LEAD FIBERS, A METHOD OF PRODUCING SAME AND RADIATION SHIELDING MATERIALS COMPRISING SAME

Abstract: Lead fibers with very small fiber diameters are produced by melt-spinning the fibers of lead containing minimal amounts of tin only. The lead fibers according to the invention are characterized in that the tin content of the lead is within the range of 50 to 500 ppm, and the average fiber diameter is preferably less than 60 microns. Radiation shielding materials made of such lead fibers are impervious to X-rays, highly flexible and resistant to corrosion. The inventive method allows for the production of lead fibers without any problems to do with the spinneret holes clogging up, which is a benefit of the low tin content.

Description: LEAD FIBERS, A METHOD OF PRODUCING SA*t AND RADIATION SHIELDING MATERIALS COMPRISING SAME FIELD OF THE INVENTION The present invention relates to lead fibers having an extremely small diameter as well as to a method of meltspinning such lead fibers. The present invention further relates to radiation shielding materials comprising these lead fibers. BACKGROUND OF THE INVENTION The prior art teaches methods of melt-spinning lead. These lead fibers are mainly used as radiation and sound shielding materials. The diameter of lead fibers made by conventional meltspinning methods ranges from 150 to 200 microns. Fibers having a considerably smaller diameter are not available. Fibers with smaller diameters cannot be produced because when the fine lead fibers are extruded the small holes clog within a very short time, or the output decreases sharply. This problem is particularly noticeable in the process of spinning high-purity lead, such as lead with a purity of more than 99.98. Japanese Patent Application No. 49-16168 discloses a method of avoiding the above-mentioned difficulties involved in the spinning of lead fibers. According' to this method, the spinneret holes are prevented from clogging by the addition of alloy elements to the lead. Considerable amounts of an alloy element are used, namely 0.1 to 40% (by weight) of tin. Naturally, the addition of considerable amounts of tin results in a lower lead content in the fiber. This lower lead content is very disadvantageous because it is of vital importance that the generation of the slightest amount of characteristic X-rays be inhibited when such a material is used for radiation shielding. In adcition, since the higher the leas content of the fiber is, the higher the radiation shielding effect is, it is not desirable to use ! such lead fibers containing considerable amounts of steer reta tin, elements for radiation shielding materials. SUMARY OF THE INVENTION The present invention is based on the concept that lead finders with verv small fiber diameters can be produced by melt-spinning the fibers of lead containing minimal amounts of tin only. This leads to high purity lead fibers witn conspicuous advantages. The lead fibers according to the invention, melt-spun of lead containing ti, are characterized in that the tin content of the lead is within the range of 50 to 500 ppm. The average fiber diameter is preferably less than 60 microns. The radiation shielding materials according to the inven ticn comprise such lead fibers in which the tin content of the lead is within the range of 50 to 500 ppm. The lead fibers according to the invention are extremely flexible due to their very small diameter, and are therefore suitable for a variety of applications. The fibers are also highly resistant to corrosion. With the inventive method, the lead fibers can be pro duced without any problems to do with the spinneret holes clogging up, which is a benefit of the low tin content. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1 to 3 show X-ray photographs of Sheets A, B and C, respectively, made according to Example 7. DETAILED DESCRIPTION OF THE INVENTION The lead fibers of the present invention contain' 50 to 500 ppm tin, and preferably 100 to 200 ppm tin. it the thin fibers according to the present invention extremely thin sheets as well as very flexible sheets in spite of their dense construction can be produced. The lead fibers are produced by melt-spinning. Tin is added to the lead so that the amount of tin is within the range of 50 to 500 ppm. Since tin melts at about 2320C the mixture can be melted completely at 3270C, the melting point of lead. The fluid mixture is spun into fibers by being passed through spinneret holes having a desired diameter. Since the spinneret holes can be prevented from clogging by the small amount of tin, holes with a very small diameter can be used without any problem. As a result, lead fibers with a