Patent Number: 
Section: description

The present invention will be illustrated more in detail. As epoxy resin for the epoxy resin composition of this invention, any kind of epoxy resin, which has more than 1.8 numbers of epoxy groups in one molecule and is liquid in the form at the ambient temperatures can be used. As typical examples of such kind of epoxy resin, aliphatic epoxy resins such as bisphenol A type epoxy resins, bisphenol F type epoxy resins, bisphenol S type epoxy resins, phenolnovolac type epoxy resins, alicyclic epoxy resins, heterocyclic epoxy resins, glycidylester type epoxy resins, glycidylamine type epoxy resins, brominated epoxy resins, cyclohexyl ring containing type epoxy resins or propyleneglycol type glycidylethers and urethane modified epoxy resins can be mentioned. One of these epoxy resins can be used alone or used together with each other more than two different kinds of these. Mono-functional and di-functional epoxy resins such as butylglycidylether, phenylglycidylether, credilglycidylether or glycidylether of aliphatic alcohol may be added if needed. Furthermore, suitable quantity of any epoxy resin which is solid in the form at ambient temperatures may be added. When higher level of neutron shielding performance will be necessary, epoxy resins with a lot of the number of hydrogen atoms in their one molecule are favorably in use. Aliphatic type epoxy resins, cyclohexyl ring containing type epoxy resins such as 4,4xe2x80x2-isopropylidenecyclohexanol type epoxy resins or alicyclic epoxy resins are typical of these. In the case of aliphatic type epoxy resins with higher content of hydrogene atoms, cured products of these resins have both of low heat resistance and mechanical strength. On the other hand, aromatic ring containing epoxy resins have good function of heat resistance and mechanical strength, but neutron shielding performance does not go well. As epoxy resins which are well-balanced in neutron shielding capability, heat resistance and mechanical strength, alicyclic epoxy resins or cyclohexyl ring containing epoxy resins can be preferable for this invention. The epoxy resin composition of this invention can be prepared by blending a ambient temperature curing type of epoxy resin hardener with above mentioned epoxy resin hardeners to hard infusible thermoset networks. The type of epoxy resins which are used for this invention, reacts with epoxy resins to the above networks at ambient temperatures (15 to 45xc2x0 C.) and in general, these give 10 minutes to several hours of pot life and need several 10 minutes to 10 days for curing, and are not restricted to the above description. Typical hardeners are figured in the following: (1) ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylene pentamine, hexamethylenediamine, polyoxypropylenediamine, iminobishexyl amine etc. (2) Alicyclic compounds with more than 2 amino groups which have active hydrogen atom to react with epoxy group at ambient temperatures as alicyclic and/or imino groups in a molecule as alicyclic polyamines such as bis(amino)cyclohexane, N-aminoethylpiperazine 3,9-bis(3-aminopropyle)2,4,8, 10-tetraoxapyro(5,5) undecan, m-xylenediamine, 1-3bis(aminomethyl)cyclohexane, N-aminoethylpiperazine, bis(4-aminocyclohexyle)methane, Isohoronediamine and norbornadiamine (3) Further derivatives of above mentioned compounds, modified alicyclic polyamines such as ethyleneoxide adducts compound of aliphatic polyamines and epoxy resins addition compound, modified aliphatic polyamine modified polyethylenepolyamines, modified heterocyclic diamines, monoglycidilether adducts of alicyclic polyamines, epoxy resin adducts of acrylonitrile or aliphatic glycidilesters, polyamideamines and their modified ones that mean polycondensation products of polyethylenepolyamines and fatty acids or dimer acids in the amine system of polyethylene polyamines and xylirendiamines. One kind of these hardeners or a mixture of more than two different kinds of these hardeners can be used. When higher level of neutron shielding performance will be required, hardeners with higher content of hydrogen atoms in their one molecule. From this point of view, the use of aliphatic polyamines, alicyclic diamines and cyclic amines will be effective, however, especially in case of using aliphatic amines, since the heat generation by curing reaction is radical, it is important to select such a kind of amines of which curing reaction proceeds more slowly because these amines have tendency to generate heat of curing quickly. Therefore, use of the hardeners which rate of curing reaction has been managed by modificating reaction will be more effective. In any case of using hardeners, transparency of any cured epoxy resin product is the essential point for the selection of hardeners. Generally, transparency of the cured epoxy products is measured by luminous flax density. In case where the above cured products are applied to front parts of a special-purpose vehicle, level of the luminosity through the products is required to be higher than a regulation limit of luminosity by the Road Traffic Control Law of Japan. In the present invention, when illuminance under an adequate light source will be over 50%, the above products are assumed to have transparency. The blending ratio of the above mentioned ambient temperature curing type epoxy resin hardener to epoxy resin to be used can be selected suitably to the kind of hardener to be used, and in general, desirable range of blending ratio of the hardener is 10 to 200 weight parts, favorably 20 to 100 weight parts per 100 weight parts of epoxy resin to be used. In the present invention, curing accelerators, which can control the curing rate can be also used, and the chemical compounds which are generally