CA1116781A - Neutron shielding material and a process for producing the same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A neutron shielding maternal comprising a polymerization product of a monomer mixture composed of a base monomer in-regredient containing as an essential ingredient at least one monomer selected from the group consisting of alkyl methacrylates containing 1-4 carbon atoms in the alkyl group and styrene, and a boric acid ester which contains as an essential constituent component at least one polyol having at least two alcoholic hydroxyl groups in the molecule thereof and contnlnlng 3-16 carbon atoms and in which a molar ratio A of said polyol to boron atoms in said boric acid ester formed from said polyol being in the range of 0.6 < A < 4, the boron content B (wt%) in the total monomer mixture being in the range of 1 ? B < 6.

Description

1L67~

The invention relates to a neutron shielding material with an excellent optical transparency and excellent mechanical strength and also to a process for producing the same.
In recent years, syntheti~ resins have found wide application in various ~ields and thus there is an increasing tendency that such resins are required to be imparted with a diversity of characteristic performances. It is well known that polymethylmethacrylate is widely used due to its excellency in transparencyO It has been proposed (in United States Patent No. 4,129,524) that the polymethylmethacrylate is imparted with a radiation shielding performance keeping its excellent transparency and mechanical strength by copolymerizing methyl methacrylate and lead acrylate or methacrylate in a SpQCifiC
mixing ratio in the presence of a lead carboxylate. The lS radiation shielding material of this type exhibits a satisfactory shieldin~ effect against radiations such as X-rays a y-ray , ~-ray, and ~-ray but its shieldin~ effect against a neutron beam is not adequate.
It is also well known that boron has a neutron shi~lding ability. Accordingly, there have been proposed a variety of neutron shielding materials using boron such as boron-containing glass, boron-containing polyethylene and the like. However, there is not proposed yet a material which is excellent in both transparency and mechanical strength and which is easy to process.
On the other hand, polymethylmethacrylate compositions containing boron compounds are known in the art ~Japanese Patent Publication No. 46-31,847 and Japanese Laid-open Patent Publication No. 52-102,362)~ However, these compositions are incorporated witll such boron compounds merely as a modifier ~ ' 11167~

for the polymer so as to impart anti-static property to the polymer. That is, the boron content in the polymer is in the range of 0.05 - 0.67 wt% and thus such compositions hardly show any ability or shielding the neutron beam. Though a boron-containing synthetic resin is obtainable by kneading a boron compound into a transparent resin or by polymerizing a boron compound-containing monomer, there is a limit in kind and amo~mt of the boron compound to be added in order to ensure satisfactory levels of mechanical strength and neutron shielding ability without a sacrifice of excellent transparen-cy inherent in polyalkylmethacrylates or polystyrene. Resin moldings incorporated with boric acid esters which show good transparency at the time of production may often gradually turn to exhibit an opaque white o~ opaque appearance, as such esters are easily hydrolyæed with water or even with moisture in the air.
It is accordingly an object of an aspect of the present invention to provide a novel material which exhibits ian excellent optical transparency ànd an excellent mechanical strength and which contains boron in high concentration and thus shows a good neutron shielding performance.
It is an object of an aspect of the present~inven-tion to provide a process for producing the just-mentioned type OI a novel neutron shielding material.
In accordance with one aspect of this invention there is provided a process for producing a neutron shielding material comprising polymerizing a monomer mixture composed of a base monomer containing as an essential ingredient at least one monomer selected from the group consisting of alkyl methacrylates containing l - 4 carbon atoms in the alkyl group and styrene, and a boric acid ester which contains as an essential constituent component at least one polyol having at least two alcohollc hydroxyl groups in the molecule thereof and containing 3 - 16 carbon atoms and in which a molar ratio A of said polyol to boron atoms in said boric acid ester formed from said polyol being in the range of 0.6 <A< 4, the boron content B (wt~) in the total monomer mixture being in the range of 1 C B ~ 6.

