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.
- supercritical CO2
Supercritical fluid (SCF) is defined as a substance for which both its pressure and temperature are above the critical values simultaneously . 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 .
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 . 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 .
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, 7, 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.
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) . 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 . 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 . 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 . 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 these agents based on the mechanisms explained above . 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 . The same principle works for deodorizing and antimicrobial agents as well.
3. Solubility of functional agents 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, 13, 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 , 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 . 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 . 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 . Thus, future research should focus on studying the solubility of functional compounds in scCO2.
4. Functionalization in scCO2
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 environmental pollution and increases production cost. Due to this, a new dyeing and impregnation process has been developed in which scCO2 is used as a solvent and transport media owing to unique and important properties as explained earlier. In this section, attempts that have been made so far to functionalize different textile fibers and polymers using scCO2 impregnation technique are reviewed. The functional finishing agents used in impregnating polymers are categorized as functional dyes based on natural and synthetic origin, silicon and fluoropolymer-based, natural functional compounds, and organometallic-based agents.
4.1 Functional dyes
One strategy that has been followed to functionalize textiles in scCO2 is by using different dyes having additional functional property. In this method, the functional dyes are either prepared by modifying them to contain functional groups through molecular design or those dyes which inherently possessed the required functional property are directly used. In most of the cases, disperse dyes are modified to contain functional groups based on the needed functionality, and some of them are presented in this section. Fluorescence functional dyes, such as disperse fluorescent yellow 82 were used to dye polyester in scCO2 with the aim to manufacture protective clothing . Results showed that polyester fabric was successfully dyed in scCO2 medium exhibiting better photostability and fastness properties, and no morphological change was detected. Abou Elmaaty et al.  synthesized new hydrazonopropanenitrile dyes and applied the new species to polyester fabric using scCO2 for potential antimicrobial application. Efficient dyeing and excellent antimicrobial and fastness properties were obtained using scCO2 dyeing procedure. A series of disperse azo dyes with potential antibacterial activity were also applied to nylon 6 fabric using scCO2 technique and compared with aqueous dyeing . The comparison showed that samples dyed under scCO2 medium had excellent antibacterial efficiency and better color fastness properties compared with the conventional exhaust dyeing with the advantage of the elimination of auxiliary chemicals. Impregnation of polyester (PET) films and poly(hydroxybutyrate) (PHB) granules with curcumin natural dye in scCO2 has been reported . In this study, the impregnation process was successfully developed with different amounts of curcumin add-on depending on the dyeing conditions and no significant detrimental effect observed on the material properties. More recently, curcumin has been used to dye and functionalize polyester in scCO2 in our research group . Dyed samples exhibited excellent color strength and fastness properties with improved antibacterial, antioxidant, and UV protection properties. Thus, the strategy of utilizing functional dyes which are suitable for scCO2 process is a promising approach toward the production of colored and functional material in a single step.
4.2 Silicon and fluoropolymer-based functional agents
As stated earlier, silicon and amorphous fluoropolymers are known to have appreciable solubility in scCO2 solvent. Due to this, functional agents based on these compounds have been employed to functionalize various textiles, polymers, and films. Mohamed et al. used a modified dimethyl siloxane terminated with silanol groups (DMS) to functionalize cotton fabric in scCO2 . Different crosslinking agents were used for covalently bonding silicon and cellulose. The results confirm that scCO2 medium provides good coating (thickness between 1 to 2 × 10−6 m) of the cotton surface with a 3D network of DMS compound and crosslinker. Chen et al.  synthesized CO2-philic silicon-containing quaternary ammonium salt (QAS) and applied to cotton in scCO2 to prepare antimicrobial fabric. The treated fabric exhibited potent antimicrobial activity with good durability against washing and UV irradiation. They also synthesized silicone-containing 2,2,6,6-tetramethyl-4-piperidinol (TMP)-based N-chloramine and applied to polyethylene (PE) fiber via scCO2 impregnation technique. A uniform coating of TMP-based N-chloramine reaching up to 70 × 10-9 m was obtained using 28 × 106 Pa pressure. The obtained PE modified with TMP-based N-chloramine imparted powerful and durable biocidal activity . The same research group synthesized a CO2-philic biocidal fluorinated pyridinium silicon and applied to cotton yarn using scCO2 impregnation medium. Up to 50 × 10-9 m thickness of biocidal layer with pyridinium groups segregated on the top surface was attained at 24 × 106 Pa and 50°C. The obtained material provided higher biocidal efficiency . Furthermore, polyester fabric was treated with low molecular weight polytetrafluoroethylene in scCO2 medium and a high degree of water repellency was consistently obtained [29, 30]. Xu and co-workers  prepared a water/oil repellent polyester fabric using a solution of organic fluorine in scCO2. A uniformly distributed fluorine could be obtained with good water/oil repellency keeping good air permeability and improved strength. Recently, perfluoroalkyl methacrylate/hydroxyalkyl methacrylate and a crosslinking agent (diisocyanate) have used to treat nylon fabric in scCO2 medium to fabricate a durable water and oil repellent coatings . A uniform, highly repellent, and durable coating was obtained by scCO2 treatment compared with a coating deposited from a liquid solvent. These studies show that silicon and fluoropolymer-based materials have been playing a key role in the application of scCO2 processing method for functionalization of textiles and polymeric materials.
