Article pubs.acs.org/Biomac
Turning Hardwood Dissolving Pulp Polysaccharide Residual Material into Barrier Packaging Soheil Saadatmand,† Ulrica Edlund,† Ann-Christine Albertsson,*,† Sverker Danielsson,‡ Olof Dahlman,‡ and Katarina Karlström‡ †
Fibre and Polymer Technology, Royal Institute of Technology (KTH), Teknikringen 56-58, SE-100 44 Stockholm, Sweden Innventia AB, Drottning Kristinas väg 61, Box 5604, SE-114 86 Stockholm, Sweden
‡
S Supporting Information *
ABSTRACT: Birch chips were subjected to pilot-scale pre-hydrolysis under various sets of conditions to mimic a pre-hydrolysis step in a dissolving pulp process. The process generates residual process liquor, a wood hydrolysate, and the treated chips may be directly utilized in a dissolving process. The wood hydrolysates were rich in xylan and utilized in the production of fully renewable films that provide very good oxygen barrier function and mechanical integrity also at high relative humidity. Membrane filtration had an effect in enriching higher molecular weight fractions from the hydrolysates, but noteworthy, a hydrolysate used in the crude state without any membrane filtration performed just as well as upgraded fractions in forming films providing acceptable tensile properties and a good barrier against oxygen permeation.
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INTRODUCTION The pulping industry, long established and profitable, is facing an unprecedented challenge with a weakening market for printing and writing paper following the growth of digital media in developed countries and spreading globally. There is an increasing need for change, new approaches and new products to continuously make use of the cheap, abundant, chemically versatile, and perfectly renewable matrix of raw material that wood constitutes. On the other hand, “peak cotton” (future supply constrains), high cotton prices, and the environmental pollution caused by cotton farming are driving a strong turnaround in the demand of dissolving pulp. Dissolving pulp is a collective name for chemical pulps with a very high content of cellulose with typical end uses in the textile industry, being cellulose derivatives such as viscose pulp. Hardwood-based prehydrolysis kraft pulping as a production process of dissolving pulp is increasing worldwide, and hydrolysates, process liquors with dispersed and soluble wood components, are becoming more and more available. The dissolved wood components are today only used as a heat source, but their potential to generate components or to be utilized directly in new materials is still unused. The process liquors are rich in dissolved and dispersed poly- and oligosaccharides, mainly of the heterogeneous type referred to as hemicelluloses, and contain some lignin. Hemicelluloses have a low energy value and from both an economic standpoint and a biorefinery perspective it would be far more valuable to utilize these materials before incineration. Such new materials are renewable and can replace existing oilbased materials. The pulping industry may increase their profit margin in existing processes, while new processes that are not © 2013 American Chemical Society
cost-effective based on the cellulosic product only can be enabled if other wood components are also utilized and sold as valuable new products. We have previously developed a method for extracting a hemicellulose-rich hydrolysate from wood by means of a pre-hydrolysis approach including the steps of hightemperature water treatment, fractionation, and upgrading to a high molecular mass fraction.1 This strategy was demonstrated to be viable for softwood extraction and for the production of films with excellent oxygen barrier properties.2 Other routes to extract noncellulosic oligo- and polysaccharides from woody biomass include using microwave aqueous heat fractionation,3 a pressurized hot water flow-through system,4,5 alkaline extraction,6 or hydrothermal treatment in acidic buffer.7 Integrating a pre-hydrolysis step, extracting valuable noncellulosic components from the woody raw material2,8 as a pretreatment stage before dissolving pulping, could hence be very beneficial for the pulping industry in increasing the profit margin in the existing processes. Among possible applications, films and coatings have in particular been explored for hemicelluloses derived from wood and plants.9−13 Soluble carbohydrate components in the monomeric or polymeric state may also be utilized as a feedstock for further derivatization and production of bulk chemicals as well as other products, including furfurals, xylitol, succinic acid, and bioethanol. A facile recovery of valuable components from the liquor after hydrolysis of wood may however be impaired by the concomitant release of lignin Received: June 10, 2013 Revised: July 13, 2013 Published: July 17, 2013 2929
