The etherification activity of cellulose was studied by kneading machine and stirring reactor respectively, and hydroxyethyl cellulose and carboxymethyl cellulose were prepared by chloroethanol and monochloroacetic acid respectively. The results showed that the etherification reaction of cellulose was carried out by stirring reactor under the condition of high intensity agitation. Cellulose has good etherification reactivity, which is better than the kneader method in improving etherification efficiency and enhancing the light transmission of the product in aqueous solution.) Therefore, improving the stirring intensity of the reaction process is a better way to develop substituting homogeneous cellulose etherification products.
Key words: etherification reaction; Cellulose; Hydroxyethyl cellulose; Carboxymethyl cellulose
In the development of refined cotton cellulose ether products, solvent method is widely used and kneading machine is used as reaction equipment. However, cotton cellulose is mainly composed of crystal regions where molecules are arranged neatly and closely. When kneading machine is used as reaction equipment, the kneading arm of kneading machine is slow during reaction, and the resistance of etherifying agent to enter different layers of cellulose is large and the speed is slow, resulting in long reaction time, high proportion of side reactions and uneven distribution of substituent groups on cellulose molecular chains.
Usually the etherification reaction of cellulose is a heterogeneous reaction outside and inside. If there is no external dynamic action, etherifying agent is difficult to enter the crystallization zone of cellulose. And through the pretreatment of refined cotton (such as using physical methods to increase the surface of refined cotton), at the same time with stirring reactor for reaction equipment, using rapid stirring etherification reaction, according to reasoning, cellulose can strongly swelling, the swelling of cellulose amorphous area and crystallization area tends to be consistent, improve the reaction activity. The homogeneous distribution of cellulose ether substituents in heterogeneous etherification reaction system can be achieved by increasing the external stirring power. So it will be the future development direction of our country to develop high quality cellulose etherification products with stirred type reaction kettle as reaction equipment.
1. Experimental part
1.1 Refined cotton cellulose raw material for test
According to the different reaction equipment used in the experiment, the pretreatment methods of cotton cellulose are different. When the kneader is used as the reaction equipment, the pretreatment methods are also different. When the kneader is used as the reaction equipment, the crystallinity of the refined cotton cellulose used is 43.9%, and the average length of the refined cotton cellulose is 15~20mm. The crystallinity of refined cotton cellulose is 32.3% and the average length of refined cotton cellulose is less than 1mm when stirring reactor is used as reaction equipment.
1.2 Development of carboxymethyl cellulose and hydroxyethyl cellulose
The preparation of carboxymethyl cellulose and hydroxyethyl cellulose can be carried out by using 2L kneader as reaction equipment (the average speed during reaction is 50r/min) and 2L stirring reactor as reaction equipment (the average speed during reaction is 500r/min).
During the reaction, all raw materials are derived from the strict quantitative reaction. The product obtained from the reaction is washed with w=95% ethanol, and then dried by vacuum for 24h under the negative pressure of 60℃ and 0.005mpa. The moisture content of the sample obtained is w=2.7%±0.3%, and the product sample for analysis is washed until the ash content w < 0.2%.
The preparation steps of kneading machine as reaction equipment are as follows:
Etherification reaction → product washing → drying → grated granulation → packaging is carried out in kneader.
The preparation steps of stirring reactor as reaction equipment are as follows:
Etherification reaction → product washing → drying and granulation → packaging is carried out in a stirred reactor.
It can be seen that the kneader is used as reaction equipment for the preparation of the characteristics of low reaction efficiency, drying and grinding granulation step by step, and the quality of the product will be greatly reduced in the grinding process.
The characteristics of the preparation process with stirred reactor as reaction equipment are as follows: high reaction efficiency, the product granulation does not adopt the traditional granulation process method of drying and grinding, and the drying and granulation process are carried out at the same time with the undried products after washing, and the product quality remains unchanged in the drying and granulation process.
1.3 X-ray diffraction analysis
X-ray diffraction analysis was performed by Rigaku D/max-3A X-ray diffractometer, graphite monochromator, Θ Angle was 8°~30°, CuKα ray, tube pressure and tube flow were 30kV and 30mA.
1.4 Infrared spectrum analysis
Spectrum-2000PE FTIR infrared spectrometer was used for infrared spectrum analysis. All the samples for infrared spectrum analysis had a weight of 0.0020g. These samples were mixed with 0.1600g KBr, respectively, and then pressed (with a thickness of < 0.8mm) and analyzed.
1.5 Transmittance detection
The transmittance was detected by 721 spectrophotometer. CMC solution w=w1% was put into a 1cm colorimetric dish at 590nm wavelength.
1.6 Degree of substitution detection
The HEC substitution degree of hydroxyethyl cellulose was measured by standard chemical analysis method. The principle is that HEC can be decomposed by HI hydroiodate at 123℃, and the degree of substitution of HEC can be known by measuring the decomposed substances ethylene and ethylene iodide produced. The degree of substitution of hydroxymethyl cellulose can also be tested by standard chemical analysis methods.
2. Results and discussion
Two kinds of reaction kettle are used here: one is kneading machine as reaction equipment, the other is stirring type reaction kettle as reaction equipment, in heterogeneous reaction system, alkaline condition and alcoholic water solvent system, etherification reaction of refined cotton cellulose is studied. Among them, the technological characteristics of kneading machine as reaction equipment are: In the reaction, the kneading arm’s speed is slow, the reaction time is long, the proportion of side reactions is high, the utilization rate of etherifying agent is low, and the uniformity of substituting group distribution in etherizing reaction is poor. The research process can only be limited to relatively narrow reaction conditions. In addition, the adjustability and controllability of main reaction conditions (such as bath ratio, alkali concentration, kneading arm’s speed of kneading machine) are very poor. It is difficult to achieve the approximate uniformity of etherification reaction and to study the mass transfer and penetration of etherification reaction process in depth. The process features of stirring reactor as reaction equipment are: fast stirring speed in reaction, fast reaction speed, high utilization rate of etherizing agent, uniform distribution of etherizing substituents, adjustable and controllable main reaction conditions.