In accordance with another aspect of this invention there is provided a neutron shielding material comprising a polymerization product of a monomer mixture composed of a base monomer ingredient containing as an essential ingredient at least one monomer selected from the group consisting of alkyl methacrylates containing 1 - 4 carbon atoms in the alkyl group and styrene, and a boric acid -3a-, 7~

ester which contains as an essential constituent component at least one polyol having at least two alcoholic hydroxyl groups in the molecule thereof and containing 3 - 16 carbon atoms and in which a molar ratio A of said polyol to boron atoms in said boric acid ester formed from said polyol being in the range of 0.6 < A < 4, the boron content B (wt~) in the total monomer mixture being in the range of 1 c B < 6.
It has been unexpectedly found that the material according to the present invention which contains high concentration of boron as obtained by polymerizing the monomer mixture of the base monomer and the boric acid ester of the specific type in a specified mixing ratio is held with high level of transparency as well as satisfactory level of mechanical strength. It is very important in industrial and medical points of view that there is provided, accordinq to the invention, a material of a practical and satisfactory neutron shielding performance excellent both in the mechanical strength and in the optical transparency.
In the drawings, Fig. 1 shows the relations between transmitted neutron dose and the thickness of the neutron shielding material o~
the invention; and Fig. 2 shows the relations between transmitted neutron~dose and the thickness of a polymethylmethacrylate

2~ plate.
Briefly stating, as seen from Fig. 2, the pol~methylmethacrylate plates are able to moderate the fast neutron but can hardly absorb the thermal neutron produced as the result o~ the moderation of the fast neutron, thus being not satisfacotr~ as a neutron shielding material. In this i7~3~

connection, however, the results of Fig. 1 reveal t~a~ the boron-containing material according to the invention shows a satisfactory thermal neutron shielding performance and is thus excellent as a neutron shielding material.
The alkyl methacrylates to be used in the present invention are those haivng 1 - 4 carbon atoms in the alkyl group and include, for example, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacryalte, sec-butyl methacryalte, tert-butyl methacrylate and thelike. Of these, methyl methacrylate is most preferable. The alkyl methacrylates and styrene may be used singly or in combination of two or more as the base monomer.
The boric acid ester used herein means esters of boric acid or metaboric acid and polyols having in the molecule thereof at least two alcoholic hydroxyl groups and containing

3 - 16 carbon atoms (hereinlater referred to simply as boric acid ester). The boric acid ester can be obtained by the esterification reaction between the polyol and boric acid or metaboric acid or by the ester interchange reaction between the polyol and a lower alkyl ester of boric acid.
Where hte boric ~id ester is prepared by the esterification reaction, it is feasible similarly to the case of a usual esterification reaction. In order that the reaction is made to proceed conveniently by removing watex produced during the course o~ the esterification reaction, the esterification reaction is conducted in a solvent capable of forming an azeotrope with water under a normal pressure or reduced pressure at a~ azeotropic temperature of the solven~
and water, by which the water is removed azeotropically.

. , .. _. .

After completion of the reaction, the solvent i5 removed and separated. In this connection, if an alkyl methacrylate such as methyl methacryalte is used as the azeotropic solvent, the monomer mixture necessary for the polymerization can be obtained only by adjusting its concentration by removing part of the alkyl methacryalte after the completion of the esterification reaction.
The essential atomic groups constituting the polyol molecule are at least two, preferably 2 - 3, alcoholic hydroxyl groups and an aliphatic or aromatic hydrocarbon residue co~taining 3 - 16, preferably 3 - 13 carbon atoms. Aside from the above polyols, there may be used polyols having an ether group, an ester group, a double bond or the like.
Boric acid usually reacts with three hydroxyl groups to form a triester thereof. It may form a diester by reaction with two hydroxyl groups or may form by reaction with an excess of hydroxyl groups a tetraester of the complex compound type expressed, for example, by the following formula ( R / B R ) H+

