Cellulo nanofiber-reinforced polylactic acid
Atsuhiro Iwatake,Masaya Nogi,Hiroyuki Yano *
darma
Rearch Institute for Sustainable Humanosphere,Kyoto University,Uji,Kyoto 611-0011,Japan
a r t i c l e i n f o Article history:
Received 18September 2007
Received in revid form 4March 2008Accepted 18March 2008
Available online 26March 2008Keywords:A.Fibers
A.Nano composites
B.Stress–strain curves
B.Thermo-mechanical properties
a b s t r a c t
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The reinforcement of polylactic acid (PLA)using microfibrillated cellulo (MFC,mechanically fibrillated pulp,mostly consisting of nanofibers)is reported,with the goal of making sustainable ‘green-compos-ites’.The production procedure to attain uniform dispersion of MFC in a PLA compound was assd,and then mechanical and thermo-mechanical properties of the sheets after hot-pressing of the com-pounds were studied.Needle-leaf Bleached Kraft Pulp (NBKP)and refiner-treated NBKP were also ud to study the effects of filler morphology.When MFC was premixed with PLA using organic solvent and the mixture was kneaded after the removal of the solvent,the MFC was uniformly disperd in the PLA.The MFC incread Young’s modulus and tensile strength of PLA by 40%and 25%,respectively,with-out a reduction of yield strain at a fiber content of 10wt%.On the other hand,NBKP reduced the yield strain by 30%and reduced the strength by 15%at a fiber content of 5wt%.
Ó2008Published by Elvier Ltd.
1.Introduction
The plant cell wall consists of nanofibers called cellulo micro-fibrils.Since the nanofibers are compod of extended cellulo chains forming a mi-crystalline structure,their thermal expan-sion
is as low as that of quartz [1],and their tensile strength is esti-mated to be about five times that of mild steel,bad on the tensile test of kraft pulp single fibers [2].
Recently,we demonstrated that microfibrillated cellulo (MFC),which consists of mechanically fibrillated pulp into nano to submicron wide fibers forming a web-like network,shows much promi as reinforcement of composites [3–6].MFC sheet-molded phenolic resin composites with 80–90wt%fiber content exhibited strength equivalent to that of mild steel or magnesium alloy.Becau of the high performance of the composites and the fact that cellulo nanofibers are the most abundant and renewable biomass resource on earth,MFC has attracted attention as poten-tial reinforcement of materials for u in automobiles,buildings,mobile computers,and many other products.
pie是什么意思In this study,the potential of MFC reinforcement of thermoplas-tic polymer subjected to compression and injection moldings was studied.To develop fully bioresource-bad nanocomposites,we ud polylactic acid (PLA)as the resin.
The reinforcement of PLA using plant and pulp fibers has been studied with the aim of developing sustainable ‘green-composites’[7,8].Although the Young’s modulus of the PLA could be incread sig
nificantly with an increa of plant fiber content,the yield strain decread,and as a result,the strength decread.To overcome
the drawbacks,reinforcement with cellulo nanowhiskers or microcrystalline cellulo has been studied by melt extrusion com-pounding using twin screw extruders [9,10].However,since the kneading process resulted in the flocculation of the microcrystal-line cellulo and whiskers as well as deterioration of the PLA,composites exhibiting higher strength than neat PLA could not be obtained.
Considering that phenol-formaldehyde (PF)resin-impregnated MFC sheet moldings demonstrated a large yield strain [4,5]and the casting film prepared from MFC and polyvinyl alcohol (PVA)solution resulted in a two-fold increa in strength over that of neat PVA along with high yield strain [11],it appears that uniformly disperd cellulo nanofibers in PLA can increa the strength of PLA composites.Thus,we first studied the pro-duction procedure to attain uniform dispersion of cellulo nanofibers in a PLA compound,and we then found out that the prepared sheet at a fiber content of 10wt%,after kneading and hot-pressing of the compounds,exhibited 25%higher tensile strength than that of a neat PLA resin sheet without a reduction of yield strain.2.Experimental 2.1.Materials
Polylactic acid (LACEA H-280,Mitsui Chemicals,Inc.,Japan)was ud as matrix.Microfibrillated cellulo (MFC,Celish KY110G,water slurry containing 10wt%fiber,Daicel Chemical Industries,Ltd.,Japan)was ud as filler.NBKP (Needle-leaf Bleached Kraft Pulp)and refiner-treated NBKP (eight pass)were also ud to investigate the effects of filler morphology.
