Ind. Eng. Chem. Prcd. Res. Dev. 1886, 25, 572-577
572
Fluoroepoxy Resin for Moisture Vapor Barrier Coating and Other Applications Sheng Yen Lee' NASA Goward Space Flight Center, &embelt, Marylrtnd 20771
James R. Grifflth Naval Research Laboratory, Washlogton,
D.C. 20375
A new fluoroepoxy resln has been developed that can be processed as a Conventional thermoset resin yet possesses some of the unique properties of the well-known thermoplastic fluoropolymers. Its moisture vapor transmission rate and mdsture absg>tkn were found to be unusually low. In general, the transmission rates were inversely proportional to the fh" content of the materials tested. The fluoroepoxy was shown to be an excellent moisture vapor barrier coating or sealant and an effective adhesive to bond Teflon without etching. It could be foamed by a new and simple foaming process. Other potential applications suggested are conformal coating,
encapsulation, and potting for electronics.
Introduction The moisture absorption of plastics is undesirable for many uses. Moisture may cause degradation of mechanical strength, electrical properties, and dimensional stability. Plastic encapsulation of integrated circuits (ICs) is common because it is convenient in fabrication and cost-effective for production. In consideration of high reliability, however, hermetic sealing by metal casing is usually required for ICs by the military since no plastic is totally moisture impermeable and all plastics absorb moisture to a certain extent. Graphite-epoxy composites are indispensible for spacecraft structure and space instruments due to their high specific strength and practically zero coefficient of thermal expansion when they are properly laminated. Here moisture absorption, small as it may be because of its usual 60 to 70% content of non-moisture-absorbing graphite fiber, remains a serious concern, especially when it is used to fabricate space antennas, space telescopes, or instrument decks. Structural dimensions may change as a result of desorption in space of the moisture absorbed on earth. Therefore, it has been a challenge to find a matrix resin preferably with the mechanical strength and processing method comparable to the present epoxy system yet with zero or near zero moisture affinity. In the meanwhile, the search for a moisture vapor barrier coating or shield appears to be an alternative means. It is particularly practicable when the spaceware is not exposed constantly to a high-humidity environment on earth, and the coating will slow down moisture absorption and may reduce the extent of moisture absorbed on earth to a significant degree. For this purpose, a special tin-indium eutectic coating was developed by General Dynamics (Haskins, 1979). In order to ensure the eutectic coverage, the composite surface preparation, the thickness of copper undercoat, and the temperature control of the eutectic coating bath were all shown to be critical. The metal coating should be impermeable if pinholes, blisters, and contaminants can be completely eliminated. Another method was reported by Staebler and Simpers of Grumman Aerospace Corp. (Lubin, 19821, which utilizes aluminum foil bonded to composite laminate as a shield. Moisture pickup by the laminate was reduced by up to
65% after humidity exposure and thermal cycling. Further studies also found that even greater improvement was obtained with the foil painted. This finding indicates, in contrast to the results of the previous studies at Grumman and elsewhere, that organic coatings or paintings do help provide the moisture barrier needed for the composite. In addition to the process difficulties, metal coating or shielding also produces a weight increase undesirable for space program$. In some cases, an electrically conductive coating may not be acceptable in the first place. On the other hand, conventional organic coatings remain attractive for their ease of application. Polymers with the lowest moisture permeation and absorption, such as fluoropolymers, should be the choice. However, thermoplastic fluoropolymers such as Teflon and others are impractical because no solvents can dissolve them, and high temperature and high pressure are needed for their processing. Therefore, our attention has turned to thermoset fluoropolymers, which would be inherently close to Teflon in properties due to their fluorine content yet may be processed conveniently as conventional thermosets. For more than a decade, Griffith et al. at the US.Naval Research Laboratory (NRL) have worked on the synthesis of fluoropolymers and have developed a basic fluorodiol structure from which a series of fluoropolyurethanes, fluoroepoxies, and fluoroacrylates were produced. Experiments that used a fluoropolyurethane spray coating on US.naval vessels demonstrated positively the improvement in the reduction of marine fouling, the ease of removing the foulants, and the prevention of corrosion in general (Griffith and Bultman, 1978; Hunston et al., 1978). It is the objective of this work to investigate the potential of the NRL thermoset fluoropolymers to be used as moisture vapor barrier coatings for spacecraft structures, as well as for other applications including conformal coating, encapsulation, potting, and sealing.
