DDT Formulations

In its undiluted form DDT is not suitable for use as an insecticide. It would be uneconomical for most purposes to use so potent an agent without diluting it greatly and the physical properties of DDT make it unsuited for direct application. DDT is practically insoluble in water and by itself will not form a satisfactory suspension in water. Its crystals are so soft that they lump or cake easily, and they cannot be ground alone to make a satisfactory dust. When DDT is mixed with talcs, clays, pyrophyllite, or various other diluents, however, it can be ground to a fine powder, which can be applied as an insecticide dust or after treatment with a wetting agent can be suspended in water for use as a spray. DDT can be dissolved in various petroleum oils, xylene, or other organic solvents for making spray solutions or water emulsions. It has been incorporated also in insecticidal aerosols.

Dust mixtures of DDT usually are prepared by grinding technical DDT with talc, clay, pyrophyllite, sulfur, or another diluent. Because technical DDT softens at a temperature somewhat below 89 C., care must be exercised during grinding to avoid overheating the mill. In this way dusts that contain a wide range of DDT concentrations can be obtained readily. By grinding DDT with a conditioning agent, such as certain grades of silica gel, preparations containing up to 90 percent of DDT can be made that are fine and free flowing and can be diluted with any of the diluents mentioned. For most purposes the final concentration of DDT used in dusts ranges from 1 to 10 percent. By the addition of a wetting agent to the dusts, they also can be dispersed or suspended in water for use as sprays. When certain types of clays are used as the diluent, water-dispersible powders containing as much as 50 percent of DDT can be prepared without the addition of any wetting agent. The concentration of DDT in sprays prepared from water-dispersible powders is 0.1 to 5 percent.

Dusts also have been prepared by the impregnation of the diluent with a solution of DDT in a volatile solvent such as acetone or benzene. The solution may be mixed with the dust diluent, the solvent allowed to evaporate, and the mixture then ground, or the solution may be gradually sprayed into the diluent during the grinding process. Solutions of DDT in nonvolatile solvents such as certain alkylated naphthalenes from petroleum may be incorporated in the diluent provided that the amount of solvent required is not so great as to impair the dusting qualities of the mixture.

Solutions of DDT in various petroleum oils, such as kerosene, have been used for spraying or painting walls, furniture, screens in buildings, and the like. A solution containing 5 percent of DDT is most commonly used. The solubility of DDT varies greatly with the type of oil used. In the case of kerosenes the solubility is higher in those of naphthenic base than in those of paraffinic base. If the purified, deodorized type of kerosene generally used in household insect sprays is used as the solvent, it is desirable to add about 15 percent of an auxiliary solvent (for example, certain petroleum products rich in alkylated naphthalenes) to increase the solubility of the DDT, especially if the product is to be stored at low temperature. More volatile solvents, such as acetone, xylene, and dry-

cleaning solvents, have been used for the preparation of DDT solutions to impregnate clothing and other fabrics.

Aqueous emulsions prepared from solutions of DDT in solvents that are substantially insoluble in water have proved very useful. Water-miscible solvents should not be used for the purpose. Two types of emulsions have been used those in which the DDT is dissolved in a volatile solvent, such as xylene, which evaporates after spraying to leave a deposit of DDT crystals, and those in which the DDT is dissolved in a relatively nonvolatile solvent, such as a petroleum oil, which leaves the sprayed surface coated with a solution of DDT in oil after evaporation of the water. A great variety of emulsifying agents is available for the preparation of DDT emulsions. The use of excessive amounts of emulsifier should be avoided in order to prevent excessive runoff of the spray and to avoid coating of the DDT deposit by the emulsifier after evaporation of the water and solvent. A convenient type of preparation for many purposes is an emulsion concentrate, consisting of a xylene solution of DDT containing an aralkyl polyether alcohol as emulsifier, from which emulsions ranging from 0.1 to 10 percent in DDT content can be prepared by mixing with water.