WO1998053461A1

WO1998053461A1 - Products containing heavy metals

Info

Publication number: WO1998053461A1
Application number: PCT/GB1998/001437
Authority: WO
Inventors: David William Lodge; John Kenward
Original assignee: Kemmetech Limited
Priority date: 1997-05-19
Filing date: 1998-05-19
Publication date: 1998-11-26
Also published as: GB9710152D0

Abstract

A plastics material comprising a heavy metal dispersed in a curable solvent-free polyurethane system is provided. The material is useful as a protective or insulating shield against radiation and also as a physical barrier to protect against shrapnel.

Description

PRODUCTS CONTAINING HEAVY METALS

The present invention relates to products containing one or more heavy metals (which expression is used herein wherever the context permits to include heavy metal -containing alloys and heavy metal compounds) , for example lead-containing products and more particularly chlorine-free, lead-containing products which are safe to handle.

For the purpose of protecting people who, in their day to day activities, are exposed to radiation (particularly to low level radiation) , it is convenient to make use of the shielding properties of heavy metals such as lead. In nuclear installations such as power stations, research institutes and nuclear submarines and ships, flexible lead-containing blankets are often used to lag pipes, walls, etc in order to protect workers in close proximity to radiation sources, for example those workers engaged in operation, maintenance or repair services . The workers themselves may wear lead- containing protective clothing.

Lead-containing products are also regularly used as radiation shields in hospital and dental theatres where there may be a risk of exposure to X-rays, for example as a protective layer on operating theatre tables and in protective curtains, protective aprons for radiographers and the like.

Given the toxicity of lead and environmental concerns over its use, it is necessary to provide lead- containing products in a form which is safe to handle. One such safe form is lead-containing polyvinylchloride (PVC) which is utilised widely in products for use as radiation shields in nuclear power stations and hospital X-ray departments .

Lead-containing PVC of this type can conveniently be prepared by a process similar to that used to prepare floor coverings and other plastic sheet materials. A coating of lead-containing PVC plastisol is spread on to a silicone release paper, passed through a curing oven and cooled, whereafter the release paper may be removed. The ability of a PVC sheet material prepared in this way to carry up to 80-85% by weight of lead is an important factor, as is the requirement for lead to be evenly distributed throughout the PVC sheet material. A plurality of lead-containing PVC sheets prepared in this way can be assembled to build up a thickness appropriate for the protection required and can if desired be sandwiched between lead-free PVC sheets which render the product easier to handle whilst providing a wipe clean surface.

The presence in lead-containing PVC of chlorine based polymers is however considered undesirable and preferably avoided for certain purposes. The corrosive nature of chlorine and its toxicity are well known and the fact that it is prone to attack stainless steel is one reason why the exclusion of chlorine-containing products is generally sought after, particularly in the nuclear power industry. Thus there is a general need for chlorine-free, lead-containing products which are safe to handle .

The present invention is based on the finding that certain polyurethanes can provide a suitable polymer system for lead and other heavy metals, and that such polymer systems have important advantages over the PVC systems which have hitherto been used for the purpose.

Thus according to one feature of the present invention there is provided a heavy metal -containing plastics material for use in providing a protective or insulating shield e.g. against radiation in which the heavy metal in particulate form is dispersed in a curable solvent-free polyurethane system.

According to a further feature of the present invention, there is provided a heavy metal-containing polyurethane shield comprising a cured plastics material according to the invention as hereinbefore defined.

According to a still further feature of the present invention there is provided a method of manufacturing a heavy metal -containing polyurethane shield comprising the steps of :

(i) spreading a heavy metal -containing plastics material according to the invention as hereinbefore defined to form a coating on a release paper; and

(ii) curing the coating at elevated temperature to obtain a heavy metal-containing polyurethane shield.

