Note: Descriptions are shown in the official language in which they were submitted.
CA 02231008 1998-03-04
POLYMERIC COMPOSITION FOR RADIATION SHIELDING
FIELD OF THE INVENTION
The present invention relates to material for
radiation shielding, and more particularly to radiation
shielding material made of resinous binders with
radiation absorbing material dispersed homogeneously
therethrough.
BACKGROUND PRIOR ART
It is well known that radiation resulting from
medical, industrial, military, and other sources can be
harmful. Care must be taken to shield living organisms
and certain non-living objects from exposure to such
harmful radiation. It is well known to use plates of
radiation absorbing metals, such as lead plates, for
radiation shields. However, such plates are often
heavy, difficult to work with and apply, and often
susceptible to damage, such as denting, through normal
usage. Therefore, some focus has been directed towards
using resinous binders or plastic materials having
radiation absorbing material, such as lead, dispersed
therethrough for radiation shielding material. Such
material is easier to work with and mold into desired
shapes, and is often more durable. However, it has
often been a problem maintaining a homogeneous
dispersion of the radiation absorbing material
throughout the resinous binder. Often the absorbing
material will settle to the lower portions of the
resinous binder due to gravitational forces. This
settling occurs rapidly in the time period before the
resinous binder cures or sets during the production of
the shielding material. AS a result, the dispersion of
radiation absorbing material is not homogeneous
throughout the shielding material and areas of low
radiation shielding result.
It has also been found that if sufficient
radiation absorbing materials are incorporated to
achieve the required shielding, the product is
CA 02231008 1998-03-04
frequently not strong enough to support its own weight,
and also tends to tear or split when it is flexed.
Conversely, if the amount of radiation absorbing
material is adjusted so as to obtain a product which is
self supporting and/or has adequate flexibility, its
shielding capability is inadequate or is only adequate
if the material is used in a thickness which creates
problems due to its bulk and also restricts the ability
of the material to flex.
It is known to disperse large quantities of lead
throughout a plasticizer resinous matrix to form a
moldable putty shielding material, as is shown in U.S.
Patent No. 2,162,178. However, the problem with such
putty substances is that they do not cure to a hardened
state, and do not exhibit the strength and structure
necessary for many shielding uses.
Laminates have been developed which incorporate
radiation absorbing material and laminate such material
with layers of structural supporting material.
However, such laminates can be difficult and expensive
to manufacture, and are restricted in shapes and usage.
SUMMARY OF THE INVENTION
The invention provides an improved radiation
shielding material.
One object of this invention is to provide an
improved polymeric composition for radiation shielding.
Another object of the invention is to provide a
polymeric composition with a high lead equivalency for
radiation shielding.
Another object of the invention is to provide a
polymeric radiation shielding material that has a
substantially homogeneous dispersion of radiation
absorbing material therethrough with minimal areas of
low shielding capability.
Another object of the invention is to provide a
polymeric radiation shielding material with a high
CA 02231008 1998-03-04
concentration of radiation absorbing material while
maintaining a high level of strength and stability.
Another object of this invention is to provide a
polymeric radiation shielding material that is
relatively injection moldable.
Another object of the invention is to provide for
a polymeric radiation shielding material that is
extrudable.
Another object of the invention is to provide a
radiation shielding material that can be prepared and
applied to an area in a liquid or putty state which
then cures or hardens to a solid state having a high
level of strength and stability.
Another object of the invention is to provide a
polymeric radiation shielding material that is
relatively lightweight and easy to work with.
Other features and advantages of the invention
will become apparent to those skilled in the art upon
review of the following detailed description and
claims.
One embodiment of the invention provides a
polymeric composition having a high radiation shielding
effectiveness containing up to about 66% by volume
radiation absorbing material. The composition includes
a resinous binder, a radiation absorbing material
suspended substantially homogeneously throughout the
resinous binder, and a thixotropic agent dispersed in
the resinous binder to act as a suspending agent. The
thixotropic agent is present in an amount sufficient to
substantially suspend the absorbing material in the
resinous binder.
