Ruijuan Zhang,Yanqun Su,*,Jingang Liu

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,we investigated the barrier properties of different kinds of microfibrillated cellulose(MFC)coating layers.The air,oxygen,and water vapor permeability,as well as the water contact angles(WCA),were measured to quantify the barrier efficacy of the applied coatings.The WCA data showed that the surfaces of MFC-coated cardboards are more hydrophilic than those of uncoated cardboards.However,different MFC coatings realize different oxygen transmission rates(OTRs)and water vapor transmission rates(WVTRs).The MFC coating derived from bleached bamboo pulp subjected to carboxyethylation pretreatment(MFCCBP)gave the best oxygen and water vapor barrier performances.The OTR of the virgin cardboard(>16500 cm3/(m2·24 h))decreased to 4638 cm3/(m2·24 h)after coating with the MFCCBP.The WVTR similarly decreased from 1016.7 g/(m2·24 h)to 603.2 g/(m2·24 h).

Keywords:microfibrillated cellulose;coating performance;surface properties;barrier properties

1 Introduction

In recent years,the packaging industry has experienced rapid growth,owing largely to the increased demand for food and beverage packaging.To meet the requirements for food packaging,paper and paperboards are often compounded with other materials,such as plastics or aluminum,to improve their barrier properties[1].However,as is well known,plastics and aluminum are non-biodegradable and remain in the soil for a long time,causing environmental problems[2].Numerous studies have focused on the development of alternative materials with better performance,sustainability,biodegradability,and recyclability[3-4],to address environmental concerns relating to sustainability and end-of-life disposal challenges.

Microfibrillated cellulose(MFC)is an ideal component candidate for packaging materials owing to its biodegradability,renewability,and excellent mechanical properties[5].MFC containing many hydrogen bonds can be separated from various wood and plant sources by chemical or enzymatic pretreatment followed by mechanical treatment[6].The hydrogen bonding ability of the material enables the formation of a strong,dense network,which can prevent the passing of oxygen and water vapor[7-9].Hence,MFC offers a promising alternative to plastics or aluminum in providing barrier properties to packaging paper and paperboard.

Kumar et al[10]fabricated a material for barrier applications using a roll-to-roll process,in which paperboard is coated with a cellulose nanofiber(CNF)suspension.They found that the CNF-coated paperboards were excellent barriers against air,grease,and mineral oil.Gicquel et al[11]investigated the application of a 100% cellulose nanocrystal(CNC)coating layer onto base paper materials and found that the coating improved the air impermeability and grease resistance of the paper materials.At a CNC coating weight of~2.5 g/m2,the air resistance of the coated material increased by 91%.Hult et al[12]investigated the enhancement of the barrier properties of paper and paperboard realized by MFC and shellac coatings.They found that the oxygen transmission rate(OTR)and water vapor transmission rate(WVTR)of paper and paperboard decreased dramatically after being coated with MFC and shellac.The OTR and WVTR of a paper sample with a 2.21-μm-thick MFC coating were 35275 mL/(m2·24 h)and 70.89 g/(m2·24 h),respectively.These studies indicate that nano-or microfibrillated cellulose is a promising alternative barrier layer for food packaging paper and cardboard.

In this study,we systematically compared the characteristics of MFC derived from corncob,hardwood,bamboo,and softwood which were subjected to carboxymethyl pretreatment,or carboxyethyl pretreatment,with mechanical treatment.The different kinds of MFC were used to coat the cardboard surfaces to evaluate the resulting barrier properties,such as air permeability,OTR,and WVTR.Furthermore,the surface properties of the MFC coatings were investigated.

2 Materials and methods

2.1 Materials

The five MFC suspensions were all prepared in our laboratory using different kinds of raw materials,i.e.,unbleached corncob pulp,bleached hardwood pulp,bleached bamboo pulp,and bleached softwood pulp.These raw materials were pretreated and/or mechanically treated to obtain five MFC samples,labeled as MFCCP,MFCCCP,MFCCHP,MFCCBP,and MFCCSP.The detailed preparation methods are shown in Table 1.The base paper material was cardboard with a basis weight of 180 g/m2used for food packaging.

