Evaluating Perf orrnance of Fiber
Glass Laminates
he use of glass fiber-reinforced resins for the fabrica-
Ttion of corrosion-resistant equipment and structures for severely corrosive environments has shown a phenomenal growth since the advent of the chemically resistant resins. However, one of the major deterrents to the greater use of glass fiber-reinforced resins as materials of construction in such applications lies in the interpretational difficulties associated with their test coupon exposures. Unfortunately, the criterion for corrosion attack employed in predicting the probable longevity of metals in particular environments, mils per year penetration, is not applicable in evaluating the performance of test coupons of such materials. The normal procedure in the evaluation of plastic materials after exposure is to determine through casual examination any abnormal surface or color changes, softening or hardening, crazing, delaminating effects, exposure of fibers, detrimental swelling or shrinkage, prohibitive weight changes, or other things which may be indicative of an immediate or impending failure. The completely unacceptable materials are readily disclosed and eliminated from further consideration at this time. This is not as simple for those cases in which the attack appears insignificant. These require techniques or procedures for measuring changes in physical properties which can be correlated with the degree of degradation of the exposed sample, and used in making a fairly accurate estimate of the future performance of the specific resin system in the particular environment. The literature is replete with reports detailing these techniques in many laboratories (7-4,7, 9, 70, 72).
The relationship between flexural modulus and flexural strength of these materials leads to a method of testing corrosion resistance and of following performance of vessels in plant use
0.
R. A. CASS H.F E N N E R
VOL. 5 6
NO. 8 A U G U S T 1 9 6 4
29
F+e
1, MomJlu-manglh rtlalh
polyester conraining Phoxpd 282
.
&?ut acid
2.
This study is concerned chiefly with the correlation between the changes in flexural strength and flexural modulus of those glass fiber laminates which are exhibiting deterioration. Most literature references relate to resins and laminates which are c o m i o n resistant, generally neglecting failures. I t is the purpose of this paper to demonstrate how to predict posible fdures by following the course of the degradation, and to show the methods employed in following this degradation so that other investigators and engineers may profit from and employ similar means to avoid borderline, unsuitable applications. The standard procedure employed in most laboratory and field evaluations is to expose the test panel within the particular environment and later to remove ~eetions for destructive testing at specific periods to follow the performance of the glass fiber resin laminate. An Instron instrument was used in the present study to determine the residual flexural strength and the flexural modulus of the test sample in just that manner. Thii article also describes the employment of a portable device which enables a flexural modulus reading to be obtained on the walls of glass fiber-reinforced structures during normal operations without damage to the process equipment. The same method was applied in the laboratory to obtain nondestructive flexural modulus evaulations of glass fiber resin laminates under one-sided exposure to corrosion environments. Examination of flexural modulus determinations on test coupons and plant structures subjected to similar environmental conditions, as determined by these procedures, indicates a fair degree of correlation between the destructive and nondestructive testing techniques.
is Corrosion Sjecialist fm the John F. Qumy plan&, The Monsunfo Go., St. Louis, Mo. R. A . Cars is Research Chemist at the same location.
AUTHOR P. H. F m w
50
INDUSTRIAL A N D ENGINEERING CHEMISTRY
I
.
Figwt 3. Expord to s l w y in water, 60-55' C.
