THE EFFECTS OF CORE SHELL IMPACT MODIFIER ON PROPERTIES AND CRYSTALLIZATION BEHAVIOUR OF POLY(LACTIC) ACID
by
ADIBAH BINTI BORHAN
Thesis submitted in fulfillment of the requirements for the degree of
Master of Science
August 2018
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ACKNOWLEDGEMENT
Glory and Praise be to Allah S.W.T. for His Guidance and infinite Bounties to us. Peace be upon the last messenger, Muhammad S.A.W., his family members, his companions and those who follow him.
First and foremost I would like to express my sincere gratitude to my respected supervisor, Assoc. Prof. Dr. Razaina Mat Taib for her continuous guidance, supervision and support. The dedication, motivation and immense knowledge from her had encouraged me to step further in the research field.
My special and utmost thanks to all the School of Materials and Mineral Resources Engineering staffs for their technical supports especially, Polymer Engineering division whom had rendered their help during the period of my research.
This research would never have been completed without their assistance.
My appreciation to my beloved parents and siblings; without them I would not be able to stand where I am today. They always have faith in me for whatever decisions I have made. Their love, thoughts and sacrifices always be my courage to step beyond my limit.
Last but not least, my thankfulness to my fellow friends for their supports through my thick and thin journey. I also would like to convey my thanks to every other individual who had made this research work possible but not mentioned personally.
Finally, special appreciation to the Ministry of Higher Education, Malaysia and Fundamental Research Grant Scheme (FRGS) for granting the research fund for this project (Project No. 203.PBAHAN.6071287).
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TABLE OF CONTENTS
Page
ACKNOWLEDGEMENT ii
TABLE OF CONTENTS iii
LIST OF TABLES vi
LIST OF FIGURES vii
LIST OF ABBREVIATIONS xii
LIST OF SYMBOLS xiii
ABSTRAK xiv
ABSTRACT xvii
CHAPTER ONE: INTRODUCTION1
1.1 Introduction 1
1.2 Problem statements 3
1.3 Research objectives 5
1.4 Scope of research 5
1.5 Dissertation overview 6
CHAPTER TWO: LITERATURE REVIEW7
2. 1 Polylactic acid 7
2.1.1 Thermal properties 10
2.1.2 Mechanical properties 11
2.1.3 Modification of PLA 12
2.2 Core shell impact modifier 15
2.2.1 Synthesis of the core shell impact modifier 17 2.2.2 Toughening mechanism of elastomer-based and CSIM 19
2.3 Polymer crystallization 21
2.3.1 Theory of polymer crystallization 22
2.3.2 Formation of crystals 23
2.3.3 Kinetics of crystallization 27
2.4 Summary 35
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CHAPTER THREE: MATERIALS AND METHODOLOGY36
3.1 Materials 36
3.1.1 Polylactic acid 36
3.1.2 Core shell impact modifier 36
3.2 Preparation of materials 37
3.2.1 Melt blending 37
3.2.2 Compression molding 38
3.3 Characterization 38
3.3.1 Fourier transform infrared spectroscopy (FTIR) 38
3.3.2 Thermogravimetric analysis (TGA) 39
3.3.3 Differential scanning calorimetry (DSC) 40
3.3.4 Polarized optical microscopy (POM) 41
3.3.5 Dynamic mechanical analysis (DMA) 42
3.3.6 Tensile test 42
3.3.7 Impact test 42
3.3.8 Scanning electron microscopy (SEM) 43
3.3.8.1 Field Emission Scanning Electron Microscopy 43 (FESEM)
3.3.8.2 Extreme High Resolution Field Emission 43 Scanning Electron Microscopy (XHR-FESEM)
3.3.9 Transmission electron microscopy (TEM) 43
3.3.10 Color analysis 44
CHAPTER FOUR: RESULTS AND DISCUSSION45
4.1 Core shell impact modifier (CSIM) characterization 45 4.1.1 Transmission electron microscopy (TEM) 45
4.1.2 Fourier transform infrared (FTIR) 47
4.1.3 Differential scanning calorimetric (DSC) 49
4.2 PLA/CSIM blends characterization 50
4.2.1 Differential scanning calorimetric (DSC) 50
4.2.2 Dynamic mechanical analysis (DMA) 54
4.2.3 Thermogravimetric analysis (TGA) 57
4.2.4 Tensile properties 61
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4.2.5 Impact properties 69
4.2.6 Scanning electron microscopy (SEM) 76
4.2.7 Color analysis 80
4.3 Isothermal crystallization of PLA/CSIM blends 82 4.3.1 Differential scanning calorimetry (DSC) 82
4.3.1.1 Melting behaviour after isothermal crystallization 94 4.3.2 Polarized optical microscopy (POM) 104
CHAPTER FIVE: CONCLUSIONS AND RECOMMENDATIONS119
5.1 Conclusions 119
5.2 Recommendations for future Research 121
REFERENCES APPENDICES
Appendix 1: Abstract submitted for poster presentation at Scientific Conference of Microscopy Society Malaysia (SCMSM 2015), 2nd – 4th December 2015, Hotel Avillion, Melaka, Malaysia.
