In-Plane Compression and Biopolymer Permeation ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

In-Plane Compression and Biopolymer Permeation Enable Superstretchable Fiber Webs for Thermoforming toward 3‑D Structures Alexey Khakalo,∥ Jarmo Kouko,‡ Ilari Filpponen,*,∥,† Elias Retulainen,‡ and Orlando J. Rojas*,∥,§ ∥

Department of Bioproducts and Biosystems, School of Chemical Engineering, Aalto University, Vuorimiehentie 1, FI-00076 Espoo, Finland ‡ VTT Technical Research Centre of Finland Ltd., P.O. Box 1603, Koivurannantie 1, FI-40101 Jyväskylä, Finland † Alabama Center for Paper and Bioresource Engineering, Department of Chemical Engineering, Auburn University, 212 Ross Hall, Auburn, Alabama 36849-5127, United States § Departments of Chemical and Biomolecular Engineering, North Carolina State University, 2820 Faucette Drive, Raleigh, North Carolina 27695, United States S Supporting Information *

ABSTRACT: The typically poor ductility of cellulosic fibers and ensuing bonded networks and paper webs set limits on any effort to produce associated three-dimensional structures without relying on chemical, often unsustainable, approaches. To address this challenge, we report on a facile and green method that combines mechanical and biopolymer treatment: in-plane compression and aqueous solution permeation via spraying. The first enabled network extensibility while the second, which relied on the use of either food-grade gelatin, guar gum, or polylactic acid, improved network strength and stiffness. As a result, an unprecedented elongation of ∼30% was achieved after unrestrained drying of the fiber web. At the same time, the structures experienced a significant increase in tensile strength and stiffness (by ∼306% and ∼690%, respectively). Such simultaneous property improvement, otherwise very difficult to achieve, represents a substantial gain in the material’s toughness, which results from the synergistic effects associated with the mechanical response of the network under load, fiber intrinsic strength, and interfiber bonding. The level of plasticity developed in fiber webs upon biaxial compaction (longitudinal followed by lateral compaction), which was performed to reduce property anisotropy, allowed the synthesis of 3-D packaging materials via direct thermoforming. Moreover, the formability was found to be temperature and humidity dependent (strain and creep compliance after creep/recovery cycles in dynamic mechanical analyses). Overall, an inexpensive, green, and scalable approach is introduced to expand the properties spaces for paper and related non-wovens that allows 2-D and 3-D formability of in-plane compacted fiber networks. KEYWORDS: Toughness, Formability, Biopolymer spraying, In-plane compaction, Extensibility, Paper, Packaging materials, 3-D structures

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INTRODUCTION Increased environmental awareness has led to a growing interest in renewable and biodegradable packaging. For instance, the production of plastics alone reached 322 million metric tons in 2015, with packaging being the top market that accounted for 40% of the annual consumption. Consequently, a large volume of post-consumer plastic waste is generated as waste streams (25.8 million tonnes in 2015). From this amount, ∼69% is recovered through recycling and energy recovery processes, while ∼31% is directed to landfills.1 Therefore, it is highly desirable, if not mandatory, for packaging materials that are in current use, to be replaced with multifunctional alternatives, with no penalties on added costs. Moreover, a recent study from Smithers Pira, commissioned by Pro Carton, © 2017 American Chemical Society

indicated a resounding evidence of the importance of sustainability in packaging and highlighted folding cartons as one of the most sustainable options.2 In addition to being extremely important in a sustainable bioeconomy, cellulosic fibers represent a versatile platform for numerous modifications targeting advanced material properties. This demands the development of formability and thermoplasticity, which nowadays is of high interest not only for the paper industry but also from the perspective of materials science. Thus, thermoformable cellulose-based networks are at the center of interest, Received: June 21, 2017 Revised: August 8, 2017 Published: August 10, 2017 9114

