EFFECT OF INCLUSION OF NANOFIBERS ON ROLLING RESISTANCE AND
FRICTION OF SILICONE RUBBER
A Thesis
Presented to
The Graduate Faculty of The University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
Chapin Hutama
May 2019
EFFECT OF INCLUSION OF NANOFIBERS ON ROLLING RESISTANCE AND
FRICTION OF SILICONE RUBBER
Chapin Hutama
Thesis
Approved: Accepted:
______
Advisor Interim Dean of the College Dr. Shing-Chung “Josh” Wong Dr. Craig Menzenmer
______Co-Advisor or Faculty Reader Dean of the Graduate School Dr. Jiang Zhe Dr. Chand Midha
______Co-Advisor or Faculty Reader Dr. Kwek-Tze Tan
______Department Chair or School Director Date Dr. Sergio Felicelli
ii
ABSTRACT
With the rapid technology development these days, silicone rubber has become an important material to support our lives from house to industrial scale applications.
Moreover, tire industries also considering the superior properties of silicone rubber in their product in applications that require high-temperature resistance to perform. This demand leads material science researchers to search for solutions by intensively studying the potential of nanotechnology to boost the mechanical performance of the materials to make them suitable to perform in extreme and specific conditions.
Therefore, this research aims to explore an electrospinning method as a process to generate second phase materials with purpose to reinforce in a form of nanofibers into a room temperature vulcanized (RTV) silicone rubber together with study of the mechanical properties of reinforced RTV silicone rubber such as rolling resistance and the static friction of the material.
The reinforcement process by electrospinning method was conducted by using polyether-based thermoplastic polyurethane as the reinforcement material that exhibits excellent temperature flexibility, abrasion resistance and strength to reinforce directly to the liquid silicone rubber to generate several specimens with a different composition ratio of matrix and reinforcement phase in the composite. Furthermore, the experiment was carried out by observing the TPU infused into silicone rubber and measuring the rolling
iii
resistance properties by using a wooden roller based device inspired by the invention of
Dr. Alan Gent [1] hardness test by following standard ASTM D2240, tensile test and static friction of the material by using the inclined surface coefficient (ASTM D4918) of friction tester to understand the influence of TPU on the RTV silicone rubber
iv
ACKNOWLEDGMENTS
First of all, I would like to thank Dr. Shing-Chung “Josh” Wong, for the honor of letting me be a part of his research group and work under his direction, leadership, as well as guidance, knowledge, and wisdom, shared to me during my master’s degree.
Secondly, I would like to thank Dr. Jiang Zhe and Dr. Kwek-Tze Tan as my committee member for being part of my academic journey, and wisdom during my master’s degree career in the mechanical engineering department.
Moreover, I would like to thank Omar Ali Blandon and Dr. Manigandan Kannan, a mentor as well as a best friend who changed my life, giving me the opportunity for me pursuing my master’s degree as well as supports that are valuable to me to be an ideal person for my future.
Furthermore, I would like to thank my close friends; Elisha Dale, Daryl George
Philip, Joseph Elrassi, Chong Zhong, Jiawei Wu, and Xiaoxiao Liu for their valuable support during my college life.
In addition, I would like to thank Juliani Soegandi and her family as well as
Indonesian communities in Cleveland area for opening their hearts and take me as their own child and give me all the support and love during my stay in the United States.
Lastly, I would like to thank my family for always being there in the most critical moments and being light on my path.
v
TABLE OF CONTENTS LIST OF TABLES………………………………………………………………..……...xv
LIST OF GRAPHS……………………………..……………………………………....xvii
LIST OF FIGURES……………………………………………………………...……...xix
CHAPTER
I. INTRODUCTION…………………………………………………………….………1
II. LITERATURE REVIEW……………………………………………………………..3
2.1 Composite Overview……………………………………………………………..…...3
2.1.1 Particle-Reinforced Composites………………………………………………...…..6
2.1.1.1 Large-Particle Composites………………….……………………………..7
2.1.1.2 Dispersion-Strengthened Composite…………………………………...…9
2.1.2 Fiber Reinforced Composites……………………………………………………….9
2.1.2.1 Influence of Fiber Length………………………………………..…..…..10
2.1.2.2 Influence of Fiber Orientation and Concentration…………………....….11
2.1.2.3 Continuous and Aligned Fiber Composites……………………….……..12
2.1.2.4 Discontinuous and Aligned Fiber Composites……………………….….13
2.1.2.5 Discontinuous and Randomly Oriented Fiber Composites……………...14
vi
2.1.2.6 The Fiber Phase…………………………………………………………..14
2.1.2.7 The Matrix Phase……………………………………………………...…15
2.1.2.8 Polymer-Matrix Composites………………………………………….….15
2.1.2.8.1 Glass Fiber Composites………………………………………..15
2.1.2.8.2 Aramid Fiber Composites……………………………………...16
2.1.2.8.3 Carbon Fiber Composites……………………………………...16
2.1.2.9 Metal-Matrix Composites……………………………………………..…17
2.1.2.10 Ceramic-Matrix Composites…………………………………………....18
2.1.2.11 Carbon-Carbon Composites…………………………………………….19
2.1.3 Processing of Composites………………………………………………………….19
2.2 Electrospinning Process…………………………………………………………...…21
2.2.1 Parameters………………………………………………………………………….23
2.2.1.1 Solution Parameters…………………………………………………...…23
2.2.1.1.1 Solvent……………………………………………………...….23
2.2.1.1.2 Concentration…………………………………………………..25
2.2.1.1.3 Viscosity…………………………………………………...…..26
vii
2.2.1.1.4 Molecular Weight……………………………………………...27
2.2.1.1.5 Surface Tension………………………………………………..28
2.2.1.1.6 Solution Conductivity………………………………………….30
2.2.1.2 Processing Parameters…………………………………………………...32
2.2.1.2.1 Voltage………………………………………………………....32
2.2.1.2.2 Taylor Cone and Jet Formation………………………………...33
2.2.1.2.3 Flow Rate……………………………………………………....34
2.2.1.2.4 Collector………………………………………………………..35
2.2.1.2.5 Collector Distance…………………………………………...…39
2.2.1.3 Environmental Parameters…………………………………………...…..40
2.2.2 Electrospinning Application…………………………………………………….…42
2.3 Silicone Rubber……………………………………………………………………....44
2.4 Plastics…………………………………………………………………………….…45
2.4.1 Polymeric Materials………………………………………………………………..46
2.4.1.1 Thermoplastic Material…………………………………………………..47
2.4.1.2 Thermosetting Plastics…………………………………………………...50
viii
2.4.2 Plastic Available to Designer……………………………………………………....50
2.4.2.1 Engineering Plastics…………………………………………………...…50
2.4.2.2 Thermosets……………………………………………………………….52
2.4.2.3 Composite………………………………………………………………..52
2.4.2.4 Structural Foam…………………………………………………………..53
2.4.2.5 Elastomers………………………………………………………………..54
2.4.2.6 Polymer Alloys…………………………………………………………..54
2.4.3 Semi-crystalline Plastics………………………………………………………...…56
2.4.3.1 Low-Density Polyethylene (LDPE)……………………………………...56
2.4.3.2 Linear Low-Density Polyethylene (LLDPE)…………………………….57
2.4.3.3 High-Density Polyethylene (HDPE)……………………………………..57
2.4.3.4 Cross-linked Polyethylene (XLPE)……………………………………....57
2.4.3.5 Polyproopylene (PP)……………………………………………………..58
2.4.3.6 Polyamides (nylon)……………………………………………………....58
2.4.3.7 Acetals……………………………………………………………………59
2.4.3.8 Polytetrafluoroethylene (PTFE)…………………………………….……59
ix
2.4.3.9 Thermoplastic Polyesters…………………………………………….…..59
2.4.3.10 Polyetheretherketone…………………………………………………...60
2.4.4 Amorphous Plastics………………………………………………………………..60
2.4.4.1 Polyvinyl Chloride (PVC)…………………………………………….....60
2.4.4.2 Polymethyl Methacrylate (PMMA)…………………………………..….61
2.4.4.3 Polystyrene (PS)………………………………………………………….61
2.4.4.4 Acrylonitrile-Butadiene-Styrene (ABS)……………………………...….61
2.4.4.5 Polycarbonates…………………………………………………………...62
2.4.4.6 Polyethersulphone………………………………………………………..62
2.4.4.7 Modified Polyphenylene Oxide (PPO)…………………………………..63
2.4.5 Thermoplastic Rubbers………………………………………………………….....63
2.4.6 Thermosetting Plastics……………………………………………………………..65
2.4.6.1 Aminos………………………………………………………………...…66
2.4.6.2 Phenolics………………………………………………………………....66
2.4.6.3 Polyurethanes………………………………………………………….…66
2.4.6.4 Polyesters………………………………………………………………...67
x
2.4.6.5 Epoxides………………………………………………………………….67
2.4.7 Selection of Plastics………………………………………………………………..67
2.5 Existing Reinforced RTV Rubber Method…………………………………………..68
2.5.1 Polymer Solution Intercalation………………………………………………….…69
2.5.2 Melt Intercalation…………………………………………………………………..69
2.5.3 Surface Modification………………………………………………………………70
2.5.4 Ultrasonicatioon…………………………………………………………………...71
2.6 Rolling Resistance…………………………………………………………………...72
2.7 Hardness……………………………………………………………………………...76
2.7.1 Durometer………………………………………………………………………….77
2.8 Friction……………………………………………………………………………….79
2.8.1 Parameters………………………………………………………………………….81
2.8.1.1 Pressure…………………………………………………………………..81
2.8.1.2 Roughness……………………………………………………………..…82
2.8.1.3 Micro- Displacement…………………………………………………….83
2.9 Silicone Rubber in Industry……………………………………………………….…84
xi
2.9.1 Transportation……………………………………………………………………...85
2.9.1.1 Automotive…………………………………………………………...….85
2.9.1.2 Mass Transportation……………………………………………………...88
2.9.1.3 Aerospace-Aircraft…………………………………………………...…..88
2.9.2 Building and Construction…………………………………………………………89
2.9.3 Medical Industry………………………………………………………………...…91
2.9.4 Electronics……………………………………………………………………….…92
2.10 Silicone Rubber Product Failure…………………………………………………....93
2.10.1 Temperature………………………………………………………………………95
2.10.2 Fluids……………………………………………………………………………..97
2.10.3 Weathering………………………………………………………………………..98
2.10.4 Ionizing Radiation………………………………………………………………...99
2.10.5 Biological Attack………………………………………………………………..100
2.10.6 Fatigue………………………………………………………………….……….101
2.10.7 Set, Stress Relaxation and Creep……………………….…………………...….102
2.10.8 Abrasion…………………………………………………….………………..…102
xii
2.10.9 Electrical Stress………………………………………………………………….103
2.11 Existing of Reinforced Silicone Rubber………………………………………...... 103
III. EXPERIMENT WORK……………………………………………………….…...105
3.1 Materials…………………………………………………………………….….…..105
3.1.1 Thermoplastic Polyurethane (TPU)………………………………………..……..105
3.1.2 RTV Silicone Rubber…………………………………………………………..…105
3.2 Electrospinning of Thermoplastic Polyurethane (TPU)………………………...... 106
3.3 RTV Silicone Rubber Preparation……………………………………………….…107
3.4 Rolling Resistance Test……………………………………………………………..107
3.5 Static Friction Test………………………………………………………..……..….111
3.6 Hardness Test……………………………………………………………………….111
3.7 Scanning Electron Microscope (SEM) Test………………………………………..112
3.8 Tensile Test………………………………………………………………………....113
IV. RESULT AND DISCUSSION…………………………………………………….114
4.1 SEM Analysis of Thermoplastic Polyurethane………………………………….….115
4.2 Hardness Test of Reinforced RTV Silicone Rubber………………………………..119
xiii
4.3 Static Friction Test……………………………………………………………….....121
4.4 Rolling Resistance Test……………………………………………………………..124
4.5 Tensile Strength Test……………………………………………………………….128
V. CONCLUSIONS…………………………………………………………………….132
5.1 Conclusions………………………………………………………………………....132
5.2 Future Work………………………………………………………………………...134
REFERENCES…………………………………………………………………………135
xiv
LIST OF TABLES
Table 1: A classification of various composites types ...... 10
Table 2: Axial tensile properties of carbon fibers ...... 21
Table 3: Axial tensile properties of carbon fibers ...... 22
Table 4: Major composite processing ...... 24
Table 5: Commonly used solvents ...... 28
Table 6: Examples of thermoplastics ...... 51
Table 7: Comparison between amorphous vs crystalline structure ...... 52
Table 8: Examples of amorphous and crystalline thermoplastics ...... 53
Table 9: Main classes of new plastics material ...... 55
Table 10: Typical plastic alloys ...... 59
Table 11: Physical characteristics of thermoplastic rubbers ...... 67
Table 12: Type of durometer ...... 82
Table 13: Common automotive silicone rubber applications ...... 89
Table 14: Specific properties and applications of silicone in the construction industry ...93
xv Table 15: Type of degradation agents ...... 97
Table 16: Universal maximum continuous use temperature for various rubbers ...... 99
Table 17: Gamma radiation resistance of a range of various rubbers ...... 103
Table 18: Classification of samples ...... 118
Table 19: Hardness test comparison between samples ...... 123
Table 20: Static friction coefficient comparison between samples ...... 126
Table 21: Rolling resistance coefficient data of prepared specimens ...... 130
xvi LIST OF GRAPHS
Graph 1: Average fiber diameter of PEO nanofibers with concentrations from 4-7% [67]
...... 25
Graph 2: Surface tension and viscosity as functions of the mass ratio of ethanol/DMF
[71] ...... 29
Graph 3: The diameter of nanofiber as a function of polypyrrole content. Solutions of Ppy with 2.5 wt% PEO (o), 1.5 wt% PEO (□), and 1.5 wt% PEO with 0.5% wt% Triton-X100
(◊) were used by Chronakis et al [73] as a carrier ...... 31
Graph 4: The distribution of pore diameter on 190,000 g/mol PS/THF electrospun fibers at varying humidity ranges: (a) 31-38%, (b) 40-45%, (c) 50-59%, (d) 60-72% [88] ...... 40
Graph 5: Tangential displacement (x-axis) vs friction force (y-axis) [115] ...... 79
Graph 6: Effect of surface roughness [118] ...... 81
Graph 7: Coefficient of static friction (μs) vs limiting displacement (δ_1) at various pressure. 1) 1.55 Mpa 2) 4.5 Mpa 3) 6.5Mpa 4) 10 Mpa [118] ...... 82
Graph 8: Example of time vs displacement collected by using motion sensor during resistance measurement ...... 109
Graph 9: Hardness of prepared samples ...... 119
Graph 10: Static friction coefficient of prepared samples ...... 121
xvii Graph 11: Non--reinforced silicone rubber (BLUE) vs reinforced silicone rubber
(PURPLE) ...... 124
Graph 12: Rolling resistance coefficient graph of prepared specimens ...... 126
Graph 13: Stress-strain curve non-reinforced vs reinforced silicone rubber ...... 129
xviii LIST OF FIGURES Figure 1: Schematic representation of the matrix phase and dispersed phase in the composite………………………………………………………………..………………...5
Figure 2: Schematic drawing of a particle-reinforced composite…………………………6
Figure 3: Schematic drawing of concrete [22]…………………………………………….8
Figure 4: Schematic drawing of (a) continuous and aligned, (b) discontinuous and aligned, and (c) discontinuous and randomly oriented fiber reinforced composites…………………………………………………………….…………………11
Figure 5: Electrospinning setup………………………………………………………….21
Figure 6: SEM graphs of polyvinyl alcohol (PVA) for different molecular weights
(M_w). (a) 9,000-10,000 g/mol; (b) 13,000-23,000 g/mol; (c) 31,000-50,000 g/mol
[69]……………………………………………………………..…...... 27
Figure 7: TEM Graph of PVP solution with fixed 4% concentration from (a) ethanol; (b)
DMF; (c) MC as solvents [71] ...... 29
Figure 8: Jet Formation: (A) Needle tip, (B) Solution droplet, (C) Taylor Cone, (D) Jet formation, I Nanofibers, (F) Terminal state [60] ...... 33
Figure 9: The influence of flow rate with various flow rates; (a) 0.1 mL/hr, (b) 0.5 mL/hr,
(c) 1.0 mL/hr, and (d) 1.5 mL/hr [62] ...... 34
xix Figure 10: Static collector; (A) syringe, (B) needle, (C), Taylor Cone, (D) nanofibers, (E) ground collector (F) High voltage power supply (positive), (G) high voltage power supply (negative) [60] ...... 35
Figure 11: Electrospinning collection methods; (a) rotating drum, (b) rotating disk, (c) parallel electrodes [43] ...... 36
Figure 12: Wired rotating drum; (A) Syringe, (B), Nanofibers, (C) Wired rotating drum
[60] ...... 37
Figure 13: Electrospinning technique using a stainless steel pin as a collector; (A)
Syringe needle, (C) A view of the pin; (M1) Motor 1, (M2) Motor 2 [82] ...... 38
Figure 14: SEM picture of ultrafine PSF with 20% PSF/DMAC solution at 10kV, flow rate 0.40 mL/h with distance; (a) 10cm and (b) 15cm [89] ...... 39
Figure 15: SEM graph of PA-6-32 solutions with 20% w/v concentration in 85% v/v formic acid at temperature: (a) 30° C and (b) 60° C (magnification = 10,000X, scale bar =
1 μm) ...... 41
Figure 16: Chemical Structure of polydimethylsiloxane (PDMS) ...... 43
Figure 17: Schematic 2D drawing of the concept of rolling resistance measurement device setup ...... 71
Figure 18: Force method to calculate rolling resistance (NHTSA,2009) [112] ...... 72
Figure 19: Torque method to calculate the rolling resistance of a tire [112] ...... 73
Figure 20: Schematic drawing of lab scale rolling resistance measurement test [112] ....73
xx Figure 21. Damped oscillations of a pendulum ...... 74
Figure 22: Schematic drawing of a hardness test by using a durometer instrument...... 76
Figure 23: Forces acting on the material in sliding motion ...... 79
Figure 24: Downward electrospinning setup ...... 105
Figure 25: RTV silicone rubber Part B and A respectively ...... 106
Figure 26: TPU reinforcement by electrospinning process ...... 107
Figure 27: 3D printing mold container ...... 108
Figure 28: Wooden rolling resistance measurement design ...... 108
Figure 29: Static Coefficient of Friction Test by using PTFE ...... 110
Figure 30: Scanning Electron Microscopy ...... 111
Figure 31: Instron E300 ...... 112
Figure 32: SEM Picture of 10 wt%(a), 15wt% (b) and 20 wt% (c)of TPU Estane
58315...... 114
Figure 33: Non-reinforced silicone rubber (a) and TPU reinforced silicone rubber ...... 115
Figure 34: Surface characterization of reinforced silicone rubber ...... 116
Figure 35: Nanofiber in the silicone rubber matrix ...... 117
Figure 36: Tensile Test of Specimen ...... 128
xxi CHAPTER I
INTRODUCTION
The tire development begun back then in 1847 when the first tire was made of solid rubber. [145-148]. With continuous development, tires today have been improved in both physical and mechanical properties. In the beginning of the development, carbon black was selected as a filler in rubber compound due to its excellent reinforcing properties [149].