2.2 Instruments and equipment

Automatic potentiometric titrator(916 Ti-Touch,Metrohm,Switzerland);scanning electron microscopy(SEM,S-3400N,Hitachi,Japan);granulometer(2000MU,Malvern,UK);oxygen transmission rate testing(Y110,Guangzhou Inter-standard,China);rotational rheometer(Bohlin Gemini 2,Malvern,UK);supermasscolloider(MKCA6-2J,Masuko Sangyo,Japan);contact angle analyzer(DSA20,Kruss Gmbh,Germany);Gurley air permeability meter(GPI Inc.,USA).

Table 1 MFC samples prepared using different raw materials and treatments

2.3 Coating preparation

The MFC suspension at 2 wt% was coated onto cardboard samples(A4 size)using a bar coating process.The coated cardboards were then dried with a non-contact drying system at 85℃for 3 min.These steps were repeated three times to obtain paper samples with three MFC layers and a coating weight of 6.5 g/m2.

2.4 Characterization techniques

2.4.1 Carboxyl content determination

The carboxyl contents of the MFC suspensions were determined using conductometric titration,as described by Araki et al[13].The MFC(50 mg)was diluted to 0.1%(w/V)before 2.0 mL 0.1 mol/L HCl was added to substitute the Na+ions bounded to the carboxyl groups with protons.To promote dynamic equilibrium,1.0 mL 1.0 mmol/L NaCl was added and the resulting slurry was sufficiently stirred for 90 min before titration.Once a stable suspension was obtained,it was titrated with 0.1 mol/L NaOH at a rate of 0.1 mL/min.The carboxyl contents of the samples were calculated from the conductivity and pH value curves,where the strong acid corresponded to the excess added HCl and the weak acid was assumed to be the carboxyl group.

2.4.2 SEM

A freeze-dried sample of each MFC suspension was treated with gold spraying before its surface morphology was observed by SEM operated at an accelerating voltage of 5 or 15 kV.

2.4.3 Rheological tests

Rheological measurements of the MFC suspensions were performed using a rheometer.A parallel-plate geometry was selected for the measurements at a constant temperature of 25℃.The shear rates investigated were between 0 and 18000 s−1.A plate clamp configuration was adopted.

2.4.4 Water contact angle(WCA)measurements

Static contact angle measurements were performed at room temperature(25℃)using a DSA20 system;deionized water was served as the osmotic reagent.

2.4.5 Air permeance measurements

Air permeance was measured using a Gurley air permeability meter according to the ISO 5636-5(2005)standard method.

2.4.6 Determination of the OTR

Oxygen barrier measurements were performed using an OTR testing system following the GB/T 19789—2005 test standard.The sample was exposed to 100%oxygen on one side and oxygen-free nitrogen on the other side;the difference in the O2pressure between the two sides were measured overnight.The permeability was determined based on the increase in the O2peak area as a function of time.The OTR is reported as the mean value of two determinations for each sample type and expressed as cm3/(m2·24 h).

2.4.7 Determination of the WVTR

WVTR tests were performed according to the GB/T 2679.2—2015 test standard.An allochroic silica gel was placed into a cup to maintain relative humidity(RH)at 0 inside the cup.The test sample was placed over the mouth of the cup,and then a gasket was applied to seal the sample to the rim of the cup.The cup was placed in a climate-controlled room(25℃,75% RH),and the increase in weight of the cup after a certain time interval,due to water vapor permeation through the test sample,was used to calculate the WVTR value.The average of three parallel measurements is reported as the WVTR.

3 Results and discussion

3.1 Properties of different MFC suspensions

The properties of MFC suspensions are key factors in the coating process.The carboxyl contents of the MFC suspensions are presented in Table 2.

Carboxyl groups significantly affect the dispersion state of MFC in water and contribute to the bonding of fibrils[14].As shown in Table 2,the carboxyl contents of MFCCHP,MFCCBP,and MFCCSPare similar,while those of MFCCPand MFCCCPare relatively low.The MFCCPand MFCCCPwere both derived from corncob pulp,but the carboxyl content of the MFCCPis much lower than thatof the MFCCCPbecause it was not chemically pretreated before mechanical dissociation.

