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B. I. Zolin and J. G. Green, E. I. du Pont de Nemours & Co., Inc.
Epoxy Resins — An Engineering Material for the Chemical Industry — Part II More case histories of the application of epoxy resins to new equipment
HEN the epoxy resin evaluation Wprogram was initiated 3 years ago, the information available then was so limited that the empirical approach was mandatory. At that
time the only available information described the standard short-time physical properties and the resist ance of the resin to various chemicals. The standard tests for strength pro
CASE HISTORY 7
Figure 3 GPX DUCT JOINT DETAIL Figure 4 RIGHT ANGLE JOINT
vided no information about the effect of stress applied for extended periods of time—i.e., stress to rupture and creep strength. The chemical resistance data did not indicate how
EPOXY ADHESIVE APPLICATION
PROBLEM : A plywood exhaust duct coated with asphalt mastic was in need of replacement because of general deterioration of the wood and leakage of condensate through the joints. A more durable but low-cost material was desired for this duct. The cross section of the duct ranged from 4 square feet to 8 square feet, and the total surface of the duct was 6000 square feet. ENVIRONMENT: Air is saturated with moisture and contains dilute sulfuric acid mist and hydrogen sulfide. The maxi mum temperature of the exhaust stream is 50° C. SOLUTION : Georgia Pacific plastic-faced plywood was selected to replace mastic-coated exterior grade plywood. To seal the plywood panels, a thixotropic epoxy-Thiokol adhesive was used. The curing agent was triethylenetetramine and the resin was cured at ambient temperature. The type of joint construction is shown in Figure 3 and an external view of the complete duct is shown in Figure 4. RESULTS : The initial exhaust duct system, using plasticfaced plywood and an epoxy-Thiokol adhesive, has been in service for 3 years and is in excellent condition. Since then an additional 6000 square feet of exhaust ducts have been installed. A cost analysis revealed that the two plastic-faced plywood ducts could be installed at a cost of $10,000 less than the cost of the mastic-coated plywood ducts.
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SPLICE JOINT
Ο HISTORY 8. EPOXY CALKING COMPOUND APPLICATION 8 ο PROBLEMCASE: Numerous crevices in several stainless steel humidifier chambers were possible sites where stress corro sion cracking could originate. · Welding of the crevices was not possible, so some other method of calking was required. ο ENVIRONMENT: The walls of the chambers were wet with water
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containing an organic acid and traces of chloride ions. The maximum water temperature was 85° C. SOLUTION: A thixotropic epoxy resin was formulated that would not sag on vertical surfaces. The epoxy calking com pound was applied to the crevices without difficulty by an unskilled person. RESULTS : The calked stainless steel seams have been in service for 1 year and are satisfactory.
the physical properties were af fected by immersion in the chemi cals. In addition, no data were available to indicate the effect of stress on a sample immersed in a given chemical solution at a given temperature. Once the nature and limitations of the data were understood, the information at hand was used as a guide in determining whether equip ment prototypes should be construc ted. Since expensive molds are not necessary for fabricating trial parts, testing of the structure under actual service conditions was found to be an efficient and satisfactory pro cedure. At the outset of the evaluation VOL. 4 9 , N O . 3
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EQUIPMENT AND DESIGN CASE HISTORY 9.
program, suitable epoxy resin com pounds could not be purchased. Therefore, it was necessary to de velop special compounds as required. As a result of these experiments, there is one formulation (Table II) that may be used for a wide variety of casting, calking, adhesive, and coating applications. Slight changes in the amount of Cab-O-Sil, the thixotropic agent, determines the fluidity or thixotropy of the com pounds. T h e use of epoxy resins for thick-
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CASE HISTORY 10.