Since then, different industrial and scientific researchers have tried different types of fillers in order to optimize the tire life and performance. [150-153]. This topic allowed me to study the tire reinforce using fibrillar fillers at the nano-scale size that have not been deeply explored.
The importance of this research work is based on the possibility of studying the change in the behavior of silicone rubber once the fibrillar nano-filler is incorporated to the matrix. Investigating the behavior of the composite in this research could lead to different path of future research that could potentially revolutionize the tire industry or the silicone rubber industry.
Nanotechnology has been already introduced to the tire industries using carbon nanotubes as nanofillers by replacing the carbon black with carbon nanotubes. In addition, by adding carbon nanotubes, several properties of tire are improved such as reduced abrasion and
1 improved skid resistance [154]. Oh et al [155] demonstrated the improvement in the properties of the material using carbon nanotubes that are able to reduce the abrasion by
0.5 times compared to regular carbon black fillers. On the other hand, carbon nanotuubes are considered as expensive material and complex to manufacture as well as to produce it in industrial quantities which leads to new challenge for tire industries to find the alternative way. With the use of the electrospinning process by adjusting the process as well as solution parameters, the output material can go from nano to micro scale easily.
Based on the results of previous researchers and their success of using carbon nanotubes, I would like to explore the possibility of using nanofibers as nanofillers for tire due to the ease of mass production of the material as well as the simplicity of the manufacturing process compared to carbon nanotubes.
This study serves as a new exploration of reinforcing rubber-based matrix by electrospun fibers. The reinforcement is expected to reinforce rubber tire to increase its mechanical properties, particularly in static friction and rolling resistance. The polymeric electrospun fibers investigated here are made using polyurethane. This polymer has been developed in the past 50 years as a tire material for limited applications such as lift trucks because its load capacity greatly exceeds other rubber material, as well as its abrasion, cut, and tear resistance. Hence, this thesis aims to investigate the effect of inclusion of thermoplastic polyurethane in silicone rubber matrix and investigate the mechanical properties, such as rolling resistance and friction performance, in order to understand the influence of reinforcement in increasing the properties of tire materials. By generating nanofibers using electrospinning, it gives the advantages of the size dependent mechanical
2 properties of the electrospun fibers that can be successfully applied to the silicone rubber industry.
Nanofibers are allowed to have an intimate contact with the surface or phase they interact due to their size. By taking advantage of this mechanical property, the nanofiber reinforcing the silicone rubber face will develop an entanglement between two faces due to the intimate interaction between two materials. In addition, the nanofibers embedded in the rubber matrix will work as a nano thread that will increase the friction on the surface of the reinforced matrix improving its traction. It is shown in this research work that by using nanofibers as nanofiller in a silicone rubber matrix, the hardness, the static friction coefficient as well as the rolling resistance of the material are improved. Therefore, this could lead into potential solution for tire industries that are looking for new reinforcement method besides carbon black and silica.
3 CHAPTER II
LITERATURE REVIEW
2.1 Composite Overview
The conventional metal alloys, ceramics, and polymeric materials are not sufficient to satisfy the requirement of materials with unusual combinations of properties such as in aerospace, underwater, and transportation applications [14]. These unusual requirements attract many researchers to develop new materials such as composite. A composite is a material consists of at least two different materials bonded together to form one matrix and designed to show the best characteristics of each component materials [15]. The rapid development and use of the composite materials in the United States was occurring in the
1940s where the demand for military industry to have a high-strength and lightweight materials became the first driving force of the use of composite materials. The second driving force was coming from polymer industries which tried to expand their market of plastics to a variety of applications. Lastly, the rapid growth of composite development was the discovery of new material such as glass fibers which has high theoretical strength that leads to the rapid development of composite materials [16], [17].
4 Furthermore, most of the composite materials are composed by two-phase that is called matrix phase and dispersed phase. Matrix phase is continuous and surrounds the dispersed phase. The dispersed phase has various geometrical and spatial characteristics of particles that can influence the properties of the composites [18].
Figure 1: Schematic representation of the matrix phase and dispersed phase in the
composite
In addition, the composite is mainly classified into three main divisions based on the differences in dispersed phase such as particle-reinforced, fiber-reinforced, and structural composite.
Table 1: A classification of various composites types[14]
Divisions Type Sub-type
Large-particle Composites Particle- Dispersion- reinforced strengthened
5 Continuous
Aligned Fiber Reinforced Discontinuous Randomly
Oriented
Laminates Structural Sandwich panels
2.1.1 Particle-Reinforced Composites
Figure 2: Schematic drawing of a particle-reinforced composite
Particle-reinforced composite is a composite that contains particle shape dispersed phase where it does not have a long dimension. Furthermore, particle-reinforced composites are divided by two types such as large-particle and dispersion-strengthened composites due to the difference in the reinforcement mechanism that in the large-particle,
6 the interaction between matrix and the particle cannot be treated on a molecular level [19].
On the other hand, dispersion-strengthened composites are a composite that contains particles smaller than particles in large-particle with a diameter between 10 and 100nm
[20].
2.1.1.1 Large-Particle Composites
Large-particle composite is a composite that uses continuum mechanics as a reinforcement mechanism to develop the interaction between the matrix and the particles.
To obtain the effective reinforcement result, the particles have to be small and evenly distributed in the matrix phase where the volume fraction of the matrix and dispersed phase affect the mechanical and behavior properties of the composite.
A theoretical method to estimate the elastic modulus on the volume fraction such as the rule of mixture method can be used with composite materials containing multiple phases. In addition, the rule of the mixture is applied to predict the elastic modulus should fall between an upper bound that is represented by
퐸푐(푢) = 퐸푚푉푚 + 퐸푝푉푝 (Equation 1) and a lower bound by
퐸푚퐸푝 퐸푐(푙) = (Equation 2) 푉푚퐸푝+ 푉푝퐸푚 where E represents the elastic modulus, V represents the volume fraction, c, m, and p represent composite, matrix, and particle.
7 Moreover, Li Guoqiang et al [21] used the rule of mixture formula together with the theory of elasticity and Eshelby’s equivalent medium theory to achieve the effective result of Young’s modulus estimation of concrete.
2.1.1.1.1 Concrete
Figure 3: Schematic drawing of concrete [22]
Concrete is one of the examples of the large-particle composite where both matrix and dispersed phases are ceramic materials. The most common concrete is made with
Portland and asphaltic cement where Portland cement concrete is mostly used for structural building material, whereas asphaltic cement concrete is commonly used for paving material
[14]. The composition of Portland cement concrete is sand, gravel, and water where both sand and gravel acts as a filler material [23].
8 2.1.1.2 Dispersion-Strengthened Composite
Dispersion-strengthened composite is a material which the strength and other properties of the material are increased by adding of fine particles that are uniformly dispersed in the matrix. In addition, the dispersed phase can be metallic or nonmetallic where oxides materials are common to be used and strengthening mechanism works based on the dislocations within the matrix and the interaction between the particles [14].
Moreover, the effect of the strengthening is similar to precipitation hardening but not so strong. Furthermore, the dispersion strengthening method can retain the strengthening at the elevated temperature at a short time due to dispersed particles are chosen to be unreactive with the matrix phase.
Benjamin [20] successfully developed a new process that is called “mechanical alloying” that was able to produce homogeneous composite particles by combining dispersion strengthening and age-hardening method in the high-temperature alloy. In the study, yttrium oxide and gamma prime hardening in a complex nickel-based superalloy were successfully combined with the new method that can obtain stress rupture properties at 40,000 psi for 100 hours at 1400°F with excellent sulfidation and cyclic oxidation resistance.
2.1.2 Fiber Reinforced Composites
Fiber reinforced composites is a composite that contains fiber as its dispersed phase.
The purpose of infusing fibers into the matrix phase is to increase the strength and stiffness of the material that is expressed in the term of specific strength and specific modulus [14].
9 In addition, the fiber-reinforced composite is divided into two types based on the length of the fiber such as continuous (aligned) and discontinuous (short).
2.1.2.1 Influence of Fiber Length
In the fiber-reinforced composite materials, the properties of the fiber together with the degree to which an applied load is applied to the fiber by the matrix phase and the magnitude of the interfacial bond between the matrix phase and the fibers [14].
The critical length is important to determine the effective strengthening and stiffening of the composite material. The mathematical model for critical length can be described as:
휎푓푢푟푓 퐿푐 = (Equation 3) 휏푖
where 퐿푐 is the critical length, 휎푓푢 , 푟푓 , and 휏푖 are the ultimate strength of the fiber and fiber radius, and interfacial shear stress between matrix and fibers respectively. The term continuous is used when the length of fiber (퐿) is much larger than the critical length of fiber (퐿푐), whereas the term of discontinuous fiber is used when the length of fiber (퐿) is significantly less than the critical length of fiber (퐿푐).
Moreover, Yun Fu et al [24] studied the influence of the fiber length and orientation distribution in short fiber reinforced polymer (SFRP). It is shown that the increase of the mean fiber length at small mean fiber lengths increase the strength of SFRP and the composite strength increase with the decrease of the critical fiber length.
10 2.1.2.2 Influence of Fiber Orientation and Concentration
The fiber orientation and concentration have a significant influence on the strength and other properties in the composite material.
Figure 4: Schematic drawing of (a) continuous and aligned, (b) discontinuous and
aligned, and (c) discontinuous and randomly oriented fiber reinforced composites
Banakar et al [25] studied the influence of fiber orientation and the thickness on tensile properties of epoxy resin composites reinforced with glass fiber. The composites were designed where the reinforcements are oriented at ±30°, ±45°, and ±90° and it is shown that the tensile strength of the composite is superior in case of 30° orientation.
11 2.1.2.3 Continuous and Aligned Fiber Composites
Continuous-fiber composites are often made by stacking single sheets of continuous fibers in specific orientation into laminates to obtain the desired mechanical properties [26]. Moreover, due to the quantity and the type of fiber reinforcement, the continuous-fiber composite can achieve the highest strength and modulus compared to the discontinuous-fiber composite where the fiber is aligned in a random orientation that reduces its strength and modulus. In addition, the elastic behavior of continuous and aligned fiber composites can be determined based on the loading is applied.
In longitudinal loading, the load is applied in the direction of fiber alignment direction and the modulus of elasticity can be calculated by:
퐸푐푙 = 퐸푚푉푚 + 퐸푓푉푓 표푟 퐸푐푙 = 퐸푚(1 − 푉푓) + 퐸푓푉푓 (Equation 4)
where 푉푚 + 푉푓 = 1 and 퐸푐푙 represents the elastic moduli for the composite in longitudinal loading.
Furthermore, in transverse loading, the load is applied at a 90° angle to the direction of the fiber alignment. Where in this situation, the stress for composite together with both phases (matrix and dispersed) are same where (휎푐 = 휎푚 = 휎푓 = 휎) and the term for this condition is called isostress state [14]. The modulus of elasticity in the transverse direction can be calculated by:
1 푉푚 푉푓 = + (Equation 5) 퐸푐푡 퐸푚 퐸푓
where 퐸푐푡 is the modulus of elasticity of the composite in transverse loading.
12 2.1.2.4 Discontinuous and Aligned Fiber Composites
Discontinuous and aligned fiber composite is a term used for the composite that is reinforced by a chopped size fiber where the orientation of the fiber is aligned. In addition, the discontinuous fiber composite will have lower strength properties compared to continuous and less expensive to manufacture. In theory, a discontinuous fiber composite with a sufficient fiber length that is perfectly oriented will approach the strength and stiffness of a continuous-fiber composite. However, there are many challenges to control the orientation in discontinuous fiber composite due to its size [26].
Several methods have been developed using electrical, magnetic, and pneumatic techniques [27], [28] where the methods are relying on the conductivity of the fibers for both electrical and magnetic. In addition, pneumatic methods are simpler, but the degree of alignment is lower compared to the electric method that can have 70% of fibers within the range of ± 20° [29].
Moreover, Yu et al [30] developed a novel manufacturing method by utilizing a low-viscous medium such as water and an orientation head that consists of parallel plates with narrow gaps where the water jets are aimed onto the orientation plates and the fiber are aligned transversely to the water jet direction by the momentum change of the liquid.
With this method, Yu et al successfully produce highly aligned short fiber composites with
65-67% of fibers with the range of ± 3° that is superior to conventional alignment method.
13 2.1.2.5 Discontinuous and Randomly Oriented Fiber Composites
Discontinuous and randomly oriented fiber composites mainly use short and discontinuous fiber as a reinforcement in the composite. The modulus of elasticity of the composite can be calculated by:
퐸푐푙 = 퐾퐸푓푉푓 + 퐸푚푉푚 (Equation 6)
퐸푚 where K is a fiber efficiency parameter that depends on ratio and 푉푓. 퐸푓
Furthermore, when a stress is applied to the discontinuous and randomly oriented fiber composite, the stress that is applied to the matrix phase will transfer by shear across the interface of the fiber [26]. Therefore, to achieve a maximum reinforcing efficiency, a strong interfacial bond is needed.
2.1.2.6 The Fiber Phase
Fiber is commonly defined as a material that has a longer axis compared to its diameter. Moreover, the aspect ratio between fibers lengths divided by the diameter of the fiber is commonly used to describe what type of the fibers [14].
Based on the character and diameter, fiber is divided into three different classes such as whiskers, fibers, and wires.
Whiskers are mainly in size between 0.1 to 10 microns in diameter. Moreover, whiskers are similar in diameter to fibers but shorter in length to diameter ratios. In theory, whiskers have a high degree of crystalline perfection due to their size. Therefore, the smaller they are, the stronger they [31], [32].
14 Fibers have a high length-to-diameter ratio where normally greater than 100 and are classified as either polycrystalline or amorphous materials with small diameters such as polymers and ceramics where glass, polymer aramids, carbon are common to use.
The wire is a term when the materials have relatively higher in diameter that is most commonly used as reinforcement in a radial tire by using steel material.
2.1.2.7 The Matrix Phase
The matrix phase can be a metal, polymer, or ceramic and the purpose of matrix phase in a composite is to bind the fibers together and protect them from the environmental influence such as mechanical abrasion or chemical reaction from the environment [14],
[26]. In addition, it is important to consider the adhesion bonding between matrix and fiber which has a high adhesive bonding force to minimize fiber pull-out.
2.1.2.8 Polymer-Matrix Composites
Polymer-matrix composite is a composite that consists of a polymer resin as the matrix together with fiber as the reinforcement phase. Moreover, polymer-matrix composites are popular to be used in various applications due to the ease of fabrication and cost [14].
2.1.2.8.1 Glass Fiber Composites
Glass fiber composites are widely used in different applications due to their low material costs compared to aramid and carbon fibers. In addition, glass fiber composite especially E-glass has good tensile strength at a low material cost. On the other hand, glass
15 fiber composites have a relatively low modulus, high density and sensitive to fatigue loading compared with aramid and carbon [26].
2.1.2.8.2 Aramid Fiber Composites
Aramid composites were popular to use when it was introduced in the 1970s that has similar tensile properties with carbon fiber composite at that time. Furthermore, aramid fiber composite achieves a good tensile property at a lower density than glass fiber composites but at a higher cost.
Moreover, aramid composite is commonly known to have good toughness, impact resistance, creep resistance, and stable at high temperature. On the other hand, their compressive properties are poor that cause them to limit their applications in tensile- dominated design [14], [26].
2.1.2.8.3 Carbon Fiber Composite
Carbon fiber composite is a type of polymer-matrix composite which carbon is well known to have a high specific modulus and specific strength compared to other reinforcing fibers.
Table 2: Axial tensile properties of carbon fibers [33]
Precursor Tensile Strength Tensile modulus Elongation at
(Gpa) (Gpa) break (%)
PAN 2.5-7.0 250-400 0.6-2.5
16 Mesophase pitch 1.5-3.5 200-800 0.3-0.9
Rayon ≈1.0 ≈50 ≈2.5
In addition, carbon fiber has great corrosion stress resistance at room temperature compared to glass and organic polymer fibers. Therefore, this property leads carbon to be used in applications where strength, stiffness, lower weight, high temperature, high damping, chemical inertness, and fatigue resistance are required such as in aerospace and nuclear applications [34].
