Jinghuan Chen,Jingang Liu,Leilei Hou,Zehong Xu

1.China National Pulp and Paper Research Institute Co.,Ltd.,Beijing,100102,China

2.National Engineering Lab for Pulp and Paper,Beijing,100102,China

Abstract:Carboxyethylation pretreatment was used to prepare microfibrillated cellulose(MFC)in this study.In order to evaluate the adaptability of this pretreatment method,carboxyethylated MFC was prepared from six different cellulosic materials.The carboxyl content,degree of polymerization,water retention value,charge density,chemical structure,size distribution,and micromorphology of the materials before and after pretreatment and grinding were studied and compared.The viscosity,ultraviolet(UV)transmittance,and thermal stability of the MFC samples at a certain concentration were determined.The results showed that the carboxyl content,water retention value,charge density,degree of polymerization,size distribution,and micromorphology of the pretreated and ground samples varied with those of the raw materials.The initial viscosity varied based on the type of raw material used.The MFC suspension prepared from cotton linter pulp had the highest UV transmittance,while the MFC prepared from bleached softwood kraft pulp had the highest viscosity at a low shear rate.After thermal degradation,the amount of residual char from the MFC prepared with the thermo-mechanical pulp was slightly higher than that of the other MFCs.This study demonstrates that carboxyethylation is an effective pretreatment method for different cellulosic materials.

Keywords:carboxyethylated pretreatment;cellulosic raw materials;MFC;properties

1 Introduction

In recent years,researchers have found that the pretreatment of raw materials before mechanical treatment can greatly reduce the energy consumption for the production of functional materials.One of the most commonly used chemical pretreatment methods for the preparation of microfibrillated cellulose(MFC)is the oxidation of cellulose with 2,2,6,6-tetramethylpiperidine 1-oxyl radical or TEMPO[1-2].This oxidation system has a good selectivity for primary hydroxyl groups,and it can oxidize the hydroxyl groups at the C6 of cellulose chains into carboxyl groups.Another commonly used method to introduce carboxyl groups into cellulose molecules is carboxymethylation pretreatment[3-4],which can introduce carboxymethyl groups onto the surface of cellulose fibers.Carboxymethylation is characterized as a Williamson synthesis reaction. High concentrations,high temperatures,and long reaction times are favorable to this reaction,but the presence of water is not.Thus,the solvents used in this reaction system are usually organic solvents,such as absolute ethanol, isopropanol, dimethylformamide, and dimethyl sulfoxide[5-8].In addition,other chemical pretreatment methods such as periodate oxidation[9-10],phosphorylation[11-12], sulfonation[13-14], and quaternization[15-16]have also been reported.

Carboxyethyl reactions can also introduce carboxyl groups onto cellulose chains,and they have been used to prepare water-and alkali-soluble carboxyethyl ethers of cellulose[17].Our group has previously demonstrated the feasibility of using this reaction as a pretreatment method for the preparation of MFCs[18].Water was used as the reaction medium,and the reaction conditions were relatively mild.In this study,the effect of the type of raw materials on the production and properties of MFC products was studied.This study provides a comprehensive understanding of the carboxyethyl pretreatment and enriches the theoretical basis for the preparation and application of MFCs.

2 Materials and methods

2.1 Materials and reagents

Six different cellulosic materials were used as raw materials for the preparation of carboxyethylated MFC.Bleached bamboo kraft pulp(BP)was supplied by Lee& Man Paper Manufacturing,Ltd.(China).Bleached hardwood(Acacia)kraft pulp(HP)was produced by the Riau Andalan Pulp & Paper Company(Indonesia).Bleached softwood(Southern pine)kraft pulp(SP)was purchased from Northwood Pulp and Timber,Ltd.(Canada).Cotton linter pulp(CP)was prepared by Jiangsu Longma Green Fiber Co.,Ltd.(China).Softwood spruce chemi-thermomechanical pulp(CTMP)was obtained from SCAÖstrand Pulp Mill(Sweden).Corncob residue cellulose(CR)was obtained from Ji'nan Shengquan Group Share Holding Co.,Ltd.(China).Sodium hydroxide,acrylamide,hydrochloric acid,and sodium chloride were purchased from Sinopharm Chemical Reagent Co.,Ltd.A copper ethylenediamine solution was prepared in the laboratory.All chemicals and reagents were used without further purification.