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fractions to the aqueous phase leading to clogging effects.14,15 Hence, careful determination of the lignin contents in the hydrolysate and, if necessary, the removal of those when developing a pre-hydrolysis process are called for. Hemicelluloses are known as good gas barriers, and the limitations and challenges that need to be overcome for future successful implementation lie in the limited film-forming ability stemming from their typical brittleness and the deteriorating properties in humid environments due to their inherent hydrophilicity. Following extraction, much effort is typically devoted to the extensive upgrading and purification of the recovered components, often involving precipitation and/or delignification and requiring large amounts of organic solvents and other chemicals. However, an economically and environmentally sound transition of the pulp industry from a core product facility to a multiproduct biorefinery also calls for a new way of thinking in terms of purity. The use of crude, or less refined, fractions will save energy and time in the production value chain and reduce the consumption of support chemicals. Noteworthy, we have recently shown that spruce wood hydrolysates that were not extensively purified produced barrier films with oxygen permeability much lower than that of the corresponding formulation based on the highly purified spruce hemicellulose, O-acetyl galactoglucomannan.12,16 The synergistic effects of a complex mixture of dissolved and dispersed components in the wood extracts on the properties of said extracts when utilized as a material is an insight well worth further exploration. Taken together, many factors, not in the least the strong drive for sustainability, points to the strong potential of converting existing pulp processes so that new products can be produced, while at the same time added value is given from utilization of side streams in renewable materials that may replace fossil-based materials. This will demand new steps and/ or unit operations in the existing processes, and to enable implementation, we need a solid and detailed understanding of the new process steps and the parameters that govern the yield of a useful product. Furthermore, investigations need to be performed under realistic conditions and in larger scales. An indepth understanding of the influence of process parameters on the products generated is a core challenge that need to and in this work will be addressed to improve the quality of the raw materials. The aim is to develop a pre-hydrolysis treatment of birch in pilot-plant scale that generates a hemicellulose-rich extract, a hydrolysate, and to turn this product to fully renewable films that provide good barrier function and mechanical integrity also at high relative humidity.
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Table 1. Carbohydrate and Lignin Composition of Dried Birch Wood Chips component
%
arabinose galactose glucose xylose mannose lignin, Klason lignin, acid sol total
0.3 0.7 36.3 18.2 1.6 21.2 4.2 82.5
temperature of the circulated liquid was 25 °C and was increased by 18 °C/min to a final temperature 155 °C by indirect steam through a heat exchanger. The hydrothermal treatment times for the different batches are given in Table 2. After the first treatment (60−120 min), the free
Table 2. Pre-hydrolysis Yield for Three Different Hydrolysis Times Yielding Samples BWH1−BWH3
sample
hydrolysis time (min)
wood chips after wood hydrolysis (% of charged wood)
BWH1 BWH2 BWH3 BWH4a
120 + 120 90 + 90 60 + 60 60 + 60
69 70 73 73
dry content hydrolysate (%)
dry substance in retentate ( g/kg charged wood)
1.8 2.1 2.1 2.1
130 105 90 nab
a Sample BWH4 was subjected to pre-hydrolysis under the same conditions as sample BWH3 but was not membrane filtrered. bNot applicable.