2.1.2.9 Metal-Matrix Composites
Metal-matrix composite is a type of composite that metal is used as its matrix and this type of material is common to be used in a high-temperature application which is one advantage of metal-matrix composites compared to polymer matrix composite.
Table 3: Axial tensile properties of carbon fibers [35]
Fiber Matrix Fiber Density Longitudinal Longitudinal
Content (g/cm3) Tensile Tensile
(vol %) Modulus Strength
(Gpa) (Mpa)
Carbon 6061 Al 41 2.44 320 620
17 Boron 6061 Al 48 -- 207 1515
SiC 6061 Al 50 2.93 230 1480
Alumina 380.0 Al 24 -- 120 340
Carbon AZ31 Mg 38 1.83 300 510
Borsic Ti 45 3.68 220 1270
However, the limitation of a metal-matrix composite is that the material is more expensive than polymer-matrix composite. Furthermore, there are two common processing steps in order to produce metal-matrix composites such as consolidation or synthesis and followed by a sharpening operation. Moreover, metal-matrix composites are widely used in automobile manufacturer and aerospace industry where require high-temperature material for most of the engine components.
2.1.2.10 Ceramic-Matrix Composites
Ceramic-matrix composite is a type of composite that is widely used due to its oxidation and deterioration resistance at the elevated temperature that makes this material is suitable for high-temperature applications in automobile and aircraft gas turbine engine.
In addition, ceramic-matrix composites are able to be generated by several fabrication methods such as hot pressing, hot isostatic pressing, and liquid phase sintering techniques.
18 2.1.2.11 Carbon-Carbon Composites
Carbon-carbon composite is a term of the composite which both reinforcement and matrix phase is carbon which has high strength, toughness and versatility properties and is relatively new and expensive due to its complicated processing techniques. Moreover, this type of composite is very limited to be used in military and aerospace applications.
2.1.3 Processing of Composites The processing mechanism of major polymer matrix composite fabrication can be divided into two categories such as thermoset composites processing and thermoplastic composites processing [26]. Thermoset composite processing for continuous fiber composite can be generated by lay-up, filament winding, liquid molding, pultrusion, vacuum bagging, and curing process.
Moreover, for short fiber composites, processes such as injection molding, compression molding, liquid molding, and spray-up are common to be used. In addition, couple methods also can be used for thermoplastic composites processing which short-fiber composites can be generated by injection and compression molding; and continuous fiber composite for thermoplastic composite processing can be generated by lay-up, thermoforming, and compression molding process.
Table 4: Major composite processing
Injection Molding Composite Short-Fiber Composites Processing Compression Molding
19 Liquid Molding
Spray-up
Thermoset Lay-up Composites Liquid Molding Processing Continuous-Fiber
Composites Filament Winding
Pultrusion
Compression Molding Short-Fiber Composites Injection Molding Thermoplastic
Composites Thermoforming
Processing Continuous-Fiber Lay-up Composites
Compression Molding
2.2 Electrospinning Process
There are many methods to generate nanofibers such as drawing [36], template synthesis [37], [38], phase separation [39], self-assembly [40], [41], and electrospinning
[42], [43].
The drawing process has the similarity with the dry spinning method in fiber- forming industries which are able to generate very long single nanofibers. However, the technique is very limited to viscoelastic materials that are capable to sustain strong
20 deformation. Moreover, template synthesis is a method by using a nanoporous membrane as a template in order to generate nanofibers with the features that can be used to fabricate various raw materials such as metals, carbon, and semiconductors. In opposition, the technique is unable to generate continuous nanofiber due to the size limitation of the template. Phase separation is another method of generating nanofibers consist of dissolution, gelation, extraction, and drying steps.
The process requires a long period of time in order to transfer a solid polymer into a nanoporous foam. The self-assembly process is a popular technique in biology application where the pre-existing components organize themselves into the desired pattern. This technique also requires a long period of time similar to the phase separation process. In conclusion, the electrospinning method is a suitable process due to the efficiency of the technique for generating polymer nanofibers.
Figure 5: Electrospinning setup
Electrospinning is a process of generating nanofibers by an electrostatic force that has been developed and patented by Formhals from 1934-1944 [44]–[47] that explain an experimental setup for generating nanofibers by using an electrostatic force from the
21 solution. The electrospinning is a straightforward process consists of three main components such as: (1) a high voltage power supply, (2) an electrically conducting spinneret, and (3) a collector separated at a defined distance that has been widely used in late 20th and built significant interest for both scientific communities and industries [48].
In addition, the electrospinning method has been emerged and used in many applications due to its advantages compared to other processing methods. Many applications such as filtration system, composite reinforcement, medical prosthesis, and tissue template. For instance, SCJ Wong [30] patented and introduced a novel method of forming a dry adhesive fabricating an electrospun non-woven of a scannable polymer that extends the electrospinning technique in adhesion field [49]–[59]. Furthermore, the electrospinning technique is well developed to reinforce the material. Engineering fibers such as Kevlar, carbon, and glass are the popular material that has been used in the composite field as a reinforcement [52].
2.2.1 Parameters
There are three main parameters in electrospinning technique that control the process such as (1) solution parameters, (2) processing parameters, (3) and environmental parameters [60].
2.2.1.1 Solution Parameters
The role of solution parameters in electrospinning process such as the solvent, polymer concentration/viscosity, molecular weight, surface tension and solution
22 conductivity [61] control directly to the morphology of the nanofibers, physical, and mechanical properties.
2.2.1.1.1 Solvent
The solvent is considered as an important parameter that is able to change the formation of the smoothness and beadles's nanofibers. In addition, there are two important keys to determine the solvent selection for electrospinning [62].
First, the ease-soluble solutions are preferred because a completely soluble solution is required for an electrospinning process. Second, the medium boiling point of the solution is preferred due to the volatility of a solvent. Volatile solvents are favored due to its high evaporation rates that will easily to get evaporated during their movement from needle tip to the collector. On the other hand, high volatile solvents should be avoided due to their low boiling points and high evaporation rates that will lead to a solidification of the solution at the needle tip. This phenomenon will block the needle tip that will cause a failure in the electrospinning process.
In addition, low volatile solvents that have high boiling rates will prevent the drying during the electrospinning process and will cause beaded nanofibers formations [63].
Table 5: Commonly used solvents
Solvent Boiling Dielectric Safety Concern
Point Constant
Water 100 78.4 None
23
Ethanol 78 24.5 Flammable; irritating to the eyes,
respiratory system, and skin
Acetone 56 20.6 Flammable; irritating to the eyes;
vapors may cause drowsiness or
dizziness
Tetrahydrofuran 66 7.58 Flammable; harmful if swallowed;
irritating to the eyes, skin, and
respiratory system
Ethyl acetate 78 6.02 Flammable; irritating to the eyes;
vapors may cause drowsiness or
dizziness
Cyclohexane 81 1.89 Flammable; may damage lungs if
swallowed; vapors may cause
drowsiness or dizziness
2.2.1.1.2 Concentration
The concentration of the solution is vital to the electrospinning that influences the formation and morphology of the nanofibers. Moreover, a power law relationship for electrospun polymer fibers was introduced where the average diameter of the nanofibers is related to the solution concentration [64].
24
Moreover, low concentration of the solution will lead to the electrospray instead of electrospinning process where the nano-droplets will be collected but nanofibers where the jet breaks into droplets. On the other hand, a high concentration of the solution will generate beads formation in the nanofibers [65].
Beachley et al [66] showed the significant increase in nanofiber diameter with increasing polymer concentration at a significant level in PCL nanofibers by comparing solution between 8% and 17%, 11% and 17%, and 11% and 2% PCL concentration.
Graph 1: Average fiber diameter of PEO nanofibers with concentrations from 4-7% [67]
In addition, Jacobs et al [67] compared the diameter of Polyethylene oxide (PEO) electrospun nanofiber (Mw= 9 x 105 g/mol) with a range of 4-7 % of solution concentration and shows the fiber diameter increase proportionally with the percentage of the solution concentration at the constant distance (15cm) and applied voltage (10kV).
25
2.2.1.1.3 Viscosity
The viscosity of the solution plays important roles in solution parameters where determines the morphology of the nanofibers. In addition, the viscosity and concentration of the solution work proportionally that viscosity can be improved by tuning the concentration of the solution [60].
Furthermore, since the diameter of nanofibers depends on the concentration and polymer chain in solution, Terada et al [68] were able to express the terms of Berry’s number (Be) that was determined as the dimensionless product of the concentration I and intrinsic viscosity (η) of the solution given as:
Be = C [η] (Equation 7)
Moreover, Zargham et al [62] were able to manipulate the morphology of Nylon 6 nanofibers by changing the solution viscosity and concentration will generate different diameter size of the nanofibers.
Similar effects from changing viscosity of the solution have been reported by Doshi et al [42] that developed a smooth Polyacrylonitrile (PAN) polymer by changing its viscosity where if the viscosity is lower than 800 centipoise, the solution was easy to dilute.
On the other hand, when the solution viscosity is higher than 4000 centipoise, the drying of the solution at the tip will occur and the fiber is difficult to form.
26
In conclusion, both works show that viscosity has a direct influence on the diameter and capability of generating nanofibers by electrospinning that shows the larger diameter of nanofibers can be obtained by increasing the viscosity of the solution [42], [64], [65].
2.2.1.1.4 Molecular Weight
The molecular weight of the solution has a direct influence on the length of the polymer chain which affects the viscosity of the solution. When the polymer with higher molecular weight is dissolved in a solvent, the viscosity of the polymer is higher compared to the same polymer with lower molecular weight due to the entanglement of the polymer chain. Furthermore, the stability of the jet is determined by the entanglement of the polymer chain which leads to bead formations in the nanofibers [67]. Koski et al [69] reported that the molecular weight of the polyvinyl alcohol (PVA) has a significant impact on bead formations in the nanofibers.
Figure 6: SEM graphs of polyvinyl alcohol (b)(PVA) for different molecular weights (Mw).
(a) 9,000-10,000 g/mol; (b) 13,000-23,000 g/mol; (c) 31,000-50,000 g/mol [69]
27 From the figure above, it shows the difference of nanofiber morphology where the bead formations of nanofiber were visible at 푀푤 = 9,000-10,000 g/mol and the smoother nanofibers were achieved at 푀푤 = 31,000-50,000 g/mol.
2.2.1.1.5 Surface Tension
The morphology of the nanofibers is influenced by surface tension that determines the extent of stretching of the solution under electrostatic forces.
Doshi et al [42] described that the surface tension of polymer solution is able to determine the electric field strength at the apex of the cone using equation (8).
4훾 퐸 = √ (Equation 8) 휀0푅 where E is representing the electric field strength, 훾 is surface tension of the polymer solution, 휀0 is the permittivity of the free space, and R is the radius of curvature of the rounded off cone apex.
Furthermore, Fong et al [65] explained the importance of balancing the surface tension and viscoelastic force. The competition between the electrical force and viscoelastic force can determine the morphology of the nanofibers that by decreasing the surface tension and increasing the viscosity of the solution, the electrospinning process will generate smooth nanofibers.
Moreover, the influence of surface tensions on the morphology has been conducted by Yang et al [70], [71] in 2004 using PVP solution 4 wt.% as a model with ethanol, DMF,
28 and MC as solvents. By reducing the surface tension of the solution, the bead formations in the nanofibers can be reduced and generate smooth nanofibers.
Figure 7: TEM Graph of PVP solution with fixed 4% concentration from (a)
ethanol; (b) DMF; (c) MC as solvents [71]
In addition, they also demonstrated by controlling the mass ratio of the solvent mix, both surface tension and viscosity of the solution can be adjusted [71].
Graph 2: Surface tension and viscosity as functions of the mass ratio of ethanol/DMF
[71]
29
2.2.1.1.6 Solution Conductivity
The conductivity of the solution is vital in electrospinning technique to determine the ability of the solution to be generated into nanofibers because the solution is being stretched due to the electrical charged on its surface [72]. Therefore, it is often that the polymers are prepared using solvents that contain higher conductivity or salt. Furthermore, higher solution conductivity is able to generate smooth and thin nanofibers.
Uyar et al [72] studied the influence of the solution conductivity using polystyrene fibers and dimethylformamide (DMF) as a solvent. In his research, he revealed that the uniformity of the nanofibers is very dependent on the solution conductivity. Similar research is conducted by Chronakis et al [73] by using polypyrene (Ppy) with PEO as a solvent. The Ppy with 2.5 wt%, 1.5 wt%, and 1.5 wt% + Triton-X100 as a carrier to show the decrease of the diameter of the nanofibers when the solution of conductivity was increased.
30 Graph 3: The diameter of nanofiber as a function of polypyrrole content. Solutions of Ppy with 2.5 wt% PEO (o), 1.5 wt% PEO (□), and 1.5 wt% PEO with 0.5% wt% Triton-X100
(◊) were used by Chronakis et al [73] as a carrier
2.2.1.2 Processing Parameters
Processing parameters such as the electrospinning voltage, feed rate of the polymer solution, collector distance and collection method of the electrospinning process are involved in the electrospinning process that has effects on the morphology of an individual electrospun and its properties.
2.2.1.2.1 Voltage
Voltage parameter is vital in the electrospinning process due to its several influences on the electrospinning result. In the electrospinning process, the power source is connected to generate an electric field between the tip of the needle and a grounded collector. The drop of solution in the tip of the needle is held due to the surface tension.
Moreover, as the voltage is increased, the electric forces will overcome the viscoelastic
31 forces and create a deformity where the jet will be generated from the apex of a conical surface, known as “Taylor” cone [74].
The required voltage to generate nanofibers are varied depends on the solvent, polymer conductivity and concentration. Lee et al [75] show that by applying a higher applied voltage, it is expected that there will be the higher electric force that generates a stretching deformation of the solution that will lead to thinner fibers.
Furthermore, the beads formations in nanofibers are influenced by the applied voltage. The flight duration of the electrospinning is reduced when the voltage is increased which leads to less time given for a solution to stretch to smooth fibers and develop beads in the nanofibers. Deitzel et al [50] confirm the concept in his work by controlling the applied voltage and keep the solution properties constant, the area density of bead defects is increased.
2.2.1.2.2 Taylor Cone and Jet Formation
Taylor Cone is a term of a structure where the formation was developed when the electric field reaches a critical value and the solution at the tip of the capillary tube elongates and form a structure then progressively initiate polymer jet [76].
In addition, experimental research that was conducted by Sir Geoffrey Ingram
Taylor [77], [78] in the 1960s shows the critical voltage can be calculated by deriving the condition for the critical electric that is needed to generate the droplet of liquid into a cone with the equation as
퐻2 2퐿 3 푉 2 = 4 (ln − ) (0.117 휋훾푅) (Equation 9) 푐 퐿2 푅 2
32
Where 푉푐 is the critical voltage, H is the distance between the capillary exit and the ground, L is the length of the capillary with R as radius, and 훾 is the surface tension of the solution.
Figure 8: Jet Formation: (A) Needle tip, (B) Solution droplet, (C) Taylor Cone, (D) Jet
formation, I Nanofibers, (F) Terminal state [60]
Moreover, Sir Geoffrey Ingram Taylor derived the formulation based on his research where the equilibrium between the surface tension and electrostatic force can be achieved when the angle of the cone which is known as Taylor angle is 49.3° [77]. On the other hand, the value of the Taylor angle may differ for a different solution. Yarin et al [79] demonstrated both numerically and experimentally to prove that there is another shape to represent a unique critical shape besides Taylor cone where he was able to obtain half angle of 33.5° rather than a Taylor cone of 49.3° with polyethylene oxide.
33 2.2.1.2.3 Flow Rate
The flow rate of the polymer solution plays a crucial role in the electrospinning process where by having high solution flow, the result will lead to a thick diameter of the nanofibers together with bead formations appearance in the nanofibers. On the other hand, low solution flow will lead to a thinner diameter of nanofibers [80]. Zargham et al
[62] proved the effect of flow rate influence the morphology and diameter distribution of the Nylon 6 nanofibers in the electrospinning process. At a low flow rate, the narrow fiber diameter distribution is achieved. On the other hand, at a high flow rate, an electrospray process is occurring due to insufficient solvent evaporation that leads to formation with defects.
Figure 9: The influence of flow rate with various flow rates; (a) 0.1 mL/hr, (b) 0.5 mL/hr,
(c) 1.0 mL/hr, and (d) 1.5 mL/hr [62]
34
2.2.1.2.4 Collector
The collector methods in the electrospinning have a significant impact on the mechanical properties, the morphology of the arrangement of the nanofibers, and the productivity of the electrospinning. Many collecting methods have been developed by researchers such as static collector, rotating drum, rotating disc, wired drum, independent electrodes, and pins [60].
The static collector is considered as a basic collector setup in the electrospinning process which consists of a syringe with electrified needle and a planar grounded collector.
Figure 10: Static collector; (A) syringe, (B) needle, (C), Taylor Cone, (D) nanofibers, (E)
ground collector (F) High voltage power supply (positive), (G) high voltage power
supply (negative) [60]
Moreover, Wong et al [52], [54], [56] have described the use of rotating disk and parallel electrodes [52]. Rotating disk and drum are considered a popular method to collect nanofibers in the electrospinning process due to the capability to control the deposition and
35
alignment of the nanofibers by generating a rotating movement during the electrospinning process.
Figure 11: Electrospinning collection methods; (a) rotating drum, (b) rotating disk, (c)
parallel electrodes [43]
In addition, different collecting method is proposed by Chase et al [81] to collect nanofibers using a wired rotating drum where the setup consists of plexiglass disks with a set of copper wires that are parallel to the axis of the rotation and have 1 cm distance between wires.
