Microfibrillated cellulose reinforced semi-crystalline polylactic acid composites: Thermal and mechanical properties

L. Suryanegara, A.N. Nakagaito*) and H. Yano

Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan

*) Present address: Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, 4-101 Koyama-cho Minami, Tottori, 680-8552, Japan

INTRODUCTION Polylactic acid (PLA) is a versatile polymer made from renewable agricultural raw materials that are fermented to produce lactic acid. The PLA can be processed similarly as polyolefines or other thermoplastic, although the thermal stability could be better. In a crystallized state, PLA has thermo-mechanical properties at high temperatures superior to PLA in an amorphous state. However, a long annealing time is required to fully crystallize PLA. In this study, the thermo-mechanical properties of composites based on microfibrillated cellulose (MFC) and partially crystallized PLA were evaluated, with the goal of reducing the time required to fabricate PLA parts while improving the thermo-mechanical properties. MATERIALS AND METHODS To evaluate the effect of reinforcement, PLA and MFC at fiber content of 0, 3, 5, 10, and 20 wt% were mixed in an organic solvent. After the solution was dried at room temperature followed by vacuum-drying, a kneader was used to obtain a homogenous compound which was hot pressed. The melted samples were immediately quenched in liquid N2 to bring them to a fully amorphous state. To obtain fully crystallized state, the amorphous sheets were annealed at 100 °C for 1 h. To attain samples with various crystallinities, the amorphous sheets at a fiber content of 10 wt% were annealed at 80 °C for different lengths of time. The thermo-mechanical properties of composites were studied with DSC, tensile test, DMA and creep test. The deforming behavior of composites was performed by a cantilever beam test in a forced convection oven. Rectangular samples were clamped at one end as cantilevers and subjected to a temperature of 110 °C for 3 h. RESULTS AND DISCUSSION Fig. 1 shows the effect of MFC reinforcement on the storage modulus of PLA in amorphous and fully crystallized states. In the amorphous state, the addition of MFC at a fiber content of 20 wt% improved the modulus of neat PLA by 50% from 3508 MPa to 5223 MPa at 20 °C (glassy state), and interestingly, by 27 times from 4 MPa to 109 MPa at 80 °C (rubbery state). This immense difference in modulus improvement at 80 °C compared to that at room temperature is explained by the fact that the matrix becomes extremely soft in the rubbery state and the reinforcement becomes much more noticeable at high temperatures. In the crystallized state, the addition of MFC at a fiber content of 20 wt% improved the storage modulus of crystallized neat PLA from 293 MPa to 1034 MPa at 120 °C. It should be emphasized that such a high storage modulus under high temperature could not be achieved by amorphous PLA/MFC composites or semi-crystalline PLA without MFC reinforcement. These results confirm that the combination of MFC reinforcement and crystallization of PLA in the matrix contributes to better heat resistance of the composite.

Fig. 1. Effect of MFC contents (wt%) on the temperature dependency of the storage modulus under (a) amorphous and (b) crystallized states

Table 1 reveals that the annealing time required to obtain PLA composite with fiber content of 10 wt% at a crystallinity (Xc) of 17% was only around one-seventh of the time needed to fully crystallize neat PLA (Xc: 41%). Interestingly, both materials had comparable rigidity above the glass transition temperature (Tg) and creep deformation at around Tg. Similar results were observed in the cantilever beam test, as seen in Fig. 2. The composite (Xc: 17%) did not show downward deformation during heating at 110 °C for 3 h. On the other hand, the amorphous composite (Xc: 0%) exhibited the highest downward deformation, which gradually decreased as the crystallinity increased up to 9%. The addition of fiber restricted the mobility of the polymer chains, and the presence of the crystalline structure of PLA also acted as reinforcement, decreasing the mobility of the PLA amorphous regions, and as a result increasing the composite’s stiffness.

Fig. 3 shows the typical stress–strain curves of crystallized neat PLA and the PLA/MFC composites with different degrees of crystallinity. The tensile strength (maximum stress) of amorphous PLA/MFC composite at a fiber content of 10 wt% was 60.8 MPa, a value similar to that of the fully crystallized neat PLA. The Young’s modulus of the composite was 4.22 GPa which is 20% higher than that of the fully crystallized neat PLA. In other words, the addition of 10 wt% MFC to amorphous PLA could increase the strength to a value as high as that of fully crystallized PLA with the advantage of a better modulus. Furthermore, the crystallization of PLA in the composites improved the mechanical properties. These results demonstrated that the addition of MFC in PLA reduces the time to crystallize PLA and improves the thermo-mechanical properties. In order to close to real condition in industrial application, we are trying to mold PLA/MFC composites by using injection molding (on going research). REFERENCES [1] Iwatake A, Nogi M, Yano H. Cellulose nanofiber reinforced polylactic acid. Compos Sci Technol 2008;68:2103-2116. [2] Suryanegara L, Nakagaito AN, Yano H. The effect of crystallization of PLA on the thermal and mechanical properties of microfibrillated cellulose- reinforced PLA composites. Compos Sci Technol 2009;69(7-8):1187-1192. [3] Suryanegara L, Nakagaito AN, Yano H. Thermo-mechanical properties of microfibrillated cellulose-reinforced partially crystallized PLA composites. Cellulose 2010;17:771-778.

