February, 1924
I N D UXTRIAL A N D ENGINEERING CHEMISTRY
155
PIGMENT CHARACTERISTICS AND PHYSICAL PROPERTIES OF OTHERMEANSOF IMPROVING THE DISPERSION OF PIGMENTS COMPOUNDED RUBBER The results obtained with glue make it appear probable Now, although a general classification of the factors in the reinforcement of rubber can be made in terms of the physical properties of the pigments, it is not quite such a simple matter to dissociate the influence of each factor from that of the others, with respect to definite physical properties of the vulcanized products. It appears, however, that for any given toughening pigment the tensile strength is largely influenced by the volume of pigment incorporated, its particle size (specific surface as developed in the initial dispersion), and wetting or adhesion. The ultimate elongation is reduced by increasing volume ratio of pigment to rubber, and, in general, pigments of smaller particle size give lower elongation than is imparted by coarser fillers in the same proportions (within certain limits). The stiffness (rigidity) of the rubber is influenced by flocculation of the pigment, by its particle size, and probably by the degree of wetting by the rubber. Since the pigment particles are themselves of much greater rigidity than the rubber mass, they impart to it, depending upon their shape and degree of attachment to the matrix, a portion of their own rigidity, as shown by the fact that even coarse, nonflocculated pigments stiffen rubber to a limited extent where incorporated in large amounts. Probably the uniformity of the pigment also has some effect on the stiffness of the vulcanizate. The resilient energy (work) capacity of compounded rubber probably depends upon all the following factors: particle size, adhesion, flocculation tendencies, uniformity, and particle shape of the pigment, as well as the volume incorporated, since resilient energy is a composite index of the physical properties above named. It may be supposed that the same thing i s true, in a different degree, for the property of abrasion resistance. The iaportance of a high degree of attachment between the particles and the rubber after vulcanization, in its influence on physical properties, is clearly indicated by the fact that lithopone and colloidal barium sulfate, although of very small particle size and satisfactory uniformity, do not increase tensile strength, resilient energy, or abrasion resistance, and influence rigidity much less than would be expected from their particle size, because they lack high adhesion in the vulcanizate. EXPLANATION OF BEHAVIOR OF GLUE WITH LIGHT MAGNESIUJ~ CARBONATE The explanation offered for the observed effects of glue, in enhancing the toughening effect of light magnesium carbonate, is that it effects an improved initial dispersion of the pigment due to increased wetting of the particles by the rubber. I n milling the pigment, especially with the larger volume proportions, the increased ease of incorporation and lessened formation of flakes between the rolls are quite noticeable. An improved initial dispersion implies a smaller effective‘particle size and an increased specific surface. The improved wetting in the unvulcanized mass may, quite probably, be associated with an enhanced degree of adhesion between the particles and the rubber after vulcanization. Since no marked increase in rigidity has been noted, it seems improbable that the glue has any great effect on the tendency of the magnesia to form flocculates during vulcanization, Improved dispersion of the pigment means a smaller number of aggregates of particles, which, in turn, implies a superior uniformity of particle size of the pigment as it exists in the vulcanized product., There would, of course, be no effect upon particle shape except in so far as the dispersion of aggregates would make the average shape more nearly approach that of the (average) ultimate particle.
that other substances have a similar effect in improving the dispersion of bulky pigments and thus enhancing their toughening effect. It is well known, for instance, that softeners, which increase the “tackiness” and wetting power of the unvulcanized rubber, facilitate the incorporation of “dry” pigments, implying also that their dispersion on the mass is correspondingly improved. Most softeners, however, if used in proportions sufficient to increase greatly the ease of incorporation of the pigment, have a disaggregating effect on the rubber due to their solution in or swelling power for rubber, so that the lowering in tensile strength and other mechanical properties which results is more than enough to offset any beneficial effect from the improved dispersion of the pigment. An exception to this, however, is mineral rubber (blom;n asphalt), which, although it has no marked effect on the stress-strain curve, does increase tensile strength up to 7 volumes per 100 of rubber. It may be expected, therefore, that small amounts of mineral rubber will improve the dispersion of magnesium carbonate and gas black and enhance the physical properties of the compounds. The use of mineral rubber has not been studied in this investigation, but it seems to merit additional experimentation. Another possibility lies in the fact that certain accelerators, of the type of aniline, ethylidene aniline, etc., perform a double function in the mix, since they also act to a certain extent as softeners. It seems probable that such accelerators, if added in the breaking down of the rubber, will improve the dispersion of pigments subsequently incorporated, and that their disaggregating effect will then be largely offset by the increase in tensile strength resulting from their effect in shortening the time of cure. Some extremely interesting and valuable compounding data might be obtained by studying the influence of each reinforcing pigment, in varying proportions, upon the reinforcing effects of others.
A Simple Distinction between Citric a n d Tartaric Acids1 By Hoyt Stevens UNITSDDRUGCO.,ST. LOUIS, M O .
This test from the methods book of an analyst for a large drug house has proved to be a quickand dependable distinction between these two acids which are used in large quantities in the business. About 0.2 gram of the sample is placed on a small spatula and held in a flame until it ignites. Then the spatula is removed and the ignition observed. I n case of tartaric acid, the burning mass draws up into a dry ball and burns with a blue flame, the ball shrinking in size until only a small residue of carbon is left on the spatula. The citric acid, when ignited, spreads out on the spatula, remaining in a liquid state while burning with a yellow flame. It burns in this manner until all is consumed excepting a brownish black residue spread out on the spatula. The burning is accompanied with considerable spattering. 1 Received
January 10, 1924.
The American Pharmaceutical Association has available a
sum amounting to $450 which will be expended after October 1, 1924, for the encouragement of research. Investigators desiring financial aid in their work are requested to communicate before March 1 with H. V. Amy, chairman, A. Ph. A. Research Committee, 115 West 68th St., New Pork, N. Y., giving their past record and outlining the particular line of work for which the grant is desired.
