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Isolation and Characterization of Acetone-Insoluble Substances in Cellulose Acetate Prepared by an Acetic Acid Acetylation Process

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Isolation and Characterization of Acetone-Insoluble Substances in Cellulose Acetate Prepared by an Acetic Acid Acetylation Process CELLULOSE CHEMISTRY AND TECHNOLOGY ISOLATION AND CHARACTERIZATION OF ACETONE-INSOLUBLE SUBSTANCES IN CELLULOSE ACETATE PREPARED BY AN ACETIC ACID ACETYLATION PROCESS X. L. CHEN, *,** S. W. XU,* W. J. PENG,* J. ZHANG * and Y. JIANG ** *Nantong Cellulose Fibers Co. Ltd, 27, East Zhongxiu Road, Nantong, Jiangsu, China ** College of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu, China ✉Corresponding author: Xiaolu Chen, [email protected] Received March 25, 2013 Cellulose acetate fibers are commonly made by a dry-spinning process, in which cellulose acetate is firstly dissolved in acetone and then the cellulose acetate acetone solution is extruded through spinneret orifices that have extremely small diameters. Any insoluble substances in acetone solutions could cause disruptions in fiber extrusion, and are not tolerated. In this work, acetone-insoluble substances in cellulose acetates prepared from several kinds of pulps with varying cellulose content were isolated by ultracentrifugation, and characterized by using chemical, spectroscopic and chromatographic techniques. According to the results, acetone-insoluble substances are enriched in hemicellulose acetates, lower in the degree of substitution and higher in crystallinity in comparison with corresponding cellulose acetates. Based on analysis, a possible mechanism for the formation of insoluble substances is proposed. Hemicelluloses residues in pulps and insufficient acetylation of crystalline cellulose play important roles in forming the insoluble substances in cellulose acetate acetone solutions. Keywords : cellulose acetates, insoluble substances, formation mechanism INTRODUCTION Cellulose acetate (CA), one of the most the compositions and formation of insoluble commercially important cellulose derivatives, has substances in cellulose acetate solutions. been utilized in a wide variety of applications, Some literatures have reported researches on the such as coatings, fibers, optical films, filtration insoluble substances in cellulose acetate membranes, composites, laminates, and medical solutions.10-16 For example, Russo et al . prepared and pharmaceutical products.1-6 Among these cellulose acetate from lower purity wood pulps applications, cellulose acetate fibers are and characterized the soluble and insoluble commonly spun from acetone solutions in a so- fractions of cellulose acetate in acetone. Ueda et called “dry extrusion process”, in which cellulose al . characterized the acetone-insoluble substances acetate is firstly dissolved in acetone and then the in cellulose acetate and proposed a possible model cellulose acetate solution is extruded through of the gel in acetone, indicating that the gel is in spinneret orifices that have extremely small molecular aggregates of cellulose acetates with regions of cellulose diacetate and cellulose diameters and spun in hot air with resultant acetone vaporization and cellulose acetate triacetate I. Fleury et al . analyzed the solutions of coagulation. Any insoluble substances in polymer cellulose diacetate by size exclusion chroma- solutions would reduce the filterability, cause tography, and characterized the series of humps in disruptions in fiber extrusion and are not the chromatogram. In spite of their pioneering works, the exact nature of acetone-insoluble tolerated, and these industrial problems should be substances is not fully understood, and little even worse when some low-grade cellulose related literature has been published in recent resources are used to make cellulose acetate.7-9 years. Therefore, it seems quite important to investigate Cellulose Chem . Technol ., 48 (5-6), 477-483 (2014) X. L . CHEN et al . In this work, cellulose acetates were prepared 3.0% solution. The solution is then transferred into by an acetic acid acetylation/ripening process ultracentrifugation tubes and spun in a Sigma 3-18K from several different pulps with varying Sartorius high speed centrifuge at 10000 rpm for 2-3 o cellulose content. Acetone-insoluble substances in hours at 5 C to collect acetone-insoluble substances. the cellulose acetates were collected by The obtained insoluble substances are washed with acetone 3 times and dried at 60 oC for 24 hours prior to ultracentrifugation, and characterized by using characterization. chemical, spectroscopic and chromatographic techniques, such as titration method, ion Characterization chromatography (IC), nuclear magnetic resonance Solution properties (NMR), X-ray diffraction (XRD) etc. Based on Haze measurements of the cellulose acetate acetone analysis, influences of hemicelluloses and the solutions