diameter of 60 or 50 microns or even preferably below 30 microns can easily be spun. The lead fibers produced are extremely resistant to corrosion due to the characteristics of the tin. These positive effects lay become significant with the small amount of 50 ppm of tin in the lead fiber. If ne tin content is low SC ppm, however, the spinneret holes cannot properly be prevented from clogging. On the other and, if the id fibers contaIn more than 500 ppm of tin, the advantageous effect as mentioned above is not increased, but the higher amount of tin is disadvantageous in view of the generation of characteristic X-rays. The tin can be mixed with the lead either in elemental form or as a compound. The lead fibers according to the invention, since they are very fine and flexible, are especially suitable for radiation shielding material. The lead fibers can easily be fabricated into sheets which ry be tin, thick, dense or coarse in their composi tio. The sheets are extremely resistant to corrosion and provide easy handling due to their flexibility. The lead fibers according to the invention are also es pecially suitable for producing composite materials compri sl the lead fibers in chopped form and resin. It has been found that the extremely fine lead fibers according to the invention should be used in shorter length, preferably with an average fiber length of between 0.5 and 1.3 mm, compared to conventional thick lead fibers because the fine fibers entanole easily and form lumps, leading to non-uniform dispersich in the composite material. If the average length of the fine lead fibers is more than 0.3 mm, the fibers easily aggregate and form lumps when slightly pressed during production. During the mixing process with the resin, these fibers also form lumps resulting in a very poor dispersibility. On the other hand, when the fibers have an average length of less than 0.5 mm the fibers fly up in the air easily and are thus very inconvenient to handle. Preferably, the spun fibers are cut immediately after solidification during spinning, using a rotary cutter next to the spinning facilities. They are then filtered through a sieve adjacent to the rotary cutter and only those lead fibers having the predetermined length are selected. The fibers of the present invention are extremely wellsuited for use in radiation shielding material, having extremely high X-ray shielding power in spite of the thin fabric construction. Conventionally, garments or aprons as commercially available are made up of a number of X-ray shielding sheets which contain powder of lead metal or a lead compound dispersed in a high polymer. A fine powder of lead or a lead compound cannot be blended into a resin to achieve a specific gravity greater than 0.2. If more powder is added the sheets become too weak to be used properly. Even in a sheet with a specific gravity of 3.5, the lead equivalent amounts only to a value of 0.08 (mm Pb) at the thickness of 0.5 mm, according to the JIS Z-4501 standard test method. This value is too low for proper shielding of X-rays. In addition, an environmental contamination by fine lead powder cannot be prevented completely, and it is oh- vious that such lead powder is extremely hazardous to the human body. On the other hand, a composite material mainly used for sound shielding is known which comprises lead fibers with diameters extending from a minimum of 100 microns arc, and normally in the range of 200 to 500 microns. Compared to lead powder, these fibers allow a significantly her leac content to be achieved when incorporated in a 1 high polymer, so that values of 5.5 of specify gravity can be reached. It has been found, however, that a composite material comoosen of lead fibers with a large diameter is fatally detective or an application in X-ray shielding materials due to innumerable pin-holes These pin-holes can be ser. in X-ray photographs due to the penetration o tne X-rays. The radiation shielding material according to the present invention, being preferably a composite material composed of lead fibers blended with synthetic resin, wherein tne lead fibers have an average diameter of less than 60 microns and the composite material has a ininimum specific gravity of 4 0, does not show such a pin-hold effect. The lead equivalent is often used to compare X-ray shielding Dowers of various materials, whereby the lead equivalent is represented by "the thickness of lead substrate to which the X-ray shielding power is equivalent". The greater the lead equivalent is, the higher the X-ray shielding power is. In order to obtain maximum X-ray shielding power, the lead equivalent value must be increased by providing