Review - Chemical Functionalization of Cotton Fabrics by Selective Oxidation

BULETINUL INSTITUTULUI POLITEHNIC DIN IAŞI Publicat de Universitatea Tehnică „Gheorghe Asachi” din Iaşi Volumul 71 (75), Numărul 3, 2025 Secţia CHIMIE şi INGINERIE CHIMICĂ DOI: 10.5281/zenodo.17357520 REVIEW - CHEMICAL FUNCTIONALIZATION OF COTTON FABRICS BY SELECTIVE OXIDATION BY ROMEO PRUNEANU, VASILICA POPESCU and MELINDA PRUNEANU “Gheorghe Asachi” Technical University of Iași, Faculty of Industrial Design and Business Management, Iași, Romania Received: June 19, 2025 Accepted for publication: September 12, 2025 Abstract. This documentary study presents the possibilities of selective oxidation by sodium or potassium periodate and/or 2,2,6,6-tetramethylpiperidin1-oxyl (TEMPO) of cotton textiles, the reaction mechanisms, the assessment and the analytical and instrumental determination of the degree of oxidation of the hydroxyl groups in the C2, C3 and C6 positions of the anhydroglucosidic unit (AGU), with aldehyde/carboxylic groups. The effects of selective oxidation on the molecular and morphological structure of cellulose are also briefly presented, since oxidation depending on the reaction conditions can significantly affect both the amorphous and crystalline areas of cellulose, at the microfibril level, which leads to a decrease in mechanical resistance, which is undesirable in the case of textiles with long-term use or even single-use. Selective oxidation in this case is a compromise between functionality (expressed by the content of aldehyde/carboxylic groups) and moderate degradation of the molecular and morphological structure of cotton cellulose. Keywords: oxidized cellulose, sodium or potassium periodate, TEMPO, aldehyde and carboxyl groups, degree of oxidation. Corresponding author; e-mail: ro[email protected] Corresponding author; e-mail: m[email protected] 86 Romeo Pruneanu et al. 1. Introduction The interest in functionalization and in particular in the selective oxidation of cellulose is in the attention of many researchers, as evidenced by the increase in the last 10-15 years in the number of scientific publications and patents reported. The oxidation of cellulosic materials is a reaction widely used in the wood industry, but also in the bleaching of cellulosic textiles. In the chemical finishing of textile materials, oxidative bleaching is essential to decolorize the natural pigments found in the morphological structure of the cotton fibre, resulting in a material with a significant degree of whiteness, so necessary to dye the material in any colour. Conventional oxidizing agents (NaClO, H2O2, HClO4, etc.) used for bleaching are non-selective, acting chemically on the chemical structure by the formation of aldehydic and carboxylic groups, but also on the morphological structure of the cellulosic fibres, action strongly dependent on the intensity of the applied treatments (Knill et al., 2003). A number of biomedical uses of selectively oxidized cellulose are reported in the literature, as oxidized cellulose is biodegradable (Hao et al., 2018), readily bioresorbable (Patel et al., 2017), has hemostatic, wound healing properties due to its increased adhesion to the skin, (Pinho and Soares, 2018; Shaohua et al., 2020; Wang et al., 2021), has antibacterial effect (Jiang et al., 2022; Ghosh et al., 2019), has the ability to sorb and retain polyamine compounds responsible for the odor of chronic wounds (Wen et al., 2023). A number of studies and applications of selectively oxidized cellulose are also reported in the pharmaceutical field as a carrier for a number of active ingredients (Sharma et al., 2021), food (Singh et al., 2020) as well as in the field of water purification as adsorbent membranes with excellent retention properties of some heavy metals, organic compounds (Khan et al., 2018), and in the greening of the dyeing operation of cellulosic fibres by reducing the pollutant load of dye baths (Wu et al., 2023). The selective oxidation leads to a number of structural changes, affects the intraand inter-molecular bonds in the cotton fibre, leads to some degradation of the fibres by dislocation of amorphous zones and to a small extent also acts on the crystallinity of the oxidized fibres, but the latter action is dependent on the parameters of the oxidation reaction which is also reflected in the newly acquired properties such as tensile strength, elongation, high temperature resistance, moisture sorption capacity (Liu et al., 2018), increased reactivity and affinity towards chemical compounds (Zhang et al., 2020, Zhang et al., 2017). All these properties are largely due to the chemical and morphological structure of cellulose, briefly described below, but also to the reaction parameters (concentration of oxidizing agents, pH and temperature of the reaction medium, oxidation time) that control the degree of oxidation, and the evolution of the Bul. Inst. Polit. Iaşi, Vol. 71 (75), Nr. 3, 2025 87 degree of polymerization, often decreasing due to the damage of the cellulosic structures (Coseri, 2017; Potthast et al., 2007). The aim of this review is to present the current state of knowledge related to the selective oxidation of cotton textile materials, the analytical and instrumental methods used to quantify the functional groups obtained after oxidation, as well as the morphological structure changes that it undergoes during the selective oxidation reaction. The cellulosic structures to which reference will be made are selectively oxidized cotton yarns, knits and fabrics, and only in the part of the analysis of the chemical and morphological structure, where the