Materials and Methods NRL Fluorinated Prepolymers. The fluorodiepoxides and the fluoropolyol were made by Du Pont Co. and Allied Corp. for the Naval Research Laboratory under contract. The nominal hydroxyl equivalent weight of the fluoro-
This article not subject to US. Copyright. Publlshed 1986 by the American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 573
Table I. Reference Material material cure epoxy Epon 815/V140, RT gelation plus 50150 by w t 70 OC)3 h and 80 OC/1 h
remarks
V140-Versamid 140
of General Mills, popular adhesive, coating, sealant epoxy Epon 8281V140, RT gelation plus popular adhesive, 50150 by w t 70 OC/2 h coating, sealant polyurethane per manufacturby Furane Producta; Uralane 5750 LV er's instructions excellent conformal coating polyurethane per manufacturexcellent conformal Uralane 5753 LV er's instructions coating and staking; solventless polyurethane per manufacturby Conap; excellent Conathane EN11 er's instructions coating and potting polyurethane per manufacturby Thiokol, Solithane 113/300 er's instructions S-113/C113-300; formula I, coating Teflon PFA by Du Pont; thermoplastic Teflon FEP by Du Pont; thermoplastic
poly01 is 448. C6-FlUOrOepOXy adduct amine was prepared at NRL by the following method To a stirred solution of 110 mL of ethylenediamine (ca. 99 g or 1.65 mol) in 300 mL of methanol in a round-bottomed flask a solution of 72 g of C6-fluorodiepoxide(0.086 mol) in 60 mL of methanol was added slowly from a dropping funnel. The mixture was stirred at room temperature for about 1h with a NaOH-tube seal and then at 65 "C for 4 h. The solvent and the excess diamine were first removed by distillation, followed by evaporation with a rotary evaporator under vacuum at about 65 "C. The thick liquid product, clouded with some white solid precipitate, was dissolved in Freon T F (bp 48 "C), and the insoluble solid was filtered. The filtrate was evaporated with a rotary evaporator under vacuum at a temperature fiially up to 80 "C to ensure the complete removal of all the solvents and the volatiles. The liquid product weighed 82 g (nearly 100% yield). Infrared spectra of the product showed the complete disappearance of the original epoxide peak at 910 cm-' and the presence of the new NH bands near 3300 and 1590 cm-'. Formulation of Fluoropolyurethane. Fluoropolyurethane GFU series was formulated with NRL fluoropoly01 and Mobay's isocyanate Desmodur N100, catalyzed by dibutyltin dilaurate. The nominal equivalent ratio of hydroxyl to isocyanate was varied from 1.0/1.0 to 1.7/1.0. The optimum ratio was found to be 1.3/1.0. A solvent mix of cx,a,a-trifluorotoluene, THF, and acetone (10/70/20 by volume) was used to prepare solution formulation. The cure cycle was room temperature (RT) gelation plus 80 "C for 3 h. The degree of cure was followed by infrared spectrometry as well as. by differential scanning calorimetry. Formulation of Fluoroepoxy GFE-AD600 Series. The series was formulated stoichiometrically as follows: C8-fluorodiepoxide 100.00 by wt, and C6-fluoroepoxyadduct amine 34.04 by wt. For solution spray coating, solutions of ca. 50% by w t were made with a solvent mix of l,l,l-trichloroethane, THF, methylene chloride, and methanol in a ratio of 20/20/30/30 by volume. The cure cycle was room temperature gelation followed by heating, 2 h at 70 "C or 2 h a t 80 "C for solution formulation. Reference Materials. For property evaluation, a number of commercial polymeric materials were used as listed in Table I. Moisture Vapor Permeation Rate Determination. The rate was determined according to the method of Am-
