Reinforced Polyester in the Chemical Process A. F. TORRES
The digester blow,pipe, left above, .in a paper . . mill carhes steam, sulfur dioxide, carbon .:. dioxide, formic'arid, and methanol bt temperatures up to
212' 'F. ,The reinforced' plastic bleach tank, center, handles chlorine and sodioq hypochlorite
It costs less thanthe unit i t repiaced and has performed longer with no deteriotation.
The de-
misters, right, have carried 70 tons per day of
:
saturated chlorine gas for over .two, years with no '
'signs of deterioration
32
,
.
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
I
tquipment Industry
Comparative purchase prices compiled by Webster ( I I) for several equipment items are shown in Table I. The comparison is based on his personal experience and is expressed in relative costs for various materials. As expected, carbon steel is the most inexpensive material. However, when the media handled are corrosive to carbon steel, other materials must be used. Under these conditions, the experience of the Du Pont Company indicates that reinforced plastics are most economical. Reinforced plastic equipment is currently available in both ordinary and fire-retardant grades. There are about a dozen fabricators in the United States who are experienced and have the ability to design and fabricate serviceable items of equipment. The long history of use for reinforced plastics available within such major companies as Du Pont, Monsanto, and Union Carbide indicates that such use is rapidly reaching maturity. Perhaps the second most important use of reinforced plastics is the lining of new process equipment or the reconditioning of used equipment. In many cases such applications have been highly successful. However, there are a number of instances where linings have failed either because of a lack of understanding of critical application requirements or exposure of the lining to excessive temperatures, and to corrosive materials which were too extreme for the lining. In contrast to an
upper l i t of 112' C. for total plastic structures, the average upper permissible limit for linings should be 60' C. to 80' C., with few exceptions. Corrosion resistant tables designed to show suitable environments for total plastic structures should not be used to judge the resistanceof linings unless the person making the judgment has intimate knowledge of this type of operation. These statements should not be construed as indicating that linings are not dependable. Rather they are intended to indicate that linings should be employed with caution and that the best advice available should be considered before applying a lining under extreme corrosive conditions. Considerable research (2,s)has been expended on linings recently and definitive test methods have been devised and published. Greatly improved new combinations of materials and new application techniques have been and are now on field test. I t is only a matter of a short time until sufficient knowledge and experience are available to make lining of process equipment a routine matter. Many tanks have been repaired and returned to service by the simple expedient of liming them with glass mat or cloth and a suitable resin. Surface preparation and elimination of air bubbles and pin holes are very important in this type of application. hlore recently there has been considerable activity in the use of OwensCorning's flake glass with the bisphenol A-fumarate resin. This type of lining is sprayed on, and is thus easier and less expensive to apply than the mat or cloth lining technique, which is essentially a hand operation. The flake glass also seems to produce better results, probably because of the flake's ability to provide a multilayer vapor barrier. Linings on metal substrates have been most popular to date, and, therefore, are best known and most thoroughly evaluated. Considerable work has also been done on the lining of concrete structures. While further development work is needed before such lining becomes routine, there is evidence that reinforced plastic linings for concrete tanks will prove of great benefit to the process industry. One of the most widely discussed applications of reinforced plastics is in the production of plastic pipe. The popularity of the idea, is due to the large sales potential in the chemical process industry. There are numerous installations of large diameter reinforced pipe, particularly in waste disposal systems, effluent lines, etc. A great deal of work is now in progress on the lining of both metallic and nonmetallic pipe with both reinforced and unreinforced plastic materials. Within two years several major manufacturers will probably be on the market with process pipe which utilizes the older materials for strength and a plastic internal lining for corrosion resistance. During the same period totally reinforced plastic pipe will probably also improve its market position. Other Uses for Plastics
The chemical industry has recognized the need for a monolithic flooring compound which can he applied VOL 5 6
NO. 6
JUNE 1 9 6 4
33
TABLE I.
PURCHASE PRICE COMPARISON FACTORS FOR UNINSTALLED PROCESS EQUIPMENT
Process tank, 1000 gal.
Carbon Steel
Reinforced Plastic
Reinforced PlasticLined Steel
Elastomer Lined Steel
Lead-Lined Steel
1.9
1.2
2.4
3.7
3.8
4.5
Brick-Lined Steel Single
Process tank, 5000 gal.
