nanomaterials

Article Preparation of Transparent and Thick CNF/Epoxy Composites by Controlling the Properties of Cellulose Nanofibrils

Shin Young Park 1, Simyub Yook 1, Sooim Goo 1, Wanhee Im 2 and Hye Jung Youn 1,2,* 1 Department of Forest Sciences, College of Agriculture and Life Sciences, Seoul National University, Seoul 08826, Korea; [email protected] (S.Y.P.); [email protected] (S.Y.); [email protected] (S.G.) 2 Research Institute of Agriculture and Life Sciences, Seoul National University, Seoul 08826, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-2-880-4787

 Received: 13 March 2020; Accepted: 26 March 2020; Published: 28 March 2020 

Abstract: Cellulose nanofibrils (CNFs) have been used as reinforcing elements in optically transparent composites by combination with matrices. In this study, strong, optically transparent, and thick CNF/epoxy composites were prepared by immersing two or four layers of CNF sheets in epoxy . The morphology of the CNF, the preparation conditions of the CNF sheet, and the grammage and layer numbers of the CNF sheets were controlled. The solvent-exchanged CNF sheets resulted in the production of a composite with high transparency and low haze. The CNF with smaller width and less aggregated fibrils, which are achieved by carboxymethylation, and a high number of grinding passes are beneficial in the production of optically transparent CNF/epoxy composites. Both the grammage and number of stacked layers of sheets in a composite affected the optical and mechanical properties of the composite. A composite with a thickness of 450–800 µm was prepared by stacking two or four layers of CNF sheets in epoxy resin. As the number of stacked sheets increased, light transmittance was reduced and the haze increased. The CNF/epoxy composites with two layers of low grammage (20 g/m2) sheets exhibited high light transmittance (>90%) and low haze (<5%). In addition, the composites with the low grammage sheet had higher tensile strength and elastic modulus compared with neat epoxy and those with high grammage sheets.

Keywords: carboxymethylation; cellulose nanofibrils; composite; solvent exchange; transparency

1. Introduction The use of -based has caused serious environmental problems, increasing the need for alternative eco-friendly materials. Diverse bio-resources, including , grass, and algae, have been used for producing new sustainable materials. Among them, cellulose nanofibrils (CNFs) have recently been vaunted as a promising eco-friendly material. CNFs are nano-sized cellulose materials of a few nanometers in width and a few micrometers in length. CNFs can be obtained by the mechanical treatment of pulp fibers with or without mechanical, chemical, or biological pretreatment [1–3]. Owing to their high strength, thermal stability, large surface specific area, and biodegradability, CNFs have been used as sustainable reinforcing elements in various matrices, including [4–8], [9,10], and rubber [11,12]. Because CNFs are a few nanometers in width, the light scattering of the CNF-containing materials can be reduced [13–16]. Many studies have reported that CNF can be used to produce optically transparent composites by itself or by mixing with other transparent [17–23]. Optically transparent CNF composites are expected to be applied in packaging, display, optoelectronics, or automobiles by replacing or plastics owing to its high transparency, outstanding mechanical

Nanomaterials 2020, 10, 625; doi:10.3390/nano10040625 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 625 2 of 13 properties, greater safety, and sustainability. Within the previous studies, the preparation of the composites by mixing CNF with transparent such as acrylic resin or epoxy resin has drawn much interest [18,21,24–26]. As these resins have a similar refractive index to that of CNF, the light scattering of the composites could be minimized [13,24]. CNF/epoxy composites can be simply manufactured by and impregnation of CNF sheet with epoxy resin, and mixing and extrusion of the CNF and epoxy resin. In the case of coating of the CNF sheet with epoxy resin [26,27], the resin covers the surface and smoothens the roughness of the CNF sheet, which increases the transparency of the composite [26]. In addition, the CNF/epoxy composite is prepared by immersing the CNF sheet into the epoxy resin [18,24]. The resin penetrates into the sheet and can reduce the light scattering of the CNF sheet [13,20]. Although many studies have revealed that CNF and transparent resin can form reinforced transparent composites, most have focused on the improvement of the mechanical strength of the composite. The composites studied had relatively thin thicknesses below 200 µm, which is only useful in limited applications. In addition, a detailed investigation of the relationship between CNF properties and the optical properties of composites is required. Therefore, this study aimed to prepare thick and optically transparent CNF/epoxy composites by controlling the properties of the CNF and CNF sheets. The CNF/epoxy composites were prepared by impregnating CNF sheets with epoxy resin. During preparation, several factors, including the CNF morphology, the CNF sheet preparation method, and the grammage and number of the CNF sheets, were controlled. The effects of the properties of the CNF and CNF sheets on the characteristics of the composites were investigated in terms of light transmittance, haze, and tensile properties. CNF morphology was controlled by carboxymethylation pretreatment and the number of grinding passes. By comparing the dried CNF sheet and the solvent-exchanged sheet during the immersion process, the effects of the miscibility and porosity of the CNF sheet on optical properties of the composite were investigated. Through this study, the key factors in preparing thick, strong, and transparent CNF/epoxy composites were determined.

