Article Preview
Top1. Introduction
The chemical stripping method prevalently used today cleans materials by dipping them into a strong acidic or alkaline solution in order to remove metal film. Because so much more toxic solvent is used than needed, excessive expense is incurred in treating the waste by-products. Also, because a drying process is required, the current method incurs more time and labor expenses. Stains are also produced as well. However, when supercritical carbon dioxide is used as a solvent, separating the solvent from the materials is much easier (but only with depressurization after stripping). Furthermore, the carbon dioxide can be recovered for reuse. Moreover, since a drying process is not required, surface defects, such as stains, can be reduced and energy expenses saved. Therefore, a process using carbon dioxide is far more environment-friendly and more economical.
Carbon dioxide has excellent properties it is economical and can be used in a low critical condition. In its use as a surfactant, it is environment-friendly and can be used efficiently to remove surface contaminants with the only requirement being the adjustment of pressure and temperature. Moreover, since supercritical carbon dioxide is very dense, like a liquid, it can effectively clean broad surfaces. But it also has a low viscosity and high diffusivity, like a gas, which enables it to penetrate and effectively clean deep into micro pores. Therefore it has advantages appropriate for surface treatment of precision parts.
However, carbon dioxide is a non-polar element that is very poor at dissolving polarized materials. Many scientists are researching ways to increase the dissolving power of carbon dioxide against polar materials. To date, there are basically the surface chelating method and carbon-dioxide + microemulsion method. The former is widely utilized in the extraction of metal ions in that technique, a surfactant is formulated or utilized to combine strongly with metal ions, as well as maintain high solubility within the carbon dioxide after combination. The latter method is used for dissolving polar contaminants within carbon dioxide by using polar materials within a micelle, whereby soluble materials depend upon which material is within the micelle. Water is used mainly as the material in the micelle. Salt, metal nano-particles, metal ions, and proteins can be dissolved in water. Recently, acids, such as nitric acid, have been heavily used instead of water. In this case, since all materials soluble in acid can be extracted from the carbon dioxide, nitric acid can be applied to various fields such as decontamination and surface treatment of radioactive contaminants in materials much the same as chemical stripping.
In this study, the characteristics of metal film removal of supercritical carbon-dioxide-micro emulsion, which utilizes nitric acid having very high applicability, were investigated using QCM.