US 2639997 A
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Patented May 26, 1953 METALLIZATION OF NONMETALLIC SURFACES Cyril Francis Drake, London, England, assignor to International Standard ElectricCorporation, New York, N. Y;, a corporation of Delaware No Drawing. Application April 22, 1948, Serial No. 22,700. In Great Britain April 24, 1947 1 This invention relates to the metallisation of non-metallic surfaces and more particularly to a process of metallising such surfaces whereby only a desired area of asurface may be coated with metal.
The process of silvering glass by immersing the glass in an ammoniacal solution of silver nitrate, it may be after having treated the glass with an activating agent such as a solution of stannous chloride in hydrochloric acid, and thenadding. a reducing agent to the solution, is well 'known.' This process suffers from'the disadvantages that silver is deposited upon areas other than those required to be silvered so that it cannot be used for silvering only a portion of the surface immersed in the solution, and, even if the deposit which occurs upon the sides of the vessel and other unwanted parts can be recovered (w'hidhis not alwayspossible to secure), a considerably larger amount of silver salt must be used than is necessary to form a coating of given thickness on a glass of given area. In addition there is formed a considerable deposit of silver in the solution itself which later settles out.
It is the object of the present invention to provide a process of metallising that does not suffer. from these disadvantages and has in consequence a wider field of application than the known method.
-According to the present invention a process of metallising non-metallic surfaces (other than hydrocarbon polymers) by reduction of a metal salt in solution is characterised in that the reduction of the metal salt in solution is effected in the presence of an anionic surface active agent.
The invention finds its principal application in the silvering of non-metallic surfaces and will accordingly be explained in relation thereto.
In carrying out the invention in relation to the silvering of non-metallic surfaces, an anionic surface active agent is added to an ammoniacal solution of silver nitrate. The preferred surface active agent is a wetting compound in the form of an alkali metal or alkaline-earth-metal salt of a sulphonated long chain fatty alcohol; specifically the sodium salt of sulphonated lauryl alcohol has been found satisfactory and the proportion added was 0.05% by weight of the solution. On now adding a reducing agent such as formaldehyde, no apparent change takes place in the solution.
Metallic silver is formed in the same way as in the absence of the surface active agent but the particles of silver remain of such small dimensions that the liquid is transparent and the silver 7 Claims. (01. 117-35) 2- remains in suspension. Since the silver particles produced by the reduction of a silver salt carry a, negative charge, it is necessary for the production of such a result that the surface active agent should be anionic. The proportion of surface active agent added should not be so great that the silver will be permanently held in suspension nor that the silverwhen deposited will be discoloured. Thus in the case of the sodium salt of sulphonated lauryl alcohol the proportion should not exceed 0.2% by weight.
In order that a non-metallic surface shall be silvered on being immersed in the liquid it is necessary that the surface shall have been activated in known manner, e. g. by immersion in a solution of stannous chloride in hydrochloric acid. If a glass surface so activated be exposed to the liquid above mentioned containing silver in suspension, the silver is deposited immediately upon the activated surface and forms an'adherent mirror coating.
The surfaces of other non-metallic substances may be equally well silvered in this manner, provided that the surface has polar groups present. Thus surfaces of glass, mica, quartz and some synthetic resins such as those of the acrylic acid group may be silvered in this way, but adherent deposits are not formed on the surface of hydrocarbon polymers such as polystyrene or solid polymers or ethylene.
It is clear that the activated surface to be silvered can be placed in the ammoniacal solution of silver nitrate containing the surface active agent either before or after the reducing agent has been added thereto. It is, however, generally convenient to add the reducing agent to the solution before exposing the surface to be silvered thereto. It is clear also that it is not necessary to add all at once an amount of reducing agent sufficient to reduce all the silver salt in solution, but that it is possible to add first only sufiicient formaldehyde to precipitate the silver required to silver a certain number of surfaces and then to expose those surfaces to the liquid, after which a further amount of formaldehyde may be added and further surfaces silvered.
Since silver is only deposited upon areas that have been activated the invention can be used for silvering only a required area on a surface. Thus electrodes of any desired shape and area can be formed upon plates of quartz piezo-electric crystals by activating only the parts of the quartz desired to be silvered.
The thickness of silver coating obtained can be varied from a transparent coating upwards to any desired thickness according to the time of immersion, though it is obviously desirable if very thick coatings are required, e. g. sufficient for solderin or connecting lead, to immerse the surface being silvered in more than one successive bath.
The present invention provides the only means known to us of uniformly silvering the interior surface of a narrow core or long length cylindrical tube of glass or similar material. By washing the interior surface of the tube with is 5.01 tion of stannous chloride in hydrochloric acidand then drawing through the tube a solution corrtaining finely divided silver in suspension as described above, a uniform and adherent silver deposit is formed on the interior surface of the tube only.
What is claimed is:
1. Process for producing an adherent coating of metallic silver upon a surface of an article formed of a non-metallic substance chosen from the class consisting of glass, mica, quartz, and synthetic resins having polar groups present in the molecule, that comprises applying then removing, a solution of stannous chloride hydrochloric acid to the surface to be coated, then treating the surface with an ammoniacal solution of silver nitrate containing an anionic surface active agent in the presence of a reducing agent.
'2. Process as claimed in claim 1 wherein the surface active agent is a substance chosen vfrom the class consisting of the alkali-metal salts and the alkaline-\earth-metal salts of a sulphonated long chain fatty alcohol.
,3. Process as claimed in claim ,1 wherein the surface active agent is the sodium salt of sulphonated lauryl alcohol.
.4.- Process of silvering a surface of an electrical insulating material having polar groups present which comprises activating the surface by treatment with a solution of stannous chloride in hydrochloric acid and thereafter immersing the surface in a liquid containing silver in suspension and an anionic surface active agent.
5. Process of silvering a given area of a nonmetallic surface Of the kind thathaspolar groups 'on :the surface which comprises treating that given area with a solution of stannous chloride in hydrochloric acid to activate it, then treating it with an ammoniacal solution of silver nitrate med thereto a small percentage of the sadium salt of sulphonated lauryl alcohol together withformaldehyde in an amount sufllcient to reduce the silver nitrate to silver.
6. Pmocess-of rsilvering the interior surface 01 a glass tube that comprises activating the said suniace by treatment with a hydrochloric acid solution of stannous chloride and thereafter drawing through the said tube an ammoniacal solution of silver nitrate containing a reducing agent and a small percentage of an anionic surface active agent of the class consisting of alkali metal and alkaline earth-metal salts of 's'ulionaied long-chain aliphatic alcohols.
7. Process according to claim 3, in which the sodium salt ,of sulphonated lauryl alcohol comprises'0.0 5% to 0.2% by'weight of theammoniacal solution.
CYRIL FRANCIS References Cited in the file .9! this UNITED cram mums I Date