Note: Descriptions are shown in the official language in which they were submitted.
NEUTRON-S~rELDING FABRIC AND COMP ~E FIBER
D '~E METHOD O~ MANUFAC'~URE THEREOF
FIELD OF '~HE INVRNTION
This invention relates to woven, non-woven and knitted f.abrics
and composite fibers for use therein having extremely effective
neutron shielding properties, particularly against thermal neutrons,
combining low emission of secondary radiation with high flexibility,
as well as to the method of manufacturing these fabrics and
neutron~shielding fibers.
BACKGROUND OF '~E INVENTION
In view of the recent significant development of the nuclear
industry, a variety of problems have emerged with respect to
potential ha~ard and exposing workers to radioactive materials an~
radiation in nuclear plants. During the periodic maintenance and
repair work carried out in nuclear power stations, it is absolutely
necessary not only to protect workers from intense radiations such
as gamma (~) rays, but also from exposure to even the slightest
amount of neutrons, which can radiate from the nuclear reactor in
the event of an emergency.
me nuclear inrlustry has thus urgently desired to have
available neutron shielding materials of high flexibility and
desirable operative properties for incorporation .into garments, so
that workers in nuclear plants and industrial sites can wear
protective garments made of neutron-shielding material.
The importance of this invention has recently increased in view
of the various experiments relating to the application of neutron
radiation to medical treatment, such as neutron capture therapy,
where certain amount of neutrons is irradiated to a cerebral tumor
so that only the tumor portion is effected a~d can thus be removedO
During this surgical procedure, it is essential to protect the ~est
of the patient'~ body fr.om ~he neutron radiation itself as well as to
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the secondary radiation produced when neutrons strike their target.
In this case, it is an urgent need in the art to fabricate
neutron-shieldin~ materials in the shape of fabrics such as
bandages~ gauzes and blankets. This demand can potentially be
satisfied only by the use of fiber materials having the necessary
neutron shielding properties.
Further, in the event that neutron weapons are used in warfare,
although the neutron-shielding fabrics and fibers of this invention
will not shield against any of the fast neutrons emitted therefrom,
if people wore protective robes of the neutron-shielding fibers of
this invention and were situated in shelters that provide the effect
of reducing the energy of fast neutrons 50 as to slow them to
thermal neutrons, people's lives m y be saved by the shielding effect
provided by the fabrics of this invention. The neutron-shielding
materialsin accordance with this invention potentially have any
possible utility relating to the protection of humans, animals and
ir~ni~ate objects from thermal neutron radiation.
Conventional neutron-shielding materials are in the form of
boards composed of cadl~um and boron compound. However, such
neutron-shielding boards are physically rigid and have no
flexibility at all; furthermore, since cadmium yields high secondary
ga~ma rays upon absorbing neutrons, it is not suitable for use in
shields for protecting the human body against neutron radiation.
Japanese laid-open patent applications nos~ 52 127597 and
52-131097 disclose neutron shielding materials formed in sheets of
various klnds of plastics with boron and/or lithiwm compounds
therein, which are disclosed to yield low levels of secondary gamma
radiation in the occasion of neutron absorption. However~ these
products are not flexible enough for use in any of the protective
clothing and the like contemplated by this invention. Anotner
Japanese laid-open patent application, no. 53-21398, discloses a
method of manufacturing neutron shielding fibers which consist
either of ion exchange fibers which have absorbed therein ionized
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compounds of boron and lithium or of staPle fiber-like polymers containing there-
in boron and/or lithium compounds. In the case of the ion exchangP fibers, the
finished products cannot satisfac-torily achieve the intended shielding pr.operties
to either the incomplete absorption and fixation oE the neutron-shielding ionized
compounds into the ion exchange fibers, or to the possible releasing of the once-
fixed ionized compounds from the fibers during the washing and rinsing the
fabrics emkodying these fibers.
In the case of staple fibers, the finished products thus obtained by means
of jet-spinning of this mixture of neutron-shielding inorganic compounds and
fiber forming polymers can physically retain the fibrous form. ~lowever, these
products are not suitable for processing with any of the yarn-spinning and
knitting or texturizing machines due to insufficient tensile strength, elong-
ationt and textured styles. In addition, the finished products thus obtained
usually have those neutron-shielding compounds exposed on the surface, and thus
they can easily be stripped off fron the surface, thus inevitably resulting in
degraded shielding properties.
