Method for Treating Plastic Substrates Prior to Plating

In a typical chemical plating procedure for polymeric plastic substrates, the plastic part may be first cleaned of surface grime and the like in an aqueous alkali soak solution, the cleaned part may then be contacted with an organic solvent medium which can be either a single-phase system or an admixed water-organic solvent emusion, and thereafter followed with a thorough water rinse of the part. The part is then contacted with an aqueous acid solution containing hexavalent chromium ions to etch the surface of the plastic, followed by one or more rinses in water and/or solutions containing chromium-reducing or chromium-extracting agents. The surface of the substrate is then contacted with an acid tin-palladium complex which generally is an activator containing palladium chloride, stannous chloride and dilute hydrochloric acid, and the polymeric substrate is then carefully rinsed. Thereafter, in accordance with this invention, the activated surface of the plastic is accelerated using particular molar concentrations of an inorganic acid, a metal salt preferably selected from the group consisting of nickel, cobalt and ruthenium and desirably a compound capable of ionizing to produce fluoride ions, that is, monovalent negative fluoride ions. The electroless plating procedure is then normally completed by a water rinse, and immersing or otherwise contacting the substrate surface with a chemical plating solution containing a reducible salt of the metal to be deposited on the surface, such as nickel, cobalt, copper or the like. The metalized surface is then rinsed with water and is now ready for conventional electroplating.

Acclerating a substrate surface after activation is of course a generally well-known procedure. The use of this step is theorized on the assumption that during activation of the substrate not only is palladium or another catalytic material laid down to provide the necessary initiating foci for the reduction of metal ions in the electroless plating solution, but excess stannous ions and/or other impurities which are also present in known activating solutions are also deposited on or at least adhere to the surface of the substrate, and more particularly, during the water rinsing subsequent to the activating step there occurs hydrolysis of palladous chloride and stannous choride which are entrained in microscopic cavities in the surface of the polymeric surstrate. These stannous ions in the form of stannous hydroxide, as well as other impurities, are deterrents to the subsequent deposition of metal, just as are residual adherent hexavalent chromium ions from the etching step. This removal of impurities is the primary function of the usual accelerating solution. The problem presented is one of promoting removal of these "poisons" preferentially to palladium hydroxide foci, which upon reduction become the catalyzing sites. The known accelerating solutions are quite effective in removing stannous ions, but their use is critical in that they must be accurately controlled in order to avoid also removing excess amounts of the palladium and thus impeding successful accomplishment of electroless plating.

It has been found by applicants that if there is employed an accelerating solution comprised of an inorganic acid in the amount of approximately 0.025 to 1.00 moles, a metallic salt desirably selected from the group consisting of nickel, cobalt and ruthenium, in an amount of about 0.002 to 0.2 moles, and in certain instances a compound capable of ionizing to produce monovalent negative fluoride ions, the latter compound being present in an amount of apporximately 0.004 to 0.6 moles, each of the above-mentioned problems and the disadvantages of prior art procedures are essentially completely overcome. To be more specific, by this invention excess stannous hydroxide is rapidly removed and the palladium hydroxide remains to perform an important catalytic function in the electroless plating step. Further, by proceeding in the manner herein disclosed, the stainless steel rack contacts are activated, that is to say, they are cleaned so that there is essentially no adsorbed metal oxides such as trace amounts of tin, and there remains on the rack contacts catalytic metal in the form of reduced nickel or cobalt, depending upon the particular mentioned metallic salts employed. Also, there is accomplished by this invention substantially complete removal of stannic hydroxide colloid S.sub.n (OH) .sub.4, which results from oxidation of stannous hydroxide by reason of the presence of oxygen in the air or in the water solution caused by air agitation in the rinsing step.

An exemplary activator solution comprises approximately 200 mg/1 palladium chloride (PdCl.sub.2), about 13 g/1 SnCl.sub.2 and 100 g/1 of hydrochloric acid. It is theorized that when these materials are combined the reaction product is substantially as follows: ##STR1##

During the rinsing step following activation it is also postulated that there is formed:

and

The water during rinsing also apparently reacts with the mentioned palladous chloride and stannous chloride in generally the following manner:

Relatively speaking, stannous hydroxide has approximately the same degree of solubility as palladium hydroxide, and it is apparently for this reason that prior attempts to accelerate the activated surface of the plastic by use of a dilute solution of an acid or alkali have not been completely successful.

