Fabrication and Characterization of Biocomposites from Polylactic Acid and Bamboo Fibers

108 Pages·2017·3.82 MB·English

by  Sarah E. Royse

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Mechanical & Materials Engineering,
Engineering Mechanics Dissertations & Theses
Department of
8-2012
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Sarah E. Royse
University of Nebraska-Lincoln, [email protected]
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FABRICATION AND CHARACTERIZATION OF BIOCOMPOSITES FROM
POLYLACTIC ACID AND BAMBOO FIBERS

By

Sarah E. Royse

A THESIS

Presented to the Faculty of
The Graduate College at the University of Nebraska
In Partial Fulfillment of the Requirements
For the Degree of Master of Science

Major: Engineering Mechanics
Under the Supervision of Professor Joseph A. Turner
Lincoln, Nebraska
August, 2012
Fabrication and Characterization of Biocomposites from Polylactic Acid and Bamboo
Fibers

Sarah Royse, M.S.
University of Nebraska, 2012

Adviser: Joseph A. Turner
Environmental concerns have been motivating research in the field of
biodegradable materials, especially those from biological sources. Polylactic acid (PLA)
is one biopolymer with the potential to replace some standard synthetic polymers. PLA is
currently used for fibers, medical sutures, and some packaging, but is still used very little
compared with synthetic polymers. One thing that can be done to expand the potential
applications of PLA is to add fibers to create composite materials. Bamboo is a good
choice for these fibers because it is abundant in many parts of the world, grows very
quickly, and is widely unused.
Composites were created from PLA and short bamboo fibers, and their
mechanical and thermal properties were examined. Samples were fabricated by mixing
PLA pellets and bamboo fibers and extruding the mixture with a single-screw extruder.
The extruded material was collected and compression molded. Bamboo fibers were alkali
treated with sodium hydroxide (NaOH) before extrusion. Samples were created with four
different treated fiber concentrations: 5%, 10%, 20%, and 40% by weight. Samples with
20% untreated fibers and samples of pure PLA were created with the same fabrication
parameters and tested for comparison. The mechanical properties of the bulk composites
were analyzed with three-point bending and Charpy impact tests, as well as dynamic
mechanical analysis. Thermogravimetric analysis and dynamic mechanical analysis were
used to examine thermal properties. Nanoindentation and scanning electron microscopy
were used to study the properties of the embedded fibers and the morphology of the
matrix-fiber interface. Some theoretical modeling was also conducted to better
understand the experimental results.
The results show no significant improvement in properties was achieved through
the addition of bamboo fibers to the PLA. The poor composite performance is attributed
to weak interfacial strength between neighboring fibers and inefficient fiber length.
iv

ACKNOWLEDGEMENTS
I would like to thank all of the people who helped and supported me throughout
the pursuit of my master’s degree. Dr. Tuner, thank you for your guidance and expertise
and for always being available to help your students. Dr. Saiter, I’d like to thank you for
your guidance as well and for inspiring this project. I’d like to thank all the people who
made the EMME program possible, especially Dr. Negahban and Dr. Saiter. It has been
an enriching and enjoyable experience.
I had the pleasure of working in two labs with helpful, supportive people. I want
to express my gratitude to everyone in LECAP, Dr. Turner’s lab, and the BM3 who
assisted me with my experiments. Antonella, thank you for your guidance and
collaboration and for conducting DMA measurements for me. Larisa, thank you for
introducing me to this project and helping me get my research started. Khadidja, thank
you for helping me create samples. Severine, Celine, and Elham, thank you for your
willingness to help me with any problem, especially with the nanoindenter. Tim, thank
you for encouraging and supporting me. I appreciate all the times you helped me,
especially with the SEM. Finally, I’d like to thank my family. I could never have gotten
to this point without your support.
This work was supported by the Nebraska Center of Energy Sciences Research,
USA and by Région Haute Normandie, France. Financial support was also provided by
the US Department of Education FIPSE-ATLANTIS, grant number P116J070028 and by
the European Union through the Erasmus Mundus program.
v

CONTENTS
List of Figures .................................................................................................................. viii
List of Tables ................................................................................................................... xiii
Introduction ......................................................................................................................... 1
1.1 Environmental Concerns .......................................................................................... 2
1.2 Fiber-reinforced Polymer Composites ..................................................................... 3
1.3 Bamboo Fibers ......................................................................................................... 4
1.4 Polylactic Acid ......................................................................................................... 9
1.5 Previous Work with PLA/Bamboo Fiber Composites ........................................... 10
1.6 Fiber Treatments .................................................................................................... 11
1.7 Thesis Organization................................................................................................ 12
Materials and Methods ...................................................................................................... 14
2.1 Materials ................................................................................................................. 14
2.2 Macro Scale Characterization ................................................................................ 17
2.2.1 Three-point Bending ........................................................................................ 17
2.2.2 Impact .............................................................................................................. 20
2.2.3 Hardness .......................................................................................................... 23
2.2.4 Density ............................................................................................................. 24
2.2.5 Dynamic Mechanical Analysis ........................................................................ 24
vi