methods of analysis applied for characterization are comparable, are those applied to the cellulosic forms of wood. 2. Cotton, cellulosic fibre Cotton cellulose is a highly valuable natural biopolymer of polysaccharide nature is a major component of plant walls and is one of the most abundant resources for biomaterials due to its biocompatibility and plant origin (Klemm et al., 2005; Chen et al., 2015; Luo and Zhang, 2010). The cellulose molecule is an insoluble linear homopolymer formed of β(1 →4)-Dglucopyranosidic β(1→4)-glucosyl units, the glucosidic bond is formed between the carbons in positions 1 and 4, as shown in Fig. 1a, called anhydroglucosidic anhydroglucosidic unit (AGU). The authors (Blackwell, 1982; Atalla et al., 1984) consider the repeating structural unit of cellulose, kelobiose, which actually consists of 2 AGU units adjacent to, but rotated by 180⁰ around the main catenary axis of cellulose, as shown in Fig. 1b. The ends of the chain contain a free hemiacetal group or free aldehydic group at the C1 position, with a reducing role, and at the other end at the C4 position, it contains a non-reducing group, a free hydroxyl group (Credou and Berthelot, 2014; Wohlert et al., 2022), as shown in Fig. 2. Fig. 1 ‒ Schematic representation of a cellulose chain (a) Repeating unit of cellulose molecular structure (AGU) (b) Cellobiose repeating unit. The figures have been drawn by the authors with ChemSketch. 88 Romeo Pruneanu et al. The three hydroxyl groups, primary at the C6 position and secondary at the C2 and C3 positions of the AGU are positioned in the ring plane (Tonozuka et al., 2014), as shown in Fig. 1a and Fig. 2 The excellent properties of cotton cellulose (hydrophilicity, increased adsorption, flexibility, mechanical strength, etc.) described by (Marjory, 1986; Gupta, 2013; Liu et al., 2018; Wohlert et al., 2022) are due to the composition, chemical and morphological structure of cellulose. Fig. 2 ‒ Schematic representation of the cellulose chain, on the right the reducing end (free aldehyde group) and on the left the non-reducing end (free hydroxyl group). The figures have been drawn by the authors with ChemSketch. Figure 3 illustrates the chemical composition and morphological structure of cotton fibre (Gallo and Almirall, 2009). As can be seen cotton fibre has a hierarchical structure (Krakhmalev and Paiziev, 2006; Madhushree et al., 2024) consisting on the outside of the cuticle, which is a thin protective layer, consisting predominantly of non-cellulosic components, proteins (1.0-2.1%), waxes (0.4-1.7%), pectins (0.4-1.9%), inorganic substances (0.7-1.8%) and other substances such as resins, pigments, hemicelluloses (1.5-2.5%). This chemical composition is the general composition of raw cotton fibres but it may vary depending on the type, origin, fibre maturity, geographical area and agricultural conditions during the cultivation of the cotton plant (Stathakos et al., 2006; *** Cotton Morphology and Chemistry). Fig. 3 ‒ Morphological structure and chemical composition of cotton fibre. (Image adapted from Dochia et al., 2012). Bul. Inst. Polit. Iaşi, Vol. 71 (75), Nr. 3, 2025 89 The primary wall is a thin layer of randomly oriented, disordered cellulosic fibrils (amorphous zone) with less than 30% cellulose content, explains the high flexibility, adsorption capacity and high reactivity of cotton fibre. The secondary wall is the main layer, which contains 87 - 90% cellulose of the weight of the fibres, it is thick, consisting of fibrils arranged in concentric layers in which are distinguished the crystalline areas ordered, dense, responsible for the mechanical strength of the fibre. In the central part is the lumen, which occupies about one-third of the thickness of the fibre and is responsible for storing and transporting nutrients to the cell nucleus and protoplasm (Broadbent, 2001). Fibrils are made of microfibrils, which in turn consist of long chains of cellulose molecules with a degree of polymerization in the case of cotton between 9000 - 15,000 and a crystallinity index of about 73% (Blackwell, 1982). Degree of polymerization (DP) represents the number of glucose units in a polymer chain as defined by (Credou et al., 2014) and the degree of crystallinity, degree of polymerization, moisture content of the fibre is dependent on the intermolecular hydrogen bonds between the cellulose chains and influences the physico-mechanical and chemical properties of cotton fibre, it is an important aspect when the textile is designed for long or single use, regardless of the area of its applicability. 3. Selective oxidation functionalization of cellulose cotton fibres Cotton oxidation, traditionally applied after conventional pre-treatment processes, such as scouring to remove the tacking agent, alkaline or enzymatic cleaning is an essential operation that completely removes the cuticle, fatty substances, proteins and other non-cellulosic components, contributes to the increase of hydrophilicity of textile materials (Stanescu et al., 2010). Bleaching removes the natural pigments from the cellulose fibres, it is carried out immediately after cleaning, in alkaline conditions and in the presence of oxidizing agents, most often hydrogen peroxide is used. The study developed by (Nassif, 2019) states that all pre-treatment operations depending on the aggressiveness of chemical processes affect in the sense of decreasing air permeability, breaking strength, and according to the authors the influence of cleaning operation is more pronounced than bleaching. A considerable increase in elongation at break is observed with cleaning followed by