0266-3538/$-e front matter Ó2008Published by Elvier Ltd.doi:10.pscitech.2008.03.006
*Corresponding author.Tel.:+81774383669;fax:+81774383658.E-mail address:yano@rish.kyoto-u.ac.jp (H.Yano).Composites Science and Technology 68(2008)
2103–2106
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2.2.Preparation of PLA and MFC mixture
MFC slurry containing3gfiber was mixed with270g of acetone to make1wt%MFC suspension.When the MFC was thoroughly disperd in the solvent by stirring,adequate amount of PLA was added gradually and stirring was kept for5h at ambient tempera-ture.The acetone and water of the mixture was evaporated under vacuum at70°C.The mixture from which water and acetone had been removed was kneaded by a twin rotary roller mixer(Labo Plastomill,Toyo Seiki Seisaku-sho,Ltd.,Japa
n).The compounding was carried out for12min at a rotary speed of40rpm at140°C. We hereafter call it the solvent method.
As a comparison,MFC water slurry containing10wt%fiber was added directly to the melted PLA,and the mixture was kneaded at 140°C,which we hereafter refer as the direct mixing method.
The compound was crushed into small pieces and compresd in a die at160°C and0.5MPa for5min followed by1MPa for another5min.The resulting sheet was0.3mm thick.Specimens 40mm long and5mm wide were prepared from the sheet.
2.3.Mechanical testing
Tensile properties of the neat PLA and composite materials were measured using a universal mechanical testing machine(Instron 3365).The specimen gage length was20mm and the testing speed was t to1mm/min.Six composite specimens were tested for each t of samples.
2.4.Dynamic mechanical thermal analysishayden
Temperature dependency of dynamic viscoelastic properties of the composites were measured by the forced vibration method in tensile mode(RHEOVIBRON DDV-25FD,Orientec A&D Co.Ltd., Japan)
,with a chuck distance of20mm,preload of5g,frequency of1Hz,and heating rate of1.5°C/min.
3.Results and discussion
3.1.Premixing of MFC and PLA using acetone
The tensile stress–strain curves of the sheets prepared from the compounds by the solvent method and direct mixing method are compared in Fig.1.MFC content in the sheets was5wt%.As can be en from Fig.1,the Young’s modulus of the MFC/PLA compos-ite prepared by the solvent method is higher than that of a neat PLA sheet.In addition,contrary to the previous results of rearch on the microcrystalline cellulo-reinforced PLA composite[7],the MFC/PLA composites maintained the yield strain of neat PLA.As a result,the MFC/PLA composites exhibited higher tensile strength than that of neat PLA.Specifically,the average values of Young’s modulus and tensile strength of the PLA incread from3.4GPa and56.2MPa to4.3GPa and66.0MPa,respectively.Meanwhile, the MFC/PLA composites made by the direct mixing method did not show any improvement in Young’s modulus.In addition,the yield strain of the composites was smaller than that of neat PLA, resulting in the reduction of tensile strength by about10%.
Fig.2shows the microscopic images of composites prepared by the solvent method and the direct mi
xing method.The image of the composite made by the solvent method shows that cellulo nanofibers were disperd uniformly in the PLA,while many agglomerations were obrved in the composite made by the direct mixing method.The reduction in yield strain of PLA in the latter ca can be attributed to the agglomerates.
The sheet prepared by the solvent method but without the kneading process exhibited a Young’s modulus of4.2GPa and a tensile strength of60.7MPa,indicating that the cellulo nano-fibers were more uniformly disperd by
kneading.
Fig.1.Typical stress–strain curves comparing the solvent method,direct mixing
method compounding,and neat
PLA.
Fig.2.Microscopy images of composites prepared by the solvent method(upper)
and the direct mixing method(lower).
2104 A.Iwatake et al./Composites Science and Technology68(2008)2103–2106
3.2.Effects offiller morphology on the mechanical properties of PLA composites
To investigate the effects of microfibrillation of pulp on the mechanical properties of PLA composites,three types offiller,that is,NBKP,refiner-treated(eight pass)NBKP,and MFC were mixed with PLA by the solvent method.Thefiller content was5wt%.
anosaFig.3shows typical morphology of thefillers.Pulp has a smooth surface with30–50l m in diameter,refiner-treated pulp has a fibrillated surface with a diameter similar to pulp and MFC is com-pletely disintegrated into nano to submicron widefibers forming a network.
catalogAs shown in Fig.4,the addition of pulp slightly incread the Young’s modulus,but reduced the yield strain by30%and reduced the strength by15%.Thefibrillation of the pulp’s surface(refiner-treated)imp
大学英语四级准考证号查询roved the Young’s modulus as well as the yield strain, resulting in strength increa by10%.Significant improvement was obrved in the MFC-reinforced PLA.Since MFC attained the in-crea of Young’s modulus,as much as25%,without a reduction of the yield strain,the strength improved by20%compared to neat PLA.
