H. DUDLEY EARTON
henolic resins were discovered as early as 1872 but did not reach s u c d u l commercial application until Dr. Leo Baekeland tookout his first patent in 1907. The reaction producing the resin is essentially a condensation between phenol and formaldehyde. The so-called on-stage resins are thermosetting resins requiring only further heat to produce solid, infusible, and very corrosion resistant materlal. In reviewmg the phenolic resins and their corrosion resistance, a look at the polymerized molecule is informative. The phenolic resin has the benzene ring, one of the more stable chemical structures, which is weakly acidic when the hydroxyl group replaces one of the hydrogens. The OH- of the phenol molecule causes a polarization of the molecule resulting in more strongly negative bonds at the para and two ortho positions and weaker, less negative bonds at the two meta positions (relative to OH-). These weaker bonds are reactive sites which will readily lose their hydrogen to a formaldehyde molecule, splitting out water and formmg rigid cross-liiks. The polymerized phenolic molecule with strong carbon-to-carbon bonds is not easily attacked by acids, salts or most organic solvents. Phenolics are subject to attack by strong alkaline materials, since the phenol is a weak acid. A fitting companion material for the phenolic products are those resins made from furfuryl alcohol or furfuryl alcohol and formaldehyde. Such “furan” materials are much newer. The first patents on furan polymers by Miner and Trickey date only to 1928. Although furans were known before then as products which could be made into resins, the reaction was difficult to control. These compounds offer several striking similarities and significant differences which make comparisons with phenolics worthwhile. The furan molecular structure also indicates inertness in chemical exposure. The furan molecule should have good resistance to the same acids, salts, and organic solvent compounds men-
Tall OM+ECL l m r oj phenolic asbmtos conpodion with c x t m d amonng OJ epoxy filommt-woundfihcr glass @mi
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tioned above, but because it does not have the susceptible hydroxyl group, it shows strong resistance to alkaline compounds. Both molecules are vulnerable to oxidizing chemicals such as hot concentrated sulfuric, nitric, and chromic acids. Chemical laboratory tests and some very unhappy experiences have proved this to be all too hue. Nevertheless, some successes have been reported with 66’ N.HISO, at room temperature and with very low concentrations of nitric or chromic acid. This article will not discuss other furan materials such as furfuryl alcohol-furfuraldehyde. Initial attempts to resinify these compounds resulted in materials with poor physical properties and no additional benefits sufficient to make them commercially desirable. Reinforcing Agents
Nearly all corrosion resistant construction materials of reinforced phenolic and furan plastics have been made by combining these resins with some form of silica as a reinforcing agent. By a wide margin, most of this equipment has been asbestos reinforced. When asbestos is used, selection and treatment play an important role in its corrosion resistance. Many asbestos compounds are available. Chrysotile, a frequently used form of asbestos, is approximately 40% to 50oJ,soluble in many mineral acids, and therefore will be seriously affected when exposed to these services. Other forms of asbestos have a much reduced soluble content-in some cases, less than 10%. These offer a much improved hished product for chemical exposure. For widerange corrosion resistance, even the better grades of asbestos must be treated by digesting with boiling hydrochloric acid to reduce to the absolute minimum any p w i ble contaminants or vulnerability to chemical exposure. This treatment is important in producing superior corrosion-resistant products. With the advent of glass in fiber form, glass fiber has frequently been used with many resins to produce c o d o n resistant products for the chemical indusq. Phenolics and furans are no exception. Reinforcing materials of the glass fiber variety have weakneses similar to those described abwe concerning asbestos. The most frequently used glass fiber is an electrical grade of fiber glass, used for structural reinforcing of many types of resins. This electrical or E-glass is about 4050% soluble in hydrochloric acid. T o improve the chemical resistance of a plastic material, a different type of glass called “chemical mat” or “veil mat” is frequently used on the surface of glass-reinforced equipment. It holds larger amounts ofresin and thereby increaseschemical resistance at the surface. The chemical mat, Cglass, has a different composition and a much lower acidsoluble content. Another surface treatment used to enhance the corrwion resistance is the reinforcement of the surface layer of the laminate with organic fiber cloth such as pokyeste~or acrylic fiber. This reinforcement offers resistance to alkaline and to fluorine compounds. The use of organic fibers throughout the laminate is normally prohibited by the lack of strength of such a laminate. An improved version of silica fiber-reinforcing is
now being used in plastic compounds being manufactured for ablation-resistant materials required in defense programs. It is made by e x p i n g glass fiber to treatment similar to that by which asbestos is upgraded. This product is not yet widely used in the chemical industry since its price is higher than other fillers. Since all of the reinforcing agents mentioned so far are silica compounds of one form or another, it might be suspected that all of these plastics would be attacked by fluorine compounds and strong alkaline materials. Since phenolic resins themselves are readily attacked by alkaline materials, this is partially hue. However, hydrofluoric acid does not readily attack phenolic asbestos and furan asbestos materials in concentrations of less than 1%. If there is any appreciable amount of hydrofluoric acid, asbestos will be selectively attacked. In furan asbestos materials, apparently because of
