Soy Proteins As a Sustainable Solution to Strengthen Recycled Paper

Research Article pubs.acs.org/journal/ascecg

Soy Proteins As a Sustainable Solution to Strengthen Recycled Paper and Reduce Deposition of Hydrophobic Contaminants in Papermaking: A Bench and Pilot-Plant Study Ali H. Tayeb,† Martin A. Hubbe,† Pegah Tayeb,† Lokendra Pal,† and Orlando J. Rojas*,†,‡ †

Department of Forest Biomaterials, North Carolina State University, 2820 Faucette Drive, Raleigh, North Carolina 27513, United States ‡ Department of Bioproducts and Biosystems, School of Chemical Engineering, Aalto University, Vuorimiehentie 1, Espoo 00076, Finland

ABSTRACT: Hydrophobic contaminants (stickies) incorporated with recycled fibers cause severe papermaking processing and product quality problems, which lead to low runnability and increased production cost. Stickies negatively affect paper strength and many other properties. In this work, we propose a sustainable approach by the application of soy protein isolate (SPI), soy flour (SF), and soybean lipoxygenase (LOX) as agents to combat hydrophobic contaminants. Tests at the bench and pilot-plant scales and under conditions similar to industrial operations demonstrated the reduction of associated challenges and the improvement of a paper’s dry strength. The soy agents were applied to aqueous dispersions of lignin-free recycled fibers (dosage levels of 1−2% based on the fiber dry weight), which contained additives typically used in papermaking (fillers, sizing agent, and others). Talc, a common detackifier, was applied in similar systems that were used as reference. The proteins were added under both high and low shear conditions, and their effect in reducing paper tackiness and increasing internal bonding was confirmed. The maximum reduction in tacky particles count was achieved with SF (2% addition level based on fiber) under mild agitation and 30 min contact time. Remarkably, synergistic effects on the fiber electrostatic charges and paper porosity and formation were observed upon the addition of the soy proteins. The obtained results indicate that residual soy products represent an inexpensive, sustainable, and environmentally benign solution to enhance papermaking performance relative to conventional and more expensive agents that are in current use. KEYWORDS: Soy protein, Lipoxygenases, Deposits, Detackification, Dry strength, Papermaking, Fiber internal bonding

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mers.5−7 These components can stick together or accumulate on papermaking surfaces and adversely affect the product quality, machine runnability, and production costs.2 Generally, detackification and fixation onto fibers are possible solutions in this context,3,8 which imply the use of additives such are bentonite,4 talc,9 surfactants,4 polymers,10 enzymes,11−13 proteins,14 fungi,15 and cyclodextrins.16 Another issue in the use of recycled fibers is their poor dry strength, both intrinsic to the fibers and their networks. Paper internal strength is mostly related to fiber−fiber bonds that form as water is removed,

INTRODUCTION Papermaking is a sustainable process that consumes large quantities of lignocellulosic materials to manufacture paper and paperboard. Even though the use of virgin fibers in this process results in high quality products, alternative use of recycled fibers has important implications in waste management as well as cost and energy savings. However, several drawbacks in such applications need to be considered, such as those associated with the limited strength of the resultant paper products,1 the presence of more contaminants,2 and the need to handle sticky particles.3 Sticky contaminants can enter the process through the raw materials and chemicals additives.4 They are typically composed of hot melt binders, pressure sensitive adhesives (PSAs), styrene−butadiene rubber (SBR), latex, and elasto© 2017 American Chemical Society

