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  The effect of chemical composition on microfibrillarcellulose films from wood pulps: water interactionsand physical properties for packaging applications Kelley L. Spence  ã Richard A. Venditti  ã Orlando J. Rojas  ã Youssef Habibi  ã Joel J. Pawlak Received: 29 January 2010/Accepted: 29 April 2010/Published online: 18 May 2010   Springer Science+Business Media B.V. 2010 Abstract  The interactions with water and the phys-ical properties of microfibrillated celluloses (MFCs)and associated films generated from wood pulps of different yields (containing extractives, lignin, andhemicelluloses) have been investigated. MFCs wereproduced by combining mechanical refining and ahigh pressure treatment using a homogenizer. Theproduced MFCs were characterized by morphologyanalysis, water retention, hard-to-remove water con-tent, and specific surface area. Regardless of chemicalcomposition, processing to convert macrofibrils tomicrofibrils resulted in a decrease in water adsorptionand water vapor transmission rate, both importantproperties for food packaging applications. Afterhomogenization, MFCs with high lignin content hada higher water vapor transmission rate, even with ahigher initial contact angle, hypothesized to be due tolargehydrophobicporesinthefilm.Asmallamountof paraffin wax, less than 10%, reduced the WVTR to asimilar value as low density polyethylene. Hard-to-remove water content correlated with specific surfacearea up to approximately 50 m 2  /g, but not with waterretention value.Thedryingrateofthe MFCsincreasedwith the specific surface area. Hornified fibers fromrecycled paper also have the potential to be used asstarting materials for MFC production as the physicaland optical properties of the films were similar to thefilms from virgin fibers. In summary, the utilization of lignin containing MFCs resulted in unique propertiesand should reduce MFC production costs by reducingwood, chemical, and energy requirements. Keywords  Water interactions   Lignin-containing microfibrillated cellulose (MFC)   Specific surface area    Hard-to-remove water   Nanofibrillar cellulose (NFC) Introduction Food packaging has become significantly morecomplex during recent years, mainly due to increaseddemands on product safety, shelf-life extension, cost-efficiency, environmental issues, and consumer con-venience. In order to improve the performance of packaging to meet these varied demands, innovativemodified and controlled packaging materials arebeing developed and optimized for potential com-mercial use. Currently these materials are largelyproduced from fossil-derived synthetic plastics, butwith increasing environmental concerns, materials K. L. Spence    R. A. Venditti ( & )    O. J. Rojas   Y. Habibi    J. J. Pawlak Department of Forest Biomaterials, North Carolina StateUniversity, Campus Box 8005, Raleigh, NC 27695-8005,USAe-mail: richard_venditti@ncsu.eduO. J. RojasDepartment of Forest Products Technology, Facultyof Chemistry and Materials Sciences, Helsinki Universityof Technology, P.O. Box 3320, 0215 Espoo, Finland  1 3 Cellulose (2010) 17:835–848DOI 10.1007/s10570-010-9424-8  derived from renewable resources are being exten-sively investigated as potential replacements. Thesematerials must provide protection for products toobtain a satisfactory shelf life at the same levels asthose obtained with petroleum-derived ones (Rhim2007; Rhim and NG 2007). Indeed, applicable materials must have adequate mechanical properties,and provide a sufficient barrier to oxygen, watervapor, light, bacteria and/or other contaminants inorder to prevent food deterioration. Although chem-ically modified biopolymers such as cellulose deriv-atives or thermoplastic starches have been widelyused in packaging, renewable biopolymers are cur-rently of central interest as there is the potential toreplace conventional petroleum-derived polymerstypically used in food packaging (de Vlieger 2003).Cellulosic fibers, as paper and paperboard, havetraditionally been used in packaging for a wide rangeof food categories such as dry food