Appendix 2: Abstract submitted for poster presentation at 5th International Conference on Solid State Science& Technology (ICSSST 2015), 13th – 15th December 2015, Bayview Hotel, Langkawi, Kedah, Malaysia.
Appendix 3: Abstract submitted for oral presentation at National Symposium Polymeric Materials (NSPM 2016), 10th – 12th November 2016, Science and Engineering Research Centre (SERC), USM, Penang, Malaysia.
Appendix 4: Polarized optical micrographs of impinged spherulites: (A) PLA/CSIM1, (B) PLA/CSIM3, (C) PLA/CSIM5, (D) PLA/CSIM10, (E)
PLA/CSIM20, and (F) PLA/CSIM30 that were crystallized at specified temperatures:
a) 95 °C, b) 100 °C, c) 105 °C, d) 110 °C, e) 115 °C, f) 120 °C, and g) 125 °C, respectively.
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LIST OF TABLES
Page
Table 3.1 Mechanical properties of PLA 3051D 36
Table 3.2 Designation of the prepared PLA/CSIM blends specimens 38 Table 4.1 Comparison of the size ranges of core, interphase, and shell
CSIM between present study and other studies
47
Table 4.2 Comparison of FTIR spectra region for CSIM between present study and other study
49
Table 4.3 Thermal properties of CSIM determined by DSC (second heating at the heating rate of 3 °C/min)
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Table 4.4 Thermal properties of neat PLA and PLA/CSIM determined by DSC (second heating at the heating rate of 5 °C/min)
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Table 4.5 DMA properties of neat PLA and PLA/CSIM blends 55 Table 4.6 Degradation temperature and % char of neat CSIM, neat PLA,
and PLA/CSIM
58
Table 4.7 Tensile properties of neat PLA and PLA/CSIM blends 65 Table 4.8 Impact properties of neat PLA and PLA/CSIM blends 72 Table 4.9 Total color difference values for neat PLA and PLA/CSIM
blends
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Table 4.10 Overall crystallization kinetic data based on the Avrami equation
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Table 4.11 Thermal properties of the neat PLA and PLA/CSIM after isothermal melt crystallization of the PLA component, at the specified temperatures (heating rate of 10 °C/min)
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Table 4.12 Values of 𝑇𝑚°, Kg, σe and q for neat PLA and PLA/CSIM blends
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LIST OF FIGURES
Page Figure 2.1 The cycle of PLA in nature (Xiao et al., 2012) 8 Figure 2.2 Synthesis of PLA from L- and D-lactic acids (Lim et al.,
2008)
9
Figure 2.3 Mechanical properties of PLA and other commodity plastics:
(a) Young’s modulus, (b) Tensile strength, and (c) Elongation at break (Balakrishnan et al., 2012)
12
Figure 2.4 The schematic representation for the seed emulsion polymerization process of the CSPN latex (Fu et al., 2015)
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Figure 2.5 Possible mechanism by which ACR toughen PLA (Song et al., 2014)
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Figure 2.6 Schematic representation of the detailed structure of spherulites (Callister and Rethwisch, 2011)
23
Figure 2.7 Schematic illustration of the variation of free energy with nucleus size (Dhanvijay and Shertukde, 2011).
