FI66437B - FOERFARANDE FOER STROEMLOES METALLUTFAELLNING - Google Patents

Description

66437

Currentless metal deposition method

The invention relates to a flow-free metal deposition method.

It is known to spray aqueous solutions of metal salts and suitable reducing agents on substrate surfaces to deposit metal layers thereon. In this way, gold, silver, copper, nickel, cobalt, chromium, platinum-10 and palladium coatings, for example, are now available for a wide variety of substrates. Likewise, compared to general galvanic metal deposition, non-current methods have the advantage that the substrate surfaces to be coated do not have to be electrically conductive or electrically conductive. In addition, the distribution of the precipitated metal over the entire substrate surface to be coated is usually more uniform, since it does not depend on the current density, which can be very different, especially on surfaces that are not completely uniform, also on three-dimensionally adjacent surface portions.

However, also in electroless metal deposition, which on an industrial scale predominates by spraying metal precipitation baths on a substrate, the layer thickness is not always as uniform in all places as would be desirable to economically utilize the metal salts contained in the baths.

Commonly used metal precipitation baths are sprayed with two or more sub-baths -30 one with a metal salt, the other with the necessary reducing agent and possibly other additives such as complexing agents, stabilizers and / or glossing agents - simultaneously but separately, to avoid metal deposition before coating. faced. However, if air-spraying nozzles are used for this spraying 2 66437, a very fine spraying of the partial baths and thus good mixing on the surface to be coated is achieved; but at the same time, the exhaust devices for the exhaust air necessary in this working-5 method also absorb considerable amounts of small mist droplets, which cause the metal to precipitate in the exhaust ducts, where it is undesirable and often has to be removed at great expense. For this reason, in the more frequently used spraying, substantially larger spray droplets are obtained from the wet spray nozzles (Airless method). Precipitation of the metal on the substrate surface now begins everywhere at the points where the jets 15 of the metal salt sub-bath coincide with the droplets of the reducing sub-bath.

In this case, the mixing ratios are correct at many points for the onset of adhesive metal deposition, but at the other intermediate points the concentrations of the reaction parties 20 required for precipitation are reached only somewhat later. In this case, only different fractions of a second, the onset times of metal deposition, initially cause a point precipitation, whereby these points then strengthen and spread until they grow together into a surface-adhesive coating. In this way, however, a macroscopically uniform surface coating is formed, which, however, has a granular structure in the microarray, in which the layer thicknesses in the adjacent microareas can be distinguished from each other by their multiplicity. The quality of each metal coating is generally determined by the points with the lowest layer thickness; therefore, in order to achieve an overall satisfactory coating quality, substantially more metal coating baths are used than the required minimum layer thickness. Particularly in precious metal coatings, for example in the manufacture of mirrors with 3,664,37 silver plating, the amount of metal that has been excessively precipitated at high layer thicknesses means a considerable loss.

The object of the present invention was to provide a method for precipitating vir-5 Raton metal, by spraying a metal precipitation bath, in which method the metal is deposited with as uniform a layer thickness as possible.

It has now been found that a very even metal layer can be obtained by spraying an aqueous metal deposition bath onto the substrate surface when the metal precipitation bath is diluted with water at the moment of impact. The rate of precipitation is essentially determined by the concentrations of the reactants in the coalescing sprayed droplets. As the concentration decreases as a result of the addition of water at the time of encounter, the time for the onset of metal deposition is longer. During this time, complete mixing of the droplets sprayed on the surface can take place, so that the metal deposition does not start in a point-like manner, but regionally in the form of a very thin but closed layer 20. The same effect is achieved when initially heavily diluted metal salt-sub-bath and baler-sub-bath solutions are sprayed through different nozzles to form a well-mixed, highly diluted metal precipitation bath.

The invention thus relates to a method for electrolessly precipitating a flat metal layer by spraying a metal precipitation bath containing a metal salt and a reducing agent and possibly other additives on a substrate surface, characterized in that by lowering the bath concentration initially , which has a standard concentration, is fixed to the desired thickness.

It is true that U.S. Pat. No. 3,983,266 discloses glass, 66437 in electroless silver plating for the manufacture of a mirror, before spraying the silver plating bath, spraying the substrate with water at about 50 ° C. In this method, however, spraying with warm water takes place only to heat the glass sheets before silvering and to rinse off the excess activating solution. It is not apparent from this patent that water is sprayed for the purpose of initially diluting the silver plating solution to be sprayed thereafter, nor is it a surprising effect that it results in particularly uniform silver layers.

The method according to the invention is in principle useful in all metal coatings which are deposited in the usual manner. Because it allows clear savings on metal salts, it is preferably used in the application of precious metal layers, for example gold, silver, platinum or palladium layers. For each metal precipitated by the process according to the invention, the usual reducing agents can be used, for example hypophosphite for gold, formaldehyde, sugar or sugar derivatives for silver, hydrazine hydrate for platinum or palladium.

