GB1603855A

GB1603855A - Resin-impregnated sintered silver articles

Info

Publication number: GB1603855A
Application number: GB20002/77A
Authority: GB
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 1978-05-10
Filing date: 1978-05-10
Publication date: 1981-12-02
Also published as: FR2390230A1; ATA341778A; DE2820974A1; IT1094654B; JPS5415406A; IT7823314A0

Description

(54) RESIN-IMPREGNATED SINTERED SILVER ARTICLES (71) We, JOHNSON, MATTHEY & CO., LIMITED, a British Company, of 43 Hatton Garden, London, EC1N 8EE do hereby declare the invention for which we pray that a Patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the formation of a resin-impregnated sintered metallic article such as an article of costume jewellery.

Casting processes for the formation of metallic articles are well-known in the art but suffer from certain disadvantages, particularly in the production of articles having complex shapes or precise dimensions. It has, in the past, been the usual practice in such cases to cast an approximate shape somewhat oversize and then to grind or otherwise machine to the actual shape and within the tolerances required.

According to the present invention, a process for forming a metallic article comprises first forming a sintered porous shaped mass from silver particles and then impregnating the voids therein with a resin by evacuating the pores of the shaped mass, introducing the resin in liquid form so that it is drawn into the said pores, subjecting the mass to elevated pressure to complete the impregnation, and curing the resin.

Preferably, the porous shaped mass is formed by dissolving a meltable binder in a solvent, forming a slurry of the solution and metal particles, evaporating the solvent to give a mass or powder-like agglomerate of binder and metal particles, compacting and extruding said mass to give a shaped article and heating said article to remove said binder and sinter the metal particles.

In an alternative embodiment of the invention, the extruding step may be replaced with an injection moulding step.

The voids of the sintered article, which is of course porous, are impregnated with a resin material to render the article essentially non-porous. The impregnated article may, if desired, be subjected to any final machining, coining or polishing operations.

The proportion of metallic particles to binder is preferably selected so that the binder will substantially completely fill the voids between the particles at the compacted stage. If too much binder is used, the particles will not have sufficient mutual contact to provide a coherent form after sintering. On the other hand, too little binder leads to difficulties in extrusion or injection moulding and also tends to give a nonuniform density of product. The volume of binder relative to the volume of metallic particles is selected, subject to the overall practical limitations referred to above, according to the desired volume of said resin material which it is intended shall replace the binder in the finished article.

It is desirable that the metallic particles form an essentially homogeneous dispersion throughout the binder prior to extrusion or injection moulding and to this end a dispersion agent or wetting agent may be added to the slurry.

Typically, the meltable binder comprises paraffin wax, the solvent comprises carbon tetrachloride and a suitable dispersion agent comprises oleic acid. The metallic particles are not generally of any particular shape, for example spherical, and are typically but not essentially in the size range from 5-8 microns.

Preferably the resin comprises a polyester, an acrylic or an epoxy resin.

The invention will now be described by way of reference to the following example A porous metallic sample containing 96% Britannia silver was prepared as follows: 1. A supply of Johnson Matthey Type EC silver powder particulate silver having an average particle size of 8 microns to 12 microns when a minus 200 mesh fraction of such a powder is measured on a Fisher Sub Sieve Sizer.

2. Using a conventional powder metal compacting press a powder compact was produced at a relatively low pressure of 1 to 10 tons per square inch. The pressure should be adjusted to ensure that the compact is made to a density of about 6.0 gs/cc, i.e. 57% of the theoretical density of silver which is 10.5 gs/cc. If desired the powder may be precoated with synthetic resin or similar substance to improve the strength and precision of form.

3. Heat treat the compact for a suitable time at a convenient temperature, e.g. 34 minutes at 600"C to strengthen the compact further.

The porosity is then filled by impregnatin the article with a plastic resin.

By this means an article may be made of material which is predominantly silver by weight, e.g., 96% (Britannia) or 92.5% (Standard) and containing only a minor proportion by weight of resin, i.e. 4% or -/.5% respectively.

