KURITA Hiroki，BERNARD Chrystelle，LAVROVSKY Agathe，NARITA Fumio

1.Department of Frontier Sciences for Advanced Environment，Graduate School of Environmental Studies，Tohoku University，Sendai，Japan；2.Frontier Research Institute for Interdisciplinary Sciences，Tohoku University，Sendai，Japan；3.ELyTMaX UMI3757，CNRS?Université de Lyon?Tohoku University，International Joint Unit，Tohoku University，Sendai，Japan；4.Department of Materials Science and Engineering，National Institute of Applied Sciences of Lyon（INSA?LYON），Villeurbanne，F(xiàn)rance

Abstract: Biodegradable polymers are highly attractive as potential alternatives to petroleum-based polymers in an attempt to achieve carbon neutrality whilst maintaining the mechanical properties of the structures. Among these polymers，polylactic acid（PLA）is particularly promising due to its good mechanical properties，biocompatibility and thermoplasticity. In this work，we aim to enhance the mechanical properties of PLA using mechanically-defibrated cellulose nanofibers（CNFs）that exhibit remarkable mechanical properties and biodegradability. We also employ fused deposition modeling（FDM），one of the three-dimensional printing methods for thermoplastic polymers，for the low-cost fabrication of the products. Mechanically-defibrated CNF-reinforced PLA matrix composites are fabricated by FDM. Their tensile properties are investigated in two printing directions（0°/90° and +45°/?45°）. The discussion about the relationship between printing direction and tensile behavoir of mechanically-defibrated CNF-reinforced PLA matrix composite is the unique point of this study. We further discuss the microstructure and fracture surface of mechanically-defibrated CNF-reinforced PLA matrix composite by scanning electron microscope.

Key words：cellulose nanofiber（CNF）；polylactic acid（PLA）；tensile property；fused deposition modeling

0 Introduction

Polymers are widely used due to their flexibili?ty，low fabrication cost，workability and so on.However，nowadays，polymers have been pointed out for the enormous plastic pollution they generate in oceans and landfill. In addition，their recycling and incineration are often complex and expensive.Therefore， interest in biodegradable polymers，such as polylactic acid（PLA），polybutylene adi?pate terephthalate and polybutylene succinate［1］，has surged as they may be used as replacements for pe?troleum-based polymers，thereby assisting achiev?ing carbon neutrality. In particular，PLA attracts at?tention due to its strength，stiffness and biocompati?bility，although it also possesses low toughness and biodegradability，and is expensive［2-3］.

Cellulose nanofibers（CNFs）are regarded as promising next-generation fibers owing to their sus?tainability，outstanding mechanical properties，bio?degradability and so on［4-5］. CNFs are generally ob?tained by either chemical treatment or mechanical defibration. 2，2，6，6-tetramethylpiperi-dinyloxy（TEMPO）oxidation is one of the most well-known chemical treatments to obtain CNFs［6-7］. However，it has been reported that TEMPO is toxic and pol?lutes the environment［8］. In contrast，aqueous coun?ter collision（ACC）methods based on mechanical defibration can obtain CNFs by using only water，al?though CNF bundles（as opposed to single nanofi?bers）are often produced［9］.

Three-dimensional（3D）printing is a promising approach for the fabrication of complex products at low cost. Various methods，such as fused deposition modeling（FDM）and stereolithography for thermo?plastic polymers，additive manufacturing for metals and direct ink writing for ceramics，have been stud?ied［10］. In particular，F(xiàn)DM is a widely distributed 3D printing method for thermoplastic polymers，due to its low fabrication costs，capability of printing largesized products，easy maintenance，and variation of raw filaments. The fabrication and mechanical prop?erties of carbon fiber-reinforced thermoplastics by FDM have been recently reported by Li et al［11］.

Xie et al.［12］reported that epoxy resin can be en?hanced by the molecular chain cross-linkage of ep?oxy with extremely low CNF addition. This strengthening mechanism shifts to a rule of mixture（i.e. mechanical enhancement）with higher CNF ad?dition. Moreover，Narita et al.［13］revealed that ag?glomerated CNFs（i.e. a CNF bundle）can be re?garded as single CNF with a small aspect ratio by fi?nite element analysis.

In this study，we fabricate CNF-reinforced PLA matrix composites by FDM. Considering envi?ronmental load，we select mechanically-defibrated CNFs by an ACC method. We investigate the ten?sile properties of the 3D-printed CNF-reinforced PLA matrix（PLA-CNF）composites in two print?ing directions of 0°/90° and +45°/?45°. Further?more，we discuss these tensile properties on the ba?sis of scanning electron microscopy（SEM）images.

