contact pressure between flange faces is a t least equal to or greater than the internal pressure. Like rubber, compressed asbestos, and other gasket materials, plastics tend to lose residual contact pressure by extrusion of material between the Range faces, This phenomenon has variously been referred to in the literature as “creep,” “cold flow,” and “stress decay.” It is well established that the tendency for plastics to creep is accelerated rapidly by increased temperature. Only meager data are available to predict rate of creep of plastics. However, if the total elastic deformation of a 2-inch, 300-pound flanged joint resulting from bolt stresses of 30,000 pound per square inch is of the order 0.003 inch, one might reasonably expect leakage if the creep rate of the gasket material exceeds 0.001 inch per inch per month. Because all plastics have a high rate of creep, they have not been very successful as O-ring static seals. The tendency for thermoplastics to creep or extrude when used as gaskets can be overcome for all practical purposes by using adequate reinforcement or filler materials. For example, a woven asbestos gasket, treated with 40% by weight of Teflon dispersion and subsequently baked, can withstand alternate heating with 150 pounds of steam and quenching with cold water without appreciable creep or cold flow (Figures 2 and 3). Another method for confining the plastic is to use it as a filler in a spiral-wound or metal-jacketed gasket construction. Both methods have been used very successfully. Influence of High Coefficient of Expansion and Low Thermal Conductivity. If thermoplastic gaskets are used under cyclic temperature conditions, they should be installed in a tongueand-groove-type joint or the mechanical equivalent. It was pointed out earlier in this paper that solid plastic rings could not
be used successfully’as rod packings because of high rates of thermal expansion combined with poor thermal conductivity. These same factors influence thermoplastic gaskets and O-ring seal applications adversely because the plastic tends t o be extruded or forced out between flange or sealing faces when subjected to cyclic temperature variations. Elastic Retraction Modulus. Despite the fact that unoriented thermoplastics show an apparent low elastic modulus when subjected to a compressive stress, the retraction modulus-Le., the slope of the stress-strain curve when the stress is removed-is quite high. For example, Teflon shows an apparent elastic modulus as determined by ASTM Method D 638467 of 58,000 pounds per square inch, whereas the retraction modulus is about 500,000 pounds per square inch as measured by subjecting a gasket to a compressive load and then releasing it. This means that only a slight amount of creep of the bolts or flanged structure will result in a rapid decrease in gasket contact pressure. CONCLUSION
The use of plastics, particularly the thermoplastic type, for components or mechanical equipment is in its infancg. Experience, though limited, indicates clearly that plastics will play an increasingly important role in the mechanical equipment component field. More data and experience verification are needed by the engineer in arriving a t optimum design. I n the meantime, a progressive attitude toward the use of plastics, seasoned with a searching engineering approach, will pay substantial dividends both to the manufacturer and to the users of his equipment. RECEIVED for review September 17, 1954.
ACCEPTEDApril 27, 1955.
Vessels The selection of suitable plastic materials for vessel construction is determined by several factors which include operating temperature, environment, mechanical and electrical properties. Thermosetting materials in common use include phenolic resins, furan resins, polyesters, epoxy resins, and hard rubber. Commonly used thermoplastics are polyethylene, poly(viny1 chloride), saran, and poly(methy1 met hacrylate).