4.2
5.6
6.1
8.2
8.5
10.5
Storage tank, 5000 gal.
4.8
6.7
7.1
10.6
1 1 .o
...
Rotary vacuum filter, 18 sq. ft. filter area‘
...
7.1
...
12.8
...
...
Centrifugal ventilation fan, 49” wheel, 15,000 s.c.f.m. @ 5” static p r e s b
...
2.5
...
2.9
...
...
Vapor duct, 4 ” diam.
4.2
3.5
7.2
12.0
9.2
...
Vapor duct, 12” diam.
12.6
6.3
20.0
34.0
27.0
...
a
Complete with drioe and motor.
2
4
6 MONTHS
8
10
~
Hastelloy
6.0
6.0
...
14.5
12.3
...
.
I
’.’
I
...
I
c
15.3 21.5 ” ’
7.0 21.0
...
INDUSTRIAL A N D ENGINEERING CHEMISTRY
a new choice in materials. The new flexibilized polyester offers applicators a choice between epoxies and rigid polyesters. Toppings formulated from flexible propoxylated bisphenol A-fumarate polyesters offer excellent resistance to thermal shock, toughness, and wear resistance. Polyesters can be formulated to cure according to a variety of time schedules, while epoxies are limited to a narrow range of curing times. I n some cases, a polyester floor can be walked on in a matter of a few hours and be put into service while epoxies can take considerably longer. At the present time there are a number of lesser applications for reinforced plastics as well as for special coating compounds and materials. These are specialty uses developed by individual equipment manufacturers. The bright future for reinforced plastics can be summed up in a statement by Monsanto Chemical Co. after over 11 years of experience with these materials. Accordingly, lightweight, reinforced plastics could replace metals today as a superior structural material for an estimated 20% of all chemical processing equipment a t substantially lower purchase and maintenance costs. T h e factors cited for this analysis include low maintenance and long service life, low initial cost, ease of fabrication and repair, electrical nonconductivity without galvanic or stray current corrosion, elimination of painting needs, lack of contamination of food chemicals, translucency, impact resistance, and lightweight. Equipment fabricated from reinforced polyester, it should be stressed, will function properly only when constructed properly from the correct components. The engineer in the chemical process industry needs a broad understanding of all the variables involved in the fabrication of reinforced polyester equipment to properly specify such units. The variables most likely to influence the performance of the equipment are the type of resin selected, the type of reinforcing fibers, the method
17 -
Figure 7 . Flexural strength retention of resins after being exposed to sodium hyfochlorite f o r one year at 210’ I;: 34
~
Stainless Ty)e 304
Double
Without motor.
directly over old or new concrete and which is resistant to attack by spilled chemicals. Much of the monolithic flooring now in service is based on epoxy resins. While offering advantages over even the most corrosion resistant polyesters, the epoxies exhibit a number of serious deficiencies. This picture should clarify with each resin finding its proper use and should result in greater use of compounds based on the bisphenol resins. Among the first resinous binders for monolithic floor toppings designed for use in corrosion resistant applications was a series of rigid general purpose polyesters. They produced a n initially satisfactory troweling mixture, but due to the rigidity of the polyesters employed, the toppings had a tendency to crack and lift from the substrate. The addition of external plasticizers or flexible polyesters minimized these problems, however, and corrosion resistance then became the limiting factor. Poor resistance to alkali and acidic environments soon eliminated polyesters from this application area and epoxies filled the void. T h e major objections to epoxy resins are: higher cost; high viscosity which increases the troweling drag; dermatitic effects of the polyamine curing agents ; poor resistance to oxidizing acids and organic acids. With the introduction of the corrosion resistant propoxylated bisphenol A-fumarate polyesters, interest in polyesters for topping compounds has been revived. These polyesters have provided the process industry with
0
~
AUTHOR A . F. Torres is a Group Leader with Atlas Chemical Industries, Inc., Wilmington, Del.
of fabrication, and, in addition, in linings, preparation of the substrate surface.