erican Society of Testing Materials ASTM D1653-72. Fisher/Payne permeation cups were used, each of which has a 10-mL capacity and a 10-cm2test film area. Teflon films were die-cut from the film products provided by Du Pont. Films of other reference materials were cast either on a Teflon plate controlled with a Doctor blade or by spray coating on a tin-plated steel plate substrate. The cured film was removed from the substrate with the aid of mercury amalgamation. For the fluoroepoxy film preparation, spraying was done with compressed helium as carrier gas to avoid gas trapping and the appearance of orange peel, which were observed when nitrogen was used. Two-stage spraying was applied to build up the desired f i i thickness. Solvent evaporation was allowed after each stage by letting the coating stand at room temperature for 1 h and exposing it for 0.5 h consecutively to 40 and 50 "C forced-air circulation in an oven. The curing was completed with 2 h at 80 "C heating. In the rate test, the cup was filled with 10 mL of distilled water, covered, and clamp-sealed with the film to be tested. The sealed cup was placed in a desiccator. The weight loss of the cup was monitored daily for a week or until a steady state was reached. The weight loss is due to saturated moisture vapor in the cup permeating through the 10-cm2film to the desiccator and is promoted by the relative humidity gradient from 100% to 0%. Adhesive Tests. The specimens for testing the shear strength of adhesives, with or without the fluoroepoxy sealing of the bond line, were made with aluminum panels, 4 X 1X in., according to ASTM D1002-72. The panels were etched with chromic acid solution per ASTM D2651 Method A. To test the tensile strength of the fluoroepoxy as an adhesive, Teflon and aluminum rods were machined generally following the direction of ASTM D2095-72. Both kinds of rods were etched with chromic acid solution as described above (Teflon was actually cleaned). The bonded rods were held in a spring-loaded fixture during the room temperature gelation period and the heat-curing time.
Results and Discussion Fluorinated Prepolymers and an Outgassing Problem. The basic fluorodiol developed by the Naval Research Laboratory is l-perfluoroalkyl-3,5-bis(2hydroxyhexafluoro-2-propy1)benzene(I). The fluorine content of the diol can be varied at will in synthesis by choosing the size of the perfluoroalkyl group, Rf. R.
R
I
II
Compound I may be epoxidized with epichlorohydrin to form fluorodiepoxide (11) (Griffith and O'Rear, 1975) or may react with acryloyl chloride to give a fluoroacrylate (Griffith and O'Rear, 1981). A fluoropolyol prepolymer as shown by the typical structure IV can be prepared by reacting Co-fluorodiolI with epichlorohydrin and olefinic fluorodiol 111 together in one step (Field and Griffith, 1979).
