HE Jie, WANG Dali, LONG Lirong, HUANG Yali, CUI Chunxiang, YI Jiajia,YANG Shengyang, WANG Yuchang

(1. College of Chemistry and Chemical Engineering, Beifang Minzu University, Yinchuan 750021, China; 2. Institute of Physical Education,Lanzhou City University, Lanzhou 730070, China)

Abstract: We aimed at producing sodium carboxymethylcellulose (CMC) from waste paper cellulose.For this etherification, the raw material was waste paper, the cellulose was initially alkalized with NaOH,the etherifying agent was sodium chloroacetate, and the reaction medium was water or ethanol. The method provided by us, i e, a method for preparing CMC from waste paper, was environment-friendly, could be easily implemented, and could be conveniently applied to make waste paper efficiently used with high profit, and to expand the range of usable raw materials for CMC production. We successfully synthesized CMC and prepared CMC plastic membrane. This practice changes waste into valuables, which is beneficial to our living environment. For preparation of CMC, one of the crucial factors is appropriate pretreatment of the cellulose from waste paper. The pretreatment was done with a self-built hydrolysis method. We experimentally examined the effects of the mass ratios of reactants, reaction temperature, time, and reaction environment of homogeneous or heterogeneous on CMC yield. The innovative points of this research could be stated as follows: the reaction activity of cellulose was improved by pre-hydrolysis; synthesizing CMC with cellulose from waste paper changes waste into valuables is beneficial to our living environment; and a freezing treatment for the cellulosealkali mixture was innovatively added. The effects were exhibited by a desired final conversion efficiency.

Key words: CMC; cellulose; etherification; cellulose ether; membrane

1 Introduction

Cellulose derivatives, such as carboxymethylcellulose (CMC), differ from cellulose in many terms (eg, solubility) and have many applications in different industrial areas[3-6]. CMC is the most widely used cellulose ether today. The slurry process is used for large scale production of CMC[7]. In this method,the CMC synthesis was carried out in two steps.The first step is the basification in which cellulose is suspended in alcohol-water-NaOH system. The second step is the etherification with monochloroacetic acid2.The synthesis of CMC from paper industry effluents containing SCF was investigated[8]. Incubation after the etherification reaction slightly increased the yield and degree of substitution during the first 12 h[8]. The obtained CMC was darker and had lower hydration properties than commercial samples[8]. Trace elements content suggests that the product could be only used in paint factories or building materials industries[8].

CMC synthesis was generally performed in alcoholic solutions or solvents like ionic liquids[9,10].Although nearly complete etherification can be achieved with these methods, use of organic solvents for the production can cause environmental pollution.Synthesis of CMC in aqueous solutions without organic solvent has not been executed frequently.However, recent studies in field of green chemistry mostly focus on water as a solvent having the features of being cheap, easily accessible, and non-toxic.Carboxymethylation of cellulose in aqueous solution may yield low-substituted CMC samples, appropriate for use in different industrial fields, especially where low or medium degree of substitution are required.

Aim of this work was to synthesize CMC in aqueous solution by using recycled paper cellulose.Characterizations of obtained CMC were made in terms of its physical and spectral properties. For preparation of CMC, one of the crucial factors is appropriate pretreatment of the cellulose from waste paper. The pretreatment was done with a self-built hydrolysis technology. In this etherification process, the cellulose was initially alkalized with NaOH, the etherifying agent was sodium chloroacetate, and the reaction medium was water or ethanol. The provided method by us,ie,a method for manufacturing CMC from waste paper,is environment-friendly, could be easily implemented,and could be conveniently applied to make waste paper efficiently used with high profit, and to expand the range of usable raw materials for CMC production. We successfully synthesized CMC and made CMC plastic film.

