Moisture leads to absorption and swelling, which may eventually induce leaning. It is also common practice with wet stacks to employ pressurized membranes to prevent condensation onto the cold external steel surface. This also can be expensive.