bleaching. Selective oxidation of cellulosic fibres is an important method for the functionalization of cotton textile materials such as yarns, gauze, nonwovens and fabrics. Depending on the oxidizing agent used, carboxylic and/or aldehydic groups are generated by substituting the hydroxyl groups in the 6-position of the carbon of the pyranose ring with carboxylic groups when using the oxidizing agent 2,2,6,6,6-tetramethylpiperidin-N-oxyl (TEMPO), or a selective cleavage of the vicinal diols in the C2 and C3 positions of the AGU takes place, generating reactive aldehydic groups. The combination of periodate oxidation with the 90 Romeo Pruneanu et al. oxoammonium cations formed during TEMPO oxidation results in tricarboxylic celluloses (Sulaeva et al., 2015a; Coseri et al., 2013; Strnad et al., 2008; Diankova and Doneva, 2009). The positions of the three hydroxyl groups, primary position C6 and secondary positions C2 and C3 of the AGU unit are shown in Fig. 1a. 3.1. Periodate and TEMPO selective oxidation of cellulosic textile materials 3.1.1. Mechanism of periodate oxidation The oxidation of cellulose in aqueous medium with sodium or potassium periodate (NaIO4 or KIO4), the Malaprade reaction, is a commonly used procedure for the modification of cellulosic textile materials, is a highly selective oxidation method, occurs through a specific reaction on vicinal diols (C2-C3), following which bond cleavage at C2-C3 positions occurs and consists in the substitution of hydroxyl groups at C2-C3 positions in AGU by aldehydic and/or ketone groups (Kim et al., 2000). This chemical functionalization significantly contributes to improve some functional properties of cellulosic textile materials, and moreover, further allows various chemical modifications of post-oxidation, sulfonation or reductive amination, an additional functionalization for applications requiring increased hydrophilicity or hydrophobicity, high adsorption and sorption capacity, antimicrobial, hemostatic effect (Matsumura et al., 2000; Sulaeva et al., 2015a; Sun and Feng, 2024). The reaction mechanism consists in the formation of a cyclic intermediate between periodate and diol, the periodate reacts with the hydroxyl groups at the C2 and C3 positions, forming a cyclic periodate-cellulose complex intermediate, initiated by nucleophilic attack of the periodate (Sun and Feng, 2024), then followed by cleavage of the C2-C3 bond and release of the iodide. The selective oxidation reaction with periodate is shown in Fig. 4, the two aldehyde groups are formed at the C2 and C3 positions, and the cellulose thus oxidized is referred to in most studies as dialdehyde cellulose, generally denoted DAC, aldehyde functionalized cellulose (AFC) or oxycellulose (OC). Fig. 4 ‒ The main selective oxidation reaction of cotton cellulose with sodium periodate. The figures have been drawn by the authors with ChemSketch. Bul. Inst. Polit. Iaşi, Vol. 71 (75), Nr. 3, 2025 91 Only in the case of cellulose solubilization design, with a high degree of oxidation, close to 100 %, it can be considered that each AGU of the oxidized cellulose contains two aldehyde groups instead of the initial hydroxyl groups in the C2 and C3 positions. The aldehyde groups generated are reactive and can be found depending on the parameters of the oxidation reaction in various masked forms, which often makes it difficult to quantitatively estimate the amount of aldehyde groups by analytical and instrumental methods. Along with the oxidized forms with free carbonyl groups, hydrated aldehyde structures can be formed, hemialdal between C2/C3 and inter and intramolecular hemiacetal between C2/C6 and C3/C6 structures are possible within the glycosidic units, as shown in Fig. 5 (Nypelö et al., 2021; Sulaeva et al., 2015b). Fig. 5 – Formation of intermediate structures (hydrated cellulose, interand intramolecular hemiacetal, hemialdal). The figures have been drawn by the authors with ChemSketch. The recent study by Brault (Brault et al., 2005), shows that the effect of side reactions, which may develop during oxidation under mild reaction conditions, numerous side reactions may occur due to the limited stability of the active form of the periodate molecule, which is dependent on the pH, concentration and temperature values used. The authors emphasize the effects of light and temperature on the instability of the hypervalent sodium periodate IO4- (+VII), which can be decomposed into different reduced forms: iodate IO3- (+V), iodide IO2- (+III), iodide I- (-I), or molecular iodine I2 (0). Side reactions of the metaperiodate may also occur with cellulose, in particular by oxidation of more than two vicinal hydroxyl groups in the AGU units at the chain end. These socalled non Malaprade reactions lead to the formation of soluble oxidized byproducts such as formic acid and formaldehyde. 92 Romeo Pruneanu et al. Simon and co-workers in the paper (Simon et al., 2021; Simon et al., 2023) recommend in the case of periodate oxidation to avoid any contamination with other compounds, a simple water wash to remove unreacted periodate. They specify that oxidation with periodate in aqueous solution often leads to hydrated, hemialdal, hemiacetal cellulose structures in equilibrium (Fig. 5) and only a small amount of the generated adehyde groups at the C2 and C3 positions are found in free form, which is why instrumental spectroscopic techniques (FTIR, NMR, Raman) do not detect aldehyde content, unlike analytical titration techniques (Amer et al., 2016; Kim et al., 2000; Simon et al., 2022). 