Heavy metals other than lead for which solvent-free polyurethane systems can be used in accordance with the present invention include copper, tin, bismuth and tungsten. Bismuth for example is known to provide protection from X-rays and tin from low energy X-rays. Thus tin is particularly suitable as a shield against radar. Heavy metal-containing alloys may also usefully be incorporated in such polymer systems, for example lead/tungsten and lead/tin alloys. In a further alternative, heavy metal compounds may be incorporated into such polyurethane polymer systems examples including galena (a lead sulphide base ore) which is useful in sound insulation systems, barium compounds which are useful for sound insulation systems and carbonyl iron which is useful for microwave reflectivity and protection against magnetic fields. A further use of the heavy-metal containing plastics material of the^" present invention is in providing a protective shield against physical attack using weapons. For example tungsten may be particularly suitable for use in making protective shields for military purposes, for example in armoured tanks and other vehicles, thereby providing a shield against shrapnel. Tungsten-containing protective shields are also suitable for use in other circumstances than for military purposes, in particular to protect from splinters or flying debris in any situation where it may arise, e.g. in mine-blasting, quarrying, etc.

The heavy metal -containing plastics materials according to the invention can for example be in the form of sheets, blocks or blankets having regard to the uses for which they are intended.

Preferred plastics materials according to the invention contain lead as the heavy metal and are for use in providing a shield against radiation. The lead content in such plastics material according to the present invention will be an amount suitable for effecting the desired shielding from radiation and is advantageously up to 95% by weight, preferably in the range of 50-95%, more preferably 60-85%, still more preferably 70 to 85% and yet still more preferably 75- 85% by weight .

The % weight contents for heavy metals other than lead are in general similar to those specifically mentioned above for lead and the above-stated preferences in respect of lead therefore apply also in respect of other heavy metals. Generally speaking, the more dense metals will preferably be present in percentages by weight towards the upper end of the ranges specified above or higher; less dense metals will in general preferably be present in lower percentages by weight. It will be appreciated that the use to which the heavy metal containing product is to be put will have an important bearing on the preferred amounts of heavy metal to be used.

The mean particle size and particle size distribution of lead in lead-containing plastics materials according to the invention has been found to be an important factor in determining the stability of the overall structure, particularly with regard to the maintenance of a relatively even distribution of the lead throughout the structure. A preferred mean particle size for the lead is less than 100 microns, more preferably less than 60 microns and still more preferably less than 40 microns. An advantage of using fine lead particles with a mean particle size below 40 microns is that there is provided a large surface area upon which polyurethane polymer may be adsorbed to give a strong supporting structure. However, if the lead particles are too small, there is an increased tendency for the lead-containing polymer to take on a dough-like texture during processing. Thus lead powder with particle sizes in the range of 30 to 60 microns, more preferably 35 to 50 microns and still more especially 35 to 45 microns is particularly preferred.

As regards particle size distribution, it is preferable to have a high percentage of lead particles within the preferred particle size ranges referred to above. It is thus preferred to have at least 50-60% by weight, more especially at least 70-80% by weight and still more especially at least 85-95% by weight of particles with particle sizes of 30 to 60 microns, more especially 35 to 50 microns and still more especially 35 to 45 microns.

The production of lead and other heavy metal - containing polyurethane sheets according to the invention can be carried out largely in the manner known for production of lead-containing PVC sheets referred to hereinbefore. In a first stage, lead or other heavy metal of an appropriate mean particle size and particle size distribution is incorporated into a curable solvent-free polyurethane system. The mixture is then spread to form a coating on a release paper (e.g. silicone release paper) and passed through a curing oven typically at a curing temperature between 120°C and 200°C to effect curing of the polyurethane. Thereafter the product can if desired be embossed by contact with an embossing roller, cooled for example by passing between cooling rollers and, if desired, the release paper removed . If desired the heavy metal-containing polyurethane sheet may be reinforced with a scrim, which may conveniently be made of polyester or fibreglass, or any other suitable material . This may for example be done by laying the scrim into the wet paste immediately after the spreading blade, and then the sheet may be cured in the usual manner. The scrim should be introduced into the heavy metal -containing polyurethane sheet at a suitable point during the manufacturing process, preferably so that the scrim is in a layer or layers situated within, e.g. as a central layer or layers, of the final product. An advantage of incorporating a scrim into the final product is that it can serve to reduce any tendency for curling of the sheet.

