Wood fiber-based packaging materials, as renewable materials, have growing market potential due to their sustainability. A new breakthrough in cellulose-based packaging requires some improvement in the mechanical properties of paper. Bleached softwood kraft pulp was mechanically treated, in two stages, using high- and lowconsistency refining, sequentially.
Carbohydrate Polymers 186 (2018) 411–419 Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol The effect of oxyalkylation and application of polymer dispersions on the thermoformability and extensibility of paper T ⁎ Jarmo Koukoa, , Harri Setäläb, Atsushi Tanakab, Alexey Khakalob, Jarmo Ropponenb, Elias Retulainena a b VTT Technical Research Centre of Finland Ltd, P.O Box 1603, FI-40101 Jyväskylä, Finland VTT Technical Research Centre of Finland Ltd, P.O Box 1000, FI-02044 VTT, Finland A R T I C L E I N F O A B S T R A C T Keywords: Biopolymer spraying Consumer packages Paper extensibility Starch acetate Strength of paper Thermoformable web Wood fiber-based packaging materials, as renewable materials, have growing market potential due to their sustainability A new breakthrough in cellulose-based packaging requires some improvement in the mechanical properties of paper Bleached softwood kraft pulp was mechanically treated, in two stages, using high- and lowconsistency refining, sequentially Chemical treatment of pulp using the oxyalkylation method was applied to modify a portion of fiber material, especially the fiber surface, and its compatibility with polymer dispersions including one carbohydrate polymer The results showed that the compatibility of the cellulosic fibers with some polymers could be improved with oxyalkylation By adjusting mechanical and chemical treatments, and the thermoforming conditions, the formability of paper was improved, but simultaneously the strength and stiffness decreased The results suggest that the formability of the paper is not a direct function of the extensibility of the applied polymer, but also depends on the fiber network structure and surface energy Introduction cellulosic fibers strong and stiff However, mechanical treatment at high consistency possibly combined with a low consistency refining phase has been shown to improve the elongation potential of paper (Khakalo, Vishtal, Retulainen, Filpponen, & Rojas, 2017; Sjöberg & Höglund, 2005; Zeng, Vishtal, Retulainen, Sivonen, & Fu, 2013) In this study, the influence of combined mechanical high consistency – low consistency treatment, chemical (oxyalkylation) treatment, and application of thermoplastic and carbohydrate polymer dispersions on the formability of bleached kraft softwood pulp, was investigated The objective of the treatments was to modify the bonding ability of the fiber surface and change the fiber shape and morphology in order to improve the elongation and bonding ability of the fibers Elongation of some thermoplastic polymers can reach 400–800% and therefore, it is reasonable to expect that the addition of such polymers to the fiber network will improve the formability of the paper (Waterhouse, 1976) Bio-based thermoplastic polymers are generally not hazardous to health and are also bio-degradable, which makes them suitable for use in food packages Challenges of polymer applications to the pulp suspension are low retention in the fiber network and insufficient adhesion to the fibers On the other hand, in cases where the polymer is applied on a formed fiber network, the retention is a less severe problem, but difficulties arise in the limited penetration of the polymer into the fiber network, and possibly in the limited adhesion Paper-based packaging materials, as renewable materials, have a growing market potential due to their sustainability However, the development of new packaging concepts requires improvement in the mechanical properties of paper High extensibility is one of these properties Highly extensible papers would have the potential to replace certain kinds of plastics used in packaging Formability can be defined as the ability of a material to deform without breaking However, formability is not a specific mechanical property, but can be regarded as a generic term for explaining how well paper deforms during a particular forming process (Vishtal, 2015) In this study, formability was mainly estimated on the basis of a 2D experimental test method that simulated the process conditions in a fixed blank thermoforming process (Vishtal & Retulainen, 2014) In the fixed blank process, the formability is determined by the extensibility and tensile strength of paper (Östlund, Borodulina, & Östlund, 2011; Vishtal, Hauptmann, Zelm, Majschak, & Retulainen, 2013) As yet, the fixed blank forming process