The thixotropic agent acts to help produce the
high radiation shielding effectiveness throughout the
shielding material by acting to maintain a homogeneous
dispersion of large amounts of absorbing material
before, during, and after the resinous binder sets or
cures. In some embodiments, the thixotropic agent also
acts as a supporting or reinforcing agent, thereby
CA 02231008 1998-03-04
adding to the strength and stability of the shielding
material after it cures. In certain embodiments, the
absorbing material can comprise up to about 95% by
weight of the total composition, and due to the
thixotropic agent, this large amount of absorbing
material is homogeneously dispersed throughout the
binder, and the shielding material maintains strength
and stability. In some formulations, the shielding
material may be applied by hand to encapsulate sources
of radiation or to repair dents or holes in existing
shielding material. In other formulations, the
shielding material may be injected into molds or fed
into extruders to obtain desired shapes.
In another embodiment of the invention, the
resinous binder includes a thermoset epoxy resin binder
comprising an epoxy base component and a curing agent
including cycloaliphatic polyamine. The radiation
absorbing material includes lead powder suspended
substantially homogeneously throughout the resinous
binder, and the thixotropic agent includes aramid
fibers dispersed in the resinous binder to act as a
suspending agent for the lead powder, and to act as a
reinforcing agent. The lead powder can comprise up to
about 95% by weight of the total composition, or about
665 by volume of the total composition, and the
shielding material maintains good strength and
stability. Prior to setting, the shielding material
may be applied by hand to encapsulate sources of
radiation or it may be injected into molds or extruded
to obtain desired shapes.
Another embodiment of the invention includes an
epoxy base component. The epoxy base component is for
use in forming a polymeric shielding material
containing up to about 66% by volume radiation
absorbing material and therefore having a high
radiation shielding effectiveness. The epoxy base
component is formulated such that when the epoxy base
component is mixed with a curing agent component, the
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polymeric shielding material is formed. The epoxy base
component includes an epoxy resin, a thixotropic agent,
and a radiation absorbing material.
Another embodiment of the invention includes a
curing agent component. The curing agent component is
for use in forming a polymeric shielding material
containing up to about 66% by volume radiation
absorbing material and therefore having a high
radiation shielding effectiveness. The curing agent
component is formulated such that when said curing
agent component is mixed with an epoxy base component,
the polymeric shielding material is formed. The curing
agent component includes a curing agent, a thixotropic
agent, and a radiation absorbing material.
Another embodiment of the invention includes a
polymeric composition including a resinous binder, and
a high level of a radiation absorbing material
suspended substantially homogeneously throughout the
resinous binder. The radiation absorbing material
comprises from about 55% to about 66% by volume of the
total composition. In certain embodiments the
radiation absorbing material comprises from about 91%
to about 95% by weight of the total composition. In
some embodiments, the radiation absorbing material is
lead powder.
Before embodiments of the invention are explained
in detail, it is to be understood that the invention is
not limited in its application to the details of the
construction and the arrangements of components set
forth in the following description. The invention is
capable of other embodiments and of being practiced or
being carried out in various ways. Also, it is
understood that the phraseology and terminology used
herein is for the purpose of description and should not
be regarded as limiting.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention provides a composition designed as
shielding to absorb and shield against radiation. The
composition includes polymeric materials having
radiation absorbing material and a thixotropic agent
homogeneously dispersed therethrough. The radiation
absorbing material and thixotropic agent are
homogeneously dispersed throughout a resinous binder.
The thixotropic agent is included in the formulation to
act as a suspending agent for the radiation absorbing
material. The thixotropic agent maintains the
radiation absorbing material in suspension sufficiently
well to allow for the maintenance of a homogeneous
dispersion of the absorbing material throughout the
binder.
