FR2567155A1 - Process for metallising nonconductive particles - Google Patents

Abstract

Process for metallising nonconductive particles such as macroporous crosslinked polystyrene beads, of the type resulting from the immersion of the latter in an acidic bath of chemical nickel, known per se, followed by reduction of the nickel ions, released in an alkaline medium, with a sodium borohydride solution. This reduction is stopped after a relatively short period of time, of the order of 2.5 min, the beads thus treated are recovered and then washed and subjected to a new, easily controllable reduction stage, in an alkaline medium, by means of a slowly added aqueous solution containing 40 g/l of sodium hypophosphite, until the gas release accompanying the reduction slows down, the thickness of nickel metal thus obtained being increased by heating the above bath and progressive addition of a commercial acidic chemical nickel solution or of recovered industrial acidic nickel-plating baths for metal articles. Application to the metallisation of nonconductive particles.

Description

"METHOD FOR METALLIZING NON-CONDUCTIVE PARTICLES."
The present invention relates to a metallization process in particular with nickel and without the use of precious metal, of non-conductive particles such as beads of polystyrene crosslinked with divinyl benzene, preferably sulfonated, macroporous, of form H.

Such balls are generally used in electrochemical generators of the type described for example in French patent 74/37 259 (SORAPEC).

The above device comprises in particular a cathode compartment and an anode compartment, the active material of which is kept in suspension in a liquid electrolyte, in the form of beads comprising a core of inert synthetic material, covered with an active coating, in l of zinc deposited on a metallic intermediate layer of nickel
Several methods of metallization of synthetic supports are already known, capable of giving rise to the previous anode particles.

One of them, for example, consists in encapsulating the plastic balls with a layer of nickel, which therefore only adheres to the support by a purely mechanical adhesion, and on which zinc is subsequently deposited .

This can be achieved by using the following set of treatments
- immersion of the balls in a degreasing solution,
- creation of ionic tin seeds on the surface of the
synthetic material by immersion in an acid bath of
stannous chloride,
- after intensive rinsing, sensitization by means of a
deposit of traces of ionic palladium constituting
germs on which, after a new rinsing can
take place,
- a deposit of chemical nickel, if necessary reinforced by
- an electrochemical nickel deposit, finally followed by
- a zinc deposit obtained by any conventional means.

However, in addition to the relatively weak mechanical adhesion already mentioned between the synthetic support and the layer of nickel encapsulating it, the use of palladium is very expensive.

Another technique, which overcomes the above drawbacks, has been developed, which consists in reducing the nickel on the surface of the plastic support, by using sodium borohydride.

This process is carried out by immersing the beads at: revstibs for about thirty: -minutes-, in a; acid solution: -from: -chloride of nickel at 200 gJl. In which a solution of sodium borohydride at 35 g / l and 2 N sodium hydroxide is added in an amount of 100 ml. every 5 min. and this for an amount of treated beads of approximately 5 liters.

However, we note that after a very short time the reaction gets carried away; the balls are covered with a layer of shiny black nickel; the bath is completely black and the container in which the reaction took place is completely metallized.

The reaction is therefore poorly controlled and not very reproducible, the sodium borohydride being an excessively powerful reducing agent.

The present invention overcomes the above drawbacks and gives rise to very uniform nickel deposits, having an adhesion to the support in synthetic material greater than that resulting from the previous processes, and not requiring the consumption of precious metals such as palladium. In addition, the method used is very reproducible and allows good control of the reduction of nickel.

It also has the advantage of allowing the use, for the supply of nickel, of used baths applied to industrial acid chemical nickel plating of metal parts. Indeed, as and when the consumption of nickel resulting from the operation of the above baths above, new compensatory contributions should be made, in particular nickel salts
From the above, it results in an accumulation over time of degradation products of the bath, thus becoming too rich in certain salts such as hypophophites, phosphites, sodium and anpnium sulfates, then tending to slow down the speed of bath reaction.

9 then becomes necessary to change it, which results in a double loss because
- that the bath is still rich in nickel, around 7 g / l.

- removal and depollution costs, in particular
to remove residual nickel.

The nickel-plating process according to the invention consists of a prior impregnation of the non-conductive particles with an acidic solution of ionic nickel, then a release of the latter in an alkaline medium, and its reduction controlled in two stages acting inside the layers. superficial so-called particles ma eropoeucs in which the nickel in the metallic state will cling.