!i
f
J. A. NEUMANN
F. J. BOCKHOFF
American Agile Corp., Bedford, Ohio
Fenn College, Cleveland, Ohio
I
N SELECTING a suitable plastic material for vessel construction, several factors must be considered. These factors include operating temperature, mechanical property requirements, electrical property requirements, chemical environment, and ease of fabrication. Initial cost of fabrication, as well as maintenance cost, is an important factor in making a choice among different plastic materials. One must not allow initial cost to become an obstacle to the selection of the proper corrosion-resistant material. The decreased maintenance costs for suitable materials will more than compensate for Q higher initial investment THERMOSETTING MATERIALS USED IN VESSEL CONSTRUCTION
The commercial use of thermosetting resins as materials of chemical construction preceded that of thermoplastic resins b y a few decades. Small scale production of phenolic resins was already under way in 1907, under the auspices of the Bakelite Co.,
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founded by Leo H. Baekeland. Phenolic materials a t that time were primarily used as insulating varnishes for the electrical industry. Much the same type of casting resin is used extensively today in the production of large, chemically resistant selfsupporting structures. The filled phenolic resins used today are generally condensation polymers of phenol and formaldehyde, having a specific gravity in the cured state of about 1.7 and a maximum recommended operating temperature of 265 O F. These resins are applied with either asbestos or graphite fillers. The asbestos-filled phenolic material exhibits fair shock resistance and excellent resistance t o inorganic salts and acids, including hot hydrochloric acid. It is not, however, recommended for oxidizing agents or strong caustics, which would include strong alkaline salts. Because of the incorporation of asbestos as a filler, this resin cannot be recommended for use in contact with hydrofluoric acid or fluorides, since these chemicals attack asbestos. Graphite-filled phenolic resins are specifically designed to overcome this lack of resistance toward fluorides and hydrofluoric
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Plastics Construction Materials acid. A few disadvantages, however, are inherent in the use of graphite as a filler for phenolic resins. The shock resistance of graphite-filled phenolics is much lower than that for asbestosfilled phenolics. I n addition, the electrical properties are poor, and tanks constructed of graphite-filled phenolics must be insulated when used for electrolytic work. The mechanical properties of the filled phenolic resins are generally fair. Tensile strengths vary from 3000 to 5000 pounds per square inch.
COURTESY AMERICAN AOILE GORP.
Figure 1. Welded 16-gallon polyethylene kettle used in food processing
Materials widely used for similar applications are the furan resins, produced by the acid catalyzed polycondensation of furfuryl alcohol. These resins are also generally filled with various types of reinforcing ingredients. The specific gravity of the furan resins varies from 1.1 to 1.5, and the maximum operating temperature is about 280" F.
Figure 2. Fabricated unplasticized PVC anolyte tank used in electrolytic chrome processing
Although asbestos is commonly used as a filling material for furans, fiber glass is often used where lighter installations are required. I n addition, laminated fiber-glass sheet tends to impart greater strength and impact properties to the cured resins. As in the phenolic resins, graphite filler is used where chemical reJuly 1955
sistance to hydrofluoric acid and the fluorides is required. Furan resins are quite different from phenolic resins in their resistance to alkalies, either hot or cold. Furans are not, however, recommended for oxidizing agents such as sodium hypochlorite, nitric acid, chromic acid, or concentrated sulfuric acid. A peculiar reaction in furan resins sometimes produces excessive brittleness. This is due to a postcuring of the resin. It appears that the final stages in the polycondensation of furfuryl alcohol takes place at a rather low reaction rate. The strength properties of the furan resins are not quite as good as those for the phenolic resins. Tensile strength values generally lie in the region of 2000 to 2500 pounds per square inch. The furan resins, as well as the phenolics, can easily be machined and fabricated with ordinary wood- and metalworking equipment. Fabrications are generally made from cast sections which are bolted together a t the installation site. Thermosetting materials of chemical construction, which are presently experiencing great popularity, are the glass-reinforced polyesters. The specific gravity of these resins is in the range 1.3 to 1.6, and their maximum operating temperature is approximately 200" F. They are generally used for nonoxidizing acid solutions, toward which they exhibit fair cliemical resistance. They are, however, less resistant to organic solvents than phenolics or furans. I n addition, they are not recommended for alkalies, alkaline salt solutions, oxidizing agents, or