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Cellulose Nanofiber-reinforced Unsaturated Polyester as a Potential Substitute for Glass Fiber-reinforced Plastics.
A. N. Nakagaitoa,b, S. Satoa,c, A. Satoa,d and H. Yanoa
a Research Institute for Sustainable Humanosphere, Kyoto University, Japan b Department of Chemistry and Biotechnology, Tottori University, Japan c NIPPON PAPER Chemicals CO., LTD., Japan d SEIKO PMC Corp., Japan
ABSTRACT
Nanocomposites of unsaturated polyester (UP) reinforced with nanofibers obtained by applying shear stress to
pulp fibers through extrusion were produced. Paper-like sheets of nanofibers were impregnated with UP,
stacked in layers and hot pressed at 1.5 MPa, a compressing pressure typical of plywood manufacturing. When
nanofibers extracted from needle-leaf unbleached pulp (NUKP) fibers were used, composites with 14 wt% UP
content exhibited flexural modulus of 13.6 GPa and strength of 260 MPa, with an Izod impact strength of 38
kJ/m2.
INTRODUCTION
Cellulose nanofibers are composed of amorphous and crystalline portions, the latter possessing tensile modulus
of 138 GPa [1]. Despite the presence of amorphous portions along their length, the practical tensile strength of
these nanofibers is supposed to be well above 2 GPa, based on the strength of pulp single fibers [2]. However
the extraction of nanofibers from plant fibers has been the bottleneck in terms of cost since nanofibrillation is
highly time and energy demanding, significantly raising the cost of originally low cost raw materials. Here we
present a new approach to nanofibrillate pulp that significantly reduces the energy input and increases yield, by
the application of shear forces in a twin-screw extruder.
MATERIALS AND METHODS
The raw materials from which the cellulose nanofibers were obtained consisted of needle-leaf unbleached pulp
(NUKP). The resin was unsaturated polyester dissolved in styrene and benzoyl peroxide was used as the free
radical initiator. Pulp fibers were fibrillated by an intermeshing co-rotating twin-screw extruder equipped with
a screw pattern optimized to apply shear force to the fibers. The pulp fibers had the moisture content adjusted
to approximately 30 wt% before feeding and the extruder’s barrel was cooled to prevent the temperature to
exceed the boiling point of water. The screw rotation speed was set to 400 rpm. Hand sheets were obtained by a
papermaking process according to TAPPI T 205 sp-02. The sheets were cut into 40 mm by 30 mm rectangles
and dried in a convection oven at 105°C for 1 hour. Unsaturated polyester (UP) was mixed with 1 wt% benzoyl
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peroxide and the sheets were dipped in the UP solution and maintained at reduced pressure for 30 minutes. The
impregnated sheets consisting of about 20 layers were stacked inside a metal mold and compressed at 1.5 MPa
and 90°C for 30 minutes.
RESULTS AND DISCUSSION
The fabrication of cellulose nanofiber-reinforced UP composites required relatively low compressing pressures.
Differently from nanocomposites fabricated with another thermoset resin, phenol formaldehyde, which
demanded pressures in the order of 50 MPa [3], UP-based nanocomposites required pressures less than 10
times lower. Higher pressures tend to squeeze out the impregnated UP resin from the sheets and the composites
fail by delamination. As shown in Table I the flexural modulus and strength of the composites were
significantly improved over the neat resin UP, however the impact strength was not striking (up to 38 kJ/m2).
Interface modification is still needed to enhance this property. The refining pre-treatment prior to extrusion
fibrillation seems not to affect the performance of the composites, nevertheless nanofibrillation by extrusion
significantly enhances the mechanical properties of the composites relative to the solely refined pulp
fiber-based composites.
Table I. Flexural properties of neat UP and NUKP nanofiber-reinforced UP composites.
Reinforcing fiber Compres. pres. (MPa) Resin cont. (wt%) E (GPa) σb (MPa)
Neat UP, no reinforcement - 100 3.07±0.09 126±6
NUKP 4 passes refiner 1.5 20.7 9.5±0.3 136±3
NUKP 0 pass refiner, extruded 1.5 15.5 12.2±0.2 254±2
NUKP 4 passes refiner, extruded 1.5 13.9 13.6±0.2 260±4
ACKNOWLEDGEMENTS
The authors are indebted to the New Energy and Industrial Technology Development Organization (NEDO) for
funding the project that made this study possible.