As mentioned previously, a number of types of polyesters are available today; not all, however, are suitable for the type of corrosion found in chemical processing. The “general purpose” resins are used mostly in the manufacture of boats, auto bodies, and other consumer items where heat distortion and chemical resistance, in our terms, is not a significant factor. A second variety of polyester resins is the isophthalic type, These two types of polyesters have not found wide use in equipment handling acids and alkalies or other solutions a t elevated temperatures. For these applications, a bisphenol A-fumarate resin (70) has found a place. Figure 1 shows the flexural strength retention of the three resin types when exposed for one year a t 210’ I?. Although only one corrosive agent is shown here (sodium hypochlorite), the comparison is typical. General purpose resins are quite poor under the severe conditions in the chemical process industry. The propoxylated bisphenol A-fumarate resins proved the best of all polyesters in a wide variety of media. Epoxy resins give excellent results in some media, particularly solvents, but are no more a “universal material” than any other metallic or plastic material. It is still necessary to select materials in terms of anticipated immediate service conditions, and build in a safety reserve for possible future changes in the corrosive media. A series of studies (9) has also shown that the type of reinforcing fiber and the reinforcing system used in a laminate will strongly influence its strength and cor-
rosion resistance. These tests bore out the need for a surface mat in all corrosion-resistant structures. When composed of a chemically resistant-type glass, the surface mat will isolate and protect the reinforcing glass of the laminate. When the surface mat is missing, bundles of fibers may be exposed a t the surface of the laminate. A resulting wicking action conducts the solution into the laminate interior where it attacks the glass reinforcement and hastens deterioration of the structure. The fabricating technique will also determine the ultimate performance of the equipment. One of the most important factors is the procedure of assembling the various reinforcing layers that make up the laminate. This procedure illustrated in Figure 2, should form an important part of every specification. As stated previously, surfacing mat is vital; this mat should be saturated with resin. Beneath the resin-rich surfacing mat lies the inner structure, consisting of several layers of glass mat and cloth. These should contain a minimum of 30% glass. The presence of bubbles and voids or a soft surface indicating incomplete cure of the polyester is a sign of substandard fabrication and will result in poor performance. Recognizing the importance of proper materials and laminating techniques, a number of major chemical companies have written complete specifications based upon existing knowledge, and they are using these standards routinely as a guide in their purchases of reinforced plastic equipment. If the chemical process engineer recognizes all the possible pitfalls in specifications of a reinforced polyester unit, where can he obtain reliable information about the various materials comprising the structure? His most reliable method to ascertain the serviceability
Figure 2. Proper laminate construction of Jooring sealants f o r corrosive service. The Jirst seven layers are necessary for all service, with
additional reinforcing layers depending upon strength requirements. The layers are exaggerated in thickness
Types of Polyesters
VOL. 5 6
NO. 6 J U N E 1 9 6 4
35
of a particular material would be to immerse samples in his specific environment for a minimum period of one year at operational temperatures. Test coupons of laminates are readily available from resin manufacturers and leading fabricators. However, this procedure is not always practical. His alternative is to depend on resin manufacturers and fabricators. H e should insist, ho\vever, that their tests reflect the results of at least 6 months of contact with the corrosive media, preferably at elevated temperatures. The importance of long-term testing was established in an Atlas study, which proved that the popular 24-hour tests are completely inconclusivea material that has withstood 24 hours of immersion in a corrosive solution admirably, might fall apart after a one-week test. It was largely because of the early acceptance of the 24-hour test that a number of equipment failures took place several years ago. Long-term immersion tests, on the other hand, are conclusive evidence of the ability of a laminate to tolerate a particular corrosive solution. Through such a study, the existence of wide variations in the corrosion resistance of the three classes of polyesters was discovered. The samples were immersed a t 210’ F. for one year or more, and then tested for flexural strength retention. Depending on the resin and the corrosive solution, some samples deteriorated radically in a one-week to twomonth period. Others lost flexural strength, but leveled off after a year, indicating that the material stabiliied and would remain intact indefinitely. Variation in corrosion resistance among the various types of polyester resins is, of course, a function of their chemical make-up. Although there is still much to be learned about the specific behavior of the various molecular configurations under certain conditions, it is possible to cover several general aspects of the influence of chemical structure on corrosion resistance. Formulation and Corrosion Resistance
