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Ind. Eng. Chem. Prod. Res. Dev.
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
1986,25, 328-332
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
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986 329
Table I. Recommended Storage Lives for Premixed PR-1422 Sealants (PRC,1972, 1973) storage life, days temp, "C class A-2 and above' class B-2 and above" -29 3 15 -40 7 30 a Class A sealants are brush applied; class B are applied by extrusion gun or spatula.
cessitates the use of materials qualified to MIL-S-83430, which requires retention of integrity up to 121 "C under continuous operating conditions and to 182 "C (360 O F ) for intermittent, short periods. Aircraft sealants frequently need to be replaced as a result of fuel or air leaks or through rework involving replacement of sealed components. After the service life of fuel tank sealants is exceeded, an entire resealing program may be necessary. Whereas small repairs may be accomplished by hand mixing, for major operations it is more convenient to mechanically premix a large batch of sealant that can be packaged in a plastic cartridge, snap frozen, and stored at low temperatures (-20 to -40 "C) until required. Thirty minutes equilibration at ambient temperature is sufficient to thaw the frozen sealant. Apart from the reduction in mixing time, this procedure has the added advantage that quality control tests are required for only the one batch. At subzero temperatures the sealant curing reaction continues, but at a much slower rate than under ambient conditions. In addition, fillers or other additives may concentrate or settle out under storage. The mixed sealant therefore slowly changes character over its storage life. Knowledge of safe storage limits of premixed sealants is therefore essential for large repair programs. The little data available, however, are of a general nature and do not relate to commercial formulations. The effect of storage has been examined by Thiokol for four "typical" sealant formulations which were held at temperatures ranging from -40 to 49 "C (-40 to 120 O F ) after mixing (Thiokol, 1958). For these sealants of undisclosed composition,the storage life at -40 "C was found to be in excess of 3 months. Products Research and Chemical Corp. (PRC) data sheets indicate that the application life of polysulfide sealant doubles for every 6 "C (10 O F ) drop in temperature and is reduced by 45 min as a result of freezing and thawing operations. High humidity also attenuates application life (PRC 1972, 1973). Additional details provided for PR-1422, a PRC product qualified to MIL-S-8802, are given in Table I. No information on the effect of low-temperature storage is provided for Coast Pro-Seal aircraft sealants. Neither the MIL specifications nor the related FMS-1044 specification (General Dynamics, 1975) demands that sealants meet refrigerated storage requirements. MIL-S-83430, however, recognizes that it is acceptable to snap freeze sealant of classes 3-2-B-6 prior to carrying out particular tests. This procedure is described, but the maximum allowable storage times are set cautiously at only 4 days. As no quantified storage life data for premixed sealants exist, maintenance facilities have developed arbitrary arrangements which require such sealants to be discarded after conservative storage times. In practice if the demand for sealants is large, the need to reject time-expired material rarely arises, but for continuing operations that involve moderate quantities of sealants or that require procurement of these limited shelf life materials from overseas, reliable data on storage properties are essential. The effects of storage at various time intervals and temperatures on the performance of three premixed com-
mercial polysulfide sealants, PR-1750, PR-1422, and ProSeal 899, have therefore been determined and are now reported.
