Decorative electroplates for plastics

Abstract

A process for electroplating ABS plastic with decorative nickel chrome plate using an essentially all nickel system involving depositing a sublayer of low strength nickel onto a plastic surface which has been made conductive, depositing over said sublayer a super levelling nickel and depositing chromium on the surface. When the ratio of the thickness of the nickel sublayer to the thickness of the super-levelling nickel is at least about 2, the plated plastic is characterized by good appearance, by excellent resistance to thermal cycling and by excellent resistance to the deleterious effect of corrosive media.

Description

The present invention is directed to electroplating of plastics and more particularly to the electroplating of plateable ABS type plastic using essentially an all nickel system underlying a surface chromium layer.

It is well known to electroplate ABS plastics using a process which essentially involves a preplating technique to provide electroconductivity on the surface of the plastic and then electrodepositing metal on the thus prepared surface. While the prior art contains disclosures of preplating of ABS plastics followed by electrodeposition of nickel to attain a significant thickness and then followed by the ordinary chromium deposition on the surface the consensus of the art is that such a process is not commercially satisfactory. The art generally requires a layer of copper to be plated immediately adjacent the preplate treated plastic.

Reference is made to an article entitled "Electroplated Plastics Environmental Accelerated Corrosion Studies" by Hepfer et al, PLATING, Apr., 1971 pages 333 to 338 et seq. The authors of this article indicate on page 337 that it is necessary to include a layer of copper adjacent to plastic in order to develop the levelling needed for bright chromium plating deposit and for assistance in meeting the thermal cycling test. The authors point out that copper does not add to corrosion protection and as a matter of fact gives a particular type of undesirable green corrosion when exposed to marine atmospheres.

Reference is also made to an article "Corrosion and Thermal Cycling Tests on Plated ABS Plastics" by V.E. Carter, appearing in the Transactions of the Institute of Metal Finishing, Vol. 48 (1970), Conference Issue Part 2, Pages 58 to 63.

In this article, the author points out that when copper undercoats were present, brown stains appeared on test panels over a long term to atmosphere exposure. The author goes on to state on page 61, "it would be desirable, therefore, to omit copper undercoats in order to achieve a better appearance when corrosion occurs: on the other hand, their presence is probably essential for resistance to thermal changes as shown previously both at the BNF.sup.2 and other workers.sup.3-5."

From the foregoing it is evident that it would be advantageous to provide an all nickel system for electroplating ABS plastics. The desirability for such a system also evident in light of the disclosures by D. Luch in copending application Ser. No. 408,410, wherein it is disclosed that for a particular novel type of preplating system it is essential that a Group 8 metal be plated as a first layer over the preplate treated surface.

It has now been discovered that by means of special controls and interrelationships a novel essentially all nickel plating system for plating ABS plastics can be provided.

It is an object of the present invention to provide a novel all-nickel electrodepositing system suitable for use in plating plastics.

It is a further object of the present invention to provide a novel plated plastic made using the process of the present invention.

Other objects and advantages will become apparent from the following description.

Generally speaking, the present invention contemplates a process for electroplating plastics and particularly ABS plastics wherein a plastic, having been caused to become electrically conductive at least on the surface, is electrolytically coated with a first deposit of nickel said nickel being of a character having a tensile strength less than about 105,000 pound per square inch (psi) e.g., about 88,000 to about 105,000 psi when deposited in layers of at least about 1 mil thick and is then further electrolytically coated with a second nickel deposit having the capability of levelling the surface to a surface roughness of less than about 4 micro inch (.mu. inch) and is then still further coated with a non-continuous electrodeposit of chromium which may be discontinuous or micro-cracked. In accordance with the invention the total thickness of the layers of nickel is about 0.9 to about 1.6 mils and the thicknesses of these layers are interrelated so that the ratio of thickness of the first layer to the thickness of the second layer is at least about 2.

For the purposes of this specification and claims the term ABS plastic includes acrylonitrile-butadiene styrene resins such as sold under the trademarks LUSTRAN (Monsanto), CYCOLAC (Marbon) and KRALASTIC (Uniroyal) as well as compositions and mixtures of such resins with stabilizers, fillers etc.

