Production and Characterization of Laminates of Paper and Cellulose

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Production and characterization of laminates of paper and cellulose nanofibrils Niloofar Yousefi Shivyari, Mehdi Tajvidi, Douglas W. Bousfield, and Douglas J. Gardner ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07655 • Publication Date (Web): 02 Sep 2016 Downloaded from http://pubs.acs.org on September 11, 2016

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ACS Applied Materials & Interfaces

Production and characterization of laminates of paper and cellulose nanofibrils 1

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Niloofar Yousefi Shivyari , Mehdi Tajvidi , Douglas W. Bousfield , Douglas J. Gardner

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1

School of Forest Resources and Advanced Structures and Composites Center, University of Maine, Orono, Maine, USA 2

Department of Chemical and Biological Engineering, University of Maine, Orono, Maine, USA

KEYWORDS: cellulose nanofibrils, paper, mechanical properties, laminate, binder ABSTRACT: A novel laminate system comprising of sheets of paper bound together using cellulose nanofibrils (CNF) is manufactured and characterized. Bonding properties of CNF were first confirmed through a series of peeling tests. Composite laminates were manufactured from sheets of paper bonded together using CNF at two different consistencies, press times, and press temperatures. Mechanical properties of the laminates in tension and bending were characterized and the results were statistically analyzed. Elastic modulus and strength results met or exceeded those of a short glass fiber reinforced polypropylene and various natural fiber-filled polypropylene composites as well as some wood and paper based laminates. Stiffness properties, assuming perfect bonding within the laminates, were successfully estimated through a classical laminated plate theory (CLPT) with only 2-10% variation compared to experimental results. Laminates, together with CNF peeled surfaces were observed and qualitatively analyzed by SEM imaging. Physical properties, namely water absorption and thickness swelling were measured. Swelling was controlled by the addition of a small percentage of a crosslinking additive. INTRODUCTION: In the past few decades, a growing environmental awareness towards “green” materials together with depletion of fossil fuels, have led to various attempts to produce alternative bio-based products 1-3. The automotive industry, in particular, has a primary motivation in the advancement of green composites, as these nature-based products can provide alternative solutions to various disadvantages of conventional materials currently used in the industry 1, 4. Bio-based composites can offer mechanical strength, lower weight and cost, ecological sustainability, low energy requirements for production, safe end of life disposal and carbon dioxide neutrality that have made them particularly attractive to many industries 1, 4-5. Therefore, several studies have focused on introducing renewable materials into automotive parts 4-5, mostly by incorporating natural fibers into the formulation of commodity non-degradable polymers like polypropylene, either as a filler or as reinforcement6-10. Similarly, the packaging industry has received considerable attention from researchers by developing entirely biodegradable packaging materials obtained from sustainable resources. A noticeable number of recently published papers in the field of green materials are concentrated on development of renewable packaging systems with emphasis on potential application in food packaging, both rigid and flexible. These packaging systems are produced by using biodegradable polymers (e.g., polylactic acid) or natural polymers (e.g., cellulose) 11-13 in various forms. Among all “green” materials, nanocellulose can be referred to as the “wonder” bio-material attributable to its

many exceptional properties. It is simply derived from cellulose, the most abundant bio-polymer on earth, by mechanical or chemical disintegration of the fibers into nano-scale dimensions 1-2. The resulting nanomaterial is called cellulose nanocrystals (CNC) if the disintegration is conducted chemically (lower aspect ratio) or cellulose nanofibrils (CNF) if disintegration is done mechanically (higher aspect ratio) 2, 14. Being renewable, sustainable, carbon neutral, non-petroleum based, non-hazardous, low cost, light, and having high aspect ratio and specific strength and stiffness, nanocellulose is widely regarded as a promising nanomaterial to find applications in the production of composites and laminates 1-5, 11-13, 15-28 nanopapers and films14, 25, 29-31 and foams and aerogels 15, 21, 32. In one particular study, cellulose nanofibrils were added to a papermaking pulp suspension, which resulted in an enhancement of the strength properties of the resulted paper without simultaneously deteriorating the drainage33. In another study27, CNF films were laminated onto wood flakes resulting in 200% and 300% increases in transverse young modulus and tensile strength of wood flake laminates, respectively. The traditional approach towards using cellulose nanomaterials as additives in the formulation of polymer composites requires a drying step that with current technologies available causes the loss of the nano-dimensions of cellulose nanomaterials 34 and difficulties with redispersion. Drying also imposes a considerable additional energy cost. The water-based nature of nanocellulose in the form of low consistency suspensions or gels coupled with high energy requirements of drying these materials

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provide enough reasons to expect that the optimal applications at least energy-wise, can be found where water does not have to be removed prior to the production of the final product 5. For this reason, considerable attention has been focused on the applications of these materials in aqueous systems such as cement reinforcement 26-28, 35-36, paperboard strengthening applications 21-22, 24, as well as applications in paper coatings for print quality improvement 23. Wet applications of nanocellulose and in particular CNF provide a closer to market approach towards commercialization of these highly interesting materials. In this study, we report on the production of high density composite laminates of paper sheets bound together using CNF as the sole binder. Given the exceptionally high mechanical properties (strength and stiffness) of CNF 26 and excellent hydrogen bonding between cellulose nanoparticles and pulp fibers 23, it is possible to envision a laminate system in which thin laminae of cellulose nanoparticles are used to bind sheets of paper to form a laminate with desired properties. As the major fiber direction of paper is in the direction the web of paper travels through the paper machine, machine made paper sheets have different mechanical properties in machine direction (MD) and cross direction (CD) making them an orthotropic material 27. Taking advantage of this, cross directional composite laminates were produced and evaluated for physical and mechanical properties. Such laminate systems, hereafter called “Cellubound” 5 , can be engineered into the desired physical and mechanical property requirements and will find applications as alternative packaging systems, or interior automotive parts, which can be 100% renewable, biodegradable and environmentally friendly. The paper sheets can also be sourced from waste paper streams, which will provide additional motivation for the industry to adopt an even greener product. Although binding effects of cellulose nanomaterials have been previously reported in very limited publications 37-39, no reports are available where laminated composites based on paper sheets and CNF are produced nor such products have been evaluated. A recent publication of our research group has introduced the concept of Cellubound production 5. Detailed information on production, properties and adhesion mechanism are reported here. This study provides new insights into the green-inspired materials development and opens avenues into further investigations on such laminated systems. EXPERIMENTAL SECTION: Materials: Cellulose nanofibrils (CNFs), originally obtained as a 3% solids suspension, were produced through mechanical refining of softwood bleached pulp by a low energy consumption method on a pilot-scale by the Process Development Center of the University of Maine. Pulp is fed through a hopper into the refiner system where it is