dF3
CF3
111
dF3
I
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-
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OH
IV
Fluoropolyol IV was used by the Navy, for the shippainting experiments, to formulate fluoropolyurethane
574
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986
(PU) with a commercial isocyanate, Desmodur Nl00 made by Mobay. Since IV was available in drum quantities, fluoro-PU was made and evaluated for this project for its outgassing property according to ASTM E595-84. Outgassing is one of the most important tests for spacecraft applications. The allowed outgassing level is 1% total mass loss (TML) (ca. lo4 Torr) when the material is heated at 125 "C for 24 h under high vacuum. Also, the collected volatile condensable materials (CVCM) must not be more than 0.1 % . The TML outgassing level of the fluoro-PU was found to be over 3% even at temperatures below 125 "C. No improvement could be made by adjusting either the ratio of the fluoropolyol to isocyanate or the curing conditions. Gel permeation chromatography analysis showed that the fluoropolyol consisted of ingredients of different functionalities and various molecular sizes. It contains only about 17% fractions whose molecular weight was above 890, the theoretical minimum represented by the formula IV. Removal of the low molecular weight ingredients is technically possible but economically impracticable. Therefore, it became necessary to abandon the attempt of making use of the ready supply of the fluoropolyol. Effort was turned to the development of fluoroepoxy resins. Synthesis of Curing Agents for the Fluorodiepoxides. Fluorinated curing agents are preferable for fluorodiepoxides I1 in consideration of their compatibility and the total fluorine content in the cured epoxy material. Early work demonstrated that 1,3-bis(y-aminopropyl)tetramethyldisiloxane, a commercially available a,u-diaminosiloxane, could be made compatible with C6-fluorodiepoxide by heating the mix to 50 "C (Christou et al., 19791, but the system is handicapped by the high sensitivity of the amine to carbon dioxide in the air as well as by the concern that the siloxane structure may compromise the low moisture permeation property desired for the polymer. A fluoroanhydride, 4-(2-hydroxyhexafluoro-2propy1)phthalic anhydride, was also synthesized earlier as a curing agent. Like other anhydrides, curing with the fluoroanhydride could be completed only at high temperature (Griffith and O'Rear, 1977). In order to achieve near room temperature curing for coating application, a new adduct amine was synthesized by reacting a fluorodiepoxide I1 with an excess of ethylenediamine (Griffith, 1984). Both c6-and C8-fluorodiepoxideswere tried in the synthesis. C6-Fluoroepoxyadduct amine (V) was obtainable by a simple preparation procedure with practically theoretical yield, while C,-fluorodiepoxide reacted with side reactions.
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Fluoroepoxy Resin Cured with the Adduct Amine. Outgassing and General Properties. Fluoroepoxy material formulated with C8-fluorodiepoxide I1 and c6fluoroepoxy adduct amine V, coded as GFE-600 series, became our primary target for this investigation after it was established that the material can meet the NASA outgassing requirement tested according to the ASTM method. Thermogravimetrical analysis also showed less than 1% weight loss when the material was heated to 250 "C at a heating rate of 10 "C per minute. The onset temperature of the weight loss was about 130 "C. Figures 1 and 2 compare the thermograms of the fluoroepoxy with those of a polyurethane and a common epoxy material. The coefficient of thermal expansion of the fluoroepoxy (Figure 1)is very close to that of the popular conformal coating Uralane 5750LV in a wide temperature range. Its modulus is intermediate between those of Uralane and epoxy Epon 815/V140 (Figure 2). These properties are of interest to applications where thermomechanical stress may be a great concern. The fluoroepoxy has a specific gravity of 1.66. Its shore hardness was determined to be 65D (ASTM Method D2240), not as hard as epoxy Epon 815/V140 (measured as 80D) and not as soft as Uralane (measured as 30D or 80A). When the electrical properties were compared by the measurements of their dielectric constants, volume resistivity (at 1000 V), and high-voltage partial discharge (up to 1000 V/mil) (Bartnikas and McMahon, 1979), the fluoroepoxy and Uralane 5753LV were found to be generally equal, and both are far superior over the common epoxy Epon 828/V140 with respect to their electrical insulation feature. Uralane 5753 and also Uralane 5750 are
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 575 11 .o