A factor of importance in achieving a good distribution of lead particles throughout the polymer system and accordingly throughout the polyurethane sheet to be produced is the viscosity of the polyurethane polymer system. A preferred viscosity range (measured at 25°C by Brookfield viscometer) is 15,000 to 90,000 cps, particularly 30,000 to 70,000 cps for the solvent- free polyurethane system prior to incorporation of the lead. The viscosity of the polyurethane rises once lead is incorporated therein. Viscosity modifiers such as white spirit can be conveniently added to the polymer system to adjust the viscosity as required for spreading .

The maximum _.thickness of a lead or other heavy metal -containing polyurethane sheet prepared in accordance with the present invention is dictated by a tendency for the release paper to flute during processing resulting in an uneven thickness in the sheet (known as ribbing) .

Thus, with the lead-containing polyurethane sheets prepared in accordance with the present invention, the maximum thickness of a single sheet which can conveniently be prepared is generally up to about 1.0 mm lead equivalent and the single sheets thus preferably have a thickness of from 0.10 mm to 0.75 mm lead equivalent, more preferably from 0.10 to 0.50 mm lead equivalent, especially preferably about 0.25 mm lead equivalent. It will be appreciated that the above thicknesses are expressed as lead equivalents, that is as the thickness of a 100% lead coating capable of providing the equivalent amount of protection against X- rays as that provided by the relevant thickness of the lead-containing plastics material concerned.

Homogeneous sheets of increased thickness can if desired be built up by repeating the steps of spreading, curing and cooling but there is nonetheless a limit to the thickness of a homogeneous sheet which can be achieved. Thus for repeating the spreading, curing and cooling steps four times applying the especially preferred thickness of about 0.25 mm lead equivalent on each occasion provides an advantageous process for producing a homogeneous sheet having a thickness of about 1.00 mm lead equivalent but for greater thickness it is in general preferred to utilise an assembly of two or more sheets.

Lead or other heavy metal -containing polyurethane sheets according to the present invention can be assembled together to form a multi-ply structure which can if desired, be sandwiched between polyurethane sheets free of heavy metal. In one such embodiment, the polyurethane sheets are adhered together using a polyurethane adhesive to form a multi-ply structure. Alternatively a plurality of sheets which are not adhered together can be contained within an outer encasing sheet. The outer sheet may simply take the form of a plastic cover which is stitched around the edge so as to contain the assembly of lead-containing sheets within. This gives the overall product greater flexibility than can in general be achieved by the use of an adhesive bonded multiply structure. Flexibility is an important consideration in the manufacture of lead-containing products such as blankets which may be needed to adapt to awkwardly shaped areas such as pipes .

The total thickness of lead or other heavy metal - containing polyurethane sheets in the final product will depend upon the intended purpose. For example, for nuclear power stations the total thickness of the blanket is preferably up to 6.5 mm lead equivalent. For radiation shields for use for example on hospital operating theatre tables, the total thickness is preferably up to 1.0 mm lead equivalent. For protective curtains or aprons, the total thickness is preferably up to 0.5 mm lead equivalent .

One embodiment of the invention is a lead blanket for use in nuclear power stations. This embodiment will now be described further with reference to Figure 1 of the accompanying drawings, in which a cross section of the blanket is shown. Thus in Figure 1, a blanket 1 has a plurality of lead-containing polyurethane sheets 4 each having a thickness of 0.50 mm lead equivalent held together by a lead- free polyurethane outer cover 3 having a thickness of 0.50 mm which has stitching 5 around its outer edge so as to contain the sheets 4 within. The outer cover is provided with eyelets 2 so as to assist the holding of the blanket in position when serving as a radiation shield. The lead-free polyurethane sheet material to provide the outer cover can be- prepared by a process similar to that described above for the lead-containing polyurethane sheet material with omission of the lead ingredient.

It should be noted that the present invention makes use of a solvent-free polyurethane system. It has thus been found that with solvent -based polyurethanes there is an undesirable tendency for a heavy metal such as lead to settle out after spreading to form a coating and as curing commences. Likewise water-based polyurethane systems have been found to give unsatisfactory results. On the other hand, the solvent-free polyurethane systems used in accordance with the present invention have surprisingly been found to provide a polymer system which can be used to produce very satisfactory products in accordance with the present invention.