has not been widely applied in industry for paperboard (Ford, Trott, Simms, & Hartmann, 2014; Vishtal 2015) Pulp fibers constitute the load-bearing components of paper Kraft pulp fibers primarily consist of cellulose and hemicellulose Cellulose is crystalline, strong and stiff material with low extensibility making ⁎ Corresponding author E-mail address: jarmo.kouko@vtt.fi (J Kouko) https://doi.org/10.1016/j.carbpol.2018.01.071 Received 12 October 2017; Received in revised form 12 January 2018; Accepted 20 January 2018 0144-8617/ © 2018 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/) Carbohydrate Polymers 186 (2018) 411–419 J Kouko et al Fig 13C CP/MAS NMR spectra of the H-substituted (A) and L-substituted (B) pulps and compatibility of the polymer with the fiber Therefore, fibers were also chemically modified by using etherification with butyl glycidyl ether, which produces a substituent with a longer side-chain, 3-butoxy2-hydroxypropyl This kind of substituent has been observed to improve the thermo-mechanical behavior of cellulose fibers (Zhang, Li, Li, Gibril, & Yu, 2014) and was also expected to improve the compatibility of the fiber surfaces with the applied polymers Several thermoplastic polymers were applied onto, and into, the prepared undried fiber networks and then the influence of the application on the formability of the paper was investigated Temperature is known to increase the elongation and decrease the tensile strength and tensile stiffness of paper (Back & Andersson, 1992; Kouko, Retulainen, & Kekko, 2014; Salmén & Back, 1980; Salmén, 1993) In this study, the influence of temperature on the formability of paper was measured using an experimental method that simulated the process conditions in thermoforming The influence of the moisture content of the paper was controlled, but not varied, though moisture is known to act as a softener, increasing the elongation and decreasing tensile strength and tensile stiffness (Andersson & Berkyto, 1951) The objective was to produce a thermoformable network composed of cellulosic fibers and bio-based thermoplastic polymers with high formability This study presents the results of laboratory scale methods to improve the extensibility of paper The extensibility of paper was tested with a tailor-made laboratory scale device that simulated process conditions in thermoforming 2.2 Oxyalkylation of the pulp Chemical modification, an oxyalkylation treatment, was applied to modify the fiber material, especially the fiber surface, to introduce substituents with longer side chains The treatment was carried out onto two levels of substitution in the alkali reaction conditions, using methods similar to those published by Tian, Ju, Zhang, Duan, and Dong (2015) This was done in order to modify the properties of the fibers and their compatibility with polymer dispersions 800 mL of deionized water and 850 mL of 90% aqueous tBuOH were added to a L glass reactor 500 g of pulp containing 182 g of cellulose (1.12 mol of anhydroglucose units, AGU) were added to the reactor, and then 160 mL of 10 M NaOH was added The reaction mixture was stirred overnight at 45 °C 400 mL (2.80 mol) or 926 mL (6.47 mol) of BGE was added for the preparation of the higher (H) or lower (L) substituted fiber samples The reaction mixture was again stirred overnight at 45 °C The reaction mixture was cooled down to room temperature and neutralized with 37% HCl The fibers were filtrated, then washed with L of 95% ethanol, L of 50% aqueous ethanol, L of 20% aqueous ethanol, and finally two times with L of deionized water A small portion of the products was freeze-dried for analytical purposes The degree of substitution (DS) of 3-butoxy-2-hydroxypropyl group was determined using 13C CP/MAS acquired with a 600 MHz Agilent NMR spectroscope The NMR spectra of H-substituted (DS 0.12) and L-substituted (DS 0.05) fiber samples are presented in Fig As a reference (R), the high and low consistency refined BSKP was also studied without the chemical treatments Investigated pulp samples are presented in Table S1 Experimental 2.1 Raw materials for the pulp preparation 2.3 Paper sheet preparation and spraying of the polymers Bleached softwood kraft pulp (BSKP) from a Finnish mill was used as the fiber raw material The BSKP was mechanically treated, in two sequential stages, using a high-consistency mechanical treatment (with a wing defibrator) and a low-consistency refining method (with a Valley beater), as it is known to improve the extensibility of paper (Khakalo et al., 2017; Sjöberg & Höglund, 2005; Zeng et al., 2013) Butyl glycidyl ether (95%, BGE), tert-butanol (99%, tBuOH), 37% hydrochloric acid solution, and sodium hydroxide solution (10 M, NaOH) were