FORTIFY AGAINST CORROSION
PLASTICS
ARCHIE J. WEITH Bakelite Corporation, Bloomfield, N. J.
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turer utilizes spray nozzles and splash plates of phenolic molded and piping of phenolic laminated to handle hot brine solutions, dilute acetic acid, or zinc chloride solutions. Bubble caps of phenolic laminated have replaced iron in applications involving boiling ester, alcohols, paraffins, cold weak mineral acids, and mild alkaline liquids. Special plastics have been designed to give the maximum in chemical resistance, and one compounded of phenolic resin and acid-washed asbestos fiber has been found eminently suitable for the construction of a wide variety of chemical-resista n t p i p e . This same material is u t i l i z e d in the m a n u f a c t u r e of vessels as large as 9 feet in diameter a n d 10 f e e t i n height. Absorption towers resistant to hot hydrochloric acid and evaporator p a n s for concentrating alums have been constructed of this material and have served admirably the needs of chemi-
ESISTANCE to moisture, to alkalies and acids, to oils
and most common organic solvents-these factors have determined the use of synthetic plastics in the chemical industry. Not all plastics satisfy these requirements and, of those that do, not all can be readily fabricated. Most widely used are the phenolics, chlorinated rubbers, vinyl esters, pyroxylin, and, if predictions come true, the methacrylates. Of the many chemical industries which have benefited by the use of plastics, the textile, paper, petroleum, fermentation, and photographic industries are outstanding. From phenolic materials the textile industry has fashioned rayon spinning pots which operate continuously 24 hours a day, 7 days a week, in contact with sulfuric acid, sulfur, and carbon disulfide, In similar fashion, phenolics have been utilized for improved godet wheels, rollers and C-guides, dipping arms, etc. Machines and tanks have been protected against the humid atmosphere of textile mills by resistant coatings of chlorinated rubber. Paper mills employ suction and head boxes of phenolic laminated and doctor blades of similar material; chlorinated r u b b e r c o a t i n g s p r o t e c t machinery against the ravages of chlorine fumes. Phenolic-laminated torpedo and bomb shells are used in the shooting of oil wells, and pyroxylin is employed to supplement the protection of a tar coat on the outside of oil lines. Beer and wine vat interiors are made resistant to acids and ferments by special coatings with a phenolic resin base; beer in cans is possible because of the protective coating of vinyl ester. Photographic film developing tanks and developing trays are molded from phenolic materials, and phenolic laminated protects table tops against the corrosive action of photographic chemicals. Oil-resistant printing rolls of alkyd resin are r e p l a c i n g the familiar glue and glycerol combinations; in the steel industry heavy-duty phenolic laminated bearings have proved superior to theusual metal bearings. Agriculture benefits through the propagation of Japanese-beetle-destroying nematodes in culture trays coated with phenolic resin. Last, but by no means least, the chemicalmanufac-
(TOP)
SYNTHANE
LAMINATED B u cK ET FOR HANDLING CORROSIVE MATERIALS In certain cases metal b u c k e t 8 d e t e r i h e 6.0 rapidly a8 t o make their w e extremely impractical. Despite a aomewhat higher initial oost, this Bake!ite bonded bucket offers important economies because of its longer life. Synthane Courtesy. Corporation
(Bottom) STEEPTANK W H O R EI N T E R I O R COATING 18 BASEDON PHENOLIC RESIN Courtesv,
Litheow Corporation
380
TUBES,
(Right) PAPER FOR USE IN A TEXTILE MILL, WATERPROOFBD WITH BAKELITE VARNISH The tubes are wound to form and impregnated with the varnish. they become rigid and have 8 hard resfstant surface. These tubes are used for winding yarn which is t o be package-dyed. Courtesy, Pairpoint Corporation
(Center)
resistance, but which form of that plastic is best adapted to the intended use. Cast p h en o 1i c resins have b e e n w i d e l y used for the fabrication of beakers, gradua t e s , b u r e t s , oil gages, etc. Because of the difficulties inv o l v e d i n c a s t i n g certain shapes, beakers and graduates were constructed with thick walls and with the bottoms c e m e n t e d i n , while burets were f a b r i c a t e d from long rods bored out only with difficulty. Today, improvements in materials have made it possible to employ transparent phenolic molding materials for the production of thin-walled beakers, crystallizing dishes, graduates, cover glasses, etc. And in their molded form, plastics provide greater freedom
CHEMICAL 4R E s I R T A N T NOZZLE
This Bakelite-molded unit is for service wherever resistance to corrosive chemicals is important: it is mechanically strong and possesses a permanent finish. Courtesy, Spraco, Inc.