products, frozenor liquid foods and beverages, and even fresh foods(Kirwan 2003; Kirwan and Strawbridge 2003). Cel- lophane, which is regenerated cellulose obtainedfrom wood pulp through a physico-chemical process,is also extensively used as a coating material for foodpackaging (Kirwan 2003; Kirwan and Strawbridge2003). An interesting form of cellulosic materialscalled microfibrillated celluloses (MFCs), first devel-oped in 1983 by Turbak et al., has emerged recentlyas a potential packaging material because it exhibitsmany of the barrier and mechanical propertiesrequired in packaging, in addition to the potentialutilization of an abundant fiber precursor, renewabil-ity and biodegradability. Previous research hasfocused on various aspects of MFC production, suchas chemical pretreatments for reducing energy con-sumption (for example, enzymatic hydrolysis (Hen-riksson et al. 2007) and TEMPO-mediated oxidation(Saito et al. 2007), and utilization for packagingapplications (Erkisen et al. 2008; Henriksson et al.2008) and composite reinforcements (Yano andNakahara 2008). These studies, however, havefocused on MFCs from bleached fibers; this studyinvestigates MFCs from fibers containing lignin.Previous studies have shown that the strength of MFC-based films for packaging applications is arequisitethat canbe easily met given that, ata 35 g/m 2 basis weight, MFC films were found to have suitablemechanical properties: tensile index of 146 Nm/g,elongation of 8.6%, and an elastic modulus of 17.5 GPa (Syverud and Stenius 2009) and low oxygentransmission rates, 17 ml/m 2 day, which were com-parable to synthetic packaging based on orientedpolyester ethylene vinyl alcohol. For MFC-basedfilms, the porosity, which is an important criterionfor packaging and barrier properties, is modifiable bydrying from different solvents, creating a tunablefeature that provides an advantage over melt-formedplastics. For example, the porosity for MFC-basedfilms dried from water was as high as 28%, in contrastto films dried from solvents such as methanol, ethanol,and acetone that had porosities of up to 40% (Henri-ksson et al. 2008). Also, when used as a coating layeron paper, it was shown that 10% MFC significantlyreduced air permeability by reducing the surfaceporosity (Syverud and Stenius 2009).For ease of processing, MFCs are usually pro-duced from purified cellulose fibers, which express ahydrophilic character and, consequently, the resultingMFC-based films have poor barrier properties againstwater vapor. This factor constitutes the main draw-back for the application in some categories of foodpackaging and, in order to overcome this issue,several strategies including chemical modificationsand the addition of hydrophobic substances have beenexplored. The motivation for the present study is toinvestigate the use of MFCs containing aromaticlignin, the polymer that occurs in nature intimatelylinked to native cellulose fibers. Lignin is a randomnetwork polymer found in the cell walls of woodyplants; it is considered to be the joining material thatholds together the other two major biopolymercomponents in natural fibers (cellulose and hemicel-luloses). Therefore, the use of lignin-containingcellulosic fibers is expected to result in less hydro-philic MFCs. In a previous study by Spence, Venditti,and co-workers (2010), the feasibility of producingMFCs from wood pulps having various chemicalcompositions, mainly different amounts of lignin,was demonstrated. The processing of such pulpsrequired a mechanical pretreatment in order to softenand make smaller the fibers prior to the main step of disintegration/individualization with homogenization.Lignin-containing MFCs (produced at a compara-tively high yield) could provide opportunities tolower the operational costs with the potential benefitsof better strength and barrier properties. The objec-tive of this study is to further elucidate the effect of pulp type (lignin content) on the physical and water 836 Cellulose (2010) 17:835–848  1 3  interaction properties of MFCs and correspondingfilms so that the employment of such materials, eitheralone