Table 2 Carboxyl contents of different MFC suspensions

SEM images of the MFC samples used to assess the structural and dimensional appearance of the microfibrils are presented in Fig.1.The images show that all of the microfibrils display web-like networks consisting of randomly entangled sub-micron fibrils,and even membranes in some parts,due to the strong hydrogen bonding between MFC fibrils.It is also evident that the studied MFC samples subjected to different preparation methods have different fibrillar morphologies.The MFCCPand MFCCSPsamples exhibit porous and bulky fibrillar networks,whereas the rest of the samples exhibit a more even layer of fibrils.In addition,the fibrils of the MFCCPsample are significantly shorter than those of the other samples,especially the MFCCBPand MFCCSP.These film formation behaviors and fibrillar structures could be related to the barrier properties of the MFC samples[14].

The rheological characteristics of the MFC suspensions were investigated and their rheological curves were fitted using the Carreau model,which is based on Equation(1).

whereηis the viscosity,Pa·s;η∞is the limit viscosity,Pa·s;η0is the zero-shear viscosity,Pa·s;kis time constant,s;γis shear rate,s−1;nis power-law index.

The fitted curves are shown in Fig.2 and the associated parameters are listed in Table 3.In Table 3,the correlation coefficients(R2)of the five MFC samples are all above 0.996,which means that this model is appropriate for describing the rheological characteristics of the MFC suspensions.

Table 3 The parameters of MFC suspensions accord⁃ing to the rheological model

Fig.2 shows the rheological power-law behavior of the MFC suspensions studied as lg-lg curves of viscosity versus shear rate.Viscosity describes the MFC suspension's internal resistance to flow[14].It can be clearly seen in the diagram that all of the MFC samples exhibit the characteristics of a pseudoplastic fluid,supporting their shear thinning behavior which is beneficial to coating applications[15].According to the rheological parameters shown in Table 3,the order ofη0is MFCCP>MFCCCP>MFCCHP>MFCCSP>MFCCBP,and there is a significant difference between theη0values of the two corncob-derived MFC suspensions and those of the other MFC suspensions.This may be because the MFCCPand MFCCCPcontain lignin that increase the hydrophobicity of the MFC suspensions,resulting in higherη0values[16].The carboxymethylation pretreatment of MFCCCPintroduced more negative charges in the MFCCCPrelative to the untreated MFCCP,which increased the electrostatic repulsion between fibrils and decreased the viscosity of the MFC suspension,resulting in anη0value that is lower than that of the MFCCP[17].Parameternis also called the non-Newtonian index and represents the dependency of the viscosity of MFC suspensions on shear rate.Table 3 shows that thenvalues of the MFCCBPand MFCCSPare smaller than those of the other samples,indicating greater pseudoplastic behavior which benefits to the coating process.

3.2 Surface properties of MFC coatings

The influence of the MFC coatings on the surface morphology of the cardboards was examined by SEM(Fig.3).The surfaces of the coated cardboards appear significantly denser and smoother than the uncoated sample.Based on a comparison of the coated samples,the MFCCBP-coated sample exhibits the smoothest surface,whereas the surface of MFCCP-coated sample is the roughest.These surface properties may result from the combined influence of the fibril morphology and film-forming properties of the MFC coatings.

We observed the wetting behavior of the sample surfaces using the sessile drop method.Surfaces producing a WCA>90° exhibit hydrophobic behavior while surfaces with a WCA<90° exhibit hydrophilic behavior[18].Table 4 and Fig.4 show the WCAs of the different samples.The uncoated cardboard is slightly hydrophobic,while the MFC-coated cardboards demonstrate hydrophilic behavior,albeit to varying degrees.This is due to the hydrophilicity of the MFC itself.Unexpectedly,the MFCCCP-coated cardboard is the most hydrophilic,possibly due to the degradation of the lignin in the MFC during pretreatment and its rough surface.Wenzel et al and Ting et al[19-20]found that the rougher the surface of a material,the greater its hydrophilicity because the effective solid-liquid contact area is larger than the measured geometric contact area of the rough surface.The other sample surfaces(Fig.3)yielded results consistent with this theory.For example,cardboards coated with MFCCHP,MFCCBP,and MFCCSPexhibit similar hydrophilic behaviors,but the MFCCBPcoated cardboard achieved the larger WCA,possibly due to its smoother surface.