3.1.2. Mechanism of nitroxyl radical oxidation (2,2,2,6,6-tetramethylpiperidin-N-oxyl or TEMPO) Selective oxidation with TEMPO is a method, which utilizes nitroxyl radicals, is a reaction taking place at the primary hydroxyl groups in the C6 position of the AGU unit with formation of carboxyl groups. In the case of cellulosic textile materials selective oxidation with TEMPO in the presence or absence of NaBr is reported in the scientific work by Toshikj (Toshikj et al., 2019; Isogai et al., 2011). The reaction mechanism consists in the in-situ generation of the nitrozonium cation in the first step of the process, then in the presence of oxidizing agents such as sodium hypochlorite and bromide salts, hypobromide anions are produced, which further oxidize the nitrozonium cation to N-hydroxy2,2,6,6-tetramethylpiperidine. The reaction medium has a pH of around to 1010.5, provided by the presence of NaOH, which also neutralizes the carboxyl groups formed by oxidation. The reaction mechanism is shown in Fig. 6, according to Duceac (Duceac et al., 2022). Fig. 6 ‒ Selective oxidation mechanism of cellulose in the presence of TEMPO, NaClO and NaBr (Duceac et al., 2022). Bul. Inst. Polit. Iaşi, Vol. 71 (75), Nr. 3, 2025 93 As shown in Fig. 6 the intermediate species that are formed during the oxidation process are: N-hydroxy-2,2,6,6,6-tetramethylpiperidine (the reduced TEMPO form); the nitrotium radical (the oxidized form of TEMPO); the nitrotium cation (the active oxidizing species in cellulose); and the sodium salt of aldehyde hydrate. In the first stages of oxidation, the primary hydroxyl groups, C6 position are oxidized, aldehyde groups are formed, but as in the case of periodate oxidation there is the possibility of formation of hemiacetal compounds, by intraand intermolecular reactions between hydroxyl groups and aldehyde groups, but also hydrated forms (Kato et al., 2003; Duceac et al., 2022). In the final stage the aldehyde hydrates and/or hemiacetal hydrate forms are oxidized by oxoammonium cations, resulting in sodium salts of carboxylic acid fragments and the intermediate, reduced form of TEMPO. The authors De Nooy and Isogai (De Nooy et al., 1994; Isogai et al., 2011) show that in TEMPO/NaBr/NaClO oxidation of native cellulose with type I cellulose crystal structure no change in morphological structure, especially on the crystalline zone, is observed, even when using large amounts of reagents. The authors note only the attack on the microfibrillar surface, which leads to low oxidation degrees, an objective often pursued in the case of cellulosic textile materials with long use (Bragd et al., 2000). 3.1.3. Kinetics of selective oxidation reactions Sultana N. (Sultana et al., 2024) reviews the kinetics of periodate oxidation and provides important insights related to the optimization of the oxidation process. They consider that, the reaction parameters, temperature and pH, as well as high concentrations of periodate IO4 - (above 1 mol/L), considerably influence the obtaining of DAC, but also of the hydrated intermediate forms, hemiacetal and aldal. The authors show that the formation of DAC follows the pseudo-first order kinetic pattern, and that driving the oxidation reaction at high temperatures and high periodate concentrations leads to a significant growth and rapid formation of the DAC form, but also to a significant degradation of the cellulose structure in the crystalline zone. They show schematically the three reactions with rate constants for DAC formation (k1), cellulose degradation (k2) and DAC degradation (k3) as shown in Fig. 7. The kinetic model developed by the authors shows that increasing the temperature has a more pronounced effect on k1 than increasing the periodate concentration, and also any increase in temperature by 10°C leads to an increase of about 5% in DAC formation, but also to an increase of about 15% in degradation of the cellulosic structure. Sun and co-workers confirm in the study undertaken (Sun et al., 2015) that, temperature plays a significant role in the formation of DAC, but the increase in temperature is detrimental to the accelerated degradation of the cellulosic 100 Romeo Pruneanu et al. triphenyltetrazolium formazan (TTF), insoluble in water, red in colour, according to the reaction in Fig. 11 (Ahmad, 2021). After separation of the formazan and its solubilization in dimethylsulfoxide (DMSO) or absolute ethyl alcohol, the absorbance of the samples is measured spectrophotometrically. The content of aldehyde groups can be quantified based on a calibration curve drawn for known concentrations of aldehydes such as glyoxal. The method has been validated on compounds with known aldehyde content, such as formaldehyde, acetaldehyde and glutaraldehyde by Marjamaa (Marjamaa et al., 2024). Fig. 11 ‒ Reaction mechanism of 2,3,5, triphenyltetrazolium chloride (TTC) with DAC and formation triphenyltetrazolium formazan (TTF). The figures have been drawn by the authors with ChemSketch. The study (Fras Zemljič et al., 2009) describes the method for determining aldehyde groups with TTC according to Szabolcs (Szabolcs et al., 1961), which suspends approximately 0.6-4.6 g of dry cellulose fibre in a 0.1% TTC solution and a 0.2 M KOH solution, at 100º, for 10 minutes, during which the coloured product, formazan, is formed. The content of reduced aldehyde groups under these conditions is determined spectrophotocolorimetrically at 546 nm, based on the determination of the formazan formed (Fras Zemljič et al., 2009) or at 482 nm as shown (Ahmad et al., 2021), determination at the UV-VIS spectrometer of the reacted TTC. The study developed by (Toshikj et al., 2017), comparatively analyses the 2 methods, the iodometric and the