Received: May 7, 2017 Revised: June 10, 2017 Published: June 24, 2017 7211

DOI: 10.1021/acssuschemeng.7b01425 ACS Sustainable Chem. Eng. 2017, 5, 7211−7219

Research Article

ACS Sustainable Chemistry & Engineering

Scheme 1. Hydrogen Bonding (Dashed Lines) Occurring within the Cellulose Hydroxyl Groups and Hydrophilic Domains of Protein Leading to Higher Strength in Fiber Networks (paper)

giving rise to strong contact and molecular interactions.17 Refining of the fibers18 and application of starch (unmodified and cationic),19 carboxymethyl cellulose, methyl cellulose,20 and cellulose nanofibrils (CNF, NFC)21,22 can benefit paper strength while preserving other properties. Additionally, savings on the operational and production costs may be attained through reduction in the basis weight for a given dry strength agent. Utilization of starch has been favored due to its low price and relatively good performance; however, at high usage levels it increases the biological oxygen demand (BOD) and reduces fiber retention in the consolidating web. Recently, biomacromolecules such as soy-derived proteins (SP), used alone or in combination with other polymers, have been suggested as environmentally benign and inexpensive alternatives in paper mills, for example, to enhance the process performance.23−26 Soy products have been employed previously in the formulations of adhesives,27,28 hydrogels,29 paper coatings, paints and films,30 foams,31,32 aerogels,33 and emulsions.34,35 Three main soy protein grades, soy isolate (90% protein), soy concentrate (70% protein), and soy flour (50% protein)26 have been considered, the latter of which is attractive to the industry due to its low processing cost. Soy proteins may be effective to increase the mechanical integrity of paper, given their strong interaction with cellulose, through a network of hydrogen bonds (Scheme 1).23,24 This is specially the case in the presence of residual lignin.36 SPs are also considered as a source of lipoxygenases (LOXs), which are able to oxidize stickies.12,37 Therefore, it is conceivable that the treatment of fibers with soy proteins would be effective to simultaneously increase the dry strength while addressing the deleterious effects of hydrophobic contaminants. We note that other proteins (e.g., caseins) have been used in papermaking, but such usage have been mainly limited to coating applications.38 The goal of the present work is to assess the feasibility of using SPs as green additives to increase dry strength and minimize the effects of stickies. While only limited studies are available in related applications, no attempts have been reported at the pilot scale, most relevant for adoption in industry. Even though we are mostly motivated by the environmental and sustainable aspects of the use of soy protein, it is economically beneficial to substitute with soy protein products the two relevant additives, talc and dry strength agents, which are used to avoid deposit formation and enhance the mechanical properties. However, for a better evaluation regarding the economic viability of the soy protein

application in recycling mills, a separate cost analysis needs to be performed. Therefore, we conducted a series of trials at the bench and pilot scales under conditions similar to industrial operations, to evaluate the effect of soy protein as a bifunctional colloidal additive in papermaking. The protein was applied in aqueous dispersions consisting of recycled fibers in the presence of other typical papermaking wet-end chemicals. The effects of SPs was compared to that of talc and isolated lipoxygenases, used here as references. Also, the synergistic effects of protein on the fibers electrostatic charge, paper porosity, and formation uniformity were evaluated. Overall, the obtained results indicate excellent prospects for adoption of soy protein in papermaking with recycled fibers.

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MATERIALS AND METHODS

Cellulosic fibers (Kappa number of 15.2) with sticky contents of 145 mm2/kg was provided by Resolute Forest Products recycling mill in Fairmont, West Virginia. The pulp was a deinked office paper grade. Commercial soy flour, SF (7B defatted, >53% protein), and soy protein isolate, SPI (Pro-Fam 955, >90% protein), were generously provided by ADM (Archer Daniels Midland). SPI is isolated through pH changes (alkaline treatment followed by a mild acidic condition) based on the protein isoelectric point. SF was produced by simply grinding defatted soy beans. Lipoxygenase (LOX) Type B-1 (activity = 50 000 units per mg of solid) was obtained from Sigma-Aldrich and used as received. Laboratory grade NaOH, HCl, KOH, and H3BO3 were purchased from Sigma and were used for pH and conductivity adjustment. Alkyl ketene dimer (AKD) sizing agent (Hercon 100) and cationic poly acrylamide (C-PAM) for use as a retention aid (Accurac 90) were supplied from Solenis and American Cyanamid Company, respectively. Ground calcium carbonate (GCC) with a particle size of 1.4 μm, a specific gravity of 2.7 g/cm3, and a surface area of 5.5 m2/g was a donation from Omya Inc. Morplas blue dye was supplied from Standard Colors Inc. Lab-Scale Experiments. Fiber Treatment with Protein. The desired levels of enzyme and soy protein were applied to a 20 mL liquid scintillation vial under nitrogen gas, where enzyme or protein was dissolved to homogeneity in a 0.2 M borate buffer with constant stirring at 200 rpm for 5 min. The addition level was determined to be 1% for enzyme (LOX) and 2% for soy protein based on the fiber dry weight. Also 2% talc was added in separate experiments. Recycled fibers were dispersed in water (solids content of 3%), where the desired amount of retention aid (0.05% cationic polyacrylamide, CPAM) was added to the slurry. The dilute dispersion of enzyme/ protein was applied to the fiber dispersion and left stirring for 2 h before the subsequent washing step. A parallel experiment was carried out under the same conditions except that no protein was added. At the end, both control and enzyme-treated fiber samples were washed with deionized water. Part of the slurry was kept for fiber total charge measurement, and the rest was used to make paper handsheets. 7212


