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sistance data of Figures 10-13. While a trend is not apparent from the data in Table VI, the scatter in Figure 14 indicates a random deviation of perhaps 0.3% volatile. The linear coefficient of expansion as a function of maleic anhydride content is shown in Figure 15. These values did not appear dependent on molecular weight. It is interesting to note that the coefficient of expansion decreases about 20% while going from 0% to 33% MA. This could be of importance in applications where plastics and metals or glass are used for separate parts in an assembly and thermal cycling is involved. Some advantages in fiberglass modified composites would also be anticipated. Since the fibers and polymer are heated during processing, the subsequent cooling and shrinking of the composite creates undesirable “prestressing” of the interface. Reducing the relative shrinkage should produce a stronger composite structure.
Summary A series of copolymers with six levels of maleic anhydride were prepared (each at three characteristic solution viscosity levels) continuously in well-stirred reactors. These polymers provide a “grid” which allowed both the effect of molecular weight and MA content on physical properties to be determined. This paper reports molecular weight data, high-shear melt viscosity, thermal stability, heat distortion temper-
atures (Vicat and DTUL, Tg)and linear coefficient of expansion on these polymers.
Acknowledgment
I extend my thanks to L. C. Chamberland and A. W. Hanson for their generous support and encouragement; to W. E. O’Connor, H. Mashue, R. Carlson, L. Ciezeck, R. Owens, R. Salisbury, M. Fryer, V. Cook, and C. Pawloski for synthesis assistance; to L. E. Smith I11 and T. R. Wayt for characterization; to W. Charlesworth for melt viscosity and physical properties; to E. T. Wagoner for thermal gravimetric analysis, differential thermal analysis, glasstransition temperature, and gel permeation chromatography; to C. Boyd for high-speed osmometry measurement of M,; and to K. Dennis and W. Alexander for solution viscosity MA determination and other helpful assistance. Registry No. (styrene) (maleic anhydride) (copolymer), 9011-13-6; polystyrene, 9003-53-6. Literature Cited Hanson. A. W.; Zimmerman, R. L. Ind. Eng. Chem. 1057,49,1803-1806. Karam, H. J.; Cleereman, K. J.; Williams, J. L. Mod. Plast. 1055,32(7),129. Moore, E. R. “Reactivity Ratios from Pilot Plant Data”; part of a symposium of polymerization methods and kinetics presented at the National AIChE meeting, Boston, Sept 1975. Zimmerman, R. L.; O’Connor, W. E. US. Patent 3336267,Aug 15, 1967.
Received for review May 24, 1985 Accepted December 9, 1985
Water Immersion of Polysulfide Sealants. 1. Effect of Temperature on Swell and Adhesion Peter J. Hanhela, Robert H. E. Huang, and D. Brenton Paul* Department of Defence, Defence Science and Technology Organlsation, Materiels Research Laboratorles, Ascot Vale, Victoria 3032, Australia
The suitability of polysulfide sealants for appllcations involving contact with hot water has been assessed through
changes in swell, permeability, peel strength, and lap shear strength that result from immersion at 25-90 O C . Peel and lap shear strengths decline with prolonged contact times, but this is ascribed to swelling rather than thermal effects. A dichromate-cured sealant (PR-1422) gave a standard saturation time-swell curve, whereas with manganese dioxide cured materials (PR-1750, Pro-Seal 899) a linear relationship existed over wide time (>100 days) and swell (160%) ranges. The generality of this behavior was established by examination of laboratory sealants prepared from various combinations of curing agents and polysulfide prepolymers. Permeability studies confirmed that continual swelling of manganese dioxide cured sealants leads to a volume swell level, between 80 and 120%, where water transmission occurs freely and sealing fails. Dichromate-cured materials were consequently more effective sealants in hot water than those cured with manganese dioxide.