fluorides. The lack of fluoride resistance is not due to the polyester resin itself but to the glass which is used as the reinforcing agent. Here again we face a problem similar to that encountered in the use of asbestos as a filler material. The mechanical properties of glass-reinforced polyesters are outstanding. Tensile strength values may vary from 7000 to 50,000 pounds per square inch, depending on the type of reinforcement. The impact strength of this type of resin is also excellent. Experience has shown, however, that these short-time properties may be unreliable criteria for many applications involving corrosive solutions and elevated temperatures. One of the traditional thermosetting materials for chemical construction is hard rubber, which is certainly still used to B very large extent today. Hard rubber is extremely variable in its physical and chemical properties. It is produced by cross linking or vulcanizing any one of the several types of synthetic or natural rubbers available today. I n addition, it can be produced by vulcanizing blends of two or more of these available elastomeric
Figure 3. Hydrochloric acid rinse tank welded i n unplasticized PVC Size, 50 X 35 X 30 inches
materials. For this reason it is difficult to present specific properties for hard rubber. Generally, however, hard rubber has a specific gravity of 1.2 to 1.3 and a maximum recommended operating temperature of 220" F. Chemical resistance to acids and inorganic salt solutions is
INDUSTRIAL AND ENGINEERING CHEMISTRY
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CDURTESY MAURICE A . KNlOHT CO
generally good. However, hard rubber is not resistant to oxidizing agents and many solvents. Solvent resistance of certain types of rubber can be made quite excellent. I n this respect butadiene-acrylonitrile synthetics have proved to be outstanding. The mechanical properties of hard rubber are again quite variable, with tensile strength varying from 2500 to 9500 pounds per square inch. The impact strength of hard rubber decreases considerably as the netting index or degree of vulcanization increases; 100% vulcanized rubber generally has a rather poor impact strength. The method of fabrication involved in the production of vessels from polyester resins, phenolic resins, and furan resins are almost identical. The technique involves casting the resin and the filler in low pressure molds and allowing a cure to take place. This technique, however, is discussed in more detail later. THERMOPLASTIC RESINS USED IN VESSEL CONSTRUCTION
Generally, thermoplastic resins have greater corrosion resistance and higher impact strength than do the thermosetting resins. Polyethylene and poly(viny1 chloride) are the most commonly used thermoplastics in vessels, particularly large vessels, although other resins such as poly(methy1 methacrylate), poly (vinylidene chloride), and styrene-butadiene-acrylonitrile blends are used in some smaller vessels. Polyethylene, a widely expanding material of construction in American chemical industry, owes its initial growth and development to English technology, and the early support of the Imperial Chemical Industries, Ltd. Recently lifted government controls on polyethylene should allow much more of this material to be utilized in the future. Polyethylene is one of the few plastic materials having a specific gravity of less than 1. The specific gravity of commercial polyethylene is 0.92, a factor which allows polyethylene to be utilized where extreme lightness in weight is desired. The maximum recommended operating temperature for polyethylene, in the range of most thermoplastics, is 150" F., although the softening point of polyethylene is in the vicinity of 230" F. Polyethylene exhibits excellent general corrosion resistance.
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There is no known solvent for this material at room temperature. It is completely resistant to practically all inorganic chemicals, except very strong oxidizing agents. It is not recommended for aromatic hydrocarbons, chlorinated hydrocarbons, esters, ketones, and certain ethers. These solvents, although they will not cause degradation or complete failure in polyethylene structures, will cause the material to swell comiderably. Weathering resistance ofpolyethylene can be made excellent by the incorporation of 2'/470 or more of channel black in the material. Polyethylene exhibits fair tensile strength, ranging from 1600 to 1900 pounds per square inch for most commercial grades. More useful are its unique flexibility and toughness over a wide range of temperatures. Polyethylene is unbreakable on impact on standard impact testing machines at room temperature or above. A much expanded field for polyethylene is predicted in the food and pharmaceutical industries because of its complete lack of taste, odor, or toxic effects. The utilization of unplasticized poly(viny1 chloride) as a material of construction was developed considerably in Germany during World War I1 under pressure of acute metal shortages. Much information regarding fabrication is therefore to be obtained from German literature over the past 15 years. American experience with unplasticized