36
Figure 12: Wired rotating drum; (A) Syringe, (B), Nanofibers, (C) Wired rotating drum
[60]
Furthermore, a novel electrospinning setup was proposed by Sundaray et al [82] that used a sharp thin stainless steel as a collector. The stainless steel pin is mounted vertically below the syringe needle then wrapped it with a 1mm thick polyester to the cylinder frame to collect the nanofibers. The cylinder frame will be rotated by M2 (Motor
2) that works as a substrate. Moreover, M1 (Motor 1) will serve as a step motor in order to move M2 as a lead screw arrangement. By using a pin as a collector, the formation of the nanofibers was aligned and nearly parallel to each other [82].
37
Figure 13: Electrospinning technique using a stainless steel pin as a collector; (A)
Syringe needle, (C) A view of the pin; (M1) Motor 1, (M2) Motor 2 [82]
2.2.1.2.5 Collection Distance
Diameter and morphology of the nanofibers are influenced by the distance between the tip of the needle and the collector. Similar to viscosity and flow rate, the distance between the collector and the tip of the needle can be varied with the polymer system [13].
Moreover, the morphology of the nanofiber is affected by the distance due to the deposition time where the process of solidification of the nanofiber during the electrospinning will not complete when the distance between the tip of the needle and the collector is short and generate large-diameter of nanofibers. On the other hand, the electric field force can be reduced when the distance of the collector and the tip of the needle is long.
38
Therefore, the critical distance needs to be obtained in order to generate smooth and uniform electrospun fibers [83]. In addition, the generation of beads shows when the distance between the tip and collector are too small and too large. [84]–[88]
Furthermore, a research was conducted by Yuan et al [89] demonstrated that by increasing the distance between tip and collector, the diameter of the nanofibers is reduced.
(a) (b)
Figure 14: SEM picture of ultrafine PSF with 20% PSF/DMAC solution at 10kV, flow
rate 0.40 mL/h with distance; (a) 10cm and (b) 15cm [89]
2.2.1.3 Environmental Parameters
Environmental parameters such as humidity, temperature, and gas composition also affect the morphology of the nanofiber. Recent studies [85], [86] investigated the role of humidity in the electrospinning process as a function of the nanofiber diameter [87]. By increasing the ambient humidity, the diameter of polyethylene oxide (PEO) is decreasing and the formation of beads is occurring. In addition, Casper et al [88] investigated that the low ambient humidity may increase the evaporation rate of the solvent. On the other hand, the high ambient humidity during the electrospinning process will increase the nanofiber
39 diameter due to the charge on the jet is neutralized and the stretching force will become weak [60].
Graph 4: The distribution of pore diameter on 190,000 g/mol PS/THF electrospun fibers
at varying humidity ranges: (a) 31-38%, (b) 40-45%, (c) 50-59%, (d) 60-72% [88]
Furthermore, Mit-uppatham et al [89] conducted research to investigate the influence of the temperature in the electrospinning process by electrospinning a solution of a polyamide-6 (PA 6-32) in 85% v/v formic acid with 20% w/v concentration.
Moreover, by using the warm water to control the temperature of the solution, they were able to conduct an electrospinning process at 30, 40, 50, and 60°C. In addition, it was found that the temperature influences the diameter size of the nanofiber where the
40 nanofiber diameter was decreased approximately 4.32% in every incensement of 10°C with the smallest diameter of nanofiber was obtained at 60°C.
(a) (b)
Figure 15: SEM graph of PA-6-32 solutions with 20% w/v concentration in
85% v/v formic acid at temperature: (a) 30° C and (b) 60° C (magnification =
10,000X, scale bar = 1 μm)
2.2.2 Electrospinning Application
Electrospinning process is widely used due to its capability to generate nanofibers with a high surface area to volume ratio that can be applied in certain applications.
Furthermore, there are many materials that can be used for the electrospinning process such as polymers, metal, and ceramic that attract many researchers to use the technique. In addition, the cost of electrospinning setup only requires three main components such as high voltage power supply, collector, and spinneret which makes the electrospinning process cost is very low compared to other methods.
In the biomedical field, Wong et al [90] generated a filtration process by using electrospinning with the PEO/lecithin fiber onto the surface of [poly (ethylene glycol) methyl ether methacrylate] [P (PEGMEMA)]-modified SEBS to develop blood contacting
41
biomaterials with long-term antihemolysis capability. Furthermore, the electrospinning technique can be applied in tissue engineering application where the material has to be biocompatible. The electrospinning technique is favorable to use due to its ability to generate an aligned electrospun fibrous mat that has a specific and controlled pore size that will allow the cell to migrate and infiltrate an electrospun scaffold [91].
Moreover, Wong et al [59] have patented an electrospinning process as a method to form a non-woven dry adhesive whereby applying the concept of gecko’s feet where the gecko’s locomotion on slippery surfaces against gravity as well as firmly attach onto and detach with ease from rough substrates.
In a filtration field, Sundarrajan et al [92] gathered many studies regarding the application of electrospinning process by generating a nanofiber membrane to increase the filtration efficiency compared to the conventional filters to remove VOC’s nanoparticles and bacterial contaminations and extend the protection duration of the filter. In addition,
Gopal et al [93] generated polyvinylidene fluoride nanofibers by electrospinning method into membranes and studied the structural characterization that has similar properties to a conventional microfiltration membrane where the result shows the electrospun membranes were successful in rejecting more than 90% of micro-particles.
Electrospinning process also can be applied in nanocomposite as a reinforcement.
Carbon nanotubes (CNT) is widely used in nanocomposite due to its unique mechanical properties and there is a challenge to align CNT to the material [13] that affect the properties of the nanocomposite itself. Therefore, several researchers [94], [95] developed
42
a method by using the electrospinning process to incorporate CNT into polymer nanofibers to align CNT along the fiber direction.
2.3 Silicone Rubber
Silicone rubber is a type of rubber that is made from silicone elastomers has special features such as “organosiloxanes polymer” that developed uniquely from its molecular structure that carries both inorganic and organic properties which makes silicone rubber unique compared to organic rubbers [96].
Figure 16: Chemical Structure of polydimethylsiloxane (PDMS)
Polydimethylsiloxane (PDMS) is a type of silicone-based organic polymer that is commonly used in various applications due to its viscoelastic properties. For housing applications, silicone rubber can be used for high voltage outdoor insulation due to the unique characteristic of hydrophobicity that provides a high surface resistance against moisture and contamination [97]. In sensor applications, silicone rubber is favorable to be used for micro-machined mechanical and chemical sensors where the silicone rubber can be applied in an ion-sensitive field-effect transistor (ISFET) as the ion selective membrane and in accelerometers as its spring material [98].
43
Room Temperature Vulcanizing silicone rubber or is known as RTV silicone rubber is liquid silicone rubber that is made by combining two different components (Liquid
Silicone and curing agent) and cures at room temperature to form a flexible rubber.
Moreover, RTV silicone rubber is widely used in art-related that RTV silicone rubber can be used to cast materials and 3D Print [99]. For industrial applications, RTV silicone rubber is commonly used in aerospace [100], microelectronics due to its unique mechanical properties such as high-temperature resistance and aging resistance.
2.4 Plastics
Nowadays, plastics become an integral part of everyone’s lifestyle in several applications from common end product for the customer to sophisticated scientific and medical instrument. Due to that reason, designers and engineers start to focus on plastics because they offer combinations of properties that may not be available in other materials.
Plastics offer advantages such as resistance to corrosion, resilience, lightness, ease of processing, etc. that make them favorable to be used in various applications. In addition, plastics are considered as part of the larger family called polymers which are different from metals in terms that their structure consists of very long chain-like molecules. Silk, bitumen, rubber, shellac, and cellulose are considered as natural polymer due to their type of structure. In the 19th century, the first synthetic polymer was successfully developed based on cellulose that is called Parkesine based on the name of the inventor, Alexander
Parkes [101].
44
2.4.1 Polymeric Materials
Monomers are a description of synthetic large molecules that are generated by joining together thousands of small molecular units and the joining process is called polymerization. There is the difference between polymers and plastics which the polymer is the pure material that developed from the process of polymerization and pure polymer are used when additives are present that the term plastic is applied. Antistatic agents, coupling agents, fillers, flame retardants, lubricants, pigments, plasticizers, reinforcement, and stabilizers are the reason why polymer contain additives and needed to be used in plastics.
The antistatic agent is required because most of the polymer are poor conductors of current and build up a charge of static electricity. Therefore, Antistatic agents are needed in order to attract moisture from the air to the plastic surface. This improvement can improve the surface conductivity and reducing the possibility of a spark or discharge in plastics.
Furthermore, couple agents are required to improve the bonding of the plastic to inorganic filler materials such as glass fibers. Some fillers such as flakes, or short fibers of inorganic material also have capabilities to improve the mechanical properties of plastic.
Most organic material polymer-based are flammable, additives can be used as flame retardants which contain chlorine, bromine, phosphorous or metallic salts that are able to reduce the possibility of occurrence of combustion.
45 Moreover, additives in plastics also can be used as lubricants such as wax or calcium stearate to reduce the viscosity of the molten plastic and improving the forming characteristic of the material as well as pigments that are used to produce colors in plastic.
Reinforcement is considered as the reason why main additives are used in plastics with the ability to improve the strength and stiffness of the polymers by adding glass fibers or carbon.
In addition, preventing deterioration of the polymer due to environmental factors are the reason additives are used as stabilizers.
2.4.1.1 Thermoplastic Material
Plastic is commonly divided by two important classes; thermoplastic materials and thermosetting plastics. Thermoplastic material has long chain-like molecules that are held together by Van der Wall’s forces that are relatively weak. In addition, intermolecular forces are weakened when the material is heated and becomes soft, flexible and eventually melt. However, the material can be reverted back to the solid state when they are allowed to cool. This cycle of softening by heating and solidifying on cooling become major advantages of in thermoplastic material, especially in a common processing method.
Table 6: Examples of thermoplastics
Examples Thermoplastics Polyethylene
46
Polyvinyl Chloride
Polystyrene
Nylon
Cellulose Acetate
Acetal
Polycarbonate
Polymethyl
Methacrylate
Polypropylene
Furthermore, thermoplastics polymer is divided into two groups in terms of their structure; a crystalline (ordered) or an amorphous (random). Some plastics such as acrylic and polystyrene are considered amorphous, whereas polyethylene and nylon have a high degree of crystallinity but are accurately described as partially crystalline or semi- crystalline because, in practice, it is impossible for molded plastic to have a completely crystalline structure because of the complex physical nature in the molecular chains.
Table 7: Comparison between amorphous vs crystalline structure [101]
Amorphous Crystalline
47
Broad softening range Sharp melting point
Usually transparent Usually opaque
Low shrinkage High shrinkage
Low chemical resistance High chemical resistance
Poor fatigue and wear Good fatigue and wear
Table 8: Examples of amorphous and crystalline thermoplastics [101]
Amorphous Crystalline
Polyvinyl Chloride (PVC) Polyethylene (PE)
Polystyrene (PS) Polypropylene (PP)
Polycarbonate (PC) Polyamide (PA)
Acrylic (PMMA) Acetal (POM)
Acrylonitrile-butadiene-styrene (ABS) Polyester (PETP, PBTP)
Polyphenylene (PPO) Fluorocarbons (PTFE, PFA, FEP, and
ETFE)
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2.4.1.2 Thermosetting Plastics
Thermosetting plastic is fabricated by a chemical reaction that contains two stages.
The first stage of the process results in a formation of long chain-like molecules similar to thermoplastic material and capable of further reaction. Moreover, the second stage of the process will occur during molding commonly performed in high temperature and pressure.
In this stage, the long molecular chains have been interlinked by strong bonds in order to constrain the material to be softened again by the application of heat. By applying high heat to the material, they will char and degrade. Due to strong chemical bonds, thermosetting materials characteristically have rigid properties and the mechanical properties of the material are not heat sensitive which commonly found in phenol formaldehyde, melamine formaldehyde, urea formaldehyde, epoxies, and couple polyesters.
2.4.2 Plastic Available to the Designer
Due to a wide spectrum of variation in plastics properties, a thoughtful consideration has to be applied in order to achieve a suitable design for the applications that are important for researchers to have awareness of properties change of the plastics.
Therefore, good design always involves a considered selection of material from the range of the material available.
2.4.2.1 Engineering Plastics
Nowadays, many thermoplastics are accepted as engineering materials because of the ability to support loads more or less indefinitely based on thermoplastic properties such
49
as low density, resistance to several chemical reactions, and easy processability. Due to those properties, plastics are favorable to be applied in engineering applications to give the balance of properties.
Currently, engineering plastics group such as nylon, acetal, polycarbonate, modified polyphenylene oxide (PPO), thermoplastic polyesters, polysulphone, and polyphenylene sulfide are common to be used in life applications and become commercial.
Moreover, in recent years, the new generation of high-performance plastics are developed and commercialized to fulfill the extreme requirements in life applications. The superior properties of these plastics such as high-temperature performance attract many automotive and aerospace industries to start using these materials.
Table 9: Main classes of new plastic material [101]
Polyarylethersulphones (PES)
Polyphenylene Sulphide (PPS) Polyarylethers and Polyethernitrile (PEN) Polyarylthioethers
Polyetherketones (PEK and
PEEK)
Polyetherimide (PEI) Polyimides and
Polybenzimidazole Thermoplastic Polyimide (PI)
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Polyamide-imide (PAI)
Fluorinated Ethylene Propylene
(FEP) Fluoropolymers
Perfluoroalkoxy (PFA)
2.4.2.2 Thermosets
In past years, thermosetting material has declined due to the slow production methods and the arrival of high-temperature engineering plastics that causes thermosetting material became not favorable. However, the improvement of thermosetting materials in color and ease-flow molding materials with a superior range of properties improve the demand for this material nowadays.
By using phenolic molding materials with developed easy-flowing granular thermosetting materials that based on urea, melamine, unsaturated polyester (UP) and epoxide resins, many technical applications can be supported by thermosetting material with superior material properties such as non-melting, high thermal and chemical resistance, surface hardness, dimensional stability, stiffness, and low flammability.
2.4.2.3 Composite
The attractiveness of plastic for engineering applications also supported by a factor of possibility for plastics to enhance the properties of another material through fiber reinforcement. Due to of this factor, the demand of the plastic for aerospace and automobile
51 industries is increasing significantly where currently in the USA, these industries utilize over 100,000 tonnes of reinforced plastics out of total consumption of over million tonnes.
Both thermoplastic and thermoset can be used as a reinforcement even they are aiming for different market sectors due to their fundamental differences in nature in terms of properties and processing characteristics.
Thermosetting systems that inhibited by the brittleness of crosslinked matrix have turned to the use of long continuous fiber reinforcement and on the other hand, it is able to use the low viscosity state to impregnate and promote the maximum utilization of fiber properties [101].
On the other hand, thermoplastics material has an advantage such as toughness with limitation in the fiber length that has concentrated on the short fiber. Nowadays, these two different types of plastics are focusing on the route to move to each other’s territory. For example, thermoset that is commonly to be applied as long fiber is trying to reduce the fiber length in order to achieve high productivity of injection molding, while thermoplastic try to advance its properties by extending the fiber length which can be found in advanced polymer composite (APC) developed by ICI and the stampable glass mat reinforced thermoplastics (GMT) developed in the US.
2.4.2.4 Structural Foam
A new concept that excites for engineers is structural foam. Many plastics can be formed by using a blowing agent in order to provide a cellular rigid foam core with a solid
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tough skin when the material structure was molded. In addition, this type of structure offers excellent strength to weight ratio and is very efficient in material terms.
Most of the applications of this structure are used in housings for business equipment and domestic appliances. Polycarbonate, polypropylene, and modified PPO are popular materials for structural foam molding and applicable in applications that require component parts to be absolute minimum including vehicle body panels and furniture.
2.4.2.5 Elastomers
Conventional rubber is also considered as a member of polymer family due to the long chain-like molecules that are consisted in their structure which are coiled and twisted in random orientation and provides flexibility to allow the material to accept large deformations. In order to prevent this phenomenon, a curing (vulcanization) process is required to anchor the molecules together. Vulcanized rubbers contain a range of desirable properties such as resistance to oils, greases, and ozone, resilience, and flexible at low temperature. However, careful processing and a huge amount of energy are required to facilitate molding and vulcanization that are considered as disadvantages. In addition, those disadvantages push researchers to develop thermoplastic rubbers (elastomers) that exhibits the desirable physical characteristics of rubber but with an ease-processing capability.
2.4.2.6 Polymer Alloys
Polymer alloys such as ABS, an alloy of acrylonitrile, butadiene, and styrene are considered as a new breakthrough that caused a lot of excitement in recent years. Several factors such as the demand for sophisticated techniques for combining plastics and market
53
competition in the new market area such as automobile bumpers, body panels, etc. pushes the development of this new material. In designing an alloy, several candidates of resins have been selected based on the properties, cost, and/or processing characteristics required at the end of the product and continue with the testing process.
Several polymers such as ABS, polycarbonate, polyurethanes have been selected as standard building blocks of the polyblends to improve the impact strength as well as polyphenylene oxide, polysulphone, PVC, polyester (PET and PBT) and acrylic to improve the heat resistance. In addition, barrier properties can be improved by using plastics such as ethylene vinyl alcohol (EVA).