TTC method, and states that the TTC method allows for a more sensitive detection of aldehyde groups after oxidation of cotton yarns with periodate, and has a higher accuracy than the iodometric method. Bul. Inst. Polit. Iaşi, Vol. 71 (75), Nr. 3, 2025 101 4.1.4. Elemental analysis The previously listed analytical methods applied for the quantification of functional groups can be a challenge most of the time in the case of yarns, cellulose fibres as stated (Vicini et al., 2004). They offer a simpler alternative for determination, namely elemental analysis, which can be applied in the case of the hydroxylamine hydrochloride method to determine, for example, the nitrogen content of oxime-modified aldehydes. Elemental analysis is also reported by (Fras Zemljič et al., 2009) in the case of oxidation of cotton yarns with 0.01 M KIO4, at 20°C for 12 hours, by determination e.g. by XPS. The recorded spectra show for the unoxidized samples an elemental composition of (% O - 39.8; % C – 60.3; % N -0; % Si0.6), and for the samples oxidized with KIO4 (% O - 36.7; % C – 62.5; % N -0.2; % Si0.4). 4.1.5. Method for determining the cupric index The content of aldehyde groups can also be determined spectrophotometrically by the cupric number (cooper number), which is defined according to TAPPI T430 (TAPPI Official Test Method T430, 1994), and by (Diankova et al., 2009), as the number of grams of metallic copper, Cu2O, that can be reduced from a solution of copper (II) sulfate by 100 grams of cellulose fibre. The principle of the method is based on the reaction of aldehyde groups with copper ions (Cu2+). The authors state that, in the case of oxidation of cotton cellulose materials (gauze, knitwear, nonwovens) with sodium periodate under conditions of pH=3.0, temperature of 20℃, for 20 hours, high values of copper number show the formation of aldehyde groups in the C2 and C3 positions of the AGU monomer, in contrast to oxidation in the presence of H2O2 and NaClO, at pH=9.0-9.5 where low values of copper number confirm the formation of ketone groups to the detriment of aldehyde and carboxyl groups. These non-selective oxidizing agents produce ketocelluloses (Diankova et al., 2009). 4.1.6. UV-VIS spectrophotometric method Another method is based on UV-VIS spectrophotometric determination of the residual (unreacted) periodate concentration. It is an indirect method for determining the carbonyl groups formed. The periodate consumption is monitored using UV-VIS spectroscopy by measuring the absorbance of periodate at 222 nm, a method applied by (Han et al., 2010; Simon et al., 2022) or at 290 nm by (Wen et al., 2023). The concentration of aldehyde groups is quantified by the difference between the initial periodate concentration and the residual concentration. The periodate concentration is calculated from the recorded absorbance, based on a calibration curve for known periodate concentrations. 102 Romeo Pruneanu et al. 4.1.7. FTIR-ATR spectroscopic method (Fourier transform infrared spectrometry using attenuated total reflectance) Identification of aldehyde groups in oxidized cellulose species using infrared (IR) analysis is often used, even though it is often not conclusive. In the case of cotton textiles, FTIR-ATR was used to analyse the changes in the chemical structure following oxidation with sodium periodate. The typical band of aldehyde groups appears at wavelengths of 1730-1740 cm-1 (Sun et al., 2015; Kim et al., 2000; Durán et al., 2018) and two absorption bands are assigned at wavelengths of 1740 cm-1 and 880 cm-1 for the carbonyl group, respectively for the hemiacetal form or for the hydrated aldehyde forms. Kim (Kim et al., 2000) shows that only at an oxidation degree of around 12% is the well-defined peak at 1740 cm-1 recorded, and for the hemiacetal form a diffuse peak appears only at oxidation degrees exceeding 20%. Identifying changes in chemical structure by selective oxidation with acceptable accuracy may only be possible at relatively high degrees of oxidation. In the case of oxidation of textile materials, in order to obtain quantifiable data, most of the time FTIR-ATR analysis is correlated with analytical methods for determining new functional groups or by correlation with near infrared spectroscopy (NIR (Simon et al., 2022). The authors (Toshikj et al., 2019) in agreement with (Sivakova et al., 2008) confirm that, in the case of cotton oxycelluloses, the identification of aldehyde groups is difficult, due to the coexistence of fully hydrated or hemiacetal forms, which is why broad regions between 1720 -1780 cm-1 are identified in the FTIR-ATR spectra. Oh and co-authors (Oh et al., 2005) calculate the relative absorbance ratio (A4000–2995/A993), as a criterion for assessing the decrease in hydrogen bond intensity by decreasing absorbance. They also mention the absorbance at 1430 cm-1 and 987–893 cm-1, calculate the ratio (A1430/A900), which is attributed to the adsorption of the crystalline area and used to calculate the crystallinity index of the cellulose I structure. 