poly(viny1 chloride) has been limited to approximately the last 5 years, although it promises to be one of the large tonnage materials within the next 5 years. The specific gravity of poly(viny1 chloride) is 1.4, and it has a maximum operating temperature of 160"F. Poly(viny1chloride) (PVC) is generally used in unplasticized form for maximum corrosion resistance. Unplasticized PVC exhibits excellent resistance to many solvents, and to all inorganic solutions. It is particularly useful for oxidizing agents, and in this respect is definitely advantageous to many of the other plastic materials of construction. Unplasticized PVC is also extremely useful where strict tolerances are required, since the dimensional stability of this material is outstanding. For many applications, PVC is modified by the addition of a plasticizer to produce a more flexible, elastomeric material. I n
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-Plastics the plasticized form, PVC is easily applied as a lining since excellent cements are generally available for this purpose, whereas the cementing of unplasticized PVC is more difficult. The corrosion resistance of plasticized PVC is not, however, as good as that of unplasticized PVC. The impact strength of unplasticized poly(viny1 chloride) is only fair, b u t it can be considerably improved b y blending with elastomeric materials such as natural or synthetic rubber. This blend, known as high impact PVC, exhibits, however, a somewhat decreased resistance to certain chemicals when compared to virgin or unplasticized poly(viny1 chloride). Whether unplasticized or high impact, the mechanical properties of PVC are excellent. Tensile strength values vary from 5700 to 8700 pounds per square inch. Poly(viny1idene chloride), more commonly known as saran, is used only to a limited extent in the construction of vessels. The specific gravity of saran varies from 1.6 to 1.7, and the maximum recommended operating temperature is 170' F. Saran is recommended for use with corrosive acids and some solvents. Most alcohols, esters, aldehydes, and ketones do not affect saran. It is, however, not recommended for strong alkalies, especially ammonium hydroxide. Neither is it recommended for aromatic hydrocarbons or halogenated hydrocarbons. A serious disadvantage in the use of saran for self-supporting structures is its brittleness, or low impact strength, especially a t lower temperatures. I t s use is limited from a practical standpoint to liners and small constructions. Fabrications in saran cannot conveniently be cemented together because of the chemical resistance of the material. In this respect saran is similar to PVC. Construction is generally carried out b y hot gas welding: saran was one of the first plastics to be welded. Another material used to a limited extent in vessel construction is poly(methy1 methacrylate), which has a specific gravity of 1.2, and a maximum operating temperature of 170' F. Poly (methyl methacrylate) easily handles inorganic acids up to 50% concentration at room temperatures. It is not recommended
Construction Materials
for use with very strong acids or oxidizing acids nor for most organic liquids. A specific advantage of poly(methy1 methacrylate) in the construction of chemical vessels is its excellent optical clarity. This is particularly useful for small vessels of an experimental nature, in which apparatus must be observed for evaluation. TYPES OF VESSELS AND METHODS O F FABRICATION
For convenience in discussing the use and fabrication of vessels, an arbitrary size classification is utilized. Small vessels are classified as those containing up to 20 gallons. This type of vessel can usually be handled by one man. Medium vessels range from 20 t o 100 gallons, and those vessels having capacities of more than 100 gallons are classified as large. Small vessels are almost invariably constructed as self-supporting structures. I n this group would be included not only the rigid containers which are handled for transportation, but also those containers used for other services. I n mentioning B few of these services where plastics are doing an outstanding job one would include acid-handling equipment such as pails, buckets, crocks, electrochemical baskets, crystallization jars, floats, small tanks in both cylindrical and rectangular forms, and utility kettles. Because of their self-supporting nature a strength safety factor of four or larger, based on long term physical properties, is generally employed for design purposes. A typical utilization of plastics for small vessels is illustrated by a container that is used for the purpose of handling and transporting concentrated sulfuric acid, This container was previously made from lead and had a gross weight of 78 pounds. Imagine how much less effort the operator expends, when for the same amount of liquid he lifts only 25 pounds when the container is made of polyethylene. I n addition, the price of a 5-pound container in polyethylene is approximately one third the price of the lead container of 58 pounds. Thermoplastics fabrications in small vessels are generally produced by injection molding when large quantities are involved.