As a general rule, molded objects made of plating grade ABS plastic, after having been subjected to a preplating regimen involving etching, sensitizing and either electroless nickel or electroless copper deposition, have a surface roughness of about 2 .mu. inch. With material of this character it has been found that the aforementioned criteria of the process of the present invention can be achieved by initially electrodepositing thereon nickel from a low chloride Watts bath. Nickel from such a bath generally exhibits a tensile strength of less than about 105,000 psi when deposited in layers of sufficient thickness, e.g., at least about 1 mil, in order to minimize the surface effects on the tensile test.

In order to achieve bright chromium deposits and at the same time achieve the ratio of deposit thicknesses required by the present invention and still at the same time remain within a commercially practical plating thickness limit, it is necessary to employ for the second electrodeposition of nickel a plating bath which in the trade is termed a super levelling nickel plating bath. A fully satisfactory example of such a bath is a proprietary bath sold by M & T Chemical, Inc., in the U.S. under the trade designation Superlume II. Other baths can be formulated to provide the same results as indicated by a bath of fully disclosed composition made available to applicant and set forth hereinafter. For purposes of a good corrosion resistance of the complete plating system, the nickel deposited from the super-levelling nickel plating bath should contain about 800 to about 1,800 parts per million (ppm) of sulfur. Below 800 ppm of sulfur the internal stress of the nickel deposit will be too high. Above 1,800 ppm of sulfur the deposit will be too brittle. A sulfur content of about 1,200 ppm i.e., about 1,000 to about 1,400 ppm is highly advantageous.

The initial soft nickel layer deposited on a preplate treated plastic is advantageously deposited from a Watts nickel bath having a composition and operated under conditions within the ranges set forth in Table I.

TABLE I ______________________________________ Broad Ingredients Range Advantageous ______________________________________ Nickel Sulfate (hydrated) (gpl) 260-440 300 Nickel Chloride (hydrated) (gpl) 15-37 30 Boric Acid (gpl) 15-45 30 Conditions Temperature (.degree.C) 50-68 55 pH 2.5-4.5 3-4 Current Density (amp/ft.sup.2) 20-80 50 ______________________________________

This bath can be used with sulfur-depolarized nickel anodes or any other depolarized anode. If relatively inactive anode material is used, the chloride ion content should be on the high side. It was noted that bath purity is most important. A newly made up nickel plating bath in accordance with Table I must be treated with activated carbon to remove undesirable impurities. It was discovered that a newly formulated bath without carbon treatment electrodeposits nickel having a tensile strength at a thickness of 1.2 mils of 150,000 psi. After carbon treatment a similar nickel electrodeposit produced under the same conditions exhibited a tensile strength of 94,700 psi. It is also advantageous to operate a bath continuously, since continuous operation favors low strength and minimal internal stress on the nickel deposit. With regular use the internal stress of deposits made from a bath within the ranges set forth in Table I is in the range of 20,000 to 25,000 psi tensile. If the bath is allowed to lie idle however, the initial nickel deposit after placing the bath back into service will have an internal stress of perhaps 10,000 psi tensile greater. After sometime in operation however, the internal stress of the deposit drops back to its original level.

In order to give those skilled in the art a better understanding and appreciation of the invention the following examples of essentially all-nickel electrodeposits were made upon molded plaques of plating grade ABS plastic having gone through a preplate cycle consisting of the steps set forth in Table II.

Table II ______________________________________ PREPLATING CYCLE USED FOR ABS ______________________________________ 1. CLEANER 3 Minutes 135.degree.F Double Rinse 2. ETCHANT 6 Minutes 140.degree.F Double Rinse 3. NEUTRALIZER 1/2 Minute R. Temp. Double Rinse 4. CATALYST 2 Minutes R. Temp. Double Rinse 5. ACCELERATOR 1 Minute 115.degree.F Double Rinse 6. ELECTROLESS NICKEL 5 Minutes R. Temp. Double Rinse ______________________________________

Those skilled in the art will appreciate that a preplating cycle using electroless copper could be readily substituted for the cycle set forth in Table II.