Introduction In applications involving water contact, such as the sealing of joints in reservoirs, polysulfide sealants are required to maintain a sealing function for many years. Studies of the effect of long-term water immersion on properties of polysulfide sealants have, however, been virtually restricted to evaluation of adhesive performance at ambient temperature (Karpati, 1980; Lee, 1982). Situations in which sealants are required to function in hot water are not common, but one example concerns the operation of the environmental control system in some
combat aircraft. This involves the use of a water-filled integral fuselage tank to cool the heat exchange unit, and during flight the water can attain temperatures approaching 100 OC. Although sealing of such tanks has been attempted using polysulfides approved for fuel tank use, leakage has occurred more frequently than from the fuel cells. Polysulfide sealanta that are available for sealing aircraft fuel tanks generally meet the requirements of either MIL-S-83430 (high-temperature resistant sealants) or MIL-S-8802,for which the thermal resistance criteria are
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less stringent. Scant information concerning the effect of immersion in hot water on the performance of these polysulfide sealants is available. It has been reported that immersion of three typical Thiokol polysulfide sealants in water at 27 "C for 30 days produces volume swell of between 2 and 4% (Panek, 1962). Technical data sheets also indicate that exposure of some commercial manganese dioxide cured polysulfide sealants to water vapor at elevated temperatures has little effect on performance: the hardness of Pro-Seal 899 and PR-1750 are suggested to undergo only a marginal decline when maintained respectively at 93 "C and 95% RH for 28 days (Coast ProSeal, 1971) or 71 "C and 95% RH for 120 days (Products Research and Chemical Corp., 1971). In these laboratories, however, it has been found that long exposure times at 90 "C and 100% RH cause a significant softening of polysulfide sealants, with the dichromate-cured PR-1422 showing the greatest resistance and PR-1750 and Pro-Seal 899 each losing about 70% hardness over 60 days (Hanhela and Paul, 1984). Volume swell results showed a similar trend with increases of 23%, 42%, and 48% being recorded for PR-1422, Pro-Seal 899, and PR-1750, respectively, over the 60-day exposure period. Recently the effect of hot water on one manganese dioxide cured polysulfide sealant was reported (Usmani et al., 1981). At 60 "C it was noted that the weight increase was linear with immersion time over the first 20 days, and it was considered that chemical relaxation could promote the high water gain which was observed. Preliminary experiments with commercial polysulfide sealants in boiling water indicated that marked increases in swell, accompanied by decreases in hardness, could be expected. Such behavior would result in a reduction in cohesive strength, greater permeability, and impairment of adhesion at the sealant-substrate interface. In practice these consequences may be significant in relation to sealing performance since vibration during flight will create shearing forces in the sealants which, if acting on a weakened matrix, could result in adhesion failure. In an endeavor to identify a suitable sealant for hot-water applications, the relative resistance of some polysulfide aircraft sealants has been studied in detail with emphasis on the dependence of volume swell and tear strength on water temperature. This approach was extended to uncured and partially cured sealants in order to assess the penalties should an aircraft be flown before the lengthy cure period had elapsed. In this report the different behavior of manganese dioxide and dichromate cured sealants when immersed in hot water is identified; a mechanistic rationalization of this observation is to be presented subsequently (Hanhela et al, 1985).
Experimental Section General. A Perkin-Elmer 580B double-beam, ratiorecording infrared spectrometer was used to determine thiol contents of the polysulfide polymers. Hardness of cured sealants was monitored with a Rex indentor Model 1700, Type A. Peel and lap shear tests were carried out by using an Instron Model 1026 testing machine. Materials. (a) Commercial Sealants. The following two-part manganese dioxide cured polysulfide sealants, qualified to MIL-S-83430, were examined: PR-1750 B-2 and B-6 (Products Research Corp.) and Pro-Seal899 B-2 and B-6 (Essex Chemical Corp., Coast Pro-Seal Division). These sealants were produced through interaction of a polysulfide liquid polymer filled with calcium carbonate and a cure paste of manganese dioxide, carbon black, and at least one activator in a hydrogenated terphenyl oil. The MIL-S-8802 sealant (PR-1422 B-2 (PRC)), which com-
prised calcium carbonate filled polymer cured with a clay-filled ammonium dichromate-dimethylacetamidewater system, was also evaluated. All sealants were mixed according to manufacturer's recommendations in a Semco pressure mixer, Model S-1350, and were cured with a cycle of 24 h at 25 "C followed by 3 h at 50 "C and a further period at 70 "C until constant hardness was attained. (b) Laboratory-Prepared Sealants. Sealants were prepared from various prepolymers and cure pastes to allow comparative assessments of volume swell in water. Liquid polysulfides used were unfilled LP-2 and LP-32 (Thiokol, average MW 4000, 2% and 0.5% cross-linking agent, respectively) together with the PR-1750 B-2 and PR-1422 B-2 prepolymers, both in the normal filled state and without fillers. Sealants prepared by using liquid polymers from which fillers were removed have been designated as "unfilled". Such prepolymers were obtained by suspending the filled polysulfide in chloroform and repeatedly centrifuging the mixture until no further solids settled out. The curing agents were (1)the pastes from PR-1422 B-2 and PR-1750 B-2, (2) a reagent prepared by stirring ammonium dichromate (1.0 g), dimethylformamide (3.5 g), and water (0.1 g) until a solution was obtained, and (3) a paste produced by grinding manganese dioxide (Riedel-de Haen, Type C) in a mortar with an equal weight of dioctyl phthalate (di-2-ethylhexyl phthalate; CSR Chemicals Corflex 810). The requisite weight of curing agent was calculated by use of either thiokdichromate 6:l or thiol:Mn02 2:l. The thiol contents in the various polysulfide liquid polymers were determined by infrared analysis (Davidson and Mathys, 1984), which gave the following concentrations: PR-1750, 2.13%; PR-1422, 1.54%; LP-2,1.69%; and