DOI: 10.1021/acssuschemeng.7b02025 ACS Sustainable Chem. Eng. 2017, 5, 9114−9125

Research Article

ACS Sustainable Chemistry & Engineering especially if they display properties that are mostly distinctive of plastics, such as high toughness. In turn, this can make paper suitable for new generation packaging materials with advanced 3-D structures. The term “formability” describes the ability of a material to undergo plastic deformation without damage, and therefore it is strongly associated with the extensibility under the processing conditions (temperature, moisture, strain rate, heat, and water transfer). The main properties of fiber-based products to meet the requirements of 3-D shaping have been a subject of recent investigation by Vishtal et al.,3,4 and methods to improve the respective material extensibility (formability) have been highlighted.5 In brief, extensibility of fiber networks was found to depend on three main aspects: single fiber properties, interfiber bonding (affected by the nature and composition of contact or crossing points), and the structure of the network.6 For instance, mechanical treatment of fibers was effective in improving the toughness of fiber networks,7 and an increase in extensibility and strength was achieved, by a factor of 3.2 and 2.7, respectively. A method for forming molded fiber bottles was developed in which a wet paper preform shaped the structure. Following, a fiber slurry was applied to the mold, where moisture was eliminated through drainage channels and the container was dried by pressing a thermoplastic preform against it, using blow molding.8 Among chemical modifications, hydroxypropylation of cellulose9 and heterogeneous partial derivatization to dialcohol cellulose10,11 have been used to develop highly ductile and thermoplastic materials. Paper for 3D forming was obtained by application of polyelectrolyte multilayers on the surface of the fibers.12 A straightforward chemical treatment based on biopolymers was also proposed to improve formability.13−15 Moreover, structural modification of fiber networks via negative straining (shrinkage upon unrestrained drying) or in-plane compression (compaction and creping) are known to facilitate extensibility and ductility. In practice, the high extensibility required in paper such as those for sack and bag grades, can be achieved by compacting moist webs in the machine direction (MD). This has been performed in the drying section, at locations where the dry solids content is ∼60−65%.16 For this purpose, industrial units such as the Clupak (compaction between a moving rubber blanket, steel roll, and static nip bar, see Figure 1) or Expanda (compaction between a steel roll covered with a rubber blanket and a heated steel roll) have been used. Here, the tensile energy absorption (TEA) is increased so that the product is able to withstand, without breaking, the sudden stresses involved during filling and handling. Compaction in MD induces extensive deformations like micro-creping through kinking and curling of the fibers18 and incorporation of microcompressions to fibers, all together leading to highly localized compressive strain in the fiber wall.6 The gain in paper extensibility and ensuing tensile energy adsorption are attributed to the straightening of the deformations before the rupture under MD load of the fiber network. In addition, extensibility depends on the extension of the creases between the fiber joints upon reaching the yield point during stretching.19 Therefore, an additional benefit of compaction is the reduced axial stiffness of the fibers, which enhances the drying shrinkage.20 Most important to this investigation, however, is the fact that compaction, while an excellent solution toward extensibility, reduces the loadcarrying ability under tensile load in the MD. This is because the introduced deformations and disturbed interfiber bonding

Figure 1. Principle of the Clupak extensible unit (reprinted with permission from BioResources5). Here the paper web continuously passes through the nip between a moving rubber blanket and a rotating dryer cylinder. The rubber blanket is pressure-loaded by a static nip bar making the gap in the nip smaller than the thickness of the rubber blanket. As a result, the surface of the rubber blanket is stretched through bending over the nip bar, and it is further stretched following the Venturi effect. The stretched rubber surface then comes into contact with the moist paper web. When the stressed rubber has passed through the center of the nip, the outer surface of the rubber begins to recoil due to the deceleration and bending of the rubber in the opposite direction. This forces the paper web to follow the dimensional changes of the rubber surface due to friction forces between paper and the rubber and a simultaneous slippage of the paper on the cylinder surface. Consequently, the paper is subjected to a longitudinal compressive forces without any major buckling or creping. The final compaction level can be adjusted by controlling the speed difference of the paper web between inlet and outlet of the nip,17 by varying the stretch of the rubber blanket, z-pressure exerted by a static nip bar, and moisture content of the paper entering the nip.

after straightening the micro-creeped structure, in other words, the stiffness and breaking strength of the web, are reduced. The impairment of tensile strength and stiffness after mechanical compaction is a major factor that limits applications in packaging. Some solutions to address this issue have used different dryness of paper and z-pressure during compaction.21 However, wrinkling becomes a problem. At last, 4-fold increase in tensile stiffness of compacted paper was reported upon straining (to a 75% of the maximum strain) via the strain hardening effect.22 Another approach to restore the tensile strength of compacted paper is the addition of various bio-based water dispersions. For instance, guar gum is an effective dry strength additive for paper23,24 as well as for cellulose nanofibril films.25 We reported on the use of gelatin protein to reinforce fiber networks,13 and Svensson et al. improved paper strainability and relative bonded area by spreading a polylactide latex on the surfaces of the fiber.26 Moreover, tensile strength properties are known to contribute to the extensibility of the paper.6 This is especially the case when the stretch potential of deformed paper structures is fully realized and, thus, the load is evenly distributed within the network. Achieving high toughness (strain and strength) is a major challenge that needs to be addressed if cellulosic fibers are to be used in high-performance packaging materials and 3-D structures. Here we propose a synergistic approach toward advanced paper formability by combining in-plane compaction and biopolymer permeation. For this purpose, a custom-made 9115


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ACS Sustainable Chemistry & Engineering

