Historical Articles

June, 1952 issue of Plating

 

Clear Protective Coatings for Copper-Chromium Plate

Wayne R. Fuller
Director of Research, Grand Rapids Varnish Corporation, Grand Rapids, Mich.

OF THE PROBLEMS created by our program of military preparedness the one posed by the prohibition of the use of nickel for most civilian purposes was only a minor dislocation in terms of the National economy. To the electroplater, however, it was a major catastrophe. It meant that a chromium finish could no longer be produced by plating, consecutively, copper, nickel and chromium. And the electroplater knew that when chromium is plated directly on copper the resulting finish is sadly deficient in protective ability. The automobile manufacturer also knew this and was gravely concerned about the plating on the zinc-base die castings used so lavishly for the ornamentation of automobiles.

Among all of the types of plate, only nickel-chromium or copper-nickel-chromium combine to a satisfactory degree adequate protection and the appearance that the public prefers. The only apparent method of preserving the appearance of the chromium plate and at least partially compensating for the protection lost by the elimination of nickel undercoating was to apply a clear organic coating;

Up to that time organic coatings for use on chromium plate had been mainly enamel products, and their purpose had been decoration of such small parts as emblems, radiator ornaments and door handles. Service requirements had not been severe. The main problem had been to secure adhesion to the very smooth, chemically almost inert surface, and this had been solved satisfactorily by minor modification of conventional baking-enamel formulas based on alkydresin-amine formaldehyde-resin vehicles.

REQUIREMENTS
A clear coating for protection was an entirely different matter. The electroplater specified that’ this coating must: (13 bake at a temperature not exceeding 300° F (149° C), preferably 275° F (135° C), to avoid blistering of the plate on zinc-base diecastings, (2) have excellent adhesion, (3) have abrasion resistance of a high order, (4) show no discoloration or dulling of the bright finish, (5) impart good salt-fog resistance to copper-chromium plate, and (6) have satisfactory weathering properties. Also mentioned were resistance to water immersion, humidity, grease, and perspiration. Inasmuch as this sounded like a rather large order, coating manufacturers were quick to realize that it called for evaluation of all of the resins and resin combinations that might conceivably prove advantageous, with emphasis on the newer developments that had not been thoroughly tested in chromium coatings.

DEVELOPMENT WORK
The resins tested are of three general types, as follows:
A. Thermoplastic
1. Nitrocellulose
2. Vinyl copolymer
3. Methacrylate
B. Oxygen convertible
1. Phthalic alkyds
2. Fatty acid ester of Epon (epichlorhydrin bis phenol)
C, Heat convertible
1. Amino-formaldehyde

Phenol-formaldehyde resins were ruled out by their tendency to discolor.

In most cases each sub-group was represented by more than one specific resin, amino-formaldehyde resins by six. With one exception, each formula contained two or more resins. A majority of the formulas contained only two resins, which differed in type. Thirty separate formulas were included in one series of preliminary, or screening, tests by one coating manufacturer.

In this series of screening tests, the coatings were applied on copper-chromium plated test panels supplied by the Doehler-Jarvis Corporation and cleaned by it. The coatings were sprayed to a dry-film thickness of approximately 1 mil (25 ), and baked 20 minutes in a convection oven at an air temperature of 300° F (149° C). The films were tested only for hardness, toughness, and adhesion, with a view to elimination of all materials that were deficient in these properties. Only three materials showed promise: (1) a combination of alkyd and amino resin, (2) a combination of Epon ester and amino resin, (3) a combination of methacrylate resin and nitrocellulose. The latter was dropped from further consideration because of its low solids content at spraying consistency and the extremely thin dry film produced. Thus, two types of formulas remained for more complete evaluation.

In the above-mentioned screening tests the baking had been done on a single schedule, which had been chosen somewhat arbitrarily. At this point it was decided to determine the optimum baking schedule before conducting further evaluation of the materials. The alkyd-amino resin formula that had given best results was chosen for this purpose. It was necessary to establish some method and standard for determination of satisfactory conversion, or cure. By correlating salt-fog results with measurements of film hardness, it was found that a hardness of H by the pencil method was adequate for excellent performance. (H hardness means that the coating cannot be scratched with an H pencil but can be scratched with a 2H pencil.) The coating was sprayed on heavy test panels to a dry-film thickness of approximately 1 mil (25 µ) and baked in an electric convection oven at six different air temperatures ranging from 275 to 400° F (135 to 204° C) until a hardness of H was obtained. The resulting optimum baking schedules are found in Fig. 1. Inasmuch as a baking time of 40 minutes is excessive, and temperatures above 300° F (149° C) are likely to cause some blistering of plate on zinc-base alloy, it was decided to adopt a schedule of 30 minutes at 300° F (149° C) for the remainder of the testing program.