LP-32,1.52%. Where necessary, curing ratios were adjusted to allow for removal of fillers and sometimes modified slightly to optimize the cure. The stoichiometric ratios of LP-32 and LP-2 to the curing agents of PR-1422 and PR-1750 were calculated after allowance for the different thiol and filler content of the prepolymerswith which these cure systems were originally matched. All sealants were cured by using the standard cycle. ( c )Panels. Clad aluminum panels for the volume swell and peel and lap shear tests qualiied to QQ-A-250/13 (T6) and were degreased before use with a cleaning solution conforming to MIL-C-38736. Coatings of alodine (MILC-5541A) and PR-1560 (PRC, corrosion-preventing polyurethane, MIL-C-27725B) were applied to peel and lap shear panels before use. Volume Swell Measurements. The volume swell of PR-1750 B-2 and PR-1422 B-2 in organic solvents was determined by the method of ASTM D-471. Procedures in which solid specimens are immersed in the swelling medium were impractical for examination of partially cured elastomers. The following modification was therefore used for the measurements in water. Degreased aluminum panels (uncoated, 50 X 25 X 1mm) were weighed in air and in water. A bead of sealant (approximately 2 g) was extruded onto the surface of each panel, and the weight of sealant was measured in both air and water. Each sealant was examined separately in the freshly mixed, tack-free and fully cured state. Triplicate specimens of each cure condition were totally immersed in glass vessels containing distilled water preheated to the appropriate temperature and stored in temperature-controlledcabinets maintained at 25,50, 70, and 90 "C ( f l"C). A t intervals the weight increases of the sealant samples were monitored. It was established that no differences resulted between the continued use of the same specimen over the duration of
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the test compared with separate measurements on different samples at each sampling interval. Repetitions with the same material were therefore used. It was necessary to minimize air entrapment in the cured sealants to reduce experimental scatter. Reproducibility from a given batch agreed to within 12% of the absolute values of volume swell (mean of triplicate measurements). Results for each material were derived with specimens obtained from a single large sample of sealant. Some variations in swell occurred with different batches, but the relative shapes of the time-swell plots did not change. Values of volume swell are, in general, quoted to the nearest 5 70. Peel Strength Measurements. Sealants were cast on polyurethane-coated aluminum panels, cured appropriately, and conditioned in distilled water maintained at 25, 50, 70, or 90 "C. For comparison, specimens were also conditioned in air at 50 and 90 "C. At suitable intervals the panels were removed and tested according to MIL-S83430. Difficulties were sometimes experienced due to the reinforcing mesh tearing through sealants that were weakened as a consequence of swelling. Lap Shear Measurements. Lap shear specimens of the sealants (25 X 25 X 0.95 mm) were prepared by using polyurethane-coated aluminum panels which were then conditioned in distilled water maintained at 25, 50,70, or 90 "C. Specimens were tested at ambient temperature at a crosshead speed of 50 mm/min. Sets of quadruplicate specimens were separately examined to obtain representative data since for manganese dioxide cured sealants a high proportion of low lap shear strength values were recorded due to total adhesion failure. The frequency of such failures was significant at both 70 and 90 "C. Permeabilities of Polysulfide Sealants. (a) Sealant Preparation. Membranes of sealant that were essentially void free were obtained as follows. Mixed polysulfide sealant was precured to close to the tack-free state (3 h at 50 "C or overnight standing at 25 "C) and placed in a steel mold (152 X 152 X 1.5 mm). Teflon sheets were inserted between the cover plates, and the sealant film was cycle pressed 10 times (200 kg/cm2) at 40 "C and maintained under pressure overnight. The mold was then removed and heated for 4 h at 50 "C and a further 3 h at 70 "C to achieve full cure. A viscous sealant mix was used to allow entrapped air to be effectively removed during the pressing sequence. Electron micrographs of sections of these membranes revealed good homogeneity with the few voids having diameters of only 3-10 pm. No voids were evident from visual examination of the sections. (b) Permeability Measurements. The ASTM E96-66 method was used but with a recent modification which involves securing disks (56-mm diameter) of the elastomeric membrane to the anodized aluminum permeability cup by use of a threaded retaining ring (Oldfield and Symes, 1981). Prior to testing, the samples were scrubbed to remove remaining Teflon release agent. Permeabilities were conducted on both normal and swollen membranes. The latter was obtained by immersing the sealant sheets for 28 days in water at either 50 or 90 "C. A sample of PR-1750 with volume swell of 120% was also prepared by prolonged immersion in water at 70 "C. After withdrawal, surface water on the sheets was removed by wiping with tissue paper. Disks were cut from the same sheet of sealant, and before insertion in the cups the thicknesses of the membranes were measured in at least five places. Tests were commenced by inverting the cup so that the inner surface of the membrane was covered with water (20 mL). The assembly was then placed in a desiccator, con-
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taining activated silica gel, maintained at 25 "C. Each cup was removed and weighed at intervals of 2 days. The weight losses were plotted against elapsed time, and measurements were continued until a straight line could be drawn through four consecutive points. Comparison of Sealants in Simulated Sealing Experiments. A series of punctures were introduced in the sides of tin-plated steel containers (65-mm diameter X 45-mm height). The area of the inner surface in the vicinity of the apertures was then cleaned with solvent, and the holes were sealed with a layer (1mm) of polysulfide sealant. After the sealant was cured the containers were filled with equal volumes of distilled water and the lids were firmly replaced. A line was drawn below the apertures with a water-soluble marking pen on the outer surface of the containers that were then conditioned at 70 "C, and the time of sealant failure was indicated by water leakage through the indicator mark.
