**2. Impregnation mechanism in scCO2**

Various studies utilizing scCO2 as impregnation medium of textiles and polymers have been reported in the literature. Generally, scCO2 impregnation of additives can be performed based on two mechanisms. The first mechanism works if the solute molecule is readily soluble in scCO2 solvent. When polymers are introduced into scCO2 bath containing solutes, the small CO2 molecules penetrate to the free volume of the amorphous region and swell the material creating additional free volumes. This causes plasticization of the material due to a decrease in the glass transition temperature (Tg) [9]. Then, the dissolved solutes are transported to the fiber surface and subsequently penetrate and diffuse into the swollen polymer matrix. Finally, upon depressurization, the CO2 molecules are removed by the shrinking polymer, and the impregnate molecules are trapped inside the polymer matrices [10]. The second mechanism applies for solute molecules, which are poorly soluble but having high affinity to the polymer. In this case, the solute molecules partition preferably toward the polymer matrix than the fluid because of their higher affinity to the polymer. This is the key mechanism by which polar dye molecules are incorporated into the polymer matrix in scCO2 dyeing and impregnation of drug molecules into polymers [8]. Therefore, the impregnation process is feasible when the active principle (solute) is soluble in scCO2 or the partition coefficient is favorable toward the polymer charging enough solute, and the polymer itself is well swollen by the scCO2 solvent [6]. The general steps of impregnation of polymeric fibres in scCO2 are illustrated in **Figure 1.**

Functional active principles such as functional dyes, antimicrobial agents, flame retardant, antioxidants, fragrances, pharmaceutical drugs, and others can be impregnated into a polymer by exposing the polymer to scCO2 medium containing

**183**

*Impregnation of Materials in Supercritical CO2 to Impart Various Functionalities*

these agents based on the mechanisms explained above [5]. It has been shown that pharmaceutical drugs can be impregnated into a swollen polymer matrix at operating temperature low enough to avoid thermal degradation of temperature-sensitive drugs. After impregnation and depressurization, the impregnated drug materials slowly diffuse out from the polymer matrix at a slower rate than the rate it was diffused into the polymer which can be used to form a novel controlled release of drugs [11]. The same principle works for deodorizing and antimicrobial agents as well.

*Schematic of impregnation mechanism of polymeric fibres with functional agent in scCO2.*

The most important property for the design of processes in scCO2 medium is the solubility of the compounds in scCO2 fluid. For this reason, solubility data of many compounds including dyes are available in the literature [12–14]. The properties of the compounds such as molecular structure, size, and polarity are the main factors determining their solubility in scCO2 solvent. The solvent character of scCO2 is very much like a hydrocarbon solvent such as n-hexane [15], in which polar compounds are poorly soluble and nonpolar molecules such as disperse dyes have relatively higher solubility [16, 17]. To improve the solubility, polar co-solvents (also called entrainer or modifier), such as acetone or alcohols, are usually added to the scCO2 bath. Furthermore, the solvent power of scCO2 is a function of its density, and this density can be fine-tuned by changing the pressure and temperature of the system [18]. Disperse dyes are among the most investigated compounds, owing to acceptable solubility and suitable molecular size for dyeing polyester and other synthetic fibers in scCO2 [13]. However, the solubility data of functional finishing agents commonly used for textile finishing are still scarce in the literature. According to reports, several non-ionic, low molecular mass organic materials are soluble in scCO2, but only two classes of polymeric materials such as fluoropolymers and silicones showed appreciable solubility in scCO2 at a readily accessible temperature and pressure [19]. Thus, future research should focus on studying the solubility of

In the conventional process, functional finishing agents are usually applied at the end of the process during dyeing or finishing stages. The common problems with these conventional finishing processes are the requirement of a higher amount of water, energy, and auxiliary chemicals, which generates toxic wastewater causing

*DOI: http://dx.doi.org/10.5772/intechopen.89223*

**Figure 1.**

**3. Solubility of functional agents in scCO2**

functional compounds in scCO2.

**4. Functionalization in scCO2**

*Impregnation of Materials in Supercritical CO2 to Impart Various Functionalities DOI: http://dx.doi.org/10.5772/intechopen.89223*

**Figure 1.**

*Advanced Supercritical Fluids Technologies*

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

Various studies utilizing scCO2 as impregnation medium of textiles and polymers have been reported in the literature. Generally, scCO2 impregnation of additives can be performed based on two mechanisms. The first mechanism works if the solute molecule is readily soluble in scCO2 solvent. When polymers are introduced into scCO2 bath containing solutes, the small CO2 molecules penetrate to the free volume of the amorphous region and swell the material creating additional free volumes. This causes plasticization of the material due to a decrease in the glass transition temperature (Tg) [9]. Then, the dissolved solutes are transported to the fiber surface and subsequently penetrate and diffuse into the swollen polymer matrix. Finally, upon depressurization, the CO2 molecules are removed by the shrinking polymer, and the impregnate molecules are trapped inside the polymer matrices [10]. The second mechanism applies for solute molecules, which are poorly soluble but having high affinity to the polymer. In this case, the solute molecules partition preferably toward the polymer matrix than the fluid because of their higher affinity to the polymer. This is the key mechanism by which polar dye molecules are incorporated into the polymer matrix in scCO2 dyeing and impregnation of drug molecules into polymers [8]. Therefore, the impregnation process is feasible when the active principle (solute) is soluble in scCO2 or the partition coefficient is favorable toward the polymer charging enough solute, and the polymer itself is well swollen by the scCO2 solvent [6]. The general steps of impregnation of polymeric

Functional active principles such as functional dyes, antimicrobial agents, flame retardant, antioxidants, fragrances, pharmaceutical drugs, and others can be impregnated into a polymer by exposing the polymer to scCO2 medium containing

the most relevant ones for this chapter are presented.

**2. Impregnation mechanism in scCO2**

fibres in scCO2 are illustrated in **Figure 1.**

**182**

*Schematic of impregnation mechanism of polymeric fibres with functional agent in scCO2.*

these agents based on the mechanisms explained above [5]. It has been shown that pharmaceutical drugs can be impregnated into a swollen polymer matrix at operating temperature low enough to avoid thermal degradation of temperature-sensitive drugs. After impregnation and depressurization, the impregnated drug materials slowly diffuse out from the polymer matrix at a slower rate than the rate it was diffused into the polymer which can be used to form a novel controlled release of drugs [11]. The same principle works for deodorizing and antimicrobial agents as well.
