Reducing problems of cyclic trimer deposits in supercritical carbon dioxide polyester dyeing machinery

Abstract The present paper describes an alternative procedure for the reduction or elimination of oligomeric polyester residues, in particular the cyclic trimer (CTR), in supercritical fluids (SCFs). Polyethylene terephthalate is the largest, (by volume) man-made synthetic fiber produced in the world owing to its favorable properties, such as durability, strength, stability during heat setting, abrasion resistance, and resistance to sunlight, acids, alkalis, and bleaches. In addition, polyester fibers have very good crease recovery and are durable to washing. Because of these characteristics, polyester has many important uses including home furnishings, apparel fabrics, automotive interior fabrics, and carpeting (Ind. Eng. Chem. Res. 39 (2000) 4806). Due to the large volume of polyester dyed, fundamental research has given attention to alternatives for conventional aqueous processes. The application of SCFs, especially supercritical carbon dioxide (SC-CO2), in the textile industry has recently become an alternative technology for developing a more environmentally friendly dyeing process. SCF dyeing technology has the potential to overcome several environmental and technical issues in many commercial textile applications such as yarn preparation, dyeing and finishing. SCFs represent a potentially unique media for either transporting chemical into or out of a polymeric substrate, because of their thermo-physical and transport properties. SCFs exhibit gas-like viscosities and diffusivities and liquid-like densities. Carbon dioxide is appealing for use as a SCF because it is inexpensive, non-toxic, non-flammable, environmentally friendly, and chemically inert under many conditions (J. Org. Chem. 49 (1984) 5097). In order to improve efficiency and address some of the environmental concerns with SCF technology, researchers at North Carolina State University (NCSU), College of Textiles, have constructed a single-package-pilot-plant system for dyeing polyester using SC-CO2. Based in part on data gathered from this investigation, the technical and economic feasibility of this process has been demonstrated and SCF dyeing appears to be on the leading edge of emerging technologies. However, it has been shown that the removal of precipitated oligomers mainly from the inside walls of all parts of the dyeing machine (i.e. vessels, spindle tube, sight glasses, valves, tubing, and fittings) is highly desirable. Experimental pressure measurements across a few sections of the SC-CO2 dyeing machine show that significant pressure losses can occur where oligomer, predominately CTR, builds up. Consequently, the maximum CO2 volume flow rate in the dyeing machine can decrease 30–35% (Conf. Eng. Note (1998); Conf. Eng. Note (1999)). A preliminary investigation shows that highly insoluble CTR has a reduced affinity to adhere to these stainless steel surfaces at higher SC-CO2 pressure. Although the knowledge and expertise base in this new textile research area has been increased considerably, an economic removal procedure for CTR in SC-CO2 machinery has not been found (Proc. 6th Conf. Supercrit. Fluids Appl. (2001) 571).