Chaoxian Zhou,Mingchang Gao,Gengmei Liu,Chen Zhang,Yang Liu,Qingxian Miao

College of Material Engineering,Fujian Agriculture and Forestry University,Fuzhou,Fujian Province,350100,China

Abstract:To improve the hydrophobicity of bagasse hemicellulose-based films,glutaraldehyde was applied when preparing films of original and cationic bagasse hemicellulose with the addition of polyvinyl alcohol and sorbitol.The results showed that the cationic modification could increase the hydrophobicity of the hemicellulose-based film,and the hydrophobicity of hemicellulose-based films crosslinked with glutaraldehyde also increased.However,cationic modification of hemicellulose decreased the stress of the hemicellulose-based film.While crosslinking with glutaraldehyde increased the stress of both the original and cationic hemicellulose-based films.Macrophotography indicated that the film formability of the original hemicellulose was better than that of cationic hemicellulose.Through SEM observation,the degree of bonding of different components of the films was found to be increased due to crosslinking with glutaraldehyde.The crosslinking reaction between glutaraldehyde and hemicellulose was further confirmed by FT-IRspectroscopy.

Keywords:bagasse;hemicellulose;film;hydrophobicity;mechanical property

1 Introduction

China is one of the main agricultural producersin the world,and its production output of sugarcane ranks third after those of Brazil and India.Bagasse is the main byproduct of the sugar industry and has great potential for use due to its widespread source,large output,renewability,and ease of degradation.It is reported that theoutput of bagasse in South China can reach more than 20 million tonsper year[1].

Hemicellulose is one of the three major cell wall components of lignocellulosic resources,with content of the hemicellulose second only to that of cellulose[2].Hemicellulose is also a significant renewable resource with abundant reserves and low cost.Approximately 60 billion tons of hemicelluloses are produced annually[3].Hemicellulose can be dissolved in common solvents and easily physically or chemically modified owing to its branched amorphous structure[4].To replace traditional non-renewable fossil-based products,the development of biodegradable materials with good biocompatibility has drawn increasing attention[5−6].As an abundant and biodegradable natural resource,hemicelluloses may be an alternative to other biodegradable materials. Hemicellulose-based materials have good biocompatibility,biodegradability,and air-barrier property,and can be applied in many fields such as packaging materials,coatings,drug release,and platform chemicals[7].

Producing packaging films has been regarded as a potential high-value application of hemicelluloses[5,8−9].However, the films prepared from original hemicellulose have high hydrophilicity because of the numerous hydroxyl and carboxyl groups distributed along the original hemicellulose backbone and side chains,whichrestrictstheirwidepracticalapplications[10].Chemical modification of hemicelluloses via hydroxyl groups on their molecular chains,using techniques such as gas-phase fluorination or esterification,can decrease the hydrophilicity of the films[11−13].In addition,the low molecular weight and short molecular chain of typical hemicelluloses results in poor filmforming properties and mechanical properties of hemicellulose-based films[14−15].Polyvinyl alcohol has free hydroxyl groups on its molecular chain,and it is usually applied to improve the film-forming property and strength of hemicellulose-based film via hydrogen bonding with the hydroxyl groups of hemicellulose[16].Polyvinyl alcohol was therefore used in the present investigation. Poor flexibility of the original hemicelluloses is also a shortcoming of the hemicellulose-based films.Some plasticizers with low molecular weights,such as glycerol and xylitol,have usually been used to improve the flexibility of the films[17−18]. A prepared film with quaternized hemicelluloses and carboxymethyl cellulose exhibited better mechanical properties[19].However,the effect of quaternization on the hydrophobicity of the resultant hemicellulose-based films was not investigated.Crosslinking with polycarboxylic acids or aldehydes is regarded as an efficient method for improving the properties of hemicellulose-based films[20−21].Crosslinking with polycarboxylic acids could increase the hydrophobicity of hemicellulose-based films,while crosslinking with glutaraldehyde(GA)could increase the mechanicle properties.The effect of crosslinking with GA on the hydrophobicity of hemicellulose-based filmshasnot been reported to date.

GA,which has bifunctional cross-linking aldehyde groups,is generally used as the crosslinking agent because it is inexpensive and readily available in commercial quantities.In the present study,the quaternization of bagasse hemicellulose and crosslinking with GA,which can decrease the amount of hydroxyl groups,was employed to increase the hydrophobicity of the hemicellulose-based films.This method has the merits of simple operation and low cost.The physical strength of the films was also investigated.The results of this study will promote the efficient utilization of hemicelluloses from lignocellulosic biomassand itsfilms.

2 Experimental

2.1 Materials

Bagasse hemicellulose was prepared in the laboratory through the alkaline treatment of holocellulose combined with ethanol precipitation.The weightaverage molecular weight of the obtained hemicellulose was 1.5×105g/mol.The main sugar component of the bagasse hemicellulose was xylose,with a content of 92.34%.3-chloro-2-hydroxy-propyl trimethyl ammonium chloride was used to prepare thecationic hemicellulose under alkaline conditions in the laboratory,and its degree of substitution was 0.025.All other reagents used in this study,including polyvinyl alcohol(PVA),sorbitol,and GA(50 wt%)were of analytical grade and purchased from Aladdin(Shanghai,China).

