Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-11-16
2003-04-15
Yoon, Tae H. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C523S138000, C523S205000, C523S351000, C524S434000, C524S439000
Reexamination Certificate
active
06548570
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for producing a radiation shielding material, in particular from a polymer that is mixed with a filler material.
Radiation shielding materials protect especially radiology technicians in medical and technical professions from undesired radiation exposure. Used as shielding materials are elastic substances that are mixed with elements with high atomic numbers or compounds thereof and that can be processed into protective clothing because of their elastic properties. Lead is added to these materials in order to absorb the X-ray radiation; however, the good radiation absorption properties of lead are disadvantageously offset by its toxicity.
Servant et al. U.S. Pat. No. 5,247,182 describes a shielding material that is worked into an apron. Said apron, attached to the body of the patient who is to be examined using fastening devices, is regarded as cumbersome because it is very heavy.
Lilley et al. U.S. Pat. No. 5,278,219 describes a flexible compound with a high filler content consisting of a synthetic thermoplastic elastomer. For example, vinyl acetate or blends with copolymers with ethylene/propylene elastomers can be mixed with radiation absorbing fillers, copper, lead, tin, tungsten, lead sulfide or mixtures thereof; the filler material represents at least 90 weight percent. This filler material is available in the form of a powder with grain sizes of between 38 &mgr;m and 150 &mgr;m. Grain size distribution and particle form are important parameters for achieving the desired flexibility with the maximum amount of filler material. However, this patent does not disclose the necessary steps for avoiding the lumping together of the metallic filler material or how to ensure its homogenous distribution within the elastomer.
Finally, U.S. Pat. No. 4,563,494 describes a compound consisting of a synthetic elastomer (on the basis of methacrylate with added boron); lead is added for radiation protection, and boron is added for improving the neutron protection. In addition, this patent describes the method for manufacturing the compound.
The content of U.S. Pat. No. 4,563,494 also provides that lead be replaced with rare earth metals; at least one of the rare earth metals is used as a compound, preferably in the form of oxides, hydroxides, salts of inorganic or organic acids and complex compounds. For production purposes the compounds, which contain at least one rare earth metal, are dissolved or dispersed in an elastomer binding material from the group with at least one vinyl monomer, selected from among a group consisting of acrylic acid or methacrylic acid or their esters, styrene and substituted styrene compounds; this preparation is then mixed with another co-polymerizable vinyl monomer and finally with a partially polymerized product thereof, and polymerized. These lanthanide compounds are added to the elastomer in a dough-like form, then finely and homogeneously dispersed, which is possible with low-viscosity elastomers, such as PVC. However, this patent does not disclose how such compounds can be added in a finely dispersed and homogeneous manner if the elastomer is not a low-viscosity elastomer.
European Document 0 371 699 proposes a lighter material with equivalent radiation absorption properties, preferably in the range of high-energy radiation to reduce the weight of the protective clothing in contrast to the traditional lead or lead vinyl aprons. Serving as a basic material is a polymer that contains 7 to 30 weight percent of a polar elastomer, 0 to 15 weight percent of a flexibilizer and 70 to 93 weight percent of an inorganic compound for absorbing the radiation. The latter consists of at least two elements or of compounds thereof that are adjusted in terms of the energy range, with lead providing the highest level of protection. Thermoplastic materials are listed as suitable polymers, in particular copolymers of ethylene with at least one partner of vinyl acrylate, alkyl acrylate, alkyl methacrylate, glycidyl methacrylate, acrylic acid, methacrylic acid, mixtures thereof, ionomers of such copolymers that are composed of one of the previously referred to monomers and ethylenized, unsaturated carboxylic acid and carboxylic anhydrides and other derivatives thereof. The polymeric base material should have a “flexural module” (ASTM D-790) in the range of 1 MPa to 100 MPa. Suitable inorganic components for absorbing the radiation are elements with higher atomic numbers, such as actinoids, antimony, barium, bismuth, bromine, cadmium, cerium, cesium, gold, iodine, indium, iridiurn, lanthanide, lead, mercury, molybdenum, osmium, platinum, polonium, rhenium, rhodium, silver, strontium, tantalum, thorium, tin, tungsten, uranium and zircon.
The amount for each of these two elements is at least 5 weight percent. When selecting the elements, their respective absorption capacities are taken into consideration to ensure that their absorption properties complement each other, at least in the range of the radiation spectrum from 10 keV to 200 keV. The specific mass is to be calculated in such a way that the lead equivalent of 0.1 mm is achieved. The density of the filler material is between 2.8 g/cm
3
and 6.5 g/cm
3
. The components are placed into a melting-mixer apparatus or into a similar device (for example, a two roller mixer apparatus, a Banbury mixer, a Farrel mixer, a Buss kneader, a Gelimat mixer or a similar apparatus) in order to effect the assemblage. The mixture is then brought into the desired shape, in particular by way of extruding, calendering, compression molding, etc. In compounds with a high metal content it is difficult to produce a homogenous mixture, therefore, in such instances it is necessary to employ intensively working mixing aggregates. It can be advantageous if concentrated mixtures of the polymer with a flexibilizer and/or the (metallic) fillers are initially produced which are subsequently “diluted” by adding polymer, a process which cannot be used in connection with tin containing mixtures.
It is disadvantageous, though, that in particular metallic fillers cannot be blended with the polymers and provide sufficient homogeneity; obviously, these problems are caused by the differences in densities and the clumping tendency of these fillers. These difficulties complicate the economical manufacturing of radiation shielding materials to the point of impossibility. The previously referred to patent documents do not disclose how mixtures containing lead replacement materials can be manufactured economically on an industrial scale in ways that make these replacement materials available in fine and even distributions.
Based on this state of the art, the object of the present invention is, therefore, to develop a method that will allow the economical manufacturing of a radiation shielding material with homogenous distribution of the radiation-absorbing components on an industrial scale.
PREFERRED EMBODIMENT OF THE INVENTION
As described below, a radiation shielding material is made from a polymer that is blended with a radiation-absorbing filler material.
Advantageously used as a polymer is a thermoplastic, vulcanizable elastomer, known in the art from European Document 0 371 699, such as natural rubber (NR) and/or polychloroprene (CR) and/or mitrile-butadiene (NBR) and/or styrene-butadiene rubber (SBR) and/or butadiene rubber (BR) and/or ethylene propylene terpolymer (EPDM) and/or ethylene propylene copolymer (FPM) and/or polyurethane (PU) and/or isobutylene-isoprene rubber ([IIR]CSM) and/or silicone rubber (VMO and MQ) and/or ethylene vinyl acetate rubber (EVA) and/or blends thereof, or a non-vulcanizing elastomer, such as polypropylene/EPDM (PP/EPDM) and/or styrene-ethylene-butylene-styrene (SEBS) and/or polyvinyl-chloride-nitrile-butadiene (NBR/PVC) and/or ethylene propylene copolymer (EPM) and/or blends thereof.
A powder of a metal with a high atomic number or a compound thereof representing more than 80 weight percent is added to this elastomer as a radiation absorbing filler material. Th
Arntz Beteiligungs GmbH & Co. KG
Yoon Tae H.
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