liquid was withdrawn from the digester into a collection vessel. The withdrawn volume was replaced with the same volume of fresh, deionized water (around 9 L) at room temperature. With newly charged fresh deionized water, the temperature had decreased to 80 °C in the digester, and the temperature was increased to 155 °C a second time at 18 °C/min. The total time for the liquid exchange including time to reach final temperature was 30 min. The hydrothermal treatment of the wood continued for an additional 60−120 min after which the second hydrolysate liquid was withdrawn into the collection vessel. The volume of the second withdrawal was around 8.5 L, and each hydrolysis was carried out three times using fresh chips to reach a total volume of 53 L. The longer the hydrolysis time, the more material is dissolved, seen as a reduced amount of remaining wood chips in Table 2. The dry content of a dilute solution is tricky to determine accurately. However, the lower dry content for the hydrolysis with the longest hydrolysis time, BWH1, could be explained by a generation of volatile molecules that escape detection as degradation reactions of carbohydrates proceed. The conditions were chosen as appropriate for the elaboration of a sound strategy for pilotscale hydrothermolysis. In industrial scale, the settings may have to be revisited, as for instance a liquid to wood ratio of 4:1 would be more reasonable than 6:1. Membrane Filtration. Three of the liquid wood hydrolysates generated in the hydrothermal treatments were ultrafiltered through ceramic membranes purchased from Orélis with a 10 kDa cutoff. The fourth sample was not subjected to membrane filtration to allow for a comparative assessment of the role of ultrafiltration. The membrane filtration unit included a feed vessel with a volume of 30 L, and the liquid was circulated back to the vessel. The circulation pump capacity was 10 m3/h. The permeate flux was determined by logging the weight of the permeate vessel. The total volume of 53 L hydrolysate was reduced to 15 L of retentate resulting in a VCF (volume concentration factor) of 3.5, meaning a 3.5-fold reduction in volume. The permeate was removed and not used further in this study. Diafiltration of the retentate phase was carried out twice through addition of 15 L of
MATERIALS AND METHODS
Materials. Industrially screened birch chips were received from Södra Cell AB and dried at room temperature to approximately 90% dryness. The chemical composition, shown in Table 1, was determined as described in the Characterization section. Carboxymethylcellulose (CMC) sodium salt with a medium viscosity of 400−1000 mPa s, 2% in water at 25 °C (CAS 9004-324) was used as received from Sigma Aldrich. The degree of substitution of CMC was 0.3 as determined by 1H NMR. CMC had a weight-average molecular weight (Mw) of 550,000 g/mol and a polydispersity index of 3.8 as determined by SEC. Hydrothermal Treatment of Birch Chips. A laboratory circulation digester was charged with 2 kg of bone dried birch chips, at 94% dry content. Deionized water in an amount of 11.9 kg was added to provide a total liquid to wood ratio of 6:1. The initial 2930
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atmospheric pressure. Freeze-dried wood hydrolysates were dissolved in dimethyl sulfoxide DMSO-d6 (Larodan Fine Chemicals AB) at concentrations of 6 mg/mL. CMC was dissolved in D2O. A total of 128 individual scans were run, and the spectra were interpreted using MestReNova 7 software. NMR tubes with an outer diameter of 5 mm were used. Size exclusion chromatography (SEC) was used for determination of hydrolysate molecular weights using two different systems. The molecular mass parameters for wood hydrolysates were first determined by employing aqueous SEC with 50 mM ammonium acetate pH 7 as the eluent system. Prefiltered hydrolysates (containing approximately 1 mg of dry matter) were injected into the SEC column system, which consisted of a Waters 625 HPLC pump and three columns containing