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INDUSTRIAL A N D ENGINEERING CHEMISTRY
United States Bureau of Standards during the World War under the guidance of A. V. Bleininger. The best results were obtained with a mixture containing about 10 per cent magnesium oxide, 27 per cent alumina, and 63 per cent silica. This was fired a t about 2600" F. (1427" C.) and pulverized. It was then intimately mixed with pure clays and pulverized artificial mullite. The difficulty from variable firing treatment was, however, B barrier to commercial success and the continuous tunnel kiln was used to correct this difficulty, as it has a relatively small cross-sectional area and can be maintained at a much more uniform temperature than the larger peiiodic kiln. Furthermore, the ware passes through the kiln a t a definite rate and a practically definite heat treatment is assured for all wares passing through the kiln. The product is our present-day spark plug which is so successfully meeting the demands of the automotive engineer. Requirements of Ideal Spark-Plug Insulator The successful spark-plug insulator must have satisfactory electrical insulating properties a t the temperatures at which it operates. It must withstand sudden temperature change, because a cold motor must reach its temperature of maximum efficiency in a minimum time. It must have a low coefficient of expansion, because it extends from the hottest part of the motor to the outside, where it is scarcely above atmospheric temperature. It must withstand a reasonable amount of mechanical shock because it is likely to receive rather rough treatment in installation. Probably as many sparkplug porcelains are broken by careless handling while being installed in motors as by any other accident to which they are exposed. All these demands point toward a dense conglomerate as the ideal insulator, but the conglomerate must be designed and developed with due consideration for the following: (1) The raw mixture must be capable of being molded into the deslred shape without developing internal strains. ( 2 ) It must dry without sufficient shrinkage to cause sepa-
Yol. 19, KO. 10
ration of the plastic portions from the faces of the non-plastic portions which do not shrink. (3) I t must mature a t a uniform rate throughout, without developing segregation or causing the glassy constituents to sweat toward the surface, causing a structural difference between the inner and the outer portions of the insulator, (4) I t must be coated with a glaze that will mature a t the maturing temperature of the body of the insulator and form a perfectly smooth surface free from microscopic pits or craters which can harbor carbon development and cause dirty plugs. This glaze must be in complete physical harmony with the body of the insulator a t all temperatures, and not craze even though subjected to abrupt temperature shock. (5) The various constituents of the conglomerate mass constituting the body of the insulator must be in complete harmony after it has been fired. hTo strain can exist between the glassy matrix and the crystalline material with which it is loaded. (6) A perfect bond must exist between matrix and crystallite and there can be no cavities of any sort if the porcelain is t o possess its maximum value. The amount of glassy matrix must be just sufficient t o enclose and bond the crystalline material and the dielectric strength of the matrix must be as high as possible. The distribution of crystallite in the matrix must be uniform, as any dielectric strength possessed by the mass depends on the dielectric strength of the glassy portion, which is the only constituent that is continuous. Electrical resistivity of such materials is roughly proportional to the distance and, assuming that the crystallite has a higher dielectric value than the glass bond, the increased length of the path of the electric current due to the introduction of the needle crystals must raise the dielectric strength of the insulator proportionately. This is only true if no electrical charge develops on the surface of the crystals, hence the necessity for perfect union between matrix and crystallite.
Finally, we have developed in the United States with the aid of government bureaus our present spark-plug insulator, which when ideally produced is undoubtedly superior to any other in the world. If a change in service demands develops, further development in spark-plug insulation may be asked. Much research with materials other than those here mentioned has been conducted and new developments are always possible. There is little danger that the field of sparkplug insulation will ever again constitute a barrier to progress in automotive engineering.
