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Biomacromolecules 2010, 11, 2532–2538
Barrier Films from Renewable Forestry Waste Ulrica Edlund, Yingzhi Zhu Ryberg, and Ann-Christine Albertsson* Fibre and Polymer Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Received July 9, 2010; Revised Manuscript Received July 21, 2010
Biobased free-standing films and coatings with low oxygen permeability were designed from a wood hydrolysate according to a recovery and formulation procedure that provides added value to wood converting industrial processes. Wood components released to the wastewater in the hydrothermal treatment of spruce wood were recovered and converted to an oligo- and polysaccharide-rich, noncellulosic fraction that was utilized in film formulations in a range of concentrations and compositions. Free-standing smooth and transparent films as well as coatings on thin PET were prepared and characterized with respect to oxygen permeability, tensile properties, structure, and water vapor transmission. With oxygen permeabilities as low as below 1 cm3 µm m-2 day-1 kPa-1 and with adequate mechanical properties, the films and coatings show promising property profiles for renewable packaging applications.
Introduction The formulation of products from and the utilization of green materials is becoming an increasingly acknowledged alternative for future material production in a more sustainable society. Using renewable resources reduces the need for fossil fuel as a feedstock for materials in packaging and other single-use applications. Abundance, availability, and being nonedible are sought-after characteristics of eligible biopolymers, and in this respect, the noncellulosic fractions of wood stand out as resources of high potential. The hemicelluloses form a class of heteropolysaccharides that constitute up to a third of the biomass of annual and perennial plants. They are released to the liquid stream during many forest and agricultural processing operations, such as pulping or fiber board production. Typically, these hemicelluloses are rejected from the process streams and disposed as an organic waste or burned for energy recovery. Value prior to pulping (VPP) strategies anticipate the isolation and extraction of wood components for subsequent production of chemicals and/or fermentation into bio fuel, realizing the idea of the forest as a biorefinery.1,2 The recovery of hemicelluloses from waste streams is possible through a series of filtration steps in which monosaccharides, other extractives, and some lignin are removed from the polysaccharide fraction.3 Such upgraded hemicellulose-rich wastewater was recently shown to be a viable resource for the design of functional hydrogels.4 As isolated and highly purified biopolymers, hemicelluloses have not yet been systematically exploited and commercialized despite their abundance and although several products have been designed and demonstrated, such as hydrogels,5 wound management products,6 binders in papermaking,7 and films.8 Many polysaccharides, hemicelluloses included, have a long tradition of being known and utilized as good barriers to oxygen.9 While being sensitive to moisture and thus regressing their barrier properties at higher relative humidities (RHs), hemicelluloses have attracted attention as possible resources for thin film design. Mannan-type hemicelluloses, such as GGM from spruce10,11 and konjac glucomannan,12 have been used in the preparation of packaging films. Likewise, xylans such as arabinoxylan from corn,13 and xylans from cotton stalk,14 or hardwood15 have been explored as film constituents for potential * Corresponding author. E-mail:
[email protected].
packaging applications. All of these purified hemicelluloses demonstrate good film-forming properties and oxygen barrier properties. However, enabling the production of functional products from less refined resources and mixed reagents rather than a highly purified biopolymer is crucial from an implementation and commercialization perspective. A rather complex and expensive process is needed to obtain pure hemicelluloses from less refined resources such as wastewater. It is essential to develop methods to turn cheap resource into functional materials without the time-consuming and costly processes for consecutive refining. Our aim was a barrier film from renewable forestry waste. We designed film and coating formulations based on a wood hydrolysate generated as wastewater after hydrothermal treatment of wood chips in a pulping process. Such a noncellulosic wood hydrolysate presents a resource of great potential for barrier film production without being extensively upgraded to generate a pure constituent. Instead, the wood hydrolysate is simple membrane filtrated to generate a high molecular weight fraction containing mainly oligo- and polysaccharides and some lignin. Film and coating formulations were accordingly prepared from such a wood hydrolysate in combination with a second component from the polysaccharide family, carboxymethyl cellulose (CMC), and chitosan, and the viability of the resulting products was verified with respect to oxygen permeability (OP), mechanical performance, and water vapor transmission (WVT).
