FR2520880A1 - Compsn. which absorbs ionising radiation similarly to living tissue - comprises polypropylene, alumina and aromatic polyamide fibres - Google Patents

Abstract

Material having similar properties of absorption of ionising radiation as living tissue, comprises a thermoplastic polypropylene matrix contg. a powdered alumina filler and an aromatic polyamide contg. benzene rings in the chain, to introduce a predetermined amt. of nitrogen into the compsn. The polyamide is esp. polyaramide or polymetadiphenylene isophthalamide. The nitrogen additive is e.g. in the form of fibres. The compsns. are useful in test methods for measuring the radiation absorption levels of living tissue, esp. human tissue, and in particular test method publication FR2068732, which employs an aliphatic polyamide and which suffers from some incompatibility problem in the compsn. Use of aromatic polyamides in fibre form gives high strength materials with good dispersion of the nitrogen cpd. The fibres have high strength and heat resistance and have a higher m.pt. than polypropylene, so that they maintain their integrity during compounding. Suitable materials are available commercially as 'Kelvar' (RTM), 'Nomex' (RTM) (Du Pont) and 'X 113' (RTM Celanese Corp.).

Description

MATERIAL PRESENT, WITH REGARD TO
THE ABSORPTION OF IONIZING RADIATION,
PROPERTIES SIMILAR TO THAT OF LIVE TISSUES
The present invention relates to a mixed material comprising a thermoplastic component, the composition of which allows it to have, with regard to the absorption of ionizing radiation, in particular, properties similar to those of living organic tissue, such as human tissue, in order allow an apparatus for measuring these radiations, in particular the outer walls of which are made using this material, to provide an indication of their biological effects. Such an apparatus is called a dosimeter, the most effective of which are ionization chambers which contain inside a sealed envelope made of this material and filled with gas, two electrodes polarized using a DC voltage source so that the average current produced by the ionization of the gases contained in the - develops under the effect of radiation passing through it, is an indication of the dose absorbed by a human body. Such a material is generally called "fabric equivalent material".

 In the publication FR-AI 360 381, it has been proposed to use, on the one hand, a mixture of polytetrafluoroethylene (between 50 and 60%) and carbon and on the other hand, a mixture of polyethylene (66%) , polyformaldehyde (11%) and magnesia (23%). In an addition thereto, FR-E-85 924, a mixture of polyethylene (71.3%) and magnesium fluoride has been proposed as a fabric equivalent material.

In a subsequent publication FR-A-1 453 818, a ternary mixture has been proposed composed of polypropylene or polyethylene (approximately 58 or 65%), polyoxymethylene or polyacetal (approximately 12%) and optionally hydrated magnesia (29 or 22% about). In the publication FR-A- 1466 400, the proportions of the organic polymer components of the last mixture were changed and the magnesia replaced by alumina (19%) In the publication FR-A- 2 068 732, the fabric equivalent material proposed comprises, in addition to polypropylene and alumina or magnesia, another thermoplastic material containing nitrogen which is constituted by a polyamide of aliphatic type (such as a polyamide 6, 11, 12, 6 - 6 or 6 - 10) in a proportion by weight greater than 25 and less than 50 percent (for the use of polyethylene with polyamide, see also the article by ROSSI & FAILLA in the American journal
NUCLEONICS, Vol. 14, NO 2, pp. 32 dc ss., Of February 1956). This kind of polyamides have relatively clear melting points between 180 and 255 "C depending on the type used, while the polypropylene which constitutes the matrix of the mixture, has melting temperatures between 165 and 176 OC and a softening point (Vicat) of around 1490 C. For certain blends, the polyamide melts before the polypropylene, which can create inhomogeneities.

 Blending a polyolefin such as polypropylene or polyethylene with a polyamide (or even another thermoplastic) has proven difficult to achieve without deterioration of the pressure molded material over time (peeling), due to their incompatibility.

 It has therefore been proposed, in publication FR-A-2 402 681, to add to the polypropylene, in addition to the mineral filler (alumina, magnesia or silica), a pulverulent organic filler containing nitrogen in the form of a resin previously thermoset and finely ground, such as an aminoplast of the melamine-formaldehyde or urea-formaldehyde type. The two pulverulent charges (mineral and organic) make up up to 50 percent of the weight of the material thus obtained, which in no way adds to the solidity of the detector walls, exposed to significant variations in temperature and / or of ambient pressure.

The invention proposes a new material making it possible to avoid the incompatibility of the mixture proposed in the publication
FR-A-2 068 732 cited above, replacing the aliphatic polyamide thereof, with an aromatic polyamide.

 The subject of the invention is a new fabric equivalent material containing polypropylene as a thermoplastic matrix and an alumina mineral filler, with another additive containing nitrogen.

 According to the invention, this additive consists of an aromatic polyamide, such as polymetadiphenylene isophthalamide (or polyaramide, marketed under the registered trademarks of KELVAR in the form of fibers, and NOMEX in the form of fiber paper, by the American company DU PONT DE NEMOURS).

This aromatic polyamide (or polyaramide) does not, due to the presence of the benzene rings in the chain it forms, have a melting temperature, but it decomposes at temperatures above 3000C and below 5000C. It is therefore thermostable and slightly deformable in molten polypropylene around 1700C
In addition, it can be produced in the form of fibers of small diameter and great strength, which makes the composite material mechanically more resistant. These fibers can be cut to different lengths, depending on whether you want greater homogeneity of the material (short fibers) or greater strength (longer fibers). Other aromatic polyamides, such as the product resulting from the polycondensation of hexamethylenediamine and terephthalic acid, called "X 113" by its manufacturer which is the American company CELANESE CORPORATION and whose melting temperature is around 375 OC, can also be used in the mixture.

 The proportions of the mixture are calculated in a conventional manner (indicated in some of the aforementioned publications) to obtain an absorption coefficient with regard to gamma radiation, electrons (beta) or neutrons, having a constant relationship with that of the tissue. human for a wide range of energies.

Claims (3)

 1. Material having, as regards the absorption of ionizing radiation, properties similar to those of living tissue, comprising a thermoplastic component constituting the matrix, made of polypropylene and a mineral filler in powdered alumina, characterized in that it comprises , in addition, an aromatic polyamide, the chain of which comprises benzene rings, making it possible to introduce a predetermined proportion of nitrogen into the mixture.  2. Material according to claim 1, characterized in that the aromatic polyamide consists of the substances designated by polyaramide or polymetadiphenylene isophthalamide.  3. Material according to one of the preceding claims, characterized in that the nitrogen additive is used in the form of fibers.