2 Experimental

2.1 Synthesis of carboxymethylcellulose

2.1.1 Paper recovery and pretreatment

Paper is mainly composed of cellulose. Removal of solubilized impurities was done to obtain recycled cellulose from waste paper. The A4 papers as raw material were firstly torn to pieces, soaked with water long enough, and then fully smashed and washed with a multifunctional food blender, followed by vacuum filtration. The operations were repeated to ensure the paper cellulose totally cleaned. The obtained filter cake was dried. After drying, the agglomerated hard paper pieces were unstuck and triturated. The paper powder was divided into several parts and hydrolyzed with 4.0vol%, 6.0vol%, 8.0vol%, 10.0vol%, and 12.0vol% dilute H2SO4at room temperature for 2 days,respectively. After pre-hydrolysis, the pulp was filtered.The filter cake was spread out in the sun, to be aged,dried, and was finally ground into powder.

Fig.1 Preparation of paper cellulose raw material

2.1.2 Synthesis of carboxymethylcellulose

Pre-prepared 18wt% NaOH aqueous solution of 10 mL was poured into 1.0 g recycled cellulose.The mixture was then frozen at about -10 ℃ for 12 h to make the cellulose swelled. CMC synthesis was subsequently done using pre-activated cellulose and sodium chloroacetate in aqueous alkaline medium. Themass ratio of sodium chloroacetate to cellulose was about 1 : 1. Carboxymethylation was carried out at 80℃ for 2 h. Obtained product was washed with 70vol%EtOH-H2O solution, then with absolute ethanol 3 times,and was dried naturally. The quantity was recorded asm1. The dried particles were put into a beaker, and were dissolved with appropriate amount of water,under strong stirring. The solution was filtered through a piece of filter cloth with vacuum filtration. The filtrate was CMC aqueous solution. The filter cake was dried at room temperature and weighed. The quantity was recorded asm2. The CMC aqueous solution was viscous. We made a small quantity of sticky CMC aqueous solution evenly spread on a smooth surface,and a plastic film was formed after the layer of solution left dried.

They went in, and inside the cottage was a tiny hall, and a beautiful sitting-room4, and a bedroom in which stood a bed, a kitchen and a dining-room all furnished with the best of everything, and fitted up with every kind of tin and copper5 utensil6

Fig.2 CMC preparation block flow diagram

2.1.3 Spectral characterization and chemical analysis

FTIR analysis was performed on Perkin Elmer Spectrum One FTIR spectrometer in the range of 4 200-200 cm-1with KBr pellets. The CMC yield and output-input ratio were calculated by:

3 Results and discussion

3.1 Recovery, pretreatment, and characterization of cellulose from waste paper

FTIR analyses were performed to characterize both starting and hydrolyzed cellulose materials and CMC products. FTIR spectrum of starting paper cellulose showed signals belonging to O-H stretching vibrations at 3 350 cm-1, C-H stretching vibrations at 2 905 cm-1, H-O-H deformation vibration at 1 637 cm-1,various C-C stretching, and C-O-H, H-C-H bending vibrations between 1 500 and 1 250 cm-1, and various C-O stretching, C-O-C, C-C-O bending vibrations between 1 250 and 900 cm-1(Fig.3)[11].

Fig.3 FTIR spectra of starting (paper cellulose) and hydrolyzed materials

3.2 Synthesis and characterization of carboxymethylcellulose

The carboxymethylation is generally proceeded in an alkaline reaction system. It is a method of Williamson ether synthesis. Typically it involves the reaction of an alkoxide ion with a primary alkyl halide via an SN2 reaction. The general reaction mechanism is as follows:

In addition, the following side reactions may occur:

With regard to the aim of the study, carboxymethylation was done in aqueous alkaline medium by using sodium chloroacetate (Scheme 1).

