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Effect of Long Term Humid Aging on Plastics


Effect of Long Term Humid Aging on Plastics
JOHN R. MARTIN and ROBERT J. GARDNER

The Foxboro Company Foxboro, Massachusetts 02035
The warm humidity resistance of more than thirty plastic formulations has been evaluated in a program which aged specimens for periods ranging from 10 months to 3 years. Both thermoset and thermoplastic products were included in this study. All of these materials can be used in high humidity at moderate temperatures for short periods of time except where water absorption lowers the glass transition temperature enough to cause distortion. Several of these commercial products exhibit good long term resistance to humidity at temperatures approaching 100°C.However, some are degraded by long term exposure to warm humidity. This hydrolysis effect can result in useful lifetimes which are considerably shorter than would be expected from projections which are based on conventional dry oven aging tests. This broad data base provides a basis for selecting plastics for use in warm, humid environments.

INTRODUCTION
sage of plastic components in industrial instruments U h a s increased substantially during the last 5-10 years. Many of the early applications were in areas that were exposed to low mechanical and environmental stresses. However, recent applications have often been much more demanding. Questions regarding material selection arise whenever a product is introduced or redesigned. Unfortunately, many capital equipment components are fabricated from metal or occasionally from expensive plastics simply because the long term durability of most plastics in harsh industrial environments is unknown. It is impractical to simulate every industrial condition in laboratory tests. However, experience shows that warm, humid environments are encountered in most of the process industries. In order to establish performance levels and limits for a broad range of plastics, 32 commercial products were aged under dry and humid conditions for periods which extended from 10 months to 3 years. The present paper summarizes the effects observed on all of the materials included in the program. In a few areas, the work was extended to provide a more fundamental understanding of the degradation processes or experimental verification of relative humidity effects. These detailed portions of the program are either still in progress or have been previously reported (1, 2). There have been a number of studies which address water absorption and swelling mechanisms in polymers (3-9). Other investigators have attempted to relate mechanical properties or transition temperatures to either water immersion or humidity exposure. These latter studies have centered on epoxy composites (10-17) and urethanes (18-22) although scattered results have
POLYMER ENGINEERING AND SCIENCE, JUNE, 1981, Vol. 21, No. 9

been reported on some thermoplastics (23-30). After reviewing the literature, it was apparent that long term performance guidelines d o not exist for many plastics in humid environments. Where results have been published, the data are often limited to one temperature or to weight/volume measurements. In principle, one would expect water immersion test results to correlate with measurements made at 100percent relative humidity (RH). This is indeed often observed but discrepancies d o exist (21).

TEST PROCEDURE
The thermoplastics were dried according to the manufacturers’ recommendations. Unless otherwise indicated, standard ASTM tensile bars were injection molded on a 30 ton Newbury machine. These specimens were aged at 100 percent relative humidity by suspending them over distilled water in sealed Pyrex jars at 66, 82 and 93°C (150, 180 and 200°F). To identify humidity effects, control samples at 0 percent RH were suspended over Type 13X molecular sieves in sealed jars and placed in the same ovens. As noted i n the text, afew of the materials were examined at intermediate humidity levels or at additional test temperatures. Samples were removed periodically from the jars, left at room temperature for at least 24 h, then tested for tensile properties according to ASTM D-638 at 0.5 cm/min (0.2 in./min). Variations in dimensions, hardness and color were also noted when appropriate. Structural foam materials evaluated in this study were cutfrom 0.6 cm (0.25 in.) thick plates that were supplied by the manufacturer. The SMC specimens were machined either from compression molded sheets or from a flat portion of instrument cases that had been compression molded on pro557

John R. Martin and Robert J . Gardner

duction tooling. The phenolic and DAP samples were compression molded on a 50-ton PHI press. Exposure conditions and test procedures were identical to those used on the thermoplastics.