Table 11. Solvent Absorption and Desorption by Plastics" (% weight gain) fluoroepoxy Conathane GFE-AD635 ENll exposure i-PrOH, CH2C12, i-PrOH, CH2Cl2, condition % % % % absorption (immersion) 24 h 0.32 9.4 19.5 disintegrated 48 h 0.49 10.5 19.7 12 h 0.63 10.4 19.6 desorption (room air drying) l h 0.55 9.2 14.2 4h 0.49 8.0 8.9 72 h 0.36 3.7 -0.48
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known to have excellent moisture resistance and electrical insulation properties. The fluoroepoxy is resistant to solvents such as acetone, ethanol, and methylene chloride, but is vulnerable to the fluorinated solvent Freon TI?.Table II records the solvent absorption and desorption of two materials for comparison. Methylene chloride, a very strong solvent, swelled the fluoroepoxy and disintegrated EN11. The latter is a polyurethane potting compound similar to Uralane in properties. In the desorption stage, the loss of solvent was very fast for EN11, but very slow for the fluoroepoxy-an indication of slow solvent diffusion or permeation for the fluoroepoxy. Moisture Vapor Tramission and Absorption. A low moisture vapor transmission rate is the key to a good moisture vapor barrier material. The rates of the fluoroepoxy film and the films of a number of reference materials were determined according to an ASTM method. The data collected are given in Table 111. Generally, the rates are inversely proportional to the fluorine content of the materials. The rate of fluoroepoxy GFE-AD611is higher than the highly fluorinated Teflon, but is only about one-twelfth of that of the four polyurethane coatings or potting materials popular in the market. Uralane and Conathane E N l l are polybutadiene-diol-based polyurethanes and are noted for their long-term moisture resistance and high electrical resistance. A spray coating method was used to prepare the fluoroepoxy films. In industrial application, a heated twocomponent spray gun, which is commercially available, would be used to spray viscous coating materials such as the fluoroepoxy. In the laboratory for the rate determination, a solution formulation of the fluoroepoxy was developed to make spray coating. Spray-coated films were then separated from the tin-plated metal substrate.
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Figure 3. Moisture absorption (plastic disk specimens, 2.28441. diameter).
Solvent trapping was a problem, which is a dilemma expected to be encountered with a good vapor barrier material. To reduce trapping, a considerable volatile solvent system was developed for the fluoroepoxy coating solution. Further improvement was made by building the film thickness with multiple light sprayings, allowing time for solvent evaporation between sprayings, and programming the curing temperature to facilitate the release of solvents. With these precautions, it was possible to keep the 125 "C vacuum outgassing level of the cured film at about 1.5%. The level could be brought down to 1%or less by baking the film at 100 "C in order to meet the NASA requirement. An attempt was made to reduce the viscosity of the fluoroepoxy compound by incorporating a small amount of the a,o-diaminosiloxane mentioned before in the discussion of curing agents. As noted for GFE-AD613 in Table 111, the transmission rate of the adulterated film seems to have been adversely affected, while the viscosity was not improved by the aminosiloxane. Closely related to moisture transmission is moisture absorption. Three materials were compared in the moisture absorption experiment conducted at room temperature and 97% relative humidity (RH). The results shown in Figure 3 indicate the overwhelming superiority of the
Table 111. Moisture Vapor Transmission Rates of Polymer Films (at 33 "C, ASTM D1653-72) av film transmission rate, thickness, mgmm/(cm2.24 h) mm (mil) material fluorine content, wt % fluoroepoxy GFE-AD611 56 0.045 (1.8) 0.101 fluoroepoxy GFE-AD613 56 0.060 (2.4) 0.139 fluoropolyurethane GFU9 fluoropolyurethane GFUll Teflon PFA Teflon FEP polyurethane Solithane 113/300 polyurethane Uralane 5750LV polyurethane Uralane 5753LV polyurethane Conathane EN11
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576
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1980 0.7
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fluoroepoxy and polyurethane E N l l over epoxy Epon 815/V140. Plotting in a different scale in Figure 4 illustrates that the moisture absorption of the fluoroepoxy was essentially leveled at 0.26% in 27 weeks, while that of ENll was approximately 2 times higher and the difference increases continuously with time. Moisture Absorption of the Coated GraphiteEpoxy Composite. It is of prime interest for this project to test the effect of the fluoroepoxy as a coating to protect the graphite-epoxy composite from moisture absorption since the results presented above have clearly established the superiority of the fluoroepoxy in terms of its extremely low transmission rate and low moisture