The two formulas that had survived the screening tests were then subjected to the following tests
1. Water immersion at 100° F (38° C) for a minimum of 168 hours, with the films being scored to the plate
2. Salt fog at 95° F (35° C), using a 5 per cent solution, for a minimum of 168 hours. The films were again scored to the plate
3. Weather-O-Meter test for 250 hours
4. Exposure to intense ultra-violet light for 24 hours
5. Exposure to 50 per cent acetic acid for 4 hours, to obtain an indication of resistance to perspiration
6. Impact test for adhesion
7. Abrasion resistance by the Tabor Abraser, using CS 10 wheels and 1000-g load

In the foregoing tests the alkyd-amino resin formula proved definitely superior to the Epon ester-amino resin formula in color retention under ultra-violet light, resistance to abrasion, and resistance to salt fog, and equal in all other properties. It met the’ most severe specifications of automobile manufacturers. Against a salt-fog requirement of 168 hours, it showed no creepage from the score line or other failure after 250 hours. In a later test it was subjected to five cycles of the following:

(a) 8 hours at 175° F (79° C)
(b) 8 hours at -40° F ( -40° C)
(c) 8 hours in Weather-O-Meter

After the five cycles, the finish was subjected to salt fog for 250 hours, and there was no failure.

This description of how coating manufacturers solve a problem may have created the impression that the final answer is simply to mix an alkyd resin and an amino resin. Any such impression needs be corrected. Alkyd resins come in a very wide range of compositions and properties, and it is important to use the particular alkyd resin or mixture of alkyds that is best for a clear coating over chromium. In like manner, the amino resin requires careful selection for best performance in the particular formula. Finally, the proportion of alkyd resin to amino resin must be closely adjusted to give optimum results.

PERFORMANCE FACTORS
It is safe to say that no electroplater has been able to obtain consistently the best performance that is inherent in the coatings and reflected by the laboratory test results that have been described. The use of these coatings certainly raises many problems, and the results are governed in large measure by the attention and understanding which are applied to these problems.

Treatment of the Chromium
The first factor that calls for consideration is the cleaning and conditioning of the parts before the clear coating is applied. Occasionally chromium plate requires buffing, and buffing compounds may contain wax. A recent analysis of three kinds of buffing sticks that were represented as wax-free disclosed that all three contained wax. An alkali wash at 185° F (85° C) is reported to be effective. The temperature is important; it should be higher than the melting point of the wax. Following the alkali wash and a cold-water rinse to remove excess alkali, the prevailing practice is to rinse with very dilute chromic acid and finally with ‘deionized water. The clear coating should be applied as soon as feasible after the prepaint treatment, preferably on the same day, as contamination from the air may reduce the adhesion and affect other properties.

Coating Thickness
Next in order is the application of the coating. The main point here is that it should be applied uniformly and in adequate film thickness. As a rule dipping is eliminated by the shape of the parts, leaving spraying as the only feasible method of application. The difficulty of properly controlling the film thickness is aggravated by the variable shapes of the parts and by the transparent nature of the coating, which makes it difficult for a hand-spray operator to judge the thickness being applied. Parts from production lines have shown a dry film thickness as low as 0.2 mil (5 µ) and as high as 3 mils (76 µ). The only means for proper control seems to be automatic spraying. Uniformity of film thickness over the entire surface of the individual parts can be improved further by electrostatic spraying with suitable equipment. Another advantage of electrostatic spraying is that it offers possibilities of reducing the loss by overspray, which at best is large, by 50 percent or more. Both the automatic sprayer and the electrostatic equipment must be carefully chosen and engineered to fit the job. One automobile manufacturer specifies a dry film thickness of 1± 0.1 mil (25 ± 2.5 µ). This is a fine goal, but the electroplater will be doing well if he maintains a range of 0.7 to 1.2 mil (18 µ to 30), which will insure good performance. There is little chance of controlling the film thickness without frequent determinations, and this calls for both a wet-film thickness gage and a dry-film thickness gage. These instruments will pay for themselves many times over by reduction in the number of rejects.