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Scheme 2. Simplified View of the Pilot-Scale Papermaking Process Used to Study the Effect of Soy Protein Addition As a Multifunctional Colloidal Agenta

a

The addition points of the soy proteins included the chest tank and the headbox, with the latter being closer to the formation section (relatively short contact time and low shear).

Handsheet Making. Handsheets were prepared according to TAPPI standard method T-205 sp-95 (cylinder mold Robert Mitchel, Inc., Quebec, Canada). Suspensions of fibers at 3% solids were diluted to 0.3% with water and stirred for 15 min. The forming screen was thoroughly cleaned with a steam hose to ensure proper drainage. Handsheets of 60 g/m2 dry basis weight were prepared and pressed for 5 min first and then for 2 min at 50 psig in a testing machine (Inc., handsheet press). After pressing, they were air-dried and conditioned overnight under 50% humidity and 23 °C. Charge Measurement. The total charge of the fibers was measured according to the methodology developed by Fras et al.39 A fiber dispersion (0.1% solids) in distilled water was used for this purpose; the total charge was determined using acid−base titration (pH 5.0− 7.0). Solutions of 0.01 N HCl and NaOH were used. The volume of base used to neutralize the OC−OH in the fibers was recorded to measure the fiber charges. Pilot-Scale Experiments. Furnish Preparation. Recovered paper bales were dispersed in water at room temperature in a hydrapulper of approximately 250 gallon capacity, and the fiber content was adjusted to 3% before the subsequent beating step. The pulp’s original freeness was 400 mL CSF, and it was refined to 240 mL in a twin-disc refiner for 5 min under 90 kW. Ground calcium carbonate (GCC) filler was dispersed in water to a solids content of 12% before addition to the refined fibers. Alkyl ketene dimer (AKD) emulsion sizing (17% AKD mass) was applied to the fiber dispersion at a dose levels of 1.36 kg/ ton sizing agent OD fiber (equivalent to 0.13% of the fiber mass). Cationic poly acrylamide (C-PAM, Accurac 90) was used as retention aid for fibers, calcium carbonate and other additives were used after vigorous dispersion in water for 1 h, to obtain a solution with 0.25% CPAM (0.05% based on fiber mass). A talc suspension was prepared in water and stirred for 30 min before addition to the fiber slurry. Protein Preparation. Soy flour (Bakers 7B defatted soy flour) dispersions were prepared with tap water before addition to the fiber dispersions. Two protein concentrations were used (1% and 2%) in the chest tank (CT, 12.5 m3 capacity) where the slurry was stirred gently for 30 min for each treatment. As shown in Scheme 2, a separate trial was performed in which 2% soy protein was added through a small tank next to headbox where more agitation and less contact time was applied so as to consider runs at zero time. Talc was also added under the same conditions. Measurements of Stickies. A 0.68 g portion of “Morplas Blue” (an oil-based dye) was dissolved in 1000 mL heptane, mixing vigorously and then the solution was filtered (Whatman filter paper no. 4). The final concentration of the solution was 0.067%. Then, paper samples were immersed in the dye solution for 10 s and placed in the hood overnight for drying. Then, the samples were rinsed in heptane bath to remove the excess dye. They were stained and left in the hood for 2 h to dry. The stickies retain the blue color after rinsing and increased the contrast for the subsequent optical analysis. An Apogee Image Analysis System equipped with a high resolution scanner (HP Scan Jet 4C scanner at 600 dots per inch), was employed to count the dyed areas