Experimental Section Materials. The softwood hydrolysate used for film and coating design was retrieved according to the following protocol:16 Industrial spruce chips were provided by So¨dra Cell AB and first screened by passing a laboratory screen grid at 8 mm but not 7 mm holes. The chips were then subjected to hydrothermal treatment. Chips were first steamed at 110-120 °C for 45 min in a batch autoclave, after which preheated water was added to a liquid/wood ratio of 6:1 (volume/mass ratio). The heating time was typically 40 min. The treatment temperature was then kept at 150-170 °C for ∼60 min. The resulting liquid phase had a pH from 3.3 to 4.0. The liquid phase collected after hydrothermal treatment was then subjected to membrane filtration using a tangential flow filtration cartridge unit equipped with a regenerated cellulose membrane (PLAC Prepscale, Millipore) with a nominal cutoff 1000 Da. After ultrafiltration, the retentate phase was collected, whereas the
10.1021/bm100767g 2010 American Chemical Society Published on Web 08/04/2010
Barrier Films from Renewable Forestry Waste monosaccharide, extractives, and lignin-rich permeate were discarded. The retentate was diluted with water and once again subjected to ultrafiltration (diafiltration). The resulting retentate phase, herein referred to as the wood hydrolysate, was finally freeze-dried and stored dry until use. CMC sodium salt with a medium viscosity of 400-1000 mPa s, 2% in H2O (25 °C) was used as received from Sigma-Aldrich. Chitosan from crab shells with a molecular weight of 150 000 g/mol was used as received from Fluka. Acetic acid was purchased from BioUltra >99.5% (GC/T). Commercially used poly(ethylene terephtalate) (PET) films were kindly provided by TetraPak Packaging Solutions AB. Film Preparation. Films were prepared from wood hydrolysate mixed with a polysaccharide cocomponent, either CMC or chitosan. First, individual solutions were prepared from (1) wood hydrolysate in deionized water, (2) CMC in deionized water, and (3) chitosan in deionized water with acetic acid (1% v/v). These solutions were homogenized on a shaking board at a shaking rate of 200 min-1 for at least 3 h. Hydrolysate and CMC were then fully mixed by combining the solutions in a beaker on a shaking board at rate of 200 min-1 for 12 h. Chitosan and hydrolysate solution mixtures were homogenized using a high intensity ultrasonic processor with a 400/600 W model and microprocessor control. The pulse was set to 1.0 s on/1.0 s off at amplitude of 60% for 2 to 5 min. The mixtures were prepared in three concentrations, each with a wood hydrolysate/cocomponent ratio of 1:1: 0.2 g/14 mL, 0.4 g/14 mL, and 0.6 g/14 mL, respectively (dry matter/solvent). A series of mixtures with the concentration of 0.4 g/14 mL and with cocomponent weight contents of 0, 10, 20, 30, 40, and 50%, respectively, were also prepared. Each solution was cast in flat Petri dishes having an inner diameter of 8.7 cm (area of 60 cm2). The solvent was allowed to evaporate slowly at room temperature and at a RH of 50% for at least 36 h producing thin, dry films that later were manually removed from the Petri dishes. Coating Preparation. Mixtures of wood hydrolysate and CMC or chitosan were prepared as previously described and with the same concentrations and ratios as for film preparation. The mixtures were applied by a brush onto a 12 × 23 cm2 PET film. The coated films were then dried in a condition room at 50% RH and 23 °C for at least 48 h. Characterization. Ion-Exchange Chromatography. Ion-exchange chromatography (IC) was employed for sugar composition analyses. The sugar composition and the lignin content of the wood hydrolysate was determined16 by first hydrolyzing the sample according the conditions described in the TAPPI standard method (T 249 cm-00: Carbohydrate Composition of ExtractiVe-Free Wood and Wood Pulp by Gas Chromatography; TAPPI Press: Atlanta, 2000). The sugar residues thus obtained were then analyzed by IC using a Dionex DX500IC analyzer equipped with a gradient pump (Dionex, GP50), electrochemical detector (Dionex, ED40), Dionex, PA1 separation column. As an eluent, a NaOH/acetate gradient was used. For the determination of monosaccharide content, a sample was injected directly into the IC analyzer without any prior acid depolymerization. The quantity of acetyl residues was used to calculate the degree of substitution of acetyl groups for the oligo- and polysaccharides in the wood hydrolysate.16 Wood hydrolysate was then kept at 80 °C for 1 h in 2 mL of NaOH (1.0 M) and then filtered through a Teflon filter (pore diameter: 0.2 µm). Filtrate (0.20 mL) was diluted with water to 10 mL, analyzed with respect to acetate ion content using a Dionex ICS-2000 ion chromatography system (with an electrochemical detector), Dionex GA15 guard column, SA15 separation column, and a potassium hydroxide (35 mM) buffer eluent. Water Size Exclusion Chromatography. Water size exclusion chromatography (SEC) was performed on a system consisting of a Waters 515 HPLC pump (Milford, MA), a Rheodyne 7725i (Rohnert Park, CA) manual injector, three TSK-gel columns (Tosoh Bioscience, Tokyo, Japan) coupled in a series, G3000PW (7.5 × 300 mm, 10 µm particle size), G4000PW (7.5 × 300 mm, 17 µm particle size), and G3000PW, a Waters 410 refractive index (RI) detector, and a Waters 2487 dual