Relolion8 between Flexural Shngth Modulus
In polymers, and in the case of polyesters in particular, there are several major types of attack: Swelling Destruction of chemical bonds by: Hydrolysis Oxidation Depolymerization (from heat) Radiation, and so forth Combinations of these mechanism One of the most destructive types of attack appears to be that due to solvent effectsor permeation. Figure 1 illustrates this effect on a polyester through the addition of a phosphate ester (6). The relative changes in the flexural modulus and the flexural strength of the polyester are essentially of the same magnitude and proportional through this dilution with the phosphate ester. This is an ideal situation in which the diluent is added before polymerization, and behaving as a plasticizer, is distributed throughout the polyester upon curing. This is likened to a plastic structure in which a surface is exposed to the environmental liquid. The degree of absorption of the environmental liquid will be a maximum at the exposed surface and approach zero at a point within the structure. The structure will then have different physical properties from the original at every location which has absorbed a part of the environmental liquid. Any measurement of the physical properties made on the structure then will be some sort of average of the compdte structure, the result of absorbing a part of the environmental liquid. Figure 5 shows the relation between the flexural strength and modulus for a polyester3lasa fiber laminate exposed under swelling conditions. Here again, the flexural strength and modulus are proportional. It is indicated by our observations that the firm relation between the flexural strength and modulus does not hold true for clear casting in an environment in which
the destruction of chemical bonds occurs. This is evidenced in part by the wide scattering of the data plotted for dear castings subjected to 10% sulfuric acid (Figure 2). This is probably the d t of surface attack in which a few bonds are broken and the material then becomes notch sensitive. The flexural strength in specimens which show no visible evidence of attack, as well as in those which do, may be much lower than predicted from the flexural modulus. C a s and Fenner (5) showed that a relation exists between the flexural modulus and the flexural strength of a glas fiber laminate, that if the ariginal strength and modulus were considered to be loo%, any degradation occurring would result in some lower value for each. A definite, repetitive pattern is observed in the performance of the glass fiber laminates when exposed to corrosive environmenu by plotting the residual per cent of flexural strength against the flexural modulus. In order to validate the use of the modulus of elasticity as a measure of
material performance, data are submitted showing this retention of flexural strength of various laminates to the flexural modulus. This is shown in Figures 3, 4, and 5 for specific field exposurea of one epoxy and five different polyester laminates over a three-year period. Additional data for other environments are detailed in the aforementioned reference (5). Studies by Wmger and Toellner (73)disclased a like effect of high temperature on the relation between the flexural strength and modulus of laminates which follows a similar curve. Their data indicated also that such correlation did not follow with the unlaminated molding compound. With po1yester-gla.w fiber laminate exposed to an oxidizing environment, the destruction of the chemical bonds does not inffuence the proportional relation between the flexural strength and modulus. Figure 6 illustrates exposure of the laminate to 10% boiling "01, which brings about almost complete destruction in 37 days.
V O L 5 6 NO. 8
A U G U S T 1964
31
Figure 9. Laboratory apparatus for one-sided exposure, nondestructive testing of G F R resin laminates
Figure 7 shows the effect of dilute cold HNO, on polyester-glass fiber laminate over a two-year period. Here again, in both situations, the flexural strength and modulus are proportional. Frequently it is desirable to measure the modulus of elasticity of a material, inasmuch as this can be a nondestructive procedure. The modulus of elasticity is an extremely valuable tool since it can be measured on a tank while in service, and at the temperature of operation of the tank, thus obtaining information invaluable for the engineering design and predicted performance of glass fiber-reinforced resin laminate structures. Accelerated corrosion tests have been run on laminates in the laboratory, simulating tank operation, and the moduli which are determined are shown to follow those obtained under long-range field tests. Fisher and Gackenbach ( 8 ) have submitted data concerning the time-strength relationship which emphasizes the fact that after the initial decrease in strength there is a leveling off at a relatively constant value. Laboratory Testing
If the laboratory testing of glass fiber-resin laminates is to be of much value, the testing must be carried out in a manner which resembles the actual application. Most testing is of the immersion type; the complete specimen is immersed in the liquid environment and at varying intervals the samples are removed from immersion, cut up, and tested. This type of testing leaves much to be desired in that the results are not readily translated to actual experience or design. In addition, because of their ultimate destruction, a large number of specimens is required. For this reason a system of testing has been employed by us which is felt to give a more precise comparison with performance given by the equipment in the field. The testing apparatus consists of a 4-inch-diameter glass cylinder, 8-10 inches long with one or two ground glass joint openings placed in line perpendicular to the axis of the cylinder. The ends of the glass cylinder are closed off with the resin glass laminate to be tested and 32
INDUSTRIAL AND ENGINEERING
CHEMISTRY
Figure 70. Determination of j e x u r a l modulus of GFR resin structure under normal operations TABLE I .
CHANGES I N MODULUS O F ELASTICITY WITH T l M E
Temg.,
' C.
E. p.s.i.
24 60 70 80 90 100 110
2 . 1 5 X 106 2.07 1.97 1.95 1.92 1, 9 0 1.86
,
I
TABLE I I
1
Date
Experimental E,p.s.i. (90' C.)
1
Corrected E,p.s.i. (24' C.)