Figure 2. Top: Compaction device shown in operation in the strained (a) and non-strained (b) positions. Bottom: Stress−strain curves of compacted paper in MD (c) and CD (d). “2×” means compaction was performed twice; “1+1” means biaxial compaction was performed, first in MD and then in CD; “45°” means mechanical properties measured at 45° relative to the direction of compaction. Note the different scales on the horizontal X-axes for the strain. obtain the data. Cumulant analysis was used to give the intensity mean value of size, i.e., hydrodynamic diameter. Samples for size measurements were prepared by dissolving agar and gelatin at 85 and 45 °C, respectively, for 6 h to yield 0.1 mg/mL polymer concentration. Experiments were carried out at 85 and 45 °C for agar and gelatin, respectively. The average values of these measurements are presented. Paper Sample Preparation. Handsheets were prepared according to ISO 5269-1:2005 standard except that the grammage of the handsheets was ∼70 g/m2 and the handsheets were dried without restraint (23 °C and 50% RH) between two synthetic wire fabrics with a gap of 1−3 mm, which allowed a free shrinkage of the paper without extensive cockling. The prepared handsheets were stored under controlled conditions (23 °C and 50% RH) for further investigations. In-Plane Compaction of Paper. In-plane compressive treatment of paper was performed by using a tailor-made compaction unit developed by VTT in Jyväskylä (VTT, Jyväskylä) (see Figure 2). In this device, paper is placed between two strained rubber bands (Poisson’s ratio is 0.49), and the bands are pressed together by a piston-driven plate. Pressing force was adjusted to allow rubber bands to slide between the supporting plates and also to provide sufficient friction between the paper and the rubber. The pressure level was kept constant at 0.4 MPa in all experiments. Once paper was pressed between the bands, tension was released, and the bands started to spring back to restore their original length. Consequently, paper was contracted along the deformed rubber bands. The strain and the ensuing strain recovery of the rubber bands was 13%. Prior to compaction, dry handsheets were moisten by spraying deionized water until ∼40% water content was reached and then kept in sealed plastic bags overnight for homogenization. In this study, MD refers to the compaction direction in the device since only randomly oriented papers from a laboratory sheet-former could be obtained. The samples

MD compaction apparatus was employed together with spraying of aqueous, food-grade biopolymers. The obtained materials were tested in humid and high-temperature conditions with the press forming unit to demonstrate their excellent performance and to improve extensibility, thus allowing 3-D forming.

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MATERIALS AND METHODS

Materials. Bleached, once-dried softwood kraft fibers (cellulose 80%, xylan 10%, glucomannan 8%, and total lignin 15 mm) by pressing with a custom-made razor blade cutter. The storage modulus (E′) was determined using dynamic setup at nitrogen atmosphere (constant frequency 1 Hz, amplitude 15 μm, preload force 0.5 N, temperature range from 25 to 200 °C at a heating rate of 2 °C/min). Samples were conditioned at least for 1 h under nitrogen flow to attain same level of 9117


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Table 2. Mechanical Properties of In-Plane Compacted Paper Treated with 4 wt% of Bio-Based Aqueous Dispersionsa sample

apparent density (kg/m3)

strain to failure (%)

shrinkage (%)

tensile strength (MPa)

tensile strength index (N·m/g)

Compaction PLA + Com GG + Com Gel + Com Com + Gel

444 541 479 498 640

14.0 14.6 15.9 16.7 14.6

± ± ± ± ±

1.4 0.5 1.5 0.9 0.8

Mechanical Properties in Longitudinal Direction (MD) 11.3 ± 1.1 10.6 ± 1.4 23.8 ± 3.1 1.7 ± 0.2 10.9 ± 0.6 12.4 ± 0.8 22.8 ± 1.6 1.7 ± 0.1 10.0 ± 1.2 18.7 ± 1.8 38.9 ± 3.8 3.4 ± 0.5 9.7 ± 0.9 26.7 ± 2.2 53.6 ± 4.3 4.3 ± 0.4 8.3 ± 0.8 43.1 ± 2.4 67.3 ± 3.7 5.4 ± 0.4

0.22 0.43 0.53 0.63 1.74

± ± ± ± ±

0.04 0.06 0.17 0.15 0.33

0.10 0.23 0.25 0.32 1.11

± ± ± ± ±

0.01 0.03 0.08 0.07 0.21

Compaction PLA + Com GG + Com Gel + Com Com + Gel

444 541 479 498 640

5.6 4.9 6.9 8.9 8.3

± ± ± ± ±

1.2 0.4 0.9 1.1 0.5

Mechanical 1.6 ± 0.7 −0.5 ± 0.3 1.3 ± 0.5 1.5 ± 0.6 2.1 ± 0.3

2.22 3.00 2.2 2.24 3.40

± ± ± ± ±

0.49 0.10 0.23 0.54 0.46

0.99 1.63 1.16 1.11 2.19

± ± ± ± ±

0.22 0.06 0.12 0.27 0.29

Properties in Lateral Direction (CD) 15.9 ± 2.2 35.6 ± 4.9 17.9 ± 1.1 33.8 ± 2.1 24.6 ± 2.8 46.6 ± 4.4 29.6 ± 2.8 59.7 ± 4.7 52.2 ± 3.2 81.1 ± 4.9

TEA index (J/g)

1.3 1.1 2.0 3.1 3.8

± ± ± ± ±

0.3 0.1 0.3 0.5 0.2

tensile stiffness index (kN·m/g)

Young’s modulus (GPa)

The abbreviation “Com” is used to indicate compacted. Polylactic acid (PLA) latex and guar gum galactomannan (GG) were added by spraying before compaction. Gelatin (Gel) was introduced either before (“Gel + Com”) or after (“Com + Gel”) compaction. Biopolymer addition amounted to 4 wt% with respect to dry fibers. ± values correspond to standard deviations from 10 replicates. Shrinkage denotes to the dimensional changes induced by the drying shrinkage and compaction.

a

Figure 3. Stress−strain curves of polymer-modified, in-plane compacted paper in MD (a) and CD (b). Reference: unmodified paper. Polylactic acid (PLA) latex and guar gum galactomannan (GG) were added by spraying before compaction. Gelatin (GEL) was introduced either before (“GEL 4% + Compaction”) or after (“Compaction + GEL 4%”) compaction. Biopolymer addition amount was 4 wt% with respect to dry fibers. Note the different scales on the horizontal X-axes for the strain.