Results and Discussion Volume Swell of Polysulfides. The volume swell in water of the sealants PR-1750 B-2 and B-6, Pro-Seal 899 B-2 and B-6, and PR-1422 B-2 was monitored at 25, 50, 70, and 90 "C. Water resistance data quoted in ASTM D-2000 for various elastomers including silicones, natural rubber, and urethanes (but not polysulfides) generally falls in the volume swell range of f5-10% after 3 days immersion at 100 "C. For the purposes of this investigation longer swelling times were desirable, and at the lower temperatures measurements were continued for more than 200 days. At 70 and 90 "C, however, substantial changes occurred within 60 days. In all cases the materials were examined in the freshly mixed, tack-free, and fully cured states. The time-swelling plots, each based on a minimum of seven samplings, are shown in Figures 1-3, and for convenience only the experimental points for the cured samples are indicated. The data can be presented to show swell-temperature relationships at fixed swelling times, and this is depicted in Figure 4 for the immersion period of 35 days, during which period linear regression analysis
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986 160 1
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of the curing agent (Panek, 1962; Bertozzi, 1968). In the time scale of the swelling experiments, therefore, the g 80 partially cured samples would rapidly reach a full cure 3 0 under the combined influences of heat and solvation. The > results suggest that in the initial stages when the sealant 8 40 40 is only partially cured the degree of cross-linking has little L influence on swell. Given that for the same material the 0 0 level of swell is an indicator of cross-link density, it is 0 80 160 240 0 80 160 240 evident that a similar degree of cure is achieved regardless TIME, d a y s TIME, d a y s of the condition of the sealant when immersed. The Figure 2. Swelling of PR-1750 B-2 sealant in water. possible extraction of water-soluble components, however, cannot be ignored. The time-swelling curves for the manganese dioxide cured sealants PR-1750 B-6 and Pro-Seal 899 B-6 were essentially the same as those of the respective B-2 grades shown in Figures 1 and 2. Small differences between grades were no greater than those expected from batch BO variation. Significant variations in response, however, were i Y observed between the individual sealants (Figures 1-3). 3 0 With Pro-Seal 899 B-2 at 70 "C, 100% swell was registered > in 30 days, whereas under the same conditions PR-1750 B-2 required 50-55 days to swell to the same degree. At constant temperatures, the volume swell of both sealants increased linearly with immersion time. In contrast, the 0 40 80 120 0 40 80 120 dichromate-curedPR-1422 B-2 exhibited swelling behavior TIME, d a y s TIME, d a y s typical of rubbers, with a moderately rapid increase in the first 15-20 days of immersion followed by only minor changes over a prolonged period to produce a characteristic saturation curve (Figure 3). A t temperatures above am120 50°C 120 25OC bient this sealant was markedly more resistant to swell than those cured by manganese dioxide. At 25 "C, however, the extent of swell after 200-days immersion was Tack Free approximately 20% for all three sealants. These differCured ences in performance indicate that chemical stress relaxation effects (Usmani et al., 1981) cannot be invoked to ae 4 o k , , h 3c4 /I ,./--'-,----------explain the high water uptake of particular sealants. A further difference was exemplified by the formation 0 of pronounced voids in the swollen Pro-Seal 899 and PR0 80 160 240 0 80 160 240 1750 sealants following immersion at 70 "C and above for TIME, d a y s TIME, d a y s prolonged periods. Moreover, adhesion was so poor that Figure 3. Swelling of PR-1422 B-2 sealant in water. light finger pressure was sufficient to dislodge these indicated that a linear relationship held. Correlation sealants from the panels. The formation of the voids occoefficients of a t least 0.98 were obtained for the curred gradually: they could be detected visually after 35 days but did not become excessively large until about straight-line graphs in Figures 1,2, and 4. For those cases 100-days immersion. In contrast, however, such cavities in which the line of best fit did not pass through the origin, experimental error is assumed. were not produced from PR-1422. Formation of linear Differences in swell between the partidy and fully cured time-swelling curves as occurs with PR-1750 and Pro-Seal specimens were insignificant and within the expected limits 899 is abnormal and is considered to be related to the of experimental scatter. Addition of small volumes of formation of such voids. The volume swell procedure is water (5-1070) to manganese dioxide cured polysulfides not applicable to cellular or porous elastomers since the is recognized to accelerate curing due to increased solvation cavities will eventually fill with solvent during testing to 3
