**Abstract**

Supercritical CO2 (scCO2) impregnation has attracted growing interest due to its unique properties such as high diffusivity, low surface tension, and ease of solvent removal at the end of the process. In addition, scCO2 is the most environmentally acceptable solvent possessing many advantages compared with the conventional aqueous and solvent-based processing. scCO2 impregnation has a wide range of applications mainly used to incorporate various active principles such as pharmaceuticals, functional finishing agents, colorants, and other agents into a polymeric matrix. This chapter reviews some studies carried out so far about the application of scCO2 as impregnation medium to develop various functional materials and it is intended to stimulate further research into the application of scCO2 to textile functionalization. It mainly focuses on applications related to textiles and some polymeric films.

**Keywords:** supercritical CO2, impregnation, functionalization, dyeing

## **1. Introduction**

Supercritical fluid (SCF) is defined as a substance for which both its pressure and temperature are above the critical values simultaneously [1]. SCFs have been applied in many areas such as extraction, dyeing, impregnation, cleaning, polymerization, fractionation, formation of powdered polymers, and so on [2, 3]. Among the SCFs, carbon dioxide (CO2) is the most popular as it offers several advantages including low toxicity, ready availability, low cost, non-flammability, environmental sustainability, and it is chemically inert under many conditions. In addition, CO2 has an easily attainable critical temperature of 31°C and critical pressure of 7.4 × 106 Pa which are lower compared with other SCFs such as water (critical temperature > 374°C and pressure > 22 × 106 Pa) and other organic solvents. Moreover, at least 90% of the CO2 introduced can be recovered and recycled at the end of the procedure, which is attractive from waste minimization viewpoint. This also reduces the production cost and avoids the undesirable solvent residue in the produced material [4].

Impregnation is the process of infusing or depositing solute molecules dissolved in a solvent into a polymer matrix to modify the property of the material by physically or chemically binding or absorbing impregnates to a bulk or surface [5].

The conventional aqueous or solvent-based impregnation processes have many drawbacks such as low diffusion rates, high temperature, limited penetration depth, very long contact time, use of hazardous solvents, consumption of high energy, water, solvents, and other additives. To solve these problems, several techniques have been developed, and it has been shown that supercritical CO2 (scCO2) is an attractive alternative to conventional organic solvents used in polymer impregnation [6].

scCO2 has appeared to be the appropriate candidate to replace conventional impregnation using organic solvents due to several unique properties suitable for impregnation of polymeric materials. It has high diffusivity and low viscosity allowing faster penetration of molecules to the polymer matrix than in water. The absence of surface tension also improves the penetration of molecules into polymeric structures and avoids the unwanted distortion of delicate materials during processing. In addition, the possibility to recover high purity and dry product free from residual solvent is one key advantage especially important when considering the production of food and pharmaceuticals [6–8]. Furthermore, scCO2 reduces the environmental pollution and the associated cost incurred for the removal of the residual solvent, cost of freshwater input, and wastewater treatment. Due to these important attributes, today, scCO2-assisted impregnation has been used in many fields and it is a promising candidate to replace organic solvents in the future.

In this chapter, studies involving scCO2 dyeing and impregnation processes to develop products for various functional applications are reviewed. The chapter focuses on studies related to scCO2 impregnation of textile fibers and polymers and some polymeric films, made of similar polymers. The references used are not exhaustive, as many articles are published covering the same subject area, but only the most relevant ones for this chapter are presented.
