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Патент USA US2403794

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Patented July 9, 1.946
’ -
Richard M. Goldrick, Arlington; and Barnard
Marks, Newark, N. J ., assignorsto E. I. du Pont
de Nemours & Company, Wilmington, Del., a
corporation of Delaware
No Drawing. Application November 19, 1943,
Serial No. 510,916
9 Claims. (Cl. 260-41)
aromatic acid salts, e. g., mercury o-benzoyl
This invention relates to transparent plastics
and, more particularly, to X-ray opaque, visible
benzoate and thallium benzoate; organo-m‘etallic
light transparent methyl methacrylate plastics.
compounds such as mercury diphenyl, and heavy
metal salts of ortho- and pyro-alkyl phosphoric
An object of the present invention is to provide
a methyl methacrylate plastic which is opaque
to X-ray radiations but which is transparent to
visible light. A further object is to provide a
process of preparing such plastic which process
may be employed on a commercial scale without
acids’ such as dibutyl lead orthophosphate.
addition certain organic iodine containing com
pounds will impart vX-ray opaquecharacteristics
to acrylictype plastic compositions'e. g.,‘triiodo
benzene and‘ ethyl iodobenzoate; However, it has
recourse to extreme precautions against indus 10 been'found that none of theseicompoiu‘nds are
really “satisfactory 'for utilization in a commercial
trial poisoning. More speci?c objects of the in
process for the production of X'-ray opaque poly-'
vention are to provide a plastic sheet which may
meric compositions comprising polymethyl meth-i
be used as a shield for protection against X-ray
acrylate because’such' compounds, although they
radiation and to provide a method whereby ob
.iects made of methyl methacrylate plastic may be 15 may. be ‘soluble in monomeric methyl methac
ryl'ate, are insoluble ‘in polymeric methyl methac;
detected and identi?ed. Other objects will be
rylate and hence render the polymeric composi
apparent from the description of the invention
given hereinafter.
tions opaque to visible‘light, and/or such com
nitrate, bismuth nitrate, and bismuth trichloride,
pounds impart visible color characteristics to the
positions are unstable and undergo, decomposi-~
The above objects are accomplished according
to the present invention by dissolving an in 20 tion 'wne??ccmpecnaed in polymethyl‘methac
rylate plastic compositions, and/or such com
organic salt from the group consisting of thorium
polymeric~ compositions rendering‘ them non
colorless, and/or such- compounds are too‘r'toxic
More speci?cally, the 25 for ; utilization in 'a commercial production
in monomeric methyl methacrylate and. there
after subjecting the resulting solution to poly
merization conditions.
process of preparing the plastic of the present
invention includes the step of ?ltering the mono
meric methyl methacrylate containing the. in
On the contrary, the three inorganic salts used
inthe present invention-have. the requisite solu
bility characteristics with’ respect to both mono
organic salt dissolved therein, through activated
carbon prior to subjecting the methyl methac 30 mericand polymeric methyl methacrylate so that
transparent plastics opaque to X-ray radiations
rylate to polymerization conditions.
may be obtained. Further, these salts are satis
The present invention resides in part in the
factorily stable when compounded in polymeric
discovery that thorium nitrate, bismuth nitrate,
methyl methacrylate compositions, do not impart
and bismuth trichloride all have a uniquely ad
vantageous combination of properties in that 35 visible color characteristics to the plastics, and
are not toxic to a degree making it dangerous to
they are soluble in monomeric methyl methac
use vthem in a commercial scale operation.
rylate and, also, soluble in or compatible with
The present invention further resides in the
polymeric methyl methacrylate in proportions
discovery that ?ltering the solution of the in-.
such that they may be used to impart X-ray
opaqueness to visible light transparent sheetsand 40 organic salt in monomeric methyl methacrylate
through activated carbon permits the production
other articles of polymeric methyl methacrylate.
of a plastic completely haze-free whereas no
In addition, these inorganic salts possess certain
other means of accomplishing this has ‘been
other properties such as stability and relative
found. While other ?ltering media have been
non-toxicity, which make them highly suitable
tried, activated carbon appears to be unique in"
for use in the present invention.