24
Figure 2.8 Trends in polymer crystal growth dependence on temperature (Hynštová and Jančář, 2010)
26
Figure 2.9 Regime transitions from I to III in the Hoffman – Lauritzen theory (Chen, 2013)
34
Figure 3.1 TGA thermogram showing Tonset, T3 and T5, and R550 40 Figure 4.1 TEM micrograph of a neat CSIM particle 46 Figure 4.2 Schematic illustration of the functional structure of PMMA
(Song et al., 2009) and PBA (Yang et al., 2017)
48
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Figure 4.3 FTIR spectra of neat CSIM 48
Figure 4.4 DSC thermograms recorded during the second heating at the rate of 3 °C/min for neat CSIM
50
Figure 4.5 DSC thermograms recorded during the second heating at the rate of 5 °C/min for neat PLA and PLA/CSIM blends
51
Figure 4.6 Schematic illustration of the interaction between matrix PLA and PMMA shell (Wang and Li, 2000)
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Figure 4.7 Storage modulus curves of neat PLA and PLA/CSIM blends 55 Figure 4.8 Loss modulus curves of neat PLA and PLA/CSIM blends 57 Figure 4.9 TGA curves of neat PLA, neat CSIM, and PLA/CSIM 58 Figure 4.10 DTG curves of neat PLA, neat CSIM, and PLA/CSIM 61 Figure 4.11 Image of the tensile test specimens of neat PLA and
PLA/CSIM blends
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Figure 4.12 Tensile stress-strain curves of neat PLA and PLA/CSIM blends
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Figure 4.13 Tensile strength of neat PLA and PLA/CSIM 64 Figure 4.14 Tensile modulus of neat PLA and PLA/CSIM 66 Figure 4.15 Elongation at break of neat PLA and PLA/CSIM 67 Figure 4.16 SEM micrographs of tensile fractured surfaces of: a) neat
PLA, b) PLA/CSIM1, c) PLA/CSIM3, d) PLA/CSIM5, e) PLA/CSIM10, f) PLA/CSIM20, and g) PLA/CSIM30
68
Figure 4.17 Image of the notch impact test specimens of neat PLA and PLA/CSIM
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Figure 4.18 Image of the un-notch impact test specimens of neat PLA and PLA/CSIM blends
70
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Figure 4.19 Impact strength of neat PLA and PLA/CSIM blends of notched test samples
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Figure 4.20 Impact strength of neat PLA and PLA/CSIM blends of notched test samples
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Figure 4.21 SEM micrographs of notched impact fractured surfaces of: a) neat PLA, b) PLA/CSIM1, c) PLA/CSIM3, d) PLA/CSIM5, e) PLA/CSIM10, f) PLA/CSIM20, and g) PLA/CSIM30
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Figure 4.22 SEM micrographs of freeze fractured surfaces of: a) neat PLA, b) PLA/CSIM1, c) PLA/CSIM3, d) PLA/CSIM5, e) PLA/CSIM10, f) PLA/CSIM20, and g) PLA/CSIM30
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Figure 4.23 XHR-FESEM micrograph of freeze-fractured surfaces of PLA/CSIM20 at different magnification: a) 10K X and B) 100K X
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Figure 4.24 Images of the neat PLA and PLA/CSIM films produced 80 Figure 4.25 DSC traces for the isothermal crystallization of neat PLA and
PLA/CSIM blends at different crystallization temperatures
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Figure 4.26 Plots of relative degree of crystallinity (Xt) versus crystallization time (t) for the isothermal crystallization of neat PLA and PLA/CSIM blends at the different crystallization temperatures
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Figure 4.27 Plots of log (-ln (1-Xt)) versus log (t) for the isothermal crystallization of neat PLA and PLA/CSIM blends at the different crystallization temperatures (regime temperature of about 10 - 90% conversion in the curves for all samples)
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Figure 4.28 Plots of crystallization rate (τ½) versus crystallization temperature (Tc) of neat PLA and PLA/CSIM blends
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Figure 4.29 Plots of crystallization rate (τ½) versus CSIM contents of PLA/CSIM blends