EXAMPLE I

Preplate treated plastics of ABS plastic were plated with nickel to a thickness of about 0.94 mil from a 10 liter Watts nickel bath containing 66 grams per liter (gpl) of nickel as the sulfate, 9 gpl of chloride ion, 26 gpl of boric acid held at a temperature of about 126.degree.F and a pH of 4.0. The current density used was 50 asf. This Watts nickel deposit was followed by a bright nickel deposit of 0.31 mil deposited out of a proprietary bright nickel bath sold under the trade name Superlume II by M & T Chemical, Inc. which bath was held at a temperature of about 140.degree.F at a pH of about 4.5 with the plating conducted at a current density of 50 amperes per square foot (asf). The second nickel deposit was topped with a conventional chromium deposit 15 micro inch (.mu. inch) thick. The plated plaques were then subjected to a thermal cycling test comprising 185.degree.F for 1 hour, 70.degree.F 1 hour, -40.degree.F 1 hour, and 70.degree.F 1 hour with the cycle being repeated four times. The plaques routinely passed these tests without blistering or cracking.

EXAMPLE II

A second series of plaques were plated under the same conditions as set forth in Example I to provide a soft Watts nickel layer of 0.86 mil, a high activity bright nickel layer 0.29 mil thick (sulfur 1200 ppm), a third nickel layer 0.1 mil thick and containing fine, inert particles to produce micro discontinuous chromium and a top chromium layer of 15 .mu. inch. These plaques were subjected to a thermal cycle comprising 180.degree.F 2 hours, 72.degree.F 1 hour, -30.degree.F 2 hours, 72.degree.F 1 hour and a CASS Corrosion test of 16 hours, the full cycle including the corrosion test being repeated three times. The plaques as tested according to this regimen typically exhibit ASTM ratings of 10/7.5 as evaluated in accordance with the procedure recommended in ASTM B537-10 (American National Standard G53-38 -- 1972 approved Apr. 20, 1972). It is believed that in order to achieve fully satisfactory corrosion resistance it is highly advantageous to employ the chromium outer layer as micro discontinuous or microcracked chromium. The formation of such chromium layers is well known to those of ordinary skill in the art and is described in general in the text Nickel Plating, R. Brugger, Rob't Draper Ltd 1970 on page 238, as well as in U.S. Pat. No. 3,471,271 issued on Oct. 7, 1969 to Brown et al. A further observation has been made that reasonably good corrosion resistance can be obtained by depositing about 0.3 mil of bright acid copper between the Watts and the Bright nickel layer. The corrosion resistance of this combination is not that outstanding to justify the complications of the plating sequence.

Additional tests have shown that when using an all nickel system having a total thickness of 1.25 mils the increase in tensile strength of the nickel directly adjacent the pretreated plastic is highly detrimental. Results of these tests are shown in Table III.

TABLE III ______________________________________ Tensile Strength of Metal Plated Next to Plastic Defect Score (Total Thickness 1.25 Mils) Blistered Area (cm.sup.2) psi .times. 10.sup.-.sup.3 Crack Length (cm) ______________________________________ 100 0 100 to 130 2 - 6 130 to 160 6 - 12 200 complete failure ______________________________________

Results of still further tests with various combinations of plated metals on preplated ABS plastics are set forth in Table IV, the ASTM ratings set forth therein being made after three cycles of testing as set forth in Example II.

TABLE IV (A) ______________________________________ Bright Dur CASS TEST Cu** Ni Ni* Cr ASTM Rating ______________________________________ (after (Mil) (Mil) (Mil) ( inch) 3 cycles) 0.94 0.31 -- 10 5/3.5 0.94 0.31 0.10 10 4/4 TABLE IV (B) Watts Bright Dur CASS TEST Ni Ni Ni* Cr ASTM Rating (after (Mil) (Mil) (Mil) ( inch) 3 cycles) 0.94 0.31 -- 10 8/4 0.94 0.31 0.10 10 10/6.5 TABLE IV (C) Semibright Bright Dur CASS TEST Cu** Ni. Ni Ni Cr ASTM Rating (After (Mil) (Mil) (Mil) (Mil) ( inch) 3 cycles) 0.70 0.50 0.30 -- 10 3/1.3 0.70 0.50 0.30 0.10 10 10/6.5 ______________________________________ *A nickel deposit containing inert particles (see U.S. Patent No. 3,471,271). ** Plus initial strike of 0.1 mil. Watts nickel.