used as curing accelerator for epoxy resins, such as imidazole, tertiary amines and phenols can be used, but not intended to be limited to them. The use of the above mentioned epoxy resins and hardeners is essential for the epoxy resin composition to be used in the present invention. Besides the above curing accelerators, various kinds of additives, coupling agents, defoarming agents and/or coloring agents such as dyes can be added or blended if required, in the limitation not to the transparency, which is one of important aims of this invention. Regarding the epoxy resin composition used in the present invention, its viscosity is also important. It is desirable that the composition""s viscosity is adjusted to be level lower than 7000 mPaxc2x7s at ambient temperatures. When its level is over 7000 mPaxc2x7s, there are cases often where foams get caught in the melt composition and it will result in disadvantageous situation for making composition. On the other hand, when the viscosity is smaller than 500 mPaxc2x7s at ambient temperatures, chance for foams to get caught in the composition can be prevented from and subsequently excellent composition is made effectively. Another aim of this invention is that the above-mentioned epoxy resin composition is molded by a casting method and then cured with the above-mentioned hardener to obtain a transparent neutron shielding material. Regarding the mold to be used for the molding process, it is not necessary to use special some of it, and any conventional mold is available in molding applications. Therefore, any metal type of mold or any organic transparent material type of it can be used. In case where a metallic mold is used, since it is necessary to decast the molded product from the mold, a parting agent must be previously coated over the surface of the mold, or it must be blend in the epoxy resin composition before its molding. In case where organic transparent mold is used, there are cases where the mold itself will be one body with the molded product, and as material for said transparent mold, a thermoplastic or thermosetting resin can be used. In this case, the kind of material is not restricted to the above mentioned, and any kind of transparent materials can be used. As the concrete examples, acrylic resin, vinylchloride resin, styrene resin, PMMA resin and polycarbonate resin are described, but not restricted to them. Further, to prevent molded product from any contamination of dusts or particles in the working atmosphere, it is desirable to carry out molding procedure in a clean bench or a clean room with a clean level over class 100000. Shape of this invention""s molded product is not especially restricted, and it is able to meet any requirement of it if the molded product realizes the designed shielding performance for neutron, and the product is not restricted by the shape of the mold. The present invention is effective especially to obtain a large size of molded product with over 50 mm thickness. For example, a plate type of molded product can be mentioned (in the form 60xc3x97600xc3x971000 mm). The shape of molded product can be designed to any kind of shapes, such as square, rectangular, cone, pyramid shapes or sphere one. Therefore, it is possible to design any desired shape for the structure to which the molded product will be set and to meet the requirement of neutron shielding performance. The present invention will be illustrated by concretely through the following Examples, however not intended to be limited to them. Typical formulation figures for epoxy resin compositions are shown together with their physical and chemical properties in Table 1. Shielding boards of Examples 1 to 6 are molded by the process mentioned below, and neutron shielding performances and other properties are summarized in Table 4. According to the formulation figure in Table 1, 3.0 kg weigh epoxy resin A and heat them to 34 to 35xc2x0 C. Warmed 1.2 kg hardener at 20xc2x0 C. and the resin A were mixed so that the temperature of the mixture was to 28 to 30xc2x0 C. The viscosity of the mixture was 2500 mPaxc2x7s. Then the mixture was defoamed for 1 minute by a type of vacuum defoamer. A transparent mold made of acrylic resin (5 mm thick acrylic panel is used; rectangular shape mold with 50 mmxc3x97590 mmxc3x97990 mm inner size) was set in a clean bench. The mold which has been kept slant by use of a jack was inspected whether there is any contamination or not. (1st step) The results showed that the mold itself has no contamination. 4.5 kg of the mixture prepared in the 1st step were casted to the mold along the wall of gate of the mold during 5 minutes, paying attention not to any foam getting caught in the melt mixture at the end of gate. Then the mold was transferred into a bath for cooling and cooling water at 15xc2x110xc2x0 C. was poured into the bath to the same height as resin mixture one inside the mold. (2nd step) The 1st process and the 2nd step were repeated 12 times. The height of cooling water was risen up to keep position slightly higher than it of the above mixture to be casted during every casting it. The mold was filled with the above mixture after all casting steps, and height of cooling water was kept at the same level as before, and curing reaction has been continued for one over night (8 to 12 hours). Cooling water was circulated to keep temperature of the mixture at 15xc2x110xc2x0 C. during the curing. (3rd step) Next morning, the mold was taken out of the above bath to the said clean bench. Thus a rectangular type for shielding board having dimension of 50 mmxc3x97590 mmxc3x97990 mm was obtained. The obtained board is a composite material composed of a mold made of 5 mm thick acrylic plate and a rectangular board of cured