These are numerous other metallic salts and;
organo-metallic compounds of heavy metal ele
ments which will impart X-ray opaqueness to
giving “a ?ltered sciatica which can be'Pdly
Iiierizéd to‘ an é?tifely‘liaz'é-fl‘ee plastic. Never
theless', plastics of a quality entirely satisfactory
for many purposes can be produced without this
amples of such salts and compounds include the 50 step.
In preparing the plastic according to the pres-1
lead, bismuth or other heavy metal salts. of sulfo
ent invention, the inorganic salt may be directly
succinic acid esters, e. g., lead dibutyl sulfo suc
polymeric compositions of the acrylic type. E'x
cinate; heavy metal salts of higher fatty. acids,
e. g., lead laurate, mercuric oleate and the like;
lead trlchloroacetate; uranyl nitrate; metallic
dissolved in monomeric methyl méthacrylat'e or’
in a syrup‘ of polymeric methy1._methacrylate
dissolved in monomeric methyl methacrylate, the
solution then preferably being ?ltered through
The step of ?ltration through activated carbon
activated carbon, and thereafter the solution is
as illustrated in this example is preferred since it
insures that the resulting polymeric structure will
be completely haze-free. Thus, an article pre
subjected to polymerization conditions. It is pre
ferred, however, to dissolve the inorganic salt in
an alcohol such as ethanol or methanol, then
pared by a procedure comparable to that of this
example, but which omitted ‘this step, although
?lter the solution through activated >carbon,'add
monomeric methyl methacrylate to’the' ?ltered
solution, and to distill off the alcohol thereby
being in all cases transparent, may possess a slight
Example II
obtaining a solution of the inorganic salt in - ~
monomeric methyl methacrylate which solution 10. A solution of 3.6 parts of bismuth trichloride in
is ?ltered through activated carbon. This ?l
96.4- parts of monomeric methyl methacrylate is
tered solution is then admixed in proper propor- »
tions with more methyl methacrylate or partially '
polymerized methyl methacrylate or a syrup of
polymer dissolved in monomer, and the resulting
_ prepared by subjecting a mixture of the two in
gredients to agitation at a slightly elevated tem
perature for a su?‘icient length of time to produce
complete solution. The resulting solution is ?l
mixture is polymerized to yield the polymeric 1,5 tered
through an activated carbon ?lter and is
X-ray opaque plastic. The ?ltering of the solu
then cast in the form of an article possessing a
tion of the inorganic salt in alcohol th'rOughac»
thickness of one-half inch, by subjecting the
tivated carbon is of advantage but it is not e?ec
polymerization conditions while con
tive in eliminating haze from the ?na1 polymeric 20 ?ned in to
a suitable mold for a suiiicient length
product to anything like the extent that the sec
of time to completely polymerize the same. The
ond ?ltering step is effective for this purpose. '
article produced thereby exhibits satisfactory
The polymerizable mixtures containing the
characteristics with regard to transparency to
opacifying inorganic salt may be polymerized and
light and opacity to X-ray radiations.
cast by any established polymerization procedure 25
Example III
using any of the well known polymerization cata
lysts employed therefor. X-ray opaque sheets
of bismuth nitrate penta-v
comprising polymeric methyl methacrylate may
hydratc in 9‘%-5 parts Oi monomeric methyl. meth
be prepared using the herein discussed composi
acrylate is made‘lup, treatedjand polymerized in
tions with any established sheet casting process,
a fashion similar to that disclosed in Example
the speci?c manner in which the polymerization
is carried out forming no part of the present in
II. _'Ifhe_polymeric~sstructure produced thereby
possesses _charact”eristics~which are, completely
satisfactory with respect to transparency to ‘visi
The following examples in which all parts are
ble light and opacity‘ to X-ray radiations._
given by weight unless otherwise noted, illustrate
‘ ;
It"wi1l be understood thatitheabove examples‘
are merely illustrative and that broadly the pres-i
speci?c embodiments of the present invention:
ent invention comprises the production of a visié
Example I ‘c
ble light transparent plastic relatively opaque to
X-ray radiations by dissolving one of the above
discussed inorganic salts in monomeric‘ methyl
A solution. of 500 parts of thorium nitrate and
500 parts of ethanol is prepared by heating the
indicated ingredients for a time su?icient to proq 40 methacrylate and subjecting the resulting-solu
duce substantially complete solution.