2.2 Methods

First,a 20 wt% sodium hydroxide solution was prepared and cooled to room temperature.The cellulose raw materials,which were torn into small pieces and labeled as CBP,CHP,CSP,CCP,CCTMP,and CCR,correspondingly,were dipped and stirred into the sodium hydroxide solution until they were dispersed evenly.Thereafter,a 22.5% acrylamide solution was prepared and mixed with the suspensions.After stirring for 5 min,the suspensions were concentrated into a 20 wt% slurry.The excess liquid was used to impregnate the next batch of cellulose samples.The slurry was loaded into a high-concentration mixing reactor(Quantum Mark V,Quantum Technologies,USA)and allowed to react at 70℃for 3 h.The slurry was then washed repeatedly with water until the filtrate was neutral.The obtained carboxyethylated cellulose samples were labeled as CCBP,CCHP,CCSP,CCCP,CCCTMP,and CCCR,correspondingly.The pretreated samples were finally diluted with water to a solid content of 5 wt% and fibrillated using a supermasscolloider-type disk mill(MKCA6-2J,Masuko Sangyo,Japan)with a gap clearance of-100µm at 1500 r/min until no visible long fibers were observed under an optical microscope.The obtained samples were labeled as MFCBP,MFCHP,MFCSP,MFCCP,MFCCTMP,and MFCCR,correspondingly.

2.3 Characterizations

The carboxyl content of the cellulose raw materials(CX)and pretreated samples(CCX)was determined through a conductometric titration method according to a previous work[19].The degree of polymerization(DP)of the CX,CCX,and ground samples(MFCX)was measured using the viscosity method[20].The water retention value(WRV)of CX,CCX,and MFCXwas measured through a method similar to the standard ISO 23714-2014 procedure.The electric charge of MFCXwas measured with a charge titrator(Mütek PCD-T3,BTG Instruments,Germany)using 0.001 mol/L poly(diallyldimethylammonium chloride)as the cationic polyelectrolyte solution[21].The dry weights of CXand MFCXwere determined after heating them in an oven at 105℃for 4 h,and these were used to calculate the sample yield after pretreatment and grinding.The microstructures of CX,CCX,and MFCXwere observed through scanning electron microscopy(SEM,S-3400N,Hitachi,Japan)at the acceleration voltage of 15 kV.The chemical structures of CXand CCXwere analyzed using a Fourier transform infrared spectrometer(FT-IR,Tensor 37,Bruker,Germany)with an attenuated total reflectance accessory in the range of 700-4000 cm−1.The length and width distribution of CXand CCXwere measured using an L&W Fiber Analyzer(912.1E,AB Lorentzen & Wettre,Sweden).The particle size distribution of MFCXwas determined through laser diffraction analysis(Mastersizer 2000 Hydro,Malvern Instruments Ltd.,UK).The rheological property of MFCXwith a concentration of 0.1 wt% was evaluated using a rotational rheometer(Bohlin Gemini 2,Malvern Instruments Ltd.,UK).The transmittance of MFCXwith a concentration of 0.1 wt% after ultrasonic treatment was measured using an ultraviolet(UV)spectrophotometer(UV-1800,Shimadzu,Japan)in the range of 200-800 nm.The thermal properties of CX,CCX,and MFCXwere investigated through thermogravimetric (TG) and derivative thermogravimetric(DTG)analyses with a DTG-60(Shimadzu,Japan)and a simultaneous DTA-TG apparatus at a heating rate of 10℃/min from 30℃to 800℃under an inert nitrogen gas atmosphere.