THERMOPLASTICS Acrylonitrile-Butadiene-Styrene (ABS) Embrittlement of ABS after oven aging is well documented. It is apparently caused by morphology changes in the glass phase as well as oxidation of surface layers of the butadiene phase (31, 32). However, plasticization due to water absorption apparently offsets the thermal annealing effect in the one grade of ABS (Cycolac KJB, Borg Warner) which was included in this test series. After 18 months at 60°C and 100 percent RH the tensile and yield strengths were unchanged. The dry samples showed typical oven aging effects as tensile and yield strengths gradually increased by 10and 30 percent respectively over the first 10 mos. Beyond this point the tensile strength of the dry specimens dropped as shown in Fig. 1.Figure Ib suggests that the plasticizing effect of absorbed water offsets the decrease in elongation which is customarily observed in dry oven aging. Although the moisture effect can be attributed to plasticization, the actual volume change was small: length, width and thickness of these tensile bars changed by -0.4,0, and +2 percent respectively. Modulus change was also small (-13 percent). This material distorts within a month when tested at 82°C and 100 percent RH, so the higher temperature tests were terminated early.

Acetal Unreinforced grades of both acetal homopolymer (Delrin 500, DuPont) and acetal copolymer (Celcon M-90,Celanese) were evaluated in this program. As expected, both materials were undected by the dry aging conditions. Neither showed substantial degradation at 100percent RH when tested at 66°C. Experimental results on these two products are included in Figs. 2 and 3 . The data in Figs. 2 and 3 illustrate the difficulty that one encounters in attempting to project long term performance from short term tests. Consider the fact that the homopolymer showed only a 6 percent decrease in tensile strength after one month of humid aging at 99°C. Nineteen weeks at 82°C produced only a 3 percent drop. Yet this same material was untestable when aged for 10 mos at 82°C and 100 percent RH. Similar effects are also evident in the copolymer where tensile strength was unchanged after 10 mos of exposure at 82°C and 100 percent RH. After 18 mos of exposure at these conditions, the material had lost half of its original'strength. Although there are several possible explanations for this delayed response, definitive answers will require GPC analysis of hydrolyzed acetal resins. The 100 percent RH test jars which contained the acetal specimens had a strong odor offormaldehyde each time they were opened. This suggests that acetal resins exposed to hot humidity degrade by an unzipping mechanism at the
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Fig. 2. Effect of aging conditions vn tensile properties of an acetal homopolymer.
POLYMER ENGINEERING AND SCIENCE, JUNE, 1981, Vol. 21. No. 9

Effect of Long Term Humid Aging on Plastics

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tially in humidity resistance. Measurements on Amidel are included in Fig. 4 . Tensile strength increased due to an annealing effect at the higher test temperatures. Even after 10 mos at 93°C and 100 percent RH this product was as strong as the original molding. Changes are occurring, however, because we see that the elongation of the 93°C samples has decreased from 170 percent down to 12 percent after 4 mos. The decrease is due to both a thermal annealing effect and degradation because a similar change of lesser magnitude also occurs in specimens that are aged at 0 percent RH. The presence of humidity apparently accentuates these structural changes. For example, the 0.2 percent yield strength increases 21 percent after 4 mos of aging at 93°C. Dry specimens at the same temperature exhibit an increase of only 13 percent even after 10 mos. By contrast, Trogamid T was rather sensitive to humid aging. At 66"C, elongation dropped from 145 percent to 7.8 percent after 10 mos. Tensile strength and yield strength increased 14 and 44 percent respectively. Plasticization by moisture apparently enhances morphological changes because these increases in tensile strength and yield strength were less noticeable in samples aged at 0 percent RH. At 82"C, moisture plasticization lowered the glass transition temperature enough to allow deorientation of the Trogamid samples. At 93°C and 100 percent RH the material turned to a taffy-like product that could not support its own weight during the first few weeks of testing. Chlorinated Polyvinyl Chloride (CPVC) The CPVC specimens used in this test were machined from extruded 1/8 in. sheet stock. As expected, this material was relatively insensitive to the high humidity