Figure 7. Polyethylene tubs used in chlorine dioxide dispenser July 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
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When, however, custom jobs are required, or short runs are to be made, the vessels are conveniently welded from molded, extruded, or cast sections. The hot gas welding method is a reliable method for producing strong and leak-tight joints. I n constructing vessels of filled phenolic resins or filled furan resins the materials are cast in inexpensive wood or metal molds using ordinary or slightly elevated temperatures and pressures. After pouring, the catalyst, resin, and filler are allowed to cure anywhere from 7 to 10 hours. Glass-reinforced polyester resins are usually formed from resin impregnated glass mat a t low pressures and moderate temperatures. The cure can be made to go to completion a t room temperature through the proper choice of accelerator-catalyst system. Medium vessels (20 to 100 gallons) can be fabricated in cylindrical or rectangular form, with flat, sloped, conical, or hemispherical bottoms. Medium vessels in thermoplastic materials are usually fabricated from cast, molded, or extruded sections by hot gas welding. This would include fabrications in poly (vinyl chloride), polyethylene, saran, and, to a very limited extent, poly( methyl methacrylate). American acrylics fabrications are generally cemented, although English fabricators claim quite effective results in the hot gas welding of acrylic vessels. Medium vessels can also be cast in thermosetting resins such as the phenolics, furans, and polyesters in the same manner as for small vessels. These vessels are generally self-supporting, and, therefore require no external construction, although in certain cases this support is desirable from a safety factor standpoint. Large vessels (over 100 gallons) generally involve the use of thermoplastics as linings for metal tanks. Materials particularly suited for this purpose are polyethylene, poly(viny1 chloride), and natural and synthetic rubber. The size of this type of unit is practically unlimited, since the base tank can be welded to any convenient size, and the lining can be installed after the unit is erected. Here again it might be mentioned that the design
temperature is quite important when installing a lining having a thermal expansion coefficient much different from that of the base metal, unless the lining itself has enough flexibility to absorb the consequent stresses. Large units in phenolic and furan resins can be constructed by bolting and flanging together small cast units. The largest individual units constructed presently contain from 600 to 700 cubic feet, some weighing up to 2 tons. Large tanks in phenolics and furans are generally reinforced with steel angles or wooden staves around the periphery. An alternate method for utilizing phenolic and furan resina involves troweling the casing resin onto a mesh welded to the interior of the tank. This, of course, represents a lined installa, tion, HOT G A S WELDING METHOD
At several points, reference to hot gas welding as a method of plastics fabrication has been mentioned, and perhaps a few worda expIaining the method would be helpful. Hot gas welding is similar to the gas welding of steel. It is particularly applicable to such plastics as polyethylene, poly (vinyl chloride), saran, and poly(methyl methacrylate). The particular advantages of hot gas welding over other types of joint fabrication are several. The use of adhesives is completely eliminated, allowing the joint to have the same corrosion resistance as the parent material, Crevices such as those produced in riveted joints are completely eliminated; this removes the possibility of solution entrenchment in these voids. I n addition, the use of hot gas welding as a method of fabrication theoretically allows infinite structure size, since units can be built one upon the other and joined a t the point of installation. Hot gas welding of plastics differs from that of metals in t h a t complete fusion is not obtained; in fact, this is actually undesirable. The method of welding, which is thoroughly discussed elsewhere ( 1 , d ) , involves practically the same type of weld joint preparation as is encountered in steel welding: The filler rod is composed of material identical in composition to that of the parent sheet or similar stock t o be welded. I n making the weld, the filler rod and surfaces to be joined are simultaneously heated with a directed blast of hot air (nitrogen is used with polyethylene) which melts only the surfaces to be bonded. Under these conditions the weld bead is laid with hand pressure and allowed to cool. It has been found that in the butt welding of sheet stock, the tensile strength of the weld is from 85 to 95%
C O U R T E W AMERICAN AOILE GORP.
Figure 9. Self supporting lh-inch polyethylene liner for steel tank
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Figure 10.