The boric acid esters use~ul in the invention include all the compounds of the above-mentioned typ~s. Further, at least two hydroxyl groups of the polyol ordinarily takes part in the esterification with the same boric acid molecule~ but may be esterified with two different boric acid molecules. -The boric acid esters used herein Include the both types o the esters.
The boric acid esters of the invention may further include those in which part of the three acid groups of boric 713~

acid having an ester-forming ability is esterified with a monoalcohol having one alcoholic hydroxyl group in the molecule thereof and the remaining acid groups of the resulting partial ester are esterified with the polyol to form completely esterified boric acid esters. If there is used, as th~
monoalcohol, one having a double bond such as hydroxylalkyl acrylate, hydroxyalkyl methacrylate or the like, the double bond takes part in the copolymerization with the alkyl acrylate or styrene thereby forming a copolymer containing the boric acid ester as its constituent component.
In case where the carbon atoms of the polyol constituting the boric acid ester used in the invention is 2, the resulting neutron shield.ing material becomes poor in water resistance and is liable to be turned white in appearance, thus not giving a material with good trancsparency. On the other hand, the polyol having greater than 1,7 carbon atoms will lower the content of boron atoms so much and deteriorate the neutron shielding performance o the material, making it difficult to fully achieve he purpose of the invention, coupled with another disadvantage that the mechanical strength of the material is lowered. In order to keep the transparency and the mechanical strength of the material at practical levels, the molar ratio A of the polyol to the boron atoms in the boric acid ester should satisfy the following condition:
0.6 ~ A ~ 4 and preferably 0.6 < A < 2. The amount of the boric acid ester also gives a great influence on the properties of the material and thus the boron conten~ B (wt%) in the total monomer mixture should be controlled as follows:
1 < B < 6 and preferably 1 ~ B ~ 4. Less amount of the boron content B than 1 wt~ will reduce the neutron shielding i78~

performance, while larger amount than 6 wt% will undesirably increase the flexibility of the pol~mer material due to the plasticizing effect o~ the boric acid ester, resultin~ in a loss of dimensional stability.
Typical polyols are 1,2-propanediol, 1,3-propanediol, glycerol, 2-methyl-1,2-propanediol, 2-methyl-1,3-propanediol, 2-methylene-1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1-butene-3,4-diol, 2-butene-1,4-diol, 2~methyl-2,4-butanediol, 3-methyl-1,3-butanediol, 3-methyl-`
1,2-butanediol, 2-methyl-1,2-butanediol, 2-methyl-1,4-butanediol, 2-methyl-1,3 butanediol, 2-methyl-1-butene-3,4-diol, 2-methyl-2-butene-1,4--diol, 1,3-pentanediol, 2,2-dimethyl-1,3-propanediol, 2-methyl-2~-ethyl-1,3-propanediol, 3-methyl-1,5-pentanediol, 3-methyl-1,3,5-pentanetriol, 2,2-diethyl-1,3-propanediol, 2-methyl-2-propyl-1,3-propanediol, 2-methyl-2,4-pentanediol, 2,4-dimethyl-2,4-pentanediol, 2-ethyl-1,3-hexanediol, ~-methyl-2-butyl-1,3-propanediol, 2-ethyl-2-propyl-1,3-propanediol, 2,2,4-trimethyl-1,3-pentanediol, 2,4-dimethyl-2,4-hexanediol, 3-ethyl-2-isobutyl-1,3-propanediol, 2-ethyl-2-(1-butenyl)-1,3-propanediol, 2-propyl-2-butyl-1,3-propanediol, 2,4-~imethyl-2-ethyl-1,3-hexanediol~ 2,2-dibutyl-1,3-propanediol, 2-pentyl-2-(l-heptenyl)-1,3-propanediol, 2-methyl-2-dodecyl-1,3-propanediol, 1,2-pentanediol, 1,2-hexanediol, 1,2-heptanediol, 1,2-octanediol, 1,2-dodecanediol, 1,2-cetanediol, trimethylolpropane, pentaerythritol, diethylene glycol, triethylene glycol, tetraethylene glycol, glycerol monoacetate, glycero:L
monopropionate, glycerol monobutyrate, glycerol monoisobutyrate, glycerol monovalerate, glycerol monoisovalerate, glycerol monocaprylate 9 glycerol monoacrylate, glycerol monomethacrylate, glycerol monobenzoate, glycerol monoc~nnamate, trimethylolpropane monoacetate, trimethylolpropane monopropionate, trimethylolpropane monobutyrate, trimethylolpropane monoisobutyrate, trimethylolpropane monoacryrate, trimethylolpropane monomethacryrate, trimethylolpropane monobenzoate, trimethylolpropane monocinnamate, 3-methyl-1,3,5-pentanetriol monoacrylate, 3-methyl-1,3,5-pentanetriol monomethacrylate, 3-methyl-1,3,5~pentanetriol monocinnamate, pentaerithrythritol monoacetate, pentaerythritol diacetate, pentaerythritol monopropionate, pentaerythritol dipropionate~
pentaerythritol monobutyrate, pentaerythritol dibutyrate, pentaerythritol monoisobutyrate, pentaerythritol diisobutyrate, pentaerythritol monovalerate, pentaerythritol monoisovalerate, pentaerythritol monocaprilate, pentaerythritol monoacrylate, pentaerythritol diacrylate, pentaerythritol monomethacrylate, pentaerythritol dimethacrylate, pentaerythritol monocinnamate, pentaer~thritol monobenzoate, 1-phenylethane-1,2-diol, ethylene ~lycol n~onoglycolate, et]lylene glycol diglycolate, 1,2-propanediol monoglycolate, 1,:2-propanediol diglycolate, 1,3-propanediol monoglycolate, 1,3-propanediol diglycolate, 1,2-psopanediol monolactate, },2-propanediol dilactate, 1,3-propanediol monolactate, 1,3-propanediol dilactate and .~ like.
Among tlle abovs mentioned polyols, 1,3-glycol having the following general foxmula I