2. Materials and Methods

2.1. Raw Materials Hardwood bleached kraft pulp (HwBKP) for CNF preparation was supplied by Moorim P&P (Ulsan, Korea). For the carboxymethylation process, monochloroacetic (Sigma Aldrich, St. Louis, MO, USA), sodium hydroxide, and isopropanol (Samchun Chemicals Co. Ltd., Seoul, Korea) were used. Epoxy resin (KDS 8128, Kukdo Chemical, Seoul, Korea) and a hardener (MH 700G, New Japan Chemical Co. Ltd., Osaka, Japan) were used for the preparation of the composite. 2,4,6-Tris(dimethylaminomethyl) (Sigma Aldrich) was used as a catalyst for epoxy hardening.

2.2. Preparation of Untreated CNF and Carboxymethylated CNF Sheets

2.2.1. Carboxymethylation of Pulp Carboxymethylation of HwBKP as a pretreatment for CNF production was conducted according to the method described by Im et al. [28]. The pulp (dry weight of 80 g) was added to a 2 L solution of sodium hydroxide (NaOH) in isopropanol and reacted at 35 ◦C for 30 min. Next, monochloroacetic acid was added to the suspension. To control the carboxyl content of the carboxymethylated pulp, the quantity of monochloroacetic acid added was controlled at 0.25 to 1 mmol/g based on the oven-dried weight of the pulp. The pulp was then reacted at 65 ◦C for 1 h. Following the reaction, the pulp was adequately washed with deionized water. The carboxyl group content of each pulp was measured by a conductometric titration method in accordance with SCAN-CM 65:02 (total acidic group content). Nanomaterials 2020, 10, 625 3 of 13

2.2.2. Preparation of Untreated CNF and Carboxymethylated CNF Untreated CNF (U-CNF) and carboxymethylated CNF (CM-CNF) were prepared by grinding untreated and carboxymethylated pulp suspensions, respectively, using a Supermasscolloider (Masuko Sangyo, Kawaguchi, Japan). The U-CNF was prepared by grinding a 1.5% pulp suspension 24 times. In the case of CM-CNF, the carboxymethylated pulp suspension passed through the grinder 4 times and 10 times, respectively, to investigate the effect of the grinder pass number on the properties of the composite. The rotational speed and gap size during the grinding process were 1500 rpm and 80 µm, − respectively. A scanning (SEM, SUPRA 55VP, Carl Zeiss, Oberkochen, Germany) was used to examine the morphology of the CNF. A diluted CNF suspension (0.005%) dropped onto Cu grid was dried under room temperature and coated by Pt with 5 nm thickness, and the observation was conducted at 2 kV. SEM images were used for measuring the fibril width using Image J program. As the CNF samples were coated by Pt for SEM observation, the width of fibrils can be affected by coating thickness. The fibril width would be larger under SEM measurement compared with measurement using atomic force microscope or transmission electron microscope. Pt coating was conducted in the same manner for all kinds of CNFs, however, the effect of coating thickness was ignored in this work. The average width and width distribution were obtained from the measurements on at least 100 fibrils for each kind of CNF.

2.2.3. Preparation of Dried- and Solvent-Exchanged-CNF Sheets To evaluate the effect of miscibility and porosity of the sheet on the optical properties of the composite, the dried sheets were prepared using the U-CNF, and solvent-exchanged wet sheets were prepared using both the U-CNF and CM-CNF suspensions, respectively. For the dried CNF sheet, 0.5% CNF suspension was de-watered on a mixed cellulose ester membrane filter (Advantec, Vernon Hills, IL, USA) by vacuum filtration. After filtration, the sheet was pressed under a pressure of 30 bar for 5 min and hot-pressed at 105 ◦C and 25 bar for 15 min. For solvent-exchanged sheets, 0.5% suspension was filtrated by vacuum, and the wet sheet was pressed at 30 bar for 10 s. The sheet was then immersed in an ethanol solvent for more than 12 h. The solvent-exchanged sheet was pressed for 10 s, and then used for composite preparation without drying. To investigate the effect of sheet grammage on the properties of the composite, solvent-exchanged CM-CNF (550 µmol/g) sheets were prepared with grammage values of 20, 30, and 40 g/m2, respectively, by adjusting the amount of CM-CNF suspension.

2.3. Preparation of CNF/Epoxy Composite The epoxy resin, hardener, and catalyst were mixed in a weight ratio of 100:50:0.1 and degassed in a vacuum oven. Each CNF sheet was immersed in the epoxy resin mixture for 30 s until the resin-penetrated sheet became transparent. After removing the impregnated CNF sheet and eliminating the excessive resin from its surface, two or four layers of the resin-impregnated sheets were stacked to produce a thick composite. Then, the sheets were cured at 105 ◦C and 30 bar in a hot press for 1 h. The overall process for the preparation of CNF/epoxy composites is shown in Scheme1. NanomaterialsNanomaterials 20192020, 9, x, 10FOR, 625 PEER REVIEW 4 of 134 of 13