Polyethylene 1100-gallon storage tank
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Plastics Construction Materials of the sheet itself, although in some cases weld values of over 100% have been observed, even though the weld had been dressed previous to testing. These figures, of course, apply only to experienced welders since it has been found that good welds are argely dependent on the skill and training of the operator. Weld strength values of from 60 to 75% are not uncommon for novice workers. Consistent weld strengths of 90 to 95% are the rule with experienced welders. Through the utilization of hot gas welding, a marked development in plastic containers'and vessels is in sight for the next 5 years, although it must be regretted that American technology is still somewhat behind German technology along these lines. Germany has three welding schools in full operation a t the present time. TYPICAL INSTALLATIONS AND FABRICATIONS
As mentioned previously, one particular advantage of polyethylene in the food industry is its extremely low toxicity and freedom from taste and odor. Figure 1illustrates a polyethylene bucket typical of vessels used in food processing. The capacity of the kettle is 16 gallons, and the wall thickness is '/z inch. Reinforcement bands are welded a t the top and bottom of the kettle. The kettle is completely fabricated by welding of centrifugally cast tubing and sheet stock. This vessel is used for the transfer of a wide variety of food products into various processing machines for cookers. The ingredients handled include everything that goes into the making of different soups. This vessel is not used for storage of ingredients except over weekends, and the ingredients stored are not necessarily the same from veekend t o weekend. These vessels have to withstand very rough treatment. After they are emptied into cookers by hand, they are tossed aside to a platform or to the floor about 4 feet below. The next operation performed on these buckets is washing, and this is done in a conveyor-type washing machine operating with caustic soda solution a t a temperature of approximately 180' F. Stainless steel buckets heretofore used have not proved entirely satisfactory. A typical fabrication in unplasticized poly(viny1 chloride) is shown in Figure 2. This vessel is an anolyte tank used in an electrolytic chrome process. The plastic is subjected to solutions containing 28% sulfuric acid and 6% chromic acid. This tank is 7 feet 11 inches long and 2l/2 inches wide. The height a t the center is 44 inches, and this dimension tapers to 221/, inches a t either end of the tank. I n operation, the tank is immersed to a point 3 inches below its top. The operating temperature of the submerged section is 140' F., and the ambient temperature above the solution is 80" F. This tank is completely fabricated in welded construction. Another type of vessel constructed of unplasticized poly(viny1 chloride) is shown in Figure 3. This tank is welded from s/r inch material throughout, except the motor base which is l / 2 inch thick. The dimensions of the tank are 50 X 35 X 30 inches. It ie used as a hydrochloric acid rinse tank. I n order to minimize vibrations, the tank is reinforced on the side where the motor is mounted. Figure 4 shows a custom-built tank in filled furan resin with a dished top and flat bottom. The capacity of t,his tank is 1400 gallons. Custom tanks as large as 2000 gallons capacity can be built with dished, conical, or flat bottoms and tops. I n addition, flanged, bell-type, or loose covers may be supplied as the situation warrants. A similar tank constructed in filled furan resins and having an open top is illustrated in Figure 5. This tank is 5 feet 6 inches in diameter by 6 feet deep and weighs approximately 900 pounds. The capacity of the tank shown is 1000 gallons. Similar tanks are constructed in standard sizes up to 1200-gallon capacity. These tanks can handle most acids, alkalies, and solvents at temperatures up to 280" F. Figure 6 illustrates a television tube waahing and etching July 1955
COURTESY AMERICAN AGILE CORP.
Figure 11. Unplastioized PVC liner for 400-gallon wooden tank
substructure, 105 inches in diameter by approximately 20 inches high, fabricated entirely from 1/2-inch polyethylene sheet stock. Intimate details of the process are not available, but it is essentially a glass etching operation in which a dilute hydrofluoric acid solution (approximately 10%) is injected through the neck of the television tube under sufficient pressure and for a sufficient period of time to provide a very highly etched glass surface on which a sensitive coating is subsequently applied. This tub is divided into two sections, each one having a drain on the bottom; one section is used for etching and the other for washing. A series of polyethylene threaded studs are found along the inner periphery, of the smaller cylinder; these are the connections for each washing station. The approximate weight of this fabrication is 600 pounds. Polyethylene tubs of 171/~-inchdiameter are a t present being conveniently used in the construction of chlorine dioxide dispensers for the purpose of maturing flour in mills. This type of apparatus is shown in Figure 7 . The wall thickness of these vessels is approximately 1 / 2 inch. The short tub in the foreground is used to prepare a solution of chlorine dioxide in water. This is accomplished by filling the tub with water and introducing a block of chlorine dioxide hydrate which then dissolves on agitation of the water. It is interesting to note that the chlorine dioxide hydrate (consisting of 1 molecule of chlorine dioxide and 10 molecules of water) is stored and transported in I-gallon polyethylene containers. These containers of hydrate are stored in a deep-freeze box a t approximately 0' F. to prevent evaporation losses. After preparing the solution of chlorine dioxide in water, it is transferred to the larger dispensing vessel in the background. From there it is metered to individual stripping towers that ensure an accurate rate of injection. The chlorine dioxide is picked up in a stream of air and is carried to the point of application in the flour agitator.