rate- rial with higher specific gravity and especially the X-ray absorbing material, which is the lead, must be uniformly dispersed. The uniform dispersion is essential because even if the lead equivalent value is increased, if there are pin-holes which allow the X-rays to penetrate the shielding sheet locally, such sheet is fatally defective for X-ray shielding purposes. The extremely fine lead fibers of the present invention can easily be homogeneously dispersed into resin and; even though a large amount of the lead fibers is added, the strength and the flexibility of the sheet remaThunaffected. If the diameter of the lead fibers exceeds 60 microns, the homogeneous dispersibility is lower, which then leads to a number of pin-holes which allow the X-rays to penetrate easily the shielding sheet itself. Any of the thermoplastic and thermosetting resins that are usually applicable for conventional composite materials may also be used with the lead fibers of the present invention. Suitable thermosetting resins include, for example: epoxy, phenol, unsaturated polyester and polyimide resins. Crosslinking agents, catalysts or any other additives may be used with any of these resins. Suitable thermoplastic resins are, for example: polyvinyl chloride, polyolefin, polyamide and polyester. Any suitable additives such as plasticizers, fillers, thermostabilizers, flame retarders, pigments, etc. may be used with these resins. The composite materials comprising chopped lead fibers and resin of the present invention normally have a specific gravity of more than 4.0 and particularly more than 4.5. The lead fibers are homogeneously dispersed. Due to the excellent homogeneity, the lead equivalent values measured by the JIS Z-4501 standard test method were signijicantly higher than those measured for any of the con ventional lead powder blended composite material, and vet the composite material comprising the lead fibers is totallv free of pin-holes which would allow X-rays to penetrate. ven a composite material comprising any of the thermosetting resins, if mace into a ver thin sheet having a thick- ness of less than 0.5 mm, provides sufficient shielding power against ?-rays. In addition, it is very easy to handle owing to its excellent flexibility. Thus, a vast variety of appli- cations is possible. preferred radiation shielding sh-- can, for example, be made by any of the following methods, without being limited thereto. First, the lead fibers are mixed, for example, with polyvinyl chlorine resin in any desired proportion. A suitable plasticizer is added before the mixture is completely blended, thereby using a Bandbury mixer. Next, the blended material is pressed between rolls that are provided with a predetermined gap so that the blended material can be shaped into a sheet. It is also possible for woven, knitted or spun-bonded fabric composed of either the natural fiber or a rn-race fiber to be laminated on or in the composite material so that the sheet strength can be increased. The invention is further illustrated by way of preferred examples. Example 1 Tin metal (Sn shot, made by Fukuda Kinzoku Hakufun Rogyo KK) was acted to moltenlead metal of 99.9% minimum purity. The sum of the portent of Sb and Sn was less than 10 ppm so that the amount of the Sn in the lead fiber had the values as shown in the following table ben the mixed metals were melted at 3850C and fed to spinning equipment provided with a spinneret comprising five holes each having a diameter of 0.05 mm. The material was then extruded through the spinneret at 3800C by pressurized inert gas. Lead fibers with a diameter of 30 to 40 microns were produced in this manner. The conditions of the spinning of each example were closely observed for two hours. For comparison, , results of spinning operations with 33 ppm tin and without tin are included in the table. As can be clearly seen from the table, lead fibers having very high purity can stably be produced although a very minimal amount of tin is present. TABLE Spinning Sn Content Time Until the Nozzle Clogged Output Characteristics Example 1 50 ppm no clogging occurred at all slightly lowered satisfactory Example 2 140 ppm no clogging occurred at all remained constant satisfactory Example 3 500 ppm no clogging occurred at all remained constant satisfactory Comparative less than all holes clogged within drastically totally Example 1 4 ppm 5 min. lowered impossible Comparative 30 ppm no clogging occurred at all greatly lowered poor Example 2 Example 4 First, tin metal was added to lead metal with a minimum purity of 99.9t, whereby the sum of the content of Sb and Sn was less than 10 ppm at 345 C so that the Sn content