Abstract
Cleaning method for the containers which contained inorganic particles was investigated by rigorous chemical leaching. The important condition for removing the particles from plastics is that the leaching solution diffuses into the plastics and it reacts with the particles.
About 0.1 percent of nitric acid and about 0.02 0.03 percent of hydrochloric acid were absorbed in Teflon but water was scarcely absorbed in it. However, after dealing by nitric acid leaching, the absorption of hydrochloric acid and water in Teflon increased up to 0.05 percent. In the case of polyethylene, about 0.1 percent of hydrochloric acid and about 0.05 percent of water were absorbed but nitric acid reacted with it and decomposed it.
Pieces of Teflon sample were placed in each leaching solution (1: nitric acid, 2 : hydrochloric acid, 3: three-to-one mixture of concentrated nitric acid and hydrochloric acid) under boiling, and after every an adequate period, the samples were taken out and the change of the particles was observed under a microscope. This experiment showed that nitric acid was more active to the particles than hydrochloric acid. The degree of the dissolution of the particles by the leaching solutions decreased in an order, nitric acid, mixture of nitric acid, hydrochloric acid and hydrochloric acid. Some of the particles were not dissolved by any acids. However, the quantity of such particles was rather small, especially by the dissolution with the mixture of nitric acid and hydrochloric acid.
The dissolution of particles can be divided into three steps, the first is the diffusion (introduction) step, where the solution reaches the particles, the second is the reaction step, where the crack between particles and plastic is occupied by the dissolved solution, and the final is the growth of branch pattern step where the dissolved solution diffuses into plastic and a branch pattern grows slowly and steady. A leaching period up to five days was necessary in order to remove large particles (200 microns). The recommended procedure is as follows.
Teflon bottles, washed by water are immersed in a three-to-one mixture of concentrated nitric acid and hydrochloric acid for two days and then immersed in concentrated nitric acid for three days. (The presence of hydrochloric acid may be favorable in order to dissolve some metal oxide particles.) The leached Teflon bottles are then immersed in boiling water for three days (the water is changed every day) and are finally dried under vacuum.