2 13 l5 Rl- C - I - C 6 (I) in which Rl, R2, R3, R5 and R6 are independently H or an alkyl group containing 1 - 10 carbon atoms, R~ is H or an alkyl or alkenyl group containing 1 - 10 carbon atom~

7~3 ~

and R3 and R~ may represent a methylene group in combination thereof, and the total number of carbon atoms is in the range of 3 - 13 inclusive gives an especially preferable result.
The reason why the 1,3-glycol gives the superior effect is not known exactly, but the 1,3-glycol is seen to form a stable borate in the polymer by reacting with boric acid or metaboric acid. Further, the 1,3-diol having the formula I, wherein Rl, R2 and R6 are CEI3, R3 and R~ are H, and R5 is H or CH3, i.e~ 2-methyl-2,~-pentanediol and 2,4-dimethyl-2,4-pentanediol are the most preferable.
Further, monoalcohols to be reacted with boric acid or metaboric acid in order to form a partial ester of the boric acid are methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol, l-pentanol, l-hexanol, ~5 l-octanol, 2-ethyl hexanol, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl acrylate, 3--hydroxypropyl methacrylate, diethylene glycol monoacrylate, diethylene glycol monomethacrylate, triethylene glycol monoacrylate, triethylene glycol monomethacrylate, dipropylene glycol monoacrylate, dipropylene glycol monomethacrylate, allyl alcohol, methallyl alcohol and the lilce.
The basa monomer useful in the present invention may include, aside from the afore-mentioned essential monomers, 2S other polymerizable monomers within such a range of amount as not to give any adverse affection to the effect of Lhe invention. Crosslinkable monomers may be used as said polymerizable monomers so as to impart hardness and dimensional stability to the polymer. Examples of such crosslinkable monomers include ethylene glycol t propylene glycol, polyethylene glycol 7~.