Table 4 WCA of MFC coated cardboards

3.3 Air/oxygen resistance performance

Air/oxygen-barrier properties of food packaging materials are very important[21].To serve as a barrier layer,an MFC coating must either increase the shelf life of foods by preventing the permeation of oxygen and other gases,or conversely,prevent the escape of food odors[22].The air/oxygen-barrier properties of the five MFC-coated cardboards and uncoated cardboard were evaluated under the same conditions,and the results are listed in Table 5.

Table 5 Air/oxygen barrier properties of MFC-coated cardboards

The uncoated cardboard demonstrates an air permeance of 956 nm/(Pa·s)while the air permeance results of the cardboards coated with MFCs are 3-4 orders of magnitude lower,a dramatic difference.This can be attributed to the closure of cardboard surface pores by the very dense network structure formed by the microfibrils.According to literature[7],the pores within substrates serve as the main pathways for permeating air molecules.The complex dense network structures formed by microfibrils increase the tortuosity of the permeation pathways and thereby decrease the permeability of materials[23].Moreover,the lowest air permeance value of the MFC-coated cardboards is achieved by the MFCCBP-coated sample;it is one order of magnitude less than that of the MFCCP-coated sample.This is because the coating structure formed by MFCCBPsuspension is much denser than those formed by other MFC suspensions,especially MFCCP,as shown in the SEM images(Fig.3).

In this study,a remarkable linear correlation was found between the air permeance of MFC-coated cardboards and theirη0values,as shown in Fig.5.As discussed above,the highη0values of certain samples are likely due to the complex inner structures that prevent the fibrils from dispersing uniformly in MFC suspensions in the steady state.Thus,the suspensions used to prepare MFC-coated samples with lowη0values achieved greater dispersion of the single fibrils to form uniform and dense coatings,resulting in relatively low air permeance.It is worth pointing out that the OTRs of most of the MFC coated samples could not be analyzed because the values were too high(OTR≥16500 cm3/(m2·24 h)),except for the MFCCBP-coated sample,which is consistent with the preceding discussion.In this case,the obtained oxygen-barrier performances are not as good as expected.The unexpected results may be explained by the incomplete closure of the surface pores of the base cardboards by the MFC coatings,due to the formation of inhomogeneities in the coating layers[7].

3.4 Water vapor resistance performance

Table 6 shows the WVTRs of the MFC-coated cardboards and uncoated sample.The results suggest that the MFC coatings provide good resistance to water vapor.The WVTRs of the coated cardboards are 26%-40% lower than that of the uncoated sample owing to the barrier of the dense network structures formed by the MFC fibrils[24].However,the relatively large decrease observed in the air permeance of the coated cardboards,relative to the uncoated cardboard,does not translate to the water vapor barrier properties,which can be ascribed to the high affinity between water and MFC fibrils.The hydrophilic nature of the MFC coatings are demonstrated by the WCAs listed in Table 4.The WVTR of MFCCP-and MFCCCP-coated samples is slightly higher than that of the other coated samples,possibly due to the smaller fibril size derived from the raw material and the presence of lignin,considering that the WCAs of the coating films are clearly reduced when lignin is present[25].

Table 6 WVTR of MFC-coated cardboards

4 Conclusions

This study focuses on the potential of microfibrillated cellulose(MFC)coatings,derived from different raw materials and prepared using different treatments,to provide barrier properties for fiber-based packaging materials.The fabricated MFC coatings demonstrate significantly different morphologies,carboxyl contents,and rheological behaviors.After being coated with different MFC suspensions,the cardboard surfaces appeared denser and smoother,and exhibited weaker hydrophobicity.In addition,barrier properties,in terms of air/oxygen and water vapor resistance performance,were significantly improved.In particular,the cardboard coated with MFC derived from bleached bamboo pulp subjected to carboxyethylation pretreatment achieved the best oxygen and water vapor barrier performances.Although the reported coating process did not realize an oxygen barrier and water vapor barrier suitable for high-barrier packaging,the results demonstrate the potential of MFC coatings as alternative barrier layers for sustainable packaging materials.