As used herein, "resinous binder" means a
synthetic organic resin or a man-made high polymer
including thermoset and thermoplastic compounds that
are moldable in an uncured or unset state, and that are
compatible with the radiation absorbing material and
thixotropic agent used. Most any resinous binder that
does not interfere with the purpose of the radiation
shielding material can be used. The desirable
characteristics of the resinous binder include
mechanical strength, chemical stability and
compatibility with the absorbing material and
thixotropic agent. Choice of the resinous binder will
for the large part depend on the expected conditions of
use and exposure of the radiation shielding material.
It is preferable to use a resin binder that has
properties that are appropriate to the environment in
which it will be used. For example, it is often
desirable to use a resinous binder that is resistant to
the effects of heat and radiation.
Suitable resinous binders include, but are not
limited to: thermoset epoxies, phenolic resins,
unsaturated polyesters, polyurethanes, polyureas,
polyvinyl chloride homo- and co-polymers, polyolefins,
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vinylesters, methacrylates, polycarbonates, polyethers,
polyamides, co-polyetheresters, polyphenylene ethers,
co-polyestercarbonates, polyaryl ethers, polyamide
imides, polystyrenes, acrylonitrile butadiene styrene
copolymers, and mixtures thereof. Thermoset epoxy
resin based systems are preferred, particularly those
employing a liquid epoxy resin base component cured
with a cycloaliphatic polyamine resin because such
systems are worker friendly in that they are non-
critical in handling, cure well over a wide temperature
range and are quite tolerant of imperfectly prepared
substrates. Furthermore the epoxy
resin/cycloaliphatic polyamine mixture employed in the
most preferred embodiments has been shown to be
particularly resistant to the effects of intense
ionizing radiation during DBA testing for nuclear
applications. A particularly suitable and commercially
available epoxy resin is Ciba Geigy #6005, marketed by
Ciba Geigy Corporation, Three Skyline Drive, Hawthorne,
New York 10532. A particularly suitable and
commercially available cycloaliphatic polyamine resin
curing agent is Air Products #2280, marketed by Air
Products and Chemicals, Inc., 7201 Hamilton Blvd.,
Allentown, PA 18195-1501.
As used herein, ~radiation absorbing material"
generally means a material that efficiently absorbs
incident high energy radiation such as X-ray, gamma and
beta radiation so that appreciable attenuation occurs
within a relatively short path. Other desirable
attributes for most general applications include
commercial availability in the physical form desired
for reasonable cost, chemical and physical stability
and manageable toxicity. Suitable radiation absorbing
materials include materials which exhibit a substantial
radiation absorbing property, and are compatible with
the resinous binder and the thixotropic agent.
Suitable radiation absorbing materials include: lead,
bismuth, tungsten, depleted uranium, tin, copper,
CA 02231008 1998-03-04
silver, nickel stainless steel, and mixtures thereof.
More preferably, due to their high radiation shielding
effectiveness, the radiation absorbing material is one
of the following: lead, bismuth, tungsten, depleted
uranium, and mixtures thereof. Due to its
availability, high shielding effectiveness, and low
cost, the most generally preferred radiation absorbing
material is lead.
Although the radiation absorbing material may be
in many different physical forms, such as in fibers or
flakes, it is preferred that the radiation absorbing
material be in a generally spherical powder form. Due
to the reduction in physical interaction between powder
particles as compared to the physical interaction
between flakes or fibers in the mixture, more powder
can be put into the composition, thereby increasing the
shielding ability of the shielding material. It is
preferred that the radiation absorbing material powder
comprises generally spherical particles having an
average diameter ranging from about 50 to about 250
microns. It is more preferable that the absorbing
material powder comprises generally spherical particles
having an average diameter ranging from about 100 to
about 150 microns. Such particles permit a high amount
of radiation material loading, storage stability and
ease of manufacturing and use. A particularly suitable
commercially available lead powder is Lead powder-150
micron, a product marketed by Vulcan Lead Products,
Inc., 1400 West Pierce Street, Milwaukee, WI 53204.