Plastic molding materials vary widely in their properties. Many, such as the cellulose and vinyl esters and the methacrylates, are thermoplastic and therefore not suitable for uses which involve temperatures exceeding 100" C. These thermoplastic materials are ordinarily used without fillers and, in general, possess a high resistance to impact and high tensile strength. They are quite moisture resistant (particularly the methacrylates), unaffected by most mineral acids and most alkalies, and insoluble in alcohol and straight-chain hydrocarbons. They may be sawed, cut, bored, turned, drilled, etc., but are most easily fabricated by injection moldPhenolic materials, on the other hand, are thermosetting. They are supplied with wood flour fabric or mineral fillers, and the design of equipment to be molded from either of these materials should take into account the different properties which they possess. Ordinary wood-flour-filled materials usually possess flow sufficient to permit fabrication into intricate designs, whereas fabric-filled materials are more deficient in flow, but the finished molding possesses much greater resistance to shock. Generally the flow of phenolic molding materials is in inverse ratio to the impact strength of the finished piece; and if maximum strength is required, the finished piece should be designed as simply as possible,
CASTRESINOID BEAEBRS AND GRADUATES These rtocurately calibrated chernictal utensils are produced from a epecial amber-hued Bakelitecast resinoid which is high1 chemical-resistant and is unaffected by the iestructive action of hydrofluoric acid. Courtesy, Eimer and Amend
industry, Recent developments in this material during the last two years include the production of standard split flanged pipe and fittings, standard bell-end fume ducts and fittings, standard flanged fume ducts and fittings in sizes ranging in inside diameter from 0.5 to 30.25 inches, and in length up to 10 feet. Tanks are either round or square and with either flat or cone bottoms. Towers up to 20 feet high have been constructed, and recently sieve plate and bubble cap designs have been introduced.
Design This question of design for molded plastic parts is sufficiently important to discuss in some detail. Obviously, the more complicated the shape, the more costly the resulting piece. The designer should bear this factor in mind and strive for simple shapes in order t o keep mold and production costs as low as possible. It should not be assumed that complicated and intricate pieces cannot be produced-they can beand are being turned out regularly-but the production cost and the cost of molds are commensurate with their complexity. The first principle to remember is that the piece must be designed so that it can be removed easily from the mold after the molding operation is completed. When this point is
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Selection of Type and F o r m Not only is there a wide variety of plastic materials to choose from, but they are utilized either with fillers as molding and laminated materials, or without fillers as cast materials, or in solution as coating compositions. Therefore, in deciding which. type of plastic to use, the engineer must consider not only which type of plastic offers the greatest chemical 381
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Courtesy, General Electric Company
PRINTING PRESSES EQUIPPED WITH GLYPTAL PRINTING ROLLS
overlooked, as it has been frequently, molds are useless for quantity production work. Obviously, production costs are greatly increased. A common fault in plastic design is the requirement of nonuniform wall thicknesses. A piece may have wall sections which will normally require about a 3-minute molding cycle to attain maximum strength and chemical resistance. To obtain a certain effect, the designer has a t one point a section with a larger wall thickness that will require approximately a 6-minute cycle. Consequently, if the minimum molding cycle is used for such a piece, the thick-walled section will not have maximum chemical resistance; if the maximum molding cycle is used, the thin-walled section will be deficient in resistance to shock. Molding materials, just as other raw materials such as iron and steel, shrink after they are molded. A heavy section tends to shrink more than a thin section b e c a u s e of its greater mass. A combination of thick and t h i n s e c t i o n s should be avoided wherever possible because unbalanced stresses will set up, due to variation in shrinkage between the sections. Often this will cause the molded piece to warp or distort. A fin or parting line is left on molded pieces by the mold parts where they meet in the closing of the mold. Hence, the piece should always be designed around the fin because a fin must be removed after the piece is molded. Therefore, a piece designed properly will not only reduce the cost of finishing, but enhance its appearance and endurance.