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986 325 o c
Table 1. Dimensional Recovery of Swollen PR-1750 Sealants Following Heat Cycling in Water B-2 B-6 volume swell (%) after 23 days at 90 "C + 15 days at 43.1 41.3 25 O C (intermittent) volume swell (%) after 23 days at 90 "C (continuous) 44.7 46.5 2.2 2.3 average recovery (%) of original volume after each 3 days at 25 O C
produce a modified matrix. In the present cases it has been shown that a chemical breakdown of the polysulfide polymer takes place which then leads to the development of voids (Hanhela et al., 1985). A plot of log (time to 20% swelling) against 1/T X lo3 (Figure 5) shows that, even in the early stages of swelling, both PR-1750 and Pro-Seal 899 behave differently from PR-1422, and this is interpreted as indicating that the degradation process leading to void formation commences immediately upon immersion. Analysis of Figures 1-3 showed a linear relationship between volume swell of the sealants and temperature. In the case of PR-1422,this dependence held only for the first 35-40 days of immersion, but with the manganese dioxide cured sealants linearity was maintained for much longer swelling times. Results for immersion for 35 days are shown in Figure 4. As the swelling, degradation, and void formation processes occur concurrently, the generation of linear temperature-swell and timeswell plota is considered to be fortuitous. Nevertheless they can be used empirically to allow prediction of sealant performance. The conditions used for assessing volume swell are severe in relation to the requirements to seal water tanks in aircraft. In service, exposure of the sealants to hot water would be for short intervals (1-2 h) interspersed between longer periods during which the water temperature would be at or below 20 "C. The possibility that contraction of the swollen sealants occurs during periods of contact with
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cold water was examined by using a temperature cycle of 3 days at 90 "C followed by 3 days at 25 "C over a total immersion period of 38 days. Only 2% recovery resulted during the periods of immersion in cold water. Typical results for PR-1750 are given in Table I. This is understandable since the gradual formation of voids in the sealant matrix is an irreversible process. Effect of Curing Agents. The influence of different curing systems on volume swell was further investigated
Table 11. Changes in Swell and Hardness of Polysulfide Sealants After Immersion in Water at 90 "C components" volume swell, % hardnessb polymerc cure agentd 3 days 24 days initial 24 days PR-1750 PR-1750 (1) 11 45 55 28 PR-1750 Mn02 (1.16) 45 80 52 22 PR-1750 MnOz (0.75) 40 85 56 21 dichromate (0.85) PR-1750 4 17 68 54 PR-1750 dichromate (1) 3 17 65 54 PR-1750 PR-1422 (2.34) 2 11 71 73 PR-1750 PR-1422 (1.50) 5 30 73 56 PR-1750 NF PR-1750 (1.52) 16 85 33 10 PR-1750 NF MnOz (1.16) 55 170 39 6 PR-1750 NF dichromate (1) 5 30 42 32 PR-1422 PR-1422 (1.33) 20 50 67 31 PR-1422 MnOz (0.65) 35 75 36 11 PR-1422 MnOz (0.44) 50 130 32 6 PR-1422 PR-1750 (1) 50' 140 37 2 PR-1422 PR-1750 (1.5) 140' 160 38 0 PR-1422 NF PR-1422 (1.97) 17 75 45 17 PR-1422 NF dichromate (1) 10 45 32 15 PR-1422 NF dichromate (1.6) 20e 35 43 28 PR-1422 NF PR-1750 (0.86) 14 65 17 7 PR-1422 NF PR-1750 (1.52) 25 110 19 1 PR-1422 NF Mn02 (0.65) 60 230 25 0 LP-32 NF dichromate (1) 100 41 10 LP-32 NF MnOz (1.16) 230 41 0 LP-32 NF PR-1422 (2) 25 95 52 12 LP-32 NF PR-1750 (1.5) 50 170 38 6 LP-2 NF dichromate (1) 85 41 15 LP-2 NF MnOz (1.16) 190 43 1 LP-2 NF PR-1422 (1.77) 25 85 46 12 LP-2 NF PR-1750 (1.54) 45 160 39 6 ORatio (w/w) of cure agent for 10 parts of polymer given in parentheses. bRex A. 'NF indicates no fillers present in the polymer. dMnOz and dichromate indicate laboratory-prepared manganese dioxide and ammonium dichromate curing agents, respectively. e After 14-days immersion.
