Evaluation of removal of model particulate and oily soils from poly(ethylene terephthalate) films by microscopic image analysis.

The soil removal behavior from poly(ethylene terephthalate) (PET) films was investigated using a microscopic image analysis system. Carbon black or stearic acid as a model soil was deposited onto a PET film. The PET film was cleaned in various aqueous and non-aqueous solutions by applying stirring or frequency-modulated ultrasound as a mechanical action of soil removal. The amounts of soil deposited on the PET film before and after cleaning were obtained via binary processing of microscopic images, from which the removal efficiency was calculated. Most of the carbon black was deposited on the PET film as submicron aggregates and ultrasound removed them efficiently in a short time, even for relatively smaller aggregates. The removal efficiencies with stirring were less than ca. 10% in all solutions, whereas the removal using ultrasound had high efficiencies that exceeded 80% in the surfactant-free systems. In the case of stearic acid, the removal efficiency with stirring was below 30% in the aqueous solutions, although stearic acid was removed completely in ethanol and n-decane. For ultrasonic cleaning, the removal efficiencies of stearic acid in aqueous solutions became 2-3 times as large as those with the stirring action. To improve soil release in aqueous solutions, the PET film was treated by the dry processing using an atmospheric pressure plasma jet (APPJ) equipment. The wettability and the surface free energy of the PET film were found to increase due to surface oxidation via the APPJ treatment, which resulted in enhanced removal of carbon black and stearic acid in any aqueous solutions.