or polypropylene glycol having an average degree of polymerization of 2 - 23, diacrylates or dimethacrylates of diols such as linear or branched polyethylene glycol having 3 - 32 carbon atoms, trimethylolpropane dimethacrylate, trimethylolpropane diacrylate, trimethylolpropane trimethacrylate, trimethylpropane triacrylate, divinyl benzene and the like.
The neutron shielding material according to the invention may be prepared by any of production processes provided tha~ there is ultimately obtained either a polymer composition composed of a polymer or copolymer containing the afore-said essential monomer or base monomer and the boric acid e~ter, or a copolymer of the essential or base monomer and the copolymerizable boric acid ester. Conveniently, the monomer mixture is polymerized in a mold or extruder in the presence of an initiator for radical polymerization. The reaction is cond~cted at a temperatuxe ranging generally 0 - 200C, preferably 20 - 180C. ~here a cast plate is made, a multi-stage polymerization process may be used in which process pre-polymerization is effected at 20 - 100C
as usual to convert most of the monomers into a polymer, after which the reaction temperature is increased to 100 -180C to convert remaining monomer into a polymer. ~his multi-stage process is also encompassed within the scope o~
the invention as its preferred embodiment.
~5 The initiator for radi~al polymerization is used generally in the range of 0.001 - 5 wt%, preferably 0.01 - 1 wt%, of the total mcnomer used. Typical examples of the initiator include lauroyl peroxide, tert-butyl peroxyisopropyl carbonate, benzoyl peroxide, dirumyl peroxide, tart-butyl peroxyacetate, tert-butyl peroxybenzoate, di-tert-butyl peroxide, azo-bis-isobutyronitrile and the like.
When the boric acid ester used has no double bonds, the produced neutron shielding material is obtained as a polymer - solution uniformly dispersing the boric acid ester in the produced polymer or copolymer since the ester exists as the base monomer solution upon the polymerization. On the other hand, in case where the boric acid ester has a double bond, the material is obtained in the form of a copolymex of the base monomer and the copolymerizable boric acid ester.
The present invention will be particularly illustrated by way of examples, in which parts and percent are by weight unless otherwise specified.
Examples 1 - 22 Synthesis of Boric Acid Ester so:Lutions 61.8 parts of boric acidl, polyols in different amounts .indicated in Table 1, and 250 parts of methyl methacrylate were fed into a reaction distillator and reacted under reduced pressure at 60C, produced water being continuously removed azeotropically with methyl methacryla~e. The reaction was stopped when a predetermined amoun~ of water was produced. The methyl methacrylate was distilled off so as to adjust th~ content of boron to a desired level thereby giving an about 2 - 6 % boric acid ester solution in methyl methacrylate (in case of high concentration, precipitation of crystals sometimes took place upon cooling). q~he test results are shown in Table 1. It is to be noted that Examples 18 - 22 are for comparative purpose.
Example 23 Preparation of Boric Acid Solution 61.8 parts of boric acid, 118 parts of 2-methyl-2,4-pentanediol and 250 parts of methyl methacrylate ~1 E;7~

were charged into a reaction distillator and reacted under reduced pressure at 60C, 35 parts of produced water being continuously removed azeotropically with methyl methacrylate.
To the reaction mixture was ~urther added 130 parts of 2-hydroxyethyl methacrylate and 18 parts of produced water wascontinuously aæeotropically eliminated, after which methyl methacrylate was distilled off to give a boric acid ester solution having a boron content of 3.0 ~.
Example 24 Pre~aration of Boric Acid Ester Solution Example 23 was repeated except that 144 parts of 2-hydroxypropyl methacrylate was used instead of 2-hydroxyethyl methacrylate thereby obtaining a boric acid ester solution having a boron content of 2.0 %.
Example 25 Preparation of Borlc Acid Ester So~lution Example 18 was repeated using 58 parts of allyl alcohol instead of 2-hydroxyethyl methacrylate, thereby obtaining a boric acid ester solution having a boron content of 3.0 ~.
Example 26 - 52 Fabrication of Cast Plate . _ . .
To the monomer mixtures obtained by admixing the methyl methacrylate solutions of boric acid esters prepared in Examples 1 - 25 with various types of vinyl monomers in amounts~ indicated in Table 2 was added azo-bis-isobut.~yronitrile : as a radical polymerization initiator to dissolve in 0.015 parts per 100 parts of the monomer mixture. Each solution was cast into a cell assembled with two glass plates and a vinyl chloride resin gasket in a thickness o 20 mm and then subjected to polymerization in a nitrogen atmosphere at 80C

~L1167~1 for 4 hours and then at 120C for 2 hours. The properties of the cast sheets thus obtained are shown in Table 2.
From the Table 2, it will be appreciated that the materials according to the invention are boron-containing synthetic resins which are excellent in transparency, mechanical strength and neutron shielding performance. It will be noted That Examples 48 - 25 are for comparison.

