Sc\;.. 1919
T H E J O U R N A L O F I N D C S T R I A L A N D ENGINEERILVG C H E M I S T K Y
quirement. might have helped in some instances ; i t would have been easy to take out ten patents instead of one except for the added expense and it would have been nearly as hard to decide between ten patents as it is to decide between ten examples in one patent. Moreover, we should remember that to require greater precision weakens the protection offered, because it makes patentable by others, modifications which would have been covered by the basic patent under the existing system. Whether this is desirable or not, I do not pretend to say, only I would have you remember that if we amend the law to force foreign firms to disclose more, the new requirements will apply to ourselves equally. In conclusion, I wish to state that patents have been extremely valuable to us because the protection afforded by them forced the German firms to publish a t least something. Without the patents, the prospect for the newer colors would be much worse than it is. I believe that we can find our way through the maze; there are methods by which the problem can be solved. Just what these are, I prefer to leave unsaid, because by doing so I may persuade others to find still better methods than those I have worked out. One thing I am sure of is that those who have the necessary information can hardly do a greater service t o the dye industry of this country than to engage in the very fascinating task OF which I have given you this very brief sketch. B. 1. DU
CHEMICAL DEPARTMENT FONT DE NEMOURS & COMPANY DELAWARE WILMINOTON,
EXPLOSIBILITY A N D INFLAMMABILITY OF DYES By BURRHUMISTON, w. s. CALCOTT AND E. c. LATHROP The following work was taken up with the view of eliminating, or at least reducing, the danger of fire and explosion in the finishing of dyes and intermediates. Although the du Pont Company has not experienced any serious difficulty occasioned by fire or explosion, we have been informed that a number of accidents have occurred throughout the country and because of the risks involved considerable work was carried out along this line. Aside from eliminating the loss of life, there are also the possibilities, of course, of eliminating the loss of material and increasing the capacity of the various units used in finishing operations. Very little data of any sort were available from which i t could be predicted under what conditions dyes or intermediates were likely to explode or decompose, other than a certain amount of plant experience. The problem was, therefore, attacked from the laboratory point of view, with the idea of developing methods by which the element of risk in each step of the finishing operation could be accurately determined. The dyes or intermediates, as delivered to the finishing department, consist of a wet mass, usually a press cake, with a widely varying amount of moisture. To prepare it for the market it is neceqsary that it be dried, ground to a fineness depending upon the individual material, and standardized, that is, brought to the correct shade and strength. These three operations combined are referred to as the finishing of the material. For the sake of clearness, each of these three operations will be taken up separately. DRYING
In drying, little danger is encountered from moistures of dust and air, as very few of the dryers used for dyes or intermediates cause the formation of any appreciable amount of dust, except in wacuo. Any trouble encountered in this operation is therefore likely to be due to the effect of the temperature upon the mass of the material. This effect may manifest itself as an oxidation, which may cause the material to become heated, or it may consist Qf a decomposition which may be accompanied by the evolution Qf gases. This latter may also be sufficiently rapid to become explosive. A good illustration of the oxidizing type of dye is
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sulfur black. This in the moist form oxidizes so readily that it is almost sure to fire if dried in the presence of air. In fact, it will frequently oxidize sufficiently to materially injure the shade on simply storing in paste form, if the amount of water present is insufficient to form a slime. If, however, it is dried and cooled in the absence of air, it becomes quite stable and may be kept indefinitely without any change in shade. Victoria green, on the other hand, is a good illustration of the type of material which decomposes without taking fire if overheated a t temperatures above 100' C. In the case of Victoria green, an exothermic reaction sets in, the dyestuff being reduced to the leuco base by the oxalic acid present, and carbon dioxide mixed with some carbon monoxide is given off freely. As the reaction is strongly exothermic, it could quite easily become explosive in a confined system. This investigation did not include slight oxidation of the materials concerned, but only cases in which the oxidation was sufficiently rapid to cause the materials to take fire. The apparatus used for determining both these points and the liability to decomposition without oxidation consisted essentially of a large test tube enclosed in an air bath and capable of being heated to a temperature of 500' C. A small amount of the dye or intermediate is placed in the test tube, a thermocouple immersed in the dye, a slow current of air passed, and the temperature gradually raised. The point a t which the dyestuff takes fire is determined, although, as a rule, smoke or fumes can be noted before ignition takes place. The results can be checked with a fair degree of accuracy. For determining the danger of decomposition of the dye, with or without the evolution of gases, a similar apparatus is used, excepting that no air is passed through and a delivery tube is connected to the test tube so that any gases given off are conducted to a gas burette. The temperature of the test tube is raised until decomposition begins, and it is then noted whether the reaction is endothermic or exothermic, as indicated by the development of higher temperatures in the dye than in the air bath with which it is heated. The point of initial