2.2.6 Thermogravimetric Analysis ........................................................................... 26
2.3 Micro/Nano Scale Characterization ....................................................................... 28
2.3.1 Ultramicrotomy ............................................................................................... 28
2.3.2 Nanoindentation............................................................................................... 35
2.3.3 Scanning Electron Microscopy ........................................................................ 42
Experimental Results ........................................................................................................ 44
3.1 Composite Fabrication ........................................................................................... 44
3.2 Macro Scale Characterization ................................................................................ 44
3.2.1 Three-point bending ........................................................................................ 44
3.2.2 Impact .............................................................................................................. 47
3.2.3 Hardness .......................................................................................................... 53
3.2.4 Density ............................................................................................................. 53
3.2.5 Dynamic Mechanical Analysis ........................................................................ 54
3.2.6 Thermogravimetric Analysis ........................................................................... 59
3.2.7 Discussion of Macro Scale Results ................................................................. 63
3.3 Micro/Nano Scale Characterization ....................................................................... 65
3.3.1 Nanoindentation............................................................................................... 65
3.3.2 Scanning Electron Microscopy ........................................................................ 69
3.3.3 Discussion of Micro/Nano Scale Results ........................................................ 71
vii

Theoretical Modeling ........................................................................................................ 72
4.1 Hirsch Model .......................................................................................................... 73
4.2 Modified Rule of Mixtures Model ......................................................................... 77
4.3 Directional Average Model .................................................................................... 82
4.3.1 Jackson and Cratchley model .......................................................................... 82
4.3.2 Averaging Technique ...................................................................................... 83
4.4 Discussion of Theoretical Modeling ...................................................................... 85
Conclusions and Future Work .......................................................................................... 87
References ......................................................................................................................... 90
viii

LIST OF FIGURES
Figure 1: Bamboo culm with node and internode locations. Bamboo fibers are found
inside the internodes. (image from Wikipedia)................................................................... 7
Figure 2: Cross-section of a bamboo internode. The bamboo fibers surround the vascular
bundles. (adapted from [18])............................................................................................... 8
Figure 3: Bamboo vascular bundle. The fibers, 1, are caps on the vascular bundles.
Groups of fibers are surrounded in parenchyma, 2. The vascular parts of the plant are 3-7.
(adapted from [19]). ............................................................................................................ 8
Figure 4: Schematic of a single-screw extruder with four heating zones. The central screw
rotates, pushing molten polymer through the barrel from the hopper (left) to the shaping
die (right). ......................................................................................................................... 16
Figure 5: Schematic of the three-point bending test configuration. The stress roller
deforms the specimen, creating a stress response in the specimen. .................................. 18
Figure 6: Ideal stress vs. strain graph for a ductile material. The material strength is the
maximum stress achieved, and the modulus is the slope of the portion of the curve
corresponding to elastic deformation. ............................................................................... 19
Figure 7: Schematic of impact test pendulum. The pendulum impacts the specimen at the
bottom of its swing. The energy absorbed during impact can be inferred from the
difference in height between the release point of the pendulum and the highest point it
reaches after impact. ......................................................................................................... 21
Figure 8: Top view of specimen during impact test. The notch opposite the impacter
controls the location of fracture. ....................................................................................... 22
ix

Figure 9: Charpy impact test specimen. The area under the notch is used to calculate the
fracture toughness from the impact energy. ...................................................................... 23
Figure 10: Sinusoidal applied force and material response (bold) in a dynamic mechanical
analysis test. The parameters of the signals are used to determine the dynamic mechanical
properties of the specimen. ............................................................................................... 25
Figure 11: Idealized specimen degradation curve from a thermogravimetric analysis test.
The onset temperature of degradation is determined from the slopes of the degradation
and pre-degradation portions of the curve. ....................................................................... 27
Figure 12: LKB Ultrotome III main unit with knife and specimen (a) entire unit; (b)
close-up of specimen arm and knife. ................................................................................ 29
Figure 13: LKB Ultrotome III control unit. This unit includes specimen elevation and
automation controls. .......................................................................................................... 30
Figure 14: LKB Ultrotome III macro feed knob for manual knife movement. The inner
and outer knobs are turned together for coarse movement. The outer knob is used for fine
movement. ......................................................................................................................... 31
Figure 15: Nanoindenter image of a PLA - bamboo fiber bundle interface area selected
for modulus mapping (50μm, 1.00 Hz scan rate). ............................................................ 39
Figure 16: Nanoindenter image of bamboo fiber bundle area selected for nanoindentation
(20μm, 1.00 Hz scan rate). ................................................................................................ 40
Figure 17: Nanoindenter image of bamboo fiber bundle area with quasi-static indentation
points selected for piezoautomation. (color altered for visibility). ................................... 41