4.3 Natural functional compounds
Supercritical CO2 has also been used to impregnate polymers with natural functional compounds to impart different functionalities. Zizovic and co-workers widely investigated the application of thymol to various textile-based substrates in scCO2 to develop different functional materials. They studied the solubility of thymol in scCO2 and its impregnation on cotton gauze , cellulose acetate [34, 35], corona modified polypropylene non-woven material , and polycaprolactone (PCL) and polycaprolactone hydroxyapatite (PCL-HA) composites. Thymol has been shown soluble in scCO2 solvent, and impregnation process was successful. All the samples prepared using scCO2 impregnation exhibited strong antimicrobial effect against a wide range of bacteria strain. The same research group also used scCO2 impregnation medium for loading cellulose acetate beads with carvacrol in order to fabricate a biomaterial with antimicrobial properties obtaining considerable antibacterial effect . Thymol has also been used to modify polylactic acid (PLA)  and linear low-density polyethylene (LLDPE)  films using scCO2 impregnation technique with the aim to prepare active materials for a wide range of applications such as food packaging and others. Furthermore, thymol was applied along with quercetin, a natural bioactive compound, to a film and foam-like structure N-carboxybutyl chitosan (CBC) film and agarose (AGR) using scCO2 impregnation technique to fabricate wound dressing material. Impregnation was performed with the help of ethanol as a co-solvent, and higher impregnation yield was obtained at higher pressure and temperature. The obtained materials also exhibited a sustained release profile based on the release kinetic study . Goñi et al. used Eugenol, a well-known natural antioxidant, and antimicrobial agent, to impregnate LLDPE films to fabricate active food packaging material . The obtained film presented a good level of antioxidant property with some degree of heterogeneity and a decrease in crystallinity when higher treatment pressure was used. In another study, Eugenol was also used to impregnate polyamide fibers to fabricate antibacterial dental floss, and inhibition of more than 99.99% has been achieved . Recently, Pajnik and co-workers impregnated pyrethrum extract to polypropylene, polyamide, and cellulose acetate in the form of films and beads using scCO2 to fabricate functionalized materials with repellent properties . In addition, chitosan and derivatives have been used to impregnate polyester in scCO2 bath . Results showed that low molecular weight chitosan and chitosan lactic acid salt were successfully impregnated whereas no chitin could be impregnated. More recently, very low molecular weight chitosan and chitosan lactate have also been successfully incorporated to polyester fabric using scCO2 dyeing technique in our research group obtaining good antibacterial activity . Overall, natural-based functional agents have shown a huge potential for the fabrication of various functional materials using scCO2 impregnation technique.
4.4 Organometallic-based functional agents
In addition to the agents mentioned above, impregnation of organometallic compounds into polymer matrices using scCO2 has also been widely studied for various functional applications. Antifungal textiles have been produced via scCO2 impregnation of cotton with silver, Ag (hepta), and Ag (cod), demonstrating measurable inhibition . Boggess et al. produced highly reflective polyimide films for aerospace application with silver-containing additive using scCO2 infusion and subsequent curing at 300°C . They have demonstrated that silver additive was incorporated into a polyimide film creating a reflective surface on both sides of the film. Chiu et al.  produced a wearable photocatalytic device via integration of Ni-P/TiO2 onto silk fabric using scCO2 impregnation technique. Co-deposition of photocatalytic TiO2 and electrically conductive Ni-P metallization layer was achieved through scCO2-assisted electroless plating and silk fabric with higher corrosion-resistant, and photocatalytic activity was achieved. Metallization of silk with platinum (Pt) was also conducted in scCO2 medium obtaining a smooth and compact layer with improved adhesion promoted by sccCO2 metallization . The results demonstrated its applicability in medical and wearable devices. Cotton fabric has been impregnated with palladium (II) hexafluoroacetylacetonate to fabricate conductive fabrics . Hematite nanoparticles were loaded to cellulosic fiber under scCO2 to fabricate a water repellent composite fiber . Peng et al. used silver nanoparticles to coat wool fabrics in scCO2, and the coated fabric exhibited excellent catalytic, antistatic, and antibacterial activities . Polycarbonate has been impregnated with silver nitrate in scCO2, resulting up to 99.9% bacteria reduction . Belmas and co-workers [54, 55, 56] have used scCO2 process to impregnate a range of organometallic complexes in a synthetic polymer prior to electroless copper plating to improve the adhesion of copper to the polymer. The adhesion between the copper and polymer was much improved after scCO2 impregnation of the organometallic complexes. Polyacrylate has been impregnated with copper (II) hexafluoroacetylacetonate in scCO2 followed by thermal decomposition of the copper. The formation of copper oxide was evident ensuring improved wear resistance of polyacrylate . In conclusion, owing to nanoscale metal microparticles, organometallic compounds have been successfully used to modify polymers in scCO2 solvent for various functional applications and might be one potential area that needs further investigations in the future.
From the studies reviewed in this chapter, it has been shown that scCO2 is a viable technique for the fabrication of various functional materials if appropriate agents suitable for the process are used. It can be an attractive alternative to traditional aqueous or organic solvents as it avoids toxic auxiliary chemicals and the use of water. Further studies are still required in selecting suitable functional agents which works best under scCO2 solvent through investigation of their solubility, compatibility, and process optimizations. Due to its environmental advantages, the scientific community and the industrial compartment would expect an increase in research in this area as scCO2 has the potential to replace the current water and solvent-based textile chemical processes in the future.
All the universities collaborating on the program are gratefully acknowledged.
Conflict of interest
The authors declare no conflict of interest.
silver agarose carboxybutyl chitosan carbon dioxide dimethyl siloxane linear low-density polyethylene meter nitrogen phosphorus pascal polycaprolactone polycaprolactone hydroxyapatite polyethylene polyester polylactic acid platinum quaternary ammonium salt supercritical carbon dioxide glass transition temperature titanium dioxide 2,2,6,6-tetramethyl-4-piperidinol
linear low-density polyethylene
quaternary ammonium salt
supercritical carbon dioxide
glass transition temperature