Scheme 1. Overall process for the preparation of cellulose nanofibril (CNF)/epoxy composites. Scheme 1. Overall process for the preparation of cellulose nanofibril (CNF)/epoxy composites. 2.4. Characterization of the CNF/Epoxy Composite 2.4. CharacterizationThe optical of propertiesthe CNF/Epoxy of the Composite CNF/epoxy composite were evaluated using a UV-Vis (-visible)The optical properties spectrophotometer of the CNF/epoxy (Cary composite 100, Agilent, were Santaevaluated Clara, using CA, a USA).UV-Vis The (ultraviolet light - transmittance of each composite was measured over the wavelength of visible light from 400 to visible) spectrophotometer (Cary 100, Agilent, Santa Clara, CA, USA). The light transmittance of each 800 nm. Haze was measured in accordance with ASTM-D1003-11 (Standard Test Method for Haze and composite was measured over the wavelength of visible light from 400 to 800 nm. Haze was Luminous Transmittance of Transparent Plastics), and four values (T1–T4) were measured for each measured in accordance with ASTM-D1003-11 (Standard Test Method for Haze and Luminous sample. T1 represents only incident light intensity with reflectance standard, and T2 represents the Transmittance of Transparent Plastics), and four values (T1–T4) were measured for each sample. T1 total light transmitted by a specimen. T3 is the intensity of the light scattered by the instrument, and 2 representsT4 is the only intensity incident of the light light scatteredintensity by with both reflectance instrument andstandard, specimen. and From T represents these values, the the total total light transmittedluminous by transmittance a specimen. (T Tt),3 diisff theuse intensity transmittance of the (Td light), and scattered haze were by calculated the instrument, using Cary and WinUV T4 is the intensityColor ofApplication the light software scattered (Agilent, by both Santa instrument Clara, CA, USA), and specimen.according to From the following these values, equations: the total luminous transmittance (Tt), diffuse transmittance (Td), and haze were calculated using Cary WinUV = Color Application software (Agilent, Santa Clara,Tt TCA,2/T 1USA), according to the following equations(1) : T = [T T (T /T )]/T (2) d 4푇−푡 =3푇22/푇1 1 1 (1) Haze (%) = T /Tt 100 (3) d × 푇푑 = [푇4 − 푇3(푇2⁄푇1)]/푇1 (2) The tensile properties of the composite were evaluated using a universal testing machine (Instron, Norwood, MA, USA). The composite samples were cut into specimens of 50 mm length and 5 mm Haze (%) = 푇푑/푇푡 × 100 (3) width. Tests were performed at 20 mm span length, and the strain speed was 20 mm/min. More than fiveThe specimens tensile properties for each sample of the were composite measured. were evaluated using a universal testing machine (Instron, Norwood, MA, USA). The composite samples were cut into specimens of 50 mm length and 5 mm3. width. Results Tests and Discussionwere performed at 20 mm span length, and the strain speed was 20 mm/min. More than3.1. five E ffspecimensect of Solvent-Exchange for each sample Process were of the measured. Sheet on the Composites Properties

3. ResultsComposites and Discussion of U-CNF sheets and epoxy were prepared by stacking two layers of dried and solvent-exchanged U-CNF sheets, which were immersed into epoxy resin, respectively. The grammage of each sheet was 20 g/m2. Figure1 shows the U-CNF sheet (single layer) before epoxy immersion and 3.1. Effect of Solvent-Exchange Process of the Sheet on the Composites Properties Composites of U-CNF sheets and epoxy were prepared by stacking two layers of dried and solvent-exchanged U-CNF sheets, which were immersed into epoxy resin, respectively. The grammage of each sheet was 20 g/m2. Figure 1 shows the U-CNF sheet (single layer) before epoxy immersion and U-CNF/epoxy composites prepared with two-layer dried sheets and the solvent- exchanged sheets. Compared with the U-CNF single sheet, the two-layered U-CNF sheet/epoxy

