Jinghuan Chen,Jingang Liu,*,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:In this study,lignin-containing microfibrillated cellulose(MFC)was prepared from corncob residue after xylose extraction via co-grinding with calcium hydroxide.The product was then compared with the MFC obtained by direct grinding and applied to strengthen paper.The chemical composition and morphological structure analysis results showed that the corncob residue can be used to prepare lignin-containing MFC and does not require further purification.Moreover,the co-grinding with calcium hydroxide is easier to fibrillate corncob residue.The MFC obtained by cogrinding with calcium hydroxide had a higher aspect ratio,and its surface was coated with calcium carbonate nanoparticles.MFCs obtained by both the methods mentioned above had an obvious strengthening effect on paper.Compared with the paper without MFC,the tensile index,elongation,burst index,and folding strength of the paper with MFC obtained by co-grinding with calcium hydroxide significantly increased by 17.5%,22.1%,19.5%,and 157.1%,respectively.This study provides a novel idea for the utilization of corncob residue,which may enhance the value and promote the comprehensive utilization of corn by-products.

Keywords:corncob residue;microfibrillated cellulose;calcium hydroxide;co-grinding;paper reinforcement

1 Introduction

Corn is an important food crop around the world[1].Global corn output was estimated to be 1.108 billion tons and China ranked second with an output of 261 million tons,accounting for 23.5%of the global corn production in 2020.Corncob is the main by-product,and the ratio of corncob to maize is approximately 0.21[2].Therefore,the annual corncob output in China was approximately 54.81 million tons in 2020.The main components of corncob are cellulose(32%-36%),hemicellulose(35%-40%),lignin(20%-25%),and a small amount of ash(2%)[3-4].Because of its rich xylan content,high yield,and low price,corncob has become the main raw material for xylose production[5-6].However,it is estimated that approximately 9-11 tons of corncob residue are released per ton of xylose produced.Most of the corncob residue is stacked in open air or burned to recover heat energy and generate electricity.Direct combustion not only harms the environment,but also wastes resources.In fact,the waste residue of hemicellulose products such as furfural and xylose oligosaccharides is rich in biomass resources,and its main components are cellulose and lignin[7].Therefore,if corncob residue can be transformed into fine chemical products,it will have markedly economic and environmental benefits.

In recent years,xylose and other hemicellulose products have been extracted from corncobs via multiproduct co-production.For example,corncob residue after lignin removal has been prepared into nanocellulose[8-9]and fuel ethanol[10-12].The separated lignin has been used to prepare phenol monomer[13-15],phenolic resin,lignosulfonate,and other products[16-19].In addition,the direct resource utilization of corncob residue without separation and purification is also one of the research directions.Qu et al[7]used corncob furfural residue as the precursor to prepare porous carbon through one-step activation and simple heat treatment.After ash removal,the porous carbon has a high surface area and excellent electrochemical properties and can be used as an electrode material for supercapacitors.The direct resource utilization of corncob residue eliminates the need for purification and separation processes,which is conducive for reducing costs as well as the use of chemicals.Considering that corncob residue is hydrolyzed and has an obvious porous structure for chemicals to permeate,which is conducive to the fibrillation of microfibrils.Therefore, the preparation of lignin-containing microfibrillated cellulose(MFC)from corncob residue has great potential.

At present,the preparation of MFC mainly involves mechanical treatment or chemical pretreatment combined with mechanical treatment.Mechanical treatment requires large amounts of energy.When chemical pretreatment is combined with mechanical treatment,energy consumption is significantly reduced;however,the pulp usually needs to be filtered and washed after chemical pretreatment[20-21].In this study,lignin-containing MFC was prepared from corncob residue via two methods,direct grinding and cogrinding with calcium hydroxide.When the calcium hydroxide co-grinding method is adopted,the entire process does not require washing;only carbon dioxide is introduced to neutralize the MFC suspension,and finally,MFC coated with calcium carbonate is obtained.The properties of MFCs obtained by direct grinding and co-grinding with calcium hydroxide and their enhancement effects on paper were compared.This study provides a novel method for the green preparation of MFC.Therefore,it contributes to the high added-value of corncob residue,reduces the impact of agricultural waste on the environment,and promotes the comprehensive utilization of agricultural waste.

