抄録
In the food industry, sodium hypochlorite (NaOCl) has been used as a reliable disinfectant for more than 100 years. It fulfills many requirements of the ideal disinfectant and furthermore has excellent cleaning actions. In a diluted NaOCl solution, over the pH range of 4 to 10, there is an equilibrium between two forms, undissociated hypochlorous acid (HOCl) and the dissociated hypochlorite ion (OCl-). It is known that HOCl is the potent microbicidal species, whereas OCl- has the strong cleaning power. The microbicidal activity of HOCl is attributed to its ability to penetrate the hydrophobic lipid bilayer of the plasma membrane by passive diffusion facilitated by its electrical neutrality. In recent years, therefore, the application of pH-controlled NaOCl solutions has been expanding to strengthen the disinfecting and cleaning actions of NaOCl solution. On the other hand, the frequent use of pH-controlled NaOCl solutions causes deterioration and damage to elastomeric gaskets, resulting in the decrease in sealing ability and physical contamination of food by flaking or fragmenting. For the maintenance of the stability and duration of gasket, it is necessary to clarify the mechanism of deterioration of gaskets exposed to NaOCl solutions. In our recent studies, it was found that the permeation of chlorine into carbon black (CB)-filled ethylene propylene diene terpolymers (EPDMs) depends on the HOCl concentration. At low temperatures, HOCl diffuses into EPDM without changes in surface appearance but with marked decrease in tensile strength. At relatively high temperatures, diffusion of HOCl is accelerated, and morphological changes and fragmentation of EPDM surfaces or edges occur. As a result of actions of OCl-, CB particles become removed and suspended in NaOCl solution. The behavior of HOCl and OCl- at EPDM/water interfaces is in line with their disinfecting and cleaning actions. In this article, deterioration of EPDM in pH-controlled NaOCl solutions is discussed with reference to actions of HOCl and OCl-.