Nanomaterials 2020, 10, 625 5 of 13

NanomaterialsU-CNF/epoxy 2019, composites 9, x FOR PEER prepared REVIEW with two-layer dried sheets and the solvent-exchanged sheets.5 of 13 Compared with the U-CNF single sheet, the two-layered U-CNF sheet/epoxy composites became compositestransparent. became In particular, transparent the composite. In particular, prepared the withcomposite the solvent-exchanged prepared with the sheet solvent demonstrated-exchanged a sheethigher demonstrated transparency than a higher that with transparency the dried than sheet. that It is with notable the that dried two-layered sheet. It is U-CNF notable sheet that/epoxy two- layercompositesed U-CNF look sheet/epoxy more transparent composites than the look single more U-CNF transparent sheet, even than though the single the composites U-CNF sheet had, largereven thoughthickness. the The composi penetrationtes had of epoxy larger resin thickness. into the The sheets penetration contributed of to improvingepoxy resin the into transparency the sheets of contributedcomposites. to Table improving1 presents the the transparency optical properties of composites. of the U-CNF Table sheet 1 presents and U-CNF the optical/epoxy properties composites. of theThe U dried-CNFU-CNF sheet and sheet U-CNF had/epoxy light transmittances composites. The close dried to 90% U-CNF and sheet 76.0% had of haze.light transmittances Although the closeU-CNF to sheet 90% had and high 76.0% transmittance, of haze. Although it appeared the U translucent-CNF sheet because had high of a transmittance, high haze value. it Whenappeared the translucentU-CNF sheets because were of impregnated a high haze with value. epoxy, When U-CNF the U/-epoxyCNF sheets composites were impregnated appeared more with transparent, epoxy, U- CNFdespite/epoxy having composites a lower transmittanceappeared more than transparent U-CNF, sheets. despite This having may a belower attributed transmittance to a lower than haze U- CNFvalue, sheets. because This epoxy may resinbe attributed penetrated to a into lower the haze sheets. value As,epoxy because and epoxy CNF resin have penetrated a similar refractive into the sheets.index, theAs epoxy-impregnatedepoxy and CNF have CNF a similar sheets exhibited refractive low index, light the scattering, epoxy-impregnated which leads CNF to a higher sheets exhibitedtransparency low [light13,20 scattering,,26]. In addition, which thelead hazes to a of higher the composite transparency was further[13,20,26]. reduced In addition, when preparedthe haze ofwith the the composite solvent-exchanged was further sheets. reduced This when can beprepared explained with by the two solvent reasons,-exchanged one of which sheets. is the This improved can be explainedmiscibility by between two reasons, the CNF one and of which epoxy is matrix. the improved CNF is hydrophilicmiscibility between polymer, the and CNF mixing and withepoxy a matrix.hydrophobic CNF is material hydrophilic is diffi polymer,cult. However, and mixing solvent-exchange with a hydrophobic treatment material decreases is difficult. the hydrophilicity However, solventof the CNF,-exchange which treatmentresults in good decreases compatibility the hydrophilicity between the of fiber the and CNF, matrix. which The resultssecond reason in good is compatibilitythat solvent-exchanged between the CNF fiber sheets and matri couldx. be The immersed second reason in epoxy is that resin solvent without-exchanged drying. ThisCNF meanssheets couldthat the be solvent-exchangedimmersed in epoxy resin CNF without sheet could drying. avoid This the means aggregation that the solvent of fibrils-exchanged or the formation CNF sheet of couldhydrogen avoid bonding the aggregation betweenfibrils, of fibrils which or the can formation occur during of hydrogen the drying bonding process. between Thus, the fibrils structure, which of cathen solvent-exchangedoccur during the drying CNF process. sheet became Thus, the more structure porous of and the less solvent packed-exchanged compared CNF with sheet that became of the moredried CNFporous sheet and (Figure less packed2). This compared open structure with of that the of solvent-exchanged the dried CNF sheet CNF sheet (Figure could 2). contribute This open structureto easier penetrationof the solvent of- epoxyexchanged resin intoCNF the sheet sheet, could and contribute ultimately to result easier in penetration higher transparency of epoxy ofresin the intocomposite the sheet [25,]. and ultimately result in higher transparency of the composite [25].

Figure 1. Appearance of ( a)) a single untreated cellulose nanofibril nanofibril ( (U-CNF)U-CNF) sheet, ( b) a two-layeredtwo-layered dried U U-CNF-CNF/epoxy/epoxy composite, and and ( cc)) a two two-layered-layered solvent solvent-exchanged-exchanged U U-CNF-CNF/epoxy/epoxy composite. composite. Pictures were taken at a distance of a few centimeters from the background printedprinted paper.paper.

Nanomaterials 2020, 10, 625 6 of 13

Table 1. Optical properties of the untreated cellulose nanofibril (U-CNF) sheet and the U-CNF/epoxy composites.

Sample Transmittance at 600 nm (%) Haze (%) Dried U-CNF sheet (single layer) 89.6 76.0 Two-layered dried U-CNF/epoxy composite 83.9 30.4 Two-layered solvent-exchanged 83.9 26.2 NanomaterialsU-CNF 2019, 9,/ epoxyx FOR PEER composite REVIEW 6 of 13

Figure 2. SScanningcanning electron electron microscope microscope (SEM (SEM)) images images of of(a) (thea) the dried dried (hot (hot-pressed)-pressed) U-CNF U-CNF sheet sheet and and(b) the (b )solvent the solvent-exchanged-exchanged U-CNF U-CNF sheet. sheet.Image of Image the solvent of the-exchanged solvent-exchanged sheet was sheet taken was after taken air- afterdrying. air-drying. 3.2. Effect of CNF Morphology on the Composite Properties Table 1. Optical properties of the untreated cellulose nanofibril (U-CNF) sheet and the U-CNF/epoxy Tocomposite evaluates. the effect of the carboxyl group content of CNF on the optical property of CNF/epoxy composites, U-CNF and CM-CNF sheets with different carboxyl group contents were used for Transmittance at composite preparation. U-CNFSample had a carboxyl group content of 50 µmol/g, whichHaze was (%) originated 600 nm (%) from hemicelluloses or created during the pulping and bleaching processes [28,29]. All CM-CNFs used in this sectionDried wereU-CNF prepared sheet (single by passing layer) the grinder four89.6 times. The grammage76.0 of each CNF sheet wasTwo 20 g-layer/m2.ed The dried composites U-CNF were/epoxy prepared composite by stacking two83.9 layers of solvent-exchanged30.4 sheets Two-layered solvent-exchanged after immersion in the epoxy resin. The morphology of each CNF83.9 with different carboxyl26.2 contents is shown in Figure3.U As-CNF the/epoxy carboxyl composite content of the CNF increased, the average fibril width decreased. The carboxyl group content of the pulp affected the morphology of the CNF. It is known that pulp 3.2. Effect of CNF Morphology on the Composite Properties fibers with a higher carboxyl content result in CNF with a more uniform and narrower width compared withTo untreated evaluate or thelower effect carboxyl of the content carboxyl fibers group [28]. Thecontent optical of propertiesCNF on the of the optical composites property with of CNF/epoxy composites, U-CNF and CM-CNF sheets with different carboxyl group contents were used for composite preparation. U-CNF had a carboxyl group content of 50 μmol/g, which was originated from hemicelluloses or created during the pulping and bleaching processes [28,29]. All CM-CNFs used in this section were prepared by passing the grinder four times. The grammage of each CNF sheet was 20 g/m2. The composites were prepared by stacking two layers of solvent- exchanged sheets after immersion in the epoxy resin. The morphology of each CNF with different carboxyl contents is shown in Figure 3. As the carboxyl content of the CNF increased, the average fibril width decreased. The carboxyl group content of the pulp affected the morphology of the CNF. It is known that pulp fibers with a higher carboxyl content result in CNF with a more uniform and narrower width compared with untreated or lower carboxyl content fibers [28]. The optical properties