3 Results and discussion

3.1 Carboxyethyl pretreatment of different raw materials

In order to evaluate the adaptability of the carboxyethyl pretreatment to cellulose raw materials and the effect of the type of raw material on the pretreatment process,six different cellulosic materials were carboxyethylated under the same conditions.The properties of the materials before and after pretreatment were measured.As shown in Table 1,the contents of carboxyl groups in the six cellulosic materials were relatively low before pretreatment,ranging from 0.1 to~0.4 mmol/g.The carboxyl content of CCPwas the lowest,while that of CCTMPwas about three times more than that of CCP.The carboxyl groups of plant fiber materials are mainly found in their hemicellulose,while that of pulps exist partly in their oxidized cellulose,which is the degradation product of cellulose,and partly in their hemicellulose and its degradation products.CCPis a dissolving pulp used to prepare regenerated cellulose fibers.It has a highα-cellulose content with hemicellulose being largely removed,which may be the reason for its relatively low carboxyl content.CCTMPis prepared through a mild chemical treatment and mechanical fiber separation with a yield of 80%-90%.The hemicellulose and lignin contents of CCTMPare generally higher than those of bleached pulps;hence,the carboxyl content of CCTMPwas the highest among the six raw materials.The carboxyl contents of all materials increased significantly after carboxyethylation,among which the carboxylated CCCTMPhad the highest carboxyl content,approximately1.6 mmol/g,while the other five samples had carboxyl contents in the range of 0.9-1.2 mmol/g.This indicates that the carboxyethyl pretreatment successfully introduced carboxyl groups into the cellulose raw materials,and the hemicellulose and lignin in the raw materials could also be modified during the pretreatment process.

Table 1 Properties of different raw materials before and after pretreatment and grinding

In addition to carboxyl content,other properties of the raw materials before and after pretreatment were also analyzed.As shown in Table 1,the DP of the pretreated materials decreased(after pretreatment),possibly due to the degradation of cellulose caused by the alkaline conditions of carboxyethylation.In addition,the DP decreased the most in CBP(40.6%),followed by CSP(30.7%)and CHP(17.3%),while that of CCR(6.6%)and CCP(5.3%)decreased the least.These results show that the higher the initial DP of the raw materials,the greater the decrease in the DP after pretreatment.WRV is an index that characterizes the degree of swelling of pulp fibers.As can be seen that after carboxyethylation,the WRV of most raw materials increased by 2-4 times,and the WRV of CCRincreased by a factor of 6.The main reason for this increase in the WRV could be the increase in the carboxyl content of the cellulosic materials.Carboxyl groups on cellulose fibers can ionize under wet conditions,adsorb ions and molecules with opposite charges,and form an adsorption double layer,i.e.,a water layer,on the surface of the fiber.With the increase in carboxyl content,the intensified ionization on the fibers leads to the increase in the water layer thickness,resulting in an increase in the WRV of the material.However,it was observed that the WRVs of the materials were not proportional to their carboxyl contents.This is because,in addition to the carboxyl content,the WRV is also related to the polar hydroxyl content,crystallinity,and other properties of materials.

After carboxyethyl pretreatment,the morphology and size distribution of the different raw materials changed.As shown in Fig.1,after pretreatment,the fiber diameter of most of the raw materials decreased,and their surface became smoother.The microscopic morphology of CCRchanged the most,as the original material,which was massive,became fibrous after pretreatment(CCCR).This result may be attributed to the swelling and partial dissolution of the carboxyethylated cellulose in the alkali solution due to the increase in the carboxyl content.During the washing process, the partially dissolved carboxyethylated cellulose in the periphery of the fiber was likely re-precipitated due to the decrease in the pH value.In addition,the swelled fibers shrunk with the loss of water during the drying process,resulting in a significant change in the morphology of the materials.

The size distribution of the cellulose raw materials before(CX)and after(CCX)carboxyethyl pretreatment is shown in Fig.2.

