EFFECT OF IN-SITU VULCANIZATION OF STYRENE MODIFIED NATURAL RUBBER IN
ADHESIVE AND RUBBER TOUGHENED POLYSTYRENE APPLICATIONS
by
NEOH SIEW BEE
Thesis submitted in fulfilment of the requirements for the Degree
of Doctor of Philosophy
JUNE 2010
ACKNOWLEDGEMENTS
First and foremost, I would like to thank my supervisor, Dr. Azura A. Rashid for her insightful guidance and encouragement throughout this work. Without her incisive advice, I would not have been able to proceed and bring the research to a satisfactory competition. I would also like to extend my deepest gratitude to my co-supervisor, Professor Azanam S Hashim, who has guided me and supported me with his vast experience knowledge and experience for the entire duration of my project. I am grateful to Dr Mas Rosmal Hakim from School of Chemistry Sciences for allowing the use of FTIR and NMR machine.
Sincere thanks are also extended to all other lecturers in Polymer Engineering Section and to all my postgraduate colleagues for their assistance towards the success of this undertaking.
The constructive advice and opinions obtained from the lab assistants namely Mr.
Gnanasegeram a/l N.B.Dorai, En. Mohd.Hasan, En.Mohd Zandar, En. Faizal, En.Rokman, En.Shahril, En.Fitri, Pn.Fong and En.Azam are greatly appreciated.
I would also like to thank my family members especially my father Mr. Neoh Yong Seng, my mother Mdm Lim Beng Choo and sister Ms. Neoh Siew Chin who provide continuous support and encouragement during past five years while working on this thesis. My special thanks go to by beloved husband Lt Cdr Gan Chin Keat, the
commanding officer of KD Perdana (2009) and my son Gan Chern Fong for their patience and greatest company. Last but not least, I wish to thank my father in law Lt Col (rtd) Dr Gan Boon Hooi KMN and mother in law Mdm Kong Suat Keow for their understanding and patience.
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS ii
TABLE OF CONTENTS iv
LIST OF TABLES x
LIST OF FIGURES xii
LIST OF NOTATIONS xvi
LIST OF ABBREVIATIONS xix
ABSTRAK xxiii
ABSTRACT xxv
CHAPTER 1 : INTRODUCTION
1.1 Latex based adhesive 1
1.1.1 Pressure sensitive adhesive 4 1.1.2 The effect of nature of adherend surface 5
1.1.3 The effect of the high humidity and liquid water upon strength of adhesive bonds 6
1.2 Rubber toughened plastic 6
1.2.1 Compatibility effect test
1.2.2 Graft-copolymerization reaction 7
1.3 Problems statement 14
1.4 Objectives of studies 17
CHAPTER 2 : LITERATURE REVIEW
2.1 Emulsion polymerization 18
2.1.1 Smith-Ewart theory 20
2.1.2 Chain growth polymerization 21
2.2 Latex compounding Ingredients 24
2.2.1 Vulcanization agents 25
2.2.1.1 Sulfur 25
2.2.1.2 Thiuram Polysulfides 26
2.2.2 Accelerators 26
2.2.2.1 Dithiocarbamates 26
2.2.2.2 Thiuram sulfides 27
2.2.3 Activator -Zinc oxide 27
2.2.4 Stabilizer-Anionic Surface Active Substance 28
2.2.5 Antioxidant 28
2.3 Type of vulcanizations 29
2.3.1 Sulfur vulcanization 29
2.3.1.1 Mechanism of sulfur vulcanization 31
2.3.2 Peroxide vulcanization 33
2.3.3 Dynamic vulcanization 33
2.3.4 Radiation induced crosslinking 34
2.4 Rubber Toughened Polymer 35
2.4.1 High Impact Polystyrene (HIPS) 37
2.4.1.1 Effect of particle size 38
2.4.1.2 The effect of crosslinking in rubber toughened plastic 39
2.4.2 Interpenetrating polymer network (IPN) 40
2.4.2.1 IPN of PS and NR 40
2.4.3 Latex interpenetrating polymer network (LIPNs) 41 2.4.4 Polymer blend-Rubber toughened plastic and TPE 42
2.4.4.1 The effect of compatibilizer 43
2.4.5 Toughening mechanisms 45
2.4.6 Classical beam theory-unnotched specimen 47 2.4.6.1 Internal elastic energy 47 2.4.7 Linear Elastic Fracture Mechanics theory (LEFM) 51
2.5 Styrene- natural rubber studies 54
CHAPTER 3 : EXPERIMENTAL
3. 1 Materials 57
3. 2 Emulsion polymerization of styrene onto DPNR 59
3.3 SNR sample preparation 61
3.4 SNR pre-vulcanization for adhesive 61
3.5 Test for adhesive applications 63
3.5.1 Preparation of substrates 63
3.5.2 Dead load test 63
3. 6 SNR vulcanizates for rubber toughened plastic 64
3.7 PS/Rubber blends preparation 68
3.7.1 Compression molding 70
3. 8 Characterization and analysis 71
3.8.1 Degree of conversion 71
3.8.2 Soxhlet extraction 71 3.8.3 Fourier-transform infrared spectroscopy (FTIR) 72 3.8.3.1 Percentage of grafting calculation 73 3.8.3.2 Absorbance values for different polymerization time 73 3.8. 4 Scanning Electron Microscopy (SEM) analysis 74
3.8. 5 Optical Microscopy 74
3.8.6 Thermal properties analysis 75
3.8.6.1 Differential scanning calorimetry (DSC) analysis 75
3.8.6.2 Dynamic mechanical analysis (DMA) 75
3.9 Mechanical properties 75
3.9.1 Tensile test 75
3.9.2 Flexural test 76
3.9.3 Fracture toughness 76
3.9. 4 Impact test 76
3.9.5 Aging test 77
CHAPTER 4 : RESULTS AND DISCUSSION
4.1 Polymerization of SNR 79
4.1.1 FTIR spectroscopy 79
4.1.2 Degree of conversion 87
4.2 Application of SNR as adhesive 90
4.2.1 Pre-vulcanization system of SNR latex films 90 4.2.1.1 The effect of three pre-vulcanization system on the 90
tensile properties of SNR latex films
4.2.1.2 The effect of pH modification on semi-EV SNR latex films 93 4.2.2 Effect of SNR formulation on adhesive performance 95 4.2.3 Effect of pH of SNR latex on adhesion performance 98
4.3 Rubber toughened plastic 100
4.3.1 The effect of crosslinking agent and accelerator in SNR 101
4.3.1.1 Tensile properties 102
4.3.1.2 Impact property 104
4.3.1.3 Morphology 105
4.3.2 The effect of sulfur loading 107
4.3.2.1 Tensile properties 108
4.3.2.2 Impact property 109
4.3.2.3 Morphology 110
4.3.3 Various techniques of sulfur vulcanization 113
4.3.3.1 Tensile properties 113
4.3.3.2 Impact property 115
4.3.3.3 Morphology 116
4.3.4 Blend composition 124
4.3.4.1 Tensile properties 124
4.3.4.2 Impact property 126
4.3.4.3 Flexural properties 129
4.3.4.4 Thermal Properties Analysis: Differential Scanning 132 Calorimetry
4.3.4.5 Dynamic mechanical analysis 133
4.3.4.6 Fracture toughness 142
4.3.4.7 Morphology 143
4.3.4.8 Aging properties 149
CHAPTER 5: CONCLUSIONS AND FURTHER WORK
5.1 Conclusions 153
5.2 Suggestions for further work 156
REFERENCES 158
PUBLICATION LIST 170
APPENDICES
A.1 (Abstract)- Morphology study of styrene-modified natural rubber 171 as rubber toughened material. Malaysia Journal Microscopy.