tion to polymerization conditions.
parts of urea, the purpose of which is to neu
Since thesopacity to X-ray radiations of the
tralize any excess nitric acid contained in the
plastic'of this invention is a relative matter, it
thorium nitrate, are then added to 300 parts of 45 will
be ‘appreciated that the concentration of the
this alcohol salt solution and the mixture is heated
inorganic salt inthe plastic will__ vary, consider;
at re?uxing temperature for three hours. The
ably depending upon the conditionsin-any given
mixture is then allowed to cool to room tem
case.‘ Obviously, the thickness of the plastic sheet
perature after which it is ?ltered through an
or article being made will a?ect the X-ray opac
activated carbon ?lter. The ?ltered solution is
mixed with an equal quantity of monomeric 60 ity, the concentration of the inorganic ‘salt in‘the
plastic being inversely proportional to the thick
methyl methacrylate and the total mixture is
ness of the sheet for a given degree of opacity.
charged into a suitable distillation apparatus.
Again, the opacity varies ‘for a: given concen
The alcohol component of this salt mixture is
tration of inorganic salt, depending upon whether
then removed by subjecting the mixture to a vac
uum distillation. The resulting methyl methac 55 a thorium salt or a bismuth salt is used.
To illustrate the X-ray" screening properties of
rylate salt solution is ?ltered through an activated
these inorganic salts it ‘should be'noted that a
carbon ?lter. This ?ltrate is admixed with a suf
concentration of at least 4% by weight of the
?cient quantity of partially polymerized methyl
polymer in the plastic is required in the case 'of
methacrylate to bring the total weight of the mix
ture to 1500 parts. The polymeric syrup thus 80 thorium nitrate, calculated as thorium, to impart
X-ray screening properties in- a polymeric methyl
produced, which possesses the following composi
methacrylate sheet 0.250 inch in thickness com
parable. to the X-ray screening properties pos
sessed by the bones of the human‘ body. For a
Methyl methacrylate (monomer and poly
1355 65 sheet only 0.125 inch‘in thickness this proportion
of thorium nitrate would ‘have to be' doubled to
Thorium nitrate tetrahydrate ___________ .._
obtain equivalent X-ray screen properties,
Benzoyl peroxide (polymerization catalyst)- 1.61
the case of bismuth nitrate or bismuth trichlo-l
is polymerized to form a polymeric sheet by cast
ride, a concentration of at least 4.7%, calculated
ing between glass plates separated by a ?exible 70 as bismuth, by weight of ‘the polymer in'the plas
ticis required'to impart Xera'ytscreen properties
The resulting polymeric sheet which‘ possesses
in a polymeric methyl methacrylate sheet 0.250‘
inch in thickness comparable‘to the'X-ray screen
properties‘possessed by the bones of the ‘human
a thickness of 0.250 inch is relatively opaque to
X-rays as compared to the opacity of bones of
the human body.
cbmpositiions atelier‘ stipend to lead glass that
Whilev the ~lower~lirnit of the: concentration’ ‘of
the opacifying'iinorganici salt ‘is; of course} impor
manyl compoundls‘shapessuch.asabout'
sheets mayi-be readily v‘formed
parts ‘in
tant in‘ that a certairr minimum concentration
is necessary to insure‘that-thelplastic has theTde
sired X-ray screening-property; the: upper limit
is not so important, always-providiligvlth'at it‘does'
the’ human body.
