Transparent neutron shielding material

Abstract

Provided is a neutron shielding material having excellent transparency and high neutron shielding ability. In this neutron shielding material, light transmittance at wave length of 400 to 700 nm is 80% or greater, and the thickness of a 1/10 value layer of a neutron generated from Californium 252 is 14 cm or less.

Claims

1. A neutron shielding material whose light transmittance at a wavelength of from 400 nm to 700 nm is 80% or greater, and whose thickness of 1/10 value layer of neutron beam generated from Californium 252 is 14 cm or less.

2. The neutron shielding material of claim 1, wherein the neutron shielding material is a cured product of an epoxy resin composition.

3. The neutron shielding material of claim 2, wherein the number density of hydrogen atom of the epoxy resin composition is 6.78×10.sup.22 atoms/cm.sup.3 or more.

4. The neutral shielding material of claim 3, wherein the epoxy resin of the epoxy resin composition possesses an alicyclic skeleton.

5. The neutron shielding material of claim 4, wherein the epoxy resin possessing an alicyclic skeleton is an epoxy resin obtained by epoxidation of a cyclic olefin.

6. The neutron shielding material of claim 4, wherein the epoxy resin possessing an alicyclic skeleton is an epoxy resin obtained by hydrogenation of an aromatic epoxy resin.

7. The neutron shielding material according to claim 2, wherein the curing agent of epoxy resin is an amine possessing an alicyclic skeleton or an aliphatic amine.

8. The neutron shielding material according to claim 1, wherein the neutron shielding material is produced by a molding method.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The present invention will be discussed below in more detail.

(2) Starting Material: Epoxy Resin

(3) The epoxy resin used in this invention is an epoxy resin possessing an alicyclic skeleton.

(4) As the epoxy resin possessing an alicyclic skeleton, an epoxy resin selected from a group composed of an epoxy resin obtainable by epoxidation of a cyclic olefin and epoxy resin obtainable by hydrogenation of an aromatic epoxy resin is desirable.

(5) The following are examples of an alicyclic epoxy resin obtained by epoxidation of a cyclic olefin:

(6) 3, 4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexanecarboxylate 1,2-epoxy-vinylcyclohexene, bis(3, 4-epoxycyclohexylmethyl) adipate, 1-epoxyethyl-3,4-epoxycyclohexane, limonenediepoxide, oligomer type alicyclic epoxy resin (product name of Daicel Chemical Industries Ltd.; Epolead GT300, Epolead GT400, EHPE-3150). Among these products, 3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexanecarboxylate is desirable, and by blending this alicyclic epoxy resin, the viscosity of the epoxy resin composition can be dropped, and, accordingly, efficiency of work can be improved.

(7) The following are examples of an epoxy resin obtained by hydrogenation of an aromatic epoxy resin: bisphenol A epoxy resins, bisphenol F epoxy resins, 3,3′,5,5′-tetramethyl-4,4′-bisphenol epoxy resins, biphenyl epoxy resins such as 4,4′-biphenol epoxy resins, phenol-novolac epoxy resins, cresol-novolac epoxy resins, bisphenol A novolac epoxy resins, naphthalenediol epoxy resins, tris-phenylolmethane epoxy resin, tetrakisphenylolethane epoxy resins or epoxy resins prepared by hydrogenation of aromatic ring of aromatic epoxy resin such as phenoldicyclopentadienenovolac epoxy resins. Among these compounds, bisphenol A epoxy resins, bisphenol F epoxy resins or epoxy resins prepared by hydrogenation of aromatic ring of biphenol epoxy resins are desirable because epoxy resins having a high hydrogenation ratio can be obtained by these compounds.

(8) The hydrogenation ratio of hydrogenated epoxy resins obtained by hydrogenation of these aromatic epoxy resins is desirably from 90 to 100%, and more desirably from 95 to 100%. When the hydrogenation ratio is smaller than 90%, the resin absorbs short wavelength light and deterioration of the resin is caused by time elapse, and is not desirable. Said hydrogenation ratio can be measured by finding a change of absorbancy (wavelength: 275 nm) using an absortiometer.

(9) With respect to the above-mentioned alicyclic epoxy resins, one kind can be used alone or used together with other kinds.