superior resistance to alkaline compounds of the furan molecule, the material has proved not subject to appreciable attack even on the asbestos fiber. Where silica compounds are not suitable, carbon and graphite can be used as alternate fillers or reinforcing materials. Graphite is a useful material for heat transfer in chemical service and is impregnated with the phenolic, epoxy, and furan resins. This application of phenolic and furan materials has had wide use in chemical equipment. This material is available in the form of tubes or shelland-tube heat exchangers, piping, re-
AUTHOR H. W l e y Barton is Gmral Sales Managn of HOIJE~ Cmp., Wilmington, Del. ,
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action columns, pumps, entrainment separators, and so forth. These products are not elaborated on in this article, and should be distinguished from graphite- or carbon-filled resins. As a reinforced plastic, we must consider phenolic and furanresins with coke flour or amorphous graphite as its filler-reinforcer. These materials are frequently used where fluorine compounds preclude the use of glass or asbestos reinforcing. Since the graphite used is not a fibrous material, it contributes physical properties which differ from any of the other fillers discussed. Although still a poor conductor, graphite-filed plastics exhibit improved heat transfer over asbestos- or silica.liUed materials. This affects thermal gradient through the material, and results in lower thermal stresses than the silica-reinforced counterparts. Modulus of elasticity is higher and other mechanical properties lower. Because this material is more rigid and less strong, large equipment should always be reinforced externally. The types of carbon black used in rubber compounding have not been important, in my experience, as components of corrosion resistant materials. In the ablative shields being produced for missile hardware, the development of carbon or graphite cloth as reinforcing for organic resins offers an ever improving future for reinforced plastics at higher temperatures and more severe conditions than present materials allow. Use in the Chemical Industry
Phenolic resins with glass fiber reinforcement have not been economically or chemically succesoful for process equipment. Current development, to my knowledge bf glass-reinforced phenolics, does not show promise of widening the usable limits of phenolic equipment. Furfuryl alcohol resins have been used for chemical equipment with glass reinforcing. Even catalysis of the polymerization of furan glass structures of varying thicknem has proved to be a problem. The higher catalyst requirement for room temperature curing generally results in local hot spots, which cause brittle resin areas. Dependable, even polymerization of furans is a requirement for the across-the-board chemical resistance for which the furans are noted. This is particularly true in those areas of solvent resistance where furan materials should be useful. If the polymerization is ta be completed at room temperature, it must be done with 100% active catalyst. This procedure incnases the production of “chaii stoppers,” limiting the flexibility and chemical resistance of the product. A preferable method would be to use part active and part latent catalyst; the latent catalyst would become active a t higher patcure temperatures. Curing at elevated temperature is preferable since at room temperature the catalyst is subject to variations in activity owing to slight variations in ambient temperature. Furan glass, because of its light weight and high strength glass reinforcing, is particularly suitable for ductwork and other g a s handling applications. Tanks are currently offered by some manufacturers, and dual laminate tanks have 68
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also been produced using furan fiber glass for the chemically exposed surface and other structural plastics to supply the necessary strength at reduced cost. I do not have data on the success or cost of these materials. I do know the problems in achieving adequate polymerization of furan fiber glass material. Superior workmanship is very critically necessary. Although this is certainly available, the requirement increases the likelihood of unsuccessful performance. The reputation and technical competency of the supplier should receive tint consideration in selecting equipment of this type. Asbestos-reinforced furan and phenolic materials are currently available in all shapes and descriptions of equipment from ‘/pinch diameter pipe to process equipment as large as 15 ft. to 20 ft. in diameter. These products, in addition to their corrosion resistance, have the advantage that they exhibit the properties of homogeneous materials. This means that surface abrasion, drilling, or machining operations will have no effect on corrosion resistance. Both phenolic and furan asbestcw, as well as furan glass material, can be erected in the field using prefabricated sections and joining with a field molding compound. This raises the limits of size over equipment that was formerly available. Some equipment in severe services made of phenolic and furan resins reinforced with asbestos fiber is worth mentioning here. In use today are hydrochloric acid absorbers 141/1 ft. in diameter by 26 ft. high, with liquid distribution systems that would strain the water supply of some communities. In use today are stacks 5 ft. in diameter by 170 ft. high, at least one of which has lasted five times as long as stadcs constructed of material previously in use (a corrosion resistant metal product). Pregsure vessels such as HC1 stripping columns, some of which operate at pressures of 20 to 25 p.s.i.g. and temper-
WALL THICKNESS, t