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ground by the mechanical attrition of refiner plates in a continuous circulating flow until a fine fraction of over 95% is achieved. No chemical treatments are carried out leading to surface charge-free CNF 5. The length scale of the nanofibrils is in the range of hundreds of nanometers whereas the widths can range anywhere from a few nanometers to 100 nanometers, giving the material a spaghetti-like look with very high aspect ratio. The 3% CNF suspension was diluted to 0.5% and 1% suspensions by simply adding water and then mechanical stirring until a homogenous suspension was obtained. Paper sheets used in the laminates were 75 g/m2 white, acid free recycled copy paper, a product of Boise Inc. (Boise, Idaho). Polycup™, which is formaldehyde free, water based crosslinking resin (polyamide epichlorohydrin), a product of Hercules Incorporation (Wilmington, Delaware), was used in very small proportions to decrease hydrophilic properties (e.g. water absorption) of some of the composites. Two commercial paper adhesives, solid and liquid, both products of Elmer’s Products Inc. (Westerville, Ohio) were used as reference bonding material in the peeling test. Preparation of nano-laminates: The production procedure is demonstrated in Figure 1. Strips of paper measuring 7 cm by 27 cm cut along the MD direction were dipped into a CNF bath for an optimized time of 1 minute, and then the two strips were placed on top of each other and folded cross-directionally. The dipping time was optimized by conducting a series of preliminary experiments by dipping layers of paper in a CNF bath of constant consistency for 3 different periods of 1, 2, and 3 minutes and then drying them in an oven to calculate % of CNF absorbed by paper. Time did not play a role in CNF absorption as long as the papers were totally wet and therefore the minimum time was selected. To reach the target density and thickness, folding was continued until a 25 layer laminate was obtained. After dewatering in cold press, the wet laminates were pressed in a hot press (Carver Inc. Model 4886, Wabash Indiana), at two different temperatures as listed in Table 1. This table summarizes the laminate production variables. CNF content of the bath, press temperature and press time were the processing variables whereas press pressure was fixed at 10 MPa, which is applicable in laboratory and also not too high to hinder scalability. Table 1. Formulations and processing variables Sample code 180-0.5-3 180-0.5-5 200-0.5-3 200-0.5-5 180-1-3 180-1-5 200-1-3 200-1-5

Temperature (°C) 180 180 200 200 180 180 200 200

CNF % 0.5 0.5 0.5 0.5 1 1 1 1

Press Time (min) 3 5 3 5 3 5 3 5

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Figure 1. Laminate manufacturing procedure: 1 and 2. Cutting papers into strips 3. Mixing the desired amount CNF suspension with water (CNF bath) 4a. Cross-folding method: dipping strips of paper into CNF bath then folding them on top of each other cross-directionally to the desired thickness. 4b. Rolling method: dipping strips of paper into CNF bath then rolling them around a polyethylene cylinder of the desired diameter. 5a. Folding method resulting in a rectangular shaped wet cross-plied laminate 5b. Collapsing the roll into a rectangular shape 6. Dewatering the laminate by cold pressing between wire meshes and steel plates 7. Hot pressing for the desired time 8. Cutting into specimens using a table saw 9.Specimens ready to be tested. chine (Instron 5966, 10 KN capacity, Norwood, MassachuThe target thickness and density of nano-laminates were setts). The peeling test is normally used to evaluate peel 1.6 mm and 1.15 g/cm3, respectively. Three laminates of strength of adhesives 40 (Figure 3). each formulation were made to verify repeatability of the To prepare the samples for the peeling test, three differresults. There can be a lot of flexibility in the production ent concentrations of CNF, and also two types of commethod but our proposed production procedures for the laminates are depicted in Figure 1. mercial paper adhesives were applied to the same copy Preparation of samples for mechanical modeling and addipaper used to make the laminated composites. Strips of tive effect evaluation: Using the rolling method described paper (15 cm in length) cut along the MD direction were dipped into 0.5%, 1% CNF, 3% CNF suspensions for 1 miin Figure 1, three 15- layer unidirectional paper plies were made and then assembled on top of each other at 0°, 90° nute to let the CNF deposit on the surfaces evenly. Two and 0° to form a 3-ply cross laminated composite (Figure types of commercial solid and liquid paper adhesives were used as reference. For paper adhesive samples, a layer of 2). Samples were made under similar conditions of 1% CNF consistency, 1 minute soaking time, 5 minutes hot the adhesive was applied evenly on both strips using a press time and 10 MPa press pressure. The samples with glass slide. additive had 3 pph (based on 1% CNF suspension) solid A piece of aluminum foil (10 cm in length) was placed based Polycup in their soaking bath. between the two strips of paper to preclude the papers from sticking to each other, remaining only 5 cm of adhered surfaces. A weight (1.5 kg) was placed on the strips while they were being air dried in the lab to result in partially adhered specimens. The peeling test process is schematically shown in Figure 3. The same procedure was done for paper strips bonded with the adhesives.

