Abstrict A method and a composition are provided for molding low density
desiccant syntactic foam articles. A low density molded desiccant
article may be made as a syntactic foam by blending a thermosetting
resin, microspheres and molecular sieve desiccant powder, molding
and curing. Such articles have densities of 0.2-0.9 g/cc, moisture
capacities of 1-12% by weight, and can serve as light weight structural
supports.
Claims What is claimed is:
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 BACKGROUND OF THE INVENTION
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. 3545622. Such
articles generally have a density of about 1 g/cc, and compressive
strengths of about 8000 psi. Higher density structural desiccants
are reported made from molecular sieve, glass frit and bentonite,
as reported in, e.g., U.S. Pat. No. 3235089.
Others have made materials having flexibility and high tensile
strengths, as disclosed in, e.g., U.S. Pat. Nos. 4036360 4239516
and 3704806. 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. 2882243 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.
TABLE 1 ______________________________________ Typical Properties
of Type 3A Powder Desiccant Property Value ______________________________________
Nominal Pore Diameter (.ANG.) 3.0 Density (g/cm.sup.3) Hydrate Wet
2.03 Activated Dry 1.57 Particle Diameter (.mu.m) 10.0 Weight (Percent)*
Equilibrium H.sub.2 O Capacity 23 Water Content (as Shipped) <2.5
______________________________________ *Grams H.sub.2 O/100 grams
activated desiccant at 17.5 mm Hg and 25.degree. C.
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. 3562223.
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. |