2 Experimental

2.1 Materials and reagents

Corncob residue(CR)was obtained from Ji'nan Shengquan Group Share Holding Co.,Ltd.(China).Before use,it was diluted to a concentration of 2 wt%,stirred at 1000 r/min for 10 min,and then passed through an 18-mesh sieve to remove large particle impurities.Calcium hydroxide,sulfuric acid,sodium hydroxide,hydrochloric acid,and other chemicals and reagents were of analytical or chemical grade and were used without further purification.

2.2 Preparation of MFC by direct grinding

The corncob residue was diluted to 5 wt%with water 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(XWY-VII-A,Zhuhai Hualun Technology Co.,Ltd.,China).The obtained samples were labeled as MFC-Ax,wherexindicates the number of grinding passes.

2.3 Preparation of MFC by co-grinding with calcium hydroxide

The corncob residue and 10 wt%calcium hydroxide(based on corncob residue)were diluted to 5 wt%with water and fibrillated using the Supermasscolloider-type disk mill mentioned above with a gap clearance of-100μm at 1500 r/min until no visible long fibers were observed under an optical microscope.Carbon dioxide was introduced during stirring of the MFC suspension,until the suspension changed from alkaline to neutral.The final samples were labeled as MFC-Bx,wherexindicates the number of grinding passes.

2.4 Preparation of paper with and without MFC

Hardwood pulp(beating degree 35°SR)and softwood pulp(beating degree 37°SR)with a mass ratio of 80:20 were mixed,and 5 wt%(based on the mass of mixed pulp)MFC was added.The mixture was diluted with water to a concentration of 1 wt%and stirred using a dredger for 5 min.Before papermaking,the mixture was further diluted with water to a concentration of 0.3 wt%.The wet paper formed was vacuum-dried at 95℃.Paper without MFC was denoted as P-0,paper with MFC-A was marked as P-A,and paper with MFC-B was marked as P-B.

2.5 Characterization

The contents of cellulose,hemicellulose,and lignin in the corncob residue were calculated from holocellulose,pentosan,acid insoluble lignin,acid soluble lignin, and ash contents, which were determined in accordance with GB/T 742—2008,GB/T 2677.8—1994,GB/T 2677.9—1994,GB/T 2677.10—1995,and GB/T 10337—2008,respectively.The chemical structure of the corncob residue was analyzed using a Fourier transform infrared (FT-IR)spectrophotometer(Tensor 27,Bruker,Germany)with an attenuated total reflectance accessory in the range of 500-4000 cm-1.The length distribution,mean length,and width of the corncob residue were measured using an L&W fiber analyzer(912.1E,AB Lorentzen&Wettre,Sweden).

Changes in the morphology of the corncob residue during the grinding process were observed using the optical microscope mentioned above. The microstructure of the corncob residue before and after grinding was observed by a scanning electron microscopy(SEM,S-3400N,Hitachi,Japan)at an acceleration voltage of 5 kV.Prior to observation,the samples were diluted to 0.5 wt%,freeze-dried,and coated with gold-palladium using a sputter coater(E-1010,Hitachi,Japan).

The particle size distribution of the obtained MFC was determined through a laser diffraction analysis(Mastersizer 2000 Hydro,Malvern Instruments Ltd.,UK).The length,thickness,and aspect ratio(length to diameter)of MFC was measured using an image particle size analyzer(FC200S,Ochio,USA)at a concentration of 0.1 wt%.

The thickness of the obtained papers was measured using a thickness gauge(PTT-01,Shandong Winish Electronic Technology Co.,Ltd.,China)according to GB/T 451.3—2002.The grammage of the obtained papers was determined according to GB/T 451.2—2002.The compactness of the obtained papers was calculated from their thickness and grammage according to GB/T 451.3—2002.

The paper samples were placed at 23℃and 50%relative humidity for 8 h before testing.The brightness of the paper samples was measured using a brightness meter(CTPC,Technidyne,USA).The tensile strength,breaking length,and elongation were measured using a tensile strength tester (DCP-KZ1000, Sichuan Changjiang Equipment Factory,China)according to the GB/T 12914—2018 standard.The width of the paper samples was 1.5 cm,and the tensile rate was 10 mm/min.Each sample was tested 10 times to obtain the average value.The tearing strength of the paper samples was measured by using a tear meter(RHSL1000,Guangzhou Runhu Instruments Co.,Ltd.,China)according to GB/T 455—2002.The stiffness of the papers was measured using a stiffness meter(TD-10A,Sumspring,China)according to GB/T 22364—2018.The bursting strength was measured using a burst tester(ZB-NPY1600,Hangzhou Zhibang Automation Technology Co.,Ltd.,China)according to GB/T 454—2020.The folding strength was measured according to GB/T 457—2008 with a folding tester(PY-H608,Shenzhen Puyun Electronic Co.,Ltd.,China).