Scheme 1 Simplified reaction scheme for derivatization of cellulose with sodium chloroacetate

Intensities of the 1 430 and 1 336 cm-1bands were influenced to various extents by other vibrations,such as C-H stretching and anomeric skeleton, while the 1 260 cm-1band was affected by C-O-H vibrations.Although, it was difficult to find an IR band attributed only to the carboxymethyl groups, band intensities of signals at 1 336 and 872 cm-1could be used to estimate DS of the sample[11]. FTIR analyses of samples showed also C=O stretching vibrations of carboxylate groups at 1 616 cm-1proving introduction of carboxymethyl moieties onto cellulose (Fig.4). Various C-O stretching and C-O-C, C-C-O bending vibrations were observed between 1 250 and 900 cm-1giving a unique pattern for polysaccharides[12]; this region was also very sensitive to side-groups and conformation changes. Since the carboxymethylation took place in basic medium, the obtained CMC samples were in sodium salt form(-COONa) rather than in acid form (-COOH). There were increases in two other IR peaks at 1 430 and 1 336 cm-1in all CMC samples. These bands were attributed to C=O stretching in COO-ions. FTIR analyses proved that carboxymethylation reactions were done successfully, and the product had carboxymethyl moieties at different ratios regarding to reaction conditions.

Fig.4 FTIR spectra of CMC products

3.3 Effects of synthesis conditions on CMC yield

3.3.1 Effect of concentration of dilute H2SO4

It can be observed from Fig.5, for preparation of CMC, with the increase of the concentration of dilute H2SO4used for paper cellulose pre-hydrolysis,the yield of CMC and output-input ratio increase first and then decrease. This is because with the increase of dilute H2SO4concentration, the molecular weight of raw paper cellulose decreases, the reactivity of cellulose increases, and the yield of CMC increases subsequently. When it was increased to about 8.0vol%H2SO4, the yield of CMC reached to the maximum(about 80wt%), and the output-input ratio reached to the maximum (0.4) too. When it exceeded 8.0vol%H2SO4, the yield of CMC decreased instead with the drastic degradation of cellulose.

Fig.5 Effect of concentration of dilute H2SO4 used for paper cellulose pretreatment on CMC yield (original paper cellulose 1.0 g)

3.3.2 Effect of heating time on CMC yield

It is seen from Fig.6, with the increase of heating time, the yield of CMC increases first, and then decreases obviously, and finally begins to level off. The reason is that, because of the short heating time, the alkalized cellulose can not fully reacted with ClCH2COONa, resulting in incomplete etherification and lower yield of CMC. When it increases to 5.0 h, the yield of CMC (about 80wt%) and the outputinput ratio (about 1.2) all reach to the maximum values, respectively. Prolonging the heating time over 5.0 h would make the efficiency in etherifying agent utilization decreased. The CMC yield will be decreased also. No significant change in CMC yield is observed,when the heating time continuously increases.

Fig.6 Effect of heating time on CMC yield (original paper cellulose 1.0 g)

3.3.3 Effect of amount of ClCH2COONa on CMC yield

As shown in Fig.7, the yield of CMC decreases first, and then increases, with the increase of etherifying agent sodium chloroacetate. The proper mass ratio of sodium chloroacetate to cellulose is about 0.8-1.2, in view of saving consumption of chemicals. CMC yield 70wt%-80wt% could be ensured. Excessive sodium chloroacetate may aggravate some side reactions and cause decrease in CMC yield.

Fig.7 Effect of the amount of ClCH2COONa on CMC yield

3.3.4 Effect of concentration of NaOH on CMC yield

Fig.8 shows that the yield and output-input ratio decreased firstly and then increased with the increase of NaOH concentration. The maximum values appeared when the concentration of NaOH was about 20wt%.The CMC yield could reach to about 70wt%, and the output-input ratio, to about 0.8. The alkalization and activation degree of cellulose was lowered, when the concentration of NaOH was less than 15wt%.The etherification agent in the early stage of reaction was excessive, which very likely made etherification agent hydrolyzed itself, and the side reactions increased, which resulted in the low yield of CMC.With the increasing of the concentration of NaOH,the permeation of alkali solution into cellulose was enhanced, and the yield of CMC would be increased.When cellulose was combined with alkali, swelling occurred, and chemical reagents could more easily diffuse into the structure of cellulose. However, when the concentration of NaOH is too high, the long chain of cellulose will be possibly destroyed. At the same time, it is easy to cause side reactions and to reduce the yield of CMC. 18wt%-20wt% NaOH would be proper.