._ ...

Table 1 Amount of Content Example Polyol Removed of Boron No Amount Molar Compound (parts) Ratio A l(atet ) (-%) 1 1,2-propanediol 152 2 52 6.0 2 1,3-butanediol 180 2 50 4.0 3 1,4-butanediol 270 3 53 2.0

4 3-methyl-1,3-butanediol 208 2 46 4.0 2,2-dimethyl-1,3- 208 2 49 4.1 propanediol 6 3-methyl-1,5-pentanediol 236 2 47 2.1 7 2-methyl-2,4-pentanediol 236 2 48 2.0 8 2-methyl-2,4-pentanediol 177 1.5 46 4.0 9 2-methyl-2,4-pentanediol 118 1 35 5.9 2-ethyl-1,3-hexanediol 438 3 $4 2.0 11 glycerol ].84 2 52 4.0 12 trimethylolpropane X68 2 52 2.0 13 3-methyl-1,3,5-pentane- 268 2 Sl 1.9 triol 14 3-methyl-1,3,5-pentane- 134 1 50 6.0 triol 2,3-dihydroxypropyl 292 2 43 2.0 acrylate 16 2,3-dihydroxypropyl 320 2 48 2.0 methacrylate 17 3-methyl-3,5-dihydroxy- 303 1.5 45 2.1 pentyl methacrylate 18 n-butanol 222 3 54 2.0 19 2-hydroxyethyl methacry- 390 3 52 2.1 ~late , ~` 20 ethylene glycol 124 2 53 2.0 21 3-methyl-1,3-bu~anediol 416 4 53 2.0 22 2-nethyl-2,4-pentanediol 118 1 36 6.2 .

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Example 53 Cast sheets having the same composition as in Example 36 but different thicknesses were each vertically irradiated with fission neutron from 252Cf of 1 Ci at a distance of 163 cr,~ from the neutron source so as to detect a transmitted neutron beam by means of a BF3 counter which was located beh.ind the cast sheet and at a distance of 239 cm from the neutron source. The BF3 counters were covered with polyethylene sheets with thicknesses of 0, 1.4, 3 and 5 cm and thus were controlled to show maximal sensitivities to thermal neutron, epithermal neutron, neutron of 100-1,000 eV, and neutron of above 100 KeV, respectively. The relations between the thickness of the cast sheet and a transmitted neutron dose are shown in Fig. 1.
For comparison, the abovle measurement was repeated using polymeth~l methacrylate sheets instead of the cast sheets of Example 36 thereby obtaininc~ the results shown in Fig. ~.
By comparison between Figs. 1 and 2, it will be understood that the cast sheets of Example 36 are much more excellent in neutron attenuating ability than the polymethyl methacrylate sheets. It will be noted that, in ~igs. 1 and 2, the curves A and E are for the polyethylene cover thickness o~ 5 cm, the curves B and F are for the thickness of 3 cm, the curves C
and G are for the thickness of 1.4 cm, and the curves D and H are for no polyethylene covering.
; Example 54 A neutron from the same type of the neutron source as used in Example 53 was irradiated against A laminated sheet of a 25 cm thick polymethyl methacrylate sheet and a 10 cm thick cast sheet with the same composition as used in Example 36 7~3~

located in the same manner as in Example 53. The lami.nated sheet was set so that the acryl sheet was faced to the neutron source. In the cast sheet were embedded thermoluminescence dosimeters (T~D) using a LiF element along the axis of the irradiation neutron at positions of 0, 5 and 10 cm from a boundary between the polymethyl methacrylate sheet and the cast sheet. After the irradiation for 72 hours, the TLDs were taken out to measure the neutron doses. As a result, it was found that relative doses of thermal neutron detected with the respective TLDs which were set at distances of 0 cm, 5 cm and 10 cm from the boundary with the polymethyl methacrylate sheet were, respectively, 1 and 9 x 10 2 and 2.8 x 10 2. From the results, it will be appreciated that the thermal neutron is intensely absorbed in the cast sheet of the invention.