The six cellulosic materials differed greatly in length,diameter,and distribution.The average fiber length of the six materials ranged from 0.76 to 2.55 mm.CSPhad the longest average fiber length,followed by CCTMPand CBP.The average fiber diameter of the six materials ranged from 19.0 to 41.8μm.CCRand CCTMPhad the largest average diameters,followed by CSPand CCP.The CHPfibers had the smallest average length and average diameter.In terms of size distribution,the CSPfibers had the widest length distribution,while the CCPfibers have the widest diameter distribution.The CHPfibers had the narrowest diameter and length distributions,indicating the size uniformity of this material.After carboxyethyl pretreatment,the size distribution of the materials changed significantly,except for CCHP.The average length of the CCCRfibers increased significantly,but their average diameter decreased.In contrast,the average fiber length of the other materials(excluding CCHP)decreased,but their average diameter increased.Moreover,the length distribution of CCCRbecame slightly wider,while that of other materials became narrower.In addition,the diameter distributions of the CCBP,CCCP,and CCCTMPfibers became significantly wider,while that of CCSPand CCCRbecame narrower.These results show that the morphology and size distribution of cellulosic materials after carboxyethyl pretreatment varied with the type of the raw material.

FT-IR analysis was used to determine the chemical structure of the cellulosic materials before and after carboxyethyl pretreatment.As shown in Fig.3(a),all six materials showed the characteristic absorption peaks of cellulose,specifically the stretching vibration absorption peak of O—H and the hydrogen bond peak at 3335 cm−1,the stretching vibration absorption peak of C—H at 2883 cm−1,the stretching vibration absorption peak of C=C at 1650 cm−1,the skeleton vibration absorption peak of C—H at 1411 cm−1,the bending vibration absorption peak of C—H at 1369 cm−1,the swing vibration absorption peak of H—C—H at 1317 cm−1,the stretching vibration absorption peak of C—O and C—C at 1056 cm−1,the bending vibration absorption peak of C—O—H at 1031 cm−1,the stretching vibration absorption peaks of C—O—C at 1159 and 898 cm−1,and the out-of-plane bending vibration absorption peak of O—H at 667 cm−1.In addition,the stretching vibration absorption peaks of the lignin aromatic rings at 1604 and 1508 cm−1and the C=O characteristic absorption peaks of hemicellulose at 1725 and 639 cm−1were also observed in the FT-IR spectra of CCTMP.After carboxyethyl pretreatment,all the products exhibited an absorption peak at 1567 cm−1(Fig.3(b)),which corresponds to the characteristic asymmetric stretching vibration peak of C=O in carboxyl groups(—COO),indicating that carboxyl groups were successfully introduced into the cellulose molecules of the different raw materials.

3.2 MFCs obtained from different raw materials

MFC products were obtained after grinding the pretreated materials.The yields of the MFC products ranged from 80%to 90%,as shown in Table 1.The loss mainly occurred during the carboxyethyl pretreatment due to the degradation of cellulose and the dissolution of hemicellulose and lignin.Fig.1 shows that the diameters of the obtained MFC products were about several hundred nanometers.MFCSPand MFCCRhad the smallest diameters at around 100 nm,and the uniformity of the MFCSPfibers diameter was better.In addition,the dissociation of CCCTMPduring grinding resulted in poor dimensional uniformity of the MFC product due to the effect of its hemicellulose and lignin content.The size distribution of the MFC samples was measured using a Malvern laser particle size analyzer.As shown in Fig.4,the size distribution of the MFCs obtained from the six cellulosic materials was relatively wide,ranging from several microns to thousands of microns.This is because of the difficulty in ensuring that each fiber is subjected to the same force when the materials pass through and are crushed and dissociated between the two rotating mills,resulting in the large difference in the diameter and length of the obtained MFC fibers.In addition,the type and properties of the cellulosic materials could have also affected the size distribution of the MFC fibers when using the same grinding method.Large MFC fibers were obtained from large coarse materials.Therefore,the particle sizes of MFCSPand MFCBPwere large,while those of MFCCRand MFCCPwere small.Meanwhile,CCCPeasily dissociated during grinding process due to its low hemicellulose content and DP;thus,the obtained MFCCPfibers had small and homogeneous sizes.