Figure 2. A three- ply cross laminated composite depiction (0°, 90°,0°). Each unidirectional ply consisted of 15 layers and was produced via the rolling method. Peeling test: Bonding property of CNF was investigated through a 180° peeling test, using an Instron testing ma-

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Figure 3. Peeling test procedure. 1. Separately dipping two paper strips in CNF bath with desired consistencies. 2. Peeling test sample preparation. 3. Representation of bonding phenomenon. 4 & 5. Peeling test. Testing was carried out with an Instron testing machine at a cross head motion rate of 5 cm/min and was terminated when the two layers of paper were fully separated. Load required to peel the samples over the width of the samples was recorded. Mechanical Testing: Tension tests and 3-point bending tests were conducted according to ASTM D638-14 and ASTM D790-15 41-42, respectively with the same Instron testing machine used for the peeling tests. For the tension test, the nominal specimen size was 80*13*1.6 mm3 and the cross head speed was 5 mm/min. To see if the results are repeatable, 3 replicates (two specimens each) were tested for each formulation (total of 6). An extensometer was mounted on tensile specimens to measure strain values over a gauge length of 2.54 cm. For the bending tests, specimen size of 80*13*1.6 and cross head speed of 1.27 mm/min was used. Total number of repeats for each formulation was three and in each repeat, two specimen were tested. Density values were calculated by measuring the weight of each specimen with a laboratory scale and dividing it by its volume, which was calculated after dimensions were measured by a digital caliper. Physical Testing: A series of long term water absorption and thickness swelling tests, based on ASTM D-570 specification 43 with modification were carried out on specimens measuring 20*13*1.6 mm to evaluate the effect of exposure to water. For samples with Polycup, the same procedure was repeated on samples produced under the same conditions, with and without the additive, to compare the effect of the additive on controlling water absorption and thickness swelling. Testing was terminated after specimens reached saturation. Statistical Analysis: A balanced factorial test based on completely randomized design was carried out using IBM SPSS Statistics 22 (IBM, North Castle, New York) on tensile elastic modulus, tensile strength, bending elastic modulus and bending strength to verify if processing var-

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iables (temperature, time, and CNF content) had statistically significant effects on mechanical properties. Mathematical modeling: A 3-ply cross laminated composite was produced as mentioned in previous sections (Figure 2). Using the rolling method, a 45 layered laminate consisting of three 15-layer unidirectional plies was made. The plies were made individually by rolling strips of paper around a polyethylene cylinder and assembleing on top of each other at 0°, 90° and 0° to form a 3-ply cross laminated composite, which was then hot pressed. Samples were made under similar processing conditions of 1% CNF consistency, 1 minute dipping time, 5 minutes press time and 10 MPa pressure. Tensile and bending moduli of one single ply and the laminate were experimentally measured via Instron and then modeled with the classical laminated plated theory (CLPT) 44, using Wolfram Mathematica (Version 10.3) software. The CLPT is used here for two purposes: First, it assumes perfect bonding between laminae and therefore should the theoretical values obtained from the model agree with experimental values, good bonding that results in adequate stress transfer between layers can be concluded 44. Second, we intended to show that the properties of the final laminate can be easily predicted having the information about the mechanical properties of single layers. This will allow engineering mechanical properties to meet the final application requirements.45 For this purpose, the CNF-paper laminate was assumed as an equivalent homogeneous anisotropic solid because of the symmetric structure. Properties of a single ply, generated under similar conditions to the 3-ply laminate were measured and used as the input to compute the reduced stiffness using Eq. 1.

. . . 0

.

.

.

0

0 0 Gxy

Eq. (1)

where Q is reduced stiffness matrix, Ex is the apparent elastic modulus in the x direction, Gxy is the shear modulus in the x-y plane, and νxy is the Poisson’s ratio measuring contraction in the y direction due to uniaxial loading in the x direction 46. The stress-strain relationship can also be expressed as apparent engineering moduli as: εxx

εyy εxy , ,

,

,

σxx σyy . σxy

Eq. (2)

where ηzxy is the coefficient of mutual influence of the first kind which characterizes shear in the x-y plane caused by normal stress in the z direction. For the laminated composite modeled in this paper, η is equal to zero

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as the composite is symmetric 45. Also the transformation matrix is defined in Eq.(3). C a"# S a"# C a". S a"

S a"# C a"# S a". C a"

2. S a". C a" 2. S a". C a"& C a"# S a"#

Eq. (3)

in which C(α) and S(α) are the cos and sin of angle α of the fibers (in this case paper) with respect to x axis. This matrix is used for transforming stresses and strains. For transforming the reduced stiffnesses value the below equation (Eq. 4) can be used 45-46:

Q(α)= [T(α)]-1 [Qij] [R] [T(α)]

Eq. (4)

where T-1 is the inverse T matrix, R is the Reuters matrix which is defined as follows 44-45: 1 ' (0 0

0 1 0

0 0* 2

Eq. (5)