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RESULTS AND DISCUSSION

demonstrate the repeatability of the in-plane compaction process. As it is apparent from Figure 2, application of in-plane compaction increased the MD strain while the strength and stiffness indices were reduced. Typically, a 10% gain in MD strain is associated with 1−2% increase in CD strain.28,29 Therefore, it was somewhat surprising to obtain a reduction in CD strain after the MD compaction and slight shrinking of the sample (Table 1). This may be associated with the alignment of induced deformations. Moreover, compaction may have facilitated a partial bond disruption, which in turn oriented, at least partially, the fibers toward the lateral direction of the compaction. Drying likely improved the bonding between the oriented fibers, as can be concluded from the higher strength and stiffness values measured in CD. The increase in strain and the decrease in tensile strength index became even more pronounced when the compaction was performed twice (“Compaction 2×”). However, an expansion (negative CD shrinkage) of paper was observed after the second compaction.

Mechanical Performance of Paper after In-Plane Compaction. Typical stress−strain curves of the networks modified by in-plane compaction are presented in Figure 2, including measurement in both the machine (compaction) (MD) and cross-machine (CD) directions. Note: only randomly oriented papers from a laboratory sheet former were used, and therefore MD and CD refer here to the direction of compaction; i.e., they are not to be confused with the dominant direction of the fibers in the paper network. The most relevant mechanical properties are reported in Table 1, where untreated (non-compacted) reference paper is included for evaluation. To facilitate comparisons, some data were normalized by using the apparent density, i.e., presented as indexed values (tensile, TEA, and stiffness indices). The extent of compaction was followed by shrinkage measurements. The narrow standard deviations associated with the shrinkage values indicate a good reproducibility of this method and also 9118


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fiber structure, at least if no activation (hot pressing) is applied. Therefore, it was reasonable to postulate that the mechanical properties of PLA-containing compacted paper largely depended on the deformations induced during the compaction. As can be seen from Table 2, the mechanical properties in MD were almost the same as those for the compacted sample without any polymer treatment. The deterioration of the extensibility in CD can be attributed to the recovery of Poisson contraction of the rubber bands during compaction. Moreover, the disruption of paper structure, as indicated by slightly lower tensile strength, might have also contributed to the poor extensibility in CD. Guar gum (GG), on the other hand, afforded a clear improvement of the mechanical properties of the compacted paper, even in both directions. It is important to note here that the application of GG restricted the MD shrinkage of paper during in-plane compaction (10.0% versus 11.3%), suggesting that the GG-reinforced structure somewhat resisted the deformations caused by compaction. Nevertheless, as indicated in Table 2, compared to biopolymer-free systems, compaction of GG-containing paper resulted in 13% and 63% increases in MD extensibility and strength, respectively. If measured in the CD direction, the aforementioned properties improved by 23% and 31%, respectively. Compared to the control sample (compaction without biopolymer addition), in-plane compaction of gelatin-treated paper resulted in a significant improvement of the strain to failure and tensile strength, i.e., 20% and 125% increase in MD, respectively. In CD, these properties improved by 60% and 70%. Moreover, notable enhancements in stiffness, elastic modulus, and TEA index were also obtained. Compared to the reference, non-compacted paper, the addition of gelatin offset the loss of tensile strength caused by in-plane compaction. The better performance of gelatin compared to GG may be explained by the lower molecular mass of the former polymer (up to 100 kDa) relative to that of GG (220 kDa).33 In addition, the Z-average hydrodynamic radii of gelatin and guar gum assessed at the given conditions were 228.5 ± 4 and 3203.0 ± 202 nm, respectively. Thus, in addition to filling the voids between fibers, gelatin possibly penetrated more readily into the fiber cell wall given that refining opens or expands pores of different sizes.34,35 In the case of GG, cellulose−guar gum interactions are known to be mainly governed by hydrogen bonding between hydroxyl groups on the cellulose chain and polar groups on the polysaccharide, which is manifested by similar conformations of the mannose backbone in GG and glucose in cellulose.36,37 In contrast, in the case of gelatin, besides H-bonding, electrostatic and other non-specific interactions are involved in cellulose− gelatin complex formation, due to the amphoteric nature of gelatin.13 An additional benefit of using water-soluble biopolymers stems from bonding upon drying (compaction was performed at 60% dry fiber content). Therefore, an attempt was made to further reinforce the compacted paper structure, i.e., by introducing gelatin after compaction (sample “Compaction + Gel”). Table 2 indicates that application of gelatin onto in-plane compacted structures further improved the mechanical properties, both in MD and CD (Figure 3). This is already a remarkable result but even more impressive is the increase in paper tensile stiffness when compared to that of the control sample (compaction without biopolymer addition). Improvements of 690% in MD and 50% in CD were noted, respectively.

