**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 [20]. 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. [21] 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 [22]. 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 [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 scCO2 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 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 [25]. 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<sup>−</sup><sup>6</sup> m) of the cotton surface with a 3D network of DMS compound and crosslinker. Chen et al. [26] synthesized CO2-philic silicon-containing quaternary ammonium salt (QAS) and applied to cotton in scCO2 to prepare antimicrobial fabric. The treated

**185**

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

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

modified with TMP-based N-chloramine imparted powerful and durable biocidal activity [27]. 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

higher biocidal efficiency [28]. 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 [31] 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 [32]. 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 functional-

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 [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 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 [37]. Thymol has also been used to modify polylactic acid (PLA) [38] and linear low-density polyethylene (LLDPE) [39] 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 [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

Pa pressure. The obtained PE

Pa and 50°C. The obtained material provided

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

the top surface was attained at 24 × 106

ization of textiles and polymeric materials.

**4.3 Natural functional compounds**

reaching up to 70 × 10-9 m was obtained using 28 × 106

*Impregnation of Materials in Supercritical CO2 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 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 [27]. 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 [28]. 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 [31] 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 [32]. 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**

*Advanced Supercritical Fluids Technologies*

**4.1 Functional dyes**

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,

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 [20]. 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. [21] 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 [22]. 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 [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 scCO2 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 scCO2 process is a promising approach

natural functional compounds, and organometallic-based agents.

toward the production of colored and functional material in a single step.

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 [25]. 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<sup>−</sup><sup>6</sup>

the cotton surface with a 3D network of DMS compound and crosslinker. Chen et al. [26] synthesized CO2-philic silicon-containing quaternary ammonium salt (QAS) and applied to cotton in scCO2 to prepare antimicrobial fabric. The treated

m) of

**4.2 Silicon and fluoropolymer-based functional agents**

**184**

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 [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 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 [37]. Thymol has also been used to modify polylactic acid (PLA) [38] and linear low-density polyethylene (LLDPE) [39] 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 [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 scCO2 to fabricate functionalized materials with repellent properties [43]. In addition, chitosan and derivatives have been used to impregnate polyester in scCO2 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 scCO2 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 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 [46]. Boggess et al. produced highly reflective polyimide films for aerospace application with silver-containing additive using scCO2 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/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 [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 scCO2 to fabricate a water repellent composite fiber [51]. Peng et al. used silver nanoparticles to coat wool fabrics in scCO2, and the coated fabric exhibited excellent catalytic, antistatic, and antibacterial activities [52]. Polycarbonate has been impregnated with silver nitrate in scCO2, resulting up to 99.9% bacteria reduction [53]. Belmas and co-workers [54–56] 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 [57]. 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.