The outputs, theoretical predictions of tensile and bending moduli of the laminate, were then calculated and compared with the experimental results obtained from bending and tension tests on the laminate. SEM imaging: The peeled surfaces of the peeling test specimens, along with the structure of the single layers of paper and laminates, with and without CNF, all coated by 15 nm of gold, were observed via a scanning electron microscope, AMRAY 1820 (AMRAY, Bedford, MA), at 10 kV. RESULTS AND DISCUSSION: Mechanical properties: Representative tensile and bending stress-strain curves of the laminates, for eight different formulations produced by the folding method are presented in Figure 4. Elastic modulus values were variable ranging from 2.3 to 3.6 GPa for bending and 7.3 to 9.0 GPa for tension. Strength values ranged from 27 to 43 MPa for bending and 42 to 52 for tension. Summary of average elastic modulus and strength values that are presented in Table 2, as well as the coefficients of variation, indicate that both moduli and strength values are slightly higher for samples with 0.5% CNF content compared to those

made with 1% CNF content. The effect of CNF content proved to be statistically significant using statistical tests. This could be attributed to better dispersion of CNFs at 0.5% concentration on the surface and their penetration into bulk of the paper leading to less agglomeration, which could cause stress concentration areas and also result in imperfect bonding between the layers. Based on the statistical test results, effects of time and temperature were not significant, however, a higher press temperature corresponded to a lower press time, meaning that when a higher temperature was used at a lower press time the properties were higher and vice versa. Another observation was the better consistency of tensile data for all samples compared to bending data (Figure 4) which can be attributed to existence of shear stresses in bending test which can result in partial delamination 46 in weakly bonded areas and thus result in more inconsistent curves for the bending properties. In tensile testing, no shear deformation exists therefore delamination does not occur. Delamination was more frequently observed for bending samples when 1% CNF content, lower press time, and lower press temperature were used. Results of the statistical analysis indicated that there were no significant effects of press time or temperature on mechanical properties. The only independent parameter with significant effect on properties was % CNF in the dipping bath suspension. A significant interaction was also observed between press time and temperature suggesting that either longer press time or higher temperature could be used to achieve optimum mechanical properties.

Table 2. Average mechanical properties of laminates (Coefficient of variation (%) in parenthesis) Sample code 180-0.5-3 180-0.5-5 200-0.5-3 200-0.5-5 180-1-3 180-1-5 200-1-3 200-1-5

BS(MPa)

BM(GPa)

TS(MPa)

TM(GPa)

33 (21) 34 (26) 39 (10) 36 (19) 28 (35) 38 (10) 36 (25) 29 (17)

3.0 (6) 3.1 (22) 3.4 (12) 3.6 (8) 2.3 (39) 3.3 (9) 3.1 (19) 2.9 (3.4)

52 (7) 50 (5) 48 (7.5) 50 (5) 43 (12) 45 (9) 48 (7.5) 49 (3.5)

8.6 (13) 8.9 (9) 7.8 (23) 8.0 (14) 7.3 (12) 8.5 (9) 8.0 (6) 8.0 (12)

*TS, Tensile strength; TM, Tensile elastic modulus; BS, Bending strength; BM, Bending elastic modulus.

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60

60

50

50

Bending Stress (MPa)

40 30 20 10

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180-1-3 180-1-5 200-1-3 200-1-5

40 30 20

180-0.5-3 180-0.5-5

10

200-0.5-3

0

200-0.5-5

0 0

2

4

6

% Strain

0

2

4

6

% Strain

Figure 4. Tensile (left) and bending (right) stress-strain curves for 8 paper/CNF laminates made under different press time, CNF consistency, and press temperature conditions

Strength and stiffness properties of the laminates are comparable to properties of many natural fiber filled polypropylene composites 6-10, other paper based 47-48 and CNF based laminates 49 as presented in Table 3, with bending strength values slightly lower for our laminates. Also, the highest tensile strength value (52 MPa) was comparable to that of a 25% short glass fiber filled and a 25% short carbon fiber filled polypropylene composite (51 MPa and 58 MPa, respectively); maximum tensile modulus value (9.0 GPa) was slightly higher than short glass fiber filled PP (8.8 GPa) but much lower than short carbon fiber filled PP (~15.0 GPa) 50. In another study47, continuous jute fibers were used to reinforce layers of thick unbleached Kraft paper and thin bleached Kraft paper, glued together with white corn flour glue. Maximum results showed tensile strength and normalized tensile modulus of 43 MPa and 1.2 GPa, respectively for thick unbleached Kraft paper. The tensile strength of 32 MPa and normalized tensile modulus of 1.8 GPa was reported for thin bleached Kraft paper. Both results were lower than the properties reported here. Table 3. Comparison of modulus and strength values with those reported in the literature Material CNF/CopyPaper (this study) KraftPaper/glue/JuteFiber 47 (0.05g) I6 PP/Sisal 20%/MAPP 7 PP/Nettle 20% 10

PP/Coir 45% II 8 PP/CB 10% 48 PP/CopyPaper/MAPP 48 PP/CopyPaper Soduim Alginate49 Film/CNF(10%)

TS* (MPa) 52

TM* (GPa) 9.0

BS* (MPa) 43

BM* (GPa) 3.6

43

1.8

--

--

38 31

4.2 3.1

55 55

3.7 2.5

48 30 80 64 30

1.0 0.9 6.1 5.3 1.4

51 ----

1.1 ----

*TS, Tensile strength; TM, Tensile elastic modulus; BS, Bending strength; BM, Bending

Bonding properties of CNF: Figure 5 presents the results for the peel strength values according to ASTM D1876-08. Peel strength is defined as the average load per unit width of bond line required to separate bonded materials when the angle of separation is 180° 40. Average peel strength values were 1.0, 1.7 and 2.2 N/cm for samples dipped into 0.5%, 1% and 3% CNF suspensions, respectively. With increasing the amount of CNF in the suspension from 0.5% to 1% which is still a small proportion, the peel strength increased nearly twice as much. Without using CNF as a binder, papers simply dipped in water showed no bonding strength, which confirms the bonding properties of CNF. The increasing trend in peel strength when CNF% is increased is in contrast with the results of mechanical testing where lower properties were obtained at higher CNF contents. The difference in the nature of peeling test with what happens in an actual bending test between laminate layers can explain this observation. The peeling test exerts forces perpendicular to the adhesion plane resulting in separation of the bonded layers in z direction whereas in bending, the interlaminar shear forces act parallel to the adhesion plane. 3.5 Peel Strength (N/cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Tensile Stress (MPa)


