Patent Number: 059088842
Section: description

EXAMPLES Example 1 Eighty eight % by weight of tungsten powder having an F.s.s.s. particle size of 3 .mu.m and 12% by weight of unvulcanized fluoro rubber containing a suitable amount of peroxide as a vulcanizer are weighed, and the tungsten powder and the unvulcanized fluoro rubber with a vulcanizer were mixed in an open roll mill for 15 minutes. Then, a 1 mm-thick vulcanized rubber sheet (hereinafter referred to as sample 1) was produced by pressing the mixture. On the other hand, 95% by weight of tungsten powder having the same F.s.s.s. particle size as described above and 5% by weight of unvulcanized EPDM rubber (ethylene-propylene rubber, hereinafter referred to as EPDM) containing a suitable amount of sulfur as a vulcanizer are weighed, and the latter was dispersed in the former in the same manner as described above to thereby prepare another sample (hereinafter referred to as sample 2). The respective sections of these samples 1 and 2 were observed by using an SEM (scanning electron microscope, hereinafter referred to as SEM). As a result, it was confirmed that tungsten powder was dispersed in a matrix of the vulcanized rubber substantially evenly. The specific gravity of the shielding material in each of the samples 1 and 2 was about 9 as a whole. As an example of radiation shielding ability, radiation absorbing characteristic in an X ray of 6 MV was measured. As a result, the radiation absorbing characteristic of each sample was about 96% of that of a lead alloy plate having the same thickness and was twice as much as that of an available lead-containing sheet (specific gravity: about 4) having the same thickness. That is, it was confirmed that each sample had radiation shielding ability which was substantially equal to that of the lead alloy and superior to that of the lead-containing sheet. The tensile strength measured in each of the samples 1 and 2 was not smaller than 60 Kg/cm.sup.2. It was confirmed that each sample was prevented from being hung or deformed by its own weight in use. Further, the extensibility (G.L.=100 mm, hereinafter the same rule is applied) was not smaller than 200%, that is, each sample had elastic deformability. It was confirmed that these samples 1 and 2 could be cut easily compared with a lead plate having the same thickness, and these samples 1 and 2 had elastic deformability in which each sample could be made to come close to a fine curved surface. When bending was repeated, the lead plate having the same thickness was broken by fatigue in the case where bending at 90 degrees was repeated 50 times (one reciprocating bending was counted as one time), whereas there was no influence on the samples 1 and 2. When an iron ball having a weight of 5 kg was naturally dropped onto each of the samples 1 and 2 from a position 2 m-higher than the position of the sample, there was no breaking such as cracks, or the like, in each sample. When a tungsten plate having the same size was subjected to the same dropping test as described above, cracks occurred. The samples 1 and 2 were exposed to air while the temperature of the air was being changed variously. As a result, the extensibility and tensile strength of the sample 1 were kept at least for 56 days at 200.degree. C. The extensibility and tensile strength of the sample 2 were lowered only in 1 day at 200.degree. C. but were kept at least for 56 days at 100.degree. C. The samples were immersed in various kinds of chemicals at room temperature. As a result, the sample 1 was little swollen by chemicals except ketones such as methylethyl ketone, and the like, that is, it was confirmed that the sample 1 was not dissolved at all. The sample 2 was little dissolved in chemicals except gasoline and benzene. The samples 1 and 2 and the lead alloy were left under the environment of a temperature of 60.degree. C. and a humidity of 90% for 100 hours. As a result, the occurrence of corrosion was observed in the lead alloy, whereas there was no occurrence of corrosion in the samples 1 and 2. Example 2 Fifteen % by weight of tungsten powder having an F.s.s.s. particle size of 1 .mu.m and 85% by weight of tungsten powder having an F.s.s.s. particle size of 8 .mu.m were mixed in advance. Then, the mixture powder was weighed by 90% by weight and unvulcanized fluoro rubber containing a suitable amount of peroxide as a vulcanizer was weighed by 10% by weight, and they were further mixed in an open roll mill for 15 minutes. Then, a 1 mm-thick vulcanized rubber sheet (hereinafter referred to as sample 3) was produced by pressing. On the other hand, the aforementioned mixture powder was weighed by 96% by weight and unvulcanized EPDM rubber containing a suitable amount of sulfur as a vulcanizer was weighed by 4% by weight, and they were used in the same manner as described above to thereby prepare a further sample (hereinafter referred to as sample 4). The respective sections of these samples 3 and 4 were observed by using an SEM. As a result, it was confirmed that tungsten powder was dispersed in a matrix of the vulcanized rubber