chain ends. Commercial acetals are end capped so the rate of unzipping, and hence the rate of property change, may be quite low until after random chain scission creates uncapped ends. Actually, the unzipping mechanism, by itself, is sufficient to explain the delayed response of mechanical properties to hydrolytic degradation. Previous work on many polymers has shown a strong correlation between average molecular weight and tensile strength (1, 33). At h g h values of the weight average molecular weight ( M w ) , changes in t h e molecular weight of a product do not influence tensile strength. However, once the Mw drops below a certain minimum level min) rapid reductions in tensile strength are observed. In the present case, one would presume that both of these resins had molecular weights above this minimum. Thus, unzipping of the chain ends would cause no substantial loss of strength until the u n z i p i n g progressed to a point where the M,fell below the M,, min. The concept of an min has also been associated with elongation measurements. There is some evidence to suggest that the minimum values based on either elongation or impact strength are higher than those observed in tensile strength correlations (1, 33). The early drop in elongation (Figs.2b and3b)for both of these resins is consistent with this evidence.

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Amorphous Polyamides Two types of unfilled "amorphous nylons" were evaluated: Amidel (Union Carbide Corp.) and Trogamid T (Dynamit Nobel). These two products differ substanPOLYMER ENGINEERING AND SCIENCE, JUNE, 1981, VoI. 21, No. 9

Fig. 4 . Effect of uging conditions on tensile properties (if an amorphous polyamide.
559

John R . Martin and Robert]. Gardner
environment. Tensile strength decreased by less than 10 percent and yield strength showed virtually no change after 10 mos at 93°C and 100 percent RH. Dry annealing at 93°C gradually increased the tensile modulus from 330,000 psi to 440,000 psi after 10 mos. This effect was also present in the initial stages of the humidity tests. However, after going through a maximum of 350,000400,000 psi at 1-4 mos, the tensile modulus of these samples decreased to 330,000-270,000 psi for the 9366°C temperature span after 10 mos. Although the elongation of specimens aged at 93°C in humidity decreased from 64 to 17 percent after 10 mos, most of this change can be attributed to thermal effects (at 0 percent RH, elongation drops to 24 percent). When the temperature is reduced to 66°C in the humidity test, only minor changes in elongation are observed (decrease from 64 to 54 percent after 10 mos). Modified Polyphenylene Oxide An unreinforced grade (Noryl SE-I), a 30 percent glass fiber reinforced grade (Noryl SE-1 GFN3) and a structural foam (Noryl FN215) were examined in this study. These General Electric products are flame retardant compositions. Experimental results for the reinforced version are included in Fig. 5 along with comparative data from the literature on a non-flame retardant product which had been immersed in water at 82°C (27). Test results on Noryl SE-1 were included in an earlier report (26). Examination of the data generated on
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these products indicates that modified PPO is quite resistant to high humidity exposure. After 18 mos at 93°C and 100 percent RH the glass reinforced grade still retains 3/4 of its initial tensile strength and one half of its initial elongation. Property reductions can be largely attributed to oxidative attack and/or morphological changes rather than hydrolysis. Nylon Three grades of nylon were evaluated. Two of the formulations were based on nylon 66 (Zytel 101-L and Zytel70G-33 HR-L). The first of these DuPont products is a standard unreinforced nylon 66. The second one is reinforced with 33 percent glass fibers. It also includes stabilizers for added heat and hydrolysis resistance. The third nylon included in this study is a 30 percent glass reinforced nylon 12 (Thermocomp SF-1006, LNP). As shown in Fig. 6 , unreinforced nylon 66 is not suitable for long term exposure to 100 percent RH at temperatures in the neighborhood of 66°C or above. Addition of fiberglass and stabilizers substantially improves the performance of nylon 66 in humid environments (Fig. 7 ) . Although half of the strength of this product is lost after 2 mos in humidity at 66"C, useful properties are retained even after 18 mos in this environment. At this point, tensile modulus was reduced to 47 percent of the initial value. Notice that the additive package in the glass reinforced product also enhanced thermal and oxidative stability. For example, the strength of unreinforced nylon 66 was substantially reduced by long term aging at 93°C even in 0 percent
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POLYMER ENGINEERING AND SCIENCE, JUNE, 1981, Vol. 21, No. 9

Effect of Long Term Humid Aging on Plastics
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Fig. 8 . Effect of ugingconditions on tensile properties of u glass reinforced nylon 12.