purchased from Sigma-Aldrich All chemicals were used as received The treated pulps were made into 60 g/m2 laboratory sheets using EN ISO 5269-3 Laboratory sheets were wet pressed at 350 kPa pressure (the EN ISO 5269-3) The selected polymer dispersions (see Table 1) were sprayed onto the top side of the wet sheets using a commercial, high-volume, lowpressure (HVLP) gravity feed air spray gun For spraying, a wet sheet was placed on a rigid plastic plate that was on a gravimetric scale The targeted amount of dispersion for a sheet was 20%, and it was adjusted by controlling the wet weight of the sheet after spraying based on the known consistency of the dispersion The targeted amount of a polymer 412 Carbohydrate Polymers 186 (2018) 411–419 J Kouko et al (PPC) at a 25wt% dispersion Miyoshi Oil & Fat co Ltd supplied the polylactic acid Landy-3000 emulsion (PLA) at 40wt% dispersion (the average particle size was μm) Sigma Aldrich’s gelatin (GL) from cold water fish skin (G7041) was diluted with water to a 2wt% dispersion Oriola Oy’s citric acid (CA) was obtained as a powder and diluted with water to a 40wt% solution The addition of gelatin has been found to improve paper formability (Khakalo et al., 2014), which was the motivation for using it with PLA (GL-PLA) On the other hand, citric acid is generally known to act as a softener for PLA and therefore it (PLA-CA) was expected to improve the extensibility of the polymer-impregnated fiber network Emerald Performance Materialsđ supplied the nitrile latex Nychemđ 1561 ì 604 (NLAT) as a 33wt% emulsion Pure water was used as a reference for the polymers All of the sprayed polymer dispersions and their application consistencies are presented in Table Table The sprayed polymers Polymer Abbreviation Polymer Name Consistency, % WREF PU-DL* PU-EPO PPC PLA GL-PLA Water (reference) Polyurethane (Impranil® DL 519) Polyurethane (Epotal® P 100) Polypropylene carbonate PLA (LANDY PL-3000) Gelatin (2% of dry wt.)+ PLA (18% of dry wt.) PLA + citric acid (4:1 mixture) Starch acetate Nitrile latex – 40 39 25 40 and 40 PLA-CA ST-AC NLAT 40 7.8 33 The dry weights of the sprayed polymer mixtures were equal to 20% of dry fiber amount, except gelatin was first sprayed equal to 2% and after that PLA equal to 18% of the dry fiber amount * Polymer sprayed onto the board samples (in the Supplementary material) 2.6 Dynamic mechanical analysis (DMA) of the polymers and papers Polymer film samples were made from the dispersions studied in order to determine their softening behavior Dynamic mechanical analysis (DMA) was performed using a DMA/SDTA 861 E Dynamic Mechanical Analyzer (Mettler-Toledo Inc.) at VTT The storage modulus and phase lag values were measured in shear mode as a function of temperature The temperature range in this investigation was from −40 to 120 °C The maximum of the loss modulus at the studied temperature range can be roughly regarded as a softening temperature of a tested material Paper samples were tested in tensile mode was then sprayed, as evenly as possible, onto the sheet Reference sheets were sprayed with an equivalent amount of pure water (WREF) The sprayed samples were set on a vacuum table covered with a fabric wire; a vacuum level of approximately 5–10 kPa was applied under the sample The aim of the vacuum treatment was to impregnate the polymers into the wet fiber network and obtain the potential advantages of not only a paper-polymer composite, but also of a fiberpolymer composite structure The sheets were dried, without restraint, in between two polymer wires with a gap of approximately millimeters The applied drying method enabled free shrinkage of the sheet structure during drying in the in-plane direction, while simultaneously preventing severe cockling and curling of the sheets The purpose of the unrestrained drying was to impose a high level of extensibility in the formability tests The obtained polymer amounts were estimated based on the basis weight of the dry paper samples 2.7 Mechanical tests of the papers The basis weight of the paper (the board sample described in the Appendices A1) samples was determined according to ISO 536:1995 The thickness was determined according to ISO 534:1998, and the density was determined based on the measured values of the basis weight and thickness The tensile strength and the strain at break were determined with a Lloyd tensile tester, in accordance with ISO 5270:1998 Paper samples were conditioned and all testing of the samples took place at a temperature of 23 °C and at 50% relative humidity 2.4 Preparation of starch acetate (carbohydrate) dispersion for spraying The acetylated potato starch acetate DS 2.8 was obtained from the VTT Technical Research Centre