The Automotive Storage Battery By W. L. Reinhardt WILLARD STORAGE BATTERYCOMPANY, CLRVELAND, OHIO
T
HE lead-acid storage battery had its beginning in 1859 with the pioneer experiments of Plant6. This original Plant6 battery consisted of sheets of pure lead which were electrolyzed in dilute sulfuric acid, the surface of the anode being converted into lead peroxide and that of the cathode to finely divided metallic lead. The development of the industry was retarded for several years by the slow and expensive methods available for forming Plant6 plates. I n order to secure satisfactory capacity it was necessary to charge and discharge the battery a large number of times, and it often required as long as two years to produce a highcapacity storage battery. At this time the only available method for charging batteries was by means of the primary cell. The next important advance in the industry was the development of the paste plate battery in 1881. This differed from the Plant6 battery in that the active materials were formed from lead oxides, the oxides having been applied to a framework or grid of lead-antimony alloy in the form of a paste, which was later converted into the active materials electrolytically. This paste plate battery was an improvement over the Plant6 battery in that it gave much higher
ampere-hour capacity per unit weight and could be manu'factured in less time. At about this time the industry was given impetus by the invention of the electric dynamo, which furnished a more economical and satisfactory means of charging the batteries. By 1900 storage batteries were finding a variety of uses in the industry, such as telephone service, electric vehicles, railwaytrain lighting, and stand-by service in power stations. Possibly the greatest development in the storage battery industry has been the application of the storage battery to starting and lighting service on the automobile. The first commercial applications of starting and lighting service were made in 1911. From this time the growth has been rapid, until there are now approximately twenty million automobiles in this country with electrical starting and lighting equipment. Construct ion In the manufacture of these twenty million batteries the following materials were required : approximately 350 million pounds of lead, 28 million pounds of antimony, 140 million pounds of container material, 165 million pounds of sulfuric acid electrolyte, 200 million square feet of insulator or sepa-
October, 1927
INDUSTRIAL AND ENGINEERIWG CHEMISTRY
1125
is possibly the most common and satisfactory one for control work. This determination is purely an empirical one and, therefore, applicable only in comparing oxides made by a given method of manufacture. This method measures the weight of a given volume of material, this weight per unit volume being dependent upon particle size. Acid absorption measurements-that is, the amount of sulfuric acid absorbed by a definite weight of lead oxidehave found a use in measuring the particle size indirectly. In other words, the amount of acid absorbed is dependent on the available surface exposed, which is inversely proportional to the particle size. The effects of temperature are extremely critical upon the results of this method, requiring carefully standardized conditions and procedure. As a means of investigating this material still further, various classification methods have been employed, separating the oxides in portions of various sized particles and making microscopic examination of the classified portions. This method has been of considerable use in determining the process by which the oxide can be used to the best advantage. None of these methods alone gives the required information, Adaptation to Varying Conditions but by a combination of them it is possible to determine the Previous to the application of the storage battery to auto- value of an oxide for use in storage batteries. The earlier mobile starting and lighting, the service required was usually practice in the industry was to build a process by the trialat low discharge rates and in most cases a t uniform and and-error method for a particular oxide, then by rigid controls moderate temperatures. With its application to the auto- in the manufacture of the oxides to keep it as near the original mobile the battery was called upon to function at high dis- material as possible. The tendency a t present, however, is charge rates and under practically all temperature conditions much the opposite, in that after the processes have been from 0" to 120" F. In addition to the variety of discharge established the oxide is secured which will be suitable for that rates it was required to stand a practically continuous charging particular method of manufacture. It is now possible to varying with the car speed These demands were met by secure from more than one oxide manufacturer a material the chemist, who supplied the improvements which enabled which will be suitable for a given process. There is still the battery to stand up under these new conditions. work to be done in obtaining more satisfactory methods of Standardization of Materials determining the size and shape of oxide particles. LEAD-ANTIMONY ALLOY-The material ranking next to I n the storage battery industry, as in many others, the most valuable contributions of the chemist have been the lead oxides in importance in the manufacture of storage standardization of raw materials and the simplification of batteries is the lead-antimony alloy used in making the grid. factory processes. The establishment and maintenance of a This alloy usually contains 6 to 10 per cent antimony. Early standard of purity for all materials used in the storage battery in the industry much variation existed in the casting qualities has been absolutely essential. We must have from day to of lead alloy used. I n order to produce uniformly satisday, year in and year out, lead