A.2 (Abstract)- Effect of in-situ polymerization of styrene onto 172 natural rubber on adhesion properties of styrene-natural
rubber adhesives. (Accepted by Journal of Adhesion: ISSN:0021-8464)
A.3 (Abstract)- Comparison of the Different Vulcanization 173 Techniques of Styrene Modified Natural Rubber (SNR) as an Impact
Modifier of Natural Rubber-Based High Impact Polystyrene
(NRHIPS). (Accepted by Polymer-Plastics Technology and Engineering:
LPTE-2010-0712)
A.4 (Abstract)- The effect of sulfur content on styrene –modified 175 natural rubber as special rubber and rubber toughened material,
Proceeding of the 7th National Symposium on Polymeric Materials (2007) A.5 (Abstract)- Morphology study of styrene-modified natural 176
rubber as rubber toughened material. Proceeding of the 6th Asean Microscopy conference (2007)
A.6 (Abstract)-. Effect of accelerator and crosslink agent on 177 mechanical properties of PS /PS –modified natural rubber blends,
International Rubber Conference (2008)
A.7 (Abstract)- Natural rubber-based high impact polystyrene blends, 179 the 8th National Symposium on Polymeric Materials (2008)
A.8 (Abstract)- Effect of thermal treatment on mechanical properties 180 of natural rubber based high impact polystyrene, the 8th National
Symposium on Polymeric Materials(2008)
LIST OF TABLES
1.1 Typical formulation for graft-coplymerization of methyl methacrylate 10 and of styrene in ammonia-preserved natural rubber latex
(Bloomfield , 1952)
2.1 Properties of styrene-butadiene latex (Rander 2006) 20 2.2 Composition of conventional, semi-EV & EV system (Bansal et al,1988) 30 2.3 The advantages and disadvantages of peroxide vulcanization 33 (Kaneko,1980)
2.4 Impact strength of dynamically vulcanized NR/PS blends (J/m) 34 (Asaletha et.al,1999)
2.5 Functional trends to adjust technical properties of HIPS (Riew,1989) 37 2.6 Typical recipe for rubber toughened polystyrene (Bucknall,1977) 38
3.1 Specifications of DPNR 57
3.2 Properties of PS (HH35) and HIPS (HT50) 58
3.3 Recipe for emulsion polymerization of styrene onto DPNR 59 (Nguyen, 2002)
3.4 Recipe of pre-vulcanization systems of SNR latex 62 3.5 Coagulum properties based on chloroform number 62 3.6 Formulation of SNR vulcanizates with different crosslinking agent and 65
accelerators
3.7 Formulation of SNR vulcanizates using conventional method 66
3.8 Formulation of SNR vulcanizates based on sulfur vulcanization 67 using mixed system
3.9 Formulation of SNR vulcanizates based on sulfur vulcanization 67 using in situ vulcanization
3.10 Formulation for in situ vulcanization of SNR vulcanizates 68
3.11 Processing parameter for PS/SNR vulcanizates blends 69 in Brabender chamber
3.12 Blend composition of PS/SNR vulcanizates 69
3.13 Blend ratio of PS/SNR vulcanizates compared 70
with actual PS/NR content
3.14 Processing parameter of PS, HIPS, PS/SNR vulcanizates 70 and PS/*DPNR for compression molding
3.15 Setting parameter of FTIR 72
4.1 Anchorage of various SNR adhesive formulations 96 4.2 Effect of pH on the anchorage of the semi-EV latex adhesive 100 4.3 Tensile properties of PS/SNR vulcanizates blends 103 4.4 Tensile properties of in situ vulcanization of PS/SNR vulcanizates 108 4.5 Tensile properties of PS/SNR vulcanizates (various technique of sulfur 115
vulcanization ), PS/*DPNR and HIPS
4.6 Tensile properties of PS/SNR vulcanizates with rubber 125 loading from 10% to 30%, PS/DPNR and HIPS
4.7 The type of failure, elastic energy (Uc , Uy or Uy*) and deflection 132 at break of PS-SNR blends (with curatives) and HIPS.
4.8 Tg of rubber portion, PS portion and intermediate Tg obtained 133 from DSC analysis of SNR9H,PS/SNR20% and PS/SNR30%
4.9 Percentage aging retention properties of PS/SNR vulcanizates 150 blends and HIPS
LIST OF FIGURES
1.1 Preparation of U-DPNR-g-PS and E-DPNR-g-PS copolymers 12 (Pukkate et. al., 2007)
1.2 Content of styrene units and conversion of styrene for U-DPNR-g-PS and 13 E-DPNR-g-PS copolymers (Pukkate et. al.,2007)
1.3 Value of grafting efficiency of styrene for U-DPNR-g-PS and 13 E-DPNR-g-PS copolymer (Pukkate et. al.,2007)
2.1 Free radical emulsion polymerization (Rander, 2006) 19 2.2 The reactions of double bonds with various types of initiating species 22 (Roderic et.al, 2005)
2.3 Kinetics for chain polymerization by free radical mechanism 23 (Roderic et.al, 2005)
2.4 Generalized structures in sulfur vulcanized natural rubber 30 (Thanaka , 1991)
2.5 Generalized mechanism of sulfur vulcanization 32 ( Morrison & Porter, 1984)
2.6 Molecular and morphology parameters that influence technical 36 properties of rubber toughened plastics (Riew,1989)
2.7 Interaction of crazes and micro-shear bands in PMMA and 46 polycarbonate (Kinloch and Young, 1983)
2.8 (a) Simply supported rectangular beam subjected to central loading 49 specimen geometry; (b) first yield; (c) general yield. Yield zone
are shaded (Bucknall,1977)
2.9 Load – deflection curve: ductile behavior type A 51 2.10 Load – deflection curve: ductile behavior type B 51
2.11 Geometry of the SEN-T specimen used 52
2.12 Load displacement curve for notched test specimen (ISO 13586 2000) 53
3.1 Reaction Vessel 60
3.2 Preparation procedure for SNR samples 60
3.3 T-peel specimens used for adhesive testing 63
3.4 A1 absorbance peak at 1600cm-1 and A2 refer to minimum 74 absorbance on base line at 1610cm-1
3.5 Outline of the work done 78
4.1 FTIR spectra of DPNR latex 80
4.2 FTIR spectra for DPNR, styrene monomer and SNR9H 81 4.3 FTIR spectra of SNR at 3 hours, 6 hours and 9 hours 83
polymerization time.