Ultrahydrogel 120, 250, and 500 (Waters Assoc., USA), respectively. The columns were linked in series to each other and to a refractometer (Waters 410 refractive index detector, Waters Assoc., USA). The signal from the refractometer was processed on a standard PC using the PL Caliber SEC software and interface (Polymer Laboratories Ltd., UK). Using this software, the molar mass parameters, i.e., the number-average molecular weight (Mn), weightaverage molecular weight (Mw), and polydispersity index (PDI), of the entire hemicellulose extract were calculated from the SEC profile, together with the MALDI-MS analyses. The hydrolysates were also analyzed using a Shimidzu RID-10A SEC equipped with a refractive index detector and three PLgel 20 μm Mixed-A (300 mm × 7.5 mm) columns with an injection volume of 200 μL. Dimethyl acetamide (DMAc) with 0.05% w/w LiCl was used as the mobile phase (0.5 mL/ min). The measurements were performed at 80 °C. The SEC system was calibrated with pullulan standards with molecular weights ranging from 342 to 800,000 g/mol. Before injection, samples were dissolved for 24 h at room temperature in DMAc (0.05% w/w LiCl) and filtered (0.45 μm, Millipore). LC Solution software from Shimatzu was used for data acquisition and calculations. The molecular weight of CMC was estimated using a SEC Dionex Ultimate 3000 equipped with a reflective index (RI) and a UV detector and 3 + 1 columns (one guard column) with the injection capacity of 100 μL. Five millimolar NaOH was used as the mobile phase, and the measurements were performed at 40 °C. The SEC system was calibrated with pullulan standards with molecular weights ranging from 342 to 800,000 g/mol. Before injection, a sample was dissolved for 24 h at room temperature in deionized water (5 mg/mL) and filtered (0.45 μm, Millipore). Chromeleon 7 software from Dionex was used for data acquisition and calculations. MALDI-TOF-MS analysis were done with a Bruker Daltonik Microflex LT instrument and evaluated with Bruker Daltonik Flex series software package. The matrix compound employed was 2,5dihydrobenzoic acid (DHB). IR microscopy was carried out using a Perkin-Elmer Spotlight 400 connected to a Perkin-Elmer spectrum 400 spectrometer, equipped with dual sources (FT-IR/FT-NIR) and dual detector. A dualcassegrain system in Spotlight 400 microscope enabled measurements down to 6.25 μm IR pixel size through reflectance and transmission modes. The spectra were gathered from a 200 μm × 200 μm area of the films in the transmission image mode, and SpectrumIMAGE software was used to gather the relative intensity of desired peaks within the range of 4000−750 cm−1 over the corresponding image captured by the Spotlight 400 microscope. Dif ferential scanning calorimetery (DSC) analyses were carried out on a Mettler Toledo DSC 820 module with 5−10 mg samples in 100 μL aluminum cups. Samples were subjected to a heating−cooling cycle from −50 to 250 °C at a rate of 5 °C/min under nitrogen atmosphere with a flow rate of 50 mL/min. Data were collected and analyzed by Mettler STARe software . Thermo-gravimetric analyses (TGA) were carried out on a Mettler Toledo TGA/STDA 851e. A 2−5 mg portion of each sample was inserted in ceramic crucibles and analyzed from room temperature to 900 °C with a heating rate of 10 °C/min under nitrogen atmosphere with a flow rate of 50 mL/min. Data were collected by Mettler STARe software and analyzed by Origin 8.
water, and in each case the total retentate volume was reduced to 15 L. The temperature throughout the filtrations was kept at 30 °C. Finally, the upgraded hydrolysates were freeze-dried (LyoLab 3000 instrument). Film Formation. To form films of CMC and lyophilized birch wood hydrolysates (BWH), 1 g of CMC was added to 40 mL of deionized water, and hydrolysate (250−1500 mg) was added in different ratios according to the protocol in Table 3. The beakers were
Table 3. Composition of Blends Based on Birch Wood Hydrolysates (BWH) and CMC for Barrier Film Formation sample name
