Abstrict A method of forming a desiccating part including the steps of:
(a) blending a composition comprising: at least 60 wt % desiccant,
up to 10 wt % wetting agent, up to 5 wt % processing aid, and 10-30
wt % thermosetting resin; (b) forming the composition blended in
step (a) into a part or shape; and (c) heating the part or shape
of step (b) to crosslink the resin. Also included is a desiccating
part comprising at least 70 wt % desiccant and a thermosetting binder
resin.
Claims 1. A method of forming a desiccating part comprising the steps
of: (a) blending a composition comprising: at least 60 wt % desiccant,
up to 10 wt % wetting agent, up to 5 wt % processing aid, and 10-30
wt % thermosetting resin; (b) forming the composition blended in
step (a) into a part or shape; and (c) heating the part or shape
of step (b) to crosslink the resin.
2. The method of claim 1 wherein the heating step (c) comprises
heating the part or shape for a time sufficient to crosslink the
resin and activate the desiccant.
3. The method of claim 1 wherein the blending step (a) comprises
adding a molecular sieve as the desiccant.
4. The method of claim 1 wherein the blending step (a) comprises
adding a desiccant which is a molecular sieve powder having a size
selected from the group consisting of 3A, 4A, 5A, and 10A molecular
sieve powders, and combinations thereof.
5. The method of claim 1 wherein the wetting agent is isopropyl
alcohol.
6. The method of claim 1 wherein the thermosetting resin is selected
from the group consisting of: phenolic resins, alkyd resins, amino
resins, polyester resins, epoxide resins, melamine resins, urea-formaldehyde
resins, phenol-formaldehyde resins, polyurethane resins, and silicone
resins.
7. The method of claim 1 wherein the thermosetting resin is not
self-curing, and the method further comprises the step of adding
a curing agent between steps (b) and (c).
8. The method of claim 1 further comprising the step: (d) machining
the crosslinked part or shape from step (c).
9. A desiccating part comprising: at least 70 wt % desiccant; and
a thermosetting binder resin.
10. The desiccating part of claim 9 wherein the binder resin is
present at 5-30 wt %.
11. The desiccating part of claim 9 wherein the binder resin is
selected from the group consisting of: phenolic resins, alkyd resins,
amino resins, polyester resins, epoxide resins, melamine resins,
urea-formaldehyde resins, phenol-formaldehyde resins, polyurethane
resins, and silicone resins.
Description FIELD OF INVENTION
[0001] The present invention relates to desiccants and more specifically
to desiccant parts that can be molded.
BACKGROUND OF INVENTION
[0002] It has been known to incorporate desiccants of varying types
into resins that can then be molded or formed into parts which are
capable of adsorbing moisture. One such type of resin has been the
various thermoplastics. When a thermoplastic part has adsorbed its
capacity of moisture, however, the part cannot be easily reactivated
so that it could be used again, because the part would melt, or
at least deform, at the temperatures needed to drive sufficient
moisture out to effectively reactivate the desiccant disposed therein.
Moreover, binders such as thermoplastics, waxes, and clay cannot
withstand the high activation temperatures needed to activate the
desiccant disposed within them. This makes parts and shapes made
of these binders usable, but not recyclable or re-activating.
SUMMARY OF INVENTION
[0003] The present invention includes a method of forming a desiccating
part comprising the steps of: (a) blending a composition comprising:
at least 60 wt % desiccant, up to 10 wt % wetting agent, up to 5
wt % processing aid, and 10-30 wt % thermosetting resin; (b) forming
the composition blended in step (a) into a part or shape; and (c)
heating the part or shape of step (b) to crosslink the resin.
[0004] Also included is a desiccating part comprising at least
70 wt % desiccant and a thermosetting binder resin. Preferably,
the part includes a binder resin present at 5-30 wt %. A preferred
binder resin is selected from the group consisting of: phenolic
resins, alkyd resins, amino resins, polyester resins, epoxide resins,
melamine resins, urea-formaldehyde resins, phenol-formaldehyde resins,
polyurethane resins, and silicone resins.
DETAILED DESCRIPTION OF INVENTION
[0005] The present invention provides a moldable desiccant material
capable of being pressed or otherwise molded into a shape or other
dimensionally stable part, such as a tablet or bar, that is then
capable of being cured to form a dimensionally stable part. This
cured part can be activated, used, and reactivated by heating at
high temperatures without the part or shape becoming disfigured
or destroyed. This aspect of the invention is achieved by using
a thermosetting resin as the binder, which is crosslinked during
curing to bind the desiccant therein in a form which is capable
of being activated and reactivated at high temperatures while still
maintaining the physical shape and properties of the form.