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by comparison of sealants formulated from the liquid polymers LP-2 and LP-32 and those used in PR-1422 and PR-1750 (Table 11). Although PR-1422 and PR-1750 sealants were examined in both filled and unfilled states, conclusions are based mainly on the results from the unfilled sealants which are not complicated by effects of additives. The prepolymers were each cured with the PR-1422 and PR-1750 curing agents together with the laboratory-prepared reagents, ammonium dichromate dissolved in dimethylformamide and manganese dioxide dispersed in di-2-ethylhexyl phthalate. A significant feature of these results is apparent from the relative volume swell of sealants obtained from the same prepolymer but separately cured with dichromate and with manganese dioxide. For example, with filled PR-1750 the swell after 24 days in water at 90 OC was 17% with the dichromate-cured system, 45 70with the normal PR-1750 cure paste, and 8 0 4 5 % with the laboratory manganese dioxide reagent. For unfilled PR-1750 the volume swells with these curing systems were 30%, 85'70, and 170%, respectively. Similar trends were noted with sealants prepared from LP-2 and LP-32 prepolymers. By contrast the swell and hardness measurements of sealants prepared from the dichromate paste and the PR-1422 curing agent showed little difference in performance. For PR-1422, replacement of the dichromate cure paste by either the PR-1750 curing agent or manganese dioxide in di-2-ethylhexyl phthalate resulted in markedly higher swelling in hot water. However, these results, while complementing those of the dichromate system, can only be considered as indicative since these manganese dioxide cured sealants were of relatively lower hardness and may not represent optimally cured products. For cases where curing was slow, mix ratios were modified in an attempt to obtain a harder product. The inferior performance of products obtained by using the manganese dioxide in di2-ethylhexyl phthalate reagent is ascribed to incomplete cure due to the absence of an activator. Commercial cure pastes are matched to their polysulfide prepolymers in such a manner as to provide sealants with controlled performance characteristics. Direct interchange of sealant components therefore would not necessarily lead to satisfactory curing, and this is the case for PR-1422 prepolymer-PR-1750 cure paste mixtures. With PR-1750 prepolymer and PR-1422 cure paste, however, curing occurred in the normal manner. On the basis that resistance to swell provides a guide to relative cross-link densities, it can be argued that with the prepolymers used in this study the dichromate curing systems were more effective than those based on manganese dioxide. In addition, for sealants prepared from both the compounded and uncompounded base polymers of PR-1750 and PR-1422 with ammonium dichromate, those from PR-1750 were the more resistant to swell in hot water. This suggests that the cross-link density in PR-1750 is greater. It is significant that the PR-1750-ammonium dichromate product, in which cross-link density and cure efficiency are maximized, provided the greatest resistance to swell in hot water. A more general indication of the effectiveness of the dichromate curing systems may be obtained from the hardness of the fully cured material. For sealants obtained from the same base polymer, the dichromate-cured product was several degrees harder than that obtained by using manganese dioxide (Table 11), which again implies (Bertozzi, 1968) that more effective cross-linking is achieved by using the dichromate cure. For the cases of the commercial sealants PR-1750, Pro-Seal 899, and PR-1422 the efficiencies of the curing
Table 111. Effect of Organic Solvents on Commercial Sealants volume swel1,O % final hardnessb solvent PR-1422 PR-1750 PR-1422C PR-1750d chloroform 540 470 5 15 dichloromethane 370 330 18 15 dioxane 315 250 5 15 benzene 200 160 25 30 toluene 100 85 30 30 methyl ethyl ketone 70 55 18 35 carbon tetrachloride 60 50 30 35 acetone 45 35 25 35 dibutyl phthalate 25 20 45 45 jet reference fluide 13 10 50 45 1-propanol 3 1 60 55 "Samples immersed for 30 days at 25 "C. bRex A. 'Initial hardness: 61. Initial hardness: 52. e Mixture of (v/v) toluene (30), cyclohexane (60), iso-octane (lo), di-tert-butyl disulfide (1) together with tert-butylmercaptan (0.015 wt YO).
