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0METHODS OF INCREASING THE SENSITIVITY AND REACTIVITY IN THE CHEMICAL PROCESSING OF CELLULOSE SAMPLES BASED ON FIBER WASTE OF COTTON GAINING ENTERPRISES
1Rakhmonov J., 2Bozorov O.
1,2University of Economics and Pedagogy, Karshi, Uzbekistan https://doi.org/10.5281/zenodo.13994983
Abstract. During chemical treatment of cellulose samples based on fibrous waste from cotton gins (FWCG), depending on the chemical composition of the cellulose, related preparations often react differently, esterify at different rates, and yield products of different quality. This raises the question of the reactivity of cellulose. In cellulose preparations with non-uniform reactivity, the fibers react to different degrees during esterification, resulting in fibers in the resulting product having different solubility.
Keywords: temperature, cellulose, component, macromolecules, cooking, homogeneous, purification, reaction, cooking, autoclave, structural, complex.
INTRODUCTION
Often, reactivity refers to the rate of the esterification process and the quality of the resulting products - their complete solubility, homogeneity, transparency, and the ability to form high-concentration, filterable solutions. In this context, it is not the rate of chemical interaction of cellulose macromolecules that is evaluated, but the solubility of the resulting xanthates in alkali, which in turn affects the filtration of viscose solutions. However, these characteristics are not usually referred to as reactivity [1]. Thus, the concept of cellulose reactivity is ambiguous. Therefore, it is advisable to use two terms instead of one: reactivity (or chemical reactivity) and processability. The chemical reactivity of cellulose describes the rate of the esterification process of cellulose. The rate of esterification of various cellulose preparations varies widely. This is primarily determined by the heterogeneity of the chemical reactions taking place and the non-uniformity of the cellulose materials. The hydroxyl groups in the elementary units of the cellulose macromolecule have different reactivities; elementary units in the cellulose macromolecule may differ from each other by the presence of certain functional groups (carbonyl, carboxyl groups) and may have different conformations ("chair," "boat"). Like other high-molecular compounds, cellulose is a polydisperse product, with macromolecules containing varying numbers of units and thus showing some isomeric differences [2]. The packing density of cellulose macromolecules into several types of crystalline structures can also vary. Finally, there is no morphological uniformity. All of these factors determine the reactivity of cellulose. Cellulose intended for chemical processing must exhibit high and homogeneous reactivity. Non-uniform reactivity during cellulose esterification can lead to incomplete esterification, an excessively long process, or premature termination of the reaction. The most reactive part of cellulose reacts much faster than the bulk of the material, while the main portion reacts at a certain defined rate. Additionally, the less reactive part reacts significantly slower than the main portion. Finally, there may be fractions in the preparation that do not react under these conditions at all [3].
The roles of these components vary when obtaining different cellulose derivatives. In general, this division is conditional, as these fractions of cellulose cannot be precisely determined. The homogeneity of cellulose in terms of reactivity is particularly important when obtaining partially substituted derivatives (such as xanthates). In this case, cellulose with a more uniform degree of substitution gives more homogeneous, fully soluble products, while the presence of fibers that are difficult to react can behave similarly to fibers that do not react under these conditions at all. To obtain fully soluble products from cellulose with non-uniform reactivity, esterification is required up to a higher average degree of substitution, resulting in greater consumption of esterifying agents. Thus, the solubility (i.e., filterability) of cellulose derivatives as the consumption of esterifying agents' changes can serve as a criterion for evaluating cellulose quality. During esterification, fibers from non-uniformly reactive cellulose preparations will react at different rates, resulting in products with fibers of varying solubilities. Correspondingly, the swelling of the fibers before dissolution will also vary. Therefore, observing the swelling pattern of esterified fibers can serve as a criterion for evaluating cellulose homogeneity while also characterizing the fiber structure. The rate of reactivity of various parts of cellulose preparations can be characterized by different methods. The kinetics of the esterification process can be studied by determining the degree of substitution in the obtained samples or by monitoring the physicochemical properties of the reaction mixture (temperature, transparency, electrical conductivity, etc.) as they change during esterification [4].
The methods for sampling during the reaction and subsequently determining the degree of substitution primarily describe the reaction rate of the main part of the product, as does calorimetry in the case of exothermic reactions [5]. The esterification rate of the less reactive part, in reactions that terminate in a homogeneous medium, can be measured by changes in the transparency of the solutions. The non-reactive portion significantly affects the quality of the final products, including their turbidity and filterability. A comprehensive characterization of cellulose used for chemical processing is best achieved by employing a combination of methods.
As mentioned earlier, cellulose intended for chemical treatment must not only exhibit high-quality indicators but also possess sufficient reactivity for esterification.
Cellulose reactivity is a complex concept that reflects various aspects of the chemical processing process, including the kinetics of heterogeneous reactions, the uniformity of esterification, the degree of dissolution, the quality of solutions, and the final products. Reactivity is directly related to the molecular, supramolecular, and morphological structure of cellulose. The reactivity of cotton cellulose has driven the development of methods to increase reactivity in response to the expansion of insufficient raw material bases. This involves the activation of cellulose through targeted structural modifications [6].