[0006] A general advantage of the present invention is that the
finished product or shape (such as a tablet) has increased strength
(as compared to products bound by thermoplastics) and that they
are not deformed when heat is later introduced to the part. Once
the thermosetting resin binder has been cured, heat cannot soften
the part. The only way the part could be destroyed is if the part
is heated above about 1000.degree. F., or, in some cases, 1500.degree.
F. (depending upon the resin used), which is the point at which
a typical thermoset binder would turn to ash. A typical crosslinked
resin used in accordance with the present invention, such as a phenolic
crosslinked resin, would withstand temperatures up to about 1100.degree.
F. Parts made with the method of the present invention using thermosetting
resin binders can be reactivated after they have absorbed their
capacity of water. This means that the desiccant parts can be reused
or recycled. When a thermoplastic part has adsorbed its capacity
of moisture the part cannot be reactivated to be used again because
it would melt at temperatures necessary to reactivate the desiccant.
[0007] As is the case with the formation of thermoplastic parts,
desiccant parts that are made with a thermosetting resin adsorb
moisture during the processing steps from mixing to forming to heating
and curing. Parts made with a thermosetting binder would be able
to be activated at a high temperature to drive off moisture, whereas
parts made with the thermoplastic resin would not be able to be
heated to these high temperatures because they would melt or deform.
For example, parts made with a thermoplastic resin and a molecular
sieve would contain 5-10% moisture at best, which moisture could
not be driven off during activation. This of course decreases the
capacity of the part and its usefulness. On the other hand, when
a thermosetting resin is used as the binder in accordance with the
present invention, the resin can tolerate the higher temperatures
used for activation of the molecular sieve and water can be removed
down to less than 2% moisture. This aspect allows the parts made
with the thermosetting resin in accordance with the invention to
have a higher capacity for adsorption and hence be more useful.
[0008] It is also noted that because the present invention, in
one embodiment, uses a thermosetting resin which cures/thermosets
with heat, volatiles are coming off the resin during this process
as the resin is crosslinking. Due to this process the resin/binder
becomes more porous than a thermoplastic resin would which results
in a part that will be better able to adsorb moisture. In other
words, the binder, and thus the part formed from it, will be more
porous which allows moisture and other materials to be adsorbed
faster.
[0009] Almost all thermosetting resins, whether self-curing or
catalyzed, can be used with the present invention. A thermosetting
resin is a resin that cures or crosslinks when heated. Some of the
types of thermosetting resins that do not require a catalyst are
phenolics, alkyds, amino resins, polyesters, epoxides, melamines,
urea-formaldehyde resins, phenol-formaldehyde resins, polyurethanes
and silicones. The term, thermosetting, also applies to materials
where additive-induced crosslinking is possible. The crosslinking
reaction of the molecular constituents can be induced by heat, radiation
or a catalyst which is also known as a curing agent. For example,
linear polyethylene can be crosslinked to a thermoset material either
by radiation or chemical reaction. As one example, there is a type
of phenolic resin known as a novolak resin which is not self curing
but is self curing with the addition of a catalyst such as hexamethylenetetramine.
[0010] As noted above, the invention includes sorbent/desiccant
particles that are blended with the binder and then pressed into
a part or shape in a press. This feature of the present invention
is advantageous in part because it allows for high volume manufacturing.
These parts can also be formed by other methods such as in a mold.
After the part has been pressed the part is heated and thereby cured
in an oven at an elevated temperature to crosslink the thermoset
binder. The heating process serves a second purpose of activating/reactivating
the desiccant. By activating the desiccant, moisture absorbed by
the desiccant during processing is driven off. The curing and the
activating can be two different steps or one step depending on the
process. Optionally, a vacuum oven is used for the activation process
to drive off more moisture.
[0011] In one example of the present invention, 3402 grams of a
3A molecular sieve powder was added to 652 grams of a phenolic resin
known as Durez 29-733 (Durez is a registered trademark of Durez
Corporation of Addison, Tex. for resins including phenol-formaldehyde
resins) in a mixer and mixed for five minutes. Then, 81 grams of
a mold lubricant, in this case a vegetable oil known commercially
as Sterotex (Sterotex is a registered trademark of Abitec Corporation
for powdered vegetable stearine used as a lubricant), was added
and the resulting mixture was mixed for 10 more minutes. Then, 410
grams of a wetting agent, in this case isopropyl alcohol (suitable
other wetting agents would be known to those skilled in the art),
was added and the resultant wet mix was mixed for 10 more minutes.
Finally, 455 grams of a 4A molecular sieve powder was added and
the resultant mixture mixed for an additional 10 minutes. After
that, the blend was laid out on a flat surface and allowed to dry
for about 20 minutes to let some of the isopropyl alcohol evaporate.
Parts were then pressed from this material.