2.2 Preparation of hemicellulose-based films

0.5 g of bagasse hemicellulose was placed into 50 mL of deionized water and dissolved at 85℃ under magnetic stirring.24%of PVA and 16%of sorbitol(based on oven dried(o.d.)hemicellulose dosage)were then added to the hemicellulose solution in sequence.After homogeneous mixing,the mixture was cooled to 60℃.Thereafter,GA solution was added and the mixture was allowed to react for 30 min.When the reaction was complete,the mixture was poured into a Teflon watch glass.The Teflon watch glass containing film-forming liquid was first dried to about a state with moisture of 50%in a 40℃oven and then allowed to form a film in a room with constant temperature(25℃)and 50%humidity.

2.3 Characterization of hemicellulose-based films

2.3.1Determination of contact angle

A Harke-Spca static contact goniometer(Beijing Harke Experimental Equipment Co.,Ltd.,China)was used to determine the contact angle of the films.The volume of deionized water used as the osmotic reagent was 5μL,and the contact time was 5 s.The values of contact angle were measured for five spotson a film sample.

2.3.2Determination of mechanical strength

The hemicellulose-based films were first cut into sheets of 15 mm×50 mm,and the mechanical strength of all samples was determined using a universal tester(KJ-1065,Kejian Testing Instrument Co.,Ltd.,China).The initial space between the two clamps was 25 mm.The stretching speed was set to 2 mm/min,and the load was 100 N.Each sample wastested in triplicate.

2.3.3FT-IRspectroscopy

The chemical structures of all the film samples were determined using an FT-IR instrument(Brüker,Vertex 70,Germany).The samples were scanned from 400 to 4000 cm-1.The samples were crushed together with KBr powder prior to analysis.

2.3.4Observation of microstructure and macrostructureof films

The microstructural morphology of the films was examined using an SU8010 scanning electron microscope(SEM,Hitachi,Japan).The samples were dried and coated with gold prior to SEM observations.To examine the cross-section of the films,they were first wetted-off with liquid nitrogen and then freezedried.The macrostructure of the films was examined using a camera.

3 Results and discussion

3.1 The contact angel of hemicellulose-based films

The contact angle is usually regarded as an important parameter for evaluating the hydrophobicity of films.A contact angle of less than 90°indicates that the material is hydrophilic,and if it exceeds 90°,the material is hydrophobic[22].As shown in Fig.1,the contact angles of both kinds of films could be increased by the addition of GA.The contact angles of both hemicellulose-based films reached a maximum value when GA dosage was 2.5%(based on the o.d.hemicellulose mass).The contact angles of the original and cationic hemicellulose-based films increased from 85°and 90°to 108°and 124°,respectively.The hydrophobicity of the films increased obviously due to the addition of GA.Fig.1 also shows that the hydrophobicity of the hemicellulose-based films can be increased through cationic modification of the original hemicellulose,which is mainly due to the replacement of hydrophilic hydroxyl groupswith amino groups.

For the original hemicellulose,numerous hydroxyl groups and some carboxyl groups located on its molecular chains are responsible for the hydrophilicity of the films.It is known that the aldehyde groups of GA are liable to react with hydroxyl groups to form acetal or hemiacetal bonds under acidic conditions[4],which reduces the number of hydroxyl groups and forms a network structure.The combination of water molecules with hydroxyl groups located on themolecular chains of hemicelluloses was reduced,and thus the hydrophobicity of the hemicellulose-based films was improved.For cationic hemicelluloses,the replacement of hydrophilic hydroxyl groups with amino groups,the Schiff base reaction between amino groups and aldehyde groups,and the acetal bonding between aldehyde groups and the residual hydroxyl groups on the molecular chains of hemicelluloses led to an increase in the hydrophobicity of the cationic hemicellulose-based films.

However, the intermolecular density can be increased because of the continuously increased dosage of GA.Due to steric hindrance,the crosslinking reaction of GA with hydroxyl and amino groups decreases,and thus the hydrophobicity of the films decreases(Fig.1)[21].In addition,the increased dosage of GA may lead to the intramolecular crosslinking reaction of GA molecules,which influenced the reaction of GA with hydroxyl or amino groups,resulting in the exposure of hydroxyl and amino groups in the films.As a result,the hydrophobicity of the hemicellulose-based composite filmsdecreased.