3 2.5 2 1.5 1 0.5 0 0

Extension (mm)

CNF 3% CNF 1%

50 Liquid Glue CNF 0.5%

100 Solid Glue

Figure 5. Peeling test curves for samples made with 0.5% (light blue), 1% CNF (purple), 3% CNF (dark blue), solid paper adhesive (green) and liquid paper adhesive (red)

elastic modulus. I Maleic Anhydride II Bagasse Cellulose

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120 110 100 90 80 70 60 50 40 30

b 120 110 100 90 80 70 60 50 40 30

180-1-3 180-1-5

Thickness Swelling %

Water Absorption %

a

0

200

400

200-1-3 200-1-5 180-0.5-3 180-0.5-5 200-0.5-3 0

600

200

Time (hr)

400

600

Time (hr)

d120

120 110 100 90 80 70 60 50 40 30 0

200

400

600

Time(hr)

200-0.5-5

180-1-3-3*

110 100 90 80 70 60 50 40 30

200-1-3-3

Thickness Swelling %

c Water Absorption %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


180-1-3-0 200-1-3-0

0

200

400

600

Time (hr)

Figure 6.Top: Water absorption (a) and thickness swelling (b) of 8 different samples generated via folding method. Comparison of water absorption (c) and thickness swelling (d) properties for samples generated via rolling method with and without addition of Polycup (*sample code: Press temperature-%CNF-Press time-Additive pph)

The mechanism of peeling was different for the sample with 0.5% and those with 1% and 3% CNF. For 0.5% CNF, the peeling was observed at the interface of the two bonded papers where the CNF was applied leaving two strips with similar thickness after the peeling was done. However, for samples with 1% and 3% CNF, peeling occurred within the bulk of one of the paper strips and not at the interface. This, along with data collected from the peeling test can be an indication of higher adhesion properties when higher concentrations of CNF are used. The highest peel strength was achieved for the sample bonded with 3% CNF suspension, showing that the bonding strength of CNF could be even higher than commercial adhesives (0.9 and 1.4 N/cm for solid and liquid adhesive samples, respectively). This can be in part attributed to the reinforcing effect of CNF contributed to the z direction tensile strength of paper. Physical properties: Water absorption and thickness swelling of eight samples (2 specimens each) were measured through a long term ASTM D570 test. The properties of samples made in similar conditions via the rolling method

with and without Polycup (crosslinking additive) were also measured. The laminates without any additive, show relatively high thickness swelling and water absorption, which is attributed to the nature of cellulose, a highly hydrophilic material, and porosity of the laminate (Figure 6). Samples made at higher press temperature (200 °C), regardless of their press time and CNF content, showed relatively lower water absorption and thickness swelling behavior. This can possibly be related to higher hornification of cellulose fibrils at higher temperatures leading to better dimensional stability and lower water absorption51. The difference in mentioned properties for the samples made through the two proposed production methods, folding and rolling, without any additional additives was negligible. The rolling method laminates showed a slightly lower water absorption and thickness swelling. However, samples generated through rolling method, upon addition of only 3 pph Polycup to the CNF suspension bath, showed a remarkable decrease in water absorption and thickness swelling compared to those made through the same method without the additive (Figure 6-c and d).

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Water absorption decreased from 85% to 60% and thick-

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ness

a

b

c

d

e

f

Figure 7. Top. Surface of paper: dipped in water (a), dipped in 1% CNF suspension (b). Laser blade cross section of a paper laminate bonded with CNF (c). Bottom. Peeling test: 0.5% CNF bound peeled paper surface (d), 1% CNF bound peeled paper surface- thinner side (e), and 1% CNF bound peeled paper surface-thicker side (f)

swelling from 75% to 50%. This could be attributed to the crosslinking of Polycup resin upon curing which provides stronger bonding between layers of paper 52. Another interesting observation during physical testing was the ability of the laminates not to disintegrate even at long exposure times (up to 500 hours). Although the laminates absorbed a lot of water and swelled to a great extent, they were able to maintain their integrity and did not delaminate. This, shows the bonding effect of the CNFs and their ability to maintain the laminates structure in long exposure to moisture. Application of Classical Laminated Plate Theory: Table 4 presents comparison of experimental and theoretical initial elastic modulus (linear elastic region) values for tension and bending tests. Longitudinal and transverse poison ratios derived from the literature 53 for copy paper along with initial modulus properties for one single unidirectional ply (consisting of 15 layers of paper) were used to compute Eq.1. The CLPT 44 appears to closely predict the laminate properties. Experimental modulus data of the tension test are very close to computed data, with only 3-6% error. As for bending modulus properties, the errors are slightly higher, 4-10%. As the CLPT assumes perfect bonding between the layers, these errors could be attributed to existence of some imperfections in the structure of the laminate (e.g. CNF agglomeration), which can cause stress concentration areas and consequently decrease the properties in practice.

The successful estimation of stiffness of the laminates using CLPT modeling provides the opportunity to design laminate systems for the desired mechanical performance in the final application by knowing the properties of single laminae. As mentioned earlier, there is wide flexibility in the design of the laminate system where laminae thickness, orientation, number and density can be engineered to produce laminates with preferred combination of properties. Table 4. Experimental and CLPT elastic modulus values for a 3-ply cross laminated paper/CNF composite *

Tension x Tension y** Bending x Bending y *

Experimental 4.9 GPa 4.1 GPa 5.6 GPa 3.9 GPa

CLPT 5.1 GPa 4.3 GPa 5.8 GPa 3.50 GPa

Difference% 3% 6% 4.4% -10%

Longitudinal paper direction (MD)

** Transverse

paper direction (CD)

