INDUSTRIAL A N D ENGINEERING CHEMISTRY
February, 1941
clock which gives the number of hours the circuit was stopped.
This instrument measures the actual number of hours during which the panels are wet with dew, both normally and on the dew cabinet. A second series of tests was then started. One panel was exposed on the regular racks 24 hours per day. A duplicate panel was exposed to the weather but was also subjected to excess dew conditions. A third panel of the same lacquer was exposed on a box where dew could not form. This was accomplished by raising the temperature of the panels slightly. Each panel was allowed to remain on its respective test until such a time as the lacquer had reached a condition of chalk which was considered objectionable. In other words, all three panels had reached the same degree of chalking when removed from the racks, regardless of the number of days required. The various conditions that prevailed for each panel are shown below: Exposure Dags Dew box panel Regular exposure N o dew
8
23 30
Heat Content B . t. u. 8,874 24,069 32,307
W. J. McCORTNEY Chrysler Corporation, Detroit, Mich.
N
Rain Inches
1.60
4.84
5.33
Dew Hours 90.0 10.5 None
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T o determine the effect of rainfall as compared to dew, a panel on regular exposure was subjected to showers of water four times a day. This panel did not chalk so rapidly as the same material on the dew box, and the chalking, when i t did develop, was different in color from that on the other panels. Panels have been exposed from noon to midnight, from midnight to noon, from 6 A. M. to 6 P. M., and from 6 P. M. to 6 A. M.; in every case panels that received dew followed by sunshine showed more rapid failure than any, others. It is realized that the evidence collected thus far is not enough to warrant any definite conclusions and that much work remains to be done. However, the data obtained are presented in the hope that they may present a new method of attack on the problem of producing more durable paint materials.
Literature Cited (1) Gardner, H. A., "Physical and Chemical Examination of Paints, Varnishes, Laoquers and Colors", 6th ed., pp. 343,346(1933). (2) Mougey, H.C., Div. Paint and Varnish Chem.. Am. Chem. SOC., Washington, D. C., 1924.
There is no decorative plastic that even approaches engineering standards for a perfect material. As a result the automotive industry uses most of the commercially available plastics for the various parts of the automobile. A detailed description of the good and bad qualities of these plastics is given, along with improvements that must come in the future for greater utilization of plastics in the automobile.
0 SINGLE decorative plastic even approaches engineer-
ing standards for satisfactory use in the automobile. However, most of the characteristics desired can be found separately in the many classes of plastic now commercially available. Many of these plastics have a number of the desirable characteristics, but no single plastic has all. Until the universal plastic is developed, the engineer must be careful to select the one that has the particular qualities necessary for the application. To be of any value, this paper must name the materials now used in the automotive industry and discuss the good and bad characteristics of each. We use practically all of the plastics now in general production, and are interested in other materials not yet produced on a commercial basis. To understand the conditions these plastics must withstand, i t should be recalled that automobiles are used in ambient temperatures below -40" F. Also, the instrument panels and plastic parts in the interior of the car heat up to approximately 60" E'. over out-of-door temperatures. This condition is most critical when a car is left standing in tlie sun with tlie wiiidows closed. Exterior car temperatures on the flatter surfaces may be 70" F. in excess of outdoor temperatures, but projecting parts not normal to the rays of the sun are considerably cooler. When synthetic paints that require baking are used on automobiles, plastics often have to go through touch-up orens that may reach 190" to 200" F. Many of the plastics are molded over metal inserts and
fastened to the metal in such a way that shrinkage and distortion subject the piece to excessive strain. We have found that it is almost impossible to specify plastics by scientific means. Instead, we endeavor to specify the conditions that plastics must fulfill in our laboratories and in mdst of our testing on completed parts. I n comparing new plastics it is customary to mold them into parts whose shrinkage and warping characteristics are already known. Figure 1 shows various plastics molded into the bezel of the 1940 Chrysler instrument cluster. Accurate measurements were taken on the various dimensions before and after subjecting the pieces to 175" F. for 24 hours, -10" for 24 hours, and then 175" for 24 hours a t 70 per cent relative humidity. It is readily seen that these measurements of some parts were of little significance after such a cycle. Figure 2 shows 1940 Chrysler glove-box doors that were subjected to the same treatment. This demonstrates the shrinkage problems involved when metal inserts are used. The plastics in this case were molded to sheet metal stampings.