As will be brought out in the examples to follow, a preferred accelerating solution contains from about 10 to 30 grams per liter of hydrochloric acid or a general concentration range of from about 3 to about 10% by volume, although this is not at all times critical, 1/2 to forty grams per liter of nickel chloride hexahydrate, and when required approximately one fourth to two grams per liter of sodium bifluoride (NaHF.sub.2). However, for the acid there may be substituted fluoroboric acid (HBF.sub.4) at about the same concentration or sulfuric acid at about the same concentration. The nickel chloride hexahydrate also can be replaced by cobalt chloride hexahydrate, ruthenium chloride, or any member from the platinum group of the Periodic Chart which has a reduction potential less than that of iron. The source of the fluoride ion could also be ammonium acid fluoride (NH.sub.4 HF.sub.2), sodium fluoride (NaF), lithium fluoride (LiF), potassium fluoride (KF), fluosilic Acid (H.sub.2 SiF.sub.6), hydrofluoric acid (HF) or any compound capable of ionizing to produce monovalent negative fluoride ions. The time for effecting acceleration of the activated plastic substrate is generally between about 30 seconds and 3 minutes, and the temperature range is approximately 115.degree. to 140.degree. F.

A wide variety of plastic materials can be treated in accordance with the novel concepts of this invention, such as polypropylene, phenolic, epoxy and polysulfone polymers as well as co-polymers such as acrylonitrile-butadiene-styrene (ABS), or any other usually chemically platable plastics. Particularly good results have been achieved to date with ABS and a polymer which is understood to be a combination of a polyaryl ether, ABS and a polysulfone, this being a product of Uniroyal Inc., identified by the trademark ARYLON.

A series of tests utilizing the instant novel composition were conducted as follows. An acidic accelerator containing 15 g/1 hydrochloric acid and 1 g/1 of nickel chloride hexahydrate had added thereto 200 ppm of bivalent tin in the form of stannous chloride, and when employing an immersion time of about 30 seconds at a temperature of between about 125.degree. and 130.degree. F, severe skipping or lack of continuity in the coverage of the substrate by the electroless deposited metal was noted. However, when about 2 g/1 of sodium bifluoride was added, the accelerator solution was restored to normal operation. As stated previously, during water rinsing or an air transfer, a portion of the bivalent tin hydroxide retained on the plastic surface is oxidized to quadravalent tin hydroxide. It would normally be expected that the hydrochloric acid would dissolve the tin hydroxide, however, colloidal quadravalent tin hydroxide is relatively slow to react with the dilute hydrochloric acid present in the accelerator. Should the concentration of hydrochloric acid be increased, there will be removed not only the tin hydroxides, but palladium which is necessary as a catalyst in the electroless plating step. Thus, one aspect of this invention, there may be employed a compound cabable of ionizing to produce a source of fluoride ions to thereby promote rapid removal and dissolution of colloidal quadravalent tin hydroxide. Further, nickel ions or other metal ions which are capable of being reduced to the metallic state by an iron or stainless steel surface, that is, metal salts desirably selected from the group consisting of nickel, cobalt and ruthenium, act as catalysts for electroless nickel and when added to the acidic accelerator give an immersion deposit on the stainless steel contacts. This leaves the contacts active and allows electroless nickel to deposit on the contacts, as well as on the parts, thereby giving a low and uniform contact resistance. In another test, an acidic accelerator containing 15 g/1 of hydrochloric acid and 1 g/1 of nickel chloride hexahydrate had added thereto about 1 ppm quadravalent tin colloid, prepared by dissolving 1.45 grams of SnCl.sub.4 . 5H.sub.2 O in 500 ml distilled water. This solution was stable for 8 to 12 hours before stannic hydroxide particles separated out as an insoluble precipitate. Severe skipping resulted but when 2 g/1 of sodium bifluoride was added, the solution was restored to normal operation. Thereafter, the addition of up to 5,000 ppm of quadravalent tin colloid did not interfere with the operation of the acidi accelerator.

In a further test an accelerator containing 15 g/1 hydrochloric acid, 1 g/1 nickel chloride hexahydrate and 1.84 g/1 of ammonium acid fluoride had added thereto about 500 ppm of quadravalent tin colloid. Plastic test parts were run immediately, and the plating quality was excellent.

Additional runs were made, in the first the accelerator comprised 15 g/1 HCl, 0.65 g/1 HF and 1.0 g/1 NiCl.sub.2 . 6H.sub.2 O. In a further run the make-up was 15 g/1 HCl, 40 g/1 NiCl.sub.2 . 6H.sub.2 O and 1.84 g/1 ammonium acid fluoride. The quality of plating on the parts in both runs was excellent.