or contained in hydrophobic matrices (forexample in packaging and composite manufacture)can be realized. Experimental MaterialsKraft wood pulps obtained after different chemicaltreatments and a thermo-mechanical pulp (TMP) wereobtained from pulp mills in the Southeastern UnitedStates and were used as received. Pulp chemicalcompositions were determined using TAPPI standardmethods (T204 1997; T222 1998; T249 2000) and a Dionex-ICS 3000 (Dionex Corporation, Sunnyvale,California, USA). The respective fiber characteristicsweredeterminedwithaFiberQualityAnalyzer—FQA(OP Test Equipment, Hawkesbury Ontario, Canada)using length weighted averages of about 3000 fibers.Fines were considered to be cell wall elements with alength between 0.05 and 0.20 mm, according to FQAtests. Pulp pH was ascertained using TAPPI standardmethod T252 (T252 1998).Microfibrillated cellulosesBefore high pressure homogenization, pulps weresubjected to a pretreatment step. Pulps were dispersedin water at a solids content of 2% and thenmechanically refined in a laboratory scale Valleybeater (Valley Iron Works, Appleton, Wisconsin,USA) for a total refining time of 3 h utilizing a 5503gram weight. The resulting fiber slurries were storedat 4   C in cold storage until needed.Homogenization of the refined fiber slurries wasperformed with a 15MR two-stage Manton-Gaulinhomogenizer (APV, Delavan, WI, USA) at approxi-mately 0.7% solids content. The operating pressurewasmaintainedat55 MPa,butthetemperaturewasnotcontrolled. Typically, homogenization temporarilyceased when the temperature of the stock reachedapproximately 90   C, to prevent pump cavitation.Processing then recommenced when the samples hadcooled to approximately 45   C. Samples were col-lected and tested after 20 passes through the homog-enizer and stored at 4   C in cold storage until needed.In order to determine if wetting/drying cyclesimpact the production and properties of MFCs,chemically-pulped, bleached fibers (softwood andhardwood samples) were subjected to a differentpretreatment, hornification; the respective sampleswere first refined using the Valley beater at 2% solidsuntil a freeness of 300 CSF was achieved (T227 1999)(approximately 40 min) and then samples were driedat 105   C for 3 days to ensure complete drying. Driedsamples were re-suspended in water at 2% solidscontent using a TAPPI disintegrator, and then sub- jected to the remaining refining time (approximately2 h and 20 min) and homogenization procedurespreviously described.Characterization of MFCsImaging of fibers and the determination of theresulting fibril diameter distribution was performedusing an Olympus BH-2 optical microscope (Olym-pus, Center Valley, Pennsylvania, USA) and a fieldemission scanning electron microscope (FE-SEM)JEOL 6400F (JEOL, Peabody, Massachusetts, USA),respectively. Approximately 100 total measurementsof microfibrils or fibrils were measured from severalSEM images to determine the average and distribu-tion of each sample.Specific Surface Area (SSA) was determined usingthe Congo red adsorption method (Goodrich andWinter 2007; Ougiya et al. 1998). Samples were adjustedto a pH of 6 and treated with varying amountsof Congo red at a final solids content of 0.7%. Thesesamples were incubated at 60   C for 24 h, and thencentrifuged at 12,000 rpm (14,000 rcf) for 15 min.Measurements of UV–Vis absorption (Perkin Elmer,Waltham, MA, USA) at 500 nm of the supernatantsamples were taken to determine Congo red concen-tration using Langmuir isotherms, according to Eq. 1: ½ E  ½  A  ¼  1 K  ad   A max þ ½ E    A max ð 1 Þ where [ E  ]isthesolutionconcentration ofCongoredatadsorption equilibrium in mg/ml, [  A ] is the adsorbedamount of Congo red on the cellulose surface in mg/g(that reached a maximum value equivalent to  A max ,themaximum adsorbed amount), and  K  ad  is the equilib-rium constant. The specific surface area was deter-mined using the following equation: Cellulose (2010) 17:835–848 837  1 3  SSA  ¼  A max    N     SAMW    10 21  ð 2 Þ where  N   is Avogadro’s constant,  SA  is the surfacearea of a single dye molecule (1.73 nm 2 ), and  MW   isthe molecular weight (696 g/mole) of Congo red.Waterretentionvalue(WRV)wasdeterminedusingthe TAPPI Useful Method with a centrifugal force(Eppendorf North America, Hauppauge, New York,USA) of 900 rcf (2,400 rpm) for 30 min (UM2561981).Hard-to-remove water content (HRW) was deter-minedwithaQ500thermogravimetricanalyzer(TGA,TA Instruments, New Castle, Delaware, USA) fol-lowing the procedure proposed by Park et al. (2006a).Homogenizer sampleswere tested at1%solids since itwas difficult to thicken the MFC slurry to 10% assuggested in the procedure. A heat and hold programwas used to isothermally heat each sample to 90   Cand hold the temperature for 90 min.MFC filmsMFC slurry was de-aerated under vacuum for 10 minin an ultrasound bath followed by manual shaking. Aportion of the slurry was slowly poured into a plasticpetri dish to produce films with a basis weight of 30 g/m 2 after drying. Dried films were conditioned at23   C and 50% ambient relative humidity. Typicaltime required for drying and conditioning was 5 days.Films from TMP were produced using Teflon petridishes, as the materials could not be removed fromthe plastic ones. Samples were oven dried at 50   Cwith an approximate drying time of 24 h.Film thickness and roughness were determined byusingstandardmethods(T4111997;T5551999)witha Lorentzen and Wettre Micrometer 51 and a Lorentzenand Wettre Parker Print Surface Tester (L&W, Stock-holm, Sweden), respectively. Roughness was measuredon both the air and dish side surfaces with a clamppressure of 3.4 kPa. The weight per unit area (or basisweight) was determined using TAPPI standard T410(T410 1998) and the apparent film density was calcu-lated using the thickness and measured basis weight.Optical properties (opacity, color, ISO brightness,and scattering coefficient) were measured using aTechnidyne Color Touch 2 ISO Model (TechnidyneCorporation, New Albany, Indiana, USA) (T4521998; T519 1996; T527 1994). A humidity trial was performed by placing homog-enized film samples in a desiccator containing phos-phorouspentoxideat0%relativehumidityfor1 week.Samples were tested for tensile properties immedi-ately upon removal using an Instron 4411 (Instron,Norwood Massachusetts, USA) with a modifiedTAPPI standard testing procedure (T404 1992). Sam-ples were 15 mm wide and the clamp span wasmodified to be 25.4 mm. Crosshead speed was alsomodified to 4 mm/min.Film water absorption was determined by placing afour cm diameter circle of the MFC film in a petri dishof 30 ml containing deionized water. The weight of the film before and after 10 min immersion in waterwas obtained to determine the amount of waterabsorbed. Water vapor transmission rate (WVTR)was determined usinga wet cup method. Film sampleswere cut into 4 cm diameter circles and restrainedabove 50 ml of water in a closed container. Thecontainer was placed on a dynamic wetting apparatusinterfaced with a computer for data acquisition. Datawere taken every 3 s and the slope of the generatedweight loss curve and film thicknesses were used tocalculate the specific WVTR for each sample.Theinitialanddynamicwatercontactangle(WCA)were determined using a Phoenix 300 contact angleanalyzer (SEO Co. Ltd, Lathes, South Korea) for bothair and dish side film surfaces. To determine the effectof extractives on WCA, films were extracted for 24 husing a benzene-ethanol (1:2) mixture with refluxcondensation. Films were air dried for 2 weeks afterthe extraction process and then measured with theDCA. Results and discussion Wood pulpsFor ease of discussion, the different wood pulpsemployed were labeled as reported in Table 1 andsamples in tables were ordered by increasing lignincontent. The main morphological and chemical char-acteristics were also provided in Table 1. As reportedin previous work (Spence et al. 2010), cellulosecontents were high for the bleached and unbleachedpulps and the lignin and extractive contents wererelatively low. The hemicellulose content was around20%forallchemicallypulpedfibertypes.Thethermo- 838 Cellulose (2010) 17:835–848  1 3
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