It should be emphasized that the reinforcement of refiner-treated pulp slightly increas the Young’s modulus of PLA, whereas MFC increas Young’s modulus of PLA by25%,as shown in Fig.4.Favier et al.[12]studied the effect of cellulo nanowhis-ker reinforcement of latex of poly(styrene-co-butyl acrylate)and found that the prence of percolated nanowhiskers,that is,the formation of a rigid network resulting from strong interactions between adjacent whiskers by hydrogen bonding,enabled effective reinforcement of the matrix and achieved a drastic increa of shear modulus at a whisker content of6wt%.Furthermore,Samir et al.[13]proved that the percolation effect is more pronounced when cellulo nanofiber is ud as reinforcement due to the addi-tional entanglement effect of the elements.Thus,as can be specu-lated bad on the comparison of SEM images(Fig.3),the fibrillation of pulp’s surface is not enough to create a strong network at a lowfiller content.On the other hand,microfibrillated cellulo creates afine network even at a lowfiller content such as 5wt%,restraining polymer deformation.
3.3.Effects of MFC content on the mechanical and thermo-mechanical properties of PLA composites
The effect offiller morphology indicated the importance of net-work formation in improving mechanical properties.Hence,the reinforcement effect of MFC on the mechanical and thermo-mechanical properties of PLA composites was studied as a function of MFC content.
Fig.5shows the stress–strain curves of tensile test of MFC/PLA composites with differentfiber contents.As can be en,the3wt% MFC/PLA composite showed a stress–strain curve similar to that of neat PLA.In other words,the effect of the addition of MFC could not be obrved.However,5wt%MFC/PLA composites evidenced a reinforcing effect.Young’s modulus incread by25%compared to neat PLA while retaining the same yield strain of2.5%.Hence, the tensile strength of PLA incread from about50MPa to 70MPa.Similar results were obrved in temperature dependency of storage modulus at1Hz(Fig.6).The storage modulus of3wt% MFC/PLA composites did not show a significant difference com-pared to PLA above100°C.On the other hand,5wt%
MFC/PLA Fig.3.Filler morphology of Needle-leaf Bleached Kraft Pulp(NBKP),refiner-treated(eight pass)NBKP,and MFC.Scale bar:10l
m.
Fig.4.Effects of morphological changes offiller on stress–strain
curves.
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Fig.5.Effects of MFC content(wt%)on stress–strain curves.
A.Iwatake et al./Composites Science and Technology68(2008)2103–21062105
composites showed a higher Young’s modulus than that of neat PLA.
Additional improvements in Young’s modulus and strength were obrved when the MFC content incread from 5wt%to 10wt%as shown in Fig.5.The average Young’s modulus and tensile strength of 10wt%MFC/PLA composites attained 4.7GPa and 75.0MPa,respectively,without a reduction of yield strain.It is worthy to note that the addition of 10wt%MFC could improve the Young’s modulus of PLA by 40%and the strength by 25%.However,a 15wt%or 20wt%addition of MFC made the compos-ites brittle and decread their strength.Fracture initiating points such as flocculated nanofibers incread and dominated the mechanical properties of MFC/PLA composites.The improvement of the procedure for making high fiber content compounds is necessary to obtain further increments in strength as well as Young’s modulus of PLA.
The constant storage modulus of 10wt%MFC/PLA composites above glass transition temperature (T g )of PLA,that is,from 70°C to 120°C should be emphasized (Fig.6).Similar phenomena were obrved in cellulo nanowhisker reinforcement of latex of poly(styrene-co -butyl acrylate)[12]and MFC reinforcement of amylopectin-glycerol blend [14].The constant storage modulus suggests that the cellulo fiber network interconnected by hydro-gen bonds resists the applied stress independently of the softening of PLA.The result shows clearly the advantage of MFC over plant and pulp fibers in the reinforcement of thermoplastic polymer.4.Conclusions
The reinforcement of PLA using microfibrillated cellulo (MFC)was studied to know the potential of reinforcement by a nanofiber
network,with the goal of making sustainable ‘green-composites’.MFC was premixed with PLA using organic solvent and the mixture was kneaded to attain uniform dispersion of MFC in PLA.The uni-formly disperd MFC reinforcement incread the Young’s modu-lus and tensile strength of PLA by 40%and 25%,respectively,without a reduction of yield strain at a fiber content of 10wt%.Fur-thermore,the storage modulus of the composites was kept con-stant above glass transition temperature of matrix polymer.MFC is a promising reinforcement of PLA composites.Acknowledgements
The authors would like to thank Dr.A.N.Nakagaito,Rearch Institute for Sustainable Humanosphere,Kyoto University for valu-able suggestions.
This rearch was supported by a Grant-in-Aid for Scientific Re-arch (B)(No.1538012,2003.4–2007.3)from the Ministry of Edu-cation,Culture,Sports,Science,and Technology,Japan,and a Grant-in-Aid for Rearch and Development for regional innova-tion consortium (No.17S5018,2006.9–2007.3)from the Ministry of Economy,Trade and Industry,Japan.References
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Fig.6.Effects of MFC contents (wt%)on the temperature dependency of storage modulus.
2106 A.Iwatake et al./Composites Science and Technology 68(2008)2103–2106