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Figure 4.30 DSC thermograms at the heating rate of 10 °C/min for the isothermal crystallization of neat PLA and PLA/CSIM blends
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Figure 4.31 DSC scans at 10 °C/min, showing the effect of the crystallization temperature (Iannace and Nicolais, 1997)
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Figure 4.32 Application of Hoffman-Weeks analysis to samples for determining equilibrium melting temperatures (𝑇𝑚°). Melting peak II as a function of the crystallization temperatures was used
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Figure 4.33 Polarized optical micrographs for the isothermal crystallization of neat PLA at the specified temperatures
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Figure 4.34 Polarized optical micrographs for the isothermal crystallization of PLA/CSIM1 blend at the specified temperatures
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Figure 4.35 Polarized optical micrographs for the isothermal crystallization of PLA/CSIM3 blend at the specified temperatures
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Figure 4.36 Polarized optical micrographs for the isothermal crystallization of PLA/CSIM5 blend at the specified temperatures
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Figure 4.37 Polarized optical micrographs for the isothermal crystallization of PLA/CSIM10 blend at the specified temperatures
109
Figure 4.38 Polarized optical micrographs for the isothermal crystallization of PLA/CSIM20 blend at the specified temperatures
110
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Figure 4.39 Polarized optical micrographs for the isothermal crystallization of PLA/CSIM30 blend at the specified temperatures
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Figure 4.40 Polarized optical micrographs of impinged spherulites of neat PLA crystallized at specified temperatures: a) 95 °C, b) 100
°C, c) 105 °C, d) 110 °C, e) 115 °C, f) 120 °C, and g) 125 °C, respectively.
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Figure 4.41 Polarized optical micrographs of neat PLA and PLA/CSIM blends isothermally crystallized at 125 °C for 8 min with different CSIM contents a) neat PLA, b) PLA/CSIM1, c) PLA/CSIM3, d) PLA/CSIM5, e) PLA/CSIM10, f) PLA/CSIM20, and g) PLA/CSIM30
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Figure 4.42 Hoffman-Lauritzen plots for the estimation of nucleation parameters of neat PLA and PLA/CSIM blends (symbols:calculated data; solid lines: fitting curves).
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LIST OF ABBREVIATIONS
ATR Attenuated total reflection
CIE Commission Internationale de l'Eclairage
CSIM Core shell impact modifier
CSPN Core shell acrylic nanoparticles DGEBA Diglycidyl ether of bisphenol-A
DMA Dynamic mechanical analysis
DSC Differential scanning calorimetry DTG Derivative thermogravimetric analysis
FESEM Field emission scanning electron microscopy FTIR Fourier transform infrared spectroscopy
HDPE High density polyethylene
HRTEM High resolution transmission electron microscope
MDS Material data sheet
MFI Melt flow index
MMT Montmorillonite
OMMT Organoclay-montmorillonite
PBA Polybutyl acrylate
PBT Polybutyl terepthalate
PC Polycarbonate
PE Polyethylene
PHA Polyhydroxyalkanoate
PLA Polylactic acid
PMMA Polymethyl methacrylate
POM Polarized optical microscopy
PP Polypropylene
PS Polystyrene
SEM Scanning electron microscopy
TEM Transmission electron microscopy
TGA Thermogravimetric analysis
XHR- FESEM Extreme High Resolution Field Emission Scanning Electron Microscopy
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LIST OF SYMBOLS
Mw molcecular weight
°C degree celcius
K Kelvin
Tm melting temperature
Tg glass transition temperature
wt% weight percentage
Tcc cold crystallization temperature