Decorative Plastics in Contact with Lacquer The following is a typical example of a specification on plastics: 1. This specification covers decorative plastics for interior use in locations where they will be in contact with, or less than l/s inch from, lacquer coated parts.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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tainable in a good range of colors with the exception of water-white, and is usually handled by injection molding. It will not withstand outdoor exposure, high humidity, or excessive heat and cold, especially when molded over large metal inserts. The tensile strength of cellulose acetate is only moderate, but it is extremely tough and nonbrittle. It cannot be used in large sections without other support. Most acetates should not be used less than l/8 inch away from painted sheet metal. I n such locations the various slightly volatile plasticizers condense on the painted metal, softening paints and lacauers with which thev come in contact. For' this reason we dd not use cellulose FIGURE 1. EFFECT OF TEXPERATURE TREATMENTS ON VARIOUS PLASTICS acetate in such places but confine its use MOLDED INTO THE BEZEL OF .4x INSTRUMER'T CLUSTER more or less to handles, knobs, and hardware trim. 2. Plastic materials covered by this specification shall not Cellulose acetobutyrate has largely replaced the acesoften, discolor, or otherwise damage the finish of nitrocellulose tates on such parts as are in close proximity to lacquered lacquered parts with which they are in close contact, A quick and painted surfaces. The different plasticizers used for test for this property may be made by placing a sample of the plastic material on a lacquered panel in an oven for 24 hours at acetobutyrate have no effect on paints and lacquers. Cellu1750 F. lose acetobutyrate is also more resistant t o shrinkage in heat 3. The colors of all plastics shall be referred to by Chrysler clue to slight loss of plasticizer, and in the cold it is tough plastic color numbers. Approved color chips for these colors enough at the lower temperatures to resist cracking when apmay be obtained from the Chrysler Plastics Laboratory. All samples of plastic materials submitted t o match a particular plied over large inserts. Cellulose acetobutyrate is also a Chrysler color must be approved through the Plastics Laboragood weathering plastic and can readily be used on parts tory. Such an approval must also be secured by the molder for exposed to the weather. each individual part before a production quantity is made. Methyl methacrylate resins are used particularly where 4. Materials covered by this specification shall have a minimum hardness reading of 70 at room temperature and 60 at water-white plastic is desired for an edge-lighting effect. 158" F. on the Shore Needlepoint type D durometer in the Cast methacrylates will withstand any temperatures enChrysler laboratory. countered by the automobile body. Compression-molding 5. All hished parts must withstand a hot and cold test, where compounds have been improved so that they will withstand they are subjected to a temperature of -40' F. for 24 hours, 175" dry heat for 24 hours, another 24 hours at -4O", and finally 24 hours at 175' with 65-70 per cent relative humidity. During t,hiscycle, the piece must not shrink in excess of 2 per cent, warp, discolor, or crack under the load or stress required in product.ion usage. The surface shall not crane, become wavy, tacky, or otherwise unsightly. 6. All finished parts must have the flash smoothly buffed off and the visible surfaces highly polished. 7. The flow hardness of the material to be used on any particular part is not specified. Each molder must, however, report to the Chrysler Engineering Laboratories the trade name, composition, and flow hardness of the particular material finally approved for each part. Subsequent changes must also be reported. 8. When finished plastic parts are exposed to ultraviolet light in the Atlas color fadometer at a distance of 10 inches from the source of light for 100 hours, they shall not show fading or change in color greater than that of an approved color sample. DOORSAFTER TEXPERATURE TREaTMENTS FIGURE2. GLOVE-BOX 9. Parts made from these plastic materials shall be so processed, when molded by either the compression or injection method, that no flow lines or welding seams are visible on the completed most of the conditions. Injection-molded methacrylates parts. however, are only fair for heat resistance and should be used 10. I t shall be permissible to use only the following approved Only With the greatest Of care. It can be said?however, that plastic molding materials for parts covered by this speclficathese two are improving rapidly and within a short time tion:'** 11. Parts covered by this specification shall be purchased should be satisfactory for many more uses in the injectionmolded form. only from the molders or fabricators listed below:*** 1verealize that this is only the beginning of plastics control Styrene resins hare many desirable characteristics in that they are stiff and of higher heat resistance than most of the and that greater improvements must be made. thermoplastic materials. Although somewhat more brittle than thermoplastics in general, we can still find great use in Plastics in Use the automotive industry for a strong, clear, transparent or The decorative plastic now enjoying the greatest use is light colored plastic. Styrene resins are used now only in cellulose acetate. It has average resistance to heat, is obplaces where high heat resistance is required and where the
February, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