Fig.8 Effect of the concentration of NaOH on CMC yield

Fig. 9 Effect of reaction medium on CMC yield

3.3.5 Effect of reaction medium on CMC yield

Fig.9 reveals that the CMC yield of about 80wt%could be got with EtOH as the reaction medium, and a high output-input ration of about 1.1 will be also obtained. When organic solvents such as ethanol and ethylene glycol used as the reaction medium, the effect of carboxymethylation of the alkalized cellulose is better than that of water as the reaction medium,and the weaker the polarity of solvent, the higher the efficiency of carboxymethylation[13]. Acid medium,such as acetic acid, lactic acid, or acrylic acid, will react with free alkali, thus reducing the etherification degree, and then the yield of CMC will be low[13]. It is worth mentioning that cellulose viscose, prepared by dissolving cellulose with low temperature NaOH-urea mixed solvent, was used as raw material solution to make cellulose more efficiently carboxymethylated in a homogeneous reaction system. Dissolving cellulose with low temperature NaOH-urea mixed solvent was an innovative and well accepted method[14].However, our original intention to provide a low-cost homogeneous etherification environment suffered an initial failure. With the reaction, the dissolved cellulose will be precipitated from NaOH-urea mixed solvent again, during the heating process, and the effect of homogeneous reaction will not be achieved. Further improvements are still under way.

3.3.6 Orthogonal experiment

In accordance to the key factors for the preparation of CMC, concentration of dilute H2SO4in prehydrolysis, amount of sodium chloroacetate, and reaction medium,L9(33) (three factors, and three levels)orthogonal experiments were designed (as shown in Table 2), and performed to find an optimal conditions combination. The results of orthogonal experiments show that in the experiment of making CMC from waste paper cellulose, the influencing intensities in decreasing sequence are as follows: amount of sodium chloroacetate ＞ reaction medium ＞ concentration of dilute H2SO4in cellulose prehydrolysis. The optimal conditions combination are: A3 (8.0vol% dilute H2SO4)B3 (sodium chloroacetate 1.0 g) C1 (medium EtOH).The highest yield of CMC was 90wt% obtained by adopting the optimal conditions got from the orthogonal experiments. It is interesting that the CMC yield could reach to about 85wt%, with H2O as the reaction medium.

Table 1 Instruments and chemicals

Table 2 L9(33) orthogonal experimental design and results

Fig.10 CMC film from self-made CMC by casting method

3.4 Application-CMC film

The CMC film as shown in the diagram was formed by a casting method. This membrane is naturally brittle. This is consistent with that of commodity CMC. A breakthrough is to make naturalsynthetic composite polymer materials. We have been conducting a study in this area. In addition, CMC is a good product itself and a promising chemical raw material. Therefore, it is of great significance to produce CMC from waste paper cellulose.

4 Conclusions

The provided method by us,i e, a method for manufacturing CMC from waste paper, is environmentfriendly, could be easily implemented, and could be conveniently applied to make waste paper efficiently used with high profit, and to expand the range of usable raw materials for CMC production. The used cellulose usually comes from wood or straw pulp and cotton etc.We successfully synthesized CMC from waster paper cellulose and prepared CMC film. The yield of CMC could be about 80wt% obtained by adopting the optimal conditions got from the orthogonal experiments. This practice changes waste into valuables, and is beneficial to our living environment. For preparation of CMC,one of the crucial factors is appropriate pretreatment of the cellulose from waste paper. The pretreatment was done with a self-built technology. We examined the effects of reactants mass ratios, reaction temperature,time, and reaction environment of homogeneous or heterogeneous on CMC yield through various experiments. The innovative points of this research could be stated as follows: the reaction activity of cellulose was improved by pre-hydrolysis; synthesizing CMC with cellulose from waste paper changes waste into valuables, and is beneficial to our living environment; and a freezing treatment for the cellulosealkali mixture was innovatively added. The effects were exhibited by a desired final conversion efficiency.