SEM Imaging: The presented micrographs in Figure 7 reveal full coating of surface of paper dipped into 1% CNF suspension (b) compared to surface of paper dipped in water (a). Nanofibrils seem to be distributed randomly on the surface of the paper (no orientation is observed) with nano-ranged widths and micron-ranged lengths which is in line with what is expected from CNF 1. Figure 7(c) shows the cross section of a paper/CNF laminate cut delicately with a razor blade. Layers of paper are

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subtly distinguishable but a uniform structure and bonding is observed. No CNFs can be seen at this magnification. Figure 7 (d,e, and f) images show the surfaces of the peeled strips of paper after the peeling test. The image on the left is for 0.5% CNF bonded paper strips. As mentioned earlier, for 0.5% CNF samples, peeling occurred at the bonded interface, leaving two strips of paper of similar thickness. CNFs are observed at the interface at 5000 x magnification. The surface is fully covered with CNFs similar to the strip of paper dipped into a CNF bath (Figure 7(b)). Figure 7(e) and figure 7(f) show the surfaces of the two strips of 1% CNF peeling test samples, thinner and thicker strips respectively. As mentioned earlier, for samples bonded with 1% CNF suspension, because of stronger interfacial adhesion, the peeling happened in the bulk of strips of paper, leaving two strips with different thicknesses. The SEMs show the bulk of the paper and look similar to each other, which again shows the fact that the sample is torn apart in the bulk of one of the strips. Only a few CNFs (white arrows) can be observed in the bulk as most of them probably remained at the interface as we predicted (Figure 8(b)). Proposed adhesion mechanism of CNF: Results of the peeling test proved that there is good adhesion among CNF and paper fibers. As CNF and paper both are from the same material, cellulose1, hydrogen bonding as well as mechanical interlocking are expected to dominate adhesion mechanisms where no chemical reaction would be involved 5,54. Figure 8 shows the suggested representation of what happens at the interface of the two paper layers bonded with CNF. Figure 8(a) is what was originally expected. In practice, an interesting phenomenon was observed when CNF was applied to the surface of the paper where it penetrated into the porous structure of the paper layers (Figure 8(b)). CNF nanoparticles seemed not to remain only on the surface but to partially impregnate the paper layers in a manner more similar to what is presented in Figure 8(b).

longitudinal (MD) and transverse (CD) properties of machine-made paper making it an orthotropic material. A nature-based nanomaterial, CNF, was used both as a “binder” to bond sheets of paper together and as a “reinforcement” to reinforce the resulting laminate composite at least in the z direction. Through a peeling test and SEM analysis it was shown that CNF had adhesive properties. No severe delamination happened within the laminates during testing and the resulting laminates properties were proved to meet or exceed other currently used composite material properties. The laminates’ integrity was maintained when exposed to water for a long period of time. Water absorption and thickness swelling properties were proved to be controllable by adding a small proportion of a green cross-linking resin. Tensile and bending elastic moduli of the laminates were predicted with only 2-10% deviation from experimental data. SEM imaging was used to qualitatively explore surfaces and cross sections of the laminates using which coupled with peeling test results a binding mechanism for CNF in the laminated system was proposed. This novel laminated system offers theoretically predictable properties, flexibility in production method, CNF content, density, and thickness and various possible combinations of angled, cross-directional, and unidirectional composite layups providing a wide range of opportunities for future developments. The production method can be easily scaled up in the rolling method to produce laminates of larger sizes.

AUTHOR INFORMATION Corresponding Author: Mehdi Tajvidi, email: [email protected], Tel: 207-581-2852

ACKNOWLEDGMENT This project was funded by the U.S. Department of Agriculture's Agricultural Research Service (USDA ARS Agreement No.58-0202-4-003). The authors would like to thank Dr. Roberto Lopez-Anido, professor of Civil Engineering at the University of Maine for his help with CLPT analysis.

ABBREVIATIONS Figure 8. a) Proposed adhesion mechanism: All CNFs (green) remain only at the interface of two layers of paper (brown) b) Most CNFs remain at the interface, but some move through the thickness and impregnate the bulk of a single layer of paper thereby providing strong bonding.

CNF, Cellulose nanofibril; CLPT, Classical laminated plate theory; TS, Tensile strength; TM, Tensile elastic modulus; BS, Bending strength; BM, Bending elastic modulus.

CONCLUSIONS: We introduced the production steps of a new class of laminated composites based on renewable and 100% nature based materials. The cross-directional laminates were generated out of sheets of paper, taking advantage of

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REFERENCES 1. Kalia, S.; Kaith, B. S.; Kaur, I., Cellulose Fibers: Bio- and Nano-Polymer Composites ; Green Chemistry and Technology. Springer: Berlin;Heidelberg;New York;, 2011. 2. Isogai, A., Wood Nanocelluloses: Fundamentals and Applications as New Bio-Based Nanomaterials. J. Wood Sci. 2013, 59 (6), 449-459. 3. Ashori, A., Wood–Plastic Composites as Promising Green-Composites for Automotive Industries. Bioresour. Technol. 2008, 99 (11), 4661-4667. 4. Koronis, G.; Silva, A.; Fontul, M., Green Composites: a Review of Adequate Materials for Automotive Applications. Composites Part B 2012, 44 (1), 120. 5. Tajvidi, M.; Gardner, D. J.; Bousfield, D. W., Cellulose Nanomaterials as Binders: Laminate and Particulate Systems. J. Renewable Mater. 2016. 6. Sun, Z.-Y.; Han, H.-S.; Dai, G.-C., Mechanical Properties of Injection-Molded Natural FiberReinforced Polypropylene Composites: Formulation and Compounding Processes. J. Reinf. Plast. Compos. 2010, 29 (5), 637-650. 7. Bajpai, P. K.; Singh, I.; Madaan, J., Comparative Studies of Mechanical and Morphological Properties of Polylactic Acid and Polypropylene Based Natural Fiber Composites. J. Reinf. Plast. Compos. 2012, 31 (24), 1712-1724. 8. Luz, S.; Del Tio, J.; Rocha, G.; Gonçalves, A.; Del’Arco, A., Cellulose and Cellulignin from Sugarcane Bagasse Reinforced Polypropylene Composites: Effect of Acetylation on Mechanical and Thermal Properties. Composites Part A 2008, 39 (9), 1362-1369. 9. Popa, M. I.; Pernevan, S.; Sirghie, C.; Spiridon, I.; Chambre, D.; Copolovici, D. M.; Popa, N., Mechanical Properties and Weathering Behavior of Polypropylene-Hemp Shives Composites. Journal of Chemistry 2013, 2013. 10. Zaman, H. U.; Khan, M. A.; Khan, R. A., Comparative Experimental Measurements of Jute Fiber/Polypropylene and Coir Fiber/Polypropylene Composites as Ionizing Radiation. Polym. Compos. 2012, 33 (7), 1077-1084. 11. Reddy, J. P.; Rhim, J.-W., Characterization of Bionanocomposite Films Prepared with Agar and Paper-Mulberry Pulp Nanocellulose. Carbohydr. Polym. 2014, 110, 480-488. 12. Aulin, C.; Ström, G., Multilayered Alkyd Resin/Nanocellulose Coatings for Use in Renewable