It has been discovered by the applicants that at this point in the process cycle an important, although heretofore unrecognized, condition prevails. Consider a rack or work fixture carrying a multiplicity of plastic parts adjacent to one another, as is common in any commercial application of the art. If the metallized plastic parts are not of a relatively low and uniform level of electrical resistance with respect to the rack or work fixture, they begin to electroplate in conventional solutions in a distinctly particular manner. Parts exhibiting a low electrical resistance between their surface and the rack begin to plate very quickly, whereas those having a high resistance between their surfaces and the rack start to plate at a significantly decreased initial rate. It follows from this that the parts which begin to plate faster soon attain a much higher surface current density than those which start to plate more slowly. As a consequence of this, it may obtain that the effective electrical resistance for the induced bipolar charge distribution on the surface of a part of high resistance is considerably less than that resulting from the direct circuit connection to the cathodic pole of the electroplating power supply. It then logically follows from elementary physics that certain areas of the surfaces of the metallized plastic parts will be anodic with respect to the electrolyte, and the metallic coating thereon will be electrolytically dissolved. The result of this sequence of events will be the stripping of certain parts on the rack of their metallic coating and skipping or misplate subsequent to the complete electroplating processing. That such high and nonuniform part to rack resistances does occur when using only an inorganic acid as an accelerator, as is common in the prior art, is portrayed in attached Table I. It is the function then, in the instant invention, of a metal salt selected from the group consisting of nickel, cobalt and ruthenium in combination of the other ingredients herein disclosed to deposit by immersion on the stainless steel contacts a metallic film which is catalytic to and promotes the deposition of the subsequent electroless metallization step, in order that the electrical resistance between the plastic surface and the rack or work fixture is decreased to a low and uniform level. The effectiveness of this method is clearly demonstrated by the data included in attached Table II, wherein there is set forth the results of incorporating the instant invention into the total process cycle.

An acidic accelerator was prepared containing 15 g/1 hydrochloric acid and 1 g/1 nickel chloride hexahydrate. When incorporated into the total process cycle for plating on plastic, complete coverage was obtained. Then a solution of stannic colloid was prepared by dissolving 1.45 g/1 stannic chloride pentahydrate in 500 ml of distilled water. This solution was stable for 8 to 12 hours before stannic hydroxide particles settled out as an insoluble precipitate. The addition of 1 ppm quadravalent tin colloid to the acidic accelerator caused severe skipping on test panels and parts. The addition of 2 g/1 sodium acid fluoride returned the solution to normal operation. Thereafter, the addition of up to 5000 ppm of quadravalent tin colloid did not interfere with operation of the acidic accelerator or complete coverage on parts and panels.

This accelerator, used in the complete process cycle yield complete coverage on parts and panels. The addition of up to 2000 ppm quadravalent tin colloid did not interfere with operation of the solution. The addition of 1.8 g/1 sodium acid fluoride had no deleterious effect on the quality of the work processed through this solution.

To Illustrate the invention further, a solution of 5% by volume of HCl (22 g/1 HCl) was prepared and utilized at 110.degree. F for 30 seconds as an acidic accelerator in a normal cycle for the metalization of ABS plastic parts. A rack of 7 plastic parts was processed, and the ohmic resistance was measured between the phase of the part and rack and between the back and front of the part only. Resistances, in ohms, were as follows:

Thereafter, 100 mg/1 of ruthenium chloride, RuCl.sub.3, (0.1 g/1 RuCl.sub.3) was added to the above solution and a identical test conducted. In this case, the ohmic resistance for each part to rack was measured to be one ohm, the same as the resistance on the part only. The use of RuCl.sub.3 in the acidic accelerator decreased the contact resistance to essentially zero. As is appreciated to those skilled in the art, the contact resistance represents a difference between the "part on rack" and "part only".

It may be seen from the foregoing that applicants have provided an accelerator solution and method of treating a plastic substrate therewith whereby there is effectively removed excess stannous ions in a rapid fashion while leaving behind the palladium ions for catalysis in the electroless plating step. Further, by the presence of nickel ions or other metal ions that act as catalysts for electroless nickel, there is obtained an immersion deposit on the stainless steel contacts which leaves them active and permits the deposition of electroless nickle on the contacts, as well as on the parts, thereby giving a low and uniform contact resistance. The accelerator and process of this invention can be used with the wide variety of plastic parts, and as has been noted, there are a number of inorganic acid substitutes as well as variations in the metal salts which are capable of being reduced to the metallic state by iron or stainless steel surfaces and which are generally taken from the group consisting of nickel, cobalt and ruthenium. Additionally, many compounds capable of ionizing to produce fluoride ions have been mentioned. These and other changes may of course be practiced without departing from the spirit of the invention or the scope of the subjoined claims.