substantially evenly. The specific gravity of the shielding material in each of the samples 3 and 4 was about 10 as a whole, so that the radiation shielding ability of each of the samples 3 and 4 was improved by about 10% compared with the samples 1 and 2 produced in the same manner by using only powder having particles of the same particle size. Further, the tensile strength, extensibility, heat resistance, chemical resistance and other characteristic of the samples 3 and 4 were substantially the same as those of the samples 1 and 2. Example 3 Further, tungsten powder having an F.s.s.s. particle size of 1 .mu.m and tungsten powder having an F.s.s.s. particle size of 10 .mu.m were mixed with various mixture proportions in advance. This mixture powder and unvulcanized fluoro rubber containing a suitable amount of peroxide as a vulcanizer were weighed and mixed in an open roll mill for 15 minutes. Then, a 1 mm-thick vulcanized rubber sheet (hereinafter referred to as sample 5) was produced by pressing. Here, the mixture proportion of the mixture powder and fluoro rubber was determined so that the tensile strength and extensibility of the sample 5 thus produced were the same as those of the sample 1 (the tensile strength was not smaller than 60 Kg/mm.sup.2 and the extensibility was not lower than 200%). Here, the particle size distribution of tungsten powder remaining after removal of the rubber component in the sample 5 was measured, and the percentage by weight of powder having a particle size in a range of from 4 .mu.m to 100 .mu.m was represented by X and the percentage by weight of powder having a particle size smaller than 4 .mu.m was represented by Y. The value of X, the value of Y, the mixture proportion of the mixture powder and fluoro rubber and the specific gravity thereof were as shown in Table 1. TABLE 1 ______________________________________ mixture fluoro X Y powder rubber specific extensibility (wt. %) (wt. %) (wt. %) (wt. %) gravity (%) ______________________________________ 30 70 84.1 15.9 7.8 200 or more 55 45 88.2 11.8 9.2 200 or more 60 40 88.9 11.1 9.5 200 or more 65 35 89.3 10.7 9.7 200 or more 70 30 89.7 10.3 9.8 200 or more 75 25 90.0 10.0 10.0 200 or more 80 20 90.2 9.8 10.1 200 or more 85 15 90.0 10.0 10.0 200 or more 90 10 89.7 10.3 9.8 200 or more 95 5 89.0 11.0 9.5 200 or more 97 3 88.5 11.5 9.3 200 or more 100 0 87.9 12.1 9.1 200 or more ______________________________________ From the aforementioned result, in the samples having the same tensile strength and extensibility as those of the sample 1, the specific gravity was not smaller than 9.5 in each and every sample containing 60% by weight to 95% by weight both (X value in the above table) of powder having a particle size in a range of from 4 .mu.m to 100 .mu.m, and 5% by weight to 40% by weight (Y value in the above table) of powder having a particle size smaller than 4 .mu.m. As an example of the radiation shielding ability of these samples, radiation absorbing characteristic was measured in an X ray of 6 MV. As a result, it was confirmed that these samples exhibited absorptivity obtained by multiplying the absorptivity of a lead alloy having the same thickness by a factor of from 1 to 1.1 and had more excellent radiation shielding ability than that of the lead alloy. Example 4 Further, a suitable amount of carbon black was mixed with a mixture of 92% by weight of tungsten powder having an F.s.s.s. particle size of 3 .mu.m and 8% by weight of unvulcanized SBR rubber (general synthetic rubber of styrene and butadiene) containing a vulcanizer in order to give electrical conductivity to the mixture. These materials were mixed in an open roll mill for 5 minutes. Then, a 1 mm-thick vulcanized rubber sheet (hereinafter referred to as sample 6) was produced by pressing. The measured radiation shielding ability of the sample 6 was reduced by about 20% compared with that of the sample 1, but electromagnetic wave shielding ability was obtained newly by giving electrical conductivity to the sample 6. That is, it was confirmed that electromagnetic wave shielding ability could be added to the radiation shielding ability in the material according to the present invention. Incidentally, tantalum (specific gravity: 16.6), rhenium (specific gravity: 21.0), osmium (specific gravity: 22.5), compounds or alloys thereof, etc., (whose .gamma.-ray absorption coefficient (cm.sup.-1) is in the range of about 0.7 to 1.2 when the energy of .gamma.-rays is 1.5 MeV) other than tungsten, tungsten compounds and tungsten based alloys may be used singly or in combination as the material of high radiation absorptivity. The kind of the rubber material, the kind of the vulcanizer and the mixture proportion thereof can be selected suitably correspondingly to the kind of powder such as tungsten powder, or the like, required radiation shielding ability, specific gravity and physical properties, and so on. In addition, carbon powder, or the like, can be added to the material according to the present invention in order to perform coloring, changing of physical properties or characteristic, etc.