Carboxymethyl cellulose CMC was prepared by kneader reaction equipment and stirring reactor reaction equipment respectively. When the kneader was used as the reaction equipment, the stirring intensity was low and the average rotation speed was 50r/min. When the stirring reactor was used as the reaction equipment, the stirring intensity was high and the average rotation speed was 500r/min. When the molar ratio of monochloroacetic acid to cellulose monosaccharide was 1:5:1, the reaction time was 1.5h at 68℃. The light transmittance of CMC obtained by kneading machine was 98.02% and the etherification efficiency was 72% due to the good permeability of CM in chloroacetic acid etherifying agent. When the stirring reactor was used as the reaction equipment, the permeability of the etherifying agent was better, the transmittance of CMC was 99.56%, and the etherizing reaction efficiency was increased to 81%.
Hydroxyethyl cellulose HEC was prepared with kneader and stirring reactor as reaction equipment. When kneader was used as reaction equipment, the reaction efficiency of etherizing agent was 47% and water solubility was poor when the permeability of chloroethyl alcohol etherizing agent was poor and the molar ratio of chloroethanol to cellulose monosaccharide was 3:1 at 60℃ for 4h. Only when the molar ratio of chloroethanol to cellulose monosaccharides is 6:1, can the products with good water solubility be formed. When the stirring reactor was used as the reaction equipment, the permeability of chloroethyl alcohol etherification agent became better at 68℃ for 4h. When the molar ratio of chloroethanol to cellulose monosaccharide was 3:1, the resulting HEC had a better water solubility, and the etherification reaction efficiency was increased to 66%.
The reaction efficiency and reaction speed of the etherizing agent chloroacetic acid are much higher than that of chloroethanol, and the stirring reactor as etherizing reaction equipment has obvious advantages over the kneader, which greatly improves the etherizing reaction efficiency. The high transmittivity of CMC also indirectly indicates that the stirring reactor as etherizing reaction equipment can improve the homogeneity of etherizing reaction. This is because the cellulose chain has three hydroxyl groups on each glucose-group ring, and only in a strongly swollen or dissolved state are all of the cellulose hydroxyl pairs of etherifying agent molecules accessible. The etherification reaction of cellulose is usually a heterogeneous reaction from the outside to the inside, especially in the crystalline region of cellulose. When the crystal structure of cellulose remains intact without the effect of external force, etherifying agent is difficult to enter the crystalline structure, affecting the homogeneity of the heterogeneous reaction. Therefore, by pretreating the refined cotton (such as increasing the specific surface of the refined cotton), the reactivity of the refined cotton can be improved. In the large bath ratio (ethanol/cellulose or isopropyl alcohol/cellulose and high-speed stirring reaction, according to the reasoning, the order of the cellulose crystallization zone will be reduced, at this time the cellulose can strongly swell, so that the swelling of the amorphous and crystalline cellulose zone tends to be consistent, Thus, the reactivity of amorphous region and crystalline region is similar.
By means of infrared spectrum analysis and X-ray diffraction analysis, the etherification reaction process of cellulose can be understood more vividly when stirring reactor is used as etherification reaction equipment.
Here, infrared spectra and X-ray diffraction spectra were analyzed. The etherification reaction of CMC and HEC was carried out in a stirred reactor under the reaction conditions described above.
The infrared spectrum analysis shows that the etheration reaction of CMC and HEC changes regularly with the extension of reaction time, the degree of substitution is different.
Through the analysis of X-ray diffraction pattern, the crystallinity of CMC and HEC tends to zero with the extension of reaction time, indicating that the decrystallisation process has basically been realized in the alkalization stage and the heating stage before the etherification reaction of refined cotton. Therefore, the carboxymethyl and hydroxyethyl etherification reactivity of refined cotton is no longer mainly restricted by the crystallinity of refined cotton. It is related to the permeability of etherifying agent. It can be shown that the etherification reaction of CMC and HEC is carried out with stirring reactor as reaction equipment. Under high speed stirring, it is beneficial to the decrystallization process of the refined cotton in the alkalization stage and the heating stage before etherification reaction, and helps the etherification agent to permeate into the cellulose, so as to improve the etherification reaction efficiency and substitution uniformity.
In conclusion, this study emphasizes the influence of stirring power and other factors on the reaction efficiency during the reaction process. Therefore, the proposal of this study is based on the following reasons: In the heterogeneous etheration reaction system, the use of large bath ratio and high stirring intensity, etc., is the basic conditions for the preparation of approximately homogeneous cellulose ether with substituent group distribution; In a specific heterogeneous etheration reaction system, high performance cellulose ether with approximately uniform distribution of substituents can be prepared by using stirring reactor as reaction equipment, which shows that cellulose ether aqueous solution has a high transmittance, which is of great significance to expand the properties and functions of cellulose ether. The kneading machine is used as reaction equipment to study etherification reaction of refined cotton. Due to the low stirring intensity, it is not good for the penetration of etherification agent, and has some disadvantages such as high proportion of side reactions and poor distribution uniformity of etherification substituents.