4.4 Normalized value of absorbance at peak1600 cm-1, Ao , at 84 difference polymerization time.
4.5 FTIR spectra for SNR9H and DPNR with styrene monomer 85 (blend ratio: 90:10;80:20; 75:25; 70:30)
4.6 FTIR spectra for SNR9H and DPNR with styrene monomer 86 (with and without soxhlet extraction) at blend ratio 75:25.
4.7 Absorbance versus polymerization time for SNR compare with 88 styrene monomer at wave number 1600 cm-1
4.8 Percentage of grafting for SNR at various polymerization time 89 4.9 Conversion-time curves of the polymerization of styrene onto DPNR 89 4.10 Tensile strength of the pre-vulcanization SNR latex films 91 4.11 Elongation at break of the pre-vulcanization SNR latex films 92 4.12 Stress-strain of the SNR latex films of (a) pure SNR, 92
(b) semi-EV, (c) EV and (d) CV pre-vulcanized systems
4.13 Effect of pH on the (a) tensile strength and (b) elongation-at-break 94 of the semi-EV SNR latex films
4.14 Mean maximum load supported by various adhesive formulations 97 4.15 Average time-to-fail for various adhesive formulations 97
4.16 Effect of pH modification on the maximum load of the SNR adhesive 99 4.17 Effect of pH modification on the average time-to-fail of the 99
SNR adhesive
4.18 Impact strength of PS/SNR vulcanizates blends with 20% 105 of rubber content
4.19 Impact fractured surface of PS/SNR vulcanizates blends 106 4.20 Impact strength of PS/SNR (in situ vulcanization) blends 110
with 20% of rubber content
4.21 SEM micrograph of (A) PS/SNR1 (B) PS/SNR4 (C) PS/SNR2 111
(D)PS/SNR 5, (E) PS/SNR3 (F) PS/SNR6 (G) PS/SNR7
4.22 Impact strength of PS/SNR vulcanizates (various technique 116 of sulfur vulcanization), PS/*DPNR and HIPS
4.23 SEM micrograph of (a)PS/R1,(b)PS/R2,(c)PS/R3 119 (d) PS/Ref.R,( e)PS/*DPNR and (f) HIPS(Bucknall,1977)
4.24 Light microscopy photograph of (a) Stained-PS/Ref.R 122 (b) Stained-PS/R3 (c) Stained-PS/*DPNR (d)stained-HIPS
(Bucknall, 1977)
4.25 Impact strength of PS/DPNR20% and PS/SNR vulcanizates 128 with difference rubber loading in comparison with HIPS
4.26 Flexural strength of PS/SNR vulcanizates with difference rubber 130 loading compare with HIPS
4.27 Flexural modulus of PS/SNR vulcanizates with difference rubber 130 loading compare with HIPS
4.28 DSC spectrum of DPNR 135
4.29 DSC spectrum of SNR after 9 hours emulsion polymerization(SNR9H) 136
4.30 DSC spectrum of PS/SNR20% 137
4.31 DSC spectrum of PS/SNR30% 138
4.32 Temperature dependence of the storage modulus for PS and PS/SNR 141 vulcanizates blends with rubber loading 10%, 20% and 30%
4.33 Temperature dependence of the tan δ for PS and PS/SNR vulcanizates 141 blends with rubber loading 10%, 20% and 30%
4.34 Composition dependence of fracture toughness and maximum 143 displacement for PS/SNR vulcanizates blends and HIPS
4.35 SEM micrograph of (a) PS/DPNR (b) PS/SNR10% (c) PS/SNR 20% 145 (d) PS/SNR30%
4.36 Light microscopy photograph of (a) Stained-PS/DPNR20% 147 (b) Stained-PS/SNR10% (c) Stained-PS/SNR20% (d) Stained-PS/SNR30%
4.37 Tensile strength of PS/SNR vulcanizates and HIPS before 151 aging and after 3 days and 7days aging
4.38 Elongation at break of PS/SNR vulcanizates and HIPS before 151 aging and after 3 days and 7days aging
4.39 Modulus of PS/SNR vulcanizates and HIPS before 152 aging and after 3 days and 7days aging
4.40 Impact strength of PS/SNR vulcanizates and HIPS before 152 aging and after 3 days and 7days aging
LIST OF NOTATIONS
[ I ] Concentration of initiator [ S] Concentration of surfactant
[M] Concentration of monomer in the monomer-polymer particles
60 Co Cobalt-60
a Total notch length
A The absorbance peak height of stretching vibration of carbon- carbon double bond of aromatic group in styrene at 1600 cm-1, which appear in SNR samples and
A1 Mass of styrene monomer used for the polymerization reaction A2 Mass of non-reacted styrene
a’ Intrinsic flaw size
A0 The absorbance peak at 1600 cm-1, which appear in the monomer styrene.
B Sample depth
E Young’s modulus
E’ Storage modulus
E” Loss modulus
Ef Flexural modulus
F Maximum force in the force-deflection trace Fmax Load at crack growth initiation
I Initiator ( Chapter 2, section 2.1.2) I Impact strength (Chpter2, section 2.4.6)
Kc Fracture toughness
kp propagation rate constant
M Monomer
Mj· Propagating free radical
N Number of the particles per unit volume
P Polymer
P*n Polymer chain with reactive site (*) and degree of polymerization of n
P*n+1 Polymer chain with a reactive site (*) and degree of polymerization of n+1
Pc Load at failure
Pgy The load at the general yield point
phr Part per hundred
R· Radical
Ri Rate of initiation
Rp Rate of propagation
Rt Rate of termination S Initiation stiffness (Figure 2.12)
S Beam span (Figure 2.8 a)
sec. seconds
T Temperature
t90 Optimum cure time
U Elastic energy
Uc Elastic energy at critical stage
Up Elastic energy corresponds to the area under the rectangle as shown in Figure 2.10
Uy Elastic energy at yield point
Uy* Elastic energy for ductile behavior Type B V1 Value of the property after aging
V2 Value of the property before aging
W Sample width
Wp Work done in deforming the bar plastically
wt. Weight
X Deflection at the center of the beam
X/S Deflection to span-ration
Xc Rupture of the bar occurs at a critical deflection to-span ration Xn Number-average number of units per chain
Xp The deflection subsequent to general yield
γ gamma
σc Critical stress in a centrally loaded and simply supported beam(three point bending)
σmax Maximum stress
σy Yield stress in a centrally loaded and simply supported beam(three point bending)
LIST OF ABBREVIATIONS
ABS Acrylonitrile-butadiene-styrene ATR Attenuated total reflectance
AXO Antioxidant used is butylated reaction product of p-cresol and dicyclopentadiene
CBS N- cyclohexyl benzothiazole 2-sulpheamide CMC Carboxymethylcellulose (Section 2.5.1) CMC Critical micelle concentration
CV Conventional vulcanization system
DCP Dicumyl peroxide
DMA Dynamic mechanical analysis DPG Diphenylguanidine
DPNR Deproteinized natural rubber DR Diffuse reflectance
DRC Dry rubber content
DSC Differential scanning calorimeter
EB Elongation at break
E-DPNR Enzymatic-deproteinized natural rubber EPDM Ethylene propylene diene methylene
EV Efficient vulcanization system
FTIR Fourier-transform infrared spectroscopy
H Allylic hydrogen
H2O Water
HANR High ammonia natural rubber HIPS High impact polystyrene
IPN Interpenetrating polymer network
IPPD N-Isopropyl-N-phenyl-p-phenyllendiamine KBR Potassium bromide
KOH Potassium hydroxide
LEFM Linear elastic fracture mechanics LIPN Latex interpenetrating polymer network MBT 2-mercautobenzothiazole
MBTS Dibenzothiazol disulfide MMA Methyl methacrylate MMT Montmorillonite clay N2H8O8 S 2 Ammonium persulfate n-BA n-butyl acrylate
NH3 Ammonia
NPSBR Nano powdered styrene-butadiene rubber
NR Natural rubber
OsO4 Osmium tetraoxide PBA Poly (n-butyl acrylate)
PE Polyethylene
PI Polyisoprene
PIB Polyisobutylene
PMMA Polymethyl methacrylate
PP Polypropylene
PS Polystyrene
PSA Pressure sensitive adhesive
R Rubber chain
R1 Refers to mixed system where potassium oleate (aqueous) and antioxidant(aqueous) is added by using in situ process in SNR latex, and the dry SNR film is subsequently mix with sulfur (solid) by two roll mill.