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wavelength absorbance detector (Milford, MA). NaOH (10 mM) was used as the mobile phase. The columns were calibrated with poly(ethylene glycol) (PEG) and poly(ethylene oxide) (PEO) standards with specific molecular weights ranging from 1500 to 400 000 g/mol. Sample solution (20 µL) was injected, and the UV absorbance at 280 and 200 nm was recorded together with the RI. The absorption peaks at these wavelengths were analyzed through a Millenium 2 software. Fourier Transform Infrared Spectrometry. Fourier transform infrared spectrometry (FTIR) was carried out on a Perkin-Elmer Spectrum 2000 FTIR with an attenuated total reflectance (ATR) crystal accessory (Golden Gate). All spectra were calculated by means of 16 individual scans at 2 cm-1 resolution in the 4000-600 cm-1 interval with corrections for atmospheric water and carbon dioxide. Field Emission Scanning Electron Microscopy. Field-emission scanning electron microscopy (FE-SEM) to assess film and coating topography was done on a Hitachi S-4800 field emission scanning electron microscope operated at 0.5 kV with magnifications of 1500 and 500. Samples were mounted on stubs, and a Cressington 208HR high-resolution gold/palladium sputter was used to coat samples with a 10 nm layer of Au/Pd. Tensile Strength Testing. Tensile strength testing was conducted on an Instron 5566 with Bluehill 2 software used for test-control and result analysis. Testing was performed at 50% RH according to the ASTM D 638M-89 standard17 with a load cell of 100 N capacity, a pulling speed of 5 mm/min, and a gauge length of 25 mm. Samples were cut into rectangular-shaped samples with a width of 5 mm and stored at 50% RH for at least 5 days prior to analysis. The samples thicknesses were determined on a Mitutoyo micrometer. Sample results are calculated by means of at least five individual discontinuous measurements. Oxygen Permeability and Oxygen Transmission Rate. OP and oxygen transmission rate (OTR) of films and coatings were measured on a Mocon Oxtran 2/20 (Modern Controls, Minneapolis, MN) apparatus equipped with a coulometric sensor following ASTM standard D398505.18 Samples were cut and sealed in aluminum foil with a round shape open area of 5 cm2 at 50% RH, 23 °C, and at atmosphere pressure (760 mmHg). The sample thickness was measured by a Mitutoyo micrometer by taking the average of five discontinuous spots. Each sample was conditioned at 50% RH for at least 1 week before testing. The OP is given in the units cm3 µm m-2 day-1 kPa-1, whereas the OTR is given as cm3 m-2 day-1. Water Vapor Transmission. WVT was assessed according to standard ASTM E 96/E 96M-05.19 Aluminum cups were used for wet-cup tests. The cups had a round opening area with inner diameter of 7 cm and were filled with distilled water to a level that leaves a 13-15 mm gap to the films. The films or coatings were sealed above the cup with epoxy glue; in the case of coatings, the coated side faced upward to air. Pure and coated PET films were conditioned for at least 1 week at 23 °C and 50% RH prior to analysis, and their thicknesses were measured by a micrometer (Mitutoyo). Kept at 23 °C and 50% RH, the water-loaded cups covered with films or coatings were weighted by the use of a scale (accurate to 0.0001 g) two or three times a day, continuously for 4 days. Three individual tests were done for each sample. WVT was calculated according to the following equation
water vapor transmission ) weight loss (g) /time (day) Results and Discussion Recently, increasing efforts have been devoted to develop green materials such as polysaccharides from plants for the use in consumer products, typically disposable items and packaging. As a nonedible and vastly occurring group of biopolymers, the hemicelluloses are attractive as a renewable resource, but unfortunately, the upgrading of wood-derived process water into pure hemicelluloses is typically a time-consuming and expensive process, which also involves organic solvents. This limits the
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Edlund et al.
Scheme 1. Representative Structures of (a) O-Acetylated Galactoglucomannan (AcGGM), (b) Carboxymethyl Cellulose (CMC), and (c) Chitosan
application of hemicelluloses in the food packaging industry, a field that demands safe and economical processes. Herein, we propose the recovery and use of wood hydrolysate from wastewater in the pulping industry for barrier film and coatings. The wood hydrolysate is obtained by a simple and economic treatment of process water, and a polysaccharide-rich noncellulosic fraction is recovered as a brownish powder and used without further purification. The wood hydrolysate contains (with respect to dry matter): 2.1% of monosaccharides, 88.8% of oligo- and polysaccharides, 9.1% of lignin, and