1
2-17-63 4-26-63 6-5-63 9-7-63
I
TABLE I l l
1
Date
E,g.s.i. (24' C.)
1
4-1-61 11-1-61 4-11-62 6-5-63
1 . 9 9 X 106 1.60 1.38 1.42
1
,
I
TABLE IV. Type of
~-
Laminate Epoxy Polyester Polyester Polyester Polyester Polyester
A B C D E
TY!p of Lamznate Epoxy Polyester Polyester Polyester Polyester Polyester
A B C D E
Initial
I
Extrapolated E, p .s.i. (90' C.) 1.79 1.44 1.24 1.27
~
BENZYL CHLORIDE-25'
x-z.
C.
Flexural Modulus ( X 106 p.s.i.)
1
I Mo.
9 Mo.
2.21 2.04 1.44 0.67 1.05 0.88 (Delaminated) 1.63 0.97 1.02 0.83
1.96 1.94 1.68 1.84 1.99 1.92
1
Initial __38.3 35.7 32.4 33.7 40.0 36.6
16 Mo.
2.17 0.42 0.58
...
0.57
1
~~
36 Ado.
1
2.06 0.36 10.51
1
0.67 (28mo.) 0.34
Flexural Strength ( X ?O3p.s.i.) 1 Mo.
9 Mo.
45.5 41.5 25.4 8.6 15.7 12.5 (Delaminated) 35.5 14.9 18.8 12.4
16 LWO.
...
7.3
1
36 Mo.
'8.7(28mo.) 5.6
1
TABLE V.
TYp of Lamznate EPOXY Polyester Polyester Polyester Polyester
~
11
32.4 33.7 36.6
2.10 1.80 1.87 1.77 1.71
1
1
EPOXY Polyester A Polyester B
TABLE V I I.
7 Mo.
-I
36 Mo.
2.14 1.86 1.90 1.97 1.77
2.20 1.87 1.85 1.93 1.90
1
2.02 1.70 1.83 1.81 1.93
42.0 33.5 30.8 27.8 33.1
1 ~
1
36 Mo.
9 Mo.
1
15 Mo.
41.5 35.1 36.0 33.4 34.1
39.4 30.8 33.3 31.8 35.4
43.9 31.2 32.9 30.5 35.1
STEAM CONDENSATE AT pH 4 AND 90" C.
~ ~ 1.98 i 1.84
1.96 1 .94 1 .68 1.84 1 .92
A B C D
15 Mo.
Flexural Strength ( X 10ap.s.i.) Initial
Polyester B Polyester C Polyester D
TABLE VI.
9 Ma. ~
1.96 1 .94 1 .68 1.84 1.92
38.3
Lamznate Type Of
1 Mo.
____
EPOXY
EPOXY Polyester Polyester Polyester Polyester
C.
Flexural Modulus ( X lo6$.si.) Initial
A B C D
Type of Laminate
37% FORMALDEHYDE-25'
_ 1.48 2.10 1.08 1.01 1.57
1.35
_ 1.58 0.86 0.95 1.30 1.49
Flexural Strength ( X 703p.s.i.) 0
1
35,7 32.4
7 Ma.
I
70 Ma.
I
77 Mo.
27.3 20.2 20.4 16.6 30.2
23.5
1
36 Mo.
17.0 8.3 10.9 13.0 19.3
18.6 11.1 12.7 13.7 21.8
sealed with suitable gaskets, allowing one-side exposure only to the particular environment. The resin glass laminate is backed up by a suitable flange which serves as a stand and a mount for the testing instrument (Figure 9). The newly created tank with glass side walls and resin glass laminate ends is filled with the environmental liquid required for the specific test on the resin glass laminate. The environmental cell is heated with a heating mantle to maintain the required temperature. With an instrument called the Deflectron, which is shown in place on the right side of the equipment, the modulus of the resin glass laminate can be followed at the temperature of the environment rather than at room temperature. This is accomplished by the simple method of applying a definite load and determining the extent of deflection as indicated on a micrometer gage. This procedure permits the comparison of two different laminate resin systems, with one-side exposures, under identical environmental conditions at the same time. The test panels may be removed on completion of the _ evaluation, and flexural strength and flexural modulus determined under more precise conditions on an Instron or similar tensile testing apparatus. Figure 8 shows the effect of the length of exposure upon the modulus of elasticity of glass fiber-reinforced polyester laminates in which complete immersion and one-side exposure to 90" C. water were evaluated. It is evident that one-side exposure is less deteriorating than complete immersion. This fact must be taken into consideration in predicting the probable life of a tank on complete exposure of coupons within a system in the field. The modulus of elasticity as measured by the deflection can be determined in the following manner ( 7 7) :
NONDESTRUCTIVE DEFLECTION FIELD TESTS
F
=
KPa? t?d
Environment and Equipment
A@lied Load (Pounds)
Start
5 10
0.00125 0.0028
0 I002 0.0038
5 10
0.0015 0.0035
0.0042 0.0080
DeJection, In. 14 Mo.