Table 10: Typical plastic alloys [101]
Alloy Features
PVC/acrylic Tough with good flame and chemical
resistance
PVC/ABS Easily processed with good impact and
flame resistance
Polycarbonate/ABS Hard with high heat distortion
temperature and good notch impact
strength
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ABS/Polysulphone Less expensive than unmodified
polysulphone
Polyphenylene oxide/HIPS Improved processability, reduced cost
SAN/olefin Good weatherability
Nylon/elastomer Improved notched impact strength
Modified amorphous nylon Easily processed with excellent surface
finish and toughness
Polycarbonate/PBT Tough engineering plastic
2.4.3 Semi-crystalline plastics
Semi-crystalline plastics such as Low-density polyethylene (LDPE), Linear Low-
Density Polyethylene (LLDPE), High-Density Polyethylene (HDPE), etc. are commonly have higher order molecular structure and do not soften as the temperature rises.
2.4.3.1 Low-Density Polyethylene (LDPE)
LDPE is widely used plastic and characterized by density in the range of 918-935 kg/m3. This plastic is considered as very tough plastic as well as flexible. Moreover, the material is common to be applied in packaging film as well as an electrical insulator due to its outstanding dielectric properties. Other applications such as domestic ware, tubing, squeeze bottles, and cold-water tanks also use LDPE in order to achieve both toughness and flexibility required.
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2.4.3.2 Linear Low-Density Polyethylene (LLDPE)
LLDPE is considered as a new type of polyethylene that was introduced in 1977.
This material is commonly produced by a low-pressure process and contains a regular structure with short chain branches. Moreover, based on the cooling rate from the melt, the material develops a structure in which the molecules are linked together. Therefore,
LLDPE is stiffer, more ductile and exhibits higher yield strength than LDPE.
2.4.3.3 High-Density Polyethylene (HDPE)
High-density polyethylene (HDPE) has a density in the range of 935 – 965 kg/m3 and is considered more crystalline than LDPE. In addition, this material is more expensive compared to LDPE, though it has higher strength and stiffness than LDPE. Metallocene- based polyethylenes are one the example of HDPE that has been recognized since the 1950s as a suitable catalyst for the manufacturer of polyethylenes.
Moreover, the advantage of having one single site catalysts provides a similarity result in molecular size during production that leads to an array of superior properties.
2.4.3.4 Cross-linked Polyethylene (XLPE)
There are several developments to alter the structure of the material. Several thermoplastic materials such as polyethylene can have their structure modified in order to develop a cross-link in molecular chains that leads the material to behave similarly to a thermoset. Several methods such as radiation, silanes, and peroxides are commonly used to perform the cross-linking process.
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The material that is cross-linked will achieve several benefits effects such as improved stress crack resistance, better chemical resistance, improved creep resistance, improved toughness, and better general thermos-mechanical stability.
2.4.3.5 Polypropylene (PP)
Polypropylene is considered as a versatile plastic that can be found in many grades and as a copolymer. Moreover, Polypropylene has the lowest density compared to other thermoplastics with 900kg/m3. Besides that, high strength, stiffness, excellent fatigue, and chemical resistance make polypropylene suitable in many applications including crates, small machine parts, car components, and cabinets for TV, chair shells, tools, etc. By having excellent fatigue resistance, polypropylene also can be applied in the molding of integral hinges.
2.4.3.6 Polyamides (nylon)
There are several different types of nylon such as nylon 6, nylon 66, and nylon 11.
That several nylons received a reputation as engineering plastics due to their characteristics such as strength, stiffness, and toughness. In addition, nylon that considered a notorious strong material in fibers is popular to be applied in small gears, bearings, bushes, sprockets, terminal blocks, slide rollers, and housings for power tools.
However, nylon has a tendency to absorb moisture which will affect its properties and dimensional stability that has to be considered by designer and researchers. By combining with glass reinforcement, the properties of nylon can be enhanced to produce an extremely strong, impact resistant material.
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2.4.3.7 Acetals
Acetals also considered as an engineering plastic due to its superior properties such as strength, stiffness, and toughness. Acetals have higher density compared to nylon and can be used for the same types of light engineering applications. With its notorious properties, acetals are favorable to be applied in water plumbing applications and as the body for electric kettles.
2.4.3.8 Polytetrafluoroethylene (PTFE)
Polytetrafluoroethylene is famously known for its excellence in chemical resistance and extremely low in the coefficient of friction that makes it favorable to be applied in bearings application, particularly extreme conditions as well as insulating tapes, gaskets, diaphragms, pumps and famously being used as a coating on nonstick cooking utensils.
2.4.3.9 Thermoplastic Polyesters
Thermoplastic polyesters are highly crystalline plastic that has high strength, toughness, abrasion resistance, low friction, chemical resistance, and low moisture absorption. Polyethylene terephthalate (PET) is one example that has been available and commercialized for many years in the shape of fibers.
The material was less attractive as a molded material due to the complicated processing and eventually solved by introducing polybutylene terephthalate (PBT). In addition, gears, bearings, impellers, housings, switch parts, pulleys, extensions are several
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applications that are supported by thermoplastic polyesters material as well as a replacement for glass in beverage bottles.
2.4.3.10 Polyetheretherketone
Polyetheretherketone known as PEEK is a new generation of plastic that offers a superior high service temperature, high resistance to attack from alkalis, acids, and organic solvents due to nature crystalline that is contained in this material. Furthermore, PEEK is easily processed and able to be used in 200-degree C continuously and offers high fatigue resistance as well as abrasion resistance, low flammability, and toughness. Due to notorious properties in PEEK, the plastics are commonly applied in several applications such as wire coatings, electrical connections, fibers, impellers, fans, etc.
2.4.4 Amorphous Plastics
Amorphous plastics are considered as the opposite side of the semi-crystalline plastic which rather than being rigid, their molecular structure is randomly oriented and move across to each other when the polymer is pushed or pulled which also provides the flexibility and elasticity of the material.
2.4.4.1 Polyvinyl Chloride (PVC)
Amorphous plastics that are widely used in life applications is polyvinyl chloride.
Moreover, polyvinyl chloride is available in two forms such as plasticized and unplasticized. In addition, both types offer excellent electrical insulation properties, good weathering resistance, good surface properties, and has a capability to self-extinguish.
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For plasticized Polyvinyl chloride, the material tends to be more flexible and is applied in several applications such as wire covering, floor tiles, toy balls, rainwear, and gloves. On the other hand, unplasticized PVC( uPVC) is considered as a tough, hard, and strong material that is widely suitable in building industry such as pipes, gutters, window frames, and wall claddings as well as in finance industry as a credit card material.
2.4.4.2 Polymethyl Methacrylate (PMMA)
Polymethyl methacrylate is famously known because of the exceptional optical clarity and resistance to outdoor exposure such as alkali, detergent, and oils. Due to the exceptional optical clarity properties, it is useful to illuminate notices, control panels, dome-lights, baths, lighting diffusers, lenses, nameplates, and display models.
2.4.4.3 Polystyrene (PS)
Polystyrene is available in a wide range of grades from brittle to very tough in impact strength. In addition, due to the low cost and ease-fabrication, PS is commonly used as model aircraft kits, vending cups, light fittings, yogurt containers, relays, coils, casings for ballpoint pens, and disposable syringes as well as ceiling tiles due to its excellence as packaging material and thermal insulator.
2.4.4.4 Acrylonitrile-butadiene-styrene (ABS)
Acrylonitrile-butadiene-styrene or known as ABS is considered as engineering plastic due to its superior in strength stiffness, and toughness properties to many plastics.
Moreover, ABS is comparable with nylon and acetal in several applications and less expensive as its strong point compared to nylon and acetal. ABS is widely known in the
60 3D printing field as their common material to be used as well as housings for TV sets, telephones, hairbrush handles, luggage, helmets, and lining for refrigerators.
However, the weak point of ABS can be spotted that the material is vulnerable to chemical attack by chlorinated solvents, esters, ketones, acids, and alkalis.
2.4.4.5 Polycarbonates
Polycarbonates are included as engineering plastics due to its extreme toughness properties. Besides that, polycarbonates have transparent and good temperature resistance which suitable to be applied in several applications such as vandal-proof street lamp covers, baby feeding bottles, machine housings, and guards, camera parts, electrical components, compact discs, and safety equipment. On the other hand, polycarbonates have a weakness that it is susceptible to alkaline solutions and hydrocarbon solvents.
2.4.4.6 Polyethersulphone
Polyethersulphone is considered as a new material that can sustain high- temperature service that is commonly used for load-bearing applications up to 180-degree
C. In addition, polyethersulphone also offers low flammability properties even without flame retardants. Moreover, the material itself can be easily processed on conventional molding equipment and favorable to be applied in several applications including aircraft heating ducts, terminal blocks, engine manifolds, bearings, grilles, non-stick coatings, and grilles.
61 2.4.4.7 Modified Polyphenylene Oxide (PPO)
The modified polyphenylene oxides offer a range of properties that make it attractive for a whole range of applications. The word modified in this material is a term to describe the inclusion of high impact polystyrene to improve the processability of the material and reduce the cost of the basic polyphenylene oxide (PPO). With the outstanding properties, the material can be used in 100-150-degree C where it is rigid, strong and tough with a good hydrolytic stability and creep resistance in several applications such as business machine parts, headlight parts, flow values, engine manifolds, grilles, pimp casings, fascia panels, hair dryer housings, etc.
2.4.5 Thermoplastic Rubbers
Thermoplastic rubbers are known to have five different types available that are based on (i) Olefinics (e.g. Alcryn, Santoprene) (ii) Polyurethanes (e.g. Elastollan, caprolan, Pellethane) (iii) Polyesters (e.g. Hytrel, Arnitel) (iv) Styrenics (e.g. Solprene,
Cariflex) and (v) Polyamides (e.g. Pebax) [101].
Table 11: Physical characteristics of thermoplastic rubbers [101]
Type Olefinic Polyurethane Polyester Styrenic Polyamide
Hardness
(Shore A- 60A to 40D to
D) 60D 60A to 60D 72D 30A to 45D 40D to 63D
62 Resilience
(%) 30 to 40 40 to 50 43 to 62 60 to 70 -
Tensile
Strength
(MN/m^2) 8 to 20 30 to 55 21 to 45 25 to 45 -
Resistance
Poor/Excellen
Chemicals Fair Poor/Good Excellent Excellent t
Oils Fair Excellent Excellent Fair -
Poor/Excellen
Solvents Poor/Fair Fair Good Poor t
Poor/Excellen
Weathering Excellent Good Excellent t Excellent
Specific
Gravity 0.97-1.34 1.11-1.21 1.17-1.25 0.93-1.0 1.0-1.12
Service temperatur -50 to 130 -40 to 130 -65 to 130 -30 to 120 -65 to 130
63 e (Degree
C)
2.4.6 Thermosetting Plastics
Thermosetting plastics offer high-end performance combination of chemical resistance, thermal stability, and structural integrity. They are also common to be applied in a wide range of industries such as an automotive, appliance, electrical, energy, and lightning markets due to excellent thermal and chemical stability together with high performance in strength, moldability, and hardness.
Unlike thermoplastics that has the capability to melt and disfigure when the heat is applied, thermosetting plastics become set in their chemical and physical properties after initial heat treatment and cannot be affected by additional heat exposure.
The advantages for a manufacturer such as various choice of color, outstanding dielectric strength, high-strength-to-weight ratio and performance, low thermal conductivity, and resistance to corrosion have gained in popularity among manufacturers.
On the other hand, several disadvantages of thermosetting plastics such as cannot be recycled, difficulty in surface finish, poor thermal conductivity for housing replacement and the rigidity of the material that can result in failure becomes a consideration for designers and engineers to select thermosetting plastics as the material for their specific applications.
64 2.4.6.1 Aminos
Aminos can be divided into two types – urea formaldehyde and melamine formaldehyde. Both types are considered as hard, rigid materials with good abrasion resistance and can perform continuously at moderate temperature (up to 100-degree C). In addition, Urea formaldehyde is considered inexpensive but has poor dimensional stability due to its ability to absorb moisture. Moreover, the material usually can be found in bottle caps, electrical switches, plugs, trays, and utensil handles. On the other hand, melamine formaldehyde has lower water absorption, chemical resistance and can sustain at a higher temperature compared to urea formaldehyde that usually can be found in tableware, laminated worktops, and electrical fittings.
2.4.6.2 Phenolics
Phenol-formaldehyde (Bakelite) is considered as one of the oldest synthetic materials available. Properties such as strength, hard, brittle material with good creep resistance and excellent electrical properties have gained its popularity among several applications such as domestic electrical fittings, saucepan handles, fan blades, pump parts, and smoothing iron handles. However, the material is only available in dark colors and vulnerable against alkalis and oxidizing agents that can be a consideration for engineers and designers in their material selections
2.4.6.3 Polyurethanes
Polyurethanes consist of three forms such as rigid foam, flexible foam, and elastomers. In addition, they are characterized based on their superior strength and
65 chemical and abrasion resistance. Furthermore, the rigid foam of polyurethanes is commonly applied for insulation material, the flexible foam is widely used for furniture as a cushion material, and the elastomeric material is specifically applied in the tire and automotive industry as shock absorbers.
2.4.6.4 Polyesters
In its application, polyesters are well known as a matrix for glass fiber reinforcement that can takes many forms and widely known as a DIY type material that is used in small boat manufacturer, tanks and repair kits for cars, chemical containers and extensively in clothing industries.
2.4.6.5 Epoxides
Epoxy resin is considered as an expensive material compared to other thermosets such as polyesters due to the higher performance of the material that outperforms other thermosets candidates in toughness, the ability to less shrinkage during the curing process, superior weatherability, and low moisture absorbance. Due to those properties, epoxides are commonly used in the aircraft industry to offer a combination of properties when they are reinforced with fibers
2.4.7 Selection of Plastics
Based on several types of plastics described above, a material selection is a key for engineers and researchers to select the suitable material for their requirements. In the design process, it is important to define clearly the purpose and function of the product that
66
is proposed as well as identify the service environment and followed by assessing the suitability of the spectrum of candidate materials.
Moreover, there are several characteristics that have to be considered for most engineering components such as mechanical properties of the material including strength, stiffness, specific strength and stiffness, toughness and fatigue, as well as the influence of high or low temperatures where the material is located. In addition, corrosion susceptibility and degradation of the material has to be studied when the material is exposed to any chemical or oxidation as well as wear resistance and frictional properties, especially in bearing applications. Furthermore, special properties such as thermal, optical, electrical and magnetic properties, damping capacity, etc. have to be calculated in order to confirm the suitability of the material as well as the method of fabrication and the cost attributable to the selected material and manufacturing route for manufacturers to make sure that the fabrication processes are suitable for the material to avoid any failures when it reaches to the customers and highly efficient to fabricate the material with limited time and production target.
2.5 Existing Reinforced RTV Rubber Method
Due to the demand for material improvement, many researchers tried to reinforce
RTV silicone rubber to increase its properties to achieve certain requirements for different applications purposes. The most common type reinforcement is fillers that are added to the polymer to improve specific properties and to reduce the cost of the material. The challenge of using fillers is when the filler content is too high, the hydrophobic properties of the material will decrease. Moreover, the reinforcement parameters such as filler concentration
67 and morphology also become factors that must be considered to improve the properties of the composite.
2.5.1 Polymer Solution Intercalation
Intercalation can be defined as a method of insertion of a molecule into materials with a layered structure. In addition, polymer solution intercalation is mainly to be used to synthesize nanophase organic-inorganic hybrids. The polymer solution intercalation is using a solvent system in which the polymer is soluble, and the silicate layers are swellable
[102]. The silicate layers will be immersed in a solvent and the polymer will be mixed together. After the mixing process, the solvent will be removed commonly by placing the mixture in a vacuum oven at a suitable temperature. Shen et al [103] use solution intercalation to generate a polymer-clay nanocomposite of poly(ethylene oxide)/Na- montmorillonite (PEO/MMT) by using water, toluene, and chloroform and placed it in a vacuum oven to remove the solvent from the composite.
2.5.2 Melt Intercalation
Melt intercalation is another technique for synthesizing thermoplastic polymer nanocomposites that were developed and became popular in the 1990s [104]. In addition, the method involves heat treatment process of the polymer matrix at high temperature then the filler is added. After that, the mixing process is needed in order to achieve uniform distribution of fillers in the matrix.
Furthermore, this technique is favorable due to its environmentally friendly compared to solution intercalation since melt intercalation method does not use solvent.
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The method itself is applicable in industrial processes such as injection molding and extrusion. On the other hand, high temperatures that are used in the process can damage the surface modification of the filler which makes optimization of the processing conditions is very important to control in order to achieve good dispersion and exfoliation.
Moreover, Wang et al [105] studied the properties of silicone rubber/organ montmorillonite hybrid nanocomposite by using melt intercalation process in order to synthesize the material. Another researcher, Shen et al [103] proposed a study of comparison of the solution and melt intercalation of polymer-clay nanocomposites
PEO/MMT that is synthesized by using melt intercalation process that was accomplished by annealing the pressed pellets in an electrical resistance furnace at 85 to 95° C for 8 hours.
2.5.3 Surface Modification
To improve the reinforcement distribution in the matrix, several techniques other than mixing has been developed [106] such as surface modification on the nanoparticles/fillers by a physical and chemical technique using surfactants [107]. Kim et al [108] studied the mechanical properties of a silicone rubber/carbon fiber composite that was synthesized by using surface modification of the carbon fiber. In the beginning, the carbon fiber was chopped into staples of 6 mm in length and burnt in a furnace with air flow for 2 hours at 400°C in order to remove the coating chemicals from the carbon fiber.
Moreover, the carbon fiber was added to 90mL of 60% HNO3 and the solution was subjected to ultrasonication process for 30 min and stirred for 12 hours. In addition, the
69 nitric acid treatment of CF will be mixed together with the PDMS in the compounding process together shows the interaction between CF and RTV was greatly enhanced.