of Finland (Rajamäki, Finland) The TECOSA was prepared in a 3:2 ratio from triethyl citrate (TEC) and noctenyl succinate anhydride (OSA) TEC was purchased from Reilly Chemicals, Hautrage, Belgium and OSA from Pentagon Chemicals, Cumbria, USA All chemicals were used as received The aqueous starch acetate (carbohydrate) dispersion (ST-AC, see Table 1) was prepared as described by Mikkonen, Peltonen, Heikkilä, and Hamara (2000) 100.0 g of an acetylated potato starch (DS 2.8), Mowiol (12.0 g), TECOSA (80.0 g) and 12 g of water were melted at 100 °C for h, in a flask equipped with a mechanical stirrer Then, the temperature was adjusted to 95 °C for 1.5 h Stirring was continued for the next 1.5 h, while 40.0 g of water at 80 °C was slowly added The dispersion was allowed to cool afterwards to room temperature, while under continuous stirring The dispersion was allowed to stand overnight without stirring, after which the final dispersion was obtained at 61% solids content In order to enable lower viscosity for the spraying, the starch acetate (ST-AC) was diluted with water to a dispersion at 7.8 wt% consistency 2.8 2D formability test The formability strain and force of the paper samples were measured using a 2D-formability tester at VTT The measurement procedure set-up was described by Vishtal and Retulainen (2014) and illustrated in Fig S1 In this investigation, the velocity of the forming press was mm/s and the width of the paper sample was 20 mm The paper samples were set in the tester so that the sprayed surface was not in contact with the heated press The temperature of the paper samples with 60 g/m2 was measured with the lower infrared thermometer Paper sample temperatures during the formability tests were typically only 2–5 °C lower than the presented set temperature, which means there was only a minor temperature gradient through the paper thickness 2.9 Contact angle and surface energy tests Surface-free energy of papers made from modified pulp was determined from contact angle (CA) measurements with water, formamide, diiodomethane, and ethylene glycol The CA of sessile drops was determined using a KSV CAM200 optical contact angle goniometer (KSV Instruments) Surface-free energy parameters for the probing liquids used for the CA measurements are available in the literature (van Oss, Good, & Chaundhury, 1988) The CA values (average of three measurements) were then used to calculate the dispersive and polar contributions to the surface energy of the samples, according to the acid–base theory (Good, Girifalco, & Kraus, 1958; Good & Girifalco, 1960) 2.5 Description of the other sprayed polymers Two commercial polyurethane water dispersions were used in this study The Impranil® DL 519 polyurethane dispersion (PU-DL) was supplied by Bayer AG (now Covestro) as a 40wt% dispersion in water (the average particle size was 110 nm) The Epotal® P 100 Eco (PU-EPO) was kindly supplied by BASF SE as a 40wt% dispersion in water the average particle size distribution was below 100 nm Empower Materials supplied the polypropylene carbonate QPAC® 40 emulsion 413 Carbohydrate Polymers 186 (2018) 411–419 J Kouko et al Table The Softening of the polymers by DMA measurement Polymer abbreviation PU-DL PU-EPO PPC PLA ST-AC NLAT Polymer name Estimated softening temperature and value of the correspondent phase lag (tan δ), °C ® Polyurethane (Impranil DL 519) Polyurethane (Epotal® P 100) Polypropylene carbonate PLA (LANDY PL-3000) Starch acetate Nitrile latex Storage modulus, MPa °C δ, ° 23 °C 80 °C 78 60 39 74 45 38 21.8 27.5 58.5 19.3 39.7 45.0 53 86 147 130 ∼40 265 4.65 0.13 0.16 1.94 0.13 0.89 Table S2 The fiber properties were measured using an L&W STFI FiberMaster The chemical modification treatment slightly decreased the fiber length from 1.80 mm to 1.71 mm, increased the fiber curl and stiffness (see shape factor from 85.4 to 83 and bendability from 6.65 to 5.4, respectively) and increased the amount of fines fraction from 8.5% to 14% Both the shape factor and the amount of fines have the potential to increase fiber network elongation 2.10 Nip peeling test The delamination resistance of double layer handsheets glued together with selected polymers was measured using a nip peeling test (Tanaka, Kettunen, Niskanen, & Keitanniemi, 2000) Two synchronized rolls, 25 mm in radius, were attached to an ordinary tensile tester, which rotates the upper roll Adhesive tape was used at the beginning of the specimen to adhere the first 10–20 mm of the specimen to the rolls After the beginning, peeling proceeded without the tape The nip peeling test was performed for the three pulps (R, L and H), but only with the PU-DL and PU-EPO polymers PU was sprayed on a dry sheet and, after spraying, another