- 20 ~

Claims (20)

What is claimed is:

1. A neutron shielding material comprising a polymerization product of a monomer mixture composed of a base monomer ingredient containing as an essential ingredient at least one monomer selected from the group consisting of alkyl methacrylates containing 1-4 carbon atoms in the alkyl group and styrene, and a boric acid ester which contains as an essential constituent component at least one polyol having at least two alcoholic hydroxyl groups in the molecule thereof and containing 3-16 carbon atoms and in which a molar ratio A
of said polyol to boron atoms in said boric acid ester formed from said polyol being in the range of 0.6 < A < 4, the boron content B (wt%) in the total monomer mixture being in the range 1 ? B < 6.

2. A material as defined in Claim l, wherein said polyol is one which has 2 - 3 alcoholic hydroxyl groups and 3 - 13 carbon atoms.

3. A material as defind in Claim 2, wherein said polyol is an aliphatic polyol.

4. A material as defined in Claim 3, wherein said polyol is 1,3-glycol expressed by the general formula (I) (I) in which R1, R2, R5 and R6 are independently ~ or an alkyl group containing 1 - 10 carbon atoms, R4 is H or an alkyl alkenyl group containing 1 - 10 carbon atoms and R3 and R4 may represent a methylene group in combination thereof and the total number of carbon atoms is the range of 3 - 13, inclusive.

5. A material as defined in Claim 4, wherein R1, R2 and R6 are CH3, R3 and R4 are H, and R5 is H or CH3.

6. A material as defined in Claim 1, wherein the essential monomer is an alkyl methacrylate.

7. A material as defined in Claim 6, wherein said alkyl methacrylate is methyl methacrylate.

8. A material as defined in Claim 1, wherein said material is a composition composed of a polymer of the base polymer and said boric acid ester.

9. A material as defined in Claim 1, wherein said material is a copolymer or the base monomer and the boric acid ester.

10. A process for producing a neutron shielding material comprising polymerizing a monomer mixture composed of a base monomer containing as an essential ingredient at least one monomer selected from the group consisting of alkyl methacrylates containing 1 - 4 carbon atoms in the alkyl group and styrene, and a boric acid ester which contains as an essential constituent component at least one polyol having at least two alcoholic hydroxyl groups in the molecule thereof and containing 3 - 16 carbon atoms and in which a molar ratio A
of said polyol to borom atoms in said boric acid ester formed from said polyol being in the range of 0.6 < A < 4, the boron content B (wt%) in the total monomer mixture being in the range of 1 ? B < 6.

11. A process as defined in Claim 10, wherein said polyol is one which has 2 - 3 alcoholic hydroxyl groups and 3 - 13 carbon atoms.

12. A process as defined in Claim 11, wherein said polyol is an aliphatic polyol.

13. A process as defined in Claim 12, wherein said polyol is a 1,3-glycol of the general formula (I) (I) in which R1, R2, R3, R5 and R6 are independently H or an alkyl group containing 1 - 10 carbon atoms, R4 is H or an alkyl or alkenyl group containing 1 - 10 carbon atoms, R3 and R4 may represent a methylene group in combination thereof, and the total number of carbon atoms is in the range of 3 - 13 inclusive.

14. A process as defined in Claim 13, wherein R1, R2 and R6 are CH3, R3 and R4 are H, and R5 is H or CH13.

15. A process as defined in Claim 10, wherein the polymerization reaction is conducted at a temperature of 0° to 200°C in the presence of an initiator for radical polymerization.

16. A process as defined in Claim 15, wherein the polymerization reaction is conducted at a temperature of 20°
to 180°C in the presence of an initiator for radical polymerization.

17. A process as defined in Claim 10, wherein the essential monomer is an alkyl methacrylate.

18. A process as defined in Claim 17, wherein said alkyl methacrylate is methyl methacrylate.

l9. A process as defined in Claim 10, wherein said material is a composition composed of a polymer of the base monomer and the boric acid ester.

20. A process as defined in Claim 10, wherein said material is a copolymer of the base monomer and the boric acid ester.