According to the method of the invention, it is thus possible to work wherever a conventional aqueous metal precipitation bath per se is sprayed on the base surface. The dilution step according to the invention can then take place in different ways. Thus, for example, the metal precipitation bath can be sprayed onto a potentially moving thin aqueous layer on the substrate surface, the amount of water being selected so that at the moment the shower encounters the water layer, the bath or at least the metal salt sub-bath decreases to 50-5%. , s 66437

In a preferred embodiment, the method according to the invention is used in practice in conventional metal deposition devices, in which the substrate is conveyed on a conveyor belt by a plurality of spray nozzles arranged in succession on the conveyor belt, the beam According to the invention, such a device is used to spray clean water from the first nozzle - viewed in the conveying direction of the conveyor belt - at such a pressure that a water wave is formed when it hits the substrate, regardless of the belt conveying speed and the spraying material. The second and possibly other nozzles or nozzle pairs from which the metallization bath, possibly also in the form of partial baths, is sprayed are then arranged so that their jet cones partially overlap with the water nozzle jet cone, with the center of the second nozzle above. However, it may also be appropriate to position the spray cone of the water nozzle (s) so that it is completely covered by the spray cone (s) of the first nozzle or the first pair of nozzles. If the metal precipitation bath 30 is sprayed from the second and third nozzles or the second and third nozzle pairs in the form of two sub-baths, these are set so that their spray cones on the substrate surface completely coincide. The other nozzles or nozzle pairs are then always sprayed alternately on each of the partial baths so that the spray cones partially overlap. In this way, the concentration of the metal precipitation bath 6 66437 on the substrate gradually increases as it is conveyed until, after passing the last nozzle, a bath concentration corresponding to the desired layer thickness is reached. After the reaction time characteristic of the respective metal precipitation bath 5, the substrate surface is then released in a manner known per se by rinsing with water from the bath waste and residues.

When the metal precipitation bath can be sprayed in a premixed form - this is possible, for example, in cases where the metal precipitation is first catalyzed by an activator on the substrate surface - it is advantageous to spray baths diluted from the second nozzle and increasingly concentrated from the third and subsequent nozzles.

15 In this case, a concentration gradient is formed in the direction of travel of the substrate, which results in a particularly uniform metal coating. Also in this embodiment of the method according to the invention, the spray pressure and the spray angle of the nozzles are preferably set so that the initial concentration of the metal precipitation bath on the substrate surface is 50-5%, in particular 10-30% of the maximum concentration.

Example a) Apparatus 25 Apparatus for making silver mirrors comprises a 140 cm wide conveyor belt for conveying glass sheets which have been cleaned and activated with tin (II) saline in a manner known per se at a conveying speed of between 100 and 260 cm / min under a nozzle beam 30 which is 20-40 cm: n at a height above the conveyor belt makes 18 reciprocating oscillations per minute over the entire width. Four pairs of nozzles are arranged in succession on the nozzle beam at an adjustable angle (nozzles 1-4), from which a metal precipitation bath or sub-35 baths can be sprayed at a pressure of 3-6 bar. At a distance of 5 cm before the first pair of nozzles, another angularly adjustable nozzle with a double performance (nozzle W) is placed in the nozzle beam and from which water can be sprayed onto the glass plates passing by the conveyor belt.

5 b) Substances and working methods

For the mirror coating, aqueous ammonium-acchi silver nitrate solutions (S) at concentrations ranging from 0.5 to 3.5% and in each case equivalent amounts of a commercially available reducing agent (R) based on 10 sugar derivatives were used. These solutions are sprayed from passing nozzles onto passing glass plates. After 2 minutes in each case, measured from the impact of the first jet on the activated glass, this is rinsed with clean water from the rear nozzles and dried. The resulting mirrors were evaluated by determining the amount of silver precipitated per unit area (mean + scatter). In this case, work was carried out at some times according to the invention with the preceding water spray and in others according to the state of the art 20 without this operation. The results are summarized in the following table: β 66437 o O O o

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9 66437

The table clearly shows that with increased water spray through the nozzle W (runs 5-8), as a result of which the initial concentration of the bath on the glass plates decreases from 3.5% to about 1.5% or from 1.75% to about 0, At 15%, 5 substantially smoother silver mirrors are obtained than without this measure. This is also clearly noticeable when looking at the manufactured silver mirror in the through-exposure, resulting in times 1-4! in the silver layers there is clearly a granular structure 10 which cannot be detected in the mirrors made in time 5-8. In addition, the adhesive strength of silver layers having a granular structure is substantially lower; here the silver layer can be removed by scraping with a nail, while this is not possible with mirrors made in time 5-8.

Claims (5)

10 66437 Claims. A method for flow-free precipitation of a uniform metal layer by spraying an aqueous metal precipitation bath containing a metal salt and a reducing agent, characterized in that by lowering the bath concentration, a uniform thin metal coating is first applied to the substrate surface, which coating is then reinforced to the desired thickness. is the usual concentration. A method according to claim 1, characterized in that when the metal precipitation bath sprayed at the beginning of the precipitation first encounters the substrate surface, the concentration of the sub-bath 15 containing at least the metal salt is reduced to about 50-5% of the concentration of the subsequent metal salt sub-bath. A method according to claim 1, characterized in that the concentration of the metal precipitation bath at the site of the first encounter is reduced to about 50-5% of the concentration in the shower of the metal salt partial bath. Method according to Claims 1 to 3, characterized in that the dilution is carried out by spraying water on the surface of the substrate from a nozzle placed in front of the first metal-precipitation bath or 25-sub-bath nozzle. Method according to Claim 4, characterized in that the spray cones of the water and the first metal-precipitation bath or sub-bath nozzles overlap.