1 Materials and Methods

As starting materials，we prepared PLA pel?lets（Terramac? TE-2000，Unitika Ltd.，Japan）and mechanically-defibrated dry CNFs（BiNFi-s?WFo-UNDP，Sugino Mashine Ltd.，Japan）. The PLA pellets and CNF were mixed at 12 000 r/min using a mixer（TM-8100，TESCOM Denki Co.，Ltd.，Japan）and the CNF volume fraction was con?trolled between 0 and 0.5 %（volume fraction）. Af?ter mixing，PLA-CNF filaments were prepared from the mixed powder at 168 ℃by an extruder（Filabot Ex2，F(xiàn)ilabot，USA）.

Tensile testing was performed in compliance with JIS K 7161 1BA［14］. Tensile specimens（Fig.1）were printed by a 3D printer（L-DEVO M2030TP，F(xiàn)usion technology Co.，Ltd.，Japan）with a printing speed of 30 mm/s，a filling density of 100%，a lamination pitch of 0.2 mm，a nozzle temperature of 200 ℃and a nozzle hole diameter of 0.4 mm in two printing directions of 0°/90° and+45°/?45°（Fig.1）. Fig.2 shows the dimension of the tensile specimens. The stage temperature was controlled between 40 ℃and 70 ℃. Tensile testing was carried out by using a universal testing machine（AutographTMAG-Xplus，Shimadzu Corporation，Japan）with a crosshead speed of 1 mm/min. We as?sumed that the volume is constant（i.e. the crosssectional area proportionally decreases by applying the load）and that the true strain（εT）and true stress（σT）were estimated as

Fig.1 3D-printed specimens using the fused deposition modeling in the 0°/90°and +45°/-45°directions

Fig.2 Dimension of tensile specimens (JIS K 7161 1BA[14])

where εNand σNare the nominal strain and the nomi?nal stress，respectively.

After testing，the fracture surface of the PLACNF composite was analyzed by SEM （JSM-7800F，JEOL Ltd.，Japan）. The preparation of the specimens consisted of the following steps：（1）Em?bedding 3D-printed PLA-CNF composites in acrylic resin；（2）polishing them with waterproof SiC paper of #600，#1000，#1500，#2400 and diamond slur?ries of 6 and 1 mm；（3）polishing the specimen sur?face by argon-ion spattering，before（4）coating with platinum. The microstructure and fracture surface of the 3D-printed PLA-CNF composites were ob?served by SEM with an accelerating voltage of 5 kV.

2 Results and Discussion

The CNFs were not burned during extrusion and white PLA-CNF filaments were obtained. The apparent CNF agglomeration was not visually ob?served. Fig.3 shows the relationship between speci?men shape and stage temperature. The specimens were deformed because of thermal shrinkage when the stage temperature was high.Therefore，we deter?mined 50 ℃as the stage temperature in this study.Fig.4 shows the printed PLA-CNF 0.5 %（volume fraction）tensile specimen in the 0°/90° direction. It is well known that voids are formed along the printing direction inside the fabricated products by FDM and the printing direction inside the tensile specimens was also visible［15］. Fig.5 shows the SEM images of CNFs inside the PLA-CNF specimen. The CNFs were agglomerated in PLA at any CNF volume frac?tion and the number of agglomerated CNF clusters was proportionally increased with the CNF volume fraction. The PLA/CNF interface was not intimate，possibly because PLA is hydrophobic while CNF has hydrophilic groups on its surface［16-17］. This result implies that the load transfer efficiency at the PLA/CNF interface is low. Furthermore，the CNF orien?tation along the fiber direction was not observed.