Baking Cycle
Care n metal preparation and control of film thickness will both be wasted unless there is equal attention to the baking cycle. The coatings that have proved best for protecting chromium plate are at least in part of the heat-convertible type. This means that their good properties are not developed unless they are baked in’ a manner that will accomplish the conversion. A partially cured coating is likely to exhibit only a small fraction of the protection of the same coating when adequately cured. It is obvious that a thin coating on metal will be substantially the same temperature as the metal and, therefore, that the metal temperature controls the rate of cure. When heating is by convection there is a considerable lag between the air temperature and the metal temperature, the amount of lag depending on the thickness of parts and the rate of air circulation. When heating is by radiation instead of by convection, the metal temperature normally rises faster than the air temperature. Some of the newer ovens combine the convection and radiation principles of heat transfer in various degrees. These few facts should make it obvious that it is impossible to specify the cure in terms of air temperature and length of time. It could be specified as a given metal temperature for ;a given time period, but the measurement of metal temperature with a thermocouple is hardly feasible for daily control of production. This brings us back to the point that our real object is a certain degree of cure, which can be determined by measuring the hardness of the film, most feasibly by the pencil method. It has been found that a hardness of H assures good performance of typical clear coatings on chromium. When a convection oven is used it is only necessary (1) to determine the air temperature and time required in the particular oven to produce a minimum hardness of H and (2) to maintain these conditions. When baking is by infra-red the arrangement of lamps and the conveyor speed should be adjusted so that a hardness of H be obtained. In either case the hardness should be checked at frequent intervals. As a further check on conveyorized convection ovens, a recording thermometer may be sent through at intervals. A typical satisfactory schedule for convection ovens is 30 minutes at 300° F (149° C). This temperature is safe for zinc-base diecastings provided the castings are dense and sound and the plating process has been carried out properly. If these conditions have not been met, it may be necessary to drop the temperature to 275° F (135° C) in order to avoid blistering of the plate.

STRIPPING
In even the best regulated plants, a small proportion of the finished work will be rejected. This creates the problem of stripping. When fully heat cured the better clear coatings for chromium now in use are remarkably resistant to strippers. In fact, when these coatings made their advent, none of the strippers then available were satisfactory for the purpose. After months of work and several field trials some stripper manufacturers succeeded in developing effective materials. It is now possible to strip in as little as 5 minutes by immersion in a stripper solution at 200° F (93° C). When strippers are evaluated consideration should be given to the physical condition in the stripping bath of the coating that has been removed. Some strippers leave the coating as small particles that tend to cling to the stripped parts; others cause the coating to form into balls and permit the parts to come out of the bath clean.

While creating a stripper problem, the use of clear coatings for chromium solved another problem, that of rack coatings. During the spraying of the parts on a rack, the rack is also coated completely. This coating when baked withstands both the cleaning baths and the plating baths. Normally the racks receive a new coating during each complete cycle, and no other rack coating is required. Racks that have been in production for many months have required no attention to the coating. The flaking or powdering in the cleaning cycle has been sufficient to prevent excessive build up.

COATING REPAIR
By comparison with straight metallic coatings, the greatness weakness of organic coatings is that they are much more susceptible to becoming scraped off by the accidental bumping and rubbing that may occur in service. When this happens the local damage needs to be repaired. It is obvious that the coating for this purpose should be suitable for application by brushing or spraying and should not require baking. Coatings of this type have been developed and are now available through automotive retail outlets. In nature they are synthetic varnishes. In performance they fall considerably short of the baked coatings, as would be expected of the air-drying or oxygen-convertible type.

COATINGS ON ZINC
The baked coatings that were developed- for chromium have also been tested on buffed zinc-base diecastings and on zinc plated zinc-base diecastings. It is well known’ that zinc is chemically more reactive than chromium, and hence its need for protection from the atmosphere is even greater. On both buffed zinc alloy and zinc plate the coatings showed good adhesion and good resistance to salt spray. It is understood that zinc requires a different prepaint conditioning than chromium. The time between conditioning and coating should be as short as possible, preferably no longer than two hours, if good adhesion is to be obtained. Should the use of chromium coatings be prohibited for most civilian purposes, zinc with baked organic coatings can take their place at least in some cases.

Much of the credit for the clear coatings that are being used today on chromium goes to the electroplaters who gave unstinted cooperation to its development, along with constant and vigorous prodding. The work of the coating chemist is never finished, although he cannot claim that he never dies. By continued cooperation the plater can look forward with assurance to still better coatings in the future.

ACKNOWLEDGMENT
The author expresses appreciation of helpful information and criticisms from Dennis 17. Roelofs, of the Grand Rapids Varnish Corporation, and Lyman B. Sperry and Edward W. Gross, of the Doehler-Jarvis corporation.