decomposition can be quite readily determined, provided gases are given off under decomposition, or if the decomposition is exothermic. Dyes which decompose exothermically are, of course, much more dangerous to handle than those which decompose endothermically, as the dyes, being in the dry state, are very poor conductors of heat, and being surrounded by the heated vessel, the conditions are very favorable to the development of high temperatures, which result in fires or explosions. Of course, after the point a t which decomposition or rapid oxidation sets in has once been determined, it is a simple matter to eliminate this element of risk in the plant by working a t a sufficiently low temperature. This should, in general, be not less than 25' to 30' C. below the temperature of exothermic decomposition or ignition, allowing for variations i n the quality of the various batches of dyes and for faulty temperature control. I
GRINDING
In grinding, in addition to the effect of temperature on the mass of the material, there must also be considered the effect of temperature upon the mixture of dust and air which is invariably present in grinding and drying solids, and also the probability of the occurrence of sparks in the mill. A certain amount of heat must be developed in any grinding operation, and the work done in reducing the size of the particles must reappear in the form of heat, unless provisions are made for removing this heat as fast as liberated, which is not usually the case. There will be a rise of temperature in the grinder, the extent of the rise depending, of course, on the material being ground and on the mill being used for the purpose, so that it is difficult to fix any limits on the temperatures to be expected in grinding operations. I t may be stated safely, however, that temperatures up to 75' C. may be experienced in the ball mill
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T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
type of grinder, and temperatures well over 100' in the impact crusher type. In the ball mill the possibility of sparks can be largely reduced, as suitable choice of the material of the mill itself and of the balls, copper mills with copper balls being almost free from risk of this sort. In the impact crusher type of mill, however, for example, where the dye is disintegrated by being submitted to blows from a revolving beater arm, the accidental intrusion of foreign particles can easily cause sparking through being struck by the rapidly revolving beater. Sparks due to static electricity must also be considered in grinding. The method for determining the effect of heat on the mass of dye in grinding is, of course, identical with that used for the corresponding factor in drying. It is obviously necessary, however, in grinding, to determine the effect of the additional factors introduced, the effect of temperature on the mixture of dust and air. I n order t o determine the stability of the dust and air mixture, there is employed the apparatus developed by the Bureau of Mines for the determination of the explosibility of coal dust and air mixtures. This consists essentially of a 1000 ec. bulb containing a small, hollow cylinder of platinum, heated from the interior by means of a platinum coil, the temperature of the platinum cylinder being accurately determined by means of a thermocouple. The dye under investigation is placed in a funnel a t the bottom of the bulb and the dust cloud is carried by blowing air upward through the funnel. This delivers the dust cloud around the heated platinum cylinder. The explosion point is taken as the temperature a t which a noticeable development of pressure takes place, which is registered ofi a gauge connected with the apparatus. At this temperature a visible flame is seen to develop and spread through the bulb. By a slight modification of this apparatus, the dust doud is subjected to the effect of sparks from an induction coil or from a small emery wheel. Little work has been done on this latter method of testing, however, but from the former, that is, the use of the platinum cylinder, results ranging from 400' to 1100' C. have been obtained as the explosion points of dusts with varying amounts of air, and it is possible to check these results within about 50'. The results obtained are in fair agreement with plant experience, as regards the relative safety of the different dyes so far investigated. This apparatus, however, has the weak point that the dust-air mixture used is not necessarily that most sensitive or most likely to explode. Somewhat more reliable results are therefore given by a modification of the apparatus for determining the decomposition temperature. This apparatus consists essentially of a large test tube immersed in an air bath with a tube for admitting air to the bottom of the test tube. The test tube is gradually heated up, while passing through it is a current of air sufficiently rapid to keep the tube completely filled with dust. In this manner dust clouds of widely varying densities, and therefore sf widely varying sensitivities, are secured, ranging from that at the bottom of the test tube, which consists almost entirely of dye, to that a t the extreme top of the tube, which contains only a few particles of dye, consisting chiefly of air. The temperature of spontaneous ignition is determined by inspection. Usually the decomposition is very noticeable, being detected by change in color and development of fumes. The increased sensitivity of this apparatus over the Bureau of%Minestype is shown by the fact that the temperatures at which explosions occur in the Bureau of Mines apparatus range from 400' to IIOO' C., while the temperature range for the same series of dyes in this apparatus is from 2 5 0 ° to 550' C. The data obtained from the two forms of apparatus, however, give a very clear idea of the relative stability or instability of the dye, as the Bureau of Mines apparatus gives the relative ease with which the different dyestuffs will propagate an explosion, once started, whereas the so-called spontaneous ignition temperature gives fairly definitely the order of stability of the different dyes
Vol.