(10) <Curing Agent>

(11) As a curing agent which is used in the present invention, an amine possessing an alicyclic skeleton, specifically a compound represented by the following general formula (1) or an aliphatic amine can be desirably used.

(12) ##STR00001##
(in the formula, R.sup.1 is one selected from the group consisting of a direct bond, methylene group, —C(CH.sub.3).sub.2—, —O— or —SO.sub.2—, R.sup.2 and R.sup.3 independently is hydrogen atom or alkyl group of carbon number 1-4)

(13) R.sup.1 is one selected from the group consisting of a direct bond, methylene group, —C(CH.sub.3).sub.2—, —O— or —SO.sub.2—, desirably is a methylene group or —C(CH.sub.3).sub.2—. R.sup.2 and R.sup.3 independently is a hydrogen atom or an alkyl group of carbon number 1-4 and desirably is an alkyl group of carbon number 1-2.

(14) An amine possessing an alicyclic skeleton to be used is not specifically restricted, however, for example, 1,2-diaminocyclohexane, 1,4-diaminocyclohexane, hydrogenated orthotoluenediamine, hydrogenated metatoluenediamine, hydrogenated metaxylilenediamine (1,3-BAC), isophoronediamine or isomer thereof, norbornanediamine, 3,3′-diethyl-4,4′-diaminodicyclohexyl-methane can be mentioned, and especially 3,3′-diethyl-4,4′-diaminodicyclohexyl-methane is desirable.

(15) As an example of a compound represented by said general formula (1), concretely, 3,3′-dimethyl-4,4′-diaminodicyclohexylmethane, 3,3′-diethyl-4,4′-diaminodicyclohexylmethane, bis(4-amino-3-methyl-5-ethylcyclohexyl) methane, 3,3′-diethyl-4,4′-diaminodicyclohexylmethane or 4,4′-diamino-dicyclohexylmethane can be used, and especially 3,3′-dimethyl-4,4′-diaminodicyclohexylmethane is desirable.

(16) As an example of an aliphatic amine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, hexamethylenediamine, metaxylilenediamine, trimethylhexamethylene diamine, 2-methylpenta-methylenediamine, diethylaminopropylamine, polyoxypropylene diamine, polyoxypropylenetriamine or N-aminoethylpiperazine or combination of these compounds can be used.

(17) Further, a modified reactant of these polyamines with epoxy resin, a modified reactant of polyamines with a monoglycidil compound, a modified reactant of polyamines with epichlorohydrin, a modified reactant of polyamines with alkyleneoxide of carbon number 2-4, an amide oligomer obtained by chemical reaction of polyamines with a multifunctional compound possessing at least one acyl group or an amide oligomer obtained by chemical reaction of polyamines with a multifunctional compound possessing at least one acyl group and monovalent carboxylic acid and/or a derivative thereof can be used as a curing agent of epoxy resin. The above-mentioned amine possessing an alicyclic skeleton and an aliphatic amine can be used alone or can be used together.

(18) In the present invention, blending an amount of a curing agent at an ordinary temperature curing epoxy resin can be properly selected according to the kind of curing agent. However, generally, the blending amount of the curing agent is 10-200 mass parts, desirably 20-100 mass parts to 100 mass parts of an epoxy resin.

(19) <Other Additives>

(20) The first essential point of the present invention is to reduce the energy which neutrons possess as generated by elastic collision of neutrons with hydrogen atoms, and as a result, to shield neutrons. That is, neutron causes nuclear reaction with specific nuclide and captured. As a neutron capturing agent, boron is well known.

(21) In the present invention, a borate compound can be further added to the epoxy resin with which the above-mentioned curing agent is blended. Powder of borate compounds represented by B.sub.4C, BN, B.sub.2O.sub.3 and B(OH).sub.3 can be added within the range so as not to spoil the effect of the present invention when necessary.

(22) The shielding effect against γ-ray can be provided by adding boron glass (borosilicate glass) frit as one example of a powder of the borate compounds. Regarding the boron atom, although the presence of 14 kinds of isotopes of mass number from 6 to 21 is known, the stable isotopes are .sup.10B and .sup.11B, and natural abundance of each is 18.8% and 80.2%. Neutron causes nuclear reaction with .sup.10B and captures neutron. For the practical use of the present invention, nature boron compound is desirable from an economical view point. Further, although there are various boron compounds such as oxide, sulfide, nitride or halide, boron glass (borosilicate glass) frit is desirable in the present invention. Boron glass (borosilicate glass) can be obtained by adding boric acid to glass, and the softening point and hardness of it are improved. Further, the term “frit” means a powder of glass.