R2 Refer to the system where sulfur (aqueous), potassium oleate (aqueous) antioxidant (aqueous) is added by using in situ vulcanization.
R3 Refer to SNR4 recipe in Table 3.11, basically is the sulfur vulcanization formulation of the in situ vulcanization.
Ref.R Refer to Table 3.8. SNR is mixed by two roll mixed, and subsequently melt blending with PS. This type of mixing sequence is known as modified dynamic vulcanization.
RSxR Initial polysulphidic crosslinks RSyX Rubber bound intermediate
S Sulfur
SAN Styrene-co-acrylonitrile
SB Styrene butadiene
SBR Styrene butadiene rubber
SEM Scanning Electron Microscopy analysis Semi-EV Semi-efficient system
SNR Styrene modified natural rubber
TBSS N-tert-butyl-2-benzothiazylsulphenamide TMTD Tetramethyhhiuram disulphide
TPE Thermoplastic elastomer
TS Tensile strength
TSC Total solid content
U-DPNR Urea-deproteinized natural rubber UFPR Ultra fine powdered rubber
X Accelerator residue
ZDC zinc dithiocarbamates
ZDEC Zinc-diethyldithiocarbonate ZDEDC Zinc diethyldithiocarbamate ZDMC zinc dimethvldithiocarbamate.
ZnO Zinc oxide
*DPNR Is a control batch contain DPNR, prepared by in situ vulcanization with sulfur vulcanization system
13CNMR Carnon-13 Nuclear magnetic resonance
1HNMR Hydrogen-1 Nuclear magnetic resonance
KESAN PEMVULKANAN IN SITU BAGI GETAH ASLI TERUBAHSUAI DENGAN STIRENA SEBAGAI APLIKASI-APLIKASI PELEKAT DAN
POLISTIRENA DIPERKUAT GETAH ABSTRAK
Kajian ini mengenai pencapaian penyediaan formulasi pemvulkanan bagi getah asli terubahsuai dengan stirena (SNR) dengan teknik pemvulkanan in situ . Sintesis bagi SNR telah disediakan dengan pempolimeran emulsi. Getah asli teryahprotein (DPNR) digraftkan dengan monomer stirena dengan penambahan amonium persulfat sebagai pemula. SNR vulkanizat telah disediakan dengan kaedah pemvulkanan dan agen-agen pemvulkanan yang berbeza. Peringkat pertama kajian ini melibatkan penyediaan SNR dengan pempolimeran emulsi dalam nisbah monomer stirena dan DPNR sebanyak 25%:75%. Untuk aplikasi pelekat, kesan tiga sistem pra-pemvulkanan, pemvulkanan lazim (CV), pemvulkanan separa cekap (semi-EV) dan penvulkanan cekap (EV) terhadap sifat-sifat tensil SNR pra-vulkanizat diperhatikan. Sistem pemvulkanan semi-EV menunjukkan ciri-ciri yang sesuai sebagai pelekat SNR pra-vulkanizat. Sistem pemvulkanan semi-EV menunjukkan keputusan yang sama pada pemanjangan takat putus (EB) berbanding dengan sistem pemvulkanan CV dengan kekuatan tensil yang rendah. Kesan pengubahsuaian pH dalam sistem pemvulkanan semi-EV menunjukkan sifat pelekatan yang baik dalam SNR pada pH 12 dan sesuai untuk digunakan dalam aplikasi pelekat. Bagi aplikasi bahan diperkuat getah, proses pemvulkanan dengan sulfur sebagai agen sambung-silang telah meningkatkan kekuatan hentaman adunan PS/SNR vulkanizat.
Keputusan menunjukkan adunan dengan kandungan getah 20% mempunyai kekuatan hentaman hampir sama dengan HIPS manakala adunan dengan kandungan getah sebanyak 30% adalah lebih liat daripada HIPS. Apabila komposisi getah dalam adunan ditingkatkan dari 10% hingga 30%, tenaga dalaman dan takat pemutusan pelenturan juga konsisten dengan kekuatan hentaman yang diperolehi. Morfologi pada PS/SNR20% dan PS/SNR30% menunjukkan fasa getah adalah dalam taburan berterusan dan sekata berbanding dengan adunan PS/DPNR. Kalorimetri pembezaan penskanan (DSC) menunjukkan terdapat pencangkukkan berlaku pada SNR9H. Walau bagaimanapun, adunan PS/SNR20% and PS/SNR30% adalah tidak serasi dan fasa terpisah wujud dalam adunan. Analisis dinamik mekanikal (DMA) menunjukkan adunan PS/SNR30%
mempunyai interaksi yang baik antara SNR dengan matrik PS, dengan meningkatkan maksimum tanδ dalam kawasan getah. Pada fasa kaca PS di bahagian suhu (90 0C-150
0C), adunan PS/SNR30% mempunyai modulus simpanan dinamik terendah berbanding dengan PS/SNR10% dan PS/SNR20%. Kehadiran kandungan getah yang tinggi dalam adunan telah mengurangkan kekakuan rantaian PS. Keliatan rekahan bagi adunan PS/SNR20% mempunyai nilai Kc yang rendah berbanding HIPS tetapi menunjukkan keliatan yang tinggi kerana mempunyai takat putus kelenturan yang lebih tinggi.
Rintangan penuaan bagi adunan PS/SNR dengan 20% dan 30% kandungan getah telah menunjukkan kekuatan hentaman yang lebih baik daripada HIPS selepas prosess penuaan. SNR yang disediakan dengan pemvulkanan in situ menunjukkan peningkatan yang baik dalam sifat-sifat pelekat sensitif tekanan dan kekuatan hentaman. Aplikasi- aplikasi SNR sebagai pelekat dan pengubahsuaian hentaman telah tercapai.
EFFECT OF IN SITU VULCANIZATION OF STYRENE MODIFIED NATURAL RUBBER IN ADHESIVE AND RUBBER TOUGHENED POLYSTYRENE
APPLICATIONS ABSTRACT
In this research, establishment of vulcanization formulation of styrene modified natural rubber ( SNR ) vulcanizates by using in situ vulcanization technique was investigate. The synthesis of SNR was prepared through emulsion polymerization. Deproteinized natural rubber latex (DPNR) was grafted with styrene monomer with the addition of ammonium persulfate as initiator. SNR vulcanizate was prepared with different vulcanization methods and vulcanization agents. The first stage of the work involved in established the emulsion polymerization of SNR with ratio of styrene monomer and DPNR ratio 25%:
75%. For adhesive application, the effect of three pre-vulcanization systems, conventional vulcanization (CV), semi-efficient vulcanization (semi-EV), and efficient vulcanization (EV) on tensile properties of SNR pre-vulcanizates were observed. The semi-EV vulcanization system showed suitable properties as SNR pre-vulcanizates adhesives. It showed similar trend in elongation at break (EB) compared to CV system with low tensile strength (TS). The effect of pH modification on semi-EV pre-vulcanized system showed the SNR with good anchorage ability at pH 12 suitable for adhesive application. For rubber toughened material application, in situ vulcanization process with sulfur as crosslinking agent had improved the impact strength of the PS/ SNR vulcanizates. The results showed at 20% of rubber content has comparable impact strength with HIPS while 30% of rubber content showed more ductile than HIPS.