XI,
No. I I
or intermediates when heated in the presence of air. Little difficulty has been experienced in obtaining concordant results with this apparatus, checking within IO' to 15'. STANDARDIZING
In the standardization of dyes, explosion risks are very much the same as in grinding, although, of course, to a much lesser extent, as the temperatures encountered are much lower and the risks of striking sparks in the mixture are, of course, much less than the risks of striking sparks in the grinding, whether of the ball mill or impact type. An additional factor is introduced, however, in that standardizing reagents are usually added to bring the mixture to the proper strength. Stability of these reagents, and the possibility of a reaction between the reagent used and the dye or intermediate has to be considered; for instance, the accidental addition of soda ash to Victoria green would probably result in a vigorous reaction between the oxalic acid of the Victoria green and the soda ash, with the possibility of raising the temperature to the decomposition point of Victoria green. This investigation is still in the preliminary stages and the methods used are tentative. They are being used, however, pending the development of more satisfactory methods for covering these points, or other points which may be brought up. They are believed to be of considerable value, however, as each method has been developed to answer some definite problem known to have occurred, and only after having given a satisfactory answer to an original problem have they been applied to the solution of similar problems for other materials. One difficulty which has been encountered is the lack of satisfactory standards with which to compare the laboratory results. Certain materials, as for example alizarine yellow, are known to be unsafe, while certain others are believed t o be safe, and in default of more satisfactory data, these materials have been temporarily taken as standards in carrying out this work. It is realized, of course, that the mere fact that the material has been safe t o date does not prove that i t will remain safe indefinitely, and that some of the dyes now passed as perfectly safe may eventually be discovered to be dangerous. However, it is believed that by continuing this investigation along these lines, it will be possible t o establish fairly definite limits as regards temperature and the proper type of mill for grinding each material, and in cases where the element of risk warrants it, t o state which of these materials should be segregated or handled in small lots, or in extreme cases handled in the paste condition. An incidental advantage which has already been gained to some extent has been the selection of the proper temperatures in drying, so as to give the maximum speed of drying with the minimum of danger, thereby increasing the capacity of the drying units. We hope later to communicate more fully on this subject. JACKSON L A B O R A T O R Y
E. I.
NEMOURS 82 COMPANY WILMINGTON, DELAWARE
DU P O N T DE
SOME PROBLEMS IN THE IDENTIFICATTON OF DYES By
E. F. HITCH AND I. E. KNAPP
At the present stage in the development of the dye industry in the United States, the American manufacturers must necessarily follow the Germans. No matter how much we may dislike to be followers and not pioneers, we must, in the first few years, confine our efforts in this field largely to the manufacture of colors that have already been produced by foreign manufacturers. No matter how optimistic we may be, or how confident we are in the ability of the American chemist to produce results, we cannot overlook the fact that we are several decades behind the German chemists in our knowledge of the preparation of dyes. This handicap can be overcome only by an immense amount of very diligent work on the part of the American chemists and manufacturers.