identical sheet was gently set on the sprayed sheet and set under a metal plate The amount of sprayed PU was equal to a 40% wt of the dry weight of both of the laboratory sheets The samples were dried using a drum dryer at 70 °C 3.2 Polymer contents The polymer contents in the laboratory sheets are presented in Fig S2 In the L-substituted pulp samples, the polymer amount was systematically 6–8wt% higher than in the unmodified reference (R) or the H-substituted samples Therefore, the WREF (water sprayed reference) samples had a key role, when the different samples were compared The estimated polymer amounts were based on the mechanical properties of the paper samples that are presented in Table S3 Polymer amounts were very similar for the PU (polyurethane) and NLAT (nitrile latex) sheets Additionally, the polymer content was similar for all of the PLA sheets (PLA, GL-PLA and PLA-CA) The sheets with PPC (polypropylene carbonate) and ST-AC (starch acetate) had polymer amounts that were significantly lower in comparison to the other sheets 2.11 SEM imaging The SEM imaging was carried out with a Merlin FE-SEM (Carl Zeiss NTS GmbH, Germany) with gold sputter coating at 20 mA for 30 s First, the sample was attached onto an aluminium specimen stub with a double-sided carbon adhesive tape The imaging was performed using 1.5 keV electron energy using a secondary electron detector The image pixel resolution was 2048 × 1536 The SEM imaging was performed for the three pulps (R, L and H), but only with the water (reference) and starch acetate spraying, excluding the L-substituted sample with water spraying 3.3 Dynamic mechanical analysis (DMA) of the polymers The films made of the applied polymers were studied using DMA in shear mode Increasing temperature softened the polymers and caused significant changes in the storage modulus, loss modulus and phase lag values The estimated softening temperature with the corresponding phase lag and storage modulus values at 23 °C and 80 °C are presented in Table The storage modulus at 80 °C was expected to have a connection with the behavior of the polymers in paper samples during the thermoforming process The observed softening temperatures of the polymers were between 38 °C and 78 °C and the observed phase lag values were between 19.3° and 61.7° As is well-known, a perfectly elastic solid material and a purely viscous fluid have phase lags 0° and 90°, respectively In case of a gel, the phase lag should be less than 45° Polypropylene carbonate (PPC) and PU-EPO (at 120 °C) had the highest phase lags (approximately 60°) indicating that those polymers can reach the most fluid-like behavior of the studied polymers The fluidization of the polymer by temperature may be very important for the sliding of individual fiber contacts and overall flexibility of the fiber network during the thermoforming A DMA test was also performed on the pulp samples, but the oxyalkylation treatment had no effect on the log-linear decay of the storage modulus, and there was no maximum for the phase lag Results and discussion 3.1 Chemical modification of pulp fibers The reaction efficiency of oxyalkylation reaction was rather low (only approximately 2%) and DSes were only 0.05 (L) and 0.12 (H) However, the reaction efficiency and DS were similar to results reported earlier When Vehviläinen et al (2015) derivatized cellulose fibers using allyl glycidyl ether under similar heterogeneous reaction conditions and Qi, Liebert, & Heinze (2012) in homogeneous reaction conditions like in NaOH/urea, the reaction efficiencies were only 1–3% and DSallyl were approximately 0.2 They also observed that when DSallyl was 0.22–0.29, the cellulose derivatives were only swelling and when DSallyl was above 0.50 they were already soluble in water Nishimura, Donkai, & Miyamoto (1997) have prepared similar 3-butoxy-2-hydroxypropyl cellulose derivatives and they have also observed good solubility in water when DS was sufficiently higher such as 0.4-1.0 Due to those results, the target of DS in this study was under 0.2 to keep the product in a fibrous form Because both of the DSes were rather low (< 0.15), the modified cellulose products L and H were still in a fibrous form However, already this rather light chemical modification and low-level DSs can, evidently, change fiber properties and, for example, to improve their compatibility with other polymeric materials The fiber properties of the chemically treated pulps are presented in 3.4 Surface energy of the pulps The surface energy of the pulps was expected to affect the fibers and their bonding ability and their compatibility with the polymers The 414 Carbohydrate Polymers 186 (2018) 411–419 J Kouko et al test results, as