We have carried out extensive experiments with fibers composed of certain
fiber-forming polymers, each having ce.rtain grading and neutron-shielding
properties. As a result, we found that a variety of critical problems potent-
ially existed. For e~ample, certain neutron-shielding compounds deposited and
existed on the surface and the adjacent po.rtions of the fibers were found to
be s-tripped off in processing, thus causing damage on the surfaces of guide
rollers and other rollers due either to staining or to friction of the fibers
against them. Consequently, not only can the production of the neutron-shield-
ing fibers of stable quality not be achieved, but the finished fibers will have
poor mechanical properties. In addition, neutron-shielding garments made of
said compound fibers exhibit eventual stripping off of the deposi-ted neutron-
shielding compounds during and after -~ash and from friction of the fabric against
objects.
We also found that, when these prior finished products composed of
neutron-shielding fibers were exposed to neutron rays, certain secondary radio-
active materials were generated by the nuclear reaction. For example, when
lithium (Li) compounds were applied to the neutron-shielding compound, the
lithium compounds exposed to the irradi.ated thermal neutron rays onto the fiber
surface then generated a certain amount of tritium which then started to diffuse
in the atmosphere.
SUMMARY OF THE INVENTION
The invention provides a neutron-shielding composite fiber compris-
ing (a) a core component of fiber-forming polymer (A) containing at least 5
weight percent of an isotope compound in particles with maximum diameter of 25
microns, capable of shielding against thermal neutrons, and (b) a sheath com-
ponent of a fiber-forming polymer (B) which is capable of bonding to said fiber-
forming polymer (A). The invention also provides a method of manufacturing
such a fiber comprising the spinning of core and sheath fibers with melt vis-
cosity ratio of sheath component to core component from about 0.2 to 0.9.
This invention involves incorporating certain fiber-forming polymers
(A) in the core which essentially contains at least 5% by weight, preferably
w:ithin a range from 10% to 60% by weight, of certain compounds having ef-fecti.ve
properties to screen thermal neutrons, each having a maximum particle diameter
of about 25 mi.crons, preferably a maximum of 15 microns whereas the other com-
ponent consists of at least one kind of fiber-forming polymer ~B~ as the sheath-
componen~ that essentially bonds the above core-component (A), so that both com-
ponents are made into the composite fibers incorporating the neutron-shielding
properties. The invention also includes the method of manufacturing said com-
posite fibers.
Relating to this invention, more preferably, the sheath-component
should consist of fiber-forming polymers having a viscosity ranging from 0.2 to
0.9 of that of said core-component polymer.
This invention has enabled us -to manllfacture neutron-shielding fibers
that sufficiently satisfy the various practical requirements in shielding from
the neutrons, minimizing the gamma rays secondarily generated, and yet providing
said fibers with sufficient mechanical properties without causing any of the
neutron-shielding compounds to go off from the surface of said fibers so tha-t
the neutron-shielding properties will remain stable. As a result, this inven-
tion has
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enabled us to manufacture neutron-shielding fabrics with sufficient
flexibîlity based on said fibers, so that garments effective in
shielding against thermal neutrons may be made that will retain
their neutron-shielding.
DETAILED DESCRIPTION OF THE INVENTION
Compounds preferred for use in this invention having the
desired properties in effective shielding from the neutrons and
being contained in the core-component polymer (A) that constitutes
the Composite Fiber by this Invention should be chemically stable
and physically capable of absorbing therm31 neutrons and min~izing
or voiding radioactive rays such as secondary gamma rays. Such
compounds should preferably be selected from any of the elements
containing isotopes, specifically, such as 6Li and/or 10B.
Conventionally, these natural isotopes exist at a rate of about
7% to 8% in the case of the isotope 6Li and about 19% to 20% in the
case of the isotope 10B. In order to implement this invention, it
is preferable to select those naturally available lithium compounds
and/or boron compounds containing said isotopes, for example, such
as lithium carbonate, lithium fluoride, boric acid, boron carbide,
boron nitride, etc. It is more advantageous to use certain com-
pounds composed of artificially separated and enriched isotopes. I
When adding said neutron-shielding compounds to the core- !
component polymer (A), it is in~ortant that the particles of said
neutron-shielding compounds should essentially have a m~ximum size
of not more than 25 microns in diameter, preferably fine particles
having diameters less than 15 microns.
If these ranges are not correctly satisfied, the mixed compound
is diffic~t to be spun into fîbers, thus eventually resulting in
poor mechanical properties of the made-up fibers.