Fig.3 Relationship between specimen shape and stage tem?perature

Fig.4 Printed PLA-CNF 0.5% (volume fraction) tensile specimen in the 0°/90°direction

Fig.5 CNF agglomeration in PLA-CNF 0.1% (volume fraction)tensile specimen

Fig.6 Typical stress-strain curves of PLA-CNF compos?ites printed at 0°/90°and +45°/?45°

Fig.6 shows typical stress-strain curves of the PLA-CNF composites printed in the 0°/90° and +45°/?45° directions. The PLA-CNF composites printed in +45°/?45° showed an apparent plastic deformation，while the PLA-CNF composites print?ed in 0°/90° showed a proportional relationship be?tween tensile stress and strain，thus being looked like to exhibit a brittle behavior. Fig.7 shows the CNF volume fraction versus ultimate tensile strength（UTS）and fracture elongation of the PLACNF composites. The UTS and fracture elongation of the PLA-CNF composites printed in +45°/?45°were higher than the ones of the PLA-CNF compos?ites printed in 0°/90°. The reason lies in the fact（Fig.8）that in the PLA-CNF composites printed at 0°/90°，the filament in the direction perpendicular to the loading direction does not contribute to bearing the applied tensile load；thus，the stress concentrates in the void at the intersection between the two direc?tions. In contrast，for PLA-CNF composites printed at+45°/ ?45°，the filaments in both directions bear the applied tensile load. Moreover，it seems that the stretching of filaments to loading directions provides the apparent plastic deformation and higher fracture elongation. A CNF addition of only 0.1%（volume fraction）increased the UTS and fracture elongation of PLA printed at +45°/?45° by 8% and 11%.However，it was inclined that a CNF addition higher than 0.3%（volume fraction）decreased the UTS and fracture elongation of PLA.

Fig.7 CNF volume fraction versus ultimate tensile strength and fracture elongation of PLA-CNF composites printed in 0°/90°and +45°/?45°

Fig.8 Mechanical behavior of PLA-CNF composites print?ed a 0°/90°and +45°/?45°under tensile load

Fig.9 shows the SEM fractographs of the print?ed PLA-CNF composites. Many pores were ob?served between the printed filaments and the frac?ture surface appears flat（i.e. a typical fracture sur?face of brittle materials）（the image in the first row in Fig.9）. Therefore，these fractographs support the stress-strain curves of the PLA-CNF composites.Few distributed CNFs were observed on the frac?ture surface（the three images in the second row in Fig.9）. These distributed CNFs were exposed with?out breaking. This result implies that the distributed CNFs were pulled out after bearing the applied ten?sile load. However，the agglomerated CNF clusters were broken and large pores were observed around them（Fig.10）. The number of agglomerated CNF clusters increased with the CNF volume fraction. It seems that the agglomerated CNF clusters decrease the tensile properties of PLA rather than increase them. Therefore，it is likely that homogeneously dispersed CNFs significantly increase the tensile properties of PLA.

Fig.9 SEM fractographs of PLA-CNF 0.1 % (volume frac?tion)composite printed in the 0°/90°direction.

Fig.10 CNF agglomeration on fracture surface of PLACNF 0.3 % (volume fraction) composite printed in the 0°/90°direction.

3 Conclusions

Mechanically-defibrated CNF-reinforced PLA matrix composites were fabricated by FDM. The tensile properties of the 3D-printed CNF-reinforced PLA matrix（PLA-CNF）composites were investi?gated in two printing directions：0°/90° and +45°/?45°. The microstructure and fracture surface were also analyzed by SEM.

Several voids were observed inside the printed specimens along the printing direction，as reported in previous studies. CNFs were agglomerated in PLA and a number of agglomerated CNF clusters was proportionally increased with the CNF volume fraction. The delaminated PLA/CNF interface at?tributed to the mismatch of hydrophilicity implies that the load is not effectually transferred from PLA to CNFs at the PLA/CNF interface. The CNF ori?entation along the printing direction was not ob?served.

The UTS and fracture elongation of the PLACNF composites printed at +45°/?45° were high?er than the ones of the PLA-CNF composites print?ed at 0°/90°. In the PLA-CNF composites printed at 0°/90°，the filament in the direction perpendicular to the loading direction does not likely contribute to bearing the applied tensile load and the stress con?centrates to the void between the filament. In con?trast，in PLA-CNF composites printed at +45°/?45°，the load is beared by the filaments printed in both directions. The stretching of the filaments and their re-orientation in the loading direction induce ap?parent plastic deformation and higher fracture elon?gation.

The exposition of distributed CNFs without breaking，the failure of CNF clusters and large pores around CNF clusters were observed on the fracture surface. Hence，the agglomerated CNF clusters decrease the tensile properties of PLA rath?er than increase them. Consequently，it was re?vealed that homogeneously dispersed CNF signifi?cantly increase the tensile properties of PLA.

Consequently， the mechanically-defibrated CNF reinforced PLA composite，which can be fab?ricated eco-friendly and in low-cost，has a potential to be used as a green composite with further studies to understand its strengthening mechanism and ef?fective enhancement.