Addition of more rubber contents from 10% increased to 30% showed an increased in internal energy and deflection at break which consistent with the impact strength
obtained. The stained photographs of PS/SNR20% and PS/SNR30% showed co- continuous rubber phase and more homogenous than PS/DPNR blend. Differential scanning calorimetry (DSC) analysis showed the occurring of grafting in SNR9H, however the blends of PS/SNR20% and PS/SNR30% showed incompatible and phase separated exist in the blends. Dynamic mechanical analysis (DMA) showed the PS/SNR30% had more interaction between SNR and PS matrix hence the maximum tan δ increases in the rubber region. At glassy PS temperature region (90 0C-150 0C), PS/SNR30% had lowest dynamic storage modulus compared to PS/SNR10% and PS/SNR20%. This large amount of rubber molecule had reduced the rigidity of the PS chain. Fracture toughness of PS/SNR20% had lower Kc value than HIPS but showed better ductility with higher deflection at break. Aging retention property of PS/SNR blends contained 20% to 30% of rubber content showed better impact strength after aging compared to HIPS. In situ vulcanization of SNR had shown great improvement in pressure sensitive adhesion properties and impact property. The applications of SNR as adhesive and impact modifier had achieved.
CHAPTER 1 INTRODUCTION 1.1 Latex based adhesive
The bonding agents used in paper product are known as adhesive which mostly is latex based. Other than paper products, leather goods and textiles, latex based adhesives also had been used in rigid substrates such as wood, floors, metal, glass, plastic and ceramic.
The advantages of latex based adhesive are as follow:
• Reduced cost of production.
• Able to formulate adhesive with a wide range of total solid contents and viscosities, such as adhesive with high total solid content at relatively low viscosity for easy handling.
• Absence of flammable and toxic solvents
• Utilizing polymer of high molecular mass
• Superior resistance to deterioration during aging.
• Ease of wetting on solid substrate and to penetrate on porous substrate.
Generally the copolymer lattices are more effective used as based for latex adhesive than the unmodified polymer lattices. Adhesive with graft copolymer or a block should adhere well to different adherend. The adhesive need to fulfill the condition where the degree of separation of the two (or more) types of the repeat unit in a copolymer should have sufficiently large domain with two types of polarity. Secondly, the block of the copolymer should able to migrate from each other to enable strong adhesive bonds can be formed at the respective adherend surfaces. The copolymer need to have adequate size with no extensive crosslinking that can separate at the respective adherend surfaces.
Basically the adhesive bond formed from the latex based adhesive have to withstand the influence exposure such as sunlight, hydrocarbon oil, ozone and heat in which these bonds are strongly depends on the polymer component. Furthermore, latex based adhesive bond able to withstand high humidity and water through nature and the amounts of hydrophilic substances in the latex. Some polymers used for the production of latex- based adhesive are inherently capable of being crosslinked. The crosslinking are formed if appropriate reagents are included in the adhesive formulation and physical conditions for crossliking are established, or the reagents can migrate into the adhesive films from the adherend substrate. The advantages of crosslinking are to improve the resistance of adhesive towards aging, reduced sensitivity of bond strength and flexibility on changes temperature, improve the resistance to deterioration by water and organic solvents. There are also adhesion modifiers in latex based adhesive such as aqueous solution and dispersions of resin, tackifiers, plasticizer, crosslinking agents, fillers, thickeners and other additives (Blackley, 1997)
Among the other substances are added in the latex based adhesive are:
• Surface active substances
• Antioxidants
• Anti-forming agents, anti-freezes and freeze-thaw stabilizers
• Fungicides, corrosion inhibitors, flame-retarders
• Colorants, de-odorants and re-odorants.
Gazeley and Mente (1985) describe the preliminary investigation of tackifying additives in reducing the molecular mass of the natural rubber. In the case of pressure sensitive adhesive, the adhesive has to be sufficiently soft in the dry state, and able to deform
under low pressure so good surface contact can be achieved. However, the sufficient cohesive strength is also required for adhesive application. Unfortunately both tack and cohesive are inversely correlated. So higher level of resin is needed for development of good tack compared to equivalent solution-based pressure sensitive adhesive.
In some latex-containing adhesive, the latex component is present in only small amount. The function of adhesive is to improve or develop existing properties possessed by the adhesive, rather than to convey distinctive characteristics of its own (Blanckley,1997).
Characteristic and process of vulcanized NR based adhesive:
• May used chemical catalysts/accelerators at ambient temperatures or heat curing to vulcanize the adhesive, as to improve strength and temperature resistance.
• Additives such as tackifiers, fillers and plasticizer and antioxidants are often used to improve the ageing of adhesive
• Can be set by solvent/ water evaporation/ vulcanization.
• Process involved NR emulsion contains stabilizers, wetting agents and other component. Adhesive may cured by heat or at room temperature, provided a suitable accelerator is used.
According to Petrie (2006), unvulcanized adhesive tends to lose its strength at temperature 66oC and the vulcanized adhesive has maximum service temperature at 93 oC.
Caution must be taken as exposure of adhesive to higher temperatures can cause permanent softening.
1.1.1 Pressure sensitive adhesive
Pressure-sensitive adhesives (PSA) are fluid applied. These types of adhesives are viscoelastic material and do not undergo a chemical reaction. PSA remains in the gel state which is tackiness capable of being removed rather than a permanent bond after wetting the substrates. In order to have good surface contact, PSA has to be sufficiently soft in the dry state to deform under low pressure and has sufficient cohesive strength contact to react as adhesive. However, in most cases the tack and cohesive strength are inversely correlated (Blackley, 1997).
In the 19th century, the discovery of natural rubber as the first solvent based PSA and its usage are widely recognized in tapes and labels industries. Recent development such as control of adhesive properties through structured particles design of water –borne PSA are studied by Andrew et. al.(2009). For further improvement over the joint strength, the mixed adhesive joint technique can offer a good combination of strength and ductility (Silva & Lopes, 2009).
Nanocomposite PSAs also one of the new growth adhesive materials which are popular among the studies of researcher. There is published study that deals with synthesis of acrylic polymer/montmorillonite (MMT) clay nanocomposite PSAs by suspension polymerization (Kajtna & Sebenik, 2009). For medical grade application, the design of new water soluble PSA for patch preparation are reported by Minghetti et al.(2003). Mixture of polyisobutylene (PIB) and sodium carboxymethylcellulose (CMC)
are physiologically inert and both yield a special moisture absorbing PSA, thus suitable for medical application as patch preparation. The rheological properties of PIB and CMC are studied by Piglowski and Kozlowski (1985).
Polymer mostly use as PSAs are block copolymers of elastomer with styrene, natural rubber, polyacrylate, random polymer butadiene-styrene rubber or butyl rubber (Andrew & Khan,1990; Satas,1989). For example, carboxylated butadiene-styrene rubber and butadiene-styrene rubber are used as base for PSA and styrene-2-ethyl hexyl acrylate copolymer containing 14 mass% of styrene is used as PSA modifier (Florian and Novak,2004).