(sticky contaminants). The experiments were performed on 5 paper specimens (6-in. round), air and wire side, by using 0.02 mm2 as the smallest speck counted. A value of 80% was used as the threshold of the average gray scale to detect specks. Peeling Test. A polyacrylate-based pressure sensitive adhesive (PSA, Carbotac 26171) was used as for attaching the paper samples on a glass plate. A diluted adhesive concentration of 25% was used. The peeling test was carried out based on the protocol developed by Pelton et al.40 Paper strips of 2.5 × 15 cm were attached to the PSA-covered glass slide, precleaned, and dried by sonication for 10 min in such a way that 5 cm of sample overhang remained beyond the tape. Excess fluid was shaken off, and the strips were flattened to remove air bubbles. The air-dried slides were attached to an Instron 4443 tensile tester with a 50 N load cell and to perform a 180° peel test under constant temperature (23 °C) and humidity (50%). An average of 5 strips measurements was reported for each treatment, and the peeling force was recorded as millinewtons per millimeter width of the strip. Formation and Brightness Analysis. A Paper Perfect Formation Analyzer (PPF, Op-Test Equipment Inc.) was used to determine paper formation. A camera with high depth of field captures the images while the paper was illuminated with a diffused quartz halogen light. The paper formation was compared to PPF or a reference sheet that reflects the uniformity of the sheet. Papers brightness was determined according to the TAPPI brightness standard test (T 452 om-08), using a Color Touch 2 Model IOS instrument. Thermal Gravimetric Analysis (TGA). The thermogravimetric analyzer used in this study was a TGA Q500. A nitrogen atmosphere followed by oxygen was applied to determine the ash content of the sheet. Heating was performed from 30 to 700 °C at 5 °C/min followed by an isothermal at 600 °C. Fourier Transform Infrared Spectroscopy (FTIR). The infrared absorption spectra of the air-dried papers with and without protein treatment were obtained using a PerkinElmer FTIR spectrometer to identify the sticky particles. Wavenumbers in the range 600−4000 cm−1 were used with the 4 cm−1 resolution, and each sample was scanned 32 times. Mechanical Properties. The tensile properties of papers were determined using a Lorentzen and Wettre device according to the TAPPI-494 standard method. Samples (100 × 15 mm) were placed between the clamps with the initial gap of 150 mm and pulled until failure at a cross head speed of 10 mm/min. The force was recorded in units of Newtons max force and tensile indices (N m/g) were determined by dividing the strength values (N/m) by the corresponding basis weight (g/m2). Burst strength (100 × 100 mm, TAPPI-T810) was also performed using a Lorentzen and Wettre tester. The average value of 10 replicates was used for all the mechanical tests. Porosity and Roughness. A Gurley Densimeter was used to determine the porosity of the papers (T 460 om-02). Samples were cut into 50 mm × 100 mm pieces and marked on both sides for easy identification. The Gurley seconds were recorded for each sample. 7213


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Figure 1. Sticky particle count per square meter (a) and paper brightness for different furnishes (b): (CT) protein was added in the chest tank; (HB) protein was added in the headbox; (TC) talc; (SF) soy flour; (SPI) soy protein isolate. Two conditions are reported, as indicated, bench- (lab) and pilot-scale experiments. (c) Examples of paper samples taken by the Apogee image analyzer: (c1) dyed paper from a reference furnish; (b) paper sample collected after application of soy protein. Roughness of the samples was tested with a Lorentzen and Wettre tester according to TAPPI T 538 standard.