This may be explained by the Poisson contraction of the rubber bands used in the unit and their corresponding expansion in CD. Moreover, such treatment resulted in more profound micro-creeped structure, which had a lower load-carrying ability and higher stretch potential due to the increased deformations (Figure S1). This can also be concluded from the shape of the stress−strain curve, where the section of the profile that corresponds to the straightening of the micro-creeped structure30 became more pronounced, i.e., the strain-hardening decreased and the length increased. In order to favor a lower anisotropy upon compaction, the operation was performed in both directions (biaxial compaction, also referred to as “Compaction 1+1”). This approach improved the extensibility in both directions while maintaining the strength properties at satisfactory levels. It is noteworthy that the second compaction step contributed to a higher degree to the extensibility. In addition to two-directional compaction, mechanical properties were also measured at 45° relative to the direction of compaction. This was carried out in order to geometrically equalize the contributions of both MD and CD deformations. However, the extensibility from such measurement was higher only for CD. The results presented thus far point to the fact that compaction is a powerful technique to improve the extensibility of paper but, unfortunately, the strength properties were compromised. Therefore, we hypothesize that such effects can be compensated via permeation of an aqueous dispersions of biopolymers, applied either before or after compaction. This is based on the possibility that tensile strength may contribute to the extensibility of the paper if the load is more evenly distributed within the network;6 thus, it is possible that the stretch potential of deformed paper structures can be fully realized. Mechanical Properties of Biopolymer-Treated, InPlane Compacted Paper. Polymer-permeated papers were obtained by spraying aqueous biopolymer solutions (only 4 wt % biopolymer mass with respect to dry cellulose fibers) either on freshly prepared handsheets before wet pressing or after compaction. The results for the mechanical properties are summarized in Table 2, and the stress−strain profiles of respective samples are shown in Figure 3. Due to addition of the biopolymers, the grammage of the handsheets was slightly higher than the initial value, in the range of 75−78 g/m2. This indicates that the biopolymers were adsorbed on the fiber surfaces and were likely to fill the free volume of the web; thus, it was expected that they increased the interfiber contact area and bonding. PLA-treated samples were hot-pressed in order to activate the latex. In order to subject all samples to the same conditions used for PLA-treated systems, therefore, allowing better comparison, all the samples were hot-pressed. (Note: hotpressing was found to have a negligible effect on the mechanical properties; data not shown.) Activation of latex by hot pressing facilitated spreading of the latex on the fiber surface and, consequently, was assumed to improve the relative bonded area in the network. In fact, the results indicated improved mechanical properties of the sample, while the structural density was not affected (Figure S2). However, the effect of PLA latex on the mechanical properties of compacted paper was negligible in MD and somewhat even negative in CD (Table 2). This was likely the result of the presence of hydrophobic, preactivated PLA latex31,32 which prevented or undermined the formation of new bonds within the compacted 9119


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ACS Sustainable Chemistry & Engineering It should be noted that the tensile stiffness, which contributes to the bending stiffness, is a desired property in packaging materials. However, strain to failure in this case improved only slightly, which could be related to a more limited extent of compaction (8.3% vs 11.3%) associated with this approach. In order to better understand the behavior of the system, additional efforts were carried out using a different sequence of protein addition. At first, compacted paper with dry solids content of ∼60% was sprayed with gelatin solution (4 wt% with respect to dry cellulose fibers) which further lowered the paper dry solids content. Afterward, the sprayed structure was wetpressed in order to facilitate the bonding between gelatin and fibers. It was hypothesized that the deformations induced by compaction were partly diminished during the wet pressing, leading to a lower shrinkage. Therefore, the underpinning assumption was that a significant toughness could be achieved by designing a methodology that eliminated the structural deformations during pressing, which was realized in this work. The results presented thus far point out that the in-plane compaction of paper in combination with the biopolymer permeation (proteins, polysaccharides, etc.) improved the stiffness and strength of compacted paper. Moreover, biopolymer permeation contributed to an improved paper extensibility. By varying the sequence used for polymer addition, it was possible to modify the tension behavior of the modified structures since bonds that were created during drying were strained first upon loading. Therefore, the combination of in-plane compaction and biopolymer treatment was carried out in two different ways: (1) Polymer addition before compaction (wet end, spray on wet paper, etc.), which increased the stiffness at the latter part of the stress−strain curve. The bonds that were created before the compaction were loaded when the compressed zones of fibers/paper were straightened out, i.e., close to the fracture of the sample. (2) Compaction followed by polymer addition (e.g., spraying on wet paper, etc.) that stiffened and reinforced the compacted paper structure, such that the bonds were strong and stiff. Therefore, the elastic modulus and tensile stiffness, which were measured at the beginning of the tensile test, were found to increase (the first part of stress−strain curve). After the bond stretching, the straightening of the compacted paper structure was initiated. Overall, it is possible to considerably modify the mechanical behavior of fiber networks by optimizing the mechanical and polymer treatments in the wet-end or in the drying section. Effects of Humidity and Temperature on the Mechanical Properties of Biopolymer-Modified, InPlane Compacted Paper. To investigate the potential for thermoforming, both dynamic and static DMA experiments were conducted in order to monitor the structural response of modified networks to temperature and humidity. The storage modulus profiles as a function of temperature for dry papers are presented in Figure 4. The storage modulus indicates the material’s ability to store energy and also reflects its stiffness. Paper that has been compacted by traditional methods (Clupak) has been reported to display a ∼2.5 times lower stiffness compared to that of the unmodified reference paper.30 As illustrated in Figure 4, PLA-containing sample, tested at room temperature, displayed a high storage modulus, which significantly decreased upon heating. This can be associated with the glass transition temperature of PLA (55−65 °C). Moreover, at high temperatures (>100 °C), when the PLA was softened, the storage modulus was found to be even lower than that of the compacted, control sample. This suggests that the