Table 1. Degrees of crystallinity of PLA and PLA/MFC composites at a fiber content of 10 wt% with different annealing times

Fig. 2. Cantilever beam test of PLA and PLA/MFC 10wt% composites with different degrees of crystallinity at 110 °C for 3 h

Fig. 3 Typical stress–strain curves of crystallized neat PLA (Xc:41%) and PLA/MFC 10 wt% composites at crystallinities of: (a) 0%, (b) 17%, (c) 25%, and (d) 43%

Microfibrillated celluloseMicrofibrillated cellulose--reinforced reinforced semisemi--crystalline PLA composites:crystalline PLA composites:

The thermal and mechanical properties The thermal and mechanical properties

L. Suryanegara, A.N. Nakagaito and H. Yano

Laboratory of Active Bio-based MaterialsRISH -- KYOTO UNIVERSITY

Background

Bio-based polymer(PLA, PBS, PHB)

(PP, PE, PS, PVC) Petroleum-based

polymer

1. the ease of processing2. high productivity3. cost reduction

• Low heat resistance (amorphous)

• Slow crystallization speed

The drawbacks of PLA:

The advantages of PLA:

• Young’s Modulus (3 GPa)

• Strength (60 MPa)

• Similar process with PP

Fully Crystallized

PLA

20%

10%5%

3%

PLA

20%

10%5%3%

Amorphous

MFC ReinforcementCompos Sci Technol 2009(69):1187

A long annealing time is required to fully crystallized PLA composites

To evaluate the thermal and mechanical properties of MFC

reinforced partially crystallized PLA with the goal of reducing the time

required to fabricate PLA parts

Objective

The slow crystallization Main drawback

Materials

MFC: Celish KY-100G(10 wt% fiber content

slurry)Daicel Chemical Industries, Ltd.

PLA: LACEA H10099% L-lactic acidMitsui Chemicals, Inc.

Drying

Preparation of sample (PLA/MFC 10wt%)

MFC (6 g)

PLA (56 g)

Kneading

Hot pressing180°C 1MPa

0.5x4x40 mm

Mixing in dichloromethane

+50°C 24 h

160°C 40 rpm

Quenched by Liquid

N2

Annealed at 80oC

Annealing time at 80 °C vsDegree of crystallinity

Annealing Crystalline (%)

Time (min) PLA PLA/MFC 10wt%1 0 4

3 0.4 17

5 1.6 32

10 25 4220 40 43

Tensile test(PLA/MFC 10 wt%)

dc b a

PLA, Xc:40% (3.5 GPa)

Crystallinity: a) 0% (4.2 GPa)b) 17% (4.1 GPa) c) 25% (4.3 GPa)d) 43% (4.5 GPa)

1 mm/min

f

ba PLA

(Xc:40%)

d

c

Crystallinity:a) 0%, b) 9%, c) 17% d) 32%, e) 43%

DMA of PLA/MFC 10 wt%(span 21 mm, 1 Hz, 2 °C/min)

Creep Test(in tensile mode at 60°C, 1 MPa)

a

d

b

e

c

f g

PLA (40%)

Crystallinity:a) 0%,b)4%, c)9%, d)17%, e) 25%, f)32%,g) 43%

Cantilever Beam Test(Heating in oven at 110°C)

How to accelerate injection molding cycle

of PLA?

“the combination of nucleating agent and MFC reinforcement”

In industrial applications Holding 3min

Longer molding cycle

www.technologystudent.org/equip1/inject1.htm

PLA

PLA/PPA-Zn

PLA/PPA-Zn/MFC

PLA/Talc

PLA/MFC

Effect of nucleating agent(DSC isothermal at 130 °C)

Preparation of sample (PLA/PPA-Zn 1wt%/MFC 10wt%)

PLA + PPA-Zn + MFC

Oven drying, 50°C, 12 hrs

evaporated at room temp.

mixingDichloromethane

specimen

Injection molding

Vacuum-driedat 50°C, 24 hrs

T Barrel: 150 – 170 °CT Mold: 95 °C

Injecting time 11 s, holding time 10 s

Injection Molded Samples(Mold Condition: 95°C, 10 s)

PLA

PLA/PPA-Zn PLA/

MFC

PLA/PPA-Zn/MFC

Rigidity of injection molded samples(Mold condition: 95°C, 10 s)

PLA/PPA-Zn(Xc:10%)

PLA/PPA-Zn/MFC (Xc:15%)

PLA (Xc:2%)

PLA (Xc:40%)

Cantilever Beam Test(Heating in oven at 110°C 1h)

40% 2% 15% 15% 18% 23%

PLA PLA/PPA-Zn/MFC

a)

PLA/PPA-Zn

PLA

PLA/PPA-Zn/MFCPLA/PPA-Zn

40% 2% 15% 15% 18% 23%

b) After heating

Before heating

Conclusion

Cellulose nanofibers reinforcement:1.Improved the thermal and mechanical properties of semi-crystalline PLA2.Accelerated injection molding cycle of semi-crystalline PLA in the present of nucleating agent

Thank YouThank You

Acknowledgements1. Mitsui Chemicals, Inc., Japan for supplying the PLA

2. Dr. Takeshi Semba and Hiroaki Okumura

3. The Ministry of Education, Culture, Sports, Science

and Technology Japan.