humidity. No such effect is observed in the stabilized glass reinforced product. Humidity resistance is substantially improved when glass reinforced nylon 12 is used in place of nylon 66. As shown in Fig. 8, nylon 12 loses about 113 of its strength immediately due to water absorption. However, further decreases at 66°C are quite small. Even at 93"C, glass reinforced nylon 12 would still be serviceable in most applications after 10 mos at 100 percent RH. The stiffness of nylon 12 shows a rather peculiar behavior in humidity environments. During the initial exposure, stiffness decreases due to water plasticization by absorbed water. After that initial drop, modulus gradually increases in a manner similar to that observed in specimens which were aged at low humidity. This modulus increase is insignificant at 66°C. Polycarbonate Results on both unreinforced Lexan 141 (General Electric) and 10 percent glass reinforced Lexan 500 were reported previously (26). This data, along with an analogous series of measurements on a polycarbonate structural foam (Lexan FL900) indicates that polycarbonate is sensitive to hot moisture. A more extensive analysis of Lexan 141 correlates the mechanical property changes with a reduction in molecular weight (1). In summary, these 18 mo tests indicated that a ductilePOLYMER ENGINEERING AND SCIENCE, JUNE, 1981, Vol. 21, No. 9

brittle transition will occur at low strain rates after 5 years exposure to 38°C (100°F) and 100 percent RH. Hydrolytic degradation results in a loss of toughness before the strength properties are affected. The rate of hydrolysis is reduced by a factor of 2 when relative humidity is decreased from 100 to 75 percent at 82 and

93°C.
Thermoplastic Polyesters Eighteen month tests conducted on 5 injection molding grades of poly(buty1ene terephthalate), or PBT, indicated that these products are quite sensitive to warm humidity, but unaffected by low humidity oven aging at 66-93°C (26). Follow-up tests on a low internal stress PBT structural foam (Valox FV-600, General Electric) confirmed the fact that the degradation was not substantially accelerated by molding stresses. Hydrolytic degradation of polyesters occurs because chain scission at the ester linkage results in a progressive reduction in molecular weight (23, 28-30). The initial tests showed excellent performance at 0 percent RH, but limited life at 100 percent RH, so 3 grades of PBT were exposed to relative humidities of 11-100 percent in a 3 year test program. These products were an unreinforced PBT (Tenite GPROA, Eastman), a flame retardant unreinforced PBT (Valox 310-SEO, General Electric) and a 30 percent glass reinforced flame retardant PBT (Valox 420-SEO). Details of this program are reported elsewhere (2). In summary, the rate of hydrolysis was found to increase with increasing relative humidity.
56 1

John R . Martin and Robert J . Gardner Equations were developed for predicting the tensile strength half life at any combination of temperature and humidity. For example, at 50°C and 100percent RH the three grades of PBT examined will lose half of their tensile strength after 3-4 years. Decreasing the humidity level to 50 percent increases the half-life to more than 10 years. It should be emphasized that these lifetimes pertain only to non-impact applications because PBT products became brittle long before their tensile halflife was reached. More conservative designs might be based upon an impact test or upon the time required for PBT to lose 25 percent of its tensile strength, i.e., the quarter-life. An Arrhenius plot showing the quarter-life of an unreinforced PBT as a function of temperature and humidity is included in Fig. 9. The combined effects of temperature and humidity shown in this figure can be expressed as: In tIl4= 11'940 - 1.38 In (R) - 29.95 T
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where tl,* = quarter-life, days; T = temperature, O K ; and R = percent relative humidity/lOO. The tensile strength quarter-life of this PBT (3 years at 50"C, 100 percent RH) is only slightly shorter than its half-life because strength decrease was puite rapid after the molecular weight dropped below M,, min. Slightly different behavior was observed in other PBT products as discussed in Ref. (2) and in a report which is now in preparation. It should be noted that designs based on
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tensile strength quarter-life are also not suitable for impact applications because failures at this point were all brittle in nature. The only other thermoplastic polyester evaluated in this program was an aromatic copolyester (Ekkcel 1-2000, formerly produced by Carborundum). Tensile specimens were molded by the supplier and tested as received. Humidity resistance of these expensive high temperature polyesters was significantly better than PBT. After 4 mos at 93°C and 100 percent RH, Ekkcel still retained 1/3of its initial elongation (4.3 percent) and 1/2 of its initial tensile strength (12,100 psi). The test was discontinued at this point because the humidity aged tensile samples became embrittled. This caused the specimens to break in the grips rather than in the test section when they were pulled on the Instron Tensile Tester. Polyphenylene Sulfide (PPS) Ryton R4 (Phillips Petroleum), a 40 percent glass reinforced PPS, demonstrated a high degree of resistance to warm humidity. After 18 mos at 93°C and 100 percent RH this product retained 77 percent of its tensile strength, 62 percent of its elongation, and 78 percent of its modulus. Some of the test curves are included in Ref. (26). Dimensional stability was also outstanding. During the exposure period noted above, the length of the Ryton R4 specimens decreased by 0.07 to 0.04 percent as the exposure progressed. At 66 and 82°C length changes were -0.01 and -0.025 percent respectively. Polypropylene
As shown in Fig. 10, the long term influence of humid-

