Method and composition for molding low density desiccant syntactic foam articlesDesiccant Abstract Desiccant Claims 1. A process for molding to size a desiccant syntactic foam article having a density of 0.2-0.9 g/cc and a moisture capacity of 1-12% by weight, comprising the steps of: (a) charging a mold with a powdery mixture of an activated molecular sieve desiccant, microspheres and a thermosetting resin, the amount of the desiccant being sufficient to provide the required moisture capacity, and the amounts of the microspheres and resin being such that the microspheres/desiccant volume fraction exceeds the packing factor by an amount sufficient to substantially avoid shrinkage without causing excessively high molding pressures; (b) covering the mold and heating the covered mold to a temperature and for an amount of time sufficient to melt the resin; and (c) tightly closing the mold and heating the closed mold to a temperature and for an amount of time sufficient to cure the resin, and removing the resultant desiccant syntactic foam article from the mold. 2. The process of claim 1, wherein the microspheres are glass microbubbles. 3. The process of claim 2, wherein the glass microbubbles have diameters of 10-150 .mu.m and a bulk density of 0.16-0.38 g/cc. 4. The process of claim 3, wherein the glass microbubbles have a bulk density of 0.18-0.22 g/cc. 5. The process of claim 1, wherein the thermosetting resin is a polybenzimidazole resin, a polyphenylene resin, a phenolic resin or a polyimide resin. 6. The process of claim 5, wherein the resin is a polyimide resin. 7. The process of claim 1, wherein the desiccant is Type 3 A molecular sieve having a nominal pore size of about 0.3 nm. 8. The process of claim 1, wherein the microspheres/desiccant volume fraction exceeds the packing factor by up to about 25%. 9. The process of claim 8, wherein the microspheres/desiccant volume fraction exceeds the packing factor by about 5-15%. 10. A desiccant syntactic foam article having a density of 0.2-0.9 g/cc and a moisture capacity of 1-12% by weight, produced by the process of claim 1. 11. The desiccant syntactic foam article of claim 10, having a compressive strength of at least about 750 psi. 12. A desiccant syntactic foam article of claim 11, wherein the compression strength is at least about 1000 psi. 13. A desiccant syntactic foam article according to claim 1, having a density of 0.2-0.9 g/cc, a moisture capacity of 1-12% by weight, and a compressive strength of at least about 750 psi, comprising a filler of molecular sieve desiccant and microspheres, and a cured thermoset resin binder. 14. The desiccant syntactic foam article of claim 13, wherein the compressive strength is at least 1000 psi. Description The present invention relates to a method and a composition for molding to size low density desiccant syntactic foam articles, and the articles produced thereby. It is known that molded desiccant articles may be made from various desiccant materials in combination with binders. Prior art articles of this type include molded blocks comprising a desiccant and an epoxy resin, as disclosed in, e.g., U.S. Pat. No. 3,545,622. Such articles generally have a density of about 1 g/cc, and compressive strengths of about 8,000 psi. Higher density structural desiccants are reported made from molecular sieve, glass frit and bentonite, as reported in, e.g., U.S. Pat. No. 3,235,089. Others have made materials having flexibility and high tensile strengths, as disclosed in, e.g., U.S. Pat. Nos. 4,036,360, 4,239,516 and 3,704,806. However, none of these references disclose a low density syntactic foam article nor a method for molding such an article to size. OBJECTS OF THE INVENTION One object of the present invention is to provide a method for molding to size low density desiccant syntactic foam articles with substantially no shrinkage and without excessively high molding pressures. Another object of the invention is to provide low density molded desiccant articles which, in addition to adsorbing moisture, are strong enough to serve as a structural support yet light enough to be used in applications where excess weight is detrimental. A further object of the present invention is to provide low density molded desiccant articles which are sufficiently stable to heat to permit repeated activation without degradation. Yet another object of the invention is to provide a powdered composition which is stable indefinitely and which can be molded in a relatively short time and by simpler procedures than those used to manufacture the higher density molded desiccants currently in use. Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art. SUMMARY OF THE INVENTION These and other objects of the invention are achieved by a process for molding to size a desiccant syntactic foam article having a density of 0.2-0.9 g/cc and a moisture capacity of 1-12% by weight, comprising the steps of: (a) charging a mold with a powdery mixture of an activated desiccant, microspheres and a thermosetting resin, the amount of the desiccant being sufficient to provide the required moisture capacity, and the amounts of the microspheres and resin being such that the microspheres/desiccant volume fraction exceeds the packing factor by an amount sufficient to substantially avoid shrinkage without causing excessively high molding pressures; (b) covering the mold and heating the covered mold to a temperature and for an amount of time sufficient to melt the resin; and (c) tightly closing the mold and heating the closed mold to a temperature and for an amount of time sufficient to cure the resin, and removing the resultant desiccant syntactic foam article from the mold. In a composition of matter aspect, the present invention provides desiccant syntactic foam articles, and a composition of matter for use in molding the same. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plot of the density and the packing factor of a mixture of glass microbubbles and desiccant as a function of the weight ratio. FIG. 2 is a plot of the volume fraction of three glass microbubble/desiccant mixtures as a function of the weight ratio, showing the intersection of those curves with the packing factor curve. DETAILED DISCUSSION The desiccant used in the present process is a powdered solid having a substantial capacity for moisture adsorption. Preferably, molecular sieve is used having a nominal pore size sufficiently large to admit water molecules. Molecular sieves are crystalline metal alumino-silicates, as disclosed, e.g., in U.S. Pat. No. 2,882,243, which is incorporated herein by reference. Any molecular sieve may be used that retains moisture, e.g., Types 3 A, 4 A, 5 A and 13X. Type 3 A will be discussed hereinafter for illustrative purposes. It is a potassium alumino-silicate crystal. "Type 3 A" refers to the nominal pore size of 3 Angstroms (0.3 nm), which admits molecules such as water, ammonia and methanol having effective diameters of 3 .ANG. or less, while excluding molecules with larger effective diameters such as nitrogen. Typical properties of Type 3 A powdered desiccant are shown in Table 1.
It will be seen that Type 3 A molecular sieve has an effective moisture adsorbing capacity (Cm) of about 20.5%, which represents the equilibrium water capacity minus the water content of the activated desiccant, as shipped. This is typical of activated molecular sieves, which normally have effective moisture capacities of about 20-23% by weight. Other desiccants having equivalent properties may also be used in the present process. The microspheres used to prepare the low density desiccant syntactic foam articles of the invention may be glass microbubbles (GMB), ceramic microballoons, carbon microspheres or other chemically stable, water resistant and non-porous hollow spheres having a low density. Commercially available microspheres normally have bulk densities of about 0.16-0.4 g/cc. They are often floated to eliminate broken microbubbles and/or surface treated, which may enhance resin adhesion and prevent the balloons from caking during storage. A preferred type of microspheres is thin walled, hollow glass microbubbles having diameters of about 10-150 .mu.m, e.g., those produced by the 3M Company. These GMB may be purchased in densities ranging from about 0.16-0.38 g/cc. As an illustration, 3M A20/1000 GMB will be exemplified in the discussion which follows, although this is not limitative of the invention. Its nominal density is 0.20 g/cc, and it has a nominal crush strength of 6900 kPa. A mixture of the microspheres, e.g., the GMB, and the desiccant, e.g., Type 3 A molecular sieve, comprises the filler used to prepare the syntactic foam of the invention. A third component is a thermosetting resin which, when cured, binds the desiccant and microsphere particles to form a rigid foam having a relatively high void volume. The resin occupies a very small volume in the syntactic foam and the void volume can range up to about 40%. Because the filler itself has a low density, this permits production of a desiccant syntactic foam article which has a density of 0.2-0.9 g/cc and a moisture capacity of 1-12% by weight, preferably 1.5-12%. It will be understood that higher moisture capacity implies higher minimum density, so that 12% moisture capacity can be achieved with a minimum density of about 0.5 g/cc. The desiccant content of the articles will thus be 4.35-60% by weight. These articles nevertheless have compressive strengths of at least about 750 psi, and usually at least about 1000 psi. They are strong enough to serve as a structural support despite their light weight. The thermosetting resin used in the present process for producing the low density syntactic foam articles of the invention should be stable at room temperature. It is advantageous to use a resin which cures by a process which does not evolve a volatile by-product to avoid bubble formation during cure. Suitable such resins include polyimide resins, polybenzimidazole resins, e.g., Imidite X386 (Whittaker), polyphenylene resins, e.g., H-resin (Hercules), phenolic resins, e.g., Plyothen 24-655 (Reichhold), and the like. The cured resin preferably is stable at temperatures of about 230.degree.