EXPERIMENTAL PART
To enhance the reactivity of cellulose, we first treated the cellulose with an aqueous solution of ammonium carbonate. As a result of the electrolytic dissociation of ammonium carbonate in water, ammonium (NH4+) and carbonate (CO32") ions are formed, which easily penetrate the structural elements of the secondary cellulose wall, specifically into the voids (interlayer spaces and amorphous regions) of the microfibril areas. When these ions interact with water (hydrolysis), weak electrolytes are formed - ammonium hydroxide and carbonic acid, which decomposes into gaseous substances (NH3 and CO2);
1) (NH4+)2CO3 2-+2H2O = 2 NH4OH+H2CO3; 2) NH4OH= NH3Î+H2O; 3) H2CO3=CO2Î+H2O.
RESULT AND DISCUSSION
The resulting gaseous substances facilitate the disruption of hydrogen bonds between cellulose macromolecules and contribute to an increase in the pore volume of the fiber. The dependence of the pore sizes of activated cellulose samples on the concentration of (NH4)2CO3 has been determined. Table 1. The radius and distribution of capillaries in various cellulose specimens treated with
an ammonium carbonate solution
PC Density, g/cm3 Capillary Radius, Â
33-101 101-258 258-1001 1001-3989 3989-12987
Capillary Volume, cm3/g
5.9 1.4950 0.0236 0.0114 0.0066 - -
9.8 1.5272 0.0269 0.0069 0.0142 0.0049 0.0214
19.5 1.5183 0.0119 0.0138 0.0163 0.0144 0.0238
20.4 1.5252 0.0108 0.0147 0.0167 0.0154 0.0249
The study of the capillary structure and density of regenerated cellulose samples revealed that as the cellulose quality index increases, a decrease in density and an increase in the radius and volume of capillaries are observed [8]. In the initial cellulose sample, the volume of capillaries with a radius of 260-1000 Â is only 0.0064 cm3/g, whereas in the sample treated with a 0.5% ammonium carbonate solution, the volume of such capillaries reaches 0.0142 cm3/g. In the initial sample, there are no pores with a radius of 1000-4000 Â, but for cellulose with a PC (quality index) of 10.5, the volume of such capillaries is 0.0152 cm3/g, and for a PC of 20.4, it is 0.0168 cm3/g. Increasing the solution concentration has little effect on the capillary volume. A similar trend is observed in capillaries with radii of 4000-12000 Â. X-ray diffraction patterns and IR spectra were obtained for the activated cellulose samples. Based on this, the crystallinity degree and absorption area of hydroxyl groups bonded through hydrogen bonds with the S-OH group were calculated. The results are presented in Table 2.
Table 2. The effect of activation treatments on cellulose DP (Degree of Polymerization) and S-
OH groups
№ Concentration (NH4)2CO3, % CC S-OH
1 - 74 74
2 0,23 74 59
3 0,48 74 57
4 0,9 74 54
As can be seen from Table 2, treatment with an aqueous solution of ammonium carbonate does not affect the crystalline regions of the microfibrils; it only breaks the hydrogen bonds between cellulose macromolecules, resulting in a decrease in the SON value. An increase in the (NH4)2CO3 concentration beyond 0.5% does not significantly affect the SON value.
The dependence of the activation process of cellulose on temperature was studied. For this purpose, cellulose samples were treated with 0.25% concentration ammonium carbonate solutions
at various temperatures for 1 hour, using 0.5% and 1% solutions. Subsequently, the activated cellulose samples were acetylated using the Malm method, and the quality indicators of the syrups obtained by TAC were determined (Figure 1).
Figure 1. The dependence of TAC syrup PC on the temperature of the (NH)2CO3 solution at
various concentrations: 1-0.25%, 2-0.5%, 3-1%.
As can be seen from Figure 1, with an increase in temperature (up to 35-40°C) and concentration (up to 1%), the quality indicators improve, though the rate of improvement slows slightly above 40°C. It is evident that as the temperature increases, the rate of the hydrolysis reaction of (NH4)2CO3 also increases, and the decomposition of the resulting ammonium and carbonate acids reaches a maximum.
The graph also shows that the quality indicators of TAC obtained from cellulose activated with 0.5% and 1% (NH4)2CO3 differ slightly. Therefore, a 0.5% solution of (NH4)2CO3 can be considered the most effective concentration for cellulose activation [9].
We also studied the dependence of TAC solutions' PC on the treatment time for cellulose activation using a 0.5% concentration of (NH4)2CO3 (Figure 2).
Figure 2. The dependence of TAC syrup PC on the treatment duration of cellulose activation with a 0.5% concentration of (NH4)2CO3 at different temperatures: 1-25°C, 2-35°C, 3-40°C.
As can be seen from the curves in the diagram, when cellulose is activated at 40°C, the value reaches its maximum in 15 minutes, while at 25°C, it reaches its maximum in 35-40 minutes.
CONCLUSION
From this, we can conclude that the activation treatment of cellulose should be carried out in a 0.5% concentration solution of ammonium carbonate (NH4)2CO3 for 40 minutes. During this process of treating cellulose with an ammonium carbonate solution, the hydrogen bonds break, and the pore size of the fiber increases. These changes occur up to a (NH4)2CO3 concentration of
0.5.. Further increases in concentration do not have a significant effect. It was determined that the breaking of hydrogen bonds occurs only between the layers and void spaces of the microfibrils. Moreover, after the gaseous substances are removed from the pores, the pores remain empty, resulting in an increase in the reactivity of the cellulose.
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