3 Results and discussion

3.1 Characterization of corncob residue

The corncob residue was sourced from the residue after the production of xylose by acid hydrolysis of the corncob.The content of each component in the corncob residue is shown in Table 1.The main component of the corncob residue was cellulose followed by lignin.Because the acid hydrolysis is performed to produce xylose,most of the hemicellulose is removed;thus,the hemicellulose content in the corncob residue is low.In addition,the corncob residue contained a certain amount of ash.Because of the high content of cellulose and lignin in the corncob residue,it can be used to prepare lignin-containing MFC.

Table 1 Content of each component in corncob residue %

The FT-IR spectrum of the corncob residue is shown in Fig.1.The absorption peaks of cellulose and hemicellulose are located at 3335(—OH stretching vibration),2887(—CH stretching vibration),1160,1030,and 898 cm-1.The absorption peak at 1699 cm-1originates from the C=O stretching vibration of the hemicellulose acetyl group. The characteristic absorption peaks of lignin are located at 1603 and 1512(C=C stretching vibration),1454(—CH3asymmetric bending vibration), 1426 (C—H deformation vibration),1317(C—O vibration on the lilac base ring),and 1202(C—O stretching vibration on the guaiac base or lilac base ring)cm-1.The characteristic absorption peaks of cellulose,hemicellulose,and lignin can be clearly observed in the FT-IR spectrum,indicating that the corncob residue contains these three components,consistent with the results of the component analysis.

Fig.1 FT-IR spectrum of corncob residue

The micromorphology of corncob residue is shown in Fig.2.Corncob residue contains both slender fibers and a large number of short and coarse blocks.In contrast to other fiber raw materials,the surface of corncob residue has dense pores with different sizes and uneven distribution.This structure may promote the penetration of water or calcium hydroxide into corncob residue during mechanical fibrillation.

The fiber size distribution of corncob residue is shown in Fig.3.Corncob residue mainly consists of short fibers with a length of less than 0.6 mm.The mean length is 0.481 mm,and the mean width is 38.0μm,which corresponds to the micromorphology in Fig.2.Generally,raw materials with high content of short fiber are not suitable for the preparation of paper;however,they provide an opportunity for the preparation of MFC.

Fig.2 Micromorphology of corncob residue at different magnifications

Fig.3 Fiber size distribution of corncob residue

3.2 MFC prepared by direct grinding or calcium hydroxide co-grinding

The fibrillation processes of corncob residue by direct grinding and co-grinding with calcium hydroxide are shown in Fig.4 and Fig.5,respectively.It can be observed that with an increase in grinding passes,the fibers and blocks in the corncob residue are gradually dissociated and broken until no obvious large fibers are observed.In this state,the grinding passes of the direct grinding method is 40(MFC-A40),whereas that of the co-grinding with calcium hydroxide method is only 28(MFC-B28). Therefore, the addition of calcium hydroxide greatly reduces the grinding energy consumption.A small part of calcium hydroxide can be dissolved in water,which provides an alkaline environment and can promote the swelling of corncob residue. Besides, the solid calcium hydroxide suspended in water plays an auxiliary role in the grinding process.In addition,the optical microscopy images of MFC obtained by co-grinding with calcium hydroxide before and after carbonization were also compared.The results showed that no notable difference was found,that is,no obvious calcium carbonate particles were observed.The possible reason may be minimal calcium carbonate production(calcium hydroxide content is only 10 wt%of corncob residue)or possibly small particle generated which cannot be observed under an optical microscope.

Fig.4 Fibrillation process of corncob residue by direct grinding

Fig.5 Fibrillation process of corncob residue by co-grinding with calcium hydroxide

The particle size distributions of MFC-A40and MFCB28were characterized using a laser particle size analyzer.The results are shown in Fig.6.It can be seen that the median particle sizeD(0.5)of MFC-B28(19.6μm)was smaller than that of MFC-A40(29.3μm).A small number of large-sized fibers can be seen in the particle size distribution diagram of MFC-A40,which may be non-dispersible flocs formed by multiple MFC bonded together.Therefore,calcium hydroxide cogrinding can reduce energy consumption,obtain finer MFC,and prevent its bonding flocculation.