The following example illustrates, without limitation, the process of the invention.

The non-conductive particles consist of partially or fully sulphonated macroporous polystyrene H-shaped beads, immersed in an acidic nickel solution initially containing 10 μg. lower this metal and a pH / a 4, provided moreover with all the usual salts used in acidic nickel-plating solutions, such as stabilizers, complexing agents, etc.

It is advantageous to use used solutions of acidic chemical nickel plating of metal parts mentioned above.

Starting from a volume of beads of 0.5 liters and a volume of bath of 1 liter, it will be noted that after 30 minutes of immersion the beads have absorbed 50% of the nickel present in the bath and after 45 minutes, 75% of this nickel.

The beads are then thoroughly rinsed, then immersed in deionized water.

A first reduction phase then occurs, due to the addition of 10 to 50 ml of a sodium borohydride solution at 35 gfl. and 2N sodium hydroxide. The resulting alkaline medium determines a progressive rejection of the nickel ions absorbed by the polystyrene beads; as soon as these ions come into contact with the borohydride of the bath, they are immediately reduced on attachment sites, formed by the surface of the pores and this up to 1 and 5 microns below the surface of the beads.

The start of this reaction is easy to detect because there is a significant release of gas; or let it continue for a relatively short period of time, of the order of 2 to 5 minutes. The beads thus treated are then removed from the previous solution and washed thoroughly until the rinsing water is clear. They are then provided with seeds of nickel grains firmly anchored. Then, the beads are subjected to a second reduction phase in a 10 g / l solution. of soda, in which an aqueous solution containing 110 g / l is added dropwise. sodium hypophosphite until the reaction, which always results in gas evolution, slows down. This signals the end of the rejection of nickel by the beads
During this second operation of reduction of the nickel ions by the hypophosphite, there is a controlled precipitation of metallic nickel from the ions initially absorbed by the beads during their phase of immersion in acid medium, and now released in alkaline medium. ; this deposit is then firmly anchored on the nickel seeds originating from the previous reduction with sodium borohydride.

In order to increase the thickness of the nickel layer on the beads, the above solution is slowly heated, while slowly adding a commercial acidic or recovery chemical nickel solution as described above.

The pH of the solution decreases rapidly. The nickel deposition is continued until the thickness planned for the layer is reached.

The phenomenon of reduction of nickel ions respectively by sodium borohydride, then by sodium hypophosphite, occurring as soon as they are released in an alkaline medium, therefore even inside the pores of the polystyrene beads, it follows that the deposition of metallic nickel is partially formed inside the balls and therefore the bonding of the nickel layer is very greatly improved compared to a conventional metallization.

In the previous example, it can be seen that for a layer of nickel 2.5 microns thick, 1 micron of this layer is located inside the ball.

It will be noted the advantage of the combination of the two successive reductions of nickel ions by sodium borohydride and by sodium hypophosphite, the first9 difficult to control in its duration allowing the formation of metallic nickel well anchored in the deep sites of the beads of polystyrene, the second, easy to control, but being able to give rise to a deposit of nickel only on an already conductive surface, obtained by the prior reduction of the nickel ions by sodium borohydride.

Claims (4)

1. Method for metallizing non-conductive particles, such as macroporous crosslinked polystyrene beads, of the type resulting from the immersion of the latter in an acidic nickel chemical bath known per se, then reduction of the nickel ions, released in an alkaline medium , with a sodium borohydride solution, characterized in that this reduction is interrupted after a relatively short period of time, of the order of 2.5 minutes, that the beads thus treated are recovered, then washed and subjected to a new easily controllable reduction phase, in an alkaline medium, using a slowly added aqueous solution of 110 g / l. of sodium hypophosphite, until the gas evolution accompanying the reduction slows down, the thickness of metallic nickel thus obtained being increased by heating the previous bath and gradual addition of an acidic commercial chemical nickel solution. 2. A method of metallizing non-conductive particles according to claim1, characterized in that the acidic chemical nickel solution used to increase the thickness of the nickel layer at the end of the process consists of a bath for recovering an acidic solution chemical nickel plating used for metal parts. 3. A method of metallizing non-conductive particles according to claim 1, characterized in that the macroporous cross-linked polystyrene beads are partially sulfcnées. 4. A method of metallizing non-conductive particles according to claim 1, characterized in that the beads are completely sulfonated.