When mixing said neutron-shielding cornpounds with the core-
component polymer (A), it is also important that said
neutron-shielding compounds should be mixed into said core-con~onent
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polymer at a ratio of at least and more than 5% by weight,
preferably within a range between 10% to 60% by weight. If said
mixture ratio is below 5% by weight, the eventually obtained
neutron-shielding properties will be lower than needed. Conversely,
if the neutron-shielding compounds are mixed with the core-cornponent
polymers by more than 60% by weight, even though the eventually
obtained neutron~shielding properties will be promoted, the
texturizing process will become difficult, thus evenkually resulting
in poor mechanical properties of the fibers themselves and garments
from which they are made.
The core-cor~onent (A) that essentially composes the Composite
Fiber rnay be chosen ~rom a variety of known fiber-rnaking raw
rnaterials, for example, such as polyester, polyamide, polyolefin
polymers, etc. In this invention, it is preferred to select any of
the suitable polymers that can be spun into yarns in order to have
the neutron-shielding compounds evenly mixed and dispersed into the
selected polymers.
Taking stabili-ty against the neutron rays into consideration,
it is more advantageous to choose polyethylene and certain
co-polymers mainly composed of polyethylene, such as with less than
10 mol. % of vinylacetate, propylene alph-olefin (l-butene or
l-penten) and vinylcarbazole,for suitable core-cornponent polymers.
This invention does not specifically define any particular
material to be used for the sheath-colrlponent (B) that also
constitutes said Composite Fiber provided that the used rnaterial can
properly be bonded with the core cornponent (A). It ls however
preferable that the sheath~component (B) falls under the same
category as the core-cornponent (A). Further, it is preferable in
implernenting this invention that the composite ratio of the
core-cornponent against the sheath-cornponent polymer rernains within a
range from 0.5 up to 10. That is to say, if the actual cornposite
ratio does not meet the desired range, for example~ if the
core-versus-sheath composite ratio exceeds a m~ximlm of 10, the
covering property of` the sheath-component polymer will then become
unstable, and may eventually cause the core-component polymer to
bare itself.
The neutron-absorbing compounds in the bare core con~onent
polymer may fall ~rom the fibers during spinri~lg, or they may fall
off later, possibly diffusing into the environment the radioactive
materials secondary generated during exposure to neutrons.
Conversely, if the core-versus-sheath composite ratio is below
0.5, since the core-component polymer containing the neutron-
shielding compounds will decrease based on the sectional areas of
the composite fibers, the originally-aimed neutron-shielding
properties will eventually lower, causing undesired results.
We have finally found that the core-versus-sheath polymer
composite ratio should preferably remain within a range of 1 to 4,
thus enabling the sheath-component polymer to cover the
neutron-shielding compounds sufficiently without any dropping off at
all and thus sealing even the slightest amount of the radioactive
materials generated by the irradiated neutron rays inside the
core-component polymer without any fear of their emission into the
atmosphere, and at the same time, in so doirlg, this invention has
eventually enabled us to obtain the core-and-sheath-integrated
composite fibers that are sufficiently capable of shielding from
neutrons.
As one of the significant characteristics in process of manu-
facturing the core-and-sheath composite fibers based on this
invention, we have also f`ound that, when certain spun yarns made of
core-and-sheath composite fibers are applied to the implementation
of this Invention by using a spinneret for the composite spinning of`
conventional synthetic fibers, the ratio between the melt-viscosity
X of the core-component (A) containing the neutron-shielding
com~ounds and the melt-viscosity Y of the sheath-component ~B) plays
a very im~ortant role. That is to say, when a certain
melt-viscosity ra.tio was provided under the optim~m spinning
temperature conditions, i.e.~ when the value of Y/X was within the
range of 0.2 to 0.9, in particular, when this value satisfied a
range between 0.3 to 0.7, it was found that the core-and-sheath
composite fibers could stably be spun into the intended textured
yarns.
If the mRlt-viscosity ratio does not meet the recom~Rnded rar.ge
as referred to above, spinning of such composite fibers is difficult
to stably be performed, and the spun-fibers will o~ten be cut during
the spinning process, thus making it difficult to satisfactorily
perform the spinning operation.
It is, however, not certain why such a phenomenon should occur,
although this is considered due to one of the potentially peculiar
phenomena incidental to core-and-sheath composite fibers where
relatively large amount of neutron-shielding particles is added to
the core-component.