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ity on 30 percent glass reinforced polypropylene (Thermocoinp MFX-l006HS, LNP) is rather small. After initial drops in both tensile strength and elongation, the product stabilizes and no further change occurs. Modulus increased by 5 percent after 10 mos for both the humid and the dry specimens. Polysulfone Humid aging appears to accelerate an annealing effect that occurs in polysulfone (Udel, P-1700, Union Carbide Corp.). For example, the tensile strength of polysulfone increased by 6-16 percent after 18 mos in both the dry and the humidity tests. This effect appeared to occur faster in the humidity aged samples. It was also more pronounced at the higher temperatures. Examination of the elongation data indicates that the humidity aged samples dropped from 90 percent elongation to 6-7 percent during the first few months. No further change occurred after that time. The dry specimens which were aged at 93°C alsodropped to this level, but only after 18 mos. Lesser changes occurred in the dry samples exposed at lower temperatures, as shown in the test curves included in Ref. (26). Polysulfone also demonstrated excellent dimensional stability. The increase in length (0.05 percent) which was measured after one month at 93°C at 100 percent RH was unchanged during the remainder of the test. Length increases of 0.02 percent were measured at the lower test temperatures. Length changes in the dry samples were typically one half as great as those measured at 100 percent RH.
POLYMER ENGINEERING AND SCIENCE, JUNE, 1981, Vol. 21, No. 9

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Effect of Long Term Humid Aging on Plastics
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Fig. 10. Effect of aging conditions on tensile properties of a glass reinforced polypropylene.

Styrene Acrylonitrile (SAN) When tested at 66"C, the humid and dry samples of S A N were virtually indistinguishable after 10 mos. In both cases the 0.2 percent yield point increased by 15 percent and elongation increased from 3.7 to 4.2percent. This material deformed during the first few weeks in humidity at 82°C and in both the wet and dry environments at 93°C. THERMOSET PLASTICS Diallyl Phthalate (DAP) Dry aging of glass reinforced DAP (RX1366, Roger Corp.) evidently results in substantial increases in tensile strength as indicated in Fig. 11. This is presumably the result of stress relieving rather than increased crosslinking because modulus changes were less than 10 percent and elongation at break actually doubled (0.7 to 1.3 percent) after 10 mos at 93°C. Similar changes of lower magnitude also occurred in specimens aged in 100 percent RH. A small decrease in modulus (-10 percent) occurred after 1 mo at 93°C in the humidity test. This value was unchanged during the remainder of the test.
POLYMER ENGINEERING AND SCIENCE, JUNE, 1981, Vol. 21, No. 9