Figure 4. Storage modulus of biopolymer-treated, in-plane compacted networks measured under nitrogen atmosphere. Polylactic acid (PLA) latex and guar gum galactomannan (GG) were added by spraying before compaction. Gelatin (GEL) was introduced either before (“GEL 4% + Compaction”) or after (“Compaction + GEL 4%”) compaction. Biopolymer addition amount was 4 wt% with respect to dry fibers.

interactions between cellulose and PLA were severely affected at high temperatures. Therefore, in order to improve material stability, an interfacial modification should be attempted as a way to restrict the motion of PLA chains. It is evident from Figure 4 that GG-modified paper displayed ∼25% higher storage modulus (compared to compacted sample, without biopolymer addition), which was likely due to enhanced interactions between cellulose and GG, within the whole temperature range. Upon gelatin modification, the storage modulus of compacted papers increased significantly, which can be attributed to the pronounced reinforcing action of gelatin. The temperature-dependent storage modulus of polymertreated, in-plane compacted papers measured at 50% and 75% RH are shown in the Figure S3a,b, respectively. In general, the storage modulus of paper in humid environment followed the same tendency as that measured in dry conditions, i.e., gelatin-modified papers displayed significantly higher values than the reference, PLA- and GG-modified papers. Effect of Humidity on Strain and Creep Compliance. 3-D forming is a process where the materials simultaneously undergo multiple external stresses (load, temperature, moisture), which in turn alter the original physico-mechanical properties and affect the product deformation. Deformation under stress can be monitored as changes in strain and creep compliance, which is related to the sample’s readiness to change its shape, a feature that is highly beneficial for thermoforming and can be calculated as an inverse of the modulus.38 DMA creep experiments were performed to further investigate the effect of humidity on the mechanical properties of modified papers. Figure 5a illustrates the creep/recovery cycles of compacted and gelatin-modified compacted networks. It can be seen that the deformation of compacted paper was more pronounced (higher strain) at low humidity, which indicates that the material was more susceptible to the applied stress than the gelatin-containing sample. Compared to the gelatin9120


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Figure 5. (a) Strain of compacted and gelatin-modified, compacted paper after multiple creep/recovery cycles (5 × 120 min cycles with stress of 5 MPa and 60 min of creep recovery), with a dynamic change of humidity from 10% to 90% RH and a constant temperature of 25 °C. (b) Relative changes in storage modulus of biopolymer treated, in-plane compacted papers as a function of increased humidity measured at 25 °C.

under the applied stress of 5 MPa. This explains the higher strain recoveries observed for the gelatin-modified samples. Moreover, it is known that gelatin chains are prone to partially recover the original triple-helix structure of collagen upon cooling, through a sol−gel transition. Thus, it is possible that gelatin formed a group of physically interconnected triple helices, which were brought together as fibrillar structures by intermolecular hydrogen bonding.39 Incorporation of gelatin’s fibrillar structures on the compacted paper may have also increased the relaxation ability of gelatin-modified, compacted paper. Upon increasing the RH to 30%, the deformability of the inplane compacted paper was significantly decreased. This may be due to the water-induced plasticization of cellulose, which increased the fiber flexibility and therefore released the tension exerted during the first creep cycle. The onset of the declining ability to recover the original strain of gelatin-modified, compacted paper occurred at a relative humidity of 50%. It can be seen that the relaxation ability of both samples after four creep/recovery cycles was comparable. Furthermore, at 90% RH, creep compliance and strain of gelatin-modified, compacted paper reached the maximum values, 16755 μm2/ N and 8.3%, which exceeded those recorded for the compacted paper. Improved deformation of gelatin-containing samples at elevated RH might be explained by the formation of strong interactions between the gelatin and cellulose. Moreover, the relative change in storage modulus, as shown in Figure 5b, indicated that the gelatin-modified paper exhibited a smaller loss in relative modulus when subjected to a stepwise change in relative humidity, from 30% RH to 90% RH (Figure S4 includes the values of the storage modulus). One possible explanation for these observations is that the penetration of water molecules was hindered by the hydrophobic residues of gelatin, which led to relatively stable gelatin−cellulose interactions, even at high humidity. The highest creep of 441% was measured at 75% RH for gelatin-modified, in-plane compacted paper. 2-D Formability of Biopolymer-Modified, In-Plane Compacted Samples. A 2-D formability unit was employed to simulate the process conditions of thermoforming. The formability strains of modified paper samples were examined as a function of the forming temperatures. As evident from Figure