∆Hcc enthalpy of cold crystallization
∆Hm enthalpy of fusion
Xc degree of crystallinity
ΦPLA weight fraction of PLA
Tc crystallization temperature
L* lightness
a* red/green coordinate
b* yellow/blue coordinate
ΔE* total color difference
Tonset onset temperature
T3% temperature 3% weight loss
T5% temperature 5% weight loss
TP DTG peak temperature
R550 percentage char/residue at temperature 550 °C Xt relative degree of crystallinity
N the Avrami exponent
K the crystallization rate constant
R2 the coefficient of determination for the Avrami fit t½ the half-time of crystallization
τ½ crystallization rate
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KESAN-KESAN PENGUBAHSUAI HENTAMAN KELOMPANG TERAS TERHADAP SIFAT-SIFAT DAN KELAKUAN PENGHABLURAN
POLI(LAKTIK) ASID
ABSTRAK
Polilaktik asid (PLA) telah dicampur leburan dengan kandungan yang berlainan (0 hingga 30% berat) pengubahsuai hentaman kelompang teras (CSIM) yang boleh didapati secara komersil. PLA tulen dan PLA/CSIM campuran telah disediakan melalui pengadun dalaman dan pemampat acuan yang dibentuk menjadi spesimen. Sifat-sifat termal, mekanikal dan morfologi campuran telah dikaji. CSIM tulen menunjukkan bahagian kelompang, antara-fasa dan teras apabila diperhatikan di bawah mikroskopi penghantaran elektron (TEM). Setiap bahagian di dalam CSIM tulen menunjukkan suhu peralihan kaca yang berbeza. Spektrofotometri inframerah transformasi Fourier (FTIR) mengesahkan bahawa kelompang dan teras CSIM tulen masing-masing terdiri daripada polimetil metakrilat (PMMA) dan polibutil akrilat (PBA). Peningkatan dalam kandungan CSIM menurunkan keupayaan PLA untuk menghablur/penghabluransemula, dan suhu penghabluran sejuk beralih kepada suhu yang lebih tinggi. Matrik PLA menunjukkan keserasian yang baik dengan kelompang PMMA pada CSIM. Hal ini telah disahkan oleh ujian dinamik mekanikal analisis (DMA). Dalam ujian gravimetri terma (TGA), kedua-dua PLA dan CSIM tulen menunjukkan penguraian langkah tunggal. Setelah penambahan CSIM, lengkungan tegasan – terikan menunjukkan bahawa kerapuhan PLA tulen meningkat tanpa penambahbaikan dalam kemuluran. Kekuatan tensil, kekuatan modulus, dan pemanjangan takat putus PLA menurun; namun begitu, kekuatan hentaman bertakuk dan tanpa takuk meningkat apabila kandungan CSIM meningkat. Mikrograf
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mikroskopi pengimbasan elektron (SEM) menunjukkan dengan penambahan kandungan CSIM daripada 5 hingga 30% berat permukaan patah menjadi lebih kasar berbanding PLA tulen. Lompang-lompang dapat diperhatikan berkemungkinan disebabkan oleh tarik keluar CSIM semasa patah. Kejelasan PLA diuji menggunakan kromameter. Penambahan CSIM tidak menjejaskan kejelasan PLA kecuali pada kandungan 30% berat daripada CSIM. Kinetik penghabluran isoterma PLA tulen dan PLA/CSIM campuran dikaji menggunakan kalorimetri pengimbasan perbezaan (DSC) dan data yang diperolehi dianalisa menggunakan persamaan Avrami.
Keputusan menunjukkan bahawa kadar penghabluran dikawal oleh suhu penghabluran dan penambahan kandungan CSIM. Kadar penghabluran maksimum, τ½ dapat dilihat apabila PLA diadun dengan 1% berat CSIM dan penghabluran secara isoterma pada 105 °C. Trend hasil untuk darjah penghabluran (Xc) hampir bersamaan dengan kadar penghabluran (τ½) berbanding suhu penghabluran (Tc). Peningkatan pada Tc ≥ 110 ° C yang lebih tinggi telah menjadikan pergerakan segmen molekul PLA menjadi mudah untuk menghasilkan peningkatan Xc. Berdasarkan persamaan Hoffman-Weeks, nilai suhu kesetimbangan (𝑇𝑚°) untuk PLA tulen dicatatkan pada 201.08 °C. Kadar pertumbuhan spherulit PLA tulen dan PLA/CSIM campuran diukur dan dianalisis dalam julat suhu 95 hingga 125 °C oleh mikroskop polarisasi optik (POM). Kadar pertumbuhan spherulit PLA dipengaruhi oleh Tc dan kandungan CSIM dalam campuran PLA/CSIM. Apabila Tc meningkat (100 °C≤ Tc ≤ 110 °C), saiz spherulit meningkat dan pembentukan kepadatan nukleus menjadi lebih padat.