1.1.2 The effect of nature of adherend surface
Latex based adhesives usually contain substances of widely different polarities, which some of the substances are hydrophobic and others are hydrophilic. This is common in latex based adhesive due to minor amount of various substances, such as surface active substances and hydrocolloids which essentially hydrophilic in nature. It is known that latex based adhesives are hydrophobic colloidal dispersions which contain at least two phases which are aqueous phase and polymer particles. There is a dispersed phase in latex based adhesives which can be occurred due to the filler particles. In principle, predominantly polar surface of adherend encourage the polar component of adhesive to accumulate at the interface between the adherend and the adhesive. Similarly, the surface which is predominantly non polar encourage the non polar component of the adhesive at the interface between the adherend and the adhesive (Comyn et.al,1992).
.
1.1.3 The effect of the high humidity and liquid water upon strength of adhesive bonds
Water is highly polar small molecule. The present of the water will further weaken the adhesive bond strength between the adhesive film and the adherend surface. If the adhesive film is in contact with water, surface active substances presence in adhesive may dissolve in the contiguous aqueous phase. Thus, the surface free energy of the water and the thermodynamic work of adhesion of the adhesive bond are reduced (Comyn et.al, 1993). So when produce a new adhesive, the adhesive which is least affected by the humidity is favored when strong adhesive bonds is desired.
1.2 Rubber toughened plastic 1.2.1 Compatibility effect Test
In order to determine whether a polymer-polymer mixture has separated into two phases, light scattering, microscope and measurement of glass transition temperatures are often used as a standard test method for compatibility. For most binary pairs, in which the polymers are completely incompatible, these method can distinguish by difference in refractive index and glass transition temperature (Tg). However, it is difficult to distinguish a single homogeneous phase from a fine dispersion of one polymer in another.
The existing analytical methods differ in their ability to make this distinction and give contradictory results. As a result, calorimetry may indicate a single glass transition for a sample while dynamic mechanical testing detects two separate transitions. Sometime, electron microscopy can resolve this problem provided there is sufficient electron contrast between the two components. If there are differences in chemical reactivity which enable one constituent to be stained or etched preferentially, then it is a useful aid
to analysis the compatibility. Kinetic effect further complicated the problem. In principle, the concept of compatibility is refer to thermodynamic and related to the equilibrium state of the mixture. Caution must be taken as mixing and demixing of polymeric systems are diffusion-controlled process which can take longer time to reach equilibrium. For second kinetic effect, the partially miscible system between its bimodal and spinodal compositions can exist indefinitely as a metastable homogeneous phase in the absence of a nucleation mechanism. Thus, some caution is necessary to prevent wrong interpreting experimental evidence as failure to mix is not necessary an indication of thermodynamic incompatibility, nor is the existence of an homogeneous phase proof of complete thermodynamic compatibility (Bucknall,1977).
1.2.2 Graft-copolymerization reaction
A specialized type of block copolymer in which blocks of one monomer units are covalently bonded to a main-chain polymer comprising exclusively units derived from other monomer is known as graft copolymer. It is said to be grafted on to the main chain polymer when the monomer units constituting the attached blocks. The aims of producing graft-copolymerization reaction in natural rubber latex are for production of self- reinforced and thermoplastic natural rubber.
A few of published reports (Bloomfiled, 1956; Merrett & Wood, 1957; Allen et al.,1959; Sekhar,1958; Ceresa,1973; Pendle,1973) stated that the amount of grafting of a second polymer such as PS and polymethyl methacrylate (PMMA) onto NR backbone is low (less than 50%). This is due to the presence of protein layer which prohibit the graft copolymerization of vinyl monomers onto natural rubber. In this case, deproteinized
natural rubber latex (DPNR) is more suitable as a grafted polymer compared to high ammonia natural rubber latex (HANR). This is due to the removal of protein layer in natural rubber and replacement of surfactant in lattices which increase the degree of grafting and furthermore increase the physical properties of copolymer (Ceresa, 1962;
Allen, 1963; Ceresa, 1973; Pendle, 1973).
Emulsion polymerization can be used to produce grafted copolymer. For emulsifier-free emulsion polymerization, the minimum number of component essential for the creation of an aqueous emulsion polymerization reaction system is three, rather than four: monomer(s), water and initiator. The present of colloid stabilizer is unnecessary. The initiator used in the emulsion polymerization reaction will generate radical –anions end groups which provide colloid stability at the surface of the polymer particles (Blackley, 1997). According to Nguyen (2000), during emulsion polymerization, if the DPNR used as main chain polymer for grafting and with already contains amount of surfactant which lower than critical micelle concentration (CMC) value during its manufacturing, hence, the used of addition surfactant in the system can be neglected. Such system will avoid the formation of micelles and has high degree of grafting which known as emulsifier-free emulsion polymerization.
The reaction time in emulsion polymerization depends on the reaction temperature used in the system. Some studies on modification of HANR latex with vinyl monomer had reported using temperature range from 50-70oC for different initiator and surfactant system. According to Nguyen (2000), modification of DPNR latex with styrene monomer with balance of properties can be achieved in the modified DPNR films
by using 60oC of reaction temperature together with initiator concentration of 2% by weight of styrene in the system,.
Based on study by Nguyen (2000), the total solid content (TSC) of 40% is recommended for the emulsion polymerization reaction of grafted styrene on natural rubber for ratio of natural rubber to monomer styrene at 75%:25%. Above the 40% of TSC, the rubbers tend to collide more frequently due to the distance between the rubber particles reduce and thus the system will easily coagulated.
Another established graft copolymer example was self reinforced rubber obtained from graft-copolymerization with methy-methacrylate (Bloomfield 1952). It is well known as ‘Heveaplus MG’ in industry. Typical formulation for graft-coplymerization of methyl methacrylate and of styrene in ammonia-preserved natural rubber latex using a hydroperoxide-polyamine initiation system are shown in Table 1.1.According to Bloomfield (1952), significant extents of grafting occur if hydroperoxide-polyamine combination or dibenzoyl peroxide is used as initiator. However, if azobisisobutyronitrile and peroxodisulphates are used as initiator, almost no grafting occurs for the former, and low grafting occurs for the latter. The dependence of the extent of grafting upon the nature of the initiator is inconsistent with the reaction mechanisms in which grafting occurs principally hydrogen-abstraction by interaction between a propagation polymer chain and a rubber macromolecule or copolymerization; neither of these reactions would be expected to depend upon the nature of the free radical which initiated the polymerization. Thus, it is believed that grafting occurs primarily by interaction between rubber molecules and the primary radicals which form from the initiator.
Table 1.1: Typical formulation for graft-coplymerization of methyl methacrylate and of styrene in ammonia-preserved natural rubber latex (Bloomfield ,1952)
Part by mass
Methyl methacrylate Styrene
Ingredient Dry Actual Dry Actual
Natural rubber(as 30%m/m latex,0.4% m/m ammonia
100 333 100 333
Non-ionogenic stabilizer (as 20% aqueous solution)
- - 3 15
Methyl methacrylate 33 33 - -
Styrene - - 55 55
Tert-butyl hydroperoxide 0.18 0.18 0.25 0.25
Tetraethylenepentamine
(as 10% m/m aqueous solution)
0.21 2.1 0.1 1.0
Time of polymerization/hour 3 6.5
Polymerization temperature/oC 12 55
Conversion/% 90 95
A combination of tert-butyl hydoperoxide and tetraethylenepentamine showed rapid initiation and smooth polymerization of both methyl methacrylate and styrene in ammonia-preserved natural rubber latex. Bloomfield (1952) indicated that methyl methacrylate could be used to polymerize natural rubber latex with the used of dibenzoly peroxide only if the ammonia was removed, the temperature need to nearly 80oC, and a substantial quantity of a non-inorganic stabilizer need to add to prevent colloidal destabilization.