EPOXY COATING OVER ALUMINUM
ο PROBLEM : An alum scrubber constructed of an aluminum alloy bacame severely pitted. The location of the tank was such that installation of a new one would shut down the operation for 1 week. 9 ο ENVIRONMENT : Alum crystal dust is collected by a water spray in the scrubber at ambient temperature. SOLUTION: A thixotropic epoxy resin that would cure at room temperature was formulated so that it could readily be trowelled onto a surface. Application of the resin coating consisted of three main steps. After sandblasting, the sur face was coated with a thin layer (0.025 inch) of activated resin. Then a layer of open weave glass cloth was placed over the resin layer and forced into the resin. The final step consisted of trowelling a thicker layer (0.060 inch) of thixotropic epoxy onto the resin-impregnated glass cloth. After the resin cured, it was subjected to a 12,000volt spark test to detect the presence of pinholes. Several small defects were observed and these areas were patched with resin. In an application of this type, a spark test is necessary to determine the quality of the coating. RESULTS : To date the coating has doubled the life of the original aluminum tank. It is expected that the coating will extend the service life of.the tank by a factor of 10. ·
EPOXY COATING OVER BRICK
PROBLEM : A brick wall was removed because the mortar as well as brick had been weakened by chemical attack. Because of surface irregularities of the wall, conventional protec tive coatings did not afford adequate protection. A heavy plastic coating was desired that could be applied to the new replacement wall. ENVIRONMENT : The relative humidity of the air was above 90%. Droplets of dilute sulfuric acid and aqueous sulfate salts were entrained in the air. SOLUTION : A thixotropic epoxy resin, similar to the one used for the alum scrubber application (Case History 9 ) , was used except for a slight modification. The resin was formulated so it would have a greater degree of resistance to sagging when applied to a vertical surface. A coating of resin, approximately 0.060-inch thick, was trowelled onto the newly constructed brick wall. The epoxy coating was applied in one step. RESULTS : The coating is in excellent condition after 1 year's service. More extensive epoxy coating applications of this type are planned.
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layer of resin is an effective method of extending the use of some of the more common low-cost engineering materials. Thick epoxy coatings may also be expected to be used in place of the more conventional sheet lining material such as rubber,
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ο filmed, solvent-free protective coat ings is one type of application that will find increased use. Case His tories 9 and 10 are typical examples. Protection of concrete on brick structures by a V32- to W i n c h
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CASE HISTORY 11.
EQUIPMENT REPAIR WITH EPOXY RESINS
PROBLEM : Molded phenolic buckets were being rejected be cause of chipped areas on the periphery of the buckets. A method of filling in these defects was required. ENVIRONMENT : The buckets are in contact with dilute sul furic acid and aqueous sulfate solutions at 50° C. SOLUTION: A silica-filled, room temperature curing resin was applied to the defective area. The exact contour of the bucket was achieved by subsequent machining. RESULTS : The repair method has been used successfully for 4 years.
12 ο CASE HISTORY 12. PIPELINE REPAIR WITH EPOXY RESINS : A broken d r a i n l i n e made of lead and a leaking ο sPROBLEM t e e l l i n e were to be repaired as quickly as p o s s i b l e w i t h
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out replacing the pipes ENVIRONMENT: The lead l i n e handled d i l u t e s u l f u r i c acid and the s t e e l conveyed steam condensate. The s e c t i o n s of pipe to be repaired were located in areas t h a t made maintenance difficult SOLUTION: After cleaning and drying, the areas t o be r e paired were wrapped with woven g l a s s tape impregnated with a room temperature curing epoxy r e s i n . The thickness of the f i n a l wrapping was approximately 1/8 inch. RESULTS : The use of r e s i n impregnated g l a s s tape proved successful. In both cases, t h i s method of r e p a i r i n g leaking l i n e s enabled the plant t o continue without a c o s t l y s h u t down.