the contribution ratio was around 30% (see Frigon and Mathews (1997) for contribution ratio) This means that the systematically better strain at break of L-substituted samples has been partly caused by the higher polymer level in the sheets It is known that the shape factor of fibers and fines content influence the elongation of paper In this investigation, the shape factor and fines content were changed with the chemical modification of the pulp (see Table S2.), which could be studied by comparing the WREF samples However, strain at break of the R, L and H pulps did not seem to have the correlation with the fines content and shape factor The WREF of the H-substituted pulp had the lowest elongation, which indicates that the chemical modifications seem to contribute to the formability of the paper at room temperature The tensile index and elastic modulus were clearly decreased with increased oxyalkylation levels (presented in Figs and S3, respectively) A simple regression analysis (independent variables were pulp type, chemical type and chemical amount) showed that the pulp type, alone, explained around 70% of the statistical variation of the tensile index and elastic modulus Probably the reason for the reduction was the reduced strength of inter-fiber bonding For the formability potential of paper, the importance of the tensile index, and especially the elastic modulus, can be regarded as secondary, compared to strain at break The tension-strain curves of the WREF paper samples are presented in Fig The presented curves are repetitions that closely represent the measured averages of the strain at break and tensile index The tensionstrain curves of the sprayed L-substituted pulp sheets are presented in Fig S4 The samples containing PLA generally showed good tension levels, compared to the average performance of the samples Moreover, incorporation of only 2wt% of gelatin (sample L-substituted GL-PLA) leads to even further improvement of mechanical properties and extensibility Beneficial action of gelatin towards paper formability improvement was described in more detail in our previous communication (Khakalo et al., 2014) Table The surface energy of the pulps Pulp sample γLW γ+ γ− γAB γS Reference L-substituted H-substituted 42.7 28.4 23.2 0.1 0.4 4.2 50.3 0.3 3.5 0.7 0.5 46.2 29.1 23.7 γLW – dispersive component (hydrophobic interactions) γAB – polar component (hydrophilic interactions) γS – total surface energy surface energies of the studied pulps gave a clear indication that the surface character of the modified fibers had been changed into direction of a higher hydrophobicity as presented in Table The corresponding polar and dispersive contributions to the surface energy shown in Table indicate that the major change in the fiber surfaces was associated with an almost total elimination of the polar contribution, although the dispersive counterpart was also critically reduced These results indicate that the oxyalkylated fibers may have better compatibility with thermoplastic polymers 3.5 Tensile properties at standard test room conditions Fig shows that the lower (L-substituted) oxyalkylation level yielded, on average, approximately 12% strain at break (i.e., strain at the maximum load), which was the best average among the studied pulps (R, L, and H) The unmodified reference (R) and the higher (Hsubstituted) oxyalkylation samples had an average strain at break of approximately 9% However, the water sprayed (WREF) unmodified reference (R) pulp sample clearly had a higher strain at break compared to the WREF of the H-substituted sample, which indicates that a high level of oxyalkylation is not beneficial for extensibility and formability at room temperature Strain at break of the WREF unmodified reference (R) pulp and modified L-substituted pulp was on the same level as the best polymers of their groups On the other hand, the starch acetate (ST-AC), which is an agro-based carbohydrate polymer, had a poor strain at break, compared to the other trial points, in all three cases However, a simple linear regression analysis (the independent variables were the pulp type, chemical type and chemical amount) showed that the amount and type of the sprayed chemical had a significant influence on the tensile 3.6 2D formability strain 2D formability strain was studied at four different press temperatures (23 °C, 60 °C, 90 °C and 120 °C) Figs 5, S5 and S6 show that the maximum formability strain was typically obtained at 60 °C temperature, whereas 120 °C clearly seemed to be above the optimal temperature Fig Strain at break of the paper samples 415 Carbohydrate Polymers 186 (2018) 411–419 J Kouko et al Fig Tensile index of the paper samples The water sprayed reference