Impregnation of Materials in Supercritical CO 2 to Impart Various Functionalities

Supercritical CO 2 (scCO 2 ) 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, scCO 2 is the most environmentally acceptable solvent possessing many advantages compared with the conventional aqueous and solvent-based processing. scCO 2 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 scCO 2 as impregnation medium to develop various functional materials and it is intended to stimulate further research into the application of scCO 2 to textile functionalization. It mainly focuses on applications related to textiles and some polymeric films.


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 (CO 2 ) 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, CO 2 has an easily attainable critical temperature of 31°C and critical pressure of 7.4 × 10 6 Pa which are lower compared with other SCFs such as water (critical temperature ˃ 374°C and pressure ˃ 22 × 10 6 Pa) and other organic solvents. Moreover, at least 90% of the CO 2 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].

Impregnation mechanism in scCO 2
Various studies utilizing scCO 2 as impregnation medium of textiles and polymers have been reported in the literature. Generally, scCO 2 impregnation of additives can be performed based on two mechanisms. The first mechanism works if the solute molecule is readily soluble in scCO 2 solvent. When polymers are introduced into scCO 2 bath containing solutes, the small CO 2 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 (T g ) [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 CO 2 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 scCO 2 dyeing and impregnation of drug molecules into polymers [8]. Therefore, the impregnation process is feasible when the active principle (solute) is soluble in scCO 2 or the partition coefficient is favorable toward the polymer charging enough solute, and the polymer itself is well swollen by the scCO 2 solvent [6]. The general steps of impregnation of polymeric fibres in scCO 2 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 scCO 2 medium containing Impregnation of Materials in Supercritical CO 2 to Impart Various Functionalities DOI: http://dx.doi.org /10.5772/intechopen.89223 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.

Solubility of functional agents in scCO 2
The most important property for the design of processes in scCO 2 medium is the solubility of the compounds in scCO 2 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 scCO 2 solvent. The solvent character of scCO 2 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 scCO 2 bath. Furthermore, the solvent power of scCO 2 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 scCO 2 [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 scCO 2 , but only two classes of polymeric materials such as fluoropolymers and silicones showed appreciable solubility in scCO 2 at a readily accessible temperature and pressure [19]. Thus, future research should focus on studying the solubility of functional compounds in scCO 2 .

Functionalization in scCO 2
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 Advanced Supercritical Fluids Technologies environmental pollution and increases production cost. Due to this, a new dyeing and impregnation process has been developed in which scCO 2 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 scCO 2 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.

Functional dyes
One strategy that has been followed to functionalize textiles in scCO 2 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 scCO 2 with the aim to manufacture protective clothing [20]. Results showed that polyester fabric was successfully dyed in scCO 2 medium exhibiting better photostability and fastness properties, and no morphological change was detected. Abou Elmaaty et al. [21] synthesized new hydrazonopropanenitrile dyes and applied the new species to polyester fabric using scCO 2 for potential antimicrobial application. Efficient dyeing and excellent antimicrobial and fastness properties were obtained using scCO 2 dyeing procedure. A series of disperse azo dyes with potential antibacterial activity were also applied to nylon 6 fabric using scCO 2 technique and compared with aqueous dyeing [22]. The comparison showed that samples dyed under scCO 2 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 scCO 2 has been reported [23]. 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 scCO 2 in our research group [24]. 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 scCO 2 process is a promising approach toward the production of colored and functional material in a single step.

Silicon and fluoropolymer-based functional agents
As stated earlier, silicon and amorphous fluoropolymers are known to have appreciable solubility in scCO 2 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 scCO 2 [25]. Different crosslinking agents were used for covalently bonding silicon and cellulose. The results confirm that scCO 2 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. [26] synthesized CO 2 -philic silicon-containing quaternary ammonium salt (QAS) and applied to cotton in scCO 2 to prepare antimicrobial fabric. The treated Impregnation of Materials in Supercritical CO 2 to Impart Various Functionalities DOI: http://dx.doi.org/10.5772/intechopen.89223 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 scCO 2 impregnation technique. A uniform coating of TMP-based N-chloramine reaching up to 70 × 10 -9 m was obtained using 28 × 10 6 Pa pressure. The obtained PE modified with TMP-based N-chloramine imparted powerful and durable biocidal activity [27]. The same research group synthesized a CO 2 -philic biocidal fluorinated pyridinium silicon and applied to cotton yarn using scCO 2 impregnation medium. Up to 50 × 10 -9 m thickness of biocidal layer with pyridinium groups segregated on the top surface was attained at 24 × 10 6 Pa and 50°C. The obtained material provided higher biocidal efficiency [28]. Furthermore, polyester fabric was treated with low molecular weight polytetrafluoroethylene in scCO 2 medium and a high degree of water repellency was consistently obtained [29,30]. Xu and co-workers [31] prepared a water/oil repellent polyester fabric using a solution of organic fluorine in scCO 2 . 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 scCO 2 medium to fabricate a durable water and oil repellent coatings [32]. A uniform, highly repellent, and durable coating was obtained by scCO 2 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 scCO 2 processing method for functionalization of textiles and polymeric materials.