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are becoming more and more familiar with the advantages and limitations of plastic materials of construction. Without a doubt, as this knowledge grows, the use of plastics in chemical construction will certainly expand a t a parallel rate. FUTURE OF PLASTICS IN VESSELS
COURTESY APEX ROTAREX GORP.
Figure 12. High pressure tank (13-inch diameter) of laminated fiber glass impregnated with epoxy resin
The 2-inch diameter black pipes visible along the back wall of this equipment are the individual stripping towers. Black poly(vinylidene chloride) pipe is used in these towers rather than polyethylene. Experiences with threaded polyethylene pipe in this application have not been completely satisfactory because of leakage a t the threaded roots of the pipe, since this is the point a t which hydrostatic stresses are concentrated. On the other hand, flexible polyethylene pipe with clamps and pressure fittings have appeared to function very satisfactorily. A unique reactor construction completely weld fabricated in polyethylene is shown in Figure 8. The capacity of this reactor is 110 gallons, and it has a wall thickness of '/z inch. This reactor is used for hydrofluoric acid and its solutions, in nuclear reactions. This particular tank has a special hinged top and conical-shaped bottom, with various appurtenances built to specifications. Much has previously been mentioned regarding the application of plastic materials as liners for steel tanks. Figure 9 shows a self-supporting polyethylene liner being set into a steel tank 36 X 36 X 36 inches, I n installations such as this, the extreme ruggedness of steel is combined with the excellent corrosion resistance of plastic materials. Figure 10 illustrates the application of polyethylene in the construction of large storage tanks. The capacity of the tank shown is 1100 gallons. The vessel is 10 feet long by 4 feet a t the widest point, tapering to 3 feet a t the narrow end. The tank is 4 feet deep. Unplasticized PVC can also be used in tank lining applications. Figure 11 shows a '/*-inch tank liner, 48 inches in diameter and 53 inches high, welded entirely of unplasticized PVC. The approximate capacity of this tank is 400 gallons. I n operation, the PVC liner slips into a supporting wooden tank. The contents used in this specific tank were aqueous solutions of pharmaceuticals containing alcohol and various organic cosmetic preparations. Certainly the installations mentioned are but a few of the many types presently employed in chemical and allied processing equipment throughout the country. As time progresses, development and production engineers involved in processing work 1334
Plastics production in general is riding on the crest of a great wave. It would not be surprising if the average production of all plastics more than double in the next 5 years. I n a few specific instances, predicted increases are even greater-for example, polyethylene production is expected to triple in the next 5 years. This material, may be expected to be the first plastic to reach an annual production of 1 billion pounds. I n addition, new fabrication processes are on the horizon. An extremely interesting development of this sort is cathode ray irradiation of polyethylene, which induces a degree of cross linking into the molecular structure of otherwise linear polyethylene. As a result the heat distortion point of polyethylene can be raised considerably. I n addition, developments indicate that irradiated polyethylene will be completely free from the annoyance of stress corrosion ( 3 ) . This process certainly opens the door to entirely new concepts in fabrication. The recent development of high pressure tanks in fiber glass impregnated with epoxy resin foretells a future use of plastics in varied types of high pressure installations. Figure 12 illustrates the previously mentioned tank, which is a sphere 13 inches in diameter, having a wall thickness of a/4 inch. This spherical tank has been experimentally incorporated as a high pressure air reservoir in the starting system of the Republic Aviation Corp. F-84F Thunderstreak. Reports indicate that trials have been successful enough to warrent installation on further Thunderstreaks. The 16-pound tank has a capacity of 900 cubic inches, and holds 8 pounds of air a t 3000 pounds per square inch. I n comparison, the fiteel unit required to do the same job, weighs 25
OOURTESI AMERICAN AOlLE GORP.