samples (WREF) of the unmodified (R) pulp and modified L-substituted (oxyalkylated) samples had a clearly higher formability strain compared to the H-substituted (oxyalkylated) sample, which once again indicated that high oxyalkylation level may not be beneficial for high formability The influence of polymer spraying on the 2D formability strain of the chemically unmodified reference (R) pulp was generally quite negligible Only PU-EP and NLAT seemed to improve the 2D formability strain of the reference pulp On the other hand, the three PLA trial points, and especially the PPC and ST-AC (the only carbohydrate polymer) trial points, had a significantly lower amount of polymers than the PUs and NLAT Several polymers improved the 2D formability strain of the L-substituted (oxyalkylated) samples, compared to the water sprayed WREF sample Both of the PUs (PU-DL and PU-EP), all three PLA mixtures (PLA, GL-PLA and PLA-CA) and the nitrile latex (NLAT) improved the 2D formability strain However, the PLA sheets contained 4–6% wt less polymer than the PUs and NLAT sheets, and therefore, PLA and PU may have similar potential for improving the formability of paper Fig Tension-strain curves of the WREF paper samples, of the investigated pulps In the legend, the samples are presented in the order that matches the order of magnitude at 6% strain Fig Formability strain of the paper samples with L-substituted (oxyalkylated) pulp 416 Carbohydrate Polymers 186 (2018) 411–419 J Kouko et al with increasing temperature (23 °C, 60 °C, 90 °C and 120 °C) Maximum formability forces were measured at the lowest 23 °C temperature, while the 2D formability forces were 25–75% of the maximum value at 120 °C The maximum formability strain was obtained at around 60 °C and therefore, the 2D formability test results were compared at that temperature in the following analysis The influence of the sprayed polymers was minor on the 2D formability force of the reference (R) samples However, the average level (150 N at 60 °C), of the reference (R) sample 2D formability force, was higher compared to the L- and H-substituted (oxyalkylated) samples In the case of the L-substituted and H-substituted samples, the sprayed polymers increased the formability force The average 2D formability forces were 115 N and 60 N for the L- and H-substituted pulps at 60 °C, respectively Both temperature and polymers had a major influence on the 2D formability force The 2D formability test results showed that by choosing the treatment method, the chemical composition drying method and thermoforming conditions, the extensibility of a dried BSKP sample reached the 16% level The results also indicated that the formability of the paper is not a direct function of the extensibility of the applied polymer Fig Peeling energy of the PU sprayed samples Most of the sprayed polymers improved the 2D formability strain of H-substituted samples However, the average 2D formability strain of the H-substituted samples was significantly lower compared to the reference (R) and the L-substituted samples Figs S7–S9 show that the 2D formability force strongly decreased Fig SEM images of untreated (R), lower (L) and higher (H) substituted (oxyalkylated) BSKP samples Images a–b present water sprayed (WREF) and c-e present starch acetate sprayed (ST-AC) samples 417 Carbohydrate Polymers 186 (2018) 411–419 J Kouko et al 3.7 Delamination resistance Acknowledgements The compatibility of the pulps with polyurethane was studied by testing the delamination resistance of two-ply handsheets bonded together with polyurethane The results presented in Fig show that the peeling energy of the H-substituted (oxyalkylated) pulp was clearly lower than that of the chemically unmodified reference (R) and the Lsubstituted (oxyalkylated) pulp The L-substituted pulp had the highest peeling energy The difference between the PU grades was minor, compared to the standard deviation of the measurements The results, once again, indicated that the L-substitution treatment improved the compatibility of the PU and the fibers, i.e., adhesion, while an excessively heavy chemical treatment, was unfavorable for the formability of the paper This work was a part of the ACel program of the Finnish Bioeconomy Cluster CLIC Innovation Funding by the Finnish Funding Agency for Technology and Innovation (TEKES) is gratefully acknowledged Completion of the publication was supported by the ExtBioNet project funded by the Academy of Finland Dr Oleg Timofeev is thanked for producing the polymer film samples and Ms Mirja Nygård for conducting the DMA measurements from them Appendix A Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.carbpol.2018.01.071 References 3.8 SEM imaging