oxides of definite physical factory grids it was necessary to make a thorough investiproperties and of the highest purity obtainable. We must gation of the physical and chemical properties of these also have lead-antimony alloy, pig lead, sulfuric acid, sepa- alloys. I n deciding upon the alloy to use two important rators, sealing compound, and containers of the very highest items were to be considered-the mechanical strength and the resistance to corrosion. The positive plate, being the standard in quality. LEAD OxIms-Probably the most important materials anode when the battery is charging, is subjected to extremely used in the manufacture of storage batteries are the lead severe corrosion due .to electrolytic oxidation. As starting oxides, which are converted into the active materials of the and lighting batteries are continually overcharged, it is plates. It is quite possible that these oxides have received essential that these grids withstand the corrosion better than more attention than any other material used in storage bat- batteries in other types of service. This investigation of teries. First the materials must be free from impurities in lead-antimony alloy has led to the selection of the proper order to insure the battery against self discharge when not in composition for withstanding this chemical corrosion, and use. We know that some impurities may be tolerated up to for supplying the mechanical strength to resist buckling, 0.1 per cent, while some others, such as platinum and man- and which proves most suitable for molding into grids. ganese, are not permissible even in the smallest traces. After determining the proper composition of lead-antimony Some impurities are not detrimental when present alone, but alloy, it was necessary to limit the amount of allowable irnare troublesome in combination with others. purities. As a large percentage of the alloy is a t present Upon the uniformity of particle size and shape depend, to a made from reclaimed material, it is always necessary to great extent, the battery capacity and life. It is known that exercise a rigid control over these impurities. Such imputhe finer oxides give greater capacity and shorter life, whereas rities as tin, copper, zinc, bismuth, nickel, silver, iron, and coarser oxides give less capacity and longer life. Whether arsenic affect the resistance to corrosion, the self discharge of these oxides are composed of round particles or flat crystals the battery, and the molding properties of the alloy. All determines the characteristics of the finished battery plates, impurities do not adversely affect the casting properties of the as well as the processes in which the oxide is used. To alloy. Small amounts of tin or arsenic may be desirable, insure this uniformity the chemist has devoted considerable while zinc, on the other hand, cannot be tolerated even in time to the methods of determining both directly and in- very small quantities. In the control of these impurities directly this particle size. Several methods have been it is necessary to sample and analyze every shipment of suggested and used, of which the apparent density method material received. rator material, and 7 million pounds of asphalt sealing compound. The automotive storage battery today consists usually of three or six cells connectkd in series, which will furnish 6 or 12 volts, respectively, assembled in a container with the required number of compartments. These containers must be made of acid-resisting, non-conductive material, generally of hard rubber or asphalt composition. Each cell contains several plates, varying in number from five to twenty-five, assembled in two units called the positive and negative groups. I n these groups the plates are welded to a lead connecting strap much in the form of a comb, so that by combining the two groups a positive and negative plate alternate, separated by an insulator. This assembly is called an element. This element then inserted in a cell container filled with electrolyte which is sulfuric acid of approximately 1.270 to 1.300 specific gravity for starting and lighting batteries, connected in series by a lead cell connector with two or more similar cells, forms our present automotive storage battery.
1126
INDUSTRIAL A N D ENGINEERING CHEMISTRY Manufacturing Process
Not only in the chemical examination of the raw material is the storage battery industry dependent upon the chemist, but in every step of its manufacture is his influence felt. MIXINGPASTE-one of the most important operations in the manufacture of the storage battery is mixing the paste which forms the active material of the plate. It is generally known that a soft paste produces a high-capacity plate with correspondingly short life, while a stiff paste produces a plate of lower capacity and longer life. Early in the industry this mixing operation was carried out in small batches by hand, consequently with considerable variation between different operators. Furthermore, the type of mixes used was such that they varied with the humidity and temperature of the atmosphere, so that it was practically impossible to obtain a paste of uniform consistency throughout the day’s production. With the application of machine mixing and improvement in type of mixes used, it has been possible to increase the size of the mix and by controlling the materials and temperatures it is possible to produce repeatedly a storage battery paste of the same consistency. In carrying out the investigations necessary to establish the proper consistency and proper type of mix, a large amount of work was required, which included capacity and life tests on the finished battery and thorough investigation of the oxides in regard to particle size and impurities. PREPARATION FOR INITIAL FORMATION--After the plate has been pasted, the next step in its manufacture is preparing it for its initial formation. This formation is the first conversion of