significant. It is possible that only a fraction of the sticky components were affected by soy protein and application of excess levels did not contribute further for detackification. Interestingly, addition of protein in the “time zero” condition (addition in the headbox, HB, SF-HB, see Scheme 2) was also effective in reducing the sticky particles count: the ppm value decreased to 93 ppm. Similar results were obtained when talc was used (TC-HB). Apparently, protein had a better effect when allowing more contact time. The same trend was seen when performing laboratory-scale experiments. The treatment of the fiber slurry with SPI and SF was as effective as that with isolated enzyme (LOX). It has been revealed that LOX can catalyze the hydroperoxygenation of unsaturated fatty acids having at least one cis-1-4 pentadiene structure.42 Such fatty acids in pulp come from residual extractives present in wood.43 Therefore, it may be concluded that part of the sticky reduction in the LOX-treated samples was due to the oxygenation of unsaturated fatty acids such as linoleic acid and its derivatives. The possible impact of protein on the optical properties of the fibers was monitored (Figure 1b). When LOX was used in bench experiments, the brightness was increased slightly (up to ∼1 point). A possible explanation for this observation is the effect of co-oxidation of residual lignin present in the recycled fibers with lipid, carbon, or oxygen-based radicals induced by the LOX enzyme, which could slightly affect lignin chromophores. However, results from the pilot scale experiments showed a slight reduction in brightness with protein addition, for reasons that are not clear. Peeling Tests. The required force to initiate the detachment of the paper strips from the surface was assumed as the paper’s peeling strength. A 180° peeling geometry was used in

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RESULTS AND DISCUSSION In order to thoroughly evaluate the potential of soy protein as a dry strength and sticky-control additive, the protein systems were applied to the recycled fibers along with typical papermaking chemicals, under industrial conditions. The main addition point was the machine chest, under mild mixing; however, protein was also applied via a separate small tank with higher agitation, providing no effective contact time between the protein and fibers prior pumping to the head box (Scheme 2). To determine the original filler content in the fiber slurry and the retention of the added filler, a thermogravimetric analysis (TGA) was performed under nitrogen and oxygen atmospheres. The residual char and ash content was increased upon the addition of C-PAM, which indicated a higher fiber, fines, and calcium carbonate retention on the web. Tackiness Measurements. Among various methods to identify the tacky particles in paper,25,41 we used three tests to quantitatively and qualitatively detect the stickies within the sheets: optical evaluation, paper tackiness and Fourier transform infrared (FTIR) analysis. To perform the optical test, papers were dyed by a lipophilic dye to enhance the contrast between the natural fibers and the hydrophobic particles (Figure 1, c1−2 show examples of paper scans taken by the Apogee image analyze before and after SP application). As observed in Figure 1a, the highest level of tacky particles (158 ppm) was observed in untreated samples. In contrast, the lowest count value (68 ppm) was determined for treatment with SF (2% concentration, 2% SF-CT; upon mixing for 30 min). The difference between 1 and 2% protein level was not 7214


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Figure 2. (a) Peeling strength as a function of different additives used. The tests were conducted with sheets attached on a glass plate and using Carbotac at 25% solid content. Two conditions are reported, as indicated, bench- (lab) and pilot-scale experiments. (b) Instron test setup under a 180° peel test geometry.