Nanomaterials 2020, 10, 625 7 of 13 Nanomaterials 2019, 9, x FOR PEER REVIEW 7 of 13 theseof CNFs the composites are presented with in these Table CNF2. Thes are composite presented in prepared Table 2. withThe composite a high carboxyl prepared content with a CM-CNF high carboxyl content CM-CNF demonstrated the highest light transmittances near 90%, and the lowest demonstrated the highest light transmittances near 90%, and the lowest haze value at approximately haze value at approximately 10%. According to previous studies, the size of fibrils greatly affects the 10%. According to previous studies, the size of fibrils greatly affects the light scattering properties of light scattering properties of the CNF sheet. An increase in fibril width results in an increase in the CNFforward sheet. light An scattering increase inand fibril leads width to the results formation in an of increaseoptically inhazed forward CNF lightsheets scattering [20,30,31]. andThus, leads a to the formationhigher carboxyl of optically content hazed in the CNFCNF results sheets in [20 an,30 improvement,31]. Thus, ain higher optical carboxyl properties, content that is, inhigher the CNF resultslight in antransmittance improvement and inlower optical haze. properties, that is, higher light transmittance and lower haze.

FigureFigure 3. SEM 3. SEM images images of (ofa) ( U-CNFa) U-CNF (50 (50µ molμmol/g/g carboxyl carboxyl content), (b (b) carboxymethylated) carboxymethylated (CM (CM)-CNF)-CNF with 280 μmol/g carboxyl content, (c) CM-CNF with 550 μmol/g carboxyl content, and (d) fibril width with 280 µmol/g carboxyl content, (c) CM-CNF with 550 µmol/g carboxyl content, and (d) fibril width distribution of CNF with various carboxyl contents. distribution of CNF with various carboxyl contents.

TableTable 2. Optical 2. Optical properties properties of of two-layered two-layeredsolvent-exchanged solvent-exchanged CNF CNF sheet sheet/epoxy/epoxy composites composites with with different carboxyl contents of CNF. CM-CNF, carboxymethylated CNF. different carboxyl contents of CNF. CM-CNF, carboxymethylated CNF. Carboxyl contents of CNF Transmittance at CNF TypeCNF CarboxylType contents of CNF (µmol/g) Transmittance at 600H nmaze (%) (%) Haze (%) (μmol/g) 600 nm (%) U-CNFU-CNF 5050 83.9 83.926.2 26.2 280280 84.6 84.621.1 21.1 355 86.3 17.2 CM-CNF 355 86.3 17.2 CM-CNF 470 88.3 15.0 470 88.3 15.0 550 89.8 10.6 550 89.8 10.6

CNFCNF morphology morphology can also can be also controlled be controlled by the by number the number of passes of passes through through a grinder. a grinder. To investigate To the effinvestigateect of grinding the effect pass of grind numbersing pass of the numbers CM-CNF of the on CM the-CNF transparency on the transparency of the CM-CNF of the CM/epoxy- composite,CNF/epoxy CM-CNF composite, (550 µCMmol-CNF/g carboxyl (550 μmol/g content) carboxyl was content) ground was 4 times ground and 4 times 10 times, and 10 respectively, times, respectively, using a grinder. The SEM images of 4 and 10 passes of the CM-CNF are shown in Figure using a grinder. The SEM images of 4 and 10 passes of the CM-CNF are shown in Figure4. After passing 4. After passing through the grinder four times, most fibers were nanofibrillated, but some micro- through the grinder four times, most fibers were nanofibrillated, but some micro-sized fibrils were sized fibrils were observed. Conversely, after grinding 10 times, micro-size fibrils were not observed, observed.and all Conversely, fibers were after deconstructed grinding 10 to times, nanofibrils micro-size with fibrils nano-scale were widths. not observed, The two and-layered all fibers CM- were deconstructedCNF/epoxy to composites nanofibrils were with prepared nano-scale using widths. these CM The-CNF two-layered sheets (20 CM-CNF g/m2 for each/epoxy sheet) composites after were prepared using these CM-CNF sheets (20 g/m2 for each sheet) after solvent exchange. The optical properties of the CM-CNF/epoxy composite are presented in Figure5. Higher pass numbers correlated to increasing transparency of the composite. The light transmittance increased slightly from 89.8% to 90.9%, and the haze decreased significantly from 10.6% to 4.1%. As mentioned above, fibrillar size is a Nanomaterials 2019,, 9,, xx FORFOR PEERPEER REVIEWREVIEW 8 of 13 Nanomaterials 2020, 10, 625 8 of 13 solvent exchange. The optical properties of the CM-CNF/epoxy composite are presented in Figure 5. Higher pass numbers correlated to increasing transparency of the composite. The light transmittance crucialincreased factorfor slightly thetransparency from 89.8% to of90.9%, the compositeand the haze [20 decre,30].ased Because significantly some microfibrils from 10.6% to remained 4.1%. As after four passesmentioned through above, the fibrillar grinder, size is relatively a crucial factor high for light the scatteringtransparency occurred of the composite throughout [20,30 the]. Because composite, some microfibrils remained after four passes through the grinder, relatively high light scattering increasingsome microfibrils its haze. However, remained afterin the four case passes of 10 through passes, the micro-size grinder, relatively fibrils disappeared high light scattering by complete occurred throughout the composite, increasing its haze. However, in the case of 10 passes, micro-size nanofibrillation, which led to an increase in light transmittance and a decrease in the haze. Therefore, fibrils disappeared by complete nanofibrillation, which led to an increase in light transmittance and this indicatesa decrease that in the the haze. morphology Therefore, of this CNF indicates is an important that the m factororphology affecting of CNF the is opticalan important properties factor of the composite,affecting particularly the optical properties its haze. of the composite, particularly its haze.