-300.degree. C., the temperature at which molecular sieves are reactivated. It will be noted that most epoxy resins do not tolerate prolonged or repeated treatment at these temperatures. It is also convenient to use a resin which melts at a temperature sufficiently below the cure temperature that the mold may be closed tightly before cure begins to any substantial extent. Otherwise, more careful timing is necessary during molding. Preferred resins are polyimide resins, especially those that cure by an addition reaction and without outgassing. Illustrative of such polyimide resins is Kerimid 601, made by Rhone Poulenc, a fully imidized powder of the type disclosed in U.S. Pat. No. 3,562,223. The present process will be exemplified with the use of this resin, although other thermosetting resins may also be used. Molding a low density syntactic foam to size requires that the packing factor of the filler component be known. The packing factor represents the maximum volume fraction the filler will occupy within the finished article, and is the ratio of the tap density to the true density. If the packing factor is exceeded, the mold will be difficult to close. If GMB is the filler and the packing factor is exceeded, some GMB will be broken. The filler component for a low density syntactic foam desiccant is the microspheres/desiccant combination, e.g., GMB/Type 3 A molecular sieve desiccant. It is advantageous to plot a curve of the packing factor for various microsphere/desiccant combinations, by blending different weight ratios of microspheres and desiccant and then measuring tap density and true density. The tap density, true density and packing factor are measured according to ASTM D3101. The packing factor generally does not vary linearly with the change in weight ratio. FIG. 1 shows the packing factor for various combinations of GMB/desiccant, where the GMB is 3M A20/1000 GMB and the desiccant is Type 3 A molecular sieve, with the properties shown in Table 1. FIG. 1 also shows a plot of the density of this filler as a function of the GMB/desiccant weight ratio. Where a molded desiccant part is to be molded to size, the volume of the part, the desired density and the desired moisture adsorption capacity are generally specified. These parameters will determine the amount of desiccant required in the final article, and thus the amount of desiccant required in the formulation used to mold it to size. This amount is constant, regardless of the remaining constituents of the formulation. To determine the weights of resin and microspheres in the formulation, an arbitrary microspheres/desiccant weight ratio is selected, the weight of microspheres is calculated from the known desiccant weight, and the resin weight is then the difference between the sum of the microspheres and desiccant weights and the total part weight, determined by the density and volume specifications. For any specific formulation, the volume fraction of the microspheres/desiccant combination is calculated by dividing the combined volumes of microspheres and desiccant by the part volume. The combined volumes are calculated by dividing the combined weights by the density for the chosen weight ratio, e.g., as shown in FIG. 1. Given a constant dessicant weight and part volume, the microspheres/desiccant volume fraction will vary with the microspheres/desiccant weight ratio. It is useful to plot the volume fraction, determined as described above, as a function of the microspheres/desiccant weight ratio. An illustration of such a plot is shown in FIG. 2. The appropriate portion of the packing factor curve of FIG. 1 is also shown in FIG. 2. The intersection of these lines is significant and indicates where a specific formulation will exceed the microspheres/desiccant packing factor. Formulations with volume factor at or above the packing factor are used when an article is to be molded to size. Formulations below the packing factor will result in shrinkage to various degrees, depending upon the amount by which the formulation is lower than the packing factor. Formulations for molding low density desiccant syntactic foam articles to size are advantageously based on microspheres/desiccant volume fractions of up to about 25% higher than the packing factor, preferably about 5-15% more than the packing factor. This value results in articles that have a small number of broken microspheres, which are