~ another application-‘for the present-‘process is -'
for characterizing ‘plastic compositions so that
they ~_mayj be identified-1hr} 'ide'tected." Thus; the
process may be 1'lJSedf-f01‘ ‘marking such structures
not exceed the proportiomof'inorganic- salt that
will remain compatible 'withthe *polymeri'c'mate
rial; the opacifyingf 'salt'iinfproportions higher
as ,dentures'emade of-polymethyl methacrylate ‘so
that the’ identity and source of :the material from
than those‘compatibleiwithv the: polymeric ->methyl
which ' they‘ are made,‘ may :be traced.‘ = Likewise,
methacrylate willhobviouslyvresult‘i in "a plastic
which is not perfectlyitransparentitovisiblelight;
Although the upperelimi'tzof thexc'oncentrationi of
the inorganic salt is not important,‘ nevertheless
the'gproce'ss‘l-“inay{be-used :to impartX-ray- opaque
qualities 1' Ftoi polymethyl '- methacrylate sheeting
suchvas that- used on-combataircraft'in order
it is desirable to employ substantially-‘no more of 15 that fragments of the same whichibeco'me rimbedded-‘in parts of the human body may be
the salt than is. necessarytqtimpart the'desired
opaci?cation in . any 1 particular. . application‘ 1 bee
cause high'concentration of the salt in the plastic
may tend to undergo, slight decomposition "when
the plastic is exposedto radiationsfor prolonged
:"ii'j ‘fl 1..
1As- many apparently widely‘ different embodi
ments of‘ this invention may be made without de
parting from the spirit and scope thereof, it is-to
belu'n'derstood that the invention is not limited
to the speci?c embodiments'ithereof except as
The chief reason th, titis preferred to dissolve
the inorganic salt in alcohol, then-add monomeric ' de?ned in the-appendedclaims.
methyl methacrylate to the resulting solution and
distill off the alcohol to get the solution of the salt 25 Vii/A visible light ‘transparent plasticfsheet 5at
in monomeric methyl methacrylate, is that in
this manner a higher proportion of the salt may
be dissolved in the monomer than in the case
where the salt is dissolved directly in'the mono
mer. In some instances it may be desirable to 30
least as opaque to X-ray radiations as the bones
of the human body, said plastic comprising poly
meric methyl methacrylate and, in compatible
admixture therewith, thorium nitrate, the pro
portion of said thorium nitrate for an 0.250 inch
employ a greater proportion of the salt than can
be dissolved directly in the monomer, at least
under practical conditions, and then the use of
this preferred method is necessitated. Instead
of ethanol or methanol, other volatile organic sol 35
thickness of said sheet being at least 4% by
weight of said polymeric methyl methacrylate,
vents such as acetone or methyl acetate may be
used to dissolve the opacifying salt. However, as
nesses of said sheet.
ethanol and methanol are both relatively inex
pensive and readily available and, of great im
when calculated as thorium, and the proportion
of said thorium nitrate being in inverse propor
tion to the thickness of said sheet for other thick
2. A visible light transparent plastic sheet at
least as opaque to X-ray radiations as the bones
portance, are so easily separated from mono
of the human body, said iplastic comprising poly
meric methyl methacrylate and, in compatible ad
meric methyl methacrylate by distillation, they
normally would be greatly preferred as the sol
tion of said bismuth nitrate for an 0.250 inch
mixture therewith, bismuth nitrate, the'propor
thickness of said sheet being at least 4.7% by
weight of said polymeric'methyl methacrylate,
The present invention provides a method for
the X-ray opaci?cation of polymeric structures 45 when calculated as bismuth, and the proportion
of said bismuth nitrate being in inverse propor-.
comprising polymethyl methacrylate. This'meth
tion to the thickness'of said sheet for other thick
ad is especially suited for utilization on a com
nesses of said sheet.
