J. of Supercritical Fluids 37 (2006) 409–416 Supercritical cleaning of rollers for printing and packaging industry G. Della Porta ∗ , M.C. Volpe, E. Reverchon Dipartimento di Ingegneria Chimica e Alimentare, Universit` a di Salerno, Via Ponte Don Melillo, 84084 Fisciano (SA), Italy Received 8 June 2005; received in revised form 24 January 2006; accepted 26 January 2006 Abstract Printing or film coupling processes use engraved rollers whose surface is formed by microscopic cells that carry inks or adhesives on the film. During their use, cells are progressively filled up of residual dry ink and/or adhesive that reduce their efficiency. Cells cleaning is very complex, due to their microscopic dimensions. A new technique based on the use of supercritical mixtures (carbon dioxide and organic solvents) to clean engraved rollers was developed. To study the feasibility of this process, various combinations liquid solvent–supercritical CO 2 and the effect of some operating parameters was investigated, as operating pressure, temperature and residence time. Due to supercritical fluid mixture characteristics as, near zero surface tension and high diffusivity, a fast and complete removal of the dried residue from the microscopic cells was obtained. An almost complete cleaning was obtained in 40 and 60 min operating with a supercritical solution of carbon dioxide plus 80% w/w of N-methyl pyrrolidone at 150 bar and 40 ◦ C for poly-urethane adhesive and polychlorovinyl resin red ink, respectively. © 2006 Elsevier B.V. All rights reserved. Keywords: Engraved roller; Cleaning; Supercritical fluids; Supercritical mixtures 1. Introduction Printing or coupling processes use engraved rollers, that typ- ically are reduced in three different kinds of cylinders. The first is a ceramic coating, the second is a chrome-plated surface and the third is a laser engraved ceramic surface. Each cylinder con- tains thousands of microscopic cells on the surface to carry ink or adhesive from a reservoir to the polymeric substrate. The cells size determines the amount of ink or adhesive each roll can carry and the resolution of the printed image increases as cell size decreases. Cells have different diameters (ranging from 5 to 100 m), depths, shapes and placement angles to meet the needs of the particular printing or coupling process. During the ordinary operation, small quantities of dried ink or adhesive plug the cells. As a consequence, the volume of ink or adhesive transferred is altered and the quality of the printing is reduced [1,2]. In a medium size industry, near 200 rollers have to be cleaned daily; therefore, it is easy to understand that rollers cleaning is an important problem for the printing and coupling industry. This ∗ Corresponding author. Tel.: +39 089964104; fax: +39 089964057. E-mail address: gdellaporta@unisa.it (G. Della Porta). operation is very difficult due to the microscopic size of the cells and their depth on the roller surface. Several methods and appa- ratuses are used for rollers cleaning. For example, ultrasonic cleaners whereby rollers are partially or completely submerged in a tank containing a highly caustic fluid. High frequency sound waves are generated in the liquid medium to create microscopic air bubbles that impact with the cells. The explosions force the caustic fluid into the cells to remove dried ink and other residues. This cleaning system is extremely expensive and time consum- ing. High pressure cleaning is also used. The roller is placed in a stream of a medium ejected from a high pressure device. The medium can be formed by particles such as small microscopic plastic particles or sodium bicarbonate or soda. In some media blast systems, the roller is entirely submerged in the fluid media. Alternatively, some pressure wash systems rotate the roller in an environment whereby a cleaning solution is ejected from nozzles toward the surface of the roller. Fluid pressure washing is less expensive in comparison with other media blast systems and if compared to ultrasonic systems. However, media blast and pressure wash systems are still relatively expensive and require a complex maintenance. Some cleaning systems utilize pressurized vapour injection, wherein the rollers are placed in a stream of a chemical vapour. 0896-8446/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2006.01.018