Fig.6 Particle size distributions of MFC-A 40 and MFC-B28

The geodesic length,thickness,and aspect ratio of MFC-A40and MFC-B28were further measured using an image particle size analyzer.The area average particle size(D[3,2])and volume average particle size(D[4,3])were obtained using quantity and volume as weight factors,respectively.The results are presented in Table 2.Compared with MFC-A40,MFC-B28had a longer ISO geodesic length and smaller ISO fiber thickness;thus,its aspect ratio was notably higher than that of MFCA40.Therefore,the presence of calcium hydroxide is essential to obtain slender MFC.This may be because calcium hydroxide renders the grinding environment alkaline.Under alkaline conditions,the corncob residue was swelled,resulting in the easy dissociation of the fiber.Simultaneously,the fiber became soft and elastic.These aspects are pertinent in promoting the peeling of the fiber along the fiber direction when it is ground and reducing the radial cutting and fracture of the fiber.Thus,the co-grinding with calcium hydroxide yields MFC with a larger aspect ratio.

Table 2 Geodesic length,fiber thickness,and aspect ratio of MFC-A40 and MFC-B28

The micromorphologies of MFC-A40and MFC-B28are shown in Fig.7.The surface of MFC-A40was relatively smooth, and the thinnest part was approximately 200 nm,but the fibers were easy to bond together.However,for the MFC-B28obtained by co-grinding with calcium hydroxide,it could be observed that particles with a diameter of approximately 200 nm were adsorbed on its surface.These particles were calcium carbonate nanoparticles produced by the reaction of calcium hydroxide with carbon dioxide during carbonization.Owing to the coating of these particles,the obtained MFC was less bonded,and the network structure was denser.Therefore,calcium hydroxide not only helps in grinding,but also isolates the newly fibrillated fibers,which can prevent their adhesion,thereby promoting the formation of MFC.This result is also consistent with the results of particle size distribution analysis.In addition,the SEM images verify the previous inference that the generated calcium carbonate particles are very small.

Fig.7 Micromorphologies of MFC-A 40 and MFC-B28

3.3 Enhancement effect of MFC on paper

To evaluate the enhancement effect of MFC-A40and MFC-B28on paper,5 wt%of MFC(based on the dry weight of pulp)was added to the mix pulp of hardwood pulp and softwood pulp to make papers and compared with the blank paper sample without MFC.The results are presented in Table 3.Although corncob residue is not considered a good raw material for papermaking,the MFC prepared from this residue has a significant impact on some indices of paper.Compared to the blank paper sample P-0,the tensile index,elongation,burst index,and folding strength of the papers with MFC-A40(P-A) and MFC-B28(P-B) increased significantly,but the brightness,tearing index,and stiffness decreased slightly.The enhancement of MFCB28was particularly obvious.Compared with P-0,the tensile index,elongation,burst index,and folding strength of P-B increased by 17.5%,22.1%,19.5%,and 157.1%,respectively.However,the brightness,tearing index,and stiffness of the paper decreased by 20.6%,3.6%,and 6.5%,respectively.This is because that the corncob residue containing lignin is brown,and the MFC prepared from it is also brown.Owing to the presence of calcium carbonate in MFC-B28,the brightness of P-B was slightly higher than that of P-A.Therefore,MFC with better performance and lower energy consumption can be obtained by the calcium hydroxide co-grinding method,which may help broaden the application potential of corncob residue,increase the added value of products,and promote the comprehensive utilization of corn by-products.

Table 3 Physical properties of paper samples P-0,P-A,and P-B

4 Conclusions

In this study,two types of microfibrillated cellulose(MFC)were prepared from corncob residue by direct grinding and co-grinding with calcium hydroxide method, and used to strengthen paper. The composition and structure analysis results showed that the corncob residue was suitable for the preparation of lignin-containing MFC.The presence of calcium hydroxide can greatly reduce the grinding energy consumption and yield MFC with a higher aspect ratio. The results of the paper strengthening experiment showed that both types of MFC could significantly improve the tensile strength,elongation,burst strength,and folding strength of paper,especially MFC obtained by the co-grinding with calcium hydroxide method.This study summarizes a promising application of corncob residue,thereby promoting the efficient utilization of corn by-products and enhancing their added value.

Conflict of interest

The authors declare no financial conflict of interest.

Acknowledgments

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