4.2. Quantification of carboxyl groups An advantage of the periodate oxidation method is the possibility of subsequently oxidizing the aldehyde groups to carboxylic groups in the presence of e.g. NaClO2 shown in the reaction in Fig. 12. The content of aldehyde groups is calculated by the difference between the carboxyl groups before and after postoxidation, especially when textile materials previously bleached by a nonselective oxidation procedure, such as e.g. with NaClO, H2O2 or HClO4, are subjected to selective oxidation with periodate and/or TEMPO, processes by which already before oxidation with periodate or TEMPO certain primary and/or Bul. Inst. Polit. Iaşi, Vol. 71 (75), Nr. 3, 2025 103 secondary hydroxyl groups in AGU are oxidized to ketone, carbonyl or carboxyl groups as shown (Diankova and Doneva, 2009). Fig. 12 ‒ Oxidation of DAC with sodium chlorite. The figures have been drawn by the authors with ChemSketch. The same principle is applied in several studies (Fras et al., 2004; Liimatainen et al., 2012; Toshikj et al., 2019, Kumar and Yang, 2002) for the determination of new functional groups. The determination of the content of aldehyde groups is carried out by analysing the group content of the carboxylic acid derivatives of the dialdehyde polysaccharide before and after oxidation with sodium chlorite. A recent study by (Brault et al., 2005) considers that both oxidative steps, Fig. 12, are known to be particularly slow and can lead to several secondary reactions, which are responsible for the discrepancies systematically observed in the determination of the DO. The methods for titrating carboxylic groups in DCC before and after oxidation with sodium chlorite are not conclusive, significant differences are obtained in the values obtained for quantifying aldehyde groups compared to the values obtained by established analytical methods for determining aldehyde groups (e.g. titration with hydroxylamine hydrochloride, iodometric or with TTC), most likely due to the consumption of chlorite only at a single aldehyde group and not at both groups, and this probably occurs due to the intramolecular formation of hemiacetal-type structures. The most used determination methods in this case are the Ca-acetate method, followed by titration with sodium hydroxide (NaOH), or complexometrically by titrating with ethylenediaminetetraacetic acid (EDTA) in the presence of murexide or Eriochrome Black T. 4.2.1. Ca-acetate and sodium hydroxide method (Ca-acetate-NaOH) The authors (Toshikj et al., 2016) describe this method as simple, very easy to conduct for the determination of carboxyl groups, consisting of treating oxidized cellulose fibres with a Ca-acetate solution, which is maintained at room temperature under stirring for 12 hours to facilitate ion exchange, then filtered and titrated with NaOH solution, in the presence of phenolphthalein. 104 Romeo Pruneanu et al. 4.2.2. Complexometric method Ca-acetate and ethylenediaminetetraacetic acid (Ca-acetate - EDTA) This method has a greater applicability than the Ca-acetate - NaOH method, ensuring greater reproducibility and sensitivity as stated (Fras et al., 2004; Toshikj et al., 2017). The method consists of immersing cotton fibres in distilled water and Ca-acetate solution at a pH=10.0, provided by the ammonia buffer solution under frequent stirring for 12 hours, then filtering, and the filtrate is titrated with EDTA solution in the presence of the Eriochrome Black T indicator. 4.2.3. Methylene blue (MB) method In order to determine the carboxyl groups, the UV-VIS spectrophotometric method can be applied at a wavelength between 600 and 660 nm, in the case of determining the carboxyl groups by the method of staining the oxidized samples with methylene blue (MB), a cationic dye, which binds to the carboxyl groups of the oxidized cellulose (Ewa et al., 2001; Chen, 2002). This method is based on the principle of light absorption by a coloured compound, in this case unabsorbed, unreacted MB. It is a relatively simple and efficient technique for quantifying carboxylic groups by measuring the absorption of electromagnetic radiation. The authors (Fras et al., 2004) studied the selective oxidation of cotton yarns with periodate and the non-selective oxidation with perchloric acid and concluded that both methods, the MB staining method and the Ca-acetate EDTA complexometric titration methods, are in excellent correlation. They state that complexometric titration induces smaller systematic errors and higher accuracy than the methylene blue spectrophotometric method, which is recommended only as a reference method to confirm any trend in the results obtained by the complexometric method. 4.2.4. Conductometric method The study developed by (Fras et al., 2005; Toshikj et al., 2019) refers to the conductometric determination of carboxylic groups, according to the method described by (Saito and Isogai, 2004), in which oxidized cellulose samples are transformed into protonated forms by treatment with hydrochloric acid (pH=2.5), a step that is followed by immersing the samples in a sodium chloride solution, necessary to control the ionic strength of the solution, then titrated with a NaOH solution to a pH = 11. Bul. Inst. Polit. Iaşi, Vol. 71 (75), Nr. 3, 2025 105 4.2.5. FTIR-ATR spectroscopy FTIR-ATR spectroscopy also provides information on the formation of carboxyl groups. The peaks recorded at 1750 cm-1 and 1650 cm-1 correspond to the stretching of the C=O bonds in the carboxylic group in the protonated form, respectively to the vibrational stretching of the C=O bonds in the carbonyl groups of the carboxylate ions, as shown (Wang et al., 2024). Also, an intact structure of the cellulose macromolecule is confirmed by the presence of characteristic peaks at 3460 cm-1 (stretching vibration of O-H bonds), 2900 cm-1 stretching of the CH bond and 1050 cm-1 stretching of the C-O-C bonds in the cellulose skeleton, values confirmed by (Wang et al., 2024; Calvini et al., 2006; Liu et al., 2018). 