Phenolic The initial increase in strength of a glass reinforced phenolic (RX611, Rogers Corp.) which was aged under dry conditions was reversed after the first few months. Except for this short term effect, the 0 and 100 percent RH test results are quite similar (Fig. 12). Little change occurred in either the strength or the elongation of these phenolic specimens after 18 mos of exposure. The specimens aged in humidity did show signs of plasticization since modulus decreased somewhat at 100 percent RH (maximum change was 25 percent after 10 mos at 93°C). . Polyester Sheet Molding Compounds (SMC) Four SMC formulations were evaluated. Since there are many variables which can be altered in an SMC formulation, it is difficult for the end user to identify the cause of outstanding or substandard performance. Figures 13 and 14 illustrate the experimental results on Vibrin-Mat 5304L and R1637 (Marco Chem.), respectively. These specimens were machined from instrument cases that had been exposed to 0 and 100 percent RH. Since the specimens were cut from molded shapes rather than test plaques, the actual test data should not be compared with standard data sheet values. The product represented in Fig. 13 is based on an orthophthalic
563

John R. Martin and Robert J . Gardner
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Fig. 13. Effect of aging conditions on tensile properties of a sheet molding compound bawd on un orthophthalic polyester.

polyester resin reinforced with fiberglass and calcium carbonate. It was untestable after 10 mos in humidity at 93°C and significantly deteriorated at 82°C. The second product from the same manufacturer is based on an isophthalic polyester resin. I t is reinforced with fiberglass and contains substantial amounts of hydrated alumina as a flame retardant. After 10 mos, relatively little degradation of this product occurred at either 0 or 100 percent RH for temperatures up to 93°C. It is not clear whether the improvement is d u e to changes in the filler, the resin, or other components. In the orthophthalic formulation, humidity dissolved one of the components in the compound and precipitated it as a white crystalline material on the surface. No real attempt was made to identify this product, but it appeared to befrom-a clay-like constituent. Towards the end of the aging period, a few viscous amber droplets formed on the surfaces of the isophthalic samples. This material may be hydrolyzed polyester, but once again, n o analysis was attempted. Test results on a flame retardant SMC based upon a bisphenol A polyester resin (Premi-Glas, 4202-22, Premix) were similar to the isophthalic based product cited above. A British SMC was also evaluated. This product had glass fibers which were longer and slightly larger in diameter than those used in the other 3 SMC formulations. As a result, moldings are unusually tough. Macroscopically, the material yields because the long glass fibers can support and bridge local cracks, crazes and
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POLYMER ENGINEERING AND SCIENCE, JUNE, 1981, Vol. 21. No. 9

Effect o Long T e r n Humid Aging on Plastics f
resin debond areas. Properties after aging, however, were inferior to both Vibrin-Mat R1637 and Premix 4202-22. Failure mode appeared to be degradation of the resin with delamination occurring between the layers of glass. Surface blistering also occurred. In comparison, the weakest link in Vibrin-Mat R1637 was the bond at the glass-resin interface. Resin degradation was much less apparent.

REFERENCES
1 R. J. Gardner and J. R. Martin,J. Appl. Polym. Sci., 24, 1269 .

SUMMARY From the results of this study we can conclude the
following: All of the plastics included in this study can be safely used in high humidity and moderate temperatures for short periods of time except where water absorption lowers the glass transition temperature enough to cause distortion. There are a number of plastics which exhibit good to outstanding long term resistance to humidity at temperatures approaching 100°C. Short term tests currently in use d o not adequately predict long term performance of plastics in warm humidity. In several instances, materials which showed little change in tensile strength after a month were severely degraded within a year under the same conditions. Accelerated aging is normally conducted in dry ovens. This type of test environment does not simulate the degradation mechanisms which lead to failure in moisture sensitive plastics. As a result, projected “lifetimes” based on such tests will be misleading for any application where moisture is present. Water absorption is not a good indicator of long term durability in moist environments. Several of the products evaluated are known to exhibit low water absorption, yet these same products were among those most sensitive to mechanical property degradation after humidity exposure. Modulus, yield strength, hardness, and tensile strength were among the last properties to change in those materials that were hydrolyzed by moisture. Test specimens became embrittled long before major changes in these parameters occurred. Thus elongation, or perhaps impact strength, should be monitored in instances where hydrolysis resistance is important. Lifetime of a hydrolysis sensitive plastic increases substantially as humidity is reduced. Thus reduction of relative humidity from 100 to 75 percent will typically double life expectancy.

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POLYMER ENGINEERING AND SCIENCE, JUNE, 1981, Vol. 21, No. 9

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