Radii or fillets should be used in place of sharp corners wherever practical. This does not apply, however, where the fin or parting line of the piece is involved. Such corners should be left square so that the finishing operation will be simple and will in no way affect the finish of the piece. Fillets generally simplify mold construction and consequently reduce the cost. They also tend to strengthen the mold and the molded piece wherever used. Often a sharp corner on a molded piece will require a great deal of handwork in some obscure section of the mold, whereas a fillet may be produced easily with a machine cutter. Shelllike wall sections around inserts should always be avoided because cracking will undoubtedly result, owing to the difference in the coefficient of expansion between the molding material and the metal inserts. Since the fundamental motion of the mold is down and up, side holes and deep undercuts can be created only by the use of split molds, removable pins, sliding bars, or plates, which are drawn from the mold with the work and later removed by the operator. Although this construction is possible and practical, it is expensive; less c o m p 1i c a t e d designs often serve just as well. Dimensions closer than plus or minus 0.005 inch should not be specified unless really necessary. It is not economy to stint on the amount of molding material to be incorporated in a molded piece. Parts withvery thin walls will be fragile and their production will be difficult. Some concerns believe that they can save by decreasing the amount of raw mate-
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rials used in the molding of a product. This is not true because, as rejects increase, so do molding costs. The chemical resistance of phenolic molded, as compared to the unfilled phenolic resin, is naturally lessened because of the inclusion of a cellulosic filler. Its water absorption will be higher and its resistance to acids considerably lower. Cellulose is affected less by alkalies, however, and consequently phenolic molded, although apparently markedly corroded by caustic, still retains a surprising amount of its original strength. Phenolic laminated possesses t h e maximum strength of this type of plastic and is usually used in sheet form or a s t u b i n g . It can be sawed, cut or machined to shape and ordinarily molded into simple forms. A more recent development in the fabrication of laminated parts is the combination of laminated rod stock and resinoidimpregnated fabric sheets. By this process some of the flow limitations in the production of laminated mater i a l s are overcome, and more complicated forms are possible . Phenolic laminated offers TOWER AND TANK OF HAVEQ greater resistance to chemiRESIN cal action than p h e n o l i c Courtesy, Haveg Corporation molded, and its resistance is greatly i m p r o v e d by coating the machined piece with a resin solution, which is then hardened by heat.
Application as Coating For those cases requiring plastics as coating materials, choice of the correct plastic involves not only the most resistant plastic but also the question of its application to the surface which it is desired to protect. A major requirement is, therefore, that the plastic have good adherence; otherwise, its chemical resistance will be of no consequence. Properly plasticized chlorinated rubber paints adhere excellently to
most metals and are specially s u i t e d for the coating of concrete floors and walls, as well as for plaster, s t u c c 0 , brick, etc., b e c a u s e of t h e i r inertness in the presence of alkalies. Phenolic materials adhere well to wood but for coating metals a suitable primer may be required over which the phenolic material is applied. Vinyl esters are best applied on a suitably primed surface. Surfaces may also require an alkaline, acid, or solvent wash, sandblasting, etc., and consideration must also be given to the fact t h a t c e r t a i n plastics are most resistant if given a heat treatment. Quite recently means were devised whereby phenolic resin coatings are applied to hrge vessels and then h a r d e n e d by heated air blasts. Such a method has proved particularly successful in the coating of ferm e n t a t i o n vats, “steep” tanks, and beer s t o r a g e tanks. An i n t e r e s t i n g application of this kind is a steep tank located in the plant of a large g l u c o s e manufacturer. The tank is constructed with a conical cast-iron bottom, 14 feet in diameter a t the base of the cone and 9 feet high. The sides of the tank are of wood, about 40 feet high. Steep water contains from 0.1 to 0.4 per cent sulfur dioxide and is e x t r e m e l y corrosive. Although the coating of this tank required baking, it was acMEASUR~NQ CYLINDEROF BAKELITE TRANSPARENTcomplished without injury MOLDINGCOMPOUND or shrinkage to the wooden staves of the tank. This coating has stood the test for a full year without being affected, whereas normally in that time the cast iron would be worn away at least l/8 inch.