atures of 250' F. or more, are as large as 4 ft. in diameter by 40 ft. high. Alum reactors up to 18 ft. in diameter and pickling tanks 100 ft. in length are not only possibilities but realities in handling today's severe corrosion problems. Much of the equipment has logged 10, 15, or 20 years service in these severe conditions. Although these materials should be recommended only to 300' F., severe but temporary exposure to very high temperatures in chemical fires has shown that reinforced phenolic and furan equipment will not contribute to the spreading of these chemical fires. I n one substantiated case, the wooden staves which were formerly used for external support were literally burned from the exterior of a tank. After a charred surface was sanded from the exterior of the tank and staves and hoops were replaced, the tank remained in useful service for several years. Corrosive Service
We previously looked at the corrosion resistance of each individual part of these materials; now let us sum up corrosion resistance by listing some specific areas in which each product can be used successfully. The phenolic asbestos equipment offers excellent corrosion resistance (all the way to 300' F. if necessary) to inorganic acids and their salts with the exception of nitric, chromic, and concentrated sulfuric (above 70%). This combination has specific use in hydrochloric, sulfuric, and phosphoric acids, and successfully handles many chlorinated aromatic compounds. It is severely susceptible to attack by organic bases such as aniline or pyridine and is disasterously affected by sodium hydroxide and sodium hypochlorite. Furan asbestos and furan glass materials have equally good resistance where inorganic acids and their salts are concerned. The furan material has similar weakness with oxidizing compounds and organic bases. It is attacked by phenol, cresol, or xylenol. It has slightly less resistance (it is subject to swelling) in the halogen acids, but has an enviable resistance to alkaline materials, even to hot concentrated sodium hydroxide. Methods of Manufacture
The asbestos-reinforced materials are made by mixing a viscous resin under a vacuum with an approximately equal weight of medium fiber asbestos which has been digested in boiling hydrochloric acid. The resulting compound then can be rolled, tamped, or spun into various shapes as might be required. A one piece cylindrical tank, for instance, is available at 12 ft. diameter by 15 ft. in height. After molding, the material is autoclaved to complete the polymerization, after which it can be machined in any way that metal can be machined to provide the shape and size product required. I t is possible, as a secondary operation, to mold additional parts onto already cured products, or to join two completely polymerized parts together by using soft unpolymerized material. Furan fiber glass material is normally made by hand layup practices and room temperature catalysis of resin. On some occasions it is postcured at elevated tempera-
tures. Because of the methods by which it is made, it is more comparable to polyester and epoxy laminates than to phenolic and furan asbestos compounds. A room temperature-setting resinous cement is available for making field alterations or repairs to phenolic asbestos, furan asbestos, and furan glass material. In general, these compounds are the same as those used in the original fabrication of the equipment. They are hardened by a catalyst which should be added just prior to use, but in almost all cases repairs or alterations should not be attempted unless the temperature can be maintained at 55' F. or above. Vessel Design
The design of plastic material has become a specialty and the standards vary with the producers and designers and the type of plastic. There is, however, a unique design problem and a unique method of solving it in the reinforced phenolic and furan products. I n pressure vessels made of these products, the proper wall thickness design is stated in an oversimplified version broken down in three sparate design requirements which must be met. First, of course, is the pressure stress (S,) from the internal pressure. A simple way of stating the thickness required for this stress alone is S, = p r / t which is Barlow's formula for thin wall cylinders. In this formula S, is the stress in the wall, p the internal pressure, r the radius, and t the wall thickness. Another stress (S,) is caused by the absorption of moisture on the internal surface of the material. This can be shown as the linear function of a constant times the wall thickness: S, = Ct. This stress is directly proportional to the thickness, as is the third stress (S,) which is linear and also proportional to the wall thickness. It is caused by the temperature differential between the inside and outside of the wall. This can be shown as the equation So = Kt. The sum of these three stresses (S,) can be represented by a modified hyperbola. The graphic representation of the sum of these three equations (see graph) shows very vividly that there is a definite wall thickness at which the stress in the wall will be minimized. Since wall thickness cannot be adjusted to a set safety factor, the use of this optimum wall thickness makes it desirable to add modulus to pressure vessels by using a filament winding technique. By this method, helical winding of monofilament glass fibers reinforced with epoxy resins contributes modulus to the predefined wall. Thus through use of this type of composite design, complete all-plastic vessels capable of withstanding higher pressures have become a reality, To sum up, we can say that although no longer young, phenolics and furans are dependable construction materials when used within the wide limits of their chemical, thermal, and physical properties.
This is the nfth in a series of articles on plastics in corrosion control, based on a symposium presented by the ACS Division of Organic Coatings and Plastic Chemistry at the New York National Meeting, September 1963. The entire series will be available in reprints.
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