FigureFigure 4. SEM 4. SEM images images of of CM-CNF CM-CNF sheets sheets(550 (550 µμmol/gmol/g carboxyl carboxyl group group content): content): (a) 4 (a passes) 4 passes and ( andb) 10 ( b) 10 passespasses through through the the grinder. grinder.

Figure 5. Appearances and optical properties of two-layered solvent-exchanged CM-CNF sheet/epoxy composites:Figure (5. a ) compositeAppearances with and four-pass optical CM-CNF, properties (b of) compositetwo-layerlayered with solvent 10-pass-exchanged CM-CNF, CM and-CNF (c) light transmittancesheet/epoxy and composites: haze of each (a) composite composite. with The four carboxyl-pass CM- contentCNF,, ((b) of composite the CM-CNF with 10 was-pass 550 CMµ-molCNF/g,, and eachand CM-CNF (c) light sheet transmittance had a grammage and haze of of each 20 g composite./m2. The carboxyl content of the CM-CNF was 550 μmol/g and each CM-CNF sheet had a grammage of 20 g/m22.. 3.3. Effects of Sheet Grammage and the Number of Sheet Layers on the Composite Properties

The effects of the grammage of each CM-CNF sheet and the number of stacked layers in the composites on the optical and physical properties of the composite were investigated. These experiments could determine the proper conditions for the preparation of highly transparent, thick, and strong CM-CNF/epoxy composites. The solvent-exchanged CM-CNF (carboxyl content of 550 µmol/g) sheet Nanomaterials 2019, 9, x FOR PEER REVIEW 9 of 13

3.3. Effects of Sheet Grammage and the Number of Sheet Layers on the Composite Properties The effects of the grammage of each CM-CNF sheet and the number of stacked layers in the Nanomaterialscomposites2020, 10 on, 625 the optical and physical properties of the composite were investigated. These9 of 13 experiments could determine the proper conditions for the preparation of highly transparent, thick, and strong CM-CNF/epoxy composites. The solvent-exchanged CM-CNF (carboxyl content of 550 was usedμmol/g) for sheetthe preparation was used for of composites. the preparation The of optical composites. properties The optical of solvent-exchanged properties of solvent CM-CNF- sheet/exchangedepoxy composites CM-CNF withsheet/epoxy different composites sheet grammage with different values sheet and grammage numbers values of sheet and layers numbers are of shown in Figuresheet6 . layers The CM-CNF are shown/epoxy in Figure composites 6. The exhibitedCM-CNF/epoxy similar composites light transmittance exhibited (above similar 90%) light and highertransmittance haze values (above compared 90%) and with higher neat haze epoxy. values There compared was anwith increase neat epoxy. in the There composite’s was an increase haze that correspondedin the composite’s with the haze increased that corresponded sheet grammage. with the This increased may result sheet from grammage. increased This CNF-epoxy may result from interface area,increased or porosity CNF formation-epoxy interfaceas a result area, of incomplete or porosity epoxy formation penetration as a result into the of higher incomplete grammage epoxy CNF sheets.penetration For the layered into the composites, higher grammage as the numberCNF sheets. of stacked For the sheetslayered increased, composites, light as the transmittance number of was stacked sheets increased, light transmittance was reduced and the haze increased. This may be reduced and the haze increased. This may be expected to occur if the interfaces between the CNF expected to occur if the interfaces between the CNF sheets are imperfect (e.g., having larger defects, sheetsdelamination, are imperfect porosity) (e.g., having, which largerwould defects,increase light , scattering. An porosity), increase which in thickness would of increasethe CNF light scattering.sheet was An likely increase to be in one thickness of factors of increasing the CNF sheet the haze was value. likely However, to be one an of increase factors in increasing the interfaces the haze value.appeared However, to an be increasea more important in the interfaces factor to appeared the haze. to Thus, be a more in terms important of the factortransparency to the haze.of the Thus, in termscomposite, of the transparencyit appears that of an the increase composite, in the grammage it appears of that sheets an increase is less detrimental in the grammage than increasing of sheets is less detrimentalthe number of than stacked increasing layers. the number of stacked layers.