(23) The borosilicate glass frit to be used in the present invention is not restricted, and any kind of product available on the market can be used.

(24) The particle size of the borosilicate glass frit to be used in the present invention is from 0.1 μm to 1000 μm, and desirably from 1 μm to 500 μm.

(25) When the added amount of borosilicate glass frit is large, much the shielding effect will be improved. However, transmittance becomes bad. Therefore, it is necessary to determine the proper ratio for the addition of the borosilicate glass frit.

(26) A desirable ratio for adding the borosilicate glass frit is from 0.1 to 13 wt %, and more desirably from 1 to 10 wt %.

(27) Regarding the adding method of borosilicate glass frit, there is no restriction. However, a method which achieves a good dispersion state is desirable.

(28) Further, Fe, Ni, Cu, W, Pb or high-density metal powder, such as an oxide of these metal elements, can be used as a γ-ray shielding agent within a range that does not spoil the effect of the present invention.

(29) Other agents such as an antioxidant, a stabilizer, a reactive or nonreactive diluent, a plasticizer, a mold-releasing agent, a flame retardant, a pigment or a fluorescent substance can be added to the curable epoxy resin composition of the present invention within a range so as not to spoil the effect of the present invention when necessary. Further, for the purpose of improving the physical properties, such as thermal expansion coefficiency, hardness or thixotropy, fillers such as silica (fumed silica, colloidal silica or sediment silica) can be added. With respect to a glass, staple fiber glass, filament glass, woven glass fiber or non-woven fiber can be used and not limited by their form. With respect to the kind of glass, any kind of glass such as E glass, T glass, D glass or NE glass can be used.

(30) Preparation Method of Molded Product

(31) In the curing reaction of an epoxy resin composition, it is necessary to cure the product by controlling the generated reaction heat. In the present invention, in a case when a large amount of inorganic subject of high heat capacity is not added at all, it is indispensable to reliably control and remove heat generated by the curing reaction for the molding process. If heat of reaction cannot be controlled, molding strain will be caused. Accordingly, deterioration of a see-through feature originated from non-uniformity of the molded product will be caused. Further, in a case when a transparent organic shuttering is used, the shuttering itself is transformed and accordingly the molded product transforms too. Furthermore, bubbles, which become a cause of deterioration of neutron shielding ability, are contained in the molded product and cause serious defects for the shielding ability. Such a product cannot be used practically. Accordingly, in the present invention, the above-mentioned problems are solved by following the molding method. That is, for the molding process, the epoxy resin composition is previously defoamed, the mixture is divided and poured into a shuttering intermittently. Preventing rolling up of bubbles at the bottom of the gate of the shuttering, heat generated by curing is removed by outer cooling of the shuttering and performs the curing process under an ordinary temperature.

(32) Mixing of starting materials: Components to be blended are weighted respectively and mixed. The mixer to be used for the mixing process is not specifically restricted. However, a mixer in which stirring and defoaming can be simultaneously carried out is desirable.

(33) As the typical example, Chemical Mixer, a product of Aicohsha Co., Ltd. can be used.

(34) Defoaming: The obtained mixture is defoamed using a specific defoamer. Since the required characteristics of the molded product of the present invention are neutron shielding ability and light transmissivity, establishment of a manufacturing technique which removes bubbles contained in the molded product as much as possible is indispensable. As a defoamer, Vacuum Deforming Apparatus of Otsuka Factory Co., Ltd. can be used. Defoaming time is decided considering the data of ascending temperature of the reacting heat of the mixture composed of a selected epoxy resin and a curing agent, and curing time.

(35) Ordinary necessary defoaming time is 1 to 120 minutes and practically adjusted to 7-60 minutes.

(36) Molding: Method for molding is not restricted, and a molding method characterizing to form a shuttering according to a necessary shape of the molded product and to pour the defoamed mixture to the shuttering can be used. After molding, the shuttering is placed under room temperature and the curing reaction progresses sufficiently. By measuring the temperature of the molded product, the end point of the curing can be detected.