4.2.6. Values of the content of functional groups (aldehyde and/or carboxyl) of selectively oxidized textile materials Values of the content of functional groups (aldehyde and/or carboxylic) of selectively oxidized textile materials depending on the reaction parameters and the analytical determination method are listed in Table 1. The reaction conditions, the oxidizing agents used were described in subsection 3.2. Table 1 Content of functional groups (aldehyde and/or carboxylic) of selectively oxidized cotton textiles depending on the nature of the textile material and the analytical determination method Fabric type Method Functional groups mmol/kg Reference Aldehyde Carboxylic Cotton yarn Alkaline scouring H2O2 bleaching HCl demineralization KIO4 oxidation MB - 48.25 - 50.44 Fras et al., 2004 Ca acetate-EDTA - 21.36 - 26.91 Cotton yarn without pretreatment Hydroxylamine hydrochloride 75 -100 Pejic et al., 2015 Raw cotton yarn without pretreatment Oxidation with NaIO4 0.2% -0.4% (w/v) Ca acetat -NaOH - 50 - 99.2 Nikolic et al., 2011 Cotton pulp (alkaline treatment) Oxidation with NaIO4 0.4 g/g cellulose or 8 g/L Conductometric titration - 714.9 Wang et al., 2024 106 Romeo Pruneanu et al. Cotton pulp (alkaline treatment) TEMPO/NaBr/ NaClO Conductometric titration 700.7 Cotton yarn Alkaline scouring and mercerization TTC 216.5 - Toshikj et al., 2017 I 90.3 - MB - 11.5 Ca acetate-EDTA - 25 Ca acetate-NaOH - 53.6 Cotton yarn Alkaline scouring, bleaching with H2O2 TTC 99 - Fras Zemljič et al., 2009 Cotton yarn Alkaline scouring, bleaching with H2O2 KIO4 oxidation +TEMPO/NaBr/ NaClO Conductometric titration 88 793 Toshikj et al., 2019 Ca acetate -NaOH 101 747 Cotton yarn Alkaline scouring, bleaching with H2O2 KIO4 oxidation + TEMPO/ NaClO Conductometric titration 134 684 Ca acetate -NaOH 179 640 Cotton knitwear Hydroxylamine hydrochloride 113.3-187.4 - Hao et al., 2018 Viscose knitwear 1284 - Raw Cellulose Fibre Oxidation TEMPO/NaBr/ NaClO Ca-acetate-NaOH 80 - 321 123 - 795 Praskalo et al., 2009 Lyocell Oxidation TEMPO/NaBr/ NaClO 40 - 634 180 - 700 Gauze, cotton knit Viscose/PES nonwoven Alkaline scouring Cupric index Gauze - 24.74 g I2/100g Knit - 23.84 g I2/100g Nonwoven - 1.7 g I2/100g - Diankova and Doneva, 2009 I Gauze - 513.1 Knit – 491.7 Nonwoven – 23.4 - Bul. Inst. Polit. Iaşi, Vol. 71 (75), Nr. 3, 2025 107 4.3. Degradation analysis of the morphological structure of the oxidized cellulose macromolecule 4.3.1. Loss of cellulose substance Cellulose loss is determined directly gravimetrically on cellulosic samples dried at constant pond, before and after oxidation. Aggressive oxidative attack on cellulosic materials can lead to significant cellulosic loss (Nikolic et al., 2011; Praskalo et al., 2009). Nikolic and co-authors (Nikolic et al., 2011) show that for raw cotton yarns oxidized with 0.2% and 0.4% (w/v) sodium periodate, the loss of cellulose is 2.5% and occurs in the first 30 minutes, after which it remains constant and is independent of increasing oxidizing agent concentration or oxidation time. Praskalo (Praskalo et al., 2009) reports in the case of oxidation of cotton yarns with TEMPO/NaBr/NaClO at the lowest concentration used of 0.3 mmol/g after 4 hours of oxidation a loss of cellulose of 2.78%, and at the maximum concentration of 9.67 mmol/g the same duration, a loss of 11.34%. 4.3.2. Determination of the degree of polymerization by intrinsic viscosity measurement The degree of polymerization (DP) of oxidized cellulose decreases with increasing degree of oxidation and is due to DAC depolymerization. Also, the degree of polymerization decreases with increasing number of aldehydic or carboxyl groups. states (Hao et al., 2018). DP by viscometric method is determined according to ISO 5351-2010. DP evolution of cellulosic forms is followed by dissolving cotton textile fibres. The authors (Fras et al., 2004; Toshikj et al., 2017) dissolve the cellulosic fibre in the sodium salt of ferric tartaric acid (EWNN) and determine the intrinsic viscosity (ɳ) of the solutions by the modified Ubbelohde method (capillary length 78 mm, capillary diameter 0.75 mm, 0.7 cm3). DP was calculated using the Kuhn, Mark, Houwink relation, eq. (7): 𝐷𝑃ɳ ɑ=1 𝐾𝑝ɳ (7) where: Kp =33 10-3 [dL/g] and ɑ =0.74. In the study by Wang and co-authors (Wang et al., 2024) cellulosic fibres are dissolved in cupric ethylenediamine (CED) and the intrinsic viscosity is determined on a viscometer. The relation he uses to calculate the DP is given by eq. (8): 𝐷𝑃0.905 = 0.75 ɳ (8) 108 Romeo Pruneanu et al. According to (Wang et al., 2024) the cotton fibres selectively oxidized with TEMPO/Na Br/NaClO (0.0025 g TEMPO/fibre) and sodium periodate (0.4 g NaIO4/g cotton fibre) respectively have DP values of 101.86 and 731.77 respectively, compared to the non-oxidized cotton fibre, which has DP= 1872.82, showing a degradation of cellulosic fibres, periodate oxidation has a lower effect on depolymerization. The study on periodate oxidation (1 g/L KIO4, 60℃, 1 hour) of mercerized and alkaline-scoured cotton yarns (Toshikj et al., 2017) obtained a DP for oxidized yarns of 206.28, compared to 1185.51 for non-oxidized fibres. 4.3.3. Differential Scanning Calorimetry (DSC) method Differential Scanning Calorimetry (DSC) is an important method that measures the heat flux associated with amorphous or crystalline, transitioning structural changes of materials as a function of temperature and time in a controlled atmosphere. DSC provides quantitative and qualitative information about physical and chemical changes in endothermic or exothermic processes. The authors of the paper (Xu and Du, 2011) subjected cotton fibres to periodate oxidation at different concentrations (0.1 - 1.0 mg/L) and durations (0 - 3 hours). As the authors state the DSC curves for the samples oxidized in the endothermic stages decomposed at a higher temperature than the non-oxidized samples up to 350℃, however, with increasing oxidation degree, the endothermic decomposition peak of oxidized cotton cellulose shifted rapidly, indicating the decrease in decomposition temperature. 