The invention provides that the radiation
absorbing material can comprises up to about 95% by
weight of the total composition, or up to about 665 by
volume of the total composition. The loading of the
absorbing material used is largely dependent upon the
most desirable viscosities and shielding ability for
the applications intended. As more absorbing material
is added to a formulation, the better the shielding
capabilities, but the more viscous the mixture becomes.
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For better shielding properties of the composition
while still allowing workability of the composition
before curing or setting, it is preferred that the
absorbing material comprises from about 50% to about
95% by weight of the total composition, or from about
20% to about 66% by volume of the total composition.
It is more preferred that absorbing material comprises
from about 75% to about 94% by weight of the total
composition, or from about 40% to about 65% by volume
of the total composition. It is even more preferred
that the radiation absorbing material comprises from
about 85% to about 94% by weight of the total
composition, or from about 50% to about 60% by volume
of the total composition. It is most preferred that
the radiation absorbing material comprises about 93% by
weight of the total composition, or about 57% by volume
of the total composition. In some embodiments, the
radiation absorbing material comprises about 95% by
weight of the total composition. In other embodiments,
the radiation absorbing material comprises about 66% by
volume of the total composition.
As used herein, ~thixotropic agent~ means
compounds which, when combined with resinous binders,
act to form gels that liquify when agitated and return
to the gel form when at rest, and act to maintain and
suspend the dispersion of radiation absorbing material
within the resinous binder. The thixotropic agent is
dispersed in the resinous binder to act as a suspending
agent for the absorbing material thereby maintaining a
homogeneous dispersion of the absorbing material
throughout the shielding material. The thixotropic
agent acts to maintain a homogeneous dispersion of the
absorbing material before and during the resinous
binder curing or setting process. Therefore, during
transportation, storage and handling of the uncured or
unset mixture including the resinous binder, the
absorbing material remains substantially homogeneously
dispersed within the resinous binder. During the
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curing or setting of the binder, the thixotropic agent
maintains the homogenous dispersion of the absorbing
material.
Most thixotropic agents that are compatible with
the resinous binder and the absorbing material are
suitable for use in the formulation. Suitable
thixotropic agents include: aramid fibers, polyolefin
fibers, bentonite clay, derivatives of bentonite clay,
hydrogenated castor oil, derivatives of hydrogenated
castor oil, pyrogenic silica, and mixtures thereof.
Aramid fibers, polyolefin fibers, or mixtures thereof
are preferred due to the high structural support
provided by such fibers, with aramid fibers being most
preferred due to their resistance to heat and added
structural support. Of the aramid fibers, Kevlar~
fiber, marketed by Du Pont Company, is most preferred.
Kevlar~ fiber has been found to offer the best overall
mix of properties and, additionally, serves to toughen
and reinforce the shielding material.
The aramid fibers acting as a thixotropic agent
generally average between about 0.3 millimeters and
about 10 millimeters in length. For better texture and
consistency of the formulation, it is preferred that
the aramid fibers average between about 0.5 millimeters
and about 5 millimeters in length.
Generally, the thixotropic agent comprises from
about 0.05% to about 1.0% by weight of the total
composition, depending upon the amount of material
present, the amount of structural support desired, and
the desired viscosity of the uncured mixture. For an
uncured mixture having a moldable putty viscosity, the
thixotropic agent more preferably comprises from about
0.10% to about 0.5% by weight of the total composition.
Even more preferably, the thixotropic agent comprises
from about 0.2~ to about 0.25% by weight of the total
composition, and most preferably comprises about 0.22%
by weight of the total composition. To produce an
uncured mixture having more of a pourable liquid
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viscosity, the thixotropic agent more preferably
comprises from about 0.05% to about 0.15% by weight of
the total composition. Even more preferably, the
thixotropic agent comprises from about 0.08% to about
0.12% by weight of the total composition, and most
preferably comprises about 0.11% by weight of the total
composition.