(37) Ordinary necessary curing time is 1 to 168 hours, and practically is 6 to 72 hours.

(38) Estimation of a shielding material can be carried out as follows. Several pieces of a specimen of the same thickness are prepared and by piling up these specimens, the thickness of the shielding material can be adjusted.

(39) Construction of measuring system. The neutron shielding ability can be measured as follows. Thickness of 1/10 value layer can be obtained from neutron shielding ratio calculated by dividing neutron incidence numbers to a shielding material with neutron transmission numbers through the shielding material.

(40) As a neutron beam source, americium 241-Be, americium 241-Li or californium 252 are known, and it is desirable to sham energy spectral of a neutron to be shielded. Especially, regarding californium 252, since radiation dose isostere average energy is 2.40 MeV and energy spectral of neutron indicates Maxwell's distribution, can be used desirably.

(41) For the measurement of neutron, a neutron survey meter on the market can be used.

EXAMPLES

(42) The present invention will be illustrated more in detail by Examples. However, the invention is not intended to be restricted to the Examples.

(43) Manufacturing Method of Molded Product

(44) First Process

(45) 1.7 kg of hydrogenate (epoxy equivalent 200 g/eq, total chlorine amount 1400 ppm) obtained by polycondensation of epoxy resin (Product of Mitsubishi Chemical Corporation Product name: jER YX8000), 4,4′-isopropylidenediphenol with 1-chloro-2,3-epoxypropane, curing agent (Product of Mitsubishi Chemical Corporation Product name: jER cure 113), 4,4′-methylenebis(2-methylcyclohexane amine), 3,3′-dimethyl-4,4′-diamino dicyclohexylmethane and 0.8 kg of laromin C diamine (amine value: 98 mgKOH/g) are weighted and stirred for 20 minutes at ordinary temperature (23.7° C.) using a mixer. At the end of the stirring process, the temperature of the mixture is 27.3° C. This mixture is defoamed by a defoamer for 50 minutes. At the end of the defoaming process, the temperature of the mixture is 30.6° C. Specifications of a mixer and defoamer are mentioned below.

(46) (1) Chemical Mixer

(47) Maker: Aicohsha Co., Ltd.

(48) Type: ACM-30LVT (special specification)

(49) Specification: Originally three phase altering current, 200 volt is changed to single phase, 100 volt for the purpose to make fine adjustment of rotating number possible at low and middle rotating speed range. Stirrer is biaxial (spiral hook type: SCS13 type) With vacuum defoaming function at stirring process and with specific piping function.
(2) Defoaming Machine

(50) Maker: Otsuka Factory Co., Ltd.

(51) Type: Vacuum defoaming machine corresponding to pail can (with specific piping function.

(52) Specification: 201 pail can corresponding type with a sensor (with chemical mixer connecting function)

(53) Second Process

(54) Shuttering for molding (200 mm×200 mm×20 mm) made of a transparent acrylic resin board (2 mm thickness) is prepared. The mixture obtained by the first process is slowly poured into the shuttering obliquely placed on a working table with a 15 degree angle along with side surface of the shuttering. Pouring is continued by changing the angle horizontally. The above-mentioned process is repeated 3 times and all of the mixture is poured into the shuttering. After the pouring process, temperature is measured 4 times at every 30 minutes and no abnormal phenomenon is detected. After the pouring process, the mixture is left for one week and the molded product specimen is obtained.

(55) Measurement of Neutron Shielding Effect

(56) The specimen is a transparent board of 200 mm×200 mm×20 mm. The dose rates of every thickness are measured by piling up the board and the neutron shielding ability is estimated. At a 1.2 m height position, the radiation source and the measuring apparatus are placed so that the distance between the radiation source and the measuring center of the measuring apparatus is 50.8 cm. In the case to set the specimen between and not to set the specimen measurement is repeated 10 times. Shielding ratio is calculated by averaging the values obtained by 10 measurements.

(57) Radiation source is californium 252 (nominal value: 3.7 MBq) and Neutron Survey Meter TPS-451 of Aloka Co., Ltd. is used as a measuring apparatus.

(58) Thickness of the specimen that indicates 90% shielding ratio is measured and obtained 12 cm thickness of the shielding board of 1/10 value layer of a neutron ray.