absorption. Quasiisotropic 16-ply laminate boards (f45°/002/f450/00/900)s were generously supplied by Hercules, Inc., for the test. The boards were thoroughly cleaned and dried at about 70 "C under vacuum for 5 months (about 0.2%total weight loss). Those to be coated had their edges and corners rounded to assure an all-around coating coverage. The fluoroepoxy coating was applied by solution-spraying. A thickness of ca. 0.0025 in. was built up by multiple passes of light spraying. Both the coated and the uncoated board specimens were exposed to an environment of 97% RH at room temperature. The moisture pickup was measured periodically over 30 weeks. The results are plotted in Figure 5. It should be expected that the initial rapid moisture pickup (ca. 0.2%) would be mainly moisture adsorption on the surface, and the difference of adsorption on the two kinds of surface was unrecognizable as shown. However, when the absorption process soon prevailed, the effectiveness of the low-permeation coating became obvious and the difference in moisture pickup was consistently augmented with time as shown. The magnitude of the difference may not look as dramatic as we would have liked, but the effect is, in fact, remarkable when it is recognized that the coating works effectively on a composite material which has very low moisture absorption itself because of its 60% content of non-moisture-absorbing graphite fiber. The effectiveness of the moisture vapor barrier coating is evidently positive and becomes more important as the exposure time is prolonged. Probably, it is the best available plastic coating as a moisture vapor barrier coating for the composite laminate. Fluoroepoxy Resin as a Sealant for an Adhesive Bondline and as an Adhesive for Teflon. The moisture barrier property of the fluoroepoxy may be further utilized by applying it as a sealant for an adhesive bondline. It is widely recognized that moisture may slowly penetrate
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Figure 5. Moisture absorption of graphite-epoxy composite laminate boards.
through the adhesive bondline and gradually deteriorate the adhesive bond strength. Improvement may be made by using adhesives with the least moisture affinity or by using a good moisture vapor sealant to cover the bondline. To illustrate the use of the fluoroepoxy, aluminum panels were overlap-bondedwith a common epoxy adhesive Epon 828/V140. The bondline of one set of the bonded panels was sealed with the fluoroepoxy. The sealed set was aged together with an unsealed set in a humidity chamber kept at 40 "C and 92% RH. After 75 days, the lap shear bond strength of the aged panels was tested. The results are as follows: with the fluoroepoxy sealing, 1546 psi; without the fluoroepoxy sealing, 1363 psi. Evidently, the sealing has helped retain 13% higher strength in 75 days. The effectiveness should be more pronounced if the aging time is extended and when the moisture diffusion is more extensive through the bondline without sealing. Like other epoxy resins, the fluoroepoxy may be used as an adhesive itself. It would be particularly attractive if it can be demonstrated to bond Teflon. Teflon and other fluorinated polymers have low surface energy and are chemically inert in general. To bond Teflon, the surface has to be etched with a special, strong etching agentsodium naphthalene complex solution, which is corrosive and may be hazardous in use. In many cases, it is undesirable or simply impossible to conduct such an etching operation. Therefore, an adhesive for Teflon and other fluorinated polymers has been highly in demand. Being chemically close to Teflon, the fluoroepoxy should have a considerable affinity to Teflon. To test its effectiveness, aluminum and Teflon rod adherends were made according to an ASTM method. One set of them was bonded with the fluoroepoxy,while the other set was bonded with Epon 828/V140 for comparison. The adhesive tensile strengths shown in Table IV demonstrate that the bond strength of the fluoroepoxy to aluminum is only 60% of that of Epon 8281V140, yet its strength to Teflon is 2 times that of the latter. The sodium naphthalene etching of Teflon improves the bond strength of the latter to a level only 40% higher in average than that achievable by the fluoroepoxy without etching. In addition, the etched surface is very
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 577
Table IV. Tensile Strennth of Adhesives (ASTM D2095) fluoroepoxy epoxy Epon GFE-AD604 828/V140 strength, failure strength, failure rod adherend psi mode psi mode aluminum 3470 f 330 adhesive 7030 f 258 adhesive/ cohesive Teflon 640 f 48 adhesive 330 f 16 adhesive Teflon, etched 912 f 180 a ~
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"All failed by the separation of the etched surface layer. Strengths varied from 710 to 1120 psi.