plastic is shielded from any possible contact with gasoline or cleaning fluids-for example, instrument pointers and dials. Hundrpds of safety solvents are sold over the country and all of them will change styrene polymers into a gummy mass on contact. Gasoline has less effect. We have been assured that there is a good chance that this weakness will be overcome, and if such is the case, styrene can become a great competitor for the clear or light colored plastics. Molded phenolics are used in places where brittleness is not too great a factor and where darker colors or painted parts can be successfully used. Phenolics are also used as inserts for the brighter thermoplastics. Urea-formaldehyde plastics were probably the first light colored plastics to be employed in the automotive industry, but were originally discarded because they could not be injection-molded and were not available in the depth of color that we now desire. These resins are now available in a good
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range of colors and are only awaiting new methods for economically molding thermosetting plastics. Cellulose nitrate, the old timer of thermoplastics, still finds important usage in the automotive industry. Its heat resistance is comparatively good and i t is available in an excellent range of colors. It is used in instrument and clock dials, and until recently we were careful not to use it in places where i t might come in contact with a careless match or cigaret. Only quite recently have nitrate samples been flameproofed to such an extent that they can be used almost anywhere on the interior of the automobile. However, they still exhibit extreme dimensional changes with moisture and heat. Thus, the engineer should be thoroughly familiar with the characteristics of the plastic material before recommending its use on any definite part of an automobile. Until such time as a perfect plastic is developed, i t will be necessary to use a variety of plastics in the automotive industry.
R. C. KRUEGER, L. W. SCHAEFER, AND J. S . LONG Jonas-Dabney Company, Louisville, Ky.
Pimpling results where adhesion, usually of primer to metal, is overcome by the resultant of the swelling forces induced by absorption of water by the three-coat system. Pimpling is often localized to areas or even tiny spots where adhesion is poor or to areas where the merging of the three coats (primer, surfacer, and enamel) produces some local condition or irregularity. Pimpling is a result depending upon primary factors-permeability, swelling, and adhesion. Each of these depends on variablese. g., ad$esion on the nature of the surface and its preparation, and nature of primer vehicle. Permeability and swelling depend on film thickness, composition of material, both vehicle and pigment, with special emphasis on particle size, pigment volume concentration, temperature of baking, final film structure, etc. Tables show the influence of a number of such factors.
EARCH of the literature failed to reveal any data on pimpling of baked enamel systems. Some papers on blistering of house paint Nms were found, and the primary underlying factors of permeability and swelling are covered. Several articles are especially pertinent (1, 8, 4-7). This paper deals primarily with variables in the design of surfacers in baked enamel systems, especially automotive types. Reference is made, however, to other variables, such as the importance of preparation of surface to receive the primer and to variables in the primer and enamel. The whole matter of pimpling of these systems and the design of systems to minimize pimpling involve a large number of interrelated variables. A few tables are given to show the influence of the factors when vaned one at a time. It should be noted in the design of a surfacer or primer that a change in one variable or factor will involve changes in others or in the system as a whole.
S
The design of a surfacer, primer, or enamel coat and the resulting system requires consideration of the large number of interdependent variables. In general, fine pigment particle size and fine grinding are important in primers and surfacers. The advantages of oleoresinous vehicles seem to outweigh their disadvantages to a greater extent than do the advantages of alkyds outweigh their disadvantages. Phenol-modified alkyds combine some features of each. What might in general be called “film structure” is an important variable. The pigment, through its particle size, directly affects the film structure. This covers the general idea of relatively fine pigment particles and preferably uniformly fine ones strongly attracted to and thoroughly wet by the vehicle so that a structure is produced having strong cohesive forces.
Systematic variation of the interrelated factors has enabled us to draw certain conclusions and t o present a theoretical picture of the mechanism of pimpling. We found such a guide helpful in showing how to compensate or adjust for any variables entering the picture from some consideration encountered in practice on the line. Despite large quantities of data already in hand, we feel that much additional work is warranted to afford security in this important field. The studies along this line are being continued in our laboratories. We have brought together measurements on the permesbility, swelling, and pimpling of the films made from the same coating materials a t the same time under as controlled conditions as practicable, and studied the resultant data for correlations.
Materials and Methods Primers, surfacers, and enamels were made up from commercial pigments and vehicles, including both oleoresinous