this oxide paste into the active material of a battery, sponge lead and lead peroxide for the negative and positive plates, respectively. This step may be carried out by any one of many methods. For example, the plates may be formed wet, or they may be hardened by immersion in dilute sulfuric acid and then dried, or it is possible to dry them directly without hardening in acid. In any method chemical control of temperature, humidity, paste consistency, etc., is absolutely essential for obtaining a satisfactory finished product. DRYING-In the earlier processes the drying operations were given very little consideration. In some cases drying was done a t ordinary room temperature with no attention to humidity control. As a result the product was never uniform. This system has been supplanted by modern methods in which the humidity and temperature are very carefully controlled and which insure a uniform plate under the most unfavorable weather conditions. After this formation the plates are practically fully charged, and wet. The positive plate may readily be dried in air, retaining a large portion of its charge. The negative plate, however, when dried in air, readily undergoes oxidation, the sponge lead being converted back to lead oxide, thereby losing a large part of its charge. This meant in the early days that before a battery could be used it must be given an initial charge, this charge requiring as much time as 40 to 90 hours. In order to preserve the charge and eliminate the inconvenience of getting a battery ready for service, a method has been devised for drying the negative plates without oxidation, thus preserving the sponge lead in its active condition. It is practicable to use this dry and charged plate only where satisfactory dry separators are available. SEPARATORS-The storage battery separators used in starting and lighting batteries may be divided into three classes-wood, wood with perforated sheet of hard rubber, and threaded rubber insulation. The most desirable properties of a separator are porosity and sufficient mechanical strength to withstand the tendency of the plates to buckle. The high porosity is a desirable
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property to insure proper voltage for cranking the engine and the strength to increase the life of the battery. The earlier wood separators were usually made from wood such as poplar and bass. These s6parators gave good voltage characteristics because they were more or less porous before being subjected to the usual treatment. They did, however, lack the necessary mechanical strength to insure satisfactory life. To overcome this lack of strength certain coniferous woods were later investigated and found to be satisfactory. Those types of woods which eventually found use in storage batteries were Port Orford cedar, California redwood, cypress, and to a limited extent Douglas fir. Wood separators are treated preparatory for use in storage batteries by hydrolyzing in either a dilute caustic soda solution or dilute sulfuric acid solution. This separator treatment has two objects: (1) to remove any material which would interfere with storage battery action, and (2) to hydrolyze a part of the cellulose, thereby increasing the porosity of the wood. The constituents present in wood which interfere with storage battery action are resins, oils, and organic acid residues, which corrode the metallic parts of the positive group. A great deal of this corrosion is caused by the acetic acid hydrolyzed from the wood of improperly treated separators by the sulfuric acid of the electrolyte. It is important, therefore, that the majority of these organic acid residues be removed from the separators. The proper treatment of the separator has been one of the outstanding factors that have contributed to increasing storage battery life. The life of the wood separator has been increased in automotive batteries by the addition of a perforated or slotted rubber sheet, which is placed next to the positive plate. It is the nascent oxygen liberated from the positive plate during charging that causes the decomposition of the wood, and the rubber sheet largely eliminates this decomposition. It does, however, have a tendency to increase the internal resistance of the cell. The third type of separator, which is known as the “threaded rubber insulator,” consists of a ribbed hardrubber sheet which has several hundred thousand small cotton threads passing through the sheet of rubber from one side to the other. This insulator has sufficient porosity to insure low internal resistance of the cell and also enough mechanical strength to prevent the buckling of the plates. It also has an extremely high resistance to chemical action, and, therefore, assures a long life. CoNTAINERs-Until the past few years storage battery containers have been a source of considerable annoyance. The original practice was to build the cells into thin-walled hardrubber jars, and to assemble these jars into a wood case. The wood case, made of the very best hardwoods obtainable and regardless of the acid-resisting coatings applied, was more or less attacked by sulfuric acid which leaked from the cells. This difficulty has been eliminated by the use of molded unit containers. These containers at the present time are of two types-hard rubber, which is possibly more universally used, and the asphalt compositions, which are a more recent development. These new containers have given the automobile owner more miles of uninterrupted storage battery service. Future Problems for Chemists The future of the storage battery industry is almost wholly dependent upon the chemist. He will be called upon to devise new processes and methods of manufacture, to increase the quality of his product, to search for new and better materials, to eliminate waste in manufacturing operations, and to meet the exacting demands of the rapidly advancing automotive industry.