Fig.1 Effect of GA dosage on the contact anglesof hemicellulose-based films

3.2 The mechanical properties of hemicellulose-based films

The stress represents the load on an object per unit area,whereas the strain represents the change in length per unit area.As shown in Fig.2,compared with the original hemicellulose-based film, the cationic hemicellulose-based film decreased the stress while increasing the strain of the film.The replacement of hydroxyl groups with amino groups led to a decrease in hydrogen bonds between inter-hemicelluloses and PVA,which resulted in a decrease in the bonding strength of different components and the stress of the cationic hemicellulose-based film. However, the flexibility of the films improved after cationic modification.It should be noted from Fig.2 that the stress and strain of both the original hemicellulosebased films and cationic hemicellulose-based films were greatly increased after crosslinking with GA.This may be due to the crosslinking reaction of GA with hydroxyl groups and the Schiff base reaction with amino groups,resulting in the formation of many uniform three-dimensional network structures inside and on the surface of the hemicellulose-based films.The formed covalent bonds enhanced the interactions of different molecules[21,23].

Fig.2 Mechanical properties of different hemicellulose-based films

3.3 FT-IRspectroscopy of hemicellulose-based films

As shown in Fig.3,the peaks at 3446,2924,1463,1166,1043,and 894 cm-1are typical infrared absorption peaks of the original hemicelluloses.Among them,the peak at 894 cm-1is the characteristic absorption peak of theβ-glycosidic bond between different sugar units[24−25].The peak at 1043 cm-1is ascribed to the ether bond between different sugar units.The bands between 1166 cm-1and 1000 cm-1are the characteristic absorption peaks of xylan.Comparedwith the original hemicellulose,the absorption peak of ether bonds moved to the high frequency of 1047 cm-1after cationic etherification of hemicellulose,which indicated that the number of ether bonds increased.The peak at 1462 cm-1is the signal peak of—CH2and the methyl groups on the substituent[22],and the peak at 1417 cm-1is the peak of C—N stretching vibration[26],which indicates the success of cationic modification for theoriginal hemicellulose.

Fig.3 FT-IRspectra of different hemicellulose-based films

Compared with the hemicellulose-based film without the addition of GA,a new absorption peak appeared at 1708 cm-1after crosslinking with GA.This is the peak of the C＝O stretching vibration of the aldehyde group,indicating an acetal reaction between the hydroxyl groups of hemicelluloses and aldehyde groups of GA.For cationic hemicellulose-based films, a new absorption peak at 1643 cm-1was observed after crosslinking with GA,which was caused by the stretching vibration of C＝N,indicating that the Schiff base reaction occurred between cationic hemicellulose and GA[27].

3.4 Macro morphology of different hemicellulosebased films

As shown in Fig.4,the film formability of cationic hemicellulose is not as good as that of the original hemicellulose,and more cracks can be found in the cationic hemicellulose-based film.This may be due to the decreased amount of hydrogen bonds between the hydroxyl groups,which were replaced by the amino groups.However,the film formability of both the original and cationic hemicelluloses improved after crosslinking with GA.It should be noted that the toughness of the cationic hemicellulose-based film was better than that of the original hemicellulose-based film after crosslinking with GA.This result was also consistent with the mechanical properties of the hemicellulose-based films(Fig.2).

Fig.4 Macroscopic images of different hemicellulose-based films

3.5 SEM morphology of different hemicellulose-based films

It can be observed from the SEM morphologies in Fig.5 that without crosslinking with GA,the surface was rough and contained grooves for hemicellulosebased films(Fig.5(a)and Fig.5(c)).The surface of the films crosslinked with GA was smooth and compact(Fig.5(b)and Fig.5(d)).This indicated that the crosslinking reaction occurred both within and on the surface of the films after the addition of GA.The crosslinking reaction of aldehyde groups with hydroxyl and amino groups helps to form a stable crosslinked structurebetween different molecules.

Fig.5 SEM imagesof different hemicellulose-based films

Fig.6 shows the cross-section SEM images of the different hemicellulose-based films.Some bulges can be clearly observed on the cross-section of the hemicellulose-based films without crosslinking with GA,and the cross-section appeared rough.The crosssectional structures of both the original and cationic hemicellulose-based films after crosslinking with GA appear to be more compact and smoother.The results also illustrate that the bonding was strengthened by crosslinking with GA.

Fig.6 Cross-section SEM imagesof different hemicellulose-based film

4 Conclusions

The effect of crosslinking with glutaraldehyde(GA)on the hydrophobicity of bagasse hemicellulose-based films was investigated.Cationic modification can improve the hydrophobicity of the hemicellulose-based films.The addition of GA can increase the contact angles of both the original and cationic hemicellulosebased films,whose contact angles increased from 85°and 90°to 108°and 124°,respectively,when the GA dosage was 2.5% (based on the oven dried hemicellulose mass).However,the stress of the hemicellulose-based film decreased after the cationic modification.Crosslinking with GA can improve the mechanical properties of both the hemicellulose-based films.The macro and micro morphologies of the films showed that the film formability of cationic hemicellulose was inferior to that of the original hemicellulose.Crosslinking with GA improved the film formability and bonding performance between different componentsof the films.