BWH content (% w/w)
CMC (% w/w)
BWH1-CMC80 BWH1-CMC67 BWH1-CMC50 BWH1-CMC45 BWH1-CMC40 BWH2-CMC80 BWH2-CMC67 BWH2-CMC50 BWH2-CMC45 BWH2-CMC40 BWH3-CMC80 BWH3-CMC67 BWH3-CMC50 BWH3-CMC45 BWH3-CMC40 BWH4-CMC80 BWH4-CMC67 BWH4-CMC50 BWH4-CMC45 BWH4-CMC40
20 33 50 55 60 20 33 50 55 60 20 33 50 55 60 20 33 50 55 60
80 67 50 45 40 80 67 50 45 40 80 67 50 45 40 80 67 50 45 40
covered and shaken for 2 days at room temperature on a shaking board at 200 rpm. Then, 100 mm diameter plastic Petri dishes were used as molds for casting. An aliquot of 20 mL of the well-dissolved slurries was added to each Petri dish, and they were all left to dry in a conditioning room at a relative humidity (RH) of 50% and 23 °C to achieve more control over the drying conditions. The free-standing films were completely dry after 3 days and were removed from the Petri dishes for further analyses. Characterization. Carbohydrate analysis was done according to the hydrolysis conditions described in the TAPPI standard method 249. However, the sugars obtained after the hydrolysis step were determined by ion exchange chromatography (IC) employing a Dionex DX500 IC analyzer equipped with a gradient pump (Dionex, GP50), electrochemical detector (Dionex, ED40), Dionex, PA1 separation column and applying a sodium hydroxide/acetate gradient buffer eluent. Lignin analysis of wood chips and hydrolysates was done by gravimetrically determining the acid insoluble rest (Klason lignin), together with the acid soluble rest, which was determined spectrophotometrically at 205 nm, by employing the TAPPI standard method 222. Oligo- and polysaccharide composition, including uronic acid residues, was determined by enzymatic hydrolysis followed by capillary electrophoresis (CE), employing a method described in detail previously.17 The CE system utilized was a Beckman P/ACE MDQ capillary electrophoresis system (Beckman Coulter, Fullerton, CA, USA) equipped with a diode-array UV detector. Prior to the CE analysis, the sugars in the hydrolysates were derivatized with the UV chromophore 4-aminobenzoic acid ethyl ester so that absorption at 306 nm could be used for quantitation. 1 H NMR spectroscopy was used to determine the chemical compositions of the hydrolysates using a Bruker 400. 1H NMR spectra were recorded at 400 MHz at room temperature and 2931
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Tensile testing was performed on an INSTRON 5944 module according to the ISO 37 standard.18 Strips with width of 5 mm and thickness of 55−120 μm were cut and stored in a conditioning room (23 °C and 50% RH) for 48 h prior to analysis. The specimens were mounted in pneumatic grips with a gauge length of 25 mm, and each test was run at a pulling speed of 10 mm/min with a 50 N load cell. For each sample, 5 individual specimens were tested in 5 individual analyses, and the reported data are calculated means. Test control and data storage were managed by Bluehill 2 software. Dynamic mechanical analysis (DMA) was determined in temperature scans using a TA Q-800 DMA 7 operating in tensile mode. Samples of 5 mm width were mounted in grips with a gauge length of 5 mm; sample thickness varied between 0.03 and 0.06 mm. A measuring frequency of 1 Hz and an amplitude 15 μm were used. Temperature scans with initial drying and conditioning sections were used with a ramp rate of 3 °C/min in the interval 10−280 °C. Oxygen permeability (OP) was measured on a Mocon OXTRAN 2/ 20 instrument from Modern Controls inc., USA, equipped with a coulometric sensor, according to ASTM standard D-3985-05.19 Samples were sealed between aluminum foils with an open area of 5 cm2 and then stored in a conditioning room for 1 week (23 °C and 50% RH) prior to analysis. Each sample thickness was determined using a Mitutoyo digital micrometer by taking the average of 5 discontinuous spots. Permeability measurements were carried out twice for each film at 23 °C and 50% or 80% RH at atmospheric pressure, and OP data were reported in the unit of cm3 μm day−1 m−2 kPa−1.