Andersson, O., & Berkyto, E (1951) Some factors affecting the stress–strain characteristics of paper Svensk Pappers Tidning, 54(13), 437–444 Back, E L., & 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Technology and Science, 27(9), 677–691 http://dx.doi.org/10.1002/pts 2056 wileyonlinelibrary.com SEM images of BSKP sheet surfaces are presented in Fig 7a–e Fig 7c–e showed that the starch acetate did not penetrate the cell wall of fibers, but rather accumulated on the surface Also, starch seems to be more prone to spread onto the fiber surface of the reference (DS 0) sample (Fig 7c), whereas no spreading of starch acetate was observed onto L- (DS 0.05) or H-substituted (DS 0.12) (oxyalkylated) samples (Fig 7d–e, respectively) The behavior of starch acetate (ST-AC) in these SEM images can probably be explained by the surface energy measurements, which showed that hydrophobicity of the sheet surface was increased with oxyalkylation, i.e., the substitution DS level Consistency of the starch acetate in spraying was 7.8%, whereas the amount of water was over 90% As a result of hydrophobicity of the BSKP sheet surface and high water content, the starch acetate was probably forced to form from spherical particles after drying The starch acetate was slightly able to spread on the untreated BSKP surface (Fig 7c) In case of the starch acetate, this observed behavior probably explains the inability of starch acetate to improve formability of BSKP sheets The compatibility of the other polymers with BSKP sheet may also have been influenced by the high water content of the spray dilutions Conclusions Bleached softwood kraft pulp was mechanically treated, in two stages, using high- and low-consistency refining, sequentially Chemical treatment of pulp using the oxyalkylation method was applied in order to modify the fiber material, especially the fiber surface, and its compatibility with polymer dispersions including one carbohydrate polymer Oxyalkylation of the BSKP to the lower substitution (DS 0.05) level increases hydrofobication and also improves the extensibility of the fiber network, but not the tensile strength The chemical oxyalkylation modification increases surface energy of the fiber network, which can improve the compatibility of fiber material and polymers On the other hand, high hydrophobicity of a surface can prevent penetration of polymer molecules in the sheet, in case high water content of a polymer dilutes In this investigation, the extensibility and formability of the BSKP fiber network was increased by an average of 4% with the polymer additions via the use of the spray Fiber network properties dominate the mechanical behavior of the structure unless a polymer penetrates into fiber network or forms a uniform phase The investigated agro-based carbohydrate polymer, starch acetate, could not improve extensibility or strength of the BSKP sheet, because of the poor penetration into the fiber network The results show that extensibility of the fiber network has an optimum temperature, which is emphasized when polymers are present Polymer softening and adhesion play a role in the extensibility 418 Carbohydrate Polymers 186 (2018) 411–419 J Kouko et al 8(1), 472–486 Zhang, Y., Li, H., Li, X., Gibril, M E., & d Yu, M (2014) Chemical modification of cellulose by in situ reactive extrusion in ionic liquid Carbohydrate Polymers, 99, 126–131 http://dx.doi.org/10.1016/j.carbpol.2013.07.084 Östlund, M., Borodulina, S., & Östlund, S (2011) Influence of paperboard structure and processing conditions on forming of complex paperboard structures Packaging Technology and Science, 24(6), 331–341 http://dx.doi.org/10.1002/pts.942 Vishtal, A (2015) Formability of paper and its improvement Doctoral thesis Tampere University of Technology VTT Technical Research Centre of Finland VTT Science 94 108 p Waterhouse, J (1976) The deformation characteristics of polymer reinforced fiber networks Tappi, 59(7), 106–109 Zeng, X., Vishtal, A., Retulainen, E., Sivonen, E., & Fu, S (2013) The elongation potential of paper—how should fibres be deformed to make paper extensible? BioResources, 419 ... applied onto, and into, the prepared undried fiber networks and then the influence of the application on the formability of the paper was investigated Temperature is known to increase the elongation and. .. have a connection with the behavior of the polymers in paper samples during the thermoforming process The observed softening temperatures of the polymers were between 38 °C and 78 °C and the observed... with polymer dispersions including one carbohydrate polymer Oxyalkylation of the BSKP to the lower substitution (DS 0.05) level increases hydrofobication and also improves the extensibility of the