order to minimize the sensitivity of the strip to the peeling direction and velocity (Figure 2). As shown in Figure 2, papers that were treated by SF and LOX in bench experiments presented a weaker attachment to the surface. The addition of talc reduced the stickiness, as was expected. Compared to other treatments, SPI caused slightly less reduction in the paper stickiness. However, relative to the control, a significant reduction in tackiness was evident. The effects were more evident in the pilot-scale study, where SF was used to confirm a large reduction in tackiness. The high surface area the additives can attach to the stickies and make larger, nonsticky particles, which are susceptible to breakage in the high-shear fields. The mechanism of SF detackification is not well understood, but it possibly involves attachment to the particles to produce less tacky agglomerates, which can either be retained or washed away during the pulping step. Soy proteins may also increase the wettability of sticky components and affect their tackiness.25 Besides, due to having both hydrophobic and hydrophilic domains, it can act as a dispersant in the white water of the tacky contaminants. In a relevant study, it was reported that a portion of protein can remain in the water phase and hence be removed through water discharge.16 This possible pathway is undesirable since it induces slime buildup. However, this unretained part is small and has a negligible contribution to biological growth. Moreover, soy protein has been also considered as a source of enzymes such as lipoxygenase,44 which under optimum conditions can oxidize a portion of the hydrophobic component and subsequently cause their removal from the fiber slurry. FTIR Spectra of Detackified Papers. FTIR was used to identify the type of stickies, based on their functional groups vibrations of various bonds at the given wavelengths (Figure 3). The spectra shown were normalized based on the wide band at 3320 cm−1 associated with cellulose’s primary OH groups, in order to provide a relative, qualitative assessment of the effects associated with the application of soy proteins. The wide band at 3320 cm−1 is related to the stretching vibration of the OH groups in cellulose. A distinct peak at 2890 cm−1 is due to C−H stretching, and the signal at 1422 cm−1 can be associated with the bending of CH2. Asymmetric stretching of C−O−C was observed at 1160 cm−1, and a strong peak at around 1030 cm−1

Figure 3. FTIR spectra obtained from papers carrying stickies at various levels, depending on the treatment: (a) untreated papers (rich in sticky) and (b) detackified sample with 2% talc. Profiles c and d correspond to samples treated with 1 and 2% soy flour (SF), respectively.

is assigned to ether bonds.45,46 A band in 895 cm−1 is associated with the β-(1 → 4) glycosidic linkage known as an “amorphous” absorption band of cellulose.47,48 This peak is related to the cellulose component, while the absorption at 2958, 2916, 2850, 1732, and 1588 cm−1 are assigned to the sticky-containing samples. The absorbance at 2850 and 2916 cm−1 is likely related to C−H stretching of methyl and methylene groups, which are present in many oils.49,50 The weak signal at 1732 cm−1 is related to the CO stretching of ester bonds, which are typical in triglycerides.51−53 In another study, bands at 2850 and 2916 cm−1 were seen in recycled papers and attributed to the ethylene vinyl acetate (EVA).54 EVA is a thermoplastic hot melt adhesive that is often found as a contamination in recycled paper. Samples treated with talc (Figure 3b) indicated the presence of such a signal at 2916 and 2850, but that at 1732 was absent. The spectra from soy-treated samples, indicated a reduced peak intensity at 2850, 2916, and 2958 cm−1 that can be an indication of partial elimination of stickies. The small peak that at 2850 cm−1 (Figure 3c and d) was anticipated from the C−H of cellulose, 7215


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enhancement obtained with 1% SF in the chest tank (1% SFCT) was more promising compared to that of 2% SF added in the head box (2% SF-HB). Most likely, the higher shear force and the limited contact time between the protein and the fibers caused a loss in retention (some of the protein is lost in the white water). Hydrodynamic shear forces are produced when fluid layers move at different velocities, which tends to increase under higher mixing speeds. It is believed that some of the induced attachment between protein and fibers may be broken or, otherwise, no attachment occurs at certain level of hydrodynamic shear stress. A similar trend was observed for the burst indices, but to a lower extent. In the bench-scale experiment, no filler was added, and thus the overall strengths were higher. The best result was achieved when using 2% SPI, which may be related to the presence of groups in protein’s residues (NH2, C−OOH), which create effective bonds within the fibrous networks. Soy protein has a large number of amine and carboxylic groups that can bring about hydrogen and ionic bonding with hydroxyl group of cellulose and lignin (Scheme 3). It was also observed in the past that the amino groups on chitosan could react with cellulose’s aldehydes and subsequently produce covalent bonds.57 If there are enough active aldehydes in the fibers, similar reactions may take place between protein primary amines and cellulose’s aldehydes. As shown in Scheme 3, three possible interactions, including covalent and noncovalent attachments, were considered between protein molecules and cellulose fibers; (1) noncovalent hydrogen bonding between protein amine groups and cellulose primary alcohols; (2) a weak ionic attraction between the ions from both systems and; (3) a covalent imine bond that can occur between the protein primary amines and cellulose aldehydes, all of which can lead to an effectively better strength of the paper web. A direct reaction between the carboxylic acids of cellulose and primary amines in soy proteins might be unlikely (amine can deprotonate the acid and produce the unreactive carboxylate salt); however, under the conditions of heat an amide might form,58,59 as illustrated (Scheme 3, bottom). Sticky components lead to weak attachments between fibers and undermine the overall paper strength. It is conceived that oxidative enzymes in the soy protein such as lipoxygenase (LOX) can degrade a fraction of these components,12,60 and open more spaces for effective fibers-fiber joints. Moreover, (LOXs) can produce aldehydes as a secondary degradation product,61 which can then combine with the cellulose aldehydes