Page 10 of 13

Packaging Solutions with a High Level of Moisture Resistance. Ind. Eng. Chem. Res. 2013, 52 (7), 25822589. 13. Khan, A.; Huq, T.; Khan, R. A.; Riedl, B.; Lacroix, M., Nanocellulose-Based Composites and Bioactive Agents for Food Packaging. Crit. Rev. Food Sci. Nutr. 2014, 54 (2), 163-174. 14. Fukuzumi, H.; Saito, T.; Iwata, T.; Kumamoto, Y.; Isogai, A., Transparent and High Gas Barrier Films of Cellulose Nanofibers Prepared by TEMPO-mediated Oxidation. Biomacromolecules 2008, 10 (1), 162-165. 15. Sehaqui, H.; Zhou, Q.; Berglund, L. A., HighPorosity Aerogels of High Specific Surface Area Prepared from Nanofibrillated Cellulose (NFC). Compos. Sci. Technol. 2011, 71 (13), 1593-1599. 16. Lavoine, N.; Desloges, I.; Dufresne, A.; Bras, J., Microfibrillated Cellulose–Its Barrier Properties and Applications in Cellulosic Materials: A Review. Carbohydr. Polym. 2012, 90 (2), 735-764. 17. Siró, I.; Plackett, D., Microfibrillated Cellulose and New Nanocomposite Materials: A Review. Cellulose 2010, 17 (3), 459-494. 18. Cao, X.; Chen, Y.; Chang, P.; Muir, A.; Falk, G., Starch-Based Nanocomposites Reinforced with Flax Cellulose Nanocrystals. eXPRESS Polym. Lett. 2008, 2 (7), 502-510. 19. Klemm, D.; Schumann, D.; Kramer, F.; Heßler, N.; Koth, D.; Sultanova, B. In Nanocellulose Materials– Different Cellulose, Different Functionality, Macromol. Symp., Wiley Online Library: 2009; pp 60-71. 20. Azeredo, H.; Mattoso, L. H. C.; AvenaBustillos, R. J.; Munford, M. L.; Wood, D.; McHugh, T. H., Nanocellulose Reinforced Chitosan Composite Films as Affected by Nanofiller Loading and Plasticizer Content. J. Food Sci. 2010, 75 (1), N1-N7. 21. Turbak, A. F.; Snyder, F. W.; Sandberg, K. R., Microfibrillated cellulose US Patents: 1983. 22. Habibi, Y.; Lucia, L. A.; Rojas, O. J., Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications. Chem. Rev. 2010, 110 (6), 3479-3500. 23. Joseleau, J.-P.; Chevalier-Billosta, V.; Ruel, K., Interaction Between Microfibrillar Cellulose Fines and Fibers: Influence on Pulp Qualities and Paper Sheet Properties. Cellulose 2012, 19 (3), 769-777. 24. Sehaqui, H.; Zhou, Q.; Berglund, L. A., Nanofibrillated Cellulose for Enhancement of Strength in High-Density Paper Structures. Nord Pulp Pap. Res. J. 2013, 28 (2), 182-189. 25. Hamada, H.; Bousfield, D. W., Nanofibrillated Cellulose as a Coating Agent to Improve Print Quality of Synthetic Fiber Sheets. Tappi J. 2010, 9 (11), 25-29. 26. Reising, A. B.; Moon, R. J.; Youngblood, J. P., Effect of Particle Alignment on Mechanical Properties of Neat Cellulose Nanocrystal Films. J. Sci. Technol. For Prod. Process 2012, 32-41.