FigureFigure 6. The 6. The (a) ( lighta) light transmittance transmittance at at 600 600 nm nm and ( (bb) ) haze haze of of neat neat epoxy epoxy and and CM CM-CNF-CNF/epoxy/epoxy composites.composites. The The carboxyl carboxyl content content of of the the CM-CNF CM-CNF was 550 550 μmol/g.µmol/g.

The thicknessThe thickness and densityand density of each of each composite composite are shown are shown in Figure in Figure7. As 7. the As grammage the grammage and and number of layersnumber of the of layers sheets of increased, the sheets theincreased, thickness the thickness of the composite of the composite also increased. also increased. However, However, the thickness the of thethickness composite of th wase composite not proportional was not proportional to the layer to numberthe layer andnumber grammage and grammage of the sheet.of the sheet When. the sheetsWhen were the stacked sheets inwere two stacked layers, in the two composites layers, the com wereposites approximately were approximately 450–550 µ450m– thick.550 μm Those thick. with four layeredThose with sheets four hadlayer aed thickness sheets had of a 650–800 thicknessµ m.of 650 Thus,–800 aμm. thicker Thus, composite a thicker co couldmposite be could prepared be by stackingprepared the CM-CNF by stacking sheets. the InCM the-CNF case sheets. of the In composite’s the case of density, the composite’s higher sheet density, grammage higher sheetand sheet grammage and sheet layers led to an increase in density. layers led to an increase in density. The tensile properties of the composites are shown in Figure8. The tensile strength of the composites made from 20 g/m2 sheets was 1.5 times greater than neat epoxy. Additionally, the CM-CNF/epoxy composites exhibited a lower breaking strain than neat epoxy. The elastic modulus of the composite increased three times higher than that of neat epoxy because of the effect of the CNF [18,26]. The four layers of sheets led to a further increase in strength compared with the two-layer sheet composites. However, as the sheet grammage increased from 20 to 40 g/m2, the composite became weak and brittle, and the breaking strain decreased. The tensile strength of the composite prepared using 40 g/m2 sheets decreased severely, and the elastic modulus was hard to determine as they were quickly broken. The effect of CNF sheets on mechanical properties of CNF/epoxy composites could be explained by the amount of CNF in the composite, and the penetration and of epoxy resin into the CNF sheet [26,32]. Saba et al. [32] prepared a CNF/epoxy composite by adding freeze-dried CNF powder into epoxy resin and reported that the tensile properties of the CNF/epoxy composite were increased by CNF loading up to a certain content, and then decreased owing to inhomogeneous dispersion of CNF in the epoxy matrix. Although the composite preparation process was different from the literature, it revealed that the reinforcing effect of CNF seemed to diminish Nanomaterials 2020, 10, 625 10 of 13 with the increasing amount of CNF in the composite. The weight portions of the CNF sheet were 8% and 16% for lower-grammage (20 g/m2) and higher-grammage (40 g/m2) sheets, respectively. Figure9 shows cross-sectional images of the four-layered composites with sheets of different grammage values. For lower-grammage sheets, it was observed that resin could penetrate deeper into the sheet. Therefore, the interfaces (red arrow) between the CNF sheet layers and the epoxy layers were hard to distinguish in the cross-sectional image of the composite (Figure9a). On the contrary, it appears that epoxy resin could not penetrate deeply into the higher-grammage sheet as distinct layers of CNF sheets can be observed in the cross-sectional image (Figure9b). Therefore, the di fferent penetration behaviors of epoxy resinNanomaterials into CNF 2019, 9 sheets, x FOR PEER might REVIEW cause differences in the mechanical properties of the composites.10 of 13

Figure 7.FigureThe 7. (a The) thickness (a) thickness and and (b) (b density) density of of eacheach CM-CNFCM-CNF/epoxy/epoxy composite composite with withdifferent diff sheeterent sheet grammageNanomaterialsgrammage values 2019 , values9 and, x FOR numbers and PEER numbers REVIEW of sheetof sheet layers. layers. 11 of 13