and the two spectra, possibly belonging to EVA and cellulose, overlapped. Moreover the signal at 1422 cm−1, assigned to CH2 bonds, lost most of its intensity upon the protein application. This signal was ascribed to a trace of polyvinyl acetate (PVA) in the recycled papers.55 Sheet Strength. Paper strength has been used as an important criterion in paper quality (related to formation, interfiber contact area, bonding strength, and other factors). A key here is the linkage between fibers and the number of joints in a given area rather than the intrinsic strength of the fiber.56 A molecular-level adhesion through hydrogen bonds is necessary to build a good fiber−fiber bond and eventually achieve a strong paper web. Hydrogen bonding is mostly responsible for paper’s internal strength. In recycled papers, there is less effective fiber bonding due to the presence of more fines, broken and less flexible fibers, and poorer conformability in the wet state. Therefore, dry strength agents are needed to compensate for the weaker bonds anticipated in recycled paper. Several aspects of mechanical properties were examined, with the tensile index being the main criterion; data in Table 1 Table 1. Mechanical Properties, Electrostatic Charge, and the Roughness of Treated Papers Obtained from Bench and Pilot-Scale Experimentsa

a

treatment

tensile index (N m2/g)

control (I) 2% SF 2% SPI 1% LOX

42.1 45.5 46.2 44.2

control (II) 2% SF-HB 1% SF-CT 2% SF-CT

28.9 29.4 31.1 34.7

burst index (KN/g)

roughness (SU)

Bench-Scale Experiments 0.40 35.4 0.41 33.0 0.44 33.8 0.43 33.8 Pilot-Plant Experiments 0.13 25.8 0.14 22.5 0.15 20.6 0.17 18.3

porosity (Gurley s)

fiber total charge (μequiv/g)

25.0 28.2 29.4 29.0

39 49 62 50

19.2 23.7 24.4 29.5

The machine direction was used in pilot-plant experiments.

showed an overall improvement in sheets strength after application of the soy protein additives. Loading of SF at 2% either in the head box (HB) or in the chest tank (CT) caused a rise in the tensile indices. However, results indicated a more pronounced improvement when protein was added in the machine chest tank, using 30 min of mixing time. The

Scheme 3. Possible Interactions between Cellulose Fibers and Soy Protein’s Amine Groups (top) and Formation of Amide Bonds between Carboxylic Acid Groups of Cellulose and the Amine Groups of the Proteins (bottom)

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Figure 4. PPF values of sheet formation (relative to a reference paper) before and after the addition of C-PAM, soy protein, and talc (a) and relative index of fiber formation compared to reference (copy paper) after the addition of same additives (b).