[0012] In a preferred embodiment, some 4A molecular sieve was added
to improve flowability of the blend, as compared to a blend having
only 3A molecular sieve added as the desiccant. Depending on the
intended use or application, other desiccants and sizes could be
used, such as silica gel, or molecular sieves of varying sizes,
such as 5A or 10A. This flowability can be important, depending
on processing parameters, because a relatively free-flowing blend
can be desired in some cases such as where a small die cavity is
filled with the blend to be pressed or molded. For example, if the
die cavity is not filled correctly the part will not experience
proper pressure and will not have enough strength after pressing.
An additional problem would involve error with respect to the weight
and size of the part molded, which could yield product with voids
or stress points that could result in tool damage or breakage.
[0013] Prior to curing, the parts do exhibit good green strength
compared to prior art compositions. This is due in part to the wetting
agent. A preferred curing process starts at 99.degree. C. (210.degree.
F.) and the temperature is increased to 193.degree. C. (380.degree.
F.) over a period of 12 hours. The wetting agent evaporates in large
part, if not completely, during this process. After the parts were
cured the parts were activated at 210.degree. C. (410.degree. F.)
with a minimum of 27 inches of vacuum over 24 hours. The activation
process was conducted to remove any residual moisture and/or wetting
agent from the molecular sieve.
[0014] The curing process is important depending on the use of
the final part. If the resin is not cured properly, and in particular
if it is cured too fast, the part can crack. An exemplary curing
cycle has been used with success on sample parts, and includes a
multi-step process where the part is cured for a short time at a
lower temperature, and then cured further at a higher temperature
for a longer relative time. One such example is a curing cycle comprising
curing the part for 2 hours at 210.degree. F., and then for 8 hours
at 380.degree. F. The temperature was raised gradually from 210.degree.
F. to 280.degree. F. Additionally, and as discussed above, a part
is preferably further treated to drive off moisture and activate
it. An exemplary activation step would be to place the part in a
vacuum oven at, for example, 60 torr for 22 hours at 210.degree.
F.
[0015] Using a thermoset resin also makes possible the production
of a mix that is totally dry and uses no wetting agent. An exemplary
such formulation was made using 83% molecular sieve, 15% thermosetting
resin, and 2% lubricant as a processing aid to aid in flowability.
Depending upon processing parameters, however, even less lubricant,
down to zero lubricant, could be used to produce a product in accordance
with the present invention.
[0016] An additional aspect of the present invention is that the
crosslinked parts can be further machined to form parts which have
the above noted advantages, but which would be otherwise not easily
formed through conventional molding or pressing operations. For
example, a grinding operation would produce heat which would destroy
or deform thermoplastic parts, but could easily be performed on
parts made in accordance with the present invention. Moreover, where
a particular part or shape could not be formed through molding or
pressing alone, some machining would be necessary. The parts formed
in accordance with the invention allow for this machining to occur,
without the drawbacks of the compositions of the prior art.
ADDITIONAL EXAMPLES
Example 1
[0017] The following were mixed in a laboratory blender: 830 grams
of a 3A molecular sieve powder; 210 grams of phenolic resin, namely
Durez resin 29-733 20 grams of Sterotex vegetable oil; and 100
grams isopropyl alcohol. This blend did not flow as well the blends
noted below, but is consistent with the present invention, and can
be used if the flowability is adequate for the intended process.
Example 2
[0018] The following were mixed in a laboratory blender: 830 grams
of a 3A molecular sieve powder; 159 grams of a phenolic resin, namely
Durez resin 29-733 20 grams of Sterotex vegetable oil, 100 grams
isopropyl alcohol, and 53 grams of a 4A molecular sieve powder.
This blend flowed well and parts were formed therewith.
Example 3
[0019] The following were mixed in a laboratory blender: 415 grams
of a 3A molecular sieve powder; 75 grams of phenolic resin, namely
Durez resin 29-733 10 grams of Sterotex vegetable oil; and 100
grams of a 4A molecular sieve.
[0020] Parts were made from this mixture on a rotary press and
pressed at pressures of 3 tons, 5 tons, and 9 tons. Example parts
were formed as discs having a 1/2 inch diameter by 5/32 inch thickness.
These parts were cured and the desiccant activated in accordance
with the above described procedure.
[0021] There are many uses for such parts, which parts can be pressed
into any number of shapes suitable for use in specialized applications.
Examples of such applications include pressing the material into
bars for placement into electro-optic devices such as night-vision
scopes or equipment.
[0022] Although the invention is illustrated and described herein
with reference to specific embodiments, the invention is not intended
to be limited to the details shown. Rather, various modifications
may be made in the details within the scope and range of equivalents
of the claims and without departing from the invention. |