During autohydrolysis (pre-hydrolysis using water), the balance between wood component dissolution and hemicellulose degradation must be tackled. The formation of furfural from xylose is acid-catalyzed, and the hydrolysate must be withdrawn before severe degradation reactions occur. A treatment time of aspen wood of 4.5 h at 150 °C has been suggested as the most suitable for hemicellulose extraction with high yield and without significant degradation of xylose to furfural.20 However, no molecular weight distributions were reported, and it is likely that acid hydrolysis results in low molecular weights. The solubility of polymeric xylan may restrict the dissolution in autohydrolysis, and in order to be soluble the xylan polymer must first be degraded to below 25 xylose units.21 In the present study, three different treatment times were studied with subsequent ultrafiltration. After one of the treatments (60 + 60 min) a hydrolysate sample (BWH4) was withdrawn prior to ultrafiltration for comparison. The aim was to produce a high performance barrier material with high yields. In our earlier work on spruce hydrolysates, membranes with different cutoffs were tested.2 The effect on separation and yield was however small. In the present study, a rather sparse membrane with cutoff 10 kDa was used while the treatment times were varied. From a process economy point of view, a sparse membrane is preferred as long as it manages the separation, as it usually results in a process with higher membrane fluxes. The yield is given in Table 2, and the fluxes through the membrane for the different hydrolysates are shown in Figure 1. With increasing hydrolysis time the yield on wood is decreased as expected. The amount of dry substance in the retentate, which should be seen as the total yield of final product from wood, is also increased by increased prehydrolysis treatment times. A substantial part of the charged wood is removed into the hydrolysis liquor, resulting in 69− 73% of charged wood being available for further kraft pulping, which is the preceding process step in mind to produce dissolving pulp, a high cellulose grade pulp. The withdrawn hydrolysate can be used as such, without ultrafiltration, for production of barrier material in packaging, represented by sample BWH4. When ultrafiltration (10 kDa ceramic membrane) is used, large amounts of low molecular material is separated, which can either be upgraded to other products or used as an energy source. It is clear that the hydrolysis time affects the membrane filterability significantly. In the case of the shortest prehydrolysis time BWH3, the initial flux was very low, and the filtration had to be interrupted and the membrane cleaned before continuation of the membrane filtration. This is the reason for the early increase in flux as shown in Figure 1. The ease of filtration for the three hydrolysates can be ranked in the following order: BWH1 > BWH2 > BWH3, where BWH3 is the most difficult to membrane filter. The reason for this is likely differences in clogging material. As the hydrolysis time is increased, the amount of clogging material seems to decrease, probably due to dissolution of extractives and micelle formation and degradation. Such fouling is a well-known issue when filtrating process and waste streams from wood hydrolysis processes, and the most straightforward strategy is to dismantle the setup and clean the membrane, although this costs time and money. Alternative pretreatments of the membrane, e.g., plasma, to suppress fouling have been viable but also demand additional, costly steps.22 The autohydrolysis of birch chips is typically run at a lower temperature than what is used for spruce chips. Comparing the membrane filtration fluxes to the
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RESULTS AND DISCUSSION Birch chips were subjected to pilot-scale pre-hydrolysis under various sets of conditions to mimic a pre-hydrolysis step in a dissolving pulp process. The process liquors were utilized for the production of renewable barrier films, and the treated chips may be directly utilized in a dissolving process. A schematic illustration of the process chain is shown in Scheme 1. To Scheme 1. Summary of the Integrated Pre-hydrolysis Approacha
a
The wood-based products in this case are dissolving pulp with high cellulose content and barrier material with high hemicellulose content. Lignin is mainly used as energy source.
provide new insights into the production−structure−property relationships governing the ultimate formation of a high quality product, the hydrolysis yield, contents, and properties were assessed and related to the process parameters. The films were formulated to perform also under high humidity conditions, which is challenging for any polysaccharide-based material due to their inherent hygroscopic nature. 2932
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Figure 1. Fluxes through the membrane for the three different hydrolysis liquids, BWH1−BWH3.
Table 4. Carbohydrate and Lignin Composition (%) of Freeze-Dried Ultrafiltration Retentates BWH1−BWH3 Obtained from Birch Wood Hydrolysates and Wood Hydrolysate BWH4 (Unfiltered BWH3) sample
arabinose
galactose
glucose
xylose
mannose
lignin, Klason
lignin, acid sol
total
BWH1 BWH2 BWH3 BWH4
0.3 0.5 0.5 0.1
1.9 2.1 1.9 0.3
2.4 2.9 2.0 0.2
43.7 42.8 37.9 37.6
3.7 3.3 2.7 0.2
20.7 17.0 18.0 12.0
5.5 11.0 4.8 15.5
78.1 79.5 67.8 73.5
Table 5. Relative Composition (%) of Neutral Saccharide and Uronic Acid Residues for the Oligo- and Polysaccharides in the Freeze-Dried Ultrafiltration Retentates BWH2 and BWH3 Obtained from Birch Wood Hydrolysates sample
xylose
glucose
mannose
arabinose
HexA
galactose
4-O-MeGlcA
BWH2 BWH3
85.9 85.4
3.3 2.4
3.8 4.2