+Figure13. Molded polyethylene hollow sphere weighing 340 pounds Diameter, 53 inches and wall thickness, 1 inch
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Plastics Construction Materials pounds, 1 pound more than the combined weight of the plastic tank and the air load. Burst strength of the unit is 7000 pounds per square inch. The reservoir is manufactured by winding a continuous roving of fiber glass impregnated with epoxy resins onto a mold made of low melting point alloy. The aluminum alloy air inlet and outlet connections are cast into the mold. After a 2-hour cure a t 240’ F. the mold material is melted out and re-used. The future also points toward a much wider utilization of the highly chemical-resistant fluorinated hydrocarbons. Expanding processing methods must be developed, and much work is presently being done along these lines. Poly(tetrafluoroethylene), for example, has the greatest chemical resistance of all the plastics, even a t higher temperatures. With this material, service temperatures above 550’ F. are possible. The only materials which will attack this plastic are molten alkali metals or fluorine above 300’ F. No solvent will dissolve or even swell it. I n addition the impact strength of poly(tetrafluoroethy1ene) is so good that standard impact testing machines will indicate no failure a t room temperature or above. The greatest drawback in wider application of this material is the extremely high melt viscosity of the molten resin which makes difficult its utilization in many of the fabrication methods presently employed. One-piece fabrication of large vessels and structures in thermoplastics, such as polyethylene, appears quite promising as techniques in molding constantly improve. Experimental moldings of surprising size, such as the hollow polyethylene sphere shown in Figure 13, have been produced for evaluation. The spherical vessel shown is 53 inches in diameter and has a-wall
thickness of 1 inch. The weight of the entire sphere is 340 pounds. I n fabrication, two molded single piece hemispheres are welded together to produce the final sphere. I n molding, walls can be made thick enough to provide for selfsupporting vessels as compared to today’s conventional welded and lined structures. Cost savings in fabrication time should be realized, although mold costs per unit will, of course, be high on small runs. It appears entirely feasible that one-piece moldings in polyethylene as large in weight as 1000 pounds can and will be produced within the next 5 years, paving the way for further applications of plastics as materials of chemical construction for large vessels. I n conclusion, it should again be emphasized that plastics are not substitute materials of construction, nor are they materiah representing a panacea for all chemical construction difficulties. It is just as important that one acquaint himself with the limitations of plastic materials as it is that one study the advantages of these materials. An honest recognition of today’s limitations is the best foundation for research toward tomorrow. LITERATURE CITED
(1) Haim, G., and Neumann, J. A., “Manual for Plastic Welding,” Vol. 11, Industrial Book, Cleveland, Ohio, 1954. (2) Haim, G., and Zade, H. P., “Welding of Plastics,” Crosby Lockwood, London, 1947. (3) Bockhoff, F. J., and Neumann, J. A., SPE Journal, 10, No. 5, 17 (1954). RECEIVED for review September 17, 1954.
ACCEPTED April 26, 1955.
Piping, Valves, and Ducts T h e use of plastics as materials of construction for the production of piping, valves, and ducts has required a sound technical viewpoint in contrast to the simple production approach associated previously with the manufacture of plastic novelties. Manufacturers of plastic materials of construction can no longer depend on their suppliers for the solution of technical problems in their specific plants. Those producing pipes, valves. and ducts must recognize the advantages and disadvantages of all types of plastics and select, design, and fabricate structures in accordance with the principles adopted by the leaders in the plastics industry. In the meantime, conservative design and proper selection have made it possible to construct successful pipelines in excess of‘ 10 miles in length, valves operating at temperatures above 400° F., and exhaust systems with areas greater than 50,000 square feet handling corrosive fumes which would destroy almost all other known materials of construction. RAYMOND B. SEYMOUR Atlas Mineral Products Co., M e r t z t o w n , Pa.
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PECIFIC thermosetting and thermoplastic materials have been used successfully for over 15 years as piping, valves, and ducts. Both type products have been used extensively for many years in Europe, but until recently American experience for these applications was limited primarily to filled or laminated thermosetting plastics. As indicated by the data in Table I, the use of plastics in corrosion-resistant applications has not paralleled the growth of plastic materials to date. However, the rate of growth of this segment of the plastics industry is now greater than that of the
July 1955
entire plastics field. The propertiea of available plastic laminates and typical thermoplastic structural materials are compared in Tables I1 and 111. Physical properties and practical values for plastic pipe are compared in Tables IV and V. The comparative chemical resistance of plastic pipe is given in Table VI. Proper attention to this type of information will aid engineers in the selection of appropriate material for any specific service. Additional information t h a t can be of assistance in the selection, design, fabrication, and installation of plastic materials of
INDUSTRIAL AND FNGINEERING CHEMISTRY
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