Natural functional compounds
Supercritical CO 2 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 scCO 2 to develop different functional materials. They studied the solubility of thymol in scCO 2 and its impregnation on cotton gauze [33], cellulose acetate [34,35], corona modified polypropylene non-woven material [36], and polycaprolactone (PCL) and polycaprolactone hydroxyapatite (PCL-HA) composites. Thymol has been shown soluble in scCO 2 solvent, and impregnation process was successful. All the samples prepared using scCO 2 impregnation exhibited strong antimicrobial effect against a wide range of bacteria strain. The same research group also used scCO 2 impregnation medium for loading cellulose acetate beads with carvacrol in order to fabricate a biomaterial with antimicrobial properties obtaining considerable antibacterial effect [37]. Thymol has also been used to modify polylactic acid (PLA) [38] and linear low-density polyethylene (LLDPE) [39] films using scCO 2 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 scCO 2 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 [40]. Goñi et al. used Eugenol, a well-known natural antioxidant, and antimicrobial agent, to impregnate LLDPE films to fabricate active food packaging material [41]. 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 [42]. Recently, Pajnik and co-workers impregnated pyrethrum extract to polypropylene, polyamide, and cellulose acetate in the form of films and beads using scCO 2 to fabricate functionalized materials with repellent properties [43]. In addition, chitosan and derivatives have been used to impregnate polyester in scCO 2 bath [44]. 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 scCO 2 dyeing technique in our research group obtaining good antibacterial activity [45]. Overall, naturalbased functional agents have shown a huge potential for the fabrication of various functional materials using scCO 2 impregnation technique.

Organometallic-based functional agents
In addition to the agents mentioned above, impregnation of organometallic compounds into polymer matrices using scCO 2 has also been widely studied for various functional applications. Antifungal textiles have been produced via scCO 2 impregnation of cotton with silver, Ag (hepta), and Ag (cod), demonstrating measurable inhibition [46]. Boggess et al. produced highly reflective polyimide films for aerospace application with silver-containing additive using scCO 2 infusion and subsequent curing at 300°C [47]. 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. [48] produced a wearable photocatalytic device via integration of Ni-P/TiO 2 onto silk fabric using scCO 2 impregnation technique. Co-deposition of photocatalytic TiO 2 and electrically conductive Ni-P metallization layer was achieved through scCO 2 -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 scCO 2 medium obtaining a smooth and compact layer with improved adhesion promoted by sccCO 2 metallization [49]. The results demonstrated its applicability in medical and wearable devices. Cotton fabric has been impregnated with palladium (II) hexafluoroacetylacetonate to fabricate conductive fabrics [50]. Hematite nanoparticles were loaded to cellulosic fiber under scCO 2 to fabricate a water repellent composite fiber [51]. Peng et al. used silver nanoparticles to coat wool fabrics in scCO 2 , and the coated fabric exhibited excellent catalytic, antistatic, and antibacterial activities [52]. Polycarbonate has been impregnated with silver nitrate in scCO 2 , resulting up to 99.9% bacteria reduction [53]. Belmas and co-workers [54][55][56] have used scCO 2 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 scCO 2 impregnation of the organometallic complexes. Polyacrylate has been impregnated with copper (II) hexafluoroacetylacetonate in scCO 2 followed by thermal decomposition of the copper. The formation of copper oxide was evident ensuring improved wear resistance of polyacrylate [57]. In conclusion, owing to nanoscale metal microparticles, organometallic compounds have been successfully used to modify polymers in scCO 2 solvent for various functional applications and might be one potential area that needs further investigations in the future.

Conclusions
From the studies reviewed in this chapter, it has been shown that scCO 2 is a viable technique for the fabrication of various functional materials if appropriate Impregnation of Materials in Supercritical CO 2 to Impart Various Functionalities DOI: http://dx.doi.org /10.5772/intechopen.89223 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 scCO 2 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 scCO 2 has the potential to replace the current water and solvent-based textile chemical processes in the future. silver