Allen et.al. (1959) reported that the polystyrene, having a greater tendency to mix with natural rubber than with polymethyl methacrylate, which less tendency to phase-
separated to form micro-aggregates. Thus the distribution of monomer styrene within the composite latex particles is expected to be more uniform.
Recently, Pukkate et. al. (2007) study the graft-copolymerization of styrene onto natural rubber in order to form nano matrix structure. Nano-matrix structure is formed by graft-copolymerization of styrene onto urea-deproteinized natural rubber (U-DPNR) latex.
The grafted U-DPNR is characterized by Fourier-transform infrared (FT-IR) spectroscopy, Hydrogen-1 nuclear magnetic resonance (1H NMR) spectroscopy and transmission electron microscopy. Conversion and grafting efficiency of styrene are more than 90% under the best condition of the graft-copolymerization. In transmission electron micrograph of film specimen stained by OsO4, it is found that natural rubber particle of about 0.5 µm in diameter is dispersed in polystyrene matrix of about 15 nm in thickness.
The conversion and grafting efficiency for the grafted U-DPNR are compared with those for a control sample prepared from enzymatic deproteinized natural rubber (E-DPNR) with styrene. Figure 1.1 shows the preparation of grafted styrene-copolymer. Graft- copolymerization of U-DPNR and E-DPNR are carried out with tert-butyl hydroperoxide /tetrethylenepentamine as an initiator in latex stage. The highest conversion and grafting efficiency of styrene for U-DPNR-g-PS copolymer is achieved at 1.5 mol/kg-rubber feed of styrene to be about 90 and 90 w/w%, respectively, as shown in Figure 1.2 and Figure 1.3.
Figure 1.1: Preparation of U-DPNR-g-PS and E-DPNR-g-PS copolymers (Pukkate et. al., 2007).
HANR
Incubation
Room temperature, 60min
Incubation 305K, 12 hours U-DPNR
urea Proteolytic
enzyme
E-DPNR
30 w/w% DRC DPNR latex Dilution
Condition
Temperature: 303K Reaction Time: 2 hours
Initiator
Styrene monomer Gross Polymer
Soxhlet extraction
(Acetone/2-butanone(3/1)
Insoluble fraction Soluble fraction
Graft copolymer PS
Figure 1.2: Content of styrene units and conversion of styrene for U-DPNR-g-PS and E- DPNR-g-PS copolymers;( ∆ ), conversion of styrene for U-DPNR-g-PS copolymer; (●), conversion of styrene for E-DPNR-g-PS copolymer; (○ ), content of styrene for U- DPNR-g-PS copolymer; (▲), content of styrene for E-DPNR-g-PS copolymer(Pukkate et.
al.,2007).
Figure 1.3: Value of grafting efficiency of styrene for (□ ),U-DPNR-g-PS and (○ ), E- DPNR-g-PS copolymer (Pukkate et. al.,2007).
Graft copolymer also can be prepared by radiation. Asaletha et.al.(1998) prepared graft copolymer of NR and PS (NR-g-PS) by polymerizing styrene in rubber latex using
60 Co γ- radiation as initiator. Styrene monomer is added into emulsion which is then mixed with NR latex. The dose rate is 0.1166 MRadmin-1. The free homopolymers natural rubber and polystyrene are removed by extraction with petroleum ether and methyethylketone.
1.3 Problems statement
High ammonia natural rubber latex (HANR) is commonly used in latex dipped products but has allergy issue (Dairlymple & Audley, 1992; Yip et. al.,1995; Pendle,1993).Thus, deproteinized natural rubber latex (DPNR) become the focus of studies to solve the protein issue. The production of DPNR is by subjecting the natural rubber latex (NR) to enzymatic treatment and centrifugation (Ichikawa et.al.,1993). Most of the proteinaceous substances are removed from NR after the deproteinization process. Protein plays an important role in stabilizing the rubber particles and film forming properties of latex. In order to maintain the stability of DPNR, surfactant is added into DPNR to stabilize the lattices. In comparison of HANR and DPNR, the former is stabilized by protein and lipid layer (Gazeley et.al., 1988), as the latter is virtually stabilized by surfactant.
Most graft-copolymerization process favors the used of DPNR than HANR.
According to Nakason et.al (2003), the grafting efficiency percentage decrease with an increase of MMA concentration when the DPNR or HANR is grafted with methyl methacrylate (MMA). In comparison with the two type of lattices, DPNR provides higher grafting efficiency which contains larger quantity of grafted poly (methyl
methacrylate),a larger average particle size, and few free natural rubber molecules are observed in the grafting system. These differences are attributed to the removal of proteins. The protein layer can act as free-radical scavengers and terminate the free- radical species during graft copolymerization. Thus DPNR becomes the focus of the recent studies.
The discovery that brittle plastics can be toughened by using minor portion of rubber has led to the commercialization of high impact thermoplastics. One of the leading commodity thermoplastic materials is high-impact polystyrene (HIPS). However, the commercial HIPS were prepared by mass suspension polymerization of poly-butadiene with styrene which is a copolymer of styrene and butadiene usually has a rubber content of 8-14 wt% (Bucknall, 1977).
PS is known to be difficult to compatibilize with natural rubber (NR). Thus, it could be expected that the mechanical properties of PS/NR blend could be further enhanced via the incorporation of a suitable compatibilizer. Natural rubber /polystyrene (NR/PS) blends with the addition of compatibilizer which is NR-g-PS had improved the mechanical properties of NR/PS blends (Chuayjuljit et al, 2005). Research on dynamic vulcanization of NR/PS blends are well established by Asaletha et.al.(1999). Dynamic vulcanization of the blends is carried out by different curing agent, ie: sulfur, peroxide (DCP) and mixed system (sulfur with peroxide). All blends are prepared by melt mixing and solution casting technique.
The study reported here is an investigation of PS-modified NR (SNR) prepared by using emulsion polymerization. Instead of vulcanized the SNR by dynamic vulcanizaion, in situ vulcanization in SNR latex is recommended to prepare the SNR vulcanizate. In
situ vulcanization process is carried out immediately after the emulsion polymerization of
SNR latex in room temperature. SNR vulcanizate is left to dry in room temperature and then leached with deionized water to remove all the water soluble impurities in SNR vulcanizate. The formulation recommended for in situ vulcanization is sulfur based vulcanization. This SNR vulcanizate is ready to be used as impact modifier.
Part of the study involved the application of SNR vulcanizate as pressure sensitive adhesive (PSA). The ratio of accelerator to sulfur used from three type of pre- vulcanization system which are conventional vulcanization system (CV), semi-efficient system (semi-EV) and efficient vulcanization system (EV). Pre –vulcanization were carried out immediately after the emulsion polymerization at 60oC. The focus in this work was to develop a PSA with good performance in anchorage properties, good mean maximum load results and longer average time to fail.
Another application of SNR vulcanizate in the industry is rubber toughened material. The vulcanization system used is semi-EV vulcanization system. Polystyrene (PS) make up a large proportion of total tonnage of plastic currently being used mainly for consumer products or non load bearing application. In order to improve the toughness property of PS glassy polymer and divert the application of PS to high impact applications, small amount of rubber as impact modifier is recommended to add in to PS matrix. SNR vulcanizate which contains highly grafted PS portion is believed to have better interaction with PS in PS/SNR vulcanizate blend and act as impact modifier.