For assistance in assuring complete mixing of the
components, it is desirable, but not strictly
necessary, to add pigment to the formulation. Suitable
pigments include: titanium dioxide, black iron oxide,
carbon black, lampblack, metallic aluminum flake, and
phthalocyanine blue or green, and mixtures thereof.
Other commonly used colored pigments could also be
used.
For an embodiment of the invention being made from
an epoxy base component and a separate curing agent
component, it is desirable, but not strictly necessary,
to add pigment to each component with different color
pigments in order that a third shade results from their
mixing.
According to the invention, it would be possible
to homogeneously disperse an absorbing material and a
thixotropic agent into a molten thermoplastic resin
which would be allowed to solidify for storage and
transportation and which would be remelted immediately
prior to application or injection into a mold or
feeding into an extruder. After the mixture is
applied, molded or formed into a desired shape, it
would be allowed to set or cure, resulting in a
radiation shielding material. The homogeneous
dispersion of the radiation absorbing material would be
substantially retained throughout each of these steps,
and produces a final radiation shielding material
having a substantially homogeneous dispersion of
radiation absorbing material.
Also according to the invention, it is possible to
disperse the absorbing material and the thixotropic
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agent into an epoxy base component and a separate
curing agent component. The two components remain
separate for storage and transportation. When it is
desired to produce a polymeric shielding material, the
two components are mixed. Prior to setting, the mixed
material is applied by hand to encapsulate sources of
radiation or it is injected into molds or extruded to
obtain desired shapes. The homogeneous dispersion of
the absorbing material is substantially retained
throughout each of these steps, and produces a final
radiation shielding material having a substantially
homogeneous dispersion of radiation absorbing material.
The following examples are intended to exemplify
embodiments of the invention and are not to be
construed as limitations thereof.
EXAMPLES
EXAMPLE NO. 1: RADIATION SNIELDING MATERIAL THAT IS
MOr.n~Rr.~ PUTTY PRIOR TO SETTING/CURING
A first shielding material in accordance with the
invention was prepared. This particular shielding
material was a moldable putty in an uncured state, and
hardened as the resinous binder cured. The shielding
material was prepared by first admixing lead powder,
color pigment, and a thixotropic agent, Kevlar~, into
an epoxy base component, (CIBA GEIGY #6005), and into a
separate curing agent component, cycloaliphatic
polyamine resin (AIR PRODUCTS #2280). The ingredients
were added in amounts in accordance with the weight
percentages given in tables 1 and 2, respectively for
each component. The two components remained separate
until it was desired to produce the shielding material,
at which time the two components were mixed to form a
moldable putty. Prior to setting, the mixed material
can be applied by hand or by other means to encapsulate
sources of radiation, repair existing shields, or may
be molded or formed into desired shapes. In this case,
the material was applied by hand to form a sheet. Upon
CA 02231008 1998-03-04
curing of the binder, a polymeric radiation shielding
material was produced having a homogeneous dispersion
of radiation absorbing material therethrough, and
maintaining a high level of strength and stability.
As shown in Table 3, the final composition
contains about 90.89% radiation absorbing material
(lead) by weight of the total composition. The
composition of the base component is listed in Table 1,
the composition of the curing agent is listed in Table
2, and the mixing ratio of the two components along
with the composition of the mixed components, or the
final product, is listed in Table 3.
The resinous portion of this formula was derived
from a formulation which performs very well in Design
Basic Accident testing (hereinafter "DBA test"). The
final radiation shielding composition passed the ASTM
D-3911 DBA test. Testing at 2.2 x 106 rads/hour to a
total dose of 1.1 X 109 rads showed the exceptional
performance of this system. The final radiation
shielding material is hard, tough, and has a very
uniform distribution of lead across the matrix.
TABLE 1
EPOXY BASE COMPONENT:% W/W S. G.VOL .
CIBA GEIGY #6005 8.66 1.167.466
KEVLAR FIBER #lF5430.21 1.440.146
SYNTH. BLACK IRON OXIDE 0.22 4.50 0.049
LEAD POWDER - 150 MICRON 90.9111.30 8.045
100.00 (6.37)15.706
TABLE 2
CURING AGENT COMPONENT: % w/w S. G . VOL .