systems and the relative proportions of cross-linking sites in the prepolymers tend to counterbalance. A guide to the degree of cross-linking in these sealants was obtained by measurement of volume swell in organic solvents (Hanhela and Paul, 1984). With dimethyl sebacate it was shown that the volume swell of PR-1422 reached 370% after 19 days, whereas those of Pro-Seal 899 and and PR-1750 were 270% and 195%, respectively. A similar trend was observed with ethanol, for which the respective figures were 25%, lo%, and 5% after 42-days immersion. Moreover, with these solvents the typical saturation curve, as in Figure 3, was produced with each sealant. Additional swell values in various other solvents have now been determined (Table 1111, and these also suggest a higher cross-link density in PR-1750 than in PR-1422. The more efficient curing system in the PR-1422 system therefore does not offset the lesser number of cross-linking sites available in the prepolymer. Effect of Hot Water on Peel Strength of Polysulfides. The integrity of the bond between the sealant and the coated aluminum surface is also a primary consideration. The effect of prolonged immersion in hot water on the peel strength of the polysulfides was therefore examined. Preliminary studies revealed mechanical difficulties since failures occurred not only through adhesive or cohesive breakdown but also from the reinforcing mesh tearing through the softened sealant. Replicate measurements eventually provided the data shown in Figure 6. At 25 and 50 "C the loss in peel strength of the sealants occurs slowly and failure is entirely cohesive, but at 70 "C and above both PR-1750 and Pro-Seal 899 undergo a rapid loss in strength and a combination of both cohesive and adhesive failures occur. The decline with PR-1422 is less severe, and for immersions greater than 6 days at the higher temperatures its peel strength, although below specification requirements (89 N), exceeds those of PR1750 and Pro-Seal 899. Sealants conditioned in air at elevated temperatures declined only slowly in peel strength over an equivalent period. In 14 days the initial strength of PR-1750 decreased from 180 to 140 N at 50 "C and to 131 N at 90 O C . Under corresponding conditions no detectable change was recorded with PR-1422. The major losses in peel strength that result from the immersion of the polysulfides are therefore attributed to interactions with water rather than to thermal effects. Lap Shear Strength Studies. A lap shear procedure was also examined since this would simulate the stresses experienced by faying surface sealants during flight. As for peel strength, the procedure provides a measure of the
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tear strength of the sealant but avoids complications associated with the mesh. The general applicability of the procedure was established through replicate testing of ten specimens from the same batch of PR-1422 B-2 which gave an average lap shear strength of 1.36 f 0.07 MPa at 25 "C. The effect of varying the sealant thickness was also assessed using PR-1422 B-2. Average lap shear strengths (mean of three measurements) and sample thickness were as follows: 0.95 mm, 1.32 MPa; 1.1mm, 1.17 MPa; 2.5 mm, 0.89 MPa; and 3.2 mm, 0.79 MPa. For convenience, in subsequent experiments a thickness of 0.95 mm was used. Variations in lap shear strength of the sealants with time and temperature are given in Figure 7. Since results for both B-2 and B-6 grades of the same sealant were similar, only values for B-2 materials are shown. Immersion of all sealants in water at 50 "C or below had little effect on lap shear strength apart from a slight post-curing which led
Permeability units: g cm-' d-l. 28-days immersion in water at 50 OC. 28-days immersion in water at 90 OC. 50-days immersion in water at 70 OC.
to generally higher results at 50 "C. The surface area of the sealants exposed to hot water in this examination was relatively small and may account for the slower decline in strength relative to the peel measurements. Only at 70 "C and above was evidence of penetration of the sealant by water apparent, and at these temperatures a combination of both cohesive and adhesive failures occurred with loss of adhesion responsible for >40% of the failure. Adhesion failure, which was more prevalent with PR-1750 and Pro-Seal 899, commenced around the perimeter of the joints. In addition, examination of cohesively failed sealant revealed the presence of voids. For PR-1422 the initial tensile shear strength is not as high as the other materials, but unlike the other sealants its slow rate of decline with time is maintained over the entire immersion period (40 days). Both PR-1750 and Pro-Seal 899 exhibit a similar decline for up to 25-30 days, but this is followed by a marked fall in shear strength, and the failure modes indicate a large contribution due to loss of adhesion. Reference to Figures 1and 2 shows that this sudden deterioration corresponds to a time at which these sealants reach a volume swell of 80-90 ?& . Permeability Measurements. A consequence of prolonged contact of polysulfides with hot water is the increased permeability of the swollen sealant matrix which provides a path to the sealant-primer interface and facilitates adhesion failure. A guide to the increased degree of water transport through swollen polysulfides was obtained by conducting permeabilities at 25 "C on sealant membranes both in their normal state and after preconditioning for 28 days in water at either 50 or 90 "C. The variations in permeability that occurred with swelling of the sealant membranes are shown in Table IV. The values obtained with unswollen PR-1422 and PR-1750 were consistent with those quoted (Panek, 1962; Bertozzi, 1968) for polysulfides compounded from LP-2, LP-31, and LP-32
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to 1.1 X g cm-' day-'), and our polymers (4 X marginally lower values may reflect differences in type of filler and a presumably reduced void content. In line with the earlier survey, the permeability results show virtually no discrimination between sealants of different cross-link density. The permeabilities of the swollen membranes, however, were 5-6 times greater than those of the unswollen sealants, and no significant differences were revealed between membranes with volume swell ranging from 45 to 80%. A dramatic increase in permeability was observed, however, with a membrane of PR-1750 that had been preconditioned in water at 70 "C until a volume swell of 120% had been obtained. With this material the permeability was nearly 140 times greater than that of unswollen PR-1750, and during the experiment the formation of water droplets on the external surface of the membrane could be observed. There is clearly a threshold level of swell that when surpassed leads to complete breakdown of sealing efficiency: for PR-1750 this level must fall in the range 80%-120%. It is probable that at this degree of swell the formation of voids in the sealant has produced a porous matrix and the transport mechanism changes from diffusion to direct water flow. Relative Sealing Performances of Polysulfides. An indication of the relative performance of sealants in hot water was also obtained through comparisons of their efficiency in sealing punctured metal containers conditioned at 70 OC. Since water tanks in the aircraft are subjected to a small positive pressure, these experiments resembled the practical situation. The average times to sealant failure were 41 days for PR-1750 B-2, 38 days for PR-1750 B-6, 32 days for Pro-Seal 899 B-2,35 days for Pro-Seal 899 B-6, and >80 days for PR-1422 B-2. The most effective seal under these conditions was provided by PR-1422, which is as expected if resistance to volume swell is the primary factor influencing sealing efficiency. It can be ascertained from Figures 1and 2 that the time to failure for PR-1750 and Pro-Seal 899 in this study again corresponds to the
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apparently critical volume swell range 80-100%.