and the supernatant after centrifugation were degree of substitution (DS) on the formation of performed on a spectrophotometric colorimeter at acetone-insoluble substances are discussed, and a room temperature. In addition, the undissolved micro- possible mechanism for the formation of insoluble particles and their diameters in solution were analyzed substances is proposed in this article, which can with a HIAC/ROYCO 3000A (Mcount-200/MC-05) help us understand what the acetone-insoluble particle sizer. substances are, how they form and how to reduce Ion Chromatography them. This is beneficial to cope with the industrial The compositions of different sugars in pulps, problems in the spinning process and is cellulose acetates and the corresponding acetone- meaningful for enabling the use of low-grade insoluble substances were analyzed with Ion cellulose resources, such as bamboo fibers and Chromatography (DIONEX ICS-5000). Firstly, pulp agricultural by-products. samples were fluffed in a blender, cellulose acetates and acetone-insoluble substances samples were ground EXPERIMENTAL into powder, and dried overnight. Then a hydrolysis Materials process was carried out as follows: a sample prepared Cotton linter, Rayonier hardwood pulp, Borregaard above (0.355±0.005 g) was immersed in 3.0 ml 72% softwood pulp and paper pulp have been purchased sulfuric acid with periodical agitation in a 30 oC water commercially. Their main components are shown in bath for 2-3 hours, until the sample was dissolved. Table 1. Acetic anhydride, acetic acid, sulfuric acid, After the sample was removed from the water bath, and anhydrous magnesium acetate have been 84.0 ml of deionized water was added to the sample, purchased from Nanjing Chemical Reagent Co., Ltd. and then heated in an autoclave at 120 oC for 10 All above materials were used without purification. minutes. After the sample was cooled to room temperature, a 50% sodium hydroxide solution was Cellulose acetates preparation and insoluble added to neutralize the sulfuric acid, and then the substances isolation sample was filtered with a 0.45 micron ion Cellulose acetates have been prepared by an acetic chromatography filter. The filtrate was analyzed with a acid process with several different pulps, 18 cotton linter Dionex ICS 5000, using a CarboPac PA20 analytical (C-CA), Rayonier hardwood pulp (R-CA), Borregaard column. softwood pulp (B-CA) and paper pulp (P-CA). In this acetic acid process, a pulp (1 part) is firstly swelled in Degree of substitution acetic acid (0.4 parts) at room temperature, and then Degrees of substitution of cellulose acetates acetylated to cellulose triacetate with acetic anhydride prepared from several different pulps were determined (3 parts) in the presence of sulfuric acid (0.07-0.15 by a titration method. Firstly, a sample of 2 g dried parts) as catalysts at 40-55 oC for 1 hour. After powder was dissolved into 150 ml acetone with neutralization of sulfuric acid, a ripening is done with periodical agitation. Subsequently, 30 ml sodium the addition of water to make cellulose acetate with a hydroxide solution (1 mol/L) was added and the degree of substitution of approximately 2.4 in 2-3 resulting solution was kept at 25 oC for 30 minutes. hours. Then, the solution is poured into distilled water Then the solution was titrated with 0.5 mol/L sulfuric to precipitate the cellulose acetate. Finally, the acid solution with a small amount of phenolphthalein resulting cellulose acetate is washed thoroughly with as an indicator until the red color faded to obtain a distilled water and dried in a vacuum oven at 60 oC for volume of sulfuric acid (A). A blank experiment was about 24 hours. also tested to obtain the volume of sulfuric acid (B). To isolate insoluble substances, the obtained The mass of acetic acid and degree of substitution of cellulose acetate is dissolved in acetone to prepare a cellulose acetate were calculated from the following 478 Cellulose acetate equations: determined by 1H nuclear magnetic resonance spectra. Insoluble substances were first reacted with propionic AV=( B - A)× CH2 SO 4 × 0.06005 ×⁄ 100 W anhydride using pyridine as solvents for 24 hours in = ×⁄ −× DS162.14 AV( 6005 42 AV ) order to convert hydroxyl groups on the molecules of where AV is acetic acid content of cellulose acetates the insoluble substances to propionyl groups. 1H (%), A is the volume of sulfuric acid for cellulose nuclear magnetic resonance spectra were recorded of acetates (ml), B is the volume of sulfuric acid for blank the reaction product on a Varian Unity spectrometer in (ml), CH2SO 4 is the concentration of sulfuric acid chloroform-d at room temperature with (mol/L), W is the weight of sample in grams (g). tetramethylsilane as an internal standard. DS of acetone-insoluble substances were Table 1 Main components of four pulps Pulps Cotton linter Hardwood pulp Softwood pulp Paper α-Cellulose content ~98%
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