The resin impregnation step may be carried out as follows:1. Place heat treated article in an autoclave.

2. Close autoclave, operate vacuum sump and reduce pressure to a vacuum of 28t to 29" of mercury. Hold vacuum for 20 minutes.

3. Still under vacuum draw resin into autoclave to cover the article.

4. Shut off vacuum pump, allow interior to return to atmospheric pressure and then admit air under pressure until autoclave is at 90-100 p.s.i. Hold pressure for 30 minutes.

5. Resin is returned to reservoir, pressure released and article is then drained of resin.

Open autoclave and remove article.

6. Rinse article in cold water and dry.

7. Cure resin by heating article for 1 hour at 1300C.

Articles made in accordance with the above from Britannia silver exhibited the following characteristics: 1. Density-determined by weighing, measuring and calculation - 8.13 gs/cc. This compares well with the theoretical density of a material composed of 86.5% silver and 3.5% of resin, viz., 8.22 gs/cc.

The resin was said to have a density of 1.1 glcc.

The theoretical density of a conventional silver copper alloy containing 96.5% silver is 10.44 cc. Thus the silver resin product was of approximately 28% greater volume than an equivalent weight of silver copper of the same silver content.

2. The silver content determined by analysis was 96.5% 3. The silver content by volume calculated from the two determinations above was 75%.

In an alternative method according to the invention, the metal particles may be coated with a wax. After forming a compact by extrusion or injection moulding the wax is removed and the compact is sintered and impregnated with resin.

This alternative method overcomes the difficulties usually associated with compaction.

We have found that articles of complex form may easily be made by compaction of pretreated particles as follows: 1. To a supply of silver particles was added a proportion of paraffin wax so that the volume proportion of wax relative to the silver metal is the same as will be needed of resin in the finished article - approximately 6% by weight of wax in this example.

One method used to add or coat the metal particles with wax is to dissolve paraffin wax in carbon tetrachloride, add the metal particles and evaporate the solvent.

2. With a suitable set of tooling or dies in an extrusion press or in an injection moulding machine preforms of the required size and shape were made from the waxed particles.

3. The preforms, after removal of feeders and runners, were placed in a furnace or an oven and then heated to vaporise the wax and subsequently heat further to sinter the preform.

4. The pores of the preforms were then filled with resin in a similar manner to that described for the compacts. This resulted in an article containing 92.5 wt% standard (Sterling) silver and balance a resin of low density.

If desired, the resin may be loaded with fine metal or other particles or a colourant.

The waxes may be any suitable animal, mineral, vegetable or synthetic wax such as ester, beeswax and the higher molecular weight paraffins.

The resin used in the above example was a polyester thermosetting resin sold under the Registered Trade Mark METALSEAL V15 by Industrial Impregnations Ltd. of Colnbrook, England.

However, other resins may be used such as acrylics, epoxys, polystyrene, polymethylmethacrylate with or without colourants.

further the resins may be used in the form of solutions or dispersions.

WHAT WE CLAIM IS: 1. A process for forming a metallic article comprising first forming a sintered porous shaped mass from silver particles and then impregnating the voids therein with a resin by evacuating the pores of the shaped mass, introducing the resin in liquid form so that it is drawn into the said pores, subjecting the mass to elevated pressure to complete the impregnation, and curing the resin.