normally not detrimental to the function of the part, and does not cause excessively high molding pressures. A different value for the amount by which the volume fraction exceeds the packing factor may be appropriate for other types of formulations, and the illustrated value of 5% should not be considered limitative, but merely an empirical guideline for GMB/Type 3 A molecular sieve fillers. The three components of a typical molding formulation are blended to achieve a homogeneous mixture, e.g., in a V-shell blender. The mixture should be stored in a moisture-free environment such as dry nitrogen until ready for use. It will be noted that this type of formulation can be stored in a dry atmosphere at room temperature for an indefinite period of time, in contrast to liquid epoxy/desiccant powder mixtures used to prepare prior art desiccant articles, which must be used within 48 hours. A mold is charged with the formulation, and it is necessary to spread the charge evenly to prevent lower density areas which will crack. The material will now flow to compensate for uneven material loading. When the charge is evenly distributed, the top of the mold is gently set in place and the entire mold is placed in a preheated oven at a temperature slightly above the melting point of the thermosetting resin. The resin melts and coats the low density filler, which remains solid. After the resin melts, the mold is tightly closed and placed in an oven set at the cure temperature, for a time sufficient to cure the thermosetting resin. After this time, the mold is removed from the oven and disassembled. It is advantageous to remove the molded article from the mold while both are hot, since removal after the mold has cooled is more difficult because the mold contracts more than the part. The times and temperatures at which melting and curing of the resin are effected will vary for different resins, and these are adjusted as needed. After completion of cure and demolding of the article, it is advantageous to effect an additional activation of the syntactic foam article under standard conditions to insure consistent quality. Low density desiccant syntactic foam articles molded to size according to the invention may be machined, if necessary, but the article can be molded to size with a minimum of machining. In contrast, prior art desiccant articles molded from a desiccant and a liquid epoxy resin must have the outer surfaces machined off, since the liquid resin migrates to the surface during cure. Furthermore, this type of article requires long curing times, on the order of 30 hours, while the cure time for, e.g., polyimide resin, which is a preferred component of the present formulation, is short, on the order of 3 hours. The molded articles produced by the present process are characterized by low density, i.e., 0.2-0.9 g/cc, and by a moisture capacity of 1-12% by weight. Furthermore, these articles have a sufficiently high compressive strength so that they are strong enough to serve as structural supports even though they are light enough to be used in applications where excess weight is detrimental. For example, a typical molded article according to the present invention will have a compressive strength of at least about 750 psi, preferably at least about 1000 psi, and routinely about 1700 psi. Articles according to the invention are rigid enough to withstand machining operations. In addition, where a polyimide resin is used, or another resin which is also stable to at least 300.degree. C., the desiccant article may be repeatedly activated without substantial degradation. Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. These examples illustrate the molding of syntactic foam articles using the formulation containing Type 3 A molecular sieve desiccant powder having the properties shown in Table 1, 3M A20/1000 GMB and Rhone Poulenc Kerimid 601 polyimide resin. In the following examples, all temperatures are set forth uncorrected in degrees Celsius; unless otherwise indicated, all parts and percentages are by weight. EXAMPLE 1 A molded part was required having a moisture capacity of 10 g, a volume of 280 cc and a maximum density of 0.5 g/cc. This corresponds to a moisture capacity of at least 7.14% by weight. Given an effective moisture capacity for the desiccant of 20.5% by weight (23.0%-2.5% water content as shipped), the amount of desiccant required is 10 g/0.205=48.8 g. This amount of desiccant is constant, regardless of the formulation. For various GMB/desiccant weight ratios, the tap density, true density and packing factor are measured and plotted, and the density of these filler combinations is also plotted, as shown in FIG. 1. The volume fraction is then calculated for the particular part volume, and plotted against a convenient range of GMB/desiccant weight ratios.
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