2000-gal. vertical tank, aromatic acid water slurry, 80° C., p H 3.5 Nitrobenzene sulfonic acid, sulfuric acid, p H 2.0, 65-75" C. 4000-gal. horizontal stg. tank, 85-95" C., phthalate and glycollate ester wash water, p H 2.0-3.5 1000-gal. vertical stg. tank, 85-95' C., phthalate and glycollate ester wash water, pH 2.0-3.5 1500-gal. chlorine absorber, 'I calcium hypochlorite, lime, 25-80' C. 5000-gal. vertical stg. tank, 85-95' C., mother liquors, HCl and aromatic carboxylic acids, p H 2.0 Methyl alcohol, top columr head
5 10
0.001 0.0022
0.0012 0.0024
5 10
0.0015 0.0045
0.004 0.0079
5 10
0.0008 0.003
0.0027 0,0053
5 10
0.002 0.0052
0.004 0.008
5 10
0.0015 0.0035
Residual aromatic sulfonamides, 65-90" C., p H 5.0
5 10
0.0015 0.0035
0.0030 0.0059 (7 months application) 0.0020 0.0041 (7 months application )
where :
E = modulus of elasticity, p.s.i. K = constant P = load, lb. a = radius of exposed section t = thickness of exposed section d = deflection If the modulus is determined by other means at the start, and an initial value for d obtained, the value for K may be calculated. O r simplified, since the modulus is inversely proportional to d, the initial modulus and the initial deflection are the only values required to app!y the above formula to determine other moduli from deflections resulting from applied loads. Temperature is important in the evaluation of the test data because as the temperature increases the modulus decreases. This is verified by the set of experimental figures given in Table I. These experimental figures may serve to extrapolate modulus data between various temperature points as desired for design calculations. vot. 5 6
NO.
a
AUGUST 1964
33
I t is indicated that the decrease in modulus with temperature is not only measurable but, in fact, is quite significant and must be taken into consideration when designing glass fiber-reinforced resin laminate tanks for elevated temperature service. Studies show that there is a general lowering of the modulus of a glass fiber-reinforced resin laminate as it is held in a specific environment at an elevated temperature for an extended period of time. This change may be slight under noncorrosive conditions but will be quite pronounced in aggressive environments. The data in Table I1 show the results of testing by one-surface exposure over an extended period of time, on HBPA polyester laminate containing 65% glass cloth in distilled water at 90” C. In contrast, the data in Table I11 show the results obtained by the conventional immersion testing in steam condensate water at 90” C. over a period much longer than that tested in Table 11. I t should be noted that significant errors may be introduced whenever the modulus figures are used without taking the temperature of operation into consideration. The results of this three-year period disclose that the modulus us. time curve is exhibiting a flattening out similar to that indicated to occur in prolonged studies of the strength-time relationship.