mercial scale for a number of reasons. First and
a. A visible light transparent plastic sheet at '
foremost of these reasons is the fact that this
method may be employed as an improvement in 50 least as opaque to X-ray radiations as the bones
of the human body, said plastic comprising poly
already established commercial procedures for
meric methyl methacrylate and, in compatible
the production of polymeric structures for poly
admixture therewith, bismuth trichloride, the
methyl methacrylate, Thus, it is possible to
proportion of said bismuth trichloride for an
modify existing formulations comprising plasti
cizers, ?llers, pigments, dyes or other copoly 55 0.250 inch thickness of said sheet being at least
4.7% by weight of said polymeric methyl meth
merized materials provided such formulations
acrylate, when calculated as bismuthrand?the
comprise at least ‘70% methyl methacrylate by
proportion of said bismuth trichloride'bein'g in
weight. Existing polymerization techniques, cast
inverse proportion to the thickness of said sheet
ing, molding, syruping and sheet casting pro
cedures may be employed without recourse to any 60 for other thicknesses of said sheet.»
4. Process of preparing a product as de?ned
in claim 8, which comprises dissolving an inor
substantial charge. Furthermore, the polymeric
compositions derived thereby may be molded or
formed according to established procedures. The
ganic salt from the group consisting of thorium
nitrate, bismuth nitrate, and bismuth‘ trichloride,
agents, their general availability and non-toxic 65 in monomeric methyl methacrylate and subject
relative low cost of the discussed opacifying
nature also are reasons for the adaptability of
the present process to a commercial scale.
_ ing the resulting solution to polymerization con
,5. Process of preparing a product as de?ned
in claim 8, which comprises dissolving an inor-V
tion may be employed for substantially all uses
for which polymethyl methacrylate plastic com 70 ganic. salt fromthe group consisting of thorium
nitrate, bismuth nitrate, and bismuth trichloride,
positions were known to be useful. A particular
in monomeric methyl methacrylate, ?ltering the
use to which these polymeric materials may be
The polymeric products of the present inven
placed is as a substitute for lead glass used as
resulting solution through activated carbon, and
thereafter subjecting the ?ltered solution to DOly
screening or shielding materials on X-ray equip
ment and X-ray works. In this respect these 75 merization conditions.
- 6- Process of preparing aproduct as de?ned in
claimB, which comprises dissolving an inorganic
salt from the group consistingof thorium nitrate,
bismuth nitrate, and bismuth trichloride, in a
lower saturated monohydric alcohol-having from
1’; ‘to 2, carbon atoms, inclusive, adding mono
merimmethyl methacrylate to the resulting solu
tion, distilling off said alcohol from" said solution
to obtain‘ a solutiongof said inorganic salt in
ganic salt
monomeric ;methyl,-__metha,crylate,
?ltering: this latter solution through activated
carbon, adding a quantity of liquid polymerizable
methyl methacrylate to said- solution, and there
after subjecting the resulting composition‘ to pol
8. Avisible light'transparent plastic comprising
polymeric methyl methacrylate and, in compat
ible admixture therewith, an inorganic salt from
monomeric methyl; methacrylate, ?ltering this 10 the group ‘consisting ofjthorium nitrate, bismuth
latter solution, through» activated carbon, 1 adding
nitrate and bismuth ,trichloride, said. inorganic
a quantity, of; liquid polymerizable methyl meth
salt being‘ present in the plastic in an amount
acrylate to said ,solutionhand thereafter sub
jecting the resulting composition to polymeriza
tion ‘conditions.
~ 7. Process of preparing a product as de?ned in
su?icient‘ to substantially reduce its ability to
transmit X-ray radiations.
9. A visible light transparent plastic compris
ing polymeric methyl methacrylate and, in com
claim 8, which comprises dissolving an inorganic
patible admixture therewith, an inorganic salt
salt from the group consisting of thorium nitrate,
from the group consisting of ‘thorium nitrate,
bismuth nitrate, and bismuth trichloride, in a
bismuth nitrate, and bismuth 'trichloride, said
lower saturated monohydric alcohol containing 20 inorganic
salt being'present'in the plastic in an
from 1 to ,2 carbon atoms, inclusive, ?ltering the
to render said plastic at least
resulting solution through activated carbon, add
radiations as the bones of
ing monomeric methyl methacrylate to the ill
the human body.
tered solution, distilling off said alcohol from
said solution to obtain a solution of said inor 25.
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