AIR PRODUCTS #2280 7.93 1.067.481
KEVLAR FIBER #lF5430.23 1.440.160
RUTILE TITANIUM DIOXIDE 0.99 4.00 0.248
LEAD POWDER - 150 MICRON 90.8511.30 8.040
100.00 (6.28)15.929
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TABLE 3
MIXING RATIO:
EPOXY BASE COMPONENT/CURING AGENT COMPONENT:
1,155.1/756.8 = 1.53/1.00 w/w
181.3/120.5 = 1.50/1.00 v/v
MIXED COMPONENTS: % w/w S.G. VOL.
CIBA GEIGY #6005 5.23 1.16 4.509
AIR PRODUCTS #2280 3.14 1.06 2.962
KEVLAR FIBER #lF5430.22 1.44 0.153
SYNTH. BLACK IRON OXIDE 0.13 4.50 0.029
RUTILE TITANIUM DIOXIDE 0.39 4.00 0.098
LEAD POWDER - 150 MICRON 90.89 11.30 8.043
100.00 (6.33) 15.794
EXANPLE NO. 2: RADIATION SHIELDING MATERIAL THAT IS
PUMPABLE/POURABLE PRIOR TO SETTING/CURING
A second shielding material in accordance with the
invention was also prepared. This particular shielding
material was a pourable liquid in an uncured state, and
hardened as the resinous binder cured. The shielding
material was prepared by first a~m;x;ng lead powder,
and a thixotropic agent, Kevlar~, into an epoxy base
component, (CIBA GEIGY #6005), and into a separate
curing agent component, cycloaliphatic polyamine resin
(AIR PRODUCTS #2280). The ingredients were added in
amounts in accordance with the weight percentages given
in tables 4 and 5, respectively for each component.
The two components remained separate until it was
desired to produce the shielding material, at which
time the two components were mixed to form a pourable
liquid. Prior to setting, the mixed material may be
poured or pumped into an injection mold or extruder, or
may be applied by hand to form a desired shape of a
shielding material. In this case the mixed material
was poured into a prepared void within a cylindrical
form to form a radiation shield suitable for, say, a
pipe or other cylindrical shape. Upon curing of the
binder, a polymeric radiation shielding material was
procured having a homogeneous dispersion of radiation
material therethrough, and maintaining a high level of
strength and stability.
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As is shown by Table 6, the final composition
contains about 89.30% radiation absorbing material,
(lead) by weight of the total composition. The
composition of the base component is listed in Table 4,
the composition of the curing agent is listed in Table
5, and the mixing ratio of the two components along
with the composition of the mixed components, or the
final product, is listed in Table 6.
This formula also performs very well in the ASTM
D-3911 DBA test. Testing at 2.2 x 106 rads/hour to a
total dose of 1.1 X 109 rads showed the exceptional
performance of this system.
TABLE 4
EPOXY BASE COMPONENT:% w/w S.G. VOL.
CIBA GEIGY #6005 10.92 1.16 9.414
KEVLAR FIBER #lF5430.11 1.44 0.076
LEAD POWDER - 150 MICRON 88.97 11.30 7.873
100.00 (5.76)17.363
TABLE 5
CURING AGENT COMPONENT: % W/W S.G. VOL.
AIR PRODUCTS #228010.08 1.06 9.509
KEVLAR FIBER #lF5430.11 1.44 0.076
LEAD POWDER - 150 MICRON 89.81 11.30 7.948
100.00 (5.70)17.533
TABLE 6
MIXING RATIO
EPOXY BASE COMPONENT/CURING AGENT COMPONENT:
9,458.9/5,731.7 = 1.65/1.00 w/w
995.3/653.4 = 1.52/1.00 v/v
MIXED COMPONENTS: %w/w S.G. VOL.