2.5.4 Ultrasonication
Ultrasonication is considered as new technology to disperse nanoparticles into a matrix phase. The concept behind this method is that when the ultrasonic wave passes through a liquid medium, a large number of microbubbles will form, grow, and collapse in a few microseconds [109]. During the ultrasonication process, the cycle of pressure (low and high pressure) form thousands of microscopic vacuum bubbles in the solution and when the bubbles collapse into the solution is commonly known as cavitation. This phenomenon causes powerful waves of vibration that release huge energy force in the cavitation field that will disarrange the molecular interactions between the particles and the solvent which will facilitate the mixing process.
Moreover, Frogley et al [110] studied the properties of single-wall carbon nanotubes (SWNTs) as a filler in RTV silicone rubber by ultrasonication method. In the preparation process, RTV silicone rubber was prepared by dissolving it in toluene (1mg/ml) to reduce the viscosity and separately dispersing the fillers in toluene by ultrasonication.
Moreover, the two dispersions were mixed and another ultrasonication process is taken for
10 minutes. Furthermore, the toluene was removed by evaporation at 50°C over several days with the continuous stirring process.
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2.6 Rolling Resistance
Rolling resistance can be defined as the energy that is consumed by a tire per unit of distance covered [111]. In addition, rolling resistance is considered important especially in tire industry where the rolling resistance of the tire has a close correlation with the fuel consumption of a vehicle where there are very deep researches in order to discover the solution to improve fuel consumption knowing that rolling resistance amounts to almost 6-
10% of the fuel consumption [112].
In the tire industry, it is common for the research and development department to have a rolling resistance measurement device. The tire is mounted on a free rolling spindle and loaded against a test drum which usually has a larger diameter compared to the tire and commonly powered by an electric motor.
Figure 17: Schematic 2D drawing of the concept of rolling resistance measurement
device setup
71 Moreover, the National Highway Traffic Safety Administration (NHTSA) explained two different methods for measuring the rolling resistance such as by force method and torque method.
Figure 18: Force method to calculate rolling resistance (NHTSA,2009) [112]
By following ISO 28580, rolling resistance can be calculated by force method as
휏 퐹 = 퐹 [1 = ( 퐿)] − 퐹 (Equation 10) 푟 푡 푅 푝푙
where Ft is spindle force, 휏퐿 is the ratio of loaded tire radius, R is the test wheel radius and
Fpl as the skim load.
On the other hand, the torque method performs a torque cell that measures the torque required for the drum to rotate.
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Figure 19: Torque method to calculate the rolling resistance of a tire [112]
As shown in the figure above, the drum is connected to a torque cell that is powered by an electric motor that is very popular for many tire companies to use and require very huge space and hence they are not suitable for laboratory set up at university lab scale.
Figure 20: Schematic drawing of lab scale rolling resistance measurement test [112]
The use of hydrostatic bearing and a belt which is pressed against the tire has been patented [113]. The difference in pressure is used to determine the rolling resistance of the
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tire. This method is commonly used for direct application to the vehicles running on the highway.
Furthermore, Gent [1] developed a simplified technique to measure the rolling resistance of the surface material by designing a heavy steel roller that is kept in contact with the material under consideration. The heavy steel roller is connected through an arm to a small weight which will create pendulum whenever the heavy roller rolling back and forth above the specimen. Moreover, the heavy roller is set in motion with the help of a control arm which is used to set the initial position. In addition, an acoustic emission device is used to measure both the frequency and damping of the roller. This device will be connected to a computer where it records the sound waves reflected from the pendulum that is attached to the roller. Based on this data acquisition, the damping of the signal can be calculated to determine rolling resistance as
Figure 21. Damped oscillations of a pendulum
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푌(푡 ) log( 푖 ) 푌(푡푗) 휉 = (Equation 11) ∆푡∗ 휔0
where 휉 is the normalized damping coefficient of the system, 푌(푡푖) is displacement envelope of the pendulum oscillations, 휔0 is the natural frequency developed by transforming the oscillations in frequthe ency domain, and ∆푡 is the time change between time 푡푖 and 푡푗.
2.7 Hardness
Hardness is defined as the resistance of a material to deformation, penetration, or indentation and commonly to be measured by hardness tests such as Brinell, Knoop,
Rockwell, or Vickers. Furthermore, hardness testing is essential to analyze, develop, and improve the materials and technology in the scope of basic research such as material science, materials engineering, and materials diagnostics. The characteristic of hardness values is crucial for assessing the use of materials in the industry such as their suitability.
In addition, each test expresses its result in its unique measure due to unviability of the standard hardness scale. Moreover, the hardness of pliable materials such as rubbers and plastics is measured by using instruments such as durometer.
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2.7.1 Durometer
Figure 22: Schematic drawing of a hardness test by using a durometer instrument
Durometer is an instrument for testing the hardness of various plastic and rubbers by following ASTM D2240 (Standard Test Method for Rubber Property). In addition, the test method is able to cover twelve types of rubber hardness.
The concept of durometer instrument is similar to many other hardness tests which measure the depth of an indentation in the material that is created by a given force on a standardized presser foot. Moreover, the depth is dependent on the hardness of the material, viscoelastic properties, the duration of the test, and the shape sensing pin.
ASTM D2240 is used as a standard reference to do an accepted measurement method to measure the initial hardness or the indentation hardness after a given period of time. The basic part of the method is applying the force in a consistent manner, without shock. There are several important requirements such as the material under test should be a minimum of 6.44mm (.25inch) thick. In addition, reading below 10 and above 90 are not
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considered reliable and should be discarded by following per the ASTM Standard D2240.
Therefore, it is important to determine the appropriate scale that will provide results between 10-90 units.
Table 12: Type of durometer [114]
Durometer Type/ Common Materials Tested Durometer Hardness
Scale
Type A Soft vulcanized rubber, flexible 20-90 A
polyacrylics, leathers
Type B Moderately hard rubber, paper Above 90 A Below 20D
products, fibrous materials
Type C Medium-hard rubber, Above 90 B Below 20 D
thermoplastics, medium-hard
plastics
Type D Hard rubber, rigid thermoplastics Above 90 A
Type DO Moderately hard rubber Above 90 C Below 20 D
Type M Specific for measuring small cross- +
section O-rigs and thin pieces of
rubber (less than 1.25m thick)
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Type O Soft rubber, very soft plastics Below 20 DO
Type OO Extremely soft rubber Below 20 O
Type OOO Very soft materials +
Type OOO-S Viscoelastic polymers such as +
motorcycle seats, medical pads,
wheelchair cushions
Type CF Composite foam materials ASTM F1957
Type SL Foam rubber, sponge rubber
2.8 Friction
Friction can be defined as a resistance to motion that is experienced when a material is moving over another. In addition, friction can be divided into two different concepts, i.e. the static friction and the dynamic friction. Static friction concept explains that the friction force will increase with the increase of tangential displacement up to the value necessary to initiate the macro-sliding movement of the material against contact material [115].
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Graph 5: Tangential displacement (x-axis) vs friction force (y-axis) [115]
Static friction has been explored and observed a long time ago by Leonardo da Vinci (1452-
1519) and the static friction can be calculated as
µ = 퐹푓/푁 (Equation 12)
Figure 23: Forces acting on the material in sliding motion
The force that is preserving the body from sliding down on a tilted plane is the static friction force. The force that is required to initiate a sliding movement on the material is
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commonly called the maximum static friction force Fs. In addition, the force that is required to sustain motion is called the dynamic friction force Fd. Moreover, the coefficient of friction is defined as the tangent of the angle of the inclined plane that can be calculated as
퐹 푊.sin 훼 휇 = 푠 = = tan 훼 (Equation 13) 푠 푁 푊.cos 훼
Typically, the coefficient of static friction is larger than the dynamic friction. However, is it possible to have coefficient static friction equal to the coefficient dynamic friction.
2.8.1 Parameters
Static friction of rubber-rigid contact was not widely studied. However, there are various parameters such as pressure, micro-displacement, and roughness that have been discussed in several literatures.
2.8.1.1 Pressure
Barquins and Roberts [116] conducted an experiment of static friction between a flat rubber surface on glass lenses. Based on their experiment, the coefficient of static friction shows a decrease if the normal load increases. In addition, a relationship between normal load N and maximum static friction force Fs was obtained by Tarr et al [117] by conducting an experiment on a filled phenolic resin/cast iron material. From their result, a formula was obtained as
훽 퐹푠 = 휇푠. 푁 (Equation 14)
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where 훽 can vary from 1.03 to 1.41 for organic material such as phenolic resin that is reinforced with asbestos and filled with minerals and 1.1 to 1.37 for semi-metallic materials.
2.8.1.2 Roughness
The roughness of the material is one parameter that is important to consider due to its influence on static friction that is related to limiting displacement and the coefficient of static friction.
Bogdanovich [118] investigated the effect of the surface roughness on the limiting displacement between a polymer plate and a steel cylinder. Bogdanovich concluded that the limiting displacement decreases with an increase in average roughness of Ra of the steel surfaces. In addition, a correlation was founded between the surface roughness and coefficient of static friction of the counter body.
Graph 6: Effect of surface roughness [118]
The behavior of surface roughness was explained using the graph by using the adhesion and deformation components of friction. In region I, it is shown that the adhesion effects decrease when the Ra increases that was resulted due to the reduction of the number
81 and size of the asperities in contact which leads to a decrease in friction level. Moreover, in region II, the friction is increased together with Ra which leads to an increase in the coefficient of static friction and limiting displacement.
2.8.1.3 Micro-displacement
An experiment that is performed by Johnson [119] in the flat end of a hard steel roller with a steel ball to investigate the static friction that shows an increase linearly with preliminary displacement. Moreover, it is shown that the dependence becomes non-linear when the material is close to the point of sliding. Therefore, the increasing of the normal load will lead to an increase in preliminary displacement and static friction force.
Furthermore, another result from Bogdanovich [118] on steel cylinders against epoxy polymer plates.
Graph 7: Coefficient of static friction (μs) vs limiting displacement (δ_1) at various
pressure. 1) 1.55 Mpa 2) 4.5 Mpa 3) 6.5Mpa 4) 10 Mpa [118]
In the graph from Bogdanovich’s experiment, it is shown that at low temperature, the limiting displacement (훿1) increase linearly with the coefficient of static friction (휇푠),
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then becomes non-linear at higher pressure which has sima ilar result with Adachi et al
[120] in terms of the behavior of friction by using a rubber hemisphere loaded against a glass plate.
2.9 Silicone Rubber in Industry
The use of silicone rubber material can be vastly found in almost all industries. In the modern lifestyle, the importance of silicone rubber is undeniable due to its mechanical properties such as elongation, high tear strength, good thermal conductivity, and high- temperature resistance that can be applied for a different variety of tasks and has become to be widely used in essential products, appliances, and equipment. Moreover, it is particularly important for various specialized industrial processes, as well as for household purposes.
The first industrial research began in 1933 and the first patent describing a silicone elastomer was issued in 1943. From that movement, the study regarding silicone rubber became worldwide.
Nowadays, silicone rubber in the industry becomes very broad. Since silicone is easily manipulated and tailored, with high-temperature stability and age resistance, it often operates cooperatively with other materials to purify, adhere, insulate, or protect a product.
In addition, silicone-based material commonly is used to improve the appearance, transfer energy, and aid in reconstruction.
For the application where a product has to adhere to another, silicone can enable bonding between the materials. Whereas, when a coating and an adhesive layer need to
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remain independent of one another in specific applications, silicone can be tailored to restrain adhesion and keep each layer separate and prevent the adhesion. Due to the broad range of different form of silicone rubber, many industrial applications use it effectively such as in transportation, building and construction, medical, and electronics
2.9.1 Transportation
The transportation industry is very intensive in terms of using silicone rubber to their product. Automotive, mass transportation and aerospace-aircraft need a material that can deliver strength, resistance, and durability.
2.9.1.1 Automotive
In the automotive industry, consumers demand an increase in power and performance of their product that will increase of heat that is generated. Therefore, engineers will rely heavily on the quality and the performance of rubber materials to control the amount of heat in the automotive industry. Silicone rubber has been common to be used in this industry due to the inability of EPDM (ethylene propylene diene monomer) rubber that has a 140°C maximum temperature. With high-temperature resistance of silicone rubber from -60°C to 230°C and outstanding weathering properties, the silicone rubber can extend the service life of automotive assemblies.
Table 13: Common automotive silicone rubber applications
Use Purpose
EV Battery Seals Sealing
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Airbags Reduce the heat and protect the fabric
Engine Gaskets Protection from weather and heat
Headlamps Sealing
Ignition Cables Protection from heat and tear
HT Cables Insulation
Grommets Protection from chemical
Radiator Seals Insulation
Performance Hoses Protection from heat and tear
Vibration Dampening Reduce the noise
Absorb vibration and shock from the
Shock Absorbers energy
Spark Plug Boots Protection
Ventilation flaps Protection from weather
Cushioning Reduce the noise and vibration
Load Bearing Absorb the compressive force
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Vibration Suppression Reduce the noise and vibration
Protective Shock Absorption in Car
seats and Dashboards Absorb shock from the energy
Moreover, reinforced silicone rubber plays a pivotal role in helping tire manufacturers to obtain ideal tire balance and sustainable quality due to its ability to withstand high temperatures and pressures.
Since tires are made from natural rubber or polyurethane, it has a tendency to stick to the metal at the curing/molding process when they are pushed directly into a hot metal mold without a protective layer. In addition, talcum powder was commonly used as a mold release agent and this powder leaves white traces on the outside of the tire and showed unappealing tire optics. Therefore, a silicone film with its anti-stick properties and excellent surface slip is preferable. The silicone film is applied in a thin layer (5 to 10 microns) on the metal molds and allow the tires to be separated from the metal mold easily.
Besides the ability of being a release agent for the tire, the silicone film is considered important of silicone rubber to flow correctly into all the groves of the mold to ensure a perfect tire molding which intricately copies all the details of the mold to the tire sidewall and tread surface and the slip function will allow tire to effortlessly slide out of the hot tire mold.
In addition, silicone rubber is favorable to be applied as a coating layer in the process between a rubber bladder and tire. The resistance to heat and pressure will perform flawlessly to protect the tire from tire curing bladder and separate both rubber parts.
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2.7.1.2 Mass Transportation
In mass transportation application, a material such as silicone rubber is required due to the specific requirement for this application. For mass transportation, having a material that can reduce the vibration and noise, reliability, and fire safety are very demanding across this industry.
Due to that demands, silicone rubber that has outstanding durability, temperature resistance, and exceptional longevity is popular to be used. Moreover, silicone seals are distinguished for preventing moisture, water, and dust ingress which can reduce the risk of failure and meet the safety requirement including flame and smoke toxicity for the mass transportation industry.
2.7.1.3 Aerospace-Aircraft
Aerospace-aircraft industry is actively seeking an outstanding material that can withstand incredible stresses and temperature extremes. Due to the fluctuation in temperature experienced when an aircraft is in operation, from the hot temperature on the ground to freezing temperature at altitude, material such as silicone rubber that is able to cope with such change without compromising performance is needed. Moreover, silicone rubber is well known for its resistance against extreme temperature and aggressive aerospace fluids, including engine lubrication oils, hydraulic fluids, jet fuels, oxidizers, and rocket propellants. For applications in aerospace that demand outgassing or applications that exist in a vacuum, silicone rubber is a good material to choose due to the physical properties of silicone rubber that can remain the same, but volatiles is minimized.
Furthermore, silicone coatings have gained popularity in the aircraft industry for their
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effective ice-release characteristics [121]. In the events of ice buildup, silicone is able to remain elastic at low temperatures that can outperform other materials that are commercially available and marketed as ice-release materials.
2.9.2 Building and Construction
The use of silicone rubber in the construction industry was popularized in the early
1960s and started to become valuable material for the industry due to important properties such as their thermal stability, unusual surface properties, water repellency, high permeability, oxidative stability and ultraviolet resistance [122]. In the construction and building industry, silicones deliver three major technologies to the construction industry in the form of sealants, coatings, and water repellents.
Additionally, sealants are commonly used as a connection and movement joints to ensure important structural materials stay in place. The sealants also add the flexibility to the building structure that allows materials to absorb stress and movement caused by wind or earthquakes as well as preventing humidity and hot or cold air from coming inside through joints and cracks. Moreover, silicone coatings are used for mainly for applications where water repellency and low-temperature flexibility is desired. As well as providing excellent sealing properties from water, dust, and dirt ingress, silicone materials are widely used in vibration management of buildings due to the ability of silicone to absorb vibration, stresses, and movement in buildings, especially taller structures that are sensitive to movement from wind or ground vibrations.
Table 14: Specific properties and applications of silicone in the construction industry
[122]
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Product Properties Application areas
Durability Glazing sealants
Resistance to heat & cold Construction sealants (neutral cure
water systems)
Resistance to ozone and Sanitary sealants ultraviolet radiation Sealants
Adhesion to a variety of
substrates
High recovery
Color stability
Water repellency Masonry structures
Permeability to vapor Natural stones Water repellents Frost resistance Protection of concrete
Durability
Coatings Permanent flexibility Protection of monuments
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Ability to bridge small A weather-resistant coating for building
cracks materials
Resistance to weathering,
aging, radiation, ozone, and Rust and corrosion resistance coating on
temperature metallic rooves and tank surface, etc.
2.9.3 Medical Industry
Due to silicone rubber unique combination of thermal stability and low risk of unfavorable biological reactions, silicone rubber is very popular to be applied in high technology applications in the medical industry such as prostheses, materials for reconstructive surgery, contact lenses, intravaginal rings, and other implantable devices for drug delivery and urinary catheters. To increase the mechanical properties of the silicone rubber for the medical industry, it is very common to be produced by the hydrolysis process of appropriate silanes and then cross-linking. The method for crosslinking elastomers can be executed by the irradiation curing process [123], [124].