became 100 ppm. The melted metal was then spun through holes each having a diameter of 0.05 mm. From the lead fibers produced having a diameter of 30 to 40 microns, a web with a surface density of 7.5 kg/m2 was made. Next, four sheets of the web were superposed and put between two sheets of woven cloth to be quilted. In this way a quilted mat having a weight of 30 kg/m2, a width of 30 cm and a length of 100 cm was produced. This mat was then rolled onto a pipe with a diameter of 10 cm. Owing to its high flexibility, the mat could be tightly rolled onto the pipe. This high flexibility is very advantageous because such sheets can be used for radiation shielding of radioactive pipes. Example 5 A light metal mixture was prepared for spinning according to Example 4. After being melted at 3450C, the fluid metal was spun using a spinneret having holes with a diameter of 0.05 mm. With these lead fibers having a diamater of 30 to 40 microns, a web with a weight of 7.5 kg/m2 was produced. Using this lead fiber web, a radiation shielding garment having a net weight of 25 kg was produced. Although it was fairly heavy to wear, the waist portions could be tightened easily with a belt due to the good flexibility of the garment. Therefore, the garment could be supported on the hip with the belt, reducing the burden on the shoulders. In this way its wearability was significantly improved. Example 6 Very fine lead fibers having a diameter of 30 to 40 microns produced according to Example 4 were cut by a cutter in order to obtain chopped fibers. Chopped fibers of several lengths were produced by varying the diameter of the cutter screen. the relationship between the physical characteristics of said lead fibers and the dispersibility in resin is shown in the tabe below. It can be seen from the values that lead fibers having an average length in the range of 0.5 to 1.3 for show excellent dispersibility and are good to handle. Average Length Physical Dispersibility Characteristics in Resin 1.0 mm no aggregation occurs excellent even under finger pressing 1,5 mm aggregation occurs poor, formation of lumps 0.3 mm no aggregation occurs, excellent but easy to fly Example 7 Using varying amounts of lead fibers, a mixture was first prepared by adding the fibers to polyvinyl chloride resin. After adding a Plasticizer, the mixture was sufficiently stirred by a Banbury mixer. The mixture was then extruded through a gap between rollers, klereby two sheets having a specific gravity of 4.2 (sheet A) and 4.7 (sheet B) were produced. The JIS Z-4501 standard test method revealed a lead equivalent of 0.14 (sheet A) and 0.17 (sheet B). Photographs were made by 100 KVP X-ray tube voltage. They showed that the sheets were of a homogeneous quality without containing any pin-holes at all and providing totally effective radiation shielding characteristics. In addition, lead fibers having a diameter of 70 microns and a length of 1 mr were mixed with polyvinyl chloride resin and a sheet having a thickness of 0.5 mm was prepared (sheet C). The specific gravity was 4.7 and the lead equivalent value 0.14. As can be seen in Figure 3, the X-ray photographs showed a number of pin-holes, which evidences that X-rays penetrate through the sheet. In addition, lead lumps are clearly noticeable. These results show that lead fibers with an average fiber diameter of less than 60 microns are much better suited for the production of radiation shielding material.

Claims: CLAIMS 1. Lead fibers melt-spun of lead containing tin, charac terized in that the tin content of the lead is within the range of 50 to 500 ppm. 2 Lead fibers according to claim 1, characterized in that the tin content of the lead is within the range of 100 to 200 ppm. 3. Lead fibers according to claim 1 or 2, characterized in that the average fiber diameter is less than 60 microns. 4. Lead fibers according to any of claims 1 to 3, cnaracte- rived in that the average fiber length is between 0.5 and 1.3 mm. 5. A method of melt-spinning lead fibers of lead containing tin, characterized in that said fibers are spun of molten lead containing 50 to 500 ppm tin. 6. A method according to claim 5, characterized in that said fibers are spun with a fiber diameter of less than 60 microns. 7. A method according to claim 5 or 6, characterized in that said fibers are spun with an average fiber length of between 0.5 and 1.3 mm. 8. Radiation shielding materials, characterized in that they comprise lead fibers according to any of claims 1 to 4.

Inventors: OUCHI, SHIGEHIRO, SHOJI, HISATAKA
Applicants: TORAY INDUSTRIES, INC.
Application year: 1983
Main classification: B22D11
IPC codes: B22F1, G21F3, B22D11, B22F9