Supercritical Fluid Cleaning of Banknotes

With nearly 150 billion new banknotes being manufactured and printed every year around the world, the replacing of unfit currency is approaching $10 billion annually. In addition, central banks must also deal with the environmental challenge of annually disposing of nearly 150,000 tons worth of notes unfit for recirculation. Seminal work by the De Nederlandsche Bank (DNB) has identified that soiling is primarily a yellowing of the notes due to the accumulation of oxidized sebum. We show that supercritical CO2 (SCCO2) can be effectively utilized to remove sebum and other oils and contaminants, including common bacterial colonies, from both paper and polymer banknotes without destroying the costly and sophisticated security features employed by central banks to prevent counterfeiting. SCCO2 cleaning at 60 degrees C and 5000 psi was shown to be effective in cleaning conventional straps of 100 banknotes, extracting nearly 4% of the initial strap weight. Measurements of note soiling distributions on a banknote sorting machine running at 10 banknotes per second showed a significant shift in soiling levels after cleaning, supporting the claim that processing of SCCO2-cleaned notes would result in significantly fewer notes being classified as unfit due to soiling and shredded.