modified and compacted paper, the compacted paper exhibited about 190% higher creep compliance (Table 3). Table 3. Creep Properties of Compacted Paper in the Presence and Absence of Gelatin after Multiple Creep/ Recovery Cycles (5 × 120 min Cycles with Stress of 5 MPa and 60 min of Creep Recovery) as a Function of Dynamic Changes of Humidity from 10% to 90% RH and a Constant Temperature of 25 °Ca property creep compliance (μm2/N) recoverable compliance (μm2/N) relaxation ability strain (%) strain recovery (%) creep (%)

creep compliance (μm2/N) recoverable compliance (μm2/N) relaxation ability strain (%) strain recovery (%) creep (%) a

cycle 1, RH 10% 6826

cycle 2, RH 30%

cycle 3, RH 50%

Compaction 4825 5559

cycle 4, RH 70%

cycle 5, RH 90%

8506

12388

2171

2034

1941

1796

1800

0.32 4.12 31.81 86.33

0.42 2.85 42.15 137.9

0.35 3.23 34.92 194.2

0.21 4.80 21.12 296.7

0.15 6.65 14.53 129.4

Gel 4% + Compaction 2365 2787 4008

8860

16755

1270

1397

1574

1877

2109

0.54 1.32 53.69 124.6

0.50 1.54 50.11 164.1

0.39 2.19 39.27 247.1

0.21 4.69 21.18 441.3

0.13 8.34 12.59 158.1

Standard deviation is less than 5%.

Moreover, the relaxation ability (calculated as the ratio of recovery and creep compliance) of samples during five creep cycles was assessed. The maximum value for the relaxation ability was unity. The obtained data indicated that, compared to the compacted paper (without gelatin), the gelatin-modified structure, after 60 min relaxation, had ∼22% lower relaxation ability. Moreover, the application of gelatin resulted in roughly 3-fold increase in MD stiffness and elastic modulus (Table 2), which in turn may have restricted the sample’s deformability 9121


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Figure 6. Top: (a) 2-D formability tester and (b) 2-D formability strain of modified papers at different temperatures. Reference: unmodified paper. Polylactic acid (PLA) latex, guar gum galactomannan (GG), and gelatin (GEL) were added by spraying before compaction. Biopolymer addition amount was 4 wt% with respect to dry fibers. Samples were conditioned overnight at 75% RH. Bottom: Photographs of samples manufactured by the press forming method with a fixed blank by using the sample “Gel 4% + Compaction 1+1” (c). The table shows average peak depths and corresponding material strain of produced rectangular trays as a function of different in-plane compaction methods. “2×” indicates compaction was performed twice; “1+1” indicates compaction was performed in both directions, first in MD followed by compaction in CD. Paper samples had a basis weight ∼220 g/m2 and were conditioned overnight at 75% RH. The female mould temperature during forming was 120 and 160 °C. Standard deviation is less than 5%.

improvement in strain may be ascribed to the gelatin−cellulose interaction, as previously discussed, and, in addition, to the fact that gelatin remained relatively stiff, regardless the water content in the sample (Figures 6b and S3b). It is also worth to mention that for most of the samples, 60 °C appears to be the temperature at which the maximum strain is achieved. 3-D Structures by Press Forming of BiopolymerModified, In-Plane Compacted Samples. The results presented thus far indicate that in-plane compaction is an attractive strategy toward improved paper extensibility. Moreover, gelatin was found to effectively improve both the strength and the extensibility of compacted paper. Therefore, gelatin treatment was combined with paper in-plane compaction to produce advanced 3-D shapes by direct press forming, which eliminated the pre-creasing step. It was found that the paper formability in fixed blank forming processes was mainly governed by the extensibility and tensile strength of paper. Therefore, the performance of the material was assessed by

6b, compaction was an effective strategy to improve the paper formability. A 2-fold increase in formability strain was observed (from ∼9% to ∼20%). PLA modification, however, was found to limit the formability strain of compacted paper. This may be explained by the poor interfacial adhesion between the PLA and moistened cellulose (samples were conditioned at 75% RH prior to testing). In addition, DMA trials performed at 75% RH revealed lower storage modulus for PLA-modified samples (Figure S3b). Modification of compacted paper with guar gum did not improve the formability strain, which was surprising considering the previously observed impact of GG on the mechanical properties of the treated paper. This observation may be due to the moisture sensitivity of cellulose−GG interactions, which in turn promote the initiation of a fracture (Figure 6b). Interestingly, the application of gelatin significantly improved the formability strain of compacted paper; values up to 3-fold higher than those of the non-compacted reference sample were recorded (from ∼9% to ∼27%). Such an 9122


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Figure 7. Proposed approach to improve paper extensibility/formability via combined mechanical and polymer treatments.