INDUSTRIAL AND ENGINEERING CHEMISTRY
o t h e r elastomers, a n d plasticizcd vinyls. Ease of a p p l i c a t i o n of a liquid epoxy m a t e r i a l , especially to c o m p l i c a t e d shapes, is o n e of t h e m a i n reasons w h y this m e t h o d of coating will c o m p e t e on a cost basis w i t h sheet linings. C h e m i c a l stability of a reinforcing fiber is necessary for t h e m a i n t e n a n c e of t h e initial l a m i n a t e properties after long periods of immersion in a c h e m i c a l solution. T h i s is shown in T a b l e I I I w h e r e epoxy resins, reinforced w i t h glass a n d O r i o n fabrics, were i m m e r s e d in a 3 0 % sulfuric acid solution a t 50° C . for
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CASE HISTORY 13. FLANGE REPAIR WITH EPOXY RESINS PROBLEM : A 9-foot diameter distillation tower, consisting of more than 20 flanged sections, developed leaks at many of the flanged joints. A method of preventing leaks without dismantling the tower was desired. :S0LUTI0N: The leaking of the flanges was prevented by casting a silica-filled epoxy resin around the flange. A Zshaped, sheet metal form was strapped to the tower as shown in Figure 5, and the resin was poured into the resulting cavity. Considerable experimentation was necessary to arrive at a suitable resin formulation—i.e., one that would not crack during curing. The formula finally used was 100 parts of Epon 828, 10 parts of triethylenetetramine, 100 parts of 100-mesh silica, and 100 parts of 325-mesh silica. RESULTS : This method has successfully stopped the leaks.
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SHEET STEEL FORM
Figure 5
Table II.
Epoxy Resin Formulation
Parts by Weight Comments Ingredient Epon 828 or equivalent 100 Liquid resin with viscosity of 100 to 160 poises. Triethylenetetramine 8 to 12 To ensure an adequate cure, use at least 10 parts. 100 Other fillers, such as graphite, carbon, or alumina, Silica, pigment grade 219, may be used. The amount used will have to be Whittaker-CIark-Daniels, determined. or equivalent 2 to 8 The amount should be adjusted for each application. Cab-O-Sil or equivalent The thickening effect increases proportionally with the addition of Cab-O-Sil. Table III.
Effect of 3 0 % Sulfuric Acid on Properties of Reinforced Epoxy Laminate
Property Flexural strength Flexural modulus Compressive strength Thickness
Original Values of Laminates, Lb./Sq. Inch Orion cloth Glass cloth 19,500 64,200 0.73 X 10« 2.3 X 10» 21,500 55,900 0.146 (inch) 0.123 (inch)
% Change after Immersion at 50° C. for 6 Months Orion cloth Glass cloth -10 -65 -70 -21 -60 -17 + 18 + 2
Resin system, Epon 828-CL
14 CASE HISTORY 14. REPAIR LEAKING STEAM CHEST PROBLEM : A steam chest had a number of leaks, and a rapid, temporary method of preventing leaks was desired. ENVIRONMENT: The steam pressure in the chest was 175 SOLUTION: The leaks were stopped by putting three layers of epoxy-impregnated glass cloth over the porous areas. The patch was put on the steam side of the chest. RESULTS : The patches prevented leaks for approximately 1 year.
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6 months. Prior to tests the flexural strength of the glass-epoxy laminate exceeded that of the Orlon-cpoxy laminate by a factor of 3. After a 6-month exposure in the acid, the strength of the glass-epoxy laminate was only 30% greater than Orlonepoxy laminate. The main disadvantage of epoxy resins reinforced with synthetic fibers is the relatively low strength properties compared to glass fiber-reinforced laminates. Where strength of a structure is important, then the use of an Orlon-epoxy surface layer approximately 0.015 inch thick over the glass-epoxy laminate is beneficial. This method protects the glass reinforcing fibers from direct contact with the chemical environment and hence prevents or minimizes deterioration of the glass. In effect, the use of an overlay is analogous to a protective coating applied over steel surfaces. When the strength of the structure is not of primary importance or when the geometry of the structure is such that the glass cannot be adequately protected from the chemical environment, the use of reinforcing fibers other than glass should be considered. Two large-scale evaluation programs are now in progress in the categories defined above. For electrical insulation applications, epoxy resins reinforced with synthetic fibers are of interest. The dielectric constant and dissipation factor retains low values throughout a wide frequency range.