CIBA GEIGY #6005 6.62 1.16 5.707
AIR PRODUCTS #22803.97 1.06 3.745
KEVLAR FIBER #lF5430.11 1.44 0.076
LEAD POWDER - 150 MICRON 89.30 11.30 7.903
100.00 (5.74)17.431
Mixing and dispersion of the ingredients to form
each of the separate components for examples 1 and 2
were accomplished using conventional "High Speed
CA 02231008 1998-03-04
Dispersion~ equipment or similar mixing equipment. In
this case the reference to ~high speed~ refers to the
ease and quickness of production rather than the speed
of any moving parts. The dispersion equipment is no
more than an electric motor connected to a vertical
shaft through pulleys and a variable speed drive. The
shaft is fitted with a disc shaped blade at its base.
The blade has "sawteeth" up and down-turned around the
edge set at not quite a tangent to the disc. One
10 source of such a disperser is Hockmeyer, IncThe
equipment was run under low speed, high torque,
conditions to uniformly disperse the lead material,
fibers and color pigments throughout the individual
components. During the course of raw material
additions to the resinous ingredients the temperature
of the batch becomes elevated due to the mechanical
work being performed. A mixing procedure was carried
out separately for each of the separate epoxy base and
curing agent components in the above examples. For
each separate component, the ingredients were
separately loaded into a simple cylindrical mixing
vessel and mixes as described below:
a) Add the resin ingredients (either #6005
liquid epoxy resin or the #2280 curing agent).
b) Move the mixing vessel under the disperser
and with the disperser running at minimum speed slowly
add the pre-weighed Kevlar #lF543 fiber. Mix until
evenly dispersed throughout the mix - about 10 minutes.
c) Pre-weigh and add any color pigment required
by the formulation and continue mixing.
d) Pre-weigh and add the lead powder under slow
speed agitation. As the lead is added the mixture will
gradually thicken. Add lead as quickly as the
disperser can disperse the powder into the resinous
mixture. It may be necessary to assist mixing towards
the end of the addition by moving the mixture with
paddles and/or by adjusting the height of the mixing
blade and/or its speed of rotation.
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The mutually reactive components for each example
were stored separately and then mixed immediately
before use to form the final shielding material.
Both of the final shielding material compositions
of examples 1 and 2 comprise a large quantity of finely
divided lead powder uniformly dispersed in an
epoxy/curing agent resin mixture. The resinous
ingredients in each example is highly loaded with lead
powder in order to mAx;m;ze the effectiveness of the
mixture as a radiation shield. The amount of lead used
in each application has been largely developed to yield
the most desirable viscosities and shielding
capabilities for the applications intended. It is
possible to obtain concentrations of approximately 50%
by volume of radiation lead powder in the total
formulation while retaining a fluid, easily workable
viscosity. The major difference between the two
formula given in examples 1 and 2 is the amount of lead
powder and Kevlar~ present in the formulations, thus
effecting the viscosity of the formulations. Example 1
produces an uncured mixture that is a moldable putty,
while example 2 produces a mixture that is a pourable
liquid until curing results in an infusible solid.
The Kevlar0 fibers reinforce the formulation and
assist in suspending the lead powder to retard
settlement. The Kevlar~ fibers also enhance the
mechanical properties of the composition and contribute
to absorbing material suspension. The Kevlar~ also
yields a stiff, thixotropic paste which assists
application in thick layers. The heat resistance of
Kevlar~ is extremely high with essentially no
degradation in properties to almost 800~F.
The titanium dioxide and synthetic black iron
oxide in example 1 are present as pigments to highlight
areas of unmixed components which show up as streaks of
light or dark material in the mix. In formulations of
the type given as examples, the individual components
are stable indefinitely and, if improperly mixed, may
CA 02231008 1998-03-04
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never cure. Once mixed with each other in the correct
ratio the two components proceed with their curing
reaction in a very definite and predictable way.
Various of the features are set forth in the
following claims.