measuring the peak depth of rectangular trays that were fabricated with the systems (Figure 6). It is evident from Figure 6 that the highest peak depths and corresponding strains were achieved by using the biaxial compaction, 1+1 method. It is likely that the transversal compaction enhanced the formability of the paper due to the uniform distribution of the elongation forces. The poorer performance of compacted 2× samples can be explained by the insufficient CD extensibility, which led to premature failure of the sample. In addition, the samples containing gelatin displayed the highest potential for 3-D structures and impressive peak depth, and strain values were recorded. The optimal processing temperature and humidity were found to be 120 °C and 75% RH (moisture content of ∼9%), respectively. Moreover, the developed 3-D shapes (trays) presented smooth edges, Figure 6, which indicated that the material is able to sustain vacuum sealing or deposition of an additional barrier layer as a post processing steps toward preparation of packaging materials. Exploiting the Synergies toward Paper Extensibility/ Formability. To summarize, and to give perspective about the impact of our results, we consider the factors that contributed to the extensibility and formability of paper at different structural levels. Thus, a combined approach to improve paper extensibility is proposed, as illustrated in Figure 7. First, precursor wood fibers can be processed with a judicious combination of low and high consistency treatments under moderate temperatures. The long fibers (e.g., softwood kraft fibers) are preferred in this case since they tend to form stronger networks and a larger amount of deformations per fiber can be created during the high-consistency mechanical treatment. However, the proposed methods can be extended to short fibers as well. After forming the network, a subsequent inplane compaction can be conducted. It is expected that the tensile strength and stiffness are significantly reduced after this process. Following, the compacted paper can be treated with water-soluble biopolymers (e.g., gelatin) and dried without restraint. Here, the emphasis is to use cost-efficient, sustainable

approaches that use food-grade, biobased polymers instead of synthetic, acrylamide-base or otherwise conventional paper additives, which can be associated with certain toxicity concerns.40 The application of water-soluble, food-grade polymers facilitates the bonding between the fibers, which in turn improves the tensile strength and stiffness of in-plane compacted paper. Moreover, the polymers can be used to promote the paper shrinkage during the unrestrained drying, which further improves the paper extensibility. Finally, a proper softening of polymers during the forming step can be promoted by controlling the moisture and temperature, depending on the type of forming unit. The proposed approach contains a certain degree of flexibility; i.e., some of the treatment steps can be modified, interchanged, or even replaced. For example, water-soluble polymers may be replaced by thermoplastics. Moreover, certain chemicals may be applied before the in-plane compaction in order to enhance the paper extensibility, followed by the addition of strength additives after the compaction to further reinforce the structure. As presented here, well-designed chemical treatments before or after the in-plane compaction is a promising approach to produce strong and extensible fiber networks. Moreover, the proposed approach to improve fiber network extensibility/ formability can be implemented within the existing papermaking environments without addition of major capital costs.

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CONCLUSIONS We propose a combination of in-plane compression and biopolymer treatment for the preparation of extensible fiber networks. Compaction effectively contributed to network extensibility in the direction of compression. However, it led to significant reduction of tensile strength and stiffness. Moreover, extensibility in the direction across the compaction was impaired. Therefore, biaxial compaction (longitudinal followed by lateral compaction) was performed to reduce property anisotropy. Application of water-soluble polymers, guar gum and gelatin, resulted in further improvement of paper extensibility. Remarkably, the strength of the compacted paper 9123



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was also improved. For instance, the extensibility of paper increased from 8.5% for untreated sample to 14.0% for the compacted paper and 16.7% for the compacted paper with the addition of gelatin; likewise, the tensile strength index changed from ∼50 to 24 N·m/g and became ∼54 N·m/g for the compacted paper with the addition of gelatin. PLA, on the other hand, did not produce major benefits, possibly due to poor adhesion to cellulose. DMA experiments indicated superior stability of gelatin-loaded samples to temperature and humidity. The 2-D formability strain of compacted paper upon gelatin addition reached values as high as 27%. Finally, press forming experiments of gelatin-loaded, compacted paper evidenced 21 mm deep rectangular trays prepared with the fixed blank process, which corresponds to unprecedented material extensibility of ∼29%. The proposed approach provides an attractive platform for the preparation of advanced 3-D shapes using fiber-based, biodegradable materials. Furthermore, the processability of gelatin-treated, compacted paper is expected to allow other types of advanced forming processes. The proposed paper-based, thermoformable materials are biodegradable/compostable and will add to the efforts to grow the future circular economy.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02025. Figure S1, photographs showing the structure of unmodified and in-plane compacted paper; Figure S2, stress−strain curves of biopolymer-treated paper; Figure S3, temperature-dependent storage modulus of modified papers at 50% and 75% RH; Figure S4, storage modulus for modified papers after the relative humidity was changed stepwise from 30% to 90% RH (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: ilari.fi[email protected]. *E-mail: orlando.rojas@aalto.fi. ORCID

Alexey Khakalo: 0000-0001-7631-9606 Ilari Filpponen: 0000-0003-0538-6523 Orlando J. Rojas: 0000-0003-4036-4020 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was a part of the ACel program of the Finnish Bioeconomy Cluster (FIBIC LTD) under the Finnish Funding Agency for Technology and Innovation (TEKES). The authors also acknowledge support from the Academy of Finland’s Project “3D-manufacturing of novel biomaterials” under the Biofuture 2025 program and the Center of Excellence “Engineering of Biosynthetic Hybrid Materials Research”, HYBER (2014−2019). The work made use of the Aalto University Nanomicroscopy Center (Aalto-NMC). We are also thankful to Sami-Seppo Ovaska (Lappeenranta University of Technology LUT) for assisting in press forming experiments. Stora Enso is gratefully acknowledged for providing the wood fibers. 9124


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