The composite fibers produced by this invention and their
secondary products, for example, those fabrics typically represented
by woven fabrics, knitted fabrics and non-woven fabrics are provided
with very excellent properties in neutron shielding, particularly in
thermal neutron shielding without generating intense secondary
radioactive rays, being totally free from stripping of the fi~ed
neutron-shielding compounds from the processed fabrics which are not
only highly durab~e in neutron-shielding properties but can also
effectlvely be applied to garments for the protecting humans against
the attack o~ neutrons owing to their fiber characteristics which
can provide such garments with mechanical properties common to any
of the conventional fibers and with high flexibility.
As a result, such human-protective garments made of the
neutron-shielding composite fibers will effectively provide very
advantageous performance and useful values in the nuclear indust~yO
This invention is described by some examples shown below.
Example No. 1
First, a total amount of 500 grams of fine LiF powder contain-
ing more than 95% of the enriched isotope of lithium6, where the
particles of said powder had a maximum diameter of about 8 micron,
and about 2.5 micron of the mean volume diameter, was mixed with a
total amount of 750 grams of high-density polyethylene po~der
(typically, "HIZEX"*2100 GP, a product of Mitsui Petrochem~cal
Company, Japan) by means of a Henschel mixer.
The mixed material was then subjected to kneadlngs three times
repeatedly by mRans of an extruder (having a 30 mm cylindrical
diamRter and a 500 mm screw length), employing a 60 rpm screw
revolution and at temperatures ranging I~rOm 250 to 280C.
After these procedures wère completed, an amount of mixture was
obtained, which consisted of polyethylene chips mixed with fine LiF
powder, where the net content of said 6LiF was measured at 38.5% by
weight. Separately, the melt-viscosity of said polyethylene chip
was measured at 260C. by means of the "K0KA'b~type flow-tester
r~nufactured by Shimazu Seisakusho, Ltd., Japan, showing a
- melt-viscosity of 2,520 poises.
Using said 6Li~-contai~ing chips as the core-component and a
certain amount of high-density polyethylene (typically, "HIZEX"
1300J, a product of Mitsui Petrochemical Company, Japan) as the
sheath-component, the melt-viscosity of which ~as measured at 1,760
poises under the same test conditions as abo~e, we carried out the
sp ~ing of the core-and-sheath composite fibers by rneans of
concentric composite spinnerets each having 12 holes of 0.65 mm
diameter. The spinning operation was stably perfo~med under the
prepared operative conditions so that 12 grams per rninute of the
output of the core-component and 5 grams per r~nute of the output of
the sheath-component were obtained at 260C. and at a take~u~ speed
of 450 meter per minute.
We then observed the mono-filarnent sections of the spun-yarns
through an optical microscope under the light penetration. As a
result5 we could confirm that the spun-yarns thus obtained had
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evenly concentric core-and-sheath composite fibers where the
core-component contained a specific amount of said LiF fine
particles.
In the following test carried out by us, these composite fibers
were elongated to a draw ratio of 5.0 on a plate heated to 95C. We
thus successfully obtained the desired continuous filaments made of
the core-and-sheath composite fibers.
These filaments were eventually found useful enough in
mechanical characteristics with their tensile strength of 2.5 grams
per denier and 25% elongation.
Example No. 2
The continuous filaments obtained by the preceding procedure of
Example l were then integrated so that each of the integrated yarns
contained 60 filaments, which were then processed by a knitting
machine in order to experimentally make tubular knitted fabrics.
After the knitting, the kni-t fabric had a 1.30 mm thickness and a
density of 490 gra~s per square meter of area.
The shielding properties of these knit fabrics against the
thermal neutrons were then evaluated. Tests were carried out in the
thermal neutron standard field based on the Maxwellian distribution
by means of the ~ ~ Aeavy water facilities, where the shielding
effect of these knit fabrics against the broad thern~l neutron rays
were measured by activated gold (Au) foils. Test results for the
neutron-shielding properties are shown in Tab]e l below.
Table No. l
The thermal neutron-shielding properties of the knitted fabrics
composed of 6LiF-mi-xed filaments.