Di dalam julat Tc tersebut, jelas bahawa spherulit telah mengalami pelanggaran lengkap dalam masa 5 minit untuk kandungan CSIM yang rendah seperti PLA/CSIM1 dan PLA/CSIM3 campuran. Walau bagaimanapun, peningkatan dalam suhu penghabluran (Tc ≥ 115 °C) telah menurunkan pembentukan ketumpatan
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nukleus manakala saiz spherulit meningkat. Pada Tc yang sama, kandungan CSIM yang semakin meningkat hampir tidak menjejaskan saiz spherulit. Berdasarkan teori Hoffman-Lauritzen, nilai tenaga diperlukan untuk pembentukan saiz kritikal nukleus (Kg) PLA adalah 6.69 x 105 K2 dan rejim III telah digunakan untuk menyesuaikan data eksperimen.
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THE EFFECTS OF CORE SHELL IMPACT MODIFIER ON PROPERTIES AND CRYSTALLIZATION BEHAVIOUR OF POLY(LACTIC) ACID
ABSTRACT
Polylactic acid (PLA) was melt-blended with different contents (0 to 30 wt%) of a commercially available core shell impact modifier (CSIM). Neat PLA and PLA/CSIM blends were prepared via an internal mixer and compression molded into test specimens. Thermal, mechanical and morphological properties of neat PLA and PLA/CSIM blends were studied. The neat CSIM showed shell, interphase, and core regions when observed under transmission electron microscopy (TEM). Each region in the neat CSIM presented different glass transition temperature. The Fourier transform infrared spectroscopy (FTIR) confirmed that the shell and core of the neat CSIM was made of polymethyl methacrylate (PMMA) and polybutyl acrylate (PBA), respectively. An increased in CSIM content slightly decreased the ability of PLA to crystallize and/or re-crystallize, and the cold crystallization temperature shifted to higher temperatures. The matrix PLA showed good compatibility with the PMMA shell of CSIM. This was confirmed by the dynamic mechanical analysis (DMA) tests. In the thermogravimetric analysis (TGA), both of the neat PLA and CSIM displayed single step decomposition. Upon addition of CSIM, the stress-strain curves presented that the brittleness of neat PLA increased without the improvement in ductility. Tensile stress, tensile modulus, and elongation at break of PLA decreased;
yet, the notched and un-notched impact strength increased as CSIM content increased. Scanning electron microscopy (SEM) micrographs revealed that with the addition of CSIM contents of 5 to 30 wt%, the fractured surface became rougher than neat PLA. Voids were observed which likely caused by the pull-out of CSIM during
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fracture. The clarity of the PLA was tested using chromameter. Incorporation of CSIM did not affect the clarity of the PLA except at 30 wt% contents of CSIM. The isothermal crystallization kinetics of neat PLA and PLA/CSIM blends were studied by differential scanning calorimetric (DSC) and data obtained were analyzed with the Avrami equation. The results showed that the crystallization rate is controlled by the crystallization temperature and the incorporation of CSIM content. The maximum crystallization rate, τ½ was observed when PLA was blended with 1 wt% CSIM and isothermally crystallized at 105 °C. The result trend for degree of crystallinity (Xc) are comparable to the crystallization rate (τ½) versus crystallization temperature (Tc).