4.3.4. Determination of mechanical resistance Also, in order to assess the degree of degradation of the cellulosic matrix, mechanical strengths such as tensile strength, elongation at break (Nypelö et al., 2021) are determined, which are performed according to the current standards EN ISO 2062: 2009. Related to the mechanical strength of oxidized cellulosic fibres (Toshikj et al., 2017), reports in the case of periodate oxidation, a tensile strength and elongation at break lower by only 23% and 15.8%, respectively, compared to non-oxidized samples, which shows that, a moderate degradation of the cellulosic macromolecule occurs. In the study also developed by Toshikj (Toshikj et al., 2019) the selective oxidation with KIO4 (0.1 - 1.0 g/g cellulose) and TEMPO of alkaline-cleaned cotton yarns bleached with hydrogen peroxide leads to a 65% decrease in breaking strength at the maximum concentration and 33% at the minimum concentration. In the case of TEMPO-mediated oxidation without NaBr, a 77.3% decrease is reported for 120 minutes under the same conditions, but in the presence of NaBr the decrease is 73.2%. The decrease in values is due to the Bul. Inst. Polit. Iaşi, Vol. 71 (75), Nr. 3, 2025 109 presence of NaClO which accelerates the degradation, NaBr has an additional effect in favouring the degradation of the cellulose molecular chain. 4.3.5. FTIR-ATR spectroscopy FTIR-ATR spectroscopy can also give information on the integrity of the cellulosic macromolecule and the degradation of the cellulosic macromolecule (Chinkap et al., 2004; Youn et al., 2005). Wang and co-authors (Wang et al., 2024) attribute the peaks recorded at 1750 cm-1 and 1650 cm-1 to the stretching of the C=O groups in the protonated form of the carboxylic groups, respectively, to the vibration of the C=O bond in the carbonyl groups in the carboxylated form. The peak corresponding to absorption at wavelength 1050 cm-1 indicates cleavage of the glycosidic bond in the cellulosic macromolecular skeleton. Simon in the study (Simon et al., 2022), indicates the absorption bands at wavelengths 2900 cm-1 and 1030 cm-1 respectively indicate C-H bond stretching and C-O-C bond vibration in the pyranosidic ring. The absence of these peaks may confirm the intact structure of the cellulosic macromolecule, which is characterized by peaks at 3460 cm-1 (OH bond stretching vibration), 2900 cm-1 C-H bond stretching) and 1050 cm-1 (CO-C bond stretching in the cellulosic skeleton), values also attributed by (Wang et al., 2024). 4.4. Analysis of the morphological structure of cellulose macromolecule and properties acquired after selective oxidation 4.4.1. X-ray diffraction (XRD) X-ray diffraction is a frequently used method for evaluating structural changes in cellulose. The crystallinity index (CrI) is a valuable indicator, expressed in % and calculated by the area ratio method of the crystalline and amorphous peaks. Depending on the reaction conditions, periodate oxidation induces some rearrangement of the cellulose macromolecular chain, with a slight perturbation of the crystallinity of the fibre. In case of cotton fibres crystallinity index is an essential parameter to characterize the internal, fine structure of the fibres, it is calculated by the peak area ratio method as mentioned by (Xu and Du, 2011). They state that, in the case of cellulose, the most commonly chosen crystalline peak is the 002 reflection, which is present between 22-24° at 2θ, usually choosing the intensity at about 18° (the minimum between peaks). Wang and co-authors (Wang et al., 2024) report for TEMPO-oxidized cellulose and periodate the crystalline peak at 22.8⁰ at 2θ, while for the noncrystalline (amorphous) value, the values 14.9⁰ and 16.7⁰, respectively. They state that, XRD is an empirical method, it can only provide relative values, because the 116 Romeo Pruneanu et al. 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Zhao H., Heindel N.D., Determination of degree of substitution of formyl groups in polyaldehyde dextran by the hydroxylamine hydrocloride method, Pharm Res., 8, 400-402 (1991). *** Cotton Morphology and Chemistry, Cotton Incorporated, Available from (https://www.cottoninc.com/quality-products/nonwovens/cotton-fiber-techguide/cotton-morphology-and-chemistry/. REVIEW - FUNCȚIONALIZAREA CHIMICĂ A MATERIALELOR TEXTILE CELULOZICE DIN BUMBAC PRIN OXIDARE SELECTIVĂ (Rezumat) Acest studiu documentar prezintă posibilitățile de oxidare selective mediate de periodat de sodiu sau potasiu și/sau de 2,2,6,6-tetrametilpiperidin-1-oxil (TEMPO) ale materialelor textile din bumbac, mecanismele de reacție, aprecierea și modul de determinare analitică și instrumentală a gradului de oxidare ale grupărilor hidroxilice din pozițiile C2, C3 și C6 a unității AGU cu grupări aldehidice/carboxilice. Se prezintă efectele oxidării selective asupra structurii moleculare și morfologice ale celulozei, deoarece oxidarea în funcție de condițiile de reacție poate afecta semnificativ atât zona amorfă, cât și zona cristalină a celulozei, la nivel de microfibrile, ceea ce conduce la scăderea rezistențelor mecanice, fapt nedorit în cazul materielor textile cu utilizare îndelungată sau chiar de unică folosință. Oxidarea selectivă în acest caz este un compromis între funcționalitate (exprimat prin conținutul de grupări aldehidice/carboxilice) și degradarea moderată a structurii moleculare și morfologice a celulozei din bumbac.