In closing, the pilot-plant experiments were performed to simulate the actual operations used in paper mills and also to confirm the observations from the bench-scale studies. A strong correlation was found between the results obtained for both experimental approaches. A clear reduction of hydrophobic contaminants and enhancement of paper dry strength were realized in the two systems considered.

to create imine bonds with SP primary amines. It has been shown that treating OCC, NSSC, and Kraft pulp with soy flourdiethylenetriaminepentaacetic acid (DTPA)-chitosan additive could increase the tensile and burst indices to a significant extent.59 Therefore, it is conceivable that the presence of soy protein leads to a better interfiber attachment through two different mechanisms at the same time; as a source of enzyme and as a bonding agent. Roughness, Porosity, and Fiber Charge. The change in anionic charge of the fiber dispersions was characterized after the addition of SF, SPI, and LOX to the system. As expected, the addition of negatively charged soy protein increased the anionic charge density (Table 1). The porosity test (Gurley numbers) revealed that the application of all the protein types used in this study led to a slightly less porous structure in the paper. It is plausible that the proteins fill the void spaces within the fibers and act as a barrier for air. This can also be an indication of protein retention. Papers with less pores are smoother and for this reason the roughness values declined. Protein-treated samples under a gentle agitation and 30 min mixing time (2% SP-CT), showed the least porosity. Formation Test. Paper uniformity highly depends on the fiber spread and a bad formation can affect paper strength, optical properties, and printability.62 In order to monitor the formation, usually the variation of local basis weight in sheets is estimated through a light transmission method. The uniformity of components with the smaller size (1−10 mm) may impact the printability, and those with bigger size (10−30 mm) can affect the mechanical properties.62 Here, the relative formation test determines the difference between sheets formation and a reference. The closer values of the paper sample to that of the reference paper, the better the formation. As shown in Figure 4b, the addition of cationic retention aid (C-PAM) increased the filler retention, and it adversely affected the formation. However, application of 2% SF under mild mixing conditions improved the formation, to some extent. It has been noticed that formation and retention perform reversely, meaning that the higher filler retention causes worse formation.63 They are both critical for papermaking; while a good formation improves the internal strength and printability, higher filler retention improves the additive efficiency and effluent loading. It seems that protein improved the formation to some extent, especially when retention aid was present. The PPF values (Figure 4a) showed that protein-treated samples had slightly more smallsize flocs, which is indicative of a slight improvement in the sheets’ printability.

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CONCLUSIONS Considering the experiments conducted at the bench and pilot scales, it can be concluded that soy proteins (SP) in various forms (mainly soy flour, SF, and soy bean lipoxygenases, LOX) offer great potential as environmentally sound solutions to address the challenges related to tacky contaminants in pulp and paper mills. This is especially the case under the current efforts to avoid talc, used as a typical detackifier agent. In the pilot-scale study, a 56% reduction in sticky particles was achieved when 2% SF was applied under mild agitation, which compared to talc (44% reduction), represents a higher sticky removal from the recycled fibers. A similar trend was also observed in the bench-scale experiments. Furthermore, SPs were further beneficial since they improved paper strength. The addition of SF increased the tensile index of papers formed at the bench and pilot scales by 4.1 and 5.8 N m2/g, respectively. Moreover, improvements related to paper roughness, porosity, and formation were achieved. Importantly, highly purified soy protein products were not required to reach the desired levels of fiber−fiber bonding, Thus, application of less refined soy flour can be as beneficial. The obtained results from the pilotplant study highlight the favorable performance that can be attained when biodegradable SP products are used as a potential dry strength and detackifier agents. It is proposed that soy-based additives can be used in the near future in pulp and paper mills, given the fact that in the US alone, for example, 63.5% of the annually produced papers are from recycled fibers.64 The proposed approach offers promise since it is costeffective, achieves higher paper quality, and may result in less down times, which are often associated with sticky deposition.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: orlando.rojas@aalto.fi. ORCID

Orlando J. Rojas: 0000-0003-4036-4020 7217


Research Article

ACS Sustainable Chemistry & Engineering Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the United Soybean Board (USB) under Project Number 1640-512-5276 and HYBER’s Academy of Finland’s Centers of Excellence Program (2014-2019). We thank Dr. Keith Wing and Kevin Wise for fruitful discussions and advice.

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REFERENCES

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Soy Proteins As a Sustainable Solution to Strengthen Recycled Paper

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