The increased in higher Tc ≥ 110 °C has made the mobility of PLA molecular segment become easy to facilitate resulting in the increase of Xc. Based on the Hoffman-Weeks equation, the equilibrium temperature (𝑇𝑚°) value for neat PLA was recorded at 201.08 °C. The spherulitic growth rate of neat PLA and PLA/CSIM blends was measured and analyzed in the temperature range of 95 to 125 °C by polarized optical microscopy (POM). The spherulitic growth rate of PLA was influenced apparently by the Tc and the CSIM content in the PLA/CSIM blends. As the Tc increased (100 °C ≤ Tc ≤ 110 °C), the spherulites size increased and the formation of nuclei density became denser. At this Tc range, it is apparent that the spherulites have undergone complete impingement within 5 minutes for low CSIM contents such as PLA/CSIM1 and PLA/CSIM3 blends. However, further increased in crystallization temperature (Tc ≥ 115 °C) has lowered the formation of nuclei density while spherulites size increased. At the same Tc, the increased in CSIM contents almost did not affect the spherulites size Based on the Hoffman-Lauritzen theory, the energy require for the formation of nuclei’s critical size (Kg) value of PLA was 6.69 x 105 K2 and regime III was applied to fit the experimental data.
1
CHAPTER ONE INTRODUCTION
1.1 Introduction
Plastics have been the most used material in the world, playing central role in modern industrial economies. However, the growing reliance on petroleum-based polymers such as polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET) (Balakrishnan et al., 2010) has raised several environmental issues (Molinaro et al., 2013). For example, the persistence of petroleum-based polymers in the environment which is not readily biodegradable and resistance to microbial degradation has led towards the scarcity of landfill space (Avérous and Pollet, 2012, Abdelwahab et al., 2012). Moreover, the decrement of petroleum resources and the concerns over emissions of toxic gases during incineration have driven efforts to develop biodegradable polymers from renewable resources (Abdelwahab et al., 2012).
Biodegradable polymers are expected to be an alternatives for petroleum- based plastics due to the limited sources and increased in petroleum price which will restrict their usage in the near future (Balakrishnan et al., 2010). Biodegradable polymers are also can be degraded through the action of enzymes or chemical deterioration associated with living organisms (Vroman and Tighzert, 2009). There are several biodegradable polymers that has been developed. Aliphatic polyesters is a part of the biodegradable polymer groups which consists of polylactic acid (PLA), polyglycolic acid (PGA), polycaprolactone (PCL), and polyhydroxybutyrate (PHB).
However, PLA which is a linear aliphatic biodegradable polyester derived from
2
biomass through bioconversion and polymerization has become a potential candidate for various large-scale industrial applications (Kumar et al., 2010).
PLA has several advantages such as renewable resources, biodegradation, biocompatibility, excellent thermal and mechanical properties, and superior transparency of the processed materials (Wu and Wu, 2006). The production cost is high during the early development of PLA. Hence, it has been primarily used only for medical applications such as internal sutures, implant devices, and tissue scaffolds (Molinaro et al., 2013) due to its high biocompatibility and biodegradability in the human body (Murariu et al., 2007). Nowadays, PLA must possess good mechanical properties and stabilities to prevent degradation and maintain high molecular weight to be processed in a mass scale into packing, textile and general plastic materials (Zhang and Wang, 2008). Unfortunately, PLA shortcomings such as brittleness, and lower impact resistance at room temperature which results in splitting and other handling problems during sheet manufacturing (Afrifah and Matuana, 2010); are preventing its large scale competition with petroleum-based polymers (Fowlks and Narayan, 2010).
Several modifications such as plasticization, copolymerization, and blending of PLA with biodegradable and non-biodegradable polymers have been suggested to enhance the mechanical properties of the neat PLA (Kumar et al., 2010). The toughening of PLA by blending with rubber will result in an expensive blend.
However, blending the PLA with inexpensive non-biodegradable polymers such as polyethylene and polyisoprene will require compatibilizers to improve the miscibility between the polymers (Kumar et al., 2010). Addition of suitable impact modifier will improve the toughness of PLA. Arkema has introduced core shell impact modifier
3
under the trade name Biostrength 282. These core shell impact modifier (CSIM) are designed to improve the toughness of the PLA and to maintain the clarity of the PLA.
1.2 Problem statements
PLA are readily toughened by blending with rubber provided that an appropriate rubber particle is incorporated during mixing and there is adequate adhesion between PLA and rubber phases (Jaratrotkamjorn et al., 2012, Bitinis et al., 2011). There are some factors influence the adhesion such as physical interactions or chemical reactions between the two phases which also generally have a strong effect on blend morphology. Hence, it is usually impossible to vary interphase adhesion and particle size in a totally independent way (Lu et al., 1996). Moreover, the absolute and relative rheological characteristics of the matrix and rubber phases are important factors during mixing that determines the final rubber particle size in the blends (Lu et al., 1996).