sensitive to moisture. The strength gained by the etching varied greatly with the degree of etching. Either overetching or underetching resulted in so drastic a chemical change of the etched surface layer that it was weakened and separated in all the rod specimens in the tensile test. In contrast, the fluoroepoxy adhesive offers a new convenient way to bond Teflon with a considerable strength free from risk. Fluoroepoxy Foam by a New Foaming Process. Teflon has been widely used for electrical insulation and as structural components due to its excellent electrical and other properties. For many applications, Teflon and other thermoplastic fluoropolymer foams are particularly desirable because they render reductions in weight, dielectric constant, and dissipation factor. There are many patented processes to make Teflon foam. However, all of them require a minimum temperature of 240 "C and a pressure of up to 1500 psi or much higher. These methods employ volatile or chemical blowing agents and nucleating agents which may leave undesirable residues in the foam products (Port et al., 1971; Kamata, 1972; Randa et al., 1983). The main reason for the required extreme conditions is that the available thermoplastic fluoropolymers are plasticable only at such high temperatures, and the high temperature dictates the use of high pressure to control foaming. Consequently, these requirements can constitute a prohibitive barrier to many applications. With thermoset fluoroepoxy resins, regular epoxy foaming processes may be practical. One novel property observed in working with the fluoroepoxy is that the highly fluorinated two-component formulated compound may be viscous but has minimal tendency to trap the air introduced in mixing the two components. In other words, the usual vacuum deaeration step in the mixing process is easy and fast. Just as it is easy to get the air out under vacuum, gas can also be readily dispersed in the fluoroepoxy compound under a moderate pressure. Perhaps this may be attributed to the unusual low molecular forces between fluorinated molecules. It is this unusual property that offers a new foaming process. The process is simple. A chosen unreactive gas is dispersed at or near room temperature in the fluoroepoxy compound before gelation by
exposing the compound to the gas under a moderate pressure. The dispersed gas will be trapped in the compound upon gelation. The compound will expand to form a foamy or cellular structure when it is heat-cured (Lee, 1985). The process is contrary to the common pressure potting method, which removes any trapped gas and eliminates voids in the potting compound by exposing the compound to an unreactive gas under a moderate pressure for a period of time or until gelation. The depth of the gas dispersion in the fluoroepoxy compound depends on the magnitude of the gas pressure, and the cellular structure may be controlled by the degree of gelation or cure and the temperature programming in the heat-cure step. Usually a pressure of 3 atm or 45 psi suffices to make a foam sheet 3 mm thick. The foam cell is essentially closed. The freedom to choose any unreactive gas for foaming is practically a freedom to store a chosen gas in the cellular material. Air and nitrogen will be used for ordinary purposes. Oxygen, argon, krypton, sulfur hexafluoride, and many others may be the choice when some special properties form the gas may be desired. The stored gas may remain in the cell for a long duration because of the low permeation rate anticipated, and the gas pressure in the cell is probably slightly negative. The low molecular weight gases such as hydrogen and helium may not be suitable because they may diffuse too fast to make a good foaming.
Acknowledgment The assistance provided by Joe A. Colony, Carroll H. Clatterbuck, Joanne M. Uber, and Renate S. Bever in analysis, sample preparation, and testing is gratefully acknowledged. Registry No. I1 (homopolymer), 104215-81-8;V, 104291-07-8.
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Received for review January 14, 1986 Accepted March 31, 1986