Folkman and Long [123], [125] proposed a delivery method of low molecular weight drugs such as digoxin by using silicone rubber. This method was performed by implanting the device into the myocardium of dogs for therapeutic purposes.
Moreover, silicone rubber has been active to be used for decades as a suitable material for substitution of stiffer tissues such as the nose, chin, ear, even a controversial application such as breast implant. The reason of using silicone rubber especially PDMS
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in this field is that the wide range of biomedical applications in silicone rubber that they have good blood compatibility, low toxicity, and good oxidative stability [126].
Furthermore, silicone rubber can be applied in drug delivery fields due to its suitable material as matrix, membrane, and coating that is important in drug delivery systems [127]. In addition, implantation in the medical industry is favorable to use silicone when the administration of a low dose of the drug over a long period of time is desired due to the ability of silicone rubber to be generated easily by using different methods such as extrusion, injection or molding.
2.9.4 Electronics
Nowadays, with the rapid technology improvement, silicone rubber has widely to be used in the electronic field due to its extreme diversity of silicone polymer chemistry and architectures for meeting technical challenges
The electronics industry is the industry that requires specific properties of the material to be able to resist to extreme temperature which silicone rubber is favorable due to its ability to be used at a temperature above 150 °C. In addition, silicone rubber is a good fit for electrical insulation applications due to their high dielectric strength.
In addition, silicone rubber is often to be used to seal the inner circuits and processors of most electronic gadgets, protecting them from heat, moisture, corrosion and various conditions that can cause wear and tear. In the cable industry, silicone rubber is favorable to be used due to the excellent flexibility, chemical resistance and good fire resistance that made them suitable for fire situations.
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2.10 Silicone Rubber Product Failure
Silicone rubber is widely manufactured for different applications which failure of the product has to be considered to maximize the material’s properties for a specific purpose. For the manufacturer, it is important to minimize the number of defective products and random failures through careful quality control and thorough testing at the design and prototypes stage to reduce the failure rate. Three basic requirements in failure analysis such as correct observation of evidence and gathering of facts, a logical sequence of measuring, reasoning, and deduction, knowledge of material properties and behavior are important to be performed in order to avoid failures in the end-product.
In addition, the basic reason of why a product such as silicone rubber can fail is because of the error in design, inappropriate material, manufacturing fault, incorrect installation, unexpected service conditions, and deliberate or accidental misuses [128].
Besides that, during its lifetime, silicone rubber will be exposed to one, or usually several degradation agents (table 15).
Table 15: Type of degradation agents [128]
Agent Type of aging or effect
Light Photo-oxidation
Temperature Thermo-oxidation, additive migration,
crosslinking
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Ionizing radiation Radio-oxidation, crosslinking
Bio-organisms Decomposition, mechanical attack
Humidity Hydrolysis
Fluids (gases, liquids, vapors) Chemical degradation, swelling, cracking
Mechanical stress Fatigue, creep, stress relaxation, abrasion,
adhesive failure
Electrical stress Local rupture
Moreover, during its lifetime, a silicone rubber product will be exposed to several degradation agents and the contribution of the degradation agents above is often complicated to evaluate as they are not related to each other. Furthermore, the properties of silicone rubber will change at different rates due to the environment such as fatigue life that often degenerates more rapidly than tensile strength.
2.10.1 Temperature
Temperature is one of the main factors of silicone rubber’s failure. Low temperatures can cause stiffening and eventually brittleness which is considered the prime of the cause of failure in some applications. In addition, repeated temperature cycling such as contraction and thermal expansion are a reversible short-term effect of temperature that can be very important in some applications. Furthermore, oxidative degradation can occur at a high temperature that is considered a vital problem in the use of silicone rubber at high
93 temperature. By using silicone rubber at inappropriate service conditions, thermal degradation will occur and lead to the failure of the silicone rubber.
Moreover, the performance of the silicone rubber will be affected if the material is applied to extreme conditions without considering the maximum continuous use temperatures of the silicone rubber. Wright [129] developed a list of maximum continuous use temperature (MCUT) for several elastomers to show the importance of temperature in rubber failure.
Table 16: Universal maximum continuous use temperature for various rubbers
[129]
Material Designation MCUT(°C)
Bromobutyl BIIR 120
Butadiene 60
Butyl IIR 100
Butyl (resin cured) IIR 130
Chlorinated PE CPE 120
Chlorobutyl CIIR 120
Chloroprene CR 90
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Chlorosulphonyl CSM 120
Ebonite 80
Epichlorohydrin CO 130
EPDM (ulphur cured) EPDM 120
EPDM (resin cured) EPDM 150
Ethylene vinyl acetate EVM 110
Ethyl acetate ACM 150
Fluoroelastomer FPM 210
Fluorosilicone FVMQ 200
Isoprene IR 60
Natural rubber NR 60
Nitrile ( <20% I) NBR 110
Nitrile ( >20% I) NBR 120
Nitrile/PVC polyblend PNBR 90
Nitrile (carboxylated) XNBR 110
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Nitrile (hydrogenated) HNBR 150
Perfluoroelastomer FFKM 260
Styrene-butadiene SBR 70
Urethane (ester) AU 75
Urathane (ester) EU 75
2.10.2 Fluids
Fluids such as chemicals in the form of both liquids and gases can interact with the silicone rubber in several ways in the applications. In addition, there are several forms on how the fluids will affect the performance of the silicone rubber. Common phenomenon such as fluids can be absorbed and develop swelling process into the rubber will lead to failures in the material due to soluble constituents of the material got extracted or direct chemical effects that generate catalyze oxidation in the material.
The situation of silicone rubber that is exposed to liquids can be tested by swelling tests that measure the change in volume. In addition, the volume change is an important measurement for general resistance of rubber to a liquid. By having a high degree of swelling indicates that the material is not suitable for use in that specific environment.
There are several instances where failure was the result of a silicone rubber being used in a condition that the material was not sufficiently resistant to liquids encountered.
A specific form of this problem is when the liquid in the applications becomes
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contaminated. For example, water stops swelled because there was an oil leakage during flooding.
In gases form, the number of applications is involved is relatively small compared to liquids form. However, when the high pressure is involved, it is required for engineers to have an extra consideration due to the possibility of explosive decompression because of absorbed gas rupturing the silicone rubber when the pressure is released.
2.10.3 Weathering
Weathering is considered as exposure to the atmosphere that includes sunlight, temperature, precipitation, ozone and any pollutants in the air. It is important for engineers to understand the intensity and spectral distribution of sunlight that will have direct contact with the material. By having different intensity and spectral distribution of sunlight variations based on the location, time of year, atmospheric conditions, and the angle of the sun; the resistance of the material will vary that cause difficulty in predicting weathering performance from accelerated tests.
Moreover, the effect of light on silicone rubbers has generally been considered to be much less important than it is for plastics. However, failures in rubber attributed to UV light has to be considered. The effect of the UV light itself is not generally serious such as a color change in the material. However, the temperature reached by exposure of UV light in outdoor applications can be surprisingly high and cause a much greater effect with consideration if the silicone rubber is in an insulated backing. Therefore, based on the aspect of the product, the lifetimes of the material could be expected to be very different.
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2.10.4 Ionizing Radiation
Atomic and nuclear particles such as gamma rays, electrons neutron are considered as ionizing radiation. The intensity of ionizing radiation at the earth’s surface will not affect the silicone rubber. However, radiation exposure in nuclear plant applications has to be considered and the possibility of any applications that use radiation to induce crosslinking for sterilization purposes.
In addition, several types of radiation have a unique characteristic of interacting with the material and transferring its energy. For example, alpha radiation has less significant power and to the surface layers of a material and can be considered when a surface is contaminated by an alpha emitter. On the other hand, gamma radiation and neutrons are very penetrating that is common to be used in sterilization of medical device.
Table 17: Gamma radiation resistance of a range of various rubbers [130]
Rubber Type Insignificant damage (radiation dose, Gy)
Butyl rubber Up to 10,000
Acrylic rubber Up to 100,000
Silicone rubber Up to 100,000
Chlorosulphonated rubber Up to 100,000
Nitrile rubber Up to 100,000
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Fluorocarbon rubber Up to 100,000
Polychloroprene rubber Up to 100,000
Styrene-butadiene rubber Up to 500,000
Ethylene-propylene rubber Up to 500,000
Polyurethane rubber Up to 500,000
2.10.5 Biological Attack
A biological attack is considered a rare factor in silicone rubber material even though silicone rubbers or other additives can prove the attraction to living organisms. In tropical countries, attack by insects or other animal is likely to be a serious problem.
Moreover, there have been several concerns in Europe with rubber pipe seal product that are suspected to fail due to microbiological degradation.
2.10.6 Fatigue
Fatigue is one of the main factors of failure in silicone rubber product and can occur when the product only has little or no service lifetime. In addition, a mechanical failure such as fatigue occurs after a certain period of time as a result of environmental degradation. In several applications when the material has to perform under repeated cyclic deformation will result in a change in stiffness, a loss of mechanical strength and will ultimately lead to a rupture phenomenon. Moreover, fatigue can be accelerated by thermal
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degradation and oxidation and the manner of the degradation will vary depending on the several variables such as geometry used, type of stress, and the environmental conditions.
Most common fatigue situation that occurs in silicone rubber can be divided into two types. The first type is when the repeated cyclic deformation leads to heat buildup in the material which causes heating of the material. Furthermore, the second type is where cracks are induced and propagated without any significant heating phenomenon that is commonly called “flex cracking”. For the first type, the fatigue situation is commonly occurring with a thick or bulky object such as a tire. On the other hand, the second type of fatigue situation is usually to be found in any types of product that is repeatedly stressed.
When the heat buildup comes to play, there is a change in stiffness of the material and lead to rupture. This condition can be avoided by measuring the temperature that is likely to be reached through dynamic stretching to avoid overload heating.
Furthermore, fatigue is often to be the cause of failure in bonded joints which is relatively easy to spot by performing quasi-static tests to determine good or bad bonds.
2.10.7 Set, Stress Relaxation and Creep
Failure from the set, stress relaxation and creep are often to be found in seals and gaskets applications since the most critical attribute for material in this application is the ability to maintain the sealing force (stress relaxation) instead of the ability to recover (set).
In addition, majority failure that occurs in seals is due to the loss of sealing force compared to loss due to mechanical strength. Brown [131] discovered that in mild temperature, the effect of aging may be small considerable set or stress relaxation will occur through
100 physical processes. Moreover, the research has been conducted by the Rapra long term aging programme that shows thermal aging was considered small for various materials even after 40 years of exposure. However, the environmental conditions especially temperature and presence of fluids are huge factors that influence the degree of stress relaxation, set, and the useful life of the material.
2.10.8 Abrasion
Abrasion is clearly an important factor in the failure of specific silicone rubber such as shoe soles and rubber flooring. The mechanism of abrasion is when there is a moving contact between two or more materials and principal factors such as cutting, and fatigue is involved. Moreover, there are a couple of categories of wear including abrasive wear, fatigue wear, and adhesive wear that have different mechanisms from each other. In some applications, there will be more than one mechanism of wear involved but one may predominate.
Abrasive wear requires sharp, hard cutting edges and high friction. Furthermore, fatigue abrasion occurs with a blunt and rough surface and does not require very high friction. In addition, adhesive wear is considered less common mechanism but can occur on a smooth surface. Based on those three different mechanisms of wear, the rate of wear can change quite sudden based on several parameters such as contact pressure, speed, and temperature.
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2.10.9 Electrical Stress
Electrical stress is only needing to be considered as a factor that causes silicone rubber only when the applications is related to electrical. Mostly short-term failure is caused due to flaws in the product assuming that the material is not properly evaluated for specific conditions. Therefore, it is very critical for industrial production to have a product 100% proof tested.
On the other hand, in the longer term of use, the electrical stress can exceed the breakdown level if the material contains local flaws or voids. For example, water treeing that use polyethylene insulated cables in wet conditions will form fine channels due to water diffuses into the insulation
2.11 Existing of Reinforced Silicone Rubber
With continuous research development in reinforced silicone rubber, there has been a various breakthrough in reinforced silicone rubber. Sui et al [132] developed a novel fiber-reinforced silicone rubber composite with Al particles in order to enhance the dielectric and thermal properties of the material. In his study, by using surface modification method, the silicone rubber was filled with several Al such as spherical Al, flaky Al, and spherical Al that was treated by a coupling agent.
Moreover, Kumar et al [133] studied the effect of thinner on RTV silicone rubber composite that is reinforced by graphitic nanofillers(GR) and carbon nanotubes (CNT) in order to increase the stiffness properties of the material. In order to prepare the nanocomposite, the RTV silicone rubber was normalized in a homogenizing machine for
102 30 minutes and mixed thoroughly with CNT and GR hybrids for 15-20 minutes to generate a reinforced silicone rubber specimen.
In addition, with the rapid technology development, reinforced silicone rubber has been discovered and promoted to end-consumer directly or industrial to achieve specific properties in their end-product. Reinforced silicone rubber is very common to be found in the shape of a sheet that offers high tensile strength and dimensional stability. A thin silicone coated fabric, EMI sheet, chemical resistant PTFE and fluorosilicone sheets have engineered a material that popular to be used both for end-consumer or industrial.
Furthermore, fiber-reinforced liquid silicone rubber has been developed as a new generation of adhesives and sealants material that has high modulus compared to normal silicone rubber which suitable for applications where a silicone elastomer is required to withstand pressure and high standard safety requirement in the part.
In addition, fiberglass reinforced silicone hoses are introduced widely due to the race-proven performance for a superior temperature range from -65-degree F to 392- degrees F. Besides that, the reinforcement in the hoses increase the strength and reliability of the product that will not easily swell or burst under extreme conditions.
103 CHAPTER III
EXPERIMENT WORK
3.1 Materials
The polymer used in this study was a polyether-based thermoplastic polyurethane
(TPU) and the room temperature vulcanized (RTV) silicone rubber. RTV silicone rubber was used as the matrix material and reinforced by TPU. The composite materials were prepared and the effect of TPU reinforcement on the RTV silicone rubber was assessed.
3.1.1 Thermoplastic Polyurethane (TPU)
Thermoplastic polyurethane is an elastomer that is fully thermoplastic, elastic and melt-processable that are commonly processed on extrusion as well as injection, blow and compression molding processes.
In this study, Estane 58315 TPU was supplied by Lubrizol with thermal properties
-50 °C to 135 °C, the hardness of 82-88 A (ASTMD2240) and tensile test 7,010 psi (ASTM
D412) was used as a reinforcement material processed by electrospinning process.
3.1.2 RTV Silicone Rubber
Liquid silicone rubber (two components, Type A and B ratio are 1:1 by volume) in this study was provided by Smooth-On with their product, OOMOO 30 which has 30
104 minutes pot life and six hours cure time with a shore A hardness at 30A and tensile strength at 240 psi (ASTM D412).
3.2 Electrospinning of Thermoplastic Polyurethane (TPU)
TPU Estane 58315 were prepared with 15 wt% by dissolving TPU in
Dimethylformamide (DMF) and Tetrahydrofuran (THF) with a ratio of 1:3 respectively.
The samples were electrospun at 0.5ml/h under an applied voltage of 18kV and 23% humidity with a downward electrospinning setup contained several parts which the needle that is connected was positioned 15cm from the collector i.e. aluminum plate. The container was purposely designed in circular shaped inscribed on squares collector in order to increase the efficiency of nanofibers to be generated by exposing four sides of collectors.
Figure 24: Downward electrospinning setup
105 3.3 RTV Silicone Rubber Preparation
The RTV silicone rubbers were purchased from Smooth-On Company arrived in two components (part A and B). Due to short pot-life of the silicone rubber, part A and B were poured together with ratio 1:1, measured by the weight and then placed in a container and placed in an electrospinning setup.
Figure 25: RTV silicone rubber Part B and A respectively
3.4 Composite preparation
In composite preparation, the prepared RTV silicone rubber was placed in an electrospinning setup and the 1-step electrospinning process successfully generated TPU nanofibers directly to the RTV silicone rubber.
106
Figure 26: TPU reinforcement by electrospinning process
After the electrospinning process, the RTV silicone rubber was mixed thoroughly and measured by weight to calculate the ratio between the matrix and reinforcement phase in the composite.
In addition, the mixture of silicone rubber and TPU were placed in a vacuum oven in order to reduce the number of bubbles and pores in the specimen. Moreover, the mixture of the composite was poured in a specific mold container made by 3D Printing of ABS thermoplastic material by University of Akron 3D Print Lab to meet the specification in length, width and thickness of the material.
107
Figure 27: 3D printing mold container
3.4 Rolling Resistance Test
In performing rolling resistance in the lab scale, the device used to measure rolling resistance was inspired by Gent in his patent [1]. A wooden roller was designed for the purpose as shown in figure 28.
Figure 28: Wooden rolling resistance measurement design
The wooden roller was set on a platform to measure the rolling resistance and calculate the rolling force by following the formula as
퐹푟 = 푚푔퐿(cos 휃푒푣푎푙 − cos(휃푠푡))/푑 (Equation 15)
108
where 휃푒푣푎푙 is the angular position of the pendulum bar after N oscillations, 휃푠푡 is the angular position of the bar when the pendulum is released, and d is the distance covered in
N cycles.
Graph 8: Example of time vs displacement collected by using motion sensor during
resistance measurement
109
3.5 Static Friction Test
Figure 29: Static Coefficient of Friction Test by using PTFE
A coefficient of static friction test was performed by using PTFE by following standard ASTM D4918 when an inclined plane is increased at a specific rate per second by an electric motor until the test block (PTFE) begins to slide. In addition, the initial movement of the sled will become the slide angle or coefficient of static friction of the specimen. Several factors such as abrasion, coatings, varnishes, printing, and humidity will affect the result.