Experimental Section Materials. The following sealants, qualified to MILS-83430, were examined: PR-1750 B-2 and B-6 (both PRC materials) and Coast Pro-Seal 899 B-2, B-6, and A-2 (Essex Chemical Corp.). In all cases the liquid polymer component was filled with calcium carbonate (approximately 35% for B-2 grades), and the curing systems were based on a paste of manganese dioxide, carbon black, and at least one activator in a hydrogenated terphenyl oil. In addition, the MIL-S-8802 sealant, PR-1422 B-2 (PRC), which comprised a 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 in order to ensure sample uniformity, minimize entrapment of air, and allow direct charging into 70-g (2.5-02)polyethylene cartridges. Control samples were taken directly after mixing and tested to determine compliance with the relevant specification. This also provided a guide to the tolerance of the batch within the specification. The filled cartridges were immediately encased in polyethylene bags, snap frozen by immersion in a dry ice and ethanol bath for 30 min, and then refrigerated at controlled temperatures of -20 f 2, -30 f 2, and -40 f 2 "C. Thawing Procedures. The frozen cartridges were thawed by either of two methods. The first procedure involved equilibration of the cartridges at 23 f 1 "C for 30 min. Zero time was taken as the time of removal of the cartridge from cold storage. The second method, as described in MIL-S-83430, required the frozen cartridges to be stabilized at -55 f 1 "C for 2 h. Thawout was accomplished by immersion of the frozen cartridges, still encased in the polyethylene bags, in a water bath at 49 f 1 "C for 18 min. The end of the 18-min period was taken as time zero. Test Methods. For each type and class of sealant at each temperature, duplicate cartridges were withdrawn at intervals, thawed by the alternative methods, and examined as follows. Aluminum panels (1 X 70 X 152 mm) conforming to temper T6 of specification QQ-A-250113 were degreased with MIL-C-38736 cleaner and inserted into poly(tetrafluoroethy1ene)-coatedmetal frames of suitable dimensions to form molds. Sealants were then cast to the required depth and cured at 25 f 1 "C and 50 f 5% relative humidity for the specified time. Tack-free properties, application time, standard cure rate (hardness), and peel strength were examined according to MIL-S-83430 or MIL-S-8802E as appropriate. Peel test panels were first coated with alodine (MIL-C-5541A)and a corrosion-prevention coating (Products Research Corp. PR-1560 MK polyurethane; MIL-C-27725B) before sealant was applied. Degreased aluminum wire (20-40 mesh) was used in this test. After cure the panels were equilibrated for 2 h at 25 "C prior to examination. Results were obtained by using an Instron Model 1026 tensile machine and are given as numerical averages of the peak loads; failures of the sealing compound to the wire mesh were not included. Hardness readings (average of 10) were taken on a doubled backto-back 3.2 mm thick specimen. To maintain consistent results, the Shore maximum reading durometer attached to a Conveloader was used. Extrusion rates for B-class sealants were determined by using a Semco Model 250 sealant gun and a constant air pressure of 90 f 5 psig.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
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Table 11. Variation in Extrusion Rate with Storage Time for Premixed Polysulfide Sealants extrusion rate after 2 h," g/min storage PR-1422 B-2 PR-1750 B-2 Pro-Seal 899 B-2 PR-1750 B-6 Pro-Seal 899 B-6 time, days -20°C -30°C - 4 0 ° c -20°C -30°C -40°C - 2 o o c -30°C -40°C -20°C -30°C -40°C -20°C -30°C -40°C ~~
~
10 15 30 50 70
16 16 15 11 6
23 22 20 16 11
25 26 27 26 25
21 16 7
27 23 14
33 32 30 27 24
37 26 14
46 41 37 36 37
49 46 43 41 40
14 11
17 16
8
14 12
6
21 19 17 15 15
10
46 41 34 29 27
51 47 42
40 40
57 53 48 45 44
"MIL-S-83430, MIL-S-8802 requirements: minimum extrusion rate of 15 g/min after 2 h (B-2 sealants) or 6 h (B-6 sealants). Table 111. Laboratory-Determined Storage Lives of Premixed Polysulfide Sealants a t Various Temperatures storage life, days sealant -20 "C -30 "C -40 "C >47 >70 PR-1422 B-2 11 Pro-Seal 899 B-2 28 >>I10 >>110 >>lo0 >>loo Pro-Seal 899 B-6 7 100 13 35 >>47 PR-1750 B-2 PR-1750 B-6 7 21 >50 >20 >>20 Pro-Seal 899 A-2 17
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Results and Discussion Storage Temperatures. Domestic freezers operate at approximately -17 "C and if equipped with a fast-freeze cycle can achieve -30 "C for short periods. The freezing compartments of two-door refrigerators can be held at -25 "C, and commercial freezers are available that can maintain -40 "C on continuous operation. For the purposes of this survey, storage temperatures of -20, -30, and -40 "C were selected to cover the range of options available from such refrigeration systems. The acceptability of premixed sealant under storage was followed by monitoring critical
L 0
20
40
60
80
100
120
TIME, Days
Figure 3. Variation of peel strength with storage time at -20 to -40 "C for class B-2 sealants. Results are the mean of measurements at -20, -30, and -40 "C.
performance and application properties (tack-free time, application life, cure rate, and peel strength) as a function of storage time. Cure Rate and Peel Strength Measurements. Individual components of two-part polysulfide sealants undergo chemical changes when stored at room temperature and as a consequence can be guaranteed to remain within
Ind. Eng. Chem. Prod.Res. Dev., Vol. 25, No. 2, 1986 331
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