Conclusions Dichromate-cured polysulfide aircraft sealants are more resistant to swelling in hot water than manganese dioxide cured materials, and consequently in these conditions PR-1422 was more effective than PR-1750 or Pro-Seal 899. Continued expansion of the manganese dioxide cured sealants eventually leads to a level of swell (80-120%) where water transport through the matrix proceeds with little resistance, the sealing function is no longer present, and chemical attack on the sealant-primer interface causes adhesion failure. The nature of the curing system must therefore be considered when polysulfide sealants are compounded for resistance to hot water. Registry No. PR-1750 B-2, 100908-84-7; PR-1750 B-6, 100908-85-8;Pro-Seal 899 B-2, 100908-87-0;Pro-Seal 899 B-6, 100908-88-1;PR-1422 B-2, 100908-83-6;MnOp, 1313-13-9; (NH,),Cr2O7, 7789-09-5.
Literature Cited Bertozzi, E. R. Rubber Chem. Techno/. 1988, 4 7 , 114-160. Coast Pro-Seal. High-temperature sealant Pro-Seal 899, technical data sheet, June 1971. Davidson, R. G.;Mathys, G. I . Anal. Chim. Acta 1984, 160, 197-204. Hanhela, P. J.; Huang, R. H. E.; Paul, D. B.; Symes, T. E. F. Materials Research Laboratories, Melbourne, Austraila, unpublished work, 1985. Hanhela, P. J.; Paul, D. B. MRL Report No. 658, "Interactions Between F-111 Fuselage Fuel Tank Sealants, Part 2", 1984. Karpati, K. K. J . Coat. Techno/. 1980, 5 0 , 66-69. Lee, T. C. P. PRI Symposium, "Water and Adhesion", City University, London, 1982. "Elastomers in Underwater Oldfield, D.: Symes, T. E. F. MRL Report No. Applications", 198 1. Panek, J. R. I n Polyethers, Par! I I I ; Gaylord, N.G., Ed.; Interscience: New York, 1962; pp 115-224. Products Research and Chemical Corp. Sealing compound PR-1750, interim technical data sheet, Oct 1971. Usmani, A. M.; Chartoff, R. P.; Warner, W. M.; Butler, J. M.; Salyer. I . 0.; Miller, D. E. Rubber Chem. Techno/.1981, 5 4 , 1081-1095.
Received for review March 20, 1985 Revised manuscript received September 23, 1985 Accepted December 11, 1985
Effect of Subzero Storage Temperatures on Properties of Premixed Polysulfide Sealants John W. Barber, Peter J. Hanhela, Robert H. E. Huang, and D. Brenlon Paul' Defence Science and Technology Organisation, Materials Research Laboratories, Ascot Vale, Victoria 3032, Australia
Premixed commercial polysulfide aircraft sealants stored at -20, -30, and -40 OC were tested for conformity to cure rate, application time, tack-free time, and peel strength requirements. With manganese dioxide cured products no significant change in hardness or peel strength occurred after prolonged storage at -20 OC and below; a minor decline in peel strength was observed with a dichromate-cured sealant. Application properties were maintained for extended periods, and at -40 OC tests terminated only through consumption of samples after storage times (7-1 6 weeks) that far exceeded published recommendations. Batch variations and stricter observation of class requirements would influence storage times, but at -40 O C even materials with llttle margin in application life rating could be safely stored in a premixed state for at least 10 weeks. Both A- and B-class sealants were able to be stored in this manner provided their application lives were 2 2 h.
Introduction Polysulfide sealants are used extensively in modern aircraft for sealing integral fuel tanks, crew modules, and canopies. For many y e m performance requirements were 0196-4321/86/1225-0328$01.50/0
defined by the specification MIL-S-8802, which stipulates that the sealants maintain properties to temperatures of 121 "C (250 OF). With modern combat aircraft, however, aerodynamic heating resulting from high-speed flight ne0 1986 American Chemical Society