The experiment results of the MFC fibers size were also verified by measuring their UV transmittance at the same concentration.Generally,small MFC fibers tend to have a higher UV transmittance.Fig.5 shows the UV transmittance curves of the obtained MFC products at a concentration of 0.1 wt%.It was observed that the transmittance of MFCCPand MFCHPat 600-800 nm was the highest,both above 80%,indicating that the dimensions of MFCCPand MFCCHPfibers were the smallest.MFCCTMPhad the lowest UV transmittance,as it contains lignin,which can absorb UV light.

The quantitative relationship between the strain and stress of an object under external forces can be determined through rheology.This strain(flow or deformation)is related to the nature and internal structure of the object,as well as the relative motion state between the particles in the object.Fig.6 shows the rheological properties of the six MFC samples at the concentration of 1.0 wt%.All six MFC samples exhibited obvious shear thinning characteristics,and they were characterized as non-Newtonian pseudoplastic fluids.When the MFC samples were at rest or at low shear rates,the fibers remained hooked and entangled with each other,resulting in a high viscosity.However,as the shear rate increased,the relatively scattered MFC fibers rotated and shrunk into clumps due to the shear stress between the troposphere,thus reducing the entanglements and leading to the phenomenon of shear thinning.In addition,MFCSP,MFCHP,and MFCBPhad higher viscosities at a low shear rate because of their larger fiber size,which more easily undergoes entanglement.Meanwhile,the fibers of MFCCR,MFCCP,and MFCCTMPwere smaller in size,so their viscosities were lower at a low shear rate.

The thermal stability of the cellulose raw materials,carboxyethyl-pretreated samples,and MFC samples is shown in Fig.7.After carboxyethyl pertreatment and grinding,the thermal degradation behavior of the CCXand MFCXsamples changed significantly.The initial decomposition temperature and maximum weight loss temperature of the six MFC samples all decreased comparing with CCXand CX,indicating that the thermal stability of the MFC fibers was reduced.This is because parts of the crystalline zone in the cellulose were damaged during carboxyethyl pretreatment and mechanical grinding.The increased disorder and accessibility of cellulose rendered it more susceptible to thermal decomposition,resulting in the decrease in thermal stability.In addition,the TG and DTG curves of MFCCTMPwere significantly different from those of other MFC samples due to the presence of hemicellulose and lignin.The amount of residual char of MFCCTMPwas the highest.Therefore,the thermal stability of the MFC products was affected by the composition of the raw materials.

4 Conclusions

The preparation method of microfibrillated cellulose(MFC)via carboxyethyl pretreatment combined with mechanical grinding was adopted for six kinds of raw materials.It was demonstrated that the carboxyl content,degree of polymerization,water retention value,yield,micromorphology,size distribution,ultraviolet transmittance,rheological property,and thermal stability of the obtained MFC samples were affected by the inherent properties of the different raw materials.The characteristic absorption peak of the carboxyl group was visible in all pretreated samples.All the MFC suspensions showed a large decrease in viscosity with increasing shear rate.At the same concentration,MFC suspensions prepared from bleached softwood(Southern pine)kraft pulp(SP),bleached hardwood(Acacia)kraft pulp(HP),and bleached bamboo kraft pulp(BP)had very high initial viscosities,while that of the other MFC suspensions were much lower.The MFCCPsuspension(obtained from cotton linter pulp(CP))had the highest ultraviolet transmittance.The amount of residual char of MFCCTMP(obtained from softwood spruce chemi-thermomechanical pulp(CTMP))after thermal degradation was slightly higher than that of the other MFCs.

Acknowledgments

The authors are grateful for the financial support from the National Key Research and Development Program of China(2017YFB0307901,2017YFE0102500).