(59) Measurement of Light Transmissivity

(60) Spectrophotometer U-2010, which is the product of Hitachi High Tech Science Co., Ltd., is used and light transmissivity is measured based on JISK7361 (Plastic-Determination of the total luminous transmittance of the transparent materials).

(61) Neutron shielding effect of the Example is shown in Table 1.

Comparative Example 1

(62) 1451 g of polycondensation product (epoxy equivalent 224 g/eq, total chlorine amount 47450 ppm) of epoxy resin (ST-3000 of Nippon Steel and Sumikin Chemical Co., Ltd.), 2,2′-bis(4-hydroxycyclohexylpropane) and 1-chrolo-2,3-epoxypropane, curing agent (HL-107 of Nippon Steel and Sumikin Chemical Co., Ltd.) and 581 g of denatured heterocyclic diamine are weighted, and molded product of 200 mm×200 mm×50 mm is obtained by the same process as used for the Example. Specimen of prescribed thickness is prepared by combining these molded products and provided to the measurement of shielding effect.

(63) Neutron shielding effect of the Comparative Example 1 is shown in Table 2.

(64) From the above-mentioned data, thickness of 1/10-value layer is 16 cm.

(65) Measuring results of light transmissivity are summarized in Table 3.

(66) Light transmissivity of the Comparative Example 1 is deteriorated from 500 nm (green color), and indicates 79.7% at 450 nm and 51.6% at 400 nm, that is, transmitted light is largely decreased at the blue-violet range and colored to a yellowish-brown color. On the contrary, in the Example, remarkable absorption cannot be observed by 450 nm, and at 450 nm indicates 84.9%, that is, high transmissivity is maintained. Coloring is not observed by naked eyes of the operator, that is, no colorless and transparent neutron shielding material is obtained.

(67) Density and hydrogen atom number density are mentioned in Table 4. Regarding a conventional acrylic board, these values are mentioned for reference.

(68) It is understood from Table 4 too that the Example shows a higher hydrogen atom number density and the neutron shielding effect is superior.

(69) TABLE-US-00001 TABLE 1 Thickness of material (cm) Shielding ratio (%) 0 0 2 32.3 4 56.4 6 70.9 8 80.7 10 86.1 12 90.4 14 92.9 16 95.1

(70) TABLE-US-00002 TABLE 2 Thickness of material (cm) Shielding ratio (%) 0 0 5 51.85 10 77.93 15 89.39 20 94.53 25 97.29 30 98.49 35 99.19 40 99.56

(71) TABLE-US-00003 TABLE 3 Transmissivity (%) Wave length (nm) Example Comparative Example 800 91.3 90.0 750 91.1 88.8 700 91.3 90.4 650 91.2 89.9 600 91.2 89.3 550 91.0 88.2 500 90.7 85.7 450 90.1 79.7 400 84.9 51.6 350 57.7 0.5 300 3.4 0.5 250 0.1 0.2

(72) TABLE-US-00004 TABLE 4 Comparative Reference Example Example Example 1 (PMMA) (C.sub.5O.sub.2H.sub.8)n Density (g/cm.sup.3) 1.06 1.13 1.18 hydrogen atom 6.83 6.77 5.67 number density (atoms/cm.sup.3) × 10.sup.22

Comparative Example 2

(73) Neutron shielding material is prepared by the same procedure as to Example 1 except it is maintained for 24 hours at 40° C. after the molding process. Transmissivities of the obtained shielding material are shown in Table 5.

Comparative Example 3

(74) Neutron shielding material is prepared by the same procedure as to Example 1 except it is maintained for 24 hours at 60° C. after the molding process. Transmissivities of the obtained shielding material are shown in Table 5.

(75) TABLE-US-00005 TABLE 5 Wave Comparative Example 2 Comparative Example 3 length (nm) 40° C. cured 60° C. cured 800 90.2 90.8 750 89.4 90.1 700 90.9 91.9 650 90.7 91.7 600 90.6 91.7 550 90.1 91.4 500 88.9 90.6 450 86.2 87.5 400 72.7 59.4 350 23.1 6.8 300 0.0 0.0 250 0.0 0.0

(76) The neutron shielding agent of the present invention has transparency and has high neutron shielding ability, and therefore, is preferably used in various hot laboratories as an excellent neutron shielding material.