The solvent-free polyurethane systems used to prepare the products of the invention are commercially available and known for other applications, for example as coating compositions for fabrics. One preferred system for use in the present invention comprises a hexamethylene diisocyanate/polyether polyol (the polyether being a low molecular weight linear prepolymer with blocked isocyanate terminations) and a polyamine cross-linking agent. With such a system, it has been found advantageous to have present an excess of isocyanate over the polyamine cross-linking agent for the reason that this facilitates the bonding of separately formed sheets to produce a multi-ply construction. The preferred commercially available solvent -free polyurethane systems are based on Larithane™ HS1200 (a high solid polyurethane used in foam, split finishing and tarpaulins manufactured by Coim SpA, Offanengo, Crema-CR, Italy) . The Larithan™ HS1200 system is preferably used with the polyamine Larithane™ HS075 (manufactured by Coim SpA, Offanengo, Crema-CR, Italy) , the amount of polyamine cross-linking agent preferably being from 7.5 to 8.5 parts per 100 parts of HS1200, and more preferably from 7.75 to 8.25 parts per 100 parts of HS1200.

An additional ingredient which may usefully be present in the solvent -free polyurethane system is an anti-settling agent such as silica which has the effect of absorbing the polymer and of helping to provide a supporting structure for the lead filler. Colouring additives may also be added where aesthetic considerations are important. In some applications where thin flexible sheets are required (e.g. for protective curtains and protective aprons), tear resistance can be an important factor. The desired tensile strength of sμch product is typically in the range 1 to 4 MW/m², more usually 2 to 3 MW/m² but an enhanced tensile strength (i.e. tear resistance) can be achieved when needed using a reinforcing additive such as fibreglass or woven polyester.

The invention will now be illustrated in a non- limiting manner with reference to the following Example:

EXAMPLE I

Preparation of lead-containing polyurethane shield

Lead powder [150.0 kg] having a mean particle size of about 40 microns with more than 95% by weight having a particle size within the range 35 to 45 microns is placed in a mixing vessel and synthetic amorphous silica (1.0 kg) added. Larithane™ HS1200 (37.5 kg) is then added and the composition mixed using a planetary mixer for 15 minutes at slow speed. At the end of this time the mixer is stopped and all partly mixed material on the mixer blades and sides of the vessel scraped back into the bulk of the mix. Larithane™ HS075 (3.0 kg) is then added and the composition mixed at fast speed under vacuum for 30 minutes to remove all air bubbles. 5 kg of white spirit is added at this mixing stage to adjust the viscosity and make the paste more easily handled and filtered. The material is then filtered before use. The material is then spread at a thickness of 0.25 mm lead equivalent on to a silicone release paper and cured for 4 minutes at 160°C in a curing oven. Having exited the oven, the product is first contacted with an embossing roller, then passed between cooling rollers and finally wound on to a further roller where it is collected.

The spreading curing, embossing and cooling steps are repeated to provide a total thickness of 0.50 mm lead equivalent .

The silicone paper is then removed and the lead- containing polyurethane sheet material used in the construction of a blanket as described above with reference to the accompanying drawing.

EXAMPLE II

Preparation of a tungsten-containing polyurethane shield

A tungsten-containing polyurethane shield is made in exactly the same manner as described above for the lead-containing polyurethane shield under Example I above. The relative amounts by weight of each component in the mixture are as follows:

Tungsten powder 100 parts

Larithane HS1200 25 parts

Larithane HS075 2 parts

Silica 0.66 parts

The mean particle size of the tungsten powder is about 40 microns with more than 95% by weight of the particles within the particle size range 35 to 45 microns .

Claims

Claims :

1. A heavy metal -containing plastics material for use in providing a protective or insulating shield, wherein said heavy metal is in particulate form and is dispersed in a curable solvent-free polyurethane system.

2. The plastics material of claim 1, wherein said heavy metal is lead, tungsten, copper, tin, iron or bismuth.

3. The plastics material of claim 1 wherein said heavy metal is lead.

4. The plastics material of claim 3 wherein the lead has a mean particle size of less than 100 microns.

5. The plastics material of claim 4 wherein at least 50% by weight of the lead has a particle size within the size range 30 to 60 microns.