The tensile properties of the composites are shown in Figure 8. The tensile strength of the composites made from 20 g/m2 sheets was 1.5 times greater than neat epoxy. Additionally, the CM- CNF/epoxy composites exhibited a lower breaking strain than neat epoxy. The elastic modulus of the composite increased three times higher than that of neat epoxy because of the effect of the CNF [18,26]. The four layers of sheets led to a further increase in strength compared with the two-layer sheet composites. However, as the sheet grammage increased from 20 to 40 g/m2, the composite became weak and brittle, and the breaking strain decreased. The tensile strength of the composite prepared using 40 g/m2 sheets decreased severely, and the elastic modulus was hard to determine as they were quickly broken. The effect of CNF sheets on mechanical properties of CNF/epoxy composites could be explained by the amount of CNF in the composite, and the penetration and adhesion of epoxy resin into the CNF sheet [26,32]. Saba et al. [32] prepared a CNF/epoxy composite by adding freeze-dried CNF powder into epoxy resin and reported that the tensile properties of the CNF/epoxy composite were increased by CNF loading up to a certain content, and then decreased owing to inhomogeneous dispersion of CNF in the epoxy matrix. Although the composite preparation process was different from the literature, it revealed that the reinforcing effect of CNF seemed to diminish with the increasing amount of CNF in the composite. The weight portions of the CNF sheet were 8% and 16% for lower-grammage (20 g/m2) and higher-grammage (40 g/m2) sheets, respectively. Figure 9 shows cross-sectional images of the four-layered composites with sheets of different grammage values. For lower-grammage sheets, it was observed that resin could penetrate deeper into the sheet. Therefore, the interfaces (red arrow) between the CNF sheet layers and the epoxy layers were hard to distinguish in the cross-sectional image of the composite (Figure 9a). On the contrary, it appears that epoxy resin could not penetrate deeply into the higher-grammage sheet as distinct layers of CNF sheets can be observed in the cross-sectional image (Figure 9b). Therefore, the different penetration behaviors of epoxy resin into CNF sheets might cause differences in the mechanical properties of the composites.

FigureFigure 8. Mechanical 8. Mechanical properties properties of of multilayered multilayered CM- CM-CNFCNF/epoxy/ epoxycomposites: composites: (a) strain–stress (a) strain–stresscurve curve ofof thethe composites with with two two layer layerss of CM of-CNF CM-CNF sheets, ( sheets,b) tensile ( bstrength,) tensile (c) strength, strain at break, (c) strain and (d at) break, and (d)elastic elastic modulus. modulus.

Figure 9. Cross-sectional images of CM-CNF/epoxy composites with (a) four layers of 20 g/m2 sheets and (b) four layers of 40 g/m2 sheets (red arrows indicate the interfaces between the CM-CNF sheet layer and epoxy layer).

4. Conclusions The CNF/epoxy composites in this study were prepared by impregnating epoxy resin into the CNF sheets, which differed in morphology, sheet preparation methods, sheet grammage, and number of sheet layers. Composite properties were evaluated in terms of light transmittance, haze,

Nanomaterials 2019, 9, x FOR PEER REVIEW 11 of 13

Figure 8. Mechanical properties of multilayered CM-CNF/epoxy composites: (a) strain–stress curve

Nanomaterialsof the composites2020, 10, 625 with two layers of CM-CNF sheets, (b) tensile strength, (c) strain at break, and (d11) of 13 elastic modulus.

2 Figure 9. CrossCross-sectional-sectional images of CM-CNFCM-CNF//epoxyepoxy composites with (a)) fourfour layerslayers ofof 2020 gg/m/m 2 sheets 2 and (b) fourfour layerslayers ofof 4040 gg/m/m 2 sheets (red(red arrowsarrows indicateindicate thethe interfacesinterfaces between the CM-CNFCM-CNF sheet layer and epoxy layer).layer). 4. Conclusions 4. Conclusions The CNF/epoxy composites in this study were prepared by impregnating epoxy resin into the The CNF/epoxy composites in this study were prepared by impregnating epoxy resin into the CNF sheets, which differed in morphology, sheet preparation methods, sheet grammage, and number CNF sheets, which differed in morphology, sheet preparation methods, sheet grammage, and of sheet layers. Composite properties were evaluated in terms of light transmittance, haze, and tensile number of sheet layers. Composite properties were evaluated in terms of light transmittance, haze, properties. It was revealed that solvent exchange of the CNF sheet could improve the transparency of the composite by reducing a haze. The CNF morphology was shown to affect the light transmittance and haze properties of the composite. The smaller fibrils and no-fibril bundles, which were obtained by carboxymethylation and high numbers of grinding passes, produced a composite with high transmittance and low haze. Low CM-CNF sheet grammage (20 g/m2) resulted in the composite with a higher light transmittance (>90%) and low haze value (<5%), and also demonstrated a reinforcing effect for the composite. However, a higher sheet grammage value resulted in decreased tensile properties and lower transparency of the composite. As the number of sheet layers increased, the tensile strength and thickness of the composite increased, whereas the transparency decreased owing to an increase in the haze of the composite. Consequently, a thick CM-CNF/epoxy composite with improved mechanical properties without a severe decrease in transparency was prepared by immersing solvent-exchanged low-grammage CM-CNF sheets with narrow and uniform fibrils into epoxy resin and stacking them.

Author Contributions: S.Y.P. and H.J.Y. designed the experiments and analyzed and discussed the data; S.Y.P., S.Y., W.I., and S.G. carried out the experiments; S.Y.P. carried out the preparation and characterization of the U-CNF, CM-CNF, CNF sheet, and the CNF/epoxy composite; S.Y. and W.I. carried out carboxymethylation and the SEM examination of the sheet and evaluation of the composite; S.G. carried out the evaluation of the composite; S.Y.P. wrote the original draft and revised; H.J.Y. supervised the whole study, reviewed the original draft, and revised. All authors have read and agreed to the published version of the manuscript. Funding: This research was partially supported by the Technological Innovation Program funded by the Ministry of Trade, Industry, & Energy (Project No. 10062717). Conflicts of Interest: The authors declare no conflict of interest.

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