10


Page 11 of 13

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27. Liu, J.-C.; Moon, R. J.; Rudie, A.; Youngblood, J. P., Mechanical Performance of Cellulose Nanofibril Film-Wood Flake Laminate. Holzforschung 2014, 68 (3), 283-290. 28. Ardanuy, M.; Claramunt, J.; Arévalo, R.; Parés, F.; Aracri, E.; Vidal, T., Nanofibrillated Cellulose (NFC) as a Potential Reinforcement for High Performance Cement Mortar Composites. BioResources 2012, 7 (3), 3883-3894. 29. Henriksson, M.; Berglund, L. A.; Isaksson, P.; Lindstrom, T.; Nishino, T., Cellulose Nanopaper Structures of High Toughness. Biomacromolecules 2008, 9 (6), 1579-1585. 30. Sehaqui, H.; Zhou, Q.; Ikkala, O.; Berglund, L. A., Strong and Tough Cellulose Nanopaper with High Specific Surface Area and Porosity. Biomacromolecules 2011, 12 (10), 3638-3644. 31. Sehaqui, H.; Liu, A.; Zhou, Q.; Berglund, L. A., Fast Preparation Procedure for Large, Flat Cellulose and Cellulose/Inorganic Nanopaper Structures. Biomacromolecules 2010, 11 (9), 2195. 32. Svagan, A. J.; Berglund, L. A.; Jensen, P., Cellulose Nanocomposite Biopolymer Foam Hierarchical Structure Effects on Energy Absorption. ACS Appl. Mater. Interfaces 2011, 3 (5), 1411-1417. 33. Taipale, T.; Österberg, M.; Nykänen, A.; Ruokolainen, J.; Laine, J., Effect of Microfibrillated Cellulose and Fines on the Drainage of Kraft Pulp Suspension and Paper Strength. Cellulose 2010, 17 (5), 1005-1020. 34. Peng, Y.; Gardner, D. J.; Han, Y., Drying Cellulose Nanofibrils: in Search of a Suitable Method. Cellulose 2012, 19 (1), 91-102. 35. Claramunt, J.; Ardanuy, M.; Arévalo, R.; Parés, F. In Mechanical Performance of Ductile Cement Mortar Composites Reinforced with Nanofibrillated Cellulose, 2nd International RILEM Conference on Strain Hardening Cementitious Composites (SHCC2Rio), RILEM Publications SARL: 2011; pp 131-138. 36. Ardanuy Raso, M.; Claramunt Blanes, J.; Tolêdo Filho, R. D. In Evaluation of Durability to Wet/Dry Cycling of Cement Mortar Composites Reinforced with Nanofibrillated Cellulose, Brittle Matrix composites, Woodhead Publishing Limited: 2012; pp 39-41. 37. Kojima, Y.; Minamino, J.; Isa, A.; Suzuki, S.; Ito, H.; Makise, R.; Okamoto, M., Binding Effect of Cellulose Nanofibers in Wood Flour Board. J. Wood Sci. 2013, 59 (5), 396-401. 38. Kojima, Y.; Isa, A.; Kobori, H.; Suzuki, S.; Ito, H.; Makise, R.; Okamoto, M., Evaluation of Binding Effects in Wood Flour Board Containing LignoCellulose Nanofibers. Materials 2014, 7 (9), 6853-6864.

39. Bilodeau, M. A.; Bousfield, D. W., Composite Building Products Bound with Cellulose Nanofibers. US Patents: 2014. 40. ASTM D1876-08(2015)e1, Standard Test Method for Peel Resistance of Adhesives (T-Peel Test), ASTM International, West Conshohocken, PA, 2015, www.astm.org. 41. ASTM D790-15e2, Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials, ASTM International, West Conshohocken, PA, 2015, www.astm.org. 42. ASTM D638-14, Standard Test Method for Tensile Properties of Plastics, ASTM International, West Conshohocken, PA, 2014, www.astm.org. 43. ASTM D570-98(2010)e1, Standard Test Method for Water Absorption of Plastics, ASTM International, West Conshohocken, PA, 2010, www.astm.org. 44. Khandan, R.; Noroozi, S.; Sewell, P.; Vinney, J., The Development of Laminated Composite Plate Theories: a Review. J. Mater. Sci.. 2012, 47 (16), 59015910. 45. Aboudi, J.; Arnold, S. M.; Bednarcyk, B. A., Micromechanics of Composite Materials: a Generalized Multiscale Analysis Approach. ButterworthHeinemann: 2012. 46. Gürdal, Z.; Haftka, R. T.; Hajela, P., Design and Optimization of Laminated Composite Materials. John Wiley & Sons: 1999. 47. Verma, B. B., Continuous Jute Fibre Reinforced Laminated Paper Composite and Reinforcement-Fibre Free Paper Laminate. Bull. Mater. Sci. 2009, 32 (6), 589-595. 48. Prambauer, M.; Paulik, C.; Burgstaller, C., Evaluation of the Interfacial Properties of Polypropylene Composite Laminates, Reinforced with Paper sheets. Composites Part A 2016, 88, 59-66. 49. Deepa, B.; Abraham, E.; Pothan, L.; Cordeiro, N.; Faria, M.; Thomas, S., Biodegradable Nanocomposite Films Based on Sodium Alginate and Cellulose Nanofibrils. Materials 2016, 9 (1), 50. 50. Fu, S. Y.; Lauke, B.; Mäder, E.; Yue, C. Y.; Hu, X., Tensile Properties of Short-Glass-Fiber- and ShortCarbon-Fiber-Reinforced Polypropylene Composites. Composites Part A 2000, 31 (10), 1117-1125. 51. Fernandes Diniz, J. M. B.; Gil, M. H.; Castro, J. A. A. M., Hornification—Its Origin and Interpretation in Wood Pulps. Wood Sci. Technol. 2004, 37 (6), 489494. 52. Mohr, B. J.; Nanko, H.; Kurtis, K. E., Aligned Kraft Pulp Fiber Sheets for Reinforcing Mortar. Cem. Concr. Compos. 2006, 28 (2), 161-172. 53. Yokoyama, T.; Nakai, K., Orientation Dependence of In-Plane Tensile Properties of Paperboard and Cardboard: Experiments and Theories.

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J. Jpn. Soc. Exp. Mech. 2010, 10 (Special_Issue), s146s151. 54. Gardner, D. J.; Oporto, G. S.; Mills, R.; Samir, M. A. S. A., Adhesion and Surface Issues in Cellulose

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and Nanocellulose. J. Adhes. Sci. Technol. 2008, 22 (5), 545-567.

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Graphics for the abstract 83x63mm (300 x 300 DPI)


Production and Characterization of Laminates of Paper and Cellulose

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