Number of plies of
knitted fabric. 1 4 6 lO
mickness (mm) of
th~ knitted fabric.1.30 5.20 7.80 13.0
Transmittance of
thermal neutrons.6.4x10 11.5x10 14.8x10 21.4x10 2
Example No. 3
As was done in the preceding Example No. 1, a total of 750
grams of fine particles "B4C" (typically, "DENKA BORON'~ No. 1200, a
product of Denki Kagaku Kogyo K.K., Japan) graded by dry separation
to have a m2Yimun diameter of 10 microns diameter and 3.2 microns of
mean volume diameter was mixed with a total of 1,000 grams of
high-density polyethylene powder (typically, "HIZEX" 2100GP, a
product of Mitsui Petrochemical Company, Japan), then the mixture
was Icneaded by an extruder, thus producing an amount of polyethylene
chips containing uniformly dispersed fine powder B4C. Our analysis
indicated that the polyethylene chips contained 42% by weight of
this B4C component. Based on the same method as was done in the
Example No. 1 procedure, the melt-viscosity of said mixture was
measured to be 2,690 poises at 260C.
Using this B4C-containing polyethylene chip as the
core-component and a certain amount of middle-density polyethylene
(typically, "NEoZEX"*45300~ a product of Mitsui Petrochemical
Company, .Japan) as the sheath-component, the melt-viscosity of which
was measured at 1,000 poises under the same test condition as above~
the spinning of the core-and-sheath composite fibers ~as carried out
by employing the concentric composite spinnerets each having 12
holes of 0.50 mm diameterO The spinning operation was stably
performed under the prepared operative conditions so that 10 grams
per minute of the output of the core-component and 4.5 grams per
minute of the output of the sheath-component were obtained at 260&. and
at a take-up speed of 400 meters per minute.
The mono-filament sections of the spun yarns were then observed
through an optical microscope under the light penetration. As a
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result, it was confirmed that the spun-yarns thus obtained showed
evenly concentric core-and-sheath composite f'ibers where the
core-component contained a specific amount of said fine B4C particles.
In the following test carried out by us, these composite fibers
were elongated at a draw ratio of 5.5 on a plate heated to 95C.
m e desired continuous filaments were thus made of the
core-and-sheath composite fibers.
These filaments were eventually found useful enough with their
tensile strength 2.3 grams per denier and 21% elongation.
Example No. 4
The continuous filaments obtained by the preceding procedure of
Example 3 were then integrated so that each of the integrated yarns
contained 48 filaments, which were then processed by a knitting
machine in order to experimentally make tubular knitted fabrics.
After the knitting, the knit fabric had a 1.25 mm thickness and a
density of 430 grams per square meter of area.
The shielding properties of these knit fabrics against the
thernal neutrons were then evaluated. Tests were carried out with
the same site and withthe same methods as were used for the Example 2
tests. Test results for the neutron-shielding properties are shown
in Ilable 2 below.
Table No. 2
Number of plies of
knitted fabric. 1 4 6 10
Thickness (mm) of'
the knitted fabric.1.25 5.0 7-5 12.5
Transmittar-,ce of
thermal neutrons.6.0xlO 1l.lxlO 14.4xlO 2l.lxlO 2
Example No. 5
As was done in the preceding Example l and using the same
methods as Example 1, a certain amount of boron nitride fine powder
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(typically, a boron nitride product made by Denki Kagaku Kogyo K.K.,
Japan) is mixed and kneaded with a certain amount of high-density
polyethylene powder (typically, "HIZEX" 1300J, a product of Mitsui
Petrochemical Company, Japan) by means of a Henschel mixer, and as a
result, a certain amount of polyethylene chips conta1ning 55% by
weight of boron nitride was obtained, which had a 2,900 poise
melt-viscosity at 250C.
Using these polyeth~lene chips containing ~oron nitride as the
core-component and an amount of said high-density polyethylene
powder without containing boron nitride having a 2,000 poise
melt-viscosity at 250C., core-and-sheath composite fibers were
spun. The spinning operation was stably performed at 250C. and at
a take-up speed of 500 meters per minute so that the output ratio of
the core component polymer to sheath component polymer became almost
2, and as a result, it was confirmed that the spun-yarns thus
obtained had evenly concentric core-and-sheath composite fibers.
After the following procedure in elongating the composite
fibers 4.5 times the original length on the plate heated at 95C.,
very satisfactory continuous filaments having a 3.0 grams per denier
tensile strength and 32% elongation were obtained.
me Inventors then processed the obtained filaments into a
taffeta of 0.50 mn thickness and a density of 250 grams per square
meter of area. me thermal--neutron~shielding properties of the
taffeta were tested at the same slte as that was used for the
Example 2 tests. When 10 pieces of said taffeta were piled up,
forming 5 mm of total thickness, the amount of the thermal neutrons
actually penetrating was was measured at 2.0 x 10 2-
3o
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