3.6 Hardness Test
Hardness test was performed by following standard ASTM D2240 type A scales for the specimen that was molded with a thickness of 6mm for the hardness test purpose.
In addition, e-Asker Type – A Durometer SUPER EX was selected and used to develop an indentation to the specimen with an equivalent pressure.
110
3.7 Scanning Electron Microscope (SEM) Test
The sample of the specimen are sputter-coated with silver by using K575x, Emitech for 1 minute and 30 seconds at 55mA then a scanning electron microscope (SEM) test was performed in order to examine the surface topography of the material.
Figure 29: Scanning Electron Microscopy
111
3.8 Tensile Test
Figure 30: Instron E300
A tensile test was performed by using Instron E3000 in order to measure the load applied to the material to obtain a stress-strain curve and observe the change in the strength of both non-reinforced and reinforced nanomaterial. The test was performed with a frequency of 5Hz and 6mm/min for the strain rate.
112
CHAPTER IV
RESULT AND DISCUSSIONS
Different types of samples were analyzed on how their performance in the hardness, rolling resistance, static friction and tensile test.
Table 18: Classification of samples
Sample List Description
Sample 1 RTV silicone rubber without any reinforcement
Sample 2 RTV silicone rubber reinforced by 15 wt% TPU in 10 minutes
Sample 3 RTV silicone rubber reinforced by 15 wt% TPU in 20 minutes
Sample 4 RTV silicone rubber reinforced by 15 wt% TPU in 30 minutes
Sample 5 RTV silicone rubber reinforced by 15 wt% TPU in 60 minutes
Sample 6 RTV silicone rubber reinforced by 15 wt% TPU in 120 minutes
The amount of polymer distributed into the matrix can be calculated in order to determine the fiber density inside the silicone rubber matrix. The comparison study was
113
performed and discussed between non-reinforced silicone rubber with reinforced rubber with various fiber density by increasing the duration of the electrospinning process.
4.1 SEM Analysis of Thermoplastic Polyurethane
The morphology of thermoplastic polyurethane Estane 58315 was analyzed using
SEM and Amscope (Figure 33&34)
Figure 31: SEM Picture of 10 wt%(a), 15wt% (b) 20 wt% (c)of TPU Estane 58315
Figure 32 above shows that nanofibers of TPU were successfully fabricated by using the electrospinning process. By controlling parameters such as applied voltage, the viscosity of the solution, the distance between the needle and the collector as well as the
114 solvent selection, TPU with a concentration of 10 wt% showed a collection of fibers with thickness averagely in 1.5 μm with several beads exposed. Moreover, TPU with a concentration of 15 wt% showed a uniform and smooth nanofiber with a diameter of approximately 2.5 μm. Furthermore, TPU with a concentration of 20 wt% showed a diameter approximately 4 μm. However, the solution at 20 wt% showed a varied diameter which leads into the inconsistency of the nanofiber. In addition, it was shown that by increasing the concentration of the solution, the diameter of a nanofiber can be increased.
Fiber
(a) (b)
Figure 32: Non-reinforced silicone rubber (a) and TPU reinforced silicone rubber
Based on figure 33 above, non-reinforced silicone rubber and TPU reinforced silicone rubber were characterized by Amscope. In figure 33 (a), the non-reinforced silicone rubber showed a smooth surface with little porosity in the silicone rubber due to poor sampling preparation. On the other hand, figure 33(b) showed a reinforced silicone rubber that contains randomly oriented nanofibers inside the silicone rubber matrix. The crystal color that was observed in figure 33(b) is the reinforced fibers bonded to the rubber matrix.
115 In addition, several porosities were observed which leads to a defect in the material.
The presences of defect will affect the performance of the material in strength and fracture properties. The porosity is occurred due to uneven mixing and pouring process into the mold that led to bubble development inside the specimen. In order to reduce the number of bubbles inside the specimens, a vacuum oven was utilized.
Fiber
Rubber
Figure 33: Surface characterization of reinforced silicone rubber
In figure 34, the reinforced silicone rubber showed several nanofibers that were exposed to the surface of the material that developed a rough surface in nanoscale level but soft and smooth when it was observed by the naked eye.
116
The inclusion nanofibers on the surface will be observed further in static friction test in order to investigate the roughness of the nanocomposite.
Figure 34: Nanofiber in the silicone rubber matrix
Since the strength of nanofibers is higher than the silicone rubber, the nanofibers were assumed to improve the mechanical properties of the nanocomposite. During the pull test, it was shown that the nanofibers carried most of the load until it breaks shown in
Figure 35. The fiber density of the nanocomposite increased by extending the duration of
117
electrospinning process. The nanofiber distribution was enlarged and diffused throughout the rubber matrix resulting in mechanical properties improvement.
4.2 Hardness Test of Reinforced RTV Silicone Rubber
Table 19: Hardness test comparison between samples
Sample Fiber Density (g/cm3) Type A Durometer (Shore-A)
1 0 30
2 2.1 x 10-3 40.5
3 4.3 x 10-3 41
4 6.4 x 10-3 43
5 13 x 10-3 45
6 26 x 10-3 47
Thermoplastic polyurethane is considered a material that has higher hardness value compared to the silicone rubber and this will contribute to the increase of the composite hardness compared to silicone rubber without reinforcement. Furthermore, advantages of nanofibers that are generated by electrospinning such as has a larger surface area and intimate contact with silicone rubber is strong also become factors of the improvement in the hardness of the material since the material with reinforcement are harder to impede.
118
Graph 9: Hardness of prepared samples
Both the silicone rubber samples with and without reinforcement were indented by a “Durometer A” type indenter following the ASTM standard D2240 measuring the resistance of the material towards the indenter. The hardness was recorded and plotted in graph 9.
In addition, the test was conducted by taking the average of measurement in 10 different specimens for each sample. Graph 9 above shows that the average of rubber without reinforcement was obtained at 30A. For specimen number 2, the silicone rubber was reinforced by TPU for 10 minutes and the hardness test value was obtained at 40A.
Therefore, by increasing the fiber density inside the composite, the significant improvement of hardness was achieved since in silicone rubber, the hardness of the
119
material is related to the degree of crosslinking as well as the amount of filler and plasticizers. Therefore, by increasing the amount of filler in the matrix, the hardness of the silicone rubber will be increased that was confirmed by steadily increased shown in graph
9.
By increasing the duration of electrospinning process to reinforce the specimen by an increment of 10 minutes for sample number 3 and number 4, a steady increase was obtained and recorded at 41A for sample number 3 and 43A for sample number 4 due to the increment of the fiber density. In addition, for sample number 5, the duration of the electrospinning process was doubled from 30 min to 60 min to observe the increase of the hardness. The hardness for sample number 5 was obtained at 45A. For sample number 6, the duration of electrospinning 120 min was performed, and the result was obtained at 47A.
Furthermore, the improvement of the hardness was obtained from this research due to the mechanism of deformation when the force was applied in order to indent and measure the material resistance, the force that was applied will create deformation inside the material and move to the force direction. Moreover, at some point the deformation will face the inclusion of nanofiber inside the material and since the nanofiber has a higher value of hardness, the resistance of the nanofiber material
4.3 Static Friction Test
A static friction test was performed by following standard ASTM D4918 in order to measure the angle of repose when the mass initially slide. Six samples were prepared with different amount of fiber density. For each sample, ten specimens were tested and the average of static friction coefficient of each sample was collected.
120
Table 20: Static friction coefficient comparison between samples
Static Sample The Angle of Repose Fiber Density (g/cm3) Friction Number (degree) Coefficient
Sample 1 0 42 0.9004
Sample 2 2.1 x 10-3 60 1.73205
Sample 3 4.3 x 10-3 72 3.07768
Sample 4 6.4 x 10-3 75 3.73205
Sample 5 13 x 10-3 84 9.51436
Sample 6 26 x 10-3 85 11.4301
Graph 10: Static friction coefficient of prepared samples
In the initial test, sample number 1 was tested by measuring the angle of repose right at the time when the mass (Polytetrafluoroethylene / PTFE) started to slide. By increasing the
121
angle of repose, the PTFE mass started to slide averagely at 40 degrees. In order to avoid gradually wear abrasion that was not discussed in this thesis, each test was performed for
1 specimen only. The result was observed that in this case, the sliding friction was mostly caused by weak electromagnetic forces between molecules that are called Van der Waal bonds where if two electrically neutral, then non-polar atoms are brought very close together, they induce electric dipole moment between them and attract to each other. Due to this mechanism, the two objects with surfaces touching to each other will attracted in molecular size and increase the static friction coefficient and make it hard for PTFE to slide along the non-reinforced rubber.
The next sample, number 2, the silicone rubber with TPU reinforcement were tested with 2.1 x 10-3 g/cm-3 and obtained the angle of repose higher compared to non-reinforced silicone rubber at 60 degrees in average. This result was caused due to the small nanofibers that were exposed on the surface of the nanocomposite that was observed during Amscope characterization.
By having the nanofibers that were exposed to the surface of the nanocomposite, the roughness of the material is increased in nanoscale level. The roughness of the nanocomposite that was developed due to the inclusion of the TPU on the surface of the silicone rubber influence the limiting displacement between the PTFE and the nanocomposite that affect the increase in the static friction coefficient of the silicone rubber with was reinforced with TPU.
Furthermore, by increasing the fiber density in the silicone rubber matrix, the increase of the static friction coefficient was obtained. Due to the advantages of nanofiber
122 of having a high surface area, the increase in contact area was expected and the significant increase was obtained from 40 degree for non-reinforced rubber to 85 degree for reinforced rubber with 26 x 10-3 g/cm3 fiber density.
4.4 Rolling Resistance Test
It is very common to have a rolling resistance coefficient lesser than static friction coefficient due to the differences in the surface of contact which rolling resistance coefficient has a smaller surface contact compared to static friction. Moreover, the rolling resistance will depend on the deformation of the body that related to its rigidity which can be confirmed by having less pressure in the tire will lead to high friction and energy consumption. On the other hand, the overly high-pressure tire will affect the traction performance due to less surface area.
123
Graph 11: Non--reinforced silicone rubber (BLUE) vs reinforced silicone rubber
(PURPLE)
Based on the graph above, a damped oscillation of pendulum was recorded and compared. The oscillating system experienced a decrease in amplitude because of rolling resistance friction in the material. Based on the damped oscillation graph above, the displacement, a number of cycles and the damping ratio can be found.
Furthermore, the rolling resistance calculation can be obtained by integrating the concept of the potential energy of the pendulum where
푃퐸 = 푚푔퐿 ( 1 − 퐶표푠 (휃)) (Equation 16) and when the 휃 = 90°, the pendulum will be located at its highest point and energy potential will be 0. By understanding this energy potential concept, the force of rolling resistance (퐹푟) can be calculated by
퐹푟 = 푚푔퐿 (푐표푠( 휃푒푣푎푙) − 푐표푠(휃푠푡))⁄푑 (Equation 17)
124
where the 휃푒푣푎푙 is the angular position of the pendulum bar after N oscillations, 휃푠푡 is the angular position of the bar when the pendulum is released, d is the distance covered in N cycles.
In addition, the force of rolling resistance is the total energy potential of the pendulum divided by the distance that covered in N cycles
∆퐸 퐹 = (Equation 18) 푟 푑 and the rolling resistance coefficient of the material can be calculated by
퐹 퐶 = 푟 (Equation 19) 푟 푁
Table 21: Rolling resistance coefficient data of prepared specimens
Fiber Density Rolling Resistance Specimen (g/cm3) Coefficient
1 0 0.00193
2 2.1 x 10-3 0.00186
3 4.3 x 10-3 0.00175
4 6.4 x 10-3 0.0015
5 13 x 10-3 0.00126
6 26 x 10-3 0.00099
125
Graph 12: Rolling resistance coefficient graph of prepared specimens
Rolling resistance coefficient was calculated and plotted in graph 12 above. For sample 1, the rolling coefficient of the material was obtained at 0.00193. Since silicone rubber is considered as soft rubber, the deformation of the material will affect the rolling resistance coefficient which the deformation of the material leads to the energy lost.
In addition, sample 2 that contains 2.1 x 10-3 g/cm3 fiber density was tested and the decrease of rolling resistance coefficient was observed at 0.00186. By increasing the fiber density of the nanofibers, the rolling resistance of the material was steadily reduced.
This phenomenon occurred due to the stiffness of the material was increased. By infusing the nanofibers into the matrix, the modulus of the material increases to the less deformation of TPU nanofibers compared to the silicone rubber. Moreover, by increasing
126
the stiffness of the material, the energy loss due to deformation was decreased which leads to fuel efficiency in the tire.
Moreover, rolling resistance of the tire material is affected by bending and compressive deformation that will result in viscoelastic energy loss in the material. This energy loss in bending is related to the loss modulus E’’ and loss compliance C’’.
Therefore, the best correlation between tire rolling loss and loss tangent can be calculated as
퐸′′ 푡푎푛 훿 = (Equation 20) 퐸′ where E’ is the storage modulus of the tread compound.
4.5 Tensile Strength Test
A tensile strength test was performed at 6mm/min strain rate and the data was collected in 5Hz (5 points/ second) to observe the changes of the material properties and compared the difference between non-reinforced rubber and reinforced rubber based on the stress strain curve obtained from the test.
127
Figure 35: Tensile Test of Specimen
The specimens were prepared with 6mm thickness with small notch 0.3mm to ensure crack initiation in the center of the specimen. Samples with a variety of fiber density were tested observing the effect of the fiber on the mechanical properties of the material.
The nanofibers were exposed when the crack occurred during the tensile test as shown in Figure 36. Since the strength of silicone rubber is much lower than the nanofibers, the crack propagated through the matrix first causing the majority of the load to be carried by the fibers. Therefore, it can be concluded that the TPU nanofiber reinforcement increases the strength of the material before the break.
128
From the tensile test, data were presented in a stress-strain curve comparing non- reinforced silicone rubber to reinforced silicone rubber (Graph 13).
Graph 13: Stress-strain curve non-reinforced vs reinforced silicone rubber
Based on the graph above, the strength and stiffness of the material increased as the fiber density of the material increased. By comparing the reinforced to non-reinforced material, the stress to rapture was increased by 23% for reinforced material as shown in
Graph 13. This phenomenon explains that reinforced material absorbed higher energy to reach higher stress. On the other hand, the strain obtained by reinforced material was lower than that obtained in non-reinforced material that clearly justifies that the increase in nanofiber density improved the material stiffness and strength based on the graph.
In addition, the toughness of the reinforced silicone rubber was increased which is the area under the curve (Graph 13) of the reinforced silicone rubber was higher compared
129
to non-reinforced silicone rubber. Therefore, as toughness went up, the ductility of the reinforced silicone rubber went down compared to non-reinforced silicone rubber. This can be verified from the graph which for non-reinforced silicone rubber. The curve shows a longer strain compared to reinforced silicone rubber that leads to higher ductility.
Furthermore, for the non-reinforced silicone rubber, both elastic and plastic region were seen. However, by increasing the nanofiber density in the material. The material behaved mainly as brittle material since the fibers were carrying the majority of the load which leads to the disappearance of the plastic region in graph 13.
Moreover, the elastic modulus of the material can be determined based on the slope of the stress-strain curve. The reinforced silicone rubber has higher elastic modulus compared to non-reinforced silicone rubber. Because of higher elastic modulus from the reinforced silicone rubber leading to higher stiffness due to the amount of stress required to elastically deform the material.
Therefore, the tensile strength verified the statement that the TPU nanofibers reinforced improved in several mechanical properties of the silicone rubber in strength, hardness, and stiffness.
130
CHAPTER V
CONCLUSIONS
5.1 Conclusions
After analyzing and comparing several composite samples that were prepared, it is possible to conclude the following:
• Electrospinning is a low-cost and efficient method to fabricate nanofibers and
successfully implemented in the research in order to generate nanofibers directly
into liquid silicone rubber.
• Nanofibers of thermoplastic polyurethanes, specifically Estane 58315, has been
successfully generated with specific controlled parameters.
• The composite of RTV silicone rubber with TPU as a reinforcement agent showed
a unique hardness result that behaves correspondingly with the strength of the
material with a different ratio of reinforcement agent. It is shown that by increasing
the fiber density of TPU in the nanocomposite, the hardness of the material was
able to be improved from 30A to 40A.
• The static friction coefficient of the composite surface has been studied that the
random-oriented nanofibers in the composite increased the roughness on the
131 • surface of the material in nanoscale that increased the static friction coefficient up
to 11.43 against PTFE.
• The rolling resistance of the reinforced RTV silicone rubber was successfully
measured by using a crafted wooden roller that shows a comparison result between
RTV silicone rubber with reinforced RTV silicone rubber. A steady decrease of
rolling resistance coefficients was recorded as well as a confirmation of
improvement in stiffness in the material due to less deformation behavior in the
material.
• The tensile test was performed to verify the effect of inclusion of the TPU
nanofibers in the material that could improve several mechanical properties of the
material such as strength, hardness, and stiffness of the material
5.2 Future Work
With the development of promising RTV silicone rubber containing TPU as a fundamental reinforcement agent, the research has introduced a solid initial foundation to investigate further as well as to incorporate the product into commercial applications. In addition, continuous research is important to be held to develop various possibilities of material development for humankind. Knowing that electrospinning can be performed in several methods, the fiber infusion process can be studied and applied with several electrospinning processes such as needless electrospinning process for mass production goals to reach commercial applications. Furthermore, the adhesion bond between the silicone rubber and TPU nanofibers has to be investigated further. By increasing the concentration of TPU, the diameter of nanofibers will increase and further research on how
132 diameter affect the performance of the material in rolling resistance and friction will need to be investigated.
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