TABLE I. REPR~SENTATIVE APPLICATIONS OF PLASTICS IN CREMICAL AND ALLIEDINDUSTRIES Industry:
Chemical
Plastics? Uses :
PF, H, C PF, H, C A Beakers, bearings, bub- Bobbins, godet Printing ble caps, buckets, buwheels, guides, rolls rets, centrifugal basrollers, spinkets, conveyor links, ning buckets, drums, drying trays, winding tubes evaporator pans, filter press plates, gears, graduates, kettles, pipe lines, plating barrels, pumps, sieve n P F = phenol-formaldehyde: H = Haveg; V = vinyl; P = pyroxylin: C = chlorinated plates, splash plates, rubber: A = alkyd. spray nozzles, tanks, towers, ventilators
Textile
Paper
Petroleum Fermenta- Photographic Steel tion PF, H, C PF, C, P PF, V PF PF Bearings, doc- Bomb Beer a n d Developing Bearings tor blades, roll- casings, wine contanks, taers, suction pipe lines, tainers, beer ble tops, boxes pump storage trays valves, tanks, s t o r a g e steep tanks, tanks tank cars, vats
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Printing
Agriculturd PF A p p 1e washers, frames, pipe lines
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Courtesy, General Plastics, Inc.
TEXTILEBOBBINS,PULLEYS,AND THREAD GUIDES AND CHEMICAL-RESISTMADEOF DUREZFRICTIONINQ MATERIAL
Large beer storage tanks have been similarly coated with excellent results. In many cases these coatings were baked on in cellars where the temperature had to be maintained between 32" and 40OF. in order to protect the beer in adjacent tanks. When completed, these protective coatings are then subjected to normal operating temperatures of 32" F., but in several instances they have withstood occasional steam sterilization at temperatures as high as 330" F. without adverse effect. These coatings have been approved and accepted by the laboratories of many of the largest breweries from the standpoint of protection and taste and the elimination of iron or metal turbidity. Because of the extremely smooth and glossy surface which they possess, the cost of cleaning tanks is only 30 per cent of the former cost of cleaning the pitch-lined coated tanks. The film is smooth, glossy, nonporous, hard, and yet tough enough to withstand metal distortion, including
VOL. 29, NO. 4
tank car railroad service. A life expectancy of such coatings is estimated a t 7 to 10 years, compared to a life of 2 years for the ordinary type of coating. Another interesting use of a plastic coating based on phenolic resin has been to protect the exterior of pipes immersed in 1 to 1.5 per cent hydrochloric acid solution maintained at 110OF. Hot water or steam is passed through the inside of the pipe to heat the solution. I n one case where pipes were immersed in the hot solution for 59 days, during which time the coated pipe was used as a heating core for over 10 days, the coating had blistered but had not failed. This behavior is unusual because blisters are usually porous and mean early failure. However, a fact often overlooked in considering the use of plastics, either as coatings or as actual materials of construction, is the surprising retention of strength exhibited by a supposedly badly corroded piece. Thus, a canvas-base phenolic-laminated ring, in contact with a 10 to 15 per cent sodium hydroxide solution a t elevated temperature, appeared to be badly corroded after 2 years, but it has retained its strength and shape and is still in use. Laboratory tests have also confirmed this fact and have indicated that phenolic laminated, after as much as 2 months of immersion in caustic, shows only approximately 20 per cent diminution in strength, although the appearance of the piece would lead to the belief that it has practically failed. Chemists and chemical engineers, therefore, cannot judge the effect of chemical action on plastics by mere appearance, or even by weight losses; for, unlike the corrosion of metals, the effect of corrosive agents on most plastics is to increase their weight, rather than diminish it; nor can the suitability of a certain plastic be determined unless the application of that plastic is carefully considered in relation to the problem at hand. But with the present different types of plastics (more are being introduced each day) with each of these types available for application in a wide variety of forms, and with the new present-day methods of application, consideration of plastics as materials for chemical resistant equipment should not be overlooked by the wide-awake engineer.
Acknowledgment The author wishes to express his thanks to the manufacturers of the plastics here mentioned for their help in gathering the information presented. R ~ C E I V EFebruary D 19. 1937.
Courleay. Westinghouse Electric and Manufacturing Company
CAPSOF MICARTA WHICHHAVEBEEN IN SERVICE23 MONTES