References
1. Nishino, T., Takano K., Nakamae, K., “Elastic Modulus of the Crystalline Regions of Cellulose
Polymorphs”, J. Polym. Sci., Part B: Polym. Phys., 33, pp. 1647-1651 (1995).
2. Page, D. H., El-Hosseiny, F., “The Mechanical Properties of Single Wood Pulp Fibres. Part VI. Fibril
Angle and the Shape of the Stress-Strain Curve”, J. Pulp Pap. Sci. 84, pp. 99-100 (1983).
3. Nakagaito, A. N., Yano, H., “Novel high-strength biocomposites based on microfibrillated cellulose having
nano-order-unit web-like network structure”, Appl. Phys. A, 80, pp. 155-159 (2005).
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Cellulose Nanofiber-reinforced Unsaturated Polyester as a
Potential Substitute for Glass Fiber-reinforced Plastics
A. N. Nakagaitoa,b, S. Satoa,c, A. Satoa,d and H. Yanoa
a Research Institute for Sustainable Humanosphere, Kyoto University, Japan
b Department of Chemistry and Biotechnology, Tottori University, Japan
c NIPPON PAPER Chemicals CO., LTD., Japand SEIKO PMC Corp., Japan
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Problem and proposed solution
• Disposal of GFRP;• Make composites with cellulose and UP
with mechanical properties comparable to GFRP;
• Bio-based nanofiber as substitute for glass fiber;
• Low cost nanofibrillation by extrusion.
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Intermeshing co-rotating twin-screw extruder
Screw pattern: continuous version of a kneader
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SEM of nanofibers obtained by twin-screw extruder
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Experimental
• Sheets obtained according to TAPPI T 205 sp-02
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Experimental
30 min., vacuum
90°C, 30 min., ≤ 5 MPa
Nanofiber sheets
UP + benzoyl peroxide
Metal mold CompositeRC 10~20%
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Effect of compressing pressure (E)
048
121620
1.5 5 10
Compressing pressure (MPa)
E (G
Pa)
UPPF
0
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Effect of compressing pressure (σb)
0
100
200
300
400
1.5 5 10
Compressing pressure (MPa)
σb (M
Pa)
UPPF
0
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Effect of impregnation time (E)
0
4
8
12
16
0.5 1 2
Impreg. time (h)
E (G
Pa)
NUKP
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Effect of impregnation time (σb)
050
100150200250
0.5 1 2
Impreg. time (h)
σb (M
Pa)
NUKP
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Effect of nanofibrillation by extrusion (E)
0
4
8
12
16
NO YES
Extrusion
E (G
Pa)
NUKPNBKP
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Effect of nanofibrillation by extrusion (σb)
050
100150200250
NO YES
Extrusion
σb (M
Pa)
NUKPNBKP
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Effect of refining pre-treatment (E)
0
4
8
12
16
0 1 4
Refiner passes
E (G
Pa)
NUKP
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Effect of refining pre-treatment (σb)
050
100150200250300
0 1 4
Refiner passes
σb (M
Pa)
NUKP
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Neat UP vs. NUKP/UP nanocomposites (E)
02468
10121416
neat UP NUKP/UP
E (G
Pa)
neat UPNUKP/UP
~4 x
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Neat UP vs. NUKP/UP nanocomposites (σb)
050
100150200250300
neat UP NUKP/UP
σb (M
Pa)
neat UPNUKP/UP
~2 x
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Neat UP vs. NUKP/UP nanocomposites (Izod)
0
10
20
30
40
50
neat UP NUKP/UP
kJ/m
2 neat UPNUKP/UP
~6 x
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Conclusions
• Nanofibrillation by twin-screw extruder;• Refiner pre-treatment unnecessary;• Compressing pressure ≤ 5 MPa;• Pulp nanofiber/UP composites:
E = 13.6 GPaσb = 260 MPa
Unnotched Izod impact strength = 38kJ/m2
• Expected water absorbance.
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Acknowledgement
New Energy and Industrial Technology Development Organization (NEDO),
Japan