1.4 Objectives of studies
The focus of the study reported here is concerned with the application of SNR as pressure sensitive adhesive and rubber toughened PS. Both applications are using chemical modified DPNR with styrene monomer. The main objectives of this study are:
1. To investigate the in situ vulcanization system for SNR vulcanizates by using sulfur based vulcanization. The effect of the vulcanization systems will be studied for both applications.
2. To study the effect of SNR pre-vulcanizate as pressure sensitive adhesive in PS- PS, PS-NR and NR-NR substrates. The formulation of SNR pre-vulcanizate with optimum improvement in anchorage properties, good mean maximum load results and longer average time to fail are to be determined.
3. To investigate the effect of SNR vulcanizate as impact modifier in PS blends. The optimum rubber composition of PS/SNR blends are to be determined in order to achieve higher reinforcement and toughening effect on PS matrix.
4. To compare the PS/SNR vulcanizate with commercial HIPS and to study the compatibility of PS/SNR vulcanizate blends.
CHAPTER 2 LITERATURE REVIEW 2.1 Emulsion polymerization
There are four components in conventional reaction system for the aqueous emulsion polymerization: monomer, water, initiator and colloidal stabilizer. Sodium-n-dodecyl sulphate is colloidal stabilizer (emulsifier or surfactant) of non-polar monomers of low solubility such as styrene. Initiator is water soluble and functions as free radical generator.
The common water soluble initiator used is peroxodisulphates (persulphates) of monovalent cations, such as potassium, ammonium and sodium. When the surfactant is added into the aqueous solution, it will saturate the water phase and then aggregated to form micelles, thus, critical micelle concentration (CMC) occur. According to Gerrens &
Hirsch (1975), CMC has to be above 2.6 g/l H2O, to ensure that micelles are formed for polymerization (Flory, 1956; Gordon, 1970; Blackey, 1975; Eliseeva et.al .,1981; Rosen, 1982; Odian, 1991; Painter & Coleman, 1994; Kumar & Gupta,1998).
In processing the styrene butadiene as example, both styrene and butadiene monomer added will diffuse through the water phase and into the micelles until equilibrium is obtained. Most polymerization occurs within the monomer-swollen micelles. The polymerization begins after the addition of initiator. Initially, the free radicals are formed with the presence of initiators. Free radical reacts with the monomer double bonds, and the chain growth began. The hydrophobic chain migrates to the swollen micelles with further increase of molecular weight is observed. Majority of polymerization occurs in the swollen micelles (Rander, 2006). Figure 2.1 shows concept of the free radical emulsion polymerization
Figure 2.1: Concept of free radical emulsion polymerization (Rander, 2006)
The application of styrene-butadiene (SB) latex in coating industry such as paperboard coating, textile coating, as binder and coating for flooring felts, and as carpet backing.
Typical properties of SB latex are shown in Table 2.1
Table 2.1: Properties of styrene-butadiene latex (Rander 2006)
Property Value
Solids pH
Brookfield viscosity Average particle diameter Surface tension
Specific gravity (at 25oC) Styrene/butadiene ratio Film properties:
Tensile strength (at break) Elongation
50% wt.
7-9
< 500cp at 25oC 20,000 45 dynes/cm
1.01 50:50
550psi 520%
2.1.1 Smith-Ewart theory
An ideal emulsion polymerization occurs when radicals entering individual latex particles successively initiate and terminate the growing chains. At any given time, the number of growing chains will be one-half the number of particles. It is to be noted, the high radical concentration does not affect the radical lifetime. The number of polymer particles depends on both the initiator concentration and the surfactant concentration (Roderic et.al.,1994),
N α [I] 2/5 [S]3/5 (2.1) Where,[ I ] = Concentration of initiator,[ S ] = Concentration of surfactant
In the case of the diene, where the classical method of photoinitiation poses difficulties, a rather elegant method for obtaining the absolute value of the propagation rate constant kp from emulsion polymerization system (Roderic et.al.,1994), as
Rp = kp [M] N/2 (2.2) Where, N = Number of the particles per unit volume, Rp = Propagation rate constant, [M]
= Concentration of monomer in the monomer-polymer particles,
2.1.2 Chain-growth polymerization
Chain growth polymerization occurs when there is an addition of monomer to reactive sites on the growing chain molecules.
Where, P*n = Polymer chain with reactive site (*) and degree of polymerization of n, M=
Monomer unit, P*n+1= polymer chain with a reactive site (*) and degree of polymerization n+1
The reactive species which initiate such chain reactions must be capable of opening one of the bonds in the monomer and may be a radical, an electrophile, a nucleophile, or an organometallic species. Hence this polymerization may proceed by a variety of possible mechanisms depending on the electronic nature of the chain-carrying species, viz., free radical, cationic, anionic, and coordination, as illustrated in Figure 2.2 (Roderic et.al.,2005).
P*n + M P*n+1 +M (2.3)
Figure 2.2 : The reactions of double bonds with various types of initiating species.
(Roderic et.al, 2005)
The general kinetics for chain propagation by free radical mechanism involve three primary steps, i.e., initiation, propagation, and termination as shown in Figure 2.3 (Roderic et.al, 2005). Where I= initiator, M=monomer, R= initial free radical, and Mj· = propagating free radical.
This sequence of steps then leads to the following simple kinetic treatment:
Rate of initiation Ri= 2ki [I] (2.4) Rate of propagation Rp= kp [Mj · ] [M] (2.5) Rate of termination Rt= 2kt [Mj · ]2 (2.6) Assuming a steady-state condition where the rate of formation of radicals is equal to their rate of disappearance, i.e., Rj =Rt,
[Mj · ]= ki1/2
kt -1/2
[I] ½ (2.7) And
Rp = kp ki1/2 kt -1/2 [M][I]1/2 (2.8)
Equation 2.8 thus illustrates the dependency of the overall rate of polymerization on the concentration of initiator and monomer. Another important aspect of the free radical polymerization is the dependency of the number-average degree of polymerization on initiator concentration, increases the rate of polymerization but decreases the degree of polymerization, Xn, which corresponds to the number-average number of units per chain.
Xn= kp ki -1/2
kt -1/2
[M][I]-1/2 (2.9)
Figure 2.3: Kinetics for chain polymerization by free radical mechanism (Roderic et.al, 2005)
2.2 Latex Compounding Ingredients
Lattices require addition of compounding ingredients for a finished product. The range of compounding ingredients used for latex are divided into the following categories (Howard,1999; Blackley,1997):
(a) Vulcanization agents: These agents are necessary for vulcanization as the chemical crosslinking reaction can improve the physical properties of the latex compound.
(b) Accelerators: The function of these chemical with the combination of vulcanizing agents will reduce the vulcanizing time (cure time) or increase the rate of vulcanization. In most cases, the physical properties of the products are also improved.
(c) Activators: These ingredients form chemical complexes after react with accelerators.
These chemical complexes further increase vulcanization rates and improve the final product properties.
(d) Stabilizer including surfactants: These chemicals are used to reduce the surface free energy of aqueous media against air, and the interfacial free energy of aqueous media against immiscible organic liquids. This is due to the majority of lattices of industrial has aqueous dispersion media.
(d) Antioxidants: To increase the ageing characteristic of the latex compound.
(e) Fillers: To stiffen the product obtained from latex and also reduce the cost of final product.
(f) Viscosity modifiers (thickeners): To enhance the colloidal stability and modify the flow behaviors of latex compound.