A discussion of the formation and characteristics or polyesters will help explain the mechanisms involved in their relative abilities to withstand potentially corrosive agents. Consider the ester resulting from the reaction of carboxylic acid and an alcohol in the presence of an acid catalyst. Of great importance in this discussion is the fact that a simple ester generally has relatively lo~v hydrolytic stability. The esterification process is reversible, and the ester may be hydrolyzed by refluxing with a n excess of water plus a mineral acid, or by refluxing in the presence of alkali and water. LVhen a dibasic acid is reacted with a dihydric alcohol, the product is referred to as a polyester. Although polyesters may be saturated or unsaturated, it is only the latter that is used in fabricating reinforced plastic equipment. The various combinations of reactants that can produce unsaturated polyesters are numerous, but this discussion will be limited to those candidate plastics for the construction of corrosion-resistant chemical process equipment. The three tvpes of polyesters commonly used for fabricating equipment are general purpose orthophthalic, general purpose isophthalic, and corrosion 36
INDUSTRIAL AND ENGINEERING CHEMISTRY
resistant prodendro-bisphenol-A polyester. A general purpose polyester may be the reaction product of propylene glycol, maleic anhydride, and either phthalic anhydride or isophthalic acid. The saturated acids serve primarily to reduce the unsaturation of the polyester and thus reduce reactivity, shrinkage, and cost of the polyester. Use of isophthalic acid in place of phthalic anhydride results in a polymer of higher molecular weight and improved hydrolytic stability. I t is not necessary to use phthalic or isophthalic acids to reduce unsaturation since the high molecular weight glycols, such as the prodendro bisphenol ti-glycol, provide a satisfactory level of unsaturation for easy cross-linking with styrene. Thus, the corrosion resistance of the various polyesters is directly related to the number of ester groups per molecular unit: the larger the number of ester groups, the more prone the polymer is to hydrolysis and breakdown. Both of the general purpose types have four ester groups in the repeating molecular unit, the bisphenol A-fumarate polyester has but two such groups exposed to hydrolytic attack. I n addition, both of the ester groups are in close proximity to a crosslink when subsequently reacted with styrene. The steric effect of the crosslinks near the acid groups protect the ester linkage from hydrolytic attack. The ester groups on the phthalic fragment of general purpose polyester are removed from the crosslinks and thus are deprived of this additional protection. Other factors, such as the large aromatic nucleus of the bisphenol A-polyester also have a significant effect on resistance to aqueous attack. These theoretical considerations help explain the differences in corrosion resistance of the various polyesters. Corrosion, however, is a complex problem and it \\-auld be folly to attempt to predict positively. a priori, the resistance of a particular polymer to a specific agent based solely upon its molecular confipration. Resistance of an)- polyester to corrosion must take into account the combination of molecular weight, glycol structure, type of dibasic acid employed, etc. .411 interpretive discussion must then give way, in the final analysis, to the results of practical laboratory tests and field experience. LITERATURE CITED ( I ) Arndt, F. \V,,Corrosion 16, No. 11, 14 (1960). (2) Report, Chem. Eng. 6 9 , No. 8 (1962). (3) Report, Chem. Process. 24, No. 6 (1961). (4) Report, Iron Age 187, No. 17 (1960). (5) Report, M o d . Plaslics 37, Yo. 9 (1061). (6) Report, Paper Trade J . 145, No. 33 (1061). ( 7 ) Sorrell, G., Chem. Eng. 66, No. 6. 149 (1959). ( 8 ) Torres, A. F., Feuer, S. S., “ T h e Development of an Improved Method for Evaluating Tdnk Lining Syntems,” 18th Ann. Conf., Nat. Assn Corrosion Eng.. hIarch 1962. (9) Torres, A. F., Feuer, S. S., ” T h e Effect of Glass 51at Reinforcement Variables on the Corrosion Resistance of Laminates,” 18th .4nn. Conf. Soc. Plastics I n d . , Frbsuary 1963. (10) U. S. Patent 2,634,251 (to Arlss Chemical Industries, Inc.) (11) \Vehster, R. M., Chern. Eng. 6 8 , KO. 28, (1961). (12) IVheeler, T. J., Chem. Pmcess. 24, No. 9 (1961).
rosion control, based on a symposium presented by the ACS Division of Organic Coatings and Plastics Chemistry at the N e w York National Meeting, September 1963. The entire series will be available in reprints.