Instead of using a rubber that has to be broken up during blending process, it is also possible to work with pre-shaped particles. These materials are supplied as watery emulsions or as precipitated agglomerates. To prevent the rubbery particles stick together, they are given a hard shell. In the blending process, these core shell impact modifiers (CSIM) have only to be deagglomerated (Gaymans and Werff, 1994). Good physical interaction, or even miscibility of the chains forming the shell with those of the matrix often permit this ideal to be achieved (Lu et al., 1996). The CSIM are frequently used in the toughening of polyesters (Gaymans and Werff, 1994).
4
The study related to the crystallization phenomena is vital in polymer processing, for several reasons. For example, in the final stage of the polymer processing, the control of the temperature profile during cooling determines the development of a specific morphology which influences the final properties material (Iannace and Nicolais, 1997). However, the crystallization rate of the PLA is extremely low in comparison with other commercial thermoplastics despite of having many desirable properties. The high degree of crystallinity in PLA is difficult to achieve. In this case, the amorphous content of PLA plays a very important role on the final properties of the articles. The presences of additives in a neat polymer resin can influence the crystalline morphology and kinetics. The additives can either provide nucleating sites for initiating the crystallization or increase the polymer chain mobility and thus enhancing the crystallization rate (As’habi et al., 2013).
The study of crystallization behaviour of polymers can be carried out either in isothermal or non-isothermal crystallization kinetics. This study is focus on isothermal crystallization kinetics of PLA. An understanding of the kinetic of the crystallization process is important for the selection of processing parameters such as mold temperature and hold time during injection molding. Isothermal crystallization is a popular method to obtain kinetics data by rapidly cooling the sample from the melt to the crystallization temperature and measuring the heat evolved while the sample is held isothermal (Foreman and Blaine, 1995). To the best knowledge, no work study has been reported on the isothermal crystallization behaviour of the PLA/CSIM blends system. This was achieved through calorimetric analyses and examination of the spherulitic evolution through polarized optical microscopy (POM).
5 1.3 Research objectives
The present study investigates the effect of incorporation core shell impact modifier in PLA matrix system. Mechanical, thermal and crystallization of the PLA/CSIM blends were studied. The objectives of the present study are as follows:
1. To characterize the core shell impact modifier.
2. To study the effect of the core shell impact modifier at different contents on the thermal and mechanical properties of PLA and PLA/CSIM blends.
3. To investigate the effect of different isothermal crystallization temperature and CSIM contents on the crystallization behaviours of neat PLA and PLA/CSIM blends.
1.4 Scope of research
The present study focuses on the fabrication of the neat PLA and PLA/CSIM blends. The first phase study highlights on characterization, thermal, and mechanical properties of the neat PLA and PLA/CSIM blends by using TGA, DSC, DMA, tensile and impact testing machine. The second phase concentrates on the study of the isothermal crystallization kinetics of the neat PLA and PLA/CSIM blends at different CSIM contents and various crystallization temperatures by using DSC. The third phase emphasizes on the crystal growth of the neat PLA and PLA/CSIM blends. This study was carried out by using POM.
6 1.5 Dissertation overview
Chapter 1 starts with the introduction of the project. It covers brief introduction about research background, problem statements, research objectives, scope of researches and dissertation overview.
Chapter 2 contains the literature review on several related topics which includes PLA, CSIM, and crystallization behaviour.
Chapter 3 describes materials specifications, equipment used, and experimental procedures that have been carried out in this research such as tensile test and DSC for the isothermal crystallization kinetics study.
Chapter 4 reports the characterization of raw materials used in the blending preparations. It is divided into three parts. The first part describes the characterization of raw materials and blends. The second part discusses the isothermal crystallization kinetics of PLA and PLA/CSIM blends. The last part explains the crystal growth of the PLA and PLA/CSIM blends.
Chapter 5 concludes the findings from this research and also some recommendations for future works in this related field.