epoxy resin product with dimension of 50 mmxc3x97590 mmxc3x97990 mm. The obtained shielding board did not contain any foams or fisheyes, and was uniform, and transparent without any stain, and its dimension was the same as designed one. By use of the same process as Example 1 but using blending recipe of Table 1, composite materials of Example 2 to 6 and Comparative Examples composed of a mold made of 5 mm thick acrylic plate and a rectangular board of cured epoxy resin product with dimension of 50 mmxc3x97590 mmxc3x97990 mm were obtained. Polyethylene is used as comparative example. Each of obtained transparent shielding materials for neutron was evaluated by the measuring methods to be mentioned below and the evaluation results are summarized in Table 4 (appearance, neutron shielding performance, luminous flax density, continuos irradiation test by use of a headlight and mechanical strengths). (1) Appearance The obtained shielding board is watched through from the viewpoint of its thickness direction through the naked eyes of one inspector. Its shape and dimension of foam inside the board were inspected. (2) Evaluation of neutron shielding performance (method for calculation of neutron moderation) For the evaluation of neutron shielding performance, a ray source, a shielding board, cast iron and detector are arranged as shown in FIG. 1, and the shielding performance of the shielding board of Example 1 is calculated by Monte Carlo method (refer to page 157 of Atomic Power Handbook, edited by Tadakazu Asada, Ohm Co., Ltd., Japan, 1976). Simulation results for shielding effect are shown in Table 2. Meanwhile, under the condition of practical use for shielding, the neutron shielding performance for Examples and Comparative Example are measured in accordance with the following measuring method: That is, 252Cf with 1.45 MBq is used as neutron ray source. Two pieces of the board with dimension of 60xc3x97600xc3x971000 mm obtained in each Example are piled up in front part of the driver""s seat of a special vehicle, and the ray source, a shielding board and a detector are arranged like the layout shown in FIG. 2. Then neutron is irradiated. {circle around (1)} is the results from the case where a shielding board is arranged, and {circle around (2)} is the results from the case where a shielding board is not arranged. Rate of {circle around (1)}/{circle around (2)} is calculated. The shielding performance for Example 1, which is the detailed example, is obtained by use of measured values listed in Table 3. The present invention has used the limitation to shielding performance, whose the level is smaller than ⅕VT. (3) Illuminance The vehicles of Mitsubishi type E-E74A of 1995 (headlamp; 4 beams) and Toyota type E-SV30 (headlamp; 2 beams) are used. Illuminance of the main travelling beam of said two vehicles are measured. Two pieces of transparent shielding board obtained in each Examples and Comparative Example are arranged together and set to front part of the main travelling beam and their illuminances are measured. Illuminance rate is defined by the ratio of illuminance value for no shielding board and the one with two pieces of piled shielding boards. Further, the illuminance for the above vehicles is prescribed by the Road Traffic Control Law (Japan) as follows: That is, in the case of 4 beams system having head lamp vehicle whose main travelling beam and a sub travelling beam or a pass travelling beam do not make light at the same time, the illuminance per one beam is assumed to be over 15000 cd. And in the case of other 4 beams system having headlamp vehicle, the illuminance of the main beam is assumed to be over 12000 cd. Further, in the case of any vehicle except 4 beams system having headlamp, the illuminance per one beam is assumed to be over 15000 cd. Another essential requirement is that the illuminance for all above cases has to meet the prescribed value by the Road Traffic Control Law (Japan). The present invention has used the limitation to the illuminance whose the rate is over 50%. (4) Heat Resistance Considering so called softening phenomenon during a long term irradiation to the shielding material by head light, its heat resistance is measured on the basis of Bicut softening temperature testing method prescribed in JIS K7206 in the following. Measuring condition: Testing load: A method, 1 kgf Temperature rising rate: 50xc2x0 C./hr Shielding effectiveness 1/7.69 VT value of shielding material obtained in Example 1 is calculated by data in Table 3. Shielding effectiveness for Example 1 is calculated as follow; {circle around (1)}/{circle around (2)}=0.08/0.61=1/7.69 VT and is smaller than ⅕ VT, which is a level of practice values. By the same method as the above, measured results are calculated and the results of Examples 2 to 6 and Comparative Example are obtained. (5) Some Mechanical Strength As some mechanical strength of a shielding material, compressive strength, compressive elasticity, flexural strength and hardness were measured in accordance with JIS K6911. Characteristic figure of shielding board obtained in Example 1 except Table 4 are shown in Table 5. Attenuation for shielding effects for water and shielding board of Example 1 by Monte Carlo method CNP4A are shown in Table 6. Effectiveness of the Invention As mentioned above, in the present invention, ambient temperature curing epoxy resin composition to which consists of epoxy resin not including any opaque inorganic compound having neutron shielding capability and a hardener is used. And when the viscosity of the composition of the above mentioned epoxy resin will be lower than 7000 mPaxc2x7s at ambient temperatures, a non-distorted and transparent neutron shielding material with any distortion can be obtained.