2. A process according to claim 1 in which the resin is a polyester, an acrylic or an epoxy resin.

3. A process according to any preceding claim in which the resin is loaded with fine metal particles or a colourant.

4. A process according to claim 1 in which the porous shaped mass is formed by the steps of dissolving a meltable binder in a solvent, forming a slurry of the solution and metal particles, evaporating the solvent to get a mass or powder-like agglomerate of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. may be precoated with synthetic resin or similar substance to improve the strength and precision of form. 3. Heat treat the compact for a suitable time at a convenient temperature, e.g. 34 minutes at 600"C to strengthen the compact further. The porosity is then filled by impregnatin the article with a plastic resin. By this means an article may be made of material which is predominantly silver by weight, e.g., 96% (Britannia) or 92.5% (Standard) and containing only a minor proportion by weight of resin, i.e. 4% or -/.5% respectively. The resin impregnation step may be carried out as follows:1. Place heat treated article in an autoclave. 2. Close autoclave, operate vacuum sump and reduce pressure to a vacuum of 28t to 29" of mercury. Hold vacuum for 20 minutes. 3. Still under vacuum draw resin into autoclave to cover the article. 4. Shut off vacuum pump, allow interior to return to atmospheric pressure and then admit air under pressure until autoclave is at 90-100 p.s.i. Hold pressure for 30 minutes. 5. Resin is returned to reservoir, pressure released and article is then drained of resin. Open autoclave and remove article. 6. Rinse article in cold water and dry. 7. Cure resin by heating article for 1 hour at 1300C. Articles made in accordance with the above from Britannia silver exhibited the following characteristics: 1. Density-determined by weighing, measuring and calculation - 8.13 gs/cc. This compares well with the theoretical density of a material composed of 86.5% silver and 3.5% of resin, viz., 8.22 gs/cc. The resin was said to have a density of 1.1 glcc. The theoretical density of a conventional silver copper alloy containing 96.5% silver is 10.44 cc. Thus the silver resin product was of approximately 28% greater volume than an equivalent weight of silver copper of the same silver content. 2. The silver content determined by analysis was 96.5% 3. The silver content by volume calculated from the two determinations above was 75%. In an alternative method according to the invention, the metal particles may be coated with a wax. After forming a compact by extrusion or injection moulding the wax is removed and the compact is sintered and impregnated with resin. This alternative method overcomes the difficulties usually associated with compaction. We have found that articles of complex form may easily be made by compaction of pretreated particles as follows: 1. To a supply of silver particles was added a proportion of paraffin wax so that the volume proportion of wax relative to the silver metal is the same as will be needed of resin in the finished article - approximately 6% by weight of wax in this example. One method used to add or coat the metal particles with wax is to dissolve paraffin wax in carbon tetrachloride, add the metal particles and evaporate the solvent. 2. With a suitable set of tooling or dies in an extrusion press or in an injection moulding machine preforms of the required size and shape were made from the waxed particles. 3. The preforms, after removal of feeders and runners, were placed in a furnace or an oven and then heated to vaporise the wax and subsequently heat further to sinter the preform. 4. The pores of the preforms were then filled with resin in a similar manner to that described for the compacts. This resulted in an article containing 92.5 wt% standard (Sterling) silver and balance a resin of low density. If desired, the resin may be loaded with fine metal or other particles or a colourant. The waxes may be any suitable animal, mineral, vegetable or synthetic wax such as ester, beeswax and the higher molecular weight paraffins. The resin used in the above example was a polyester thermosetting resin sold under the Registered Trade Mark METALSEAL V15 by Industrial Impregnations Ltd. of Colnbrook, England. However, other resins may be used such as acrylics, epoxys, polystyrene, polymethylmethacrylate with or without colourants. further the resins may be used in the form of solutions or dispersions. WHAT WE CLAIM IS:

1. A process for forming a metallic article comprising first forming a sintered porous shaped mass from silver particles and then impregnating the voids therein with a resin by evacuating the pores of the shaped mass, introducing the resin in liquid form so that it is drawn into the said pores, subjecting the mass to elevated pressure to complete the impregnation, and curing the resin.

binder and metal particles, compacting or forming said mass to give a shaped article and heating said article to remove said binder and sinter the metal particles.

5. A process according to claim 1 in which the porous shaped mass is made by compacting metal powder at a pressure in the range -10 tonnes per square inch.

6. A process according to claim 1 in which the porous shaped mass is formed by extrusion.

7. A process according to claim 1 in which the porous shaped mass is formed by injection moulding.

8. An article of manufacture comprising a sintered particulate metallic body having impregnated within voids between the metallic particles a cured resin material, and produced by a process as claimed in any preceding claim.

9. An article of manufacture produced substantially as hereinbefore described with reference to the example.