The results in Table IV comprising sections A and B clearly show the superiority of an all-nickel plating system over one including copper when the plating thicknesses are equal. Table IV (C) shows that excellent results can be attained with a copper, nickel, microcontinuous chromium system at the cost however of some 26 percent greater thickness of plating. The data shows however that without microdiscontinuity of the chromium the thicker copper-containing deposit was significantly inferior to the thinner all nickel deposit. Table IV shows that the ASTM rating based upon the thermal cycle corrosion test as set forth in Example II is relatively poor for the plating combinations specified in Table IVA compared to the advantageous results obtained in employing the present invention.

While the data has shown that a relatively thick copper, nickel microdiscontinuous chromium system can achieve results akin to those achieved by the thinner all nickel systems, further advantages can accrue using the all-nickel system. The all-nickel system requires two fewer baths which simplifies the plating operation and three fewer rinsing operations which cuts the volume of water that must be treated to meet pollution standards.

While the results set forth in Examples I and II were achieved on samples which owed their brilliant finishes at least in part to the use of commercially available, proprietary super-levelling nickel electroplating bath, equivalent results are attained using a super-levelling nickel bath of the composition and under the conditions set forth in Table V.

TABLE V ______________________________________ Nickel (sulfate hydrate) (gpl) 292 Nickel (chloride hydrate) (gpl) 39 H.sub.3 BO.sub.4 (gpl) 51 Butynediol (gpl) 0.6 Benzosulfamic Acid (gpl) 2.5 pH 4 Temp. (.degree.C) 54 C.D. (a.sf) 50 ______________________________________

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

Claims

I claim:

1. In the process of decorating plating plastics wherein the plastic is first caused to become electrically conductive at least on the surface and thereafter metal is electrodeposited thereupon, the improvement comprising initially electrodepositing on the electrically conductive plastic surface nickel having a tensile strength less than about 105,000 pounds per square inch, as measured in deposits at least about 1 mil thick, subsequently electrodepositing prior to a non-continuous outer layer of chromium a layer of sulfur-containing nickel having the capability of levelling the surface to a surface roughness of less than about 4 microinches, the sulfur content of said sulfur containing nickel being about 800 to about 1,800 parts per million and the ratio of thickness of said first mentioned nickel deposit to said second mentioned nickel deposit being at least about 2 whereby is produced a decorative plated plastic having good resistance to the deleterious effects of thermal cycling.

2. A process as in claim 1 wherein the electrodeposits except for the non-continuous chromium outer layer are nickel electrodeposits whereby is produced a decorative plated plastic having good resistance both to corrosion and to the deleterious effects of thermal cycling.

3. A process as in claim 1 wherein the plastic is an ABS plastic.

4. A process as in claim 1 wherein the first mentioned nickel layer has a tensile strength of about 88,000 to about 105,000 psi.

5. A process as in claim 1 wherein the initial nickel electrodeposit is deposited from a Watts bath.

6. A process as in claim 2 wherein the total thickness of the electrodeposited nickel layers is 0.9 to about 1.6 mils.

7. A process as in claim 1 wherein an electrodeposit of bright acid copper is interposed between the two nickel layers.

8. A process as in claim 1 wherein a thin nickel layer containing small inert particles is deposited immediately prior to the chromium layer to produce microdiscontinuities in the chromium layer.

9. A plated plastic having adjacent to the plastic a layer of nickel having a tensile strength, as measured on sample greater than 1 mil thick, less than about 105,000 pounds per square inch, having an outer layer of non-continuous decorative chromium and having a layer of super levelling nickel containing about 800 to about 1800 parts per million of sulfur lying between the outer chromium layer and the first mentioned layer of nickel, the total thickness of said nickel layers being about 0.9 to about 1.6 mils and the ratio of the thickness of said first mentioned nickel layer to the thickness of said second mentioned nickel layer being at least about 2.

10. A plated plastic as in claim 9 wherein the plastic is ABS plastic.