6. The plastics material of any of the preceding claims which contains 50 to 95% of the heavy metal.

7. The plastics material of claim 6 which contains 60 to 85% of the heavy metal.

8. The plastics material of claim 7 which contains 70 to 85% of the heavy metal.

9. The plastics material of any one of the preceding claims wherein said solvent-free polyurethane system comprises a hexamethylene diisocyanate/polyether polyol and a polyamine cross-linking agent.

10. The plastics material of claim 9 wherein the solvent-free polyurethane system has a viscosity (measured at 25°C by Brookfield viscometer) in the range of 15,000 to 90,000 cps prior to dispersion therein of the heavy metal .

11. The plastics material of any one of the preceding claims wherein said solvent-free polyurethane system includes one or more additives selected from the following: a viscosity modifier, an anti-settling agent, a colouring additive and a reinforcing additive.

12. The plastics material of any one of the preceding claims, wherein said material is reinforced with a fibreglass or polyester scrim.

13. A heavy metal -containing polyurethane shield comprising a cured plastics material as claimed in any of the preceding claims .

14. The polyurethane shield of claim 13 in the form of a sheet having a thickness of from 0.1 mm to 0.75 mm.

15. A protective blanket comprising a plurality of the polyurethane sheets of claim 14 assembled together to form a multi-ply structure.

16. The protective blanket of claim 15 wherein the plurality of polyurethane sheets of claim 14 are sandwiched between polyurethane sheets which are free of heavy metal .

17. A method of manufacturing a heavy metal -containing polyurethane shield as claimed in any of claims 13 to 16 comprising the steps of:

(i) spreading a heavy metal -containing plastics material as claimed in any of claims 1 to 12 to form a coating on a release paper; and

(ii) curing the coating at elevated temperature to obtain a heavy metal -containing polyurethane shield.

18. The method of claim 17 wherein the surface of the coating is embossed after curing by contact with an embossing roller.

AMENDED CLAIMS

[received by the International Bureau on 26 October 1998 (26.10.98); original claims 1-18 replaced by amended claims 1-17 (3 pages)]

1. A heavy metal -containing plastics material for use in providing a protective or insulating shield, wherein said heavy metal is in particulate form and is dispersed in a curable solvent-free polyurethane system, and wherein said plastics material contains 50 to 95% of the heavy metal .

3. The plastics material of claim 1 wherein said heavy metal is lead.

6. The plastics material of any of the preceding claims which contains 60 to 85% of the heavy metal.

7. The plastics material of claim 6 which contains 70 to 85% of the heavy metal.

8. The plastics material of any one of the preceding claims wherein said solvent-free polyurethane system comprises a hexamethylene diisocyanate/polyether polyol and a polyamine cross-linking agent.

9. The plastics material of claim 8 wherein the solvent -free polyurethane system has a viscosity (measured at 25°C by Brookfield viscometer) in the range of 15,000 to 90,000 cps prior to dispersion therein of the heavy metal .

10. The plastics material of any one of the preceding claims wherein said solvent-free polyurethane system includes one or more additives selected from the following: a viscosity modifier, an anti-settling agent, a colouring additive and a reinforcing additive.

11. The plastics material of any one of the preceding claims, wherein said material is reinforced with a fibreglass or polyester scrim.

12. A heavy metal-containing polyurethane shield comprising a cured plastics material as claimed in any of the preceding claims.

13. The polyurethane shield of claim 12 in the form of a sheet having a thickness of from 0.1 mm to 0.75 mm.

14. A protective blanket comprising a plurality of the polyurethane sheets of claim 13 assembled together to form a multi-ply structure.

15. The protective blanket of claim 14 wherein the plurality of polyurethane sheets of claim 13 are sandwiched between polyurethane sheets which are free of heavy metal .

16. A method of manufacturing a heavy metal-containing polyurethane shield as claimed in any of claims 12 to 15 comprising the steps of:

(i) spreading a heavy metal -containing plastics material as claimed in any of claims 1 to 11 to form a coating on a release paper; and

17. The method of claim 16 wherein the surface of the coating is embossed after curing by contact with an embossing roller.