Field Testing
T o further determine any correlation over extended periods of time between the changes in flexural modulus and flexural strength of glass fiber-reinforced resins, the following investigation has been followed for the past few years. The laminates tested were prepared from 10 layers of Garon finish glass cloth and contained 65% glass. The laminates were prepared by a low pressure process. The mold was vented to the atmosphere, and pressure only to close the mold to the stops was employed. The resin was cured in the mold at 100’ C., until gelation, followed by a 1-hour cure at 125” C. in an oven. The exposed laminates consisted of pieces 5 inches wide by 10 inches long by l / 8 inch thick. These samples were suspended in groups, each separated from the other by small gaskets, for adequate circulation of the environment over the entire surfaces, in process equipment covering eight highly corrosive environments and subjected to normal operating conditions. Each test station was blanketed with five different polyester resins and an epoxy laminate. The laminates were removed and sections cut off for testing after various periods of time up to a three-year exposure. The bottom ‘ 1 4 inch was sawed off with an abrasive wheel and the next inch cut off for testing. The laminate was then returned to the environment for additional exposure. The flexural strength and modulus were both determined on the Instron tensile tester. The results of a few cases in this study are shown in Tables IV, V, and VI, where various polyesters were 34
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
exposed to benzyl chloride, 37y0 formaldehyde, and acidic steam condensate for 3 years in the field. Coincidentally with this investigation which required the destruction of a portion of the glass fiber-resin laminate to obtain the necessary data on the Instron instrument, flexural modulus evaluations were obtained upon certain wall sections of glass fiber-reinforced resin equipment by periodic nondestructive field measurements with the portable unit (Figure 10). Data covering the results of these checks on seven glass fiber-reinforced structures are shown in Table VII. Each set of figures shows a definite increase in deflection under the identical applied load over the period of the investigation indicating a continuing degradation of the resin laminate wall to date.
Conclusions
Laboratory and field data show the close correlation between the changes in flexural strength and flexural modulus with those glass fiber-reinforced resin laminates which are experiencing deterioration. Procedures, techniques, and equipment are available whereby the changes in flexural modulus of a glass fiber reinforced resin may be monitored and evaluated under actual operations in the field or laboratory by nondestructive means. The modulus data may be correlated with flexural strength data and employed in the engineering design of glass fiber-reinforced resin structures to obtain the desired performance. REF ER ENCES (1) Adams, W. H., Lebach, H. H., “New corrosion test for plastics,” Chem. ERE. 5 6 , No. 7 (1959). ( 2 ) Arndt, F. W., “Applying reinforced plastics in corrosive environments of the process industry,” Corrosion 16, No. 11 (1960). (3) Atkinron, H. E., “Reinforced plastics for chemical process equipment,” ASME paper 61-WA-267 (1961). (4) Cass, R . A , Fenner, 0. H., “.4ccelerated test procedure for evaluation of fiber reinforced resin equipment in the chemical industry,” Corrosion 17, No. 1 (1961). (5) Cass, R . A , , Fenner, 0. H., “Effect of chemical formulation of GFR resin laminates upon performance in corrosive environments,” 18th Annual Conf. Nat. Assoc. Corrosion Engrs., Kansas City, Mo., 1962. (6) Cass, R. A , , Raether, L. O., “Evaluation of new esters of phosphorus for flameproofing polymethyl methacrylate,” ACS Division of Organic Coatings & Plastics, Los Angeles, March 1963. (7) Feuer, S. S., Torres, A. F., “Comparative study of the corrosion resistance of a bisphenol-a polyester resin, a general purpose polyester resin, and an isophthalic polyester resin, 15th Annual Meeting, Reinforced Plastics Div., SOC. Plastics Ind., Chicago, Ill., 1960. (8) Fisher, J. J., Gackenbach, R . E., “Reinforced plastics in the chemical industry,” 15th Annual Meeting of Reinforced Plastics Div., SOC.Plastics Ind., Chicago, Ill. (9) Gackenbach, R . E., “Reinforced polyesters enjoy continued success” 18th Annual Conf. Nat. Assoc. of Corrosion Engrs., Kansas City, Mo., 1962. (10) M a t t 2 Ji F.,, utwater, 0. J., “Nature, origin and effectsofinternalstresses in reinforce p astic aminates,” SOC.Plastics Engrs. Trans., October 1962. (11) Roark, R . J., “Formulas for Stress and Strain,” 3rd ed., McGraw Hill, N. Y . , 1954. (12) VanDelinder L. S “Evaluation of rigid plastics for chemical service,” 17th Annual CoAf., Kay. Assoc. Corrosion Engrs., Buffalo, N. Y . , 1961. (13) Salinger and Toellner, “New high strength molding compound,” 13th Annual Tech. Conf., Soc. Plastics Ind., 1958.
9
This i s the 6th in a series of articles on plastics in corrosion control, based on a symposium presented by the ACS Division of Organic Coatings and Plastic Chemistry at the N e w York National Meeting, September 1963. The entire series will be available in reprints.
